Exciplex formation quantum yields

Various spectroscopic techniques and probes have been used to investigate solubilization of probe molecules, mostly using UV/visible spectroscopy, fluorescence spectroscopy, ESR spectroscopy [64, 74, 217, 287] and NMR-spectro-scopy [367-369]. Fluorescence spectroscopy is particularly versatile [370], as various static and dynamic aspects can be covered by studying excitation and emission spectra, excimer or exciplex formation, quantum yields, quenching, fluorescence life-times, fluorescence depolarization, energy transfer etc. 

The quantum yield for the formation of the cycloaddition product has been found to be temperature dependent, increasing by a factor of approximately three as the temperature is lowered from 65 ( = 0.24) to 5°C ( = 0.69). Photolysis of mixtures of the olefin and f/my-stilbene in the presence of sensitizers yielded no cycloaddition product (42) but rather only m-stilbene. This suggests that the cycloadduct is produced via a singlet reaction. This conclusion is supported by the fact that tetramethylethylene quenches fluorescence from the /rans-stilbene singlet. A plot of l/ (42) vs. 1/[TME] (TME = tetramethylethylene) is linear. The slope of this plot yields rate constants for cycloadduct formation which show a negative temperature dependence. To account for this fact, a reversibly formed exciplex leading to (42) was proposed in the following mechanism<82) ... 

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. 

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). 

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. 

Cycloaddition is a singlet state reaction, triplet quenching yielding only stilbene isomerization. In the limit of high t-1 concentration, the quantum yield for the formation of 89 and 90 is 0.66 and no c-1 is formed. Nonradiative exciplex decay is proposed to occur by partitioning at the pericyclic minimum (Fig. 2) between products and reactants. In the limit of high c-1 concentration, 91 is formed with a quantum yield of 0.05 and the predominant exciplex decay pathway is dissociation to yield f-c, which decays to a mixture of t-1 and c-1. 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

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)... 

A kinetic scheme was proposed [122] with the fluorescent exciplex as precursor of the photoproducts (ortho as well as meta adducts). Quantum yields of adduct formation, exciplex emission, and benzene fluorescence were measured as a function of alkene concentration. The kinetic data fit the proposed reaction scheme. The authors have also attempted to prove the intermediacy of the exciplex in the photoaddition by adding a quencher to the system benzene + 2,2-dimethyl-... 

In cyclohexane, no exciplex emission is observed. Leismann et al. [182] have measured the quantum yields of meta and ortho adduct formation from benzene and 2,2-dimethyl-l,3-dioxole in cyclohexane at various concentrations of the... 

Farid et al. reported the formation of two types of (4 + 2) photocycloadduct of 9,10-dicyanoanthracene (DCA) with 3-carbomethoxy-l,2-diphenylpropene [192,193], The product ratio depends on the solvent polarity. In benzene, exo-125 is selectively obtained via exciplex (Scheme 38). In acetonitrile, endo-125 is obtained as a sole product via the radical ion pair. Photochemical reactions of DCA with 1,2-diarylcyclopropanes gave (4 + 3) cycloadducts [194,195], In degassed acetonitrile solution, (4 + 3) photocycloaddition occurred to give cis and trans cycloadducts in a 3 1 ratio in good chemical yields, although the quantum yields... 

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. 

In anthracenocryptand 23, exciplexes are formed between the anthracene and nitrogen ion pairs.60,61,63,65,140 In MeOH, the quantum yield dramatically decreases due to the formation, via exciplex intermediates, of nonfluorescent radical ions. Upon addition of an excess of K+, Ag+, or Tl+ to methanolic solutions of 23, 1 1 cryptate-type complexes are formed.61,65 Complexation causes drastic changes in the spectroscopic properties. Cations such as Na+, for example, decrease the intensity of the exciplex emission and increase the intensity of the structured anthracene emission. Heavy-metal ions (Ag+, Tl+) interact strongly with the central ring of anthracene as shown by an exciplex-type emission observed for the Ag+ complex of 23. 

A most interesting example of intramolecular exciplex formation is afforded by 1-naphthy 1-pentamethydisilanes (Shizuka et al., 1981). Exciplex fluorescence is observable in solvents of widely differing polarity. Presumably the disilane cannot adopt a sandwich configuration and therefore the apparently high quantum yields for exciplex formation are somewhat surprising. 

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