Separate Triplet Research Articles

Enhancement on the selective flotation separation of carbon coated LiFePO4 and graphite electrode materials

With the rapid development of lithium-ion batteries, the recycling of spent lithium-ion batteries will be a big challenge in the future, among which the recycling of spent positive and negative electrode materials powder is a difficult point. Flotation has been able to separate triplet materials from graphite to some extent, but there are still many problems to be solved in the separation of LiFePO4 from graphite. In this study, a novel flotation reagent regime was proposed to enhance the flotation separation performance of LiFePO4 and graphite electrode materials by carbonaceous surface selective inhibition, which were achieved by H acid as a depressor. The flotation results showed that under the flotation conditions of pH value 9.3 ± 0.1, 10.0 mg/L n-dodecane, 7.5 mg/L MIBC, and 250 mg/L H acid, the recovery of LiFePO4 and graphite were 11.95 % and 94.28 %, respectively. The adsorption kinetic model showed that the adsorption speed of LiFePO4 electrode material is higher than graphite, the adsorption isothermal model showed that the adsorption capacity of LiFePO4 electrode material is significantly higher than graphite. The results of X-ray photoelectron spectroscopy showed that the interaction between H acid and LiFePO4 or graphite electrode material is mainly π-π interaction. Raman spectroscopy analysis showed that the carbon disorder degree and skeleton form of the graphite and LiFePO4 are significantly different, which makes H acid more easily adsorbed on the surface of LiFePO4. This study provided a new method for the efficient separation and recovery of LiFePO4 and graphite electrode materials from spent lithium iron phosphate batteries.

Modulating the intersystem crossing mechanism of anthracene carboxyimide-based photosensitizers via structural adjustments and application as a potent photodynamic therapeutic reagent.

Herein, a series of compact anthracene carboxyimide (ACI) based donor-acceptor dyads were prepared by substituting bulky aryl moieties with various electron-donating ability to study the triplet-excited state properties. The ISC mechanism and triplet yield of the dyads were successfully tuned via structural manipulation. Efficient ISC (ΦΔ ≈ 99%) and long-lived triplet state (τT ≈ 122 μs) was observed for the orthogonal anthracene-labeled ACI derivative compared to the Ph-ACI and NP-ACI dyads, which showed fast triplet state decay (τT ≈ 7.7 μs). Femtosecond transient absorption study demonstrated the ultrafast charge separation (CS) and efficient charge recombination (CR) in the orthogonal dyads and ISC occurring via spin-orbit charge transfer (SOCT) mechanism (AN-ACI: τCS = 355 fs, τCR = 2.41 ns; PY-ACI: τCS = 321 fs, τCR = 1.61 ns), while in Ph-ACI and NP-ACI dyads triplet populate following the normal ISC channel (nπ* → ππ* transition), no CS was observed. We found that the attachment of suitable aryl donor moiety (AN- or PY-) to the ACI core can ensure the insertion of the intermediate triplet state, resulting in a small energy gap among charge separated state (CSS) and triplet state, which leads to efficient ISC in these derivatives. The SOCT-ISC-based AN-ACI dyad was confirmed to be a potent photodynamic therapeutic reagent; an ultra-low IC50 value (0.27 nM) that was nearly 214 times lower than that of the commercial Rose Bengal photosensitizer (57.8 nM) was observed.

(Invited) Vibronic Effect of Donor-Acceptor Interaction Determines Fate of Mutiexciton Spins Generated By Singlet Fission

