Département de chimie moléculaire

The electrochem. and spectroelectrochem. properties of N,N-diphenyl-1,4-phenylenediamine (PDA) were studied in the absence and in the presence of 18-crown-6-ether (18C6) or dibenzo 24-crown-8-ether (DB24C8), in a soln. of Bu4NPF6 (TBAPF6) in MeCN and in the presence of HO2CCF3 (TFA) only for 18C6. In neutral MeCN, PDA undergoes two reversible oxidn. processes, which lead 1st to the formation of the cation-radical considered as mixed valence (MV) compd., and then to the dicationic species. When 18C6 is added in the medium and depending on 18C6 concn., cyclic voltammetry shows a marked shift to more cathodic potentials of the current waves of the 2nd redox process only. This is attributed to a strong interaction between the PDA+2 dication and two 18C6 mols., giving a supramol. complex with an assocn. const. value Ka = 7.0 × 107 M-2. The interaction of 18C6 with PDA+2 dication has a direct effect on the PDA+. cation-radical corresponding to a decrease in the lifetime of the MV compd. and of the intramol. electron transfer rate when 18C6 is present. Indeed, it results in a large decrease in the intervalence charge transfer (IV-CT) between the two amine centers in the MV compd. (kth = 1.35 × 1010 s-1 in 18C6-free neutral soln. contg. 5.0 × 10-4 M PDA, and kth = 3.6 × 109 s-1 in the same medium at [18C6]/[PDA] = 20/1). And the comproportionation const. Kco falls from 6.0 × 106 in 18C6-free soln. to 1.6 × 103 at [18C6]/[PDA] = 20/1. In acidified MeCN and when TFA concn. is increased, PDA still shows the two successive and reversible oxidn. processes, but both are shifted to more anodic potentials. However, when 18C6 is added, the two oxidn. waves shift to more cathodic potentials, indicating an interaction of all protonated PDA redox states with 18C6, giving supramol. complexes. In the presence of TFA, the value of Kco is decreased to 4.3 × 104, but it remains unchanged when 18C6 is added, indicating no change in the lifetime of the MV compd. In this medium, IV-CT in the MV compd. is greater with 18C6 (kth = 2.3 × 1010 s-1 for [18C6]/[PDA] = 20/1) than without (kth = 1.4 × 109 s-1), which indicates a more important IV-CT rate when 18C6 is present. Is it possible to control the IV-CT rate, through the lifetime and the potential range where the MV compd. is the most important. This control is not obtained as usual by chem. modification of the structure of the starting mol., but by varying either the acidity or the 18C6 concn. as external stimuli, which lead to reversible formation/dissocn. of a supramol. complex species. Also, the authors also studied the electrochem. properties of PDA in the presence of wider crown ether such as DB24C8. PDA undergoes the same electrochem. behavior with DB24C8 than with 18C6 in neutral org. medium (Ka = 2.9 × 103 M-1). Probably the complexation between the electrogenerated PDA+2 dication and the crown ethers may occur through face-to-face mode rather than rotaxane mode even with DB24C8 which probably forms inclusion complexes. [on SciFinder(R)]