Chiral metal dithiolene complexes represent a family of chiral precursors, which can give rise to molecular materials with properties resulting from the interplay of chirality with conductivity, magnetism, and photophysics. We describe herein the first examples of chiral metal diimine dithiolene complexes, by the use of a platinum(II) centre coordinated by 2,2’-bipyridine and helicene-dithiolene ligands. Straightforward synthesis of racemic and enantiopure complexes allows the preparation of luminescent Pt(bipy) [4] and [6]helicene compounds for which the solid-state structure was determined as well. TD-DFT calculations support the assignment of the low energy bands observed in the UV-vis absorption spectra as mixed metal-ligand-to-ligand charge transfer transitions and confirm that the emission band results from the T1 excited state. Interestingly the enantiopure [6]helicene complexes show CPL activity at room temperature in acetonitrile solutions with anisotropy factors of 3×10^-4.

Ultrafast time-resolved infrared spectroscopy of [Ru(bpy)₃]²⁺ (bpy = 2,2’-bipyridine), [Ru(mbpy)₃]²⁺ (mbpy = 6-methyl-2,2’-bipyridine), and [Ru(mphen)₃]²⁺ (mphen = 2-methyl-1,10’-phenanthroline) in deuterated acetonitrile serves to elucidate the evolution of the system following pulsed excitation into the ¹MLCT band at 400 nm. Whereas for [Ru(bpy)₃]²⁺ no intermediate state can be evidenced for the relaxation of the corresponding ³MLCT state back to the ground state, for [Ru(mbpy)₃]²⁺ and [Ru(mphen)₃]²⁺ an intermediate state with a lifetime of about 400 ps is observed. The species associated IR difference spectra of this state are in good agreement with the calculated difference spectra of the lowest energy ³dd state using DFT. The calculated potential energy curves for all the complexes in the triplet manifold along the metal-ligand distance show that for [Ru(bpy)₃]²⁺ the ³dd state is at higher energy than the ³MLCT state and that there is a substantial barrier between the two minima. For [Ru(mbpy)₃]²⁺ and [Ru(mphen)₃]²⁺, the ³dd state is at lower energy than the ³MLCT state.

The ligand 3-chloro-6-dipicolylamino-1,2,4,5-tetrazine (Cl-TTZ-dipica) 1, prepared by the direct reaction between 3,6-dichloro-1,2,4,5-tetrazine and di(2-picolyl)-amine, afforded a series of four neutral transition metal complexes formulated as [Cl-TTZ-dipica-MCl₂]₂, with M = Zn(II) 2a, Cd(II) 2b, Mn(II) 2c and Co(II) 2d, when reacted with the corresponding metal chlorides. The dinuclear structure of the isostructural complexes was disclosed by single crystal X-ray analysis, clearly indicating the formation of [M^II-(m-Cl)₂M^II] motifs and the involvement of the amino nitrogen atom in semi-coordination with the metal centers, thus leading to distorted octahedral coordination geometries. Moreover, the chlorine atoms, either coordinated to the metal or as substituent on the tetrazine ring, engage respectively in specific anion-p intramolecular and intermolecular interactions with the electron poor tetrazine units in the solid state, thus controlling the supramolecular architecture. Modulation of the emission properties is observed in the case of the Zn(II) and Cd(II) complexes when compared to the free ligand. A striking difference is observed in the magnetic properties of the Mn(II) and Co(II) complexes. An antiferromagnetic coupling takes place in the dimanganese(II) compound (J = -1.25 cm^-1) while the Co(II) centers are ferromagnetically coupled in the corresponding complex (J = +0.55 cm^-1), the spin Hamiltonian being defined as H = -JS_A^.S_B.

The role of ligand-field states for the photophysical properties of d⁶ systems has been discussed in a large number of publications over the past decades. Since the seminal paper by Houten and Watts, for instance, the quenching of the ³MLCT luminescence in ruthenium(II) polypyridyl complexes is attributed to the presence of the first excited ligand-field state, namely a component of the ³T₁(t_2g⁵e_g¹) state, at similar energies. If this state lies above the ³MLCT state, the luminescence is quenched via thermal population at elevated temperatures only. If it lies well below, then the luminescence is quenched down to cryogenic temperatures. In this contribution we present transient absorption spectra on non-luminescent ruthenium polypyridyl complexes such as [Ru(m-bpy)₃]²⁺, m-bpy = 6-methyl-2,2’-bipyridine, in acetonitrile at room temperature, which reveal an ultra-rapid depopulation of the ³MLCT state but a much slower ground state recovery. We propose that in this and related complexes the methyl groups force longer metal-ligand bond lengths, thus resulting in a lowering of the ligand-field strength such that the ³dd state drops to below the ³MLCT state, and that furthermore the population of this state from the ³MLCT state occurs faster than its decay to the ground state. In addition we demonstrate that in this complex the luminescence can be switched on by external pressure, which we attribute to a destabilisation of the ligand-field state by the pressure due to its larger molecular volume compared to the ground state as well as the ³MLCT state.

Ultrafast transient absorption spectroscopy serves to identify the ³dd state as intermediate quencher state of the ³MLCT luminescence in the non-luminescent ruthenium complexes [Ru(m-bpy)₃]²⁺ (m-bpy = 6-methyl-2,2′-bipyridine) and [Ru(tm-bpy)₃]²⁺ (tm-bpy = 4,4′,6,6′-tetramethyl-2′,2′-bipyridine). For [Ru(m-bpy)3]²⁺, the population of the ³dd state from the ³MLCT state occurs within 1.6 ps, while the return to the ground state takes 450 ps. For [Ru(tm-bpy)₃]²⁺, the corresponding values are 0.16 and 7.5 ps, respectively. According to DFT calculations, methyl groups added in the 6 and 6′ positions of bipyridine stabilize the ³dd state by ∼4000 cm^–1 each, compared to [Ru(bpy)₃]²⁺.