Photochemical and electrochemical properties of zinc chlorin-C60 dyad as compared to corresponding free-base chlorin-C60, free-base porphyrin-C60, and zinc porphyrin-C60 dyads

Symmetry Effects in Photoinduced Electron Transfer in Chlorin‐Quinone Dyads: Adiabatic Suppression in the Marcus Inverted Region

In donor–acceptor dyads undergoing photoinduced electron transfer (PET), a direction or pathway for electron movement is usually dictated by the redox properties and the separation distance between the donor and acceptor subunits, while the effect of symmetry is less recognized. We have designed and synthesized two isomeric donor–acceptor assemblies in which electronic coupling between donor and acceptor is altered by the orbital symmetry control with the reorganization energy and charge transfer exothermicity being kept unchanged. Analysis of the optical absorption and luminescence spectra, supported by the DFT and TD‐DFT calculations, showed that PET in these assemblies corresponds to the Marcus inverted region (MIR) and has larger rate for isomer with weaker electronic coupling. This surprising observation provides the first experimental evidence for theoretically predicted adiabatic suppression of PET in MIR, which unambiguously controlled solely by symmetry.

Triplet photosensitizer-nanotube conjugates: synthesis, characterization and photochemistry of charge stabilizing, palladium porphyrin/carbon nanotube conjugates

The ability of a triplet photosensitizer to generate long-lived charge separated states, in contrast to traditionally used singlet photosensitizers, in covalently functionalized single-walled carbon nanotube hybrids has been investigated. Enriched single-walled carbon nanotubes with two diameters, namely (6,5) and (7,6), were covalently modified to carry a charge-stabilizing triplet photosensitizer derived from a palladium porphyrin. The nanohybrids were fully characterized and the presence of intramolecular interactions between the porphyrin and nanotubes was established from various spectroscopic, imaging, electrochemical and thermochemical studies. Photoluminescence of palladium porphyrin was found to be quantitatively quenched in the presence of covalently appended SWCNTs and this quenching is due to excited state charge separation and has been established by femtosecond transient absorption studies. Owing to the presence of the triplet photosensitizer, the charge separated states lasted over 3 ns, i.e., much longer than those reported earlier for singlet photosensitizer-derived nanotube hybrids. The nanohybrids also exhibited efficient photocatalytic behavior in experiments involving electron pooling of one-electron reduced methyl viologen in the presence of a sacrificial electron donor. Higher yields of photoproducts were achieved from the present donor-acceptor nanohybrids when compared with those of singlet photosensitizer-derived nanohybrids, more so for (6,5) nanotube derived hybrids compared to (7,6) nanotube derived hybrids. The present findings highlight the importance of triplet photosensitizer derived nanohybrids in artificial photosynthesis of charge separation and photocatalytic applicatons.

Molecular-scale engineering of the charge-transfer excited states in non-covalently bound Zn–porphyrin and carbon fullerene based donor–acceptor complex

Tailoring charge-transfer through selective pyrrole ring hydrogenation in a novel Zn–porphyrin and PCBM based donor–acceptor complex has been investigated using quantum chemical computations.

Carbon nanoonion-ferrocene conjugates as acceptors in organic photovoltaic devices

Many macromolecular systems, including carbon nanostructures (CNs), have been synthesized and investigated as acceptors in photovoltaic devices. Some CNs have shown interesting electrochemical, photophysical and electrocatalytic properties and have been used in energy and sustainability applications. This study focuses on the covalent functionalization of carbon nanoonion (CNO) surfaces with ferrocene moieties to obtain donor-acceptor systems involving CNOs as acceptors. The systems were synthesized and characterized by infrared, Raman, UV-vis and fluorescence spectroscopies, thermogravimetric analysis, scanning electron microscopy, nitrogen adsorption and electrochemical measurements. The HOMO-LUMO levels were calculated to evaluate the possibility of using these systems in photoactive devices. In this study, for the first time, the CNO-based derivatives were applied as acceptors in the active layer of photovoltaic devices. This study is the first to use large CNO-based derivatives as acceptors in organic photovoltaic devices, and a power conversion efficiency as high as 1.89% was achieved.

Mimicry and functions of photosynthetic reaction centers

The structure and function of photosynthetic reaction centers (PRCs) have been modeled by designing and synthesizing electron donor–acceptor ensembles including electron mediators, which can mimic multi-step photoinduced charge separation occurring in PRCs to obtain long-lived charge-separated states. PRCs in photosystem I (PSI) or/and photosystem II (PSII) have been utilized as components of solar cells to convert solar energy to electric energy. Biohybrid photoelectrochemical cells composed of PSII have also been developed for solar-driven water splitting into H2 and O2. Such a strategy to bridge natural photosynthesis with artificial photosynthesis is discussed in this minireview.

