explain the reasons for the abnormal values of quantum yields of photoreactions?

Answer 1

Although it is "forbidden" by selection rules, you may get a singlet to triplet transition.

In reality, this is forbidden strictly by using the electric dipole as the operator, but is allowed w/ other operators (i.e. magnetic dipole)

Once you get to a triplet state, you can get some unexpected products, and some very different yields.

That's one example. You may get also charge-transfer reactions w/ solvents, polymerization, etc.

Answer 2

photoreactions are very complex reaction compared to "normal" thermal reactions, because they occur through the excited electronic states of the molecule.
There are rules known as the Woodward-Hoffman selection rules that tell us when a photoreaction will take place and when the reaction is possible thermally, however about 50% of the predictions are wrong! (some rule, eh?)
This is usually explained by the presence of something known as a conical intersection. This is a point in the potential energy surface that is shared by the excited state and ground state of a molecule and it is believed to be an efficient funnel to the ground state of the product or reactant, thus increasing by many factor the quantum yield of the reaction. Bare in mind that not all reactions take place through conical intersections so this does not explain all abnormal values of quantum yields, but when there are abnormal values for quantum yields a conical intersection should always be suspected.

Answer 3

because light is both particle and wave. The energy doesn't fit nicely into a quantifiable equation.