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1. Resolving electron injection from singlet fission-borne triplets into mesoporous transparent conducting oxides

   https://doi.org/10.1039/d1sc03253d  

   Gish, Melissa K. ; Raulerson, Emily K. ; Pekarek, Ryan T. ; Greenaway, Ann L. ; Thorley, Karl J. ; Neale, Nathan R. ; Anthony, John E. ; Johnson, Justin C. ( January 2021 , Chemical Science)

   null (Ed.)

   Photoinduced electron transfer into mesoporous oxide substrates is well-known to occur efficiently for both singlet and triplet excited states in conventional metal-to-ligand charge transfer (MLCT) dyes. However, in all-organic dyes that have the potential for producing two triplet states from one absorbed photon, called singlet fission dyes, the dynamics of electron injection from singlet vs. triplet excited states has not been elucidated. Using applied bias transient absorption spectroscopy with an anthradithiophene-based chromophore ( ADT-COOH ) adsorbed to mesoporous indium tin oxide ( nanoITO ), we modulate the driving force and observe changes in electron injection dynamics. ADT-COOH is known to undergo fast triplet pair formation in solid-state films. We find that the electronic coupling at the interface is roughly one order of magnitude weaker for triplet vs. singlet electron injection, which is potentially related to the highly localized nature of triplets without significant charge-transfer character. Through the use of applied bias on nanoITO : ADT-COOH films, we map the electron injection rate constant dependence on driving force, finding negligible injection from triplets at zero bias due to competing recombination channels. However, at driving forces greater than −0.6 eV, electron injection from the triplet accelerates and clearly produces a trend with increased applied bias that matches predictions from Marcus theory with a metallic acceptor. 

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2. Enhancing Organic Semiconductor Molecular Packing Using Perovskite Interfaces to Improve Singlet Fission

   https://doi.org/10.1002/adfm.202303232  

   Conley, Ashley M. ; Dziatko, Rachel A. ; Westendorff, Karl S. ; Sviripa, Anna ; Guthrie, Stephanie M. ; Chen, Zhuo ; Stone, Kevin H. ; Bragg, Arthur E. ; Paolucci, Christopher ; Choi, Joshua J. ; et al ( September 2023 , Advanced Functional Materials)

   Singlet fission, a process by which one singlet exciton is converted into two lower energy triplet excitons, is sensitive to the degree of electronic coupling within a molecular packing structure. Variations in molecular packing can be detrimental to triplet formation and triplet–triplet separation, ultimately affecting the harvesting of triplets for electricity in organic photovoltaic devices. Here, six phase‐pure molecular packing structures of 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene) with varying optoelectronic properties are isolated using 2D lead halide perovskites as tunable, crystalline surfaces for crystallization. Transient absorption spectroscopy reveals that while triplet formation is fast (<100 fs) regardless of template structure, the increased ordering in perovskite‐templated samples speeds up triplet–triplet separation and recombination, providing evidence that the benefits of phase‐purity offset minor variations in molecular packing. Molecular dynamics modeling of the interface reveals that perovskite‐templating allows for closer packing of TIPS‐pentacene molecules for all perovskite templates. With an extensive number of organic molecule‐perovskite pairings, this work provides a methodology to use ordered, periodic surfaces to elucidate structure–property relationships of small organic molecules in order to adjust structural or optoelectronic responses, such as molecular packing and singlet fission.

    

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3. Revealing the Cooperative Relationship between Spin, Energy, and Polarization Parameters toward Developing High‐Efficiency Exciplex Light‐Emitting Diodes

   https://doi.org/10.1002/adma.201904114  

   Wang, Miaosheng ; Huang, Yi‐Hsuan ; Lin, Kai‐Siang ; Yeh, Tzu‐Hung ; Duan, Jiashun ; Ko, Tzu‐Yu ; Liu, Shun‐Wei ; Wong, Ken‐Tsung ; Hu, Bin ( September 2019 , Advanced Materials)

