Intermolecular Charge Transfer Interaction Research Articles

Photomodulation of Charge Transfer through Excited‐State Processes: Directing Donor‐Acceptor Binding Dynamics

AbstractModulating charge transfer (CT) interactions between donor and acceptor molecules may give rise to unique dynamic changes in physicochemical properties, exhibiting great importance in supramolecular chemistry and materials science. In this work, we demonstrate the first instance of reversible photomodulation of donor‐acceptor (D−A) CT interaction in the solid state. Pyridinium‐based chromophore featuring π‐conjugated D−A structures can not only function as a good electron acceptor to undergo photoinduced electron transfer (ET) or engage in intermolecular CT interaction, but also exhibit unique dual emission depending on the excitation wavelengths. The rotatable C−C single bonds within D−A pairs enhance the tunability of molecular structure. Through the synergy of a photoinduced ET and an excited‐state conformational change, the intermolecular CT interaction can be switched on and off by alternate light irradiation to enable reversibly modulation of the affinity between donor and acceptor molecules, accompanied by visual color switching and fluorescence on‐off as feedback signals.

Selective Formation of Homochiral Dimers by Intermolecular Charge Transfer on a hBN Nanomesh.

In this study, a comprehensive characterization was conducted on a chiral starburst molecule (C57H48N4, SBM) using scanning tunneling microscopy. When adsorbed onto the hBN/Rh(111) nanomesh, these molecules demonstrate homochiral recognition, leading to a selective formation of homochiral dimers. Further tip manipulation experiments reveal that the chiral dimers are stable and primarily controlled by strong intermolecular interactions. Density functional theory (DFT) calculations supported that the chiral recognition of SBM molecules is governed by the intermolecular charge transfer mechanism, different from the common steric hindrance effect. This study emphasizes the importance of intermolecular charge transfer interactions, offering valuable insights into the chiral recognition of a simple bimolecular system. These findings hold significance for the future advancement in chirality-based electronic sensors and pharmaceuticals, where the chirality of molecules can impact their properties.

Appending Coronene Diimide with Carbon Nanohoops Allows for Rapid Intersystem Crossing in Neat Film

AbstractThe development of innovative triplet materials plays a significant role in various applications. Although effective tuning of triplet formation by intersystem crossing (ISC) has been well established in solution, the modulation of ISC processes in the solid state remains a challenge due to the presence of other exciton decay channels through intermolecular interactions. The cyclic structure of cycloparaphenylenes (CPPs) offers a unique platform to tune the intermolecular packing, which leads to controllable exciton dynamics in the solid state. Herein, by integrating an electron deficient coronene diimide (CDI) unit into the CPP framework, a donor‐acceptor type of conjugated macrocycle (CDI‐CPP) featuring intramolecular charge‐transfer (CT) interaction was designed and synthesized. Effective intermolecular CT interaction resulting from a slipped herringbone packing was confirmed by X‐ray crystallography. Transient spectroscopy studies showed that CDI‐CPP undergoes ISC in both solution and the film state, with triplet generation time constants of 4.5 ns and 238 ps, respectively. The rapid triplet formation through ISC in the film state can be ascribed to the cooperation between intra‐ and intermolecular charge‐transfer interactions. Our results highlight that intermolecular CT interaction has a pronounced effect on the ISC process in the solid state, and shed light on the use of the characteristic structure of CPPs to manipulate intermolecular CT interactions.

Appending Coronene Diimide with Carbon Nanohoops Allows for Rapid Intersystem Crossing in Neat Film.

The development of innovative triplet materials plays a significant role in various applications. Although effective tuning of triplet formation by intersystem crossing (ISC) has been well established in solution, the modulation of ISC processes in the solid state remains a challenge due to the presence of other exciton decay channels through intermolecular interactions. The cyclic structure of cycloparaphenylenes (CPPs) offers a unique platform to tune the intermolecular packing, which leads to controllable exciton dynamics in the solid state. Herein, by integrating an electron deficient coronene diimide (CDI) unit into the CPP framework, a donor-acceptor type of conjugated macrocycle (CDI-CPP) featuring intramolecular charge-transfer (CT) interaction was designed and synthesized. Effective intermolecular CT interaction resulting from a slipped herringbone packing was confirmed by X-ray crystallography. Transient spectroscopy studies showed that CDI-CPP undergoes ISC in both solution and the film state, with triplet generation time constants of 4.5 ns and 238 ps, respectively. The rapid triplet formation through ISC in the film state can be ascribed to the cooperation between intra- and intermolecular charge-transfer interactions. Our results highlight that intermolecular CT interaction has a pronounced effect on the ISC process in the solid state, and shed light on the use of the characteristic structure of CPPs to manipulate intermolecular CT interactions.

