The Jablonski Diagram

absorption of a photon of sufficient energy by an organic ground state molecule results in promotion of one bonding or non-bonding electron to a vibrational level of the first (Si) or second (S2) excited singlet state, depending on the 

So-H heat (10 M0 s) ISC inter system crossing T, - So -t heat (10 -10 s) ISC S, T, -H heat (10- -10 s). P, Reaction product from the singlet state (intramolecular) P2 (intermolecular) reaction product from the triplet state. The reactions from Sj and T] may also include electron or energy transfer reactions. The arrows in the boxes represent the spin orientation of the electrons in the participating MOs. 

Absorption of UV/VIS radiation in the solid state is different from UV/VIS absorption in the liquid or gaseous phase with respect to photophysical processes taking place in the crystal lattice and to the metallic, semiconductor (SC) or insulator properties of the absorbing solid (Bottcher, 1991). In crystals, multiple atomic or molecular orbitals are combined to form broad energy bands, i.e. a valence band (vb) fully occupied by electrons and a conduction band (cb) unoccupied or only partly occupied by electrons. Conduction bands and valence bands have different energetic positions relative to one another depending on the specific substrate. In a SC cluster, electronic transitions between the valence band and the conduction 

UV/VIS Radiation as a Specific Reagent Quantum Yield, Quantum Efficiency, Actinometiy and Photoreaction Kinetics 

The quantum yield 0 of a photophysical or photochemical event is a quantitative measure of the overall efficiency of this process (Braun et al., 1991). It is a unitless constant, which usually ranges from zero to one. However, some authors express 0 in units of mol einstein, which in fact is unit-less, because an einstein is defined as one mol of photons. Quantum yields greater than one indicate photo-induced chain reactions, which may involve radical species or photo-generated catalysis. Commonly used definitions of 0 are collected in Tab. 3-7. These definitions describe quantum yields of photophysical events and of photochemical reactions with regard to the reactant diminution or to the formation of the photoproduct Quantum yields may be dependent on the wavelength of the absorbed UV/VIS radiation, but many photochemical systems exist that have a constant quantum yield 0 over a defined wavelength range. Such chemical systems can be 

The various unimolecular photophysical processes may be envisaged in a rather illuminating way with the help of the Jablonski diagram shown in Fig- 

Emission or luminescence is referred to as fluorescence or phosphorescence, depending on whether it corresponds to a spin-allowed or a spin-forbidden transition, respectively. Similarly, radiationless transitions between states of the same multiplicity and of a different multiplicity are known as internal conversion (1C) and intersystem crossing (ISC), respectively. 

Photon-induced excitations of molecules also include vibrations of nuclei. This fact can be visualized with the aid of the Jablonski diagram (see Fig. 1.3). 

---

Fluorescence is a process that occurs after excitation of a molecule with light. It involves transitions of the outermost electrons between different electronic states of the molecule, resulting in emission of a photon of lower energy than the previously absorbed photon. This is represented in the Jablonski diagram (see Fig. 6.1). As every molecule has different energy levels, the fluorescent properties vary from one fluorophore to the other. The main characteristics of a fluorescent dye are absorption and emission wavelengths, extinction... 

The most populated energy state of chemical species at room temperature is the ground state. Once a molecule has absorbed energy in the form of electromagnetic radiation, it returns to the ground state, which can occur via several routes, some of which are shown in the Jablonski diagram (Fig. 9). 

The Jablonski diagram for thermally-activated delayed fluorescence is shown in Figure 4.12. 

Nickel B. (1997) From the Perrin Diagram to the Jablonski Diagram. 

Nickel B. (1998) From Wiedemann s discovery to the Jablonski Diagram EPA Newsletter 64, 19-72. 

The primary photophysical processes occuring in a conjugated molecule can be represented most easily in the Jablonski diagram (Fig. 1). Absorption of a photon by the singlet state So produces an excited singlet state S . In condensed media a very fast relaxation occurs and within several picoseconds the first excited singlet state Si is reached, having a thermal population of its vibrational levels. The radiative lifetime of Si is in the order of nanoseconds. Three main routes are open for deactivation ... 

More complex Jablonski diagrams can include vibrational sub-levels of the electronic states. Rotational sub-levels are not shown because they are so closely spaced as to form a near continuum. One important feature of the Jablonski diagram is that spin-allowed transitions only are shown by vertical lines any transition which has a horizontal component is spin forbidden. 

The wavy arrows in the Jablonski diagram of Figure 3.23, p. 50, correspond to the non-radiative transitions of internal conversion (ic) and the short arrows to intersystem crossing (isc) the former are spin allowed, as they take place between energy states of the same multiplicity the latter are spin forbidden and are therefore much slower. The rate constants of ic and isc span extremely large ranges because they depend not only on the spin reversal (for isc) but also on the energy gap between the initial and final states. 

Figure 4.62 shows the Jablonski diagram of the oxygen molecule, restricted to the first few states relevant to photo-oxidation processes. The phosphorescence Sj-Tq is very weak and is difficult to detect because it comes in the NIR at 1270 nm. There are however two other emissions at 634 nm and 703 nm which are due to a biphotonic process... 

Figure 5.9 The Jablonski diagram describing absorption and emission processes.

The Jablonski diagram is also called the electronic transitions diagram, since electrons of chromophores and/or fluorophores are responsible for the different described transitions. 

Dynamic quenching occurs within the fluorescence lifetime of the fluorophore, i.e., during the excited-state lifetime. This process is time-dependent. We have defined fluorescence lifetime as the time spent by the fluorophore in the excited state. Collisional quenching is a process that will depopulate the excited state in parallel to the other processes already described in the Jablonski diagram. Therefore, the excited-state fluorescence lifetime is lower in the presence of a collisional quencher than in its absence. 

De-excitation of a fluorophore occurs via different competitive mechanisms described in the Jablonski diagram. The global rate constant, which is the inverse of the fluorescence lifetime, can be considered equal to the sum of the different rates of the competitive mechanisms. Thus, one can write ... 

The various light-emission process are best described with reference to the Jablonski diagram. The diagram for a simple carbonyl compound is given in Fig. 5. The absorption process leads to the formation of excited singlet states (Sj, 2, etc.). A very rapid depopulation of the upper excited states occurs by internal conversion through vibrational relaxation processes. After deactivation to the first excited singlet state (Si), several procrases are possible for the molecule to reduce... 

---