Pressure effects on the photophysical properties of the platinum(II) dimer Pt₂(POP)₄^4- (POP = μ-η²-H₂P₂O₅^2-) and related d⁸-d⁸ dinuclear complexes in solution

The effect of hydrostatic pressure (0.1-300 MPa) upon phosphorescence lifetimes in fluid solution has been examined for four dinuclear d⁸-d⁸ transition-metal complexes: A, Pt₂(POP)₄^4- (POP = μ-η²-H₂P₂O₅^2-); B, Rh₂(br)₄²⁺ (br = μ-η²-1,3-diisocyanopropane); C, Ir₂(pz)₂(COD)₂ (pz = μ-η²-pyrazolate, COD = η⁴-1,4-cyclooctadiene); D, Rh₂(TMB)₄²⁺ (TMB = μ-η²-2,5-dimethyl-2,5-diisocyanohexane). In acetonitrile solutions, phosphorescence lifetimes of A and B proved to be relatively insensitive to pressure, and the rates for nonradiative deactivation displayed small, negative volumes of activation ΔV^‡_n.. In contrast triplet-state lifetimes for both C and D lengthened with increasing pressure, the result of substantial positive ΔV^‡_n values in these cases. These observations and the correlations between pressure and previously determined temperature effects on emission lifetimes are interpreted in terms of weak-coupling mechanisms dominating nonradiative deactivation from ³[A*] and ³[B*] and of strong-coupling mechanisms dominating the nonradiative deactivation from the triplet states of the less tightly bridged C and D.