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Hydrogen Bonding-Controlled Photoinduced Electron and Energy Transfer

  • Yu-Zhe ChenEmail author
  • Li-Zhu Wu
  • Chen-Ho Tung
  • Qing-Zheng YangEmail author
Chapter
  • 1k Downloads
Part of the Lecture Notes in Chemistry book series (LNC, volume 88)

Abstract

Photoinduced electron and energy transfer processes play an important role in photosynthesis and optoelectronic conversion. The investigation of photoinduced electron and energy transfer, wherein donor and acceptor are assembled via noncovalent interactions, has attracted much interest. Among these noncovalent interactions, H-bonding interaction has emerged as a powerful tool to construct high array of supramolecular architectures owing to their tunable binding constant, high directionality and selectivity. This chapter provides an overview of the recent developments in H-bonding-based energy/electron transfer. Advances in this research field, including (1) the basic theories for energy transfer and electron transfer processes, (2) recent progresses of energy transfer and electron transfer based on H-bonding, (3) their applications in constructing optoelectronic devices, light-harvesting systems, wide-range color display, and storage materials, are presented.

Keywords

Electron Transfer Energy Transfer High Occupied Molecular Orbital Lower Unoccupied Molecular Orbital Electron Transfer Reaction 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

1.1 Introduction

Photosynthesis is a natural energy conversion system that converts solar energy into chemical energy, the primary processes of which are a cascade of photoinduced excitation energy transfer to the reaction center (RC) and the subsequent rapid electron transfer to generate charge-separated state. Over the past decades, a series of significant results have been obtained for the study of photosynthesis: (1) The structure of the protein subunits in the photosynthetic RC has been ascertained. The RC consists of a pair of chloroplast–protein complex and various quinone derivatives with electron transfer chains, which are located in the grana membrane. Its periphery is the light-harvesting (antenna) complexes made up of a large number of protein-embedded pigments of chlorophyll and carotenoid [1]. Deisenhofer, Huber, and Michel were awarded jointly by the Nobel Prize in Chemistry in 1988 “for the determination of the three-dimensional structure of a photosynthetic reaction centre.” (2) Research revealed that photosynthesis initiates with the absorption of light by pigments bound to the antenna, which is transferred over nanometer distances to RC through efficient long-distance energy transfer, where it is converted into chemical energy via charge separation produced by rapid electron transfer [2, 3, 4, 5, 6, 7]. (3) A series of theories on energy transfer and electron transfer have been established, such as Förster [8, 9] and Electron Exchange mechanism [10] of energy transfer, as well as Marcus theory [11, 12, 13, 14, 15] and Rehm-Weller equation [16, 17] of electron transfer. These theories will provide certain directions for photosynthesis studies.

The success and importance of photosynthesis have inspired researchers to develop artificial molecular systems for solar energy conversion based on efficient energy/electron transfer processes, which require the strictest control of the direction and efficiency of the transductions. Such a complex goal can be achieved by integrating energy/electron transfer donor–acceptor systems in a single molecular structure [18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34] or by self-assembling them through highly selective and directional supramolecular interactions [35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51]. However, the donor–acceptor system based on covalent bond connection has become more and more complex, which will make the synthesis much more complicated. In this context, the investigation of the photoinduced energy and electron transfer, wherein donor and acceptor are assembled via noncovalent interactions, has attracted much interest. Among various noncovalent interactions (π–π stacking, hydrogen (H)-bonding, ion–ion, ion–dipole, dipole–dipole, electrostatic interactions), H-bonding interaction has emerged as a powerful tool to construct high array of supramolecular architectures [52, 53, 54, 55, 56]. In particular, the energy and electron transfer in biological photosynthesis in nature are regulated through a network of H-bonds. H-bonding is directional and has a wide range of interaction energies (4–120 kJ mol−1) that are tunable by adjusting the number and type of H-bonds, their relative orientation, and their position in the overall structure. Donor–acceptor chromophores undergoing photoinduced energy and electron transfer are well organized through H-bonding. Studies on the energy/electron transfer process within H-bonding system will give insights into how various factors, such as driving force, H-bonding pathways, and interchromophore orientations can influence the transfer rate, and thus regulate its biological charge separation processes.

This chapter is intended to describe the major developments and breakthroughs in the H-bonding-based energy/electron transfer in the last two decades. We will start with the introduction of the basic theory for energy transfer and electron transfer processes, then review the most recent progress of energy transfer and electron transfer based on H-bonding and their applications.

1.2 Basic Theories for Photoinduced Electron and Energy Transfer

In this section, a brief discussion of electron transfer and energy transfer theories is presented. The purpose of this short exposition is to provide the basis for a discussion regarding the features of theory that are addressed by current H-bonding systems for photoinduced electron and energy transfer.

1.2.1 Photoinduced Electron Transfer (PET) Processes

Electron transfer can formally be described in terms of electronic motion between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of donor and acceptor. PET is a one-electron reaction in which an electron on excited-state donor jumps from LUMO to the LUMO of ground-state acceptor, or electron on ground-state donor jumps from HOMO to the HOMO of excited-state acceptor (Fig. 1.1). In either case, electron transfer reaction between donor and acceptor leads to a radical ion pair or a charge transfer complex.
Fig. 1.1

Schematic representation of photoinduced electron transfer process (D is electron donor, A is electron acceptor, and * denotes an excited state)

The feasibility of electron transfer is dictated by the overall change of free energy (∆G) during the process. Generally, an electron transfer reaction will be thermodynamically allowed when it is exothermic (∆G < 0). It can be expressed by the simplified Rehn–Weller equation [17]:
$$\Delta G = E_{\text{ox}} \left( {\text{D}} \right) - E_{\text{red}} \left( {\text{A}} \right) - E^{*} - e^{2} /\left( {\varepsilon R} \right)$$
(1.1)
where E ox(D) and E red(A) are the oxidative and reductive potential of donor and acceptor, respectively; R is the distance between the ions; ε is the solvent dielectric constant; e 2R is the Coulombic energy which is related with the solvent polarity, and E* is the excited energy of a donor or acceptor. It can be seen from the equation that electron transfer depends on (1) the oxidative potential of a donor, (2) the reductive potential of an acceptor, (3) the excited energy of a donor or acceptor, (4) solvent polarity and distance between the ions. The electron transfer reaction is feasible when ∆G is negative.
Equation 1.2 predicts a parabolic relationship between the free energy of activation and thermodynamic driving force of electron transfer, which was developed by Marcus [11, 12, 13, 14, 15, 57]:
$$\Delta G^{\ddag } = \left( {\uplambda +\Delta G} \right)^{2} /\left( {4\uplambda} \right)$$
(1.2)
where ∆G is the free energy of activation; ∆G is the overall free energy difference between reactant and product states; λ represents the intrinsic barriers corresponding to the bond length changes and solvent reorganization.
It can be predicted that the rate of electron transfer should increase as the reaction becomes more exothermic until a certain value of ∆G is reached where the rate begins to fall again. The range of free energy values where the rate increases with increasing driving force is known as the “normal” free energy region (Fig. 1.2). A plot of ∆G versus ∆G in the “normal” region gives a slope of 0.5. The very negative free energies where the rate is predicted to diminish have been described as the Marcus “inverted” region.
Fig. 1.2

The relationship between ΔG and ΔG. Reprinted with the permission from Ref. [57]. Copyright 1986 American Chemical Society

1.2.2 Photoinduced Energy Transfer Processes

It has been generally accepted that two main mechanisms are involved by which a donor (D) in the excited state can pass its energy to a proximal acceptor (A) in the ground state, which results in the decrease of the emission intensity of the donor and transfers the energy to the fluorescent or nonfluorescent acceptor: Fluorescence resonance energy transfer (FRET, also named as Förster mechanism) [8, 9] and electron exchange (Dexter interactions) [10].

