Photoinduced electron transfer in Langmuir-Blodgett monolayers of porphyrin-fullerene dyads

A series of electron donor-acceptor (DA) dyads, composed of a porphyrin donor and a fullerene acceptor covalently linked with two molecular chains, were used to fabricate solid molecular films with the Langmuir-Blodgett (LB) technique. By means of the LB technique, the DA molecules can be oriented perpendicular to the plane of the substrate. In DHD6ee and its zinc derivative hydrophilic groups are attached to the phenyl moieties in the porphyrin end of the molecule; while in the other three dyads, TBD6a, TBD6hp, and TBD4hp, the hydrophilic groups are in the fullerene end of the molecule. This makes it possible to alternate the orientation of the molecules in two opposite directions with respect to the air-water interface and to fabricate molecular assemblies in which the direction of the primary photoinduced vectorial electron transfer can be controlled both by the deposition direction of the LB monolayer and by the selection of the used DA molecule. This was proved by the time-resolved Maxwell displacement charge measurements. The spectroscopic properties of the DA films were studied with the steady-state absorption and fluorescence methods. In addition, the time correlated single photon counting technique was used to determine the fluorescence properties of the dyad films.     

Photoelectrodes based on acceptor–donor assemblies in functionalized polypyrrole films

The capability to act as molecular photoelectrodes under visible light irradiation of optically transparent electrodes (ITO) modified by thin films of polypyrrole containing several kind of electron acceptor–donor assemblies has been examined. Photolysis of electrodes coated by thin films of polypyrrole substituted by a reversible electron donor (phenothiazine) in the presence of an irreversible electron acceptor (tropylium cation) in acetonitrile gives weak photocurrents. In contrast, appreciable photocurrents can be obtained using a symmetrical arrangement; viologen as reversible electron acceptor, benzilate anion as irreversible donor. The photoresponses result from the photo-induced charge separation of the charge transfer complexes created in the film. The measured photocurrents are markedly larger (up to five times), with films of polypyrrole substituted by a reversible electron acceptor (viologen) covalently linked with a donor (phenothiazine, triphenylamine or benzidine) than with the unimolecular immobilized system in similar experimental conditions. The greater efficiency of these materials is attributed to the formation of an intramolecular charge transfer complex occurring inside films between the two molecular entities. Markedly weaker photocurrents are obtained with polypyrrole films based on bilayers of the two independent components than those with the unimolecular design, while films based on copolymers arrangements give moderately weaker photoresponses.     

Symmetry Effects in Photoinduced Electron Transfer in Chlorin-Quinone Dyads: Adiabatic Suppression in the Marcus Inverted Region

In donor–acceptor dyads undergoing photoinduced electron transfer (PET), a direction or pathway for electron movement is usually dictated by the redox properties and the separation distance between the donor and acceptor subunits, while the effect of symmetry is less recognized. We have designed and synthesized two isomeric donor–acceptor assemblies in which electronic coupling between donor and acceptor is altered by the orbital symmetry control with the reorganization energy and charge transfer exothermicity being kept unchanged. Analysis of the optical absorption and luminescence spectra, supported by the DFT and TD-DFT calculations, showed that PET in these assemblies corresponds to the Marcus inverted region (MIR) and has larger rate for isomer with weaker electronic coupling. This surprising observation provides the first experimental evidence for theoretically predicted adiabatic suppression of PET in MIR, which unambiguously controlled solely by symmetry.     

胆红素有序分子膜的行为研究

研究了不同亚相表面胆红素(BR)单分子膜和LB膜的性质,讨论了胆红素分子在有序分子膜中的堆积密度、分子伸展和金属离子配位。在气-水界面,BR与金属离子的配位导致BR单分子截面积、崩溃压和可见紫外光谱的变化。原子力显微镜表明BR-Cu单分子膜的厚渡为1.23 nm。     

Elimination of cross-talk and modulation of function in an organized heterosupramolecular assembly

