Ultrafast Decay Dynamics of Excited and Charged States in α

1510

J. Phys. Chem. B 1997, 101, 1510-1519

ARTICLES Ultrafast Decay Dynamics of Excited and Charged States in r-Sexithienyl Film As Revealed by Femtosecond Transient Absorption and Picosecond Fluorescence Spectroscopy Kazuya Watanabe, Tsuyoshi Asahi, Hiroshi Fukumura, and Hiroshi Masuhara* Department of Applied Physics, Osaka UniVersity, Suita, Osaka 565, Japan

Kouji Hamano and Tetsuyuki Kurata AdVanced Technology R&D Center, Mitsubishi Electric Corporation, 1-1 Tsukaguchi-Honmachi 8-chome, Amagasaki, Hyogo 661, Japan ReceiVed: March 27, 1996; In Final Form: December 16, 1996X

Primary processes of photoexcited states in an R-sexithienyl film are investigated by femtosecond transient absorption spectroscopy and picosecond time resolved fluorescence spectroscopy. Four species are observed in the transient absorption measurement. A broad absorption that shows a very rapid relaxation (within a few picoseconds) is ascribed to a singlet Frenkel exciton state. An oscillating structure is also apparent immediately after excitation and is ascribed to the Stark effect induced by a charged species (ion pair state). The latter state decays within 200 ps and is replaced by a different oscillating structure that is due to a thermal effect induced by a dissipation of excess energy. Another band owing to a triplet state appears via a very rapid conversion from a higher singlet exciton state. Fluorescence decay curves are well fitted with lifetimes of the charged state, which indicates that most of the emission is brought about by the charge recombination process, and furthermore, a charge-transfer (CT) emission band that has not been reported is observed.

1. Introduction Recently, significant efforts have been devoted to elucidate optical and electronic properties of π-conjugated organic systems. R-Sexithienyl, hereafter abbreviated as 6T, and its derivatives are known as good model compounds of polythiophene and exhibit significant conducting behavior1 and optical nonlinear properties,2 which indicates the potential of the compounds to be a promising material for optoelectro devices. The fundamental properties of the optically excited state of 6T have been studied with great detail in the solid film state3 as well as in solution4 by one- and two-photon absorption spectroscopy, photoluminescence spectroscopy, electroabsorption spectroscopy,5 and so on. Also, charged states of 6T have attracted much attention6 because of polaron or bipolaron formation in the solid phase, which is an important process in polythiophene films.7 Investigations of an ultrafast response to a very short optical pulse are important not only from a fundamental physical viewpoint but also for studying the potential as optical devices. Transient absorption and emission spectroscopy are well-known as powerful methods for investigating the dynamics of transient species, which controls the ultrafast response of the materials. There have been some reports on transient measurements of 6T such as nanosecond transient absorption measurements on film states,8 picosecond photoluminescence in solution,4 picosecond dichroism measurements in solution9 and the film state,10 picosecond anisotropic transient absorption in a film,11 as well as picosecond13 and femtosecond12 transient absorption measurements in solution. In solution, isolated 6T shows an unstructured broad absorption band around 430 nm. Rotational conformers around the X

Abstract published in AdVance ACS Abstracts, February 1, 1997.

S1089-5647(96)00928-5 CCC: $14.00

inter-ring σ bond and their rotational vibrations of neighboring thiophene rings may cause the structureless band.13 Also, there is a possibility of aggregation effects on the spectrum. On the other hand, fluorescence spectrum from isolated 6T molecule shows the vibronic structure clearly, and this is ascribed to a quinoidic planar structure in the excited state.4 Not only transient absorption spectra of its singlet excited state but also those of the triplet state have been observed so far.6a,9,13 Also, absorption spectra of chemically prepared6c and photogenerated cationic states6a of the molecule have been obtained already. By use of femtosecond transient absorption spectroscopy, the ultrafast relaxation dynamics of its higher electronic excited states has been investigated in solution.12 In the solid phase, the situation becomes more complicated. With a picosecond transient dichroism measurement,10 the formation of the triplet excited state has been observed. Another study of photoinduced absorption has given absorption spectra similar to the dichroism measurement and has ascribed it to a polaron state.14 Thus, the assignments of the transient signals are still under argument and the primary processes of electronically excited states are ambiguous. In solid films, ground-state absorption spectra shift to lower energy compared with thosein solution, and some structures appear, which change rigorously depending on the molecular alignment.3 The ground-state absorption spectra have been interpreted with molecular exciton theory, and the electronic level ordering of the 21Ag exciton band and the 11Bu exciton band has been discussed.3 On the other hand, some evidence for a photoinduced intermolecular charge separation, brought about by the excitation to higher electronic levels, have been pointed out,15 as well as a generation of a charge-transfer exciton © 1997 American Chemical Society

Ultrafast Decay Dynamics state.5 Such a characterization of the electronic excitation could be provided with the ultrafast spectroscopic analysis of the film. As for a solid film, studies of the electronically excited state with transient spectroscopy have not been done in detail, and there have been no attempts of femtosecond absorption spectroscopy and picosecond photoluminescence spectroscopy on 6T film as far as we know. Recently, a report on a femtosecond transient absorption experiment on a film of polythiophene derivative has been given,16 and the results are interpreted as a generation of a free exciton state, which is succeeded by a formation of a self-trapped exciton state. Though 6T is considered as a model compound of polythiophene with respect to the electronic properties, exciton states in 6T film might show relaxation dynamics different from that in polythiophene, since a different dependence on molecular periodic alignment and lattice vibration is expected. In this report, we apply ultrafast spectroscopy to vacuumevaporated 6T films. Transient absorption spectroscopy over a wide wavelength region from 300 to 800 nm is performed, and very complicated temporal absorbance changes are observed. Assignments of transient species are attempted by combining absorption spectral data with the picosecond fluorescence measurement. As a result, a formation of a charged state is found and a contribution of a fast heat formation to the transient signal is confirmed. 2. Experimental Section A. Transient Absorption Spectroscopy. The principle of our transient absorption measurement is the same as that described before on picosecond 17 and femtosecond 18 transient absorption spectroscopic systems. Here, we used a Ti:sapphire laser system, which is different from the one in the previous report,18 and the details are reported elsewhere.19 We briefly summarized the experimental condition below. An excitation light pulse was obtained by amplifying a femtosecond Ti: sapphier laser (Coherent Mira Basic) pulse with a nanosecond YAG-pumped conventional regenerative amplifier (Continuum TR70-10). The amplified pulse (780 nm, 3-4 mJ/pulse, fwhm 170 fs, 10 Hz) was frequency-doubled and used as an excitation light source (center wavelength 390 nm). The residual amplified fundamental light was focused into a H2O cell to generate a white continuum (longer than 400 nm) as a probe light. In the case of measuring in the near-ultraviolet wavelength region (300-500 nm), the second harmonic light was divided into halves and a probe light pulse was obtained by focusing the half into the H2O cell. The white continuum was divided into two; one was used as a probe of the sample and the other was used as a reference in order to compensate for the scatter of the intensity and the change of the spectral shape of each white continuum pulse. The white continuum was detected by multichannel photodiode arrays (Hamamatsu C4351). Each transient spectrum was obtained by averaging 100-300 shots of probe light. When picosecond and femtosecond transient absorption spectroscopic measurements were performed, the chirping (dispersion) of the white continuum becomes a problem because it brings the wavelength-dependent temporal distribution of the probe light. In 1983, we reported for the first time the direct measurement of the temporal distribution of a picosecond continuum20 and have established the correction method for this effect. In this study, we estimated the temporal dispersion of the white continuum by measuring the time correlation profile between the white continuum and the excitation light with a

