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 (Eu³⁺) concentration in the submicromolar concentration range. In the presence of 0.25 equiv of Eu³⁺ (5 μM), LH forms a luminescent 4:1 complex ([Eu³⁺(L^–)₄]⁻) exhibiting dual emission at 357 and 613 nm resulting from the local excitation of the carbazole ring and ligand-sensitized luminescence from the Eu³⁺-β-diketonate unit, respectively. The 4:1 complex begins to convert into a 2:1 complex ([Eu³⁺(L^–)₂]⁺) via a 3:1 complex [Eu³⁺(L^–)₃] above a molar ratio ([Eu³⁺]/[LH]) of 0.25, which provides the opportunity for binding of solvent methanol molecules to the vacant site of the Eu³⁺ ion in the complex ([Eu³⁺(L^–)₂(MeOH)_<i>n</i>]⁺). The OH oscillators of coordinated methanol molecules facilitate the nonradiative pathway of the Eu³⁺ emission; hence the emission at 613 nm almost disappears above the 0.50 equivalent of Eu³⁺ (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>₂‑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>₂-symmetrical alternating circular helicate with the general formula [(<i>R</i>)- or (<i>S</i>)-Ph-Pybox]₄(Ln^III)₄(BTP)₆. The lanthanide­(III) Ln^III assemblies (Ln^III₄-<i>RRRR</i> and Ln^III₄-<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^III₂L₂ units, where both show (<i>M</i>)-helicity for Ln^III₄-<i>RRRR</i> and (<i>P</i>)-helicity for Ln^III₄-<i>SSSS</i>, where π–π stacking interaction is present between the side arm of (<i>R</i>)-Ph-Pybox (Ph₁) and the adjacent BTP ligand around the Eu^III metal center (<i>d</i>_ππ = 3.636 Å: the diketonate plane···Ph₁ distance). The newly obtained circular lanthanide­(III) helicate exists as single and homochiral diastereomers in solution (Ln^III₄-<i>RRRR</i> and Ln^III₄-<i>SSSS</i>), exhibiting circularly dichroism (CD) and circularly polarized luminescence (CPL). Conversely, the circular helicate favors the heterochiral arrangement (i.e., Ln^III₄-<i>RRRR</i>/Ln^III₄-<i>SSSS</i>)

Ligand-to-Ligand Interactions That Direct Formation of <i>D</i>₂‑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>₂-symmetrical alternating circular helicate with the general formula [(<i>R</i>)- or (<i>S</i>)-Ph-Pybox]₄(Ln^III)₄(BTP)₆. The lanthanide­(III) Ln^III assemblies (Ln^III₄-<i>RRRR</i> and Ln^III₄-<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^III₂L₂ units, where both show (<i>M</i>)-helicity for Ln^III₄-<i>RRRR</i> and (<i>P</i>)-helicity for Ln^III₄-<i>SSSS</i>, where π–π stacking interaction is present between the side arm of (<i>R</i>)-Ph-Pybox (Ph₁) and the adjacent BTP ligand around the Eu^III metal center (<i>d</i>_ππ = 3.636 Å: the diketonate plane···Ph₁ distance). The newly obtained circular lanthanide­(III) helicate exists as single and homochiral diastereomers in solution (Ln^III₄-<i>RRRR</i> and Ln^III₄-<i>SSSS</i>), exhibiting circularly dichroism (CD) and circularly polarized luminescence (CPL). Conversely, the circular helicate favors the heterochiral arrangement (i.e., Ln^III₄-<i>RRRR</i>/Ln^III₄-<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>_lum) 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>_lum) 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 derivativean axially chiral π-extended compound in which two perylene subunits fused to 1,1′-binaphthyl scaffoldhas 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^–1 cm^–1 at 449 nm, |<i>g</i>_CD| = 5.8 × 10^–3 at 453 nm), high fluorescence quantum yield (Φ_f = 64%), and strong circular polarized luminescence (CPL) (|<i>g</i>_CPL| = 5 × 10^–3 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