Dye Aggregation and Complex Formation Effects in 7-(Diethylamino

Article pubs.acs.org/JPCC

Dye Aggregation and Complex Formation Effects in 7‑(Diethylamino)-coumarin-3-carboxylic Acid Xiaogang Liu,† Jacqueline M. Cole,*,†,‡ Philip C. Y. Chow,† Lei Zhang,† Yizhou Tan,† and Teng Zhao† †

Cavendish Laboratory, Department of Physics, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom ‡ Argonne National Laboratory, 9700 S Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: 7-(Diethylamino)-coumarin-3-carboxylic acid (1) has been used as a laser dye, fluorescent label, and biomedical inhibitor in many different applications. Although this dye is typically used in the solution phase, it is prone to molecular aggregation, resulting in many inconsistent optoelectronic properties being reported in the literature. In this paper, the ultraviolet−visible absorption and fluorescence spectra of 1 are investigated in three representative solvents: cyclohexane [nonpolar and non-hydrogen bonding (NHB)], ethanol (moderately polar and hydrogen bond accepting and donating), and dimethyl sulfoxide (DMSO) (strongly polar and hydrogen bond accepting). These experimental results, in conjunction with (time-dependent) density functional theory (DFT/TD-DFT)-based quantum calculations, have led to the identification of the J-aggregates of 1 and rationalized its different aggregation characteristic in cyclohexane in contrast to that of another similar compound, coumarin 343. We show here that these aggregates are largely responsible for the anomalous optoelectronic properties of this compound. In addition, DFT calculations and 1H NMR spectroscopy measurements suggest that the intramolecular hydrogen bond in 1 could be “opened up” in hydrogen bond accepting solvents, affording significant molecular conformational changes and complex formation effects. The comprehensive understanding of the molecular aggregation and complex formation mechanisms of 1 acquired through this work forms a foundation for the knowledge-based molecular design of organic dyes with tailored aggregation tendencies or antiaggregation characteristics catering to different optoelectronic applications.

1. INTRODUCTION Molecular aggregation is a common phenomenon with profound implications on the photophysical, photochemical, and biomedical properties of optoelectronic materials. In some applications, aggregation causes effects that are so detrimental to a system that they should be avoided. For example, the aggregation of coumarin and xanthene dyes reduces their lasing and fluorescence efficiencies;1,2 J-aggregation of dyes generally enhances unfavorable charge recombination and consequently decreases the efficiency of dye-sensitized solar cells (DSSC);3,4 and the aggregation of the β-amyloid peptide in the brain is related to Alzheimer’s disease.5 In some cases, molecular aggregation turns out to be useful. For example, the molecular aggregation of silole changes this dye from a weak luminophor in its isolated molecular state to a strong emitter as an aggregate.6 This mechanism can be used to develop sensors for the detection of biomolecules and bioactivities.7,8 H-aggregation of dyes also boosts charge injection yield and increases DSSC efficiency in some cases,9 and the aggregation of protein LHCIIb forms part of a critical route for plant leaves to nonphotochemically dissipate excess photons required for photosynthesis and regulate light harvesting.10 The selfassembly engineered aggregation of molecules can be © XXXX American Chemical Society

developed into useful optical and electronic applications, such as recording media, organic solar cells, and sensors.11,12 Understanding the nature and molecular origins of dye aggregation is therefore an important step toward tailoring materials for favorable optoelectronic device performance. Although seldom reported, 7-(diethylamino)-coumarin-3carboxylic acid (1; Figure 1) is very prone to molecular aggregation. This dye is predominantly used in the solution phase in a broad range of applications, such as biomedical inhibitors,13 fluorescent dyes,14 laser dyes,15 and organic sensitizers in dye-sensitized solar cells.15,16 Its optoelectronic properties have been studied by several research groups.17−19 However, considerably inconsistent results are reported in the literature. For example, the ultraviolet−visible (UV−vis) peak absorption wavelength (λabs max) of 1 in ethanol has been reported at 393 nm18 and 422 nm.19 Such a large difference is significantly beyond typical systematic differences due to UV−vis absorption instrumentation and clearly indicates some unusual behavior of 1 in the solution phase. This dye Received: September 21, 2013 Revised: March 20, 2014

