Tetrathiafulvalene-fused porphyrins via quinoxaline linkers: Symmetric and asymmetric donor-acceptor systems

A tetrathiafulvalene (TTF) donor is annulated to porphyrins (P) via quinoxaline linkers to form novel symmetric P-TTF-P triads 1 a-c and asymmetric P-TTF dyads 2 a,b in good yields. These planar and extended π-conjugated molecules absorb light over a wide region of the UV/Vis spectrum as a result of additional charge-transfer excitations within the donor-acceptor assemblies. Quantum-chemical calculations elucidate the nature of the electronically excited states. The compounds are electrochemically amphoteric and primarily exhibit low oxidation potentials. Cyclic voltammetric and spectroelectrochemical studies allow differentiation between the TTF and porphyrin sites with respect to the multiple redox processes occurring within these molecular assemblies. Transient absorption measurements give insight into the excited-state events and deliver corresponding kinetic data. Femtosecond transient absorption spectra in benzonitrile may suggest the occurrence of fast charge separation from TTF to porphyrin in dyads 2 a,b but not in triads 1 a-c. Clear evidence for a photoinduced and relatively long lived charge-separated state (385 ps lifetime) is obtained for a supramolecular coordination compound built from the ZnP-TTF dyad and a pyridine-functionalized C₆₀ acceptor unit. This specific excited state results in a (ZnP-TTF)^•+⋯(C ₆₀py)^•- state. The binding constant of Zn ^II⋯py is evaluated by constructing a Benesi-Hildebrand plot based on fluorescence data. This plot yields a binding constant K of 7.20×10⁴ M^-1, which is remarkably high for bonding of pyridine to ZnP.