The intramolecular [2+2] cycloaddition of 1,3-dienes under visible light irradiation investigated by Yoon and his co-workers shows remarkably high yield and stereoselective differences under different photocatalysts. The reaction was speculated to be induced by energy transfer. However, the origin for these phenomena is still unclear. In this scene, the detailed mechanism for the [2+2] cycloaddition of 1,3-dienes under visible light has been investigated using density functional theory B3LYP and TPSSTPSS methods. The result shows that the reaction not only can be induced by energy transfer between photocatalysts and reactants, but also can be induced by electron transfer between them. The [2+2] cycloaddition induced by energy transfer is carried out along the potential energy surface (PES) of triplet excited states (T₁) firstly, and then goes back to the singlet ground state (S₀) via MECPs (minimum energy crossing points) between the PESs of the S₀ and T₁ states, forming the product in the S₀ state. The [2+2] reaction induced by electron transfer proceeds along the doublet state PES of the cation radical reactant and the neutral four-membered ring product could be obtained by electron transfer from the corresponding reactant or reduced photocatalyst. The origin of stereoselectivity of the [2+2] reaction is attributed to the reaction mechanism difference under different photocatalysts.

Yoon及其同事研究了在可见光照射下1,3-二烯的分子内[2 + 2]环加成反应，在不同的光催化剂下，其收率和立体选择性差异显着。推测该反应是由能量转移引起的。但是，这些现象的根源仍不清楚。在此场景中，已使用密度泛函理论B3LYP和TPSSTPSS方法研究了可见光下1,3-二烯[2 + 2]环加成的详细机理。结果表明，该反应不仅可以通过光催化剂与反应物之间的能量转移来诱导，而且可以通过它们之间的电子转移来诱导。沿着三重激发态（T的势能面（PES））进行由能量转移引起的[2 + 2]环加成反应。₁）首先，然后通过S ₀和T ₁状态的PES之间的MECP（最小能量交叉点）返回到单重态基态（ S ₀），形成S ₀状态的乘积。由电子转移引起的[2 + 2]反应沿着阳离子自由基反应物的双峰态PES进行，中性四元环产物可通过电子转移从相应的反应物或还原的光催化剂中获得。[2 + 2]反应的立体选择性的起源是由于在不同的光催化剂下反应机理的不同。