Evidence for a pterin-derivative associated with the molybdenum cofactor of Neurospora crassa nitrate reductase

Abstract— Following the method of Johnson and Rajagopalan (1982) for obtaining Form B of molybdopterin cofactors, we observed a prominent fluorescence band at480–482 nm in purified NR of Neurospora crassa mutant albino-band after boiling the enzyme at acidic pH and readjusting the sample to alkaline pH. This fluorescence band is maximally excited at 410 nm and maximally emitting at pH 11 (“F-480_pH11”); at pH4–7 only a featureless fluorescence band of low intensity remains (“F-480_pH5”). The fluorescence ΔF-480 = F-480_pH11 - F-480_pHS is examined here. ΔF-480 is associated specifically with NADPH-dependent and MVH-dependent nitrate reduction activities and with cytochrome b-557 absorption. In a protease-digested preparation lacking NADPH-dependent NR activity, ΔF-480 is associated with MVH-dependent nitrate reduction. The ΔF-480 signal is followed during the course of purification of NR. Its size increases with increasing purity of the enzyme. In partially purified NR preparations and especially in aqueous extracts from mycelia of N. crassa, a second, strong fluorescence signal with a pH-dependent emission maximum at around 450 nm (maximally excited at350–370 nm) was found beside ΔF-480. This “unspecific” signal was lost during NR purification. A procedure is developed to demonstrate AF-480 also in presence of the unspecific (350-370/450 nm) signal as well as flavins. We deduce that the ΔF-480 component is part of the Mo cofactor of N. crassa NR and that the signal is caused by a pterin derivative. From calculations of total content of the AF-480 component in mycelia it is likely that in vivo it is shared also by other enzymes.     

Tuning Excited State Isomerization Dynamics through Ground State Structural Changes in Analogous Ruthenium and Osmium Sulfoxide Complexes

The complexes [Ru(bpy)₂(pyESO)](PF₆)₂ and [Os(bpy)₂(pyESO)](PF₆)₂, in which bpy is 2,2′‐bipyridine and pyESO is 2‐((isopropylsulfinyl)ethyl)pyridine, were prepared and studied by ¹H NMR, UV–visible and ultrafast transient absorption spectroscopy, as well as by electrochemical methods. Crystals suitable for X‐ray structural analysis were grown for [Ru(bpy)₂(pyESO)](PF₆)₂. Cyclic voltammograms of both complexes provide evidence for S→O and O→S isomerization as these voltammograms are described by an ECEC (electrochemical‐chemical electrochemical‐chemical) mechanism in which isomerization follows Ru²⁺ oxidation and Ru³⁺ reduction. The S‐ and O‐bonded Ru^3+/2+ couples appear at 1.30 and 0.76 V versus Ag/AgCl in propylene carbonate. For [Os(bpy)₂(pyESO)](PF₆)₂, these couples appear at 0.97 and 0.32 V versus Ag/AgCl in acetonitrile, respectively. Charge‐transfer excitation of [Ru(bpy)₂(pyESO)](PF₆)₂ results in a significant change in the absorption spectrum. The S‐bonded isomer of [Ru(bpy)₂(pyESO)]²⁺ features a lowest energy absorption maximum at 390 nm and the O‐bonded isomer absorbs at 480 nm. The quantum yield of isomerization in [Ru(bpy)₂(pyESO)]²⁺ was found to be 0.58 in propylene carbonate and 0.86 in dichloroethane solution. Femtosecond transient absorption spectroscopic measurements were collected for both complexes, revealing time constants of isomerizations of 81 ps (propylene carbonate) and 47 ps (dichloroethane) in [Ru(bpy)₂(pyESO)]²⁺. These data and a model for the isomerizing complex are presented. A striking conclusion from this analysis is that expansion of the chelate ring by a single methylene leads to an increase in the isomerization time constant by nearly two orders of magnitude.     

