Pyrene excimer formation

In order to study the molecular dynamics of the outer segments of a dendrimer, one pyrene moiety was selectively and covalently attached to one dendron of poly(aryl ester) dendrimers by Adams (in total three pyrene molecules per dendrimer) [24]. The fluorescence decay of pyrene in the THF solution of the labeled dendrimers provided details of the pyrene excimer formation, such as the excimer formation rate, the excimer decomposition rate constant and the equilibrium constant of the excimer formation. These parameters were utilized to evaluate the diffusional mobility of the dendrimer branches. 

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... 

ZAGROBELNY BRIGHT Investigation of Pyrene Excimer Formation... 

Figure 10. Pyrene excimer formation on a decanol covered silica...

Figure 11. Effect of lowering the temperature on the pyrene excimer formation. The silica gel is covered with decanol (1 x 10"3 mol/g SI02). Pyrene iSo 2. 5 mg/g Si02. Emission spectrum of pyrene at 200° K. The excitation wavelength 345 nm.

Figure 8 Potassium-ion-induced pyrene excimer formation of 81 in the presence of y-...

The CD architecture is particularly fitting for excimer-based reporter sites. As long as the bucket is sufficiently large to accept two guests, monomers will cofacially organize within the interior of a CD to form excimer. Early spectroscopic studies revealed an enhanced excimer emission upon the addition of y-CD to solutions of naphthalene [361], a result that was subsequently observed for anthracene [362] and pyrene [363,364], Pyrene excimer formation has been examined in particular detail. Proper concentrations of pyrene and y-CD afford 2 1 or 2 2 excimer inclusion complexes. In both cases, pyrenes interact in a cofacial manner and excimer efficiently forms. Alcohol addition can disrupt the excimer by the preference to form a 1 1 1 complex of alcohol, pyrene, and CD. 

The formation of triplets via excimers is still an area which is little explored. Medinger and Wilkinson (1966) have shown that pyrene excimer formation leads to a decrease in the quantum yield of triplet formation. A similar result has been obtained for benz(a)anthracene (Heinzelmann and Labhart, 1969). On the other hand, benzene excimer formation leads to an increased triplet yield (Cundall et al., 1972 Hentz and Thibault, 1973). There has been a recent theoretical study of intersystem crossing in molecular pairs (Bowman and Norris, 1978). 

W2 has been reported as 2.9, 3.9, and 6.8 KJ/mol for three solvents, n-nonane, cyclohexane, and i-propanol, respectively [25]. Considerably smaller W2 than that of pyrene excimer formation, and the large deviation from Stokes - Einstein flow shown by the small value of x is probably relevant to the small rotor size. This chromophore will therefore be suitable to detect a small free volume change in highly viscous media. 

There have been studies in which pyrene alone has been used as the fluorescent probe for in situ polymerization (Pekcan et al, 1997, 2001, Pekcan and Kaya, 2001). It is assumed that the observed enhancement of fluorescence intensity or increase in singlet-state lifetime of the monomeric pyrene as the network forms arises Irom a decrease in bimolecular quenching events as the viscosity increases and Atq decreases (Equations (3.28) and (3.29)). It is noted that the spectra showed no evidence of pyrene excimer formation, suggesting that the pyrene is isolated in the network at the concentrations employed. 

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 ". 

A study has been made of both naphthalene/trimethylamine exciplex and pyrene excimer formation in supercritical COj and ethylene fluids. This is a most... 

In these solutions, K-(A -X)/(Ai-X2), F-K/(Ai-A2), A-(Ai X)/(X-Y), X-ki+k2+k3, Y-k4+k5, and Aj and A2 are given by Eqn. 16. For several well known (30) limiting cases, A3 and A2 are equivalent to and r2, the lifetimes of the pyrene singlet state and excited state complexes, respectively (see Eqns. 9-11). Activation parameters for pyrene excimer formation were calculated by two Independent methods. Since kx+k2 is known to be virtually temperature independent and k4 and ky are negligible(31), the ratios of fluorescent intensity maxima from the pyrene excimer and monomer maxima (Ig/Itl) the inverse of temperature yield the activation energy for pyrene excimer formation, E3. A similar experiment for the pyrene-CA system was not possible since its exciplex is not emissive. Activation parameters for the excimer and exciplex were also obtained from temperature and phase dependent pyrene fluorescent lifetime data. In the 1iquid-crystalline and isotropic phases of M, all pyrene decays were single exponential and the excimer decays could be expressed as the difference between two exponentials. 

In addition to energy transfer, pyrene excimer formation has also been used to detect DNA hybridization. In this case DNA probes were synthesized with pyrene attached to the S and 3 ends (Figure 21.22). It is wdl known that one excited pyrene molecule can form an excited-state complex with anotho- ground-state [ rene. This com dex is called an excimer and rliqrlays an unstructured emission near SOO nm as compared to the structured emission of pyrene imnomer near 400 nm. 

P. Lianos and R. Zana, Use of pyrene excimer formation to study the effect of NaCl on the... 

Zana et al. [10a] found that the pyrene excimer formation was similarily slowed down in SDSA EO-aggregates. They proposed that an interaction between pyrene and PEO was the probable reason, and rejected an explanation based on increased microviscosity from the fact that the depolarization of diphenylhexatriene fluorescence was not affected. 

Nakamura and Thomas have studied a stable aqueous suspension of the hexadecyltrimethyl-ammonium chloride (Ci63CiNCl)/laponite system using pyrene as a probe (126). Their suspension contained 2 mmol of Ci63CiNCl and 1 g of laponite, which was able to dissolve 0.1 mmol pyrene. The kinetics of pyrene quenching and pyrene excimer formation reactions suggests that the Ci63CiNCl forms a cluster-like double layer on the clay surface. Since the amount of CieSCiNCl used is twice the CEC of laponite, the effects of Cl on the photophysics of adsorbed pyrene cannot be excluded. 

Figure 27 Intermolecular pyrene excimer formation in [Zn(114)] + mediated by the interaction with DTP.

Fig. 10 Steady-slate spectra depicting the pyrene excimer formation in 1,4-dioxane as a fimctitm of pyrene concentration. In case of pyrene dissolved in a fairly polar 1,4-dioxane, the/i//3 ratio in pyrene monomer spectrum is higher than 1, but the excimer formation affects its value, which decreases in excimer containing solutions...

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