Effect of Electron-Donating Substituents and an Electric\nField on the Δ<i>E</i><sub>ST</sub> of Selected Imidazopyridine\nDerivatives: A DFT Study

A series of thermally activated delayed\nfluorescent (TADF) molecules\nhaving an imidazopyridine acceptor, a benzene linker, and a 9,9-dimethyl-9,10-dihydroacridine\ndonor are designed and examined using a quantum chemical approach.\nThe above framework spatially separates the highest occupied molecular\norbital (HOMO) and lowest unoccupied molecular orbital (LUMO), minimizing\ntheir overlap, ultimately resulting in a reduced energy gap between\nthe excited singlet and triplet states (Δ<i>E</i><sub>ST</sub>). The impact of electron-donating substituents (-Me, -Et,\n-<i>t</i>-Bu, -OMe, and -NMe<sub>2</sub>) on the donor moiety\nof 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.\nThe calculated results revealed that for a given substituent, the\npara-substituted derivatives exhibit relatively less Δ<i>E</i><sub>ST</sub>, compared to that of the respective ortho-\nand meta derivatives. The value of Δ<i>E</i><sub>ST</sub> decreased with an increase in the electron-donating capacity of\nthe substituent. Additionally, the Δ<i>E</i><sub>ST</sub> of the disubstituted derivatives is found to be less than that for\nthe monosubstituted derivatives. The charge transport studies revealed\nthat molecules with strong electron-donating substituents act as electron\ntransporters. The effect of an external electric field (EEF) on Δ<i>E</i><sub>ST</sub> of the parent molecule Ac-CNIMPY and its\nderivative is also examined and revealed that the Δ<i>E</i><sub>ST</sub> can be further reduced by applying an electric field\nof appropriate strength in a direction perpendicular to the dipole\nmoment of the molecule and in the plane of the acceptor moiety.