Optische Absorptions-Spektroskopie an ionischen Ozoniden

Optical Absorption Spectroscopy on Ionic Ozonides Electronic transitions of ionic ozonides of alkali-metal and tetraorganylonium cations have been investigated in the range from 200 to 2000 nm. Optical absorption measurements of diffuse reflexion on powder and transmission on single crystals displayed a broad absorption at ∼480 nm and 490 nm, respectively. Optical spectra of the O₃^–-radical in solution gave a well resolved vibronic finestructure with spacings of ∼790 cm^–1, symmetrically to the maximum of the absorption band at 460 nm, and uneffected of the counter ion and solvent used. The finestructure allows a correlation to the ν_s-vibration of the ozonide ion in the excited state. The observed electronic transition is assigned to ²B₁ → ²A₂, based on MO-observations and ab initio calculations. Earlier reports of a strong absorption band at low energy for O₃^– cannot be reconfirmed.     

Dynamics of carbocation formation in the photolysis of 1,2,2,3-tetramethyl-1,2-dihydroquinoline in alcohols

Dynamics of the formation of the carbocation in the ground state as a result of photoinduced proton transfer from a solvent to the excited state of 1,2,2,3-tetramethyl-1,2-dihydroquinoline (3MDHQ) in MeOH and 2,2,2-trifluoroethanol (TFE) was registered by pump-probe laser photolysis (λ_pump = 310 nm) with femtosecond time resolution. The lifetimes of the excited singlet state of 3MDHQ τ = 115 and 780 ps were determined in TFE and MeOH, respectively. The transient species with absorption spectrum corre-sponding to the spectrum of the carbocation from 3MDHQ (λ_max = 480 nm) is generated at time delays lower than 500 fs from the unrelaxed excited singlet state.     

Excited‐State Dynamics of the Metal String Complex Co3(dpa)4(NCS)2 from Femtosecond Transient Absorption Spectra

Transient absorption spectroscopy is used to study the excited‐state dynamics of Co₃(dpa)₄(NCS)₂, where dpa is the ligand di(2‐pyridyl)amido. The ππ*, charge‐transfer, and d–d transition states are excited upon irradiation at wavelengths of 330, 400 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution and the transient species show weak absorption. We thus propose that a low‐lying metal‐centered d–d state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal rapid conversion from the ligand‐centered ππ* and the charge‐transfer states to this metal‐centered d‐d state within 100 fs. The excited molecule then crosses to a second d–d state within the ligand‐field manifold, with a time coefficient of 0.6–1.4 ps. Because the ground‐state bleaching band recovers with a time coefficient of 10–23 ps, we propose that an excited molecule crosses from the low‐lying d–d state either directly within the same spin system or with spin crossing via the state ²B to the ground state ²A₂ (symmetry group C₄). In this trimetal string complex, relaxation to the ground electronic surface after excitation is thus rapid.     

Bromdioxid: Spektroskopische Eigenschaften des Radikals

Pulse radiolysis of oxygen-saturated bromobenzene yields a transient absorption with λ_max = 480 nm at pulse end, which is identified as due to the BrO₂-radical. To confirm this assignment the same radical was produced by flash photolysis of a Br₂/O₂-gas mixture and by chemical synthesis at low temperature. The absorption spectra of these BrO₂-samples present a maximum between 480 and 380 nm dependent on the solvent. The influence of complex-formation on this absorption band is discussed. The ESR.-spectrum of chemically synthesized BrO₂ is given.     

