Intramolecular Charge Transfer of p-(Dimethylamino)benzethyne: A Case of Nonfluorescent ICT State

Abstract

The effect of solvent polarity and external heavy atom on the steady state and time-resolved fluorescence properties of the locally excited (LE) state of p-(dimethylamino)benzethyne (DMABE) has been studied. It has been observed that in less polar environments, intersystem crossing (ISC) is the main nonradiative channel of deactivation for the excited species. However, at higher solvent polarity (ET(30) ≥ 44) the activation barrier dependent intramolecular charge transfer (ICT) reaction is the main nonradiative process dominating over ISC. The results reveal that DMABE undergoes an ICT reaction in polar environments to reach a nonfluorescent intramolecular charge transfer (ICT) state. The efficiency of formation of the ICT state of DMABE is compared with that of p-(dimethylamino)benzonitrile (DMABN). The present work settles the controversy raised by two previous works with conflicting results: a theoretical one, predicting that DMABE will undergo ICT reaction even under isolated conditions (Sobolewski, A. L.; Domcke, W. Chem. Phys. Lett. 1996, 259, 119), and an experimental one, excluding the possibility of the ICT process for the same molecular system (Zachariasse, K. A.; Grobys, M.; Tauer, E. Chem. Phys. Lett. 1997, 274, 382).