BIPHOTONIC IONIZATION PROCESSES
2229
may be formed in either of two ways, (15) or (16). It follows then that the ratio of the corresponding formation constants, Kls and K16, and the first-order disso-
x. + Y - Z X Y x- + Y . -XYJ
(15) (16)
ciation rate constant, IC-15 and k-16, will depend on whether X. or Y . is the better oxidant; i.e.
As an example consider the case of IBr-. Clearly Br. is a far more powerful oxidant than I., and from the estimates in Table I11 we find [Bra ] [I-]/ [Br-1 [I.1 N 1.7 X Thus IBr- can only be formed from the reaction of Bra with I- and would be expected decay
into Br- and I. with a large first-order rate constant. For example, we find that when C O ( N H ~ ) ~ is B flash ~~+ photolyzed in the presence of I- only the IC transient is observed. Finally, the mere observation of radical anions of the XY- type helps bracket the potential of the Y./Ycouples. Thus the observation of Schoneshofen and Henglei@ that both (1SCN)- and (BrSCN)- have appreciable lifetimes indicates that the NCS /NCScouple is pretty much central (E" N 1.8 V) between the I./I- and the Br./Br- couples. On the other hand, our observations that flash photolysis of Co(NH3)sNa2+ 420 in I- produces 12-, and a new transient (A, nm), presumably BrN3-, in 1.0 M Br- suggests that IS3.is only a little poorer oxidant than Br..
-
-
(53) (a) -M.Schoneshofen and A. Henplein, Ber. Bunsenges. Phys. Chem., 74, 393 (1970); (b) ibid., 73, 289 (1969).
Biphotonic Ionization Processes in Nonpolar Solutions of N,N,N',N'-Tetrameth yl-p-phenylenediamine by A. Alchalal, M. Tamir, and M. Ottolenghi* Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel
(Received January 10, 1078)
Publication costs assisted by the U . S. National Bureau o j Standards
Photoconductivity experiments, using pulsed nitrogen-laser excitation a t 3371 1,are carried out in fluid hydrocarbon solutions of T M P D (N,N,N',N'-tetramethyl-p-phenylenediamine) . The effects of light intensity and of added quenchers indicate that T M P D undergoes ionization in ci biphotonic process involving both excited singlet and triplet states as photoactive intermediates. T M P D solutions containing naphthalene and biphenyl as fluorescence quenchers are found to be photoconductive, exhibiting a second power dependence on the laser light intensity. The photocurrents are attributed to light absorption by the resulting fluorescent exciplexes. The details of this process are discussed in terms of the exciplex absorption spectrum.
Introduction Excited aromatic molecular systems in fluid solutions may undergo ionization via two alternative paths represented schematically by
interaction between an excited donor and a solute acceptor molecule (or vice versa) leading, in polar solvents, t o the solvated D + and A- radical ions.3
D* +D + 3. e,- (or R-)
(1) J. Jortner, M.Ottolenghi, and G. Stein, J . Amer. Chem. Soc., 85, 2712 (1963); J. Eloranta and H. binschitz, J . Chem. Phys., 38, 2214 (1963); L. I. Grossweiner and H. I. Joscheck, J . Amer. Chem. Soc., 88, 3261 (1966); H . S. Pilloff and A. C. Albrecht, Nature ( L o d o n ) , 212, 499 (1966). (2) C. Chachaty, D. Schoemaker, and R. Bensasson, Photochem. Photobiol., 12, 317 (1970); M. Ottolenghi, Chem. Phys. Lett., 12, 339 (1971). (3) H . Leonhardt and A. Weller, 2. Phys. Chem. (Frankfurt a m M a i n ) , 29, 277 (1961); Ber. Bunsenges. Phys. Chem., 67, 791 (1963); H . Knibbe, D. Rehm, and A. Weller, ibid., 72, 257 (1968); K . H . Grellmann, A. R. Watkins, and A. Weller, J . Luminescence, 1,2, 678 (1970).
