Effect of ionic liquid on diffusion in P123 gel: fluorescence correlation spectroscopy

The effect of two room-temperature ionic liquids (RTILs) on the diffusion of three fluorescent dyes in the gel phase of a triblock copolymer, (PEO)(20)-(PPO)(70)-(PEO)(20) [Pluronic P123; poly ethylene oxide (PEO), poly propylene oxide (PPO)], was studied by using fluorescence correlation spectroscopy (FCS). We used three dyes, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), coumarin 480 (C480), and coumarin 343 (C343). By field-emission scanning electron microscopy (FESEM), it was observed that the macroscopic structure of the P123 gel remained unaffected upon addition of RTIL. In the absence of RTIL, the diffusion coefficient (D(t)) of the hydrophobic dye DCM (1 μm(2) s(-1) at the core) is smaller than that of the other two hydrophilic dyes (7 μm(2) s(-1) for C480 and C343). On addition of RTIL, the D(t) values of all of the dyes increase, indicating a decrease in local viscosity (η(eff)). The η(eff) of the core of the RTIL-P123 gel estimated from the D(t) of DCM is lower than that of both the P123 gel (at the core η=90 cP) and RTIL (η=110 cP). It is shown that the RTIL affects the structure of the gel by modifying the size of the micellar aggregates and by penetrating the core.     

Microviscosity in multiple regions of complex aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)

Aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO109-PPO41-PEO109) copolymers are nonionic surfactants that self-organize to form aggregate structures with increasing temperature or concentration. We have studied two concentrations over a range of temperatures so that the copolymers are in one of three microphases: unimers, micelles, or hydrogels formed from body centered cubic aggregates of micelles. Three different coumarin dyes were chosen based on their hydrophobicity so that different aggregate regions could be probed independently-water insoluble coumarin 153 (C153), hydrophobic coumarin 102 (C102), and the hydrophilic sodium carboxylate form of coumarin 343 (C343-). Fluorescence anisotropy experiments provide detailed information on the local microviscosity. C153 experiences a fourfold increase in reorientation time and hence microviscosity with increasing temperature through the microphase transition from unimers to micelles. C102 also shows an increase in microviscosity with temperature but smaller in magnitude and with the microphase transition shifted to higher temperature relative to C153. C343- shows only a slight sensitivity to the microphase transition. For any of the three coumarin probes, fluorescence anisotropies do not show any correlation with the microphase transition to form cubic hydrogels.     

Systematic study of the fluorescence decays of amino‐coumarin dyes in polymer matrices

We measured the fluorescence decays of seven different amino‐coumarin dyes in polymer films of poly(methyl methacrylate) (PMMA), poly(styrene) (PS), and ethylene‐butene rubber (EBR); as well as in the small molecule analogs ethyl acetate and toluene. Many of the dye‐solvent and dye‐polymer combinations exhibited single exponential decays with lifetimes ranging from 2.3 to 3.9 ns. Small deviations from single exponential behavior occurred for most of the dyes in EBR. Significant deviations from single exponential behavior occurred for 7‐(diethylamino)‐2‐oxo‐2H‐1‐benzopyran‐3‐carboxylic acid (coumarin‐3) in ethyl acetate and in all polymer matrices and 2,3,6,7‐tetrahydro‐11‐oxo‐1H,5H,11H‐[1]benzopyrano[6,7,8‐ij]quinolizin‐10‐carboxylic acid (coumarin‐343) in all of the polymer matrices. Time‐resolved fluorescence spectra indicated the presence of two different excited states for coumarin‐3 and coumarin‐343 in PMMA; these spectra were qualitatively different from the time‐resolved spectra of coumarin‐3 in ethyl acetate. We rationalize these results in terms of the chemical functionalities of the various dyes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2333–2343, 2007     

Theoretical investigation of the excited states of coumarin dyes for dye-sensitized solar cells

