J . Phys. Chem. 1990, 94, 1404-1408
1404 8600
800
1000
800
1000
I
1200 ,8
7
6
3
2
11
600
I1 1200
T, K Figure 5. Comparison of the kinetic isotope effect x ( T ) = k , ( T ) / k 4 ( T ) between experimental and models: experimental best fit (-); ICVT/
SCSAG ( - - - - - ) : CTST/N (---); CTST/W (---); CTST/E
(---).
surface as for reaction 1 and making changes based only on mass and zero-point energy differences.s The results are shown in Figure 4. In the 590-1220 K range, the agreement with the experimental k4(T ) of Marshall and FontijnS is better than f 6 0 % for CTST/N and better than the f30% error limits of that work for CTST/W and CTST/E. This shows that the tunneling model chosen is not
critical for the transfer of the heavier D atoms As with reaction 1, ICVT/SCSAG systematically underestimates k4( T). In Figure 5, the kinetic isotope effect, x ( T ) = k , ( T ) / k 4 (T), from the experimental data and from the four pairs of theoretical calculations is shown. Combining the error limits in k l ( r ) and k4( T ) of about f25% and f30%, respectively, we estimate the confidence limits for the experimental x( T ) to be about f40%. Within these error limits, x ( T ) from the three CTST models agree with the experimental x( T ) except for CTST/N at T < 750 K and CTST/W at T < 650 K. It is interesting to note that although ICVT/SCSAG underestimates k,( T ) and k4(T ) systematically, the x ( T ) is in good accord in the 650-1220 K range. The closest agreement to the strong curvature in x( T ) observed experimentally is from CTST/E. We conclude that the In k , ( T ) vs T I plot does not curve as strongly as our previous results, suggested and that both CTST and VTST calculations can approximate the shape of this plot. These experimental data combined with the CTST calculations presented confirm the conclusion reached earliers that a single potential energy surface and conventional transition-state theory with a simple tunneling model can simultaneously fit the rate coefficients for the H + NH3 and D ND, reactions.
+
Acknowledgment. This work was supported by the US.Army Research Office. We thank W. F. Flaherty for technical assistance. We also thank Drs. B. C. Garrett, M. L. Koszykowski, and C. F. Melius for providing the ICVT/SCSAG results prior to publication and Drs. J. V. Michael and K. Mahmud for helpful discussions. Registry No. H, 12385-13-6; NH,, 7664-41-7; D,, 7782-39-0.
Anomalous Solvent Eftects on the Twisted Intramolecular Charge Transfer Fluorescence of Ethyl 4-(N,N-Dimethylamino)benzoate in Chlorinated Solvents Rong Kun G U O , *Noboru ~~ Kitamura, and Shigeo Tazuket Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta. Midori- ku, Yokohama 227, Japan (Received: January 27, 1989; In Final Form: August 28, 1989)
It was found that the TICT (twisted intramolecular charge transfer) fluorescence of the title compound was very different in chlorinated solvents in comparison with other solvent systems. The emission energy from the a* state (charge transfer state) is higher and the intensity ratio, R = & / I b where I, and Ib are the emission intensity from the a* state and from the local excited state, is lower at room temperature in CHCI, and butyl chloride than in nonchlorinated solvents. The activation energies E , and E2 for the forward (b* a*) and backward (b* a*) reactions are also higher in chlorinated solvents.
-
-
The origin of the anomalous solvent effects was attributed to donor-acceptor type solutesolvent interaction. This interaction was thought to affect the potential energy hypersurface along the reaction path: b* a*. An anomalous red-shift of the a* band of polymer-bound TICT chromophore in chlorinated solvents was discussed in terms of reduced solvation. -+
Introduction
Detailed understanding of twisted intramolecular charge transfer (TICT) fluorescence as a function of molecular environment benefits the application of this interesting group of compounds as fluorescence probes. Several reports have already proved the usefulness of TICT compounds as probes for molecular motion in polymer^.^-^ In the course of studying the title compound (DMABE) as a reference model compound for polymer study, we found anomalous solvent effects on fluorescence in chlorinated solvent^.^ The anomaly of TICT fluorescence in chlorinated solvents Seems to be a general phenomenon for TICT compounds having dialkylamino groups as electron-donating subunits. Examples are Deceased July I 1 , 1989.
