Sequential Energy and Electron Transfer in an Artificial Reaction Center: Formation of a Long-Lived Charge-Separated State

Towards multielectron photocatalysis: a porphyrin array for lateral hole transfer and capture on a metal oxide surface

“An artificial photosynthetic model system is reported consisting of a porphyrin monolayer on a SnO₂ surface that enables oxidizing equivalents to be efficiently delivered to a thermodynamic trap via lateral hole transfer.”

Energy flow dynamics within cofacial and slip-stacked perylene-3,4-dicarboximide dimer models of π-aggregates

Robust perylene-3,4-dicarboximide (PMI) π-aggregates provide important light-harvesting and electron-hole pair generation advantages in organic photovoltaics and related applications, but relatively few studies have focused on the electronic interactions between PMI chromophores. In contrast, structure-function relationships based on π-π stacking in the related perylene-3,4:9,10-bis(dicarboximides) (PDIs) have been widely investigated. The performance of both PMI and PDI derivatives in organic devices may be limited by the formation of low-energy excimer trap states in morphologies where interchromophore coupling is strong. Here, five covalently bound PMI dimers with varying degrees of electronic interaction were studied to probe the relative chromophore orientations that lead to excimer energy trap states. Femtosecond near-infrared transient absorption spectroscopy was used to observe the growth of a low-energy transition at ~1450-1520 nm characteristic of the excimer state in these covalent dimers. The excimer-state absorption appears in ~1 ps, followed by conformational relaxation over 8-17 ps. The excimer state then decays in 6.9-12.8 ns, as measured by time-resolved fluorescence spectroscopy. The excimer lifetimes reach a maximum for a slip-stacked geometry in which the two PMI molecules are displaced along their long axes by one phenyl group (~4.3 Å). Additional displacement of the PMIs by a biphenyl spacer along the long axis prevents excimer formation. Symmetry-breaking charge transfer is not observed in any of the PMI dimers, and only a small triplet yield (<5%) is observed for the cofacial PMI dimers. These data provide structural insights for minimizing excimer trap states in organic devices based on PMI derivatives.

Light-driven hydrogen evolution system with glutamic-acid-modified zinc porphyrin as photosensitizer and [FeFe]-hydrogenase model as catalyst

An intermolecular light-driven hydrogen evolution system with free glutamic-acid-modified zinc tetra(p-phenyl) porphyrin (Glu-ZnP) as a photosensitizer and [Fe2(CO)6(μ-adt)C6H5] [μ-adt = N(CH2S)2] (Badt) as a catalyst has been constructed. Using phenylmercaptan (BSH) as electron donor and acetic acid (HOAc) as proton source, hydrogen was obtained after irradiation with visible light for 2 h; the efficiency is comparable to that of the similar intramolecular dyad. Steady-state and time-resolved spectroscopy and cyclic voltammetry show that both the first and the second electron transfer from singlet 1*Glu-ZnP to Badt and reduced Badt are thermodynamically feasible. However, the competition of electron transfer from singlet 1*Glu-ZnP to Badt with intersystem crossing from singlet 1*Glu-ZnP to triplet 3*Glu-ZnP, inefficient electron transfer from triplet 3*Glu-ZnP to Badt, and the lower energy of triplet 3*Glu-ZnP and possible 3*Badt to that of yielded charge-separated state of Glu-ZnP+·-Badt−· were believed to be the obstacles for efficient hydrogen evolution.

Influence of the energy of charge transfer on non-covalent interactions between fullerenes and a designed bisporphyrin

The present paper reports the spectroscopic and theoretical investigations on the formation of supramolecular complexes of a designed bisporphyrin (1) with C(60) and C(70) in toluene. Absorption spectrophotometric studies establish appreciable amount of ground state electronic interaction between fullerenes and 1. The interaction is facilitated through charge transfer (CT) transition as evidenced from well defined CT absorption bands in the visible region of the electronic spectra. In our present case, the CT interaction may be claimed as one of the rare findings, especially on account of interaction between fullerenes and bisporphyrin in a non-polar solvent. Other than fullerenes C(60) and C(70), various other electron acceptors, viz., 2,3-dichloro-5,6-dicyano-p-benzoquinone, tetracyanoethylene, o-chloranil and p-chloranil form CT complexes with 1. Utilizing the CT transition energies for various electron donor-acceptor complexes of 1, vertical ionization potential (I(D)(v)) of 1 is determined to be 6.37 eV in solution. Estimation of degrees of CT, oscillator and transition dipole strengths evoke that the fullerene-1 non-covalent complexes are of neutral character in ground state. Higher magnitude of electronic coupling elements for the C(70)-1 complex compared to C(60)-1 complex indicates strong binding between C(70) and 1. Steady state fluorescence studies elicit efficient quenching of the fluorescence of 1 in presence of fullerenes. Both UV-Vis and steady state fluorescence measurements reveal large value of binding constant (K) for C(70)-1 system (∼6.94 × 10(4)dm(3)mol(-1)) than that of C(60)-1 system (K∼2.1 × 10(4)dm(3)mol(-1)). Time resolved emission studies establish charge-separated state for the fullerene-1 systems. Transient absorption measurements in the visible region establish the formation of 1(+) and fullerene(-) in toluene medium. Molecular mechanics calculations employing force field method in vacuo evoke the single projection structures of the fullerene-1 complexes and interpret the stability difference between C(60) and C(70) complexes of 1 in terms of heat of formation values of the respective complexes.

