Effects of High Pressure on Photochemical Reactivity of Organic Molecular Materials Probed by Vibrational Spectroscopy

Chemical transformations of molecular materials induced by high pressure and light radiation exhibit novel and intriguing aspects that have attracted much attention in recent years. Particularly, under the two stimuli, entire transformations of molecular species can be realized in condensed phases without employing additional chemical constraints, e.g., the need of solvents, catalysts or radical initiators. This new synthetic approach in chemistry therefore satisfies increasing need for production methods with reduced environmental impacts. Motivated by these promises, my Ph. D thesis focuses on this state-of-the-art branch of high-pressure photochemistry. Specifically, high pressure is employed to create the necessary reaction conditions to transform molecular materials, whereas monochromatic light is applied to trigger and direct the chemical reaction according to selective paths. Systematic studies on selective molecular hydrocarbon materials provide new insights into the understanding of different effects that is achieved by the combined pressure-light tuning and demonstrate significant feasibility and controllability of the method in material synthesis. Using optical microscopy and vibrational spectroscopy, I firstly studied pressure effects on production of energetic materials from laser-induced decomposition of fluid ethylene glycol and mixture of 2-butyne and water. The work demonstrated that type of reactions and quantity of products as well as the associated kinetics were highly pressure dependent. Next, I examined pressure effects on photochemical phase transitions of fluid (Z)-stilbene. The study showed that increasing pressure not only tunes the photoisomerization type phase transitions but also opens a new reaction type, and thus allowing the production of novel crystal and liquid material, respectively. Finally, I explored polymeric transformations from three unsaturated hydrocarbon monomers under high pressure and/or UV radiation. In these studies, single reaction channel permits the quantitative analysis of polymerization kinetics and the pressure-dependence, so that correlations between rate constant, activation volume and pressure can be obtained. Moreover, physical states of matter accessed by compression significantly influence the polymerization kinetics, selectivity and microstructures of products. Overall, these studies provide important contributions in discovering and understanding the high-pressure photochemical behaviors of molecular materials and show profound implications of using the combined pressure photon tunable power to produce controlled molecular materials of potential new applications

Electronic Spectroscopy of Jet-Cooled Benzylidenecyclobutane, a Sterically Hindered Styrene

The electronic spectrum of the styrene derivative, benzylidenecyclobutane, seeded in a supersonic jet expansion has been recorded using resonantly enhanced two-photon ionization spectroscopy. The main vibronic features in the spectrum are associated with a low frequency progression assigned to the torsional motion of the phenyl ring. Analysis of the observed torsional levels reveals an excited state potential energy surface characteristic of a planar equilibrium geometry which undergoes large amplitude motion and a ground state surface having a minimum at a torsional angle of 25° between the phenyl and vinyl groups. Ab initio calculations of the ground state torsional potential surface predict a minimum in the range of 28°-26°, depending on the size of the basis set. In these structures the cyclobutane ring adopts a puckering angle between 17° and 19°. Deuterated isotopomers have also been synthesized and their corresponding photoionization spectra analyzed to reveal the mixing between the torsion and other low frequency modes such as cyclobutane ring puckering. The extent of this mixing is found to be sensitive to the sites of deuteration on the molecule. © 1996 American Institute of Physics

Electronic Spectroscopy of Jet-Cooled Benzylidenecyclobutane, a Sterically Hindered Styrene

The electronic spectrum of the styrene derivative, benzylidenecyclobutane, seeded in a supersonic jet expansion has been recorded using resonantly enhanced two-photon ionization spectroscopy. The main vibronic features in the spectrum are associated with a low frequency progression assigned to the torsional motion of the phenyl ring. Analysis of the observed torsional levels reveals an excited state potential energy surface characteristic of a planar equilibrium geometry which undergoes large amplitude motion and a ground state surface having a minimum at a torsional angle of 25° between the phenyl and vinyl groups. Ab initio calculations of the ground state torsional potential surface predict a minimum in the range of 28°-26°, depending on the size of the basis set. In these structures the cyclobutane ring adopts a puckering angle between 17° and 19°. Deuterated isotopomers have also been synthesized and their corresponding photoionization spectra analyzed to reveal the mixing between the torsion and other low frequency modes such as cyclobutane ring puckering. The extent of this mixing is found to be sensitive to the sites of deuteration on the molecule. © 1996 American Institute of Physics

