Fluorescence quantum yield, substituent effect

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... 

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. 

Substituent groups have a marked effect on the fluorescence quantum yield of many compounds. Electron-donating groups such as -OH, -NH2 and -NR.2 enhance the fluorescence efficiency, whereas electron-withdrawing groups such as -CHO, -C02H and -N02 reduce the fluorescence quantum yield, as shown by naphthalene and its derivatives in Table 4.3. 

Table 4.3 The effect of substituent groups on fluorescence efficiency of naphthalene and its derivatives. Fluorescence quantum yields measured in fluid solution at room temperature...

Substituents have considerable influence on emission characteristics of aromatic compounds. Heavy atom substituents tend to reduce the fluorescence quantum yield 4>f in favour of phosphorescence emission f. In halogen series the effect increases in the order F < Cl < Br < I. In Table 5.1 are recorded experimental data for halogen substituted Naphthalenes. 

From a systematic study of bichromophoric compounds 97-99, the importance of substituents and solvent polarity in intramolecular deactivation processes of photoexcited anthracenes by nonconjugatively tethered, and spatially separated, aromatic ketones in their electronic ground state is apparent. For 97a-d, in which the electron acceptor properties of the aromatic ketone moiety have been varied by appropriate p-substitution of the phenyl ring (R is methoxy, H, phenyl, and acetyl, respectively), the longest-wavelength absorption maximum band lies at 388 nm, i.e., any ground state effects of substitution are not detectable by UV spectroscopy. Also, the fluorescence spectra of 97a-d in cyclohexane are all related to the absorption spectra by mirror symmetry. However, the fluorescence quantum yields for 97a-d in cyclohexane dramatically are substituent dependent (see Table 19), ranging from 0.20 for the methoxy derivative to 0.00059 for the acetyl compound [33,109],... 

In a study on substituted terthiophenes, if methoxy substitution induces a bathochromic shift of the absorption and emission bands, it has little or no effect on fluorescence quantum yields and lifetimes of the excited states (93JPC513). The bathochromic shifts induced by the end disubstitution by bromine atoms can be rationalized as previously described for the methoxy groups and thus this substituent effect is only limited at the five first terms of the oligothiophene series. 

Only the nitro-substituted oligothiophenes display large bathochromic shifts, large Stokes shifts, high fluorescent quantum yields, and long lifetimes for excited states. As for the other substituents, the trend is mostly noticeable for the short oligomers like terthiophenes and seems to disappear for sexithiophenes. As can be inferred from their solvatochromic effect, an intramolecular charge transfer takes place in the excited states of these molecules. 

Aryl substitution has a dramatic effect on the lifetimes and quantum yields for the xanthyl cations. Lifetimes vary from the subnanosecond range for the m-OMe-substituted cation to the tens of nanoseconds for the 9-phenyl- and p-F-substituted cations. A similar trend is seen with the fluorescence quantum yields, those cations that exhibit short lifetimes are also more weakly fluorescent [11,30]. Implications of the substituent effects on cation photophysical parameters will be discussed in Section IV. 

Absorption and emission spectra of 20 trans-4,4 -disubstituted stilbenes have been measured in four solvents cyclohexane (CH), chlorobenzene (CB), 2-butanone (methylethylketone, MEK), anddimethylsulfoxide (DMSO) at room temperature [24]. Fluorescence quantum yields (4>f) and fluorescence lifetimes (Xf) have been measured for these stilbenes. Substituent effects on the Stokes shift were described by a spectroscopic Hammett equation... 

Table3.1 Sensitivity p) ofthe Stokes shift, fluorescence decay rate constant, fluorescence quantum yield, and radiative deactivation rate constant to intramolecular substituent effects for two different groups of trans-4,4 -disubstituted stilbenes [87].

The substituents may be essential for the chemical reaction of the hydrazide oxidation itself the excitation step may depend on them or they may affect the fluorescence efficiency of the product. It would appear that all three influences are at work both quantitatively and qualitatively. For example, substitution may result in general instability towards oxidation as in the cases of 3,6-diamino- or 3-amino-6-methoxy phthalhydrazide [8]. The ease of oxidation on the chemiluminescent pathway is certainly significant, although it is difficult to separate this from the influence on the population of the excited state. An example of this effect is that of the 4-dialkylamino-phthalhydrazides. Differences in chemiluminescence quantum yields of up to 600-fold were observed, whereas the fluorescence quantum yields of the corresponding 4-dialkyl-amino phthalates only differ by a factor of about 10 [9]. 

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... 

The effects of substituents and solvent polarity on the luminescence properties also have been evaluated for of a series of bichromophoric anthronyl-substituted anthracenes 98 and 99. It can be concluded from the quantum yield data summarized in Table 20 for spiro-substituted compounds 98a e that, dependent on solvent polarity, two different modes of intramolecular interactions between the electronically excited anthracene chromophore and the ground state ketone typically are operative, and both types of interaction result in fluorescence quenching. In nonpolar solvents, fluorescence quenching apparently involves endothermic intramolecular... 

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