Luminescent Homoatomic Exciplexes in Dicyanoargentate(I) Ions Doped in Alkali Halide Crystals. 1. “Exciplex Tuning” by Site-Selective Excitation

Abstract

The photoluminescence behavior of dicyanoargentate(I) ions doped in KCl host crystals has been studied. Several ultraviolet and visible emission bands are observed in this system. Each emission band becomes dominant at a characteristic excitation wavelength; that is, the energy of the emission can be tuned by site-selective spectroscopy. Both the experimental and the theoretical results suggest the formation of Ag−Ag-bonded excimers and exciplexes between adjacent Ag(CN)2- ions in the host lattice. The experimental evidence includes the broadness, the absence of detailed structure, the large Stokes shifts, and the very low band energies of the luminescence bands. Meanwhile, ab initio calculations show that the LUMO is bonding with respect to Ag−Ag bonds while the HOMO is antibonding. Moreover, excited state extended Hückel calculations indicate the formation of exciplexes with shorter Ag−Ag distances, higher binding energies, and higher Ag−Ag overlap populations than the corresponding ground-state oligomers. The emission bands are assigned to different *[Ag(CN)2-]n luminescent exciplexes. Tuning of the emission over the 285−610-nm range has been achieved by site-selective excitation in a single KCl:Ag(CN)2- crystal. The results in this work give rise to a new optical phenomenon we call “exciplex tuning”.