Excimer fluorescence, compounds

Replacing the nitrile group by a benzothiazole produces an important subclass of fluorescent compounds represented by thioflavin T (25, Fig. 10). It is not clear if this compound undergoes deactivation via intramolecular rotation that would meet the criterion for a molecular rotor. The steady-state absorption and emission properties of thioflavin T has been attributed to micelle formation [53, 54], dimer and excimer formation [55, 56], and deactivation through intramolecular rotation [57]. 

Compounds Exhibiting Excimer Fluorescence in Solutions at Room Temperature11... 

Chandross and Ferguson64 find that the absorption spectra of dimers, produced65 by photolytic cleavage of photodimers of anthracene and mono-derivatives in a rigid methylcyclohexane glass at 77°K, are consistent with a symmetrical sandwich configuration these dimers also emit the characteristic excimer fluorescence. On the other hand, it is necessary to assume a 60° rotation of one component about the intermolecular axis of the 9,10-di-chloroanthracene dimer (as in the crystalline compound) to account for the observed resonance splittings of both absorption bands.64... 

How is the monomer fluorescence of aryl vinyl polymers or intramolecular excimer-forming compounds distinguished from that of monochromophoric compounds ... 

A similar but smaller intramolecular quenching effect was seen by Phillips and co-workers 44,4S) for 1-vinylnaphthalene copolymers incapable of excimer fluorescence. The monomer fluorescence lifetime of the 1-naphthyl group in the methyl methacrylate copolymer 44) was 20% less than the lifetime of 1-methylnaphthalene in the same solvent, tetrahydrofuran. However, no difference in lifetimes was observed between the 1-vinylnaphthalene/methyl acrylate copolymer 45) and 1-methylnaphthalene. To summarize, the nonradiative decay rate of excited singlet monomer in polymers, koM + k1M, may not be identical to that of a monochromophoric model compound, especially when the polymer contains quenching moieties and the solvent is fluid enough to allow rapid intramolecular quenching to occur. 

The inter-ring separation in [4.4] paracyclophane has been calculated to be 3.73 A, assuming normal bond angles and planar benzene rings. At this distance, there is no ground-state overlap, and the UV absorbance does not extend past 280 nm. Nevertheless, the peak of the excimer fluorescence intensity of [4.4] paracyclophane is red-shifted 1900 cm"1 relative to the peak of the solution excimer of toluene at 31,300 cm-1. Neither the excimer lifetime nor the excimer fluorescence response function have been reported for any of the exrimer-forming paracyclophanes, so little is known about the kinetics of excimer formation in these compounds. 

While the photodimerization of bis(l-naphthylmethyl) ether was acknowledged somewhat earlier 39), the photodimers were first characterized and the quantum yield of the dimerization determined by Todesco et al. U2). Both the syn- and anti-photodimers were formed in roughly equal amounts, and the quantum yield for formation of the anti-dimer was independent of solvent. However, the quantum yields for formation of the syn-dimer and for excimer fluorescence were found to vary with solvent such that their sum was independent of solvent. The fact that irradiation of l,3-bis(l-naphthyl)-1-propanol yields only the syn-photodimer 113> indicates that the conformational properties of oxygen are largely responsible for anii-dimerization in the ether compound. The possibility of photodimerization was unfortunately not considered in the fluorescence studies of protonated bis (1-naphthylmethyl) amine 115>, l,3-bis(4-methoxy-l-naphthyl) propane 116>, and meso-bis( 1 -(1 -naphthyl-ethyl) ether 13). 

All the above compounds yield excimer fluorescence when excited in room-temperature solution. However, because the rotational potential of the C—X bond and the nonbonded interactions of the substituents of the X atom differ from those of the C—C bond 126), the amount of excimer fluorescence from R(C—X—C)R differs from that of R(C—C—C)R. The heteroatom X can also influence the rotational state of the side groups R, as illustrated by the formation of the anti-photodimer in bis(l-naphthylmethyl)ether u2), but not in l,3-bis(l-naphthyl)propane 10). Finally, compounds having n 3 may exhibit excimer fluorescence, if the linkage contains one or more heteroatoms. For example, the—C—O—C—C— linkage in a,to-bis(2-naphthyl) compound allows excimer fluorescence to be observed in room-temperature solution39). 

There is evidence both for and against the contention that only (me excimer fluorescencepeakandlifetimeispossibleforbis(l-naphthyl)compoundshavingn = 3. Studies of l,3-bis(l-naphthyl)propane9,107,lls,129) and bis(l-naphthylmethyl)ether 39.ii4.iis) jjj various solvents over a range of temperatures have found only one excimer fluorescence peak and decay rate (even though there appear to be two possible excimer structures in the ether compound 114). On the other hand, fluorescence peaks attributed to two excimer types have been recorded at 28,200 and 26,700 cm-1 for meso-bis( 1 -(1 -naphthyl)-ethyl)ether13), and at 27,000 and 24,400 cm-1 both for the compound l,3-bis(4-methoxy-l-naphthyl)propane and for l,3-bis(4-hydroxy-l-naphthyl)propane U6). 