A

dx.doi.org/10.1021/jp409435v | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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70, 65, and 80 °C for 30 min in cyclohexane, ethanol, and DMSO, respectively, in order to ensure that the monomer− aggregate coupled systems reached equilibrium in the solution of 1. These samples were then cooled to various temperatures prior to UV−vis absorption measurements. Steady-state fluorescence spectra of 1 in cyclohexane, ethanol, and DMSO were collected on a Varian Cary Eclipse fluorescence spectroscope at room temperature. The uncertainties in all peak fluorescence wavelength (λflu max) readings are ±1 nm. For measurements of fluorescence decay dynamics, a pulsed diode laser (PicoQuant LDH400) of 407 nm was used as the excitation source. The emission of 1 was measured using a microchannel plate photomultiplier from Hamamatsu coupled to a monochromator and time-correlated single-photon counting (TCSPC) electronics from Edinburgh Instruments (Lifespec-ps and VTC900 PC card). 2.2. Solution-State 1H NMR Spectroscopy. For 1H NMR spectroscopy experiments, 1 was dissolved into deuterated DMSO in high concentration (∼1 mM). NMR spectra were collected using a Bruker Avance 500 Cryo Ultrashield NMR spectrometer at four different temperatures: 25, 35, 45, and 55 °C. Chemical shifts were measured relative to an internal tetramethylsilane reference. 2.3. DFT and TD-DFT Calculations. Quantum-chemical calculations were performed on 1 using Gaussian 09.25 Becke’s three-parameter and Lee−Yang−Parr hybrid functional (B3LYP)26−28 and a 6-31+G(d,p) basis set29 were used in all calculations, unless stated otherwise. The geometries of three conformationally representative monomers and three exemplary dimers of 1 were first optimized in vacuo and in solution using the SMD model30 to act as an implicit solvent incorporation. Cyclohexane, ethanol, and DMSO solvents were modeled in this way. Analogous calculations of 1 were also performed using explicit DMSO/ethanol solvent molecules in the structural model in order to understand solute−solvent hydrogen bond interactions. TD-DFT calculations were subsequently carried out to determine the peak absorption wavelengths of the monomers and dimers of 1. In all calculations, frequency checks were performed after each geometry optimization to ensure that minima on the potential energy surfaces were found. During the modeling of the dimers and complexes of 1, the D3 version of Grimme’s dispersion correction with Becke−Johnson damping (GD3BJ)31 was applied.

Figure 1. (a) Molecular structure of 1. (b) Two dimer models of 1, denoted as b-b and c-c, according to their different −COOH conformations. (c) A high-order aggregate model: a cluster of ten molecules extracted from the crystal structure of 1, representing a firstorder approximation to high-order aggregates in solution.24 The arrows indicate the dipole moment of 1.

has also been used as a fluorescence lifetime reference based on the monomer assumption.20 The demonstration of the widespread aggregation of 1 will thus have an important implication for the reassessment of this dye’s characteristics and its suitable applications. Moreover, we have shown that coumarin 343 experiences serious molecular aggregation in the solution phase in previous studies.21−23 Owing to the similar molecular structures of coumarin 343 and 1, one may expect that analogous dye aggregation formation effects also occur for 1. However, it turns out that a small molecular structural difference causes considerably different aggregation phases in them. In this paper, the UV−vis absorption and fluorescence spectra, as well as the fluorescence decay dynamics of 1, are reported in three representative solvents, namely, cyclohexane [nonpolar and non-hydrogen bonding (NHB)], ethanol [moderately polar and hydrogen bond accepting and donating (HBA/D)], and dimethyl sulfoxide (DMSO; strongly polar and HBA). On the basis of these data, the molecular aggregation of 1 in the solution phase is analyzed. Density functional theory (DFT) and time-dependent DFT (TD-DFT) based quantum calculations, in conjunction with 1H NMR spectroscopy, are also employed to understand the aggregation formation mechanisms of 1, as well as potential conformational changes and complex formation effects in its monomer form. The knowledge gained through this study provides deeper insights into the impact of molecular aggregation and complex formation effects on the optoelectronic properties of 1, from which one can build an important foundation for the formulation of new coumarin molecular-design strategies with tailored aggregation or antiaggregation characteristics.

3. RESULTS AND DISCUSSION 3.1. Solvatochromism of 1. The molecular aggregation of 1 in the solution phase is indicated by a fitting of its λabs max values to an empirical solvatochromic model. Solvatochromic models are powerful tools for interpreting solvent effects on the spectral shifts of dyes.21,32 These models allow one to split the shift of peak UV−vis absorption and/or emission wavelengths (λmax) into different categories, such as nonspecific and specific solvent effects. A nonspecific solvent effect concerns the “global” solvent effect acting as a continuous and uniform dielectric. In contrast, specific solvent effects deal with a directional, nonuniform distribution of the “local” molecularlevel interactions between solvents and solutes, such as hydrogen bond interactions.33 One of the most popular solvatochromic models is the Taft−Kamlet solvatochromic comparison method (eq 1).34−37 In this model, ν represents the

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Solution-State Optical Spectroscopy. Compound 1 was supplied by Sigma-Aldrich and used without further purification. For solvatochromism studies, dilute solution samples of 1 (with absorption below 10% of the incident light) in cyclohexane, ethanol, DMSO, acetone, acetonitrile, diethyl ether, ethyl acetate, ethylene glycol, methanol, and 2propanol were tested on a Hewlett-Packard G1103A spectrophotometer. The uncertainties in all peak UV−vis absorption wavelength (λabs max) readings are ±1 nm. More samples of 1 in cyclohexane, ethanol, and DMSO at various concentrations (from ∼1 to ∼50 μM) were heated at B



