Facial complexation of naphthalene to {[3,5-(CF3)(2)Pz]Agl(3)(Ag}(3) gives rise to a stacked binary adduct (1) that exhibits phosphorescence corresponding to T, monomer emission of naphthalene. Crystals and powders of T-1 exhibit bright-green phosphorescence at room temperature with a 830 mu s lifetime, whereas cooling to cryogenic temperatures increases the intensity, lifetime, and vibronic resolution. The binary adduct exhibits a drastically shorter phosphorescence lifetime of 6.7 mu s versus free naphthalene (2.4 s) in a frozen dichloromethane matrix, which results from the external heavy-atom effect of silver. Adduct I represents a new class of phosphors containing lighter but more benign silver than mercury atoms in trinuclear d(10) pi-acid complexes as arene triplet sensitizers.