12 research outputs found
Nanoparticle-doped electrospun fiber random lasers with spatially extended light modes
Complex assemblies of light-emitting polymer nanofibers with molecular
materials exhibiting optical gain can lead to important advance to amorphous
photonics and to random laser science and devices. In disordered mats of
nanofibers, multiple scattering and waveguiding might interplay to determine
localization or spreading of optical modes as well as correlation effects. Here
we study electrospun fibers embedding a lasing fluorene-carbazole-fluorene
molecule and doped with titania nanoparticles, which exhibit random lasing with
sub-nm spectral width and threshold of about 9 mJ cm^-2 for the absorbed
excitation fluence. We focus on the spatial and spectral behavior of optical
modes in the disordered and non-woven networks, finding evidence for the
presence of modes with very large spatial extent, up to the 100
micrometer-scale. These findings suggest emission coupling into integrated
nanofiber transmission channels as effective mechanism for enhancing spectral
selectivity in random lasers and correlations of light modes in the complex and
disordered material.Comment: 22 pages, 6 figure
Diverse regimes of mode intensity correlation in nanofiber random lasers through nanoparticle doping
Random lasers are based on disordered materials with optical gain. These
devices can exhibit either intensity or resonant feedback, relying on diffusive
or interference behaviour of light, respectively, which leads to either
coupling or independent operation of lasing modes. We study for the first time
these regimes in complex, solid-state nanostructured materials. The number of
lasing modes and their intensity correlation features are found to be
tailorable in random lasers made of light-emitting, electrospun polymer fibers
upon nanoparticle doping. By material engineering, directional waveguiding
along the length of fibers is found to be relevant to enhance mode correlation
in both intensity feedback and resonant feedback random lasing. The here
reported findings can be used to establish new design rules for tuning the
emission of nano-lasers and correlation properties by means of the
compositional and morphological properties of complex nanostructured materials.Comment: 30 pages, 10 figure
The Role of Triplet Exciton Diffusion in Light-Upconverting Polymer Glasses
Light upconversion (UC) via triplet-triplet annihilation (TTA) by using noncoherent photoexcitation at subsolar irradiance power densities is extremely attractive, particularly for enhanced solar energy harvesting. Unfortunately, practical TTA-UC application is hampered by low UC efficiency of upconverting polymer glasses, which is commonly attributed to poor exciton diffusion of the triplet excitons across emitter molecules. The present study addresses this issue by systematically evaluating triplet exciton diffusion coefficients and diffusion lengths (LD) in a UC model system based on platinum-octaethylporphyrin-sensitized poly(methyl methacrylate)/diphenylanthracene (emitter) films as a function of emitter concentration (15-40 wt %). For this evaluation time resolved photoluminescence bulk-quenching technique followed by Stern Volmer-type quenching analysis of experimental data was employed. The key finding is that although increasing emitter concentration in the disordered PMMA/DPA/PtOEP films improves triplet exciton diffusion, and thus LD, this does not result in enhanced UC quantum yield. Conversely, improved LD accompanied by the accelerated decay of UC intensity on millisecond time scale degrades TTA-UC performance at high emitter loadings (\u3e25 wt %) and suggests that diffusion-enhanced nonradiative decay of triplet excitons is the major limiting factor
Thermal and Optical Properties of Red Luminescent Glass Forming Symmetric and Non Symmetric Styryl-4H-Pyran-4-Ylidene Fragment Containing Derivatives
Dyes with amorphous structure deposited from organic solvents and having good fluorescence properties
show potential for photonic device applications. Organic glass-forming symmetric and non symmetric
styryl- derivatives of 2(2,6-substituted-4H-pyran-4-ylidene)-malononitrile (it has backbone of known
laser dye 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran), 2(2,6-substituted-
4H-pyran-4-ylidene)-1H-indene-1,3(2H)-dione and 2(2,6-substituted-4H-pyran-4-ylidene)-pyrimidine-
2,4,6(1H,3H,5H)-trione were synthesized and investigated. Glass transition temperatures higher than
110 C were achieved. The absorption bands in dichloromethane solution cover the spectral region from
450 nm to 600 nm with fluorescence maxima between 580 nm and 690 nm. Photoluminescence quantum
yields of the compounds in solution are between 0.3 and 0.54, which is reduced by one order in thin
amorphous film prepared from volatile organic solvents. Incorporation of bulky trityloxyethyl groups in
the derivatives results in significant reduction of aggregate formation. Thus fluorescence concentration
quenching is reduced, enabling higher doping levels as compared to the unsubstituted 4-(dicyanomethylene)-
2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran dye
Synthesis and Properties of Ditriazolylpurine Nucleosides
The synthesis of new 2,6-bis-ditriazolylpurine nucleosides is described and reactions with different nucleophiles studied
Structure Properties Relationship of Donor–Acceptor Derivatives of Triphenylamine and 1,8-Naphthalimide
Solution-processable donor–acceptor molecules
consisting
of triphenylamine core and 1,8-naphthalimide arms were designed and
synthesized by palladium-catalyzed Heck reaction. Dilute solutions
of the synthesized compounds show strong absorption peaks in the visible
wavelength range from 400 to 550 nm, which can be ascribed to the
intramolecular charge transfer. Fluorescence quantum yields of dilute
solutions of the synthesized materials range from 0.45 to 0.70, while
those of the solid samples are in the range of 0.09–0.18. The
synthesized molecules exhibit high thermal stability with the thermal
degradation onset temperatures ranging from 431 to 448 °C. The
compounds form glasses with glass-transition temperatures of 55–107
°C. DFT calculations show that HOMO and LUMO orbitals are almost
entirely localized on the donor and acceptor moieties, respectively.
