Photosensitized electron transfer processes of nanocarbons applicable to solar cells

Excited Charge Transfer Promoted Electron Transfer in all Perylenediimide Derived, Wide-Band Capturing Conjugates: A Mimicry of the Early Events of Natural Photosynthesis

Fundamental discoveries in electron transfer advance scientific and technological advancements. It is suggested that in plant and bacterial photosynthesis, the primary donor, a chlorophyll or bacteriochlorophyll dimer, forms an initial excited symmetry-breaking charge transfer state (1CT*) upon photoexcitation that subsequently promotes sequential electron transfer (ET) events. This is unlike monomeric photosensitizer-bearing donor-acceptor dyads where ET occurs from the excited donor or acceptor (1D* or 1A*). In the present study, we successfully demonstrated the former photochemical event using an excited charge transfer molecule as a donor. Electron-deficient perylenediimide (PDI) is functionalized with three electron-rich piperidine entities at the bay positions, resulting in a far-red emitting CT molecule (DCT). Further, this molecule is covalently linked to another PDI (APDI) carrying no substituents at the bay positions, resulting in wide-band capturing DCT-APDI conjugates. Selective excitation of the CT band of DCT in these conjugates leads to an initial 1DCT* that undergoes subsequent ET involving APDI, resulting in DCT +-APDI - charge separation product (kCS~109 s-1). Conversely, when APDI was directly excited, ultrafast energy transfer (ENT) from 1APDI* to DCT (kENT~1011 s-1) followed by ET from 1DCT* to PDI is witnessed. While increasing solvent polarity improved kCS rates, for a given solvent, the magnitude of the kCS values was almost the same, irrespective of the excitation wavelengths. The present findings demonstrate ET from an initial CT state to an acceptor is key to understanding the intricate ET events in complex natural and bacterial photosynthetic systems possessing multiple redox- and photoactive entities.

Fabrication and Photovoltaic Conversion Performances of Imidazolyl and Fumaric Acid Composite Langmuir-Blodgett Membranes

Novel organic small molecule structures have received increasing attention in the preparation of multifunctional thin film materials and have become the subject of research in many cutting-edge directions. In this work, imidazolyl and transbutylene glycolic acid molecules and dye molecules were designed and prepared as composite films by supramolecular self-assembly in the LB technique, and their morphological features and spectral properties were analyzed. The results showed that the prepared LB films presented different aggregation states. In addition, their photoelectrochemical properties, on ITO sheets were tested, all of which showed good optoelectronic properties, and their binding energies, structure optimization, and electrostatic potentials were theoretically calculated by DFT simulations, which proved that the prepared films have good optoelectronic properties, and have the potential to become optoelectronic multifunctional ultrathin film devices.

Photoinduced Energy and Electron Transfer in a 'Two-Point' Bound Panchromatic, Near-Infrared-Absorbing Bis-styrylBODIPY(Zinc Porphyrin)2 - Fullerene Self-Assembled Supramolecular Conjugate

Structurally well-defined self-assembled supramolecular multi-modular donor-acceptor conjugates play a significant role in furthering our understanding of photoinduced energy and electron transfer events occurring in nature, e. g., in the antenna-reaction centers of photosynthesis and their applications in light energy harvesting. However, building such multi-modular systems capable of mimicking the early events of photosynthesis has been synthetically challenging, causing a major hurdle for its growth. Often, multi-modularity is brought in by combining both covalent and noncovalent approaches. In the present study, we have developed such an approach wherein a π-extended conjugated molecular cleft, two zinc(II)porphyrin bearing bisstyrylBODIPY (dyad, 1), has been synthesized. The binding of 1 via a 'two-point' metal-ligand coordination of a bis-pyridyl fulleropyrrolidine (2), forming a stable self-assembled supramolecular complex (1 : 2), has been established. The self-assembled supramolecular complex has been fully characterized by a suite of physico-chemical methods, including TD-DFT studies. From the established energy diagram, both energy and electron transfer events were envisioned. In dyad 1, selective excitation of zinc(II)porphyrin leads to efficient singlet-singlet excitation transfer to (bisstyrly)BODIPY with an energy transfer rate constant, kEnT of 2.56×1012 s-1. In complex 1 : 2, photoexcitation of zinc(II)porphyrin results in ultrafast photoinduced electron transfer with a charge separation rate constant, kCS of 2.83×1011 s-1, and a charge recombination rate constant, kCR of 2.51×109 s-1. For excitation at 730 nm corresponding to bisstyrylBODIPY, similar results are obtained, where a biexponential decay yielded estimated values of kCS 3.44×1011 s-1 and 2.97×1010 s-1, and a kCR value of 2.10×1010 s-1. The newly built self-assembled supramolecular complex has been shown to successfully mimic the early events of the photosynthetic antenna-reaction center events.

