Oligoquinane Hydrocarbons Display Dual Fluorescence & Phosphorescence Anti-Kasha Emissions

A series of molecules containing an oliquinane structural motif have been prepared. These oligoquinanes are constructed by connecting azulene units through the five-membered rings, resulting in a ladder-type structure of azulene oligomers. These oligoquinanes form a sequence of oligofulvene units which endow them with cross-conjugated character. In addition, through the 1-position of the central azulene unit, one carbazole is appended. A complete synthesis, solid-state studies, which document the formation of porous solids, and spectroscopic experimental characterization are carried out. The optical properties are analyzed as a function of the temperature in steady-state mode and by transient absorption. The whole study is guided and comprehensively rationalized by quantum chemical calculations. The hybridization of azulene and carbazole into an oligoquinane backbone confers the dual capacity of anti-Kasha fluorescence (azulene-like) and intersystem crossing (carbazole-like), enabling the synergistic emergence of anti-Kasha phosphorescence at 80 K. The observation of anti-Kasha phosphorescence is particularly notable, given that these are nearly pure hydrocarbons, i.e., heavy-atom-free organic chromophores. Furthermore, the design of new chromophores disclosing fluorescence and phosphorescence dual emissions in anti-Kasha mode is very appealing in organic photonics. In general, the discovery of modes of inferring additional properties to actuate from high energy lying excited states is of special interest in fields such as photochemistry, catalysis and photovoltaics.