Boosting the photothermal conversion performance of a dumbbell-shaped molecule with coordinated methanol

Abstract

Dumbbell-like molecules have attracted substantial attention because of their unique structural features, light-collecting properties, and potential applications. Designing and constructing novel dumbbell-like molecules can enrich the diversity of these molecular species with favorable properties. The first dumbbell-like molecule based on the [Cu^II(cyclam)]²⁺ (cyclam = 1,4,8,11-tetraazacyclotetradecane) macrocyclic building block has been synthesized and comprehensively characterized using single-crystal X-ray crystallography (SXRD), electrospray ionization mass spectrometry (ESI-MS), infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and so forth. Single crystal X-ray structural analysis reveals that {[cyclam₂Cu₂(1,4-NDC)(CH₃OH)]·(BF₄)₂} (I, 1,4-NDC = 1,4-naphthalenedicarboxylic acid) consists of two [Cu^II(cyclam)]²⁺ building blocks at both ends serving as the bulky “bell” and one 1,4-NDC linker in the middle acting as the connecting “bar”, with one methanol molecule attached. Upon heating sample I to remove the coordinated methanol, {[cyclam₂Cu₂(1,4-NDC)]·(BF₄)₂} (II) is formed. Both samples exhibit similar optical absorption capabilities both in the solid state and in solution. However, they display markedly different photothermal conversion performances in the solid-state under the simulated sunlight and in a CH₃CN solution when irradiated with a 635 nm laser. Comparative studies of their structure–activity relationships reveal that the coordinated CH₃OH molecule in sample I enables efficient intramolecular motions, which in turn promote photothermal conversion. These findings indicate that the unprecedented synthesis of a dumbbell-like molecule based on [Cu^II(cyclam)]²⁺ is feasible and offer valuable insights for the design of new complexes with desired photothermal conversion characteristics.