Using time-dependent density functional theory (TD-DFT), configuration interaction single (CIS) method, and approximate coupled cluster singles and doubles (CC2) method, we investigated the absorption spectra of coumarin derivative dyes (C343, NKX-2388, NKX-2311, NKX-2586, and NKX-2677), which have been synthesized for efficient dye-sensitized solar cells. The CC2 calculations are found in good agreement with the experimental results except for the smallest coumarin dye (C343). TD-DFT underestimates the vertical excitation energy of the larger coumarin dyes (NKX-2586 and -2677). Solvents (methanol) are found to induce a red shift of the vertical excitation energies, and their effects on the molecular geometry and the electronic structure are examined in detail. The deprotonated form of coumarin is also investigated, where a blue shift of the vertical excitation energies is observed.     

Ultrafast electron transfer in a nanocavity. dimethylaniline to coumarin dyes in hydroxypropyl gamma-cyclodextrin

Photoinduced electron transfer (PET) from N,N-dimethylaniline (DMA) to four coumarin dyes (C151, C481, C153, and C480) inside the cavity of hydroxypropyl gamma-cyclodextrin (hpCD) is studied using femtosecond upconversion. The rate of PET is found to be nonexponential and to vary significantly with the coumarin dyes. The rate for C481 is 100 times faster than that for C480. The PET rate displays a bell-shaped dependence on the free energy change and thus reveals a Marcus-type inverted region. The anisotropy decay of the four dyes in hpCD are found to be very similar, and hence the observed variation in the rate of PET is not due to variation in rotational diffusion of the acceptors (coumarin dyes).     

Phosphorylated 3‐Heteroarylcoumarins and Their Use in Fluorescence Microscopy and Nanoscopy

Photostable and bright fluorescent dyes with large Stokes shifts are widely used as markers in far‐field optical microscopy, but the variety of useful dyes is limited. The present study introduces new 3‐heteroaryl coumarins decorated with a primary phosphate group (OP(O)(OH)₂) attached to C‐4 in 2,2,4‐trimethyl‐1,2‐dihydroquinoline fragment fused with the coumarin fluorophore. The general synthetic route is based on the Suzuki reaction of 3‐bromocoumarines with hetarylboronic acids followed by oxidation of the methyl group at the C?C bond with SeO₂ (to an aldehyde), reduction with NaBH₄ (to an alcohol), and conversion into a primary phosphate. The 4 position in the coumarin system may be unsubstituted or bear a methyl group. Phosphorylated coumarins were found to have high fluorescence quantum yields in the free state and after conjugation with proteins (in aqueous buffers). In super‐resolution light microscopy with stimulated emission depletion (STED), the new coumarin dyes provide an optical resolution of 40–60 nm with a low background signal. Due to their large Stokes shifts and high photostability, phosphorylated coumarins enable to combine multilabel imaging (using one detector and several excitation sources) with diffraction unlimited optical resolution.     

Separation and analysis of dynamic Stokes shift with multiple fluorescence environments: coumarin 153 in bovine beta-lactoglobulin A

We use time-dependent fluorescence Stokes shift (TDFSS) information to study the fluctuation rates of the lipocalin, beta-lactoglobulin A in the vicinity of an encapsulated coumarin 153 molecule. The system has three unique dielectric environments in which the fluorophore binds. We develop a method to decompose the static and dynamic contributions to the spectral heterogeneity. This method is applied to temperature-dependent steady-state fluorescence spectra providing us with site-specific information about thermodynamic transitions in beta-lactoglobulin. We confirm previously reported transitions and discuss the presence of an unreported transition of the central calyx at 18 degrees C. Our method also resolves the contributions to the TDFSS from the coumarin 153 centrally located in the calyx of beta-lactoglobulin despite overlapping signals from solvent exposed dyes. Our experiments show dynamics ranging from 3-1200 ps. The analysis shows a decrease in the encapsulated dye's heterogeneity during the relaxation, which is taken as evidence of the breakdown of linear response.     

