Exciton Diffusion and Annihilation in an sp² Carbon-Conjugated Covalent Organic Framework

Abstract

To optimize the optical and optoelectronic functionalities of two-dimensional (2D) covalent organic frameworks (COFs), detailed properties of emissive and nonradiative pathways after photoexcitation need to be elucidated and linked to particular structural designs. Here, we use transient absorption (TA) spectroscopy to study the colloidal suspension of the full sp2 carbon-conjugated sp2c-COF and characterize the spatial extent and diffusion dynamics of the emissive excitons generated by impulsive photoexcitation. The ∼3.5 Å stacking distance between 2D layers results in cofacial pyrene excitons that diffuse through the framework, while the state that dominates the emissive spectrum of the polycrystalline solid is assigned to an extended cofacial exciton whose 2D delocalization is promoted by CC linkages. The subnanosecond kinetics of a photoinduced absorption (PIA) signal in the near-infrared, attributed to a charge-separated exciton, or polaron pair, reflects three-dimensional (3D) exciton diffusion as well as long-range exciton–exciton annihilation driven by resonance interactions. Within our experimental regime, doubling the excitation intensity results in a 10-fold increase in the estimated exciton diffusion length, from ∼3 to ∼30 nm, suggesting that higher lattice temperature may enhance exciton mobility in the COF colloid.

Designing Single‐Atom Active Sites on sp2‐Carbon Linked Covalent Organic Frameworks to Induce Bacterial Ferroptosis‐Like for Robust Anti‐Infection Therapy

“…[ 25 ] The robust CC bond not only endues the framework with excellent stability under harsh conditions but also ensures efficient electron transfer via extensive π ‐conjugation throughout the framework. [ 26 ] Nevertheless, due to the poor reversibility of CC bond formation, the preparation of sp 2 ‐carbon conjugated COFs maintains a tremendous challenge. Fortunately, under solvothermal conditions (tetrahydrofuran/0.1 m Cs 2 CO 3 = 20/1 v/v, 3 days, 120 °C), the topology‐directed polycondensation of C 3 ‐symmetric 4,4′,4″‐(1,3,5‐triazine‐2,4,6‐triyl)tribenzaldehyde (TA) as a knote and C 2 ‐symmetric linear 2,6‐dicyanomethylbenzo[1,2‐d:4,5‐d′]bisthiazole (BTHAN) as a linker yielded a sp 2 c‐COF (Figures S1–S4, Supporting Information).…”

Advances in organic semiconductors for photocatalytic hydrogen evolution reaction

“…Page 22 of 41 EES Catalysis Investigating exciton dynamics in COFs for photocatalytic hydrogen production presents challenges. These challenges arise from the poor optical quality of COF samples, caused by their insolubility and the difficulty in obtaining colloidal suspensions, which complicates optical characterization, particularly the acquisition of absorption spectra 101 . Nevertheless, a universal model of the electronic processes and optical properties of a series of imine-linked…”

Customizable 2D Covalent Organic Frameworks for Optoelectronic Applications

“…have elucidated detailed properties of emissive and nonradiative pathways after photoexcitation and linked this property to specific structural designs by transient absorption spectroscopy. [ 190 ] The study of exciton diffusion and annihilation processes in sp 2 C conjugated COFs will be beneficial for optimizing the optical and optoelectronic functions of 2D COFs.…”