1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Electro, Physical & Theoretical Chemistry Polyazulenes and Polynaphthalenes: Prediction and Computational Study Alexandre Costa* and A. López-Castillo [a] The geometries, the electronic structures and the aromaticity of the[n]-azulene and [n]-naphthalene polymers were studied, by using the Density Functional Theory (DFT) and the Møller– Plesset (MP2) Pertubation Theory, for the different multiplicities (M = 2S + 1): singlet (S = 0, closed and open shell), triplet (S = 1) and quintet (S = 2). The ground-states of the [n]-azulene polymers were a singlet (closed shell) for any values of n (n � 10). The ground-states of the [n]-naphthalene polymers were a singlet (closed shell) for n � 6 and triplet for 7 � n � 10. The electric dipole moment of the odd [n]-azulene polymers varied with the length of the polymer chain, while exhibiting a local minimum for [5]-azulene. The dipole of the even [n]-azulene and the (even and odd) [n]-naphthalene polymers were null by symmetry. The HOMO-LUMO gap was estimated at 0.70 eV for [n]-azulene polymers with large chain. All of the polymers had electronic transition peaks in the visible region and their maximum was red-shifted for the increasing chains. The nucleus independent chemical shift (NICS) calculations have shown that ring tension was an important factor in the aromaticity loss, as shown, for example, for the flat, the cycle, and the Möbius strip [20]-polymers. The Aromatic Stabilization Energies (ASEs) that were based on the homodesmotic and isodesmic reactions were also obtained. Introduction The azulene and naphthalene molecules are structurally similar, with the same number of carbon and hydrogen atoms and with 10 π-electrons. However, the properties of the azulene monomers and the polymers differ from their correspondent naphthalene isomers in several respects. [1–3] The azulene molecule consists of two fused rings (five and seven mem- bered) and its aromaticity can be obtained approximately from the resonance stabilization of tropylium cation and cyclo- pentadienide anion. [2-4] The importance of the contributions fromthesechargedresonancesissupportedbythehighdipole moment μ ∼ 1D and with an intense blue color. [5,6] The naphthalene molecule consists of two fused six-membered rings with six electrons in each ring, where there is no occurrence of a charge transfer, which could concur with their aromatic stability,by maintaining a nullelectric dipole moment and its colorless characteristic. [7,8] Since azulene (C 2v symmetry) and naphthalene (D 2h symmetry) are planar molecules,a π- electron delocalization is favored. [9,10] Organic conjugated polymers are a well-known class of materials and they possess an extended π-orbital system in their backbone structure. [11] The π (bonding) and the π* (antibonding) orbitals render delocalized valence and electric conduction, which can support mobile charge carriers. [12] There are potential commercial interests for applying materials based upon these polyazulenes and polynaphthalenes, due to their electrochemical and nonlinear optical properties for sensors, batteries, and electrochromic and electroluminescence devices. [13–19] Polyazulenes and polynaphthalenes with different structures than those that were studied in this work have been prepared by electrochemical polymerization or chemical poly- merization. [20-25] The present work will describe theoretical studies that can help to predict their structures and properties, as well as to also stimulate the synthesis of a new class of polymers that are based on azulene. The naphthalene polymers were considered mainly as a reference system in order to compare them with the azulene polymers. Methods Computational method In the present work, polymers that were based on azulene and naphthalenemoleculesofupto 10 monomersinaC 1 symmetry were studied, in accordance with Figure 1. The calculations were performed by considering the Density Functional Theory (DFT), when using the functional hybrid of the three parame- ters of Becke, together with the correlation functionality of Lee, Yang, and Parr (B3LYP) [26] and the Møller–Plesset Perturbation Theory at second order (MP2). [27] The 6-311G(d,p) basis set was used for the atoms (C and H). [28] The numerical integration for the DFT calculations were carried out by considering a grid step average precision of 10 -7 au. The MP2 geometry optimizations were considered for the compounds up to n = 5. The single point calculations were conducted at the MP2/6- 311G(d,p) level, by using the B3LYP optimized geometries for [a] A. Costa, Prof. A. López-Castillo Chemistry Department, Universidade Federal de São Carlos (UFSCar), São Carlos, SP 13560-970, Brazil E-mail: alexandre.c@ufscar.br Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201802711 Full Papers DOI: 10.1002/slct.201802711 11779 ChemistrySelect 2018, 3,11779–11790 ©2018Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim