David W. Thompson

(Ru(bpy) 3 ) 2+ * and other remarkable metal-to- ligand charge transfer (MLCT) excited states*

In 1974, the metal-to-ligand charge transfer (MLCT) excited state, [Ru(bpy)3]2+*, was shown to undergo electron transfer quenching by methylviologen dication (MV2+), inspiring a new approach to artificial photosynthesis based on molecules, molecular-level phenomena, and a “modular approach”. In the intervening years, application of synthesis, excited-state measurements, and theory to [Ru(bpy)3]2+* and its relatives has had an outsized impact on photochemistry and photophysics. They have provided a basis for exploring the energy gap law for nonradiative decay and the role of molecular vibrations and solvent and medium effects on excited-state properties. Much has been learned about light absorption, excited-state electronic and molecular structure, and excited-state dynamics on timescales from femtoseconds to milliseconds. Excited-state properties and reactivity have been exploited in the investigation of electron and energy transfer in solution, in molecular assemblies, and in derivatized polymers and oligoprolines. An integrated, hybrid approach to solar fuels, based on dye-sensitized photoelectrosynthesis cells (DSPECs), has emerged and is being actively investigated.

Supramolecular ‘flat’ Mn9 grid complexes—towards functional molecular platforms

Flat, quantum dot like arrays of closely spaced, electron rich metal centres are seen as attractive subunits for device capability at the molecular level. Mn(II)9 grids, formed by self-assembly processes using tritopic pyridine-2,6-dihydrazone ligands, provide easy and pre-programmable routes to such systems, and have been shown to exhibit a number of potentially useful physical properties, which could be utilized to generate bi-stable molecular based states. Their ability to form surface monolayers, which can be mapped by STM techniques, bodes well for their possible integration into nanometer scale electronic components of the future. This report highlights some new Mn(II)9 grids, with functionalized ligand sites, that may provide suitable anchor points to surfaces and also be potential donor sites capable of further grid elaboration. Structures, magnetic properties, electrochemical properties, surface studies on HOPG (highly ordered pyrolytic graphite), including the imaging of individual metal ion sites in the grid using CITS (current imaging tunneling spectroscopy) are discussed, in addition to an analysis of the photophysics of a stable mixed oxidation state [Mn(III)4Mn(II)5] grid. The grid physical properties as a whole are assessed in the light of reasonable approaches to the use of such molecules as nanometer scale devices.

1,8-Pyrenylene−Ethynylene Macrocycles

A concise, highly regioselective synthesis of 1,8-dibromo-4,5-dialkoxypyrenes has been developed and exploited in the synthesis of some 1,8-pyrenylene-ethynylene macrocycles. The (1)H NMR data and NICS calculations indicate that there is little or no macrocyclic ring current. Concentration-dependent UV-visible studies indicate no aggregation at low concentration, but 8b forms dimers with voids suitable for intercalation of small molecules in the solid state.

Highly π-Extended TTF Analogues with a Conjugated Macrocyclic Enyne Core

The synthesis of a class of highly pi-extended tetrathiafulvalene derivatives (1a and 1b) was explored using Sonogashira macrocyclization as a key step. The solid-state structure of 1b was characterized by X-ray single crystallography, showing a substantially bent, S-shaped molecular backbone and an ordered packing geometry in a pi-alkyl-alkyl-pi stacking fashion. Electronic and redox properties of 1b were investigated by UV-vis absorption, fluorescence spectroscopy, and cyclic voltammetry.

H-shaped OPE/OPV oligomers: a new member of 2D-conjugated fluorophore cores.

A new class of two-dimensional (2D), pi-conjugated, skewed H-shaped co-oligomers of phenylene vinylene and phenylene ethynylene (11a- d, termed H-mers) were synthesized using Sonogashira coupling and Wittig-Horner reactions as the key steps. Electronic and spectroscopic properties of the H-mers were investigated by UV-vis absorption, fluorescence spectroscopic techniques, and ab initio calculations (HF/3-21G). Two amino-containing functional H-mers (11b and 11d) were found to exhibit appealing fluorescence sensitivity to selected Brønsted acids and transition-metal ions.

