Croconaine dyes with intermediate diradical character exhibiting intense one- and two-photon absorption in the short-wavelength infrared region†

Chalcogenopyrylium-based croconaine dyes with intermediate diradical character exhibit intense one-photon absorption (OPA) and two-photon absorption (TPA) in the short-wavelength infrared (SWIR) region. These properties arise from the synergistic effects of enhanced diradical character via substitution with heavier chalcogen atoms (O, S, Se) and π-conjugation extension through sphenyl group incorporation. Chalcogenopyrylium-based croconaine dyes with intermediate diradical character exhibit intense one-photon absorption (OPA) and two-photon absorption (TPA) in the short-wavelength infrared (SWIR) region. These properties arise from the synergistic effects of enhanced diradical character through substitution with heavier chalcogen atoms (O, S, Se) and π-conjugation extension via phenyl substitution. 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 in the order O < S < Se as the atomic number of the chalcogen element increases. Correspondingly, the OPA band shifts to lower energy regions. Notably, dyes incorporating sulfur and selenium exhibit 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 findings demonstrate 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.