The substituent effect on the photophysical and charge transport properties of non-planar dibenzo[a,m]rubicenes

The photophysical and charge transport properties of five non-planar cyclopenta-fused polycyclic aromatic hydrocarbons including dibenzo[a,m]rubicene (molecule 1) and its derivatives are studied through density functional theory (DFT). The absorption and emission spectra (lambda(ab) and lambda(em)) are calculated at the B3LYP/6-31+G(d,p), PBE0/6-31+G(d,p), BMK/6-31+G(d,p), M062X/6-31+G(d,p) and CAM-B3LYP/6-31+G(d,p) levels in dichloromethane solvent. The results show that the BMK and M062X functionals can reproduce experimental lambda(ab) and lambda(em) more accurately than other functionals, respectively. The charge mobilities can be effectively tuned through introducing silyl groups of different sizes (molecules 2, 3 and 4) and the 1,3-dimethoxy benzene group (molecule 5) to dibenzo[a,m]rubicene. The relationship between the structures of five dibenzo[a,m]rubicene derivatives and a set of relevant electronic properties including the molecular geometries, frontier molecular orbitals, ionization potentials, electron affinities, reorganization energies and transfer integrals have been established. The results show that these molecules possess prominent charge carrier mobilities, although they all possess non-planar geometries. The charge transport properties could be effectively modulated via introducing different substituents to molecule 1. The high hole mobilities indicate that all five molecules facilitate hole transport, which is in accordance with the experimental results. Intriguingly, the electron mobility is enhanced maximally by five times through introducing silyl groups of different sizes from 0.06 cm(2) V-1 s(-1) (molecule 1) to 0.30 cm(2) V-1 s(-1) (molecule 4). Particularly, molecules 4 and 5 possess balanced electron and hole mobilities, which could be used as bipolar semiconductors and show promising use in optoelectronic devices.