Wurster blue cation

The energy of a single photon is obviously insufficient to ionize an organic compound. As early as the nineteen forties (3, 4), however, it -was observed that Wurster blue cation radical is produced by photoirradiation of 3-methylpentane glass containing N,N-tetramethyl p-phenylenediamine (TMPD) at 77° K. The recent detailed study of this system by electric conductivity measurement (5, 6) and electronic spectroscopy (7) provided conclusive evidence that the ionization is brought about via excitation to the triplet state followed by successive photoabsorption at the triplet state. This mechanism is supported by the facts that the life-time of the photochemical intermediate is identical with that of phosphorescence and the formation of Wurster blue, and that phosphorescence is inhibited in the presence of triplet scavengers. 

Fig. 3.4 ESR spectrum of the Wurster blue cation, synthesized from experimental parameters...

Even more complex ESR spectra can occur for nitrogen-containing aromatic radicals of the type in Fig. 3.4. A stickplot analysis is not easily performed. At this level of complexity a combination of ESR and ENDOR measurements, simulation of the observed spectra, and theoretical calculations of hyperfine coupling constants is often applied to obtain a reliable assignment. The procedure to obtain the coupling constants from the ENDOR spectrum of the Wurster blue cation is indicated in Exercise E3.3. ENDOR lines due to can be observed more easily than signals due to " N, a phenomenon that is quite typical in CW-ENDOR studies. The hyperfine couplings due to N nuclei may therefore have to be deduced from simulation of the ESR spectrum. 

E3.3 The ENDOR lines of the Wurster blue cation discussed in the text (Section... 

Schematic X-band ENDOR spectrum for Wurster blue cation in solution with lines due to two inequivalent H and two equivalent nuclei...

In one of the first experimental studies where ion radical annihilation in solution was considered as an emissive possibility, Yamamato, Nakato, and Tsubomura61 found that Y,Y,Y, Y -tetramethyl-p-phenyl-enediamine (TMPD) and pyrene when irradiated in the ultraviolet in a glass at low temperature formed Wurster s blue cation radical, pyrene anion radical, and solvated electrons. When the glass was warmed, thermoluminescence was observed. A similar emission was observed when a previously irradiated mixture of TMPD and 2-methylnaph-thalene was warmed. The emission in both instances was ascribed to charge-transfer fluorescence resulting from combination of a cation radical with an anion radical. 

Exciplex chemiluminescence has been observed by Weller and Zachariasse128 from the flow reactions of aromatic hydrocarbon anions M " and Wurster s Blue Cations Q +. Since the processes... 

Another interesting species is WWAf W -tetramethyl-p-phenylenediamine, the parent neutral of the well-known radical cation, Wurster blue. Its enthalpy of formation as a solid75 is 39.6 3.9 kJ mol 1. Equations 16 and 17 might be expected to be thermoneutral. The calculated enthalpy of reaction of the former is 5.9 kJmol-1 and that of the latter is —7.1 kJ mol 1, adequate indication that the enthalpies of formation are essentially accurate. 

Formation of dicationic dimers (90) has been longer known than formation of the monocationic type (89), no doubt because of the availability of and interest in the very stable Wurster Salts. Hausser and Murrell (1957) proposed that the long wave-length absorption band (near 800 nm) of Wurster s Blue perchlorate in ethanol at —90° was caused by two associated, cation radicals lying in parallel planes. Since that time a considerable number of workers have explored the dimerization of Wurster and analogous cation radicals, (e.g. Kawamori et al., 1966 Kimura et al., 1968). Not only does Wurster s Blue cation radical (i.e. TMPD +) associate with itself, but it also forms a spin paired dimer with p-phenylenediamine cation radical (PD +). In fact, Takimoto et al. (1968) conclude from absorption spectroscopy that solutions of TMPD + and PD + in ethanol-ether at —195° contain (PD"+)2 and (PD"+-TMPD +)2 but very little of (TMPD +)2. Dimerization of unlike cation radicals is known in other systems too. Perylene" and naphthacene4-each forms an (M"+)2 dimer in sulfuric acid at reduced temperatures (Kimura et al., 1971). Mixtures of the two cation radicals in sulfuric acid leads to a mixed dimer too, (Perylene "+, naphthacene +), the heat of formation of which (—7-7 kcal mole-1) incidentally, lies between that of the perylene"+ (—8-8) and naphthaccne + (— 5 6) dimers (Yamazakiand Kimura, 1972). 

One of the most studied examples of complete electron transfer in a complex where both donor and acceptor are organic species is the tetramethyl-p-phenylenediamine-chloranil case. In polar solvents (see also the section on solvent effects, p. 101) the absorption spectrum is a superposition of what one observes separately for the Wurster s blue cation radical (III) and chloranil radical anion (IV). A recent report contains a survey of the older literature concerning electron-spin resonance studies of this complex. Often the free-radical species initially produced in such reactions undergo further slow chemical changes, not infrequently to intractable products. 

