Three-State Fluorescence of a 2-Functionalized Pyrene-Based RNA Label

Area of Science:

• Photophysics
• Molecular Spectroscopy
• Biophysical Chemistry

Background:

• RNA structural dynamics are crucial for biological function.
• Fluorescence labels are essential tools for studying biomolecules.
• Pyrene-based probes offer unique photophysical properties.

Purpose of the Study:

• To investigate the unusual wavelength-dependent fluorescence of the pyrene-based RNA label 2-(2-pyrenylethynyl) adenosine (2PyA).
• To elucidate the excited-state dynamics and emission mechanisms of 2PyA in solution.
• To correlate fluorescence behavior with molecular structure and excitation conditions.

Main Methods:

• Steady-state and time-resolved emission spectroscopy.
• Ultrafast transient absorption spectroscopy.
• Quantum chemical calculations (CC2 and DFT).

Main Results:

• 2PyA displays triexponential fluorescence strongly dependent on excitation wavelength.
• Excitation into the S2 state yields structured, long-lived fluorescence shifted hypsochromically.
• Emission arises from simultaneous mesomeric (MICT) and twisted (TICT) intramolecular charge transfer states.
• Populations of these states are influenced by excitation energy, rotational momentum, and molecular geometry (distance and tilt angle).

Conclusions:

• The complex fluorescence of 2PyA is attributed to simultaneous emission from distinct intramolecular charge transfer states.
• Excitation wavelength critically controls the population pathways and observed fluorescence characteristics.
• Molecular geometry, specifically adenine-pyrene distance and tilt, dictates substate stabilization.