The process of electronic absorption involves participation by the nuclei through molecular vibrations that act as either accepting modes or promoting modes. Accepting modes are also known as Franck-Condon active modes and they are totally symmetric vibrations. There is an excited state displacement along these modes due to the change the charge distribution the excited state configuration. The transition involving Franck-Condon active modes is a vertical transition because the excitated by electromagnetic radiation is much faster than any nuclear vibration. Thus, the molecule ends up on slope of the excited state potential energy profile. The result of the nuclear displacement is a shift potential energy profile, which means that the ground state wavefunction will not be in a stationary state when it is project onto the excited state. Instead, the wave function oscillate in the excited state manifold. because of a displacement in the nucleuar coordinate along all Franck-Condon active modes. The Franck-Condon active modes are called accepting modes since the vertical transition leads to dyanmics in the excited state manifold. The promoting modes are known as vibronically active modes and they are non-totally symmetric modes.

Fluoresence occurs only after an excited state relaxation to a new equilibrium position. Then the molecule can return to the ground state by a vertical transition`. The Franck-Condon factors for the downward transition are the same as those for the upward transition. It is the excited state displacement that determines the nuclear overlap factors and it does not matter whether light is being absorbed or emiitted. This fact gives rise to the mirror image relationship between absorption and emission (fluorescence) spectra. The 0-0' is the same for both absorption and emission. However, the other lines (e.g. 0 - 1', 0 - 2', etc.) are spaced to higher energy for absorption and to lower energy (e.g. 0' - 1, 0' - 2, etc.) for the emission. This is exaplained in greater detail under "mirror image relationship" on the right.

Fluorescence quantum yield

A key point for the emission of light is to determine whether the emission yield is sufficiently high that a fluorecent molecule has application as a molecule probe. Appications of fluorescence have become important in cell imaging, protein orientation, binding and structural changes, materials science and many other specific areas of research. In order to consider applications of fluorescence we have started by studying its line shape as determined by the Franck-Condon factors. However, precisely as is the case for absorption, the square of the transition dipole moment determines the absorption and emission intensity. The Einstein relations show that there is a fixed relationship between the probability of spontaneous emission and the probability for absorption. Incidentally, Einstein also showed that there probability for stimulated emission is equal to that for absorption, which helps to define the characteristics of a laser.

Applications of fluorescence spectroscopy

1. Fluorescence quenching
2. Fluorescence anisotropy
3. Fluorescence resonance energy transfer (FRET)
4. Fluorescence recovery after photobleaching (FRAP)
5. Fluorescence correlation spectroscopy (FCS)

Practice as you go...

Electronic part

Electronic factor problems

Nuclear part

Franck Condon problems

Extinction coefficient

Beer's law problems

Fluorescence

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