A close-packed monolayer of TiO2 nanocrystals was deposited on a conducting glass support using Langmuir-Blodgett (LB) techniques and fired. A close-packed mixed monolayer of eicosyl phosphonic acid (I) and the viologen. 1,1'-dieicosyl-4,4'-bipyridinium dichloride (II) was then deposited on the TiO2 substrate, also using LB techniques. At sufficiently high dilutions of II in II a single viologen molecule is adsorbed with a known orientation at the surface of each nanocrystal. The resulting assembly was incorporated as the working electrode in an electrochemical cell. Under open circuit conditions, bandgap excitation of a TiO2 nanocrystal results in electron transfer to a viologen molecule. No electron transfer between the viologen molecules adsorbed at different nanocrystals is observed. At a positive applied potential, electron transfer following bandgap excitation is largely suppressed. Considered are the implications of these findings for the development of practical devices based on modulatable function addressable on the nanometer scale.     

Photoinduced intramolecular electron transfer and charge separation in zinc phthalocyanine-viologen linked system

Photoinduced electron transfer and charge separation processes in zinc phthalocya-nine-viologen linked system have been studied and the distance effect of donor/acceptor on electron transfer reaction is discussed. It is indicated that the fluorescence from the zinc phthalocyanine moiety is appreciably quenched and the life-time of singlet excited state is reduced by the pendant viologen. Time-resolved transient absorption spectra measurements show that intramolecular quenching of the triplet state of zinc phthalocyanine by the attached viologen results in charge separation giving reduced viologen radical alive for a rather long period with hundred microsecond duration. The effect of the carbon chain length on the electron transfer rate constant and charge separation efficiency suggests that upon excitation, the zinc phthalocyanine and viologen groups tend to take closer conformation with the increase of the carbon chain examined. The rate constant for the intramolecular electron transfer ket with n = 3     

Synthesis and characterization of monolayers and Langmuir-Blodgett films of an amphiphilic oligo(ethylene glycol)-C60-hexadecaaniline conjugate

A novel amphiphilic oligo(ethylene glycol)-C60-hexadecaaniline (A16) tricomponent conjugate, C60>(A16-EG43), possessing a well-defined number of repeating aniline donor units and a hydrophilic ethylene glycol oligomer chain was synthesized. The compound is composed of a covalently bound donor-acceptor chromophore structure. Molecular self-assembly of C60>(A16-EG43) at the air-water interface formed a densely packed Langmuir monolayer with all highly hydrophobic fullerene cages located above the liquid interface. The monolayer can then be transferred onto a glass substrate via Langmuir-Blodgett (LB) deposition. LB multilayered thin films formed by multiple deposition of the monolayer yielded broadened optical absorption peaks extending beyond 600 nm into the 950 nm region, suggesting strong intermolecular interactions among the C60 cages and the A16 moieties. An X-ray reflectometry study clearly reveals that the Langmuir film at the air-water interface consists of a C60 top layer and a bottom layer containing hexcadecaaniline and oligo(ethylene glycol) with gradually decreasing electron density over a distance of approximately 130 A above bulk water. The pressure isotherm shows that the packing density of the C60>(A16-EG43) monolayer, corresponding to a molecular area of approximately 95 A2/molecule, is similar to that of the surface area of the C60 monolayer. This result suggests that C60 packing plays a dominant role in guiding the formation of the monolayer structure. Further photoexcitation of hexadecaaniline moieties of aligned (C60>)-A16 layers by a flash light source induces cross linking between adjacent A16 segments forming an interlinked A16 array. Our results have demonstrated a unique fabrication method for preparing the aligned donor-acceptor array using strong intermolecular interactions between fullerenes as the molecular orientation guiding force in the Langmuir-Blodgett technique.     

