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Highly Fluorescent Open-Shell NIR Dyes: The Time-Dependence of Back Electron Transfer in Triarylamine-Perchlorotriphenylmethyl Radicals Alexander Heckmann,†,| Stefan Du¨mmler,‡ Jutta Pauli,§ Markus Margraf,‡ Juliane Ko¨hler,‡ Dominik Stich,‡ Christoph Lambert,*,†,| Ingo Fischer,*,‡ and Ute Resch-Genger*,§ Institut fu¨r Organische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, Institut fu¨r Physikalische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, Bundesanstalt fu¨r Materialforschung und -pru¨fung, Richard-Willsta¨tter-Strasse 11, D-12489 Berlin, Germany, and Wilhelm Conrad Ro¨ntgen Research Center for Complex Material Systems, Wu¨rzburg, Germany ReceiVed: September 1, 2009; ReVised Manuscript ReceiVed: October 5, 2009
Triarylamine-perchlorotriphenylmethyl radicals (TARA-PCTM) may be viewed as open-shell mixed valence donor-acceptor compounds that exhibit strong charge-transfer (CT) bands in the visible to NIR spectral region. While open-shell molecules generally do not fluoresce at RT, we observed a surprisingly strong fluorescence from the highly polar excited CT state of the TARA-PCTM radicals in the visible and NIR spectral region. The fluorescence quantum yield is enhanced by a factor of up to 150 compared to the unsubstituted perchlorotriphenylmethyl radical. The enhancement depends on the donor strength of the TARA moiety which was tuned by small substituents (OMe, Me, Cl, CN, and NO2) attached to the phenyl groups, thus forming a series of donor-acceptor species that mainly differ by the free energy difference of the excited CT state and the ground state. The reorganization parameters of the CT process were extracted by Bixon-Jortner fits to either the absorption or the fluorescence bands. The dynamics of the nonradiative back-electron transfer were investigated by time-resolved fluorescence and transient absorption spectroscopy in the ps to ns time regime. We observed a strong deviation of the back-electron transfer rate from the expected inverted Marcus behavior which might be due to anharmonic effects. Introduction Recently, we have investigated the charge transfer behavior of a series of neutral mixed-valence (MV) compounds such as 1.1,2 These compounds, which contain two redox centers in different oxidation states, are useful model systems for the exploration of basic electron (or hole) transfer phenomena. The triarylamine (TARA) moiety acts as the electron donor and the perchlorinated triphenylmethyl (PCTM) radical as the electron acceptor.3-6 In previous work, we focused on the investigation of the steady state optical properties and of their solvent dependence. Recently, time dependent measurements in the fs to ps time domain demonstrated that the solvent dynamics govern the back electron transfer after optical excitation into the CT () charge transfer) state.7,8 In this paper, we concentrate on the dependence of back electron dynamics on substituent effects in compounds of type 2-8 in which the donor strength of the TARA moiety is varied. Investigations of charge transfer phenomena are driven by the desire to better understand fundamental principles of charge transfer as well as by the demand for development of new (opto)electronic devices for signal storage, signal transduction, light emission or solar energy conversion.9-13 Among these studies, small molecular donor-acceptor (D-π(σ)-A) systems14 are used to address questions such as the role of solvent dynamics,15 substituent effects, and conformational aspects16 or * To whom correspondence should be addressed. E-mail: lambert@ chemie.uni-wuerzburg.de. † Institut fu¨r Organische Chemie, Universita¨t Wu¨rzburg. ‡ Institut fu¨r Physikalische Chemie, Universita¨t Wu¨rzburg. § Bundesanstalt fu¨r Materialforschung und -pru¨fung. | Wilhelm Conrad Ro¨ntgen Research Center for Complex Material Systems.
