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.     

Nanosecond time-resolved studies of long-lived photoinduced charge separation in the dyad fluorescein-anthraquinone-methyl ester adsorbed on TiO2 colloids

A dyad composed of fluorescein and 2-methyl-anthraquinone (FL-AQ) was synthesized and its photophysical properties were examined by absorption, fluorescence spectroscopy, and fluorescence lifetime. The charge-separated state formed by photoinduced intramolecular electron transfer was detected by nanosecond transient absorption spectroscopy for the first time. When FL is excited in solution, the photoinduced electron transfer from FL to AQ proceeds efficiently. The rate constant and the efficiency of intramolecular electron transfer are 3.95 x 10(9) s(-1) and 95%, respectively. Its charge-separated state lifetime is too short to detect by transient absorption spectroscopy. Adding nanometer colloidal TiO(2) to an FL-AQ ethanol solution prolongs the lifetime of the charge-separated state, so its transient absorption signal is recorded significantly. The lifetimes of FL(+). at 480 nm and AQ(-). at 560 nm in the FL-AQ/TiO(2) colloidal system are 11.1 and 8.93 mivros, respectively.     

Charge transfer complexes of 9-vinyl-carbazole with pi acceptors in homogeneous and in micellar solutions

The molecular association of 9-vinyl-carbazole (CBZ) with three electron acceptors, p-chloranil (CHL), 2,7-dinitro-9-fluorenone (FL), and tetracyano-p-quinodimethane (TCNQ), is studied in acetonitrile and in micellar aqueous solution of sodium dodecyl sulfate (SDS). In both media, stable charge transfer (CT) complexes are formed with association constants in the range of 8-500 M(-1). CBZ and FL form a 1:2 complex in acetonitrile, but in SDS micelles the association is 1:1 due to size restriction and occupancy statistics in the host aggregates. The combination of absorption and fluorescence emission spectroscopy data indicates that the bimolecular CT complex of CBZ with TCNQ is stabilized in two distinct environments of the SDS micelles providing then two separated CT absorption bands.     

芴酮对聚醚侧链聚芴体系的影响: 合成, 表征及光物理性质的研究

利用Yamamoto聚合反应, 通过调节单体2,7-二溴9,9'-二-(三乙氧基甲基)芴和2,7-二溴芴酮的比例, 合成了侧链为极性聚醚链, 芴酮含量逐渐增加的聚芴系列. 通过聚合物溶液及固态薄膜的紫外荧光谱图, 深入研究芴酮作为能量受体的能量转移过程及其对聚合物光物理性质的影响. 结果表明: 稀溶液中体系呈现聚芴本征态的荧光发射, 能量转移对溶液浓度具有依赖性; 固态薄膜中能量转移效率随芴酮含量的增加而快速增长, 退火后这种现象更加明显.     

Spectroscopy and photophysics of self-organized zinc porphyrin nanolayers. 2. Transport properties of singlet excitation

Exciton diffusion has been studied in 5-25-nm-thick films of zinc tetra-(p-octylphenyl)-porphyrin (ZnTOPP) spin-coated onto quartz slides by intentional doping with quenchers using steady-state as well as time-resolved fluorescence spectroscopy. The fluorescence spectra of the films are very similar to those of solutions, indicating emission from localized exciton states. From the dependence of the fluorescence quenching on the quencher concentration and fluorescence lifetime measurements, the exciton diffusion can be concluded to be quasi-one-dimensional with an exciton diffusion length of 9 +/- 3 nm and an intrastack energy-transfer rate constant of 10(11)-10(12) s(-1). From fluorescence anisotropy decay measurements, we conclude that neighboring stacks aggregate in a herringbone structure, forming ordered domains that are randomly oriented in the substrate plane. These measurements indicate an interstack energy-transfer rate constant of (7 +/- 2) x 10(10) s(-1).     

Aggregation-Induced Emission with Large Redshift in 2,7-Diphenyl-fluorenone:Reality or Artifact?

