Tuning the Linear and Nonlinear Optical Properties of Pyrene-Pyridine Chromophores by Protonation and Complexation to d(10) Metal Centers

The linear and second-order nonlinear optical (NLO) properties of two pyrene-pyridine chromophores, namely, 4-(pyren-1-yl)pyridine (L-1) and 4-(2-(pyren-1-yl)ethyl)pyridine (L-2), were investigated and modulated by performing protonation/deprotonation cycles or by complexation to d(10) metal centers such as Zn(II) and Cu(I) to form the monomeric [Zn(CH3CO2)(2)(L-1)(2)] complex and the [CuI(L-2)](n) coordination polymer, respectively. The structures of L-1, L-2, [Zn(CH3CO2)(2)(L-1)(2)] and [CuI(L-2)](n) were determined by means of single-crystal X-ray diffraction studies. The NLO response, measured by the electric-field-induced second harmonic generation (EFISH) technique, was positive for both chromophores and showed an inversion of the sign after exposure to HCl vapors. This process was completely reversible and the original values were restored by simple exposure to NH3 vapors. Coordination of L-1 to Zn(II) also resulted in a negative NLO response, although smaller in magnitude compared to the protonated form, due to the weak Lewis acidity of the "Zn(CH3CO2)(2)" fragment. The results were also interpreted on the basis of DFT/TDDFT calculations.