Monomer-excimer pair

Pyrene excimer formation still continues to be of interest and importance as a model compound for various types of study. Recent re-examinations of the kinetics have been referred to in the previous section. A non a priori analysis of experimentally determined fluorescence decay surfaces has been applied to the examination of intermolecular pyrene excimer formation O. The Kramers equation has been successfully applied to the formation of intermolecular excimer states of 1,3-di(l-pyrenyl) propane . Measured fluorescence lifetimes fit the predictions of the Kramer equation very well. The concentration dependence of transient effects in monomer-excimer kinetics of pyrene and methyl 4-(l-pyrenebutyrate) in toluene and cyclohexane have also been studied . Pyrene excimer formation in polypeptides carrying 2-pyrenyl groups in a-helices has been observed by means of circular polarized fluorescence" . Another probe study of pyrene excimer has been employed in the investigation of multicomponent recombination of germinate pairs and the effect on the form of Stern-Volmer plots ". 

Birks 68) has proposed that the only change between the unexcited and excited pyrene pair is a reduction in the interplanar distance from 3.53 to 3.37 A, i.e. that the pyrene excimer is not a completely eclipsed sandwich pair either in solution or in the crystal. This proposal is consistent with the observed similarity of the excimer band position for the crystal and solution environment, and with the emission of excimer fluorescence from the crystal even at 4 K. For naphthalene, the greater separation and the nonparallel structure of nearest-neighbor pairs in the crystal apparently prohibits the formation of the sandwich excimer during the naphthalene singlet monomer lifetime. Thus, no excimer fluorescence is observed from defect-free naphthalene crystals. 

Figure 8 shows a pair of typical time-resolved fluorescence decay traces for 100 / M pyrene in supercritical CO2 (Tr = 1.02 pr = 1.17). Note that the ordinate is logarithmic. The upper and lower panels show results for selective observation in the monomer (400 +. 10 nm) and excimer (460 + 10 nm) regions of the pyrene emission spectrum. Several interesting features are apparent from these traces. First, both decay processes are not single exponential. Second, the excimer emission has a significant contribution from a species that "grows in" between 30 - 75 ns this is a result of the excimer taking time to form (i.e., k in Figure 1). Third, the fits between the experimental data and the model shown in Figure 1 are good. Detailed analysis of these decay traces (10,11,21-26) yields the entire ensemble of photophysical kinetic parameters for the pyrene excimer in supercritical C02.

Bodenant and coworkers [387] have taken advantage of the affinity of protons for the nitrogen lone pair of hydroxamate in the design and synthesis of 53 and 54. Two hydroxamic acids, each bearing a pyrene, were covalently attached at the 1,3-positions of the lower rim of calix[4]arene. Methanol and diethyl ether solutions of 53 exhibit signatures of both pyrene monomer and excimer fluorescence. As is the case for 37 through 40, the intensity ratio of... 

In the case of fluorescent monomers, monitoring the change of fluorescence upon association directly yields the equihbrium constant [149,182]. Alternatively, it is possible to label the monomer with a suitable excimer forming chromophore [24] or with a pair of chromophores for fluorescence resonance energy transfer studies [200]. The clear advantage of fluorescence... 

Weller and Zachariasse thoroughly investigated exciplex formation and luminescence for donor acceptor systems in THF [18]. A particularly interesting result from their work came from an examination of the temperature dependence of radiative charge recombination between 9,10-dimethylanthracene anion (DMA") and TPTA+ in THF [19]. They found that both exciplex emission and fluorescence from DMA were observed in solution at low temperature (ca. —50°C). As the solution temperature is raised, the excimer emission decreases in relative intensity, and at room temperature the emission is nearly completely DMA fluorescence. The monomer-to-exciplex emission intensity ratio as a function of temperature follows Arrhenius kinetic behavior and yields an activation barrier that is nearly the same as the energy gap between the exciplex and the DMA states. Thus, their model consisted of reaction of the solvent-separated ions to form an intimate emissive ion pair which could dissociate to yield the singlet anthracene derivative. 

Achiral or racemic anthracenes in fact frequently pack in centrosymmetrical pairs with pronounced overlap between the aromatic rings. This motif is easily detectable because of its characteristic excimer emission the monomers photodimerize almost quantitatively, yielding sparingly soluble dianthracenes which can be split back to monomers simply by heating them to their melting points (Scheme 10). In contradistinction, resolved samples can only pack in the herringbone or stack motif, and thus are monomer-emitting and photostable. 

Moreover, it should be noted that the same pair of decay times appears in the monomer as well as in the excimer decay, see [3] and... 

Figure 8.1 Schematic potential energy diagram of an excimer-forming pair of moiecuies. The iower curve shows both molecules in the ground state. The upper curve shows the excimer formation on the approach between an excited molecule and a molecule in the ground state. AEfm is the excitation energy of the monomer, AEfd is the excitation energy of the excimer, and 6 is the excimer-binding energy.

As with anthracene, no delayed excimer fluorescence is observed for 1,2-benzanthracene under these conditions (225), and the deviation of Ps from Vi> may well be due to decay from the relaxed doubly excited singlet pair state as shown in Scheme 5. A relatively smallp = 0.092 value for 1,2-benzanthracene has been estimated from photodimerization results in cyclohexane (229). The 1,2-benzanthracene case illustrates some of the difficulties associated with the evaluation of published TTA data. Undoubtedly, depending upon the availability of quintet, triplet, and excited singlet monomer states at or below the energy of initially formed triplet encounter pairs, exceptions to Eq. 70 and Scheme 5 will be found. The details of the mechanism proposed here may also need to be modified even for anthracene and 1,2-benzanthracene. 

---