Radical cation of anthracene

On the contrary, the radical cation of anthracene is unstable. Under normal volt-ammetric conditions, the radical cation, AH +, formed at the potential of the first oxidation step, undergoes a series of reactions (chemical -> electrochemical -> chemical -> ) to form polymerized species. This occurs because the dimer, tri-mer, etc., formed from AH +, are easier to oxidize than AH. As a result, the first oxidation wave of anthracene is irreversible and its voltammetric peak current corresponds to that of a process of several electrons (Fig. 8.20(a)). However, if fast-scan cyclic voltammetry (FSCV) at an ultramicroelectrode (UME) is used, the effect of the follow-up reactions is removed and a reversible one-electron CV curve can be obtained (Fig. 8.20(b)) [64], By this method, the half-life of the radical cat-... 

The radical cation of 9,10-diphenylanthracene is much more stable than that of anthracene, because, with 9,10-diphenylanthracene, the 9- and 10-positions, which are reactive because of the high unpaired-electron densities, are masked by phenyl groups and the unpaired electrons are delocalized. The stabilization of the radical cation of anthracene also occurs by introducing other substituents like -NH2 and -OCH3 in the 9,10 positions. 

Figure 1.6 Simulated fluid solution EPR spectrum of the radical cation of anthracene, v = 9.5 GHz, llnewldth = 0.15 G, 0, = 4.89G, fl2 = 1-81 G.

The addition of anthracene to maleic anhydride (Fig. 11) was reported to be accelerated by sonication. From a mechanistic study in the presence of electron carriers, an electron transfer process was ruled out. These results could not be reproduced, and no difference between the sonochemical and thermal rates and yields was observed (adduct formation in 30% after 1 h, 50% after 3 h, with or without sonication). In the presence of monoelectronic oxidizers such as ferric chloride,or tris(4-bromophenyl)aminyl hexachloroantimonate (TBPA),28a,c change was noted in these figures, although the radical cation of anthracene was formed.43 This radical cation is not involved in the reaction pathway. 

Luche and coworkers [34] investigated the mechanistic aspects of Diels-Alder reactions of anthracene with either 1,4-benzoquinone or maleic anhydride. The cycloaddition of anthracene with maleic anhydride in DCM is slow under US irradiation in the presence or absence of 5% tris (p-bromophenyl) aminium hexachloroantimonate (the classical Bauld monoelectronic oxidant, TBPA), whereas the Diels Alder reaction of 1,4-benzoquinone with anthracene in DCM under US irradiation at 80 °C is slow in the absence of 5 % TBPA but proceeds very quickly and with high yield at 25 °C in the presence of TBPA. This last cycloaddition is also strongly accelerated when carried out under stirring solely at 0°C with 1% FeCh. The US-promoted Diels Alder reaction in the presence of TBPA has been justified by hypothesizing a mechanism via radical-cation of diene, which is operative if the electronic affinity of dienophile is not too weak. 

So far, only a few examples of cationic photopolymerizations using PET corresponding to Scheme 3 have been described [10,13,165]. In the ternary system cyclohexene oxide, 9.10-dicyano anthracene and polynuclear aromatics, the polymerization of the former is initiated by the radical cations of the aromatic hydrocarbons formed via the PET with the dicyano compound. 

In the field of hydrocarbons, radical cations of extended 7t systems tend to form 7t dimers under appropriate reaction conditions. This was established for classical aromatics such as naphthalene or anthracene. In cyclophanes, the geometric restrictions force the aromatic constituents to interact through space. 

Figure 4 (a) Transient absorption spectrum of the cation radical from anthracene at -40 ps following the 332 nm CT excitation of the OSO4 complex with a 30 ps (fwhm) laser pulse. The inset is the steady-state spectrum of Ar obtained by spectroelectrochemical generation, (b) Appearance and decay of the radical cation from anthracene by following the change of the absorbance at X 742 nm. The inset shows the first-order plot of the absorbance decay subsequent to the maximum at -20 ps... 

Photoinduced electron transfer between amines and aromatic hydrocarbons occurs to generate radical cations of amines and radical anions of aromatic hydrocarbons. Pac and Sakurai reported the photoaddition of N,N-dimethylaniline to anthracene via photoinduced electron transfer [60]. In benzene, the 4n + 4n) photocyclodimer of anthracene is produced as a sole isolable product, although an emission due to the exciplex formed from anthracene and JV,N-dimethylaniline is observed. In acetonitrile, the addition of dimethylaniline to anthracene occurs via their radical ions to give 9,10- dihydro-9-(4 -dimethylaminophenyl)anthracene as the major product. However, the photoamination on anthracene takes place even in benzene when iV-methylani-line is used as an electron donor. Sugimoto and his coworkers reported the intramolecular photoaddition of anilines to aromatic hydrocarbons to give cyclic amino compounds (Scheme 16) [61-63]. 