NIR‐II Organic Photothermal Cocrystals with Strong Charge Transfer Interaction for Flexible Wearable Heaters

Comprehensive SummaryThe organic cocrystal strategy has provided a convenient and efficient platform for preparing organic photothermal materials. However, the rapidly directional preparation of cocrystals with desirable photothermal properties remains challenging due to a lack of suitable design ideas. Here, two new photothermal cocrystals, MTC and MFC, based on acceptor molecules (TCNQ and F4TCNQ) with different electron‐withdrawing capacities were quickly prepared by the coprecipitation method, aiming to explore the effect of charge transfer (CT) interaction on photothermal properties. Compared with MTC, the stronger intermolecular CT interaction in MFC facilitates extending the absorption range (from the NIR‐I to the NIR‐II region) and enhancing the non‐radiative transition process. Under the 808 nm laser irradiation, the photothermal conversion efficiency (PCE) of MFC is 54.6%, whereas MTC displays a mere 36.8%. The MFC cocrystal was further combined with a flexible polymer substrate (HPDMS) to prepare a flexible wearable heater (HPDMS@MFC), which exhibits excellent NIR‐II photothermal performance. This work points out a research direction for the rapid assembly of efficient photothermal cocrystals and additionally provides an extensive application prospect for organic photothermal cocrystals in the field of wearable devices.

Experimental and theoretical interpretations of structural, optical, mechanical and thermal properties of 2-aminopyridinium phthalate single crystals

Nonlinear optical good quality single crystals of 2-aminopyridinium phthalate single crystals are synthesized by solution growth technique. Structural parameters are elucidated by Single Crystal X-ray diffraction method which reveals that the material crystallizes in non-centrosymmetric crystalline framework. Vibrational assignments are interpreted by Fourier Transform Infrared spectral analysis. The thermal, mechanical and laser stability are explored by TG/DTA, microhardness and Laser Damage threshold analysis. The material has good emission nature in the violet emission region. The linear optical properties are elucidated with the help of UV–visible spectral analysis. The intermolecular charge transfer interactions are analyzed by HOMO-LUMO and MESP maps by using Gaussian ’09 software program. The second harmonic generation efficiency of the synthesized material are predicted by Kurtz Perry powder technique.

Acid/base responsive pseudo[3]rotaxanes from amine naphthotubes and bis-pyridinium/isoquinolinium guests.

A novel cooperative pseudo[3]rotaxane system was successfully constructed by the inclusion complexation of two identical amine naphthotubes with a bis-pyridinium/isoquinolinium guest. Single crystal structure analysis revealed that weak Csp3-H⋯O hydrogen bonds between the two hosts are responsible for the positive cooperativity during the formation of pseudo[3]rotaxanes. Moreover, intermolecular charge-transfer interactions between the electron-rich host and the electron-poor guests were observed. The pseudo[3]rotaxanes showed pH-controllable association/dissociation processes with naked-eye color changes in solution.

Near-infrared OLEDs with high radiance of ∼105 mW Sr−1 m−2 for single-wavelength oxygen saturation detection

Real-time in-home monitoring oxygen saturation (SpO2) is of great significance for early diagnosis and treatment of severe COVID-19, however, which should be based on multi-wavelength technology and complicated equipment. Herein, as a proof of concept, 740 nm is chosen as singlet light source for single-wavelength SpO2 detection, since absorbance differences between oxyhemoglobin (HbO2) and other interferences are the most rational. To controllably optimize excited-state characteristics of a near-infrared (NIR) thermally activated delayed fluorescence (TADF) emitters, an “extra conjugation” method is demonstrated by CNQP-TPA, whose 2,3-dicyanoquinoxaline phenanthroline (CNQP) acceptor forms strong intramolecular hydrogen bonds with two phenyls of triphenylamine (TPA) donors, which not only reduce excited-state relaxation-induced energy lose, but also provide more space for accurate modulation of intermolecular charge transfer (CT) interactions. As consequence, at the suitable doping concentration, CNQP-TPA endowed its device with a high radiance reaching ∼105 mW Sr−1 m−2 and desired electroluminescence peak at 744 nm. This device is further used as light source to detect SpO2 variation under single-wavelength mode. These results indicate the potential of NIR TADF materials and devices for in-home SpO2 monitoring and pre-diagnosis and prevention of severe COVID-19.

