Intramolecular Charge Transfer Research Articles

Structural Fine‐Tuning to Achieve Highly Fluorescent Organic and Water‐Soluble Thiazolo[5,4‐d]thiazole Chromophores

AbstractFluorescent molecular systems are important for various applications such as sensing of analytes, probes for biologically relevant processes and as optoelectronic materials. Achieving high fluorescence quantum yield across the spectrum of solvent polarity and in solid‐state is challenging in molecular materials. Herein, we present a strategy to achieve strongly fluorescent molecular materials based on weak intramolecular charge‐transfer (ICT) in a family of unsymmetrical donor‐thiazolo[5,4‐d]thiazole‐acceptor systems (both neutral and cationic). Detailed photophysical studies reveal that the delicate balance between the donor and acceptor result in high solution‐state fluorescence quantum yield (>80 %) in both polar protic and apolar solvents. Quantum chemical computations uncover a hitherto unappreciated insight that the extent of ICT is aptly represented by the change in Mulliken charges between the ground and excited state for different fragments rather than the classical approach of monitoring the change in dipole moment for the entire molecule. This insight rationalizes the observed photophysical properties and can have implications in the design of tuneable donor‐π‐acceptor systems.

Molecular Design Motifs of Non‐Fullerene Acceptors for Mitigating All Inherent Non‐Ideal Energy Losses in Organic Photovoltaics

AbstractNon‐fullerene acceptors (NFAs) have demonstrated a great potential for the transcendence of performance limits in organic photovoltaics. Despite the tremendous efforts, however, synthetic control of non‐ideal recombination losses on the photovoltage are still elusive and thus lag far behind the inorganic counterparts. Here, a rational design strategy of non‐fullerene acceptors is presented to overcome the limitations derived from the inherent excited state properties of organic molecules. Spectroscopic studies on a series of chemical analogs of ladder‐type fused acceptors reveal that the radiative efficiency of non‐fullerene acceptors is predominantly determined by the inherently strong electron‐vibration coupling of their bound excited states, rather than the redistribution of molecular orbitals itself. In this sense, regardless of intramolecular charge transfer characteristics, the incorporation of rigid, and planar moiety with wide π plane is found to promote the dominance of long‐lived and weakly‐bound excited states in the photophysical process, in which the mitigated non‐radiative decays of NFAs can be directly translated into that of charge‐transfer states. Furthermore, the incorporation of such chemical moiety can minimize the evolution of subgap broadening in the bulk heterojunction blends, lowering the contribution of non‐ideal radiative losses. As a result, the model systems demonstrate the non‐radiative loss down to 180–190 meV, without sacrificing photocurrent generations, reaching that of inorganic counterparts.

Exploring solvatochromism: a comprehensive analysis of research data of the solvent -solute interactions of 4-nitro-2-cyano-azo benzene-meta toluidine

This study investigates the solvatochromic behavior of a new D1 disperse dye, focusing on the 4-nitro-2-cyano-azo-Benzene-metaToluidine type. Using UV-visible spectroscopy, we analyze the dye’s performance in single solvents and binary mixtures. Our analysis includes the evaluation of solvent polarity parameters and dye structure parameters, with emphasis on the longest wavelength intramolecular charge transfer absorption band. We identify nonlinear and linear patterns in the electronic transition energies plots with respect to solvent composition. Statistical analysis considers correlation coefficients, root mean squared error, mean absolute percentage error, and other parameters, providing insights into data accuracy and precision. This experimental and statistical study explores the absorption spectrum, wavelength characteristics, absorption energy, and polarity of a novel azo dye across various solvent and solute environments. These findings contribute to a comprehensive understanding of the dye’s photophysical and photochemical properties, potentially enabling applications in optical sensors and advanced materials.Graphical abstract

Colorimetric sensors for discriminatory detection of arsenite ions: Application in milk, honey and water samples and molecular logic gates

