Triplet C60 Research Articles

Optical Initialization of Molecular Qubit Spin States Using Weak Exchange Coupling to Photogenerated Fullerene Triplet States.

The ability to initialize an electron spin qubit into a well-defined state is an important criterion for quantum information applications. To achieve this goal, a chromophore photoexcited to its triplet state is used to strongly spin polarize a nearby stable radical in a series of C60 fullerene derivatives containing a covalently linked α,γ-bisdiphenylene-β-phenylallyl (BDPA) radical. Selective photoexcitation of C60 results in up to 20-fold enhancement of the BDPA spin polarization observed by pulse electron paramagnetic resonance spectroscopy at room temperature. The sign of the spin polarization depends on the nature of the molecular spacer between C60 and BDPA. In addition, transient absorption spectroscopy and pulse-EPR measurements reveal that the BDPA spin polarization is derived from spin polarization transfer from the C60 triplet state by weak exchange coupling over a 1 nm distance.

Direct detection of spin polarization in photoinduced charge transfer through a chiral bridge.

It is well assessed that the charge transport through a chiral potential barrier can result in spin-polarized charges. The possibility of driving this process through visible photons holds tremendous potential for several aspects of quantum information science, e.g., the optical control and readout of qubits. In this context, the direct observation of this phenomenon via spin-sensitive spectroscopies is of utmost importance to establish future guidelines to control photo-driven spin selectivity in chiral structures. Here, we provide direct proof that time-resolved electron paramagnetic resonance (EPR) can be used to detect long-lived spin polarization generated by photoinduced charge transfer through a chiral bridge. We propose a system comprising CdSe quantum dots (QDs), as a donor, and C60, as an acceptor, covalently linked through a saturated oligopeptide helical bridge (χ) with a rigid structure of ∼10 Å. Time-resolved EPR spectroscopy shows that the charge transfer in our system results in a C60 radical anion, whose spin polarization maximum is observed at longer times with respect to that of the photogenerated C60 triplet state. Notably, the theoretical modelling of the EPR spectra reveals that the observed features may be compatible with chirality-induced spin selectivity, but the electronic features of the QD do not allow the unambiguous identification of the CISS effect. Nevertheless, we identify which parameters need optimization for unambiguous detection and quantification of the phenomenon. This work lays the basis for the optical generation and direct manipulation of spin polarization induced by chirality.

Charge transfer dynamics in CsPbBr3 perovskite quantum dots-anthraquinone/fullerene (C60) hybrids.

An advantage of colloidal quantum dots, particularly perovskite quantum dots (PQDs), as photoactive components is that they easily form complexes with functional organic molecules, which results in hybrids with enriched photophysical properties. Herein, we demonstrate the formation of stable ground state complexes of CsPbBr3 PQD with two widely used molecular electron acceptors, fullerene (C60) and anthraquinone, (AQ) which contain carboxylic anchor groups. Dynamics of the photo-induced electron transfer in the hybrids were compared. The use of carboxylic groups for binding results in stable complex formation and their photophysical properties depend on the ratio of components but not the absolute concentrations (up to micromolar concentrations). Time-resolved transient absorption (TA) spectroscopy shows that in both cases, a charge separated (CS) state is formed. Data analysis was aimed to evaluate the CS time constant in ideal one-to-one complexes and was found to be in the range of 30-190 ps. The CS state of PQD-AQ complexes recombines directly to the ground state in roughly one microsecond. Recombination of the CS state of PQD-C60 is more complex and points to strong inhomogeneity of these complexes. Majority of the CS states relax by first forming the C60 triplet state.

On the regioselectivity of the Diels-Alder cycloaddition to C60 in high spin states.

