PHOTOELECTROCHEMICAL STUDY OF CHLOROPHYLL-a MULTILAYERS ON SnO2 ELECTRODE

Photoelectrochemical behaviour of Chl- a multilayers, built up on SnO₂ optically transparent electrode by means of the Langmuir technique, was investigated in an electrolyte solution with or without hydroquinone as a reducing agent. Quantum efficiencies of the anodic photocurrents in the presence of hydroquinone tended to decrease with increasing number of layers. In the electrolyte without hydroquinone, the cathodic photocurrents were observed in the presence of dissolved oxygen. A possibility of a reaction of water with the oxidized Chl- a is suggested.     

PROPERTIES OF THE CHLOROPLAST FILM ELECTRODE IMMOBILIZED ON AN SnO2-COATED GLASS PLATE

Abstract— Type C chloroplasts were deposited on the surface of an SnO₂ optically transparent electrode glass plate with polyvinyl alcohol plus bovine serum albumin as immobilizing supports. This electrode, on illumination of 250 J/m² in an electrolyte solution, generated anodic photo current more than 150 nA per 10 μg chlorophyll/cm² of the SnO₂ glass plate at a potentiostatic condition of + 0.5 V against a saturated calomel electrode, and gave rise to an open circuit potential up to 300 mV. The photocurrent output was enhanced as high as 60-fold under the short circuit condition by the addition of an artificial electron carrier, l-methoxy-5-methylphenazinium methyl sulfate, to the electrolyte solution. With the electrode poised at +0.5 V against a saturated calomel electrode, the enhancement effect was exhibited as high as 13-fold in the presence of 2,6-dichlorophenol indophenol. A photocurrent spectrum coincides well with an absorption spectrum of the chloroplast film electrode. Effects of heat-treatment, photosynthetic inhibitors, and electrolyte's pH on the magnitude of the photocurrent were studied in detail. Water molecule, a primary electron donor in the chloroplast photosystems, contributes to the large majority of photocurrent generation. A minor output was observed with the electrode coated with completely inactivated chloroplasts, probably due to the chlorophyll photosensitization.     

LIGHT-INDUCED ELECTRON TRANSPORT ACROSS SEMICONDUCTOR ELECTRODE/REACTION-CENTER FILM/ELECTROLYTE INTERFACES

Abstract— Reaction center (RC) complexes isolated from the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26 were dried as a film onto platinum and semiconductor (SnO₂) electrodes. The light-induced primary charge separation which occurs across the biological complex couples electrically with the SnO₂ but not with the metal electrode on the time scale of observation. As the working electrode in a two-electrode photoelectrochemical cell, RC-coated SnO₂ generated photovoltages as high as 80 mV and photocurrents as high as 0.5µA·cm² when exposed to light of λ >600nm. The number of quinone molecules per RC strongly influences the photovoltage and photocurrent observed. Photo-effects generated by RC electrodes persist after several days of storage; however, the kinetics and polarity of the effects are subject to change. The potential use of RC electrodes lies more as a new probe of photosynthetic electron transport rather than as a solar energy conversion device because modification to the RCs and their environment affect the electrical properties of the cell. An energy-level model is proposed to explain how the photoelectrochemical cell functions.     

PHOTOVOLTAIC PROPERTIES OF MESO-TETRAARYLPORPHYRINS

Abstract— The photovoltaic properties of meso -tetraphenylporphyrin and its derivatives with various para -substituents, meso-tetrakis(2,4-dimethoxyphenyl)porphyrin and meso -tetrakis (2-fluorenyl)porphyrin have been investigated. The forward dark current-voltage characteristics of Al/porphyrin/Ag cells are attributed to the MIS Schottky barriers consisting of Al/Al₂O₃/porphyrin. The barrier parameters such as the apparent diffusion potential V ₀ the barrier width w₀ and the density of ionized impurity N are estimated by using the capacitor discharge method. The action spectra of photocurrents closely follow the optical absorption spectra of the porphyrin films. The photocurrents vary as i _pα I ^γ, where I is the incident light intensity, and the Sight exponents y range between 0.83 and 1.0. The sublinear difference from unity could be related to the exponential distribution of hole traps in films of the porphyrins. No obvious correlation between the photocurrent and the fluorescence quantum yields of the sublimed porphyrin films is found. The electron donating substituents such as OCH₃ and CH₃ strikingly increase the photocurrents. The photocurrent quantum yields correlate exponentially with the first ring oxidation potentials of the porphyrins and also with the substituent constants for the Hammett linear free-energy relationship. The current quantum yields estimated for the porphyrins studied, range in the order of 10^-4–10^-2 with power conversion efficiencies 10^-7–10^-3.     

