Issue 10 - Volume MA2016-01 - ECS Meeting Abstracts (2024)

The photovoltaic community is currently experiencing the revolution of the solar cells based on perovskite materials. Since the first reports based on a solid hole-transporting layer appeared, a great variety of organic molecules have been employed as electron or hole-selective contacts in regular and/or inverted perovskite-based solar cells (PSCs) (Figure 1 A and C, respectively). Fullerene and their derivatives are playing a very important role as electron-selective contact (ESC) in inverted solar cells as well as in self-assembled monolayers of fullerene derivatives on TiO2 in regular devices. Moreover, the interest of fullerenes in PSCs has been increased by the recent report of Snaith's group, which describes the successful incorporation of [60]fullerene instead of the usual TiO2 as ESC in regular PSCs architectures (Figure 1B).

In the present comunication, careful photovoltaic studies following a well-described experimental protocol have been carried out with PSC fabricated with [70]fullerene, [60]fullerene and [60]fullerene derivatives as Electron Transport Material in regular perovskite solar cell architecture, in order to study their suitability as ETM for PSC. The obtained results indicate that the use of these materials as ET materials in PSC allow the obtention of highly efficient devices (≥10%), due to the introduction of the "fullerene saturation approach".

Figure 1

We have recently predicted that when the dielectric constant of organic semiconductors is increased from 2-4 to >10, a dramatic increase in (maximal) power conversion efficiency of organic solar cells, based on these materials as active layer constituents, can be achieved.^1-3 Binding energies of initial excitons, of charge transfer excitons, and between charges in any type of recombination process are diminished with increasing electric permittivity of the medium.

In order to design and develop new molecular materials with higher dielectric constant, the next challenge is to make the connection between the dielectric constant, as a macroscopic property, and molecular structure.

One approach is to connect suitable substituents to known (opto)electronically functional molecular moieties. We report here on the design and preparation of molecules bearing substituents that are to enhance the dielectric properties without diminishing other crucial parameters (like charge carrier mobility, bandgap, stability, etc.).^4,5 Another approach is to design inherently more polarizable/polar chromophores. A new class of conjugated polymers, based on this principle is presented.⁶

Through a computational chemistry approach, we investigate the influence of molecular moieties on the relative energies of various charge separated states resulting from photo-induced charge transfer between molecular donors and acceptors. We report on a system in which the energies of the charge separated states are tuned to a favorable situation by proper placement of dipolar substituents.⁷

1. L. J. A. Koster, S.E. Shaheen, J.C. Hummelen, Adv. Energy Mater. 2012, 2, 1246.

2. R.C. Chiechi, J.C. Hummelen, ACS Macro Lett. 2012, 1, 1180.

3. R. C. Chiechi, R. W. A. Havenith, J. C. Hummelen, L. J. A. Koster, M. A. Loi, Materials Today 2013, 16, 281.

4. F.Jahani, S. Torabi, R.C. Chiechi, L. J. A. Koster, J. C. Hummelen Chem. Commun. 2014, 50, 10645.

5. S. Torabi, F.J.Bahnamiri, I. Van Severen, C. Kanimozhi, S. Patil, R.W.A. Havenith, R.C. Chiechi, L. Lutsen, D.J.M. Vanderzande, T.J. Cleij, J.C.Hummelen, L.J.A.Koster, Adv. Funct. Mater. DOI 10.1002/adfm.201402244 (2014)

6. T. P. Voortman, H. D. de Gier, R.W. A. Havenith, R. C. Chiechi, J. Mater. Chem. C, 2014, 2, 3407

7. H. De Gier, R. Broer, R.W.A. Havenith, Phys.Chem.Chem.Phys., 2014, 16, 12454

Spontaneous molecular assembly in solution usually proceeds under thermodynamic control and thus provides a reliable approach to creating materials with high positional and/or orientational order of the constituent molecules. If supramolecular building blocks could be purposely designed, one may control the alignment of functional molecular units so that the resulting assembly can take full advantage of the inherent properties of the functional units. This presentation will focus on the controlled assembly of C₆₀ using several supramolecular building blocks. When C₆₀ is attached to a particular type of hexabenzocoronene capable of forming a nanotubular assembly, a coaxial nanotube results, where the wall consists of hole-transporting hexabenzocoronene arrays, while inner and outer surfaces are covered by an electron-transporting monolayer of C₆₀ [1]. We have also shown that an amphiphilic design of an oligothiophene-C₆₀ dyad provides a rational strategy for tailoring bicontinuous electron donor and acceptor arrays [2]. We recently found that a tripodal triptycene self-assembles into a 2D hexagonal structure by nested packing. The interpenetration of the triptycene part may suppress structural fluctuation, thus enabling the formation of organic films with a remarkably long-range structural order [3]. Using this tripodal triptycene as a supramolecular building block, a C₆₀ unit can be assembled into a two-dimensional sheet-like structure. The synthesis and properties of these materials with controlled C₆₀assemblies will be discussed.

References:[1] Y. Yamamoto, G. Zhang, W. Jin, T. f*ckushima, N. Ishii, A. Saeki, S. Seki, S. Tagawa, T. Minari, K. Tsukagoshi, T. Aida, Proc. Natl. Acad. Sci. U.S.A.2009, 106, 21051–21056. [2] W.-S. Li, Y. Yamamoto, T. f*ckushima, A. Saeki, S. Seki, S. Tagawa, H. Masunaga, S. Sasaki, M. Takata, T. Aida, J. Am. Chem. Soc.2008, 130, 8886­–8887. [3] N. Seiki, Y. Shoji, T. Kajitani, F. Ishiwari, A. Kosaka, T. Hikima, M. Takata, T. Someya, T. f*ckushima, Science2015, 348, 1122–1126.

Phthalocyanines (Pcs) are planar, two-dimensional aromatic macrocycles which possess outstanding electrical and optical properties that make them perfect building blocks for their incorporation into multifunctional materials.Similarly, the excellent electron-accepting property of C₆₀ fullerenerenders this carbon nanostructure an ideal molecular partner for photo- and electroactive molecular systems such as Pcs.

Although several reports have appeared on the preparation of Pc-based, mono-functionalized fullerenes,not many examples have been reported concerning the double functionalization of C₆₀ fullerene by Pcs, which remains problematic due to the formation of a large number of regio- and/or stereoisomers.

