Molecular Origins of Optoelectronic Properties in Coumarins 343

Article pubs.acs.org/JPCC

Molecular Origins of Optoelectronic Properties in Coumarins 343, 314T, 445, and 522B Xiaogang Liu,† Jacqueline M. Cole,*,†,‡,§ Paul G. Waddell,†,‡ Tze-Chia Lin,† and Scott McKechnie† †

Cavendish Laboratory, Department of Physics, University of Cambridge, J. J. Thomson Avenue, Cambridge, CB3 0HE, United Kingdom ‡ Department of Chemistry and §Department of Physics, University of New Brunswick, P.O. Box 4400, Fredericton, NB, E3B 5A3, Canada S Supporting Information *

ABSTRACT: The relationships between the structure and laser dye properties of four coumarin derivatives are investigated to assist in knowledge-based molecular design of coumarins for various optoelectronic applications. Four new crystal structures of coumarins 343, 314T, 445, and 522B are determined at 120 K and analyzed via the empirical harmonic−oscillator−stabilization− energy and bond-length−alternation models, based on resonance theory. Results from these analyses are used to rationalize the optoelectronic properties of these coumarins, such as their UV− vis peak absorption wavelength, molar extinction coefficient, and fluorescence quantum efficiency. The specific molecular structural features of these four coumarins and the effects on their optoelectronic properties are further examined via a comparison with other similar coumarin derivatives, including coumarins 314, 500, and 522. These findings are corroborated by density functional theory (DFT) and time-dependent DFT calculations. The structure−property correlations revealed herein provide a foundation for the molecular engineering of coumarins with “dialup” optoelectronic properties to suit a given device application.

1. INTRODUCTION

General relationships between the molecular structures of 1 4 and their optoelectronic properties, including UV−vis peak absorption wavelength (λabs max) and molar extinction coefficient (ε), are studied via the empirical harmonic−oscillator− stabilization−energy (HOSE) model,9 bond-length−alternation (BLA) analysis10 and (time-dependent) density functional theory (DFT/TD-DFT) calculations. The influence of the specific molecular structures and crystal-packing environment features of 14 on their optoelectronic properties is also analyzed via a comparison with other similar coumarin derivatives (Scheme 1); namely, coumarin 314 (5; alternative name: coumarin 504; C 18 H 19 NO 4 ), coumarin 500 (6; C12H10F3NO2), and coumarin 522 (7; alternative name: C8F; C14H12F3NO2).

Coumarin derivatives play important roles in many different applications, such as pharmaceutical agents,1 solution dynamic probes,2 fluorescent metal ion sensors,3 laser dyes,4 and, more recently, organic sensitizers in high-efficiency dye-sensitized solar cells.5 By attaching different substituents to the coumarin fragment, the optoelectronic properties of these derivatives can be tuned to meet different application requirements. Understanding the structure−property relationships of these coumarins is thus essential to facilitate the design of new coumarin dyes with enhanced optoelectronic performance. Furthermore, such understanding also helps us to rationalize the complex photophysics and photochemistry of coumarins. To this extent, our previous work has revealed the effect of substituents on intramolecular charge transfer (ICT) enhancement, wavelength tuning, and molar extinction coefficient improvement in coumarins.6 Four new crystal structures of coumarins are reported (Scheme 1): coumarin 343 (1; alternative name: coumarin 519; C16H15NO4), coumarin 314T (2; alternative name: coumarin 504T; C22H27NO4), coumarin 445 (3; C12H13NO2), and coumarin 522B (4; C17H18NO2F3). In particular, the molecular formula of 4 is reported for the first time because it is not listed in the Chemical Abstracts Service.7 It turns out that the actual structure of this compound is different from its presumed configuration, as previously reported.8 © 2013 American Chemical Society

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Single-Crystal X-ray Diffraction Experiments. All coumarins were supplied by the Exciton chemical company.11 Crystals of 1 and 4 suitable for single-crystal X-ray diffraction were grown via slow solvent evaporation in acetone, at room temperature and 0 °C, respectively; crystals of 2 and 3 were obtained using the same method but with ethanol at room temperature. Received: January 18, 2013 Revised: April 25, 2013 Published: June 17, 2013 14130

dx.doi.org/10.1021/jp400614e | J. Phys. Chem. C 2013, 117, 14130−14141

The Journal of Physical Chemistry C

Article

Scheme 1. Molecular Structures of Seven Coumarins (17)a

a

Their common coumarin fragment is highlighted in blue.

For 14, X-ray diffraction data were collected at 120 K on a Rigaku Saturn 724+ CCD diffractometer equipped with a molybdenum X-ray source (λ Mo,Kα = 0.71073 Å) and accompanying SHINE Optics. An open-flow nitrogen Oxford Cryosystems CryostreamPlus was used to cool the sample. Rigaku CrystalClear-SM Expert 2.0 software was used for all data collection, cell refinement, and data reduction procedures.12 The data were corrected for absorption effects by comparing equivalent reflections with SORTAV;13 SHELXS9714 and SHELXL9714 were used, respectively, to solve and refine the structures (Figures 1−4). Unless otherwise stated, all hydrogen

Figure 2. Crystallographic asymmetric unit of 2 at 120 K with anisotropic displacement ellipsoids drawn at the 50% probability level.

