[2.2]Paracyclophane-Based Chiral Platforms for Circularly Polarized Luminescence Fluorophores and Their Chiroptical Properties: Past and Future

Optical Property Control by the Interligand Charge Transfer Excited State in Brominated Homoleptic and Heteroleptic Aluminum Dinuclear Triple-Stranded Helicates

The utilization of aluminum, an abundant and inexpensive element, for the synthesis of novel functional complexes is extremely important, but the design and control of photofunctionality are still unexplored. In this study, we focused on our previously developed dinuclear triple-stranded helicates incorporating two aluminum ions (ALPHY) to synthesize both homoleptic and heteroleptic complexes with bromine atoms at the 3-position of the pyrrole moiety in the Schiff base ligands. The brominated Schiff base ligands were reacted with AlCl3 to synthesize homoleptic complexes, while different ligands were mixed to prepare heteroleptic complexes. Single-crystal X-ray structural analysis revealed the structures of these novel complexes. We found that increasing the degree of bromination resulted in a tunable emission color, shifting progressively from 550 (yellow) to 566 nm (orange). Optical resolution of the complexes facilitated the observation of mirror-image circular dichroism and circularly polarized luminescence. Furthermore, employing ultrafast spectroscopy techniques, we have elucidated that the optical properties are governed by the interligand charge transfer (ILCT) among the three ligands. The formation of heteroleptic complexes induces the ILCT state even in nonpolar environments, thereby accelerating nonradiative decay and intersystem crossing. These findings mark significant advancements in photofunctional materials based on multinuclear complexes.

Water-Regulated Evolution of Inversion, Reinversion, and Amplification of Circularly Polarized Luminescence of Supramolecular Organogels Based on Glutamide-Cyanostilbene Amphiphile

Water incorporated with supramolecular building blocks in organic solvents can play a key role in the circularly polarized luminescence (CPL) inversion and amplification of supramolecular assemblies. Herein, we demonstrate that fine-tuning the water content regulated the assembly structure evolution and made the circular dichroism and CPL sign of the system undergo intriguing inversion, reinversion, and amplification processes based on a unique and interesting glutamide-cyanostilbene system, as supported by morphology, spectroscopic observations, and time-dependent density functional theory calculation.

Linker-dependent control of the chiroptical properties of polymethylene-vaulted trans-bis[(β-iminomethyl)naphthoxy]platinum(II) complexes

The effects of polymethylene bridges on the chiroptical properties of trans-bis[(β-iminomethyl)naphthoxy]platinum(II) platforms were examined both experimentally and theoretically using newly designed planar chiral Pt analogues (1) having three-dimensional superstructures. A series of optically pure polymethylene-vaulted Pt complexes (R)- and (S)-1 were synthesized and characterized with regard to the chiroptical behaviour of the trans-bis[(β-iminomethyl)naphthoxy]platinum(II) platforms. These complexes were found to exhibit structure-dependent chiroptical characteristics in solution, such that the absolute values of specific rotation, the circular dichroism dissymmetry factor (gabs) and the circularly polarized luminescence dissymmetry factor (glum) all increased upon shortening the polymethylene bridges. Density functional theory and time dependent density functional theory calculations were used to analyse vaulted and non-vaulted complexes, which demonstrated that the present linker-dependent chiroptical properties resulted from constraint-induced changes in the square planar Pt coordination centres rather than from chiral distortion along the coordination platforms.

Synthesis and characterization of one-handed helical oligo(o-phenylene)s: control of axial chirality by planar chiral [2.2]paracyclophane

Optically active oligo(o-phenylene)-layered molecules were synthesized from planar chiral enantiopure [2.2]paracyclophane. Their structures and optical properties were characterized by experimental and theoretical approaches. The axial chiralities between phenylene rings of the oligo(o-phenylene)s were controlled by the planar chirality to form one-handed helical structures. The o-quinquephenyl-layered molecule was emissive, and circularly polarized luminescence was observed with a high anisotropy factor (|glum| value) of 0.012.

