Efficient Photoswitching of the Nonlinear Optical Properties of Dipolar Photochromic Zinc(II) Complexes

General Procedure. All manipulations were performed using Schlenk techniques under an Ar atmosphere. All solvents were dried and purified by standard procedures. NMR spectra were recorded on Bruker DPX-200, AV 300 or AV 500 MHz spectrometers. 1H and 13C chemical shifts are given versus SiMe4 and were determined by reference to residual 1H and 13C solvent signals. Assigments of carbon atoms were based on HMBC, HMQC and COSY experiments. High resolution mass spectra (HRMS) were performed on a MS/MS ZABSpec TOF at the CRMPO (Centre de Mesures Physiques de l’Ouest) in Rennes. Elemental analyses were performed by Muriel Escadeillas at the CRMPO. UV/vis absorption spectra were recorded using a UVIKON 9413 or Biotek Instruments XS spectrophotometer using quartz cuvettes of 1 cm pathlength. Compound 3 was prepared according to a reported procedure, namely by bromination of 5-methyl-thiophene-2-carboxaldehyde, and was subsequently converted into its dimethyl acetal 3-(OMe)2 for further use.1 Compounds 5a,b 2 were prepared by a Suzuki coupling reaction between the appropriate arylboronic acid and 2,4-dibromo-5-methylthiophene1 (Scheme S1). 4,4’-bis(diethylphosphonomethyl)-[2,2’]-bipyridine 4 was synthesized using published procedure. Zinc acetate dihydrate was obtained from Acros Organics and used as received. Spectroscopic grade chloroform and dichloromethane were used for all optical measurements. Photoisomerization experiments and kinetics were performed by using a Hamamatsu UV Spot Light Source (Xe-Hg lamp) as an excitation light source equipped with bandpass filters (0.63 mW.cm-2 at 365 nm; 18.6 mW. cm-2 at 588 nm) and an optical fiber whil