Excited Charge Separation in a π-Interacting Phenothiazine-Zinc Porphyrin-Fullerene Donor-Acceptor Conjugate

We have designed, synthesized, and characterized a donor-acceptor triad, SPS-PPY-C60, that consists of a π-interacting phenothiazine-linked porphyrin as a donor and sensitizer and fullerene as an acceptor to seek charge separation upon photoexcitation. The optical absorption spectrum revealed red-shifted Soret and Q-bands of porphyrin due to charge transfer-type interactions involving the two ethynyl bridges carrying electron-rich and electron-poor substituents. The redox properties suggested that the phenothiazine-porphyrin part of the molecule is easier to oxidize and the fullerene part is easier to reduce. DFT calculations supported the redox properties wherein the electron density of the highest molecular orbital (HOMO) was distributed over the donor phenothiazine-porphyrin entity while the lowest unoccupied molecular orbital (LUMO) was distributed over the fullerene acceptor. TD-DFT studies suggested the involvement of both the S2 and S1 states in the charge transfer process. The steady-state emission spectrum, when excited either at porphyrin Soret or visible band absorption maxima, revealed quenched emission both in nonpolar and polar solvents, suggesting the occurrence of excited state events. Finally, femtosecond transient absorption spectral studies were performed to witness the charge separation by utilizing solvents of different polarities. The transient data was further analyzed by GloTarAn by fitting the data with appropriate models to describe photochemical events. From this, the average lifetime of the charge-separated state calculated was found to be 169 ps in benzonitrile, 319 ps in dichlorobenzene, 1.7 ns in toluene for Soret band excitation, and ∼320 ps for Q-band excitation in benzonitrile.

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.

Switchover from singlet oxygen to superoxide radical through a photoinduced two-step sequential energy transfer process

The competitive nature of type II photosensitizers in the transfer of excitation energy for the generation of singlet oxygen (1O2) presents significant challenges in the design of type I photosensitizers to produce the superoxide anion radical (O2˙-). In this study, we present an efficient method for the direct transformation of type II photosensitizers into type I photosensitizers through the implementation of an artificial light-harvesting system (ALHSs) involving a two-step sequential energy transfer process. The designed supramolecular complex (DNPY-SBE-β-CD) not only has the ability to generate 1O2 as type II photosensitizers, but also demonstrates remarkable fluorescence properties in aqueous solution, which renders it an efficient energy donor for the development of type I photosensitizers ALHSs, thereby enabling the efficient generation of O2˙-. Meanwhile, to ascertain the capability and practicality of this method, two organic reactions were conducted, namely the photooxidation reaction of thioanisole and oxidative hydroxylation of arylboronic acids, both of which display a high level of efficiency and exhibit significant catalytic performance. This work provides an efficient method for turning type II photosensitizers into type I photosensitizers by a two-step sequential energy transfer procedure.

Zinc Tetrapyrrole Coordinated to Imidazole Functionalized Tetracyanobutadiene or Cyclohexa-2,5-diene-1,4-diylidene-expanded-tetracyanobutadiene Conjugates: Dark vs. Light-Induced Electron Transfer

Using the popular metal-ligand axial coordination self-assembly approach, donor-acceptor conjugates have been constructed using zinc tetrapyrroles (porphyrin (ZnP), phthalocyanine (ZnPc), and naphthalocyanine (ZnNc)) as electron donors and imidazole functionalized tetracyanobutadiene (Im-TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded-tetracyanobutadiene (Im-DCNQ) as electron acceptors. The newly formed donor-acceptor conjugates were fully characterized by a suite of physicochemical methods, including absorption and emission, electrochemistry, and computational methods. The measured binding constants for the 1 : 1 complexes were in the order of 104 -105  M-1 in o-dichlorobenzene. Free-energy calculations and the energy level diagrams revealed the high exergonicity for the excited state electron transfer reactions. However, in the case of the ZnNc:Im-DCNQ complex, owing to the facile oxidation of ZnNc and facile reduction of Im-DCNQ, slow electron transfer was witnessed in the dark without the aid of light. Systematic transient pump-probe studies were performed to secure evidence of excited state charge separation and gather their kinetic parameters. The rate of charge separation was as high as 1011  s-1 suggesting efficient processes. These findings show that the present self-assembly approach could be utilized to build donor-acceptor constructs with powerful electron acceptors, TCBD and DCNQ, to witness ground and excited state charge transfer, fundamental events required in energy harvesting, and building optoelectronic devices.

