New Squaraine‐based two‐component initiation systems for UV‐blue light induced radical polymerization: Kinetic and time‐resolved laser spectroscopy studies

In this article, the ability of two‐component photoinitiator systems for efficient polymerization of 2‐ethyl‐2‐(hydroxymethyl)?1,3‐propanediol triacrylate was presented. The photophysics and photochemistry of squaraine dyes in the presence of an electron donor as well as an electron acceptor was investigated, and it was found that the photosensitizer in an excited state might act as an electron acceptor or an electron donor. The excited states of squaraines may be quenched by tetramethylammonium n‐butyltriphenylborate ( B2 ), diphenyliodonium chloride ( I1 ), and N‐methoxy‐4‐phenylpyridinium tetrafluoroborate ( NO ). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 471–484     

Efficient solution-processed bulk heterojunction solar cells by antiparallel supramolecular arrangement of dipolar donor-acceptor dyes

A series of dipolar donor–acceptor (D –A) chromophores with aminothiophene donor and different heterocyclic acceptor units is reported. By modulation of the acceptor strength, absorption bands over the whole visible spectrum are accessible as well as adjustment of the frontier molecular orbital levels. The performance of the chromophores in blends with fullerene acceptors in solution‐processed bulk heterojunction solar cells was studied and related to the molecular properties of the dyes. In particular, the effect of the large ground‐state dipole moments of these dyes was investigated by single crystal X‐ray analysis, which revealed antiparallel dimers, resulting in an annihilation of the dipole moments. This specific feature of supramolecular organization explains the excellent performance of merocyanine dyes in organic solar cells. With blends of HB366 :PC₇₁BM, the most efficient solar cell with a V_OC of 1.0 V, a J_SC of 10.2 mA cm^?2, and a power‐conversion efficiency of 4.5 % was achieved under standard AM1.5, 100 mW cm^?2 conditions. Under reduced lighting conditions, even higher efficiencies up to 5.1 % was obtained.     

Synthesis,electrochemical, and optical properties of low band gap homo‐ and copolymers based on squaraine dyes

A broad series of monomeric and polymeric squaraines was synthesized to investigate the impact of electron‐donating bridges, such as unsaturated triarylamine, carbazole, and saturated piperazine groups, on the spectroscopic and redox properties. These bridges were attached to both standard trans‐indolenine squaraines and dicyanomethylene‐substituted cis‐indolenine squaraines. The conjugates were investigated by absorption, steady‐state, and time‐resolved fluorescence spectroscopy and cyclic voltammetry. While addition of the donors resulted in significant redshift of the absorption of the model compounds, hardly any further shift or broadening was observed for the copolymers. Also the redox properties remained nearly unchanged compared with the model dyes. In contrast, immense broadening and redshift was observed for homopolymers. This behavior is explained by mostly excitonic coupling of localized squaraine transitions. The increasing distance of the chromophores determined by the bridges led to a decrease of the exciton coupling energy. We also performed semiempirical CNDO/S2 calculations on AM1 optimized structures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 890–911     

Efficient NIR‐Light Emission from Solid‐State Complexes of Boron Difluoride with 2′‐Hydroxychalcone Derivatives

This article describes a series of nine complexes of boron difluoride with 2′‐hydroxychacone derivatives. These dyes were synthesized very simply and exhibited intense NIR emission in the solid state. Complexation with boron was shown to impart very strong donor–acceptor character into the excited state of these dyes, which further shifted their emission towards the NIR region (up to 855 nm for dye 5 b , which contained the strongly donating triphenylamine group). Strikingly, these optical features were obtained for crystalline solids, which are characterized by high molecular order and tight packing, two features that are conventionally believed to be detrimental to luminescence in organic crystals. Remarkably, the emission of light from the π‐stacked molecules did not occur at the expense of the emission quantum yield. Indeed, in the case of pyrene‐containing dye 4 , for example, a fluorescence quantum yield of about 15 % with a fluorescence emission maximum at 755 nm were obtained in the solid state. Moreover, dye 3 a and acetonaphthone‐based compounds 1 b , 2 b , and 3 b showed no evidence of degradation as solutions in CH₂Cl₂ that contained EtOH. In particular, solutions of brightly fluorescent compound 3 a (brightness: ε×Φ_f=45 000 M ^?1 cm^?1) could be stored for long periods without any detectable changes in its optical properties. All together, these new dyes possess a set of very interesting properties that make them promising solid‐state NIR fluorophores for applications in materials science.     

