Browsing by Author "Uthe, Brian"
Now showing 1 - 7 of 7
Results Per Page
Sort Options
Item Engineering giant excitonic coupling in bioinspired, covalently bridged BODIPY dyads(Royal Society of Chemistry, 2023-02-22) Ansteatt, Sara; Uthe, Brian; Mandal, Bikash; Gelfand, Rachel S.; Dunietz, Barry D.; Pelton, Matthew; Ptaszek, MarcinStrong excitonic coupling in photosynthetic systems is believed to enable efficient light absorption and quantitative charge separation, motivating the development of artificial multi-chromophore arrays with equally strong or even stronger excitonic coupling. However, large excitonic coupling strengths have typically been accompanied by fast non-radiative recombination, limiting the potential of the arrays for solar energy conversion as well as other applications such as fluorescent labeling. Here, we report giant excitonic coupling leading to broad optical absorption in bioinspired BODIPY dyads that have high photostability, excited-state lifetimes at the nanosecond scale, fluorescence quantum yields of nearly 50%. Through the synthesis, spectroscopic characterization, and computational modeling of a series of dyads with different linking moieties, we show that the strongest coupling is obtained with diethynylmaleimide linkers, for which the coupling occurs through space between BODIPY units with small separations and slipped co-facial orientations. Other linkers allow for broad tuning of both the relative through-bond and through-space coupling contributions and the overall strength of interpigment coupling, with a tradeoff observed in general between the strength of the two coupling mechanisms. These findings open the door to the synthesis of molecular systems that function effectively as light-harvesting antennas and as electron donors or acceptors for solar energy conversion.Item Highly Spherical Nanoparticles Probe Gigahertz Viscoelastic Flows of Simple Liquids Without the No-Slip Condition(ACS Publications, 2021-05-06) Uthe, Brian; Collis, Jesse F.; Madadi, Mahyar; Sader, John E.; Pelton, MatthewSimple liquids are conventionally described by Newtonian fluid mechanics, based on the assumption that relaxation processes in the flow occur much faster than the rate at which the fluid is driven. Nanoscale solids, however, have characteristic mechanical response times on the picosecond scale, which are comparable to mechanical relaxation times in simple liquids; as a result, viscoelastic effects in the liquid must be considered. These effects have been observed using time-resolved optical measurements of vibrating nanoparticles, but interpretation has often been complicated by finite velocity slip at the liquid–solid interface. Here, we use highly spherical gold nanoparticles to drive flows that are theoretically modeled without the use of the no-slip boundary condition at the particle surface. We obtain excellent agreement with this analytical theory that considers both the compression and shear relaxation properties of the liquid.Item Optical measurement of the picosecond fluid mechanics in simple liquids generated by vibrating nanoparticles: a review(IOP Science, 2022-10-17) Uthe, Brian; Sader, John E; Pelton, MatthewStandard continuum assumptions commonly used to describe the fluid mechanics of simple liquids have the potential to break down when considering flows at the nanometer scale. Two common assumptions for simple molecular liquids are that (1) they exhibit a Newtonian response, where the viscosity uniquely specifies the linear relationship between the stress and strain rate, and (2) the liquid moves in tandem with the solid at any solidliquid interface, known as the no-slip condition. However, even simple molecular liquids can exhibit a non-Newtonian, viscoelastic response at the picosecond time scales that are characteristic of the motion of many nanoscale objects; this viscoelasticity arises because these time scales can be comparable to those of molecular relaxation in the liquid. In addition, even liquids that wet solid surfaces can exhibit nanometer-scale slip at those surfaces. It has recently become possible to interrogate the viscoelastic response of simple liquids and associated nanoscale slip using optical measurements of the mechanical vibrations of metal nanoparticles. Plasmon resonances in metal nanoparticles provide strong optical signals that can be accessed by several spectroscopies, most notably ultrafast transient-absorption spectroscopy. These spectroscopies have been used to measure the frequency and damping rate of acoustic oscillations in the nanoparticles, providing quantitative information about mechanical coupling and exchange of mechanical energy between the solid particle and its surrounding liquid. This information, in turn, has been used to elucidate the rheology of viscoelastic simple liquids at the nanoscale in terms of their constitutive relations, taking into account separate viscoelastic responses for both shear and compressible flows. The nanoparticle vibrations have also been used to provide quantitative measurements of slip lengths on the single-nanometer scale. Viscoelasticity has been shown to amplify nanoscale slip, illustrating the interplay between different aspects of the unconventional fluid dynamics of simple liquids at nanometer length scales and picosecond time scales.Item Solvent-dependent energy and charge transfer dynamics in hydroporphyrin-BODIPY arrays(AIP, 2020-08-17) Uthe, Brian; Meares, Adam; Ptaszek, Marcin; Pelton, MatthewArrays of hydroporphyrins with boron complexes of dipyrromethene (BODIPY) are a promising platform for biomedical imaging or solar energy conversion, but their photophysical properties have been relatively unexplored. In this paper, we use time-resolved fluorescence, femtosecond transient absorption spectroscopy, and density-functional-theory calculations to elucidate solvent-dependent energy and electron-transfer processes in a series of chlorin- and