Supramolecular Complexation and Collective Optical Properties Induced by Linking Two Methyl Salicylates via a ?-Bridge

Supramolecular complexes or polymers, formed bynoncovalent intermolecular forces such as pi-pi and dipole-dipoleinteractions, have the potential to render collective opticalproperties brought about by excitons spreading over multiplemolecules, as seen in J-aggregates. In this respect, molecules with alarge pi-system and dipole moment are advantageous. However, wereport here that methyl salicyate (MS) dyad-type molecules,synthesized by connection of two MSs via a Sigma-bridge, are effectivefor forming stable aggregates with collective optical properties. Theself-association of MS-dyads occurs in a CHCl3solution at a highconcentration of over 10-2M, which is recognized by theappearance of an absorption band (lambda max= 464 nm) bathochromi-cally shifted beyond 8300 cm-1from the band in the dilutesolution (lambda max= 334 nm). Upon excitation of this band, an intense greenfluorescence is observed without aggregation-causedquenching. The absorption andfluorescence bands, both of which have well-resolved vibronic progressions, are in a near-mirrorimage relationship, yielding a small Stokes shift of 600 cm-1. A reasonable explanation for these characteristic optical properties isprovided from theoretical considerations on the aggregate model constructed based on the results of single-crystal X-ray analysis.The1H NMR measurements suggest that unconnected MSs also form aggregates at high concentrations, although the absorptionmeasurements do not provide any evidence for this. It is thus presumed that the connection of MSs stabilizes the MS stackingstructure of the aggregates, leading to the generation of an excited state delocalized over multiple molecules

Automated summary

This study explores the application of stable aggregates formed through noncovalent interactions in optical properties. The researchers discovered that methyl salicyate dyad-type molecules, synthesized by connecting two methyl salicyate molecules, can form stable aggregates with collective optical properties at high concentrations.