J. Phys. Chem. B 2001, 105, 7139-7144
7139
Formation and Characterization of the π-Radical Cation and Dication of π-Extended Tetrathiafulvalene Materials Dirk M. Guldi,*,† Luis Sa´ nchez,‡ and Nazario Martı´n*,‡ Radiation Laboratory, UniVersity of Notre Dame, Indiana, 46556, and Departamento de Quı´mica Orga´ nica, Facultad de Quı´mica, UniVersidad Complutense, E-28040 Madrid, Spain ReceiVed: January 4, 2001; In Final Form: April 13, 2001
The π-radical cation as well as the dication species of a series of quinonoid π-extended TTF have been generated by means of time-resolved and steady-state radiation chemical techniques. The spectral characteristics of the π-radical cation revealed fingerprint absorptions, which are predominantly in the red region of the spectrum (570-690 nm). The maxima vary markedly upon altering (i) the substitution pattern of the TTF rings (i.e., H, S-CH3 and S-CH2-CH2-S) and (ii) the different backbone structures (i.e., anthracene, tetracene, and pentacene). In accordance with the electrochemical studies, which give rise to a single two-electron oxidation step forming directly the π-extended TTF dication, the π-radical cation intermediate is short-lived and decays on a time scale of several tens of milliseconds. Clean second-order decay dynamics, indicative for a disproportionation reaction of the π-radical cation into the corresponding dications, govern the instability of the π-radical cation. By contrast the dication species is stable without showing a significant degradation over minutes. All the generated dications show transitions that are significantly shifted to the blue (455-490 nm) relative to the π-radical cation intermediates and are dominated by the polyacenic unit.
Introduction The reactivity of tetrathiafulvalenes (TTF) attracted considerable attention, since they emerged as an important category of organic electron donors. One of the most dominant reasons for this interest is that TTF undergoes two reversible and wellseparated one-electron oxidation steps, yielding the stable π-radical cation (TTF•+) and dication (TTF2+), respectively.1 The low potential values, at which these two oxidation processes occur, stem primarily from the high-lying HOMO of -6.81 eV (HF/6-31G*), in combination with the gain of aromaticity that the oxidized donor reveals. These attractive characteristics evoked their use in the preparation of TTF-containing sensors2 and supramolecular switches.3 Hereby, the three stable forms, namely, TTF, TTF•+, and TTF2+, have all been characterized in detail by electrochemistry as well as by absorption spectroscopy.1,4 Much less is known about p-quinodimethane analogues of TTF (π-extended TTF 1-9), in which aromatic arenes that possess larger aromatic stabilization energies have been added to the heteroaromatic rings. Incorporating extended polyacenic units as π-conjugated spacers between the dithiole rings leads to a more sophisticated class of TTF analogues with strong electron donor attributes.5 In particular, lower oxidation potentials, better charge delocalization, and lower Coulombic repulsion are important incentives guiding the design of novel donor π-systems. Not surprisingly, these materials have been used with great success as electron-donating building blocks in artificial photosynthetic models6 or as materials with marked nonlinear optical properties.7 Another interesting application implicates crown-annelated π-extended TTFs, in which the crown ether moiety is located at the dithiole rings, as complexing agents for the recognition of metal ions8 and for the synthesis of electroactive donor-acceptor dyads.9 † ‡
University of Notre Dame. Universidad Complutense.
