Cyclodextrin Inclusion Complexes with a Solvatochromic Fluorescent Probe

摘要

Solvent polarity can have a dramatic effect on the emission color of certain fluorescent probes. Such solvent-sensitive-solvatochromic-probes have as a defining feature the simultaneous presence of electron-donating and electron-withdrawing groups in resonant positions in a conjugated system. When a solvatochromic molecule absorbs a photon, the dipole moment increases as a result of charge traveling along the system, intramolecular charge transfer (see Figure 1). The energy of this excited state, S_1, which is no longer in equilibrium with the surrounding solvation shell, can be lowered, or relaxed, by the reorganization of polar solvent molecules which create favorable interactions with the increased dipole of the fluorescent molecule. The underpinnings of the solvent-induced spectral shift rely on this solvent reorganization known as solvent relaxation. If solvent relaxation occurs prior to the radiative transition to the groun state, S__0, the result is a lower energy excited state and a concomitant red shift in the fluorescence emission (Fl_r). In less-polar solvents this relaxation process is less effective resulting in blueshifted emission (Fl_b) relative to that in polar media. The classic example of such a donor-acceptor molecule exhibiting solvatochromism is 6-propinoyl-2-(N,N-dimethylamino) naphthalene, PRODAN (1), whose ground- and excited-state dipole moments are 2.8 D and 10 D (2), respectively. The emission maximum of PRODAN shifts 150 nm between solutions of cyclohexane and water (Figure 2). The tremendous sensitivity of PRODAN emission to solvent polarity has been used to investigate the binding sites of proteins (1,3,4) and enzymes (5), interactions with cell membranes and phospholipid vesicles (6-8), the structure of reverse micelles (9,10), and cyclodextrin inclusion complexation (11).