Discerning the multi-color fluorescence in donor-π-acceptor molecules by femtosecond transient absorption spectroscopy

Time-resolved femtosecond transient absorption (fs-TA) spectroscopy was used to measure the multi-color fluorescence of two donor-π-acceptor (D-π-A) models based on (2-(4-vinylbenzylidene)malononitrile (VBM) derivatives. 2-[[4-[2-(2-benzothiazolyl)ethenyl]phenyl]methylene]-Propanedinitrile (α-VBM) in DMSO presents a sequential red-shift fluorescence (80 nm) changing from 460 nm (blue) to 540 nm (green) within 669.2 fs, corresponding the twisted→planar conformational torsion. The delay prolongs to 1.406 ps and 2.26 ps respectively with glycerin fractions at 30% and 50% in the glycerin/DMSO mixed solutions. Theoretical emission spectra attribute the shifts to a rotation of the hole-electron acceptor axis of A-Ph-A-C(CN)2 during charge transfer, and represents the characteristics of a twist in orientation of the more distal acceptor A-C(CN)2 (along t3 in Fig. 1) in preference to the rotation of the two more proximal D-π-A axes (along t1 and t2 in Fig. 1). This interpretation is supported by calculations showing t3's favorable energy barrier and lower Laplacian bond orders (t3 (1.477) < t1 (1.536) < t2 (1.549)). The exchange of the nitrile groups by the weaker electron deficiency of the aldehyde group in 4-[2-(2-benzothiazolyl)ethenyl]-benzaldehyde (collectively β-VBM) produces a smaller fluorescence emission shift (30 nm) compared to VBM derivatives. In contrast to previously reported D-π-A rotation axes, this work stresses the major contribution of the more distal A-Ph-Asubstituent on multi-color fluorescence.