Exciplex formation parameter

The separation of binding site and fluorophore by a nonconjugating spacer opens the path to other mechanisms of communication, most prominently ET and exci-mer/exciplex formation. In the first case, the electronic nature of both fluorophore and receptor unit and the steric nature of the spacer are the important parameters for signal generation. In the second case, for most systems the electronic nature of the fluorophores and the steric nature of the receptor as well as its change upon analyte binding determine the signal. 

The theory of geminate recombination experienced a similar evolution from primitive exponential model and contact approximation [19,20], to distant recombination carried out by backward electron transfer [21], However, all these theories have an arbitrary parameter initial separation of reactants in a pair, / o. This uncertainty was eliminated by unified theory (UT) proposed in two articles published almost simultaneously [22,23], UT considers jointly the forward bimolecular electron transfer and subsequent geminate recombination of charged products carried out by backward electron or proton transfer. The forward transfer creates the initial condition for the backward one. This is the distribution of initial separations in the geminate ion pair/(ro), closely analyzed theoretically [24,25] and inspected experimentally [26,27], It was used to specify the geminate recombination kinetics accompanied by spin conversion and exciplex formation [28-31], These and other applications of UT have been covered in a review published in 2000 [32],... 

Gorman, A. A., Gould, I. R., Hamblett, I., Time resolved Study of the Solvent and Temperature Dependence of Singlet Oxygen ( Ag) Reactivity toward Enol Ethers Reactivity Parameters Typical of Rapid Reversible Exciplex Formation, J. Am. Chem. Soc. 1982, 104, 7098 7104. 

Following a similar approach to that of pyrene exclmer formation, activation energies for pyrene- 4. exciplex formation can be obtained from expression of Eqn. 9 in an Arrhenius form and differentiation by 1/T. ki+k2 are obtained from data taken in eyelohexane (32), and A3 and E3 from the lifetime taken at f CA1 - 0. E3 can also be obtained from the slope of the phase dependent dynamic Stern-Volmer plots. As seen in Table 1 the data from each method are in good agreement. The small differences in activation parameters measured in the cholesteric and isotropic phases probably reflect changes in viscosity that accompany phase transitions. 

TABLE 1. Activation parameters for pyrene- exciplex formation in CM ---------------------------------------------------------------... 

In contrast, the very significant difference between activation parameters for pyrene-CA exclplex formation in cholesteric and Isotropic CM is compatible with the geometric requirements for exclplex formation the bulky exclplex must disturb cholesteric order, thus increasing the entropy of activation to displace ordered solvent molecules during exciplex formation, energy additional to that required in the isotropic phase must be infused. 

Our investigation of the mechanism of intramolecular adduct formation employed the technique of arene-amine exciplex quenching by primary amines, which had been developed in earlier investigations of exciplex quenching. These experiments provided evidence for the occurrence of adduct formation via proton transfer in the fluorescent exciplex. In the case of 39, activation parameters for exciplex formation and proton... 

Activation parameters for the quenching of Cu(2,9-Me2-phen)2 by a series of Lewis base donors have provided support for exciplex formation in these reactions of Cu(I) complexes. The data for a series of Lewis base quenchers are collected in Table 2.11. These data show that the fastest quenching rates are found for strong donor ligands such as DMF and DMSO. It is also apparent that the activation... 

Polymers are not homogeneous in a microscopic scale and a number of perturbed states for a dye molecule are expected. As a matter of fact, non-exponential decay of luminescence in polymer systems is a common phenomenon. For some reaction processes (e.g, excimer and exciplex formation), one tries to fit the decay curve to sums of two or three exponential terms, since this kind of functional form is predicted by kinetic models. Here one has to worry about the uniqueness of the fit and the reliability of the parameters. Other processes can not be analyzed in this way. Examples include transient effects in diffusion-controlled processes, energy transfer in rigid matrices, and processes which occur in a distribution of different environments, each with its own characteristic rate. This third example is quite common when solvent relaxation about polar excited states occurs on the same time scale as emission from those states. Careful measurement of time-resolved fluorescence spectra is an approach to this problem. These problems and many others are treated in detail in recent books (9,11), including various aspects of data analysis. 

The formation of excimers and exciplexes are diffusion-controlled processes. The photophysical effects are thus detected at relatively high concentrations of the species so that a sufficient number of collisions can occur during the excited-state lifetime. Temperature and viscosity are of course important parameters. 

Experimental data on the formation, thermodynamic and kinetic decay parameters, and the multiplicity of exciplexes and excimers involving tetrapyrrole complexes are summarized in Tables 1 and 2. Based on these data and on information on the spectral properties and chemical behavior, some conclusions... 

Table 1. Formation, thermodynamic and kinetic decay parameters (stability constant K, energy and entropy values, decay rate constant k, lifetime x, quantum yield < >) of exciplexes (A — Q) involving tetrapyrrole complexes A (energy values expressed in kJmol 1 entropy in Jmol"1 K-1 k and x in s 1 and s, respectively)... 

Bromobiphenyl undergoes photoreduction from the triplet state375. The dependence of the quantum yield upon the concentration of the substrate does indicate the formation of an excimer. Since cpisc = 0.98, it may be concluded that this excimer is formed via the triplet state. The linear solvation energy parameters indicate a weak polarization of the excimer, suggesting a weak radical anion and cation character in the two moieties. The charge separation is smaller than in the exciplex formed from 4-bromobiphenyl and tri-ethylamine. 

PET-sensitization means that the desired radical pair D + A is produced indirectly by first generating another radical pair D + X using an auxiliary sensitizer X and then exchanging X for A by a thermal electron transfer. X is chosen such that the photophysical parameters and redox potentials bar all other pathways except the PET-sensitized one. Of particular significance for the above mechanistic question is that neither D nor A are excited hence, an exciplex (D A) cannot be formed. Chart 9.2 juxtaposes the direct and the PET-sensitized formation of D- + A-. ... 

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. 

The apparent bimolecular rate parameters > STS characteristic of the external heavy-atom quenching of the molecular singlet and triplet states of anthracene (A) (solvent = cyclohexane, quenchers = bromobenzene and ethyl iodide) and 9,10-dibromoanthracene (DBA) (solvent = ethanol, quencher = KI) have been determined and used to describe the sensitivity of the external heavy-atom effect acting on the non-radiative Sx - Tx and T -> S0 processes.141 142 The interesting observation that ethyl iodide and benzene increase the fluorescence efficiency from excited dibromoanthracene has been explained in terms of the formation of photoassociation product E (exciplex ).141b This species then undergoes its own characteristic photophysics. The dominant processes for the ethyl iodide case are ... 

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