Competing electron-transfer pathways in hydrocarbon frameworks: short-circuiting through-bond coupling by nonbonded contacts in rigid U-shaped norbornylogous systems containing a cavity-bound aromatic pendant group

This work explores electron transfer through nonbonded contacts in two U-shaped DBA molecules 1DBA and 2DBA by measuring electron-transfer rates in organic solvents of different polarities. These molecules have identical U-shaped norbornylogous frameworks, 12 bonds in length and with diphenyldimethoxynaphthalene (DPMN) donor and dicyanovinyl (DCV) acceptor groups fused at the ends. The U-shaped cavity of each molecule contains an aromatic pendant group of different electronic character, namely p-ethylphenyl, in 1DBA, and p-methoxyphenyl, in 2DBA. Electronic coupling matrix elements, Gibbs free energy, and reorganization energy were calculated from experimental photophysical data for these compounds, and the experimental results were compared with computational values. The magnitude of the electronic coupling for photoinduced charge separation, /V(CS)/, in 1DBA and 2DBA were found to be 147 and 274 cm(-1), respectively, and suggests that the origin of this difference lies in the electronic nature of the pendant aromatic group and charge separation occurs by tunneling through the pendant group, rather than through the bridge. 2DBA, but not 1DBA, displayed charge transfer (CT) fluorescence in nonpolar and weakly polar solvents, and this observation enabled the electronic coupling for charge recombination, /V(CR)/, in 2DBA to be made, the magnitude of which is approximately 500 cm(-1), significantly larger than that for charge separation. This difference is explained by changes in the geometry of the molecule in the relevant states; because of electrostatic effects, the donor and acceptor chromophores are about 1 A closer to the pendant group in the charge-separated state than in the locally excited state. Consequently the through-pendant-group electronic coupling is stronger in the charge-separated state--which controls the CT fluorescence process--than in the locally excited state--which controls the charge separation process. The magnitude of /V(CR)/ for 2DBA is almost 2 orders of magnitude greater than that in DMN-12-DCV, having the same length bridge as for the former molecule, but lacking a pendant group. This result unequivocally demonstrates the operation of the through-pendant-group mechanism of electron transfer in the pendant-containing U-shaped systems of the type 1DBA and 2DBA.     

On the endo/exo stereoselectivity of intramolecular Diels-Alder reactions of hexadienylacrylates: an interesting failure of density functional theory

[formula: see text] A combined experimental and computational study of endo/exo stereoselectivity in a series of IMDA reactions of hexadienylacrylates has found that DFT makes erroneous predictions when the endo and exo transition states possess differing degrees of conjugation. These problems are overcome by carrying out calculations at the MP2 level of theory, which gives remarkably accurate Boltzmann distributions of products. These findings are used to predict ways to obtain either endo- or exo-cycloadducts exclusively.     

Theoretical studies of stereoselective hydroborations

Asymmetric hydroborations of alkenes are outstanding examples of reactions which proceed with high acyclic stereoselection to give synthetically useful functionality. We have undertaken com- putational studies of these reactions as a part of our more general studies of stereoselective organic reactions. We wish to develop understanding of those factors which control stereoselectivity in known cases, and to develop both qualitative and quantitative methods to predict stereoselectivities of cases not yet investigated. Here, we detail our progress in understanding hydroborations, and in subsequent publications, we will report similar studies for other types of organic reactions, such as intermolecular and intramolecular cycloadditions, epoxidations, radical reactions, nucleophilic additions, and aldol condensations.     

Double diels-alder reactions of linear conjugated tetraenes

[reaction: see text] Linear conjugated tetraenes are shown to participate effectively as bis-dienes in sequences involving either two intermolecular Diels-Alder reactions or an intramolecular followed by an intermolecular Diels-Alder reaction. Thus, simple tetraenol 1 is transformed into tetracyclic products 5, 6, and 9 in high yielding and highly stereoselective sequences with maleic anhydride involving the formation of three rings, four C-C bonds and one C-O bond, and eight stereocenters. In the latter case, the one-pot reaction protocol is very simple, and furnishes a single diastereoisomeric product in essentially quantitative yield. Linear conjugated tetraenes exhibit complete terminal site selectivity in reactions with dienophiles and computational investigations reveal that two discrete pi-conjugative interactions are the origin of this unexpected reactivity. B3LYP/6-31G(d) transition structures also allow an explanation of unexpectedly high pi-diastereofacial selectivities witnessed during these transformations, through the identification of preferred C1-C diene conformations and nonbonded interactions. These new experimental and computational findings encourage the use of linear conjugated polyenes in domino sequences.     

Unusually rapid heterogeneous electron transfer through a saturated bridge 18 bonds in length

Ferrocene modified self-assembled monolayers of a novel saturated norbornylogous bridge, 21.3 A long, demonstrated unusually rapid rates of heterogeneous electron transfer, three orders of magnitude faster than the equivalent length alkanethiol.     

Pendant unit effect on electron tunneling in U-shaped molecules

The electron transfer reactions of three U-shaped donor-bridge-acceptor molecules with different pendant groups have been studied in different solvents as a function of temperature. Analysis of the electron transfer kinetics in nonpolar and weakly polar solvents provides experimental reaction Gibbs energies that are used to parameterize a molecular solvation model. This solvation model is then used to predict energetic parameters in the electron transfer rate constant expression and allow the electronic coupling between the electron donor and electron acceptor groups to be determined from the rate data. The U-shaped molecules differ by alkylation of the aromatic pendant group, which lies in the ‘line-of-sight’ between the donor and acceptor groups. The findings show that the electronic coupling through the pendant group is similar for these molecules.     

Protein modulation of electrochemical signals: application to immunobiosensing

Herein we present a label-free immunobiosensor based on the modulation of amperometric signals of surface bound redox species when immersed in a protein environment which is applicable to either the detection of antibodies or the detection of small molecules such as drugs or pesticides.     

Charge separation in mesoporous aluminosilicates

EPR spectroscopy has been used to investigate spontaneous and/or photo-induced electron transfer between adsorbed organic molecules and the mesoporous aluminosilicate MCM-41 host. Spontaneous electron transfer occurs from the host to electron acceptor molecules with sufficiently favourable reduction potentials (TCNE, TCNQ, 1,4-benzoquinone, 1,4-naphthaquinone and 1,4-anthraquinone), provided the MCM-41 contains aluminium and the radical anion yield correlates with the aluminium content of the host. The semiquinone radical anions are interacting strongly with exposed Al³⁺ sites, whereas the TCNE and TCNQ radical anions are loosely bound and can be washed from the host. Radical cation formation is observed when electron donor molecules with favourable oxidation potentials are adsorbed in MCM-41 containing aluminium, and the radical cations formed interact with exposed Al³⁺ sites. This work shows that aluminium-containing MCM-41 contains both electron donating and electron accepting sites which may intervene in intra-molecular charge separation processes in adsorbed organic molecules.