Large Molar Absorption Research Articles

Effect of Side Substituents Incorporated into π‐Bridges of Quinoxaline‐Based Sensitizers for Dye‐Sensitized Solar Cells

Two new metal‐free donor (D)–acceptor (A)–π–A‐type sensitizers with 2,3‐dihexylquinoxaline (HQ) and 2,3‐diphenylquinoxaline (PQ) moieties as additional acceptors, respectively, coded as ZW002 and ZW003, are synthesized and applied in dye‐sensitized solar cells (DSSCs). Both dyes have broad absorption and large molar absorption coefficients. The sensitizer with the HQ core effectively restrains intermolecular aggregation due to steric hindrance of numerous alkyl chains. A thienyl unit with or without a hexyl branch is used as a π‐bridge. Incorporation of only thienyl groups as a π‐bridge into the sensitizer increases the dye‐loading amount on the TiO2 film due to the smaller molecular size. Using the ZW002 dye with the Co (II/III) redox couple results in DSSCs that exhibit a high photovoltaic conversion efficiency of 8.23% and short‐circuit current density (Jsc) of 12.43 mA cm−2, an open‐circuit voltage (Voc) of 960 mV, and a fill factor (FF) of 0.69. Although ZW003 shows a wider absorption range than that of ZW002, the two additional phenyl groups on the quinoxaline unit contribute to poorer performance (power conversion efficiency [PCE] of 7.43%) due to more serious dye aggregation.

Conjugated Polymer Nanoparticles as Unique Coinitiator-Free, Water-Soluble, Visible-Light Photoinitiators of Vinyl Polymerization.

The use of conjugated polymer nanoparticles (CP NPs) of poly(9,9-dioctylfluorene-alt-benzothiadiazole) and poly(9,9-di-n-octylfluorenyl-2,7-diyl) as efficient photoinitiator systems (PIS) of vinyl polymerization in water is reported herein. CP NPs are biocompatible, excitable with blue commercial LEDs and, unlike visible light Type II PIS, do not need co-initiators to trigger a monomer chain reaction. CP NPs photoinitiate polymerization of a variety of acrylic monomers with initiation rates comparable to those observed for well-known Type II PIS. Given the extraordinarily large molar absorption coefficients of CP NPs (≈108 m-1 cm-1 ) very low particle concentration is required for effective polymerization. Additionally, CP NPs behave as conventional macrophotoinitiators significantly reducing contamination risks due to leaching of low molecular weight byproducts. These combined features make CP NPs PIS suitable to synthesize polymeric materials for many healthcare and biomedical applications including drug delivery, tissue engineering, prosthetic implants, and food/medicine packaging. These CP NPs PIS are also used to synthesize nano-hydrogels with a relatively narrow and controlled size distribution in the absence of surfactants. It is proposed that polymerization is initiated at the CP NPs surface by photogenerated free polarons, in close analogy to the mechanism previously described for PIS based on inorganic semiconductor NPs.

Near‐Infrared Emitting Heterobimetallic Zn‐4f Schiff Base Complexes with Visible Excitation Wavelength

Herein, we report the synthesis and characterization of heterobimetallic Zn‐4f complexes formed by the “valnap” ligand (L1) with extended aromaticity, thereby acting as visibly absorbing antenna for generating the near‐infrared (NIR) luminescence of three different lanthanide(III) ions (Ln3+). The series of new Zn(py)L1Ln(NO3)3 complexes [Ln = Yb (1), Nd (2), Er (3)] have been characterized by single‐crystal X‐ray diffraction and were found to be isotypic, with a variation in the Ln3+ coordination number (CN = 9–11) in line with increasing atomic radius. Absorption spectra recorded for L1, ZnL1, and complexes 1–3 display ligand‐based absorption bands in the UV (λmax ≈ 320 nm) and visible regions (λmax ≈ 430 nm) with molar absorption coefficients up to 6.3 × 104 and 3.9 × 104 m–1 cm–1, respectively. Upon excitation into ligand‐centered absorption bands in the visible range, characteristic NIR emission signals of Yb3+ (1), Nd3+ (2), and Er3+ (3) have been obtained. Quantum yields measured under visible excitation at 430 nm (QLnL) and observed luminescence lifetimes (τobs) reflecting the excited states attributed to the 2F5/2 (Yb3+), 4F3/2 (Nd3+) and 4I13/2 (Er3+) levels have been determined. These data quantifying the luminescence properties of this family of complexes and absorption bands in the visible range with large molar absorption coefficients make complexes 1–3 of potential interest for biological applications where UV light is inappropriate.

