Photoinduced charge transfer in porphyrin-C60 oligomer

Photoinduced electron transfer (ET) between C₆₀ and porphyrin (P) in a new polymer containing porphyrin, poly(p-phenylenevinylene), and pendant fullerene units has been investigated by nanosecond transient absorption and phosphorescence spectroscopy. Compared to the physically doping material systems, binding porphyrin/C₆₀ through chemical bonds in a polymer detains the formation of the triplet states of porphyrins and C₆₀. The formation of intermediate charge transfer state (CSS) of P⁺-C₆₀ ^? was observed, which led to the delayed formation of triplet states of porphyrins and C₆₀. The reduced opto-electronic properties, such as optical limiting performance, were also observed, which resulted from the delayed formation of triplet states. The results presented in this article are significant in understanding the complicated spectral characteristics of the triplet state and charge transfer of the porphyrin and C₆₀ complexes, and are therefore related to the controllable performance of the new materials in applications.     

Photoinduced charge transfer in poly(phenylene-vinylene) and its derivatives

Photoinduced absorption measurements on poly(phenylenevinylene), PPV, or its derivatives, illuminated with photons of energy past the absorption edge revealed that some percentage of the photons create a long-lived excitation rather than the expected excitons. We present the evidence that these excitations are polaron pairs, which are essentially excimers. The reasons why this percentage varies from sample to sample are discussed. Also discussed are the reasons why polaron pairs in some derivatives of PPV emit light while in others they do not. Calculations with a relatively simple Hamiltonian can account well for the peaks in the photoinduced absorption produced by these pairs.     

Photoinduced charge transfer in helical polypeptides

Investigation of electron transfer in synthetic polypeptides provides an important probe of how charge entrainment is mediated in redox-active proteins, including photosynthetic reaction centers. Interest in this field has focused increasingly on experimental probes of photoinduced electron transfer kinetics and thermodynamics, and the influence of various features of polypeptide templates (e.g. the hydrogen bonding network, the permanent dipole moment for α-helices) that assemble redox groups for long range charge transfer. A review of the various approaches is presented here with attention to heliogenic peptides and the mediation of photoinduced charge entrainment.     

Photoinduced intramolecular charge transfer in 4-(dimethyl)aminobenzonitrile--a theoretical perspective

Recent advances in time-dependent density functional theory (TDDFT) have led to computational methods that can predict properties of photoexcited molecules with satisfactory accuracy at comparably moderate cost. We apply these methods to study the photophysics and photochemistry of 4-(dimethyl)aminobenzonitrile (DMABN). DMABN is considered the paradigm of photoinduced intramolecular charge transfer (ICT), leading to dual fluorescence in polar solvents. By comparison of calculated emission energies, dipole moments, and vibrational frequencies with recent results from transient spectroscopy measurements, a definitive assignment of the electronic and geometric structure of the two lowest singlet excited states of DMABN is possible for the first time. We investigate the mechanism of the ICT reaction by means of minimum energy path calculations. The results confirm existing state-crossing models of dual fluorescence. Our study suggests that analytical TDDFT derivative methods will be useful to predict and classify emissive properties of other donor-acceptor systems as well.     

Photoinduced charge transfer in short-distance ferrocenylsubphthalocyanine dyads

Two new ferrocenylsubphthalocyanine dyads with ferrocenylmethoxide (2) and ferrocenecarboxylate (3) substituents directly attached to the subphthalocyanine ligand via the axial position have been prepared and characterized using NMR, UV-vis, and magnetic circular dichroism (MCD) spectroscopies as well as X-ray crystallography. The redox properties of the ferrocenyl-containing dyads 2 and 3 were investigated using the cyclic voltammetry (CV) approach and compared to those of the parent subphthalocyanine 1. CV data reveal that the first reversible oxidation is ferrocene-centered, while the second oxidation and the first reduction are localized on the subphthalocyanine ligand. The electronic structures and nature of the optical bands observed in the UV-vis and MCD spectra of all target compounds were investigated by a density functional theory polarized continuum model (DFT-PCM) and time-dependent (TD)DFT-PCM approaches. It has been found that in both dyads the highest occupied molecular orbital (HOMO) to HOMO-2 are ferrocene-centered molecular orbitals, while HOMO-3 as well as lowest unoccupied molecular orbital (LUMO) and LUMO+1 are localized on the subphthalocyanine ligand. TDDFT-PCM data on complexes 1-3 are consistent with the experimental observations, which indicate the dominance of π-π* transitions in the UV-vis spectra of 1-3. The excited-state dynamics of the dyads 2 and 3 were investigated using time-correlated single photon counting, which indicates that fluorescence quenching is more efficient in dyad 3 compared to dyad 2. These fluorescence lifetime measurements were interpreted on the basis of DFT-PCM calculations.     

