Optimizing the excited state processes of conjugated polymers for improved sensory response

Abstract

Conjugated polymers exhibit useful and interesting electrical and optical properties. We exploit the wandering excitons produced after photoexcitation for chemosensory applications. By sampling many sites in a polymer film, the excitation has a greater chance to encounter an analyte, such as 2,4,6 trinitrotoluene (TNT), electrostatically poised to induce non-radiative decay. The result is attenuation of the fluorescence signal characteristic of these bright polymers. Because energy migration is responsible for the amplification of sensory response, we sought to augment this migration by integrating chromophores with long-lived excited states into the polymer backbone. The first chromophore we targeted, triphenylene, has a symmetrically-forbidden ground state transition, resulting in a long excited state lifetime. Chapter 2 describes the synthetic incorporation of triphenylene into conjugated polymer backbones, and Chapter 3 details the spectroscopic interrogation of these materials. We demonstrate that lifetime extension is universal to all triphenylene-containing polymers. The longer excited state lifetimes are then correlated with increased energy migration through polarization spectroscopy. In Chapter 4, we extend this paradigm for elongating energy migration in conjugated polymers to several other systems. Unique polymers with symmetric, aromatic chromophores are investigated. These materials allow us to look more rigorously at the variations of effective conjugation pathways and their implications before and after chromophore cyclization. The novel dibenzo[g,p]chrysene, triphenylene and thiophene-based systems afforded us a more complete understanding of the interplay of rigidification, symmetrization, lifetime, and energy migration in conjugated polymers.

(cont.) In the final chapter, we exploit another excited state process in conjugated polymers, stimulated emission, to provide additional amplification of sensory response. We demonstrate that lasing action in optically-pumped conjugated polymer thin film structures can be inhibited by exposure of samples to trace amounts of electron deficient aromatic analytes such as TNT. Analyte exposure introduces non-radiative pathways in the polymer, increasing the lasing threshold. Because lasing is a non-linear phenomenon, it provided two orders of magnitude greater sensitivity to TNT. In combination, we hope that the developments described in this thesis will serve to improve current dernining teclmology in the near future.