Aggregation in conjugated polymers is a well-recognized driver of performance in organic opto-electronic devices. In particular, good device performance is correlated with processing methods that also minimize non-radiative traps that quench emission in thin films. Here we compare the efficacy of various solvent systems in producing weakly versus highly emissive aggregates in the well-studied polymer poly(3-hexylthiophene) or P3HT and show that many systems that appear highly quenched in bulk solution are strongly emissive in the solid state. Microscopy, transient dynamical measurements, and structural studies to probe the electronic and structural differences between weakly and highly emissive aggregates are described.
Helical conjugated polymers are of great interest for their potential as sources of circularly polarized luminescence and as magnetic field sensors. Due to their relatively strong absorption cross sections and high emissivity in the visible wavelength range, these materials permit a detailed investigation of how the transition between helical and random coil forms are driven by polymer structural features such as chain length and chemical defects as well as environmental properties such as solvent and temperature. Bulk methods such as circular dichroism, absorption, and fluorescence as well as single-particle microscopy is used to probe the helix-to-coil phase transition in a model chiral polyfuran and to determine whether the conformations favored in solution are retained in the solid state.
While fluorescent conjugated polymers are seeing increased use in optoelectronics, aggregation remains a roadblock due to emission quenching. It is therefore important to more fully understand the conditions that drive the emissive behavior. To properly interpret the spectral changes of P3HT from solvent poisoning, we employed a variety of single-molecule fluorescence techniques. In general, is impossible to assign spectral features in bulk emission spectra to the monomer versus aggregate or a mixture of different aggregate types without additional information. However, by measuring the difference in something like diffusion time as a function of emission wavelength, the spectral features that correspond to monomeric and aggregated chains can be assigned. Notably, these aggregates are highly emissive in the solid state though not in solution. The results suggest solvent poisoning provides a simple method of producing highly emissive aggregates from otherwise weakly emissive materials.
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.
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