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

The photophysical characterisation of bis-porphyrins consisting of two porp hyrins bridged by either a quinoxaline Troger's base (1 and 2) or a biquino xalinyl (3) spacer are reported. Efficient intramolecular electronic energy transfer (EET) between the rigidly linked free-base porphyrins in 1 and fr om the zinc(ii) porphyrin to the free-base porphyrin in 2 has been investig ated by steady-state absorption and emission spectroscopy, time-resolved fl uorescence 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 via the quinoxaline Trog er's base linker is responsible for the enhanced energy transfer rates in t hese systems. Strong quenching of the fluorescence intensity observed in 3 is interpreted as arising from long-range (> 18 Angstrom) through-biquinoxa linyl bridge mediated photoinduced electron transfer from the free-base por phyrin to the gold(iii) porphyrin. These systems provide useful models for the arrangements of the primary donor-acceptor pair in photosynthetic react ion centres, and for elucidating the role of the connecting bridge in elect ron and energy transfer processes.