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

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.


Experimental Methods
Sample preparation S1 Time-resolved EPR S1 Pulse EPR S1 Spectral analysis S2 Computational methods S2 Additional figures S3 General chemical synthesis details S5 Synthetic Procedures and Characterization S8 References S13

Sample preparation
The zinc porphyrin oligomers (P1 to P6) and the porphyrin ring (c-P6) depicted in Figure 1 of the main text were synthesized as described below. The EPR measurements were performed on 50-200 μM solutions in MeTHF: pyridine 10:1. The measurements on the oligomers bound to the template were performed in toluene, since coordinating solvents interfere with the binding of the template. The solutions were degassed by several freeze-pump-thaw cycles and frozen in liquid N 2 .

Time-resolved EPR
The time-resolved EPR experiments were performed at X-band on a Bruker Elexsys 680 spectrometer equipped with a helium gas-flow cryostat from Oxford instruments. Laser excitation at 532 nm was provided by the second harmonic of an Nd:YAG laser (Surelite Continuum) with a repetition rate of 10 Hz. Light depolarized with an achromatic depolarizer was used unless otherwise stated. TR-EPR experiments were performed by direct detection with the transient recorder without lock-in amplification; the microwave power was 0.2 mW. The laser background signal was removed by 2D baseline-correction determined based on the offresonance transients. The spectra were integrated over the first 2 μs after the laser flash. Experiments were typically performed at 20 K, but no changes in the spectra were observed at temperatures between about 100 and 5 K.
Experiments with different excitation wavelengths were performed with an Opotek Opolette Optoparametric Oscillator (OPO) tunable laser (20 Hz repetition rate) at Figure S1. Room temperature UV-Vis spectra recorded in MeTHF:pyridine 10:1. wavelengths corresponding to the region of the Q-bands in the UV-Vis spectra (see Figure S1). For the magnetophotoselection measurements the light was polarized with a Glan-Thompson polarizer.
Pulse EPR X-band pulse EPR measurements were performed on a Bruker Elexsys 680 spectrometer with a Bruker EN 4118X-MD4 resonator. The measurements were performed at 20 K and with laser excitation as previously described.
The repetition rate of the pulse experiments was determined by the laser repetition rate of 10 Hz (20 Hz for measurements with the OPO). Radical signals were removed with the use of a saturating microwave pulse preceding the laser flash. 1 H Mims ENDOR spectra were recorded with the pulse sequence − τ − echo with mw pulse lengths of t π/2 =24 ns, τ =120, 160, 200 ns and a radiofrequency pulse length of 15 µs; the RF power was adjusted based on a nutation experiment. The ENDOR spectra were recorded at the canonical field positions of the triplet state EPR spectrum; spectra were recorded for three different τ values (120, 160 and 200 ns) and summed to prevent distortions by blind spots.

Spectral Analysis
The spin-polarized powder triplet state spectra were simulated using EasySpin's pepper routine. 1 The zero-S2 field splitting parameters D and E as well as the relative population probabilities at zero-field were determined by least-square fitting of the experimental transient EPR data. The energy ordering of the triplet sublevels was chosen as |Z|>|X|>|Y|.
The relative orientations of the zero-field splitting tensor orientations and the optical transition dipole moments was determined based on the polarization ratio calculated from the magnetophotoselection data. The polarization ratio is defined as: [2][3] where I i ∥/⊥ are the intensities of the derivative EPR signal for excitation with light polarized parallel or perpendicular to the magnetic field at the field positions corresponding to the X, Y or Z orientation of the ZFS tensor. The polarization ratio was calculated by integration of the low and high field canonical regions of the derivative spectra; standard deviations were estimated by considering different regions for the integration (derivative signal maximum ±0.05-0.40 mT). The values of the proton hyperfine couplings along the principal axes of the ZFS tensor were determined by Gaussian deconvolution of the ENDOR spectra.

