Magic Angle Spinning Solid-State 13C Photochemically Induced Dynamic Nuclear Polarization by a Synthetic Donor-Chromophore-Acceptor System at 9.4 T.

Solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) is a nuclear magnetic resonance spectroscopy technique in which nuclear spin hyperpolarization is generated upon optical irradiation of an appropriate donor-acceptor system. Until now, solid-state photo-CIDNP at high magnetic fields has been observed only in photosynthetic reaction centers and flavoproteins. In the present work, we show that the effect is not limited to such biomolecular samples, and solid-state 13C photo-CIDNP can be observed at 9.4 T under magic angle spinning using a frozen solution of a synthetic molecular system dissolved in an organic solvent. Signal enhancements for the source molecule larger than a factor of 2300 are obtained. In addition, we show that bulk 13C hyperpolarization of the solvent can be generated via spontaneous 13C-13C spin diffusion at natural abundance.

A 2D chiral microcavity based on apparent circular dichroism.

Engineering asymmetric transmission between left-handed and right-handed circularly polarized light in planar Fabry-Pérot (FP) microcavities would enable a variety of chiral light-matter phenomena, with applications in spintronics, polaritonics, and chiral lasing. Such symmetry breaking, however, generally requires Faraday rotators or nanofabricated polarization-preserving mirrors. We present a simple solution requiring no nanofabrication to induce asymmetric transmission in FP microcavities, preserving low mode volumes by embedding organic thin films exhibiting apparent circular dichroism (ACD); an optical phenomenon based on 2D chirality. Importantly, ACD interactions are opposite for counter-propagating light. Consequently, we demonstrated asymmetric transmission of cavity modes over an order of magnitude larger than that of the isolated thin film. Through circular dichroism spectroscopy, Mueller matrix ellipsometry, and simulation using theoretical scattering matrix methods, we characterize the spatial, spectral, and angular chiroptical responses of this 2D chiral microcavity.

Harvesting electrons and holes from photodriven symmetry-breaking charge separation within a perylenediimide photosynthetic model dimer.

Understanding how to utilize symmetry-breaking charge separation (SB-CS) offers a path toward increasingly efficient light-harvesting technologies. This process plays a central role in the first step of photosynthesis, in which the dimeric "special pair" of the photosynthetic reaction center enters a coherent SB-CS state after photoexcitation. Previous research on SB-CS in both biological and synthetic chromophore dimers has focused on increasing the efficiency of light-driven processes. In a chromophore dimer undergoing SB-CS, the energy of the radical ion pair product is nearly isoenergetic with that of the lowest excited singlet (S1) state of the dimer. This means that very little energy is lost from the absorbed photon. In principle, the relatively high energy electron and hole generated by SB-CS within the chromophore dimer can each be transferred to adjacent charge acceptors to extend the lifetime of the electron-hole pair, which can increase the efficiency of solar energy conversion. To investigate this possibility, we have designed a bis-perylenediimide cyclophane (mPDI2) covalently linked to a secondary electron donor, peri-xanthenoxanthene (PXX) and a secondary electron acceptor, partially fluorinated naphthalenediimide (FNDI). Upon selective photoexcitation of mPDI2, transient absorption spectroscopy shows that mPDI2 undergoes SB-CS, followed by two secondary charge transfer reactions to generate a PXX•+-mPDI2-FNDI•- radical ion pair having a nearly 3 µs lifetime. This strategy has the potential to increase the efficiency of molecular systems for artificial photosynthesis and photovoltaics.

Exciplex Emission and Förster Resonance Energy Transfer in Polycyclic Aromatic Hydrocarbon-Based Bischromophoric Cyclophanes and Homo[2]catenanes.

Energy transfer and exciplex emission are not only crucial photophysical processes in many living organisms but also important for the development of smart photonic materials. We report, herein, the rationally designed synthesis and characterization of two highly charged bischromophoric homo[2]catenanes and one cyclophane incorporating a combination of polycyclic aromatic hydrocarbons, i.e., anthracene, pyrene, and perylene, which are intrinsically capable of supporting energy transfer and exciplex formation. The possible coconformations of the homo[2]catenanes, on account of their dynamic behavior, have been probed by Density Functional Theory calculations. The unique photophysical properties of these exotic molecules have been explored by steady-state and time-resolved absorption and fluorescence spectroscopies. The tetracationic pyrene-perylene cyclophane system exhibits emission emanating from a highly efficient Förster resonance energy transfer (FRET) mechanism which occurs in 48 ps, while the octacationic homo[2]catenane displays a weak exciplex photoluminescence following extremely fast (<0.3 ps) exciplex formation. The in-depth fundamental understanding of these photophysical processes involved in the fluorescence of bischromophoric cyclophanes and homo[2]catenanes paves the way for their use in future bioapplications and photonic devices.

