Fluorescence Spectroscopy as a Tool for Investigating the Self-Organized Polyelectrolyte Systems - Self Organized Nanostructures of Amphiphilic Block Copolymers - page 187

2 Principles of the Fluorescence Techniques Used

2.1 State Diagram and the Characteristics of Time-Dependent

Fluorescence

The most comprehensive description and explanation of all processes that affect the
fluorescence and the role of additional external factors is provided by the Jablonski
diagram (Fig. 1 ) , which represents the scheme of energy levels of electronic and
vibrational states of a molecule and the possible transitions [ 1 ] .
The vertical axis indicates increasing energy of different quantum states in vacuo
or the Gibbs free energy of the fluorophore in condensed systems (e.g., in solutions).
Individual states corresponding to the optimum molecular geometry are depicted by
horizontal lines. The arrows indicate possible transitions between different states.
Typical values of rate constants of all processes have been also included.
Prior to excitation, the molecule is in the lowest vibrational state of the lowest
electronic state (ground state), S 0 . The absorption of a photon is governed by opti-
cal selection rules [ 2 ]. Its probability is proportional to the square of the transition
dipole moment. The most severe restriction concerns the spin conservation. Fur-
ther restrictions reflect the symmetry and overlap of corresponding wave functions.
Regardless of the probability (reflected by the molar absorption coefficient), the sin-
gle act of transition to a higher excited state due to absorption of a photon belongs
to the fastest processes that occur in nature (except nuclear processes) and proceeds
on timescales shorter than 10 − 15 s[ 3 ] . In this short time, neither the position of

Fig. 1 The Jablonski diagram: the energies of the ground electronic singlet state S 0 , excited singlet
S 1 , and triplet T 1 are depicted by bold horizontal lines ; vibrational states by narrow lines ;the
most important transitions are depicted by arrows and wavy lines ; the typical values (in orders of
magnitudes) of rate constants of the processes have been also included