J . Phys. Chem. 1988,92, 3394-3399
3394
-.5
0
.5 1 Time (ps)
1.5
2
Figure 3. Plots of the correlation function @ ( t ) for the large (top, 504 TIP4P waters) and small (bottom, 212 TIP4P waters) boxes for the 4to 12-D sample. The total relaxation time measured is essentially unaffected by the change in box size. Similar results are found for the 4- to -4-D flip.
spherical cutoff of the solute interactions is set at 8.5 A (close to the shell boundary of 8.8 A), and hence the third shell does not "feel" the solute dipole directly but rather feels it through the solvent surrounding it. The data for the first two shells clearly demonstrate that the interactions between neighboring solvent
molecules are important in determining the relaxation time. In the above simulations the solute was not allowed to rotate. In general, rotational diffusion of the solute is expected to effect the solvent relaxation dynamics. This problem has been addressed theoretically6within a continuum approximation, and experimental data on the solvation dynamics of Coumarin 153 in alcohols" suggests that rotational dynamics could be contributing to the response. However, for most of the time-dependent Stokes shift measurements of nonequilibrium solvation, the rotational relaxation time of the probe molecules are at least an order of magnitude longer than the solvation times. Thus, the solute undergoes essentially no rotational diffusion during the solvation process. Fixing the solute in the simulations can be considered to be a good first approximation to the experimental studies. From Figure 2, it is clear that change in the outermost solvent shell contributes to the overall solvation. This result suggests that the finite size of the simulated system could affect the relaxation time obtained. Even though the relaxation dynamics per molecule in the third shell is small, it is likely that additional shells will show limited but nonnegligible relaxation. In order to address the effects of the finite box size, we have calculated + ( t ) for a smaller system containing 212 TIP4P water molecules. The total relaxation functions of the small and large boxes for the 4- to 12-D sample are plotted for comparison in Figure 3. The two curves differ slightly. The calculated relaxation times are, however, nearly identical. This is not surprising because more than half of the polarization response occurs in the first solvent shell. Similar results were observed for the 4- to -4-D flip. Further simulations, including larger water samples, are being pursued in order to provide a more quantitative picture of the nonequilibrium solvation process.
Acknowledgment. This research was supported by NSF through the Presidential Young Investigator program. We acknowledge PYI matching support from Ford Motor Co. (A.D. J.H.) and Sun Microsystems (J.D.S.). J.D.S. thanks Professors Dan Kivelson and J. T. Hynes for many stimulating discussions.
Role of Charge-Transfer Interactions in Photoreactions. 4. Photophysical Study of Exciplexes between trans -9-Styrylphenanthrene and Aminesti$ G. G. Aloisi,* F. Masetti, F. Elisei, and U. Mazzucato Dipartimento di Chimica, Uniuersitd di Perugia, I-06100 Perugia, Italy (Received: June 19, 1987; I n Final Form: January 4, 1988)
The fluorescence quenching of trans-9-styrylphenanthreneby the electron donors diethylaniline (DEA) and tributylamine (TBA) has been studied in methylcyclohexane at different temperatures by stationary and pulsed techniques. The rate parameters for the formation and deactivation of the exciplexes have been determined as well as the related activation energies and thermodynamic parameters. The comprehensive photokinetic scheme thus obtained has allowed the different quenching efficiency of the two quenchers to be discussed comparatively and further studies on the effect of complexation on trans cis photoisomerization to be initiated.
-
The role of excited-state charge-transfer (CT) complexes (exciplexes) in the quenching of fluorescence and in photochemical reactions is a subject widely investigated in the past decade.'-" Our interest has been centered around the exciplexes of transstilbene-like molecules with amines or other electron donors because they play an important role in the geometrical isomerization of the trans olefins, as has been brought out in previous 'Paper presented at the XI JUPAC Symposium on Photochemistry, Lisboa, July 1986, abstract p 180. *Dedicated to Prof. Giovanni Semerano on the occasion of his 80th birthday.
0022-3654/88/2092-3394$01.50/0
In particular, enhanced isomerization has been observed for these fluorescent molecules when the quencher is a heavy-atom anion (1) The Exciplexes; Gordon, M. S., Ware, W. R., Eds.; Academic: New York, 1975. Davidson, R. S. In Molecular Association; Foster, R., Ed.; Academic: London, 1975; Vol. 1, pp 215-334. Mataga, N.; Ottolenghi, M. In Molecular Association; Foster, R., Ed.; Academic: London, 1979; Vol. 2,
P 1.
