Excited-State Intramolecular Charge-Transfer Dynamics in 4-Dimethylamino-4′-cyanodiphenylacetylene: An Ultrafast Raman Loss Spectroscopic Perspective

Photo-initiated intramolecular charge transfer (ICT) processes play a pivotal role in the excited state reaction dynamics in donor-bridge-acceptor systems. The efficacy of such a process can be improved by modifying the extent of pi-conjugation, relative orientation/twists of the donor/acceptor entities and polarity of the environment. Herein, 4-dimethylamino-4'-cyanodiphenylacetylene (DACN-DPA), a typical donor-pi-bridge-acceptor system, was chosen to unravel the role of various internal coordinates that govern the extent of photo-initiated ICT dynamics. Transient absorption (TA) spectra of DACN-DPA in n-hexane exhibit a lifetime of > 2 ns indicating the formation of a triplet state while, in acetonitrile, a short time-constant of similar to 2 ps indicates the formation of charge transferred species. Ultrafast Raman loss spectroscopy (URLS) measurements show distinct temporal and spectral dynamics of Raman bands associated with C C and C=C stretching vibrations. The appearance of a new band at similar to 1492 cm(-1) in acetonitrile clearly indicates structural modification during the ultrafast ICT process. Furthermore, these observations are supported by TD-DFT computations.