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.

Silicon-RosIndolizine fluorophores with shortwave infrared absorption and emission profiles enable in vivo fluorescence imaging.

In vivo fluorescence imaging in the shortwave infrared (SWIR, 1,000-1,700 nm) and extended SWIR (ESWIR, 1,700-2,700 nm) regions has tremendous potential for diagnostic imaging. Although image contrast has been shown to improve as longer wavelengths are accessed, the design and synthesis of organic fluorophores that emit in these regions is challenging. Here we synthesize a series of silicon-RosIndolizine (SiRos) fluorophores that exhibit peak emission wavelengths from 1,300-1,700 nm and emission onsets of 1,800-2,200 nm. We characterize the fluorophores photophysically (both steady-state and time-resolved), electrochemically and computationally using time-dependent density functional theory. Using two of the fluorophores (SiRos1300 and SiRos1550), we formulate nanoemulsions and use them for general systemic circulatory SWIR fluorescence imaging of the cardiovascular system in mice. These studies resulted in high-resolution SWIR images with well-defined vasculature visible throughout the entire circulatory system. This SiRos scaffold establishes design principles for generating long-wavelength emitting SWIR and ESWIR fluorophores.

Switch-on near infrared emission in albumin behind dark fabric: toward application in forensic latent bloodstain detection.

Latent bloodstain detection remains imperative for crime scene investigators. Widely used luminol offers high sensitivity to human blood, but can produce untrustworthy results from a bleach-cleaned crime scene or in a room not dark enough. Furthermore, dark pigments impede imaging bloodstains covered by dark materials with previously reported bloodstain detection agents. A novel on/off human albumin-sensing dye (SO3C7) is reported herein with a longer emission wavelength (942 nm) than previous materials that allows imaging behind ∼5 mm of black fabric. The switch-on emission of SO3C7 is selective and sensitive to human albumin and lasts longer than luminol (24-48 hours). Emission studies, transient absorption spectra (TAS), and near-infrared (NIR) photographs herein describe the albumin sensing properties of the dye.

Symmetric dicyanobenzothiadiazole (DCBT) dyes with a 1.5 eV excited state reduction potential range.

Strong molecular photooxidants are important in many disciplines including organic synthesis and renewable energy. In these fields, strongly oxidizing chromophores are employed to drive various transformations from challenging bond formations to energy storage systems. A range of photooxidant strengths are needed to drive these processes. A series of 8 symmetrically bisarylated 5,6-dicyano[2,1,3]benzothiadiazole (DCBT) dyes were studied for their tunability toward breadth of light absorption and photooxidant strength. The dye oxidation strength and light absorption tunability is the result of appending various aryl substituents on the periphery of the DCBT core which shows remarkable tunability of the final chromophore. The dyes are studied via steady-state absorption and emission, time-correlated single photon counting, computational analysis, and cyclic voltammetry. In changing the peripheral aryl substituents via electronics, sterics, and π-conjugation length, a series of dyes are arrived at with a dramatic 1.5 eV range in oxidizing strength and >200 nm (0.95 eV) absorption maxima tunability. Furthermore, two dyes in the series exhibit strong oxidizing strength while still approaching red light absorbance (>650 nm onset) which provides unique opportunities for the use of lower energy light to affect chemical transformations. Ultimately, this series provides options for photooxidations that allow for energetic tuning and selectivity for a given chemical transformation.

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.

Latent bloodstain detection using a selective turn-on NIR fluorescence dye responsive to serum albumin.

