J. Phys. Chem. 1986, 90, 6415-6479 facile reaction of O H with the other HOz as in reaction j or with H C ( 0 ) O O may deplete the O H concentration so rapidly that no OH can be detected in our experiments. Therefore, mechanism I is most consistent with our observations. However, mechanism IV involving H C ( 0 ) O O radical should account for some SO2to-SO, conversion with CO, as a product. The results summarized in Table I11 show that A(CO)/AF and A(CO,)/AF remain constant within experimental error, although mechanism I should increase the value of A(CO)/AF and decrease the value of A(CO,)/AF when H2C0.S02 species are photooxidized. This discrepancy can be rationalized if we include mechanism IV and/or if we recognize that the observed product yields in samples 5 and 6 represent contributions from not just H 2 C 0 . S 0 2 but from two sources, Le., -40% from monomeric/dimeric H 2 C 0 and -60% from H2CO.SO2 complex. Since the reaction of H 0 2 with SO2 is known to be very slow in the gas cm3 s-I (see Table IV), we need to justify phase, k, 5 1 X our use of reaction c as the primary oxidation route (in mechanism I) for converting SO2 to SO3 in solid 0, at 12 K. It can be speculated that the nascent H0, generated from photolysis of the H 2 C 0 that is complexed to SO2in solid 0, could be more reactive than thermalyzed HO,. It is also possible that the S02-to-S03 conversion process is aided by secondary photolysis of either H 0 2 or HC(0)OO in our experiments. If this is the case, the efficiency of the secondary photolysis must be very high to justify the observed value of AS(VI)/AC = 0.7. The UV absorption threshold wavelength of H 0 2 is below 270 nm in the gas phase at room t e m p e r a t ~ r e . ' ~However, our photolysis occurs at wavelength above 270 nm, and therefore the extent of the secondary photolysis of H 0 2 to give H O and 0 should be negligible. Unfortunately, we know nothing about the UV absorption of HC(0)OO. Although at present we cannot clarify these uncertainties, we conclude that mechanism I together with a minor contribution from mechanism IV may explain an efficient conversion of SO2 to SO3 by RO, in the photolysis of H 2 C 0 in solid 02. F. Implication to Atmospheric Conversion of SOz to S( VI). Hashimoto et al. did not observe SO3nor H2S04in their matrix (13) (a) Paukert, T. T.; Johnston, H. S. J . Chem. Phys. 1972, 56, 2824. (b) For solution, see: Hochnedel, C. J.; Ghormely, J. A,; Ogren, P. J. J . Chem. Phys. 1972, 56, 4426.
6475
isolation study of the gas-phase reaction involving OH, SO,, and O2in Ar.9 According to homogeneous gas kinetic rate constants listed in Table IV, reaction c which involves a reaction of H 0 2 with SO2is at least 5 orders of magnitude slower than reactions 1 and m and involving H O SO,. Therefore it appears odd that reaction c is the key reaction in mechanism I. Recent experimental results support an SO2 oxidation mechanism (via homogeneous gas-phase reactions 1-3 plus 12) which involves HO, H 0 2 , and
+
H02
+ NO
-+
+ NO2
(12)
SO3 + NO2
(13)
HO
NO radicals, and gives a net reaction
SO2 + NO
+0 2
---*
It is interesting to find that this HO, cycle in the gas phase is somewhat similar to the HO, cycle in solid 0, (mechanisms 11-IV). The reaction scheme involved in our 0, matrix/photolysis system could also be operative in oxygenated liquids containing dissolved SO, and HzCO or other small molecules which can potentially provide HO, species upon photolysis. Since SO, and H 2 C 0 (or other polar molecules) may form molecular complexes, light absorption characteristics such as absorption threshold and absorption coefficient can be significantly modified by chargetransfer phenomena. In addition, such complex formation can significantly modify photolysis characteristics, and "cage effects" can enhance the photooxidation efficiency of SO2. Such effects may provide an important pathway for SO, photooxidation (or nonphotochemical oxidation) in droplets present in clouds, polluted urban atmosphere, etc. We have not yet simulated any such experiments, but the present study provides a firm base for future experiments to investigate the role that molecular complexes play in homogeneous and heterogeneous processes.
