Excited-State Energy Transfer and Ground-State Hole/Electron Hopping in p-Phenylene-Linked Porphyrin Dimers

Abstract

The ground- and excited-state properties of a series of p-phenylene-linked porphyrin dimers have been examined using a variety of static and time-resolved spectroscopic techniques. The dimers consist of a zinc porphyrin and a free base (Fb) porphyrin (ZnFbΦ), two zinc porphyrins (Zn2Φ), or two Fb porphyrins (Fb2Φ). In each array, the porphyrins are joined by the p-phenylene linker at one meso position, with the nonlinking meso positions bearing mesityl groups. Three analogous dimers in which the mesityl groups are replaced with pentafluorophenyl groups (F30ZnFbΦ, F30Zn2Φ, and F30Fb2Φ) were also synthesized and characterized. The excited-state energy-transfer rate from the photoexcited Zn porphyrin to the Fb porphyrin is (3.5 ps)-1 for ZnFbΦ and (10 ps)-1 for F30ZnFbΦ. The quantum yields of excited-state energy transfer are ≥99% for both complexes. The energy-transfer rates in the p-phenylene-linked dimers are considerably faster than those observed for the analogous dimers containing a diphenylethyne linker ((24 ps)-1, ZnFbU; (240 ps)-1, F30ZnFbU). At these distances, both through bond and through space contributions to the electronic coupling are important. The faster energy-transfer rates in the p-phenylene- versus diarylethyne-linked dimers are attributed to enhanced electronic coupling between the porphyrins in the former dimers arising primarily from the shorter inter-porphyrin separation. The electronic coupling in the p-phenylene-linked dimers is sufficient to support ultrafast energy transfer in both ZnFbΦ and F30ZnFbΦ, but is not so large as to significantly perturb the redox or inherent lowest excited-state photophysical properties of the porphyrin constituents. Electronic perturbations resulting from fluorination have little effect on the energy-transfer rates in the p-phenylene-linked dimers, but the rates of room-temperature ground-state hole/electron hopping processes in the corresponding monocation radicals of the bis-Zn analogues of the p-phenylene-linked dimers (≥(0.05 μs)-1, [Zn2Φ]+; ≤(2.5 μs)-1, [F30Zn2Φ]+) are significantly influenced by the fluorination-induced changes in the electronic structure. Collectively, these characteristics make these constructs attractive candidates for incorporation into extended multi-porphyrin arrays for a variety of molecular photonics applications.

Intramolecular and intermolecular energy transfers in donor-acceptor linear porphyrin arrays

“…8,[48][49][50][51][52][53] In both phenylethyne-linked and phenylenelinked Zn͑II͒ porphyrin and free-base porphyrin dyads, it was concluded that through bond interaction is the major contribution. 8,53 For the phenylene-linked porphyrin arrays, however, the calculated Förster rate and the observed rate are similar, 19 indicating that through space interaction is also significant. Since the phenyl linker is perpendicular to the porphyrin plane due to the steric hindrance of the o-hydrogen atoms, conjugative link is broken to result in less through bond interactions between Zn and A.…”

“…When a longer linker such as p , pЈ-diphenylethyne or a linker has no steric hindrance, through bond interaction was found to be the major contribution. 8 When the through bond interaction is important, the energy transfer from Q e to Q y A should also occur as well as the energy from Q m to Q y A . The time-resolved fluorescence spectra, however, support the conclusion that the energy transfer occurs mostly from Q m to Q y A .…”

“…Among these, porphyrins and porphyrin oligomers linked by various spacers are two of the most attractive systems due to their advantageous characteristics such as rigid planar geometry, high stability, intense electronic absorption, and small highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap. [7][8][9][10] The porphyrin system is also one of the candidates in the fabrication of artificial light harvesting antenna system and molecular photonic devices. [11][12][13][14][15][16] The energy deposited on a molecule by the absorption of a photon can dissipate by various processes.…”

Interaction Between Terminal Zinc–Porphyrins in a U‐Shaped Trimeric Porphyrin Array Bearing Chiral Amino Acid Tails

“…Split Soret absorptions appear for 5ZFZ at 427 (ε/ -1 cm -1 = 661 000) and 435 nm (646 000). The split absorption of 5ZFZ is probably due to the exciton coupling between the porphyrins, [1,22] in which the close existence of the zinc-porphyrins was shown in the 1 H NMR spectrum as described above. The Soret band of 5ZFZ is red-shifted from the absorptions of the reference monomeric components (420 nm for both the free-base template αα-4 and the monomeric zinc complex 10Z, see Supporting Information).…”

Evidence for reverse pathways and equilibrium in singlet energy transfers between an artificial special pair and an antenna

“…The resulting k ET (S 1 ) values at 77 K (k ET (FB* → [Zn 2 ]) = k 1 = 0.31 (ns) -1 ; k ET ([Zn 2 ]* → FB) = k -1 = 1.8 (ns) -1 ) appears to be drastically slow comparatively to that reported for dyad 4 (k ET (FB* → [Zn 2 ]) = 196 (ns) -1 , the only process that could be studied) [11], and dyads 6 (290 (ns) -1 ) and 7 (100 (ns) -1 ; Fig. 5), for which these ET processes are only in one direction (D* → A) [43]. This 2 or 3 order of magnitude difference can clearly not be accounted for using the Forster's theory (Equation 1).…”