Michael N. Paddon-Row

Distance dependence of photoinduced electron transfer through non-conjugated bridges

Abstract Picosecond time-resolved emission studies of a series of molecules containing an electron donor-acceptor pair interconnected by a series of rigid non-conjugated bridges reveal the occurrence of very fast photoinduced intramolecular electron transfer. The length of the bridge was varied to provide donor-acceptor centre-to-centre separations ranging from 8.1 to 13.3 A (edge-to-edge 5 to 10.2 A). At centre-to-centre separations up to 10.7 A the rate of photoinduced electron transfer exceeded 5×10 10 s −1 (τ 10 s −1 (τ = 68 ps).

Formation of Efficient Electron Transfer Pathways by Adsorbing Gold Nanoparticles to Self-Assembled Monolayer Modified Electrodes

The influence of the length of a self-assembled monolayer (SAM) linker on the electrochemical performance of electrode-linker-gold nanoparticle molecular constructs is investigated. Electrodes were first modified with amino-1-alkanethiols of four different lengths (C=2, 6, 8, and 11). The SAM showed progressively greater blocking ability to ruthenium hexamine as the length of the alkyl chain increased to the point where no significant Faradaic peak was observed for the amino-1-undecanethiol SAM. Upon the attachment of gold nanoparticles, distinct Faradaic electrochemistry of the ruthenium hexamine was observed for all four length SAMs with the electrochemistry being similar to that observed on a bare electrode. The charge transfer resistance to this Faradaic process was observed to be insensitive to the length of the intervening SAM, indicating it is electron transfer between the redox species and the nanoparticles, rather than tunneling across the SAM, which is the rate-limiting step. Some comments on the mechanism of charge transfer are provided. When forming multilayers of the linker-nanoparticle constructs, fabricated in a stepwise manner, whenever the distal species was the SAM the Faradaic process was blocked and whenever it was the nanoparticle a distinct Faradaic process was observed. With up to five layers of linker-nanoparticles, there was little increase in charge transfer resistance and again the charge transfer resistance was insensitive to the length of the linker.

Charge-transfer absorption and emission resulting from long-range through-bond interaction; exploring the relation between electronic coupling andelectron-transfer in bridged donor-acceptor systems.

Abstract The electronic absorption- and emission spectra of seven (D)onor-(A)cceptor systems are studied with the general structure D-bridge-A, where the bridge consists of an extended, rigid, saturated hydrocarbon skeleton that separates D and A by distances ranging from 3 to 12 CC σ-bonds. Across bridges with a length up to six σ-bonds sufficient electronic interaction occurs to cause a detectable perturbation of the electronic absorption spectra and for lower homologues this leads to the appearance of discrete intramolecular charge-transfer absorptions with an intensity that is strongly enhanced by intensity borrowing from symmetry-matched local transitions. In the fluorescence spectra discrete charge-transfer (CT) type emission has been detected for bridge lengths up to ten σ-bonds. The radiative transition probability of this CT emission provides a direct measure for the electronic coupling matrix element (H da ) between the charge-separated- and the groundstate. The magnitude of H da is found to decrease exponentially with the number of intervening σ-bonds from 850 cm −1 at 3-bond separation to 17.6 cm −1 at 10-bond separation. Furthermore the rate of charge-recombination in the compounds studied is found to be proportional to the square of H da , thus providing an experimental verification of this often implied “golden rule” relation.

Strong effects of the bridge configuration on photoinduced charge separation in rigidly linked donor-acceptor systems

Abstract Photoinduced electron-transfer rates are reported for two pairs of rigid bichromophoric molecules 1 (6)/ 2 (6) and 1 (8)/ 2 (8). In the first pair electron donor and acceptor are separated by six, in the second pair by eight, carbon—carbon σ bonds. While these σ bonds provide an all-trans coupling path in 1 (6) and 1 (8), that path contains s-cis elements in 2 (6) and 2 (8), which - as shown by X-ray structure data and by spectroscopic evidence - leads to a slight decrease in the effective, spatial donor-acceptor separation. Nevertheless, photoinduced electron transfer in each of the “stretched” compounds is about one order of magnitude faster than in the corresponding “bent” compound. This remarkable effect is interpreted as resulting from the unique ability of an all-trans array of σ bonds to mediate electronic through-bond interaction (TBI). Interestingly the solvent dependence of the rate of photoinduced electron transfer is significantly larger in the “bent” systems, thus indicating that superexchange via solvent molecules becomes competitive with TBI if an all-trans array is not available.

Long-range exchange contribution to singlet-singlet energy transfer in a series of rigid bichromophoric molecules

Abstract Intramolecular singlet-singlet energy transfer is reported in a series of compounds containing a 1,4-dimethoxy-naphthalene chromophore as the energy donor and a cyclic ketone as the energy acceptor connected by rigid, elongated, saturated hydrocarbon bridges with an effective length of 4, 6, and 8 CC σ bonds. The rate of energy transfer is found to be proportional to the spectral overlap - as varied by solvent variation - and to show an exponential distance dependence while its magnitude significantly exceeds that predicted for a dipole-dipole coupling mechanism. From this it is concluded that energy transfer occurs predominantly via an exchange mechanism. Exchange integrals of 60, 10, and 2.5 cm −1 across 4, 6, and 8 σ bonds are calculated. The magnitude of these is proposed to signify through-bond exchange interaction between symmetry-matched donor (ππ*) and acceptor (nπ*) states.