Anchoring of carboxyl-functionalized porphyrins on MgO, TiO2, and Co3O4 nanoparticles

Hybrid materials consisting of functional organic molecules on metal oxide nanomaterials are key components in emerging technologies, for example in energy conversion and molecular electronics. In this work, we present the results of a comparative study of carboxyl-functionalized porphyrins on different oxide nanomaterials. Specifically, we investigated the interaction of 5(3-carboxyphenyl)-10,15,20-triphenyl-21,23H-porphyrin (2H-3-MCTPP) and 5(4-carboxyphenyl)-10,15,20-triphenyl-21,23H-porphyrin (2H-4-MCTPP), on MgO, TiO2, and Co3O4 nanoparticles (NPs) using isothermal and temperature-programmed diffuse reflection infrared Fourier transform spectroscopy (DRIFTS). We show that both porphyrins bind to the NPs, yielding stable monolayer films consisting of tilted surface carboxylates. In all cases, anchoring through the carboxylic acid group suppresses self-metalation of the porphyrin unit. Upon annealing, all anchored porphyrin films undergo metalation. The position of the acid group has no major influence on the reactivity. The same is true for the nature of the metal oxide, suggesting that the observed behaviour is general for most anchored porphyrin films on oxide nanomaterials.

Excitation-dependent electron exchange energy and electron transfer dynamics in a series of covalently tethered N,N-bis(4'-tert-butylbiphenyl-4-yl)aniline - [C60] fullerene dyads via varying π-conjugated spacers

Femtosecond time-resolved fluorescence and transient absorption studies are reported for three newly synthesized covalently linked N,N-bis(4'-tert-butylbiphenyl-4-yl)aniline (BBA) and pyrrolidinofullerenes (C60)-based donor-π conjugated bridge-acceptor dyads (D-B-A) as functions of the bridge length (7.1, 9.5 and 11.2 Å for Dyad-1, Dyad-2 and Dyad-3), dielectric constants of the medium and pump wavelengths. In polar solvent, ultrafast fluorescence quenching (kEET ≥ 2 × 1012 s-1) of the BBA moiety upon excitation of the BBA moiety (320 nm) is observed in the dyads and is assigned to a mechanism involving electron exchange energy transfer (EET) from 1BBA* to C60 followed by electron transfer from BBA to 1C60*. Cohesive rise and decay dynamics of conjugated BBA˙+-C60˙- anion pairs confirm the involvement of a distance independent adiabatic charge-separation (CS) process (kCS ≥ 2.2 × 1011 s-1) with near unity quantum efficiency (φCS ≥ 99.7%) and a distance-dependent non-adiabatic charge-recombination (CR) process [kCR ∼ (1010-108) s-1]. In contrast, excitation of the C60 moiety (λex = 430 to 700 nm) illustrates photoinduced electron transfer from BBA to 1C60*, involving non-adiabatic (diabatic) and distance-dependent CS (kCS in the range of 0.59-1.78 × 1011 s-1) with 98.86-99.6% (Dyad-3-Dyad-1) quantum efficiency and a CR process with kCR values [kCR ∼ (1010-108) s-1] up to three orders greater than kCS of the respective dyads. Both the processes, CS and CR, upon C60 excitation and the CR process upon BBA excitation show distance dependent rate constants with exponential factor β ≤ 0.5 Å-1, and electron transfer is concluded to occur through a covalently linked conjugated π bridge. Global and target analysis of fsTA data reveal the occurrence of two closely lying CS states, thermally hot (CShot) and thermally relaxed (CSeq) states, and two CR processes with two orders of different rate constants. Careful analysis of the kinetic and thermodynamic data allowed us to estimate the total reorganization energy and electronic coupling matrix (V), which decrease exponentially with distance. These novel features of the distance independent adiabatic CS process and the distance-dependent diabatic CR process upon donor excitation are due to extending the π-conjugation between BBA and C60. The demonstrated results may provide a benchmark in the design of light-harvesting molecular devices where ultrafast CS processes and long-lived CS states are essential requirements.

Hot kinetic model as a guide to improve organic photovoltaic materials

The model yields that the most promising ways to increase the OSC performance are decreasing the reorganization energy, increasing the dielectric permittivity and enhancing the charge delocalization.

