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.     

Dynamics of electron injection in DNA hairpins

The dynamics of electron injection has been investigated in DNA hairpins possessing a stilbenediether electron donor linker by means of femtosecond transient absorption spectroscopy. Ultrafast electron injection and charge recombination are observed with neighboring cytosine or thymine bases; however, guanine-guanine base pairs are not reduced, permitting the investigation of the distance dependence of charge injection.     

Influence of structural dynamics on charge recombination rates in photogenerated radical ion pairs: Evidence from EPR spectroscopy and computation

In order to better understand the dependence of charge recombination rate vs. temperaturek _CR(T) within a linear donor-chromophore-acceptor (D-C-A) molecular triad, the structural dynamics of the cation radical D⁺-C is studied individually using variable-temperature electron paramagnetic resonance (EPR) spectroscopy and electronic structure calculations. Here, the donor D isp-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, and the acceptor A is naphthalene-1,8∶4,5-bis(dicarboximide). The EPR spectra of D⁺-C exhibit marked changes in their overall shape throughout the 190–295 K temperature range. These spectra have hyperfine splittings that are strikingly well simulated with a model that includes methoxy group rotation, which occurs at a rate of 2.6 · 10⁴ s^?1 at 210 K and speeds up to 1.25 · 10⁷ s^?1 at 295 K, corresponding to an energy barrier of 38 kJ/mol. This considerable barrier reflects the partial conjugation between MeO and the aromatic ring and is confirmed by the calculated energy of a series of D^{+ ·}-C rotamers. The simulations also reveal that inversion of the anilino N center emerges atT > 250 K and can be represented by a planar and a pyramidal conformation with the equilibrium constantK = [pyramidal]/[planar] increasing from 0.029 at 250 K to 0.56 at 295 K. In the same temperature range, the charge recombination rate of D^{+ ·}-C-A^{? ·} accelerates abruptly and can be separated into two components, according to the above planar/pyramidal equilibrium. Thek _CR (T) of the pyramidal conformation has an activation energy of 41 kJ/mol, virtually the same as the barrier of MeO rotation. These results show that the intramolecular structural dynamics of the radical cation within D^+·-C-A^{? ·} control the overall charge recombination reaction with this radical ion pair.     

Selectively metalated doubly cofacial porphyrin trimers. New models for the study of photoinduced intramolecular electron transfer.

The preparation of a doubly cofacial porphyrin trimer consisting of a Zn mesoporphyrin - Zn mesoporphyrin - mesoporhyrin stack is described. The optical absorption and fluorescence properties of the molecule as a function of solvent are also reported.     

Dynamics of inter- and intrastrand hole transport in DNA hairpins

The dynamics of photoinduced electron transfer has been investigated in DNA hairpins possessing a stilbenedicarboxamide (Sa) electron acceptor, a guanine (G) primary donor, and two adjacent guanines (GG) as secondary donors. Hole transport from G to GG across a single A is more rapid than across AA or T by factors of 20 +/- 7 and 40 +/- 15, respectively. Intrastrand hole transport across a single A is more rapid than interstrand transport by a factor of 7 +/- 3.     

Biomimetic multifunctional porous chalcogels as solar fuel catalysts

Biological systems that can capture and store solar energy are rich in a variety of chemical functionalities, incorporating light-harvesting components, electron-transfer cofactors, and redox-active catalysts into one supramolecule. Any artificial mimic of such systems designed for solar fuels production will require the integration of complex subunits into a larger architecture. We present porous chalcogenide frameworks that can contain both immobilized redox-active Fe(4)S(4) clusters and light-harvesting photoredox dye molecules in close proximity. These multifunctional gels are shown to electrocatalytically reduce protons and carbon disulfide. In addition, incorporation of a photoredox agent into the chalcogels is shown to photochemically produce hydrogen. The gels have a high degree of synthetic flexibility, which should allow for a wide range of light-driven processes relevant to the production of solar fuels.     

Gated Electron Sharing Within Dynamic Naphthalene Diimide‐Based Oligorotaxanes

The controlled self‐assembly of well‐defined and spatially ordered π‐systems has attracted considerable interest because of their potential applications in organic electronics. An important contemporary pursuit relates to the investigation of charge transport across noncovalently coupled components in a stepwise fashion. Dynamic oligorotaxanes, prepared by template‐directed methods, provide a scaffold for directing the construction of monodisperse one‐dimensional assemblies in which the functional units communicate electronically through‐space by way of π‐orbital interactions. Reported herein is a series of oligorotaxanes containing one, two, three and four naphthalene diimide (NDI) redox‐active units, which have been shown by cyclic voltammetry, and by EPR and ENDOR spectroscopies, to share electrons across the NDI stacks. Thermally driven motions between the neighboring NDI units in the oligorotaxanes influence the passage of electrons through the NDI stacks in a manner reminiscent of the conformationally gated charge transfer observed in DNA.