Reproduction and interpretation of the UV–vis spectra of some flavonoids

The UV–vis spectra of 15 naturally occurring flavonoids, including flavones, flavonols, and isoflavones, were simulated and compared to literature experimental results. For this purpose, the TDDFT method in conjunction with the B3LYP-D3 and B3LYP functionals was applied. In spite of some discrepancy between the structural predictions, the differences between the TDDFT/B3LYP and TDDFT/B3LYP-D3 results are negligible. The performance of both functionals in reproducing the UV–vis spectra of the selected flavonoids is satisfactory. The NLMO clusters, i.e. molecular moieties characterised with distinguished electron density, were created. The combined results from the TDDFT and NLMO approaches were employed to perform deep analysis of the spectra. It was found that the transitions from the bonding NLMO clusters to the LUMO cluster are favoured at longer wavelengths, whereas the transitions from the HOMO and HOMO−1 clusters to the antibonding NLMO clusters are favoured at shorter wavelengths. All flavonoids show the HOMO → LUMO and HOMO−1 → LUMO transitions at long wavelengths, and majority of them show the HOMO → LUMO+1 transition at moderate wavelengths. This investigation confirms our earlier finding that the TDDFT and NBO approaches are complementary, implying that the results from the two theories can be combined to better understand the redistribution of electron density upon excitation.