J. Phys. Chem. B, Vol. 101, No. 9, 1997 1511

Figure 1. (a) Transient absorption spectra of perylene in isooctane at ca. 10-4 M concentration. The excitation wavelength was 390 nm, and the excitation power is indicated in the figure. (b) Same spectra as in (a) normalized at ca. 700 nm.

Kerr gate method using CCl4 as a Kerr medium. The spectra shown in this paper were corrected using this time dispersion curve. We demonstrated a test measurement to show the reliability of our femtosecond transient absorption measurement system. Figure 1a shows time-resolved absorption spectra of perylene at 100 ps after excitation in isooctane at a concentration of ca. 10-4 M. The apparent features are due to S1-Sn absorption and bleaching of the ground state of perylene. The transient absorbance changes almost linearly with the excitation power, and when they are normalized (Figure 1b), the spectrum corresponds well to each other within the measurement error. Thus, it is clearly shown that, at least within this absorbance range (up to 0.3), we can measure a transient absorbance change quite accurately with our system. B. Time-Resolved Fluorescence Measurement. Timeresolved fluorescence measurements were done by using a mode-locked femtosecond Ti:sapphier laser (Coherent Mira 900) operating at 76 MHz. The laser pulses were picked up at 4.75 MHz by a pulse picker (Coherent 9200) and frequency-doubled (center wavelength of 360 nm) by a KDP crystal. The resulting excitation pulse was 170 fs fwhm, and its intensity was on the order of nanojoules. Temporal changes of fluorescence spectra were detected by a picosecond streak camera system (Hamamatsu streak scope C4334). The instrument response was several tens of picosecond because of unremovable jitter of the detecting system. The fluorescence was detected under the magic angle polarization condition. Temporal change of the fluorescence spectra was obtained by accumulating ca. 6 × 104 shots. All the measurements in this paper were performed at room temperature and in air. To prove the accuracy and the reliability of our system, timeresolved fluorescence spectra of perylene in ethylene grycol are measured and shown in Figure 2. Two spectra at different delay times are shown, and they are normalized at around 450 nm.

1512 J. Phys. Chem. B, Vol. 101, No. 9, 1997

Watanabe et al.

Figure 2. Time-resolved fluorescence spectra of perylene in ethylene glycol. The observation time window is indicated in the figure.

Since only the perylene molecule contributes to the emission spectra, no temporal spectral change is expected, and actually, the two spectra correspond within the measurement error. The maximum count of the perylene fluorescence at 15-16.5 ns (dots in Figure 2) was only about 70, and from the spectrum we estimated that the spectral uncertainty is within 15 counts in this measurement. C. Sample Preparation. R-Sexithienyl was obtained by oxidation coupling of the thiophene trimer in benzene solution mixed with Fe(Cl)2. Sample films were prepared with an organic molecular beam deposition apparatus using a Knudsen Cell under a condition of 10-7 Torr.21 Fused silica glass plates were used as substrates. The temperature of the substrates was maintained at room temperature during the deposition. The deposition rate was controlled to be several hundreds Å/min to obtain the isotropic films, and the film thickness was ca. 300 or 100 nm. It has already been reported that the 6T molecules were highly oriented perpendicularly in the films fabricated with a low deposition rate.21,22 The isotropy of the film in this experiment was confirmed by polarized UV-vis spectra for oblique incidence. Studies of the crystal structure of evaporated 6T films have been reported by several authors,23 and from their analysis, our film is considered to be random microcrystalline with the molecules taking a monoclinic arrangement in each microcrystal.

Figure 3. Transient absorption spectra of 6T film. The excitation wavelength was 390 nm, and the delay times are indicated in the figure. The absorbance at 770-790 nm is erased, since it is not reliable because of the laser light at 780 nm. The spectral change can be characterized by four features (see text).

3. Results A. Transient Absorption Spectroscopy. The transient absorption spectra of 6T film are given in Figure 3, which show quite complicated dynamics. The absorbance in the region of 770-790 nm was not reliable because of a leaked fraction of 780 nm laser light. The observed spectral change can be characterized by these four features (features a-d): (a) a very fast decay component with a broad band over a wide wavelength region (400-800 nm, and the same decay component was also observed at 330 nm); (b) an oscillating structure that appears immediately after excitation and changes its shape up to 100 ps (300-560 nm); (c) a component that decays at a rate of ca. 1-2 ns (650-800 nm); (d) an oscillating structure observed in a few nanoseconds time range (300-500 nm). Indications of these features are given in Figure 3. Absorption decay curves to 7 ps at various wavelengths are shown in Figure 4 on a semilog scale. The transient absorbance change at the bleaching position is also plotted with a multiplication factor of -1. The decay kinetics are represented by double-exponential functions plotted in the figure (solid curves). The decay time of the two lower curves are 1 and 20 ps, and they are capable of representing the experimental curves at 600 and 375 nm with a change in the relative amplitudes of the two components. The decay times of the uppermost curve

Figure 4. Semilog plot of transient absorption decay curves of 6T film to 7 ps. These curves are arbitrarily offset. The observation wavelength is indicated in the figure. The absorbance at 355 and 375 nm is negative but displayed as positive by multiplying by -1. The solid curves are double-exponential functions with the decay times of 1 ps (40%) and 30 ps (60%) (upper curve), 1 ps (70%) and 20 ps (30%) (middle curve), and 1 ps (83%) and 20 ps (17%) (lower curve). (The percentage in parentheses indicates the relative amplitude.)

are 1 and 30 ps, and this function reproduces the absorption curve at 750 nm well. The faster component apparent in every decay curve corresponds to feature a, and the decay time is ca.

