Photoinduced energy and electron transfer in bis-porphyrins with quinoxaline Tröger's base and biquinoxalinyl spacers

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Abstract
The photophysical characterisation of bis-porphyrins consisting of two porphyrins bridged by either a quinoxaline Tröger's base (1 and 2) or a biquinoxalinyl (3) spacer are reported. Efficient intramolecular electronic energy transfer (EET) between the rigidly linked free-base porphyrins in 1 and from the zinc(II) porphyrin to the free-base porphyrin in 2 has been investigated by steady-state absorption and emission spectroscopy, time-resolved fluorescence spectroscopy and semi-empirical calculations. A resonance dipole–dipole mechanism alone cannot account for the rate of EET in both 1 and 2. It is demonstrated that a superexchange mechanism ia the quinoxaline Tröger's base linker is responsible for the enhanced energy transfer rates in these systems. Strong quenching of the fluorescence intensity observed in 3 is interpreted as arising from long-range (>18 Å) through-biquinoxalinyl bridge mediated photoinduced electron transfer from the free-base porphyrin to the gold(III) porphyrin. These systems provide useful models for the arrangements of the primary donor–acceptor pair in photosynthetic reaction centres, and for elucidating the role of the connecting bridge in electron and energy transfer processes.

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