The Effects of Pi-Pi Stacking on the Controlled Radical Polymerization of Vinyl Aromatics

The atom transfer radical polymerization (ATRP) of styrene (St) was conducted in the presence of varying equivalence (eq) of hexafluorobenzene (HFB) and octafluorotoluene (OFT) to probe the effects of pi-pi stacking on the rate of the polymerization and on the tacticity of the resulting polystyrene (PSt). The extent of the pi-pi stacking interaction between HFB/OFT and the terminal polystyrenic phenyl group was also investigated as a function of solvent, both non-aromatic solvents (THF and hexanes) and aromatic solvents (benzene and toluene). In all cases the presence of HFB or OFT resulted in a decrease in monomer conversion indicating a reduction in the rate of the polymerization with greater retardation of the rate with increase eq of HFB or OFT (0.5 eq to 1 eq HFB/OFT compared to St). Additionally, when aromatic solvents were used instead of non-aromatic solvents the effect of the HFB/OFT on the rate was minimized, consistent with the aromatic solvent competitively interacting with the HFB/OFT. The effects of temperature and ligand strength on the ATRP of St in the presence of HFB were also probed. It was found that when using N,N,N’,N’,N’’-pentamethyldiethylenetriamine (PMDETA) as the ligand the effects of HFB at 38o were the same as at 86oC. When tris[2-(dimethylamino)ethyl]-amine (Me6TREN) was used as the ligand at 38o there was a decrease in monomer conversion similar to the analogous PMDETA reaction. When the polymerization was conducted at 86oC there was no effect on the monomer conversion with HFB present compared to when HFB was absent. To investigate the pi-pi stacking effect even further, the reverse pi-pi stacking system was observed by conducting the ATRP of pentafluorostyrene (PFSt) in the presence of varying eq of benzene and toluene, which in both cases resulted in an increase in monomer conversion compared to when benzene or toluene were absent; in summary the rate of the ATRP of PFSt increases when benzene or toluene waas present in the reaction. The pi-pi stacking interaction between the HFB/OFT and the dormant alkyl bromide of the polymer chain was verified by 1H-NMR with 1-bromoethylbenzene as the alkyl bromide. Also verified by 1H-NMR was the interaction between HFB/OFT and St and the interaction between PFSt and benzene. In all 1H-NMR spectra a perturbation in the aromatic and/or vinyl peaks was observed when the pi-pi stacking agent was present compared to when it was absent. The tacticity of the PSt formed in the presence of 1 eq of HFB was compared to the PSt formed in the absence of HFB by observing the C1 signal in their 13C-NMR spectra, but no change in shape or chemical shift of the signal was observed indicating that there was no change in tacticity.

Solvatochromic and Thermochromic Studies of Tryptanthrin

Solvatochromism and thermochromism describe how a solvent or environment affects the photophysical behavior of a photoluminescent solute. The most common use of solvatochromism is as a probe in which the polarity of a solvent in which a solvatochromic solute is dissolved can be spectroscopically measured. Solvatochromic and thermochromic studies of tryptanthrin in several different solvents are reported. Absorption and corrected emission spectra for tryptanthrin at ~10-6 M concentrations are reported in four aprotic and nine alcoholic solvents. The absorption spectra are relatively unaffected by changes in solvent polarity and by differences in the hydrogen bonding ability of the alcoholic solvents. The emission spectra are much more affected by changes in solvent polarity and hydrogen bonding ability. In aprotic solvents, emission energy decreases and emission intensity increases with increasing solvent polarity. In the alcoholic solvents, emission energy also decreases with increasing solvent polarity. However, emission intensity for the alcoholic solvents varies significantly from the aprotic solvents over similar polarity ranges. This suggests that in the alcoholic solvents, hydrogen bonding ability correlates better than polarity to emission energy and intensity trends. The absorption and emission data in the aprotic solvents were also used to estimate the ground and emitting excited state dipole moments for tryptanthrin. The value obtained for the ground state dipole moment (2.37 D) agrees with theoretical results (2.06 D) and a previously reported experimental value (2.0 D). Attempts to explain previously reported results and conclusions with respect to the solvatochromic behavior of the aromatic carbonyls fluorenone and benzo(b)fluorenone were explored in an attempt to understand the solvatochromic response of tryptanthrin. Such attempts include models dependent on non-radiative decay pathways like intersystem crossing, internal conversion, and hydrogen bonding interactions.