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit with power conversion efficiencies (PCE) ~33 % of single junction organic solar cells (OSC) because two separated triplet excitons (T+T) can be produced from one excited singlet state (S1S0) sharing its excitation energy with a neighboring ground-state chromophore. The intramolecular SF (iSF) dynamics have widely been studied using the transient absorption spectroscopic methods together with theoretical modeling taking into account the vibronic effects in the ultrafast regimes. The correlated intermolecular triplet pair 1(TT) generated with the singlet character is known to be converted to the quintet (Q) state as 5(TT). These TT pairs separate into individual triplets as the T+T state. In the previous study, we reported the iSF dynamics and subsequent separations to T+T in the pentacene dimers (PcD) bridged by a phenylene at ortho and meta positions by a combination of the ultrafast transient absorption spectroscopy and the time-resolved electron paramagnetic resonance (TREPR) method.1 Before the spin-conversions to the 5(TT) state, the initially populated 1(TT) is strongly coupled in the four unpaired spins of the two triplet excitons, i.e. the entangled spin-state by large spin-spin exchange coupling (J) with the large orbital overlap produced at femtoseconds regions. The modulations of the J-couplings are thought to be essential for the 1(TT) -> 5(TT) conversions. In an iSF system, we interpreted that the quintet ESPs were caused by the anisotropic spin conversions due to singlet-quintet spin-relaxations to the five sublevels at m S = +2, +1, 0, -1 and -2 in accordance with the anisotropy of the ZFS coupling which is highly dependent on the relative orientation of one triplet exciton (TB) with respect to the other triplet (TA) in the TATB multiexciton.2 Internal fluctuations in the J-coupling were concluded to be crucial on the quintet generations. Moreover, we reported the treatment of the EPR transitions in the weakly coupled T+T states in the framework of the spin-correlated triplet pair (SCTP) for the very weak exchange coupling including the superpositions of the nine basis functions of 5(TT) - 3(TT) - 1(TT), as treated in the spin-correlated radical pair model. 3,4 The above pioneering interpretations of the ESP mechanisms may pave a new avenue to clarify how the molecular conformations and their motions drive the spin-conversions and the ultimate T+T generations, both of which are essential but unknown for the photon-to-energy conversion process. In particular, because the J-coupling is anticipated to be sensitive to the TATB conformations, it is expected that the specific molecular motions or phonon modes may play roles for the ultimate T+T de-coupling, as the vibronic effects. So far, details of the TATB geometries, the vibration modes and frequencies, and the electronic couplings are unresolved in the iSF systems for generating the quintet multiexcitons and the subsequent T+T dissociations.We have characterized the molecular geometries, conformation dynamics and J-couplings of the intramolecular multiexcitons after the photoinduced iSF with the proposal of the density matrix analysis of ESP observed by the TREPR method. It is concluded that the J-coupling is determined by the donor-acceptor electronic coupling in the multiexciton and that terahertz TA-TB conformation dynamics plays a significant role for generating the quintet multiexcitons and for the individual T+T states caused by the J-fluctuation in the covalently linked systems.4 Sakai, H.; Inaya, R.; Nagashima, H.; Nakamura, S.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T., J. Phys. Chem. Lett. 2018, 9, 3354-3360.Matsui, Y.; Kawaoka, S.; Nagashima, H.; Nakagawa, T.; Okamura, N.; Ogaki, T.; Ohta, E.; Akimoto, S.; Sato-Tomita, A.; Yagi, S., et al., J. Phys. Chem. C 2019, 123, 18813-18823.Matsuda, S.; Oyama, S.; Kobori, Y., Chem. Sci. 2020, 11, 2934-2942.Kobori, Y.; Fuki, M.; Nakamura,; S. Hasobe, T. J. Phys. Chem. B 2020, 124, 9411–9419.