Synthesis,Structural Characterization,Photophysics, and Broadband Nonlinear Absorption of a Platinum(II) Complex with the 6‐(7‐Benzothiazol‐2′‐yl‐9,9‐diethyl‐9 H‐fluoren‐2‐yl)‐2,2′‐bipyridinyl Ligand

A platinum complex with the 6‐(7‐benzothiazol‐2′‐yl‐9,9‐diethyl‐9H‐fluoren‐2‐yl)‐2,2′‐bipyridinyl ligand ( 1 ) was synthesized and the crystal structure was determined. UV/Vis absorption, emission, and transient difference absorption of 1 were systematically investigated. DFT calculations were carried out on 1 to characterize the electronic ground state and aid in the understanding of the nature of low‐lying excited electronic states. Complex 1 exhibits intense structured ¹π–π* absorption at λ_abs<440 nm, and a broad, moderate ¹M LCT/¹LLCT transition at 440–520 nm in CH₂Cl₂ solution. A structured ³π–π*/³M LCT emission at about 590 nm was observed at room temperature and at 77 K. Complex 1 exhibits both singlet and triplet excited‐state absorption from 450 nm to 750 nm, which are tentatively attributed to the ¹π–π* and ³π–π* excited states of the 6‐(7‐benzothiazol‐2′‐yl‐9,9‐diethyl‐9H‐fluoren‐2‐yl)‐2,2′‐bipyridine ligand, respectively. Z‐scan experiments were conducted by using ns and ps pulses at 532 nm, and ps pulses at a variety of visible and near‐IR wavelengths. The experimental data were fitted by a five‐level model by using the excited‐state parameters obtained from the photophysical study to deduce the effective singlet and triplet excited‐state absorption cross sections in the visible spectral region and the effective two‐photon absorption cross sections in the near‐IR region. Our results demonstrate that 1 possesses large ratios of excited‐state absorption cross sections relative to that of the ground‐state in the visible spectral region; this results in a remarkable degree of reverse saturable absorption from 1 in CH₂Cl₂ solution illuminated by ns laser pulses at 532 nm. The two‐photon absorption cross sections in the near‐IR region for 1 are among the largest values reported for platinum complexes. Therefore, 1 is an excellent, broadband, nonlinear absorbing material that exhibits strong reverse saturable absorption in the visible spectral region and large two‐photon‐assisted excited‐state absorption in the near‐IR region.     

Picosecond time-evolution of the fluorescence spectrum of a large aromatic hydrocarbon molecule in solution at 300K

A new Nd³⁺-YAG laser configuration has been used to obtain fluorescence spectra of a large aromatic molecule with about 12 ps time-resolution. The data presented are both in the form of time-profiles at selected wavelengths and more importantly fluorescence spectra recorded at 6.7 ps intervals around the irradiation event. The sample molecule was 3,4,9,10-dibenzpyrene in hexane at 300 K. The major result was the observation of two short-lived transients in the region 380–480 nm characterized by rather diffuse spectra. One, which included a strong “hot-band”, was clearly pulse- limited and is assigned to the second excited singlet state. The second transient was considerably weaker and survived beyond the irradiation pulse. This had characteristics of a vibrational quasicontinuum in the S₁ state. Out of these evolved the familiar structured spectrum characteristic of thermalised S₁ → S₀ fluorescence. In a related experiment, the ultraviolet fluorescence spectrum (230–340 nm) of two-photon-excited higher states of the same molecule was shown to depend on the time-delay between two excitation pulses.     

Relaxation Kinetics of Excitonic States in ZnSe Quantum Dots: A Femtosecond Laser Spectroscopy Study

Relaxation processes in ZnSe quantum dots upon excitation by a 30-fs pulse at a wavelength of 360 nm have been studied by broadband femtosecond absorption spectroscopy. The diameter of ZnSe nanoparticles was 3.7 ± 0.6 nm. A colloidal solution of ZnSe in cyclohexane was used. In the differential spectra, a bleaching band at the edge of the excitonic absorption band of ZnSe, an absorption band of the biexcitonic transition with a peak at about 420 nm, and a broad structureless absorption band in the region from 440 to 750 nm have been revealed. From the analysis of the absorption and luminescence spectra, the shift of the excitonic luminescence band δ_XX = 127 meV has been measured. From the femtosecond photolysis data, an estimate of the biexcitonic interaction Δ_XX ≈ 75 meV has been obtained. It has been shown that the relaxation kinetics of the differential spectra is described by three-exponential dependences with time constants and corresponding amplitude contributions of 1 ps (42%), 13 ps (22%), and 91 ps (17%). The kinetic component of 1 ps (42%) is presumably due to hole transport to surface traps. The kinetic components of 13 ps (22%) and 91 ps (17%) apparently describe the processes of electron transport to shallow and deep traps.     

