Synthesis and characterization of blue‐light emissive carbazole containing perfluorocyclobutyl arylene ether polymers

A series of N‐alkyl/aryl carbazole 3,6‐substituted arylene trifluorovinyl ether (TFVE) monomers were synthesized in high purity and yield from a concise four‐step synthesis using carbazole as a starting material. Condensate‐free, step‐growth chain extension of the monomers afforded perfluorocyclobutyl (PFCB) arylene ether homo‐ and copolymers as solution processable, optically transparent blue‐light emissive materials. Arylene TFVE monomers and conversion to PFCB arylene ether polymers were structurally elucidated and purity confirmed by high resolution mass spectroscopy, NMR (¹H, ¹³C, and ¹⁹F) spectroscopy, gel permeation chromatography, and attenuated total reflectance Fourier transform infrared analysis. Thermal analysis by differential scanning calorimetry and thermogravimetric analysis revealed glass transition temperatures >150 °C and onset of decomposition in nitrogen >410 °C with 40 wt % char yield up to 900 °C. Optical and electrochemical studies included solution (tetrahydrofuran) and solid state (spin cast thin film) UV–vis/fluorescence spectroscopy and cyclic voltammetry which showed structure dependence of these blue emissive systems on the nature of the N‐alkyl/aryl carbazole substitution in either homo‐ or copolymer configurations. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 552–560     

Conformational and Intermolecular Interaction Dynamics of Photolyase/Cryptochrome Proteins Monitored by the Time‐Resolved Diffusion Technique

Cryptochrome (CRY), a blue light sensor protein, possesses a similar domain structure to photolyase (PHR) that, upon absorption of light, repairs DNA damage. In this review, we compare the reaction dynamics of these systems by monitoring the reaction kinetics of conformational change and intermolecular interaction change based on time‐dependent diffusion coefficient measurements obtained by using the pulsed laser‐induced transient grating technique. Using this method, time‐dependent biomolecular interactions, such as transient dissociation reactions in solution, have been successfully detected in real time. Conformational change in (6‐4) PHR has not been detected after the photoexcitation by monitoring the diffusion coefficient. However, the repaired DNA dissociates from PHR with a time constant of 50 μs, which must relate to a minor conformational change. However, CRY exhibits a considerable diffusion change with a time constant of 400 ms, which indicates that the protein–solvent interaction is changed by the conformational change. The C‐terminal domain of CRY is shown to be responsible for this change.     

Synthesis and characterization of highly stable blue‐light‐emitting hyperbranched conjugated polymers

Polyfluorene homopolymer ( P1 ) and its carbazole derivatives ( P2 – P4 ) have been prepared with good yield by Suzuki coupling polymerization. P2 is an alternating copolymer based on fluorene and carbazole; P3 is a hyperbranched polymer with carbazole derivative as the core and polyfluorene as the long arms; P4 is a hyperbranched polymer with carbazole derivative as the core and the alternating fluorene and carbazole as the long arms. These polymers show highly thermal stability, and their structures and physical properties are studied using gel permeation chromatography, ¹H NMR, ¹³C NMR, elemental analysis, Fourier transform infrared spectroscopy, thermogravimetry, UV–vis absorption, photoluminescence, and cyclic voltammetry (CV). The influence of the incorporation of carbazole and the hyperbranched structures on the thermal, electrochemical, and electroluminescent properties has been investigated. Both carbazole addition and the hyperbranched structure increase the thermal and photoluminescent stability. The CV shows an increase of the HOMO energy levels for the derivatives, compared with polyfluorene homopolymer ( P1 ). The EL devices fabricated by these polymers exhibit pure blue‐light‐emitting with negligible low‐energy emission bands, indicating that the hyperbranched structure has a strong effect on the PLED characteristics. The results imply that incorporating carbazole into polyfluorene to form a hyperbranched structure is an efficient way to obtain highly stable blue‐light‐emitting conjugated polymers, and it is possible to adjust the property of light‐emitting polymers by the amount of carbazole derivative incorporated into the polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 790–802, 2008     

Primary events in the blue light sensor plant cryptochrome: intraprotein electron and proton transfer revealed by femtosecond spectroscopy