As far as the redox chemistry of TTF and π-extended TTF is concerned, significant differences are seen: In contrast to the redox reactivity of the parent TTF, π-extended TTF moieties exhibit only a single, quasireversible oxidation wave involving, however, the transfer of two electrons to form the stable dicationic species.10a Moreover, the oxidation potential measured for lead compound 1 (E1ox ) 0.44 V vs SCE) is slightly higher than the first oxidation potential reported for the parent TTF (E1ox ) 0.34 V vs SCE). Support for these experimental findings is lent from the calculated MO energies (1: HOMO ) -6.89 eV), which predict that the HOMO in 1 lies slightly below the HOMO of TTF (-6.81 eV). This feature has been theoretically rationalized by the higher degree of stability that the dication exhibits relative to that of the π-radical cation, which subsequently explains the coalescence of the two one-electron oxidation potentials under the same oxidation wave.10a In fact, optimization of the two-electron oxidized state reveals that, in contrast to the parent TTF, p-quinodimethane analogues (1-9) exhibit a major structural change in the oxidation process. In particular, the central anthracenediylidene ring, which has a butterfly shape in the neutral molecule, transforms into a planar and fully aromatic anthracene unit, with the heteroaromatic 1,3dithiolium cations in an almost orthogonal disposition.5,10a In the context of donor-acceptor ensembles, considerable progress has been made toward improving the lifetime of the charge-separated state via the selection of π-extended TTF donor moieties. One of their features, that is, to be nonaromatic in the ground state, but to transform into an aromatic π-radical cation or dication upon oxidation, led to the following observation: The gain of aromaticity and planarity associated with the oxidation of the π-extended TTF electron donor stabilizes the photolytically generated radical pair in a C60-(π-extended) TTF dyad by several orders of magnitude relative to dyads whose donor molecules lose rather than gain aromaticity upon oxidation.
10.1021/jp010031w CCC: $20.00 © 2001 American Chemical Society Published on Web 06/28/2001
7140 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Guldi et al.
Despite the broad interest in these p-quinodimethane analogues of TTF, no spectroscopic characterization (i.e., transient absorption spectroscopy) has yet been reported dealing with the formation and stability of the π-radical cation or dication in these π-extended TTF derivatives.11 In this work, these critical issues are addressed through a series of steady-state and timeresolved pulse radiolytic investigations. Here, the overriding objective is to evaluate the optical and kinetic data, as they evolve following the initial, radical-induced oxidation of π-extended TTF 1-9. A fundamental advantage of the radiation chemical technique chosen is the identification and characterization of stable products as well as short-lived reactive intermediates. Results and Discussion The inaccessibility of the π-extended TTFs radical cation by, for example, chemical oxidation techniques or conventional cyclic voltametry prompted us to probe the selective oneelectron oxidation by radiation chemical methods. In particular, the reaction under investigation is the one-electron transfer from the ground state of π-extended TTFs 1-9 to the [CH2Cl2]•+ radical cation and the two different peroxyl radicals, •OOCH2Cl and •OOCHCl2 (vide infra), i.e.
π-extended TTF + [CH2Cl2]•+ f (π-extended TTF)•+ + CH2Cl2 (1) π-extended TTF + •OOCH2Cl/•OOCHCl2 f (π-extended TTF)•+ + -OOCH2Cl/-OOCHCl2 (2) The technique chosen for experimental verification was timeresolved pulse radiolysis following a short electron pulse (∼50 ns), known to be one of the most powerful tools to investigate reactive intermediates, complemented by additional steady-state γ-radiolysis experiments.12 Applying high solute concentrations, typically on the order of ∼0.1 mM, under typical pulse radiolysis conditions, which produces around 8 µM radicals or less, should limit the radical-induced oxidation of any given substrate to a single one-electron oxidation step. By contrast, subsequent oxidation of the generated π-radical cation to the corresponding dication, such as (π-extended TTF)2+, is implausible. The pseudo-first-order conditions, that is, a large excess of the starting material, hinder this second oxidation. The electron-transfer studies were carried out in oxygenated CH2Cl2, since this medium provides excellent solubility for the π-extended TTFs. More importantly, upon radiolysis, the generation of the short-lived, strongly oxidizing solvent radical cation ([CH2Cl2]•+) takes place (eq 3a).13 The radiolysis of dichloromethane also leads to the carbon-centered •CH2Cl and •CHCl radicals, particularly in the absence of a radical cation 2 scavenger (eqs 3b/3c). These are formed via dissociative electron capture and proton loss, respectively. In oxygenated solvents the radical formation is followed by a rapid reaction with molecular oxygen, yielding the respective oxidizing •OOCH2Cl and •OOCHCl2 peroxyl radicals (eqs 3d/3e). These latter species may also contribute to the oxidation of the π-extended TTFs, although probably to a lesser extent, as can be concluded from several reports on the peroxyl radical induced oxidation of sulfur organic molecules.14 It is important to note that [CH2Cl2]•+ itself does not react with molecular oxygen. radiolysis
CH2Cl2 98 [CH2Cl2]•+ + e-sol
(3a)
Figure 1. π-extended TTF derivatives (1-9) used in this study.