Assessment of Alcohol Reward in Prairie Voles (Microtus ochrogaster)

The purpose of our study was to assess whether prairie voles find alcohol rewarding. Prairie voles have recently become a species of interest for alcohol studies, which have traditionally used other rodent model species including several different strains of mice and rats. The prairie vole is one of only two known rodent species that readily administers high levels of unsweetened alcohol, implicating it as a potentially effective animal model for studying alcohol abuse. However, voluntary consumption does not necessarily imply that prairie voles find it rewarding. Therefore the purpose of our study was to investigate if alcohol has rewarding properties for prairie voles using three different approaches: place conditioning, flavor conditioning, and immunohistochemistry. Furthermore, we sought to characterize their reward profile and compare it to other commonly used rodent models ¿ C57BL/6 mice, DBA/2J mice, and Sprague-Dawley rats. Place and flavor conditioning are behavioral methods that rely on the learned association between a stimulus and the effects of a drug; the drug of interest in these studies is alcohol. To assess whether prairie voles will demonstrate a conditioned preference for alcohol-paired stimuli, seven place conditioning studies were run that investigated a range of different doses, individual conditioning session durations, and trial durations. Video analysis revealed no difference in the amount of time spent on the alcohol-paired floor, suggesting no conditioned place preference for alcohol. Two flavor conditioning tests were conducted to assess whether voles would demonstrate a preference for an alcohol-paired flavored saccharin solution. Voles demonstrated reduced consumption of the alcohol-paired flavored saccharin solution, regardless of dose or flavor, when alcohol administration occurred after conditioning sessions (p=<0.001). When alcohol was administered before conditioning sessions, no difference in consumption of the alcohol-paired and saline-paired flavored saccharin solutions was seen (p=0.545). Previous studies that have documented similar behavior have hypothesized that this is an example of an anticipatory contrast effect. This theory proposes that prairie voles reduce their intake of a hedonic solution (flavored saccharin solution) in anticipation of later drug administration (alcohol). However, conditioning-based behavioral methods of studying alcohol reward are highly sensitive to the parameters of the conditioned stimulus, thus it is possible that voles will not show preference for alcohol-related stimuli, even if they do find alcohol rewarding. Immunohistochemical analysis supplemented this behavioral data by allowing us to identify specific neural regions that were directly activated in response to the acute administration of alcohol. No difference in the number of activated c-Fos neurons in the Nucleus Accumbens (NAc) core or shell was seen (p=0.3364; p=0.6698) in animals that received an acute injection of alcohol or saline. There was a significant increase in the number of activated c-Fos neurons in the Paraventricular Nucleus of the Hypothalamus (PVN) in alcohol-treated animals compared to saline-treated animals (p=0.0034). There was no difference in the pixel count of activated c-Fos neurons or in the % area activated in the Arcuate Nucleus between alcohol and saline-treated animals (p=0.4523; p=0.3304). In conclusion, the place conditioning studies that were conducted in this thesis suggest that prairie voles do not demonstrate preference or aversion towards alcohol-paired stimuli. The flavor conditioning studies suggest that prairie voles do not demonstrate aversion but rather avoidance of the alcohol-paired flavor in anticipation of future alcohol administration. The preliminary immunohistochemical data collected is inconclusive but cannot rule out the possibility of neuronal activation patterns indicative of reward. Taken together, our data indicate that prairie voles hav