RESUMEN

A computational study (ωB97X-D/6-31G(d)) of the Mallory photocyclization reaction has revealed that the well-established dihydrophenanthrene (DHP) intermediates can adopt either closed-shell (CS) or open-shell-diradical (OS) singlet ground states. A detailed study of the properties of DHPs allowed their classifications as OS, borderline-OS, borderline-CS, or CS intermediates. The triplet electronic state and higher energy CS* isomer of all the OS singlet diradicals were computationally located, and the expected relationship between the diradical index, yo, and the triplet energy and the OS-CS* energy gaps was established. The importance of aromaticity in stabilizing the OS singlet diradicals was confirmed by using the Harmonic Oscillator Model of Aromaticity (HOMA). The thermal decompositions of DHPs by cycloreversions to regenerate the Mallory starting materials were also studied. The cycloreversion mechanism was described as a homolytic cleavage characterized by an anchimeric assistance continuum promoted by bis-ß-homolytic cleavage.

RESUMEN

The fundamental aspects of aromatic endoperoxide chemistry are reviewed including their synthesis and reactions. The discussion will focus on factors that will both enhance and prevent the formation of aromatic endoperoxides and on structural features that will provide control over their ability to release singlet oxygen. This approach recognizes the dual use of aromatic hydrocarbons as both precursors of endoperoxides and as valuable materials for incorporation in electronic and photonic devices. Improvement of the existing methods and development of new methods for the synthesis of endoperoxides is necessary as a result of the demand to improve existing and to create new applications for these valuable materials. On the contrary, prevention of endoperoxide formation is crucial to inhibit irreversible oxidative degradation of aromatic hydrocarbons and to extend their lifetimes as useful organic semiconductors.

RESUMEN

The syntheses of isomeric helical viologens that have potential applications in supramolecular chemistry and catalysis have been developed. The structures of the molecules and their solid-state packing motifs have been determined by X-ray crystallography. Computational studies demonstrate that the magnitude of their racemization barriers is primarily determined by the identity of the helical scaffold and is insensitive to the placement of the viologen functional group. The isomers are similar in their photophysical behavior but very different in their photochemical behavior.

RESUMEN

The first helical viologen (4,4'-bipyridinium salt) has been prepared and characterized. Its reduction to the radical cation at -0.22 V vs SCE makes it the most easily reduced redox-active helicene known. It exhibits absorption at 397 nm for the S1 ← S0 transition, and it is luminescent allowing measurement of both its singlet (59.3 ± 0.1 kcal/mol) and triplet (54 ± 1 kcal/mol) energies. In contrast to neutral helicenes, it is not aromatic π-stacked in the crystal and has a shortest interdication distance of 4.977 Å. Its racemization barrier is calculated to be a sensitive function of its redox state.

RESUMEN

A series of new dicationic sensitizers that are hybrids of pyrylium salts and viologens has been synthesized. The electrochemical and photophysical properties of these "pyrylogen" sensitizers are reported in sufficient detail to allow rationale design of new photoinduced electron transfer reactions. The range of their reduction potentials (+0.37-+0.05 V vs SCE) coupled with their range of singlet (48-63 kcal mol(-1)) and triplet (48-57 kcal mol(-1)) energies demonstrate that they are potent oxidizing agents in both their singlet and triplet excited states, thermodynamically capable of oxidizing substrates with oxidation potentials as high as 3.1 eV. The pyrylogens are synthesized in three steps from readily available starting materials in modest overall 11.4-22.3% yields. These sensitizers have the added advantages that: (1) their radical cations do not react on the CV timescale with oxygen bypassing the need to run reactions under nitrogen or argon and (2) have long wavelength absorptions between 413 and 523 nm well out of the range where competitive absorbance by most substrates would cause a problem. These new sensitizers do react with water requiring special precautions to operate in a dry reaction environment.

RESUMEN

The synthesis and characterization of a new mesityl ring-substituted pyrylogen with a substantially decreased rate of reaction with water is reported. Computational and experimental data are presented that suggest that addition of water to the pyrylium ring of this highly sterically shielded pyrylogen is reversible. On the other hand, experimental data suggest that the overall hydrolysis of this new sterically shielded pyrylogen, but not the parent pyrylogen, is irreversible. Two potential explanations for this behavior are presented and discussed. These results provide important new information that can be used to design and synthesize new electron transfer sensitizers that can be used even in highly aqueous environments.

RESUMEN

The first synthesis of the two-electron reduction product of a pyrylogen is reported. The magnitude of the experimentally determined disproportionation constant for a pyrylogen radical cation was used to advantage in order to provide compelling evidence for formation of this reduction product. Computational studies were used to provide verification of these results and to provide additional insight into the pyrylogen redox system.

RESUMEN

The syntheses and characterizations of the 4,2'-regioisomers of the dicationic pyrylogen electron transfer sensitizers are reported. The electrochemical and photophysical properties of these sensitizers are compared to the previously reported 4,4'-pyrylogens.

RESUMEN

A direct comparison of photooxygenation at sulfur and selenium centers is reported, and the reactivity of 1,5-thiaselenocane is compared to that of 1,5-dithiacyclooctane.

RESUMEN

The synthesis and photophysical properties of a new series of dicationic electron transfer sensitizers have been reported. These new materials, pyrylogens, are hybrids of pyrylium cations and Viologen dications. Electron transfer reactions of neutral organic substrates using these new sensitizers generate radical-cation/radical-cation pairs whose repulsive (repellent) interaction is designed to compete with energy wasting return electron transfer (RET) by enhancing diffusive separation and formation of solvent separated ions.

