Abstract
Triplet formation by charge recombination is a phenomenon that is encountered in many fields of the photo-sciences and can be a detrimental unwanted side effect, but can also be exploited as a useful triplet generation method, for instance in photodynamic therapy. In this Perspective we describe the various aspects that play a role in the decay of charge separated states into local triplet states. The observations and structures of a selection of (pre-2015) molecular electron donor-acceptor systems in which triplet formation by charge recombination occurs are reported. An overview is given of some more recent systems consisting of BODIPY dimers, and BODIPYs attached to various electron-donor units displaying this same triplet formation process. A selection of polymer–fullerene blends in which triplet formation by (non-geminate) charge recombination has been observed, is presented. Furthermore, in-depth information regarding the mechanistic aspects of triplet formation by charge recombination is given on spin dephasing, through hyperfine interactions, as well as on spin–orbit coupling occurring simultaneously with charge recombination. The limits and constraints of these factors and their role in intersystem crossing are discussed. A pictorial view of the two mechanisms is given and this is correlated to aspects of the selection rules for triplet formation, the so-called El-Sayed rules. It is shown that the timescale of triplet formation by charge recombination is indicative for the mechanism that is responsible for the process. The relatively slow rates (CRkT ~ 1 × 108 s−1 or slower) can be correlated to proton hyperfine interactions (also called the radical pair mechanism), but substantially faster rates (CRkT ~ 1 × 109 up to 2.5 × 1010 s−1 or faster) have to be correlated to spin-orbit coupling effects. Several examples of molecular systems showing such fast rates are available and their electron donor and acceptor orbitals display an orthogonal relationship with respect to each other. This orientation of (the nodal planes of) the π-orbitals of the donor and acceptor units is correlated to the mechanisms in photodynamic agents and photovoltaic blends
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W. Rutsch, K. Dietliker, D. Leppard, M. Köhler, L. Misev, U. Kolczak and G. Rist, Recent development in photoinitiators, Prog. Org. Coat., 1996, 27 ,227–239, DOI: 10.1021/jp001348j.
L. B. Rocha, F. Schaberle, J. M. Dąbrowski, S. Simões and L. G. Arnaut, Intravenous Single-Dose Toxicity of Redaporfin-Based Photodynamic Therapy in Rodents, Int. J. Mol. Sci., 2015, 16 ,29236–29249, DOI: 10.3390/ijms161226162.
M. Wainwright, Photodynamic antimicrobial chemotherapy (PACT), J. Antimicrob. Chemother., 1998, 42 ,13–28, DOI: 10.1590/S0100-879X2010007500141.
A. van Dijken, J. J. A. M. Bastiaansen, N. M. M. Kiggen, B. M. W. Langeveld, C. Rothe, A. Monkman, I. Bach, P. Stössel and K. Brunner, Carbazole Compounds as Host Materials for Triplet Emitters in Organic Light-Emitting Diodes: Polymer Hosts for High-Efficiency Light-Emitting Diodes, J. Am. Chem. Soc., 2004, 126 ,7718–7727.
D. N. Congreve, J. Lee, N. J. Thompson, E. Hontz, S. R. Yost, P. D. Reusswig, M. E. Bahlke, S. Reineke, T. Van Voorhis and M. A. Baldo, External Quantum efficiency above 100% in a Singlet-Exciton-Fission––Based Organic Photovoltaic Cell, Science, 2013, 340 ,334–337.
W. Shockley and H. J. Queisser, Detailed Balance Limit of Efficiency of p-n Junction Solar Cells, J. Appl. Phys., 1961, 32 ,510–519, DOI: 10.1063/1.1736034.
A. Rao, P. C. Y. Chow, S. Gélinas, C. W. Schlenker, C.-Z. Li, H. L. Yip, A. K.-Y. Jen, D. S. Ginger and R. H. Friend, The role of spin in the kinetic control of recombination in organic photovoltaics, Nature, 2013, 500 ,435–439, DOI: 10.1038/nature12339.
