Heavy-atom effect internal

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 . 

In general, substitution with electron-donating groups induces an increase in the 

10) The energy difference between Si and the accepting triplet state has to be small enough for intersystem crossing to compete effectively with fluorescence. This is not the case with perylene, as demonstrated by the fact 

The absorption and emission characteristics of phenols and aromatic amines are pH-dependent. These aspects will be discussed in Section 4.5. 

In general, substitution with electron-donating groups induces an increase in the molar absorption coefficient and a shift in both absorption and fluorescence spec- 

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The effect of the substitution of a heavy-atom directly onto the nucleus of aromatic compounds (internal heavy-atom effect) on intercombinational radiative and nonradiative processes can be seen by examination of experimental data obtained for naphthalene and its derivatives. The data obtained by Ermolaev and Svitashev<104) and analyzed by Birks(24) to obtain individual rate constants for the various processes are collected in Table 5.9. 

Table 5.9. Internal Heavy-Atom Effect on Naphthalene<24 1 4> ...

Plummer and Ferree(89) have utilized the photoaddition of 5-bromo-acenaphthylene to cyclopentadiene to compare external and internal heavy-atom effects in the acenaphthylene system ... 

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... 

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

Table 4.4 The effect of the internal heavy atom effect on the fluorescence efficiency of naphthalene and its derivatives. Fluorescence quantum yields determined in solid solution at 77K...

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. 

The intensity of singlet-triplet transitions can be increased by the external and internal heavy-atom effect. It has been noticed by Kearns 8) that the Tn.n - So transitions were enhanced by a factor of about 2 on passing from an ordinary low-temperature glass, such as a 2 1 1 mixture of ether, ethanol, and toluene, to a heavy-atom glass, such as a 2 2 1 1 mixture of ethyl iodide, ether, ethanol, and toluene. 

Figure 3.8 Heavy atom effect in the T - S0 transitions in halonaphthalenes. A. External heavy atom effect 1-chloronaphthalene with (a) ethyl iodide, (b) xenon (143 atml, (c) oxygen (30 atm) and (d) pure 1-chloronaphthalene B. Internal heavy atom effect (ej 1-chloronaphthalene, (f) 2-iodonaphthalene, (g) 1-iodonaphthalene.

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 introduction of trifluoromethyl (H2TCF3P) or phenyl groups (H2TPP) in the four meso-positions of porphin leads to relatively small and nearly identical blue shifts of the Q and B bands. However, the fluorescence lifetime is reduced by nearly one order of magnitude from H2TPP to H2TCF3P, and the fluorescence quantum yields follow the same trend (34). This is consistent with the internal heavy-atom effect to be discussed in the next section. 

The extinction coefficient of triplet benzophenone has been remeasured as 7220 320 dm moF cm at 530 nm and this has been used to measure the triplet yield for zinc tetraphenylporphine. The sensitization of naphthalene by benzophenone has been examined in different hydrocarbon or ethanol-ether glasses.Internal heavy-atom effects on the T, states of monochloro-... 

On the other hand, it is known that substitution by a heavy atom such as Br or I (internal heavy atom effect) decreases rT. This is also reflected by the shorter rT and rp values of 4-bromo- and 4-iodostilbene compared to others, for example, carbonyl- or nitro-substituted stilbenes (Tables 15a and... 

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