5147
J . Phys. Chem. 1985, 89, 5 147-5 149
Two-step Laser Excitation Fluorescence Study of the Reactions of the I-Naphthylmethyl Radical Produced in the 248-nm KrF Laser Photolysis of I-(Chloromethy1)naphthalene in Hexane at Room Temperature Kunihiro Tokumura,* Masahiro Udagawa, and Michiya Itoh* Faculty of Pharmaceutical Sciences, Kanazawa University, Takara-machi, Kanazawa 920, Japan (Received: June 26, 1985; In Final Form: September 17, 1985)
The second laser pulse (365-386 nm) excitation of the transient 1-naphthylmethyl radical, formed by the first laser pulse (248 nm) induced fragmentation of 1-(chloromethy1)naphthalene in deoxygenated hexane at room temperature, affords an orange-red fluorescence of the radical. The radical was observed to exhibit a second-order decay by the variable-delay technique of the two-step laser excitation (TSLE) fluorescence spectroscopy. In addition to the fluorescence spectrum due to the 1-naphthylmethyl radical, two kinds of TSLE fluorescence spectra in the UV (A, = 340 nm) and visible (A,, = 490 and 525 nm) regions were observed. The intensities of these fluorescence spectra increase in accord with the decrease of the 1-naphthylmethylradical. The former may be ascribed to a,a-dinaphthylethane formed by a-a coupling of 1-naphthylmethyl radicals, while, the latter may be ascribed to 1-naphthylmethylene-substituted1(4H)-naphthylidene formed in a-para and a-ortho couplings of the radicals.
Introduction Recently, the doublet-doublet fluorescence of several aromatic free radicals in solution at room temperature has been studied by laser pulse excitation of the unstable free radical produced in laser p u l ~ e l -or~ accelerated electron pulse4s5excitation of a suitable precursor. By means of a consecutive two-color picosecond pulse experiment, Rentzepis and his co-workers1*2observed the 355-nm = 610 nm; pulse-induced orange-red fluorescence spectrum (A, Tf= 13 ns) of 1- and 2-naphthylmethyl radicals which were formed by the 266-nm pulse-induced dissociation of 1- and 24halomethy1)naphthalene in hexane at room temperature. The rapid (C15 ps) photodissociation of these halomethylnaphthalenes was directly confirmed by the determination of the fluorescence intensity of the naphthylmethyl radicals (fragmentation product) at various picosecond delay times between the two laser pulses. They proposed intersystem crossing from the S2(7r,n*) state into the T(u,u*) state of the C-X bond in halomethylnaphthalene followed by fragmentation to a naphthylmethyl radical and a halogen. However, the decay processes of the naphthylmethyl radical could not be examined in detail by picosecond spectroscopy using two laser pulses with a maximum delay time of ca. 3 ns. To the best of our knowledge, little has been reported on the coupling reaction of the naphthylmethyl radical, although it has been demonstrated that the coupling of benzyl radicals generates not only bibenzyl but also semibenzenes.68 The purpose of the work described in this paper is to elucidate the mechanism of the coupling reaction of the 1-naphthylmethyl radical in the nanoto microsecond time domain by two-step laser excitation spectroscopy, which has been recently demonstrated to be a novel and versatile technique in the study of the kinetics and dynamics of photochemically unstable molecule^.^-^^ (1) Kelley, D. F.; Milton, S.V.; Huppert, D.; Rentzepis, P. M. J. Phys. Chem. 1983,87, 1842. (2) Hilinski, E. F.; Huppert, D.; Kelley, D. F.; Milton, S. V.; Rentzepis, P. M. J. Am. Chem. SOC.1984, 106, 1951. (3) Naaaraian, V.; Fessenden. R. W. Chem. Phvs. Lett. 1984. 112. 207. (4) Brgmbe-rg, A.; Schmidt, K. H.; Meisel, D. j . Am. Chem. SOC.1984, 106, 3056. (5) Brombcrg, A,; Schmidt, K. H.; Meisel, D. J. Am. Chem. SOC.1985, 107, 83. ( 6 ) Langhals, H.; Fischer, H. Chem. Ber. 1978, 111, 543. (7) Skinner, K. J.; Hochster, H. S.; Mcbride, J. M. J. Am. Chem. SOC. 1974, 96, 4301. (8) Dannenberg, J. J.; Tanaka, K. J. Am. Chem. SOC.1985, 107, 671. (9) Itoh, M.; Adachi, T.; Tokumura, K. J . Am. Chem. SOC.1984,106,850. (10) Itoh, M.; Adachi, T. J. Am. Chem. SOC.1984, 106, 4320. (1 1) Tokumura, K.; Watanabe, Y.; Itoh, M. Chem. Phys. Lett. 1984,111, 379. (12) Itoh, M.; Fujiwara, Y. J. Am. Chem. SOC.1985, 107, 1561.
