NLO chromophores containing dihydrobenzothiazolylidene and dihydroquinolinylidene donors with an azo linker: Synthesis and optical properties

Dyes and Pigments 98 (2013) 82e92

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NLO chromophores containing dihydrobenzothiazolylidene and dihydroquinolinylidene donors with an azo linker: Synthesis and optical properties Mohamed Ashraf a, Ayele Teshome b, Andrew J. Kay a, *, Graeme J. Gainsford a, M. Delower H. Bhuiyan a, Inge Asselberghs b, Koen Clays b a b

Photonics, Industrial Research Ltd., P.O. Box 31-310, Lower Hutt 5040, New Zealand Department of Chemistry, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 December 2012 Received in revised form 6 January 2013 Accepted 7 January 2013 Available online 24 January 2013

We report the synthesis of a series of nonlinear optical chromophores containing either a dihydrobenzothiazolylidene or dihydroquinolinylidene donor with an azo linker. The results demonstrate the versatility of coupling a diazonium salt to a donoremethylidene nucleus in order to access novel materials with good thermal stabilities and high nonlinear optical responses. From hyperRaleigh scattering studies it was found that the use of the well-known TCF acceptor yields the best performing chromophores, one of which has a dynamic first hyperpolarizability of 1900
1030 esu at 800 nm. The results from this study suggest the use of pro-aromatic donors such as dihydrobenzothiazolylidene or dihydroquinolinylidene results in compounds with higher nonlinear optical responses than those containing donors based on either indoline or aniline. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Nonlinear optics Chromophore Hyperpolarizability Hyper-Raleigh scattering Azo dyes X-Ray crystallography

1. Introduction Organic molecules and materials possessing large second-order optical nonlinearities continue to be of interest due to their potential use in optical communications, information storage, optical switching and photonic imaging and sensing [1,2,3]. The basic design strategy successfully used by many researchers involves the attachment of strong donor (D) and acceptor (A) groups at opposing ends of a p-conjugated bridge. This results in highly polarizable molecules which exhibit large molecular optical nonlinearities. Over the past two decades a number of different p-conjugated bridges have been investigated in order determine the extent to which they impact both the nonlinear optical (NLO) response, as well as other parameters such as thermal stability, photochemical stability and processability/solubility. Common conjugated interconnects include olefins [4,5], acetylenes [6], aromatic and heteroaromatic rings [7,8,9], oxadiazole systems [10] and azo groups [11]. Thus, there is often a trade-off to be made, insofar as developing pushepull polyenes with large first hyperpolarizabilities (b) * Corresponding author. Tel.: þ64 4 931 3210; fax: þ64 4 931 3306. E-mail address: [email protected] (A.J. Kay). 0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dyepig.2013.01.010

typically results in a reduction in the inherent stability or processability due to the presence of either open chain double bonds or aryl groups respectively. However, despite the vast range of possibilities, there are some strategies for designing effective NLO materials that consistently give good results. Among these are the incorporation of azo linkers into the conjugated interconnect. Consequently a number of such DepeA systems have been investigated, with many azo-containing systems showing improved nonlinear optical performance [12] and thermal stability [13] when compared to the olefinic analogues. Furthermore, over the past two decades, azobenzene/azoheterocycle containing polymers have been the subject of intensive research in optical switching [14], and digital and holographic storage applications [15]. Thus they represent a useful class of compound to study as they hold promise for applications beyond just nonlinear optics. With the above in mind, we recently described the synthesis and optical and nonlinear optical properties of a suite of NLO chromophores containing an indoline donor and azo linker, e.g. 1ae c [16]. It was found that the compounds had dynamic first hyperpolarizability values, bzzz, of between 210 and 1640
1030 esu when measured at a fundamental wavelength of 800 nm. In particular the highest value was found for 1c, which contains the very

M. Ashraf et al. / Dyes and Pigments 98 (2013) 82e92

well studied “TCF” acceptor system, 4,4,5-trimethyl-3-cyano2(5H)-furanylidenepropanedinitrile. Another interesting feature was that both the static (b0 ¼ 900
1030 esu) and dynamic (bzzz ¼ 1640
1030 esu at 800 nm) hyperpolarizability values measured for 1c were significantly higher than an analogous compound, 2, which contains an aniline e as opposed to indoline e donor unit (b0 ¼ 343
1030 esu and bzzz ¼ 682
1030 esu at 800 nm). This was somewhat surprising given the conjugation length in the indoline systems is shorter by two sp2 hybridised carbon atoms, and we proposed that it was likely due to the aromatic versus non-aromatic nature of the aniline and indoline nuclei respectively. Based on these results it was a logical progression for us to explore the effect of replacing the indoline ring in series 1 with other donor units, and in particular pro-aromatic systems. Consequently in this paper we report on the synthesis and properties on a series of chromophores analogous to 1aec that have either a benzothiazolylidene (BTZ) or 4-quinolinylidene (QUIN) donor. Indeed these are particularly useful comparators. The BTZ ring is structurally very similar to that of indoline, save for the sulfur atom that will now confer “pro-aromatic” character. Likewise the QUIN donor has one more double bond than the indoline ring, but an identical conjugation length to aniline species 2; however once again the QUIN nucleus is pro-aromatic. In order to ensure valid comparisons with previously reported compounds we have retained the azo-phenyl-vinylidene spacer between the donor and acceptor moieties.

2. Experimental 2.1. Reagents and procedures All reagents were obtained from Aldrich and were used without further purification. Analytical grade solvents were used and also without additional purification. Thin-layer chromatography (TLC) was performed on pre-coated plates (Merck aluminium sheets, silica gel 60F 254, 0.2 mm). Column chromatography was conducted on Merck silica gel 9385 (230e400 mesh) with the stated solvent systems.

