Time-resolved Electron Paramagnetic Resonance Spectra Research Articles

Charge Transfer and Intersystem Crossing in Compact Naphthalenediimide-Phenothiazine Triads: Synthesis and Study of the Photophysical Property with Transient Optical and Electron Paramagnetic Resonance Spectroscopic Methods.

In order to obtain a long-lived charge separation (CS) state in compact electron donor-acceptor molecular systems, we prepared a series of naphthalenediimide (NDI)-phenothiazine (PTZ) triads, with phenylene as the linker between the donor and acceptor. Conformation restriction is imposed to control the mutual orientation of the NDI and PTZ units by attaching methyl groups on the phenylene linker to tune the electronic coupling between the donor and the acceptor. Moreover, the PTZ moiety was oxidized to sulfoxide to tune the ordering of the CS state and the 3LE state (LE: locally excited state). UV-vis absorption spectra indicate electronic coupling between NDI with the phenylene linker as well as the PTZ units, manifested by the appearance of a charge-transfer (CT) absorption band, whereas this coupling is devoid in the triads with conformation restriction imposed. Fluorescence is strongly quenched in the triads compared to the reference compound, indicating electron transfer upon photoexcitation. Femtosecond transient absorption spectra indicate that the CS takes 0.8 ps, and then the 3LE state is formed by charge recombination in 83 ps. Nanosecond transient absorption (ns-TA) spectra show that the 3NDI state was observed in nonpolar solvents such as cyclohexane (triplet state lifetime: 95.7 μs), whereas the CS state was observed in more polar solvents. The CS state lifetimes are up to 1.2 μs (in toluene). Time-resolved electron paramagnetic resonance spectra of the triads in toluene consist of two types of signals: CS states (narrower signals, ∼10 mT) and 3LE states (broader signals, ∼50 to 200 mT). In the spectra of the triads containing PTZ, the CS state signals dominate, whereas for the triads containing oxidized PTZ, the 3NDI signals (zero-field splitting D ≈ 2000 MHz) prevail, both observations being in agreement with the ns-TA spectral studies. The electron spin polarization phase pattern of the 3NDI states of the triads indicates that the intersystem crossing (ISC) mechanism is spin-orbit charge-transfer ISC. Considering the 3CS state as ion pairs, the electron-exchange energy (J) is determined to be -39 to -59 MHz, and the electron spin dipolar interaction is 83-92 MHz.

Triplet Excited State Mechanistic Study of meso‐Substituted Methylthio Bodipy Derivative: Time‐Resolved Optical and Electron Paramagnetic Resonance Spectral Studies

AbstractUnderstanding the intersystem crossing (ISC) mechanism of organic compounds is essential for designing new triplet photosensitizers. Herein, we investigated the ISC mechanism of a heavy atom‐free Bodipy derivative with thiomethyl substitution (S−BDP). A long‐lived triplet state was observed with nanosecond transient absorption spectroscopy with lifetime of 7.5 ms in a polymer film and 178 μs intrinsic lifetime in fluid solution, much longer as compared with what was previously reported (apparent triplet lifetime=15.5 μs). Femtosecond transient absorption studies retrieved an ISC time constant of ∼3 ns. Time‐resolved electron paramagnetic resonance (TREPR) indicated a special triplet electron spin polarization phase (ESP) pattern (a, e, a, e, a, e) for S−BDP, different from the ESP (e, e, e, a, a, a) typical for the spin‐orbital coupling (SOC) mechanism. This indicates that the electron spin selectivity of the ISC of S−BDP is different from that of the normal SOC effect in iodo‐Bodipy. Simulations of the TREPR spectra give a zero‐field‐splitting D parameter of −2257 MHz, much smaller as compared to the reference 2,6‐diiodo‐Bodipy (D=−4380 MHz). The computed SOC matrix elements (0.28–1.59 cm−1) and energy gaps for the S1/Tn states suggest that the energy matching between the S1 and T2/T3 states (supported by the largest kISC ∼109 s−1) enhances the ISC for this compound.

A Rational Way to Control the Triplet State Wave Function Confinement of Organic Chromophores: Effect of the Connection Sites and Spin Density Distribution-Guided Molecular Structure Design Principles in Bodipy Dimers.