0022-3654/90/2094- 1404$02.50/0
presented later. The general understanding of solvent effects on TICT fluorescence is as follows. The polarity of the solvent is considered to determine the height of the energy barrier for the b* a* process4 as well as the stabilization of the a* state, and therefore the emission energy, the relationship between the energy barrier of the TICT formation and solvent polarity, was establ i ~ h e d . ~Solvent viscosity is thought to control the rate of the TICT formation if the process requires no energy
-
( 1 ) Visiting fellow from Institute o f Photographic Chemistry, Academia Sinica, Beijing, China. (2) Tazuke, S.;Guo,R. K.; Hayashi, R. Macromolecules 1988,21, 1046. ( 3 ) Tazuke, S . ; Guo, R. K.; Hayashi, R. Macromolecules 1989, 22,729. (4) Lippert, E.; Rettig, W.; Bonacic-Koutecky, V.; Heisel, F.; Miehe, J. A. Ado. Chem. Phys. 1987, 68, 1, and many references therein. ( 5 ) Hicks, J. Chem. Phys. Left. 1985, 116, 18.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1405
TICT Fluorescence
TABLE I: Some Physical Properties of Solvents and the Absorption and Emission Energy of the TICT State for DMABE at 298 K
150
20 0
25 0
Wave number x 10.3 cm-'
300
Figure 1. Fluorescence spectra of DMABE in EAc (-) and BuCl (---) at 298 K, excitation at 300 nm. The a* and b* bands were separated by assuming a Gaussian distribution of the a* band.
However, the anomalies in chlorinated solvents cannot be attributed to these two factors, since BuCl is similar to EAc and THF in polarity, viscosity, and other physical properties (see Table I in the text).8 It seems necessary to consider some specific solute-solvent interactions. Experimental Section DMABE was prepared by esterification of the corresponding benzoyl chloride and purified by sublimation before use. All solvents were refined by standard methods and checked to be nonluminescent at or above 300 nm before use. Absorption spectra were obtained by a Hitachi UV-320 spectrophotometer. Corrected fluorescence spectra were recorded on a Hitachi MPF-4 spectrofluorometer under argon atmosphere and calibrated for the instrumental response by a connected Oki IF-800 microcomputer. The separation of the a* band from the b* band was carried out on a PC-9801 computer (NEC) using the simplex methods9 On the assumption that the shape of the a* band can be simulated by a Gaussian distribution, the a* and b* bands were separated. The emission intensity ratio R and the maximum wave number vmaX of the two bands were given by the computer automatically. Quantum yields were determined by a Hitachi F-4000 spectrofluorometer using a 1 N H2S0, solution of quinoline. IH N M R spectra were obtained by a JNM-GX-270 (270 MHz, JEOL Inc.) N M R spectrometer. Fluorescence measurement below room temperature was conducted with the use of a cryostat (Oxford Model DN1704) and its temperature controller (Model 3120). A Yamato BH-71 thermocontrol unit was used for fluorescence measurement above room temperature. The temperature range for the measurement was between -100 and +60 OC depending on the freezing/boiling points of the solvents. The experimental error was within f0.5 OC. No deterioration of sample during measurement was confirmed by absorption and fluorescence spectroscopy. Results and Discussion Higher Emission Energy of the a* Band in Chlorinated Soloents. An example of the anomalous solvent effect of chlorinated solvent is shown in Figure 1. Although the dielectric constant t is nearly the same for EAc and BuCl as shown in Table I, the a* band is strongly blue shifted in BuCl (-40 nm). Furthermore, the relative intensity ( R value) in BuCl is about one-fourth of the value in EAc at room temperature. A simple evaluation on solvatochromism is provided by the Lippert-Mataga equation.1° A linear relationship between the Stokes shift (vab - ve,) of DMABE and the solvent polarity parameter Af (Af = (E - 1)/(2t + 1) - (n2 - 1)/(2nZ + 1)) has been established.' Later, an improved equation with modified Af (Af') was suggested by Mataga (eq la) for cases when the dipole moment of the excited (6) Rettig, W. J. Lumin. 1980, 26, 21. (7) Wermuth, G.; Rettig, W.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. .. 1981, 85, 64. (8) Riddick, J. A.; Bunger, W. B. Organic Soluents, Techniques of Chemistry; Wiley-Interscience, Inc.: New York, 1970; Vol. 11. (9) Neder, J . A.; Mead, R. Compur. J. 1963, 6, 163. (10) Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1957.61.962; Magata, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 115, 465.
