8855
2008, 112, 8855–8858 Published on Web 07/03/2008
Stable Aromatic Dianion in Water Elijah Shirman,† Alona Ustinov,† Netanel Ben-Shitrit,† Haim Weissman,† Mark A. Iron,‡ Revital Cohen,‡ and Boris Rybtchinski*,† Departments of Organic Chemistry and Chemical Research Support, Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed: April 6, 2008; ReVised Manuscript ReceiVed: June 10, 2008
Perylene diimide (PDI) bearing polyethylene glycol substituents at the imide positions was reduced in water with sodium dithionite to produce an aromatic dianion. The latter is stable for months in deoxygenated aqueous solutions, in contrast to all known aromatic dianions which readily react with water. Such stability is due to extensive electron delocalization and the aromatic character of the dianion, as evidenced by spectroscopic and theoretical studies. The dianion reacts with oxygen to restore the parent neutral compound, which can be reduced again in an inert atmosphere with sodium dithionite to give the dianion. Such reversible charging renders PDIs useful for controlled electron storage and release in aqueous media. Simple preparation of the dianion, reversible charging, high photoredox power, and stability in water can lead to development of new photofunctional and electron transfer systems in the aqueous phase. Doubly reduced aromatic compounds, aromatic dianions, have been extensively studied due to their fundamental importance in understanding aromaticity, π-delocalization, and electron transfer.1–13 Photophysical properties of the aromatic dianions are exceptional for achieving high energy transformations. Thus, photoexcitation of the dianions, most of which absorb strongly in the visible spectrum, results in high energy excited states, which are more stable and long-lived than those of the radical anions, providing access to high energy electron transfer reactions.14–17 However, aromatic dianions readily decompose, as they are very reactive toward a broad range of compounds, e.g., various oxidants, chlorinated hydrocarbons, protic solvents (especially water), etc.2 Such lack of stability is prohibitive for utilization of aromatic dianions. Herein, we report on a perylene diimide (PDI) aromatic dianion that is remarkably stable in aqueous solution. The advantageous photophysical characteristics of the dianion, its simple preparation, reversible discharge of excess electrons, and stability in water can lead to development of new photofunctional and electron transfer systems in the aqueous phase. Combined experimental and theoretical studies provide insight into the fundamental properties of the PDI dianion, whose stability is due to the extensive charge delocalization and high degree of aromaticity.
* To whom correspondence should be
[email protected]. † Department of Organic Chemistry. ‡ Department of Chemical Research Support.
addressed.
E-mail:
10.1021/jp8029743 CCC: $40.75
Figure 1. Left: UV-vis spectra for the titration of 1 with Na2S2O4. Right: Aromatic region of the 1H NMR spectrum of 12- in D2O.
PDIs have been utilized as industrial dyes, electronic materials, photovoltaics, and building blocks for artificial photosynthetic systems.18–21 PDIs feature an extended aromatic system attached to electron-withdrawing imide groups, which can result in the stabilization of the dianion. To investigate the possibility of obtaining PDI dianion in water, we chose compound 1. This PDI derivative decorated with polyethylene glycol groups (PDI-PEG) has been recently reported to possess excellent solubility in water, in which it self-assembles into small aggregates (4-6 nm).22 The solubility of 1 in water allows for use of simple reducing agents and precise control over the stoichiometry of reduction. We found that compound 1 undergoes facile reduction in water with sodium dithionite under an inert (N2) atmosphere. When an aqueous solution of 1 is titrated with Na2S2O4, its color changes from red to blue, and typical PDI radical anion peaks appear in the UV-vis spectrum at 726, 812, and 980 nm16 (Figure 1). Formation of 1•- was confirmed by EPR (see the Supporting Information). Further titration results in the appearance of signals characteristic of PDI dianion absorption as observed in nonprotic solvents:16,23 a strong band 2008 American Chemical Society
8856 J. Phys. Chem. B, Vol. 112, No. 30, 2008
Figure 2. Absorption (solid line), emission (dashed line), and excitation (dotted line) spectra of 12- in water.
