Double J-Coupling Strategy for Near Infrared Emitters

Dual-modal imaging-guided agent based on NIR-II aggregation-induced emission luminogens with balanced phototheranostic performance

Phototherapy has garnered considerable interest for its potential to revolutionize conventional cancer treatment. Organic materials with near-infrared II (NIR-II, 1000-1700 nm) fluorescence and photothermal effects are key for precise tumor diagnosis and treatment, yet optimizing their output for higher resolution and reduced photodamage remains a challenge. Herein, a multifunctional NIR-II photosensitizer (LSC) has been developed using the aggregation-induced emission (AIE) technology. The utilization of thieno[3,2-b]thiophene as an electron-rich and bulky donor/acceptor bridge has allowed for the elongation of conjugation length and distortion of the AIE main chain. This strategic modification effectively enhances the electron push-pull effect, endowing the LSC with a Stokes shift of over 400 nm and AIE characteristics. We have successfully built-up stable nanoparticles called FA-LSC NPs using a nano-precipitation method. These nanoparticles exhibit high NIR-II fluorescent brightness (ε × QY = 1064 M-1 cm-1) and photothermal conversion efficiency (41%). Furthermore, the biocompatible FA-LSC NPs demonstrate effective tumor accumulation and exceptional photothermal therapeutic efficacy both in vitro and in vivo. These nanoparticles were applied to fluorescence-photothermal dual-mode imaging-guided photothermal ablation in a HeLa tumor xenograft mouse model, resulting in favorable photothermal clearance outcomes.

Electron-Withdrawing Substituents Enhance the Type I PDT and NIR-II Fluorescence of BODIPY J Aggregates for Bioimaging and Cancer Therapy

Organic dyes with simultaneously boosted near-infrared-II (NIR-II) fluorescence, type I photodynamic therapy (PDT), and photothermal therapy (PTT) in the aggregate state are still elusive due to the unclear structure-function relationship. Herein, electron-withdrawing substituents are introduced at the 5-indolyl positions of BODIPY dyes to form tight J-aggregates for enhanced NIR-II fluorescence and type I PDT/PTT. The introduction of an electron-rich julolidine group at the meso position and an electron-withdrawing substituent (-F) at the indolyl moiety can enhance intermolecular charge transfer and the hydrogen bonding effect, contributing to the efficient generation of superoxide radicals in the aggregate state. The nanoparticles of BDP-F exhibit NIR-II fluorescence at 1000 nm, good superoxide radical generation ability, and a high photothermal conversion efficiency (50.9%), which enabled NIR-II fluorescence-guided vasculature/tumor imaging and additive PDT/PTT. This work provides a strategy for constructing phototheranostic agents with enhanced NIR-II fluorescence and type I PDT/PTT for broad biomedical applications.

Introducing a π-Skeleton Perpendicular to the Central Methylene Carbon in Alkanediamides: Design of Supramolecular Polymers with an Offset π-Stacking Arrangement

The self-assembly behavior of a heptanediamide derivative that contains a four-ring fused π-skeleton on its central methylene carbon atom has been examined. This molecule, which also contains two octyl chains, gelated the nonpolar solvent methylcyclohexane through the formation of fibrous nanostructures with hydrogen-bonding networks through a cooperative nucleation-elongation process. The supramolecular polymerization is accompanied by bathochromic shifts of both the absorption and fluorescence bands while maintaining a fluorescence quantum yield comparable to that of the monomeric state. Theoretical calculations provided an energetically stable structure, in which the π-skeletons are stacked with an offset of more than 8.0 Å, replicating the experimentally observed absorption change due to exciton coupling. Moreover, a slow transition with an inversion of the chiral arrangement of the π-conjugated moieties was induced by replacing the octyl chains with chiral alkyl chains. Our molecular-design strategy was further applied to a five-ring fused π-skeleton, which also forms an offset π-stacking arrangement and exhibits more effective chiral exciton coupling in the aggregated state.

