Efficient Photoinduced Electron Transfer Process Research Articles

Excellent Nonlinear Optical Limiting Behavior of Tetra (C60) Lanthanum Phthalocyanine in Solid Poly(methyl methacrylate) Films

This work reports on the exceptional optical limiting performance of a novel D–A-type compound, namely, LaPc-4(C60), wherein the lanthanide phthalocyanine is used as a donor and four C60 are used as acceptors. In addition, phenyl ether bonds were introduced into the molecule, making it semirigid and increasing its solubility. At the same time, the phenyl ether bond could act as a π-electron bridge, which increases the electron transfer pathway, expands the π-conjugated system, and reduces the spatial site resistance of the molecule. Furthermore, the uniform poly(methyl methacrylate) (PMMA) composite film (LaPc-4(C60)/PMMA) was prepared by a simple solution casting method. In comparison with individual C60 or LaPc(CHO)4, both LaPc-4(C60) in solution and in the solid PMMA composite film exhibited larger third-order nonlinear absorption coefficients and lower limiting thresholds at 532 nm. In particular, LaPc-4(C60)/PMMA exhibits huger nonlinear absorption coefficient (2900 cm/GW) and larger third-order susceptibility (4.91 × 10–9 esu), which can be attributed to the synergistic effect of the different non-linear absorption mechanisms and efficient photoinduced electron transfer process between C60 and LaPc.

Cp2TiIIICl Catalysis in a New Light

AbstractVisible‐light‐mediated photogeneration of Cp2TiIIICl has opened new avenues for titanium catalysis. The synergistic utilization of dual photoredox and Cp2TiIIICl catalysis uses Hantzsch ester (HE) as an electron donor instead of traditional super‐stoichiometric metallic reductants. This new paradigm has found numerous applications in synthetic organic chemistry. The highly efficient photoinduced electron transfer process between light‐sensitive Cp2TiCl2 and an excited photocatalyst is the key to the success of such a photocatalytic system and ensures overall catalyst turnover. Comparisons between the metal reduction approach and photo reduction approach have been made.

(Invited) Chiral-at-Metal Fullerene Hybrids for Catalysis

Fullerenes have found their main application in organic photovoltaic devices thanks to their ability to give rise a high efficient photo-induced electron transfer process and to their behaviour as n-type charge carriers.[1] On the other hand, in face of the great interest for carbon based materials in catalysis, the use of fullerenes in this field has been completely neglected as probably due to the difficult to endow fullerene cages with suitable active sites. In this regards, as a first step to address this issue, we have reported recently a wide range of asymmetric metal mediated or organocatalytic methodologies for a precise stereocontrol in the fullerenes functionalizations.[2]Herein, we present the enantioselective catalytic synthesis of fullerene hybrids where C60 is endowed with metal atoms such us iridium, rhodium and ruthenium with high enantiomeric excesses. A complete control on the stereochemistry of all the asymmetric centers was achieved, even on the stereogenic metal center.[3]The catalytic and photo-electrocatalytic properties of these new fullerene derivatives will be discussed together with their ability to transfer chirality in hydrogen transfer processes.[4][1] (a) B. C. Thompson, J. M. J. Fréchet, Angew. Chem. Int. Ed. 2008, 47, 58; (b) J. L. Delgado, P.-A. Bouit, S. Filippone, M. A. Herranz, N. Martin, Chem.Commun. 2010, 46, 4853. [2] (a) Maroto, E. E.; Izquierdo, M.; Reboredo, S.; Marco-Martínez, J.; Filippone, S.; Martín, N. Acc. Chem. Res. 2014, 47, 2660. (b) Filippone, S.; Maroto, E. E.; Martín-Domenech, A.; Suárez, M.; Martín, N. Nat. Chem. 2009, 1, 578. (c) Maroto, E. E.; Mateos, J.; Garcia-Borràs, M.; Osuna, S.; Filippone, S.; Herranz, M. Á.; Murata, Y.; Solà, M.; Martín, N. J. Am. Chem. Soc. 2015, 137, 1190. (d) Marco-Martínez, J.; Reboredo, S.; Izquierdo, M.; Marcos, V.; López, J. L.; Filippone, S.; Martín, N. J. Am. Chem. Soc. 2014, 136, 2897. (e) Marco-Martínez, J.; Marcos, V.; Reboredo, S.; Filippone, S.; Martín, N. Angew. Chem. Int. Ed. 2013, 52, 5115. [3] J. Marco-Martínez, S. Vidal, I. Fernández, S. Filippone and N. Martín, Angew. Chem. Int. Ed., 2017, 56, 2136. [4] R. Maria Giron, J. Marco-Martinez, S. Bellani, A. Insuasty, H. Comas Rojas, G. Tullii, M. R. Antognazza, S. Filippone and N. Martin, J. Mater. Chem. A, 2016, 4, 14284. Figure 1

