Kinetics of Weak Molecular Exciplex Formation. Electron Donor

J. Phys. Chem. 1989, 93, 671-677

671

Kinetics of Weak Molecular Exciplex Formation. Electron Donor-Acceptor Systems of Tetracyanobenzene J. Dresner, J. Prochorow,* and W. Ode Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Al. Lotnikow 32/46, Poland (Received: November 13, 1987; In Final Form: July 15, 1988)

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The kinetics and thermodynamics of tetracyanobenzene-benzene, -toluene, and -p-xylene exciplexes were studied in solvents of different polarity. Equilibrium between formation and feedback dissociation processes is found to be strongly dependent on the solvent polarity and the donor ionization potential. For the systems with the highest rate of feedback dissociation, the process of formation and decay of an exciplex involves a three-state rather than a two-state kinetic scheme. Experimental evidence is provided by a quantitative study of time-resolved fluorescence spectra.

1. Introduction Electron donor-acceptor (EDA) complexes formed between tetracyanobenzene (TCNB), an electron acceptor, and various substituted derivatives of benzene (electron donors) have attracted wide research interest for many years, as typical examples of weak charge-transfer (CT) molecular complexes. Ground-state interactions that lead to the complex formation and to the characteristic charge-transfer absorption spectra have been studied for many TCNB systems as early as 20 years ago.14 Quantum mechanical treatment of their electronic structure and ground-state properties have also been proposed by various authors.@ Essential knowledge of C T excited-state properties of TCNB complexes has also been collected due to the fact that their CT fluorescence is relatively efficient in rigid and liquid media. Therefore, a large body of fundamental data concerning kinetic and dynamic behavior of the excited, (A-D+)*, C T state of EDA complexes of TCNB with various aromatic electron donors, created upon direct charge-transfer transition from the ground state, (AD) (AD) hVCT (A-D+)* is readily a ~ a i l a b l e . ~ - ' ~ It is known, however, that the C T state of EDA systems of TCNB can also be reached via an alternative, exciplex mechanism in the course of molecular luminescence quenching reactionI5 A* + D (A-D+)*

+

-

-

where A* and D are the excited state of the acceptor and the ground state of the donor, respectively. An exciplex formation reaction in EDA systems is by no means limited to complexes of TCNB; it seems to be a common process for weak EDA systems,I6 resulting generally in the same spectroscopic properties of the excited C T state, (AD)*, independent of its way of creation (1) or (2), though these ways, in any respect (physical interaction, thermodynamics, etc.), must be different. Gas-phase luminescence and kinetic studies"*'* have shed some light on the dynamics of exciplex formation, especially with respect to the role of inter- and intramolecular vibrations in nonradiative transitions, but the thermodynamics of the excited state remains obscure. Whereas exciplex formation in the gas phase seems to be a nonreversible process, it is not the case in solution. Our previoqs reports on time-resolved studies of excited-state pseudoequilibrium between exciplex and A* D in the TCNB-toluene system in low-polarity s o l ~ e n t have s ~ ~clearly ~ ~ ~ demonstrated the importance of the so-called feedback dissociation reaction (A-D+)* A* D. In this paper we present kinetic and thermodynamic data on TCNB exciplexes with a series of electron donors: benzene, toluene, and p-xylene in solvents of low and medium polarity. These data have been gathered by combined steady-state and nanosecond time-resolved studies that made it possible to perform an analysis of the extended photokinetic scheme of formation and

+

-

+

*To whom all correspondence should be addressed.

relaxation of weak molecular exciplexes.

