Ruthenium complexes bearing terpyridyl ligands of distinct donor-acceptor configuration for solar energy conversion

A novel series of heteroleptic ruthenium complexes employing a tbutyl-functionalized terpyridine (tpy) as a donor and tpy functionalized with up to three carboxylic acid groups as acceptors are prepared. Structural tailoring of these complexes helped not only to tune the photo-excited-state lifetimes but also aided in accumulating electron density through the anchoring groups. Wider separation between the triplet metal-to-ligand charge transfer (3MLCT) state and the metal-centered states is achieved while maintaining a reasonable barrier between the S0 ground and the 3MLCT states. This electronic state alignment facilitated the electron injection into TiO2 photoanodes from the excited states of complexes. The structural configuration comprising a tbutyl-tpy donor and three carboxylate-functionalized tpy acceptors around the ruthenium center ensured faster interfacial electron transfer (IET) and charge dissipation into the TiO2 clusters to longer diffusion lengths. This work presents a facile synthetic route to access these push-pull complexes and evaluates their use as molecular photosensitizers via a combined experimental and computational approach. Quantum dynamics simulation results demonstrate the extent of vectorial electron transfer from these complexes into the TiO2 cluster.

Factorizing the Nephelauxetic Effect in Heteroleptic Molecular Rubies

The interest in chromium(III) complexes has been renewed over the past decade for the design of efficient earth-abundant phosphorescent red-to-near-infrared spin-flip emitters and photocatalysts with long excited state lifetimes. In this context, we report the energy tuning of spin-flip excited states based on heteroleptic bis(tridentate) polypyridine chromium(III) complexes [3X,Y]3+, namely, [3NMe,CH2]3+, [3NMe,S]3+ and [3CH2,S]3+ with the tridentate ligands LX and LY [X/Y = NMe, N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine; X/Y = CH2, 2,6-bis(2-pyridylmethyl)pyridine and X/Y = S, 2,6-bis(pyridine-2-ylthio)pyridine]. The heteroleptic complexes [3X,Y]3+ are obtained via a novel synthetic approach toward the required intermediate labile triflato complexes Cr(LX)(OTf)3 (2X) from the respective chlorido precursors CrCl3(LX) (1X) using trimethylsilyl trifluoromethanesulfonate. Spin-flip energies were experimentally detected by vis/near-infrared absorption and emission spectroscopy as well as computationally derived by multireference calculations. Together with the known homoleptic molecular ruby complexes, the three resulting series of luminescent complexes [3X,X]3+/[3X,Y]3+/[3Y,Y]3+ allow delineation of an additive nephelauxetic effect of the ligands with chromium(III) ions and thus prediction of spin-flip emission energies of derived molecular rubies.

Bridge editing of spin-flip emitters gives insight into excited state energies and dynamics

Six-coordinate chromium(iii) complexes with high spin-flip (SF) photoluminescence quantum yields and lifetimes (molecular rubies) have attracted huge interest in the past years due to their applicability in sensing, photocatalysis or circularly polarised emission. However, clearcut design rules for high quantum yields and lifetimes are still lacking due to the multidimensional problem of the non-radiative decay of the SF states. Based on an isostructural series of complexes differing in the ligand backbone, we disentangle decisive structural and electronic features for SF excited state energies and non-radiative decays promoted by spin-orbit coupling, Jahn-Teller distortions and (thermally activated) multiphonon relaxation. This analysis goes beyond the classical increasing of the ligand field strength or the metal-ligand covalency to reduce non-radiative decay or to tune the SF energy. The results underscore the utility of the combination of near-infrared absorption, variable temperature emission and fs-transient absorption spectroscopy as well as photolysis and high-level quantum chemical calculations to obtain a comprehensive picture of the excited dynamics on ultrafast and long timescales.

