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Crystallization-induced light-emission enhancement of diphenylmethane derivatives
Tetrahedron 69 (2013) 9329e9334
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Tetrahedron journal homepage: www.elsevier.com/locate/tet
Crystallization-induced light-emission enhancement of diphenylmethane derivatives ~o Rocha a, Artur M.S. Silva b, * Samuel Guieu a, b, *, Joa a b
CICECO, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal QOPNA, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 May 2013 Received in revised form 6 July 2013 Accepted 29 July 2013 Available online 29 August 2013
A family of diphenylmethane derivatives has been synthesized and their luminescence properties characterized. While in solution the compounds are weakly emissive, showing no aggregation-induced emission enhancement, the crystals of three dialkyl 5,50 -methylenebis(2-hydroxybenzoate) samples exhibit intense emission. This emission enhancement upon crystallization is ascribed to particular molecular packing, which stiffens the structure of the compounds via hydrogen bonds, preventing consecutive pep interactions. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Diphenylmethanes Luminescence Crystalline state Molecular packing
1. Introduction
2. Results and discussion
Emissive solids are important materials for the development of new devices,1 such as organic light emitting diodes, photoelectric converters or lasers. Most fluorophores are luminescent in dilute solutions, or as dopants in a glassy matrix, but become nonemissive in the solid state, due to exciplex formation and nonradiative decay through thermal relaxation.2 Diphenylmethane is a highly flexible core, which is present in some natural molecules.3 Due to the ability to absorb UV light,4 the diphenylmethane derivatives have been widely used as additives in polymers, preventing their photo-degradation, with bisphenol A being a well-known example. Not much attention has been given to their emissive properties,5 however, and to the best of our knowledge no study is available on their room temperature luminescence. Herein is reported the synthesis and luminescence properties of six diphenylmethane derivatives. Some of them exhibit surprisingly high quantum yields in the solid state at room temperature, which has been rationalized by the study of their crystal structure.
2.1. Synthesis
* Corresponding authors. Tel.: þ351 234 370714; fax: þ351 234 370084; e-mail addresses: [email protected] (S. Guieu), [email protected] (A.M.S. Silva). 0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2013.07.107
Diphenylmethane derivatives are usually synthesized using double condensation of formaldehyde with the corresponding phenyl derivative, catalyzed by a strong acid or base.6 This allows an easy access to diphenylmethane derivatives bearing similar substituents on both phenyl rings. Compounds 1e3 were obtained in reasonable yields using salicylic acid,7 salicylaldehyde8 or acetophenone9 (Scheme 1, i). Methylene disalicylic acid 1 was subsequently esterified using the corresponding alcohol as solvent with a catalytic amount of sulfuric acid. Dimethyl ester 4,10 diethyl ester 5,11 and dipropyl ester
Scheme 1. Synthesis of compounds 1e6. Reagents and conditions: i) 0.5 equiv (CH2O)n, catalytic concd H2SO4, glacial AcOH, 90 C, 2e4 h; ii) MeOH, EtOH or PrOH, catalytic concd H2SO4, 100 C, 12 h.
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612 were obtained (Scheme 1, ii).
in reasonable yields after
purification
2.2. UVevis absorption and fluorescence spectra The absorption and emission spectra of compounds 1e6 in THF are presented in Fig. 1, and the key features summarized in Table 1. The absorption maxima are observed at ca. 320 nm for 1 and 4e6, and ca. 340 nm for 2 and 3. The molar extinction coefficients are in the range 20,000e30,000 dm3 mol1 cm1 for all compounds. The fluorescence maximum (Fmax) is observed at ca. 460 nm for diacid 1, with a large Stroke shift of ca. 140 nm. Fmax are red shifted for dialdehyde 2 and diketone 3, ca. 485 and 520 nm, with larger Stroke shifts of ca. 150 and 180 nm, respectively. The emissions of the three diesters 4e6 are very similar, with Fmax ca. 470 nm. The large Stroke shifts are indicative of intramolecular charge transfer in the excited state, which for compounds 1e6, could be explained by the tautomerisation-transfer of the labile proton from the hydroxyl to the carbonyl group.13,14
The normalized solid state emission spectra of compounds 1e6 are shown in Fig. 2. All compounds are luminescent under UV irradiation, except for compound 2 whose emission is very weak. The emission of compounds 1 and 3 are blue shifted from solution to solid state. Diacid 1 has a similar quantum yield in solution and in the solid state, around 0.10. The emission efficiency of 3 is greatly enhanced in the solid state, with a quantum yield of ca. 0.18, compared to <0.01 in solution. Diesters 4e6 present a similar emission maximum in solution and in the solid state, but their quantum yield is much larger in the solid state.
Fig. 2. Normalized solid-state emission spectra (excitation at 280 nm for 1 and 2, at 320 nm for 3e6).
2.3. Aggregation-induced emission enhancement test
Fig. 1. Absorption spectra (solid lines) measured at 3105 mol L1 and emission spectra (dotted lines, lex¼310 nm) measured at 3103 mol L1 in THF at 20 C.
Table 1 Absorption and emission data for compounds 1e6 Compounds
1 2 3 4 5 6
In THF
Solid state
lmaxa (εmax) nm
Fmaxb nm
ffc
SSd nm
Fmaxb
ffe
317 337 337 317 317 318
458 486 519 472 476 471
0.10 <0.01 <0.01 0.02 0.02 0.03
141 149 182 155 159 153
428 542 498 474 465 469
0.07 <0.01 0.18 0.28 0.68 0.54
(32,600) (19,800) (25,700) (17,000) (25,700) (19,200)
Measured at the concentration of 3105 mol L1 at 25 C. Excitation wavelength was 310 nm. c Determined by comparison with fluorescein in 0.01 mol L1 NaOH in water (ff¼0.90). d Stroke shift. e Determined by absolute PL quantum yield measurement in the powder form (compound 1) or in the crystalline form (compounds 2e6). a
b
The luminescence of the different compounds in THF ranges from almost non-emissive (2 and 3) and weakly luminescent (diesters 4e6, ff¼0.02e0.03) to a maximum quantum yield of ca. 0.10 (diacid 1). This order for the increasing luminescence of compounds 1e6 may be explained by the increasing strength of the intramolecular hydrogen bond between the hydroxyl proton and the carbonyl oxygen atom, which rigidifies the structures inhibiting non-emissive thermal relaxation.14
Aggregation-induced emission enhancement (AIEE) tests were performed in THF/water mixed solvents. For all compounds, the solution turned cloudy at 80% water. For diacid 1, the emission maximum shifts from 458 to 417 nm when the water content increases but no enhancement in the emission is observed. This is an expected result because the quantum yield is roughly the same in the solution and solid state. For dialdehyde 2, the emission maximum remains the same at ca. 500 nm, with a constant very weak emission. As the quantum yields are very small both in solution and solid state, this result is not surprising. For the diketone 3, the emission remains very weak with increasing water content, as a broad band at ca. 500 nm (no emission enhancement even when the solution turns cloudy). The results for the three diesters 4e6 are similar, and Fig. 3 depicts the AIEE test for diester 6 as an example. As the water content increases, the emission maximum remains unchanged at ca. 470 nm. The emission intensity decreases from 0% to 60% water, increasing from 80% to 95% water by a factor 2, too small to allow concluding that the diesters 4e6 exhibit a clear AIEE effect. Other mixed solvents systems were used to test the AIEE properties of compounds 3e6, such as ethanol/water and methanol/water, which gave results similar to THF/water, and dichloromethane/hexane and THF/hexane, for which no aggregate formed and the emission intensity decreased slightly. None of the compounds show a clear AIEE, which is at odds with the quantum yield increase observed from the solution to solid state for compounds 3e6. In order to rationalize this feature, we looked at the organization of the molecules in the solid state. 2.4. Solid state structure and single-crystal X-ray diffraction Single crystals suitable for X-ray diffraction (XRD) were obtained for compounds 2e5 by slow evaporation of a solution in
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Fig. 3. AIEE test for the dipropyl ester 6 in THF/H2O mixtures, at 0.5104 M, excitation at 310 nm.
