Singlet fission as a polarized spin generator for dynamic nuclear polarization

Organic assemblies' photo-excited states offer unique opportunities in optoelectronics, leveraging singlet and triplet molecular excitons. Singlet fission (SF), generating two triplet excitons from one singlet exciton, presents unique electron and spin functionalities. While extensively studied for photovoltaics as an exciton multiplier, its spin degree of freedom remains largely unexplored. SF's potential surpasses the theoretical limit of single-junction solar cells if split excitons are harvested as free electrons and holes. The basic SF process involves a singlet exciton (S1) transitioning to a triplet-pair state (TT) with singlet multiplicity, followed by intersystem crossing (ISC) to a quintet triplet-pair (5TT). In larger assemblies, triplet-pair states can dissociate into free triplets. The multiexcitonic nature allows for quintet multiplicity due to four half-filled orbitals. SF efficiently creates spin-polarized quintet states without heavy metals. However, applications of this unique quintet state are underexplored. This research investigates SF's spin degree of freedom for quantum technologies. Specific quintet sublevels can be preferentially populated, notably the (5TT)0 state in chromophore dimers, achieved through sufficient exchange interaction between parallel chromophores aligned with the Zeeman field. This model explains experimental results in oriented crystalline samples. Organic spin materials, with their nanoscale size and biocompatibility, are ideal for quantum information science (QIS) and quantum biotechnologies. Dynamic nuclear polarization (DNP) of biomolecules is a key area where polarized electron spins are crucial. NMR and MRI, while indispensable, suffer from low sensitivity. Current clinical trials use cryogenic temperatures to transfer polarization from radical electron spins to nuclear spins of bioprobes. However, the equipment is expensive and complex. A need exists for DNP at higher temperatures using polarized electron spins. While triplet excited states from spin-preferential ISC have been used, their polarization is typically far less than 100%. SF, however, offers the potential for ultimate polarization by preferentially populating the (TT)0 spin sublevel through control over chromophore dimer structure and orientation.

The literature extensively covers singlet fission (SF) in organic materials for its potential in photovoltaic applications, focusing on its ability to generate multiple excitons from a single photon. Studies have explored the mechanism of SF, including the ultrafast dynamics of exciton generation and the role of intermolecular interactions. However, the exploitation of the unique spin properties of SF-generated multiexcitons, specifically the quintet state, has received limited attention. Existing work highlights the potential for preferential population of specific spin sublevels within the quintet state, but practical applications in areas like dynamic nuclear polarization (DNP) are yet to be fully realized. The application of polarized electron spins in DNP, particularly for enhancing the sensitivity of NMR and MRI, is well-established. Methods utilizing cryogenic temperatures and radical electron spins have shown significant improvements but are limited by their complexity and cost. The use of photoexcited triplet states as polarization sources has also been explored, but these typically suffer from lower polarization efficiencies. This study builds upon the existing knowledge of SF and DNP, aiming to bridge the gap between the fundamental understanding of SF spin dynamics and its application in a practically relevant DNP system.

This study employs a pentacene derivative, sodium 4,4'-(pentacene-6,13-diyl)dibenzoate (NaPDBA), as the representative chromophore exhibiting SF. Two strategies are used to construct discrete assemblies of pentacene moieties in a water-glycerol matrix: supramolecular assembly of an amphiphilic NaPDBA and complexation with cyclodextrin (CD). A 1:1 water-glycerol mixture maintains the glassy state at low temperatures required for DNP. Molecular dynamics (MD) simulations were performed to model the supramolecular assemblies. Ultrafast pump-probe transient absorption spectroscopy (TAS) and time-resolved electron spin resonance (ESR) measurements were conducted to investigate the SF process and evaluate the spin polarization. Femtosecond (fs) and nanosecond (ns) TAS measurements were performed at 143 K to monitor the ultrafast generation of triplet states through SF. Global analysis of the TAS data was used to determine the time constants of SF and other relevant processes. Time-resolved ESR measurements were conducted to detect the spin polarization generated by SF. The ESR spectra were simulated using a geometric fluctuation model to assess the population of quintet and triplet sublevels. Dynamic nuclear polarization (DNP) experiments were performed using the integrated solid effect (ISE) sequence. The Hartmann-Hahn condition was satisfied by simultaneously sweeping the magnetic field and applying microwave irradiation. The DNP efficiency was evaluated by measuring the enhancement of the 1H NMR signal intensity. The dependence of DNP on magnetic field and microwave power was investigated. Various controls using β-cyclodextrin (BCD) complexes, which do not exhibit SF, were used to confirm the origin of DNP enhancement. The NaPDBA was dispersed in methanol before use to prevent aggregation. The water-glycerol samples were prepared by mixing and rapid cooling before measurements. The MD simulations were run using AMBER18 and TIP4P-Ew water model. The absorption spectra were analyzed for evidence of aggregate formation in water-glycerol. Time-resolved ESR measurements were conducted to identify quintet and triplet signals. For DNP experiments, a 5-mm ESR tube was used, with a solvent mixture of deuterated glycerol, D2O, and H2O (5:4:1). The DNP sequence involved microwave irradiation and magnetic field sweeping to match the Rabi frequency of electron spins and the Larmor frequency of nuclear spins.

