Croconaine Dyes with Intermediate Diradical Character Exhibiting Intense One- and Two-Photon Absorption in the Short-Wavelength Infrared Region

Abstract

Intense one-photon absorption (OPA) and two-photon absorption (TPA) in the short-wavelength infrared (SWIR) region are achieved with chalcogenopyrylium-based croconaine dyes with intermediate diradical character by the synergistic effects of an enhanced degree of diradical character through substitution with heavy chalcogen atoms (O, S, and Se) and the extension of π-conjugation via the incorporation of phenyl groups. X-Ray crystal structure analysis, temperature-dependent ¹H-NMR, and ESR spectra revealed the intermediate diradical character of the croconaine dyes, with the contribution of the diradical form increasing with the atomic number of the chalcogen elements, in the order O < S < Se. As the atomic number of the chalcogen element increases, the OPA band shifts to lower energy regions. Dyes with S and Se as chalcogen elements, in particular, demonstrate strong OPA in the SWIR region, with absorption maxima at 1046 nm and 1096 nm, respectively. Furthermore, TPA spectra measured using the Z-scan technique reveal that dyes containing O, S, and Se exhibit large TPA cross sections of 1008 GM at 1255 nm, 1011 GM at 1395 nm, and 1177 GM at 1445 nm, respectively. The absorption bands shift to lower energy regions, similar to the OPA bands, with the cross section increasing with the atomic number of the chalcogen element. These results indicate that chalcogen atoms not only play a crucial role in modulating the intermediate diradical character of croconaine dyes but also significantly influence their OPA and TPA properties.