dichloromethane. Despite all our efforts, crystals suitable for X-ray diffraction could not be obtained for compound 1 and 6. Compound 1 seems amorphous, and compound 6 seems crystalline, but a single crystal could not be isolated. The crystal structure of compounds 2,9 3,9 and 410b was already reported. Based on the unit cell dimensions obtained from the single crystals we grew, the structures we obtained are the same as the published ones, so the published data have been used in the discussion below. The XRD structure of dialdehyde 29 is shown in Fig. 4. The two salicylaldehyde moieties are not related by symmetry. Intramolecular phenolic OeH/O hydrogen bonds with carboxyl O-atom acceptors are present in both salicylaldehyde moieties. The molecules are arranged in a chain fashion through two types of interactions: on one side, a centrosymmetric cyclic intermolecular OeH/O hydrogen bonding association; on the other side, a CeH/H contact through Van der Waals interactions is combined with p stacking. These chains are aligned and connected through p stacking interactions (see front view). Thus, compound 2 has consecutive pep stacking interactions.
Fig. 5. Single-crystal XRD structure of diketone 3.
Fig. 4. Single-crystal XRD structure of dialdehyde 2.
The XRD structure of diketone 39 is shown in Fig. 5. The two 20 hydroxyacetophenone moieties are related by crystallographic symmetry. Intramolecular phenolic OeH/O hydrogen bonds with carboxyl O-atom acceptors are present in both moieties. The molecules are organized in sheets, with one dimensional OeH/O hydrogen bonded chains on each extremity. The sheets interconnect via CeH/O contacts between the methyl and alcohol groups forming another chain structure orthogonal to the sheets (see side view). No p stacking is present.
The XRD structure of diester 410b is shown in Fig. 6. The two methyl salicylate parts are related by crystallographic twofold rotational symmetry. Intramolecular OeH/O hydrogen bonds are present and centrosymmetric cyclic intermolecular OeH/O hydrogen bonds link the molecules into infinite chains. These chains are parallel and cross-linked by CeH/O hydrogen bonds between the methyl group and the methoxy oxygen. Along the b axis, the molecules seat on top of each other, with the shortest contact between the carbon of the COOMe and the CeCH2 carbon being ca. 3.38(0) A (see front view). The top view reveals that the phenyl rings, even if close to each other and parallel, do not seat on top of each other, and therefore there is no p stacking. The single-crystal X-ray crystallography of diester 515 is shown in Fig. 7. The ethyl salicylate moieties are related by crystallographic twofold rotational symmetry. Intramolecular OeH/O hydrogen bonds are present and the molecules are arranged in a chain fashion through intermolecular CeH/O hydrogen bonds. These chains have approximately a square section and are aligned down the c axis. They are in contact through Van der Waals interactions. The side view and top view reveal that the phenyl rings do not seat on top of each other, and therefore there is no p stacking in the crystal.
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Fig. 6. Single crystal XRD structure of diester 4.
hydrogen bonds also force the molecules to organize without pep interactions, which also enhances their fluorescence intensity. We wondered why the AIEE test failed with these compounds, and investigated the solid-state structure of the aggregate. None of the compounds gave crystals by slow evaporation of a solution in THF/water mixture, and only amorphous solids were obtained. The FTIR spectra of the crystal obtained by slow evaporation of a solution in dichloromethane and of the solid obtained by slow evaporation of a solution in THF/water mixture were compared. They all show similar features and the spectra of diester 4 are depicted in Fig. 8, as an example. The amorphous solid displays two sharp bands at ca. 3650 and 860 cm1, which are absent from the spectrum of the crystal, and are attributed to free OeH vibrational and stretching vibration, respectively. The hydrogen bonded OeH stretching is still present at ca. 3200 cm1, while the carbonyl band shifts. The aromatic CeH vibration bands also appear stronger. All this indicates that water molecules are present in the solid not as free molecules but interacting with the oxygen atoms of compound 4. As a consequence, the solid-state packing should be different from the crystal. The hydrogen-bonded chains are probably disrupted and the polar environment may favor the pep interactions between the molecules, which would explain why the emission is quenched in the aggregate form.
Fig. 7. Single-crystal XRD structure of diester 5.
The intensity of the emission in the solid state depends on the packing motif and on the stiffening of the molecular structure, enhancing or hindering the thermal relaxation. Consecutive pep interactions dramatically reduce the solid-state fluorescence intensity.16 In contrast, strong hydrogen bonding, rigidifying the structure, and preventing the torsional vibration of phenyl rings17 or carbonyl groups,14 may promote the solid-state emission. Although none of the compounds presented here show AIEE, compounds 3e6 have higher quantum yields in the solid state than in solution. Compound 1 is an amorphous solid, probably lacking the organization needed to stiffen its structure. Compound 2 has consecutive pep interactions and, thus, it is fair to expect that its solidstate emission intensity is low. Compounds 3e5 have chain or sheets structures with strong hydrogen bonds and no pep interactions, and it is likely that the compound 6 has a similar organization. Each carbonyl accepts two hydrogen bonds. The excited state is likely the result of the tautomerisation-transfer of the labile proton from the hydroxyl to the carbonyl group. Two strong intra and inter-molecular hydrogen bonds are still present, restricting the torsional vibrations of the protonated carbonyl and of the aromatic rings, thus preventing the thermal relaxation of the excited state through non-emissive processes. These particularly strong
Fig. 8. FTIR spectra of diester 4 crystallized from dichloromethane and precipitated from a THF/water mixture.
3. Conclusion While diphenylmethane derivatives have been used mainly for their UV absorption properties in dilute conditions, this study shows that some of them are highly emissive in the crystalline state (ff up to 0.63). When water is present, the compounds do not exhibit AIEE due to the disruption of the crystal packing, but they show crystallization-induced emission enhancement. This is attributed to particular forms of crystal packing efficiently hampering the molecular motion and preventing consecutive pep interactions.
