Anti-Dissipative Strategies toward More Efficient Solar Energy Conversion.

In natural and artificial photosynthesis, light absorption and catalysis are separate processes linked together by exergonic electron transfer. This leads to free energy losses between the initial excited state, formed after light absorption, and the active catalyst formed after the electron transfer cascade. Additional deleterious processes, such as internal conversion (IC) and vibrational relaxation (VR), also dissipate as much as 20-30% of the absorbed photon energy. Minimization of these energy losses, a holy grail in solar energy conversion and solar fuel production, is a challenging task because excited states are usually strongly coupled which results in negligible kinetic barriers and very fast dissipation. Here, we show that topological control of oligomeric {Ru(bpy)3} chromophores resulted in small excited-state electronic couplings, leading to activation barriers for IC by means of inter-ligand electron transfer of around 2000 cm-1 and effectively slowing down dissipation. Two types of excited states are populated upon visible light excitation, that is, a bridging-ligand centered metal-to-ligand charge transfer [MLCT(Lm)], and a 2,2'-bipyridine-centered MLCT [MLCT(bpy)], which lies 800-1400 cm-1 higher in energy. As a proof-of-concept, bimolecular electron transfer with tri-tolylamine (TTA) as electron donor was performed, which mimics catalyst activation by sacrificial electron donors in typical photocatalytic schemes. Both excited states were efficiently quenched by TTA. Hence, this novel strategy allows to trap higher energy excited states before IC and VR set in, saving between 100 and 170 meV. Furthermore, transient absorption spectroscopy suggests that electron transfer reactions with TTA produced the corresponding Lm•--centered and bpy•--centered reduced photosensitizers, which involve different reducing abilities, that is, -0.79 and -0.93 V versus NHE for Lm•- and bpy•-, respectively. Thus, this approach probably leads in fine to a 140 meV more potent reductant for energy conversion schemes and solar fuel production. These results lay the first stone for anti-dissipative energy conversion schemes which, in bimolecular electron transfer reactions, harness the excess energy saved by controlling dissipative conversion pathways.

A photoinduced mixed valence photoswitch.

The ground state and photoinduced mixed valence states (GSMV and PIMV, respectively) of a dinuclear (Dp4+) ruthenium(II) complex bearing 2,2'-bipyridine ancillary ligands and a 2,2':4',4'':2'',2'''-quaterpyridine (Lp) bridging ligand were investigated using femtosecond and nanosecond transient absorption spectroscopy, electrochemistry and density functional theory. It was shown that the electronic coupling between the transiently light-generated Ru(II) and Ru(III) centers is HDA ∼ 450 cm-1 in the PIMV state, whereas the electrochemically generated GSMV state showed HDA ∼ 0 cm-1, despite virtually identical Ru-Ru distances. This stemmed from the changes in dihedral angles between the two bpy moieties of Lp, estimated at 30° and 4° for the GSMV and PIMV states, respectively, consistent with a through-bond rather than a through-space mechanism. Electronic coupling can be turned on by using visible light excitation, making Dp4+ a competitive candidate for photoswitching applications. A novel strategy to design photoinduced charge transfer molecular switches is proposed.

MLCT Excited-State Behavior of Trinuclear Ruthenium(II) 2,2'-Bipyridine Complexes.

Four trinuclear ruthenium(II) polypyridyl complexes were synthesized, and a detailed investigation of their excited-state properties was performed. The tritopic sexi-pyridine bridging ligands were obtained via para or meta substitution of a central 2,2'-bipyridine fragment. A para connection between the 2,2'-bipyridine chelating moieties of the bridging ligand led to a red-shifted MLCT absorption band in the visible part of the spectra, whereas the meta connection induced a broadening of the LC transitions in the UV region. A convergent energy transfer from the two peripheral metal centers to the central Ru(II) moiety was observed for all trinuclear complexes. These complexes were in thermal equilibrium with an upper-lying 3MLCT excited state over the investigated range of temperatures. For all complexes, deactivation via the 3MC excited state was absent at room temperature. Importantly, the connection in the para position for both central and peripheral 2,2'-bipyridines of the bridging ligand resulted in a trinuclear complex (Tpp) that absorbed more visible light, had a longer-lived excited state, and had a higher photoluminescence quantum yield than the parent [Ru(bpy)3]2+, despite its red-shifted photoluminescence. This behavior was attributed to the presence of a highly delocalized excited state for Tpp.

Efficient Convergent Energy Transfer in a Stereoisomerically Pure Heptanuclear Luminescent Terpyridine-Based Ru(II)-Os(II) Dendrimer.

