Long-lived Photoinduced Charge Separation Research Articles

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.

Harnessing Intermolecular Interactions to Promote Long-Lived Photoinduced Charge Separation from Copper Phenanthroline Chromophores

Facilitating photoinduced electron transfer (PET) while minimizing rapid charge-recombination processes to produce a long-lived charge-separated (CS) state represents a primary challenge associated with achieving efficient solar fuel production. Natural photosynthetic systems employ intermolecular interactions to arrange the electron-transfer relay in reaction centers and promote a directional flow of electrons. This work explores a similar tactic through the synthesis and ground- and excited-state characterization of two Cu(I)bis(phenanthroline) chromophores with homoleptic and heteroleptic coordination geometries and which are functionalized with negatively charged sulfonate groups. The addition of sulfonate groups enables solubility in pure water, and it also induces assembly with the dicationic electron acceptor methyl viologen (MV2+) via bimolecular, dynamic electrostatic interactions. The effect of the sulfonate groups on the ground- and excited-state properties was evaluated by comparison with the unsulfonated analogues in 1:1 acetonitrile/water. The excited-state lifetimes for all sulfonated complexes are similar to what we expect from previous literature, with the exception of the sulfonated heteroleptic complex whose metal-to-ligand charge-transfer (MLCT) lifetime in water has two components that are fit to 10 and 77 ns. For the sulfonated complexes, we detected reduced MV+• in both solvent environments following MLCT excitation, but control measurements in 1:1 acetonitrile/water with the unsulfonated analogues showed no PET to MV2+, indicating that electrostatically driven supramolecular assemblies of the sulfonated complexes with MV2+ facilitate the observed PET. Additionally, the strength of the intermolecular interactions driving the formation of these assemblies changes drastically with the solvent environment. In 1:1 acetonitrile/water, PET occurred from both sulfonated complexes with quantum yields (ΦET) of 2-3% but increased to a remarkable 98% for the sulfonated heteroleptic complex with a 3 μs CS-state lifetime in water.

(Invited) Organic/Inorganic Hybrid Interfaces between Perovskite Semiconductors and Semiconducting SWCNTs

Low-dimensional semiconductors provide a rich tapestry of tunable optical and electrical properties for a wide variety of 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. While organic energy harvesting devices often employ interfaces between distinct organic species such as polymers, fullerenes, and carbon nanotubes, hybrid interfaces between organic and inorganic semiconductors have unique properties that are relatively unexplored.In this talk, I will discuss our recent efforts at constructing novel hybrid interfaces between s-SWCNTs and low-dimensional perovskite-based semiconductors. I will highlight time-resolved spectroscopy results that illustrate long-lived photoinduced charge separation across such interfaces, demonstrating utility for photovoltaics and photodetectors. Interestingly, some systems demonstrate additional transient processes that are exceptionally slow and may indicate ion movement within the hybrid systems. Complementary device studies demonstrate exceptional efficiencies for conversion of photons into stable photocurrent, and I will highlight several novel applications in which these interfaces can be employed.

Anionic Fluorinated Zn-porphyrin Combined with Cationic Endohedral Li-fullerene for Long-Lived Photoinduced Charge Separation with Low Energy Loss.

Here we report an anionic meso-tetrakis(4-carboxymethylthio-2,3,5,6-tetrafluorophenyl) zinc porphyrin (ZnTF4PPTC4-) to form a supramolecular complex with a cationic lithium endohedral [60]fullerene (Li+@C60). The supramolecular ZnTF4PPTC4-/Li+@C60 complex formed by strong electrostatic attraction with a large binding constant generates a long-lived charge-separated (CS) state with low energy loss by photoinduced electron transfer from ZnTF4PPTC4- to Li+@C60. The anionic fluorinated zinc porphyrin with high oxidation potential reduces the energy loss associated with the charge separation and enhances the energy level of the CS state. The energy level of the CS state determined by electrochemical measurements is at 0.94 eV, which is much higher than that of a similar supramolecular complex using an anionic meso-tetrakis(sulfonatophenyl) zinc porphyrin (ZnTPPS4-) at 0.55 eV. Time-resolved transient absorption spectroscopy demonstrates that ZnTF4PPTC4-/Li+@C60 generates a long-lived CS state with a lifetime of 0.29 ms in a binary solvent of acetonitrile and chlorobenzene. The lifetime of the CS state is comparable to that of ZnTPPS4-/Li+@C60 in benzonitrile.

