Persistent radical anions in the series of peri-arylenes: broadband light absorption until far in the NIR and purely organic magnetism

Abstract Stable radicals in organic conjugated molecules are of great interest due to their magnetic signals and broad optical absorptions. In this paper, we report on naphthalene, benzoperylene, perylene, terrylene, and quaterrylene carboximides, reduced under controlled conditions, where stable metal-free solid salts of radical anions could be obtained forming darkly colored solutions with line-rich UV/Vis/NIR spectra and exhibiting special magnetic properties. The most bathochromic shift of the absorption maxima extend from 760 until 1700 nm. Persistent paramagnetic properties of the solids were observed and temperature-dependent susceptibilities are measured. Graphical abstract


Introduction
Absorption edge of organic compounds is in the ultraviolet (UV) because of large energetic gaps between the highest occupied molecular orbital (HOMO) and highest occupied molecular orbital (LUMO). Shifting light absorption edge into the visible requires molecular design and band gap engineering [1]. An extended π-system increases the absorption wavelength to obtain colored materials with lower energetic gaps [2][3][4]. On the other hand, smaller differences in energy levels are expected within the energetic band structure of the occupied and the unoccupied π-orbitals of complex aromatic and heteroaromatic systems. Thus, the removing or addition of one electron to an extended π-system would make these energetic sublevels of π-band and π*-band, respectively, accessible for electronic transitions to enable light absorption even until the near infrared (NIR) region. These may be extended far into the IR for sufficiently large electronic systems. However, there is an inherent tendency of such free radicals for dimerization recombination. This may be counteracted by the introduction of charge because of electrostatic repulsion between the molecules. We prefer radical anions avoiding the combination of the electrophilic radical 1 3 cations and aromatics. As a consequence, suitable electron deficient structures are required for the uptake of electrons.
The peri-arylenebiscarboximides [5] 1 according to Scheme 1 are known for n = 1 absorbing in the UV [6,7] to n = 4 and, finally, n = 6 [8] absorbing in the NIR and are attractive starting materials for the preparation of radical anions 2 because of their high stability, electron depletion by four carbonyl groups and broad region of tuning of light absorption.
The radical anions 2 (n = 1) of the naphthalenebiscarboximides 1 (n = 1) were many times reported as intermediates in photochemical processes. The electrochemical reduction of derivatives of 1 (n = 1) to 2 (n = 1) was indicated by ESR signals [9]. The same method allowed the preparation of a dark crystalline electrically conducting, however, not further characterized material [10] attributed to 2; this material seemed to be insoluble in polar aprotic solvents because of washing with acetonitrile was described. UV/Vis/NIR spectra of electrochemically prepared 2 were indicated [11]. Chemical reduction of 1 (n = 1) was described by means of strongly alkaline dithionite [12] solution without further characterization. UV/Vis spectra of radical anions 2 were obtained by flash photolysis and spectroscopically compared [13] with the products from reduction with dithionite and with cobaltocene [14]. A most bathochromic absorption at 777 nm in dichloromethane can be taken from the graph of the published spectra for the latter. Finally, cyclic triradical trianions were described [15][16][17].

Results and discussion
Here, we studied the reduction of the naphthalenebiscarboximides 1 (n = 1) for the synthesis of the radical anions [18] 2 on a preparative scale and applied hydroxyacetone in alkaline media (NaOH) as an efficient, versatile reducing agent where there are well-defined products of the oxidation of the reductone. The preparation of the radical anions 2 (n = 1) was successful in various solvents for both for R = alkyl and R = aryl and was indicated by the bathochromic UV/Vis absorption with a maximum at 761 nm corresponding to the absorption of the electrochemically prepared radical anions.