Photoinduced electron transfer in a new Bis(C60)-phthalocyanine triad

[Structure: see text] A novel covalently linked bis([60]fullerene)-phthalocyanine triad is reported, exhibiting long-lived photoinduced charge separation both in solution and in the solid state. The first demonstration of a working solar cell using triad 1 as the active material is also presented.     

Photoinduced intramolecular charge transfer of sodium 4-(N,N-dimethylamino) benzenesulfonate

A new dual fluorescent N,N-dimethylaniline derivative, sodium 4-(N,N-dimethylamino)-benzenesulfonate (SDMAS), is reported. In SDMAS, the electron acceptor is linked to the phenyl ring via a sulfur atom at the para-position of the electron donor. It was found that SDMAS emits dual fluorescence only in highly polar solvent water but not in organic solvents such as formamide, methanol and acetonitrile. In organic solvents only a single-band emission at ca.360 nm was observed in the short wavelength region. The dual fluorescence of SDMAS in water was found at 365 and 475 nm, respectively. Introduction of organic solvent such as ethanol, acetonitrile, and 1,4-dioxane into aqueous solution of SDMAS leads to blue-shift and quenching of the long-wavelength emission. Measurements of steady-state and picosecond time-resolved fluorescence indicate that the long wavelength fluorescence is emitted from a charge transfer (CT) state that is populated from the locally excited (LE) state, with the latter giving off the     

Photoinduced energy transfer and charge transfer on squarylium cyanine dyes

A series of squarylium cyanine dyes (Sqs) were synthesized to explore their applications in functional devices.Preliminary investigation on the mechanism involved in these devices was carried out.Spectroscopic behavior of Sqs with porphyrin (P),8-hydroxyquinolium aluminum (Alq) and ruthenium bipyridyl complex (Ru(bipy)) in solution,in film and on nanocrystalline TiO2 was investigated,respectively.A mechanism including photoinduced energy transfer and charge transfer processes was suggested in the corresponding practical devices.By means of doping,a red organic electroluminescent device (ELD) using Sq-doped Alq as the emission layer (EML) has been developed,and the total light to electricity efficiency of nanocrystalline TiO2 electrode based on using Sq-doped Ru(bipy) as photosensitizer has been improved greatly in the whole visible region,particularly in the red area above 600 nm.     

Photoinduced charge transfer processes along triarylamine redox cascades

In this paper, we describe the synthesis and photophysical properties of a series of acridine-triarylamine redox cascades. These cascades were designed in order to promote photoinduced hole transfer from an acridine fluorophore into an adjacent triarylamine. The excited dipolar state then injects a hole into the triarylamine redox cascade. Subsequently, the hole migrates along the redox gradient which was tuned by the substituents attached to the triarylamine redox centers. The rate of hole migration was determined by fluorescence lifetime measurements and is in the ns regime and depends strongly on the solvent polarity. The photophysical processes were also investigated by femtosecond broadband pump-probe spectroscopy. Our studies reveal different dynamic processes in the cascades depending on the solvent polarity, e.g., direct charge separation after photoexcitation vs a two step hole transfer mechanism.     

