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

Abstract

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 (λmax = 464 nm) bathochromically shifted beyond 8300 cm–1 from the band in the dilute solution (λmax = 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.

Supramolecular Chirality Achieved by Assembly of Small π-Molecules of Octahydrobinaphtols with Axial Chirality

“…Thus, the hbNaph aggregate emits blue fluorescence and the intensity increases as aggregation progresses. Its fluorescence and excitation spectral characteristics are similar to those of the methylsalicylate aggregates we recently reported, 34 in that both spectra have a vibrational progression and their energy difference (i.e., Stokes shift) is very small, about 600 cm −1 . We attempted to detect CPL from this blue fluorescent solution but unfortunately could not obtain adequate signals (Figure S2).…”

“…We therefore attempted a regression fit of the data using the isodesmic model (see the Supporting Information for fitting details) and obtained a good fit at concentrations below 10 –1 M, providing an estimated K value of 2.71 M –1 , comparable to the K values for other small aromatic molecules . The deviation from the isodesmic model above 10 –1 M is probably because the crystal nucleation starts and proceeds rapidly …”

“…38 The deviation from the isodesmic model above 10 −1 M is probably because the crystal nucleation starts and proceeds rapidly. 34 We also measured the 1 H NMR spectra of bP (Figure S4), whose fluorescence spectra suggested the formation of aggregates similar to those of hbNaph. The signals associated with aromatic and OH protons of bP also shifted to the upfield and downfield side, respectively, with increasing concentration.…”

“…For example, cyanine and perylenebisimide (PBI) dyes form head-to-tail arranged aggregates (i.e., J-aggregates) with unique optical properties, including chiroptical properties. , In contrast, if molecules bear substituents that are effective for forming intermolecular hydrogen bonds, including small molecules whose π-systems consist of a single benzene ring, these molecules can self-assemble into one-dimensional columnar structures that exhibit morphological and chiroptical properties. ,− However, since intermolecular π–π interactions are absent in small π-systems, such systems are unlikely to exhibit the assembly-induced absorption and fluorescence spectral changes seen in larger π-systems like cyanine and PBI dyes. We recently found an exception: methyl salicylate (MS) can form aggregates with optical properties characteristic of excitons spread over multiple MS molecules, despite being a small π-system . The connection of two MS molecules via a flexible dialkoxy bridge stabilizes high concentrations so that MS aggregates in solution, giving rise to unique assembly-induced absorption and fluorescence spectra.…”

“…We recently found an exception: methyl salicylate (MS) can form aggregates with optical properties characteristic of excitons spread over multiple MS molecules, despite being a small π-system. 34 The connection of two MS molecules via a flexible dialkoxy bridge stabilizes high concentrations so that MS aggregates in solution, giving rise to unique assemblyinduced absorption and fluorescence spectra. This suggests that even small π-molecules can form aggregates that exhibit collective optical properties, depending on conditions such as compatible intermolecular orbital mixing and high solubility in solvents.…”