Molecules exhibiting nonlinear optical (NLO) properties have received significant attention because of their possible applications as NLO devices in the fields of spectroscopy, telecommunications, optical data storage and processing, ingredients in UVB sunscreen etc. In general, second-order NLO-active chromophores consist of strong electron donating(D) and electron-withdrawing (A) moieties capping a π-conjugated spacer. Such kinds of D -π˗bridge -A combinations are commonly referred to as “push-pull” systems. Moreover, a push-pull molecules were assessed for application in p-type dye sensitized solar cells, where power conversion efficiency was observed to be higher [1]. A combined application of femtosecond transient absorption and CWUV-Vis spectroscopy has been used to probe directly the dynamics, nature, formation and decay paths of the excited states of derivative of 1,5-bis(trifluoromethyl)-3-[4-dimethylaminophenyl]methylene-2,4-pentanedione, in solvent with different viscosity. The absorption spectra taken for both solutions reveal the presence of two main electronic transitions attributed to the locally excited state (LE) and at the twisted intramolecular charge transfer (TICT) state, respectively [2]. The intensity ratio between these two bands changes drastically going from a non-viscose solution to a viscose one. Photoluminescence spectra have been also taken and reveal that in a viscous solution both states can emit in an efficient way. Furthermore, by means of femtosecond pump-probe spectroscopy we were able to temporally resolve the formation of the TICT state after pumping the LE state. Transient absorption spectra show the instantaneous creation of a stimulated emission band that within few hundred femtoseconds undergoes change and a new photoinduced absorption band appears. This new band is assigned to the twisted intramolecular charge transfer (TICT) state. We temporally resolve the complete formation of this band following the blue shift associated to the twisting of the –aryl group around its own axis group, which takes place on time scale of few ps, dependent on solvent viscosity.
1. J. Warnan et al. Dyes and Pigments 105 (2014) 174-179
2. C. Chan et al.Phys. Chem. Chem. Phys.13 (2011) 16306-16313