J . Phys. Chem. 1986, 90, 1055-1058
1055
Pressure Effects on Exciplex Formation for the Pyrene-p -Dicyanobenzene System in Solution Masami Okamoto,* Fujio Tanaka, and Hiroshi Teranishi Technical College and Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan and College of Integrated Arts and Science, University of Osaka Prefecture, Mom-umemachi, Sakai-city 591, Japan (Received: July 1, 1985; In Final Form: October 2, 1985)
The pressure dependence of the exciplex fluorescence for the pyrene-p-dicyanobenzene system has been measured in various solvents up to 350 MPa. The fluorescence growth and decay curves of the exciplex were analyzed to give the rate parameters for the exciplex formation and dissociation processes, and its unimolecular decay as a function of pressure at 25 OC. The rate constant for the exciplex formation process decreased with increasing pressure, which was interpreted in terms of the solvent viscosity change induced by pressure. The rate constant for the dissociation process was strongly decreased with increasing pressure. The volume changes for exciplex formation were -38.7 cm3/mol in toluene and -18.3 cm3/mol in ethyl acetate. From the solvent effect on the volumes of activation for the rate processes, the solvent participation in exciplex formation and dissociation was discussed.
Introduction The nature of exciplexes has been of great concern to photochemists and photobiologists. In partiiular, exciplex photokinetics has been actively studied by the steady-state and transient fluorescence measurements.'-' A simple mechanism has been developed to explain the exciplex formation and dissociation processes and their dependence on solvent, temperature, and the nature of the donor and acceptor. For some exElplex systems, it has been shown that the process of exciplex formation is diffusion The pressure effect on fluorescence intensities of the exciplexes in solution has been The pressure dependence of both the fluorescence spectrum and the solvent viscosity has given volume changes on exciplex formation ranging from -15 cm3/mol to -32 cm3/mol for 1,2-benzanthracene-l,2,4-trimethoxybenzene for pyrene-N,N-diethylaniline in nonpolar solvents. These high-pressure studies have been limited to observations from measurements of the relative intensity, and hence the individual pressure dependence of the relevant rate parameters is not clear. The present work was carried out to study the diffusion processes associated with the exciplex formation mechanism by transient fluorescence measurements under high pressure. We chose the pyrene-p-dicyanobenzene system, in which the exciplex fluorescence has been observed in various solvents.I0 From the individual pressure dependence of the rate parameters associated with the exciplex mechanism, we will discuss the formation and the dissociation processes of the exciplex.
Experimental Section Pyrene was purified as described previously." p-Dicyanobenzene (DCNB) was recrystallized from benzene twice, followed by vacuum sublimation. The solvents used were spectroscopic grade and used without further purification. A block diagram for high-pressure transient luminescence measuring system and the associated experimental techniques have been described elsewhere." A nitrogen laser (8-11s pulse width) was used as the exciting light source. The fluorescence was detected by a Hamamatsu R 9 2 8 photomultiplier through a monochromator. The output signal was digitized by an Iwatsu TS 8 1 2 3 Stragescope, which provides 51 2 channels for the time axis and a resolution of 256 for the vertical axis, and transferred into a microcomputer. The signal was accumulated at least eight times and averaged. The sample solution was deoxygenated by bubbling with dry nitrogen under a nitrogen atmosphere. The concentration change of DCNB with bubbling was corrected by weighing the sample solution. The concentration of pyrene was between 6 X lo4 and
* Address correspondence to this author at the Technical College, Kyoto Institute of Technology.