Generation of Long-Lived Photoinduced Charge Separation in a Supramolecular Toroidal Assembly.

Efficiencies of artificial photosynthetic and photocatalytic systems depend on their ability to generate long-lived charge-separated (CS) states in photoinduced electron transfer (PET) reactions. PET, in most cases, is followed by an ultrafast back electron transfer, which severely reduces lifetime and quantum yield of CS states. Generation of a long-lived CS state is an important goal in the study of PET reactions. Herein, we report that this goal is achieved using a hierarchically self-assembled anthracene-methyl viologen donor-acceptor system. Anthracene linked to two β-cyclodextrin molecules (CD-AN-CD) and methyl viologen linked to two adamantane units (AD-MV2+-AD) form an inclusion complex in water, which further self-assembled into well-defined toroidal nanostructures. The fluorescence of anthracene is highly quenched in the self-assembled system because of PET from anthracene to methyl viologen. Irradiation of the aqueous toroidal solution led to formation of a long-lived CS state. Rational mechanisms for the formation of the toroidal nanostructures and long-lived photoinduced charge separation are presented in the paper.

(Invited) Organic/Inorganic Hybrid Interfaces between Perovskite Semiconductors and Semiconducting Swcnts for Energy Harvesting and Conversion

Low-dimensional semiconductors provide a rich tapestry of tunable optical and electrical properties for a wide variety of 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. While organic energy harvesting devices often employ interfaces between distinct organic species such as polymers, fullerenes, and carbon nanotubes, hybrid interfaces between organic and inorganic semiconductors have unique properties that are relatively unexplored.In this talk, I will discuss our recent efforts at constructing novel hybrid interfaces between s-SWCNTs and low-dimensional perovskite-based semiconductors. I will highlight time-resolved spectroscopy results that illustrate long-lived photoinduced charge separation across such interfaces, demonstrating utility for photovoltaics and photodetectors. Interestingly, some systems demonstrate additional transient processes that are exceptionally slow and may indicate ion movement within the hybrid systems. Complementary device studies demonstrate exceptional efficiencies for conversion of photons into stable photocurrent, and I will highlight several novel applications in which these interfaces can be employed.

(Invited) Organic/Inorganic Hybrid Interfaces with Swcnts for Energy Harvesting and Conversion

Low-dimensional semiconductors provide a rich tapestry of tunable optical and electrical properties for a wide variety of 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. While organic energy harvesting devices often employ interfaces between distinct organic species such as polymers, fullerenes, and carbon nanotubes, hybrid interfaces between organic and inorganic semiconductors have unique properties that are relatively unexplored. In this talk, I will discuss our recent efforts at constructing novel hybrid interfaces between s-SWCNTs and low-dimensional inorganic semiconductors. I will highlight time-resolved spectroscopy results that illustrate exceptionally long-lived photoinduced charge separation across such interfaces. These studies provide insight into some of the remarkable properties of these interfaces that can enable efficient solar energy harvesting and conversion to fuels.

Long-Lived Photoinduced Charge Separation in a Trinuclear Iron-μ3-oxo-based Metal–Organic Framework

The presence of long-lived charge-separated excited states in metal–organic frameworks (MOFs) can enhance their photocatalytic activity by decreasing the probability that photogenerated electrons and holes recombine before accessing adsorbed reactants. Detecting these charge-separated states via optical transient absorption, however, can be challenging when they lack definitive optical signatures. We investigate the long-lived excited state of a MOF with such vague optical properties, MIL-100(Fe), composed of Fe3-μ3-oxo clusters and trimesic acid linkers, using Fe K-edge X-ray transient absorption (XTA) spectroscopy to unambiguously determine its ligand-to-metal charge-transfer character. Spectra measured at time delays up to 3.6 μs confirm the long-lived nature of the charge-separated excited state. Several trinuclear iron μ3-oxo carboxylate complexes, which model the trinuclear cores of the MOF structure, are measured for comparison using both steady-state X-ray absorption spectroscopy and XTA to furthe...

Self-Assembled Ruthenium(II)Porphyrin-Aluminium(III)Porphyrin-Fullerene Triad for Long-Lived Photoinduced Charge Separation.

