Several BOPHY derivatives with and without ferrocene fragments, and with electron-withdrawing ester groups appended to the β-pyrrolic positions have been prepared and characterized by NMR, UV/Vis near-infrared (NIR), high-resolution mass spectrometry, and fluorescence spectroscopy, as well as X-ray crystallography. The redox properties of new BOPHYs were probed by electrochemical (cyclic and differential pulse voltammetry) and spectroelectrochemical methods. In an attempt to prepare BOPHY derivatives with a cyano group at the bridging position using a similar approach for BODIPY cyanation, adducts from the nucleophilic attack of the cyanide anion on the bridging position in BOPHY have been isolated and characterized by spectroscopic methods. Oxidation of such adducts, however, resulted in formation of either the starting BOPHYs, or partial extrusion of the BF fragment from the BOPHY core, which was confirmed by spectroscopy and X-ray crystallography. DFT and TDDFT calculations on all target materials correlate well with the experimental data, and suggest the dramatic reduction of the nitrogen atom basicity at the hydrazine bridge of the BOPHY upon introduction of the cyano group at the bridging-carbon atom.

The electrochemistry of pyridines in acidic solution is dominated by a 'weak acid' reduction on the cyclic voltammetry timescale. Here we show that electrochemical hydrogenation of a benzannulated pyridine, phenanthridine (1), to the biomimetic hydride donor 1,2-dihydrophenanthridine (1-H2) can occur selectively at glassy carbon electrodes over longer timescales of potentiostatic electrolysis.

The synthesis, characterization, and coordination chemistry of a doubly π-extended bipyridine analogue, 6,6′-biphenanthridine (biphe), is presented. The structure of the molecule has been determined in the solid state by X-ray diffraction, showing an angle of 72.6°between the phenanthridine planes. The free, uncoordinated organic molecule displays blue fluorescence in solution. It can be singly protonated with strong acids, and the protonated form displays more intense yellow emission. The effect of acid on the excited states is interpreted with the aid of TDDFT calculations. Two Ru(II) coordination complexes, tris(6,6′biphenanthridine)ruthenium(II) dichloride, [Ru(biphe) 3 ]Cl 2 , and bis-(2,2′-bipyridine)(6,6′-biphenanthridine)ruthenium(II) tetraphenylborate, [Ru(bpy) 2 (biphe)](BPh 4 ) 2 , are also reported and their structures determined in the solid state by X-ray diffraction. Both complexes display emission at 77 K that is strongly bathochromically shifted by almost 200 nm compared to that of the archetypal 3 MLCT emitter [Ru(bpy) 3 ] 2+ . Such a red shift is consistent with the more extended conjugation and lower-energy π* orbitals associated with the biphe ligand, lowering the energy of the 3 MLCT excited state, as revealed by TDDFT calculations. The efficient non-radiative decay that is typical of such low-energy emitters renders the phosphorescence extremely weak and short-lived at ambient temperature, and rapid ligand photodissociation also competes with radiative decay, especially in the heteroleptic complex. Electrochemical analysis illustrates the effect of biphe's stabilized vacant π* manifold, with multiple reversible reductions evident at much less negative potentials than those observed for [Ru(bpy) 3 ] 2+ .

Zn2+ templating enables synthesis of redox ‘non-innocent’ diimine pyridine ligands with strong electron-withdrawing groups, allowing construction of iron complexes with multiple ligand-based reductions for application in redox flow batteries.