Acknowledgment. We thank J. G. Calvert, C. J. Howard, J. R. Sodeau, K. C. Clemitshaw, and A. Beeby for helpful discussions. This research has been supported by the National Science Foundation grant ATM-83-16572. Registry No. H2C0, 50-00-0; 02,7782-44-7; SO2, 7446-09-5; H,O, 7732-18-5; CO, 630-08-0; SO3, 7446-1 1-9; H02, 3170-83-0; HC(O)OO, 56240-83-6; H2SO4, 7664-93-9; 03, 10028-15-6.
Solvent Dynamics and Twisted Intramolecular Charge Transfer in Bis( 4-aminophenyl) Sulfone Shyh-Gang Su and John D. Simon*+ Department of Chemistry, University of California-San Diego, La Jolla, California 92093 (Received: May 21, 1986; I n Final Form: July 14, 1986) Picosecond time-resolved fluorescence spectroscopy is used to probe the formation kinetics of the twisted intramolecular charge-transfer state of bis(4-aminophenyl) sulfone in acetonitrile and alcohol solvents. Emission kinetics were measured at 20-nm intervals from 400 to 560 nm, covering the entire visible region of the emission spectrum. Wavelength-independent rise times were observed in methanol and acetonitrile. In all the longer chain alcohol solvents examined (ethanol to hexanol), wavelength-dependent rise times were observed; longer rise times were observed with decreasing emission energy. In addition, multiple decay components for wavelengths higher in energy than the maximum of the TICT emission maximum,, ' ::A: were observed in all solvents. Only the red edge of the emission (A > A):'i revealed rises and decays that could be described by single exponentials. The observed kinetics are interpreted in terms of solvent restructuring to stabilize the charge-separated state. Excellent agreement between the fitted rise times of the emission at 500, 540, and 560 nm and the solvent longitudinal relaxation time, T ~ is, observed.
Introduction Since the observation of dual fluorescence for dimethylaminobenzonitrile (DMABN) in polar solvents, numerous investigations into the details of this phenomenon have appeared.'-" 'Presidential Young Investigator 1985-1990.
0022-3654/86/2090-6475$01 S O / O
Solvent studies as well as recent picosecond experiments conclude that the Stokes-shifted emission arises from a twisted intramo(1) Rettig, W.; Wermuth, G.; Lippert, E . Ber. Bunser-Ges. Phys. Chem. 1979, 83, 692. (2) Rettig, W . ;Lippert, E. J . Mol. Srruct. 1980, 61, 17.
0 1986 American Chemical Society
6476
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986
Su and Simon I
SCHEME I A
-, h"
k1
LE
k-l
TICT 4
lecular charge-transfer (TICT) state. Twisted intramolecular charge transfer has since been invoked to account for dual emission observed in several molecule^.'^-'^ However, few have been examined with time-resolved techniques. The general kinetic scheme invoked to describe the kinetics of the formation and decay of TICT intermediates is shown in Scheme I. This scheme proposes that emission arises from two distinct electronic states, the local excited state, LE, and the TICT state. In the case of TICT formation in DMABN, time-gated emission spectra" as well as streak camera kinetic measurementslO support this conclusion. Dual emission has recently been observed for bis(4-aminophenyl) sulfone (APS) (1) and related compounds15 in alcohol and nitrile solvents. The reported steady-state spectroscopy
1
supports the formation of a TICT state in this molecule. Formation of the TICT state in APS involves rotation of the anilino group, creating an intermediate with perpendicular *-systems, involving a full intramolecular charge transfer. Unlike the corresponding ketones, the ground-state geometry of the sulfone has both phenyl rings perpendicular to the N-S-N plane,18 resulting from the participation of the sulfur d-orbital in the bonding. Thus, formation of the TICT state in this molecule involves rotation of an anilino group through a full 90°. Formation of TICT states usually increases the magnitude of the permanent dipole moment of the molecule. For APS, the dipole moment of the excited state is estimated to be greater than 15 D.15 The degree of charge separation, and hence the dipole moment created, should depend on the angle of rotation between the anilino group and the rest of the molecule. Two limiting effects could be envisioned. If internal rotation and charge separation occurs rapidly compared to solvent relaxation, the solvent will not be in orientational equilibrium with the excited molecule. Nonequilibrium solvation will result in a blue shift of the emission from the equilibrium spectrum. As a result, relaxation and restructuring
(3) Rettig, W.; Bonaccic-Koutecky, V. Chem. Phys. Lett. 1979, 62, 115. (4) Heisel, F.; Miehe, J. A. Chem. Phys. Lett. 1983, 100, 183. (5) Rettig, W. J . Phys. Chem. 1982, 86, 1970. (6) Wang, Y.; Eisenthal, K. B. J . Chem. Phys. 1982, 77, 6076. (7) Eisenthal, K. Laser Chem. 1983, 3, 145. (8) Struve, W. S.; Rentzepis, P. M.; Jortner, J. J . Chem. Phys. 1973, 59, 5014.
