Computational study of the geometry and electronic structure of triazolephthalocyanines R. S. Iglesias, a M. Segala, a M. Nicolau, b B. Cabezo ´n, b V. Stefani,* a T. Torres* b and P. R. Livotto* a a Instituto de Quı ´mica, Universidade Federal do Rio Grande do Sul, Av. Bento Gonc ¸alves, 9500, Caixa Postal 15003, Porto Alegre, RS 91501-970, Brazil. E-mail: vstefani@iq.ufrgs.br, livotto@iq.ufrgs.br b Dpto. de Quı ´mica Orga ´nica, Universidad Auto ´noma de Madrid, 28049 Madrid, Spain. E-mail: tomas.torres@uam.es Received 29th August 2001, Accepted 30th January 2002 First published as an Advance Article on the web 20th March 2002 Semiempirical molecular orbital methods were used to simulate the molecular structure and electronic spectra of a series of triazolephthalocyanines (Tpcs). All molecular studies are in agreement with an aromatic macrocycle where a hypothetical substitution on both triazole and meso nitrogens of the core causes a partial breakdown in the Gouterman’s four level model. Minor effects are predicted for the substitution in the isoindole subunit. Comparison between the experimental and theoretical UV–VIS spectra of different metal substituted triazolephthalocyanines is also made. Introduction The search for advanced materials with potential technological applicability, especially in the optoelectronic and photonic fields, has activated interest in phthalocyanines 1,2 and related compounds. These azaporphyrin systems are considered as targets in materials science not only because of their unique physical and chemical properties, such as chemical versatility, thermal stability, 3 semiconductor behaviour 1 and intense nonlinear-optical responses, 4–7 but also because of their intrinsic capability to self-assemble providing supramolecular devices. 8–12 Most of the specific properties of phthalocyanine-like com- pounds are founded on the aromatic character, the p-electron distribution and the macrocyclic skeleton structure of the Pc-core. Therefore, the development of theoretical calculations for these macrocycles is of great interest to elucidate their molecular geometry and electronic based parameters such as redox potentials, electronic conductivity properties, optical absorptions, etc. Many theoretical studies have been carried out on several phthalocyanine compounds, 13–17 such as metal-free phthalo- cyanine, various extended conjugation analogues, 18,19 and metal phthalocyanines. 20–22 Different calculation methodolo- gies have been reported and these studies have proved to be useful in both explaining and predicting experimental data for phthalocyanine derivatives. Although theoretical studies on symmetrical phthalo- cyanines are quite well documented, there are few reports on non-centrosymmetrical phthalocyanines and Pc analogues. 23,24 Pc analogues are core-modified phthalocyanines where one or two benzo-fused rings have been replaced by another hetero- aromatic system. The theoretical study of these core-modified macrocycles is of great interest since this change significantly modifies the electronic structure of the Pc macrocycle, clearing the path to understanding the nature of aromaticity in these systems and to allow the computer design of azaporphyrin- based new materials. As a consequence of the interest of our group in the theo- retical understanding of phthalocyanine related systems, the molecular electronic analysis of different subphthalocyanines has been reported recently. 25 MNDO calculations on the triazolehemiporphyrazine 26 (Thp), a four-unit macrocycle which bears two opposite-faced isoindole units and two 1,2,4-triazole moieties all bound through aza bridges, explained why it was impossible to obtain the aromatic triazolehemipor- phyrazines which was owing to the antiaromatic character of the triazolinediimine substructure. Recently, we have focused our interest on triazolephthalo- cyanines (Tpc), 27,28 isoelectronic core-modified phthalocya- nines in which one isoindole ring has been formally replaced by a 1,2,4-triazole subunit. These compounds can be syn- thesised by two synthetic approaches: a statistical procedure useful to prepare triazolephthalocyanines with the same substitution pattern in the three isoindole subunits, 29 and a stepwise route, 30 that allows the synthesis of Tpcs with a differently substituted isoindole opposite to the triazole moiety. These unsymmetrical macrocycles have shown comparable properties with phthalocyanines; they are able to be organised in Langmuir–Blodgett films 31,32 and have shown liquid-crystal behaviour, 33 high conductivity measurements, 34 important second and third non-linear optical properties, 35 and high thermal stability. 3 Previous Tpc theoretical studies have been reported on the metal free triazolephthalocyanine. 36 The geometry optimiza- tion that was carried out using a semiempirical AM1 method showed that the macrocycle should be visualised as an 18 p-carbon-nitrogen macrocycle with peripheral N–N and benzene moieties. The electronic structure was calculated at the HF ab initio 3-21G level. However, there were no experi- mental data available to compare with the theoretical results. The comparison of experimental and computational data is a good way to check the validity of the theoretical calculations, and also experimental data, especially absorption data can be predicted. It opens the way to a more effective design of triazolephthalocyanine materials based on required properties assisted by computer tools. In this article, a theoretical chemistry study, based on semiempirical molecular orbital calculations on triazolephtha- locyanines has been carried out. A comparative study of the results yielded by spectroscopic calculation methods and 1256 J. Mater. Chem., 2002, 12, 1256–1261 DOI: 10.1039/b107790m This journal is # The Royal Society of Chemistry 2002