Mechanistic studies on the photogeneration of o- and p-xylylenes from α,α′-dichloroxylenes

Abstract

Two-colour two-laser techniques have unambiguously proved that photolysis of the o-/p-(chloromethyl)benzyl radical leads to the sequential two-photon generation of o-/pxylylene from a,aA-dichloro-o-/p-xylene.Xylylenes are molecules of considerable theoretical and synthetic interest, which have been detected and characterized by different spectroscopic methods. 1 Although benzylic dichlorides have been used as very simple and readily accessible precursors, the involved mechanisms are not completely understood. 2 It has been recently reported 3 that 266 nm laser irradiation of a,aA-dichloro-o-xylene (1a) produces the dissociation of both C-X bonds via a two-photon process; however, the nature of the intermediate species which absorbs the second photon is uncertain. Three possibilities have been considered: the S 1 and T 1 states of 1a and the o-(chloromethyl)benzyl radical (2a). Based on the short lifetime of the singlet state of the dichloride and the apparent relative yields of the observed transients, the undetected T 1 state of the dichloride has been suggested as the photochemical precursor of o-xylylene.Our aim was to explore the photochemistry of the benzyl radical 2a produced after dissociation of one C-X bond of a,aAdichloro-o-xylene, using two-laser two-colour techniques, in order to obtain new data to support or reject the intermediacy of 2a in the formation of o-xylylene. We report here our findings and compare them with those obtained in the photolysis of a,aAdichloro-p-xylene. Based on the obtained results, a mechanism for the formation of o-and p-xylylenes from their corresponding dichloro precursors is presented.Laser flash photolysis of deaerated 1.5 mm solutions of 1a in cyclohexane at 266 nm (Nd: YAG laser, fourth harmonic, < 10 ns, @20 mJ pulse 21 ) yielded the transient absorption spectra shown in Fig. 1. Two transients with different lifetimes were obtained. According to the literature, 3 they were assigned to 2a (with a maximum at 330 nm) and o-xylylene (3a) (maximum at 360 nm, e max = 3 3 10 3 dm 3 mol 21 cm 21 ), 4 generated through one-and two-photon processes, respectively.When irradiating with a 266 nm laser it is useful to place a beam diffuser (which eliminates 'hot' spots in the laser beam) close to the sample in order to get a better observation of monophotonic transients. 5 Actually, a new spectrum obtained under these conditions showed a higher ratio of radical 2a to the two photon intermediate, 3a (Fig. 1).To confirm the radical nature of the transient at 330 nm, oxygenated samples were examined, showing that, while the band at 330 nm was quenched at close to the diffusion controlled limit (see insert Fig. 1), the lifetime of the transient at 360 nm appeared insensitive to the presence or absence of oxygen (data not shown).Furthermore, to study the photobehaviour of 2a, two-colour two-laser flash photolysis experiments 6 were carried out using 266 nm laser pulses (20 mJ pulse 21 ) to photolyze 1a and a 308 nm excimer laser (90 mJ pulse 21 ) to irradiate 2a. The two pulses were ty...

Influence of Single-Walled Carbon Nanotubes Enriched in Semiconducting and Metallic Tubes on the Vibrational and Photoluminescence Properties of Poly(para-phenylenevinylene)

“…According to Figure S1, an absorption band peak at 338 nm was not observed during the electrochemical reduction of TBPX in the potential range from 0 to −2 V vs Ag/AgCl. The absorption band of TBPX is peaked at 270 nm (4.6 eV), which is in good agreement with ref . The UV–vis spectrum of the PPV film electrochemically synthesized onto ITO support shows an absorption edge at 3.23 eV, which is assigned to the π–π* electronic transition .…”

o-Quinodimethanes: Efficient Intermediates in Organic Synthesis

“…Mechanistic studies on the photogeneration of oQDM have been recently carried out. 11,26 Miranda et al have further explored the photochemistry of the benzyl radical 15 produced after dissociation of one C-Cl bond of 14 by using twolaser two-color techniques to obtain new data to support or reject the intermediacy of 15 in the formation of oQDM. 26 Thus, 266 nm laser pulses (20 mJ/pulse) were used to photolyze 14, and a 308 nm excimer laser (90 mJ/pulse) was used to irradiate 15.…”

“…20 Other less general photochemical generation methods of oQDM have also been used. Furukawa reported not long ago on the generation of oQDM by photolysis of 8,13-dihydrobenzo 26). The decomposition of 77a,b takes place quantitatively when exposed to sunlight in the presence of air to give an oQDM dimer (80) and naphtho [1,8-c,d]-1,2diselenole (78).…”

Benzophenones in the higher triplet excited states

“…The two-colour two-laser excitation technique is very useful for studying transient phenomena of reactive intermediates in the excited state. [1][2][3][4][5][6] For example, molecules in the higher triplet excited states (T n , n ≥ 2) can be easily studied because of the relatively long lifetimes of the lowest triplet excited state (T 1 ). 7 The first laser excites molecules in the ground state (S 0 ) to the T 1 state, and then the second laser excites molecules in the T 1 state to the T n state.…”