Squaraine Molecules Research Articles

Understanding device-structure-induced variations in open-circuit voltage for organic photovoltaics.

We investigate the structural influences on the device performance, especially on open-circuit voltage (V(OC)) in squaraine (SQ)/fullerene (C60) bilayer cells. Simply changing the SQ thickness could lead to 40% variation in V(OC) from 0.62 to 0.86 V. The ionization potential (IP) of SQ films and recombination at the anode surface as well as donor/acceptor (D/A) interface sensitively vary with film thicknesses, which account for the shifts in V(OC). The anode recombination can be effectively suppressed by preventing direct contact between C60 and the anode with a buffer layer, delivering an elevated V(OC). Through polarized infrared-multiple-angle incidence resolution spectroscopy measurement, the molecular structure of SQ films is found to gradually evolve from lying-down on indium-tin oxide substrates with noncentrosymmetric orientation at low thicknesses to random structure at high thicknesses. The different molecular orientation may yield different strengths of electronic coupling, which affects the charge-carrier recombination and thus V(OC). Moreover, the oriented SQ films would spontaneously compose aligned dipole moments at the D/A interface because of the strong dipolar effects in SQ molecules identified by density functional theory calculations, whereas no aligned interfacial dipole moment exists in the random structure. The resulting interfacial dipole moments would form an electric field at the D/A interface, leading to variations in the IP and thus impacting V(OC). Our findings demonstrate that V(OC) in organic photovoltaic cells is critically associated with the molecular orientation that affects the charge-carrier recombination and interfacial dipole alignment, which should be seriously taken into consideration for the design of organic molecules and optimization of the cell efficiency.

A two-step electron-driven proton transfer mechanism for the solution-phase nonradiative decay process of squaraine molecule

The strong solvent polarity dependence observed in the nonradiative decay rates of Bis[4-(dimethylamino)phenyl]squaraine (SQ-DMA) had previously been explained by the twisted intramolecular charge transfer mechanism. However, our recent theoretical study did not support this mechanism. We theoretically study the possibility of electron-driven proton transfer (EDPT) processes between an alcohol solvent molecule and the CO− groups of SQ-DMA using a complex model. The results actually support the EDPT mechanism that proceeds via two nonadiabatic transitions: the first from the locally excited (π→π*) state of SQ-DMA to the charge transfer (CT) state, and the second from the CT to S0 states.

Coevaporated Bisquaraine Inverted Solar Cells: Enhancement Due to Energy Transfer and Open Circuit Voltage Control

There is currently enormous interest in the development of small molecule organic solar cells (SMSC), as in principle, these systems offer advantages over both conventional Si photovoltaics and organic polymer solar cells. Here, we report Förster Resonance Energy Transfer (FRET) enhanced inverted SMSC fabricated by coevaporating two different squaraine donors, a symmetrical squaraine (SQ, 2,4-bis-4-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine), and an asymmetrical squaraine (ASSQ, 2,4-bis-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyliminio) squaraine). ASSQ absorbs blue light (λmax 540 nm) and emits from 550 nm to the near-infrared region, which overlaps with SQ absorption (λmax 690 nm). Therefore, by utilizing a thin film containing the two squaraine donors as the active layer in a SMSC, we can both broaden the photovoltaic absorption spectrum, and reduce recombination loss as a result of FRET. This strategy has resulted in SMSC with power conversion efficiencies (PCE) which are up to 46% greater than those obtained by using a single squaraine donor. Ultrafast time-resolved photoluminescence and transient absorption spectroscopy provide clear evidence of FRET between the small molecules, with a rapid energy transfer time of ∼1 ps. At optimal blending, which correlates to the highest PCE measured, the efficiency of energy transfer is as high as 85%. Furthermore, in the devices containing two different squaraine molecules, the open circuit voltage (VOC) is proportional to the fraction of the two donors in the blend, allowing us to predict the VOC as the ratio of the two donors is changed. SMSC with inverted structures also demonstrate long-term stability in ambient conditions compared to devices employing a conventional architecture.