Controlling the regioselectivity in the exohedral functionalization of fullerenes and endohedral metallofullerenes is essential to produce specific desired fullerene derivatives. In this work, using density functional theory (DFT) calculations, we show that the regioselectivity of the Diels-Alder (DA) cycloaddition of cyclopentadiene to 2S+1C60 changes from the usual [6,6] addition in the singlet ground state to the [5,6] attack in high spin states of C60. Changes in the aromaticity of the five- and six-membered rings when going from singlet to high spin C60 provide a rationale to understand this regioselectivity change. Experimentally, however, we find that the DA cycloaddition of isoindene to triplet C60 yields the usual [6,6] adduct. Further DFT calculations and computational analysis give an explanation to this unanticipated experimental result by showing the presence of an intersystem crossing close to the formed triplet biradical intermediate.

Synthesis and Properties of Axially Symmetrical Rigid Visible Light-Harvesting Systems Containing [60]Fullerene and Perylenebisimide.

Two visible light-harvesting perylenebisimide (PDI)-[60]fullerene (C60) systems, dyad P1 with one C60 unit and triad P2 with two C60 units, have been synthesized. Both systems are axially symmetrical with a rigid biphenyl linker, ensuring a relatively fixed spatial distance between the donor and acceptor, preventing through-space interaction, and enhancing energy transfer. Steady-state and transient spectroscopy, electrochemistry, as well as theoretical calculations have been used to investigate the electrochemical and photophysical properties of the two systems. Steady-state and time-resolved spectroscopy demonstrate that the excited state is featured by an efficient intramolecular energy transfer from PDI to C60. Then, the high efficient intrinsic intersystem crossing of C60 eventually leads to the production of the triplet C60. The extensive visible light absorption of PDI in the range of 400-650 nm and the final localization of the excited energy at the triplet C60 make these compounds ideal singlet oxygen inducers. Further investigation shows that the photooxidation capability for both compounds is significantly enhanced with respect to either PDI or C60 and even better than that of the commonly used triplet photosensitizer methylene blue (MB). The double C60 moieties in P2 display a better result, and the photooxidation efficiency of P2 increases 1.3- and 1.4-fold compared to that of P1 and MB, respectively. The combination of a light-harvesting unit with an intersystem crossing unit results in a highly efficient photooxidation system, which opens up a new way to triplet photosensitizer design.

Large area self-ordered parallel C60 molecular nanowire arrays on Si(110) surfaces

We report that the large-scale self-organization of well-ordered parallel C60 molecular nanowire arrays with tunable structures on Si(110) surfaces can be achieved through a site-selective anchoring method. Our scanning tunneling microscopy investigations show that the preferential trapping of C60 molecules atop Si pentagons on upper terraces of the C60-induced Si(110) reconstructed surface leads to the formation of long-range ordered C60 molecular nanowires with a C60 triplet as a repeat unit along each Si upper terrace (i.e., a Si nanowire). Scanning tunneling spectroscopy measurements show a large energy gap of ∼3.0 eV between the highest occupied molecular orbital and the lowest unoccupied molecular orbital of the C60 molecules within a C60 triplet, indicating a relatively weak C60–Si(110) interaction that is the main driving force for the site-selective anchoring atop Si pentagons via the geometric matching. The formation of ordered parallel C60-tetramer nanowire arrays upon further C60 adsorption confirms this self-ordering mechanism. Such mesoscopically ordered hybrid C60/Si nanojunction arrays with the unmodified C60 on periodic Si nanowires were not yet observed before. The ability to grow the large-area parallel fullerene nanowire arrays with tunable structures on Si(110) represents an important step toward the engineering of advanced Si-based molecular nanoelectronics and quantum computation.

Photoinduced electron transfer in a dynamic supramolecular system with curved π-structures.

Photoinduced electron-transfer processes in a carbonaceous supramolecular combination of a tubular host and a C60 guest were investigated with time-resolved transient absorption spectra upon laser flash photolysis. Following the formation of triplet charge-separated species via electron transfer from the host to the guest, a rapid back electron transfer proceeded to afford triplet C60.