Exceeding the limitation of molecular dynamics, the steric requirement of trimethylsilyl groups can cause drastic changes both in structure and of molecular properties of organosilicon compounds. For illustration, the so-called "Wurster s-Blue11 radical ions are selected On one-electron oxidation of tetramethyl-p-phenylenediamine, its dark-blue radical cation, detected as early as 1879 [11a], is gene-... 

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

Light emission occurs during the reaction of numerous radical anions of aromatic hydrocarbons with radical cations such as Wurster s red 103, Wurster s blue 104 or radical cations derived from triarylamines of the type 105, 106. 

One-electron oxidation of aniline derivatives gives a radical-cation in which the unpaired electron is distributed over both the nitrogen atom and the aromatic system. The further reactions of these intermediates more resemble those of aromatic compounds than of aliphatic amines. Some of the radical-cations are very stable in solution Wurster s blue, prepared by oxidation of tetramethyl-1,4-pheny ene-diamine [152], and Wurster s red from N,N-dimethyI-l,4-phenylenediamine [153] have been known since 1879. They were recognised as radical-cations by Mi-chaelis [154]. 

Some easily formed cation radical salts, particularly those of the Wurster s blue type, may be prepared and isolated in protic media.36 Such cation radical salts may then be dried and slurried in aprotic solvents (their solubility is usually very low in such media) and will produce emission when caused to react with etheral solutions of aromatic anion radicals and other energetic reductants.15,24... 

The photooxidation of p-phenylenediamine to the Wurster s Blue radical cation apparently proceeds by photoionization of the excited triplet state of the neutral molecule,219 and it has been suggested that the delayed fluorescence of perylene may be partly due to photoionization of its triplet state and slow subsequent recombination of... 

A particularly stable cation-radical of the semiquinone type is formed by mild oxidation of N,N,N, N -tctramethyl-l, 4-bcnzencdiamine. The cation, which is isolable as a brilliant-blue perchlorate salt, 2, is called Wurster s Blue ... 

Exercise 26-17 Write resonance structures that account for the stability of the cation of Wurster s salts, such as Wurster s Blue, 2. Explain why N,N,N, N -2,3,5,6-octamethyl-1,4-benzenediamine does not form a similarly stable cation radical. 

Bard etaL 5S6>5571 and Visco etaL 558) have quantitatively analyzed the intensity of pulsed ECL of 9,10-diphenylanthracene, tetraphenylpyrene and rubrene. By computer simulation of the electrode process and the subsequent chemical reactions the rates for chemical decay of the radical ions could be determined. Weaker ECL with fluorescence emission 559 or electrophosphorescence S60) occurs if the radical anion R - reacts with a dissimilar radical cation R,+ of insufficient high oxidation potential to gain enough energy for fluorescence emission, that is, if ht fluorescence) >23.06 (Ej >+. -Ej -.), e.g., in the annihilation of the anthracene radical anion with Wurster s blue. For these process the following schemes are assumed (Eq. (242) ) ... 

The starting point is the Wurster s Blue radical cation, discovered in 1879 and thoroughly investigated since then The twofold N pyramidal IVWW. /V -tetraalkyl-p-phenylenediamine precursors are completely flattened on two-electron oxidation as proven by crystal structure analyses of the resulting redox salts (Scheme 8a). 

Using the experimental setup and conditions mentioned earlier, Jean-maire and Van Duyne (77) studied one-electron oxidation of N,N,N, N -tetramethyl-/7-phenylenediamine (TMPD) to its radical cation (TMPD +, Wurster s Blue). 

Optical absorption spectroscopy of organic radical cations was pioneered by Hoijtink and coworkers and others before the advent of PE spectroscopy, but it was limited for a long time to aromatic amines, polycyclic aromatic hydrocarbons and similar compounds whose radical cations could be generated under stable conditions by chemical oxidation. It was observed that many colourless neutral compounds give rise to intensely coloured radical cations which indicates that excited states of these reactive species lie generally at much lower energy than those of the neutral parent molecules (the most famous example is perhaps Wurster s blue, the radical cation of the colourless tetramethyl-p-phenylenediamine). 

D-Xylitol pcniaiulraie with teiramethyl-p phenylenc diaminc (TMPD) gives a blue colour due to the formation of Wurster cation after which another charge-transfer complex is formed by 2 moles of xylitol pentanitrate and 1 mole of TMPD dication formed from Wurster cation [13-17],... 