(1)
D*+A+D++A(2) The first process involves electron transfer from an excited donor molecule (D*) to the solvent, followed by the formation of the D + radical cation and a solvated electron (e,-). I n some cases2 dissociative electron capture may occur, leading to the formation of solvent radical fragments (R-). Process 2 involves the
The Journal o j Physical Chemistry, Vol. 76, N o . 16, 1978
A. ALCHALAL, M. TAMIR, AND M. OTTOLENGHI
2230 When excitation is carried out with near-uv light in solvents of very low polarity (e < 3), the energy requirements for both processes can be satisfied only by assuming that at least two light quanta are involved in each one-electron ionization. This has been recently confirmed for process 1 with D = TRIPD, by a pulsedlaser photoconductivity study,4 showing that the two quantum process involves the consecutive absorption of two photons, rather than the interaction between two one-photon excited molecules. Process 2 has not yet been distinguished in nonpolar systems. It is the purpose of the present work to elucidate the mechanism and to identify the photoactive intermediates in the two quantum ionization of T M P D via processes 1 and 2 in nonpolar liquids. Conductivity detection methods are applied since their higher sensitivity, relative to absorbance measurements, is essential for monitoring the low yield photoionization products in nonpolar solvents.
t12v
L
M C I445
r - L '
0;SV GATE
Figure 1. Fast response photoconductivity circuit.
Experimental Section The pulsed Nz laser photolysis techniques with absorbance or conductivity detection methods have been previously described. 4,5 A fast response (-15 nsec) detection system (Figure 1) for transient conductivity signals, employing a Motorola MC 1445 amplifier, was employed when high time resolution was required. All experiments were carried out using singIe J. The pulse reproshots with energy of -5 X ducibility was better than 5%. The laser beam cross section after focusing was of about 6 mm2. Maximum peak photocurrents measured under an applied field of 0.5 kV, with a lo4D load resistor, were about 5 PA. Eastman Kodak TMPD, naphthalene, and biphenyl were zone refined. Matheson spectrograde solvents were used without further purification. All experiments were carried out at room temperature. Results and Discussion
(1) T M P D in Isopentane. In a previous work, applying a pulsed Nz laser photoconductivity technique, we have shown that TMPD in isopentane undergoes photoionization via the consecutive absorption of two 3371-A photon^.^ It was suggested that in such a process both lowest singlet and triplet states may act as photoactive intermediates. With the purpose of clarifying the exact roles of these tlvo states in the photoionization process, we have now carried out pulsed laser experiments using selective quenchers of singlet and triplet states. From absorbance lifetime measurements in 3-methylpentane (3MP) (Figure 2 ) , we have established that piperylene efficiently quenches the TMPD triplet state (3T*) with a rate constant of 1.1f 0.4X 1O1O M-' sec-l. However, the fluorescence of TMPD was found to be unaffected by piperylene concentrations as high as 0.1 M. Figure 3 shows the effect of piperylene on the photoconductivity after pulsing a The Journal of Physical Chemistry, Vol. 76, N o . 16,1974
0
2 ~ 0 - ~ [PIPERYLENE] , M
~ x ! O - ~
I
Figure 2. The effect of piperylene on the half-life of the triplet state in a deaerated 10-3 M solution of TMPD in 3MP.
deaerated 1.5 X M solution of TMPD in 3MP. The photoconductivity signal is constant as long as M. the piperylene concentration is below -5 X At higher concentrations the photocurrent decreases, reaching a constant limiting value when [piperylene] < -0.1 M . An important conclusion from the data of Figure 3 is that, even when substantial quenching of *I?* takes place, as long as the triplet lifetime is long compared to the duration of the laser pulse (-10 nsec) , the photocurrent is independent of the piperylene concentration. In other words, the photocurrent starts to be affected only when the triplet lifetime ( T ~ )becomes comparable to the duration of the laser pulse (T,). A residual value is observed when T t < ~,/10. Such a behavior is entirely consistent with a biphotonic ionization involving the triplet state, i.e. ST* h', T+ + e,excluding, in the present low polarity system, triplettriplet annihilation6 as a possible path of charge (4) M. Tamir and M. Ottolenghi, Chem. Phus. Lett., 6 , 369 (1970). ( 5 ) C. R. Goldschmidt, M. Ottolenghi, and G . Stein, Israel J. Chem.,
8 , 29 (1970). (6) L. P. Gary, K. de Groot, and R. C . Yarganin, J . Chem. Phys., 49, 1577 (1968).