Au/PATP/PANI膜电极和Au/PATP/PANI/TiO2膜电极的光电化学

聚苯胺（PANI）是一种结构、 性质不同于聚乙炔和聚吡咯的新型导电高聚物,有着十分广泛的应用[1,2].近年来利用光电化学研究聚苯胺的电子结构、 掺杂情况引起人们的重视[3～5].在酸性溶液中电聚合制备聚苯胺的循环伏安曲线上出现两对氧化还原峰,其峰值电位分别为E11/2=0.13 V和E21/2=0.7 V(对饱和甘汞电极).通过改变电极电位,可获得部分氧化态、还原态、氧化态等3种状态的聚苯胺.部分氧化态具有金属导电性,还原态和氧化态均为绝缘体.本文测量3种状态聚苯胺膜电极的光电响应,首次得到其光电流谱,发现聚苯胺一些新的光电化学实验结果.提出了覆盖绝缘体的金属内发射的光电化学模型.同时,在聚苯胺膜上电沉积纳米TiO2微粒膜,得到广谱区的复合光电材料.     

The role of n–p junction electrodes in minimizing the charge recombination and enhancement of photocurrent and photovoltage in dye sensitized solar cells

The composite electrode comprising n-type TiO₂ and p-type NiO oxides when sensitized with Ru-dye showed short-circuit photocurrent (I_sc) of 17 mA/cm² and open-circuit photovoltage (V_oc) of 730 mV compared to I_sc of 12 mA/cm² and 700 mV for TiO₂ electrodes. Formation of a n–p junction between TiO₂ and NiO oxide layers contributes to the enhanced photocurrent, photovoltage, fill factor and efficiency. In addition to the junction effect, NiO acts as a barrier for charge recombination leading to higher cell performance. The efficiency of the NiO coated TiO₂ solar cell is 30% more than that of bare TiO₂. The negative shift of the flat-band potential of the NiO coated TiO₂ electrode compared to TiO₂ also could be one of the reasons for higher photovoltage observed for TiO₂/NiO electrode. The highest cell efficiencies were obtained immersing TiO₂ thin films in Ni²⁺ solution and converting them to NiO by firing and the optimum NiO coating thickness was found to be only a few angstroms. The energy levels of the excited dye and the band positions of TiO₂ and NiO suggest that the electron transfer from the excited dye to the underlying n-type oxide layer occurs by tunneling through the p-type NiO layer.     

TiO2光电化学电池催化氧化甲基红

以钛基TiO₂薄膜为光阳极，研究了光电化学电池中阳极光催化降解偶氮染料甲基红的动力学. 结果表明，短接光电化学电池分隔了光催化过程的阴、阳极反应，有利于抑制光生载流子的复合，提高光催化氧化速率. 相同实验条件下短路光电流越大，则甲基红降解速率越高. 在基底和TiO₂薄膜之间夹层SnO₂得到组装电极Ti/SnO₂/TiO₂，进一步提高了光生载流子的分离效率；同时采用电化学阻抗谱（EIS）评价了电极的光催化性能.     

溴化银晶体光谱敏化光电流的研究和染料增感性能的判据

本文以AgBr晶体和菁染料为对象,对AgBr晶体的光电流进行了研究。实验发现,AgBr晶体经其本征吸收波长的光辐照后,可产生新的诱导吸收带。用诱导吸收带波长范围内的光辐照AgBr晶体,在非本征吸收区可测量到自敏化光电流。若染料吸附在AgBr晶体片上,则其敏化光电流将叠加在自敏化光电流上。增感染料对AgBr晶体的阳极和阴极光电导均有不同程度的敏化作用。本文在实验研究的基础上,提出了对染料增(减)感性能的判据S_D,比现有的一些判据更具有综合性。利用S_D—λ曲线可以较直观地对染料的增(减)感性能做出预测。本文利用S_D判据对一些实用增感染料及其组合作了分析。     

Dye‐Controlled Interfacial Electron Transfer for High‐Current Indium Tin Oxide Photocathodes

