The origin of the unusual broad and intense visible absorption of tetrathiafulvalene-annulated zinc porphyrazine: A density functional theory study

The vertical excitation energies of tetrathiafulvalene (TTF)-annulated zinc porphyrazine (ZnPzTTF) were investigated using time-dependent density functional theory (TDDFT) calculations and compared to the experimental UV–vis spectra. To examine the effects of the aza substitutions and TTF groups on the molecular properties, zinc complexes of porphyrin (ZnP), porphyrazine (ZnPz) and tetraTTF-annulated porphyrin (ZnPTTF) were also selected for comparison. It was shown that numerous electronic transitions with TTF-to-porphyrin or porphyrazine charge transfer character exist and the Q band of ZnPzTTF is dominated by TTF-to-porphyrazine charge transfer transition mixed with porphyrazine core unit itself except for classic porphyrazine π → π* transitions. The Q band of ZnPzTTF mixes with other configurations, which breaks down the Gouterman's classic four-orbital model for the spectral interpretation. The data suggest that TDDFT/SAOP performs best for Q and B bands of ZnPzTTF with the maximum error in excitation energy being 0.17 eV. The CAM-B3LYP, ωB97XD and M06-2X calculations qualitatively predict that the low-lying electronic transitions of ZnPzTTF with TTF-to-porphyrazine charge transfer character located below the Q band. The broad and intense red-shifted Q band suggests that ZnPzTTF can be a candidate for dye-sensitized solar cells.

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► The Q band of ZnPzTTF is dominated by TTF-to-porphyrazine charge transfer mixed with porphyrazine core unit itself except for classic porphyrazine π → π* transitions.
► ZnPzTTF has a broad and red-shifted Q band with strong intensity, and so has potential as a sensitizer for dye-sensitized solar cells.
► TDDFT/SAOP performs best for the Q and B bands of ZnPzTTF in terms of agreement with experimental values.
► The low-lying CT states of ZnPzTTF occur below the Q band.