Plasmonically Enhanced Photostability in Organic Fluorophores

Organic semiconductors hold promise for the development of numerous applications including next-generation solid state lighting, flexible displays and organic-based lasing. However, a critical factor limiting their utility is their inherently low stability relative to inorganic materials. Organic molecules are susceptible to oxygen damage in an irreversible
process known as photo-bleaching. This effect not only limits device lifetimes but is also exceedingly problematic in fluorescence-based imaging and in biological assays that rely on sensitive fluorescence detection. Substantial challenges remain despite efforts to enhance stability of organics through modification of the chemical structure, device architecture, developing oxygen scavenger systems and encapsulation methods. The increasing demand for enhanced stability of organics requires new strategies beyond these chemical approaches. In addition to addressing stability issue, another important factor to
improve the performance of optoelectronic devices and fluorescence detection assays is efficient emission collection from organic fluorophores. Directional control of emission can significantly enhance signal collection from otherwise weak signal due to inherently omni-directional nature of fluorescence. The thesis here focuses on the use of plasmonic effects to enhance the photo-stability of a conjugated organic molecule as well as control their fluorescence direction. We have achieved over one order of magnitude increase in stability for organic fluorophores with a careful engineering of the metal film properties. These results were attained using a metal film that is prepared via a simple, fast, and inexpensive single-step physical vapor deposition method. As an example, we have successfully demonstrated this effect for poly [2-methoxy-5(2'-ethylhexyloxy)-1,4-phenylene]vinylene (MEH-PPV) and
for model oligomers of this polymer. The stability results for organic films are presented in chapter 3, while the single molecule photostability enhancement is explored in chapter
5. The single molecule study is important for getting mechanistic information and will also be useful for single molecule imaging applications. We also successfully demonstrate highly directional doughnut emission from MEHPPV deposited on the ultra-thin Au films as presented in chapter 4. In addition to achieving the results, under lying photo-physical interactions of organic-fluorophore and plasmon are also studied in detail using various
fluorescence microscopy techniques. Our photostability enhancement results are superior to those reported in the literature in terms of the magnitude of the stability enhancement, the simplicity of the method, and its
potential to be broadly generalizable to a wide variety of emitters for applications including, but not limited to, next generation opto-electronic devices, organic lasing.