J-Aggregation Strategy toward Potentiated NIR-II Fluorescence Bioimaging of Molecular Fluorophores

Molecular fluorophores emitting in the second near-infrared (NIR-II, 1000-1700 nm) window with strong optical harvesting and high quantum yields hold great potential for in vivo deep-tissue bioimaging and high-resolution biosensing. Recently, J-aggregates are harnessed to engineer long-wavelength NIR-II emitters and show unique superiority in tumor detection, vessel mapping, surgical navigation, and phototheranostics due to their bathochromic-shifted optical bands in the required slip-stacked arrangement aggregation state. However, despite the preliminary progress of NIR-II J-aggregates and theoretical study of structure-property relationships, further paradigms of NIR-II J-aggregates remain scarce due to the lack of study on aggregated fluorophores with slip-stacked fashion. In this effort, how to utilize the specific molecular structure to form slip-stacked packing motifs with J-type aggregated exciton coupling is emphatically elucidated. First, several molecular regulating strategies to achieve NIR-II J-aggregates containing intermolecular interactions and external conditions are positively summarized and deeply analyzed. Then, the recent reports on J-aggregates for NIR-II bioimaging and theranostics are systematically summarized to provide a clear reference and direction for promoting the development of NIR-II organic fluorophores. Eventually, the prospective efforts on ameliorating and promoting NIR-II J-aggregates to further clinical practices are outlined.

Recent advances in fluorescence imaging-guided photothermal therapy and photodynamic therapy for cancer: From near-infrared-I to near-infrared-II

Phototherapy (including photothermal therapy, PTT; and photodynamic therapy, PDT) has been widely used for cancer treatment, but conventional PTT/PDT show limited therapeutic effects due to the lack of disease recognition ability. The integration of fluorescence imaging with PTT/PDT can reveal tumor locations in a real-time manner, holding great potential in early diagnosis and precision treatment of cancers. However, the traditional fluorescence imaging in the visible and near-infrared-I regions (VIS/NIR-I, 400-900 nm) might be interfered by the scattering and autofluorescence from tissues, leading to a low imaging resolution and high false positive rate. The deeper near-infrared-II (NIR-II, 1000-1700 nm) fluorescence imaging can address these interferences. Combining NIR-II fluorescence imaging with PTT/PDT can significantly improve the accuracy of tumor theranostics and minimize damages to normal tissues. This review summarized recent advances in tumor PTT/PDT and NIR-II fluorophores, especially discussed achievements, challenges and prospects around NIR-II fluorescence imaging-guided PTT/PDT for cancers.

Exploration of NIR Squaraine Contrast Agents Containing Various Heterocycles: Synthesis, Optical Properties and Applications

Squaraine dye is a popular class of contrast near-infrared (NIR) dyes. Squaraine dyes have shown the ability to be modified with various heterocycles. The indole moiety is the most notable heterocycle incorporated in squaraine dyes. A tremendous amount of work has gone into developing indole-based squaraine dyes and determining their applications. The optical properties of squaraine dyes containing an indole moiety facilitate high quantum yields and molar absorptivity, but the absorbance maxima is capped near 700 nm. This is the major limitation of indole-based squaraine dyes. In comparison, other heterocycles with larger conjugated systems such as quinoline and perimidine have demonstrated promising optical properties and immense potential for modifications, albeit with limited development. Quinoline- and perimidine-based squaraine dyes have molar extinction coefficients over 100,000 M-1 cm-1 and absorbances over 800 nm. This report will look at indole-, quinoline-, and perimidine-based squaraine dyes. Due to the sheer number of reported dyes, the search for indole-based squaraine dyes has been limited to reports from the past five years (2018-2023). For quinoline- and perimidine-based squaraine dyes, a holistic search was performed to analyze the optical properties and applications, due to the abovementioned limitation. This report will evaluate the three different classes of squaraines: indole-, quinoline-, and perimidine-based, to evaluate their optical properties and applications, with the goal of encouraging the exploration of other heterocycles for use in squaraine dyes.