Here, we report a novel example of controlled bisaddition over C₆₀ fullerene in which the occurrence of π-π supramolecular interactions between structurally-rigid, Pcs addends in solution led to the formation of a Pc₂-C₆₀ bisadduct triad in a regio- and stereocontrolled fashion.In the Pc₂-C₆₀ triad, the two Pc units are interacting through intramolecular π-stacking interactions, as demonstrated by several techniques (i.e., DABCO titration experiments, ground- and excited state photophysical studies, HPLC analysis, electrochemical studies, etc.).

Curved π-conjugated molecules have attracted considerable interest because of their unique properties originating from their curved π surface. Besides their figurative beauty, the curved psurface generates unique functions such as chiroptical properties, anisotropic electron transitions, and dynamic motion in solution and solid states. However, the synthesis of such distorted molecules requires harsh conditions, which hamper an easy access to heteroatom-containing curved π-systems.

Buckybowls represent important curved π-conjugated molecules, which can be precursors for the bottom-up synthesis of fullerenes and nanotubes. The nitrogen-embedded bowl-shaped molecules have been sought as model compounds for azafullerenes and nitrogen-doped carbon nanotubes. However, the synthesis of buckybowls with internal nitrogen atoms has been still challenging.

Here we report the synthesis of a π-extended azacorannulene with nitrogen in its center. The oxidation of 9-aminophenanthrene provides tetrabenzocarbazole, which is converted to the azabuckybowl through palladium-catalyzed intramolecular coupling reaction. The structure was unambiguously elucidated by X-ray diffraction analysis, showing its bowl-shaped conformation. The electron-donating nature and curved π surface of the azabuckybowl enable its tight association with C₆₀ in solution and solid states. The titration experiments with C₆₀ by electronic absorption and emission spectroscopic analysis indicate the presence of intermolecular charge-transfer interaction between them. In addition, high charge carrier mobility was observed for the azabuckybowl/C₆₀ assembly in solid state. This new compound would be a novel molecular entity in the field of curved π systems as fullerene hosts, anisotropic π donors, and precursors to nitrogen-containing nanocarbon materials.

Figure 1

A Graphene quantum dots have received great attention for a potential role in the development of unique optoelectronic materials. In the development of optoelectronic devices, the preparation of GQDs that are controlled with their size and dimensions and are un-oxidized at the sp² surfaces is needed. We developed GQDs functionalized with bulky dendritic wedges at the GQD periphery. The single-layered and size-regulated structures of the dendronized GQDs were revealed by atomic force microscopy. The edge-functionalization of the GQDs gave rise to uncommon white light emission.

In this presentation, I will describe the synthesis and characterization of newly developed graphene quantum dots, and the unique photochemical features.

1) Sekiya, R.; Uemura, Y.; Murakami, H.; Haino, T. Angew. Chem. Int. Ed.2014, 53, 5619-5623.

Figure 1

Metal chalcogenide clusters are structural motifs utilized, for example, in enzymatic active sites, catalysts, and optical/optoelectronic materials. Among non-biomimetic cluster complexes, those consisting of cobalt and sulfur are particularly attractive due to their potential as catalysts, electrocatalysts, semiconducting materials, and so forth. Moreover, intriguing properties have been reported for cobalt sulfide nanoparticles, giving rise to interest in cobalt–sulfur cluster complexes. However, synthetic routes to cobalt–sulfur cluster complexes are quite limited and their structural diversity is very low. Thus, development of facile cobalt–sulfur sources and terminal ligands is strongly desired. In this presentation, we discuss the selective synthesis of the well-defined Co₈S₁₅ cluster complex starting from a cobalt trisulfide complex. The precursor cobalt trisulfide complex served as a cobalt–sulfur source as well as a powerful sterically templating terminal ligand. A corner-sharing double-cubane-like cobalt–sulfur cluster was selectively synthesized from an easily prepared cobalt trisulfide complex of pentaaryl[60]fullerene. The obtained cluster contained two Co₄S₅ cubane-like units coordinated by thiolate and disulfide ligands. The bowl-shaped steric template as well as the metastable cobalt trisulfide moiety played important roles in the exclusive cluster formation. Crystallographic, electrochemical, and magnetic measurements elucidated a mixed-valence nature of the cluster with two high-spin cobalt(II) centers. The present study will provide new opportunities for development of artificial functional metal–sulfur cluster complexes for various applications.

Figure 1

Given the large number of possible non-equivalent bonds of Endohedral MetalloFullerenes (EMFs), the elucidation of the preferred addition sites for a target reaction is tremendously challenging.¹ The development of a set of simple rules able to rationalize the preferred regioselectivities by solely inspecting the fullerene structure would be extremely appealing for the whole fullerene community. This communication describes a set of predictive criteria called Predictive Aromaticity Criteria (PAC), to a priori identify the most reactive bonds of a given EMF towards the Bingel-Hirsch (BH) cycloaddition based on a simple evaluation of the cage structure, and without need to perform any expensive theoretical calculation or experimental measure.²

We take as starting point the investigation of the BH addition of diethylbromomalonate on all non-equivalent bonds of Sc₃N@D_3h-C₇₈ using density functional theory calculations. From the Sc₃N@D_3h-C₇₈ analysis, a set of rules that we called Predictive Aromaticity Criteria (PAC) is derived, which are further applied to predict the most favorable BH adducts for the challenging case of Sc₃N@D_5h-C₈₀. The predictions based on PAC are afterwards fully confirmed by both the computational and experimental results of the complete study of the regioselectivity of BH reaction to Sc₃N@D_5h-C₈₀. These results provide conclusive evidence on the reliability of the PAC, indicating that these rules are general and applicable to any BH reaction on IPR EMFs. Moreover, this work provides clear evidence of the key role played by the local aromaticity of the cage in EMF functionalization.^2-5

References:

[1] M. Garcia-Borràs, S. Osuna, M. Swart, J.M. Luis and M. Solà,Chem. Soc. Rev., 2014, 43 (14), 5089

[2] M. Garcia-Borràs, M. R. Cerón, S. Osuna, M. Izquierdo, J.M. Luis, L. Echegoyen, M. Solà, submitted for publication, 2015.

[3] M. Garcia-Borràs, S. Osuna, M. Swart, J.M. Luis and M. Solà, Chem. Commun., 2013, 49 (12), 1220

[4] M. Garcia-Borràs, S. Osuna, M. Swart, J.M. Luis, L. Echegoyen, Miquel Solà,Chem. Commun.,2013, 49 (78), 8767

[5] M. Garcia-Borràs, S. Osuna, M. Swart, J.M. Luis and M. Solà,Angew. Chem. Int. Ed., 2013, 52 (35), 9275

Figure 1

Polycyclic aromatic hydrocarbons (PAHs) are receiving a great deal of attention because of their increasingly better performance in organic electronic applications. Among these, PAHs containing N atoms (N-PAHs) are particularly interesting since their electronic structure, stability, solubility, and supramolecular organization can be modulated by varying the number and position of N atoms.