Photoreceptors are chromoproteins that undergo fast conversion from dark to signaling states upon light absorption by the chromophore. The signaling state starts signal transduction in vivo and elicits a biological response. Therefore, photoreceptors are ideally suited for analysis of protein activation by time-resolved spectroscopy. We focus on plant cryptochromes which are blue light sensors regulating the development and daily rhythm of plants. The signaling state of these flavoproteins is the neutral radical of the flavin chromophore. It forms on the microsecond time scale after light absorption by the oxidized state. We apply here femtosecond broad-band transient absorption to early stages of signaling-state formation in a plant cryptochrome from the green alga Chlamydomonas reinhardtii. Transient spectra show (i) subpicosecond decay of flavin-stimulated emission and (ii) further decay of signal until 100 ps delay with nearly constant spectral shape. The first decay (i) monitors electron transfer from a nearby tryptophan to the flavin and occurs with a time constant of τ(ET) = 0.4 ps. The second decay (ii) is analyzed by spectral decomposition and occurs with a characteristic time constant τ(1) = 31 ps. We reason that hole transport through a tryptophan triad to the protein surface and partial deprotonation of tryptophan cation radical hide behind τ(1). These processes are probably governed by vibrational cooling. Spectral decomposition is used together with anisotropy to obtain the relative orientation of flavin and the final electron donor. This narrows the number of possible electron donors down to two tryptophans. Structural analysis suggests that a set of histidines surrounding the terminal tryptophan may act as proton acceptor and thereby stabilize the radical pair on a 100 ps time scale.     

Characterization of the Blue–Light‐Activated Adenylyl Cyclase mPAC by Flash Photolysis and FTIR Spectroscopy

The recently discovered photo‐activated adenylyl cyclase (mPAC from Microcoleus chthonoplastes) is the first PAC that owes a light‐, oxygen‐ and voltage‐sensitive (LOV) domain for blue‐light sensing. The photoreaction of the mPAC receptor was studied by time‐resolved UV/vis and light‐induced Fourier transform infrared (FTIR) absorption difference spectroscopy. The photocycle comprises of the typical triplet state LOV₇₁₅ and the thio‐adduct state LOV₃₉₀. While the adduct state decays with a time constant of 8 s, the lifetime of the triplet state is with 656 ns significantly shorter than in all other reported LOV domains. The light‐induced FTIR difference spectrum shows the typical bands of the LOV₃₉₀ and LOV₄₅₀ intermediates. The negative S‐H stretching vibration at 2573 cm^?1 is asymmetric suggesting two rotamer configurations of the protonated side chain of C194. A positive band at 3632 cm^?1 is observed, which is assigned to an internal water molecule. In contrast to other LOV domains, mPAC exhibits a second positive feature at 3674 cm^?1 which is due to the O‐H stretch of a second intrinsic water molecule and the side chain of Y476. We conclude that the latter might be involved in the dimerization of the cyclase domain which is crucial for ATP binding.     

BLUE AND ULTRAVIOLET-B LIGHT PHOTORECEPTORS IN PARSLEY CELLS

Abstract— Ultraviolet-B (UV-B) and blue light photoreceptors have been shown to regulate chalcone synthase and flavonoid synthesis in parsley cell cultures. These photoreceptors have not yet been identified. In the present work, we studied UV-B photoreception with physiological experiments involving temperature shifts and examined the possible role of flavin in blue and UV-B light photoreception. Cells irradiated with UV-B light (0.5–15 min) at 2°C have the same fluence requirement for chalcone synthase and flavonoid induction as controls irradiated at 25°C. This is indicative of a purely photochemical reaction. Cells fed with riboflavin and irradiated with 6 h of UV-containing white light synthesize higher levels of chalcone synthase and flavonoid than unfed controls. This effect did not occur with blue light. These results indicate that flavin-sensitization requires excitation of flavin and the UV-B light photoreceptor. The in vivo kinetics of flavin uptake and bleaching indicate that the added flavin may act at the surface of the plasma membrane. In view of the likely role of membrane-associated flavin in photoreception, we measured in vitro flavin binding to microsomal membranes. At least one microsomal flavin binding site was solubilized by resuspension of a microsomal pellet in buffer with high KPi and NaCl concentrations and centrifugation at 38000 g. The 38000 g insoluble fraction had much greater flavin binding and contained a receptor with an apparent K_D of about 3.6 μM and an estimated in vivo concentration of at least 6.7 × 10–⁸M. Flavin mononucleotide, roseoflavin, and flavin adenine dinucleotide can compete with riboflavin for this binding site(s), although each has lower affinity than riboflavin. Most microsomal protein was solubilized by resuspension of the microsomal pellet in non-denaturing detergents and centrifugation at 38 000 g ; however, this inhibited flavin binding, presumably because of disruption of the environment of the flavin receptor. The parsley microsomal flavin binding receptor(s) have a possible role in physiological photoreception.     

