Multiple emission in coumarins with heterocyclic substituents

The photophysical behavior of three coumarin dyes with heterocyclic substitution in 3-position and an electron-withdrawing cyano group in 4-position, Scheme lb, have been studied in solvents of different polarity. The results are compared with those for rotation-inhibited coumarin 153 and coumarin 6, Scheme Ia. It is concluded that the dyes undergo an excited-state equilibrium reaction from an emissive intramolecular charge transfer (ICT) state to a twisted intramolecular charge transfer (TICT) state with forbidden emission in strongly polar solvents. In coumarins I and II this reaction acts as nonradiative decay involving the rotation about the bond joining the coumarin and the heterocyclic substituent in the 3-position because the TICT state reached is nonemissive. In contrast, coumarin III shows emission from two species with the longer wavelength emitting species being preferred in polar solvents. This is characteristic for an emissive TICT state. The donor strength of the heterocyclic substituent in 3-position plays a major role in increasing the CT character of coumarins I and II when compared to C153. Coumarin III possesses significantly larger Stokes shifts and a correspondingly higher excited state dipole moment than the comparable coumarin 6 lacking the electron-withdrawing cyano group in 4-position.