Gas-Phase Spectroscopy of Laser-Desorbed Acedan and Proline-Acedan

Abstract

Acedan (2-acetyl-6-dialkylaminonaphthalene) and prolineacedan (A and B in Scheme 1, respectively) are precursors for two-photon fluorescent probes such as ANO1, which turns on its fluorescence when bound to nitric oxide (NO). In spite of their similar molecular structures, A and B have different photophysical properties including absorption coefficient and fluorescence quantum yield. In order to prove the hypothesis that their different fluorescence properties stem from their different conformational structures, we tried to measure their conformation-specific UV and IR spectra in gas phase. The experimental setup has been described in previous publications. In-house synthesized powder sample of A or B was mixed with carbon black powder and the mixture was deposited on the lateral face of a graphite disk. The sample was desorbed by 1064 nm radiation of a pulsed Nd:YAG laser in vacuum, and the desorbed molecules were subsequently cooled down by pulsed supersonic expansion of Ar gas at 40 bar. UV spectra were measured by two-color resonance-enhanced multiphoton ionization (REMPI) with frequency-doubled output of Nd:YAG pumped dye lasers and a time-of-flight mass spectrometer. IR laser was provided by difference frequency generation of a dye laser and an Nd:YAG laser. IR-UV spectroscopy was used to record conformer-specific IR spectrum while the IR was scanned, and conformer-specific UV spectrum while the UV was scanned. Figure 1 shows 1+1' REMPI spectra of B (m/z = 283) and its fragments (m/z = 182, 241) with the second UV at 266 nm. Even though laser-desorption was employed to vaporize the sample, significant fragmentation was observed in the photoionization mass spectrum. The fragment of B at m/z = 241 (241) shows much stronger ion signal than the molecular ion (B, m/z = 283). As B and 241 showed similar vibronic bands in this region, it was assumed that 241 is issued from dissociative ionization of B, and the strong REMPI spectrum of 241 was presumed to reflect that of B. Figure 2 shows the 1+1' REMPI spectrum of 241 at lower