ARTICLES Three-Color Three-Laser Photochemistry of Di(p-methoxyphenyl)methyl Chloride Michihiro Hara, Sachiko Tojo, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan ReceiVed: September 16, 2002; In Final Form: February 12, 2003 Three-color three-laser photochemistry of di(p-methoxyphenyl)methyl chloride ((p-CH 3 OC 6 H 4 ) 2 CHCl ) An 2 CHCl) was studied by three-step excitation using 308-, 355-, and 495-nm lasers with delay times of 100 ns to 3 μs. Di(p-methoxyphenyl)methyl radical (An 2 CH • ) was produced together with An 2 CH • in the excited state (An 2 CH • *) and di(p-methoxyphenyl)methyl cation (An 2 CH + ) in quantum yields of 0.09, 0.12, and 0.12, respectively, after a laser flash during 308-nm laser (first laser) photolysis of An 2 CHCl in acetonitrile. Excitation of An 2 CH • with a 355-nm laser (second laser) resulted in formation of transient absorption of An 2 CH • * and An 2 CH + and fluorescence of An 2 CH • * with a peak at 550 nm. The formation of An 2 CH + from An 2 CH • requires two-photon energy at 355 nm and proceeds by resonant two-photon ionization (RETPI) of An 2 CH • through sequential excitation of An 2 CH • *. Excitation of An 2 CH + with a 495-nm laser (third laser) produced fluorescence with a peak at 560 nm. Although the fluorescence of An 2 CH + was also observed without the second laser excitation because of the initial formation of An 2 CH + during the first 308-nm laser photolysis, the fluorescence intensity of An 2 CH + increased approximately 1.2 times with the second 355-nm laser excitation of An 2 CH • . Therefore, the second laser excitation can perform the conversion of An 2 CH • to An 2 CH + through RETPI within the laser flash duration, and the fluorescence intensity of An 2 CH + can be controlled by the second irradiation. Introduction Multilaser chemistry is defined as chemistry involving multi- excitation with more than two lasers, in which short-lived species such as excited molecules and reactive intermediates are generated by the first excitation of starting molecules, and used as starting molecules for the second excitation. Short-lived species generated by the second excitation are used as starting molecules for the third excitation. In other words, photochem- istry of short-lived species can be investigated by the multistep excitation method using more than two lasers. Properties of lasers such as the monochromaticity, fine directivity, high intensity, and short-pulse duration are certainly used in the multilaser photochemistry in which selective excitation of a short-lived molecule, spatially selective excitation, time control of irradiation, and excitation at high laser fluence are carried out. As the simplest multilaser chemistry, two-laser chemistry of short-lived species such as excited molecules and reactive intermediates in liquid phase has been extensively investigated by various groups. For example, Miranda et al. have reported that heterolytic cleavage of C-Cl bond proceeds to give R-chloroalkylbenzyl cation and chloride ion during the first 266- nm laser flash photolysis of dichlorodiphenylalkanes, 1,2 followed by dehydrochlorination of the cation to give R-E-allylbenzyl cation, and that the R-E-allylbenzyl cation isomerizes to the Z-isomer with the second laser excitation at 480 or 532 nm. Ouchi et al. have reported that two-step homolytic cleavages of two C-Br or C-Cl bonds proceed to give a biradical during two-color two-laser photolyses of 1,8-bis(bromomethyl)- and 1,8-bis(chloromethyl)naphthalenes using the first 193-, 248-, or 308-nm and second 351-nm excimer lasers. 3 It has been reported by Redmond et al. that hydrogen abstraction of a triplet ketone proceeds to give ketyl radical during the first 308-nm laser flash photolysis of substituted benzophenone, and that the ketyl radical is excited to give fluorescence during the second 337- or 532- nm laser flash photolysis. 4 It has been also reported by Goez et al. that photoinduced electron transfer (PET) between ketones such as 4-carboxybenzophenone and 1,5-anthraquinonedisul- fonate and donor molecules occurs to give the radical anion and cation during the first 308-nm laser flash photolysis, and electron detachment from the radical anion proceeds to give the ketone and solvated electron during the second 387.5-nm laser flash photolysis. 5,6 Wang et al., Johnston et al., McGimpsey et al., and Goez et al. have reported that the excited singlet or triplet state molecules are generated during the first 266-, 308-, or 355-nm laser flash photolysis of anthracene, diphenylamine, (4′-(phenanthrene-9-carboxylate)methyl-4-methylnaphthoate)- biphenyl, and dimethylaminobenzonitrile and Ru(bpy) 3 2+ , re- spectively, and that the radical cations are produced during the second 355-, 480-, 532-, or 590-nm laser flash photolysis of the excited-state molecules. 7-11 It has been reported by Wang et al. that resonant two-photon ionization (RETPI) of all-trans- 1,6-diphenyl-1,3,5-hexatriene (t-DPH) proceeds to give the * To whom correspondence should be addressed. Telephone: Japan +6-6879-8495. FAX: Japan +6-6879-8499. E-mail: majima@ sanken.osaka-u.ac.jp. 4778 J. Phys. Chem. A 2003, 107, 4778-4783 10.1021/jp027002l CCC: $25.00 © 2003 American Chemical Society Published on Web 05/24/2003