Recent update on the electroactive oligopyrrolic macrocyclic hosts with a Bucky-ball heart

Supramolecular chemistry is a multidisciplinary research area mostly associated with the investigation of host-guest interactions within intricate three-dimensional (3D) molecular architectures held together reversibly by various non-covalent interactions. Continuous efforts to develop such kinds of complex host-guest systems with designer oligopyrrolic macrocyclic receptors are a rapidly growing research domain, which is deeply involved in applied supramolecular chemistry research. These host-guest supramolecular complexes can be constructed by combining suitable electron-rich oligopyrrolic donors (as a host) with complementary electron-poor guests (as acceptors), held together by the ionic force of attraction triggered by intermolecular charge/electron transfer (CT/ET) transitions. Some of these resulting CT/ET ensembles are potential candidates for the construction of efficient optoelectronic materials, optical sensors, molecular switches, etc. In this Feature Article we aim to focus on these supramolecular ensembles composed by size and shape complementary electroactive oligopyrrolic molecular containers, which are suitable for spherical guest (e.g., buckminsterfullerene) complexation. We also provide a "state-of-the-art" overview on plausible applications of these particular host-guest systems. Our aim is to cover only specific electron-rich tetrathiafulvalene (TTF)-based oligopyrrolic receptors, e.g., TTF-calix[4]pyrroles, TTF-cryptands, TTF-porphyrins and exTTF-porphyrin-based molecular motifs reported to date, along with a brief outlining of their "functional behaviour" in materials chemistry research.

Panchromatic Light-Capturing Bis-styryl BODIPY-Perylenediimide Donor-Acceptor Constructs: Occurrence of Sequential Energy Transfer Followed by Electron Transfer

Two wide-band-capturing donor-acceptor conjugates featuring bis-styrylBODIPY and perylenediimide (PDI) have been newly synthesized, and the occurrence of ultrafast excitation transfer from the 1 PDI* to BODIPY, and a subsequent electron transfer from the 1 BODIPY* to PDI have been demonstrated. Optical absorption studies revealed panchromatic light capture but offered no evidence of ground-state interactions between the donor and acceptor entities. Steady-state fluorescence and excitation spectral recordings provided evidence of singlet-singlet energy transfer in these dyads, and quenched fluorescence of bis-styrylBODIPY emission in the dyads suggested additional photo-events. The facile oxidation of bis-styrylBODIPY and facile reduction of PDI, establishing their relative roles of electron donor and acceptor, were borne out by electrochemical studies. The electrostatic potential surfaces of the S1 and S2 states, derived from time-dependent DFT calculations, supported excited charge transfer in these dyads. Spectro-electrochemical studies on one-electron-oxidized and one-electron-reduced dyads and the monomeric precursor compounds were also performed in a thin-layer optical cell under corresponding applied potentials. From this study, both bis-styrylBODIPY⋅+ and PDI⋅- could be spectrally characterizes and were subsequently used in characterizing the electron-transfer products. Finally, pump-probe spectral studies were performed in dichlorobenzene under selective PDI and bis-styrylBODIPY excitation to secure energy and electron-transfer evidence. The measured rate constants for energy transfer, kENT , were in the range of 1011  s-1 , while the electron transfer rate constants, kET , were in the range of 1010  s-1 , thus highlighting their potential use in solar energy harvesting and optoelectronic applications.

Far-Red Excitation Induced Electron Transfer in Bis Donor-AzaBODIPY Push-Pull Systems; Role of Nitrogenous Donors in Promoting Charge Separation

