H. Masuhara, T. Hino, and N. Mataga
Ionic Photodissociation of Excited Electron Donor-Acceptor Systems. 1. An Empirical Equation on the Relationship between the Yield and the Solvent Dielectric Constant Hiroshi Masuhara,* Tetsuro Hino, and Noboru Mataga Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received September 30, 1974)
Ionic photodissociation yields of some 1,2,4,5-tetracyanobenzenecomplexes and of the pyrene-N,N-dimethylaniline exciplex system in various solvents were obtained by using the nanosecond laser photolysis method. On the basis of the results obtained, a new empirical equation representing the relationship between the yield and the solvent dielectric constant was proposed, which was discussed from the viewpoints of three different theories: (i) Horiuchi-Polanyi’s relation for the activation energy; (ii) Onsager’s ion recombination model; and (iii) phenomenological theory of reaction rates based on Fick’s law. With i and ii the empirical rule was verified qualitatively but not with iii, which seems to mean that the ions produced begin to diffuse with excess energy from the excited state nonrelaxed with regard to solvent orientation. The correlation of the rate constant of dissociation with various solvent polarity parameters was also examined. The solvent effect on ionic photodissociation seems to be well explained in terms of dielectric constant.
Introduction The excited singlet and triplet states of electron donoracceptor (EDA) complexes, which are stable in the ground state, and exciplexes are all called in this paper the excited EDA systems. These systems are suitable to clarify the relationship between photophysical as well as photochemical primary processes and molecular interactions, and some detailed investigations have been performed by conventional flash photolysis, laser photolysis, and transient photocurrent measurements. Ionic photodissociation to donor cation and acceptor anion in polar solvents is one of the most important primary processes, which is directly related to radiationless transitions or to fluorescence quenching.14 Moreover, results on this process have been referred to in many fields of chemistry. In fact, some investigations have been made which relate ionic photodissociation to photoinduced ionic polymerization,5 mechanistic organic photochemistry! electr~chemistry,~ and photosynthesis.8 Ionic photodissociation processes are sensitive to the nature of solvent, which is the subject of the present paper. At present, solvent effects on ionic photodissociation can be classified into two groups. The first is the solvent effect on the nature of the dissociative states. Excited singlet and triplet states are known as dissociative states of EDA comp l e ~ e s , 2 , while ~ - ~ the thermalized exciplex state as well as the excited state nonrelaxed with respect to solvent orientation may be important for exciplex s y s t e m ~ . l *Which ~ state is dissociative is determined by the surrounding environment, namely, by the nature of the solvent and also by the temperature. The second is the solvent effect on quantum yields of ionic photodissociation. In general, the yield changes from solvent to solvent even when the dissociation is occurring from the same kind of state. It increases as the solvent polarity increases, which has been qualitatively explained from the viewpoint of the lowering of free energy of dissociated ionic states.l In the present paper the second solvent effect will be studied and a new empirical equation on the relationship The Journal of Physical Chemistry, Vol. 79, No. 10, 1975
between the yield and solvent dielectric constant will be proposed. The mechanism of ionic dissociation and the proposed empirical equation will be discussed on the basis of three different theories. Moreover, the correlation of the rate constant of dissociation with various solvent polarity parameters will be examined. Although there have been many investigations on stable ions and ion pairs in various solvents, studies concerning the solvent effect upon the dynamical processes of ionic dissociation in the excited state is rather scarce.
Experimental Section The laser photolysis method used was the same as reported previously.1av2bSolid samples were the same as used before.1*2All the solvents were spectrograde (Nakarai or Wako) and used without further purification. Solutions were completely degassed by freeze-pump-thaw methods.
