Photophysical Characterization Of Photorefractive Polymers PDF Download

"The unique photophysical and electronic properties of conjugated polymers (CPs) make them promising candidates for applications in displays, biosensors, field-effect transistors and photovoltaic devices. The photophysical properties of CPs are strongly dependent on the chain morphology. However, the understanding of the morphology-property correlation is still obscured both due to inhomogeneities in ensemble experiments and even due to multichromophoric properties in single chain experiments. In this dissertation, we constructed a single molecule spectroscopy apparatus and home-built MATLAB code together to correlate simultaneous measurements of intensity, lifetime and spectrum to learn more about proper photophysical models to describe the properties of CPs, the effect of O2, and the role of energy transfer in CPs. Bulk experiments in mixed solvents where chain conformations can be varied were also conducted to understand the CPs from a macroscopic perspective, where chain-chain interactions also play an important role. Especially, we selected three CPs to study, Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), Poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) and Poly(9,9-dioctylfluorene-alt-biselenophene) (F8Se2). In Chapter 3, data from mixed solvent experiments are known to produce biomodal distributions of spectra where chains emitting red light are thought to be in regions where chromophore aggregation is important. We showed in single molecule experiments that MEH-PPV follows the J-aggregate model where the properties of the chromophore are closely related to the conjugation length. Despite the similarity of the single molecule spectra to those observed in the bulk, fluorescence dynamics and intensities of red emitters are similar to those of blue emitters in the single molecule case. O2 mainly causes reversible quenching ("blinking") on the single MEH-PPV chains. Unfortunately, we could not reproduce flaring observed in a previous thesis and, although we had improved N2 protection and better signal to noise (S/N) ratio, we are not sure why our results are different. In Chapter 4 where we study F8T2, we also observe two distinct types of chromophores in the bulk experiments depending on the solvent quality. The photophysical properties of the F8T2 chains were well explained by the Spano HJ model. Even though spectral lineshapes of single F8T2 chains resemble those observed in the bulk, we concluded on the basis of very different emission intensity and lifetime behavior that the origin of the emission spectra is different and could be ascribed to bent chains. We argue that the propensity to bend in the F8T2 case and what makes it different than MEH-PPV derives from the presence of five-membered rings in F8T2 and we note behavioral similarities to polythiophenes that share this property. In Chapter 5, we showed that analogous CPs F8Se2 with selenium instead of sulfur in the monomer units exhibit strongly reduced fluorescence and dramatic reduction in singlet lifetime in the bulk due to heavy atom induced intersystem crossing to the triplet. Surprisingly, however, we observe substantial fluorescence and relatively strong emission in the single chain case for F8Se2. On the basis of a red shift in the spectrum and the long lifetimes relative to the bulk, we believe that the emission we are observing in this case is primarily from a photooxidized species similar to that observed in fluorene polymers. Overall, this work is a step towards looking more carefully at the complex correlation between morphology and photophysical properties. We show that bending of single chains can mimic the spectroscopy of aggregation and that it is necessary to consider fluorescence lifetime and efficiency data to distinguish between these effects. We have also used single chain spectroscopy to look at how chain morphology plays a significant role in energy transfer. In particular, we see little evidence for energy transfer in chains deposited from good solvents in contrast to some work where chains can behave as single chromophores when deposited under conditions where chains are able to collapse. We have also introduced a new way of looking at the relative propensity of different chains within a sample to blink and/or permanently photooxidize through studies correlating intensity drops and rises with spectra"--Pages x-xii.