ChemComm View Article Online FEATURE ARTICLE View Journal | View Issue Covalent non-fused tetrathiafulvalene–acceptor systems Cite this: Chem. Commun., 2016, 52, 7906 Flavia Pop†* and Narcis Avarvari* Covalent donor–acceptor (D–A) systems have significantly contributed to the development of many organic materials and to molecular electronics. Tetrathiafulvalene (TTF) represents one of the most widely studied donor precursors and has been incorporated into the structure of many D–A derivatives with the objective of obtaining redox control and modulation of the intramolecular charge transfer (ICT), in order to address switchable emissive systems and to take advantage of its propensity to form regular stacks in the solid state. In this review, we focus on the main families of non-fused TTF–acceptors, which are classified according to the nature of the acceptor: nitrogen-containing heterocycles, BODIPY, Received 29th February 2016, perylenes and electron poor unsaturated hydrocarbons, as well as radical acceptors. We describe herein the Accepted 25th April 2016 most representative members of each family with a brief mention of their synthesis and a special focus on Creative Commons Attribution 3.0 Unported Licence. DOI: 10.1039/c6cc01827k their D–A characteristics. Special attention is given to ICT and its modulation, fluorescence quenching and switching, photoconductivity, bistability and spin distribution by discussing and comparing spectroscopic www.rsc.org/chemcomm and electrochemical features, photophysical properties, solid-state properties and theoretical calculations. 1. Introduction many organic materials, with applications in fields such as photovoltaics, nonlinear optics and molecular electronics.4 Covalent donor–acceptor systems have long attracted significant One of the most important electron donors, especially in the interest due to their involvement in electron transfer processes field of molecular materials, is the tetrathiafulvalene (TTF) This article is licensed under a in biology1–3 and as they constitute valuable precursors for unit,5 which has provided many organic metals and super- conductors6 and, more recently, multifunctional materials combining conductivity and magnetism,7 conductivity and chirality,8 Universite´ d’Angers, CNRS, Laboratoire MOLTECH-Anjou, UMR 6200, UFR Sciences, 9 Open Access Article. Published on 25 April 2016. Downloaded 9/27/2021 10:32:14 PM. Baˆt. K, 2 Bd. Lavoisier, 49045 Angers, France. E-mail: [email protected] or based on luminescent single molecule magnets or electro- 10 † Present address: School of Chemistry, The University of Nottingham, NG72RD active ligands and metal complexes. TTF has also been Nottingham, UK. E-mail: [email protected] associated with electron poor units and thus has been explored Flavia Pop obtained her PhD in Narcis Avarvari was born in 2009 (Babes--Bolyai University, Romania. He received his PhD in Cluj-Napoca, Romania and Chemistry in 1998 at the Ecole University of Angers, France) under Polytechnique, France. After one- the joint supervision of Prof. I. Grosu year postdoctoral stay at the ETH and Dr J. Roncali. She continued her Zu¨rich, he obtained in 1999 a research with a postdoc position in permanent research position the field of molecular materials with the CNRS in Nantes, and (with Dr N. Avarvari), focusing on then he moved in 2001 to the tetrathiafulvalene-based chiral University of Angers, laboratory conducting materials, covalent MOLTECH-Anjou. He was awarded donor–acceptor systems and chiral the 2007 Prize of the French Flavia Pop supramolecular aggregates. Flavia Narcis Avarvari Chemical Society, Coordination is currently a postdoctoral Chemistry Division. In 2010, he researcher working on chiral chromophores and their aggregation was promoted to CNRS director of research. He is currently heading for active materials in solar cells (with Prof. D. B. Amabilino, a research team dealing with molecular materials, crystal engineering The University of Nottingham, UK). and coordination chemistry, as well as self-assembly and chirality. 7906 | Chem. Commun., 2016, 52, 7906--7927 This journal is © The Royal Society of Chemistry 2016 View Article Online Feature Article ChemComm in donor–acceptor (D–A) systems. The manifold interest in II mixed valence species thanks to communication through the TTF–acceptor systems is primarily at the stage of fundamental TTF bridge. research, as in such compounds there is, for example, the Across this short historical saga of TTF–TCNQ, we wanted possibility to modulate the intramolecular charge transfer with to point out a very illustrative scientific process, originating the oxidation state of TTF, which can afford electrochromic from a hypothesis that evolved towards other numerous other and/or tuneable emission properties, or can allow obtaining directions, constituting the molecular