Squaraine Chromophore Research Articles

Squaraine Appended with Benzodipyrrole for Fluorescent Sensing of Methanol: Exciton Coupling Controls Photophysical Properties.

The photophysical properties of dyes composed of two squaraine chromophores fused with a benzodipyrrole central moiety (BS1 and BS2), were investigated using steady-state absorption, fluorescence, and transient absorption spectroscopy. The dyes exhibit solvent-independent split electronic absorption due to exciton-coupling. Interestingly significant solvent-dependent fluorescence properties were observed. In toluene, they emit from the lowest excited state, while in methanol, they show weak emission in the higher energy region. In the low-temperature glass matrix, emission from the lowest excited state dominates similarly to that in toluene. The transient absorption spectra exhibit similar ground-state bleaching in toluene and methanol, revealing the formation of delocalized excited states by exciton coupling independent of solvent. However, the excited state deactivates rapidly in ultrafast time scale in methanol, likely due to solvent interaction, leading to rapid non-radiative deactivation. The PEG film doped with the exciton-coupled bis-squaraine shows a distinct fluorescence response to methanol vapor.

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers.

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies gabs of up to 2.6 × 10-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

Synthesis, Photophysical and Electrochemical Properties of Bis-Squaraine Dyes Fused on Isomeric Benzodipyrrole Central Units.

Exciton interactions are not only observed in assembled molecules but also in compounds with multiple chromophores referred to as superchromophores. We have developed isomeric bis-squaraine dyes as superchromophores in which two squaraine chromophores are fused onto the isomeric benzodipyrrole skeleton so as to regulate conformations and to reduce distances between two chromophores. The dyes with benzo[1,2-b:3,4-b']dipyrrole and benzo[1,2-b:5,4-b']dipyrrole moieties exhibited split electronic absorption originated from the intramolecular exciton interaction. The intensity of the split absorption bands varies in correlation with the orientation of chromophores. The isomeric dye with benzo[1,2-b:4,5-b']dipyrrole moiety exhibited a near-infrared absorption associated with the resonance throughout two chromophores. Their electrochemical and spectroelectrochemical properties are distinct from those of monomeric dyes owing to electronic interactions between the two chromophores. Thus, the structural isomerism of the central skeleton significantly affects their optical properties as well as their electrochemical properties.

Axially chiral indolenine derived chromophore dimers and their chiroptical absorption and emission properties.

Yamamoto homocoupling of two chiral oxindoles led to the atropo-diastereoselective formation of an axially chiral oxindole dimer. This building block served as the starting material for the syntheses of axially chiral squaraine and merocyanine chromophore dimers. These dimers show pronounced chiroptical properties, this is, outstandingly high ECD signals (Δε up to ca. 1500 M-1 cm-1) as a couplet with positive Cotton effect for the P-configuration around the biaryl axis and a negative Cotton effect for the M-configuration. All investigated dimers also exhibit pronounced circularly polarised emission with anisotropy values of ca. 10-3 cgs. Time-dependent density functional calculations were used to analyse the three contributions (local one electron, electric-magnetic coupling, and exciton coupling) to the rotational strength applying the Rosenfeld equation to excitonically coupled chromophores. While the exciton coupling term proves to be the dominant one, the electric-magnetic coupling possesses the same sign and adds significantly to the total rotational strength owing to a favourable geometric arrangement of the two chromophores within the dimer.

