Non radiative intersystem crossing

Then, the carotenoid triplet state decays via non-radiative intersystem crossing ... 

Deactivation of the State in Solids. In frozen solutions (say, at 77 K) or in a dry solid at room temperature, Ti molecules are not quenched instead, they return to the So state by two slower pathways (1) phosphorescence emission, and (2) non-radiative intersystem crossing. In the latter pathway, excess electronic energy is converted to vibrational energy as the Tj molecule crosses over to some higher vibrational level (GV ) of the ground state. This is faster than the internal conversion pathway for Si molecules because much less electronic energy is involved. 

Thus we see that in molecules possessing ->- 77 excited states inter-combinational transitions (intersystem crossing, phosphorescence, and non-radiative triplet decay) should be efficient compared to the same processes in aromatic hydrocarbons. This conclusion is consistent with the high phosphorescence efficiencies and low fluorescence efficiencies exhibited by most carbonyl and heterocyclic compounds. 

The possible fate of excitation energy residing in molecules is also shown in Figure 2. The relaxation of the electron to the initial ground state and accompanying emission of radiation results in the fluorescence spectrum - S0) or phosphorescence spectrum (Tx - S0). In addition to the radiative processes, non-radiative photophysical and photochemical processes can also occur. Internal conversion and intersystem crossing are the non-radiative photophysical processes between electronic states of the same spin multiplicity and different spin multiplicities respectively. 

In solution at room temperature, non-radiative de-excitation from the triplet state Ti, is predominant over radiative de-excitation called phosphorescence. In fact, the transition Ti —> S0 is forbidden (but it can be observed because of spin-orbit coupling), and the radiative rate constant is thus very low. During such a slow process, the numerous collisions with solvent molecules favor intersystem crossing and vibrational relaxation in So-... 

The natural radiative lifetime is the longest (average) lifetime of an excited molecule. This lifetime is seldom observable in practice because there are other deactivation processes which compete with the luminescence emission. These can be intramolecular, non-radiative transitions (internal conversion or intersystem crossing) or intermolecular quenching processes these are considered in the next sections. 

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. 

Upper excited states are extremely short-lived. When the molecule is promoted to an excited singlet state beyond S1 the non-radiative deactivation by internal conversion is much faster than the spin-forbidden intersystem crossing to any triplet state. Therefore, the first excited singlet state is formed with near unit quantum yield. If an upper triplet state could be reached, it would also deactivate very rapidly to T1 and no singlet excited state would be formed. The extremely short lifetime of all upper excited states Sb(m>1) and Tb(w>1) means that luminescence emission and chemical reaction are, as a rule, not observed from such states. There are some exceptions to this rule, but there are many more mistaken reports of chemical reactions from short-lived upper excited states. Any such report... 

Figure 4.74 Jablonski diagram of a metal complex with three d electrons. The wavy arrows show non-radiative transitions. Note that intersystem crossings between higher states can be important as a result of the heavy atom effect...

Some fluorescence lifetimes are observed in ps times, although these are unusual cases. In organic molecules the Sj—S0 fluorescence has natural lifetimes of the order of ns but the observed lifetimes can be much shorter if there is some competitive non-radiative deactivation (as seen above for the case of cyanine dyes). A few organic molecules show fluorescence from an upper singlet state (e.g. azulene) and here the emission lifetimes come within the ps time-scale because internal conversion to S and intersystem crossing compete with the radiative process. To take one example, the S2-S0 fluorescence lifetime of xanthione is 18 ps in benzene, 43 ps in iso-octane. 

Figure 1. Jablonski diagram for a four-level system depicting absorption, non-radiative (wavy arrows) and radiative processes between singlet (total spin S = 0) and triplet (total spin S = 1) states. Emissions respect Kasha s rule. IC internal conversion. ISC intersystem crossing.

Non-radiative processes can also be distinguished on the basis of the spin multiplicity of the initial and final electronic states. Internal conversion (IC) is the non-radiative crossover between two states of identical multiplicity, while intersystem crossing (ISC) is a process in which spin is not conserved. In both instances, crossover between the states is isoenergetic, regardless of the multiplicity. Subsequent vibrational relaxation (VR) occurs to release excess vibrational energy (see Figure 2.12). 

---