Fluorescence heavy atoms

In addition to the processes that can compete with fluorescence within the molecule itself, external actions can rob the molecule of excitation energy. Such an action or process is referred to as quenching. Quenching of fluorescence can occur because the dye system is too warm, which is a very common phenomenon. Solvents, particularly those that contain heavy atoms such as bromine or groups that ate detrimental to fluorescence in a dye molecule, eg, the nitro group, ate often capable of quenching fluorescence as ate nonfluorescent dye molecules. 

The dosimeter can detect various polynuclear aromatics at the pph level after 1 hour of exposure. It has been shown that the RTF of aza-arenes can he enhanced by using mercury(II) chloride as a heavy atom (21). Also, sensitized fluorescence spectrometry with a solid organic substrate can be used to detect trace amounts of polynuclear aromatic compounds (22). 

Thus we see that we have three processes which can compete for deactivation of the excited singlet fluorescence, internal conversion, and intersystem crossing. If we increase the rate of the latter by adding a heavy atom, this should result in a decrease or quenching of the fluorescence intensity ... 

The ratio of the fluorescence quantum efficiency in the absence of the heavy atom to that in the presence of the heavy atom is given by... 

The best evidence for a charge-transfer exciplex (hetero excimer) has been provided by Thomaz and Stevens.<148,149) They note a reduction in fluorescence yield of pyrene with increasing heavy-atom concentration and proposed the following set of reactions to explain their results ... 

Figure 5.16. Plot of data for the external heavy-atom quenching of pyrene fluorescence in benzene at 20°C. Polaro-graphic half-wave reduction potentials Ein are used as a measure of the electron affinity of the quencher containing chlorine (O), bromine ( ), or iodine (3). From Thomaz and Stevens<148) with permission of W. A. Benjamin, New York.

Midinger and Wilkinson<54> have used flash photolysis and fluorescence quenching by heavy atoms to determine the intersystem crossing efficiencies of anthracene and a number of its derivatives. As discussed in Section 5.2b, heavy atoms present as molecular substituents or in the solvent serve to promote multiplicity forbidden transitions. When anthracene is excited the following processes can occur ... 

Based on analogies we have cited, the kinetic scheme proposed for heavy-atom fluorescence quenching is reasonable and would predict the following relationship for fluorescence quenching ... 

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... 

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). 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

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... 

In spite of the heavy atom, compound (32) is sufficiently fluorescent for this to be used as an analytical tool to examine localization and pharmacokinetics. In EMT-6 murine tumors, (32) localizes initially on lysosomes, with selectivity for tumor over surrounding normal tissue, and with evidence for apoptotic cell kill.137 Fluorescence studies using a hamster cheek pouch model show a maximum emission in 2-3 h, with selectivity for the tumor (x 1.5 over normal tissue) after 24 h the photosensitizer is no longer detectable.138 Lutetium texaphyrin (32) has been compared... 

Unless otherwise specified, both qy and x refer to the same process, indicated by f for fluorescence and p for phosphorescence. For naphthalene, qy is for fluorescence and T is for phosphorescence. bNote heavy atom effect in phosphorescence. 

Finally, in many of the perturbation calculations of the effect of substituents and other structural changes, an important tacit assumption is made and it is far from obvious that it is always fulfilled. As already discussed, the physical argument on which the calculation is based is that the value of the initial slope, or the height of a small barrier along the way, determine the rate at which the photochemical reaction occurs. However, the experimental value with which comparison is made usually is not the reaction rate but the quantum yield, which of course also depends on rates of other competing processes and these may be affected by substitution as well. For instance, the rate at which fluorescence occurs is related to the absorption intensity of the first transition, the rate of intersystem crossing may be affected by introduction of heavy atoms... 

Assign differences in fluorescence quantum yield to differences in the electronic configuration of SI, substituent effects, molecular rigidity and the presence or absence of heavy atoms. 

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...

The heavy atom effect offers certain possibilities for fluorescence lifetime-based analytical exploitation which will not be discussed here. The reader is referred to [38] for more details. 

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 . 

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