The luminescence property of 2, 7-diphenyl-fluorenone (DPFO) was previously reported to be very unusual with a large aggregation-induced effect associated with a fluorescence redshift of 150 nm. The phenomenon is reexamined in this work. It is found that the abnormal observations are caused by the presence of a trace amount of impurity 2, 7-diphenyl-fluorene (DPF) in the as-synthesized DPFO. The pure DPFO molecule does have an intense fluorescence (FL) in solid (528 nm), about 4-5 times larger than in its dilute dichloromethane solutions (542 nm), but with a blueshifted rather than redshifted FL wavelength in solid. The enormous FL enhancement and redshifted FL wavelength of the as-synthesized DPFO solid are due to the presence of impurity DPF. The FL of DPF is much stronger than that of DPFO in dilute solutions and it also has shorter FL wavelengths. In a dilute solution of DPFO with a trace amount of DPF (~1%), the dominant FL peaks are from DPF. Because the electronic absorption peaks of DPF overlaps with DPFO, the electronic energy of DPF can transfer to DPFO. The energy transfer is faster with the increase of concentration because DPF and surrounding DPFO molecules become closer, which quenches the FL of DPF (356 and 372 nm) and enhances the FL of DPFO (542 nm in solution and 528 nm in solid). Therefore, at high concentrations or in solids, only peak at about 542 or 528 nm shows up, and peaks at 356 and 372 nm disappear.     

A Tri-state Fluorescent Switch with “Gated” Solid-state Photochromism Induced by an External Force

Mr. Chen Qian, Dr. Zhimin Ma, Mr. Jianwei Liu, Mrs. Xue Zhang, Prof. Shitao Wang and Prof. Zhiyong Ma. In this article, we report a newly designed molecule composed of a dihydroazulene (DHA) group and a phenothiazine (PTZ) moiety, which achieves aggregation-induced emission enhancement (AIEE), mechanochromism and “gated” solid-state photochromism upon stimulation by an external force. Grinding loosens intermolecular interactions in the crystal and causes a red-shift of fluorescence from 570 nm to 600 nm. Meanwhile, the ring-opening reaction of DHA unit is activated by grinding and a remarkable photochromism could be observed from the grinded powder. The reddish emission of the grinded powder peaked at 600 nm weakened gradually and finally became dark, and a new absorption band at 470 nm emerged in the absorption spectra. Time-dependent density functional theory (TD-DFT) calculation results reveal that the intramolecular intramolecular charge-transfer (ICT) process is replaced by a locally excited (LE) emission on the DHA group, which leads to the quenching of fluorescence. Its impressive photochromic property inspired us to a simple but effective way to develop an encryption system which can let the correct information be displayed upon external stimulation.     

Spectroscopic properties of cyclophane/anthracene and cyclophane/9-fluorenone complexes in dichloromethane

Host/guest interactions of cyclophane/anthracene (C/A) and cyclophane/9-fluorenone (C/F) complexes in dichloromethane, where the cyclophane molecule is the host, are investigated. The stability constants, log K_a, for the C/A and C/F complexes are determined by absorption and fluorescence spectroscopy. For the C/A system, log K_a is 4.2±0.2 as determined from absorption (at 325 nm) and emission (at 382, 403 and 427 nm) spectroscopic data. The analogous measurements yield 3.6±0.2 from absorption (at 309 nm) and emission (at 505 nm) spectroscopic data for the C/F system. Heats of formation of these complexes were determined by measuring the complex association constants at 25, 29 and 32 °C. These results reveal that binding of the anthracene guest by this cyclophane molecule is thermodynamically favored over that for a 9-fluorenone guest. Excited state lifetimes of these systems are also determined.     

基于分子内荧光能量转移的碘聚合膜荧光传感器

合成了基于分子内荧光能量转移的蒽（An）-四苯基卟啉（TPP）双发色团碘荧光探针1．由于An的荧光光谱与TPP的S吸收带具有较好的重叠,供体An与受体TPP之间可以发生有效的分子内荧光能量转移,以An的最大吸收波长作为激发波长时,由于分子内荧光能量转移,受体TPP发出荧光．当碘与探针分子中的识别基团An作用时,导致探针分子的荧光转导基团TPP荧光淬灭．与An、TPP和An＋TPP混合物作敏感材料相比,将探针1固定在PVC膜中制备的敏感膜对碘选择性高、灵敏度好．另外,敏感膜具有很好的重现性、可逆性和稳定性,响应时间小于60S．除Cr2O7^2-和MnO4^-外,食品中常见的无机离子和可能存在的干扰物质不影响碘的测定．在最优条件下,传感器的线性范围为2．04×10^-6-2．36×10^-2mol／L,检出限为3．30×10^-8mol／L．本方法应用于加碘食盐中碘含量的测定,结果满意．     

