The structural substitution effects of different intramolecular pi-stacking molecules at 9-position of diarylfluorenes were analyzed. Ground (excited-state) geometric conformations were optimized at B97-D/TZVP (TD-B97D/6-31+G*) level and their intramolecular pi-pi interactions were investigated by non-covalent interactions (NCI) theory. UV-vis and photoluminescence (PL) spectra were obtained on the optimized structures at TD-B97D/TZVP level. The results show that the strong pi-pi interactions mainly focus on the middle area of substituent groups, whereas the outer is the weak pi-pi interactions. The pi-pi stacking interactions have different effect on the HOMOs and LUMOs in the diarylfluorenes and diaryldiazafluorenes, resulting in the distinct optical properties. (c) 2013 Elsevier B.V. All rights reserved.