Spectroscopic properties of aromatic dicarboximides Part 4: N-alkyl- and N-cycloalkyl-substituted 1,2-naphthalimides

Abstract

The photophysics of a series of N-alkyl- and N-cycloalkyl-substituted 1,2-naphthalimides has been investigated. Fluorescence spectra, fluorescence quantum yields and decay times as well as triplet yields are determined in a wide temperature range. The rate coefficients for fluorescence are independent of temperature. However, the rate of non-radiative processes shows characteristic temperature dependence, consisting of a temperature independent and a temperature-dependent component: k nr= k o nr+ k nr T = k o nr + A nr exp(− E nr/ RT), where nr designates either intersystem crossing or internal conversion. The temperature-independent component of internal conversion can be associated with a direct process in which the electronic energy is dissipated by a single mode, probably an aromatic vibrational mode. Vibrational coupling between the two lowest excited states is expected to occur for compounds with N-alkyl- (or N-cycloalkyl-) groups of high electron donating character, and is expected to increase as the solvent polarity decreases. This results in an efficient and temperature-dependent internal conversion to the ground state (pseudo-Jahn—Teller effect or proximity effect). The temperature-independent component of intersystem crossing may be identified with a barrierless transition from the lowest singlet to a lower-lying 4(ππ *) triplet state, while the thermally activated isc process is probably a transition to a higher 3( nπ *) triplet state.