280 research outputs found
Breakdown of the mirror image symmetry in the optical absorption/emission spectra of oligo(para-phenylene)s
The absorption and emission spectra of most luminescent, pi-conjugated,
organic molecules are the mirror image of each other. In some cases, however,
this symmetry is severely broken. In the present work, the asymmetry between
the absorption and fluorescence spectra in molecular systems consisting of
para-linked phenyl rings is studied. The vibronic structure of the emission and
absorption bands is calculated from ab-initio quantum chemical methods and a
subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is
achieved. A clear relation can be established between the strongly anharmonic
double-well potential for the phenylene ring librations around the long
molecular axis and the observed deviation from the mirror image symmetry.
Consequences for related compounds and temperature dependent optical
measurements are also discussed.Comment: 12 pages, 13 Figure
Bent-core liquid crystalline cyanostilbenes : fluorescence switching and thermochromism
The authors from ICMA greatly appreciate financial support from the Spanish Government (MINECO-FEDER project MAT2012-38538-C03-01), the Aragon's Government and FSE (project E04) and the Jae PreDoc-CSIC (M. M.-A.) fellowship program.Fluorescent bent-core molecules, bearing one or two cyanostilbene units in the lateral structure and different positions of the cyano group (α- or β-isomers), are described with the aim of modulating the molecular packing and fluorescence properties. These compounds give rise to a variety of crystal polymorphs and bent-core liquid crystalline phases (SmCP, Colr and B6), offering the unique chance to study the fluorescence properties of the cyanostilbene structure in different phases. Experimental and computational studies elucidate geometrical and electronic properties of these bent-core structures but especially the fluorescence properties (spectral positions, quantum yields and decay curves), in a detailed comparison between diluted solutions, in dichloromethane (DCM) or poly(methylmethacrylate) (PMMA), and condensed phases. Quantum yields as high as 70% have been obtained in some diluted solutions (PMMA) and condensed phases. Remarkably, the quantum yield values depend on the position of the cyano group, being higher for β- than for the α-isomers due to the higher radiative rates and lower non-radiative rates of the former. The photophysical characterization in the condensed phase focuses on RT studies with solid samples and different processing, and show that, upon aggregation, interactions between the cyanostilbene groups result in changes of the emission spectra and dynamics compared to the diluted systems in DCM and PMMA, giving rise to H-aggregations of varying strength. Furthermore, the compounds exhibit thermochromism, showing a green-yellow fluorescence in the pristine crystalline phase that changes to blue on heating to the liquid crystal phase. This journal isPostprintPeer reviewe
Efficient deep-red light-emitting electrochemical cells based on a perylenediimide-iridium-complex dyad
A two-layer light-emitting electrochemical cell device based on a new perylenediimide-iridium-complex dyad is presented emitting in the deep-red region with high external quantum efficiencies
(3.27%).Costa Riquelme, Ruben Dario, [email protected] ; Orti Guillen, Enrique, [email protected] ; Bolink, Henk, [email protected] ; Gierschner, Johannes, [email protected]
Highly Efficient Photocatalytic Hydrogen Evolution Using a Self-Assembled Octupolar Molecular System
Structure, Photophysics and the Order-Disorder Transition to the Beta Phase in Poly(9,9-(di -n,n-octyl)fluorene)
X-ray diffraction, UV-vis absorption and photoluminescence (PL) spectroscopy
have been used to study the well-known order-disorder transition (ODT) to the
beta phase in poly(9,9-(di n,n-octyl)fluorene)) (PF8) thin film samples through
combination of time-dependent and temperature-dependent measurements. The ODT
is well described by a simple Avrami picture of one-dimensional nucleation and
growth but crystallization, on cooling, proceeds only after molecular-level
conformational relaxation to the so called beta phase. Rapid thermal quenching
is employed for PF8 studies of pure alpha phase samples while extended
low-temperature annealing is used for improved beta phase formation. Low
temperature PL studies reveal sharp Franck-Condon type emission bands and, in
the beta phase, two distinguishable vibronic sub-bands with energies of
approximately 199 and 158 meV at 25 K. This improved molecular level structural
order leads to a more complete analysis of the higher-order vibronic bands. A
net Huang-Rhys coupling parameter of just under 0.7 is typically observed but
the relative contributions by the two distinguishable vibronic sub-bands
exhibit an anomalous temperature dependence. The PL studies also identify
strongly correlated behavior between the relative beta phase 0-0 PL peak
position and peak width. This relationship is modeled under the assumption that
emission represents excitons in thermodynamic equilibrium from states at the
bottom of a quasi-one-dimensional exciton band. The crystalline phase, as
observed in annealed thin-film samples, has scattering peaks which are
incompatible with a simple hexagonal packing of the PF8 chains.Comment: Submitted to PRB, 12 files; 1 tex, 1 bbl, 10 eps figure
Shear-triggered crystallization and light emission of a thermally stable organic supercooled liquid
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