Third-Order Nonlinear Optical Effects in Organic Nickel Complexes and Triarylmethyl Cations

Abstract

The second molecular hyperpolarizability, γ, has been determined at 1064 nm by Third- Harmonic Generation (THG) using the Maker fringe technique, for a family of triarylmethyl cations and for a familiy of organic Nickel complexes as guests in PMMA thin films. For the metal complexes it is a well established notion that the low-lying transition with ligand to metal charge transfer character is important for the nonlinear optical properties(1). However, ambiguity arises due to large discrepancies between different measurements(2-5), as well as difficulties in assessing the exact contribution to γ of the ligand to metal charge transfer transition(2,6). To assess the latter question by experimental means, we present here a comparison between a family of Nickel complexes, and triarylmethyl cations. The electronic structure of the triarylmethyl cations resemble that of the metal complexes in the sense that intramolecular charge transfer from the periphery to the central atom takes place upon excitation in the first electronic band. This is shown by semi-empirical PM3 calculations on the three members of the family shown in figure 1. For the amino substituted compound 1 the calculations reveal a significant charge transfer from the amino moiety to the central carbon atom. For the molecules 2 and 3 this effect decreases due to the less efficient donor substituents (2) or forced planarity (3) resulting in more delocalized electronic states both in the HOMO and the LUMO. The observed γ values (table 1) can be correlated with the PM3 calculations in the way that the greater the amount of charge moved and the longer the spatial distance over wich it is moved, the greater is γ. The calculated static γ values, using the semi-empirical PM3/Finite-Field method follow the same trend although much smaller values are obtained.