Rotationally resolved UV spectroscopy of the rotamers of indole-4-carboxylic acid: Evidence for charge transfer quenching

Photoenhancement of the C≡N Stretching Vibration Intensity of Aromatic Nitriles

The C≡N stretching vibration is a versatile infrared (IR) reporter that is useful for a wide range of applications. Aiming to further expand its spectroscopic utility, herein, we show that, using 4-cyanoindole and 4-cyano-7-azaindole as examples, photoexcitation can significantly shift the frequency (ν_CN) and enhance the molar extinction coefficient (ε_CN) of this vibrational mode of aromatic nitriles and that, for these indole derivatives, the enhancement factor can reach 13. Moreover, we find that while solvent relaxation at the excited electronic state(s) always leads to an increase in ε_CN, its effect on ν_CN depends on the solute and the solvent. Taken together, these results demonstrate that solvent relaxation can differently affect the local environment of the nitrile group and its conjugation with the indole ring and, more importantly, that the C≡N stretching vibration can serve as a sensitive IR probe of charge and electron transfer processes in which an aromatic nitrile is involved.

Interleaved electro-optic dual comb generation to expand bandwidth and scan rate for molecular spectroscopy and dynamics studies near 1.6 µm

A chirped-pulse interleaving method is reported for generation of dual optical frequency combs based on electro-optic phase modulators (EOM) in a free-running all-fiber based system. Methods are discussed to easily modify the linear scan rate and comb resolution by more than three orders of magnitude and to significantly increase the spectral bandwidth coverage. The agility of the technique is shown to both capture complex line shapes and to magnify rapid passage effects in spectroscopic and molecular dynamics studies of CO₂. These methods are well-suited for applications in the areas of remote sensing of greenhouse gas emissions, molecular reaction dynamics, and sub-Doppler studies across the wide spectral regions accessible to EOMs.

Tryptophan as a Template for Development of Visible Fluorescent Amino Acids

Most biological systems, at both molecular and cellular levels, are intrinsically complex, diverse, and nonfluorescent. Therefore, studying their structures, dynamics, and interactions via fluorescence-based methods requires incorporation of one or multiple external fluorophores that would not significantly affect any native property of the system in question. This requirement necessitates the development of a diverse set of fluorescence reporters that differ in chemical, physical, and photophysical properties. Herein, we offer our perspective on the need for, recent progress in, and future directions of developing tryptophan-based fluorescent amino acids.

High-resolution UV spectroscopy of 1-indanol

We report on rotationally resolved laser induced fluorescence (LIF) and vibrationally resolved resonance-enhanced multiphoton ionization (REMPI) spectroscopy of the chiral molecule 1-indanol. Spectra of the S₁← S₀ electronic transition are recorded in a jet-cooled, pulsed molecular beam. Using two time-delayed pulsed lasers, the lifetimes of the S₁ state of the two most stable conformers, referred to as eq1 and ax2, have been determined. The S₁← S₀ origin bands of these conformers as well as the transition to a vibrationally excited level in the S₁ state of eq1 are recorded with full rotational resolution (25 MHz observed linewidth) by measuring the LIF intensity following excitation with a tuneable, narrowband cw laser. On selected rotationally resolved electronic transitions, Lamb-dips are measured to confirm the Lorentzian lifetime-contribution to the observed lineshapes. The rotationally resolved S₁← S₀ origin band of a neon-complex of eq1 is measured via LIF as well. The fit of the rotationally resolved LIF spectra of the origin bands to those of an asymmetric rotor yields a standard deviation of about 6 MHz. The resulting spectroscopic parameters are tabulated and compared to the outcome of ab initio calculations. For both conformers as well as for the Ne-eq1 complex, the geometric structures in the S₀ and S₁ states are discussed. For all systems, the transition dipole moment is mainly along the a-axis, the contributions along the b- and c-axes being about one order of magnitude smaller.