PHASE-TRANSITION PATTERN OF 2,5-DIPHENYLOXAZOLE/GAMMA-CYCLODEXTRIN (PPO/GAMMA-CD) SELF-ASSEMBLY AGGREGATES

The molecule 2,5-diphenyloxazole (PPO) organizes into non-covalently bonded self-assembled aggregates of guest-host complexes with gamma-cyclodextrin (gamma-CD) in aqueous solutions. This structure is held together by weak interactions between the PPO and the gamma-CD molecules. Therefore a small activation energy allows deterioration of these self-assembled aggregates. In this study, we report the effect of temperature and pH on this unique ensemble of aggregates. We report a phase transition pattern in which the structure disassembles from long to short (i.e. basic unit) aggregates. The relative fluorescence intensities of the excimer and monomer of PPO have been used to study this transition. Earlier investigations reported that these self-assembled aggregates were destroyed by an increase in temperature up to 80 degrees C. In this study, we examined the effect of temperature on this structure and found a phase transition pattern of the temperature-dependent structure. Both the intensity and polarization of the steady state fluorescence reveal similar values of the critical temperature at 46 +/- 8 degrees C and 49 +/- 7 degrees C for the monomer and excimer fluorescence respectively. In addition, earlier studies proposed that hydrogen bonding between the guest and host molecules may play an active role in stabilizing the linear beads. We examined this hypothesis by changing the pH to an acidic or basic medium. A change in the aggregates was observed at approximately pH 12, which is the pK(a) value of gamma-CD. For the pH range 10-13, a phase transition is centered at about pH 12.