Phospha-RosIndolizine Dye with Shortwave Infrared (SWIR) Absorption and Emission.

Shortwave infrared (SWIR, 1000-1700 nm) absorbing and emitting dyes are needed for infrared diodes and sensors used in a wide variety of industrial and medical applications. Herein, an electron-withdrawing phosphine oxide (P═O) substituted xanthene is coupled with strong indolizine donors to produce a SWIR absorbing (λabs = 1294 nm in DCM) and emitting (λemis = 1450 nm in DCM) dye called PRos1450. The unique properties of this dye are characterized via photophysical, electrochemical, and computational analyses.

Molecular Engineering of Stabilized Silicon-Rosindolizine Shortwave Infrared Fluorophores.

Fluorescence-based biological imaging in the shortwave infrared (SWIR, 1000-1700 nm) is an attractive replacement for modern in vivo imaging techniques currently employed in both medical and research settings. Xanthene-based fluorophores containing heterocycle donors have recently emerged as a way to access deep SWIR emitting fluorophores. A concern for xanthene-based SWIR fluorophores though is chemical stability toward ambient nucleophiles due to the high electrophilicity of the cationic fluorophore core. Herein, a series of SWIR emitting silicon-rosindolizine (SiRos) fluorophores with emission maxima >1300 nm (up to 1550 nm) are synthesized. The SiRos fluorophore photophysical properties and chemical stability toward nucleophiles are examined through systematic derivatization of the silicon-core alkyl groups, indolizine donor substitution, and the use of o-tolyl or o-xylyl groups appended to the fluorophore core. The dyes are studied via absorption spectroscopy, steady-state emission spectroscopy, solution-based cyclic voltammetry, time-dependent density functional theory (TD-DFT) computational analysis, X-ray diffraction crystallography, and relative chemical stability over time. Optimal chemical stability is observed via the incorporation of the 2-ethylhexyl silicon substituent and the o-xylyl group to protect the core of the fluorophore.

Photocatalytic Hydrogen Evolution by a De Novo Designed Metalloprotein that Undergoes Ni-Mediated Oligomerization Shift.

De novo metalloprotein design involves the construction of proteins guided by specific repeat patterns of polar and apolar residues, which, upon self-assembly, provide a suitable environment to bind metals and produce artificial metalloenzymes. While a wide range of functionalities have been realized in de novo designed metalloproteins, the functional repertoire of such constructs towards alternative energy-relevant catalysis is currently limited. Here we show the application of de novo approach to design a functional H2 evolving protein. The design involved the assembly of an amphiphilic peptide featuring cysteines at tandem a/d sites of each helix. Intriguingly, upon NiII addition, the oligomers shift from a major trimeric assembly to a mix of dimers and trimers. The metalloprotein produced H2 photocatalytically with a bell-shape pH dependence, having a maximum activity at pH 5.5. Transient absorption spectroscopy is used to determine the timescales of electron transfer as a function of pH. Selective outer sphere mutations are made to probe how the local environment tunes activity. A preferential enhancement of activity is observed via steric modulation above the NiII site, towards the N-termini, compared to below the NiII site towards the C-termini.

Selective Near-Infrared Blood Detection Driven by Ionic Liquid-Dye-Albumin Nanointeractions.

Due to its abundance in blood, a great deal of research has been undertaken to develop efficient biosensors for serum albumin and provide insight into the interactions that take place between these biosensing molecules and the protein. Near-infrared (NIR, >700 nm) organic dyes have been shown to be effective biosensors of serum albumin, but their effectiveness is diminished in whole blood. Herein, it is shown that an NIR sulfonate indolizine-donor-based squaraine dye, SO3SQ, can be strengthened as a biosensor of albumin through the addition of biocompatible ionic liquids (ILs). Specifically, the IL choline glycolate (1:1), at a concentration of 160 mM, results in the enhanced fluorescence emission ("switch-on") of the dye in the presence of blood. The origin of the fluorescence enhancement was investigated via methods, including DLS, ITC, and molecular dynamics. Further, fluorescence measurements were conducted to see the impact the dye-IL system had on the fluorescence of the tryptophan residue of human serum albumin (HSA), as well as to determine its apparent association constants in relation to albumin. Circular dichroism (CD) spectroscopy was used to provide evidence that the dye-IL system does not alter the secondary structures of albumin or DNA. Our results suggest that the enhanced fluorescence of the dye in the presence of IL and blood is due to diversification of binding sites in albumin, controlled by the interaction of the IL-dye-albumin complex.

