Surface-Enhanced Raman Spectroscopy: Investigations at the Nanorod Edges and Dimer Junctions

Raman signal enhancement of two analyte molecules, containing bipyridine and phenyl moieties, were investigated by linking them (i) onto the edges of Au nanorods using monothiol derivatives and (ii) at the junctions of two Au nanorods using dithiol derivatives. Edges of Au nanorods are regions of high electric field, and specific interaction of the thiol molecules on the {111} planes at the edges resulted in an enhanced Raman signal. When two Au nanorods are brought together in a linear fashion through dithiol linkages, their longitudinal plasmon oscillations couple each other, creating regions of enhanced electric field (hot spots) at the junctions. Interestingly, dimerization leads to a spontaneous enhancement in the intensity of Raman signals (enhancement factor of ∼1.4 × 10⁵) due to the localization of molecules at the junctions of Au nanorod dimers

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

Upconverted Chiral Emission in Hybrid Photonic Nanomaterials: Toward Amplified Circularly Polarized Luminescence with Tunable Chirality

Circularly polarized luminescence (CPL), an emergent technique to study the excited-state chirality in molecules and materials, is finding potential applications in the field of data encryption, anticounterfeiting, and photonic technologies. In this regard, upconverted chiral luminescence holds great promise due to the inherent advantages offered by the upconversion nanophosphors (UCNPs), which include excellent photochemical stability, a large anti-Stokes shift, and high penetration depth. However, the generation of upconverted circularly polarized luminescence (UC-CPL) has remained a great challenge. Herein, we demonstrate a facile approach for the synthesis of chiral luminescent self-standing photonic films that display UC-CPL with intense luminescence dissymmetry. Cellulose nanocrystals (CNCs) that exhibit lyotropic liquid-crystalline properties provide a robust platform as a chiral host, whereas a set of lanthanide-based UCNPs that emit different colors upon excitation using a near-IR laser act as achiral guests. While the chiral emission color could be tuned by controlling the nature of the activator/sensitizer in UCNPs, the sign of CPL is modulated through physical and chemical methods such as varied sonication time and electrolyte addition. The judicious choice of the electrolyte and duration of ultrasonication assisted in regulation of the helical sense of the nematic phase of CNCs, thereby influencing the selective reflection properties of the film. The approach adopted herein for the fabrication of materials exhibiting intense UC-CPL holds the potential to be expanded to a variety of nanosystems that can ultimately find application in display technology and circularly polarized light-emitting devices

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

Two-Step Synthesis of Boron-Fused Double Helicenes

Novel boron-fused double [5]­helicenes were synthesized from hexabromobenzene in two steps via Hart reaction and demethylative cyclization. The parent helicene shows excellent ambipolar conductivity, which can be explained by unique 3D π-stacking with a brickwork arrangement. Moreover, the introduction of four <i>tert</i>-butyl groups suppresses racemization, enabling optical resolution. The enantiomerically pure helicene shows deep blue fluorescence with Commission Internationale de l′Eclairage coordinates of (0.15, 0.08) and circularly polarized luminescence activity

Highly Fluorescent [7]Carbohelicene Fused by Asymmetric 1,2-Dialkyl-Substituted Quinoxaline for Circularly Polarized Luminescence and Electroluminescence

A new 1,2-dialkylquinoxaline-fused [7]­carbohelicene ([7]­Hl-NAIQx) was designed and synthesized by asymmetrically introducing two alkyl chains onto the quinoxaline unit. Direct alkylation of the quinoxaline ring of quinoxaline-fused helicene leads to discontinuity in the conjugated structure. In the single-crystal analysis, the parent quinoxaline-fused [7]­carbohelicene ([7]­Hl-Qx) was found to have a helical structure formed by two phenanthrene units and a nonplanar twisted angle between the phenanthrene and quinoxaline units. In contrast, [7]­Hl-NAIQx possesses an almost planar aromatic structure between the alkyl-quinoxaline and phenanthrene units (torsion angle: 179°), in addition to the similar helical structure between the two phenanthrene units. The steady-state absorption, fluorescence, and circular dichroism (CD) spectra of [7]­Hl-NAIQx were significantly red-shifted compared to those of [7]­Hl-Qx and [7]­carbohelicene ([7]­Hl). These spectral changes were mainly explained by electrochemical measurements and density functional theory calculations. Moreover, the absolute fluorescence quantum yield (Φ_FL) of [7]­Hl-NAIQx was 0.25, which is more than 10 times larger than that of the reference [7]­Hl (Φ_FL = 0.02). Such a large enhancement of the fluorescence of [7]­Hl-NAIQx has provided excellent circularly polarized luminescence (CPL). The value of the anisotropy factor <i>g</i>_lum (normalized difference in emission of right-handed and left-handed circularly polarized light) was estimated to be 4.0 × 10^–3. The electroluminescence of an organic light-emitting diode utilizing [7]­Hl-NAIQx was successfully observed

Bis(dipyrrinato)zinc(II) Complex Chiroptical Wires: Exfoliation into Single Strands and Intensification of Circularly Polarized Luminescence

One-dimensional (1D) coordination polymers (CPs) experiences limitations in exfoliation into individual strands, which hamper their utility as functional 1D nanomaterials. Here we synthesize chiral 1D-CPs that feature the bis­(dipyrrinato)­zinc­(II) complex motif. They can be exfoliated into single strands upon sonication in organic media, retaining lengths of up to 3.19 μm (ca. 2600 monomer units). Their chiroptical structure allows the exfoliated wires to show circularly polarized luminescence at an intensity 5.9 times that of reference monomer complexes