Millions of people are exposed to dangerous arsenic levels in drinking water, highlighting the urgent need for affordable, continuous on-site arsenic monitoring methods. It is crucial to specifically detect arsenite among inorganic arsenic anions because it is more poisonous than arsenate. Addressing these concerns, the present study developed 5-(4-nitrophenyl)-2-furaldehyde based two colorimetric chemosensors, N5R3 and N5R4, with different signaling groups for the selective detection of arsenite anions over arsenate in DMSO/H2O (6:4, v/v). The red-shift in the UV–Vis absorption spectra supported the distinct color changes of sensors N5R3 and N5R4 displayed upon binding with arsenite. Sensors demonstrated stability over a pH range of 6 to 12 and exhibited stability over a considerable time period. Among the chemosensors, N5R3 exhibited the lowest detection limit of 7.41 ppb with a high binding constant of 2.9976 × 106 M−1 for arsenite. The 1:1 binding interactions between the chemosensors and arsenite were confirmed using B-H plot and Job’s plot analysis. The intramolecular charge transfer (ICT) mechanism for detecting arsenite was proposed through UV and 1H NMR titrations, electrochemical studies, mass spectral analysis and DFT calculations. The interactions between the sensor and arsenite anions were further analyzed using global reactivity parameters (GRPs). Practical applications were demonstrated through the utilization of test strips and molecular logic gates. Both chemosensors efficiently recognized arsenite in real water, honey, and milk samples.

Twisted Intramolecular Charge Transfer-Based Viscosity-Responsive Probe Reveals Lysosomal Degradation Process of Endocytosed Foreign Bodies.

The optimization of nanomedicines requires a thorough understanding of nanocarrier attrition during lysosome-mediated biological processes. Real-time monitoring of endocytosis provides valuable insights into the lysosomal effects on nanocarriers and the release of nanodrugs. We report the development of a coresponsive probe that detects changes in the spatial viscosity of the intracellular domain caused by lysosomal degradation of foreign bodies. The probe, based on a benzofuro[2,3-d]pyrimidine structure, exhibits torsional intramolecular charge transfer (TICT) and responds to ambient viscosity changes with a sensitive fluorescence intensity. The antidiffused fluorescence transition of the probe in the spatially restricted domain serves as a key indicator for real-time monitoring. When encapsulated with diverse foreign bodies and emitted into macrophages by endocytosis, the probe forms nanoparticles. Lysosomes uptake these materials for intracellular digestion, causing alterations in the aggregation or depolymerization state of the nanoparticles, leading to viscosity changes manifested by the probe's fluorescence. By studying the spatial viscosity changes caused by lysosomal degradation of foreign bodies, our monitoring strategy contributes to understanding the digestion or escape capabilities of potential pharmaceutical-carrying nanocarriers, providing guidelines to design more effective nanocarriers that navigate lysosomal degradation to achieve precise drug payloads and release.

Regulating Charge Transfer in Cyanine Dyes: A Universal Methodology for Enhancing Cancer Phototherapeutic Efficacy.

ConspectusDue to the advantages of spatiotemporal selectivity and inherent noninvasiveness, cancer phototherapy, which includes both photodynamic therapy (PDT) and photothermal therapy (PTT), has garnered significant attention in recent years as a promising cancer treatment. Despite the commendable progress in this field, persistent challenges remain. In PDT, limitations in dyes manifest as low intersystem crossing (ISC) efficiency and oxygen-dependent photoactivity, resulting in unsatisfactory performance, particularly under hypoxic conditions. Similarly, PTT encounters consistent insufficiencies in the photothermal conversion efficiency (PCE) of dyes. Additionally, the suboptimal phototherapeutic efficacy often exhibits a limited immune response. These factors collectively impose significant constraints on phototherapy in oncological applications, leading to limited tumor inhibition, tumor recurrence, and even metastasis.Unlike strategies that rely on external assistance with complicated systems, manipulating excited-state deactivation pathways in biocompatible dyes offers a universal way to systematically address these challenges. Our group has devoted considerable effort to achieving this goal. In this Account, we present and discuss our journey in optimizing excited-state energy-release pathways through regulating molecular charge transfer based on cyanine dyes, which are renowned for their exceptional photophysical properties and harmonious biocompatibility. The investigation begins with the introduction of amino groups in the meso position of a heptamethine cyanine dye, where the intramolecular charge transfer (ICT) effect causes a significant enlargement of the Stokes shift. Subsequently, ICT induced by introducing functional electron-deficient groups in cyanines is found to decrease the overlap of electron distribution or narrow the energy gaps of molecular frontier orbitals. Such modifications result in a reduction of the energy gaps between singlet and triplet states or an improvement in internal conversion, ultimately promoting phototherapy efficacy in both primary and distant tumors. Furthermore, with the intensification of the charge transfer effect aided by light, photoinduced intramolecular electron transfer occurs in some cyanines, leading to complete charge separation in the excited state. This process enhances the transition to the ground or triplet states, improving tumor phototherapy and inhibiting metastasis by increasing the PCE or the yield of reactive oxygen species, respectively. Shifting focus from intramolecular to intermolecular interactions, we successfully constructed and explored cyanines based on intermolecular charge transfer. These dyes, with excited-state dynamics mimicking natural photosynthesis, generate radicals and facilitate oxygen-independent hypoxic tumor PDT. Finally, we outlined the existing challenges and future directions for optimizing phototherapeutic efficacy by regulating molecular charge transfer. This Account provides molecular-level insights into improving phototherapeutic performance, offering valuable perspectives, and inspiring the development of functional dyes in other application fields.