However, three topics still remain for the improvement of the synthesis: (i) Over reduction in the redox equilibrium proceeds to the bis-anions 3 (n = 1) under these conditions and is indicated by an absorption at 612 nm (overlapping with the absorption of 2). The ratio of 2 and 3 can be controlled by solvent effects where 3 dominates in ethanol, 2 exceeds in acetone, and about a 1:1 mixture of 2 and 3 is found in toluene. The equilibrium can be shifted to 2 by the addition of diacetyl as a weak oxidant corresponding to the oxidized hydroxyacetone. Thus, solutions can be obtained essentially containing 2. (ii) The required alkaline medium causes slow hydrolysis of carboximides limiting the stability of solutions; hydrolysis is inhibited if R means a 3-hydroxypropyl group as was found in preceding work [19]; see Scheme 2. Thus, the series of compounds 4 proved to be very resistant concerning alkaline hydrolysis. For example, the refluxing of 4b or 4e with 23 equivalents of 85% KOH in tert-butylalcohol for 6 h was not successful for hydrolysis, but only caused slow ring contraction [20] where 4e seems to react even more slowly. The formation of intramolecular hydrogen bonds with a more rigid structure may be, therefore, responsible and are indicated as well by the 1 H NMR coupling of the hydroxy hydrogen atoms to form triplets excluding a fast intermolecular proton exchange as sharp bands for the OH groups between 3480 and 3550 cm −1 . A slight shift of the carbonyl absorption of naphthalimides with aliphatic substituents at about 1705 and 1666 cm −1 by 5 cm −1 to lower wavenumbers may be a further indicator. Such intramolecular hydrogen bonds to the carbonyl groups can be verified by means of quantum chemical calculations as shown in Fig. 1.
The application of the non-ionic base DBU (1,8-diazabicyclo [5.4.0]undec-7-ene) [21][22][23] instead of NaOH is further favorable for a long-term stability the system. (iii) The solubility of 4 is limited constricting general applications and should be enhanced; as a consequence, we introduced geminal long-chain alkyl groups in 4a to 4d for increasing solubilization according to Ref. [24].
Considering topics (i)-(iii), we could prepare stable solutions of 5a-5e in ethanol. We precipitated the radical anion by means of an isoionic addition in excess of a salt. Counterions of minor importance for solutions of 5, however, fundamentally influence [25] the properties of solid materials. Best results for precipitation were obtained with tetrabutylammonium bromide where the addition of diacetyl proved to be not necessary because the lower solubility of salts of Scheme 1 the radical anions 5 compared with the bis-anion 6 allows its selective precipitation along with a shift of the redox equilibria. The solid tetrabutylammonium salts of 5 are black solids stable at room temperature for years, if atmospheric oxygen is rigorously excluded; even a brief handling in air is possible because re-oxidation proceeds only slowly. In the IR spectrum, the carbonyl absorption of 5 between 1621 and 1632 cm −1 and for the second band between 1580 and 1587 cm −1 are appreciably shifted to lower wavenumbers compared with 4 and can be taken as an indicator for a weakening of the C=O-double bond by the delocalization of the unpaired electron in 5. Even the C=C-vibration was shifted to lower wavenumbers and found at 1556 and 1520 cm −1 . In the 1 H NMR spectrum the signals of the tetrabutylammonium cation could be observed and signals from the periphery of the anion whereas the resonances of the aromatic protons could not be detected; this was attributed to appreciable line broadening and can be taken as a proof for the unpaired electron in 5. A further proof was given by high-resolution anion mass spectroscopy. The solid radical salts can be re-dissolved to obtain reddish brown solutions containing exclusively the radical anion. Such solutions can be stored without decomposition for many months if atmospheric oxygen is rigorously excluded (handling becomes Scheme 2 Fig. 1 Quantum chemical (DFT B3LYP) calculated structure of 4 with R=CH 3 (one rotamer is shown); the proximity of the OH and carbonyl groups can be clearly seen simplified by a stabilization with hydroxyacetone and DBU concerning traces of oxygen). Obviously, disproportionation is kinetically inhibited.