Figure 1. Crystallographic asymmetric unit of 1 at 120 K with anisotropic displacement ellipsoids drawn at the 50% probability level. Figure 3. Crystallographic asymmetric unit of 3 at 120 K with anisotropic displacement ellipsoids drawn at the 50% probability level.

atoms were positioned geometrically and refined via a riding model based on their parent atom locations, using C−H = 0.980 Å and Uiso(H) = 1.5Ueq(C) for methyl H atoms, CH = 0.950 Å and Uiso(H) = 1.2Ueq(C) for aromatic H atoms, and CH = 0.990 Å and Uiso(H) = 1.2Ueq(C) for other types of hydrogen atoms. Hydrogens attached to heteroatoms [H3O in 1 (Figure 1) and H1N in 3 (Figure 3)] were located directly from the Fourier difference map of the experimental structural factors against those of the structural model. The coordinates of H3O and H1N were refined freely to understand the hydrogen-bond interactions between O2 and O3 (to which H3O is attached) in 1 and the nature of the hybridization of N1 (to which H1N is attached) in 3, respectively.

ORTEP-3 for Windows15 was employed to prepare molecular graphics, while publCIF16 was executed for preparing publication material. A summary of crystal, data collection, and refinement details is given in Table 1. 2.2. Solution-State UV−vis Absorption Spectroscopy. UV−vis peak absorption wavelengths, λabs max, for 24 were measured on a Hewlett-Packard G1103A spectrophotometer in solutions of absolute ethanol (24). Extinction coefficients, ε, for 24 at their λabs max in ethanol were calculated using the Beer− Lambert law according to the best linear plot of the peak 14131



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geometry optimization to ensure that minima on the potential energy surfaces were found. The DFT-optimized molecular structures were then used as a starting point for TD-DFT calculations that employed the hybrid B3LYP functional with the 6-311++G(2d,2p) basis set to afford molar extinction coefficients of 14 in vacuo.

3. RESULTS AND DISCUSSION 3.1. Four New Crystal Structures of 14. 50% probability anisotropic displacement ellipsoidal plots of 14 are given in Figures 14, respectively. Selected bond lengths and torsion angles for these compounds are available in Table 2, while full structural data for all four coumarins can be found in the Supporting Information. It should be noted that the crystal structure of 1 at room temperature has already been published.21 However, our data were collected at 120 K to reduce atomic thermal vibrations and to ensure internal consistency herein by having a common data collection temperature for all of our data. The most significant difference between the asymmetric units of these two sets of data for 1 lies in the conformation of the julolidyl moiety (Scheme 2a). Whereas our data show an anti structure [C14 bends up and C15 bends down; Scheme 2b], the room-temperature structure of 1 displays a different anti conformation [C14 bends down and C15 bends up; Scheme 2c].21 Other than that, these two data sets agree with each other very well, with the largest bond-length difference being 0.023(4) Å for C12−C15 and the largest bond-angle variance being 0.8(3)° for C12−C15−C16; that is, these relatively significant geometric differences all pertain to the region of the molecule exhibiting the conformational variation. The asymmetric unit of the room-temperature structure has been assumed to be the only absolute configuration of 1 in some TD-DFT calculations.22 However, we would like to emphasize that the two asymmetric units are essentially mirror images of each other, with respect to the plane of the coumarin fragment, known as enantiomers. Because 1 is in a centrosymmetric space

Figure 4. Crystallographic asymmetric unit of 4 at 120 K with anisotropic displacement ellipsoids drawn at the 50% probability level. 4 lies on a plane with mirror symmetry; for this reason, only two fluorine atoms in the −CF3 group are shown in the asymmetric unit. In addition, there are 14 carbon atoms, C11C17 and C11′C17′, which are disordered about the mirror plane.

absorbance (A) plotted against different coumarin concentrations (c). The λabs max of 1 and 7 and ε of 1 in ethanol were imported from the literature.2a,17 2.3. DFT and TD-DFT Calculations. Quantum-chemical calculations were performed on 17 using Gaussian 09.18 The geometries of these molecules were optimized with DFT using the Becke’s three-parameter and Lee−Yang−Parr hybrid functional (B3LYP)19 and a 6-311++G(2d,2p) basis set20 for all of these compounds except for 2 and 6. Because of convergence errors involving diffuse or polarization functions, the basis set was reduced to 6-311G(2d,2p) for 2 and 6-311G for 6. For 2 and 6, single-point energy calculations were subsequently performed on the optimized structures using B3LYP/6-311++G(2d, 2p), to be consistent with the rest of the calculations using this basis set. In all calculations, frequency checks were performed after each Table 1. Single-Crystal X-ray Diffraction Experimental Details

