Irregular behaviour of fluorescence and laser properties of 9 and 9,10 derivatives of anthracene

Author(s): N.Nijegorodov, V.Vasilenko, P.Monowe, D.P.Winkoun, J.G.King

The absorption, fluorescence and laser properties of 9 specially selected 9,10-derivatives of anthracene are studied quantum-chemically and experimentally at room temperature (293 K). The quantum yield of fluorescence , fluorescence decay time  f, of dearated and non-dearated cyclohexane solutions are measured. The main laser parameters are determined. The oscillator strength fe of the S0  Sp transition, fluorescence rate constant kf, natural lifetime T 0  , and intersystem crossing rate constant kST, are found for each compound. The orbital nature of the lowest excited state singlet S1 state and direction of polarization of the S0  S1 transition are determined using the PPP-CI method. The investigation shows that the substitution of CH3 group(s), phenyl ring(s) or Cl and Br atom(s) into position “9” or into position “9” and “10” causes the inversion of the Sp and Tâ states. The inversion of these states suppresses the decisive Sp T channel of depopulation of Sp state, and improves the fluorescence and laser properties. It is found out that the kST of the decisive Sp T  B u 1 1    u B2 3 mixing process in anthracene is almost equal to 14.5  107 s-1, that is almost 96% of the whole kST value. It is shown that the internal heavy atom effect depends not only upon the molecular symmetry, but also upon the position of the Tâ level. It is found out that the compound 9,10-bis(phenyl-ethynyl)anthracene is not planar in the S0 state due to specific interactions of -electrons and that its laser ability, which was not studied before, is excellent, especially if XeCl laser is used for pumping.

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