Binuclear cyclometalated Pt(II) complexes of the form C∧NPt(μ−pz′)2PtC∧NC^{\wedge}NPt(\mu-pz')_2PtC^{\wedge}N, whose intramolecular Pt···Pt distance is set synthetically by the steric bulk of two bridging pyrazolate ligands, were introduced by Ma, Li, Djurovich, Yousufuddin, Bau and Thompson (JACS 2005) as a family in which the emissive triplet state can be tuned from ligand-centered/metal-to-ligand charge-transfer character to a metal–metal-to-ligand charge-transfer (3^3MMLCT) state by compressing the Pt···Pt contact from 3.38 Å to 2.83 Å. The paper deferred four linked questions: electroluminescent device evaluation; direct time-resolved measurement of the excited-state Pt···Pt bond contraction; the microscopic timescale and pathway of the fluid-solution 'collapse' into the 3^3MMLCT potential-energy well; and the proposed use of the contraction as a photoactuated molecular nanohinge/nanotweezer.

Background

Square-planar d8 Pt(II) centers with filled dz2 orbitals can stack so that the two metals interact through overlap of their d orbitals, giving a weak metal–metal (metallophilic) bond; in binuclear complexes with two bridging pyrazolate ligands (pyrazolate = the deprotonated form of pyrazole, a five-membered N,N-heterocycle) the two Pt(II) centers are held at a distance that depends on the steric bulk of the substituents at the 3- and 5-positions of the bridging rings. Cyclometalation here means that each Pt(II) is bound through one carbon and one nitrogen of a 2-(2,4-difluorophenyl)pyridyl ligand (C^N), an arrangement standard in phosphorescent Pt(II) emitters. When the Pt···Pt distance is short, the filled antibonding combination σ* of the two dz2 orbitals becomes the HOMO (highest occupied molecular orbital), and a transition from σ* (metal–metal) into a ligand π* orbital becomes the lowest-energy excitation: a metal–metal-to-ligand charge-transfer (MMLCT) state. Because σ* is depopulated in that state, the excited molecule is predicted to shorten its Pt···Pt distance relative to the ground-state geometry — a light-driven intramolecular contraction that was proposed as the motion element of a 'molecular nanohinge' or 'nanotweezer'. Ma et al. (JACS 2005) synthesized the series C∧NPt(μ−pz′)2PtC∧NC^{\wedge}NPt(\mu-pz')_2PtC^{\wedge}N with pz' = pyrazolate, 3,5-dimethylpyrazolate, 3-methyl-5-tert-butylpyrazolate, and 3,5-bis(tert-butyl)pyrazolate (complexes 1–4), giving crystallographic Pt···Pt spacings of 3.3763(7), 3.1914(9), 3.0457(7), and 2.8343(6) Å and a fold of the two Pt square planes from about 85° to about 55°. Emission at 77 K shifted from blue (1, 2) to green (3) to red (4) as the Pt···Pt distance decreased, the emissive triplet changing from a mixed ligand-centered/metal-to-ligand charge-transfer state (1, 2) to a 3^3MMLCT state (3, 4). In rigid matrices the complexes emit from their unrelaxed geometry, while in fluid 2-methyltetrahydrofuran solution they reorganize toward the shortened Pt···Pt geometry and emit red-shifted 3^3MMLCT phosphorescence on microsecond timescales; the 2005 Communication did not directly measure the excited-state geometry, quantify the collapse kinetics, evaluate devices, or demonstrate any mechanical actuation. Relevant experimental tools developed in the same period include time-resolved wide-angle X-ray scattering (TR-WAXS), which reconstructs photoexcited molecular structures in dilute solution from pump–probe X-ray scattering differences, X-ray transient absorption (XTA) at the Pt LIII-edge (the L3 core-level absorption edge of platinum, which reports the electronic and local structure of the short-lived state), and femtosecond transient absorption with coherent vibrational wavepacket analysis, which tracks nuclear motion along the Pt–Pt stretch; these are the natural instruments for the deferred structural and dynamical questions.

Problem Statement

For the pyrazolate-bridged binuclear cyclometalated Pt(II) complexes C∧NPt(μ−pz′)2PtC∧NC^{\wedge}NPt(\mu-pz')_2PtC^{\wedge}N (C∧NC^{\wedge}N = 2-(2,4-difluorophenyl)pyridyl, abbreviated F2ppy; pz′pz' = pyrazolate, 3,5-dimethylpyrazolate, 3-methyl-5-tert-butylpyrazolate, 3,5-bis(tert-butyl)pyrazolate, i.e. complexes 1–4 of Ma et al., JACS 2005, with ground-state Pt···Pt distances 3.376–2.834 Å), the single overarching objective left open by the source was to establish and exploit the photoinduced Pt···Pt contraction associated with population of the 3^3MMLCT (metal–metal-to-ligand charge-transfer, σ∗(Pt−Pt)→π∗\sigma^{*}(\mathrm{Pt}-\mathrm{Pt})\to\pi^{*}) excited state. Its four linked components are: (1) evaluate the complexes as phosphorescent emitters in organic light-emitting diodes (architectures, efficiencies, lifetimes); (2) determine directly, by time-resolved structural measurement, the magnitude and dynamics of the excited-state Pt···Pt contraction, rather than inferring it from emission shifts; (3) quantify the microscopic kinetics and pathway by which the photoexcited molecule in fluid solution reorganizes geometrically and electronically from its ground-state geometry into the 3^3MMLCT potential-energy well; and (4) demonstrate and characterize the proposed exploitation of this contraction as a photoactuated molecular nanohinge/nanotweezer, i.e. mechanical motion, force generation, and work output under optical excitation. The problem posed here, faithful to the source's outlook, is component (4): whether the photoinduced 3^3MMLCT Pt···Pt contraction of a pyrazolate-bridged binuclear Pt(II) complex can be realized in a system that performs measurable mechanical work — the system physically realized and its mechanical response measured by experiment, through the measured displacement and the measured force or work per photon, together with the response time, quantum efficiency, and reversibility over repeated cycles — and whether the achievable force and work per photon of this contraction mechanism are sufficient for practical actuation, established against a stated practical load or actuator benchmark and traced to the excited-state energy and displacement available rather than to engineering limitations.

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