at 541 nm with unresolved vibronic structure and a weaker band at 609 nm. The formation of the anions is concomitant with the gradual disappearance of the neutral PDI-PEG absorption. Upon addition of two or more equivalents of Na2S2O4, a strong magenta color appears, and the UV-vis spectrum shows exclusive formation of PDI dianion 12- (Figure 1), which is EPR-silent, as is typical of most aromatic dianions.2 The diamagnetic nature of 12- allows measurement of the 1H NMR spectrum, which shows typical PDI aromatic signals (Figure 1, J ) 10.0 Hz), indicating the aromatic character of 12- (see below). A standard charge redistribution test also confirmed the dianionic nature of 12-: upon addition of 1 to 12-, immediate formation of 1•- takes place, as observed in the UV-vis spectra. Dianion 12- was also produced using electrochemical reduction in water; its characteristics are identical to those of the chemically generated 12-. Various carbonyl dyes, e.g., indigo and anthraquinone dyes, undergo two-electron reduction in water that is followed by protonation (quinone-hydroquinone type redox system). If this reduction is carried out under basic conditions, the anionic leuco form is produced, in which the negative charges are localized on the oxygen atoms.18 The behavior of PDI is distinctly different: over a broad pH range (4-12), identical PDI dianion spectra, typical of PDI dianions in aprotic media,16,23 are observed, indicating that reduction of PDI produces the nonprotonated charge-delocalized aromatic dianion. Dithionite/base treatment of PDI dyestuffs is a subject of several patents,24,25 in which the existence of the “leuco form” rather than aromatic dianion has been implied. This species has not been characterized. Apparently, formation of PDI radical anions, and not dianions, must occur in these systems, since equivalent PDI/ Na2S2O4 ratios were employed. Generation of a naphthalene diimide dianion in water26 and perylene diimide dianion in water/ethanol solution27 has been mentioned in the literature. These species have been characterized by UV-vis, but their stability and properties have not been studied. To the best of our knowledge, PEG-PDI2- is the first well-characterized aromatic dianion in water, showing distinct chemical and photophysical properties in aqueous media. Compound 12- shows a relatively strong luminescence (λmax ) 622 nm) with a quantum yield of 10% and a lifetime of 6.5 ns (Figure 2), while 1 and 1•- are not luminescent in water. The emission appears to originate from the excited singlet state of 12-, as evidenced by a small Stokes shift and a short lifetime.21 Such emission properties are comparable to the ones of disaggregated PDIs (in the aggregated PDI systems, the emission is normally quenched, as in the case of 1). The appearance of the absorption bands (very similar to the ones observed in organic solvents), fluorescence typical of disaggre-
Letters
Figure 3. Left: Cyclic and differential pulse voltammograms of 1 in various solvents. Right: Energy levels of PDI estimated from electrochemical measurements and optical gaps.
