Programming intramolecular charge transfer in NDI-pyrene paracyclophanes through distance and linker design.

Abstract

Through-space charge transfer (TSCT) in electron donor-acceptor (D-A) molecules is a powerful strategy for controlling excited-state processes. However, the influence of molecular architecture on TSCT dynamics remains poorly understood. Herein, we report a series of naphthalene diimide (NDI)-pyrene (Py) D-A paracyclophanes (PCPs) with systematically varied interchromophore distance and spatial arrangement via different linkers: phenylene (NDI-Ph-Py), benzylamine (NDI-BnNH-Py), and adamantylamine (NDI-AdaNH-Py). All PCPs exhibit low-energy CT absorption bands absent in the corresponding reference compounds, confirming efficient intramolecular TSCT interactions. Combined spectroscopic, electrochemical, and computational studies reveal that the TSCT strength and energies are highly sensitive to linker structure, rigidity, and NDI-Py separation. Furthermore, femtosecond transient absorption spectroscopy shows multiple CT states and distinct relaxation pathways governed by linker properties. NDI-AdaNH-Py exhibits CT-state formation on the sub-picosecond timescale, facilitated by the short NDI-Py separation distance and substantial orbital overlap. NDI-Ph-Py, with the shortest interchromophore distance, strongest electronic coupling, and rigid structure, further promotes the formation of an exciplex state. These findings demonstrate that linker engineering is an effective means of modulating TSCT interactions, providing valuable design principles for the development of next-generation functional organic materials.