S. D. Dimitrov, S. Wheeler, D. Niedzialek, B. C. Schroeder, H. Utzat, J. M. Frost, J. Yao, A. Gillett, P. S. Tuladhar, I. McCulloch, J. Nelson and J. R. Durrant, Polaron pair mediated triplet generation in polymer/fullerene blends, Nat. Commun., 2015, 6 ,6501–6508, DOI: 10.1038/ncomms7501.
F. Etzold, I. A. Howard, N. Forler, A. Melnyk, D. Andrienko, M. R. Hansen and F. Laquai, Sub-ns triplet state formation by non-geminate recombination in PSBTBT:PC70BM and PCPDTBT:PC60BM organic solar cells, Energy Environ. Sci., 2015, 8 ,1511–1522, DOI: 10.1039/c4ee03630a.
I. A. Howard, N. C. Greenham, A. Abrusci, R. H. Friend and S. Westenhoff, Charge Transfer and Charge Separation in Conjugated Polymer Solar Cells, in Nanostructured conductive polymers, ed. A. Eftekhari, Wiley, ch. 13, 2010, pp. 531–561 (see especially page 555).
D. W. Gehrig, Unraveling Efficiency-Limiting Processes in Organic Solar Cells by Ultrafast Spectroscopy – Impact of Chemical Structure and Morphology on Photophysics and Efficiency, PhD Dissertation, Johannes Gutenberg-Universität, Mainz, 2015. page 185. http://pubman.mpdl.mpg.de, /.
X.-K. Chen, T. Wang and J.-L. Brédas, Suppressing Energy Loss due to Triplet Exciton Formation in Organic Solar Cells: The Role of Chemical Structures and Molecular Packing, Adv. Energy Mater., 2017, 7 ,1602713–1602721, DOI: 10.1002/aenm.201602713.
See for example: N. J. Turro, V. Ramamurthy and J. C. Scaiano, Principles of Molecular Photochemistry: An Introduction, University Science Books, ch. 7, 2009; R. M. Williams, Introduction to Electron Transfer, 2007, DOI: 10.13140/RG.2.2.16547.30244; R. M. Williams, Introduction to Photo-induced Electron Transfer in “Mathematica”-V5. Classical Marcus Equation Calculator, 2019, DOI: 10.13140/RG.2.2.20531.89125/2 and references therein.
M. Lor, J. Thielemans, L. Viaene, M. Cotlet, J. Hofkens, T. Weil, C. Hampel, K. Müllen, J. W. Verhoeven, M. Van der Auweraer and F. C. De Schryver, Photoinduced Electron Transfer in a Rigid First Generation Triphenylamine Core Dendrimer Substituted with a Peryleneimide Acceptor, J. Am. Chem. Soc., 2002, 124 ,9918–9925.
N. J. Turro, The triplet state, J. Chem. Educ., 1969, 46 ,2–6.
N. J. Turro, V. Ramamurthy and J. C. Scaiano, Principles of Molecular Photochemistry: An Introduction, University Science Books, ch. 2 and 3, 2009.
E. A. Margulies, J. L. Logsdon, C. E. Miller, L. Ma, E. Simonoff, R. M. Young, G. C. Schatz and M. R. Wasielewski, Direct Observation of a Charge-Transfer State Preceding High-Yield Singlet Fission in Terrylenediimide Thin Films, J. Am. Chem. Soc., 2017, 139 ,663–671.
Y. Hou, X. Zhang, K. Chen, D. Liu, Z. Wang, Q. Liu, J. Zhao and A. Barbon, Charge separation, charge recombination, long-lived charge transfer state formation and intersystem crossing in organic electron donor/acceptor dyads, J. Mater. Chem. C, 2019, 7, 12048–12074, DOI: 10.1039/C9TC04285G.
T. Okada, I. Karaki, E. Matsuzawa, N. Mataga, Y. Sakata and S. Misumi, Ultrafast Intersystem Crossing in Some Intramolecular Heteroexcimers, J. Phys. Chem., 1981, 85 ,3957–3960.