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2.2. Measurements and instrumentation Melting points were recorded using an EZ-Melt automated melting point apparatus and are uncorrected. The melting point values given represent the onset of melting. 1H and 13C NMR spectra were recorded on a Bruker AVANCE 300 MHz or 500 MHz spectrometer and proton multiplicities are defined by the usual notations. Accurate mass measurements were made on a Micromass Q-Tof Premier Mass Spectrometer operating in the positive ion mode. The UVeVisible (UVeVis) absorption spectra were recorded by a PerkineElmer Lambda 900 spectrometer at room temperature. Hyper-Rayleigh scattering (HRS) measurements were performed at 800 nm in both THF and DMF. Crystal violet dissolved in methanol was used as an octupolar external reference (bxxx ¼ 338
1030 esu at 800 nm). For all of the HRS measurements, a series of dilute solutions were measured and compared to the reference concentration series. To correct for the differences in the solvent between the chromophores and the reference compound, local field correction factors were applied [(n2 þ 2)/3]3 where n is the refractive index of the solvent at the sodium D line, with n(DMF) ¼ 1.43, n(THF) ¼ 1.41 and n(MeOH) ¼ 1.32. The calculations of the dynamic first hyperpolarizability were performed by taking the ratio of the slopes of the sample and the reference compound. For dynamic first hyperpolarizability values obtained at 800 nm it is necessary to consider the appropriate tensor components (dipolar versus octupolar geometry) as the

reference compound crystal violet is octupolar whereas the chromophores under investigation are dipolar. The static first hyperpolarizability is derived from the simple two-level model as previously described [17]. The apparatus and experimental procedures for the HRS measurements are exactly the same as those described before [18]. 2.3. Preparation of chromophores The methods used to prepare the chromophores are shown in Schemes 1e4. 4-Aminobenzaldehyde and compounds 11b and

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Scheme 1. Methodology used to prepare the donoremethylidene precursor compounds.

11c were prepared according to the literature procedures [19]. As part of this study we also attempted to obtain X-ray crystallographic data for the target chromophores, but we were not able to grow suitable crystals (possibly due to the presence of the Ndecyl group). However we found that when we replaced the Ndecyl group in 12b with a N-pentyl group to give 14 we were able to grow X-ray quality crystals. The synthesis of 14 is also described. 2.4. Preparation of quarternary salts 6 and 8 The appropriate donor moiety, 5 or 7 (250 mmol), was dissolved in dry acetonitrile (100 mL) to which an iododecane (69.68 g, 260 mmol) was then added. The reaction mixture was heated under reflux for 3 days in the case of 2-methylbenzothiazole (5) and overnight with 4-methylquinoline (7). The solvent was removed under reduced pressure and the residual solid slurried with diethyl ether. This was collected by filtration, washed 2e3 times with diethyl ether and dried under vacuum to afford a white solid in quantitative yield. The products were purified by recrystallization from ethanol.

2.4.1. 3-Decyl-2-methylbenzothiazolium iodide (6) M.p. 176.8  C. (Found: Mþ m/z 290.1156 C18H28NS requires Mþ m/z 290.1160; D ¼ 1.8 ppm). 1H NMR (500 MHz, DMSO-d6): d 8.52 (d, 1H, J 10 Hz), 8.38 (d, 1H, J 10 Hz), 7.91 (m, 1H), 7.82 (m, 1H), 4.75 (m, 2H), 3.28 (s, 3H), 1.88 (m, 2H), 1.46e1.34 (m, 14H), 0.89 (t, 3H). 13 C NMR (75.4 MHz, DMSO-d6): d 16.46, 19.99, 24.44, 30.25, 30.67, 52.03, 119.62, 127.39, 130.75, 131.74, 132.07, 143.07, 179.59. 2.4.2. 1-Decyl-4-methylquinolinium iodide (8) M.p. 198.3  C. (Found: Mþ m/z 284.1600 C20H30N requires Mþ m/ z 284.1596; D ¼ 1.9 ppm). 1H NMR (500 MHz, DMSO-d6): d 9.53 (d, 1H, J 10 Hz), 8.66 (d, 1H, J 10 Hz), 8.58 (d, 1H, J 10 Hz), 8.30 (m, 1H), 8.13 (d, 1H, J 10 Hz), 8.09 (m, 1H), 5.07 (m, 2H), 3.05 (s, 3H), 1.99 (m, 2H), 1.42e1.33 (m, 14H), 0.88 (t, 3H). 13C NMR (75.4 MHz, DMSOd6): d 16.46, 19.99, 24.44, 30.25, 30.67, 52.03, 119.62, 127.39, 130.75, 131.74, 132.07, 143.07, 179.59. 2.5. Preparation of methylidene donor precursors (3) and (4) To a stirred solution of the appropriate quaternary salt (50 mmol) was added distilled water (100 mL). After all the solid

Scheme 2. Coupling of the diazonium salt of 4-aminobenzaldehyde to the donor-methylidene units.

M. Ashraf et al. / Dyes and Pigments 98 (2013) 82e92

85

Scheme 3. Coupling of acceptor units to aldehyde containing donor-linker systems.

had dissolved, the solution was filtered to remove any traces of insoluble material. To the filtrate was added dropwise a solution of NaOH (4 g, 100 mmol) in distilled water (20 mL) and the resulting mixture was stirred at room temperature for 5 h. To this was added a further portion of 20% NaOH in water (10 mL) and the solution stirred for another 2 h. The product, which formed a reddish or violet oil, was extracted with ether, the solution dried (Na2SO4) and the solvent removed under reduced pressure. The oily product was

purified by column chromatography using 1:1 dichloromethane:light petroleum as eluent. 2.5.1. 3-Decyl-2-methylene-2,3-dihydro-benzothiazole (3) Dark red oil (60%). (Found: MHþ m/z 290.1961 C18H28NS requires MHþ m/z 290.1960; D ¼ 0.5 ppm). 1H NMR (500 MHz, CDCl3): d 6.81 (t, 1H), 6.73 (d, 1H, J 5 Hz), 6.63 (m, 1H), 6.31 (d, 1H, J 5 Hz), 4.55 (s, 2H), 3.02 (t, 2H), 1.90 (m, 2H), 1.50e1.30 (m, 14H), 0.88 (t, 3H). 13C

Scheme 4. Preparation of compound 14 for X-ray crystallographic studies.