Photophysical properties of new bis-perylene dyads for potential upconversion use

Two new bis-perylenes, BPC with a meta-disubstituted benzene linker, and BPD with a p,p′-disubstituted biphenyl linker, have been synthesized and their photophysical parameters measured. Their singlet and triplet energies decrease incrementally in the order perylene, BPC, BPD, making them ideally matched with C60 for triplet–triplet energy transfer in sensitized photon upconversion schemes. Following photosensitization by triplet C60, BPC exhibits strong fluorescence upconversion by triplet–triplet annihilation, indicating that these bis-perylene dyads (and the multimers that can be constructed from them) will be interesting candidates for use in organic devices such as bulk-heterojunction and dye-sensitized solar cells employing non-coherent photon upconversion.

Effects of Light Energy and Reducing Agents on C60‐Mediated Photosensitizing Reactions

Many biomolecules contain photoactive reducing agents, such as reduced nicotinamide adenine dinucleotide (NADH) and 6-thioguanine (6-TG) incorporated into DNA through drug metabolism. These reducing agents may produce reactive oxygen species under UVA irradiation or act as electron donors in various media. The interactions of C60 fullerenes with biological reductants and light energy, especially via the Type-I electron-transfer mechanism, are not fully understood although these factors are often involved in toxicity assessments. The two reductants employed in this work were NADH for aqueous solutions and 6-TG for organic solvents. Using steady-state photolysis and electrochemical techniques, we showed that under visible light irradiation, the presence of reducing agents enhanced C60 -mediated Type-I reactions that generate superoxide anion (O2(.-)) at the expense of singlet oxygen ((1)O2) production. The quantum yield of O2(.-) production upon visible light irradiation of C60 is estimated below 0.2 in dipolar aprotic media, indicating that the majority of triplet C60 deactivate via Type-II pathway. Upon UVA irradiation, however, both C60 and NADH undergo photochemical reactions to produce O2(.-), which could lead to a possible synergistic toxicity effects. C60 photosensitization via Type-I pathway is not observed in the absence of reducing agents.

Theoretical study on energy transfer from the excited C60 to molecular oxygen

Singlet oxygen is used as an important oxidizer for various organic reactions, on the other hand, it has toxicity for the human body, since it destroys biological molecules such as DNA. Fullerene C60 is known to act as a photosensitizer of singlet oxygen molecule, and it has been understood that energy transfer from the excited triplet C60 to the ground triplet O2 forming the singlet O2 is a biologically important reaction. In the present study, we have analyzed these reaction pathways by means of the DFT (density functional theory) calculation.

Azobenzene-Linked Porphyrin−Fullerene Dyads

A new group of porphyrin-fullerene dyads with an azobenzene linker was synthesized, and the photochemical and photophysical properties of these materials were investigated using steady-state and time-resolved spectroscopic methods. The electrochemical properties of these compounds were also studied in detail. The synthesis involved oxidative heterocoupling of free base tris-aryl-p-aminophenyl porphyrins with a p-aminophenylacetal, followed by deprotection to give the aldehyde, and finally Prato 1,3-dipolar azomethineylide cycloaddition to C60. The corresponding Zn(II)-porphyrin (ZnP) dyads were made by treating the free base dyads with zinc acetate. The final dyads were characterized by their 1H NMR, mass, and UV-vis spectra. 3He NMR was used to determine if the products are a mixture of cis and trans stereoisomers, or a single isomer. The data are most consistent with the isolation of only a single configurational isomer, assigned to the trans (E) configuration. The ground-state UV-vis spectra are virtually a superimposition of the spectral features of the individual components, indicating there is no interaction of the fullerene (F) and porphyrin (H2P/ZnP) moieties in the ground state. This conclusion is supported by the electrochemical data. The steady-state and time-resolved fluorescence spectra indicate that the porphyrin fluorescence in the dyads is very strongly quenched at room temperature in the three solvents studied: toluene, tetrahydrofuran (THF), and benzonitrile (BzCN). The fluorescence lifetimes of the dyads in all solvents are sharply reduced compared to those of H2P and ZnP standards. In toluene, the lifetimes of the free base dyads are 600-790 ps compared to 10.1 ns for the standard, while in THF and BzCN the dyad lifetimes are less than 100 ps. For the ZnP dyads, the fluorescence lifetimes were 10-170 ps vs 2.1-2.2 ns for the ZnP references. The mechanism of the fluorescence quenching was established using time-resolved transient absorption spectroscopy. In toluene, the quenching process is singlet-singlet energy transfer (k approximately 10(11) s-1) to give C60 singlet excited states which decay with a lifetime of 1.2 ns to give very long-lived C60 triplet states. In THF and BzCN, quenching of porphyrin singlet states occurs at a similar rate, but now by electron transfer, to give charge-separated radical pair (CSRP) states, which show transient absorption spectra very similar to those reported for other H2P-C60 and ZnP-C60 dyad systems. The lifetimes of the CSRP states are in the range 145-435 ns in THF, much shorter than for related systems with amide, alkyne, silyl, and hydrogen-bonded linkers. Thus, both forward and back electron transfer is facilitated by the azobenzene linker. Nonetheless, the charge recombination is 3-4 orders of magnitude slower than charge separation, demonstrating that for these types of donor-acceptor systems back electron transfer is occurring in the Marcus inverted region.

