Kunihiro Tokumura

Doublet-doublet fluorescence of benzyl, p-methylbenzyl and p-chlorobenzyl radicals in solution

Abstract The doublet-doublet fluorescence spectra of benzyl, p -methylbenzyl and p -chlorobenzyl radicals were detected in the 450–650 nm region upon the double-pulse (248 nm→308 nm) excitation of corresponding benzyl chlorides in hexane at room temperature. The respective fluorescence lifetimes were determined to be ≈ 1 ns or less (benzyl), 14 ns ( p -methylbenzyl) and 81 ns ( p -chlorobenzyl). Extensively temperature-dependent non-radiative relaxation was confirmed for these benzyl radicals with close-lying lowest doublet excited states.

Time-resolved infrared study of ground-state phototautomer formed in the excited-state proton transfer of 7-hydroxyquinoline in methanol

Time-resolved infrared absorption spectra of 7-hydroxyquinoline (7-HQ) in methanol were measured to investigate the relaxation processes following S1 → S′1 proton transfer tautomerization. Deuterium and nitrogen isotope effects were observed for the transient infrared spectra of 7-HQ-14N and -15N in MeOD and MeOH. The 1644 (1628) cm−1 band in MeOH (MeOD) is ascribed to the H(D)-bonded C = O stretching of the phototautomer in the ground state (St0). Transient absorption decay exhibits a remarkable deuterium isotope effect. It is thus demonstrated that the ground-state reverse proton transfer of S′0 is responsible for the observed transient decay.

A transient absorption and two-step laser excitation fluorescence study of the double proton transfer of 7-azaindole H-bonded dimers in 3-methylpentane

Abstract For the 7-azaindole dirner (ND) in 3-methylpentane, the metastable ground-state dimeric tautomer generated through an excited- state double proton transfer and its subsequent relaxation was detected by transient absorption and two-step laser excitation fluorescence spectroscopies. The second laser excitation of the ground-state dimeric tautomer produces different fluorescence character from that of the one-step laser excitation of ND, suggesting the formation of a monomeric tautomer.

Phototautomerization of 3-hydroxyflavone in the lowest triplet state

Abstract Selective excitation of benzil in the presence of 3-hydroxyflavone (3HF) in fluid solution results in diffusional triplet energy transfer from benzil to 3HF, and the Tn←T1 absorption spectrum of 3HF (acceptor) was determined by transient absorption spectroscopy. It is demonstrated that red probe pulse excitation of the lowest triplet state (T1) of the normal tautomer yields green fluorescence from the phototautomer in the lowest excited singlet state (S′1). Generation of S′1 upon the excitation of T1 may be ascribed to an efficient Tn→T′n proton transfer followed by Tn′→T′2 relaxation and T′2→S′1 reverse intersystem crossing.

Para-substituent effect upon the oxygenation reaction of the benzyl radical in solution

Abstract The oxygenation rate constants ( k O 2 ) of the ground-state para-substituted benzyl radicals ( p -X-benzyls) were obtained by Stern-Volmer plots for the decay of the transient D n ←D 0 absorption in the presence of various concentrations of dissolved molecular oxygen. A remarkable substituent effect, a sharp drop in the k O 2 value in the order of p -chlorobenzyl, p -phenylbenzyl, p -bromobenzyl, p -nitrobenzyl and p -cyanobenzyl, was confirmed in contrast to almost the same k O 2 values of (3.0±0.4) × 10 9 M −1 s −1 for p -X-benzyls (X = H, CH 3 , OCH 3 and F).

Steady-state and nanosecond spectroscopic studies of triplet sensitized reaction of K-region arene oxides

Abstract Photorearrangement reactions of K-region arene oxides, 9,10-epoxy-9,10-dihydrophenanthrene ( 1a ), 3-acetyl-9,10-epoxy-9,10-dihydrophenanthrene ( 1b ), and 3,4-epoxy-3,4-dihydropyrene ( 1c ) in dichloroethane (DCE) solution were investigated by steady irradiation and nanosecond transient spectroscopy. Photorearrangements producing substituted oxepins, 2 occur via the singlet excited state of these compounds, while the phenolic products, 9-hydroxyphenanthrene ( 3a ), 3-acetyl-9-hydroxyphenanthrene ( 3b ), and 4-hydroxypyrene ( 3c ) are formed via the triplet state. Phenol 3 formation from the triplet 1 sensitized by the triplet 3 (i.e. product sensitization) is proposed for the photorearrangement reactions of 1a and 1c , and this process is the only way phenol ( 3a ) is formed because of the negligible intersystem crossing probability of 1a . No product sensitization occurs in the photorearrangement reaction of 1b .

Reverse intersystem crossing from higher triplet to excited singlet in 2,2′-bipyridine-3,3′-diol phototautomer

Abstract The S 1 ′→S 0 ′ fluorescence of 2,2′-bipyridine-3,3′-diol was detected on red probe pulse excitation of the tautomeric triplet (T 1 ′), which was generated via S 1 ′→S 1 ′ double proton transfer followed by S 1 ′→T 1 ′ intersystem crossing. The excitation spectrum of the two-step laser-induced fluorescence corresponds satisfactorily to the transient T n ′←T 1 ′ absorption spectrum in the 600–800 nm region. It is demonstrated that the T 2 ′(n,π*)→S 1 ′(π,π*) reverse intersystem crossing is responsible for the generation of S 1 ′ on excitation of T 1 ′. The probability of reverse intersystem crossing was estimated to be 0.0024.

Photodeoxygenation of transient p-methoxybenzyl-peroxy radical in solution at room temperature

Abstract Transient absorption and two-step laser-induced-fluorescence measurements confirmed that the transient p-methoxybenzyl-peroxy radical generated via the oxygenation of the p-methoxybenzyl radical releases molecular oxygen upon the 308 nm pulse excitation. In the air-saturated n-hexane and cyclohexane at room temperature, the respective lifetimes of the peroxy radical were determined to be 120 and 187 μs, which are sufficiently longer than the lifetime (110–130 ns) of the p-methoxybenzyl radical. It was assumed that the peroxy radical is subjected to H-abstraction from the solvent molecule in the dark.

Triplet-sensitized doublet-doublet fluorescence of p-methoxybenzyl radical in solution at room temperature

Abstract Unusually long-lived (500 ns) doublet-doublet fluorescence of p -methoxybenzyl radical was observed in the KrF-laser photolysis of p -methylanisole in hexane at room temperature, in spite of the intrinsic fluorescence lifetime (120 ns) of the radical. It was confirmed that the triplet-doublet electronic energy transfer from p -methylanisole to p -methoxybenzyl is responsible for the long-lived fluorescence. The rate constant of the energy transfer was determined to be 1.2×10 10 M −1 s −1 .

A possible oxaziridine intermediate in the photorearrangement reaction of 6-cyanophenanthridine 5-oxide

Abstract Nanosecond spectroscopy and low temperature matrix stabilization technique present evidence of a possible oxaziridine intermediate in the photorearrangement reaction of 6-cyanophenanthridine 5-oxide in ethanol solution to form 5-ethoxyphenanthridine.