Rate, internal conversion intersystem crossing

A molecule could also relax from S, or T, to S0 by emitting a photon. The radiational transition Sj —> S0 is called fluorescence (Box 18-2), and the radiational transition T, —> S0 is called phosphorescence. The relative rates of internal conversion, intersystem crossing, fluorescence, and phosphorescence depend on the molecule, the solvent, and conditions such as temperature and pressure. The eneigy of phosphorescence is less than the energy of fluorescence, so phosphorescence comes at longer wavelengths than fluorescence (Figure 18-14). 

In order to elucidate a mechanism, one must first consider the nature of the states initially formed by photoexcitation as well as the natures of other expected states eventually populated by internal conversion/intersystem crossing. Although it is by no means universally true, many transition metal complexes, when excited, undergo efficient relaxation to a bound, lowest energy excited state (LEES) or an ensemble of thermally equilibrated LEESs from which the various chemical processes lead to photoproducts. In such systems, the simplest model of which is illustrated by Figure 9, one can comfortably apply transition state theory to the rates and consider pressure effects in terms of the mechanisms of the individual decay LEES processes. In this case, the quantum yield of product formation would be defined by the ratio of rate constants by which the various chemical and photophysical paths for ES decay are partitioned. For Figure 9, in the absence of a bimolecular quencher Q, this would be... 

Fig. 14. Schematic representation of energy levels and transitions for fluorescence and related processes kic, rate constant for interval conversion fcF, rate constant for fluorescence fcISC, rate constant for intersystems crossing fc[cp> rate constant for internal conversion from triplet state kp, rate constant for phosphorescence S, energy level for the first excited singlet state after solvent rearrangement for a polarity probe in a polar solvent.

From kinetic considerations each can be further subdivided according to observed values of rate constants fcIC, the rate constant for internal conversion and kIsc, the rate constant for intersystem crossing. 

Rate constant for internal conversion Rate constant for intersystem crossing Rate constant for non-radiative decay Observed decay rate constant... 

Fluorescence quantum yields (Of), lifetimes (tj), radiative rate constant ky.), rate constants for intersystem crossing (/ isc) and for internal conversion (kjc) intersystem crossing quantum 5ueld (Ot), bimolecular rate constants for triplet quenching by O2 ( 02) and ground state (kos), for 1 in various media. Data from Ref. [2]... 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

Thus we see that we have three processes which can compete for deactivation of the excited singlet fluorescence, internal conversion, and intersystem crossing. If we increase the rate of the latter by adding a heavy atom, this should result in a decrease or quenching of the fluorescence intensity ... 

The quantum efficiency of fluorescence of a molecule is decided by the relative rates of fluorescence, internal conversion and intersystem crossing to the triplet state. Up to the present time it has proved impossible to predict these relative rates. Thus, whilst it is now possible to calculate theoretically the wavelengths of maximum absorption and of maximum fluorescence of an organic molecule, it remains impossible to predict which molecular structures will be strong fluorescers. Design of new FBAs still relies on semi-empirical knowledge plus the instinct of the research chemist. 

The much larger energy difference between Si and S0 than between any successive excited states means that, generally speaking, internal conversion between Si and S0 occurs more slowly than that between excited states. Therefore, irrespective of which upper excited state is initially produced by photon absorption, rapid internal conversion and vibrational relaxation processes mean that the excited-state molecule quickly relaxes to the Si(v0) state from which fluorescence and intersystem crossing compete effectively with internal conversion from Si. This is the basis of Kasha s rule, which states that because of the very rapid rate of deactivation to the lowest vibrational level of Si (or Td, luminescence emission and chemical reaction by excited molecules will always originate from the lowest vibrational level of Si or T ... 

The competing intramolecular photophysical processes that can occur from Si(v0) are fluorescence, intersystem crossing and internal conversion, with first-order rate constants of kf, kisc and kic, respectively (Figure 3.3). 

When a molecule is in the S v = 0) state, fluorescence emission is only one of the several competing physical processes by which the molecule can return to the ground state. A molecule in Si(v = 0) can undergo fluorescence, intersystem crossing or internal conversion, which have rate quantum yields < >f, (j) sc and respectively and ... 

Understand the importance of the overlap of vibrational probability functions and the energy gap law in determining the rate of internal conversion and intersystem crossing. 

In Chapters 2 and 4, the Franck-Condon factor was used to account for the efficiency of electronic transitions resulting in absorption and radiative transitions. The efficiency of the transitions was envisaged as being related to the extent of overlap between the squares of the vibrational wave functions, /2, of the initial and final states. In a horizontal radiationless transition, the extent of overlap of the /2 functions of the initial and final states is the primary factor controlling the rate of internal conversion and intersystem crossing. 

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