Any time you bombard an aromatic species with light, you will frequently produce some quantity excited triplet states. If oxygen is present, this can photosensitize the production of singlet oxygen, which is a very potent oxidative species. It will not be surprising to see deleterious photochemistry/bleaching under these conditions.
As for heating the solution, it depends on many things, including the light source and the nature of the experimental setup. But probably you will see very little temperature change.