The synthesis of new cyclic thioquinone derivatives

New bis(thio)substituted, S‐,O‐substituted, and S‐,S‐substituted benzoquinone compounds were synthesized from the reaction of p‐chloranil ( 1 ) with S‐,O‐substituted thiols, dithiols, and monothiols. The ¹³C NMR spectra and the IR spectra of heterocyclic compounds 3 , 4 and 7 , 8 showed different behavior; that of 3 , 7 showed a carbon signal and a >CO group band for the carbonyl group and that of 4 , 8 showed two carbon signals and split bands for the carbonyl group. The structures of the novel compounds were characterized by microanalysis, FT‐IR, ¹H NMR, ¹³C NMR, MS, and UV–vis spectroscopy. The crystal structure of 2,3,5,6‐tetrakis(4‐fluorobenzylthio)cyclohexa‐2,5‐diene‐1,4‐dione ( 15 ) was determined by the X‐ray diffraction method. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:446–452, 2010; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.20634     

Color‐Discriminating Retinal Configurations of Sensory Rhodopsin I by Photo‐Irradiation Solid‐State NMR Spectroscopy

SRI (sensory rhodopsin I) can discriminate multiple colors for the attractant and repellent phototaxis. Studies aimed at revealing the color‐dependent mechanism show that SRI is a challenging system not only in photobiology but also in photochemistry. During the photoreaction of SRI, an M‐intermediate (attractant) transforms into a P‐intermediate (repellent) by absorbing blue light. Consequently, SRI then cycles back to the G‐state. The photoreactions were monitored with the ¹³C NMR signals of [20‐¹³C]retnal‐SrSRI using in situ photo‐irradiation solid‐state NMR spectroscopy. The M‐intermediate was trapped at ?40 °C by illumination at 520 nm. It was transformed into the P‐intermediate by subsequent illumination at 365 nm. These results reveal that the G‐state could be directly transformed to the P‐intermediate by illumination at 365 nm. Thus, the stationary trapped M‐ and P‐intermediates are responsible for positive and negative phototaxis, respectively.     

Cryptochrome signaling in plants

Cryptochromes are blue light receptors that mediate various light-induced responses in plants and animals. They share sequence similarity to photolyases, flavoproteins that catalyze the repair of UV light-damaged DNA, but do not have photolyase activity. Arabidopsis cryptochromes work together with the red/far-red light receptor phytochromes to regulate various light responses, including the regulation of cell elongation and photoperiodic flowering, and are also found to act together with the blue light receptor phototropins to mediate blue light regulation of stomatal opening. The signaling mechanism of Arabidopsis cryptochromes is mediated through negative regulation of COP1 by direct CRY-COP1 interaction through CRY C-terminal domain. Arabidopsis CRY dimerized through its N-terminal domain and dimerization of CRY is required for light activation of the photoreceptor activity. Recently, significant progresses have been made in our understanding of cryptochrome functions in other dicots such as pea and tomato and lower plants including moss and fern. This review will focus on recent advances in functional and mechanism characterization of cryptochromes in plants. It is not intended to cover every aspect of the field; readers are referred to other review articles for historical perspectives and a more comprehensive understanding of this photoreceptor.     

Synthesis of W‐doped TiO2 by low‐temperature hydrolysis: Effects of annealing temperature and doping content on the surface microstructure and photocatalytic activity

A series of tungsten‐doped Titania photocatalysts were synthesized using a low‐temperature method. The effects of dopant concentration and annealing temperature on the phase transitions, crystallinity, electronic, optical, and photocatalytic properties of the resulting material were studied. The X‐ray patterns revealed that the doping delays the transition of anatase to rutile to a high temperature. A new phase W_yTi_1‐yO₂ appeared for 5.00 wt% W‐TiO₂ annealed at 900 °C. Raman and diffuse reflectance UV–Vis spectroscopy showed that band gap values decreased slightly up to 700 °C. X‐ray photoelectron spectroscopy showed that surface species viz. Ti³⁺, Ti⁴⁺, O^2?, oxygen‐vacancies, and adsorbed OH groups vary depending on the preparation conditions. The photocatalytic activity was evaluated via the degradation of methylene blue using LED white light. The degradation rate was affected by the percentage of dopants. The best photocatalytic activity was achieved with the sample labeled 5.00 wt% W‐TiO₂ annealed at 700 °C.     

A Comparative 13C-NMR. Study on Various Reduced Flavins

Various two-electron reduced flavin derivatives have been investigated by natural abundance ¹³C-NMR. spectroscopy. Some selectively ¹³C-enriched compounds were synthesized to ensure the assignment of some of the quaternary C-atoms of the flavin molecule. Addition of two electrons to oxidized flavin leads to upfield shifts of all resonances except for those due to C(5a), C(9) and C(10′α). The largest upfield shift is observed for C(4a). Also some direct and two-bond coupling constants are reported. Theoretical calculations by INDO show that a rather good correlation exists between the calculated π-electron densities and the observed chemical shifts of the two-electron reduced molecule. For the oxidized molecule, the correlation is less satisfactory. Most substitution effects are additive, but some deviations in some compounds are observed indicating structural differences between the compounds in question. The chemical shifts are also discussed in terms of the chemical reactivity of the oxidized and reduced flavin molecule.     