Solvent Effects on Spectral Property and Dipole Moment of the Lowest Excited State of Coumarin 343 Dye

Steady-state absorption and fluorescence spectra, and time-resolved fluorescence spectra of coumarin 343 (C343) were measured in different solvents. The effect of the solvent on the spectral properties and dipole moment of the lowest excited state of C343 were investigated. It was found that the absorption and fluorescence spectra red-shifted slightly and strongly with increasing solvent polarity, respectively, because the charge distribution of the excited state leaded to the increasing difference between the absorption and fluorescence spectra with increasing solvent polarity. The dipole moment of the lowest excited state of C343was determined from solvatochromic measurements and the quantum chemical calculation, and the results obtained from these two methods were fully consistent. Investigations of the time-resolved fluorescence of C343 in different solvents indicated that the fluorescence lifetimes increased nearly linearly with increasing solvent polarity from 3.09 ns in toluene to 4.45 ns in water. This can be ascribed to the intermolecular hydrogen bonding interactions between C343 and hydrogen donating solvents.     

Shape-persistent macrocycles functionalised with coumarin dyes: acid-controlled energy- and electron-transfer processes

We have investigated the spectroscopic properties (absorption spectra, emission spectra, emission lifetimes) of three triads in CH(2)Cl(2): C2-M-C2, C343-M-C343, and C2-M-C343, in which M is a shape-persistent macrocyclic hexagonal backbone composed of two 2,2'-bipyridine (bpy) units embedded in opposing sides, and C2 and C343 are coumarin 2 and coumarin 343, respectively. All the components are strongly fluorescent species (Phi=0.90, 0.79, and 0.93 for M, C2, and C343, respectively, as established by investigating suitable model compounds). In each triad excitation of M leads to almost quantitative energy transfer to the lowest coumarin-localised excited state. Upon addition of acid, the two bpy units of the M component undergo independent protonation leading to monoprotonated (e.g., C2-MH(+)-C2) and diprotonated (e.g., C2-M2 H(+)-C2) species. Further addition of acid leads to protonation of the coumarin component so that each triad is involved in four protonation equilibria. Protonation causes strong (and reversible, upon addition of base) changes in the absorption and fluorescence properties of the triads because of inversion of the excited-state order and/or the occurrence of electron-transfer quenching processes.     

Effect of anisotropic and isotropic solvent on the dipole moment of coumarin dyes

The ground state (μ(g)) and the excited state (μ(e)) dipole moments of two coumarin laser dyes, coumarin 440 and 460, were studied at room temperature in various solvents, viz., general solvents, alcohols and liquid crystals at 298 K. In this work, we report dipole moment of laser dyes in different anisotropic (liquid crystal) and isotropic environments for understanding the effects of environments on the molecular dipole moment and comparing them. Ground and excited state dipole moments of coumarin dyes were evaluated by means of solvatochromic shift method. It was observed that dipole moment values of excited states (μ(e)) were higher than the corresponding ground state values (μ(g)) in all media.     

4‐Trifluoromethyl‐Substituted Coumarins with Large Stokes Shifts: Synthesis,Bioconjugates, and Their Use in Super‐Resolution Fluorescence Microscopy

