30 research outputs found
Photophysical Properties of a Terarylene Photoswitch with a Donor–Acceptor Conjugated Bridging Unit
A terarylene
photoswitch composed of an electron-donating thienothiophene
unit which is connected to the central bridging imidazole was designed.
The electronic and geometrical structures of the central π-conjugation
unit was controlled by chemical modifications on the imidazole unit,
in which the distribution of frontier molecular orbitals was modulated
between the neutral state and its protonated or quaternized cationic
forms. These electronic and geometrical changes modulated the mixing
of the closely lying excited state potential energy surfaces, resulting
in the excitation wavelength dependent photocyclization performance.
The chemical modifications on the imidazole ring also had an effect
on the fluorescence property in the open-ring forms, which is explained
by the formation of different push–pull π-conjugation
systems
OFF–ON–OFF Dual Emission at Visible and UV Wavelengths from Carbazole Functionalized β‑Diketonate Europium(III) Complex
This
work demonstrates dual emission “OFF–ON–OFF”
switching at visible and UV wavelengths of a carbazole functionalized
β-diketone (LH) by a simple change of a europium(III) ion (Eu<sup>3+</sup>) concentration in the submicromolar concentration range.
In the presence of 0.25 equiv of Eu<sup>3+</sup> (5 μM), LH
forms a luminescent 4:1 complex ([Eu<sup>3+</sup>(L<sup>–</sup>)<sub>4</sub>]<sup>−</sup>) exhibiting dual emission at 357
and 613 nm resulting from the local excitation of the carbazole ring
and ligand-sensitized luminescence from the Eu<sup>3+</sup>-β-diketonate
unit, respectively. The 4:1 complex begins to convert into a 2:1 complex
([Eu<sup>3+</sup>(L<sup>–</sup>)<sub>2</sub>]<sup>+</sup>)
via a 3:1 complex [Eu<sup>3+</sup>(L<sup>–</sup>)<sub>3</sub>] above a molar ratio ([Eu<sup>3+</sup>]/[LH]) of 0.25, which provides
the opportunity for binding of solvent methanol molecules to the vacant
site of the Eu<sup>3+</sup> ion in the complex ([Eu<sup>3+</sup>(L<sup>–</sup>)<sub>2</sub>(MeOH)<sub><i>n</i></sub>]<sup>+</sup>). The OH oscillators of coordinated methanol molecules facilitate
the nonradiative pathway of the Eu<sup>3+</sup> emission; hence the
emission at 613 nm almost disappears above the 0.50 equivalent of
Eu<sup>3+</sup> (11 μM), while the UV emission at 357 nm remains
mostly constant over the whole concentration range
Charge Transfer Emission of T‑Shaped π‑Conjugated Molecules: Impact of Quinoid Character on the Excited State Properties
We investigate the impact of quinoid
character of a π-conjugation
system on the emission properties of T-shaped cross-conjugated molecules.
Three π-conjugated systems with different quinoid nature including
benzothiophene, 2-phenylthiophene, and 2-phenylthieno[3,2-<i>b</i>]thiophene were connected orthogonally to a π-conjugated
bis(phenylethynyl)arylene with an acid responsive <i>N</i>-methylbenzimidazole junction. The enhancement of quinoid character
of a vertical π-system effectively suppressed the twisted intramolecular
charge transfer (TICT) emission, leading to a more planar ICT state
with enhanced emission intensity as well as a shortened Stokes shift
Ligand-to-Ligand Interactions That Direct Formation of <i>D</i><sub>2</sub>‑Symmetrical Alternating Circular Helicate
This
work demonstrates that ligand-to-ligand interactions between
achiral bis-β-diketonate (BTP) and chiral bis(4-phenyl-2-oxazolinyl)pyridine
[(<i>R</i>)- or (<i>S</i>)-Ph-Pybox] are successfully
directed to the fabrication of a <i>D</i><sub>2</sub>-symmetrical
alternating circular helicate with the general formula [(<i>R</i>)- or (<i>S</i>)-Ph-Pybox]<sub>4</sub>(Ln<sup>III</sup>)<sub>4</sub>(BTP)<sub>6</sub>. The lanthanide(III) Ln<sup>III</sup> assemblies (Ln<sup>III</sup><sub>4</sub>-<i>RRRR</i> and Ln<sup>III</sup><sub>4</sub>-<i>SSSS</i>) have a nanometer-size squarelike grid (interatomic
distances > 10 Å). X-ray structure analysis revealed that
the
circular helicate contains two double helicate Ln<sup>III</sup><sub>2</sub>L<sub>2</sub> units, where both show (<i>M</i>)-helicity
for Ln<sup>III</sup><sub>4</sub>-<i>RRRR</i> and (<i>P</i>)-helicity for Ln<sup>III</sup><sub>4</sub>-<i>SSSS</i>, where π–π stacking interaction is present between
the side arm of (<i>R</i>)-Ph-Pybox (Ph<sub>1</sub>) and
the adjacent BTP ligand around the Eu<sup>III</sup> metal center (<i>d</i><sub>ππ</sub> = 3.636 Å: the diketonate
plane···Ph<sub>1</sub> distance). The newly obtained
circular lanthanide(III) helicate exists as single and homochiral
diastereomers in solution (Ln<sup>III</sup><sub>4</sub>-<i>RRRR</i> and Ln<sup>III</sup><sub>4</sub>-<i>SSSS</i>), exhibiting
circularly dichroism (CD) and circularly polarized luminescence (CPL).