In general, PAHs are planar structures but they can adopt twisted conformations as the result of the steric strain induced by overcrowding or congestion in key positions of the aromatic core. twisted-PAHs have shown enhanced solubility and unique optoelectronic and chiroptical properties as an effect of their distorted molecular structure.

We have developed a general strategy that provides access to a new family of twisted-N-PAHs (0D) with different twist angles by introducing silyl groups with different size and rigidity, providing direct experimental correlation between twist size and properties. In addition, this methodology has been successfully implemented in the preparation of 1D and 2D materials. The most recent advances of these materials including synthetic routes, optoelectronic properties, self-organising properties, and potential applications will be discussed.

References

[11] L. Jiang, A.C. Papageorgiou,* S. Cheol; Ö. Sağlam, J. Reichert,* D.A. Duncan, Y.-Q. Zhang, F. Klappenberger; S. More, R. Bhosale, A. Mateo-Alonso* and J.V. Barth ACS Nano 2016,DOI: 10.1021/acsnano.5b06340. [10] A. Mateo-Alonso Chem. Soc. Rev.2014, 43, 6311-6324. [9] A.B. Marco, D. Cortizo-Lacalle, C. Gozalvez, M. Olano, A. Atxabal, X. Sun, M. Melle-Franco, L.E. Hueso, and A. Mateo-Alonso Chem. Comm. 2015, 51, 10754-10757. [8] R. Garcia, M. Melle-Franco and A. Mateo-Alonso Chem. Comm. 2015, 51, 8037-8040.[7] R. García, S. More, M. Melle-Franco, A. Mateo-Alonso Org. Lett.2014, 16, 6096−6099. [6] G. Tregnago, C. Fléchon, S. Choudhary, C. Gozalvez, A. Mateo-Alonso, F. Cacialli App. Phys. Lett., 2014, 105, 143304. [5] S. More, R. Bhosale, A. Mateo-Alonso Chem. Eur. J., 2014, 20, 10626-10631. [4] S. More, S. Choudhary, A. Higelin, I. Krossing, M. Melle-Franco, A. Mateo-Alonso Chem. Comm., 2014, 50, 1976-1979. [3] S. Choudhary, C. Gozalvez, A. Higelin, I. Krossing, M. Melle-Franco, A. Mateo-Alonso Chem. Eur. J., 2014, 20, 1525-1528. [2] S. More, R. Bhosale, S. Choudhary, A. Mateo-Alonso Org. Lett.2012, 14, 4170-4173. [1] N. Kulisic, S. More, and A. Mateo-Alonso Chem. Comm.2011, 47, 514-516.

[60]Fulleroindolines can accept two electrons to form the corresponding dianions, which involve the heterolytic cleavage of the C₆₀－N bond. The ring-opened structures are perfect building block for the further functionalizations. The synthesis of both tetrafunctionalized and hexafunctionalized products with unusual addition patterns have been achieved.^[1]

References

[1]. Xiao, Y.; Zhu, S.-E; Liu, D.-J.; Suzuki, M.; Lu, X.; Wang, G.-W. Angew. Chem. Int. Ed. 2014, 53, 3006-3010.

Protonation of a free-base porphyrin (H₂P) has been widely investigated because of the crucial influences on characteristic redox and photophysical properties of H₂P. Protonation of H₂P under strongly acidic conditions resulted in formation of diprotonated porphyrin (H₄P²⁺) without any detectable monoprotonated porphyrin (H₃P⁺) as an intermediate. Thus, there are only a few reports on the formation of H₃P⁺ because of the higher basicity of saddle distorted H₃P⁺ rather than that of planer H₂P. On the other hand, dodecaphenylporphyrin (H₂DPP),¹ shows a saddle-distorted structure due to the steric repulsion among the phenyl rings. Previously, we reported a crystal structure of a monoprotonated dodecaphenylporphyrin (H₃DPP⁺) by using 2-anthracene sulfonic acid (2-AN-SO₃H) as a proton source.² Herein, we report selective formation of H₃DPP⁺ in an acetone solution with a polar protic solvent such as methanol. Hydrogen bonding between H₃DPP⁺ and methanol has been revealed to be important for stabilization of the monoprotonated porphyrin. In this presentation, we will focus on photoinduced electron-transfer (ET) reactions from ferrocene derivatives to the triplet excited state of H₃DPP⁺ (³[H₃DPP⁺]*), which have been scrutinized by femto- and nano-second laser flash photolysis for the first time.

Protonated products of saddle-distorted H₂DPP with trifluoroacetic acid (TFA) in acetone were revealed to be controlled by addition of methanol as a protic polar solvent. Addition of 1 eq of TFA to an acetone solution containing H₂DPP in the presence of 3% methanol (v/v) resulted in selective formation of a monoprotonated form (H₃DPP⁺) as confirmed by spectroscopic measurements and X-ray crystallography. The crucial role of methanol for the selective H₃DPP⁺ formation was interpreted as stabilization of H₃DPP⁺ by hydrogen bonding with methanol; because a methanol molecule was found to form two-point hydrogen bonding with an NH proton and the non-protonated nitrogen atom of H₃DPP⁺in the crystal.

The reduction potential (E_red, V vs Fc/Fc⁺) of H₃DPP⁺(CF₃COO^–) was determined to be –1.05 V in acetone/methanol (3%) containing 0.1 M [(n-butyl)₄N]BPh₄ as an electrolyte at 298 K by electrochemical measurements. The E_red value of H₃DPP⁺(CF₃COO^–) was lower than that of H₄DPP²⁺(CF₃COO^–)₂ (–0.83 V) and higher than that of H₂DPP (–1.41 V). Then, absorption and emission spectra of H₃DPP⁺(CF₃COO^–) were measured to determine the energy levels of the singlet and triplet excited states of H₃DPP⁺(CF₃COO^–) in acetone/methanol (3%). As a result, the energy levels of ¹[H₃DPP⁺]* and ³[H₃DPP⁺]* were determined to be 1.66 eV and 1.44 eV, respectively.

