Singlet fission as a polarized spin generator for dynamic nuclear polarization

Abstract

Singlet fission (SF), converting a singlet excited state into a spin-correlated triplet-pair state, is an effective way to generate a spin quintet state in organic materials. Although its application to photovoltaics as an exciton multiplier has been extensively studied, the use of its unique spin degree of freedom has been largely unexplored. Here, we demonstrate that the spin polarization of the quintet multiexcitons generated by SF improves the sensitivity of magnetic resonance of water molecules through dynamic nuclear polarization (DNP). We form supramolecular assemblies of a few pentacene chromophores and use SF-born quintet spins to achieve DNP of water-glycerol, the most basic biological matrix, as evidenced by the dependence of nuclear polarization enhancement on magnetic field and microwave power. Our demonstration opens a use of SF as a polarized spin generator in bio-quantum technology.

Fig. 1. Schematic illustration of DNP using SF-born quintet electron polarization.

A Nuclear spins in the thermal equilibrium state. The red and blue arrows indicate α spin state and β spin state, respectively, and the gray circles indicate the populations of each spin state. B Polarization transfer from electron spins in the quintet state (|Q₀〉 state, green arrows) generated by photo-induced SF to nuclear spins and the subsequent diffusion of hyperpolarized nuclear spins. The green circles indicate the populations of polarized quintet state. DNP increases the α spin population (red circle) and decrease the β spin population (blue circle), resulting in the hyperpolarized nuclear spin state (red square). C Pulse sequence of quintet/triplet-DNP. D Molecular structures of NaPDBA and γ-cyclodextrin (γCD) and supramolecular assembly of only NaPDBA and the NaPDBA-γCD inclusion complex. E Absorption spectra of NaPDBA in water-glycerol at 143 K (black), NaPDBA-γCD in water-glycerol (1:1) at 143 K (blue), and NaPDBA in methanol at room temperature (red). The concentrations of NaPDBA and γCD were 1 and 5 mM, respectively.

Fig. 2. MD simulation of the supramolecular assemblies.

A, B MD simulation snapshots of NaPDBA ([NaPDBA] = 1 mM) in water-glycerol (1:1) at 300 K. Parallel oriented dimers are shown in yellow. C, D MD simulation snapshots of NaPDBA and γCD ([NaPDBA] = 1 mM, [γCD] = 5 mM) in water-glycerol (1:1) at 243 K.

Fig. 3. fs-transient absorption spectroscopy (TAS) measurements of the supramolecular assemblies.

Overview of fs-TAS analysis of A–E NaPDBA and F–J NaPDBA-γCD in water-glycerol (1:1) at 143 K ([NaPDBA] = 1 mM, [γCD] = 5 mM). A, F Pseudo-2D plots of experimentally observed fs-TAS (excitation: 635 nm for NaPDBA and 600 nm for NaPDBA-γCD), B, G spectral evolution of the TAS, and C, H temporal change of transient absorption at selected wavelengths and fitting curves from global analysis. D, I Evolution-associated spectra (ESA) and E, J corresponding concentration kinetics obtained from global analysis based on sequential models. EAS1, EAS2, and EAS3 indicate the first, second and third components of EAS, respectively.

Fig. 4. Time-resolved ESR measurements of the supramolecular assemblies.

Time-resolved ESR spectra of A NaPDBA and B NaPDBA-γCD in water-glycerol (1:1) at 143 K ([NaPDBA] = 1 mM, [γCD] = 5 mM) just after photoexcitation at 527 nm and simulated spectra of C NaPDBA and D for NaPDBA-γCD, attributing transitions between each energy level of the quintet in the ESR spectra. The fitting parameters of the ISC-born triplet and SF-born quintet are summarized in Supplementary Tables 1 and 2, respectively.

Fig. 5. DNP using SF-born quintet electron spin polarization.

A, B ¹H-NMR signals under thermal conditions (black lines, 5 scans every 10 min) and after quintet-DNP (red lines, ISE sequence for 5 min, 1 scan) of water-glycerol (glycerol-d₈:D₂O:H₂O = 5:4:1) containing A NaPDBA and B NaPDBA-γCD at 100 K ([NaPDBA] = 1 mM, [γCD] = 5 mM). The photo-excitation wavelength and frequency were 527 nm and 500 Hz, respectively. DNP was performed by matching the magnetic field to the quintet peaks. The microwave power and frequency were 20 W and 17.30 GHz, respectively, the laser powers were 2.7 W for A and 1.5 W for B. The magnetic field sweep width was 10 μs. Magnetic field dependence of the signal intensity of the ¹H NMR by DNP and time-resolved ESR spectra in water-glycerol containing C NaPDBA, D NaPDBA-γCD, and E NaPDBA-βCD ([NaPDBA] = 1 mM, [βCD] = [γCD] = 5 mM). Water-glycerol glass (glycerol-d₈:D₂O:H₂O = 5:4:1) was used for the DNP measurement at 100 K. ISE sequence for 20 s (C, E) and 30 s (D) and 1 scan; microwave power and frequency were 20 W and 17.25 GHz, respectively; laser power: 1.5 W; magnetic field sweep width: 10 μs. Water-glycerol glass (glycerol:H₂O = 5:5) was used for the time-resolved ESR measurements at 143 K. ESR spectra were integrated for 10 μs after photoexcitation in order to compare the DNP profile with the ISE sequence for 10 μs. F Microwave power dependence of DNP enhancement. The gray dashed line indicates the microwave power when the ¹H-NMR signal is at its maximum. The red arrow indicates the peak top shift from triplet-DNP to quintet-DNP. Triplet-DNP was performed at 27.4 MHz (ISE sequence for 10 s and 4 scans with a laser power of 2.7 W, microwave frequency of 17.30 GHz and sweep width of 25 μs). Quintet-DNP was performed at 26.9 MHz (ISE sequence for 10 s and 10 scans with a laser power of 2.7 W, microwave frequency of 17.30 GHz and sweep width of 10 μs).