Factors controlling the transfer of biogenic organic species from seawater to sea spray aerosol.

Ocean waves transfer sea spray aerosol (SSA) to the atmosphere, and these SSA particles can be enriched in organic matter relative to salts compared to seawater ratios. A fundamental understanding of the factors controlling the transfer of biogenic organic matter from the ocean to the atmosphere remains elusive. Field studies that focus on understanding the connection between organic species in seawater and SSA are complicated by the numerous processes and sources affecting the composition of aerosols in the marine environment. Here, an isolated ocean-atmosphere system enables direct measurements of the sea-air transfer of different classes of biogenic organic matter over the course of two phytoplankton blooms. By measuring excitation-emission matrices of bulk seawater, the sea surface microlayer, and SSA, we investigate time series of the transfer of fluorescent species including chlorophyll-a, protein-like substances, and humic-like substances. Herein, we show the emergence of different molecular classes in SSA at specific times over the course of a phytoplankton bloom, suggesting that SSA chemical composition changes over time in response to changing ocean biological conditions. We compare the temporal behaviors for the transfer of each component, and discuss the factors contributing to differences in transfer between phases.

Dye Encapsulation in Polynorbornene Micelles.

The encapsulation efficiency of high-Tg polynorbornene micelles was probed with a hydrophobic dye 2,6-diiodoboron-dipyrromethene (BODIPY). Changes in the visible absorption spectra of aggregated versus monomeric dye molecules provided a probe for assessing encapsulation. Polynorbornene micelles are found to be capable of loading up to one BODIPY dye per ten polymers. As the hydrophilic block size increased in the polymeric amphiphiles, more of the dye was incorporated within the micelles. This result is consistent with the dye associating with the polymer backbone in the shell of the micelles. The encapsulation rate varied significantly with temperature, and a slight dependence on micellar morphology was also noted. Additionally, we report a 740 µs triplet lifetime for the encapsulated BODIPY dye. The lifetime is the longest ever recorded for a BODIPY triplet excited state at room temperature and is attributed to hindered triplet-triplet annihilation in the high-viscosity micellar shell.

Resonance Raman Spectroscopy of the T1 Triplet Excited State of Oligothiophenes.

The characterization of triplet excited states is essential for research on organic photovoltaics and singlet fission. We report resonance Raman spectra of two triplet oligothiophenes with n-alkyl substituents, a tetramer and hexamer. The spectra of the triplets are more complex than the ground state, and we find that density functional theory calculations are a useful starting point for characterizing the bands. The spectra of triplet tetrathiophene and hexathiophene differ significantly from one another. This observation is consistent with a T1 excitation that is delocalized over at least five rings in long oligomers. Bands in the 500-800 cm(-1) region are greatly diminished for an aggregated sample of hexathiophene, likely caused by fast electronic dephasing. These experiments highlight the potential of resonance Raman spectroscopy to unequivocally detect and characterize triplets in thiophene materials. The vibrational spectra can also serve as rigorous standards for evaluating computational methods for excited-state molecules.

Photogeneration and Quenching of Tryptophan Radical in Azurin.

Tryptophan and tyrosine can form radical intermediates that enable long-range, multistep electron transfer (ET) reactions in proteins. This report describes the mechanisms of formation and quenching of a neutral tryptophan radical in azurin, a blue-copper protein that contains native tyrosine (Y108 and Y72) and tryptophan (W48) residues. A long-lived neutral tryptophan radical W48• is formed upon UV-photoexcitation of a zinc(II)-substituted azurin mutant in the presence of an external electron acceptor. The quantum yield of W48• formation (Φ) depends upon the tyrosine residues in the protein. A tyrosine-deficient mutant, Zn(II)Az48W, exhibited a value of Φ = 0.080 with a Co(III) electron acceptor. A nearly identical quantum yield was observed when the electron acceptor was the analogous tyrosine-free, copper(II) mutant; this result for the Zn(II)Az48W:Cu(II)Az48W mixture suggests there is an interprotein ET path. A single tyrosine residue at one of the native positions reduced the quantum yield to 0.062 (Y108) or 0.067 (Y72). Wild-type azurin with two tyrosine residues exhibited a quantum yield of Φ = 0.045. These data indicate that tyrosine is able to quench the tryptophan radical in azurin.

