J. Phys. Chem. 1082, 86, 1623-1628
1623
Reactions of Triplet States of a Porphyrin Measured by Delayed Fluorescence J. Feltelson+and D. Mauzerall' The Rockefeller University, New York, New York 10021 (Recelveo: August 24, 1981; I n Final Form: November 24, 1981)
The delayed fluorescence of zinc octaethylporphyrin is a very sensitive M) measure of the triplet state of this molecule. The activation energy of the delayed fluorescence,2750 cm-', agrees with the singlet-triplet energy gap of 3000 cm-' estimated by the difference in fluorescence and phosphorescence emissions of this molecule. The rate constants for the reaction of the triplet with ground-state porphyrin, with another triplet, and with benzoquinone were determined. The latter two reactions are encounter limited. This method and that of ion conductivity are currently the only means with sufficient sensitivity,when applicable, of determining triplet reactions unobscured by the fast triplet-triplet reactions.
Introduction reagent grade) and 1,2-dichloroethane (certified ASC, Fisher Scientific Co., lot 700068) were purified by fractional The finding that porphyrin anions and cations can be distillation from 3-A molecular sieves. [Caution: One of formed in high yield by reactions of porphyrins and their us (J.F.) developed a very unpleasant contact dermatitis triplet states' prompted the measurement of the triplet toward dichloroethane.] Benzoquinone was resublimated reactions via delayed fluorescence. The high yield of these and kept in the dark. Spectrograde hexanes (a mixture reactions, particularly of the ground state with triplet-state of isomers; Matheson Coleman and Bell) were fractionally reactions, brought into question the energy level of the redistilled from molecular sieves. A lo4 M stock solution triplet in solution. The redox free energy of the porphyrin of ZnOEP in purified 1,2,-dichloroethane was kept in the cation plus anion in polar solvents, the difference of redox dark and under refrigeration. Samples (100-200 pL) of potentials determined by polarography,2is 2.2 eV, whereas this solution were diluted with freshly distilled toluene in the energy of the triplet is about 1.8 eV. We believe that the optical cell assembly to the final 3-9 pM concentration. the deficit is made up by entropic effects,' but the energy Benzoquinone was dissolved in hexane (6 X M) and level of the triplet required verification by thermal analysis. diluted to 2.2 X M as determined spectrophotometThe porphyrins are ideal molecules for this study. rically. Samples (50 and 15 pL) of this solution, when Emission measurements on zinc mesoporphyrin indicate a fluorescence yield of 5% and a lifetime of 2 n ~ a , ~ added to 5 mL of toluene containing ZnOEP, gave 2.5 X and 0.94 X lo-' M solutions in benzoquinone. A phosphorescence yield of 1.5% and a lifetime of 50 m ~ , ~ Spectrosil 1-cm optical path fluorescence cell fitted with and a singlet-triplet gap of about 3000 cm-l (ref 3 and 4) a side-arm bulb and a Teflon stopcock was used for deall in ether-pentane-alcohol glass at 77 K. The efficiency oxygenation and measurements. The cell assembly with of intersystem crossing is very high, -90%." Our finding about 4 mL of solution and a glass-encased magnetic stirrer that the quantum yield of delayed fluorescence at room in the side arm was attached to the vacuum line and detemperature is appreciable (-0.13%, see below) raised a gassed by three freeze-pump-thaw-stir cycles. The requestion about the experimental separation of singlet and sidual oxygen in solution was estimated at less than 5 x triplet emissions and thus of the energy gap. PhosM. An Oxford Instruments cryostat fitted with phorescence of porphyrins in liquid solution has been three-way windows and a home-built temperature conobserved.s Parker and Joyces observed delayed fluorestroller were used for measurements in the -50 to +25 "C cence from chlorophyll a and b in ethanol. They identified range. two routes to the excited singlet: thermal from the triplet The light source was a Molectron UVlOOO nitrogen laser (E type), and reaction by encounter between two triplet emitting at 337 nm, 1-MW peak power, 9-ns fwhm. The states (T type). laser pulse was attenuated by use of calibrated neutral In the present study the long-lived emissions from density filters. The fairly homogeneous middle section of well-deoxygenated solutions of zinc octaethylporphyrin the slightly diverging laser beam was directed into the (ZnOEP) were measured with a gated photomultiplier solution. The pulse intensity was measured at the location following laser flash excitation. Both polar (acetonitrile) of the optical cell by a YSI Kettering Model 65 radiometer. and nonpolar (toluene) solvents were used because of the The excitation light was separated from the fluorescence measured strong dependence of ion escape on solvent polarity.' In a'nonpolar solvent the yield of ions is negligibly small. It is, however, likely that ions are formed, (1)Ballard, S. G.; Mauzerall, D. J. Chem. Phys. 1980, 72, 933-47. but they recombine before escaping. This would mean a (2)Fuhrhop, J. H.;Kadish, K. M.; Davis, D. G. J. Am. Chem. SOC. loss of energy of the triplet state, the degree of quenching 1973,95,5140. indicating the probability of initial ion formation.lJO This (3)Mokeyeva, G.A.;Sveshnikov, B. Ya. Opt. Spektrosk. 