solventa Et20 BuAc EAc THF PrCN MeCN CHCI, BuCl DCM DCE
4.335 (20 "C) 5.01 (20 "C) 6.02 (25 "C) 7.58 (25 "C) 27.2 (30 "C) 37.5 (20 "C)
AfIb 0.262 0.270 0.293 0.309 0.382 0.391
ANc 3.9
4.806 (20 "C) 7.39 (20 "C) 8.93 (25 "C) 10.36 (25 "C)
0.255 0.308 0.319 0.327
23.1
€
10.7 8.0 16.7 19.3
20.4 21.0
10-3~,~,
10-3~,*,
cm-' 32.77 32.48 32.48 32.36 32.26 32.15
cm-I 22.58 21.52 20.97 21.22 19.82 19.42
32.26 32.78 32.36 32.56
23.07 22.53 21.77 21.62
Et20, diethyl ether; BuAc, butyl acetate; EAc, ethyl acetate; THF, tetrahydrofuran;PrCN, propionitrile;MeCN, acetonitrile; BuCI, butyl chloride; DCM, dichloromethane; DCE, dichloroethane. For the definition of Af', see text, eq 2. CThevalues of A N are from ref 17 except those of EAc, PrCN, and DCE. A N of EAc is from the relation of A N and Dimroth-Reichart ET,'' A N of PrCN and DCE are obtained from correlation between A N and the absorption energy of cis-dicyanobis( 1 , l O-phenanthroline)ruthenium(II).22
0.35 0.40 Af' Figure 2. Plots of ,v and (vat* -),v of TICT fluorescence for DMABE vs Af'at 293 K, excitation at 300 nm: 1, CHCI,; 2, BuCI; 3, CH2CI,; 4, DCE; 5, (CIEt),O; 6, Et& 7, BuAc; 8, EAc; 9, THF; 10, PrCN; 1 1 , MeCN.
0.25
0.30
state is much larger than that of the ground state, such as in the case of TICT compoundsll (kc - PLg)z Vab
-
Vem
=2 (kc
= -2
hca3
- WgFC)kc
Af'
+ constant
(14
+
(1b)
Af' constant hca3 In eq 1b, constant means vem in the gas phase v,,
Since the absorbing state does not correspond to the transition (1 1 ) Mataga, N. In Introduction of Photochemistry; Kyoritsu Shupan, Inc.: Tokyo, 1975.
Guo et al.
1406 The Journal of Physical Chemistry. Vol. 94, No. 4, 1990 TABLE 11: Thermodynamic and Speetrogcopic Data for DMABE in Various Solvents“ E , of solvent solvent E, hvl E2 Tmx? K THF 6.7 4.3 10.8 250-260 243-250 BuAC 8.6 5.1 16.8 242-248 EAc 6.8 5.0 11.3 270-276 PrCN 5.9 4.3 7.1 204-2 14 BuCl 6.0 6.6 18.2 268-270 DCE 7.6 7.5 23.6 22 0-2 23 DCM 5.8 7.2 19.2
+
4”
6b*
13.6 20.1 24.1 15.0 25.1 20.0 21.1
0.005 b 0.007 0.002 0.026 0.021 0.028
68, 0.035 b 0.063 0.022 0.076 0.079 0.062
cm-’. bNot determined.