Ultrafast Decay Dynamics

Figure 5. Transient absorption decay curves of 6T film to 200 ps. The observation wavelengths were 750 nm (closed circles) and 560 nm (triangles). Open circles are a difference between the curves at 760 and 730 nm, which corresponds to a time dependent spectral change (see text). A solid curve indicates a triple-exponential function with three decay times: 20, 45, and 200 ps (see text).

Figure 6. Transient absorption decay curves of 6T film to 5 ns. The observation wavelengths were 750 nm (closed circles) and 520 nm (triangles).

1 ps. Since the calculated curves are not convoluted with the instrument response function, the actual decay time would be faster than 1 ps. At 375 nm, a recovery of the bleaching occurs with the same time constant of the species giving feature a, and afterward, a succeeding recovery, which agrees with the decay curve at 565 nm, was observed. At 750 nm, the contribution of feature a is rather small, and the behavior is similar to the bleaching recovery at 355 nm. The temporal absorption changes of the slower species were plotted to 200 ps in Figure 5, where the absorbance change before 10 ps is omitted. The decay dynamics of the feature b is present in the decay curve at 560 nm, and it drops off during this time range. The decay curves of features c and d in the later stage are given in Figure 6. The absorbance at 750 nm decays almost exponentially in the range greater than 500 ps, while the bleach absorbance does not show a significant temporal change in this region (at 520 nm). Figure 7 shows the excitation laser power dependence of the 750 and 650 nm absorption at 10 and 0 ps, respectively. The observed absorbance changes almost linearly with the excitation power. Because the contribution of the features a and c are most dominant in the absorption at 650 and 750 nm, respec-

J. Phys. Chem. B, Vol. 101, No. 9, 1997 1513

Figure 7. Excitation power dependence of the transient absorption of 6T film (log-log scale) at 650 nm (0 ps, 4) and at 750 nm (10 ps, 0) after excitation. The solid line corresponds to a slope of 1, which indicates a linear relationship between the excitation power and the transient absorption.

Figure 8. Time-resolved fluorescence spectra of 6T film. The excitation wavelength was 360 nm. Each spectrum was normalized at the peaks, and the delay times are depicted in the figure. The lowest spectrum plotted with dots is a subtraction of the earliest (-20 to 20 ps) time fluorescence spectrum from the latest (420-920 ps) time fluorescence spectrum with normalization around 600 nm.

tively, it is concluded that the observed transients a and c were generated with one photon process. Also, it was confirmed that the feature b contributes linearly with the exciting power. B. Time-Resolved Fluorescence Spectroscopy. Figure 8 shows a temporal change of fluorescence spectra of 6T film. Up to several hundreds of picosecond, structured fluorescence spectra were observed with a strong peak at 595 nm and humps at both sides of the peak. Figure 9 shows decay curves of the fluorescence intensity at various wavelengths with the response function of the system. All the decay curves show nonexponential behavior, and the two decay curves at the peaks of shorter wavelengths (560 and 600 nm) show similar behavior, while those at longer wavelength have slower components. Steady-state fluorescence spectra of 6T films at 12 K with an excitation at a wavelength shorter than 530 nm, which show a structure similar to the structure of our spectra, have been reported,3c and according to the assignment, the present strong peak at 595 nm corresponds to the emission from the lower

1514 J. Phys. Chem. B, Vol. 101, No. 9, 1997

Figure 9. Fluorescence decay curves of 6T film at various wavelength regions. These curves are normalized at the peaks and shifted arbitrarily along the vertical axis. The observation wavelengths are indicated in the figure. A pulse shape curve plotted with dots is the instrumental response function. The excitation wavelength was 360 nm.

11Bu Frenkel exciton state. The hump around 650 nm is considered to be a progression of some vibrational modes (whose frequencies are around 1500 cm-1) coupled to the lower 11Bu Frenkel exciton state. According to a very high-resolution emission spectroscopy of tetrathiophene,24 there exist some vibrational modes that have frequencies around 1500 cm-1, such as the thiophene ring-stretching mode (1478 cm-1), the CdC stretching mode (1531 cm-1), and the coupling of a deformation mode to these vibrational modes. The assignment of the hump around 560 nm is not easy. In ref 3c, it is proposed that the fluorescence component at 560 nm is a band from the 21Ag Frenkel exciton state coupled to some u vibrational modes. On the other hand, Zamboni et al.3b reported on the fluorescence spectra of 6T film at 4 K excited at 532 nm, which show three peaks at 588, 640, and 700 nm, and they ascribed these peaks to the emission from the 11Bu exciton band. Zamboni et al. did not observe the band around 560 nm. They also performed one-photon and two-photon fluorescence excitation spectroscopy and found that the origin of the 11Bu exciton band is located at 573 nm and that of 21Ag exciton band at 544 nm. They claimed that the 21Ag exciton state rapidly relaxes to the 11Bu exciton state and that a radiative transition from the 21Ag exciton state does not occur. Though it is plausible that the fluorescence radiation occurs from the lowest excited state, the component at 560 nm, which is higher than the origin of the 11Bu exciton state (573 nm) by ca. 400 cm-1, is difficult to explain with this model. One possible explanation of the component observed in our measurement is a thermal population to the higher vibronic level in the 11Bu exciton state because our measurement was done at room temperature. However, the appearance of the same component at 560 nm in ref 3c at 12 K cannot be attributed to this effect. Another candidate is the effect of some defects at grain boundaries or something that would have energy levels different from the others. Nevertheless, as is shown in Figure 9, the fluorescence decay curves at around 560 and 595 nm correspond to each other. This indicates that the two bands originate from the same species. Finally, we cannot come to a conclusion on the assignment of this 560 nm fluorescence band at this stage of the investigation. The spectra showed a temporal change of its shape; the lower energy components became relatively stronger at later times. This temporal change of the spectral shape is not an artifact due to, for example, a faint background light, which is clearly demonstrated by the test measurement in Figure 2. The

Watanabe et al.

