Tuning ‘Stokes Shift’ and ICT Character by Varying the Donor Group in Imidazo[1,5 a]pyridines: A Combined Optical, DFT, TD‐DFT and NLO Approach

Abstract

AbstractImidazo[1, 5a]pyridine based novel donor‐acceptor dyes (2a‐e) with the large Stokes shift (ca. 140–200 nm) were synthesized and characterized. We report on the steady‐state and nanosecond time‐resolved emission studies of 2a‐e in different microenvironments which reveal that the dyes 2a‐c (average lifetime of 0.65, 1.36 and 1.63 ns respectively) perform reversible proton motion in chloroform due to variable donor group present at the para position of substituted phenyl group attached to imidazopyridine core as an acceptor group whereas 2d and 2e (average lifetime of 3.50 and 4.06 ns respectively) compete with 2a‐c in agreement with considerable intramolecular charge transfer (ICT) characteristics. Solvent polarity plots viz. Lippert‐Mataga, McRae, and Bakhshiev equation furnish the confirmation of ICT character. Density functional theory (DFT) calculations [(B3LYP/6‐311++G(d, p)] provide the information of structural as well as the electronic properties of dyes 2a‐e. Natural bond orbital (NBO) analyses enlighten the nature of the charge or proton transfer within the selected donor‐acceptor orbital interaction with the large energy of stabilization (the donor‐acceptor orbital interactions increases in the order 2a > 2b > 2c > 2d > 2e). The large difference in dipole moment (ca. 1.57‐21.55 D) results in a strong non‐linear optical (NLO) properties. The NLO properties were evaluated theoretically as well as spectroscopically in solvents of different polarities and the resulting hyperpolarizabilities fall within the expected limiting values. These results bring new findings in coupled ICT and proton transfer reactions which will be useful for further research and promising applications in sensing polarity or H‐bonding of microenvironments similar to that of the biological ones.