Bloodstain detection can provide crucial information and evidence at a crime scene; however, the ability to selectively detect bloodstains in a non-destructive manner with high sensitivity and low background is limited. This study reports a fluorescent dye (sulfonate indolizine squaraine, SO3SQ) for bloodstain visualization under near-infrared (NIR) irradiation. While the dye itself is minimally fluorescent in aqueous solution, it exhibits a "turn-on" mechanism upon binding with human serum albumin (HSA) as the fluorescence intensity increases over 160 times with strong absorption and emission at 693 nm and 758 nm, respectively. Bloodstains can be visualized on a surface even after being diluted 1000 times, and washed latent bloodstains can be detected with high sensitivity. Further, the turn-on fluorescent emission lasts for a minimum of seven days, allowing adequate time for detection. This study also indicates that the SO3SQ can sensitively detect bloodstain after the bloodstain aged for one week. Furthermore, the detection of latent blood fingerprint patterns from colorful backgrounds is demonstrated using this non-destructive method. The selective turn-on fluorescent dye with NIR excitation and emission is highly suitable in forensic science for bloodstain visualization.

Fused Double Donor Design with a Cross-Conjugated Dibenzosilin for Dye-Sensitized Solar Cells.

Dye-sensitized solar cells (DSCs) can provide a clean energy solution to growing energy demands. In order to have devices of high performance, sensitizers that are able to absorb in the near-infrared region (NIR) are needed. Stronger electron donors are needed for intramolecular charge-transfer sensitizers to access longer wavelength photons. Thus, two novel organic dyes with a cross-conjugated dibenzosilin double donor design are studied herein. The double donor delocalizes multiple filled orbitals across both amine donors due to the fused design that planarizes the donor as observed computationally, which improves intramolecular charge-transfer strength. The dyes are studied via density functional theory (DFT), optical spectroscopy, electrochemistry, and in DSC devices. The studies indicate that the dye design can reduce recombination losses, allowing for improved DSC device performances relative to a single arylamine donor. The reduction in recombination losses is attributed to the six alkyl chains that are incorporated into the donor, which offer good surface protection.

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.

Heteroacene-Based Polymer with Fast-Switching Visible-Near Infrared Electrochromic Behavior.

The electrochromic properties and application of electronically conducting polymers (ECPs) (PTRPZ-EDOT) consisting of a 3,4-ethylenedioxythiophene (EDOT) and the heteroacene-based molecular scaffold, 6H-pyrrolo[3,2-b:4,5-b'] bis [1,4] benzothiazine (TRPZ), are reported. Known for its high electron mobility and conducting properties, the novel TRPZ scaffold was synthesized to possess two EDOT molecules termini affording TRPZ-EDOT. Electropolymerization of TRPZ-EDOT resulted in remarkable spectroscopic and conductive properties suitable for electrochromic device fabrication. Using atomic force microscopy (AFM), the average surface roughness and surface topography of PTRPZ-EDOT polymer thin films were determined. Spectroelectrochemical data showed that the polymer achieved switching times of 4.07 (coloration) and 0.47 s (bleaching) at 539 nm. The PTRPZ-EDOT film exhibits an optical contrast of 36-44% at 539 nm between its neutral and colored states, respectively. The NIR region from 1000 to 1700 nm shows the appearance of charge carrier bands with a 0-1 V potential range. An electrochromic device was successfully fabricated from PTRPZ-EDOT, showcasing the potential and applicability of the polymer material for advanced technologies such as smart windows, flexible electrochromic screens, and energy storage devices.

An indolizine squaraine-based water-soluble NIR dye for fluorescence imaging of multidrug-resistant bacteria and antibacterial/antibiofilm activity using the photothermal effect.