Photoinduced electron transfer in xerogel fabricated by covalently bound polycondensation of chromophore-contained alkoxysilanes

A series of alkoxysilane derivatives containing chromophores were chosen as pairs of donor and acceptor in organic-inorganic hybrid material (OIHM), in which photoinduced electron transfer (PET) emerged. The modified Rehm-Weller equation was employed to decide the free energy change of PET reaction where parameters such as reduction potential and zero-zero transition energy of acceptor, and the oxidation potential of donor were measured. The occurrence of PET in OIHM was observed by fluorescence quenching of acceptor chromophore after photo-excitation, and this process was affected by the molar ratios between donor and acceptor. Such efficient PET process can only be observed when donor and acceptor moieties were covalently bound in OIHM. A controlled experiment showed that the PET was difficult to occur when polycondensation reaction would not take place between donor and acceptor. This sol-gel method played an important role to covalently bind two chromophores together and facilitate PET process. Kinetic measurement of fluorescence decay determined the rate of PET, where the efficiency of PET was up to 86% when donor/acceptor = 1/2. This hybrid system provided further understanding of electron transfer reaction within solid state material and gave the potential usage of hybrid materials in optoelectronics.

Photosubstitution of Dicyanoarenes by Hypervalent Allylsilicon Reagents via Photoinduced Electron Transfer

Abstract Photosubstitution of dicyanoarenes with hypervalent allylsilicon reagents in the presence of anthracene efficiently proceeded to afford the allyl-substituted product in moderate to good yields. The efficiency for the present photoreaction was much higher than that for the tetracoordinate allylsilanes, because hypervalent allylsilicon reagents possess lower oxidation potential than the tetracoordinate ones, which resulted in more efficient photoinduced electron-transfer process.

Photoinduced electron transfer of zinc porphyrin–oligo(thienylenevinylene)–fullerene[60] triads; thienylenevinylenes as efficient molecular wires

Two novel donor-bridge-acceptor arrays (ZnP-nTV-C60) with zinc porphyrin (ZnP) and fullerene (C60), covalently connected by oligo(thienylenevinylene) (nTV) molecular wires (n = 3 and 8; ), have been prepared in a multistep convergent manner. The influence of the nTV-length on the electrochemical and electronic properties of the ZnP-nTV-C60 triads has been revealed. Interestingly, an efficient photoinduced electron transfer process occurs in both triads with formation of intermediate radical-ion pairs (namely, ZnP˙(+)-nTV-C60˙(-) and ZnP-nTV˙(+)-C60˙(-)) as confirmed by the nanosecond transient absorption measurements in the visible and NIR regions. In polar and nonpolar solvents, the rate constants of charge-separation processes (kCS) via(1)ZnP*-nTV-C60 were found to decrease from ca. 1.2 × 10(10) s(-1) for n = 3 (RDA = 20 Å) to (5-7) × 10(9) s(-1) for n = 8 (RDA = 60 Å) on the basis of fluorescence lifetime measurements of the ZnP moiety. From these data, together with those previously obtained ones for n = 4 in the related ZnP-nTV-C60 systems, a low attenuation coefficient was evaluated for the nTV molecular wires.

Photoinduced Electron Transfer Processes of Functionalized Nanocarbons; Fullerenes, Nanotubes and Graphene

To solve the energy demands for a sustainable society, it is crucial to use clean solar energy. Thus, finding methods for efficient light energy conversion is an important scientific goal. For this aim, molecular devices employing composites of nanocarbons such as fullerenes, single wall carbon nanotubes (SWCNTs), and graphene are promising. In this review, we survey light-induced electron-transfer processes of these nanocarbons hybridized with photosensitizers, which have proved to be more favorable than ordinary molecules. Fullerenes connected with suitable electron donors yield characteristic long-lived radical ion pairs, suitable for constructing artificial photosynthetic systems. For SWCNTs, a combination of photosensitizers is also crucial to achieve efficient photo-induced electron transfer processes. Furthermore, in the case of diameter-sorted semi-conductive SWCNTs, each tube can be treated as a “molecule”, thus, allowing molecular orbital calculations to probe the geometry as well as the electronic structures of the photosensitizernanotube hybrids. The two-dimensional graphene has afforded a new reaction field with a wide π-system for the attached molecules. For the confirmation of electron transfer processes, transient absorption methods in the wide wavelength regions have been used effectively. The kinetic data obtained in solution are quite useful to understand the electron transfer mechanisms, which afford useful guiding principles to construct efficient light-energy harvesting devices such as photoelectrochemical and photovoltaic solar cells.

