2. Photoinitiator of photopolymerization.

PI plays an extremely important role in photopolymerization reaction, it undergoes a series of complex physical or chemical changes in initiating process.[17-19] As is shown in Figure 2, PI absorbs light, then is excited from ground state (S0) to singlet state (S1) which could be returned to ground state by fluorescence emission or nonradiative deactivation, also may reach to triplet state (T1) by intersystem crossing (ISC). Similarly, PI of triplet state can return to ground state by phosphorescence or nonradiative deactivation as well. When active species are generated by cleavage or hydrogen abstraction of PI in triplet state, photopolymerization can be initiated.