CH2Cl2 + e-sol f •CH2Cl + Cl-
(3b)
[CH2Cl2]•+ f •CHCl2 + H+
(3c)
•
CH2Cl + O2 f •OOCH2Cl
•
CHCl2 + O2 f •OOCHCl2
(3d) (3e)
Via the above outlined reaction sequence, clean experimental conditions are given that yield only oxidizing entities, without, however, affecting or involving directly the substrate species or its chemical nature, as taking place in photoexcitation or photoionization experiments. For the current work we selected a wide array of π-extended TTFs with (i) different substitution pattern of the TTF rings (i.e., H, S-CH3 and S-CH2-CH2-S) and (ii) different backbone structures (i.e., anthracene, tetracene, and pentacene). Changing these two parameters allows fine-tuning of the oxidation potentials over a wide range, typically between +0.39 and +0.65 V versus SCE. Compounds 1-9 (Figure 1) were synthesized according to the methods reported in the literature.5,10a Thus, a Wittig-Horner reaction of the respective quinones with differently substituted 1,3-dithioles endowed with phosphonate esters in the presence of butyllithium at -78 °C affords the corresponding π-extended TTFs (1-9) as air-stable orange solids in moderate to good yields. The structure of the π-extended donors 1-9 was supported by their analytical and spectroscopic data and the UV-vis spectra show a low-energy absorption band appearing above 400 nm, which has been assigned to an intramolecular charge transfer from the 1,3dithiol-2-ylidene moieties to the laterally fused aromatic rings acting as the acceptor.10a Upon pulse radiolytic oxidation of an oxygen-saturated dichloromethane solution of 1, strong absorption changes, were noted throughout the entire visible part of the spectrum. A net decrease of the absorption centered around 430 nm, a region which is governed, in large, by the strong ground-state absorption of the π-extended TTF moiety. Although these spectral changes occurred principally in two steps with a fast (∼2 µs) and a slow (∼140 µs) component, the product is the same (similar to Figure 2b). This suggests, in principle, consumption of 1 as a result of a reaction with [CH2Cl2]•+ (fast reaction) and •OOCH2Cl/•OOCHCl2 radicals (slow reaction) according to eqs 1 and 2, respectively. In addition, spectral analysis in the red gave evidence for a spectral fingerprint: In particular, a broad maximum centered around 660 nm was detected for 1 (Figure 2a), whose formation also took place in a two-step process (Figure 2b). Important is the observation that the decay and grow-in dynamics at these wavelengths are virtually identical, which leads to the important conclusion that the nature of the species is unequivocally the same. Taking these evidences
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J. Phys. Chem. B, Vol. 105, No. 29, 2001 7141 TABLE 1: Spectral and Kinetic Properties of π-Extended TTF 1-9
compd
ground state λmax (nm)
π-radical cation λmin (nm)
π-radical cation λmax (nm)
koxidation (M-1 s-1)
dication λmax (nm)
TTF 1 2 3 4 5 6 7 8 9
428 434 446 400 420 433 403 407 419
430 435 445 400 420 435 400 410 425
430, 580 660 675 690 620 620 630 570 575 595
4.2 × 108 a 5.2 × 108 0.43 × 108 0.85 × 108 7.2 × 108 0.69 × 108 0.35 × 108 0.41 × 108 0.33 × 108 0.47 × 108
353 470 455 480 460 473 487 485 485 490
a
Figure 2. Radiolytic oxidation of 1 in oxygenated dichloromethane solutions. (a) Differential absorption spectrum (UV-vis spectrum) monitored by pulse radiolysis, 200 µs after the pulse (i.e., after completion of the one-electron oxidation reaction). (b) Time-absorption profile at 660 nm.
