Acridinium ion

Another photochemically driven proton transfer occurs in the system 46. This molecule on sublimation at low temperatures crystallizes in form 46a having infinite chains of intermolecular hydrogen bonds connecting zwitterionic molecules. Its fluorescence at such temperatures is essentially that of the acridinium ion. However, on prolonged UV irradiation, conversion to 46b occurs and the... 

It is well known that disproportionation of 154 occurs via direct hydride ion transfer from the 9-position of the pseudobase 155 to the 9-position of the acridinium ion, to form acridone 156 and acridan 157 (77JCS(P1)1966 84JCS(P2)661) (Scheme 26). 

The photosensitization of this aquation can also be initiated by energy nsfer from biacetyl and acridinium ion in the same solvent. The energy fer reactions have helped to identify the details of this aquation... 

Acridinium ion as sensitizer fluoresence quenched both NH3 and NCS- released although NHS predominates in presence of 02, NCS-release decreased, hence triplet state of the sensitizer is involved nh / ncs-, 33 for quartet state, 8 for doublet state. 

Methylpyridinium ions (181) react reversibly with hydroxide to form a small proportion of the pseudo-base (182). The term pseudo is used to designate bases that react with acids measurably slowly, not instantaneously as for normal acid-base reactions. Fused benzene rings reduce the loss of resonance energy when the hetero ring loses its aromaticity and hence pseudo-bases are formed somewhat more readily by 1-methylquinolinium, 2-methylisoquinolinium and 10-methylphenan-thridinium, and much more readily by 10-methylacridinium ions. Pseudo-bases carrying the hydroxy group in the a-position are usually formed preferentially, but acridinium ions react at the y-position. 

J. Lappe, R. J. Cave, M. D. Newton and I. V. Rostov, A theoretical investigation of charge transfer in several substituted acridinium ions, J. Phys. Chem. B, 109 (2005) 6610-6619. 

Photoexcitation of a deaerated PhCN solution of Acr+-Mes by a nanosecond laser light flash at 430 nm results in the formation of Acr -Mes+ with a quantum yield close to unity (98 %) via photoinduced electron transfer from the mesitylene moiety to the singlet excited state of the acridinium ion moiety ( Acr -Mes) [54]. The decay of Acr -Mes+ obeyed second- rather than first-order kinetics at ambient temperature as observed in the case of Fc+-ZnP-H2P-C60 , when the bimo-lecular back electron transfer predominates owing to the slow intramolecular back electron transfer (see above) [50]. In contrast, the decay of Acr -Mes+ obeys first-order kinetics in PhCN at high temperatures (e.g. 373 K). This indicates that the rate of the intramolecular back electron transfer of Acr -Mes4 becomes much faster than the rate of the intermolecular back electron transfer at higher tempera-... 

The reactivity of 2- and 3-methoxy-NMAHs towards -acceptors and the cobalt(III) reagent have also been compared (Colter et al., 1984). The 3-methoxy substituent better stabilizes the acridinium ion, while 2-methoxy-NMAH is the better one-electron donor. With a series of quinones in acetonitrile, the 3-methoxy-NMAH is between 4 and 10 times more reactive than its isomer although absolute rates vary by more than 105. With the cobalt(III) oxidant, the 2-methoxy-NMAH is more than 50 times more reactive, suggesting that the quinones react with these acridans uniformly by one-step hydride transfer. 

Steric enhancement of reaction rate has been reported in cycloaddition of styrene to acridinium ion (74JOC1172) and of 2(lH)-pyridone to dimethyl butynedioate (79CC501) when methyl groups are either ortho or peri (Scheme 45). 

Abraham, Wlosnewski, Buck, and Jacob [95] invented a new type of photoswitchable rotaxane based on the above principles where the diaryl-methoxy-cycloheptatriene unit is replaced by 9-aryl-9-methoxy-acridanes (Fig. 9b4) that undergo photoheterolysis (313 nm light) with formation of acridinium ions (Fig. 9b5). Duo, Jacob, and Abraham [96] demonstrated that such devices can be deposited and can operate on gold nanoparticles. 

The reductive dimerisation of N-methyl acridinium ion to give (125) proceeds under ultra-violet light illumination in aqueous methanol or aqueous acetonitrile in the presence of triphenyl-phosphine here electron transfer from the triphenylphosphine to the excited state of the acridinium ion to produce the N-methylacridinyl radical is implicated. 

Matsuo and Mayer [33], on the other hand, found that the same reactant in acetonitrile solution, upon treatment with Ru O, quicMy generated the acridinium ion, as might have been anticipated for a simple hydride-transfer reaction, but in only 40-50% yield. Relatively slowly thereafter the acridinium compound and the remaining reactant were converted to the acridinium leuco-base (hydroxide-ion adduct). Matsuo and Mayer concluded that the process shovm in Eq. (4.5) was occurring ... 

The photochemical reaction of the acridinium ion 8.203 in methanol is an example of a general reaction type in which radical ions and radicals are created by electron transfer to or from the excited state. As we saw earlier in the discussion about Fig. 8.10, the HOMO of an excited state is a low-energy, half-filled orbital, into which another electron can be fed from a filled orbital of suitably close energy. In this case, the lone pair in methanol transfers an electron to the HOMO of the excited state 8.204, giving the radical 8.205 and the radical cation 8.206. The latter easily loses a proton to give the radical 8.207, and the C—C coupling of this... 

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