Enantiopure J-Aggregate of Quaterrylene Bisimides for Strong Chiroptical NIR-Response

Chiral polycyclic aromatic hydrocarbons can be tailored for next-generation photonic materials by carefully designing their molecular as well as supramolecular architectures. Hence, excitonic coupling can boost the chiroptical response in extended aggregates but is still challenging to achieve by pure self-assembly. Whereas most reports on these potential materials cover the UV and visible spectral range, systems in the near infrared (NIR) are underdeveloped. We report a new quaterrylene bisimide derivative with a conformationally stable twisted π-backbone enabled by the sterical congestion of a fourfold bay-arylation. Rendering the π-subplanes accessible by small imide substituents allows for a slip-stacked chiral arrangement by kinetic self-assembly in low polarity solvents. The well dispersed solid-state aggregate reveals a sharp optical signature of strong J-type excitonic coupling in both absorption (897 nm) and emission (912 nm) far in the NIR region and reaches absorption dissymmetry factors up to 1.1 × 10^-2. The structural elucidation was achieved by atomic force microscopy and single-crystal X-ray analysis which we combined to derive a structural model of a fourfold stranded enantiopure superhelix. We could deduce that the role of phenyl substituents is not only granting stable axial chirality but also guiding the chromophore into a chiral supramolecular arrangement needed for strong excitonic chirality.

Anti-Quenching NIR-II J-Aggregates of Benzo[c]thiophene Fluorophore for Highly Efficient Bioimaging and Phototheranostics

Molecular fluorophores with the second near-infrared (NIR-II) emission hold great potential for deep-tissue bioimaging owing to their excellent biocompatibility and high resolution. Recently, J-aggregates are used to construct long-wavelength NIR-II emitters as their optical bands show remarkable red shifts upon forming water-dispersible nano-aggregates. However, their wide applications in the NIR-II fluorescence imaging are impeded by the limited varieties of J-type backbone and serious fluorescence quenching. Herein, a bright benzo[c]thiophene (BT) J-aggregate fluorophore (BT6) with anti-quenching effect is reported for highly efficient NIR-II bioimaging and phototheranostics. The BT fluorophores are manipulated to have Stokes shift over 400 nm and aggregation-induced emission (AIE) property for conquering the self-quenching issue of the J-type fluorophores. Upon forming BT6 assemblies in an aqueous environment, the absorption over 800 nm and NIR-II emission over 1000 nm are boosted for more than 41 and 26 folds, respectively. In vivo visualization of the whole-body blood vessel and imaging-guided phototherapy results verify that BT6 NPs are excellent agent for NIR-II fluorescence imaging and cancer phototheranostics. This work develops a strategy to construct bright NIR-II J-aggregates with precisely manipulated anti-quenching properties for highly efficient biomedical applications.

Excited-State THz Vibrations in Aggregates of PtII Complexes Contribute to the Enhancement of Near-Infrared Emission Efficiencies

The exploration of deactivation mechanisms for near-infrared(NIR)-emissive organic molecules has been a key issue in chemistry, materials science and molecular biology. In this study, based on transient absorption spectroscopy and transient grating photoluminescence spectroscopy, we demonstrate that the aggregated PtII complex 4H (efficient NIR emitter) exhibits collective out-of-plane motions with a frequency of 32 cm-1 (0.96 THz) in the excited states. Importantly, similar THz characteristics were also observed in analogous PtII complexes with prominent NIR emission efficiency. The conservation of THz motions enables excited-state deactivation to proceed along low-frequency vibrational coordinates, contributing to the suppression of nonradiative decay and remarkable NIR emission. These novel results highlight the significance of excited-state vibrations in nonradiative processes, which serve as a benchmark for improving device performance.

Maximal emission beyond 1200 nm dicyanovinyl-functionalized squaraine for in vivo vascular imaging

The first maximum emission wavelength beyond 1200 nm acceptor-substituted squaraine fluorophore with ultra-high brightness and photostability has been developed. It can be co-assembled with bovine serum albumin to form an excellent biocompatible dye-protein nanocomplex with significant fluorescence enhancement for high-resolution vascular imaging.

