Abstract
We report the synthesis and physical study of a series of 1,1- dicyano-4-[4-(diethylamino)phenyl]buta-1,3-dienes in which the number and position of additional CN substituents along the 1,1-dicyanobuta-1,3-dienyl fragment is systematically varied. While X-ray analysis provided unambiguous infor- mation about molecular geometries in the crystal, UV/Vis and electrochemical measurements, by cyclic voltammetry (CV) and rotating disk voltammetry (RDV), revealed that in- troduction of additional cyano groups in the C2- and C4-posi- tions most affected the optical properties of these molecules in solution, in terms of intramolecular charge-transfer ab- Introduction π-Conjugated donor–acceptor (D–A) chromophores have been investigated for quite some time,[1,2] but have re- cently attracted renewed interests for potential applications in the fabrication of opto-electronic materials.[2–4] The en- ergy and intensity of their characteristic intramolecular charge-transfer (ICT) transitions depend on the strength of the electron donor and acceptor moieties and the nature of the π-conjugated spacer.[5–8] While the nature of the donor, acceptor, and π-conjugated spacer have been systematically varied,[9] the number and positioning of the push/pull sub- stituents along the π-conjugated spacer backbone has only been addressed in a few cases.[10] Our group has observed strong electro-optical effects associated with the increasing number of cyano groups in push–pull chromophores;[10e,11] [a] Laboratorium für Organische Chemie, ETH Zürich, Hönggerberg, HCI, 8093 Zürich, Switzerland Fax: +41-44-632-1109 E-mail: diederich@org.chem.ethz.ch [b] Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide UMR 7177, CNRS, Université de Strasbourg, 4, rue Blaise Pascal, CS 90032, 67081 Strasbourg Cedex, France [c] Advanced Technology Institute and Department of Physics, University of Surrey, Stag Hill, Guildford GU2 7XH, Surrey, United Kingdom [d] Department of Physics and Center for Optical Technologies, Lehigh University, 415 Lewis Lab, 16 Memorial Dr. East, Bethlehem, PA 18015, USA Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.201200111. sorption energy and intensity. A comparison with structurally related chromophores indicates that the shift of the anilino donor from position 2/3 to 4 along the butadiene scaffold re- sults in a remarkable bathochromic shift of the ICT absorp- tion maxima, mainly due to the higher planarity in the pres- ent series. These findings are further corroborated by density functional theory calculations. Preliminary nonlinear optical (NLO) measurements confirm the promise of the new push- pull chromophores as third-order nonlinear-optical molecular materials.