The reduction of perylenes 1 (n = 2) was studied as the next higher homologues of the peri-arylenes. Radical anions of perylenebiscarboximides 2 (n = 2) were firstly described [26] in 1978 and electrochemically generated [27][28][29] by the reduction of 1 (n = 2). A further electron depletion by means of trifluoromethyl groups allowed the isolation of salts of radical anions [30][31][32][33][34]. In this work, the generally pigment-like perylenecarboximides 1 (n = 2) were solubilized by means of long-chain sec-alkyl groups such as the 1-hexylheptyl group at their nitrogen atoms and reduced with hydroxyacetone in alkaline solution to form 2 (n = 2): Mostly over-reduction to the bis-anion 3 (n = 2) and slow hydrolysis of the carboximide groups proceeded. Here, we applied the same strategy for the preparation of the more stabilized 8 as was described for 5. Thus, perylene tetracarboxydiimides 7 were protected against hydrolysis [19] with 3-hydroxyalkyl groups analogously to 4, and reduced with hydroxyacetone and NaOH to form surprisingly stable products which are now the focus of the following discussion. Over reduction proceeded for 7a-7e under these conditions to mainly form the bis-anions 9a-9e, whereas only some radical anions 8a-8e are found in the equilibrium; this may be a consequence of the electron-withdrawing effect of the hydroxy groups in the side chains of 7 stabilizing the higher charged bis-anion. The equilibrium can be shifted to the radical anions 8 by the addition of the weak oxidant diacetyl. The radical anions 8 can be precipitated from the reduced solution by the addition of tetra-n-butyl ammonium bromide. The preferred precipitation of the 8 compared with the bis-anions 9 may be a consequence of the more efficient lowering of the solubility by the isoionic addition of the ammonium salt for 8 than for the bis-anions (compare the reduction of 4). The precipitated radical anions 8 form bluish dark solids and seemingly can be stored indefinitely under protective atmosphere such as argon. 8a and 8b, respectively, can be even briefly handled in air; however, slow oxidation proceeds to convert the substance back to the staring materials 7a and 7b, respectively. Essentially, solutions of the radical anions 8 are obtained on re-dissolution of the solids being comparably stable; disproportionation of 8 to 7 and 9 seems to be kinetically inhibited. The re-oxidation of the dissolved 8 with air to the starting material 7 proceeds much more quickly than for the solids. A trapping of the dissolved anions 8 by electrophiles such as protons, acetic anhydride or trimethylsilyl chloride was not successful because the starting materials 7 were spontaneously formed after the contact with any such reagents. Even an addition of concentrated solutions of NH 4 + , Mg 2+ , Ca 2+ , Ba 2+ , Li + , Na + , K + , and Zn 2+ salts caused the formation of the starting materials 7. The NMR spectra of dissolved 8 are typical for paramagnetic materials because only signals of the n-butyl groups of the counter ions were found and signals from the aliphatic periphery, whereas the signals of the aromatic core of radical anion were lost by strong line-broadening because of unfavorable relaxation processes. Spin and charge of radical anions 8 seem to be fully delocalised because both the carbonyl and C=C valence frequencies are lowered in the IR spectrum from 1688 and 1638 for the starting material [32] to 1601 and 1542 cm −1 for 8b and even for the frequencies of the C=C vibrations from 1595 and 1579 to 1560 and 1492 cm −1 for 8b indicating a weakening of these bonds. The radical-type character of the solids 8 is univocally indicated by their ESR spectra both of the solids and the re-dissolved material where signals with no hyper-fine structures were found; see below. The negative charge of 8 is indicated by anion mass spectrometry.
The more bathochromically shifted absorbing terrylenecarboximides [35] 1 (n = 3) were targeted as the next higher homologue; see Scheme 3. The formation of radical anions 2 (n = 3) was reported for the electrochemical reduction [36] of 1 (n = 3) and indicated by their UV/Vis/NIR absorption. Thus, we studied the isolation of 2 (n = 3) and found a minor pronounced tendency for hydrolysis compared with the lower homologues, presumably because of the higher electron density of 1 (n = 3). As a consequence, we substituted the nitrogen atoms of 1 (n = 3) with the highly solubilising 1-hexylheptyl group (swallow tail substituent) to prepare [37,38] 10. However, no 11 was obtained by a reduction with hydroxyacetone because a complete over reduction proceeded to the bis-anion 12 absorbing at 694 nm; see Fig. 4. The reducing aptitudes of hydroxyacetone and DBU could be balanced with the addition of diacetyl to the formation of the radical anion 11 indicated by a color change from deep blue to greenish blue. Finally, a precipitation with tetran-butylammonium bromide was successful to obtain the salt of 11 as a dark blue solid.