Up- and Down-Converting Photons in Molecular Carbon Materials

The Shockley-Queisser limit places an upper bound on solar conversion efficiency for a single p-n junction solar cell at slightly more than 30%. To surpass this limit, multi-exciton generation is being explored in inorganic semiconductors, while singlet fission (SF) is being investigated in arrays of conjugated organic molecules. In an optimal SF process, the lowest singlet excited state of one molecule (S1) that is positioned next to a second molecule in its ground state (S0) is down-converted into two triplet excited states (T1) each residing on one of the two adjacent molecules. The two triplet states initially form a correlated pair state 1(T1T1), which then evolves into two separated triplet states (T1 + T1). As such, the energetic requirement for SF is E(S1) ≥ 2 ´ E(T1).We have set our focus in recent years on intramolecular SF in molecular carbon materials and their studies in solution rather than on intermolecular SF investigations in crystalline films.Implicit in intramolecular SF is a resonant, direct excitation of the SF material. In dimers of molecular carbon materials linked by a myriad of molecular spacers, SF takes place with quantum yields of up to 200%. In addition, all key intermediates in the SF process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons, have been identified. This approach is, however, limited to the part of the solar spectrum, where, for example, the dimers of carbon materials feature a significant absorption cross-section. To employ the remaining part of the solar spectrum necessitates non-resonant, indirect excitation of the SF materials via either up- or down-conversion. For example, the up-conversion approach is realized with singlet excited states , which are accessed by two-photon absorptions (TPA). TPA is then followed in the second step of the sequence by an intramolecular SF – similar to what is seen upon resonant, direct excitation. Quite different is the down-conversion approach, which is based on an intramolecular Förster resonance energy transfer (FRET) and thereby the (photo)activation of the SF material. FRET requires the use of a complementary absorbing chromophore and enables funneling its excited state energy unidirectionally to the SF performing pentacene dimer. Again, SF completes the reaction sequence.

Role of nanosize and defect trapping upon singlet fission yield and singlet fission dynamics of 1,6-Diphenyl-1,3,5-hexatriene nanoaggregates

Nanoaggregate formation of 1,6-Diphenyl-1,3,5-hexatriene (DPHT) has been observed to introduce energy perturbation which results nearly isoergic singlet fission (SF), as compared to the endoergic SF in the crystalline forms of DPHT. It results prompt triplet pair production with a time constant ∼11.4 ps and subsequently it goes for production of separated triplet excitons with time constant of ∼152 ps. The separated triplet exciton yield is estimated to be 115 ± 10 %. The reverse process of singlet exciton production i.e. triplet fusion or triplet triplet annihilation process is also energetically feasible in this material due to the isoergic nature of SF process. It causes generation of back transferred singlet excitons which can again participate in SF process or it may decay through defect trapping. The defect trapping of photogenerated as well as re-generated singlet excitons plays detrimental role in the overall triplet yield as compared to that of the crystalline forms of same molecule.

Singlet fission in nanoaggregate of bis(phenylethynyl) derivative of benzene (BPEB): High energy triplet exciton generation with >100 % yield

1,4-bis(phenylethynyl)benzene (BPEB) is observed to show a prominent ∼0.17 eV red shift of absorption band due to monomer-aggregate conversion. Along with the absorption band shift it also causes nearly one order reduction in both emission yield and singlet lifetime. The ultrafast transient absorption measurements show annihilation free decay of singlet excitons with ∼40 ps lifetime for this nanoaggregate. Concomitant growth of triplet excitonic absorption and ground state bleach signal with similar time constant ensures presence of singlet fission (SF) process in the present system. Flash photolysis study indicates separated triplet exciton yield of ∼139 % and its decay with ∼4.4 ms lifetime. Observation of triplet-triplet annihilation (TTA) and resultant delayed fluorescence process in this NA also confirms the near-isoergic or endoergic nature of SF process. It also indicates the presence of high energy triplet excitonic state which can be utilized to boost the silicon solar cell efficiency or sensitize other materials having application in photodynamic therapy or near-IR phosphorescence based bio-imaging.

Dynamic emissive signatures of intramolecular singlet fission during equilibration to steady state revealed from stochastic kinetic simulations.

We investigate the ability of dynamic fluorescence probes to accurately track populations of multi-excitonic states in molecular dyads based on conjugated acenes capable of intramolecular singlet fission (iSF). Stochastic simulations of reported photophysical models from time-resolved spectroscopic studies of iSF dyads based on large acenes (e.g., tetracene and pentacene) are used to extrapolate population and fluorescence yield dynamics. The approach entails the use of repetitive rectangular-shaped excitation waveforms as a stimulus, with durations comparable to triplet lifetimes. We observe unique dynamics signatures that can be directly related to relaxation of multi-exciton states involved over the entire effective time of singlet fission in the presence and absence of an excitation light stimulus. In particular, time-dependent fluorescence yields display an abrupt decay followed by slower rise dynamics appearing as a prominent "dip" feature in responses. The initial fast decrease in the fluorescence yield arises from the formation of triplet pairs and separated triplets that do not produce emission resembling a complete ground state bleach effect. However, relaxation of one separated triplet allows the system to absorb, and in some cases, this increases the fluorescence yield, causing rise dynamics in the emissive response. Our approach also permits extrapolation of all multi-exciton state population dynamics up to steady state conditions in addition to the ability to explore consequences of alternative relaxation channels. The results demonstrate that it is possible to resolve unique signatures of singlet fission events from dynamic fluorescence studies, which can augment detection capabilities and extend sensitivity limits and accessible time scales.