Figure 10. Temperature dependence of the ground-state absorption spectra of 6T film. The inset shows the enlarged spectra and the temperatures. A thin solid spectrum is a red-shifted (energetically lower) spectrum. The energy shift is 400 cm-1.

maximum count of our time-resolved spectrum at the latest time range (420-920 ps) in Figure 8 is about 130, while the spectral uncertainty is within 15 counts. Hence, we can conclude safely that the temporal change of the fluorescence spectra in Figure 8 is not an artifact and reflects a change of the electronic state, which gives the fluorescence. Since the employed excitation power in the fluorescence measurement is much lower than that in the absorption measurement, the heating effect of the film by the dissipation energy is not considerable as a cause of the fluorescence spectral change. 4. Discussion A. Thermal Origin Signal. First, we discuss the origin of the oscillating structure (feature d). Because at the wavelength region 300-500 nm the ground-state 6T has a significant absorption coefficient, this structure may be due to a transient change of the absorption spectrum of the ground-state 6T molecules. A temporal heating of the sample film by an irradiation of the laser pulse is the possible cause of it. To examine this hypothesis, we measured the ground-state absorption spectra at various temperatures and summarized some of the results in Figure 10. A slight change of absorption spectra was observed, and the reversibility of this thermal spectral change has been confirmed up to 60 °C. Together with the transient absorption spectrum at 2 ns, Figure 11a shows difference spectra (afterward called the temperature difference spectrum) obtained by subtracting the high-temperature spectrum from that at 25 °C. The correspondence between the temperature difference and the transient absorption spectra was fairly good so that we can conclude that the oscillating structure originates from a temporal thermal effect in the film. It is not strictly accurate to deduce that the actual transient temperature of the film at 2 ns was 40 °C from our result because the gradient of the temperature to the direction of thickness should be considered, but maybe we can use the value to estimate the degree of this temperature change. The line shape of the ground-state absorption is considered to be determined by a coupling with low-frequency intermolecular vibrational modes (up to a few hundred cm-1) whose population would be changed by a temperature change of a few tens of degrees. Cheng et al.8 have reported on a nanosecond transient absorption measurement in some oligothiophene films and observed oscillating

Ultrafast Decay Dynamics

Figure 11. (a) Temperature difference spectrum of 6T film between 40 and 24 °C (dots) and between 60 and 24 °C (broken line). A transient absorption spectrum at 2 ns, which was normalized to the temperature difference spectrum between 40 and 24 °C at around 350 nm, was also plotted with a solid line. (b) Subtraction spectra of a ground-state absorption spectrum from red-shifted (energetically lower) ground-state absorption spectra. The energy shift value is indicated in the figure.

structures similar to those in our spectra. They also ascribed the spectra to the thermal origin spectra, but its formation process was obscure. In our analysis, the oscillating structure in transient absorption spectra changes its form up to 100 ps and, furthermore, no spectral change was observed up to 5 ns. Thus, we can conclude that the film reaches the quasiequilibrium temperature in 100 ps after excitation. In this paragraph, we mention sample stability upon laser excitation. In this study we used excitation power less than ca. 5 mJ/cm2. We found that irradiation with an intensity more than that value induces an ablation of the sample film. Although no sample damage was observed under this power, even a few mJ/cm2 seem to be very high power. The ground-state absorbance at 390 nm was 0.65 with a film thickness of ca. 100 nm, and assuming the density of the film as 1.5 g/cm3, an irradiation at 5 mJ/cm2 may excite about 40% of the molecules in the film. The reason no photothermal damage occurs is considered to be due to the low repetition rate (10 Hz) of the laser irradiation. The thermal diffusion constants of typical organic crystals are on the order of 10-3 cm2/s,25 and the thermal diffusion length during 1 µs reaches up to a few hundreds of nanometers, which is close to the film thickness in our study. Thus, the generated heat can diffuse into the substrate during the interval of the excitation (100 ms). For the measurement, ca. 104 shots of excitation pulses were used to obtain one sequence of the spectral change. Under this condition, no thermal degradation in the film was observed. B. Stark Effect Induced by a Transient-Charged Species. Now we discuss the oscillating structure up to 100 ps (feature b). The oscillating structure at several picoseconds is slightly shifted compared to that at 2 ns (Figure 3) and cannot be fitted by the temperature difference spectra. Dippel et al. have reported on the energy dependent branching between fluorescence and an electron-hole pair generation in 6T film by means of the fluorescence yield and the photocurrent yield measurements from 2 to 3 eV.15 They observed a rapid drop of fluorescence yield above the S0-S1 absorption edge and ascribed it to an electron-hole pair formation induced by a torsional

J. Phys. Chem. B, Vol. 101, No. 9, 1997 1515

Figure 12. New, composed spectra (solid line) by subtracting a factored ground-state absorption spectrum from the subtraction spectrum obtained by 400 cm-1 energy shift shown in Figure 11b (see text). The new, composed spectra are normalized to the transient absorption spectra (broken line) at various delay times.

disorder of each 6T molecule. Furthermore, an electroabsorption study has revealed that a charge-transfer (CT) exciton band lies in a region greater than 2.7 eV.5 CT exciton states in molecular crystals have been studied theoretically,26 and their existence has been confirmed experimentally in various molecular crystals such as anthracene, tetracene, pentacene, and phthalocyanine.27 In these crystals, it has been confirmed that a strong mixing between a CT state and a neutral Frenkel exciton state occurs and the CT transition possesses a large oscillator strength by intensity borrowing from the Frenkel exciton states. Because the excitation photon energy in our measurement is 3.18 eV, the generation of a CT exciton state and the succeeding formation of an intermolecular ion pair state induced by lattice deformation are plausible. Since the densely formed charged state (ion pair state) will polarize the environment molecules, we examined the possibility of a Stark effect on the neighboring molecules near the charged species in a 6T film. Figure 10 also shows a ground-state absorption spectrum, which is energetically shifted to the lower side by 400 cm-1, and Figure 11b shows the subtraction spectra, which were composed by subtracting the ground-state absorption spectrum of the film from the energetically shifted spectra. The energy shift value was varied and indicated in the figure. The subtraction spectra show oscillating structures whose amplitudes vary with the shift amount. With the shift amount used in Figure 11 (100-600 cm-1), the subtraction spectra approximately correspond to each other when they are normalized, though their shapes are expected to vary with the energy shift. The subtracted spectra alone cannot reproduce the experimental results. The observed transient absorption spectrum is considered to be composed of the bands due to the charged species, the depleted spectrum of the ground state caused by all the transient species, and the shift due to the Stark effect. If the first component is located atother wavelength, the transient spectrum should correspond to that obtained by subtracting further the ground-state absorption spectrum from the above subtraction spectrum. This new spectrum can reproduce the transient absorption spectra up to several tens of picoseconds surprisingly well (Figure 12). This procedure is described by eq 1,

Com(E) ) a(Sub(E) - bG(E))

