Transient EPR Reveals Triplet State Delocalization in a Series of Cyclic and Linear π-Conjugated Porphyrin Oligomers

Abstract

The photoexcited triplet states of a series of linear and cyclic butadiyne-linked porphyrin oligomers were investigated by transient Electron Paramagnetic Resonance (EPR) and Electron Nuclear DOuble Resonance (ENDOR). The spatial delocalization of the triplet state wave function in systems with different numbers of porphyrin units and different geometries was analyzed in terms of zero-field splitting parameters and proton hyperfine couplings. Even though no significant change in the zero-field splitting parameters (D and E) is observed for linear oligomers with two to six porphyrin units, the spin polarization of the transient EPR spectra is particularly sensitive to the number of porphyrin units, implying a change of the mechanism of intersystem crossing. Analysis of the proton hyperfine couplings in linear oligomers with more than two porphyrin units, in combination with density functional theory calculations, indicates that the spin density is localized mainly on two to three porphyrin units rather than being distributed evenly over the whole π-system. The sensitivity of the zero-field splitting parameters to changes in geometry was investigated by comparing free linear oligomers with oligomers bound to a hexapyridyl template. Significant changes in the zero-field splitting parameter D were observed, while the proton hyperfine couplings show no change in the extent of triplet state delocalization. The triplet state of the cyclic porphyrin hexamer has a much decreased zero-field splitting parameter D and much smaller proton hyperfine couplings with respect to the monomeric unit, indicating complete delocalization over six porphyrin units in this symmetric system. This surprising result provides the first evidence for extensive triplet state delocalization in an artificial supramolecular assembly of porphyrins.

Figure 1

Molecular structures of the linear porphyrin oligomers, PN (N = 1–6), of the linear porphyrin trimer bound to the template T6 (PN·T6) and of the six-membered porphyrin ring (c-P6) with template indicated in gray (R = n-hexyl, Ar = phenyl rings with SiR₃ substituents at the meta positions).

Figure 2

Experimental X-band transient EPR spectra of linear porphyrin chains (P1–P6) in MeTHF:pyridine 10:1 recorded at 20 K as average up to 2 μs after the laser pulse with unpolarized light at 532 nm. Simulations with the parameters reported in Table 1 are compared to the experimental data. The ordering of the triplet sublevels was chosen as |Z| > |X| > |Y|, and the six canonical positions are indicated for P1 and P2. For P3–P6, the same assignments as shown for P2 are valid (A = absorption, E = emission). The inset shows the orientation of the ZFS tensor in the molecular frame for the oligomers P2–P6.

Figure 3

Experimental X-band transient EPR spectra of the zinc (A) and free-base (B) linear porphyrin oligomers P1–P4 in MeTHF:pyridine 10:1 recorded as average up to 2 μs after the laser pulse at 20 K. The spectra were recorded after excitation at wavelengths corresponding to the planar conformations (645, 750, 800, and 830 nm for the zinc porphyrins and 680, 740, 780, and 810 nm for the free-base porphyrins, see UV–vis data in the Supporting Information). At shorter wavelengths, the contribution of different conformations affects the spin polarization of the EPR spectrum.

Figure 4

(A) Experimental Mims ENDOR spectra of P1–P6 recorded at the high-field Y position at 20 K. (B) Hyperfine couplings of the H₁ protons along the Y axis of the ZFS tensor (A_Y) determined by Gaussian fitting of the principal hyperfine peak in the experimental ENDOR spectra as a function of oligomer size; the error bars indicate the full width at half-maximum (fwhm). The gray line corresponds to the theoretical N^–1 relationship for the hyperfine couplings in case of complete delocalization. The change of the position of the hyperfine peak with respect to the Larmor frequency between P1 and P2 was explained by a change in the sign of D. The orientation of the ZFS tensor for the linear oligomers is shown in the inset; in P1, the X and Z axes are exchanged.

Figure 5

Spin density distributions in the first excited triplet state calculated at B3LYP/EPRII level for the optimized geometries of P3 and P4. The spin density distributions of the longer oligomers are shown in Figure S2 of the Supporting Information.

Figure 6

Transient EPR spectra recorded at 20 K up to 2 μs after the 532 nm laser pulse for the linear oligomers P2, P3, P4, and P6 in toluene:pyridine 10:1 and of the same oligomers bound to the T6 template in toluene without pyridine.

Figure 7

Mims ENDOR spectra recorded at the high-field Y position for the free and T6-bound P2 and P3 in toluene solution. Excitation at 532 nm was used in both cases.

Figure 8

(A) Transient EPR spectra recorded at 20 K for c-P6, c-P6·T6, and free-base c-P6. The spectra are compared to the EPR spectrum of P1 in the inset. (B) Mims ENDOR spectra recorded at 20 K at a magnetic field of 354.1 mT (high-field Z transition) for c-P6 and c-P6·T6. The spectra are compared to the ENDOR spectrum of P1 (high-field Y position, corresponding to the same molecular orientation along the phenyl rings) in the inset.

Figure 9

Bottom panel: Transient EPR spectra recorded for c-P6·T6 after excitation with light at 810 nm polarized parallel or perpendicular to the magnetic field. The contributions of the m_S = −1 → m_S = 0 and m_S = 0 → m_S = +1 transitions to the spectrum are shown for comparison. The simulation parameters are reported in Table 4. Top panel: The polarization ratios are shown as a function of field position above the spectra.