Quaterrylene tetracarboxybisimides [39,40] 1 (n = 4) being known to be very bathochromically absorbing were rendered soluble [41,42] by the N-(1-hexylheptyl) substituents to obtain 13 and reduced according to 4. Complete over reduction proceeded to the slightly more bathochromically absorbing bis-anion 15 at 803 nm; see Fig. 5. The reducing aptitude of hydroxyacetone was controlled by the addition of diacetyl finally to obtain 14. This radical anion could be precipitated by the addition of tetra-n-butylammonium bromide to obtain the radical salt of 14 as a dark green solid. The solid material can be stored under inert atmosphere without decomposition.
We extended the application of the method of reduction to even larger laterally conjugated systems. Thus, we studied the reduction of benzoperylenehexacarboxytrisimides [43] analogously to 1 to form 2 and tried to protect the starting material against alkaline hydrolysis analogously to 7 with 3-hydroxyalkyl groups. However, the Scheme 3 synthesis of the staring material by the Clar variant of the Diels-Alder reaction of 7e with maleic anhydride was not successful, presumably because of the still low solubility. As a consequence, we allowed to react the better soluble perylene derivative 1 (n = 3, R = 1-heyxlheptyl) to form the core-extended anhydride and tried to condense it with 2-aminomethyl-2-octyldecan-1-ol to form 16 to protect the more labile five-membered ring; Scheme 4. However, the condensation of the anhydride group with hydroxy amines by means of imidazole under standard conditions [44] was not successful, but could be achieved with DCC and TFA in chloroform [45] to form the benzoperylene hexacarboxytrisimide 16 with two efficiently solubilising [46] swallow-tail substituents (R) and one hydrolysis protecting group (R′) attached to the more labile five-membered ring causing an acceptable stabilization concerning hydrolysis. The benzoperylenehexacarboxytrisimide was reduced by the application of hydroxyacetone and precipitated with tetrabutylammonium bromide to form the salt of 17 as a yellowish dark green solid being much more sensitive versus oxygen than 8. The precipitated 17 can be stored if oxygen is carefully excluded. The material is appreciably less stable in solution than 8 and rapidly decomposes during the UV/Vis spectroscopic measurements. The band-rich UV/Vis absorption spectrum of 5c with the most bathochromic maximum at 761 nm is reaching NIR and is shown in Fig. 2. Fluorescence could not be detected as was expected.
The solids of 8 form characteristically dark blue solutions in solvents such as acetonitrile or acetone where the color is not caused by the most bathochromic electron transition, but by a transition to higher electronic levels, and thus presents an example of colors of second order [47][48][49][50]. The most bathochromic absorption maximum is far in the NIR at 957 nm and there are many bands in the spectrum; see Fig. 3. The UV/Vis absorption of 8 corresponds to the previously reported absorption of the electrochemically [18,29,36] generated transient coloration by derivatives of 8. Solid-state UV/Vis/NIR spectroscopy is used to confirm independently the radical character of 8 because a strong bathochromic absorption at 957 nm is observed being identical with respect to the dissolved material.
The terrylene-based radical anion 11 absorbs as bathochromic as 1320 nm; see Fig. 4. The absorption of the latter is mainly in the NIR, so that color is caused by residual bands in the visible.
Finally, the quaterrylene-derived precipitated radical anion 14 was re-dissolved where the solution in acetone absorbs in the UV/Vis/NIR as bathochromic as 1699 nm being not far away from the region of vibration spectra; see Fig. 5.
The lateral extension of the perylenes to benzoperylenes causes a hypsochromic shift such as that shown in Fig. 6 for 16. The UV/Vis spectrum of the radical anion 17 obtained by the reduction of 16 still extends to the NIR at 867 nm.

