Excited-State Bond Contraction and Charge Migration in a Platinum Dimer Complex Characterized by X-ray and Optical Transient Absorption Spectroscopy

Abstract

Interactions between metal centers in dimeric transition metal complexes (TMCs) play important roles in their excited-state energetics and pathways and, thus, affect their photophysical properties relevant to their applications, for example, photoluminescent materials and photocatalysis. Here, we report electronic and nuclear structural dynamics studies of two photoexcited pyrazolate-bridged [Pt(ppy(1) and [Pt(NDI-ppy)(μ-Ph 2 pz)] 2 (2, NDI = 1,4,5,8-naphthalenediimide), both of which have distinct ground-state Pt−Pt distances. X-ray transient absorption (XTA) spectroscopy at the Pt L III -edge revealed a new d-orbital vacancy due to the one-electron oxidation of the Pt centers in 1 and 2. However, while a transient Pt−Pt contraction was observed in 2, such an effect was completely absent in 1, demonstrating how the excited states of these complexes are determined by the overlap of the Pt (d z 2 ) orbitals, which is tuned by the steric bulk of the pyrazolate R-groups in the 3-and 5-positions. In tandem with analysis of the Pt−Pt distance structural parameter, we observed photoinduced electron transfer in 2 featuring a covalently linked NDI acceptor on the ppy ligand. The formation and subsequent decay of the NDI radical anion absorption signals were detected upon photoexcitation using optical transient absorption spectroscopy. The NDI radical anion decayed on the same time scale, hundreds of picoseconds, as that of the d-orbital vacancy signal of the oxidized Pt−Pt core observed in the XTA measurements. The data indicated an ultrafast formation of the charge-separated state and subsequent charge recombination to the original Pt(II−II) species.

Ligand-Structure-Dependent Coherent Vibrational Wavepacket Dynamics in Pyrazolate-Bridged Pt(II) Dimers

“…During the first a few picoseconds of the excitation, the rates of the internal conversion (IC) processes in Pt-bzq and Pt-piq within the triplet manifold are comparable to the same process in relaxation processes, as observed in related pyrazolatebridged Pt(II) dimers. 15,16 In Pt-piq, the relaxation of SADS2 with a time constant of 2.85 ps was slightly delayed relative to that seen in Pt-bzq (1.76 ps). Nevertheless, the spectral change from SADS2 to SADS3 was similar to that in Pt-bzq, implying that the transition of SADS2 to SADS3 involves the relaxation of the MMLCT triplet excited state.…”

“…Bimetallic transition metal complexes (TMCs) bridged by organic ligands have gained attention because of their potential applications as photocatalysts, − light-emitting materials, , and photoluminescent sensors. , Among the various types of bimetallic TMCs, Pt­(II) dimers have been considered as promising light-emitting materials because of their long-lived phosphorescence and high emission quantum yields. − Pt­(II) dimers constructed from pyrazolate bridges and conjugated aromatic cyclometalating (CM) ligands feature metal–metal d 8 –d 8 interactions displaying prominent absorption and emission properties that depend on the Pt–Pt distance. − Previous studies of CM Pt­(II) dimers with sufficiently short Pt–Pt distances (<2.95 Å) produce metal–metal-to-ligand charge-transfer (MMLCT) excited states upon photoexcitation into the lowest-energy visible absorption bands. ,− This MMLCT transition predominantly promotes an electron from the highest occupied molecular orbital (HOMO) (mainly Pt­(5d z 2 )–Pt­(5d z 2 ) dσ*) to the lowest unoccupied molecular orbital (LUMO) (mainly N^C ligand-centered π*), which depletes electron density in the antibonding orbital of dσ*, resulting in an increase of the Pt–Pt bond order by ∼0.5, contracting the Pt–Pt distance as shown in Scheme . Because of this photoinduced charge transfer and its subsequent electronic dynamics related to the associated structural rearrangement, some of these Pt­(II) dimers have been extensively studied through various spectroscopic and theoretical approaches. − …”

Ultrafast branching in intersystem crossing dynamics revealed by coherent vibrational wavepacket motions in a bimetallic Pt(ii) complex

“…In particular, the bond contraction in the MMLCT states has been directly observed by the X-ray transient absorption and scattering measurements of the structurally related pyrazole-bridged Pt( ii ) dimers. 24,25,41…”

“…In particular, the bond contraction in the MMLCT states has been directly observed by the X-ray transient absorption and scattering measurements of the structurally related pyrazole-bridged Pt(II) dimers. 24,25,41 In Fig. 4b, the normal mode calculations of the ground state showed the Pt-Pt stretching vibration at a frequency of 114.7 cm À1 .…”

Beyond structural insight: a deep neural network for the prediction of Pt L2/3-edge X-ray absorption spectra

“…5c; ppy = 2-phenylpyridine; t Bu 2 pz = 3,5-di-tert-butylpyrazolate) -a complex previously studied by Lockard et al 100 Indeed, there has been extensive work carried out using time-resolved X-ray techniques on diplatinum transition metal complexes with a view towards elucidating the electronic and geometric structural changes upon photoexcitation. 66,[101][102][103][104] Fig. 8a shows the Pt L 3 edge XANES spectrum of [Pt(ppy)(m-t Bu 2 pz)] 2 in the electronic ground state.…”