To study the intersystem crossing (ISC) and the spatial confinement of the triplet excited states of organic chromophores, we prepared a series of Bodipy dimers. We found that the connection position of the two units has a significant effect on the absorption and fluorescence. Singlet oxygen quantum yields of 3.8-12.4% were observed for the dimers, which are independent of solvent polarity. Nanosecond transient absorption spectra indicate the population of long-lived triplet excited states with lifetimes (τT) of 45-454 μs. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that the T1 triplet states are essentially delocalized, which is different from the case for the previously reported Bodipy dimers. The TREPR spectra of the triplet states imply that the delocalization over the whole dimer essentially depends on the electron density of the carbon atoms at the connection sites. This property may become a universal rule for controlling the T1 state confinement in multichromophore organic molecules.

Electron transfer and intersystem crossing in the coumarin-anthracene electron donor-acceptor dyads

A series of compact coumarin (Cou)-anthracene (An) electron donor-acceptor dyads were prepared, in which the two units are directly connected at either 2- or 9- position of the An unit and the 3-position of the Cou unit to study the relationship between the molecular configuration, electronic coupling and the intersystem crossing (ISC) efficiency. Single crystal X-ray diffraction results indicate that the dihedral angles are 88.0° and 85.7°, respectively, in the dyads, in which the donor and acceptor are connected by 9- position of the An unit (Cou-9An and Cou-9AnCN). However, for Cou-2An the optimized ground state geometry indicates a more coplanar configuration with a dihedral angle of 38.6°. The solvent polarity-dependent singlet oxygen quantum yields (ΦΔ) of the orthogonal dyads (Cou-9An and Cou-9AnCN) are 39% (in acetonitrile) and 64% (in dichloromethane), respectively. whereas ΦΔ is negligible for the coplanar dyad (Cou-2An). These results indicated that orthogonal geometry is beneficial to spin−orbit charge transfer ISC (SOCT-ISC) mechanism. Nanosecond transient absorption spectra demonstrated that the lowest triplet state was mainly localized on An unit in Cou-9An and Cou-9AnCN, with the intrinsic triplet state lifetime of 234 and 207 μs, respectively. Triplet state delocalization was observed for Cou-2An, which prolongs the triplet state lifetime to 742 μs. Femtosecond transient absorption spectra results indicated fast charge separation (CS, 0.36−0.41 ps) and slow charge recombination (CR, 2.1−2.8 ns) in the orthogonal dyads. Time-resolved electron paramagnetic resonance (TREPR) spectra further confirmed the SOCT-ISC mechanism in the orthogonal dyads (Cou-9An and Cou-9AnCN), however, the SOCT-ISC and spin orbit ISC (SO-ISC) mechanism are both responsible for the triplet state formation in the coplanar dyad (Cou-2An). Electron spin polarization (ESP) pattern of the triplet state TREPR spectra of Cou-9An and Cou-9AnCN is (a,e,a,e,a,e), however, ESP phase pattern of (e,a,e,a,e,a) and (e,e,e,a,a,a) were observed for the two triplet states required for the simulation of the TREPR spectrum of Cou-2An, which indicated that ESP patterns of triplet state not only depend on the molecular geometry, but also on the structure of the electron donor and acceptor.

Stepwise generation of spin-polarized radicals and their reactions in the photolysis of an oxime ester compound as studied by pulsed EPR spectroscopy

To generate spin-polarized free radicals, the photolysis of 1,2-octane-dione, 1-[4-(phenylthio)-2-(o-benzoyloxime)] (OXE01) was investigated using time-resolved electron paramagnetic resonance (TR-EPR) and pulsed EPR. This photolysis proceeds through α-cleavage of the N-O bond of OXE01 to yield iminyl and benzoyloxy radicals, the latter of which is converted to a phenyl radical by decarboxylation. The phenyl radical promptly abstracts hydrogen atoms from the surrounding solvent molecules to produce a solvent dehydrogenated radical (solvent radical). The spin polarization created in the initial OXE01 triplet excited state was preserved during these series of reactions, which enhanced EPR sensitivity. This enabled us to observe the TR-EPR spectra of the intermediate free radicals, except phenyl. The decarboxylation rate of the benzoyloxy radical was measured in various solvents by probing the free induction decay signal of the product solvent radicals using pulsed EPR. This new method for preparing free radicals with spin-polarization was applied to determine the addition reaction rate constants of the cyclohexyl radical, a solvent radical, to olefins using pulsed EPR monitoring of the electron spin echo signals of the radical.