NIR Absorbing Squaraines by Extension of the Conjugation with (Aminothiazolyl)ethenyl Groups

The bis{4‐{2‐[2‐(dialkylamino)thiazol‐5‐yl]ethenyl}‐2,6‐dihydroxyphenyl}squaraines 12a , b were synthesized from ethyl carbonochloridate ( 1 ) in six steps (Scheme). The donor–acceptor–donor systems 12a , b are dark blue dyes with absorption maxima in the NIR region, unless the measurements are performed in the presence of EtOH. In the latter case, the long‐wavelength band disappears, and the absorption in the UV region is strongly enhanced. The λ_max values in CHCl₃ and CHCl₃/EtOH differ by more than 450 nm. The completely reversible effect can be rationalized by the reversible degradation of intramolecular H‐bonds and a consequent torsion between the acceptor and the donor moieties.     

Rational Design of Fluorescent Bioimaging Probes by Controlling the Aggregation Behavior of Squaraines: A Special Effect of Ionic Liquid Pendants

We herein present an effective strategy to create water‐soluble fluorescent bioimaging dyes by introducing the imidazolium‐based ionic liquid (IL) pendants into a fluorescent skeleton. A new type of water‐soluble imidazolium‐anchored squaraine dye was synthesized accordingly. The relationship between the aggregate of squaraines and their fluorescent cell imaging application was elucidated in detail. Firstly, the aggregation behavior of squaraines in water solutions could be suppressed by varying the alkyl chain attached to the imidazolium unit. Secondly, the capability of cellular uptake and staining of dyes was also dramatically enhanced upon increasing the length of the paraffinic chain. These squaraine dyes displayed an excellent photostability that could permit real‐time fluorescence bioimaging experiments to be monitored over a long time period with constant sample irradiation. Additionally, we designed for the first time an Fe^II‐ion probe on the basis of an attack of the hydroxyl radical to the four‐membered ring of squaraine. The results demonstrated that the imidazolium‐anchored squaraines could perform “naked‐eye” detection of the Fe²⁺ ion over a wide range of other interfering metals in aqueous media. More surprisingly, this process showed a fluorescence “turn‐off” and “‐on” response through the regeneration of squaraines in cells.     

Fluorescence lifetime properties of near-infrared cyanine dyes in relation to their structures

Structurally diverse near-infrared (NIR) absorbing polymethine dyes were prepared and their fluorescence lifetimes (FLT) were evaluated in relation to their structural features. Comparative FLT analysis based on the modification of methine chain length and heterocyclic system showed that indolium or benzo[e]indolium heptamethine dyes exhibited longer FLT than the benzo[c,d]indolium trimethine dye. Modification of heterocyclic system alone with an intact chain length showed that indolium-based heptamethine dyes showed approximately 30% longer FLT than the benzo[e]indolium-based dyes. In general, the FLT of polymethine dyes increased from polar to non-polar solvents. In addition, correlation study between the theoretical and the experimental FLT for indocyanine green (ICG) suggests that the lack of structural rigidity for these cyanine dyes is primarily responsible for the loss of the excited state energy via non-radiative pathway.     