Green emission from end-group-enhanced aggregation in polydioctylfluorene

Green emission in polyfluorenes (PFs) has been attributed to aggregation or excimer emission, but recently it was reassigned as an on-chain fluorenone defect. We show here that, in dialkyl-substituted PFs that is hydrogen-free at the 9'-position of the fluorene, blue emission with very weak green emission is observed from end-capped polydioctylfluorene (PFO) for both photoluminescence and electroluminescence spectra, while the low-energy green emission at 507 nm is very pronounced only in uncapped PFO (PFOun). The facts that there is no detectable infrared absorption at around 1721 cm(-1) due to >C=O stretching vibration in PFOun and no charge-trapping occurring in the light-emitting device from PFOun are in contrast with those found in the literature-reported copolymers with fluorenone units, which have detectable infrared absorption at 1721 cm(-1) and charge-trapping in devices. We found that this green emission at around 507 nm originates from the end-group-enhanced aggregation by use of UV-vis absorption, photoexcitation spectra, and steady-state photoluminescent and electroluminescent spectra. The end-group-enhanced aggregation is much weaker in other PFs with less-ordered structures.     

Photophysical processes in aromatic polyimides. Studies with model compounds

Emission mechanism in an aromatic polyimide, PI(BPDA/PDA), derived from biphenyltetracarboxylic dianhydride and p-phenylene diamine were studied with ultraviolet visible absorption and fluorescence spectra of a series of the model compounds. The excitation spectrum of the intermolecular charge-transfer (CT) fluorescence peaking around 550 nm of PI(BPDA/PDA) thin film was completely consistent with the absorption spectrum, indicating that the intermolecular CT fluorescence emission of PI(BPDA/PDA) film is not caused by direct excitation of the CT absorption band, but by light absorption due to structural units in the polymer backbone. The UV-vis. absorption spectra of the model compounds corresponding to the structural units in PI(BPDA/PDA) showed that the longest wavelength absorption band is due to the biphenylbisimide moiety. The band was assigned as π, π* transition with the polarization spectrum of the model compound. The fluorescence spectra of the model compounds changed sensitively depending on the conformation around N-phenyl bond. The lifetime measurement for the model compounds suggested that intramolecular CT process occurs very rapidly. © 1993 John Wiley & Sons, Inc.     

水体系中含卟啉基团的两亲高分子的自组装行为和性质的研究

在文中合成了两种不同的卟啉单体5-(4-acryloyloxyphenyl)-10,15,20-tris(4-carboxylphenyl)porphyrinate zinc(Ⅱ)(ZnAOTCPP)和5-(4-acrylo-yloxyphenyl)-10,15,20-tris(4-methoxycarboxylphenyl)porphyrinate zinc(Ⅱ)(ZnMeAOTCPP),它们分别与丙烯酰胺(Acrylamide,AM)聚合得到含有卟啉基团的两亲高分子.与对应的单体相比,含有三羧酸卟啉基团的高分子在水中形成了一个新的紫外吸收峰和一个新的荧光发射峰,而含有三羧酸甲酯卟啉基团的高分子除此之外在更长波的方向上还另外出现了一个新的紫外吸收峰和一个新的荧光发射峰.随着高分子水溶液浓度的提高,高分子中卟啉基团的光谱性质的变化趋势显示高分子在水中的自组装行为可以分为分子间缔合和分子内缔合的两种情况.此外,实验结果显示含有三羧酸甲酯卟啉基团的高分子相对于含有三羧酸卟啉基团的高分子更加有利于卟啉缔合物的形成.     

