Aromatic hydrocarbons excimer formation

Excimer formation is observed quite frequently with aromatic hydrocarbons. Excimer stability is particularly great for pyrene, where the enthalpy of dissociation is A// = 10 kcal/mol (Fbrster and SeidI, 1965). The excimers of aromatic molecules adopt a sandwich structure, and at room temperature, the constituents can rotate relative to each other. The interplanar separation is 300-350 pm and is thus in the same range as the separation of 375 pm between the two benzene planes in 4,4 -paracyclophane (13), which exhibits the typical structureless excimer emission. For the higher homologues, such as 5,5 -paracylophane, an ordinary fluorescence characteristic of p-dialkyl-benzenes is observed (Vala et al., 1965). 

The formation of such excimers, which only exist in the excited state, is commonplace among polynuclear aromatic hydrocarbons, the simple potential energy diagram for which is shown in Figure 6.4. 

Excimer formation can serve as a sensitive probe of group proximities Excimers make evident the interaction of an excited molecule M, (typically an aromatic hydrocarbon), with a molecule in the ground state M producing an excited dimer Mf (or D ). The dimer must be formed within the lifetime of the excited species (e.g., for pyrene derivatives, about 100 nsec). For molecules such as pyrene, excimer formation and fluorescence are contingent on attainment of a well-defined steric arrangement in the dimer.41... 

There is a substantial entropy decrease AS associated with intermolecular excimer formation, as given in the tabulation by Birks 71). For all solvents (except 95% ethanol 75), the value AS ss —20 cal/mole-K was observed for naphthalene and its derivatives. For comparison, the entropy of fusion of unsubstituted aromatic hydrocarbons such as naphthalene falls in the range of —8 to —15 e.u. The large loss of entropy in the intermolecular excimer formation process indicates a very constrained symmetric structure. 

Excimer fluorescence has been observed in a variety of systems in which intermolecular diffusion does not play a role in excimer formation. Five such systems involving the naphthyl chromophore will be discussed (1) Crystals of aromatic hydrocarbons ... 

Creed, 1978a). It was found that as the charge-transfer character in the transition state increased the rate constant for cycloaddition decreased. The oxidation of crystal violet to its cation radical can be initiated by reaction of the dye with the excited singlet states of many polycyclic aromatic hydrocarbons. This reaction was found to be far less efficient for polymer-bound pyrene than for free pyrene and this was attributed to excimer formation occurring in the polymer system which ultimately led to energy wastage (Tazuke et al., 1979). 

Di-(l-naphthylmethyl)sulphone forms an excimer but does not react to give an intramolecular cycloaddition product like the corresponding ether but rather fragments to give sulphur dioxide and (l-naphthyl)methyl radicals (Amiri and Mellor, 1978). I-Naphthylacetyl chloride has a very low quantum yield of fluorescence and this is possibly due to exciplex formation between the acyl group and the naphthalene nucleus (Tamaki, 1979). Irradiation leads to decarbonylation. It is known that acyl chlorides quench the fluorescence of aromatic hydrocarbons and that this process leads to acylation of the aromatic hydrocarbon (Tamaki, 1978a). The decarboxylation of anhydrides of phenylacetic acids [171] has been interpreted as shown in (53), involving... 

The other aromatic hydrocarbons capable of intermolecular excimer formation (10-14), i.e. benzene, naphthalene, 9,10-al)cylated anthracenes,1,2-benzanthracene and also perylene, have considerably smaller values for the ratio shorter fluorescence lifetimes... 

Exciplexes are complexes of two different molecules usually of 1 1 stoichiometry. Their fluorescence phenomena are similar to those described for excimers, but their formation is not restricted to aromatic systems. If the sum of effective rate constants of the non radiative processes is so high such that the lifetime of emission is undetectable, these molecules do not necessarily luminesce. In contrast to the excimers, which are non polar, the exciplexes are polar entities. It was shown by Beens and co-workers [13] that exciplexes from aromatic hydrocarbons and aromatic tertiary amines demonstrate the charge-transfer character of the complexes as reported by Knibbe and co-workers [14], and their dipole moments were greater than 3.3 x 10 C m (10 D). 

Aromatic hydrocarbons such as pyrene have also been employed as a luminescence probe of polarity and microviscosity in a variety of organized assemblies (109). Pyrene is a good excimer-forming probe due to the long lifetime of fluorescence and formation of excited-state dimers (excimers) at low concentration. Figure 9 shows an example pyrene luminescence spectrum. The ratio of excimer to monomer fluorescence intensity is often utilized as a measure of pyrene mobility and proximity. The vibronic fine structure of the pyrene monomer is sensitive... 

Aromatic hydrocarbons such as pyrene, naphthalene, perylene or other related compounds are known to undergo excimer formation reactions in the excited state. For intermolecular excimer formation, the kinetics fall in the category of two-state systems (Scheme 15.7), as well as for the intramolecular case when the interconnecting chain is sufficiently long. With short connecting chains, two excimer conformations may occur, leading to three excited state species (three-state system, see below). 

From the above-mentioned aromatic hydrocarbons, pyrene is for sure the most widespread excimer forming fluorescent probe. The fluorescence spectra of pyrene are known to display the characteristic vibronically resolved pyrene band with a maximum at 375 nm, together with a stractureless long-wavelength band (ca. 480 nm). Typically, only at concentrations of pyrene above ca. 10 mol dm , intermolecular excimer formation is clearly observed. For intramolecular excimer formation (concentration independent kinetics) the long-wavelength emission band can be observed for concentrations as low as 10 mol dm . ... 

Excimer formation has been found to be most prominent for small organic aromatic molecules such as benzene [13],p-xylene [14], naphthalene [15], anthracene [16], pyrene [17], perylene [18], stilbene [19], and others [20]. Some polymers that contain aromatic groups, such as polystyrene [21] and poly (ethylene terephthalate) (PET) [22], and polynucleotides such as cytosine and thymine [23] have also been shown to exhibit excimer emission. Excimer emission is, in fact, widely observed in aromatic hydrocarbons [20]. 

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