FRET is relatively long-range (10–100 Å) through-space dipole–dipole interaction, in which the electron on LUMO in the excited state of donor returns to its HOMO, simultaneously causing an electron in the HOMO of acceptor to go into its corresponding LUMO (Fig. 1.3a). The process finishes as that electron returns accompanying emission or heat dissipation of acceptor. According to the Förster theory, the energy transfer rate k ET(r) is given by:
$$k_{\text{ET}} \left( r \right) = 1/\tau_{\text{D}} \left( {R_{0} /r} \right)^{6}$$
(1.3)
in which τ D is the fluorescence lifetime of the donor in the absence of acceptor, r is the center-to-center donor–acceptor distance, and R 0 is the Förster distance at which 50 % of the excited donor decays. R 0 can be calculated as follows:
$$R_{0} = 9.78 \times 10^{3} \left[ {K^{2} n ^{- 4}\Phi J\left( \lambda \right)} \right]^{1/6} \, \left( {{\text{in {\AA}}}} \right)$$
(1.4)
where K 2 describes the transition dipole orientation. It usually assumes to be 2/3 for a random distribution of interacting dipoles, n is the refractive index of the medium, Φ is the quantum yield of the donor in the absence of acceptor, and J(λ) is the integral of the spectral overlap between donor emission and acceptor absorption. The FRET rate depends on the spectral overlap between the emission spectrum of donor and the absorption spectrum of acceptor, the distance between the donor and acceptor molecules, the quantum yield of the donor, and the relative orientation of the donor and acceptor transition dipoles. FRET has been widely used due to its favorable distances (10–100 Å), which is comparable with the size of most biological macromolecules.
Fig. 1.3

Schematic representation of photoinduced energy transfer processes. a FRET mechanism. b Electron Exchange Mechanism. D is energy donor, A is energy acceptor, and * denotes an excited state

Electron exchange is the short-range (<10 Å) through-bond mechanism involving simultaneous electron exchange between the HOMO of *D and A and between the LUMO of D* and A. As shown in Fig. 1.3b, the electron in the LUMO of the excited donor transfers to the LUMO of acceptor. Next the acceptor transfers an electron back to the HOMO of the donor, resulting in the acceptor in an excited state. The rate of exchange energy transfer has been shown by Dexter in the following equation:
$$K_{\text{dexter}} = 2\pi /\hbar \times KJ \times { \exp }{( - 2R/L)}$$
(1.5)
where K as an experimental factor is related to specific orbital interactions, J is the normalized spectra overlap integral between the emission of donor and absorption of acceptor, R is the distance between D and A, and L is the sum of donor-to-acceptor van der Waals radius. From the energy transfer rate expression of electron exchange, we can find that electron exchange is similar to FRET in the aspect of the spectral overlap dependence and donor-to-acceptor energy transfer consequence. However, unlike the R −6 dependence of FRET, the rate of electron exchange energy transfer decays exponentially [exp(−2R/L)] as the donor–acceptor distance increases. Electron exchange is almost non-observable when R is larger than the sum of van der Walls radius of D and A.
Energy transfer mechanisms strongly depend on the multiplicity of the electron spin states of donor and acceptor. There are two types of spin-allowed energy transfer processes: singlet–singlet energy transfer (SSET) and triplet–triplet energy transfer (TTET). Singlet–singlet energy transfer means that an electronically excited donor in its singlet state produces an electronically excited acceptor in its singlet state. The expression is given by Eq. 1.6. According to the Wigner spin conservation rule, SSET can happen through FRET (Fig. 1.3a) or electron exchange mechanism (Fig. 1.3b). Triplet–triplet energy transfer is given by Eq. 1.7. According to the Wigner spin conservation rule, this energy transfer type only happens when undergoing the electron exchanges. FRET is not involved in TTET since it is a spin-forbidden process.
$$^{1} {\text{D}}^{*} +^{1} {\text{A}} \to^{1} {\text{D}} +^{1} {\text{A}}^{*}$$
(1.6)
$$^{3} {\text{D}}^{*} + {^{1}\text{A}} \to {^{1}\text{D}} + {^{3}\text{A}}^{*}$$
(1.7)

It can be seen that the rate and efficiency of the electron/energy transfer is directly related to the distance, their relative spatial orientation, and the conformation of the bridge between donor and acceptor, as well as the direct orbital overlap between the components. Integrating energy/electron transfer donor–acceptor systems by covalent bonds has allowed reasonably precise knowledge of the above information. Such studies have contributed a great deal to our understanding of the fundamental photophysical processes in photosynthetic center and the development of artificial photosynthesis. However, the donor–acceptor assemblies have evolved from binary, ternary to multiple systems. The preparation of large, sophisticated species by covalent bonds will make the synthesis much more complicated, sometimes beyond the scope of more conventional synthetic method. Moreover, nature relies on supramolecular methods of assembly. In natural light-harvesting assemblies, the crucial parameters, such as distance and orientations, are satisfied by organizing key pigment molecules in a fixed spatial relationship by the surrounding medium with the help of noncovalent interactions. In this context, self-assembling donor and acceptor through highly selective and directional supramolecular interactions has attracted much interest, in which H-bonding is undoubtedly the most compelling and well-studied supramolecular motif. In the following sections, we will focus on photoinduced electron and energy transfer processes based on various H-bonds, as well as their applications in constructing optoelectronic devices, light-harvesting system, wide-range color display, and storage materials.

1.3 Hydrogen Bonding-Controlled Photoinduced Electron Transfer

More and more attentions have been paid to H-bonding-based electron transfer in recent years. These systems have been extended to the donor–acceptor assemblies linked by two-point H-bonds, triple H-bonds, or multiple H-bonds. In this section, we will first present the important mechanistic progress in the last two decades and then discuss recent representative examples based on H-bonding.

1.3.1 Mechanistic Studies

It has been long recognized that H-bond is not only the bridge between electron donor and acceptor in biological matrix, but also involved directly in the electron transfer process and plays an important role in mediating electronic tunneling. However, the binding strength of a single H-bond is rather weak compared to those of the popularly studied covalent bond. It has been always a primary focus whether H-bonding is an appropriate electronic coupling tunnel. This question was solved until three supramolecular bischromophoric model systems 13 were synthesized by Therien to evaluate the relative magnitudes of electronic coupling provided by H, σ, and π bonds [58]. (Porphinato)zinc donors and (porphinato)iron(III) chloride acceptors in 13 are separated by a virtually constant distance, thus establishing uniform driving force and reorganization energy for the ET reaction. Measurement of the photoinduced electron transfer rate constants enables a direct comparison of how well these three types of chemical interactions facilitate electron tunneling due to their exactly the same number of bonds and identical electron tunnelings. The electron transfer rates and driving forces are k ET = 4.3 × 109 s−1, ∆G 1 = –0.87 eV for σ bond interface in 1; k ET = 8.8 × 109 s−1, ΔG 2 = –0.87 eV for π bond interface in 2; and k ET = 8.1 × 109 s−1, ΔG3 = –0.70 eV for H-bond interface in 3 respectively. The surprising result is the electronic coupling modulated by the carboxylic acid/H-bond interface is superior to that provided by an interface composed of an analogous number of σ bonds, despite its relatively smaller driving force (ΔG 3 = –0.70 eV) than σ bonds (ΔG 1 = –0.87 eV). These results demonstrated the key role played by H-bonds in biological electron transfer processes and helped to analyze the through-protein electron transfer rate data as well as to strengthen the theory in predicting the path traversed by the tunneling electron in a biological matrix.