Charge Separation and Intersystem Crossing in Homo- and Hetero-Compact Naphthalimide Dimers.

Naphthalimide (NI) homo- and hetero-dimers adopting orthogonal geometry were prepared to study photo-induced symmetry-breaking charge transfer (SBCT) and charge recombination (CR)-induced intersystem crossing (ISC). The two moieties in the dimer are connected either at the 3-C or 4-C position of the NI unit. The photophysical properties of the dimers were studied with steady-state and transient absorption spectroscopic methods. Significant CT only occurs for the hetero-dimer, in which one NI unit has a 4-amino substituent and the other NI unit is without it. The CR-induced ISC is most efficient for this dimer (singlet oxygen quantum yield ΦΔ = 50.3%). For the homo-dimer, in which both NI units did not present amino substitution, SBCT was not observed. Based on the electrochemical studies, we propose that the absence of SBCT for the homo-dimer is attributed to its high oxidation potential and low reduction potential. Femtosecond transient absorption (fs TA) spectra show that there is no charge separation (CS) for the homo-dimer. Nanosecond transient absorption spectroscopy indicate the formation of a triplet state with electron delocalization for the homo dimer, with a lifetime of 72.0 μs, while for the hetero dimer a triplet state with an intrinsic lifetime of 206.4 μs is observed. CS (11.6 ps) and slow CR-induced ISC (>1.5 ns) were observed for the hetero-dimer. Time-resolved electron paramagnetic resonance spectra give the zero-field splitting parameters (|D| = 1894 MHz and |E| = 111 MHz) and electron spin polarization patterns (e, e, e, a, a, a) for the triplet state of the hetero-dimer, inferring that the triplet state of the hetero-dimer is confined on the amino-substituted NI moiety.

Triplet-radical spin entanglement: potential of molecular materials for high-temperature quantum information processing

Recently, spin-bearing molecules have been experimentally demonstrated to have great potential as building blocks for quantum information processing due to their substantial advantages including tunability, portability, and scalability. Here, we propose a theoretical model based on the theory of open quantum systems for spin dynamics in a molecule containing one radical, which can interact with the triplet state arising from another part of the molecule owing to optical excitation and intersystem crossing. With the initial state being a classical mixture of a radical frac{1}{2}-spin, the exchange interaction between the radical and the triplet produces a spin coherent state, which could potentially be used for a qubit-qutrit quantum entangling gate. Our calculations for the time-resolved electron paramagnetic resonance spectra showed good qualitative agreement with the related experimental results for radical-bearing molecules at high temperature (~77 K, the boiling point of liquid nitrogen). This work therefore lays a solid theoretical cornerstone for optically driven quantum gate operations in radical-bearing molecular materials, aiming toward high-temperature quantum information processing.

Geometries and Terahertz Motions Driving Quintet Multiexcitons and Ultimate Triplet–Triplet Dissociations via the Intramolecular Singlet Fissions

Importance of vibronic effects has been highlighted for the singlet-fission (SF) that converts one high-energy singlet exciton into doubled triplet excitons, as strongly coupled multiexcitons. However, molecular mechanisms of spin conversion processes and ultimate decouplings in the multiexcitons are poorly understood. We have analyzed geometries and exchange couplings (singlet-quintet energy gaps: 6J) of the photoinduced multiexcitons in the pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as o-(Pc)2 and m-(Pc)2] by simulations of the time-resolved electron paramagnetic resonance spectra. We clarified that terahertz molecular conformation dynamics play roles on the spin conversion from the singlet strongly coupled multiexcitons 1(TT) to the quintet multiexcitons 5(TT) and on the intramolecular decouplings in the 6J to form spin correlated triplet pairs (T+T). The strongly coupled 5(TT) multiexcitons are revealed to possess entirely planar conformations stabilized by mutually delocalized spin distributions, while the intramolecular decoupled spin-correlated triplet pairs generated at 1 μs are also stabilized by distorted conformations resulting in two separately localized biradical characters.