Syntheses and excitation transfer studies of near-orthogonal free-base porphyrin-ruthenium phthalocyanine dyads and pentad

A new series of molecular dyads and pentad featuring free-base porphyrin and ruthenium phthalocyanine have been synthesized and characterized. The synthetic strategy involved reacting free-base porphyrin functionalized with one or four entities of phenylimidazole at the meso position of the porphyrin ring with ruthenium carbonyl phthalocyanine followed by chromatographic separation and purification of the products. Excitation transfer in these donor-acceptor polyads (dyad and pentad) is investigated in nonpolar toluene and polar benzonitrile solvents using both steady-state and time-resolved emission techniques. Electrochemical and computational studies suggested that the photoinduced electron transfer is a thermodynamically unfavorable process in nonpolar media but may take place in a polar environment. Selective excitation of the donor, free-base porphyrin entity, resulted in efficient excitation transfer to the acceptor, ruthenium phthalocyanine, and the position of imidazole linkage on the free-base porphyrin could be used to tune the rates of excitation transfer. The singlet excited Ru phthalocyanine thus formed instantly relaxed to the triplet state via intersystem crossing prior to returning to the ground state. Kinetics of energy transfer (k(ENT)) was monitored by performing transient absorption and emission measurements using pump-probe and up-conversion techniques in toluene, respectively, and modeled using a Förster-type energy transfer mechanism. Such studies revealed the experimental k(ENT) values on the order of 10(10)-10(11) s(-1), which readily agreed with the theoretically estimated values. Interestingly, in polar benzonitrile solvent, additional charge transfer interactions in the case of dyads but not in the case of pentad, presumably due to the geometry/orientation consideration, were observed.

Single-wall carbon nanotube porphyrin nanoconjugates

We describe the synthesis, microscopic and spectroscopic characterization of a novel donor-acceptor prototype, namely, a single wall carbon nanotube (SWNT) nanoconjugate bearing a covalently linked free base porphyrin ( H 2 P ). In the characterized SWNT- H 2 P nanoconjugate the electronic features of SWNT are largely retained, when compared to pristine SWNT. In fact, carefully controlled reaction conditions lead to a low degree of SWNT functionalization, as evidenced by the preservation of the SWNT van Hove singularities. As a consequence, the overall SWNT loading with H 2 P is so low that in the ground state, the H 2 P features are barely seen, which is different from the excited state, where features of H 2 P are clearly discernable. For example, fluorescence and transient absorption characteristics corroborate, unequivocally, the formation of the H 2 P singlet excited state, which is, however, impacted by the presence of SWNT.

Synthesis and photophysical and electrochemical properties of a binuclear Ru(bpy)3-Cu(III) corrole complex

A novel binuclear Ru - Cu complex, composed of a copper(III)-corrole and a ruthenium(II) tris(bipyridine) moiety linked by an amide bond, has been synthesized and characterized by 1 H NMR, UV-vis and mass spectrometry. The steady-state emission and the electrochemical properties were investigated. Compared to the parent [ Ru ( bpy )3]2+, the emission of the desired complex was substantially quenched when the MLCT of [ Ru ( bpy )3]2+ was selectively photoexcited.

Synthesis and the electron transfer reaction of (metallo)-porphyrin linked with a fullerene through an (L)-lysine spacer

A novel visible-light induced electron transfer system composed of the free-base or Zn porphyrin, C60 molecule, and an (L)-lysine was synthesized and characterized. The porphyrin and C60 were covalently linked by the amide bonds through a relatively long and flexible spacer, (L)-lysine. The 1H NMR spectrum of the Zn porphyrin/(L)-lysine/ C60 conjugate (3Zn) suggested that C60 caused the slow molecular motion of this molecule. The steady-state absorption and the emission spectra of 3Zn revealed the intramolecular excited-state electron transfer from the Zn porphyrin moiety to the C60 moiety. The fluorescent lifetime measurement of the excited-state Zn porphyrin in 3Zn indicated the quenching by C60 . The intermolecular electron transfer from the excited-state zinc porphyrin to the viologen derivative in the solution was examined. The Zn porphyrin/(L)-lysine/ C60 conjugate and the free-base porphyrin/(L)-lysine/ C60 conjugate were more effective catalysts than the tetraphenylporphyrin Zn complex, suggesting the intermediacy of the charge separation state, ZnP·+-C60·- .