Cellulose-based fluorescent chemosensor with controllable sensitivity for Fe3+ detection

Polymer-based sensors, particularly those derived from renewable polymers, are gaining attention for their superior properties compared to organic small molecules. However, their complex preparation and poor, uncontrollable sensitivity have hindered further development. Herein, cellulose-based polymer photoluminescence (PL) chemosensors were fabricated using a straightforward and adjustable strategy. Specifically, water-soluble cellulose acetoacetate (CAA) was used as the substance for the in-situ synthesis of 1,4-dihydropyridine (DHPs) fluorescent rings on cellulose chains via a catalyst-free, room-temperature Hantzsch reaction. Benefiting from the synergetic through-space conjugation of DHPs rings and semi-rigid cellulose chains with heteroatoms, the sensors exhibit bright and stable PL properties. Based on this performance, the cellulose-based sensor excels in the specific recognition of Fe3+ in aqueous systems, showing exceptional selectivity, stability, and anti-interference performance due to the synergy between the inner filter effect (IFE) and intramolecular charge transfer (ICT). Theoretical calculations confirm the role of the extended π-conjugated structure at the DHPs-4 position in modulating the sensor sensitivity, achieving a low limit of detection (LOD) of 0.48 μM. Furthermore, the versatility of the Hantzsch reaction shows the potential of this strategy for developing a new generation of biomass-based polymer portable sensors for real-time and on-site detection.

Synthesis of a dual-response signal sensor and naked-eye recognition of hypochlorite in real water samples

An innovative dual-modal colorimetric and fluorometric probe MDXM is synthesized based on the dihydro-1H-xanthene motif via Knoevenagel reaction. The incorporation of a malononitrile group into the dihydro-1H-xanthene motif via a double bond results in an extensive macroconjugated π-system, which endows MDXM with a pronounced molar absorptivity and effectively quenches the intramolecular charge transfer (ICT) effect. Upon exposure to hypochlorite, the C = C bond in MDXM undergoes cleavage, resulting in the formation of an aldehyde group. This process restores the ICT effect and manifests as a clear fluorescent ''turn-on'' signal at 530 nm, with a remarkably linear response across the ClO− concentration range of 0–60 μM. The probe achieves a notably low detection limit of 7.14 nM, reflecting its exceptional sensitivity. In the presence of 15 common metal ions and oxidizing agents, MDXM can specifically identify hypochlorite with excellent anti-interference ability. The response mechanism of MDXM to hypochlorite was confirmed through Nuclear Magnetic Resonance (NMR) spectroscopy and computational Density Functional Theory (DFT) calculations. The practical applicability of MDXM is underscored by its successful implementation in analyzing hypochlorite in 5 varied real water samples. This showcases MDXM's promise for environmental and analytical chemistry, providing a robust tool for hypochlorite detection in diverse settings.