gated PDI, and sharp NMR signals suggest that dianion species are mostly disaggregated. Remarkably, 12- is very stable in water: no decomposition was observed after keeping aqueous solutions of 12- (deoxygenated and protected from light) for two months at ambient temperature. Furthermore, deuteration of 12- in D2O does not occur, ruling out reversible protonation of the aromatic system. The cyclic voltammogram of 1 in water (Figure 3) shows fully reversible reduction, attesting to the stability of 12-. While completely inert in water, 12- is extremely sensitive to oxygen. Thus, upon exposure to air, 12- is instantly converted to the neutral parent compound 1 (upon careful oxidation, 1•- can be observed as an intermediate), which can be reduced again in an inert atmosphere with sodium dithionite to give 12-. Such oxidation/reduction cycles can be repeated at least 10 times without any noticeable decomposition of 1, allowing fully reversible chemical charge/discharge of excess electrons in aqueous medium. Notably, the 1 h 1•- h 12- transformation is characterized by entirely reversible switching of the absorption, fluorescence, and magnetic properties in water, providing access to multiple color, emission, and spin modes, all based on a single molecular system. To address the stability of 12- in water, we compared the reduction potentials of 1 in various solvents (Figure 3). The second reduction potential for PDI-PEG in water (-0.46 V vs SCE) and methanol (-0.49 V) is considerably less negative than that in dimethyl formamide (-0.71 V) and dichloromethane (-0.76 V). Thus, protic solvents appear to stabilize the PDI dianion. This specific effect is not due to the difference in polarities, as DMF and methanol have similar dielectric constants (εDMF ) 38, εMeOH ) 33), suggesting that the stabilization may be due to hydrogen bonding between water molecules and the negatively charged PDI dianion. This and the electrostatic repulsion may be responsible for the mostly disaggregated state of PDI dianions in water. In order to gain insight into the properties of the PDI dianion, we performed electronic structure calculations on PDI and PDI2models (bearing H substituents at imide positions) using DFT at the B3LYP/6-31++G** level of theory (see the Supporting Information for computational details). Both molecules have planar D2h geometries, yet their π-electron distributions differ significantly, resulting from the dissimilar natures of the HOMOs (Figures 4 and 5). Comparison of the NPA charges in PDI and PDI2- reveals that most of the excess charge in the dianion is localized on the oxygen atoms and the three peripheral carbon atoms of the aromatic rings adjacent to the imides (see Figure 4 and the Supporting Information), consistent with the change in the frontier orbitals (Figure 5). TDDFT calculations reveal two main electronic transitions for PDI2-: 538 nm (f ) 1.049) and 570 nm (f ) 0.165), which is in good agreement with the
Letters
J. Phys. Chem. B, Vol. 112, No. 30, 2008 8857
Figure 4. Left: AICD plots. Clockwise direction of AICD arrows indicates diatropic current (aromaticity); counter-clockwise direction indicates paratropic current (antiaromaticity). Right: Excess charge distribution (NPA charge difference between PDI2- and PDI).
Figure 5. Orbital diagrams of PDI and PDI2- model systems.
experiment (peaks at 541 and 609 nm). Optical spectroscopy, electrochemistry, and TDDFT calculations show that the excited state of 12- lies at an energy ca. 2.0 eV higher than that of neutral PDI, implying a high photoredox power for PDI2(Figure 3). Stability toward protonation, spectral properties, and DFT calculations suggest that 12- has a considerable aromatic character. In order to visualize delocalized electrons, we calculated the anisotropy of the induced current density (AICD),28 which revealed significant π-delocalization in the dianion that involves the entire PDI system (Figure 4), whereas in the neutral PDI the two π-electron systems of naphthalene units are somewhat detached from each other. To estimate aromaticity, we calculated the nucleus-independent chemical shifts, NICS.29 The four aromatic rings adjacent to the imide show decreased aromaticity in PDI2- (NICS(1): -8.27 for PDI, -4.05 for PDI2-), while the central ring transforms from nonaromatic in PDI (NICS(1) ) 1.65) to strongly aromatic in PDI2- (NICS(1) ) -9.04). The cyclic imide moieties transform from nonaromatic in PDI (NICS(1) ) -0.08) to somewhat aromatic in PDI2- (NICS(1) ) -4.64), indicating significant involvement of the imides in charge delocalization. This aromaticity trend is further supported by the changes in bond lengths and bond orders of PDI and PDI2- (see the Supporting Information) and by AICD plots (Figure 4). Thus, PDI remains mostly aromatic following the acceptance of two electrons, reflecting the tendency of charged aromatic systems to gain aromaticity through reorganization of their π-electron delocalization mode.9 Similarly to the reported pericondensed polycyclic aromatic systems,11 neutral PDI features dominant delocalization over local arrays (two naphthalene systems), while