Consequently, the frontier orbital energies for the three synthesized
compounds are similar and practically do not depend on the number
of 1,8-naphthalimide moieties. Ionization potentials of the solid
samples (5.75–5.80 eV) are comparable. The charge-transporting
properties of the synthesized materials were studied using xerographic
time-of-flight method. Hole mobilities in the layers of the compounds
having one and two 1,8-naphthalimide moieties exceed 10<sup>–3</sup> cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> at high electric fields at room temperature. The differences on
the hole mobilities between the three synthesized compounds are discussed
in the frame of Marcus theory by comparing the reorganization energy
and electronic coupling parameters
Proof of principle of a purine D–A–D′ ligand based ratiometric chemical sensor harnessing complexation induced intermolecular PET
A comprehensive photophysical study of a series of purines, doubly decorated at C2 and C6 positions with identical fragments ranging from electron acceptor to donor groups of different strengths, is presented. The asymmetry of substitutions creates a unique molecular D-AD 0 structure possessing two independent electronic charge transfer (CT) systems attributed to each fragment and exhibiting dual-band fluorescence. Moreover, the inherent property of coordination of metal ions by purines was enriched due to a presence of nearby triazoles used as spacers for donor or acceptor fragments. New molecules present a bidentate coordination mode, which makes the assembly of several ligands with one metal cation possible. This property was exploited to create a new concept of a ratiometric chemical fluorescence sensor involving the photoinduced electron transfer between branches of different ligands as a mechanism of fluorescence modulation
Proof of principle of a purine D–A–D′ ligand based ratiometric chemical sensor harnessing complexation induced intermolecular PET
A comprehensive photophysical study of a series of purines, doubly decorated at C2 and C6 positions with identical fragments ranging from electron acceptor to donor groups of different strengths, is presented. The asymmetry of substitutions creates a unique molecular D-AD 0 structure possessing two independent electronic charge transfer (CT) systems attributed to each fragment and exhibiting dual-band fluorescence. Moreover, the inherent property of coordination of metal ions by purines was enriched due to a presence of nearby triazoles used as spacers for donor or acceptor fragments. New molecules present a bidentate coordination mode, which makes the assembly of several ligands with one metal cation possible. This property was exploited to create a new concept of a ratiometric chemical fluorescence sensor involving the photoinduced electron transfer between branches of different ligands as a mechanism of fluorescence modulation
V‑Shaped Hole-Transporting TPD Dimers Containing Tröger’s Base Core
V-shaped hole transporting
materials based on <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetraarylbenzidine (TPD)-type
moieties conjoined by Tröger’s
base core were synthesized and investigated. These hole transporting
materials were obtained by a three-step synthetic method, are fully
amorphous, and demonstrate high glass transition temperatures and
good thermal and morphological stability. Relatively high charge mobility
(up to 0.036 cm<sup>2</sup> V <sup>–1</sup> s<sup>–1</sup>) was measured in these hole transporting materials, exceeding that
of corresponding methyl and methoxy substituted TPD analogues without
TB core by more than 2 orders of magnitude. Determined ionization
potential and charge mobility values permit use of the synthesized
compounds as hole transporting materials in fabrication of perovskite
solar cells