Hydrogen-Bonding Motifs and Proton-Transfer Dynamics in Electronically Excited 6-Hydroxy-2-formylfulvene

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Low-barrier hydrogen bonding (LBHBing) influences molecular dynamics and reaction pathways.
• Understanding proton transfer mechanisms is crucial in chemical and biological systems.

Purpose of the Study:

• To investigate the impact of LBHBing on hydron migration dynamics in the excited state of 6-hydroxy-2-formylfulvene (HFF).
• To elucidate the changes in reaction coordinates and vibrational landscapes upon electronic excitation.

Main Methods:

• Synergistic application of fluorescence-based laser spectroscopy and quantum-chemical calculations.
• Probing the ùB₂-X̃¹A₁ (π* ← π) absorption system of HFF and its monodeuterated isotopolog (HFF-d) under free-jet expansion.

Main Results:

• The excited state exhibits a notably aplanar transition-state configuration, contrasting with the planar ground state.
• Surprisingly regular vibronic structure was observed in the ùB₂ state, lacking signatures of efficient proton transfer.
• A significant decrease (>1000-fold) in excited-state hydron migration was quantified, indicated by a reduced tunneling-induced bifurcation of the vibrationless ùB₂ level.

Conclusions:

• The excited-state ùB₂ manifold of HFF shows a pronounced quenching of hydron migration.
• This quenching is attributed to kinematic penalties associated with heavy-atom motion along an aplanar reaction coordinate in the excited state.