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.