Efficient inclusion complexation and intra-complex excitation energy transfer between aromatic group-modified β-cyclodextrins and a hemicyanine dye

A hemicyanine dye, 4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (4-ASP) forms inclusion complexes with β-cyclodextrin (β-CD) and modified β-CDs having a sulfonaphthyl (β-CD-NS) or a pyrenyl group (β-CD-py) at the primary face. The complexation constants in water were determined by fluorescence titration and found to be 4.7 × 104 M−1 with β-CD-NS and 3.6 × 104 M−1 with β-CD-py, while it is 730 M−1 with β-CD. Compared to 4-ASP in water, the emission maximum of 4-ASP of the inclusion complexes is blue-shifted by about 20 nm, and emission intensity is higher by 6.4-fold for β-CD, 14-fold for β-CD-NS and as much as 56 fold for β-CD-py complexes. These are much greater than the 3 nm blue shift and 3.3-fold enhancement of emission intensity when the solvent medium is changed to CH3OH. Good overlap between the emission bands of β-CD-appended aromatic groups and absorption band of 4-ASP results in the excitation energy transfer from the excited aromatic groups to 4-ASP and the efficiency of the intra-complex transfer is near 100%. 1H NMR spectra and molecular modeling of the 4-ASP complexes with β-CD-NS and β-CD-py indicated that N-methyl-pyridinium moiety of 4-ASP is outside the primary face of β-CD cavity. The interaction between the N-methyl-pyridinium group and the appended aromatic groups appear to result in the high stability of the complexes. The high fluorescence intensity of 4-ASP in the complexes with β-CD-NS and β-CD-py was explained in terms of hindrance of the formation of the non-fluorescent twisted intramolecular charge transfer (TICT) states by the interaction and confinement in cavity.