Intra- and Intermolecular Charge Transfer in Aggregates of Tetrathiafulvalene-Triphenylmethyl Radical Derivatives in Solution Judith Guasch, †,⊥ Luca Grisanti, §,# Manuel Souto, † Vega Lloveras, † Jose ́ Vidal-Gancedo, † Imma Ratera, † Anna Painelli, § Concepció Rovira, † and Jaume Veciana* ,† † Institut de Cie ̀ ncia de Materials de Barcelona (ICMAB-CSIC)/CIBER-BBN, Campus Universitari de Bellaterra, E-08193 Cerdanyola del Valle ̀ s (Barcelona), Spain § Dipartimento Chimica, GIAF, Parma University/INSTM-UdR, I-4310 Parma, Italy *S Supporting Information ABSTRACT: An extensive investigation of aggregation phenomena occurring in solution for a family of electron donor−acceptor derivatives, based on polychlorotriphenyl- methyl radicals (PTM) linked via a vinylene-bridge to tetrathiafulvalene (TTF) units, is presented. A large set of temperature and/or concentration dependent optical absorp- tion and electron spin resonance (ESR) spectra in a solution of dyads bearing different number of electrons and/or with a hydrogenated PTM residue offer reliable information on the formation of homo dimers and mixed valence dimers. The results shed light on the reciprocal influence of intramolecular electron transfer (IET) within a dyad and the intermolecular charge transfer (CT) occurring in a dimer between the TTF residues and are rationalized based on a theoretical model that describes both interactions. ■ INTRODUCTION The current interest on developing molecule-based spin- electronic devices such as switches, memories, or spin valves has stimulated the study of responsive bistable molecular materials with magnetic and/or conducting properties. 1−5 Organic electron donor−acceptor (D−A) dyads are excellent building blocks for bistable materials, due to their ability to switch between neutral and charge-separated states via an intramolecular electron transfer (IET) process in response to an external stimulus (temperature, pressure, light, solvent, magnetic and electrical fields). 6 Nonetheless, for most D−A dyads the absence of strong cooperative interactions makes the reverse IET process very fast, thus preventing a genuine bistability. 7−10 Cooperative and collective phenomena are driven in molecular aggregates by strong intermolecular interactions, 11,12 and therefore the design of new switchable D−A dyads with a propensity toward functional self-assembly is highly desirable. Tetrathiafulvalene (TTF) derivatives are excellent electron-donor molecules showing remarkable self- assembling phenomena both in solution and at solid state. 13−15 In fact, TTF derivatives are among the most widely utilized compounds for the development of organic metals, 16,17 where the self-assembly of neutral TTFs and their mono-oxidized cation-radicals (TTF + ·) originate partially filled band structures and conductivity in solid state. 18 Likewise, TTF + · molecules self-assemble in solid state leading to Mott insulators and diamagnetic solids. Recently, Kochi and Rosokha presented a very detailed study of the self-assembly of unsubstituted TTFs in solution. 19 They unequivocally established the thermody- namic framework for aggregation, shedding light on the self- assembly of this important family of compounds in solution. In this article we analyze the TTF-driven supramolecular aggregation of a recently reported organic D−A dyad 1, formed by a TTF unit, which acts as the electron-donor, conjugated through a vinylene bridge to a polychlorotriphenylmethyl radical (PTM) that acts as an acceptor. Dyad 1 can be found in two stable states in solution, the neutral 1a and the zwitterionic 1b states, depicted in Scheme 1, whose relative concentration can be modified by moderate changes in the molecular Received: January 10, 2013 Published: March 21, 2013 Scheme 1. Neutral 1a and Zwitterionic 1b States of Dyad 1 a a The two states are interchanged by an IET process between the TTF and the PTM subunits. Article pubs.acs.org/JACS © 2013 American Chemical Society 6958 dx.doi.org/10.1021/ja400281b | J. Am. Chem. Soc. 2013, 135, 6958−6967