Results 1. Dissociative States of the TCNB Complexes and the Pyrene-DMA Exciplex System. Ionic dissociation processes have been considerably elucidated based on transient absorption and photocurrent measurements with the laser photolysis method. In the present paper some 1,2,4,5tetracyanobenzene(TCNB) complexes and the pyrene-N,N- dimethylaniline (DMA) system are dealt with as typical examples of excited EDA systems. It is necessary to identify the dissociative state of the present systems, since several different dissociative patterns (dissociations from excited singlet or triplet states, and from the excited states relaxed or nonrelaxed with respect to solvent orientation) have been d e m o n ~ t r a t e d . ~In , ~ ?the ~ , ~case of TCNB complexes all photocurrent and absorption rise curves due to the ions produced are identical with a time-integrated function of an exciting pulse, namely, quite rapid formation of ions was observed. Hence, the dissociation of these complexes is deemed to occur from the excited CT singlet state in various solvents. The further problem of these
Ionic Photodissociation of Excited EDA Systems complexes is the nature of the excited CT singlet state, which depends upon the surrounding environment.1° Namely, in the case of liquid donor solutions a 2:l (Dz+A-) complex is formed, while a 1:l (D+A-) complex is formed with low toluene concentration or in a rigid matrix. Which is the dissociative state should be now clarified, and for this purpose the measurement of fluorescence spectra in mixed solvents may be useful. When the volume ratio of toluene to dichloromethane was changed, the fluorescence spectra showed a peculiar change as shown in Figure 1. As the concentration of toluene is increased, the peak of broad band shifts to shorter wavelength, which may be due to the decrease of electrostatic interactions between the complex and the solvent. When the volume ratio of toluene exceeds 25%the shoulder at the longer wavelength region cannot be neglected, and dual emission was clearly observed in the case of a 1:1 volume ratio. This behavior cannot be explained in terms of the simple bulk solvent effect on fluorescence spectra and the red-shifted band is considered to be due to the (Dz+A-) complex.lOJ1 On the other hand, the solvent effect on the fluorescent state of the TCNB-toluene complex was examined, fixing the volume ratio (-5%) of toluene to the solvent. The results, which were analyzed by the usual electrostatic theory of solvent effect, showed that the fluorescent state is a contact ion pair of the (D+A-) complex.12 Therefore, in the present work the solvent effect on ionic dissociation should be examined under the condition of low toluene concentration (lower than 25% volume ratio), where the dissociation occurs from the 1:l (D+A-) complex. In the case of exciplex systems, the observed ionic species (ion pair or free ions) are produced in the course of the quenching of pyrene fluorescence due to electron transfer. Accordingly the problem of dissociative states is not on the spin state but on the excited states relaxed or nonrelaxed with respect to solvation. Both rapid and slow photocurrent rise curves were first observed in pyridine solutions by Taniguchi and Mataga.lb The same results on dichloromethane and 1,2-dichloroethane solution are given in the present work. Since the rapid formation of ions is completed before decay of the thermalized exciplex, the dissociation was concluded to occur from the excited state, nonrelaxed with respect to solvent reorientation, competing with exciplex formation. The rise of the slow component does not correspond to the decay of the exciplex emission and such slow formation cannot be recognized in transient absorption measurements. I t may be due to the dissociation from the ion-pair state relaxed with respect to solvation. On the other hand, only the rapid dissociation process was observed in polar solvents. Although the real nature of the nonrelaxed dissociating state is not very clear a t the present stage of investigation, it seems to be an encounter complex immediately after the electron transfer. 2. Solvent Dependence of t h e Quantum Yields of Ionic Photodissociation. The quantum yield of dissociation of the present excited EDA systems depends significantly on the nature of solvents. This fact was confirmed by measuring transient absorption spectra as well as the laser-induced photocurrent. The transient spectra of the TCNBtoluene complex are characterized by the TCNB anion band a t 462 nm,13J4 and the solvent effect was examined by changing the volume ratio of toluene to dichloromethane, as given in Figure 2. The optical density decreases as the volume percentage of toluene increases. The observed relationship between the absorbance of ions and the dielec-
995
Figure 1. Corrected fluorescence spectra of the TCNB-toluene-dichloromethane system. The concentration of TCNB was adjusted to give an almost constant absorbance (-1.0) of the CT band at 350 nm, which is an excitation wavelength. Numbers in the figure represent the volume percent of toluene.
Figure 2. Dependence of ionic photodissociation of the TCNB-toluene-dichloromethane system on the solvent composition. The vertical scale represents the absorbance of ion radical bands at 100 nsec after excltation: (0)at 462 nm of TCNB anion. The sample solutions were the same as used in Figure l . tric constant (6) of solvents is given in Figure 3, calculating t values of mixed solvents by using the equation, tmix = attoluene + besolvent. Coefficients a and b represent the volume fraction of toluene and the solvent, respectively. Similar results for the TCNB-toluene complex in various solvents are also shown in Figure 3, where the volume ratio of toluene was fixed at 5%. Curves A and B are similar in shape, except that they have different slopes. Ionic dissociation of the pyrene system quenched by DMA was inyestigated by transient photocurrent measurem e n t ~ Since . ~ ~ the photocurrent shows a rapid rise in the nanosecond time region and a slow decay in the microsecond time region,l the peak value is deemed to be proportional to the concentration of the initially produced ions. The relationship between the concentration of the ions produced and t is given in Figure 4. Since in moderately polar solvents the ion radicals are produced via two states, the maximum peak photocurrent value attained after the slow rise process and the value obtained immediately after The Journal of Physical Chemistry, Vol. 79, No. IO, 1975
H. Masuhara, T. Hino, and N. Mataga
996
TABLE I: Quantum Yields of Ionic Photodissociation and Free Energies of the TCNB-Toluene Complex in Vaiious Solvents
____
Solvents
E
Ethyl acetate Methyl acetate Tetrachloroethane Dic hloromethane 1,2-Dichloroethane Methyl ethyl ketone Acetone Figure 3. Solvent dependence of quantum yields of ionic photodissociation of the TCNB-toluene complex. The vertical line represents the absorbance of the TCNB anion band as well as th8 quantum yields (see text). The volume percent of toluene was fixed at 5 (A). The solvent composition was varied in the case of TCNB-toluenedichloromethane systems (B). The concentration of TCNB was adjusted to give an almost constant absorbance of the CT band.