Femto- and nano-second laser flash photolysis was applied to elucidate the photodynamics of intermolecular photoinduced ET reactions from ferrocene derivatives as one-electron donors to H₃DPP⁺ as an electron acceptor. Femto-second laser flash photolysis of H₃DPP⁺ in acetone/methanol (3%) with laser excitation at 500 nm revealed a transient absorption spectrum with a peak at 575 nm assigned to the singlet excited state of H₃DPP⁺ (¹[H₃DPP⁺]*). The decay time profile at 575 nm exhibited a mono-exponential decay with the lifetime of 200 ps, which is ascribed to intersystem crossing from ¹[H₃DPP⁺]* to the triplet excited state of H₃DPP⁺ (³[H₃DPP⁺]*). The decay time profiles of ³[H₃DPP⁺]* were measured by nano-second laser flash photolysis in the absence and presence of ferrocene derivatives as one-electron donors. As a result, we obtained the lifetime of ³[H₃DPP⁺]* (τ = 42 μs) and second-order rate constants (k_et) of the ET reactions, respectively. The k_et values were analyzed in light of the Marcus theory of electron transfer (Figure 1). The reorganization energy (λ) of electron transfer was determined to be 1.85 eV, which is slightly larger than that of H₄DPP²⁺ in acetonitrile (1.69 eV),³ due to the larger structural change after electron transfer than that of H₄DPP²⁺.

In conclusion, we have succeeded in selective formation of H₃DPP⁺ in the presence of methanol. We have also revealed the photodynamics of H₃DPP⁺ in photoinduced ET by analyzing intermolecular ET from ferrocene derivatives as electron donor to ³[H₃DPP⁺]* as an electron acceptor to determine the λ value of intermolecular ET for H₃DPP⁺to be 1.85 eV. This is the first report on a reorganization energy of a monoprotonated porphyrin in ET.

Figure 1. Driving-force dependence of log k_et for ET from electron donors to ³[H₃DPP⁺]* and a fitting curve based on the Marcus theory of ET (black). Gray: H₄DPP²⁺(Cl^–)₂ in acetonitrile.³1: Fc, 2: Me₂Fc, 3: Me₅Fc, 4: Me₈Fc, 5: Me₁₀Fc.

References

• Medforth, C. J.; Senge, M. O.; Smith, K. M.; Sparks, L. D.; Shelnutt, J. A. J. Am. Chem. Soc.1992, 114, 9859-9869.

• Honda, T.; Kojima, T.; f*ckuzumi, S. Chem. Commun.2009, 4994-4996.

• Nakanishi, T.; Ohkubo, K.; Kojima, T.; f*ckuzumi, S. J. Am. Chem. Soc.2009, 131, 577–584.

Figure 1

The molecular heterojunction triphenylamine/C₆₀ is a potential material to construct high-efficient dye-sensitized solar cells. We have carried out a combined molecular dynamics and time-dependent density functional theory study to analyze charge separation and exciton delocalization in excited states of two constitutional isomers of this interface.^[1] Our results reveal the presence of exciton states characterized by partial charge-transfer character (hybrid states); which are suggested to promote the direct charge separation process by excitation. These states may have a significant impact on the efficiency of the light-harvesting ensembles. That is to say, a greater amount of such hybrid states is found at short distances between the triphenylamine fragment and the C₆₀ cage; as a result, charge transfer reactions can occur faster. This trend resembles the experimental evidence since rate constants for photoinduced charge transfer reactions k_ct are bigger for the constitutional isomer with the shortest distance between the triphenylamine fragment and the C₆₀ cage.^[2] We have also evaluated the performance of U-CAM-B3LYP/6-31G* and TD-CAM-B3LYP/6-31G* level of theories to compute the different parameters affecting k_ct; that is the reorganization energy λ, the electronic coupling V_ij, and free energies ΔG.^[3] Our study is done under the theory of nonadiabatic electron transfer. By using the standard Marcus formulation, our results show that the outer-sphere contribution to λ and ΔG for the recombination reaction are the parameters that more uncertainty introduces to the estimation of k_ct. We implemented a low-cost methodology based on U-DFT for the evaluation of the outer-sphere part of λ and ΔG; wherein Franck-Condon and relaxed charge-transfer states are conveniently modeled, including properly the solvent reorganization energy, by U-CAM-B3LYP/6-31G* coupled to continuum solvation models. Finally, it is worth to mention that the replacement of the C₆₀ cage by I_h-Sc₃N@C₈₀ generates longer-lived charge-transfer states; which is attributed to the lower ΔG of the charge recombination reaction.^[2] Our group is currently analyzing this trend for the whole family of endohedral metallofullerenes; for which dynamics is expected to be in the normal region of the Marcus parabola.

[1] J. P. Martínez, S. Osuna, M. Solà, A. Voityuk, Theor. Chem. Acc.2015, 134, 12.

[2] J. R. Pinzón, D. C. Gasca, S. G. Sankaranarayanan, G. Bottari, T. Torres, D. M. Guldi, L. Echegoyen, J. Am. Chem. Soc.2009, 131, 7727–7734.

[3] J. P. Martínez, M. Solà, A. Voityuk, 2015, in preparation.

Figure 1

Photoinduced electron transfer (ET) is one of the most fundamental processes in physics, chemistry, and biology. In the reaction center of natural photosynthesis, photoinduced ET generates a long-lived charge separation (CS) state with ~100% efficiency, leading to light-to-chemical energy conversion. In contrast, photoinduced CS at the interfaces of organic photovoltaic cells and dye-sensitized solar cells generates electron-hole pair, eventually achieving light-to-electricity conversion. However, the interfaces of semiconductor/dye and donor/acceptor (D/A) in artificial photosynthesis and organic photovoltaic cells often suffer from the partial or even large loss of the CS state at the early stage, which has still been controversial and not unveiled owing to inevitable inhom*ogeneous spatial distribution of D-A components.

In this context D-A linked systems have generated remarkable interest for the last few decades. The D-A covalent linkage can eliminate complex factors arising from diffusion in solutions and assess the photoinduced ET properties precisely with the help of hom*ogenous spatial distribution of the D-A components. So far these systems have provided fundamental information on photoinduced ET. Impact of various ET parameters, i.e., driving force, electronic coupling, reorganization energy, and temperature, on ET rate has been evaluated by using elaborated D-A linked molecules. In particular, a well-defined D-A linked molecule with a rigid bridge has allowed us to shed light on photoinduced ET more accurately. Here we show unprecedented dependence of the final CS efficiency on D-A interaction (i.e., electronic coupling) that can be changed systematically in the D-A linked models with a one-dimensional (1D) nonconjugated bridge. We have thoroughly examined the photoinduced ET properties by using femtosecond to microsecond time-resolved transient absorption (TRTA) and electron paramagnetic resonance (TREPR) spectroscopies.

