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Two novel dinuclear Pt(ii) complexes featuring 8-hydroxyquinoline bridges were synthesized and found to have photophysical properties dominated by ligand-centered transitions mirroring that of a mononuclear model system, [Pt(8HQ)2].

Ultrafast excited state processes of transition metal complexes (TMCs) are governed by complicated interplays between electronic and nuclear dynamics, which demand a detailed understanding to achieve optimal functionalities of photoactive TMC-based materials for many applications. In this work, we investigated a cyclometalated platinum(II) dimer known to undergo a Pt-Pt bond contraction in the metal-metal-to-ligand-charge-transfer (MMLCT) excited state using femtosecond broadband transient absorption (fs-BBTA) spectroscopy in combination with geometry optimization and normal mode calculations. Using a sub-20 fs pump and broadband probe pulses in fs-BBTA spectroscopy, we were able to correlate the coherent vibrational wavepacket (CVWP) evolution with the stimulated emission (SE) dynamics of the

The performance of a perylene monoimide annihilator is evaluated in a photochemical upconversion composition. It is found to perform up to five times better than the commonly employed rubrene annihilator at low excitation intensity, but suffers from a low annihilation singlet yield which hinders its performance under strong excitation. Upconversion action spectroscopy under broadband bias reveals that under one sun illumination, an upconversion composition employing the perylene monoimide utilizes more than 12% of the generated triplet states to generate emissive, excited singlet states. In a suitable medium, this composition could enhance the energy conversion efficiency of high band gap solar cells.

The steady-state and ultrafast to supra-nanosecond excited state dynamics of fac-[Re(NBI-phen)(CO)3(L)](PF6) (NBI-phen = 16H-benzo[4′,5′]isoquinolino[2′,1′:1,2]imidazo[4,5-f][1,10]phenanthrolin-16-one) as well as their respective models of the general molecular formula [Re(phen)(CO)3(L)](PF6) (L = PPh3 and CH3CN) has been investigated using transient absorption and time-gated photoluminescence spectroscopy. The NBI-phen containing molecules exhibited enhanced visible light absorption with respect to their models and a rapid formation (

A quaternary segmented flow regime for robust and flexible continuous biphasic chemical processes is introduced and characterized for stability and dynamic properties through over 1500 automatically conducted experiments. The flow format is then used for the continuous flow ligand exchange of cadmium selenide quantum dots under high intensity ultraviolet illumination for improved photoluminescence quantum yield.

In this work, a series of nine Re(i) diimine dicarbonyl complexes of the general molecular formula cis-[Re(N^N)2(CO)2]+ (N^N are various 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen) derivatives) were prepared and spectroscopically investigated to systematically evaluate the photophysical consequences of various substituents resident on the diimine ligands. These panchromatic absorbing chromophores were structurally characterized, evaluated for their electrochemical and spectroelectrochemical properties, and investigated using static and dynamic electronic absorption, photoluminescence (PL), and infrared spectroscopy from ultrafast to supra-nanosecond time scales. The ultrafast time-resolved infrared (TRIR) analysis was further supported by electronic structure calculations which characterized the changes within the two C[triple bond, length as m-dash]O vibrational modes upon formation of the metal-to-ligand charge transfer (MLCT) excited state. The MLCT excited state decay of this series of dicarbonyl molecules appears completely consistent with energy-gap law behavior, where the nonradiative decay rate constants increase logarithmically with decreasing excited state - ground state energy separation, except in anticipated cases where the substituents were phenyl or tert-butyl.

Many fundamental questions remain in the elucidation of energy migration mechanisms across the interface between semiconductor nanomaterials and molecular chromophores. The present transient absorption study focuses on PbS quantum dots (QDs) of variable size and band-edge exciton energy (ranging from 1.15 to 1.54 eV) post-synthetically modified with a carboxylic acid-functionalized TIPS-pentacene derivative (TPn) serving as the molecular triplet acceptor. In all instances, selective excitation of the PbS NCs at 743 nm leads to QD size-dependent formation of an intermediate with time constants ranging from 2–13 ps, uncorrelated to the PbS QD valence band potential. However, the rate constant for the delayed formation of the TPn triplet excited state markedly increases with increasing PbS conduction band energy, featuring a parabolic Marcus free energy dependence in the normal region. These observations provide evidence of an indirect triplet sensitization process being inconsistent with a concerted Dexter-like energy transfer process. The collective data are consistent with the generation of an intermediate resulting from hole trapping of the initial PbS excited state by midgap states, followed by formation of the TPn triplet excited state whose rate constant and yield increases with decreasing quantum dot size.

