Phosphorescence of platinum(II) octaethyl porphyrin (PtOEP), which has been used in organic light emitting diodes to overcome the efficiency limit imposed by the formation of triplet excitons, is studied by time-dependent (TD) density functional theory (DFT). The spin-orbit coupling (SOC) effects and the phosphorescence radiative lifetime (tau(p) (r)), calculated by the TD DFT method with the quadratic response technique, are analyzed for a series of porphyrins in order to elucidate the internal heavy atom effect on tau(p) (r). While the significance of the d(pi) orbital admixture into the lowest unoccupied molecular orbital e(g)(pi(*)), proposed by Gouterman et al. [J. Chem. Phys. 56, 4073 (1972)], is supported by our SOC calculations, we find that the charge-transfer (CT) mechanism is more important; the CT state of the (3)A(2g) symmetry provides effective SOC mixing with the ground state, and a large (3)A(2g)-(3)E(u) transition dipole moment gives the main contribution to the radiative phosphorescence rate constant. The IR and Raman spectra in the ground state and first excited triplet state (T(1)) are studied for proper assignment of vibronic patterns. An orbital angular momentum of the T(1) state is not quenched completely by the Jahn-Teller effect. A large zero-field splitting is predicted for PtP and PtOEP which results from a competition between the SOC and Jahn-Teller effects. A strong vibronic activity is found for the e(g) mode at 230 cm(-1) in PtP phosphorescence which is shifted to 260 cm(-1) in PtOEP. This out-of-plane vibration of the Pt atom produces considerable change of the SOC mixing. The role of charge-transfer state of d(pi)pi(*) type is stressed for the explanation of the electroluminescent properties of the dye doped light emitting diode.