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transition energy (or the reciprocal of λmax) and v0 is its value in cyclohexane. π*, α, and β describe the solvent polarity, solvent hydrogen bond donor (HBD) acidity (the ability of the solvent to donate a proton to a solvent-to-solute hydrogen bond), and solvent hydrogen bond acceptor (HBA) basicity (the ability of the solvent to accept a proton from a solvent-to-solute hydrogen bond), respectively. Note that eq 1 works only for nonaromatic and nonchlorinated solvents; additional correction terms are required for aromatic and chlorinated solvents. The values of π*, α, and β have been experimentally determined through various means (Table 1).38 Consequently, by Table 1. Solvent Properties in the Taft−Kamlet Solvatochromic Comparison Method and the Associated a λabs max of 1 solvent

π*

α

β

λabs max (nm)

acetone acetonitrile cyclohexane diethyl ether DMSO ethanol ethyl acetate ethylene glycol methanol propan-2-ol

0.71 0.75 0 0.27 1.00 0.54 0.55 0.92 0.60 0.48

0.08 0.19 0 0 0 0.86 0 0.90 0.98 0.76

0.43 0.40 0 0.47 0.76 0.75 0.45 0.52 0.66 0.84

427 430 415 418 420 421 424 420 422 425

Figure 2. (a) Normalized UV−vis absorption spectra of (a) 1 and (b) coumarin 343 in cyclohexane.22 The solutions are prepared via two different methods (Table 2). (c) Compound 1 in cyclohexane prepared via method B before (left) and after (right) bubbling N2 gas.

Solvent parameters for the Taft−Kamlet model (π*, α, and β) were retrieved from ref 38. Relatively dilute solution samples (with absorption below 10% of the incident light) were used for all UV− vis absorption spectroscopy experiments, but without special attention to the dye aggregation effects. a

(Figure 2b),22 one may also assign these three peaks to monomers, dimers, and J-aggregates of 1, respectively. However, it turns out that the small molecular structural variance between 1 and coumarin 343 leads to a large difference in their aggregation phases. In fact, the three peaks of 1 are all related to its J-aggregates: the two least significant of these peaks, centered at 393 and 405 nm, correspond to the vibration bands of the main J-aggregate peak at 415 nm. This assignment is further justified below. It is interesting to point out that the solution of 1 and coumarin 343 in cyclohexane has been prepared by two methods, A and B (Table 2). In method B, 1 is first dissolved in acetonitrile and then transferred into cyclohexane. Because acetonitrile and cyclohexane are immiscible, the transferred acetonitrile solution of 1 is separated into small “domains” within cyclohexane (Figure 2c). Molecules of 1 are likely to stay inside these acetonitrile domains, owing to preferential solvation. However, as acetonitrile molecules evaporate away such that their domain sizes become smaller, the local concentration of 1 within these domains increases such that the formation of aggregates is enhanced. Moreover, J-aggregates form in acetonitrile (Section 3.2.2); they eventually transfer into the cyclohexane when all acetonitrile molecules evaporate (Figure 2c), in addition to a small amount of other supramolecular species. In the case of coumarin 343 prepared via method B, we have shown that there are three peaks in its UV−vis absorption spectra and the relative weights of these peaks vary as the solution concentration increases (Figure 2b); these peaks at 406, 419, and 430 nm have been assigned to the monomers, dimers (formed via intermolecular hydrogen bonds at their −COOH groups), and J-aggregates of coumarin 343, respectively.22 Moreover, we have shown that dimers of

measuring λmax (or v) of a dye in various solvents, s, a, and b can be derived via multiple linear regression; their values quantify the contributions from distinctive solvent properties (π*, α, and β) upon a change in v. Previous studies show that the solvatochromism of a number of coumarin derivatives could be described very well by the Taft−Kamlet solvatochromic comparison method with excellent goodness-of-fit (R2 close to 1).34−36,39 v = v0 + sπ * + aα + bβ

(1)

λabs max

On the basis of the data of 1 (Table 1), the statistical best-fit to the Taft−Kamlet model is also obtained (eq 2). However, the goodness-of-fit parameter is surprisingly low, with R2 = 0.222. This is a strong indication that some unusual solvent effect is involved, which is unaccounted in the Taft− Kamlet model; one of the most likely causes concerns dye aggregation. Hence, the molecular aggregation effects of 1 will be examined in selected solvents in the next sections. v = (23.92 ± 0.39) − (0.40 ± 0.60)π *

(2)

