Theoretical study of charge-transport and optical properties of organic crystals: 4,5,9,10-pyrenediimides

This work presents a systematic study of the conducting and optical properties of a family of aromatic diimides reported recently and discusses the influences of side-chain substitution on the reorganization energies, crystal packing, electronic couplings and charge injection barrier of 4,5,9,10-pyrenediimide (PyDI). Quantum-chemical calculations combined with the Marcus-Hush electron transfer theory revealed that the introduction of a side chain into 4,5,9,10-pyrenediimide increases intermolecular steric interactions and hinders close intermolecular pi-pi stacking, which results in weak electronic couplings and finally causes lower intrinsic hole and electron mobility in t-C-5-PyDI (mu(h) = 0.004 cm(2) V-1 s(-1) and mu(e) = 0.00003 cm(2) V-1 s(-1)) than in the C-5-PyDI crystal (mu(h) = 0.16 cm(2) V-1 s(-1) and mu(e) = 0.08 cm(2) V-1 s(-1)). Furthermore, electronic spectra of C5-PyDI were simulated and time-dependent density functional theory calculation results showed that the predicted fluorescence maximum of t-C-5-PyDI, corresponding to an S-1 -> S-0 transition process, is located at 485 nm, which is close to the experimental value (480 nm).