Exciplex singlet

The photophysical investigation of the exciplex formed between 4,4, 4"-tris[3-methylphenyl(phenyl)amino] triphenylamine (m-MTDATA) and 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-l,3,4-oxadiazole (PBD) in a 50 50 blended film showed that the mechanism behind extra singlet production was consistent with the photoluminescence being enhanced via thermally activated delayed fluorescence (E-type nature). Measurements of the emission intensity change with temperature were used to estimate the exciplex singlet-triplet energy splitting to be around 5 meV. 

If a triplet-state molecule (A ) meets a singlet-state molecule (B ), a short-lived complex can be formed (an exciplex). In the latter, the molecules exchange energy, returning to its singlet state (A ) and B raised to its triplet state (B ). If the new triplet state is relatively long-lived, it can serve to produce the population inversion needed for lasing, as in the He/Ne laser. 

These reactions are believed to proceed through a complex of the alkene with a singlet excited state of the aromatic compound (an exciplex). The alkene and aromatic ring are presumed to be oriented in such a manner that the alkene n system reacts with p orbitals on 1,3-carbons of the aromatic. The structure of the excited-state species has been probed in more detail using CAS-SCF ab initio calculations. ... 

It is evident from the exceptions noted that the mechanism proposed above does not fully capture the pathways open to the Patemo-Biichi reaction. A great deal of effort has been devoted to deconvoluting all of the possible variants of the reaction. Reactions via singlet state carbonyls, charge-transfer paths, pre-singlet exciplexes, and full electron transfer paths have all been proposed. Unfortunately, their influence on product... 

The irradiation is usually carried out with light of the near UV region, in order to activate only ihc n n transition of the carbonyl function," thus generating excited carbonyl species. Depending on the substrate, it can be a singlet or triplet excited state. With aromatic carbonyl compounds, the reactive species are usually in a Ti-state, while with aliphatic carbonyl compounds the reactive species are in a Si-state. An excited carbonyl species reacts with a ground state alkene molecule to form an exciplex, from which in turn diradical species can be formed—e.g. 4 and 5 in the following example ... 

The primary interaction of singlet oxygen, produced by energy transfer from the excited sensitizer, with the diene can give rise to an exciplet that then collapses to peroxide, to a 1,4-biradical or to a 1,4-zwitterion alternatively, the adduct is the result of a concerted action without the involvement of an intermediate. Detailed kinetic Diels-Alder investigations of singlet oxygen and furans indicate that the reactions proceed concertedly but are asynchronous with the involvement of an exciplex as the primary reaction intermediate [63]. 

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) ... 

The products consistently exhibit retention of the original olefin stereochemistry and are probably formed in a concerted manner. An exciplex formed from singlet excited benzene and the ground state olefin (allowing relaxation of the orbital symmetry requirements for concerted 1,3- and 1,4-cycloaddition) has been proposed to account for these products/126 Srinivasan and Hill reported an unusual photochemical addition to benzene to form cycloadduct (52)<74) ... 

A singlet naphthalene or a singlet exciplex is thought to be the reactive species in this reaction since the quantum yield of cycloaddition parallels the quenching of naphthalene fluorescence by acrylonitrile. 

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). 

The [2 + 2] photocycloaddition reaction of enones with allenes was first reported in 1966. A diradical intermediate is formed from a triplet enone via an exciplex. The triplet diradical cyclizes to the product after spin inversion to the singlet state [31,32]. 

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

A. Weller, Singlet- and triplet-state exciplexes, in The Exiplex (M. Gordon and W. R. Ware, eds.), p. 23, Academic Press, New York, San Francisco, London, (1975). 

In 1971, adrenodoxin, an iron-sulfur protein with a single tyrosine residue and no tryptophan was shown to fluoresce at 331 nm upon 280-nm excitation at neutral pH/20 1 On cooling from room temperature to 77 K, the emission maximum shifts to 315 nm. The redox state of the iron does not have any effect on the tyrosine emission. From these results, an exciplex between the excited singlet state of tyrosine and an unidentified group was suggested as the cause of the anomalous emission energy/2031 Later studies have shown that the excitation spectrum is a red-shifted tyrosine spectrum, that removal of the iron to form the apoprotein has no effect on the emission, and that heat, low pH, guanidine hydrochloride, urea, and LiCl all cause the emission... 

The stereoselectivity of anti-Markovnikov adducts (161) and (162) produced through photo-induced electron-transfer reaction of (160) with MeOH in MeCN depends on the optimum structures and stabilities of the corresponding radical and carbanion intermediates (163) and (164). In PhH, steric hindrance in an exciplex, comprising an excited singlet sensitizer and (160), forced cis addition of MeOH to (160) to give trans-isomer (161) as the major addition product. 

Gorman and coworkers found ° that the reactions of singlet oxygen with a variety of substrates have negative activation enthalpies and concluded that a reversible formed exciplex is the intermediate. They pointed out that none of the experiments clearly distinguishes between formation of a perepoxide or a geometrically similar exciplex. 

The most recent work210 on sensitized decomposition of peroxides dealt primarily with ketones (cf. Section III.C.3) although it was also demonstrated that anthracene could sensitize decomposition of benzoyl peroxide. In view of the earlier work206,209 and the fact that benzoyl peroxide quenches anthracene fluorescence at a rate of 1.0 x 1010M 1 sec-1 in benzene,211 a singlet exciplex mechanism is strongly implicated. 

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