Electronic absorption spectral analysis of chlorin-based dyad sensitizers by TD-DFT calculations

As part of our studies of chlorophyll-based sensitizers for solar energy applications, we recently synthesized two dyad panchromatic chromophores of chlorin-indoline (Chl-Ind) and chlorin-carotenoid (Chl-Car) conjugates aiming at the wider range of sunlight utilization. By comparing the absorption spectra of these dyad sensitizers in various solvents, we found that each chromophore unit in Chl-Ind showed drastic red-shift when measured in polar solvents, while the spectra of Chl-Car showed lesser solvent effects. To elucidate the origin of such solvatochromism by time dependent density functional theory (TD-DFT) calculations with molecular dynamics studies, we decided to compare the estimation results of TD-DFT/CAM-B3LYP (M06-2X and omega B97X-D)/6-31(d,p) using polarizable continuum model (H 2 0 and THF) with the absorption spectra for Chl-Ind and Chl-Car, together with their sole chromophore units methyl trans-3(2)-carboxy-pyropheophorbide-a (Chl), methyl ester of indoline dye D102 (Ind), and beta-apo-8'-carotenoic acid ethyl ester (Car). The results revealed that the unusual peak shifts of Chl and Ind moieties in Chl-Ind dyad could be most properly reproduced at omega B97X-D density functionals. This is likely because omega B97X-D contains dispersion-corrected and long-range corrected functionals. The destabilized HOMO-1 is expected to contribute to the red-shift of Ind peak in the dyad. Our results would provide useful information for the selection of computational methods to anticipate spectral characteristics of covalently-linked multi-chromophores in advance.