Study of the self-association of molecular tweezers bearing two different arms: influence of the stereoelectronic effects of the arm substituents

In the general field of molecular recognition, the self-assembly of a tweezer bearing a R(+) usnic acid arm and a substituted 1,8-naphthalimide moiety as a second arm was investigated through a 1 H NMR study. Two self-association modes were evidenced: either the usnic acid arms (mode A) or the naphthalimide ones (mode B) were face to face in the center of the dimer. The existence of a third non-symmetrical mode (mode C) has not been evidenced experimentally. The behavior of these tweezers was discussed taking into account the electronic density of the naphtalimide arms and the slow mode A/mode B exchange rate (EXSY experiments). Only the mode A was observed when naphthalimide was either non-substituted ( 1 ) or substituted by Br ( 2 ). With one or two electrodonating substituents (OCH 3 ( 3 ) or (OCH 3 ) 2 ( 4 ) or N(CH 3 ) 2 ( 5 )) both modes A and B occured in a 50/50 ratio. This ratio depended on temperature for tweezer 3 .