(1)

where E stands for the transition energy, Com(E) is the new

1516 J. Phys. Chem. B, Vol. 101, No. 9, 1997 composed spectrum, Sub(E) is the subtraction spectrum, G(E) is the ground-state absorption spectrum, and a and b are parameters adjusted to fit the experimental results. The shift energy for the good reproduction of the spectra in Figure 12 was 400 cm-1. The multiplication factor b in eq 1 is 1/6.5 for the spectrum at 0.5 ps, 1/10 at 3 ps, and 1/12 at 5 ps. The decrease of the b value could be represented with a decay time of ca. 1 ps, which corresponds to the decay of feature a. At 0.5 and 3 ps, the agreement of the simulated spectra and the experimental results is not so good especially around 330 nm, and this may be due to an overlapping of feature a in this region. This is apparent also in the decay curves in Figure 4. Here, the Stark shift energy is estimated. X-ray analyses of 6T crystal revealed its form as a monoclinic (a ) 45, b ) 7.9, c ) 6.0 Å)23 in which each molecule has an angle of about 23° to the a axis. As reported in ref 5, the CT excitation is found to be anisotropic and to have a preferential oscillator strength along the a axis. Thus, the dipole change with the formation of the CT state (hole-electron pair) is considered to be a vector nearly parallel to the a axis. For this CT transition, the energy shift is mainly due to a first-order Stark effect, ∆E ) ∆µ‚F, where ∆µ is considered to be 10-30 D and F is the vector of the electric field generated by a neighboring charge. Since the vector F, which is nearly parallel to ∆µ, has a large contribution to the Stark shift, the electric field generated by a point charge placed on a diagonal lattice point along the a axis is important. The strength is estimated to be 1 MV/cm with a dielectric constant of 2.5. Assuming the angle between ∆µ and F to be about 15° in this case, the amount of the Stark shift is estimated to be about 4 × 102 cm-1. The value used in the spectral composition in Figure 12 is accidentally the same as this estimation. It is claimed in ref 5 that a transition with an energy less than 2.7 eV (460 nm) generates a Frenkel exciton state, where electrons are tightly bound on the molecule, instead of a CT exciton state. For this case, the second-order Stark shift is dominant and is estimated as ∆E ) (1/2)∆RF2, where ∆R is the difference in the polarizability between the ground state and the excited state of the 6T molecule, and the value of ∆R at 2.56 eV was reported as 120 Å3 in ref 5. Since ∆R has been found to be almost isotropic, the electric field produced by a charge on the nearest molecule along the b or c axis may contribute significantly. The amount of the Stark shift by this electric field is estimated as 170 cm-1. The expected amplitude of the difference spectra by this Stark shift would be smaller than that expected based on the 400 cm-1 shift (see Figure 11b). This may be one of the causes of the slight discrepancies apparent in the region of the energy less than 2.7 eV (460 nm) between the transient absorption spectra and the composed spectra (Figure 12). The composed spectra in Figure 12 are multiplied by certain values to fit the experimental data (a in eq 1). The values of a are 1/1.8, 1/3, and 1/3 for fitting the data at 0.5, 3, and 5 ps, respectively. Because the subtracted data in Figure 11b before multiplication correspond to the difference spectra induced when whole molecules contained in the film have shifted their transition energy, the multiplication factors indicate the percentage of the molecules that suffered the Stark effect. As has been estimated above, the percentage of the excited molecules in the film is ca. 40% at 5 mJ/cm2 excitation energy. This very high concentration of the excited state may lead to the rather high contribution of the Stark effect at the early stage of the relaxation in Figure 12. It is very surprising that our simple analysis explains the experimental results well. The correspondence of the spectra

Watanabe et al.

Figure 13. (a) Transient absorption spectra of 6T film at 10 ps (solid line) and at 400-500 ps (broken line) after excitation normalized at 760 nm. The absorbance at 770-790 nm is erased, since it is not reliable because of the laser light at 780 nm. (b) Transient absorption decay curves at 730 nm (b) and at 760 nm (4). These are normalized at 500 ps after excitation.

in Figure 12 indicates that the present consideration is enough to reveal the origin of the signal: the Stark effect by the charged species. A similar analysis of the electric field effect on a cartenoid molecule induced by a neighboring charge in solution has been reported,28 and the validity of such an analysis of the electric field effect has been confirmed. For molecular solid states, few studies on the transient Stark effect of neighboring charges have been reported as far as we know. Thus, this may be the first work that points out the possibility of a new detection scheme of the dynamics of charged species. C. Absorption and Fluorescence Band Due to the Charged Species. If our analysis is correct, there should be some observable bands due to electronic absorption of the charged state (ion pair state). The broad absorption band around 650800 nm shows a slight temporal change of its form in a few hundreds of picoseconds time range (Figure 13a). This spectral change means another species, which decays during 100 ps, contributes in this wavelength region. We can see a difference between the decay curves, corresponding to this spectral change, in Figure 13b, where the absorption decay curves at 730 and 760 nm are plotted. At 730 nm, where the transient absorption signal contains a greater contribution of the new species apparent in Figure 13a, the absorption decays faster than that at 760 nm. After 500 ps, both decays became identical. We obtained the temporal profile of this spectral change by subtracting the decay curve at 730 nm from that at 760 nm in Figure 13b. This time profile of the spectral change plotted in Figure 5 (open circle) shows an excellent agreement with the absorption decay curve at 560 nm. At 560 nm, the absorbance change that is due to the Stark effect is significant, and furthermore, the signal induced by the thermal effect does not have a large contribution. Thus, the decay curve at 560 nm corresponds to the decay of the contribution of the Stark effect, and the agreement between this curve and the subtracted curve indicates that the spectrum, which appears in Figure 13a as a difference between the two transient spectra, is ascribed to the charged species (ion pair

Ultrafast Decay Dynamics

J. Phys. Chem. B, Vol. 101, No. 9, 1997 1517

TABLE 1: Time Constants for the Best Fit to the Fluorescence Decay Curves, and χ2 Value of the Fittinga wavelength/nm 540-580 580-620 640-680 680-780

decay time/ps (amplitude) 21 (44%) 40 (28%) 100 (28%) 22 (48%) 51 (34%) 120 (18%) 22 (50%) 55 (35%) 150 (15%) 40 (60%) 90 (30%) 260 (10%)

χ2 1.01 1.04 1.02 1.07

a

Percentages in parentheses are the relative amplitudes of each decay component.

