Efficient Bimolecular Mechanism of Photochemical Hydrogen Production Using Halogenated Boron-Dipyrromethene (Bodipy) Dyes and a Bis(dimethylglyoxime) Cobalt(III) Complex

A series of Boron-­dipyrromethene (Bodipy) dyes were used as photosensitizers for photochemical hydrogen production in conjunction with [CoIII(dmgH)2pyCl] (where dmgH = dimethylglyoximate, py = pyridine) as the catalyst and triethanolamine (TEOA) as the sacrificial electron donor. The Bodipy dyes are fully characterized by electrochemistry, x-­‐ray crystallography, quantum chemistry calculations, femtosecond transient absorption and time-­‐resolved fluorescence, as well as in long-­‐term hydrogen production assays. Consistent with other recent reports, only systems containing halogenated chromophores were active for hydrogen production, as the long-­‐lived triplet state is necessary for efficient bimolecular electron transfer. Here, it is shown that the photostability of the system improves with Bodipy dyes containing a mesityl group versus a phenyl group, which is attributed to increased electron donating character of the mesityl substituent. Unlike previous reports, the optimal ratio of chromophore to catalyst is established and shown to be 20:1, at which point this bimolecular dye/catalyst system performs 3-­‐4 times better than similar chemically linked systems. We also show that the hydrogen production drops dramatically with excess catalyst concentration. The maximum turnover number of ~700 (with respect to chromophore) is obtained under the following conditions: 1.0 × 10­‐4 M [Co(dmgH)2pyCl], 5.0 × 10-6 M Bodipy dye with iodine and mesityl substituents, 1:1 v:v (10% aqueous TEOA):MeCN (adjusted to pH 7), and irradiation by light with λ \u3e 410 nm for 30 h. This system, containing discrete chromophore and catalyst, is more active than similar linked Bodipy – Co(dmg)2 dyads recently published, which, in conjunction with our other measurements, suggests that the nominal dyads actually function bimolecularly

Longer lifespan in male mice treated with a weakly estrogenic agonist, an antioxidant, an α-glucosidase inhibitor or a Nrf2-inducer

The National Institute on Aging Interventions Testing Program (ITP) evaluates agents hypothesized to increase healthy lifespan in genetically heterogeneous mice. Each compound is tested in parallel at three sites, and all results are published. We report the effects of lifelong treatment of mice with four agents not previously tested: Protandim, fish oil, ursodeoxycholic acid (UDCA) and metformin – the latter with and without rapamycin, and two drugs previously examined: 17-α-estradiol and nordihydroguaiaretic acid (NDGA), at doses greater and less than used previously. 17-α-estradiol at a threefold higher dose robustly extended both median and maximal lifespan, but still only in males. The male-specific extension of median lifespan by NDGA was replicated at the original dose, and using doses threefold lower and higher. The effects of NDGA were dose dependent and male specific but without an effect on maximal lifespan. Protandim, a mixture of botanical extracts that activate Nrf2, extended median lifespan in males only. Metformin alone, at a dose of 0.1% in the diet, did not significantly extend lifespan. Metformin (0.1%) combined with rapamycin (14 ppm) robustly extended lifespan, suggestive of an added benefit, based on historical comparison with earlier studies of rapamycin given alone. The α-glucosidase inhibitor, acarbose, at a concentration previously tested (1000 ppm), significantly increased median longevity in males and 90th percentile lifespan in both sexes, even when treatment was started at 16 months. Neither fish oil nor UDCA extended lifespan. These results underscore the reproducibility of ITP longevity studies and illustrate the importance of identifying optimal doses in lifespan studies

Excited state dynamics, molecular interactions, and electron transfer in systems for the photochemical production of hydrogen