2. Experimental Section TCNB was recrystallized from 1,2-dichloroethane and sublimated in vacuo. Toluene and p-xylene of spectrograde quality were used without further purification, and benzene was distilled twice. The solvents hexane, cyclohexane, dichloroethane, and dichloromethane of spectrograde quality (Merck UVASOL) were used without further purification. Measurements were carried out in mixed solvents of different polarity. The dielectric constant of the solvent was varied through the change of the volume ratio of the polar solvent (dichloroethane or dichloromethane) to the nonpolar one (n-hexane or cyclohexane). Hereafter, the dielectric constant values refer to 20 OC. Samples were prepared by dissolving a proper amount of the donor in solution of acceptor of constant concentration (ca. M) for the whole series studied. All samples were degassed by the freeze-pump-thaw technique and sealed off in a 1 X 1 cm Suprasil cuvette. Steady-state measurements were performed with a PerkinElmer 5 12 spectrofluorometer equipped with a thermostat for temperature measurements. Nanosecond time-resolved studies were carried out with the use of a setup consisting of a homemade, N2-pumped, frequency-doubled dye laser, an emission prism monochromator (COBRABiD), an EM1 98 13 photomultiplier, and a Model 280 boxcar (1) Zweig, A,; Lehnsen, J. E.; Hodgson, W. G.; Jura, W. H. J . Am. Chem. SOC.1963, 85, 3937. (2) Bailey, A. S.;Henn, B. R.;Langton, J. M. Tetrahedron 1963, 19, 161. (3) Foster, R.; Thomson, T. Trans. Faraday. SOC.1963, 59, 2287. (4) Iwata, S.; Tanaka, J. S.; Nagakura, S. J . Am. Chem. SOC.1966,88, 894. (5) Iwata, S.;Tanaka, J.; Nagakura, S.J. Am. Chem. Soc. 1%7,89,2813. (6) Niimura, N.; Ohashi, Y.; Saito, Y. Bull. Chem. SOC.Jpn. 1968, 41, 1815. ( 7 ) Deperasinska, I.; Kwiatkowski, J. S.;Smentek, L. Acta Phys. Pol. A 1973, A44, 71. (8) Masuhara, H.; Mataga, N. Z . Phys. Chem. (Munich) 1972,80, 113. (9) Iwata, S.; Tanaka, J.; Nagakura, S. J. Chem. Phys. 1967, 47, 203, 2203. (10) Mataga, N.; Murata, Y. J . Am. Chem. SOC.1969, 91, 3144. (1 1) Kobayashi, T.; Yoshihara, K.; Nagakura, S.Bull. Chem. SOC.Jpn. 1971,44, 2603. (12) Kobayashi, T.; Nagakura, S. Bull. Chem. SOC.Jpn. 1972,45, 987. (13) Nagakura, S. In Excited States; Lim, E.C . , Ed.; Academic: New York, 1975; Vol. 2, p 321. (14) Mataga, N.; Ottolenghi, M. In Molecular Association; Foster, R., Ed.; Academic: New York, 1979; p 1. (15) Gaweda, E.; Prochorow, J. Chem. Phys. Lett. 1975, 30, 155. (16) Itoh, M.; Mimura, T. Chem. Phys. Lett. 1974,24, 551. Itoh, M. J . Am. Chem. SOC.1974, 96, 7390. (17) Prochorow, J.; Okajima, S.; Lim, E. C. Chem. Phys. Lett. 1979,66, 590. (18) Okajima, S.Ph.D. Thesis, Wayne State University, Detroit, MI, 1982. (19) Dresner, J.; Prochorow, J. J . Lumin. 1981, 24/25, 539. (20) Prochorow, J.; Dresner, J. Acta Phys. Pol. A 1987, A71, 833.

0022-3654/89/2093-0671$01.50/00 1989 American Chemical Society

672

.

The Journal of Physical Chemistry, Vol. 93, No. 2, 1989

I

Dresner et al.

t

I

' 100 -

If1

50 -

031 0

390

470

550 h Inrn

Figure 1. Fluorescence spectrum of TCNB-toluene exciplex system in liquid solution (e = 3.31); excitation within the molecular absorption band of TCNB (295 nm). The inset shows the fluorescence excitation spec-


trum (fluorescence monitored at 480 nm). integrator (ZWG). Decay curves were deconvoluted with the aid of reiterative convolution method. A least-squares optimization program based on the Marquardt algorithmZ1allowing evaluation of the single- and double-exponential decays was run on a microcomputer. Time-resolved spectra were recorded by scanning the emission monochromator with the fixed time window. The time resolution of the setup was found to be 0.5 ns according to short-lived fluorescence standards.