Acid-Triggered Switchable Near-Infrared/Shortwave Infrared Absorption and Emission of Indolizine-BODIPY Dyes.

Fluorescent organic dyes that absorb and emit in the near-infrared (NIR, 700-1000 nm) and shortwave infrared (SWIR, 1000-1700 nm) regions have the potential to produce noninvasive high-contrast biological images and videos. BODIPY dyes are well known for their high quantum yields in the visible energy region. To tune these chromophores to the NIR region, fused nitrogen-based heterocyclic indolizine donors were added to a BODIPY scaffold. The indolizine BODIPY dyes were synthesized via microwave-assisted Knoevenagel condensation with indolizine aldehydes. The non-protonated dyes showed NIR absorption and emission at longer wavelengths than an aniline benchmark. Protonation of the dyes produced a dramatic 0.35 eV bathochromic shift (230 nm shift from 797 nm to 1027 nm) to give a SWIR absorption and emission (λmaxemis = 1061 nm). Deprotonation demonstrates that material emission is reversibly switchable between the NIR and SWIR.

Xanthene-Based Nitric Oxide-Responsive Nanosensor for Photoacoustic Imaging in the SWIR Window.

Shortwave infrared (SWIR) dyes are characterized by their ability to absorb light from 900 to 1400 nm, which is ideal for deep tissue imaging owing to minimized light scattering and interference from endogenous pigments. An approach to access such molecules is to tune the photophysical properties of known near-infrared dyes. Herein, we report the development of a series of easily accessible (three steps) SWIR xanthene dyes based on a dibenzazepine donor conjugated to thiophene (SCR-1), thienothiophene (SCR-2), or bithiophene (SCR-3). We leverage the fact that SCR-1 undergoes a bathochromic shift when aggregated for in vivo studies by developing a ratiometric nanoparticle for NO (rNP-NO), which we employed to successfully visualize pathological levels of nitric oxide in a drug-induced liver injury model via deep tissue SWIR photoacoustic (PA) imaging. Our work demonstrates how easily this dye series can be utilized as a component in nanosensor designs for imaging studies.

Guest-Host Raman under Liquid Nitrogen Spectroscopy for the Acquisition of Improved Vibrational Spectra of Solids.

Guest-host Raman under liquid nitrogen spectroscopy (GHRUNS) is introduced whereby solid-state guest molecules are isolated inside cage-like host environments for the facile acquisition of their Raman spectra. This convenient method features reduced fluorescence, the analysis of populations in their ground states, and increased signal to noise ratios. Samples are also preserved through the reduction of thermal degradation and oxidation. To demonstrate the benefits of this new method, Raman spectra of the ubiquitous molecule C60 inside a cage of water ice are presented. Using this technique, a new normal mode of C60 is elucidated. The GHRUNS methodology is of interest to those seeking to acquire and characterize the vibrational spectra, structure, and properties of emissive, air-sensitive molecules.

Design and Synthesis of RhodIndolizine Dyes with Improved Stability and Shortwave Infrared Emission up to 1250 nm.

The design of shortwave infrared (SWIR) emissive small molecules with good stability in water remains an important challenge for fluorescence biological imaging applications. A series of four SWIR emissive rhodindolizine (RI) dyes were rationally designed and synthesized to probe the effects of nonconjugated substituents, conjugated donor groups, and nanoencapsulation in a water-soluble polymer on the stability and optical properties of the dyes. Steric protecting groups were added at the site of a significant LUMO presence to probe the effects on stability. Indolizine donor groups with added dimethylaniline groups were added to reduce the electrophilicity of the dyes toward nucleophiles such as water. All of the dyes were found to absorb (920-1096 nm peak values) and emit (1082-1256 nm peak values) within the SWIR region. Among xanthene-based emissive dyes, emission values >1200 nm are exceptional with 1256 nm peak emission being a longer emission than the recent record setting VIX-4 xanthene-based dye. Half-lives were improved from ∼5 to >24 h through the incorporation of either steric-based core protection groups or donors with increased donation strength. Importantly, the nanoencapsulation of the dyes in a water-soluble surfactant (Triton-X) allows for the use of these dyes in biological imaging applications.