(For eq 2, the numbers in parentheses represent 95% confidence limits for their corresponding fitting coefficients.) 3.2. Molecular Aggregation of 1. The tendency of 1 to aggregate and its associated characteristics were studied in three different solvents: cyclohexane, ethanol, and DMSO. 3.2.1. Molecular Aggregation of 1 in Cyclohexane. The UV−vis absorption spectra of 1 in cyclohexane exhibit three peaks at 393, 405, and 415 nm (Figure 2a). Following its similarity to the UV−vis absorption spectra and the associated peak assignments of a very similar compound, coumarin 343 C



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Table 2. Two Solution Preparation Methods for 1 in Cyclohexane Solvent method A 1 2 3

Add 1 directly into cyclohexane. Because of its low solubility, saturated cyclohexane solution and precipitates of 1 are formed. Keep the sample for several hours, as the dissolution of 1 in cyclohexane is a very slow process. Take the saturated solution and dilute it to different ratios with more cyclohexane.

method B 1 2 3

coumarin 343 are more energetically stable than monomers.22 Consequently, by directly dissolving coumarin 343 into cyclohexane via method A, a single dominant dimer peak at 419 nm is observed (Figure 2b). Furthermore, the dimers of coumarin 343 may form stacked-aggregates, owing to their highly planar molecular structure. As a result, dimers of coumarin 343 are largely preserved and become dominant over monomers and J-aggregates. abs In contrast, such preparation-method dependent λmax variation is not found in the case of 1. For example, three distinct peaks are found in the UV−vis absorption spectra of 1, and both solution preparation methods lead to the same λabs max, which is attributed to the J-aggregates (Figure 2a). This difference between 1 and coumarin 343 suggests that the aggregation phases of 1 may be different from those of coumarin 343. In fact, we have also collected the UV−vis absorption spectra of several similar coumarin derivatives, including coumarin 521, coumarin 314T, coumarin 337, and coumarin 498 (Figure 3). These compounds do not possess a

Add a known amount of 1 into acetonitrile. Compound 1 has high solubility in this solvent. Take a small amount of the prepared acetonitrile solution and add it into cyclohexane in order to achieve the required concentration. Bubble nitrogen gas until acetonitrile evaporates from the cyclohexane solution.

Figure 4. (a) UV−vis absorption spectra of 1 in cyclohexane at 25 °C with various concentrations: ∼1.2, ∼2.5, ∼5, and ∼10 μM. (c) UV−vis absorption spectra of 1 in cyclohexane at various temperatures (concentration ≈10 μM). Panels (b) and (d) show the normalized UV−vis absorption spectra corresponding to panels (a) and (c).

absorbance in the long-wavelength range, caused by the dissolution of some supramolecular species in this lowsolubility solvent (Figure 4c). Nevertheless, the wavelengths of all three peaks and their relative weights remain unchanged (Figure 4d). Steady-state fluorescence measurements of 1 in cyclohexane show that its λflu max value is independent of excitation wavelength (λexc) and tops out at 420 nm (Figure 5a,b), with two secondary peaks at ∼431 and ∼444 nm. The normalized emission spectra of 1 match each other very well (Figure 5b).

Figure 3. Normalized UV−vis absorption spectra of (a) coumarin 334, (b) coumarin 314T, (c) coumarin 337, and (d) coumarin 498 in cyclohexane.

carboxylic acid group and, as such, are not able to form dimers via intermolecular hydrogen bonds like coumarin 343. However, each of these UV−vis absorption spectra exhibit three peaks, similar to that of 1. To gain more knowledge about the first two peaks at 393 and 405 nm, the concentration and temperature dependences of the UV−vis absorption spectra of 1 have been investigated. Henceforth, all samples are prepared via method A, as both solution preparation methods lead to similar UV−vis absorption profiles for 1. The normalized UV−vis absorption spectra of 1 at various concentrations match each other almost perfectly (Figure 4a,b). As temperature increases, the absorbance of all three peaks increase at the expense of the

Figure 5. (a) Steady-state fluorescence spectra of 1 in cyclohexane excited at various λexc. (c) Fluorescence excitation spectra of 1 in cyclohexane monitored from λem = 420 to 490 nm at a step size of 10 nm. Panels (b) and (d) show normalized spectra corresponding to panels (a) and (c), respectively (concentration ≈5 μM). D



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Moreover, the fluorescent excitation spectra of 1 have been probed at various emission wavelengths (λemi) (Figure 5c,d). These spectra exhibit three peaks at 393, 405, and 415 nm, in excellent agreement with the UV−vis absorption spectra of 1; the relative weights of these three peaks remain constant, as reflected by the normalized fluorescent excitation spectra (Figure 5d). Interestingly, the UV−vis absorption and fluorescence excitation spectra and the emission spectra of 1 are approximately mirror-images of one another. These results suggest that the three peaks in the UV−vis absorption spectra of 1 correspond to the same species. In other words, the two peaks appearing at the shorter wavelengths are two vibrational bands of the J-aggregates but not monomers and dimers of 1. When photoexciting 1 at the only excitation wavelength available in our TCSPC setup, 407 nm, and probing the emission decay dynamics in cyclohexane at 450 and 515 nm, it was found that both decays fit to a biexponential function; the emissions arise mainly from J-aggregates, but there is also a minor contribution of short-lived fluorescence, probably owing to supramolecular species formed in cyclohexane, especially in the long-wavelength region (Table 3).

