Fast Internal Conversion, Dual Fluorescence and Intramolecular Charge Transfer in 9-Cyano-10-(Dimethylamino)Anthracene#

Abstract

The deactivation of the singlet excited state of 9-cyano-10-(dimethylamino)anthracene (CDA) in a variety of solvents predominantly takes place by way of internal conversion (IC), with a yield of ~0.97 in n-propylcyanide, whereas intersystem crossing and intramolecular charge transfer (ICT) only play a minor role. The ICT reaction leads to dual fluorescence, from a locally excited (LE) and a charge transfer (CT) state. The CT/LE fluorescence quantum yield ratio Φ'(CT)/Φ(LE) becomes larger with increasing solvent polarity, from ~0.1 in cyclopentane to 3.7 in n-propylcyanide at -92 °C. The LE and CT fluorescence decays, with shortest decay times of several tens of picoseconds, are measured as a function of temperature and a growing-in of the CT fluorescence is observed at -110 °C in n-propylcyanide. The occurrence of IC is connected with the amino twist angle relative to the anthracene plane of CDA in the ground state. The S 2 fluorescence reported for CDA in the literature is attributed to the presence of a small amount of a highly fluorescing impurity.