Abstract 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.
Ultrafast infrared and electronic spectroscopy are used to examine the dynamics of triplet pair separation following singlet fission in amorphous and crystalline films of the model singlet fission chromophore, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn). Probing of correlated triplet pair intermediates directly through their unique vibrational frequencies and infrared electronic transitions and indirectly through their visible triplet absorptions reveals that triplet pair separation occurs on similar picosecond time scales in both amorphous and crystalline films despite their markedly different average intermolecular coupling strengths. Although triplet pair separation occurs on similar time scales in both environments, measurements of diffusion-controlled triplet–triplet annihilation reveal that the diffusivity of triplet excited states is an order of magnitude lower in amorphous films. The data reveal the presence of sparse triplet traps in the amorphous environment that inhibit the transport of triplet excitons in comparison to crystalline films. These observations inform recent efforts to develop disordered and polymeric singlet fission sensitizers that contain amorphous regions. In particular, the data suggest that it may be possible to nanostructure amorphous or polymeric singlet fission sensitizers to allow ultrafast triplet pair separation and harvesting in photovoltaic and light-emitting applications despite their low triplet exciton diffusivity.
Nadia Korovina opened discussion of the paper by David N. Beratan: Does the excited state energy of the bridge affect the coupling between the donor and the acceptor? How does the rate of energy transfer through the bridge scale with respect to the bridge energy? David N. Beratan
Abstract Redox is emerging as an alternative modality for bio‐device communication. In contrast to the more familiar ionic electrical modality: (i) redox involves the flow of electrons through oxidation–reduction reactions; (ii) the aqueous medium is an “insulator” to this electron flow since free electrons do not normally exist in water; and (iii) redox states are intrinsically digital (oxidized and reduced). By exploiting these unique features, a catechol‐based molecular memory film is reported. This memory is fabricated by electrochemically grafting catechol to a chitosan–agarose polysaccharide network to generate a redox‐active but non‐conducting matrix. The redox state of the grafted catechol moieties serves as the 2‐state memory. It is shown that these redox states: can be repeatedly switched by diffusible mediators (electron shuttles); can be easily read electrically or optically; are stable for at least 2 h in the absence of energy; are sensitive to biologically relevant oxidizing and reducing contexts; and can be switched enzymatically. This catechol‐based molecular memory film is a simple circuit element for redox linked bioelectronics.
Free carrier dynamics in organo-halide perovskites can directly reveal information about their carrier lifetimes and indirectly reveal information about trap state distributions, both of which are critical to improving their performance and stability. Time-resolved photoluminescence (TRPL) spectroscopy is commonly used to probe carrier dynamics in these materials, but the technique is only sensitive to radiative decay pathways and may not reveal the true carrier dynamics. We used time-resolved infrared (TRIR) spectroscopy in comparison to TRPL to show that photogenerated charges relax into free carrier states with lower radiative recombination probabilities, which complicates TRPL measurements. Furthermore, we showed that trapped carriers exhibit distinct mid-infrared absorptions that can be uniquely probed using TRIR spectroscopy. We used the technique to demonstrate the first simultaneous measurements of trapped and free carriers in organo-halide perovskites, which opens new opportunities to clarify how charge trapping and surface passivation influence the optoelectronic properties of these materials.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Dye-sensitized electron injection into cerium oxide (CeO2) nanoparticles is studied using femtosecond transient absorption spectroscopy. Following the 430 nm photoexcitation of coumarin 343 molecules adsorbed on CeO2 nanoparticles, mid-infrared (mid-IR) transient absorption signals appear within 500 fs. Mid-IR signals are assigned to electrons injected into broadly distributed trap or defect states found roughly midway between the Ce 5d conduction band edge and the bottom of the empty band of Ce 4f states. These states are proposed to have Ce 5d character. In contrast, mid-IR signals are not observed when electrons in bare CeO2 nanoparticles are promoted from the valence band to the band built from Ce 4f orbitals by UV excitation. Transient absorption signals from the oxidized dye rise within 300 fs and decay somewhat more slowly than the mid-IR signals, suggesting that electron relaxation occurs within CeO2 in competition with back-electron transfer to the oxidized dye. These results reveal elementary events important for understanding photocatalysis using CeO2 nanoparticles.
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