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NMR (75.4 MHz, CDCl3): d 14.03, 22.53, 27.52, 27.73, 29.17, 29.26, 29.91, 32.60, 44.66, 53.46, 80.90, 108.14, 117.47, 119.71, 126.87, 132.85, 143.66, 149.28. 2.5.2. 1-Decyl-4-methylene-1,4-dihydro-quinoline (4) Dark violet oil (30%). (Found: MHþ m/z 284.1906 C20H30N requires MHþ m/z 284.1913; D ¼ 2.3 ppm). 1H NMR (500 MHz, CDCl3): d 7.21 (t, 1H), 6.98 (d, 1H, J 5 Hz), 6.82 (m, 1H), 6.41 (d, 1H, J 5 Hz), 6.21 (d, 1H, J 5 Hz), 5.88 (d, 1H, J 5 Hz), 5.38 (d, 2H, J 5 Hz), 4.55 (m, 2H), 1.94 (m, 2H), 1.50e1.22 (m, 14H), 0.88 (t, 3H). 13C NMR (75.4 MHz, CDCl3): d 14.04, 24.11, 26.86, 29.10, 29.30, 29.44, 30.21, 31.78, 49.37, 57.99, 109.23, 109.96, 116.08, 117.04, 119.03, 127.02, 129.04, 132.57, 142.73, 158.03.

2.6.2. 4-[(1-Decyl-1H-quinolin-4-ylidenemethyl)-azo]benzaldehyde (10) Dark violet oil (48%). (Found: MHþ m/z 416.2598 C27H34N3O requires MHþ m/z 416.2596; D ¼ 0.4 ppm). 1H NMR (500 MHz, CDCl3): d 9.97 (s, 1H), 9.32 (d, 1H, J 5 Hz), 8.21 (m, 2H), 7.84 (d, 2H, J 10 Hz), 7.67 (m, 2H), 7.64 (d, 2H, J 10 Hz), 7.24 (m, 1H), 6.80 (d, 1H, J 5 Hz), 4.01 (m, 2H), 1.82 (m, 2H), 1.42e1.22 (m, 14H), 0.88 (t, 3H). 13C NMR (75.4 MHz, DMSO-d6): d 14.00, 22.12, 26.02, 26.48, 27.00, 28.49, 28.84, 31.21, 46.04, 57.08, 112.18, 112.90, 113.86, 117.32, 119.67, 120.90, 121.62, 122.39, 124.07, 125.82, 127.22, 128.10, 129.65, 130.27, 132.90, 141.59, 191.59. 2.7. Preparation of chromophores 12aec and 13aec

2.6. Diazotisation: general procedure for preparation of azo-aldehydes 9 and 10 To concentrated sulfuric acid (4 mL) was added 4-aminobenzaldehyde (0.302 g, 2.5 mmol) and the reaction mixture cooled to 0e5  C. A solution of sodium nitrite (206 mg, 3 mmol) in water (2 mL) was added slowly and the reaction stirred at 0e5  C for 30 min. To this was added a solution of either 5 or 7 (2 mmol) in glacial acetic acid (10 mL), the mixture stirred for an additional 2 h and then poured into water and neutralized with aqueous sodium carbonate. The resulting oil was extracted with dichloromethane, dried (MgSO4) and concentrated under reduced pressure. The final product was obtained following purification by column chromatography by using 1:1 dichloromethane:light petroleum as an eluent. 2.6.1. 4-[(3-Decyl-3H-benzothiazol-2-ylidenemethyl)-azo]benzaldehyde (9) Dark red oil (60%). (Found: MHþ m/z 422.2263 C25H32N3OS requires MHþ m/z 422.2266; D ¼ 0.7 ppm). 1H NMR (500 MHz, CDCl3): d 9.95 (s, 1H), 7.87 (d, 2H, J 10 Hz), 7.72 (d, 2H, J 10 Hz), 7.52 (s, 1H), 7.38e7.32 (m, 2H), 7.21 (m, 1H), 7.18 (m, 1H), 4.01 (m, 2H), 1.82 (m, 2H), 1.42e1.22 (m, 14H), 0.88 (t, 3H). 13C NMR (75.4 MHz, DMSO-d6): d 13.88, 22.02, 25.92, 26.48, 27.00, 28.60, 28.84, 31.21, 45.97, 57.20, 112.21, 117.32, 119.67, 121.62, 122.45, 124.07, 125.79, 127.97, 129.65, 130.27, 130.86, 132.00, 132.90, 141.69, 191.59.

The appropriate aldehyde 9 or 10 (1.1 eq) and the corresponding acceptor 11aec (1 eq) were dissolved in methanol (c. 50 mL per gram of acceptor) and a catalytic quantity of triethylamine added. The mixture was refluxed for 6 h, and resulted in a change in the colour of the mixture from either red to purple or dark blue depending on the acceptors used. The methanol was then removed under reduced pressure, the residue washed with water and taken up into dichloromethane and dried (MgSO4). The solvent was removed at reduced pressure the resulting solid was purified by column chromatography. 2.7.1. 2-{4-[(3-Decyl-3H-benzothiazol-2-ylidenemethyl)-azo]benzylidene}-malononitrile (12a) Brownish-violet solid (61%), purification by column chromatography on silica gel (dichloromethane:petroleum ether 4:1); m.p. 121.9  C. (Found: MHþ m/z 470.2380 C28H32N5S requires MHþ m/z 470.2378; D ¼ 0.4 ppm). 1H NMR (500 MHz, CDCl3): d 7.72 (d, 2H, J 10 Hz), 7.55 (d, 2H, J 10 Hz), 7.42 (d,1H, J 5 Hz), 7.35e7.30 (m, 2H), 7.18 (d, 1H, J 5 Hz), 7.11 (m, 1H), 6.75 (s, 1H), 4.01 (m, 2H), 1.82 (m, 2H), 1.42e1.22 (m, 14H), 0.88 (t, 3H). 13C NMR (75.4 MHz, DMSO-d6): d 13.88, 20.91, 22.02, 25.98, 26.93, 28.61, 28.83, 31.22, 46.20, 57.20, 78.9, 113.40, 116.80, 118.10, 118.40, 118.82, 119.42, 127.17, 127.50, 127.90, 129.21, 129.38, 130.20, 135.20, 141.77, 146.50, 156.70, 165.00. lmax (DMF) 484, log10ε 4.37.

