© Copyright 1999 by the American Chemical Society
VOLUME 103, NUMBER 40, OCTOBER 7, 1999
LETTERS Laser-Induced Fluorescence of 1,3,5-Trimethoxybenzene Radical Cation in Solution at Room Temperature Nobuyuki Ichinose,*,†,‡ Tomoko Tanaka,†,§ Shunichi Kawanishi,† Tomoe Suzuki,| and Kazunaka Endo| Kansai Research Establishment, Japan Atomic Energy Research Institute, 25-1 Mii-minamimachi, Neyagawa, Osaka 572-0019, Japan and Department of Chemistry, Faculty of Science, Kanazawa UniVersity, Kakumacho, Kanazawa, Ishikawa 920-1192, Japan ReceiVed: May 12, 1999; In Final Form: August 11, 1999
Laser excitation at 532 nm of 1,3,5-trimethoxybenzene radical cation (1•+), which is generated by photoinduced electron transfer in aerated acetonitrile containing 1.4-dicyanonaphthalene and biphenyl (BP) as a sensitizercosensitizer system by the use of XeCl laser (308 nm), gave fluorescence around 620 nm. The assignment to that from the excited state of 1•+ (1•+*) is based on the agreement of the rates for the formation of 1•+ obtained by laser-induced fluorescence and absorption spectroscopies. The fluorescence quantum yield for 1•+ is ca. 2 × 10-3. There was no observable fluorescence from other di- and trimethoxybenzenes. It was revealed by molecular orbital and configuration interaction calculations that the fluorescence corresponds to the D2-D0 transition of 1•+. The high quantum yield of 1•+ would be attributable to the large energy separation between the emissive D2 and a non-emissive D3 states of 1•+ preventing their mixing which leads to the non-radiative decay.
The photochemistry of chemical intermediates such as excited radicals or higher excited states has attracted much interest from physical and synthetic points of view.1 Radical ions have been of interest because their excitation affects their reactivity through the enhancement of redox properties or weakening of chemical bonding.2-4 However, the physical properties of radical ions, such as fluorescence, have been investigated less because of the experimental requirement of a pair of pulsed lasers or a pulsed accelerator and a laser with a temporal control for the * Corresponding author. E-mail:
[email protected]. † Japan Atomic Energy Research Institute. ‡ Present address. The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. § Present address. Advanced Telecommunications Research Institute International, Adaptive Communications Research Laboratories, 2-2 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0288, Japan. | Kanazawa University.
generation of radical ions and their excitation. Recent studies on excited radical ions in solution have revealed short lifetimes (