c-c (Figure 1b), have energies lower than those of monomers of 1 in cyclohexane as a result of intermolecular hydrogen bond interactions within these dimers. Note that the difference between these two dimers lies in the dihedral angles between their −COOH groups and the coumarin framework which differ by ca. 180°. On the other hand, while the exact size and structure of these J-aggregates are unknown, the molecular packing of 1 in its crystal-lattice environment provides a very good approximation (Figure 1c); dimer a-a represents a basic unit of the J-aggregate, in which molecules of 1 are connected in a head-to-tail fashion. Indeed, the calculations show that a-a has the lowest energy. Furthermore, dimer a-a exhibits close packing and its energy is likely to reduce when it grows into a large J-aggregate. In contrast, owing to the bending of the diethyl-amino group caused by steric hindrance, it is difficult for b-b and c-c to join other dimers so as to accumulate into a larger aggregate. Consequently, J-aggregation is predominant in the solution of 1 in cyclohexane. Our calculations also show that the intermolecular dipole− dipole interactions in J-aggregates further enhance the dipole moments and intramolecular charge transfer (ICT) of 1. As a result, TD-DFT calculations demonstrate that dimer a-a has a λabs max that is relatively longer than that of monomer 1a, i.e., 391.0 nm (a-a) versus 385.8 nm (1a). 3.2.2. Molecular Aggregation of 1 in Ethanol. DFT calculations show that J-aggregation formation of 1 is also energetically favorable in both ethanol and DMSO (Table S1 of Supporting Information). Indeed, although 1 has good solubility and the absolute concentration of 1 is kept relatively low in ethanol (≤53.84 μM), significant J-aggregation is observed. As the concentration of the ethanol solution rises, a significant red shift in λabs max from 389 to 424 nm is evident (Figure 7a,b). Furthermore, as the temperature rises from 10 to

Table 3. Single- and Double-Exponential Fittings to the Fluorescence Decay Dynamics of 1 τ1 (ratio) ⟨assignment⟩ decay 450 decay 515

at nm at nm

decay 470 decay 600

at nm at nm

decay 470 decay 600

at nm at nm

τ2 (ratio) ⟨assignment⟩

fluorescent decay dynamics in cyclohexane 2.57 ns (97%) 0.88 ns (3%) ⟨others⟩ ⟨J-aggregates⟩ 2.59 ns (74%) 0.48 ns (26%) ⟨others⟩ ⟨J-aggregates⟩ fluorescent decay dynamics in ethanol 2.43 ns (100%) ⟨monomers⟩ 2.46 ns (65%) 0.69 ns (35%) ⟨monomers⟩ ⟨J-aggregates⟩ fluorescent decay dynamics in DMSO 0.70 ns (93%) 2.56 ns (7%) ⟨monomers⟩ ⟨J-aggregates⟩ 0.73 ns (55%) 2.53 ns (45%) ⟨monomers⟩ ⟨J-aggregates⟩

χ2 1.177 1.387

1.369 1.164

1.300 1.234

In order to understand the molecular origins of J-aggregation formation, representative molecular structures of 1 and their energies were computed using DFT (Figure 6). On the one hand, these calculations show that dimers of 1, such as b-b and

Figure 7. (a) UV−vis absorption spectra of 1 in ethanol at 20 °C of various concentrations: 1.80, 3.59, 7.18, 10.77, 14.36, 21.54, 28.73, 35.91, and 53.84 μM. (c) UV−vis absorption spectra of 1 in ethanol at various temperatures (concentration, 14.36 μM). Panels (b) and (d) show normalized UV−vis absorption spectra corresponding to panels (a) and (c), respectively.

45 °C, the UV−vis spectra of 1 (concentration, 14.36 μM) experience a progressive blue shift from 401 to 396 nm (Figure 7c,d); this is attributed to the dissolution of J-aggregates at high temperatures. Further analysis shows that the molar absorptivity of 1 exhibits one isosbestic point at 400 nm (Figure 8a), indicating that two species (i.e., monomers and J-aggregates) are present

Figure 6. (a) Theoretically optimized molecular structures of a monomer and three different dimers of 1 in cyclohexane. (b) Their relative energy differences. E



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Figure 8. (a) Molar absorptivity of 1 in ethanol at 20 °C of various concentrations: 1.80, 3.59, 7.18, 10.77, 14.36, 21.54, 28.73, 35.91, and 53.84 μM. (b) Calculated molar absorptivities of the monomer and Jaggregate of 1. (c) Measured (black solid line) and calculated (green dotted line) molar absorptivity of 1 at 20 °C (concentration, 14.36 μM), in conjunction with the calculated contributions from monomers (blue dotted line) and J-aggregates (red dotted line). (d) Calculated fractions of monomers and J-aggregates of 1 as a function of concentration at 20 °C.