in concert, we assign the new transient to features associated with the one-electron oxidized product, namely, (π-extended TTF)•+. The π-radical cation absorption was stable over several hundred microseconds (i.e., 500 µs) without exhibiting a significant change on this short time scale. Further support for our assignment was lent from recent studies probing either photoionization15 of a π-extended TTF moiety or photoinduced electron transfer in a π-extended TTFcontaining donor-acceptor ensemble.6 Importantly, the characteristics of the products, evolving from these photochemically induced processes and also from our present radiation chemically induced oxidation processes are in excellent agreement with each other. This can be taken as a clear sign for the comparability of these time-resolved techniques, despite their fundamental differences. Principally, similar absorption changes were recorded in radiolytic experiments with π-extended TTF compounds 2-9. The absorption minima (i.e., bleaching of the ground state) and maxima (i.e., new transient absorption) of the π-radical cations of 1-9 are collected in Table 1. It should be noted that the absorption features are, however, strikingly different from those known for the π-radical cation of the parent TTF. The latter species, which, in contrast to those of the π-extended TTFs, is stable even on an electrochemical time scale, exhibits only one strong maximum around 430 nm, shown in Figure 3. By contrast the visible and red regions lack spectral attributes, such as bleaching or a new band, respectively. On closer inspection of Table 1, the minima of the bleaching part in the transient absorption spectra reveal an interesting direction. This is illustrated in Figure 4: Successive and gradual red shifts, which are nice mirror images to the corresponding
Taken from ref 18.
Figure 3. Differential absorption spectrum (UV-vis spectrum) monitored 200 µs after pulse radiolytic oxidation of TTF in oxygenated dichloromethane solutions.
Figure 4. Differential absorption spectrum (UV-vis spectrum) monitored 200 µs after pulse radiolytic oxidation of 1, 2, and 3 in oxygenated dichloromethane solutions.
ground-state maxima (Figure 5), were seen upon varying the substitution pattern of the TTF rings, for example, from R ) -H (430 nm) (1), R ) -S-CH3 (435 nm) (2), and R ) -S-CH2-CH2-S- (445 nm) (3). The electronic effects stemming from the substitution pattern, that is +M, -I effects, also impact the transitions of the π-radical cation maxima in the red region, leading, essentially, to parallel red shifts ((1) 660 nm, (2) 675 nm, (3) 690 nm). The tetracene- (4-6) and pentacene-based (7-9) analogous reveal the same basic trends, namely, red-shifted features in the ground and oxidized state for the S-CH3 and (-S-CH2-CH2-S-)-derivatives. Surprisingly, the trends seen for the ground state and also π-radical cation absorption of 1 (428 nm, 660 nm) in comparison to 4 (400 nm, 620 nm) and 7 (403 nm, 570 nm) contrasts the
7142 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Guldi et al. TABLE 2: Kinetic and Electrochemical Properties for the Oxidation of π-Extended TTF 1-9
b
compd
koxidation (M-1 s-1)
E1/2a (oxidation) (V vs SCE)
1 2 3 4 5 6 7 8 9
5.2 × 108 0.43 × 108 0.85 × 108 7.2 × 108 0.69 × 108 0.35 × 108 0.41 × 108 0.33 × 108 0.47 × 108
0.44b 0.52c 0.50d 0.39b 0.55b 0.52b 0.50b 0.63b 0.65b
a Taken from ref 10a-c, respectively; measured in dichloromethane. XXX. c XXX. d XXX.
Figure 5. Ground-state absorption spectra of 1, 2, and 3 in dichloromethane.
Figure 6. Plot of kobs ) ln 2/τ1/2 vs [π-extended TTF] for the pulse radiolytic oxidation of 1, 2, and 3 by a bimolecular reaction with •OOCH Cl and •OOCHCl peroxyl radicals. 2 2
features of the pristine polyacenic units, anthracene, tetracene, and pentacene. In fact, lower energies are seen typically for the pristine backbones with increasing conjugation. From this we must infer, that the interruption of the backbone’s π-system, imposed by the heteroaromatic rings and the highly distorted, butterfly-shaped structure, is indeed quite significant. This leads, in turn, to isolated benzene (1 and 4) and naphthalene rings (4 and 7) separated by the p-quinodimethane moieties. It is important to note that the laternally fused benzene and naphthalene units preserve their planarity and structural aromaticity. An alternative assignment of the bathochromic shifts of the ground-state absorption implied an intramolecular chargetransfer band at lower energies. The observed rates (kobs) for the one-electron oxidation step were linearly dependent upon the π-extended TTF concentration in a range between 7 and 270 µM. Figure 6 depicts the corresponding relationships for 1-3 in their reaction with •OOCH Cl and •OOCHCl radicals (i.e., slower component). 2 2 In general, increasing the π-extended TTF concentration resulted in an acceleration of the decay and grow-in dynamics of the corresponding π-radical cation features in the visible (350450 nm) and far visible (450-800 nm), respectively, without, however, effecting the different absorption changes. In all cases, the kinetics followed eq 4, where kobs refers to the observed first-order rate constant of the π-radical cation generation that enables us to derive the bimolecular rate constant ket as a function of the substrate concentration [S] (i.e., π-extended TTF).