Probing halogen bonding interactions between heptafluoro-2-iodopropane and three azabenzenes with Raman spectroscopy and density functional theory.

The potential formation of halogen bonded complexes between a donor, heptafluoro-2-iodopropane (HFP), and the three acceptor heterocyclic azines (azabenzenes: pyridine, pyrimidine, and pyridazine) is investigated herein through normal mode analysis via Raman spectroscopy, density functional theory, and natural electron configuration analysis. Theoretical Raman spectra of the halogen bonded complexes are in good agreement with experimental data providing insight into the Raman spectra of these complexes. The exhibited shifts in vibrational frequency of as high as 8 cm-1 for each complex demonstrate, in conjunction with NEC analysis, significant evidence of charge transfer from the halogen bond acceptor to donor. Here, an interesting charge flow mechanism is proposed involving the donated nitrogen lone pair electrons pushing the dissociated fluorine atoms back to their respective atoms. This mechanism provides further insight into the formation and fundamental nature of halogen bonding and its effects on neighboring atoms. The present findings provide novel and deeper characterization of halogen bonding with applications in supramolecular and organometallic chemistry.

Shortwave Infrared Absorptive and Emissive Pentamethine-Bridged Indolizine Cyanine Dyes.

Shortwave infrared (SWIR)-emitting small molecules are desirable for biological imaging applications. In this study, four novel pentamethine indolizine cyanine dyes were synthesized with N,N-dimethylaniline-based substituents on the indolizine periphery at varied substitution sites. The dyes are studied via computational chemistry and optical spectroscopy both in solution and when encapsulated. Dramatic spectral shifts in the absorption and emission spectrum wavelengths with added donor groups are observed. Significant absorption and emission with an emissive quantum yield as high as 3.6% in the SWIR region is possible through the addition of multiple donor groups per indolizine.

Relative energetics of CH3CH2O, CH3CHOH, and CH2CH2OH radical products from ethanol dehydrogenation.

This study has examined the relative energetics of nine stationary points associated with the three different radical isomers generated by removing a H atom from ethanol at the O atom (ethoxy, CH3CH2O), the α C atom (CH3CHOH), and the ß C atom (CH2CH2OH). For the first time, CCSD(T) geometry optimizations and harmonic vibrational frequency computations with the cc-pVTZ and aug-cc-pVTZ basis sets have been carried out to characterize two unique minima for each isomer along with three transition state structures with Cs symmetry. Explicitly correlated CCSD(T) computations were also performed to estimate the relative energetics of these nine stationary points near the complete basis set limit. These benchmark results were used to assess the performance of various density functional theory (DFT) and wave function theory methods, and they will help guide method selection for future studies of alcohols and their radicals. The structures generated by abstracting H from the α C atom have significantly lower electronic energies (by at least 7 kcal mol-1) than the CH3CH2O and CH2CH2OH radicals. Although previously reported as a minimum on the ground-state surface, the 2A″ Cs structure of the ethoxy radical was found to be a transition state in this study with MP2, CCSD(T), and a number of DFT methods. An implicit solvation model used in conjunction with DFT and MP2 methods did not qualitatively change the relative energies of the isomers, but the results suggest that the local minima for the CH3CHOH and CH2CH2OH radicals could become more energetically competitive in condensed phase environments, such as liquid water and ethanol.

Tracking the Amide I and αCOO- Terminal ν(C=O) Raman Bands in a Family of l-Glutamic Acid-Containing Peptide Fragments: A Raman and DFT Study.