donor benzene

toluene toIu en e toluene to1u en e

toluene toluene

t

3.31 2.59 2.66 2.14 3.31 3.59 4.33 2.59 3.11 3.59

ACT, nm 435 445 450 455 460 470 480 485 490 500

f 10

f 10 f5 f5 f5 f5

ns

T ~ ,

4.7' 10.0 f 0.6 8.5 0.6 10.8 f 0.5 13.0 f 0.6 10.5 f 0.5 10.0 f 0.5 26.4 f 0.5 16.0 f 0.5 10.5 f 0.5

*

3. Results It is essential in our present studies to establish a selectivity of the system to the excitation within CT and local (molecular) absorption bands. Figure 1 shows the fluorescence spectrum of a TCNB-toluene complex with a molecular (TCNB) band situated at 320 nm and a broad C T band with a maximum around 470 nm. The CT band undergoes a typical red shift when the dielectric constant of the solvent increases (Le., when the concentration of a polar component in mixed solvent increases). No quenching of acceptor (TCNB) fluorescence was observed when the concentration of the polar component in the solution was increased. One may therefore expect that possible specific solvent-solute interactions are of minor importance here. An excitation spectrum of C T fluorescence of this complex is presented in the inset in Figure 1 and shows that the C T state is effectively populated after excitation either within the local absorption band of TCNB (305-3 15 nm) or within the C T absorption band (small shoulder at 340 nm). It is known that the number of acceptor (TCNB) molecules that form an EDA complex through the ground-state interaction is very small as compared to free (uncomplexed) molecules in the solution (approximately 1:30 ratio22). Consequently, the number of excited (A-D+)* species that were reached via excitation of prepared exciplex pair" from the ground state is negligible. This is directly verified by comparison of the risetime of C T fluorescence when excited in local (TCNB) and CT absorption bands, the given in Figure 2, which demonstrates the finite risetime of exciplex fluorescence resulting from the diffusion-controlled kinetics (reaction 2) in opposition to the immediate buildup of EDA complex fluorescence (reaction 1). The well-known effect of solvent polarity on exciplex emission is a red shift of the exciplex fluorescence band upon increasing polarity of solvent.23 In the present study it is observed that in a nonpolar solvent the pasition of the maximum of the fluorescence band is red-shifted as compared to the position in the gas phase for any given exciplex." The maxima of fluorescence for each system studied are collected in Table I.

It has been established at an early stage of the investigations that the decay time of C T fluorescence of TCNB EDA complexes and exciplexes depends on the donor c o n ~ e n t r a t i o n . ' ~In * ~order ~ to avoid such effects connected with the formation of aggregates with a stoichiometric ratio different than 1:1,10~24 the measurements in this work have been performed for donor concentrations not exceeding cD = 5 X M. 3.1. Steady-State Studies. An indication of exciplex formation is the quenching of molecular (monomer) fluorescence of A* by unexcited D, accompanied by the appearance of a new exciplex fluorescence band. Quenching of molecular fluorescence usually follows the ordinary Stern-Volmer quenching lawZ5 with a quenching constant Ksv = [(ZM/ZM') - l]/cD, where ZM' and ZM are the intensities of fluorescence of acceptor in the absence and in the presence of the donor, respectively. In all solvents used the O is the fluorescence quenching rate constant kq = K ~ V / T(where decay time of acceptor in the absence of the donor) is in the range 5 X lo9 to 3 X 1O'O M-' s-l (at room temperature), which is expected for the diffusion-controlled process.26 The sensitivity of the quenching process to physical conditions (temperature, polarity, viscosity, etc.) is particularly visible in the behavior of the intensity ratio, RS= ZE/(ZMcD),of fluorescence of the exciplex, ZE, to molecular fluorescence of the acceptor, ZM. In Figure 3 some typical examples of the temperature dependence of RSare presented (in the form of Arrhenius plots). A close inspection of the nature of the temperature changes of the intensity ratio, RS,shows that they fall into three qualitatively different categories. Accordingly, we may introduce an empirically based classification of TCNB exciplex systems under study into the following three groups: (i) a group of "weak" exciplexes which includes TCNB-toluene in solvents of e < 2.74 and TCNB-

(21) Numerical Methods for Unconstrained Optimization; Murray, W., Ed.; Academic: London, 1972. (22) Gaweda, E. Ph.D. Thesis, Institute of Physics, Polish Academy of Sciences, Warsaw, 1980. (23) Beens, H.; Knibbe, H.;Weller, A. J . Chem. Phys. 1967, 47, 1183.

