Experimental and theoretical methods have been combined to investigate why 1,2,4,5-benzene tetracarboxylic dianhydride (PMDA) can extend light absorption of wide band gap carbon nitride polymer. Based on VBXPS results and detailed orbital analysis, it is most likely that LUMO of PMDA acts as doping state between CB and VB of wide band gap carbon nitride polymer and extends light absorption by using its LUMO to accept the electron from VB. In addition, the interaction between PMDA and wide band gap carbon nitride polymer influence the energies of CB and VB. (c) 2012 Elsevier B.V. All rights reserved.