Diradical Character of Benzo- and Naphtho-Annelated Thiophene ] Pyrrole Mixed Oligomer Dications Takuya Kageyama, A Shunsuke Uneme, A Masayoshi Takase, A Kotohiro Nomura, A and Tohru Nishinaga A,B A Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan. B Corresponding author. Email: nishinaga-tohru@tmu.ac.jp Benzo- and naphtho-annelated thiophene–pyrrole mixed octamers Bz8TP-C m and Np8TP-C m , comprising benzo- or naphthodithiophene and two dithienylpyrrole units, were synthesised. Density functional theory (DFT) calculations based on B3LYP/6–31G(d) with the broken symmetry method predicted that dications Bz8TP-C 1 21 and Np8TP-C 1 21 have stronger diradical character than previously investigated non-annelated thiophene–pyrrole mixed octamer 8TP-C 1 21 (R 1 ¼ R 2 ¼ H). Compounds Bz8TP-C 8 and Np8TP-C 8 showed a one-step, two-electron oxidation process based on cyclic voltammetry analysis. Reaction with SbCl 5 involved a two-electron oxidation. Bz8TP-C 8 21 and Np8TP-C 8 21 displayed similar absorption spectra to that of nonamer 9TP-C 12 21 (R 1 ¼ R 2 ¼ H), rather than that of octamer 8TP-C 12 21 , indicating the stronger diradical characters of octamers Bz8TP-C 8 21 and Np8TP-C 8 21 , as enhanced by benzo- and naphtho-annelation, and were comparable with that of 9TP-C 12 21 . Time-dependent-DFT calculations supported the conclusion obtained from the experimental results. Manuscript received: 28 September 2013. Manuscript accepted: 18 December 2013. Published online: 8 January 2014. Introduction In recent years, there has been growing interest in the study of various diradicals. [1] The diradical character of a-linked oli- gothiophene (nT) dications has been investigated as models of p-doped polythiophenes. [2–8] The diradical state of long oli- gothiophene dications, i.e. two polarons (two radical cation moieties) on a single chain, was first suggested in 1998 based on the study of 12T 21 bearing long alkyl side chains. [2] Prior to this study, it was generally thought that the bound pair of polarons i.e. bipolaron [9] (closed-shell dication) was simply formed at highly doped states, and the very weak electron spin resonance (ESR) signal of the highly doped polythiophenes was explained in terms of a bipolaron. [10,11] However, the report [2] noted that the singlet diradical state whereby two delocalised spins are aligned in an anti-parallel mode could provide another expla- nation of the inactive ESR properties. Thus, studying the rela- tionship between the molecular structure and diradical character of the dication of oligothiophenes is an interesting research topic. However, because of the intrinsic high reactivity of rad- ical species, 12T 21 with simple long alkyl side chains is inap- propriate for study, owing to insufficient stability. Therefore, to precisely investigate the electronic structure of oligothiophene dications, various structural modifications to stabilise the radi- cal cation moiety were examined. [3,5,6,8] We recently designed and synthesised thiophene–pyrrole mixed oligomer nTP-C m (R 1 ¼ R 2 ¼ H) with methylthio end- capping groups that prevent radical coupling in the oxidised states. [5f] Study of the mixed oligomer demonstrated that the insertion of N-alkylpyrrole enhanced the diradical character of the oligothiophene dications. Based on the diradical index estimated by natural orbital occupation number (NOON) analysis [12] at the B3LYP/6–31G(d) level, dication 8TP-C 1 21 (R 1 ¼ R 2 ¼ H) displayed a higher value (66%) than octithio- phene dication (8T 21 ) (46 %). [5e] In the present study, we designed and synthesised novel benzo- and naptho-annelated derivatives of thiophene–pyrrole mixed octamer, i.e. Bz8TP-C m and Np8TP-C m , to obtain a stable dication with stronger diradical character than previously reported oligomers. The annelated aromatic rings are expected to restrict the quinoidal resonance structure to the dicationic state, which would tend to divide the dication into two radical cation moieties. Herein, we report a combined theoretical and experimental study on the diradical character of Bz8TP-C m 21 and Np8TP-C m 21 (Chart 1). Results and Discussion Theoretical Evaluation of Diradical Character The geometry optimisation and estimation of the diradical index by NOON analysis [12,13] of Bz8TP-C 1 21 and Np8TP-C 1 21 were conducted at the B3LYP/6–31G(d) level using the broken symmetry method. The alkylthio groups were introduced in the b-position of the thiophene rings of the dithienylpyrrole units to stabilise the radical cation moiety and reduce p-dimerisation in CH 2 Cl 2 solution, [5i] thus allowing observation of the electronic structure of the diradicals. Furthermore, two methylthio groups were added on the benzodithiophene moiety of Bz8TP-C 1 21 to protect the carbons with a relatively high spin density (as dis- cussed below). The calculated structures and spin distributions are shown in Fig. 1. The alkylthio substituents at the b-position of the thiophene rings in the dithienylpyrrole units promote the CSIRO PUBLISHING Aust. J. Chem. 2014, 67, 722–728 http://dx.doi.org/10.1071/CH13522 Journal compilation Ó CSIRO 2014 www.publish.csiro.au/journals/ajc Full Paper RESEARCH FRONT