Lifetime Of Charge-separated State Research Articles

(Invited) Distance Dependent Electron Transfer Kinetics in Water Soluble Axially Connected Silicon Phthalocyanine-Fullerene Conjugates

The effect of donor-acceptor distance in controlling the rate of electron transfer in axially linked silicon phthalocyanine – C60 dyads has been investigated. Herein we will presented the synthesis and photophysical properties of two water soluble C60-SiPc-C60 dyads, 1 and 2, varying in their donor-acceptor distance.In previous investigation for C60-SiPc-C60 dyads where the SiPc and C60 are separated by a phenyl spacer, faster electron transfer was observed with k cs equal to 2.7 x 109 s-1 in benzonitrile. However, in the case of C60-SiPc-C60, where SiPc and C60 are separated by a biphenyl spacer, a slower electron transfer rate constant, k cs equal to 9.1 x108 s-1 was recorded. The addition of an extra phenyl spacer in 2 increased the donor-acceptor distance by ~ 4.3 Å, and consequently, slowed down the electron transfer rate constant by a factor of ~3.7. The charge separated state lasted over 3 ns, monitoring time window of our femtosecond transient spectrometer. Complimentary nanosecond transient absorption studies revealed formation of 3SiPc* as the end product and suggested that the final lifetime of charge separated state to be in the 3-20 ns range. Energy level diagram established to comprehend these mechanistic details indicated that the comparatively high-energy SiPc .+ -C60 .- charge separated states (1.57 eV) to populate the low-lying 3SiPc* (1.26 eV) prior returning to the ground state.[1]Here we will present our recent results using water soluble SiPc-C60 1-2 systems where the SiPc is substituted with pyridine units.[3]AcknowledgmentsThis work was supported by the Ministerio de Ciencia e Innovación of Spain and FEDER funds (Grants PID2020-117855RB-I00 to ASS) and National Science Foundation (Grant No. 1401188 and 2000988 to FD).References L. Martín-Gomis, S. Seetharaman, D. Herrero, P. A. Karr, F. Fernández-Lázaro, F. D’Souza, and Á. Sastre-Santos, Chem Phys. Chem., 2020, 21, 2254-2262.V. Sobrino, L. Martín-Gomis, S. Seetharaman, P. A. Karr, D’Souza, and Á. Sastre-Santos, under preparation. Figure 1

(Invited) Exciton Dissociation in Mixed-Dimensionality SWCNT-Based Heterojunctions

Quantum-confined semiconductors provide highly tunable optical and electrical properties for a wide variety of emerging applications. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have shown tremendous potential in applications ranging from digital logic, biological imaging, quantum information processing, photovoltaics, and thermoelectric energy harvesting. Energy harvesting applications rely critically upon the creation of tailored interfaces that enable the movement of energetic species (excitons, electrons, holes) in specified directions. In this talk, I will discuss the use of a variety of rationally designed “mixed-dimensionality” heterojunctions between s-SWCNTs and transition metal dichalcogenide (TMDC) monolayers to harvest visible/near-infrared photons and convert these excitations into long-lived charge-separated states. Type-II heterojunctions between s-SWCNTs and TMDCs enable rapid exciton dissociation and microsecond charge-separated state lifetimes. We have developed new TMDC/SWCNT/TMDC trilayer architectures that enable charge transfer cascades for improving charge separation yield and lifetime. These trilayer architectures allow us to probe out-of-plane carrier diffusion in the nanotube layer and the degree to which bound interfacial excitons impact the photophysics in these mixed-dimensionality heterostructures. I will also discuss the methodology by which we attempt to quantify the photoinduced exciton dissociation quantum yields in these nanoscale heterostructures. Using several heterostructure results, we are attempting to narrow in on appropriate ways to quantify the local dielectric constant and exciton size in both the SWCNT and TMDC components, each of which impact the estimated charge carrier yields. Taken together, these studies provide fundamental insights into the links between ground-state thermodynamics and excited-state dynamics for model nanoscale heterojunctions used to harvest solar energy and produce long-lived charge-separated states.

Decoupling Plasmonic Hot Carrier from Thermal Catalysis via Electrode Engineering.

