Two-Photon Absorption in Near-IR Conjugated Molecules: Design Strategy and Structure–Property Relations

Abstract

In the past few years, applications built around two-photon absorption (2PA) have emerged, which require new materials to be designed and characterized in order to discover new applications and to advance the existing ones. This chapter describes the nonlinear optical processes and characterization techniques along with design strategies and structure-property relations of cyanine and cyanine- like molecular structures with the goal of enhancing 2PA in the near-IR for multiphoton fluorescence sensing applications. Specifically, a detailed analysis of the linear and nonlinear optical properties of several classes of polymethine dyes, which include symmetrical and asymmetrical combinations of p-conjugated bridges with electron donating (D) or electron accepting (A) terminal groups, are presented. These structures are: D-p-D, A-p-A, D-p-A, and a quadrupolar type arrangement of D-p-A-p-D. The results of this research combined with the growing literature on structure- property relations in organic materials is moving us closer to the ultimate goal of developing a predictive capability for the nonlinear optical properties of molecules.