Bright and photostable fluorescent dyes with large Stokes shifts are widely used as sensors, molecular probes, and light‐emitting markers in chemistry, life sciences, and optical microscopy. In this study, new 7‐dialkylamino‐4‐trifluoromethylcoumarins have been designed for use in bioconjugation reactions and optical microscopy. Their synthesis was based on the Stille reaction of 3‐chloro‐4‐trifluoromethylcoumarins and available (hetero)aryl‐ or (hetero)arylethenyltin derivatives. Alternatively, the acylation of 2‐trifluoroacetyl‐5‐dialkylaminophenols with available (hetero)aryl‐ or (hetero)arylethenylacetic acids followed by intramolecular condensation afforded coumarins with 3‐(hetero)aryl or 3‐[2‐(hetero)aryl]ethenyl groups. Hydrophilic properties were provided by the introduction of a sulfonic acid residue or by phosphorylation of a primary hydroxy group attached at C‐4 of the 2,2,4‐trimethyl‐1,2‐dihydroquinoline fragment fused to the coumarin fluorophore. For use in immunolabeling procedures, the dyes were decorated with an (activated) carboxy group. The positions of the absorption and emission maxima vary in the ranges 413–480 and 527–668 nm, respectively. The phosphorylated dye, 9 ,CH?CH‐2‐py,H, with the 1‐(3‐carboxypropyl)‐4‐hydroxymethyl‐2,2‐dimethyl‐1,2‐dihydroquinoline fragment fused to the coumarin fluorophore bearing the 3‐[2‐(2‐pyridyl)ethenyl] residue (absorption and emission maxima at 472 and 623 nm, respectively) was used in super‐resolution light microscopy with stimulated emission depletion and provided an optical resolution better than 70 nm with a low background signal. As a result of their large Stokes shifts, good fluorescence quantum yields, and adequate photostabilities, phosphorylated coumarins enable two‐color imaging (using several excitation sources and a single depletion laser) to be combined with subdiffractional optical resolution.     

Intriguing H-aggregate and H-dimer formation of coumarin-481 dye in aqueous solution as evidenced from photophysical studies

Photophysical properties of coumarin-481 (C481) dye in aqueous solution show intriguing presence of multiple emitting species. Concentration and wavelength dependent fluorescence decays and time-resolved emission spectra and area-normalized emission spectra suggest the coexistence of dye monomers, dimers, and higher aggregates (mostly trimers) in the solution. Because of the efficient intramolecular charge transfer (ICT) state to twisted intramolecular charge transfer (TICT) state conversion, the dye monomers show very short fluorescence lifetime of ~0.2 ns. Fluorescence lifetimes of dimers (~4.1 ns) and higher aggregates (~1.4 ns) are relatively longer due to steric constrain toward ICT to TICT conversion. Observed results indicate that the emission spectra of the aggregates are substantially blue-shifted compared to monomers, suggesting H-aggregation of the dye in the solution. Temperature-dependent fluorescence decays in water and time-resolved fluorescence results in water-acetonitrile solvent mixtures are also in support of the dye aggregation in the solution. Though dynamic light scattering studies could not recognize the dye aggregates in the solution due to their small size and low concentration, fluorescence up-conversion measurements show a relatively higher decay tail in water than in water-acetonitrile solvent mixture, in agreement with higher dye aggregation in aqueous solution. Time-resolved fluorescence results with structurally related non-TICT dyes, especially those of coumarin-153 dye, are also in accordance with the aggregation behavior of these dyes in aqueous solution. To the best of our knowledge, this is the first report on the aggregation of coumarin dyes in aqueous solution. Present results are important because coumarin dyes are widely used as fluorescent probes in various microheterogeneous systems where water is always a solvent component, and the dye aggregation in these systems, if overlooked, can easily lead to a misinterpretation of the observed results.     

“Reduced” Coumarin Dyes with an O‐Phosphorylated 2,2‐Dimethyl‐4‐(hydroxymethyl)‐1,2,3,4‐tetrahydroquinoline Fragment: Synthesis,Spectra, and STED Microscopy

Large Stokes‐shift coumarin dyes with an O‐phosphorylated 4‐(hydroxymethyl)‐2,2‐dimethyl‐1,2,3,4‐tetrahydroquinoline fragment emitting in the blue, green, and red regions of the visible spectrum were synthesized. For this purpose, N‐substituted and O‐protected 1,2‐dihydro‐7‐hydroxy‐2,2,4‐trimethylquinoline was oxidized with SeO₂ to the corresponding α,β‐unsaturated aldehyde and then reduced with NaBH₄ in a “one‐pot” fashion to yield N‐substituted and 7‐O‐protected 4‐(hydroxymethyl)‐7‐hydroxy‐2,2‐dimethyl‐1,2,3,4‐tetrahydroquinoline as a common precursor to all the coumarin dyes reported here. The photophysical properties of the new dyes (“reduced coumarins”) and 1,2‐dihydroquinoline analogues (formal precursors) with a trisubstituted C=C bond were compared. The “reduced coumarins” were found to be more photoresistant and brighter than their 1,2‐dihydroquinoline counterparts. Free carboxylate analogues, as well as their antibody conjugates (obtained from N‐hydroxysuccinimidyl esters) were also prepared. All studied conjugates with secondary antibodies afforded high specificity and were suitable for fluorescence microscopy. The red‐emitting coumarin dye bearing a betaine fragment at the C‐3‐position showed excellent performance in stimulation emission depletion (STED) microscopy.     