R. M. Williams, J. M. Zwier and J. W. Verhoeven, Photoinduced Intramolecular Electron Transfer in a Bridged C60 (Acceptor)-Aniline (Donor) System; Photophysical Properties of the First “Active” Fullerene Diad, J. Am. Chem. Soc., 1995, 117 ,4093–4099.
M. H. Lee, B. D. Dunietz and E. Geva, Calculation from First Principles of Intramolecular Golden-Rule Rate Constants for Photo-Induced Electron Transfer in Molecular Donor-Acceptor Systems, J. Phys. Chem. C, 2013, 117 ,23391–23401.
P. Hudhomme and R. M. Williams, Energy And Electron Transfer In Photo- And Electro-Active Fullerene Dyads, in Fundamental and Applications of Carbon Nano Materials, World Scientific Publishing Company, 2011.
M. T. Colvin, A. B. Ricks, A. M. Scott, D. T. Co and M. R. Wasielewski, Intersystem Crossing Involving Strongly Spin Exchange-Coupled Radical Ion Pairs in Donor–bridge–Acceptor Molecules, J. Phys. Chem. A, 2012, 116 ,1923–1930.
H. van Willigen, G. Jones and M. S. Farahat, Time-Resolved EPR Study of Photoexcited Triplet-State Formation in Electron-Donor-Substituted Acridinium Ions, J. Phys. Chem., 1996, 100 ,3312–3316.
M. Wegner, H. Fischer, M. Koeberg, J. W. Verhoeven, A. M. Oliver and M. N. Paddon-Row, Time-resolved CIDNP from photochemically generated radical ion pairs in rigid bichromophoric systems, Chem. Phys., 1999, 242 ,227–234, DOI: 10.1016/S0301-0104(99)00047-6.
M. R. Roest, A. M. Oliver, M. N. Paddon-Row and J. W. Verhoeven, Distance Dependence of Singlet and Triplet Charge Recombination Pathways in a Series of Rigid Bichromophoric Systems, J. Phys. Chem. A, 1997, 101 ,4867–4871, DOI: 10.1021/jp970969i.
H. Imahori, K. Tamaki, D. M. Guldi, C. Luo, M. Fujitsuka, O. Ito, Y. Sakata and S. Fukuzumi, Modulating Charge Separation and Charge Recombination Dynamics in Porphyrin-Fullerene Linked Dyads and Triads: Marcus-Normal versus Inverted Region, J. Am. Chem. Soc., 2001, 123 ,2607–2617, DOI: 10.1021/ja003346i.
A. Loudet and K. Burgess, BODIPY Dyes and Their Derivatives: Syntheses and Spectroscopic Properties, Chem. Rev., 2007, 107 ,4891–4932.
M. A. Filatov, S. Karuthedath, P. M. Polestshuk, H. Savoie, K. J. Flanagan, C. Sy, E. Sitte, M. Telitchko, F. Laquai, R. W. Boyle and M. O. Senge, Generation of Triplet Excited States via Photoinduced Electron Transfer in meso-anthra-BODIPY: Fluorogenic Response toward Singlet Oxygen in Solution and in Vitro, J. Am. Chem. Soc., 2017, 139 ,6282–6285.
Z. Wang and J. Zhao, BODIPY–Anthracene Dyads as Triplet Photosensitizers: Effect of Chromophore Orientation on Triplet-State Formation Efficiency and Application in Triplet–Triplet Annihilation Upconversion, Org. Lett., 2017, 19 ,4492–4495.
M. A. Filatov, S. Karuthedath, P. M. Polestshuk, S. Callaghan, K. J. Flanagan, M. Telitchko, T. Wiesner, F. Laquai and M. O. Senge, Control of triplet state generation in heavy atom-free BODIPY-anthracene dyads by media polarity and structural factors, Phys. Chem. Chem. Phys., 2018, 20 ,8016–8031.
X.-F. Zhang, X. Yang and B. Xu, PET-based bisBODIPY photosensitizers for highly efficient excited triplet state and singlet oxygen generation: tuning photosensitizing ability by dihedral angles, Phys. Chem. Chem. Phys., 2017, 19 ,24792–24804.
