Photoinduced Intermolecular Charge Transfer Research Articles

Metal-free C-Se cross-coupling enabled by photoinduced inter-molecular charge transfer.

Metal-free C-Se cross-couplings via the formation of electron-donor-acceptor (EDA) complexes have been developed. The visible-light induced reactions can be applied for the synthesis of a series of unsymmetrical diaryl selenides employing aryl bromides, aryl iodides as well as aryl chlorides under mild reaction conditions. The scale-up was readily achieved. UV-Vis spectroscopy measurements provide insight into the reaction mechanism.

Photoluminescence Quenching of a Novel Electroconductive Poly(propylene thiophenoimine)-co-Poly(ethylenedioxy thiophene) Star Copolymer.

A generation 1 poly(propylene thiophenoimine)-co-poly(ethylenedioxy thiophene) (G1PPT-co-PEDOT) star copolymer, which exhibits a strong optical absorption over a broad range in the ultraviolet–visible (UV-Vis) region and with good electro/conductive properties, was chemically prepared for the first time. Synthesis of the star copolymer, G1PPT-co-PEDOT was confirmed by spectroscopic studies. Indeed, the disappearance of the very high intensity bands, C–H bending at α-position (687 cm−1), and C=N stretching (1620 cm−1) in the Fourier transform infrared spectroscopy (FTIR) of G1PPT-co-PEDOT, which were initially present in the spectrum of the thiolated starting material, G1PPT, confirmed copolymerization. Furthermore, a large bathochromic shift in the onset wavelength of the UV-Vis absorbance spectra from 367 nm in G1PPT to 674 nm in G1PPT-co-PEDOT further attests of successful copolymerization. The electrochemical analysis of G1PPT-co-PEDOT achieved a highest occupied molecular orbital (HOMO) energy level value of 5.3 eV, which is reminiscent of the value for an ideal electron-donor material. Photoluminescence quenching of up to 82% was observed in solution blends of the G1PPT-co-PEDOT star copolymer and N,N′-diisopropyl naphthalene diimide (NDI). This demonstrates the occurrence of photoinduced intermolecular charge transfer (PICT) from the electron-donating G1PPT-co-PEDOT to the electron accepting NDI, a good property, beneficial for optoelectronic and photovoltaic applications.

Photoinduced intermolecular charge transfer processes on pure and modified silica surfaces

This article briefly reviews some of our work over the past two decades devoted to the photoinduced electron-transfer processes and formation of bimolecular complexes–exiplexes between adsorbed donor pyrene and anthracene and acceptor N,N-dimethylaniline molecules on silica and silica–titania surfaces. Quenching of the fluorescence of acenes by N,N-dimethylaniline on silica surfaces is found to be diffusion controlled and results from exciplex formation at room temperature. The effect of silica–titania on the processes of photoinduced electron-transfer resulted vise-versa in increases of the fluorescence intensity from the adsorbed acenes. This phenomenon is explained by the selective adsorption of N,N-dimethylaniline on the acid “titanium” centers and a shift of the acenes to weaker adsorption sites located on the silica support. The excited molecular pair effectively yields reduced titanium ions due to an electron transfer process. This study demonstrates the important role that adsorption of organic molecules plays in photocatalytic processes on the surface of semiconductor composites. Sol–gel produced porous silica modified with benzophenone molecules leads to full photoreduction of silver and gold ions. Formation of stable Ag and Au nanoparticles embedded in porous silica films was carried out by irradiation of silver (gold) ions-modified silica in benzophenone–water–isopropanol solution. The triplet state of benzophenone takes part in the formation of the reducing agents, such as ketyl radicals and anion radicals of benzophenone and isopropanol. These systems can be used as bactericide materials, for extraction of metal ions from solution, for photocatalytic reduction of toxic transition metals and other applications.

The nature of photoinduced intermolecular charger transfer in fluorescence resonance energy transfer.

