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Frederick D Lewis

Publications and source records attributed to Frederick D Lewis.

At least 37 records · Page 2Linked to original sources
PubMed2004

DNA-mediated exciton coupling and electron transfer between donor and acceptor stilbenes separated by a variable number of base pairs.

The synthesis, steady-state spectroscopy, and transient absorption spectroscopy of DNA conjugates possessing both stilbene electron donor and electron acceptor chromophores are described. These conjugates are proposed to form nicked DNA dumbbell structures in which a stilbenedicarboxamide acceptor and stilbenediether donor are separated by variable numbers of A-T or G-C base pairs. The nick is located either adjacent to one of the chromophores or between two of the bases. Thermal dissociation profiles indicate that stable structures are formed possessing as few as two A-T base pairs. Circular dichroism (CD) spectra in the base pair region are characteristic of B-DNA duplex structures, whereas CD spectra at longer wavelengths display two bands attributed to exciton coupling between the two stilbenes. The sign and intensity of these bands are dependent upon both the distance between the chromophores and the dihedral angle between their transition dipoles [Deltaepsilon approximately Rda(-2) sin(2theta)]. Pulsed laser excitation of the stilbenediamide results in creation of the acceptor-donor radical ion pair, which decays via charge recombination. The dynamics of charge separation and charge recombination display an exponential distance dependence, similar to that observed previously for systems in which guanine serves as the electron donor. Unlike exciton coupling between the stilbenes, there is no apparent dependence of the charge-transfer rates upon the dihedral angle between donor and acceptor stilbenes. The introduction of a single G-C base pair between the donor and acceptor results in a change in the mechanism for charge separation from single step superexchange to hole hopping.

Chromatography, High Pressure Liquid↗
PubMed2004

Polyaryl anion radicals via alkali metal reduction of arylurea oligomers.

A series of N-methylated polyarylurea oligomers have been reduced with potassium metal in HMPA. These reductions result in the transient formation of arylurea anion radicals, which undergo reductive elimination of the urea linkages. The aryl moieties appear in the products as the anion radicals of oligoaryl systems. The reaction is intramolecular, and the sequencing in the polyaryl anion radical remains the same as in the polyarylureas due to the urea-enforced pi-pi stacking interactions.

Journal Article↗
PubMed2004

Positional effects of the hydroxy substituent on the photochemical and photophysical behavior of 3- and 4-hydroxystilbene.

The photophysical and photochemical behavior of meta- and para-hydroxy- and methoxystilbene are reported and compared to the aminostilbenes. Absorption spectra of all four studied compounds in organic solution display a featureless, intense long-wavelength absorption band around 300 nm. In basic aqueous solution, meta-hydroxystilbene displays a less intense band with a long-wavelength shoulder, a consequence of configuration interaction from the O(-) substituent. Emission from the meta-substituted stilbenes is much stronger than from the para isomers, with an especially large effect for the hydroxystilbenes in solvents that are good hydrogen bond acceptors. Emission from the hydroxystilbenes is weak in aqueous solution, and even weaker in basic solution, a consequence of facile nonradiative decay. The excited state lifetimes of the meta isomers are longer than those with para substituents. Ground and excited state acidity constants are reported for the hydroxystilbenes. In the ground state, the para derivative is an order of magnitude more acidic than the meta isomer, however, its short excited state lifetime precludes excited state deprotonation. In contrast, 3-hydroxystilbene does exhibit deprotonation in its excited state.

Journal Article↗
PubMed2004

Dynamics and equilibrium for the formation of fluorescent Lewis acid-base exciplexes and triplexes between 9-aminophenanthrene and aliphatic amines.

The excited singlet states of 9-aminophenanthrene and its N-aminoalkyl derivatives are strongly fluorescent in cyclohexane. Addition of low concentrations of Et(3)N, Pr(2)NH, or PrNH(2) results in a red shift of the emission maximum and moderately decreased fluorescence intensity. Analysis of the fluorescence behavior using a combination of singular value decomposition with self-modeling and kinetic analysis provides evidence for the sequential formation of 1 : 1 (exciplex) and 1 : 2 (triplex) complexes between the excited 9-aminophenanthrene and ground-state alkylamine, both of which are strongly fluorescent. Both the formation and decay of the exciplex and triplex are dependent upon the extent of amine N-alkylation. Rate constants and equilibrium constants for complex formation follow the order 1 degree approximately 2 degree > 3 degree, analogous to that for the formation of ground-state complexes between amines and the soft Lewis base HgBr(2). Similarly, N-aminoalkyl derivatives of 9-aminophenanthrene form intramolecular exciplexes. Excited-state complex formation is attributed to a Lewis acid-base interaction between the excited aminophenanthrene (lone-pair acceptor) and ground-state amine (lone-pair donor). The factors which determine the stability of excited-state Lewis acid-base complexes are characteristic of the specific excited-state acceptor. No universal scale of lone-pair donor strength can be expected to describe the formation of such complexes.