Advances in Singlet Fission Chromophore Design Enabled by Vibrational Spectroscopies

Singlet fission leads to the formation of two separate triplet T1 excitons from an initial singlet S1 exciton through 1(TT) and 1(T...T), multiexcitonic intermediates that retain singlet character....

Giant magnetic field effects in donor-acceptor triads: On the charge separation and recombination dynamics in triarylamine-naphthalenediimide triads with bis-diyprrinato-palladium(II), porphodimethenato-palladium(II), and palladium(II)-porphyrin photosensitizers.

A series of triads consisting of a triarylamine donor, a naphthalenediimide acceptor, and a palladium photosensitizer bridge was investigated for the photoinduced electron transfer processes and the spin chemistry involved. In this series, the ligand in the palladium photosensitizer was varied from bis-dipyrrinato to porphodimethenato and to a porphyrin. With the porphyrin photosensitizer, no charge separated state could be reached. This is caused by the direct relaxation of the excited photosensitizer to the ground state by intersystem crossing. The bis-dipyrrinato-palladium photosensitizer gave only a little yield (7%) of the charge separated state, which is due to the population of a metal centered triplet state and a concomitant geometrical rearrangement to a disphenoidal coordination sphere. This state relaxes rapidly to the ground state. In contrast, in the porphodimethenato-palladium triads, a long lived (μs to ms) charge separated state could be generated in high quantum yields (66%-74%) because, here, the population of a triplet metal centered state is inhibited by geometrical constraints. The magnetic field dependent transient absorption measurement of one of the porphodimethenato triads revealed a giant magnetic field effect by a factor of 26 on the signal amplitude of the charge separated state. This is the consequence of a magnetic field dependent triplet-singlet interconversion that inhibits the fast decay of the charge separated triplet state through the singlet recombination channel. A systematic comparative analysis of the spin-dependent kinetics in terms of three classical and one fully quantum theoretical methods is provided, shedding light on the pros and cons of each of them.

Up- and Down-Converting Photons in Molecular Carbon Materials

The Shockley-Queisser limit places an upper bound on solar conversion efficiency for a single p-n junction solar cell at slightly more than 30%. To surpass this limit, multi-exciton generation is being explored in inorganic semiconductors, while singlet fission (SF) is being investigated in arrays of conjugated organic molecules. In an optimal SF process, the lowest singlet excited state of one molecule (S1) that is positioned next to a second molecule in its ground state (S0) is down-converted into two triplet excited states (T1) each residing on one of the two adjacent molecules. The two triplet states initially form a correlated pair state 1(T1T1), which then evolves into two separated triplet states (T1 + T1). As such, the energetic requirement for SF is E(S1) ≥ 2 ´ E(T1).We have set our focus in recent years on intramolecular SF in molecular carbon materials and their studies in solution rather than on intermolecular SF investigations in crystalline films.Implicit in intramolecular SF is a resonant, direct excitation of the SF material. In dimers of molecular carbon materials linked by a myriad of molecular spacers, SF takes place with quantum yields of up to 200%. In addition, all key intermediates in the SF process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons, have been identified. This approach is, however, limited to the part of the solar spectrum, where, for example, the dimers of carbon materials feature a significant absorption cross-section. To employ the remaining part of the solar spectrum necessitates non-resonant, indirect excitation of the SF materials via either up- or down-conversion. For example, the up-conversion approach is realized with singlet excited states , which are accessed by two-photon absorptions (TPA). TPA is then followed in the second step of the sequence by an intramolecular SF – similar to what is seen upon resonant, direct excitation. Quite different is the down-conversion approach, which is based on an intramolecular Förster resonance energy transfer (FRET) and thereby the (photo)activation of the SF material. FRET requires the use of a complementary absorbing chromophore and enables funneling its excited state energy unidirectionally to the SF performing pentacene dimer. Again, SF completes the reaction sequence.

