The fundamental absorption edge and optical constants of some charge transfer compounds F. Yakuphanoglu a, * , M. Arslan b a Faculty of Arts and Sciences, Department of Physics, Firat University, Elazig, Turkey b Faculty of Arts and Sciences, Department of Chemistry, Sakarya University, Sakarya, Turkey Received 22 December 2003; received in revised form 21 January 2004; accepted 26 January 2004 Available online 12 March 2004 Abstract A series of the charge-transfer compounds determined as 1:1 stochiometric ratio by job method has been prepared with the interaction of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and anthracene, 9-methylanthracene, 9-bromanthracene in dichlorome- thane at room temperature. The values of the optical band gap E g , and Urbach energy E 0 were determined from the optical absorption. The optical absorption measurements indicate that the absorption mechanism is due to allowed direct transitions for the compounds and it is evaluated that the optical band gap and Urbach energy values changes with incorporating R group in the compounds. The optical constants such as refractive index n, and extinction coefficient k and real and imaginary part of dielectric constant and optical conductivity of the compounds were calculated. E g and E 0 reflect the influence of different types of disorder on the absorption spectra processes. Thus, a correlation between E g and E 0 was made. Form this correlation, G value that is pro- portional to the second-order deformation potential and, E f value that depend on local coordination, parameters are found to be 0.29 and 2.37 eV, respectively. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Charge-transfer compound; Optical constants; Optical band gap 1. Introduction The synthesis of new charge transfer complexes is considerably important for the development of metallic and semiconducting materials [1]. Studies on their elec- trical and optical properties have attracted much attention in the view of their application in electronic and optical devices. Charge transfer complexes play also an important role in chemical reactivity since they constitute in many cases the very early interaction between two substrates. In addition, also physical properties like nonlinear optical effects or electrical conductivity may strongly depend on degree and nature of the charge transfer interaction [2]. Solid state charge transfer complexes prepared by co-crystallization of organic donor and acceptor molecules have been the object of much study. 7,7,8,8-tetracyanoquinodime- thane (TCNQ) is frequently used as an electron acceptor molecule [3]. Charge transfer complexes of donor mol- ecules with the electron acceptor TCNQ have been studied extensively over the past 30 years. A great variety of such complexes has now been characterized, with electronic conductivities spanning the whole range from insulator to semiconductor to organic metal [4–6]. The study of optical absorption, particularly the absorption edge has proved to be very useful for eluci- dation of the electronic structure of the materials. It is possible to determine indirect and direct transition occurring in band gap by optical absorption spectra [7]. The data transmittance can be analyzed to determine optical constants such as refractive index, absorption index and dielectric constant. The refractive index is one of the fundamental properties of a material, because it is closely related to the electronic polarizability of ions and the local field inside the material. The evaluation of refractive indices of optical materials is of considerable importance for applications in integrated optic devices such as switches, * Corresponding author. Tel.: +90-424-273-0000; fax: +90-424-233- 0062. E-mail address: fyhan@hotmail.com (F. Yakuphanoglu). 0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.01.017 Optical Materials 27 (2004) 29–37 www.elsevier.com/locate/optmat