4 research outputs found
Solvent Effect on the Stokes Shift and on the Nonfluorescent Decay of the Daidzein Molecular System
The flavonoids have been the target
of several experimental works
due to its influence in the human health as antioxidant elements.
The fluorescence properties of these compounds have been widely studied
due to the large Stokes shifts experimentally observed and the variety
of processes that lead to the fluorescence. In the present work the
role of the solvent in the large Stokes shift experimentally observed
in the daidzein molecular system in water is theoretically studied.
Also studied is the nonfluorescent decay mechanism in a polar aprotic
solvent like acetonitrile. The solvent effect in the ground and in
the low-lying excited electronic states is taken into account by using
the sequential-QM/MM methodology. Excited state properties like equilibrium
geometries and transition energies were studied by using multiconfigurational
calculations, CASSCF and CASPT2. The excited electronic state responsible
for the fluorescence spectrum in water was identified, and the large
Stokes shift seems to be the result of the large interaction of the
system in this electronic state with the solvent. On the other hand,
spin–orbit coupling calculations, between the singlet and triplet
electronic states, indicate favorable conditions for intersystem crossing,
in agreement with the experimental result of nonfluorescence observation
Oxazole Dyes with Potential for Photoluminescence Bioprobes: A Two-Photon Absorption Study
In
this work, six π-conjugated oxazole compounds dissolved
in dichloromethane were characterized with linear and nonlinear optical
measurements. Z-scan with femtosecond laser pulses was employed to
determine the two-photon absorption (TPA) spectra. Other photophysical
parameters, such as absorbance, solvatochromism, lifetime fluorescence,
and fluorescence anisotropy, were evaluated with linear optical techniques.
The experimental TPA cross section spectra were adjusted by the sum-over-states
(SOS) model, by which important parameters such as transition dipole
moments and broadening parameters were determined. To better understand
the TPA spectra of the oxazole compounds, quantum-chemical calculations
using the response function formalism and the density functional theory
level of theory were performed. Using the results provided by the
quantum-chemical calculations and the broadening parameters estimated
through the application of the SOS model, the TPA spectra were simulated
by the superposition (summation) of individual homogeneous Lorentzian
absorption profiles
Revealing the Electronic and Molecular Structure of Randomly Oriented Molecules by Polarized Two-Photon Spectroscopy
In
this Letter, we explored the use of polarized two-photon absorption
(2PA) spectroscopy, which brings additional information when compared
to methods that do not use polarization control, to investigate the
electronic and molecular structure of two chromophores (<b>FD43</b> and <b>FD48</b>) based on phenylacetylene moieties. The results
were analyzed using quantum chemical calculations of the two-photon
transition strengths for circularly and linearly polarized light,
provided by the response function formalism. On the basis of these
data, it was possible to distinguish and identify the excited electronic
states responsible for the lowest-energy 2PA-allowed band in both
chromophores. By modeling the 2PA circular–linear dichroism,
within the sum-over-essential states approach, we obtained the relative
orientation between the dipole moments that are associated with the
molecular structure of the chromophores in solution. This result allowed
to correlate the V-shape structure of the <b>FD48</b> chromophore
and the quantum-interference-modulated 2PA strength
Experimental and Theoretical Study on the One- and Two-Photon Absorption Properties of Novel Organic Molecules Based on Phenylacetylene and Azoaromatic Moieties
This Article reports a combined experimental and theoretical
analysis
on the one and two-photon absorption properties of a novel class of
organic molecules with a π-conjugated backbone based on phenylacetylene
(<b>JCM874</b>, <b>FD43</b>, and <b>FD48</b>) and
azoaromatic (<b>YB3p25</b>) moieties. Linear optical properties
show that the phenylacetylene-based compounds exhibit strong molar
absorptivity in the UV and high fluorescence quantum yield with lifetimes
of approximately 2.0 ns, while the azoaromatic-compound has a strong
absorption in the visible region with very low fluorescence quantum
yield. The two-photon absorption was investigated employing nonlinear
optical techniques and quantum chemical calculations based on the
response functions formalism within the density functional theory
framework. The experimental data revealed well-defined 2PA spectra
with reasonable cross-section values in the visible and IR. Along
the nonlinear spectra we observed two 2PA allowed bands, as well as
the resonance enhancement effect due to the presence of one intermediate
one-photon allowed state. Quantum chemical calculations revealed that
the 2PA allowed bands correspond to transitions to states that are
also one-photon allowed, indicating the relaxation of the electric-dipole
selection rules. Moreover, using the theoretical results, we were
able to interpret the experimental trends of the 2PA spectra. Finally,
using a few-energy-level diagram, within the sum-over-essential states
approach, we observed strong qualitative and quantitative correlation
between experimental and theoretical results