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Biomedical subjects

Miguel A Garcia-Garibay

Publications and source records attributed to Miguel A Garcia-Garibay.

At least 19 recordsLinked to original sources
PubMed2006

Photophysical properties of a 1,2,3,4,5,6-hexasubstituted fullerene derivative.

The photophysical properties of a novel 1,2,3,4,5,6-hexasubstituted fullerene derivative (1) are examined in this study. In addition to the ground state absorption spectrum of 1, we report its triplet-triplet absorption spectrum and molar extinction coefficient (Deltae(T-T)), as well as the triplet quantum yield (PhiT), lifetime (tauT), and energy (ET). The saturation of a single six-member ring on the fullerene cage results in significant changes in the triplet state properties as compared to that of pristine C60. The triplet-triplet absorption spectrum shows a hypsochromic shift in long wavelength absorption, and both the triplet state lifetime and the triplet quantum yield are decreased. The triplet energy was found to be similar to that of C60. In addition, the quantum yield (PHI(delta)) of singlet oxygen generated by 1 was calculated and is found to be significantly less than in the case of C60.

Fullerenes↗
PubMed2006

Improved physical properties and rotational dynamics in a molecular gyroscope with an asymmetric stator structure.

[structure: see text] A molecular gyroscope consisting of a 1,4-diethynylphenylene rotator linked to trityl and triptycyl groups (3) showed significantly improved physical properties and faster rotational dynamics than analogous symmetric bis(trityl) (1) or bis(triptycyl) (2) structures. An activation energy of 7.9 kcal/mol for 3 was determined by 2H NMR. This is ca. 4-6 kcal/mol lower than that of compound 1. The different dynamics of the three compounds can be qualitatively understood in terms of their different packing coefficients.

Journal Article↗
PubMed2006

Large-scale photochemical reactions of nanocrystalline suspensions: a promising green chemistry method.

Photochemical reactions in the solid state can be scaled up from a few milligrams to 10 grams by using colloidal suspensions of a photoactive molecular crystal prepared by the solvent shift method. Pure products are recovered by filtration, and the use of H(2)O as a suspension medium makes this method a very attractive one from a green chemistry perspective. Using the photodecarbonylation of dicumyl ketone (DCK) as a test system, we show that reaction efficiencies in colloidal suspensions rival those observed in solution. [reaction: see text]

Journal Article↗
PubMed2006

Photocatalytic reduction of an azide-terminated self-assembled monolayer using CdS quantum dots.

Ordered, tightly packed aryl-azide-terminated, self-assembled monolayers (SAMs) were created on gold substrates from a new disulfide precursor. These monolayers were reduced at least partially in an aqueous environment using approximately 2 nm CdS quantum dots (Qdots) as photocatalysts to give mixed monolayers of arylamine- and aryl azide-terminated species. The CdS photocatalysts were made available for the reaction by exposure of the azide-terminated SAM to Qdots initially in solution or by preadsorption of the CdS nanoparticles on the SAM. In either case, X-ray photoelectron spectroscopy (XPS), grazing angle Fourier transform infrared spectroscopy (FTIR), and contact angle measurements were used to show the occurrence of the photocatalytic reduction. As further evidence for the presence of arylamine-terminated thiolate in the reduced SAM, these arylamine groups were successfully tagged with fluorescein isothiocyanate (FITC). The use of Qdot photocatalysts to functionalize surfaces may lead to a means to pattern surfaces at the nanoscale.

Journal Article↗
PubMed2006

Crystalline molecular machines: a quest toward solid-state dynamics and function.

Complex molecular machinery may be envisioned as densely packed, multicomponent, self-assembling systems built with high structural precision to control the dynamics of one or more internal degrees of freedom. With molecular gyroscopes as a test, we describe a general strategy to design crystals capable of supporting structurally programmed molecular motions, a practical approach to their synthesis, convenient strategies to characterize their solid-state dynamics, and potential applications based on polar structures responding collectively to external fields.

Journal Article↗
PubMed2005

Secondary alpha isotope effects on deuterium tunneling in triplet o-methylanthrones: extraordinary sensitivity to barrier width.

The rates of deuterium transfer in the photoenolization of triplet 1,4-dimethyl-10H-anthracen-9-one (1) with varying degrees of deuterium label in their methyl groups (1-d3, 1-d2, and 1-d) have been investigated as a function of temperature between 5 and 77 K. Measurable rate constants in the case of 1-d3 and 1-d2 were used to construct Arrhenius plots which illustrate the expected curvature and leveling off of rate constant versus temperature. The difference in tunneling rate constants of 1-d3 and 1-d2 yields a tunneling isotope effect, TIE = 2.4, which is attributed to the secondary alpha isotopic substitution. Density functional theory (DFT, B3LYP/6-31G*) calculations were carried out to obtain structural and energetic information for the H(D) transfer along the triplet state zero-point energy levels. The temperature dependence of the rate constants for each isotopologue was simulated with a model that considers the frequency of the C-D stretching mode and the quantum mechanical permeability determined from calculated energy parameters. The model suggests that a difference in barrier width of only 0.015 A between 1-d3 and 1-d2 leads to the observed 2-fold difference between tunneling rates.

