Covalent Bonding in Ground and Excited States

There may be a coupling between the locally excited states, which is the main reason for EET. In some cases, this leads to an energy lowering of several hundreds of cm in the excited state, even if the monomers are not covalently bonded. If the monomers are covalently linked, we are, in principle, talking about single molecules. 

In the excited state, the LUMO is singly occupied and since the LUMO is bonding between the central carbon atoms, there is now some conjugation between the two 

FIGURE 14.5 Butadiyne-linked zinc porphyrins in planar (a) and perpendicular (b). The triple bonds are marked out geometry. 

FIGURE 14.6 LUMO of the planar complex, showing the jc-honding contribution between the central carbon atoms of the bntadiyne gronp. 

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