Rapid cleavage of the naphthylmethyl–oxygen bond in higher triplet excited states† Xichen Cai, a Masanori Sakamoto, a Michihiro Hara, a Sachiko Tojo, a Mamoru Fujitsuka, a Akihiko Ouchi b and Tetsuro Majima* a a The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan. E-mail: majima@sanken.osaka-u.ac.jp b National Institute of Advanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Received (in Cambridge, UK) 24th July 2003, Accepted 4th September 2003 First published as an Advance Article on the web 18th September 2003 Rapid cleavage of the naphthylmethyl–oxygen bond of 1- and 2-[(4-benzoylphenoxy)methyl]naphthalenes in higher triplet excited states occurred within a laser flash of 5 ns to give 1- and 2-naphthylmethyl radicals with formation quantum yields of 0.042 ± 0.004 and 0.020 ± 0.002, respectively, during two-colour two-laser flash photolysis. Naphthylmethyl radical (NpCH 2 ·) is a typical organic radical, and has been extensively studied. 1–7 It is well established that NpCH 2 · is produced from photolysis of naphthylmethylhalides through cleavage of the naphthylmethyl–halogen bond. 2,4 Cleavage also occurs in other naphthylmethyl compounds under laser irradiation. For example, cleavage of the C–X bond of 1-naphthylmethyl compounds (1-NpCH 2 XPh; X = O, S, Se) and naphthyl-1,8-dimethyl compounds (1,8-Np(CH 2 -XPh) 2 ) has been reported with transient absorption measurements. It is found that cleavage occurs very quickly from 1-NpCH 2 -SPh and 1-NpCH 2 -SePh in the S 1 states. 5 In contrast, the lowest triplet state (T 1 ) of 1-NpCH 2 -OPh is predominantly generated from laser flash photolysis of 1-NpCH 2 -OPh with excitation at 266 nm, because the energy of the T 1 state is lower than the dissociation energy of the NpCH 2 -OPh bond. Steenken and coworkers also found that C–O bond cleavage occurred in the S 1 state but not in the T 1 state of 1-[(4-benzoylphenoxy)- methyl]naphthalene (1-NpCH 2 -OBP). 7 They assumed that C–O bond cleavage could occur if the Np moiety is excited to a higher triplet state (T n ) through further photon excitation of the T 1 state. However, no experimental result has been reported to support their hypothesis on C–O bond cleavage from the T n state. Recently, we found that naphthalene(T n ) generated from the excitation of naphthalene(T 1 ) has a lifetime of 4.5 ps and sensitises some reactions of halogenated compounds in two- colour two-laser flash photolysis experiments. 8 Similarly, Np(T 1 ) of 1-Np(T 1 )CH 2 -OBP can be excited to higher T n states. We report here the occurrence of C–O bond cleavage of 1- and 2-NpCH 2 -OBP in the T n states (1- and 2-NpCH 2 -OBP(T n )) giving 1- and 2-NpCH 2 ·, respectively. When 1-NpCH 2 -OBP (1.0 3 10 23 M) was irradiated at 355 nm using a Nd : YAG laser (3 mJ pulse 21 , laser flash duration of 5 ns) in Ar-saturated cyclohexane at room temperature, the transient absorption spectrum observed immediately after the laser flash (Fig. 1a) was coincident with that of Np(T 1 ) with a peak at 420 nm as reported previously. 5,7 Because the Np chromophore has no absorption at 355 nm, the first 355 -nm photon is absorbed by the BP chromophore to give BP(S 1 ) from which intersystem crossing occurs to give BP(T 1 ) in a quantum yield of 1.0. 9 Intramolecular triplet energy transfer from BP(T 1 ) to the Np chromophore occurs to give Np(T 1 ) within a laser flash of 5 ns. The observed energy, lifetime, and extinction coefficient (e) of 1-Np(T 1 )CH 2 -OBP were similar to those of naphthalene(T 1 ) and 1-methylnaphthalene(T 1 ). 9 No appearance of the peak at 365 nm assigned to 1-NpCH 2 · indicates that the C–O bond cleavage did not occur from 1-Np(T 1 )CH 2 -OBP as reported by Steenken et al. 7 This observation is in agreement with the triplet energy of naphthalene(T 1 ) (254 kJ mol 21 ) and C–O bond dissociation energy (285 kJ mol 21 ). 7 The absorption peak at 365 nm assigned to 1-NpCH 2 · was observed with bleaching of the 420 nm peak within a laser flash, when the second 430 nm OPO laser flash (10 mJ pulse 21 , laser flash duration of 5 ns) irradiated 1-Np(T 1 )CH 2 -OBP with a delay time of 100 ns after the first 355 nm laser flash (Fig. 1b). Because only 1-Np(T 1 )CH 2 -OBP has an absorption at 430 nm, 1-Np(T 1 )CH 2 -OBP can be excited to 1-NpCH 2 -OBP(T n ) by the second laser. The 430 nm photon (278 kJ mol 21 ) supplies sufficiently high energy into 1-Np(T 1 )CH 2 -OBP giving 1-NpCH 2 -OBP(T n ) from which rapid C–O bond cleavage occurred within the laser flash (5 ns). Together with 1-NpCH 2 ·, the p-benzoylphenoxyl radical (BPO·) should be also produced. However, the absorption peak of BPO· was not observed. This is because BPO· absorbs at 395 nm with a low e (e 395 = 1550 M 21 cm 21 ) 7 so the peak must be submerged in the strong 395 nm absorption of 1-Np(T 1 )CH 2 -OBP with e 395 = 7200 M 21 cm 21 ; e was calculated from the ratio of DO.D. 420 and DO.D. 395 (Fig. 1), and e 420 = 13200 M 21 cm 21 . 9 The time profiles of the transient absorption detected at 420 and 365 nm are shown in Fig. 2a and b, respectively. The rapid formation of the transient absorption at 420 and 365 nm after the first 355 nm laser irradiation corresponds to the formation of 1-Np(T 1 )CH 2 -OBP. The change of the transient absorption at 365 nm caused by the second 430 nm laser irradiation involves the effects of both bleaching of 1-Np(T 1 )CH 2 -OBP and the formation of 1-NpCH 2 ·. The bleaching of the transient absorp- tion of 1-Np(T 1 )CH 2 -OBP at 420 nm (DDO.D. 420 ) and the formation at 365 nm (DDO.D. 365 ) increased with increasing second 430 nm laser power. Plots of ¡DDO.D. 420 ¡ and † Electronic supplementary information (ESI) available: C–O bond cleav- age of 2-NpCH 2 -OBP(T n ). See http://www.rsc.org/suppdata/cc/b3/ b308577e/ Fig. 1 Transient absorption spectra obtained at 1.1 ms after the 355 nm first laser irradiation (a) (broken line) and at 1 ms after the second 430 nm laser irradiation (b) (solid line) during two-colour two-laser flash photolysis of 1-NpCH 2 -OBP in Ar-saturated cyclohexane at room temp. The delay time of the second laser after the first laser was 100 ns. The inset shows the spectrum obtained by (b) 2 (a) in the region of 350–380 nm. This journal is © The Royal Society of Chemistry 2003 2604 CHEM. COMMUN. , 2003, 2604–2605 DOI: 10.1039/b308577e