Rapid intersystem crossings in anti bimanes

Area of Science:

• Computational Chemistry
• Photophysics
• Quantum Mechanics

Background:

• Syn and anti bimanes are isomers with distinct photophysical properties.
• Syn bimanes are widely used as fluorophores in biological labeling.
• Anti bimanes typically exhibit phosphorescence at low temperatures.

Purpose of the Study:

• To elucidate the electronic structure differences between syn and anti bimanes.
• To explain the contrasting fluorescence and phosphorescence behaviors of the isomers.
• To investigate the pathways of intersystem crossing in bimanes.

Main Methods:

• First-principles electronic structure calculations.
• Analysis of ground and excited valence states.
• Investigation of excited singlet (S1) and triplet (T1, T2) states.
• Determination of energetic ordering and transition types.

Main Results:

• Anti bimanes possess a T2 state (π→π*) energetically close to S1, facilitating rapid intersystem crossing (ca. 10^11 s⁻¹).
• This fast intersystem crossing in anti bimanes quenches fluorescence.
• In syn bimanes, T2 is energetically inaccessible from S1, allowing efficient fluorescence.
• Minimum energy conical intersections between S0 and S1 are high in energy and distorted, not influencing S1 photophysics.

Conclusions:

• Subtle differences in the energetic ordering of excited states dictate the photophysical outcomes of syn and anti bimanes.
• The rapid intersystem crossing pathway in anti bimanes explains their lack of fluorescence.
• Syn bimanes' fluorescence is favored due to the inaccessibility of rapid intersystem crossing pathways.