Two-Dimensional Electronic Spectroscopy Reveals Vibrational Modes Coupled to Charge Transfer in a Julolidine-BODIPY Dyad.

Understanding charge transfer (CT) dynamics in molecular donor-acceptor (D-A) dyads can provide insight into developing efficient D-A molecules for capturing solar energy. Here, we characterize the excited-state evolution of a julolidine-BODIPY (Jul-BD) D-A system with an emissive CT state using time-resolved fluorescence, femtosecond transient absorption, and two-dimensional electronic spectroscopies. Comparison of these results with those from phenyl-BODIPY (Ph-BD) allows us to identify the dynamics at play during CT state formation and its subsequent conversion to either a fully charge-separated or triplet state. Photoexcitation of Jul-BD in tetrahydrofuran results in the formation of an initial emissive CT state that relaxes before fully charge-separating. In contrast, Jul-BD in toluene exhibits similar CT state dynamics, albeit at slower timescales, before decaying to a terminal triplet species. Quantum beat analysis at early times in both solvents shows several vibronic modes, which are corroborated using density functional theory (DFT) calculations. For Ph-BD, a single 220 cm-1 compression mode about the single bond linking the phenyl to BODIPY modulates their orbital overlap. Three active vibronic modes, 147, 174, and 214 cm-1, are found in Jul-BD, regardless of the dielectric constant of the medium. These motions correspond to compression and torsional motions along the single bond joining Jul to BD and are responsible for the evolution of the spontaneous and stimulated emission features in the time-resolved spectroscopic data, which is further supported by time-dependent DFT calculations of the steady-state absorption spectrum of the Jul-BD as a function of increasing D-A dihedral core angle. These findings show how torsional and compression motions can play a pivotal role in intramolecular CT between a D and an A linked by a single bond.

Singlet Fission in Perylene Monoimide Single Crystals and Polycrystalline Films.

Singlet fission (SF) is a spin-allowed process in which a photogenerated singlet exciton down-converts into two triplet excitons. Perylene-3,4-dicarboximide (PMI) has singlet and triplet state energies of 2.4 and 1.1 eV, respectively; thus making SF slightly exoergic and providing triplet excitons that have sufficient energy to raise the efficiency of single-junction solar cells by reducing thermalization losses from hot excitons formed when absorbed photons have energies higher than the semiconductor bandgap. However, PMI SF in the solid state has not been studied previously. Here, we show that 2,5-diphenyl-N-(2-ethylhexyl)perylene-3,4-dicarboximide (dp-PMI) crystallizes into a slip-stacked intermolecular morphology favorable for SF. Transient absorption microscopy and spectroscopy show that dp-PMI SF occurs in ≤50 ps in both single crystals and polycrystalline thin films with a triplet yield of 150 ± 20%. Ultrafast SF in the solid state, the high triplet yield, and its photostability make dp-PMI an attractive candidate for SF-enhanced solar cells.

Optical Spin Polarization of a Narrow-Linewidth Electron-Spin Qubit in a Chromophore/Stable-Radical System.

Photoexcited organic chromophores appended to stable radicals can serve as qubit and/or qudit candidates for quantum information applications. 1,6,7,12-Tetra-(4-tert-butylphenoxy)-perylene-3,4 : 9,10-bis(dicarboximide) (tpPDI) linked to a partially deuterated α,γ-bisdiphenylene-ß-phenylallyl radical (BDPA-d16 ) was synthesized and characterized by time-resolved optical and electron paramagnetic resonance (EPR) spectroscopies. Photoexcitation of tpPDI-BDPA-d16 results in ultrafast radical-enhanced intersystem crossing to produce a quartet state (Q) followed by formation of a spin-polarized doublet ground state (D0 ). Pulse-EPR experiments confirmed the spin multiplicity of Q and yielded coherence times of Tm =2.1±0.1 µs and 2.8±0.2 µs for Q and D0 , respectively. BDPA-d16 eliminates the dominant 1 H hyperfine couplings, resulting in a single narrow line for both the Q and D0 states, which enhances the spectral resolution needed for good qubit addressability.