TABLE I: Rate Constants in Toluene at 25 O press./ kMa/106 k3/109 k4/1O6 MPa M-1 s-l S-1 0.1 3.41 8.85 f 0.17 31.0 f 2.8 3.64 6.93 f 0.10 14.7 f 2.2 50 3.73 6.11 f 0.07 6.5 f 1.4 100 4.89 f 0.09 4.3 f 1.6 150 3.79 3.87 4.24 f 0.07 1.8 f 1 . 1 200 3.96 3.58 f 0.09 2.6 f 1.2 250 300 4.02 3.08 f 0.08 2.1 f 1.4 350 4.11 2.74f0.09 1.9f1.3
C
SKI
kdiff/ o9 M-1 s-I
18.9 f 0.5 19.2 f 0.5 17.8 f 0.3 18.1 f 0.5 17.8 f 0.7 17.0 f 0.5 16.4 f 0.6 15.4f0.7
12.0 8.8 6.7 5.1 3.9 3.0 2.4 1.8
kP/1O6
Experimental error is within 1%. TABLE 11: Rate Constants in EA at 25 OC press./ k,/ 109 k4/ 1 O6
M-1
MPa 0.1 50 100 150 200 250 300 350 (i
3.47 3.55 3.56 3.62 3.65 3.66 3.66
s-I
9.18 f 0.06 7.39 f 0.03 6.23 f 0.06 5.37 f 0.03 4.55 f 0.04 3.92 f 0.04 3.46 f 0.03 3.05 f 0.06
k p / 1O6
kdiff/
M-1
O9
s-I
s-1
SKI
3.9 f 2.0 2.5 f 1.7 1.2 f 1.0 0.8 f 0.8 0.5 f 0.9
13.9 f 0.2 14.8 f 0.2 15.1 f 0.2 15.7 f 0.2 16.2 f 0.2 16.8 f 0.3 16.9 f 0.2 17.3 f 0.5
15.5 11.4 9.0 7.2 5.7 4.6 3.7 3.0
kp/ s-I 1O6
kdifl/ M-1 s-Ilo9
Experimental error is within 1%
TABLE III: Rate Constants in THF at 25 OC press./ kMn/1O6 k3/109 k4/106
MPa
s-l
0.1 50 100 150 200 250 300 350
2.83 2.94 3.04 3.08 3.20 3.27 3.33 3.40
M-1
s-I
S-'
8.58 f 0.11 2.4 f 3.3 17.6 f 0.4 19.3 f 0.4 7.20 f 0.10 6.17 f 0.07 20.8 f 0.4 21.5 f 0.3 5.53 f 0.05 4.84 f 0.05 22.7 f 0.4 23.4 f 0.6 4.24 f 0.08 24.0 f 0.5 3.76 f 0.06 24.4 f 0.6 3.37 f 0.06
14.4
"Experimental error is within 1%. 9 X IO* M. At these concentrations the excimer formation process is negligible. The change in concentration with increasing pressure (1) N. Nakashima and N. Mataga, Z . Phys. Chem., 79, 150 (1972). (2) N. Nakashima, N. Mataga, and C. Yamanaka, Int. J . Kinet., 5, 833 (1973). (3) W. R. Ware, D. Watt, and J. D. Holmes, J . A m . Chem. SOC.,96, 7853 (1974). (4) D. V. Oconnor and W. R. Ware, J . A m . Chem. SOC.,98,4706 (1976); M. H. Hui and W. R. Ware, Ibid., 98, 4712, 4718 (1976). ( 5 ) D. V. O'conner and W. R. Ware, J . Am. Chem. SOC.,101, 121 (1979).
0022-3654/86/2090-1055$01.50/00 1 9 8 6 American Chemical Society
1056
The Journal of Physical Chemistry, Vol. 90, No. 6, 1986
TABLE IV: Rate Constants in DCE at 25 O C k,/1O6 press./ kMo/1o4 k,/109 s-I M-1 s-I s-1 MPa 7.95f0.14 1.7f1.8 0.1 5,52 7.56 f 0.21 25 6.35 f 0.22 50 6.03 6.02 f 0.17 75 100 6.42 5.66 f 0.20 125 4.78 f 0.34 150 6.78 4.53 f 0.29
Okamoto et al. m J
$,/lo6 s-I
45.5f1.0 47.3 f 1.8 52.4 f 2.2 52.9 f 2.0 52.8 f 2.6 57.3 f 5.7 55.7 f 4.5
,
kdiff/i09
M-1
s-l
-
8.46
I.,
I
I
CHANNEL NUMBER
E
0 I
100
200
I
I
1 350MPa
YExperimentai error is within 1%.
was corrected by the compressibility of toluene,12 ethyl acetate (EA),I3 tetrahydrofuran (THF),I3 and 1,2-dichloroethane (DCE).', Temperature was controlled at 25 & 0.1 "C. Pressure was measured by a manganin wire.