The majority of nosocomial infections are caused by bacteria with antimicrobial resistance and the formation of biofilms, such as implant-related bacterial infections and sepsis. There is an urgent need to develop new strategies for early-stage screening, destruction of multidrug-resistant bacteria, and efficient inhibition of biofilms. Organic dyes that absorb and emit in the near-infrared (NIR) region are potentially non-invasive, high-resolution, and rapid biological imaging materials. In this study, a non-toxic and biocompatible indolizine squaraine dye with water-solubilizing sulfonate groups (SO3SQ) is studied for bacterial imaging and photothermal therapy (PTT). PTT is efficient in eliminating microorganisms through local hyperthermia without the risk of developing drug-resistant bacteria. The optical properties of SO3SQ are studied extensively in phosphate-buffered saline (PBS). UV-Vis-NIR absorption spectra analysis shows a strong absorption between 650 nm - 1000 nm. SO3SQ allows for the wash-free fluorescence imaging of drug-resistant bacteria via NIR fluorescence imaging due to a "turn-on" fluorescence property of the dye when interacting with bacteria. Although SO3SQ exhibits no toxicity against both Gram-positive bacteria and Gram-negative bacteria, the PTT property of SO3SQ is efficient in killing bacteria as well as inhibiting and eradicating biofilms. PTT experiments demonstrate that SO3SQ reduces 90% of cell viability in bacterial strains under NIR radiation with a minimum inhibition concentration (MIC90) of >450 µg/mL. The PTT property of SO3SQ can also inhibit biofilms (BIC90 = 1000-2000 µg/mL) and eradicate both preformed young and mature biofilms (MBEC90 = 1500-2000 µg/mL) as observed by crystal violet assays.

Molecular Switch Cobalt Redox Shuttle with a Tunable Hexadentate Ligand.

Strong-field hexadentate ligands were synthesized and coordinated to cobalt metal centers to result in three new low-spin to low-spin Co(III/II) redox couples. The ligand backbone has been modified with dimethyl amine groups to result in redox potential tuning of the Co(III/II) redox couples from -200 to -430 mV versus Fc+/0. The redox couples surprisingly undergo a reversible molecular switch rearrangement from five-coordinate Co(II) to six-coordinate Co(III) despite the ligands being hexadentate. The complexes exhibit modestly faster electron self-exchange rate constants of 2.2-4.2 M-1 s-1 compared to the high-spin to low-spin redox couple [Co(bpy)3]3+/2+ at 0.27 M-1 s-1, which is attributed to the change in spin state being somewhat offset by this coordination switching behavior. The complexes were utilized as redox shuttles in dye-sensitized solar cells with the near-IR AP25 + D35 dye system and exhibited improved photocurrents over the [Co(bpy)3]3+/2+ redox shuttle (19.8 vs 18.0 mA/cm2). Future directions point toward pairing the low-spin to low-spin Co(II/III) tunable series to dyes with significantly more negative highest occupied molecular orbital potentials that absorb into the near-IR where outer sphere redox shuttles have failed to produce efficient dye regeneration.

N-Heterocyclic Carbene Gold Complexes in a Photocatalytic CO2 Reduction Reaction.

Molecular catalysts that are durable and highly selective in the photocatalytic CO2 reduction reaction (PCO2RR) are in high demand. Molecular gold complexes are underexplored in the CO2RR manifold despite heterogeneous gold-CO2 reduction catalyst counterparts being frequently studied. In this report, a series of N-heterocyclic carbene (NHC)-ligated Au complexes are evaluated in the PCO2RR with an added photosensitizer (tris(2-phenylpyridine)iridium, Ir(ppy)3). The complexes were each studied with and without an added activator used to open a coordination site on the Au complexes. Results show an example of an exceptionally durable PCO2RR catalyst lasting >100 h with high product selectivity for CO. Heterogeneity tests reveal no evidence of a nonhomogeneous active catalyst, and structure-activity relationships of the molecular complexes are discussed.

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.

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.

Dye-sensitized solar cells strike back.

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Iron Redox Shuttles with Wide Optical Gap Dyes for High-Voltage Dye-Sensitized Solar Cells.

A series of iron polypyridyl redox shuttles were synthesized in the 2+ and 3+ oxidation states and paired with a series of wide optical gap organic dyes with weak aryl ether electron-donating groups. High voltage dye-sensitized solar cell (HV-DSC) devices were obtained through controlling the redox shuttle energetics and dye donor structure. The use of aryl ether donor groups, in place of commonly used aryl amines, allowed for the lowering of the dye ground-state oxidation potential which enabled challenging to oxidize redox shuttles based on Fe2+ polypyridyl structures to be used in functional devices. By carefully designing a dye series that varies the number of alkyl chains for TiO2 surface protection, the recombination of electrons in TiO2 to the oxidized redox shuttle could be controlled, leading to HV-DSC devices of up to 1.4 V.