Chromophore‐labeled dendrimers for use in single‐layer light‐emitting diodes

Two novel chromophore‐labeled dendrimers are presented and their incorporation into two‐component, single layer organic light emitting diodes (OLEDs) is described. The photoluminescence (PL) spectra, both in solution and in the solid state, demonstrate that Forster energy transfer from the donor chromophores on the dendrimers periphery to the acceptors located at the core is highly efficient, and affords emission exclusively from the core dyes, either coumarin 343 or a benzene‐capped pentathiophene. When these dendrimers are doped into single layer OLEDs, the electroluminescence (EL) is nearly identical to the photoluminescence described above, indicating once again exclusive emission from the core chromophores.     

Coumarin-based two-photon AIE fluorophores: Photophysical properties and biological application

Based on the coumarin skeleton, we deliberately designed two groups of fluorophores, termed as Coum-R and Naph-Coum-R, using the diphenylamino group as the electron donor, which displayed long-wavelength emissions (red spectral region), large Stokes shift (up to 204 nm), superior AIE performance, and large two-photon absorbance cross-sections (as high as 365 GM). The electron-withdrawing substituents at the 3-position of these dyes could induce a significant red-shift in their emission spectra. Preliminary imaging experiments demonstrated the capability of these dyes as two-photon fluorophores for specifically staining lipid droplets in living cells.     

Coumarin derivatives for dye sensitized solar cells: a TD-DFT study

Time dependent density functional theory (TD-DFT) calculations have been carried out to study the electronic structure and the optical properties of five coumarin based dyes: C343, NKX-2311, NKX-2586, NKX-2753 and NKX-2593. We have found out that the position and width of the first band in the electronic absorption spectra, the absorption threshold and the LUMO energy with respect to the conduction band edge are key parameters in order to establish some criteria that allow evaluating the efficiency of coumarin derivatives as sensitizers in Dye Sensitized Solar Cells (DSSC). Those criteria predict the efficiency ordering for the coumarin series in good agreement with the experimental evidence. Presumably, they might be used in the design of new efficient organic based DSSC.     

Temperature dependence of solvation dynamics of probe molecules in methanol-doped ice and in liquid ethanol

We have studied the solvation statics and dynamics of coumarin 343 and a strong photoacid (pK* approximately 0.7) 2-naphthol-6, 8-disulfonate (2N68DS) in methanol-doped ice (1% molar concentration of methanol) and in cold liquid ethanol in the temperature range of 160-270 K. Both probe molecules show a relatively fast solvation dynamics in ice, ranging from a few tens of picoseconds at about 240 K to nanoseconds at about 160 K. At about 160 K in doped ice, we observe a sharp decrease of the dynamic Stokes shift of both coumarin 343 and 2N68DS. Its value is approximately only 200 cm-1 at approximately 160 K compared to about 1100 cm-1 at T >/= 200 K (at times longer than t > 10 ps). We find a good correlation between the inefficient and slow excited-state proton-transfer rate at low-temperature ice, T < 180 K, and the dramatic decrease of the solvation energy, as measured by the dynamic band shift, at these low temperatures. We find that the average solvation rate in ice is similar to its value in liquid ethanol at all given temperatures in the range of 200-250 K. The surprisingly fast solvation rate in ice is explained by the relatively large freedom of the water hydrogen rotation in ice Ih.     

