Photophysics of a phenyl-pyridinium cation with hindered rotation

• Published in: Journal of photochemistry and photobiology. A, Chemistry. Chemistry., 2000-02, Vol.131 (1), p.17-21
• Main Authors: Kharlanov, V.A, Knyazhansky, M.I, Bicherov, A.V, Rettig, W
• Format: Article
• Language: English
• Subjects: Absorption; Fluorescence spectroscopies; Internal hindered rotation; Phenyl pyridinium cations; Semiempirical calculation
• ISSN: 1010-6030; 1873-2666
• DOI: 10.1016/S1010-6030(99)00218-X

The influence of the internal rotation on the photophysical parameters of the pyridinium cation N-methyl-9-phenyl-1,2,3,4,5,6,7,8-octahydroacridinium 9PA was studied by absorption and fluorescence spectroscopy as well as by semiempirical calculations. A weak long wavelength absorption shoulder at 340 nm appears in the red edge of longest wavelength absorption band and is caused by a forbidden transition of charge transfer nature involving the phenyl group as the electron donor. The excitation of 9PA at any point of the absorption spectrum leads to a weak (Φ f = 0.03) fluorescence at 420 nm possessing a relatively large Stokes shift with respect to the weak long wavelength absorption shoulder (Δ ν a-f = 5600 cm −1) and a long fluorescence lifetime (average decay time 5.4 ns). The fluorescence corresponds to a forbidden transition from the twisted S 1 state equilibrium geometry generated from the more twisted S 1-Frank-Codon geometry.