The E-hook of ß-tubulin plays instrumental roles in cytoskeletal regulation and function. The last six C-terminal residues of the ßII isotype, a peptide of amino acid sequence EGEDEA, extend from the microtubule surface and have eluded characterization with classic X-ray crystallographic techniques. The band position of the characteristic amide I vibration of small peptide fragments is heavily dependent on the length of the peptide chain, the extent of intramolecular hydrogen bonding, and the overall polarity of the fragment. The dependence of the E residue's amide I ν(C=O) and the αCOO- terminal ν(C=O) bands on the neighboring side chain, the length of the peptide fragment, and the extent of intramolecular hydrogen bonding in the structure are investigated here via the EGEDEA peptide. The hexapeptide is broken down into fragments increasing in size from dipeptides to hexapeptides, including EG, ED, EA, EGE, EDE, DEA, EGED, EDEA, EGEDE, GEDEA, and, finally, EGEDEA, which are investigated with experimental Raman spectroscopy and density functional theory (DFT) computations to model the zwitterionic crystalline solids (in vacuo). The molecular geometries and Boltzmann sum of the simulated Raman spectra for a set of energetic minima corresponding to each peptide fragment are computed with full geometry optimizations and corresponding harmonic vibrational frequency computations at the B3LYP/6-311++G(2df,2pd) level of theory. In absence of the crystal structure, geometry sampling is performed to approximate solid phase behavior. Natural bond order (NBO) analyses are performed on each energetic minimum to quantify the magnitude of the intramolecular hydrogen bonds. The extent of the intramolecular charge transfer is dependent on the overall polarity of the fragment considered, with larger and more polar fragments exhibiting the greatest extent of intramolecular charge transfer. A steady blue shift arises when considering the amide I band position moving linearly from ED to EDE to EDEA to GEDEA and, finally, to EGEDEA. However, little variation is observed in the αCOO- ν(C=O) band position in this family of fragments.

Water-Soluble NIR Absorbing and Emitting Indolizine Cyanine and Indolizine Squaraine Dyes for Biological Imaging.

Organic dyes that absorb and emit in the near-infrared (NIR) region are potentially noninvasive, high-resolution, and rapid biological imaging materials. Indolizine donor-based cyanine and squaraine dyes with water-solubilizing sulfonate groups were targeted in this study due to strong absorptions and emissions in the NIR region. As previously observed for nonwater-soluble derivatives, the indolizine group with water-solubilizing groups retains a substantial shift toward longer wavelengths for both absorption and emission with squaraines and cyanines relative to classically researched indoline donor analogues. Very high quantum yields (as much as 58%) have been observed with absorption and emission >700 nm in fetal bovine serum. Photostability studies, cell culture cytotoxicity, and cell uptake specificity profiles were all studied for these dyes, demonstrating exceptional biological imaging suitability.

Evaluating Donor Effects in Isoindigo-Based Small Molecular Fluorophores.

Small molecular organic fluorophores have garnered significant interest because of their indispensable use in fluorescence imaging (FI) and optoelectronic devices. Herein, we designed triphenylamine (TPA)-capped donor-acceptor-donor (D-A-D)-based fluorophores having a variation at the heterocyclic donor (D) units, 3,4-ethylenedioxythiophene (EDOT), furan (FURAN), thiophene (THIO), and 1-methyl-1H-pyrrole (MePyr), with isoindigo as the core electron acceptor (A) unit. Synthesis of these fluorophores (II-X-TPA) resulted in four symmetrical dye molecules: II-EDOT-TPA, II-FURAN-TPA, II-THIO-TPA, and II-MePyr-TPA, where TPA functioned as a terminal unit and a secondary electron donor group. Photophysical, electrochemical, and computational analyses were conducted to investigate the effect of heterocyclic donor units on the II-X-TPA derivatives. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations provided insightful features of structural and electronic properties of each fluorophore and correlated well with experimental observations. Electron density distribution maps, overlapping frontier molecular orbital diagrams, and highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) electron transfer indicated intramolecular charge transfer (ICT). Theoretical studies confirmed the experimental HOMO energy trend and demonstrated its crucial importance in understanding each heterocycle's donor ability. Stokes shifts of up to ∼178 nm were observed, whereas absorptions and emissions were shifted deeper into the NIR region, resulting from ICT. Results suggest that this isoindigo fluorophore series has potential as a molecular scaffold for the development of efficient FI agents. The studied fluorophores can be further tuned with different donor fragments to enhance the ICT and facilitate in shifting the optical properties further into the NIR region.

Surface and Interfacial Interactions in Dodecane/Brine Pickering Emulsions Stabilized by the Combination of Cellulose Nanocrystals and Emulsifiers.