A very efficient metal-mediated strategy led, in a single step, to a quantitative construction of a new three-component multichromophoric system containing one fullerene monoadduct, one aluminium(III) monopyridylporphyrin, and one ruthenium(II) tetraphenylporphyrin. The Al(III) monopyridylporphyrin component plays the pivotal role in directing the correct self-assembly process and behaves as the antenna unit for the photoinduced processes of interest. A detailed study of the photophysical behavior of the triad was carried out in different solvents (CH2Cl2, THF, and toluene) by stationary and time-resolved emission and absorption spectroscopy in the pico- and nanosecond time domains. Following excitation of the Al-porphyrin, the strong fluorescence typical of this unit was strongly quenched. The time-resolved absorption experiments provided evidence for the occurrence of stepwise photoinduced electron and hole transfer processes, leading to a charge-separated state with reduced fullerene acceptor and oxidized ruthenium porphyrin donor. The time constant values measured in CH2Cl2 for the formation of charge-separated state Ru-Al+-C60- (10 ps), the charge shift process (Ru-Al+-C60- → Ru+-Al-C60-), where a hole is transferred from Al-based to Ru-based unit (75 ps), and the charge recombination process to ground state (>5 ns), can be rationalized within the Marcus theory. Although the charge-separating performance of this triad is not outstanding, this study demonstrates that, using the self-assembling strategy, improvements can be obtained by appropriate chemical modifications of the individual molecular components.

Long-Lived Photoinduced Charge Separation in Inclusion Complexes Composed of a Phenothiazine-Bridged Cyclic Porphyrin Dimer and Fullerenes

C60, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), lithium-cation-encapsulated C60 (Li+@C60), and [6,6]-diphenyl-C62-bis(butyric acid methyl ester) (bis-PCBM) were included into a phenothiazine-bridged cyclic free-base porphyrin dimer (H4-Ptz-CPDPy(TEO)) in a polar solvent (benzonitrile) with large association constants of 1.3 × 106, 6.4 × 105, 3.2 × 106, and 2.5 × 105 M–1, respectively. Based on the electrochemical data, the lowest energy levels of the charge-separated (CS) states for the inclusion complexes of H4-Ptz-CPDPy(TEO) with C60, PCBM, Li+@C60, and bis-PCBM (designated as C60⊂H4-Ptz-CPDPy(TEO), PCBM⊂H4-Ptz-CPDPy(TEO), Li+@C60⊂H4-Ptz-CPDPy(TEO), and bis-PCBM⊂H4-Ptz-CPDPy(TEO)) composed of the phenothiazine donor and fullerene acceptors were determined to be 1.30, 1.40, 0.66, and 1.51 eV, respectively. Both C60⊂H4-Ptz-CPDPy(TEO) and PCBM⊂H4-Ptz-CPDPy(TEO) underwent electron transfer upon photoexcitation of the porphyrin and fullerene chromophores, and the resultant photoinduced CS states comp...

Long-lived photoinduced charge separation for solar cell applications in supramolecular complexes of multi-metalloporphyrins and fullerenes

Monomers, dimers, trimers, dendrimers and oligomers of metalloporphyrins form supramolecular complexes with fullerene derivatives via electrostatic interactions, π-π interactions and coordination bonds. Photoexcitation of the supramolecular complexes resulted in photoinduced electron transfer from the porphyrin moiety to the fullerene moiety to produce the charge-separated states as revealed by laser flash photolysis measurements. The rate constants of photoinduced charge separation and charge recombination in supramolecular complexes of multi-metalloporphyrins and fullerenes were also determined by laser flash photolysis measurements and the results depending on the number of porphyrins in the supramolecular complexes are discussed in terms of efficiency of photoinduced energy transfer and charge separation as well as the lifetimes of charge-separated states. The photoelectrochemical performances of solar cells composed of supramolecular complexes of monomers, dimers, dendrimers and oligomers of metalloporphyrins with fullerenes are compared in relation to the rate constants of photoinduced charge separation and charge recombination.

