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Frank-Condon-Principle

Changing the electronic state of a molecule results in a change of the forces between the atomic nuclei. This can lead even to the dissociation of molecules and/or the initiation of chemical reactions. Since the nuclei are much heavier than the electrons, a change of the electronic state happens so fast that the nuclei have no time to respond to this new situation. This means that the distribution of the electron density within the molecule is changed causing vibrations of the nuclei. Because the structure of the nuclei does not change during the electronic transition, the transition is vertical. This concept is called the FRANCK-CONDON principle. The most intense transition occurs from the vibrational ground state of the electronic ground state to that vibrational state of the excited electronic state for which the overlap of the probability functions has its maximum. According to KASHA’s rule, the electron will relax quickly to the vibrational ground state of the electronic excited state. From this level it can decay to the electronic ground state. This can be associated by a photon emission (fluorescence). Again, the electron will not decay to the lowest vibrational level of the electronic ground state, but to that vibrational level, where the overlap of the wave functions is largest. Figure 1 illustrates the processes.

 Frank-Condon

Figure 1: Scheme of the electronic states S0 and S1 as potential curves and the transitions of a light-induced excitation and following relaxation of an electron.

The energy required to excite an electron depends on the energy difference between the two electronic levels, between which the electron changes. This difference depends on the chemical nature of the molecule. Typical examples of electronic transitions are ππ transitions occurring when a C––C double bond absorbs light. In this case, the electron is excited from a π orbital to an antibonding πorbital. For an isolated double bond an energy of about 7 eV is required corresponding to a wavelength of 180 nm. The required energy changes if the electronic system is changed, e.g. by a second conjugated double bond or an aromatic ring. In these cases, the energy between the electronic levels decreases and with this the required energy for the transition. Due to the vibrational relaxations, the energy is lower and accordingly the wavelength of the photons is larger for fluorescence as for absorption. A more detailed description of these processes can be found in textbooks on physical chemistry or quantum mechanics.

  1. BECKER, H.; BÖTTCHER, H.; DIETZ, F.; REHOREK, D.; ROEWER, G.; SCHILLER, K.; TIMPE, H.-J.: Einführung in die Photochemie, 3. bearb. aufl. edn., 1991, Deutscher Verlag der Wissenschaften GmbH, ISBN 978–3–3260–0604–8.
  2. WÖHRLE, D.; TAUSCH, M. W.; STOHRER, W.-D.: Photochemie, 1998, Wiley VCH Verlag GmbH, ISBN 3527295453.



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