Exciplex polarity

The failure of tertiary (AI,IV-dimethylaminoalkyl)arenes and stilbenes to undergo intramolecular addition may reflect structural differences between inter- vs. intramolecular exciplexes. Polar solvents are generally required for the observation of in-termolecular addition reactions of tertiary amine exciplexes. Equilibration between solvent-separated and radical ion pairs may be necessary in order to achieve an appropriate reaction trajectory for a-C-H proton transfer. In the case of intramolecular exciplexes with short chain linkers, electron transfer in polar solvents may occur in extended geometries which are inappropriate for proton transfer and chain folding may not compete effectively with exciplex decay. The exceptions to these generalizations, benzene and styrene, form more localized anion radicals which undergo both inter- and intramolecular reactions with tertiary amine cation radicals in nonpolar solvents. 

Droplet temperature is of interest in practical spray processes since it influences the associated heat and mass transfer, chemical reactions, and phase changes such as evaporation or solidification. Various forms of Rayleigh, Raman and fluorescence spectroscopies have been developed for measurements of droplet temperature and species concentration in sprays.16471 Rainbow refractometry (thermometry), polarization ratioing thermometry, and exciplex method are some examples of the droplet temperature measurement techniques. 

Figure 6.7 Energy diagram for exciplex formation in solvents of differing polarities...

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. 

The exciplex emission is also affected by solvent polarity, where an increase in the solvent polarity results in a lowering of the energy level of the exciplex, at the same time allowing stabilisation of charged species formed by electron transfer (Figure 6.7). Thus, in polar solvents the exciplex emission is shifted to even higher wavelength and accompanied by a decrease in the intensity of the emission, due to competition between exciplex formation and electron transfer. 

A short excursion into the physics and spectroscopy of intermolecular interactions is intended to illustrate the effects of fluorescence spectra change on the transition of dye molecules from liquid solvents to solid environments, on the change of polarity and hydration in these environments, and on the formation of excited-state complexes (excimers and exciplexes). 

When the solvent polarity increases, the exciplex band is red-shifted. The intensity of this band decreases as a result of the competition between de-excitation and dissociation of the exciplex. 

It should be noted that de-excitation of exciplexes can lead not only to fluorescence emission but also to ion pairs and subsequently free solvated ions. The latter process is favored in polar media. Exciplexes can be considered in some cases to be intermediate species in electron transfer from a donor to an acceptor (see Section 4.3). 

Photochemistry of amines and amino compounds folded conformations of the singlet exciplex intermediates in non-polar solvents31. 

The triplet reaction of 2-nitrodibenzo[fc,primary amines (n-propylamine and benzylamine) was studied110 in polar and apolar solvents. In polar solvents, the irradiation results in the formation of two isomeric compounds, (alky-lamino)hydroxynitrodiphenyl ether andiV-(alkylamino)-2-nitrophenoxazine (equation 54). In apolar solvents, only the nitrophenoxazine is obtained. In polar solvents, the exciplex formed between the 2-n i trodi benzol h,e [ 1,4]dioxin triplet state and amines dissociates to the solvated radical ions, from which the diphenyl ether arises. 1-Nitrodibenzo[fr,e][l,4]dioxin is stable even on prolonged irradiation. 

Change-transfer complexes of solute-alcohol stoichiometry 1 2 have been reported by Walker, Bednar, and Lumry3 for indole and certain methyl derivatives (M) in mixtures of associating solvents n-butanol and methanol (Q) with n-pentane these authors introduced the term exciplex to describe the emitter of the red-shifted structureless fluorescence band which increases in intensity with the alcohol content of the mixed solvent. The shift of the exciplex band to longer wavelengths as the solvent polarity is increased, described by Eq. (15), confirms the dipolar nature of the complex that must have the structure M+Q2. No emission corresponding to the 1 1 complex is observed in these systems which indicates (but does not prove) that the photo-association involves the alcohol dimer. The complex stoichiometry M+Q determined from (Eqs. 9, 10, and 12)... 