Excited-state non-radiative decay in stilbenoid compounds:an ab initio quantum-chemistry study on size and substituent effects

In the framework of optoelectronic luminescent materials, non-radiative decay mechanisms are relevant to interpret efficiency losses. These radiationless processes are herein studied theoretically for a series of stilbenoid derivatives, including distyrylbenzene (DSB) and cyano-substituted distyrylbenzene (DCS) molecules in vacuo. Given the difficulties of excited-state reaction path determinations, a simplified computational strategy is defined based on the exploration of the potential energy surfaces (PES) along the elongation, twisting, and pyramidalization of the vinyl bonds. For such exploration, density functional theory (DFT), time-dependent (TD)DFT, and complete-active-space self-consistent field/complete-active-space second-order perturbation theory (CASSCF/CASPT2) are combined. The strategy is firstly benchmarked for ethene, styrene, and stilbene; next it is applied to DSB and representative DCS molecules. Two energy descriptors are derived from the approximated PES, the Franck-Condon energy and the energy gap at the elongated, twisted, and pyramidalized structures. These energy descriptors correlate fairly well with the non-radiative decay rates, which validates our computational strategy. Ultimately, this strategy may be applied to predict the luminescence behavior in related compounds

Evidence for a Double Well in the First Triplet Excited State of 2-Thiouracil

The computationally predicted presence of two structurally distinct minima in the first triplet excited (T₁) state of 2-thiouracil (2TU) is substantiated by sub-picosecond transient vibrational absorption spectroscopy (TVAS) in deuterated acetonitrile solution. Following 300 nm ultraviolet excitation to the second singlet excited state of 2TU, a transient infrared absorption band centered at 1643 cm^–1 is observed within our minimum time resolution of 0.3 ps. It is assigned either to 2TU molecules in the S₁ state or to vibrationally hot T₁-state molecules, with the latter assignment more consistent with recent computational and experimental studies. The 1643 cm^–1 band decays with a time constant of 7.2 ± 0.8 ps, and there is corresponding growth of several further bands centered at 1234, 1410, 1424, 1443, 1511, 1626, and 1660 cm^–1 which show no decline in intensity over the 1 ns time limit of our measurements. These spectral features are assigned to two different conformations of 2TU, corresponding to separate energy minima on the T₁-state potential energy surface, on the basis of their extended lifetimes, computed infrared frequencies, and the observed quenching of the bands by addition of styrene. Corresponding measurements for the 4-thiouracil (4TU) isomer show sub-picosecond population of the T₁ state, which vibrationally cools with a time constant of 5.2 ± 0.6 ps. However, TVAS measurements in the carbonyl stretching region do not distinguish the two computed T₁-state conformers of 4TU because of the similarity of their vibrational frequencies

Mechanism of Oxidative Alkoxyamine Cleavage: The Surprising Role of the Solvent and Supporting Electrolyte

In this work, we show that the nature of the supporting electrolyte and solvent can dramatically alter the outcome of the electrochemically mediated cleavage of alkoxyamines. A combination of cyclic voltammetry experiments and quantum chemistry is used to study the oxidation behavior of TEMPO-i-Pr under different conditions. In dichloromethane, using a noncoordinating electrolyte (TBAPF6), TEMPO-i-Pr undergoes reversible oxidation, which indicates that the intermediate radical cation is stable toward mesolytic fragmentation. In contrast, in tetrahydrofuran with the same electrolyte, oxidized TEMPO-i-Pr undergoes a rapid and irreversible fragmentation. In nitromethane and acetonitrile, partially irreversible oxidation is observed, indicating that fragmentation is much slower. Likewise, alkoxyamine oxidation in the presence of more strongly coordinating supporting electrolyte anions (BF4-, ClO4-, OTf-, HSO4-, NO3-) is also irreversible. These observations can be explained in terms of solvent- or electrolyte-mediated SN2 pathways and indicate that oxidative alkoxyamine cleavage can be "activated" by introducing coordinating solvents or electrolytes or be "inhibited" through the use of noncoordinating solvents and electrolytes