Thesis (Ph. D.)--University of Rochester. Department of Chemistry, 2015.Photochemical H2 production involves the absorption of light by a chromophore, subsequent electron transfer to a catalyst, reduction of protons to form H2, and then regeneration of the chromophore to continue the cycle. While H2 is produced on the order of minutes and hours, the preceding processes occur on a much faster timescale, from seconds to femtoseconds. Electron transfer from chromophore to catalyst competes with a number of other excited state dynamics, all of which can vary wildly from system to system. This thesis will document both the H2 production and spectroscopic characterization of a series of different organic and inorganic chromophores. By measuring such rates as internal conversion, intersystem crossing, and energy and electron transfer, an understanding of the relative activities was attained, as well as intuition on how to improve upon the system. Chapter 1 describes the theory involved with this thesis. Chapter 2 details the methods used to characterize the systems studied. Chapter 3 provides an introduction to molecular chromophores currently in the literature. Chapter 4 describes a series of Boron-dipyrromethene (Bodipy) dyes, which were studied to determine the effects of the triplet state on photochemical hydrogen production. Halogens (Br, I) were added to the 2,6 positions of the parent dye in order to increase spin-orbit coupling, facilitating intersystem crossing. Ultrafast transient absorption spectroscopy (TAS) revealed the lifetime of the singlet excited state (S1) to be 3-5 ns, 1.2 ns, and 130 ps for the parent, brominated, and iodinated dye, respectively. The decrease in S1 lifetime throughout the series is attributed to an increase in the rate of intersystem crossing. Using platiized titanium dioxide (Pt-TiO2), only the halogenated dyes were active for photochemical hydrogen production, suggesting that the long-lived triplet state is necessary for bimolecular electron transfer, and thus H2 production. In Chapter 5, a series of Bodipy dyes are used in conjunction with a cobalt dimethylglyoxime (Co(dmg)) catalyst. Only halogenated Bodipy chromophores were active for H2 production, and dyes with a mesityl substituent were more stable than those with a phenyl substituent, due to the more basic nature of the aryl. Although recent literature articles describe H2 production using a Bodipy – Co(dmg) dyad, the higher activity from this study, using discrete molecules, sheds doubt that the dyad actually remains intact upon photolysis. Chapter 6 compares the activity of chalcogenorhodamine dyes, attached to TiO2, used in dye sensitized solar cells (DSSCs) and photochemical H2 production systems. While the oxygen and selenium derivative performed similarly in DSSCs, the selenium derivative greatly outperformed its oxygen counterpart in H2 production. TAS revealed ultrafast electron transfer in conditions similar to DSSCs but not in conditions similar to H2 production, making the long-lived triplet state only beneficial in H2 production. The discrepancy in electron transfer rates appears to be caused by the presence or absence of aggregation in the system, altering the coupling between the dye and TiO2. This finding demonstrates the importance of understanding the differences between, as well as the effects of the conditions for DSSCs and solar hydrogen production. Chapter 7 describes a photophysical comparison of a large group of rhodamine dyes, characterizing the deactivation pathways upon excitation. One major result was that the thienyl dyes have shorter lifetimes than their phenyl counterparts, yet they have similar activity when attached to Pt-TiO2 for photochemical H2 production. This was attributed to better coupling to the surface of TiO2, due to a smaller dihedral angle between the xanthylium core and aryl. The relevance of both the charge-separated and triplet state was also studied for the different derivatives In Chapter 8, dyads consisting of a Bodipy chromophore and a Pt(diimine)(dithiolate) complex were studied both spectroscopically and for H2 production. To produce H2 effectively, excitation of either moiety should result in electron transfer from the Pt complex to the catalyst. However, excitation of the Bodipy moiety results in singlet energy transfer to the Pt singlet excited state, rapid intersystem crossing, and then triplet energy transfer back to the Bodipy triplet state. Two methods, lowering the solvent dielectric and placing electron withdrawing groups on the diimine, were found to lower the energy of the Pt triplet state, making triplet energy transfer unfavorable. H2 production of several dyads, attached to Pt-TiO2, all showed light harvesting improvement of the dyad relative to the Pt complex by a factor of 2-4. The insensitivity to the triplet alignment is attributed to the strong coupling of the dyad to the surface of TiO2, which results in a much faster rate of electron transfer than that of triplet energy transfer