Increased attention has been directed toward generating nonequilibrium hot carriers resulting from the decay of collective electronic oscillations on metal known as surface plasmons. Despite numerous experimental endeavors, demonstrating hot carrier-mediated photocatalysis without a heating contribution has proven challenging, particularly for single electron transfer reactions where the thermal contribution is generally detrimental. An innovative engineering solution is proposed to enable single electron transfer reactions with plasmonics. It consists of a photoelectrode designed as an energy filter and photocatalysis performed with light function modulation instead of continuously. The photoelectrode, consisting of FTO/TiO2 amorphous (10 nm)/Au nanoparticles, with TiO2 acting as a step-shape energy filter to enhance hot electron extraction and charge-separated state lifetime. The extracted hot electrons were directed toward the counter electrode, while the hot holes performed a single electron transfer oxidation reaction. Light modulation prevented local heat accumulation, effectively decoupling hot carrier catalysis from the thermal contribution.

Spectral properties and photophysical processes of triphenylamine-porphyrin-pyrimidine triads

Porphyrins have attracted considerable attention as an important family of photosensitizers used for dye-sensitized solar cells (DSSCs). In this work, a novel triphenylamine-porphyrin-pyrimidine triad with a triple bond linkage was synthesized and characterized. The sensitizer presents broad absorption in solution and on TiO2 films across the UV-Vis region benefiting from the extended conjugation. The acetenyl [Formula: see text]-conjugated bridge stimulates the photoinduced electron transfer (PET) process and brings a charge-separated state lifetime of around 4 μs. DSSC devices based on the sensitizer achieve a power conversion efficiency (PCE) of 3.77% thanks to the prolonged charge-separated state lifetime and suppressed charge recombination. The understanding of the structural, optical, and electrochemical properties of triphenylamine-porphyrin-pyrimidine triads provides new insights into the rational design of porphyrin sensitizers for various applications.

Long-lived photoinduced charge separation in adamantane-bridged donor-acceptor dyads: A case of adamantane bridge conferring orthogonal donor-acceptor orientation and restricted rotation

Design of compact donor-acceptor dyads that can exhibit charge separated (CS) state lifetimes extending to the microsecond time domain remains a major challenge even today. A simple but effective design strategy that can lead to long CS state lifetimes, in at least a few compact dyads, is still elusive. Herein we propose that use of adamantane (AD) as bridge can enhance CS state lifetimes in small molecule donor-acceptor dyads. This paper reports AD-bridged dyads, AN-AD-NB and PY-AD-NB, wherein the anthracene (or pyrene) donor and nitrobenzene acceptor are attached to bridgehead positions of AD. The singlet CS states generated upon irradiation were long-lived (lifetimes >1 μs) and decayed to the ground and local triplet states, simultaneously. Electron transfer reactions of the CS state with secondary donor and acceptor established that the CS state is sufficiently long-lived to participate in intermolecular reactions. X-ray crystal structure of the AN-AD-NB dyad reveals that the donor and acceptor are orthogonally oriented, which reduces the donor-acceptor interaction, and thereby the charge recombination rate, significantly. By analysing the X-ray structure, we predict that all adamantane-bridged compact dyads will have orthogonal donor-acceptor orientation and exhibit long-lived CS states. We also report studies with bisadamantyl-substituted dyads and the results confirmed that the local triplets are formed from the CS state through hyperfine interaction. Based on our work we propose that the hyperfine interaction, which is not expected to occur in compact dyads because of the short donor-acceptor separation, can indeed occur if the CS state is sufficiently long-lived.

Multimodular Wide-Band Capturing Nanohybrids: Role of Carbon Nanotubes in Slowing Charge Recombination in Supramolecular C60-BisstyrylBODIPY-(Zinc Porphyrin)2 Donor-Acceptor Molecular Cleft.

The importance of diameter-sorted single-wall carbon nanotubes (SWCNTs) noncovalently bound to a donor-acceptor molecular cleft, 1, in prolonging the lifetime of charge-separated states is successfully demonstrated. For this, using a multistep synthetic procedure, a wide-band capturing, multimodular, C60-bisstyrylBODIPY-(zinc porphyrin)2, molecular cleft 1, was newly synthesized and shown to bind diameter-sorted SWCNTs. The molecular cleft and its supramolecular assemblies were characterized by a suite of physicochemical techniques. Free-energy calculations suggested that both the (6,5) and (7,6) SWCNTs bound to 1 act as hole acceptors during the photoinduced sequential electron transfer events. Consequently, selective excitation of 1 in 1:SWCNT hybrids revealed a two-step electron transfer, leading to the formation of charge-separated states. Due to the distant separation of the cation and anion radical species within the supramolecules, improved lifetimes of the charge-separated states could be achieved. The present supramolecular strategy of improving charge separation involving SWCNTs and donor-acceptor molecular clefts highlights the potential application of these hybrid materials for various light energy harvesting and optoelectronic applications.

