a molecule having absorption maxima at 623 nm shows direct photolysis by UV-254 nm light. what could be the possible reason?
Your molecule may be having two different chromophores. One absorbs at 254 nm and the other at 623nm. At 254 nm, photodissociation must be occurring.
Absorbed light must having an energy that matches a possible electronic transition within the molecule. The direct photolysis by UV-254 nm light ,it means the molecule is strongly absorbs UV-light at 254 nm .I think the absorption of 254 nm light associates with existence of π bonds(π- π* transition) within the molecule.
Kishore is right, the direct photolysis also yield hydroxyl radical from photodissociation of H2O, though at lower rate, and could be another possible reason for direct photolytic degradation of compounds with absorption at 623nm, Some molecules absorb close to 254nm but still no direct photolysis, all these are explained in literature
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