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Similar H-bonding-mediated PET was also observed by Nocera et al. in the Zn′′PCO2H porphyrin and 3,4-dinitrobenzoic acid electron donor–acceptor system 4 [59]. Fluorescence quenching and time-resolved absorption experiment demonstrated that photoinduced electron transfer occurred from photoexcitable Zn′′PCO2H porphyrin donor to the electron acceptor 3,4-dinitrobenzoic acid through a protonated (1-H) or deuterated (1-D) dicarboxylic acid interface. It is significant to observe the pronounced deuterium isotope effect of k H/k D = 1.7 and 1.6 for the charge separation and recombination rates, respectively. Thus, H-bonding network formed between dicarboxylic acid must have been involved in the electron transfer process. The results indicated that H-bonding interfaces are not only important in the supramolecular preorganization of acceptor/donor pairs for energy and electron transfer but also may directly mediate the electron transfer event.

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In 2000, Isied et al. reported a series of transition metal complexes with H-bonding molecular recognition sites capable of forming noncovalent donor acceptor complexes (k a = (2−5) × 105 M−1 in CH2Cl2) [60]. They determined for the first time the dependence of the rate of intramolecular electron transfer on the reaction driving force across a H-bonding interface. Fluorescence lifetime measurement indicated the electron transfer reactions from the excited states of the *MIIG to H-′MIII in MIIG–H–′MIII complexes 5. The emissions of all complexes decay biexponentially with short lifetimes corresponding to the intramolecular electron transfer (τ = 4–7 ns) and longer lifetimes corresponding to the emission of the uncomplexed *MIIG (M = Ru, τ′ = 770 ns; M = Os, τ′ = 90 ns). The forward and reverse electron transfer reaction rates of the MIIG–H–′MIII complexes 5 were plotted against their respective free energies. Despite that it is incapable of distinguishing the similar values of λel and Had obtained from the classical Marcus equation, this work demonstrated how the driving force dependence of the rates of intramolecular electron transfer reactions in these H-bonded complexes can be used to determine the electronic coupling matrix element for electron transfer. Comparison of the H-bonded systems with similar covalently bound complexes 6 at similar distances shows that an H-bonding interface can be as effective a bridge for electron transfer as covalently bound bridges.

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The above examples show that H-bonding is effective electron conduit for donor–acceptor interaction. This discovery provides strong support for the assumption that H-bonding plays an important role in the long-range electron transfer in biological system, which is a milestone for the development of electron transfer.

1.3.2 Electron Transfer Based on Double and Triple Hydrogen Bonding

The porphyrin–quinone hybrid system is among those most commonly found in natural and many elegant synthetic light harvesting systems. The investigation of PET process in porphyrin–quinone hybrid system is of current interest in the area. For example, in 2001, Branda et al. reported the regulation of PET within carboxylate–urea H-bonded porphyrin-phenoxynaphthacenequinone photochromic system 7 [61]. Irradiation of trans isomer 1t in CH2Cl2 at 365 nm could afford its ana form 1a, finally reaching its photostationary state as a 5:1 ana:trans mixture. 1t could be reformed by irradiating with light greater than 434 nm (Fig. 1.4). Cyclic voltammetry experiments and the negative values of the free energies for PET calculated clearly show that ana isomer 1a should act as a better electron acceptor than trans isomer 1t. The reversible photoisomerization of the phenoxynaphthacenequinone with H-bonded porphyrin was also confirmed under similar conditions. 1a would lead to a more significant quenching of P1 fluorescence than 1t. This study represents the first example of photoregulation of PET in a porphyrin–quinone system by reversibly changing the electronic properties of the electron acceptor.
Fig. 1.4

Structures of H-bonded porphyrin-phenoxynaphthacenequinone photochromic system 7 and its photo-controlled electron transfer

In the past decades, porphyrins and metalloporphyrins have been probed as integrative components in photosynthetic RC models due to their rich photo- and redox chemistry. They function as light harvesting dyes through most of the visible part of the solar spectrum and as donors to electron transfer and transport. On the other hand, fullerene was frequently employed as electron acceptor due to its small reorganization energy in electron transfer reactions, as well as its formation of unprecedented long-lived radical ion pair states through ultrafast charge separation together with very slow charge recombination feature. Thus, porphyrins/metalloporphyrins and fullerenes are molecular architectures ideally suited for electron donor–acceptor systems to transmit and process photoenergy. Photoexcitation of the porphyrin/metalloporphyrin by visible light is readily followed by an electron transfer to fullerenes, guaranteeing the formation of a radical ion pair state.

Guldi et al. reported a set of two-point amidinium–carboxylate H-bonded C60–porphyrin ensembles (8) [62]. 1H NMR, absorption and fluorescence spectroscopy confirmed the formation of 1:1 complex between porphyrin and C60. The binding constant deduced from nonlinear least-square analyses of the fluorescence intensity versus concentration of C60 is 2.1 × 107 M−1 in toluene. Transient absorption studies reveals that the singlet excited states of zinc porphyrin in 8b decay with rates of about 1010 s−1, from which an electronic coupling of 36 cm−1 between zinc porphyrin and C60 was calculated. A fast, efficient, and longer-lived formation of radical ion pair states (10 μs in THF) was facilitated by such strong electronic couplings. It can be seen that H-bonding is advantageous in facilitating electron transfer processes.

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Bis(zinc porphyrin)-fullerene supramolecular triad 9 could also be constructed using a diacetylamidopyridine/uracil complementary H-bonding motif [63]. The formation constant for the supramolecular triad, calculated by the Benesi-Hildebrand plot of fluorescence quenching, was 6.2 × 103 M−1 in acetonitrile/o-dichlorobenzene (6:4) with 1:1 complexation between the (ZnP)2 and C60 entities. Both zinc porphyrin entities of the dimer seem to be involved in the electron transfer process. The measured k CS and k CR values were found to depend on the positioning of the porphyrin entity with respect to the fullerene entity (near or far), thus delineating the structural importance of the studied supramolecular triad in controlling the electron transfer rates. The calculated damping factor was suggestive of through-space ET in the triad.

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Considerable efforts in Sessler group have been devoted to constructing various electron donor–acceptor systems based on cytosine–guanine base-pairing motif to model the basic electron transfer process. Ensemble 10 is an H-bonded assembly formed between a zinc porphyrin appended to guanine and quinine appended to cytosine [64]. PET was observed from the excited zinc porphyrin to quinone with k ET = 4.2 ± 0.7 × 108 s−l. However, the low association constant (3,100 ± 470 M−1) and the large degree of flexibility in the system would complicate the analyses. It is possible that electron transfer in 10 occurs by diffusional encounter between partners within a H-bonded complex. A more rigid system 11, in which the flexible alkane chain was removed, was then synthesized to avoid the problem [65]. The corresponding association constant in the ensemble was calculated to be 8,990 ± 600 M−1, which is significantly higher than that observed for the more flexible aggregate 10. Due to the rigidity, electron transfer most probably occurred via a through-bond process involving the H-bonded network with k ET = 8 × 108 s−l.

