Complexation and fluorescence of tricyclic basic dyes encapsulated in cucurbiturils

Area of Science:

• Supramolecular Chemistry
• Photochemistry

Background:

• Tricyclic basic dyes are widely used in biological staining and as fluorescent probes.
• Cyclodextrins (CDs) are known host molecules that form inclusion complexes with various guests, including dyes.
• Cucurbit[n]urils (CBs) are macrocyclic hosts with unique binding properties.

Purpose of the Study:

• To investigate the complexation behavior of tricyclic basic dyes with cucurbit[7]arene (CB[7]) and cucurbit[8]arene (CB[8]).
• To quantify the binding affinities and stoichiometries of these host-guest complexes.
• To evaluate the impact of complexation on the photophysical properties, specifically fluorescence, of the dyes.

Main Methods:

• Spectroscopic titration methods were employed to determine binding constants and stoichiometry.
• UV-Vis absorption and fluorescence spectroscopy were used to monitor changes in dye properties upon complexation.
• Analysis of complex formation between dyes like proflavine, acridine orange, pyronine Y, and cucurbit[7]arene and cucurbit[8]arene.

Main Results:

• Tricyclic basic dyes form 1:1 or 2:1 complexes with CB[7] and CB[8], respectively.
• Pyronine Y exhibits 1:1 complexation with CB[7] and 3:1 with CB[8].
• Binding constants for CB[7] complexes are in the range of 10^6–10^7 M⁻¹, while CB[8] association constants reach up to 10^16 M⁻².
• Complexation with CB[7] enhances dye fluorescence quantum yields.
• Dimeric or trimeric dye complexes in CB[8] show significantly reduced fluorescence compared to free dyes.

Conclusions:

• Cucurbit[8]arene demonstrates exceptionally high binding affinities for tricyclic basic dyes, exceeding those of CB[7] and common cyclodextrins.
• The formation of supramolecular complexes with CB[7] and CB[8] can be used to modulate dye fluorescence.
• These findings highlight the potential of cucurbiturils as hosts for controlling dye photophysics for sensing or imaging applications.