Structure-property relationship of π-extended boron-dipyrromethene derivatives towards optoelectronic applications

Abstract

A series of novel boron-dipyrromethenes bearing β-benzo-fused rings, or benzo-BODIPYs, and phenyl, thienyl and bithiophenyl meso-substituents was synthesized, and investigated for their photophysical and electrochemical properties. Results from the UV–visible spectrophotometry revealed that the boron-complexation and the presence of the β-benzo-fused rings significantly increase absorptivity of the molecules in the longer wavelength region. Moreover, according to the observed electrochemical and photophyscial properties of these compounds, the extended conjugation system and the presence of thiophene-based meso-substituents in the target benzo-BODIPYs created great impact on the energy levels of the molecular frontier orbitals, resulting in narrowing of the energy gap of the materials. We have also demonstrated the use of density functional theory calculations to evaluate the performance of these compounds as dye sensitizers in solar cells. While all the molecules studied have their frontier orbitals aligned appropriately with respect to the band gap of TiO2, they are found to differ in their charge injection properties when anchored on TiO2, and even more markedly in electron-hole separation and change in dipole moment upon excitation. Upon combining these criteria using a simple model and the experimental results, the bithiophenyl-substituted benzo-BODIPY was suggested as the optimal candidate.