An EFISH, Theoretical, and PGSE NMR Investigation on the Relevant Role of Aggregation on the Second Order Response in CHCl3 of the Push-Pull Chromophores [5-[[4'-(Dimethylamino)phenyl]ethynyl]​-​15-​[(4''-​nitrophenyl)​ethynyl]​-​10,​20-​diphenylporphyrinate] M(II) (M = Zn, Ni)

This work has produced experimental and theoretical evidence for the independence, on the nature of the metal, of the second order NLO response of [5-[[4′-(dimethylamino)phenyl]ethynyl]-15-[(4′′nitrophenyl)- ethynyl]-10,20-diphenylporphyrinate]M(II) NLO chromophores (M ) Zn, Ni). EFISH measurements, carried out at a nonresonant 1.907 μm incident wavelength and at variable concentrations in CHCl3 or in a polar and donor solvent such as DMF or by addition of an excess of pyridine to a CHCl3 solution, have shown, together with a PGSE NMR investigation, that the different second order NLO response obtained in CHCl3 for Zn(II) and Ni(II) NLO chromophores is due to a different aggregation in CHCl3 solution. Theoretical DFT/TDDFT calculations on the NLO properties of dimeric aggregates of the NLO chromophores containing the Zn(II) center have suggested, in agreement with EFISH measurements and PGSE NMR investigation, a J aggregation which induces a doubling of the second order NLO response. The PGSE NMR investigation has also suggested a much weaker dipolar interaction for the dimerization process of the NLO chromophores containing the Ni(II) center. In this latter case the antiparallel alignment of the dipole moments of the two chromophores produces a lower second order NLO response, as experimentally observed in CHCl3 solution by increasing concentration.