kobs ) kd + ket[S]
(4)
Figure 7. Radiolytic oxidation of 1 in oxygenated dichloromethane solutions. γ-Radiolytic oxidation doses: 0, 50, 150, 350, 550, and 750 Gy. The arrows indicate the direction of absorption changes during the course of irradiation.
The linearity observed for this relationship infers that the underlying process involves a reaction between the π-extended TTF and the oxidizing •OOCH2Cl and •OOCHCl2 radicals (see Figure 6). The bimolecular rate constants for 1-9 are summarized in Table 1. They are similar and range from 0.33 × 108 to 7.2 × 108 M-1 s-1. It is interesting to note that the determined values imply some activation energy and that even the highest rate constant is still markedly different from the diffusion-controlled limit in this solvent (kdiff ) 1.1 × 1010 M-1 s-1). Correlating the rate constants of the radical-induced oneelectron oxidation (ket) with the oxidation potentials of the extended TTFs reveals a clear trend (see Table 2): The rate constants for the oxidation of 1 and 4 are noticeably larger relative to those derived for 2, 3, and 5-9. This observation coincides well with the reported oxidation potentials of 1-9, revealing the lowest values for 1 (0.44 V vs SCE) and 4 (0.39 V vs SCE). With the scope to examine (i) the stability of the π-radical cation and (ii) the second one-electron oxidation step, yielding the corresponding dications of 1-9, complementary steady-state γ-radiolysis was conducted. In particular, the optical absorption spectra of π-extended TTFs 1-9 in oxygenated CH2Cl2 were recorded before and after several irradiation intervals (Figure 7). The peaks of the starting material, for example of 1, at 361 and 428 nm decayed gradually upon irradiation, while new peaks at 364 and 468 nm were formed. In general, these spectral features are markedly blue-shifted from those observed at short times after the electron pulse (140 µs). Specifically, no evidence
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J. Phys. Chem. B, Vol. 105, No. 29, 2001 7143
for the characteristic fingerprint of the π-radical cation was found in the region around 660 nm. However, the newly formed species is stable and no subsequent changes in absolute absorption, following the initial radiation, were detected over several hours. This leads us to conclude that the product may result from the following disproportionation of the short-lived π-radical cation:
2(π-extended TTF)•+ f (π-extended TTF)2+ + π-extended TTF (5) Furthermore, the good isosbestic points (at 336 and 450 nm) corroborate undoubtedly a clean radiolytic transformation of the starting ground state into the stable product. γ-Radiolysis of similar solutions of π-extended TTFs 2-9 resulted also in a gradual disappearance of the 400 nm peaks and simultaneous formation of new peaks in the red, around 460 nm (see Table 1). To confirm the assignment of the different absorption spectra and to compare their behavior with that reported in the literature, we oxidized, for example, 1 with the chemical oxidant Br2. The reaction of 1 with Br2 in dichloromethane resulted in the formation of new peaks at 364 and 468 nm, identical with those formed in the γ-radiolysis in dichloromethane and in good agreement with a previous study on the spectroelectrochemical oxidation of a π-extended TTF, where the corresponding spectrum was attributed to the dication. These results support the conception that the stable species, formed in the γ-radiolytic oxidation, is truly the (π-extended