Conversely, the circular helicate favors the heterochiral arrangement
(i.e., Ln<sup>III</sup><sub>4</sub>-<i>RRRR</i>/Ln<sup>III</sup><sub>4</sub>-<i>SSSS</i>)
Ligand-to-Ligand Interactions That Direct Formation of <i>D</i><sub>2</sub>‑Symmetrical Alternating Circular Helicate
This
work demonstrates that ligand-to-ligand interactions between
achiral bis-β-diketonate (BTP) and chiral bis(4-phenyl-2-oxazolinyl)pyridine
[(<i>R</i>)- or (<i>S</i>)-Ph-Pybox] are successfully
directed to the fabrication of a <i>D</i><sub>2</sub>-symmetrical
alternating circular helicate with the general formula [(<i>R</i>)- or (<i>S</i>)-Ph-Pybox]<sub>4</sub>(Ln<sup>III</sup>)<sub>4</sub>(BTP)<sub>6</sub>. The lanthanide(III) Ln<sup>III</sup> assemblies (Ln<sup>III</sup><sub>4</sub>-<i>RRRR</i> and Ln<sup>III</sup><sub>4</sub>-<i>SSSS</i>) have a nanometer-size squarelike grid (interatomic
distances > 10 Å). X-ray structure analysis revealed that
the
circular helicate contains two double helicate Ln<sup>III</sup><sub>2</sub>L<sub>2</sub> units, where both show (<i>M</i>)-helicity
for Ln<sup>III</sup><sub>4</sub>-<i>RRRR</i> and (<i>P</i>)-helicity for Ln<sup>III</sup><sub>4</sub>-<i>SSSS</i>, where π–π stacking interaction is present between
the side arm of (<i>R</i>)-Ph-Pybox (Ph<sub>1</sub>) and
the adjacent BTP ligand around the Eu<sup>III</sup> metal center (<i>d</i><sub>ππ</sub> = 3.636 Å: the diketonate
plane···Ph<sub>1</sub> distance). The newly obtained
circular lanthanide(III) helicate exists as single and homochiral
diastereomers in solution (Ln<sup>III</sup><sub>4</sub>-<i>RRRR</i> and Ln<sup>III</sup><sub>4</sub>-<i>SSSS</i>), exhibiting
circularly dichroism (CD) and circularly polarized luminescence (CPL).
