Crossing, intersystem heavy atom effect

Interestingly, it was possible to probe the spin-forbidden component of the tunneling reaction with internal and external heavy atom effects. Such effects are well known to enhance the rates of intersystem crossing of electronically excited triplets to ground singlet states, where the presence of heavier nuclei increases spin-orbit coupling. Relative rates for the low-temperature rearrangements of 12 to 13 were... 

Frosch(84,133) have explained the external heavy-atom effect in intersystem crossing by postulating that the singlet and triplet states of the solute, which cannot interact directly, couple with the solvent singlet and triplet states, which themselves are strongly coupled through spin-orbit interaction. Thus the transition integral becomes<134)... 

A heavy-atom effect on the photocycloaddition of acenaphthylene to acrylonitrile has also been observed.<68) The effect of heavy atoms in this case is seen as an apparent increase in the quantum yield of product formation in heavy-atom solvents as opposed to cyclohexane (the time to achieve about 42% reaction in cyclohexane is greater than that required to produce the same conversion in dibromoethane by a factor of ten). An increase in the rate of acenaphthylene intersystem crossing due to heavy-atom perturbation was proposed to explain this increase in reaction rate. 

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

For the photodiagnostic use of these compounds, a high quantum yield of fluorescence, r, is desirable. The metal complexes of the common first-row transition metals are not suitable, because they show very low 4>f values. On the other hand, porphyrin complexes of d° and d10 elements show appreciable fluorescence, although generally less than that of the metal-free compounds, presumably because of the heavy-atom effect (e.g., TPP ZnTPP, Table 5). The further operation of the heavy-atom effect, which increases the rate of intersystem crossing (/cisc) by... 

Scheme 8 gives species with extended 109 or starbust -type structures, which are strongly luminescent even with the large ligand L = PCy3.85 The diphenylfluorene derivative shows a remarkable heavy atom effect on the intersystem crossing rate.78... 

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

Intersystem crossing (i.e. crossing from the first singlet excited state Si to the first triplet state Tj) is possible thanks to spin-orbit coupling. The efficiency of this coupling varies with the fourth power of the atomic number, which explains why intersystem crossing is favored by the presence of a heavy atom. Fluorescence quenching by internal heavy atom effect (see Chapter 3) or external heavy atom effect (see Chapter 4) can be explained in this way. 

In general, the presence of heavy atoms as substituents of aromatic molecules (e.g. Br, I) results in fluorescence quenching (internal heavy atom effect) because of the increased probability of intersystem crossing. In fact, intersystem crossing is favored by spin-orbit coupling whose efficiency has a Z4 dependence (Z is the atomic number). Table 3.3 exemplifies this effect. 

However, the heavy atom effect can be small for some aromatic hydrocarbons if (i) the fluorescence quantum yield is large so that de-excitation by fluorescence emission dominates all other de-excitation processes (ii) the fluorescence quantum yield is very low so that the increase in efficiency of intersystem crossing is relatively small (iii) there is no triplet state energetically close to the fluorescing state (e.g. perylene)10 . 

The heavy-atom effect on intersystem crossing, due to spin-orbit coupling, is very well known.118-120 On the other hand, very little is known about internal conversion. This process is mostly considered to be of negligible importance, but as was pointed out by El-Sayed121 there exists little rate data to support this assumption. As far as xanthene dyes and related dyes are concerned, results indicate the occurrence of internal conversion.7,93,103,110 122... 

Several examples of heavy atom quenching of aromatic hydrocarbon states are known for example, carbon tetrabromide is an efficient quencher of the fluorescence of anthracene167 and carbon tetrachloride behaves similarly with p-terphenyl.188 Since quenching results in formation of the triplet state, it has been possible to use the heavy atom effect to measure intersystem crossing efficiencies ( ). Because of the elegance of this technique 169 and the importance of the results in photochemistry, we shall cover it in some detail. 

After exclusion of a predissociation process responsible for the lifetime shortening in complexes of benzene with noble gases, we consider the external heavy-atom effect on the intersystems crossing rate as the origin of the lifetime shortening [42]. The strong decrease of the lifetime in the... 

Besides the spin-forbidden processes of Sections VII-XII, there are a number of other spin-forbidden processes of interest. Intersystem crossing may occur in certain predissociation phenomena and in P-type delayed fluorescence.198 Also of interest are the heavy atom effect and the direct interaction of radiation with spin. 

Now, in aromatic hydrocarbons intramolecular skeletal vibrations, rather than C—H vibrations, dominate the vibronic coupling contribution to the term J m = — . Furthermore, intermolecular vibrations will have negligible effect on the coupling of the electronic states of interest. Thus, in the case of internal conversion, where the (relatively large) matrix elements are solely determined by intramolecular vibronic coupling, no appreciable medium effect on the nonradiative lifetime is to be expected. On the other hand, intersystem crossing processes are enhanced by the external heavy atom effect, which leads to a contribution to the electronic coupling term. 

The independence of luminescence quantum yields on excitation wavelength is known as Vavilov s rule. There are however many exceptions to this rule, in particular for molecules which contain heavy atoms such as Br and I, or metals (e.g. organometallic complexes). The heavy atom effect makes intersystem crossings more efficient and these can compete with internal conversions. 

Figure 4.25 Jablonski diagram of the heavy atom effect on photochemical reactivity. If excitation to S2 (hv2) is followed by intersystem crossing (isc) to T2, the quantum yield of reaction R decreases at higher excitation energies, ic = internal conversion, a = absorption, f = fluorescence, p = phosphorescence...

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...

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