References

[1] H. Imahori and Y. Sakata, Adv. Mater. (Review Article), 9, 537-546 (1997).

[2] H. Imahori and Y. Sakata, Eur. J. Org. Chem. (Microreview), 1999, 2445-2457.

[3] H. Imahori, Y. Mori, and Y. Matano, J. Photochem. Photobiol., C: Photochem. Rev., 4, 51-83 (2003).

[4] H. Imahori, Org. Biomol. Chem. (Perspective), 2, 1425-1433 (2004).

[5] H. Imahori, J. Phys. Chem. B (Feature Article), 108, 6130-6143 (2004).

[6] H. Imahori and S. f*ckuzumi, Adv. Funct. Mater. (Feature Article), 14, 525-536 (2004).

[7] H. Imahori, Bull. Chem. Soc. Jpn. (Award Accounts), 80, 621-636 (2007).

[8] T. Umeyama and H. Imahori, J. Phys. Chem. C (Feature Article), 117, 3195-3209 (2013).

Nowadays, a complete understanding of fullerenes chemical reactivity has been achieved and, in this regard, we have demonstrated that a full control in the stereoselective functionalization of fullerenes is also possible, by using different asymmetric metal- and organo-catalyzed processes.¹Furthermore, fullerenes and transition metals have given rise to a wide set of sophisticated metallo-fullerene hybrids.²However, despite the rich chemistry developed "onto fullerenes", the chemistry carried out "with fullerenes" has been almost neglected. Thus, an intriguing and still open question is whether fullerenes could be involved in promoting or catalysing chemical transformations and, in particular, if fullerene double bonds are suitable to act in transfer hydrogenation processes.

In order to address these questions, we have designed and prepared a fullerene hybridwhere C₆₀is endowed with a metal atom, such as Ir, that has shown to have a high activity in transfer hydrogenation reactions. Thus, a new hybrid iridium-fullerene has been synthesized in which the fullerene moiety, far from being innocent, plays a leading role as a multi-site carbon-based hydrogen transfer catalyst. [60]Fullerene, analogously to NAD(P)H, acts as a hydrogen borrowing molecule by the reversible uploading of hydrogen. The highly efficient reduction of ketones with isopropanol as hydrogen donor, or the N-alkylation of amines with alcohols catalyzed by iridium-fullerene represent the first examples of fullerenes as catalysts in chemical reactions.

References

1. a) Maroto, E. E.; Izquierdo, M.; Reboredo, S.; Marco-Martínez, J.; Filippone, S.; Martín, N. Acc. Chem. Res.2014, 47, 2660. b) Filippone, S.; Maroto, E. E.; Martín-Domenech, A.; Suárez, M.; Martín, N. Nat. Chem.2009, 1, 578; c)Maroto, E. E.; Mateos, J.; Garcia-Borràs, M.; Osuna, S.; Filippone, S.; Herranz, M. Á.; Murata, Y.; Solà, M.; Martín, N. J. Am. Chem. Soc.2015, 137, 1190.

2.a) Lebedeva, M. A.; Chamberlain, T. W.; Khlobystov, A. N. Chem. Rev. 2015, 115 (20), 11301;(b) Matsuo, Y.; Nakamura, E. Chem. Rev.2008, 108, 3016.

Figure 1

Molecular engineering of donor-acceptor conjugates to control their photoinduced processes to yield the much desired long-lived charge separated states has been one of the major goals in artificial photosynthesis. The charge separated states of sufficient lifetime and stored energy are key in ultimately driving the photocatalytic processes of either light-to-electricity conversion or light-to-fuel production. In simple donor-acceptor dyads, charge separation from the singlet excited donor or acceptor due to rapid charge recombination process results in a short-lived charge separated state of singlet spin character. Utilization of high energy triplet sensitizers instead of the traditionally employed singlet sensitizers could be an elegant approach to generate long-lived charge separated states. The radical ion-pairs of triplet spin character charge recombine to the ground state slowly due to spin forbidden nature of the process. However, witnessing such a process purely from the triplet excited state without the interference of the singlet excited state and other competing photochemical events such as excitation energy transfer has been challenging to establish.

Here, we report our strategy to observe electron transfer exclusively from the triplet excited state of the donor in PdP-C₆₀ dyads (see figure below). Using transient absorption spectral studies operating at different timescales and time-resolved electron paramagnetic resonance (TR-EPR) studies, photoinduced electron transfer from ¹PdP* to C₆₀ leading to the formation of PdP^•+-C₆₀^•- charge separated state of singlet spin character has been witnessed in 2 (instead of triplet-triplet energy transfer observed in 1¹) due to the close proximity of the donor and acceptor entities. In contrast, by lowering the energy of the [(TPA)₃PdP]^•+-C₆₀^•- state owing to the presence of electron rich TPA entities (facile oxidation) compared to that of ³C₆₀*, electron transfer from ³(TPA)₃PdP* to C₆₀ leading to [(TPA)₃PdP]^•+-C₆₀^•- charge separated state of triplet spin character with lifetime in the microsecond range has been observed in 3 in polar solvents. Results of these findings and its significance will be highlighted.

• C. Obondi, G. N. Lim, F. D'Souza, Triplet-Triplet Excitation Transfer in Palladium Porphyrin – Fullerene and Platinum Porphyrin – Fullerene Dyads' J. Phys. Chem. C 2015, 119, 176-185.

Figure 1

Based on the unique electronic and structural properties, carbon nanostructures and are promising candidates for the constructions of advanced multidimensional organic architectures. Therefore a detailed understanding of the chemical and physical properties of these all-carbon structures is crucial for the further and targeted development of advanced materials. The main purpose of our work is to design and investigate new classes of carbon nanostructures with defined structural, physical, and chemical properties which are suitable as functional materials, e.g. in the field of organic conductors, photonic applications, and biological applications.

Herein we report the recent progresses achieved in our laboratory on the synthesis, characterization, and physical studies of new classes of [60]fullerene-based reaction centers equipped with new chromophoric antennas.

