Energy of exciplex

Other reactions of aromatic hydrocarbon anion radicals and amine cation radicals lead to exciplex emission, particularly in nonpolar solvents [15], Luminescence from exciplexes is most definitively observed in systems for which the redox reaction is energetically unable to yield a localized excited state. The free energy of exciplex formation, jE exc, is associated with solvation and geometry optimization in the encounter complex of and A+. 

From a simple MO treatment it follows that the electron transfer leading to exciplex formation can occur either from an excited donor to an acceptor or from a donor to an excited acceptor. (See Figure 5.25.) In both cases, the singly occupied orbitals of the resulting exciplex correspond to the HOMO of the donor and the LUMO of the acceptor. Neglecting solvent effects, the energy of exciplex emission is therefore given by... 

Besides the excited molecule can interact physically with a second molecule, i.e. undergo bimolecular processes. These are either energy transfer (1.7) or exciplex formation (1.8) depending on the relative excitation energies of the molecule to be studied and its partner. 

The chemical association of the exciplex results from an attraction between the excited-state molecule and the ground-state molecule, brought about by a transfer of electronic charge between the molecules. Thus exciplexes are polar species, whereas excimers are nonpolar. Evidence for the charge-transfer nature of exciplexes in nonpolar solvents is provided by the strong linear correlation between the energy of the photons involved in exciplex emission and the redox potentials of the components. 

Weller24 has estimated enthalpies of exciplex formation from the energy separation vg, — i>5 ax of the molecular 0"-0 and exciplex fluorescence maximum using the appropriate form of Eq. (27) with ER assumed to have the value found for pyrene despite the doubtful validity of this approximation the values listed for AHa in Table VI are sufficiently low to permit exciplex dissociation during its radiative lifetime and the total emission spectrum of these systems may be expected to vary with temperature in the manner described above for one-component systems. This has recently been confirmed by Knibbe, Rehm, and Weller30 who obtain the enthalpies and entropies of photoassociation of the donor-acceptor pairs listed in Table XI. From a detailed analysis of the fluorescence decay curves for the perylene-diethyl-aniline system in benzene, Ware and Richter34 find that... 

An example of exciplex formation in the solid state may be afforded by perylene doped crystals of pyrene which emit a green structureless fluorescence in addition to the blue and orange-red excimer bands of pyrene and perylene, respectively. Hochstrasser112 has shown that the energy of the emitting species is consistent with that of a charge transfer complex of pyrene and perylene molecules in a bimolecular unit of the pyrene lattice. 

We have been concerned so far with excimers in which the partner molecules are identical. An exciplex in which M = N is different, since the orbital energies of M and N do not coincide. One of the molecules will now act as an electron donor and the other as an electron acceptor, as illustrated in Figure 4.23. 

Stabilization of the exciplex by the estimated bond energy of the amine dimer cation radical, 0.7 eV (129), would render intersystem crossing to form t endothermic by 0.2 eV as shown in Fig. 9 and enhance the rate of nonradiative decay. 

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)... 

In 1977, Scharf and Mattay [123] found that benzene undergoes ortho as well as meta photocycloaddition with 2,2-dimethyl-1,3-dioxole and, subsequently, Leismann et al. [179,180] reported that they had observed exciplex fluorescence from solutions in acetonitrile of benzene with 2,2-dimethyl-l,3-dioxole, 2-methyl-l,3-dioxole, 1,3-dioxole, 1,4-dioxene, and (Z)-2,2,7,7-tetram-ethyl-3,6-dioxa-2,7-disilaoct-4-ene. The wavelength of maximum emission was around 390 nm. In cyclohexane, no exciplex emission could be detected. No obvious correlation could be found among the ionization potentials of the alkenes, the Stern-Volmer constants of quenching of benzene fluorescence, and the fluorescence emission energies of the exciplexes. Therefore, the observed exciplexes were characterized as weak exciplexes with dipole-dipole rather than charge-transfer stabilization. Such exciplexes have been designated as mixed excimers by Weller [181],... 

In addition to the excited-state chemical reactions and the radiative and radiationless relaxations, the energy of the excited state can also be degraded by bimolecular processes. One of such processes is the formation of exciplexes (A X) or excimers (A = X). In these processes, the energy is degraded when the excimer or exciplex decays via radiative or radiationless processes (Equation 6.80).38 10... 

To shift this border to the right, one should facilitate the exciplex recombination transforming the reversible transfer into irreversible transfer. Rehm and Weller chose this very way assuming that Wr = const and is larger than k/Kecl everywhere. This is a very astonishing and unacceptable assumption since Wr (AGr) should also follow FEG law, which is common for any transfer rate. Since the energy of the excited reactant... 

There are two parallel channels of energy quenching by either contact formation of exciplex with subsequent dissociation to RIP [Eq. (3.604), scheme I] or by remote formation of RIP with subsequent association (transformation) to exciplex [Eq. (3.604), scheme II]. The last one was considered first by means of unified theory [29], which was extended later to account for both schemes together [30]. Since the results were comprehensively reviewed in Chapter IX of Ref. 32, there is no need to do the same here. It should only be noted that the theory of scheme I has been generalized to account exciplex formation, not only by encounters of excited reactants but also by a straightforward light excitation of existing complexes of the same particles [31]. 

Exciplexes formed between relatively strong donors and acceptors are preferably represented by 1(A D+), expressing the fact that the excited state is a singlet CT state [the coefficients C2, C3 and C4 in (41) are negligibly small] the emission of the complex, therefore, corresponds to the CT transition, the reverse of the CT absorption, D+A ) —> DA). The energy of the pure CT state, D+A ), in the gas phase relative to... 

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