MATLOUBI MOGHADDAM, FIROUZ, et al. “Synthesis of Bis(indolyl)methanes in Presence of Anhydrous Copper(II) Sulfate”. Asian Journal of Chemistry, vol. 20, no. 2, Mar. 2010, pp. 1063-7, https://asianpubs.org/index.php/ajchem/article/view/12528.