(24) Lim, B. T.; Okajima, S.; Chandra, A. K.; Lim, E. C. Chem. Phys. Lett. 1981, 79, 22. (25) Birks, J. B. Photophysics ofAromatic Molecules; Wiley-Interscience: London, 1970; p 441. (26) Alwattar, A. H.; Lumb, M. D.; Birks, J. B. In Organic Molecular Photophysics; Birks, J . B., Ed.; Wiley: London, 1974; Vol. 1, Chapter 8.

p-xylene p-xylene p-xylene

f5 5 f5 f5

*

Average decay time.

~~

The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 673

Kinetics of Tetracyanobenzene

1.6

-


1.0 -

3.0

3.2

Alnrn

3.4

Figure 3. Logarithmic plot of exciplex, I,, to molecular, IM, fluorescence intensity ratio R' = IE/(IMcD) for exciplexes of TCNB with (a) toluene in solvent of e = 2.59; (b) toluene, e = 2.66; (c) toluene, c = 3.31; (d) toluene, c = 3.59; and (e) p-xylene, e = 3.11.

benzene in solvents of e < 6 ; (ii) a group of "strong" exciplexes which includes exciplexes of TCNB with p-xylene in solvents of e > 2.59 and with toluene in solvents of e > 3.31; (iii) "intermediate" exciplexes consisting of TCNB-toluene in solvent of e = 3.31 and TCNB-p-xylene in solvent o f t = 2.59. Weak exciplexes are characterized by the intensity ratio RS (and the quenching constant Ksv), which is decreasing with increasing temperature. A reverse behavior is observed for strong exciplexes. In the intermediate cases only a weak temperature dependence is observed. 3.2. Nanosecond Time-Resolved Studies. Decays of molecular fluorescence of TCNB in the absence of donor (monitored in the 330-350-nm range) are exponential. Also in the presence of donors, for TCNB-toluene in solvent of t > 6 and TCNB-p-xylene in solvents o f t > 2.59 fluorescence decays can be considered as practically single exponential, as the fits of decay curves to the two-exponential model yield the ratio of preexponential factors greater than 100. In all other cases the presence of the donor leads to the double-exponential decay of quenched molecular fluorescence of the acceptor. The decay of exciplex fluorescence is a single exponential with fast risetime detectable only at low donor concentrations. Some departure from exponential decay is found for the TCNB-toluene exciplex in low-polarity solvent (e = 2.59). For TCNB-benzene exciplex fluorescence decay curves are nonexponential, and no reasonable fits were obtained with either two- or three-exponential models. Exciplex fluorescence decay times at room temperature are collected in Table I. The results of our previous temperature studies of exciplex fluorescence decay time in solvents of different polarities have revealed a distinct qualitative difference in the behavior of weak and strong exciplex systems and practically temperature-independent decay time for intermediate cases.2o A difference in the behavior of weak and strong exciplexes is particularly pronounced in the nanosecond timeresolved fluorescence spectra. Similar to the steady-state conditions (cf. Figure l), the time-resolved fluorescence spectrum of the exciplex consists of two bands which are easily identified as a molecular (TCNB) and exciplex fluorescence band, respectively. In Figures 4 and 5, time-resolved fluorescence spectra of TCNB-toluene in a solvent of e = 2.74 are given for different temperatures as typical examples of weak exciplex systems. It can be easily n o t i d that the intensity ratio of molecular to exciplex bands increases with delay time and

Figure 4. Time-resolved fluorescence spectra of TCNB-toluene exciplex in solvent of e = 2.74 at 20 "C. Individual spectra were recorded after delay times as indicated on the right (in nanoseconds).

