The gas phase structure of 2-aminophenol has been investigated using UV-UV as well as IR-UV hole burning spectroscopy. The presence of a free OH vibration in the IR spectrum rules out the contribution of the cis isomer, which is expected to have an intramolecular H-bond, to the spectra. The excited state lifetimes of different vibronic levels have been measured with pump-probe picosecond experiments and are all very short (35 ± 5) ps as compared to other substituted phenols. The electronic states and active vibrational modes of the cis and trans isomers have been calculated with ab initio methods for comparison with the experimental spectra. The Franck-Condon simulation of the spectrum using the calculated ground and excited state frequencies of the trans isomer is in good agreement with the experimental one. The very short excited state lifetime of 2-aminophenol can then be explained by the strong coupling between the two first singlet excited states due to the absence of symmetry, the geometry of the trans isomer being strongly nonplanar in the excited state.

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.

The excited state dynamics of the H-bonded 7-azaindole-phenol complex (7AI-PhOH) has been studied by combination of picosecond pump and probe experiments, LIF measurements on the nanosecond time scale and ab initio calculations. A very short S(1) excited state lifetime (30 ps) has been measured for the complex upon excitation of the 0(0)(0) transition and the lifetime remains unchanged when the ν(6) vibrational mode (0(0)(0) + 127 cm(-1)) is excited. In addition, no UV-visible fluorescence was observed by exciting the complex with nanosecond pulses. Two possible deactivation channels have been investigated by ab initio calculations: first an excited state tautomerization assisted by a concerted double proton transfer (CDPT) and second an excited state concerted proton electron transfer (CPET) that leads to the formation of a radical pair (hydrogenated 7AIH(●) radical and phenoxy PhO(●) radical). Both channels, CDPT and CPET, seem to be opened according to the ab initio calculations. However, the analysis of the ensemble of experimental and theoretical evidence indicates that the excited state tautomerization assisted by CDPT is quite unlikely to be responsible for the fast S(1) state deactivation. In contrast, the CPET mechanism is suggested to be the non-radiative process deactivating the S(1) state of the complex. In this mechanism, the lengthening of the OH distance of the PhOH molecule induces an electron transfer from PhOH to 7AI that is followed by a proton transfer in the same kinetic step. This process leads to the formation of the radical pair (7AIH(●)···PhO(●)) in the electronically excited state through a very low barrier or to the ion pair (7AIH(+)···PhO(-)) in the ground state. Moreover, it should be noted that, according to the calculations the πσ* state, which is responsible for the H loss in the free PhOH molecule, does not seem to be involved at all in the quenching process of the 7AI-PhOH complex.