Thermally Controlled Exciplex Fluorescence in a Dynamic Homo[2]catenane.

Motion-induced change in emission (MICE) is a phenomenon that can be employed to develop various types of probes, including temperature and viscosity sensors. Although MICE, arising from the conformational motion in particular compounds, has been studied extensively, this phenomenon has not been investigated in depth in mechanically interlocked molecules (MIMs) undergoing coconformational changes. Herein, we report the investigation of a thermoresponsive dynamic homo[2]catenane incorporating pyrene units and displaying relative circumrotational motions of its cyclophanes as evidenced by variable-temperature 1H NMR spectroscopy and supported by its visualization through molecular dynamics simulations and quantum mechanics calculations. The relative coconformational motions induce a significant change in the fluorescence emission of the homo[2]catenane upon changes in temperature compared with its component cyclophanes. This variation in the exciplex emission of the homo[2]catenane is reversible as demonstrated by four complete cooling and heating cycles. This research opens up possibilities of using the coconformational changes in MIMs-based chromophores for probing fluctuations in temperature which could lead to applications in biomedicine or materials science.

Alkoxy-Substituted Quadrupolar Fluorescent Dyes.

Polar and polarizable π-conjugated organic molecules containing push-pull chromophores have been investigated extensively in the past. Identifying unique backbones and building blocks for fluorescent dyes is a timely exercise. Here, we report the synthesis and characterization of a series of fluorescent dyes containing quadrupolar A-D-A constitutions (where A = acceptor and D = donor), which exhibit fluorescence emission at a variety of different wavelengths. We have investigated the effects of different electron-withdrawing groups, located at both termini of a para-terphenylene backbone, by steady-state UV/vis and fluorescence spectroscopy. Pyridine and substituted pyridinium units are also introduced during the construction of the quadrupolar backbones. Depending on the quadrupolarity, fluorescence emission wavelengths cover from 380 to 557 nm. Time-resolved absorption and emission spectroscopy reveal that the photophysical properties of those quadrupolar dyes result from intramolecular charge transfer. One of the dyes we have investigated is a symmetrical box-like tetracationic cyclophane. Its water-soluble tetrachloride, which is non-cytotoxic to cells up to a loading concentration of 1 µM, has been employed in live-cell imaging. When taken up by cells, the tetrachloride emits a green fluorescence emission without any hint of photobleaching or disruption of normal cell behavior. We envision that our design strategy of modifying molecules through the functionalization of the quadrupolar building blocks as chromophores will lead to future generations of fluorescent dyes in which these A-D-A constitutional fragments are incorporated into more complex molecules and polymers for broader photophysical and biological applications.

Aggregation-Induced Emission and Circularly Polarized Luminescence Duality in Tetracationic Binaphthyl-Based Cyclophanes.

Here, we report an approach to the synthesis of highly charged enantiopure cyclophanes by the insertion of axially chiral enantiomeric binaphthyl fluorophores into the constitutions of pyridinium-based macrocycles. Remarkably, these fluorescent tetracationic cyclophanes exhibit a significant AIE compared to their neutral optically active binaphthyl precursors. A combination of theoretical calculations and time-resolved spectroscopy reveal that the AIE originates from limited torsional vibrations associated with the axes of chirality present in the chiral enantiomeric binaphthyl units and the fine-tuning of their electronic landscape when incorporated within the cyclophane structure. Furthermore, these highly charged enantiopure cyclophanes display CPL responses both in solution and in the aggregated state. This unique duality of AIE and CPL in these tetracationic cyclophanes is destined to be of major importance in future development of photonic devices and bio-applications.

Symmetry-Breaking Charge Separation in Phenylene-Bridged Perylenediimide Dimers.