Rhenium(I) Complexes with Sulfur-Bridged Dipyridyl Ligands: Structural, Photophysical, and Computational Studies

A series of six new rhenium(I) tricarbonyl complexes [Re(CO)3(N-N)Br] bearing sulfur-bridged dipyridyl (N-N) ligands with three different oxidation states (sulfide (S), sulfoxide (SO), and sulfone (SO2)) are described. Spectroscopic studies show that changing the oxidation state of the ligands influences the photophysical properties of the complexes, with complexes 3 and 6 containing the sulfone ligand exhibiting a lower energy MLCT absorption band tailing into the visible region. Solution-state emission measurements show that these complexes exhibit readily tunable emission energies from 480 to 610 nm, depending on the oxidation state of the sulfur bridge and the presence of substituents on the pyridyl rings. Solid-state emission measurements show that the emission is significantly red-shifted upon oxidation of the sulfur bridge to sulfone with enhanced photoluminescence quantum yield.

Synthesis and Theoretical and Photophysical Study on a Series of Neutral Ruthenium(II) Complexes with Donor-Metal-Accepter Configuration

In order to construct a new type of ruthenium(II) terpyridine complexes with activated triplet metal-centered (³MC) states, as well as stabilized triplet metal-to-ligand charge transfer (³MLCT) states, conducive to fine emissive performances, Ru-1, Ru-2, Ru-3, and Ru-4 were synthesized. Compared with the [Ru(terpyridine)₂]²⁺ prototype (0.25 ns), this series of ruthenium(II) terpyridine complexes exhibit lengthened excited state lifetime (43.3 ns for Ru-1, 52.7 ns for Ru-2, 43.6 ns for Ru-3, and 53.4 ns for Ru-4). Interfragment charge transfer analysis illustrates the electron transfer direction of the four complexes, manifesting their intramolecular charge transfer characteristic. When excited, their lowest-lying triplet states are assigned as ³MLCT based on spin-density surface distribution. The singlet excited states and ³MLCT states were thoroughly studied by UV-visual absorption and nanosecond time-resolved transient absorption spectra, respectively. Photoluminescence spectra revealed their weak broadband near-infrared emission at room temperature and red phosphorescence at 77 K. The low molecular weight and the good thermal stability make Ru-1 and Ru-2 suitable for vaporization coating, while the fine solubility in common organic solvents makes Ru-3 and Ru-4 suitable for solution processing. Furthermore, the intrinsic electroneutrality and favorable energy levels endow them with new potential to be applied in the optoelectronic field.

Pushing steric limits in osmium(IV) tetraaryl complexes

Investigations into the reactivity, properties, and applications of osmium(IV) tetraaryl complexes have been hampered by their low yielding syntheses from volatile and toxic OsO4 (typically ≤34%). Here we show that...

Controlling ultralong room temperature phosphorescence in organic compounds with sulfur oxidation state

Sulfur oxidation state is used to tune organic room temperature phosphorescence (RTP) of symmetric sulfur-bridged carbazole dimers. The sulfide-bridged compound exhibits a factor of 3 enhancement of the phosphorescence efficiency, compared to the sulfoxide and sulfone-bridged analogs, despite sulfone bridges being commonly used in RTP materials. In order to investigate the origin of this enhancement, temperature dependent spectroscopy measurements and theoretical calculations are used. The RTP lifetimes are similar due to similar crystal packing modes. Computational studies reveal that the lone pairs on the sulfur atom have a profound impact on enhancing intersystem crossing rate through orbital mixing and screening, which we hypothesize is the dominant factor responsible for increasing the phosphorescence efficiency. The ability to tune the electronic state without altering crystal packing modes allows the isolation of these effects. This work provides a new perspective on the design principles of organic phosphorescent materials, going beyond the rules established for conjugated ketone/sulfone-based organic molecules.

Controlling photocatalytic reduction of CO2 in Ru(II)/Re(I) dyads via linker oxidation state

Electronic communication between the linked metal centers in Ru(ii)-Re(i) dyads is tuned using the oxidation state (S and SO2) of sulfur-bridged ligands. Higher catalytic activity is seen for the SO2-bridged dyad in the photocatalytic reduction of CO2.