Photophysical properties of carborane-containing derivatives of 5,10,15,20-tetra(p-aminophenyl)porphyrin

Absorption, fluorescence emission, fluorescence excitation spectra and fluorescence decay kinetics of carborane derivatives of 5,10,15,20-tetra(p-aminophenyl)porphyrin have been investigated. Carborane derivatives are prepared by acylation of the amino groups of 5,10,15,20-tetra(p-aminophenyl)porphyrin by 9-carboranyl acetyl chloride. From the analysis of the absorption and fluorescence spectra, it is concluded that covalent linking of carborane molecules to the tetraphenylporphyrin molecule significantly changes the self-conjugated pi-system of the porphyrin macrocycle: positions of maxima of absorption and fluorescence spectra shift to the red region by 3-8 nm; the halfwidths of these bands are broadened by 2.5-5.0 nm; the relative intensity of the bands I-IV also changes. The fluorescence decay kinetics of the carborane derivatives are biexponential. According to the experimental data and model simulation, it is concluded that the intramolecular electron transfer proceeds from the porphyrin excited part of the molecule to carboranyls with a rate constant of 415 ps(-1) and efficiency of 0.16-0.8. Recombination of separated charges occurs within 1.4 ns.     

Intramolecular excimer formation and photoinduced electron-transfer process in bis-1,8-naphthalimide dyads depending on the linker length

The photophysical properties of bis-1,8-naphthalimide (NI-L-NI) dyads with different linkers ( L = -C 3H 6-, -C 4H 8-, -C 6H 12-, -C 8H 16-, and -C 9H 18-) as well as the reference NI derivative (NI-C 7H 15) were investigated in CH 3CN and H 2O/CH 3CN (v/v = 1:9). The normal fluorescence peak of (1)NI*-L-NI was observed at 379 nm together with a broad emission at longer wavelength both in aprotic CH 3CN and in H 2O/CH 3CN, which is assigned to an excimer, (1)(NI-L-NI)*. The excimer emission maximum was blue-shifted with increasing length of the linker. The photoinduced electron-transfer process of NI-L-NI was also investigated in both solvents by using nanosecond-laser flash photolysis. The T 1-T n absorption band for (3)NI*-L-NI was observed around 470 nm in both solvents. In H 2O/CH 3CN, NI-L-NI is solvated with H 2O in the ground state to exist as solvated NI-L-NI. In the excited triplet state, the NI radical anion (NI (*-)) was generated via the intramolecular quenching of (3)NI*-L-NI by another NI moiety. The solvated NI (*-)-L-NI may undergo the proton abstraction process to give NI(H) (*)-L-NI, which can be confirmed by the transient absorption band at 410 nm. This band was not observed in pure aprotic CH 3CN.     

Fluorescence quenching of meso-tetrakis (4-sulfonatophenyl) porphyrin by colloidal TiO(2)

A stable colloidal TiO(2) has been prepared. The interaction of meso-tetrakis (4-sulfonatophenyl) porphyrin (TSPP) with colloidal TiO(2) was studied by absorption and fluorescence spectroscopy. Upon excitation of its absorption band, the fluorescence emission of TSPP was quenched by colloidal TiO(2). The bimolecular quenching rate constant (k(q)) is 1.78 x 10(11)M(-1)s(-1). The porphyrin can participate in the quenching process by injecting electrons from its excited states into the conduction band of TiO(2). The quenching mechanism is discussed on the basis of the quenching rate constant as well as the reduction potential of the colloidal TiO(2). Rehm-Weller equation was applied for the calculation of free energy change (DeltaG(et)).     

荧光素蒽醌甲酯分子内光诱导电子转移及电荷分离态的检测

合成了以荧光素为光敏剂的电子给体-受体二元化合物荧光素蒽醌甲酯(FL-AQ),用吸收光谱、荧光光谱、荧光寿命研究了该化合物在乙醇溶液中的光物理性质,并首次用纳秒级瞬态吸收光谱检测了此化合物分子内光诱导电子转移所形成的电荷分离态.在溶液中激发FL,电子可从FL有效地转移到AQ,其速率常数为3.95×10⁹s^-1,效率为95%.但由于电荷分离态寿命较短,瞬态吸收信号弱,若在此溶液中加入二氧化钛(TiO₂)纳米胶体,使FL-AQ吸附在胶体上,电荷分离态信号明显增强.480nm处FL^+·的寿命为11.1μs;560nm处AQ^-·的寿命为8.93μs.     