R. M. Williams, Distance and orientation dependence of photoinduced electron transfer through twisted, bent and helical bridges: :A Karplus relation for charge transfer interaction, Photochem. Photobiol. Sci., 2010, 9 ,1018–1026.
Y. Liu, J. Zhao, A. Iagatti, L. Bussotti, P. Foggi, E. Castellucci, M. Di Donato and K.-L. Han, A Revisit to the Orthogonal BODIPY Dimers: Experimental Evidence for the Symmetry Breaking Charge Transfer-Induced Intersystem Crossing, J. Phys. Chem. C, 2018, 122 ,2502–2511.
M. A. Filatov, S. Karuthedath, P. M. Polestshuk, S. Callaghan, K. J. Flanagan, T. Wiesner, F. Laquai and M. O. Senge, BODIPY-pyrene and perylene dyads as heavy atom-free singlet oxygen sensitizers, ChemPhotoChem, 2018, 20 ,8016–8031.
Y. Hou, T. Biskup, S. Rein, Z. Wang, L. Bussotti, N. Russo, P. Foggi, J. Zhao, M. Di Donato, G. Mazzone and S. Weber, Spin–Orbit Charge Recombination Intersystem Crossing in Phenothiazine-Anthracene Compact Dyads: Effect of Molecular Conformation on Electronic Coupling, Electronic Transitions, and Electron Spin Polarizations of the Triplet States, J. Phys. Chem. C, 2018, 122 ,27850–27865.
Z. E. X. Dance, S. M. Mickley, T. M. Wilson, A. B. Ricks, A. M. Scott, M. A. Ratner and M. R. Wasielewski, Intersystem Crossing Mediated by Photoinduced Intramolecular Charge Transfer: Julolidine-Anthracene Molecules with Perpendicular pi Systems, J. Phys. Chem. A, 2008, 112 ,4194–4201.
S. Weber, Transient EPR, eMagRes, 2017, 6 ,255–270.
J. T. Buck, A. M. Boudreau, A. DeCarmine, R. W. Wilson, J. Hampsey and T. Mani, Spin-Allowed Transitions Control the Formation of Triplet Excited States in Orthogonal Donor-Acceptor Dyads, Chem, 2019, 5 ,138–155.
M. A. El-Sayed, Effect of spin orbit interactions on the dipolar nature of the radiative microwave zero-field transitions in aromatic molecules, J. Chem. Phys., 1974, 60 ,4502–4507.
Z. E. X. Dance, Q. Mi, D. W. McCamant, M. J. Ahrens, M. A. Ratner and M. R. Wasielewski, Time-Resolved EPR Studies of Photogenerated Radical Ion Pairs Separated by p-Phenylene Oligomers and of Triplet States Resulting from Charge Recombination, J. Phys. Chem. B, 2006, 110 ,25163–25173.
S. Callaghan, M. A. Filatov, H. Savoie, R. W. Boyle and M. O. Senge, In vitro cytotoxicity of a library of BODIPY-anthracene and -pyrene dyads for application in photo-dynamic therapy, Photochem. Photobiol. Sci., 2019, 18 ,495–504.
J. Zhao, K. Chen, Y. Hou, Y. Che, L. Liu and D. Jia, Recent progress in heavy atom-free organic compounds showing unexpected intersystem crossing (ISC) ability, Org. Biomol. Chem., 2018, 16 ,3692–3701.
J. Zhao, K. Xu, W. Yang, Z. Wang and F. Zhong, The triplet excited state of BODIPY: formation, modulation and application, Chem. Soc. Rev., 2015, 44 ,8904–8939.
M. A. Filatov, Heavy-atom-free BODIPY photosensitizers with intersystem crossing mediated by intramolecular photoinduced electron transfer, Org. Biomol. Chem., 2020, 18 ,10–27.
P. C. Y. Chow, S. Gélinas, A. Rao and R. H. Friend, Quantitative Bimolecular Recombination in Organic Photovoltaics through Triplet Exciton Formation, J. Am. Chem. Soc., 2014, 136 ,3424–3429, DOI: 10.1021/ja410092n.
