In search for phototautomerization in solid dipyrido[2,3-a:3′,2′-i]carbazole

Dipyrido[2,3-a:3′,2′-i]carbazole (DPC) is known to undergo excited state double proton transfer in cyclic complexes with alcohols. This process is detected by the appearance of low-energy fluorescence, red-shifted with respect to the “normal” emission. X-ray studies show that DPC forms cyclic, but strongly nonplanar doubly hydrogen-bonded dimers in the crystalline phase. No tautomeric fluorescence could be detected for DPC crystals. However, this emission becomes readily observed for solid DPC samples that have been exposed to water vapor, or prepared on hydroxylic supports, such as wool, filtration paper, or mica. These results suggest possible use of DPC as a probe for detection of humidity and hydroxyl groups. Tautomeric emission was also observed for DPC embedded into a hydroxyl-group-containing polymer, poly(vinyl butyral-co-vinyl-alcohol-co-vinyl acetate). The ability to form cyclic, doubly hydrogen-bonded dimers and complexes with hydroxyl-group-containing partners is compared in a series of structurally related molecules: DPC, 1H-pyrrolo[3,2-h]quinoline, 7-azaindole, 1-azacarbazole. Molecular geometry dictates that the systems with the strongest propensity to form flat dimers should have the weakest tendency for the formation of cyclic solvates, and vice versa.