Mechanism of carotenoid coloration in the brightly colored plumages of broadbills (Eurylaimidae).

The plumage carotenoids of six species from five genera of broadbills (Eurylaimidae) have been examined. These plumages are crimson, violet, purple-maroon, or yellow. Two genera also have brilliant green plumages that are produced by a combination of structural coloration and unknown carotenoids. Six different carotenoids from nine different plumage patches were identified, including two previously unknown molecules, using high-performance liquid chromatography, mass spectrometry, and MS/MS fragment analysis. The yellow pigment in Eurylaimus javanicus and Eurylaimus ochromalus is identified as the novel carotenoid, 7,8-dihydro-3'-dehydro-lutein. The yellow and green plumages of Psarisomus dalhousiae contain the unmodified dietary carotenoids lutein and zeaxanthin. The brilliant green feathers of Calyptomena viridis contain a mixture of lutein and two other xanthophylls that have previously been found only in woodpeckers (Picinae). The crimson and violet colors of Cymbirhynchus, Sarcophanops, and Eurylaimus are produced by a novel pigment, which is identified as 2,3-didehydro-papilioerythrinone. The molecular structure of this carotenoid was confirmed using (1)H nuclear magnetic resonance, correlated two-dimensional spectroscopy, and two-dimensional nuclear Overhauser effect spectroscopy. Resonance Raman (rR) spectroscopy carried out at room and low temperatures was used to probe the configuration and conformation of 2,3-didehydro-papilioerythrinone in situ within crimson C. macrorhynchos and purple-red E. javanicus feathers. The rR spectra reveal that the pigment is in an all-trans configuration and appears to be relatively planar in the feathers. The likely metabolic pathways for the production of broadbill carotenoids from dietary precursors are discussed.

Vibrational and electronic spectroscopy of the retro-carotenoid rhodoxanthin in avian plumage, solid-state films, and solution.

Rhodoxanthin is one of few retro-carotenoids in nature. These chromophores are defined by a pattern of single and double bond alternation that is reversed relative to most carotenoids. Rhodoxanthin is found in the plumage of several families of birds, including fruit doves (Ptilinopus, Columbidae) and the red cotingas (Phoenicircus, Cotingidae). The coloration associated with the rhodoxanthin-containing plumage of these fruit dove and cotinga species ranges from brilliant red to magenta or purple. In the present study, rhodoxanthin is characterized in situ by UV-Vis reflectance and resonance Raman spectroscopy to gain insights into the mechanisms of color-tuning. The spectra are compared with those of the isolated pigment in solution and in thin solid films. Key vibrational signatures are identified for three isomers of rhodoxanthin, primarily in the fingerprint region. Electronic structure (DFT) calculations are employed to describe the normal modes of vibration, and determine characteristic modes of retro-carotenoids. These results are discussed in the context of various mechanisms that change the electronic absorption, including structural distortion of the chromophore or enhanced delocalization of π-electrons in the ground-state. From the spectroscopic evidence, we suggest that the shift in absorption is likely a consequence of perturbations that primarily affect the excited state of the chromophore.

Resonance Raman spectra of a perylene bis(dicarboximide) chromophore in ground and lowest triplet states.

Resonance Raman spectroscopy is employed to probe the ground (S0) and lowest triplet (T1) excited states of a perylene bis(dicarboximide) (PDI) dimer. Four bands at ~1324, 1507, ~1535, and 1597 cm(-1) are signatures of the T1 excited state; a fifth band at ~1160 cm(-1) is tentatively assigned. Density functional calculations of an asymmetrically substituted PDI monomer match the experimental bands of the PDI dimer in both S0 and T1 states. The match supports a T1 excited state that is localized on a single PDI moiety of the dimer. The normal modes of the asymmetrically substituted PDI are correlated with ones calculated for the unsubstituted PDI in the D2h point group. Patterns in the Raman intensities are consistent with an A-term mechanism of enhancement. The positions of six bands are predicted for the resonance Raman spectrum of unsubstituted PDI in its T1 excited state. The spectra and normal-mode analysis reported here are expected to facilitate future studies of singlet fission in PDI crystals or other assemblies.

Raman microspectroscopy and vibrational sum frequency generation spectroscopy as probes of the bulk and surface compositions of size-resolved sea spray aerosol particles.

