James M. Lawson

Through-bond modulation of intramolecular electron transfer in rigidly linked donor-acceptor systems

Abstract Photoinduced electron transfer rates are reported for a pair of rigid bichromophoric molecules, 1 (8) and 3 (8), in a variety of solvents. It was found that intramolecular electron transfer in 1 (8) is up to five times faster than in 3 (8). X-ray crystallography revealed that the cyclopentyl ring attached to the dicyanoethylene group is “flatter” in 3 (8) compared to 1 (8), and this results in diminished through-bond coupling in 3 (8) compared to 1 (8). This conclusion is reinforced by the observation that the calculated through-bond π, π and π * interaction energies in dienes 7 (6) and 9 (6) are larger than those in the dienes 8 (6) and 10 (6), respectively.

Identification of a totally charge separated state in completely rigid trichromophoric compounds

Two ‘triads’ 1-syn and 1-anti were investigated containing the same linear arrangement of two electron donors and an acceptor connected by fully rigid saturated hydrocarbon bridges. The systems differ in the relative orientation of the bridges and thereby in the distance between the terminal chromophores which is ≈ 14.9 and ≈ 19.4 A in the syn and anti isomer respectively. For both systems full charge separation between the terminal chromophores is shown to occur upon photoexcitation of the central donor chromophore, but the lifetime of this charge separation is dramatically longer in 1-anti.

Synthesis and structural characterization of a novel pair of rigid diastereomeric triads

Abstract A method is described for constructing totally rigid triad (nichromophoric) systems, D 2-B 1-D 1-B 2-A, in which the chromophores D 2 (= dimethylaniline), D 1 (= 1,4-dimethoxynaphthalene), and A (= dicyanovinyl) are fused to rigid hydrocarbon bridges, B 1 and B 2, comprising linearly fused norbornane and bicyclo[2.2.0]hexane units.

Investigation of the primary electron transfer processes in some rigidly bridged dyads and triads by (sub)picosecond pump–probe experiments

The electron transfer kinetics of several completely rigid dyads and triads which contain N,N-dimethylaniline (DMA) and dimethoxynaphthalene (DMN) as possible donors and the dicyanovinyl group (DCV) as acceptor were studied by means of (sub)picosecond time-resolved transient absorption spectroscopy. In the dyads DMN[n]DCV, the rate of charge separation decreases exponentially with the number of σ-bonds n in the bridge, the ‘‘damping factor’’ being 0.8 per bond in tetrahydrofuran solvent. In the triads DMA[4]DMN[8]DCV, the primary electron transfer from DMN to DCV occurs in solvents of low and medium polarity within 10 ps in both isomers (syn and anti). The rates of the secondary electron transfer step (formation of the fully charge separated state, DMA+[4]DMN[8]DCV−) and the following deactivation processes depend, however, strongly on the conformation. In acetonitrile, the primary electron transfer involves the two donor groups yielding preferentially DMA+[4]DMN−[8]DCV. In the anti-conformer this state is fairly long-lived; in the syn-conformer, however, it decays rapidly, in part to locally excited triplet states.

Electron and singlet energy transfer in rigid supramolecular systems

Abstract Fluorescence spectroscopic measurements are reported on the N,N-dimethylaniline-{polynorbornyl (4, σ-bonds)}-dimethoxynaphthalene (DMA[4]DMN[2]) dyad and the H-isomer of the trichromophore, N,N-dimethylaniline-{polynorbornyl (4, σ-bonds)}-dimethoxynaphthalene-{polynorbornyl (8, σ-bonds)}-dicyanovinyl (DMA[4]DMN[8]-DCV) to probe electron transfer (ET) and singlet energy transfer (EnT) in these novel systems. In acetonitrile, the DMA[4]DMN[2] dyad is shown to undergo rapid and complete photoinduced ET following excitation of DMA and DMN chromophores. In n-hexane, very little ET is apparent for DMA[4]DMN[2] and this allows the observation of a very efficient singlet EnT process from locally excited DMA to the lowest DMN singlet excited state. A mode of vectorial excitation EnT from locally excited DMA to DMN followed by ET from DMN to DCV is observed for DMA[4]DMN[8]DCV in n-hexane.

Synthesis and crystal structures of three rigidly linked donor–acceptor systems designed to test the effect of bridge configuration on the dynamics of long-range intramolecular electron transfer processes

The synthesis and single crystal X-ray structures of three rigid bichromophoric systems are described, namely the dicyanovinyl derivatives of 7,12-dimethoxydodecahydro-1,4:6,13-dimethanopentacen-15-one, 3, 8,13-dimethoxyhexadecahydro-1,4:5,16:6,17:7, 14-tetramethanohexacen-17-one, 4 and 6,11-dimethoxy-4b, 12b-dimethyldodecahydro-4,13;5,12-dimethano-2H-indeno-[5′,6′:3′,4′]cyclo-buta[1′,2′:3,4]cyclobuta[1,2-b]anthracen-2-one 5. Compounds 3 and 4 were synthesized from dimethanonaphthacene 8via Diels–Alder reaction with tetrachlorodimethoxycyclopentadiene 9(in the case of 3) and via successive Diels–Alder reactions, firstly with hexachlorocyclopentadiene, 14, and then with 9, in the case of 4. The synthesis of 5 was achieved through ring expansion of the dichloroketene [2 + 2] cycloadduct formed from 21. Differences in the rates of photoinduced intramolecular electron transfer in 3–5, compared with those for 1 and 2, are rationalized in terms of the differing configurations of the hydrocarbon bridges in these systems, as revealed by X-ray crystallography.

A synthetic strategy for the construction of a novel series of rigid supramolecular triads

A method is described for constructing totally rigid triad(trichromophoric) supramolecular systems, D2–B1–D1–B2–A, in which the chromophores D2(dimethylaniline), D1(dimethoxynaphthalene), and A(dicyanovinyl) are fused to bridges, B1 and B2, consisting of linearly fused norbornyl and bicycle[2.2.0]hexanyl units.

Recent Surprises in the Study of Photoinduced Electron Transfer: Covalent Versus Non-Covalent Pathways

Stepwise photoinduced charge separation in a rigid trichromophoric system with a U-shaped conformation is described. In the thus produced totally charge-separated state the plus and minus charges reside on the termini of the U-shaped molecule with a separation of about 16 A measured ‘through-space’ against about 27 A as measured via the shortest ‘through-bond’ pathway. Charge recombination to the ground-state is found to be accompanied by charge transfer fluorescence. Both from the overall rate of this recombination and from the radiative rate constant of the accompanying charge transfer fluorescence it appears that the interaction between the termini is much stronger than can be accounted for by a pure through-bond mechanism.