Y. Tamai, K. Tsuda, H. Ohkita, H. Benten and S. Ito, Charge-carrier generation in organic solar cells using crystalline donor polymers, Phys. Chem. Chem. Phys., 2014, 16 ,20338–20346, DOI: 10.1039/c4cp01820f.
M. C. Scharber, C. Lungenschmied, H.-J. Egelhaaf, G. Matt, M. Bednorz, T. Fromherz, J. Gao, D. Jarzab and M. A. Loi, Charge transfer excitons in low band gap polymer based solar cells and the role of processing additives, Energy Environ. Sci., 2011, 4 ,5077–5083, DOI: 10.1039/c1ee02181h.
R. Haberkorn, M. E. Michel-Beyerle and R. A. Marcus, On spin-exchange and electron-transfer rates in bacterial photosynthesis, Proc. Natl. Acad. Sci. U. S. A., 1979, 76 ,4185–4188.
D. W. Gehrig, I. A. Howard and F. Laquai, Charge Carrier Generation Followed by Triplet State Formation, Annihilation, and Carrier Recreation in PDBTTT-C/PC60BM Photovoltaic BlendsBlends, J. Phys. Chem. C, 2015, 119 ,13509–13515, DOI: 10.1021/acs.jpcc.5b03467.
J. R. Ochsmann, D. Chandran, D. W. Gehrig, H. Anwar, P. K. Madathil, K.-S. Lee and F. Laquai, Triplet State Formation in Photovoltaic Blends of DPP-Type Copolymers and PC71BM, Macromol. Rapid Commun., 2015, 36 ,1122–1128.
R. M. Williams, H.-C. Chen, D. Di Nuzzo, S. C. J. Meskers and R. A. J. Janssen, Ultrafast charge and triplet state formation in Diketopyrrolopyrrole Low Band Gap Polymer/Fullerene blends: influence of Nanoscale Morphology of Organic Photovoltaic Materials on charge recombination to the Triplet State, J. Spectrosc., 2017, 6867507, DOI: 10.1155/2017/6867507.
R. M. Williams, N. Vân Anh and I. H. M. van Stokkum, Triplet Formation by Charge Recombination in Thin Film Blends of Perylene Red and Pyrene: Developing a Target Model for the Photophysics of Organic Photovoltaic Materials, J. Phys. Chem. B, 2013, 117 ,11239–11248.
C. M. Kirk, ESR studies of radical ions, PhD thesis, University of Canterbury, 1975.
B. G. Segal, M. Kaplan and G. K. Fraenkel, Measurement of g Values in the Electron Spin Resonance Spectra of Free Radicals, J. Chem. Phys., 1965, 43 ,4191–4200.
B. M. Latta and R. W. Taft, Substituent Effects on the Hyperfine Splitting Constants of N,N- Dimethylaniline Cation Radicals, J. Am. Chem. Soc., 1967, 89 ,5172–5178.
V. I. Krinichnyi and E. I. Yudanova, Light-Induced EPR Study of Charge Transfer in P3HT/PC71BM Bulk Heterojunctions, J. Phys. Chem. C, 2012, 116 ,9189–9195.
V. I. Krinichnyi and E. I. Yudanova, Influence of morphology of low-band-gap PCDTBT:PC71BM composite on photoinduced charge transfer: LEPR spectroscopy study, Synth. Met, 2015, 210 ,148–155.
S. A. J. Thomson, J. Niklas, K. L. Mardis, C. Mallares, I. D. W. Samuel and O. G. Poluektov, Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy, J. Phys. Chem. C, 2017, 121 ,22707–22719.
J. W. Verhoeven, On the role of spin correlation in the formation, decay, and detection of long-lived, intramolecular charge-transfer states, J. Photochem. Photobiol., C, 2006, 7 ,40–60.
K. Maeda, K. B. Henbest, F. Cintolesi, I. Kuprov, C. T. Rodgers, P. A. Liddell, D. Gust, C. R. Timmel and P. J. Hore, Chemical compass model of avian magnetoreception, Nature, 2008, 453 ,387–390.