Photodriven electron-transfer dynamics in a series of heteroleptic Cu(I)-anthraquinone dyads.

Solar fuels catalysis is a promising route to efficiently harvesting, storing, and utilizing abundant solar energy. To achieve this promise, however, molecular systems must be designed with sustainable components that can balance numerous photophysical and chemical processes. To that end, we report on the structural and photophysical characterization of a series of Cu(I)-anthraquinone-based electron donor-acceptor dyads. The dyads utilized a heteroleptic Cu(I) bis-diimine architecture with a copper(I) bis-phenanthroline chromophore donor and anthraquinone electron acceptor. We characterized the structures of the complexes using x-ray crystallography and density functional theory calculations and the photophysical properties via resonance Raman and optical transient absorption spectroscopy. The calculations and resonance Raman spectroscopy revealed that excitation of the Cu(I) metal-to-ligand charge-transfer (MLCT) transition transfers the electron to a delocalized ligand orbital. The optical transient absorption spectroscopy demonstrated that each dyad formed the oxidized copper-reduced anthraquinone charge-separated state. Unlike most Cu(I) bis-phenanthroline complexes where increasingly bulky substituents on the phenanthroline ligands lead to longer MLCT excited-state lifetimes, here, we observe a decrease in the long-lived charge-separated state lifetime with increasing steric bulk. The charge-separated state lifetimes were best explained in the context of electron-transfer theory rather than with the energy gap law, which is typical for MLCT excited states, despite the complete conjugation between the phenanthroline and anthraquinone moieties.

Structure-enabled long-lived charge separation in single crystals of an asymmetric donor-acceptor perylenediimide cyclophane.

We report the synthesis and characterization of a covalently linked asymmetric cyclophane comprising a 1,7-di(pyrrolidin-1'-yl)perylene-3,4,9,10-bis(dicarboximide) (pyrPDI) and 1,6,7,12-tetra(4'-t-butylphenoxy)perylene-3,4,9,10-bis(dicarboximide) (tpPDI), which absorbs light from 400-750 nm. Single crystals of pyrPDI-tpPDI were analyzed by using X-ray diffraction and transient absorption microscopy. The crystal structure contains several types of intermolecular donor-acceptor interactions (pyrPDI-pyrPDI, tpPDI-tpPDI, and pyrPDI-tpPDI) in addition to the covalently installed intramolecular interaction. Following photoexcitation of the pyrPDI-tpPDI single crystal, the transient absorption data show that charge separation occurs in τ = 21 ps, which is about nine times faster than in toluene solution, while charge recombination occurs in τ > 2 μs, which is more than 400 times longer than in solution. The faster charge separation in the single crystals results from the intermolecular donor-acceptor pyrPDI-tpPDI interactions, while the greatly enhanced charge-separated state lifetime is a consequence of charge transport through the intermolecular π-stacks. These results demonstrate the utility of pre-organizing donor-acceptor structural motifs to elicit specific crystal morphologies that can lead to enhanced photogenerated charge carrier lifetimes for solar energy conversion.

Synthesis, Chemical Structure, and Ground- and Excited-State Spectral Characteristics of (Porphyrinato)(chloro)indium(III) and Its Complexes with C60 and Pyridyl-Substituted Fullero[60]pyrrolidine

New complexes of [5,10,15,20-tetra-(4-methoxyphenyl)porphinato](chloro)indium(III) ((Cl)InTPP(p-OCH3)4) with unsubstituted C60 and 1-methyl-2-(pyridin-4'-yl)-3,4- fullero[60]pyrrolidine (PyC60) were synthesized in toluene. The stability constants of 1 : 1 complexes (dyads) were determined using UV-vis and fluorescence spectroscopy. The dyads were characterized by IR and 1H NMR spectroscopy data. It was established that fluorescence of (Cl)InTPP(p-OCH3)4 is quenched upon gradual addition of fullerenes. The numerical values of the Stern–Volmer quenching constants (KSV) were determined. The most important charge transfer characteristics of dyads (the lifetime of charge-separated states and charge separation and recombination constants), needed for further consideration of dyads based on indium(III) porphyrins as photoinduced electron transfer systems, were determined using femtosecond laser spectroscopy.