Precise Molecular Engineering of Photosensitizers with Aggregation‐Induced Emission over 800 nm for Photodynamic Therapy

AbstractOwing to efficient singlet oxygen (1O2) generation in aggregate state, photosensitizers (PSs) with aggregation‐induced emission (AIE) have attracted much research interests in photodynamic therapy (PDT). In addition to high 1O2 generation efficiency, strong molar absorption in long‐wavelength range and near‐infrared (NIR) emission are also highly desirable, but difficult to achieve for AIE PSs since the twisted structures in AIE moieties usually lead to absorption and emission in short‐wavelength range. In this contribution, through acceptor engineering, a new AIE PS of TBT is designed to show aggregation‐induced NIR emission centered at 810 nm, broad absorption in the range between 300 and 700 nm with a large molar absorption coefficient and a high 1O2 generation efficiency under white light irradiation. Further, donor engineering by attaching two branched flexible chains to TBT yielded TBTC8, which circumvented the strong intermolecular interactions of TBT in nanoparticles (NPs), yielding TBTC8 NPs with optimized overall performance in 1O2 generation, absorption, and emission. Subsequent PDT results in both in vitro and in vivo studies indicate that TBTC8 NPs are promising candidates in practical application.

Photochemical degradation of halogenated estrogens under natural solar irradiance.

Halogenated estrogens are thought to be moderately potent endocrine-disrupting compounds that are formed during chlorine-based wastewater disinfection processes and may represent a significant fraction of the total amount of estrogen delivered from wastewater treatment plants to receiving waters. Yet we lack key information about the photochemical degradation of halogenated estrogens, a process that has important implications for UV-based wastewater treatment and environmental fate modeling. To better understand halogenated estrogen degradation in aquatic environments, we studied the direct photolysis of 17β-estradiol (E2), 2-bromo-17β-estradiol (monoBrE2), 2,4-dibromo-17β-estradiol (diBrE2), and 2,4-dichloro-17β-estradiol (diClE2) as well as the indirect photolysis of diBrE2 under natural solar irradiance. We found that direct photolysis rate constants increased with halogenation as pKa values decreased and molar absorptivity spectra shifted toward higher wavelengths. Compared to E2, quantum yields were threefold larger for monoBrE2, but 15-32% smaller for the dihalogenated forms. The rate of diBrE2 (pKa ∼ 7.5) photolysis was strongly influenced by pH. At pH 7, diBrE2 degraded on minute time scales due to the large red-shifted molar absorptivity values and greater quantum yields of the phenolate form. Degradation rates were only slightly different in the presence of Suwannee River Humic Acid (5 mg L-1), and quenching experiments pointed to excited triplet state dissolved organic matter (3DOM*) as the dominant reactive intermediate responsible for the indirect photolysis of diBrE2. Overall, our data suggest that halogenated estrogens are particularly susceptible to photochemical degradation at environmentally relevant pH values.

Molar Range Detection Based on Sideband Differential Absorption Spectroscopy with a Concentrated Reference.

Conventional absorption spectroscopy (CAS) with a blank reference has only a slight capacity to detect high concentrations at characteristic wavelengths owing to the corresponding large molar absorption coefficient (ε) on the scale of 103 or 104 cm-1 M-1. To monitor concentrated analytes as high as the molar range in a plating bath and on a chemical production line, we propose a new approach using sideband differential absorption spectroscopy (SDAS). SDAS is obtained by subtracting the absorption spectra of the samples, A(λ,Cx), from that of a reference containing a concentrated standard analyte, A(λ,Cref>Cx), resulting in concave spectra with peaks at the sideband of conventional spectra with generally low ε values on the scale of 100 cm-1 M-1 or less. The negative absorbance changes linearly with the sample concentration at a certain peak wavelength, obeying Lambert-Beer's law. In this work, SDAS was obtained and verified using inorganic and organic substances, such as chromate potassium, rhodamine B, and paracetamol.