4. Experimental 4.1. General The UVevis absorption spectra were recorded using a UV-2501 PC Shimadzu spectrophotometer. The photoluminescence spectra
S. Guieu et al. / Tetrahedron 69 (2013) 9329e9334
were recorded at room temperature with a modular double grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Horiba Scientific) coupled to a R928 Hamamatsu photomultiplier, using a front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. For compounds 2e5, a colorless single crystal was selected for indexing and, for compound 5, data collection at 150 K on a Noniusbased Kappa Bruker diffractometer equipped with a chargeA) radiacoupled device (CCD) area detector and MoKa(l¼0.7107 tion. Absorption corrections were applied using the multi-scan semi-empirical method implemented in SADABS.18 The structure was solved by direct methods using the program SHELXS-9719 and refined by full-matrix least-square refinement on F2 using the program SHELXL-97. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were visible on the difference Fourier map, placed in geometrically calculated positions and included in the final refinement using the ‘riding’ model with isotropic temperature factors fixed at 1.2 times that of the parent atom. 4.2. Synthesis of diphenylmethane derivatives 1e3 4.2.1. 5,50 -Methylenebis(2-hydroxybenzoic acid) 1.7 Concentrated sulfuric acid (1.0 mL, catalytic) was slowly added to a suspension of salicylic acid (10.0 g, 72.5 mmol) and formaldehyde (1.09 g, 36.2 mmol) in glacial acetic acid (30 mL) at room temperature. The solution was stirred at 90 C for 2 h. It was then poured on iced water (50 mL), the solid that formed was collected by filtration and washed with water (50 mL) and methanol (50 mL). The product was obtained as an off-white solid (8.07 g, 77%). Mp 248e250 C (lit.7b 238e240 C); 1H NMR (300.13 MHz, DMSO-d6, 25 C): d¼7.61 (d, 4JHeH 2.4 Hz, 2H, aromatic CH), 7.36 (dd, 4JHeH 2.4, 3JHeH 8.7 Hz, 2H, aromatic CH), 6.88 (d, 3JHeH 8.7 Hz, 2H, aromatic CH), 3.85 (s, 2H, CH2). 4.2.2. 5,50 -Methylenebis(2-hydroxybenzaldehyde) 2.8 Concentrated sulfuric acid (1.0 mL, catalytic) was slowly added to a suspension of salicylaldehyde (12.2 g, 100 mmol) and formaldehyde (1.5 g, 50 mmol) in glacial acetic acid (30 mL) at room temperature, and the solution was stirred at 90 C for 4 h. It was then poured on iced water (50 mL), the solid that formed was collected by filtration and washed with water (50 mL) and light petroleum (50 mL). The product was obtained as an off-white solid (6.9 g, 54%). Mp 128e130 C (lit.20 141e142 C); 1H NMR (300.13 MHz, CDCl3, 25 C): d¼10.92 (s, 2H, OH), 9.85 (s, 2H, CHO), 7.35 (dd, 4 JHeH 2.1, 3JHeH 8.4 Hz, 2H, aromatic CH), 7.32 (d, 4JHeH 2.1 Hz, 2H, aromatic CH), 6.96 (d, 3JHeH 8.4 Hz, 2H, aromatic CH), 3.96 (s, 2H, CH2). 4.2.3. 5,50 -Methylenebis(2-hydroxyacetophenone) 3.9 Concentrated sulfuric acid (0.5 mL, catalytic) was slowly added to a suspension of 20 -hydroxyacetophenone (5.0 g, 36.7 mmol) and formaldehyde (550 mg, 18.35 mmol) in glacial acetic acid (15 mL) at room temperature, and the solution was stirred at 90 C for 2 h. It was then poured on ice (50 mL), the solid that formed was collected by filtration and washed with water (50 mL) and methanol (50 mL). Silica gel flash column chromatography (eluent: dichloromethane) of the residue gave the product as a white solid (1.0 g, 19%). Slow evaporation of a concentrated solution in dichloromethane gave yellowish crystals suitable for X-ray diffraction. Mp 147e149 C (lit.9 143 C); 1H NMR (300.13 MHz, CDCl3, 25 C): d¼12.17 (s, 2H, OH), 7.50 (d, 4JHeH 2.1 Hz, 2H,
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aromatic CH), 7.28 (dd, 4JHeH 2.1, 3JHeH 8.7 Hz, 2H, aromatic CH), 6.93 (d, 3JHeH 8.7 Hz, 2H, aromatic CH), 3.91 (s, 2H, CH2), 2.60 (s, 6H, CH3). 4.3. Synthesis of the esters 4e6 4.3.1. Dimethyl 5,50 -methylenebis(2-hydroxy-benzoate) 4.10 Concen trated sulfuric acid (0.1 mL, catalytic) was slowly added to a solution of 5,50 -methylenebis(2-hydroxybenzoic acid) (2.0 g, 6.9 mmol) in methanol (100 mL) at room temperature, and the solution was stirred at 100 C for 12 h. The solvent was then evaporated under reduced pressure, and silica gel flash column chromatography (eluent: dichloromethane) of the residue gave the product as a white solid (0.84 g, 38%) and some starting material (1.02 g). Slow evaporation of a concentrated solution in dichloromethane gave colorless crystals suitable for X-ray diffraction. Mp 108e110 C; 1H NMR (300.13 MHz, CDCl3, 25 C): d¼10.64 (s, 2H, OH), 7.62 (d, 4JHeH 2.4 Hz, 2H, aromatic CH), 7.25 (dd, 4JHeH 2.4, 3JHeH 8.7 Hz, 2H, aromatic CH), 6.92 (d, 3JHeH 8.7 Hz, 2H, aromatic CH), 3.93 (s, 6H, CH3), 3.84 (s, 2H, CH2). 4.3.2. Diethyl 5,50 -methylenebis(2-hydroxy-benzoate) 5.11 Concen trated sulfuric acid (0.01 mL, catalytic) was slowly added to a solution of 5,50 -methylenebis(2-hydroxybenzoic acid) (288 mg, 1.0 mmol) in ethanol (20 mL) at room temperature, and the solution was stirred at 100 C for 12 h. The solvent was then evaporated under reduced pressure, and silica gel flash column chromatography (eluent: dichloromethane) of the residue gave the product as a white solid (43 mg, 13%) and some starting material (210 mg). Slow evaporation of a concentrated solution in dichloromethane gave colorless crystals suitable for X-ray diffraction. Mp 105e107 C (lit.11 220e222 C); 1H NMR (300.13 MHz, CDCl3, 25 C): d¼10.73 (s, 2H, OH), 7.65 (d, 4JHeH 2.4 Hz, 2H, aromatic CH), 7.23 (dd, 4JHeH 2.4, 3JHeH 8.7 Hz, 2H, aromatic CH), 6.90 (d, 3JHeH 8.7 Hz, 2H, aromatic CH), 4.39 (q, 3 JHeH 7.2 Hz, 4H, CH2eCH3), 3.85 (s, 2H, CH2), 1.41 (t, 3JHeH 7.2 Hz, 6H, CH2eCH3). 