Hydrocarbons, aromatic excimers

Different aromatic hydrocarbons (naphthalene, pyrene and some others) can form excimers, and these reactions are accompanying by an appearance of the second emission band shifted to the red-edge of the spectrum. Pyrene in cyclohexane (CH) at small concentrations 10-5-10-4 M has structured vibronic emission band near 430 nm. With the growth of concentration, the second smooth fluorescence band appears near 480 nm, and the intensity of this band increases with the pyrene concentration. At high pyrene concentration of 10 2 M, this band belonging to excimers dominates in the spectrum. After the act of emission, excimers disintegrate into two molecules as the ground state of such complex is unstable. 

A. Weller and K. Zachariasse 157-160) thoroughly investigated this radical-ion reaction, starting from the observation that the fluorescence of aromatic hydrocarbons is quenched very efficiently by electron donors such as N,N diethylaniline which results in a new, red-shifted emission in nonpolar solvents This emission was ascribed to an excited charge-transfer complex 1(ArDD(H )), designated heteroexcimer, with a dipole moment of 10D. In polar solvents, however, quenching of aromatic hydrocarbon fluorescence by diethylaniline is not accompanied by hetero-excimer emission in this case the free radical anions Ar<7> and cations D were formed. 

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. 

Many aromatic hydrocarbons such as naphthalene or pyrene can form excimers. The fluorescence band corresponding to an excimer is located at wavelengths higher than that of the monomer and does not show vibronic bands (see Figure 4.6 and the example of pyrene in Figure 4.7). 

A number of dissolved aromatic hydrocarbons exhibit both molecular and excimer bands in the delayed fluorescence spectrum6,83,116,117 that originates118 in the mutual annihilation of two molecular triplet states if this can produce both the molecular singlet state and the excimer directly according to the scheme... 

Simple MO considerations show that the dimer cation of an aromatic hydrocarbon M, with one less antibonding electron than the ground excimer configuration, should be stable with respect to its constituents M+ + M this is confirmed by esr129 and optical absorption8 studies of y-irradiated solutions of benzene, naphthalene, and anthracene in low temperature glasses. 

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. 

The rate constant kTD for fluorescence of the pyrene intermolecular solution excimer has been found to follow the relation kFD = n2(kFD)n=I, where n is the the refractive index of the solvent69 . The values of kTO for the 1-methylnaphthalene excimer in ethanol at various temperatures are also consistent with the above relation 76). The fact that (kFD)n=I is independent of solvent and temperature indicates that the excimer has a specific structure, according to Birks 69,71). Experimentally, it was observed much earlier that kFM = n2(kFM)n=i for the polycyclic aromatic hydrocarbons, and that k /kp is independent of solvent and temperature. Table 5 shows that agreement between independent investigators of the excimers of naphthalene compounds is not always good, as in the case of 1-methylnaphthalene. 

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 ... 

Figure 6.6 Excimer emission from crystalline planar aromatic hydrocarbons and types of crystal lattice.

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). 

Triplet excimers of aromatic hydrocarbons have proved very difficult to detect and hence their role in deactivation of excited states is largely speculative. However, on the basis of emission experiments (Subudhi and Lim, 1976 Okajima et al, 1977 Chandra and Lim, 1977 Webster et ai, 1981), it has been suggested that some di(l-naphthyl)alkanes form such species. It is suggested that the favoured conformation of the triplet excimer does not have the two naphthalene rings lying parallel to each other. 

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... 

There have been several recent investigations into the mechanism of photo-cyanation of aromatic hydrocarbons. The process with naphthalene, biphenyl, and phenanthrene has been subjected to a kinetic analysis the reactions in dry or aqueous methyl cyanide are shown to involve two transient species, the first of which is an ionic complex formed from a triplet excimer of the arene, or, in the presence of an electron acceptor, from a triplet exciplex. Reaction of the transient complex with the cyanide ion yields the radical ArHCN, and in aqueous methyl cyanide this second transient reacts with itself to produce dihydrocyano- and cyano-compounds. In dry methyl cyanide the radical species is oxidized to the cyano product. 

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 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... 

It is known that a new component appears in the emission spectrum of many aromatic hydrocarbons with increasing concentration [147]. This new fluorescent component is ascribed to excimers which are complexes of an electronically excited molecule with an identical molecule in the ground state. In these complexes molecules are parallel at a distance of about 0.3 nm. Excimers only exist in the excited state after de-excitation the two partners repel each other. Therefore no corresponding change is observed in the absorption spectrum. The kinetic scheme just presented must be complemented by the following steps ... 

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