(9) Wang, Y.;McAuliffe, M.; Novak, F.; Eisenthal, K. B. J . Phys. Chem. 1981, 85, 3736.
(IO) Hicks, J.; Vandershall, M.; Babarogic, 2.;Eisenthal, K. B. Chem. Phys. Letf. 1985, 116, 18. (11) Yeh, S. W.; Philips, L. A,; Webb, S. P.; Buhse, L. F.; Clark, J. H . In Ultrafast Phenomena ZV; Auston, D. H., Eisenthal, K. B., Ed.; SpringerVerlag: New York, 1984; p 359. (12) Rettig, W.; Zander, M. Chem. Phys. Lett. 1982, 87, 229. (13) Wermuth, G.;Rettig, W.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 64. (14) Siemiarczuk, A.; Grabowski, Z.; Krowczynski, A,; Asher, M.; Ottolenghi, M. Chem. Phys. Lett. 1977, 51, 315. (15) Rettig, W.; Chandross, E. A. J . Am. Chem. Soc. 1985, 107, 5617. (16) Kosower, E. M.; Dcdiuk, H . J . Am. Chem. Soc. 1978, 100, 4173. (17) Kosower, E. M.; Kanety, H.; Dodiuk, H.; Striker, G.; Jovin, T.; Boni, H.; Huppert, D. J . Phys. Chem. 1983, 87, 2479. (18) Dickenson, C.; Stewart, J. M.; Ammon, H. L. J . Chem. Soc., Chem. Commun. 1970, 920.
300
400 Wavelength
500 (nm)
600
Figure 1. Static emission spectra for APS in acetonitrile and ethanol. In nonpolar solvents, only a single narrow band is observed with an emission maximum around 330 nm. The emission spectra observed in acetonitrile and ethanol are assigned to a convolution of emission from both the local excited state and the TICT state. Similar behavior is observed in all of the alcohols studied (methanol to hexanol). With increasing chain length, the TICT emission shifts to the blue.
of the surrounding solvent will be manifested by a continual Stokes shift of the emission spectrum. Time-dependent Stokes shifts have been observed in several molecules,11*'g-21 and theoretical models have recently begun to emerge.22x23 On the other hand, if rotation and charge separation were slow compared to solvent relaxation, equilibrium solvation would always be maintained during the rotation process. This would also be manifested by a gradual shift in the emission to lower energy as more charge-transfer character is mixed into the excited state by rotation about the sulfur-anilino bond. However, in this case, the rate of TICT formation should be much slower than the relaxation times associated with solvent motion. For the alcohols, this would correspond to hundreds of picoseconds to nanosecond~.~~-~~ In order to investigate the role of solvent dynamics on the kinetics of formation and decay of the twisted intramolecular charge-transfer state, bis(4-aminophenyl) sulfone was examined in acetonitrile and alcohol solvents.