Journal Article↗
PubMed2005

Crystalline molecular machines: encoding supramolecular dynamics into molecular structure.

Crystalline molecular machines represent an exciting new branch of crystal engineering and materials science with important implications to nanotechnology. Crystalline molecular machines are crystals built with molecules that are structurally programmed to respond collectively to mechanic, electric, magnetic, or photonic stimuli to fulfill specific functions. One of the main challenges in their construction derives from the picometric precision required for their mechanic operation within the close-packed, self-assembled environment of crystalline solids. In this article, we outline some of the general guidelines for their design and apply them for the construction of molecular crystals with units intended to emulate macroscopic gyroscopes and compasses. Recent advances in the preparation, crystallization, and dynamic characterization of these interesting systems offer a foothold to the possibilities and help highlight some avenues for future experimentation.

Journal Article↗
PubMed2005

Green chemistry strategies using crystal-to-crystal photoreactions: stereoselective synthesis and decarbonylation of trans-alpha,alpha'-dialkenoylcyclohexanones.

This paper describes steps to develop green chemistry strategies to prepare compounds with adjacent quaternary centers by stereospecific photodecarbonylation of crystalline ketones bearing radical-stabilizing alkenyl substituents in their alpha-positions. Crystals of trans-2,6-dimethyl-2,6-di(benzyloxycarbonyl-trans-ethenyl)-cyclohexanone, trans-2, prepared by double Michael addition of benzyl propiolate to 2,6-dimethyl-cyclohexanone iso-butylimine (1), were investigated along with those of the dicarboxylic acid (trans-3) and a dibenzylammonium salt derivative (trans-4) to establish the best substrates for the solid-state reaction. While reactions of the ester and the acid lose CO to give radical combination products with high selectivity at low conversions, the crystalline salt gave a similar product in >97% yield. Explorative reactions carried out under sunlight and in semipreparative (1.5 g) scale highlight the potential of reactions in crystals as a viable strategy for the development of green chemistry.

Journal Article↗
PubMed2005

Effects of rotational symmetry order on the solid state dynamics of phenylene and diamantane rotators.

The rotational dynamics of phenylene and diamantane rotators in crystals of 1,9-bis(4-[3,3,3-triphenylpropynyl]phenyl)diamantane were analyzed independently within the same crystal structure. The dynamics of phenylene rotation were established by dynamic line shape analysis using 13C CPMAS NMR. The phenylene signals were selectively highlighted by deuteration of the aromatic trityls and the use of short contact times for cross polarization. The dynamics of the diamantane group were established by 1H spin-lattice relaxation under conditions where dipolar relaxation was shown to be the dominant mechanism between 250 and 425 K. A factor of 20 000 between the rates of rotation of the faster diamantane and the slower phenylene at 300 K supports expectations that higher symmetry rotors should have significantly faster dynamics.

Journal Article↗
PubMed2005

Hammett analysis of photodecarbonylation in crystalline 1,3-diarylacetones.

[reaction: see text] The relative quantum yields and chemical efficiencies of crystalline p,p'-disubstituted 1,3-diphenyl-2-propanones with 4-MeO, 4-Me, 4-F, 4-CF3, and 3,4-diMeO groups were determined by parallel irradiation of polycrystalline samples. Variations in quantum yields that span a factor of 4 are analyzed in terms of the effects of substituents on the stability of the benzylic radical. All solid-state reactions proceeded with 100% chemoselectivity and in >95% chemical yield.

Journal Article↗
PubMed2005

2H NMR and X-ray diffraction studies of methyl rotation in crystals of ortho-methyldibenzocycloalkanones.

We have used (2)H NMR lineshape analyses and single crystal X-ray diffraction (XRD) to investigate the effects of molecular structure and crystalline environment on the rotational dynamics of methyl groups in four aromatic cycloalkanones. These include two methyl-substituted anthrones, one anthraquinone and one dibenzosuberone, which are known to undergo excited state H-atom tunneling from the ortho-methyl group to the carbonyl oxygen. With experiments conducted between 100 and 300K, samples 1,4-dimethylanthrone (DMAT) and 1,4-dimethylanthraquinone (DMAQ) were shown to enter the intermediate exchange regime (k(rot) approximately <10(7)s(-1)) at ca. 120K while samples of 1,4,10,10-tetramethylanthrone (TMAT) and 1,4-dimethyldibenzosuberone (DMDBS) remained in the fast exchange regime even at ca. 100K. Single crystal XRD analyses suggest that high intramolecular hindrance is avoided by molecular distortions, and that intermolecular contacts play an important role.

Journal Article↗
PubMed2004

Crankshaft motion in a highly congested bis(triarylmethyl)peroxide.

