β,meso-Acetylenyl-Bridged, Asymmetrical, Porphyrin Dyads - Synthesis, Spectral, Electrochemical and Computational Studies

The first examples of beta,meso-acetylenyl-bridged, asymmetrical, covalently linked, porphyrin dyads, containing two different subunits, such as ZnN(4)P-N(3)OP (1), ZnN(4)P-N(3)SP (2), ZnN(4)P-N(2)SOP (4) and ZnN(4)P-N(2)S(2)P (6), were synthesized by the coupling of a beta-acetylenyl, ZnN(4) porphyrin with a meso-bromoheteroporphyrin under mild, Pd(0)-catalyzed, coupling conditions. The dyads containing different types of metal-free subunits, such as N(4)P-N(3)SP (3), N(4)P-N(2)SOP (5) and N(4)P-N(2)S(2)P (7), were synthesized by the demetallation of the corresponding dyads. The seven beta,meso-acetylenyl dyads 1-7 were characterized by NMR, MS, absorption, fluorescence and electrochemical techniques. The NMR, absorption and electrochemical studies support an electronic interaction between the subunits in all seven dyads. The steady-state fluorescence studies on dyads 1-7 support an efficient energy transfer from the donor (ZnN(4) or N(4)) subunit to the acceptor heteroporphyrin subunit upon excitation of the ZnN(4)/N(4) subunit. First-principle-based, quantum-chemical studies carried out on dyads 1, 2, 4 and 6 further support an electronic interaction between the donor and acceptor subunits. The computational studies also predict significant tuning of the electronic energy levels in these dyads with the modification of the porphyrin core of the acceptor groups. The calculations support the experimental results of efficient donor -> acceptor energy transfer in these dyads.