Picosecond and Nanosecond Fluorescence Decays of 4-(Dimethylamino)phenylacetylene in Comparison with Those of 4-(Dimethylamino)benzonitrile. No Evidence for Intramolecular Charge Transfer and a Nonfluorescing Intramolecular Charge-Transfer State

Abstract

With 4-(dimethylamino)phenylacetylene (DACET) in p-dioxane/water mixtures, efficient fluorescence quenching of the locally excited (LE) state occurs, becoming more efficient with increasing water content. This quenching does not lead to the appearance of an emission from an intramolecular charge-transfer (ICT) state, in contrast to what is found with 4-(dimethylamino)benzonitrile (DMABN). With DMABN in p-dioxane and its mixtures with water, picosecond LE and ICT fluorescence decays are observed, with a shortest decay time down to 3 ps for p-dioxane/water (50/50). A similar quenching process takes place with the planarized 1-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMCQ), for which molecule ICT has not been found. It is concluded that there is no experimental evidence or photophysical argument that ICT occurs with DACET in p-dioxane/water mixtures or other polar solvents, contrary to what was recently reported on the basis of nanosecond single-photon-counting (SPC) experiments (N. Chattopadhyay et al. J. Phys. Chem. A 2001, 105, 10025). A comparison between the results of picosecond and nanosecond SPC measurements with DMABN, DACET, and NMCQ is made.