The ground state of the starting materials only "attempts" to correlate with the second excited state as there is an avoided crossing in the middle due to the states possessing the overall same symmetry. Thus in actuality, the ground state of the reactants is transformed into the ground state of the products only after achieving a high energetic barrier. However, there is no large activation barrier if the reactants are in the first excited state. Thus this reaction proceeds easily under photochemical control, but has a very high barrier to reaction under thermal control.