10 research outputs found
Thio-Mayan-like Compounds: Excited State Characterization of Indigo Sulfur Derivatives in Solution and Incorporated in Palygorskite and Sepiolite Clays
Following what may mirror the recipe used by the ancient
Maya civilization,
consisting of a mixture of indigo and palygorskite, leading to the
organic/inorganic pigment known as Maya Blue, we have switched indigo
with thioindigo (<b>TI</b>) and a derivative (Ciba Brilliant
Pink, <b>CBP</b>) and synthesized what was Christianized as
āMaya Pinkā. The spectral and photophysical behavior
of <b>TI</b> and <b>CBP</b> was investigated in solution
and solid state (in powder and incorporated in palygorskite clays).
In solution, <b>TI</b> was investigated in different organic
solvents (benzene, toluene, and dioxane) at room and low temperatures
and further compared with indigo. <b>TI</b> displays a different
spectral and photophysical behavior when compared to indigo: fluorescence
dominating the deactivation of the first excited singlet state (50ā70%
depending on the solvent), with the S<sub>1</sub>ć°āS<sub>0</sub> internal conversion and the S<sub>1</sub>ć°āT<sub>1</sub> intersystem crossing deactivation channels representing the
remaining <i>quanta</i> loss. Moreover, whereas in the case
of indigo the proton transfer process in the excited state (between
the NāH and Cī»O groups), involving the formation of
two excited species, leads to an efficient internal conversion radiationless
process, with <b>TI</b> this process is precluded leading to
a single exponential fluorescence decay. Incorporation of <b>TI</b> and <b>CBP</b> in palygorskite and sepiolite clays has showed
different photophysical properties with decrease of the fluorescence
quantum yields, a single exponential decay for <b>TI</b> and
a multiexponential decay for <b>CBP</b> in palygorskite. Based
on molecular fluorescence data, potential locations of <b>TI</b> and <b>CBP</b> in the clay are equated
Chain Length Dependent Excited-State Decay Processes of Diluted PF2/6 Solutions
The excited-state dynamics of a series
of four polyĀ[2,7-(9,9-bisĀ(2-ethylhexyl)Āfluorene]
fractions, PF2/6, with different chain length (degrees of polymerization
DP: 5, 10, 39, and 205) was investigated in dilute solutions by steady-state
and time-resolved fluorescence techniques. Two decay components are
extracted from time-resolved fluorescence experiments in the picosecond
time domain: a chain length dependent, fast decay time (Ļ<sub>2</sub>) for shorter emission wavelengths (ranging from 30 to 41
ps), which is associated with a rising component at longer wavelengths,
and a longer decay time, Ļ<sub>1</sub> (ranging from 387 to
452 ps). The system was investigated with kinetic formalisms involving
(i) a two-state system (A and B) involving conformational relaxation
of the initially excited PF2/6 segment (A) under formation of a more
planar (B) relaxed state and (ii) a time-dependent red shift of the
emission spectrum using the Stokes shift correlation function (SSCF).
