Hydrogen bonds vs. π-stacking interactions in the p-aminophenolp-cresol dimer: an experimental and theoretical study

Area of Science:

• Physical Chemistry
• Molecular Spectroscopy
• Computational Chemistry

Background:

• Aromatic dimers exhibit complex interactions, including π-π stacking and hydrogen bonding.
• Understanding these interactions is crucial for predicting molecular behavior in various chemical systems.

Purpose of the Study:

• To investigate the gas-phase structure and excited-state dynamics of the p-aminophenol-p-cresol heterodimer.
• To elucidate the competition between π-π and hydrogen-bonding interactions in aromatic dimers.

Main Methods:

• Resonance-enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF) spectroscopy.
• Nanosecond and picosecond pump-probe experiments.
• High-level quantum chemical calculations (MP2/RI-CC2, DFT-ωB97X-D).

Main Results:

• The heterodimer exhibits a long-lived excited state (2.5 ± 0.5 ns) with a red-shifted fluorescence spectrum.
• Ground-state calculations show nearly isoenergetic hydrogen-bonded (HB) and π-stacked isomers.
• Excited-state analysis indicates the dominance of the π-stacked isomer, forming an exciplex responsible for fluorescence.

Conclusions:

• The excitation spectrum is attributed to the π-stacked isomer due to negligible Franck-Condon factors for the HB isomer.
• The observed differences between the heterodimer and homodimer structures are due to optical properties, not ground-state interactions.