Tuning the Ground-State and Excited-State Interchromophore Interactions in Porphyrin−Fullerene π-Stacks

Missing Excitons: How Energy Transfer Competes with Free Charge Generation in Dilute-Donor/Acceptor Systems

Energy transfer across the donor-acceptor interface in organic photovoltaics is usually beneficial to device performance, as it assists energy transport to the site of free charge generation. Here, we present a case where the opposite is true: dilute donor molecules in an acceptor host matrix exhibit ultrafast excitation energy transfer (EET) to the host, which suppresses the free charge yield. We observe an optimal photochemical driving force for free charge generation, as detected via time-resolved microwave conductivity (TRMC), but with a low yield when the sensitizer is excited. Meanwhile, transient absorption shows that transferred excitons efficiently produce charge-transfer states. This behavior is well described by a competition for the excited state between long-range electron transfer that produces free charge and EET that ultimately produces only localized charge-transfer states. It cannot be explained if the most localized CT states are the intermediate between excitons and the free charge in this system.

Photoinduced Electron Transfer in a Porphyrin-Fullerene Dyad at a Liquid Interface

The excited-state properties of an amphiphilic porphyrin-fullerene dyad and of its porphyrin analogue adsorbed at the dodecane/water interface are investigated by using surface second-harmonic generation. Although the porphyrin is formally centrosymmetric, the second-harmonic spectra of both compounds are dominated by the intense Soret band of the porphyrin. Polarization-selective measurements and molecular dynamics simulations suggest an angle of about 45° between the donor-acceptor axis and the interfacial plane, with the porphyrin interacting mostly with the nonpolar phase. Time-resolved measurements reveal a marked concentration dependence of the dynamics of both compounds upon Q-band excitation, indicating the occurrence of intermolecular quenching processes. The significant differences in dynamics and spectra between the dyad and the porphyrin analogue are explained by a self-quenching of the excited dyad via an intermolecular electron transfer.

A charge transfer state induced by strong exciton coupling in a cofacial μ-oxo-bridged porphyrin heterodimer

Photosensitizers with high energy, long lasting charge-transfer states are important components in systems designed for solar energy conversion by multistep electron transfer. Here, we show that in a push-pull type, μ-oxo-bridged porphyrin heterodimer composed of octaethylporphyrinatoaluminum(iii) and octaethylporphyrinatophosphorus(v), the strong excitonic coupling between the porphyrins and the different electron withdrawing abilities of Al(iii) and P(v) promote the formation of a high energy CT state. Using, an array of optical and magnetic resonance spectroscopic methods along with theoretical calculations, we demonstrate photodynamics of the heterodimer that involves the initial formation of a singlet CT which relaxes to a triplet CT state with a lifetime of ∼130 ps. The high-energy triplet CT state (3CT = 1.68 eV) lasts for nearly 105 μs prior to relaxing to the ground state.

Electron transfer in oriented donor-acceptor dyads, intralayer charge migration, and formation of interlayer charge separated states in multi-layered Langmuir-Schäfer films

Photoinduced intra- and interlayer electron transfer (ET) of doubly bridged donor-acceptor molecule, porphyrin-fullerene dyad (PF), was studied in single- and multi-layered Langmuir-Schäfer (LS) films and in LS films, where PF and an efficient electron donating polymer polyhexyltiophene (PHT) formed a bilayer PHT/PF and multi-layered PHT/PF structures. The ET through layers were investigated by a method, which measures the photovoltaic (PV) response proportional to the number of charge-separated (CS) states and to the CS distance between the electrons and holes formed in pulsed photo-excitation. Primary conclusions were, that ET starts as formations of CS dyads (P⁺F^-) in single-layers, continues as long-range intra-layer charge migrations following interlayer CS between two adjacent monolayers. Quantitative conclusions were, that the interlayer ET efficiency is 100% in the bi-layered PF structure (2PF), where two CS dyads in adjacent layers forms CS complexes (P⁺F/PF^-) and that the probability to form longer or higher order of CS complexes follows an expression of a convergent geometric series, with a converting factor of 2/3. In the PHT/PF bilayer structure the ET efficiency was one order of magnitude higher, than that for the 2PF structure due to the ET from the CS dyads to ground state electron donor PHT, with an acceptor density, much higher than that of (P⁺F^-).

