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Nile Red Fluorescence: Where's the Twist?

Camilla Gajo1, Darya Shchepanovska1, Jacob F Jones1

  • 1School of Chemistry, Cantock's Close, University of Bristol, Bristol BS8 1TS, U.K.

The Journal of Physical Chemistry. B
|November 14, 2024
PubMed
Summary

Nile Red fluorescence arises from a single excited state, not two, according to combined experimental and theoretical studies. This research clarifies the photophysics, confirming emission from a planar intramolecular charge-transfer (PICT) state.

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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.