One-Step Assembly of Visible and Near-Infrared Emitting Metallacrown Dimers Using a Bifunctional Linker.

A family of dimeric LnIII [12-MCGa(III)N(shi) -4] metallacrowns (MCs) (LnIII =Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) was synthesized using the isophthalate group (ip2- ) as a linker. The [LnGa4 ]2 complexes exhibit remarkable photophysical properties, with large molar absorptivities of ≈4×104 m-1 cm-1 , high quantum yields and long luminescence lifetimes with values of (i) 31.2(2)% and 1.410(1) ms, respectively for the visible-emitting [TbGa4 ]2 complex and (ii) 2.43(6)% and 30.5(1) μs for the near-infrared (NIR) emitting [YbGa4 ]2 in the solid state. The NIR emission was obtained not only from Yb, Nd, and Er complexes but also from the less frequently observed emitters such as Pr and Ho. In addition, emission in both visible and NIR domains could be detected for Dy and Sm MCs. ESI-MS and UV/Vis data revealed that the complexes are highly stable in dimethylsulfoxide (DMSO) solution with the 1 H- and COSY-NMR spectra of the diamagnetic [YGa4 ]2 analogue providing evidence for long-term solution stability. This new approach allows one to construct a basis for highly luminescent MCs that may be further modified to be adapted for applications such as optical imaging.

5]Helicene derivatives containing aromatic imide moiety: Synthesis, structure, and photophysical properties

A series of diaryl-substituted [5]helicene derivatives containing aromatic imide moiety were effectively synthesized. From the single crystal of 3, obviously helical structure with a torsion angle of 65.4° was detected. Compared with their tetrahydro[5]helicene analogues, the [5]helicene derivatives showed longer absorption wavelength and larger molar absorptivity, which might be attributed to the large π-conjugated skeleton that was beneficial to electron delocalization at ground state. Moreover, except for 5k, long emission wavelength (520–545nm), medium fluorescence quantum yields (14.4–21.6%) and large Stokes shifts (89–95nm) of the [5]helicene derivatives were also found, which could make them be found wide potential applications in optoelectronic materials.

Furan-based diketopyrrolopyrrole chromophores: Tuning the spectroscopic, electrochemical and aggregation-induced fluorescent properties with various intramolecular donor-acceptor spacers

A family of furan-based diketopyrrolopyrrole chromophores, which comprise different donor or acceptor spacers, was synthesized by classic Suzuki-Miyaura reactions between dibrominated diketopyrrolopyrrole intermediate and five substituted aromatic boronic acids. They have the same π-extended unit and different pyridyl, thienyl and triarylamino tails with various intramolecular donor-acceptor spacers. Furthermore, absorption and emission spectral studies reveal that all dyes show extraordinarily large molar absorption coefficients (εmax = 22 400–120 000 L mol−1 cm−1) and high luminescence quantum yields (Φs = 27–88%). Moreover, the TGA studies indicate that the triarylamino-extended compounds have excellent thermal stabilities. A systematical investigation has been carried out, including energy gap correlation among optical, electrochemical and computational data. All the targeted chromophores exhibit potential applications in luminescent materials and optoelectronic devices.

Solvatochromic Probes Displaying Unprecedented Organic Liquids Discriminating Characteristics.