I. The Synthesis, Absolute Configuration, and Stereochemistry of Thermal Decomposition of (+)-3R,5R- and (+)-3R,5S-3-Ethyl-5-Methyl-1-Pyrazoline. II. Mechanistic Investigations of the Thermal Decompositions of 2H-Azirines

I. (+)-3R,5R- and (+)-3R,5S-3-ethyl-5-methyl-1-pyrazolines (34T and 34C, respectively) have been prepared in optically active form. Their stereochemistries have been determined by correlation with (-)-R-3-hexanol and (-)-R-2-bromohexane. Pyrolysis of these pyrazolines in the gas phase at 292° allows a complete study of the stereochemistry of the 1-pyrazoline decomposition. 34T yields cis-1-ethyl-2-methylcyclopropane (48C) in nearly racemic form and trans-1-ethyl-2-methylcyclopropane (48T) 22.5% optically active with predominant inversion of the alkyl groups. 34C yields cis- 1-ethyl-2-methylcyclopropane (48C) 36.5% optically active with predominant retention of stereochemistry and trans-1-ethyl-2-methycyclopropane (48T) 14.2% optically active with predominant single inversion of the ethyl group. II. A series of phenyl-substituted 2H-azirines: 3,3-dimethyl- 2-phenyl-2H-azirine (33c), 3-methyl-2-phenyl-2H-azirine (33a), 3-ethyl-2-phenyl-2H-azirine (33b), and 2,3-dimethyl-3-phenyl-2H-azirine (33d) were synthesized and their thermal decompositions were investigated. 2-aza-1,3-butadienes were formed as primary pyrolysis products from 33a-c, which indicates that the thermal reaction proceeds via carbon-carbon bond cleavage leading to iminocarbene intermediates. 33d yields 2,3-dimethylindole as its only pyrolysis product, which indicates initial carbon-nitrogen bond cleavage leading to vinyl nitrene: intermediates. Further thermal reactions of the azabutaaienes to yield olefins and nitriles, and dihydroisoquinolines were also studied.</p

Jet-Cooled Spectroscopic Characterization of Anethole (methoxy-4-(prop-1-enyl)benzene), a Natual Styrene Derivative

The molecular structure of anethole (methoxy-4-(prop-l-enyl)benzene), was investigated using Jet-Cooled UV spectroscopy. A laser-induced fluorescence spectrum was obtained of the S₁-S₀ transition. Two 0⁰₀ transitions were observed in the LIF spectrum, separated by 69 cm⁻¹, and were assigned to the syn and anti conformers of anethole. Single vibronic level fluorescence spectra were obtained for both of the origin transitions. Bands of the S|VLF spectrum of each conformer were assigned by comparison with theoretical calculations at the DFT/B3LYP, 6-311G++(d,p) level of theory, as well as experimental information from similar molecules. In order to assign the LIF spectrum further, SVLF spectra were obtained of many of the LIF transitions, and assigned where possible. SVLF transitions of particular importance are the 63⁰₂ and the 39⁰₁ which appear reliably in almost all SVLF spectra, but at slightly different frequencies for each conformer, allowing the assignment of SVLF spectra to a specific conformation of anethole. LIF and SVLF data indicated the possibility of water-anethole van der Waals clusters, which were confirmed by adding water to the jet. Additionally, we performed potential energy scans of the vinyl and methoxy rotations of anethole, and fit these scans in order to determine parameters for anharmonicity and the barrier to rotation. By comparing to experimental fits in the literature, we determined that MP2 calculations predicted the barrier to rotation best, but HF calculations did a better job of predicting the anharmonicity. Opportunities for future work include the modeling of the potential using experimental data, further investigation of anethole-water clusters, finishing the assignment of the LIF and SVLF, and possibly investigating the spectroscopy of cis-anethole