The manipulation of the triplet excited state manifold leads to large differences in the photophysical properties within a given class of metal–organic chromophores. By the appropriate choice of ancillary ligand, large changes can be made both to the order and nature of the lowest excited states and therefore to the resulting photophysical properties. Herein, a series of four bis-2-phenylpyridine (ppy) cyclometalated Ir(III) compounds bearing two arylisocyanide ligands were synthesized and photophysically characterized to understand the effects of using ancillary ligands featuring systematic changes in π-conjugation. By varying the arylisocyanide ligands, the photoluminescence quantum yield ranged from 5% to 49% and the excited state lifetime ranged between 24 μs and 2 ms. These variations in photophysical response are consistent with lowering the triplet ligand-centered (3LC) state of the arylisocyanide ligand as the π system was extended, confirmed by 77 K photoluminescence emission spectra and ultrafast transient absorption experiments. The latter analysis gleaned detailed insight into the importance of the interplay of the 3LC state of the phenylpyridine and arylisocyanide ligands in these polychromophic Ir(III) molecules.

Mononuclear metalloenzymes in nature can function in cooperation with precisely positioned redox-active organic cofactors in order to carry out multielectron catalysis. Inspired by the finely tuned redox management of these bioinorganic systems, we present the design, synthesis, and experimental and theoretical characterization of a homologous series of cobalt complexes bearing redox-active pyrazines. These donor moieties are locked into key positions within a pentadentate ligand scaffold in order to evaluate the effects of positioning redox non-innocent ligands on hydrogen evolution catalysis. Both metal- and ligand-centered redox features are observed in organic as well as aqueous solutions over a range of pH values, and comparison with analogs bearing redox-inactive zinc(ii) allows for assignments of ligand-based redox events. Varying the geometric placement of redox non-innocent pyrazine donors on isostructural pentadentate ligand platforms results in marked effects on observed cobalt-catalyzed proton reduction activity. Electrocatalytic hydrogen evolution from weak acids in acetonitrile solution, under diffusion-limited conditions, reveals that the pyrazine donor of axial isomer 1-Co behaves as an unproductive electron sink, resulting in high overpotentials for proton reduction, whereas the equatorial pyrazine isomer complex 2-Co is significantly more active for hydrogen generation at lower voltages. Addition of a second equatorial pyrazine in complex 3-Co further minimizes overpotentials required for catalysis. The equatorial derivative 2-Co is also superior to its axial 1-Co congener for electrocatalytic and visible-light photocatalytic hydrogen generation in biologically relevant, neutral pH aqueous media. Density functional theory calculations (B3LYP-D2) indicate that the first reduction of catalyst isomers 1-Co, 2-Co, and 3-Co is largely metal-centered while the second reduction occurs at pyrazine. Taken together, the data establish that proper positioning of non-innocent pyrazine ligands on a single cobalt center is indeed critical for promoting efficient hydrogen catalysis in aqueous media, akin to optimally positioned redox-active cofactors in metalloenzymes. In a broader sense, these findings highlight the significance of electronic structure considerations in the design of effective electron-hole reservoirs for multielectron transformations.

