Crystal structure and spectroscopic analysis of the asymmetric squaraine [(2-dimethylamino-4-anilino)squaraine]

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 P21/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) Å. 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 π-stacking interaction where the interplanar and centroid–centroid distances are 3.24 and 3.58 Å, 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 ν(CC) + ν(CO) and ν(CC) + ν(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.