Experimental Section A complete experimental procedure will be published at a later date. Briefly, picosecond pulses are generated by synchronously pumping a R6G dye laser with the second harmonic of a continuous-wave mode-locked Nd3+:YAG laser (Quantronix 1 16 mode-locked at 38 MHz). Autocorrelation traces of the dye laser output (InRad 5-14) indicate that the pulses are on the order of 1-2 ps fwhm. The dye laser pulses are amplified by a three-state longitudinally pumped pulsed dye amplifier using a QuantaRay DCR-2(20) Nd3+:YAG laser (100 mJ/pulse at 532 nm). Saturable absorber jets of crystal violet are used to isolate the stages. The output pulse energy is e l mJ/pulse, &20% peak-to-peak fluctuations, with amplified spontaneous emission suppression of better than 1OO:l. The red light is frequency doubled (KDP = 5-10% efficiency) to 300 nm and used to excite the sample. The remaining red light is delayed and used as a timing marker for (19) Safarzadeh-Amiri, A. Chem. Phys. Lett. 1986, 125, 272. (20) Halliday, L. A.; Topp, M. R. Chem. Phys. Lett. 1977, 48, 40. (21) Okamura, T.; Sumitani, M.; Yoshihara, K. Chem. Phys. Lett. 1983, 94, 339. (22) van der Zwan, G.; Hynes, J. T. J . Phys. Chem. 1985, 89, 4181. (23) Bagchi, B.; Oxtoby, D. W.; Fleming, G. R. Chem. Phys. 1984, 86, 257. (24) Garg, S. K.; Smyth, C. P. J . Phys. Chem. 1965, 69, 1294. (25) Hynes, J. T.; Wolynes, P. G.J . Chem. Phys. 1981, 75, 395. (26) Davies, M. In Dielectric Properties and Molecular Behavior; Hill, N. E., Vaughan, W. E., Price, A. H., Davies, M., Ed.; van Nostrand-Reinhold: New York, 1969.
Solvent Dynamics and TICT Relaxation
0 -
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 6477
--
I I ' I 200 io0 600 0 200 400 600 time ( p i c o s e c o n d 1 time [ p i c o s e c o n d 1 Figure 2. Fluorescence data for APS in hexanol. The wavelengths Figure 3. Fluorescence data for APS in methanol. The wavelengths monitored are (-) 400 nm; (----) 420 nm; (---) 460 nm; (-) 500 nm; monitored are (-) 420 nm; (----) 460 nm; (---) 500 nm; (-.-) 560 nm. (---) 560 nm. The decays observed at 400 and 420 nm can be described The decays observed at 420, 460, and 500 nm can be described by two by two exponentials. All wavelengths appear to have a rise time. For exponentials. All wavelengths appear to have no rise time. lower emission energies, the rise time is longer. Note that there is no red emission at t = 0.
0
signal averaging of the streak camera data. Fluorescence is detected 90' from the direction of excitation and focused on the input slit of a Hamamatsu C979 streak camera. A 1-cm cell was masked so that emission over the central 0.2 cm was detected. The streak camera output is recorded by an EGG Intensified Reticon which is interfaced to an LSI-11/23+ computer. The computer software is designed so that data are accepted by the computer, the prepulse is examined for both intensity and position, and the acceptable data is shifted and added into memory at the laser repetition rate of 20 Hz. Time calibration was determined with etalons. Intensity nonlinearities were corrected by measuring decay curves of dilute R6G in methanol. Wavelength selection was accomplished with narrow-band interference filters (ESCO, 5-nm fwhm). The difference in arrival times of various wavelengths at the streak tube was calibrated with anthracene and NADH. Anthracene was also used to determine the response of the instrument to the instantaneous rise of an emission decay. From traces of the laser pulses, an instrument response of 10-15 ps is obtained. The resulting data were transferred to a Celerity C-1260 computer system for kinetic analysis. All decay kinetics were determined by convolving the desired functional form with the prepulse recorded by the streak camera. Sample concentrations were to M. All samples were degassed by bubbling nitrogen for at least 30 min, after which the cuvette was capped and sealed with Parafilm. On the average, 5000 laser shots were averaged for one decay. Absorption spectra were run before and after photolysis to ensure that no photochemical degradation of the sample had occurred. Bis(4aminophenyl) sulfone was purchased from Aldrich and recrystallized twice from ethanol. All solvents were dried over molecular sieves and checked for background fluorescence. Fluorescence spectra were recorded on a Model 1902 Spex Fluorolog.