Homogeneous aqueous solutions of photocatalytic ensembles, consisting of [Ru(bpy)3]2+ as a photosensitizer, ascorbic acid/ascorbate as the electron source, and 10 distinct Co2+-based molecular catalysts, were evaluated for visible-light induced hydrogen evolution using high-throughput screening. The combined results demonstrate that Co2+ complexes bearing tetradentate ligands yield more active photocatalytic compositions than their congeners with pentadentate ligands while operating with high catalyst stability. Additionally, molecular Co2+ catalysts with cis open coordination sites appear to be significantly more active for hydrogen evolution than those with trans open sites. As evidenced by mass spectrometric analysis of the reactor headspace and associated deuteration experiments, the H2 gas generated in all instances was derived from aqueous protons. One of the most promising cis-disposed Co2+ species, [Co(bpyPY2Me)(CH3CN)(CF3SO3)](CF3SO3) (1), engages in highly efficient hydrogen evolving photocatalysis, achieving a turnover number of 4200 (H2/Co) and a turnover frequency of 3200 (H2/Co per h) at pH 4 under simulated sunlight (AM 1.5G, 100 mW cm−2) at room temperature. At equimolar concentrations of photosensitizer and 1, the total hydrogen produced appears to be exclusively limited by the photostability of [Ru(bpy)3]2+, which was observed to decompose into an Ru(bpy)2–ascorbate adduct, as evidenced by HPLC and ESI-MS experiments. Lowering the operating temperature from 27 to 5 °C significantly attenuates bpy dissociation from the sensitizer, resulting in a net ∼two-fold increase in hydrogen production from this composition. The primary electron transfer steps of this photocatalytic ensemble were investigated by nanosecond transient absorption spectroscopy. Photoexcited [Ru(bpy)3]2+ undergoes reductive quenching by ascorbic acid/ascorbate (kq = 2.6 × 107 M−1 s−1), releasing [Ru(bpy)3]+ from the encounter solvent cage with an efficiency of 55 ± 5%. In the presence of catalyst 1, [Ru(bpy)3]+ generated in the initial flash-quench experiment transfers an electron (ket = 2 × 109 M−1 s−1) at an efficiency of 85 ± 10% to the catalyst, which is believed to enter the hydrogen evolution cycle subsequently. Using a combinatorial approach, all ten Co2+ catalysts were evaluated for their potential to operate under neutral pH 7.0 conditions. Catalyst 7, [Co(PY4MeH2)(CH3CN)(CF3SO3)](CF3SO3), was revealed to be most promising, as its performance metrics were only marginally affected by pH and turnover numbers greater than 1000 were easily obtained in photocatalytic hydrogen generation. These comprehensive findings provide guidelines for the development of molecular compositions capable of evolving hydrogen from purely aqueous media.

A new pentadentate, redox-active ligand bpy2PYMe has been synthesized and its corresponding transition metal complexes of Fe2+ (1), Co2+ (2), Ni2+ (3), Cu2+ (4), and Zn2+ (5) have been investigated for electro- and photo-catalytic proton reduction in acetonitrile and water, respectively. Under weak acid conditions, the Co complex displays catalytic onset at potentials similar to those of the ligand centered reductions in the absence of acid. Related Co complexes devoid of ligand redox activity catalyze H2 evolution under similar conditions at significantly higher overpotentials, showcasing the beneficial effect of combining ligand-centered redox activity with a redox-active Co center. Furthermore, turnover numbers as high as 1630 could be obtained under aqueous photocatalytic conditions using [Ru(bpy)3]2+ as a photosensitizer. Under those conditions catalytic hydrogen production was solely limited by photosensitizer stability. Introduction of an electron withdrawing CF3 group into the pyridine moiety of the ligand as in bpy2PYMe-CF3 renders its corresponding Co complex 6 less active for proton reduction in electro- and photocatalytic experiments. This surprising effect of ligand substitution was investigated by means of density functional theory calculations which suggest the importance of electronic communication between Co1+ and the redox-active ligand. Taken together, the results provide a path forward in the design of robust molecular catalysts in aqueous media with minimized overpotential by exploiting the synergy between redox-active metal and ligand components.

A new Ru(II) diimine complex, [Ru(5-pyrenyl-1,10-phenanthroline)]3(PF6)2 ([Ru(py-phen)3]2+), possesses a room temperature excited state lifetime of 148 ± 8 μs.

Eighteen distinct Pt(II) terpyridyl chloride cross-reactive sensor elements incorporated into microarrays with 1000 x 250 microm well dimensions generate distinctive colorimetric and luminometric responses upon exposure to a variety of volatile organic compounds (VOCs).