Designing of an artificial light energy converter in the form of short-chain dyad when combined with core-shell gold/silver nanocomposites

UV-vis absorption, steady state and time resolved fluorescence and absorption spectroscopic investigations demonstrate that the short chain dyad MNTMA when combined with gold-silver core-shell (Au@Ag) nanocomposite , forms elongated conformers in the excited state whereas for the dyad - Ag (spherical) system the majority of dyads remains in a folded conformation. In the dyad-core-shell nanocomposite system, energy wasting charge recombination rate slows down primarily due to elongated conformation and thus it may be anticipated that this hybrid nanocomposite system may serve as a better light energy conversion device.

Modulating the generation of long-lived charge separated states exclusively from the triplet excited states in palladium porphyrin-fullerene conjugates

This study demonstrates molecular engineering of a series of donor-acceptor systems to allow control of the lifetime and initial spin multiplicity of the charge-separated state. By tuning the rate of intersystem crossing (ISC) and the donor-acceptor distance, electron transfer can be made to occur exclusively from the triplet excited state of the electron donor resulting in long-lived charge separation. To achieve this, three new palladium porphyrin-fullerene donor-acceptor systems were synthesized. The heavy Pd atom enhances the rate of ISC in the porphyrin and the rates of electron and energy transfer are modulated by varying the redox potential of the porphyrin and the porphyrin-fullerene distance. In the case of the meso-tris(tolyl)porphyrinato palladium(ii)-fulleropyrrolidine, the donor-acceptor distance is relatively long (13.1 Å) and the driving force for electron transfer is low. As a result, excitation of the porphyrin leads to rapid ISC followed by triplet-triplet energy transfer to fullerene. When the fullerene is bound directly to the porphyrin shortening the donor-acceptor distance to 2.6 Å electron transfer from the singlet excited palladium porphyrin leading to the generation of a short-lived charge separated state is the main process. Finally, when the palladium porphyrin is substituted with three electron rich triphenylamine entities, the lower oxidation potential of the porphyrin and appropriate donor-acceptor distance (∼13 Å), lead to electron transfer exclusively from the triplet excited state of palladium porphyrin with high quantum yield. The results show that when electron transfer occurs from the triplet state, its increased lifetime allows the distance between the donor and acceptor to be increased which results in a longer lifetime for the charge separated state.

Potential Tuning of Nanoarchitectures Based on Phthalocyanine Nanopillars: Construction of Effective Photocurrent Generation Systems

Nanopillars composed of a photoresponsive phthalocyanine derivative have been conveniently fabricated using a continuous silane coupling reaction on a substrate. The chemical potentials of phthalocyanine nanopillars (PNs) are precisely controlled by changing the number of phthalocyanine derivatives on the substrate. In addition, photocurrent generation efficiencies have been strongly influenced by the number of phthalocyanine derivatives. High photocurrent conversion cells in a solid state have been obtained by the combination of PNs and a fullerene derivative.

Supramolecular complex formation of resorcin[4]arene-modified zinc phthalocyanine and fullerene

A resorcin[4]arene-strapped zinc phthalocyanine 1, in which two benzene rings of the upper rim of resorcin[4]arene were fused to the adjacent phthalocyanine ligand, was synthesized from two phthalonitriles. Introducing resorcin[4]arene on one side of zinc phthalocyanine strongly affects the spectroscopic property of the phthalocyanine. The Q-band of 1 in CH 2 Cl 2 is split into 694 and 740 nm due to the self-protonation of 1 on the meso-nitrogen of phthalocyanine ligand. Bowl-shaped resorcin[4]arene can capture C 60 through the non-covalent interactions, and the complexation between 1 and C 60 results in an efficient photo-induced electron transfer.