Wurster salts (Sidgwick, 1966) are usually obtained by oxidizing diaminobenzenes (p-phenylenediamines) with bromine in methanol-acetic acid solution. Ibis leads to the Wurster bromide (Wurster and Sendtner, 1879), but if oxidation is carried out in the presence of a large amount of perchlorate ion the Wurster perchlorate is formed. This is the way in which Wurster s Blue [ 1 ] perchlorate (the most widely studied cation radical salt of all) and Wurster s Red [2] perchlorate are made (Michaelis and Granick, 1943). The chlorides and iodides are also known (Oohashi and Sakata, 1973 Sakata and Nagakura, 1970). 

Wurster s Blue perchlorate has been the subject of many investigations. Its stability as a solid and in solution, its ease of preparation, and its symmetry make it an ideal model in spectroscopic and magnetic susceptibility studies. It was one of the first cation radicals to be shown to exist both as a monomer and as a dimer in solution (Hausscr, 1956 Bolton et al., 1962) and in the solid state (Duffey, 1962 McConnell and Lynden-Bell, 1962), the dimeric form being obtainable reversibly by lowering the temperature. This phenomenon is widespread among cation radicals and is discussed in section 3. 

Most oxy or halo free radicals readily oxidize phenjienediamines to radical cations called Wurster salts (12), which are stable in water-ethanol solutions at pH 3. On the other hand, carbon and sulfur radicals generally do not produce Wurster salts. The intense color of Wurster salts can be used as a quick spot test for variously substituted phenylenediamines when oxidized with bromine in carbon tetrachloride solution. For example, Al,AT-diaIkyl substitution gives red, N,N,N N gives blue, and N-alkyl-TST-aryl gives light blue or green. 

The aminoarylation of 9-methylacridinium salts with aromatic amines provides a well-established example of a SET from arylamines to the acridinium ion, as evidenced by the formation of diacridanyl and arylamine radical cation species (Scheme 62). Also, treatment of W-methylacridinium ion with W,W -tetramethyl-para-phenylenediamine, as the model compound, gave the characteristic color of the Wurster s Blue radical cation [11, 136, 137]. 

The reduction of co-ordinated oxygen by organic substrates has been investigated. Oxygen adducts of 3-methoxysalen-Co in pyridine are formed reversibly, and reaction takes place with substrates (RHg) such as thiols, ascorbic acid, and AW -tetramethyl-/ phenylenediamine (tmpd). Since the latter has no significant ligand properties and Wurster s blue radical cation is formed in the course of reaction as confirmed by e.s.r. and u.v., it is suggested that the reaction proceeds without... 

Wurster s blue perchlorate Feiylene cation (98% H2SO4) Tetracene cation (98% H2SO4) p-Benzosemiquinone anion ... 

In the gas phase, radical cations are more stable and chemically more important than radical anions because the latter can too easily lose their extra weakly bound electrons. In solution, the reverse is the case. Here radical cations are difficult to obtain and so extremely reactive that they usually disappear almost as soon as they are formed. Only in a few cases are they stable enough to survive in solution, the exceptions involving species highly stabilized by mesomerism, such as Wurster s Blue (43) or the thian-threne radical cation (44). 

Stable radical cations are rare but the famous examples known as Wurster s Red and Wurster s Blue have provided a very interesting series of reactions. The first examples of energy deficient chemiluminescence were discovered during this investigation. 

In polar solvents, however, there is no exciplex emission. The free radical ions are formed, and they can produce chemiluminescence by annihilation. The recombination reaction between perylene radical anion (14) and tetramethyl-p-phenylene diamine radical cation ( Wurster s Blue ) in 1,2-dimethoxyethan (= DME) was the first bright chemiluminescence of this type [34]. 

Of course, the same energetic requirements as in the direct production of excited singlet states ( S-route ) or the production of the latter via triplet states ( T-route ) are valid here as in ECL (see p. 130). As an example, the chemiluminescent reaction of Wurster s Blue radical cation (12) with chrysene radical anion (13) must occur via the T-route . The emission spectrum matches chrysene fluorescence, but the (12)/(13) redox reaction has an enthalpy corresponding to 2.66eV only, whereas the energy of the chrysene first excited singlet state is 3.43 eV. 

A large number of aromatic hydrocarbon radical anions has been reacted with Wurster s Blue-type radical cations, e.g. the radical anions of 1-phenylnaph-thalene, 8,8 -dimethyl-naphthalene, 1,1 -binaphthyl, p-terphenyl, chrysene, 1,2-dimethylchrysene [29]. They all represent energy-deficient systems, so that triplet-triplet annihilation had to be regarded as the mechanism for the production of the emitting singlet state. The quantum yields were in the range 10" to 10" Einstein/mol [29]. 

Quantum yields about ten times higher were measured when Wurster s Blue-type radical cations were replaced by those of tri-p-substituted triphenylamines. These radical cations are relatively stable [35]. The following triphenylamine derived radical cations were investigated ... 

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