A long-wavelength fluorescent probe based on carbazole with AIE characteristics for the detection of ONOO− and its application

Peroxynitrite (ONOO−) is an important reactive oxygen species, and excess ONOO− in the body can lead to a range of diseases. Addressing the need for sensitive detection ONOO−, in this work, a highly sensitivity and selectivity long-wavelength fluorescent probe (named CIS) was designed for tracking ONOO− in biological cells. Probe CIS featured carbazole as the fluorophore and the CC double bond with two electron withdrawing –CN groups as the recognition site. The probe CIS exhibited the typical aggregation-induced emission (AIE) and intramolecular charge transfer (ICT) characteristics with the bright red fluorescent emission in high water fraction (fw ≥ 50 %) and negligible emission in low water fraction. The fluorescence emission wavelength of probe CIS is 631 nm, with a large Stokes shift (151 nm), which can sensitively detect ONOO− in DMSO: H2O (7:3, v/v) environment. Notably, the probe CIS exhibited a wide pH detection range (pH = 3–10), a low detection limit of 36.66 nM, high selectivity and minimal interference. Furthermore, probe CIS demonstrated excellent biocompatibility and low cytotoxicity, and can be used for the detection of exogenous ONOO− in 4T1 cells (Mouse breast cancer cells). These research results indicate that probe CIS has potential application prospects in the biological aspects.

Substituent effects on the ESIPT process of H2O2 sensing product 1,3-bis(bispyridin-2-ylimino) isoindolin-4-ol: A theoretical study

H2O2 plays an important role in a variety of physiological processes, but its overproduction or accumulation can lead to oxidative damage and related diseases, so it is crucial to establish accurate H2O2 detection methods. Fluorescence sensing as a potential method, the design and modulation of its luminescent properties are crucial to the detection results. In this work, we investigated the sensing mechanism of a novel H2O2 fluorescent probe (P1) and examined the role of substituent effects in modulating the fluorescence properties of its reaction products. The results show that the non-fluorescence property of the probe is due to twisted intramolecular charge transfer (TICT) mechanism resulting from a torsion of the excited state molecular conformation, and the fluorescence turn-on upon recognition of H2O2 by the probe is due to the reaction product (BBYI1) following the excited state intramolecular proton transfer (ESIPT) process. When the substituent is an electron-donating group, the energy barrier of the ESIPT reaction is lowered, promoting luminescence, while the electron-withdrawing group has the opposite effect. In summary, the present work may offer a theoretical foundation for the development of more sensitive fluorescent probes for ESIPT-based H2O2 detection.

Red/near-infrared (NIR) difluoroboron β-diketonate derivatives with reversible mechanochromism for cellular imaging

Near-infrared (NIR) fluorophores have promoted the development of materials for bioimaging, but traditional NIR dyes usually suffer from aggregation-caused quenching (ACQ), impeding their applications. Herein, we propose two difluoroboron β-diketonate complexes TBO and TBS, consisting a donor–acceptor (D-A) structure with triphenylamine (TPA) moiety as an electron donors and difluoroboron as well as furan or thiophene building block as an electron acceptor. The theoretical calculation and optical data shows that both of them have intramolecular charge transfer (ICT) characteristics. Such ICT characteristics endow them with both solvatochromism and dual-state emission (DSE) properties. In the solvent CH2Cl2, the emission wavelength of TBO ranges from 550 nm to 750 nm, with a low fluorescence quantum yield (Φ = 7.0 %). However, in the less polar solvent hexane, the emission wavelength blue-shifts, with an increased Φ reaching up to 18 %. Moreover, TBO and TBS exhibit mechanochromic characteristics and rare multi-channel fluorescence emission phenomena at solid-state. Their solid-state samples can emit fluorescence in four spectral bands with maximum emission wavelengths at 300 nm, 400 nm, 600 nm, and 770 nm under excitation at 240 nm. These unique optical properties are expected to be utilized for detecting polarity of system and deformation. Moreover, according to the results of cell imaging and flow cytometry, TBO molecular were easily internalized into Hela cells and distributed in the cytoplasm with strong red fluorescence. Therefore, this research inspires more insight into development of NIR luminogens for biomedical imaging.

Ultrafast response of diketo-pyrrolo-pyrrole dyes with large two-photon absorption enhancement as well as AIE property