4.3.3. Dipropyl 5,50 -methylenebis(2-hydroxy-benzoate) 6.12 Concen trated sulfuric acid (0.01 mL, catalytic) was slowly added to a solution of 5,50 -methylenebis(2-hydroxybenzoic acid) (288 mg, 1.0 mmol) in propanol (20 mL) at room temperature, and the solution was stirred at 100 C for 12 h. The solvent was then evaporated under reduced pressure, and silica gel flash column chromatography (eluent: dichloromethane) of the residue gave the product as a white oily solid (45 mg, 12%) and some starting material (195 mg). Mp 78e80 C (lit. not reported); 1H NMR (300.13 MHz, CDCl3, 25 C): d¼10.72 (s, 2H, OH), 7.65 (d, 4JHeH 2.4 Hz, 2H, aromatic CH), 7.24 (dd, 4JHeH 2.4, 3JHeH 8.4 Hz, 2H, aromatic CH), 6.91 (d, 3JHeH 8.4 Hz, 2H, aromatic CH), 4.29 (t, 3JHeH 6.6 Hz, 4H, CH2eCH2eCH3), 3.86 (s, 2H, CH2), 1.80 (m, 4H, CH2eCH2eCH3), 1.02 (t, 3JHeH 7.2 Hz, 6H, CH2eCH2eCH3). Acknowledgements Thanks are due to the University of Aveiro and the Portu^ncia e a Tecnologia (FCT) for funding guese Fundac¸~ ao para a Cie the Organic Chemistry Research Unit (project PEst-C/QUI/ UI0062/2013), the CICECO Associate Laboratory (PEst-C/CTM/ LA0011/2013), and the Portuguese National NMR Network ~o for (RNRMN). The authors thank R.A.S. Ferreira and P. Branda their help in the PL measurements and XRD, respectively. S.G. also thanks the FCT for a postdoctoral grant (SFRH/BPD/70702/ 2010).
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Supplementary data These data contain the single crystal X-ray crystallographic data for compound 5. Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.tet.2013.07.107. References and notes 1. (a) Toal, S. J.; Jones, K. A.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc. 2005, 127, 11661e11665; (b) Bhongale, C. J.; Hsu, C.-S. Angew. Chem., Int. Ed. 2006, 45, 1404e1408; (c) Shirota, Y. J. Mater. Chem. 2000, 10, 1e25. 2. (a) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, UK, 1970; (b) Malkin, J. Photophysical and Photochemical Properties of Aromatic Compounds; CRC: Boca Raton, FL, 1992. 3. See, for example: (a) Ye, Y. H.; Zhu, H. L.; Song, Y. C.; Liu, J. Y.; Tan, R. X. J. Nat. Prod. 2005, 68, 1106e1108; (b) Chen, J. L.; Gerwick, W. H. J. Nat. Prod. 1994, 57, 947e952. 4. Cui, G.-J.; Xu, X.-Y.; Lin, Y.-J.; Evans, D. G.; Li, D.-Q. Ind. Eng. Chem. Res. 2010, 49, 448e453. 5. (a) Wirz, D. R.; Wilson, D. L.; Schenk, G. H. Anal. Chem. 1974, 46, 896e900; (b) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371e392. 6. (a) Coffield, T. H.; Filbey, A. H.; Ecke, G. G.; Kolka, A. J. J. Am. Chem. Soc. 1957, 79, 5019e5023; (b) Bandgar, B. P.; Kasture, S. P. Monatsh. Chem. 2000, 131, 913e915; (c) Sereda, G. A. Tetrahedron Lett. 2004, 45, 7265e7267; (d) Kumarraja, M.; Pitchumani, K. Synth. Commun. 2003, 33, 105e111; (e) Kharasch, M. S.; Joshi, B. S. J. Org. Chem. 1957, 22, 1435e1438. 7. (a) Clemmensen, E.; Heitman, A. H. C. J. Am. Chem. Soc. 1911, 33, 733e745; (b) Reddy, C. S.; Raghu, M. Chem. Pharm. Bull. 2008, 56, 1732e1734.
8. Marvel, C. S.; Tarkoy, N. J. Am. Chem. Soc. 1957, 79, 6000e6002. 9. Barba, V.; Betanzos, I. J. Organomet. Chem. 2007, 692, 4903e4908. ric, R.; Vigneron, J.-P.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1993, 10. (a) Me ~o, P.; Rocha, J.; Silva, A. M. S. Acta Crystallogr. 129e131; (b) Guieu, S.; Branda 2012, E68, o1404. 11. Nagaraj, A.; Reddy, C. S. J. Heterocycl. Chem. 2007, 44, 1357e1361. 12. Lapkin, I. I.; Orlova, L. D. Zh. Org. Khim. 1970, 6, 68e71. 13. Kozma, L.; Hornak, I.; Eroshtak, I.; Nemet, B. J. Appl. Spectrosc. 1990, 53, 851e855. 14. Hisaindee, S.; Zahid, O.; Meetani, M. A.; Graham, J. J. Fluoresc. 2012, 22, 677e683. 15. CCDC 938828 contains the supplementary crystallographic data for compound 5. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/ conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (0044) 1223-336-033; or e-mail: [email protected]. Crystal data: C19H20O6, Mw¼344.35, monoclinic, C2/ c, Z¼4, a¼11.6206(4), b¼8.7856(3), c¼16.8516(5) A, a¼g¼90 , b¼95.761(2) , Dcalcd¼1.336 g cm3, T¼150(2) K, F(000)¼728, m¼0.100 mm1, 7654 reflections were corrected, 2297 unique (Rint¼0.0541), 1787 observed (I>2s (I)), R1¼0. 0396, wR2¼0.1016. 16. (a) Matsui, M.; Shibata, T.; Fukushima, M.; Kubota, Y.; Funabiki, K. Tetrahedron 2012, 68, 9936e9941; (b) Shirai, K.; Matsuoka, M.; Fukunishi, K. Dyes Pigm. 1999, 42, 95e101; (c) Shirai, K.; Matsuoka, M.; Matsumoto, S.; Shiro, M. Dyes Pigm. 2003, 56, 83e87. 17. Guieu, S.; Rocha, J.; Silva, A. M. S. Tetrahedron Lett. 2013, 54, 2870e2873. 18. Sheldrick, G. M. SADABS V.2.01, Bruker/Siemens Area Detector Absorption Correction Program; Bruker AXS: Madison, WI, 1998. € ttingen: Germany, 2008. 19. Sheldrick, G. M. SHELXS-97; University of Go 20. Delogu, G.; Podda, G.; Corda, M.; Fadda, M. B.; Fais, A.; Era, B. Bioorg. Med. Chem. Lett. 2010, 20, 6138e6140.
