Fast Photoinduced Electron Transfer Research Articles

Effects of single-walled carbon nanotubes on the optical and photo-conductive properties of their composite films with regio-regular poly(3-hexylthiophene)

The effect of a small admixture of single-walled carbon nanotubes (SWNTs) HiPCO (high pressure carbon monoxide) (from 0.5 to 2 wt%) on the supramolecular structure in regio-regular poly(3-hexylthiophene) (RR-P3HT) thin films is studied and their optical and photoconductivity properties are investigated. It is demonstrated that the presence of such small amounts of nanotubes improves the structural organization in the films as evidenced by X-ray diffraction (XRD) studies. This is confirmed by UV–visible optical absorption investigations which clearly show a better conjugation of P3HT in the presence of nanotubes. In Raman spectra of composites, changes in intensities and frequencies of the radial breathing modes are observed upon addition of nanotubes. This can be rationalized by a modification of the resonance conditions caused by a selective dispersion and wrapping of SWNTs via π-interaction (π-stacking). As a consequence of these interactions, a dramatic photoluminescence (PL) quenching is observed which becomes more and more pronounced with increasing the nanotube content. This implies a fast photo-induced electron transfer favoured by a large area of the SWNTs/P3HT interface and strong interactions between these two components. An increase in the composite photocurrent by at least one-order of magnitude, as compared to the case of pure P3HT film, is the most pronounced effect of this electron transfer. These two effects are of crucial importance for the application of the investigated composites in bulk hetero-junction photovoltaic cells (BHJPCs) and organic photo-detectors (OPDs).

A New Near-Infrared Neutral pH Fluorescent Probe for Monitoring Minor pH Changes and its Application in Imaging of HepG2 Cells

A new near-neutral pH near-infrared (NIR) fluorescent probe utilizing a fluorophore-receptor molecular framework that can modulate the fluorescence emission intensity through a fast photoinduced electron transfer process was developed. Our strategy was to choose tricarbocyanine (Cy), a NIR fluorescent dye with high extinction coefficients, as a fluorophore, and N-methylpiperazine (MP) as a receptor. The pH titration indicated that MP-Cy can monitor the minor physiological pH fluctuations with a pKa of ∼7.10 near physiological pH, which is valuable for intracellular pH researches. The probe responds linearly and rapidly to minor pH fluctuations within the range of 3.05-7.10 and exhibits strong dependence on pH changes. As expected, the real-time imaging of cellular pH and the detection of pH in situ was achieved successfully in living HepG2 cells by this probe. It is shown that the probe effectively avoids the influence of autofluorescence and native cellular species in biological systems and meanwhile exhibits high sensitivity, good photostability, and excellent cell membrane permeability.

Improved Superiority by Covalently Binding Dye to Graphene for Hydrogen Evolution from Water under Visible-Light Irradiation

A novel nanohybrid composed of bipyridine ruthenium complex ((2,2′-bipyridyl)-4-pyridyl-chloro-ruthenium(II), Ru(bpy)2(py)Cl), covalently functionalized graphene (Ru(bpy)2(py)Cl/G), has been synthesized successfully via coordination of Ru(bpy)2Cl2 with pyridine covalently functionalized graphene (py/G), which was synthesized through 1,3-dipolar cycloaddition of azomethine ylides. Ru(bpy)2(py)Cl/G was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectra, Fourier transform infrared (FTIR), and ultraviolet–visible absorption (UV–vis). The fluorescence quenching efficiency of the nanohybrid excited at 290 and 535 nm was calculated to be 83% and 71%, respectively. The photocurrent density of the nanohybrid also significantly improved. The results revealed that fast photoinduced electron transfer from Ru(bpy)2(py)Cl moiety to graphene sheet occurred. Ru(bpy)2(py)Cl/G modified with platinum nanoparticles demonstrates remarkable enhanced photocatalytic activity for hydrogen evolution from water. Under 10 h UV–vis or visible light irradiation (>400 nm), the total amount of H2 evolution was 39.3 and 7.6 μmol mg–1, respectively. In addition, Ru(bpy)2(py)Cl/G/Pt nanohybrid shows sufficient stability for photocatalytic H2 evolution, and the hydrogen yield remains virtually unchanged in 50 h of irradiation. This study suggests that the carbon based nanohybrid composed of organic dye molecules covalently functionalized graphene is a promising candidate as a novel photocatalyst for photocatalytic hydrogen evolution.