Figure 9. (a) Steady-state fluorescence spectra of 1 (concentration, 7.18 μM) in ethanol; λexc is varied from 350 to 435 nm at a step size of 5 nm. (c) Steady-state fluorescence spectra of 1 in ethanol (λexc = 390 nm) at various concentrations: 0.45, 0.90, 1.80, 3.59, 7.18, 10.77, 14.36, 21.54, 28.73, and 35.91 μM. (e) Fluorescence excitation spectra of 1 in ethanol (concentration, 35.91 μM); λemi is varied from 430 to 530 nm at a step size of 10 nm. Panels (b), (d), and (f) show normalized spectra corresponding to panels (a), (c), and (e), respectively.

in the solution. Hence, by using the isodesmic model11 implemented via a modified version of Kubista’s chemometric analysis algorithm (Supporting Information),40 the molar absorptivities of 1 in both the monomer and J-aggregate phases were determined (Figure 8b). The calculated values demonstrate excellent agreement with experimental data (Figure 8c) and suggest that there is an increasing level of Jaggregation as the concentration of 1 rises (Figure 8d). The aggregation formation constant was determined to be ∼1.5 × 104 M−1 at 20 °C. flu During the steady-state emission measurements, λmax demonstrates a rather complicated dependence on both λexc and solution concentration. At a concentration of 7.18 μM, λflu max appears at ∼449 nm when λexc ≤ 405 nm (Figure 9a,b; Table S2 of Supporting Information). As the excitation wavelength is further shifted into the red region, fewer monomers and more J-aggregates are excited. As a consequence, λflu max shifts to 465 nm when λexc = 435 nm (Table S2 of Supporting Information). While keeping λexc at 390 nm but increasing the solution concentration from 0.45 μM to 35.91 μM, λflu max progressively shifts from 445 to 459 nm, owing to more widespread Jaggregation at higher concentrations and their associated emissions (Figure 9c,d; Table S3 of Supporting Information). The fluorescence excitation spectra of 1 also illustrate the presence of J-aggregates. Although the λabs max value of 1 lies at 421 nm at a concentration of 35.91 μM, its fluorescence excitation spectra actually peak at ∼380 nm (Figure 9e,f). This is because J-aggregates are weakly emissive, in contrast to strongly fluorescent monomers; the fluorescence excitation spectra mainly reflect the absorption of monomers. TCSPC measurements further confirm the presence of Jaggregates. For example, the fluorescence decay dynamics of 1 in ethanol (concentration, 1 μM) can be described reasonably well by a single-exponential decay at 470 nm, turning into a biexponential decay at 600 nm (Table 3), owing to the relatively more significant J-aggregate emissions, in addition to monomer emissions in the long wavelength region.

3.2.3. Molecular Aggregation of 1 in DMSO. Molecular aggregation of 1 in DMSO is similar to that in ethanol, but it turns out to be more subtle. For example, λabs max of 1 in DMSO resides at 420 nm, regardless of the solution concentration (Figure 10a,b). Moreover, λabs max is invariant over a range of temperatures from 25 to 60 °C (Figure 10c,d). The formation of J-aggregation in DMSO is reflected in the excitation-dependent fluorescence spectra of 1 (Figure 11a−d). flu When λ exc ≤ 420 nm, λ max is excitation-wavelength independent, peaking at ∼467 nm; as λexc increases to 450 nm, however, λflu max progressively moves to 478 nm (Figure 11e,f). The gradual wavelength shift in these fluorescence spectra suggests the presence of J-aggregates, which become excited and contribute to the emission when λexc moves to the red region, leading to the overall bathochromic shift of λflu max. The fluorescence excitation spectra of 1 at λemi = 470 nm was also probed (Figure S1 of Supporting Information). It is interesting to note that these spectra peak at 418 nm, demonstrating a small but consistent blue shift of 2 nm with respect to λabs max. These data also indicate the presence of weakly emissive J-aggregates, whose absorption shifts the overall λabs max of 1 to 420 nm. In TCSPC measurements (λexc = 407 nm; concentration, 1 μM), a biexponential decay is found in the emission of 1 at 470 and 600 nm (Table 3), owing to both monomer and Jaggregate emissions; the relative weight of the J-aggregate emission (with respect to that of the monomer emission) increases in the long-wavelength region (Table 3). Furthermore, the lifetime of monomer emission is unusually shorter than that of J-aggregates; this anomalous observation is F



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Figure 10. (a) UV−vis absorption spectra of 1 in DMSO at 25 °C with various concentrations: 1.4, 2.7, 8.2, 10.9, 16.4, 21.9, and 27.4 μM. (c) UV−vis absorption spectra of 1 in DMSO at various temperatures (concentration, 10.9 μM). Panels (b) and (d) show the normalized UV−vis absorption spectra corresponding to panels (a) and (c), respectively.

