J. Phys. Chem. 1995, 99, 13356-13361
13356
High-pressure Studies of the Viscosity Effects on the Formation of the Twisted Intramolecular Charge-Transfer (TICT) State in 4,4’-Diaminodiphenyl Sulfone (DAPS) Dmitry S. Bulgarevich, Okitsugu Kajimoto, and Kimihiko Hara* Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan Received: January 4, 1995; In Final Form: May I , 1995@
To investigate the dynamic solvent effects on the formation of the “twisted intramolecular charge-transfer (TICT)” state, the pressure as well as temperature effects on the fluorescence spectra of 4,4’-diaminodiphenyl sulfone (DAPS) in alcohol solvents were examined. It is concluded that solvent viscous friction makes an important contribution to the rate of TICT-state formation of DAPS in alcohols. The contribution of the dynamic solvent effect on the kinetics is well-separated from that of the intrinsic static contribution defined by transition-state theory (TST). The contribution of the solvation effect to the activation volume and activation energy of the TICT-state formation is also discussed.
Introduction Understanding the details of “dynamic solvent effects” on electronic relaxation processes involving large-amplitude molecular motion has been the recent central subject of experimental and theoretical Solvent friction is expected to play an important role in these processes. The case with a sizable activation barrier (@ > kBT has been discussed extensively following the classic work of K r a m e r ~ . ~Recently, attention has been directed toward frequency-dependent effects in solvent friction when barriers are sharp.6 The application of pressure provides a convenient way of making substantial changes in solvent viscosity while only slightly changing the solvent shell structure. Studies of reactions involving large-amplitude intramolecular reorientational motion have been particularly important in establishing ideas about viscous friction. In previous works that have examined the effect of pressure on several reactions, we suggested that the solvent viscous friction plays a vital role in determining the rates of intramolecular charge transfer of 9,9’-bianthryl (BA)738 and 4-[4-(dimethy1amino)phenyllpyridine(DAPP)? the photoisomerization of 3,3’-diethyloxadicarbocyanineiodide (DODCI),Io and the excimer formation of 1,3-di(1-pyreny1)propane (DPP).” Formation of the “twisted intramolecular charge-transfer (TICT)” state involves large conformational changes accompanying the charge-transfer process. Since the observation of dual fluorescence for (N,N-dimethylamino)benzonitde(DMABN) in polar solvents, the dynamics and energetics associated with TICT-state formation of DMABN and related molecules have The mechanism been examined by several responsible for this model, for the case of DMABN in polar solvents, requires a rotational relaxation around the dimethylamino-phenyl bond in the electronically excited SI state. It would be expected that the relative rotational motion of the dialkylamino group is dependent on the solvent viscosity. It hak been reported, however, that the magnitude of the reaction rate of the TICT formation in DMABN is insensitive to the solvent viscosity but is predominantly controlled by the polarity of the s01vent.I~ The study of the TICT formation of 4,4’-bis(dialky1amino)diphenylsulfone (DMAPS) in alcohols by chang-
* To whom correspondence @
should be addressed. Abstract published in Advance ACS Abstracts, August 15, 1995.
0022-3654/95/2099- 13356$09.00/0
ing the size of the rotating group has also suggested that solvent diffusional friction plays a minor role in determining the reaction rate.I6 For 4,4’-diaminodiphenyl sulfone (DAPS) shown below, which was chosen for this study, dual fluorescence has been observed, and it has been explained by the TICT model
stabilized by rotation around the phenyl-sulfur bond accompanying intramolecular charge transfer.” Furthermore, it has been reported that the TICT formation rates in alcohols agree with the inverse of the longitudinal relaxation times (t~-l) of alcohols. l 8 In this study, in order to investigate the role of the dynamic solvent effects on the TICT-state formation in DAPS, we examined the pressure and temperature effects on the fluorescence spectra of a series of linear alcohol solvents from methanol to 1-pentanol. Experimental Section DAPS, purchased from Aldrich, was used without further purification. Linear alcohols from methanol to 1-pentanol used as solvents were of spectroscopic grade quality, in which no impurity was detectable both in the absorption and the emission spectra within the wavelength region studied. All the measurements were obtained using M solutions, which is a low enough concentration to prevent intermolecular interaction. The solutions were not degassed in order to prevent photodissociation. The high-pressure optical cell and emission equipment for fluorescence spectra have been described previously.20 The high-pressure measurements were performed to 500 MPa at 303 K. The effect of temperature was also examined at atmospheric pressure. The sample temperature ( f l K) was controlled by circulating liquid NZ or warmed water, enabling the variation of the sample temperature from 188 to 347 K. Anisotropies for emission and absorption were also measured at 77 K in ethanol with a standard method of L format.*’ The sensitivity difference among wavelengths was corrected by using the standard emission spectra of quinine M) in 0.05 M H2S04 and 2-naphthol (2 x M) in buffer solution
0 1995 American Chemical Society
TICT State in 4,4'-Diaminodiphenyl Sulfone (DAPS)
J. Phys. Chem., Vol. 99, No. 36, 1995 13357
. u)
Wave number / 1O3 cm" Figure 1. Absorption spectrum of DAPS in ethanol at 303 K. Fluorescence and phosphorescence spectra of DAPS in ethanol at 77 K. Dashed lines represent anisotropy spectra. See text for assignments of the observed bands.
(pH = 4.65). The corrected fluorescence spectra were deconvoluted into two Gaussians by means of least-squares fittings.
