  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