2231
BIPHOTONIC IONIZATION PROCESSES
i TMPD IN 3 METHYLPENTANE
v)
ciz
a
----505 nsec nsec
>.
2a
....,.,.,.....
fa
Estimated excited singlet -singlet absorption
a
1
I2 W L1z
CT 3
V
e 0
I
a
E -
3~
2
3
-log
4
5
c
Figure 3. The effect of piperylene concentration (C in moles per liter) on the peak photocurrent in a deaerated 1.5 M solution of TMPD in 3MP. x
1
500
I
520
I
540
I
560
I
580
I
600
I
620
I
640
I
66(
X, nrn
Figure 5 . Absorbance changes in the laser photolysis of M TMPD in deaerated isopentane. The spectrum recorded 50 nsec after pulsing is that of 8T*.The spectrum after 5 nsec is the superposition of the absorptions due to I T * and 3T*. The spectrum of IT*has been estimated by substracting the triplet contribution as described by D. S. Kliger and A. C. Albrecht, J . Chem. Phys., 50,4109 (1969).
v)
r-
3
cent singlet state for which, in 3MP, T~ E! 4 nsec. To check this assumption we have investigated the effect of added bromobenzene on the photocurrent. The linear relation in Figure 4 shows that the quenching of fluorescence by bromobenzene is accompanied by a drop in the system’s photoconductivity. Piperylene was added to scavenge triplet molecules resulting from the heavy atom quenching process. These data establish the second ionization path as due to light absorption by the thermalized singlet state, Le. IT* h’, T+
FLUORESCENCE (ARBITRARY UNITS 1
Figure 4. Peak photocurrent ( P ) as function of fluorescence M solution of TMPD in isopentane. intensity ( I ) in a 2 X The fluorescence was quenched by adding bromobenzene (concentrations are reported in the figure) in the presence of 0.1 M piperylene.
carrier formation. Saturation of TMPD-isopentane solutions with air does not affect the photoconductivity. , Since in air-saturated solutions Tt S 30 nsec > T ~ this is in keeping with the proposed mechanism. The residual (-70%) photocurrent left after total quenching of the triplet should be attributed to a second absorbing species present during the laser pulse. The most plausible intermediate is the fluores-
+ e,-
In keeping with our previous con~lusions,~these results rule out the simultaneous absorption of two photons as a route leading to ionization. The biphotonic processes involving 3T* and lT* imply considerable extinction coefficients for the corresponding species at 3371 A, For *T*the value e(3371 A) E 4 X lo3has been reported.’ The absorption spectrum of IT*obtained by pulsed laser photolysis is shown in Figure 5 , Unfortunately, due to the interference of fluorescence and of the absorption of ground state TMPD, the measurements could not be (7) K. D. Cadogan and A. C. Albreoht, J. Phys. Chem., 7 3 , 1868
(1969). The Journal of Physical Chemistry, Val. 76, No. 16, 1872
A. ALCHALAL, M. TAMIR, AND M. OTTOLENGHI
2232
/
I
I
-4
I
". --
1
20 nsec
Figure 7. Oscillograms recorded in a 0.1 M biphenyl-2 X 10-8 M TMPD-methylcyclohexane solution showing: bottom, the rise of the photocurrent; top, the decay of the exciplex fluorescence at 610 nm recorded with no monitoring light (upper trace). The superposition of exciplex fluorescence and ababsorbance at 610 nm, recorded in the presence of the monitoring light beam (lower trace).