Photoinduced energy transfer coupled to charge separation in a Ru(II)-Ru(II)-acceptor triad

The bichromophoric system Ru-Ru(C)-PI ([(bpy)3Ru-Ph-Ru(dpb)(Metpy-PI)][PF6]3, where bpy is 2,2'-bipyridine, Hdpb is 1,3-di(2-pyridyl)-benzene, Metpy is 4'-methyl-2,2':6',2' '-terpyridine and PI is pyromellitimide) containing two Ru(II) polypyridyl chromophores with a N6 and a N5C ligand set, respectively, was synthesized and characterized. Its photophysical properties were investigated and compared to those of the monochromophoric cyclometalated complexes Ru(C)-PI ([Ru(dpb)(Metpy-PI)][PF6]), Ru(C)-phi-PI ([Ru(dpb)(ttpy-PI)][PF6], ttpy is 4'-p-tolyl-2,2':6',2' '-terpyridine), Ru(C)-phi ([Ru(dpb)(ttpy)][PF6]), and Ru(C) ([Ru(dpb)(Metpy)][PF6]). Excitation of the Ru(C) unit in the dyads leads to oxidative quenching, forming the Ru(C)(III)-phi-PI*- and Ru(C)(III)-Pl.- charge-separated (CS) states with k(f)(ET) = 7.7 x 10(7) s(-1) (CH3CN, 298 K) in the tolyl-linked Ru(C)-phi-PI and k(f)(ET) = 4.4 x 10(9) s(-1) (CH2Cl2, 298 K) in the methylene-linked Ru(C)-PI. In the Ru-Ru(C)-PI triad, excitation of the Ru(C) chromophore leads to dynamics similar to those in the Ru(C)-PI dyad, generating the Ru(II)-Ru(C)(III)-PI*- CS state, whereas excitation of the Ru unit results in an initial energy transfer (k(EnT) = 4.7 x 10(11) s(-1)) to the cyclometalated Ru(C) unit. Subsequent electron transfer to the PI acceptor results in the formation of the same Ru(II)-Ru(C)(III)-PI*- CS state with k(f)(ET) = 5.6 x 10(9) s(-1) that undergoes rapid recombination with k(b)(ET) = 1 x 10(10) s(-1) (CH2Cl2, 298 K). The fate of the Ru(II)-Ru(C)(III)-PI*- CS state upon a second photoexcitation was studied by pump-pump-probe experiments in an attempt to detect the fully charge-separated Ru(III)-Ru(C)(II)-PI*- state.     

Photoinduced charge transfer mediated by DNA-wrapped carbon nanotubes

We report our observation of solution photochemical reactions catalyzed by carbon nanotubes. Addition of sub-millimolar Ag+ ions into a solution of DNA-wrapped carbon nanotubes (DNA-CNT) leads to a strong charge-transfer band in the UV region of the optical absorption spectrum. Light irradiation of the Ag+/DNA-CNT mixture results in reduction of Ag+ to Ag nanoparticles and concomitant oxidation of water.     

Photoinduced intramolecular charge transfer phenomena in 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid

A donor acceptor substituted aromatic system 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid (DMAPPDA) has been synthesized and its spectral properties have been explored on the basis of steady state absorption and fluorescence spectroscopy. Spectral features point largely towards a possible occurrence of photoinduced intramolecular charge transfer process from the donor NMe2 group to the acceptor acid group. Solvent dependency of the large Stokes' shifted emission band and the calculated large excited state dipole moment support the polar character of the charge transfer excited state. Quantum yield calculations and effect of addition of acid and base on the steady state spectra were also performed to further scrutinize the excited state CT character.     

Photoinduced charge transfer at the poly(N-vinylcarbazole)—tin oxide interface

An investigation of steady state and transient photoconductivity in films of poly-(N-vinylcarbazole) equipped with SnO₂ and metal electrodes is described. The studies have shown that photoinduced charge transfer, leading to hole injection into the polymer film, takes place with photons of energy greater than 1.8–1.9 eV. Dark conductivity studies on samples equipped with grounded guard rings have shown that carriers originating in the bulk of the polymer film dominate the dark current when the metal electrode is at negative bias. Thermionic hole emission from the SnO₂ electrode is not observed.     