In the case of (i), the kinetic scheme was solved considering the
simultaneous excitation of A and B or only of A, and the rate constants
for formation [<i>k</i>ā²<sub>CR</sub> or <i>k</i>ā²<sub>CR</sub>(Ī±)], dissociation (<i>k</i><sub>āCR</sub>), and deactivation (<i>k</i><sub>B</sub><sup>*</sup>) were obtained
together with the fraction of species A and B present in the ground
state. The use of the SSCF in (ii) was found to be more adequate leading
to a decay law with a 3.4 ps component (associated with the slow part
of the solvation dynamics process) and a longer decay (43.3 ps) associated
with the conformational/torsional relaxation process with a rate constant <i>k</i><sub>CR</sub>. This longer component of the SSCF was found
to be identical to the short-living decay (Ļ<sub>2</sub>) component
of the biexponential decays, displaying an Arrhenius-type behavior
with activation energy values in the range 5.8ā8.9 kJ mol<sup>ā1</sup> in toluene and 6.5ā10.7 kJ mol<sup>ā1</sup> in decalin. From the dependence of the fast decay component (<i>k</i><sub>CR</sub> ā” 1/Ļ<sub>2</sub>) on solvent
viscosity and temperature, the activation energy for the conformational
relaxation process was found to be distinctly dependent on the chain
length, with the relaxation rate dependence with the solvent viscosity
(<i>k</i><sub>CR</sub> ā Ī·<sup>āĀ Ī³</sup>) displaying Ī³ = 1 for the oligomer fraction with DP = 5 (i.e., <i>k</i><sub>CR</sub> is associated with a pure diffusion-controlled
process) and Ī³ < 1 for the higher molecular weight PF2/6
fractions (with DP = 10, 39, 205). This happens because of a decreased
conformational barrier between nonrelaxed and relaxed states promoted
by the polymer skeleton
Excited State Characterization and Energy Transfer in Hyperbranched Polytruxenes and Polytruxene-<i>block</i>-Polythiophene Multiblock Copolymers
A comprehensive investigation of the excited state characteristics
of two hyperbranched truxene polymers [one end-terminated with polyĀ(3-hexylthiophene)
blocks, P3HT] and a bistruxene model compound has been undertaken
aiming to rationalize its inherent photophysical properties, including
the energy transfer processes between the truxene (donor) and P3HT
(acceptor) moieties. The study comprises qualitative absorption, emission,
and triplet-singlet difference spectra, together with quantitative
measurements of quantum yields (fluorescence, intersystem crossing,
internal conversion and singlet oxygen formation) and fluorescence
decay times. From the time-resolved data in solvents of different
viscosity and as a function of temperature, it was established that
with the P3HT-terminated hyperbranched polytruxene, the excited state
deactivation mainly results from energy transfer and that conformational
relaxation is absent in these systems, which gives further support
for the rigidity of these polymers both in the ground and excited
state. An energy transfer efficiency of 91% was obtained at room temperature.
From a qualitative analysis of the data, it was also seen that radiationless
processes (particularly the S<sub>1</sub>ā¼ā¼āS<sub>0</sub> internal conversion channel) mainly contribute to the excited
state deactivation of the hyperbranched polytruxenes, a behavior that
is in contrast to what was observed for the bistruxene model compound.
Spectral and fluorescence time-resolved data in thin films was also
obtained and compared with the solution data
Controlling the Fluorescence Behavior of 1āPyrenesulfonate by Cointercalation with a Surfactant in a Layered Double Hydroxide
ZnāAl
layered double hydroxides (LDHs) containing solely
1-pyrenesulfonate (PS) or 1-heptanesulfonate (HS) anions, or a mixture
of the two with HS/PS molar ratios ranging between ca. 7.5 and 82,
were prepared by the direct synthesis method and characterized by
powder X-ray diffraction, thermal and elemental analyses, scanning
electron microscopy, and FT-IR, FT-Raman, and <sup>13</sup>CĀ{<sup>1</sup>H} CP MAS NMR spectroscopies. Well-ordered intercalates were
obtained with basal spacings of 18.8 Ć
for the LDH intercalated
by PS and 19.2ā19.4 Ć
for the other materials containing
HS. The photophysics of the solids, as well as the PS probe dissolved
in water and common organic solvents (aiming to compare the behavior
of the āisolatedā molecule with that in the solid),
were investigated by steady-state and time-resolved fluorescence techniques.
