In this study, we outline successful demonstration of changing the counter-anion of propidium iodide (PI) to allow tuning of the photo-physical properties of propidium salts. The organic mono-anions used in this study to replace iodide in PI were BETI, NTf2, and TfO. These novel organic salts which were obtained after anion exchange were defined as propidium GUMBOS (Group of Uniform Material Based on Organic Salts) or PGUMBOS. The physical properties of these PGUMBOS were found to be quite diverse and very much anion dependent. For example, PBETI and PNTf2 were soluble in relatively non-polar solvents; whereas, PTfO and PI were soluble in relatively polar solvents. Examination of powder X-ray crystallographic data indicated that the morphology of PI changed from predominantly crystalline to predominantly amorphous upon ion exchange, primarily due to the low packing efficiency of PGUMBOS in the solid state. The thermal stability of PBETI and PNTf2 were found to be approximately 25% higher with respect to PI as indicated by thermo-gravimetric analysis. PGUMBOS showed negligible photo-bleaching as reflected in fluorescence kinetic studies. In contrast, PI showed approximately 8% reduction in fluorescence intensity under the same experimental conditions. Spectral characteristics such as absorption, emission, and fluorescence quantum yields of PGUMBOS were found to be diverse and dependent on the anion as well as the solvent characteristics. For example, the fluorescence lifetimes of PI and PGUMBOS were affected by solvent polarity and in the range of 0.96 (water) to 7.98 (DCM). Examination of data from cyclic voltammetry and UV-Vis absorption studies produced values of HOMO and LUMO energy levels of PGUMBOS of -5.15(±0.12) and -1.95(±0.21) eV respectively. Optical band gaps were in the range of 2.99–3.20 eV. Based on these studies, we conclude that the photo-physical and electronic properties of these novel PGUMBOS are suitable for use in optoelectronic applications such as red emissive materials in organic light emitting diodes (OLEDs).

Enhancing photo-physical properties of propidium iodide by substitution of organic anions

255th National Meeting (2018)

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