Substituent control of the ultrafast twisted intramolecular charge transfer rate in dimethylaminochalcone derivatives.

The effect of acceptor strength variation on ultrafast twisting relaxation dynamics was investigated by comparing the ultrafast relaxation dynamics in a series of dimethylaminochalcone (DMAC) derivatives. Employing femtosecond resolved transient absorption and fluorescence experiments, the twisted intramolecular charge transfer (TICT) relaxation rate is shown to vary from 2.0 picoseconds in a weak electron accepting system to 420 femtoseconds in a strong electron accepting system. The strength of the acceptor, empirically expressed as Hammett's constant, is shown to exhibit a linear free energy relationship (LFER) with the twisting rate. It is proposed that variation in the charge pulling capacity of the acceptor modifies the torsional barrier along the TICT coordinate in the S state, resulting in a tunable TICT relaxation rate, depending on the acceptor strength.