In the second category, polymers with the repeat unit [CH2— CRR —(CH2)m CRR —CH2], where R = phenyl, m = 3-6 or 10, and R = hydrogen or methyl, were synthesized by Richards et al.143). The fluorescence of the R = H compounds 144) and the R = CH3 compounds 2S 145) were studied in fluid and rigid solution and in pure films. Although no spectra were given for the R = H compounds, these were stated 144) to have no excimer emission at 330 nm in fluid solution nor in pure films. A similar report was made25) for the R = CH3 compounds in 2-methyltetrahydrofiiran solution at room temperature, and in such solutions to which methanol had been added to the point of opalescence. These results were confirmed for the R = CH3 compounds in solution, and the spectra of pure films did not show significant amounts of excimer fluorescence at 330 nm t4S). However, an extraneous emission at 310 nm in the film spectra made quantitative measurement of the 330 nm excimer band impossible. 

Despite the technical problems in the latter film study, we conclude that there is no intramolecular excimer formation in the compounds of Richards et al.143, and probably little intermolecular excimer formation in the pure films. The absence of an effect of solvent power 25) on the possible excimer fluorescence of the R = CH3 polymer may not be significant, since little change in the coil dimensions would be expected for the short ( 300 backbone atoms) polymers 143> which were studied. Additional work is needed on the fluorescence of such polymers having higher molecular weights, different aryl substituents (R = 2-naphthyl, for example), and fewer adventitious impurities. 

As noted earlier, the limiting lifetime of pyrene excimer fluorescence from concentrated solutions in PS and PMMA glasses was found to be the same as that of pyrene in cyclohexane solution. There have been no similar studies of naphthyl compounds in rigid glasses. Values of k and Q for the [2,6]-naphthalenophanes have not yet been determined for any solvent system. The bis(2-naphthyl) compounds have not been quantitatively characterized in rigid matrices, probably because excimer fluorescence is weak and difficult to detect under such conditions. Given such limited data, it can only be assumed that the values of QD and kD of 2-naphthyl excimers remain the same in rigid solution as in fluid solution. 

Excimer fluorescence from polychromophoric compounds in rigid systems, while easy to detect, is difficult to interpret. The transient response of the excimer can be empirically characterized by the limiting lifetime T p. In the absence of processes which convert D to M, this limiting lifetime is the reciprocal of k . We will examine xa]D for PS, P2VN, and other aromatic polymers to see if there is any difference between fluid and rigid solution at room temperature. 

The emission and the excitation spectra of 6-methylaminopurine and 6-dimethyl-aminopurine derivatives were found to resemble to those of the corresponding adenine derivatives (Fig. 2). The monomeric model compounds showed monomeric fluo-rescene at about 330 nm, while the dimeric model compounds and the polymers showed excimer fluorescence at about 360 nm. 

Intramolecular Excimer Fluorescence Studies in Polymers Carrying Aromatic Side Chains. Some years ago, it was shown that certain excited aromatic molecules may form a complex with a similar molecule in the ground state, which is characterized by a structureless emission band red-shifted relative to the emission spectrum of the monomer. The formation of such complexes, called "exclmers", requires the two chromophores to lie almost parallel to one another at a distance not exceeding about 3.5A° (11). Later, it was found that Intramolecular excimer formation is also possible. In a series of compounds of the type C5H (CH2)jiC H5, excimer fluorescence, with a maximum at 340nm, was observed only for n 3 -all the other compounds had emission spectra similar to toluene, with a maximum at about 280nm (12). Similar behavior was observed in polystyrene solutions, where the phenyl groups are also separated from one another by three carbon atoms (13). 

Silica/i-Octanol. The monomer and excimer fluorescence decays of 1Py(3)1Py in the system silica/octanol were fitted with three exponentials (38), double-exponential fits giving unacceptable results. The decay times at 25°C (38) for the monomer (20, 43 and 146 ns), have values in the same range as those of the excimer (27, 51 and 106 ns). As was noted in studies with 1Py(3)1Py and related compounds in homogeneous solution (20,62), the monomer decay often contains a contribution from an impurity with a lifetime similar to that of e.g. 1-methylpyrene (x ), becoming more important with increasing fluorescence quenching. This then leads to the difference observed in the longest decay times of excimer and monomer. 

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