Intramolecular charge transfer of a push–pull chromophore with restricted internal rotation of an electron donor?

Physical Chemistry Chemical Physics Pub Date: 2022-02-14 DOI: 10.1039/D1CP05541K

Abstract

Intramolecular charge transfer (ICT) of 4-(dicyanomethylene)-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)vinyl]-4H-pyran (LD688) in DMSO solution was investigated by femtosecond stimulated Raman spectroscopy (FSRS) with 403 nm excitation. The molecular structure of LD688 is similar to that of a well-known push–pull chromophore, 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), except that the internal rotation of the electron-donating dimethylamino group is restricted with the introduction of the julolidine moiety. Upon photo-excitation, LD688 shows an ultrafast (1.0 ps) ICT followed by the vibrational relaxation (3–8 ps) in the charge-transfer (CT) state. Two distinct Raman spectra of LD688 in the locally excited (LE) and CT state of the S1 state were retrieved from FSRS measurements. Based on the time-dependent density functional theory (TDDFT) simulations, a “twisted” julolidine geometry of LD688 was proposed for the ICT state, which was further confirmed in comparison to the spectral changes of several push–pull chromophores with the π-conjugated backbone of stilbene, biphenyl, styrylpyran, styrylpyridinium, and styrene in terms of the skeletal vibrational modes of ν19b,py, νC[double bond, length as m-dash]C,ph, and νC[triple bond, length as m-dash]N.

Graphical abstract: Intramolecular charge transfer of a push–pull chromophore with restricted internal rotation of an electron donor
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