Wavenumber/103cm-'
Figure 2. Fluorescence spectra of DAPS at 303 K in methanol (-), ethanol (- - -), 1-propanol (- -), 1-butanol (-
*
-), and 1-pentanol (-),
SCHEME 1 0
0
0
0
Results and Discussion Absorption and Fluorescence Spectra. The absorption spectrum in ethanol of DAPS at 298 K at atmospheric pressure (=0.1 MPa) and the emission spectra of DAPS in ethanol at 77 K at atmospheric pressure are shown in Figure 1. The peak location and the shape of the absorption spectra in liquid solutions are almost the same among all the alcohols from methanol to pentanol. In ethanol, for example, the absorption in the range of 30 000-42 000 cm-' consists of two bands with maxima of 33 600 and 38 200 cm-I, which have large extinction coefficients of 30 200 and 19 300 L mol-' cm-', respectively. It is predicted from related molecules** that the absorption spectrum of DAPS in this wavenumber region is composed of three transitions, Le., the transitions from 'A to ILa, ILb, and 'B states, as indicated in Figure 1. This assignment is reinforced by the fact that donor-acceptor-type para-disubstituted benzene derivatives exhibit almost an identical absorption spectrum, except for some modification depending on the relative strength of the donor-acceptor character of the two substituents.** The transition 'A ILb for DAPS is not well-separated from the other due to its low intensity, but it must be located in the same wavenumber region as expected from the benzene derivatives. Therefore, because of the similarity of the absorption spectra of various diphenylmethane derivatives such as 4,4'-bis(dimethy1amino)benzophenone(Michler' s ketone), the lowest excited singlet (SI)state of DAPS is likely to be a 'La-type state.17 In order to confirm this assignment, anisotropy measurements were performed. The anisotropy ( r ) is defined by r = (41 11)/(41 211), where 41 is the intensity when the observing polarizer is oriented parallel to the direction of the polarized excitation and 11 is the intensity when the polarizer is perpendicular to the excitation. As shown in Figure 1, the excitation anisotropy observed at the fluorescence peak maximum (340 nm) has a positive value of 0.23-0.16 in the lower energy region of the primary absorption band, and it decreases to approximately 0 at the higher energy side of this absorption band. Conversely, the fluorescence anisotropy, which was excited at the absorption maximum (297 nm), gives a large positive and constant value of approximately 0.2 in the entire region of the fluorescence band. From these results, it is concluded that the absorption band around 32 000-38 000 cm-' involves two different transitions and that the main part of the absorption band assigned as 'La corresponds to the lowest excited fluorescent state. The phosphorescence anisotropy recorded by the excitation to the lower energy absorption peak maximum (297 nm) was
-
+
approximately 0.07 through the whole phosphorescence band. However, it was impossible to carry out the above analysis because of the effect of depolarization even in the frozen ethanol. This is not a surprising fact considering the phosphorescence lifetime is of the order of 1 s.I9 Figure 2 shows the fluorescence spectra of DAPS at 303 K in different alcohol solvents from methanol to 1-pentanol. As has been reported previou~ly,'~ the dual fluorescence corresponding to the emissions from the local excited (LE) state (330 nm < il < 380 nm) and from the TICT state (400 nm < il < 620 nm) is observed in every alcohol. The TICT model explains the dual fluorescence as occurring from two equilibrated configurations of the same molecule. The mechanism for this phenomenon is characterized by the rotational relaxation of one phenyl ring around the phenyl-sulfur bond to 90" relative to other phenyl ring in the excited state, which leads to a highly polar TICT The possible reaction process is shown in Scheme 1. The geometries of DAPS for the ground state and the LE state possess phenyl ring planes perpendicular to the N-S-N plane, while the TICT state possesses two mutually perpendicular phenyl ring^.'^,*^ Kinetic Scheme of TICT Formation. The kinetic process of the TICT-state formation of DAPS in alcohols can be represented by the photoelementary processes shown in Scheme 1. Within the framework of this scheme, the fluorescence yield ratio of the TICT state to the LE state, @~c+DLE, can be written as
where kl and k2 represent the rate constants of the TICTstate formation and the backward reaction from the TICT to the LE state, respectively. and k: denote the radiative and nonradiative rate constants for the LE state, and kf"' and
Bulgarevich et al.
13358 J. Phys. Chem., Vol. 99, No. 36, 1995
3.0 2.5
-
2.0
2
1.5
\\\
I
-1 w
L
u *+ 1.0
24
20
16
Wave number I
IO3 cm-' Figure 4. Fluorescence spectra of DAPS in 1-butanol at 303 K at different pressures. Arrows indicate the direction of the change with
increasing pressure.
'
-0.5 2.5
'
3.0
1
3.5
4.0
4.5
5.0
TABLE 1: Activation Energy of TICT Formation ( E l ) , Activation Energy of Backward Reaction (Ed, Observed Activation Volume of TICT Formation (AVob*), Stabilization Energy (A&), and Difference in Entropy (BO)
I 5.5
1 /T / 1 o . ~ K-'
Figure 3. Plot of h(@T&@LE) against 1/T in different alcohol solvents. Solid lines represent the best fit curves to eq 4. kTICT
denote the corresponding rate constants for the TICT state. Both radiative rate constants, and k?", are assumed to be relatively independent of pressure as well as solvent and temperature.24-27 A constant value of the ratio for different solvents has been reported for DMABN and related compounds.28 In addition, we shall apply a general assumption that the temperature dependence of k;ff" is much smaller than that of kl and At high pressures and/or at low temperatures where the solvent viscosity is sufficiently large, the backward process from the TICT state to the LE state becomes negligibly small; Le., kz
