Effect of Donor–Acceptor Coupling on TICT Dynamicsin the Excited States of Two Dimethylamine Substituted Chalcones.

Significanteffect of coupling between the electron donor and acceptorgroups in intramolecular charge transfer (ICT) dynamics has been demonstratedby comparing the photophysical properties of two isomeric N,N-dimethylaminochalcone derivatives (namely,DMAC-A and DMAC-B). In the case of the DMAC-B molecule, the distancebetween the donor (N,N-dimethylanilineor DMA) and the acceptor (carbonyl) groups is larger by one ethyleneunit as compared to that in the case of DMAC-A. The excited singlet(S1) states of both the isomers have strong ICT characterbut their photophysical properties are remarkably different. In polarsolvents, fluorescence quantum yields (and the lifetimes of the S1state) of DMAC-A are more than 2 orders of magnitude lower(and shorter) than those of DMAC-B. Remarkable differences in thephotophysical properties of these two isomers arise due to occurrenceof the ultrafast twisting of the DMA group (or the TICT process) duringthe course of deactivation of the S1state of the DMAC-Amolecule, but not in the case of DMAC-B. In the later case, becauseof the presence of a large energy barrier along the twisting coordinate(s),TICT is not a feasible process, and hence, the S1stateof DMAC-B has the planar ICT structure. In the DMAC-A molecule, thestrength of coupling between the donor and acceptor groups is relativelystronger because of a shorter distance between these groups. Femtosecondtransient absorption spectroscopic measurements and DFT/TDDFT calculationshave been adopted to establish the above aspects of the relaxationdynamics of the S1states of these two isomeric chalcones. [ABSTRACT FROM AUTHOR]