Crystal Data chemical formula Mr crystal system, space group a, b, c (Å) α, β, γ (°) V (Å3) Z μ (mm−1) crystal size (mm3) Data Collection Tmin, Tmax no. of measured, independent and observed [I > 2σ(I)] reflections Rint Refinement R[F2 > 2σ(F2)], wR(F2), S no. of reflections no. of parameters Δρmax, Δρmin (e Å−3)

1

2

3

4

coumarin 343

coumarin 314T

coumarin 445

coumarin 522B

C16H15NO4 285.29 orthorhombic, Pbca 7.707 (3), 13.875 (5), 23.418 (8) 90, 90, 90 2504.1 (15) 8 0.11 3.25 × 10−3

C22H27NO4 369.45 monoclinic, C2/c 25.115 (8), 11.990 (3), 15.721 (5) 90, 126.342 (4), 90 3813 (2) 8 0.09 3.93 × 10−2

C12H13NO2 203.23 monoclinic, P21/c 13.786 (6), 5.596 (2), 15.675 (5) 90, 122.93 (3), 90 1015.0 (7) 4 0.09 1.09 × 10−2

C17H18F3NO2 325.32 monoclinic, P21/m 12.365 (6),, 7.358 (3), 16.939 (8) 90, 95.378 (6), 90 1534.3 (12) 4 0.12 4.96 × 10−2

0.983, 0.995 8873, 2841, 2318

0.962, 0.984 15132, 5762, 5382

0.977, 0.991 5000, 2299, 1866

0.946, 0.975 15532, 4004, 3807

0.046

0.035

0.052

0.030

0.069, 0.187, 1.07 2841 194 0.43, −0.33

0.052, 0.133, 1.10 5762 249 0.40, −0.24

0.057, 0.172, 1.08 2299 142 0.54, −0.26

0.054, 0.154, 1.08 4004 321 0.41, −0.29

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Table 2. Selected Bond Lengths and Torsion Angles for the Crystal Structures of 14a 1

2

3

4 coumarin 522B

coumarin 343 Bond Lengths (Å) C6−C7 C7−C8 C8−C9 C9−C10 C5−C10 C5−C6 C9−O1 C2−O1 C2−C3 C3−C4 C4−C10 C2−O2 C7−N1 Torsion Angles (deg) C6−C7−N1−C11 C8−C7−N1−C12 χN

1.433 (3) 1.428 (3) 1.371 (3) 1.415 (3) 1.419 (3) 1.371 (3) 1.386 (3) 1.373 (3) 1.437 (3) 1.380 (3) 1.398 (3) 1.237 (3) 1.362 (3) −6.4 (3) −9.8 (3) 3.4 (4)

coumarin 314T 1.4424 (14) 1.4322 (14) 1.3917 (14) 1.4051 (14) 1.4111 (14) 1.3727 (14) 1.3724 (13) 1.3945 (12) 1.4527 (15) 1.3718 (15) 1.4082 (14) 1.2112 (13) 1.3631 (13) 7.73 (15) −2.10 (15) 9.83 (21)

coumarin 445 1.422 (2) 1.404 (2) 1.370 (2) 1.409 (2) 1.408 (2) 1.376 (2) 1.3794 (19) 1.3823 (19) 1.442 (2) 1.354 (2) 1.438 (2) 1.206 (2) 1.359 (2) 3.8 (3) 2 (2)b 1.8 (20)

molecule 1

molecule 2

1.430 (2) 1.408 (2) 1.378 (2) 1.398 (2) 1.408 (2) 1.376 (2) 1.3811 (18) 1.375 (2) 1.448 (2) 1.349 (2) 1.435 (2) 1.211 (2) 1.3720 (19)

1.435 (2) 1.402 (2) 1.378 (2) 1.401 (2) 1.406 (3) 1.371 (3) 1.377 (2) 1.380 (2) 1.452 (3) 1.343 (3) 1.430 (2) 1.203 (3) 1.367 (2)

5.0 (18) 1.1 (6) 3.9 (19)

6.6 (10) 11.6 (2) 5.0 (10)

a χN denotes the absolute difference between the two torsion angles along C7N1. This angle represents the overall out-of-plane torsion angle of the two substituent branches attached to the N1 atom with respect to the plane of the coumarin framework. bThis value represents the torsion angle of C8−C7−N1−H1N. H1N was refined from the residual electron density map to determine this torsion angle accurately.

