Theoretical perspective for structural isomerism effect on photoelectric properties of organic room temperature phosphorescence molecules

Purely organic room temperature phosphorescence (RTP) materials have garnered extensive attentions in anti-counterfeiting and encryption, information display, biological imaging and organic light emitting diodes (OLEDs) due to the features of long lifetimes, low toxicity and good biocompatibility. Nevertheless, both the species and amounts of the efficient RTP molecules are far from meeting the requirements for practical applications, and the quantitative relationship between molecular structures and optoelectronic properties needs full elucidated. Herein, the photophysical properties and properties of three isomers (o-BA, m-BA and p-BA) are explored based on the first-principles calculations coupled with thermal vibration correlation function (TVCF) method and kinetic Monte Carlo simulations. The substituent effects on the molecular structures, intermolecular interactions, transition properties, Huang-Rhys factors and reorganization energies, excited state dynamics as well as transfer integrals and charge carrier mobilities are investigated in detail to clarify the structure-property relationships. Results show that o-BA achieves fast radiative rate on account of impressive transition dipole moment and oscillator strength, but equally large non-radiative decay rate leads to low RTP efficiency. Promisingly, the highest holes mobility is determined for o-BA due to the largest transfer integrals of holes originating from the strong intermolecular interactions. In addition, compared with o-BA and p-BA, m-BA could achieve fast intersystem crossing (ISC) rate with large spin orbit coupling constant and low non-radiative decay rate with small geometric structure changes. Relatively balanced holes and electrons mobilities of m-BA have been demonstrated, which contributes to the recombination of excitons and the improvement of luminescence efficiency. For p-BA, the strong intermolecular hydrogen bonding restricts the free rotation of single bonds and promotes the molecular coplanarity, which contributes to efficient emission. Furthermore, the charge mobility is calculated and the temperature dependence is investigated, the hopping mechanisms for studied isomers are illustrated. This work reveals the relationship among molecular structures, crystal packing modes, photophysical properties and charge transport properties of isomers, and provides an insight for molecular design and property prediction.