Looking for chiral recognition in photoinduced bimolecular electron transfer using ultrafast spectroscopy.

Occurrence of chiral recognition in bimolecular photoinduced electron transfer (ET) is difficult to identify because of the predominant role of diffusion. To circumvent this problem, we apply a combination of ultrafast time-resolved fluorescence and transient electronic absorption to look for stereoselectivity in the initial, static stage of ET quenching, where diffusion is not relevant. The fluorophore and electron acceptor is a cationic hexahelicene, whereas the quencher has either stereocentered (tryptophan) or axial (binaphthol) chirality. We found that, in all cases, the quenching dynamics are the same, within the limit of error, for different diastereomeric pairs in polar and medium-polar solvents. The same absence of chiral effect is observed for the recombination of the radical pair, which results from the quenching. Molecular dynamics simulations suggest that the distribution of inter-reactant distance is independent of the chirality of the acceptor and the donor. Close contact resulting in large electronic coupling is predicted to be possible with all diastereomeric pairs. In this case, ET is an adiabatic process, whose dynamics do no longer depend on the coupling, but are rather controlled by high-frequency intramolecular modes.

Bimolecular photoinduced electron transfer in non-polar solvents beyond the diffusion limit.

Electron transfer (ET) quenching dynamics in non-polar solvents are investigated using ultrafast spectroscopy with a series of six fluorophore/quencher pairs, covering a driving force range of more than 1.3 eV. The intrinsic ET rate constants, k0, deduced from the quenching dynamics in the static regime, are of the order of 1012-1013 M-1 s-1, i.e., at least as large as in acetonitrile, and do not exhibit any marked dependence on the driving force. A combination of transient electronic and vibrational absorption spectroscopy measurements reveals that the primary product of static quenching is a strongly coupled exciplex that decays within a few picoseconds. More weakly coupled exciplexes with a longer lifetime are generated subsequently, during the dynamic, diffusion-controlled, stage of the quenching. The results suggest that static ET quenching in non-polar solvents should be viewed as an internal conversion from a locally excited state to a charge-transfer state of a supermolecule rather than as a non-adiabatic ET process.

Photolabile coumarins with improved efficiency through azetidinyl substitution.

Azetidinyl substituents have been recently used to improve the fluorescence quantum yield of several classes of fluorophores. Herein, we demonstrate that other useful photochemical processes can be modulated using this strategy. In particular, we prepared and measured the quantum yield of photorelease of a series of 7-azetidinyl-4-methyl coumarin esters and compared it to their 7-diethylamino and julolidine-fused analogues. The efficiency of the photorelease reactions of the azetidinyl-substituted compounds was 2- to 5-fold higher than the corresponding diethylamino coumarins. We investigated the origin of this effect in model fluorophores and in the photoactivatable esters, and found that H-bonding with the solvent seems to be the prominent deactivation channel inhibited upon substitution with an azetidinyl ring. We anticipate that this substitution strategy could be used to modulate other photochemical processes with applications in chemical biology, catalysis and materials science.

Influence of the hydrogen-bond interactions on the excited-state dynamics of a push-pull azobenzene dye: the case of Methyl Orange.

The excited-state dynamics of the push-pull azobenzene Methyl Orange (MO) were investigated in several solvents and water/glycerol mixtures using a combination of ultrafast time-resolved fluorescence and transient absorption in both the UV-visible and the IR regions, as well as quantum chemical calculations. Optical excitation of MO in its trans form results in the population of the S2 ππ* state and is followed by internal conversion to the S1 nπ* state in ∼50 fs. The population of this state decays on the sub-picosecond timescale by both internal conversion to the trans ground state and isomerisation to the cis ground state. Finally, the cis form converts thermally to the trans form on a timescale ranging from less than 50 ms to several minutes. Significant differences depending on the hydrogen-bond donor strength of the solvents, quantified by the Kamlet Taft parameter α, were observed: compared to the other solvents, in highly protic solvents (α > 1), (i) the viscosity dependence of the S1 state lifetime is less pronounced, (ii) the S1 state lifetime is shorter by a factor of ≈1.5 for the same viscosity, (iii) the trans-to-cis photoisomerisation efficiency is smaller, and (iv) the thermal cis-to-trans isomerisation is faster by a factor of ≥103. These differences are explained in terms of hydrogen-bond interactions between the solvent and the azo nitrogen atoms of MO, which not only change the nature of the S1 state but also have an impact on the shape of ground- and excited-state potentials, and, thus, affect the deactivation pathways from the excited state.

Electron-deficient fullerenes in triple-channel photosystems.

Fullerenes of increasing electron deficiency are designed, synthesized and evaluated in multicomponent surface architectures to ultimately build gradients in LUMO levels with nine components over 350 meV down to -4.22 eV.