Investigation of near-infrared absorption properties by the GW–BSE method in heptamethine pyrylium dyes

Abstract

Many studies have focused on obtaining a longer absorption wavelength in the second near-infrared (NIR-II) region of polymethine dyes. Previous studies on heptamethine pyrylium with tert-butyl cyanine substitution (TBCY) dyes showed an intense and sharp absorption exceeding 1000 nm and a red-shift in the longest absorption wavelength by modifying the cyclic ring on the π-conjugated bridge. In this study, we used the GW–BSE method to investigate the effect of the cyclic ring structure on the longest absorption wavelength in TBCY molecules. GW–BSE successfully reproduced the absorption spectra of TBCY dyes, and this matched well with experimental results, with the longest absorption wavelengths of TBCY-5 and TBCY-6 at 1003 nm and 968 nm, respectively. The exciton binding energy and charge density in highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels showed a local charge-transfer state in the cyclic ring, revealing a correlation with the absorption spectra. Furthermore, modifying the π-conjugated bridge with an aromatic ring showed a significant shift in HOMO and LUMO energy and notable changes in the absorption spectra. The five-membered aromatic ring diminishes the longest absorption wavelength in TBCY-ar5, whereas the six-membered aromatic ring considerably changes the electronic structure and properties of TBCY-ar6. Our study provides possible explanations and viable theoretical approaches to explain the red-shift of the longest absorption wavelength toward the NIR-II region of TBCY dyes.