The first example of cofacial bis(dipyrrins)

Two series of cofacial bis(dipyrrins) were prepared and their photophysical properties as well as their bimolecular fluorescence quenching with C₆₀ were investigated.

Time dependent DFT investigation of the optical properties of artificial light harvesting special pairs

Simulated absorption spectra of (ZnTriPP)₂DPB dimer in which Q band is enhanced 50 times for visibility.

Very fast singlet and triplet energy transfers in a tri-chromophoric porphyrin dyad aided by the truxene platform

A trichromophoric dyad composed of an octa-β-alkyl-palladium(II)porphyrin (donor) and two tri-meso-aryl-zinc(II)porphyrins (acceptors) held by a truxene spacer exhibits very fast rates for triplet energy transfers at 77 (kET(T1) = 1.63 × 108 s-1) and 298 K (kET(T1) = 3.44 × 108 s-1), whereas the corresponding singlet energy transfer rates, kET(S1) = 3.9 × 1010 s-1 (77 K) and kET(S1) = 6.0 × 1010 s-1 (298 K), are also considered fast. The interpretation for these results is that the energy transfer processes proceed via a through bond Dexter mechanism (i.e. double electron exchange) supported by comparison with literature data and evidence for a moderate MO coupling between the donor and acceptor chromophores in the frontier MOs.

Interplay between singlet and triplet excited states in a conformationally locked donor–acceptor dyad

The synthesis and photophysical characterization of a palladium(ii) porphyrin – anthracene dyad bridged via short and conformationally rigid bicyclo[2.2.2]octadiene spacer were achieved.

What can we learn from artificial special pairs?

Plants and photosynthetic bacteria obtain their energy from sunlight or surrounding radiation. Their photosynthetic membranes are composed of a much elaborated series of antenna molecules based on chlorophylls or bacteriochlorophylls, carotenoids playing multiple roles, various electron transport accessories, and central special pairs. The latter components are the most difficult to mimic with exactitude because the structure−property relationship depends on many factors including interplanar distance, slip angle, substituents, metal, and axial ligand. To this list of factors to control with quasi-perfection, one should also add the thermal activation (i.e., temperature). Over the past 15 years or so (2001–2013), an intensive collaboration with Professor Roger Guilard (Université de Bourgogne, Dijon) dealt with elucidating the role of each parameter to provide the best design of artificial special pairs capable of responding or behaving like the natural special pairs, namely with regards with the antenna effect. The latest feature is one of the defence mechanisms slowing down the rate for the primary electron transfer from the special pair to the electron transport accessories. This review highlights the advances in this challenging area of mimicry of the photophysical events in biological systems, namely the artificial special pairs designed in our laboratory for the antenna processes.

Evidence for reverse pathways and equilibrium in singlet energy transfers between an artificial special pair and an antenna

A dyad, 1, built on an artificial special pair (bis(meso-nonyl)zinc(II)porphyrin), [Zn2], a spacer (biphenylene), a bridge (1,4-benzene), and an antenna (di-meso-(3,5-di(t-butyl)phenyl)porphyrin free base), FB, is prepared by Suzuki coupling and is analyzed by absorption and steady state, and time-resolved emission spectroscopy at 298 and 77 K. Using bases from the Förster theory, evidence for two pathways for S 1 energy transfer, FB* → [Zn2], and [Zn2]* → FB, along with their respective rates, k ET ( S 1)1 and k ET ( S 1)-1, are extracted from the comparison of the fluorescence decays monitored at the emission maximum. At 77 K, the unquenched (1.79 ([Zn2]) and 10.6 ns (FB)) and quenched components (<100 ps; i.e. k ET ( S 1) > 10 (ns)-1), are observed, hence, demonstrating the bidirectional paths with no back energy transfer. A 298 K, only two components are detected (0.44 ([Zn2]) and 2.64 ns (FB)) and the resulting reduced τFs indicates back energy transfer, therefore cycling and equilibrium. Their global rates are 0.31 and 1.8 (ns)-1 for k ET ( S 1)1 and k ET ( S 1)-1 at 298 K. This large temperature dependence on k ET ( S 1) is fully consistent with the participation of thermal activation. Finally, DFT calculations (B3LYP) were used to illustrate a clear correlation between the relative k ET ( S 1) s and the amplitude of the MO couplings between the artificial special pair and the antenna.

Is the special pair structure a good strategy for the kinetics during the last step of the energy transfer with the nearest antenna? A chemical model approach

A cofacial bis(Mg(II)porphyrin)-C(6)H(4)-free base ([Mg(2)]-bridge-FB) dyad shows S(1) energy transfer in both directions and much slower rates than similar monoporphyrin systems are observed.

Decoupling the artificial special pair to slow down the rate of singlet energy transfer

Trimer 2, composed of a cofacial heterobismacrocycle, octamethyl-porphyrin zinc(II) and bisarylporphyrin zinc(II) held by an anthracenyl spacer, and a flanking acceptor, bisarylporphyrin free-base ( Ar = -3,5-(t Bu )2 C 6 H 3), has been studied by means of absorption spectroscopy, "steady state and time-resolved fluorescence" and fs transient absorption spectroscopy, and density functional theory (DFT) in order to assess the effect of decoupling the chromophores' low energy MOs on the rate of the singlet, S1, energy transfer, k ET , compared to a recently reported work on a heavily coupled trimeric system, Trimer 1, [biphenylenebis(n-nonyl)porphyrin zinc(II)]-bisarylporphyrin free-base ( Ar = -3,5-(t Bu )2 C 6 H 3). The position of the 0–0 peaks of the absorption and fluorescence spectra of Trimer 2 indicates that these porphyrin units are respectively energy donor 1, donor 2, and acceptor. The DFT computations confirm that the MOs of the cofacial donor 1-donor 2 dyad are decoupled, but significant MO coupling between donor 2 and acceptor 1 is still present despite the strong dihedral angle between their respective average planes (77.5°: geometry optimization by DFT). The fs transient absorption spectra exhibit a clear S1–Sn fingerprint of the bisarylporphyrin zinc(II) chromophore and the kinetic trace exhibits a slow rise time of 87 ps, due to a S1 donor 1 → donor 2 ET. The transient species donor 2 and acceptor decay respectively in the short (~1.5) ns and 6 ns time scale.