Photophysical properties and bioimaging application of an aminonaphthalimide-squaraine non-conjugated system

An aminonaphthalimide-squaraine non-conjugated system was designed and synthetized with the purpose of preparing fluorescent molecule in the 650–700 nm region that could operate via energy transfer (ET) between covalently linked naphthalimide and squaraine chromophores. The photophysical properties of the new fluorescent system were explored with the aim of understanding the ET in one- and two-photon excitation modes. The spectroscopic techniques employed in the characterization includes; absorption, fluorescence, quantum yields and fluorescence lifetime measurements in different solvents. The effect of polarity of solvents on efficiencies of ET were evaluated using one- and two-photon excited fluorescence. The optical behavior of the non-conjugated system was compared with its individual squaraine and naphthalimide moieties. The two-photon absorption (TPA) spectrum of the molecule was obtained between 750 and 1040 nm, with the largest two-photon cross section (δTPA)above 4200 GM. Finally, the applicability of the molecule for fluorescence imaging in the one- and two-photon excitation mode was demonstrated in N13 Microglial cells. The in vitro and in vivo confocal microscopy studies indicated that the non-conjugated system efficiently accumulated in the cytoplasm suggesting it could be utilized as a subcellular probe.

Polymorphism in Squaraine Dye Aggregates by Self‐Assembly Pathway Differentiation: Panchromatic Tubular Dye Nanorods versus J‐Aggregate Nanosheets

AbstractA bis(squaraine) dye equipped with alkyl and oligoethyleneglycol chains was synthesized by connecting two dicyanomethylene substituted squaraine dyes with a phenylene spacer unit. The aggregation behavior of this bis(squaraine) was investigated in non‐polar toluene/tetrachloroethane (98:2) solvent mixture, which revealed competing cooperative self‐assembly pathways into two supramolecular polymorphs with entirely different packing structures and UV/Vis/NIR absorption properties. The self‐assembly pathway can be controlled by the cooling rate from a heated solution of the monomers. For both polymorphs, quasi‐equilibrium conditions between monomers and the respective aggregates can be established to derive thermodynamic parameters and insights into the self‐assembly mechanisms. AFM measurements revealed a nanosheet structure with a height of 2 nm for the thermodynamically more stable polymorph and a tubular nanorod structure with a helical pitch of 13 nm and a diameter of 5 nm for the kinetically favored polymorph. Together with wide angle X‐ray scattering measurements, packing models were derived: the thermodynamic polymorph consists of brick‐work type nanosheets that exhibit red‐shifted absorption bands as typical for J‐aggregates, while the nanorod polymorph consists of eight supramolecular polymer strands of the bis(squaraine) intertwined to form a chimney‐type tubular structure. The absorption of this aggregate covers a large spectral range from 550 to 875 nm, which cannot be rationalized by the conventional exciton theory. By applying the Essential States Model and considering intermolecular charge transfer, the aggregate spectrum was adequately reproduced, revealing that the broad absorption spectrum is due to pronounced donor‐acceptor overlap within the bis(squaraine) nanorods. The latter is also responsible for the pronounced bathochromic shift observed for the nanosheet structure as a result of the slip‐stacked arranged squaraine chromophores.

Innenrücktitelbild: Polymorphism in Squaraine Dye Aggregates by Self‐Assembly Pathway Differentiation: Panchromatic Tubular Dye Nanorods versus J‐Aggregate Nanosheets (Angew. Chem. 21/2021)

Supramolekularer Polymorphismus wird im Forschungsartikel auf S. 12056 von David Bialas, Kazunori Sugiyasu, Frank Würthner und Mitarbeitern für Bis(squarain)-Farbstoffaggregate beschrieben. Das Bis(squarain) assembliert in Lösung bei schneller Abkühlung zu helikalen Nanostäbchen, während langsames Abkühlen zu thermodynamisch stabilen Nanoblattstrukturen führt. Die beiden Polymorphe zeigen völlig unterschiedliche UV/Vis/NIR-Absorptionseigenschaften mit panchromatischen Merkmalen für die Nanostäbchen, während für die Nanoblätter eine schmale, bathochrom verschobene J-Bande beobachtet wird.