Sea spray aerosol (SSA) represents one of the largest aerosol components in our atmosphere. SSA plays a major role in influencing climate; however the overall impacts remain poorly understood due to the overall chemical complexity. SSA is comprised of a mixture of inorganic and organic components in varying proportions that change as a function of particle size and seawater composition. In this study, nascent SSA particles were produced using breaking waves, resulting in compositions and sizes representative of the open ocean. The composition of individual SSA particles ranging in size from ca. 0.15 to 10 µm is measured using Raman microspectroscopy, while the interfacial composition of collections of size-resolved particles is probed by sum frequency generation (SFG). Raman spectra of single particles have bands in the 980 to 1030 cm(-1) region associated with the symmetric stretch of the sulfate anion, the 2800 to 3000 cm(-1) region associated with carbon-hydrogen stretches, and from 3200-3700 cm(-1) associated with the oxygen-hydrogen stretches of water. The relative intensities of these features showed a strong dependence on particle size. In particular, submicrometer particles exhibited a larger amount of organic matter compared to supermicrometer particles. However, for external surfaces of homogeneous SSA particles (i.e. particles without a solid inclusion), and also the interfaces of mixed-phase particles, there was a strong SFG response in the aliphatic C-H stretching region for both sub- and supermicrometer particles. This finding suggests that organic material present in supermicrometer particles primarily resides at the interface. The presence of methylene contributions in the SFG spectra indicated disordered alkyl chains, in contrast to what one might expect for a surfactant layer on a sea salt particle. Changes in peak frequencies and relative intensities in the C-H stretching region are seen for some particles after the addition of bacteria, phytoplankton, and growth medium to the seawater. This study provides new insights into the bulk and surface composition of SSA particles and represents a step forward in our understanding of this globally abundant aerosol. It also provides insights into the development of model systems for SSA that may more accurately represent the organic layer at the surface.

Characterization of carotenoid aggregates by steady-state optical spectroscopy.

The carotenoids have low-lying triplet excited states and can self-assemble in some solvents to form weakly or strongly coupled aggregates. These qualities make carotenoid aggregates useful for studies of singlet fission, where an outstanding goal is the correlation of interchromophoric coupling to the dynamics and yield of triplet excited states from a parent singlet excited state. Three aggregates of zeaxanthin, two weakly coupled and one strongly coupled, are characterized by steady-state spectroscopic methods including temperature-dependent absorption, fluorescence, and resonance Raman spectroscopy. The absorption spectra for each type of aggregate are distinct; however, an analysis of band positions reveals some important shared characteristics and suggests that the strongly coupled H-aggregate contains a subpopulation of weakly coupled constituents. Temperature-dependent absorption spectroscopy indicates that one of the weakly coupled aggregates can be converted to the other upon heating. The emission spectra of the three aggregates have similar profiles that are overall red-shifted by more than 1000 cm(-1) relative to the monomer. The emission quantum yields of the aggregates are 5 to 30 times less than that of the monomer, with the lowest yield for the strongly coupled aggregate. The vibrational spectra of the chromophores support only slight perturbations from the structure of solvated monomers. Our findings support the conclusion that all three aggregates are best characterized as H-aggregates, in agreement with a prior theoretical study of lutein aggregates.

Rapid self-healing hydrogels.

Synthetic materials that are capable of autonomous healing upon damage are being developed at a rapid pace because of their many potential applications. Despite these advancements, achieving self-healing in permanently cross-linked hydrogels has remained elusive because of the presence of water and irreversible cross-links. Here, we demonstrate that permanently cross-linked hydrogels can be engineered to exhibit self-healing in an aqueous environment. We achieve this feature by arming the hydrogel network with flexible-pendant side chains carrying an optimal balance of hydrophilic and hydrophobic moieties that allows the side chains to mediate hydrogen bonds across the hydrogel interfaces with minimal steric hindrance and hydrophobic collapse. The self-healing reported here is rapid, occurring within seconds of the insertion of a crack into the hydrogel or juxtaposition of two separate hydrogel pieces. The healing is reversible and can be switched on and off via changes in pH, allowing external control over the healing process. Moreover, the hydrogels can sustain multiple cycles of healing and separation without compromising their mechanical properties and healing kinetics. Beyond revealing how secondary interactions could be harnessed to introduce new functions to chemically cross-linked polymeric systems, we also demonstrate various potential applications of such easy-to-synthesize, smart, self-healing hydrogels.