Results The fluorescence in the absence of DCNB showed a single exponential decay under all conditions of solvent and pressure. The rate constants kLIare listed in Tables I-IV. It can be seen from Tables I-IV that the values of k , increase monotonically with increasing pressure. The pressure dependence of k M is in good agreement with the results in other solvent^.^^"^^^ In the present work, exciplex fluorescence was used to determine the individual rate constants associated with the exciplex system. Figure 1 shows the fluorescence growth and decay of the exciplex in toluene at two pressures. Similar curves were observed in EA and T H F up to 350 MPa. In DCE solution the fluorescence was too weak to observe above 150 MPa. In nonpolar and slightly polar solvents, it has been confirmed that the following photokinetic scheme provides an adequate representation for the intermolecular exciplex formation process: M M * +
\
M
+
h,
Q,
A h3
%
M. 3M* M
M*
.(MQ*
x;/ \ +
Q
+
(1)
Xz -
x,
{(A2 - kv - k 3 [ Q ] ) e - X+~ '( k ,
+ k 3 [ Q ]- AZ)e-'l'J
O.1MPa
41:: I '
0.0
40.0
(3) where
/
120.0
\
160.0
NAN0 SECOND
Figure 1. A typical example of the fluorescence growth and decay curves of the exciplex for the pyrene-DCNB system in toluene a t 25 OC. The and 3.55 X concentrations of pyrene and DCNB are 6 X M. respectively.
is the fluorescence growth and decay of the exciplex. [ M * ] , is the concentration of M* at t = 0. From eq 4, one can also obtain XI
(2)
80.0
TIME
hvE products
In the present work, M, Q, and (MQ)* represent pyrene, DCNB, and the exciplex, respectively. From the reaction scheme of eq 1 the time dependence of the fluorescence intensities is given for &pulse excitation by = kl [M*I,
-
+ A2 = k v + k4 + k p + k3[Q]
X1X2
= k ~ ( k +, kp) + kpk,[QI
(5) (6)
The data obtained from the transient measurements of the exciplex fluorescence were analyzed by curve fitting eq 3 by a nonlinear least-squares method. In the analysis, 250 channels was used to determine XI and A2. After an extensive examination of the fitting procedure, it was found that the variation of the initial channel for calculation from 10 to 18 ns after the pulse leads to a difference in A, or A2 of within 5%. The analysis was therefore carried out from the channel 15 ns after the pulse to 245 channels. A typical example of the curve fitting is shown with the solid line in Figure 1. The residual at 0.1 MPa is also shown in Figure 1. Using the values of A, and A, thus obtained, we can determine the individual rate constant for the exciplex system from eq 5 and 6. Plots of XI X2 vs. [ Q ] are shown in Figures 2a-5a. The rate constants k3 were determined from the least-squares slopes in the figures (eq 5). Plots of A,A2 vs. [ Q ] are shown in Figures 2b-5b. The rate constants k , were determined from the least-squares slopes in the figures and the corresponding values of k3 (eq 6). The sum k, + k, was determined from the intercepts of the plots of A, X2 vs. [ Q ] (Figures 2a-5a) and of XlX2 vs. [ Q ] (Figures 2b-5b) using the values of k M . The values thus obtained were averaged. The rate constants k4 were calculated by subtracting k , from k4 + k,. The rate constants associated with the exciplex system at various pressures are listed in Tables I-IV for four solvents. The last column in Tables I-IV lists the values of kdIffcalculated from
+
In eq 2 and 3. I , ( t ) is the fluorescence decay of pyrene, and I E ( t ) (6) S. T. Cheung and W. R. Ware, J . Phys. Chem., 87, 466 (1983). (7) W. R. Turley and H. W. Offen, J . Phys. Chem., 88, 3605 (1984). (8) P. Pollmann and A. Weller, Ber. Bunsenges. Phys. Chem., 77. 1071 (1973). (9) P. Pollmann, D. Rehm, and A. Weller, Ber. Bunsenges. Phys. Chem., 79,692 (1975). (10) H. Been, H. Knibbe, and A . Weller, J . Chem. Phys., 47, 1183 (1967). (11) M. Okamoto and H. Teranishi, J . Phys. Chem., 88, 5644 (1984). (12) F. I . Mopsik, J . Chem. Phys., 50, 2559 (1969). (13) L. G. Schornack and C. A. Eckert, J . Phys. Chem., 74, 3014 (1970) (14) D. M. Newitt and K. E. Weale, J . Chem. SOC.,3092 (1951). (15) P. C. Johnson and H. W. Offen, J . Chem. Phys., 59, 801 (1973).