Journal Article↗
PubMed2003

Ground-state conformational equilibrium and photochemical behavior of syn and anti N,N'-dimethyl-N,N'-di-1-naphthylurea protophanes.

The structure, spectroscopy, and photochemistry of N,N'-dimethyl-N,N'-di-1-naphthylurea have been investigated and compared to the properties of the corresponding secondary diarylurea N,N'-di-1-naphthylurea and the tertiary mono arylurea N,N,N'-trimethyl-N'-1-naphthylurea. The crystal structures and solution NMR spectra of the tertiary and secondary dinaphthylureas establish that they adopt folded (E,E) and extended (Z,Z) structures, respectively, both in the solid state and in solution. In solution, the tertiary E,E-dinaphthylurea exists as a mixture of syn and anti conformations separated by a barrier of ca. 14 kcal/mol, as determined by variable-temperature (1)H NMR spectroscopy. Computational exploration of the ground-state potential energy surface suggests that the lowest energy pathway for interconversion of the syn and anti conformers requires concurrent rotation about both the nitrogen-naphthalene and the nitrogen-carbonyl single bonds. The tertiary dinaphthylurea exhibits blue-shifted absorption and red-shifted emission attributed to excitonic interactions between the naphthalene rings. The secondary dinaphthylureas and mono naphthylurea have typical naphthalene-like monomer absorption and fluorescence spectra. Dual exponential fluorescence decay is assigned to the two conformers of the tertiary dinaphthylurea. Nonlinear fitting of the fluorescence decay times provides activation parameters for singlet decay of the two conformers. The decay process is attributed to nonsynchronous naphthalene-naphthalene bonding which, in the case of the syn conformer, results in the formation of a [2+2] intramolecular adduct. The preferred E,E conformation and moderate barrier to conformational isomerization make the tertiary dinaphthylurea an attractive building block for larger self-organizing pi-stacked aromatic arrays.

Journal Article↗
PubMed2003

Stepwise evolution of the structure and electronic properties of DNA.

The optical properties of a family of duplex DNA conjugates possessing from 1 to 11 A:T base pairs and covalently attached stilbene chromophores at both ends have been investigated. As few as from two to four base bairs are needed to establish base-pair absorption and circular dichroism spectra similar to those of longer oligomers. The stilbene chromophores exhibit exciton-coupled circular dichroism, which can be observed even with 11 intervening base pairs, a distance of ca. 40 A. The dependence of the sign and amplitude of these spectra reveals the ability of B-DNA to serve as a helical ruler.

Circular Dichroism↗
PubMed2003

Face-to-face and edge-to-face pi-pi interactions in a synthetic DNA hairpin with a stilbenediether linker.

Synthetic conjugates possessing bis(2-hydroxyethyl)stilbene-4,4'-diether linkers (Sd2) form the most stable DNA hairpins reported to date. Factors that affect stability are length and flexibility of the linkers and pi-stacking of the stilbene moiety on the adjacent base pair. The crystal structure of the hairpin d(GT(4)G)-Sd2-d(CA(4)C) was determined at 1.5 A resolution. The conformations of the two molecules in the asymmetric unit differ both in the linker and the stem portions. One of them shows a planar stilbene that is stacked on the adjacent G:C base pair. The other displays considerable rotation between the phenyl rings and an unprecedented edge-to-face orientation of stilbene and base pair. The observation of considerable variations in the conformation of the Sd moiety in the crystal structure allows us to exclude restriction of motion as the reason for the absence of Sd photoisomerization in the hairpins. Conformational differences in the stem portion of the two hairpin molecules go along with different Mg(2+) binding modes. Most remarkable among them is the sequence-specific coordination of a metal ion in the narrow A-tract minor groove. The crystal structure provides unequivocal evidence that a fully hydrated Mg(2+) ion can penetrate the narrow A-tract minor groove, causing the groove to further contract. Overall, the structural data provide a better understanding of the origins of hairpin stability and their photochemical behavior in solution.

Cations↗
PubMed2003

Control of kinetics and thermodynamics of [1,5]-shifts by aromaticity: a view through the prism of Marcus theory.

The effects of aromatic stabilization on the rates of [1,5]-hydrogen shifts in a series of carbo- and heterocyclic dihydroaromatic compounds were estimated by B3LYP/6-31G computations. The aromatic stabilization energy of the product is directly translated into increased exothermicity of these reactions. Relative trends for a significant range of endothermic and exothermic [1,5]-shifts with different intrinsic activation energies are reliably described by Marcus theory. The effects of aromaticity or antiaromaticity are very large and can lead to dramatic acceleration or deceleration of [1,5]-hydrogen shifts and even to complete disappearance of the reaction barrier. Not only the activation energy but the shape and position of the reaction barrier can be efficiently controlled by changes in the aromaticity of the products, making these systems interesting models for studying hydrogen tunneling. Marcus theory can also be applied successfully to other pericyclic shifts such as [1,5]-shifts which involve chlorine and methyl transfer.

