Scientists at the University of North Carolina at Chapel Hill, U.S.A., report temperature-dependent excited-state lifetime measurements, in protic and aprotic solvents (MeOH, BuCN, MeCN) on [Ru(bpy)₂dppz]²⁺ which suggest that the light-switch effect is competitive (M. K. Brennaman, J. H. Alstrum-Acevedo, C. N. Fleming, P. Jang, T. J. Meyer and J. M. Papanikolas, J. Am. Chem. Soc., 2002, 124, (50), 15094–15098).

The dppz ligand has bpy-like and phz-like states. The bpy is associated with the bright state and phz with the dark state. The bpy-like state is similar in size to the corresponding orbital in the ³MLCT state in [Ru(bpy)₃]²⁺; it is entropically favoured and populated at high temperatures. The dark state is lowest in energy and is populated at low temperatures. The switch effect results from competition between the energetic and entropic factors, not from state reversal.