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The complementary imide–diaminotriazine triple H-bonding was also widely utilized in the construction of electron donor–acceptor system. Schenning et al. reported the collective and hierarchical self-assembly of oligo(phenylenevinylene) (OPV) and perylenediimide (PDI) into chiral fibers through diaminotriazine-diimide triple H-bonding and π–π interaction [66]. Supramolecular OPV-PDI-OPV entity 12 was initially formed via H-bonding and subsequently self-assembled into chiral stacks by π–π interaction. UV/vis, fluorescence, and CD titration experiments confirmed the formation of the 2:1 complex and its further stacking into J-aggregates with a helical screw sense. Upon photoillumination of these fibers, electron transfer from the OPV-donor to the perylene-acceptor chromophore took place, leading to charge separation within the aggregated dyes. This process was ascertained by transient absorption spectroscopy. Later, their further studies of ensemble 13 indicated that there is a distinct dependence of aggregate stability on OPV conjugation length [67, 68]. Different CD effects and electron recombination rates were observed due to different packing ways and their modes of connection (H-bonded or covalent) of the donor and acceptor dyes. Such well-defined co-aggregated dyes may serve as valuable nanoscopic functional building blocks for solid-state devices. These studies carried out with the electron donor OPV and electron acceptor PDI couple shed light on the importance of the chromophore organization in thin films for application in efficient organic photovoltaic devices.

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In 2013, Rodríguez-Morgade et al. reported the assembly of phthalocyanine and bifunctional PDI through malamine-imide triple H-bonding, to afford electron donor–acceptor array 14 [69]. In this study, 14 was probed in the ground and excited state by steady-state and time-resolved techniques. It was demonstrated that electronic interaction between phthalocyanine and PDI in 14 gave rise to intramolecular electron transfer upon selective photoexcitation of PDI, affording a several-ns-lived PDI•−/H2Pc•+ species. It is also interesting to observe the presence of the corresponding “SYN” and “ANTI” isomers in the arrays by 1H NMR.

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Braunschweig et al. reported another helical supramolecular system 15 in which PDI and diketopyrrolopyrrole (DPP) was employed as electron acceptors and donors, respectively (Fig. 1.5) [70]. The superstructures were assembled through triple H-bonding interaction and π–π stacking. Fast photoinduced charge separation was only observed upon superstructure formation, which was confirmed by variable temperature fluorescence spectroscopy and femtosecond transient absorption spectroscopy. This donor–acceptor system exemplifies how supramolecular assembly and frontier molecular orbitals can be synergistically designed to achieve emergent charge transfer in hierarchical organic superstructures.
Fig. 1.5

a Triple H-bonding brings the DPP donor (red) and the PDI acceptor (blue) together. b Superstructures arise from H-bonding and orthogonal π-stacking upon cooling. c FMO scheme indicating possible photoinduced electron and hole transfer via donor or acceptor excitation. Reprinted with the permission from Ref. [70]. Copyright 2014 American Chemical Society

Controlled assembly between CdSe quantum dots (QDs) and a fullerene (C60) derivative via complementary diamidopyridine–thymine three-point H-bonding have been studied by Rotello co-workers [71] (Fig. 1.6). Thymine-functionalized CdSe quantum dots (Thy-QDs) and diamidopyridine-functionalized fullerene (C60-DAP) were utilized as the donor and acceptor materials, respectively. Efficient charge transfer from excited CdSe QD to C60 was observed by fluorescence essays and subpicosecond transient absorption measurements. In addition, the recognition-mediated assembly also facilitated interpenetrated network morphology.
Fig. 1.6

Recognition-mediated assembly of Thy-QD and C60-DAP. Reprinted with the permission from Ref. [71]. Copyright 2013 American Chemical Society

A charge-separated (CS) state will be produced in a photoinduced electron transfer event to drive further chemical reaction in natural photosynthesis. Obtaining long-lived CS state following electron transfer is a key for improving the efficiency of solar energy conversion, which is in essence to accelerate the forward ET and to slow down the charge recombination (CR). Over the past decades, researches have been focused on designing versatile electron donor–acceptor arrays to give rise to long-lived CS states with high quantum yield upon photoexcitation.

Sessler et al. synthesized a rigid ensemble 16, in which dimethylaniline and anthracene donor–acceptor couple was constructed via noncovalent guanosine–cytidine base-pairing interaction [72, 73]. Steady-state fluorescence quenching measurements, time-resolved fluorescence quenching and transient absorption measurements indicated that photoinduced electron transfer occurred from the dimethylaniline donor to the singlet excited state of the anthracene acceptor upon excitation at 420 nm. The rate constants for photoinduced intraensemble electron transfer and subsequent charge recombination are k CS = (3.5 ± 0.03) × 1010 s−1 and k CR = (1.42 ± 0.03) × 109 s−1, respectively, which led to a short lifetime for charge-separated state as 705 ps [72]. Later, the same group developed another porphyrin–fullerene donor–acceptor ensemble 17 based on Watson–Crick hydrogen bonding paradigm to further improve the CS state [73]. Fullerene was employed here because small reorganization energy in ET reactions was required which should accelerate forward ET and to slow down back ET, resulting in the formation of long-lived CS states. Steady-state fluorescence quenching spectroscopic analyses indicated an efficient electron transfer occurs from the excited zinc porphyrin to fullerene with binding constant as (5.1 ± 0.5) × 104 M−1 in CH2Cl2. Time-resolved fluorescence measurement indicated a bi-exponential decay (τ 1 = 2.1 ns, τ 2 = 0.6 ns) of porphyrin fluorescence in the presence of fullerene, which implies the PET process within the ensemble. From the derived lifetimes, a rate constant (ca. k cs = 1.2 × 109 s−1) for the forward electron transfer was estimated. The oxidized zinc porphyrin radical cation (ZnP•+) and the fullerene radical anion (C60 •−) were observed from transient absorption spectroscopic measurements, with weak bands at 600–800 nm range and 1,000 nm, respectively. Kinetic analysis of the transient absorption data resulted in a long-lived CS state of 2.02 μs. This value is higher than those reported for related covalently linked C60-ZnP dyads as a result of the beneficial effect of the hydrogen bonds.

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Mendoza et al. reported a series of C60–TTF ensembles 1821, in which a photoexcited-state acceptor (C60) and an electroactive donor (TTF) were held together through complementary guanidinium-carboxylate H-bonding [74]. Transient absorption spectroscopy supported the formation of C60 •−·TTF•+ by the characteristic fullerene radical anion (1,000 nm) and TTF•+ (450 nm) transitions. The lifetimes of the radical pairs are in the range of hundreds of nanoseconds to microseconds, several orders of magnitude higher than those reported for covalently linked C60–TTF dyads [75, 76, 77]. Charge recombination rates in the C60·TTF ensembles are typically around 106 s−1 depending on electronic coupling between donor and acceptor. Photophyscial investigation of C60·TTF ensembles with different chemical spacers (i.e., phenyl vs. biphenyl) and two functional groups (ester and amide) supports through-space electron transfer.

