Synthesis and properties of push – pull chromophores for second-order nonlinear optics derived from p-extended tetrathiafulvalenes (TTFs) Manuel Otero, a M a A ´ ngeles Herranz, a Carlos Seoane, a Nazario Martı ´n, a, * Javier Garı ´n, b Jesu ´s Orduna, b Rafael Alcala ´ c and Bele ´n Villacampa c a Departamento de Quı ´mica Orga ´nica I, Facultad de Quı ´mica, Universidad Complutense de Madrid, Avenida Complutense 22, E-28040 Madrid, Spain b Departamento de Quı ´mica Orga ´nica, ICMA, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain c Departamento de Fı ´sica de la Materia Condensada, ICMA, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain Received 14 March 2002; revised 6 June 2002; accepted 10 July 2002 Abstract—Novel p-extended tetrathiafulvalene (exTTF) derivatives connected to a p-nitrophenyl electron-accepting unit through a p-conjugated oligoenic spacer (14a – c – 16a – c) show good second-order optical nonlinearities. The effect of the conjugation length of the oligoenic spacer and the presence of a thiophene ring in the p-conjugated bridge (20a – c) have been theoretically (PM3) and/or experimentally (EFISH technique) studied. The redox properties of the novel compounds have been investigated by cyclic voltammetry (CV) and the CV data reveal the good donor ability of the exTTF unit. Although the energy of the intramolecular charge-transfer (ICT) band displayed by these compounds and their redox potentials are fairly insensitive to the nature of the conjugated spacer linking the donor and the acceptor moieties, an increase in their second order nonlinear optical responses is observed on increasing the conjugation lengths. Compounds 20a – c bearing a thiophene ring show a remarkable thermal stability with decomposition temperatures over 4008C. A new series of D-p-A compounds (23a – c) bearing a 1,3-dithiol-2-ylidene donor unit has also been synthesized and studied. q 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction The synthesis of molecular and polymeric materials exhibiting nonlinear optical (NLO) properties is currently a major task in chemistry due to their applications in the domain of opto-electronics and photonics. 1 Molecular NLO-phores are traditionally constituted by electron donor and acceptor moieties covalently connected through a conjugated bridge, and a wide variety of p-conjugated and homo- and heterobridges have been investigated. 2 In this regard, Marder has shown that bond length alternation, i.e. the average difference in length between single and double bonds in the molecule, is the relevant parameter in the optimization of the hyperpolarizability of molecules. 3 Another important aspect in the design of molecules exhibiting NLO properties has been directed to the synthesis of systems where the loss of aromaticity in the ground state is compensated by a gain in aromaticity in the charge separated form. 4 Finally, a good thermal and chemical stability as well as low optical loss (high transparency) are important requirements for molecular organic materials to be incorporated into more practical nonlinear optical polymers. 1 In the search of new NLO-phores, during the last years, we have reported the synthesis and second order NLO proper- ties of push–pull tetrathiafulvalenes (TTFs) connected through an oligoenic bridge to different acceptor moieties (1 – 3). 5 In these systems, the degree of intramolecular charge transfer from the donor to the acceptor has been recently analyzed by means of Raman spectroscopy. 6 TTF and its derivatives are nonaromatic (14 p electrons) planar p-electron-donor molecules which are easily oxi- dized to form reversibly the radical cation and dication species. 7 Both species are thermodynamically very stable due to the aromatic character (six p-electrons) of the resulting 1,3-dithiolium cation. Therefore, TTFs fulfill the requirement of gain in aromaticity in the oxidized state. In addition, these TTF-phores show a good degree of thermal stability as evidenced by the decomposition temperatures around 2008C. 6 The effect of methyl substituents in the TTF moiety and the nature of the conjugated bridge on the NLO properties have also been recently studied. 8 0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0040-4020(02)00803-7 Tetrahedron 58 (2002) 7463–7475 * Corresponding author. Tel.: þ34-91-394-4227; fax: þ34-91-394-4103; e-mail: nazmar@quim.ucm.es Keywords: D-p-A compounds; 1,3-dithiol-2-ylidene; p-extended tetrathiafulvalene (exTTF); nonlinear optics (NLO); electrochemistry.