Intriguing Influence of −COOH-Driven Intermolecular Aggregation and Acid–Base Interactions with N,N-Dimethylformamide on the Second-Order Nonlinear-Optical Response of 5,15 Push–Pull Diarylzinc(II) Porphyrinates

A series of 5,15 push-pull meso-diarylzinc(II) porphyrinates, carrying one or two −COOH or −COOCH3 acceptor groups and a −OCH3 or a −N(CH3)2 donor group, show in N,N-dimethylformamide and CHCl3 solutions a negative and solvent-dependent second-order nonlinear-optical (NLO) response measured by the electric-field-induced second-harmonic generation (EFISH) technique, different from the structurally related zinc(II) porphyrinate carrying a −N(CH3)2 donor group and a −NO2 acceptor group, where a still solvent-dependent but positive EFISH second-order response was previously reported. Moreover, when a −N(CH3)2 donor group and a −COOH acceptor group are part of a sterically hindered 2,12 push-pull β-pyrrolic-substituted tetraarylzinc(II) porphyrinate, the EFISH response is positive and solvent-independent. In order to rationalize these rather intriguing series of observations, EFISH measurements have been integrated by electronic absorption and IR spectroscopic investigations and by density functional theory (DFT) and coupled-perturbed DFT theoretical and 1H pulsed-gradient spin-echo NMR investigations, which prompt that the significant concentration effects and the strong influence of the solvent nature on the NLO response are originated by a complex whole of different aggregation processes induced by the −COOH group