The [Cu-3(dppm)(3)OH](BF4)(2) cyclic cluster host is found to be luminescent at 298 K (lambda(max) = 540 nm; tau(e) = 89 +/- 9 mu s; Phi(e) = 0.14 +/- 0.01) in degassed ethanol solutions and at 77 K (lambda(max) = 480 nm; tau(e) = 170 +/- 40 mu s; Phi = 0.73 +/- 0.07) also in ethanol. The nature of the lowest energy excited states has been addressed theoretically using density functional theory and experimentally using UV-visible, luminescence, and polarized luminescence spectroscopy and is found to be (1,3)A(2) arising from the ...(18e)(4)(7a(2))(1)(13a(1))(1) electronic configuration. The excited state geometry optimization for the model Cu-3(PH3)(6)OH2+ compound in its T-1 state ((3)A(2)) has been performed using density functional theory and compared to its ground state structure. The Cu ... Cu bond length is expected to decrease greatly in the excited state (calculated Delta Q similar to 0.47 Angstrom), in agreement with the d(10) electronic configuration. The perturbation of the photophysical properties by the addition of two guest carboxylate anions has been investigated. From the Stern-Volmer plots, the quenching constants, k(q), are 1.65 x 10(8) and 5.10 x 10(8) M(-1) s(-1) for acetate and 4-aminobenzoate, respectively, which are also proportional to the relative binding strengths of the substrates with Cu-3(dppm)(3)OH2+ (i.e., acetate < 4-aminobenzoate).