Conversely, the circular helicate favors the heterochiral arrangement
(i.e., Ln<sup>III</sup><sub>4</sub>-<i>RRRR</i>/Ln<sup>III</sup><sub>4</sub>-<i>SSSS</i>)
Circularly Polarized Luminescence in Chiral Aggregates: Dependence of Morphology on Luminescence Dissymmetry
The self-assembly of a chiral perylene bisimide bichromophoric derivative possessing a 1,1′-binaphthalene bridge was investigated by adopting two different methodologies, leading to the formation of aggregates with dissimilar morphologies. The chiral nature of the aggregated structures was optically probed with the help of circular dichroism (CD), vibrational circular dichroism (VCD), and circularly polarized luminescence (CPL). The one-dimensional aggregates formed in methylcyclohexane (MCH) exhibited twice the value of luminescence dissymmetry factor (<i>g</i><sub>lum</sub>) when compared with the spherical aggregates formed in chloroform at higher concentration. The summation of excitonic couplings between the individual chromophoric units in an aggregated system is responsible for the remarkably high luminescence dissymmetry exhibited by the chiral aggregates. The nanostructures could be successfully embedded into polymer films, leading to the fabrication of solid-state materials with high CPL dissymmetry that can find novel applications in chiroptical sensing, memory, and light-emitting devices based on organic nanoparticles
Circularly Polarized Luminescence in Chiral Aggregates: Dependence of Morphology on Luminescence Dissymmetry
The self-assembly of a chiral perylene bisimide bichromophoric derivative possessing a 1,1′-binaphthalene bridge was investigated by adopting two different methodologies, leading to the formation of aggregates with dissimilar morphologies. The chiral nature of the aggregated structures was optically probed with the help of circular dichroism (CD), vibrational circular dichroism (VCD), and circularly polarized luminescence (CPL). The one-dimensional aggregates formed in methylcyclohexane (MCH) exhibited twice the value of luminescence dissymmetry factor (<i>g</i><sub>lum</sub>) when compared with the spherical aggregates formed in chloroform at higher concentration. The summation of excitonic couplings between the individual chromophoric units in an aggregated system is responsible for the remarkably high luminescence dissymmetry exhibited by the chiral aggregates. The nanostructures could be successfully embedded into polymer films, leading to the fabrication of solid-state materials with high CPL dissymmetry that can find novel applications in chiroptical sensing, memory, and light-emitting devices based on organic nanoparticles
Dual Transient Bleaching of Au/PbS Hybrid Core/Shell Nanoparticles
We examined the optical response of hybrid Au/PbS core/shell
nanoparticles
(NPs) using transient absorption spectroscopy. Finite-difference time-domain
(FDTD) calculations and transient absorption measurements show that
Au/PbS NPs have unique two extinction peaks: the peak at the longer
wavelength (∼700 nm) is originated from the plasmon, and that
at the shorter wavelength (550 nm) is from the local maximum of the
refractive index of PbS. The transient absorption dynamics of Au/PbS
NPs excited at 400 nm have clear oscillation behavior, which is assigned
to the breathing mode of whole particle. We observed a weak excitation-wavelength
dependence of the plasmon band. The time constant of electron–phonon
coupling of Au/PbS NPs was obtained by changing the excitation intensity.
We show that spectral properties of Au/PbS NPs are strongly altered
by the hybrid formations, while their dynamics differ only minimally
compared with those of Au NPs
Synthesis and Photophysical Properties of a 13,13′-Bibenzo[<i>b</i>]perylenyl Derivative as a π‑Extended 1,1′-Binaphthyl Analog
A 13,13′-bibenzo[<i>b</i>]perylenyl derivativean
axially chiral π-extended compound in which two perylene subunits
fused to 1,1′-binaphthyl scaffoldhas been synthesized
from 1,8-dibromophenanthrene using an anionic cyclodehydrogenation
reaction in the presence of potassium metal as the key step. The pair
of enantiomers can be separated by chiral high-performance liquid
chromatography (HPLC), which showed a strong circular dichroism (CD)
(Δε = 330 M<sup>–1</sup> cm<sup>–1</sup> at 449 nm, |<i>g</i><sub>CD</sub>| = 5.8 × 10<sup>–3</sup> at 453 nm), high fluorescence quantum yield (Φ<sub>f</sub> = 64%), and strong circular polarized luminescence (CPL)
(|<i>g</i><sub>CPL</sub>| = 5 × 10<sup>–3</sup> at 454 nm) in solution phase
Impact of Optical Purity on the Light Harvesting Property in Supramolecular Nanofibers
Supramolecular
ordering and orientation of chromophores are tremendously
accomplished in photosynthetic light harvesting complexes, which are
crucial for long-range transfer of collected solar energy. We herein
demonstrate the importance of optical purity on the organization of
chromophoric chiral molecules for efficient energy migration. Enantiomeric
bichromophoric compounds, which self-assemble into nanofibers capable
of chiral recognition, were mixed to form supramolecular coassemblies
with variable enantiopurity. The chiral molecules self-assembled into
extended fibers regardless of enantiopurity, while their morphology
was dependent on the enantiomeric excess. The optical purity of assemblies
also had an effect on the emission efficiency; the nanofibers with
higher enantiomeric excess afforded a larger emission quantum yield.
The presence of an opposite enantiomer is considered to deteriorate
the chiral molecular packing suitable for directional growth of the
nanofiber, efficient exciton migration, and chiral guest recognition