Ultrafast Energy Transfer Dynamics in a Squaraine Heterotriad

A squaraine heterotriad consisting of three different covalently linked squaraine chromophores was synthesized, and its absorption spectra were interpreted in terms of Kasha's exciton coupling theory. Using the exciton couplings derived from model dyads (ca. 700 cm-1) as the input, we were able to predict the exciton state energies of the heterotriad. Transient absorption spectroscopy with femtosecond time resolution showed that excitation of the highest exciton state populates a state mainly localized at one terminal squaraine chromophore, and energy transfer to the lowest exciton state localized at the other terminal squaraine occurs within 30 fs. Field-induced surface hopping dynamics simulations support the assumption of ultrafast energy transfer. Moreover, they show the close relationship between internal conversion and energy transfer in the intermediate to weak coupling regime. The latter is a consequence of excitation localization caused by molecular vibrations.

Design and Synthesis of Near-Infrared Mechanically Interlocked Molecules for Specific Targeting of Mitochondria.

The entrapment of squaraine (SQ) within a molecular container to form rotaxane has been shown to improve the dye stability and the fluorescence proficiency inside the mitochondria. The macrocycle provides shelter and protects the near-infrared (NIR) SQ chromophore from nucleophilic attacks made by the exposed thiol of Cys-containing mitochondrial proteins and mitochondrial glutathione. Herein a microwave-assisted template-directed clipping reaction on low-loading 2-chlorotrityl chloride resin is used to develop an NIR unsymmetrical squaraine rotaxane in high quantum yield.

An Unsymmetrical Squaraine-Based Activatable Probe for Imaging Lymphatic Metastasis by Responding to Tumor Hypoxia with MSOT and Aggregation-Enhanced Fluorescent Imaging.

Optoacoustic imaging has great potential for preclinical research and clinical practice, and designing robust activatable optoacoustic probes for specific diseases is beneficial for its further development. Herein, an activatable probe has been developed for tumor hypoxia imaging. For this probe, indole and quinoline were linked on each side of an oxocyclobutenolate core to form an unsymmetrical squaraine. A triarylamine group was incorporated to endow the molecule with the aggregation enhanced emission (AEE) properties. In aqueous media, the squaraine chromophore aggregates into the nanoprobe, which specifically responds to nitroreductase and produces strong optoacoustic signals due to its high extinction coefficient, as well as prominent fluorescence emission as a result of its AEE feature. The nanoprobe was used to image tumor metastasis via the lymphatic system both optoacoustically and fluorescently. Moreover, both the fluorescence signals and three-dimensional multispectral optoacoustic tomography signals from the activated nanoprobe allow us to locate the tumor site and to map the metastatic route.

A Self-Assembled Unit Comprising 12 Squaraine Dyes Built Up from Two Star-Shaped Hexasquarainyl-Benzene Molecules.

We describe the aggregate formation and optical properties of a star-shaped hexaarylbenzene with six squaraine chromophores (=hexasquarainyl benzene). Comprehensive concentration-dependence studies in acetone/CHCl3 mixtures reveal a strong propensity to form discrete dimeric aggregates with a high binding constant in excess of 106 m-1 . In this context, a large hypsochromic shift of almost 2700 cm-1 was found in the absorption spectrum, indicating H-type exciton coupling. The aggregate band is characterised by a very small band width of only 560 cm-1 , probably caused by exchange narrowing. Both experimental and computational methods were used to elucidate the supramolecular aggregate structure, which is assumed to consist of two stacked hexasquarainyl benzene monomers.

Intramolecular Exciton-Coupled Squaraine Dyes for Dye-Sensitized Solar Cells

V-shaped squaraine dyes with carbazole cores in the center and indolenine groups at the end were designed to broaden the light-harvesting range of dye-sensitized solar cells (DSSCs) by taking advantage of intramolecular exciton coupling. V-shaped squaraines containing carboxy group on the indolenine moiety and 4-phenyl-2-cyanoacrylic acid groups on the carbazole cores as anchoring groups for chemical adsorption onto TiO2 were synthesized by the utilization of Stille cross-coupling as a key reaction. Although conventional squaraine dyes show typically narrow absorption bands, V-shaped squaraine dyes exhibited split absorption bands due to the intramolecular exciton coupling and thereby enabled the absorption of a broad range of light. The exciton coupling between squaraine chromophores obliquely installed in the molecule was theoretically supported by the Kasha exciton coupling model, in which the coupling energy was well-fitted with the experimental absorption spectra. As the result of intramolecular exciton coupling, the DSSCs based on exciton-coupled squaraine dyes exhibited a spectral response in a much wider range compared to that of a conventional linear counterpart, giving the higher energy conversion efficiency in comparison to that of the single chromophoric counterpart.