Charge generation and energy transfer in hybrid polymer/infrared quantum dot solar cells

Conjugated polymers blended with nanocrystal quantum dots are interesting as solution processable active layers for infrared light harvesting in thin film solar cells. We study photocurrent generation processes in hybrid polymer/quantum dot photovoltaics by comparing device performance and photoinduced absorption (PIA) spectra across blends of three different conjugated polymers, poly(2,3-bis(2-(hexyldecyl)-quinoxaline-5,8-diyl-alt-N-(2-hexyldecyl)-dithieno[3,2-b:2′,3′-d]pyrrole) (PDTPQx-HD), poly[(4,4′-bis(3-(2-hexyl-decyl)dithieno[3,2-b:2′,3′-d]pyrrole)-2,6-diyl-alt-(2,5-bis(3-(2-ethyl-hexyl)thiophen-2yl)thiazolo[5,4-d]thiazole)] (PPEHTT), and poly[(4,4′-bis(2-octyl)dithieno[3,2-b:2′3′-d]silole)-2,6-diyl-alt-(2,5-bis(3-octylthiophen-2yl)thiazolo[5,4-d]thiazole)] (PSOTT) with PbS quantum dots. The PIA spectra and device performance provide evidence for long-lived photoinduced charge separation and bulk heterojunction device operation for blends of both PDTPQx-HD and PPEHTT with PbS. In contrast we find that PSOTT/PbS blends can produce viable solar cells without any evidence for long-lived charge transfer in the PIA spectra. Even so, the external quantum efficiency (EQE) spectra of PSOTT/PbS solar cells indicate that the polymer plays a significant role in light harvesting. We use photoluminescence excitation spectroscopy to confirm that the polymer funnels energy to the PbS quantum dots via energy transfer, and speculate that these blends may operate as PbS Schottky diodes sensitized by energy transfer from the semiconducting polymer host.

Chemical Control of Photoinduced Charges under Confinement in Zeolites

The organized internal porous void of dehydrated zeolites provides a suitable environment to promote long-lived photoinduced charge separation. Herein we have conducted time-resolved UV–visible absorption spectroscopy experiments from nanosecond to day time scale following nanosecond UV (266 nm) pulsed laser irradiation of trans-stilbene (t-St) occluded in channels of nonacidic M–FER, M–MFI, and M–MOR zeolites with various pore diameters, with differing framework aluminum content, and with different extraframework cations (M = Na+, K+, Rb+, and Cs+). The cation radical of trans-stilbene (t-St•+) and trapped electron (AlO4•–) have been generated directly by means of laser-induced electron transfer within the channels of medium pore zeolites. We have highlighted that the general back electron transfer processes include direct charge recombination (CR), hole transfer (HT), and finally electron–hole recombination to re-form the occluded t-St ground state without any isomerization or oligomerization. It was de...

Strong supramolecular binding of Li+@C60 with sulfonated meso-tetraphenylporphyrins and long-lived photoinduced charge separation

A supramolecular binding occurred between lithium ion encapsulated [60]fullerene (Li(+)@C(60)) and sulfonated tetraphenylporphyrins ([MTPPS](4-) M = H(2) and Zn) in a benzonitrile solution. Photoexcitation of Li(+)@C(60)/[MTPPS](4-) results in formation of a long-lived charge-separated state by photoinduced electron transfer.

-Quantum-Dot Sensitized Metal Oxide Photoelectrodes: Photoelectrochemistry and Photoinduced Absorption Spectroscopy

TiO2, ZnO nanoparticulate(-np), and ZnO-nanorod(-nr) electrodes have been modified with FeS2 (pyrite) nanoparticles. Quantum size effect is manifested by a blue shift in both absorption and photocurrent action spectra. PIA (photoinduced absorption spectroscopy), a multipurpose tool in the study of dye-sensitized solar cells, is used to study quantum-dot modified metal oxide (MO) nanostructured electrodes. The PIA spectra showed an evidence for long-lived photoinduced charge separation. Time-resolved PIA showed that recombination between electrons and holes occurs on a millisecond timescale. Incident-photon-to-current efficiencies at 400 nm are ranged between 13% and 25%. The better solar cell performance of FeS2 on ZnO-nr over ZnO-np can be ascribed to the faster, unidirectional e-transport channels through the ZnO-nr as well as the longer electron lifetimes. The lower performances of electrodes can be explained by the presence of FeS2 phases other than the photoactive pyrite phase, as evidenced from XRD study.