Interfacial properties of cellulose nanocrystals (CNC) and surfactants were studied in high ionic strength (I) brines and correlated to the stability of dodecane/brine Pickering emulsions. Bis-(2-hydroxyethyl) cocoalkylamine (CAA), dodecyltrimethylammonium bromide (DTAB), and octyl-ß-d-glucopyranoside (OGP) were adsorbed onto CNC in American Petroleum Institute (API) brine (I = 1.9 M) and synthetic seawater (SSW), with I = 0.65 M. Raman spectroscopy indicated that hydroxyl groups on the CNC surface interact with all three surfactants in high ionic strength media. Ionic interactions still play a role at the very large ionic strengths studied herein. Despite all surfactants adsorbing onto CNC, only the surface tension of CAA solutions in both brines was increased by the addition of 0.5 wt % CNC. The effect was much more prominent in API than in SSW. Contact angle measurements indicated that CAA increased the wettability of CNC by both brines in dodecane; DTAB, on the other hand, decreased wettability. Emulsion stability studies revealed that ionic strength, wettability, adsorption energy, and oil content strongly affect emulsion stability, more so than surfactant adsorption. In API, CNC aggregates alone stabilized the emulsions better compared to samples with additional emulsifiers; the same was true in SSW for oil contents below 50% v/v. For oil contents above 50% v/v in SSW, CAA was either detrimental or failed to improve emulsion stability. On the other hand, DTAB increased the stability of dodecane in SSW emulsions. Emulsions stable for over 21 months were prepared with oil contents of 75% v/v. The adsorption of CAA onto CNC limits the migration of both CNC and CAA to the dodecane/brine interface, while DTAB adsorption has the opposite effect.

Indolizine-Cyanine Dyes: Near Infrared Emissive Cyanine Dyes with Increased Stokes Shifts.

Molecular engineering strategies designed to red-shift cyanine dye absorptions and emissions further into the near-infrared (NIR) spectral region are explored. Through the use of a novel donor group, indolizine, with varying cyanine bridge lengths, dye absorptions and emissions, were shifted deeper into the NIR region than common indoline-cyanines. Stokes shifts resulting from intramolecular steric interactions of up to ∼60 nm in many cases were observed and explained computationally. Molecular brightnesses of up to 5800 deep into the NIR region were observed. Structure-property relationships are explored for the six indolizine-cyanine dyes with varying cyanine bridge length and indolizine substituents showing broad absorption and emission tunability. The dyes are characterized by crystallography, and the photophysical properties are probed by varying solvent for absorption and emission studies. Computational data show involvement of the entire indolizine π-system during light absorption, which suggests these systems can be tunable even further into the NIR region through select derivatizations.

Donor-Acceptor-Donor NIR II Emissive Rhodindolizine Dye Synthesized by C-H Bond Functionalization.

A NIR II emissive dye was synthesized by the C-H bond functionalization of 1-methyl-2-phenylindolizine with 3,6-dibromoxanthene. The rhodindolizine (RhIndz) spirolactone product was nonfluorescent; however, upon opening of the lactone ring by the formation of the ethyl ester derivative, the fluorophore absorbs at 920 nm and emits at 1092 nm, which are both in the NIR II region. In addition, 4-cyanophenyl- (CNRhIndz) and 4-methoxyphenyl-substituted rhodindolizine (MeORhIndz) could also be prepared by the C-H activation reaction.

Photocatalytic H2-Evolution by Homogeneous Molybdenum Sulfide Clusters Supported by Dithiocarbamate Ligands.

