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

Abstract

Laser photolysis studies at 355 nm have been carried out on the intramolecular hydrogen atom transfer (intra-HT) reaction of [(hydroxynaphthyl)propyl] benzophenone, BP-(CH&-ROH, and the proton-induced intramolecular ionization (intra-pion) reaction of [ (methoxynaphth yl)propyl]benzophenone, BP-(CH2)3-ROMe, in acetonitrilewater (4:l v/v) at 295 K. After 355-nm laser pulsing to BP-(CH&-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-( C H Z )~-~R O M~* on a nanosecond time scale. The former triplet molecule (BP-(CH2)3-3ROH*) undergoes intramolecular hydrogen atom transfer (intra-HT) with an efficiency (&T) of unity to yield a biradical molecule with benzophenone ketyl radical (>COH-) and naphthoxy radical (ROO) moieties (>C0H-(CH2)3-ROo). The rate constant (~H T ) for intra-HT was determined to be 9.8 X lo5 s-l and independent of [BP-(CH2)3-ROH]. The deuterium isotope effect on intra-HT was examined by using BP-(CH&-ROD. The rate constant (~D T ) for intra-DT was determined to be 5.1 X lo5 s-l. In the presence of protons, the rate constant of intra-HT is enhanced proportional to the acid concentration though the value of r#Jm (=1.0) is independent of the acid concentration. The triplet molecule (BP-(CH2)s-3ROMe*) undergoes ionization only in the presence of protons (intra-pion) to yield a biradical molecule with >COH-and methoxynaphthalene cation radical (-ROMe'+) moieties (>C0H-(CH2)3-ROMeo+) with efficiencies (&I) of 0.3 for [H2S04] = 0.005 M and 1.0 for [H2S04] = 0.5 M in acetonitrilewater (4:l v/v) at 295 K. The rate constant for intra-pion increases with an increaseof 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-pion 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.

Hydrogen-Bonding-Induced Electron Transfer from Triplet N,N-Dialkyl-1-naphthylamines to Benzophenone via Triplet Exciplexes

“…The values of φ TET for the BP-DMNA and -DENA systems in ACN were experimentally determined to be 0.74 ((0.04) and 0.61 ((0.03), respectively, with the use of eqs 2-4. [24][25][26][27][28][29][30][31] Analogously, the triplet exciplex formation between BP and 3 DMNA* (or 3 DENA*) was expected in the present system. It is often reported that the induced quenching of 3 BP* by naphthalene derivatives competes with triplet energy transfer.…”

“…[24][25][26][27][28][29][30][31]41 The rate constants for triplet energy transfer (k TET ) and induced-quenching (k IQ ) are expressed by eqs 5 and 6, respectively, since the quenching of 3 BP* consists of both triplet energy transfer and the induced-quenching processes. [24][25][26][27][28][29][30][31] Therefore, the mechanism for the triplet exciplex formation in the present system can be accounted for analogously by Scheme 1, [24][25][26][27][28][29][30][31] where k 0 and k ex are the rate constants for the decay processes of 3 DANA* and 3 (DANA‚‚‚ BP)* to the ground state (DANA abbreviated for dialkylnaphthylamine represents DMNA or DENA), respectively, k 1 and k -1 are the rate constants for the association and dissociation [ 3 BP*] max ) OD 520 / 520 The values of k TET and k IQ , which are close to the diffusion-controlled rate constant, indicate that both triplet energy transfer and the induced quenching in the present system in ACN occur in a collision process.…”

“…[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] However, little attention has been paid to the interactions of triplet-sensitized compounds with the ground state of tripletenergy donors until recently. [22][23][24][25][26][27][28][29][30][31][32] When naphthylammonium ion 22,25 and naphthol 24,26 were employed as triplet energy acceptors from triplet BP in acetonitrile (ACN) or a mixture of ACN and water (4:1 v/v) upon laser excitation at 355 nm, it was found by the production of the benzophenone ketyl radical and the corresponding naphthylamine cation and naphthoxy radicals that hydrogen atom transfer occurred from triplet naphthylammonium ion and naphthol to BP. [22][23][24][25][26][27][28][29][30][31][32] When naphthylammonium ion 22,25 and naphthol 24,26 were employed as triplet energy acceptors from triplet BP in acet...…”

“…In the case of naphthylammonium ion, the rate was suppressed by protons 25 but enhanced for naphthol. [24][25][26][27][28][29][30][31] It was shown that the triplet exciplexes had loose, sandwich-like structures 27 and weak charge-transfer character. 23, 28 The rate for electron transfer, in other words, ionization of methoxynaphthalene increased with an increase of the proton concentration.…”

“…26 On the other hand, in the BPmethoxynaphthalene system, electron transfer was found during the decay of triplet methoxynaphthalene, resulting in the production of the methoxynaphthalene cation and benzophenone ketyl radicals only in the presence of protons in ACN-H 2 O (4:1 v/v). [24][25][26][27][28][29][30][31] It can be generalized that the triplet aromatic compounds sensitized by BP would deactivate via triplet exciplexes. The reaction mechanism for the hydrogen atom and electron-transfer processes, including the proton effects, was explained in terms of triplet exciplexes between triplet naphthalene derivatives and the ground-state BP.…”