Red-to-Near-Infrared Emitting PyrrolylBODIPY Dyes: Synthesis, Photophysical Properties and Bioimaging Application

Near-infrared (NIR) fluorophores with characteristics such as deep tissue penetration, minimal damage to the biological samples, and low background interference, are highly sought-after materials for in vivo and deep-tissue fluorescence imaging. Herein, series of 3-pyrrolylBODIPY derivatives and 3,5-dipyrrolylBODIPY derivatives have been prepared by a facile regioselective nucleophilic aromatic substitution reaction (S_N Ar) on 3,5-halogenated BODIPY derivatives (3,5-dibromo or 2,3,5,6-tetrachloroBODIPYs) with pyrroles. The installation of a pyrrolic unit onto the 3-position of the BODIPY chromophore leads to a dramatic red shift of both the absorption (up to 160 nm) and the emission (up to 260 nm) in these resultant 3-pyrrolylBODIPYs with respect to that of the BODIPY chromophore. Their further 5-positional functionalization provides a facile way to fine tune their photophysical properties, and these resulting dipyrrolylBODIPYs and functionalized pyrrolylBODIPYs show strong absorption in the deep red-to-NIR regions (595-684 nm) and intense NIR fluorescence emission (650-715 nm) in dichloromethane. To demonstrate the applicability of these functionalized pyrrolylBODIPYs as NIR fluorescent probes for cell imaging, pyrrolylBODIPY 6 a containing mitochondrion-targeting butyltriphenylphosphonium cationic species was also prepared. It selectively localized in mitochondria of HeLa cells, with low cytotoxicity and intense deep red fluorescence emission.

Discovery of BODIPY J-aggregates with absorption maxima beyond 1200 nm for biophotonics

Organic dyes with absorption maxima in the second near-infrared window (NIR-II; 1000 to 1700 nm) are of great interest in biophotonics. However, because of the lack of appropriate molecular scaffolds, current research in this field is limited to cyanine dyes, and developing NIR-II-absorbing organic dyes for biophotonics remains an immense challenge. Here, we rationally designed an ethenylene-bridged BODIPY scaffold featuring excellent J-aggregation capabilities and revealed that the bridging ethylene unit is crucial for intermolecular J-coupling regulation. By integrating the electron-donating groups into the scaffold, we obtained a BODIPY dye, BisBDP2, with a J-aggregate absorption maximum of around 1300 nm. BisBDP2 J-aggregates show excellent photothermal performance, including intense photoacoustic response, and a high photothermal conversion efficiency value of 63%. In vivo results demonstrate the potential of J-aggregates for photoacoustic imaging and photothermal ablation of deep-seated tumors. This study will speed up the exploration of NIR-II-absorbing J-aggregates for future biophotonic applications.

Modified norcyanines enable ratiometric pH imaging beyond 1000 nm

Activatable fluorophores with emission beyond 1000 nm have the potential to enable high contrast imaging in complex in vivo settings. However, there are few scaffolds that can be applied to this challenge. Here we detail the synthesis and evaluation of benzo[c,d]indole-substituted norcyanines that enable pH responsive fluorescence imaging in the long wavelength (>1150 nm) range. A key component of our molecular design is the installation of a hydrophilic substituted quaternary amine in the central dihydropyridine ring system. A compound with a C4'-phenyl substituent, but not the C4'-protio homologue, exhibits absorbance maxima of 740 nm and 1130 nm in basic and acidic media, respectively, with evidence of J-aggregate-like properties. These two distinct absorbances enabled ratiometric imaging of probe internalization in a tumor model. Overall, these studies provide a new class of activatable long-wavelength responsive fluorophores with promising photophysical properties.

Charge Delocalization and Vibronic Couplings in Quadrupolar Squaraine Dyes

Squaraines are prototypical quadrupolar charge-transfer chromophores that have recently attracted much attention as building blocks for solution-processed photovoltaics, fluorescent probes with large two-photon absorption cross sections, and aggregates with large circular dichroism. Their optical properties are often rationalized in terms of phenomenological essential state models, considering the coupling of two zwitterionic excited states to a neutral ground state. As a result, optical transitions to the lowest S1 excited state are one-photon allowed, whereas the next higher S2 state can only be accessed by two-photon transitions. A further implication of these models is a substantial reduction of vibronic coupling to the ubiquitous high-frequency vinyl-stretching modes of organic materials. Here, we combine time-resolved vibrational spectroscopy, two-dimensional electronic spectroscopy, and quantum-chemical simulations to test and rationalize these predictions for nonaggregated molecules. We find small Huang-Rhys factors below 0.01 for the high-frequency, 1500 cm^-1 modes in particular, as well as a noticeable reduction for those of lower frequency modes in general for the electronic S0 → S1 transition. The two-photon allowed state S2 is well separated energetically from S1 and has weak vibronic signatures as well. Thus, the resulting pronounced concentration of the oscillator strength in a narrow region relevant to the lowest electronic transition makes squaraines and their aggregates exceptionally interesting for strong and ultrastrong coupling of excitons to localized light modes in external resonators with chiral properties that can largely be controlled by the molecular architecture.