320

400

560

480

hlnm Figure 5. Time-resolved fluorescence spectra of TCNB-toluene exciplex in solvent of e = 2.74 at 50 OC. Individual spectra were recorded after delay times as indicated on the right (in nanoseconds).

approaches a constant value after ca. 60 ns (cf. the insets in Figures OC,where the exciplex fluorescence band disappears already after 4 0 ns, with molecular fluorescence still present. A typical behavior of time-resolved spectra of strong exciplexes is exemplified in Figure 6 by the TCNB-p-xylene case. In this case the intensity of molecular fluorescence decreases and that of the exciplex increases as the time advances. After 30 ns only traces of molecular emission are left.

4 and 5). This is even more pronounced at 50

4. Discussion

A generally accepted photokinetic scheme of fluorescence quenching reaction with the formation of an exciplex is the following:27 - *4

Iko A

I

I*. (AD)

674 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989

Dresner et al. IM(t) = CI exp(-X,t)

+ C2 exp(-X2t)

IE(t) = C,[exp(-X,t) - exp(-X,t)]

(6a) (6b)

with X1,2

= 0.5(ko

+ k3C~+ k4 + k,

f [(ko

+ k3C~- k4 - k,)' + 4k3k4cD]'/') (6c)


where [A*l0 is the initial concentration of a primarily excited molecule. The time-dependent intensity ratio of exciplex to molecular fluorescence can be obtained from (6a) and (6b) as I

IE

I

I

3 20

400

480

-(t)

560

IM

k5 c3[exp(-AZt) - exp(-hlt)l =k l CI exp(-A,?) + C, exp(-A2t)

(7)

hhm Figure 6. Time-resolved fluorescence spectra of TCNB-p-xylene exciplex in solvent of L = 3.59 at 63 OC. Individual spectra were recorded after delay times as indicated on the right (in nanoseconds).

When t

The different rate constants refer to the following processes involved in this reaction scheme: ko = k, k, is the sum of molecular radiative, k,, and nonradiative, k2, decay rate constants. k3 is the rate constant of exciplex formation. k4 is the rate constants of exciplex backward dissociation leading to regeneration of the excited A* state (hereafter called a feedback dissociation, as usually found in the literature). k, = k5 + k6 is the sum of exciplex radiative, kS, and nonradiative, kb, decay rate constants. In the following we recall the form of the solutions of population equations when the two-state equilibrium (3) is assumed. 4.1. Kinetics of Two-State Equilibrium of Exciplex Formation D and and Decay. A pseudoequilibrium (3) between A* (A-D+)* excited states is described by the following population equations:

In the next section we shall discuss the temperature dependence of kinetic parameters and inspect whether it is consistent with solutions given above. 4.2. Temperature Dependence of Kinetic Parameters. Among all kinetic processes involved in scheme 3 both exciplex formation, with the rate k,, and feedback dissociation, k4, are the processes that require overcoming the finite energy barriers .E3and E4(and it is reasonable to assume an Arrhenius-type temperature dependence of the rate constants k3 and k4). If we recall eq 5c for intensity ratio Rs,which is governed by the interrelation between the feedback and inner decay channels of exciplex (k4 and k,), then two limiting cases can be considered (with the assumption that molecular and exciplex radiative decay rate constants, k, and k5, and exciplex radiationless constant, k6, are only weakly dependent on temperature):

+

+

d[A*]/dt = I , - k,cD[A*] - ko[A*]

+ k4[(A-D+)*]

d[(A-D+)*]/dt = k , c ~ [ A * ]- (k4 + k,)[(A-D+)*]

(4a) (4b)

where I, is the excitation rate of the molecular A* state and cD is the donor concentration. Under steady-state conditions one can easily find expressions for steady-state fluorescence intensities of the molecular A* state, I M = k,[A*], and the exciplex (A-D+)* state, IE:

IEmkqCDkO-l 1, =

1

k,cDko-'

(5b)

In eq 5a and 5b I M o = klI,/ko is the molecular fluorescence = k51a/kp is the exciplex intensity when cD = 0 and fluorescence intensity for cD a. The previously defined intensity ratio of exciplex to molecular fluorescence RS= IE/(IMcD) (cf. section 3.1) results immediately from eq 5a and 5b as

-

RS = k,k,/[kS(k4 + kp)l

(5c)

The general, time-dependent solutions of eq 4a and 4b have the form of biexponential functions (27) Leonhardt, H.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1963,67, 791.

