Counterion Migration Driven by Light-Induced Intramolecular Charge Transfer

Abstract

A series of D−π– A + pyridinium compounds, in which D = −NPh 2 and A + = −PyMe + are linked by various amounts of linear phenyl spacers, were strategically designed and synthesized. Their characterization revealed the presence of excited-state intramolecular charge transfer (ESICT) that triggers a corresponding response from the counterion. In medium and strong polar solvents, the fast solvent relaxation occurring after ESICT overwhelms the counterion effect, showing typical emission solvatochromism. In weakly polar solvents, ESICT induces counteranion migration for electrostatic stabilization, the time scale of which is dependent on the radius of the counteranion, the length of the π-linker, and the viscosity of the solvent. In low-viscosity organic solvents such as toluene, counteranion migration occurs within several tens to hundreds of picoseconds, resulting in a time-dependent continuous emission that can be resolved from the spectral temporal evolution. Concrete evidence for this is provided by the chemical synthesis of a D−π– A + pyridinium–sulfur trioxide – zwitterion, where anion migration is restricted due to its internally locked ion pair. As a result, only a single emission band can be observed. These comprehensive studies prove that the ion migration process may be significant for a wide range of ESICT-type ionic fluorophores. Such an ionic movement, triggered by optically pumped ESICT of the D−π– A + dyad, is similar to the molecular machine driven by the redox reaction, but with a facile access and fast response.

Dynamic Full‐Color Tuning of Organic Chromophore in a Multi‐Stimuli‐Responsive 2D Flexible MOF

“…Although most of the organic chromophores with nonplanar propeller geometry possess potential stimuliresponsiveness, [50] their luminescence responses in bulk states are weak or even imperceptible, mainly ascribed to the packing-induced rigidified molecular geometry and alignment. [25,51] In this work, given the coupled structural transformations of a 2D dynamic coordination space and the embedded chromophore, we then anticipated that this novel…”

“…A sophisticated way to circumvent this issue is utilizing the conformational changes of organic chromophores, in particular those with nonplanar propeller-like geometry. [21][22][23] With this idea, a vast array of organic oligomers [24,25] /polymers [26] and metal-organic complexes [27,28] with stimuli-responsive luminescence have been reported, but most of them suffer from narrow color-tunable range, poor reversibility, unresolved structures, and ambiguous color switching mechanisms.…”

Dynamic Full‐Color Tuning of Organic Chromophore in a Multi‐Stimuli‐Responsive 2D Flexible MOF

“…Although most of the organic chromophores with nonplanar propeller geometry possess potential stimuliresponsiveness, [50] their luminescence responses in bulk states are weak or even imperceptible, mainly ascribed to the packing-induced rigidified molecular geometry and alignment. [25,51] In this work, given the coupled structural transformations of a 2D dynamic coordination space and the embedded chromophore, we then anticipated that this novel material may exhibit distinctive stimuli-responsive luminescent properties, with the hope of surpassing other types of smart luminescent materials. Strikingly, the solid-state photoluminescence of NKU-128 presented entirely different responses within two temperature ranges (Figure 1e, f).…”

Multiple Emission of Phosphonium Fluorophores Harnessed by the Pathways of Photoinduced Counterion Migration

“…The resulting dual‐band‐like steady‐state luminescence was shown to be dependent on the length of the π‐spacer, the size of migrating anion, and the solvent viscosity. Comparison of phosphonium [10a] and pyridinium [10b] congeners also indicate the important role of the cationic group and, consequently, polarization of the spacer in the process of light‐triggered anion migration and corresponding optical response.…”