In this paper, we report time resolved fluorescence resonance energy transfer (FRET) using femtosecond ultrafast transient absorption spectroscopy. The lifetimes of FRET are strongly dependent on the molecular concentration and ratio of donor and acceptor. Also, in the FRET, photoinduced intermolecular charge transfer (PICT) is also investigated theoretically. The driving force for PICT in FRET system equals the reorganization energy, which gives barrier-less charge transfer according to Marcus theory. The rates of PICT in the FRET system can be estimated with our simplified Marcus equation. Our results of PICT in FRET system provide a new efficient way for the charge transfer in donor-acceptor system.

Physical mechanism of photoinduced intermolecular charge transfer enhanced by fluorescence resonance energy transfer.

In this paper, photoinduced intermolecular charge transfer (PICT) and fluorescence resonance energy transfer (FRET) in donor-acceptor systems have been investigated experimentally and theoretically. We attempt to investigate the natural relationship between FRET and PICT, and reveal the advantages of FRET enhanced PICT. The driving force for PICT in the FRET system equals the reorganization energy, which gives barrier-less charge transfer according to Marcus theory. The rates of PICT in the FRET system can be estimated with our simplified Marcus equation. Our results can promote the deeper understanding of the nature of FRET enhanced PICT, and benefit rational design for the use of the FRET system in organic solar cells.

Insight into external electric field dependent photoinduced intermolecular charge transport in BHJ solar cell materials

The external electric field dependent photoinduced intermolecular charge transfer in D⋯A type BHJ solar cell materials was theoretically investigated.

Photoelectron processes in heterojunctions based on organic films

The photoconductivity and photo emf have been studied in a heterostructure based on thin films of electron donor and acceptor organic compounds. It is established that the main process responsible for the photogeneration and separation of charge carriers is the decay of photoexcited molecular states at the interface between the donor and acceptor materials. The determining influence on the photoelectron processes in the organic heterostructure is produced by the energy structure of electron states at the interface. The role of the photoinduced intermolecular charge transfer in the development of photoconductivity in the given heterostructure is considered.

Photoinduced Intramolecular Charge Transfer in Donor-acceptor Dyad and Donor-bridge-acceptor Triad

The ground and excited state properties of the [60]fullerene, diphenylbenzothiadiazole-triphenylamine (PBTDP-TPA) dyad and fullerene-diphenylbenzothiadiazole-triphenylamine (fullerene-PBTDP-TPA) triad were investigated theoretically using density functional theory with B3LYP functional and 3-21G basis set and time-dependent density functional theory with B3LYP functional and STO-3G basis set as well as 2D and 3D real space analysis methods. The 2D site representation reveals the electron-hole coherence on excitation. The 3D transition density shows the orientation and strength of the transition dipole moment, and the 3D charge difference density gives the orientation and result of the intramolecular charge transfer. Also, photoinduced intermolecular charge transfer (ICT) in PBTDP-TPA-fullerene triad are identified with 2D and 3D representations, which reveals the mechanisms of ICT in donor-bridge-acceptor triad on excitation. Besides that we also found that the direct superexchange ICT from donor to acceptor (tunneling through the bridge) strongly promotes the ICT in the donor-bridge-acceptor triad.

Infrared photoexcitation spectra of conducting polymer/methanofullerene films

We report photoinduced absorption (PIA) and photoinduced reflectance (PIR) spectra of poly(3-octyl thiophene)-methanofullerene films. As a result of the efficient photoinduced intermolecular charge transfer, the PIA and PIR spectra of the composite films are significantly enhanced in magnitude over those in either of the component materials. From the PIA and PIR spectra, the corresponding changes in the complex refractive index, ΔN = Δn(ω)+iΔϰ(ω), are obtained. The results indicate that the PIA spectra are dominated by Δϰ(ω), while the PIR spectra are dominated mainly by Δn(ω). The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear optical applications of these materials.