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Kim et al. reported the formation of supramolecular complex 22 of the benzo-annulated TTF calix [4] pyrrole (TTF-C4P) as an electron donor with porphyrin as an electron acceptor in benzonitrile [78]. The TTF-C4P binds to the carboxylate moiety of the porphyrin through H-bonding with a 1:1 stoichiometry and a binding constant of 6.3 × 104 M−1 in this solvent at 298 K. Photoexcitation of complex 22 formed between these two components in PhCN at 298 K afforded the CS state, which was characterized by forward and backward intramolecular ET rate constants of 2.1 × 104 and 3.6 × 102 s−1, respectively. The triplet CS state produced upon photoirradiation of porphyrin was found to be 2.8 ms, one of the longest known lifetimes for a CS generated via a photoinduced electron transfer process within a noncovalently bound complex. It is interesting to see that free metal porphyrin acts as electron acceptor instead of donor in this case.

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1.3.3 Photoinduced Electron Transfer Based on Quadruply and Multiple Hydrogen Bonding

In 1996, Meijer et al. built the first donor–acceptor–donor–acceptor (DADA) quadruple H-bonding array by acylated triazines and pyrimidine derivatives, whose highest association constant reaches 2 × 105 M−1 [79, 80]. In 1998, they succeeded in building a self-complementary acceptor–acceptor–donor–donor (AADD) quadruple H-bonding array based on 2-ureido-4[1H]-pyrimidinone (UPy) unit, which can exist as homodimers in nonpolar solvents with the association constant higher than 107 M−1 [81]. The UPy module represents a fascinating self-complementary AADD quadruple H-bonding module. The association ability of UPy is much higher than that of the DADA array. Owing to their great binding strength and directionality, quadruple H-bonding arrays have shown extensive applications in assembling and disassembling supramolecular systems, as well as in creating H-bonded donor–acceptor dyads with efficient energy/electron transfer process.

The first example that the UPy quadruple complementary H-bonding was used as a conduit for the PET process was reported by Wu et al. [82]. In this study, porphyrins–fullerenes were employed as donor–acceptor systems owing to their rich and well-understood electrochemical and spectroscopic properties. Connected by the rigid UPy H-bonding, porphyrin and fullerene in assembly 23 and 24 were projected in the opposite direction, hence the intra-assembly collisions between donor and acceptor via the through-space mechanism was prohibited. On the other hand, due to the high association constant of UPy and long pre-exchange lifetime of UPy quadruple H-bonding unit, the intra-assembly donor–acceptor interaction was enhanced, thereby the intermolecular diffusion encounter between the electron donor and acceptor was avoided. Steady-state and time-resolved spectroscopy demonstrated that upon excitation of the porphyrin the electron transfer to the fullerene occurred with rate constants (quantum efficiency) of 1.6 × 108 s−1 (60 %) and 4.2 × 108 s−1 (44 %) for assemblies 23 and 24, respectively, and then giving rise to a long-lived CS state with a lifetime up to 9.8 μs for assembly 23 and 4.0 μs for assembly 24. Scheme 1.1 provides energy diagram for the PET and CS processes in assembly 23 and assembly 24. In assembly 23, light absorbed by the porphyrin creates its singlet excited state (1H2P*), which lies at ∼1.91 eV above the ground state (Scheme 1.1). The PET reaction takes place from the excited porphyrin of assembly 23 to the fullerene unit (C60) located ∼30 Å distant and yields the final products of the porphyrin radical cation (H2P•+) and fullerene radical anion (C60 •−) that lies 1.34 eV above the ground state. This means that the excitation energy of the porphyrin loses only 0.57 eV to reach the CS state of assembly 23 under the PET conditions, and the energy conversion efficiency is as high as 70 %. The long-lived CS state demonstrates that the quadruple complementary H-bonding plays a crucial role in mediating the intra-assembly PET and CR processes.
Scheme 1.1

Schematic energy diagram for the PET and CS processes in assembly 23 and 24 in CH2Cl2. Reprinted with the permission from Ref. [82]. Copyright 2011 American Chemical Society

Multiple H-bonding motifs, such as Barburic acid–Hamiliton receptor pairing, whose K a is in the range of 103–1012 M−1 in apolar solvents, have also been employed in constructing electron donor–acceptor assemblies. In 2007, Guldi and Hirsch et al. developed porphyrin–fullerene electron donor–acceptor systems 25 held together by a Hamilton-receptor-based H-bonding motif [83]. In this system, fullerene derivative is a monomalonate, where one branch carries a cyanuric acid moiety. The corresponding linkers are either propylene or hexylene chains. The second malonate branch bears dendritic termini to improve solubility. The Hamilton-receptor counterpart is coupled to a library of porphyrin derivatives involving either tin or zinc as central metals leading to new porphyrin building blocks. The association constants of the corresponding 1:1 complexes 25 connected by six H-bonds were determined to be in the range between 3.7 × 103 and 7.9 × 105 M−1 in CHCl3 by 1H NMR and fluorescence titration experiments. In response to visible light irradiation, 25 gave rise to a fast charge separation (k CS = 4.3 × 109 s−1) evolving from the photoexcited ZnP chromophores.

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Within this context, Guldi and co-workers further studied the electron transfer along π-conjugated spacers (p-phenyleneethynylene, p-phenylene-vinylene, p-ethynylene, and fluorine) in a series of supramolecular porphyrin/fullerene hybrids (26) through Hamilton receptor/cyanuric acid binding motif [84]. Selective photoexcitation of the porphryins (2.0 eV) triggers electron transfer in most hybrids to yield one-electron-reduced fullerenes and one-electron-oxidized porphyrins (1.4 eV). Electronic communications in the systems are controlled by the conjugated spacers (length and nature). In terms of simple distance dependence, transient absorption measurements confirm that the charge separation rate constants are 3.1 × 109 and 1.1 × 1010 s−1 for 26a and 26c, respectively. For 26b, no electron transfer activity is found on the time scale of up 3,000 ps, which suggests that incorporation of an extra p-phenylene-ethynylene shuts down electron transfer in 26b. On the other hand, the rate constants in 26a, 26d, and 26f increase with decreasing attenuation factor of the spacer. Charge recombination rates also follow the resembling trend. In p-phenylene-ethynylene-based systems, the values of 1.4 × 107 (26a) and 3.3 × 107 s−1 (26c) reflect the change in spacer length, 12.2 versus 7.9 Å. On the other hand, varying the spacer from p-phenylene-ethynylene, fluorene, to p-phenylene-vinylene (i.e., 26a, 26f, and 26d) results in steady increase in charge recombination rate constant from 1.4 × 107 to 1.9 × 107 and to 2.5 × 10−7 s−1. This study demonstrates the charge separation recombination kinetics is dependent of either the length or the β-factor of the employed spacer.

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de Cola and co-workers studied strong ground-state electron donor–acceptor (EDA) interactions in 1:1 H-bonded assemblies 27, constructed by a barbituric acid-substituted fullerene derivative and corresponding Hamiliton receptor bearing thienylenevinylene (TV) units [85]. The observed intense EDA absorption feature between the two TV and C60 moieties comes from the close proximity of the redox-active moieties within the assembly. The binding constant for the assemblies is 5,500 M−1. The photoinduced electron transfer from electron-rich thienylenevinylene subunits to the fullerene is very fast (k et = 5.5 × 1012 s−1), as determined by fs-time-resolved transient absorption spectroscopy.