对位氨基能显著提高D-A-D型化合物的双光子吸收性能

报道了具有典型D-A-D型共轭结构的反式2,5-二氰基-1,4-二(4'-甲氧基苯乙烯基)苯(MOS-CN), 2,5-二氰基-1,4-二(4'-二甲胺基苯乙烯基)苯(MAS-CN)和1,4-二(4'-甲氧基苯乙烯基)苯(MOS)的合成. 用核磁、红外和元素分析进行了表征. 测试了紫外吸收光谱、单光子荧光光谱、双光子荧光光谱、双光子吸收系数及双光子吸收截面. 在800 nm的飞秒脉冲激光激发下, 化合物MOS-CN, MAS-CN和MOS分别发出很强的绿色、黄色和蓝色上转换荧光. 化合物MOS-CN, MAS-CN和MOS的最大吸收波长、单光子发射波长、双光子诱导荧光波长、荧光量子产率、双光子吸收系数、双光子吸收截面及双光子荧光寿命各分别是393, 473, 367 nm; 470, 569, 434 nm; 475, 574, 438 nm; 0.12, 0.72, 0.21; 0.8, 5.3, 0.3 cm/GW; 270, 1790, 101 GM; 140 ps, 1.32 ns, 54 ps. MAS-CN的双光子吸收截面是MOS-CN的6.63倍, MOS-CN的双光子吸收截面是MOS的2.67倍, 表明对位氨基显著地提高了化合物的双光子吸收性能, 氰基也较大地提高了双光子吸收截面.     

Preparation and femtosecond non-linear optical properties of Ag/SiO2 composite thin films

Ag nanoparticles embedded in SiO₂ thin films (Ag/SiO₂ films) were prepared by a multitarget sputtering method. In the optical absorption spectra of the Ag/SiO₂ films, the absorption peak due to the surface plasmon resonance (SPR) of Ag particle was clearly observed at the wavelength of 394–413 nm. The imaginary part of the third-order non-linear susceptibility, Im [χ⁽³⁾], of the Ag/SiO₂ film was estimated to be ?1.1×10^?8 esu measured by the femtosecond Z-scan technique near the SPR peak. The response time of the film measured from the decay of the differential transmission of the pump-probe experiment was 1.3 ps at the SPR peak.     

Ultrafast fluorescence resonance energy transfer in a bile salt aggregate: Excitation wavelength dependence

Fluorescence resonance energy transfer (FRET) from Coumarin 153 (C153) to Rhodamine 6G (R6G) in a secondary aggregate of a bile salt (sodium deoxycholate, NaDC) is studied by femtosecond up-conversion. The emission spectrum of C153 in NaDC is analysed in terms of two spectra-one with emission maximum at 480 nm which corresponds to a non-polar and hydrophobic site and another with maximum at ∼530 nm which arises from a polar hydrophilic site. The time constants of FRET were obtained from the rise time of the emission of the acceptor (R6G). In the NaDC aggregate, FRET occurs in multiple time scales — 4 ps and 3700 ps. The 4 ps component is assigned to FRET from a donor (D) to an acceptor (A) held at a close distance (R _DA ∼ 17 ?) inside the bile salt aggregate. The 3700 ps component corresponds to a donor-acceptor distance ∼48 ?. The long (3700 ps) component may involve diffusion of the donor. With increase in the excitation wavelength (λ _ex) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (∼4 ps) increases from 3 to 40% with a concomitant decrease in the contribution of the ultraslow component (∼3700 ps) from 97 to 60%. The λ _ex dependence is attributed to the presence of donors at different locations. At a long λ _ex (435 nm) donors in the highly polar peripheral region are excited. A short λ _ex (375 nm) ‘selects’ donor at a hydrophobic location.     