Table 1 Linear, nonlinear optical and thermal properties of the compounds prepared in this study. For comparative purposes data obtained for 1aec and 2 in earlier studies are also given [16,22]. Dynamic first hyperpolarizability bzzz values were determined at a fundamental wavelength of 800 nm; static hyperpolarizability bzzz,0 values were obtained using the two level model. Calculated dipole moments and the corresponding figures of merit are also included. Compound

Solvent

lmax (nm)

Log10ε

bzzz (1030 esu)

bzzz,0 (1030 esu)

12a

THF DMF THF DMF THF DMFa THF DMF THF DMF THF DMFa THF DMF THF DMF THF DMF DMSO

477 484 556 569 637 e 539 550 520 564 572 e 540 549 525 534 590 601 e

4.35 4.37 4.79 4.76 4.86 e 4.89 4.85 4.79 5.02 4.57 e 4.95 4.69 4.79 4.67 4.75 5.04 e

370  20 320  25 800  30 600  30 1900  100 e 840  25 680  25 600  40 960  30 1280  60 e 710  20 550  25 695  25 650  25 1060  55 1640  155 682

100  5 95  5 380  15 300  15 1100  50 e 450  10 290  10 230  15 480  20 650  30 e 320  10 260  10 290  10 280  10 570  30 900  85 343

12b 12c 13a 13b 13c 1a 1b 1c 2 a b

These two compounds were not stable in DMF. MOPAC (CS Chem Draw Pro), AM1 Level using PRECISE keyword.

mb (calc) (1018 esu) 8.3 7.2 14.0 10.2 8.5 15.7 7.1 5.6 12.6 e

m(calc).bzzz (1048 esu)

Td  C

3071 2656 5760 4320 26600 e 8568 6936 5100 8160 20096 e 5014 3905 3892 3640 13356 20664 e

221.2 232.8 248.1 215.2 220.0 230.5 230.0 245.0 270.0 e

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2.7.2. 5-{4-[(3-Decyl-3H-benzothiazol-2-ylidenemethyl)-azo]benzylidene}-3-(4-hydroxy-phenyl)-2-thioxo-thiazolidin-4-one (12b) Violet solid (63%), purification by column chromatography on silica gel (dichloromethane); m.p. 221.6  C. (Found: MHþ m/z requires MHþ m/z 629.2079; 629.2073C34H37N4O2S3 D ¼ 1.0 ppm). 1H NMR (500 MHz, CDCl3) d 7.79 (s, 1H), 7.73 (d, 2H, J 10 Hz), 7.70 (s, 1H), 7.54 (d, 2H, J 10 Hz), 7.53 (s, 1H), 7.36 (m, 1H), 7.22 (m, 2H), 7.15 (d, 1H, J 5 Hz), 7.13 (d, 2H, J 10 Hz), 6.97 (d, 2H, J 10 Hz), 4.00 (m, 2H), 1.82 (m, 2H), 1.44e1.24 (m, 14H), 0.88 (t, 3H). 13 C NMR (75.4 MHz, DMSO-d6): d 13.90, 22.04, 25.95, 26.87, 28.63, 28.85, 28.88, 31.24, 45.70, 53.30, 110.92, 111.74, 115.75, 119.28, 120.04, 120.33, 120.90, 122.23, 123.63, 125.61, 126.06, 127.01, 129.69, 129.90, 132.09, 132.32, 132.58, 141.73, 155.10, 158.10, 163.66, 167.05, 193.70. lmax (DMF) 569, log10ε 4.76. 2.7.3. 2-[3-Cyano-4-(2-{4-[(3-decyl-3H-benzothiazol-2ylidenemethyl)-azo]-phenyl}-vinyl)-5,5-dimethyl-5H-furan-2ylidene]-malononitrile (12c) Brown solid (63%), purification by column chromatography on silica gel (dichloromethane:methanol 9:1); m.p. 201.4  C. (Found: MHþ m/z 603.2900C36H39N6OS requires MHþ m/z 603.2906; D ¼ 1.0 ppm). 1H NMR (500 MHz, CDCl3) d 7.74 (s, 1H), 7.68 (m, 1H), 7.62 (d, 2H, J 10 Hz), 7.56 (d, 2H, J 10 Hz), 7.40 (m, 1H), 7.25 (m, 1H), 7.19 (d, 2H, J 5 Hz), 6.95 (d, 1H, J 15 Hz), 4.07 (m, 2H), 1.85 (m, 2H), 1.79 (s, 6H), 1.48e1.28 (m, 14H), 0.88 (t, 3H). 13C NMR (75.4 MHz, DMSO-d6): d 13.88, 22.02, 25.33, 25.92, 27.04, 28.61, 28.83, 30.58, 30.82, 31.22, 45.97, 53.33, 96.91, 98.92, 111.20, 112.06, 112.21, 112.86, 113.08, 119.80, 120.44, 122.44, 123.37, 124.08, 125.86, 127.23, 131.46, 138.59, 141.71, 147.41, 147.76, 164.24, 175.02, 177.02. lmax (THF) 637, log10ε 4.86.

Fig. 1. a and b: UVevis absorption spectra of chromophores 12aec and 13aec in THF.

