| Abstract | The photophysical behavior of three coumarin dyes with heterocyclic substitution in 3-position and an electronwithdrawing
cyano group in 4-position, Scheme 1b, have been studied in solvents of different polarity. The
results are compared with those for rotation-inhibited coumarin 153 and coumarin 6, Scheme 1a. 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. |