Photodissociation of CH2I2 and Subsequent Electron Transfer in Solution

We studied photoinduced reactions of diiodomethane (CH₂I₂) upon excitation at 268 nm in acetonitrile and hexane by subpicosecond–nanosecond transient absorption spectroscopy. The transient spectra involve two absorption bands centered at around 400 (intense) and 540 nm (weak). The transients probed over the range 340–740 nm show common time profiles consisting of a fast rise (<200 fs), a fast decay (≈500 fs), and a slow rise. The two fast components were independent of solute concentration, whereas the slow rise became faster (7–50 ps) when the concentration in both solutions was increased. We assigned the fast components to the generation of a CH₂I radical by direct dissociation of the photoexcited CH₂I₂ and its disappearance by subsequent primary geminate recombination. The concentration‐dependent slow rise produced the absorption bands centered at 400 and 540 nm. The former consists of different time‐dependent bands at 385 and 430 nm. The band near 430 nm grew first and was assigned to a charge‐transfer (CT) complex, CH₂I₂^δ+???I^δ?, formed by a photofragment I atom and the solute CH₂I₂ molecule. The CT complex is followed by full electron transfer, which then develops the band of the ion pair CH₂I₂⁺???I^? at 385 nm on the picosecond timescale. On the nanosecond scale, I₃^? was generated after decay of the ion pair. The reaction scheme and kinetics were elucidated by the time‐resolved absorption spectra and the reaction rate equations. We ascribed concentration‐dependent dynamics to the CT‐complex formation in pre‐existing aggregates of CH₂I₂ and analyzed how solutes are aggregated at a given bulk concentration by evaluating a relative local concentration. Whereas the local concentration in hexane monotonically increased as a function of the bulk concentration, that in acetonitrile gradually became saturated. The number of CH₂I₂ molecules that can participate in CT‐complex formation has an upper limit that depends on the size of aggregation or spatial restriction in the neighboring region of the initially photoexcited CH₂I₂. Such conditions were achieved at lower concentrations in acetonitrile than in hexane.     

A femtosecond study of excitation wavelength dependence of a triblock copolymer-surfactant supramolecular assembly: (PEO)20-(PPO)70-(PEO)20 and CTAC

Solvation dynamics and anisotropy decay of coumarin 480 (C480) in a supramolecular assembly containing a triblock copolymer, PEO20-PPO70-PEO20 (Pluronic P123) and a surfactant, CTAC (cetyl trimethylammonium chloride) are studied by femtosecond up-conversion. In a P123-CTAC complex, C480 displays a significant (22 nm) red edge excitation shift (REES) in the emission maximum as lambda ex increases from 335 to 445 nm. This suggests that the P123-CTAC aggregate is quite heterogeneous. The average rotational relaxation time (tau rot) of C480 in a P123-CTAC complex decreases by a factor of 2 from 2500 ps at lambda ex = 375 nm to 1200 ps at lambda ex = 435 nm. For lambda ex = 375 nm, the probe molecules in the buried core region of P123-CTAC are excited and the solvation dynamics displays three components, 2, 60, and 4000 ps. It is argued that insertion of CTAC in P123 micelle affects the polymer chain dynamics, and this leads to reduction of the 130 ps component of P123 micelle to 60 ps in P123-CTAC. For lambda ex = 435 nm, which selects the peripheral highly polar corona region, solvation dynamics in P123-CTAC and P123 are extremely fast with a major component of <0.3 ps ( approximately 80%) and a 2 ps ( approximately 20%) component.     