Infrared nanoscopy of dirac plasmons at the graphene-SiO2 interface.

We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac fermions in graphene. This is achieved by confining mid-IR radiation at the apex of a nanoscale tip: an approach yielding 2 orders of magnitude increase in the value of in-plane component of incident wavevector q compared to free space propagation. At these high wavevectors, the Dirac plasmon is found to dramatically enhance the near-field interaction with mid-IR surface phonons of SiO(2) substrate. Our data augmented by detailed modeling establish graphene as a new medium supporting plasmonic effects that can be controlled by gate voltage.

Hydrogen bonding of tryptophan radicals revealed by EPR at 700 GHz.

Redox-active tryptophans are important in biological electron transfer and redox biochemistry. Proteins can tune the electron transfer kinetics and redox potentials of tryptophan via control of the protonation state and the hydrogen-bond strength. We examine the local environment of two neutral tryptophan radicals (Trp108 on the solvent-exposed surface and Trp48 buried in the hydrophobic core) in two azurin variants. Ultrahigh-field EPR spectroscopy at 700 GHz and 25 T allowed complete resolution of all of the principal components of the g tensors of the two radicals and revealed significant differences in the g tensor anisotropies. The spectra together with (2)H ENDOR spectra and supporting DFT calculations show that the g tensor anisotropy is directly diagnostic of the presence or absence as well as the strength of a hydrogen bond to the indole nitrogen. The approach is a powerful one for identifying and characterizing hydrogen bonds that are critical in the regulation of tryptophan-assisted electron transfer and tryptophan-mediated redox chemistry in proteins.

High-yield singlet fission in a zeaxanthin aggregate observed by picosecond resonance Raman spectroscopy.

We report high-yield triplet generation by singlet fission upon photoexcitation of a new aggregate of the carotenoid all-trans 3R,3'R-zeaxanthin. The yield is determined by picosecond time-resolved resonance Raman spectroscopy, which allows direct characterization and quantification of triplet excited-state signatures and ground-state depletion. The technique and analysis reveals that triplets form within picoseconds. A quantum yield of 90-200% is derived with the assumption of weak exciton-coupling in the zeaxanthin aggregate.

Spectroscopic comparison of photogenerated tryptophan radicals in azurin: effects of local environment and structure.

Tryptophan radicals play a significant role in mediating biological electron transfer. We report the photogeneration of a long-lived, neutral tryptophan radical (Az48W*) from the native residue tryptophan-48 in the hydrophobic core of azurin. The optical absorption, electron paramagnetic resonance, and resonance Raman spectra strongly support the formation of a neutral radical, and the data are consistent with direct electron transfer between tryptophan and the copper(II) center. Spectra of the long-lived Az48W* species are compared to those of a previously studied, solvent-exposed radical at position 108 to identify signatures of tryptophan radicals that are sensitive to the local environment. The absorption maxima of Az48W* display an approximately 23 nm hypsochromic shift in the nonpolar environment. The majority of the resonance Raman frequencies are downshifted by approximately 7 cm(-1) relative to the solvent-exposed radical, and large changes in intensity are observed for some modes. The resonance Raman excitation profiles for Az48W* exhibit distinct maxima within the absorption envelope. Electron paramagnetic resonance spectroscopy yields spectra with partially resolved lines caused by hyperfine couplings; the differences between the coupling constants for the buried and solvent-exposed radical are primarily caused by variations in structure. The insights gained by electronic, vibrational, and magnetic resonance spectroscopy enhance our fundamental understanding of the effects of protein environment on radical properties. Hypotheses for the proton transfer pathway within azurin and a deprotonation rate of approximately 5 x 10(6) s(-1) are proposed.

Dye-sensitized solar cell constructed with titanium mesh and 3-D array of TiO2 nanotubes.

