Luminescent homoatomic exciplexes in dicyanoargenate(I) ions doped in alkali halide crystals. ‘Exciplex tuning’ by site-selective excitation and variation of the dopant concentration

Abstract

The photoluminescence spectra of dicyanoargenate(I) ions doped in KCl host crystals show several ultraviolet and visible emission bands. Each emission band becomes dominant at a characteristic excitation wavelength; i.e. the energy of the emission can be tuned. Both the experimental and 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, and the low band energies of the luminescent bands. Ab-initio and extended Hückel calculations show that the lowest unoccupied molecular orbital (LUMO) is bonding with respect to Ag–Ag bonds while the highest occupied molecular orbital (HOMO) is antibonding. Further, the calculations indicate the existence of exciplexes with shorter Ag–Ag bond distances, higher binding energies, and larger Ag–Ag overlap populations than the corresponding ground state oligomers. The results in this study give rise to a new optical phenomenon which is called ‘exciplex tuning’. Tuning of the emission over the 285–610 nm wavelength range has been achieved in KCl/Ag(CN)2− crystals by site-selective excitation and varying the Ag(CN)2− dopant concentration.