E. L. Frankevich, A. A. Lymarev, I. Sokolik, F. E. Karasz, S. Blumstengel, R. H. Baughman and H. H. Hörhold, Polaron-pair generation in poly(phenylene vinylenes), Phys. Rev. B: Condens. Matter Mater. Phys., 1992, 46 ,9320–9324, DOI: 10.1103/PhysRevB.46.9320.
D. Veldman, S. M. A. Chopin, S. C. J. Meskers, M. M. Groeneveld, R. M. Williams and R. A. J. Janssen, Triplet Formation Involving a Polar Transition State in a Well-Defined Intramolecular Perylenediimide Dimeric Aggregate, J. Phys. Chem. A, 2008, 112 ,5846–5857.
M. Kállay, K. Németh and P. R. Surján, Triplet State Characteristics of Higher Fullerenes, J. Phys. Chem. A, 1998, 102 ,1261–1273.
D. Beljonne, Z. Shuai, G. Pourtois and J. L. Bredas, Spin–Orbit Coupling and Intersystem Crossing in Conjugated Polymers: A Configuration Interaction Description, J. Phys. Chem. A, 2001, 105 ,3899–3907.
M. A. El-Sayed, Triplet state. Its radiative and nonradiative properties, Acc. Chem. Res., 1968, 1 ,8–16.
N. J. Turro, V. Ramamurthy and J. C. Scaiano, Principles of Molecular Photochemistry: An Introduction, University Science Books, ch. 5, 2009, p. 284. Note that the second representation in Fig. 5.9(a) (top line) on page 285 is not correct: a 1(n,π*) is depicted (should be a 1(π,π*)).
B. Hu, L. Yan and M. Shao, Magnetic-Field Effects in Organic Semiconducting Materials and Devices, Adv. Mater., 2009, 21 ,1500–1516.
R. M. Williams, M. Koeberg, J. M. Lawson, Y.-Z. An, Y. Rubin, M. N. Paddon-Row and J. W. Verhoeven, Photoinduced Electron Transfer to C60 across Extended 3-and 11-Bond Hydrocarbon Bridges: Creation of a Long-Lived Charge-Separated State, J. Org. Chem., 1996, 61 ,5055–5062.
M. N. Paddon-Row, Electron and Energy Transfer, in Stimulating Concepts in Chemistry, VCH, Weinheim, 2000, pp. 267–291.
R. M. Williams, A highly soluble asymmetric perylene-bis (dicarboximide)-acceptor system incorporating a methylene bridged methoxybenzene-donor: solvent dependence of charge transfer interactions, Turk. J. Chem., 2009, 33 ,727–737.
S. I. van Dijk, P. G. Wiering, C. P. Groen, A. M. Brouwer, J. W. Verhoeven, W. Schuddeboom and J. M. Warman, Solvent-dependent switching between two dipolar excited states in a rigidly extended trichromophoric system, J. Chem. Soc., Faraday Trans., 1995, 91 ,2107–2114.
D. Veldman, S. M. A. Chopin, S. C. J. Meskers and R. A. J. Janssen, Enhanced Intersystem Crossing via a High Energy Charge Transfer State in a Perylenediimide—Perylenemonoimide Dyad, J. Phys. Chem. A, 2008, 112 ,8617–8632.
P. K. Samanta, D. Kim, V. Coropceanu and J.-L. Brédas, Up-Conversion Intersystem Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of the Nature of Singlet vs Triplet Excited States, J. Am. Chem. Soc., 2017, 139 ,4042–4051.
P. A. B. Haase, M. Repisky, S. Komorovsky, J. Bendix and S. P. A. Sauer, Relativistic DFT Calculations of Hyperfine Coupling Constants in 5d Hexafluorido Complexes: [ReF6]2− and [IrF6]2−, Chem. -Eur. J., 2018, 24 ,5124–5133.
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Gibbons, D.J., Farawar, A., Mazzella, P. et al. Making triplets from photo-generated charges: observations, mechanisms and theory. Photochem Photobiol Sci 19, 136–158 (2020). https://doi.org/10.1039/c9pp00399a
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DOI: https://doi.org/10.1039/c9pp00399a