2.7.4. 2-{4-[(1-Decyl-1H-quinolin-4-ylidenemethyl)-azo]benzylidene}-malononitrile (13a) Dark brown solid (50%), purification by column chromatography on silica gel (dichloromethane); m.p. 115.2  C. (Found: MHþ m/z 464.2690 C30H34N5 requires MHþ m/z 464.2688; D ¼ 0.4 ppm). 1H NMR (500 MHz, CDCl3): d 8.45 (m, 1H), 8.21(m, 2H), 7.86 (d, 2H, J 10 Hz), 7.78 (m, 3H), 7.38 (m, 3H), 6.62 (d, 1H, J 5 Hz), 4.01 (m, 2H), 1.84 (m, 2H), 1.42e1.22 (m, 14H), 0.90 (t, 3H). 13C NMR (75.4 MHz, DMSO-d6): d 13.88, 20.87, 21.58, 22.02, 26.01, 26.83, 28.61, 28.96, 31.22, 55.80, 78.20, 113.08, 115.98, 117.98, 118.08, 118.92, 119.42, 127.21, 127.48, 127.90, 128.20, 128.50, 129.10, 129.30, 130.20, 135.28, 141.77, 146.48, 156.65, 165.20. lmax (DMF) 550, log10ε 4.85.

Table 2 Crystallographic and structure refinement data for 14. Asymmetric unit

C58 H60 N4 O8S6

m, mm1

2.81

Moiety formula MW Temperature (K) Wavelength ( A) Crystal system, Space group Refinement method a,  A b,  A c,  A a, deg. g, deg. Volume,  A3 Z P, Mg m3

2(C29 H26 N4 O2 S3).4H2O 1189.50 123(2) 1.54184 Triclinic P-1 Full matrix least-squares on F2 11.7058(13) 15.675(2) 17.281(2) 68.779(13) 69.883(12) 2768.1(6) 2 1.427

Limiting indices Crystal size (mm) Theta range deg. Reflections collected No of unique data Rint P1, P2 coefficients of weighting schemea Absorp. Coeff. range Restraints No. of parameters b, deg. Goodness-of-fit on F2 R1b,c/data number wR2c i (all data) Largest diff. peak and hole (e A3)

13  h  13, 17  k  18, 0  l  20 0.28
0.17
0.04 3.2e75.5 8656 4647 0.24 0.166000, 3.505400 0.512, 0.896 10 772 79.145(10) 1.02 0.094, 4647 0.319 1,23 and 0.44

a b c

Weight, w ¼ 1/[s2(Fo2) þ (P1
P)2] where P ¼ (Max(Fo2, 0) þ 2
Fc2)/3. Intensities 2.0 times their standard deviations (from counting statistics). R1 ¼ SjjFoj  Fcjj/SjFoj; wR2 ¼ S[w(F2o  F2c )2]/S[(wF2o)2]]1/2.

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Table 3 Selected bond distances ( A) in 14 and related compound 1b [16]. Bond

A

B

Average 1b

N2eN3 C13eN2 N3eC14 C17eC20 C20eC21 S2eC21 S2eC22 C22eS3 C27eO2

1.312(8) 1.301(8) 1.404(9) 1.443(9) 1.348(11) 1.753(8) 1.760(8) 1.635(7) 1.347(8)

1.288(8) 1.347(10) 1.394(9) 1.450(10) 1.330(11) 1.723(8) 1.752(8) 1.638(8) 1.334(8)

1.318(11) 1.388(11) 1.412(12) 1.462(13) 1.409(13) 1.718(11) 1.763(11) 1.597(11) 1.423(12)

2.7.5. 5-{4-[(1-Decyl-1H-quinolin-4-ylidenemethyl)-azo]-benzyl}3-(4-hydroxy-phenyl)-2-thioxo-thiazolidin-4-one (13b) Brownish-pink solid (51%), Purification by column chromatography on silica gel (dichloromethane:methanol 4.5:0.5); m.p. 210.2  C. (Found: MHþ m/z 623.2376 C36H39N4O2S2 requires MHþ m/z 623.2382; D ¼ 1.0 ppm). 1H NMR (500 MHz, CDCl3) d 9.22 (d, 1H, J 5 Hz), 7.82 (s, 1H), 7.73 (d, 2H, J 10 Hz), 7.70 (s, 1H), 7.54 (d, 2H, J 10 Hz), 7.53 (s, 1H), 7.36 (m, 1H), 7.22 (m, 2H), 7.15 (d, 1H, J 5 Hz), 7.13 (d, 2H, J 10 Hz), 6.97 (d, 2H, J 10 Hz), 6.62 (d, 1H, J 5 Hz), 4.00 (m, 2H), 1.82 (m, 2H), 1.44e1.24 (m, 14H), 0.88 (t, 3H). 13C NMR (75.4 MHz, DMSO-d6): d 14.02, 22.04, 26.00, 26.87, 28.28, 28.65, 28.90, 31.24, 45.66, 53.32, 110.88, 111.68, 116.02, 119.28, 120.04, 120.30, 120.90, 122.19, 123.58, 125.61, 125.98, 127.01, 129.70, 129.90, 132.09, 132.26, 132.58, 141.73, 155.10, 158.10, 163.72, 166.98, 193.68. lmax (DMF) 564, log10ε 5.02. 2.7.6. 2-[3-Cyano-4-(2-{4-[(1-decyl-1H-quinolin-4ylidenemethyl)-azo]-phenyl}-vinyl)-5,5-dimethyl-5H-furan-2ylidene]-malononitrile (13c) Green solid (54%), Purification by column chromatography on silica gel (dichloromethane:methanol 4:1); m.p. 196.8  C. (Found: MHþ m/z 597.3300 C38H41N6O requires MHþ m/z 597.3306; D ¼ 1.0 ppm). 1H NMR (500 MHz, CDCl3) d 7.89 (d, 2H, 10 Hz), 7.82 (m, 1H), 7.68 (m, 1H), 7.62 (m, 5H), 7.58 (d, 1H, J 15 Hz), 7.28 (d, 1H, J 5 Hz), 7.01 (d, 1H, J 15 Hz), 6.64 (d, 1H, J 5 Hz), 4.07 (m, 2H), 1.85 (m, 2H), 1.80 (s, 6H), 1.48e1.28 (m, 14H), 0.88 (t, 3H). 13C NMR (75.4 MHz, DMSO-d6): d 12.98, 22.22, 24.99, 25.92, 27.14, 28.61, 28.86, 30.48, 30.98, 31.10, 45.93, 53.33, 96.85, 98.92, 100.12, 104.98, 106.22, 111.20, 112.06, 112.21, 112.98, 113.10, 119.80, 120.44, 122.44, 123.37, 124.08, 126.02, 127.23, 131.46, 138.63, 141.90, 147.41, 147.76, 163.96, 175.12, 177.02. lmax (THF) 572, log10ε 4.57.