the type of spacer (e.g., π vs σ).17 More complicated cascade molecules such as triades and pentades are employed to achieve long-lived charge separated states in order to emulate key steps in the photosynthetic reaction center.18 A common problem of most of these triades, tetrades, and pentades is their structural complexity associated with tedious multistep syntheses. Here, the synthesis of small molecular systems with long-lived charge separated states presents a simpler alternative.14 Among the various strategies employed to achieve long-lived charge separation, control of orbital symmetry,19 spin control,20 and inverted region effects14 are found. For the system 1, we found an intriguingly sintense fluorescence from the lowest CT state which, at RT, is uncommon for such π-radicals. The fluorescence quantum yields of the perchlorotriphenylmethyl radical21,22 and the radical cations of trimethoxybenzene,23 triarylamines,24 tetrathiafulvalene,25 and dimethyldihydrophenazine26 in fluid solution, for example, are on the order of 10-3 to 10-5. The striking observation that the perchlorotriphenyl-triarylamine conjugates fluoresce strongly at RT in fluid solution points to a relatively long-lived CT state and warrants further investigation of the photophysical properties of these simple donor-acceptor dyads.
10.1021/jp908425w CCC: $40.75 2009 American Chemical Society Published on Web 10/27/2009
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J. Phys. Chem. C, Vol. 113, No. 49, 2009 20959 ∞
ε/ν˜ )
e-SSj 2000Nπ2 (n2 + 2)2 2 µabs 3ε0 ln 10 9n j! j)0
∑
[
exp -
1 4πhcλokT
hc(jν˜ v + λo - ν˜ + ∆G00)2 4λokT
]
(1)
with the Huang-Rhys factor S ) λv/ν˜ v In our earlier studies, we observed that dyads of type 1 display a CT absorption band in the NIR region at 12000-14000 cm-1. Steckhan et al. showed that the redox potential of triarylamines can be tuned by more than 1 eV when changing the substituents from MeO to NO2.27 This encouraged us to synthesize compounds 2-8 where the donor strength of the triarylamine group is varied by small electron donating or withdrawing substituents. Here, our goal was to tune the excited state energy of the TARAPCTM CT state and to probe the influence of the TARA substitution pattern on the photophysical properties of 2-8, particularly on the back electron transfer rate. We used biaryl type chromophores 2-8 rather than stilbene type chromophores such as 1 because the 90° angle between the two aryl groups induced by the steric interaction of the ortho-chloro substituents leads to electronic decoupling of the donor and acceptor moiety which simplifies such a conceptual approach. Results Theoretical Model. In order to describe the photophysical properties of 2-8, we adopt the diabatic model developed by Bixon and Jortner, see Figure 1.28-32 In this approach, two diabatic (formally noninteracting) states are separated by the free energy difference ∆G00. The interaction of these diabatic states with the solvent bath is treated classically; that is, the solvent coordinate (and other low energy molecular vibrations) for the electronic ground state A and the excited CT state B are harmonic potentials with λo being the solvent reorganization energy. In contrast, high energy molecular vibrations are treated quantum mechanically with ν˜ v being an average molecular vibration frequency associated with the charge transfer and λv the vibronic (molecular) reorganization energy. Absorption bands can be viewed as processes starting from the minimum of potential A into the vibronic manifold of state B. This results in stacked transitions which are Gaussian broadened by solvent interactions. Thus, λo determines the vibronic resolution of the band. The sum of these stacked Gaussian bands yields the CT absorption band as given on the right-hand side of Figure 1. The sum of ∆G00, λo, and λv determines the absorption maximum, λv the width of the absorption band and the Huang-Rhys factor S ) λv/ν˜ v the vibronic coupling, that is, the number of vibronic levels that are excited, and thus the overall shape (asymmetry) of the band. Analogously, fluorescence is viewed as transitions starting from the minimum of the potential B into the vibronic manifold of state A. Back electron transfer is interpreted as internal conversion from B to A whose rate constant is k-ET. Within this Golden rule formulation, eqs 1-3 give results for the (reduced) absorption intensity, (reduced) fluorescence intensity, and back electron transfer rate constant. Thus, least-squares fits of eqs 1 and 2 to experimentally determined absorption or fluorescence spectra can yield the four reorganization parameters ∆G00, λo, λv, and ν˜ v. With these data and the electronic coupling, one is then able to calculate the back electron transfer rate using eq 3.
