Decoding Excimer Formation in Covalent–Organic Frameworks Induced by Morphology and Ring Torsion

Author:

Chakraborty Jeet1ORCID,Chatterjee Amrita1,Molkens Korneel234,Nath Ipsita1ORCID,Arenas Esteban Daniel5,Bourda Laurens16,Watson Geert1,Liu Chunhui17,Van Thourhout Dries34,Bals Sara4ORCID,Geiregat Pieter23ORCID,Van der Voort Pascal1ORCID

Affiliation:

1. Centre for Ordered Materials Organometallics and Catalysis (COMOC) Department of Chemistry Ghent University Krijgslaan 281‐S3 Ghent 9000 Belgium

2. Physics and Chemistry of Nanostructures Department of Chemistry Ghent University Krijgslaan 281‐S3 Ghent 9000 Belgium

3. NOLIMITS Center for Non‐Linear Microscopy and Spectroscopy Ghent University Krijgslaan 281‐S3 Ghent 9000 Belgium

4. Photonics Research Group Department of Information Technology Ghent University – imec Technologiepark‐Zwijnaarde 126 Ghent 9052 Belgium

5. EMAT‐Electron Microscopy for Materials Science Department of Physics University of Antwerp Groenenborgerlaan 171 Antwerp 2020 Belgium

6. XStruct Department of Chemistry Ghent University Krijgslaan 281‐S3 Ghent 9000 Belgium

7. NanoSensing Group Department of Chemistry Ghent University Krijgslaan 281S3 Ghent 9000 Belgium

Abstract

AbstractA thorough and quantitative understanding of the fate of excitons in covalent–organic frameworks (COFs) after photoexcitation is essential for their augmented optoelectronic and photocatalytic applications via precise structure tuning. The synthesis of a library of COFs having identical chemical backbone with impeded conjugation, but varied morphology and surface topography to study the effect of these physical properties on the photophysics of the materials is herein reported. The variation of crystallite size and surface topography substantified different aggregation pattern in the COFs, which leads to disparities in their photoexcitation and relaxation properties. Depending on aggregation, an inverse correlation between bulk luminescence decay time and exciton binding energy of the materials is perceived. Further transient absorption spectroscopic analysis confirms the presence of highly localized, immobile, Frenkel excitons (of diameter 0.3–0.5 nm) via an absence of annihilation at high density, most likely induced by structural torsion of the COF skeletons, which in turn preferentially relaxes via long‐lived (nanosecond to microsecond) excimer formation (in femtosecond scale) over direct emission. These insights underpin the importance of structural and topological design of COFs for their targeted use in photocatalysis.

Funder

China Scholarship Council

Publisher

Wiley

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