Unveiling the nature of supramolecular crown ether-C60 interactions

A series of exTTF-(crown ether)₂ receptors, designed to host C₆₀, has been prepared. The size of the crown ether and the nature of the heteroatoms have been systematically changed to fine tune the association constants. Electrochemical measurements and transient absorption spectroscopy assisted in corroborating charge transfer in the ground state and in the excited state, leading to the formation of radical ion pairs featuring lifetimes in the range from 12 to 21 ps. To rationalize the nature of the exTTF-(crown ether)₂·C₆₀ stabilizing interactions, theoretical calculations have been carried out, suggesting a synergetic interplay of donor-acceptor, π-π, n-π and CH···π interactions, which is the basis for the affinity of our novel receptors towards C₆₀.

Photophysical study of a self-assembled donor-acceptor two-layer film on TiO2

The self-assembled monolayer (SAM) technique was employed to fabricate a two-layer donor-acceptor film on the surface of TiO2. The approach is based on using donor and acceptor compounds with anchoring groups of different lengths. The acceptor, a fullerene derivative, has a carboxyl anchor attached to the fullerene moiety via a short linker that places the fullerene close to the surface. The donor, a porphyrin derivative, is equipped with a long linker that can penetrate between the fullerenes and keep porphyrin on top of the fullerene layer. The two-layer fullerene-porphyrin structures were deposited on a mesoporous film of TiO2 nanoparticles by immersing the TiO2 film sequentially into fullerene and porphyrin solutions. Transient absorption spectroscopy studies of the samples revealed that after the selective photoexcitation of porphyrin a fast (<5 ps) intermolecular electron transfer (ET) takes place from porphyrin to the fullerene layer, which confirms the formation of the interlayer donor-acceptor interface. Furthermore, in the second step of ET the fullerene anions donate electrons to the TiO2 nanoparticles. The latter reaction is relatively slow with an average time constant of 230 ps. It involves roughly half of the primary generated charges, and the second half relaxes by the interlayer charge recombination. The resulting state with a porphyrin cation and electron in TiO2 has an extremely long lifetime and recombines with an average time constant of 23 ms.

Strongest π–metal orbital coupling in a porphyrin/gold cluster system

π–Metal coupling in a porphyrin/gold cluster system at the closest distance was investigated spectroscopically; this system formed an exciplex in the excited state.

Role of charge transfer interaction and the chemical physics behind effective fulleropyrrolidine/porphyrin non-covalent interaction in solution

The present paper reports the photophysical insights on supramolecular interaction of a monoporphyrin derivative, namely, 1, with C60 pyrrolidine tris-acid ethyl ester (PyC60) in toluene and benzonitrile. The ground state interaction between PyC60 and 1 is facilitated through charge transfer interaction. Both UV-Vis and steady state measurements elicit almost similar magnitude of binding constant for the PyC60/1 complex in toluene and benzonitrile, viz., 6825 and 6540 dm(3 )mol(-1), respectively. Life time measurement evokes that rate of charge separation is fast in benzonitrile. Both hybrid-DFT and DFT calculations provide very good support in favor of electronic charge-separation in PyC60/1 system in vacuo.

Porphyrin π-stacking in a heme protein scaffold tunes gas ligand affinity

The role of π-stacking in controlling redox and ligand binding properties of porphyrins has been of interest for many years. The recent discovery of H-NOX domains has provided a model system to investigate the role of porphyrin π-stacking within a heme protein scaffold. Removal of a phenylalanine-porphyrin π-stack dramatically increased O2, NO, and CO affinities and caused changes in redox potential (~40mV) without any structural changes. These results suggest that small changes in redox potential affect ligand affinity and that π-stacking may provide a novel route to engineer heme protein properties for new functions.

