What is the ground state electron configuration of isoprene?
The ground state is a singlet, when excited by UV light a $\pi\pi^*$ singlet excited state can be produced. (Singlet has electron spin paired with one electron in $\pi$ HOMO orbital one in LUMO [or higher orbital for other excited states]). The singlet may then form a triplet or decay to the ground state by emission/ internal conversion/reaction as per normal. In some polyenes the lowest excited state may be symmetry forbidden from the ground state, you will have to look into the details for each molecule.