Synthesis and properties of functional dyes with squaraine–naphthalene diimide hybrid structure

Naphthalene diimides (NDIs) are promising candidate for electron acceptors due to their low-lying HOMOs and LUMOs. The functinalization of soluble NDIs at the 2,6-position affects the absorption and electrochemical properties. In this study, NDI-based hybrid dyes NDI-SQ-A, B fused with squaraine chromophore were designed and synthesized in order to elucidate the effects of the substitution on their optical and electrochemical properties. These dyes were successfully synthesized by Stille coupling reactions using 3-stannylcyclobutenediones and brominated NDI derivative, followed by a condensation reaction. DFT calculation predicts that the present dyes adopt distorted structures coming from a steric hindrance between semisquaraine and NDI moieties. The hybrid dyes show low-lying LUMOs due to the introduction of electron-deficient NDI moiety and broad absorption spectra in the far-red region. The absorption spectra of their thin films were bathochromically shifted relative to those in solution, indicating that hybrid dyes formed J aggregates.

Photophysical Characterization and BSA Interaction of Direct Ring Carboxy Functionalized Symmetrical squaraine Dyes

A series of far-red sensitive symmetrical squaraine dyes bearing direct –COOH functionalized indole ring were synthesized, characterized and subjected to photophysical investigations. These symmetrical squaraine dyes were then subjected to investigate their interaction with bovine serum albumin (BSA) in Phosphate buffer solutions. All the squaraine dyes under investigation exhibit intense and sharp optical absorption mainly in the far-red wavelength region from 550 nm -700 nm having very high molar extinction coefficients from 1.3 × 105 dm3.mol−1.cm−1. A very small Stokes shift of 10-17 nm indicates the rigid conformational structure of squaraine chromophore. Interaction of these dyes with BSA leads to not only enhanced emission intensity but also bathochromically shifted absorption maximum due to formation of dye-BSA conjugate. These dyes bind strongly with BSA having about an order of magnitude higher binding constant as compared to the reported squaraine dyes. Amongst the symmetrical squaraine dyes investigated in this work one bearing substituents like trifluorobutyl as alkyl chain at N-position of indole ring and carboxylic acid on benzene ring at the terminal (SQ-26) exhibited highest association with the BSA having very high binding constant 8.01 × 106 M−1.

Linear and Tripodal Squaraine Sensitizers with Triphenylamine Donor Components for Dye-sensitized Solar Cells

Linear and tripodal π-conjugated compounds consisting of squaraine chromophores and triphenylamine moieties were synthesized for application to dye-sensitized solar cells. Their optical and electro...

Coupled oscillators for tuning fluorescence properties of squaraine dyes.

Combining a squaraine (S) and a BODIPY (B) chromophore in a heterodimer (SB) and two heterotrimers (BSB and SBS) by alkyne bridges leads to the formation of coupled oscillators whose fluorescence properties are superior compared to the parent squaraine chromophore. The lowest energy absorption and emission properties of these superchromophores are mainly governed by the squaraine part and are shifted by more than 1000 cm(-1) to the red by excitonic interaction between the squaraine and the BODIPY dye. Employing polarization-dependent transient absorption and fluorescence upconversion measurements, we could prove that the lowest energy absorption in SB and BSB is caused by a single excitonic state but by two for SBS. Despite the spectral red-shift of their lowest absorption band, the fluorescence quantum yields increase for SB and BSB compared to the parent squaraine chromophore SQA. This is caused by intensity borrowing from the BODIPY states, which increases the squared transition moments of the lowest energy band dramatically by 29% for SB and 63% for BSB compared to SQA. Thereby, exciton coupling leads to a substantial enhancement of fluorescence quantum yield by 26% for SB and by 46% for BSB and shifts the emission from the red into the near-infrared. In this way, the BODIPY-squaraine conjugates combine the best properties of each class of dye. Thus, exciton coupling in heterodimers and -trimers is a valuable alternative to tuning fluorescence properties by, e.g., attaching substituents to chromophores.

