Nile Red Fluorescence: Where's the Twist?

Area of Science:

• Photophysics
• Spectroscopy
• Fluorescent Dyes

Background:

• Nile Red is a widely used fluorescent dye in bioimaging, known for its solvatochromism.
• Decades of research have debated Nile Red's fluorescence mechanism, with conflicting theories on dual excited states and the nature of the emissive state (TICT vs. PICT).

Purpose of the Study:

• To definitively resolve the long-standing questions regarding the photophysical mechanism of Nile Red fluorescence.
• To elucidate whether Nile Red exhibits dual fluorescence or emission from a single excited state.
• To determine the character of the primary emissive state (TICT or PICT).

Main Methods:

• Combined experimental and theoretical computational chemistry approach.
• Time-resolved fluorescence spectroscopy to analyze emission dynamics.
• Ultrafast pump-probe spectroscopy to detect transient excited states.
• Quantum chemical calculations (DFT) to model electronic structure and excited states.

Main Results:

• Time-resolved fluorescence measurements demonstrated single-exponential decay, indicating emission from one excited state.
• Theoretical calculations found no evidence for a low-lying twisted intramolecular charge-transfer (TICT) state.
• The calculated S₁ minimum corresponded to a planar intramolecular charge-transfer (PICT) state.
• Ultrafast spectroscopy data did not reveal signatures of an additional excited state involved in fluorescence decay.

Conclusions:

• The study refutes the hypothesis of dual fluorescence in Nile Red across various solvent polarities.
• Nile Red fluorescence definitively originates from a single excited state.
• The primary emissive state of Nile Red is confirmed to be of the planar intramolecular charge-transfer (PICT) type.