Chiroptical Response Control of Planar and Axially Chiral Polymethylene-Vaulted Platinum(II) Complexes Bearing 1,1'-Binaphthyl Frameworks

In this study, the synthesis, structure, and chiroptical response control of planar chiral polymethylene-vaulted trans-bis[(β-iminomethyl)aryloxy]platinum(II) complexes bearing axially chiral 1,1'-binaphthyl ligands are described. A series of enantiopure polymethylene (n = 4-10)-vaulted complexes were prepared in 6 steps using commercially available (R)- or (S)-BINOL as the starting material without an optical resolution process. The trans-coordination and three-dimensional vaulted structures of the platinum complexes were elucidated from X-ray diffraction (XRD) studies. The complexes were found to show structural dependence of chiroptical responses in the dilute solution state such that the absolute values of [α]D, dissymmetry factors gabs in circular dichroism (CD), and glum in circularly polarized luminescence (CPL) increased upon shortening the length of the polymethylene bridges. The enhanced chiroptical responses were theoretically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, and the results are discussed in terms of the molecular structures and transition dipole moments of the ground states. The structural dependence of the chiroptical responses was ascribed to the distortion of the coordination platforms caused by restriction of the vaulting methylene linkers.

Stepwise Solution-Interfacial Nanoarchitectonics for Assembled Film with Full-Color and White-Light Circularly Polarized Luminescence

The fabrication of chiral thin films with tunable circularly polarized luminescence (CPL) colors is important in developing chiroptical materials but remains challenging due to the lack of assembly-initiated chiral film formation methodology. Here, by adopting a combined solution aggregation and interfacial assembly strategy, we report the fabrication of chiral film materials with full-color and white-light CPL. A biquinoline glutamic acid ester (abbreviated as BQGE) shows a typical aggregation-induced emission property with blue CPL after solution aggregation. Subsequent interfacial assembly of these solution aggregates on a solid substrate leads to the formation of a CPL active film consisting of nanobelt structures. Since the BQGE molecule has a coordination site, the CPL emission of an individual BQGE film can be extended from blue to green emission upon coordination with a zinc ion, accompanied by morphology transition from nanobelts to nanofibers. Further extension to red-color CPL is successfully achieved by coassembly with an achiral acceptor dye. Interestingly, the proper combination of coordination ratio and acceptor loading ratio provides bright white-light CPL emission from the BQGE/Zn²⁺/PDA triad composite film. This work provides a new approach to fabricating chiroptical film materials with controlled microscopic morphology and tunable CPL properties.

Synthesis, structural characterization, and chiroptical properties of planarly and axially chiral boranils

2-Amino[2.2]paracyclophane reacts with salicylaldehyde or 2-hydroxyacetophenone to yield imines that then give access to a new series of boranils (8b-d) upon complexation with BF₂ . These novel boron-containing compounds display both planar and axial chiralities and were examined experimentally and computationally. In particular, their photophysical and chiroptical properties were studied and compared to newly prepared, simpler boranils (9a-d) exhibiting axial chirality only. Less sophisticated chiral architectures were shown to demonstrate overall stronger circularly polarized luminescence (CPL) activity.

Synthesis of Two Novel Optically Active #-Shaped Cyclic Tetramers Based on Planar Chiral [2.2]Paracyclophanes

This study reports the synthesis of optically active cyclic tetramers comprising four stacked π-electron systems from two enantiomerically pure [2.2]paracyclophane compounds (bis-(para)-pseudo-ortho- and bis-(para)-pseudo-meta-tetrasubstituted [2.2]paracyclophane compounds). Depending on the combination of the absolute configurations of the planar chiral pseudo-ortho- and pseudo-meta-[2.2]paracyclophane units, the cyclic tetramers formed either parallel-#- or weave-#-structures. The optical and chiroptical properties of both structures were investigated experimentally and theoretically. In particular, the weave-#-shaped cyclic tetramer exhibited good chiroptical properties and emitted circularly polarized luminescence (CPL) with a high anisotropy factor (|g_lum | value of the order of 10^-3 ) and a CPL brightness (B_CPL ) higher than 100.

Effect of Oxidation on the Chiroptical Properties of Sulfur-Bridged Binaphthyl Dimers

A series of axially chiral sulfur-bridged dimers were prepared from 1,1'-binaphthyl-2,2'-diol and subsequently oxidized to the respective sulfones. The chiroptical properties of the chiral chromophores were studied as a function of the oxidation state. Upon oxidation, an increase in quantum yields was observed for directly linked sulfur bridged binaphthyls (0.04 to 0.32), and a modest increase in dissymmetry factor was observed for diphenylsulfide-bridged binaphthyls (-8.9 × 10^-4 to -1.4 × 10^-3). Computational calculations were used to elucidate the changes in photophysical properties.

