Nature of the strong electron donor 1,3,6,8-tetrakis(dimethylamino)pyrene and ionicity of its charge transfer complexes

Abstract

The 1,3,6,8-tetrakis(dimethylamino)pyrene (TDAP) molecule is a stronger electron donor (D) than conventional donor molecules such as tetramethyltetrathiafulvalene, N,N,N′,N′-tetramethyl-p-phenylenediamine and N,N-dimethyldihydrophenazine judged from both their charge transfer (CT) bands with s-trinitrobenzene (TNB) complexes and their redox potentials. The first averaged potential of oxidation and reduction peak potentials of TDAP is +0.02 V vs. SCE in acetonitrile. The molecule easily releases four electrons by two successive redox processes with two-electron transfer at each step as previously reported. Semiempirical calculation of molecular conformation infers that the peculiar redox behavior may be ascribed to the deformed conformations of cationic species of the TDAP molecule. Complex formation with a variety of acceptor molecules (A): TCNQs, p-benzoquinones and others was examined. With TCNQ derivatives, only tetrafluoro-TCNQ affords a 1 ∶ 1 stoichiometry with an ionic ground state (D2+)(A2−). The major ratio of the other CT complexes is 1 ∶ 2, those with a strong acceptor in the TCNQ system have a completely ionic ground state; (D2+)(A−)2 while those with a weak acceptor have a partial CT with the charge of TDAP between +1 and +2; (D)+2(1 − δ)(A−(1 − δ))2 (δ ∼ 0). A very weak acceptor in the TCNQ system mainly affords the 1 ∶ 3 complex with a partial CT state. In addition, the non-substituted TCNQ molecule affords a 1 ∶ 4 complex with a partial CT state where the cationic species are deduced to be a mixture of TDAP2+ and protonated TDAP. These partial CT compounds of TCNQs are highly conductive. The 1 ∶ n (n = 2, 3, 4) complexes are found to contain anion species of TCNQs−(1 − δ) (δ ∼ 0) or a mixture of charge separated species (A−γ + A−(1 − δ) or A−(1 − δ) + A−2(1 − γ): γ, δ ∼ 0). A very weak acceptor such as C60 or TNB affords a neutral complex. On the basis of the ionicity of TDAP CT complexes with TCNQs, p-benzoquinones and others, the criterion for partial CT is discussed.