Efficient Solution‐Processed Bulk Heterojunction Solar Cells by Antiparallel Supramolecular Arrangement of Dipolar Donor–Acceptor Dyes

A series of dipolar donor–acceptor (D –A) chromophores with aminothiophene donor and different heterocyclic acceptor units is reported. By modulation of the acceptor strength, absorption bands over the whole visible spectrum are accessible as well as adjustment of the frontier molecular orbital levels. The performance of the chromophores in blends with fullerene acceptors in solution‐processed bulk heterojunction solar cells was studied and related to the molecular properties of the dyes. In particular, the effect of the large ground‐state dipole moments of these dyes was investigated by single crystal X‐ray analysis, which revealed antiparallel dimers, resulting in an annihilation of the dipole moments. This specific feature of supramolecular organization explains the excellent performance of merocyanine dyes in organic solar cells. With blends of HB366 :PC₇₁BM, the most efficient solar cell with a V_OC of 1.0 V, a J_SC of 10.2 mA cm⁻², and a power‐conversion efficiency of 4.5 % was achieved under standard AM1.5, 100 mW cm⁻² conditions. Under reduced lighting conditions, even higher efficiencies up to 5.1 % was obtained.     

An Aggregating Amphiphilic Squaraine: A Light‐up Probe That Discriminates Parallel G‐Quadruplexes

G‐quadruplexes (G4s) are peculiar DNA or RNA tertiary structures that are involved in the regulation of many biological events within mammalian cells, bacteria, and viruses. Although their role as versatile therapeutic targets has been emphasized for 35 years, G4 selectivity over ubiquitous double‐stranded DNA/RNA, as well as G4 differentiation by small molecules, still remains challenging. Here, a new amphiphilic dicyanovinyl‐substituted squaraine, SQgl , is reported to act as an NIR fluorescent light‐up probe discriminating an extensive panel of parallel G4s while it is non‐fluorescent in the aggregated state. The squaraine can form an unconventional sandwich π‐complex binding two quadruplexes, which leads to a strongly fluorescent (Φ _F=0.61) supramolecular architecture. SQgl is highly selective against non‐quadruplex and non‐parallel G4 sequences without altering their topology, as desired for applications in selective in vivo high‐resolution imaging and theranostics.     

A High-Affinity “Synthavidin” Receptor for Squaraine Dyes

Strong-binding host–guest pairings in aqueous media have potential as “supramolecular glues” in biomedical techniques, complementing the widely-used (strept)avidin-biotin combination. We have previously found that squaraine dyes are bound very strongly by tetralactam macrocycles possessing anthracenyl units as cavity walls. Here we show that replacing the anthracenes with pentacyclic 5,7,12,14-tetrahydro-5,7,12,14-tetraoxapentacene (TOP) units generates receptors which bind squaraines with increased affinities (around K_a=10¹⁰ m ⁻¹) and improved selectivities. Binding can be followed through changes to squaraine fluorescence and absorbance. The TOP units are easy to prepare and potentially variable, while the TOP-based receptor shows improved photostability, both in itself and in complex with squaraines. The results suggest that this system could prove valuable in the further development of practical “synthavidin” chemistry.     

Studies on a Vinyl Ruthenium‐Modified Squaraine Dye: Multiple Visible/Near‐Infrared Absorbance Switching through Dye‐ and Substituent‐Based Redox Processes

The bis(vinyl ruthenium)‐modified squaraine dye 1 was synthesized by treatment of [RuHCl(CO)(PiPr₃)₂] with bis(ethynyl)‐substituted squaraine 8 . Spectroscopic and electrochemical measurements on 1 and its organic precursors 6 – 8 were performed to study the effect of the vinyl ruthenium “substituents,” particularly with respect to (poly)electrochromism. Attachment of the vinyl ruthenium moieties endows metal–organic squaraine 1 with two additional oxidation waves and lowers the first two oxidation potentials by approximately 300 mV with respect to its organic precursors. Squaraines 6 , 7 , 8 , and 1 strongly absorb at 648, 663, 656, or 709 nm. Although organic dyes 6 , 7 , and 8 fluoresce, no room‐temperature emission is observed for 1 . The radical cations and anions of 6 , 7 , 8 , and 1 as well as the doubly oxidized dications have been studied by IR and UV/Vis/NIR spectroelectrochemistry, and the ?/0/+/2+ redox sequences were found to be reversible in each case. Our results indicate that the 1^2?/?/0/+/2+ redox system constitutes a polyelectrochromic switch in which absorption in the visible or the near‐infrared range is reversibly turned off or shifted deep into the NIR. They also show that radical cation 1^.+ is an intrinsically delocalized system with only little contribution from the outer vinyl ruthenium tags to the oxidation process. Dication 1²⁺ constitutes a class‐II mixed‐valent system with two electronically different vinyl ruthenium moieties and has an open‐shell singlet electronic ground‐state structure. ESR and NMR spectra of chemically prepared 1^.+ and 1²⁺ corroborate these results. It has also emerged that reduction involves an orbital that is strongly delocalized across the entire squaraine π system and strongly affects the peripheral vinyl ruthenium sites.     