1961,10,86. (4)Becker, R. S.; Allison, J. B. J . Phys. Chem. 1963,67, 2669-80. quenching could be directly measured by delayed (5)Gradyushko, A. T.; Tsvirko, M. P. Opt. Spectrosc. (Engl. Transl.) fluorescence. 1971,27,91-111. (6)Gurinovich, G. P.; Dzhagarov, D. M. In "Luminescence of Crystals, Experimental Section Molecules and Solutions"; Williams, F., Ed.; Plenum Press: New York, Acetonitrile (Eastman no. 488) was fractionally distilled 1973: D 196. (7)bzhagarov, B. M.; Cagin, E. N.; Bondarev, C. L.; Gurinovich, G. from P205into activated molecular sieves (Chemilog MoN. RLoDhv8ika 1977. 22.~565-70. .".. .- _. . ., _ .. .. lecular Sieve, 3 A, 1/16) and again fractionally redistilled @j-Kautsky, H. Ber. 1935,68,152-67. before conducting the experiment. Toluene (Merck (9)Parker, C. A.;Joyce, T. A. Photochem. Photobiol. 1967,6,395-406. On sabbatical from the Hebrew University, Jerusalem, Israel.
(10)Carapellucci, P.A.; Mauzerall, D. Ann. N.Y. Acad. Sci. 1975,244, 214-38.
0 1982 American Chemical Society
1624
Feitelson and Mauzerali
The Journal of Physical Chemistry, Vol. 86, No. 9, 1982
450'3
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175 Change time base
Photomultiplier gate closed
I50 13 Ln
Figure 1. Delayed fluorescence of ZnOEP (4.5 pM) In toluene at 24 OC. Laser intenslty: 2.5 pj pulse-' cm-'. Insert shows first-order plot of decay. Arrow indlcates change in time base.
by GB-10 (Schott) and C2-61 (Corning) filters. An EM1 9858 photomultiplier at right angle to the incident laser pulse was used to monitor the emission. The high voltage was gated on for variable time intervals after the laser pulse so as to avoid saturating the photomultiplier by the prompt fluorescent light. The gate was essentially that of DeMartini and Wachs" as modified by DeMarco and Penco.12 The current output of the photomultipler was converted to voltage by an operational amplifier with feedback time constant of 1 pS, digitized by a transient recorder (Biomation 805) and averaged in a Fabri Tek 1072. The data graphs obtained on an X-Y recorder were normalized and fitted to calculated curves using an HP 9825 computer and plotter. The spectrum of the long-lived emission at 24 "C was first obtained by the same setup and a scanning interference wedge. In addition to the two peaks of fluorescence, a low-intensity tail extending to 710 nm was observed. This emission was subsequently studied as a function of temperature under conditions which allowed its spectral resolution. A steady-state fluorometer equipped with a light chopper and photon counter (Brookdeal-oRm photon counting system 5C1) were used which made it possible to monitor the prompt fluorescence in the "light-on" intervals of the chopper and the long-lived emission during the "light-off" periods. A red-sensitive Hamamatsu R-928 photomultiplier was used to monitor the emission. ( 1 ) Low-Intensity Laser Pulse (2.5 d p u l s e - ' cm-2). ( a ) Kinetics. At this intensity and at an absorbance of 0.15 at 337 nm (ZnOEP N 5 gM),the concentration of excited ZnOEP in toluene produced per pulse is -2 X lo4 M. On flashing such a solution with the 337-nm pulse in oxygen-free solution, one observes a long-lived emission in the millisecond time range (Figure 1). The spectrum of the delayed emission corresponded to that of the prompt emission, the former being measured through a scanning interference wedge at the delayed time (At = 10 ps). Thus, the emission is that of delayed fluorescence. The timeintergrated signals at the emission maximum showed that for a delayed fluorescence of about 5 m s lifetime, the ratio of the quantum yields for the delayed (DF) to prompt fluorescence (F) is @DF/@F N 0.025 f 0.008. When the laser intensity is increased from 2.4 to 4.8 pJ pulse-' cm-2, the initial height of the delayed fluorescence signal also increases by a factor of 2. The linear increase of fluorescence with laser pulse energy and its strictly first-order decay show that at low laser intensities we are dealing with E-type delayed fluorescence? No effects of triplet-triplet (T-T) interactions are observed at this in(11) DeMartini, F.; Wachs, K. Reu. Sci. Instrum. 1967, 38, 866-70. (12) DeMarco, R.; Penco, E. Reu. Sci. Instrum. 1969, 40, 1158-60.
