Self-assembly of bis(merocyanine) tweezers into discrete bimolecular pi-stacks

Examples of a novel class of tweezer molecules have been constructed through the tethering of two dipolar merocyanine chromophores by a naphthalenedimethylene or dimethylenediphenylmethane spacer. The electrostatic-interaction-directed self-assembly of these bis(merocyanine) tweezers affords centrosymmetric bimolecular complexes with very high dimerization constants of up to >10(9) M(-1), even in the good solvating solvent chloroform. This pronounced self-association of the bis(merocyanine) tweezers is attributed to the strongly dipolar nature of the merocyanine chromophores and a unique aggregate geometry of four pi-stacked chromophores with an alternating arrangement of their dipole moments. The structural assignment of the tetrachromophoric pi-stack has been accomplished by MALDI-TOF mass spectrometry and ROESY NMR spectroscopy. Furthermore, molecular modeling studies have accounted for the relationships between the dimerization constants and optical properties of the bimolecular complexes of the present bis(merocyanine) dyes and the structure of the spacer and the positions at which the merocyanine chromophores are attached.