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

Abstract

Nature has designed efficient photosystems allowing microorganisms such as photo-bacteria to survive under particularly difficult environment due to scarcity of light [1]. In the photosynthetic membranes of these organisms, exciton (energy excitation migration), energy and electron transfers occur with impressing rates [1], and numerous research groups around the world attempted to design chemical models, which were recently reviewed by us [2]. One of the key components in both the natural and synthetic systems is the special pair [3,4]. Moreover, the various parameters controlling the rates for singlet energy transfer, k ET , in a general sense, was detailed ABSTRACT: 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-(tBu) 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, S 1 , 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-(tBu) 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 S 1 -S n fingerprint of the bisarylporphyrin zinc(II) chromophore and the kinetic trace exhibits a slow rise time of 87 ps, due to a S 1 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.

Design of Triads for Probing the Direct Through Space Energy Transfers in Closely Spaced Assemblies

“…Their amplitude (∼6.2 × 10 9 s −1 ) is similar to that recently reported for compound 14 (11 × 10 9 s −1 ; Figure 4) using time-resolved femtosecond transient absorption spectroscopy. 30 The larger global k ET (S 1 ) value for 14 over 10a,b is fully consistent with the shorter D•••A 1 separation for the former (distance C meso •••C meso = 4.98 Å for anthracenyl, 6.30 Å for DPS). 28 Consequently, D is closer to both A 1 and A 2 .…”

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

“…5), for which S 1 energy transfers were clearly established [11,36]. Efficient energy transfer are generally accompanied by a strong fluorescence quenching of the donor, clearly felt by the significant decrease in F F and lifetime, t F , and the close resemblance between the excitation and absorption spectra of the dyad or polyad.…”

“…A recent investigation indicated that faster rates of S 1 energy transfers were accompanied by stronger MO couplings between the acceptor and the donor (see compound 4 for example) [36]. Coupling is defined by atomic orbital distribution that is spread over 2 or more porphyrin macrocycles.…”

What can we learn from artificial special pairs?

“…This concept led to the idea of MO couplings and decoupling based on the proximity of the chromophores, which was also experimentally verified using comparison of k ET versus representations of the frontier MOs. 36 For compound 6, the donor fluorescence was not detected, meaning that the antenna fluorescence was completely quenched and that the rate for energy transfer must be very fast. Fortunately, the fs transient absorption spectra exhibited stimulated fluorescence (see the strong negative signal in Fig.…”

“…Representation of the frontier MOs of 6. Image modified from Harvey et al 36 and reproduced with permission from World Scientific. in (monporphyrin donor)−C 6 H 4 −(monoporphyrin acceptor)) also indicated a discrepancy where again the cofacial systems exhibit slower rates from severalfold to one order of magnitude difference.…”