The integration of furan based repeat units into conjugated systems meant for optoelectronic applications has generally been limited by the photostability of the furan unit. This limitation is due to the susceptibility of furan towards reaction with singlet oxygen, which disrupts the conjugation of the system. Here, we present a family of helical, ester-functionalized polyfurans with dramatically enhanced photostability. Within this family, the emission intensity of P3HEF is essentially non-existent while a chiral branched variant, (S)-P3EHEF, is highly fluorescent. This discrepancy is due to the difference in the compactness of the helical structure of the two polymers. Interestingly, the emission wavelength of (S)-P3EHEF can be tuned through several different techniques such as deposition speed and solvation conditions. The mechanism behind the tunability was explored using fluorescence-based techniques.
In recent decades, fluorescent conjugated polymers have been widely studied due to their ability to produce low-cost and lightweight organic electronics. However, given that their solid state packing arrangements are difficult to control or predict, maintaining the desired emissive properties of the materials is challenging. As the emission wavelengths and quantum yields of these materials are highly sensitive to their solid state packing arrangements, pre-aggregation in solution can assist in maintaining properties through the transition between solution and solid state. Here we use poly(3- hexylthiophene) (P3HT), a well-studied organic semi-conductor, to explore the impact of aggregation on the photophysical properties of the material. We have employed various bulk and single molecule fluorescence-based methods to better understand the effect of aggregation on the emission properties of the polymer system. The addition of a highly-polar solvents to induce aggregation leads to strong emission quenching, but no change in the lifetime in solution. However, in the solid state the aggregates exhibit enhanced emission intensity combined with shorter lifetimes as demonstrated by fluorescence lifetime imaging microscopy (FLIM). To better understand the differences of the aggregate properties in the solution and solid state, fluorescence anti-bunching was used to probe the degree of electronic coupling of the polymer chains. Surprisingly, fluorescence anti-bunching measurements reveal the highly collective nature of the P3HT aggregate emission, which likely accounts for its strong fluorescence intensity in the solid state. Studying the aggregated system at both the bulk and single-molecule level will yield a better understanding of the aggregate properties which will lead to better control and higher performance of organic semiconductors in device applications.
Fluorescent conjugated polymers are attractive materials to produce low-cost and lightweight displays, lighting, and organic electronics. However, when transitioning from solution to solid state, maintaining the desired emissive properties of these materials remains a challenge; the emission wavelength and quantum yield of fluorescent polymers are highly sensitive to their solid state packing arrangements which are difficult to control or predict. Additionally, their susceptibility to photo-degradation limits their widespread use. Aggregation of the polymer can protect the material from most oxidative damage by reducing the diffusivity of the oxygen through the aggregate structure. Here we employ various bulk and single molecule fluorescence-based methods to explore this aspect of a well-studied organic semi-conductor, poly(3-hexylthiophene) (P3HT). Pre-aggregating P3HT with highly-polar solvents prior to spin casting leads to aggregate structures and thin films with significantly enhanced emissive intensity and photo-stability relative to films cast without pre-aggregation. Additionally, enhanced photo-oxidative stability was seen in films formed from the pre-aggregated samples. A better understanding of aggregate properties should lead to better control and higher performance of organic semiconductors in device applications.