TTF)2+ as the stable product evolving from an oxidation of π-extended TTFs. Conversely to the ground-state and π-radical cation absorption, the transitions of the parent dicationic structures, for which an aromatic polyacenic backbone (i.e., anthracene, tetracene, and pentacene) is predicted as a basic constituent, reveal lower energies for the larger conjugated π-system. From this observation we reach the important conclusion that the two-electron oxidation process, indeed, invalidates the interruption of the π-system, transforming the quinonoid structure with laternally fused benzene and naphthalene rings into a fully conjugated polyacenic π-system. On the other hand, the +M, -I effects still prevail in effecting the maxima of the R ) -H, R ) -S-CH3, and R ) -S-CH2-CH2-S- derivatives within the 1-3 (anthracene-based), 4-6 (tetracene-based), and 5-9 series (pentacene-based). To inspect the disproportionation in question, we decided to revisit the pulse radiolysis experiments. Extending the time scale to 40 ms and beyond led to the anticipated decrease of the π-radical cation absorption (1) in the 660 nm range. Concomitant with this decay is the grow-in of a new maximum around 470 nm, as shown in Figure 8a. Most importantly, both decay (see Figure 8b; 460 and 680 nm) and grow-in dynamics (see Figure 8b; 470 nm) appear to obey mainly a second-order rate law, as inferred from the radical concentration dependence (i.e., between 5 and 15 µM). Variation of the initial radical concentration, for example, by a factor of 2 led to the expected 2-fold change in lifetime. From this we derived a second-order rate constant of 2k ∼ 103 for eq 5. Another argument in support of the proposed disproportionation mechanism implies the spectral resemblance of the newly formed 470 nm species with that of the stable product evolving during the steady-state γ-radiolysis (Figure 7). It should be noted that not only the π-radical cation and dication of the parent TTF (see Table 1) but also those of various π-extended TTF derivatives, bearing an additional vinyl group
Figure 8. Radiolytic oxidation of 1 in oxygenated dichloromethane solutions. (a) Differential absorption spectrum (UV-vis spectrum) monitored by pulse radiolysis, 40 ms after the pulse (i.e., after completion of the disproportionation reaction). (b) Time-absorption profiles at 460, 470, and 680 nm.
or a furan ring as spacers connecting the two dithiole rings, give rise to a similar blue shift of their corresponding absorptions.16,17 Conclusion In summary, we have given the first experimental evidence for the formation of the π-radical cation and dication species of a series of π-extended TTFs by using time-resolved and steady-state radiolytic techniques. The study makes use of the defined (i.e., pseudo-first-order) conditions that limit the oxidation process to a single one-electron transfer and benefits from the rather strong absorption of the π-radical cations in the red (i.e., between 570 and 690 nm). This fingerprint absorption will emerge as an important probe for their identification in more sophisticated structures used, for example, in the preparation of optoelectronic devices involving similar types of TTF analogous. Acknowledgment. We are indebted to the DGESIC of Spain (Project PB98-0818) for financial support. Part of this work has been supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. This is document NDRL-4302 from the Notre Dame Radiation Laboratory. References and Notes (1) (a) Wudl, F.; Smith, G. M.; Hufnagel, E. J. J. Chem. Soc., Chem. Commun. 1970, 1453. (b) Hu¨nig, S.; Kiesslich, G.; Schentzow, D.; Zahradnik, R.; Carsky, P. Int. J. Sulfur Chem. C 1971, 6, 109. (2) (a) Bang, K. S.; Nielsen, M. B.; Zurbarev, R.; Becher, J. Chem. Commun. 2000, 215. (b) Johnston, B.; Goldenberg, L. M.; Bryce, M. R.; Kataky, R. J. Chem. Soc., Perkin Trans 2 2000, 189. (c) Le Derf, F.; Mazari, M.; Merciev, N.; Levillain, E.; Richomme, P.; Becher, J.; Garı´n, J.; Orduna,
