EFFECTS OF ORBITAL ORDERING ON ELECTRONIC COMMUNICATION IN MULTIPORPHYRIN ARRAYS

The rational design of molecular photonic devices requires a thorough understanding of all factors affecting electronic communication among the various constituents. To explore how electronic factors mediate bo th excited- and ground-state electronic communication in multiporphyri n arrays, we have conducted a detailed static spectroscopic (absorptio n, fluorescence, resonance Raman, electron paramagnetic resonance), ti me-resolved spectroscopic (absorption, fluorescence), and electrochemi cal (cyclic and square-wave voltammetry, coulometry) study of tetraary lporphyrin dimers, The complexes investigated include both zinc-free b ase (ZnFb) and bis-Zn dimers in which the porphyrin constituents are l inked via diphenylethyne groups at the meso positions. Comparison of d imeric arrays containing pentafluorophenyl groups at all nonlinking me so positions (F(30)ZnFbU and F30Zn2U) with nonfluorinated analogs (ZnF bU and Zn2U) directly probes the effects of electronic factors on intr adimer communication. The major findings of the study are as follows: (1) Energy transfer from the photoexcited Zn porphyrin to the Fb porph yrin is the predominant excited-state reaction in F(30)ZnFbU, as is al so the case for ZnFbU. Energy transfer primarily proceeds via a throug h-bond process mediated by the diarylethyne linker. Remarkably, the en ergy-transfer rate is 10 times slower in F(30)ZnFbU ((240 ps)(-1)) tha n in ZnFbU ((24 ps)(-1)), despite the fact that each has the same diph enylethyne linker. The attenuated energy-transfer rate in the former d imer is attributed to reduced Q-excited-state electronic coupling betw een the Zn and Fb porphyrins. (2) The rate of hole/electron hopping in the monooxidized bis-Zn complex, [F30Zn2U](+), is similar to 10-fold slower than that for [Zn2U](+). The slower hole/electron hopping rate in the former dimer reflects strongly attenuated ground-state electron ic coupling. The large attenuation in excited-and groundstate electron ic communication observed for the fluorine-containing dimers is attrib uted to a diminution in the electron-exchange matrix elements that ste ms from stabilization of the a(2u) porphyrin orbital combined with cha nges in the electron-density distribution in this orbital. Stabilizati on of the porphyrin a(2u) orbital results in a switch in the HOMO from at, in ZnFbU to a(1u) in F(30)ZnFbU. This orbital reversal diminishes the electron density at the peripheral positions where the linker is appended. Collectively, our studies clarify the origin of the differen t energy-transfer rates observed among various multiporphyrin arrays a nd exemplify the interconnected critical roles of a(1u)/a(2u), orbital ordering and linker position in the design of efficient molecular pho tonic devices.