To study the effect of terminal groups substituent on the two-photon absorption (TPA) properties of [Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text] structural dyes with diketo-pyrrolo-pyrrole (DPP) as central electron acceptor group, linear/transient absorption spectroscopy, one/two-photon fluorescence spectra, open-aperture [Formula: see text]-scan, and excited states quantum chemical calculations have been performed to systematically study the intermediate product electron acceptor J0 as well as three [Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text]-[Formula: see text] structural dyes using benzene, triphenylamine and 2,3-methoxy-triphenylamine as terminal electron donors, named as J1, J2, and J3, respectively. The intermediate product electron acceptor itself exhibits a relatively large TPA response, which indicates the DPP group has a large conjugated system. J2, with strong electron-donating ability groups triphenylamine as terminal groups, shows a large TPA cross-section of 534[Formula: see text]GM, which is approximately 10 times larger than that of J1 with weak electron pushing/pulling abilities groups benzene as terminal groups (51[Formula: see text]GM). The result indicates that the electron-donating abilities of terminal groups do play a key role in TPA response. Furthermore, methoxy substituent on the terminal triphenylamine groups can further effectively increase the TPA response, J3exhibits a 15-fold enhancement compared with that of J1. Transient absorption spectra and quantum chemical calculation results are well accorded with the experimental results. Terminal substituents with strong electron donor groups are beneficial for the formation of intramolecular charge transfer (ICT) process. Besides, methoxy substituent in the terminal of triphenylamine groups can further enhance the degree of ICT and induce AIE characteristics in the molecule.

Pyrazine-based supramolecular photosensitizing assemblies as type I photosensitizers for high-efficiency photooxidation reactions

Donor-acceptor thermally activated delayed fluorescent (TADF) materials have been widely used in photocatalytic reactions as organic photosensitizers. However, the construction of TADF photosensitizers via a supramolecular self-assembly approach with the regulation of reactive oxygen species (ROS) is still a big challenge. In this study, a TADF molecule 4,4'-(6-(pyridin-4-yl)quinoxaline-2,3-diyl)bis(N,N-diphenylaniline) (TPA-PQ) by incorporating triphenylamine (TPA) act as donor and 6-(pyridin-4-yl)quinoxaline (PQ) act as an acceptor was designed and synthesized, which exhibited excellent TADF property and strong intramolecular charge transfer. Meanwhile, the TPA-PQ can not only self-assemble to form nanofiber structure in mixed solvent of H2O and THF, but also can be used as a type I photosensitizer to selectively produce superoxide anion radicals (O2•–). More interestingly, the ROS generated from TPA-PQ can be used as photocatalysts for the oxidative hydroxylation of arylboronic acids and aerobic cross dehydrogenation coupling (CDC) reactions with high yields of 92 % and 88 %, respectively. This study presents an innovative and eco-friendly approach to utilize TADF supramolecular structures for the purpose of conducting photocatalyzed organic reactions.

CALCULATION of HOMA AND BIRD AROMATICITY INDICES, NLO and NBO PROPERTIES OF BENIDIPINE

Benidipine hydrochloride, being the derivate of 1,4-dihydropyridine is a calcium channel blocker antihypertensive drug. The aromaticity, natural bond orbital (NBO) and Nonlinear Optical (NLO) parameters properties of Benidipine (BEN) and BENHCl compounds were theoretically investigated using density functional theory (DFT) electronic structure method. Among the values of common aromaticity indices, the Harmonic Oscillator Aromaticity Model (HOMA) and BIRD's (Aromaticity Index) for electronic delocalization show that the aromaticity of the nitrophenyl group is higher in BEN and BENHCl compounds, and also the aromaticity in the gas phase is higher than that in water, octanol and DMF phase. In compound BEN, it was observed that as the dielectric constant of the medium increased, the aromaticity difference increased slightly. In the compound BEN, the correlation coefficient between HOMA values and aromatic fluctuation index (FLU), para delocalization index (PDI) and para-linear response (PLR) for the nitrophenyl group is higher than that the phenyl group. Donor-acceptor transitions, stabilization energies, intramolecular charge transfer were determined by natural population analyses. NLO parameters such as dipole moment, polarisibility and first order hyperpolarizability values of the BEN and BENHCl compound were also studied.

A lipid droplet-targeted NIR fluorescent probe for viscosity detection in Parkinson's disease

Investigating lipid droplets (LDs) is crucial for elucidating their role in biological function and the pathogenesis of related diseases, including Parkinson's disease (PD). Therefore, developing a fluorescent probe to measure LDs viscosity is crucial. To address the challenges of complex biological environments, a chemical toolbox, F-675, was introduced based on the twisted - intramolecular charge transfer mechanism. F-675 featured a large Stokes shift of 158 nm and emitted near-infrared emission at 675 nm, facilitating LDs detection with minimal biological fluorescent background. Additionally, F-675 exhibited excellent selectivity and sensitivity to changes of LDs. Furthermore, F-675 effectively detected LDs changes in PD models, ranging from cells, C. elegans and Parkin null Drosophila. These findings indicate the potential of F-675 as an effective tool for PD diagnosis.