An EDTA promoted coordination induced disaggregation for specific Hg2+ detection

In this work, a novel fluorescent sensing strategy was developed for the detection of Hg2+ with excellent sensitivity and selectivity using a metal-induced disaggregation of aggregated squaraine molecules with the help of ethylenediaminetetreaacetic acid (EDTA). This EDTA promoted probe exhibited a specific “turn on” fluorescent response to Hg2+ with a detection limit of 7.6 × 10−9 M (3σ/k). Owning to the high coordinating ability of EDTA and dithiocarbamate tethered squaraine dye toward Hg2+, the disaggregation process could be improved by EDTA via the formation of squaraine-Hg2+-EDTA trinary complex, resulting in a greater fluorescent release, and this novel phenomenon has been confirmed by light scattering experiments, 1H NMR, IR and MS.

A coordination driven deaggregation approach toward Hg2+-specific chemosensors based on thioether linked squaraine-aniline dyads

Herein, we present an effective strategy based on coordination induced deaggregation signaling to create water-soluble fluorescent probes for the detection of mercury ion by introducing the thioether group linkage into squaraine-aniline dyads. Three unsymmetrical squaraine dyes tethered with thiopodand chains in different length were synthesized accordingly. The deaggregation process of the aggregated squaraine dyes triggered by Hg2+ was elucidated in detail. Firstly, the aggregation behavior of squaraine dyes with varying thioether chains in aqueous solutions has been investigated. Secondly, the complexation ability of squaraines with Hg2+ was found to be strengthened by increasing the length of the thioether chain. Additionally, an aggregated squaraine solution was designed by adding trace amount of non-ionic surfactant to solubilize the dye in pure water for the detection of Hg2+. Significantly, the aggregated state of squaraine molecules was selectively broken by Hg2+ ions despite a wide range of other interfering metals in water media. Under optimum conditions, the fluorescence recovery was linearly proportional to the concentration of Hg2+ between 0.03 and 1.8μM and the detection limit was as low as 6.6nM (1.3ppb). Further study of the mechanism by absorption and emission spectroscopy, MS, 1H NMR and IR titration showed that a 1:1 complex was formed between squaraine and Hg2+ ion.

Unambiguous determination of molecular packing in crystalline donor domains of small molecule solution processed solar cell devices using routine X-ray diffraction techniques

The crystalline phase in crystalline donor domains of squaraine-based small molecule (SM) solution processed organic solar cell (OSC) devices has been accurately characterized. The active layers consisting of solution processed bulk-heterojunction (BHJ) films of squaraine (SQ)-PC70BM blends and of pristine SQ thin films were characterized using a set of X-ray diffraction techniques, simple sample preparation methods, and routine least-square data refinement. The approach involves comparing experimental diffractograms recorded from active layers and from powdered active layers with those calculated from single crystal structures of the SQ molecule. The calculated diffractograms were fitted to the experimental powder diffractograms using least-square refinement routines, which allow assignment of crystalline phases with great accuracy, providing clear evidence that the packing in the active layer is similar in nature to that in the corresponding single crystal polymorphs. It was found that molecules of SQ exclusively pack into a P21/n phase in crystallites of BHJ active layer blends comprising PC70BM as the acceptor and into two distinct phases in the case of pristine SQ films, the P21/n phase and a polymorphic Pbcn phase. In the latter case, the predominance of one or the other phase depends on the solvent used in the deposition step, the annealing process employed and the substrate the active layer is deposited onto. This information, together with that obtained from grazing incidence X-ray diffraction GIXRD, provided precise structural information in the nano- and sub-nanometer range.

Controlled Aggregation and Enhanced Two-Photon Absorption of a Water-Soluble Squaraine Dye with a Poly(acrylic acid) Template

Controlling the aggregation behavior of organic dyes is important for understanding and exploring supramolecular assembly utilizing the specific characteristics of aggregation. Regulating J-aggregation by electrostatic interactions between anionic polyelectrolytes and cationic dyes has gained growing interest. Here, we report the formation of J-aggregates of a water-soluble cationic squaraine dye, 4-(pyridinium-1-yl)butylbenzothiazolium squaraine (SQ), using poly(acrylic acid) sodium salt (PAA-Na) as a template. Electrostatic interactions between the PAA-Na polyelectrolyte and the cationic SQ dye enhanced J-aggregation; the absorbance of the resulting J-band with the polyelectrolyte template was much sharper than the absorbance of the J-aggregate formed using a high concentration of NaCl. Significantly, removal of the polyelectrolyte PPA-Na template by the introduction of calcium ions, which can form stronger ionic binding with carboxylate groups, dissociated J-aggregates, freeing the SQ molecules back to unaggregated or lower aggregate forms. To demonstrate the reversibility of the J-aggregate formation cycle, an in situ experiment was conducted that showed 60% reversibility of the second cycle. In addition, an enhancement by ca. 23 times per repeat unit of the two-photon absorption (2PA) cross section was observed at 920 nm for the polyelectrolyte template-SQ J-aggregate compared to unaggregated or lower aggregate SQ. These results suggest a prominent role of polyelectrolyte templated SQ J-aggregation in the enhancement of 2PA efficiency and provide a means of modulating supramolecular assembly.

