9-Methylanthracene, excitation

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

In some molecules, the interaction can develop into a stronger force and the interplanar distance further reduced to form stable photodimers through covalent bonds. For example, anthracene forms a photodimer and no excimer emission is observed, whereas some of its derivatives with bulky substituents which hinder close approach give excimer fluorescence. In 9-methylanthracene both photodimer formation and excimer emission is observed. 9, 10-diphenylanthracene neither forms a photodimer nor emits excimer fluorescence due to steric hindrance. These observations are tabulated in the Table 6.3, which shows that the nature of the excited state is also important. 

The chemical deactivation of photoexcited anthracenes by dimerization usually proceeds by 4re + 4re cycloaddition [8]. However, exceptions to this rule have become known in recent years [8], and a multitude of steps, including the formation of metastable intermediates such as excimers, may actually be involved in a seemingly simple photochemical reaction such as the dimerization of 9-methylanthracene [9, 10]. Moreover, substitution of the anthracene chromophore may affect and alter its excited state properties in a profound manner for a variety of reasons. For example, in 9-tert-butylanthracene the aromatic ring system is geometrically distorted [11,12] and, consequently, photoexcitation results in the formation of the terf-butyl-substituted Dewar anthracene [13-15], The analogous photochemical isomerization of decamethylanthracene [16] probably is attributable to similar deviations from molecular planarity. 

When the concept of adiabatic photoreactions was first tested on the photolytic cycloreversion of the 9-methylanthracene photodimer 104, the formation of singlet excited 9-methylanthracene A (R=methyl) according to... 

Fig. 12. Perrin quenching radii, R, [33J vs. variations of the free energy, - AG°, of electron transfer from the excited donor molecule to the acceptor molecule for donor-acceptor pairs in vitreous /nms-l,5-decalindiol. 1, Rubrene + A/ AT-diethylamline (DEA) 2, rubrene + N,N,-Ar,Ar-tetramethyl-p-phenylenediamine (TMPD) 3, rubrene + tetrakis(dimethylaminoethy-lene) 4, tetracene + DEA 5, tetracene + TMPD 6, 9,10-dinaphthylanthracene + DEA 7, 9,10-dinaphthylanthracene + TMPD 8, perylene + DEA 9, perylene + TMPD 10, 9-methylanthracene + TMPD 11, 9,10-diphenylanthracene + TMPD 12, coronene + TMPD 13, benzo[ Ai jperylene + TMPD 14, fluoranthene + DEA 15, acridine + DEA.

Pincock and Fleming have recently summarized the mechanistic alternatives [15,25]. The probability of the fragmentation reaction has been evaluated first of all on the basis of thermochemical analyses. The energy of the singlet excited state of simple benzylic derivatives is >100kcal/mol (106 for toluene) and drops to 90 for 1-methylnaphthalene and 73 for 9-methylanthracene. The Si-Tx gap is quite large for the nn states of aromatics, and the corresponding values for the triplet are 83 (for toluene), 61,... 

Fig. 8. Transient spectrum (33 pseconds) obtained from the charge-transfer excitation at 532 nm of a solution of 0.2 AT 9-methylanthracene and 3.4 mAf (HMB)2Fe2+. The inset shows the time-resolved spectrum of the 9-methylanthracene cation radical for comparison (123).

Similar electron-transfer intermediates are observed with other (aromatic) donors that promote deligation of Ar2Fe + acceptors. For example, charge-transfer laser excitation of the EDA complex of (HMB)2pe + with 9-methylanthracene (MeANT) generates the 9-methylanthracene cation radical with its characteristic absorption centered at 700 nm (see Figure 1 IB Eq. 38). 

A study of Livingston and Wei (102) showed that the dimerization reaction of anthracene is occurring via the singlet excited state although it may be partially due to triplet reaction based on the effect of oxygen on the quantum yield of cyclomerl-zatlon. Recent results (105), however, indicate that dimerization of 9-methylanthracene occurs via excited state complex formation from the singlet excited state. 

Anthracene photodimers have been investigated extensively. A study of the photodissociation of the dimer of 9-methylanthracene, under both steady-state and flash conditions, suggests that the process occurs through an excited singlet... 

Excited states of metal-ion-containing surfactants are of very considerable current interest. Photoexcitation, of ruthenium(ii) tris-bipyridyl has been studied and in sodium lauryl sulphate micelles efficient triplet energy-transfer to 9-methylanthracene occurs within the micelle. It is governed by the equation ... 

Nimura and Kinoshita [82] developed a derivatization technique for the analysis of fatty acids that is based on 9-anthryldiazomethane (ADAM). The latter is very stable in solution and reacts with fatty acids at room temperature without a catalyst to yield strongly fluorescent esters, which can be chromatographed on an ODS phase with acetonitrile/water mixtures. The derivatization of fatty acid is carried out with a 0.1 % methanohc solution of 9-anthryldiazomethane, which is prepared via oxidation of 9-anthraldehyde-hydrazone following a procedure by Nakaya et al [83]. The fluorescence- and excitation spectra of fatty add-methylanthracene esters exhibit maxima at 412 nm and 365 nm, respectively. As an example. Fig. 7-45 shows the separation of ADAM derivatives a mixture of saturated and unsaturated long-chain... 

Previous investigations [18] showed that the photochemical reduction of viologen compounds can be efficiently performed in degassed CH2CI2 solution by using 9-methylanthracene [64] as a photosensitizer and triethanolamine (TEOA) as a sacrificial reductant. As schematically shown in Fig. 4.4, light excitation of... 

Mikami et al. investigated the photochemical Diels-Alder reaction of anthracenes with in the solid state. Irradiation of a mixture of and 9-methylanthracene led to the formation of mono- and bisadducts. No formation of 9-methylanthracene dimers as in solution was observed. The reaction did not work with less bulky anthracenes. The reaction rate depends on the ionization potential of the anthracenes. With decreasing ionization potential, the Diels-Alder reaction of proceeds much more easily. Therefore, the reaction may proceed via photoinduced electron transfer from anthracenes to the triplet excited state of Q . The energetics for the photoinduced electron transfer in the solid state are significantly different from those in solution where solvation plays an important role. Such a difference leads to the different reactivity of the anthracene derivatives in the solid state as compared to that in solution. 

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