Optoelectronic Characterization of tunable Luminescent organic molecules

Luminescent organic molecules (LOMs) are interesting compounds, as they combine the photo-induced charge carrier generation and recombination of inorganic semiconductors with the lightweight and easy fabrication techniques common to organic materials. LOMs therefore offer the potential to be used in the development of inexpensive, flexible, and tunable organic electronic and optoelectronic devices as alternatives to inorganic semiconductors. Examples of such devices include organic light emitting diodes (LEDs), solar cells, chemical and biochemical sensors, field-effect transistors, and nonlinear optical media for lasing applications. <br/>We present results of a novel class of LOMs, Benzoyl Pyraziniums, whose optical properties can be controlled by tuning the electronic and chemical structure, as well as their concentration and interaction with surrounding materials. Optical characterization of the three differently substituted (methoxy, fluorine and unsubstituted) salts of the LOMs exhibited interesting results. The photoluminescent (PL) emission intensity increased, and the emission spectra shifted to higher energies, with decreasing concentration. The recombination lifetimes of all three salts decreased as the solution concentration was increased from 0.1 to 100 mM. As pyridiniums are capable of intramolecular charge transfer, assuming that the primary concentration dependent non-radiative decay is through this pathway, these results can be explained by a corresponding increase in the number of non-radiative decay pathways with increase in the concentration. Most recently, we have discovered an optical activation pathway in these LOMs, where the absorption and emission spectra changed significantly on UV exposure. These changes are dependent on the energy of the UV excitation, time of irradiation, as well as their concentration.<br/>To study the behavior of these LOMs as composites in the mesoscale regime, we further looked at their cumulative emissions in solid state, via spin-casted thin films. While thin films also demonstrate the spectral blue-shift with decreasing concentration, the shift is only on the order of a few nanometers (nm) over the entire range of 0.1 – 100 mM with a significantly narrowed emission.<br/>Further investigations of the electrochemistry of these molecules offer unique perspectives on the charge transfer properties within them, which is used to optimize their optical emission and establish these LOMs as highly suitable candidates for organics LEDs.<br/><b>Funding:</b><br/>This work was supported with funding from: NSF-CREST: Center for Cellular and Biomolecular Machines at UC Merced (NSF-HRD-1547848 and NSF-HRD-2112675)