SPECTROSCOPIC PROPERTIES OF AROMATIC DICARBOXIMIDES .4. ON THE MODIFICATION OF THE FLUORESCENCE AND INTERSYSTEM CROSSING PROCESSES OF MOLECULES BY ELECTRON-DONATING METHOXY GROUPS AT DIFFERENT POSITIONS - THE CASE OF 1,8-NAPHTHALIMIDES

N-Phenyl-1,8-naphthalimide shows almost no fluorescence emission and a low (ca 0.1) intersystem crossing quantum yield. Its photophysical pr operties can be drastically varied by methoxy substitution at differen t locations of the molecule: (i) when it is at the para position of th e N-phenyl ring, the fluorescence remains weak but is red-shifted by 7 0 to 200 nm and, at the same time, the intersystem crossing quantum yi eld increases by a factor of five; (ii) when the methoxy group is at t he 4-position on the naphthalene ring, the fluorescence quantum yield increases to 0.87 in acetonitrile with a concomitant increase of the s inglet excited state lifetime to 9.0 ns and a consequent drop of the i ntersystem crossing yield. The effect is less pronounced in hexane sol ution. However, methoxy substitution at both positions increases the i ntersystem crossing quantum yield and induces dual luminescence, which is solvent dependent. The triplet excited state has been characterize d by phosphorescence emission and by triplet-triplet absorption. The e nergy of the lowest triplet excited state appears to be independent of the substitution, whether on the dicarboximide nitrogen atom or at th e 4-position of the naphthalene ring. All the spectroscopic changes ob served with substitution have been tentatively rationalized from the m odification of the energy levels: (i) of the close-lying S-1 and S-2 s tates (having pi,pi and charge-transfer character, respectively) and (ii) of an upper n,pi triplet state.