A spectroscopic and DFT study of thiophene-substituted metalloporphyrins as dye-sensitized solar cell dyes

A combination of density functional theory calculations, electronic absorption and resonance Raman spectroscopy has been applied to a series of beta-substituted zinc porphyrins to elucidate how the substituent affects the electronic structure of the metalloporphyrin and assign the nature of electronic transitions in the visible region. The use of conjugated beta substituents invokes a large perturbation to both the nature and energy of the frontier molecular orbitals and results in the generation of additional molecular orbitals from the parent metalloporphyrin species. A complicated electronic absorption spectra is observed which can be rationalised by an extension of Goutermans' four-orbital model. The excitations involved in the visible transitions have been determined using resonance Raman spectroscopy. This has revealed that the B band retains much of its original nature and is centred largely on the porphyrin core. Additional electronic transitions invoke population of orbitals localised on the substituent chain. The nature of the electronic transitions depends heavily on the type of beta substituent. The results of this investigation question some previously held beliefs for the rational design of metalloporphyrins for dye-sensitized solar cell applications.