“All energies in X
9
-‘E,
a
8
N
h6 x
tlj-
5 4
I ‘ 3.0
I 3.5
L.0
4.5
5.0
5.5
l / T x103 K Figure 3. Temperature dependence of R value for DMABE in BuCl (O), CH2CI2(v), DCE (a), THF (0),PrCN (A),EAc (A),and BuAc (4). Excitation was at 300 nm.
to the TICT state and consequently the physical meaning of (uah - vem) is ambiguous,12 the expression based on fluorescence alone is recommendable (eq 1b). In either case by use of eq l a or lb, the data points for chlorinated solvents fall in a distinctively different line from that for nonchlorinated solvents as shown in Figure 2. This phenomenon is not unprecedented. 4,4-(Dimethylamino)phenyl sulfone’3also exhibits a blue shift and intensity loss in BuCl at room temperature compared with the fluorescence in T H F having comparable physical properties with BuCI. (N,NDimethylanilino)anthracene,14a TICT compound exhibiting only one fluorescence band, shows a much higher emission energy in CHC13 and DCE than expected from the polarity of the solvents. Another example is found for 1-ethylindoline-5-carboxylicacid ethyl ester.’ In the plots of (vak - v,*) vs solvent polarity, the data in DCE and BuCl deviate from the linear line for the other solvents. However, these anomalies of halogenated solvents have been overlooked. Interestingly, the slope of Figure 2a is slightly but clearly smaller for chlorinated solvents than for other solvents, indicating the difference in energy gap between the TICT a* state and the Franck-Condon ground state at a certain solvent polarity in these two series of solvents. Namely, the dipole moment (pa*) is smaller in chlorinated solvents owing to the specific solvation to be discussed in the following sections. Temperature Dependence of R and Activation Energies E , and E2. Figure 3 shows the temperature dependence of R in seven aprotic solvents. It is noticeable that the slopes of the Arrhenius plots in chlorinated solvents are steeper in both lower and higher temperature ranges. In particular, the difference in the slope is phenomenal in the high-temperature range above Tmx. Although the present results suggest a higher activation energy E , for the (12) Schneider, F.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 1 1 55: 1970. 74. 624. ( 1 3 ) Rettig, W.; Chandross, E. A. J. Am. Chem. SOC.1985, 107, 5617. (14) Visser, R. J.; Varma, C. A. G. 0. J . Chem. Soc., Faraday Trans. 2 1980, 76, 453.
4
4
5
6
7
E, x 10-2cm-
8
9
Figure 4. Dependence of E, and E, on E,: 0, normal solvents; 0 , chlorinated solvents.
- -
forward reaction b* a* and a much higher E, for the backward reaction b* a* in halogenated solvents according to the kinetic scheme by Grabowski,Is the values of E2 are unreliable. The higher temperature sides of the Arrhenius plots are hardly linear for chlorinated solvents and we are not sure if the values represent E2 defined kinetically. The values of E l + hvl, “apparent” El, quantum yields and &., T,,, (the temperature at which R reaches the maximum value R,), R,, and E, (activation energy for solvent viscosity) are collected in Table 11. Usually hv, is much smaller than E , and its contribution is neglected in the following discussion. There is no systematic variation of R , and T,, in the two types of solvents. Obviously, the unusually small R at room temperature in chlorinated solvents is attributed to the steeper temperature dependence of the R value, that is, to the higher activation energies E , and E2. Usually, the solvent viscosity is considered as an important factor controlling the rate of the TICT formation.n The relationship of E , and E2 with E,,16is depicted in Figure 4. It is easy to find that not only E , but E2 is changed linearly with E,,. It is understandable that the geometric return of perpendicular a* state to the planar b* state also involves the rotational motion around the nitrogen-benzene bond and is restricted by friction with the surrounding solvents. The values of E l in chlorinated solvents are nearly comparable to the corresponding E, values (15) Grabowski, Z . R.; Rotkiewicz, K.; Rubaszewska, W.; Kirkor-Kaminska, E. Acta Phys. Pol., 1978, 54, 767. (16) E is obtained from the improved Veizen-Cardozo-Langenkamp formula (keid, R. C.; Pravsnitz, J. M.;Shewood, T. K. The Properties of Gases and Liquids;McGraw-Hill, Inc.: New York, 1977; p 627) log,, 7 = visB( 1/ T - 1/visTo), where visB and visTo are empirical parameters. The available temperature range is from 20 to 30 ‘C above freezing point to T, -0.75 ( T , = T/T,), which is wide enough for the present discussion.
