Exciton Diffusion and\nAnnihilation in an sp² Carbon-Conjugated Covalent Organic\nFramework

Abstract

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