Charge and Spin Transfer Dynamics in a Weakly Coupled Porphyrin Dimer

Abstract

The dynamics of electron and spin transfer in the radical cation and photogenerated triplet states of a tetramethylbiphenyl-linked zinc-porphyrin dimer were investigated, so as to test the relevant parameters for the design of a single-molecule spin valve and the creation of a novel platform for the photogeneration of high-multiplicity spin states. We used a combination of multiple techniques, including variable-temperature continuous wave EPR, pulsed proton electron-nuclear double resonance (ENDOR), transient EPR, and optical spectroscopy. The conclusions are further supported by density functional theory (DFT) calculations and comparison to reference compounds. The low-temperature cw-EPR and room-temperature near-IR spectra of the dimer monocation demonstrate that the radical cation is spatially localized on one side of the dimer at any point in time, not coherently delocalized over both porphyrin units. The EPR spectra at 298 K reveal rapid hopping of the radical spin density between both sites of the dimer via reversible intramolecular electron transfer. The hyperfine interactions are modulated by electron transfer and can be quantified using ENDOR spectroscopy. This allowed simulation of the variable-temperature cw-EPR spectra with a two-site exchange model and provided information on the temperature-dependence of the electron transfer rate. The electron transfer rates range from about 10.0 MHz at 200 K to about 53.9 MHz at 298 K. The activation enthalpies Δ^‡H of the electron transfer were determined as Δ^‡H = 9.55 kJ mol^-1 and Δ^‡H = 5.67 kJ mol^-1 in a 1:1:1 solvent mixture of CD₂Cl₂/toluene-d₈/THF-d₈ and in 2-methyltetrahydrofuran, respectively, consistent with a Robin-Day class II mixed valence compound. These results indicate that the interporphyrin electronic coupling in a tetramethylbiphenyl-linked porphyrin dimer is suitable for the backbone of a single-molecule spin valve. Investigation of the spin density distribution of the photogenerated triplet state of the Zn-porphyrin dimer reveals localization of the triplet spin density on a nanosecond time scale on one-half of the dimer at 20 K in 2-methyltetrahydrofuran and at 250 K in a polyvinylcarbazole film. This establishes the porphyrin dimer as a molecular platform for the formation of a localized, photogenerated triplet state on one porphyrin unit that is coupled to a second redox-active, ground-state porphyrin unit, which can be explored for the formation of high-multiplicity spin states.

Figure 1

a) Scheme of the basic elements of a spin valve, which allows tuning the current (red arrow) that flows through it by the mutual alignment of two magnetic moments (purple and light blue arrows), separated by a barrier (gray). b) Scheme of the molecular unit, with metals in purple and light blue, nitrogens in blue, and carbons color-coded depending on the functional element they belong to, green for the porphyrin elements, gray for the twisted biphenyl barrier.

Scheme 1. Synthesis of Biphenyl-Porphyrin Monomer P1 and Biphenyl-Linked Porphyrin Dimer P2 from the Symmetric Monomer P1_Sym

Reaction conditions: (i) TBAF, CHCl₃, 21 °C, 30 min, 50%; (ii) XPhos Pd G4, CuI, toluene/i-Pr₂NH (2:1), 70 °C, 3 h, P2: 45%; (iii) Pd(PPh₃)₂Cl₂, CuI, toluene/i-Pr₂NH (5:1), 45 °C, 2 h, 15% over two steps.

Figure 2

Steady-state UV–vis–NIR absorption spectra of neutral (black) and oxidized (blue) P1_Sym, P1, and P2 in CHCl₃ (298 K). The absorption spectra of the radical cations P1_Sym^•+, P1^•+, and P2^•+ were obtained by oxidation with one equivalent of BAHA (see SI Figure S27 for the full oxidation titrations). The vertical bars indicate the TD-DFT (LC-ωPBE/6-31G*; ω = 0.1) calculated wavelengths and oscillator strengths, f, for the electronic transitions of the neutral and oxidized compounds in the absence of vibronic coupling.

Figure 3

Experimental cw-EPR spectra of the radical cations P1_Sym^•+, P1^•+, and P2^•+ (black) acquired at 298 K in CD₂Cl₂/toluene-d₈/THF-d₈ 1:1:1 at X-band frequencies. The simulated cw-EPR spectra (red) of P1_Sym^•+ and P1^•+ were obtained by least-squares fitting of the isotropic ¹⁴N hyperfine interactions ^14NA_iso, and in the case of P1_Sym^•+ the isotropic hyperfine coupling ^1HA_iso to four equivalent ¹H nuclei using EasySpin. Dimer P2^•+ was simulated assuming a complete and uniform distribution of the radical spin density over both sites on the EPR time scale with ^14NA_iso(P2^•+) = 0.5·^14NA_iso(P1^•+).

