Donor-acceptor oligoenes with a locked all-trans conformation: synthesis and linear and nonlinear optical properties.

A general synthetic approach to variously polarised merocyanines and a cyanine with enhanced thermal and (photo)chemical stability by a locked all-trans conformation (derived from a rigidified hexatriene unit and a variety of common donor and acceptor groups) is presented as well as a systematic study of their (non)linear optical properties. Apart from the UV/Vis absorption and fluorescence behaviour, the ground- and excited-state dipoles, the first-, second- and third-order molecular polarisabilities were determined by electro-optical absorption measurements (EOAM) and degenerate four-wave mixing (DFWM) techniques in solution. Large values for the second- and third-order polarisability up to β0=461×10−50 C V−2 m3 (1242×10−30 esu) and |γLL|=183×10−60 C V−3 m4 (15×10−34 esu) were found. The linear and nonlinear optical properties were related to the ground-state polarisation and the resonance structure of the chromophores. In order to reveal the influence of the length of the polymethinic chain (number of π electrons within the chromophore), some lower homologues shortened by one C=C (double) bond were also taken into account. The unexpectedly high γ values of some of the merocyanines cannot be explained by a two-level model. Molecular vibrational third- order polarisabilities (calculated from absolute Raman intensities in solution) were qualitatively correlated to the DFWM results. Furthermore, the dependence of the 13C NMR chemical shifts of the polymethinic carbons within the merocyanines upon ground-state polarisation was investigated and compared to those within a corresponding cyanine.