Adsorption and Photophysics of Fullerene C60 at Liquid−Zeolite Particle Interfaces: Unusually High Affinity for Hydrophobic, Ultrastabilized Zeolite Y

Adsorption of fullerene C60 from solution to the external surface of zeolite particles has been investigated. The most intriguing result of this study was the nature of C60 adsorption to ultrastablized zeolite Y (or USY). Two commercial samples of USY were tested: CBV780 (Y780) and CBV901 (Y901). Y901 was shown in previous reports to be more hydrophobic than Y780. Higher affinity of C60 for Y901 was found relative to Y780 in a variety of hydrocarbon solvents, including toluene and cyclohexane. In these same solvents, weak or no affinity for Y901 of typical arenes such as naphthalene or pyrene was observed. In toluene, adsorption isotherms for C60 gave dissociation constants (and values of saturation binding) = 0.5 microM (5.8 micromol g(-1)) and 8 microM (1.4 micromol g(-1)) for Y901 and Y780, respectively. C60 was estimated to cover nearly one-half of the estimated external surface area of Y901 particles at saturation. Significant adsorption of C60 to the ionic zeolites NaX, NaY, and KL was observed in cyclohexane but not in toluene, consistent with the pi-cation effect as a driving force for adsorption to these materials. The main driving force for C60 adsorption to Y901 is postulated to involve the interaction of C60 with lone pair electrons of framework oxygen atoms of the 12-ring entry aperture to the supercage. In the 12-ring site, C60 is located in half-supercage bowls on the exterior particle surface. The adsorptive interaction on Y901 relies on the spherical shape of C60 and the hydrophobicity of the zeolite surface. On ionic zeolites, the presence of specific adsorption sites such as exchangeable cations and hydroxyl groups hinder the special positioning necessary for C60 interaction with the 12-ring site. The ground-state and triplet-state absorption spectrum of adsorbed C60 was solution-like on all zeolites. Quenching of the C60 triplet state was examined by using transient absorption spectroscopy. Rate constants for quenching by rubrene, ferrocene, and O2 at the Y901-toluene interface were 18, 9, and 3 times lower, respectively, relative to rate constants in solution. These differences point out that the approach of molecular quenchers to C60 at the interface is more hindered for larger molecules, an expected result for C60 located in half-supercage bowls. The high affinity of fullerenes for hydrophobic zeolite Y provides a strategy for organizing fullerenes at interfaces and for studies of fullerene photochemistry.

Fullerene Polypyridine Ligands: Synthesis, Ruthenium Complexes, and Electrochemical and Photophysical Properties

Fullerene coordination ligands bearing one bipyridine or terpyridine unit were synthesized, and their coordination to ruthenium(II) formed linear rod-like donor-acceptor systems. Steady-state fluorescence of [Ru(bpy)(2)(bpy-C(60))](2+) showed a rapid solvent-dependent, intramolecular quenching of the ruthenium(II) MLCT excited state. Time-resolved flash photolysis in CH(3)CN revealed characteristic transient absorption changes that have been ascribed to the formation of the C(60) triplet state, suggesting that photoexcitation of [Ru(bpy)(2)(bpy-C(60))](2+) results in a rapid intramolecular transduction of triplet excited state energy. The electrochemical studies on both [Ru(bpy)(2)(bpy-C(60))](2+) and [Ru(tpy)(tpy-C(60))](2+) indicated electronic coupling between the metal center and the fullerene core.

