Exciplexes fluorescence

In 1982 Wei et al. [78,79] studied the quenching of N,N-dimethyltoiuidine (DMT) fluorescence by adding the electron-accepting monomer MA or MMA and successfully observed broad and structureless exciplex fluorescences at longer wavelengths in nonpolar solvents for the first time. 

D. Decay Characteristics of Molecular and Excimer (Exciplex) Fluorescence 178... 

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

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... 

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... 

Lifetimes t°(C are available from analyses of fluorescence decay curves as described in Section II.D where, to a good approximation, 1/t (C is given as the experimental parameter Ax describing terminal decay for a system exhibiting excimer (exciplex) fluorescence only. 

The data reported for exciplex fluorescence are less extensive however, it is interesting to note that rg for the anthracene-diethylaniline system (105 nsec31) and the perylene-diethylaniline system (37 nsec34) exceed the reciprocal molecular (xLa -> A) radiative decay constants by a factor of 10. It is unlikely that the forbidden nature of the exciplex fluorescence in these cases originates in the high symmetry of the emitting species. 

Fig. 12. Variation of lifetime t° and relative yield (y )rei of anthracene-diethylaniline exciplex fluorescence with solvent dielectric constant e (after Knibbe, Rollig, Schafer, and Weller81).

Although this process (with rate constant A IM) produces no additional quenching of molecular fluorescence, the quantum yield of exciplex fluorescence is reduced to... 

Although excimer (exciplex) fluorescence is also exhibited by most dinucleotides,133 the observed phosphorescence from these systems, and from DNA, is characteristic of the lowest molecular triplet state. In the case of DNA at low temperatures this is identified132 as the triplet state of thymine which, in the absence of molecular intersystem crossing, must be populated by intermolecular energy transfer in the triplet manifold or by intersystem crossing from the XAT exciplex.134... 

Figure 4.24 Example of the solvent dependence of an exciplex fluorescence the pyrene/N,N-dime-thylaniline exciplex in cyclohexane (broken line) and tetrahydrofuran (full line). Wavelength A (XlOO) nm...

Frequency of the exciplex fluorescence maximum in nonpolar solvent. ... 

The reaction of t-1 with dimethyl fumarate is proposed to occur via the weakly fluorescent singlet exciplex intermediate (76). Increasing the solvent polarity results in a decrease in both the exciplex fluorescence intensity and the cycloaddition quantum yield, presumably due to radical-ion pair formation. The low efficiency of cycloaddition from c and the absence of triplet cycloaddition indicate that a planar stilbene chromophore is necessary for exciplex formation (see also Sections V-B and C). 

Mechanistic investigations by Chapman and co-workers (99) indicated that these reactions occurred via a nonfluorescent singlet exciplex intermediate. While the rate constant for quenching of - -t5 by 2,3-dimethy 1-2-butene is slower than the rate of diffusion (Table 8), the limiting quantum yield for cycloaddition is 1.0. Thus, highly efficient exciplex cycloaddition may account for the absence of exciplex fluorescence, as in the case of t-1 photodimerization. Photochemical [2+2] cycloaddition reactions have also been observed to occur upon irradiation of the cyclic c-1 analogues diphenylcyclobutane (7) and diphenyl-vinylene carbonate (10) with 2,3-dimethyl-2-butene (96) however, the mechanistic aspects of these reactions have not been investigated. 

Exciplex fluorescence is observed upon quenching of 3-t (but not 3-7 ) by N-methylpyrrole (117) and tertiary aliphatic amines (113-116) with < 0.90 in nonpolar solvents, but not for... 

TABLE 11. trans-Stilbene Fluorescence Quenching and Exciplex Fluorescence Data for Amine Quenchers3... 

Increasing the solvent polarity results in a red shift in the -t -amine exciplex fluorescence and a decrease in its lifetime and intensity (113), no fluorescence being detected in solvents more polar than tetrahydrofuran (e = 7.6). The decrease in fluorescence intensity is accompanied by ionic dissociation to yield the t-17 and the R3N" free radical ions (116) and proton transfer leading to product formation (see Section IV-B). The formation and decay of t-17 have been investigated by means of time resolved resonance Raman (TR ) spectroscopy (116). Both the TR spectrum and its excitation spectrum are similar to those obtained under steady state conditions. The initial yield of t-1 is dependent upon the amine structure due to competition between ionic dissociation and other radical ion pair processes (proton transfer, intersystem crossing, and quenching by ground state amine), which are dependent upon amine structure. However, the second order decay of t-1" is independent of amine structure... 