electronics field,24 peculiar architectures in the solid state with either segregation and motivated an enormous amount of experimental work25 or alternation of the donors and acceptors.11 The interest in the that eventually provided several covalent TTF–TCNQ systems. latter feature is related to the possibility of observing charge Although molecular rectification was not reported in any of the separation and photoconductivity,12 and hence has potential in latter reports, their synthesis and investigations stimulated the fields of photovoltaics13,14 or molecular electronics.11,15 along the years many attempts at the covalent association of However, the first highlighted potential application of a covalent TTF and ext-TTF units to a large diversity of acceptors within donor–acceptor derivative was the molecular rectifier,16 theoretically fused and non-fused derivatives.11,26–28 Remarkably, inspired postulated by Aviram and Ratner in a hypothetical system consisting by Aviram and Ratner’s prediction, Bryce, Perepichka and of a TTF donor and a tetracyanoquinodimethane (TCNQ) acceptor colleagues prepared a TTF–s–trinitrofluorene system29 showing separated by a s-bridge (Scheme 1).17 Because of some synthesis molecular rectification behaviour.30 TTF–acceptor systems issues, it took more than twenty years after this theoretical published before 2004 were reviewed by Bendikov, Wudl and prediction for covalently linked TTF–TCNQ or TTF–TCNAQ Perepichka,11 while the more recent reviews by Martı´n et al.27 (TCNAQ = tetracyanoanthraquinodimethane) compounds to and Liu et al.28 deal with ext-TTF–acceptors and fused TTF– be described and properly characterized. Some of the first such acceptors. In this review, we focus on several recent representative examples consist of TTF–TCNAQ systems 2 and derivatives, as families of non-fused covalently linked TTF–acceptors, classified reported by Bryce et al.,18 and 3, as reported by Liu et al.,19 while according to the chemical nature of the acceptor, with an aim to 20 Creative Commons Attribution 3.0 Unported Licence. in compound 4, as described by Martin et al., TTF was highlight some of their most peculiar features. replaced by extended-TTF (ext-TTF) and a conjugated linkage between the two units was preserved. Both flexible 5 and rigid 6 TTF–s–TCNQ systems possessing small HOMO–LUMO gaps 2. Nitrogen-containing six-membered were described by Bryce and Perepichka et al.21 and Khodor- ring acceptors kovsky et al.,22 respectively. Finally, Rovira et al. reported the first fully conjugated TCNQ–TTF–TCNQ 7,23 in which the Themostrepresentativeunitofthenitrogenbasedsix-membered radical anion obtained by a one-electron reduction was a class rings is pyridine (Py), which has been attached to TTF in various This article is licensed under a Open Access Article. Published on 25 April 2016. Downloaded 9/27/2021 10:32:14 PM. Scheme 1 Aviram and Ratner’s theoretical molecular rectifier 1 and experimental systems based on TTF and TCNQ units. This journal is © The Royal Society of Chemistry 2016 Chem. Commun., 2016, 52, 7906--7927 | 7907 View Article Online ChemComm Feature Article ways (Scheme 2). Simple pyridines are poor electron acceptors Table 1 Electrochemistry, UV-vis spectroscopy and theoretical data for and therefore they are not expected to favour massive intra- TTF–Py 8–12 molecular charge transfer when attached to TTF units. This was 1 2 E1/2,ox E1/2,ox lmax ICT DEHOMO–LUMO confirmed by the relatively high energy charge transfer bands at Compound (V vs. SCE) (V vs. SCE) (nm) (eV) l a d = 400–440 nm (Table 1), indicative of large HOMOTTF–LUMOA 8a 0.439 0.777 428 3.253 gaps, which were observed in the directly attached TTF–Py 8a-H+ 0.529 0.827 574a 2.150d 31 32 33 8b 0.529 0.804 414a 3.389d 8a–b, 8c and Py–TTF–Py 9, and in derivatives with ethenyl + a d 34,35 36 8b-H 0.603 0.851 551 2.332 or ethynyl bridges, such as mono(Py)–TTF 10 and 11 or 8c 0.494 0.797 425b 3.529e 35 37 bis(Py)–TTF 12a and 12b. 8c-H+ 0.571 0.797 561b 1.445e a e The position of these bands slightly shifts with the polarity 9 0.508 0.835 403 3.03 9-H+ 0.679 0.918 539a 1.43e of the solvent and the connectivity of pyridine in the ortho, meta 10 0.440 0.805 444a 2.858 f or para position, with the latter generally showing the strongest 10ÁPb2+ 0.510 0.885 555a a f effect. The corresponding transitions are assigned to HOMO - 11 0.484 0.839 432 2.898 11ÁPb2+ 0.520 0.879 550a LUMO excitations, with the HOMO clearly based on TTF, while 12a 0.578 0.945 440a the LUMO spans over the Py units, with a non-negligible 12aÁPb2+ 0.695 0.980 540a c g contribution from the TTF half bearing the acceptor (Fig. 1 12b 0.583 1.049 450 2.9 a b c d for 8a–b). The calculated HOMO–LUMO gaps depend on the In CH3CN. In CH2Cl2/CH3CN 1/1. In CH2Cl2. B3LYP/6-31G(d) basis set and the inclusion or not of a solvation model (Table 1) with PCM.