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 C[sbnd]C bond formation in both the ground and excited states is the presence of non-bonding centers. Hence, the nature of C[sbnd]C 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 C[sbnd]C bonding centers is substantial. Consequently, the topological fingerprints of the C[sbnd]C 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).