Interplay of monomer, intra- and intermolecular excimer fluorescence in cyclophanes and selective recognition of methanol vapours

With an objective to develop simple organic systems for optoelectronic applications, we have investigated photophysical properties of water soluble cyclophane derivatives 1, 2 and model compound 3 in solution and solid state. Of these, the symmetrical cyclophane 1 exhibited monomer emission in methanol, whereas an intramolecular excimer emission was observed in the aqueous medium. Interestingly, in the solid state it showed a highly red-shifted emission maximum when compared to those in the solution. The unsymmetrical cyclophane 2, on the other hand, exhibited monomer emission in both organic and aqueous media, while a dual emission (monomer and excimer) was observed in the solid state. Insights into these unique fluorescence properties were gained by studying the molecular packing of the cyclophanes through single crystal X-ray structure analysis. The cyclophane 1 exhibited a unidirectional sandwich-type packing pattern thereby resulting in strong face-to-face pi-pi stacking interactions between the anthracene chromophores, whereas 2 exhibited both herringbone and sandwich-type pattern, wherein the interacting anthracene moieties are arranged in an intermolecular T-shaped fashion. By virtue of different molecular packing the cyclophane 1 showed visible fluorescence colour change from yellow to blue when exposed to methanol vapours as compared to ethanol, acetonitrile, hexane and water vapours. Uniquely, this system can detect as low as 1% of methanol in ethanol solution thereby demonstrating its usefulness as an optical probe for the detection of methanol vapours.