The effects of the heteroatom and position on excited-state intramolecular proton transfer (ESIPT) of 2-[4′-(N-4,6-dichloro-1,3,5-triazi-n-2-yl)2′hydroxyphenyl]benzoxazole (4THBO) have been investigated via time-dependent density functional theory studies. The heteroatoms refer to O and S atoms, and the position effect refers to the N-4,6-trichloro-1,3,5-triazin-2-yl (TCT) substituents in the para and meta positions. The configuration of the four compounds (4THBO, 4THBT, 5THBO and 5THBT) was optimized and the bond lengths, bond angles and inf...