Journal Article↗
PubMed2003

Activated decay pathways for planar vs twisted singlet phenylalkenes.

The ground state conformation, spectroscopy, and photochemical behavior of styrene and eight of its vinyl- and ring-methylated derivatives have been investigated. Introduction of methyl groups at the alpha-position of the vinyl group or the ortho positions of the phenyl results in increased phenyl-vinyl dihedral angles. Styrenes possessing both alpha-methyl and ortho-methyl groups adopt orthogonal geometries. Decreased planarity results in a progressive blue-shift in the lowest energy allowed pi,pi transition and a decrease in the singlet lifetime. Kinetic modeling of the temperature-dependent singlet lifetimes provides activation parameters for the activated decay pathway, which is assigned to C=C torsion for styrenes with phenyl-vinyl dihedral angles, phi, < 60 degrees. Planar styrenes have large torsional barriers (7 +/- 1 kcal/mol) and decay predominantly via intersystem crossing and fluorescence at room temperature. Styrenes with values of 30 degrees < phi < 60 degrees have smaller torsional barriers and decay predominantly via C=C torsion at room temperature. Highly nonplanar styrenes decay predominantly via relatively rapid, weakly activated intersystem crossing.

Journal Article↗
PubMed2003

Dynamics and energetics of single-step hole transport in DNA hairpins.

The dynamics of single-step hole transport processes have been investigated in a number of DNA conjugates possessing a stilbenedicarboxamide electron acceptor, a guanine primary donor, and several secondary donors. Rate constants for both forward and return hole transport between the primary and secondary donor are obtained from kinetic modeling of the nanosecond transient absorption decay profiles of the stilbene anion radical. The kinetic model requires that the hole be localized on either the primary or the secondary donor and not delocalized over both the primary and the secondary donor. Rate constants for hole transport are found to be dependent upon the identity of the secondary donor, the intervening bases, and the location of the secondary donor in the same strand as the primary donor or in the complementary strand. Rate constants for hole transport are much slower than those for the superexchange process used to inject the hole on the primary donor. This difference is attributed to the larger solvent reorganization energy for charge transport versus charge separation. The hole transport rate constants obtained in these experiments are consistent with experimental data for single-step hole transport from other transient absorption studies. Their relevance to long-distance hole migration over tens of base pairs remains to be determined. The forward and return hole transport rate constants provide equilibrium constants and free energies for hole transport equilibria. Secondary GG and GGG donors are found to form very shallow hole traps, whereas the nucleobase deazaguanine forms a relatively deep hole trap. This conclusion is in accord with selected strand cleavage data and thus appears to be representative of the behavior of holes in duplex DNA. Our results are discussed in the context of current theoretical models of hole transport in DNA.

Base Pairing↗
PubMed2003

Hydroxystilbene isomer-specific photoisomerization versus proton transfer.

The ground- and excited-state acidities of 3- and 4-hydroxystilbene have been investigated in aqueous solution. Approximate pKa* values determined from the Förster equation are similar for the two isomers; however, 3-hydroxystilbene behaves as a photoacid in water, whereas 4-hydroxystilbene does not. Singular value decomposition with self-modeling is used to obtain more accurate pKa* values for 3-hydroxystilbene. The different behavior of the isomeric hydroxystilbenes is attributed to differences in their excited-state C=C torsional barriers. 3-Hydroxystilbene has a high barrier and a long singlet lifetime, permitting proton transfer to compete with other singlet decay pathways, whereas 4-hydroxystilbene has a low barrier, and thus proton transfer cannot compete with C=C torsion.

Journal Article↗
PubMed2003

Torsional barriers for planar versus twisted singlet styrenes.

Kinetic modeling of the temperature-dependent lifetimes of styrene and several methyl-substituted styrenes has been used to obtain the torsional barriers for singlet-state C=C rotation. The barrier for C=C torsion is found to be correlated with the ground-state phenyl-vinyl dihedral angle, varphi, planar styrenes having barriers of approximately 6.5 kcal/mol and moderately twisted styrenes having smaller barriers. Highly twisted styrenes undergo exceptionally rapid intersystem crossing. This unexpected dependence of excited-state behavior on varphi is attributed to a change in the character of the excited states from delocalized for planar styrenes to localized for highly twisted styrenes.

Journal Article↗
PubMed2003

N,N'-dimethyl-N,N'-diarylurea anion radicals: an intramolecular reductive elimination.