tween photoconductivity and light intensity observed, for example, for the pyrene-diethylaniline pair in log I (ARBITRARY UNITS) acetonitrile (Figure 6). Figure 6. Dependence of peak photocurrent ( P ) on laser light We have already pointed out that the photocurrents intensity ( I ) . A, 0, 2 x M TMPD and 0.1 M observed in TMPD-isopentane systems in the absence naphthalene in isopentane; 0 , 2 X M TMPD and 0.1 M of electron acceptor quenchers cannot be accounted for M pyrene and 0.1 biphenyl in isopentane. B, A, 2 X M N,N-diethylaniline in acetonitrile. by the simultaneous absorption of two photons. All ionization phenomena observed in TMPD systems in the presence of high biphenyl and naphthalene concenextended below 500 nm, so :hat at present the relative trations should thus be explained in terms of the ababsorbance of IT* at 3371 A cannot be estimated. 5 X sorbing species generated by the fast ( T (2) TMPD-Naphthalene and TMPD-Biphenyl in nsec) quenching process. We have accordingly carNonpolar Solvents. The fluorescence of TMPD in ried out an absorption pulsed-laser photolysis study of nonpolar solvents is quenched by aromatic electron both TRIPD-naphthalene and TMPD-biphenyl sysacceptors such as naphthalene (N) and biphenyl (B), tems. The transient absorbance changes upon pulsing in a process leading to the formation of the corresponding charge-transfer fluorescent e x ~ i p l e x e s . ~ ~are ~ composed of a fast decay in the nanosecond region (see Figure 7), leaving (below 450 nm) a residual absorWith the purpose of establishing the role of the chargebance of microseconds lifetime. The fast decay matches transfer interaction in possible ionization processes we the decay of the exciplex fluorescence (n,$E 60 nsec) have submitted a 2 X IOv3 M TMPD-isopentane soand is therefore attributed to the exciplex absorption. lution, in the presence of 0.1 f&l naphthalene or 0.1 M The corresponding spectra are shown in Figure 8. The biphenyl, to pulsed laser experiments. The solutions slowly decaying transients exhibit spectra identical were found to be photoconductive, the photocurrent exwith the known spectra of the triplets of naphthalene hibiting a second power dependence on the light inten( W *) and biphenyl (SB *) .lo sity (Figure 6). Thus, biphotonic ionization is inIn previous studies" it has been shown that intervolved in process 2 as in the case of process 1. Figure system crossing following excited donor-acceptor inter7 shows that the rise of photocurrent is faster than the action may be a fast process taking place in competition -15-nsec resolution of our apparatus. This is in with the population of the thermalized (relaxed) fluokeeping with the proposed biphotonic mechanism, rescent state of the exciplex, i.e. yielding additional independent evidence in excluding
-
spontaneous ionization of the thermalized fluorescent (-60 nsec) exciplex. It is interesting to note that the P 12 relation observed in the nonpolar T N P D exciplex systems is in contrast with the postulated monophotonic ionization following excited donor-acceptor interactions in polar solvents.3 The latter behavior is in fact clearly demonstrated by the linear relation be-
-
The Journal of Physical Chemistry, Vol. 76, N o . 16, 1978
(8) N. Yamamoto, Y. Nakato, and H. Tsubomura, Bull. Chem. SOC.Jap., 40,451 (1967).
(9) H.Knibbe, Ph.D. Thesis, Free University of Amsterdam, 1969. (10) Y. Novros, N. Orbach, and M. Ottolenghi, t o be published. (11) C. R. Goldschmidt, R. Potashnik, and M. Ottolenghi, J . Phys. Chem., 75, 1025 (1971); N. Orbach, R. Potashnik, and M. Ottolenghi, ibid., 76, 1133 (1972).
