The electronic structures and spectroscopic properties of the three tridentate cyclometalated Pt(II) complexes Pt((NNC)-N-and-C-and)CCPh ((NNCH)-N-and-C-and = 6-phenyl-2,2'-bipyridine) (1), Pt((NNS)-N-and-S-and)CCPh ((NNSH)-N-and-S-and = 6-thienyl-2,2'-bipyridine) (2), and Pt((NNO)-N-and-O-and)CCPh ((NNOH)-N-and-O-and = 6-furyl-2,2'-bipyridine) (3) were investigated theoretically using the density functional theory (DFT) method. The geometric structures of the complexes in the ground and excited states were explored at the B3LYP and UB3LYP levels, respectively. The absorption and emission spectra of the complexes in CH2Cl2 and CH3CN solutions were calculated by time-dependent density functional theory (TD-DFT) with the PCM solvent model. The calculated energies of the lowest singlet state and lowest triplet state in the three complexes are in good agreement with the results of experimental absorption and luminescence studies. All of the lowest-lying transitions were categorized as LLCT combined with MLCT transitions. The 623-nm emission of 1 from the (3)A' -> (1)A' transition was assigned as (LLCT)-L-3 and (MLCT)-M-3 transitions, whereas the 657- and 681-nm emissions of 2 and 3, respectively, were attributed to (ILCT)-I-3 perturbed by (MLCT)-M-3 transitions. NLO response calculations revealed that the nonzero values of the static first hyperpolarizability (beta(0)) for 1-3 are greatly enhanced through the introduction of the metal Pt(II) into the cyclometalated ligands, an effect that is determined by MLCT and LLCT transitions.