Probing impacts of π-conjugation and multiarm on the performance of two-dimensionally expanded small molecule hole-transporting materials: A theoretical investigation

Abstract Apart from the spiro-like molecule architecture, a large number of two-dimensionally expanded small molecule hole-transporting materials (HTMs) have been investigated to improve their electronic, optical and charge transport properties. Compared with the variety of synthesized molecules, the systemic studies on the effects of molecular geometry are still less reported. To further explore the useful clues for the rational design of two-dimensional small molecule materials, a series of new designed (4–8) and the model (1–3) HTMs are simulated by combining of the density functional theory and the Marcus theory of electron transfer. Our results show that, by adding oxygen-bridge in the core unit and additional electron-donor, the highest occupied molecular orbital (HOMO) levels of designed HTMs are slightly up-shifted, whereas introducing carbon-carbon single bond makes the HOMOs more stabilized due to the slight break of molecular π-conjugation. Meanwhile, by rigidifying and extending the π-conjugated core structure and adding of extra electron-donors, the reorganization energies are obviously lowered and the transfer integrals are increased. Accordingly, the hole mobilities of designed HTMs are distinctly enlarged. Moreover, our results also indicate that the weak light absorption in visible region, easy charge dissociation, and good solubility and stability can be expected for these designed molecules, and we hope that our results could be helpful for the design of high-efficient HTMs.