A red-emitting light switch based on a heteroleptic ruthenium(II) complex containing a tridentate dppz analogue.

Delocalization of the charge-transfer excitation in the series [Ru(NNN)(bpy)CN](+) [bpy = 2,2'-bipyridine; 1, NNN = 2,2':6',2''-terpyridine; 2, NNN = 2-(2'-pyridyl)-1,10-phenanthroline or php; 3, NNN = 6'-(2''-pyridyl)dipyrido[3,2-a:2',3'-c]phenazine or dppzp] proves to be an effective way of tuning photophysical properties. Red-emitting 3 functions as a DNA "light switch", shows emission from a state with charge-transfer-to-phenazine character, and exhibits a significantly enhanced emission signal relative to 1 and other dppzp-containing ruthenium(II) complexes.