Ultrafast Photoinduced Electron Transfer between an Incarcerated Donor and a Free Acceptor in Aqueous Solution

Supramolecular photoinduced electron transfer dynamics between coumarin 153 (C153) and 4,4'-dimethyl viologen dichloride (MV(2+)) across the molecular barrier of a host molecule, octa acid (OA), has been investigated with femtosecond time resolution. The ultrafast electron transfer from C153 to MV(2+) followed excitation with 150 fs laser pulses at a wavelength of 390 nm despite the fact that C153 was incarcerated within an OA(2) capsule. As a result, the photoexcited coumarin did not show any of the typical relaxation dynamics that is usually observed in free solution. Instead, the excited electron was transferred across the molecular wall of the capsuleplex within 20 ps. Likewise, the lifetime of the charge transfer state was short (724 ps), and electron back-transfer reestablished the ground state of the system within 1 ns, showing strong electronic coupling among the excited electron donor, host, and acceptor. When the donor was encapsulated into the host molecule, the electron transfer process showed significantly accelerated dynamics and essentially no solvent relaxation compared with that in free solution. The study was also extended to N-methylpyridinium iodide as the acceptor with similar results.     

Diversified Excited-State Relaxation Pathways of Donor–Linker–Acceptor Dyads Controlled by a Bent-to-Planar Motion of the Donor

Herein, we introduce the cyclic 8π-electron (C8π) molecule N,N′-diaryl-dihydrodibenzo[a,c]phenazine ( DPAC ) as a dual-functional donor to establish a series of new donor–linker–acceptor (D–L–A) dyads DLA1 – DLA5 . The excited-state bent-to-planar dynamics of DPAC regulate the energy gap of the donor, while the acceptors A1 – A5 are endowed with different energy gaps and HOMO/LUMO levels. As a result, the rate and efficiency of the excited-state electron transfer vs. energy transfer can be finely harnessed, which is verified via steady-state spectroscopy and time-resolved emission measurements. This comprehensive approach demonstrates, for the first time, the manifold of excited-state properties governed by bifunctional donor-based D–L–A dyads, including bent-to-planar, photoinduced electron transfer (PET) from excited donor to acceptor (oxidative-PET), fluorescence resonance energy transfer (FRET), bent-to-planar followed by electron transfer (PFET), and PET from donor to excited acceptor (reductive-PET).     

Fluorescence-On Response via CB7 Binding to Viologen-Dye Pseudorotaxanes

Fluorescence-on sensors typically rely on disrupting photoinduced electron transfer quenching of the excited state through binding the electron donor. To provide a more general fluorescence-on signaling unit, a quencher-fluorophore dyad has been developed in which quenching by electron transfer to a tethered viologen acceptor can be disrupted through complexation of the viologen by cucurbit[7]uril (CB7). Dyads of benzyl viologen-rhodamine B or a BODIPY fluorophore gave upon CB7 complexation 14- and 30-fold fluorescence enhancement, respectively.     

Highly selective fluorescent OFF-ON thiol probes based on dyads of BODIPY and potent intramolecular electron sink 2,4-dinitrobenzenesulfonyl subunits

Two highly selective OFF-ON green emitting fluorescent thiol probes (1 and 2) with intense absorption in the visible spectrum (molar extinction coefficient ε is up to 73?800 M(-1) cm(-1) at 509 nm) based on dyads of BODIPY (as electron donor of the photo-induced electron transfer, i.e.PET) and 2,4-dinitrobenzenesulfonyl (DNBS) (as electron acceptor of the PET process) were devised. The single crystal structures of the two probes were determined. The distance between the electron donor (BODIPY fluorophore) and the electron acceptor (DNBS) of probe 2 is larger than that of probe 1, as a result the contrast ratio (or the PET efficiency) of probe 2 is smaller than that of probe 1. However, fluorescence OFF-ON switching effects were observed for both probe 1 and probe 2 in the presence of cysteine (the emission enhancement is 300-fold for probe 1 and 54-fold for probe 2). The fluorescence OFF-ON sensing mechanism is rationalized by DFT/TDDFT calculations. We demonstrated with DFT calculations that DNBS is ca. 0.76 eV more potent to accept electrons than the maleimide moiety. The probes were used for fluorescent imaging of cellular thiols.     