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1.3.4 Applications of Hydrogen Bonding-Based Photoinduced Electron Transfer

Much effort has been devoted to applying electron transfer in H-bonding system for the construction of optoelectronic devices and organic solar cells. For example, fullerene derivative and perylene bisimide have been assembled to form a H-bonded supramolecular system 28 through triple amino-carboxylic acid interaction [86]. Under 63.2 mW/cm2 white light irradiation, the film made from the assembly on indium tin oxide (ITO) electrodes generated a steady and rapid photocurrent. The response of on/off cycling was prompt and reproducible.

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The construction of all-organic photovoltaic devices by melamine-barbituric acid triple H-bonded motif has been studied by Bassani and co-workers (Fig. 1.7) [87]. The symmetric melamine-terminated electron donor oligothiophenes were co-deposited with a complementary barbiturate-labeled electron acceptor fullerene into homogeneous films. Photovoltaic device made of these films gave a 2.5-fold enhancement in light energy to electrical energy conversion compared to analogous systems with non-H-bonding parent C60. This is ascribed to higher molecular-level ordering. Later, self-assembled monolayers bearing H-bonding molecular recognition end groups were utilized to modify gold electrode surface [88]. Further enhancement of the PV response of the corresponding functional supramolecular device was observed.
Fig. 1.7

The construction of all-organic photovoltaic devices by melamine-barbituric acid triple H-bonded motif. Reprinted with the permission from Ref. [87]. Copyright 2011 American Chemical Society

The group of Meijer and Schenning has constructed ambipolar field-effect transistors from imides-diaminotriazines H-bonded p-n dyad complexes 29 based on OPV4T in combination with PBI-2 [89]. The transistors show two independent pathways for charge transport. In contrast, processing of OPV and PBI that are not connected by H-bonds formed charge transfer donor–acceptor complexes. They showed no mobility in field-effect transistors, presumably due to an unfavorable supramolecular organization.

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Imahori and co-workers studied mixed films of porphyrin and fullerene with H-bonding on a tin oxide (SnO2) electrode and titanium oxide (TiO2) electrode to reveal their efficient photocurrent generation [90, 91]. H-bonding effect on the photoelectrochemical properties of the D-A systems were evaluated on these two electrodes. The nanostructured SnO2 and TiO2 electrodes modified with mixed films of porphyrin and fullerene composites with H-bonding exhibited efficient photocurrent generation compared to reference systems with no H-bonding (Fig. 1.8). Atomic force microscopy, infrared reflection absorption and ultraviolet-visible absorption, as well as time-resolved fluorescence lifetime and transient absorption spectroscopic measurements disclosed the relationship between the film structures and optical and photoelectrochemical properties relating to the formation of H-bonding between the porphyrins and/or the C60 moieties in the films on the electrode surface. These results showed that H-bonding is efficient for the fabrication of donor and acceptor composites on a nanostructured TiO2 electrode, which exhibits high open circuit potential relative to that of the corresponding SnO2 electrode.
Fig. 1.8

Porphyrin and fullerene derivatives used in the study

Xue et al. reported complex 30 of π gelator and fullerene derivative with photoinduced electron transfer for photocurrent generation [92]. In this study, π-gelator as an electron donor formed a complex with a fullerene derivative (electron acceptor). In the hybrid gel phase, the complex self-assembled into nanofibers in which C60CO2H and the gelator were packed into 1D superstructures being interdigitated each other. Such an ordered microstructure ensured efficient charge-carrier transport so that large photocurrents were achieved. The method provides a convenient way to fabricate active layers with high performance in a photovoltaic system by utilizing the gelation of D-A-type π-conjugated gelators.

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Qin and co-workers reported the preparation of polythiophene block copolymers (BCPs) which were selectively functionalized with diaminopyrimidine moieties (electron donor) and thymine-tethered fullerene derivative (electron acceptor) [93]. It was shown that the stability of the polymer solar cell (PSC) devices employing these copolymers (ensemble 31) was significantly improved through the “three-point” complementary H-bonding between diaminopyrimidine and thymine moieties. More interestingly, bulk heterojunction (BHJ) morphologies could be systematically adjusted by varying the blend ratio of BCPs to fullerene derivatives. However, the overall device efficiency was still quite low compared with benchmark poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) BHJ devices. To improve power conversion efficiencies (PCEs), the authors further synthesized new diblock polythiophene copolymer, P4, having a relatively shorter functionalized block carrying isoorotic acid moiety and a diaminopyridine tethered fullerene (PCBP) [94]. Self-assembly between P4 and PCBP (ensemble 32) through “three-point” complementary hydrogen bonding interactions is utilized to control and stabilize blend morphologies. Solar cells employing these materials show not only comparable PCEs with standard P3HT/PCBM devices but also much enhanced stability and tunable active layer morphologies by simply varying polymer/fullerene weight ratios.

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1.4 Hydrogen Bonding-Controlled Photoinduced Energy Transfer

1.4.1 Photoinduced Energy Transfer Based on Triple Hydrogen Bonding

Watson–Crick nucleobase pairing is one of the most popular and well-studied triple H-bonding motifs. Sessler and co-workers assembled triple H-bonding ensembles 3335 based on Watson–Crick nucleobase pairing between guanine (G) and cytosine (C) and studied their energy transfer processes [95, 96, 97]. Zinc porphyrins and free base porphyrins were employed as energy donor and acceptor. The association constant for G and C association is around 2 × 104 M−1 in CH2Cl2. In the first generation ensembles 33, photoinduced TTET was observed within the H-bonded complex following excitation of the porphyrin [95]. However, ensembles 33 proved to be very flexible so that no SSET was observed. The intraensemble diffusional encounter between the donor and acceptor in such flexible system could not be excluded for the observation of TTET. They further developed rigid ensembles 34 and 35, in which donor and acceptor are connected by a phenyl group to G and C recognition units respectively [96, 97]. Both singlet and triplet energy transfer were observed in ensembles 34 and 35. The singlet energy transfer dynamics was consistent with Förster mechanism, with transfer rate as k ET = 9 × 108 M−1 and transfer efficiency as Φ = 0.6. Due to their rigid structures and large center-to-center distance between donor and acceptor (ca. 22.5 Å), intraensemble triplet energy transfer with electron exchange mechanism occurred through H-bonding interface rather than through space. The results indicate that H-bonding is not involved in mediating FRET (Förster mechanism) process but play an important role in mediating triplet energy transfer process through electron exchange mechanism.

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Another classic triple H-bonding motif is the combination of either barbituric or cyanuric acid and melamine, which was originally described by Whitesides [98]. In 2006, Schenning and Meijer et al. reported helical co-assemblies 36 yielded by π–π stacking of H-bonded porphyrin and oligo(p-phenylene vinylene)s (OPVs) based on this motif [99]. In this study, a porphyrin derivative bearing enantiomerically pure 3,7-dimethyloctyloxy side chains was equipped with cyanuric acid (CN-Por), resulting in two sites available for H-bonding to OPVnT. The formation of 1:2 complexation was confirmed by fluorescence titration. This trimer was then organized into a helical structure by π–π stacking and H-bonding. Fluorescence exchange experiment in methylcyclohexane demonstrated the energy transfer from OPV4T to porphyrin.

