Synthesis and emission characteristics of some fluorescent derivatives of 2-(2-quinolyl)thiophen

An examination of solvent effects on the fluorescence characteristics of a series of 2-(2-quinolyl)thiophen derivatives has shown that these compounds normally fluoresce from an excited state which is mainly π–π* in character. Under acid conditions, the fluorescence quantum yield of 5-(2-quinolyl)-2,2′-bithienyl is anomalously decreased; this effect has been ascribed to a large decrease in the energy difference between the triplet n–π* and singlet π–π* levels resulting in an increased degree of intersystem crossing. The other 2-(2-quinolyl)thiophen derivatives show enhanced fluorescence in acid solution owing to elimination of n–π* character from the excited singlet state or, in the case of two formyl derivatives, possibly because of an inversion of excited energy levels to allow fluorescence from a singlet charge-transfer level.