Intersystem crossover

Such a process is called an intersystem crossover, since the molecule changes from triplet to singlet. Letting M represent a molecule M with two unpaired electrons, i.e., a triplet [T] state, and M the d same molecule... 

Here the symbol M M o represents the intermediate resonance hybrid II) at Q s Qo- An equivalent symbol for M <-> could be M -. Another notation for this intersystem crossover process is obtained if one denotes singlet [5] by f j and triplet T] by f, ... 

Note that in this particular intersystem crossing process the nuclear coordinate Q is less than Q0 in both initial and final states. We call eq. (7-12) nonadiabatic intersystem crossover, since the process initiates on the E(Y) curve and ends up on the (11) curve. We call the process (7-9) an adiabatic intersystem crossover, since it follows curve E ) only. 

If a chemical reaction passes through a biradical intermediate, intersystem crossover may occur because of the localization and consequent near degeneracy. An example is provided by the reaction of ground state... 

Intersystem crossover processes between singlet and triplet states are well known in photochemistry. Intersystem crossing processes have been of importance in interpreting benzene photochemistry. Anticipating that... 

The pyridine-sensitized cis-trans isomerization of cw-2-butene in the gas phase has been reinvestigated recently.75 If the lowest singlet states of the pyridine are assumed not to interact with the cw-2-butene in its ground state, the quantum yields of intersystem crossovers of the pyridine from these lowest singlet states could be measured by Cundall s technique.18,26 1,264 As a matter of fact, there are now some doubts about the validity of Cundall s technique for pyridine since it has been suggested that a complex between excited pyridine and the c/s-2-butene may be formed and provide a path of deactivation of the pyridine molecule. With the assumptions of Cundall, 26a 26a the quantum yield of... 

Pyridazine (1,2-diazine) exhibits a weak fluorescence in solutions at room temperature,73 but no phosphorescence appears in hydrogen and rare gas matrices at 4.2°K. The intersystem crossover was shown to be 103-105 times as slow (at low temperatures) in pyridazine as in pyrazine or pyrimidine.84,85 Very efficient radiationless processes of deactivation must be considered again. 

It is unsafe to generalize at this point, so that for purposes of record we keep (3) in the steady-state equations. We refer to (1) as an intersystem crossover and to (3) as an internal conversion. These uses of these terms are widely but not universally accepted. 

The mechanism just discussed illustrates the difficulties which may be encountered in discussing intersystem crossovers and internal conversions. In the gas phase, among those molecules carefully studied the following generalizations should be noted ... 

Another point may well be mentioned. If the processes already referred to are accompanied by parallel dissociations or rearrangements, there will usually be a decided dependence both of and of intersystem crossover on the wavelength of the incident light. 

Sigal studied the benzene d-6 sensitized isomerization of butene-2 at pressures below 0.1 mm., where the interval between collisions is long relative to the fluorescence lifetime. The butene-2 isomer concentration was monitored by long-path infrared spectrometry. The data are consistent with a kinetic scheme based on collisions being rate determining for intersystem crossover at low pressures. 

Such chemical evidence as exists, plus a strong wavelength dependence of fluorescence yield from benzene and the low yield of triplet state formation at 2400 A. all point to a competing process whose importance at 2537 A. cannot for the moment be estimated. One must state, therefore, the evidence for the effect of colhsions on the rate of crossover of benzene from excited singlet to excited triplet states is conflicting. In solid matrix, vibrations definitely play an important role in intersystem crossover for benzene. ... 

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). 

A laser flash photolysis study of the sensitized and heavy atom induced production of the acenaphthylene triplet in solution emphasizes the importance of heavy atom effects on intersystem crossing. The role of the T2 state in 9,10-dibromoanthracene excited singlet-triplet crossover has been assessed by the use of 1,3-octadiene which for this system is a selective triplet quencher for triplet states higher than the lowest, T-. 0 The T2 state mediates intersystem crossing but does not participate in any T-T annihilation process. Triplet state properties of 9-acetoxy-10-phenylanthracene derivatives have also been reported in detail. 

Ohnesorge [23] have postulated that this is the result of a crossover to a ta/CgV spin state during the lifetime of the initial excited state of the iron(II) chelates. Such a spin state is paramagnetic and undergoes rapid intersystem crossing and rapid internal conversion to the ground state before emission can occur. 

With respect to lifetimes of the light-induced HS states of the order of minutes to days, LIESST is restricted to iron(II) spin-crossover compounds with a small zero-point energy difference, between the HS and the LS state. From the point of view of the double intersystem crossing step, LIESST is more general than that. It can also be observed for LS systems provided sufficiently fast excitation and detection methods are being used for monitoring the (in this case) much faster decay of the light-induced HS state (see below). 

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