(Invited) Electron Spin Polarization Generated by Transport of Spin-Entanglements in Singlet Fissions

Applications of singlet fission (SF)1 are expected to exceed the Shockley–Queisser theoretical limit because two separated triplet excitons (T+T) can be produced from one excited singlet state (S1S0) sharing its excitation energy with a neighboring ground-state chromophore. Subsequent two charge-separations (CS) at the D:A domain interfaces after individual triplet-exciton diffusions may cause the boost of the photocurrent density of the OSCs. Applications of the SF-induced triplet generations are also promising for boosting the organic light emitting diode (OLED) efficiency. Several studies clarified the initial multiexciton (TT) generation mechanisms from the S1S0 states. More importantly, elucidation of TT T+T dissociation mechanism is highly desired. The dissociation time constants ranging from sub-picosecond to microsecond regions are reported. However little is known how the T+T dissociation or the decoupling2 occurs from the strongly coupled TT states.The strongly coupled singlet TT states are known to be initially generated from the S1S0 states. Generations of the quintet state in the triplet–triplet pair3-7 have been reported and are thought to be essential for preventing the loss of the SF-born multiexciton through the singlet channels. Using the time-resolved electron paramagnetic resonance (TREPR) method for TIPS-tetracene thin films, Weiss et al. 8 characterized the SF-induced strongly-coupled quintet of 5(TT). The quintet electron spin polarization (ESP) was detected as the microwave absorption (A) and emission (E) in 5(TT) for frozen aggregates of the SF-materials. The ESPs were interpreted using the sublevel-selective quintet conversions by the zero-field splitting (ZFS) interaction at the singlet-quintet level-crossings in the presence of the negative exchange coupling (J) during triplet exciton-diffusion and subsequent re-encounter in the highly disordered region, causing modulation of the J-coupling to result in spontaneous 5(TT) generations.4 Interestingly, transport of spin-entangled multiexcitons by the dissociative exciton diffusion have been observed using an ultrafast transient absorption imaging of pentacene crystals and by the magnetic field effect on the fluorescence time-profiles. Although such transportation of spin-entanglements is key to elucidate the dissociation mechanism and to apply in quantum information science, no direct evidence for the transportation of the spin-entanglements has been examined in the separated T+T states with the singlet and quintet characters.The present study thus focuses on how the spin-entanglements develop and affect the direct TREPR detection of the T+T state. The transport of the quantum entanglements was demonstrated in thin films. When the triplet excitons are separated from each other to result in a negligible magnitude of the exchange coupling (J 0), spin correlations in the triplet-triplet pair may occur to induce diagonalized nine spin-states as the quantum superpositions of the singlet-triplet-quintet character through the spin-spin dipolar couplings of the individual triplet species. This is quite similar to the spin-correlated radical pair (SCRP) model9, 10 in the photoinduced long-range charge-separation system generated by singlet or triplet precursors, causing the superpositions of the singlet-triplet characters when the J-coupling is small in the radical pairs. We herein demonstrate that the transportations of the SF-induced 5(TT) and 1(TT) characters can be distinguished on the T+T dissociations using the ESPs of correlated T-T states by applying TREPR spectroscopy to thin films of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn).REFERENCES 1. M. B. Smith and J. Michl, Chem. Rev., 2010, 110, 6891-6936.2. K. Miyata, F. S. Conrad-Burton, F. L. Geyer and X. Y. Zhu, Chem. Rev., 2019, 119, 4261-4292.3. D. Lubert-Perquel, E. Salvadori, M. Dyson, P. N. Stavrinou, R. Montis, H. Nagashima, Y. Kobori, S. Heutz and C. W. M. Kay, Nat. Commun., 2018, 9, 4222.4. H. Nagashima, S. Kawaoka, S. Akimoto, T. Tachikawa, Y. Matsui, H. Ikeda and Y. Kobori, J. Phys. Chem. Lett., 2018, 9, 5855-5861.5. Y. Matsui, S. Kawaoka, H. Nagashima, T. Nakagawa, N. Okamura, T. Ogaki, E. Ohta, S. Akimoto, A. Sato-Tomita, S. Yagi, Y. Kobori and H. Ikeda, J. Phys. Chem. C, 2019, 123, 18813-18823.6. T. Saegusa, H. Sakai, H. Nagashima, Y. Kobori, N. V. Tkachenko and T. Hasobe, J. Am. Chem. Soc., 2019, 141, 14720-14727.7. H. Sakai, R. Inaya, H. Nagashima, S. Nakamura, Y. Kobori, N. V. Tkachenko and T. Hasobe, J. Phys. Chem. Lett., 2018, 9, 3354-3360.8. L. R. Weiss, S. L. Bayliss, F. Kraffert, K. J. Thorley, J. E. Anthony, R. Bittl, R. H. Friend, A. Rao, N. C. Greenham and J. Behrends, Nat. Phys., 2017, 13, 176-181.9. Y. Kobori, T. Ako, S. Oyama, T. Tachikawa and K. Marumoto, J. Phys. Chem. C, 2019, 123, 13472-13481.10. J. H. Olshansky, M. D. Krzyaniak, R. M. Young and M. R. Wasielewski, J. Am. Chem. Soc., 2019, 141, 2152-2160.

