The B3LYP/6-31G^* method has been applied to the intramolecular hydrogen transfer(IHT)reaction of perylenequinonoid photosensitizer hypocrellin A(HA)and its model compounds in order to illuminate the effect of side chains.A series of compounds with similar molecular structure as that of perylenequinonoid(PQ),the active center of HA,were studied with the CIS/6-31G^* method in this article.The most important results are as follows:(1) HA executes a quick IHT reaction in the ground state at room temperature.(2)There is a little difference between IHT barrier and charge distribution ofHA and the model compounds,which indicates that side chain has little effect on IHT reaction of HA.(3)As for HA series compounds,IHT barrier is linear to the change of H-bond length orO-H bond length.(4)There is a charge separation process coupled with the IHT reaction.The Coulombic interaction dominates the IHT process,which indicates that the IHT reaction of HA is a proton transfer process in nature.(5)Conjugated structure has a vital effect on the IHT reaction of PQ and its similar molecules.As for the molecules with smaller conjugated structures,the singlet and triplet excited state potential energy profiles of IHT reaction separate from each other.The bigger the conjugated structure is,the closer the two potential energy profiles are.Considering PQ,there is an intersection part between its S₁ and T₁ profiles.So the intersystem crossing(ISC)is likely to take place during the IHT reaction of PQ leading to high triplet quantum yield eflqciency.It is well known that most photosensitization processes involve reactions in triplet excited state.So the IHT process plays a vital role in retaining the photosensitization of PQP.

Key words: perylenequinonoid photosensitizer(PQP), hypocrellin A(HA), intramolecular hydrogen transfer(IHT), quantum chemical calculation