∞
Ifl /ν˜ 3 )
e-SSj 16 × 106π3 n(n2 + 2)2 2 µfl 3ε0 9 j! j)0
∑
[
1 4πhcλokT
hc(jν˜ v + λo + ν˜ + ∆G00)2 exp 4λokT ∞
k-ET ) 4π2hc2V2
-S j
∑ e j!S j)0
[
1 4πhcλokT
exp -
hc(jν˜ v + λo + ∆G00)2 4λokT
]
(2)
]
(3)
In these equations, V is the electronic coupling between the diabatic states and µabs and µfl are the transition moments for absorption and fluorescence, respectively. Steady State Spectroscopy. Absorption spectra of 2-8 were measured in cyclohexane as this solvent provides good solubility for all of the compounds and is very apolar. This generally results in narrower bands and in a partially visible vibronic structure (asymmetry) of the absorption and fluorescence bands. This fact allows for a better fit of experimental spectra with eqs 1 and 2. The absorption spectra of 2-8 comprise several features. They display a strong band at ca. 34 000 cm-1 which is shifted to lower energies (down to ca. 27 000 cm-1) with increasing acceptor strength of the substituents attached to the TARA moiety. This band is ascribed to localized excitations within the triarylamine moiety for R ) MeO, Me, and Cl.33 The TARAlocalized transition is red-shifted for stronger electron withdrawing substituents R like CN (7) and NO2 (8) which indicates some charge transfer character. This can clearly be seen for 6 which shows both signatures of the Cl- and the CN-substituted TARA at 34 000 and 30 000 cm-1, respectively. Our interpretation is supported by a comparison with the spectra of the precursor compounds (not shown), where the radical center is replaced by CH, and which show essentially the same absorption bands. The sharp signal at 26 000 cm-1 is typical of PCTM radicals, similarly as the weak signals at ca. 17 000-22 000 cm-1.34 Our particular attention is focused on the weak and broad bands centered between 12 500 and 18 000 cm-1 (see Figure 2b and Table 1). These bands are due to CT transitions between the TARA donor and the PCTM acceptor. For 6-8, the CT band strongly overlaps with the PCTM radical absorption bands. However, both bands are almost completely separated for 2 and 3 (both equipped with electron donating substituents R), because the CT band is bathochromically shifted with increasing donor strength of the TARA donor. A stronger TARA donor stabilizes state B which leads to a decrease of ∆G00 as sketched in Figure 1. This simplified picture holds as long as the substituents R do not influence the other reorganization parameters λo, λv, and ν˜ v. While this is certainly the case for λo, it is a matter of debate for λv and/or ν˜ v as discussed below.
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Figure 1. Harmonic diabatic potential surfaces along a classic solvent coordinate. Molecular high energy vibrations ν˜ v appear as stacked parabola for the ground state A and the excited state B. Transitions into the manifold of vibrational states lead to stacked Gaussians (green) which add to the overall (red line) absorption profile. The parameters used for the plot were arbitrarily chosen as ν˜ v ) 1290, ∆G00 ) 12900, λo ) 1160, and λv ) 900 cm-1.
Figure 2. (a) Absorption spectra of 2-8 in cyclohexane. (b) Magnification of the CT bands of 2-8.