(2) Cheung, S.T.; Ware, W. R. J . Phys. Chem. 1983, 87, 466 and references therein. (3) (a) Knibbe, H.; Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73,839. (b) Weller, A. Z . Phys. Chem. (Munich) 1982, 133, 93 and references therein.
0 1988 American Chemical Society
Role of Charge-Transfer Interactions in Photoreactions
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3395
SCHEME I
lMtt h V
+
'Mt4
c
z
Iltl 'Mt + h
-Q+hVE
10 I
t
lMt because of induced intersystem crossing.%" In the case of amines as quenchers, exciplexes are formed which deactivate mainly through a radiative pathway or internal conversion, the CT-induced isomerization yield being small and markedly solvent dependent.',* The latter systems, however, are good candidates for a photophysical study which aims at investigating the photokinetics of the C T interactions in the excited states. It is likely that a knowledge of the individual rate constants for the formation and disappearance of the exciplexes and their dependence on the solvent, temperature, internal heavy atoms, or oxygen can give useful suggestions to control the photoreaction quantum yield. In this paper we report the results of an investigation on the exciplexes of trans-9-styrylphenanthrene(2-9-StPh) with amines in methylcyclohexane (MCH); the rate parameters and thermodynamic data obtained by fluorescence decay and steady-state techniques give a sufficiently comprehensive photokinetic scheme for the formation and decay of the complex. Experimental Section trans- and cis-9-styrylphenanthrene were already synthesized for previous works.12 The amine quenchers, diethylaniline (DEA) and tributylamine (TBA), and 4-bromodimethylaniline (BrDMA) were commercial products (Carlo Erba or Fluka) distilled over N a O H under reduced pressure or recrystallized before use. Methylcyclohexane (MCH, Carlo Erba RPE) was distilled and dried. The fluorescence spectra and quantum yields were measured with a Perkin-Elmer MPF-44 spectrophotofluorimeter with a rhodamine B accessory for spectrum correction. The measurements of the emission yields (mean deviation of three experiments ca. 5%) were carried out in dilute solutions (absorbance -0.1 at the excitation wavelength) using 2 4 l-naphthyl)-5-phenyl-1,3,4oxadiazole (a-NPD) in cyclohexane ($JF = 0.70)'2b,'3 as standard. The solutions were deaerated by bubbling nitrogen. For the fluorescence quenching constant determination (steady-state (4) (a) Van der Auweraer, M.; Gilbert, A,; De Schryver, F. C. J . Am. Chem. SOC.1980, 102, 4007. (b) Palmans, J. P.; Van der Auweraer, M.; Swinnen, A. M.; De Schryver, F. C. J . A m . Chem. SOC.1984,106,7721. (c) Vanderawera, P.; De Schryver, F. C.; Weller, A.; Winnik, M. A,; Zachariasse, K. A. J . Phys. Chem. 1984,88,2964. (d) Swinnen, A. M, Van der Auweraer, M.; De Schryver, F. C.; Nakatani, K.; Okada, T.; Mataga, N. J . Am. Chem. Soc. 1987, 109, 321. (5) Mattes, S . L.; Farid, S . Acc. Chem. Res. 1982, 15, 80 and references therein. Caldwell, R. A,; Creed, D. Acc. Chem. Res. 1980, 13, 45. Albini, A,: Fasani. E.: Suluizio. A. J . A m . Chem. SOC.1984, 106, 3562. (6) Hub, W.; SEhneider, S.; Dorr, F.; Oxman, J. D.; Lewis, F. D. J. A m . Chem. SOC.1984, 106, 701, 708 and references therein. (7) Aloisi, G. G.; Bartocci, G.; Favaro, G.; Mazzucato, U. J. Phys. Chem. 1980, 84, 2020. ( 8 ) Aloisi, G . G.; Mazzucato, U.; Birks, J. B.; Minuti, L. J . Am Chem. SOC. 1977, 99, 6340. (9) Mazzucato, U.; Aloisi, G. G.; Masetti, F. J. Photochem. 1982, 18, 21 1. (10) Bartccci, G.; Mazzucato, U.; Favaro, G. J . Photochem. 1979,11,79. (11) Gutierrez, A. R.; Whitten, D. G. Mol. Photochem. 1979, 9, 157 and references therein. (12) (a) Aloisi, G. G.; Mazzucato, U.; Spalletti, A,; Galiazzo, G. Z . Phys. Chem. (Munich) 1982, 133, 107. (b) Bartocci, G.; Masetti, F.; Mazzucato, U.; Spalletti, A.; Baraldi, I.; Momicchioli, F. J . Phys. Chem. 1987, 91, 4733. (c) Aloisi, G. G.; Elisei, F.; Masetti, F.; Mazzucato, U. XI JUPAC Symp. Photochem. 1986, 180. (13) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic: New York, 1971.
