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[<i>b</i>,<i>g</i>,<i>l</i>]­thiopheno­[<i>3</i>,<i>4</i>-<i>q</i>]­porphyrazine (<b>1</b>), substituted tribenzo­[<i>b</i>,<i>g</i>,<i>l</i>]­porphyrazine (<b>2</b>), and 2,3-bis­(methylcarboxyl)­phthalocyanine (<b>3</b>). 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 <b>1</b>, whereas the Q-band region of <b>2</b> and <b>3</b> 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_<i>x</i>- and Q_<i>y</i>-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.