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

Abstract

The compound 4-dimethylamino-4′-carbomethoxydiphenylacetylene (DACM-DPA) is a donor-π-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 excitedstate 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 (C�C) 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 (C�C), 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.

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