Influence of the Auxiliary Acceptor on the Absorption Response and Photovoltaic Performance of Dye‐Sensitized Solar Cells

Three new dyes with a 2‐(1,1‐dicyanomethylene)rhodanine (IDR‐ I , ‐ II , ‐ III ) electron acceptor as anchor were synthesized and applied to dye‐sensitized solar cells. We varied the bridging molecule to fine tune the electronic and optical properties of the dyes. It was demonstrated that incorporation of auxiliary acceptors effectively increased the molar extinction coefficient and extended the absorption spectra to the near‐infrared (NIR) region. Introduction of 2,1,3‐benzothiadiazole (BTD) improved the performance by nearly 50 %. The best performance of the dye‐sensitized solar cells (DSSCs) based on IDR‐ II reached 8.53 % (short‐circuit current density (J_sc)=16.73 mA cm^?2, open‐circuit voltage (V_oc)=0.71 V, fill factor (FF)=71.26 %) at AM 1.5 simulated sunlight. However, substitution of BTD with a group that featured the more strongly electron‐withdrawing thiadiazolo[3,4‐c]pyridine (PT) had a negative effect on the photovoltaic performance, in which IDR‐ III ‐based DSSCs showed the lowest efficiency of 4.02 %. We speculate that the stronger auxiliary acceptor acts as an electron trap, which might result in fast combination or hamper the electron transfer from donor to acceptor. This inference was confirmed by electrical impedance analysis and theoretical computations. Theoretical analysis indicates that the LUMO of IDR‐ III is mainly localized at the central acceptor group owing to its strong electron‐withdrawing character, which might in turn trap the electron or hamper the electron transfer from donor to acceptor, thereby finally decreasing the efficiency of electron injection into a TiO₂ semiconductor. This result inspired us to select moderated auxiliary acceptors to improve the performance in our further study.     

Squaraines as reporter units: insights into their photophysics, protonation, and metal-ion coordination behaviour

The synthesis, photophysical properties, protonation, and metal-ion coordination features of a family of nine aniline-based symmetrical squaraine derivatives are reported. The squaraine scaffold displays very attractive photophysical properties for a signalling unit. These dyes show absorption and weakly Stokes-shifted, mirror-image-shaped emission bands in the visible spectral range and there are no hints of multiple emission bands. The mono-exponential fluorescence decay kinetics observed for all the derivatives indicate that only one excited state is involved in the emission. These data stress the interpretation that squaraines can be regarded as polymethine-type dyes. From a coordination chemistry point of view, the squaraines possess four potential binding sites; that is, two nitrogen atoms from the anilino groups and two oxygen atoms from the central C(4)O(2) four-membered ring. These coordination sites are part of a cross-conjugated pi-system and coordination events with protons or certain metal ions affect the electronic properties of the delocalised pi-system dramatically, resulting in a rich modulation of the colour of the squaraines. The absorption band at around 640 nm is blue-shifted when coordination at the anilino nitrogen atoms occurs, whereas coordination to the C(2)O(4) oxygen atoms results in the development of red-shifted bands. Addition of more than one equivalent of protons or metal cations could additionally entail mixed N,O- or N,N-coordinated complexes, manifested in the development of a broad band at 480 nm or complete bleaching in the visible range, respectively. Analysis of the spectrophotometric titration data with HYPERQUAD yielded the macroscopic and microscopic stability constants of the complexes. Theoretical modelling of the various protonated species by molecular mechanics methods and consideration of some of the title dyes within the framework of molecular chemosensing and molecular-scale "logic gates" complement this contribution.     

