Enhanced near-infrared phosphorescence found in a structurally similar host–guest system†

Abstract

Longer-wavelength phosphorescence in the near-infrared (NIR) region has attracted attention in the field of biological application. Compared to widely studied shorter-wavelength green and yellow phosphorescence, achieving NIR phosphorescence is difficult because of a lack of versatile strategies using a phosphorescent platform. An NIR emission near 720 nm was observed for the benzoselenodiazole dye BSeD(OMe)–Br bearing bromine atoms and methoxy groups; nevertheless, its phosphorescence efficiency is weak. The NIR phosphorescence emission increased significantly when a trace amount of BSeD(OMe)–Br was incorporated into the one-dimensional aggregate structure composed of the corresponding benzothiadiazole dye BTD(OMe)–Br. In this host–guest BTD(OMe)–Br/BSeD(OMe)–Br system, an effective Dexter-type energy transfer with 92% efficiency occurs from the excited triplet state of the host BTD(OMe)–Br molecule to the excited triplet state of the guest BSeD(OMe)–Br molecule. In the 100 : 0.5 host–guest system, the phosphorescence intensity was enhanced by a factor of 200. The enhanced NIR phosphorescence through efficient energy transfer is attributed to a highly ordered host–guest aggregate structure arising from the same crystal packing pattern of the structurally similar host and guest molecules.