Homogeneous and Heterogeneous Self‐Assembly of Luminescent Pyromellitic Dianhydride‐Based Charge‐Transfer Complexes

AbstractUsing easily hydrolyzable brominated pyromellitic dianhydride (PMDA) as an electron acceptor, a wide variety of structurally stable binary organic charge‐transfer (CT) microcrystals that are stabilized by dominant intermolecular CT interactions is achieved. By varying the electron‐donating abilities of π‐electron compounds, the resulting single crystalline CT assemblies display tailorable fluorescence emissions spanning from green to near‐infrared. Upon implantation of a π‐electron donor anthracene (An) into fluoranthene‐PMDA (Fl‐PMDA), red and NIR emissions of ternary alloyed assemblies are substantially enhanced due to efficient energy transfer from Fl‐PMDA to An‐PMDA as well as structural complementarity between two CT complexes. Depending on the well‐matched epitaxial relationship, seeded growth of phenanthrene‐PMDA (Ph‐PMDA) onto the pre‐existing An‐PMDA microcrystals is also achieved, leading to core‐shell heterostructures with full and partial coverage. Such an epitaxial growth strategy is also applicable to the construction of microscale heterostructures of diverse CT complex combinations. The ternary Fl1−xAnx‐PMDA alloyed assemblies display composition‐dependent tailorable optical waveguiding behaviors. While An‐PMDA@Ph‐PMDA core‐shell microrods present wavelength‐dependent two‐photon excited fluorescence performances. The rational creation of these homogeneous and heterogeneous CT‐assembled architectures provides us a deep insight to investigate multicomponent functional organic cocrystals.

Investigation into Anion Effects on the Bulk and Interfacial Solvation Structure for Multivalent Ion Batteries

Zinc ion batteries (ZIB) are attractive because of the relative abundance of Zn as a core earth element, a higher volumetric energy density compared to lithium metal and it is cost effective. Despite this promise, a general lack of fundamental understanding of the electrode-electrolyte interface in ZIB, and multivalent (MV) ion technologies more broadly, hinders their wide scale deployment. In particular, the solvation structure of aqueous and non-aqueous electrolytes plays a critical role in promoting the necessary intermolecular charge transfer interactions to drive reversible plating/stripping at the metal anode. Here we examine the dynamics of the bulk and interfacial solvation structure to uncover differences in the bulk vs. the interfacial speciation as a function of anion chemistry in hopes to develop new design principles for new electrolyte development in ZIB and MV technologies. Utilizing Raman spectroscopy to probe the bulk solvation structures of Zn-based electrolytes containing one anion such as SO4 2-, bis-trifluoromethane sulfonimide (TFSI-) and trifluoromethanesulfonate (triflate, OTf-), we find three solvation environments present; a main feature corresponding to the “free” anion breathing mode and, in some cases, small amounts of mono- or bidentate contact ion pairs (CIPs). In some cases, introduction of co-anions (i.e., halides, X = Cl-, Br-, I-) results in significant extra intensity on the low frequency side of the overall Raman peak envelope that does not correspond with solvation structures observed in the single anion electrolytes. First principles density functional theory (DFT) calculations indicate the additional vibrational mode corresponds to a mixed anion CIP, suggesting anion-anion interactions can induce nominally dissociated anions to coordinate the cation in ways they otherwise would not. Raman and DFT analysis of Mg-based electrolytes indicate similar mixed anion CIP structures are formed, indicating similar solvation effects are active in other MV systems. Preliminary measurements utilizing shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERs) will also be discussed as a means of developing in situ understanding of the evolution of interfacial solvation and surface chemistry during the stripping/plating process. Overall, these results point to new possibilities in controlling the multivalent cation solvation structures present in the bulk and at the interface. This will aid in the development of highly reversible electrolytes that are stable against both divalent metal anodes and high voltage cathodes.

The host–guest inclusion driven by host-stabilized charge transfer for construction of sequentially red-shifted mechanochromic system

Developing more extensive methods to understand the underlying structure-property relationship of mechanochromic luminescent molecules is demanding but remains challenging. Herein, the effect of host-guest interaction on the mechanochromic properties of organic molecules is illustrated. A series of pyridinium-functionalized triphenylamine derivatives show bathochromic-shifted emission upon mechanical stimulation. These derivatives bind to cucurbit[8]uril to form homoternary host-guest inclusion complexes through host-stabilized intermolecular charge transfer interactions. Remarkably, the homoternary complexes exhibit longer emission than that of free guests in the solid state (even longer than ground guests), and a further bathochromic-shifted emission is observed upon grinding. Additionally, a heteroternary complex constructed through the encapsulation of pyrene (donor) and pyridinium (acceptor) guest pair in cucurbit[8]uril also displays the mechanochromic luminescent property. This work not only discloses the effect of host-guest inclusion on the mechanochromic property of organic molecules, but also provides a principle and a facile way to design the sequentially red-shifted mechanochromic materials.

