Excitation to the Franck-Condon State

Excitation of the investigated stilbene molecule from its ground state H to the Franck-Condon state occurs in a few femtoseconds. As a result, only fast electronic polarization techniques can follow a drastic change of the charge distribution around the zwitterionic exited FC state. The latter has been proved particularly by the excitation energy dependence on the solvent refractive index [23]. The first step after excitation to the Franck-Condon state of the trans-stilbene configuration is vibrational relaxation followed by solvent-solute relaxation that leads to a rapid population of the H state from which fluorescence occurs. These relaxation processes result in a Stokes shift (A ). 

Absorption and emission spectra of 20 trans-4,4 -disubstituted stilbenes have been measured in four solvents cyclohexane (CH), chlorobenzene (CB), 2-butanone (methylethylketone, MEK), anddimethylsulfoxide (DMSO) at room temperature [24]. Fluorescence quantum yields (4 f) and fluorescence lifetimes (Xf) have been measured for these stilbenes. Substituent effects on the Stokes shift were described by a spectroscopic Hammett equation 

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The time of excitation to the Franck-Condon state is approximately 1 fs (10 s). Thus, these techniques will freeze out any equilibrium that involves the actual movement of nuclei, such as a cis-trans interconversion. Electron exchange can, however, be faster than a vibration. These very fast electron transfer processes can be probed by exposing a sample to laser radiation with a pulse shorter than 10 s, i.e., the realms of femtosecond (10 s) and attosecond (10 s) spectroscopy. ... 

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