Charge transfer excitations in cofacial fullerene-porphyrin complexes

Porphyrin and fullerene donor-acceptor complexes have been extensively studied for their photo-induced charge transfer characteristics. We present the electronic structure of ground states and a few charge transfer excited states of four cofacial porphyrin-fullerene molecular constructs studied using density functional theory at the all-electron level using large polarized basis sets. The donors are base and Zn-tetraphenyl porphyrins and the acceptor molecules are C(60) and C(70). The complexes reported here are non-bonded with a face-to-face distance between the porphyrin and the fullerene of 2.7 to 3.0 Å. The energies of the low lying excited states including charge transfer states calculated using our recent excited state method are in good agreement with available experimental values. We find that replacing C(60) by C(70) in a given dyad may increase the lowest charge transfer excitation energy by about 0.27 eV. Variation of donor in these complexes has marginal effect on the lowest charge transfer excitation energy. The interfacial dipole moments and lowest charge transfer states are studied as a function of face-to-face distance.

[60]Fulleropyrrolidines bearing π-conjugated moiety for polymer solar cells: contribution of the chromophoric substituent on C₆₀ to the photocurrent

Two fullerene-terthiophene dyads without hexyl chains (3T-C₆₀) and with hexyl chains (3TH-C₆₀) on the terthiophene substituent are synthesized by 1,3-dipolar cycloaddition of corresponding azomethine ylides to C₆₀. The cyclic voltammetry studies indicate no apparent electronic communication between the terthiophene pendent group and the fulleropyrrolidine core in the ground state. However, a significant florescence quenching is observed for 3T-C₆₀ and 3TH-C₆₀, compared to their fluorescent terthiophene (3T) and 3TH precursors, respectively, suggesting the occurrence of strong intramolecular electron/energy transfers in the photoexcited state. Furthermore, these new fulleropyrrolidine derivatives are applied as electron acceptors to fabricate poly(3-hexylthiophene) (P3HT) based bulk heterojunction solar cells. The incident photon-to-current efficiency (IPCE) value of the P3HT/3T-C₆₀ device is significantly higher than that of the P3HT/PCBM cell in wavelengths of 350-420 nm. This finding provides direct evidence for the contribution of 3T excitons to the photocurrent. Replacing 3T-C₆₀ with 3TH-C₆₀ effectively improves the morphology of the photoactive layer and widens the window of optimal D/A ratios, raising the power conversion efficiency (PCE) from 2.14% to 2.54%. Importantly, these devices exhibit superior stability of PCE against high-temperature aging.

Transient states in photoinduced electron transfer reactions of porphyrin- and phthalocyanine-fullerene dyads

This paper combines the most important results on studies performed by the authors during the last decade on photoinduced electron transfer reactions of pheophytin-, phthalocyanine-, and porphyrin-fullerene dyads, in which donor and acceptor moieties are covalently linked to each other. Practically all studied molecules form an intramolecular exciplex as a transient state before the formation of the charge separation state or tight ion pair. When the center-to-center distance of the donor and acceptor pair is short (7–10 Å) both the exciplex formation and primary electron transfer are extremely fast with rate constants of 7–23 × 1012 s -1 and 40–1400 × 109 s -1, respectively. Rates become slower when the distance and orientational fluctuation increases. No systematic correlation between free energies and the rates of the formation and recombination of the exciplex and the charge separation state, respectively, were observed. The mechanism is discussed in frames of the Marcus electron transfer and the radiationless quantum transition theories.

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.

Photophysical insights into supramolecular interaction of a designed bisporphyrin with fullerenes C60 and C70

The present paper reports the photophysical investigations of a designed bisporphyrin (1), and its supramolecular complexes with C60 and C70 in toluene medium. UV-vis studies reveal appreciable ground state interaction between fullerenes and 1. The stoichiometry of the fullerene complexes of 1 is found to be 1:1. Steady state fluorescence studies elicit quenching of fluorescence of 1 in the presence of fullerenes. The binding constants of the C60/1 and C70/1 complexes are estimated to be 3760 and 31,222.5 dm3 mol(-1), respectively. Time resolved emission studies establish relatively long-lived charge separated state for the C70/1 complex. Molecular mechanics calculations in vacuo evoke the stereoscopic structures of the fullerene/1 complexes and interpret the stability difference between C60 and C70 complexes of 1 in terms of heat of formation values.

