New aspects of the photophysical properties of 1'H-2',3'-dimethoxycarbonylspiro[fluoren-9,1'-pyrrolo[2,1-a]quinoline]

In this paper we describe photophysical and mechanistic studies of spiro[fluorene-9,1'-pyrrolo[2,1-a]quinolines] (DHI) 1-4 and of their styryl homologues 5-11 (styryl DHI). DHIs 1-11 have low fluorescence quantum yields (approximately Phi(f) = 10(-3)). The position of the fluorescence emission band is temperature-dependent. The ring-opened betaines 1'-11' do not fluoresce at all. DHIs 1-11 exhibit phosphorescence, which is hypsochromically shifted by NO2 substitution of the fluorene ring. The triplet can be populated only by sensitization (with benzophenone as sensitizer) and, therefore, the reaction that occurs under direct excitation of the molecule involves the singlet excited state only. On irradiation under time-resolved conditions, two transients were seen to be formed in the picosecond and microsecond domains. Slightly tilted educt or product-like geometries are attributed to these transients. An energy diagram is proposed for the photoreaction of the model compound spiro[pyrroloquinoline] which takes into account the singlet pathway and the different transients of ca. 100 ps lifetimes formed from the singlet excited state of 1-11.