Efficient sensitized photocathodes are highly desired for solar fuels and tandem solar cells, yet the development is hindered by the scarcity of suitable p‐type semiconductors. The generation of high cathodic photocurrents by sensitizing a degenerate n‐type semiconductor (tin‐doped indium oxide; ITO) is reported. The sensitized mesoporous ITO electrodes deliver cathodic photocurrents of up to 5.96±0.19 mA cm^?2, which are close to the highest record in conventional p‐type sensitized photocathodes. This is realized by the rational selection of dyes with appropriate energy alignments with ITO. The energy level alignment between the highest occupied molecular orbital of the sensitizer and the conduction band of ITO is crucial for efficient hole injection. Transient absorption spectroscopy studies demonstrate that the cathodic photocurrent results from reduction of the photoexcited sensitizer by free electrons in ITO. Our results reveal a new perspective toward the selection of electrode materials for sensitized photocathodes.     

Light-driven OR and XOR programmable chemical logic gates

Photoelectrodes made of nanocrystalline titanium dioxide modified with various pentacyanoferrates exhibit unique photoelectrochemical properties; photocurrent direction can be switched from anodic to cathodic and vice versa upon changes in photoelectrode potential and incident light wavelength (PhotoElectrochemical Photocurrent Switching, PEPS effect). At certain potentials, anodic photocurrent generated upon UV irradiation has the same intensity as the cathodic photocurrent generated upon visible irradiation. Under these conditions, simultaneous irradiation with UV and visible light results in compensation of anodic and cathodic photocurrents, and zero net photocurrent is observed. This process can be used for construction of unique light-driven chemical logic gates.     

Energetics of Photoinduced Electron Transfer in the Indole-O2 Cluster in Gas Phase: Possible Consequences for Photoexcited Tryptophan in Solution

Abstract— A direct process for an activationless electron transfer from photoexcited tryptophan to molecular oxygen is proposed. By photodetachment of mass-selected indole-O_2- clusters in gas phase a neutral indole₊ O_2- charge-separated exciplex state is found at 2.25 0.2 eV above the neutral ground state. By theory also, the existence of an excited charge separated state at 3.05 0.2 eV is postulated. In gas phase both charge-separated cluster states are energetically below the first singlet states ₁L_b and ₁L_a and the lower even below the first triplet state T₁ of indole. In gas-phase clusters these energetics imply a very efficient quenching of photoexcited indole by fast electron transfer to oxygen. We discuss a similar mechanism for tryptophan-O₂ in aqueous environment and find it without activation barrier and presumably extremely fast. In the collisional tryptophan_*-O₂ complex the efficiency and the time scale of the charge transfer process should be mostly solvent independent. In polar solvent a complete charge separation and free superoxide formation are expected. We correlate this model with previous fluorescence and phosphorescence quenching data of excited tryptophan by O₂ and propose electron transfer to be the relevant process.     

POLYMER BOUND PYRROLE COMPOUNDS–VI. PHOTOPHYSICAL PROPERTIES OF MONOMERIC MODELS FOR POLYSTYRENE-BOUND PORPHYRINS

Abstract— Several porphyrin esters used as models for polystyrene-bound porphyrins have been prepared and their excited states have been studied by laser flash photolysis, IR phosphorescence of singlet molecular oxygen, O₂(¹Δ_g), and steady-state fluorescence. The photophysical properties of the porphyrin esters in solution are affected by the presence of nitro group(s) in the chain. In this case, an important decrease in φ_f, φ_T and φ_δ (to ca 0.7–0.4 of the value for the parent dimethyl ester) is observed. This is mainly due to intramolecular electron-transfer quenching [by the nitro group(s)] of the first excited singlet state of the porphyrin. The thermodynamic feasibility of this deactivation pathway has been confirmed polarographically. Quenching of the porphyrin triplet state and of O₂(¹Δ_g) by the nitro groups is negligible. The present conclusions explain also the results obtained previously for the photooxidation of bilirubin sensitized by the parent insoluble polystyrene-bound porphyrins. In that case the photooxidation rates were correlated directly with the quantum yield of O₂(¹Δ_g) production by the sensitizer. The consequences of these results for the use of polystyrene-bound porphyrins in sensitized photooxidation processes are discussed.     

