Simultaneous Intra- and Intermolecular Singlet Fission in Bipentacene Macrocycle Aggregates.

Singlet fission (SF) is a process where a singlet state splits into two triplet states, which is essential for enhancing optoelectronic devices. Macrocyclic structures allow for precise control of chromophore orientation and facilitate singlet fission in solutions. However, the behavior of these structures in thin films, crucial for solid-state device optimization, remains underexplored. This study examines the aggregation and singlet fission processes of bipentacene macrocycles (BPc) in thin films using molecular dynamics simulations and electronic structure calculations. Findings indicate that BPc aggregates more rapidly with less chloroform, aligning parallel to the substrate. Intramolecular singlet fission (iSF) rates are rarely changed during evaporation, but the efficiency of intermolecular singlet fission (xSF) improves due to the increase in packing domains, suggesting that orderly crystal domains are not necessary for device efficiency. This opens avenues for varied device designs and traditional solution-based methods for optimal device development.

Ester Bond: Chemically Labile Yet Mechanically Stable.

Due to the dynamic nature of ester linkages, ester-bond-containing materials are well known for their outstanding degradability and stimuli responsiveness. However, whether ester hydrolysis is affected by mechanical forces remains unclear. Here, we develop a single-molecule assay to quantitatively study the force-dependent ester hydrolysis using an engineered circular permutant protein with a caged ester bond as a model. Our single-molecule force spectroscopy results show that the ester hydrolysis rate is surprisingly insensitive to forces, with a ∼7 s-1 dissociation rate that remains almost unchanged in the force range of 80 to 200 pN. Quantum calculations reveal that the ester hydrolysis involves an intermediate state formed by either H3O+- or OH--bonded tetrahedral intermediates. The measured ester-hydrolysis kinetics at the single-molecule level may primarily correspond to the rupture of these intermediate states. However, the rate-limiting step appears to be the formation of the tetrahedral intermediates, which cannot be quantitatively characterized in our experiments. Nonetheless, based on the quantum calculations, this step is also insensitive to applied forces. Altogether, our study suggests that the ester bond is chemically labile yet mechanically stable, serving as the basis for the design of responsive materials using ester bonds as mechanically inert units.

Mechanisms of a Cyclobutane-Fused Lactone Hydrolysis in Alkaline and Acidic Conditions.

Searching for functional polyesters with stability and degradability is important due to their potential applications in biomedical supplies, biomass fuel, and environmental protection. Recently, a cyclobutane-fused lactone (CBL) polymer was experimentally found to have superior stability and controllable degradability through hydrolysis reactions after activation by mechanical force. In order to provide a theoretical basis for developing new functional degradable polyesters, in this work, we performed a detailed quantum chemical study of the alkaline and acidic hydrolysis of CBL using dispersion-corrected density functional theory (DFT-D3) and mixed implicit/explicit solvent models. Various possible hydrolysis mechanisms were found: BAC2 and BAL2 in the alkaline condition and AAC2, AAL2, and AAL1 in the acidic condition. Our calculations indicated that CBL favors the BAC2 and AAC2 mechanisms in alkaline and acidic conditions, respectively. In addition, we found that incorporating explicit water solvent molecules is highly necessary because of their strong hydrogen-bonding with reactant/intermediate/product molecules.

Excited-State Decay Pathways of Flavin Molecules in Five Redox Forms: The Role of Conical Intersections.

Flavin molecules play an important role in light-driven biological activities. They have drawn significant interest for decades because of their rich photochemistry. In addition to the well-explored FADH- (anionic hydroquinone), which is supposed to be the only catalytic active state to repair DNA lesions, other four flavin molecules (i.e., FAD, FAD·-, FADH·, and FADH2) in three redox forms combined the redox cycle of flavins. Although extensive studies have been carried out for steady-state spectroscopic properties of five redox flavins in various proteins and solutions, the photochemistry and photophysical properties of those different redox states significantly complicate the corresponding theoretical studies. In present work, we employed the ab initio wave function based CASSCF method to systematically investigate the excited state decay pathways of flavins in five redox forms through two approaches. First, the comparison of the absorption and emission spectra from both theoretical calculation and experiment allows a detailed mapping of the transition properties of different redox states in flavins. Second, we identified four kinds of conical intersections (CIs) for five different redox states as the possible deactivation mechanisms responsible for internal conversion or intersystem crossing from the initially populated excited state. The theoretical calculations provide atomic details for the photochemical and photophysical properties of flavins on photoinduced processes. Our findings highlight the indispensable effects of CIs in the excited state decay of flavin molecules and thereby provide basic theoretical information for light-driven biological activities.