BIPHOTONIC IONIZATION PROCESSES aD*
D*
+ A'
7
2233
+ A (or D + 3A*)
conductivity of the system as long as the concentration was above -2 X lo-' M . Around 1 M piperylene we have measured a drop of -35% in the photocurrent. However, a similar piperylene effect has been also observed in the TMPD-N system, where the role of 3N* has already been excluded. This makes the participation of 3B* in the photoionization quite unlikely, calling for a common explanation of the piperylene effect in both systems. We have observed that in the presence of ~1 M piperylene the initial fluorescence yield of both TMPD-B and TMPD-N exciplexes is lowered by -25% (the emission lifetime is reduced from 60 to -30 nsec). Excluding the corresponding triplets as possible photoactive intermediates and interpreting the biphotonic ionization in terms of the absorption of a second photon by the fluorescent exciplex, the effect of piperylene on the photocurrents can be at least partially accounted for by its effect in reducing the exciplex yield. I t is possible that the rest of the effect may be attributed to specific solvent effects on the ionization cross section or on ionic mobilities. ( 3 ) Photoionization of the Exciplex. ( a ) The Exciplex Absorption Spectrum. The previous analysis leads t o the process
I
(D +A-) *
It may thus be possible that 3N* and 3B* are already present during the laser pulse rather than being formed in the relatively slow processes l(T+N-)* + 3N*
+T
and l(T+B-)* -+ 3B*
+T
where '(T+N-)* and l(T+B-)* stand for the corresponding thermalized exciplexes. One should therefore consider the possibility that W *and (or) 3B* are involved in the biphotonic ionization similarly to 3T*. The participation of W *can readily be ruled out since the naphthalene triplet state in solution does not abThe role of aB*which exhibits a subsorb at 3371 lo4 13) could stantial absorption at the laser line (€3311 be excluded by carrying out an analysis using piperylene as a selective triplet state quencher as it was previously done for the single TMPD molecule in the absence of electron acceptor quenchers. For the reaction between 3B* and piperylene we have established the value piperylene) GS 5 X lo9 M-' sec-'. Thus, k(3B* piperylene concentrations of around 2 X 10+ M are needed to reduce the lifetime of 3B* so that Tt N T ~ Actually we have not observed any effect on the photo-
-
1(D+A-)* h', D +
+
.
+ A-
as the principal ionization path of donor-acceptor systems in nonpolar solvents. Further insight into the nature of the process in the system of the present work calls for an analysis of the exciplex spectra presented in Figure 8. It has been recently shown,14in the cases of various N,N-diethylaniline systems, that the corresponding exciplex absorption spectra should be interpreted in terms of four types of transitions
1. l(D+A-)* -% (D+*A-); localized excitation within the D + species 2. l(D+A-)*
-% (D+A-*);
localized excitation within the A- species
3. ~(D+A-)*-% (~D*A); electron transfer to states in which the donor is locally excited 4.
~(D+A-)*3-(D~A*) ; electron transfer
t o states in which the acceptor is locally excited
i X
The various types of transitions can best be visualized by an approximate orbital diagram such as presented in Figure 9. Transitions belonging t o types 1 and 2 lead to higher charge-transfer states of the exciplex, while
,nm
Figure 8. Exciplex absorption spectra recorded (20 nsec after triggering) in 2 X M TMPD-0.1 M biphenyl or naphthalene-methylcyclohexane solutions. The net absorbance change was obtained after substracting the contribution of fluorescence (see Figure 7).
(12) G.Porter and M. W. Windsor, Proc. Roy. Soc., Ser. A , 245, 238 (1958). (13) W. Heizelmann and H. Labhart, Chem. Phys. Lett., 4, 20 (1969). (14) R. Potashnik, C. R. Goldschmidt, M. Ottolenghi, and A. Weller, J . Chem. Phys., 55, 5344 (1971). The Journal of Physical Chemistry, Vol. 76,N o . 16,197.8
A. ALCHALAL, M. TAMIR, AND M. OTTOLENGHI
2234 I1
............ 1(2,
............................................. D*
A-
INTERMOLECULAR SEPARATION
Figure 9. Schematic diagrams of potential energy curves (I) and molecular orbitals (11)of exciplexes in nonpolar solvents. The notations (l),(2), (3), and (4) refer to the exciplex absorption transitions described in the text. (Excited exciplex states such as (D*+As-)* and (D*A*) have been omitted since their contribution to the spectrum will be far out of our experimental observation range). hv' denotes the absorption populating the lowest excited state of the TMPD donor (DI*). hv" denotes the absorption of the second photon by the exciplex. Broken arrows denote radiationless transitions, dotted arrows the exciplex fluorescence.
