A hexagonal prismatic porphyrin array: Synthesis, STM detection, and efficient energy hopping in near-infrared region

A belt-shaped hexagonal cyclic porphyrin array 2 that comprises of six meso-meso, β-β, β-β triply linked diporphyrins 3 bridged by 1,3-phenylene spacers is prepared by oxidation from cyclic dodecameric array 1 consisting of six meso-meso directly linked diporphyrins 4 with DDQ and Sc(OTf)₃. The absorption spectrum of 2 is similar to that of the constituent subunit 3 but shows a slight red-shift for the Q-bands in near-infrared (NIR) region, indicating the exciton coupling between the neighboring diporphyrin chromophores. Observed total exciton coupling energies in the absorption spectra were largely matched with the calculated values based on point-dipole exciton coupling approximation. It was found that the experimental exciton coupling strength (292 cm^-1) of the Q-band in 2 is slightly larger than the calculated one (99 cm^-1), indicating that the electronic communications are enhanced through 1,3-phenylene linkers in hexameric macromolecule. A rate of the excitation energy hopping (EEH) that occurs in 2 at the lowest excited singlet state in the near-infrared region has been determined to be (1.8 ps)^-1 on the basis of the pump-power dependent femtosecond transient absorption (TA) and the transient absorption anisotropy (TAA) decay measurements. The 2 times faster EEH rate of 2 than that of 1 (4.0 ps)^-1 mainly comes from involving through-bond energy transfer among diporphyrin subunits via 1,3-phenylene bridges as well as Förster-type through-space EEH processes. STM measurement of 2 in the Cu(100) surface revealed that it takes several discrete conformations with respect to the relative orientation of neighboring diporphyrins. Collectively, an effective EEH in the NIR region is realized in 2 due largely to the intensified oscillator strength in the S₁ state (Q-band) and the close proximity held by 1,3-phenylene spacers.