Synthesis and Excited State Charge Separation Involving Coordinated C60 of π-Extended Pyrazinepyrene Fused Zinc Phthalocyanines

Polycyclic aromatic hydrocarbons (PAHs) play an important role in organic electronics.[1] To increase the stability and the conjugation, pyrene units could be further attached through heteroatoms to improve the photo/chemical stability to oxygen or light.[2] In terms of the synthesis of these types of large acenes through pyrene, many routes have been studied and one of the best protocols is to synthesize the quinoxaline derivative from the dione and the corresponding diamino subunit.[3] Herein, we will present a series of pyrazinepyrene fused zinc phthalocyanines (ZnPc-Pyrn) as a new class of π-extended phthalocyanine systems. Bathochromically shifted absorption as a function of the number of pyrazinepyrene entities due to extended π-conjugation, and quenched fluorescence due to the presence of fused pyrazinepyrene were witnessed. The electronic structures of these phthalocyanines were probed by systematic computational and electrochemical studies while the excited state properties were examined by pump-probe spectroscopies operating at the femto- and nanosecond time scales. Like the excited singlet lifetimes, the excited triplet states also revealed diminished lifetimes with an increased number of pyrazinepyrene entities. Further, the coordinatively unsaturated zinc in these molecules was coordinated with phenyl imidazole functionalized fullerene, ImC60 to form a new series of donor-acceptor conjugates. Upon full characterization of these conjugates, the occurrence of excited-state charge separation was established by transient pump-probe spectroscopy covering wide temporal and spatial regions. With an increase in pyrazinepyrene entities, an increased rate of forward charge separation was witnessed while rate constants for charge recombination were slower resulting in the final lifetime of charge-separated states of about 40-50 ns, better than that reported earlier for a similar pristine zinc phthalocyanine derived dyad. [4]AcknowledgmentsAuthors acknowledge the support from the Spanish Ministerio de Ciencia e Innovación (MICINN/FEDER, PID2020-117855RB-I00 to ASS) and Generalitat Valenciana (CIPROM/2021/059 and MFA/2022/028 to ASS) and US-National Science Foundation (2000988 to FD). The computational work was completed at the Holland Computing Center of the University of Nebraska, which receives support from the Nebraska Research Initiative.

Promoted Charge Separation and Long-Lived Charge-Separated State in Porphyrin-Viologen Dyad Nanoparticles.

Developing light-harvesting systems with efficient photoinduced charge separation and long-lived charge-separated (CS) state is desirable but still challenging. In this study, we designed a zinc porphyrin photosensitizer covalently linked with viologen (ZnP-V) that can be prepared into nanoparticles in aqueous solution. In DMF solution, the monomeric ZnP-V dyads show no electron transfer between the ZnP and viologen units. In contrast, the ZnP-V nanoparticles in aqueous solution show fast charge separation with a CS state lifetime of up to 4.3 ms. This can be attributed to charge hopping induced by aggregation or distance modification between the donor and acceptor induced by electronic interaction. Nevertheless, the lifetime of the CS state is orders of magnitude longer than for molecular aggregates reported previously. The ZnP-V nanoparticles show enhanced photocatalytic hydrogen production as compared to the ZnP nanoparticles and still hold promise for other applications such as photovoltaic devices and photoredox catalysis.

Observation of Long-Lived Charge-Separated States in Anthraquinone-Phenothiazine Electron Donor-Acceptor Dyads: Transient Optical and Electron Paramagnetic Resonance Spectroscopic Studies.

