Any method that can enhance the fluorescence of fluorophores is highly desirable. Fluorescence enhancement accomplished by restricted Z/E photoisomerization through intramolecular steric hindrance or relatively high bond order of a C=C double bond in a S-1 excited state has rarely been studied. In this article, we used green fluorescent protein (GFP) chromophore analogues as a model to get new physical insights into the restricted Z/E photoisomerization and E/Z thermoisomerization phenomena. We found that the S-1 and S-0 potential energy surfaces (PESs) of the GFP chromophore analogues experience two dramatically different types of restricted rotation, and 2b can be a representative example. In its S-1 PES, it is not the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group but the relatively high bond order of the I-bond in the S, excited state of 2b that makes it have a higher barrier for the Z/E photoisomerization, a smaller Z/E photoisomerization quantum yield, and a higher fluorescence quantum yield. In its S-0 PES, it is not the reduced bond order of the I-bond in the S-0 ground state of 2b but the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group that makes it have an extra higher barrier for E/Z thermoisomerization and a much smaller E/Z thermoisomerization rate constant.