SYNTHESIS AND CHARACTERIZATION OF TETRACHLORODIARYLETHYNE-LINKED PORPHYRIN DIMERS - EFFECTS OF LINKER ARCHITECTURE ON INTRADIMER ELECTRONICCOMMUNICATION

The effects of incorporating chloro groups at all ortho positions of a diphenylethyne linker that bridges the zinc and free base (Fb) compon ents of a porphyrin dimer (ZnFbB(Cl-4)) have been investigated in deta il via various static and time-resolved spectroscopic methods. The exc ited-state energy-transfer rate in ZnFbB(Cl-4) ((134 ps)(-1)) is 5-fol d slower than that in the corresponding dimer having an unsubstituted linker (ZnFbU, (24 ps)(-1)) but is only modestly slower than that in t he dimer having o-methyl groups on the linker (ZnFbB(CH3)(4), (115 ps) (-1)). The ground-state hole/electron-hopping rates in the oxidized bi s-Zn analogues of all three dimers are much slower than the excited-st ate energy-transfer rates. There is no discernible difference between the hole/electron-hopping rates in the o-chloro- and o-methyl-substitu ted arrays. The similar ground-and excited-state dynamics observed for the o-chloro- and o-methyl-substituted arrays is attributed to the do minance of torsional constraints in mediating the extent of through-bo nd electronic communication. These constraints attenuate intradimer co mmunication by restricting the rotation toward coplanarity of the phen yl rings of the linker and the porphyrin rings. Thus, the o-chloro gro ups on the linker decrease electronic communication via a steric, rath er than purely electronic, mechanism.