6 -02 5.84 6.68 6.47 82 0 7.91 8 -93 8.60 10.36 9.96 18.5 17.69 20.7 19.78
0.03 0.04 0.05 0.07 0.11 0.23 0.26
3.38 3.30 3.19 3.15 3.08 2.90 2.88
3.02 3.00 2.93 2.91 2.88 2.79 2.77
TABLE 11: Quantum Yields of Ionic Photodissociation and Free Energies of the TCNB-Toluene- Dichloromethane System
0.07 3.15 2.91 8.601 8.274 3.17 2.92 0.04 2 -93 15 7.946 0.03 3.19 2.94 20 7.619 0.02 3.21 25 7.291 0.02 3.23 2.95 6.964 0.01 32 6 2.96 30a 50a 0.01 3.39 3.03 5.654 aIn this case dual fluoresence spectra were observed and the situation is different from the case of low percentage of toluene. 5 10
I
+
I
See text.
Flgure 4. Solvent dependence of quantum yields of ionic photodisso-
ciation of the pyrene-DMA system. The concentrations of pyrene and of DMA are 5 X and 8 X lo-* M, respectively. Since oscillograms in dichlorornethane, 1,2-dichloroethane, and pyridine show both rapid and slow formations of ion radicals, maximum yield (0)and the yield obtained immediately after excitation ( 0 )are given in the figure. The values in alcohols were also shown (+).
laser excitation are both plotted in Figure 4. The obtained S-shaped curve is similar to those in Figure 3. 3. Linear Relations between log [(l/$)- 11 and 1 1 6 or between log l/$ and 11~.In the first place, the absolute quantum yield of ionic photodissociation was determined for the TCNB-toluene-acetone system (5% volume ratio of toluene) by the same method as used before?b The value of the molar extinction coefficient of the TCNB anion band at 460 nm was assumed to be 1 X 104. The number of photons per exciting pulse was 1 X 10l7which was the same value as measured before.2b The quantum yield in acetone was 0.26, from which all ionic photodissociation yields of TCNB complexes have been evaluated by comparing the absorbance of the ion band. For the pyrene-DMA system, we have used the absolute yield in acetonitrile solution determined by Taniguchi et al. as a standard.16 The significant figures of calculated quantum yields are one or two, which are ascribed to the fluctuations of the output of the exciting pulse and to a low SIN ratio in the fast kinetic spectroscopy. All results are listed in Tables 1-111. Using these values, The Journal of Physical Chemistry, Vol. 79, No. IO, 1975
TABLE 111: Quantum Yields of Ionic Photodissociation and Free Energies of the Pyrene-N,N-Dimethylaniline Exciplex System i n Various Solvents AF
Solvent
LL
E
A F
(A; + (A,-.-* D,?, D,'), eV eV
____-
Acetonitrile Acetone Pyridine 1,2-Dichloroethane Dichloromethane
37.5 20.7 12.3 10.4 9.08
0.5 0.34 0.08 0.04 0.03
2.86 2.95 3.08 3.15 3.20
2.80 2.86 2.92 2.95 2.98
the following linear relations between log [(l/$)- 11 and 116 was found:
(;
log--1
) = -:+ q
where p and q are constants independent of 6 . The results on TCNB-toluene and pyrene-DMA systems are given in Figures 5 and 6, respectively. In the case of the pyreneDMA system a straight line is obtained using the values obtained immediately after excitation. If the value of the maximum yield is used, there arises a deviation from the straight line. Moreover, we have reexamined our previous data on TCNB complexes (Table IV)2bvc915and the same relation was obtained as indicated in Figure 7 . A plot of log l/$ vs. l / c was also examined and it was shown that the lin-
Ionic Photodissociation of Excited EDA Systems
997
TABLE IV: Quantum Yields of Ionic Photodissociation of Some TCNB Complexes Systems
Solvents
E
$
TCNB- naphthalene
Flgure 5. The relations between log [(l/$) - 11 and l / t ( 0 )or between log l/$ and l / t (A) for the TCNB-toluene complex in some solvents. In the case of the TCNB-toluene-dichloromethane system (+) the difference between the two plots cannot be recognized.
