Shifting UV-vis absorption spectrum through rational structural modifications of zinc porphyrin photoactive compounds

Abstract

Metalloporphyrin assemblies such as Zn–porphyrins are significant photoactive compounds with a number of applications including molecular devices and dye-sensitized solar cells (DSSC). Recent Zn–porphyrin based DSSC, which achieves significant efficiency as high as 13%, stimulates rational molecular design through computer aided chemical and structural modifications. In the present study, time-dependant density functional theory (TD-DFT) is employed to simulate the ultraviolet-visible (UV-vis) spectra of the new photoactive compounds through chemical modification of the donor (D) and π-bridge of a high performing reference Zn–porphyrin (Pzn-EDOT) dye in DSSC. It is found that substitutions with electron donating groups such as –NH₂, –OCH₃ and –N(CH₃)₂ at the meso positions of the D of the dye reduce the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap by lifting up the occupied frontier orbitals; whereas chemical modification of the π-bridge with dyad monomers such as pyrimidine (Py), thiophene (THn) and EDOT reduces the HOMO–LUMO energy gap by lowering the frontier virtual orbitals. The new dye compounds exhibit the predicted changes to the UV-vis spectra, with a splitting of the Soret band and a distinct red-shift of the Q-bands. The study demonstrates that modification of the π-bridge is an attractive approach for tuning Zn–porphyrin assemblies for DSSC applications. The present study provides a rationale to design new architectures of molecular devices and for the improvement of metalloporphyrin assemblies.