Photophysical properties of self-assembling porphinatozinc and photoinduced electron transfer with fullerenes

Photochemical and photophysical properties of self-assembling 5-(4-pyridyl)-10,15,20-triphenylporphinatozinc ( Znpyp 3) have been studied by steady-state and time-resolved absorption in addition to time-resolved fluorescence spectroscopy. Self-assembling oligomers ( Znpyp 3)n( n = 3 at 0.1 mM) were synthetically generated and assigned to a zigzag chain oligomeric structure, where n is dependant on the concentration and temperature. The lifetimes of the singlet and triplet excited states of ( Znpyp 3)n depend on n and the axial ligation. The rate-constant of ( Znpyp 3)n for the intermolecular T-T annihilation process was smaller than that of monomeric porphyrins. In the photoinduced electron-transfer to fullerenes ( C 60 and C 70), it was revealed that the rate-constants and efficiencies for ( Znpyp 3)n were essentially the same as those of the monomer. In the back electron transfer, the rate-constants of oligomers were smaller than that of the monomeric porphyrin, which suggests hole-delocalization along the porphyrin chain.

Layer-by-layer assembly of porphyrin-fullerene dyads

The fabrication of high quality, robust and photoactive ITO electrodes is reported. Electrostatic and van-der-Waals interactions have been used for the step-by-step deposition of layers containing a fullerene-porphyrin dyad as the active layer.

A charge-transfer challenge: combining fullerenes and metalloporphyrins in aqueous environments

A series of truly water-soluble C(60)/porphyrin electron donor-acceptor conjugates has been synthesized to serve as powerful mimics of photosynthetic reaction centers. To this end, the overall water-solubility of the conjugates was achieved by adding hydrophilic dendrimers of different generations to the porphyrin moiety. An important variable is the metal center of the porphyrin; we examined zinc(II), copper(II), cobalt(II), nickel(II), iron(III), and manganese(III). The first insights into electronic communication between the electron donors and the electron acceptors came from electrochemical assays, which clearly indicate that the redox processes centered either on C(60) or the porphyrins are mutually affected. Absorption measurements, however, revealed that the electronic communication in terms of, for example, charge-transfer features, remains spectroscopically invisible. The polar environment that water provides is likely to be a cause of the lack of detection. Despite this, transient absorption measurements confirm that intramolecular charge separation processes in the excited state lead to rapid deactivation of the excited states and, in turn, afford the formation of radical ion pair states in all of the investigated cases. Most importantly, the lifetimes of the radical ion pairs were found to depend strongly on several aspects. The nature of the coordinated metal center and the type of dendrimer have a profound impact on the lifetime. It has been revealed that the nature/electronic configuration of the metal centers is decisive in powering a charge recombination that either reinstates the ground state or any given multiplet excited state. Conversely, the equilibrium of two opposing forces in the dendrimers, that is, the interactions between their hydrophilic regions and the solvent and the electronic communication between their hydrophobic regions and the porphyrin and/or fullerene, is the key to tuning the lifetimes.

Triazole bridges as versatile linkers in electron donor-acceptor conjugates

Aromatic triazoles have been frequently used as π-conjugated linkers in intramolecular electron transfer processes. To gain a deeper understanding of the electron-mediating function of triazoles, we have synthesized a family of new triazole-based electron donor-acceptor conjugates. We have connected zinc(II)porphyrins and fullerenes through a central triazole moiety--(ZnP-Tri-C(60))--each with a single change in their connection through the linker. An extensive photophysical and computational investigation reveals that the electron transfer dynamics--charge separation and charge recombination--in the different ZnP-Tri-C(60) conjugates reflect a significant influence of the connectivity at the triazole linker. Except for the m4m-ZnP-Tri-C(60)17, the conjugates exhibit through-bond photoinduced electron transfer with varying rate constants. Since the through-bond distance is nearly the same for all the synthesized ZnP-Tri-C(60) conjugates, the variation in charge separation and charge recombination dynamics is mainly associated with the electronic properties of the conjugates, including orbital energies, electron affinity, and the energies of the excited states. The changes of the electronic couplings are, in turn, a consequence of the different connectivity patterns at the triazole moieties.