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Tetrahedron journal homepage: www.elsevier.com/locate/tet
Crystallization-induced light-emission enhancement of diphenylmethane derivatives ~o Rocha a, Artur M.S. Silva b, * Samuel Guieu a, b, *, Joa a b
CICECO, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal QOPNA, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 May 2013 Received in revised form 6 July 2013 Accepted 29 July 2013 Available online 29 August 2013
A family of diphenylmethane derivatives has been synthesized and their luminescence properties characterized. While in solution the compounds are weakly emissive, showing no aggregation-induced emission enhancement, the crystals of three dialkyl 5,50 -methylenebis(2-hydroxybenzoate) samples exhibit intense emission. This emission enhancement upon crystallization is ascribed to particular molecular packing, which stiffens the structure of the compounds via hydrogen bonds, preventing consecutive pep interactions. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Diphenylmethanes Luminescence Crystalline state Molecular packing
1. Introduction
2. Results and discussion
Emissive solids are important materials for the development of new devices,1 such as organic light emitting diodes, photoelectric converters or lasers. Most fluorophores are luminescent in dilute solutions, or as dopants in a glassy matrix, but become nonemissive in the solid state, due to exciplex formation and nonradiative decay through thermal relaxation.2 Diphenylmethane is a highly flexible core, which is present in some natural molecules.3 Due to the ability to absorb UV light,4 the diphenylmethane derivatives have been widely used as additives in polymers, preventing their photo-degradation, with bisphenol A being a well-known example. Not much attention has been given to their emissive properties,5 however, and to the best of our knowledge no study is available on their room temperature luminescence. Herein is reported the synthesis and luminescence properties of six diphenylmethane derivatives. Some of them exhibit surprisingly high quantum yields in the solid state at room temperature, which has been rationalized by the study of their crystal structure.
2.1. Synthesis
* Corresponding authors. Tel.: þ351 234 370714; fax: þ351 234 370084; e-mail addresses: [email protected] (S. Guieu), [email protected] (A.M.S. Silva). 0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2013.07.107
Diphenylmethane derivatives are usually synthesized using double condensation of formaldehyde with the corresponding phenyl derivative, catalyzed by a strong acid or base.6 This allows an easy access to diphenylmethane derivatives bearing similar substituents on both phenyl rings. Compounds 1e3 were obtained in reasonable yields using salicylic acid,7 salicylaldehyde8 or acetophenone9 (Scheme 1, i). Methylene disalicylic acid 1 was subsequently esterified using the corresponding alcohol as solvent with a catalytic amount of sulfuric acid. Dimethyl ester 4,10 diethyl ester 5,11 and dipropyl ester
Scheme 1. Synthesis of compounds 1e6. Reagents and conditions: i) 0.5 equiv (CH2O)n, catalytic concd H2SO4, glacial AcOH, 90 C, 2e4 h; ii) MeOH, EtOH or PrOH, catalytic concd H2SO4, 100 C, 12 h.
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612 were obtained (Scheme 1, ii).
in reasonable yields after
purification
2.2. UVevis absorption and fluorescence spectra The absorption and emission spectra of compounds 1e6 in THF are presented in Fig. 1, and the key features summarized in Table 1. The absorption maxima are observed at ca. 320 nm for 1 and 4e6, and ca. 340 nm for 2 and 3. The molar extinction coefficients are in the range 20,000e30,000 dm3 mol1 cm1 for all compounds. The fluorescence maximum (Fmax) is observed at ca. 460 nm for diacid 1, with a large Stroke shift of ca. 140 nm. Fmax are red shifted for dialdehyde 2 and diketone 3, ca. 485 and 520 nm, with larger Stroke shifts of ca. 150 and 180 nm, respectively. The emissions of the three diesters 4e6 are very similar, with Fmax ca. 470 nm. The large Stroke shifts are indicative of intramolecular charge transfer in the excited state, which for compounds 1e6, could be explained by the tautomerisation-transfer of the labile proton from the hydroxyl to the carbonyl group.13,14
The normalized solid state emission spectra of compounds 1e6 are shown in Fig. 2. All compounds are luminescent under UV irradiation, except for compound 2 whose emission is very weak. The emission of compounds 1 and 3 are blue shifted from solution to solid state. Diacid 1 has a similar quantum yield in solution and in the solid state, around 0.10. The emission efficiency of 3 is greatly enhanced in the solid state, with a quantum yield of ca. 0.18, compared to <0.01 in solution. Diesters 4e6 present a similar emission maximum in solution and in the solid state, but their quantum yield is much larger in the solid state.
Fig. 2. Normalized solid-state emission spectra (excitation at 280 nm for 1 and 2, at 320 nm for 3e6).
2.3. Aggregation-induced emission enhancement test
Fig. 1. Absorption spectra (solid lines) measured at 3105 mol L1 and emission spectra (dotted lines, lex¼310 nm) measured at 3103 mol L1 in THF at 20 C.
Table 1 Absorption and emission data for compounds 1e6 Compounds
1 2 3 4 5 6
In THF
Solid state
lmaxa (εmax) nm
Fmaxb nm
ffc
SSd nm
Fmaxb
ffe
317 337 337 317 317 318
458 486 519 472 476 471
0.10 <0.01 <0.01 0.02 0.02 0.03
141 149 182 155 159 153
428 542 498 474 465 469
0.07 <0.01 0.18 0.28 0.68 0.54
(32,600) (19,800) (25,700) (17,000) (25,700) (19,200)
Measured at the concentration of 3105 mol L1 at 25 C. Excitation wavelength was 310 nm. c Determined by comparison with fluorescein in 0.01 mol L1 NaOH in water (ff¼0.90). d Stroke shift. e Determined by absolute PL quantum yield measurement in the powder form (compound 1) or in the crystalline form (compounds 2e6). a
b
The luminescence of the different compounds in THF ranges from almost non-emissive (2 and 3) and weakly luminescent (diesters 4e6, ff¼0.02e0.03) to a maximum quantum yield of ca. 0.10 (diacid 1). This order for the increasing luminescence of compounds 1e6 may be explained by the increasing strength of the intramolecular hydrogen bond between the hydroxyl proton and the carbonyl oxygen atom, which rigidifies the structures inhibiting non-emissive thermal relaxation.14
Aggregation-induced emission enhancement (AIEE) tests were performed in THF/water mixed solvents. For all compounds, the solution turned cloudy at 80% water. For diacid 1, the emission maximum shifts from 458 to 417 nm when the water content increases but no enhancement in the emission is observed. This is an expected result because the quantum yield is roughly the same in the solution and solid state. For dialdehyde 2, the emission maximum remains the same at ca. 500 nm, with a constant very weak emission. As the quantum yields are very small both in solution and solid state, this result is not surprising. For the diketone 3, the emission remains very weak with increasing water content, as a broad band at ca. 500 nm (no emission enhancement even when the solution turns cloudy). The results for the three diesters 4e6 are similar, and Fig. 3 depicts the AIEE test for diester 6 as an example. As the water content increases, the emission maximum remains unchanged at ca. 470 nm. The emission intensity decreases from 0% to 60% water, increasing from 80% to 95% water by a factor 2, too small to allow concluding that the diesters 4e6 exhibit a clear AIEE effect. Other mixed solvents systems were used to test the AIEE properties of compounds 3e6, such as ethanol/water and methanol/water, which gave results similar to THF/water, and dichloromethane/hexane and THF/hexane, for which no aggregate formed and the emission intensity decreased slightly. None of the compounds show a clear AIEE, which is at odds with the quantum yield increase observed from the solution to solid state for compounds 3e6. In order to rationalize this feature, we looked at the organization of the molecules in the solid state. 2.4. Solid state structure and single-crystal X-ray diffraction Single crystals suitable for X-ray diffraction (XRD) were obtained for compounds 2e5 by slow evaporation of a solution in
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Fig. 3. AIEE test for the dipropyl ester 6 in THF/H2O mixtures, at 0.5104 M, excitation at 310 nm.
