Most of QM calculations were performed with Gaussian 16 software. To exclude the influence of different software and make a direct assessment of the electrostatic effect of protein environment, we performed an additional QM calculation with System III in the gas phase using ORCA. The emission energy difference obtained with different software is 0.09 eV (refer to Table ), which is much smaller compared to the large difference (0.58 eV) caused by the protein environment. Figures were prepared with Multifwn, VMD, and PyMol.