PHOTOPHYSICS OF DONOR-ACCEPTOR SUBSTITUTED STILBENES - A TIME-RESOLVED FLUORESCENCE STUDY USING SELECTIVELY BRIDGED DIMETHYLAMINO CYANOMODEL COMPOUNDS

Several selectively bridged 4-(dimethylamino)-4'-cyanostilbenes are investigated. All of them show strongly red-shifted fluorescence spectra connected with a considerable dynamical Stokes shift in polar solvents. Bridging, however, affects the photophysics (fluorescence quantum yields and lifetimes) dramatically. Blocking of the double bond twist by a sufficiently rigid bridge (-CH2-) leads to quantum yields near 1, although the anilino group is free to rotate. A flexible bridge (-CH2-CH2-), however, leads to strong fluorescence quenching with a mte constant k(nr) Similar to the unbridged compound. Bridging both single bonds leading to the phenyl rings while freeing the double bond twist (substituted "stiff stilbent"), on the other hand, increases k(nr), further by a factor of 70-100. In all cases, k(nr) decreases with increasing solvent polarity. The results can be understood within a three-state model comprising the excited states E* (planar, highly polar, fluorescent), A* (twisted anilino group, highly polar, fluorescent), and P* (twisted double bond, weakly polar, nonfluorescent funnel to the ground state).