dichloromethane. Despite all our efforts, crystals suitable for X-ray diffraction could not be obtained for compound 1 and 6. Compound 1 seems amorphous, and compound 6 seems crystalline, but a single crystal could not be isolated. The crystal structure of compounds 2,9 3,9 and 410b was already reported. Based on the unit cell dimensions obtained from the single crystals we grew, the structures we obtained are the same as the published ones, so the published data have been used in the discussion below. The XRD structure of dialdehyde 29 is shown in Fig. 4. The two salicylaldehyde moieties are not related by symmetry. Intramolecular phenolic OeH/O hydrogen bonds with carboxyl O-atom acceptors are present in both salicylaldehyde moieties. The molecules are arranged in a chain fashion through two types of interactions: on one side, a centrosymmetric cyclic intermolecular OeH/O hydrogen bonding association; on the other side, a CeH/H contact through Van der Waals interactions is combined with p stacking. These chains are aligned and connected through p stacking interactions (see front view). Thus, compound 2 has consecutive pep stacking interactions.
Fig. 5. Single-crystal XRD structure of diketone 3.
Fig. 4. Single-crystal XRD structure of dialdehyde 2.
The XRD structure of diketone 39 is shown in Fig. 5. The two 20 hydroxyacetophenone moieties are related by crystallographic symmetry. Intramolecular phenolic OeH/O hydrogen bonds with carboxyl O-atom acceptors are present in both moieties. The molecules are organized in sheets, with one dimensional OeH/O hydrogen bonded chains on each extremity. The sheets interconnect via CeH/O contacts between the methyl and alcohol groups forming another chain structure orthogonal to the sheets (see side view). No p stacking is present.
The XRD structure of diester 410b is shown in Fig. 6. The two methyl salicylate parts are related by crystallographic twofold rotational symmetry. Intramolecular OeH/O hydrogen bonds are present and centrosymmetric cyclic intermolecular OeH/O hydrogen bonds link the molecules into infinite chains. These chains are parallel and cross-linked by CeH/O hydrogen bonds between the methyl group and the methoxy oxygen. Along the b axis, the molecules seat on top of each other, with the shortest contact between the carbon of the COOMe and the CeCH2 carbon being ca. 3.38(0) A (see front view). The top view reveals that the phenyl rings, even if close to each other and parallel, do not seat on top of each other, and therefore there is no p stacking. The single-crystal X-ray crystallography of diester 515 is shown in Fig. 7. The ethyl salicylate moieties are related by crystallographic twofold rotational symmetry. Intramolecular OeH/O hydrogen bonds are present and the molecules are arranged in a chain fashion through intermolecular CeH/O hydrogen bonds. These chains have approximately a square section and are aligned down the c axis. They are in contact through Van der Waals interactions. The side view and top view reveal that the phenyl rings do not seat on top of each other, and therefore there is no p stacking in the crystal.
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Fig. 6. Single crystal XRD structure of diester 4.
hydrogen bonds also force the molecules to organize without pep interactions, which also enhances their fluorescence intensity. We wondered why the AIEE test failed with these compounds, and investigated the solid-state structure of the aggregate. None of the compounds gave crystals by slow evaporation of a solution in THF/water mixture, and only amorphous solids were obtained. The FTIR spectra of the crystal obtained by slow evaporation of a solution in dichloromethane and of the solid obtained by slow evaporation of a solution in THF/water mixture were compared. They all show similar features and the spectra of diester 4 are depicted in Fig. 8, as an example. The amorphous solid displays two sharp bands at ca. 3650 and 860 cm1, which are absent from the spectrum of the crystal, and are attributed to free OeH vibrational and stretching vibration, respectively. The hydrogen bonded OeH stretching is still present at ca. 3200 cm1, while the carbonyl band shifts. The aromatic CeH vibration bands also appear stronger. All this indicates that water molecules are present in the solid not as free molecules but interacting with the oxygen atoms of compound 4. As a consequence, the solid-state packing should be different from the crystal. The hydrogen-bonded chains are probably disrupted and the polar environment may favor the pep interactions between the molecules, which would explain why the emission is quenched in the aggregate form.
Fig. 7. Single-crystal XRD structure of diester 5.
The intensity of the emission in the solid state depends on the packing motif and on the stiffening of the molecular structure, enhancing or hindering the thermal relaxation. Consecutive pep interactions dramatically reduce the solid-state fluorescence intensity.16 In contrast, strong hydrogen bonding, rigidifying the structure, and preventing the torsional vibration of phenyl rings17 or carbonyl groups,14 may promote the solid-state emission. Although none of the compounds presented here show AIEE, compounds 3e6 have higher quantum yields in the solid state than in solution. Compound 1 is an amorphous solid, probably lacking the organization needed to stiffen its structure. Compound 2 has consecutive pep interactions and, thus, it is fair to expect that its solidstate emission intensity is low. Compounds 3e5 have chain or sheets structures with strong hydrogen bonds and no pep interactions, and it is likely that the compound 6 has a similar organization. Each carbonyl accepts two hydrogen bonds. The excited state is likely the result of the tautomerisation-transfer of the labile proton from the hydroxyl to the carbonyl group. Two strong intra and inter-molecular hydrogen bonds are still present, restricting the torsional vibrations of the protonated carbonyl and of the aromatic rings, thus preventing the thermal relaxation of the excited state through non-emissive processes. These particularly strong
Fig. 8. FTIR spectra of diester 4 crystallized from dichloromethane and precipitated from a THF/water mixture.
3. Conclusion While diphenylmethane derivatives have been used mainly for their UV absorption properties in dilute conditions, this study shows that some of them are highly emissive in the crystalline state (ff up to 0.63). When water is present, the compounds do not exhibit AIEE due to the disruption of the crystal packing, but they show crystallization-induced emission enhancement. This is attributed to particular forms of crystal packing efficiently hampering the molecular motion and preventing consecutive pep interactions.
