One of the basic rules of photochemistry is that the electronic configuration of a conjugated polyene does not change when excited thermally, but changes when photochemically excited, i.e. an electron moves from HOMO to LUMO.
Can anyone please explain this to me in terms of quantum mechanics?
(P.S.: Please do not provide explanation like thermal energy is very less to excite than photochemical, since it would imply that at very high temp (~1,00,00,000 C) the reaction is possible, which is not really the case).