Scheme 2. (a) Julolidyl Moiety (b,c) Two Different Anti Structures of the Julolidyl Moiety in 1

group, its crystal structure contains equal numbers of both types of enantiomers, and they are both valid “absolute configurations”. The same conclusion also applies to the anti structures in the julolidyl moiety of 2 (Figure 2). Another interesting feature in the asymmetric unit of 1 is an intramolecular hydrogen bond, formed between O2 (acceptor), O3 (donor), and H3O (Figure 1). The bond distance between H3O and these two oxygen atoms is 1.70(3) and 0.97(3) Å, respectively, and the resulting angle (O3H3O···O2) amounts to 153(3)°. 3.2. Quantification of Structure−property Relationships via Empirical Models That Employ Resonance Theory. The molecular charge-transfer characteristics of the structures of 14 can be elucidated by applying resonance theory. According to this theory, the overall structure of a πconjugated molecule is collectively described by all of its possible resonance structures or canonical forms.9,23 The dominant resonance structure reveals its major chemical-bonding character to a first-order approximation. A comprehensive map of possible resonance states applicable to all of these coumarin laser dyes is illustrated in Figure 5a,6 which helps to interpret the bond-length patterns in 14. Commonly used atom- and ring-labeling and positionnumbering designations in the coumarin framework are presented in Figure 5b to facilitate the discussion. A strong electron-donating amino group is attached at the seven-position

Figure 5. (a) Possible resonance structures in coumarin laser dyes. While the electron-donating substituent, D, is shown here only at the seven-position, other substituents are likely to be attached onto this framework. The blue-highlighted Ring 1 moieties represent four distinct canonical molecular fragments: Q (para-quinoidal), OQ (orthoquinoidal), K1 (Kekulé configuration 1), and K2 (Kekulé configuration 2). (b) Molecular structure of coumarin and its atom and ring labeling and position numbering designations. Green arrows indicate the direction of ICT in coumarin: the long axis ICT corresponds to S0 to S1 transition, and the short axis ICT corresponds to S0 to S2 transition (see Section 3.3).6. 14133



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Table 3. Results from the HOSE and BLA Analyses and the Optoelectronic Properties of 14 compound the position of the Ring 2 substituent Ring 2 substituent Hammett valueb σm σp HOSE Analysis K1 crystalc vacuod K2 crystal vacuo Q crystal vacuo OQ crystal vacuo BLA Analysis BLA crystal vacuo Optoelectronic Propertiese λabs max in ethanol (nm) f λabs max in vacuo (nm) calculated bandgap in vacuo (eV) measured ε in ethanol (× 104 L mol−1 cm−1)g calculated ε in vacuo (× 104 L mol−1 cm−1) Λh

1

2

3

4a

{3-sub} −COOH 0.36 0.43

{3-sub} −COOC2H5 0.37 0.45

{4-sub} −CH3 −0.07 −0.17

{4-sub} −CF3 0.43 0.54

0.1701 0.2312 0.1578 0.1708 0.4898 0.3704 0.1822 0.2277

0.2255 0.2415 0.1571 0.1622 0.3995 0.3640 0.2179 0.2323

0.2177 0.2357 0.1997 0.2208 0.3628 0.3065 0.2199 0.2370

0.2546 0.2596 0.1746 0.1692 0.3401 0.3301 0.2308 0.2411

0.037 0.062

0.059 0.063

0.086 0.090

0.095 0.091

434.9

435 395.8 3.5359 4.88 2.59 0.661

366 320.9 4.1073 2.30 1.60 0.695

409 364.0 3.6091 1.92 1.58 0.618

3.5342 4.43 2.26 0.658

a

HOSE and BLA values for the crystal structure of 4 are obtained by taking the average HOSE and BLA results for its two molecules in the asymmetric cell. bHammett values quantify the electron-withdrawing/-donating power of a substituent; a more positive Hammett’s value corresponds to a stronger electron-withdrawing power, whereas a more negative value indicates a greater electron-donating power. The electronwithdrawing/-donating strength of a substituent is determined by both the inductive effect, as approximately quantified by σm, and the resonance effect, as approximately quantified by σm − σp.27 In other words, σp jointly measures both inductive and resonance effects. cHOSE and BLA values for the crystal structures of 14 are calculated based on their bond lengths determined via crystallography: Ring 1 bond lengths are used in the HOSE analysis, and the bond lengths of C2−C3, C3−C4, and C4−C10 are used to compute the BLA values. dHOSE and BLA values for the structures of 14 in vacuo are calculated based on their theoretically optimized molecular structures via Gaussian 09 using the same method as stated in footnote c. eAll properties in vacuo are calculated via Gaussian 09, unless stated otherwise. fλabs max values in vacuo are determined via an empirical h solvatochromic model,30 that is, Catalán’s generalized solvatochromic model.31 gε values of 14 are measured at their respective λabs max. Λ is essentially an index used to quantify the overlap of HOMO and LUMO in 14; that is, a smaller Λ corresponds to less HOMO−LUMO overlap and thus a larger extent of ICT in a molecule.32

and III. Nevertheless, their contributions are relatively small and C2−C3 and C4−C10 are still more representative of a single bond. An electron-withdrawing group could be attached to Ring 2 of the coumarin framework, at either the three- or four-position, to enhance the ICT. Herein, coumarins with substituents at the three-position and four-position are denoted as {3-sub} and {4sub}, respectively. Note that 1 and 2 belong to the family of {3sub}, while 3 and 4 are members of {4-sub}. It has been shown that a {3-sub} rather than {4-sub} attachment leads to more efficient ICT and thus a larger red shift in their λabs max values, as well as approximately doubled ε values.6 This conclusion is reinforced by comparing the molecular structures and optical absorption properties of 14. This ICT enhancement is reflected by the increasing contribution of the para-quinoidal resonance structure III toward the observed coumarin geometries. This contribution can be quantified by the empirical HOSE model.9 HOSE describes the energy difference between a particular bond-length pattern in an aromatic ring and the standard Kekulé structure. It is defined as:

