Intersystem crossing radiationless processes

Figure 1. JabJonski-type diagram of the lowest energy levels of electron donor-acceptor molecules formally linked by a single bond which show dual fluorescence phenomenon. D-A, (D A), (D+-A ), (D -A ) and (D-A) denote the ground state, the primary excited and charge-transfer (CT) singlet states, and CT and locally excited triplet states, respectively. The arrows correspond to the radiative (absorption, A, fluorescence, F, and phosphorescence, Ph) and the radiationless (internal conversion, IC, and intersystem crossing, ISC) processes.

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. ... 

FIGURE 7.4 Modified Jablonski diagram showing transitions between excited states and the ground state. Radiative processes are shown by straight lines, radiationless processes by wavy lines. IC = internal conversion ISC = intersystem crossing, vc = vibrational cascade hvf = fluorescence hVp = phosphorescence. 

The radiationless transition between two states of same spin is called internal conversion, the one occuring with inversion of spin being termed intersystem crossing. In both processes the excess energy is liberated as heat. All these transitions between different electronic states are customarily preceded by vibrational relaxation, i.e. the deactivation from a higher vibronie level to the v0-level of the same electronic state (Fig. 5). 

Radiationless transitions (internal conversion and intersystem crossing) between electronic states are isoenergetic processes and are drawn as wavy arrows from the v = 0 level of the initial state to a vibrationally-hot (v > 0) level of the final state. 

The same experiment can be carried out quantitatively. By taking into account radiationless processes, namely, internal conversion fcIC, intersystem crossing isc, and bimolecular quenching kQ[Q] with a quencher Q, the time-dependent concentrations of the donor D and the acceptor A in the excited singlet state Si, [Ds,] and [As,] can be expressed as follows ... 

A, 3A and A are molecules in first excited singlet state, molecules in triplet state and in the ground state respectively. In radiationless processes such as internal conversion and intersystem crossing the excess energy is lost to the environment as thermal energy. Some of the unimolecular processes are represented by a Jablonski diagram in Figure 5.1. Radiative transitions me denoted... 

Alternatively, the molecule could cross from S, into an excited vibrational level of T,. Such an event is known as intersystem crossing (ISC). After the radiationless vibrational relaxation R3, the molecule finds itself at the lowest vibrational level of T,. From here, the molecule might undergo a second intersystem crossing to S0, followed by the radiationless relaxation R4. All processes mentioned so far simply convert light into heat. 

Radiationless transitions among electronic states of molecules represent a class of relaxation processes that are electronic in nature. The general term electronic relaxation appears to be appropriate for these processes,23 but it is convenient to divide those transitions involving a change in the bound electronic states of a molecule into two categories Transitions between states of the same multiplicity, referred to as internal conversion, and transitions between states of different multiplicity, referred to as intersystem crossing. Although there are several early experimental... 

As an example of the effect of level shifts in the crystalline state, as just described, consider the observed rates of radiationless transitions in anthracene.45 The first excited 1BSu of the isolated anthracene molecule is located about 600 cm-1 above the second triplet state. Hence, 8 < vv and the intersystem crossing process is quite rapid at room temperature. The fluorescence quantum yield is about 0.3 for this molecule in the gas phase and in solution. In the crystal the first excited singlet state is red shifted (from the gas level) by about 1880 cm- while the second triplet state is hardly affected, so that in this case the energy gap between those two states increases in the crystal. Then the coupling term, v, is smaller in the crystalline state than in solution, thereby leading to a decrease in the rate of the intersystem crossing. The result is that the fluorescence yield in the crystal is close to unity.40... 

The concept of radiationless transitions, namely internal conversion and intersystem crossing1 is one which is widely used in photochemistry today. However, the precise nature of the processes involved is elusive since direct measurement of the yields of radiationless transitions is impossible with the exception of those intersystem crossings between first excited singlet states and lower-lying triplets where the triplet state can be quantitatively estimated by chemical or physical means. In all other cases, the accepted practice is to sum the quantum yields of processes which can be estimated directly, such as decomposition and emission, and attribute those excited molecules not accounted for by such processes to radiationless transitions. 

Spin equilibria are thermal intersystem crossing processes. The ground state and the excited state lie within a few hundred wavenumbers of each other and both are thermally populated. There are two photophysical processes in excited states related to the dynamics of thermal spin equilibria. One is the radiationless deactivation of an excited state to a ground state of different spin multiplicity. The other is intersystem crossing between excited states. 

The results obtained from thermal spin equilibria indicate that AS = 1 transitions are adiabatic. The rates, therefore, depend on the coordination sphere reorganization energy, or the Franck-Condon factors. Radiationless deactivation processes are exothermic. Consequently, they can proceed more rapidly than thermally activated spin-equilibria reactions, that is, in less than nanoseconds in solution at room temperature. Evidence for this includes the observation that few transition metal complexes luminesce under these conditions. Other evidence is the very success of the photoperturbation method for studying thermal spin equilibria intersystem crossing to the ground state of the other spin isomer must be more rapid than the spin equilibrium relaxation in order for the spin equilibrium to be perturbed. 

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