Electronic spectra and excited-state dynamics of acridine and its hydrated clusters

We combine results from several different experiments to investigate the photophysics of acridine (Ac) and its hydrated clusters in the gas phase. Our findings are also compared with results from condensed phase studies. Similar to measurements of Ac dissolved in hydrocarbons, the lifetime of the first electronically excited state of isolated Ac in vacuum is too short for typical resonantly enhanced multiphoton ionization (REMPI) and laser induced fluorescence (LIF) experiments, hence no signal from REMPI and LIF can be attributed to monomeric Ac. Instead, sensitized phosphorescence emission spectroscopy is more successful in revealing the electronic states of Ac. Upon clustering with water, on the other hand, the lifetimes of the excited states are substantially increased to the nanosecond scale, and with two water molecules attached to Ac, the lifetime of the hydrated cluster is essentially the same as that of Ac in aqueous solutions. Detailed REMPI and ultraviolet-ultraviolet hole-burning experiments are then performed to reveal the structural information of the hydrated clusters. Although the formation of hydrogen bonds results in energy level reversal and energy separation between the first two excited states of Ac, its effect on the internal geometry of Ac is minimal, and all clusters with 1-3 water molecules demonstrate consistent intramolecular vibrational modes. Theoretical calculations reveal just one stable structure for each cluster under supersonic molecular beam conditions. Furthermore, different from mono- and di-water clusters, tri-water clusters consist of a linear chain of three water molecules attached to Ac. Consequently, the fragmentation pattern in the REMPI spectrum of tri-water clusters seems to be dominated by water trimer elimination, since the REMPI spectrum of Ac⁺·W₃ is largely reproduced in the Ac⁺ mass channel, but not in the Ac⁺·W₁ or Ac⁺·W₂ channel.