Four highly fluorescent derivatives of bis(phenyl-ethynyl-)-2-naphthyl (BPEN) with push-pull structures were designed and synthesized, of which azetidine was adopted as an electron-donating unit. For the electron withdrawing moiety, it varies from hydrogen, to formyl, then to the 2-ethoxyethyl derivative of dicyanovinyl, and finally to dicyanovinyl itself, and the corresponding fluorophores are denoted as A1, A2, A3, and A4, respectively. To enhance the solubility of the compounds, two n-hexadecyl residues were grafted onto the side positions of BPEN. Interestingly, introduction of azetidine not only improves the fluorescence quantum yield and enlarges the Stoke's shift of the parent compound but also endows them, in particular A2 and A4, exceptional capability to distinguish structurally relevant organic liquids, such as ethylbenzene and its isomers (o-xylenes, m-xylenes, and p-xylenes), monoalkyl-substituted benzene derivatives, gasolines of different grades, and other organic liquids. Theoretical calculation and Lippert-Mataga equation-based tests revealed the intramolecular charge transfer (ICT) nature of the solvatochromic properties of the compounds. Further quantitative analysis of the data obtained from studies of the probes/n-hexane-dioxane systems revealed the big differences in the dipole moments between the excited and ground states of A1, A2, A3, and A4, which are about 23, 29, 43, and 38 D, respectively. Moreover, the four novel fluorophores possess exceptional photochemical stability as demonstrated by the fact that more than 2 h of UV light illumination did not result in detectable reduction in the fluorescence emission of the fluorophores. It is the long wavelength absorption (>380, ≈400, >410, and >430 nm), large molar absorption coefficient (>59 000, >52 000, >39 000, and >34 000 cm-1 M-1), great color change (400-620 nm), and good solubility in common organic liquids that makes the as developed compounds, in particular A2 and A4, very competitive solvatochromic probes.

Ga(3+)/Ln(3+) Metallacrowns: A Promising Family of Highly Luminescent Lanthanide Complexes That Covers Visible and Near-Infrared Domains.

Luminescent lanthanide(III)-based molecular scaffolds hold great promises for materials science and for biological applications. Their fascinating photophysical properties enable spectral discrimination of emission bands that range from the visible to the near-infrared (NIR) regions. In addition, their strong resistance to photobleaching makes them suitable for long duration or repeated biological experiments using a broad range of sources of excitation including intense and focalized systems such as lasers (e.g., confocal microscopy). A main challenge in the creation of luminescent lanthanide(III) complexes lies in the design of a ligand framework that combines two main features: (i) it must include a chromophoric moiety that possesses a large molar absorptivity and is able to sensitize several different lanthanide(III) ions emitting in the visible and/or in the near-infrared, and (ii) it must protect the Ln(3+) cation by minimizing nonradiative deactivation pathways due to the presence of -OH, -NH and -CH vibrations. Herein, a new family of luminescent Ga(3+)/Ln(3+) metallacrown (MC) complexes is reported. The MCs with the general composition [LnGa4(shi)4(C6H5CO2)4(C5H5N) (CH3OH)] (Ln-1, Ln = Sm(3+)-Yb(3+)) were synthesized in a one pot reaction using salicylhydroxamic acid (H3shi) with Ga(3+) and Ln(3+) nitrates as reagents. The molecular structure of [DyGa4(shi)4(C6H5CO2)4(C5H5N) (CH3OH)] was obtained by X-ray analysis of single crystals and shows that the complex is formed as a [12-MCGa(III)shi-4] core with four benzoate molecules bridging the central Dy(3+) ion to the Ga(3+) ring metals. The powder X-ray diffraction analysis demonstrates that all other isolated complexes are isostructural. The extended analysis of the luminescence properties of these complexes, excited by the electronic states of the chromophoric ligands, showed the presence of characteristic, sharp f-f transitions that can be generated not only in the NIR (Sm, Dy, Ho, Er, Yb) but also in the visible (Sm, Eu, Tb, Dy, Tm). All Ln-1 complexes possess very high quantum yield values with respect to other literature compounds, indicating a good sensitization efficiency of the [12-MCGa(III)shi-4] scaffold. Especially, as of today, the Yb-1 complex exhibits the highest NIR quantum yield reported for a lanthanide(III) complex containing C-H bonds with a value of 5.88(2)% in the solid state. This work is a significant step forward toward versatile, easily prepared luminescent lanthanide(III) complexes suitable for a variety of applications including highly in demand biological imaging, especially in the NIR domain.