Role of central metal ions in hematoporphyrin-functionalized titania in solar energy conversion dynamics

In this study, we have investigated the efficacy of electron transfer processes in hematoporphyrin (HP) and iron hematoporphyrin ((Fe)HP) sensitized titania as potential materials for capturing and storing solar energy. Steady-state and picosecond-resolved fluorescence studies show the efficient photoinduced electron transfer processes in hematoporphyrin-TiO2 (HP-TiO2) and Fe(III)-hematoporphyrin-TiO2 (Fe(III)HP-TiO2) nanohybrids, which reveal the role of central metal ions in electron transfer processes. The bidentate covalent attachment of HP onto TiO2 particulates is confirmed by FTIR, Raman scattering and X-ray photoelectron spectroscopy (XPS) studies. The iron oxidation states and the attachment of iron to porphyrin through pyrrole nitrogen atoms were investigated by cyclic voltammetry and FTIR studies, respectively. We also investigated the potential application of HP-TiO2 and Fe(III)HP-TiO2 nanohybrids for the photodegradation of a model organic pollutant methylene blue (MB) in aqueous solution under wavelength dependent light irradiation. To further investigate the role of iron oxidation states in electron transfer processes, photocurrent measurements were done by using Fe(III) and Fe(II) ions in porphyrin. This work demonstrates the role of central metal ions in fundamental electron transfer processes in porphyrin sensitized titania and their implications for dye-sensitized device performance.

Structural and Photophysical Characterization of Multichromophoric Pyridylporphyrin-Rhenium(I) Conjugates

Four porphyrin-Re(I) conjugates, in which a pyridylporphyrin chromophore is directly coordinated to the electron-acceptor fragment [ fac-Re(CO) 3(bipy)] (+), were prepared: the dimeric and pentameric compounds [ fac-Re(CO) 3(bipy)(4'MPyP)](CF 3SO 3) ( 1) (4'MPyP = 4'-monopyridylporphyrin) and [ fac-{Re(CO) 3(bipy)} 4(mu-4'TPyP)](CF 3SO 3) 4 ( 2) (4'TPyP = 4'-tetrapyridylporphyrin), and the corresponding compounds with 3' rather than 4' porphyrins, [ fac-Re(CO) 3(bipy)(3'MPyP)](CF 3SO 3) ( 3) and [ fac-{Re(CO) 3(bipy)} 4(mu-3'TPyP)](CF 3SO 3) 4 ( 4). These adducts proved to be very stable in solution and were also structurally characterized in the solid state by X-ray crystallography. A detailed photophysical study was performed on the zincated adducts of the conjugates 1- 3, labeled 5, 6, and 7, respectively. In all adducts the typical fluorescence of the zinc-porphyrin unit was reduced in intensity and lifetime by the presence of the peripheral rhenium-bipy fragment(s) (heavy-atom effect). For the dyads 5 and 7 the photoinduced charge transfer process from the zinc-porphyrin to the Re(I)-bipy unit is only slightly exoergonic. Ultrafast spectroscopy experiments showed no evidence for electron transfer quenching in the dyads as such, whereas the addition of pyridine (that binds axially to zinc and thus affects the porphyrin redox potential) led to a moderately efficient photoinduced electron transfer process. In perspective, an appropriate functionalization of the bipy ligand and/or of the porphyrin chromophore might improve the thermodynamics and, thus the efficiency, of the photoinduced electron transfer process.

Photoinduced electron transfer reactions across rigid linear spacer groups of high symmetry

A series of highly symmetrical dienes 1– 3 were prepared through [2+2] cyclo-addition of norbornadiene (NBD) in an exo– trans– exo geometry. These molecules can be utilized as spacer groups in donor(D)–spacer(S)–acceptor(A) types of electronic dyads possessing strictly linear rigidity. The electronic coupling between D and A was promoted effectively by the σ-bonds through the all- trans spacer groups so that a highly efficient photoinduced electron transfer process takes place. The even–odd NBD dependent alternation of the D/A orientation is intriguing, which leads to an unusual correlation in the spacer tuning electron transfer process.