Tetrathiafulvalene vinylogues as versatile building blocks for new organic materials

Although tetrathiafulvalene (TTF) and its derivatives have been extensively studied as important organic electronic materials over the past half century, tetrathiafulvalene vinyl-ogues (TTFVs) still remain a relatively underdeveloped branch in the family of TTF derivatives. Our recent work has investigated the synthesis and characterization of a class of diphenyl-substituted TTFVs carrying alkynyl functionality. The unique conformational and redox properties of such TTFV derivatives along with the versatile chemistry enabled by acetylenic groups (e.g., metal-catalyzed coupling and click reactions) have led us to a variety of functional molecular architectures ranging from oligoynes, polymers, and molecular tweezers, to macrocycles. Property studies of these new TTFV-based molecular materials point to appealing applications in molecular electronics and optoelectronics.

Biscrown-annulated TTFAQ-dianthracene hybrid: synthesis, structure, and metal ion sensing.

A new fluorescence chemosensor (3) made up of a biscrown-annulated TTFAQ receptor and two anthracene fluorophores was designed and synthesized. Its solid-state structure was disclosed by X-ray crystallographic analysis, while fluorescence titrations indicated a high sensitivity for large hard metal cations such as Ba(2+).

TTFV-based molecular tweezers and macrocycles as receptors for fullerenes.

Hybrids of tetrathiafulvalene vinylogues (TTFVs) and planar arenes were synthesized via the click reaction to form tweezer-like and macrocyclic structures. These compounds were investigated as receptors for fullerenes (C60 and C70) by UV-vis absorption and fluorescence spectroscopy.

Click synthesized dianthryl–TTFV: an efficient fluorescent turn-on probe for transition metal ions

Tetrathiafulvalene vinylogue (TTFV) was functionalized with two anthryl fluorophores via Cu(I)-catalyzed alkyne-azide [3 + 2] cycloaddition, forming a dianthryl-TTFV hybrid to show fluorescent turn-on sensing behaviour for Cu(2+), Fe(2+), and Cd(2+) ions in THF with remarkably low detection limit down to the sub-ppm level.

Direct observation of light-driven, concerted electron–proton transfer

Significance Concerted proton-coupled electron transfer (EPT) reactions in which both electrons and protons transfer in tandem are at the heart of many chemical and biological conversions including photosystem II. We report here the direct observation of absorption bands arising from photoEPT transitions, in this case, in H-bonded complexes between N-methyl-4,4′-bipyridinium cation and biologically relevant donors including tyrosine. The importance of these observations follows from the earlier experimental observations by Taube and coworkers on intervalence transfer in mixed-valence complexes. The observation of these photoEPT transitions and the appearance of reactive radical products also points to a possible, if inefficient, role in DNA photodamage and, possibly, in the formation of reactive oxygen intermediates. The phenols 4-methylphenol, 4-methoxyphenol, and N-acetyl-tyrosine form hydrogen-bonded adducts with N-methyl-4, 4′-bipyridinium cation (MQ+) in aqueous solution as evidenced by the appearance of low-energy, low-absorptivity features in UV-visible spectra. They are assigned to the known examples of optically induced, concerted electron–proton transfer, photoEPT. The results of ultrafast transient absorption measurements on the assembly MeOPhO-H---MQ+ are consistent with concerted EPT by the instantaneous appearance of spectral features for MeOPhO·---H-MQ+ in the transient spectra at the first observation time of 0.1 ps. The transient decays to MeOPhO-H---MQ+ in 2.5 ps, accompanied by the appearance of oscillations in the decay traces with a period of ∼1 ps, consistent with a vibrational coherence and relaxation from a higher υ(N-H) vibrational level or levels on the timescale for back EPT.