Crankshaft motion has been proposed in the solid state for molecular fragments consisting of three or more rotors linked by single bonds, whereby the two terminal rotors are static and the internal rotors experience circular motion. Bis-[tri-(3,5-di-tert-butyl)phenylmethyl]-peroxide 2 was tested as a model in search of crankshaft motion at the molecular level. In the case of peroxide 2, the bulky trityl groups may be viewed as the external static rotors, while the two peroxide oxygens can undergo the sought after internal rotation. Evidence for this process in the case of peroxide 2 was obtained from conformational dynamics determined by variable-temperature (13)C and (1)H NMR between 190 and 375 K in toluene-d(8). Detailed spectral assignments for the interpretation of two coalescence processes were based on a correlation between NMR spectra obtained in solution at low temperature, in the solid state by (13)C CPMAS NMR, and by GIAO calculations based on a B3LYP/6-31G structure of 2 obtained from its X-ray coordinates as the input. Evidence supporting crankshaft rotation rather than slippage of the trityl groups was obtained from molecular mechanics calculations.

Journal Article↗
PubMed2004

Photocatalytic reduction of aromatic azides to amines using CdS and CdSe nanoparticles.

We have shown that CdS and CdSe nanoparticles can act as very efficient and highly chemoselective photocatalysts for the reduction of aromatic azides to aromatic amines. In several cases, the reaction proceeds with quantum yields near 0.5, which approaches the theoretical maximum for a two-electron process. The wide scope of the reaction was confirmed with compounds containing electron withdrawing (-NO(2), CO(2)R, COR) and electron donating groups (-OMe, -R, -Cl) at the para-, meta-, and ortho-positions. Remarkably, the reaction is relatively insensitive to the electron demands of the substituent. However, azides with meta-substituents give slightly lower yields than those with the same substituent at the ortho- or para-position.

Journal Article↗
PubMed2004

Molecular crystals with moving parts: synthesis, characterization, and crystal packing of molecular gyroscopes with methyl-substituted triptycyl frames.

We report a highly convergent synthesis for the preparation of molecular gyroscopes consisting of para-phenylene rotors linked by triple bonds to methyl-substituted triptycenes acting as pivots and encapsulating frames. The desired 1,4-bis[2-(2,3,6,7,12,13-hexamethyl-10-alkyl-9-triptycyl)ethynyl]benzenes were prepared from 2,3-dimethyl-1,3-butadiene using Diels-Alder cycloadditions and Pd(0)-catalyzed coupling as the key reactions. The main challenge in the synthesis came about in the preparation of 9-alkynyl-triptycenes by Diels-Alder reaction of benzynes and 9-alkynyl-2,3,6,7-tetramethylanthracenes. These reactions occurred with chemical yields and regioselectivities that were strongly influenced by steric and electronic effects of substituents at C10 of the anthracene core. Anthracenes with methyl, propyl, and phenyl substituents were utilized to complete the synthesis of their corresponding molecular gyroscopes, and their solid-state structures were determined by single-crystal X-ray diffraction analysis. Examination of these results indicated that, as expected, the bulky triptycyl groups encourage crystallization motifs that create more free volume around the phenylene rotor, as needed to facilitate fast gyroscopic motion in the solid state.

Journal Article↗
PubMed2004

Total synthesis of (+/-)-herbertenolide by stereospecific formation of vicinal quaternary centers in a crystalline ketone.

[reaction: see text] The sesquiterpene (+/-)-herbertenolide was synthesized in seven steps from commercial 2-bromo-4-methylanisole. In the key step, two adjacent stereogenic quaternary centers were controlled by a highly chemoselective and stereospecific photodecarbonylation reaction of crystalline methyl-trans-3-(2-methyl-5-methoxyphenyl)-1,3-dimethyl-2-oxocyclohexancarboxylate (3). An efficient generation of radical pairs and the stereochemical control exerted by the solid state suggest that this reaction may become a useful synthetic method.

Catalysis↗
PubMed2003

Molecular compasses and gyroscopes with polar rotors: synthesis and characterization of crystalline forms.

We report the highly convergent synthesis and solid-state characterization of six crystalline "molecular compasses" consisting of a central phenylene rotor with polar substituents, or compass needle, and two trityl groups axially connected by acetylene linkages to the 1,4-positions. Compounds with fluoro-, cyano-, nitro-, amino-, diamino-, and nitroamino substituents are expected to emulate the parent compound which was shown to form crystals where the central phenylene can rotate about its 1,4-axis with rate constants in the 10(3) -10(6) s(-)(1) dynamic ranges near ambient temperature, depending on crystal morphology. With data from single-crystal X-ray diffraction analysis, solid-state CPMAS (13)C NMR, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA), it is shown that a relatively small structural perturbation by a single polar group (F, CN, NO(2), NH(2)) results in isomorphous structures with analogous properties. In analogy to the parent compound, crystals grown from benzene formed clathrate structures in the space group Ponemacr; with one molecular compass and two benzene molecules per unit cell. Solvent-free crystals with the same space group obtained by a first-order phase transition between 60 and 130 degrees C were shown to be spectroscopically identical to those obtained by slow solvent evaporation from a mixture of CH(2)Cl(2) and hexanes. A qualitative analysis of the positionally disordered phenylene groups in terms of the expected solid-state rotational dynamics suggests a nonsymmetric, 2-fold rotational potential, or a process involving full 360 degrees turns.

Journal Article↗