We prepared a series of phenothiazine (PTZ)-anthraquinone (AQ) electron donor-acceptor dyads to study the relationship between molecular structures and the possibility of charge transfer (CT) and intersystem crossing (ISC). As compared to the previously reported PTZ-AQ dyad with a direct connection of two units via a C-N single bond, the PTZ and AQ units are connected via a p-phenylene or p-biphenylene linker. Conformation restriction is imposed by attaching ortho-methyl groups on the phenylene linker. UV-vis absorption spectra indicate electronic coupling between the PTZ and AQ units in the dyads without conformation restriction. Different from the previously reported PTZ-AQ, thermally activated delayed fluorescence (TADF) is observed for the dyads containing one phenylene linker (PTZ-Ph-AQ and PTZ-PhMe-AQ). The prompt fluorescence lifetime in cyclohexane is exceptionally long (τPF = 62.0 ns, population ratio: 99.2%) and 245.0 ns (93.5%) for PTZ-Ph-AQ and PTZ-PhMe-AQ, respectively (normally τPF <20 ns); the delayed fluorescence lifetimes for these two dyads were determined as τDF = 2.4 μs (6.5%) and 7.6 μs (0.8%), respectively. For the dyad containing a biphenylene linker (PTZ-Ph2Me-AQ), no TADF was observed. Charge-separated (CS) states were observed for PTZ-Ph-AQ and PTZ-PhMe-AQ, and the lifetimes were determined as 7.0 and 1.3 μs, respectively, indicating the triplet spin multiplicity of the CS state. The 3CS state lifetimes are shortened to 100 ns and 440 ns for the two dyads, respectively, in the polar solvent acetonitrile. For dyads with a longer linker, i.e., PTZ-Ph2Me-AQ, the CS state lifetime is not sensitive to solvent polarity (τCS = 1.8 and 1.3 μs in cyclohexane and acetonitrile, respectively). In reference dyads, where the PTZ unit is oxidized to sulfoxide, no CT absorption band and TADF were observed, which is attributed to the increased CS state energy (>3 eV) becoming higher than that of the AQ triplet (3AQ*) state (ca. 2.7 eV). These experimental evidence show that the presence of 1CS, 3CS, and 3LE (LE: locally excited) states sharing similar energy is essential for the occurrence of TADF. Population of the long-lived 3CS state (with a lifetime of a few μs) does not produce by itself TADF, because the ISC process of 1CS→3CS is nonsufficient. Femtosecond transient absorption spectra show that charge separation (CS) occurs readily (<5 ps) for most dyads, even in nonpolar solvents. Nanosecond pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that either a spin correlated radical pair (SCRP) is formed, with the electron exchange energy 2J = +2.14 mT, or radical pairs with stronger interaction, |2J| > 6.57 mT. These studies are useful for in-depth understanding of the CS and ISC in compact electron donor-acceptor dyads and for design of efficient TADF emitters.

Excitation Wavelength-Dependent Charge Stabilization in Highly Interacting Phenothiazine Sulfone-Derived Donor–Acceptor Constructs

Prolonging the lifetime of charge-separated states (CSS) is of paramount importance in artificial photosynthetic donor-acceptor (DA) constructs to build the next generation of light-energy-harvesting devices. This becomes especially important when the DA constructs are closely spaced and highly interacting. In the present study, we demonstrate extending the lifetime of the CSS in highly interacting DA constructs by making use of the triplet excited state of the electron donor and with the help of excitation wavelength selectivity. To demonstrate this, π-conjugated phenothiazine sulfone-based push-pull systems, PTS2-PTS6 have been newly designed and synthesized via the Pd-catalyzed Sonogashira cross-coupling followed by [2 + 2] cycloaddition-retroelectrocyclization reactions. Modulation of the spectral and photophysical properties of phenothiazine sulfones (PTZSO2) and terminal phenothiazines (PTZ) was possible by incorporating powerful electron acceptors, 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (exTCBD). The quadrupolar PTS2 displayed solvatochromism, aggregation-induced emission, and mechanochromic behaviors. From the energy calculations, excitation wavelength-dependent charge stabilization was envisioned in PTS2-PTS6, and the subsequent pump-probe spectroscopic studies revealed charge stabilization when the systems were excited at the locally excited peak positions, while such effect was minimal when the samples were excited at wavelengths corresponding to the CT transitions. This work reveals the impact of wavelength selectivity to induce charge separation from the triplet excited state in ultimately prolonging the lifetime of CCS in highly interacting push-pull systems.

Thousand-Fold Enhancement of Charge-Separated State Lifetimes Caused by an Adamantane Bridge in Dimethylaniline-Anthracene and Dimethylaniline-Pyrene Dyads.

