Date of Award

5-2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering

Committee Member

Dr. Lindsay Shuller-Nickles, Committee Chair

Committee Member

Dr. Timothy DeVol

Committee Member

Dr. Ayman Seliman

Abstract

The goal of this research is to benchmark the computational parameters necessary to accurately model the fluorescence process of seven organic scintillating molecules, including 2-(1-naphthyl)-5-phenyloxazole (αNPO), 2-(1-naphthyl)-4-vinyl-5- phenyloxazole (vNPO), 5-(4-Bromophenyl)-3-(4-ethylphenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (PZ1), 3-(4-Ethylphenyl)- 5-(4-vinylphenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (vPZ1), 3-(4-Ethylphenyl)-5-(4-fluorophenyl)-1-phenyl-4,5-dihydro-1H- pyrazole (PZ2), 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4 oxadiazole (PBD), and 2-[4-(4'- vinylbiphenylyl)]-5-(4-tert-butylphenyl)-1,3,4-oxadiazole monomer (vPBD). Organic fluorophores are utilized for scintillation detection, as the fluorophores emit detectable photons in response to energy deposition from ionizing radiation into the scintillator. Optimal detection efficiency occurs with emitted photons at the maximum response wavelength of the photodetector, which, for a conventional photomultiplier tube, is 420 nm. Identifying an optimal organic fluorophore structure through synthesis is expensive. Instead, time-dependent density functional theory (TD-DFT) can be used to predict absorption and emission wavelengths for optimal fluor structures. We report on the performance of four exchange-correlation energy functionals (B3LYP, M06-2X, CAM-B3LYP, wB97X-D) for each fluor molecule in toluene and cyclohexane solutions as implemented in the polarizable continuum model (PCM). This research has identified the M06-2X functional as the best functional for identifying distinct absorption and emission features throughout the entire corresponding spectra for the molecules of interest. Furthermore, the work presented here has determined that the 6-31G(d,p) basis set in combination with the M06-2X functional produce computational results that follow general trends produced in experimental measurements of absorption and emission wavelengths. Methods for explicitly modeling solvation were developed and compared with calculations performed within the PCM framework (i.e., implicit solvation).

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