Optical absorption and emission properties of benzene-expanded Janus AT nucleobase analogues: A DFT study

We design benzene-expanded Janus AT bases (J-AT1, J-AT2, BF1 and BF2) and investigate the structural and optical properties of these bases using DFT and TDDFT methods and also consider the effect of water solution and base pairing. The results show that J-AT1 has a nonplanar structure and its amino group is more asymmetric than the other three bases. J-AT1 and BF1 exist intramolecular H-bonds. The Janus AT base pairs exist three intermolecular H-bonds: NH···N has the largest energy, followed by NH···O and CH···O. The Janus AT base pairs retain the strength of H-bonds and maintain the stability of the base pair. The lowest absorption and emission wavelengths of the benzene-expanded Janus AT bases are all assigned to ππ* character arising from HOMO to LUMO transition, and they exhibit redshift due to the increase in effective conjugation length with the introduction of benzene. The excited state assignments in water solution are one-to-one correspondence to those in gas phase. The lowest absorption and emission wavelength of J-AT1 is blueshift, while J-AT2, BF1 and BF2 bases are redshift as compared to those in gas. The TD-B3LYP method predicts that the first excitation wavelengths of Janus AT base pairs are local excitation on the Janus AT base moieties, which is coincident with the result of CAM-B3LYP and M062X functional. Base pair can influence the excitation properties of base monomer. The results obtained from this theoretical investigation confirm that the position of benzene ring significantly influences the structure and optical properties of the Janus AT bases.