Energy-raising conjugation

Ultraviolet spectroscopy tells us about conjugation in a molecule. It uses the highest energy radiation of any of the techniques described in this book radiation at 200 nanometres is equivalent to 595 kilojoules per mole. Absorption of this energy raises an electron from a bonding orbital to an antibonding orbital. 

The basic conclusions of this analysis are that a donor substituent, D, raises the energy of the HOMO and polarizes it toward the unsubstituted carbon. Furthermore, an acceptor substituent, A, lowers the energy of the LUMO and also polarizes it toward the unsubstituted carbon. A third kind of substituent is an unsaturated group (U), such as a phenyl or vinyl group. From our previous analysis of tt systems, we know that extended conjugation will raise the HOMO and lower the LU MO (Section 14.3.3, Figure 14.14). We find polarization of the HOMO and LUMO toward the unsubstituted carbon, but the effect is much less dramatic than with a D or A substituent. 

Several effects can influence the electronic structure of Cjq upon metal complex formation. One is the removal of one double bond from the remaining 29 fullerene double bonds. As in any polyene system, this decreased conjugation is expected to raise the energy of the LUMO and therefore decreases the electron affinity of the system. Conversely, the d-orbital backbonding transfers electron density from the metal into n orbitals of the remaining double bonds, which also decreases the electron affinity. 

With incremental functionalization of the fullerene, the LUMO of the remaining conjugated framework is raised in energy. 

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