Michiya Itoh

Magnetic field effects on the fluorescence of intramolecular electron-donor-acceptor systems

Abstract The external magnetic field effects on the exciplex fluorescence of α-(4-dimethylaminophenyl)-ω-(9-phenanthryl)alkanes have been studied by photostationary, time-resolved, and magnetic field modulation fluorescence spectroscopy. The singlet-triplet degeneracy of the radical ion pair is suggested to occur at a methylene chain length of about ten.

Magnetic field effects on the intramolecular hydrogen abstraction reaction of polymethylene-linked systems

The magnetic field effect on the intramolecular hydrogen abstraction of α-(xanthone-2-carbonyloxy)-ω-(xanthene-2-carbonyloxy) alkanes (XO-n-XH,n = 2–12 ), in bichromophoric compounds, was studied using laser flash photolysis. Excited triplet XO abstracts a hydrogen atom from the XH moiety, generating a biradical of xanthenyl and xanthone ketyl radicals. The magnitude of the magnetic field effect on the lifetime of the xanthenyl radical increases with increasing chain length n and reaches a constant value at n ≈ 6.

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.

Nanosecond laser photolysis study of the magnetic field effect on p-xylosemiquinone radical in sodium dodecylsulfate micelle

Abstract The external magnetic field effect ( p -xyloquinone was studied in sodium dodecyl-sulfate micelle. The decays of a transient (λ max 415 nm) due to p -xylosemiquinone radical in the micelle consisted of two components (τ = 130 ns and ≈100μs). for which the intensity ratio decreased ≈40% at ≈400–950 G.

Intermolecular dynamics and exciplex formation of the 1‐cyanonaphthalene/triethylamine van der Waals complex

The exciplex formation dynamics of the 1‐cyanonaphthalene/triethylamine van der Waals complex upon excitation has been examined under collision free conditions. Mode‐specificity has been found in the excitation of a combination band between intra‐ and intermolecular vibrational modes in the S1 state of the vdW complex to promote the exciplex formation reaction. The specificity is lost at higher excess energies since randomization of energy among intermolecular modes is more extensive than the reaction rate. The fluorescence decays of the S1 and exciplex states have been measured as a function of excess vibrational energy above the S1 origin. The lifetime of the S1 state at the excess energy where the mode‐specific reaction is observed is different from that of the exciplex fluorescence. As the excess energy increases, the decay rate of the S1 state shows a sharp increase due to the exciplex formation while the exciplex fluorescence rate remains constant. The observed decay rate constants have been discuss...

Intermolecular exciplex induced by the formation of 1-cyanonaphthalene/triethylamine van der Waals complex in a molecular beam

Abstract We report on the first observation of the exciplex fluorescence via van der Waals complex formation of 1-cyanonaphthalene and triethylamine in a supers

Intramolecular excited‐state proton transfer in jet‐cooled 2‐substituted 3‐hydroxychromones and their water clusters

Intramolecular excited‐state proton transfer (ESPT) of 3‐hydroxychromone (3‐HC), 3‐hydroxyflavone (3‐HF) and 2‐(2‐naphthyl)‐3‐hydroxychromone (2‐NHC), and of their water clusters, was investigated in a supersonic expansion. The visible tautomer fluorescence excitation spectra of these compounds exhibit considerably well‐resolved vibrational structures, while no significant uv fluorescence excitation spectrum due to the normal form was observed. The upper limit of the rate constant of the tautomer formation was estimated to be 1.77×1012 s−1 for 3‐HC, 6.5×1011 s−1 for 3‐HF, and 1.54×1011 s−1 for 2‐NHC in supersonic expansion by simulation of the linewidth of the respective origin bands in the visible fluorescence excitation spectra. The order of rate constants of the tautomer formations in the supersonic free jet are consistent with that of the ESPT of these compounds in nonpolar solution. The fluorescence excitation and dispersed fluorescence spectra demonstrate 1:1 and 1:2 water complex formations of 3‐HF...

Photochemistry of 1‐cyanonaphthalene/triethylamine van der Waals complex: Intermolecular exciplex formation under collision free conditions

The van der Waals complex of 1‐cyanonaphthalene with triethylamine exhibits fluorescence from the intermolecular exciplex upon electronic excitation. The efficiency of exciplex formation is strongly excitation energy dependent. The excitation of the complex into the origin band shows resonant emission, while excitations into vibronic bands with excess vibrational energies >400 cm−1 lead to the exciplex formation. The exciplex formation efficiency is also dependent on isotopic substitution in 1‐cyanonaphthalene. The barrier to the exciplex formation is larger for the deuterated complex. The fluorescence excitation spectrum of the exciplex reveals that the exciplex formation process is favored over other intramolecular relaxation processes over the excess energy range 700–1500 cm−1. These results suggest that van der Waals complexes can be important intermediates in the intermolecular exciplex formation in the vapor phase.

Tryptophan fluorescence of human hemoglobin. I. Significant change of fluorescence intensity and lifetimes in the T − R transition

Abstract The fluorescence spectra and fluorescence lifetimes due to tryptophan residues in HbA, Hb Chesapeake, NES-des-Arg Hb and Hb Kempsey were determined at room temperature. The fluorescence intensity and apparent fluorescence lifetimes decrease when the deoxy or T structure in HbA changes to the oxy or R structure, while no significant difference was observed in Hb Kempsey. The difference of fluorescence behavior was ascribed to the quaternary conformational transition of T- and R-states.

CIDEP studies on the intramolecular hydrogen abstraction reaction of polymethylene linked system. Population relaxation between the ST0 mixed states

Abstract An anomalous intensity pattern in the biradical CIDEP spectrum is observed in an intramolecular hydrogen abstraction reaction system. The intensity pattern is successfully interpreted by the biradical CIDEP theory proposed by Buckley et al. (1987) and Closs et al. (1987), assuming fast population relaxation between the central two ST0 mixed states.