Potential Energy Surfaces of Environment-Sensitive Flapping Fluorophores: FLAP

Area of Science:

• Photochemistry
• Computational Chemistry
• Materials Science

Background:

• Flexible Aromatic Photofunctional (FLAP) molecules are fluorescent probes sensitive to environmental changes.
• Understanding their photophysical properties is crucial for developing advanced sensors.

Purpose of the Study:

• Investigate the potential energy surfaces (PESs) of a representative FLAP molecule, FLAP0.
• Elucidate the excited-state dynamics and emission mechanisms of FLAP0.

Main Methods:

• Density Functional Theory (DFT) calculations.
• Time-Dependent DFT (TD-DFT) for excited states.
• Analysis of potential energy surfaces and conical intersections.

Main Results:

• Upon excitation, FLAP0 accesses S2 and S3 states, transitioning to S1 via conical intersections.
• Symmetry-breaking motion, not flapping, occurs at a transition state near the S2/S1 conical intersection.
• Planarization in the S1 state is thermally induced by COT hydrogen wagging, with a low barrier.
• V-shaped and planar forms exist in thermal equilibrium, leading to structured green emission.

Conclusions:

• The observed "vibronic structure" in FLAP0 emission arises from the thermal equilibrium between distinct planar forms in the S1 state.
• This equilibrium, rather than unidirectional transformation, dictates the molecule's photophysical response.
• FLAP0's unique dynamics explain its utility as a sensitive fluorescent probe.