Gold III bisporphyrins

The electronic absorption spectrum recorded for 16 in N,N-dimethylformamide (DMF) is similar to that of the corresponding monomeric porphyrin and there was no indication of exciton interaction between the two porphyrin rings [16]. Fluorescence was not observed, presumably because the excited singlet state lifetime is shortened due to the internal heavy-atom effect, but weak phosphorescence was detected in a methanol glass at 77K with a maximum at 690 nm. Excitation of 16 with a 

30-ps laser pulse at 532 nm generated the triplet state which decayed via first-order kinetics with a lifetime of (1.5 0.2) ns to reform the ground state species. 

Immediately following excitation of 17 in DMF with a low intensity 30-ps laser pulse at 532 nm the observed transient differential absorption spectrum was considered to be characteristic of a gold(III) porphyrin triplet excited state [17]. The transient absorption decayed via biexponential kinetics with lifetimes of (170 20) ps and (2.0 0.4) ns, even at very low laser intensities. Absorption spectra recorded at different times during the decay process indicated that the triplet state was reduced to the gold(III) porphyrin neutral radical. With increasing laser intensity, the fractional contribution of the shorter-lived component increased and, whereas the longer lifetime 

The dual exponential behavior was apparent at the lowest laser intensity ( 0.5 mJ) and well below the onset threshold at which similar behavior became apparent for 16. Although the probability of depositing 2 photons onto the tetrakisporphyrin ensemble 17 is twice that for the corresponding bisporphyrin 16, the high concentration and low photon density employed should ensure monophotonic excitation of 17. Therefore, the dual exponential decay kinetics observed at low laser intensity are not due to triplet-triplet annihilation but must arise from an additional process. 

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A further improvement in the status of bisporphyrin photoredox chemistry came with the synthesis of an oblique zinc(II)/gold(III) bisporphyrin by Sauvage and coworkers [16]. This compound (Figure 7) has a constrained geometry that somewhat resembles the arrangement of the porphyrin... 

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