in PDI2- peripheral delocalization is prevailing. Both modes lead to the overall aromatic character of both PDI and PDI2-. The relatively large size of the PDI system and the presence of two terminal imide groups lead to the minimized Coulombic repulsion in the dianion,7 resulting in further stabilization of PDI2-. PDIs are readily available organic dyes, exceptionally stable and versatile: they are widely utilized in solar cells and organic electronics,21,30–38 as well as in the supramolecular artificial photosynthetic systems.20,21,35,39–42 Convenient access to stable photoactive PDI dianion in water provides a new “functionality tool-box” in such systems. As the PDI dianion strongly absorbs visible light and possesses significant emission, its high excitation energy may be utilized in the systems relevant to solar energy conversion.14,20 The use of PDI dianion-based systems as ground-state or photoexcited electron reservoirs43 in water can also be envisaged. In conclusion, we have prepared a perylene diimide aromatic dianion that is almost indefinitely stable in aqueous medium. Our experimental and theoretical studies reveal extensive charge delocalization and aromatic character of PDI dianion; these properties are the consequence of the PDI structure and are responsible for the stability of the dianion. Reversible charging renders PDIs useful for controlled electron storage and release in aqueous media, associated with change in optical and magnetic properties. The photophysical properties of the dianion can lead to applications of PDI-based systems in high energy electron transfer chemistry. The studies toward these applications are currently underway in our group. Acknowledgment. This work was supported by research grants from Israel Science Foundation and Sir Harry Djanogly, CBE. We thank Prof. M. R. Wasielewski and Ms. T. Wilson (Northwestern University) for their assistance with EPR measurements and Dr. L. Kronik (Weizmann Institute) for valuable discussions. We thank Prof. R. Herges and his group (University of Kiel) for providing a copy of AICD software and advice regarding its use. B.R. holds the Abraham and Jennie Fialkow Career Development Chair. Supporting Information Available: Experimental and computational procedures. This material is available free of charge via the Internet at http://pubs.acs.org.
8858 J. Phys. Chem. B, Vol. 112, No. 30, 2008 References and Notes (1) Katz, T. J. J. Am. Chem. Soc. 1960, 82, 3784–3785. (2) Holy, N. L. Chem. ReV. 1974, 74, 243–277. (3) Ichikawa, M.; Tamaru, K. J. Am. Chem. Soc. 1971, 93, 2079–2080. (4) Mu¨llen, K. HelV. Chim. Acta 1978, 61, 2307–2317. (5) Mu¨llen, K. HelV. Chim. Acta 1978, 61, 1296–1304. (6) Mu¨llen, K.; Huber, W.; Meul, T.; Nakagawa, M.; Iyoda, M. J. Am. Chem. Soc. 1982, 104, 5403–5411. (7) Mu¨llen, K. Chem. ReV. 1984, 84, 603–646. (8) Huber, W.; Mu¨llen, K. Acc. Chem. Res. 1986, 19, 300–306. (9) Rabinovitz, M.; Ayalon, A. Pure Appl. Chem. 1993, 65, 111–118. (10) Cohen, Y.; Klein, J.; Rabinovitz, M. J. Am. Chem. Soc. 1988, 110, 4634–4640. (11) Rabinovitz, M.; Willner, I.; Minsky, A. Acc. Chem. Res. 1983, 16, 298–304. (12) Willner, I.; Becker, J. Y.; Rabinovitz, M. J. Am. Chem. Soc. 1979, 101, 395–401. (13) Aprahamian, I.; Wegner, H. A.; Sternfeld, T.; Rauch, K.; De Meijere, A.; Sheradsky, T.; Scott, L. T.; Rabinovitz, M. Chem. Asian J. 2006, 1, 678–685. (14) Fox, M. A. Chem. ReV. 1979, 79, 253–273. (15) Frim, R.; Rabinovitz, M.; Muszkat, K. A. Chem. Commun. 1987, 10, 770–771. (16) Gosztola, D.; Niemczyk, M. P.; Svec, W.; Lukas, A. S.; Wasielewski, M. R. J. Phys. Chem. A 2000, 104, 6545–6551. (17) Shenhar, R.; Willner, I.; Preda, D. V.; Scott, L. T.; Rabinovitz, M. J. Phys. Chem. A 2000, 104, 10631–10636. (18) Zollinger, H. Color Chemistry, 3rd ed.; VCH: Weinheim, Germany, 2003. (19) Langhals, H. HelV. Chim. Acta 2005, 88, 1309–1343. (20) Wasielewski, M. R. J. Org. Chem. 2006, 71, 5051–5066. (21) Wu¨rthner, F. Chem. Commun. 2004, 1564–1579. (22) Ryu, J. H.; Jang, C. J.; Yoo, Y. S.; Lim, S. G.; Lee, M. J. Org. Chem. 2005, 70, 8956–8962. (23) Lu, W.; Gao, J. P.; Wang, Z. Y.; Qi, Y.; Sacripante, G. G.; Duff, J. D.; Sundararajan, P. R. Macromolecules 1999, 32, 8880–8885. (24) Hoch, H.; Hiller, H. (BASF AG) DE 78-2803362, 1978. (25) Hoch, H.; Hiller, H. (BASF AG) DE 78-2837731, 1980.