Photoinduced charge separation and charge recombination in [60]fullerene-ethylcarbazole and [60]fullerene-triphenylamines in polar solvents

Molecules of C60 covalently connected with N-ethylcarbazole (EtCz) and triphenylamine (TPA) have been synthesized. Photoinduced electron transfer in C60-EtCz and C60-TPA has been studied in polar and nonpolar solvents using time-resolved transient absorption and fluorescence measurements. From the fluorescence lifetimes, the excited singlet state of the C60 moiety (1C60) of C60-TPA generates predominantly C60*--TPA*+, which decays quickly to the ground state within 6 ns even in polar solvents. In the case of C60-EtCz, on the other hand, about half of the 1C60 moiety generates short-lived C60*--EtCz*+, while the other half of the 1C60 moiety is transferred to the 3C60 moiety via intersystem crossing in dimethylformamide, in which the energy level of C60*--EtCz*+ is lower than that of 3C60. Thus, the charge separation takes place via 3C60 generating C60*--EtCz*+, having a lifetime as long as 300 ns, probably because of the triplet spin character of C60*--EtCz*+. A special property of the EtCz moiety to stabilize the hole in the charge-separated state was revealed.     

Surface-assisted transient displacement charge technique. I. Photoinduced charge transfer in self-assembled monolayers

Surface-assisted photoinduced transient displacement charge (SPTDC) technique was developed in order to study light-induced charge transfer in surface-bound molecules and applied to investigation of self-assembled monolayers of 7-diethylaminocoumarin and 2,4-dinitrophenylamine. The dipole moment change measured by SPTDC correlates reasonably well with that measured in solution by standard PTDC technique and with semiempirical calculations. Shortening of the excited-state lifetime of surface-immobilized coumarin due to stimulated emission was observed in both fluorescence and dipole measurements. The dipole signal decline in low-polarity solvents indicates the importance of dipole-dipole interaction that causes reorientation of molecules upon photoexcitation.     

Photoinduced charge transfer in ZnO/Cu(2)O heterostructure films studied by surface photovoltage technique

ZnO/Cu(2)O heterostructure films were prepared by a two-step electrodeposition method in aqueous solution on fluorine-doped tin oxide (FTO) substrates. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and UV-vis transmission measurements were utilized to characterize the films. Surface photovoltage (SPV) technique was used to investigate the process of photoinduced charge transfer. The results show that there is an electric field located at the interface between ZnO and Cu(2)O film and the photoinduced electrons in Cu(2)O film inject into ZnO under the effect of interfacial electric field with visible light irradiation. While under ultraviolet light illumination, the photoinduced electrons in Cu(2)O film accumulate at the surface of Cu(2)O film instead of injecting into ZnO under the action of surface built-in electric field of Cu(2)O film. The work function measurements confirm that the direction of interfacial electric field is from ZnO to Cu(2)O. These results are help to future design of high performance heterostructure photovoltaic devices.     

Photoinduced energy and electron transfer in phenylethynyl-bridged zinc porphyrin-oligothienylenevinylene-C60 ensembles

Donor-bridge-acceptor triad (Por-2TV-C(60)) and tetrad molecules ((Por)(2)-2TV-C(60)), which incorporated C(60) and one or two porphyrin molecules that were covalently linked through a phenylethynyl-oligothienylenevinylene bridge, were synthesized. Their photodynamics were investigated by fluorescence measurements, and by femto- and nanosecond laser flash photolysis. First, photoinduced energy transfer from the porphyrin to the C(60) moiety occurred rather than electron transfer, followed by electron transfer from the oligothienylenevinylene to the singlet excited state of the C(60) moiety to produce the radical cation of oligothienylenevinylene and the radical anion of C(60). Then, back-electron transfer occurred to afford the triplet excited state of the oligothienylenevinylene moiety rather than the ground state. Thus, the porphyrin units in (Por)-2TV-C(60) and (Por)(2)-2TV-C(60) acted as efficient photosensitizers for the charge separation between oligothienylenevinylene and C(60).