Synthesis, photophysical and acidochromic properties of a series of tetrahydrodibenzo[a,i]phenanthridine chromophores

A series of luminescent 5-aryl-tetrahydrodibenzo[a,i]phenanthridines was developed through modified synthetic protocol using SnCl2-acetic acid as a catalyst. The synthesis was consistent with various functionalities and significantly improves the yields. These derivatives exhibit large molar absorption coefficients and higher Stokes shift values in the range 9739–2628 cm−1. The donor and acceptor groups significantly induced the charge transfer character leading to widespread emission properties in the range of 390–531 nm with moderate quantum yields. The protonation and quaternization studies of these derivatives revealed the formation of a new red shifted absorption band followed by hyperchromic effect in emission, which could be used as an acidochromic sensor.

Synthesis and Electroluminescent Properties of OLED Green Dopants Based on BODIPY Derivatives

Organic light-emitting diodes (OLEDs) based on the small organic molecules have gained remarkable attention for display and solid-state lighting applications over the last few years due to their well-defined molecular structures, facile syntheses and modifications, and specific structure-property correlations. One approach that has been used to improve the efficiency of the OLED devices in term of brightness and stability is the introducing of the highly fluorescent organic guest dopants into the host materials to obtain the full color display. Therefore, there is still a need for the development of new dopants with good color purity, stability and high efficiency. Difluoroboradiaza-s-indacene (BODIPY) represents an outstanding class of fluorophore with high quantum yield, large molar absorption coefficient, high chemical, thermal and photophysical stability. The development of new BODIPY derivatives has become a booming of research due to their potential applications in luminescent devices, chemical sensors, biological labeling, and photovoltaic cells. Major efforts have been devoted to obtain the welldesigned BODIPY structures by modifying of the pyrrole core, fusing some aromatic rings to the BODIPY core, and replacing the 8-carbon atom with a nitrogen atom to form aza-BODIPY. The incorporation of boron element into the π-conjugated framework may lead to the appearance of unique electronic and photophysical properties. Recently, we have successfully explored some green dopants based on arylamine 2,3-disubstituted bithiophene derivatives for OLEDs. In the course of our ongoing studies, herein we wish to report the novel green dopants based on BODIPY and two different arylamine moieties with good overall performance in the multilayer device (Figure 1). In the design of the target molecules 5a–b (Figure 1), the outstanding fluorescent BODIPY core was fused with different arylamine moieties which were expected to enhance the light emitting efficiency through the extension of the

Efficient Asymmetric Synthesis of Tryptophan Analogues Having Useful Photophysical Properties

Two new fluorescent probes of protein structure and dynamics have been prepared by concise asymmetric syntheses using the Schöllkopf chiral auxiliary. The site-specific incorporation of one probe into dihydrofolate reductase is reported. The utility of these tryptophan derivatives lies in their absorption and emission maxima which differ from those of tryptophan, as well as in their large Stokes shifts and high molar absorptivities.

Progress in the Studies of Croconium Dyes

Croconium dye is an important part in the cyanine family used as organic function dye, which has been attracted extensive attention recently because of its strong and sharp absorption, large molar absorption coefficient and good photo- chemical stability in the near-infrared region. The unique structure, and physical and chemical properties, together with the recent progress in the preparation methods of croconium dyes are reviewed. The promising application in the fields of optical materials, biology, medicine, etc. is also introduced. Keywords croconium dye; structure; property; synthesis; application

Solvent and Media Effects on the Photophysics of Naphthoxazole Derivatives

The photophysical properties of 2-phenyl-naphtho[1,2-d][1,3]oxazole, 2(4-N,N-dimethylaminophenyl)naphtho[1,2-d][1,3]oxazole and 2(4-N,N-diphenylaminophenyl) naphtho[1,2-d][1,3]oxazole were studied in a series of solvents. UV-Vis absorption spectra are insensitive to solvent polarity whereas the fluorescence spectra in the same solvent set show an important solvatochromic effect leading to large Stokes shifts. Linear solvation energy relationships were employed to correlate the position of fluorescence spectra maxima with microscopic empirical solvent parameters. This study indicates that important intramolecular charge transfer takes place during the excitation process. In addition, an analysis of the solvatochromic behavior of the UV-Vis absorption and fluorescence spectra in terms of the Lippert-Mataga equation shows a large increase in the excited-state dipole moment, which is also compatible with the formation of an intramolecular charge-transfer excited state. We propose both naphthoxazole derivatives as suitable fluorescent probes to determine physicochemical microproperties in several systems and as dyes in dye lasers; consequence of their high fluorescence quantum yields in most solvents, their large molar absorption coefficients, with fluorescence lifetimes in the range 1-3 ns as well as their high photostability.