Enhanced Intersystem Crossing Rate in Polymethine-Like Molecules: Sulfur-Containing Squaraines versus Oxygen-Containing Analogues

Two different approaches to increase intersystem crossing rates in polymethine-like molecules are presented: traditional heavy-atom substitution and molecular levels engineering. Linear and nonlinear optical properties of a series of polymethine dyes with Br- and Se-atom substitution, and a series of new squaraine molecules, where one or two oxygen atoms in a squaraine bridge are replaced with sulfur atoms, are investigated. A consequence of the oxygen-to-sulfur substitution in squaraines is the inversion of their lowest-lying ππ* and nπ* states leading to a significant reduction of singlet-triplet energy difference and opening of an additional intersystem channel of relaxation. Experimental studies show that triplet quantum yields for polymethine dyes with heavy-atom substitutions are small (not more than 10%), while for sulfur-containing squaraines these values reach almost unity. Linear spectroscopic characterization includes absorption, fluorescence, quantum yield, anisotropy, and singlet oxygen generation measurements. Nonlinear characterization, performed by picosecond and femtosecond laser systems (pump-probe and Z-scan measurements), includes measurements of the triplet quantum yields, excited state absorption, two-photon absorption, and singlet and triplet state lifetimes. Experimental results are in agreement with density functional theory calculations allowing determination of the energy positions, spin-orbital coupling, and electronic configurations of the lowest electronic transitions.

The effect of controllable thin film crystal growth on the aggregation of a novel high panchromaticity squaraine viable for organic solar cells

A description of the synthesis procedure is provided for a new squaraine molecule, DiPSQ(OH)2. This molecule is an interesting target molecule for organic photovoltaics (OPV) because of its unusually broad panchromaticity, suggesting a tighter molecular packing than would be expected, given its bulky alkyl side groups. UV–vis–NIR absorption data provides evidence for the presence of both H- and J-aggregates. Thin film X-ray diffraction (TFXRD) data supports the presence of one dominant crystal structure. Further interpretation of TFXRD results and complementary Atomic Force Microscopy (AFM) characterization for pure and blended samples provides a comprehensive explanation of how DiPSQ(OH)2 aggregates are affected by annealing and crystal growth. The described aggregation properties of donor–acceptor blends for solar cells are strongly dependent on annealing. Influencing phase separation, local crystallinity, and macroscopic crystallization, these physical properties are known to significantly impact charge dissociation, exciton diffusion rates, and bulk charge transport in devices. The strategy of systematically varying annealing times has resulted in improved bulk heterojunction crystallinity and regions of increased phase separation, both necessary to achieve the long term goal of morphological control for commercially viable NIR-active OPV devices.

Supramolecular self-assembly structures and properties of zwitterionic squaraine molecules

To further investigate the influence of H-bond combining with π–π stack interaction on the optical performances, a few rigid zwitterionic squaraine molecules, with different terminal H-bond recognition abilities, are identified and designed. They can controllably construct a series of supramolecular self-assemblies with different morphologies and dimensions via a simple solution-based self-assembly method under mild conditions. The morphologies, synergetic reinforcing effect, and optical-thermal properties are investigated and confirmed by both theoretical and experimental methods. The results indicate that the morphologies of the resultant self-assemblies can be effectively controlled by rational molecular design, whose absorption and emission spectra and thermal stabilities accordingly change with the alteration of zwitterionic squaraine molecular structures.

Dual-arm Z-scan technique to extract dilute solute nonlinearities from solution measurements

We present a technique in which small solute nonlinearities may be extracted from large solvent signals by performing simultaneous Z-scans on two samples (solvent and solution). By using a dual-arm Z-scan apparatus with identical arms, fitting error in determining the solute nonlinearity is reduced because the irradiance fluctuations are correlated for both the solvent and solution measurements. To verify the sensitivity of this technique, the dispersion of nonlinear refraction of a squaraine molecule is measured. Utilizing this technique allows for the effects of the solvent n2 to be effectively eliminated, thus overcoming a longstanding problem in nonlinear optical characterization of organic dyes.