bacteriochlorin-BODIPY arrays. Excitation of the BODIPY moiety results in ultrafast energy transfer to the hydroporphyrin moiety, regardless of the solvent. In toluene, energy is most likely transferred via the through-space Förster mechanism from the S1 state of BODIPY to the S2 state of hydroporphyrin. In DMF, substantially faster energy transfer is observed, which implies a contribution of the through-bond Dexter mechanism. In toluene, excited hydroporphyrin components show bright fluorescence, with quantum yield and fluorescence lifetime comparable to those of the benchmark monomer, whereas in DMF, moderate to significant reduction of both quantum yield and fluorescence lifetime are observed. We attribute this quenching to photoinduced charge transfer from hydroporphyrin to BODIPY. No direct spectral signature of the charge-separated state is observed, which suggests that either (1) the charge-separated state decays very quickly to the ground state or (2) virtual charge-separated states, close in energy to S1 of hydroporphyrin, promote ultrafast internal conversion.Item Unraveling the Complex Fluid Dynamics of Simple Liquids at Nanometer Length Scales and Gigahertz Frequencies(2020-01-01) Uthe, Brian; Pelton, Matthew; Physics; PhysicsThe flow of simple liquids, such as water, is often described using Newtonian fluid mechanics which predicts a purely viscous response by the liquid to the motion of an object moving in that liquid. Treating the liquid response as Newtonian and use of the no-slip boundary condition � the assumption that the tangential velocity is zero between the object and the liquid � together, are commonly used to describe the interaction of an object interacting with a liquid. These two assumptions need to be revisited for flows of simple liquids at the nanometer scale. A reduction in the size of an object causes a corresponding decrease in the mechanical response time of that structure. A nanoscale structure has a characteristic response time on the picosecond scale, which is comparable to the molecular relaxation time of simple liquids. Deviations from a purely viscous (Newtonian) response by simple liquids to the motion of nanoscale objects have been observed, with the liquid exhibiting a viscoelastic response commonly associated with complex liquids. Additionally, molecular dynamics studies predict slip lengths for wetting liquids at the single-nanometer scale and numerous experimental studies have reported sub-micrometer slip lengths. In this work, we present two studies investigating the shear and compressible flow of simple liquids at the nanoscale. We use time-resolved spectroscopy to excite and probe the gigahertz frequency dynamics of metal nanoparticle vibrations suspended in varying concentrations of glycerol in water. In the first study we excite the extensional vibrational mode of highly monodisperse gold bipyramids to generate primarily shear flow in the liquid, which is sensitive to liquid slip. We show that the inherent viscoelasticity of the liquid, at this frequency and length scale, causes a drastic enhancement in the effects of slip at the nanoparticle-liquid interface and use of the no-slip boundary condition fails to quantitatively predict the experimental damping. In the second study we excite the breathing mode vibrations of highly spherical, monodisperse gold nanoparticles which produce compressible flow in the liquid. Use of the highly spherical particles eliminates shear motion at the interface between the nanoparticle surface and the liquid ensuring that the effects of velocity slip are minimized for the nanoparticle-liquid interaction. We obtain excellent agreement between the experimentally measured damping rates and theory, thus validating the underlying compressible viscoelastic constitutive model recently developed for the liquid.Item Viscoelasticity Enhances Nanometer-Scale Slip in Gigahertz-Frequency Liquid Flows(ACS Publications, 2021-03-31) Chakraborty, Debadi; Uthe, Brian; Malachosky, Edward W.; Pelton, Matthew; Sader, John E.The interaction between flowing liquids and solid surfaces underpins many physical phenomena and technologies, such as the ability of an airfoil to generate lift and the mixing of liquids for industrial applications. These phenomena are often described using the Navier–Stokes equations and the no-slip boundary condition: the assumption that the liquid immediately adjacent to a solid surface does not move relative to the surface. Herein, we observe violation of the no-slip condition with strong enhancement of slip due to intrinsic viscoelasticity of the bulk liquid. This is achieved by measuring the 20 GHz acoustic vibrations of gold nanoparticles in glycerol/water mixtures, for which the underlying physics is explored using rigorous, theoretical models. The reported enhancement of slip revises current understanding of ultrafast liquid flows, with implications for technologies ranging from membrane filtration to nanofluidic devices and biomolecular sensing.Item Weakly conjugated bacteriochlorin-bacteriochlorin dyad: Synthesis and photophysical properties(World Scientific, 2021-06-01) Yu, Zhanqian; Uthe, Brian; Gelfand, Rachel; Pelton, Matthew; Ptaszek, MarcinDyads containing two near-infrared absorbing and emitting bacteriochlorins with distinct spectral properties have been prepared and characterized by absorption, emission, and transient-absorption spectroscopies. The dyads exhibit ultrafast (∼3 ps) energy transfer from the bacteriochlorin with the higher-energy S1 state to the bacteriochlorin emitting at the longer wavelength. The dyads exhibit strong fluorescence and relatively long excited state lifetimes (∼4 ns) in both non-polar and polar solvents, which indicates negligible photoinduced electron transfer between the two bacteriochlorins in the dyads. These dyads are thus attractive for the development of light-harvesting arrays and fluorophores for in vivo bioimaging.