   Experimental studies to reveal the cooperative relationship between spin, energy, and polarization through intermolecular charge‐transfer dipoles to harvest nonradiative triplets into radiative singlets in exciplex light‐emitting diodes are reported. Magneto‐photoluminescence studies reveal that the triplet‐to‐singlet conversion in exciplexes involves an artificially generated spin‐orbital coupling (SOC). The photoinduced electron parametric resonance measurements indicate that the intermolecular charge‐transfer occurs with forming electric dipoles (D^+•→A^−•), providing the ionic polarization to generate SOC in exciplexes. By having different singlet‐triplet energy differences (ΔE_ST) in 9,9′‐diphenyl‐9H,9′H‐3,3′‐bicarbazole (BCzPh):3′,3′″,3′″″‐(1,3,5‐triazine‐2,4,6‐triyl)tris(([1,1′‐biphenyl]‐3‐carbonitrile)) (CN‐T2T) (ΔE_ST= 30 meV) and BCzPh:bis‐4,6‐(3,5‐di‐3‐pyridylphenyl)‐2‐methyl‐pyrimidine (B3PYMPM) (ΔE_ST= 130 meV) exciplexes, the SOC generated by the intermolecular charge‐transfer states shows large and small values (reflected by different internal magnetic parameters: 274 vs 17 mT) with high and low external quantum efficiency maximum, EQE_max(21.05% vs 4.89%), respectively. To further explore the cooperative relationship of spin, energy, and polarization parameters, different photoluminescence wavelengths are selected to concurrently change SOC, ΔE_ST, and polarization while monitoring delayed fluorescence. When the electron clouds become more deformed at a longer emitting wavelength due to reduced dipole (D^+•→A^−•) size, enhanced SOC, increased orbital polarization, and decreased ΔE_STcan simultaneously occur to cooperatively operate the triplet‐to‐singlet conversion.

    

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4. Formation of Stilbene Azo‐Dimer by Direct Irradiation of p‐Azidostilbene †

   https://doi.org/10.1111/php.13659  

   Osisioma, Onyinye ; Patton, Leanna J. ; Merugu, Rajkumar ; Govorov, Dmitrii ; Milbrandt, Margaret A. ; Jarus, Cassandra ; Karney, William L. ; Gudmundsdottir, Anna D. ( July 2022 , Photochemistry and Photobiology)

   Triplet arylnitrenes may provide direct access to aryl azo‐dimers, which have broad commercial applicability. Herein, the photolysis ofp‐azidostilbene (1) in argon‐saturated methanol yielded stilbene azo‐dimer (2) through the dimerization of tripletp‐nitrenostilbene (³1N). The formation of³1Nwas verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λ_max ~ 375 nm) in cryogenic 2‐methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of1in methanol formed³1N(λ_max ~ 370 nm, 2.85 × 10⁷ s⁻¹). On shorter timescales, a transient absorption (λ_max ~ 390 nm) that decayed with a similar rate constant (3.11 × 10⁷ s⁻¹) was assigned to a triplet excited state (T) of1. Density functional theory calculations yielded three configurations for T of1, with the unpaired electrons on the azido (T_A) or stilbene moiety (T_Tw, twisted and T_Fl, flat). The transient was assigned to T_Twbased on its calculated spectrum. CASPT2 calculations gave a singlet–triplet energy gap of 16.6 kcal mol⁻¹for1 N; thus, intersystem crossing of¹1Nto³1Nis unlikely at ambient temperature, supporting the formation of³1Nfrom T of1. Thus, sustainable synthetic methods for aryl azo‐dimers can be developed using the visible‐light irradiation of aryl azides to form triplet arylnitrenes.

    

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5. Triplet Transfer Mediates Triplet Pair Separation during Singlet Fission in 6,13‐Bis(triisopropylsilylethynyl)‐Pentacene

   https://doi.org/10.1002/adfm.201703929  

   Grieco, Christopher ; Doucette, Grayson S. ; Munro, Jason M. ; Kennehan, Eric R. ; Lee, Youngmin ; Rimshaw, Adam ; Payne, Marcia M. ; Wonderling, Nichole ; Anthony, John E. ; Dabo, Ismaila ; et al ( October 2017 , Advanced Functional Materials)

   Triplet population dynamics of solution cast films of isolated polymorphs of 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS‐Pn) provide quantitative experimental evidence that triplet excitation energy transfer is the dominant mechanism for correlated triplet pair (CTP) separation during singlet fission. Variations in CTP separation rates are compared for polymorphs of TIPS‐Pn with their triplet diffusion characteristics that are controlled by their crystal structures. Since triplet energy transfer is a spin‐forbidden process requiring direct wavefunction overlap, simple calculations of electron and hole transfer integrals are used to predict how molecular packing arrangements would influence triplet transfer rates. The transfer integrals reveal how differences in the packing arrangements affect electronic interactions between pairs of TIPS‐Pn molecules, which are correlated with the relative rates of CTP separation in the polymorphs. These findings suggest that relatively simple computations in conjunction with measurements of molecular packing structures may be used as screening tools to predict a priori whether new types of singlet fission sensitizers have the potential to undergo fast separation of CTP states to form multiplied triplets.

    

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