IONIC-STRENGTH EFFECTS ON THE GROUND-STATE COMPLEXATION AND TRIPLET-STATE ELECTRON-TRANSFER REACTION BETWEEN ROSE-BENGAL AND METHYL VIOLOGEN

Abstract— The equilibrium constants, Kc, for complexation between methyl viologen dication (MV2+) and Rose Bengal, or Eosin Y, decrease with increasing ionic strength. At zero ionic strength Kc is 6500 (± 500) mol−1 dm3 for Rose Bengal and 3200 (± 200) mol−1 dm3 for Eosin Y, and these values decrease to 1500 (± 100) and 680 (± 40) mol−1 dm3, respectively, at an ionic strength of 0.1 mol dm−3. Kc is independent of pH between 4.5 and 10. ΔH is ‐25 (± 1) kJ mol−1 for complexation with either dye, whereas ΔS is ‐15 (± 3) J K−1 mol−1 for Rose Bengal, and ‐ 23 (± 3) J K−1 mol−1 for Eosin Y. The complexation constant for Rose Bengal and the neutral viologen, 4,4'‐bipyridinium‐N, N'‐di(propylsulphonate), (4,4'‐BPS), is 420 (± 35) mol−1 dm3, and independent of ionic strength. No complexation could be observed for either Rose Bengal or Eosin with another neutral viologen, 2,2'‐bipyridinium‐N,N'‐di(propylsulphonate), (2,2'‐BPS). MV2+ quenches the triplet state of Rose Bengal with a rate constant of 7 × 109 mol−1 dm3 s−1, and this rate constant decreases slightly as ionic strength increases. The cage escape yield following quenching, Φcc is very low (Φcc= 0.02 (± 0.005), and independent of ionic strength. 4,4'‐BPS quenches the triplet state of Rose Bengal with a rate constant of 2.2 (± 0.1) × 109 mol−1 dm3 s−1, and gives a cage escape yield of 0.033 (± 0.006). 2,2'‐BPS quenches the Rose Bengal triplet with a rate constant of 6 (± 1) × 108 mol−1 dm3 s−1 and gives a cage escape yield of 0.07 (± 0.01). Conductivity measurements indicate that MV2+(Cl−)2 is completely dissociated at concentrations below 2 × 10−2 mol dm−3. Copyright © 1990, Wiley Blackwell. All rights reserved