TABLE 1: Absorption and Fluorescence Data of 2-8 in Cyclohexane absorption CT band ν˜ max/cm-1 (ε/M-1cm-1)
µabs/Da
emission ν˜ max / cm-1
〈ν˜ fl-3〉av-1/cm-1b
2 3
12700 (1300) 13150 (1650)
1.21 1.23
11300
10870
1.04
4
14400 (1900)
1.31
12250
11810
1.11
5
15500 (1650)
13100
12820
1.16
6
17400 (1300)
13800
13080
1.17
7
d
14350
13520
1.08
8
d
14750
µfl/D
14090
0.96
b
Φ
c
0.04 (384 nm) 0.15 (384 nm) 0.38 (380 nm) 0.14 (380 nm) 0.11 (380 nm) e
Φ
c
0.03 (760 nm) 0.15 (700 nm) 0.33 (645 nm) 0.37 (564 nm) 0.35 (564 nm) e
The absorption transition moment was calculated by ) [(3hcε0 ln10)/(2000π N)][(9n)/(n + 2) ]∫[(ε)/(ν˜ )]dν˜ . b The fluorescence transition moments µfl were calculated by the Strickler-Berg equation35 kf ) [(16 × 106π3)/(3hε0)][(n(n2 + 2)2)/9][(gg/ge)〈ν˜-3〉av-1µfl2] where 〈ν˜ fl-3〉av-1 ) ∫If dν˜ /∫ν˜ -3If dν˜ is the mean cubic fluorescence energy. c Excitation wavelength in parentheses. d Strong overlap with energetically higher lying band. e Decomposition. a
2 µabs
While radicals and radical ions usually show no or only extremely weak fluorescence at RT,21-26 we observed an intriguingly strong fluorescence in the NIR for 2-8 at RT in cyclohexane solution (Figure 3). Much in contrast, in more polar solvents such as toluene and dibutylether, the fluorescence was
2
2
too red-shifted and too weak to be measurable with our setup. For comparison, the PCTM radical itself has a fluorescence quantum yield Φ of 0.015 when excited at 530 nm and 0.0025 when excited at 386 nm.22 To gain further insight into the spectroscopic properties of compounds 2-8 and as a prerequisite
Highly Fluorescent Open-Shell NIR Dyes
J. Phys. Chem. C, Vol. 113, No. 49, 2009 20961 TABLE 2: Data Fitted to the Selected Absorption and Fluorescence Spectra of 2-8 in Cyclohexane using eqs 1 and 2a
Figure 3. Normalized fluorescence spectra of 2-8 in cyclohexane solution. Excitation of 2 and 3 was at 384 nm and excitation of 4-6 at 380 nm, respectively.
for the eventually desired calculation of radiative rate constants for mechanistical interpretations, we measured the emission spectra and fluorescence quantum yields at two different wavelengths, i.e., at the maximum of the PCTM-localized absorption band and at the maximum of the intramolecular CT absorption band. The exact procedure for the determination of the fluorescence quantum yields is described in detail in the Experimental Section. For mechanistic considerations it is highly desirable for fluorescence quantum yields to be available with an uncertainty of 10% or less.36,37 To achieve this the wavelengthdependent spectral irradiance was measured for all of the instrument settings used and accordingly considered38,39 The corresponding fluorescence maxima of compounds 2-8 on an energy scale and the fluorescence quantum yields as determined from spectrally corrected emission spectra are summarized in Table 1. All of the compounds studied revealed identical emission spectra independent of the excitation wavelength used. However, the fluorescence quantum yields derived from excitation at the PCTM-localized and the CT absorption band differ in a substituent-dependent fashion. Compound 6 displays quantum yields of 0.14 and 0.37 upon excitation at 380 and 564 nm, respectively. A similar trend is observed for 7. Thus, for these species, energy transfer is incomplete when excited at higher energies. For compounds 3-5, the fluorescence quantum yields are more or less unaffected by excitation wavelength. For compounds 5-7, the quantum yield approaches almost 40% which is up to a factor 150 larger than the quantum yield of the unsubstituted PCTM radical (see above). We were not able to determine the fluorescence quantum yield of 2 displaying a strongly red-shifted emission, the region of which exceeds the
∆G00/cm-1
λ0/cm-1
λv/cm-1
ν˜ /cm-1
fluorescence
3 4 5 6 7 8
12100 13550 14300 14800 15500 15750
800 1350 1100 1000 1150 1050
800 600 800 850 1000 800
1150 1350b 1250 1350 1450 1350
absorption
2 3 4
10600 11900 12600
1600 1000 1400
900 850 950
1250 1200 1250
a All values were determined with a maximum error of (50 cm-1. b Maximum error of (60 cm-1.