4. Experimental 4.1. General The UVevis absorption spectra were recorded using a UV-2501 PC Shimadzu spectrophotometer. The photoluminescence spectra
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were recorded at room temperature with a modular double grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Horiba Scientific) coupled to a R928 Hamamatsu photomultiplier, using a front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and optical spectral response of the spectrofluorimeter and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. For compounds 2e5, a colorless single crystal was selected for indexing and, for compound 5, data collection at 150 K on a Noniusbased Kappa Bruker diffractometer equipped with a chargeA) radiacoupled device (CCD) area detector and MoKa(l¼0.7107 tion. Absorption corrections were applied using the multi-scan semi-empirical method implemented in SADABS.18 The structure was solved by direct methods using the program SHELXS-9719 and refined by full-matrix least-square refinement on F2 using the program SHELXL-97. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were visible on the difference Fourier map, placed in geometrically calculated positions and included in the final refinement using the ‘riding’ model with isotropic temperature factors fixed at 1.2 times that of the parent atom. 4.2. Synthesis of diphenylmethane derivatives 1e3 4.2.1. 5,50 -Methylenebis(2-hydroxybenzoic acid) 1.7 Concentrated sulfuric acid (1.0 mL, catalytic) was slowly added to a suspension of salicylic acid (10.0 g, 72.5 mmol) and formaldehyde (1.09 g, 36.2 mmol) in glacial acetic acid (30 mL) at room temperature. The solution was stirred at 90 C for 2 h. It was then poured on iced water (50 mL), the solid that formed was collected by filtration and washed with water (50 mL) and methanol (50 mL). The product was obtained as an off-white solid (8.07 g, 77%). Mp 248e250 C (lit.7b 238e240 C); 1H NMR (300.13 MHz, DMSO-d6, 25 C): d¼7.61 (d, 4JHeH 2.4 Hz, 2H, aromatic CH), 7.36 (dd, 4JHeH 2.4, 3JHeH 8.7 Hz, 2H, aromatic CH), 6.88 (d, 3JHeH 8.7 Hz, 2H, aromatic CH), 3.85 (s, 2H, CH2). 4.2.2. 5,50 -Methylenebis(2-hydroxybenzaldehyde) 2.8 Concentrated sulfuric acid (1.0 mL, catalytic) was slowly added to a suspension of salicylaldehyde (12.2 g, 100 mmol) and formaldehyde (1.5 g, 50 mmol) in glacial acetic acid (30 mL) at room temperature, and the solution was stirred at 90 C for 4 h. It was then poured on iced water (50 mL), the solid that formed was collected by filtration and washed with water (50 mL) and light petroleum (50 mL). The product was obtained as an off-white solid (6.9 g, 54%). Mp 128e130 C (lit.20 141e142 C); 1H NMR (300.13 MHz, CDCl3, 25 C): d¼10.92 (s, 2H, OH), 9.85 (s, 2H, CHO), 7.35 (dd, 4 JHeH 2.1, 3JHeH 8.4 Hz, 2H, aromatic CH), 7.32 (d, 4JHeH 2.1 Hz, 2H, aromatic CH), 6.96 (d, 3JHeH 8.4 Hz, 2H, aromatic CH), 3.96 (s, 2H, CH2). 4.2.3. 5,50 -Methylenebis(2-hydroxyacetophenone) 3.9 Concentrated sulfuric acid (0.5 mL, catalytic) was slowly added to a suspension of 20 -hydroxyacetophenone (5.0 g, 36.7 mmol) and formaldehyde (550 mg, 18.35 mmol) in glacial acetic acid (15 mL) at room temperature, and the solution was stirred at 90 C for 2 h. It was then poured on ice (50 mL), the solid that formed was collected by filtration and washed with water (50 mL) and methanol (50 mL). Silica gel flash column chromatography (eluent: dichloromethane) of the residue gave the product as a white solid (1.0 g, 19%). Slow evaporation of a concentrated solution in dichloromethane gave yellowish crystals suitable for X-ray diffraction. Mp 147e149 C (lit.9 143 C); 1H NMR (300.13 MHz, CDCl3, 25 C): d¼12.17 (s, 2H, OH), 7.50 (d, 4JHeH 2.1 Hz, 2H,
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aromatic CH), 7.28 (dd, 4JHeH 2.1, 3JHeH 8.7 Hz, 2H, aromatic CH), 6.93 (d, 3JHeH 8.7 Hz, 2H, aromatic CH), 3.91 (s, 2H, CH2), 2.60 (s, 6H, CH3). 4.3. Synthesis of the esters 4e6 4.3.1. Dimethyl 5,50 -methylenebis(2-hydroxy-benzoate) 4.10 Concen trated sulfuric acid (0.1 mL, catalytic) was slowly added to a solution of 5,50 -methylenebis(2-hydroxybenzoic acid) (2.0 g, 6.9 mmol) in methanol (100 mL) at room temperature, and the solution was stirred at 100 C for 12 h. The solvent was then evaporated under reduced pressure, and silica gel flash column chromatography (eluent: dichloromethane) of the residue gave the product as a white solid (0.84 g, 38%) and some starting material (1.02 g). Slow evaporation of a concentrated solution in dichloromethane gave colorless crystals suitable for X-ray diffraction. Mp 108e110 C; 1H NMR (300.13 MHz, CDCl3, 25 C): d¼10.64 (s, 2H, OH), 7.62 (d, 4JHeH 2.4 Hz, 2H, aromatic CH), 7.25 (dd, 4JHeH 2.4, 3JHeH 8.7 Hz, 2H, aromatic CH), 6.92 (d, 3JHeH 8.7 Hz, 2H, aromatic CH), 3.93 (s, 6H, CH3), 3.84 (s, 2H, CH2). 