However, in certain cases under photolytic conditions, spectra of the corresponding arylmercury radical cations 6 developed, whereas no mercuration occurred in dark [81] signifying collapse of the ArH +,Hg(TFA)2 radical ion pair 4, provides an alternative path way to Wheeland complex 2 and hence to ArHg(TFA) + 6. Arene radical cations can also be generated from arene and thallium(III) tris-(trifluoroacetate) in trifluoroacetic acid [82], but with a different mechanism proposed by Eberson et al. [83]. Oxidation of anthracene showed 9-trifluoroacetoxy and 9,10-bis(trifluoroacetoxy)anthracene [84, 85], benzo[a]pyrene, 7-methylbenzo-[a]pyrene and 12-methylbenzo[a]pyrene yielded radical cations of 7- and/or 12-trifluoroacetates [86], triptycene (9,10-dihydro-9,10-[l,2]benzanthracene) showed... 

With azines the situation is varied. In the radical cations of pyridine and diazines the semi-occupied orbital is largely confined to the nn orbital(s) (see Scheme 2, structure 2), while the radical cation is of the n type with monoazanaphthalenes, -phenanthrenes and -anthracenes. The situation might change with substitution. As an example, alkylpyridine radical cations are of the n type, like the parent compound, whereas for the 2,5-dimethyl, 2-chloro, and 2-bromo derivatives the structure is of the n type [13]. Likewise, with benzo[c]cinnoline the parent compound and its alkyl derivatives give an n radical cation, but with some dimethoxy derivatives a n structure is found [14] and a switch from n to 7t structure occurs also in passing from 1,2,4,5-tetrazine to its 3,6-diamino derivatives [15]. 

The redox properties of donor-rt-acceptor molecules 24 and 40-44 are collected in Table 5 <1998T13257>. The compounds display a reversible one-electron oxidation wave, which corresponds to the formation of the radical cation of the 1,3-diselenole donor moiety, and an irreversible one-electron reduction wave forming the radical anion located on the dicyanomethylene or A -cyanoimine group. The derivatives 24 and 44 are both easier to oxidize and reduce than the other compounds, which is explained by the gain of aromaticity of the spacer unit. The second one-electron reduction of these compounds is ascribed to reduction of the anthracene unit. 

The direct nature of attack of CN on radical cations of aromatic compounds has been demonstrated by CV [221]. The reversible one-electron oxidation of anthracene becomes irreversible in the presence of CN, and 2 F electrolysis gives a mixture of cyano and isocyano addition across the 9,10-position. Interestingly, it appears that cyanation of 9,10-diphenylanthracene gives the 9,10-diphenyl-9,10-dicyano-9,10-dihydroanthra-cene only [233]. 

Benchmark ab initio quantum dynamical studies are carried out for the prototypical naphthalene and anthracene radical cations of the PAH family aiming to understand the vibronic interactions and ultrafast decay of their low-lying electronic states. The broadening of vibronic bands and ultrafast internal conversion through conical intersections in the Da — — D2 electronic states of these species is... 

Naphthalene and anthracene radical cations are the two simplest members in the family of the PAH radical cations. Investigation of the photophysics and photochemistry of the latter are of major concern in contemporary chemical dynamics. The radical cations of PAHs are of fundamental importance in the chemistry of the interstellar space, environmental, biological processes and combustion [103-106]. Radical cations of PAHs are most abundant in the interstellar and extragalactic environments [41]. They absorb strong UV radiation emitted by the young stars and get electronically excited. Examination of the fate of electronically excited PAH radical cations invited critical measurements of their optical spectroscopy in the laboratory in recent years [42 4]. Attempt is made to understand the important issues like, (1) photostability and lack of fluorescence emission and (2) the origin of the enigmatic... 

Some substituted dibenzo-7-silabicyclo[2,2,l]hepta-2,5-dienes are reported to phototransfer an electron to TCNE using 2,4,6-triphenylpyrylium tetrafluoro-borate as sensitizer to give difluorosilane and anthracene as products. The MOP AC MP3 method has been used to examine the electronic features of the radical cation of the parent silane. 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

Scheme 4 describes the electron transfer photosensitization of iodonium salts by anthracene [61,70,91-94]. Singlet anthracene reacts with diphenyliodonium cation by diffusion controlled electron transfer in acetonitrile solution. In-cage decomposition of diphenyliodo radical competes with rapid back electron transfer to yield the singlet radical pair of anthracene cation... 

PET is the key feature of many other photo-oxidations involving aromatic substrates. It has been shown, for example, that pyrene and anthracene which are covalently attached to silica, gold or indium-doped tin oxide (ITO), undergo a photo-oxidation forming dihydroxy/dione derivatives. The reaction involves 02, formed by ET between excited pyrene, or anthracene, and O2, and it is suggested that the implications of such a photodegradation need to be considered when polycyclic aromatic hydrocarbons (PAHs) are used as spectroscopic probes in surface adlayers. The redox photosensitized amination of 1,2-benzo-1,3-cycloalkadienes, arylcyclopropanes, and quadricyclane with ammonia and primary amines, using 1,2,4-triphenylbenzene (1,2,4-TPB) or 2,2 -methylenedioxy-1,1 -binaphthalene in the presence of m- or p-dicyano-benzene (DCB), has been described (Scheme 51). The process involves the formation of the radical cation of 1,2,4-TPB, for example, by PET to the DCB, followed by hole transfer from the radical cation to the substrate, the latter... 

---