Supramolecular Complexation and Collective Optical Properties Induced by Linking Two Methyl Salicylates via a...

Supramolecular complexes or polymers, formed by noncovalent intermolecular forces such as π-π and dipole-dipole interactions, have the potential to render collective optical properties brought about by excitons spreading over multiple molecules, as seen in J-aggregates. In this respect, molecules with a large π-system and dipole moment are advantageous. However, we report here that methyl salicyate (MS) dyad-type molecules, synthesized by connection of two MSs via a σ-bridge, are effective for forming stable aggregates with collective optical properties. The self-association of MS-dyads occurs in a CHCl₃ solution at a high concentration of over 10^-2 M, which is recognized by the appearance of an absorption band (λ_max = 464 nm) bathochromically shifted beyond 8300 cm^-1 from the band in the dilute solution (λ_max = 334 nm). Upon excitation of this band, an intense green fluorescence is observed without aggregation-caused quenching. The absorption and fluorescence bands, both of which have well-resolved vibronic progressions, are in a near-mirror image relationship, yielding a small Stokes shift of 600 cm^-1. A reasonable explanation for these characteristic optical properties is provided from theoretical considerations on the aggregate model constructed based on the results of single-crystal X-ray analysis. The ¹H NMR measurements suggest that unconnected MSs also form aggregates at high concentrations, although the absorption measurements do not provide any evidence for this. It is thus presumed that the connection of MSs stabilizes the MS stacking structure of the aggregates, leading to the generation of an excited state delocalized over multiple molecules.