RESUMO
Recent developments in fluorescence spectroscopy have made it possible to measure both absorption and dispersed fluorescence spectra of isolated molecular ions at liquid-nitrogen temperatures. Absorption is here obtained from fluorescence-excitation experiments and does not rely on ion dissociation. One large advantage of reduced temperature compared to room-temperature spectroscopy is that spectra are narrow, and they provide information on vibronic features that can better be assigned from theoretical simulations. We report on the intrinsic spectroscopic properties of oxazine dyes cooled to about 100 K. They include six cations (crystal violet, darrow red, oxazine-1, oxazine-4, oxazine-170 and nile blue) and one anion (resorufin). Experiments were done with a home-built setup (LUNA2) where ions are stored, mass-selected, cooled, and photoexcited in a cylindrical ion trap. We find that the Stokes shifts are small (14-50 cm-1), which is ascribed to rigid geometries, that is, there are only small geometrical changes between the electronic ground and excited states. However, both the absorption and the emission spectra of darrow-red cations are broader than those of the other ionic dyes, which is likely associated with a less symmetric electronic structure and more non-zero Franck-Condon factors for the vibrational progressions. In the case of resorufin, the smallest ion under study, vibrational features are assigned based on calculated spectra.
RESUMO
Bioluminescence from fireflies, click beetles, and railroad worms ranges in color from green-yellow to orange to red. The keto form of oxyluciferin is considered a key emitter species in the proposed mechanisms to account for color variation. To establish the intrinsic photophysics in the absence of a microenvironment, we present experimental and theoretical gas-phase absorption and emission spectra of the 5,5-dimethyloxyluciferin anion (keto form) at room and cryogenic temperatures as well as lifetime measurements based on fluorescence. The theoretical model includes all 75 vibrational modes. The spectral impact of the large number of excited states at elevated temperatures is captured by an effective state distribution. At low temperature, spectral congestion is greatly reduced, and the observed well-resolved vibrational features are assigned to multiple Franck-Condon progressions involving different vibrational modes. An in-plane â¼60 cm-1 scissoring mode is found to be involved in the dominant progressions.
RESUMO
We have observed nine bimolecular hydrogen- or deuterium-bound complexes at room temperature using Fourier transform infrared (FTIR) spectroscopy. The complexes were formed using methanol or ethanol as hydrogen bond donors, as well as deuterated isotopologues of these, in order to study isotopic effects on hydrogen bonds. The complexes were formed using either a dimethylether- (O) or trimethylamine (N) acceptor, to facilitate comparison of two different types of hydrogen or deuterium bonds, OH(D)·O and OH(D)·N. For each complex, the characteristic OH- or OD-stretching fundamental band in the bimolecular complex was observed. The Gibbs energy of complex formation was determined at room temperature for each complex to compare the relative stability of hydrogen- and deuterium-bound bimolecular complexes. It is well known that deuterium-bound complexes are more stable at low temperatures because of the lower frequency of its intermolecular modes and thus a lower zero point vibrational energy. However, at room temperature, entropic contributions to the stability should also be considered. At room temperature, we find the Gibbs energy of complex formation for each pair of corresponding hydrogen- and deuterium-bound complex to be similar. The similar values of the Gibbs energies at room temperature is explained from a difference in the entropy, upon complexation, which favors the formation of the hydrogen-bound complex more than the deuterium-bound complex at higher temperatures.
