Effect of Electron-Donating Substituents and an Electric Field on the Δ<i>E</i>_ST of Selected Imidazopyridine Derivatives: A DFT Study

A series of thermally activated delayed fluorescent (TADF) molecules having an imidazopyridine acceptor, a benzene linker, and a 9,9-dimethyl-9,10-dihydroacridine donor are designed and examined using a quantum chemical approach. The above framework spatially separates the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), minimizing their overlap, ultimately resulting in a reduced energy gap between the excited singlet and triplet states (Δ<i>E</i>_ST). The impact of electron-donating substituents (-Me, -Et, -<i>t</i>-Bu, -OMe, and -NMe₂) on the donor moiety of the parent molecule 2-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)imidazo[1,2-<i>a</i>]pyridine-3,6-dicarbonitrile (Ac-CNImPy) is investigated. The calculated results revealed that for a given substituent, the para-substituted derivatives exhibit relatively less Δ<i>E</i>_ST, compared to that of the respective ortho- and meta derivatives. The value of Δ<i>E</i>_ST decreased with an increase in the electron-donating capacity of the substituent. Additionally, the Δ<i>E</i>_ST of the disubstituted derivatives is found to be less than that for the monosubstituted derivatives. The charge transport studies revealed that molecules with strong electron-donating substituents act as electron transporters. The effect of an external electric field (EEF) on Δ<i>E</i>_ST of the parent molecule Ac-CNIMPY and its derivative is also examined and revealed that the Δ<i>E</i>_ST can be further reduced by applying an electric field of appropriate strength in a direction perpendicular to the dipole moment of the molecule and in the plane of the acceptor moiety.