The observation of decreased exciplex fluorescence intensity and increased adduct formation with increasing solvent polarity (Fig. 10) led to the proposal that adduct formation proceeds via initial one-electron transfer to yield a radical ion pair, followed by proton transfer to yield a 1,2-diphenylethyl and a-di-alkylaminoalkyl radical pair, which subsequently combines to yield 63, disproportionates or diffuses apart (114). Subsequent investigation of this reaction led to the proposal that proton transfer occurs only from the initially formed exciplex or contact radical ion pair prior to solvation to yield a solvent separated radical ion pair. The detailed mechanism for reaction of It with tertiary amines in acetonitrile solution is summarized in Fig. 11 (116c). 

FIGURE 10. Relative quantum yields for exciplex fluorescence (filled symbols) and addition product formation (open symbols) versus solvent dielectric constant for trans-stilbene with di isopropyl methyl amine (0)> ethyldiisopropylamine (A), and triethylamine ( ) in hexane-ethyl acetate and ethyl acetate-acetonitrile mixed solvents. From ref. (114) with permission of the American Chemical Society. 

The last three entries in Table 13 reflect a marked preference for a-methylene versus methyl proton transfer for electron-withdrawing a substituents. These amines are also unusual in that they react with t in nonpolar solvents and do not display exciplex fluorescence. While this unusual behavior was initially attributed to a free radical hydrogen atom transfer mechanism leading to the formation of exceptionally stable "merostabilized" a-aminoallyl radicals (115), our current view is that the high kinetic acidity of the a-C-H bond of these amines when complexed with t is responsible for their behavior. 

The addition reactions of It with amines are also presumed to occur via exciplex or radical-ion pair intermediates however, exciplex fluorescence is observed only under conditions where chemical reactions do not occur. Transfer of hydrogen from the amine a-C-H (tertiary amine) or N-H (secondary amine) bond results in the formation of a radical pair which ultimately gives rise to stilbene amine adducts and other free-radical derived products. The radical-ion pairs can also be intercepted by external electrophiles and nucleophiles, leading to formation of radical-ion-derived products. 

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],... 

Leismann et al.[182] have recognized this problem in their publication of 1984, in which they describe a thorough and detailed investigation of the kinetics of formation and deactivation of exciplexes of. S) benzene or toluene and 1,3-diox-ole, 2,2-dimethyl-l,3-dioxole, and 2,2,4-trimethyl-l,3-dioxole. The evolution in time of monomer and exciplex fluorescence after excitation using a nanosecond flash lamp was analyzed, and again it was concluded that the formation of exciplexes is diffusion controlled their decay proceeds mainly (>90%) via radiationless routes. The polar solvent acetonitrile enhances radiationless deactivation, possibly by promoting radical ion formation. Because decay of benzene fluorescence is essentially monoexponential, dissociation of the exciplex into Si benzene and dioxole is negligible. 

Additional quenching experiments (1,3-cyclohexadiene quenches both 1,4-dicyanonaphthalene fluorescence and 1,4-dicyanonaphthalene-indene exciplex fluorescence) further supported the triplex mechanism outlined in Scheme 8. In addition, experiments under high pressure conditions also confirm the triplex mechanism for nonpolar or weakly polar solvents [64], However, no unequivocal proof for the triplex intermediate in these reactions exists. 

Unified Theory of Scheme I The quantum yield of exciplex fluorescence is... 

Quenching of the exciplex fluorescence by a third molecule may lead to a termolecular complex called triplex. It was first observed by Beens and Weller [47] and has since been shown to be a general phenomenon [48-53]. Each exciplex shows a marked preference for quenching by donor (DQ) or acceptor (Aq). The species approximately represented by Ds+. .. A8+ should be quenched from the D side by Dq, i.e., Dq+. .. D8+. .. A8-, or from the A side by Aq, i.e., Ds+. .. A8-... F Aq Caldwell s results showed a quenching preference when the interaction occurred with the exciplex component of lower singlet energy. Thus the exciplex of 9-cyanophenanthrene (9-(NP) and p-(isobutenyl) anisole (p-BA) clearly prefers Aq, while the exciplex of phenanthrene and FN prefers DQ. 

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