Charge-transport properties of the tetraphenylbis(indolo[1,2-a])quinoline and 5,7-diphenylindolo[1,2-a]quinoline crystals

Recently, a single-crystal field-effect hole mobility of about 1.0 cm ²/(V s) has been measured for the tetraphenylbis(indolo[1,2-a]) quinoline (TPBIQ) crystal. TPBIQ can be considered as almost the dimer of 5,7-diphenylindolo[1,2-a]quinoline (DPIQ), but the experimental hole mobilities differ markedly. Here, based on density functional theory and semiempirical calculations, the charge-transport parameters of TPBIQ crystal are studied and compared to those of the "parent" DPIQ crystal. The results indicate that hole and electron transport in the TPBIQ crystal is significant only along the π-stacking direction, while in DPIQ substantial electronic couplings are also found along other directions. The larger electronic couplings and much smaller reorganization energy calculated for the TPBIQ crystal point to a higher hole mobility in TPBIQ, which is consistent with the experimental observations. Molecular dynamics simulations are also carried out to evaluate the nonlocal electron-phonon couplings. The results suggest that the scattering of the charge carriers is more substantial in DPIQ (especially in the case of electrons) than in TPBIQ, and the relative impact of the phonon-assisted mechanism is also larger in the case of electrons in DPIQ.