Fluorescent exciplexes

Photoinduced intramolecular interaction of t-S and tertiary amine moieties linked with a polymethylene chain has also been studied24. The photoexcitation of fraws-stilbene in which a tertiary amine is attached to the ortho position with a (CH2)i-3 linker leads to fluorescent exciplexes by intramolecular electron transfer, and results in no more than trans-cis isomerization. The failure to give adducts from the intramolecular exciplexes could arise from the unfavourable exciplex geometry to undergo the necessary bond formation. 

The quenching of - -t by 1-methylpyrrole in nonpolar solvents results in the formation of a fluorescent exciplex, but no adduct formation (117). Irradiation in acetonitrile (anhydrous or aqueous) solution produces 1,2,3,4-tetraphenylbutane (65) in low yield. Formation of adducts 75 and 76 and 1,2-diphenylethane (64) is observed upon irradiation in acetic acid solution and attributed to protonation of t-lT followed by coupling of the 1,2-diphenylethyl radical and 1-methylpyrrole cation radical and deprotonation of the coupling product ... 

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-... 

Photoirradiation of phenanthrene with acetylacetonatoboron difluoride 137 in ether or dioxane irreversibly forms a fluorescent exciplex which is the precursor to the product d.v-138 [216] (Scheme 44). Trace amount of trans-138 was also obtained. In contrast, the excitation of 137 in the presence of phenanthrene gives neither exciplex emission nor any products. 

Normally w-amino-a-arylalkanes can adopt a conformation which produces exciplex fluorescence by a process involving C—C-bond rotation which is very fast. Both 3-(4-dimethylaminophenyl)-l-(9-anthracenyl)propane and 3-(4-dimethylaminophenyl)-l-(l-pyrenyl)propane form fluorescent exciplexes, which by means of picosecond time-resolved fluorescence spectroscopy have been shown to take a few nanoseconds to be formed (Migita et al., 1980, 1981). The rate of intramolecular fluorescent exciplex formation has also been shown to be dependent upon the length of the linking chain, the polarity of the solvent (the build up time decreases as solvent polarity is increased)... 

Conformational requirements for fluorescent exciplex formation It was stated earlier that the fluorescent exciplex of [36 = 2] has a sandwich conformation. This statement was based on earlier studies of exciplex formation of naphthylalkylamines (Chandross and Thomas, 1971 Brimage and Davidson, 1971). It was found that when a dimethylene and trimethylene chain was interposed between the amino and hydrocarbon groups that... 

Triplet production in the quenching of aromatic hydrocarbon fluorescence by amines has usually been monitored by flash photolysis. A particularly good example is the study of the quenching of the excited singlet state of pyrene by l,4-diazabicyclo[2.2.2]octane in cyclohexane solutions (Delouis et al., 1979). Fluorescent exciplex formation is not observed with this system but triplet formation does occur. By triplet-triplet absorption spectroscopy... 

Deuterium labelling studies have also been used to investigate the reaction of stilbenes and related compounds with amines (Lewis, 1979). It is known that tertiary amines form fluorescent exciplexes with stilbenes in nonpolar solvents and that polar solvents are necessary for chemical reaction to occur (Lewis and Ho, 1977). This suggests that radical ions are involved in product formation. When secondary amines are used, reaction occurs in solvents of widely differing polarity and this is presumably due to the acidity of the secondary N—H bond. N-deuteriated diethylamine reacts with 1,2-diphenyl-cyclobutene in benzene to give products [65], [66] and [67] incorporating deuterium (Scheme 6). For the reaction with unsymmetrically substituted... 

As discussed earlier, deprotonation of a-carbon forms a major reaction pathway for the disappearance of the amine radical cation. Studies of photoinduced electron-transfer reactions of tertiary amines by Lewis [7, 11] and by Mariano [5, 10] have contributed significantly towards our understanding of the factors that control this process. Lewis and coworkers used product-distribution ratios of stilbene-amine adducts to elucidate the stereoelectronic effects involved in the deprotonation process [5, 10, 121, 122]. In non-polar solvents, the singlet excited state of tran -stilbene forms non-reactive but fluorescent exciplexes with simple trialkylamines. Increasing solvent polarity brings about a decrease in the fluorescence intensity and an increase in adduct formation. For non-symmetrically substituted tertiary amines two types of stilbene-amine adduct can be formed, as is shown in Scheme 9, depending on whether the aminoalkyl radical adding to the stilbene radical is formed by de-... 

The potential stereoselectivity of this photocyclization process has recently been investigated for the two rigid 2-allylanilines 70 and 71169. Irradiation of compound 70 at room temperature gives a mixture of diastereomers trans-12 and cis-12 with a little stereoselectivity (equation 23). While a poor stereoselectivity is also observed for 71, the photocyclization is regioselective, where the products trans-12 and cis-12 are minor (equation 24). However, the diastereoselectivity of trans-12 vs cis-12 is increased in the case of 71 when the temperature is changed, indicating that the reaction is significantly entropy-controlled. In addition, the observation of fluorescent exciplex formation for 70 and 71 supports the electron-transfer mechanism for the photocyclization of 2-allylanilines. 

Exciplex Emission A solution to obtain white light while reducing the number of layers and dopants is to use a broadly emitting exciplex (i.e., an excited state formed by two charges located on two dissimilar molecules). Fluorescent exciplex systems exhibit low efficiency [252], but phosphorescent ones lead to broad efficient WOLED [253]. 

Our investigation of the mechanism of intramolecular adduct formation employed the technique of arene-amine exciplex quenching by primary amines, which had been developed in earlier investigations of exciplex quenching. These experiments provided evidence for the occurrence of adduct formation via proton transfer in the fluorescent exciplex. In the case of 39, activation parameters for exciplex formation and proton... 

Data on photophysics and photochemistry of intramolecular stilbene-amine exciplexes and reactions have been reported [34]. The obtained data indicated that the photophysical and photochemical behavior of a series of traws-(aminoalkyl) stilbenes in which a primary, secondary, or tertiary amine is appended to the stUbene ortho position with a Me, Et, or Pr linker. The tertiary (aminoalkyl) stilbenes formed fluorescent exciplexes and underwent trans cis isomerization but failed to undergo intramolecular N—H addition. The secondary (aminoalkyl)stilbenes did not form fluorescent exciplexes but underwent the addition to the stilbene double bond. Intramolecular reactions were highly selective, providing an efficient method for the synthesis oftetrahydrobenzazepines. Direct irradiation of the primary (aminoalkyl) stilbenes resulted only in trans-cis isomerization, while irradiation in the presence of the electron acceptor p-dicyanobenzene resulted in regioselective intramolecular N—H addition to the stilbene double bond. 

For charge transfer leading to a fluorescent exciplex, aU rate constants can be evaluated from the fluorescence decays, but particular attention should be paid to the possibility of occurrence of (1) transient effects, (2) the harpoon mechanism [70] (the electron goes first and then the exciplex is formed) and (3) ground-state charge-transfer complexes. All these phenomena lead to deviations from doubleexponential decays and/or differences between Stem-Volmer plots obtained from time-resolved (tq/t vs [Q]) and steady-state (Iq// vs [Q]) measurements. 

Formation of a fluorescent exciplex = excited complex with a compound P)... 

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