Study of Photoinduced Electron Transfer between [60]Fullerene and Proton-Sponge by Laser Flash Photolysis: Addition Effects of Organic Acid

Photoinduced electron-transfer processes between fullerene (C60) and 1,8-bis(dimethylamino)naphthalene, which is called a proton-sponge (PS), have been investigated by means of laser flash photolysis in the presence and absence of CF3CO2H. For a mixture of C60 and PS, the transient absorption spectra showed the rise of the C60 radical anion with concomitant decay of the C60 triplet (3C60), suggesting that photoinduced intermolecular electron transfer occurs via 3C60 in high efficiency in polar solvent. For a covalently bonded C60-PS dyad, photoinduced intramolecular charge-separation process takes place via the excited singlet state of the C60 moiety, although charge recombination occurs within 10 ns. For both systems, electron-transfer rates were largely decelerated by addition of a small amount of CF3CO2H, leaving the long-lived 3C60. These observations indicate that the energy levels for charge-separated states of the protonated PS and C60 become higher than the energy level of the 3C60 moiety, showing low donor ability of the protonated PS. Thus, intermolecular electron-transfer process via 3C60 for C60-PS mixture and intramolecular charge-separation process via 1C60-PS for C60-PS dyad were successfully controlled by the combination of the light irradiation with a small amount of acid.

Interaction of triplet C60with p-tert-butyl-calixarenes and their complexes with pyridine derivatives

The interaction of triplet excited C60 with p-tert-butyl-calix[n]arenes (BCXn, n = 4, 6 or 8) and with their 2,4,6-trimethylpyridine (TMP) and pyridine complexes has been studied with laser flash photolysis experiments. It has been found that in polar solvents 3C60 is quenched by BCX6 via electron transfer, producing C60−˙ radical anion with a yield of 0.45 ± 0.07 in benzonitrile. In contrast, no reaction occurs in nonpolar media or with BCX4 and BCX8. Upon the addition of TMP, the rate of quenching is enhanced considerably due to the formation of TMP-BCXn complexes. The significant deuterium isotope effect indicates that the electron movement from the calixarene to the triplet excited C60 is coupled to the proton displacement from the calixarene to the base. In the presence of pyridine, which has weaker hydrogen-bonding power, the enrichment of the solvate shell in the proton acceptor molecules may play an important role in promoting triplet C60 quenching.

New Photosensitive Polyimide System for Sealing High-Density Semiconductor Chip

A heat-resistant photosensitive polymer consisting of furan-substituted polyimide (FPI), and fullerene C60 as a photosensitizer has been developed. The FPI was cross-linked by a photo-oxidation-induced polycondensation (POP) reaction by means of the intervention of singlet oxygen generated from C60 triplet state photosensitization of oxygen molecules. Photochemical study using the laser flash photolysis method revealed that the C60 triplet state was quenched by FPI in solution. This phenomenon suggested a possibility that the POP reaction was hindered. However, in polyimide film, the effect of FPI on the quenching of the C60 triplet state was negligible. This is due to a fast diffusion rate of oxygen in polyimide film in comparison with the mobility of the FPI. It was shown by photolithographic evaluation that postexposure baking (PEB) enhanced the curing rate of the POP polyimide. Photolithography with PEB treatment at 120°C for 10 min is capable of boring pinholes of 10 µm diameter in a 12-µm-thick POP polyimide film. The thermostability of POP polyimide was found to be quite high, with the temperature needed to induce a 5% weight loss being higher than 370°C even without high-temperature annealing.

