Hydrogen bonds vs. $\pi$-stacking interactions in the p-aminophenol...p-cresol dimer: an experimental and theoretical study

TL;DR

This study combines experimental spectroscopy and theoretical calculations to explore the competition between hydrogen bonding and π-stacking in the p-aminophenol...p-cresol dimer, revealing that excited state fluorescence originates from a p-stacked isomer.

Contribution

It provides new insights into the excited state structure and optical properties of aromatic dimers, highlighting the role of π-stacking over hydrogen bonding in fluorescence.

Findings

The excited state lifetime of the heterodimer is approximately 2.5 ns.

Only the p-stacked isomer is excited in the S1 state, not the hydrogen-bonded form.

Differences in optical properties explain structural variations between heterodimer and homodimer.

Abstract

The gas phase structure and excited state lifetime of the p-aminophenol...p-cresol heterodimer have been investigated by REMPI and LIF spectroscopy with nanosecond laser pulses and pump-probe experiments with picosecond laser pulses as a model system to study the competition between p-p and H-bonding interactions in aromatic dimers. The excitation is a broad and unstructured band. The excitedstate of the heterodimer is long lived (2.5 +/- 0.5) ns with a very broad fluorescence spectrum red-shifted by 4000 cm^{-1} with respect to the excitation spectrum. Calculations at the MP2/RI-CC2 and DFT-oB97X-D levels indicate that hydrogen-bonded (HB) and p-stacked isomers are almost isoenergetic in the ground state while in the excited state only the p-stacked isomer exists. This suggests that the HB isomer cannot be excited due to negligible Franck-Condon factors and therefore the excitation spectrum is associated with the p-stacked isomer that reaches vibrationally excited states in the S1 state upon vertical excitation. The excited state structure is an exciplex responsible for the fluorescence of the complex. Finally,a comparison was performed between the p-stacked structure observed for the p-aminophenol...p-cresol heterodimer and the HB structure reported for the (p-cresol)2 homodimer indicating that the differences are due to different optical properties (oscillator strengths and Franck-Condon factors) of the isomers of both dimers and not to the interactions involved in the ground state