LASER FLASH-PHOTOLYSIS STUDIES ON INTRAMOLECULAR HYDROGEN-ATOM TRANSFER OF [(HYDROXYNAPHTHYL)PROPYL]BENZOPHENONE AND INTRAMOLECULAR PROTON-INDUCED ELECTRON-TRANSFER OF [(METHOXYNAPHTHYL)PROPYL]BENZOPHENONE VIA TRIPLET EXCIPLEXES

Laser photolysis studies at 355 nm have been carried out on the intramolecular hydrogen atom transfer (intra-HT) reaction of [(hydroxynaphthyl)propyl]benzophenone, BP-(CH2)3-ROH, and the proton-induced intramolecular ionization (intra-p-ion) reaction of [(methoxynaphthyl)propyl]benzophenone, BP-(CH2)3-ROMe, in acetonitrile-water (4:1 v/v) at 295 K. After 355-nm laser pulsing to BP-(CH2)3-ROH and BP-(CH2)3-ROMe, the intramolecular triplet energy transfer (intra-ET) reactions from the triplet moiety (3BP*) to the naphthol (ROH) and methoxynaphthalene (ROMe) moieties occur to yield BP-(CH2)3-3ROH* and BP-(CH2)3-3ROMe* on a nanosecond time scale. The former triplet molecule (BP-(CH2)3-3ROH*) undergoes intramolecular hydrogen atom transfer (intra-HT) with an efficiency (phi(HT)) of unity to yield a biradical molecule with benzophenone ketyl radical (>COH-) and naphthoxy radical (RO') moieties (>COH-(CH2)3-RO.). The rate constant (k(HT)) for intra-HT was determined to be 9.8 x 105 s-1 and independent of [BP-(CH2)3-ROH]. The deuterium isotope effect on intra-HT was examined by using BP-(CH2)3-ROD. The rate constant (k(DT)) for intra-DT was determined to be 5.1 X 10(5) S-1. In the presence of protons, the rate constant of intra-HT is enhanced proportional to the acid concentration though the value of phi(HT) (= 1.0) is independent of the acid concentration. The triplet molecule (BP-(CH2)3-3ROMe*) undergoes ionization only in the presence of protons (intra-p-ion) to yield a biradical molecule with >COH- and methoxynaphthalene cation radical (-ROMe.+) moieties (>COH-(CH2)3-ROMe.+) with efficiencies (phi(el)) of 0.3 for [H2SO4] = 0.005 M and 1.0 for [H2SO4] = 0.5 M in acetonitrile-water (4:1 v/v) at 295 K. The rate constant for intra-p-ion increases with an increase of the acid concentration. The mechanisms for intra-HT and intra-p-ion were interpreted by considering the intramolecular triplet exciplexes, 3(>CO-(CH2)3-ROH)* and 3(>CO-(CH2)3-ROMe)*, and the intramolecular protonated triplet exciplexes, 3(>+COH-(CH2)3-ROH)* and 3(>+COH-(CH2)3-ROMe)*, respectively. On the basis of the mechanism for intra-HT and intra-p-ion of the two chromophores linked with a propyrene chain, the structures of the triplet exciplex and protonated triplet exciplex are suggested to be sandwichlike with weak charge-transfer character.