Photophysics of 1-azacarbazole dimers: a reappraisal

A systematic study of 1-azacarbazole (1AZC) dissolved in 2-methylbutane (2MB) at gradually decreasing temperatures from room temperature to 77 K revealed the chromophore to exhibit four fluorescence emissions: a structured fluorescence in the UV region that is due to the 1-azacarbazole monomer, a structureless emission centered at 500 nm and assigned to the centrosymmetric dimer formed by double hydrogen bonding, an also structureless emission centered at ca. 400 nm and due to a noncentrosymmetric doubly hydrogen bonded dimer, and a fourth, structured emission at 357 and 375 nm due to a card-pack dimer. Evidence obtained from dilute solutions of 1-azacarbazole is for the first time assigned to a card-pack dimer, consistent with the photophysical behavior of carbazole in the same medium. Previously established photophysical evidence for such an interesting compound, which has been used as a model for studying light-induced double proton transfer mutational mechanisms, is completed or discussed here. The evidence obtained in this work reveals that 1AZC at a 10-4 M solution in 2MB does not exhibit doubly hydrogen bonded centrosymmetric dimer emission as the temperature decreases from room temperature up to 113 K (with a corresponding exponential increase of the solvent viscosity). At this temperature and below, however, the doubly hydrogen bonded centrosymmetric dimer emission appears. This evidence and others implemented in this work contradict the assumption of Waluk et al. that the appearance of the doubly hydrogen bonded centrosymmetric dimer is hindered by an increased viscosity of the medium.