A long-standing challenge in photoinduced electron transfer research is the design of compact donor-acceptor dyads that can generate long-lived charge-separated (CS) states for use as sensitizers in solar energy harvesting. Reports of dyads exhibiting CS state lifetimes in the microsecond time domain are very rare. Herein, we report two compact donor-bridge-acceptor dyads exhibiting lifetimes in the microsecond domain. We employed an adamantane moiety as a bridge, and the lifetimes obtained are nearly 1000-fold larger when compared to those of the same donor-acceptor dyads bridged through C3-alkyl chains. In addition to long-lived CS state decays, slow formation of acceptor triplets was also observed via nanosecond flash photolysis. The long lifetime of the CS state is attributed to the extremely small value of the electronic coupling matrix element for the charge recombination compared to charge separation.

Photoinduced asymmetric charge trapping in a symmetric tetraazapyrene-fused bis(tetrathiafulvalene) conjugate.

In fused donor-acceptor (D-A) ensembles, rapid charge recombination often occurs because the D and A units are spatially close and strongly coupled. To the best of our knowledge, a long-lived charge separated (CS) state is still elusive in such systems. The results presented here show that symmetric annulation of two tetrathiafulvalene (TTF) donors to a central tetraazapyrene (TAP) acceptor via two quinoxaline units leads to a CS state lifetime of a few ns. A detailed study of the electronic interactions between TTF and TAP units in the ground and excited states was performed and compared with the asymmetric counterpart by cyclic voltammetry, optical absorption and ultrafast transient absorption spectroscopy. The results demonstrate that the photoinduced asymmetric charge trapping between two TTFs significantly stabilizes the CS state, which is also verified theoretically.

Self-assembling cobalt(II) porphyrin - fullero [60]pyrrolidine triads. Synthesis and spectral properties in the ground and excited state

Meso‑4-Trifluoromethylphenyl substituted cobalt(II)porphin, CoT(CF3P)P, and its coordination 1: 2 complex (triad) with 1-N-methyl-2-(pyridin-4-yl)-3,4-fullero [60]pyrrolidine, (PyC60)2CoT(CF3P)P, were firstly synthesized and the properties of their molecular solutions were studied. The chemical structure of CoT(CF3P)P and triad was fully confirmed by the UV–vis, IR and 1H NMR spectroscopy and mass spectrometry methods. The triad formation mechanism as the two-step equilibrium characterized by equilibrium constant K1 (5.89 ± 1.50) × 104 M−1 and K2 (7.40 ± 1.66) × 105 M−1 was revealed. Using femtosecond pump-probe spectroscopy measurements, the photoinduced electron transfer in the synthesized triad was described. The excited state lifetime of the precursors, CoTPP, CoT(CF3P)P and the charge separated state lifetime of (PyC60)2CoT(CF3P)P compared with unsubstituted (PyC60)2CoTPP were determined. It was established that the CF3 groups in the porphyrin periphery slightly change the CoTPP/CoT(CF3P)P excited state lifetime and the electron transfer rate constant in the correspondent triads.

Chlorination Enabling a Low-Cost Benzodithiophene-Based Wide-Bandgap Donor Polymer with an Efficiency of over 17.

Three regioregular benzodithiophene-based donor-donor (D-D)-type polymers (PBDTT, PBDTT1Cl, and PBDTT2Cl) are designed, synthesized, and used as donor materials in organic solar cells (OSCs). Because of the weak intramolecular charge-transfer effect, these polymers exhibit large optical bandgaps (>2.0eV). Among these three polymers, PBDTT1Cl exhibits more ordered and closer molecular stacking, and its devices demonstrate higher and more balanced charge mobilities and a longer charge-separated state lifetime. As a result of these comprehensive benefits, PBDTT1Cl-based OSCs give a very impressive power conversion efficiency (PCE) of 17.10% with a low nonradiative energy loss (0.19eV). Moreover, PBDTT1Cl also possesses a low figure-of-merit value and good universality to match with different acceptors. This work provides a simply and efficient strategy to design low-cost high-performance polymer donor materials.

