Probing chirality fluctuations in molecules by nonlinear optical spectroscopy

Vibronic Intramolecular Resonant Energy Transfer along More than 5 nm: Synthesis of Dyads for a Re-Examination of the Distance Function of FRET

Dyads of chromophores with orthogonal transition moments and sequences of aliphatic cage-structures as spacers were prepared where resonance energy transfer (FRET) proceeded in contrast to Förster's theory even until 58 Å. The distance dependence of the efficiency was re-examined by means of various functions; the commonly used R^-6 dependence gave acceptable results, but a slightly larger exponent was more useful for practical applications.

Chalcone-based molecules: Experimental and theoretical studies on the two-photon absorption and molecular first hyperpolarizability

Five chalcone-based molecules denominated by C-3 ((E)-1-(4-methoxyphenyl)-3-phenylprop-2-en-1-one), C-4 ((E)-1,3-bis(4-methoxyphenyl)prop-2-en-1-one), C-5 ((E)-1-(benzo[d][1,3]dioxol-5-yl)-3-(4-methoxyphenyl)prop-2-en-1-one), C-6 ((E)-3-(naphthalen-1-yl)-1-phenylprop-2-en-1-one) and C-7 ((E)-1-(4-methoxyphenyl)-3-(naphthalen-1-yl)prop-2-en-1-one) were synthesized by Claisen-Schmidt reaction in solution of NaOH in water/ethanol 2:1. The aldehydes used were benzaldehyde, anisaldehyde, and β-naphthaldehyde, while the used ketones were acetophenone, p-methoxyacetophenone, and 3,4-methylenedioxyacetophenone. Z-scan and hyper-Rayleigh scattering techniques were used to study the nonlinear optical properties of these compounds in dichloromethane medium. By using Z-scan technique with femtosecond pulses, two-photon absorption cross-sections (σ^TPA) were determined, while the first molecular electronic hyperpolarizabilities (β^HRS) were evaluated by the hyper-Rayleigh scattering technique, with picosecond pulses. From the recorded two-photon absorption spectra, it was identified that compound C-7 presented the highest σ^TPA, regarding the HOMO-LUMO transition, with a value of 40 GM, while C-6 achieved the lowest value for the same transition with 13 GM. Concerning the values of the first molecular hyperpolarizability, compound C-4 presented the highest value, 38 × 10^-30 cm⁴ statvolt^-1, while C-3 presented the lowest β^HRS value of about 16 × 10^-30 cm⁴ statvolt^-1. Time-dependent density functional theory calculations were used to simulate the one- and two-photon absorption spectra, as well to predict the theoretical value of β^HRS in dichloromethane and vacuum medium.

Isolating the chiral contribution in optical two-dimensional chiral spectroscopy using linearly polarized light

The full development of mono- or multi-dimensional time-resolved spectroscopy techniques incorporating optical activity signals has been strongly hampered by the challenge of identifying the small chiral signals over the large achiral background. Here we propose a new methodology to isolate chiral signals removing the achiral background from two commonly used configurations for performing two-dimensional optical spectroscopy, known as BOXCARS and gradient assisted photon echo spectroscopy (GRAPES). It is found that in both cases an achiral signal from an isotropic system can be completely eliminated by small manipulations of the relative angles between the linear polarizations of the four input laser pulses. Starting from the formulation of a perturbative expansion of the signal in the angle between the beams and the propagation axis, we derive analytic expressions that can be used to estimate how to change the polarization angles of the four pulses to minimize achiral contributions in the studied configurations. The generalization to any other possible experimental configurations has also been discussed.