in all cases: 14. This enhances ICT from Ring 1 to Ring 2. The net effect is the stabilization of the para-quinoidal resonance structure (Figure 5a, III) because seven-substitution extends the π-conjugated bonding through the para-quinoidal structure of Ring 1 to its para-position (C4−C10). Accordingly, the bondlength patterns in Ring 1 of these coumarins reflect the predominance of resonance state III. For example, C5−C6 and C8−C9 are the shortest bonds in Ring 1 of 14 (Table 2). Many coumarin and azacoumarin laser dyes share the same pattern.6,24 Resonance structure III is also manifested in the characteristic partial double-bond character of C7N1 located at the sevenposition substituent: its bond lengths lie between those of pure single and double carbon−nitrogen bonds (Table 2), that is, 1.44 and 1.27 Å, respectively.25 Moreover, the overall out-of-plane torsion angle of the two branches of the amino 7-substituent with respect to the Ring 1 plane, χN, is much closer to 0° (for ideal sp2 hybridization) than to 60° (for ideal sp3 hybridization, Table 2).26 Resonance theory works equally well to explain the bond lengths in Ring 2. For example, C2−C3 and C4−C10, which are typically represented by single bonds, are shorter than the average single-bond length between aromatic and sp2 hybridized carbon atoms at 1.470 Å (Table 2).25 This shortening can be explained by the competing influence of resonance structures I

n

HOSE = 14134

1 1 [∑ (R r′ − R os)2 kr′ + 2 r=1

n2

∑ (R r″ − R od)2 kr″] r=1

(1)



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Figure 6. LUMO, HOMO, and corresponding band gaps of 14 as computed in vacuo using B3LYP/6-311++G(2d,2p) functional/basis set (red: positive; blue: negative; isovalue: 0.02).

where R′r and R″r are actual π-bond lengths, Rso and Rdo are reference single- and double-bond lengths, n1 and n2 are the number of single and double bonds; and kr′ and kr″ are force constants defined as: kr = a + bR r

lengths for 1 and 2, compared with those of 3 and 4, are observed (Table 3). It is worth mentioning that the Q contributions are larger in the solid-state than in vacuo, as realized by a comparison of Xray-diffraction-derived molecular structures with those calculated via gas-phase density functional theory (Table 3). This difference is due to intermolecular interactions present only in the crystal form. For example, molecules of 1 are linked by head-to-tail nonbonded contacts in the crystal structure,28 and the dipole− dipole interactions between neighboring molecules further enhance their ICT,29 leading to a more para-quinoidal molecular pattern. In the crystal structure of 3, intermolecular hydrogen bonds are present, which boost its ICT and raise the Q contribution (Section 3.4.2). When such intermolecular interactions are weak, that is, in the crystals of 2 and 4 ( Section 3.4), the differences between Q contributions in crystal and in vacuo environments become smaller. 3.3. Optically Excited Charge-Transfer and LightAbsorption Characteristics. DFT calculations also displayed a predominant ICT from Ring 1 to Ring 2, upon these vertical optical excitations, as judged by the electron density perturbations between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of 14. The extent of the ICT can be quantified by Λ, an index describing the weighted overlap of all orbitals contributing to a particular excitation.32 In 14, only the HOMO→LUMO transition is involved in the first excited state, so Λ for this state essentially measures the overlap between HOMO and LUMO; that is, a smaller Λ corresponds to less HOMO−LUMO overlap and thus a larger extent of ICT. Λ is the smallest in 4 and the largest in 3 (Table 3), in good agreement with the electron-withdrawing power of their Ring 2 substituents, that is, −CF 3 in 4 > −COOH in 1 ≈ −COOC 2H5 in 2 > −CH 3 in 3. However, in coumarins whose molecular structures have a pseudoreflective symmetry, electron transfer

(2)

where Rr is the actual bond length and a and b are constants derived based on experimental data. For carbon−carbon bonds, Rso = 1.467 Å, Rdo = 1.349 Å, a = 44.39 × 104 Pa, and b = −26.02 × 104 Pa/Å. The HOSE model can be extended to compute the relative contribution for each of the n resonance states in benzene derivatives.9 For example, the ith resonance state has a contribution of: Ci =

(HOSEi)−1 N

∑ j = 1 (HOSEj)−1

(3)