Water soluble bis(indolenine)squaraine salts for use as fluorescent protein-sensitive probes

Abstract In this study we have shown that water soluble fluorescent squaraine dyes can be produced as organic salts in combination with a range of organic cations although some cations, it was found, limit solubility. Chemical analysis of twenty-four such squaraine salts showed that only one existed in the solid-state as the pure organic salt, all others existed as hydrates with varying amounts of crystalline water, except for one that existed as a butanol solvate. Five of the twenty-four (primarily nitro-substituted cations) existed as mono-cation/sulfonic acid salts, as opposed to the bis-cationic salt state that was expected for all. Preliminary fluorescence results indicated that the increase in fluorescence verses increase of protein (bovine serum albumin) concentration was significantly hindered if the protein was stored in a phosphate buffer. Comparative experiments undertaken with the protein dissolved in water showed substantial differences in the rate of increase of fluorescence with increasing protein concentration. The largest rate of change came from the morpholinium salt. It is suggested that this is because morpholine hinders neither the hydrophobic docking of the squaraine group into the protein nor the fluorescence generation within the squaraine molecule.

Femto- to Millisecond Photophysical Characterization of Indole-Based Squaraines Adsorbed on TiO2 Nanoparticle Thin Films

In this paper, we address femto- to millisecond transient absorption studies of TiO2 nanoparticle (NP) thin films sensitized with four squaraine (SQ) molecules, with and without a deaggregating agent, chenodeoxycholic acid (CDCA). On the femto- to picosecond time scale, we determined the presence of three transient species by using singular value decomposition (SVD) analysis, i.e., S1 of the SQ monomers, S1 of the SQ H-aggregates, and the SQ radical cation formed after the electron injection. Both monomers and H-aggregates are proven to inject electrons to the TiO2 conduction band, being 5 times faster in the monomers (e.g., keimon = 5.1 × 1011 s–1 and keiH-agg = 1.1 × 1011 s–1 for SQ 41). Besides, the undesired singlet–singlet annihilation is an active process in these samples, constituting the drain of a high percentage of the absorbed photons. The coadsorption of CDCA on the TiO2 NP avoids the formation of H-aggregates, and therefore, only two transient species are present in these samples: S1 of the m...

Femto to millisecond observations of indole-based squaraine molecules photodynamics in solution

We present femto-to-millisecond studies of the photodynamics of seven types of indole-based squaraine molecules (SQs) in solvents of different H-bonding ability and viscosity. These SQs can be classified into two families: SQs with two carboxylic groups in the side indole groups (symmetrical SQs) and with only one carboxylic group (asymmetrical SQs). Steady-state absorption and fluorescence techniques show narrow absorption and emission bands, with a small Stokes shift (about 300 cm(-1)). The femtosecond transient absorption spectra give a very short (∼100 fs) dynamics (assigned to IVR) and the associated spectra show two excited species assigned to two stereoisomers. A trans-cis photoisomerization occurs in a very fast time through a conical intersection. Pico-to-nanosecond emission experiments also reveal the presence of two fluorescing trans stereoisomers whose lifetimes show similar sensitivities to the nature of solvent. For example, lifetimes of 1.72, 0.46 and 0.29 ns were determined for the trans photoisomer of the SQ 41 in triacetin, dichloromethane and acetonitrile, respectively, reflecting the short decay of the S(1) state in highly polar and low viscous solvents. Flash photolysis experiments gave the transient absorption signals of the cis photoisomer that is formed after the twisting process at S(1). The cis-to-trans photoisomerization at the ground state happens in the μs time scale (1-4 μs), and it depends on the H-bonding ability and viscosity of the solvent. Thus, combining fs-ns and ns-μs experiments suggests that in the conical intersection region, only a small fraction of the twisted trans isomers are converted to the cis ones in the excited states. These results bring detailed and global insight into the large time window photodynamics of this family of SQs in solution.