Results and Discussion In Figure 1 fluorescence spectra for bis(4-aminophenyl) sulfone in acetonitrile and ethanol are shown. In nonpolar solvents, such as hexane, only a single narrow band is observed around 330 nm. This band is assigned to emission from the Franck-Condon state excited at 300 nm. However, in both polar solvents, an additional Stokes-shifted emission band is observed. The maximum of the charge-transfer band of APS in alcohol solutions shifts to higher energy with increasing alcohol chain length. This is consistent with the decrease in polarity of the solvent with increasing chain length. A linear relationship between the TICT emission maximum and solvent polarity has been reported for APS in the series of alcohol^.'^ As a result, for longer chain alcohols, there is increasing overlap between the emission from the LE and TICT
> u rl m
0.6
C
al
u
5
0.4
0.2
0 0
I
,
200
400
-7-1
600
i picosecond I Figure 4. Fluorescence data at 400 nm for APS in (-) methanol, (----) propanol, and (- -) hexanol. This data clearly shows two decay components at 400 nm for APS in all solvents studied. In addition, with increasing chain length, a rise time is observed at this wavelength. The emission in methanol rises within the instrumental response. time
~
states, supporting the conclusion of an equilibrium between the two emitting excited states. In Figure 2 fluorescence decays for APS in hexanol for several wavelengths are presented. Several general observations can be made from this data. First of all, wavelength-dependent rise times are observed. Wavelengths characteristic of pure TICT emission are observed to rise with different rates. Second, at zero time, no red emission is observed. These observations are not unique to hexanol solutions and are observed in all alcohols except methanol. In methanol, Figure 3, all wavelengths are observed to rise with the temporal resolution of the streak camera detection system. In addition, for 420, 460, and 500 nm, complicated decay kinetics are observed. Examination of the static emission spectrum reveals that these wavelengths correspond to emission from the TICT state; no emission is expected from the LE state.I5 In Figure 4 are plotted the emission kinetics observed at 400 nm for APS in methanol, propanol, and hexanol. As mentioned above, emission in methanol is observed to rise within the time resolution of the experiment. However, for longer chain alcohols, a rise time is observed at this wavelength, the time increasing with increasing chain length. 111 Figure 5 are presented similar data for emission at 560 nm. Once again, the rise time of emission at this wavelength is found to be solvent dependent. In nonpolar solvents, only a single emission band is observed. In hexane solution, no rise time is observed at any emission wavelength and the decay kinetics for all wavelengths in the emission spectrum are identical. The data can be fit to a sin-
6478
S u and Simon
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986
TABLE I: Solvent Dielectric Data24-z6and Emission Dynamics solvent T D * Ps (0 6, methanol ethanol propanol butanol pentanol hexanol acetonitrile
48 170 430
668 927 1210 3.9
32.6 24.3 20.1 17.1 13.9 13.3 35.9
5.6 3.4 4.0 3.0 2.2 2.2 2.0
TL1
PS
8.84 33.8 93.9 132.9 173.8 236.8 0.268
Tr560,
PS
TrS4O,
0 20 55 85 120
PS
Tr5003 PS
0 15 45
80
150 0
7260
0
500“
15 30 60 85
720 800
105 140
105
1000 1100 1150
0
0
650
“Two exponentials needed to fit decay
gle-exponential decay, with a decay time of 2.0 ns. If the two-state model shown in Scheme I applied to APS dynamics in alcohols, the dynamic data could be modeled by the following kinetic equations:
0.8
ILdtJ) =
ALE(X)-
IO
a1 - a2
0 6
[ ( k - , + kf2 - al)e-alf- ( k - ) + kf2(1)
0 4
and 0 2
In the above equations, ILEand I,, are the emission intensities from the local excited state and the TICT state, respectively. The coefficients A , @ ) correspond to the intensity of the static emission spectrum at wavelength A. The constants a , and a 2 are related to the rate constants shown in Scheme I by the following expression: 1~2.1 =
y*[(k,I + kl
+ kf2 + k-1) f ( ( k , + k-l
- k f , - k1)* +
