Acridinium Salts and Acridans

The acridine nucleus has proved to be a good basis for a variety of chemiluminescent compounds. Among these are the acridinium active esters which not only provide the best understood examples of chemiluminescence to date but are of considerable value in determining the mechanism of light emission in certain bioluminescent organisms. 

Acridinium phenyl esters have been thoroughly investigated [14,19,22] with the convincing mechanism shown below as a result  

A similar mechanism applies to the acridine phenyl esters and acridine N-oxide phenyl esters [21]. Quantum yields are not quite so high, and in the former case, dipolar aprotic solvents are required, presumably to enhance nucleophilic attack at the now less activated 9-position. Quinolinium (10) and phenanthridinium (11) esters are also chemiluminescent but with a lower efficiency [21]  

Acridan active esters are particularly efficient, quantum yields in the region of 10% being obtainable. The reaction is entirely analogous to that of firefly and coelenterate luciferins. Dipolar aprotic solvents give the best results and there is good evidence for all the steps shown [21]  

These compounds all react by formation of a dioxetanone. This is the immediate precursor of the light emitting step. It is significant that the peroxide (12) is isolable, yet the dioxetanone is certainly not. The formation of the strained per-ester increases the oxidizing potential of the peroxide to the point where electron transfer from the electron rich heterocycle ensues  

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In the presence of hydrogen peroxide and base, acridinium salts lead to chemiluminescence emission. Acridans, in their reduced forms, are able to react directly with oxygen in aprotic solvents with 4>cl up to 10% . Scheme 31 shows the proposed mechanism for chemiluminescence of 9-cyano-lO-methylacridan and 9-cyano-lO-methylacridinium salt in the presence of oxidant and base, which postulates the cyclic peroxidic intermediate 44. 

In addition to their implication as reactive intermediates in bioluminescence, dioxetanones have been proposed as key intermediates in several chemiluminescent systems. Most notable are the chemiluminescent oxidation reaction of acridan esters [19] and the chemiluminescent reaction of the related acridinium salts [20] (Rauhut et al., 1965a McCapra et al., 1977). Both reactions are quite efficient at generating singlet excited states (pCE = 10% and 2% respectively) and, owing to the elegant work of McCapra and others, are among the best understood complex chemiluminescent reaction mechanisms. 

The acridinium salt (290 mg, 0.70 mM) is dissolved in 20 mis of distilled methanol. The clear, yellow solution is acidified with two drops of HCl (2M), the catalyst (25 mg) added and hydrogen bubbled into the stirred mixture at room temperature. After about 10 minutes the solution turns an intense purple colour which completely disappears to leave a suspension of the white product and the black catalyst. Chloroform (20 ml) is added to dissolve the acridan and the solution is filtered, quickly washed with water (5 ml) and evaporated at room temperature under vacuum to dryness to give an off-white solid (16 mg, 72%). Recrystallisation from ethanol gave white needles. 

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