Fluorescence detection of trace TNT by novel cross-linking electropolymerized films both in vapor and aqueous medium

Electropolymerized (EP) films with high fluorescent efficiency are introduced to the detection of trace 2,4,6-trinitrotoluene (TNT). Three electroactive materials TCPC, OCPC and OCz have been synthesized and their EP films have been demonstrated to be sensitive to TNT. Among them, the TCPC EP films have displayed the highest sensitivity to TNT in both vapor and aqueous medium, even in the natural water. It is proposed that the good performances would be caused by the following two factors: first, the cross-linking network of EP films can generate the cavities which benefit the TNT penetration, and remarkably increase the contact area between the EP films and TNT; second, the frontier orbits distribution leads the fast photo-induced electron transfer (PET) from the TCPC EP films to TNT. Our results prove that these EP films are promising TNT sensing candidates and provide a new method to prepare fluorescent porous films.

Electron Transfer Across Modular Oligo-p-phenylene Bridges in Ru(bpy)2(bpy–phn–DQ)4+(n= 1–5) Dyads. Unusual Effects of Bridge Elongation

A series of dyads of general formula Ru(bpy)(2)(bpy-ph(n)-DQ)(4+) (n = 1-5), based on a Ru(II) polypyridine unit as photoexcitable donor, a set of oligo-p-phenylene bridges with 1-5 modular units, and a cyclo-diquaternarized 2,2'-bipyridine (DQ(2+)) as electron acceptor unit, have been synthesized. Their spectroscopic and photophysical properties have been investigated in CH(3)CN and CH(2)Cl(2) by time-resolved emission and absorption spectroscopy in the nanosecond and picosecond time scale. The experimental study has also been complemented with a computational investigation carried out on the whole series of dyads. The absorption spectra of the dyads show new spectroscopic transitions, in addition to those characteristic of the donor, bridge, and acceptor fragments. DFT calculations suggest the assignment of such bands as bridge-to-acceptor (π ph(n)) → (π* DQ) charge-transfer transitions. This assignment is consistent with the solvatochromic and spectroelectrochemical behavior of the new bands. For all the dyads at room temperature in fluid solution, the typical (3)MLCT luminescence of the Ru(II) polypyridine unit is strongly (>90%) quenched, supporting the occurrence of an efficient intramolecular photoinduced electron transfer. The study has revealed, however, that the photophysical mechanism is actually more complex than presumed on the basis of a simple photoinduced electron-transfer scheme. For n = 1, very fast (few picoseconds) photoinduced electron transfer from the MLCT state localized on the substituted bpy ligand to the DQ unit has been observed, followed by slower interligand hopping and charge recombination. For n = 2-5, MLCT excited-state quenching takes place without transient detection of charge-separated product, indicating that charge recombination is faster than charge separation. This behavior can be rationalized in terms of the superexchange couplings expected through this type of bridges for the two processes. The kinetics of MLCT quenching in the dyads with n = 1-5 does not follow the usual exponential falloff with bridge length: after a regular decrease for n = 1-3, the rate constants become almost insensitive to bridge length for n = 3-5. The rationale of this uncommon behavior, as suggested by DFT calculations, lies in a switch in the MLCT quenching mechanism with increasing bridge length, from oxidative quenching by the DQ acceptor to reductive quenching by the bridge.

Differential Tuning of the Electron Transfer Parameters in 1,3,5-Triarylpyrazolines: A Rational Design Approach for Optimizing the Contrast Ratio of Fluorescent Probes

A large class of cation-responsive fluorescent sensors utilizes a donor-spacer-acceptor (D-A) molecular framework that can modulate the fluorescence emission intensity through a fast photoinduced intramolecular electron transfer (PET) process. The emission enhancement upon binding of the analyte defines the contrast ratio of the probe, a key property that is particularly relevant in fluorescence microscopy imaging applications. Due to their unusual electronic structure, 1,3,5-triarylpyrazoline fluorophores allow for the differential tuning of the excited-state energy DeltaE(00) and the fluorophore acceptor potential E(A/A(-)), both of which are critical parameters that define the electron transfer (ET) thermodynamics and thus the contrast ratio. By systematically varying the number and attachment positions of fluoro substituents on the fluorophore pi-system, DeltaE(00) can be adjusted over a broad range (0.4 eV) without significantly altering the acceptor potential E(A/A(-)). Experimentally measured D-A coupling and reorganization energies were used to draw a potential map for identifying the optimal ET driving force that is expected to give a maximum fluorescence enhancement for a given change in donor potential upon binding of the analyte. The rational design strategy was tested by optimizing the fluorescence response of a pH-sensitive probe, thus yielding a maximum emission enhancement factor of 400 upon acidification. Furthermore, quantum chemical calculations were used to reproduce the experimental trends of reduction potentials, excited-state energies, and ET driving forces within the framework of linear free energy relationships (LFERs). Such LFERs should be suitable to semiempirically predict ET driving forces with an average unsigned error of 0.03 eV, consequently allowing for the computational prescreening of substituent combinations to best match the donor potential of a given cation receptor. Within the scaffold of the triarylpyrazoline platform, the outlined differential tuning of the electron transfer parameters should be applicable to a broad range of cation receptors for designing PET sensors with maximized contrast ratios.

