Synthetic control of the cis/trans geometry of [M(cyclam)(CCR)2]OTf complexes and photophysics of cis-[Cr(cyclam)(CCCF3)2]OTf and cis-[Rh(cyclam)(CCCF3)2]OTf

Single-Molecule Junction of a Cationic Rh(III) Polyyne Molecular Wire

Single-molecule conductance studies on metal-containing inorganic and organometallic molecular wires are relatively less explored compared to those on organic molecular wires. Furthermore, conductance and transmission profiles of the metal-containing wires insensitive to the metal centers often hinder rational design for high performance wires. Here, synthesis and single-molecule conductance measurements of the bis(butadiynyl)rhodium wires with tetracarbene ligands 1^H and 1^Au are reported as rare examples for Rh(III) diacetylide molecular wires. The rhodium wires derived from the terminal acetylene and gold-functionalized precursors show comparable, high single-molecule conductance ((6-7) × 10^-3 G₀) as determined by the STM break-junction measurements, suggesting formation of virtually the same covalently linked metal electrode-molecule-metal electrode junctions. The values for the metallapolyynes are larger than those of the organic polyyne wires having the similar molecular lengths. The hybrid DFT-NEGF calculations of the model systems suggest that profiles of transmission spectra are highly sensitive to the presence and species of the metal fragments doped into the polyyne molecular wire because the conductance orbitals of the metallapolyynes molecular junctions carry significant metal fragment characters. Thus, the metallapolyyne junctions turn out to be suitable platforms for rationally designed molecular wires.

Chromium(III) Bis-Arylterpyridyl Complexes with Enhanced Visible Absorption via Incorporation of Intraligand Charge-Transfer Transitions

A series of chromium(III) bis-arylterpyridyl complexes containing intraligand charge-transfer (ILCT) excited states were prepared and characterized. These complexes show significant absorption in the visible region due to the ILCT bands. The ILCT bands are tunable across the UV and visible spectrum via incorporation of electron-withdrawing and electron-donating groups on the aryl ring. The absorption of Cr(4'-(4-methoxyphenyl)-2,2':6',2″-terpyridine)₂³⁺ (4) in particular is much stronger in the visible region (ε = 11 900 M^-1 cm^-1 at 450 nm and ε = 5090 M^-1 cm^-1 at 500 nm) than that of the parent complex Cr(tpy)₂³⁺ (tpy = 2,2':6',2″-terpyridine; ε = 2160 M^-1 cm^-1 at 450 nm, and ε = 170 M^-1 cm^-1 at 500 nm). Emission experiments on this series reveal Cr(III)-based phosphorescence with lifetimes from 140 to 600 ns upon excitation into the ILCT bands, which indicates funneling of the excitation energy from ligand-localized excited states to Cr(III)-based excited states. Cyclic voltammograms exhibit at least three reversible ligand-based reductions. The first reduction shows shifts of up to -160 mV compared to Cr(tpy)₂³⁺. The excited-state reduction potential of these complexes ranges from +0.95 to +1.04 V vs the ferrocene/ferrocenium couple, making them potent photooxidants.

Sustainable metal alkynyl chemistry: 3d metals and polyaza macrocyclic ligands

This article describes the recent development of sustainable metal-alkynyl chemistry based on the combination of 3d metals and polyaza macrocycles.