Letters (26) Rahe, N.; Rinn, C.; Carell, T. Chem. Commun. 2003, 17, 2120– 2121. (27) Marcon, R. O.; Brochsztain, S. Langmuir 2007, 23, 11972–11976. (28) Geuenich, D.; Hess, K.; Kohler, F.; Herges, R. Chem. ReV. 2005, 105, 3758–3772. (29) Chen, Z. F.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. V. Chem. ReV. 2005, 105, 3842–3888. (30) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119–1122. (31) Breeze, A. J.; Salomon, A.; Ginley, D. S.; Gregg, B. A.; Tillmann, H.; Horhold, H. H. Appl. Phys. Lett. 2002, 81, 3085–3087. (32) Yoo, B.; Jung, T.; Basu, D.; Dodabalapur, A.; Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J. Appl. Phys. Lett. 2006, 88, 082104. (33) Yakimov, A.; Forrest, S. R. Appl. Phys. Lett. 2002, 80, 1667–1669. (34) Neuteboom, E. E.; van Hal, P. A.; Janssen, R. A. J. Chem.sEur. J. 2004, 10, 3907–3918. (35) Elemans, J. A. A. W.; Van Hameren, R.; Nolte, R. J. M.; Rowan, A. E. AdV. Mater. 2006, 18, 1251–1266. (36) Oh, J. H.; Liu, S.; Bao, Z.; Schmidt, R.; Wu¨rthner, F. Appl. Phys. Lett. 2007, 91, 212107/1–212107. (37) De Witte, P. A. J.; Hernando, J.; Neuteboom, E. E.; Van Dijk, E. M. H. P.; Meskers, S. C. J.; Janssen, R. A. J.; Van Hulst, N. F.; Nolte, R. J. M.; Garcia-Parajo, M. F.; Rowan, A. E. J. Phys. Chem. B 2006, 110, 7803–7812. (38) Zhan, X. W.; Tan, Z. A.; Domercq, B.; An, Z. S.; Zhang, X.; Barlow, S.; Li, Y. F.; Zhu, D. B.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2007, 129, 7246–7247. (39) Wu¨rthner, F.; You, C. C.; Saha-Mo¨ller, C. R. Chem. Soc. ReV. 2004, 33, 133–146. (40) Sautter, A.; Kaletas, B. K.; Schmid, D. G.; Dobrawa, R.; Zimine, M.; Jung, G.; van Stokkum, I. H. M.; De Cola, L.; Williams, R. M.; Wu¨rthner, F. J. Am. Chem. Soc. 2005, 127, 6719–6729. (41) Wu¨rthner, F. Pure Appl. Chem. 2006, 78, 2341–2349. (42) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491–1546. (43) Astruc, D. Bull. Chem. Soc. Jpn. 2007, 80, 1658–1671.
JP8029743