A new unsymmetrical near-IR small molecule with squaraine chromophore for solution processed bulk heterojunction solar cells

A new unsymmetrical low bandgap push–pull squaraine chromophore bis-DMFA-Th-SQ-Th-DCA (JK216D) was synthesized and its optical and electrochemical properties were investigated.

A Ca2+ controlled thioether linked bichromophoric squaraine foldamer for “turn on” fluorescent sensing of oxalate

Several thioether linked bichromophoric squaraines 1 were prepared and characterized. Among these, the bichromophore 1c, containing three sulfur atoms, was found to selectively bind with Ca2+ and lead to a folded conformation. The selective binding of Ca2+ resulted in an absorption blue shift and fluorescence quenching due to the cation-induced folding of the bichromophore and the resultant exciton coupling between the squaraine chromophores. Job's plot confirmed a 1:1 binding between 1c and Ca2+. Anions such as oxalate can extract Ca2+ from squaraine foldamers via the formation of insoluble salts and restore the fluorescent signal. This sensing approach can be applied to the “turn on” fluorescent detection of oxalate with a low detection limit of 5.2nM (3σ/k).

Aggregation control of organic sensitizers for panchromatic dye co-sensitized solar cells

The photovoltaic properties of dye co-sensitized solar cells were compared with those of mono-sensitized devices. Co-sensitized TiO2 photo-electrodes were prepared from a phenothiazine chromophore for the RED dye and a squaraine chromophore for the BLUE dye to achieve panchromatic light absorption in dye co-sensitized solar cells (DSSCs). Co-sensitization on the TiO2 photo-electrode could reduce the aggregation of the BLUE dye adsorbed on the TiO2 surface, which led to an enhancement of the short circuit current (Jsc) of the co-sensitized solar cells. The dye co-sensitized solar cells with the RED and BLUE dyes optimized according to the dipping time showed an increase in the photon-to-current efficiency compared to that of the solar cell with a mono-sensitized photo-electrode. The photovoltaic and aggregation properties of the DSSCs were examined by measuring the current–voltage curve, incident photon-to-current efficiency, and electrochemical impedance spectra.

A bifunctional “Turn On” fluorescent probe for trace level Hg2+ and EDTA in aqueous solution via chelator promoted cation induced deaggregation signalling

Because mercury and ethylenediaminetetreaacetic acid (EDTA) are widely used materials and are commonly found together in the environment due to anthropogenic and industrial releases, there is a great need to develop a bifunctional sensor for both mercury and EDTA. A fluorescent sensor SQ-1 based on a squaraine chromophore with dithiocarbamate (DTC) side arms has been prepared, and its aggregation behavior in aqueous solution has also been investigated. The results show that increasing water concentration in the solution causes dramatic H-aggregation of the symmetric SQ-1 and consequently provides a mercury probe with a visual color change and “turn on” fluorescence via cation induced deaggregation. EDTA, a strong chelator to mercury, accelerates the deaggregation process and enhances the fluorescent emission, which not only improves the sensitivity of SQ-1 to mercury by 10-fold, with a detection limit improvement from 1.1×10−7M to 1.3×10−8M, but also provides a way for trace level detection of EDTA, with the detection limit of 8.7×10−8M. The potential application of this technique for Hg2+ and EDTA determination of river water samples is revealed by proof-of-concept experiments.