Controlling the Efficiency of Singlet Fission in TIPS-Pentacene/Polymer Composite Nanoparticles

Singlet fission (SF) is a process with the potential to increase the efficiency of solar cells by reducing losses from thermal relaxation of hot carriers. By generating two triplet excitons from one singlet exciton, the process effectively splits the energy of high-energy photons into two, providing a means to circumvent the Shockley–Queisser limit. Although the applications of SF are promising, questions remain about the mechanistic details and practicalities of implementation in photovoltaic devices that must be resolved to exploit its full potential. In this study, we present a way to investigate the effect of average intermolecular distance on SF by embedding 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) in an amorphous polymer matrix in the form of aqueous nanoparticle dispersions. By controlling the mass ratio of TIPS-Pn to the host polymer, we systematically tune the concentration of TIPS-Pn molecules in a nanoparticle and in turn, the average intermolecular separation, leading to a range of SF quantum yields. We study this system using both steady-state and ultrafast time-resolved spectroscopic techniques and fit the results to a kinetic model to decipher the observed behavior. The quantum yield of SF is shown to decrease with average intermolecular separation, which is explained by diffusion-limited SF and an increase in loss pathways through isolated sites. Additionally, we identify an intermediate species in the SF process and show that a significant proportion of this species decays nonradiatively without dissociating to form separated triplets, revealing a major loss pathway that has important implications for future research and applications.

Transient Magnetic Field Effect of Photoexcitations in Donor-Acceptor Organic Semiconductors.

We report transient photoinduced absorption (t-PA) and magnetic field ( B)-dependent t-PA (t-MPA( B)) in a pristine low band gap π-conjugated copolymer composed of donor and acceptor moieties, namely, the poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thi-eno[3,4- b]thiophenediyl]]) (or PTB7) used in photovoltaic applications. Unlike traditional π-conjugated polymers in which the primary photoexcitations are singlet excitons (SE), in pristine PTB7 we find at short times coexistence of two primary photoexcitation species, namely, SE and triplet-triplet (TT) pair. Both species are photogenerated directly from the ground state and are spin-correlated. Although the TT pair decomposes into two separate triplet excitons (TEs) in ∼100 ps, the separated TE spins are still entangled up to ∼6 μs. At longer times, the t-MPA( B) response of the surviving TEs shows transient narrowing effect, which is attributed to a distribution of the TE size.

