Quantum Chemical Calculations on Conformations and Absorption Spectra of Azo Dyes Consisting of Thiophene and 5-Membered Rings

Yoshiya KOGOa*, Sumio TOKITAa and Kichisuke NISHIMOTOb

aDepartment of Applied Chemistry, Faculty of Engineering, Saitama University
225 Shimo-Okubo, Sakura-ku, Saitama, Saitama 338-8570, Japan
bDepartment of Chemistry, Faculty of Science, Osaka City University
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan

(Received: December 12, 2006; Accepted for publication: February 2, 2007; Published on Web: April 4, 2007)

The bathochromicity of thiophene and thiazole azo dyes was studied by using quantum chemical calculations on azo compounds consisting of benzene, thiophene, furan, pyrrole, cyclopentadiene and their aza substituted rings. Conformational analyses at AM1, PM3 and B3LYP/6-31G* level of theory were performed to estimate the stable conformers of these compounds. The CNDO/S calculations including and excluding 3d-AOs on the sulfur atom confirmed that the 3d-AOs do not contribute to the bathochromicity of thiophene and thiazole azo dyes. The results of CNDO/S and TD-B3LYP/6-31+G* calculations on absorption spectra suggested that azo compounds with a 5-membered ring commonly absorb at longer wavelengths relative to the corresponding azobenzenes.

Keywords: Heterocyclic azo dye, Bathochromicity, Excitation energy, Conformational analysis, AM1, PM3, CNDO/S, TD-DFT


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