2915
J . Phys. Chem. 1992, 96, 2915-2917
LETTERS Subnanosecond Dynamics of a Naphthalene-Oxygen Exciplex Stephan L.Logaanovt and Michael A. J. Rodgem* Center for Photochemical Sciences, Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403 (Received: October 7, 1991; In Final Form: January 31, 1992)
Picosecond laser (266 nm) flash photolysis studies of oxygen-saturated solution of naphthalene (N) in cyclohexane were performed. Absorbance changes consistent with the conversion of N(S,) to N(TJ by two independent processes were observed. A slower component (T = 4 ns) was identified as the normal diffusive conversion of SI T, by induced intersystem crossing. A rapid component (T = 200 ps) was tentatively identified as a conversion of a photoexcited 3(N*.-02) triplet exciplex state to the free molecular TI state.
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Introduction The proximity of an oxygen molecule to an aromatic hydrocarbon molecule induces interesting spectroscopic effects. Thus, while the ground-state interaction between the two species is not bound, spectroscopic transitions can occur from the so-called contact ground state to higher-lying two-molecule states, viz., 3(M*:02)and 3(M*+02c).1,2 These transitions are of lower energy than the M(So) M(SJ transition and are found at the red edge of the So SIband in the case of aromatic hydrocart” Over the years such observations have led to a wealth of theoretical discu~sion,~~~” but the photophysical consequences of populating these exciplex states directly were not examined. Recently, experiments in Ogilby’s laboratory6s7have demonstrated that excitation into the CT band of several aromatic-oxygen complexes leads to the formation of 02( *As)in significant yields. In particular, Kristiansen et al. have concluded that for methyl naphthalene where E(CT) > B(3M,), direct population of 3(M*+02’-) results in relaxation to the 193(3M1-32, 0,)states prior to 02(’$) formation. In the work described herein we describe preliminary attempts to observe the dynamics of such processes related to naphthalene.
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Experimental Methods Instrument. Laser flash excitation was carried out with the fourth harmonic (266 run; 25 p) of a mode-locked Nd:YAG laser system (Quantel YG571). The basic features of the pump-continuum probe double diode array spectrography system have been described.* Recent modifications include operating with quasi-coaxial excitation/interrogation in a 2 mm path length to minimize time dispersion that occurs with right angle geometry in a 1-cm cuvette. Kinetic waveforms were obtained using a single-wavelength, double-beam arrangement in which the two white light probe beams emergent from a beam splitter crossed the cuvette in the excited and nonescited regions, respectively, prior to entering the input slit of a high-intensity (PTI Model 01-002) monochromator with vertical displacement (ca. 5 mm) between the two. A pair of silicon photodiodes (EGBG Judson UV-100BQ) at the exit slit viewed the emergent monochromated beams. The photodiode output currents were integrated by 1 A4Q load resistors, and the resultant voltages displayed, averaged, and stored in memory in a digital CRO (LeCroy 9400) prior to transfer to a personal computer for analysis. The sequence of observations that resulted in the determination of an absorption profile was simi!ar to that described for the multiple wevclength measurements *Author to whom correspondence should be addressed ‘Permanent address. Biophysics Department, Faculty of Biology, M V Lomonosov State University, 119899 Moscow, Russia.
0022-3654/92/2096-29 15$03.00/0
using the array detector^.^ The major advantage of this single-wavelength measurement proved to be the lower number of laser pulses (at 5 Hz)needed to obtain a satisfactory signal-to-noise ratio (50 shots per point gave an error of -0.003 absorbance units). The convolution of excitation and probe pulse profiles yielded a response of ca. 20 ps. Materials. Naphthalene (Baker and Adamson) and cyclohexane (Fluka) were used as received.