Phthalocyanines (Pcs) are planar, two-dimensional aromatic pophyrinoids which possess outstanding structural, optical and electrical properties as well as stability that render them perfect building blocks for the incorporation into multifunctional materials. Similarly, I_h-Sc₃N@C₈₀ fullerene – the most abundant member of the fullerenes family after C₆₀ and C₇₀ – in addition to have excellent electron-accepting properties, provides conditions for an extra stabilization of charge-separated states due to the presence of the internal Sc₃N species. This makes I_h-Sc₃N@C₈₀ an ideal molecular partner for photo- and electroactive molecular systems such as Pcs.

Although several reports have appeared on the preparation of covalent Sc₃N@I_h-C₈₀ ensembles, there is no use of I_h-Sc₃N@C₈₀ for the preparation of non-covalent assemblies. In this context, plenty of supramolecular systems arranged via coordination interactions have been successfully demonstrated for Pc-C₆₀ supramolecular dimers and trimers. Low accessability of non-covalent supramolecular systems with I_h-Sc₃N@C₈₀ is mainly caused by the more sophisticated synthesis and purification of the corresponding fullerene derivatives.

Herein, we report a novel example of donor-acceptor Pc/I_h-Sc₃N@C₈₀ supramolecular dimers assembled via metal-ligand coordination interactions between Zn(II)Pcsand pyridil-substituted I_h-Sc₃N@C₈₀ or C₆₀ fullerene. Photophysical (ground- and excited state) and electrochemical studies on these ensembles showed that depending on the electron-donor or electron-acceptor character of the ZnPcs, a switchable electron transfer reactivity occurs in the case of the I_h-Sc₃N@C₈₀ supramolecular systems, revealing the ability for this endohedral metallofullerene to behave either as electron donor or electron acceptor unit.

Photosensitivity is one of the most important physico-chemical properties of fullerenes. Photoexcited C₆₀ and C₇₀ can cause the electron transfer (type I)¹ or energy transfer (type II)^2,3 reaction under visible light irradiation in quite high quantum yields (quantitative). These reactions have been used to develop new functional materials for the application of photovoltaic devices and photodynamic therapy agents. Especially in the latter application, generation of reactive oxygen species (ROSs)⁴ such as ¹O₂ and O₂•^–5,6is used to affect the biological molecules.

The generation of ¹O₂ has been also used in the organic reaction. Using fullerenes as photocatalyst, oxidation of olefins,⁷ phenols⁸ and sulfides⁹ were reported. In this study, we used C₇₀ for the oxidation of benzylamines. Compared to C₆₀ and other commonly used photosensitizers such as tetraphenylporphyrin (TPP),^10,11 C₇₀ could catalyze the oxidation of benzylamines to corresponding imines efficiently with lower catalyst loading (0.01 mol %). The reaction worked also under the lower energy of lights using blue (max 470 nm) and green (max 520 nm) diodes. Reaction processed in clean manner without any significant amount of side product, and removal of the C₇₀catalyst was carried out easily by just simple precipitation and filtration.

(1) Arbogast, J. W.; Foote, C. S.; Kao, M. J. Am. Chem. Soc.1992, 114, 2277. (2) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys. Chem.1991, 95, 11. (3) Arbogast, J. W.; Foote, C. S. J. Am. Chem. Soc.1991, 113, 8886. (4) Tokuyama, H.; Yamago, S.; Nakamura, E.; Shiraki, T.; Sugiura, Y. J. Am. Chem. Soc.1993, 115, 7918. (5) Yamakoshi, Y.; Sueyoshi, S.; f*ckuhara, K.; Miyata, N. J. Am. Chem. Soc.1998, 120, 12363. (6) Yamakoshi, Y.; Umezawa, N.; Ryu, A.; Arakane, K.; Miyata, N.; Goda, Y.; Masumizu, T.; Nagano, T. J. Am. Chem. Soc.2003, 125, 12803. (7) Tokuyama, H.; Nakamura, E. J. Org. Chem.1994, 59, 1135. (8) Jensen, A. W.; Daniels, C. The J. Org. Chem.2003, 68, 207. (9) Latassa, D.; Enger, O.; Thilgen, C.; Habicher, T.; Offermanns, H.; Diederich, F. J. Mater. Chem.2002, 12, 1993. (10) Jiang, G.; Chen, J.; Huang, J. S.; Che, C. M. Org. Lett.2009, 11, 4568. (11) Ushakov, D. B.; Gilmore, K.; Kopetzki, D.; McQuade, D. T.; Seeberger, P. H. Angew. Chem.-Int. Ed.2014, 53, 557.

One of our research field concerns porphyrin-functionalized α-polypeptides to control the spatial organization of the chromophores and enable exciton migration through the pool of porphyrins to mimic the light harvesting antennae of the photosynthetic system.¹

In the photosynthetic system, the solar energy is collected by pigment molecules attached to the light harvesting complexes, in which the chlorophylls are held in a parallel orientation by fairly short a-helical polypeptides. When a photon hits one of the chlorophylls, the absorbed energy spreads extremely rapidly to the others until the reaction center is reached.¹ Succeeding in organizing the porphyrins in a similar way as nature does should provide information about the relation existing between the orientation of the chlorophylls and the efficiency of nature in transferring an excited state over a very long distance and with minimal loss of energy.

The right-handed 3₁₀ helical conformation adopted by our poly-L-lysine functionalized with eight porphyrins induces an overlap of the chromophores (studies realized in collaboration with Prof. T. Keiderling, University of Chicago, US), which thus present sufficient electronic coupling to promote a good exciton migration within the molecular wire.

The properties of our peptides with pendant porphyrins were studied extensively in collaboration with with Professor S. f*ckuzumi (Univeristy of Osaka, Japan) and applications were found as organic photovoltaic cells,² multiple photosynthetic reaction centers³ and for the successful purification of Single-Walled Carbon Nanotubes.⁴

Acknowledgements

This work was supported by the CNRS and the French Ministry of Research.

References

• W. Kühlbrandt, Nature1995, 374, 497-498.

• T. Hasobe, K. Saito, P. V. Kamat, V. Troiani, H. Qiu, N. Solladié, K. S. Kim, J. K. Park, D. Kim, F. D'Souza, S. f*ckuzumi, J. Mater. Chem.2007, 39, 4160-4170.

• S. f*ckuzumi, K. Saito, K. Ohkubo, V. Troiani, H. Qiu, S. Gadde, F. D'Souza and N. Solladié, Phys. Chem. Chem. Phys.2011, 13, 17019-17022.