Initial-state preparation effects in time-resolved electron paramagnetic resonance experiments.

We explain a recent experimental observation that the time-resolved electron paramagnetic resonance spectra of an organic molecule for optical excitation within a highly absorbing region of the molecule has similar intensities to the spectra for optical excitation in a nonabsorbing region [D. L. Meyer et al., J. Phys. Chem. Lett. 8, 1677 (2017)]. We demonstrate that this phenomenon is due to an initial-state preparation effect of photoexcitation that leads to similar initial populations of triplet states for both optical excitation regions. Due to the low intersystem crossing rates, the initial triplet populations are not perturbed on the time scale of the experiment, so they determine the relative intensities of the paramagnetic resonance spectra. The effect is surprising given the weak spin-orbit interactions of organic molecules. Such initial-state preparation effects are likely to occur in systems where the intersystem crossing time scales are long compared to the time scale of the experiment.

Conversion between triplet pair states is controlled by molecular coupling in pentadithiophene thin films.

In singlet fission (SF) the initially formed correlated triplet pair state, 1(TT), may evolve toward independent triplet excitons or higher spin states of the (TT) species. The latter result is often considered undesirable from a light harvesting perspective but may be attractive for quantum information sciences (QIS) applications, as the final exciton pair can be spin-entangled and magnetically active with relatively long room temperature decoherence times. In this study we use ultrafast transient absorption (TA) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy to monitor SF and triplet pair evolution in a series of alkyl silyl-functionalized pentadithiophene (PDT) thin films designed with systematically varying pairwise and long-range molecular interactions between PDT chromophores. The lifetime of the (TT) species varies from 40 ns to 1.5 μs, the latter of which is associated with extremely weak intermolecular coupling, sharp optical spectroscopic features, and complex TR-EPR spectra that are composed of a mixture of triplet and quintet-like features. On the other hand, more tightly coupled films produce broader transient optical spectra but simpler TR-EPR spectra consistent with significant population in 5(TT)0. These distinctions are rationalized through the role of exciton diffusion and predictions of TT state mixing with low exchange coupling J versus pure spin substate population with larger J. The connection between population evolution using electronic and spin spectroscopies enables assignments that provide a more detailed picture of triplet pair evolution than previously presented and provides critical guidance for designing molecular QIS systems based on light-induced spin coherence.

Competition between Singlet Fission and Spin-Orbit-Induced Intersystem Crossing in Anthanthrene and Anthanthrone Derivatives.

Singlet and triplet excited-state dynamics of anthanthrene and anthanthrone derivatives in solution are studied. Triisopropylsilyl- (TIPS) or H-terminated ethynyl groups are used to tune the singlet and triplet energies to enable their potential applications in singlet fission and triplet fusion processes. Time-resolved optical and electron paramagnetic resonance (EPR) spectroscopies are used to obtain a mechanistic understanding of triplet formation. The anthanthrene derivatives form triplet states efficiently at a rate (ca. 107 s-1 ) comparable to radiative singlet fluorescence processes with approximately 30 % triplet yields, despite their large S1 -T1 energy gap (>1 eV) and the lack of carbonyl groups. In contrast, anthanthrone has a higher triplet yield (50±10 %) with a faster intersystem crossing rate (2.7 108 s-1 ) because of the n-π* character of the S1 ←S0 transition. Analysis of time-resolved spin-polarized EPR spectra of these compounds reveals that the triplet states are primarily generated by the spin-orbit-induced intersystem crossing mechanism. However, at high concentrations, the EPR spectrum of the 4,6,10,14-tetrakis(TIPS-ethynyl)anthanthrene triplet state shows a significant contribution from a non-Boltzmann population of the ms =0 spin sublevel, which is characteristic of triplet formation by singlet fission.

Balance between Triplet States in Photoexcited Orthogonal BODIPY Dimers.

