SPECTROSCOPIC PROPERTIES OF AROMATIC DICARBOXIMIDES - PART 4 - N-ALKYL-SUBSTITUTED AND N-CYCLOALKYL-SUBSTITUTED 1,2-NAPHTHALIMIDES

The photophysics of a series of N-alkyl- and N-cycloalkyl-substituted 1,2-naphthalimides has been investigated. Fluorescence spectra, fluore scence quantum yields and decay times as well as triplet yields are de termined in a wide temperature range. The rate coefficients for fluore scence are independent of temperature. However, the rate of non-radiat ive processes shows characteristic temperature dependence, consisting of a temperature independent and a temperature-dependent component: k( nr) = k(nr)(o) + k(nr)(T) = k(nr)(o) + A(nr) exp( -E-nr/RT), where nr designates either intersystem crossing or internal conversion. The tem perature-independent component of internal conversion can be associate d with a direct process in which the electronic energy is dissipated b y a single mode, probably an aromatic vibrational mode. Vibrational co upling between the two lowest excited states is expected to occur for compounds with N-alkyl- (or N-cycloalkyl-) groups of high electron don ating character, and is expected to increase as the solvent polarity d ecreases. This results in an efficient and temperature-dependent inter nal conversion to the ground state (pseudo-Jahn-Teller effect or proxi mity effect). The temperature-independent component of intersystem cro ssing may be identified with a barrierless transition from the lowest singlet to a lower-lying(3) (pi pi) triplet state, while the thermall y activated isc process is probably a transition to a higher(3) (n pi ) triplet state. (C) 1998 Elsevier Science S.A.