Fluorescent mesomorphic pyrazinacenes

We report the synthesis of highly fluorescent pyrazinacenes (6,13-dihydrohexaazapentacene; fluorubine) containing groups at 6,13 positions that promote their solubility in non-polar organic solvents and their self-assembly properties. Hydrophilic 3,4,5-tris[2-[2-(2-methoxyethoxy)ethoxy]ethoxy]benzyl (TEG₃Bz) and hydrophobic 3,4,5-tris(1-n-dodecyloxy)benzyl (C12₃Bz) groups were incorporated in a series of compounds 1-9 where the fluorubine core was modified either by 2-bromination or by introduction of four peripheral benzo groups. Thus, 1-3 are respectively hydrophilic 6,13-(TEG₃Bz)₂, hydrophobic 6,13-(C12₃Bz)₂ and amphiphilic 6-(TEG₃Bz)(C12₃Bz) with 4-6 being the corresponding 2-brominated compounds and 7-9 the corresponding [a,c,l,n]-tetrabenzo compounds. Several of the compounds exhibit thermotropic mesophases: 4 and 7 have lamellar structures even at room temperature; 8 and 9 form columnar rectangular phases at higher temperatures. All the compounds are highly fluorescent exhibiting quantum yields (QY) up to 60% in solution. Solid state fluorescence spectra reveal that while the unmodified 6,13-dihydrohexaazapentacene compounds 1-3 maintain their large QYs in the solid state, 2-bromination (4-6) reduces the QY substantially (to 10% approx.). 7-9 give QY similar to 1-3 in solution but their QYs are attenuated in solid state due to chromophore aggregation.