RESUMEN

The methylene blue, N-methylquinolinium tetrafluoroborate, and pyrylium-cation-sensitized photooxygenations of 5H, 7H-dibenzo[b,g] [1,5]dithiocin, 1, and 1,5-dithiacyclooctane, 2, have been investigated. The methylene blue sensitized reactions exhibit all of the characteristics of a singlet oxygen reaction including isotope effects for the formation of a hydroperoxysulfonium ylide and the ability of 1 and 2 to quench the time-resolved emission of singlet oxygen at 1270 nm. The product compositions in the N-methylquinolinium tetrafluoroborate and pyrylium-cation-sensitized reactions are dramatically different and are both different from that anticipated for the participation of singlet oxygen. This argues for different reaction mechanisms for all three sensitizers. However, both the quinolinium and pyrylium-cation-sensitized reactions display all of the characteristics of electron-transfer-initiated photooxygenations. Both sensitizers were quenched at nearly diffusion-limited rates by 1 and 2. Laser flash photolysis of mixtures of either sensitizer and 1 or 2 resulted in direct observation of the reduced sensitizer and the sulfide radical cation. In addition, electron-transfer reactions involving both sensitizers were shown to be exergonic. These results are consistent with the previously proposed outer sphere electron-transfer mechanism for N-methylquinolinium tetrafluoroborate and were used to argue for a new inner sphere mechanism for the pyrylium cation reactions.

RESUMEN

The synthesis and spectral properties of a new 2,2'-bipyridinium ion, 1,1'-dimethyl-4,4'-(dimethylamino)-2,2'-bipyridinium bis(tetrafluoroborate) are reported. Rotation of the dimethylamino group is slow at room temperature on the 400 MHz 1H and 100 MHz 13C NMR time scales. Complete line shape fit of the dynamically broadened NMR spectra was used to determine the activation barriers for this process. The first complete set of UV-vis spectra for a 2,2'-bipyridinium dication and its one- and two-electron reduced products was reported. TD-DFT calculations were used to help assign the origin of the long wavelength absorptions in these species. The effect of substituents on the energies and conformational potential energy surfaces of all three species were also examined using the B3LYP/6-31G(d) computational method.

RESUMEN

Creating a stationary fluorinated environment inside the zeolite cavity can increase the reactivity observed for intrazeolite photooxidation of alkenes. Exchanging the zeolite with fluorinated organic cations is a much more effective strategy than simply using a fluorinated solvent for slurry irradiations. Use of cations containing C-F bonds is also more efficient than use of deuterated cations for creation of a singlet oxygen friendly environment where the quenching processes are slowed down. Doping the zeolite with fluoro-organic cation 4 resulted in an increase in the singlet oxygen lifetime to 12 micros.

RESUMEN

The singlet oxygen ene reactions of four allylic alcohols and for comparison an allylic ether have been examined both in solution and in zeolite Y. Brønsted acid sites in the zeolite were shown to induce decomposition of several of the allylic alcohols. Treatment of the zeolites with pyridine removed these acid sites and allowed intrazeolite reactions of the allylic alcohols without interference from decomposition. Control reactions with an allylic alcohol that is inert to decomposition provided evidence that the presence of pyridine in the zeolite labyrinth does not influence the product composition.

RESUMEN

[reaction: see text] The syntheses of three new 2,2'-bipyridinium tetrafluoroborate sensitizers are reported. Their preliminary electrochemical and photophysical properties are compared to the properties of the more widely used pyrylium cation sensitizers. In addition, the first examples of triplet-triplet absorption spectra of 2,2'-bipyridinium ions are presented.

RESUMEN

The conformationally induced electrostatic stabilization (CIES) sulfide photooxygenation mechanism was computationally examined using an ab initio model and extended to the study of new donor atoms. The MP2/6-31G(d) geometries and a natural population analysis of natural lone-pair orbitals on the donor atoms support the mechanism and reveal that oxygen and nitrogen donor groups are more stabilizing than sulfur.

RESUMEN

1,5-Dithiacyclooctane is shown to chemically react more efficiently and to remove singlet oxygen from solution more rapidly than either thiane or 1,4-dithiane. These unusual characteristics of the 1,5-dithiacyclooctane reaction were explored using ab initio quantum chemical methods. A large number of persulfoxides, thiadioxiranes, and hydroperoxy sulfonium ylides were located and their structures analyzed. The unusual efficiency of the reaction was attributed to a conformational change that electrostatically stabilized the persulfoxide and increased the potential energy barrier for physical quenching.

Asunto(s)

Safrol/análogos & derivados , Oxígeno Singlete/química , Simulación por Computador , Cristalografía por Rayos X , Cinética , Modelos Moleculares , Conformación Molecular , Estructura Molecular , Safrol/síntesis química , Safrol/química , Electricidad Estática , Estereoisomerismo

RESUMEN

[structure: see text] The optical diffuse reflectance and solution spectra of two bis-hydrazine radical cationic intervalence compounds have been compared. The results are consistent with an ion-pairing increase and an "effective polarity" in these crystals that is not far from that of acetonitrile or other polar solvents.

RESUMEN

The intrazeolite photooxygenations of four diarylethylenes have been examined. Several intermediates, including an epoxide, have been identified by comparison to independently synthesized samples. Aldehyde intermediates were shown to undergo intrazeolite Norrish type I cleavages in competition with a novel new photooxygenation/autoxidation reaction. [reaction: see text]

RESUMEN

The reactions of singlet oxygen with 2,2,6,6-tetradeuterio-1,4-dithiane have been examined in acetone and methanol, and in the interior of the zeolite NaY. The product isotope effects k(H)/k(D) have been measured for sulfoxide and, when possible, for sulfone formation. The results provide evidence for a hydroperoxysulfonium ylide intermediate in acetone, a hydrogen bonded or sulfurane intermediate in methanol, and an interesting equilibrium between two complexed forms of the substrate in NaY.