Self-assembly of partially oxidized pillar[5]arenes induced by intermolecular charge transfer interactions

Supramolecular interactions such as π-π stacking interaction and charge transfer interaction have drawn much attention in the design and construction of various supramolecular assemblies. Herein, partially oxidized pillar[5]arene (P5A), pillar[4]arene[1]quinone (P4A1Q), pillar[3]arene[2]quinone (P3A2Q), and pillar[2]arene[3]quinone (P2A3Q) were synthesized by one-step reaction. As indicated by experimental characterization data and density function theory modeling results, charge transfer interaction among partially oxidized P5A plays a significant role in host-host self-assembly behavior and corresponding packing morphology. This work provides a unique strategy for the construction of functional macrocyclic assemblies through host-host self-assembly.

Two-Dimensional Optical Waveguides at Telecom Wavelengths Based on Organic Single-Crystal Microsheets of a Charge Transfer Complex.

Organic charge transfer (CT) cocrystals open a new door for the exploitation of low-dimensional near-infrared (NIR) emitters by a convenient self-assembly approach. However, research about the fabrication of sheet-like NIR-emitting microstructures that are significant for structural construction and integrated application is limited by the unidirectional molecular packing mode. Herein, via regulation of the biaxial intermolecular CT interaction, single-crystalline microsheets with remarkable NIR emission from 720 to 960 nm were synthesized via the solution self-assembly process of dithieno[3,2-b:2',3'-d]thiophene and 7,7,8,8-tetracyanoquinodimethane. The expected sheet-like structure is conducive to achieving a two-dimensional (2D) optical waveguide with an ultralow optical loss rate of 0.250 dB/μm at 860 nm. More significantly, these as-prepared organic microsheets with tunable thicknesses (h) from 100 to 1100 nm exhibit thickness-dependent NIR optical transportation performance. These findings could pave the way to a new class of low-dimensional NIR emitters for 2D photonics at telecom wavelengths.

The future of visible light photoinitiators of polymerization for photocrosslinking applications

Photopolymerization under visible light is an active research field whose intense research activity is supported by the recent advances in 3D and 4D printing. Within this perspective article, emerging trends in visible light photopolymerization for photocrosslinking applications will be presented. Notably, the performant multicomponent photoinitiating systems extensively used in the past are now progressively discarded in favor of mono-component systems easier to handle. The reduction of side-products formed during polymerization, the reduction of oxygen inhibition, the formation of dyes by intermolecular charge transfer interactions or the development of more ecological polymerization processes using sunlight as the irradiation source have recently been the focus of intense research efforts. Furthermore, emerging trends in processes favoring the elongation of the polymer lifetime such as self-healing will be presented and this perspective article aims at presenting the most recent breakthroughs in these attempts. To end, the future vision of visible light photopolymerization for photocrosslinking applications will be discussed.

Macrocycle‐Based Crystalline Supramolecular Assemblies Built with Intermolecular Charge‐Transfer Interactions

AbstractSynthetic macrocycles have served as principal tools for supramolecular chemistry, have greatly extended the scope of organic charge transfer (CT) complexes, and have proved to be of great practical value in the solid state during the past few years. In this Minireview, we summarize the research progress on the macrocycle‐based crystalline supramolecular assemblies primarily driven by intermolecular CT interactions (a.k.a. macrocycle‐based crystalline CT assemblies, MCCAs for short), which are classified by their donor–acceptor (D‐A) constituent elements, including simplex macrocyclic hosts, heterogeneous macrocyclic hosts, and host–guest D‐A pairs. Particular attention will be focused on their diverse functions and applications, as well as the underlying CT mechanisms from the perspective of crystal engineering. Finally, the remaining challenges and prospects are outlined.

Macrocycle-Based Crystalline Supramolecular Assemblies Built with Intermolecular Charge-Transfer Interactions.

