Pyrenoimidazole fused phenanthridine derivatives with intense red excimer fluorescence in solid-state
A new series of polycyclic aromatic hydrocarbon (PAH), pyrenoimidazole (PyFPs) having different substituents were developed as fluorescence emitters for optoelectronic applications. The compounds were synthesized by means of transition metal-free, base promoted intramolecular C-C bond formation between pyrene and phenanthridine bridged by imidazole which facilitates extended conjugation throughout the molecule. PyFPs also maintains the vibronic features characteristic of PAHs with intense blue emission in the solution state with the spectral maximum around 425 nm. While the absorption spectra of the drop-casted film of PyFPs maintain marginal spectral shifts with respect to the solution, but that of fluorescence is quite phenomenal with the 25 to 107 nm red-shifted spectral maximum having intense red-emission was observed. Fluorescence, electron, and atomic force microscopic images reveal the formation of rod- and flower-like self-assembled supramolecular structures possibly aided by the strong intermolecular interactions. Indeed, the solid-state fluorescence was also observed in the red-region with the broad spectra spread over the near-infrared region. The largely red-shifted emission maximum is attributed to the excimer emission in the solid-state formed through tailored intermolecular interactions by the pyrene and phenanthridine and have potential to be used in optoelectronic applications.