Unveiling the theoretical mechanism of purely organic room temperature phosphorescence emission and heteroatomic effects on singlet-triplet intersystem crossing for isopropylthioxanthone derivatives

Abstract

ABSTRACT In this work, the density functional theory (DFT) and time-dependent density functional theory (TDDFT) method were performed to investigate the mechanism of purely organic room temperature phosphorescence emission and heteroatomic effect on the electronic structures and photochemistry for the isopropylthioxanthone (ITX) and its derivatives. We designed a series of compounds by replacing sulfur atom on the heterocyclic ring and oxygen atom on the double bond of ITX by the atoms from the same group, which were named as O-, Se-HC-ITX, and S-, Se-DB-ITX. The result shows that both different heteroatoms and different positions of heteroatoms have a prominent impact on the photochemical properties of ITX derivatives. With the change of heteroatoms and their positions, the molecular structures are distorted causing the change of electron densities distribution and energy levels of the frontier molecular orbitals. The energy gaps between lowest unoccupied molecular orbitals (LUMOs) and the highest occupied molecular orbitals (HOMOs) are red-shifted from 6.83 to 5.28 eV with the heteroatom changed from O to Se, which is relative to the energy-level configuration. As a result, the maximum absorption wavelengths of the UV absorption spectra are red-shifted from 218 to 259 nm, and the fluorescence and phosphorescence emission spectra can be tuned from the ultraviolet region to the visible region even to the infrared region with the heteroatom on the double bond changed from O to Se. Moreover, the change of heteroatoms also introduces into the heavy-atom effect which enhances the spin-orbital coupling (SOC) between singlet and triplet excited states dramatically and increases the probability of singlet-triplet intersystem crossing (ISC) happening which lead to purely organic room temperature phosphorescence (RTP). Hence, this work demonstrates that the change-heteroatom strategy can make the tunable photochemical properties realizable, which provides a new strategy for researches to design molecular systems.