Metallocyclodextrins as building blocks in noncovalent assemblies of photoactive units for the study of photoinduced intercomponent processes

Cyclodextrin cups have been employed to build supramolecular systems consisting of metal and organic photoactive/redox-active components; the photoinduced communication between redox-active units assembled in water via noncovalent interactions is established. The functionalization of a beta-cyclodextrin with a terpyridine unit, ttp-beta-CD, is achieved by protection of all but one of the hydroxyl groups by methylation and attachment of the ttp unit on the free primary hydroxyl group. The metalloreceptors [(beta-CD-ttp)Ru(ttp)][PF(6)](2), [(beta-CD-ttp)Ru(tpy)][PF(6)](2), and [Ru(beta-CD-ttp)(2)][PF(6)](2) are synthesized and fully characterized. The [(beta-CD-ttp)Ru(ttp)][PF(6)](2) metalloreceptor exhibits luminescence in water, centered at 640 nm, from the (3)MLCT state with a lifetime of 1.9 ns and a quantum yield of Phi = 4.1 x 10(-)(5). Addition of redox-active quinone guests AQS, AQC, and BQ to an aqueous solution of [(beta-CD-ttp)Ru(ttp)](2+) results in quenching of the luminescence up to 40%, 20%, and 25%, respectively. Measurement of the binding strength indicates that, in saturation conditions, 85% for AQS and 77% for AQC are bound. The luminescence quenching is attributed to an intercomponent electron transfer from the appended ruthenium center to the quinone guest inside the cavity. Control experiments demonstrate no bimolecular quenching at these conditions. A photoactive osmium metalloguest, [Os(biptpy)(tpy)][PF(6)], is designed with a biphenyl hydrophobic tail for insertion in the cyclodextrin cavity. The complex is luminescent at room temperature with an emission band maximum at 730 nm and a lifetime of 116 ns. The osmium(III) species are formed for the study of photoinduced electron transfer upon their assembly with the ruthenium cyclodextrin, [(beta-CD-ttp)Ru(ttp)](2+). Time-resolved spectroscopy studies show a short component of 10 ps, attributed to electron transfer from Ru(II) to Os(III) giving an electron transfer rate 9.5 x 10(9) s(-)(1).     

Fluorescent study of the keto-enol equilibrium of 5-fluorouracil tautomers in aqueous solutions

A spectral-luminescent study of the keto-enol tautomerism of 5-fluorouracil (FU) has been performed. A discrepancy between the absorption and fluorescence (FL) excitation spectra of aqueous FU solutions (pH 7) has been established. Photoexcitation at the long-wavelength band (340 nm) of the FU excitation spectrum made it possible to detect the fluorescence of its dienol tautomer (λ_max = 440 nm). The quenching of tryptophan fluorescence (K = 15 × 10³ l/mol) and blood fluorescence by 5-fluorouracil has been investigated.     

NEW EMISSION BAND OF TYROSINE INDUCED BY INTERACTION WITH PHOSPHATE ION

Abstract. Excitation of tyrosine solutions (pH 7.5) containing dipotassium phosphate was found to be accompanied by an appearance of a new emission band around 345 nm, with a specific quenching of tyrosine fluorescence. From the features of UV absorption and fluorescence excitation spectra, it is concluded that the new emission is due to the interaction between phosphate and tyrosine in the ground state.     

Role of nonemissive quenchers for the green emission in polyfluorene

The stability of fluorene-based compounds and polymers, especially at the bridged C-9 position under photoirradiation and thermal treatment, has claimed much attention. The emission of fluorenone formed by degradation of the 9-site is regarded as the origin of the low emission band at 2.2-2.3 eV in polyfluorene-based conjugated materials. We have investigated the role of nonemissive quenchers such as alkyl ketones, which were also one of the products of polyfluorene degradation, for the low-energy emission in polyfluorenes. The spectral characteristics of a blend system of polyfluorene/nonemissive quencher/fluorenone are found to accord well with the kinetics of actual polyfluorene degradation. Our results indicate that strong green emission in degraded polyfluorene would be not caused only by fluorenone, but also by nonemissive quenchers for their effectively quenching bulk emission.