0022-3654/85/2089-5147$01.50/0
Experimental Section
1-(Chloromethy1)naphthalene (Nakarai Chemicals) was purified by distillation. Spectral grade hexane (Nakarai Chemicals) was used as solvent without further purification. Sample solutions were degassed by repeated freeze-pump-thaw cycles. Transient absorption spectra were measured by a laser-flash apparatus which consisted of an excimer laser (Lambda Physik EMG 50E), a monitoring xenon arc beam, cut off by an electromagnetic shutter operating synchronously with the excimer laser, and a HTV R-666 photomultiplier tube. The electric signals from the tube are terminated to the Tektronix 7904 oscilloscope with 7A 13 (differential comparator) and 7B 80 plug-in units. In the two-step laser excitation (TSLE) fluorescence measurements, the 248-nm (KrF) pulse of the excimer laser was used as the first pulse to induce the fragmentation of 1-(chloromethyl)naphthalene, and several (N2-laser-pumped) dye laser pulses (386, 365, and 290 nm) were used as the second pulse to induce the emission of transients and/or products. The 290-nm pulse was obtained by inserting a frequency doubler (KDPcrystal) in the dye laser beam from an ethanol solution of rhodamine 6G. The delay time (0 ns-100 11s) between two laser pulses is variable by use of a delay circuit. TSLE fluorescence signals were collected by a photomultiplier (HTV 1P28 or R-666)-oscilloscope (IWATSU TS-8 123) system. Results and Discussion Figure 1 shows the time-resolved transient absorption spectra and two typical oscillogram traces obtained by KrF laser pulse (248 nm) excitation of 1-(chloromethy1)naphthalene (1CMN) in oxygen-free hexane at room temperature. Both long-lived UV and short-lived visible absorption bands are readily seen in the figure, although the fluorescence of l C M N prevents the observation of the UV absorption at the short delay time (0.5 M). The short-lived visible absorption spectrum (A, = 400 and 420 nm), being very similar to the T, TI absorption spectrum of 1chloronaphtbalene in hexane,13 exhibits an exponential decay (T = 0.7 ps), as shown in the upper oscillogram trace. Thus, the short-lived transient absorption spectrum may be attributed to T I absorption spectrum of 1CMN. The UV absorption the T, spectrum (A, = 340 and 365 nm) exhibits a second-order decay ( 2 k , / c = 7.6 X lo5 cm s-l at 365 nm), as shown in the lower oscillogram trace. The UV absorption spectrum at 5 - w ~delay in Figure 1 is similar to the reported absorption spectrum2obtained 50 ps after 266-nm excitation of l C M N in hexane.I4 Hilinski
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(13) Huppert, D.; Rand, S.D.; Reynolds, A. H.; Rentzepis, P. M. J . Chem. Phys. 1982, 77, 1214.
0 1985 American Chemical Society
Letters
5148 The Journal of Physical Chemistry. Vol. 89. No. 24. 1985
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Figure 2. TSLE and steady-state fluorescence spectra of a deaerated hexane solution of I-(chlorome1hyl)naphthslene (3.4 X IO” M): first laser pulse, 248 nm (KrF laser); second laser pulse, 386 nm for spectra 4 and -O-, 290 nm for spectrum 0; steady-state spectrum (-)
et al? observed an orange-red fluorescenceof the I-naphthylmethyl radical in the 355-nm excitation of this transient absorption spectrum. Thus, our ahsorption spectrum at 5-1”. delay, when the decay of T, state of I C M N has been nearly completed, may he ascribed to the I-naphthylmethyl radical in the ground state (Do). An orange-red D, -Do fluorescence spectrum (550-750 nm) was obtained by 386-nm excitation of the aforementioned UV transient absorption (D,or D, Doabsorption) at 2-ps delay from the first laser pulse (248 om) excitation of ICMN, as shown in Figure 2. This two-step laser excitation (TSLE) fluorescence spectrum is almost identical with the reported spectrum.l.’ The fluorescence lifetimes of the TSLE fluorescence in deaerated and aerated hexane were determined to be 34 and 18 ns, respectively. The latter is slightly longer than the reported lifetime of I3 m? If it is reasonably assumed that the intensity of D, Do fluorescence induced by the second laser is proportional to the Dotransient, a plot of TSLE absorbance (
