Toward Optimization of Photomodulation of Azobenzene-modified PPV Derivatives

Abstract

Photophysical characterization of a family of photoswitchable conjugated polymers is presented in this work. Additionally the instrumentation constructed for these studies, a time-correlated single photon counting spectrometer, is discussed along with the methods used to characterize the instrument. Sample data and fitting procedures are presented. Overall instrument capabilities are also presented, specifically the use of the spectrometer to measure time-resolved fluorescence anisotropy. An example study probing the interactions between charged fluorophores and surfactant vesicles is included as a demonstration of a time-resolved fluorescence anisotropy application. Understanding the effect of side chain modifications on the emission of light from conjugated polymers is useful in the design of new polymers for applications in sensing and photovoltaics. This thesis focuses on determining the photophysical interactions between a photochromic side chain, azobenzene, covalently bound to a poly(p-phenylenevinylene) (PPV) derivative. Time-resolved and steady-state fluorescence measurements were employed in these studies. The photochromic azobenzene quenches emission from the PPV backbone differentially in its' two isomeric states. Both static quenching and non-radiative energy transfer were found to play important roles in the differential quenching of PPV emission by azobenzene. These studies led to the definition of a parameter to judge degree of difference in quenching between the two isomeric forms. This parameter, modulation efficiency (Emod), serves throughout the studies as an important figure of merit for the depth of modulation observed for structurally modified derivatives of the original azobenzene-modified PPV derivative. Maximizing the modulation efficiency was the aim of the PPV studies. The results presented here elucidated the complex photophysical processes that influence the emission properties of this family of azobenzene-modified PPV derivatives. Important guidelines to maximize modulation efficiency were determined based on these results which will aid researchers in the design of photomodulated conjugated polymers.