Ultrafast photoinduced relaxation dynamics of the indoline dye D149 in organic solvents

The relaxation dynamics of the indoline dye D149, a well-known sensitizer for photoelectrochemical solar cells, have been extensively characterized in various organic solvents by combining results from ultrafast pump-supercontinuum probe (PSCP) spectroscopy, transient UV-pump VIS-probe spectroscopy, time-correlated single-photon counting (TCSPC) measurements as well as steady-state absorption and fluorescence. In the steady-state spectra, the position of the absorption maximum shows only a weak solvent dependence, whereas the fluorescence Stokes shift Δν?(F) correlates with solvent polarity. Photoexcitation at around 480 nm provides access to the S(1) state of D149 which exhibits solvation dynamics on characteristic timescales, as monitored by a red-shift of the stimulated emission and spectral development of the excited-state absorption in the transient PSCP spectra. In all cases, the spectral dynamics can be modeled by a global kinetic analysis using a time-dependent S(1) spectrum. The lifetime τ(1) of the S(1) state roughly correlates with polarity [acetonitrile (280 ps) < acetone (540 ps) < THF (720 ps) < chloroform (800 ps)], yet in alcohols it is much shorter [methanol (99 ps) < ethanol (178 ps) < acetonitrile (280 ps)], suggesting an appreciable influence of hydrogen bonding on the dynamics. A minor component with a characteristic time constant in the range 19-30 ps, readily observed in the PSCP spectra of D149 in acetonitrile and THF, is likely due to removal of vibrational excess energy from the S(1) state by collisions with solvent molecules. Additional weak fluorescence in the range 390-500 nm is observed upon excitation in the S(0)→S(2) band, which contains short-lived S(2)→S(0) emission of D149. Transient absorption signals after excitation at 377.5 nm yield an additional time constant in the subpicosecond range, representing the lifetime of the S(2) state. S(2) excitation also produces photoproducts.     

Photophysical and electrochemical characterization of new poly(arylene vinylene) copolymers containing quinoline or bisquinoline segments

Four new fluorescent conjugated vinylene‐copolymers incorporating quinoline or bisquinoline segments along the backbone were synthesized by Heck coupling. Three of them were fluorenevinylene‐copolymers and contained quinoline ( PQFV , PQFVT ) or bisquinoline segments ( PBQFV ). One of them ( PBQPV ) was phenylenevinylene‐copolymer and contained bisquinoline segments. All the copolymers were soluble in common organic solvents and had relatively low glass transition temperature (T_g = 50–56 °C for fluorenevinylenes and T_g < 25 °C for phenylenevinylene). In THF solutions, the quinoline‐containing copolymers showed absorption maxima at 411–420 nm while the bisquinoline‐containing ones exhibited maxima at 357–361 nm. The emission maxima of solutions were 465–490 nm. The copolymers showed high quantum yields up to 64%. The films exhibited absorption and emission maxima in the range of 371–437 nm and 480–521 nm, respectively. All copolymers revealed reversible reduction with electron affinity of 2.66–3.53 eV and irreversible oxidation scans with ionization potential of 5.39–5.53 eV. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3370–3379, 2009     

Characterization of Locally Excited and Charge-Transfer States of the Anticancer Drug Lapatinib by Ultrafast Spectroscopy and Computational Studies

Lapatinib (LAP) is an anticancer drug, which is metabolized to the N- and O-dealkylated products (N-LAP and O-LAP, respectively). In view of the photosensitizing potential of related drugs, a complete experimental and theoretical study has been performed on LAP, N-LAP and O-LAP, both in solution and upon complexation with human serum albumin (HSA). In organic solvents, coplanar locally excited (LE) emissive states are generated; they rapidly evolve towards twisted intramolecular charge-transfer (ICT) states. By contrast, within HSA only LE states are detected. Accordingly, femtosecond transient absorption reveals a very fast switching (ca. 2 ps) from LE (λ_max=550 nm) to ICT states (λ_max=480 nm) in solution, whereas within HSA the LE species become stabilized and live much longer (up to the ns scale). Interestingly, molecular dynamics simulation studies confirm that the coplanar orientation is preferred for LAP (or to a lesser extent N-LAP) within HSA, explaining the experimental results.     