Perylenediimides (PDIs) are important molecular building blocks that are being investigated for their applicability in optoelectronic technologies. Covalently linking multiple PDI acceptors at the 2,5,8,11 (headland) positions adjacent to the PDI carbonyl groups is reported to yield higher power conversion efficiencies in photovoltaic cells relative to PDI acceptors linked at the 1,6,7,12 (bay) positions. While the photophysical properties of PDIs linked via the bay positions have been investigated extensively, those linked at the headland positions have received far less attention. We showed previously that symmetry-breaking charge separation (SB-CS) in PDIs hold promise as a strategy for increasing photovoltaic efficiency. Here we use transient absorption and emission spectroscopies to investigate the competition between SB-CS, fluorescence, and internal conversion in three related PDI dimers linked at the headland positions with o-, m-, and p-phenylene moieties: o-PDI2, m-PDI2, and p-PDI2, respectively. It is found that o-PDI2 supports SB-CS yielding PDI•+-PDI•-, which is in equilibrium with the o-PDI2 first excited state in a polar solvent (CH2Cl2) while m-PDI2 and p-PDI2 exhibit accelerated internal conversion due to the motion of the linker along with subnanosecond intersystem crossing (ISC). Electronic coupling and structural dynamics are shown to play a significant role, with o-PDI2 being the only member of the series that exhibits significant through-bond interchromophore coupling. The pronounced o-PDI2 steric congestion prevents the free internal rotation that leads to rapid deactivation of the excited state in the other dimers.

Divergent excited state proton transfer reactions of bifunctional photoacids 1-ammonium-2-naphthol and 3-ammonium-2-naphthol in water and methanol.

This paper highlights the challenge of predicting the excited state proton transfer (ESPT) reactions of small organic compounds with multiple proton transfer sites. Aminonaphthols, naphthalene compounds with both hydroxyl and amino substituents, can be viewed as a combination of two monoprotic photoacids, naphthol and naphthylammonium. Here, the ESPT reactions of 3-ammonium-2-naphthol (3N2OH) and 1-ammonium-2-naphthol (1N2OH) were studied in water and methanol using a combination of steady-state and time-correlated single-photon counting emission spectroscopy. For 3N2OH, ESPT was observed at the OH site in water but at neither of the sites in methanol; for 1N2OH, ESPT was observed at both the OH and NH3+ sites in water but only at the NH3+ site in methanol. Evidence of ESPT at the NH3+ site is limited for aminonaphthols. The divergent dynamics of 3N2OH and 1N2OH in water and methanol are discussed; dependent on the substitution and solvent, the ESPT reactions were analysed within the frameworks of reference photoacids 2-naphthol and 1-naphthylammonium. The application of crown ether and salt to control the release of select protons in non-aqueous media is also discussed.

Divergent Hammett Plots of the Ground- and Excited-State Proton Transfer Reactions of 7-Substituted-2-Naphthol Compounds.

The rational design of photoacids requires accessible predictive models of the electronic effect of functional groups on chemical templates of interest. Here, the effect of substituents on the photoacidity and excited-state proton transfer (PT) pathways of prototype 2-naphthol (2OH) at the symmetric C7 position was investigated through photochemical and computational studies of 7-amino-2-naphthol (7N2OH) and 7-methoxy-2-naphthol (7OMe2OH). Time-resolved emission experiments of 7N2OH revealed that the presence of an electron-withdrawing versus electron-donating group (EWG vs EDG, NH3+ vs NH2) led to a drastic decline in photoacidity: p Ka* = 1.1 ± 0.2 vs 9.6 ± 0.2. Time-dependent density functional theory calculations with explicit water molecules confirmed that the excited neutral state (x = NH2) is greatly stabilized by water, with equation-of-motion coupled cluster singles and doubles calculations supporting potential mixing between the La and Lb states. Similar suppression of photoacidity, however, was not observed for 7OMe2OH with EDG OCH3, p Ka* = 2.7 ± 0.1. Hammett plots of the ground- and excited-state PT reactions of substituted 7-x-2OH compounds (x = CN, NH3+, H, CH3, OCH3, OH, and NH2) vs Hammett parameters σp showed breaks in the linearity between the EDG and EWG regions: ρ ∼ 0 vs 1.14 and ρ* ∼ 0 vs 3.86. The divergent acidic behavior most likely arises from different mixing mechanisms of the lowest Lb state with the La and possible Bb states upon substitution of naphthalene in water.