A far-red absorbing sensitizer, BF2 -chelated azadipyrromethane (azaBODIPY) has been employed as an electron acceptor to synthesize a series of push-pull systems linked with different nitrogenous electron donors, viz., N,N-dimethylaniline (NND), triphenylamine (TPA), and phenothiazine (PTZ) via an acetylene linker. The structural integrity of the newly synthesized push-pull systems was established by spectroscopic, electrochemical, spectroelectrochemical, and DFT computational methods. Cyclic and differential pulse voltammetry studies revealed different redox states and helped in the estimation of the energies of the charge-separated states. Further, spectroelectrochemical studies performed in a thin-layer optical cell revealed diagnostic peaks of azaBODIPY⋅- in the visible and near-IR regions. Free-energy calculations revealed the charge separation from one of the covalently linked donors to the 1 azaBODIPY* to yield Donor⋅+ -azaBODIPY⋅- to be energetically favorable in a polar solvent, benzonitrile, and the frontier orbitals generated on the optimized structures helped in assessing such a conclusion. Consequently, the steady-state emission studies revealed quenching of the azaBODIPY fluorescence in all of the investigated push-pull systems in benzonitrile and to a lesser extent in mildly polar dichlorobenzene, and nonpolar toluene. The femtosecond pump-probe studies revealed the occurrence of excited charge transfer (CT) in nonpolar toluene while a complete charge separation (CS) for all three push-pull systems in polar benzonitrile. The CT/CS products populated the low-lying 3 azaBODIPY* prior to returning to the ground state. Global target (GloTarAn) analysis of the transient data revealed the lifetime of the final charge-separated states (CSS) to be 195 ps for NND-derived, 50 ps for TPA-derived, and 85 ps for PTZ-derived push-pull systems in benzonitrile.

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.

Non-negligible roles of charge transfer excitons in ultrafast excitation energy transfer dynamics of a double-walled carbon nanotube

Herein, we employed a developed linear response time dependent density functional theory-based nonadiabatic dynamics simulation method that explicitly takes into account the excitonic effects to investigate photoinduced excitation energy transfer dynamics of a double-walled carbon nanotube (CNT) model with different excitation energies. The E₁₁ excitation of the outer CNT will generate a local excitation (LE) |out*〉 exciton due to its low energy, which does not induce any charge separation. In contrast, the E₁₁ excitation of the inner CNT can generate four kinds of excitons with the LE exciton |in*〉 dominates. In the 500-fs dynamics simulation, the LE exciton |in*〉 and charge transfer (CT) excitons |out^-in⁺〉 and |out⁺in^-〉 are all gradually converted to the |out*〉 exciton, corresponding to a photoinduced excitation energy transfer, which is consistent with experimental studies. Finally, when the excitation energy is close to the E₂₂ state of the outer CNT (∼1.05 eV), a mixed population of different excitons, with the |out*〉 exciton dominated, is generated. Then, photoinduced energy transfer from the outer to inner CNTs occurs in the first 50 fs, which is followed by an inner to outer excitation energy transfer that is completed in 400 fs. The present work not only sheds important light on the mechanistic details of wavelength-dependent excitation energy transfer of a double-walled CNT model but also demonstrates the roles and importance of CT excitons in photoinduced excitation energy transfer. It also emphasized that explicitly including the excitonic effects in electronic structure calculations and nonadiabatic dynamics simulations is significant for correct understanding/rational design of optoelectronic properties of periodically extended systems.

The Hunter Falls Prey: Photoinduced Oxidation of C60 in Inclusion Complex with Perfluorocycloparaphenylene

Perfluorocycloparaphenylenes (PFCPPs) are cycloparaphenylenes (CPPs) in which all hydrogen atoms have been replaced by fluorine atoms. Like CPPs, PFCPPs are highly strained, hoop-shaped π-conjugated molecules. In this article, we report a computational modeling of photoinduced electron transfer processes in the inclusion complex of PF[10]CPP with C60 fullerene. Its unique feature is the favorable electron transfer from C60 to the host molecule. The photooxidation of C60 is predicted to occur on a sub-nanosecond timescale. The PF[10]CPP⊃C60 dyad is the first nanoring-fullerene complex in which C60 acts as an electron donor in the photoinduced charge separation.

Formation and Photoinduced Electron Transfer in Porphyrin- and Phthalocyanine-Bearing N-Doped Graphene Hybrids Synthesized by Click Chemistry