Figure 4

a) Chemical structures of P1, P2, and b-P2 with colors corresponding to the cw-EPR spectra in b)–e); Ar = 3,5-bis(trihexylsilyl)phenyl and R′ = cyanopropyldiisopropylsilyl (CPDIPS) acetylene. Variable-temperature (VT) cw-EPR spectra of b) P1^•+, c) P2^•+, and e) b-P2^•+ in CD₂Cl₂/toluene-d₈/THF-d₈ 1:1:1 between 298 and 175 K in fluid solution and at 100 K in a frozen glass at X-band frequencies. At each temperature, the variable-temperature spectrum is compared to the corresponding spectrum at 298 K (gray). The variable temperature spectra of P2^•+ are superimposed in Figure S7 to highlight the exchange broadening. d) Comparison of the variable-temperature cw-EPR spectra of P1^•+ (purple) and P2^•+ (blue).

Figure 5

a) Experimental (black) and simulated (red) ¹H Mims ENDOR spectra of P1_Sym^•+, P1^•+, and P2^•+ recorded at Q-band frequencies in frozen CD₂Cl₂/toluene-d₈/THF-d₈ 1:1:1 at 80 K. The spectrum of P1_Sym^•+ was simulated using anisotropic ¹H hyperfine tensors, ^1HA, obtained from DFT calculations at the B3LYP/EPR-II level of theory. The spectra of P1^•+ and P2^•+ were simulated under identical conditions using hyperfine tensors calculated via the distributed point-dipole model for P1^•+ with a spin density ratio Δ = 96.1%, which is defined as the fraction of spin density on the porphyrin part. The individual contributions to the ¹H ENDOR spectra from the ortho- and β₁-hydrogens are highlighted in dark green and orange, respectively. b) Schematic representation of the anisotropic ¹H hyperfine tensors of P1_Sym^•+ and P1^•+. The tensors of the ortho- and β₁-protons that give rise to the highlighted transitions in a) are shown in dark green and orange, respectively; additional tensors are shown in gray as a reference. c) Root-mean-square deviation (rmsd) between the experimental and simulated ¹H Mims ENDOR spectra of P1^•+ as a function of Δ. d) Schematic visualization of the formal separation of P1^•+ into a porphyrin (purple) and biphenyl (blue) fragment.

Figure 6

a) Schematic visualization of the redistribution of spin density during the reversible intramolecular electron transfer in P2^•+. The spin population on each nucleus is represented by a sphere centered on the atom with a radius proportional to the magnitude of its assigned spin population. Red spheres represent excess spin-up and light-blue spheres indicate excess spin-down populations. The torsion angle Θ between the phenyl parts of the tetramethylbiphenyl linker is shown in the inset. Comparison of the experimental (black) and simulated (red) variable-temperature cw-EPR spectra of b) P1^•+ and c) P2^•+ at X-band frequencies in CD₂Cl₂/toluene-d₈/THF-d₈ 1:1:1 between 298 and 200 K. Simulations were performed with a two-site chemical exchange model. All monomer spectra were simulated in the slow exchange limit with k_ex = 10^–10 MHz.

Figure 7

Eyring plots of ln(k_exT^–1) versus T^–1 for P2^•+ in CD₂Cl₂/toluene-d₈/THF-d₈ 1:1:1 (black) and MTHF (blue). The data points at 200 K were omitted from the linear regression analysis because the experimental cw-EPR spectra at this temperature exhibit anisotropic broadening.

Figure 8

Comparison of the experimental (black) and simulated (red) transient EPR spectra of ³P1_Sym, ³P1, and ³P2 at X-band frequencies in a) MTHF at 20 K and b) a polyvinylcarbazole (PVK) film at 250 K. The experimental spectra are recorded as an average between 300 and 400 ns after the laser pulse with depolarized light excitation at 532 nm. Simulations were performed with the parameters reported in Table 3. The energetic ordering of the principal components of D was chosen as |D_Z| > |D_X| > |D_Y|, and the canonical field positions are indicated (A = absorption, E = emission). c) Experimental ¹H Mims ENDOR spectra of ³P1_Sym, ³P1, and ³P2 recorded at the X^–, Y^–, and Z^– field positions at 20 K in MTHF at X-band frequencies. d) DFT-calculated spin density distributions of ³P1_Sym, ³P1, and ³P2 using the B3LYP functional and the EPR-II basis set.