Theoretical prediction of chiral 3D hybrid organic-inorganic perovskites

Hybrid organic-inorganic perovskites (HOIPs), in particular 3D HOIPs, have demonstrated remarkable properties, including ultralong charge-carrier diffusion lengths, high dielectric constants, low trap densities, tunable absorption and emission wavelengths, strong spin-orbit coupling, and large Rashba splitting. These superior properties have generated intensive research interest in HOIPs for high-performance optoelectronics and spintronics. Here, 3D hybrid organic-inorganic perovskites that implant chirality through introducing the chiral methylammonium cation are demonstrated. Based on structural optimization, phonon spectra, formation energy, and ab initio molecular dynamics simulations, it is found that the chirality of the chiral cations can be successfully transferred to the framework of 3D HOIPs, and the resulting 3D chiral HOIPs are both kinetically and thermodynamically stable. Combining chirality with the impressive optical, electrical, and spintronic properties of 3D perovskites, 3D chiral perovskites is of great interest in the fields of piezoelectricity, pyroelectricity, ferroelectricity, topological quantum engineering, circularly polarized optoelectronics, and spintronics.Accepted versio

Chiral perovskite optoelectronics

Hybrid organic–inorganic perovskites (HOIPs) offer long carrier diffusion lengths, high absorption coefficients, tunable bandgaps and long spin lifetimes. The flexible crystal structure and ionic nature of HOIPs makes it possible to allow tune their material properties through rational design, including the incorporation of chiral organic ligands. Recently, chiral HOIPs have emerged as promising materials for chiroptoelectronics, spintronics and ferroelectricity. They exhibit high photoluminescence polarization (17% without an external magnetic field), good device performance (a circularly polarized photodetector had 100 times higher responsivity than one based on chiral metasurface) and high saturated polarization (~2 times higher than that of barium titanate). Here we review the latest advances in chiral HOIPs and investigate the specific benefits of combining chiral organic and inorganic components in perovskites. We discuss demonstrations of chiroptical and ferroelectric applications, and conclude with our perspective on the future opportunities for chiral HOIPs.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)National Research Foundation (NRF)Accepted versionW.G., G.K.L. and A.S. acknowledge the support from the Singapore National Research Foundation through 2015 NRF fellowship grant (NRF-NRFF2015-03), Singapore Ministry of Education via AcRF Tier2 grant (No. MOE2016-T2-2-077, No. MOE2017- T2-1-163), and A*Star QTE Programme. R.S and G.L. acknowledge the support from the Australian Research Council Centre of Excellence in Exciton Science (Funding grant number CE170100026). E.H.S. acknowledges support from the U.S. Office of Naval Research (grant award no.: N00014-17-1-2524). We thank Prof. Mingtao Zhang (Nankai University) for helpful discussions

Solvation and Rotation Dynamics in the Trihexyl(tetradecyl)phosphonium Chloride Ionic Liquid/Methanol Cosolvent System

The interactions and solvent structure in trihexyl­(tetradecyl)­phosphonium chloride ionic liquid ([P_14,6,6,6⁺]­[Cl^–], “PIL-Cl”)/methanol (MeOH) solutions across the entire range of mole fraction PIL-Cl (<i>x</i>_IL = 0–1) are discussed. Viscosity and conductivity measurements are used to characterize the bulk solvent properties. At <i>x</i>_IL < 0.1, the log­(η) data show a nonlinear dependence on mole fraction in contrast to the data for <i>x</i>_IL > 0.1 where the data vary linearly with mole fraction. Conductivity data show a maximum at <i>x</i>_IL = 0.03 in good agreement with conductivity measurements in imidazolium ILs. Steady-state and time-resolved fluorescence spectroscopies were used to measure the equilibrium, lifetime, and rotational response of coumarin 153 (C153) in neat and MeOH cosolvent modified PIL-Cl. The collective set of data depicts the formation of an increasingly aggregated solvent structure that changes in proportion to the amount of PIL-Cl present in MeOH. Average solvation and rotation times are found to scale with solution viscosity. At <i>x</i>_IL values of 0.02–0.2, the rotation times are at or near the hydrodynamic stick limit, whereas for <i>x</i>_IL > 0.2 rotation times drop to between 40 and 70% of the stick limit, consistent with the IL literature. In this cosolvent system, the most dramatic changes in solution behavior occur between 0 and 10% PIL-Cl