Graphene doped with heteroatoms such as nitrogen, boron, and phosphorous by replacing some of the skeletal carbon atoms is emerging as an important class of two-dimensional materials as it offers the much-needed bandgap for optoelectronic applications and provides better access for chemical functionalization at the heteroatom sites. Covalent grafting of photosensitizers onto such doped graphenes makes them extremely useful for light-induced applications. Herein, we report the covalent functionalization of N-doped graphene (NG) with two well-known electron donor photosensitizers, namely, zinc porphyrin (ZnP) and zinc phthalocyanine (ZnPc), using the simple click chemistry approach. Covalent attachment of ZnP and ZnPc at the N-sites of NG in NG-ZnP and NG-ZnPc hybrids was confirmed by using a range of spectroscopic, thermogravimetric and imaging techniques. Ground- and excited-state interactions in NG-ZnP and NG-ZnPc were monitored by using spectral and electrochemical techniques. Efficient quenching of photosensitizer fluorescence in these hybrids was observed, and the relatively easier oxidations of ZnP and ZnPc supported excited-state charge-separation events. Photoinduced charge separation in NG-ZnP and NG-ZnPc hybrids was confirmed by using the ultrafast pump-probe technique. The measured rate constants were of the order of 1010  s,-1 thus indicating ultrafast electron transfer phenomena.

Charge-Transfer in Panchromatic Porphyrin-Tetracyanobuta-1,3-Diene-Donor Conjugates: Switching the Role of Porphyrin in the Charge Separation Process

Using a combination of cycloaddition-retroelectrocyclization reaction, free-base and zinc porphyrins (H₂ P and ZnP) are decorated at their β-pyrrole positions with strong charge transfer complexes, viz., tetracyanobuta-1,3-diene (TCBD)-phenothiazine (3 and 4) or TCBD-aniline (7 and 8), novel class of push-pull systems. The physico-chemical properties of these compounds (MP-Donor and MP-TCBD-Donor) have been investigated using a range of electrochemical, spectroelectrochemical, DFT as well as steady-state and time-resolved spectroscopic techniques. Ground-state charge transfer interactions between the porphyrin and the electron-withdrawing TCBD directly attached to the porphyrin π-system extended the absorption features well into the near-infrared region. To visualize the photo-events, energy level diagrams with the help of free-energy calculations have been established. Switching the role of porphyrin from the initial electron acceptor to electron donor was possible to envision. Occurrence of photoinduced charge separation has been established by complementary transient absorption spectral studies followed by global and target data analyses. Better charge stabilization in H₂ P derived over ZnP derived conjugates, and in phenothiazine derived over aniline derived conjugates has been possible to establish. These findings highlight the importance of the nature of porphyrins and second electron donor in governing the ground and excited state charge transfer events in closely positioned donor-acceptor conjugates.

Synthesis of 5,15-meso-bis(2-chlorothiophen-3-yl)porphyrin as a building block for further functional materials

A Suzuki–Miyaura coupling reaction was performed to synthesize 5,15-meso-bis(2-chlorothiophen-3-yl)porphyrins in order to obtain a building block for further functional porphyrins. Photophysical and electrochemical properties were investigated to understand the effect of 2-chloro substituted thiophenes which are connected at their 3-position to the meso-position of the porphyrin. A computational calculation with counterpoise method was demonstrated to estimate the relative intermolecular interaction in order to compare solubility. Installed chloro atoms at the 2-position of thiophen-3-yl decreased in intermolecular interaction which caused an increase in solubility.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor-Acceptor Conjugates

Inspired by the high photoconversion efficiency observed in natural light-harvesting systems, the hierarchical organization of molecular building blocks has gained impetus in the past few decades. Particularly, the molecular arrangement and packing in the active layer of organic solar cells (OSCs) have garnered significant attention due to the decisive role of the nature of donor/acceptor (D/A) heterojunctions in charge carrier generation and ultimately the power conversion efficiency. This review focuses on the recent developments in emergent optoelectronic properties exhibited by self-sorted donor-on-donor/acceptor-on-acceptor arrangement of covalently linked D-A systems, highlighting the ultrafast excited state dynamics of charge transfer and transport. Segregated organization of donors and acceptors promotes the delocalization of photoinduced charges among the stacks, engendering an enhanced charge separation lifetime and percolation pathways with ambipolar conductivity and charge carrier yield. Covalently linking donors and acceptors ensure a sufficient D-A interface and interchromophoric electronic coupling as required for faster charge separation while providing better control over their supramolecular assemblies. The design strategies to attain D-A conjugate assemblies with optimal charge carrier generation efficiency, the scope of their application compared to state-of-the-art OSCs, current challenges, and future opportunities are discussed in the review. An integrated overview of rational design approaches derived from the comprehension of underlying photoinduced processes can pave the way toward superior optoelectronic devices and bring in new possibilities to the avenue of functional supramolecular architectures.