Figure 12. (a) Different molecular conformations of 1 and their complexes formed with an explicit ethanol or DMSO molecule. (b) Theoretically optimized monomers of 1 in DMSO; note that those in ethanol possess similar molecular structures. (c) Relative energy differences among different molecular conformations of 1, calculated using an implicit solvent model (SMD) when applicable. Different molecular conformations of (d) 1-ethanol and (e) 1-DMSO complexes and their relative energy differences according to DFT calculations embracing an explicit solvent model. Hydrogen atoms except the one attached to the carboxylic acid group have been omitted for clarity in panels (d) and (e).

Figure 11. Steady-state fluorescence spectra of 1 in DMSO (concentrations, 1.4, 2.7, 8.2, 10.9, 16.4, 21.9, and 27.4 μM) excited at (a) λexc = 400 nm, (c) λexc = 420 nm, and (e) λexc = 450 nm. Panels (b), (d), and (f) show normalized fluorescence spectra corresponding to panels (a), (c), and (e), respectively. Red dotted lines at λ = 467 nm are drawn to assist the visualization of the red shift of emission spectra as the excitation wavelength changes.

solvent effects, such as in vacuo and in cyclohexane environment, DFT calculations show that 1a is more stable than the other two monomer conformations, 1b and 1c, by ∼0.25−0.30 eV (Figure 12c; Table S4 of Supporting Information). Indeed, the molecular structure of 1 in the crystal-lattice environment exhibits the conformation of 1a.24 Using the SMD model to emulate solvent effects implicitly, DFT calculations show that 1a remains the most stable monomer conformation when in ethanol or DMSO (Figure 12c). However, as pointed out earlier, both nonspecific and specific solvent effects are present in these two solvents; the SMD model takes into account the former effect but treats the latter in an incomplete manner, as the charge variations on the solute cavity surface in the SMD model cannot fully describe the molecular-level solute−solvent interactions in this implicit solvation model. In contrast, previous studies have shown that specific solvent effects play a critical role in the solvatochromism of coumarins21,32,44 and the stability of their molecular conformations.22 Hence, further DFT calculations were

consistent with the formation of a nonradiative twisted intramolecular charge transfer (TICT) state in the monomers, via a rotation of the diethyl-amino group along the N1−C7 bond in polar solvents like DMSO (Figure 1a).41−43 In contrast, TICT will be prevented in J-aggregates because of steric hindrance, thus affording a longer emission lifetime. 3.3. Molecular Conformational Changes of 1 in DMSO and Ethanol. Another subtle specific solvent effect in HBA solvents concerns the opening-up of the intramolecular hydrogen bond in 1. Three representative monomer conformations of 1 are shown in Figure 12a,b: 1a, 1b, and 1c. In 1a, the −COOH group forms an intramolecular hydrogen bond with a carbonyl oxygen. In contrast, this hydrogen bond is “opened” up in both 1b and 1c. In the absence of specific G



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In fact, a similar “opening-up” mechanism of intramolecular hydrogen bonds has also been reported for several other compounds in HBD solvents.48−51 This conformational change of 1 has a significant impact on its optoelectronic properties. For example, λabs max values for monomers without intramolecular hydrogen bonds are calculated to produce a blue shift (Table 4). This significant

performed that incorporated the specific solvent effects via the explicit positioning of a solvent molecule next to 1; this enabled a study of their hydrogen bond interactions. Because the key conformational differences among 1a, 1b, and 1c concern the geometry of the H atom in the −COOH group, the explicit solvent molecule was purposely placed near this H atom at a typical hydrogen bond distance (∼1.8 Å). Nevertheless, the formation of hydrogen bonds with the explicit solvent molecule is not favored upon geometry optimization of 1a (Figure 12d,e). In contrast, 1b and 1c establish strong hydrogen bond interactions with the solvent molecule. Such interactions greatly minimize the energy of the resulting 1b− and 1c−solvent complexes. Their energies turn out to be lower than that of 1a− solvent complex in ethanol, while their energy differences are minimized from ∼0.3 eV to ∼0.15 eV in comparison to that of 1a−solvent complex in DMSO (Figure 12d,e). The effect of positioning more solvent molecules (i.e., three DMSO and four ethanol molecules) around the −COOH group of 1 was also tested. In this case, the overall energetic stability among 1a-, 1b-, and 1c-complexes were shown to be comparable; in particular, the energies of 1b− and 1c−DMSO complexes were shown to be lower than that of the 1a−DMSO complex (Figure S2 of Supporting Information). DFT calculations that incorporate specific solvent effects suggest that some monomers of 1 may open up their hydrogen bonds under certain solvent conditions. To corroborate this indication, 1H NMR spectra of 1 in deuterated DMSO were collected at four different temperatures: 25, 35, 45, and 55 °C (Figure 13 and Figures S3−S6 of Supporting Information). The

Table 4. Theoretical λabs max Values of Different Conformations of 1 in Ethanol and DMSO (Nanometers) ethanol DMSO

1a

1b

1c

401.0 399.1

397.9 395.2

397.0 393.7

conformational perturbation is unaccounted for in solvatochromic models; this could also contribute to the poor goodness-offit of the λabs max data to the Taft−Kamlet model.