The intersystem crossing (ISC) and the triplet states in two representative BODIPY orthogonal dimers were studied with time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The electron spin polarization (ESP) of the triplet state of the dimers, accessed with spin-orbit charge-transfer ISC, is different from that of the monomer (spin-orbit coupling-induced ISC). The TREPR spectra show that the triplet state initially formed by charge recombination is localized on either of two subunits, with different preference and ESP patterns. On the basis of the relative orientation of the respective zero field splitting principal axes, the Tx state on one subunit and the Tz state on another subunit in the dimer are overpopulated. The balance between the two triplet states is confirmed by the temperature dependency of the population ratio. No quintet state was detected with TREPR down to 20 K; thus, the recently proposed singlet fission ISC mechanism is excluded.

(Invited) Geometry and Dynamics of Quintet Multiexciton Studied By Time-Resolved EPR

Singlet fission (SF) is a reaction that one singlet exciton separates to two triplet excitons. Theoretical limit of the photovoltaic efficiency (Shockley–Queisser limit) can be exceeded by the SF because of a potential of generating two charge-separations from the one high-energy photon. The SF reactions have been observed in organic molecules, tetracene (Tc), pentacene (Pn) and their derivatives. 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) and 5,12-bis(triisopropylsilylethynyl)tetracene (TIPS-Tc) are representative molecules and their SF efficiencies are high (up to 200 %). Clarification of the mechanisms and guidelines for designing the SF materials are highly required in the solid state. In the solid state, large chromophore couplings were reported for SF because Pn molecules are highly ordered to form the p-stacking and herringbone structures. Recent study however showed an efficient SF in an amorphous environment.[1] It is particularly important for the OPV applications that the two triplets are efficiently separated from the correlated TT pair without going back to the S1S0and S0S0 pairs. Since these pairs are initially the singlet states, efficient production of the quintet states (5TT pair) is essential for preventing the unwanted singlet deactivations. However, SF-born spin-conversion mechanism is not fully understood. The time-resolved electron paramagnetic resonance (TREPR) spectroscopy is a powerful tool to detect unpaired spins and to investigate their electronic structures, geometries and dynamics. Recently, TREPR spectra were reported on the SF-born quintet states as correlated TT pairs.[2] The spin polarized EPR spectra were interpreted by initial population of the quintet states at a sublevel of m S= 0, i.e. |5TT0> |Q0>. The origin of the Q0population is however unclear. When the SF occurs via the spin-conservation, the quintet generation is required to be accompanied by the singlet–quintet (SQ) mixing from the singlet multiexciton. The |1TT> –> |5TT0> (SQ0) mixing is allowed when the energy difference (|E S– E Q| between Q0and S is smaller than the size of the zero-field splitting (ZFS) interaction (D) of the individual triplet. Nevertheless, it is reported that |1TT> is highly stable with respect to |5TT0>; the exchange coupling 6J(ΔE SQ= E S– E Q) is stronger than D.[2a]It was hypothesized that an intermediate TT pair with weak Jwould play a role for the Q0populations. Since deactivations occur not from the 5TT but from the 1TT in the multiexciton, clarification of the 5TT formation mechanism is a key to the OPV applications. Reported time scales (typically in order of fs – ps) of the SF and the triplet-exciton migrations are however too short to track the SF-induced spin-conversion and dissociation those of which are hidden in the multiexciton dynamics. We herein prepared disordered aggregates of TIPS-Pn and of 2-phenyl-6,11-bis(triisopropylsilylethynyl)tetracene (TIPS-Ph-Tc). The disordered aggregates were produced by freezing diluted solutions in dichloromethane (CH2Cl2). This will inhibit the rapid exciton process, allowing us to unveil natures of the multiexcitons in inhomogeneous environments of the solid-states that generally possess both the crystalline and disordered domains. The spin-state dynamics and its related energetics were clarified by a combination of the TREPR and the picosecond fluorescence measurements. We have demonstrated sublevel selective populations in the excited quintet states (|5TT0>, |5TT-1> and |5TT-2>) of triplet-triplet (TT) pairs, manifesting that the SF-born quintet state is detected as strongly-coupled trapped TT-pairs generated by singlet-quintet interconversions during the triplet-exciton diffusion within hot geminate multiexcitons in the presence of certain exchange couplings. The present fundamental characteristics on the quintet generations demonstrate the importance of controlling both molecular p-stacking “hot spot” and disordered “trapping area” for rational designs of the efficient SF systems with preventing unwanted triplet fusion processes.[3] References [1] E. Kumarasamy, S. N. Sanders, M. J. Y. Tayebjee, A. Asadpoordarvish, T. J. H. Hele, E. G. Fuemmeler, A. B. Pun, L. M. Yablon, J. Z. Low, D. W. Paley, J. C. Dean, B. Choi, G. D. Scholes, M. L. Steigerwald, N. Ananth, D. R. McCamey, M. Y. Sfeir, L. M. Campos, J. Am. Chem. Soc. 2017, 139, 12488-12494. [2] a) L. R. Weiss, S. L. Bayliss, F. Kraffert, K. J. Thorley, J. E. Anthony, R. Bittl, R. H. Friend, A. Rao, N. C. Greenham, J. Behrends, Nat. Phys. 2017, 13, 176-181; b) M. J. Y. Tayebjee, S. N. Sanders, E. Kumarasamy, L. M. Campos, M. Y. Sfeir, D. R. McCamey, Nat. Phys. 2017, 13, 182-188. [3] H. Nagashima, S. Kawaoka, S. Akimoto, T. Tachikawa, Y. Matsui, H. Ikeda, Y. Kobori J. Phys. Chem. Lett. 2018, 9, 5855-5861.