(s, 1H), 7.92 (d, 2H, J 10 Hz), 7.68 (d, 2H, J 10 Hz), 7.42 (s, 1H), 7.37e 7.33 (m, 2H), 7.21 (m, 1H), 7.18 (m, 1H), 4.30 (m, 2H), 1.79 (m, 2H), 1.40e1.30 (m, 4H), 0.88 (t, 3H). 13C NMR (75.4 MHz, DMSO-d6): d 13.75, 21.87, 28.45, 30.61, 45.70, 48.31, 111.29, 114.98, 117.71, 126.58, 129.32, 129.62, 130.79, 131.12, 134.51, 136.84, 139.86, 168.84, 192.04. 2.8.3. 3-(4-Hydroxy-phenyl)-5-{4-[(3-pentyl-3H-benzothiazol-2ylidenemethyl)-azo]-benzylidene}-2-thioxo-thiazolidin-4-one (14) Reddish-brown solid (58%), Purified by column chromatography on silica gel (dichloromethane); m.p. 227.3  C. (Found: MHþ m/z 559.1299 C29H27N4O2S3 requires MHþ m/z 559.1296; D ¼ 0.5 ppm). 1 H NMR (500 MHz, CDCl3) d 9.89 (s, 1H), 7.95 (s, 1H), 7.90 (m, 1H), 7.78 (s, 1H), 7.69 (m, 3H), 7.59 (d, 2H, J 10 Hz), 7.52 (m, 1H), 7.38 (m, 1H), 7.16 (d, 2H, J 10 Hz), 6.89 (d, 2H, J 10 Hz), 4.30 (m, 2H), 1.79 (m, 2H), 1.40e1.30 (m, 4H), 0.88 (t, 3H). 13C NMR (75.4 MHz, DMSO-d6): d 13.80, 18.82, 21.84, 27.00, 28.13, 41.80, 46.41, 51.12, 61.67, 63.23, 110.93, 112.76, 115.28, 119.73, 120.57, 122.98, 124.62, 126.06, 127.55, 129.72, 132.29, 149.46, 158.11, 162.64, 167.08, 193.79. X-ray quality crystals were grown by slow evaporation from a mixture 1:1 mixture of methanol and chloroform. 3. Results and discussion 3.1. Synthesis The synthetic methodology used to prepare chromophores 12aec and 13aec is shown in Schemes 1e3. In line with our previous work using indoline based chromophores, the diazonium salt of 4-aminobenzaldehyde was coupled to the methylidene analogue of the donor unit e in this case compounds 3 and 4 e to give the key aldehyde containing synthons 10 and 11. The success of these diazonium coupling reactions is presumably assisted by the strong electron donating ability of the donor rings, and confirms the utility of this methylidene-diazonium method as a means to easily build such “donor-linker” units. It is worth noting that compounds 3 and 4 were in turn prepared by dehydrohalogenation of the quarternary salts of 6 and 8 using 20% aqueous NaOH at room temperature (Scheme 1) and these molecules can be considered direct analogous of Fischer’s base, which we successfully used to prepare a suite of indoline containing compounds. The final chromophores were then readily accessible via the condensation of the appropriate acceptor system, 11aec with 10 or 11. The final products were all obtained as

2.8. Synthesis of N-pentyl derivative 14 Compound 14 and its precursors were prepared using exactly the same methodology described above. Thus compounds 3,6 and 9 were replaced with their N-pentyl analogues. 2.8.1. 2-Methylene-3-pentyl-2,3-dihydro-benzothiazole Starting from 2-methyl-3-pentyl-benzothiazoliumiodide [20]. Purified by column chromatography using 1:1 dichloromethane:light petroleum as eluent. Deep red oil (50%). (Found: MHþ m/z 220.1155 C13H18NS requires MHþ m/z 220.1160; D ¼ 2.3 ppm). 1H NMR (500 MHz, CDCl3): d 6.81 (m, 1H), 6.73 (d, 1H, J 5 Hz), 6.63 (t, 1H), 6.31 (d,1H, J 5 Hz), 4.50 (s, 2H), 4.30 (m, 2H),1.79 (m, 2H),1.40e1.30 (m, 4H), 0.88 (t, 3H). 13C NMR (75.4 MHz, CDCl3): d 13.77, 21.96, 29.12, 31.84, 54.90, 88.03, 113.56, 117.21, 120.79, 127.26, 133.36, 148.01, 160.05. 2.8.2. 4-[(3-Pentyl-3H-benzothiazol-2-ylidenemethyl)-azo]benzaldehyde Red oil (55%). (Found: MHþ m/z 352.1484C20H22N3OS requires MHþ m/z 352.1484; D ¼ 0.0 ppm). 1H NMR (500 MHz, CDCl3): d 10.00

Fig. 2. Structure of molecule B in 14 (with 20% probability ellipsoids) [26].

