Vibrational Spectroscopy 56 (2011) 60–65 Contents lists available at ScienceDirect Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec Effect of pyridine on zinc phthalocyanine studied by density functional theory calculations and infrared absorption spectroscopy Sukh Dev Dogra a , Sukhwinder Singh b , Sarvpreet Kaur c , S.K. Tripathi a , G.S.S. Saini a,∗ a Department of Physics, Panjab University, Chandigarh 160014, India b Department of Physics, Government College for Women, Ludhiana, India c Department of Physics, Government College for Girls, Sector 11, Chandigarh, India article info Article history: Received 12 May 2010 Received in revised form 11 August 2010 Accepted 17 August 2010 Available online 18 September 2010 Keywords: Zinc phthalocyanine Phthalocyanine pyridine interaction Infrared spectra X-ray Diffraction Density functional theory calculations abstract In the present work, we have studied the effect of pyridine on the zinc phthalocyanine by recording the infrared absorption and X-ray diffraction spectra with and without pyridine. In the presence of pyridine phase of the crystalline zinc phthalocyanine changes from ˇ to ˛. Some infrared bands show changes in their positions and/or intensities. These changes have been interpreted on the basis of coordination of the pyridine molecule with the central zinc ion. Coordinated pyridine transfers some of its charge to the  electron system of the phthalocyanine ring through zinc ion. Pyridine molecule also distorts the phthalocyanine molecule by pulling zinc ion out of the phthalocyanine plane. Density functional theory also confirms the ligation of pyridine molecule at the fifth coordination site of the central metal ion. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Phthalocyanines are well suited for electronic devices due to their excellent stability against chemicals and temperature, rich substitution chemistry and processability. Phthalocyanine (Pc) and its metal analogous (MPc) possess interesting physico-chemical properties, which have the potential for applications in the area of non-linear optic, acoustics, gas sensing, magnetic materials, pho- toconducting agents, photovoltaic cell elements, electrocatalysis, colorants in chemical industry, optical data storage, photosensi- tizers, light emitting diodes and dyes [1–7]. In these materials, excellent chemical tunability of properties can be achieved by replacing metal ions at the center of the phthalocyanine (Pc) ring and functionalizing their side groups/axial positions. Zinc phthalocyanine (ZnPc), complexes have, in particular, been inten- sively studied due to their excellent photosensitizing properties [8–11]. It is known that solvents some times leads to the aggregation of Pcs, which have distinctly different optical properties than the corresponding monomers [8,12–16]. Solvents also have profound effects on the crystalline nature of the Pcs and their transformation from one crystalline form to another is found to increase on the sol- vent dependent -electron-donating ability of Pcs [17]. Theoretical ∗ Corresponding author. Tel.: +91 172 253 4454; fax: +91 172 278 3336. E-mail address: gsssaini@pu.ac.in (G.S.S. Saini). calculations of Caro et al [18] also indicates that solvent plays a role in stabilization of some Pc molecules, while in others it has a desta- bilizing effect. However, unsubstituted Pcs are sparsely soluble in solvents other than pyridine. Recently, progress has been made towards the understanding of interaction between Pc and chemical analytes including solvent vapours at molecular level [19,20] in order to comprehend their sensing action. It is believed that the electron acceptor molecules on interaction form charge transfer complex with the Pc molecules [19]. However, situation is not very clear about the interaction of Pc with the electron donors. There are some reports which sug- gest that small amount of charge transfer also takes place in case of some electron donors such as methanol [20,21]. The purpose of this work is to elucidate the detailed mechanism of interaction between ZnPc and pyridine and to investigate the chemical changes induced by the pyridine at the molecular level. In the present work, ZnPc, dissolved in pyridine, has been investigated by infrared (IR) absorption, X-ray diffraction techniques (XRD) and density func- tional theory (DFT) calculations. Since the wave numbers of infrared bands are affected by structural changes induced by the interaction, therefore, study of the effect of pyridine on the infrared frequencies facilitates the determination of bond lengths changes on a sub-Å scale. Similar mechanism of pyridine detection may be applicable to the Pc based sensors in which the pyridine vapour molecules react with a Pc thin film onto a substrate and provokes a mea- surable change in a certain property. In recent past, the DFT has been used to calculate the equilibrium geometries of Pc molecules 0924-2031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2010.08.010