Synthesis, Electrochemistry and Photochemistry of Hypervalent Phosphorus(V) Porphyrin and Antimony(V) Porphyrin Black Dyes

In the last few years, our focus has been on push-pull type hypervalent Group 15 porphyrins with intramolecular charge transfer (ICT) properties. Insertion of P(V) or Sb(V) ion in the porphyrin cavity results in a highly electron-deficient porphyrin center, under this condition the presence of electron-rich units in porphyrin meso-positions results in charge transfer within the molecules. Recently, we extended this strategy to 5,10,5,20-tetrakis(triphenylamine)porphyrin derivative where strong electron donor triphenylamine (TPA) units are in meso-positions. Upon insertion of the P(+5) and Sb(+5) ion, the resulting hypervalent Group 15 porphyrins manifested a strong ICT from the TPA units to the central porphyrin ring. The existence of ICT makes the absorption profiles cover the entire visible region with high molar extinction coefficients. Due to this reason, the studied hypervalent Group 15 porphyrins appear as black dyes. Here, I will present these porphyrins and discuss their structural, redox, and remarkable photophysical properties. Figure 1

Light-Induced Redox Dynamics in Plasmonic Nanostructures and Photosensitizer Transition Metal Compounds

Solar energy harvesting is a nuanced scientific field that involves precise control across energy, time, and nanometer length scales. The understanding of how to harness sunlight to promote chemical reactions is vital for the advancement of renewable energy applications and a sustainable future. Limited knowledge of the precise chemical mechanisms underlying solar energy harvesting restricts widespread technological use and the development of photovoltaic devices that utilize light-harvesting biodegradable materials. My research focuses on developing a fundamental understanding of visible light-activated excited state chemistry that is of great importance in elucidating the mechanisms, structures, and design features for catalysis. In this talk, we will examine excited-state chemical structure dynamics in nanomaterials that use light-matter interactions to drive interfacial or intramolecular charge transfer. Steady-state and time-resolved optical/X-ray spectroscopy allow for the observation of these dynamics across an expansive timescale—from the seconds timescale of oxygen-evolving reactions to the ultrafast timescales of electronic and vibrational motion. Novel light-matter interactions in plasmonic nanostructures, biomimetic metal oxides, and molecular transition metal complexes will be discussed. The talk will conclude with future opportunities for expanding the scope of nanoscale photocatalysis and for designing enhanced localized control over excited-state chemistry.[1] E. A. Sprague-Klein*, X. He, M. W. Mara, B. J. Reinhart, S. Lee, L. M. Utschig, K. L. Mulfort, L. X. Chen*, D. M. Tiede*. “Photo-Electrochemical Effect in the Water Oxidation Catalyst Cobalt-Phosphate (CoPi),” ACS Energy Letters, 2022, 7, 9, 3129–3138. [2] N. L. Warren, U. Yunusa, A. B. Singhal, E. A. Sprague-Klein*. "Facilitating Excited-State Plasmonics and Photochemical Reaction Dynamics," Chemical Physics Reviews, (Accepted December 2023).[3] U. Yunusa, N. Warren, D. Schauer, P. Srivastava, E. Sprague-Klein*. "Plasmon resonance dynamics and enhancement effects in Tris(bipyridine)ruthenium(II) gold nanosphere oligomers," Nanoscale, (Under Review).

Supramolecular Assembly of Nitrogen-Containing Curved π-Conjugated Molecules to Polar Crystal