Defining a Polymethine Dye for Fluorescence Anisotropy Applications in the Near‐Infrared Spectral Range

Fluorescence anisotropy in the near‐infrared (NIR) spectral range is challenging because of the lack of appropriate NIR fluorescent labels. We have evaluated polymethine fluorescent dyes to identify a leading candidate for NIR anisotropy applications. The NIR dye LS601 demonstrated low fluorescence anisotropy values (r) as a result of its relatively long fluorescent lifetime 1.3 ns. The r value of LS601 unbound and coupled to biological macromolecules was found to have a sufficient dynamic range from 0.24 to 0.37, demonstrating the feasibility of fluorescence anisotropy in the NIR. The viability of fluorescence anisotropy using a NIR label was demonstrated by characterization of dye–protein conjugates. These results open the door to a number of applications in drug discovery, fluorescence anisotropy imaging and contrast agent development.     

Effective Amyloid Defibrillation by Polyhydroxyl‐Substituted Squaraine Dyes

With an objective to develop β‐amyloid destabilizing agents, we have investigated the interactions of a few water‐soluble near‐infrared (NIR)‐absorbing squaraine dyes 1 – 3 with lysozyme and its amyloid aggregates through photophysical and biophysical techniques. These dyes exhibited strong interactions with lysozyme and β‐amyloids in addition to serum albumins as evidenced by the absorption and emission changes. The interactions were found to be spontaneous with association constant values in the range of approximately 10⁴–10⁵ m ^?1, as confirmed through half‐reciprocal analysis and isothermal calorimetric measurements. Uniquely, such effective interactions of the dyes have led to the complete disassembly of the β‐amyloid fibrillar structures to form spherical particles approximately 350 nm in size, as confirmed through photophysical, thioflavin assay, circular dichroism (CD), atomic force microscopy (AFM), TEM, and selected‐area electron diffraction (SAED) techniques. These results demonstrate that the squaraine dyes 1 – 3 under investigation act as effective protein‐labelling and destabilizing agents of the protein amyloidogenesis.     

The Role of π Bridges in High‐Efficiency DSCs Based on Unsymmetrical Squaraines

A series of squaraine‐based sensitizers with various π bridges and anchors were prepared and examined in dye‐sensitized solar cells. The carboxylic anchor group was attached onto a squaraine dye through π bridges with and without an ethynyl spacer. DFT studies indicate that the LUMO is delocalized throughout the dyes, whilst the HOMO resides on the squaraine core. The dye that incorporates a 4,4‐di‐n‐hexyl‐cyclopentadithiophene group that is directly attached onto the π bridge, JD10 , exhibits the highest power conversion efficiency in a DSC; this result is attributed, in part, to the deaggregative properties that are associated with the gem‐di‐n‐hexyl substituents, which extend above and below the π‐conjugated dye plane. Dye JD10 demonstrates a power‐conversion efficiency of 7.3 % for liquid‐electrolyte dye‐sensitized solar cells and 7.9 % for cells that are co‐sensitized by another metal‐free dye, D35 , which substantially exceed the performance of any previously tested squaraine sensitizer. A panchromatic incident‐photon‐to‐current‐conversion efficiency curve is realized for this dye with an excellent short‐circuit current of 18.0 mA cm^?2. This current is higher than that seen for other squaraine dyes, partially owing to a high molar absorptivity of >5 000 M ^?1 cm^?1 from 400 nm to the long‐wavelength onset of 724 nm for dye JD10 .     