Structural, optical, photocatalytic, and optoelectronic properties of Zn2SnO4 nanocrystals prepared by hydrothermal method.

Zn2SnO4 (ZTO) nanocrystals are extensively studied in various fields. However, size-dependent ZTO nanocrystals are still challenging to understand their structural, optical, photocatalytic, and optoelectronic properties. ZTO nanocrystals are synthesized by a facile hydrothermal reaction method. The structural properties of the synthesized ZTO nanocrystals are studied by x-ray diffraction and transmission electron microscope. The sizes of the ZTO nanocrystals are controlled by the pH values of the precursor and the molar ratios of the Zn:Sn in the starting materials. ZTO nanocrystals with the small size of 6 nm and large size of 270 nm are obtained by our method. The Eu3+ ions are doped into ZTO nanocrystals to probe size-dependent Eu doping sites, which shows significant potential applications in light emitting diode phosphors. Moreover, the photocatalytic activity of ZTO nanocrystals on rhodamine (RhB) decoloration are investigated, and the results show that 6 nm ZTO nanocrystals show better performance in the photocatalytic decoloration of RhB compared to 270 nm nanocrystals. Most importantly, we design and fabricate optoelectronic devices to detect IR light based on our nanocrystals and a self-prepared NIR cyanine dye. The device based on small sized ZTO nanocrystals exhibits better device performance under 808 nm IR light compared to that of the large sized ZTO nanocrystals. We believe this work represents ZTO size-dependent properties in term of structural, optical, photocatalytic, and optoelectronic properties as a multifunctional material.

SWIR emissive RosIndolizine dyes with nanoencapsulation in water soluble dendrimers.

Shortwave infrared (SWIR) emission has great potential for deep-tissue in vivo biological imaging with high resolution. In this article, the synthesis and characterization of two new xanthene-based RosIndolizine dyes coded PhRosIndz and tolRosIndz is presented. The dyes are characterized via femtosecond transient absorption spectroscopy as well as steady-state absorption and emission spectroscopies. The emission of these dyes is shown in the SWIR region with peak emission at 1097 nm. TolRosIndz was encapsulated with an amphiphilic linear dendritic block co-polymer (LDBC) coded 10-PhPCL-G3 with high uptake yield. Further, cellular toxicity was examined in vitro using HEK (human embryonic kidney) cells where a >90% cell viability was observed at practical concentrations of the encapsulated dye which indicates low toxicity and reasonable biocompatibility.

Photoinduced Generation of a Durable Thermal Proton Reduction Catalyst with in Situ Conversion of Mn(bpy)(CO)3Br to Mn(bpy)2Br2.

The conversion of protons to H2 is a critical reaction for the design of renewable fuel generating systems. Robust, earth-abundant, metal-based catalysts that can rapidly facilitate this reduction reaction are highly desirable. Mn(bpy)(CO)3Br generates an active catalyst for the proton reduction reaction upon photolysis at a high, directly observed H2 production rate of 1 300 000 turnovers per hour, with a low driving force for this reaction. Through the use of FcMe10 as an electron source, a proton source (triflic acid, 4-cyanoanilinium, or tosylic acid), and MeCN/H2O as solvent, the thermal reaction at room temperature was found to proceed until complete consumption of the electron source. No apparent loss in catalytic activity was observed to the probed limit of 10 000 000 turnovers of H2. Interestingly, a catalytically competent complex (Mn(bpy)2Br2), which could be isolated and characterized, formed upon photolysis of Mn(bpy)(CO)3Br in the presence of acid.