铅电极在硫酸溶液中的光电化学行为

研究了铅电极在阳极氧化过程中光响应的产生机制。结果表明，Ｐｂ电极在光照氧化时存光活化过程，且光经过程须在电位高于０Ｖ时才能发生。采用现场测量光电压技术研究了Ｐｂ电极在硫酸溶液中的恒电流极化过程。     

PHOTOKINETIC AND PHOTOPHYSICAL MEASUREMENTS OF THE SENSITIZED PHOTOOXIDATION OF THE TRYPTOPHYL RESIDUE IN N-ACETYL TRYPTOPHANAMIDE AND IN HUMAN SERUM ALBUMIN

Abstract— The photosensitized oxidation of 10–100 μ M N -acetyl-L-tryptophanamide (NATA) in neutral aqueous solution and in the presence of various dyes proceeds by a pure O₂(¹Δ_g)-involving mechanism. Incorporation of the tryptophyl (Trp) residue into the polypeptide chain of human serum albumin (HSA) has no influence on the mechanism and efficiency of Trp photooxidation when sensitized either by methylene blue, a non-binding dye, or by rose bengal, a dye that gives non-covalent 1: 1 complexes with HSA. This is due to the location of the Trp residue in close proximity of the protein surface and, in the case of rose bengal, to the coincidence of the photophysical properties (including the quantum yield of O₂(¹Δ_g) generation) for the free and HSA-bound dye. Hematoporphyrin also binds to HSA with 1: 1 stoichiometry, although at a different site from rose bengal. Bound Hp again displays photophysical properties very similar with those of free Hp; however, the efficiency of Trp photo-oxidation in HSA is about 5-fold higher than in NATA owing to a limited rearrangement of the protein structure, induced by Hp binding, which enhances the probability of chemical quenching of O₂(¹Δ_g) by the indole ring.     

苯基磷酸联吡啶钌络合物敏化纳晶多孔TiO2薄膜电极光电性能研究

用有机光敏染料敏化半导体,通过染料分子的吸附功能基团与半导体相互作用,使染料分子与半导体表面之间建立电性耦合,进行有效的电荷转移,可以形成有机-半导体复合新型光电功能材料。联吡啶钌络合物有较强的可见光吸收、氧化还原性能可逆、氧化态稳定性高,是一类性能优越的有机光敏染料。近来许多研究发现,羧酸联吡啶钌的强吸附与TiO₂纳晶薄膜的大比表面相结合,导致光生电荷快速注入TiO₂导带达到有效的电荷分离,得到了接近100%的单色光光电流效率^[1]。为研究联吡啶钌分子的不同吸附功能基团与TiO₂纳晶薄膜表面的相互作用对提高光电性能的影响,本文报道苯基磷酸取代的联吡啶钌络合物敏化纳晶多孔TiO₂薄膜的光电性能。     

Ru(bpy)2(NCS)2染料敏化CdS/Zn2+TiO2复合半导体纳米多孔膜的光电化学

采用水热法合成了Zn²⁺离子掺杂的TiO₂纳米粒子[Zn²⁺掺杂量0.5%(物质的量的比)],并用光电化学方法研究了经Ru(bpy)₂(NCS)₂(bpy=2,2′bipyridine₄,4′dicarboxylicacid)分别敏化的掺杂Zn²⁺的TiO₂电极(简写为Zn²⁺-TiO₂)和CdS/Zn²⁺-TiO₂复合半导体纳米多孔膜电极的光电化学行为.实验证明Ru(bpy)₂(NCS)₂敏化CdS/Zn²⁺-TiO₂复合半导体纳米多孔膜电极比单独敏化Zn²⁺-TiO₂电极的光电转换效率高,且敏化Zn²⁺TiO₂电极和敏化CdS/Zn²⁺TiO₂复合半导体纳米多孔膜电极比Zn²⁺-TiO₂电极的光电流产生的起始波长都向长波方向移动.在360600nm范围内,Ru(bpy)₂(NCS)₂敏化CdS/Zn²⁺-TiO₂复合半导体纳米多孔膜电极光电转换效率最好.     

