Computational Study of Excited States of Luminescent Bimetallic 1,1-Bis(diphenylphosphino)methane complexes
Many bimetallic complexes with bridging phosphorus ligands have a unique luminescence in that they both phosphoresce and fluoresce. The low lying phosphoresce denotes a transition from what is formally three degenerate excited states, a triplet state. In reality the three states are not iso-energetic but split into what is believed to be one lower state and two upper states separated by 10’s of cm-1 in energy, a splitting that occurs because of relativistic spin-orbit coupling (SOC). What was once a prohibitive computation, quantum mechanically, because of computation limitations should now be possible for these large heavy metal complexes. The bimetallic complexes [Pt2(pop)4]4-, [Pt(CN)2Rh(tBuNC)2(Âµ-dppm)2][PF6], Pt2(CN)4(Âµ-dppm)2, [AuIrCl(CO)(Âµ-dppm)2][PF6], [AuPt(CN)2(Âµ-dppm)2][PF6], [AuRh(tBuNC)2(Âµ-dppm)2][PF6]2, where dppm = (diphenylphosphino)methane; tBuNC = terr-butyl isocyanide; pop= pyrophosphate were studied computationally utilizing density functional theory (DFT) to compute molecular orbitals and excited state energies utilizing open shell computations, B3lyp functionals, a SDD pseudopotential for the metals, and a 6-31G(d) basis set for all other atoms. Computational results showed that lowest energy excited state transition in these complexes originate from a metal-metal anti-bonding dz2 highest occupied molecular orbital (HOMO) and a metal-metal bonding pz lowest unoccupied molecular orbital (LUMO). The results are consistent with the systems ability to both fluorescence and phosphorescence because both orbitals have zero angular momentum and will not couple well with higher lying singlet states. Using the excited state energies and transitions from the computations, SOC perturbations was applied to estimate the energy splitting of the lowest triplet state and the results compared to experimental work.