Photoluminescent Organic Molecules from the Perspective of Density Functional Theory




Massaro, Richard Douglas

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I have studied the electronic structure, vibrational modes, and photophysics of methyl salicylate (MS) isomers in detail using density functional theory (DFT) and its timedependent (TDDFT) companion. I have confirmed that six isomers are stable in their ground states with the ketoB isomer being the global minimum structure. I have performed free energy calculations which show that other isomers may be energetically favorable at higher temperatures. The calculated vibrational modes of ketoB match well with experimental infrared spectra. Using TDDFT, I have confirmed that the ketoB isomer undergoes an energetically favorable excited-state intermolecular proton transfer (ESIPT) to an enol isomer. I found that the ESIPT has a small potential energy barrier when the proton transitions from the ketoB to the enol structure and a ten times larger barrier to accomplish a reverse ESIPT from enol to ketoB. The barrier asymmetry is responsible for the temperature dependent suppression of the far-blue fluorescence. I modeled the emission spectra for gas phase MS using Franck- Condon factors based on the calculated 0-0 transition and vibrational modes for the ground and excited states. The calculated spectra match well to gas phase experimental spectra. Finally, I performed detailed DFT studies on dipicolinic acid (DPA) and determined its stable structures, energetics, and vibrational modes. My calculations predict the existence of six stable isomers of gas phase DPA in the ground state. Three of these isomers are nearly energetically degenerate. I calculated several transition state reaction paths between these isomers. I performed similar calculations on five dimerized formations. By using periodic boundary conditions (PBC) on three dimerized DPA arrays containing hydrogen-bonding DPA monomers, I was able to predict three different crystal structures. I report the band structures of the resulting DPA crystals for the first time. All of them are insulators.



Density functional theory, Dipicolinic acid, Photoluminescence, Vibrational spectrum, Methyl salicylate, Time-dependent DFT