Density Functional Theory Study On Absorption Spectral Properties Oftetrapyrrole Derivatives Dyes

Recently, progress on dye sensitized solar cells (DSSCs) based on tetrapyrrolederivatives sensitizers including porphyrins, phthalocyanines, chlorins andbacteriochlorins have been reviewed. Tetrapyrrole derivatives have been extensivelystudied with respect to their good photophysical properties, which are easilycontrollable by chemical modification of the macrocycle. In this paper we study andexplore the relationship between structure and properties of tetrapyrrole derivatives byusing density functional theory (DFT) and time dependent density functional theory(TDDFT) methods. It provides the theoretical basis for designing and synthesizingnew sensitizers for DSSCs. The major works are as follows:(1) The vertical excitation energies of tetrathiafulvalene (TTF)-annulated zincporphyrazine (ZnPzTTF) were investigated using time-dependent density functionaltheory (TDDFT) calculations and compared to the experimental UV–vis spectra. Toexamine the effects of the aza substitutions and TTF groups on the molecularproperties, zinc complexes of porphyrin (ZnP), porphyrazine (ZnPz) andtetraTTF-annulated porphyrin (ZnPTTF) were also selected for comparison. It showsthat numerous electronic transitions with TTF-to-porphyrin or porphyrazine chargetransfer (CT) characteristic exist and the Q band of ZnPzTTF is dominated byTTF-to-porphyrazine charge transfer transition mixed with porphyrazine core unititself except for classic porphyrazine Ï€â†'Ï€*transitions. The Q band of ZnPzTTFmixes with other configurations, which breaks down the Gouterman’s classicfour-orbital model for the spectral interpretation. The data suggest that TDDFT/SAOPperforms best for Q and B bands of ZnPzTTF with the maximum error in excitationenergy being0.17eV. The CAM-B3LYP, ωB97XD and M06-2X calculationsqualitatively predict that the low-lying electronic transitions of ZnPzTTF withTTF-to-porphyrazine charge transfer character located below the Q band. The broadand intense red-shifted Q band suggests that ZnPzTTF can be a candidate fordye-sensitized solar cells.(2) The vertical excitation energies of the novel chlorophyll f (Chl f) with theinstinctive near-infrared (NIR) Q_yabsorption were investigated using densityfunctional theory (DFT) and time-dependent DFT (TDDFT) calculations in order toobtain the suitable and economic method for the study of the red-most Q_yabsorptionband. The absorption properties of Chl f with the aza-substitutions at the meso-position were also studied on the basis of the above calculations. It shows thatωB97or LC-ωPBE with the6-311G (2d) basis set performs best for the red-most Q_yabsorption of Chl f with the maximum error in excitation energy being0.01eV. Chlf-meso-5,10-N and Chl f-meso-5,10,20-N have the red-shifted and more intense Q_yabsorption bands in the NIR region. Because of the small gap between the unchangedHOMO and decreased LUMO level of Chl f-meso-5,10-N and Chl f-meso-5,10,20-Ncompared with Chl f, their lowest NIR Q_yabsorption bands are significant red-shifted.At the same time, the aza-substitutions at the meso-position contained5and10greatly increase y-component of transition moment, their lowest NIR Q_yabsorptionbands are further intense. The novel Chl f with the minor modification of azasubstitutions can induce red-shifted and enhanced NIR Q_yabsorption band, whichprovide guidance to design novel enhanced and red-shifted chlorophylls sensitizersfor DSSCs.