FACTORS AFFECTING NONRADIATIVE DECAY - TEMPERATURE-DEPENDENCE OF THE PICOSECOND FLUORESCENCE LIFETIME OF PT2(POP)4(4-)

The lifetime of the picosecond fluorescence of Pt2(pop)4(4-)(pop = mu-P2O5H2) has been observed to increase with decreasing temperature (e.g., from 40 ps at 290 K to 740 ps at 80 K in H2O:glycerol (1:2)). The longer lifetimes at the lower temperatures are attributed to a decrease in the rate of the nonradiative decay of the short-lived emissive state, 1A2u, to the long-lived emissive state, 3A2u, of the complex. The variation of the lifetime of the 1A2u state with temperature has been fit to two-channel expressions that include both a temperature-independent term and a term that is derived from a single-mode quantum treatment of nonradiative decay in either its normal or high-temperature limit. The temperature dependence of the lifetime in each solven yields sufficient information to define only three parameters, while the two-channel, single-mode quantum expression has five independent parameters. Thus, even when one of the five parameters is held constant, a range of values of the other four parameters gives reasonable fits to the data. When a two-channel expression with a single-mode quantum term in the high-temperature limit is used (k(obs) = k0 + (A/square-root RT) exp(-E(a)/RT)), the three independent parameters can be well defined in each solvent. In nondeuterated solvents the decay of the 1A2u state gives values of A that vary from 9.2 x 10(12) to 6.0 x 10(14) s-1, values of E(a) that vary from 890 to 1590 cm-1, values of k0 that are almost independent of solvent ((1.5 +/- 0.2) x 10(9) s-1), and low-temperature lifetimes that vary from 620 to 850 ps. Upon solvent deuteration, the low-temperature lifetime of the 1A2u state increases to 2170 ps. The temperature-dependent component of the 1A2u decay is attributed to a nonradiative pathway involving strong coupling between the fluorescent 1A2u (1d-sigma*p-sigma) state and a primarily ligand-field 3B2u (3d-sigma*d(x2-y2)) state; this pathway ultimately leads to population of the long-lived 3A2u (3d-sigma*p-sigma) state. The temperature-independent component of the 1A2u decay is dominated by a nonradiative pathway involving weak coupling between the 1A2u and 3A2u states in which the high-energy modes of the solvent are important accepting modes.