The effect of exciplex dissociation (process MC) on the over-all kinetics of molecular fluorescence decay has been examined by Ware and Richter34 for the system perylene-dimethylaniline in solvents with dielectric constants (e) varying from 2.3 to 37. In low dielectric media (e = 2.3-4) the perylene fluorescence response may be fitted to a two-component exponential curve and exciplex emission is also observed, whereas in more polar solvents (e > 12) exciplex fluorescence is absent (at ambient temperatures) and the molecular fluorescence decays exponentially. These observations are consistent with both an increase in exciplex stability toward molecular dissociation with solvent polarity (Eq. 13) and the increased probability of dissociation into solvated ions... 

The interaction of nondegenerate molecular or charge-transfer states is insufficient to describe the stability of photoassociation products of molecules with different electronic energy levels, ionization potentials, and electron affinities. On the other hand, treatments26-26 of the exciplex as a pure charge-transfer state afford a quantitative description of the shift in fluorescence peak with solvent polarity and with electron affinity of the (fluorescent) donor in the same quencher-solvent system (Eq. 13) moreover, estimated values for the dipole moment of the emitting species (Table VI) confirm its pronounced charge-transfer character. 

The red shift of the exciplex band with increasing solvent polarity (Section II.C) is accompanied by a marked decrease in its intensity, until in highly polar solvents such as acetonitrile this band is no longer observed.24,31,102 This behavior is illustrated in Figure 12 where the relative intensity of the... 

Bimolecular photoinduced electron transfer between an electron donor and an electron acceptor in a polar solvent may result in the formation of free ions (FI). Weller and coworkers [1] have invoked several types of intermediates for describing this process (Fig.la) exciplex or contact ion pair (CIP), loose ion pair (LIP), also called solvent separated ion pair. The knowledge of the structures of these intermediates is fundamental for understanding the details of bimolecular reactions in solution. However, up to now, no spectroscopic technique has been able to differentiate them. The UV-Vis absorption spectra of the ion pairs and the free ions are very similar [2], Furthermore, previous time resolved resonant Raman investigations [3] have shown that these species exhibit essentially the same high frequency vibrational spectrum. 

Thiophene on irradiation in presence of dihalogenomaleimides leads to low yields of the 2-substituted products (Scheme 89) (78TL125). When benzo[6]thiophene is irradiated in the presence of dibromomaleic anhydride, both photosubstitution and photocycloaddition are observed (Scheme 90) (77BCJ1797). The ratio of photosubstitution relative to cycloaddition increases with the temperature and polarity of the solvent. A common triplet exciplex has been suggested for both processes. 

In polar solvents such as chloroform, dichloromethane, acetone, and acetonitrile, the fluorescence quantum yields of 97a-d decrease by varying degrees (see Table 19). Moreover, in the case of the phenyl and acetyl derivatives 97c and 97d, the rather drastic decrease of the structured fluorescence from the locally excited anthracene is associated with the appearance of a structureless, red-shifted emission which is attributable to an intramolecular exciplex. For 97d, in which the electron acceptor properties of the aromatic carbonyl moiety are enhanced by p-acetyl substitution, exciplex emission is dominant even in toluene solution (see Figure 22). 

In polar solvents, the quantum yields for the emission from the locally excited state of anthronyl-anthracenes 98 and 99 decrease drastically (see Tables 20 and 21), and a structureless, red-shifted exciplex emission is observed (see Figure 23). For the parent compound 98a in dichloromethane, for example, the quantum yield of emission from the exciplex state is 0.012, but that of emission from the locally excited state has decreased to 0.00058 (cf. Tables 20 and 22). Thus, intramolecular exciplex formation between the photoexcited anthracene moiety and the aromatic ketone in its electronic ground state represents the major mode of deactivation in polar solvents. 

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