4k,k_J1:2] (3) These kinetic equations predict the following behavior for the emission dynamics. First, all wavelengths in the TICT emission band would be expected to rise with the same time constant. Second, biphasic decays would only be expected for wavelengths characteristic of the local excited state. Third, the rates corresponding to the fast decay of the local excited state should match the rise times of the TICT emission. These predictions are not consistent with the observed data. Biphasic decays are observed for wavelengths that correspond to pure TICT emission (Figure 3). In addition, as in the following section, different rise times are observed for wavelengths characteristic of pure TICT emission (Figure 2). In order to quantitatively determine the emission rise and decay kinetics, the emission data for a particular wavelength, X, was fitted to the following function:
0 I
0
I 600
t:me ( picosecond 1 Figure 5. Fluorescence curves at 560 nm for APS in (----) methanol, (---) propanol, and hexanol. The solid lines through the propanol and hexanol data are the best fit of the experimental data to eq 1. The emission in methanol rises within the instrumental response. The decay kinetics observed in methanol are poorly fit by eq I . (-e-)
that solvent and/or solute relaxation processes are important in the evolution of the TICT emission. The role of solvent relaxation on the rate of time-dependent fluorescence shifting has been the subject of recent theoretical Within the Debye continuum limit, in which the solvent dipoles relax by rotational diffusion, the following correlation between the rate of Stokes shifting and the longitudinal relaxation times of solvents, 7L, is [w(t) -
w(m)l
-
e-r:~L
[ N O ) - w(m)I
(5)
In the above expression, w(O), w ( m ) , and w(t) are the emission maximum at t = 0, t = m (fully relaxed), and 0 < t < m , respectively. The longitudinal relaxation time is related to the Debye relaxation time, 7 D , by the following e x p r e ~ s i o n : ~ ’ - ~ ~ 7L
In the above expression, T, and 7d are time constants characteristic of the emission rise and decay, respectively. G(7) is the instrument response function. The intensity is expressed as the convolution of the instrument response with the exponential rise and decay functions. The above functional form successfully describes emission in the red end of the spectrum, X > 500 nm, for several of the alcohol solvents examined; see Figure 5. For higher energy wavelengths than the maximum of the TICT emission, Le., 420 and 460 nm, an additional fast decay component is observed in all solvents (Figure 3). To fit this data, an additional exponential term needs to be included in eq 1. In Figure 5, the solid lines represent the best fits between eq 1 and the experimental data. In Table I are presented the results of the fit. In the case of ethanol, it is difficult to accurately determine the rise time at 500 and 540 nm due to the temporal resolution of the streak camera system. The data presented in Table I substantiate the earlier conclusion that the dynamics cannot be adequately described by the simple kinetics in Scheme I. The wavelength-dependent rise times observed for all the alcohol solutions except methanol suggest
400
200
+ +
2t, I =2€0 1 7 D
where to and 6- are the static and high-frequency dielectric constants, respectively. In determining 7L, the values can vary depending on the value used for the high-frequency dielectric constant, t,. Measurements of the dielectric absorption of several normal and primary alcohols at wavelengths ranging from 30 000 to 0.22 cm have been analyzed to yield three dispersion regions with relaxation times of 50-2000, 17-50, and 1.7-4 ps, respect i ~ e l y . *Each ~ of these regions has associated with it a highfrequency dielectric constant. The slow relaxation is attributed to the breaking of hydrogen bonds in molecular aggregates of alcohol molecules. The intermediate relaxation time is interpreted as being due to the rotation of monomeric alcohol molecules, while the shortest relaxation time is attributed to the rotation of the terminal OH group about the C-0 axis. The total amount of (27) van der Zwan, G.; Hynes, J. T. J . Chem. Phys. 1982, 76, 2993. (28) van der Zwan, G.; Hynes, J. T. Chem. Phys. Lett. 1983, 101, 367. (29) Hubbard, J. B.; Wolynes, P. G. J . Chem. Phys. 1978, 69. 998.