The fullerene end-capped platinum acetylide donor-acceptor triad Pt(2)ThC(60) was synthesized and characterized by using photophysical methods and photovoltaic device testing. The triad consists of the platinum acetylide oligomer Ph-[triple bond, length as m-dash]-Pt(PBu3)2-[triple bond, length as m-dash]-Th-[triple bond, length as m-dash]-Pt(PBu3)2-[triple bond, length as m-dash]-Ph (Ph=phenyl and Th=2,5-thienyl, stereochemistry at both Pt centers is trans) that contains fulleropyrrolidine moieties on each of the terminal phenylene units. Electrochemistry of the triad reveals relatively low potential oxidation and reduction waves corresponding, respectively, to oxidation of the platinum acetylide and reduction of the fulleropyrrolidine units. Photoluminescence spectroscopy shows that the singlet and triplet states of the platinum acetylide chromophore are strongly quenched in the triad assembly, both in solution at ambient temperature as well as in a low-temperature solvent glass. The excited state quenching arises due to intramolecular photoinduced electron transfer to produce a charge separated state based on charge transfer from the platinum acetylide (donor) to the fulleropyrrolidine (acceptor). Picosecond time resolved absorption spectroscopy confirms that the charge transfer state is produced within 1 ps of photoexcitation, and it decays by charge recombination within 400 ps. Organic photovoltaic devices fabricated using spin-coated films of Pt2ThC60 as the active material operate with modest efficiency, exhibiting a short circuit photocurrent of 0.51 mA cm(-2) and an open circuit voltage of 0.41 V under 100 mW cm(-2)/AM1.5 illumination. The results are discussed in terms of the relationship between the mechanism of photoinduced electron transfer in the triad and the comparatively efficient photovoltaic response exhibited by the material.

The synthesis, photophysics, electronic structure, and electrochemical characterization of 4′-tert-butylacetylene-2,2′:6′,2″-terpyridineplatinum(II) chloride (1), 4′-phenylacetylene-2,2′:6′,2″-terpyridineplatinum(II) chloride (2), and their ZnII analogs are described. The PtII complexes display interesting photophysical properties, showing vibronically resolved emission spectra at room temperature in CH2Cl2, resembling a ligand localized emission profile. The photophysics and 1O2 sensitization experiments support a triplet state assignment for these emissions which are best described as an admixture of charge transfer and ligand localized components, which decay symmetrically with time as evidenced by time resolved emission spectra. Room temperature ligand-localized fluorescence emission is observed from the zinc complexes whereas phosphorescence emission from the 3π–π* manifold was obtained at 77 K in 4 : 1 EtOH/MeOH matrices doped with 10% ethyliodide. Compounds 1 and 2 display long-lived emission at room temperature, the latter possessing a longer lifetime, higher quantum yield, and longer wavelength emission. Lowering the temperature from 298 K to 77 K induces an increase in the excited state lifetime of both platinum systems together with a blue shift in their respective emission maxima, concomitant with more pronounced vibronic structure. The data are consistent with configurationally mixed triplet excited states at room temperature which persists in 77 K glasses. The corresponding Zn(II) complexes display significantly higher energy ligand-localized phosphorescence at 77 K. This latter result suggests that the nature of the metal and/or coordination environment imparts a significant electronic pertubation into the ligand-localized triplet states of these conjugated terpyridyl structures.

Selective low energy excitation of the metal-to-ligand charge transfer (MLCT) transition in [Ru(dmb)3]2+ (dmb = 4,4′-dimethyl-2,2′-bipyridine) in the presence of anthracene or 9,10-diphenylanthracene yields easily visualized upconverted singlet fluorescence resulting from triplet–triplet annihilation at low excitation power.

Chiral molecular squares based on the Pt-alkynyl linkage were synthesized via stepwise directed assembly, and exhibit interesting dual luminescence at room temperature which is potentially exploitable for chiral sensory applications.

We describe the synthesis and photophysical properties of new Ru(II) complexes bearing different numbers of pyrenylethynylene substituents in either the 5 or 5,5′ positions of 2,2′-bipyridine. Static and dynamic absorption and luminescence measurements reveal the nature of the lowest excited states in each molecule. The 5-substituted complexes display behavior dominated by triplet intraligand π,π* excited states, generating long-lived room temperature phosphorescence in the red. While the photophysical properties in the 5,5′-substituted case are still largely influenced by triplet intraligand π,π* excited states, the data suggest the possibility of an excited state manifold composed of “mixed” intraligand and charge transfer character.