Substituent groups are widely used to take advantage of their electronic and steric proper- ties. For example, substituents with an electron-donating or electron-withdrawing nature have been utilized to design new materials with effective photophysical properties. Additionally, bulky substituents have been used to perform regioselective transformations. The effect of various substituents can be indirectly measured from observables, such as absorption/emission wavelength and chemical reaction rates.

To obtain a deeper understanding of the photophysical and chemical properties of a substituted molecule at the molecular level, a systematic theoretical investigation on both molecular and its substituent fragment would be highly beneficial. In this work, the effect of an electron-donating and electron-withdrawing substituent group on the electron distribution of several chromophores or reagents are studied us- ing a relatively simple quantum mechanics-based method based on the absolutely local- ized molecular orbitals (ALMO) for analyzing substituent effect. Our work showed that the substitution effects might be readily decomposed into electrostatics, polarization, and charge-transfer interactions between the substituent group and the substrate molecule. Specifically, in the case of oxyluciferin analogs, out-of-phase mixing of occupied- occupied orbitals can raise the HOMO energy of the chromophore, whereas in-phase virtual- virtual orbital mixing can lower the LUMO energy. Separately, the chemical reactivity trends observed in the CH activation reactions of β -TMS fluorobenzene were explained by computing the reaction barrier heights and performing the ALMO analysis.