1
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2
4
6
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Figure 2. Quenching of ZnOEP delayed fluorescence by ground-state ZnOEP (T-P reaction) and by impurity Q. Solvent: acetonitrile at 25 OC (B); toluene at 25 (A), 0 (El), -25 (0),and -50 (0)OC. Insert: temperature dependence of quenching rate constant: (a) ZnOEP, (b) impurity Q.
tensity. Triplet molecules are thermally excited to the 'S state, and the fluorescence decay measured the lifetime of the first triplet state. A dependence of the decay rate on the concentrations of ZnOEP was observed. This indicates the expected reaction between the triplet and ground-state molecules (T-P reaction). Although the yield of free P+and P ions produced photochemically in toluene is only the triplet-ground state interaction does provide a deactivating pathway for the triplet. Figure 2 shows the concentration dependence of the observed decay rate constant at different temperatures. If we write kobsd = k,' + kg(ZnOEP) where k,' is the first-order decay constant and k9 is the second-order triplet-ground state reaction rate constant (see following sections), we obtain at 24 OC a value of k , = (1.05 f 0.15) X 10' M-l s-' in toluene. This constant is comparable to that measured by conductivity.' From Figure 2 it is, however, evident that in addition to the slope, k,, the intercept, k,', also decreases with lowering temperature; i.e., the lifetime extrapolated to zero ZnOEP concentration increases, indicating another temperature-dependent quenching process which affects the triplet lifetime. We therefore denote the intercept by k,' = ko k (Q) with Q representing a quenching impurity present in tke solution. If we now assign the reasonable value of ko = 30 to the true reciprocal lifetime of the triplet in liquid solution, an Arrhenius plot yields the activation energies of E , = 1.45 kcal molW1(510 cm-') for K, and 2.4 kcal mol-' (840 cm-l) for k, (Figure 2, insert), respectively. Since the value of k , is smaller by 3 orders of magnitude than a diffusionlimited rate constant, it should not be affected by the viscosity, and thus a potential barrier has to be overcome by the quenching process. This process could contribute about 0.1 eV to the aforementioned energy "deficit" of ion formation. The activation energy of E, = 2.4 kcal mol-' found for k, indicates that the quenching of the triplet by a residual impurity (such as 0,)is viscosity (see section c) and hence diffusion dependent. ZnOEP is known to produce free positive and negative ions upon illumination in acetonitrile with a quantum yield in excess of 0.25.' Hence, the triplet-ground state P re-
-
+
Reactions of Triplet States of a Porphyrin
action should also be observable in this system. The dependence of the delayed fluorescence decay rate on the ZnOEP concentrations in acetonitrile yields a quenching constant of k , = (5 f 1) X lo7 M-1 8. The difference between this value and 2 X lo8 M-2 s-' observed in the conductivity measurements of Ballard and Mauzerall' is discussed in the conclusion. ( b ) Activation Energy for Formation of the Triplet State. Parker13has described the kinetics of the delayed fluorescence (E type) and derived its temperature dependence in terms of the population of the first excited singlet state obtained by thermal activation of the triplet:
k4 = Ae-m/RT (1) Here 4 D F = @F& k 4 / [ k 4+ k, + k8 + ke(OP)]is the yield of delayed fluorescence due to the thermal process, 4F = ~ D F / ~ F=T 4TAe-hEiRT
The Journal of Physical ChemWy, Vol. 86, No. 9, 1982 1825
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c 01
c
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+
k z / ( k l kz + k3) is the yield of the prompt fluorescence, and T = l / [ k 4 + k , + k8 + kS(OP)]is the triplet-state lifetime as measured by the delayed fluorescence decay at a given temperature. 4T = k l / ( k l + kz + k3) is the yield of the intersystem crossing which for ZnOEP is close to unity. We have used here the rate constants as defined in the following section. A plot of In &,F/(&T) against 1/T yields the activation energy of the delayed fluorescence. @DF and 4T, as defined, neglect the rapid intersystem crossing (kl)which follows the activation step (k4) and require a s m d correction term (