state) that causes the Stark effect. The absorption due to the charged species (ion pair state) may be a composition of the absorption band of the 6T cation and 6T anion (or trapped electron). Since we have no information on the absorption of the 6T anion or electron trapped in a 6T crystal, we cannot say anything about their absorption. However, the radical cation of 6T is known to show a band peaked at 780 nm,6a and at least this component should be observed. The absorption band of the charged species apparent as a difference between two spectra in Figure 13a is in the same wavelength region as the 6T radical cation band. To estimate the order of the decay time constant of the charged species, we tentatively fitted the transient absorption decay curve at 560 nm in Figure 5 with a triple-exponential function and obtained a good fitting result (Figure 5, solid line). Note that we omitted the fast component up to 10 ps in Figure 5, and here, we discuss the decay dynamics after 10 ps after excitation. The decay times are 20, 45, and 200 ps. This suggests that there are at least three kinds of ion pair states that recombine at each rate independently. By comparison of the decay behavior of the fluorescence (Figure 9) and the absorption, it is easily found that the b species (hereafter, we denote the charged state that causes feature b as “b species”) decays on a time scale similar to that of the fluorescence. In general, decay dynamics in the solid state show nonexponential behavior, reflecting an inhomogeneous broadening of each state, and analysis with a multicomponent function with many kinds of time constants would be an appropriate treatment for such a system. However, to examine the correspondence between the kinetics of the b species and the fluorescence, we tentatively attempt the analysis of the fluorescence decay dynamics with a triple-exponential function. Results of a least-squares fitting of the fluorescence decay curves are listed in Table 1. All the decay curves could be fitted well with the multiexponential functions, whose rate constants vary in each curve. The three decay times are almost common, 20, 40-50, and 100-150 ps, except for the decay curve at longest wavelength region in which the decay times are 40, 90, and 260 ps. These values obtained for the fluorescence decay curves are close to those that reproduce the decay curves of the b species in Figure 5, and it is straightforward to consider that the fluorescence was emitted by the charge recombination of the ion pair state. The proportion of the amplitudes of 20 ps: 45 ps:200 ps components of the curve plotted in Figure 5 is 1:0.6:0.4, which does not contradict that of the fluorescence decay curves. If our assignment of feature a to a Frenkel exciton state (given in section 4D) is true, the emission corresponding to this state should be observed, but its rather short lifetime

(ca. 1 ps) may prevent its detection in our fluorescence measurement system. Thus, most of the emission observed here is brought about by a charge recombination. According to the assignment of the fluorescence spectra, Frenkel exciton states are generated by this recombination during a few hundreds of picoseconds. The absence of the signal due to this Frenkel exciton state in the transient absorption spectra in this time regime is also attributed to its short lifetime; namely, appreciable transient concentration giving enough absorbance was not attained. The three decay constants apparent in the decay curve of the b species indicate that there are at least three kinds of different ion pair states that give recombination rates. The fluorescence spectrum after a few hundreds of picosecond shows a significant increase of the lower energy part, and this is interpreted as an overlapping of a structureless broad band from 600 to 800 nm. The lowest spectrum in Figure 8 (dots) is obtained by subtracting the fluorescence spectrum at the earliest time (-20 to 20 ps) from the latest one (420-920 ps) with normalization of these two around 600 nm. Considering the feature of the subtracted spectrum, such as the Stokes shifted and structureless band shape, one possible candidate for this new emission band is a CT emission. Namely, from the ion pair state, an excited state of an intermolecular CT complex is also populated. The existence of the two kinds of emission from the ion pair states may indicate that depending on the microscopic structure, some ion pair states in which the stabilization energy is sufficient are able to populate a CT complex state, and some ion pair states do not gain enough stabilization energy and bring about the Frenkel exciton state with the charge recombination. Zamboni et al. have reported on the site selective fluorescence spectra of a 6T film by exciting at ca. 2000 cm-1 below the lowest exciton state.3b By changing the excitation wavelength in the range 610-630 nm, they observed red-shifted emission depending on the excitation wavelength. They ascribed the origin of these spectra to physical defect sites (X-traps). One should consider the possibility of such a defect as an origin of the broad new emission (dots in Figure 8). If this is the case, the new band would appear as a result of an energy transfer from higher states. In a poly(p-phenylenevinylene) film, a fast (less than a few picoseconds) time dependent peak shift of the emission, which is ascribed to an energy relaxation by an incoherent random walk of excitations in an inhomogeneous density of states, is observed by a femtosecond fluorescence measurement.29 We did not observe such a peak shift of the fluorescence at the 560 and 595 nm peaks on the time scale of the spectral change apparent in Figure 8. The observed spectral change indicates that the new broad band appears independently from the structured component at shorter wavelength rather than as a result of a cascading of the excitation in the various localized sites. Thus, we consider that the CT emission is a more plausible assignment as the new broad emission. Besides, we have to take into account the possibility of concentration dependent decay dynamics, such as an excitonexciton annihilation, on comparison of the transient fluorescence and absorption measurements. However, the obtained absorption decay curves are independent of the excitation power for every species, despite the high concentration of the excited states. Thus, we can conclude that any concentration dependent dynamics do not play a dominant role in this work. The absence of the concentration dependent bimolecular decay process indicates that the generation of a free radical ion that migrates throughout the crystal is not plausible. Thus, the lifetime of the ion pair state is just determined by the charge recombination rate, and the ion pair states do not undergo

1518 J. Phys. Chem. B, Vol. 101, No. 9, 1997 dissociation to free ionic species. The relative increase of the contribution of this CT emission with delay time means that the CT complex state has a longer lifetime than the ion pair state. D. Frenkel Exciton State. As discussed above, a mixing of a CT exciton state and a Frenkel exciton state, in which the electron is tightly bound in each molecule, may occur at the excitation energy (3.18 eV). Thus, a generation of a neutral Frenkel exciton state should also be considered. Because in our experiment we have employed an excitation light whose energy exceeds the 0-0 transition energy by ca. 1 eV, a rapid relaxation process from the higher Frenkel exciton state into the lowest exciton state is one candidate for the fast component with a broad band absorption around 400-800 nm (feature a). However, we observed a recovery of bleaching at 375 nm with the same time constant, which means that the rapid process with the time constant of 1 ps is a relaxation to the ground state. Thus, we tentatively ascribe this feature a to a relaxed singlet Frenkel exciton state generated by a very rapid (faster than the time resolution of the measurement) conversion from the optically prepared higher exciton state. The broad transient spectra of feature a indicate that transitions to more than one higher excitonic band are involved in the wavelength region. Recently, a 800 fs decay of a self-trapped exciton (STE) following 70 ( 50 fs after its formation process from a free exciton state has been observed in poly(3-methylthiophene) film by transient absorption spectroscopy.16 Because of the similarity of the emission spectra of the polythiophene film and the 6T film, the singlet Frenkel exciton state in the 6T film is supposed to be an electronic state similar to the STE state in polythiophene. The lifetime of the singlet Frenkel exciton state in the 6T film observed in our measurement is about 1 ps, which is close to that of STE in the polythiophene derivative. E. Triplet State. Figures 5 and 6 show the absorption decay curve at 750 nm. The decay curve is composed of a fast component (to a few hundreds of picosecond), which reflects a superposition of a band due to the charged species (ion pair state) and a slow component whose lifetime is ca. 1.5 ns. Next we discuss this slower component observed around 650-800 nm (feature c). Poplawski et al. reported on photoinduced absorption spectra of 6T film over a wide observation range from 0.7 to 2.6 eV.14 They observed two bands with peaks at 0.8 eV (1550 nm) and 1.54 eV (800 nm) and ascribed them to the radical cation of 6T because the spectra correspond to that of 6T radical cation in solution. The latter band (peak at 1.54 eV) shows a structure very similar to feature c in our study. However, from the disagreement of the kinetics between features c and b (the Stark effect by the ion pair state), the possibility of the 6T cation state as the origin of feature c is excluded in our case. Because feature c shows slower decay kinetics than feature b (the Stark effect by the ion pair state) and fluorescence, the T1-Tn absorption is the most plausible assignment of this band. According to the discussion given in section 4C, we consider that the band around 650-800 nm is a composition of the charged state (ion pair state) absorption and the triplet state absorption during a few hundreds of picoseconds after excitation. After a few hundreds of picoseconds after excitation, the ion pair state disappears by a charge recombination and the band around 650-800 nm (feature c) is considered to be a purely T1-Tn absorption. The T1-Tn absorption band in solution has been confirmed to show a peak at 680 nm.6a Thus, if our assignment is the case, a red shift of the T1-Tn absorption band by 0.2-0.3 eV from absorption in solution to absorption in the film state should be considered. Jundt et al. has reported on a transient absorption

