| Abstract
| - We investigated spectroscopic and dynamic fluorescence properties of the S1 ← S0 transitions of threeintramolecularly hydrogen-bonded molecules, 1,8-dihydroxyanthraquinone (1,8-DHAQ), 1-aminoanthraquinone(1-AAQ), and 9-hydroxyphenalenone (9-HPA), by determining their fluorescence excitation spectra and state-selective fluorescence lifetimes under supersonic jet conditions. Moreover, ab initio calculations were performed on one-dimensional hydrogen transfer potential energy curves in both the S0 and the S1 state andon S0 and S1 minimum energy conformations and normal-mode frequencies at different levels of theory(HF/6-31G(d,p) and B3LYP/6-31G(d,p), CIS/6-31G(d,p) and TDDFT/6-31G(d,p)//CIS/6-31G(d,p), respectively). In line with calculations based on the theory of “atoms in molecules” (AIM), we suggest that thefluorescence properties of 1-AAQ are associated with a single-minimum-type potential. The nonradiativerelaxation mechanism is attributed to internal conversion to the S0 state. For 1,8-DHAQ, we suggest inagreement with previous findings that the fluorescence bands below ∼600 cm-1 are due to transitions originating in the 9,10-quinone well, whereas the bands above ∼600 cm-1 are due to transitions originating in theproton-transferred 1,10-quinone well, thus confirming the assumption that 1,8-DHAQ possesses a double-minimum-type S1 potential. On the basis of our ab initio calculations, we suggest that the fluorescenceoriginating in the 1,10-quinone well is due to vertical absorption into the 9,10-quinone well and subsequentfast ESIPT above the hydrogen transfer barrier. For 9-HPA, only the frequency-domain measurements givetentative evidence of the presence of a pronounced double-minimum-type potential. The rapid nonradiativerelaxation mechanism as revealed by fluorescence lifetime measurements is attributed to intersystem crossingto a triplet state.
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