Irradiation at 460 nm of [Mo3(µ3-S)(µ2-S2)3(S2CNR2)3]I ([2a]I, R = Me; [2b]I, R = Et; [2c]I, R = iBu; [2d]I, R = CH2C6H5) in a mixed aqueous-polar organic medium with [Ru(bipy)3]2+ as photosensitizer and Et3N as electron donor leads to H2 evolution. Maximum activity (300 turnovers, 3 h) is found with R = iBu in 1:9 H2O:MeCN; diminished activity is attributed to deterioration of [Ru(bipy)3]2+. Monitoring of the photolysis mixture by mass spectrometry suggests transformation of [Mo3(µ3-S)(µ2-S2)3(S2CNR2)3]+ to [Mo3(µ3-S)(µ2-S)3(S2CNR2)3]+ via extrusion of sulfur on a time scale of minutes without accumulation of the intermediate [Mo3S6(S2CNR2)3]+ or [Mo3S5(S2CNR2)3]+ species. Deliberate preparation of [Mo3S4(S2CNEt2)3]+ ([3]+) and treatment with Et2NCS21- yields [Mo3S4(S2CNEt2)4] (4), where the fourth dithiocarbamate ligand bridges one edge of the Mo3 triangle. Photolysis of 4 leads to H2 evolution but at ∼25% the level observed for [Mo3S7(S2CNEt2)3]+. Early time monitoring of the photolyses shows that [Mo3S4(S2CNEt2)4] evolves H2 immediately and at constant rate, while [Mo3S7(S2CNEt2)3]+ shows a distinctive incubation prior to a more rapid H2 evolution rate. This observation implies the operation of catalysts of different identity in the two cases. Photolysis solutions of [Mo3S7(S2CNiBu2)3]+ left undisturbed over 24 h deposit the asymmetric Mo6 cluster [(iBu2NCS2)3(µ2-S2)2(µ3-S)Mo3](µ3-S)(µ3-η2,η1-S',η1-S″-S2)[Mo3(µ2-S)3(µ3-S)(S2CNiBu2)2(µ2-S2CNiBu2)] in crystalline form, suggesting that species with this hexametallic composition and core topology are the probable H2-evolving catalysts in photolyses beginning with [Mo3S7(S2CNR2)3]+. When used as solvent, N,N-dimethylformamide (DMF) suppresses H2-evolution but to a greater degree for [Mo3S4(S2CNEt2)4] than for [Mo3S7(S2CNEt2)3]+. Recrystallization of [Mo3S4(S2CNEt2)4] from DMF affords [Mo3S4(S2CNEt2)4(η1,κO-DMF)] (5), implying that inhibition by DMF arises from competition for a Mo coordination site that is requisite for H2 evolution. Computational assessment of [Mo3S4(S2CNMe2)3]+ following addition of 2H+ and 2e- suggests a Mo(H)-µ2(SH) intermediate as the lowest energy species for H2 elimination. An analogous pathway may be available to the Mo6 cluster via dissociation of one end of the µ2-S2CNR2 ligand, a known hemilabile ligand type, in the [Mo3S4]4+ fragment.

Ullazine Donor-π bridge-Acceptor Organic Dyes for Dye-Sensitized Solar Cells.

A series of four ullazine-donor based donor-π bridge-acceptor (D-π-A) dyes have been synthesized and compared to a prior ullazine donor-acceptor (D-A) dye as well as a triphenylamine donor with an identical π-bridge and acceptor. The D-π-A ullazine series demonstrates an unusually uniform-in-intensity panchromatic UV/Vis absorption spectrum throughout the visible region. This is in part due to the introduction of strong high-energy bands through incorporation of the ullazine building block as shown by computational analysis. The dyes were characterized on TiO2 films and in DSC devices. Performances of 5.6 % power conversion efficiency were obtained with IPCE onsets reaching 800 nm.

Iodine binding with thiophene and furan based dyes for DSCs.

Iodine binding to thiophene rings in dyes for dye-sensitized solar cells (DSCs) has been hypothesized to be performance degrading in a number of literature cases. Binding of iodine to dyes near the semiconductor surface can promote undesirable electron transfers and lower the overall efficiency of devices. Six thiophene or furan containing dye analogs were synthesized to analyze iodine binding to the dyes via Raman spectroscopy, UV-Vis studies, device performance metrics and density functional theory (DFT) based computations. Evidence suggests I2 binds thiophene-based dyes stronger than furan-based dyes. This leads to higher DSC device currents and voltages from furan analogues, and longer electron lifetimes in DSC devices using furan based dyes. Raman spectrum of the TiO2 surface-bound dyes reveals additional and more instense peaks for thiophene dyes in the presence of I2 relative to no I2. Additionally, broader and shifted UV-Vis peaks are observed for thiophene dyes in the presence of I2 on TiO2 films suggesting significant interaction between the dye molecules and I2. These observations are also supported by DFT and TD-DFT calculations which indicate the absence of a key geometric energy minimum in the dye-I2 ground state for furan dyes which are readily observed for the thiophene based analogues.