Synthesis and Characterization of Tetrachlorodiarylethyne-Linked Porphyrin Dimers. Effects of Linker Architecture on Intradimer Electronic Communication

The effects of incorporating chloro groups at all ortho positions of a diphenylethyne linker that bridges the zinc and free base (Fb) components of a porphyrin dimer (ZnFbB(Cl₄)) have been investigated in detail via various static and time-resolved spectroscopic methods. The excited-state energy-transfer rate in ZnFbB(Cl₄) ((134 ps)^-1) is 5-fold slower than that in the corresponding dimer having an unsubstituted linker (ZnFbU, (24 ps)^-1) but is only modestly slower than that in the dimer having o-methyl groups on the linker (ZnFbB(CH₃)₄, (115 ps)^-1). The ground-state hole/electron-hopping rates in the oxidized bis-Zn analogues of all three dimers are much slower than the excited-state energy-transfer rates. There is no discernible difference between the hole/electron-hopping rates in the o-chloro- and o-methyl-substituted arrays. The similar ground- and excited-state dynamics observed for the o-chloro- and o-methyl-substituted arrays is attributed to the dominance of torsional constraints in mediating the extent of through-bond electronic communication. These constraints attenuate intradimer communication by restricting the rotation toward coplanarity of the phenyl rings of the linker and the porphyrin rings. Thus, the o-chloro groups on the linker decrease electronic communication via a steric, rather than purely electronic, mechanism.