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Guldi and co-workers assembled another supramolecular phthalocyanine (Pc)–PDI trimer 37 by using the melamine/perylenediimide triple H-bonding motif [100]. The association constant between PDI and ZnPc was calculated to be 2 × 105 M−1 in THF. Photoexcitation of the PDI component afforded transduction of singlet excited-state energy to the energetically lower lying phthalocyanine. The energy transduction process was investigated by fluorescence, time-resolved fluorescence spectroscopy, and transient absorption spectroscopy.

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1.4.2 Energy Transfer Based on Quadruply Hydrogen Bonding and Its Applications

OPVs are extensively investigated π-conjugated molecules which exhibit tunable absorption and emission properties. The modulation of the optoelectronic properties of OPVs could be realized by functionalization or supramolecular aggregation. In 2004, Schenning, Meijer, and co-workers developed a modular supramolecular approach through quadruply H-bonding and π–π interaction to create molecular stacks with mono(uriedo)triazine-functionalized oligo(p-phenylene vinylene)s MOPV3 as donor and MOPV4 as acceptor [101]. Ultrafast energy transfer was observed in mixed supramolecular stacks of MOPV in solution. Extended, highly ordered columnar aggregates of MOPV3 incorporating MOPV4 showed a very efficient quenching of the MOPV3 fluorescence, which indicates that energy transfer occurred within the supramolecular stacks from the shorter oligomer to the longer one (Fig. 1.9). Later, they presented in-depth atomistic models of the energy migration taking place along OPV-based chiral stacks based on quantum-chemical calculations, combined with transient photoluminescence measurements and polarization anisotropy data [102]. This is a nice example illustrating how slight modifications in the chemical structure of the individual molecules (chiral side chains and H-bonded units) can be used to control their packing and the resulting transport properties. The same group also studied the influence of supramolecular organization on the resonance energy transfer rates of photoexcitations along supramolecular assemblies of H-bonded OPV molecules [103, 104]. For well-defined, helical stacks of MOPVs, fast (≈50 ps) photoluminescence depolarization and excitation transfer to dopants was observed, in agreement with semi-coherent exciton diffusion. For disordered assemblies of bifunctional OPVs (BOPVs) incorporating a spacer to link adjacent molecules, depolarization and energy transfer dynamics occurred on a longer time scale (≈ns) (Fig. 1.10). Overall, their study gave detailed insights into the subtleties of the organizational demands of these artificial systems for efficient energy transfer. It should be noted that these energy donor–acceptor systems are connected by (uriedo)triazine H-bonds (DADA).
Fig. 1.9

Mixed columnar stacks of MOPV dimers in dodecane. Energy transfer (ENT) within mixed stacks is studied from MOPV3 to MOPV4. Reproduced from Ref.  [101]  by permission of John Wiley & Sons Ltd

Fig. 1.10

Molecular structures of MOPV and BOPV derivatives and schematic representation of the supramolecular structure in dodecane. Reprinted with permission from Ref.  [104]. Copyright 2008, American Institute of Physics

Energy transfer was not only observed in the supramolecular assembly with different lengths of OPV, but also among OPV and various other quadruply H-bonded functionalized chromophores. For example, Janssen and co-workers reported the synthesis of a perylenediimide with the UPy H-bonding motif (PERY-UPy) 38 and studied its photophysical properties in hetero-assemblies with UPy functionalized OPY (OPV-UPy) using fluorescence spectroscopy and femtosecond pump-probe spectroscopy [105]. The association constant of 38 is as high as 108 M−1 in toluene. Photoluminescence studies revealed that a singlet energy transfer reaction occurred after excitation of the OPV chromophore. A time constant of 5.1 ps was obtained for this reaction, which is in fair agreement with Förster theory. Although exergonic, electron transfer did not occur after photoexcitation as a result of a too weak electronic coupling between OPV and PERY chromophores in the excited state.

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Schenning and Meijer also synthesized a set of fluorene oligomers with bis-UPy at its both ends (UPy-OFn-UPy) [106]. The resulting bis-UPy-terminated oligomers can self-assemble into supramolecular chain polymers. The dimerization constant of the UPy groups is more than 100 times stronger than that for their previously reported uriedotriazine systems, permitting polymer assemblies based purely on H-bonding. Chains of H-bonded fluorenes can be simply end-capped by a variety of chain stoppers that have one UPy group. In this manner, the H-bonded fluorene chains have been end-capped with either OPV or perylene bisimide (Fig. 1.11). Energy transfer experiments in solution and the solid state demonstrated that oligofluorenes could donate energy to a variety of energy acceptors, but this energy transfer occurred most effectively when the donor fluorene was H-bonded to the acceptor.
Fig. 1.11

a Chemical structure of bis-UPy-terminated oligofluorenes (UPy-OFn-UPy), UPy-terminated oligo(p-phenylenevinylene) (UPy-OPV), and UPy-terminated perylene bismide (UPy-Pery). b Depiction of the energy transfer (ET) concept in the H-bonded oligofluorenes. Reprinted with the permission from Ref. [106]. Copyright 2005 American Chemical Society

Wu et al. also synthesized and assembled heterodimer 39, in which the chromophores were connected to the AADD module via only a methylene group [107]. Due to the rigidity, directionality, and specificity of the linker, naphthalene and anthracene were arranged side-by-side with donor-to-acceptor edge-to-edge distance being 13 Å. Excitation of the naphthalene chromophore in 39 resulted in efficient inner-assembly singlet energy transfer from naphthalene to anthracene. The energy transfer efficiency and rate constant were calculated to be 89 % and 9.8 × 108 s−1, respectively.

To examine whether the UPy H-bonding module could mediate the TTET, Wu et al. designed another UPy-bridged assembly 40 [108]. They reported that photoinduced intra-assembly TTET took place between the UPy-bridged benzophenone and naphthalene. The naphthalene group quenched the phosphorescence of the benzophenone efficiently upon selective excitation of the benzophenone at 77 K. A flash photolysis experiment at room temperature indicated that the intra-assembly TTET occurred with a rate constant of 3.0 × 106 s−1 and an efficiency of 95 % in CH2Cl2. It is known that TTET occurs via the Dexter electron exchange mechanism, and its rate constant decreases exponentially with donor–acceptor distance. When the donor–acceptor distance increases beyond the sum of their van der Waals radii, this kind of energy transfer process is generally negligible. In this case, the benzophenone and naphthalene units are arranged side-by-side with a much larger separation. Since the rigid structure prevents the donor and acceptor from any collisions through the solvent or space mechanism, the highly efficient intra-assembly TTET in 40 should occur through the H-bonds. As a result, UPy H-bonds here not only act as rigid scaffolds to fix the well-defined relative separations and orientations of the chromophores but also play a crucial role in mediating the photoinduced intra-assembly triplet–triplet energy transfer process.

Later, Wu et al. designed another UPy-bridged assembly 41, which have ferrocene and fullerene donor and acceptor, respectively [109]. Intra-assembly electron transfer from the singlet or triplet excited fullerene chromophore to the ferrocene group was too efficient to be observed, despite that ferrocene and fullerene are typically an electron transfer pair for efficient electron transfer reactions. Instead, intra-assembly triplet–triplet energy transfer occurred through the UPy quadruply H-bonding module at room temperature with a rate constant of 9.2 × 105 s−1 and an efficiency of 73 % in CH2Cl2.