4.3.2. Diethyl 5,50 -methylenebis(2-hydroxy-benzoate) 5.11 Concen trated sulfuric acid (0.01 mL, catalytic) was slowly added to a solution of 5,50 -methylenebis(2-hydroxybenzoic acid) (288 mg, 1.0 mmol) in ethanol (20 mL) at room temperature, and the solution was stirred at 100 C for 12 h. The solvent was then evaporated under reduced pressure, and silica gel flash column chromatography (eluent: dichloromethane) of the residue gave the product as a white solid (43 mg, 13%) and some starting material (210 mg). Slow evaporation of a concentrated solution in dichloromethane gave colorless crystals suitable for X-ray diffraction. Mp 105e107 C (lit.11 220e222 C); 1H NMR (300.13 MHz, CDCl3, 25 C): d¼10.73 (s, 2H, OH), 7.65 (d, 4JHeH 2.4 Hz, 2H, aromatic CH), 7.23 (dd, 4JHeH 2.4, 3JHeH 8.7 Hz, 2H, aromatic CH), 6.90 (d, 3JHeH 8.7 Hz, 2H, aromatic CH), 4.39 (q, 3 JHeH 7.2 Hz, 4H, CH2eCH3), 3.85 (s, 2H, CH2), 1.41 (t, 3JHeH 7.2 Hz, 6H, CH2eCH3). 4.3.3. Dipropyl 5,50 -methylenebis(2-hydroxy-benzoate) 6.12 Concen trated sulfuric acid (0.01 mL, catalytic) was slowly added to a solution of 5,50 -methylenebis(2-hydroxybenzoic acid) (288 mg, 1.0 mmol) in propanol (20 mL) at room temperature, and the solution was stirred at 100 C for 12 h. The solvent was then evaporated under reduced pressure, and silica gel flash column chromatography (eluent: dichloromethane) of the residue gave the product as a white oily solid (45 mg, 12%) and some starting material (195 mg). Mp 78e80 C (lit. not reported); 1H NMR (300.13 MHz, CDCl3, 25 C): d¼10.72 (s, 2H, OH), 7.65 (d, 4JHeH 2.4 Hz, 2H, aromatic CH), 7.24 (dd, 4JHeH 2.4, 3JHeH 8.4 Hz, 2H, aromatic CH), 6.91 (d, 3JHeH 8.4 Hz, 2H, aromatic CH), 4.29 (t, 3JHeH 6.6 Hz, 4H, CH2eCH2eCH3), 3.86 (s, 2H, CH2), 1.80 (m, 4H, CH2eCH2eCH3), 1.02 (t, 3JHeH 7.2 Hz, 6H, CH2eCH2eCH3). Acknowledgements Thanks are due to the University of Aveiro and the Portu^ncia e a Tecnologia (FCT) for funding guese Fundac¸~ ao para a Cie the Organic Chemistry Research Unit (project PEst-C/QUI/ UI0062/2013), the CICECO Associate Laboratory (PEst-C/CTM/ LA0011/2013), and the Portuguese National NMR Network ~o for (RNRMN). The authors thank R.A.S. Ferreira and P. Branda their help in the PL measurements and XRD, respectively. S.G. also thanks the FCT for a postdoctoral grant (SFRH/BPD/70702/ 2010).
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Supplementary data These data contain the single crystal X-ray crystallographic data for compound 5. Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.tet.2013.07.107. References and notes 1. (a) Toal, S. J.; Jones, K. A.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc. 2005, 127, 11661e11665; (b) Bhongale, C. J.; Hsu, C.-S. Angew. Chem., Int. Ed. 2006, 45, 1404e1408; (c) Shirota, Y. J. Mater. Chem. 2000, 10, 1e25. 2. (a) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, UK, 1970; (b) Malkin, J. Photophysical and Photochemical Properties of Aromatic Compounds; CRC: Boca Raton, FL, 1992. 3. See, for example: (a) Ye, Y. H.; Zhu, H. L.; Song, Y. C.; Liu, J. Y.; Tan, R. X. J. Nat. Prod. 2005, 68, 1106e1108; (b) Chen, J. L.; Gerwick, W. H. J. Nat. Prod. 1994, 57, 947e952. 4. Cui, G.-J.; Xu, X.-Y.; Lin, Y.-J.; Evans, D. G.; Li, D.-Q. Ind. Eng. Chem. Res. 2010, 49, 448e453. 5. (a) Wirz, D. R.; Wilson, D. L.; Schenk, G. H. Anal. Chem. 1974, 46, 896e900; (b) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371e392. 6. (a) Coffield, T. H.; Filbey, A. H.; Ecke, G. G.; Kolka, A. J. J. Am. Chem. Soc. 1957, 79, 5019e5023; (b) Bandgar, B. P.; Kasture, S. P. Monatsh. Chem. 2000, 131, 913e915; (c) Sereda, G. A. Tetrahedron Lett. 2004, 45, 7265e7267; (d) Kumarraja, M.; Pitchumani, K. Synth. Commun. 2003, 33, 105e111; (e) Kharasch, M. S.; Joshi, B. S. J. Org. Chem. 1957, 22, 1435e1438. 7. (a) Clemmensen, E.; Heitman, A. H. C. J. Am. Chem. Soc. 1911, 33, 733e745; (b) Reddy, C. S.; Raghu, M. Chem. Pharm. Bull. 2008, 56, 1732e1734.
8. Marvel, C. S.; Tarkoy, N. J. Am. Chem. Soc. 1957, 79, 6000e6002. 9. Barba, V.; Betanzos, I. J. Organomet. Chem. 2007, 692, 4903e4908. ric, R.; Vigneron, J.-P.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1993, 10. (a) Me ~o, P.; Rocha, J.; Silva, A. M. S. Acta Crystallogr. 129e131; (b) Guieu, S.; Branda 2012, E68, o1404. 11. Nagaraj, A.; Reddy, C. S. J. Heterocycl. Chem. 2007, 44, 1357e1361. 12. Lapkin, I. I.; Orlova, L. D. Zh. Org. Khim. 1970, 6, 68e71. 13. Kozma, L.; Hornak, I.; Eroshtak, I.; Nemet, B. J. Appl. Spectrosc. 1990, 53, 851e855. 14. Hisaindee, S.; Zahid, O.; Meetani, M. A.; Graham, J. J. Fluoresc. 2012, 22, 677e683. 15. CCDC 938828 contains the supplementary crystallographic data for compound 5. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/ conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (0044) 1223-336-033; or e-mail: [email protected]. Crystal data: C19H20O6, Mw¼344.35, monoclinic, C2/ c, Z¼4, a¼11.6206(4), b¼8.7856(3), c¼16.8516(5) A, a¼g¼90 , b¼95.761(2) , Dcalcd¼1.336 g cm3, T¼150(2) K, F(000)¼728, m¼0.100 mm1, 7654 reflections were corrected, 2297 unique (Rint¼0.0541), 1787 observed (I>2s (I)), R1¼0. 0396, wR2¼0.1016. 16. (a) Matsui, M.; Shibata, T.; Fukushima, M.; Kubota, Y.; Funabiki, K. Tetrahedron 2012, 68, 9936e9941; (b) Shirai, K.; Matsuoka, M.; Fukunishi, K. Dyes Pigm. 1999, 42, 95e101; (c) Shirai, K.; Matsuoka, M.; Matsumoto, S.; Shiro, M. Dyes Pigm. 2003, 56, 83e87. 17. Guieu, S.; Rocha, J.; Silva, A. M. S. Tetrahedron Lett. 2013, 54, 2870e2873. 18. Sheldrick, G. M. SADABS V.2.01, Bruker/Siemens Area Detector Absorption Correction Program; Bruker AXS: Madison, WI, 1998. € ttingen: Germany, 2008. 19. Sheldrick, G. M. SHELXS-97; University of Go 20. Delogu, G.; Podda, G.; Corda, M.; Fadda, M. B.; Fais, A.; Era, B. Bioorg. Med. Chem. Lett. 2010, 20, 6138e6140.