The Journal of Physical Chemistry, Vol. 94, NO. 4, 1990 1407
TICT Fluorescence whereas those in nonhalogenated solvents are somewhat lower than E,,. This is explainable by the fact that the motion leading to the formation of the a* state is rotational and the energy will probably be smaller than the energy required for the translational motion of the solvent molecules in viscous flow. The differences in the activation energies are likely to be related to specific solvation which is not interpreted within the frame work of dielectric continuum treatment." Donor-Acceptor Interaction with Chlorinated Solvents. Chlorinated solvents usually have stronger electron-accepting powers which can be evaluated by the solvent acceptor number (AN).'* For example, the ANs of CH2C12(20.4) and CHCI, (23.4) are larger than that of MeCN (19.3), in spite of much higher polarity of the latter (see Table I). Another example is a nonpolar solvent, CC14. Its AN (8.6) is even larger than that of T H F (8.0) with moderate polarity. In chlorinated solvents, for DMABE as a polar molecule composed of two subunits, solute-solvent donor-acceptor interaction is either with the dimethylamino (DMA) group with one lone electron pair at the nitrogen in the planar b* state or with the negatively polarized benzoate (BE) group in the perpendicular a* state.I5 Obviously, the donor-acceptor interaction with solvent molecules is unfavorable for the intramolecular restructuring of the electronic charges accompanying the rotational motion in the forward b* a* or backward reaction b* a* and thus leads to a rise in the height of the energy barrier in the reaction pathway. A slight decrease in pa* in chlorinated solvents (Figure 2a) might stem from this solvent effect, which causes dispersion of electronic charge located at the BE moiety. A distinct increase in the quantum yield of the b* state (&.) in chlorinated solvents (Table 11) also suggests that the conversion to the a* state is suppressed so that the emission probability of the b* state is increased. The high emission energies of the TICT fluorescence in chlorinated solvents relative to those in nonhalogenated solvents are very difficult to understand. An acceptable explanation is to assume the solvent effect on Franck-Condon ground state (So+. As shown in Figure 5, under the condition of identical absorption energy in both types of solvents and higher E l and E2 in halogenated solvents, the a* band energy should be lower in chlorinated solvents unless the SoFcstate is stabilized in the solvents. One possibility is the stabilization of the electron-accepted BE part by electrophilic chlorinated solvents when DMABE is twisted and the dispersion of electronic charge located at the BE part has been shown by a slight decrease in dipole moment pa*;therefore the SOFC state with twisted conformation should be stabilized more in chlorinated solvents. Owing to uncertainty of E2, it is difficult to put forward a conclusive discussion. There is however a piece of evidence for specific interaction of halogenated solvents with the negative amino part of DMABE. 'H NMR spectra of dodecyl4-(dimethylamino)benzoatein CDC13 and in Et,O, both having nearly identical polarity, exhibit different chemical shifts at the methyl protons of the dimethylamino group. The signal appears at 6 = 3.033 ppm in CDC13and at 2.758 ppm in E t 2 0whereas the signals other than the methyl groups are not subjected to solvent effect within fO.l ppm. The downfield shift in CDCI, is an indication of specific solvation at the amino group by highly electron-accepting solvents. This donor-acceptor interaction is likely to be more effective in the SoFcstate in which the dimethylamino group is perpendicular to the phenyl group (0 = 7r/2 in Figure 5) and the basicity of the electron-accepted BE moiety will be higher than that of amino group in the planar ground state (6 = 0). Thus, stabilization of the Franck-Condon ground state is likely. Reduced SoluteSolvent Interactions in the Polymer System. In the previous work, we have noticed an anomalous fluorescence behavior of the polymer-bonded DMAB chromophore in chlorinated solvent^.^ Figure 6 shows an example. In a broad tem-
-
-
(17) Rettig, W . J . Mol. Srruct. 1982, 84, 303. (1 8 ) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum Press: New York, 1978.
0"
90' Twisting angle(8)
Figure 5. Solvent effects on energy diagram: -, chlorinated solvents.