Chemically induced dynamic electron polarization of C60 nitroxide bisadducts and spin multiplicity of their excited states

A series of biradicals consisting of a C60 linked to two nitroxide addends have been studied by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The series includes all four trans bisadducts and the equatorial isomer. After a visible laser pulse, the bisadducts in toluene liquid solution show chemically induced dynamic electron polarization (CIDEP) effects with a complex pattern of narrow lines in emission and enhanced absorption. From the knowledge of the sign of exchange interaction between the two nitroxide moieties, it was possible to account for the CIDEP effect in terms of an intramolecular triplet-triplet annihilation. A broad signal attributed to the quintet excited state was observed at short time delay after the laser pulse. The TR-EPR spectra recorded at low temperature in glassy matrix are assigned to an excited triplet state localized on the C60, with a small interaction with the spins of the two nitroxide fragments. The assignment is based on spectral simulation and analysis of the transient nutation frequency. The small exchange interaction between excited triplet C60 and nitroxide is shown to depend on the nature of the nitroxide addend.

Solvent Dependence of Charge Separation and Charge Recombination Rates in Porphyrin−Fullerene Dyad

Photoinduced processes in zinc porphyrin−C60 dyad (ZnP−C60) in different organic solvents have been investigated by fluorescence lifetime measurements and pico- and nanosecond time-resolved transient absorption spectroscopies. Irrespective of the solvent polarity, the charge-separated state (ZnP•+−C60•-) is formed via photoinduced electron transfer from the excited singlet state of the porphyrin to the C60 moiety. However, the resulting charge-separated state decays to different energy states depending on the energy level of the charge-separated state relative to the singlet and triplet excited states of the C60 moiety. In nonpolar solvents such as benzene (εs = 2.28), the charge-separated state undergoes charge recombination to yield the C60 singlet excited state, followed by intersystem crossing to the C60 triplet excited state, since the energy level of the charge-separated state is higher than that of the C60 singlet excited state (1.75 eV). More polar solvents such as anisole (εs = 4.33) render the energy level of the charge-separated state lower than the C60 singlet excited state, resulting in the direct formation of the C60 triplet excited state (1.50 eV) from the charge-separated state, formed by the photoinduced charge separation from the porphyrin to the C60 singlet excited state as well as from the porphyrin excited singlet state to the C60. In polar solvents such as benzonitrile (εs = 25.2), where the energy level of the charge-separated state (1.38 eV) is low compared with the C60 triplet excited state, the charge-separated state, produced upon excitation of the both chromophores, decays directly to the ground state. Such solvent dependence of charge recombination processes in ZnP−C60 can be rationalized by small reorganization energies of porphyrins and fullerenes in electron-transfer processes.

C60 Thermal Lens Spectra and Its Theoretical Explanation

We presented the time-resolved thermal lens spectra of C60-benzene system under nitrogen-saturated condition. The theoretical explanation for C60 thermal lens spectra was indicated and in agreement with the experimental data. The theoretical model for C60 thermal lens spectra and the lifetime of triplet C60 (τT=10.2μs) were also given.

Coupled Electron-Proton Transfer in Interactions of Triplet C60 with Hydrogen-Bonded Phenols: Effects of Solvation, Deuteration, and Redox Potentials

The quenching of triplet C60 by phenols is greatly enhanced by addition of pyridines, which also lower the phenol voltammetric oxidation potentials. Flash photolysis shows that the products of this quenching reaction are the C60•- anion radical, neutral phenoxy (or naphthoxy) radicals, and protonated pyridines. Analysis of the second-order kinetics gives quenching rate constants and values of formation constants of hydrogen-bonded phenol−pyridine pairs. The latter agree with those derived from absorption spectra over a wide range of phenols, pyridines, and solvents. Significant deuterium kinetic isotope effects are observed, indicating the importance of both proton transfer and hydrogen bonding in enhancing the rate. Quenching rates and radical yields both increase with solvent polarity. It is concluded that this quenching process involves interaction between 3C60 and a hydrogen-bonded phenol−pyridine complex, in which electron transfer from the phenol to 3C60 is concerted with proton transfer from phenol...