Solvent Effects on Fluorescence Spectra

When excited states of a molecule are created in solution by continuous or flash excitation, the excited-state molecule interacts to a varying degree with the surrounding solvent molecules, depending on their polarity, before returning to the ground state. These excited-state solute/solvent interactions found in fluorescent molecules are often reflected in the spectral position and shape of the emission bands as well as in the lifetimes of the excited-state molecules. The solvent-dependence of the position of emission bands in fluorescence spectra is commonly included in the term solvatochromism. Sometimes, the solvent-dependence of fluorescence spectra has been called fluoro-solvatochromism [26] or solvatofluorochromism [27], Because of the close connection between spectral absorption and emission (see Figs. 6A and 6-7), there is no need for special terms for the fluorescence-based solvatochromism [16], 

When considering the solvent dependence of the position of emission bands, the finite relaxation time tr for the rearrangement of the solvent molecules surrounding the solute molecule in the Franck-Condon excited state and the finite lifetime Te of the molecule in the excited state have to be taken into account. 

In the case of tr Ts the emission will occur before any rearrangement of solvent molecules in the solvation shell takes place. The initial state of the emission process is the Franck-Condon excited state and the final state is the equilibrium ground state. Hence, the wavenumber of emission will be equal to the wavenumber of the corresponding absorption. In the case of tr Te (c/ Fig. 6-7) , reorientation of the solvent molecules can take place after electronic excitation and a relaxed excited state is obtained in which another solvation equilibrium has been established. It is from this equilibrium state that fluorescence occurs at room temperature. By analogy, there is a 

Franck-Condon ground state after emission, which persists momentarily until the solvent molecules reorganize to the equilibrium arrangement for the ground state. 

The solvent-induced shifts of absorption and emission bands can be used to calculate dipole moments of electronically excited molecules [32, 33, 47, 303], Excited-state dipole moments have also been obtained by the measurement of fluorescence polarization caused by external electric fields [32, 33], 

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