DFT STUDIES OF SMALL ORGANIC MOLECULES FOR APPLICATIONS IN MALDI MASS SPECTROMETRY AND SOLAR CELLS

Author

Chelsea Nicole Gemeinhardt, Southern Illinois University Carbondale

Date of Award

12-1-2019

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Wang, Lichang

Abstract

The focus of the work presented herein is to understand the underlying photoexcitationprocesses of small organic molecules for applications in MALDI mass spectrometry and solarcell applications through computational methods. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) calculations were performed to provide insightinto the effect of heavy atom substitution on the charge transfer properties of MALDI matrixmolecules as well as understand the underlying photophysical properties of rationally designedorganic molecules for use in Dye Sensitized Solar Cells (DSSCs) and Organic Solar Cells(OSCs).Fifth and sixth carbon ring substituted isomers of fluorinated, chlorinated, andbrominated isomers of 2,4-dihydroxybenzoic acid and their thermodynamic and photophysicalproperties were examined to understand the effect of halogenation of MALDI matrix molecules.Thermodynamics studies included the calculation of Proton Affinity (PA), Gas Phase Acidity(GPA), and Gas Phase Basicity (GPB). PA values suggested that nearly all halogenated matricesat the ground electronic state became less acidic than its unsubstituted counterpart. Additionally,the GPA values of the unsubstituted isomer revealed that the order of acidity of protons is thecarboxylic acid proton, followed closely by the two phenolic protons. Following fifth ringsubstitution, the carboxylic acid clearly became to most acidic, followed by the second phenolicproton, and lastly the fourth phenolic proton while for the sixth ring substitution, the carboxyliciiacid and second phenolic group yield similar thermodynamic values, but the fourth phenolic group has the most basic GPA value.Comparison of the UV-Vis absorbance spectra generally showed increased absorbance at λmax compared to the unsubstituted isomer, and most importantly, absorbance at MALDI laser excitation wavelength of 337nm increases after halogenation in almost all cases of halogen substitution. An increase in overlap between the absorbance and emission spectra (Stokes shift) occurred after fifth carbon ring halogenation, but not for sixth ring suggesting that the addition of a halogen on the fifth carbon of the aromatic ring results in stokes shiftan overall more rigid molecular structure, while sixth carbon substitution does not.The bond distances, angles, and Mulliken charges for the ground and excited states were tabulated and compared to understand the structural impact of halogenation at two different ring positions, which showed that not only does the exact location of the halogen affect the structure in unique ways, but also the specific identity of the attached halogen.The HOMO-LUMO contours, energies, and gaps were also obtained in the ground, first excited singlet, and triplet states for both sets of isomers. Contours were examined to quantify the role of the halogen as an electron withdrawing group, where the location of the halogen atom, and its identity, leads to different configurations of electron density. The energies of these orbitals are raised or lowered depending on the location of the halogen on the fifth or sixth carbon and whether the molecule is in the ground or excited state.IR spectra were compared for the ground, the first excited singlet, and triplet states to further elucidate the effect of halogenation on the stretching modes of the protons of interest in charge transfer during MALDI. In both fifth and sixth carbon ring substitutions in the groundiiistate, the carboxylic acid proton stretch is not affected while the phenolic protons were to varying degrees.Most interestingly is that, in the singlet state, the second phenolic stretching mode is largely weakened following nearly all halogenation at the fifth and sixth carbon ring position, which may play a role in stabilizing the carboxylate anion left after charge transfer occurs via an intramolecular hydrogen bond. Conversely, in the triplet state, halogenation on the sixth carbon ring position results in a decreased frequency intensity as well as bond strengthening for chlorine and bromine and weakening for fluorine. This in contrast to 5-X-2,4-DHB substitution, where all halogenation weakens the second phenolic group.Short-lived charged states in simple organic donor−acceptor (D−A) systems were also studied as huge improvements are needed to produce efficient photovoltaic devices. One strategy implemented in this work was to prevent back-electron transfer by forming a cascade of energy levels through the use of a donor−acceptor 1−acceptor 2 (D−A1−A2) architecture , where three systems YD-TRC, YD-TRC-AEAQ, and MHTPP-TRC-AEAQ were characterized in depth via computational and experimental methods.The DFT results indicated that YD-TRC could form a D–A1 system, YD-TRC-AEAQ, a D–A1–A2 system, and MHTPP, a D-L-A system. Computational predictions were confirmed via electrochemical measurements included in the Appendix of this work. Ambipolar systems were synthesized, as the HOMO energy of YD and MHTPP were both higher than the HOMO of AEAQ. Furthermore, the red shift exhibited in UV-vis absorption after YD was connected its first acceptor to form YD-TRC was confirmed by computational analysis as an intramolecular charge transfer.ivWhile these studies showed that the addition of a second acceptor extended the lifetime compared to its dyad counterpart, large amounts of energy are still lost during each sequential electron transfer process, as well as the difficulty of synthesizing these molecules in the laboratory. To remedy both concerns, another viable strategy involves designing simple, small Donor-Acceptor (D-A) systems with long charge separation and slow charge recombination.The effect of donor molecules on the compound’s photophysical properties was explored, by selecting multiple donor modules and keeping the accepter the same as used in previous studies. TPA and PCB donors were proposed, and the following compounds were obtained: TPA-TRC and PCB-TRC, respectively. Additionally, dimethoxy groups and dioctyloxy groups were introduced to TPA-TRC, forming the derivatives MeTPA-TRC and OeTPA-TRC, respectively, to be added to this work.In addition to confirming that these compounds are truly arranged in the typical D-A structure based on their orbital energies, we also showed that the HOMO and LUMO are located at the donor module and TRC module, respectively. Most interestingly, the LUMO and LUMO+1 energy levels of these studied systems remained essentially unchanged, while the HOMO energies were impacted, where donor modules with stronger electron donating abilities have stronger effect on the HOMO levels.This work serves to highlight the importance of utilizing computational chemistry throughout the design, screening, and application of small organic dyads and triads toward solar cell applications. As demonstrated, calculations provide insight into the optically active molecular orbitals responsible for electronic transitions, such as those within the UV-Vis range for solar cell applications while the corresponding energies of these same orbitals can be used to accurately screen a potential system for true D-A or D-A1-A2 character.