Photoinduced absorption and photoinduced reflectance in conducting polymer/methanofullerene films: Nonlinear-optical changes in the complex index of refraction.

We report photoinduced absorption (PIA) and photoinduced reflectance (PIR) spectra of poly(3-octyl thiophene)/methanofullerene films. As a result of the efficient photoinduced intermolecular charge transfer, the PIA and PIR spectra of the composite films are significantly enhanced in magnitude over those in either of the component materials. From the PIA and PIR spectra, the corresponding changes in the complex refractive index \ensuremath{\Delta}N=\ensuremath{\Delta}n(\ensuremath{\omega})+i\ensuremath{\Delta}\ensuremath{\kappa}(\ensuremath{\omega}) are obtained. The results indicate that the PIA spectra are dominated by \ensuremath{\Delta}\ensuremath{\kappa}(\ensuremath{\omega}); the subgap electronic absorptions peak at approximately 0.1 and 1.6 eV. The PIR spectra are dominated mainly by \ensuremath{\Delta}n(\ensuremath{\omega}), which exhibits a derivative spectral shape with zero crossing at 0.1 eV. The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear-optical applications of these materials. \textcopyright{} 1996 The American Physical Society.

Enhanced nonlinear absorption and optical limiting in semiconducting polymer/methanofullerene charge transfer films

Nonlinear optical absorption in solid films of poly(3-octylthiophene) (P3OT) sensitized with methanofullerene was investigated for wavelengths from 620 to 960 nm. The nonlinear absorption is enhanced over that in either of the component materials by more than two orders of magnitude at 760 nm. The large nonlinearity results from efficient photoinduced intermolecular charge transfer from P3OT to methanofullerene, followed by absorption in the charge separated excited state. P3OT/fullerene films are promising as optical limiters; the transmission clamps at an average fluence of approximately 0.1 J/cm2. The damage threshold was 15 μJ/pulse (≊1 J/cm2 in average fluence), above which there is a permanent change in the linear transmission.

Photoinduced charge transfer reactions at surfaces: CF3I on Ag(111)

The surface photochemistry of submonolayer to multilayer amounts of CF3I, adsorbed on Ag(111) at 95 K, has been studied using 248 and 193 nm pulsed laser excitation. For low doses, there is some thermally activated dissociation, limited to 30% of the first monolayer, to form adsorbed CF3 and I. The remaining CF3I adsorbs molecularly. Neither CF3 nor I is photoactive, but adsorbed CF3I is photodissociated, by C–I bond cleavage, at both 248 and 193 nm. A fraction of the resulting CF3 and I desorbs during photolysis; the remainder is retained as chemisorbed CF3 and I. The former processes were probed using time-of-flight and Fourier transform mass spectrometry. The retained products were detected by post-irradiation temperature programmed desorption and Auger electron spectroscopy. The photochemistry varied with wavelength and coverage. Regarding the mechanism, for both 193 and 248 nm, there is good evidence that both submonolayer and multilayer CF3I molecules absorb photons and dissociate into CF3 and I, i.e., direct photodissociation. There is evidence, based on time-of-flight distributions of CF3 photofragments, that I(2p1/2), electronically excited I, is produced at both wavelengths, while ground state atomic iodine is produced only at 248 nm. At both 193 and 248 nm, and for coverages up to three monolayers, there is also evidence for a charge transfer process involving hot electrons produced by photon absorption in Ag(111), i.e., substrate mediated photodissociation. These hot carriers attach to CF3I, and the resulting anion dissociates into CF3 and I−. The latter is detected by Fourier transform mass spectrometry and the former as a low-velocity component in time-of-flight mass spectrometry. At 193, but not 248 nm, there is evidence for a second, substrate independent, charge transfer process also leading to CF3 and I−. For coverages exceeding ten monolayers, approximately 80% of the reaction was through this channel, the remaining 20% occurring through the direct photodissociation channel. To account for this second charge transfer channel, photoinduced intermolecular charge transfer is proposed.