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 (ZnTF₄PPTC^4-) to form a supramolecular complex with a cationic lithium endohedral [60]fullerene (Li⁺@C₆₀). The supramolecular ZnTF₄PPTC^4-/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 ZnTF₄PPTC^4- 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 (ZnTPPS^4-) at 0.55 eV. Time-resolved transient absorption spectroscopy demonstrates that ZnTF₄PPTC^4-/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 ZnTPPS^4-/Li⁺@C₆₀ in benzonitrile.

Spectroelectrochemical Properties and Catalytic Activity in Cyclohexane Oxidation of the Hybrid Zr/Hf-Phthalocyaninate-Capped Nickel(II) and Iron(II) tris-Pyridineoximates and Their Precursors

The in situ spectroelectrochemical cyclic voltammetric studies of the antimony-monocapped nickel(II) and iron(II) tris-pyridineoximates with a labile triethylantimony cross-linking group and Zr(IV)/Hf(IV) phthalocyaninate complexes were performed in order to understand the nature of the redox events in the molecules of heterodinuclear zirconium(IV) and hafnium(IV) phthalocyaninate-capped derivatives. Electronic structures of their 1e-oxidized and 1e-electron-reduced forms were experimentally studied by electron paramagnetic resonance (EPR) spectroscopy and UV−vis−near-IR spectroelectrochemical experiments and supported by density functional theory (DFT) calculations. The investigated hybrid molecular systems that combine a transition metal (pseudo)clathrochelate and a Zr/Hf-phthalocyaninate moiety exhibit quite rich redox activity both in the cathodic and in the anodic region. These binuclear compounds and their precursors were tested as potential catalysts in oxidation reactions of cyclohexane and the results are discussed.

Charge stabilization via electron exchange: excited charge separation in symmetric, central triphenylamine derived, dimethylaminophenyl-tetracyanobutadiene donor-acceptor conjugates

Photoinduced charge separation in donor-acceptor conjugates plays a pivotal role in technology breakthroughs, especially in the areas of efficient conversion of solar energy into electrical energy and fuels. Extending the lifetime of the charge separated species is a necessity for their practical utilization, and this is often achieved by following the mechanism of natural photosynthesis where the process of electron/hole migration occurs distantly separating the radical ion pairs. Here, we hypothesize and demonstrate a new mechanism to stabilize the charge separated states via the process of electron exchange among the different acceptor entities in multimodular donor-acceptor conjugates. For this, star-shaped, central triphenylamine derived, dimethylamine-tetracyanobutadiene conjugates have been newly designed and characterized. Electron exchange was witnessed upon electroreduction in conjugates having multiple numbers of electron acceptors. Using ultrafast spectroscopy, the occurrence of excited state charge separation, and the effect of electron exchange in prolonging the lifetime of charge separated states in the conjugates having multiple acceptors have been successfully demonstrated. This work constitutes the first example of stabilizing charge-separated states via the process of electron exchange.

Synergy of Electrostatic and π-π Interactions in the Realization of Nanoscale Artificial Photosynthetic Model Systems

In the scientific race to build up photoactive electron donor-acceptor systems with increasing efficiencies, little is known about the interplay of their building blocks when integrated into supramolecular nanoscale arrays, particularly in aqueous environments. Here, we describe an aqueous donor-acceptor ensemble whose emergence as a nanoscale material renders it remarkably stable and efficient. We have focused on a tetracationic zinc phthalocyanine (ZnPc) featuring pyrenes, which shows an unprecedented mode of aggregation, driven by subtle cooperation between electrostatic and π-π interactions. Our studies demonstrate monocrystalline growth in solution and a symmetry-breaking intermolecular charge transfer between adjacent ZnPcs upon photoexcitation. Immobilizing a negatively charged fullerene (C60 ) as electron acceptor onto the monocrystalline ZnPc assemblies was found to enhance the overall stability, and to suppress the energy-wasting charge recombination found in the absence of C60 . Overall, the resulting artificial photosynthetic model system exhibits a high degree of preorganization, which facilitates efficient charge separation and subsequent charge transport.