Axle charge effects on photoinduced electron transfer processes in rotaxanes containing porphyrin and [60]fullerene

Two rotaxanes containing zinc porphyrin (ZnP) with crown ether and [60]fullerene (C(60)) with cationic and neutral axles are synthesized. Optimized structures calculated by molecular orbital methods indicate that the rotaxane with an ammonium cation in the center of the axle has a shorter distance between the C(60) and ZnP moieties than that of the rotaxane with a neutral axle because of acylation of the ammonium cation, which draws away the C(60) and ZnP moieties by releasing the interaction with the crown ether. The charge-transfer interaction is revealed by absorption spectra for the rotaxane with a short distance, but not for the rotaxane with a long distance, which strongly affects the rates and efficiencies of photoinduced electron-transfer and energy-transfer processes via the excited singlet states of the ZnP and C(60) moieties and their triplet states, as revealed by the time-resolved fluorescence and absorption measurements. The rate of the charge recombination of the radical ion pair of the rotaxane with a neutral axle is slower than that of the rotaxane with a cationic axle, due to the loose structure of the former rotaxane, which results from the long distance between the C(60) anion radical and the ZnP cation radical.

trans-2 addition pattern to power charge transfer in dendronized metalloporphyrin C60 conjugates

Coordinating different transition metals--manganese(III), iron(III), nickel(II), and copper(II)--by a dendronized porphyrin afforded a new family of redox-active metalloporphyrins to which C(60) was attached as a ground-state electron acceptor. Such a strategy introduced an additional center of redoxactivity, that is, a change of the oxidation state of the metal. Cyclic voltammetry and absorption/fluorescence measurements provided support for mutual interactions between the redox-active constituents in the ground state. In particular, slightly anodic shifted reduction potentials/cathodic shifted oxidation potentials and the occurrence of new charge transfer features in the 700-900 nm range prompt to sizable electronic coupling in the range of 300 cm(-1). Photophysical means--steady-state/time-resolved fluorescence and transient absorption measurements--shed light on the excited-state interactions. To this end, we have added pulse radiolytic investigations to characterize the radical cation (i.e., metalloporphyrins) and radical anion (i.e., fullerene) characteristics. Pi-pi stacking of the excited state electron donor and the electron acceptor is key to overcome the intrinsically fast deactivation of the excited states in these metalloporphyrins and to power an exothermic charge transfer. The lifetimes of the rapidly and efficiently generated radical ion pair states, which range from 15 to >3000 ps, revealed several important trends. First, they were found to depend on the solvent polarity. Second, the nature of the transition metal plays a similarly decisive role. It is important that the product of charge recombination, namely tripmultiplet excited states versus ground state, had a great impact. Finally, a correlation between the charge transfer rate (i.e., charge separation and charge recombination) and the free energy change for the underlying reaction reveals a parabolic dependence with parameters of the reorganization energy (0.84 eV) and electronic coupling (70 cm(-1)) closely resembling that seen for the zinc(II) and free base analogues.

Gold nanoparticle enhanced charge transfer in thin film assemblies of porphyrin-fullerene dyads

Photoinduced vectorial electron transfer in a molecularly organized porphyrin-fullerene (PF) dyad film is enhanced by the interlayer charge transfer from the porphyrin moiety of the dyad to an octanethiol protected (dcore approximately 2 nm) gold nanoparticle (AuNP) film. By using the time-resolved Maxwell displacement charge (TRMDC) method, the charge separation distance was found to increase by 5 times in a multilayer film structure where the gold nanoparticles face the porphyrin moiety of the dyad, that is, AuNP|PF, compared to the case of the PF layer alone. Films were assembled by the Langmuir-Blodgett (LB) method using octadecylamine (ODA) as the matrix compound. Atomic force microscopy (AFM) images of the monolayers revealed that AuNPs are arranged into continuous, islandlike structures and PF dyads form clusters. The porphyrin reference layer was assembled with the AuNP layer to gain insight on the interaction mechanism between porphyrin and gold nanoparticles. Interlayer electron transfer was also observed between the AuNPs and porphyrin reference, but the efficiency is lower than that in the AuNP|PF film. Fluorescence emission of the reference porphyrin is slightly quenched, and fluorescence decay becomes faster in the presence of AuNPs. The proposed mechanism for the electron transfer in the AuNP|PF film is thus the primary electron transfer from the porphyrin to the fullerene followed by a secondary hole transfer from the porphyrin to the AuNPs, resulting in an increased charge separation distance and enhanced photovoltage.