• Kenji Saito, Vincent Troiani, Hongjin Qiu, Nathalie Solladié, Takao Sakata, Hirotaro Mori, Mitsuo Ohama, Shunichi f*ckuzumi, J. Phys. Chem. C 2007, 111, 1194-1199.

Figure 1

Carbon-based nanostructured materials (fullerenes, carbon nanotubes or graphene, to name a few) are known to show electron-acceptor properties at the molecular scale, together with high values of electron mobilities in bulk. Thus, its implementation in active materials is an important issue in order to develop organic efficient full optoelectronic devices. One of the strategies employed in the improvement of the interesting optoelectronic properties of carbon nanostructures, is the doping, either in a covalent or a non-covalent way, with specially designed electron-donor/light harvesting moieties. These moieties act like light collectors, exciting themselves and promoting an electron to a higher energy state, which is rapidly deactivated through electron and/or energy transfer processes to the carbon-based nanostructure.

Here we present different strategies for the covalent and non-covalent functionalization of nanocarbon materials. In these examples, the decorating agents, specially designed perylenediimides[1-4] (Figure 1) and others, whose electronic character has been modulated through the introduction of different substituents, are successfully employed to improve the light absorption behaviour in the UV and NIR regions of the, in principle, unabsorbing carbon nanoforms, thus yielding hybrid materials with potential applicability like active layers in organic photovoltaic devices.

Acknowledgements. This work has been supported by the Spanish Ministerio de Economía y Competitividad, Generalitat Valenciana and the European FEDER funds (grants CTQ2011-26455, CTQ2014-55798-R, PROMETEO 2012/010 and ISIC/2012/008)

References

• Martín-Gomis, L.; Rotas, G.; Ohkubo, K.; Fernández-Lázaro, F.; f*ckuzumi, S.; Tagmatarchis, N.; Sastre-Santos, Á. Nanoscale 2015, 7, 7437-7444..

• Barrejón, M.; Pla, S.; Berlanga, I.; Gómez-Escalonilla, M. J.; Martín-Gomis, L.; Fierro, J. L.; Zhang, M.; Yudasaka, M.; Iijima, S.; Gobeze, H. B.; D'Souza, F.; Sastre-Santos, Á.; Langa, F. J. Mater.Chem. C. 2015, 3, 10215-10224.

• Pla, S.; Niemi, M.; Martín-Gomis, L.; Fernández-Lázaro, F.; Lemmetyinen, H.; Tkachenko, N. V.; Sastre-Santos, Á. Phys.Chem. Chem. Phys. Accepted.

• Martín-Gomis, L.; Karousis, N.; Petsalakis, I. D.; Ohkubo, K.; Fernández-Lázaro, F.; Tagmatarchis, N.; f*ckuzumi, S.; Sastre-Santos Á. Submitted.

Figure 1. Chemical structures of a) [60]Fullerene-Perylenediimide (C₆₀-PDI, DB-1,7-Pyr-PDI-C₆₀) and b) Double Wall Carbon Nanotube-Perylenedimide (DWCNT-PDI).

Figure 1

The combination of two carbon nanostructures in a single material, giving rise to new hybrids with improved mechanical, electrical, and thermal properties, has sparked a great interest in the last years.[1],[2],[3] Hybrids combining carbon nanotubes and fullerene derivatives have been described by endohedral,[4] supramolecular[5] and covalent[1] approaches, showing, in general, rather poor solubility in common solvents, which hampered the study of the resulting photophysical properties. In this regard, the synthesis of nanohybrids bearing long alkyl chains,[2]_^,[6]improve the dispersibility in organic solvents, rendering them excellent candidates for their application in the semiconducting materials field.

In this presentation, we will discuss our recent studies involving the synthesis of nanohybrids combining carbon nanotubes and C₆₀, as well as the new photophysical properties arising from the combination of both carbon nanomaterials.

[1] J. L. Delgado, P. de la Cruz, A. Urbina, J. T. López Navarrete, J. Casado and F. Langa, Carbon, 2007, 45, 2250.

[2] M. Vizuete, M. J. Gómez-Escalonilla, J. L. G. Fierro, M. Yudasaka, S. Iijima, M. Vartanian, J. Iehl, J.-F. Nierengarten and F. Langa, Chem. Commun., 2011, 47, 12771.

[3] M. Vizuete, M. Barrejón, M. J. Gómez-Escalonilla and F. Langa, Nanoscale, 2012, 4, 4370.

[4] B. W. Smith, M.Monthioux and D. E.Luzzi,Nature, 1998,396,323.

[5] D. M. Guldi, E. Menna, M. Maggini, M. Marcaccio, D. Paolucci, F. Paolucci, S. Campidelli, M. Prato, G. M. A. Rahman and S. Schergna, Chem. Eur. J.2006, 12, 3975.

[6] N. Mackiewicz, T. Bark, B. Cao, J. A. Delaire, D. Riehl, W. L. Ling, S. Foillard and E. Doris, Carbon, 2011, 49, 3998.

The symmetrical, malonodiserinolamide hexa-substituted [60]fullerene, C₆₀-ser, is a key water-soluble C₆₀ derivative of interest for the treatment of cancer by radiofrequency(RF)-induced hyperthermia due to its biocompatibility, neutral charge, lipophilicity, and ability to cross restrictive biological membranes. In aqueous solution C₆₀-ser, which is surrounded by 24hydroxyl groups, maintains relatively stable layers of closely-bound water molecules (hydration sphere), forms highly dynamic aggregates, sized between 100-400nm in diameter, which exist in equilibrium with individual, hydrated C₆₀-ser molecules. Our recent observations indicate that these dynamic C₆₀-ser aggregates are built up of a single layer of C₆₀-ser molecules, with water locked inside a hollow core structure. Using a thermal imaging camera, aqueous C₆₀-ser solutions have been shown to produce heat when irradiated with an 13.56 MHz RF field (900W) in a concentration-dependent manner with a heating rate of up to 1.0°C/sec at 10mg/ml. This result establishes that C₆₀-ser is a neutrally-charged, key [60]fullerene-based nanoparticle, that might be used for the treatment of cancer by RFH-induced hyperthermia.

We acknowledge Kanzius Foundation and Welch Foundation Grant C-0627 for financial support.