SYNTHESIS OF AMPHIPATHIC PORPHYRINS AND THEIR PHOTOINDUCED ELECTRON TRANSFER REACTIONS AT THE LIPID BILAYER-WATER INTERFACE

A one flask synthesis of cis -substituted amphipathic porphyrins is reported. These porphyrins were used to study electrostatic effects on photoinduced electron transfer across the lipid bilayer-water interface. A neutral porphyrin undergoes only dynamic interfacial electron transfer reactions irrespective of charge of the acceptor, although ionic strength effects indicate a negative charge on the porphyrin donor species. A dianionic porphyrin forms an interfacial static complex with a dicationic electron acceptor, methyl viologen, at low ionic strength. The electron transfer rate within the complex is slow, 10⁵∼ 10⁶ s^-1, which is attributed to a near orthogonal orientation between the donor and the acceptor ∼ orbitals.     

Use of UV-vis reflection spectroscopy for determining the organization of viologen and viologen tetracyanoquinodimethanide monolayers

UV-vis reflection spectroscopy has been used for proving in situ the organization of pure viologen and hybrid viologen tetracyanoquinodimethanide monolayers at the air-water interface. Other more classical measurements concerning Langmuir monolayers, including surface pressure-area and surface potential-area isotherms, are also provided. The organization of the viologen in the Langmuir monolayer was investigated upon the different states of compression, and the tilt angle of the viologen moieties with respect to the water surface was determined. A gradual transition of the viologen molecules from a flat orientation in the gas phase to a more tilted position with respect to the water surface in the condensed phases occurs. The addition of a tetracyanoquinodimethane (TCNQ) salt in the subphase leads to the penetration of TCNQ anions into the positively charged viologen monolayer forming a hybrid viologen tetracyanoquinodimethanide film where a charge-transfer interaction between the two moieties is observed. From a quantitative analysis of the reflection spectra, an organization model of these hybrid monolayers at the air-water interface is proposed, suggesting a parallel arrangement of viologen and TCNQ units with a 1:2 stoichiometry.     

取代苄叉亚胺类化合物溶液电子光谱的研究

研究了二甲氨基苄叉亚胺类化合物溶液的电子光谱。结果表明一端为电子给体另一端为电子受体的共轭型取代苄叉对硝基苯胺3,在基态下发生了分子内电荷转移,其跃迁吸收波长比化合物1,2(315 nm)红移90 nm。以乙撑基联接的非共轭型苄叉亚胺奥化合物4,5,6未观察到基态时的电荷转移现象,但在激发条件下可明显的发生分子内的电荷转移而使荧光强度大大减弱。MMA等对1的荧光有明显的猝灭作用,表明其间发生了电荷转移。丙烯酸则因会引起西佛碱的水解,而使荧光减弱。     

蒽醌、甲基紫精、N,N-二乙基苯胺与金丝桃蒽酮素光致电子转移的研究

金丝桃蒽酮素(HYP)是有多个羟基的醌类衍生物。在极性溶剂乙腈中,电子给体N,N-二乙基苯胺和电子受体甲基紫精、蒽醌均能有效猝灭其荧光。说明它既具有酚羟基的给电子性能,又有醌类化合物的受电子性能。HYP与平面构型的蒽醌可能形成基态复合物,以不同的方式与其发生光致相互作用。     

A Novel Fluorescent Probe for Lead Ions

A new fluorescent probe for lead ions, p-nitrophenyl 3H-phenoxazin-3-one-7-yl phosphoric acid (NPPA), has been synthesized by linking resorufLn (serving as a fluorophore and electron acceptor) to p-nitrophenol (serving as a fluorescence quencher and electron donor) through phosphodiester bonds. When NPPA was irradiated with light, intramolecular fluorescence self-quenching took place due to the PET (photoinduced electron transfer) from the donor to the acceptor. However, upon addition of Pb^Ⅱ, the phosphate ester bonds in the probe were cleaved and the fluorophore was released, accompanying the retrievement of fluorescence.     