A new donor–acceptor double‐cable carbazole polymer with perylene bisimide pendant group: Synthesis,electrochemical, and photovoltaic properties

We report here electrochemical synthesis of novel soluble donor–acceptor (D–A) polymer with suitably functionalized perylenetetracarboxylic diimide dye derivative covalently linked to carbazole moiety (Cbz‐PDI). The band gap, E_g was measured using UV–Vis spectroscopy and compared with that obtained by cyclic voltammetry (CV). Efficient intramolecular electron transfer from carbazole‐donor to perynediimide‐acceptor leads to remarkable fluorescence quenching of the perylene core. Furthermore, spectroelectrochemical property and surface morphology of the polymer film were investigated. Characteristic monoanion and dianion radical bands on the UV–Vis absorption spectra attributed to the electrochemical reduction of the neutral polymer were observed. During the reduction process, red color of the film turned into blue and violet, respectively. Finally, the photovoltaic performance of the D–A double‐cable polymer was checked and nearly 0.1% electrical conversion efficiency is obtained under simulated AM 1.5 solar light with 100 mW/cm² radiation power. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6280–6291, 2009     

Highly efficient and stable blue‐light‐emitting binaphthol‐fluorene copolymers: A joint experimental and theoretical study of the main‐chain chirality

In a quest for the main‐chain chiral and highly stable blue‐light‐emitting π‐conjugated polymers, a novel series of soluble conjugated random and alternating copolymers (PF‐BN) derived from fluorene and axially chiral 1,1′‐binaphthol (BINOL) were successfully synthesized by Suzuki coupling polymerization. The polymer structures, optical properties, and their electrochemical properties were investigated by ¹H NMR, TGA/DSC, UV‐Vis absorption, photoluminescence, cyclic voltammetry, circular dichroism spectroscopy, and DFT calculations. The blue‐light‐emitting BINOL‐containing copolymers with proper content of BINOL show highly efficient photoluminescence and ultra highly stable light‐emission with almost unchanged fluorescent spectra after annealing at 200 °C in air for 10 h. The joint experimental and theoretical study of the main‐chain chirality reveals that (1) the chirality of BINOL can be transferred to the polymer backbone, (2) the effective conjugation length is about one BINOL and three fluorenes, (3) the main active chiral block in the copolymers is probably composed by one BINOL with the other two or three fluorenes, and (4) the dihedral angle in the PF‐BN copolymers should be larger than 105°. The incorporation of BINOL into the polyfluorene backbone is an effective way to produce highly efficient and stable blue‐light‐emitting main‐chain chiral conjugated polymer with interesting optoelectronic properties. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3868–3879, 2010     

Exploring the Reactivity of Multicomponent Photocatalysts: Insight into the Complex Valence Band of BiOBr

The band structure of multicomponent semiconductor photocatalysts, as well as their reactivity distinction under different wavelengths of light, is still unclear. BiOBr, which is a typical multicomponent semiconductor, may have two possible valence‐band structures, that is, two discrete valence bands constructed respectively from O 2p and Br 4p orbitals, or one valence band derived from the hybridization of these orbitals. In this work, aqueous photocatalytic hydroxylation is applied as the probe reaction to investigate the nature and reactions of photogenerated holes in BiOBr. Three organic compounds (microcystin‐LR, aniline, and benzoic acid) with different oxidation potentials were selected as substrates. Isotope labeling (H₂¹⁸O as the solvent) was used to determine the source of the O atom in the hydroxyl group of the products, which distinguishes the contribution of different hydroxylation pathways. Furthermore, a spin‐trapping ESR method was used to quantify the reactive oxygen species (^.OH and ^.OOH) formed in the reaction system. The different isotope abundances of the hydroxyl O atom of the products formed, as well as the reverse trend of the ^.OH/^.OOH ratio with the oxidative resistance of the substrate under UV and visible irradiation, reveal that BiOBr has two separate valence bands, which have different oxidation ability and respond to UV and visible light, respectively. This study shows that the band structure of semiconductor photocatalysts can be reliably analyzed with an isotope labeling method.     

Dynamic Nuclear Polarization NMR Spectroscopy of Polymeric Carbon Nitride Photocatalysts: Insights into Structural Defects and Reactivity

Metal‐free polymeric carbon nitrides (PCNs) are promising photocatalysts for solar hydrogen production, but their structure–photoactivity relationship remains elusive. Two PCNs were characterized by dynamic‐nuclear‐polarization‐enhanced solid‐state NMR spectroscopy, which circumvented the need for specific labeling with either ¹³C‐ or ¹⁵N‐enriched precursors. Rapid 1D and 2D data acquisition was possible, providing insights into the structural contrasts between the PCNs. Compared to PCN_B with lower performance, PCN_P is a more porous and more active photocatalyst that is richer in terminal N?H bonds not associated with interpolymer chains. It is proposed that terminal N?H groups act as efficient carrier traps and reaction sites.