Crystal structure and spectroscopic analysis of the asymmetric squaraine [(2-dimethylamino-4-anilino)squaraine] Carlos Eduardo Silva a , Renata Diniz a , Bernardo L. Rodrigues b , Luiz Fernando C. de Oliveira a, * a Nu ´ cleo de Espectroscopia e Estrutura Molecular, Departamento de Quı ´mica, Universidade Federal de Juiz de Fora, Campus Universita ´ rio, Juiz de Fora, MG, 36036-900, Brazil b Instituto de Fı ´sica de Sa ˜o Carlos, Universidade de Sa ˜o Paulo, Sa ˜o Carlos, SP, Brazil Received 20 June 2006; received in revised form 1 August 2006; accepted 1 August 2006 Available online 15 September 2006 Abstract The asymmetric squaraine (2-dimethylamino-4-anilino)squaraine (ADTCH3) has been synthesized and investigated by single crystal X-ray diffraction and vibrational spectroscopy. This compound crystallizes in a monoclinic space group P2 1 /n and has four molecules per unit cell. The nitrogen atom of the aniline ring and the oxygen atoms of the four-membered ring are involved in a medium-strength hydrogen bonded interaction, giving rise to a dimer design where the NO distance is 2.880(2) A ˚ . Two types of molecular arrangements can be observed in the crystal packing, forming a wave design parallel to the c crystallographic axis. The cyclobutene rings are effectively involved in a p-stacking interaction where the interplanar and centroid–centroid distances are 3.24 and 3.58 A ˚ , respectively. Vibrational spectroscopy (Raman and infrared) of ADTCH3 shows that the CO stretching modes from the oxocarbon and the aniline components are little affected by molecular substitution. The Raman spectrum of ADTCH3 shows intense bands at 1604 cm 1 and 1595 cm 1 , related to the m(CC) + m(CO) and m(CC) + m(CN) vibrations, which can be used to identify the electronic delocalization in the cyclobutene ring. The infrared spectrum shows a medium-weak band at 3120 cm 1 and a broad band at 3197 cm 1 which can be tentatively assigned to in- phase and out-of-phase stretching of the NH bond of the dimer species. These results are in good agreement with X-ray crystal data which show the presence of this arrangement in the solid state. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Asymmetric squaraine; Oxocarbon derivatives; Hydrogen bond; Crystal structure; Vibrational spectroscopy 1. Introduction Oxocarbon ions, of the general formula [(C n O n ) 2 ], have a high molecular symmetry (D nh ) and effective electronic delocalization [1,2]. These characteristics are very interest- ing and important for several studies mainly related to structural and spectroscopic investigations. Replacement of the oxygen atoms by other atoms or groups in oxocar- bon ions gives rise to the well-known pseudo-oxocarbons [3]. Mesoionic derivatives of squaric acid [H 2 C 4 O 4 ], which have a nitrogen atom in their structure, are termed squa- raines. The general representation of this species is shown in Fig. 1. These species have sharp and intense absorption bands in the long wavelength range of the visible and near- infrared regions [4] and have been investigated in recent years due to their interesting chemical and physical properties, as for instance, substrates for photovoltaic investigations [5], photoconductors and photoreceptors [6], and non-linear optical applications [7]. Solid-state investigations of these species are very important for the understanding of their photophysical properties. In spite of all the spectroscopic analysis of such compounds, few vibrational investigations have been undertaken and most of these have been in the infrared. It is well known from the literature that squaraines possess intense fluorescence 0022-2860/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.08.001 * Corresponding author. Tel.: +55 32 3229 3310; fax: +55 32 3229 3314. E-mail address: luiz.oliveira@ufjf.edu.br (L.F.C. de Oliveira). www.elsevier.com/locate/molstruc Journal of Molecular Structure 831 (2007) 187–194