The fluorescence spectra of the solid samples display two bands with
maxima at 376 and 495 nm. Depending on the HS/PS ratios, the band
intensity ratio (obtained at 375 and 520 nm) changes, reflecting different
contributions from monomer and dimer species. The decays collected
at 375 nm are biexponentials with a major component (ā¼97% of
the total fluorescence) of 105 ns for the highest HS/PS ratio, which
further loses importance with an increase in the PS content. When
the decays are collected at 480 and 520 nm, the fits are triexponentials
with a major component varying from 108 to 124 ns, attributed to an
excimer. Steady-state and time-resolved measurements with PS in solution
(ethanol, methanol, DMF, DMSO, and water) were also measured, and
a comparison of the vibronic <i>I</i><sub>1</sub>/<i>I</i><sub>3</sub> ratio and lifetimes in water (65 ns) with
those in the LDHs indicates that the PS probe in the cointercalated
LDHs is surrounded by the HS surfactant
TriphenylamineāBenzimidazole Derivatives: Synthesis, Excited-State Characterization, and DFT Studies
The synthesis and comprehensive characterization
of the excited
states of four novel triphenylamineābenzimidazole derivatives
has been undertaken in solution (ethanol and methylcyclohexane) at
room temperature. This includes the determination of the absorption,
fluorescence, and tripletātriplet absorption spectra, together
with quantum yields of fluorescence, internal conversion, intersystem
crossing, and singlet oxygen. From the overall data the radiative
and radiationless rate constants could be obtained, and it is shown
that the compounds are highly emissive with the radiative decay dominating,
with more than 70% of the quanta loss through this deactivation channel.
The basic structure of the triphenylamineābenzimidazole derivatives
(<b>1a</b>) was modified at position 5 of the heterocyclic moiety
with electron-donating (OH (<b>1b</b>), OCH<sub>3</sub> (<b>1c</b>)) or electron-withdrawing groups (CN, (<b>1d)</b>). It was found that the photophysical properties remain basically
unchanged with the different substitutions, although a marked Stokes
shift was observed with <b>1d</b>. The presence and nature of
a charge-transfer transition is discussed with the help of theoretical
(DFT and TDFT) data. All compounds displayed exceptionally high thermal
stability (between 399 and 454 Ā°C) as seen by thermogravimetric
analysis
Interactions and Supramolecular Organization of Sulfonated Indigo and Thioindigo Dyes in Layered Hydroxide Hosts
Supramolecularly
organized hostāguest systems have been
synthesized by intercalating water-soluble forms of indigo (indigo
carmine, IC) and thioindigo (thioindigo-5,5ā²-disulfonate, TIS)
in zincāaluminum-layered double hydroxides (LDHs) and zinc-layered
hydroxide salts (LHSs) by coprecipitation routes. The colors of the
isolated powders were dark blue for hybrids containing only IC, purplish
blue or dark lilac for cointercalated samples containing both dyes,
and ruby/wine for hybrids containing only TIS. The as-synthesized and thermally treated materials
were characterized by Fourier transform infrared, Fourier transform
Raman, and nuclear magnetic resonance spectroscopies, powder X-ray
diffraction, scanning electron microscopy, and elemental and thermogravimetric
analyses. The basal spacings found for IC-LDH, TIS-LDH, IC-LHS, and
TIS-LHS materials were 21.9, 21.05, 18.95, and 21.00 Ć
, respectively,
with intermediate spacings being observed for the cointercalated samples
that either decreased (LDHs) or increased (LHSs) with increasing TIS
content. UVāvisible and fluorescence spectroscopies (steady-state
and time-resolved) were used to probe the molecular distribution of
the immobilized dyes. The presence of aggregates together with the
monomer units is suggested for IC-LDH, whereas for TIS-LDH, IC-LHS,
and TIS-LHS, the dyes are closer to the isolated situation. Accordingly,
while emission from the powder H<sub>2</sub>TIS is strongly quenched,
an increment in the emission of about 1 order of magnitude was observed
for the TIS-LDH/LHS hybrids. Double-exponential fluorescence decays
were obtained and associated with two monomer species interacting
differently with cointercalated water molecules. The incorporation
of both TIS and IC in the LDH and LHS hosts leads to an almost complete
quenching of the fluorescence, pointing to a very efficient energy
transfer process from (fluorescent) TIS to (nonfluorescent) IC
Multifaceted Regioregular Oligo(thieno[3,4ā<i>b</i>]thiophene)s Enabled by Tunable Quinoidization and Reduced Energy Band Gap