Results An argon-saturated solution of naphthalene (N, 7 mM) in cyclohexane when irradiated with a ca.25-ps pulse of 266-nm light (ca. 1.0 mJ) showed an instantaneously formed absorption band in the 370-450-nm spectral region. The bend had a broad maximum near 420 nm and a shoulder near 405 nm (Figure 1A). This species showed no significant decay over 9 ns (our largest time window) and is attributed to the SI S, absorption of N.I0 The same experiment camed out on an oxygen-saturated (10 mM) solution of 7 m M N in cyclohexane’’ showed the same absorption feature at the earliest times which rapidly developed into a more structured absorption which showed peaks at 390 and 414 nm (Figwe 1, S-D). The coincideme of these peaks with the T,-T, absorption spectrum of N in cyclohexane indicates that the TI state of N is very rapidly evolving from the SIin Q,-saturated solution. A measurement of the fluorescence lifetime of an 02-saturated solution of N in cyclohexane provided a value of 5.2 ns . The kinetics of the time evolution of the absorbances at 415
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(1) Tsubomura, H.; Mulliken, R. S. J. Am. Chem. SOC. 1950, 82, 5966-5974.
(2) Eirks, J. B.; Pantos, E.; Hamilton, T. D. S . Chem. Phys. Lett. 1973,
20, 544-546. (3) Kawaoka, K.; Khan, A. U.; Kearns, D. R. J . Chem. Phys. 1967,47, 1883-1884. (4) Khan, A. U.; Kearns, D. R. J . Chem. Phys. 1958, 48, 3272-3275. ( 5 ) Murrell, J. N. Mol. Phys. 1960, 3, 319-329. (6) Scurlock. R. D.: Oeilbv. P. R. J . Phvs. Chem. 1989. 93. 5493-5500. (7j Kristiansen, M.; Scurkck, R. D.; I;, K.-K.; Ogilby, P: R. J . Phys. Chem. 1991, 95, 5190-5197. ( 8 ) Shand, M. A.; Rodgers, M. A. J.; Webber, S . E. Chem. Phys. f m . 1991 177, 11-16 -. ._,. (9) Hubig, S. M.; Rodgers, M. A. J. In Handbook of Organic Photochemistry;Scaiano, J. C., Ed.; CRC Press: Boca Raton, 1989; Vol. I, Chapter 13. (10) Bebelaar, D. Chem. Phys. 1974, 3, 205-216. (1 1) We are unable to record any difference in the absorbance at 266 nm
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between oxygen-saturated and nitrogen-saturated solution of naphthalene, but since we do not know the value of the extinction coefficient of the So 7S, and )Eo )E* transition at 266 nm, we are unable to state how the incident energy is being partitioned. A 7 mM solution of N in cyclohexane had an optical density 3.5 at 266 nm in a 1-mm cell.
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0 1992 American Chemical Society
Letters
2916 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
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.
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Wmnwph, nm
al
-.-_A..121 4i8 448
0
1
1
3
4
5
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Figure 1. Difference absorbance spectrum of 7 mM naphthalene in argon-saturatedcyclohexane: (A) 0 delay between excitation and probe beam; (B) oxygen-saturated solution at 99 ps; (C) oxygen-saturated solution at 230 ps; and (D) oxygen-saturated solution at 900 ps.
time, ns
Figure 3. Ratio of absorbance changes (data from Figure 2) at 415 and 430 nm. Solid lines are (a) a simulation of eq 5 with k, = 2 X lo8 s-l (rate constant for N fluorescence decay) and (b) eq 7 with A I = 0.15, A~ = 0.85, ko = 5 x 109 s-1, k, = 2 x io* s-1.
The kinetic waveforms at 415 and 430 nm shown in Figure 2 are fitted with biexponential curves, the early component in both cases corresponding to a 200-ps lifetime and representing some 15% of the slower (4 ns). The inset to Figure 2 shows how the rising part of the 415-nm absorption superimposed on the laser pulse instrumental profile which was determined from the rising edge of the SI S, absorption in 02-freesolution. It is apparent that the response function does not distort the absorption signal after ca. 50 ps.
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.-Eln
Discussion The evidence presented in the previous section indicates that
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266-nmexcitation of an 0, saturated solution of N in cyclohexane initiates the formation of N(Tl) states by a nonexponentiai process.