The Pursuit of Shortwave Infrared-Emitting Nanoparticles with Bright Fluorescence through Molecular Design and Excited-State Engineering of Molecular Aggregates

Shortwave infrared (SWIR) fluorescence detection gradually becomes a pivotal real-time imaging modality, allowing one to elucidate biological complexity in deep tissues with subcellular resolution. The key challenge for the further growth of this imaging modality is the design of new brighter biocompatible fluorescent probes. This review summarizes the recent progress in the development of organic-based nanomaterials with an emphasis on new strategies that extend the fluorescence wavelength from the near-infrared to the SWIR spectral range and amplify the fluorescence brightness. We first introduce the most representative molecular design strategies to obtain near-infrared-SWIR wavelength fluorescence emission from small organic molecules. We then discuss how the formation of nanoparticles based on small organic molecules contributes to the improvement of fluorescence brightness and the shift of fluorescence to SWIR, with a special emphasis on the excited-state engineering of molecular probes in an aggregate state and spatial packing of the molecules in nanoparticles. We build our discussion based on a historical perspective on the photophysics of molecular aggregates. We extend this discussion to nanoparticles made of conjugated polymers and discuss how fluorescence characteristics could be improved by molecular design and chain conformation of the polymer molecules in nanoparticles. We conclude the article with future directions necessary to expand this imaging modality to wider bioimaging applications including single-particle deep tissue imaging. Issues related to the characterization of SWIR fluorophores, including fluorescence quantum yield unification, are also mentioned.

Helically Twisted Nanoribbons Based on Emissive Near-Infrared Responsive Quaterrylene Bisimides

Graphene nanoribbons (GNRs) have the potential for next-generation functional devices. So far, GNRs with defined stereochemistry are rarely reported in literature and their optical response is usually bound to the ultraviolet or visible spectral region, while covering the near-infrared (NIR) regime is still challenging. Herein, we report two novel quaterrylene bisimides with either one- or twofold-twisted π-backbones enabled by the steric congestion of a fourfold bay arylation leading to an end-to-end twist of up to 76°. The strong interlocking effect of the π-stacked aryl substituents introduces a rigidification of the chromophore unambiguously proven by single-crystal X-ray analysis. This leads to unexpectedly strong NIR emissions at 862 and 903 nm with quantum yields of 1.5 and 0.9%, respectively, further ensuring high solubility as well as resolvable and highly stable atropo-enantiomers. Circular dichroism spectroscopy of these enantiopure chiral compounds reveals a strong Cotton effect Δε of up to 67 M^-1 cm^-1 centered far in the NIR region at 849 nm.

Exciton Delocalization Counteracts the Energy Gap: A New Pathway toward NIR-Emissive Dyes

Exciton coupling between the transition dipole moments of ordered dyes in supramolecular assemblies, so-called J/H-aggregates, leads to shifted electronic transitions. This can lower the excited state energy, allowing for emission well into the near-infrared regime. However, as we show here, it is not only the excited state energy modifications that J-aggregates can provide. A bay-alkylated quaterrylene was synthesized, which was found to form J-aggregates in 1,1,2,2-tetrachloroethane. A combination of superradiance and a decreased nonradiative relaxation rate made the J-aggregate four times more emissive than the monomeric counterpart. A reduced nonradiative relaxation rate is a nonintuitive consequence following the 180 nm (3300 cm^-1) red-shift of the J-aggregate in comparison to the monomeric absorption. However, the energy gap law, which is commonly invoked to rationalize increased nonradiative relaxation rates with increasing emission wavelength, also contains a reorganization energy term. The reorganization energy is highly suppressed in J-aggregates due to exciton delocalization, and the framework of the energy gap law could therefore reproduce our experimental observations. J-Aggregates can thus circumvent the common belief that lowering the excited state energies results in large nonradiative relaxation rates and are thus a pathway toward highly emissive organic dyes in the NIR regime.