(Invited) Distance Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine-Fullerene Conjugates

The effect of donor-acceptor distance in controlling the rate of electron transfer in axially linked silicon phthalocyanine – C60 dyads has been investigated. Herein we will presented the synthesis and photophysical properties of two C60-SiPc-C60 dyads, 1 and 2, varying in their donor-acceptor distance. In the case of C60-SiPc-C60 1 where the SiPc and C60 are separated by a phenyl spacer, faster electron transfer was observed with k cs equal to 2.7 x 109 s-1 in benzonitrile. However, in the case of C60-SiPc-C60 2, where SiPc and C60 are separated by a biphenyl spacer, a slower electron transfer rate constant, k cs equal to 9.1 x108 s-1 was recorded. The addition of an extra phenyl spacer in 2 increased the donor-acceptor distance by ~ 4.3 Å, and consequently, slowed down the electron transfer rate constant by a factor of ~3.7. The charge separated state lasted over 3 ns, monitoring time window of our femtosecond transient spectrometer. Complimentary nanosecond transient absorption studies revealed formation of 3SiPc* as the end product and suggested that the final lifetime of charge separated state to be in the 3-20 ns range. Energy level diagram established to comprehend these mechanistic details indicated that the comparatively high-energy SiPc .+ -C60 .- charge separated states (1.57 eV) to populate the low-lying 3SiPc* (1.26 eV) prior returning to the ground state. [1].AcknowledgmentsThis work was supported by the Ministerio de Ciencia e Innovación of Spain and FEDER funds (Grants CTQ2017-87102-R and RED2018- 102471-T to ASS) and National Science Foundation (Grant No. 1401188 and 2000988 to FD).References L. Martín-Gomis, S. Seetharaman, D. Herrero, P. A. Karr, F. Fernández-Lázaro, F. D’Souza, and Á. Sastre-Santos, Chem Phys. Chem., 2020, 21, 2254-2262. Figure 1

Photocathodes beyond NiO: charge transfer dynamics in a \u03c0-conjugated polymer functionalized with Ru photosensitizers

A conductive polymer (poly(p-phenylenevinylene), PPV) was covalently modified with RuII complexes to develop an all-polymer photocathode as a conceptual alternative to dye-sensitized NiO, which is the current state-of-the-art photocathode in solar fuels research. Photocathodes require efficient light-induced charge-transfer processes and we investigated these processes within our photocathodes using spectroscopic and spectro-electrochemical techniques. Ultrafast hole-injection dynamics in the polymer were investigated by transient absorption spectroscopy and charge transfer at the electrode–electrolyte interface was examined with chopped-light chronoamperometry. Light-induced hole injection from the photosensitizers into the PPV backbone was observed within 10 ps and the resulting charge-separated state (CSS) recombined within ~ 5 ns. This is comparable to CSS lifetimes of conventional NiO-photocathodes. Chopped-light chronoamperometry indicates enhanced charge-transfer at the electrode–electrolyte interface upon sensitization of the PPV with the RuII complexes and p-type behavior of the photocathode. The results presented here show that the polymer backbone behaves like classical molecularly sensitized NiO photocathodes and operates as a hole accepting semiconductor. This in turn demonstrates the feasibility of all-polymer photocathodes for application in solar energy conversion.

The effect of inner-sphere reorganization on charge separated state lifetimes at sensitized TiO2 interfaces.

Improving the efficiency of photo-electrocatalytic cells depends on controlling the rates of interfacial electron transfer to promote the formation of long-lived charge separated states. Ultimately, for efficient catalytic assemblies to see widespread implementation, repeated electron transfer in the absence of charge recombination needs to be realized. In this study, a series of manganese-based transition metal complexes known to undergo charge transfer-induced spin crossover are employed to study how significant increases in inner-sphere reorganization energy affect the rates of interfacial electron transfer. Each complex is characterized by transient spectroscopic and electrochemical methods to calculate the rate of electron transfer to a model chromophore anchored to the surface of a TiO2 film. Likewise, open-circuit voltage decay measurements were used to determine the voltage-dependent lifetime of injected electrons in TiO2 in the presence of each complex. To further characterize the rates of electronic recombination, density functional theory was used to calculate the inner-sphere and outer-sphere reorganization energy for each complex. These calculations were then combined with classical Marcus theory to determine the theoretical rate of back-electron transfer from the TiO2 conduction band. These results show that, in model complexes, a significant reduction in the recombination rate constant is achieved for complexes possessing a significant inner-sphere reorganization energy.