Covalent functionalization of multi-walled carbon nanotube surfaces by conjugated polyfluorenes

The outer surface of multi-walled carbon nanotubes (MWCNTs) was successfully modified with 7-bromo-9,9-dioctylfluorene-2-carbaldehyde by using 1,3-dipolar cycloaddition of azomethine reaction to introduce bromo functional groups. The peripheral bromo functional groups can be used to further react with AB-type monomers through Suzuki polycondensation to afford the PF-functionalized MWCNTs, which are of a cable-like structure. Through covalent connection to MWCNTs, the fluorescence of polyfluorenes was completely quenched by the MWCNTs, indicating a fast photo-induced electron transfer from polyfluorenes to MWCNTs.

Viologen-Modified Platinum Clusters Acting as an Efficient Catalyst in Photocatalytic Hydrogen Evolution

A highly efficient photocatalytic system for hydrogen evolution with dihydronicotinamide coenzyme (NADH) as a sacrificial agent in an aqueous solution has been constructed by using water-soluble platinum clusters functionalized with methyl viologen-alkanethiol (MVA2+) and a simple electron-donor dyad, 9-mesityl-10-methylacridinium ion (Acr+-Mes), which is capable of fast photoinduced electron transfer but extremely slow back electron transfer. The mean diameter of the platinum core was determined as R(CORE) = 1.9 nm with a standard deviation sigma = 0.5 nm by transmission electron microscopy (TEM). As a result, the hydrogen-evolution rate of the photocatalytic system with MVA2+-modified platinum clusters (MVA2+-PtC) is 10 times faster than the photocatalytic system with the mixture of the same amount of MVA2+ and platinum clusters as that of MVA2+-PtC under otherwise the same experimental conditions. The radical cation of NADH has been successfully detected by laser flash photolysis experiments. The decay of the absorbance due to NAD*, produced by the deprotonation from NADH*+, coincides with the appearance of the absorption band due to Acr*-Mes. This indicates electron transfer from NAD* to Acr+-Mes to give Acr*-Mes, which undergoes the electron-transfer reduction of MVA2+-PtC, leading to the efficient hydrogen evolution.

Carbon Nanotubes with Covalently Linked Porphyrin Antennae: Photoinduced Electron Transfer

Single- and multiwalled carbon nanotubes have been covalently functionalized with free-base porphyrin. The quantity of porphyrin linked to the surface was determined from thermogravimetric and UV-vis analysis. A reversible protonation equilibrium between the attached porphyrin and the residual acid groups of the carbon nanotubes has been identified. Steady-state fluorescence emission spectrum of the solutions of porphyrin-linked carbon nanotubes shows that the porphyrins act as energy absorbing and electron transferring antennae, and the carbon nanotubes act as efficient electron acceptors. The porphyrin-linked carbon nanotubes show 95-100% emission quenching, indicating a fast photoinduced electron transfer.

Modulating Electron Transfer in a Simple Bichromophoric System Employing Axial-Ligation as an Organising Precept

A naphthalene diimide acceptor 1 bearing a pyridine group linked directly (a series) or through a —CH2CH2— spacer (b series) coordinated axially to a metallotetraarylporphyrin (MP) undergoes fast photoinduced electron transfer (in the case of MP = Zn(II)TTP) while the kinetically more stable ruthenium complexes (MP = Ru(CO)TPP) have been used to illustrate the correlation between the distance of probe protons from the porphyrin plane and the change in chemical shift (Δδ) upon coordination. Changes in the emission spectra at 650 nm (λex= 400 nm) upon the addition of Zn(II) ions and/or 1b to TTPH2 can be interpreted in a truth table to illustrate a NAND gate.

