Oxidation of Tetraphenylhexaazaanthracene: Accessing a Scissor Dimer of a 16π Biscyanine Georgia A. Zissimou, Christos P. Constantinides, Maria Manoli, Galatia K. Pieridou, Sophia C. Hayes, and Panayiotis A. Koutentis* Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus * S Supporting Information ABSTRACT: Tetraphenylhexaazaanthracene (TPHA), a fluorescent zwitterionic biscyanine with a closed-shell singlet ground state, on treatment with manganese dioxide or phenyliodine bis(trifluoroacetate) (PIFA), undergoes oxidative dimerization to give a near-zero dipole scissor 5,5′-dimer DI-TPHA. Both acene components of the new dimer DI-TPHA maintain their biscyanine closed-shell singlet ground state motifs, as judged by analysis of both single crystal X-ray crystallographic and density functional theory computational studies; however, unlike TPHA, DI-TPHA is only very weakly fluorescent. A cenes with zwitterionic biscyanine motifs are unusual, as their central arenes have lost their aromaticity by parting their π electrons. The first example, 5,7-diphenyl-5H,12H- quinoxalino[2,3-b]phenazine (DPTAP, a.k.a. diphenylisofluor- indine) was reported as early as 1896, but at that time the exact electronic nature of the compound was unclear. 1 While these compounds were of interest as potential textile dyes, 2 little more was reported until 1998 when Wudl prepared the zwitterionic biscyanine of tetraphenylhexaazaanthracene TPHA 3 (Figure 1). Since then, there has been increased interest in their potential use as organic field effect transistors (OFETs), 4 as well as other applications such as organic light-emitting diodes (OLEDs), memory devices, phototransistors, solar cells, photoelectrical chemical cells, sensors, and conductors. 5 Several recent developments include the improved synthesis of TPHA 6 and DPTAPs, 7 the synthesis of heptaazaanthra- cenes, 8 hexa- 9 and octaazapentacene 10 analogues, the synthesis of asymmetric acene systems, 11 and Oakley’s sulfur and selenium containing systems. 12 Computational studies on biscyanine acenes to predict the energy differences between the triplet and singlet ground states, 13 optical studies, 7a,8,9a and liquid crystalline 14 and molecular monolayer 15 behavior have been reported. Surprisingly, further synthetic chemistry is limited to N- protonation 7a,8b,9a or alkylation 7b,11a,12b-j,16 of the nitrogens on the -ve cyanines and also to the oxidation of DPTAP to afford para-quinonimine systems 16a and finally the reductive ring contraction of TPHA to afford imidazolo-fused systems. 17 Recently, we reinvestigated the synthesis of TPHA, which involved the oxidation of the bisamidrazone 1 and identified Ag 2 O as a superior oxidant that gave TPHA in near-quantitative yield (Scheme 1). 6 During screening of oxidants for the conversion of the bisamidrazone 1 to TPHA, we discovered that MnO 2 (10 equiv) in CH 2 Cl 2 at ca. 20 °C gave after 24 h in addition to the desired TPHA (74%) a low yield (2%) of the dedihydro 5,5′- dimer of TPHA namely 1,1′,3,3′,7,7′,9,9′-octaphenyl-1H,1′H- [5,5′-bibenzo[1,2-e:5,4-e′]bis([1,2,4]triazine)]-9,9′-diium-6,6′- diide (DI-TPHA). This new dark purple compound was more polar than TPHA [R f 0.25 (DI-TPHA) vs 0.44 (TPHA) in n- hexane/Et 2 O 50:50 on silica TLC], was nonfluorescent, showed sharp line NMR spectra indicating a closed-shell electronic ground state, and was thermally stable: it did not melt up to 400 °C, but differential scanning calorimetry (DSC) revealed a decomposition onset at 415.9 °C; see Sect. S7 and S8 in the Supporting Information (SI). The structure of DI-TPHA was derived from an analysis of the spectroscopic data (Sect. S1 in the SI) and further Received: January 22, 2016 Figure 1. Structures of DPTAP and TPHA with IUPAC numbering. Scheme 1. Improved Route to TPHA Letter pubs.acs.org/OrgLett © XXXX American Chemical Society A DOI: 10.1021/acs.orglett.6b00222 Org. Lett. XXXX, XXX, XXX-XXX