Photoreaction of 9,10-Anthraquinone-2,6-disulfonic Acid and Ascorbic Acid Complexed at the Cationic Bilayer Interface and Consecutive Radical Dynamics

The antioxidant ability of substances such as vitamin C has attracted wide attention, and many antioxidant effects on our body are considered to act in the biomembrane interface region. However, the detailed dynamics of the radicals at the interface have not been deeply clarified. Here, we studied the reaction of photoexcited 9,10-anthraquinone-2,6-disulfonic acid dianion (AQDS2–) and ascorbic acid in the cationic vesicle solution of didodecyldimethyl ammonium bromide using time-resolved electron paramagnetic resonance (TR-EPR) and UV–vis absorption methods. The results were compared with those of micellar and aqueous systems. Analysis of UV–vis spectra revealed the formation of an intermolecular charge-transfer complex of ascorbic acid monoanion (AscH–) and AQDS2– at the cationic bilayer interface. The TR-EPR spectra revealed a hydrogen atom abstraction reaction by the excited triplet state of AQDS2– from AscH– to form an ascorbic acid monoanion radical (Asc•–) and an anthrasemiquinone dianion radical (A...

Interaction between the end groups and the main chain of conjugated polymers by time-resolved EPR and fluorescence spectroscopy

ABSTRACTInteraction between a zinc porphyrin (ZnPor) as the end-group and poly(9,9-di-n-octylfluorene-2,7-vinylene) (PFV) as the main chain in a porphyrin end-modified fluorescent conjugated polymer, ZnPFV, was studied by time-resolved electron paramagnetic resonance (EPR) and fluorescence spectroscopy. While fluorescence from the PFV part of ZnPFV showed a spectral profile almost identical to that of a PFV oligomer without end-modification, the emission spectrum of the ZnPor part exhibited a much broader profile compared to that of the reference zinc porphyrin monomer. Based on the analysis of lifetimes and quantum yields, it was found that radiative rate constant of the ZnPor part was enhanced by nearly three times. The observed unusual enhancement in the radiative rate constant was rationalised in terms of a partial π-conjugation between the end group and the main chain, as a result of co-planarisation in fluid solution. On the other hand, the time-resolved EPR spectrum of ZnPFV at 100 K basically showed a similar spectral pattern to that of the reference zinc porphyrin, but with significant differences in zero-field spitting parameters and initial population ratios. The π-system of the excited triplet state is deduced to deviate from D4h symmetry in the end zinc porphyrin groups. The obtained results show that interaction of the porphyrin end group with the main chain of the polymer significantly influences the excited singlet state properties of the porphyrin, while its triplet state properties were affected to a lesser extent.