The stereoisomerically pure synthesis of a novel heptanuclear Ru(II)-Os(II) antenna bearing multitopic terpyridine ligands is reported. An unambiguous structural characterization was obtained by 1H NMR spectroscopy and ion mobility spectrometry (IMS-MS). The heptanuclear complex exhibits large molar absorption coefficients (77900 M-1 cm-1 at 497 nm) and undergoes unitary, downhill, convergent energy transfer from the peripheral Ru(II) subunits to the central Os(II) that displays photoluminescence with a lifetime (τ = 161 ns) competent for diffusional excited-state electron transfer reactivity in solution.

Tuning the excited-state deactivation pathways of dinuclear ruthenium(ii) 2,2'-bipyridine complexes through bridging ligand design.

A detailed photophysical investigation of two dinuclear ruthenium(ii) complexes is reported. The two metallic centers were coordinated to a bis-2,2'-bipyridine bridging ligand, connected either through the para (Lp, Dp) or the meta position (Lm, Dm). The results obtained herein were compared to the prototypical [Ru(bpy)3]2+ parent compound. The formation of dinuclear complexes was accompanied by the expected increase in molar absorption coefficients, i.e. 12 000 M-1 cm-1, 17 000 M-1 cm-1, and 22 000 M-1 cm-1 at the lowest energy MLCTmax transition for [Ru(bpy)3]2+, Dm and Dp respectively. The Lp bridging ligand resulted in a ruthenium(ii) dinuclear complex that absorbed more visible light, and had a longer-lived and more delocalized excited-state compared to a complex with the Lm bridging ligand. Variable temperature measurements provided valuable information about activation energies to the uppermost 3MLCT state and the metal-centered (3MC) state, often accompanied by irreversible ligand-loss chemistry. At 298 K, 48% of [Ru(bpy)3]2+* excited-state underwent deactivation through the 3MC state, whereas this deactivation pathway remained practically unpopulated (<0.5%) in both dinuclear complexes.

Photostable Polynuclear Ruthenium(II) Photosensitizers Competent for Dehalogenation Photoredox Catalysis at 590 nm.

Higher nuclearity photosensitizers produced dehalogenation yields greater than 90% in the reported [Ru(bpy)3]2+-mediated dehalogenation of 4-bromobenzyl-2-chloro-2-phenylacetate to 4-bromobenzyl-2-phenylacetate with orange light in 7 h, whereas after 72 h yields of 49% were obtained with [Ru(bpy)3]2+. Dinuclear (D1), trinuclear (T1), and quadrinuclear (Q1) ruthenium(II) 2,2'-bipyridine based photosensitizers were synthesized, characterized, and investigated for their photoreactivity. Three main factors were shown to lead to increased yields (i) the red-shifted absorbance of polynuclear photosensitizers, (ii) the more favorable driving force for electron transfer, characterized by more positive E1/2(Ru2+*/+), and (iii) the smaller population of the 3MC state (<0.5% for D1, T1 and Q1 vs 48% for [Ru(bpy)3]2+ at room temperature). Collectively, these results highlight the potential advantages of using polynuclear photosensitizers in phototriggered redox catalysis reactions.

Converging Energy Transfer in Polynuclear Ru(II) Multiterpyridine Complexes: Significant Enhancement of Luminescent Properties.

Ruthenium-based complexes are widely used as photocatalysts, as photosensitizers, or as building blocks for supramolecular assemblies. In the field of solar energy conversion, building light harvesting antenna is of prime interest. Nevertheless, collecting light is mandatory but not sufficient; once collected and transferred, the exciton has to be long-lived enough to be transferred to a catalytic site. If Ru(II) terpyridine complexes are prime building blocks for structural reasons, the short lifetime of their excited state prevents their use as a harvesting center in light antennae. In this paper, we present new polynuclear assemblies, based on Ru(II)-terpyridine units where delocalization of the excited state is combined with an antenna effect. As a consequence, complexes C1-C3 display long-lived excited states compared to [Ru(tpy)2]2+, making them promising efficient antenna building blocks to be connected to a final acceptor or a catalytic center.

Access to Functionalized Luminescent Multi-2,2':6',2″-terpyridine Ligands.

A one-pot synthesis of substituted multi-2,2':6',2″-terpyridines (multi-tpy) has been achieved using an acetylquaterpyridine precursor with various aryl aldehydes in basic media. This strategy enables ready access to functionalized tri-terpyridines. Utilizing a Suzuki-type cross-coupling, larger structures such as tetra- or even hexa-tpy were obtained from our tri-tpy precursor. These macromolecular units are ideal building blocks for the construction of transition-metal-based supramolecular assemblies.