We have designed and constructed dye sensitized solar cells based on new, 3-D configurations of TiO(2) nanotubes. The overall efficiency of our best cells is 5.0% under standard air mass 1.5 global (AM 1.5 G) solar conditions, and the incident photon-to-current efficiency exceeds 60% over a broad part of the visible spectrum. Unlike prior nanotube-based cells where tubes are grown vertically in a 2-D array, the anodes of the present cells consist of tubes that extend radially in a 3-D array from a grid of fine titanium wires. The nanotubes are tens of micrometers in length, and the radial nature of the anode allows the photon absorption path length to exceed the electron transport distance (nanotube length). The cells are front-illuminated and do not require a transparent conductive oxide substrate at either the anode or cathode. The use of 3-D configured nanotubes and low-resistance titanium metal substrates are expected to enhance the performance and simplify the construction of large area dye-sensitized solar cells.

Toward an n-type molecular wire: electron hopping within linearly linked perylenediimide oligomers.

A series of linearly linked perylenediimide (PDI) dimers and trimers were synthesized in which the PDI pi systems are nearly orthogonal. These oligomers and several model compounds were singly reduced, and intramolecular electron hopping between the PDI molecules was probed by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy. When the functional groups attached to the ends of the oligomers were chosen to make each PDI molecule electronically equivalent, the single electron hops between the PDI molecules with rates that significantly exceed 10(7) s(-1). Rapid electron hopping between pairs of PDI molecules having orthogonal pi systems is unexpected and may expand the possible design motifs for organic electronic materials based on PDI.

Resonance Raman characterization of a stable tryptophan radical in an azurin mutant.

Tryptophan radicals play a significant role in mediating biological electron transfer and catalytic processes. Here, we employ visible and UV resonance Raman, EPR, and absorption spectroscopy along with pH/isotope studies and calculations to probe a neutral closed-shell tryptophan and its oxidized radical counterpart in a modified azurin protein. Comparison of the resonance Raman spectra of the radical and closed-shell species combined with vibrational analysis reveals important structural differences between these two tryptophan species. We experimentally observe a significant reduction in bond order of the pyrrole ring of the radical, as evidenced by a 208 cm(-1) downshift of the W3 mode (predominantly C(2)-C(3) stretch). Analysis of the spectra acquired at acidic pH and in deuterated buffer highlights those vibrational modes of the radical that are sensitive to the hydrogen-bonding environment. The most significant change caused by the deuterated buffer is a 45 cm(-1) downshift of an indole nitrogen displacement mode (W17). Our spectra provide evidence that the radical species is a strong hydrogen bond acceptor, particularly in an acidic environment. Furthermore, the pK(a) for this tryptophan radical must be less than 4.0, which falls below previously reported values for l-tryptophan in aqueous solution. The normal mode assignments of the tryptophan radical help characterize its local environment, conformation, hydrogen bonding, and protonation state within a protein.

Controlling energy and charge transfer in linear chlorophyll dimers.

A series of linkers constructed from combinations of phenyl and ethynyl groups is shown to permit ultrafast energy transfer between two chlorophylls, while allowing control over radical cation migration between them.

Structure and dynamics of the solvated electron in alcohols from resonance Raman spectroscopy.

Resonance Raman (RR) spectroscopy is used to probe the structure and excited-state dynamics of the solvated electron in the primary liquid alcohols methanol (MeOH), ethanol (EtOH), n-propanol (n-PrOH), and n-butanol (n-BuOH). The strong resonance enhancements (>or=10(4) relative to pure solvent) of the libration, CO stretch, COH bend, CH3 bend, CH2 bend, and OH stretch reveal significant Franck-Condon coupling of the intermolecular and intramolecular vibrational modes of the solvent to the electronic excitation of the solvated electron. All enhanced bands are fully accounted for by a model of the solvated electron that is comprised of several nearby solvent molecules that are only perturbed by the presence of the electron; no new molecular species are required to explain our data. The 340 cm(-1) downshift observed for the OH stretch frequency of e-(MeOH), relative to pure solvent, strongly suggests that the methanol molecules in the first solvent shell have the hydroxyl group directed linearly toward the excess electron density. The smaller downshifts observed for e-(EtOH), e-(n-PrOH), and e-(n-BuOH) are explained in terms of a OH group that is bent 28-40 degrees from linear. The Raman cross sections and absorption spectra are modeled, lending quantitative support for the inhomogeneous origin of the broad absorption spectra, the necessity of OH local motion in all enhanced Raman modes of the alcohols, and the dominant librational response of the solvent upon photoexcitation of the electron.