-
Figure 6. The relation between log [(l/$) 11 and l / t for the pyrene-DMA system. The solid line is expected for values obtained immediately after excitation (0).The dashed line holds for the systems in alcohols (+). There is a systematic deviation from the straight line, if the value (0)of the maximum yield instead of the values immediately after the excitation are used for moderately polar solvents.
T
-
Figure 7. The relation between log [(l/$) 11 and l / t for some TCNB complexes. Almost the same relation was obtained between log l/$ and 116 The donors are naphthalene (0),toluene (X). and benzene (A). Data in ref 2 are reexamined.
ear relationship of eq 1' holds almost equally as well for all of these systems.
1 log- = e' &
E
+
q'
Ether 4.34 0.001 1,2-Dichloroethane 10.36 0.024 Butyronitrile 20.3 0.031 Acetonitrile 37.5 0.053 TCNB-tolueneb 1,2-Dichloroethane 7.7" 0.003 Acetone 14.6a 0.019 Acetonitrile 25.8n 0.1 TCNB-benzeneb 1,2-Dichloroethane 7 . 7 " 0.003 1- Met hy let hy 1 13.0' 0.014 alcohol Acetonitrile 25.8" 0.1 a Calculated dielectric constant, using the same equation as given in the text. The volume ratio of donor vs. solvent was 1 : 2 .
Discussion 1. Free Energy Consideration. Ionic dissociation may be possible when the free energy of the final ionic states is lower than that of the initial dissociative state. The free energy of ion and ion-pair states in various solvents has been estimated for the present excited EDA systems according to the usual f o r m ~ l a . ~The J ~ lvalues ~ ~ obtained are listed in Tables 1-111 and schematically shown in Figure 8, which can qualitatively explain the present t dependence of the quantum yield shown in Figures 3 and 4. The yield increases as the free energy of ions as well as ion pairs becomes lower. The rather significant decrease of energy in the region of moderate polarity ( t = 5 10) seems to correspond to the appreciable change of the yields in this range of solvent polarity. However, only slight change is observed in energy from 1,2-dichloroethane to acetone and acetonitrile, although the yield increases to some extent. This result will be explained in terms of the following theoretical considerations. 2. Theoretical Considerations. Empirically obtained eq 1 and 1' are now discussed from the viewpoints of three different theories. The first is based on the usual molecular mechanism of primary processes of excited EDA systems. Since the yield of the transient ions produced is less than unity, other deactivation processes undoubtedly compete with ionic dissociation. It is reasonable to divide primary processes of the dissociative excited state into ionic dissociation (with rate constant of hi) and other quenching mechanisms (with hq). The latter includes internal conversion and intersystem crossing processes and is now assumed to be independent of t. The yield of dissociation is given as follows:
-
The unimolecular rate constant hi, of course, depends upon solvent polarity and may be written as
k i = Z exp(-E/RT) (3) Here 2, E, and R represent the frequency factor, activation energy, and gas constant, respectively. Although we have no information on E, its change may be correlated with that of the free energy of initial and final states. In the present case, the initial state of the dissociation process of TCNB complexes is the excited CT singlet state nonrelaxed with regard to solvation. The dissociative state of the The Journal of Physical Chemistry, Vol. 79, No.
IO, 1975
H. Masuhara, T. Hino, and N. Mataga
998
TABLE V: Calculated Distance of the Ion Pair According to Onsager’s Ion Recombination Model (1) TCNB-Toluene (5%Volumc. Ratio) Systems
Solvent 92.3 23.9 6.02 5.8 Ethyl acetate 6.68 Methyl acetate 6.46 83.0 26.2 Dichloromethane 8.20 8.60 67.8 26.5 1,2-Dichloroethane 10.36 9.96 53.6 25.1 20.7 Acetone 17.7 26.8 20.1 (2) TCNB-Toluene-Dichloromethane Systems
-*
&
40
Toluene, vol %
Figure 8. Calculated free energies of ion radical (a) and ion-pair (b) states: (A) TCNB-toluene (5% volume); ( 6 ) pyrene-DMA. Dashed lines represent the energy of the dissociative states.
pyrene-DMA exciplex system is the encounter complex, which is isoenergetic with the excited pyrene. It is easily recognized that the free energy of the initial state is almost constant in various solvents while the energy of the final ion radical state is strongly dependent upon t. The energy of ion radicals as well as ion pairs can be written in the following form:16J7
+
Fion= @!e)
(4 )
where A and B are constants irrelevant of the solvent polarity. The change of this energy with static dielectric constant (from to to e ) is given by AFlo,, = A(1/E0
- I/€)
0