Porphyrin–fullerene dyad with a long linker: formation of charge transfer conformer in Langmuir–Blodgett film

A porphyrin–fullerene covalently linked dyad is studied in solutions and Langmuir–Blodgett (LB) films. The dyad is characterized by a relatively long linker. In benzonitrile, it performs a relatively slow photo-induced electron transfer (ET), k ET ≈5×10 8 s −1 . Being incorporated in an LB film the dyad forms a compact conformer, which takes the smallest surface area. For this conformer the donor–acceptor separation is much shorter as compared with that in liquid phases and the molecule perform a fast photo-induced ET, k ET ≈8×10 9 s −1 . The LB films were studied using time-resolved measurements of fluorescence, absorption, and Maxwell displacement charge.

Molecular triads of soluble oligo-2,6-naphthylenevinylenes end-capped with [60]fullerene

Novel soluble oligo-2,6-naphthylenevinylenes linked to two [60]fullerene units have been prepared from the corresponding highly fluorescent diformyl-oligo-2,6-naphthylenevinylene through 1,3-dipolar cycloadditions to C 60. The fluorescence spectrum of the triad formed shows a strong quenching of the fluorescence of the oligonaphthylenevinylene moiety thus suggesting a fast photoinduced electron transfer from the excited oligomer to the C 60 moiety.

Fast Photoinduced Electron Transfer from Polyalkyl- to Polyfluoro-Metalloporphyrins in Lipid Bilayer Membranes

Direct electrochemical evidence shows that photoinduced electron transfer from excited Mg−octaethylporphyrin (MgOEP) to a Mg (or Zn) complex of tetrakis(pentafluorophenyl)porphyrin (TFPP) in lipid bilayers occurs with a second-order rate constant ≥108 M-1 s-1. This reaction is ∼100 fold faster than the reported intermolecular rate between porphyrins, and occurs even under aerobic conditions. Electron transfer from the ground state of MgOEP to excited ZnTFPP is observable under anaerobic conditions, but the rate is ∼10-fold slower. Time-resolved photoconductivity of the lipid bilayer with the mixed metalloporphyrins suggests that the charge recombination times (τ) of the geminate ion pairs in the MgTFPP−MgOEP and ZnTFPP−MgOEP systems are 20 and 38 μs, respectively. The MgTFPP- anion reduces O2 with a second-order rate constant of ∼107 M-1 s-1, but the oxidation of ZnTFPP- anions by O2 is very slow. The differences between the two systems may arise from different redox potentials of ZnTFPP and MgTFPP. These data prove that, even containing Mg, the least electronegative element which can be stably chelated, a metalloporphyrin with poly electron-withdrawing groups is a good electron acceptor. Our results suggest that such electronegative porphyrins are useful molecular parts for assembling of porphyrin-based biomimetic energy conversion devices.

Fast photoinduced electron transfer through DNA intercalation.

We report evidence for fast photoinduced electron transfer mediated by the DNA helix that requires metal complexes that are avid intercalators of DNA. Here the donor bis(phenanthroline)(dipyridophenazine)ruthenium(II) [Ru(phen)2dppz2+] and acceptor bis(9,10-phenanthrenequinone diimine)(phenanthroline)rhodium(III) [Rh(phi)2phen3+] intercalate into DNA with Kb > 10(6) M-1. Luminescence quenching experiments in the presence of two different lengths of DNA yield upward-curving Stern-Volmer plots and the loss of luminescence intensity far exceeds the change in emission lifetimes. In the presence of a nonintercalative electron acceptor, Ru(NH3)3+(6), Ru(phen)2dppz2+ luminescence is quenched much less efficiently compared to that found for the intercalative Rh(phi)2phen3+ quencher and follows linear Stern-Volmer kinetics; steady-state and time-resolved Stern-Volmer plots are comparable in scale. These experiments are consistent with a model involving fast long-range electron transfer between intercalators through the DNA helix.

Distance dependence of photoinduced electron transfer through non-conjugated bridges

Picosecond time-resolved emission studies of a series of molecules containing an electron donor-acceptor pair interconnected by a series of rigid non-conjugated bridges reveal the occurrence of very fast photoinduced intramolecular electron transfer. The length of the bridge was varied to provide donor-acceptor centre-to-centre separations ranging from 8.1 to 13.3 Å (edge-to-edge 5 to 10.2 Å). At centre-to-centre separations up to 10.7 Å the rate of photoinduced electron transfer exceeded 5×10 10 s −1 (τ<20 ps); at the largest separation, 13.3 Å, the rate was 1.47×10 10 s −1 (τ = 68 ps).