Strong coupling and energy funnelling in an electrically conductive organic blend

Strong coupling between an exciton and a cavity photon mode offers the promise of lower lasing thresholds, which has attracted interest in organic systems working toward electrically injected lasing. However, current organic polariton lasers have yet to exhibit thresholds beyond the reach of traditional lasers. Here, we investigate the possibility of energy funnelling from host to guest in a polariton system. We construct a material blend containing a dithiophenyl diketopyrrolopyrrole dye with an electrically conductive fluorene–benzothiadiazole co-polymer matrix. We demonstrate that a polariton system can exhibit efficient host to guest energy transfer while maintaining both strong exciton–polariton coupling and polariton emission. We expect that energy funnelling will become an important tool to drive down polariton laser thresholds in organic systems

Small-Band-Offset Perovskite Shells Increase Auger Lifetime in Quantum Dot Solids

Colloidal quantum dots (CQDs) enable low-cost, high-performance optoelectronic devices including photovoltaics, photodetectors, LEDs, and lasers. Continuous-wave lasing in the near-infrared remains to be realized based on such materials, yet a solution-processed NIR laser would be of use in communications and interconnects. In infrared quantum dots, long-lived gain is hampered by a high rate of Auger recombination. Here, we report the use of perovskite shells, grown on cores of IR-emitting PbS CQDs, and we thus reduce the rate of Auger recombination by up to 1 order of magnitude. We employ ultrafast transient absorption spectroscopy to isolate distinct Auger recombination phenomena and study the effect of bandstructure and passivation on Auger recombination. We corroborate the experimental findings with model-based investigations of Auger recombination in various CQD core–shell structures. We explain how the band alignment provided by perovskite shells comes close to the optimal required to suppress the Auger rate. These results provide a step along the path toward solution-processed near-infrared lasers

Efficient Bimolecular Mechanism of Photochemical Hydrogen Production Using Halogenated Boron-Dipyrromethene (Bodipy) Dyes and a Bis(dimethylglyoxime) Cobalt(III) Complex

A series of Boron-dipyrromethene (Bodipy) dyes were used as photosensitizers for photochemical hydrogen production in conjunction with [Co^III(dmgH)₂pyCl] (where dmgH = dimethylglyoximate, py = pyridine) as the catalyst and triethanolamine (TEOA) as the sacrificial electron donor. The Bodipy dyes are fully characterized by electrochemistry, X-ray crystallography, quantum chemistry calculations, femtosecond transient absorption, and time-resolved fluorescence, as well as in long-term hydrogen production assays. Consistent with other recent reports, only systems containing halogenated chromophores were active for hydrogen production, as the long-lived triplet state is necessary for efficient bimolecular electron transfer. Here, it is shown that the photostability of the system improves with Bodipy dyes containing a mesityl group versus a phenyl group, which is attributed to increased electron donating character of the mesityl substituent. Unlike previous reports, the optimal ratio of chromophore to catalyst is established and shown to be 20:1, at which point this bimolecular dye/catalyst system performs 3–4 times better than similar chemically linked systems. We also show that the hydrogen production drops dramatically with excess catalyst concentration. The maximum turnover number of ∼700 (with respect to chromophore) is obtained under the following conditions: 1.0 × 10^–4 M [Co­(dmgH)₂pyCl], 5.0 × 10^–6 M Bodipy dye with iodine and mesityl substituents, 1:1 v:v (10% aqueous TEOA):MeCN (adjusted to pH 7), and irradiation by light with λ > 410 nm for 30 h. This system, containing discrete chromophore and catalyst, is more active than similar linked Bodipy–Co­(dmg)₂ dyads recently published, which, in conjunction with our other measurements, suggests that the nominal dyads actually function bimolecularly