DOI: 10.1002/ejoc.201600005 Full Paper Heteroacene Synthesis Mechanochemical Synthesis, Photophysical Properties, and X-ray Structures of N-Heteroacenes Prasit Kumar Sahoo, [a] Chandan Giri, [a,b] Tuhin Subhra Haldar, [a] Rakesh Puttreddy, [b] Kari Rissanen* [b] and Prasenjit Mal* [a] Abstract: The described mechanochemical methodology is an example of a proof-of-concept in which solution-based tedious, poor yielding, and difficult syntheses of pyrazaacenes are achieved under solvent-free ball-milling conditions; the method is easy, high yielding, time-efficient, and environmentally be- nign. The synthesized compounds also include pyrazaacenes (N-heteroacenes) that are octacene analogues containing pyr- ene building blocks. The compounds were sparingly soluble in Introduction The recent advances in the syntheses of extended π-conjugated acene molecules with exciting optoelectronic properties have led to significant interest in these compounds for materials sci- ence. [1] Generally, acenes are considered as the most compre- hensive class of fused polycyclic aromatic hydrocarbons and are described by the fewest localized Clar resonant sextets per number of aromatic rings. [2] The uses of these materials vary from moth repellents to precursors for synthetic dyes such as alizarin. Also, owing to their electronic properties, these mol- ecules have gained attention for applications such as field- effect transistors (FETs), organic light-emitting diodes (OLEDs), and photovoltaic cells. Unfortunately, acenes are highly unsta- ble, and the incorporation of heteroatoms increases their stabil- ity. [3] These heteroacenes display improved stability compared to their hydrocarbon analogues because it is much easier to make C–N bonds than C–C bonds. [4] Owing to their electron- deficient nature, small band gaps, and high thermal stabilities, heteroacenes are considered to be alternatives to acenes as po- tential n-type semiconducting materials. [1d,3] [a] School of Chemical Sciences, National Institute of Science Education and Research (NISER) Bhubaneswar, PO Bhimpur-Padanpur, Via Jatni, District Khurda, Odisha 752050, India E-mail: pmal@niser.ac.in http://www.niser.ac.in/users/pmal [b] University of Jyvaskyla, Department of Chemistry, Nanoscience Center, P. O. Box. 35, 40014 University of Jyvaskyla, Finland E-mail: kari.t.rissanen@jyu.fi https://www.jyu.fi/kemia/tutkimus/orgaaninen/en/research/rissanen Supporting information and ORCID(s) from the author(s) for this article are available on the WWW under http://dx.doi.org/10.1002/ejoc.201600005. Eur. J. Org. Chem. 2016, 1283–1291 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1283 common solvents, and column chromatographic purifications could be avoided after the solvent-free syntheses. The UV/Vis absorption spectra of the pyrazaacenes show intense absorp- tion bands in the near-IR region. The single-crystal X-ray analy- ses of selected pyrazaacene derivatives showed pairwise π–π interactions and some C–H···π interactions, which could ac- count for some of the photophysical features of the compounds in the solid state. Herein, we mainly focus our attention on the synthesis of pyrazaacenes or N-heteroacenes, that is, nitrogen-containing heteroacenes with either pyrene building blocks or pyrene- fused oligoazaacenes. In general, N-heteroacenes are synthe- sized by two popular methods: (1) cyclocondensation reactions between 1,2-diaminoarenes and 1,2-diketones; [5] (2) substitu- tion reactions of 1,2-diaminoarenes with dihydroxyacenes, fol- lowed by oxidation. [3,4] However, these pyrazaacenes are syn- thesized by following the cyclocondensation method, be- cause –C=N bonds are readily constructed through this reac- tion. [5b] The major disadvantages of the known syntheses of pyrazaacenes are associated with the hazardous reaction condi- tions and low yields; therefore, very few methods are available for pyrazaacene synthesis. [5a,6] Recently, as result of public interest in alternative energy in- put, [7] ball-milling mechanochemistry [8] has received significant interest from chemists as a solvent-free synthetic methodology owing to its benefits over traditional solution-based methods. [9] The advantages of mechanochemistry are substantial and well- documented. [7] This technique has huge significance to green processes and is time efficient, environmentally benign, and economical. The minimum purification required, quantitative conversions, and the production of fewer byproducts bring ex- tra importance to this method. Under the area of mechano- chemistry, we have recently reported multistep syntheses, [10] metal-free synthetic methodologies, [11] and self-sorting reac- tions. [12] Therefore, we have anticipated that solvent-free syn- thesis by the ball-milling methodology may possibly be used as a supply of mechanical energy and that reactions for the synthesis of heteroacenes could be performed in a greener way. [13] Taking into consideration all of these aspects, we have designed a straightforward mechanochemical route for the