A luminescent cyclometalated gold(III) complex [Au(C(LAMBDA)N(LAMBDA)N-dpp)Cl](X) (1a, X = C7H7SO3; 1b,X = ClO4; where C(LAMBDA)N(LAMBDA)N = C-anion of 2,9-diphenyl-1,10-phenanthroline) was prepared and characterized by X-ray crystal analysis. Crystal data for [Au(C24H15N2)Cl] (C7H7SO3).1/2CH30H : mol wt 950.97, triclinic, space group P1BAR (No. 2), a = 9.690(3) angstrom, b = 10.799(2) angstrom, c = 15.934(3) angstrom, alpha = 74.20(1)degrees, beta = 85.59(2)degrees, gamma = 68.72(2)degrees, V = 1494.6 angstrom3, Z = 2. In the solid state, two [Au(C(LAMBDA)N(LAMBDA)N-dpp)Cl]+ cations stack on each other with an intermolecular Au...Au separation of 3.6 angstrom. Complexes 1a and 1b are emissive in fluid solutions with quantum yields of approximately 10(-4) and lifetimes of 0.4-0.7 mus. The emission spectra of 1a and 1b are quite insensitive to solvents(521-670 nm) and display vibrational progression of approximately 1400 cm-1. The small spectral red-shift (1400 cm-1) of the emission of la from that of free ligand (dpp) suggests a metal-perturbed intraligand emissive state. The estimated excited state reduction potential E-degrees*(Au(III/II) of 2.2 V (vs NHE) suggests that [Au(C(LAMBDA)N(LAMBDA)N-dpp)Cl]+ is a strong photo-oxidant. This photooxidizing property has been demonstrated with the formation of the 1,4-dimethoxybenzene cation radical (DMB.+) upon UV-vis irradiation of an acetonitrile solution of [Au(C(LAMBDA)N(LAMBDA)N-dpp)Cl]+ and DMB and visible light (lambda = 406 nm) induced reduction of the gold(III) complex by tetrahydrofuran.