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

• Published in: Physical chemistry chemical physics : PCCP Vol. 18; no. 45; pp. 31260 - 31267
• Main Authors: Capello, M C, Hernández, F J, Broquier, M, Dedonder-Lardeux, C, Jouvet, C, Pino, G A
• Format: Journal Article
• Language: English
• Published: England 16.11.2016
• Subjects: Dimers; Excitation; Excitation spectra; Fluorescence; Ground state; Isomers; Mathematical models; Molecular structure
• DOI: 10.1039/c6cp06352g
• ISSN: 1463-9076

The gas phase structure and excited state lifetime of the p-aminophenolp-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 π-π and H-bonding interactions in aromatic dimers. The excitation is a broad and unstructured band. The excited state of the heterodimer is long lived (2.5 ± 0.5) ns with a very broad fluorescence spectrum red-shifted by 4000 cm with respect to the excitation spectrum. Calculations at the MP2/RI-CC2 and DFT-ωB97X-D levels indicate that hydrogen-bonded (HB) and π-stacked isomers are almost isoenergetic in the ground state while in the excited state only the π-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 π-stacked isomer that reaches vibrationally excited states in the S 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 π-stacked structure observed for the p-aminophenolp-cresol heterodimer and the HB structure reported for the (p-cresol) 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.