Electronic excitations conjugation effects

The photochemistry of conjugated polyenes has played a central role in the development of modern molecular photochemistry, due in no small part to its ultimate relevance to the electronic excited state properties of vitamins A and D and the visual pigments, as well as to pericyclic reaction theory. The field is enormous, tremendously diverse, and still very active from both experimental and theoretical perspectives. It is also remarkably complex, primarily because file absorption spectra and excited state behavior of polyene systems are strongly dependent on conformation about the formal single bonds in the polyene chain, which has the main effect of turning on or off various pericyclic reactions whose efficiencies are most strongly affected by conformational factors. 

Such a distribution has a plausible physical basis, since the driving force for phenyl rotation into the porphyrin plane provided by the electronic excitation (the eg orbital has particularly large coefficients at the meso carbon atoms ( )) encounters steric resistance from the non-bonded interactions between the protons at the ortho positions of the phenyl groups and those on the outer pyrrole carbon atoms (20). Consequently the phenyl torsion potential in the excited states may be relatively flat. Nevertheless, the vibrational frequencies are expected to be sensitive to the torsion angle for orientation close to co-planar because of the effect of conjugation. 

The pK values of phenols in singlet and triplet states are valuable guide to substituent effect in the excited states, specially for the aromatic hydrocarbons. In general, the conjugation between substituents and -electron clouds is very significantly enhanced by electronic excitation without change in the direction of conjugative substituent effect. The excited state acidities frequently follow the Hammett equation fairly well if exalted substituent constants a are used. 

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