Mixed β‑Pyrrole Substituted meso-Tetraphenylporphyrins and Their Metal Complexes: Optical Nonlinearity Using Degenerate Four Wave Mixing Technique

We have investigated the roles of structural modification and polar effects in the optical nonlinearities of a series of selectively mixed β-pyrrole functionalized tetraphenylporphyrin, MTPP(CHO)(R)2 (R = H, Br, 2-thienyl, phenyl (Ph), phenylethynyl (PE) compounds and their metal (Cu(II), Zn(II)) complexes in toluene. In the present study, we have used phase conjugation geometry of the four wave mixing process to measure the third order nonlinear susceptibility (χ(3)) and the second order hyperpolarizabilty (⟨γ⟩) with picosecond laser pulse excitation at 532 nm. An increase in the values of χ(3) and ⟨γ⟩ for electron-withdrawing groups was observed whereas an opposite trend was noticed for the electron-donating groups at the β-pyrrole positions. In the Cu(II) and Zn(II) complexes of substituted free base porphyrins, the distortion of the macrocyclic ring may be responsible for the reduction of the values of χ(3) and ⟨γ⟩. From fluorescence measurements, it has been found that the electron-donating and electron-withdrawing substituted groups at β-pyrrole positions and also the macrocyclic ring distortion of the porphyrin lead to increased radiationless transitions.