Synthetic macrocycles have served as principal tools for supramolecular chemistry, have greatly extended the scope of organic charge transfer (CT) complexes, and have proved to be of great practical value in the solid state during the past few years. In this Minireview, we summarize the research progress on the macrocycle-based crystalline supramolecular assemblies primarily driven by intermolecular CT interactions (a.k.a. macrocycle-based crystalline CT assemblies, MCCAs for short), which are classified by their donor-acceptor (D-A) constituent elements, including simplex macrocyclic hosts, heterogeneous macrocyclic hosts, and host-guest D-A pairs. Particular attention will be focused on their diverse functions and applications, as well as the underlying CT mechanisms from the perspective of crystal engineering. Finally, the remaining challenges and prospects are outlined.

Color-Tunable Upconversion-Emission Switch Based on Cocrystal-to-Cocrystal Transformation.

Cocrystal engineering, involving the assembly of two or more components into a highly ordered solid-state superstructure, has emerged as a popular strategy for tuning the photophysical properties of crystalline materials. The reversible co-assembly and disassembly of multicomponent cocrystals and their reciprocal transformation in the solid state remain challenging objectives. Herein, we report a color-tunable upconversion-emission switch based on the interconversion between two cocrystals. One red- and one yellow-emissive cocrystal, composed of an electron-deficient naphthalenediimide-based triangular macrocycle and different electron donors, have been obtained. The red- and yellow-emissive cocrystals undergo reversible transformations on exchanging the electron donors. Benefiting from intermolecular charge transfer interactions, the two cocrystals display superior two-photon excited upconversion emission. Accompanying the interconversion of the two cocrystals, their luminescent color changes between red and yellow, forming a dual-color upconversion-emission switch. This research provides a rare yet critical example involving precise control of cocrystal-to-cocrystal transformation and affords a reference for fabricating color-tunable nonlinear optical materials in the solid state.

Aqueous-phase tunable multi-color luminescent supramolecular assemblies based on cucurbit[10]uril-enhanced intermolecular charge-transfer interactions

A series of supramolecular assemblies through CB[10]-enhanced intermolecular (homodimer or heterodimer) charge-transfer interactions show their tunable multi-color luminescence.

Molecular and Aggregate Structural, Thermal, Mechanical and Photophysical Properties of Long-Chain Amide Gelators Containing an α-Diketo Group in the Presence or Absence of a Tertiary Amine Group

Three structurally related gelators, each containing octadecyl chains, an α-diketo group at the 9,10 positions, and each with a different N-amide group-isobutyl (DIBA), isopentyl (DIPA) or N-(2-(dimethylamino)ethyl) (DMEA)-have been synthesized. Their neat structures as well as the thermal mechanical, and photophysical properties in their gel states with various liquids have been investigated. The gelator networks of DIBA and DIPA in octane, hexylbenzene and silicone oil consist of bundles of fibers. These gels are partially thixotropic and mechanically, thermally (to above their melting or silicone oil gelation temperatures), and photophysically stable. They are mechanically and thermally stronger than the gels formed with DMEA, the gelator with a tertiary amine group. The lone pair of electrons of the tertiary amine group leads to an intra-molecular or inter-molecular charge-transfer interaction, depending on whether the sample is a solution, sol, or gel. Neat, solid DMEA does not undergo the charge-transfer process because its amino and diketo groups are separated spatially by a large distance in the crystalline state and cannot diffuse into proximity. However, the solution of DIPA upon the addition of triethylamine becomes unstable over time at room temperature in the dark or (more rapidly) when irradiated, which initiates the aforementioned charge-transfer processes. The eventual reaction of the gelators in the presence of a tertiary amine group is ascribed to electron transfer from the lone-pair on nitrogen to an α-diketo group, followed by proton transfer to an oxygen atom on the anion radical of the α-diketo group from a methyl or methylene group attached to the nitrogen atom of the cation radical. Finally, the formation of an α-diketyl radical leads to irreversible electronic and structural changes that are observed over time.

Hierarchical Integration of Organic Core/Shell Microwires for Advanced Photonics.

The combination of multiple components or structures into integrated micro/nanostructures for practical application has been pursued for many years. Herein, a series of hierarchical organic microwires with branch, core/shell (C/S), and branch C/S structures are successfully constructed based on organic charge transfer (CT) cocrystals with structural similarity and physicochemical tunability. By regulating the intermolecular CT interaction, single microwires and branch microstructures can be integrated into the C/S and branch C/S structures, respectively. Significantly, the integrated branch C/S microwires, with multicolor waveguide behavior and branch structure multichannel waveguide output characteristics, can function as an optical logic gate with multiple encoding features. This work provides useful insights for creating completely new types of organic microstructures for integrated optoelectronics.