Facial complexation of naphthalene to {[3,5-(CF(3))(2)Pz]Ag}(3) (Ag(3)) gives rise to a stacked binary adduct (1) that exhibits phosphorescence corresponding to T(1) monomer emission of naphthalene. Crystals and powders of 1 exhibit bright-green phosphorescence at room temperature with a 830 mus 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 ms versus free naphthalene (2.4 s) in a frozen dichloromethane matrix, which results from the external heavy-atom effect of silver. Adduct 1 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.