-
23
normal solvents; ---,
0
0
I
5 "b
ma
D U
m
m
0
222
0 0
..
W
6 21
a
.-3
0
0
E
O Q
W
0,".
20
30
35
40
45
lOOOlT K Figure 6. Temperature dependence of emission energy of the TICT a* state for poly(MMA-co-2) (0 in EAc; in BuCI), poly(MMA-co-12) (0 in EAc; in BuCI), and DMABE (0in EAc; 0 in BuCI). Excitation was at 300 nm. perature range, the emission energy of TICT fluorescence from the polymer-bound DMAB group is higher than that from the monomer model DMABE in EAc but lower in BuCI. This tendency is more prominent for poly(MMA-co-2) than for poly(MMA-c0-12).'~ For a polymer-bound chromophore, the polymer chain often acts as a protector against solvation. The number of solvent molecules around the chromophore is decreased, and furthermore, solvent motions are restricted by the polymer chain. The protecting action of the polymer chain depends on the solvent-polymer chain affinity. When a poor solvent is used, the chromophore is surrounded by the polymer chain tightly and the solvent effects on chromophore fluorescence are greatly reduced. There are many examples of polymer-bound exciplex20*21 and TICT compound^.^,^^ Since polymer chains are in general less polar than polar solvents, the usual trend is to observe a blue shift ~~
(19) PolyCMMA-co-2) and poly(MMA-co-12) are the abbreviations of
copolymers of methyl methacrylate with 2-[4-(dimethylamino)benzoyloxy]ethyl methacrylate and with 12-[4-(dimethylamino)benzoyloxy]dodecylmethacrylate, respectively. (20) Tazuke, S.;Yuan, H. L. Polym. J . 1982, 14, 215; Yuan, H. L.; Tazuke, S. Polym. J . 1983, 15, 111. (21) Tazuke, S.; Matsuyama, Y. Macromolecules 1977, 10, 215. (22) Okada, T.; Fujita, T.; Mataga, N. 2.Phys. Chem. (Munich) 1976, 101, 51.
1408
J . Phys. Chem. 1990, 94, 1408-1413
of the emission in comparison with the emission from a small molecular model compound in the same solvent. In this context the red-shift of the TICT fluorescence of polymer models in chlorinated solvents is unusual. This abnormal polymer effect is however quite understandable as a result of reduced solvation in polymeric systems. As discussed in the previous section, specific solvation by chlorinated solvents at the DMA group brings about
a blue shift relative to the solvent of comparable polarity. Reduced solvation in polymeric systems reduces the solute-solvent donor-acceptor interaction and results in a red shift on a relative scale. When the DMAB chromophore is more exposed to the solvent as in the case of poly(MMA-co-12) in which the DMAB group is bound to PMMA with a long spacer length, this polymer effect is reduced.
Time-Resolved Fluorescence Spectroscopy of a Twisted Intramolecular Charge Transfer Compound Bonded to Polymers Shigeo Tazuke,+Rong Kun Guo,*,~and Tomiki Ikeda* Research Laboratory of Resources Utilization. Tokyo Institute of Technology, 4259 Nagatsuta. Midori-ku, Yokohama 227, Japan (Received: May 31, 1989)
Time-resolved fluorescence spectroscopy with a single photon counting method in the picosecond time region (22.05 ps/channel) was performed for a twisted intramolecular charge transfer (TICT) compound, 4-(N,ro’-dimethylamino)benzoatebonded to polymers. The samples were poly(methy1 methacrylate) bearing a trace amount (1/2000) of the TICT probe via spacers of variable lengths and various poly(alky1 methacry1ate)s bearing the probe via up to 12 methylene chains. The decay rate of the initially planar b* state and the rise rate of the TICT a* state are suppressed in the polymer systems, as a function of the spacer length between the TICT fluorescence probe and the main chain, and the size of adjacent the side chain, as well as the conformation of the polymer main chain. Together with the results of temperature-controlled stationary fluorescence spectroscopy, the polymer effects were concluded to be mainly attributable to the changes in the solvation dynamics in the course of the TICT formation process.
