Theoretical calculations on chiral discrimination of organic molecules by a chiral Co(III) α,α,β,β-tetramethylchiroporphyrin (TMCP) have been performed at the time-dependent density functional theory level. Upon taking the correction of the basis set superposition errors into consideration, molecular recognition leads to a large interaction energy, 310.5 kJ/mol for discrimination of S-prolinol by Co(III)-TMCP and 241.6 kJ/mol for discrimination of (S)-2-butylamine by Co(III)-TMCP. In addition to the coordinated bond between the host Co(III)-TMCP and the guest (S)-prolinol or (S)-2-butylamine, the hydrogen bond interactions are also an important factor in enhancing the molecular recognitions. Chiral discrimination induces remarkable differences in both electronic circular dichroism and optical rotatory power. The lowest-energy band of electronic absorption spectra originates from the host-guest charge-transfer transition (π→p*) between the HOMO and the LUMO, which causes a negative/positive Cotton effect. The higher-energy band is induced by the localized π→π* transition in the porphyrin ring, and is composed of a negative Cotton effect and a positive Cotton effect. The highest-energy band stems from the p→π* transition from the substituents to the porphyrin ring, which leads to two negative Cotton effects.

Key words: molecular recognition, chiral discrimination, porphyrin, electronic circular dichroism, density functional theory