Dual fluorescence of fast blue RR and fast violet B: effects of solvents and cyclodextrin complexation

Absorption and steady-state and time-resolved fluorescence spectra of fast blue RR (FBRR) and fast violet B (FVB) were studied in solvents with different polarities and in the presence of α- and β-cyclodextrins (CDs). Dual emission observed in nonpolar solvents suggested that the energy of the intramolecular charge transfer (ICT) state is lower than that of the locally excited state. The normal Stokes-shifted band originated from the locally excited state, and the large Stokes-shifted band was due to the emission from a planar intramolecular charge transfer (PICT) state. The ratio of the PICT emission to the normal emission increased with β-CD concentration, whereas it was constant upon addition of α-CD. This behavior is in accordance with CD-dependent decay times of PICT and normal emissions, indicating the formation of different 1:1 FBRR/CD inclusion complexes. The rise time for the PICT emission increased with β-CD concentration, whereas no rise time was observed in the case of the α-CD complex. The size of the dimethoxyaniline ring suggested that the orientation of FBRR in the β-CD complex was different from that in the α-CD complex. The benzamido moiety of FBRR is deeply encapsulated in the CD cavity, whereas the aniline ring is exposed to the hydrophilic part. Semiempirical quantum-mechanical (ΔE, ΔG, ΔH, ΔS, and HOMO-LUMO) calculations were also carried out to assign the encapsulation of FBRR and FVB.