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 pi-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 pi-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.