Silicon https://doi.org/10.1007/s12633-017-9747-2 ORIGINAL PAPER Evaluation of the Optical and Structural Properties of Constructed Bis-indole Derivatives Using (Sm 2 O 3 /SiO 2 ) Catalyst Tamer K. Khatab 1 · A. M. Abdelghany 2 · N. Shaker 3 · Yasmen Osama 3 · E. M. Kandil 3 Received: 1 December 2017 / Accepted: 26 December 2017 © Springer Science+Business Media B.V., part of Springer Nature 2018 Abstract (SiO 2 /Sm 2 O 3 ) was successfully synthesized and examined using scanning electron microscopy; X-ray diffraction and Fourier transform infrared spectroscopy. Obtained data shows the formation of crystalline phase superimposed over the silicate amorphous matrix. The synthesized catalyst was used for eco-friendly synthesis of bis-indole derivatives. Two moles of indole and different aromatic aldehyde were mixed and heterocyclic aldehydes have been adopted in-expensive, recyclable, easily prepared and nontoxic catalyst (SiO 2 /Sm 2 O 3 ) under solvent free conditions. The chemical structures of the synthesized compounds have been introduced and correlated with UV/vis. optical absorption spectra in combination with (HOMO-LUMO) through experimental spectrophotometric data in both direct-indirect transitions and that obtained based of density functional theory (DFT). Keywords Indole · Aldehydes · Sm 2 O 3 · Silica · DFT 1 Introduction Catalytic heterogeneous organic synthesis using solid cat- alysts during the last ten years was considered as one of the important modern synthetic techniques [1, 2]. Silica-based resources play an important role in heteroge- neous catalytic reactions and have a lot of applications. Bis(indolyl)methanes (BIMs) are widely existing in vari- ous terrestrial and sea origin bioactive metabolites [3, 4] and have an effective role as important intermediates in synthetic organic and medicinal chemistry [5]. BIMs show various physiological and pharmacological activities, for example, it act as antibacterial, antiangiogenic [6], antimi- crobial, antifungal [7], antibiotic [8], cytotoxic [9], radical  Tamer K. Khatab tamer khatab@hotmail.com 1 Organometallic-Organometalloid Chemistry Department, Chemical Industries Research Division, National Research Centre, 33 El-Behouth St., Dokki, 12622, Cairo, Egypt 2 Spectroscopy Department, Physics Division, National Research Centre, 33 El-Behouth St., Dokki, 12622, Cairo, Egypt 3 Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt scavenging [10], antimetastatic [11–13], growth promoting [14], anti-inflammatory and analgesic [15]. Also there are several medicinal chemistry applications of BIMs such as the treatment of chronic fatigue, fibromyalgia, and irrita- ble bowel syndrome [16, 17], they can promote healthy estrogen metabolism in human as dietary supplements [18], and also have an important role in avoidance of cancer [19, 20]. They have an effective role in laser technologies and fluorescent materials for visualization of biomolecules [21]. Synthesis of BIMs derivatives by condensation of indole with a variety of aromatic aldehydes in the pres- ence of different catalysts and reagents has been reported [22]. Few years ago, there were several catalysts which have been used in this reaction such as, pentafluorophenylam- moniumtriflate [23], RuCl 3 ·3H 2 O[24], PEG-SO 3 H[25], sodium lauryl ether sulfate [26], montmorillonite K10 clay [27], sodium p-toluene sulfonate [28], NaHSO 4 · SiO 2 [29], LiClO 4 [30], Cu 1.5 PMo 12 O 40 [31]. Although these catalysts are useful but they have many disadvantages for example, expensive and need long, so we are in need of new catalysts that overcome these disadvantages. Presented study aims to introduce an eco-friendly route for catalytic synthesis of BIMs. Herein, we use Sm 2 O 3 /SiO 2 as a catalyst, which has several advantages. In addition, chemical structures of the synthesized compounds have been correlated with (HOMO- LUMO) obtained from UV/vis. optical absorption data in