ChemInform Abstract: Triplet Photosensitizers: From Molecular Design to Applications

AbstractReview: 248 refs.

Symmetric Bis-Azospiropyrans: Synthesis, Characterization and Colorimetric Study

Synthesis and characterization of some novel symmetric bis-azospiropyrans are reported in this study. These bis-azospiropyrans are bifunctional chromophores with two spiropyrans linked by a bis-azo extended aromatic system that produce more color strength (large molar absorption coefficient in mero forms) due to appending two azospiropyran chromophores on one molecule. Comparing to the molar absorption coefficients of the conventional spiropyran chromophores () and mono-azospiropyran chromophores (), the novel synthesized photochromes showed astonishingly increased molar absorption coefficients () at the same conditions. Such high molar absorption coefficients confers high sensitivity to light and more color intensity of mero form, that leads to improvement of their light sensitivity and better discrimination of spiro (OFF) form from mero (ON) ones in molecular switches. The structures were deduced from their MS, FT-IR, and -NMR spectroscopic data and CHN analysis. All the synthesized photochemically bifunctional compounds revealed fluorescent emission in their colorless form which was faded out after exposing to UV light. Fluorescence quantum yield values of the mero forms were 0.25-0.81 and two high fluorescence quantum yield values (0.60 and 0.81) were found in these series.

Multiple roles of photosynthetic and sunscreen pigments in cyanobacteria focusing on the oxidative stress.

Cyanobacteria have two types of sunscreen pigments, scytonemin and mycosporine-like amino acids (MAAs). These secondary metabolites are thought to play multiple roles against several environmental stresses such as UV radiation and desiccation. Not only the large molar absorption coefficients of these sunscreen pigments, but also their antioxidative properties may be necessary for the protection of biological molecules against the oxidative damages induced by UV radiation. The antioxidant activity and vitrification property of these pigments are thought to be requisite for the desiccation and rehydration processes in anhydrobiotes. In this review, the multiple roles of photosynthetic pigments and sunscreen pigments on stress resistance, especially from the viewpoint of their structures, biosynthetic pathway, and in vitro studies of their antioxidant activity, will be discussed.

Triplet photosensitizers: from molecular design to applications

Triplet photosensitizers (PSs) are compounds that can be efficiently excited to the triplet excited state which subsequently act as catalysts in photochemical reactions. The name is originally derived from compounds that were used to transfer the triplet energy to other compounds that have only a small intrinsic triplet state yield. Triplet PSs are not only used for triplet energy transfer, but also for photocatalytic organic reactions, photodynamic therapy (PDT), photoinduced hydrogen production from water and triplet-triplet annihilation (TTA) upconversion. A good PS should exhibit strong absorption of the excitation light, a high yield of intersystem crossing (ISC) for efficient production of the triplet state, and a long triplet lifetime to allow for the reaction with a reactant molecule. Most transition metal complexes show efficient ISC, but small molar absorption coefficients in the visible spectral region and short-lived triplet excited states, which make them unsuitable as triplet PSs. One obstacle to the development of new triplet PSs is the difficulty in predicting the ISC of chromophores, especially of organic compounds without any heavy atoms. This review article summarizes some molecular design rationales for triplet PSs, based on the molecular structural factors that facilitate ISC. The design of transition metal complexes with large molar absorption coefficients in the visible spectral region and long-lived triplet excited states is presented. A new method of using a spin converter to construct heavy atom-free organic triplet PSs is discussed, with which ISC becomes predictable, C60 being an example. To enhance the performance of triplet PSs, energy funneling based triplet PSs are proposed, which show broadband absorption in the visible region. Applications of triplet PSs in photocatalytic organic reactions, hydrogen production, triplet-triplet annihilation upconversion and luminescent oxygen sensing are briefly introduced.