Intramolecular Hydrogen Bonding in 1,8-Dihydroxyanthraquinone, 1-Aminoanthraquinone, and 9-Hydroxyphenalenone Studied by Picosecond Time-Resolved Fluorescence Spectroscopy in a Supersonic Jet

We investigated spectroscopic and dynamic fluorescence properties of the S1 ← S0 transitions of three intramolecularly 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 and on 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 the fluorescence properties of 1-AAQ are associated with a single-minimum-type potential. The nonradiative relaxation mechanism is attributed to internal conversion to the S0 state. For 1,8-DHAQ, we suggest in agreement 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 the proton-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 fluorescence originating in the 1,10-quinone well is due to vertical absorption into the 9,10-quinone well and subsequent fast ESIPT above the hydrogen transfer barrier. For 9-HPA, only the frequency-domain measurements give tentative evidence of the presence of a pronounced double-minimum-type potential. The rapid nonradiative relaxation mechanism as revealed by fluorescence lifetime measurements is attributed to intersystem crossing to a triplet state.