Figure 1

Two-dimensional nanomaterials and their association into 2-D heterostructures have been extensively studied for the last couple of years. Thus, the development of new, versatile, high-resolution visualization and placement techniques is highly desirable. It is well known that a single layer of graphene can be observed under optical microscopy on Si/SiO₂ substrates because of interference effects when the thickness of silicon oxide is optimized.^1,2However, differentiating monolayers from bilayers remains a challenge and advanced techniques like Raman mapping, Atomic Force or Scanning Electron Microscopy (AFM, SEM) are more suitable to observe monolayers of graphene. Raman mapping and AFM are relatively slow and SEM is operated in vacuum so that in all cases real-time experiments including chemical modifications are not accessible. Therefore, the development of techniques that combine the large scale and speed of SEM, the topological information of AFM and the simplicity optical microscopy may greatly facilitate the study of nanomaterials.

In our group, we are interested in particular forms of graphene derivatives which are graphene oxide (GO) and reduced graphene oxide (r-GO). While a single layer of graphene absorbs 2.3% of the incident light,³ graphene oxide monolayer exhibits a weaker light absorption that makes it almost impossible to distinguish on Si/SiO₂ surface. In 2007, Ruoff and co-workers were able to observe directly GO flakes using confocal microscopy; they even obtained higher contrast for GO than for graphene on Si/SiO₂. For that they optimized a substrate by depositing silicon nitride layers from 60 to 70 nm on silicon.⁴

Here we introduce a novel optical microscopy technique based on the use of Anti-Reflecting and Absorbing (ARA) layers yielding to ultra-high contrast reflection imaging of graphene monolayers in air or in water. We name this technique "Backside Absorbing Layer Microscopy" (BALM) and we illustrate its efficiency by in-situ imaging graphene oxide (see Figure) and its chemical modification.

Reference:

(1) Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Appl. Phys. Lett.2007,91, 063124.

(2) Roddaro, S.; Pingue, P.; Piazza, V.; Pellegrini, V.; Beltram, F. Nano Lett.2007,7, 2707-2710.

(3) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Science2008,320, 1308.

(4) Jung, I.; Pelton, M.; Piner, R.; Dikin, D. A.; Stankovich, S.; Watchrotone, S.; Hausner, M.; Ruoff, R. S. Nano Lett.2007,7, 3569-3575.

Figure 1

The recognition of fullerenes (C₆₀ and its higher analogues) is one of the most intensive research subjects in supramolecular and host-guest chemistry. Bowl-type p-conjugated molecules, for example, corannulene or sumanene, are promising host molecules because their concave surfaces closely resemble fullerene segments and can fit precisely to the convex surface of C₆₀. In particular, these molecules are attractive because of their potential to function as chiral hosts based on "bowl-chirality". However, racemization easily underwent in many of chiral bowl-type molecules. We envision a strategy for the design of a chiral concave molecule using a phosphorus-doping method, which can play a crucial role in construction of the concave structure. We now report the synthesis of the chiral concave molecule by extension of the p-surface ofphosphorus-containing concave molecule.

We synthesized the C₃-symmetric chiral concave mole-cule having a phosphorus atom at the center and resolved the enantiomers. The chiral concave structure was revealed by X-ray analysis. The concave molecule exhibited strong CD signals with a large anisotropy, the detail of which was confirmed by TD DFT calculation. In the co-crystal with C₆₀, four molecules of enantiopure perfectly wrapped the surface of C₆₀. MALDI-TOF mass and NMR spectra also supported the concave-convex interaction between the host molecule and C₆₀. Furthermore, the observa-tion of the ICD signal of the guest fullerene indicates the transfer of the chiroptical properties of the host molecule to the guest. These results suggest that the chiral host is a component of a chiral self-assembled host for C₆₀. The self-assembly of the concave molecules and the properties of the host-guest complex with fullerene will be presented.

Figure 1

The development of graphene spintronic devices requires that, in addition to its capability to passively transmit spins over long distances, new magnetic functionalities are incorporated to graphene. By growing epitaxially graphene on single crystal metal surfaces under UHV conditions [1] and either adsorbing molecules on it or intercalating heavy atoms below it, long range magnetic order or giant spin-orbit coupling, respectively, can be added to graphene.

i) Achieving long range magnetic order by a monolayer of electron acceptor molecules adsorbed on graphene /Ru(0001).

Epitaxial graphene is spontaneously nanostructured forming an hexagonal array of 100 pm high nanodomes with a periodicity of 3 nm [2]. Cryogenic Scanning Tunnelling Microscopy (STM) and Spectroscopy and DFT simulations show that TCNQ molecules deposited on gr/Ru(0001) acquire charge from the (doped) substrate and develop a sizeable magnetic moment revealed by a prominent Kondo resonance. The molecular monolayer self-assembled on graphene develops spatially-extended spin-split electronic bands. The predicted spin alignment in the ground state is visualized by spin-polarized STM at 4.6 K [3]. The system shows promising perspectives to become an effective graphene-based spin filter device.

ii) Introducing a giant spin-orbit interaction on graphene/Ir(111) by intercalation of Pb. The intercalation of an ordered array of Pb atoms below graphene results in a series of sharp pseudo-Landau levels in the differential conductance revealed by STS at 4.6 K. The vicinity of Pb enhances by four orders of magnitude the, usually negligible, spin-orbit interaction of graphene. The spatial variation of the spin-orbit coupling creates a pseudo-magnetic field that originates the observed pseudo-Landau levels [4]. This may allow the processing and controlled manipulation of spins in graphene.

References

[1] A.L. Vázquez de Parga et al, Phys. Rev. Lett. 100, 056807 (2008) [2] B. Borca et al, Phys. Rev. Lett. 105, 036804 (2010) [3] M. Garnica et al, Nature Physics 9, 368 (2013) [4] F. Calleja et al, Nature Physics 11, 43 (2015)

Using a modified scanning tunnelling microscope we measure simultaneously the thermopower and conductance of single-molecule junctions of fullerenes connected to gold electrodes. We find that in contrast with C₆₀ (1) the endohedral fullerene Sc₃N@C₈₀ (2) forms jnctions in which the magnitude and sign of the thermopower depend strongly on the orientation of the molecule and on applied pressure. We demonstrate that the origin of this exceptional behavior is the presence of a sharp resonance near the Fermi level created by the Sc₃N inside the fullerene cage, whose energetic location, and hence the thermopower, can be tuned by applying pressure. These results reveal that Sc₃N@C80 is a bi-thermoelectric material, exhibiting both positive and negative thermopower, and provide an unambiguous demonstration of the importance of transport resonances in thermoelectric performance of organic materials.

(1) C. Evangeli et al, Nanoletters 2013, 13, 2141.

(2) L. Rincon-Garcia et al, Nature Materials 2015 doi:10.1038/nmat4487.

Figure 1