Ultrafast Electronic Energy Flow in a Bichromophoric Molecule

Abstract

The intramolecular electronic excitation energy flow was investigated in a specially designed bichromophoric molecule, 2-(3-benzanthronylamino)-4-(1-pyrenylamino)-6-chloro-1,3,5-triazine (1) and was compared with the behaviour of two relevant component model compounds that closely mimic the photophysical properties of acceptor and donor sub-units in the bichromophore. Electronic absorption and fluorescence spectroscopy was applied (including fluorescence anisotropy and decay kinetics measurements with nanosecond to femtosecond time resolution) in order to resolve the energy relaxation process on a real time. An unambiguous piece of evidence is reported for an ultrafast process which leads to practically instantaneous population of the emitting state of the acceptor sub-unit after selective ≈200-fs-excitation of the donor sub-unit. This first direct observation of extremely fast energy transfer in a stiff bichromophore is significant for further development of relevant theory. Two conceptually different approaches to explaining such fast energy flow are discussed.