Tunable fluorescence of Imidazo[1,2‐a]pyridine derivatives with additional proton transfer sites Harnessing excited-state intramolecular double proton transfer: Theoretical insight

Abstract

Electronic properties and excited-state intramolecular proton transfers of a new designed series of doubly intramolecular hydrogen bonding molecules having imidazo[1,2-a]pyridines as proton acceptors with different types of proton donors (hydroxy-type, amino-type, and mixing-type) and their derivatives have been theoretically evaluated for fluorescent probes. Overall, all designed molecules are photoexcited at ∼420 nm and give fluorescence in the NIR region with large Stokes shifts. The fluorescence of NH- and Mixed-type molecules are notably longer than that of OH-type molecules, and those of the derivatives with a π-conjugated spacer are slightly longer than those of their analogues. Based on the kinetic and thermodynamic information, all designed molecules could not exhibit the double proton transfer (DPT) process but only single proton transfer (SPT) is preferred. Moreover, the dynamic simulations on the excited-state confirm that only SPT is allowed but not DPT in accordance with the predicted PT barriers and reaction energies, leading only NT* fluorescence but their NT* fluorescence maxima are still in the NIR region (600–900 nm) with large Stokes shifts (∼300 nm). The NIR fluorescence is caused by an additional PT unit and a π-conjugated spacer induce the ICT character evidenced by the EDD maps. Therefore, these designed molecules exhibiting NIR tautomer emission with large Stokes shifts could be potential candidates for NIR fluorescent probes with avoiding self-absorption and less photo-damage.