MD simulations showed that NaPDBA formed dimers and a few monomers in water-glycerol, preventing larger aggregates that hinder polarization transfer. fs-TAS measurements revealed ultrafast SF in NaPDBA aggregates and the NaPDBA-γCD complex, with time constants of 2.65 ps and 0.79 ps respectively, while no significant SF was observed in the NaPDBA-BCD complex. Time-resolved ESR measurements confirmed the generation of quintet states in both NaPDBA and NaPDBA-γCD, characterized by the observation of signals at a position and width consistent with a quintet state. The (3TT)2, (3TT)0, and (3TT)-2 sublevels were preferentially populated. The ESR spectra did not show signs of significant triplet dissociation from the quintet state. DNP experiments showed a clear increase in the 1H NMR signal intensity for NaPDBA-only and NaPDBA-γCD samples at a position corresponding to the (TT)→(TT)0 ESR peak. The DNP enhancement was significantly lower for the NaPDBA-BCD complex. A 20-fold (NaPDBA) and 6.5-fold (NaPDBA-γCD) enhancement at the quintet ESR peak was observed. The DNP enhancement showed a clear dependence on both the magnetic field and the microwave intensity. The NMR signal was maximized at a weaker microwave intensity for the quintet than the triplet due to the higher Rabi frequency of electrons in the quintet state. This confirmed the successful utilization of the SF-derived polarized quintet state for DNP.

The findings demonstrate that SF-generated quintet multiexcitons can serve as a highly effective source of spin polarization for DNP of water molecules. The successful DNP enhancement in water-glycerol, a biologically relevant matrix, highlights the potential of this approach for applications in bio-quantum technology. The observed dependence of DNP efficiency on the assembly structure underscores the importance of controlling the interactions between pentacene units for optimizing the performance. The preferential population of the (TT)0 sublevel suggests a path towards further improving DNP efficiency by controlling the orientation of the chromophores relative to the magnetic field. The lower enhancement in the NaPDBA-γCD complex compared to NaPDBA is likely due to triplet-triplet annihilation, highlighting the need for optimized inter-chromophore interactions. The results show that while triplet DNP has a larger effect than quintet DNP in this system, selective dimer synthesis and control over (TT)0 population could improve the quintet DNP performance. This work opens new possibilities for quantum sensing and quantum information science based on organic chromophore multiexcitons.

This study successfully demonstrates the application of singlet fission as a polarized spin generator for dynamic nuclear polarization, specifically achieving nuclear hyperpolarization of water molecules using SF-derived quintet electron polarization. The observed DNP enhancement is dependent on the assembly structure, indicating directions for optimization. This research opens up new avenues for quantum biotechnology and accelerates the development of quantum sensing and information science based on organic chromophores.

The study primarily focuses on pentacene derivatives in a water-glycerol matrix, limiting the generalizability to other organic systems and solvents. The molecular dynamics simulations provide a simplified model of the supramolecular assemblies, and the actual structure and dynamics in the experimental samples may deviate from the simulations. The DNP efficiency is influenced by several factors, and further optimization of the experimental conditions and molecular design is necessary for practical applications.