Synthetic and quantum mechanical studies into the N-heterocyclic carbene catalyzed (4 + 2) cycloaddition.

The N-heterocyclic carbene catalyzed (4 + 2) cycloaddition between α,β-unsaturated acid fluorides and TMS dienol ethers provides cyclohexene fused β-lactone intermediates stable below -20 °C. These can be intercepted reductively or with organolithium reagents to produce diastereomerically pure cyclohexenes (>20:1 dr) with up to four contiguous stereocenters. The mechanism has been investigated using theoretical calculations and by examining secondary kinetic isotope effects. Together these studies implicate the formation of a diastereomerically pure β-lactone intermediate by a stepwise (4 + 2) cycloaddition involving Michael addition, aldol cyclization, and lactonization.

Long-lived photoinduced charge separation in a bridged C60-porphyrin dyad

Abstract Long-range photoinduced energy and electron transfer have been investigated in a novel dyad containing a zinc tetraarylporphyrin (PZn) donor and a C60 acceptor separated by a saturated norbornylogous bridge nine sigma bonds in length (PZn-9σ-C60). In non-polar solvents singlet-singlet energy transfer from PZn to C60 is observed but in benzonitrile efficient (> 90%) charge separation occurs with a rate constant of 1 × 1010 sec−1 to yield PZn.+-9σ-C60.−. The charge separated state exhibits a remarkably long lifetime for a dyad of 420 ns. This behaviour is discussed with reference to the energetics, dynamics and orbital symmetry properties of the states involved.

Practical Synthesis of the Dendralene Family Reveals Alternation in Behavior

Back from obscurity: The practical synthesis of the first six members of the fundamental class of acyclic branched oligoalkenes has been achieved. The syntheses allow access to the target compounds on multigram scales in good yields. Members of the family with even numbers of double bonds are significantly more stable than those with odd numbers (see picture), and exhibit different chemical reactivities in Diels-Alder reactions.

Superexchange-Mediated Charge Separation and Charge Recombination in Covalently Linked Donor-Bridge-Acceptor Systems ∗

Evidence is presented in support of the concept that electron transfer (ET) between a pair of chromophores may take place efficiently over large distances (>10 A) by the mediation of an intervening saturated hydrocarbon medium. For example, ET is found to take place on a sub-nanosecond timescale through saturated norbornylogous bridges greater than 13 A in length, by a superexchange (through-bond coupling) mechanism. The dependence of the ET dynamics on the bridge length and configuration are consistent with the operation of a superexchange mechanism. The distinction between molecular wire behaviour and superexchange-mediated ET is made. The distance dependence of ET dynamics through different types of bridges—saturated and unsaturated hydrocarbon bridges, proteins, and duplex DNA—is discussed and explained. Strategies for prolonging the lifetimes of charge-separated states are explored and discussed. In general, long-lived charge-separated species have been generated using giant multichromophoric systems in which the charges are separated by large distances, often exceeding 20 A. In contrast, it is shown that very long-lived charge-separated states, possessing the triplet multiplicity, may be generated using short ‘dwarf’ dyads, in which the charges are less than 6 A apart. Charge recombination in these species is slowed by the difference in electron spin multiplicity between the charge-separated state and the ground state.

Scanning electrochemical microscopy. 59. Effect of defects and structure on electron transfer through self-assembled monolayers.

Electron transfer (ET) rate kinetics through n-alkanethiol self-assembled monolayers (SAMs) of alkanethiols of different chain lengths [Me(CH2)nSH; n=8, 10, 11, 15] on Au and Hg surfaces and ferrocene (Fc)-terminated SAMs (poly-norbornylogous and HS(CH2)12CONHCH2Fc) on Au were studied using cyclic voltammetry and scanning electrochemical microscopy (SECM). The SECM results allow determination of the ET kinetics of solution-phase Ru(NH3)63+/2+ through the alkanethiol SAMs on Au and Hg. A model using the potential dependence of the measured rate constants is proposed to compensate for the pinhole contribution. Extrapolated values of koML for Ru(NH3)63+/2+ using the model follow the expected exponential decay (beta is 0.9) for different chain lengths. For a Fc-terminated poly-norbornyl SAM, the standard rate constant of direct tunneling (ko is 189+/-31 s(-1)) is in the same order as the ko value of HS(CH2)12CONHCH2Fc. In blocking and Fc SAMs, the rates of ET are demonstrated to follow Butler-Volmer kinetics with transfer coefficients alpha of 0.5. Lower values of alpha are treated as a result of the pinhole contribution. The normalized rates of ET are 3 orders of magnitude higher for Fc-terminated than for blocking monolayers. Scanning electron microscopy imaging of Pd nanoparticles electrochemically deposited in pinholes of blocking SAMs was used to confirm the presence of pinholes.