+
(7)
Pressure Effects on the Pyrene-p-Dicyanobenzene System
The Journal of Physical Chemistry, Vol. 90, No. 6, 1986 1057 ,O;lMPa a
I
1
L
,
200 300
,
,
,
,
,
,
,
,
I
,
,
0'
13
,
N
2 0 - 9 N
.
i
l
N
x x-
5
-
A -
5-
1
/: /'
/"'
5
0
[Q] Figure 4. Plots of Xi 5
0
[Q] Figure 2. Plots of XI
10 M
/
+ X2 vs. [Q] (a) and of X,X2 vs. [Q](b) in THF.
10
M
/
+ X2 vs. [Q] (a) and of XIX2 vs. [Q] (b) in toluene.
a 100
150
30
N I
VI
$2 0
.
c
N
x
XL
5-
10
150
1
TI
I
,
0
[Q] Figure 3. Plots of X I
1
I
I
L
5 /
-L--_I_1
10
M
+ X2 vs. [Q] (a) and of X1X2 vs.
[Q] (b) in EA.
where q is the viscosity of the solvent. The viscosities for toluene and EA at high pressure were estimated from Bridgman's datal6 at 30 O C according to the equation log q = A BP + CP2,but
+
(16) P. W. Bridgman, Proc. Am. Acad. Arts Sci., 61, 57 (1926).
0
5
[Q] I
Figure 5. Plots of A,
10 M
+ h2 vs. [Q] (a) and of X,X2 vs. [Q] (b) in DCE.
the available data for THF and DCE are not known. As illustrated in Figures 2-5, the plots of X I X2 and X l X 2 vs. [Q] show a good linearity over the pressure and concentration ranges examined in this study. These results suggest that the simple exciplex kinetic model (eq 1) is held at high pressure. Thus, it will be reasonably assumed that the kinetic model is correct,
+
1058 The Journal of Physical Chemistry, Vol. 90, No. 6, 1986
Okamoto et al.
TABLE V: Activation Volumes, Volume Changes, and q p at 25 “C and 0.1 MPa for the Pyrene-DCNB Exciplex System in Various Solvents“ solvent toluene EA TH F DCE
AV3 8.1 i 0.5 7.7 i 0.2 5.7 i 0.3 7.7 i 0.5
Cb
2.38 6.02 7.58 10.36
A 1/14 46.8 i 4.7 26.0 f 1.1
A Vr -38.7 f 5.2 -18.3 i 1.3
APP 0.3 f 0.4 -2.3 f 0.2 -4.2 i 0.2 -6.8 i 1.5
Apdiff 11.5 f 1.0 9.7 i 0.8
qpc
1.58 1.19 0.96 1.15
“The units are cm3/mol for volumes and IO4 MPa-’ for qp. bDielectricconstant. CCalculatedfrom ref 12, 13, and 21 and the effects of pressure and solvent on k 3 and k4 will be discussed below. Discussion
The volumes of activation A v * for the exciple formation and dissociation processes were determined from RT(
y)
= -Av*
+ RTxAn*
T
where i is 3 or 4. In this equation, K is the compressibility of the solvent, and An* is the difference in number of molecules between the initial and the transition states. The values of An* for k3 and k4 are -1 and 0, respectively. The terms of RTKat 25 “C and 0.1 MPa were calculated from the available data.I2-l4 The activation volumes AV,* obtained from the slopes of the plots of In k , vs. pressure at 0.1 MPa are summarized in Table V, together with AV,,rf*calculated from
of the excimer in which charge-transfer interactions are relatively small is 21 cm3/mol in a toluene solution of pyrene.I9 Therefore, the largely positive volume of activation AV, may be related to the loss of solvation on going from the polar exciplex to less polar transition state. The solvent polarity dependence on k4 and AV4seems to be predicted by an electrostatic model. When we consider the continuum medium of dielectric constant e, a calculation based on the Kirkwood equation20 gives
where
q, =
-( (26
3
+ 1)*
”> dP
T
In eq 1 0 , and ~ r are dipole moment and radius, respectively, and No is Avogadro’s number. The subscripts and c refer to the transition state and exciplex. ko is the rate constant in a medium of unit dielectric constant. As the reaction proceeds through a less polar transition state, the term Ap2/r3 is expected to be negative. In fact, a plot of In k4 vs. (e - 1)/(2e + 1) in four solvents at 0.1 MPa gave a good linearity, from the slope of which Ap2/r3 is calculated to be -41 kJ/mol. As found in Table V, the change in AV4appears to correspond roughly to that in q, in accordance with eq 11 when toluene is replaced by EA. The less negative value of A P 4 in EA might be consistent with the trend predicted by the electrostatic model. Excited-State Equilibrium. Thermodynamic equilibrium is not established for the present system at high pressure since k , > k4 (Tables