On the nature of the substituent effect in slipped π-π structures

In recent years, some controversy has been running related to the origin of the substituent effect in π-π interactions. The earlier model based on ring polarisation and electrostatic interactions does not seem to be able to describe the observed effects, so a new model considering that substituent effect comes from the direct interaction between the substituent and the other ring has been proposed. In any case, the net effect of the substituent is largely dependent on other variables, and attention has been recently called upon the fact that the distance between rings can alter the observed behaviour. In a recent work (PCCP 2020, 22, 12068) we have shown that the origin of the substituent effect can change depending on the relative position of the substituent and the interacting rings. In the present study, an extension of the previous work is performed by using similar systems but changing the substituent , now located in a pyridine ring, and thus changing the relative positions of the substituent and the interacting rings.