Watanabe et al. SCHEME 1

measurement of a pentacene film, and a red shift of the T1-Tn absorption band by 0.3 eV was observed compared to that in solution.30 They said that the red shift in the film is due to the strong polarization effect in the medium. Similarly, the red shift of the T1-Tn transition energy in 6T film may be possible. Here, the formation mechanism of the triplet state is discussed. Its precursor state should be the singlet Frenkel exciton state or the charged species (ion pair state) that would bring a triplet state of 6T with a charge recombination. Since we did not observe any rise component that corresponds to the decay of the charged state (species b) in this wavelength region, the latter candidate is not plausible. Thus, the formation of the triplet state would occur directly from the singlet Frenkel exciton state. Because the intersystem crossing of 6T molecule occurs at a rate of about 1 ns-1 in solution,13 it is inconceivable that the intersystem crossing occurs within 1 ps in the film state. In such a case, a fission of a higher exciton state into two triplets is a possible mechanism. The fission of a singlet exciton state into two triplet exciton states in molecular crystals has been studied theoretically and experimentally.31 Because the relaxation of this higher Frenkel exciton state would occur within the time resolution as discussed above, this fission process also occurs within hundreds of femtoseconds. Since the fission process is spin-allowed, very rapid conversion from the higher singlet Frenkel exciton states into the triplet states may occur. Such a fission process from the higher singlet exciton state has been reported recently on the anthracene crystal.32 Nunzi et al. have reported on a photoinduced dichroism measurement of the 6T film. Spectral components similar to feature c in our study were observed by the picosecond dichroism measurement and was ascribed to a triplet state of 6T.10a They have also observed photoinduced dichroism at around 550 nm that decays within 50 ps and assigned this feature as “a singlet state of 6T”. They also observed the rise time difference between the signal due to the triplet and the signal due to the “singlet state” and considered that it corresponds to the intersystem crossing process. However, we did not observe such dynamics that indicates the intersystem crossing process. Furthermore, the dichroism signal they observed at around 550 nm is considered to correspond to the transient Stark effect by the ion pair state (b species) in our study. If a dispersion effect of the probe light was not corrected well, delay of the rise time of transient species that have a signal at a longer wavelength would occur, and this might be the reason for the discrepancy of the rise curve at two different wavelengths in ref 10a. 5. Summary Combining the above-mentioned assignments, we can describe the primary photophysics process in 6T microcrystalline film as Scheme 1. First, the optical excitation at 3.18 eV generates a higher singlet Frenkel exciton state and a CT exciton

Ultrafast Decay Dynamics state, which results in the formation of an ion pair state within the time resolution of our measurement system. The higher Frenkel exciton state relaxes into lower singlet exciton states within the time resolution competing with a fission process into triplet states. The relaxed singlet Frenkel exciton state decays with a time constant of ca. 1 ps, while the ion pair state recombines with 20, 45, and 200 ps radiative decay times. Some ion pair states emit CT fluorescence. During these relaxation processes, nonradiative relaxation with a dissipation of surplus energy also occurs, and this leads to the heating of the sample film. The triplet state decays with a 1.5 ns lifetime. These findings will be useful for understanding the relaxation dynamics of polythiophene films. Acknowledgment. This work was partly supported by a Grant-in-Aid for the Scientific Research on Priority-AreaResearch “Photoreaction Dynamics” from the Japanese Ministry of Education, Science, Sports, and Culture (06239101) and by a Grant-in-Aid from the Japanese Ministry of Education, Science, Sports, and Culture (1985). K. Watanabe is a research fellow of the Japanese Society for the Promotion of Science. References and Notes (1) (a) Horowitz, G.; Fichou, D.; Peng, X.; Xu, Z.; Garnier, F. Solid State Commun. 1989, 72, 381. (b) Horowitz, G.; Peng, X. Z.; Fichou, D.; Garnier, G. J. Appl. Phys. 1990, 19, 1489. (2) (a) Kurata, T.; Hamano, K.; Kubota, S.; Koezuka, H. Organic Thin Films for Photonic Applications; Optical Society of America: Toronto, 1993. (b) Zhao, M. T.; Singh, B. P.; Prasad, P. J. Chem. Phys. 1988, 89, 5535. (c) Thienpont, H.; Rikken, G. L. J. A.; Meijer, E. W.; ten Hoeve, W.; Wynberg, H. Phys. ReV. Lett. 1990, 65, 2141. (3) (a) Periasamy, N.; Danieli, R.; Ruani, G.; Zamboni, R.; Taliani, C. Phys. ReV. Lett. 1992, 68, 919. (b) Zamboni, R.; Periasamy, N.; Ruani, G.; Taliani, C. Synth. Met. 1993, 54, 57. (c) Deloffre, F.; Garnier, F.; Srivastava, P.; Yassar, A.; Fave, J.-L. Synth. Met. 1994, 67, 223. (d) Yaasar, A.; Valat, P.; Wintgens, V.; Hmyene, M.; Deloffre, F.; Horowitz, G.; Srivastava, P.; Garnier, F. Synth. Met. 1994, 67, 277. (4) Chosrovian, H.; Rentsch, S.; Grebner, D.; Dahm, D. U.; Birckner, E.; Naarmann, H. Synth. Met. 1993, 60, 23. (5) Blinov, L. M.; Palto, S. P.; Ruani, G.; Taliani, C.; Tevosov, A. A.; Yudin, S. G.; Zamboni, R. Chem. Phys. Lett. 1995, 232, 401. (6) (a) Wintgens, V.; Valat, P.; Garnier, F. J. Phys. Chem. 1994, 98, 228. (b) Kurata, T.; Fukada, C.; Fuchigami, H.; Hamano, K.; Tsunoda, S. Jpn. J. Appl. Phys. 1995, 34, L1464. (c) Fichou, D.; Horowitz, G.; Xu, B.; Garnier, F. Synth. Met. 1990, 39, 243. (7) (a) Chung, T.-C.; Kaufman, J. H.; Heeger, A. J.; Wudl, F. Phys. ReV. B 1984, 30, 702. (b) Vardeny, Z.; Ehrenfreund, E.; Brafman, O.; Nowak, M.; Schjaffer, H.; Heeger, A. J.; Wudl, F. Phys. ReV. Lett. 1986, 56, 671. (c) Kaneto, K.; Uesugi, F.; Yoshino, K. J. Phys. Soc. Jpn. 1988, 57, 747. (d) Kaneto, K.; Hayashi, S.; Yoshino, K. J. Phys. Soc. Jpn. 1988, 57, 1119.

