Topological bonding fingerprints in photochemically substituted [2 + 2] cycloaddition

Using the framework of the bonding evolution theory (BET), we investigated excited-state substituted [2 + 2] cycloaddition. Our findings demonstrate that the presence of non-bonding density centers during the S₁ excited state results in the electronic activation of both unsubstituted and substituted ethylene. It should be noted that these electronic rearrangements imply a very high energy barrier in the ground state, where [2 + 2] cycloaddition is forbidden by the orbital symmetry rules. A crucial bonding process that leads to CC bond formation in both the ground and excited states is the presence of non-bonding centers. Hence, the nature of CC bond formation changes when electron-withdrawing or electron-donating groups are present in the reaction center. On the other hand, the non-polar behavior in cycloadditions is associated with low differences in electron density, whereas polar effects due to hydroxy and cyano substitutions emerge when the difference in electron density between the CC bonding centers is substantial. Consequently, the topological fingerprints of the CC bond creation in the photochemically induced [2 + 2] cycloadditions can be cusp (symmetric collapse of pairing density) if the reaction center is unsubstituted or fold if the reaction center undergoes some substitution (asymmetric collapse of pairing density).