spectral range covered by the detection channel of our fluorometer (500 to 1100 nm, see the Experimental Section). The rather unique observation of strong fluorescence in 2-8 opens the chance to perform least-squares fits of the experimentally observed CT absorption and emission bands using eq 1 and 2 in order to extract the reorganization parameters ∆G00, λo, λv, and ν˜ v. This can be done for the absorption spectra of 2, 3, and 4 where the CT absorption band is well separated from other higher energy bands and for the emission spectra of 3-8. The resulting fits are displayed in Figure 4 and the fit results are given in Table 2. From Table 2 and the plot of ∆G00, λo, λv, and ν˜ v in Figure 5, it is immediately clear that it is ∆G00 that changes with substituent R while the other parameters are almost independent of R. There is generally a reasonable agreement of the data obtained from fits to the absorption spectra and to the fluorescence spectra. This demonstrates the lack of an anomalous structural reorientation between the absorption and the fluorescence process. The parameters λo, λv, and ν˜ v compare very well with those derived for, e.g., 1 in nonpolar solvents. Time-Resolved Spectroscopy. Fluorescence lifetime measurements were performed with two different spectrometers, one with a ns H2/N2 charged flashlamp excitation (IRF ca. 2 ns) at 358 nm and one with 355 nm pulsed NdYAG laser excitation (IRF ca. 9 ns) at 355 nm. The reason for using the latter lies in the only moderate spectral sensitivity of the flash-lamp setup in particular for compounds with red-shifted emission. Besides the IRF, the major difference between both measurements is the excitation intensity which is by far higher for laser excitation. Both methods gave almost identical lifetimes in the ns regime (see Table 3). The weak and red-shifted fluorescence of 2
Figure 4. Fits (black dots) of eqs 1 and 2 to selected reduced absorption (ε/ν˜ ) and fluorescence (If/ν˜ 3) spectra of 2-8 in cyclohexane.
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TABLE 3: Excited State Lifetimes (in ns) Measured by Transient Absorption and Fluorescence Spectroscopy Assuming Monoexponential Decay Kinetics at the Given Probe Wavelength transient absorption lifetime at 355 nm laser excitation (IRF 35 ps)/nsa
transient absorption lifetime at 416 nm laser excitation (IRF 9 ns)/nsa
fluorescence lifetime at 358 nm flashlamp excitation (IRF 2 ns)/nsa,b
fluorescence lifetime at 355 nm laser excitation (IRF 9 ns)/nsa,c
cyclohexane 8 7 6 5 4
11 at 490 nm 9.5 at 740 nm 4.3 at 500 nm 4.2 at 710 nm
3 2
21 22 19 21 13 3.7f
23 20 20 20 12 4.8
0.29d 0.26e toluene
8 7
8.8 at 490 nm 11 at 750 nm 3.9 at 500 nm 3.9 at 720 nm 2.3 at 490 nm 2.9 at 700 nm 2.2 at 700 nm
6 5 4 3 2
14 11
11 9.3 4.9 3.5 2.6
0.092 at 500 nm 0.096 at 670 nm 0.0064e dibutylether
7 6 5 4 3 2
1.3 at 500 nm 1.3 at 700 nm 0.87 at 500 nm 0.85 at 720 nm 0.62 at 500 nm 0.63 at 710 nm 0.082 at 510 nm 0.085 at 670 nm 0.0061e
The uncertainty is ca. 5% for lifetimes >10 ns and ca. 10% for lifetimes 0.5)36 as well as (5% (for Φf > 0.2) and ( 10% for weaker emitting dyes (for 0.2 > Φf > 0.02),37 Time-dependent fluorescence measurements in the nanosecond time regime were performed with a PTI TM-2/2003 fluorescence-lifetime spectrometer with a nanosecond flash lamp charged with H2/N2 (1:1). The instrument response (ca. 2 ns) of the nanosecond flash lamp was determined using 4-(N,Ndimethylamino)-4′-cyanostilbene (DMACS) which has a lifetime of 75 ps in cyclohexane. The fluorescence decay curves were deconvoluted with the signal of the flash lamp using the