The inverse empirical relationship between the HOSE value of the ith resonance state and its relative contribution indicates that the more stable resonance state has a larger contribution to the overall molecular structure. Our previous studies have shown the HOSE model to be a useful tool for correlating bond-length patterns to the optical properties of coumarins.6 For 14, the para-quinoidal structure (Q) carries the largest contribution among all possible resonance states (Table 3), according to the HOSE model. In addition, {3-sub} coumarins have larger Q contributions with respect to {4-sub} ones. It should be highlighted that the electron-withdrawing power of −COOH and −COOCH3, the three-substituents of 1 and 2, is actually weaker than that of −CF3, the four-substituent of 4, as shown by their respective Hammett values (Table 3).27 Yet, their substitution positions seem to have a greater impact on the ICT enhancement. Indeed, larger UV−vis peak absorption wave14135



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Figure 7. Molecular packing features in the crystal of three coumarins: (a) 1; (b) polymorph 1 of 5; (c) polymorph 2 of 5; and (d) 2. The two polymorphs of 5 were reported at 100 K.37 For clarity, only parallel molecular layers are shown in panels a−c.

of delocalization and typically a larger molar extinction coefficient. It is realized that the BLA values of {3-sub} are smaller than those of {4-sub} (Table 3). This difference at least partially explains the approximately doubled ε values for {3-sub} coumarins in ethanol (Table 3). The higher ε values of 1 and 2 can also be related to the fact that their substituents (−COOH and −COOC2H5) extend the π-conjugated network of coumarins. Nevertheless, substituent attachment position plays a critical role on ε. We have shown that even when the same substituent is attached to the coumarin framework but at different positions, the ε values of {3-sub} coumarins are approximately twice as high as those of {4-sub} coumarins.34 These results are all consistent with the aforementioned conclusions derived from the resonance theory (Section 3.2). 3.4. Comparison between 14 and Other Similar Coumarins on Their Molecular Structures, Crystal Packing Features, and Optoelectronic Properties. 3.4.1. Compounds 1, 2, and 5. 1, 2, and 5 possess very similar molecular structures: they all include a julolidyl moiety to rigidize

along the long axis corresponds to a low-energy jump (S0 to S1) compared with that of the short axis (S0 to S2; Figure 5b).6,33 Consequently, the first light absorption and emission bands are mainly determined by the long axis ICT, and chemical substituents that lie along this direction play a more significant role than those attached along the short axis.33 Although −CF 3 in 4 has the largest electron-withdrawing power, it is attached at the four-position in Ring 2, which influences the ICT along the short axis. In contrast, −COOH in 1 and −COOC 2H5 in 2 attached at the three-position of Ring 2 align more closely to the long axis and thus produce a greater impact on the optoelectronic properties. For example, the bandgaps of {3-sub} are lower than those of {4-sub} (Figure 6), and the molar extinction coefficients of {3-sub} are greater than those of {4-sub} (Table 3). The higher molar extinction coefficients, ε, of {3-sub} coumarins are revealed by their higher degree of electron delocalization. This degree is quantified by BLA values. BLA calculates the difference of average single-bond length and average double-bond length in the conjugated alkyl chain of a molecule.10 A smaller BLA value corresponds to a higher degree 14136



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Figure 8. Molecular packing features in the crystal of 3. For clarity, two molecules in the top layer are highlighted in red, while those two in the bottom layer are highlighted in green, in the top view.

Figure 9. (a) Molecular packing features in the crystal of 6. (b) Side view of the molecular packing. (c) Top view of the molecular packing. For clarity, two molecules in the top layer are highlighted in red, while those two in the bottom layer are highlighted in green, in the top view. The crystal structure of 6 was determined at room temperature.38 There is a disorder site at the ethyl group, which is attached to a nitrogen atom (N1).

power of the amino group because they are further away from N1. Therefore, the electron donors in 1, 2, and 5 carry similar donating power. Moreover, −COOH and −COOC2H5 have similar electron-withdrawing power, reflected by their similar Hammett constants.27 As a result, the bandgaps of 1 and 2 (Table 3) are very close to that of 5 (calculated to be 3.5606 eV in vacuo); their λabs max values are thus quite similar, measured to be 434.9,2a 435, and 436 nm17a in ethanol, respectively. The four methyl groups attached to the julolidyl moiety, however, lead to considerable differences in the molecular

the electron-donating amino group, limit nonemissive twisted intramolecular charge transfer (TICT), and enhance fluorescence and lasing efficiencies in these dyes;17a in addition, an electron-withdrawing group (either −COOH or −COOC2H5) is attached at the three-position to boost ICT and shift the absorption, fluorescence, and lasing spectra of these coumarins toward longer wavelengths. In particular, 2 and 5 have the same electron-withdrawing group, that is, −COOC2H5. Only 2 exhibits four methyl groups in it julolidyl moiety. These methyl groups cause very little change to the electron-donating 14137



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Figure 10. Molecular packing features in the crystal of 4. For clarity, a molecule in the top layer is highlighted in red, while the other in the bottom layer is highlighted in green, in the top view.