Molecular origins of optoelectronic properties in coumarin dyes: toward designer solar cell and laser applications

Coumarin derivatives are used in a wide range of applications, such as dye-sensitized solar cells (DSCs) and dye lasers, and have therefore attracted considerable research interest. In order to understand the molecular origins of their optoelectronic properties, molecular structures for 29 coumarin laser dyes are statistically analyzed. To this end, data for 25 compounds were taken from the Cambridge Structural Database and compared with data for four new crystal structures of coumarin laser dyes [Coumarin 487 (C(19)H(23)NO(2)), Coumarin 498 (C(16)H(17)NO(4)S), Coumarin 510 (C(20)H(18)N(2)O(2)), and Coumarin 525 (C(22)H(18)N(2)O(3))], which are reported herein. The competing contributions of different resonance states to the bond lengths of the 4- and 7-substituted coumarin laser dyes are computed based on the harmonic oscillator stabilization energy model. Consequently, a positive correlation between the contribution of the para-quinoidal resonance state and the UV-vis peak absorption wavelength of these coumarins is revealed. Furthermore, the perturbations of optoelectronic properties, owing to chemical substituents in these coumarin laser dyes, are analyzed: it is found that their UV-vis peak absorption and lasing wavelengths experience a red shift, as the electron-donating strength of the 7-position substituent increases and/or the electron-withdrawing strength of the 3- or 4-position substituent rises; this conclusion is corroborated by quantum-chemical calculations. It is also revealed that the closer the relevant substituents align with the direction of the intramolecular charge transfer (ICT), the larger the spectral shifts and the higher the molar extinction coefficients of coumarin laser dyes. These findings are important for understanding the ICT mechanism in coumarins. Meanwhile, all structure-property correlations revealed herein will enable knowledge-based molecular design of coumarins for dye lasers and DSC applications.     

Simultaneous light emission from a mixture of dendrimer encapsulated chromophores: a model for single-layer multichromophoric organic light-emitting diodes

The site isolation of two dyes capable of electronic interaction via Forster energy transfer has been studied with the two dyes coumarin 343 and pentathiophene encapsulated by dendrons containing both solubilizing and electroactive moieties. Photoluminescence studies of mixtures of the dendritic dyes show that at high dendron generation, significant site isolation is achieved with relative emission characteristics influenced by both the degree of site isolation and the emission quantum yield of the dyes. Electroluminescence studies carried out in organic light emitting diode devices confirm that color tuning may be achieved by mixing the two encapsulated dyes in a single layer. However, selective carrier trapping by one of the core component dyes can dramatically influence the effectiveness of other components in the device.     

Ultrafast fluorescence resonance energy transfer in a reverse micelle: excitation wavelength dependence

Fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to fluorescein 548 (F548) in a sodium dioctyl sulfosuccinate (AOT) reverse micelle is studied by picosecond and femtosecond emission spectroscopy. In bulk water, at the low concentration of the donor (C480) and the acceptor (F548), no FRET is observed. However, when the donor (C480) and the acceptor (F548) are confined in a AOT reverse micelle very fast FRET is observed. The time constants of FRET were obtained from the rise time of the emission of the acceptor (F548). In a AOT microemulsion, FRET is found to occur in multiple time scales--3, 200, and 2700 ps. The 3 ps component is assigned to FRET in the water pool of the reverse micelle with a donor-acceptor distance, 16 A. The 200 ps component corresponds to a donor-acceptor distance of 30 A and is ascribed to the negatively charged acceptor inside the water pool and the neutral donor inside the alkyl chains of AOT. The very long 2700 ps component may arise due to FRET from a donor outside the micelle to an acceptor inside the water pool and also from diffusion of the donor from bulk heptane to the reverse micelle. With increase in the excitation wavelength from 375 to 405 nm the relative contribution of the FRET due to C480 in the AOT reverse micelle (the 3 and 200 ps components) increases.