Long-lived charge separation and applications in artificial photosynthesis

Researchers have long been interested in replicating the reactivity that occurs in photosynthetic organisms. To mimic the long-lived charge separations characteristic of the reaction center in photosynthesis, researchers have applied the Marcus theory to design synthetic multistep electron-transfer (ET) systems. In this Account, we describe our recent research on the rational design of ET control systems, based on models of the photosynthetic reaction center that rely on the Marcus theory of ET. The key to obtaining a long-lived charge separation is the careful choice of electron donors and acceptors that have small reorganization energies of ET. In these cases, the driving force of back ET is located in the Marcus inverted region, where the lifetime of the charge-separated state lengthens as the driving force of back ET increases. We chose porphyrins as electron donors and fullerenes as electron acceptors, both of which have small ET reorganization energies. By linking electron donor porphyrins and electron acceptor fullerenes at appropriate distances, we achieved charge-separated states with long lifetimes. We could further lengthen the lifetimes of charge-separated states by mixing a variety of components, such as a terminal electron donor, an electron mediator, and an electron acceptor, mimicking both the photosynthetic reaction center and the multistep photoinduced ET that occurs there. However, each step in multistep ET loses a fraction of the initial excitation energy during the long-distance charge separation. To overcome this drawback in multistep ET systems, we used designed new systems where we could finely control the redox potentials and the geometry of simple donor-acceptor dyads. These modifications resulted in a small ET reorganization energy and a high-lying triplet excited state. Our most successful example, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), can undergo a fast photoinduced ET from the mesityl (Mes) moiety to the singlet excited state of the acridinium ion moiety (Acr(+)) with extremely slow back ET. The high-energy triplet charge-separated state is located deep in the Marcus inverted region, and we have detected the structural changes during the photoinduced ET in this system using X-ray crystallography. To increase the efficiency of both the light-harvesting and photoinduced ET, we assembled the Acr(+)-Mes dyads on gold nanoparticles to bring them in closer proximity to one another. We can also incorporate Acr(+)-Mes molecules within nanosized mesoporous silica-alumina. In contrast to the densely assembled dyads on gold nanoparticles, each Acr(+)-Mes molecule in silica-alumina is isolated in the mesopore, which inhibits the bimolecular back ET and leads to longer lifetimes in solution at room temperature than the natural photosynthetic reaction center. Acr(+)-Mes and related compounds act as excellent organic photocatalysts and facilitate a variety of reactions such as oxygenation, bromination, carbon-carbon bond formation, and hydrogen evolution reactions.

Synthesis, structural characterization and protonation/deprotonation of hydroxyl-substituted free-base tetraphenylporphyrins in nonaqueous media

A series of hydroxyl-substituted free-base tetraphenylporphyrins was synthesized and characterized by UV-vis spectroscopy, 1 H NMR and mass spectrometry. The porphyrins are represented as (HOPh) n(t BuPh )4-n PH 2, where Ph presents a phenyl group, HO and t Bu are substituents on the para-positions of the phenyl rings of the macrocycle, n = 0–4 and P represents the dianion of tetraphenylporphyrin. The UV-visible properties of each porphyrin were examined in dichloromethane (DCM), N,N′-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) before and after addition of trifluoroacetic acid (TFA) or sodium hydroxide (NaOH) to solution. Equilibrium constants for protonation ( log βn) and deprotonation [Formula: see text] of each compound were determined using standard equations. The protonations occur in a single step involving a simultaneous two proton addition at the porphyrin central nitrogens. The phenolic protons on (HOPh) n(t BuPh )4-n PH 2 are easier to deprotonate than the core nitrogen protons of the porphyrins and this reaction occurs in a single step involving the simultaneous loss of 1–4 protons on the hydroxyl groups followed by a loss of two protons from the central nitrogens. The effect of HO substituents on UV-visible spectra and the magnitude of the protonation/deprotonation constants ( log βn and [Formula: see text]) are discussed. Two of the porphyrins, (t BuPh )4 PH 2 and trans- (HOPh) 2(t BuPh )2 PH 2, are also characterized by a single-crystal X-ray analysis.

Controlling Morphology and Molecular Packing of Alkane Substituted Phthalocyanine Blend Bulk Heterojunction Solar Cells

Systematic changes in the exocyclic substiution of core phthalocyanine platform tune the absorption properties to yield commercially viable dyes that function as the primary light absorbers in organic bulk heterojunction solar cells. Blends of these complementary phthalocyanines absorb a broader portion of the solar spectrum compared to a single dye, thereby increasing solar cell performance. We correlate grazing incidence small angle x-ray scattering structural data with solar cell performance to elucidate the role of nanomorphology of active layers composed of blends of phthalocyanines and a fullerene derivative. A highly reproducible device architecture is used to assure accuracy and is relevant to films for solar windows in urban settings. We demonstrate that the number and structure of the exocyclic motifs dictate phase formation, hierarchical organization, and nanostructure, thus can be employed to tailor active layer morphology to enhance exciton dissociation and charge collection efficiencies in the photovoltaic devices. These studies reveal that disordered films make better solar cells, short alkanes increase the optical density of the active layer, and branched alkanes inhibit unproductive homogeneous molecular alignment.