lifetime, 2.6 times that of 5, and hence an increase in lasing efficiency of 24% in solution.36 3.4.2. Compounds 3 and 6. 3 and 6 have very similar molecular structures; the only difference is that the methyl group at the four-position in 3 has been replaced by a trifluoromethyl group in 6. The crystal structures of 3 and 6 also share some common molecular-packing features, although they exhibit different space group symmetry, that is, P21/c for 3 and P1̅ for 6. In both 3 and 6, the carbonyl oxygen, O2, acts as a hydrogen-bond acceptor; the hydrogen atom attached to N1 functions as a hydrogen-bond donor. Consequently, head-to-head connected dimers of 3 and 6 form in the crystal via intermolecular hydrogen bonds. In the crystal structure of 3, this hydrogen-bond length is 3.110 Å (Figure 8). Between the two intermolecular hydrogen bonds in the dimer A−B, there are also two CH···O interactions, with a C···O distance of 3.336 Å (Figure 8). This dimer is connected to its neighboring molecules via weak CH···O interaction (C···O distance =3.532 Å). That aside, as shown in the top view, the overlap between two adjacent molecular layers, which are separated by 3.405 Å, is quite small. As a result, the interactions between these layers are relatively weak. Dimers in the crystal structure of 6 demonstrate similar intermolecular hydrogen-bonding features (Figure 9a). However, because molecules A and B in a dimer do not align on the same plane (there is an offset of 0.642 Å between the two molecular planes), the hydrogen-bond distance, measured to be 3.124 Å, is slightly longer than that in 3 by 0.014 Å (Figure 9b). The dimer is connected to its neighboring molecules via both CH···O and CH···F interactions (C···F distance = 3.582 Å). Owing to the disorder of the ethyl group attached to the nitrogen

packing of 2 in the crystal. Coumarins have a relatively planar molecular configuration, which make them vulnerable to dye aggregation via close-packing and strong intermolecular interactions.2a,28 The introduction of the methyl groups at the julolidyl moiety, however, expands the molecular structures of 2 in the third dimension, increasing intermolecular distances and reducing intermolecular interactions. This effect is probably best illustrated by a comparison of the crystal packing in 1, 2, and 5. In 1, due to its planar geometry (mean deviation from the leastsquares plane of all non-hydrogen atoms = 0.2130 Å), the distance between two adjacent layers of molecules is only 3.483 Å and there is a large overlap between them, resulting in relatively strong π···π interactions [Figure 7a]. This structural feature rationalizes its propensity for dye aggregation in a wide range of solvents.28 In the crystal structures of the two polymorphs of 5, the distance between adjacent molecular layers is also relatively short, measured to be 3.473 and 3.769 Å, respectively (Figure 7b,c). In contrast, with methyl groups on the julolidyl moiety, the mean deviation from the least-squares plane of all non-hydrogen atoms in 2 has increased to 0.5707 Å, and the interlayer distance has increased to 4.331 Å in the crystal structure of 2 (Figure 7d). This increase accommodates the out-of-plane conformation of the ethyl group in the −COOC2H5 substituent observed in the crystal structure of 2, whereas the ethyl configuration in 5 lies in the same plane as the coumarin fragment (Figure 7). The methyl groups at the julolidyl moiety also have important effects on the optoelectronic properties of coumarins. Indeed, it is known that increased intermolecular distances and decreased intermolecular interactions improve emission quantum efficiencies of coumarins.35 For example, 2 offers a longer excited-state 14138



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their low fluorescence yield in the solid state. Finally, the parallel layered structure and weak interactions between such layers in coumarin 522B rationalizes its fragility to thermal shock. The structure−property correlations of coumarins 343, 314T, 445, and 522B revealed herein provide a foundation for the molecular engineering of coumarins with enhanced optoelectronic properties to better suit a given device application.

atom, the CH···O bond distance is not well-defined. Between two adjacent layers of molecules, which are separated by 3.713 Å, there is a relatively large overlap and therefore strong π···π interactions (Figure 9c). Because of the hydrogen-bonding network and strong intermolecular interactions (CH···O and/or CH···F interactions) in the crystals of 3 and 6, their solid-state fluorescence efficiencies are expected to be low.35 3.4.3. Compounds 4 and 7. 4 was developed by Exciton as a replacement for 7.11 Their optoelectronic properties are thus very similar. For example, the λabs max values of 4 and 7 are 409 and 408 nm17b,39 in ethanol, respectively; their peak fluorescence wavelengths are 510 and 515 nm, respectively.11 These similarities are corroborated by DFT calculations. For example, the bandgaps of 4 and 7 are calculated to be 3.6091 and 3.6134 eV in vacuo, respectively, and their dipole moments are 8.3569 and 7.9576 D, respectively. It should also be noted that the crystal structure of 4 comprises layers of molecules; all of these molecular layers are parallel to each other (Figure 10). Moreover, 4 includes three additional methyl groups on the ring, which acts to rigidize its molecular structure to minimize nonemissive TICT de-excitation. These methyl groups expand the planar molecular structure of 4 into the third dimension (perpendicular to the plane of the coumarin fragment) and increase the distance of adjacent molecular layers in the crystal to 3.679 Å (Figure 10). Between these layers, only π···π interactions are present. This suggests that crystals of 4 will be mechanically very fragile along the direction perpendicular to these layers. Indeed, crystals of 4 were found to be very prone to crack under thermal shock, as discovered during our X-ray diffraction experiments; care was therefore taken to ramp down the temperature at very slow cooling rate (∼100 K/h) to avoid crystal cracking. Isolated monomers with some intermolecular interactions in the crystal have medium solid-state fluorescence quantum efficiency.35 An intermediate value of this efficiency is thus expected for 4 as well, estimated to be ∼10%. A crystal packing comparison between 4 and 7 was not possible because the crystal structure of 7 remains elusive.

