N-Phenyl-1,8-naphthalimide shows almost no fluorescence emission and a low (ca 0.1) intersystem crossing quantum yield. Its photophysical properties can be drastically varied by methoxy substitution at different locations of the molecule: (i) when it is at the para position of the N-phenyl ring, the fluorescence remains weak but is red-shifted by 70 to 200 nm and, at the same time, the intersystem crossing quantum yield increases by a factor of five; (ii) when the methoxy group is at the 4-position on the naphthalene ring, the fluorescence quantum yield increases to 0.87 in acetonitrile with a concomitant increase of the singlet excited state lifetime to 9.0 ns and a consequent drop of the intersystem crossing yield. The effect is less pronounced in hexane solution. However, methoxy substitution at both positions increases the intersystem crossing quantum yield and induces dual luminescence, which is solvent dependent. The triplet excited state has been characterized by phosphorescence emission and by triplet-triplet absorption. The energy of the lowest triplet excited state appears to be independent of the substitution, whether on the dicarboximide nitrogen atom or at the 4-position of the naphthalene ring. All the spectroscopic changes observed with substitution have been tentatively rationalized from the modification of the energy levels: (i) of the close-lying S1 and S2 states (having π,π* and charge-transfer character, respectively) and (ii) of an upper n,π* triplet state.
|Number of pages||10|
|Journal||New Journal of Chemistry|
|Publication status||Published - Jan 1 1996|
ASJC Scopus subject areas
- Materials Chemistry