M. Ashraf et al. / Dyes and Pigments 98 (2013) 82e92

89

coloured solids that were readily purified by column chromatography, and this was no doubt aided by the inclusion of the decyl groups on the donor nitrogen atoms which are essential for pre-

aromatic group in the linker between the donor and acceptor as this will result in a loss of aromatic stabilisation energy should the ground state be zwitterionic.

paring dyes that are sufficiently soluble in organic solvents. The chromophores were fully characterised by NMR, high resolution mass spectrometry and UVevis data. A feature to note is that NMR data indicate that all of 12aec and 13aec exist as a single isomer and that the rotational isomerisation sometimes present when asymmetrical acceptors are used (e.g. 11b,c) is not observed [21]. In order to assess the likely stability of the chromophores their thermal decomposition temperatures (Td) were measured and the results are presented in Table 1. Pleasingly, all of the compounds studied have acceptable thermal stabilities as their Td’s are all higher than 215  C, and this confirms their suitability for poling at high temperatures, i.e. up to 180  C. Among all of the chromophores 12c is the best performed with a Td of 248  C, although this is some 22  C less than its indoline analogue (1c, Td 270  C). Moreover, combining the results of this study with our previous work there is a clear improvement in thermal stability in going from the quinoline to benzothiazole to indoline donor systems.

The absorption maxima of the BTZ compounds 12b and 12c in THF are red shifted by 36 nm and 65 nm respectively when compared to the corresponding QUIN compounds 13b and 13c. It is well established that a reduction in the energy difference between the ground and excited states typically corresponds to an increase in the molecular hyperpolarizability, b; thus for these four compounds it would be expected that the two compounds containing the BTZ donor would give the superior NLO response. However, contrary to this trend, compound 12a with benzothiazole donor and malononitrile acceptor units shows on average a 64 nm hypsochromic shift in its absorption maxima when compared to QUIN compound 13a. While the reason for this reversal in absorption maxima is not immediately obvious, it does imply that the ground state of 12a is more charge separated than 13a. This in turn will be due to a combination of factors such as the relative strength of the donore acceptors units, the aromatic stabilisation energy of a benzothiazole versus quinoline ring system and the number of carbon atoms between the donor nitrogen atoms and that of dicyanomethylidene acceptor unit (i.e. 11 for 12a and 13 for 13a). Furthermore, the comparatively high energy absorption maxima seen for 12a also suggests that it will have the lowest molecular hyperpolarisability. It should also be noted that the spectra obtained in THF (Fig. 1) show no evidence of H- or J-aggregation, as the absorption bands show none of the high or low energy shoulders typically associated with such phenomena. This is important as the polarity of THF (ε ¼ 7.5) is similar to that of the polymers in which these chromophores would ultimately need to be deployed (ε 1e9). Such polymers include amorphous polycarbonate, polyimides and polyurethanes. Hence, on this basis, it would appear that all of the chromophores will exhibit little, if any, aggregation when incorporated into host polymers. This in turn suggests they will respond well to poling and better retain a non-centrosymmetric alignment post-poling.

3.2. Linear optical properties The UVevisible spectra of all chromophores were recorded in DMF and THF, and the data is summarised in Table 1. Furthermore, for illustrative purposes, the spectra obtained in THF are shown in Fig. 1a and b. For chromophores 12a,b and 13a,b there is a slight bathochromic shift in the absorption maxima on changing from THF to the more polar DMF, and such positive solvatochromism indicates the chromophores have only a small degree of charge separation in the ground state, and cannot be considered zwitterionic. This needs to be considered alongside the fact that the use of pro-aromatic donors such as BTZ, QUIN and pyridinylidene in combination with the TCF acceptor is known to result in chromophores that display high degrees of negative solvatochromism e.g. 15e17 [21]. Furthermore, X-ray crystallographic studies on analogues of 16 and 17 confirms their zwitterionic nature due to the bond order observed across the corresponding molecular backbones [23]. As UVevis data for 12c and 13c in DMF could not be obtained e these compounds decompose in this solvent e it can only be assumed that the acceptor units deployed in 12a,b and 13b,c are insufficiently powerful to induce a reversal in bond order across the conjugated system. Furthermore, the low degree of charge separation is probably also due to the presence of an

3.3. Quadratic nonlinear optical properties The second-order NLO polarizability (or first hyperpolarizability

b), the static first hyperpolarizability bzzz,0 and the calculated dipole moments m of all the compounds under investigation are listed in

Table 1. It should be noted that there is no fluorescence contribution from the compounds. From the values given in Table 1 it is evident that most of the bzzz and bzzz,0 values are in general agreement with

90

M. Ashraf et al. / Dyes and Pigments 98 (2013) 82e92

Table 4 Hydrogen bond geometries ( A, deg.) for 14. DdH.A

DdH

H.A

D.A

DdH.A

Symmetrya

O2BeH2B...O2A O1WeH1WA...O2B O1WeH1WB...O2W O2WeH2WA...O2B O3WeH3WA...N3B O3WeH3WB...O2A O4WeH4WA...S3B O4WeH4WB...O3W C6AeH6AA...O4W C8AeH8AB...O4W C9BeH9BB...S3B C18AeH18A...O1B C18BeH18B...O1A C20AeH20A...O1B C20BeH20B...O1A C25BeH25B...O3W

0.84 0.93(10) 0.93(10) 0.92(10) 0.93(6) 0.93(12) 0.94(5) 0.9(2) 0.95 0.95 0.99 0.95 0.95 0.95 0.95 0.95

1.83 1.89(10) 1.88(10) 1.76(10) 2.05(7) 2.03(13) 2.71(13) 2.04(19) 2.57 2.43 2.84 2.41 2.42 2.33 2.40 2.42

2.542(7) 2.794(10) 2.775(10) 2.663(10) 2.939(10) 2.899(11) 3.477(11) 2.777(15) 3.426(14) 3.409(17) 3.758(10) 3.259(11) 3.294(11) 3.215(11) 3.298(11) 3.270(13)

142 164(10) 163(10) 165(10) 160(11) 155(11) 139(16) 136(14) 151 171 154 149 153 155 159 149

1  x, x, y, 1 1  x, x, y, 1 x, y, z 1  x, 1  x, 1  x, 1  x, 1  x, 2  x, 1  x, 1  x, 1  x, 1  x, 1  x,

a

y, 1  z þz y, 1  z þz y, z 1  y, z 1  y, 1  z 1  y, 1  z 1  y, 1  z 1  y, z y, z y, z y, z y, z 1  y, z

