A Building Block Approach Towards Novel Nonlinear Optical Materials.

Abstract

The purpose of the present research is to develop and apply a building block approach towards the design of novel nonlinear optical materials, capable of exhibiting enhanced Two-Photon Absorption (TPA) behavior. These materials have potential applications in biological imaging, microfabrication, sensing and photodynamic cancer therapy. Electronic structure, symmetry and intermolecular forces are vital for designing the right building block. The next step is to connect them to form macromolecules. However, besides covalent bonding, aggregation and self assembly of building blocks can also be utilized, which renders the strategies for materials design less reliant on chemical synthesis. The application of building block approach was illustrated using several examples, including rigid, two-dimensional architectures. These enabled the investigation of macrostructures that were synthetically inaccessible as well as demonstrated the influence of symmetry on TPA behavior. Electronic coupling between building blocks and excited state dynamics were the observed reasons for enhanced TPA. In an attempt to investigate strong coupling that would extend over the entire chromophore, novel “endless” nano-cavities were examined for their TPA behavior. Ultrafast spectroscopy proved complete delocalization in these materials. Another approach to harness the coupling between small building blocks in a synergistic fashion is to arrange them into branched architectures. The influence of pi-character of branching units on the charge transfer character, which in turn influences the TPA behavior, was examined. Using excited state studies, not only was it observed that alkene pi-bridging resulted in enhancement of TPA behavior over alkyne pi-bridging, but also the mechanism for cooperative enhancement upon assembly of small chromophores into branched architecture was elucidated. Such branched materials serve as building blocks for dendrimers and a series of thiophene dendrons was investigated for enhanced TPA behavior. It was also found that such dendrons could be used for light harvesting and funneling applications. Using the knowledge gained from the investigations in branched materials, a potential application in the form of a highly sensitive and selective two-photon “turn-on” sensor for the detection of zinc ions was developed. This study could serve as a guideline for developing chromophores for imaging metal ions in biological systems using multiphoton excitation.