Magnetic properties of the radical anions
The radical-type character of the solids 8 as well as in solutions is univocally indicated by their ESR spectra both of the solids and the re-dissolved material where signals with no hyper-fine structures were found; see Fig. 7.

Magnetic measurements
Organic magnetic materials are of general interest because special effects can be expected such as the ferromagnetism, giant magnetoresistance, magnetic Seebeck effect, etc.; the low heat conductivity of organic materials is important for the latter [51][52][53][54]. The unpaired spin of 5a and 8a, respectively, persists in the solid state and allows the study of magnetic ordering. The ESR signal of the solid naphthalene derivative 5a shows an irregular line shape which consists of more (up to three) overlapping lines with slightly shifted g-factors; Fig. 8, left. The g-factor of the main peak was found to be 2.0035. The signal undergoes line broadening upon cooling. We have observed change in amplitudes of individual signals after exposure to air which was partially reversible after re-evacuation. No detailed evaluation of signal amplitudes for samples exposed intentionally to air was performed due to the complex nature of their behavior. The ESR signal of the solid perylene derivative 8a shows a more regular line shape; Fig. 8, right. Unwanted passage effects led us to work at very low microwave power with attenuation set to 40 dB (20 µW), modulation amplitude 0.1 G, and modulation frequency 10 kHz to prevent any unwanted signal distortion. Perylene g-factor was found to be 2.0031. Small line broadening upon cooling was observed also in 7a but not as significant as in the naphthalene derivative 4a.
In the next step, we took a closer look at the magnetic behavior of the material. Figure 9 shows the normalized temperature dependence of doubly integrated ESR signals which corresponds to the magnetic susceptibility of the sample under study. Inset shows the temperature dependence of inverse susceptibility.
Using the Curie-Weiss law (Eq. 1) where C is a Curie constant, T is the absolute temperature in Kelvin, T C is the Curie temperature, and γ is critical exponent, it is possible to determine the character of magnetism (Fig. 10 fits). We observe that T C and γ are close to 0 (i.e., 2-3 K) and close to unity, respectively. This indicates a paramagnetic behavior. Figure 10 shows in a more illustrative way the (paramagnetic) regime for both materials as the temperature dependence of the product χ T.

Compressed solids
The solid materials 5a and 8a were compacted in vacuo by the application of pressure of 2700 bar (3 tons at 12 mm piston). The diagrams of Fig. 11 indicate paramagnetic properties for both compacted solids.
We calculated the number of free spins using the absolute intensities of the ESR signals. We found 1.5 × 10 16 free spins at room temperature, where 1.5 × 10 16 spins were found for 0.8 mg of compressed 5a (1.9 × 10 19 spins/g; 0.025 mol spins/mol) and 1.1 × 10 17 for 2.8 mg of compressed 8a (3.8 × 10 19 spins/g; 0.060 mol spins/mol). This corresponds to one free spin for 40 molecules of 5a and one free spin for 17 molecules of 8a.

Conclusion
We conclude that (bathochromically) shifted UV/Vis/NIR absorptions can be obtained by the reduction of suitable chromophores to their radical anions where an overview of peri-arylenes is shown in Fig. 12. The isolated radical anions promise many novel applications for dyestuff applications. The problems in conventional vat dying with the heavy-load of alkali being damaging delicate surfaces such as silk or human hair can be overcome by decoupling the processes of reduction and re-oxidation. The replacement of the environmentally problematic dithionite by hydroxyacetone or even electrochemical [55] processes for vat dyeing and the precipitation of the radical salts may turn such processes to green chemistry. The vat dying with the colorless 1 may be of special advantage because of sun-protection concerning UVA [56,57]. This may be useful for the protecting of fibers and other materials, and also for sun-protecting clothes. Even the radical anions may be useful for many applications because of their bathochromic shifts of light absorption, their sensitivity against atmospheric oxygen can be solved by encapsulation such as in laminated compound glass.