Analysis of time-resolved EPR spectra observed in the photolysis of disulphonated anthraquinones included in cyclodextrins – evidence of the S-T_ mixing of radical pair mechanism

ABSTRACTCyclodextrins (CDs) are well known hosts of inclusion compounds, and they can be used for molecular identification and separation. Several studies have already been reported from the point of view of photchemistry and spin-chemistry. These reports showed that the inside environment of CDs such as their bore-size works for their specific properties. In this report, the time-resolved EPR spectra observed in the photolysis of aqueous solution of disulphonated anthraquinones included in β-CD and γ-CD are carefully analysed. The spectral simulation employing Radical Pair Mechanism (RPM) of Chemically Induced Dynamic Electron Polarisation is carried out. Finally, it is concluded that the peculiar hyperfine-dependence observed in these particular systems is exclusively caused by the mixing RPM, despite the radical pairs (RPs) having small hyperfine coupling (HFC) constants (less than 3.4 mT). Another finding is the quenching of the spin polarisation of the RP conveyed from the excited triplet state of anthraquinones before mixing. This is explained as arising from slow relative diffusional motion of the RP members when included in the interior of the CDs. General expression of the and S-T+ mixing of multiple nuclei (I = 1/2) with different conditions such as the spin multiplicity of the precursor molecule and the sign of the HFC constants, is also suggested.

Charge Separation and Triplet Exciton Formation Pathways in Small Molecule Solar Cells as Studied by Time-resolved EPR Spectroscopy.

Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh2)2, DTS(F2BTTh2)2, DTS(PTTh2)2, DTG(FBTTh2)2 and DTG(F2BTTh2)2) with the fullerene derivative PC61BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh2)2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh2)2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. The higher BET triplet exciton population in the DTS(PTTh2)2 blend is in accordance with the slower charge separation dynamics observed in this blend.

(Invited) Time-Resolved EPR Study on Charge Dynamics of Electron-Hole Pairs in Lead Iodide Perovskite Thin Film

For developments of the light-energy conversion systems, it is essential to achieve efficient long-range charge-separations (CS). Light-induced electrons and holes generated by the electron-transfer (ET) reactions are thus required to escape from the electrostatic binding at the initial stage. In this respect, the lead iodide perovskite solar cells have attracted great attentions since they exhibit quite efficient energy conversion performances. This solar cell adopts the hybrid metal iodide perovskites as a photoactive layer with the general formulation of ABX3(CH3NH3PbI3), where A is the methylammonium (CH3NH3 +) cation, B is Pb2+ and M is I-. In the devise structure, this photoactive layer is sandwiched by the compact titanium oxide (TiO2) layer and by the hole-transporting layer to efficiently collect the photocarriers generated by the light irradiations of the perovskite, giving rise to ~20% power conversion efficiencies (PCE). Recent studies have clearly shown that the carrier mobilities of the electrons and the holes are significantly high in the CH3NH3PbI3 layer, resulting in micrometer ranges of the charge-diffusion lengths.[1] These efficient charge diffusions play a crucial role on the prevention of the unwanted geminate charge-recombination in the electron-hole pairs. However, little is known concerning the electronic interactions of the electron-hole pairs initially generated in the CH3NH3PbI3 layer. At lower temperature (T < 160 K), it is reported that the electron and the hole generated by the light irradiation exhibit a Wannier exciton character with a biding energy of ~30 meV.[2] Thus, low temperature investigations of the electronic characters in the electron-hole pairs are very important to understand the mechanisms of the efficient charge-conductions initiated by the primary electron-hole pairs in the CH3NH3PbI3 layer. Another importance is to understand electronic characters of the trapped-charges generated by the light irradiations of the photoactive layers. It is recently reported that the charge careers are trapped at the surface area of the crystalline CH3NH3PbI3 layer.[3] However, electronic orbital characteristics of the trapped charges are not well known in the devices. Time-resolved electron paramagnetic resonance (TREPR) and pulsed EPR methods have been powerful to obtain the electron spin-spin interactions including the spin-spin exchange coupling of the photoinduced CS states and thus have been utilized to characterize the interspin distances and the electronic couplings of the transient CS states.[4] Additionally, TREPR can detect the electron spin polarization (ESP) as an effect of the initial photochemical events. This phenomenon is referred to as the chemically induced dynamic electron polarization (CIDEP). According to the radical pair mechanism (RPM) CIDEP, the ESP is generated as a result of the coherent singlet-triplet interconversion in the presence of the exchange coupling (J) of the primary radical-pair states, producing the enhanced microwave absorption and the microwave emission in the separated radicals during the diffusive separations of the individual radicals in the radical-pair.[5] Since the ESP detected as the microwave absorption (A) or the emission (E) is thus sensitive to J, one is able to access the electronic interactions in the primary radical-pairs.[6] To unveil the above fundamental electronic characteristics for the light-induced charge conductions, we have observed low temperature TREPR spectra for the three layer thin films (Spiro-OMeTAD/CH3NH3PbI3/TiO2 on a glass substrate) composed of the organic hole transport layer (Spiro-OMeTAD), the CH3NH3PbI3 layer and the mesoporous TiO2 fabricated by spin coating method on the mesoporous TiO2 films. From the EPR spectra, we have characterized the spin density distributions of the trapped holes of the CH3NH3PbI3 layer at 110 K from the anisotropic hyperfine structures originating from the iodine atoms (I). Furthermore, from the ESP generated in the trapped hole, we estimated the J coupling in the primary electron-hole pairs which will give us an insight on the electronic coupling of the Wannier excitons. 1.﷒ Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Science 2013, 342, 344. 2.﷒ Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.; Kanemitsu, Y. J. Am. Chem. Soc. 2014, 136, 11610-11613. 3.﷒ Tachikawa, T.; Karimata, I.; Kobori, Y. J. Phys. Chem. Lett. 2015, 6, 3195-3201. 4. Kobori, Y.; Miura, T. J. Phys. Chem. Lett. 2015, 6, 113-123. 5.﷒ Kobori, Y.; Yago, T.; Akiyama, K.; Tero-Kubota, S.; Sato, H.; Hirata, F.; Norris, J. R. J. Phys. Chem. B 2004, 108, 10226-10240. 6. Kobori, Y.; Yamauchi, S.; Akiyama, K.; Tero-Kubota, S.; Imahori, H.; Fukuzumi, S.; Norris, J. R. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 10017-10022.