ACKNOWLEDGMENTS We thank the EPSRC U.K. National Service for Computational Chemistry Software (NSCCS), based at Imperial College London, and contributions from its staff in carrying out this work. Mr. Kian Sing Low and Mr. Jignesh Radia from University of Cambridge are acknowledged for their assistance in XRD experiments. X.L. is indebted to the Singapore Economic Development Board for a Clean Energy Scholarship. J.M.C. thanks the Royal Society for a University Research Fellowship, the University of New Brunswick (UNB), Canada, for The UNB Vice-Chancellor’s Research Chair and NSERC for the Discovery Grant, 355708 (for PGW). T.C.L. is indebted to the Taiwanese Ministry of Education for a partially funded Ph.D. studentship. S.M. thanks King’s College, University of Cambridge and EPSRC, U.K., for a DTA Ph.D. studentship (EP/P505445/1).

4. CONCLUSIONS The crystal structures of coumarins 343, 314T, 445, and 522B at 120 K have been reported and compared with their theoretically optimized molecular geometries in vacuo via DFT. Structural analysis has rationalized their different optoelectronic properties, such as UV−vis peak absorption wavelengths (λabs max) and molar extinction coefficients (ε). The attachment of an electronwithdrawing group at the three-position instead of the fourposition in the coumarin framework has been demonstrated to afford a larger red shift of λabs max and a greater enhancement of ε, owing to more effective ICT and electron delocalization in coumarins. This conclusion is further justified by DFT/TD-DFT calculations. The molecular structures, crystal-packing features, and optoelectronic and physical properties of these four compounds are also compared with other similar coumarin derivatives, including coumarins 314, 500, and 522. Through this comparison, the four additional methyl groups in the julolidyl moiety of coumarin 314T are shown to effectively decrease intermolecular interactions and help to increase the fluorescence quantum efficiency of this compound. The network of dimer structures connected via intermolecular hydrogen bonds in the crystals of coumarins 445 and 500, however, are responsible for

(1) (a) Kontogiorgis, C.; Hadjipavlou-Litina, D. Biological Evaluation of Several Coumarin Derivatives Designed as Possible Anti-inflammatory/Antioxidant Agents. J. Enzyme Inhib. Med. Chem. 2003, 18, 63−69. (b) Kulkarni, M. V.; Kulkarni, G. M.; Lin, C. H.; Sun, C. M. Recent Advances in Coumarins and 1-Azacoumarins as Versatile Biodynamic Agents. Curr. Med. Chem. 2006, 13, 2795−2818. (c) Deng, R. W.; Wu, J. G.; Long, L. S. Lanthanide Complexes of Di(4-hydroxycoumarinyl-3)acetic Acid and Their Anticoagulant Action. Bull. Soc. Chim. Belg. 1992, 101, 439−443. (2) (a) Correa, N. M.; Levinger, N. E. What Can You Learn from a Molecular Probe? New Insights on the Behavior of C343 in Homogeneous Solutions and AOT Reverse Micelles. J. Phys. Chem. B 2006, 110, 13050−13061. (b) Gutierrez, J. A.; Falcone, R. D.; Silber, J. J.; Correa, N. M. Role of the Medium on the C343 Inter/Intramolecular Hydrogen Bond Interactions. An Absorption, Emission, and 1HNMR Investigation of C343 in Benzene/n-Heptane Mixtures. J. Phys. Chem. A 2010, 114, 7326−7330. (3) Xu, Z.; Liu, X.; Pan, J.; Spring, D. R. Coumarin-Derived Transformable Fluorescent Sensor for Zn2+. Chem. Commun. 2012, 48, 4764−4766. (4) Duarte, F. J.; Hillman, L. W. Dye Laser Principles, with Applications; Academic Press: San Diego, CA, 1990. (5) (a) Hara, K.; Sayama, K.; Arakawa, H.; Ohga, Y.; Shinpo, A.; Suga, S. A Coumarin-Derivative Dye Sensitized Nanocrystalline TiO2 Solar

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ASSOCIATED CONTENT

S Supporting Information *

Theoretically optimized atom coordinates of 17 in vacuo, selected bond lengths, torsion angles of the optimized molecular structures, and crystallographic information files (CIFs) for 1 4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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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