A TDDFT study of the low-lying excitation energies of polycyclic cinnolines and their carbocyclic analogues

The low-lying excited states of 21 compounds of polycyclic cinnoline, monoaza-hydrocarbon and their corresponding polycyclic aromatic hydrocarbon (PAH) analogues have been investigated in the framework of time-dependent density functional theory (TDDFT). The gradient corrected BLYP, hybrid B3LYP and B3P86 functionals together with 6-31G (d) basis set have been used. The hybrid-type B3LYP and B3P86 systematically overestimate the excitation energies for states with dominating ionic character (corresponding to group III band), with a mean absolute deviation (MAD) of 0.33 eV (B3LYP) and 0.34 eV (B3P86), respectively. However, they can accurately predict the excitation energies for states with covalent character (corresponding to group II and I bands). The MAD for group II (B3LYP: 0.05 eV; B3P86: 0.05 eV) and I bands (B3LYP: 0.12 eV; B3P86: 0.12 eV) are significantly smaller. The BLYP outperforms B3LYP and B3P86 for group III band (MAD: 0.09 eV), but has a worse performance for group II (MAD: 0.15 eV) and I (MAD: 0.13 eV) bands. Comparison of the lowest-lying excited states for polycyclic cinnolines with those of the corresponding PAH analogues, the first excited states of polycyclic cinnolines mainly result from n→π0* transitions. Therefore, in non-polar solvents, the spectra of some polycyclic cinnolines exhibit an additional absorption band at longer wavelengths.