4. CONCLUSIONS The nature of molecular aggregation formation effects of 1 in the solution phase have been demonstrated in three representative solvents: cyclohexane, ethanol, and DMSO. UV−vis absorption, steady-state fluorescence, and timecorrelated single-photon counting spectroscopies enabled the optoelectronic properties of monomers and J-aggregates of 1 to be investigated. The aggregation of 1 turns out to be responsible for many inconsistent spectral data reported in the literature on this compound. Moreover, dimers of 1, formed via intermolecular hydrogen bonds, are not found in cyclohexane, unlike those of a similar compound, coumarin 343; DFT calculations suggest that the J-aggregates of 1 are more energetically favorable, which rationalizes their widespread presence in cyclohexane (as well as in ethanol and DMSO). In ethanol, the aggregation of 1 leads to a significant red shift in its UV−vis absorption spectra; using the isodesmic model and a modified version of the chemometric analysis of spectroscopic data, the molar absorptivities of 1 in both the monomer and Jaggregate phases have been determined with excellent agreement to experimental data. The aggregation formation constant was found to be ∼1.5 × 104 M−1 in ethanol at 20 °C. In contrast, the presence of J-aggregates in DMSO is very subtle, abs as the λmax values of 1 in DMSO are independent of temperature and solution concentration. However, excitationdependent emission spectra and fluorescence lifetime measurements have successfully revealed the formation of J-aggregates of 1 in DMSO. (TD)-DFT calculations and 1H NMR spectroscopy show that the monomer conformations of 1 may experience significant changes in hydrogen bond accepting solvents, such as DMSO and ethanol. Results show that the intramolecular hydrogen bond in 1 could be opened up in these solvents. This finding demonstrates the importance of considering specific solvent effects, i.e., the molecular level interactions between 1 and solvent molecules, in these types of (TD)-DFT calculations. However, our calculations are limited by the number of explicit solvent molecules included in the complex models. Given sufficient computational power, a few layers of solvent molecules around 1 would be included to obtain more insights about the specific solvent effects. Our results provide a deeper understanding of molecular aggregation and complex formation effects in 1 in the solution phase and suggest that the “monomer-interpretation” about the

Figure 13. Temperature dependence of the 1H NMR chemical shifts for the carboxylic proton resonance of 1 in DMSO.

temperature coefficient of the NMR chemical shift due to the proton in the −COOH group is calculated to be ∼5.8 ppb/K. This coefficient lies in the range of ∼1−3 ppb/K in DMSO for an intramolecular hydrogen bonded proton, cf., greater values are reminiscent of intermolecular hydrogen bonds.45−47 The relatively large temperature coefficient of 1 in DMSO illustrates the presence of some monomers with opened intramolecular hydrogen bonds. It should also be pointed out that during the 1 H NMR spectroscopy experiments, a very concentrated solution of 1 (∼1 mM) was in use. As such, the presence of J-aggregates with intramolecular hydrogen bonds in this solution is expected to bring down the overall temperature coefficient; the actual temperature coefficient due to monomers of 1 is likely to be larger than 5.8 ppb/K. H



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optoelectronic characteristics and functionalities of 1 should be used with care. The accumulation of such knowledge will also lead to the development of molecular design criteria for creating new coumarin dyes with tailored self-assembly or antiaggregation characteristics for optoelectronic applications.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Absorption, emission, fluorescence excitation, and 1H NMR spectral data; relative energy differences between different conformations of 1 as well as its dimers and complexes; and the modified version of Kubista’s chemometric analysis algorithm to implement the isodesmic model. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*E-mail: [email protected]. Tel: +44 (0)1223 337470. Fax: +44 (0)1223 373536. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Paul Waddell from the Department of Physics, University of Cambridge, U.K., for his helpful comments on this manuscript and the EPSRC UK National Service for Computational Chemistry Software (NSCCS), based at Imperial College London, for supporting this work. X.L. is indebted to the Singapore Economic Development Board for a Clean Energy Scholarship. J.M.C. thanks the Fulbright Commission for a UK-US Fulbright Scholar Award, hosted by Argonne National Laboratory where work done was supported by DOE Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357.

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Dye Aggregation and Complex Formation Effects in 7-(Diethylamino

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