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that the isomers n-propyl and isopropyl iodide produce very different fragmentation patterns when examined at the same laser power. Acknowledgment. The author thanks Professor R. N.
Zare for suggesting this study. Me1 N. Kronick of Quanta-Ray provided helpful suggestions on MPI and Larry Marshall and Robert Rorden provided the pulsed nozzle. This work received financial support from the Army Research Office, Contract DAAG 29-81-C-0023.
Flash Photolysis-Pulse Radiolysis Spectroscopy. Reactions of Photoexcited Metalloporphyrlns with Short-Lived Radicals' Haim Levanon"* and P. Neta" Radbtion Laboratory and Department of Chemistv, University of Notre Dame, Notre Dame, Indiana 46556 (Received: September 9, 196 1)
Simultaneous flash photolysis and pulse radiolysis of metalloporphyrin (MP) solutions allowed the formation of photoexcited triplets (MPT)and observation of their reactions with short-lived radicals. A light pulse of 1-5-ms duration produced a steady-state concentration of triplets and was followed by a delayed electron pulse which produced radicals from the solvent. The reactions of these radicals with the MPT were followed spectrophotometricallyover 0.1-4 ms. It is found that the reactions of triplet porphyrins with solvent radicals are highly selective and yield intermediates different than those produced from the ground-state porphyrins. Application of this double pulsing technique is demonstrated by the reactions of MPT with radicals in both nonaqueous and aqueous solutions.
Introduction In a recent study3on the formation of porphyrin cation radicals in pulse-irradiated 1,2-dichloroethane (DCE) solutions, we have noticed a strong effect of the analyzing light on the transient spectra. A bleaching signal was observed at wavelengths where the solution components did not absorb and thus had to be attributed to reactions of species formed by the light. The extent of the bleach was strongly dependent on the light intensity and its wavelength range. In order to study this phenomenon we have employed an independent pulsed light source for the excitation, combined with the conventional pulse radiolysis setup. The technique described herein bears important implications in studying reactions of photoexcited states with short-lived radicals and it is demonstrated in this Letter that it is possible to use pulse radiolysis techniques to study the reactions of photoexcited triplet porphyrins with short-lived solvent radicals. Method In principle, the method described here is a combined flash photolysis and pulse radiolysis spectrocopic experiment in which the radiation chemistry of photochemically prepared intermediates is examined. The computer-controlled pulse radiolysis apparatus used is as described before.@ It consists of an ARC0 LP-7 linear accelerator, supplying 5-50-ns pulses of 7-MeV electrons, each pulse producing 1-20 pM of radicals, and a Xe lamp as analyzing (1)The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2281 from the Notre Dame Radiation Laboratory. This work was also supported by a grant from the Israel-US. Binational Science Foundation (No. 2102/80). (2) Permanent address: Department of Physical Chemistry and Fritz Haber Research Center for Molecular Dynamics, The Hebrew University, Jerusalem, 91904 Israel. (3) Levanon, H.; Neta, P. Chem. Phys. Lett. 1980, 70, 100. (4) Patterson, L. K.; Lilie, J. Int. J. Radiat. Phys. Chem. 1974,6, 129. (5) Schuler, R. H.: Neta, P.: Zemel, H.: Fessenden, R. W. J. Am. Chem. SOC.1976,98, 3825.
light for the kinetic spectrophotometric detection. To this setup we have added, in parallel to the electron beam, an additional 1-kW Xe lamp, as an excitation light source pulsed for 1-10 ms to -50 times its steady-state intensity. The light pulse produces a transient absorption with a time profile as shown by ABC in Figure 1. The concentration of the transient increases to a steady-state level and at the end of the light pulse it decreases exponentially. The steady-state level represents -50% conversion of the porphyrin to the excited state. The decay takes place over several milliseconds, typical of metalloporphyrin triplets(see discussion below). The electron pulse is introduced when the triplet steady state is approached. This is shown by the bleaching process (starting at point B, Figure 1). Other experimental details, materials, and solution handling techniques are as described p r e v i o ~ s l y . ~ ~ ~ J ~
Results and Discussion Photolyzed solutions of porphyrins and metalloporphyrins in 1,2-dichloroethane,2-propanol, or water all gave transient absorptions (ABC in Figure 1). The excitation spectrum was examined with several systems by using various cutoff and interference narrow-band pass filters and a 470-nm analyzing wavelength. In all cases, the excitation spectrum responsible for the transient absorption was found to correspond to the Soret and the Q bands of the porphyrin used. From this result, together with the transient lifetime, and an observed efficient and rapid quenching by oxygen, it appears that the transient is the photoexcited triplet state. This was further confirmed by varying the wavelength of the analyzing light (6) Pekkarinen, L.; Linschitz, H. J. Am. Chem. SOC.1960, 82, 2407. (7) Scherz, A.; Orbach, N.; Levanon, H. Isr. J. Chem. 1974,12,1037. (8) Pileni, M. P.; Braun, A. M.; Gratzel, M. Photochem. Photobiol. 1980, 31, 423. (9) Neta, P.; Scherz, A.; Levanon, H. J. Am. Chem. SOC.1979, 101,
3625. (10) Neta, P. J. Phys. Chem. In press.