>> XI-,

this ratio approaches the limit

case a: if k4 > k, then Rs = k,k3/k5k4 Case a refers to the low-temperature limit in which exciplex deactivates much faster via its internal channels than through feedback dissociation. The intensity ratio Rsgrows with temperature as the diffusion-controlled motion of A* and D is activated. The slope of the Arrhenius-type plot of RSagainst 1 / T

yields E3, an activation energy of the formation process. Case b is a high-temperature limit; excited-state kinetics is governed by the k3/k4 ratio. When the temperature increases, a feedback dissociation overwhelms other processes, leading to the decrease of the exciplex to molecular fluorescence intensity ratio. The slope of the Arrhenius-type plot of RSparameter is proportional to the heat of exciplex formation: d(ln RS) d ~In (k3k5/k4kS)z d(l/T)

= 41/77

E4 - E3 = -AHo

(10)

Thus, following the temperature dependence of the Rs parameter from the low- to high-temperature limit should supply an activation energy of exciplex formation, E,, and its heat of formation, -AH,,, and subsequently an activation energy for the feedback dissociation process, E4. For a number of molecular exciplexes such an approach based on steady-state studies has been successfully ex-



TABLE 11: Kinetic and Thermodynamic Data for TCNB Exciplexes with Different Donors in Various Solvents E3:

donor toluene to1uen e toluene toluene toluene toluene p-xylene p-xylene p-xylene

€

kcal/ mol

2.59 2.66 2.14 3.31 3.59 4.33 2.59 3.1 1 3.59

4.2 f 0.3 3.0 f 0.8 3.5 f 0.2 3.0 f 0.5 3.2 f 0.5 2.4 f 0.3 2.9 f 0.4 3.7 f 0.5

kcal/mol

E4: kcal/mol

k4 x 108,s-' at 20 O C at 60 "C

6.0 f 0.6 5.4 f 1

5.5 f 0.4 6.9f 0.5

1.1 f 0.5 0.93 f 0.5

2.4 f 0.5 2.8 f 0.5

6.5 f 0.4 7.5 f 0.5 11.3 f 0.5

0.34 f 0.4 0.09 f 0.4 > k,), as in weak TCNB-toluene exciplexes, eq 6c predicts a decrease of fluorescence decay time with temperature, and this was in fact observed. On the other hand, if k4 2.59) do not exhibit any red shift of molecular fluorescence band in their time-resolved spectra (cf. Figure 6). The weakest exciplex system studied in this work is that of TCNB-benzene. (Its heat of formation can only be roughly estimated as not exceeding 0.5 kcal/mol.) In this case a timedependent shift of the molecular fluorescence band is very dramatic. As illustrated in Figure 8, changes in the position and shape of the molecular band can be clearly seen in the spectrum delayed by 15 ns and are completed after 30-11s time delay, resulting in the total red shift of ca. 25-30 nm. As a matter of fact, one should consider such changes as a development of a "new" fluorescence band which is due to the species different from either A* or (A-D+)*. An excited doorway state for a new red-shifted fluorescence could be formed during either the formation process of exciplex or its relaxation. In accordance with the position of this fluorescence band, one could infer that this state has more molecular than CT character, or in other words it can be considered as an A* state disturbed by the presence of the donor in its close neighborhood (presumably within the nonrelaxed solvent cage). Such a transient species (A*D) could be formed due to the feedback dissociation of the nonrelaxed (vibronically) exciplex state (A-D+)*. Hence, the following kinetic scheme would be possible: (13)

