Raman Spectroscopy Shows Interchain through Space Charge Delocalization in a Mixed Valence Oligothiophene Cation and in Its π-Dimeric Biradicaloid Dication Juan Casado,* ,† Kazuo Takimiya, ‡ Tetsuo Otsubo, ‡ Francisco J. Ramírez, † J. Joaquín Quirante, † Rocío Ponce Ortiz, † Sandra R. Gonza ´ lez, † María Moreno Oliva, † and Juan T. Lo ´ pez Navarrete* ,† Departamento de Química Física, UniVersidad de Ma ´laga, 29071-Ma ´laga, Spain, and Department of Applied Chemistry, Graduate School of Engineering, Hiroshima UniVersity, Higashi-Hiroshima 739-8527, Japan Received August 7, 2008; E-mail: casado@uma.es; teodomiro@uma.es Any new electronic insight on charge transfer processes, though subtle at first glance, can contain key information for optimal development of new organic materials. Hence how charge delo- calizes over a given molecule or substrate acquires utmost importance. Mixed-valence (MV) systems are interesting models for charge transfer events where two equal redox centers in different oxidation degrees compete for the charge via a mediating molecular bridge. 1 In particular, class III MV (Robin-Day classification) compounds are very promising in application as single-molecule devices or molecular wires 2 where the excess of charge is fully delocalized in the two redox centers over the conjugated bridge, or through-bond electronic communication. 3 The counterpart class II MV systems are characterized by the localization of the total charge in one of the donor groups. Much less attention however has been devoted to class III MV cases where through-space intercenter full charge delocalization takes place. A paradigmatic case of the latter is π-dimerization of conjugated radical cations. 4 Recently, these π-dimers have been suggested to possess very accessible magnetically active triplet states, 5 a phenomenon similar to the polaron pair versus bipolaron stability debate, occurring in extensive through-bond electron delocalized molecules (long chain oxidized oligothiophenes), owing to their biradicaloid nature. 6 Interest in electron delocalization aside, the realization of intermo- lecular charge delocalization, and magnetism is highly welcome for new functional materials. Nonetheless, in π-stacked molecules the degree of cofacial intermolecular coupling determines the extent of the interaction and the appearance of the properties mentioned above. A decathiophene 7 constituted by two parallel π-stacked pentathiophenes (5T) spaced by four methylenes spacers, [4,4] in Figure 1, is our target molecule and provides an opportunity to address the features of through- space electronic coupling in (i) class III MV systems when it is charged to the radical cation and (ii) its π-dimeric dication to explore the biradicaloid character and the presence of triplet magnetic states. We want to check the spectroscopic signals of through-space charge delocalization such as the existence of the Davidov effect or the occurrence of singlet biradicaloid species. In all these cases, Raman spectroscopy pivots a common argument with the challenge of providing the characteristic spectra of every species. As an experimental guide DFT model chemistry is used to analyze the vibrational data. The spectra of a single chain pentathiophene (i.e., 5T or R,ω- dimethyl pentathiophene in Figure S1) in the radical cation (5T •+ ) and dication (5T +2 ) states are considered as references showing the strongest Raman lines at 1438 and 1417 cm -1 , respectively (Figure S2). 8 Considering that the spectrum of neutral 5T has the strongest signal at 1481 cm -1 , the underlying structural motif causing this spectral evolution with oxidation is the quinoidization of the π-con- jugated backbone which is more pronounced for higher charged species. In oxidized [4,4] the spectra (Figure 2) were acquired by using resonance Raman spectroscopy with 785 and 1064 nm excitation wavelengths, matching the absorptions of [4,4] •+ at 760 nm and that of [4,4] +2 at 1085 nm, respectively (Figure S3). The 785 nm spectrum of [4,4] •+ shows the stronger peaks at 1464-1452 cm -1 at higher values than that in 5T •+ at 1438 cm -1 . After two-electron extraction in [4,4] +2 the band that becomes stronger in the 1064 nm spectrum is at 1426 cm -1 between 5T •+ and 5T +2 . From a structural point of view, one can argue that pentathiophene quinoidizations in [4,4] •+ /[4,4] +2 are substantially lower than the ones attained in single 5T •+ /5T +2 chains. As for the radical cations, this finding can be interpreted as a result of charge delocalization in the two chains promoted by the competition between the two donors leading to through-space mixed valence action. The radical cation can be viewed as a molecular domain of 10 like interacting thiophenes (i.e., 5T+5T) where one charge is fully delocalized, relaxing the quinoidal modification relative to 5T •+ with the subsequent Raman upshift. For [4,4] +2 , again each charge does not totally reside on one 5T chain but apparently delocalizes in the two since quinoidization is greater than that in 5T •+ and lower than that in 5T +2 . In this case the driving force for the interaction is the † Universidad de Ma ´laga. ‡ Hiroshima University. Figure 1. Chemical structure and notation. Models A and B schematize the cases where inter-5T interactions take place. Figure 2. (i) (a and c) 1064 nm FT-Raman spectra of [4,4] and [4,4] +2 and (b) 785 nm resonant Raman spectrum of [4,4] •+ . (ii) Experimental Raman spectra of the radical cations of 5T (bottom) and [4,4] (top). (iii) DFT/UB3LYP/ 3-21G* theoretical Raman spectra of 5T •+ (bottom) and [4,4] •+ (top). Published on Web 10/07/2008 10.1021/ja806207j CCC: $40.75  2008 American Chemical Society 14028 9 J. AM. CHEM. SOC. 2008, 130, 14028–14029