7144 J. Phys. Chem. B, Vol. 105, No. 29, 2001 J.; Gorgues, A.; Salle´, M. Chem. Commun. 1999, 1417. (d) Lin, H.; Liu, S.; Echegoyen, L. Chem Commun. 1999, 1493. (3) Ashton, P. R.; Balzani, V.; Becher, J.; Credi, A.; Fyfe, M. C. T.; Mattersteig, G.; Menzer, S.; Nielsen, M. B.; Rayuno, F. M.; Stoddart, J. F.; Venturi, M.; Willians, D. J. J. Am. Chem. Soc. 1999, 121, 3951. (4) (a) Hu¨nig, S.; Kiesslich, G.; Quast, H.; Schentzow, D. Liebigs Ann. Chem. 1973, 310. (b) Schukat, G.; Fangha¨nel, E. J. Prakt. Chem. 1985, 327, 767. (5) (a) Yamashita, Y.; Kobayashi, Y.; Miyashi, T. Angew. Chem., Int. Ed. Engl. 1989, 28, 1052. (b) Bryce, M. R.; Moore, A. J.; Hasan, M.; Ashwell, G. J.; Fraser, A. T.; Clegg, W.; Hursthouse, M. B.; Karaulov, A. I. Angew. Chem., Int. Ed. Engl. 1990, 29, 1450. (6) Martı´n, N.; Sa´nchez, L.; Guldi, D. M. Chem. Commun. 2000, 113. (7) Herranz, M. A.; Martı´n, N.; Sa´nchez, L.; Garı´n, J.; Orduna, J.; Alcala´, R.; Villacampa, B.; Sa´nchez, C. Tetrahedron 1998, 54, 11651. (8) Bryce, M. R.; Batsanov, A. S.; Finn, T.; Hansen, T. K.; Howard, J. A. K.; Kamenjicki, M.; Lednev, I. K.; Asher, S. A. Chem. Commun. 2000, 295. (9) (a) Christensen, C. A.; Bryce, M. R.; Batsanov, A. S.; Howard, J. A. K.; Jeppesen, J. O.; Becher, J. Chem. Commun. 1999, 2433. (b) Herranz, M. A.; Martı´n, N. Org. Lett. 1999, 1, 2005. (10) (a) Martı´n, N.; Sa´nchez, L.; Seoane, C.; Ortı´, E.; Viruela, P. M.; Viruela, R. J. Org. Chem. 1998, 63, 1268. (b) Saito, K.; Sgiura, Ch.; Tanimoto, E.; Saito, K.; Yamashita, Y. Heterocycles 1994, 38, 2153. (c) Martı´n, N.; Pe´rez, I.; Sa´nchez, L.; Seoane, C. J. Org. Chem. 1997, 62, 5690.
Guldi et al. (11) Only the parent unsubstituted 2,2′-p-quinobis(1,3-dithiole) shows two close oxidation potentials to the radical cation and dication at -0.11 and -0.04 V (CH3CN vs SCE), being a highly unstable molecule. See ref 5a. (12) Pulse radiolysis experiments were performed by utilizing 50 ns pulses of 8 MeV electrons from a Model TB-8/16-1S Electron Linear Accelerator. Steady-state irradiations were done in a Gammacell 220 60Co source with a dose rate of 1 Gy s-1. Details on such equipment, in general, and the data analysis have been described in: Hug, G. L.; Wang, Y.; Scho¨neich, C.; Jiang, P.-Y.; Fessenden, R. W. Radiat. Phys. Chem. 1999, 54, 559. (13) Shank, N. E.; Dorfman, L. M. J. Chem. Phys. 1970, 52, 4441. (14) Scho¨neich, C.; Aced, A.; Asmus, K.-D. J. Am. Chem. Soc. 1993, 115, 11376. Scho¨neich, C.; Aced. A.; Asmus, K.-D. J. Am. Chem. Soc. 1991, 113, 375. (15) The π-radical cation of the π-extended TTF (2) has been simultaneously and independently characterized. Jones, A. E.; Christensen, Ch. A.; Perepichka, D. F.; Batsanov, A. S.; Beeby, A.; Low, P. J.; Bryce, M. R.; Parker, A. W. Chem. Eur. J. 2001, 7, 973. (16) Richter, A. M.; Bauroth, J.; Fangha¨nel, E.; Kutschabsky, L.; Radeglia, R. J. Prakt. Chem. 1994, 336, 355. (17) Scho¨berl, U.; Salbeck, J.; Daub, J. AdV. Mater. 1992, 4, 41. (18) Martı´n, N.; Sanchez, L.; Herranz, M. A.; Guldi, D. M. J. Phys. Chem. A 2000, 104, 4648.