O1

1

A

(AD)

lkr

kp

A

+

D (?I

According to (1 3), exciplex deactivates via its internal channels and also undergoes a back C T reaction with (A*D) as a product. The latter process differs from a "normal" feedback dissociation discussed up to now because it leads to the (A*D) pair which is not dissociated (and solvent separated) but remains in some sort of "contact" (within the solvent cage). It is difficult to solve reaction scheme (13) in the general form, and practical solutions may be given in numerical form only.31 In the simplest case of a nonreversible reaction path ~~

~~~~

(31) Zachariasse, K. A,; Busse, R.; Duveneck, G.; Kunhle, W. J . Photochem. 1985, 28, 231



+D

A*

I

R

(A-D+)*

1

kg

A

2 (A*D)

(14)

1

Rp

(AD)

IMCO 6 0 1

A + D

the solutions of relevant population equations are of the following form: [A*I(t) = [A*], exp(-ht)

(15a)

0

exP(-Xlt) exp(-X2) + (XI - X2)(X1 - kr) (XI - X2)(kr - X2)


+

+

with XI = ko k3cD and X2 = k, k,, thus leading to the three-exponential time dependence of [ (A-D+)*]. Due to the high degree of overlap between molecular, A*, and (A*D) fluorescence bands, one may not be able to resolve them in the measurement. This may be the case in our time-resolved study. If the total fluorescence intensity, ZM + Z(pD), of both bands is actually measured, eq 8 for the time-dependent intensity ratio, R', is no longer valid. If the following condition for the rate constant present in eq 15c holds X I > k, > A2 (16) the buildup time of the (A*D) state, l/k,, is longer than that of exciplex state, 1/X2. If conditions 16, which is a consequence of the molecular-like nature of the (A*D) state, is used in simulations of time dependence of eq 15a-l5c, it leads to the result given in Figure 9. Most parameters used in this simulation were derived from the experiment: ko, k3, and k, are the same as those listed for case b in Figure 7 . k7 was chosen to reproduce the exciplex decay time [TE = (kp k7)-l]. The simulated function is of the form

+

Only two parameters, k, and k,/k5, were optimized in order to achieve the best fit with the experimental curve. It must be pointed out that as long as conditions 16 are fulfilled, variation of the optimized parameters has no qualitative influence on simulation curves. Hence, the fit presented in Figure 9 is by no means a result of a critical choice of parameters. The most important conclusion of simulations discussed here is that in order to reproduce the experimentally observed nonmonotonic time dependence of ZE/ZM, it is necessary to go beyond the two-exponential kinetic model. This result may be considered therefore as strong support for the idea of the presence of ad-

10

30

50

70

90 tlns

Figure 9. A comparison of experimentally observed time dependence of intensity ratio, ZE/(ZMCD), for TCNB-toluene exciplex, (t = 2.74, t = 20 " C ) with the theoretical simulation of ZE/[CD(ZM + I(A*D))] time dependence according to eq 15a-15c. Parameters used: ko = 0.15 ns-', k3 = 46 ns-I, k7 = 0.02 ns-l, k, = 0.17 ns-l, k 5 / k l = 0.054, k 5 / k , = 0.5. Simulated IM, ZE,and curves were convoluted with experimental excitation pulse in order to mimic the observed time dependence.

ditional transient species in the kinetic scheme of weak exciplex formation and/or decay. 5. Concluding Remarks

We made an attempt to analyze the kinetics of weak TCNB exciplexes in the framework of the two-state equilibrium model. Time-resolved study shows that the feedback process leading to regeneration of molecular-like fluorescence may actually produce a state different from the initially excited molecular (monomer) A* state. Little can be said, at the present stage, about pseudoequilibrium among molecular, exciplex, and (A*D) states, and an adequate kinetic scheme requires more kinetic data than those available at the moment. Also, an elucidation of the nature of the (A*D) excited state needs still more studies, although one may notice its close, but only formal, analogy to excited van der Waals states of C T complexes observed recently under supersonic jet condition~.~~ We point out that incorporation of the third species into the kinetic scheme (even in the simplest case described by eq 14) can explain the unexpected behavior of the weak exciplexes. It is well-established that these effects which, we believe, are closely related to the population of (A*D) state gradually disappear when more polar solvent or stronger donors are used. Registry No. Tetracyanobenzene, 2863 1-68-7; benzene, 7 1-43-2; toluene, 108-88-3; p-xylene, 106-42-3. (32) Prochorow, J.; Castella, M.; Tramer, A. J. Lumin. 1984, 31, 32, 603. Saigusa, H.; Itoh, M . Chem. Phys. Lett. 1984, 106, 391.

Kinetics of Weak Molecular Exciplex Formation. Electron Donor

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