J. Phys. Chem. C, Vol. 113, No. 49, 2009 20965 corresponding spectrometer software. The decays were fitted monoexponentially. The χ2 test, residuals and autocorrelation function served as the main criteria in the evaluation of the fit. Oxygen was removed as outlined above. The concentration of the samples was ca. 10-6-10-5 M. Transient Absorption Measurements. Nanosecond transient absorption spectra were acquired on an Edinburgh LP 920 Laser Flash spectrometer. All of the solvents were of spectroscopic grade and were used as received. Measurements were carried out in 1 cm quartz cells with an optical density between 0.1 and 1.0 at the excitation energy (concentration ca. 10-4 M). Oxygen was removed by bubbling inert gas through the solutions until a constant decay time was monitored. Samples were excited with a 5 ns duration laser pulse using the third harmonic (355 nm) of a Continuum Minilite II Nd:YAG laser operating at 10 Hz shifted to 416 nm by a H2-Raman shifter. The probe pulse was provided by a pulsed Xe flash lamp. The wavelength was selected by a monochromator, the time-dependent decay curves were recorded with a R928 photomultiplier tube and data were sampled with a fast oscilloscope (1.25 GS/s). Transient spectra were reconstructed by scanning through the absorption spectrum at 10 nm steps. The transient spectra were corrected for fluorescence contributions as detailed in ref 40. The instrument response of the Nd:YAG laser (9 ns) was determined by an empty cuvette as a scatterer. For determining the lifetime the decay curves were deconvoluted with the signal of the laser using the corresponding spectrometer software. Residuals and autocorrelation function served as the main criteria in the evaluation of the fit. With the same spectrometer, we also measured time-resolved fluorescence spectra which was done with the same samples as employed for the transient absorption measurements, but diluted to 10-5 M and with the probe lamp switched off. Also the pump energy was much lower than in the transient absorption measurements. For the ps pump probe experiments, the third (2-3 mJ) harmonic of a mode-locked Nd:YAG laser (Continuum PY61C10) served as the pump beam for sample excitation. The 1064 nm fundamental was focused into a flow cell filled with D2O to generate a white-light continuum that was employed as a probe beam. The white light was split in two parts that were directed into the sample cell by quartz fibers. While one of them served as the reference, the other one was overlapped with the pump beam in the sample cuvette at a 90° angle. The two whitelight beams were then directed by fibers into a spectrograph equipped with a diode array to analyze the spectral distribution of the absorbing excited species. The pump-probe delay was varied by means of a computer controlled delay line set on the probe beam. Typically around hundred shots were averaged per data point. The instrument response function (fwhm ) 35-40 ps) was derived from experiments with β-carotene in C2H4Cl2. To record the very fast transients of compound 2, a setup offering femtosecond time resolution was employed which is