Computational methods
DFT geometry optimizations of the triplet excited state structures for P1 to P6 and for P2·T6 to P4·T6 were performed in ORCA [4][5] with the BP86 functional and the SV(P) basis set using the RI approximation with the auxiliary SV/C basis set. The Si(C 6 H 13 ) 3 groups were replaced by hydrogen atoms and the resulting structures were optimized without symmetry constraints. Calculations of the zero-field splitting interaction were performed according to a procedure published by Sinnecker et al., 6 using the B3LYP functional and the EPRII basis set 7 and calculating the spin-spin contribution to the ZFS using UNO determinants. Calculations of the zero-field splitting parameters could only be performed for systems with up to four porphyrin units, calculations on the larger systems were computationally not feasible. The results were compared to calculations with the BP86 and BHLYP functional to evaluate the extent of the self-interaction error and no significant difference was found. The spin density distributions for the linear oligomers are shown in Figure  S2.
The hyperfine parameters were calculated with the B3LYP functional and the EPRII basis set, purposely developed for the calculation of EPR hyperfine interaction values, for the C, N and H nuclei [7][8][9] and the 6-31G(d) basis set for Zn.
The DFT calculations for P3 predict an uneven spin density distribution with increased spin density on the central porphyrin unit. The predicted ratio of spin densities on the three porphyrin units depends on the amount of exact Hartree-Fock exchange included in the DFT functional. A calculation with the GGA (generalized gradient approximation) functional BP86 predicts a spin density distribution of 0.26:0.48:0.26 over the three porphyrin units, while the hybrid functional B3LYP, containing 20% of exact exchange and 80% of DFT exchange, predicts a distribution of 0.19:0.62:0.19. Variation of the contribution of exact exchange in the B3LYP functional in the range from 10% to 40% leads to variations of the relative spin density on the central porphyrin unit from 0.54 to 0.74. The best agreement of calculated and experimental hyperfine couplings is obtained for 20% exact exchange, corresponding to the standard B3LYP functional. This amount of exact exchange almost corresponds to the optimal mixing ratio of exact and DFT exchange believed to yield results close to chemical accuracy (25%). 10 DFT also predicts uneven spin density distributions for P4, P5 and P6, with the spin density mainly localized on the two central porphyrin units for even N and on the central porphyrin unit for odd N. The ratios of spin densities resulting from B3LYP/EPRII calculations (20% exact exchange) correspond to 0.10 : 0.40 : 0.40 : 0.10 for P4, 0.04 : 0.19 : 0.54 : 0.19 : 0.04 for P5 and 0.02 : 0.10 : 0.38 : 0.38 : 0.10 : 0.02 for P6. The BP86 functional again predicts different spin density distributions, which are slightly more spread out over the porphyrin array due to the self-interaction error affecting DFT calculations with functionals only containing DFT exchange. 11 The changes in ZFS parameters for different geometries of the linear oligomers were investigated by considering the SOMOs. The quasi-restricted molecular orbitals from single-point calculations performed at B3LYP/EPRII level for the porphyrin monomer, dimer, trimer and tetramer are shown in Figure S3. The corresponding orbitals of the templated oligomers are shown in Figure S4. The SOMOs localized using the Pipek-Mezey scheme 12 are also shown and were used to separate the Coulomb and exchange contribution to the D-value.

General chemical synthesis details
Unless stated otherwise, all reagents were obtained from commercial sources and used as received without further purification. Diethyl ether, chloroform and toluene were dried by passing through activated alumina columns using a positive pressure of dry N 2 . Diisopropylamine (DIPA) was dried by distillation from CaH 2 . All uses of petroleum ether refer to the 40-60 °C fraction. Analytical gel permeation chromatography (GPC) was performed on a JAIGEL H-P precolumn, a JAIGEL 3H-A (8 mm × 500 mm) and a JAIGEL 4H-A column (8 mm × 500 mm) in series with toluene/pyridine 100/1 as eluent. Preparative gel permeation chromatography (GPC) was performed using a line of PLGel 600 mm 10 μm 500 Å, 300 mm 10 μm 500 Å, and 300 mm 10 μm 1000 Å columns (all with 25 mm ID) (Polymer Laboratories Ltd) at a flow rate of 8.5 mL min -1 . Preparative recycling GPC was performed on a JAIGEL H-P precolumn, a JAIGEL 3H (20 mm × 600 mm) and a JAIGEL 4H column (20 mm × 600 mm) in series with toluene/pyridine 100/1 as eluent. Flash column chromatography was performed on Merck silica gel 60 (40-63 μm). Alumina columns were performed using aluminum oxide, activated, basic, Brockmann I, standard grade, ~150 mesh, 58 Å from Sigma Aldrich. For TLC, Merck silica gel 60 F 254 aluminum-backed sheets were used. Size exclusion chromatography (SEC) was carried out using Bio-Beads SX-1, 200-400 mesh (Bio Rad).
NMR spectra were recorded on a Bruker AVII400 or Bruker AVIII400 (400 MHz) spectrometer. The residual solvent peak was used as internal reference (chloroform, 1 H δ = 7.26 ppm, 13 C δ = 77.2 ppm). Multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, and m = multiplet) and coupling constant(s) were reported whenever possible. MALDI-TOF-MS spectra were measured using a Waters MALDI Micro MX or at the EPSRC National Mass Spectrometry service (Swansea, Wales, UK) using the Applied Biosystems Voyager DE-STR. UV-vis absorption spectra were recorded at ambient temperature with a Perkin-Elmer Lambda 20 using quartz 1 cm cuvettes. Absorptions appearing as shoulders are annotated "sh".