M. Ángeles Herranz

Low dimensional nanocarbons – chemistry and energy/electron transfer reactions

This Minireview sheds light onto the electronic communication between, on one hand, low dimensional nanocarbons – single and multiwalled 1D carbon nanotubes and 2D graphene – and, on the other hand, a variety of electroactive species en-route to novel electron donor–acceptor conjugates and hybrids in relation to their covalent and non-covalent chemistry, respectively. A common denominator to any of the highlighted conjugates/hybrids is charge transport across different scales, that is, from individual molecular conjugates/hybrids to morphologically controlled devices.

Tetrathiafulvalene-Based Nanotweezers—Noncovalent Binding of Carbon Nanotubes in Aqueous Media with Charge Transfer Implications

Electron donor-acceptor hybrids based on single wall carbon nanotubes (SWCNT) are one of the most promising functional structures that are currently developed in the emerging areas of energy conversion schemes and molecular electronics. As a suitable electron donor, π-extended tetrathiafulvalene (exTTF) stands out owing to its recognition of SWCNT through π-π stacking and electron donor-acceptor interactions. Herein, we explore the shape and electronic complementarity between different types of carbon nanotubes (CNT) and a tweezers-shaped molecule endowed with two exTTFs in water. The efficient electronic communication between semiconducting SWCNT/multiwall carbon nanotubes (MWCNT), on one hand, and the water-soluble exTTF nanotweezers 8, on the other hand, has been demonstrated in the ground and excited state by using steady-state as well as time-resolved spectroscopies, which were further complemented by microscopy. Importantly, appreciable electronic communication results in the electronic ground state having a shift of electron density, that is, from exTTFs to CNT, and in the electronic excited state having a full separation of electron density, that is oxidized exTTF and reduced CNT. Lifetimes in the range of several hundred picoseconds, which were observed for the corresponding electron transfer products upon light irradiation, tend to be appreciably longer in MWCNT/8 than in SWCNT/8.

Donor−Acceptor Conjugates of Lanthanum Endohedral Metallofullerene and π-Extended Tetrathiafulvalene

Stable donor-acceptor conjugates (2, 3) involving an endohedral metallofullerene, La(2)@I(h)-C(80), and pi-extended tetrathiafulvalene (exTTF) have been synthesized by highly regioselective 1,3-dipolar cycloadditions of exTTF-containing azomethine ylides to the endofullerene, yielding exclusively [5,6] metallofulleropyrrolidines with C(1) symmetry in high yields (68-77%). The cyclic voltammograms (CVs) of the conjugates reveal the redox active character of the system due to the presence of both donor and acceptor groups, that is, exTTF and La(2)@I(h)-C(80), respectively. Furthermore, the electrochemically reversible character of the endofullerene confirms the presence of the [5,6] adduct. Despite the relatively close proximity between the exTTF and the endohedral metallofullerene (EMF), only a weak electronic interaction was observed in the ground state, as evidenced by absorption spectroscopy and CV measurements of 2 and 3. On the other hand, in the excited state the fast formation of a radical ion-pair state (i.e., 6.0 x 10(10) s(-1)), that is, the reduction of the electron accepting La(2)@C(80) and the oxidation of exTTF, evolves with lifetimes as long as several ns (3.0 x 10(8) s(-1)) in toluene. Transient absorption spectroscopy experiments confirmed these observations.

A Molecular Ce2@Ih-C80 Switch—Unprecedented Oxidative Pathway in Photoinduced Charge Transfer Reactivity

We report for the first time the versatile Ce(2)@I(h)-C(80) building block toward synthesizing a novel electron donor-acceptor conjugate, Ce(2)@I(h)-C(80)-ZnP (1). A systematic investigation of the charge transfer chemistry documents a reductive charge transfer (i.e., formation of (Ce(2)@I(h)-C(80))(*-)-(ZnP)(*+)) in nonpolar media (i.e., toluene/THF), while an oxidative charge transfer (i.e., formation of (Ce(2)@I(h)-C(80))(*+)-(ZnP)(*-)) dominates in polar media (i.e., benzonitrile/DMF). Reduction of the [Ce(2)](6+) cluster, which is highly localized and collinearly arranged with respect to the quaternary bridge carbon, is sufficiently exothermic in all solvents. Notably weak is the electronic coupling between the [Ce(2)](6+) cluster and the electron-donating ZnP. The oxidation of C(80)(6-) and the simultaneous reduction of ZnP, on the other hand, necessitate solvent stabilization. In such a case, the strongly exothermic (Ce(2)@I(h)-C(80))(*-)-(ZnP)(*+) radical ion pair state formation is compensated within the framework of a nonadiabatic charge transfer by a C(80)(6-)/ZnP electronic matrix element, as the sum of good overlap and short distance, that exceeds that for [Ce(2)](6+)/ZnP.

Electron transfer in Me-blocked heterodimeric α,γ-peptide nanotubular donor–acceptor hybrids

Bio-inspired cyclopeptidic heterodimers built on β-sheet-like hydrogen-bonding networks and bearing photoactive and electroactive chromophores on the outer surface have been prepared. Different cross-strand pairwise relationships between the side chains of the cyclic α,γ-peptides afford the heterodimers as three nonequivalent dimeric species. Steady-state and time-resolved spectroscopies clearly show an electron transfer process from π-extended tetrathiafulvalene, covalently attached to one of the cyclopeptides, to photoexcited [60]fullerene, located on the complementary cyclopeptide. The charge-separated state was stabilized for up to 1 μs before recombining and repopulating the ground state. Our current example shows that cyclopeptidic templates can be successfully used to form light-harvesting/light-converting hybrid ensembles with a distinctive organization of donor and acceptor units able to act as efficient artificial photosystems.

