Franck-Condon excited state dipole moment

Fig. 4.3.1. Diagrammatic representations of solvent effects on the transition energies of polar solutes in polar solvents. A refers to a reference solvent, 5 to a less solvating solvent and C to a more solvating solvent, (a) Case where the dipole moment of the Franck-Condon excited state of the solute is larger than that of the ground state. (b) Case where the Franck-Condon excited state dipole moment is less than that of the ground state.

The enrichment of the concentration of the polar solvent component in the cage and, therefore, the relative amount of the red shift of the fluorescence band is a function of viscosity, since the diffusion-controlled reaction time must be smaller than the excited-state lifetime. This lifetime limitation of the red shift is even more severe if the higher value of the excited-state dipole moment is not a property of the initial Franck-Condon state but of the final state of an adiabatic reaction. Nevertheless, the additional red shift has been observed for the fluorescence of TICT biradical excited states due to their nanosecond lifetime together with a quenching effect of the total fluorescence since the A to 50 transition is weak (symmetry forbidden) (Fig. 2.25). 

When PRODAN is excited to its lowest singlet state, it undergoes a change in dipole moment of approximately 20 Debye units (38). According to the Franck-Condon principle, absorption of a photon occurs faster than nuclei can move (31,40). Therefore, immediately following optical excitation, there will be an excited-state dipole moment produced, but this dipole is essentially surrounded by a ground-state solvent cage. That is, the system is not in equilibrium. With time, the solvent... 

The comparison of the Mfiu values with those of Mabs allows one to obtain information about the changes in the electronic structure and molecular conformation between the Franck-Condon excited state initially reached upon excitation and the solvent-equilibrated fluorescent state [14]. Electronic transition dipole moments are mainly determined by the direct interactions between the lowest CT state and the ground state (So), and by the contributions from the locally excited configurations [14, 54, 56, 57]. For example, for the fluorescent CT state one can obtain... 

When the dipole moment of the solute is greater in the excited state, then, although the solvent cage becomes strained, the Franck-Condon excited state is formed in an already partly oriented solvent cage, and the excited state is more solvated than the ground state. 

Here the ground state solvation energy results largely from dipole-dipole and ion-dipole forces, so there is an oriented solvent cage. If the solute dipole moment is increased during the transition, the Franck-Condon excited state is formed in a cage of already partly oriented... 

If the solute dipole moment decreases during the transition the Franck-Condon excited state is in a strained cage of oriented dipoles, and this will contribute a blue shift. The superimposed polarisation red shift will usually be less, so the resultant will be a shift to the blue. 

For an intermolecular transition (except charge transfer to solvent) the direction of the solvent shift is determined similarly. That is, if the Franck-Condon excited state has a larger dipole moment than the ground state, a red shift is observed with increase in solvating power of the solvent. Conversely, when the transition results in a decrease in dipole moment, there will be a blue shift contribution from solvent cage strain, which in most cases outweighs the dispersion red shift. 

In hydrocarbon solvents the spectra show a vibrational structure. Some signs of vibrational structure are discernible in the spectra of NO3 in DMF at room temperature. The red shift of n -> 7T transitions, observed with decrease in dielectric constant and hydrogen bonding ability of the solvent, is consistent with the Franck-Condon excited state having a smaller dipole moment than the ground state. This is as expected when an electron is promoted from a localised lone pair to a more diffuse tt orbital. 

TT- TT transitions may or may not result in the Franck-Condon excited state possessing a different dipole moment, as the transition may remain relatively localised, in which case little change occurs. Thus, the solvent shifts observed may be red, blue or negligible. Table 4.4.3 contains some selected examples of solvent effects on the wavelength of maximum absorption for several different species. 

S0 is the unperturbed vapour phase energy level of the molecule in the ground state, in solution it is depressed by an amount proportional to R0. On excitation, the molecule is promoted to the Franck-Condon excited te (FC). In the excited state, the dipole moment may not only have a... 

In order to allow a comparison with experimental data we first consider Franck-Condon ICT states, i.e. we calculate the dipole moments of the excited state by keeping the geometry frozen in the ground state. The results are reported in Table 7-3. 

A scheme for the treatment of the solvent effects on the electronic absorption spectra in solution had been proposed in the framework of the electrostatic SCRF model and quantum chemical configuration interaction (Cl) method. Within this approach, the absorption of the light by chromophoric molecules was considered as an instantaneous process. Tliere-fore, during the photon absorption no change in the solvent orientational polarization was expected. Only the electronic polarization of solvent would respond to the changed electron density of the solute molecule in its excited (Franck-Condon) state. Consequently, the solvent orientation for the excited state remains the same as it was for the ground state, the solvent electronic polarization, however, must reflect the excited state dipole and other electric moments of the molecule. Considering the SCRF Hamiltonian... 

Spectroscopy provides a window to explain solvent effects. The solvent effects on spectroscopic properties, that is, electronic excitation, leading to absorption spectra in the nltraviolet and/or visible range, of solutes in solution are due to differences in the solvation of the gronnd and excited states of the solute. Such differences take place when there is an appreciable difference in the charge distribution in the two states, often accompanied by a profonnd change in the dipole moments. The excited state, in contrast with the transition state discussed above, is not in equilibrium with the surrounding solvent, since the time-scale for electronic excitation is too short for the readjustment of the positions of the atoms of the solute (the Franck-Condon principle) or of the orientation and position of the solvent shell around it. 

Thus, the absorption to the excited electronic state depends on the electronic transition dipole moment, the Franck-Condon (EC) overlap between the vibrational wavefunctions in both electronic states and the vibrational excitation probability. Indeed, as seen from the schematic representation in Eigure 2.1b, the absorption spectrum represents the reflection of the wavefunction, but it is also dependent on the EC factors that lead to intensity alterations in the observed features. 

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