Photoinduced Electron Transfer and Energy Transfer Processes in a Flexible BODIPY-C60 Dyad

A new donor-acceptor dyad composed of a BODIPY (4,4'-difluoro-4-bora-3a,4a-diaza-s-indacene) donor and a fullerene C₆₀ acceptor has been synthesized and characterized. This derivative has been prepared using a clickable fullerene building block that bears an alkyne moiety and a maleimide unit. The post-functionalization of the maleimide group by a BODIPY thiol leads to a BODIPY-C₆₀ dyad, leaving the alkyne moiety for further functional arrangement. On the basis of the combination of semi-empirical and density functional theory (DFT) calculations, spectroelectrochemical experiments, and steady-state and time-resolved spectroscopies, the photophysical properties of this new BODIPY-C₆₀ dyad were thoroughly studied. By using semi-empirical calculations, the equilibrium of three conformations of the BODIPY-C₆₀ dyad has been deduced, and their molecular orbital structures have been analyzed using DFT calculations. Two short fluorescence lifetimes were attributed to two extended conformers displaying variable donor-acceptor distances (17.5 and 20.0 Å). Additionally, the driving force for photoinduced electron transfer from the singlet excited state of BODIPY to the C₆₀ moiety was calculated using redox potentials determined with electrochemical studies. Spectroelectrochemical measurements were also carried out to investigate the absorption profiles of radicals in the BODIPY-C₆₀ dyad in order to assign the transient species in pump-probe experiments. Under selective photoexcitation of the BODIPY moiety, occurrences of both energy and electron transfers were demonstrated for the dyad by femtosecond and nanosecond transient absorption spectroscopies. Photoinduced electron transfer occurs in the folded conformer, while energy transfer is observed in extended conformers.

Photoinduced electron transfer from carbon nanotubes to fullerenes: C60versus C70

Hybrid carbon materials are found to exhibit novel optoelectronic properties at their interfaces, but the related interfacial carrier dynamics is rarely explored theoretically. In this contribution, we have employed density functional theory (DFT) and DFT-based nonadiabatic dynamics methods to explore photoinduced interfacial electron transfer processes at interfaces between a single-walled carbon nanotube with chiral index (6,5) and C60 or C70 (C60@CNT65 and C70@CNT65). We have found that with low E11 excitation, electron transfer takes place from CNT65 to C60 and C70 in both heterojunctions. This process is ultrafast and completed within about 200 fs, which is consistent with recent experiments. Differently, high E22 excitation does not induce electron injection to C60 in C60@CNT65; instead, "hot" electrons produced within CNT65 will be trapped in its higher conduction band for a while because of slow inter-band relaxation. By contrast, in C70@CNT65, high E22 excitation still can lead to ultrafast electron transfer to C70, but only a comparable amount of electrons are transferred (ca. 30%). Interestingly, electrons either remaining on CNT65 or transferred to C70 are trapped in the higher conduction band for a while, similarly, due to slow inter-band relaxation. The present results could be useful to guide the design of excellent interfaces of mixed-dimensional hybrid carbon materials for various optoelectronic applications.

Emerging Scientific Field Detection Using Citation Networks and Topic Models—A Case Study of the Nanocarbon Field

In fields with high science linkage, such as the nanocarbon field, trends in academic papers are particularly important for identifying future technological trends. The use of the number of citations allows us to predict the qualitative trends on a paper-by-paper basis. At the same time, it is necessary to be able to comprehensively discuss both qualitative and quantitative aspects in the subject area. This study aimed to detect emerging areas in the nanocarbon field using network models and topic models. It was possible to not only construct a model that exceeded an 86.2% F1 measure but also to focus on an area that could not be detected by the prediction model. This was accomplished by focusing on paper units, such as the research on the chemical synthesis of zigzag single-walled carbon nanotubes. Thus, it is possible to obtain knowledge that contributes to diversified R&D strategies and innovation policies by considering the emergence of new fields from multiple perspectives.

Symmetry-Breaking Charge-Transfer Chromophore Interactions Supported by Carbon Nanodots

Carbon dots (CDs) and their derivatives are useful platforms for studying electron-donor/acceptor interactions and dynamics therein. Herein, we couple amorphous CDs with phthalocyanines (Pcs) that act as electron donors with a large extended π-surface and intense absorption across the visible range of the solar spectrum. Investigations of the intercomponent interactions by means of steady-state and pump-probe transient absorption spectroscopy reveal symmetry-breaking charge transfer/separation and recombination dynamics within pairs of phthalocyanines. The CDs facilitate the electronic interactions between the phthalocyanines. Thus, our findings suggest that CDs could be used to support electronic couplings in multichromophoric systems and further increase their applicability in organic electronics, photonics, and artificial photosynthesis.