The chemistry of curved π-conjugated molecules has been extensively studied because of the development of their synthetic approach. Among the numerous studies, we have been engaged in nitrogen-containing curved π-conjugated molecules. These molecules have extended optical properties, redox-active behaviors, and electron-donating ability due to the electron-rich nature of nitrogen atoms. Recently, we have focused on the supramolecular assembling behavior of these compounds. For example, we have reported buckycatcher consisted of two azabuckybowls, providing two-photon absorbing host-guest complexes with fullerenes. In this presentation, we would like to show the packing behavior using π-extended azahelicenes and azabuckybowls. Our group has developed π-extended aza[5]helicene derivatives, showing brilliant emissive features. Recently, we succeeded in the synthesis of unsymmetrically substituted azahelicenes. We found that their stackings in the crystal were dependent on the substituent. X-ray diffraction analysis for all products showed π-stacked conformation with each other in the crystal (Figure 1b). In particular, azahelicenes with triisopropylsilyl groupswere found to align their dipole moments to form a one-dimensional columnar structure in the crystal. Polar crystals are expected to have applications in ferroelectric, piezoelectric, and nonlinear optical response materials. This is the first example of a polar crystal with helicenes, paving the way for novel piezoelectric organic materials.The second topic is related to azabuckybowl. In the crystal, azabuckybowl forms a face-to-face dimer by concave-convex interaction. In this study, we prepared two types of azabuckybowl perylene bisimide (PBI) dyads connected with alkyne linkages. PBIs have also been reported to show columnar stacking in solution. We anticipated that the segregate stacking should be achieved by recognizing the difference in structures between the bowl and planar geometry. The UV/vis absorption spectra revealed the presence of intramolecular charge-transfer interaction. In addition, the concentration-dependent absorption spectra and 1H NMR spectra indicated PBI and azabuckybowl units stacked with each other independently in solution (Figure 1a). Here, we would like to discuss about the effect of the effect of molecular shape on the supramolecular assembly with the data obtained using these compounds. Figure 1

Nonadiabatic Molecular Dynamics Simulations Provide Evidence for Coexistence of Planar and Nonplanar Intramolecular Charge Transfer Structures in Fluorazene.

Fluorazene is a model compound for photoinduced intramolecular charge transfer (ICT) between aromatic moieties. Despite intensive studies, both spectroscopic and theoretical, a complete model of its photophysics is still lacking. Especially controversial is the geometry of its ICT structure, or structures. In order to fill in the gaps in the state of knowledge on this important model system, in the present study I report the results of nonadiabatic molecular dynamics (NAMD) simulations of its photorelaxation process in acetonitrile solution. To afford a direct comparison to spectroscopic data, I use the simulation results as the basis for the calculation of the transient absorption (TA) spectrum. The NAMD simulations provide detailed information on the sequence of events during the excited-state relaxation of the title compound. Following initial photoexcitation into the bright S2 state, the molecule undergoes rapid internal conversion into the S1 state, leading to the locally excited (LE) structure. The LE structure, in turn, undergoes isomerization into a population of ICT structures, with geometries ranging from near-planar to markedly nonplanar. The LE → ICT isomerization reaction is accompanied by the decay of the characteristic excited-state absorption band of the LE structure near 2 eV. The anomalous fluorescence emission band of fluorazene is found to originate mainly from the near-planar ICT structures, in part because they dominate the overall population of ICT structures. Thus, the planar ICT (PICT) model appears to be the most appropriate description of the geometry of the ICT structure of fluorazene.

AIE-active photosensitizers for concomitant fluorescence visualization and efficient photodynamic elimination of multidrug-resistant bacteria

Multidrug resistance (MDR) bacterial infection is a serious hazard to human health. Photodynamic therapy (PDT) has been deployed as an emerging tactic to combat pathogenic bacteria, but the development of efficient photosensitizers against MDR bacteria still remains a global challenge. Herein, two near-infrared (NIR) photosensitizers, named TPA-Th and TPA-Ph, which feature aggregation-induced emission (AIE), were constructed and successfully employed for MDR bacteria monitoring and effective elimination. TPA-Th and TPA-Ph exhibited great affinity with MDR G(+) and G(−) bacteria via the electrostatic interaction between the cationic pyridinium segment and the negative bacterial envelope together with hydrophobic forces. Meanwhile, the reinforced D-A effect of the two cationic AIEgens further accelerated the intersystem crossing (ISC) process and enhanced the intramolecular charge transfer (ICT), resulting in the boosted ROS generation capability particularly in the case of predominant hydroxyl radicals. Therefore, superb photodynamic antibacterial performances were achieved for both MRSA and MDR E.coli with a low dose of the AIE photosensitizers. This study provides guidance for the development of high-performance AIE-active PSs with no antibiotic resistance to defeat MDR bacterial infections.