Comparative Study between Dye-Sensitized and CdS Quantum-Dots-Sensitized TiO2 Solar Cells Using Photoinduced Absorption Spectroscopy

Two 8 μm thick TiO2 photoelectrodes have been sensitized separately by N719 dye molecules and CdS quantum dots for a comparison study. Photoinduced absorption (PIA) spectroscopy was employed to investigate the mechanistic properties of electrons under illumination conditions comparable to sunlight. The PIA spectrum of both electrodes (in the presence of electrolyte) is due to electrons in TiO2 and iodine radicals in the electrolyte. In the absence of redox electrolyte, both electrodes show long-lived photoinduced charge-separation with lifetime in a millisecond range (8.5 ms for Q-dot-sensitized TiO2 and 11.5 ms for dye-sensitized TiO2).

FeS2 quantum dots sensitized nanostructured TiO2 solar cell: photoelectrochemical and photoinduced absorption spectroscopy studies

Thin films of nanostructured TiO2 have been modified with FeS2 (pyrite) nano-particles by a low temperature chemical reaction of iron pentacarbonyl with sulfur in xylene. Quantum size effects are manifested by the observation of a blue shift in both absorption and photocurrent action spectra. PIA (Photoinduced absorption spectroscopy), where the excitation is provided by a square-wave modulated (on/off) monochromatic light emitting diode, is a multipurpose tool in the study of dye-sensitized solar cells. Here, PIA is used to study quantum-dot modified TiO2 nanostructured electrodes. The PIA spectra obtained give evidence for long-lived photoinduced charge separation: electrons are injected into the metal oxide and holes are left behind in the FeS2 quantum dot. Time-resolved PIA shows that recombination between electrons and holes occurs on a millisecond timescale. The Incident-Photon-to-Current Efficiency of about 23 % was obtained at 400 nm excitation. The performances of TiO2 electrodes modified with FeS2 are relatively low, which is explained by the presence of FeS2 phases other than the photoactive pyrite phase, as follows from the XRD spectrum.

Long-Lived Photoinduced Charge Separation Due to the Inverted Region Effect in 1,6-Bis(phenylethynyl)pyrene−Phenothiazine Dyad

Photoinduced electron-transfer processes in a 1,6-bis(phenylethynyl)pyrene−phenothaiazine dyad, BPEP−PT, are examined using various techniques. The BPEP singlet excited state is quenched by electron transfer from PT leading to formation of BPEP radical anion and PT radical cation. Rate constants and quantum yields of the PET processes were determined from steady-state and time-resolved fluorescence experiments, and spectroscopic identification of the radical ion products was achieved using picosecond and nanosecond flash photolysis experiments. The charge-separated (CS) state was found to be long-lived but decayed to the BPEP triplet state under the influence of external heavy atom effect. The energy level diagram constructed on the basis of experimental data revealed the existence of the local triplet state below the CS state, yet the CS state did not exhibit any tendency to decay to this level. This showed that hyperfine interaction (HFI) or spin−orbit charge-transfer intersystem crossing (SOCT-ISC) mec...

Mesostructured organosilica with a 9-mesityl-10-methylacridinium bridging unit: photoinduced charge separation in the organosilica framework

Polycondensation of 9-mesityl-10-methylacridinium-bridged organosilane in the presence of a nonionic surfactant yielded a mesostructured organosilica solid with a functional framework that exhibited long-lived photoinduced charge separation.

Ordered and Oriented Supramolecular n/p-Heterojunction Surface Architectures: Completion of the Primary Color Collection

In this study, we describe synthesis, characterization, and zipper assembly of yellow p-oligophenyl naphthalenediimide (POP-NDI) donor-acceptor hybrids. Moreover, we disclose, for the first time, results from the functional comparison of zipper and layer-by-layer (LBL) assembly as well as quartz crystal microbalance (QCM), atomic force microscopy (AFM), and molecular modeling data on zipper assembly. Compared to the previously reported blue and red NDIs, yellow NDIs are more pi-acidic, easier to reduce, and harder to oxidize. The optoelectronic matching achieved in yellow POP-NDIs is reflected in quantitative and long-lived photoinduced charge separation, comparable to their red and much better than their blue counterparts. The direct comparison of zipper and LBL assemblies reveals that yellow zippers generate more photocurrent than blue zippers as well as LBL photosystems. Continuing linear growth found in QCM measurements demonstrates that photocurrent saturation at the critical assembly thickness occurs because more charges start to recombine before reaching the electrodes and not because of discontinued assembly. The found characteristics, such as significant critical thickness, strong photocurrents, large fill factors, and, according to AFM images, smooth surfaces, are important for optoelectronic performance and support the existence of highly ordered architectures.