Electrochemical generation of triplet states: Simplified estimation of triplet energies by electrogenerated chemiluminescence based on the anodic cleavage of dimeric dihydroheteroarenes

Abstract

The chemiluminescence which is generated in the voltammetric oxidation of solutions containing a bis-dihydroheteroarene. P 2, a cation radical precursor. D, and an anion radical precursor. A, in acetonitrile/toluene is investigated at the rotating platinum disc electrode with respect to its use for an estimation of the triplet energies of D or A. In the indirect anodic cleavage of P 2 via the D/D + redox mediator, a strongly reducing intermediate, P , with an oxidation potential between −0.8 and −2.0 V is formed which reduces A to A −. The triplet state 3D or 3A arises from the homogeneous electron transfer between D + and P or A −, and is transformed into the emitting singlet state by triplet-triplet annihilation 2,4,6,2',4',6'-hexamethyl-4,4'-bi4H-pyrane, 1), the bis-[1,4-dihydropyridinyl-(4)] derivatives 2a-2h and bis-[1,2,3-trimethyl-1,2-dihydrobenzimidazolyl-(2)] (3) proved to be suitable as compound P 2. It is shown in some series of experiments that the triplet energy of A, of D, or of triplet quencher. Q. added to the solution can be determined from the luminescence-threshold value of E ox 1/2(D)- E red 1/2(A) or from the dependence of the luminescence intensity on the triplet energy difference E T(A)- E T(Q) or E T(D)- E T(Q). In comparison to the square-wave voltage methods or the rotating ring-disc electrode previously used for this purpose, the method turned out to be simplified from the experimental point of view and to be improved with respect to the lower requirements of the stability of the ion radicals involved.