types 3 and 4 are "reverse charge-transfer" transitions leading to locally excited states of low CT character. Accordingly, the exciplex absorption spectrum will exhibit a superposition of the spectra of the ground state D+ and A- radical ions. It will also include bands due to transition types 3 and 4, reflecting the energy gaps between the fluorescent state of the exciplex (D+A-)* and those corresponding approximately to higher excited (singlet) states of D and A. I n the case of TMPD-N, the contribution of the TMPD+ transitions to the exciplex spectrum is evident in the form of the broad, nonresolved, 590-nm band which covers the range of the closely spaced maxima a t 620, 580, and 540 nm, which characterize the TMPDf ab~orption.15~~6 The relatively weak absorption around 800 nm can be attributed to transitions to a (TMPD+N-*) state. (The spectrum of the naphthalene negative ion is characterized by a band in the same region.") Thc absorption in the 430-520-nm range cannot be accounted for by higher charge-transfer states and should thus be attributed to reverse charge-transfer transitions. The only possible contributor in this energy range is the locally excited state (TXIPD-N"), due to the -44.000 cm-1 S3(lBb) state of naphthalene, predicting an exciplex transition with 0-0 energy around 480 nm. The S2(IL,) naphthalene state is expected to yield an exciplex band at 850 nm, in fair agreement with the observed band around 880 nm. The fit between expected and observed spectra is The Journal of Physical Chemistry, Vol. 76, No. 16,1978
even more clear-cut in the case of the TMPD-biphenyl exciplex. Thus, all three bands (at 620, 580, and 540 nm) characterizing the TMPD+ absorption are present, slightly shifted, in the exciplex spectrum. The shoulder around 650 nm and its tail in the red, as well as the intense 410-nm exciplex band, are correspondingly attributable to the relatively broad 630-nm band and to the more intense and sharper one a t 400 nm, both characterizing the absorption of the biphenyl negative ion.'I Among the upper locally excited states only one, involving the S,('B) biphenyl state, is predicted to yield an exciplex band in the 400-900-nm range, its 0-0 energy value being expected around 450 nm. This prediction fairly coincides with the 450-nm band in the exciplex spectrum. ( b ) The Photoionization Process. A biphotonic ionization process involving the exciplex as the photoactive intermediate implies considerable exciplex absorbance around the 3371 b laser line. Unfortunately, due to the interference of emissions and of the absorption of ground state TMPD, the exciplexes absorption spectra around 3371 b could not be recorded. However, based on our previous analysis, we may predict a relatively strong contribution to the transition from the (TMPD+*A-) charge-transfer state for both exciplexes. This is due to the relatively high extinction e TMPD+ (3371 A) E lo4, which is comparable to that in the 530-620-nm region. l6 Transitions to anionic excited states (TMPD+A-*) are expected at 3371 b for both biphenyl or naphthalene exciplexes. The contributions in the two cases are expected to be similar since for both N- and B- negative ions B (3371 b) = 3 X 108.17 It is difficult to predict to what extent transitions to locally excited states (types 3 an,d4) will contribute to the exciplex absorption at 3371 A. This is due to the fact that not all excited singlet energy levels of donor and acceptors are known with accuracy, as well as to the incapacity of predicting the intensities of types 3 and 4 bands in the exciplex spectrum.14 When considering the exciplex photoionization from a purely energetic aspect, we refer to the free energy gap between the fluorescent exciplex state AF(E) and D+) recently the dissociated ion radical state AF(Aestimated by Taniguchi, Nishina, and M ~ t a g a for ,~~ the pyrene-N,N-dimethylaniline exciplex in various 2, AF(Asolvents. I n hydrocarbons with e D+) - AF(E) E 1.6 eV. A similar value is also ex-
+
+
(16) W, C. Meyer and A. C. Albrecht, J . Phya. Chem., 66, 1168 (1962). (16) N. Yamamoto, Y. Nakato, and Y. Tsubomura, BUZZ. Chem. SOC.Jup., 39, 2603 (1966); R. Potashnik, M. Ottolenghi, and R. Bensasson, J . Phys. Chem., 73, 1912 (1969). (17) J. Jagur-Grodsinski, M. Feld, S. L. Yang, and M. Szwarc, ibid., 69, 628 (1965); W. P. Weyland, Thesis, Free University of Amsterdam, 1958, p 82; J. Dieleman, Thesis, Free University of Amsterdam, 1962, p 11. (18) Y . Taniguchi, Y . Nishina, and N. Mataga, BUZZ. Chem. SOC. Jup., 45, 3 (1972).
