Structural analysis of a dithiosquarylium dye Sung-Hoon Kim a, *, Sun-Kyung Han a , Woo-Taik Lim b , Nam-Ho Heo b , Han-Chul Song a a Department of Dyeing and Finishing, Kyungpook National University, Taegu, 702±701, South Korea b Department of Industrial Chemistry, Kyungpook National University, Taegu, 702±701, South Korea Received 5 January 1999; accepted 4 February 1999 Abstract The MOPAC PM3 approach is used to study the structure of a dithiosquarylium dye (DTSQ). It was shown that the X-ray structure and the calculated results, are in good agreement. From the experimental and calculated structure of DTSQ, DTSQ-I is a more stables tructure than DTSQ-II. The structural analysis suggests that the DTSQ is extensively bond delocalized with relatively small torsion angles of 5  . # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Dithiosquarylium dye (DTSQ); MOPAC; Structural analysis; Solvatochromism 1. Introduction Squarylium dyes form a class of dyes possessing intense absorption in the visible and near infrared regions [1]. Squarylium dyes and derivatives are 1,3-disubstituted compounds synthesized from squaric acid and two equivalent of various types of electron donating carbocycles and heterocycles [2±4]. The chemistry of squaric acid has been reviewed several times [5]. This class of cyanine dyes has attracted much attention because of their potential application in xerographic organic photoreceptors [6,7], optical recording media [8] and organic solar cells [9]. We have previously reported the synthesis [10], electroluminescence properties [11] and electro- photographic properties [10] of dithlosquarylium dyes (DTSQ). As in a previous paper [12] single crystal X-ray crystallography and crystal packing examinations have been employed to determine both the structure and the important inter- molecular interactions in the solid state of DTSQ. In this work the structure of DTSQ has been solved using MOPAC PM3 semi-empirical mole- cular orbital calculations in combination with X-ray structure data. 2. Results and discussion 2.1. Comparison between experimental and MOPAC PM3 geometry An understanding of the relationship between the structure and properties of organic materials is crucial if they are to be designed and used for speci®c applications. Unfortunately, in general, organic dyes do not readily provide single crystals for X-ray structure determinations, which has Dyes and Pigments 42 (1999) 223±226 0143-7208/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0143-7208(99)00027-3 * Corresponding author. Tel.: +82-53-950-5641; fax: +82- 53-950-6617; e-mail: shokim@bh.kyungpook.ac.kr