I-IV). But the volume change for exciplex formation AVf (= A P 3 - AV4)was calculated by defining the equilibrium constant K* = k3/k4 for the excited-state reaction. The results are listed in Table V. The values of -AVf are fairly large compared with that for the excimer formation of pyrene.” The largely negative values of AVf may be related to the solvation of the polar exciplex as described in the previous paragraph. The values of AVf are comparable to those reported for other exciplex s y ~ t e m s . ~ J ~ We have demonstrated that the high-pressure study gives knowledge about the volume profile for the diffusion process associated with the exciplex formation and the interaction between the excited molecules and solvent. The pressure effect on the unimolecular decay constants is fairly small by comparison, but it is worthy of note that the apparent volume of activation calculated from the pressure dependence of k , decreased with increasing solvent polarity (Table V). These facts combined with information from transient absorption measurements at high pressure would give valuable suggestions about the mechanism of ionic photodissociation.
*
The Exciplex Formation Process. As seen from Tables I-IV,
k3 is nearly equal to kd,flin solvents used at 0.1 MPa, and is also nearly equal to kdlffin toluene and EA at high pressure. These results may lead to the conclusion that the exciplex formation process is approximately diffusion controlled. However, as Table V indicates the difference between AV3* and AVdiff*is somewhat large, whose reasons should be considered. Equation 7 predicts that the slope in In k3 vs. In 7 should be -1. In fact, the slopes are -1 for the quenching of pyrene fluorescence by oxygen and m-dicyanobenzene in polar solvents’ in agreement with the prediction of eq 7 . However, plots of the present results gave good linearities with slopes of -0.63 f 0.01 in toluene and -0.68 f 0.01 in EA. Similar observations have been found for diffusion-controlled fluorescence quenching at high pressure. The slopes are -0.65 for the quenching of naphthalene by biacetyl in methylcy~lohexane’~ and -0.90 for that of pyrene by carbon tetrabromide in hexane.” As a result, the present results in toluene and EA could not be interpreted simply by eq 7 , leading to a discrepancy in the activation volume. The Exciplex Dissociation Process. From the results at 0.1 MPa in Tables I-IV, the values of k4 decrease considerably with increasing solvent polarity, which is consistent with the observations for other exciplex system^.^^^^^ It can be seen from Table V that, whereas the volume of activation for the forward reaction is almost independent of solvent polarity, AV, for the reverse reaction is larger and decreases markedly when toluene is replaced by EA. Accordingly, the present results appear to indicate that the transition state is less polar than the exciplex. The volume of activation consists of two major contributions, one due to a structure change of activation AV(,tr)and the other due to a solvation change AVv‘,,ol~.Since the reverse reaction is associated with the process of separation into M* and Q,AP,,,,, is expected to be positive. The magnitude is not known, but bond breaking processes usually involve 2-8 cm3/mo1.’* Moreover, the activation volume for the dissociation (17) L. A. Brev, G. B. Schuster, and H. G. Drickamar, J . Chem. Phys., 67,‘5763 ( 1 971). (18) T. Asano and W. J. leNoble, Chem. Rev., 78, 407 (1978); N. S. Isaacs, “Liquid Phase High Pressure Chemistry”, Wiley, New York, 1981.
Acknowledgment. The authors are grateful to Dr. T. Takagi of the Kyoto Institute of Technology for the setup of the high pressure system. Registry No. DCNB, 623-26-7; pyrene, 129-00-0. (19) P. C. Johnson and H. W. Offen, J . Chem. Phys., 56, 1638 (1972). (20) J. K. Kirkwood, J . Chem. Phys., 2, 351 (1934). (21) H. Hartmann, A. Neumann, and A. P. Schmidt, Be?. Bunsenges. Phys. Chem., 72, 877 (1968).
