Toward panchromatic organic functional molecules: density functional theory study on the nature of the broad UV-Vis-NIR spectra of substituted tetra(azulene)porphyrins.

To achieve full solar spectrum absorption of organic dyes for organic solar cells and organic solar antenna collectors, a series of tetra(azulene)porphyrin derivatives including H₂(TAzP), H₂(α-F₄TAzP), H₂(β-F₄TAzP), H₂(γ-F₄TAzP), H₂(δ-F₄TAzP), H₂(ɛ-F₄TAzP), H₂(ζ-F₄TAzP), H₂[α-(NH₂)₄TAzP], H₂[β-(NH₂)₄TAzP], H₂[γ-(NH₂)₄TAzP], H₂[δ-(NH₂)₄TAzP], H₂[ɛ-(NH₂)₄TAzP], and H₂[ζ-(NH₂)₄TAzP] were designed and their electronic absorption spectra were systematically studied on the basis of TDDFT calculations. The nature of the broad and intense electronic absorptions of H₂(TAzP) in the range of 500-1450 nm is clearly revealed. In addition, different types of π→π* electronic transitions associated with different absorption bands are revealed to correspond to different electron density moving direction between peripherally-fused ten electron-π-conjugated azulene units and the central eighteen electron-π-conjugated porphyrin core. Introduction of electron-donating groups onto the periphery of H₂(TAzP) macrocycle is revealed to be able to lead to novel NIR dyes such as H₂[α-(NH₂)₄TAzP] and H₂[ɛ-(NH₂)₄TAzP] with regulated UV-Vis-NIR absorption bands covering the full solar spectrum in the range of 300-2500 nm. In addition, the basic designing rules for panchromatic organic functional molecules based on tetrapyrrole derivatives are proposed together with the suggestions in experiments, including low molecular symmetry and narrow gap between HOMO and LUMO/LUMO+1, which will be helpful toward the design and synthesis of new panchromatic organic functional molecules.