Solvent Polarity Governs Ultrafast Structural Dynamics: A Case Study of 4-Dimethylamino-4?-carbomethoxydiphenylacetylene

The compound 4-dimethylamino-4 '-carbomethoxydiphenylacety-lene (DACM-DPA) is a donor-7r-acceptor system that exhibits significant charge transfer character in the excited state in polar solvents. Femtosecond transient absorption measurements in n-hexane, acetonitrile, and methanol exhibit excited-state lifetimes of >2 ns, -,500 ps, and -,30 ps, respectively. Femtosecond Raman measurements of DACM-DPA elucidated the intricate structural dynamics during the excited-state evolution by revealing distinct vibrational signatures of DACM-DPA in these solvents. In the case of n-hexane solvent, vibrational bands at 2075 cm-1 (CC) and 1600 cm-1 (C=C) stretch exist throughout the excited-state lifetime. However, in the case of polar solvents, the amplitudes of vibrational frequencies at 2098 cm-1 (CC), 1618 cm-1 (C=C, acetonitrile), and 1603 cm-1 (C=C, methanol) diminish in a time period of 2.5 ps (acetonitrile) and 6.5 ps (methanol) along with the appearance of new bands at 2082 cm-1 (cumulene stretch) and 1582 cm-1 (quinoid stretch), clearly revealing the formation of an intramolecular charge-separated state. Furthermore, these experimental observations are corroborated by computations using DFT and TD-DFT.