Dipole moment exciplex

A and D are the exciplex or excimer components, denotes the primarily excited species, k is the limiting photoassociation equilibrium constant, AHat ASa, and are the thermodynamic parameters for the exciplex-excimer, and p is the excited state dipole moment of the complex. Note that the large dipole moment for the exciplex indicates almost complete charge transfer in the excited state, (D+, A-). rfc and r, are the fluorescence lifetimes for the complex and the component. 

Dipole Moments from Solvent Effect on Exciplex Fluorescence Maxima ... 

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. 

Solute-solvent interactions are of two types (1) universal interaction, and (2) specific interaction. Universal interaction is due to the collective influence of the solvent as a dielectric medium. It depends on the dielectric constant D and refractive index n of the solvent and the dipole moment g of the solute molecule. Such interactions are van der Waals type. Specific interactions are short range interactions and involve H-bonding, charge-transfer or exciplex formation. H-bonding ability may change on excitation specially for n-yxt transitions. 

Some properties of the t -dimethylhexadiene exciplex are summarized in Table 7. Its fluorescence maximum is at slightly shorter wavelength than that of the anthracene-dimethylhexadiene exciplex (435 nm) (51). While data on other unsubstituted arene-diene exciplexes are not available, t appears to be more reactive and to form more stable exciplexes with dienes than arenes of comparable electron affinity (101). The dipole moment of the - -t -dimethylhexadiene exciplex is estimated to be 7 D from the solvent dependence of its fluorescence maxima (36). This value is substantially lower than those for pure charge-transfer exciplexes (p > 15 D) and indicates that this exciplex is relatively nonpolar and might be better categorized as a hetero-excimer, than as an exciplex (83). That is, using the normal resonance description of an exciplex... 

The exciplexes with A values >0.2 eV and dipole moments which are smaller than those of the CT exciplexes are formed as a result of interactions between the CT state X(A D+) and non-polar (locally) excited complex states such as 1(A D) and 1(AD ) leading to stabilization of the CT state and to lowering of the dipole moment [116]. The coefficients Ci or C2, and C3 and C4 in the wave function (41) can be of comparable magnitude for such exciplexes. The value of A = 0.3 eV follows for the exciplex formed by the TPOB acceptor [l,3,5-tris(4-fert-butylphenyl-l,3,4-oxadiazolyl)benzene] with the TCTA donor [4,4/,4//-tri(A -carbazolyl) triphenyla-mine] based on EDOX = 0.69eV and EAred = —2.1eV, and the... 

The exciplex or CIP is treated as a dipole of radius q and dipole moment p. The last term in Eq. (15) describes the energy of this dipole, based on the Kirkwood-Onsager model (assuming formation of a spherical complex) [14]. Thus, an exciplex or CIP is stabilized by Coulombic interactions and by solvation. The solvation energy is expected to be favored by increasing solvent polarity and a large dipole... 

Some exciplexes, however, can be stabilized in polar solvents because of favorable orbital interactions, electrostatic effects, or a Kirkwood-Onsager solvation energy term [18]. For example, in polar solvents an exciplex structure with a large dipole moment can be stable with respect to solvent-separated and free ions (Fig. 8). The lifetimes of these exciplexes should then be sufficiently long to allow for observation. In fact, there is evidence to support exciplex emission even in polar solvents. Eisenthal and coworkers, for example, observed the exciplex emission of 9-anthracene-(CH2)3-iV,JV-dimethylaniline systems in acetonitrile [19] ... 

The latter mechanism is met in amine-vinyl monomer systems [41-46] (see Scheme 4). Due to the small n-acceptor ability of normal substituted vinyl monomers, an interaction in the ground-state level does not take place. The exciplexes assumed are detectable in aromatic amine-acrylonitrile (AN) systems by their emission spectra, as is shown in Fig. 1 for typical examples. The emission bands at 350 nm (by JV,JV-dimethyl-p-toluidine (DMT)) and 370 nm (by p-phenylene diamine (TMPD) result from the normal fluorescence of the isolated amine. As can be seen, the intensity of the exciplex emission is much higher in the DMT-AN system. This corresponds to the higher polymerization efficiency of that system (<)>[, by A. = 313 nm and 80 K 0.6 for DMT 0.15 for TMPD [46]). Mainly, the much higher dipole moment of DMT (1.1 D) is responsible for this result. The cation radicals [46] or neutral radicals [42] of the amines formed after PET and proton transfer have been detected by ESR measurements. As expected, the rate of photopolymerization of the systems discussed increases with increasing... 

Steric effects have also been shown to be important in intramolecular exciplex formation (Pragst et al., 1978). It was found that the intramolecular exciplex formed by 3-(N-methyl-N-p-tolyl) amino-l-(9-anthraceno)propane had a dipole moment of 12.2 D whereas 3-(N-p-tolyl)amino-l-(9-anthraceno)-propane gave an exciplex having a dipole moment of 15 D. It was suggested that in the former compound the exciplex deviates from the sandwich conformation and consequently has less charge-transfer character. 

The finding that the wavelength of exciplex emission is solvent dependent has been used to calculate the dipole moments of a variety of complexes (Beens and Weller, 1968b). Most exciplexes have dipole moments which correspond to nearly complete electron transfer. Very few systems correspond to the intermediate region. One such system is N-methylbenzimidazole plus 1-cyanonaphthalene (Davidson et al., 1977). In this intermolecular system, exciplex emission can be observed in highly polar solvents. It appears that for most systems electron transfer is so facile that use of high polarity solvents leads to complete electron transfer giving radical ions as a result, the formation of fluorescent complexes is not observed. 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

Deactivation of 2-naphthylamine singlet state by pyridines in enhanced by dipole moment and the ability to form hydrogen bonds. Picosecond laser spectroscopy shows charge transfer from the excited amine. The fluorescence of 2-iV,A -dimethylaminopyridine induced by p-nitroaniline is also caused by exciplex formation. The latter enhances triplet population of p-nitroaniline. The quenching of the fluoresence of carbazole and some derivatives by trichloroacetic acid and related compounds in fluid solutions has been studied by Johnson.A charge-transfer interaction is involved and the basicity of carbazole and derivatives determined. Charge transfer is also involved by quenching of carbazole by halocarbons. The A -isopropylcarbazole-dimethylterephthalate exciplex has been observed in PMMA films.Photoinduced electron-transfer in the p-phenylenediamine-paraquat complex yields the paraquat cation. ... 

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