A Unified Push–Pull Model for Understanding the Ring-Opening Mechanism of Rhodamine Dyes

Oxygen/carbon/silicon-rhodamine fluorophores and probes are ubiquitous in bioimaging and biosensing applications. Besides excellent brightness, photostability, and biocompatibility, these dyes possess a unique intramolecular spirocyclization equilibrium between nonemissive ring-closed and fluorescent ring-opened forms. Understanding the closed-ring/open-ring switching mechanism is critical for the rational designs of high-performance rhodamine dyes and probes. First-principles calculation-combined data search carried out herein quantitatively elucidates the importance of both substituent groups and environmental conditions in influencing the ring-opening process. Our analysis yields a unified push–pull model in elucidating the ring-opening mechanism of rhodamines. We demonstrate that this model produces excellent agreements with a broad range of experiments that involve different structural modifications in rhodamines and varied environmental conditions (i.e., solvent polarity, hydrogen bond donating strength, and acidity). We foresee that this push–pull model will provide important guidelines for understanding and designing rhodamine dyes and probes with required fluorescence-switching properties, such as autoblinking dyes and photoactivable dyes for super-resolution imaging and fluorogenic dyes for biochemical studies.