S₁/S₀ Potential Energy Surfaces Experience Different Types of Restricted Rotation: Restricted Z/E Photoisomerization and E/Z Thermoisomerization by an Out-of-Plane Benzyl Group or In-Plane m‑Pyridinium Group?

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 S₁ 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 S₁ and S₀ 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 S₁ 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 S₁ 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 S₀ PES, it is not the reduced bond order of the I-bond in the S₀ 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.