Intramolecular Energy and Solvent‐Dependent Chirality Transfer within a BINOL‐Perylene Hetero‐Cyclophane

Multichromophoric macrocycles and cyclophanes are important supramolecular architectures for the elucidation of interchromophoric interactions originating from precise spatial organization. Herein, by combining an axially chiral binaphthol bisimide (BBI) and a bay‐substituted conformationally labile twisted perylene bisimide (PBI) within a cyclophane of well‐defined geometry, we report a chiral PBI hetero‐cyclophane (BBI‐PBI) that shows intramolecular energy and solvent‐regulated chirality transfer from the BBI to the PBI subunit. Excellent spectral overlap and spatial arrangement of BBI and PBI lead to efficient excitation energy transfer and subsequent PBI emission with high quantum yield (80–98 %) in various solvents. In contrast, chirality transfer is strongly dependent on the respective solvent as revealed by circular dichroism (CD) spectroscopy. The combination of energy and chirality transfer affords a bright red circularly polarized luminescence (CPL) from the PBI chromophore by excitation of BBI.

Fluorescent cyclophanes and their applications

With the serendipitous discovery of crown ethers by Pedersen more than half a century ago and the subsequent introduction of host-guest chemistry and supramolecular chemistry by Cram and Lehn, respectively, followed by the design and synthesis of wholly synthetic cyclophanes-in particular, fluorescent cyclophanes, having rich structural characteristics and functions-have been the focus of considerable research activity during the past few decades. Cyclophanes with remarkable emissive properties have been investigated continuously over the years and employed in numerous applications across the field of science and technology. In this Review, we feature the recent developments in the chemistry of fluorescent cyclophanes, along with their design and synthesis. Their host-guest chemistry and applications related to their structure and properties are highlighted.

Tuning the aromatic backbone twist in dipyrrolonaphthyridinediones

This communication describes the photophysical behavior of three analogs of cyclophane bearing the dipyrrolonaphthyridinedione (DPND) core. In these molecules, intersystem crossing (ISC) can be successfully induced by distinct changes in the deviation from planarity within the DPND core, allowing at the same time the emission maximum to shift from the green to red region of the visible spectrum without any synthetic modifications of the chromophore structure. This finding may build the foundation for a new paradigm for inducing ISC-type transitions within other centrosymmetric and planar cross-conjugated chromophores.

Synthesis of Bridged Indigos and Their Thermoisomerization and Photoisomerization Behaviors

Bridged indigos were synthesized by bridging the two nitrogen atoms in the indigo structure with a carbon chain, and their properties were carefully examined. These bridged indigos have intrinsic planar chirality, and the enantiomers were separated using chiral high-performance liquid chromatography. When the chiral bridged indigos were subjected to thermo- and photoisomerization, the corresponding (Z)-indigo was not observed at all, and racemization was observed. This phenomenon is caused by the low activation energy of inversion due to the 1.5 bond order of the double bond of the indigo skeleton and the large energy difference between the ground states of (E)-indigo and (Z)-indigo.

Planar Chiral Analogues of PRODAN Based on a [2.2]Paracyclophane Scaffold: Synthesis and Photophysical Studies

We describe the synthesis and photophysical characterization of differently substituted planar chiral analogues of PRODAN based on a [2.2]paracyclophane scaffold. This experimental and theoretical study highlights that the (chir)optical properties of the new "phane" compounds, which incorporate an electron-withdrawing propionyl moiety and an electron-donating dimethylamino group at their para or pseudo-para positions, strongly depend on their substitution patterns. In particular, for this series of molecules, a more pronounced solvatochromism and clear chiroptical behaviors are observed when the two substituents are placed on the two rings of the pCp core in a non-"co-planar" arrangement (pseudo-para derivative). This observation may help design new pCp-based luminophores with finely tuned photophysical properties.

Chiral macrocycle-induced circularly polarized luminescence of a twisted intramolecular charge transfer dye

The host-guest binding between a chiral macrocycle and an achiral dye could suppress the twisted intramolecular charge transfer (TICT) process, leading to enhanced emission and bright circularly polarized luminescence (CPL) from the originally achiral TICT-dye.

3D Boranil Complexes with Aggregation-Amplified Circularly Polarized Luminescence

The development of small organic CPL-active molecules with large luminescent dissymmetry factors is highly demanded due to their promising applications in chiroptical devices and sensors. This work describes the design and synthesis of a new family of CPL-active BF₂ complexes, (R_p)/(S_p)-3a-3e, which were constructed by fusing a N̂O-chelated BF₂ complex with [2.2]paracyclophane. These complexes display aggregation-amplified CPL with moderate dissymmetry factors values and moderate quantum yields both in solution and in the solid state. In addition, these photophysical properties were rationalized via X-ray diffraction and TD-DFT calculations.