0 0
64
128
192
256
'-" 0
-1
64
CHANNEL
128
192
256
CHANNEL
Figure 1. Fluorescence decay curves of exciplexes of t-9-StPh with DEA (a) and TBA (b) a t 375-, and 440-, and 520-nm emission wavelengths in methylcyclohexane a t 291 K. (For A,, see Tables I and 11.)
method) the ratio of the fluorescence quantum yields was equated to the intensity ratio at the analytical wavelength where the exciplex emission is negligible. For quenching experiments and fluorescence lifetime determinations, the olefin concentration was in the range 4 X lo4 to 1 X M and the amine concentration varied up to 0.4 M. Fluorescence and photoisomerization quantum yields for the exciplexes ;6 ; and $J& respectively) were determined as previously The fluorescence lifetimes (mean deviation of three independent experiments 15%) were measured by a homemade fluorometer (Laben Electronics and Applied Photophysics source and monochromators) based on the single-photon time correlation technique. A 68000-based Cromenco C S 1D2E microcomputer was used to process data collected in the multichannel analyzer. The decay curves were analyzed by using a single- or double-exponential deconvolution program with a nonlinear least-squares fitting pr~cedure.'~ Photokinetic Scheme and Rate Parameter Notation The formation of the exciplex (]E* = '(M.Q)*) between a singlet excited molecule ('M*) and a quencher Q, its dissociation, and the decay of the excited species in nonpolar solvents can reasonably be described by Scheme I. Subscripts t and c refer to trans and cis isomers, respectively. The quenching of IM,* by Q is described, under steady-state conditions, by the Stern-Volmer (SV) equation
= 1 + Ksv[Ql = 1 + kqT~[Ql
+FM'/$FM
(1)
where $JFMo and +FM are the fluorescence quantum yields of the monomer in the absence and presence of Q, Ksv is the SV coefficient, k , is the 'M,* quenching rate parameter, and T~ is the 'M,* lifetime at [Q] = 0. In terms of rate parameters
-
where kM = 1 / ~ kE ~ is, the 'E* decay parameter when [Q] m (in this condition rE = l/kE is the 'E* lifetime), kEMand kME are the rate parameters for the formation and dissociation, respectively, and p is the quenching probability for encounter. The increase in exciplex fluorescence quantum yield with increasing quencher concentration is described by the equation +~'/+FE
-
= 1 + 1/Ksv[QI
(3)
where 4; is the fluorescence quantum yield of the exciplex for [Q] m and &E is the related value for intermediate values of [QI. The transient kinetic behavior of exciplexes is of the same type as that described for excimers, and the derivation of the related (14) (a) Bartocci, G.; Masetti, F.; Mazzucato, U.; Marconi, G . J . Chem. SOC.,Faraday Trans. 2 1984, 80, 1093. (b) Barigelletti, F.; Dellonte, S.; Orlandi, G.; Bartccci, G.;Masetti, F.; Mazzucato, U. J. Chem. SOC.,Faraday
Trans. 1 1984, 80, 1123.
3396
The Journal of Physical Chemistry, Vof. 92, No. 12, 1988
Aloisi et al.
