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-(tBu)₂C ₆H₃), 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-(tBu)₂C₆H₃). The position of the 00 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-S_n 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.