Observation of splitting of EPR spectral lines without any concomitant splitting in energy levels.

Splitting of spectral lines is generally associated with corresponding splitting of appropriate energy levels. However, here we report our observation of the splittings of hyperfine lines in the time-resolved EPR spectrum of a stable free radical that participates in the quenching of an excited molecule. The observed splitting does not arise from any splitting of the energy levels of the radical, nor due to Torrey oscillations but is a culmination of a detailed interplay of photophysical and magnetic resonance dynamics of the quenching process. In particular, sequential quenching of an excited singlet and triplet states by the free radical, generation of opposite electron spin polarization, and time-dependent EPR line width evolving according to the Bloch equations contribute to the appearance of unusual lineshapes of the observed EPR spectrum. This effect is sufficiently general and should always be observable in the time-resolved EPR experiments on free radicals involved in photophysical quenching of excited molecules. We also point out that this effect cannot be seen in Fourier transform EPR spectroscopy.

Radical-triplet pair interactions as probes of long-range polymer motion in solution.

Radical-triplet pair interactions are used to investigate the dynamics of acrylic polymers in dilute solution. Methyl methacrylate was randomly copolymerized with a small amount of an amine-containing monomer to create the polymers. The amine subunits were then oxidized to nitroxide moieties (stable free radicals). Using time-resolved electron paramagnetic resonance (TREPR) spectroscopy on the sub-microsecond time scale, competition is observed between two deactivation processes of the ester side chain photoexcited triplet state: (1) Norrish I α-cleavage, leading to a 21-line main chain polymeric free radical TREPR spectrum with emissive triplet mechanism spin polarization, and (2) spin polarization transfer or quenching by a nearby stable nitroxide radical, which gives a 3-line spectrum exhibiting emissive radical-triplet pair spin polarization. The main chain polymer radical signal dominates the TREPR spectrum if the probability of radical-triplet pair encounters is low. These competing reactions show a strong dependence on nitroxide incorporation (mol %), temperature, solvent, and acrylic polymer ester side chain structure. A comparison of steady state EPR and TREPR signals from inter- vs intramolecular processes clearly demonstrates the influence of polymer chain dynamics on the observed phenomena.