Profiling Energetics and Spectroscopic Signatures in Prototropic Tautomers of Asymmetric Phthalocyanine Analogues

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were used to explain discrepancies in UV–vis and MCD spectra of the metal-free tribenzo­[b,g,l]­thiopheno­[3,4-q]­porphyrazine (1), substituted tribenzo­[b,g,l]­porphyrazine (2), and 2,3-bis­(methylcarboxyl)­phthalocyanine (3). On the basis of gas-phase and polarized continuum solvation model (PCM) DFT and TDDFT calculations, it was suggested that both NH tautomers contribute to the spectroscopic signature of 1, whereas the Q-band region of 2 and 3 is dominated by a single NH tautomer. For all tested compounds, it was found that the combination of the BP86 exchange–correlation functional, 6-31G­(d) basis set, and TDDFT-PCM approach provides the best accuracy in energies of the Q_x- and Q_y-bands of the individual NH tautomers as well as correctly describes their relative energy differences, which are important in understanding of experimental spectroscopy of the target systems.