Aza-Cibalackrot: Turning on Singlet Fission Through Crystal Engineering

Structural Dynamic Impact of Chromophores on Singlet Fission

Singlet fission (SF) represents a unique mechanism that allows a single high-energy photon to split into two triplet excitons, significantly enhancing the quantum efficiency of photovoltaic and optoelectronic materials. Therefore, SF shows great potential for applications in solar cells and optoelectronic devices. Despite significant progress in recent years, synergistic effects of various factors that govern the structural dynamics of solvated chromophores individually or jointly and lead to the complicated dynamics of SF still require further exploration. This Perspective systematically analyzes various factors that affect and even modulate the efficiency and mechanism of SF, especially the structural dynamics of solvated chromophores, including molecular vibrations, chromophore dynamic interactions, solvent and environmental fluctuations, temperature and thermodynamic variations, and pressure and physical changes. The inherent dynamic characteristics of these factors dominate the structural and electronic properties and interchromophore interactions, thereby manipulating the energetics and kinetic pathways of SF and ultimately determining the SF efficiency and feasibility. The discussions presented in this Perspective provide dynamics insights, important foundations, and strategies for future design of materials and performance optimization of optoelectronic devices through considering dynamic factors.

In Situ Generation of Microwire Heterojunctions with Flexible Optical Waveguide and Hydration-Mediated Photochromism

Flexible heterojunctions based on molecular systems are in high demand for applications in photonics, electronics, and smart materials, but fabrication challenges have hindered progress. Herein, we present an in situ approach to creating optical heterojunctions using hydration-mediated flexible molecular crystals. These hydrated multi-component molecular solids display strong blue emitting optical waveguides with minimal optical loss (0.005 dB/μm) and excellent flexibility (elastic modulus: 3.87 GPa). The water-mediated process enables the molecular microwires with tunable elastic and plastic deformation, as well as reversible uptake and release of lattice water, facilitating the formation of flexible heterojunctions. Spectral analysis and theoretical modeling reveal that these microwires exhibit both photochromism and color-tunable dual emission (fluorescence and phosphorescence), expanding their utility in photonic information encoding. Therefore, this work introduces a hydration-mediated molecular engineering strategy for fabricating crystalline heterojunctions with on-demand processability and controllable emission sequences, enabling optical signal manipulation at the micro/nanoscale.

Machine learning photodynamics decode multiple singlet fission channels in pentacene crystal

Crystalline pentacene is a model solid-state light-harvesting material because its quantum efficiencies exceed 100% via ultrafast singlet fission. The singlet fission mechanism in pentacene crystals is disputed due to insufficient electronic information in time-resolved experiments and intractable quantum mechanical calculations for simulating realistic crystal dynamics. Here we combine a multiscale multiconfigurational approach and machine learning photodynamics to understand competing singlet fission mechanisms in crystalline pentacene. Our simulations reveal coexisting charge-transfer-mediated and coherent mechanisms via the competing channels in the herringbone and parallel dimers. The predicted singlet fission time constants (61 and 33 fs) are in excellent agreement with experiments (78 and 35 fs). The trajectories highlight the essential role of intermolecular stretching between monomers in generating the multi-exciton state and explain the anisotropic phenomenon. The machine-learning-photodynamics resolved the elusive interplay between electronic structure and vibrational relations, enabling fully atomistic excited-state dynamics with multiconfigurational quantum mechanical quality for crystalline pentacene.

Photophysics of a Monoannulated Indigo: Intra- and Intermolecular Charge Transfer

In the present work, the photoinduced charge-transfer (CT) behavior of 7-phenyl-6H-pyrido[1,2-a:3,4-b']diindole-6,13(12H)-dione (HCB) as a function of solvent polarity is reported by UV-vis absorption, steady-state and time-resolved fluorescence, and quantum chemical calculations. Calculated excited state energies of HCB at the B3PW91/6-31+G* level in vacuo and in solvents fulfill the energy requirements for singlet fission, which is the most promising path for the generation of highly efficient solar cells. The calculated potential energy curve for the compound reveals that the keto form is the predominant form in the ground state. Large bathochromic shifts in fluorescence with decreasing trends of quantum yield and lifetime indicate the occurrence of intramolecular CT from the indole bicycle to the indolinone moiety of HCB in highly polar solvents. The observed quenching of HCB fluorescence in different solvents without altering the spectral shape upon addition of a donor, triethylamine, is attributed to intermolecular CT, and it was examined in terms of the Stern-Volmer kinetics. The thermodynamics of photoinduced CT processes in HCB was analyzed using the measured photophysical data and cyclic voltammetric redox potentials via the Rehm-Weller equation. Analyses with the semiclassical Marcus theory suggest that both the CT processes fall under the Marcus normal region.