Synthesis and photovoltaic properties of functional dendritic oligothiophenes

Novel p‐type and low bandgap functional dendritic oligothiophenes bearing hole‐transporting carbazole as peripheral substituents and an electron‐withdrawing dicyanovinyl core group, namely, DCT(n)‐DCN, where n = 1 or 2 for solution‐processable photovoltaic (PV) applications have been synthesized. With electron‐donating carbazole surface‐functionalized moieties conjugated with dicyanovinyl core group, the optical bandgap of these functional dendritic oligothiophene thin‐films greatly reduces to 1.74 eV with a strong spectral broadening and a high ionization potential at ～5.5 eV as determined by UV photoelectron spectroscopy. The bulk heterojunction PV cells fabricated from these dendrimers blended with PC₇₁BM as an acceptor showed a power conversion efficiency up to 1.64% with an open circuit voltage of (V_oc) = 0.93 V in the annealed device. We have demonstrated that the desirable molecular and PV properties of dendritic oligothiophenes can be obtained/tuned by the incorporation of functional group(s) onto peripheral of the dendron and into the core. In addition, these functional dendritic oligothiophenes show superior functional properties even at low dendritic generation as compared to the unsubstituted higher generation dendritic oligothiophenes as a p‐type, low‐bandgap semiconductor for solution‐processable bulk heterojunction PV cells. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and Electronic Properties of 1,2‐Hemisquarimines and Their Encapsulation in a Cucurbit[7]uril Host

The synthesis of a new class of robust squaraine dyes, colloquially named 1,2‐hemisquarimines (1,2‐HSQiMs), through the microwave‐assisted condensation of aniline derivatives with the 1,2‐squaraine core is reported. In CH₃CN, 1,2‐HSQiMs show a broad absorption band with a high extinction coefficient and a maximum at around λ=530 nm, as well as an emission band centered at about λ=574 nm, that are pH dependent. Protonation of the imine nitrogen causes a redshift of both absorption and emission maxima, with a concomitant increase in the lifetime of the emitting excited state. Encapsulation of the chromophore into a cucurbit[7]uril host revealed fluorescence enhancement and increased photostability in water. The redox characteristics of 1,2‐HSQiMs indicate that charge injection into TiO₂ is possible; this opens up promising perspectives for their use as photosensitizers for solar energy conversion.     

Synthesis of N,N‐Dialkylamino‐nor‐Dihydroxanthene‐Hemicyanine Fused Near‐Infrared Fluorophores and Their First Water‐Soluble and/or Bioconjugatable Analogues

The effective synthesis of extended conjugated N ,N ‐dialkylamino‐nor ‐dihydroxanthene‐based fluorophores is described from diversely functionalized salicylic aldehydes. The access to these original fluorescent derivatives proceeded in two steps through a one‐pot construction of the unusual nor ‐dihydroxanthene (nor ‐DHX) scaffold followed by a diversification step providing a wide variety of nor ‐DHX‐hemicyanine fused dyes emitting in the range of 730–790 nm. The versatility of our approach has enabled a further extension to the late‐stage introduction of negatively/positively charged polar groups onto their terminal nitrogen heterocyclic subunit, thereby giving access to the first water‐soluble and/or bioconjugatable members of this emerging class of NIR fluorophores. Our water‐solubilizing method is easily implementable, and the nor ‐DHX‐hemicyanine skeleton maintains satisfying fluorescence quantum yields (5–20 %) under physiological conditions. Finally, the bioconjugation ability of fluorescent derivatives bearing a free carboxylic acid was demonstrated through the covalent labeling of a model protein, namely, bovine serum albumin.     

Theoretical Study of Origin of Triple Fluorescences of a Squaraine Dye, Bis[4-(N,N-dimethylamino)phenyl]-squaraine

A new scheme of photo‐fluorescent emission origin, described as S₀ (relaxed state)→S_n (Frank‐Condon state)→ S_m (relaxed state)→S₀ (Frank‐Condon state), is presented to explain the multiple fluorescent emissions of squaraine dyes observed experimentally according to the configuration interaction singles calculations of relaxed excited states of a model compound, bis[4‐(N,N‐dimethylamino)phenyl]squaraine (SQ). It is exhibited that all triple fluorescent emissions of SQ have their significant origin in vertical electron transitions of different relaxed excited states. In addition, some important absorption peaks appearing in higher energy region are most likely to be responsible for the higher energy band observed in solid states of many squaraine dyes.