Near-Infrared Luminescence from Transparent Thin Films of Copper(I) Thiocyanate Modified with 2‑Mercaptobenzothiazole via Excited-State Symmetry Breaking

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.