Hydrogen bonds vs. π-stacking interactions in the p-aminophenolp-cresol dimer: an experimental and theoretical study.
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-1with 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 S1state 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)2homodimer 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.
Instituto de Investigaciones en Físico Química de Córdoba (INFIQC) CONICET - UNC. Dpto. de Fisicoquímica - Facultad de Ciencias Químicas - Centro Láser de Ciencias Moleculares - Universidad Nacional de Córdoba, Ciudad Universitaria, X5000HUA Córdoba, Argentina. gpino@fcq.unc.edu.ar.
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Capello
Instituto de Investigaciones en Físico Química de Córdoba (INFIQC) CONICET - UNC. Dpto. de Fisicoquímica - Facultad de Ciencias Químicas - Centro Láser de Ciencias Moleculares - Universidad Nacional de Córdoba, Ciudad Universitaria, X5000HUA Córdoba, Argentina. gpino@fcq.unc.edu.ar.
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Hernández
Centre Laser de l'Université Paris Sud (CLUPS/LUMAT), Univ. Paris-Sud, CNRS, Institut d'Optique Graduate School, Univ. Paris-Saclay, F-91405 Orsay, France and Institut des Sciences Moléculaires d'Orsay (ISMO), CNRS, Univ. Paris-Sud, Univ. Paris-Saclay, F-91405 Orsay, France.
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Broquier
Aix Marseille Université, CNRS, PIIM UMR 7345, Marseille, 13397, France.
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Dedonder-Lardeux
Aix Marseille Université, CNRS, PIIM UMR 7345, Marseille, 13397, France.
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Jouvet
Instituto de Investigaciones en Físico Química de Córdoba (INFIQC) CONICET - UNC. Dpto. de Fisicoquímica - Facultad de Ciencias Químicas - Centro Láser de Ciencias Moleculares - Universidad Nacional de Córdoba, Ciudad Universitaria, X5000HUA Córdoba, Argentina. gpino@fcq.unc.edu.ar.
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Pino