Spin–Orbit TDDFT Electronic Structure of Diplatinum(II,II) Complexes

Abstract

[Pt2(μ-P2O5H2)4](4-) (Pt(pop)) and its perfluoroborated derivative [Pt2(μ-P2O5(BF2)2)4](4-) (Pt(pop-BF2)) are d(8)-d(8) complexes whose electronic excited states can drive reductions and oxidations of relatively inert substrates. We performed spin-orbit (SO) TDDFT calculations on these complexes that account for their absorption spectra across the entire UV-vis spectral region. The complexes exhibit both fluorescence and phosphorescence attributable, respectively, to singlet and triplet excited states of dσ*pσ origin. These features are energetically isolated from each other (∼7000 cm(-1) for (Pt(pop-BF2)) as well as from higher-lying states (5800 cm(-1)). The lowest (3)dσ*pσ state is split into three SO states by interactions with higher-lying singlet states with dπpσ and, to a lesser extent, pπpσ contributions. The spectroscopically allowed dσ*pσ SO state has ∼96% singlet character with small admixtures of higher triplets of partial dπpσ and pπpσ characters that also mix with (3)dσ*pσ, resulting in a second-order (1)dσ*pσ-(3)dσ*pσ SO interaction that facilitates intersystem crossing (ISC). All SO interactions involving the dσ*pσ states are weak because of large energy gaps to higher interacting states. The spectroscopically allowed dσ*pσ SO state is followed by a dense manifold of ligand-to-metal-metal charge transfer states, some with pπpσ (at lower energies) or dπpσ contributions (at higher energies). Spectroscopically active higher states are strongly spin-mixed. The electronic structure, state ordering, and relative energies are minimally perturbed when the calculation is performed at the optimized geometries of the (1)dσ*pσ and (3)dσ*pσ excited states (rather than the ground state). Results obtained for Pt(pop) are very similar, showing slightly smaller energy gaps and, possibly, an additional (1)dσ*pσ - (3)dσ*pσ second order SO interaction involving higher (1)dπpσ* states that could account in part for the much faster ISC. It also appears that (1)dσ*pσ → (3)dσ*pσ ISC requires a structural distortion that has a lower barrier for Pt(pop) than for the more rigid Pt(pop-BF2).

Solution Structure and Ultrafast Vibrational Relaxation of the PtPOP Complex Revealed by ΔSCF-QM/MM Direct Dynamics Simulations

“…The Pt-Pt bond in the T 1 state is found to be shorter than in the S 1 state of ∼0.01Å from the GPAW calculations. Indeed, a slightly reduced contraction in the singlet excited state with respect to the triplet has been inferred experimentally by comparing the wavenumbers of the Pt-Pt stretching progression exhibited 20 by the absorption bands of crystal (n-Bu 4 N)4 [PtPOP] relative to the S 1 (145-147 cm −1 ) and T 1 (150 cm −1 ) states,7,50 and was further confirmed by the DFT calculations performed by Záliš et al58 using the PBE0 functional, which delivered a ∆(S 1 − T 1 ) for the Pt-Pt bond of ∼0.02Å. InFigure 1(left) we report the PESs computed along the Pt-Pt coordinate for all three electronic states using GPAW with the BLYP functional.…”

“…51,[53][54][55][56][57] It is well established, based on the vibronic progression of lowtemperature UV-vis S 0 → S 1 and S 0 → T 1 absorption bands, 50 that the potential energy surfaces of S 1 and T 1 along the Pt-Pt coordinate are parallel. Moreover, these states are found, from experiments 50,51 and previous DFT studies, 58,59 to be separated by a relatively large energy gap of around 0.65 eV, and isolated from other electronic states. The electronic structure of the complex, together with the fact that direct spin-orbit coupling (SOC) between S 1 and T 1 is forbidden for symmetry reasons, 58,60 accounts for intersystem crossing (ISC) times between 11.0 and 101.5 ps, 14,50,61 depending on solvent and temperature.…”

“…From a structural point of view, we also found that in the excited state PtP 4 groups are distorted towards a local quasi-trigonal bipyramidal geometry, thus lowering the Pt 2 P 8 core symmetry to D 2d ; an aspect that had passed unnnoticed from previous DFT calculations but which could play a decisive role in determining the trends observed in the ISC rates of PtPOP and its derivatives in solution. 7,58,60 This distortion is associated to the presence of a low-frequency mode in the molecule with character of bending of the ligands, which in the gas-phase ∆SCF BOMD simulations manifests itself with oscillations with a period of ∼490 fs (68 cm −1 ). Based on this, we tentatively assigned the 40 cm −1 sideband superimposed to the Pt-Pt stretch progression in low-temperature absorption spectra of PtPOP in crystals, 51 which had so far remained uncharacterized, to this ligand distortion mode.…”

“…The mechanism of ISC from S 1 to T 1 in PtPOP is also a recurrent subject of discussion in the PtPOP literature. 7,14,58,60 All recent experimental indications seem to point in the direction of a possible involvement of a dark mode that would lower the D 4h symmetry of the Pt 2 P 8 core of the complex, allowing for direct SOC or lowering the energy of other triplet states, but this mode has never been observed experimentally.…”

Ultrafast Wiggling and Jiggling: Ir2(1,8-diisocyanomenthane)42+

“…In d 8_ d 8 complexes, ISC requires either a structural distortion to promote spin-orbit coupling (SOC) between the dσ*pσ singlet and triplet or a low-lying intermediate state allowing second-order SOC and/or a thermally activated pathway. 1,3,25,26,27,28 The ~70 ps ISC in Ir(dimen) is faster than in the structurally more rigid isocyanide-bridged complexes of Rh(I) and Ir(I) (230 ps -1.3 ns). 1,29,30 The dimen ligand flexibility likely enables ISC-promoting structural distortions through thermal fluctuations.…”