Triplet Energy Transfer Governs the Dissociation of the Correlated Triplet Pair in Exothermic Singlet Fission.

Singlet fission is a spin-allowed process of exciton multiplication that has the potential to enhance the efficiency of photovoltaic devices. The majority of studies to date have emphasized understanding the first step of singlet fission, where the correlated triplet pair is produced. Here, we examine separation of correlated triplet pairs. We conducted temperature-dependent transient absorption on 6,3-bis(tri isopropylsilylethynyl)pentacene (TIPS-Pn) films, where singlet fission is exothermic. We evaluated time constants to show that their temperature dependence is inconsistent with an exclusively thermally activated process. Instead, we found that the trends can be modeled by a triplet-triplet energy transfer. The fitted reorganization energy and electronic coupling agree closely with values calculated using density matrix renormalization group quantum-chemical theory. We conclude that dissociation of the correlated triplet pair to separated (but spin-entangled) triplet excitons in TIPS-Pn occurs by triplet-triplet energy transfer with a hopping time constant of approximately 3.5 ps at room temperature.

Simple Rule To Predict Boundedness of Multiexciton States in Covalently Linked Singlet-Fission Dimers

Because of the potential for increasing solar cell efficiencies, significant effort has been spent understanding the mechanism of singlet fission. We provide a simple connectivity rule to predict whether the through-bond coupling will be stabilizing or destabilizing for the 1(TT) state in covalently linked singlet-fission chromophores. By drawing an analogy between the chemical system and a simple spin-lattice, one is able to determine the ordering of the multiexciton spin state via a generalized usage of Ovchinnikov's rule. This allows one to predict (without any computation) whether the 1(TT) multiexciton state will be bound or unbound with respect to the separated triplets in covalently linked singlet-fission dimers. To test our hypothesis, we have performed ab initio calculations on a systematic series of covalently linked singlet-fission dimers. Numerical examples are given, and the limitations of the proposed theory are explored.

Intrafetal laser ablation for embryo reduction from dichorionic triplets to dichorionic twins.

To report the outcome of dichorionic (DC) triplet pregnancies reduced to DC twins by laser ablation of the pelvic vessels of one of the monochorionic (MC) twins. Intrafetal laser embryo reduction (ER) from DC triplets to DC twins was carried out in 61 pregnancies at 11 + 0 to 14 + 3 weeks' gestation. Pregnancy outcome was examined. Intrafetal laser was successfully carried out in all cases, but ultrasound examination within 2 weeks of the procedure demonstrated that the MC cotwin had died in 28 (45.9%) cases and was alive in the other 33 (54.1%). In the DC group, there was one miscarriage at 23 weeks, one neonatal death after delivery at 26 weeks and in the other 31 cases there were two live births at a median gestational age of 35.3 (range, 30.4-38.4) weeks. In the 28 cases in which both MC fetuses died, there was one miscarriage at 16 weeks and in the other 27 cases the separate triplet was liveborn at a median gestation of 38.2 (range, 32.2-42.1) weeks. The overall rate of miscarriage was 3.3% (2/61) and that of preterm birth (PTB) at < 33 weeks was 6.8% (4/59). In the management of DC triplet pregnancies, ER to DC twins by intrafetal laser ablation is associated with lower rates of miscarriage or early PTB, compared with expectant management or ER by fetal intracardiac injection of potassium chloride. However, about half of the pregnancies result in the birth of one rather than two babies. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

Singlet Fission in a Covalently Linked Cofacial Alkynyltetracene Dimer.

