Heavy Atom Effects on Intersystem Crossing

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... 

The bridged cyclohexenediones whose photoisomerization to cyclobutanediones have been observed are summarized in Table II where it can be seen that a wide variety of compounds undergo this reaction. In all cases where the point has been checked, quantum yields for disappearance of starting material and for formation of product were identical. The series of compounds, entries 13-17, were of interest since they involve unsubstituted (entry 13), tetrachloro (14, 16) and tetrabromo (15, 17) isomers where heavy atom effects on intersystem crossing rates might be manifested in quantum yield variations. As can be seen in the Table, the results do not permit such an interpretation. 

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. 

Quantum yields for fluorescence and phosphorescence from several 1-substituted naphthalenes as well as their observed phosphorescence lifetimes, T, are shown in Table II. The data for halogenated naphthalenes clearly demonstrate the heavy atom effect on intersystem crossing efficiency. Such data strongly suggest that substituent effects on photo-... 

A considerable amount of investigation has been carried out on the unsensitized photoisomerization of mono- and disubstituted stilbenes. The use of the heavy atom effect on intersystem crossing as an elegant tool for gaining mechanistic information has been very nicely demonstrated in this system (Dyck and McClure, 1962 Saltiel, 1964). It was found that, although irans-4-bromostilbene, in contrast to the parent hydrocarbon. 

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. 

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. 

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... 

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... 

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...

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... 

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. 

These results were interpreted with reference to an intramolecular heavy atom effect the authors concluded the carbene is formed from the excited singlet state of the molecule, in competition to intersystem crossing. A direct proof of this statement could, however not be obtained [24], It should be noted here that alternative explanations for the effect of halogen substitution on photolysis quantum yields have been put forward for the 4-haloanilines (see below). 

The photochemistry of polysilane derivatives may occur also via the triplet state 13, 30), on the basis of the observation of a weak-structured phosphorescence characteristic of a localized excited state for a number of polysilane derivatives. In principle, the halogenated additives could promote intersystem crossing via an intermolecular heavy-atom effect (39). Again, however, why the two structurally similar polysilanes should respond so differently to the presence of the additive is unclear. 

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