Metal complexes with macrocyclic ligands. Part XIX. Synthesis and Cu2+-complexes of a series of 12-, 14- and 16-membered cis- and trans-N2S2-macrocycles

A series of 12-, 14-, and 16-membered N₂S₂-macrocycles ( 9–11 and 19–21 ) with cis and trans-arrangement of the heteroatoms have been synthesized by high-dilution cyclization and subsequent reduction of the amides with B₂H₆. With these ligands the corresponding Cu²⁺-complexes were prepared and their UV/VIS spectra, their electrochemistry and their EPR properties have been studied. Generally three absorption bands at 270–320 nm, 330–370 nm and 530–620 nm can be observed in aqueous solution and these have been assigned to the N→Cu²⁺ and S→Cu²⁺ charge-transfer bands and to the d-d* transition, respectively. The cyclic voltammetry in CH₃CN shows in all cases a reversible or quasi-reversible Cu²⁺/Cu⁺-transition at potentials of 10–480 mV against SHE. The values of g_‖ and A_‖ obtained from EPR spectra indicate that the geometry of the Cu²⁺-complex of the 14-membered cis-N₂S₂-macrocycle is less distorted than that of the other complexes.     

Radical reactions of C84

Pulse radiolysis and steady-state radiolysis experiments describing the radical and electron transfer reactions of C₈₄ are reported here for the first time. C₈₄ reacts readily with radiolytically generated chloromethyl (^•CCl₃) and trichloromethylperoxyl (CCl₃OO^•) radicals in CCl₄. The formation of the radical adduct has been confirmed from its characteristic absorption in the UV (320 nm) and visible (480 nm). Radical-induced oxidation in 1,2-dichloroethane (1,2-DCE) resulted in a short lived transient absorbing at 920 nm. Reduction of C₈₄ in toluene/2-propanol/acetone could be conveniently followed by formation of an absorption band with an absorption maximum at 960 nm.     

Regulating Charge-Transfer in Conjugated Microporous Polymers for Photocatalytic Hydrogen Evolution

Bandgap engineering in donor–acceptor conjugated microporous polymers (CMPs) is a potential way to increase the solar-energy harvesting towards photochemical water splitting. Here, the design and synthesis of a series of donor–acceptor CMPs [tetraphenylethylene (TPE) and 9-fluorenone (F) as the donor and the acceptor, respectively], F_0.1CMP , F_0.5CMP , and F_2.0CMP , are reported. These CMPs exhibited tunable bandgaps and photocatalytic hydrogen evolution from water. The donor–acceptor CMPs exhibited also intramolecular charge-transfer (ICT) absorption in the visible region (λ_max=480 nm) and their bandgap was finely tuned from 2.8 to 2.1 eV by increasing the 9-fluorenone content. Interestingly, they also showed emissions in the 540–580 nm range assisted by the energy transfer from the other TPE segments (not involved in charge-transfer interactions), as evidenced from fluorescence lifetime decay analysis. By increasing the 9-fluorenone content the emission color of the polymer was also tuned from green to red. Photocatalytic activities of the donor–acceptor CMPs ( F_0.1CMP , F_0.5CMP , and F_2.0CMP ) are greatly enhanced compared to the 9-fluorenone free polymer ( F_0.0CMP ), which is essentially due to improved visible-light absorption and low bandgap of donor–acceptor CMPs. Among all the polymers F_0.5CMP with an optimum bandgap (2.3 eV) showed the highest H₂ evolution under visible-light irradiation. Moreover, all polymers showed excellent dispersibility in organic solvents and easy coated on the solid substrates.