BIPHOTONIC :IONIZATION PROCESSES
2235
pected in our TMFD-acceptor systems. Since thc energy of the 3371-A laser quantum exceeds this value by more than a factor of 2, the basic requirement for a biphotonic ionization process involving the thermalized exciplex as the photoactive intermediate is satisfied. Dealing with finer details of the exciplex photoionization process, one should consider two alternative mechanisms (D+A-)*
h”, A
+ D + + e,-
(D+A-)* h”, A-
-+A-
+ D+
+ D+
(I) (11)
I n the first case initial electron photoejection from the exciplex is followed by recapture by an acceptor molecule. I n some respects this photoejection carries analogies to the formation of solvated electrons in the photolysis of radical anions in solution. l9 Mechanism I1 involves direct formation of the separated A- and D+ ions. This implies two assumptions. (a) I n the internal conversion (wavy arrows in Figure 9) from higher exciplex states [(D+*A-), (D+A-*), and possibly (D*A) or (DAY)],back to the fluorescent state (D+A-) *, a considerable amount of electronic energy is transformed into relative kinetic energy of the solvated ions, D + and A-, so that the system is above the ionization limit of the fluorescent state in the particular solvent involved. (b) This excess in kinetic energy is lost after a few diffusive displacements, so that if thermalization is complete only after an interionic separation of a t least 20 A is achieved, then pairing due to electrostatic attraction will be small and photocurrent will be
observed. With respect t o point b, mechanism I, with a solvated electron replacing A-, appears more plausible, This is in view of the very high mobility of solvated electrons in hydrocarbon solventsz0which may help obtaining large separations between D + and e,before thermalization. I n other words, a nonthermalized e,- is more likely to escape the field of D+ than a nonthermalized A-. However, a discrimination between mechanisms I and I1 is, at present, impossible. We finally refer t o the recent work by Taniguchi, et a1.,18 who observed monophotonic photoconductivity phenomena in relatively polar solutions of pyrene quenched by N,N-dimethylaniline, but did not record either mono- or biphotonic photocurrents in a lorn polarity hydrocarbon solvent such as n-hexene. Their failure to observe biphotonic ionization may be due to relatively weaker light intensities or, alternatively, to a low absorbance of the particular exciplex involved. This calls for a more detailed and comprehensive study of photoionization phenomena in intermolecular chargetransfer systems.
Acknowledgment. This work has been carried out under the sponsorship of the U. S. National Bureau of Standards. The authors are indebted t o a referee for suggesting the process of electron photoejection from the exciplex. (19) J. Eloranta and H. Linschitz, J . Chem. Phys., 38, 2214 (1963); L. J. Giling, J. G. Kloosterboer, R. P. H. Rettschnick, and J. D. W. Van Voorst, Chem. Phys. Lett., 8 , 4 5 7 (1971); M. Fisher, G. Riimme, S. Claesson, and M. Szwarc, ihid., 9, 306 (1971). (20) J. T. Richards and J. K. Thomas, ihid., 10, 317 (1971).
Th.e Journal of Physical Chemistry, Vol. 76, No. 16, 1972