Singlet fission is a process in which a singlet exciton converts into two triplet excitons. To investigate this phenomenon, we synthesized two covalently linked 5-ethynyl-tetracene (ET) dimers with differing degrees of intertetracene overlap: BET-X, with large, cofacial overlap of tetracene π-orbitals, and BET-B, with twisted arrangement between tetracenes exhibits less overlap between the tetracene π-orbitals. The two compounds were crystallographically characterized and studied by absorption and emission spectroscopy in solution, in PMMA and neat thin films. The results show that singlet fission occurs within 1 ps in an amorphous thin film of BET-B with high efficiency (triplet yield: 154%). In solution and the PMMA matrix the S1 of BET-B relaxes to a correlated triplet pair (1)(T1T1) on a time scale of 2 ps, which decays to the ground state without forming separated triplets, suggesting that triplet energy transfer from (1)(T1T1) to a nearby chromophore is essential for producing free triplets. In support of this hypothesis, selective excitation of BET-B doped into a thin film of diphenyltetracene (DPT) leads to formation of the (1)(T1T1) state of BET-B, followed by generation of both DPT and BET-B triplets. For the structurally cofacial BET-X, an intermediate forms in <180 fs and returns to the ground state more rapidly than BET-B. First-principles calculations predict a 2 orders of magnitude faster rate of singlet fission to the (1)(T1T1) state in BET-B relative to that of crystalline tetracene, attributing the rate increase to greater coupling between the S1 and (1)(T1T1) states and favorable energetics for formation of the separated triplets.

Correlated Pair States Formed by Singlet Fission and Exciton-Exciton Annihilation.

Singlet fission to form a pair of triplet excitations on two neighboring molecules and the reverse process, triplet-triplet annihilation to upconvert excitation, have been extensively studied. Comparatively little work has sought to examine the properties of the intermediate state in both of these processes-the bimolecular pair state. Here, the eigenstates constituting the manifold of 16 bimolecular pair excitations and their relative energies in the weak-coupling regime are reported. The lowest-energy states obtained from the branching diagram method are the triplet pairs with overall singlet spin |X1⟩ ≈ (1)[TT] and quintet spin |Q⟩ ≈ (5)[TT]. It is shown that triplet pair states can be separated by a triplet-triplet energy-transfer mechanism to give a separated, yet entangled triplet pair (1)[T···T]. Independent triplets are produced by decoherence of the separated triplet pair. Recombination of independent triplets by exciton-exciton annihilation to form the correlated triplet pair (i.e., nongeminate recombination) happens with 1/3 of the rate of either triplet migration or recombination of the separated correlated triplet pair (geminate recombination).

Singlet fission of hot excitons in π-conjugated polymers.

We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(ω)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot excitons into two separated triplet excitons, in two luminescent and non-luminescent π-conjugated polymers. From the high energy step in the triplet EXPA(ω) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E≈2E(T) (=2.8 eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8 eV above the lowest singlet exciton energy. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2E(T), which shows typical SF response. This process is missing in DOO-PPV solution, showing that it is predominantly interchain in nature. By contrast, the triplet EXPA(ω) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset at E=E(g) (≈2.25 eV). From this, we infer that intrachain SF that involves a triplet-triplet pair state, also known as the 'dark' 2A(g) exciton, dominates the triplet photogeneration in PDA polymer as E(g)>2E(T). The intrachain SF process was also identified from the MPA(B) response of the triplet PA band in PDA. Our work shows that the SF process in π-conjugated polymers is a much more general process than thought previously.

A Simple Kinetic Model for Singlet Fission: A Role of Electronic and Entropic Contributions to Macroscopic Rates

A simple three-state model for the dynamics of the singlet fission (SF) process is developed. The model facilitates the analysis of the relative significance of different factors, such as electronic energies, couplings, and the entropic contributions. The entropic contributions to the rates are important; they drive the SF process in endoergic cases (such as tetracene). The anticipated magnitude of entropic contributions is illustrated by simple calculations. By considering a series of three acenes (tetracene, pentacene, and hexacene), we explained the experimentally observed 3 orders of magnitude difference in the rate of SF in tetracene and pentacene and predicted that the rate in hexacene will be slightly faster than in pentacene. This trend is driven by the increased thermodynamic drive for SF (Gibbs free energy difference of the initial excitonic state and two separated triplets). The model also explains experimentally observed fast SF in 5,12-diphenyltetracene. Consistently with the experimental observations, the model predicts weak temperature dependence of the multiexciton formation rate in tetracene as well as a reduced rate of this step in solutions and in isolated dimers.