Tetrapyrrole singlet energies

Figure 20. Absorption (upper curve) and corrected fluorescence excitation (lower curve) spectra for carotenopyropheophorbide 27 in toluene. The spectra have been normalized over the 6(X)-630 nm region. The excitation spectrum is virtually identical in shape to the absorption spectrum of methyl pyropheophorbide-a, and singlet-singlet energy transfer from the carotenoid to the tetrapyrrole is therefore minimal (<7%).

Fig. 1. Approximate energy levels for dyads 1 and 4 illustrating the two pathways for singlet energy transfer from the carotenoid to the tetrapyrrole moiety. The carotenoid S levels have not been measured and the small difference between the levels of tetrapyrroles 2 and 5 is not shown.

As is known, the triplet and singlet tetrapyrrolic pigment states are caused by excitation of delocalized ir electrons of the heteroaromatic macrocycle (1 ). The most favourable structure of dimeric complexes for metal 1oporphyrin TTA seems to be a sandwich dimer as in the case when molecular orientation of plane to plane electronic shells reaches the maximum overlap. Quantum chemistry calculations of metalloporphyrin dimers indicates that for the dimeric emission process the 2 state is split into states of higher energy (SJ) and lower energy ( 2). In the case of a sandwich dimer of 04 symmetry, the following states are seen ... 

Generally, the differences in Gibbs energy, AG, of a solvated exciplex (A — Q) and its solvated constituents A and Q are not very large. The values of the formation constants Kf for the exciplexes involving a tetrapyrrole complex are of the order 103- 10s (for triplet exciplexes) and of some order lower for singlet exciplexes. 

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