Thiophene-based
materials have occupied a crucial position in the
development of organic electronics. However, the energy band gaps
of oligo- and polythiophenes are difficult to modulate without resorting
to pushāpull electronic effects. We describe herein a new series
of monodisperse oligoĀ(thienoĀ[3,4-<i>b</i>]Āthiophene) derivatives
with well-defined regioregular structures synthesized efficiently
by direct CāH arylation. These compounds show a unique palette
of colors and amphoteric redox properties with widely tunable energy
band gaps. The capacity to stabilize both cations and anions results
in both anodic and cathodic electrochromism. Under excitation, these
compounds can produce photoionized states able to interconvert into
neutral triplet or form these through singlet exciton fission or intersystem
crossing. These features arise from a progressive increase in quinoidization
on a fully planar platform making the largest effective conjugation
length among hetero-oligomers. OligoĀ(thienoĀ[3,4-<i>b</i>]Āthiophene)Ās might represent the more distinctive family of oligothiophenes
of this decade
Multifaceted Regioregular Oligo(thieno[3,4ā<i>b</i>]thiophene)s Enabled by Tunable Quinoidization and Reduced Energy Band Gap
Thiophene-based
materials have occupied a crucial position in the
development of organic electronics. However, the energy band gaps
of oligo- and polythiophenes are difficult to modulate without resorting
to pushāpull electronic effects. We describe herein a new series
of monodisperse oligoĀ(thienoĀ[3,4-<i>b</i>]Āthiophene) derivatives
with well-defined regioregular structures synthesized efficiently
by direct CāH arylation. These compounds show a unique palette
of colors and amphoteric redox properties with widely tunable energy
band gaps. The capacity to stabilize both cations and anions results
in both anodic and cathodic electrochromism. Under excitation, these
compounds can produce photoionized states able to interconvert into
neutral triplet or form these through singlet exciton fission or intersystem
crossing. These features arise from a progressive increase in quinoidization
on a fully planar platform making the largest effective conjugation
length among hetero-oligomers. OligoĀ(thienoĀ[3,4-<i>b</i>]Āthiophene)Ās might represent the more distinctive family of oligothiophenes
of this decade
Self-Assembly of Poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} with Oppositely Charged Phenylenevinylene Oligoelectrolytes
The
interaction of the water-soluble conjugated polyelectrolyte
(CPE) polyĀ{1,4-phenylene-[9,9-bisĀ(4-phenoxy-butylsulfonate)]Āfluorene-2,7-diyl}
(PBS-PFP) (degree of polymerization, DP, ā¼3ā6) with
various concentrations of a homologous series of oppositely charged
amphiphilic phenylenevinylene oligomers was investigated in water:dioxane
mixtures and in aqueous micellar solutions of the non-ionic surfactant <i>n</i>-dodecylpentaoxyethylene glycol ether. The excellent spectral
overlap between the CPE fluorescence and the conjugated oligoelectrolyte
(COE) absorption indicates that energy transfer between these is a
highly favored process, and can be tuned by changing the COE chain
length. This is supported by time-resolved fluorescence data. The
overall results provide support for different types of self-assembly,
which are sensitive to the solvent environment and to the size of
the phenylenevinylene oligoelectrolyte chain. It is suggested that
large aggregates are formed in water:dioxane mixtures, while decorated
coreāshell structures are present in the surfactant solutions
Modulating the Self-Assembly of Calix[4]azacrowns to Design Materials with Improved Emission and Stimuli-Responsive Behavior
1,3-[Ethylene-bisĀ(aminocarbonylmethyl)]<i>-p-tert-</i>butylcalixĀ[4]Āarene is analyzed and its optical properties
are investigated, showing luminescence upon aggregation at low temperature
or in the solid state described as aggregation-induced blue emission.
The shift of the self-assembly from nanoporous to helical framework
in the crystal is associated with a remarkable blue-shift and 33-fold
increase in the fluorescence intensity. The purple-blue light emission
is highly efficient (Ī¦<sub>F</sub> ā¼ 60%) and is switched
to thermally activated yellow emission as a stimuli-responsive behavior
of the helical assembly. The emitting material has been characterized
by single crystal and powder X-ray diffraction, ATR-FTIR spectroscopy,
and DSC and PLTM analysis, and its optical properties are investigated