Solvent Dynamics and TICT Relaxation
0
50
150
100 I /
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 6479
200
250
'
Figure 6. Emission rise times, T , (Table I), are plotted as a function of the solvent longitudinal relaxation time, T ~ The . emission data are de-
termined by modeling the observed decays to eq l . The wavelengths plotted are 560 nm (0),540 nm (X), and 500 nm (+). absorption in the low dispersion region (slowest relaxation time) is much greater than that in the other two regions, indicating the proportion of free monomeric alcohol capable of independent rotation and possessing non-hydrogen bonded OH groups that can rotate is relatively small. As a result, solvent relaxation phenomena should correlate with the time scale corresponding to hydrogen bond restructuring, and the high-frequency dielectric constant used should correspond to this area of dispersion. These values are different than those calculated with the expression em = n2, where n is the index of refraction. Due to overlapping areas of dielectric dispersion, the assignment of 6- is difficult. The values used were obtained by examining the original data published by Smyth and c o - w o r k e r ~ .The ~ ~ values used and the resulting T~ values are also given in Table I. In order to examine the possible contribution of solvent relaxation to the observed emission dynamics, the wavelength-dependent rise times were assumed to be proportional to the rate of Stokes shifting of the emission spectrum [w(t) - w(m)I
[ N O )- w(m)1
a &r
(7)
Within this assumption, a linear correlation is predicted between T, and T ~ In . Figure 6 are plotted the rise times of the emission observed for low-energy emission wavelengths as a function of T~ for the series of alcohols. The emission kinetics correlate extremely well with the solvent longitudinal relaxation time, strongly supporting the conclusion that the dynamics associated with wavelengths to the red of the emission maximum arise from solvent relaxation dynamics. Wavelength-independent rise times were observed for APS in both methanol and acetonitrile. These observations do not contradict the above conclusion. The longitudinal relaxation time for methanol and acetonitrile is 8.8 and 0.27 ps, respectively. When the results in Figure 5 are used to calculate the expected rise time, emission rises under 5 ps are predicted. Dynamics on this time scale are too rapid to be recorded by our detection system. Recently it has been proposed that polarity plays a significant role in the kinetics of formation of the TICT state in DMABN.I0 Our data indicate that in the case of APS solvation dynamics play
an important role in the observed emission dynamics. Our data do not determine whether or not excited-state equilibrium between the local and TICT emitting states exists. Studies based on the steady-state spectroscopy in the series of alcohols support an excited-state equilibrium. We are currently modifying our streak camera system so that the UV emission dynamics can be recorded. This will allow us to directly determine if the kinetics of Scheme I can be applied to the local excited-state emission and whether charge separation in this molecule is an activated event. It is interesting to note that charge separation does not occur in a nonpolar solvent. If the solvent structure around the local excited state is similar to the ground state, it is likely that even in polar solvents the charge-transfer state is energetically unfavorable in the initial solvent configuration. Only through solvation dynamics2*is the system able to evolve into the lower energy TICT geometry. The complicated dynamics observed for wavelengths to the blue of the TICT emission maximum suggest that additional factors along with solvent restructuring contribute to the dynamics. It is important to stress that the fast and slow decays observed at 420 and 460 nm in the series of alcohols cannot arise from decays of the local excited state and TICT state, respectively, as might be expected from either Scheme I or in comparison with results observed for DMABN. Comparison of the static emission spectrum in polar and nonpolar solvents clearly demonstrates that the local excited state does not emit at these wavelengths. Several effects could account for the rapid decays observed at these wavelengths. First, in addition to a time-dependent spectral shift, the line shape of the emission spectrum may be changing with time. Second, relaxation of the blue edge of the emission spectrum may be controlled by faster components of the dielectric relaxat i ~ n Third, . ~ ~ due to the large size of the rotating group, it is conceivable that emission in this part of the spectrum reflects solute dynamics, in which the evolution of the emission spectrum reflects the dynamics of twisting of the anilino group around the sulfur-anilino bond. We are currently examining derivatives of APS [bis(4-(methylamino)phenyl) sulfone and bis(3-aminophenyl) sulfone] in order to further understand the role of the solvent in the dynamics of TICT formation in these systems.
Conc1usions The emission dynamics of bis(4-aminophenyl) sulfone have been examined in alcohol and acetonitrile solutions. Wavelength-dependent rise times were observed in all alcohol solvents except methanol. In addition, no wavelength-dependent rise times were observed in acetonitrile. Emission kinetics recorded for the red edge of the spectrum were well described by single-exponential rises and decays. The calculated rise times for emission at 500, 540, and 560 nm were found to correlate with the longitudinal relaxation times of the solvent. In addition, the complicated rise and decay kinetics observed for wavelengths higher in energy than the maximum of the TICT emission suggest that both solvent reorganization as well as solute motion (twisting dynamics and charge separation) may contribute to the emission dynamics. Acknowledgment. We are grateful for support from Research Corp., the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation. We thank Professors Michael Fayer and Charles Harris for their help and advise in the construction of the laser system. We are indebted to Dr. A1 Cross who graciously provided his data-fitting and plotting routines. We also thank the Chemistry Department at UCLA for use of their fluorimeter, purchased by funds provided by NSF NO. MPS75-06135.