Pyrene-tetrathiafulvalene supramolecular assembly with different types of carbon nanotubes

Pyrene-tetrathiafulvalenes reveal strong interactive forces with different types of carbon nanotube (CNT) samples—ranging from single-walled carbon nanotubes (SWNTs) to multi-walled carbon nanotubes (MWNTs). Notable, although subtle, are the differences that emerged while preparing, handling, and characterizing these composite materials. Stronger are the differences seen in the context of photoinduced charge transfer. In particular, charge injection into the conduction band of CNT probes afforded stable radical ion pair states (i.e., on the time scale of our femto/picosecond investigation) only for MWNTs, while the lifetimes for SWNTs are shorter. A likely rationale involves the multiple electron acceptor levels in MWNTs, when compared to SWNTs or double-walled carbon nanotubes (DWNTs).

Supramolecular chemistry of π-extended analogues of TTF and carbon nanostructures

The shrewd combination of complementary electroactive molecular fragments through weak, non-covalent forces can be exploited to construct stimuli-responsive assemblies or to achieve self-ordered arrays of electron donor and/or acceptor moieties to be utilized in optoelectronic devices. Among the electron donors, tetrathiafulvalene (TTF) has been particularly fashionable in this field. Comparatively, the supramolecular chemistry of its π-extended analogues was severely underdeveloped. Herein we present a summary of some recent results in the exploration of the non-covalent chemistry of π-extended analogues of tetrathiafulvalene.

Bowl-shape electron donors with absorptions in the visible range of the solar spectrum and their supramolecular assemblies with C60

We describe the synthesis, electronic, optical and photophysical properties of a family of three electron-donor bowl-shaped organic molecules that absorb light in the whole range of the visible spectrum (up to 800 nm in one case), and associate C60 in solution with binding constants in the range of 104–102 M−1 as measured from both UV-vis and fluorescence titrations in several solvents. These molecules are π-extended derivatives of tetrathiafulvalene, based on a truxene core to which two or three units of dithiole are covalently attached. The inclusion of the bulky dithiole groups is responsible for their bowl-shape geometry, which allows them to associate with C60, and their electron-donor character. The symmetric derivative 1, with three dithiole units, absorbs light in the 370–520 nm range. Exchanging one of the dithiole groups by an electron-withdrawing group, ketone (2) and dicyanomethylene (3), results in an intramolecular push–pull effect that extends the absorption to nearly 700 nm in the case of 2, and up to 800 nm in the case of 3. Transient absorption measurements, supported by spectroelectrochemical and radiolytical experiments, reveal that upon photoexcitation of the 1∙C60 associate the fully charge-separated state 1•+∙C60•− is generated, with lifetimes of hundreds of picoseconds. Molecular-level understanding of the electronic and supramolecular properties of 1–3 is provided by density functional theory calculations.

An Endohedral Metallofullerene as a Pure Electron Donor: Intramolecular Electron Transfer in Donor–Acceptor Conjugates of La2@C80 and 11,11,12,12-Tetracyano-9,10-anthra-p-quinodimethane (TCAQ)

An endohedral metallofullerene, La(2)@C(80), is covalently linked to the strong electron acceptor 11,11,12,12-tetracyano-9,10-anthra-p-quinodimethane (TCAQ) by means of the Prato reaction, affording two different [5,6]-metallofulleropyrrolidines, namely 1a and 2a. 1a and 2a were isolated and fully characterized by means of MALDI-TOF mass, UV-vis-NIR absorption, and NMR spectroscopies. In addition, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) corroborated the unique redox character of 2a, that is, the presence of the electron-donating La(2)@C(80) and the electron-accepting TCAQ. Although a weak electronic coupling dictates the interactions between La(2)@C(80) and TCAQ in the ground state, time-resolved transient absorption experiments reveal that in the excited state (i.e., π-π* centered at La(2)@C(80)) the unprecedented formation of the (La(2)@C(80))(•+)-(TCAQ)(•-) radical ion pair state evolves in nonpolar and polar media with a quantum efficiency of 33%.

Tetrathiafulvalene: A Paradigmatic Electron Donor Molecule

Abstract Tetrathiafulvalene (TTF) and its derivatives are exceptional building blocks in many areas of organic, supramolecular, and materials chemistry. Since the discovery ca. 30 years ago of the first “organic metal” tetrathiafulvalene-tetracyano-p-quinodimethane (TTF-TCNQ), a huge number of TTF derivatives have been synthetized.     Although initial efforts were directed to enhance the electron-donating ability of TTF analogues to improve the conductivities of salts and charge-transfer (CT) complexes derived from them, the developments in synthetic TTF chemistry have made it possible to incorporate TTF into more sophisticated structures such as materials exhibiting intramolecular charge-transfer and nonlinear optical properties, sensors, molecular shuttles and devices.     Compounds in which TTF and electron-accepting molecules, especially C 60 , are covalently tethered exhibit outstanding photophysical properties leading, upon photoexcitation, to charge-separated (CS) states showing remarkable lifetimes. In these systems, the gain of aromaticity upon oxidation of the TTF moiety has been used as a new concept for improving the stability of the charge-separated state, and, therefore, are of interest for the preparation of artificial photosynthetic systems as well as photovoltaic devices.