Optically Active Cyclic Oligomers Based on Planar Chiral [2.2]Paracyclophane

Optically active cyclic dimer, trimer, and tetramer, in which the p-arylene-ethynylenes were stacked, were prepared using enantiopure 4,7,12,15-tetrasubstituted [2.2]paracyclophane as the chiral building block. All molecules exhibited clear Cotton effects in their absorption bands with mirror image spectra, and the signals of the longest wavelengths of the (R_p )- and (S_p )-isomers were positive and negative, respectively. Their circularly polarized luminescence (CPL) signs corresponded with the those of the first Cotton effect. For all molecules, molecular orbitals were localized in one of the stacked p-arylene-ethynylenes in the excited states, resulting in a similar photoluminescence behavior. Although the cyclic dimer did not emit CPL, the cyclic trimer and tetramer exhibited intense CPL emissions with a relatively high dissymmetry factor in the order of 10^-3 . Their optical and chiroptical properties were reproduced by time-dependent density functional theory calculations.

Helical Oligophenylene Linked with [2.2]Paracyclophane: Stereogenic π-Conjugated Dye for Highly Emissive Chiroptical Properties

A stereogenic π-system based on dimer (2) and trimer (3) of [2.2]paracyclophane (PC) and biphenyl was prepared and its structural, photophysical, and chiroptical properties were investigated. X-ray analysis revealed that the quaterphenyl moieties in 2 adopt a double helical structure anchoring [2.2]PC from both sides. Furthermore, 3 forms a isosceles triangle structure with a large chiral cavity. A homodesmotic reaction using DFT calculations revealed that 2 has a larger strain energy than 3 owing to its highly twisted phenylene linkers. Electronic and circular dichroic (CD) spectra were recorded in CH₂ Cl₂ solution. The spectra of both 2 and 3 are similar, and their longest absorption band accompanying a remarkable Cotton effect is attributed to the transition from HOMO to LUMO, which is delocalized to the quaterphenyl moiety. These compounds exhibit fairly high fluorescence quantum yields (ϕ=0.70-0.83) and moderate dissymmetry factor (|g_CPL |=1.6×10^-3 ) in circularly polarized luminescence (CPL).

Diversity-Oriented Synthesis of [2.2]Paracyclophane-derived Fused Imidazo[1,2-a]heterocycles by Groebke-Blackburn-Bienaymé Reaction: Accessing Cyclophanyl Imidazole Ligands Library

This report describes the synthesis of a [2.2]paracyclophane‐derived annulated 3‐amino‐imidazole ligand library through a Groebke‐Blackburn‐Bienaymé three‐component reaction (GBB‐3CR) approach employing formyl‐cyclophanes in combination with diverse aliphatic and aromatic isocyanides and heteroaromatic amidines. The GBB‐3CR process gives access to skeletally‐diverse cyclophanyl imidazole ligands, namely 3‐amino‐imidazo[1,2‐a]pyridines and imidazo[1,2‐a]pyrazines. Additionally, a one‐pot protocol for the GBB‐3CR by an in situ generation of cyclophanyl isocyanide is demonstrated. The products were analyzed by detailed spectroscopic techniques, and the cyclophanyl imidazo[1,2‐a]pyridine was confirmed unambiguously by single‐crystal X‐Ray crystallography. The cyclophanyl imidazole ligands can be readily transformed to showcase their useful utility in preparing N,C‐palladacycles through regioselective ortho‐palladation.

Solvent-regulated chiral exciton coupling and CPL sign inversion of an amphiphilic glutamide-cyanostilbene

The chiral exciton couplings within a Y-shaped amphiphilic glutamide-cyanostilbene (GCS) could be significantly biased by solvent polarity and hydration effects, which led to sign inversion of both the circular dichroism and circularly polarized luminescence of the GCS assemblies.

Influence of Positional Isomerism on the Chiroptical Properties of Functional Aromatic Oligoamide Foldamers

A series of functionalized quinoline-based aromatic oligoamide foldamers were prepared in their two enantiomeric forms, comprising an enantiopure terminal camphanyl chiral inducer, which governed the adjacent (P-/M-) helical-handedness. Hierarchical chirality transfer was further investigated in chromophore-appended variants via a range of electronic and vibrational spectroscopic techniques, including circularly polarized luminescence, vibrational circular dichroism and fluorescence. Intense total and polarized photoluminescence (up to Φ_lum =0.39, g_lum =1.5×10^-3 ) was observed in the visible region from these modular multicomponent architectures and a significant influence of positional isomerism was evidenced. The optimal position of a fluorophore substituent on the quinoline hexamers was determined as being position 2 over position 6, as stronger chiroptical features were systematically observed with the 2-positioned derivatives.