J. Phys. Chem. B, Vol. 101, No. 9, 1997 1519 (8) Cheng, X.; Ichimura, K.; Fichou, D.; Kobayashi, T. Chem. Phys. Lett. 1991, 185, 286. (9) Charra, F.; Fichou, D.; Nunzi, J.-M.; Pfeffer, N. Chem. Phys. Lett. 1992, 192, 566. (10) (a) Nunzi, J.-M.; Pfeffer, N.; Charra, F.; Fichou, D. Chem. Phys. Lett. 1993, 215, 114. (b) Fichou, D.; Nunzi, J.-M.; Charra, F.; Pfeffer, N. AdV. Mater. 1994, 6, 64. (11) Lanzani, G.; Danieli, R.; Muccini, M.; Taliani, C. Phys. ReV. B 1993, 48, 15326. (12) Grebner, D.; Lap, D. V.; Rentsch, S.; Naarmann, H. Chem. Phys. Lett. 1994, 228, 651. (13) Grebner, D.; Helbig, M.; Rentsch, S. J. Phys. Chem. 1995, 99, 16991. (14) Poplawski, J.; Ehrenfreund, E.; Cornil, J.; Bredas, J. L.; Pugh, R.; Ibrahim, M.; Frank, A. J. Synth. Met. 1995, 69, 401. (15) Dippel, O.; Brandl, V.; Ba¨ssler, H.; Canieli, R.; Zamboni, R.; Taliani, C. Chem. Phys. Lett. 1993, 216, 418. (16) Kobayashi, T.; Yoshizawa, M.; Stamm, U.; Taiji, M.; Hasegawa, M. J. Opt. Soc. Am. B 1990, 7, 1558. (17) Miyasaka, H.; Masuhara, H.; Mataga, N. Laser Chem. 1983, 1, 357. (18) Tamai, N.; Masuhara, H. Chem. Phys. Lett. 1992, 191, 189. (19) Asahi, T.; Matsuo, Y.; Masuhara, H. Chem. Phys. Lett. 1996, 256, 525. (20) Masuhara, H.; Miyasaka, H.; Karen, A.; Uemiya, T.; Mataga, N.; Koishi, M.; Takeshima, A.; Tsuchiya, Y. Optics Commun. 1983, 44, 426. (21) Hamano, K.; Kurata, T.; Kubota, S.; Koezuka, H. Jpn. J. Appl. Phys. 1994, 33, L1031. (22) Ostoja, P.; Guerri, S.; Rossini, S.; Servidori, M.; Taliani, C.; Zamboni, R. Synth. Met. 1993, 54, 447. (23) (a) Horowitz, G.; Bachet, B.; Yassar, A.; Lang, P.; Demanze, F.; Fave, J.-L.; Garnier, F. Chem. Mater. 1995, 7, 1337. (b) Porzio, W.; Destri, S.; Mascherpa, M.; Rossini, S.; Bru¨ckner, S. Synth. Met. 1993, 55-57, 408. (24) Birnbaum, D.; Fichou, D.; Kohler, B.-E. J. Chem. Phys. 1992, 96, 165. (25) (a) Lange’s Handbook of Chemistry; Dean, J.-A., Ed.; McGrawHill, Inc.: New York, 1973. (b) CRC Handbook of Thermophysical and Thermochemial Data; Lide, D.-R., Kehiaian, H.-V., Eds.; CRC Press Inc.: Boca Raton, FL, 1994. (26) Hernandez, J. P.; Choi, S. J. Chem. Phys. 1969, 50, 1524. (27) (a) Tokura, Y.; Koda, T.; Iyechika, Y.; Kuroda, H. Chem. Phys. Lett. 1983, 102, 174. (b) Sebastian, L.; Weiser, G.; Ba¨ssler, H. Chem. Phys. 1981, 61, 125. (c) Bounds, P. J.; Petelentz, P.; Siebrand, W. Chem. Phys. 1981, 63, 303. (28) Gosztola, D.; Yamada, H.; Wasielewski, M.-R. J. Am. Chem. Soc. 1995, 117, 2041. (29) Kersting, R.; Lemmer, U.; Mahrt, R. F.; Leo, K.; Kurz, H.; Ba¨ssler. H.; Go¨bel, E. O. Phys. ReV. Lett. 1993, 70, 3820. (30) Jundt, C.; Klein, G.; Sipp, B.; Le Moigne, L.; Joucla, M.; Villaeys, A. A. Chem. Phys. Lett. 1995, 241, 84. (31) (a) Johnson, R. C.; Merrifield, R. E. Phys. ReV. B 1970, 1, 896. (b) Berk, N. F. Phys. ReV. B, 1978, 18, 4535. (c) Swenberg, C. E.; Geacintov, N. E. In Organic Molecular Photophysics; Birks, J. B., Ed.; Wiley: New York, 1973; Vol. 1, Chapter 10. (d) Tavan, P.; Schulten, K. Phys. ReV. B 1987, 36, 4337. (32) Katoh, R.; Kotani, M. Chem. Phys. Lett. 1992, 196, 108.