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Quadruply H-bonding-mediated energy transfer processes have also been explored in constructing various materials with certain functions, such as wide-range color display, light harvesting, and storage. In these materials, energy transfer events constitute a fundamental mechanism to harvest and convey the excitation energy between different dye molecules. In this context, Meijer and Schenning constructed white light-emitting H-bonded supramolecular copolymers based on π-conjugated oligomers (Fig. 1.12) [110]. In this study, three different π-conjugated oligomers (a blue-emitting OF, a green-emitting OPV, and a red-emitting PERY) are functionalized with UPy units at both ends. The molecules self-assembled in solution and in the bulk, forming supramolecular polymers. When mixed together in solution, random noncovalent copolymers were formed which contained all three types of chromophores, resulting in energy transfer upon excitation of the oligofluorene energy donor. At a certain mixing ratio (UPy-OF3-UPy/UPy-OPV5-UPy/UPy-Pery-UPy = 59:33:8), a white emissive supramolecular polymer was created in solution due to partial energy transfer. The relatively high ratio of UPy-OPV5-UPy to UPy-OF3-UPy suggested that cascade energy transfer took place in which UPy-OPV5-UPy also acted as an energy donor for UPy-Pery-UPy. In contrast to unfunctionalized counterparts, bis-UPy-chromophores could easily be deposited as smooth thin films on surfaces by spin coating, giving rise to white fluorescence by more efficient energy transfer with a mixture of the chromophores in a ratio of UPy-OF3-UPy/UPy-OPV5-UPy/UPy-Pery-UPy = 84:10:6. Light-emitting diodes based on these supramolecular polymers have also been prepared from all three types of pure materials, yielding blue, green, and red devices, respectively, or from their mixtures, yielding an electroluminescence spectrum close to the CIE coordinates of white light. This modular approach with di-UPy is a promising strategy for creating multicomponent systems that can be used as functional optoelectronic materials.
Fig. 1.12

Chemical structures of the di-UPy functionalized chromophores and a schematic illustration of the creation of white photoluminescence. Reprinted with the permission from Ref. [110]. Copyright 2009 American Chemical Society

Wong et al. reported the construction of highly luminescent hollow nanospheres from three aggregated π-conjugated oligomers (4244) [111]. The most remarkable characteristic of the system is that, by mixing the oligomers in a suitable ratio through quadruple H-bonding interactions, any luminescence color, including white light emission, could be formed from the aggregated objects. In detail, emission of 42 (10−4 M solution in THF) is efficiently quenched by the addition of micromolar amounts of 43 or 44, which is assigned to singlet energy transfer on the basis of the favorable overlap between the emission spectrum of 42 and the absorption shell of 43 or 44. Along similar lines, the emission spectrum of the individual vesicle-like aggregates could be tuned by adjusting the composition of the solution from which they were dropcast. To demonstrate this, the color coordinates of a series of samples in which the proportion of 42, 43, and 44 were varied were measured, and the vesicle-like aggregates thus formed gave the expected progression of colors from blue to yellow. Thanks to the possibility of combining three different colors, the gamut obtainable is very large. They cover more than 75 % of the gamut of a standardized red-green-blue (RGB) liquid crystalline color display.

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Wang also prepared compound 45 in which two UPy units are connected with a photochromic dithienylethene unit [112]. It exists as a cyclic monomer at low concentrations and undergoes concentration-dependent ring-opening polymerization process (Fig. 1.13). To explore the potential applications of the supramolecular polymer, a small amount of mono-UPy-terminated fluorescent dye F1 was added to a solution of 45 to end-cap the linear polymer and then the mixture was spin coated to fabricate a smooth mixed film. The closed form of the diarylethene can absorb 4-aminonaphthalimide fluorescence emission, whereas the open form cannot. The open form and closed form of the diarylethene can be transformed into each other under UV and visible light irradiation respectively. Nearly 90 % of the fluorescence could be quenched by UV irradiation and regenerated by visible light irradiation of the thin film. As a result, the film fluorescence could be switched by UV/Vis light, presenting a fluorescent switch with nondestructive readout ability for data storage and high-resolution imaging technology.
Fig. 1.13

a Structures of bi-UPy photochromic dithienylethene 45 and mono-UPy-terminated fluorescent dye F1; b Depiction of the energy transfer (ET) process of the thin film spin coated from solutions of 97:3 45F1 under UV/Vis irradiation. (green open form of 45; purple closed form of 45; yellow F1) Reproduced from Ref.  [112]  by permission of John Wiley & Sons Ltd

Yang and co-workers prepared water-dispersible nanospheres of quadruply H-bonded supramolecular polymers with well-defined shape and size using the miniemulsion method (Fig. 1.14) [113]. They constructed brightly fluorescent light-harvesting nanospheres from supramolecular copolymers containing 46 as an energy donor and 47 as acceptor. The energy transfer from the donor to the acceptor was confirmed by steady-state and time-resolved fluorescence spectroscopy. As shown in Fig. 1.15, an increase of the acceptor-to-donor molar ratio from 1/352 to 1/44 lowered the intensity of the donor emission at 430 nm while enhancing that of the acceptor at 496 nm when the donor was selectively excited at 375 nm. Time-resolved fluorescence measurements showed that the fluorescence decay of 46 accelerated after co-assembling with 47, indicating an efficient energy transfer. With different molar ratios between the donor and acceptor, the acceptor emission amplified significantly as the ratio of the donor increased. Eventually, the energy transfer was saturated at the 1:352 ratio of acceptor and donor, when the maximum acceptor emission amplification reached a factor of 35, which is much higher than that of other artificial light-harvesting systems.
Fig. 1.14

Chemical structures of monomer chromophore 46 (donor) and 47 (acceptor) and graphical representation for preparation of water-dispersible light-harvesting nanospheres of hydrogen bonded supramolecular polymers. Adapted from Ref.  [113]  by permission of The Royal Society of Chemistry

Fig. 1.15

Fluorescence spectra of nanospheres dispersed in water with different molar ratios between donor (D) and acceptor (A). [D] = 49.7 mM. [D] to [A] molar ratio is 352:1, 176:1, 88:1, 58:1, 44:1 from bottom to top. λex = 375 nm. Adapted from Ref.  [113]  by permission of The Royal Society of Chemistry

1.5 Summary and Outlook

In summary, the principles, examples, and applications of H-bonding-controlled photoinduced electron and energy transfer have been reviewed. H-bonding clearly offers an attractive way to assemble donor and acceptor, to facilitate photoinduced electron/energy transfer processes, as well as to increase the charge separation lifetime. These systems are of great importance to mimic and understand the photosynthetic system in nature. H-bonding is not only the bridge between donor and acceptor in biological matrix, but also involved directly in the electron transfer/exchange process and plays an important role in mediating electronic tunneling. Electron/energy transfer based on H-bonding systems have found application in promising fields such as in constructing optoelectronic devices and organic solar cells, light-harvesting system, wide-range color display, and storage material. This is a thriving topic that will furnish future avenues as an outstanding assuming motif in nanoscience and nanotechnology. More applications might also be found in aqueous media and in biological matrix such as cells.

Notes

Acknowledgments

We are grateful for the financial support from the 973 Program (2013CB933800, 2013CB834505), the National Natural Science Foundation of China (91027041 21222210).

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© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  1. 1.Key Laboratory of Photochemical Conversion and Optoelectronic MaterialsTechnical Institute of Physics and Chemistry, Chinese Academy of SciencesBeijingChina

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