A Convenient Organic–Inorganic Hybrid Approach Toward Highly Stable Squaraine Dyes with Reduced H‐Aggregation

AbstractA generic and effective approach for solving the aggregation effect observed with optical materials in solid state or in a solution with a poor solvent was explored by designing two types of squaraine‐containing polyhedral oligomeric silsesquoixane (POSS)‐based hybrids. It is expected that incorporation of “huge” inorganic POSS nanoparticles into optical materials via covalent bonding can effectively decrease the strong dipole–dipole and π–π stacking interactions, inhibit intermolecular charge transfer between adjacent squaraine molecules, and improve optical, thermal and chemical stability of the resultant materials. Both theoretical calculations and experimental results indicate that the molecular design strategy is rational and efficacious. The resultant organic–inorganic hybrid optical materials effectively eliminate the aggregation of organic optical chromophoric groups by hindering intermolecular charge transfer and decreasing dipole–dipole and π–π stacking interaction between the chromophores, and exhibit good optical stability, i.e., the absorption peaks of H1 and H2 display only a slight blue‐shift, even in the solid. Simultaneously, the hybrids also show significantly enhanced thermal, and chemical stabilities in comparison with the precursor organic optical materials.

Surface Modification of ZnO Nanorods with Small Organic Molecular Dyes for Polymer–Inorganic Hybrid Solar Cells

ZnO nanorod arrays modified with dye molecules demonstrate specific light-harvesting and charge-collecting properties which are promising for the enhancement of the characteristic performance of hybrid solar cells based on ZnO/poly(3-hexylthiophene). The properties of dyes commonly used for dye-sensitized solar cells were investigated in relation to the performance of polymer–inorganic hybrid photovoltaic devices. The use of indoline dye D205, which has dipole moments directing away from the ZnO surface, was found to suppress the reverse saturation dark current density and charge recombination and to consequently lead to higher open-circuit voltage and improved power conversion efficiency (PCE) from 0.22% to 0.71%. Derivatized squaraine molecules were synthesized and were found to improve device performance by extending the light-harvesting range to the near-infrared region, leading to increased short-circuit current density and the highest PCE of 1.02%.

A combined molecular dynamics and computational spectroscopy study of a dye-sensitized solar cell

An organic dye-sensitized solar cell consisting of a squaraine molecule attached to a TiO2 surface is modeled using first-principles molecular dynamics and time-dependent density functional theory. The system is surrounded by solvent molecules that are treated at the same level of theory as the dye molecule and the surface. The effect of the solvent on optical properties is investigated by computing many absorption spectra for various configurations along a molecular dynamics trajectory. It is shown that the dynamical effects induced by thermal fluctuations have a strong effect on the optical properties and that satisfactory agreement with experiments is achieved only when those thermal effects are accounted for explicitly.

Optical properties of dye sensitized TiO2 nanowires from time‐dependent density functional theory

AbstractWe present a computational study based on time‐dependent density functional theory of the optical absorption spectra of TiO2 nanowires sensitized with organic dye molecules. We concentrate on catechol and squaraine dyes. For those molecules, we compute adsorption geometries and energies and investigate the optical properties of the combined dye– nanowire system. We find that although the molecules have qualitatively different optical spectra in the gas phase, both lead to an enhancement of the absorption in the visible frequency range when adsorbed on a nanowire.magnified imageA TiO2 nanowire with attached squaraine and catechol dye molecules.(© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Near-Unity Quantum Yields for Intersystem Crossing and Singlet Oxygen Generation in Polymethine-like Molecules: Design and Experimental Realization

The photophysical properties of two pairs of squaraine molecules where two oxygen atoms in a squaraine bridge are replaced with sulfur atoms are investigated. This substitution leads to an inversion of the lowest singlet π−π* electronic transition by an n−π* transition, effectively reducing the energy difference between singlet and triplet states and significantly increasing the intersystem crossing efficiency without the use of “heavy atoms”. Experimental results, in agreement with quantum chemical calculations, show near-unity values for triplet quantum yield and singlet oxygen generation quantum yield for sulfur-containing squaraines, which are potentially useful for two-photon photodynamic therapy.

Influence of molecular structure of squaraine dyes on their aggregation in aqueous solutions

The influence of the molecular structure of several squaraine dyes on their aggregation in aqueous solutions and its spectral manifestation were studied. The results are analyzed using quantum-chemical modeling. It is shown that a transverse dipole moment in squaraine molecules contributes to aggregation with an antiparallel orientation of the molecules whereas a longitudinal dipole moment leads to formation of J-type aggregation with a longitudinal orientation of the molecules. In some instances both aggregate types are present in the solution over a wide concentration range.