Effects of Orbital Ordering on Electronic Communication in Multiporphyrin Arrays

Abstract

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 both excited- and ground-state electronic communication in multiporphyrin arrays, we have conducted a detailed static spectroscopic (absorption, fluorescence, resonance Raman, electron paramagnetic resonance), time-resolved spectroscopic (absorption, fluorescence), and electrochemical (cyclic and square-wave voltammetry, coulometry) study of tetraarylporphyrin dimers. The complexes investigated include both zinc-free base (ZnFb) and bis-Zn dimers in which the porphyrin constituents are linked via diphenylethyne groups at the meso positions. Comparison of dimeric arrays containing pentafluorophenyl groups at all nonlinking meso positions (F30ZnFbU and F30Zn2U) with nonfluorinated analogs (ZnFbU and Zn2U) directly probes the effects of electronic factors on intradimer communication. The major findings of the study are as follows: (1) Energy transfer from the photoexcited Zn porphyrin to the Fb porphyrin is the predominant excited-state reaction in F30ZnFbU, as is also the case for ZnFbU. Energy transfer primarily proceeds via a through-bond process mediated by the diarylethyne linker. Remarkably, the energy-transfer rate is 10 times slower in F30ZnFbU ((240 ps)-1) than in ZnFbU ((24 ps)-1), despite the fact that each has the same diphenylethyne linker. The attenuated energy-transfer rate in the former dimer is attributed to reduced Q-excited-state electronic coupling between the Zn and Fb porphyrins. (2) The rate of hole/electron hopping in the monooxidized bis-Zn complex, [F30Zn2U]+, is ∼10-fold slower than that for [Zn2U]+. The slower hole/electron hopping rate in the former dimer reflects strongly attenuated ground-state electronic coupling. The large attenuation in excited- and ground-state electronic communication observed for the fluorine-containing dimers is attributed to a diminution in the electron-exchange matrix elements that stems from stabilization of the a2u porphyrin orbital combined with changes in the electron-density distribution in this orbital. Stabilization of the porphyrin a2u orbital results in a switch in the HOMO from a2u in ZnFbU to a1u in F30ZnFbU. This orbital reversal diminishes the electron density at the peripheral positions where the linker is appended. Collectively, our studies clarify the origin of the different energy-transfer rates observed among various multiporphyrin arrays and exemplify the interconnected critical roles of a1u/a2u orbital ordering and linker position in the design of efficient molecular photonic devices.

An Amphiphilic BODIPY‐Porphyrin Conjugate: Intense Two‐Photon Absorption and Rapid Cellular Uptake for Two‐Photon‐Induced Imaging and Photodynamic Therapy

“…Al inear dependence of the emission intensity on the square of the powero ft he incident laser beam (Figure 3B inset) was seen, thus confirming that it is at wo-photon excitation process. [21] [d] Singleto xygen was measured in deoxygenatedC HCl 3 relativet ot etraphenylporphyrin in benzene( H2TPP, F D = 0.55). [22] [e] Measured by the Zscan method (GM = 10 À50 cm 4 sphoton À1 molecule…”

Triplet–Triplet Energy Transfer Controlled by the Donor–Acceptor Distance in Rigidly Held Palladium‐Containing Cofacial Bisporphyrins

“…[45] Based on the absorption (Figure 3c) and emission data (Figure 7), the donor and acceptor are 641 [18] 636 [18] 0.0040 [18] 0.012 [18] Pd, 2H [27,42,63] and the quantum yield for H 2 TPP (0.11) at 77 K was verified by reference to (Pd)TPP (0.17; 77 K; MCH). [43,64] [b] The uncertainties in the values of l max are AE 1 nm.…”

“…Each absorbance value was measured five times to gain better accuracy in the measurement of the quantum yields. The quantum yield of H 2 TPP (F = 0.11) was used as a reference for the quantum yields measured at 298 K. [27,42,63] The quantum yield of H 2 TPP (F = 0.11) was also used as reference for the quantum yields measured at 77 K, which itself was obtained by using (Pd)TPP (F = 0.17; 77 K; MCH = methylcyclohexane) as reference. [43,64] Crystallography: Red, single crystals of (Pd)H 2 DPX, which exhibit a plate morphology, were grown from CH 2 Cl 2 /heptane diffusion.…”

Time‐Resolved Spectroscopic Study on Photoinduced Electron‐Transfer Processes in Zn(II)porphyrin–Zn(II)chlorin–Fullerene Triad

“…The Zn(II)porphyrin moiety, which is of the TPP-type, has the HOMOs of a 2u ;and the HOMOs of Zn(II)chlorin are a 1u orbitals. [24] Therefore, the electron density of Zn(II)porphyrin HOMO is higher at the meso-carbon positions and the HOMOs of Zn(II)chlorin have higher electron density at the b-carbon positions (Figure 4). According to the four orbital theory, in the Q-excited states that are composed of a configuration interaction (CI) admixture of a 1u e g and a 2u e g , each HOMO in the respective moieties has the strong configuration interaction with the e g (LUMO) orbital and contributes more to the electron density distribution in the Q-excited states than the next molecular orbital just below the HOMO (a 1u for Zn(II)porphyrin and a 2u for Zn(II)chlorin).…”

“…[24] The extent of electronic coupling can be provided by Coulombic interactions and electron exchange interactions, which are mainly related to through-space and through-bond energy transfer, respectively, although their relative contributions are not certain. [25] Since the absorption spectrum of the triad 6 was not significantly perturbed compared with the sum of the absorption spectra of its constituent molecules, the through-space interaction belongs to the weak coupling case as the Fˆrster-type energy transfer mechanism.…”