0022-365418112085-3754$0 1.2510 0 1981 American Chemical Society
Letters
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Figure 1. Transient triplet-triplet absorption curve (AB) and the bleaching process (ED) upon electron irradiation of a solution of ZnTPP in i-PrOH. These kinetic profiles were taken at Amadtor = 470 nm and with Aexcitation > 420 nm.
and finding a spectrum which corresponds to that of the triplet.6-8 The high-energy electrons act only on the solvent to produce various radicals. In all cases, these radicals were found to bleach the triplet absorption (BD in Figure 1). However, at the time of the electron pulse the solutions contain both ground state and triplet porphyrin and the reactions of the radicals with both forms have to be considered. The initial solutions usually undergo -50% ground-state depletion (from the level of absorption and the extinction coefficients of the ground-state porphyrin and of its triplet).6 We chose to exemplify this technique by describing the reactions of ZnTPPT and ZnTPPST (zinc tetraphenyl- and tetra-(4-~ulfonatophenyl)porphyrin) in nonaqueous and aqueous solutions, respectively. The transient spectra are shown in Figure 2. In Figure 2a, the spectrum produced upon reaction of solvent radical cation S+- with ZnTPP (without light excitation)
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ZnTPP S+. ZnTPP+. + S (1) is in agreement with that reported previ~usly.~ When the excitation light is introduced the transient spectrum is different. Bleaching of the triplet absorption at 450-500 nm is observed ZnTPPT + S+- products (2)
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Reaction 2 may include more than S+. as neutral solvent radicals may also react with the triplet. Apparently, reaction 2 does not yield the cation radical (ZnTPP)+.. This is somewhat surprising, since electron transfer from a triplet state is usually expected to be more favorable than its ground state. We are led to conclude that reaction 2 proceeds by a route more favorable than electron transfer and that this involves a reaction between a triplet and reactive free radicals produced from the solvent in the course of electron irradiation, thus resulting in the addition of S+- to a double bond (either P-pyrrole or phenyl). In aqueous solutions ground-state ZnTPPS react with (CHJ2COH to produce ZnTPPS-..'O With light excitation
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A-nm Flgure 2. Difference absorption spectra of pulse-irradiated solutions, monitored 120 ps after the electron pulse, with (0)and without (A) light excitation: (a) 3.0 X lo4 M ZnTPP in N,-saturated DCE contalnlng 1 % pyridine; (b) 2.0 X lod M ZnTPPS in N,O-saturated H,O, pH 11.3, containing 2 % t-BuOH.
similar spectral changes to those described above are noticed. On the other hand, the cH,C(CH,),OH radical does not transfer an electron and is usually quite inert." It does not react with ZnTPPS under the pulse radiolysis conditions as demonstrated in Figure 2b. However, this radical is found to react rapidly with (ZnTPPS)T and to bleach its absorption at 480,580, and 670 nm. This bleach corresponds to the triplet-tri let ab~orption.~J The triplet is thus being quenched by H2C(CHJ20Hwithout producing the anion or cation radicals and again involves an addition to the double bond. The radical adducts decay to final products at longer timescales (2-4 ms). This detection method, which appears to be general, opens the possibility for studying reactions between radicals and photoexcited tripleh. Results with other systems will be reported in more detail elsewhere.
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Acknowledgment. We are indebted to Professor R. H. Schuler for his continuous interest and many helpful discussions in the course of this work. (11) Swallow, A. J. Prog. React. Kinet. 1978, 9, 195.
