Ferrocenyl substituted triphenylamine based donoreacceptor molecular systems; synthesis, photophysical, electrochemical, and theoretical studies Ramesh Maragani, Rajneesh Misra * Department of Chemistry, Indian Institute of Technology Indore, Indore 452 017, India article info Article history: Received 10 February 2014 Received in revised form 27 March 2014 Accepted 28 March 2014 Available online 3 April 2014 abstract A series of pushepull molecular systems 5e14 were designed, and synthesized by the Sonogashira cross- coupling, Knoevenagel condensation, and cycloaddition reactions. The donor ‘ferrocenyl substituted triphenylamine’ was kept constant, whereas the acceptors were varied (malononitrile, indanone, and tetralone). The electronic absorption spectra of the ferrocenyl substituted triphenylamine based do- noreacceptor (DeA) compounds 5e9 exhibit intense intramolecular charge transfer (ICT) band in the visible region (415e502 nm). The incorporation of the tetracyanoethylene (TCNE) group in compounds 5e9 results in new DeA system 10e14, which exhibits the ICT band in the region 630e700 nm. The electrochemical studies suggest considerable donoreacceptor interaction. The computational studies reveal strong DeA interaction, and show good agreement with the experimental results. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction There has been increasing interest in the design, and synthesis of pushepull molecular systems because of their applications in nonlinear optics (NLO), and organic photovoltaics (OPV). 1,2 In pushepull systems the electron donating group is attached to the electron accepting group via p-conjugated spacer (DepeA). The photonic properties of the pushepull systems can be tuned by varying the strength of donor/acceptor groups, and the p-spacer. 3 Our group is interested in the design, and synthesis of pushepull systems for variety of applications. 4 We have explored ferrocene as a strong electron donor. 5 The triphenylamine (TPA) moiety is also a strong electron donor. 6 Therefore attaching the ferrocene unit on the periphery of the triphenylamine will increase its donor strength. Francois Diederich has explored tetracyanoethylene (TCNE), as a strong electron acceptor. 7 TCNE undergo rapid [2þ2] cycloaddi- tion followed by electrocyclic ring opening with electron donating acetylenes, and exhibit intramolecular charge transfer. 8 Triphenylamine based donoreacceptor systems have been widely studied for organic photovoltaic devices. To the best of our knowledge, there are no reports, where ethynyl ferrocene substituted triphenylamine is attached with the acceptor. In this article we report synthesis, photophysical, electrochemical, and computational properties of the ferrocenyl substituted triphenyl- amine based donor connected to the acceptors; malononitrile (a), indan-1,3-dione (b), 1-indanone (c), 1-tetralone (d), tetracyano- ethylene (e). 2. Results and discussion The ferrocenyl substituted triphenylamine based donoreacceptor molecular systems 5e14 (Chart 1), were designed, and synthesized by the Pd-catalyzed Sonogashira cross-coupling, Knoevenagel con- densation, and cycloaddition reactions. The triphenylamine based intermediate 2 was synthesized by the VilsmeiereHaack formylation reaction of triphenylamine 1 . 9 The monoformylation reaction selectively occurred at the para- position of the phenyl ring, which resulted 4-(diphenylamino)- benzaldehyde 2 in 80% yield. The iodination reaction of 2 in the presence of potassium iodide (KI), potassium iodate (KIO 3 ), and acetic acid as a solvent resulted 4-(bis(4-iodophenyl)amino)- benzaldehyde 3 in 85% yield (Scheme 1). 10 The Pd-catalyzed Sonogashira cross-coupling reaction of 3 (4- (bis-(4-iodophenyl)-amino]-benzaldehyde) with ethynyl ferrocene resulted mono substituted 4, and di-substituted ferrocenyl triphe- nylamine aldehyde 5 in 20% and 65% yields, respectively (Scheme 2). The ferrocenyl substituted triphenylamine based donoreacceptor compounds 6e9 were synthesized by the Knoevenagel condensation reaction of compound 5, with the corresponding active methylene compounds; malononitrile (a), indan-1,3-dione (b), 1-indanone (c), * Corresponding author. Tel.: þ91 7312438710; fax: þ91 7312361482; e-mail address: rajneeshmisra@iiti.ac.in (R. Misra). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2014.03.096 Tetrahedron 70 (2014) 3390e3399