described in detail elsewhere.52,53 Part of the amplified 800 nm fundamental radiation of a Ti:sapphire (Ti:Sa) laser system, amplified at a 1 kHz repetition rate pumped an optical parametric generator which served as a tunable source for the pump-beam. A supercontinuum probe beam was generated by focusing a small part of the residual Ti:Sa fundamental into a 3 mm thick sapphire plate. It was sent into the sample, where it was overlapped with the pump beam. The pump pulse energy was less than 1 µJ. It was varied to ensure a linear power dependence of the transient absorption spectrum. The zero in time t0 was determined by recording the Kerr-effect signal in acetone. The
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pump-probe cross correlation measured by the same method was found to be around 150 fs. Acknowledgment. We dedicate this work to Prof. W.-D. Schenk on the occasion of his 65th birthday. The groups in Wu¨rzburg are indebted to the Deutsche Forschungsgemeinschaft for financing this project within the Graduiertenkolleg GRK 1221 and by the DFG grant La 991/12-1. We thank T. Hertel/ Wu¨rzburg for the opportunity to use their TCSPC setup. References and Notes (1) Heckmann, A.; Lambert, C. J. Am. Chem. Soc. 2007, 129, 5515– 5527. (2) Heckmann, A.; Lambert, C.; Goebel, M.; Wortmann, R. Angew. Chem., Int. Ed. 2004, 43, 5851–5856. (3) PCTM radicals also form donor-acceptor dyads in combination with ferrocene donors; see refs 4 and 5. (4) Ratera, I.; Ruiz-Molina, D.; Renz, F.; Ensling, J.; Wurst, K.; Rovira, C.; Guetlich, P.; Veciana, J. J. Am. Chem. Soc. 2003, 125, 1462–1463. (5) Ratera, I.; Sporer, C.; Ruiz-Molina, D.; Ventosa, N.; Baggerman, J.; Brouwer, A. M.; Rovira, C.; Veciana, J. J. Am. Chem. Soc. 2007, 129, 6117–6129. (6) D’Avino, G.; Grisanti, L.; Guasch, J.; Ratera, I.; Veciana, J.; Painelli, A. J. Am. Chem. Soc. 2008, 130, 12064–12072. (7) Maksimenka, R.; Margraf, M.; Koehler, J.; Heckmann, A.; Lambert, C.; Fischer, I. Chem. Phys. 2008, 347, 436–445. (8) Duemmler, S.; Roth, W.; Fischer, I.; Heckmann, A.; Lambert, C. Chem. Phys. Lett. 2005, 408, 264–268. (9) Wasielewski, M. R. J. Org. Chem. 2006, 71, 5051–5066. (10) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Miller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. J. Phys. Chem. B 2003, 107, 6668–6697. (11) Balzani, V.; Credi, A.; Venturi, M. ChemSusChem 2008, 1, 26– 58. (12) Guldi, D. M. Chem. Soc. ReV. 2002, 22–36. (13) Electron Transfer in Chemistry; Balzani, V. , Ed.; Wiley-VCH: Weinheim, Germany, 2001. (14) Verhoeven, J. W.; Ramesdonk, H. J. v.; Groeneveld, M. M.; Benniston, A. C.; Harriman, A. ChemPhysChem 2005, 8, 2251–2260. (15) Heitele, H. Angew. Chem., Int. Ed. 1993, 32, 359–377. (16) Benniston, A. C.; Harriman, A. Chem. Soc. ReV. 2006, 35, 169– 179. (17) Albinsson, B.; Eng, M. P.; Pettersson, K.; Winters, M. U. Phys. Chem. Chem. Phys. 2007, 9, 5847–5864. (18) Gust, D.; Moore, T. A.; Moore, A. L. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim Germany, 2001; Vol. 3. (19) Oliver, A. M.; Paddonrow, M. N.; Kroon, J.; Verhoeven, J. W. Chem. Phys. Lett. 1992, 191, 371–377. (20) Verhoeven, J. W. J. Photochem. Photobiol. C 2006, 7, 40–60. (21) Fox, M. A.; Gaillard, E.; Chen, C. C. J. Am. Chem. Soc. 1987, 109, 7088–7094. (22) Ruberu, S. R.; Fox, M. A. J. Phys. Chem. 1993, 97, 143–149.
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