4
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I O
0.00
~-
7
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400
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900
1400
delay, p s Figure 2. Absorbance changes in oxygen-saturated solution of 7 mM naphthalene in cyclohexane at 415 nm (closed circles) and 430 nm (open circles). The curves are double-exponential fits to the data. Inset: absorbance changes of 10 mM N at 415 and 430 nm over a 200-ps time scale.
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nm (TI T,) and 430 nm (SI S,,)are shown in Figure 2. The rise at 415 nm and the decay a t 430 nm are nonexponential; an early fast component precedes a component that corresponds to an exponential change with a time constant of ca. 4 ns. Since this lifetime is close to that of the measured fluorescence lifetime of the same solution (vide supra), we attribute thii to the formation of the N(TI) state from the sum of elementary processes 1-3: N(Si) -.+ N(So) + ~ Y F (1)
Both reactions 2 and 3 represent random diffusional processes. Reaction 3 corresponds to an oxygen-induced intersystem crossing which is allowed for N on spin and energetic grounds, the SI-, TI energy gap in N being ca. 8 kcal/mol greater than the 'Ag 3Z; gap in molecular oxygen.
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This is contrary to what is anticipated from a consideration of processes 1 through 3, which clearly requires exponential decay of N(SJ and growth of N(T,). The kinetic analysis requires an early component that is unrelated to the diffusive interaction between N(SJ and 02.To demonstrate this effect more clearly, the plot in Figure 3 is presented. The filled circles in the plot are values of the ratio of h A 4 1 5 : h A 4 u ) vs time where and a 4 3 0 are the measured optical absorbances at 415 and 430 nm, respectively. As seen in Figure 1, A and B, both N(Sl) and N(Tl) absorb strongly at 415 nm, whereas the TI absorption at 430 nm is relatively weak, Le., ~ 4 1 5 ( S I ) z 6 4 M ( S I ) ; 6430(sI) >> c430(T1).On the simple exponential model of reactions 1-3, we can describe and A A 4 3 0 as follows the time dependence of U415(f)
= t415s1[Slloe-kcr + ( t 4 1 5 T l ) e d S 1 1 0 ( 1 - e 9
U 4 3 0 ( 1 ) = %OS'[Sll@-kcr where [Silo is the number of excited states, and k, = k l + (k2 k3)[021r and ( e 4 1 5 ~ l ) e f f= % i s T ' ( k 3 [ 0 2 1 / k , h if k 3 [ 0 2 1 >> k i ,
k 2 [ 0 2 1 , (t415T')cf~
7 C415T'*
When the ratio
IS given
by
Bebelaar's worklo showed becomes
~415(SI)
=
'/26415(TI).
Thus (4)
Thus the ratio of the absorptions at the two indicated wavelengths will experience an exponential increase with time when processes 1, 2, and 3 are the only contributes to TIformation. If N(Tl) is also formed independently of processes 1-3 (seebelow) the time
J . Phys. Chem. 1992, 96, 2917-2920 dependence of
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and AA430(t) will be more complex:
&15(t)
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adventitious collisions. A 266-nm photon has sufficient energy to induce the N(So) N(S,) transition as well as the transition from the complex ground state to an exciplex state in which the N(SJ state combines with an 02(3Z,-) molecule. In the notation used by Birks,12this is a 3E,, 3E5*transition. The jE5*exciplex state will also be produced in the bimolecular process
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A A ~ ~=~t415S~[S1]O(Ale-kor ( ~ ) A2e-9 c ~ ~ ~ -~ Ale-kor ~ [ -SA2e-!9 ~ ] ~(6a)( ~
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AA430(t) = ~ ~ ~ ~ ~ 1 [ S ~ A] ~2 e( 9A ~ e -(6b) ~ o ~ where ko is the rate constant of TI production in the independent process and A I and A2 are the relative concentrations proceeding down the two parts. Equation 4 becomes u 4 1 5 -(r)
+
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,Es*
>> ko-l, e-kor