A NEW METHOD FOR THE DETECTION OF SINGLET OXYGEN IN AQUEOUS SOLUTIONS

Abstract— In experiments on the interception of reactive intermediates of strongly oxidizing character in dye (S) sensitized photooxidations using p -nitrosodimethylaniline (RNO) as a selective scavenger, it has been observed that some substrates (A) or ¹O₂ acceptors (like imidazole derivatives) induce the bleaching of RNO as followed spectrophotometrically at 440 nm. Since singlet oxygen (¹O₂) does not react chemically with RNO, this bleaching is a consequence of ¹O₂ capture by the imidazole ring which results in the formation of a trans-annular peroxide intermediate [¹O₂] capable of inducing the bleaching of RNO (-RNO). In the absence of RNO, [¹O₂] decomposes or rearranges into the final oxygenation product ¹O₂: ¹Δ_g Thus, the system imidazole plus RNO can be used as a sensitive and selective test for the presence of ¹O₂ in aqueous solutions. The method can also be applied in the presence of sensitizing dyes which, under visible irradiation, can partially bleach RNO even in the absence of imidazole derivatives. In such a case, the bleaching of RNO is strongly increased by the presence of imidazoles with a characteristic dependence on their concentration. The separation of the product of RNO bleaching by thin layer chromatography can serve as additional proof of the presence of ¹O₂ in the system. The imidazole plus RNO method has been applied to a number of sensitizing and non-sensitizing dyes.     

EFFECT OF AGGREGATION ON THE HEMATOPORPHYRIN-SENSITIZED PRODUCTION OF SINGLET MOLECULAR OXYGEN

Abstract— The hematoporphyrin-sensitized production of singlet molecular oxygen, O₂(¹Δ_g), has been investigated in methanol and in aqueous solution. The quantum yield for formation of O₂(¹Δ_g) (Φ_Δ) has been measured by both steady-state (oxygen consumption) and time-resolved (near-infrared luminescence) methods. In methanol, both techniques indicate that Φ_Δ= 0.76 and the value remains independent of sensitizer concentration over a wide range. This finding is consistent with the dye persisting in a monomelic form in methanol solution. In contrast, Φ_Δ decreases markedly with increasing sensitizer concentration in water due to dimerization of the dye. Analysis of the steady-state data indicates Φ_Δ values of 0.74 and 0.12, respectively, for monomer and dimer. It is further shown that the efficiency whereby quenching of the triplet state by O₂ results in generation of O₂(¹Δ_g) is substantially lower for the dimer than for the corresponding monomer. Because monomer and dimer possess quite different absorption spectral profiles, the efficacy for photodynamic action with hematoporphyrin exhibits a pronounced wavelength dependence.     

汞溴红敏化TiO2纳米多孔膜电极的光电化学性质研究

用汞溴红(Mercurochrome)作敏化剂敏化TiO2纳米多孔膜电极,UV-Vis,光电流作用谱和SPS谱表明,该敏化剂能有效地敏化TiO2电极,并且染料聚集体能扩大染料的敏化区间,增加了太阳光的利用效率,对电极施加不同的偏压,测试电极产生的瞬态光电流,研究了偏压对光生电荷转移及复合速率的影响,从不同方向照射电极,对阳极光电流影响显著,结合电化学、光电化学原理对这些现象进行了讨论.     

COMPETITION BETWEEN THE SINGLET OXYGEN AND ELECTRON TRANSFER MECHANISMS IN THE PORPHYRIN-SENSITIZED PHOTOOXIDATION OF l-TRYPTOPHAN AND TRYPTAMINE IN AQUEOUS MICELLAR DISPERSIONS

Abstract— Kinetic studies of the hematoporphyrin–sensitized photooxidation of l -tryptophan and tryptamine at pH 10 in either homogeneous aqueous solutions or in aqueous dispersions of Triton X–100 and cetyltrimethylammonium bromide micelles indicate that the indole substrates are attacked via a mixed type I (electron transfer from triplet dye)/type II (¹O₂-involving) mechanism. Both reactive intermediates, generated by micelle-solubilized hematoporphyrin, can diffuse to attack substrate molecules located in either the bulk aqueous phase or a different micelle. In particular, incorporation of the substrate into a micelle has only minor effects on its reactivity toward¹O₂, although the ¹O₂—indole interaction appears to be more efficient in cationic micelles owing to a favourable orientation of the target with respect to the attacking species. On the other hand, the electron transfer from triplet porphyrin to a micellized substrate is virtually non-operative when the latter is located in an anionic micelle, whereas in neutral or cationic micelles, the efficiency of the process is again controlled by the substrate orientation. Studies of tryptamine photooxidation sensitized by meso-tetra-(4-sulfonato-phenyl) porphine in the presence of sodium dodecylsulphate micelles lend further support to the abovementioned conclusions.