+
TABLE I: Short-Lived ( T ns) ~ ~and Long-Lived ( T ~ ns) , Components of the Fluorescence Decay for the t-9-StPh DEA System as a Function of Amine Concentration and Temperature in Methylcyclohexane (Awc = 355 nm) 291.1 K 298.6 K 307.2 K 321.4 K 328.0 K 334.4 K [DEAI, 10-3 M 0 6.25 12.5 18.75 25.0 3 1.25 37.5
71
4.07 3.18 2.73 2.26 2.02 1.785 1.61
72
71
32.1 32.4 32.5 32.9 33.0 32.8
4.05 3.13 2.61 2.24 1.93 1.74 1.58
72
71
28.75 29.65 29.5, 29.9 29.5 29.8
4.01 3.00 2.55 2.15 1.84 1.63 1.46
72
Ti
29.9 27.3 27.3 28.2 27.4 30.0
3.90 2.86 2.30 1.98 1.68 1.47 1.34
72
71
26.7 27.7 28.0 29.3 28.6 27.8
3.87 2.82 2.21 1.86 1.61 1.41 1.25
72
71
72
24.5 26.7 26.6 28.4 27.6, 28.6
3.76 2.52 2.13 1.76 1.44 1.28 1.16
23.7 26.5 26.2 26.64 27.7 26.0,
+
TABLE I 1 Short-Lived ( T ~ ns) , and LongLived ( T ~ ns) , Components of the Fluorescence Decay for the t-9-StPh TBA System as a Function of = 335 nm) Amine Concentration and Temperature in Methylcyclohexane (bXc 291.1 K 298.0 K 307.5 K 314.6 K 320.3 K 334.1 K P A ] , low2M
72
71
0 1.68 2.1 2.52 3.36 4.20 4.33 4.5 6.3 6.63 8.4 8.92 9.5 10.5 11.3 12.6
4.07 3.27 2.89 2.65 2.48 2.02 1.75 1.49 1.52 1.40
71
72
4.05
72
71
3.99 2.43
18.8 3.04
17.5
2.42
18.4
19.8 19.2 19.2 2.42
16.1
2.02
16.7
19.9 1.89
18.6
1.69
18.6
20.0 20.0 20.0 20.2
1.50
19.1
1/72 = k M
1.50
16.5
1.41
16.9
+ ~ E M [ Q+I ME + k ~ )
72
71
72
3.90
TI
T2
3.76
14.8
equations has been reported p r e v i o ~ s l y . ~ * ~ *From - ’ ~ the known analysis method, the following relationships are obtained: 1/71
71
3.94
(4)
The kMvalue was determined from monomer fluorescence decay ~ ~obtained from the decona t [Q] = 0 while l / r l and 1 / were volution of the fluorescence decay curves at various quencher concentrations.
Results Fluorescence Spectra, Lifetimes, and Derived Parameters. The general features of absorption and emission spectra of the sty-
+
rylphenanthrene amine systems are consistent with the formation of singlet excited molecular complexes. The addition of DEA quenches the monomer fluorescence, according to the SternVolmer (SV) equation ( k , = 9.2 X lo9 M-’ s-’), and gives rise to a new emission at longer wavelenths (Am, = 460 nm), ascribed to the exciplex. Fluorescence quenching by TBA is less efficient (k, = 3.4 X lo9 M-’ s-l ), a nd the related exciplex emission is 30 nm red-shifted (A, = 490 nm), the other spectral characteristics being the same as with DEA. Fluorescence lifetime measurements gave in general two-component decay curves (see Figure 1) from which the parameters T~ (short-lived component) and r2 (long-lived component) were obtained by using a double-exponential deconvolution program (Tables I and 11). The decay curves were in practice monoexponential when the emission was observed (i) at the longer wavelength tail of the exciplex band with the lifetime of ’M,* being short compared to that of IE* and (ii) in the region where only the monomer emits and the complex dissociation constant kME being much lower than the sum of the rate constants for the (1 5) Birks, J. B. In Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970. (16) Weller, A. In The Exciplexes; Gordon, M., Ware, W. R., Eds.; Academic: New York, 1975. (17) Mataga, N.; Kubota, T.In Molecular Interactions and Electronic Spectra; Marcel Dekker: New York, 1970.
2.77
13.1
2.63
12.2
2.49
8.7
2.21
13.2
2.10
12.3
1.81
9.3
1.92
14.0
1.66
13.6
1.44
10.6
1.55
15.4
1.43
14.9
1.26
11.3
1.35
14.6
1.20
14.2
1.02
12.0
+
TABLE III: Photophysical Parameters for the t-9-StPh DEA System as a Function of Temperature and Related Equilibrium Constant (KF*) and AGO in Methylcyclohexane parameter TM, ns kEM,109 M-I s-1 kME, IO6 s-’
kE, 106 s-1 7~ (=l/kE), nS ICE*, M-’ -AGO, kcal mol-’
91rn FE
kFE, lo6 S-I kNRE, lo6 S-’ k,, 109 M-1 s-I k E d , lo9 M-’ SKI
291.1 K
298.6 K
307.2 K
321.4 K
328.0 K
334.4 K
4.07 9.9 2.0, 29.7 33.7 4829 4.91 0.55 16.0 13.0 9.21 9.26
4.05 10.2 2.34 32.8 30.5 4359 4.97
4.01 11.4 3.2 33.9 29.9 3562 5.00
3.90 12.8 4.2 34.1 29.4 3048 5.12
3.87 13.8 12.7 31.1 32.0 1087 4.56
3.76 15.4 8.7 35.0 28.6 1770 4.97
exciplex deactivation (kME
