For the excited states, the optimisation calculations were performed using TD-DFT with root = 2. To analyse the charge transfer between excited states, single-point calculations were performed on the natural bond orbital (NBO) population. To obtain molecular electrostatic potential (MEP) maps, post Hartree-Fock energy calculations were performed on charges from electrostatic potential using a grid-based method (CHELPG) population at the MP2 level. MEP and molecular orbital (MO) surfaces were generated using GaussView with surface isovalues of 0.0004 and 0.0300, respectively. Finally, the density of states (DOS) spectra were plotted using GaussSum software . All the calculations were performed using the Gaussian 09 software package.