Multiple-Photon Chemistry in the Laser-Jet:  Photochemistry and Time-Resolved Laser-Flash Spectroscopy of the 1-Naphthylmethyl Radical

Abstract

In the laser-jet (LJ) photolysis (λ exc 333, 351, and 364 nm, high intensity) of 1-[(4-benzoylphenoxy)methyl]naphthalene (1), a two-photon process is observed in CCl 4 to yield 1-(chloromethyl)naphthalene (3a) as the main product, while the one-photon products of the conventional photolysis (low intensity) are 1,2-di(1-naphthyl)ethane (2), 1,1,1-trichloro-2-(1-naphthyl)ethane (3b), and 1-naphthaldehyde (5). In MeOH or EtOH, however, our results suggest even a three-photon reaction in which 1-naphthylmethyl methyl ether (4c) and 1-naphthylmethyl ethyl ether (4b) are produced in addition to the one-photon product 2. The three-photon mechanism is based on a competition experiment in which ether 1 was irradiated in mixtures of CCl 4 and methanol. The ratio of the two high-intensity products ether 4c and 1-(chloromethyl)naphthalene (3a) showed a strong intensity dependence. We propose that the electronically excited 1-naphthylmethyl radical 1-NpCH 2 • * is photochemically ionized to 1-naphthylmethyl cation 1-NpCH 2 + under the high-intensity LJ photolysis conditions and the latter trapped by the respective alcohols to give the 1-naphthylmethyl ethers 4c and 4b. With the help of time-resolved laser flash photolysis (248 nm) of 1-[(4benzoylphenoxy)methyl]naphthalene (1), it was established that homolysis of the C-O bond to yield 1-naphthylmethyl (1-NpCH 2 • ) and 4-benzoylphenoxy (ArO • ) radicals takes place. The authentic 1-NpCH 2 • and ArO • radicals were generated independently by pulse radiolysis of 1-(bromomethyl)-or 1-(chloromethyl)naphthalene in 2-propanol and of 4-hydroxybenzophenone (6) in water (1% tBuOH); in the latter case, SO 4 •-was used as oxidant. In the LJ photolysis, the triplet state of the 1-naphthylmethyl moiety is probably further excited to yield the observed products.

Rapid cleavage of the naphthylmethyl–oxygen bond in higher triplet excited states

“…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.…”

“…Naphthylmethyl radical (NpCH 2 •) is a typical organic radical, and has been extensively studied. [1][2][3][4][5][6][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.…”

“…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.…”

“…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.…”

Stepwise Photocleavage of C−O Bonds of Bis(substituted-methyl)naphthalenes with Stepwise Excitation by Two-Color Two-Laser and Three-Color Three-Laser Irradiations

“…The transient absorption spectra of all ROCH 2 Np in the T 1 states [ROCH 2 Np(T 1 )] showed the characteristic absorption of Np(T 1 ) with two peaks around 425 and 400 nm. , Therefore, BP, Ph, and CH 3 substituents have little effect on the electronic property of Np(T 1 ), indicating that the excitation energy of the T 1 state is localized on the Np moiety. Because of the higher C−O bond dissociation energy (285 kJ mol -1 ) than the excitation energy of Np(T 1 ) (254 kJ mol -1 ), no C−O bond cleavage occurred from ROCH 2 Np(T 1 ). , …”