Single-molecule fluorescence photoswitching of a diarylethene-perylenebisimide dyad: non-destructive fluorescence readout

Single-molecule fluorescence photoswitching plays an essential role in ultrahigh-density (Tbits/inch(2)) optical memories and super-high-resolution fluorescence imaging. Although several fluorescent photochromic molecules and fluorescent proteins have been applied, so far, to optical memories and super-high-resolution imaging, their performance is unsatisfactory because of the absence of "non-destructive fluorescence readout capability". Here we report on a new molecular design principle of a molecule having non-destructive readout capability. The molecule is composed of acceptor photochromic diarylethene and donor fluorescent perylenebisimide units. The fluorescence is reversibly quenched when the diarylethene unit converts between the open- and the closed-ring isomers upon irradiation with visible and UV light. The fluorescence quenching is based on an electron transfer from the donor to the acceptor units. The fluorescence photoswitching and non-destructive readout capability were demonstrated in solution (an ensemble state) and at the single-molecule level. Femtosecond time-resolved transient and fluorescent lifetime measurements confirmed that the fluorescence quenching is attributed to the intramolecular electron transfer.     

Biomolecular photodiode consisting of cytochrome c-adsorbed hetero-Langmuir–Blodgett films

Photoinduced electron transfer in biomolecular photodiode consisting of protein-adsorbed hetero-Langmuir–Blodgett (LB) films was investigated. Four kinds of functional molecules, cytochrome c, viologen, flavin, and ferrocene, were used as a second electron acceptor, a first electron acceptor, an electron sensitizer, and an electron donor, respectively. The hetero-LB film was fabricated by subsequently depositing ferrocene, flavin, and viologen onto the pretreated ITO glass. Cytochrome c-adsorbed hetero-LB films were prepared by soaking the hetero-LB films into the phosphate buffer solution containing cytochrome c. To verify the optimal adsorption conditions of cytochrome c molecules onto the viologen LB layers, the UV-absorption spectrum and atomic force microscopy observations of LB films were performed. Finally, the metal/insulator/metal structured molecular device was constructed by depositing aluminum onto the surface of the cytochrome c-adsorbed hetero-LB films. For photoelectric response properties, the current–voltage characteristic and photoswitching effect of the proposed molecular photodiode were investigated. To verify the charge shift, transient photocurrent of the molecular photodiode was measured.     

Fabrication of densely packed titania nanosheet films on solid surface by use of Langmuir-Blodgett deposition method without amphiphilic additives

Densely packed exfoliated nanosheet films such as Ti0.91O2, Ti0.8M0.2O2 (M = Co, Ni), Ti0.6Fe0.4O2, and Ca2Nb3O10 on solid substrates were prepared by the LB transfer method without any amphiphilic additives at the air-water interface. Nanosheet crystallites covered nearly 95% on the solid surface with minimum overlapping of nanosheets. The LB transfer method of the Ti0.91O2 nanosheet monolayer film is applicable for not only hydrophilic substrates such as quartz, silicon, indium-tin oxide (ITO), and glass but also the hydrophobic Au surface. On the basis of these points, the LB transfer method has advantages compared to the alternating layer-by-layer method, which makes use of oppositely charged polyelectrolytes such as poly(ethylenimine) (PEI). Adsorption of hydrophobic Ti0.91O2 nanosheets at the air-water interface is responsible for this LB transfer deposition method. The addition of tetrabutylammonium bromide into the subphase assisted the adsorption, causing an increase in the adsorbed amount of Ti0.91O2 nanosheets at the air-water interface.     

Porphyrin light-harvesting arrays constructed in the recombinant tobacco mosaic virus scaffold

We have demonstrated the construction of multiple porphyrin arrays in the tobacco mosaic virus (TMV) supramolecular structures by self-assembly of recombinant TMV coat protein (TMVCP) monomers, in which Zn-coordinated porphyrin (ZnP) and free-base porphyrin (FbP) were site-selectively incorporated. The photophysical properties of porphyrin moieties incorporated in the TMV assemblies were also characterized. TMV-porphyrin conjugates employed as building blocks self-assembled into unique disk and rod structures under the proper conditions as similar to native TMV assemblies. The mixture of a ZnP donor and an FbP acceptor was packed in the TMV assembly and showed energy transfer and light-harvesting activity. The detailed photophysical properties of the arrayed porphyrins in the TMV assemblies were examined by time-resolved fluorescence spectroscopy, and the energy transfer rates were determined to be 3.1-6.4x10(9) s(-1). The results indicate that the porphyrins are placed at the expected positions in the TMV assemblies.