The Heavy Atom Effect

We have seen that transitions between electronic states of different spin quantum numbers are in principle forbidden by the law of conservation of angular momentum. In practice these transitions take place only through the compensation of two simultaneous changes in angular momentum represented by the orbital quantum number L and the spin quantum number S their sum J=L + S remains constant while L and S vary in opposite directions. 

The link between L and S is provided by the spin-orbit coupling which increases with the atomic number of the atoms electrons move faster around nuclei which carry large positive charges, so that the interaction between the electron currents and the related magnetic fields increases with atomic number. This is the basis of the important processes known as the heavy atom effects which enhance the rates of formally spin-forbidden radiative and non-radiative transitions. 

The heavy atom effect can rely on the presence of an atom of high atomic number either within the molecule itself (the internal heavy atom effect) or in the solvent (external heavy atom effect). In both cases the fluorescence 

The question then arises what is a heavy atom Br and I can be considered as heavy atoms in organic molecules, but Cl is a borderline case. Most metals qualify as heavy atoms and the photophysical properties of metal complexes are related to this increased spin-orbit coupling. Some noble gas heavy atoms like Xe show important external heavy atom effects. 

The presence of so-called heavy atoms such as bromine or iodine in either the parent molecule (internal heavy atom effect) or the solvent 

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Thus it can be seen that the heavy-atom effect, so useful in spectroscopy, can also be an important tool in photochemistry, not only to facilitate the study of a reaction mechanism, but also to control the major reaction product. This general system and other related 2w + lit photoaddition reactions will be considered in more detail in Chapter 10. 

On the other hand, the introduction of halide substituents at the C-2 and C-6 position decreases fluorescence quantum yields and gives a bathochromic shift of emission maxima. For example, bromine at the C-2 and C-6 position in compound 14b deteriorates fluorescence quantum yields from 0.95 (14a) to 0.45 and the emission maximum is red-shifted by 42 nm. Moreover, iodine at the C-2,6 position in compound 14d gives the similar bathochromic shift to bromine (14b, 44 nm) and more dramatic reduction in quantum yields (almost nonfluorescent, photophysical properties were interpreted as the heavy atom effect of halides on a BODIPY core skeleton. The bathochromic shift of BODIPY dyes without dramatic decrease in quantum yield was observed by the introduction of vinyl substituents at the C-2 and C-6 position. The extension of conjugation... 

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... 

The palladium(II) phthalocyanines are satisfactory singlet oxygen generators. Thus, f>A values for tetra-t-butylphthalocyanine ((27), with stated replacement for Mg) are 2H, 0.22 Zn, 0.34 Pd, 0.54, i.e., increasing in line with the heavy atom effect.180 For palladium(II) tetra-t-butylphthalocyanine ((27), Pd instead of Mg) in benzene at 290 K, 4>f= 0.048 and X = 0.49.180 However, palladium(II) 2,3-dihydroxy-9,16,23-tri-t-butylphthalocyanine was synthesized by a mixed condensation (1 9 mix of appropriate dinitriles) as a likely amphiphilic sensitizer, but did not show bioactivity in an enzyme assay, possibly because of aggregation.180... 

The heavy atom effect can show itself as the internal heavy atom effect, where incorporation of a heavy atom in a molecule will enhance S0 —> Tx absorption due to spin-orbit coupling. For example, 1-iodonaphthalene has a much stronger S0 —> Ti absorption than 1-chloronaphthalene... 

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 . 

It is obvious that there is a basic difference between assemblies of second-and third-row substituents (OCH3, F, Cl Category I) and those of higher rows (Br, I Category II). In the latter the signals are not shifted less downfield than expected (see below) by additional substitution, but rather upheld, culminating in a chemical shift of 8 = —292.3 in CI4 (281). This latter phenomenon has been designated the heavy-atom effect (57). 

Similarly, ab initio calculations on the thermal reaction of propene forming methyl-cyclopentane suggested a three-step biradical reaction with 1,4-biradical and 1,5-biradical as intermediates. Quantum-chemical calculations have been carried out for the cyclization of the neocarzinostatin chromophore cyclonona-l,2,3,5-tetraen-7-yne to 1,5-didehydroindene biradical. The degree of stereoselectivity of the Diels-Alder reaction of 2-methylfuran and maleic acid in water has been found to reduce significantly in the presence of heavy atoms. Taking into account the relatively low concentration (3.5-7 m) of heavy-atoms, and the rapid fall off of the heavy-atom effect with distance, these results show that a large portion of the Diels-Alder reaction occurs via diradical intermediates. " ... 

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... 

Values of kisc for singlet para-substituted phenylnitrenes are given in Table 11.2. The ISC rate constant for singlet p-bromo phenylnitrene is about seven times larger than that of parent 33s and the p-fluoro and p-chloro analogues. This difference is easily attributable to a small heavy atom effect. The heavy atom effect of iodine is even larger than that of bromine, as expected, and raises the ISC rate by more than a factor of 20, relative to parent 33s. 

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. 

The remainder of this section considers several experimental studies of reactions to which the Smoluchowski theory of diffusion-controlled chemical reaction rates may be applied. These are fluorescence quenching of aromatic molecules by the heavy atom effect or electron transfer, reactions of the solvated electron with oxidants (where no longe-range transfer is implicated), the recombination of photolytically generated radicals and the reaction of carbon monoxide with microperoxidase. 

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. 

For the photoionization of predominantly metal nd orbitals there is generally a significant increase in cross section as the principal quantum number n increases (172). This phenomenon is referred to as the heavy atom effect. ... 

The first two bands in the UPS of Ni(PF3)4 (Fig. 26) are remarkably similar to those of Ni(CO)4 and hence can be assigned to the 2 7 2 and 2 E ionic states, which are produced by electron elimination from the t2 and e MOs of predominantly Ni(3d) character (20). The UPS of Pd(PF3)4 and Pt(PF3)4 can be assigned analogously (Table XXVII and Fig. 26) (20). The heavy atom effect (172) is clearly operative in this triad in the sense that the relative intensities of the spectral peaks are in the order Pt > Pd > Ni. The next two bands in the UPS of Ni(PF3)4 correspond to the ionization of the metal-phosphorus a bonds of symmetries t2 and a,. The latter ionization is not detectable in the UPS of the heavier metal compounds and, presumably, is obscured by peaks of higher intensity. [Note that there are some differences between the preliminary reports (152, 182) and a subsequent full paper (20) regarding the spectra and assignments. For example, the weak 14.7 eV band of Ni(PF3)4 was not detected in one report (152). This band was detected in... 

Carbon-13 shifts of representative phosphines [364], phosphonium salts [365], phospho-nium ylides [365, 366], diphosphines [367], phosphonates [368], phosphorous and phosphoric acid derivatives [369] are summarized in Table 4.49. I3C shift data of some group V organoelement compounds are compared in Table 4.50. It turns out that a sp3 carbon nuclei of phosphines and arsines are shielded (0-25 ppm) relative to those of amines (30-60 ppm), as expected from the heavy atom effect, sp2 carbons of CC double bonds behave correspondingly, as shown for the triphenyl derivatives in Table 4.50, with... 

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. 

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