Contrasting Solid-State Fluorescence of Diynes with Small and Large Aryl Substituents: Crystal Packing Dependence and Stimuli-Responsive Fluorescence Switching

There has been a significant current
interest in solid state luminescence
of organic molecules and their stimuli responsive fluorescence switching
behavior. Although small organic derivatives with olefinic, acetylenic,
phenylenevinylenic, phenyleneethynylenic spacers are widely documented
as solid state emitters in the literature, the solid state photophysics
of organic derivatives with “butadiyne” spacer still
remains unexplored. We provide detailed investigation on the solid
state fluorescence properties of a series of butadiynyl fluorophores.
Replacement of a phenyl ring, which is at periphery of the butadiyne
bridge, with a large moiety such as pyrenyl group furnishes contrasting
emissions in the solid state. While the butadiyne bridged phenyl derivatives
show a blue shift of emission maxima in the solid powder with respect
to monomer spectra in solution state, the butadiyne bridged pyrenyl
derivatives exhibit a red shift in the solid state. The blue shift
of the emission maxima of the butadiyne bridged phenyl derivatives
in the solid powder is attributed to allowed excitonic transition
in aggregates with nearly parallel transition dipoles. On the other
hand, formation of pyrenyl excimer accounts for the red shift of the
butadiyne bridged pyrenyl derivatives in the solid powder. In addition
to that, the solid state fluorescence of the pyrenyl analogues is
reversibly switched between two aggregate forms through external heating
and rubbing stimuli.