Persubstituted Triphenylamine Bearing Zinc Porphyrin to Host Endohedral Fullerene, Sc3N@C80: Formation and Excited State Electron Transfer

A persubstituted porphyrin with eight entities of triphenylamines at the β-pyrrole positions of a zinc tetraphenylporphyrin, 1, was newly synthesized and characterized. Due to the severe nonplanar distortion caused by the peripheral, electron rich substituents, the zinc porphyrin was able to comfortably bind a relatively large endohedral fullerene, Sc₃N@C₈₀, to form a new class of donor-acceptor-type host-guest complex. Spectral, computational, and electrochemical studies were systematically performed to evaluate the binding, spatial geometry, and redox properties of the host-guest system. Further, free-energy calculations were performed to seek the thermodynamic feasibility of excited state charge transfer. Finally, transient absorption spectral studies at different time scales were performed to secure evidence and kinetic information on excited state charge transfer leading to the 1^•+:Sc₃N@C₈₀^•- charge separated species. The present unprecedented, highly functionalized material with electron rich substituents carries zinc porphyrin as a photoactive host to large endohedral fullerenes, and its ability to undergo excited state electron transfer opens up new avenues to build photoactive host-guest systems relevant to light energy conversion and optoelectronic applications.

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.

Light induced intramolecular energy and electron transfer events in carbazole–corrole and phenothiazine-corrole dyads

We report two corrole based donor–acceptor (D–A) dyads, Cbz-Cor and Ptz-Cor to understand the energy/electron transfer reactions. In these D–A systems, the donor, either carbazole (Cbz) or phenothiazine (Ptz), is covalently connected at the meso-phenyl position of 10-(phenyl)-5,15-bis-(pentafluorophenyl)corrole (Ph-Cor) by C–N linkage. Both the dyads were characterized by 1H NMR, MALDI-TOF MS, UV-vis, electrochemical, computational methods, study state fluorescence and TCSPC techniques. A comparison of absorption spectra with their reference monomeric compounds (Cbz-Ph, Ptz-Ph and Ph-Cor) revealed minimal ground-state interactions between chromophores in both dyads. Fluorescence studies suggested that singlet–singlet energy transfer from 1Cbz* to corrole is the major photochemical pathway in the Cbz-Cor dyad with a quenching efficiency of [Formula: see text]99%. Detailed analysis of the data suggests that Forster’s dipole–dipole mechanism does not adequately explain this energy transfer. However, at a 410 nm excitation, florescence quenching is detected in Ptz-Cor (49%) supporting a photo induced electron transfer (PET) process from the ground state of PTZ to the excited state of corrole macrocycle. The electron-transfer rates ([Formula: see text] of Ptz-Cor are found in the range [Formula: see text] to [Formula: see text] and are concluded to be solvent dependent.

Exclusive triplet electron transfer leading to long-lived radical ion-pair formation in an electron rich platinum porphyrin covalently linked to fullerene dyad

The formation of a high-energy, long-lived radical ion-pair by electron transfer exclusively from the triplet excited state, is demonstrated in a newly synthesized platinum porphyrin-fullerene dyad, in which the porphyrin ring is modified with three electron rich triphenylamine entities. The spin selectivity of the electron transfer leading to the formation of the radical ion-pair is demonstrated using time-resolved optical and EPR spectroscopic techniques.

Supramolecular ultrafast energy and electron transfer in a directly linked BODIPY-oxoporphyrinogen dyad upon fluoride ion binding

A directly linked BODIPY-oxoporphyrinogen dyad has been newly synthesized and occurrence of sequential photoinduced energy and electron transfer upon fluoride anion binding to oxoporphyrinogen has been demonstrated by spectral, electrochemical and femtosecond transient absorption studies.

Light-responsive nanozymes for biosensing

Using light as an external stimulus plays a key role not only in modulating activities of nanozymes, but also in constructing efficient biosensing systems.

Caged-Electron States in Endohedral Li Fullerenes

By employing large-scale high-level EA-EOM-CCSD calculations, we have computed and analyzed the low-lying states of neutral Li@C₆₀. Apart from one state, all states are found to be charge-separated states of the type Li⁺@C₆₀^-. The new state is the first reported non-charge-separated state in endohedral alkali fullerenes. This caged-electron state is analyzed in detail. Arguments are given that in larger highly symmetric endohedral fullerenes the caged-electron state can be the electronic ground state of the system. HF and DFT calculations on Li@C₁₈₀ indeed find that the caged-electron state is the ground state and that in its equilibrium geometry Li sits at the center of the cage. Applications are mentioned.