In previous papers, we have shown that the twisted intramolecular charge transfer (TICT) phenomenon is a useful fluorescence probe to study polymer segment mobility in solutionsz4 and in solid mat rice^.^ When a trace amount of (dimethylamino)benzoate (DMAB) chromophore is bonded to polymethacrylates, the ease of the TICT formation expressed by the emission intensity ratio ( R ) of the TICT a* state to the initially planar b* state is used to express the local segment mobility, R is controlled by the distance to the main chain3 and by the structure of the neighboring side chains, as well as the conformation of the polymer chain.4 The thermodynamic parameters obtained from temperature-dependent TICT fluorescence suggest that the polymer effect on TICT formation is mainly entropical, particularly in good solvents? However, the origin of the polymer effects has yet to be further investigated by direct determination of the formation rate of the a* state in the piscosecond time range. Picosecond dynamics of TICT formation was discussed by several the kinetic model of intramolecular charge transfer suggested by Grabowski et al. was further proved’ and the important role of solvents in the electronic relaxation process was demonstrated by picosecond fluorescence lifetime measurement.*q9 However, when we try to determine the decay rate of the b* state and the rise rate of the a* state by fluorescence lifetime analysis in the present polymer systems, we encountered a number of difficulties. Firstly, we have reported that the activation energy E , of the b* a* process is smaller than or close to that of solvent viscosity E,, not only for the monomer model compound, ethyl p-(dimethy1amino)benzoate (DMABE), but also for all polymer samples in most of the cases: indicating the b* a* process to be a barrierless adiabatic photoreaction. According to the Smoluchowski equation involving a bistable potential,lOJ1the zero or low barriers lead to a time-dependent reaction rate and has been proved by the picosecond fluorescence lifetime measurement of p(dimethy1amino)benzonitrile (DMABN).9 Such a timedependent rate is difficult to be determined since the fluorescence decay profile is to be analyzed by a nonexponential function.
-
-
‘Deceased July 1 1 , 1989.
0022-3654/90/2094-1408$02.50/0
CHART I --tCH2-
CH3
CH3
C F C H Z -
Cfn
,
I
Y
X
abbreviation
M ~ x I O - ~
c=o
c=o
1
2
poly(MMA-to-2)
0
0
1
4
p o l y (MMA-co-4)
5. 5
l
8
poly(MMA-cc-8)
10 0
1
12
poly(MMA-co-12)
10. 0
12
poly(BMA-co-12)
5. 8
12
poly(DDMA-co-12)
5. 0
,
( y X
CYHZY+l
0
-
10.0
0
Secondly, a certain angular distribution of ground-state conformation exists for the polymer-bonded DMAB chromophore even in very dilute solutions shown by the red edge effect (REE).3,4 Thus, the fluorescence decays must be described by multi- or non(1) Visiting fellow from the Institute of Photographic Chemistry, Academia Sinica, Beijing, China. October 1986 to March 1989. (2) Hayashi, R.; Tazuke, S.; Frank, C. W. Macromolecules 1987,20,983. ( 3 ) Tazuke, S.;Guo, R. K.; Hayashi, R. Macromolecules 1988,2I, 1046. (4) Tazuke, S.;Guo, R. K.; Hayashi, R. Macromolecules 1989, 22, 729. (5) Guo, R. K.; Tazuke, S.Macromolecules, in press. ( 6 ) Tazuke, S.;Guo, R. K. Macromolecules, in press. (7) Huppert, D.; Rand, S.D.; Rentzepis, P. M.; Barbara, P. F.; Struve, W. S.; Grabowski, Z. R. J . Chem. Phys. 1981, 75, 5714. (8) Wang, Y.;Eisenthal, K. B. J. Chem. Phys. 1982, 77, 6076. (9) (a) Heisel, F.; Miehe, J. A. Chem. Phys. Letr. 1983, 100, 183. (b) Chem. Phys. 1985, 98, 233. (IO) Hong, K. M.; Nmlandi, J. Surf. Sci. 1978, 75, 561. (1 1) Lippert, E.; Rettig, W.; Bonacic-Koutecky, V.; Heisel, F.; Miehe, J. A . Ado. Chem. Phys. 1987, 68, 1.
0 1990 American Chemical Society
