Near-Infrared Luminescence from Transparent Thin Films of Copper(I) Thiocyanate Modified with 2-Mercaptobenzothiazole...

Near-infrared (NIR) emission in molecular materials is typically achieved by narrowing the band gap through extended π-conjugation or heavy-atom substitution. Here, we report an unconventional route─excited-state symmetry breaking─that enables NIR emission while preserving a wide band gap. Copper(I) thiocyanate (CuSCN), a transparent semiconductor with optical activity normally observed only in the ultraviolet region, forms a dimeric complex with 2-mercaptobenzothiazole (MBTz). In thin films, CuSCN-MBTz complex exhibits NIR photoluminescence at 800 nm, while retaining a large optical gap (>3 eV), resulting in a transparent NIR-emissive material with a large Stokes shift of ∼15,000 cm^-1. Structural and spectroscopic analyses reveal that, unlike the rigid crystalline phase, the film state allows molecular reorganization of Cu₂(SCN)₂(MBTz)₄ dimers, which induces symmetry breaking and lowers the energy of the emissive state without reducing the band gap, in contrast to typical Cu(I) emitters. This work provides a new photophysical mechanism for generating long-wavelength emission from wide-band gap, earth-abundant Cu(I) systems and widens the functional scope of CuSCN materials.