The one-electron reduction of tertiary N,N'-dimethyl-N,N'-diarylureas (aryl = phenyl, beta-naphthyl, alpha-naphthyl), in HMPA, results in anion radicals that undergo novel intramolecular reductive elimination reactions leading to the formation of the anion radicals of the corresponding biaryls. These results are due to face to face pi-pi stacking interactions involving the two aromatic rings in the urea systems. The overlapping p(pi)() orbitals on the ipso carbons of opposing aryl groups evolve into a sigma bond leading to the formation of the biaryl anion radical. In the case of the N,N'-dimethyl-N,N'-di-2-pyrenylurea system, there is a node in the LUMO of the number 2 carbon, and the parent anion radical remains intact.

Journal Article↗
PubMed2003

Conformer-specific photoisomerizaton of some 2-vinylbiphenyls.

The ground-state conformations, spectroscopy, and photochemistry of 2-vinylbiphenyl and three of its vinylsubstituted derivatives have been investigated. The ground state exists as a mixture of syn and anti conformers in which the vinyl group is directed either toward or away from the unsubstituted phenyl ring. Both conformers are highly non-planarn having large phenyl phenyl and phenyl vinyl dihedral angles. The vinylbiphenyls are found to undergo conformer-specific photochemistry. The syn rotamers are non-fluorescent and undergo rapid and efficient cyclization to 8a,9-dihydrophenanthrene intermediates, which undergo rapid thermal sigmatropic rearrangements to yield 9,10-dihydrophenanthrenes. The intermediates have been generated at 77 K and detected by absorption spectroscopy. The singlet states of the anti rotamers are fluorescent, and undergo competing intersystem crossing and activated C=C torsion. The torsional barriers are of the order of those for styrenes with similar vinyl substituents.

Journal Article↗
PubMed2002

Formation and decay of localized contact radical ion pairs in DNA hairpins.

The dynamics of charge separation and charge recombination in synthetic DNA hairpins possessing diphenylacetylene-4,4'-dicarboxamide linkers have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. The lowest excited singlet state of the linker is capable of oxidizing nearest neighbor adenine as well as guanine. A large wavelength shift in the transient absorption spectrum accompanies the conversion of the singlet linker to its anion radical, facilitating the investigation of electron-transfer dynamics. The rate constants for charge separation are dependent upon the oxidation potentials of the neighboring nucleobase donors but not upon the identity of nonnearest neighbors. Thus, the charge separation processes yield a contact radical ion pair in which the positive charge is localized on the neighboring nucleobase. Rate constants for charge recombination are dependent upon the identity of the first and second nearest-neighbor nucleobases but not more remote bases. This dependence is attributed to stabilization of the contact radical ion pair by interaction with its nearest neighbor. The absence of charge migration to form a base-pair separated radical ion pair is a consequence of Coulombic attraction in the contact radical ion pair and the low effective dielectric constant (epsilon < 7) experienced by the contact radical ion pair. Photoinduced charge injection to form a base-pair separated radical ion pair is necessary in order to observe charge migration.

Acetylene↗
PubMed2002

Symmetry-enforced conformational control of photochemical reactivity in 2-vinyl-1,3-terphenyl.

The photocyclization of o-vinylbiphenyl to 9,10-dihydrophenanthrene occurs with very low quantum yield because of the low ground-state population of the reactive syn rotamer. 2-Vinyl-1,3-terphenyl exists as a single rotamer which undergoes highly efficient photocyclization. Irradiation at 77 K in a rigid glass permits observation of the 8a,9-dihydrophenanthrene which rearranges to the 9,10-dihydrophenanthrene upon warming of the glass. The barriers for photocyclization and the thermal sigmatropic hydrogen shift are both remarkably small.

Journal Article↗
PubMed2002

Synthesis, structure, and photochemistry of exceptionally stable synthetic DNA hairpins with stilbene diether linkers.

The structure and properties of 18 hairpin-forming bis(oligonucleotide) conjugates possessing stilbene diether linkers are reported. Conjugates possessing bis(2-hydroxyethyl)stilbene 4,4'-diether linkers form the most stable DNA hairpins reported to date. Hairpins with as few as two T:A base pairs or four noncanonical G:G base pairs are stable at room temperature. Increasing the length of the hydroxyalkyl groups results in a decrease in hairpin thermal stability. On the basis of the investigation of their circular dichroism spectra, all of the hairpins investigated adopt B-DNA structures, except for a hairpin with a short poly(G:C) stem which forms a Z-DNA structure. Both the strong fluorescence of the stilbene diether linkers and their trans-cis photoisomerization are totally quenched in hairpins possessing neighboring T:A and G:C base pairs. Quenching is attributed to an electron-transfer mechanism in which the singlet stilbene serves as an electron donor and T or C serves as an electron acceptor. In contrast, in denatured hairpins and hairpins possessing neighboring G:G base pairs the stilbene diether linkers undergo efficient photoisomerization.

Circular Dichroism↗