Symmetry to bring the A atom into contact.

the data obtained from the linear optical properties. Thus the compounds with the lowest energy absorption maxima in THF (12c and 13c) also have the highest dynamic and static hyperpolarizabilities. This supports the two-level model equation insofar as a red-shift in absorption maxima is associated with lower absorption energy and this in turn enhances the b value [17]. Also from Table 1 it is seen that the three compounds containing a TCF acceptor (1c, 12c and 13c) undoubtedly have superior nonlinear optical responses, thus reinforcing the consistently high performance of NLO molecules containing this moiety. Furthermore, with the exception of 12a, there is little difference in NLO response between those compounds containing either a dicyanomethylidene or hydroxyphenylrhodanine acceptor; again this is entirely consistent with the expected trends obtained from the linear absorption data. There is no definitive trend when comparing the various donor units as the highest NLO response (in THF only) is clearly found for the BTZ derivative 12c while for the other two types of acceptor the QUIN donor, on average, is preferable. In addition, both the linear and second-order NLO properties of compounds with a QUIN donor (13a,b) can be more readily tuned by varying the dielectric constant of the surrounding medium than compounds with either a BTZ or IND donor unit (12a,b and 1a,b). In the latter compounds there is a smaller shift in the absorption maxima but its impact on the static first molecular hyperpolarizability is generally not significant. It is also evident that regardless of the acceptor system used, for the most part, the new compounds reported here have superior static and dynamic hyperpolarizabilities when compared to the known aniline containing chromophore 2 (Table 1). However, there is no obvious trend when comparing 12aec and 13aec with their indoline analogues 1aec. Thus, while 1a,b exhibit the second highest hyperpolarizabilities when compared to 12a,b and 13a,b, the use of TCF as acceptor results in 12c and 13c being clearly superior to 1c. As the TCF containing compounds consistently give the highest values this is perhaps the most important comparison and confirms the efficacy of using either a BTZ or QUIN donor. From Table 1 it is also seen that there is a reasonable correlation between the polarity of the solvent used and the magnitude of the b value, and with the exception of 1c and 13b, a decrease in solvent polarity results in an increase in the dynamic and static hyperpolarisability. As noted above, this is an encouraging trend as utilisation of the chromophores would ultimately require their deployment in nonpolar environments such as hosteguest polymer matrices. Furthermore, for the most part, the values of the dynamic hyperpolarizability bzzz are between of 500e1000
1030 esu at 800 nm which is a very respectable range. Coupled with the

calculated dipole moment values, this suggests these compounds are likely to have high figures of merit of approximately 5,000e 25,000
1048 esu, where the figure of merit is simply the product of the dipole moment and first hyperpolarizability, viz. m.b. 3.4. X-ray crystallography We have managed to obtain adequate X-ray crystallographic data for compound 14; Table 2 contains the refinement data whereas Table 3 contains a summary of key bond lengths. The crystals contained water solvent of crystallization (Table 2). The asymmetric unit of 14 contains two independent copies (labeled A & B) of the compound (Fig. 2 shows molecule B) and four water solvate molecules. In molecule A, the butyl atom chain adopts two conformations for the last 3 atoms (10-C12). Excluding the terminal

Fig. 3. Crystal packing of 14 [27]; some of the main attractive contacts are shown as dotted lines (see Table 4). Atom H3A is H3WA. Symmetry (i): 1  x, 1  y, 1  z.

M. Ashraf et al. / Dyes and Pigments 98 (2013) 82e92

4-hydroxyphenyl ring, the molecules are approximately planar, with both lying parallel to the (2,2,1) plane. The terminal 4hydroxyphenyl rings make angles of 67.1(4) (in A) and 65.9(4) (B, Fig. 2) to the 5-membered thioxo rings (S1,C20eC22,N4). The root mean square bond and angle fits between the two molecules, excluding the butyl atoms, are 0.018  A and 1.33 [24]. These similar dimensions (Table 3) confirm that within the limited resolution provided by the analysis there are no significant deviations between the two copies, or with a previously reported compound (1b) [16]. The molecules are not exactly superimposable with molecule A twisting further from planarity around the central C14eC19 ring: the interplanar angles with the 5-membered thioxo rings are 15.9(4) and 10.1(4) respectively (molecules A & B). The molecular contents are bound together via a comprehensive set of Oe H...O(water & hydroxyl), O(water)eH...N(azo), O(water)eH...S, and CeH...O(carbonyl & water) interactions (Table 4; Fig. 3 shows the conventional hydrogen bonding set of these interactions). There are bifurcated interactions involving two acceptors, the hydroxyl atom O2B and water O4W. Molecules lying “side by side” are linked by weaker CeH...O(]C) interactions completing R12(6), R12(7) and R22(10) ring motifs [25] The overall packing is thus based on a full 3dimensional interaction set through the water solvate molecule interactions. Lastly it needs to be noted that the molecules of 14 are packed centrosymmetrically in the crystal structure, meaning that the crystals will not exhibit a macroscopic NLO response. Rather 14 e and presumably all the other chromophores e will need to be incorporated into polymeric films and subjected to poling in order to have the desired macroscopic response. 4. Conclusions We have successfully synthesised a series of novel NLO chromophores containing either a dihydrobenzothiazolylidene or dihydroquinolinylidene donor with an azo linker. The ease of synthesis demonstrates that our previously reported method of coupling the diazonium salt of 4-aminobenzaldehyde to a donoremethylidene nucleus can be readily extended. Linear absorption studies show the compounds to be free from aggregation in low polarity media. Hyper-Raleigh scattering results once again show that the TCF acceptor is preferred and that the combination of this accepter with a benzothiazole donor yields a chromophore with a dynamic first hyperpolarizability of 1900
1030 esu at 800 nm. Furthermore, for the most part, the NLO responses of the chromophores prepared in this study are higher than corresponding compounds containing an indoline donor, and this is most likely a reflection of the proaromatic nature of the BTZ and QUIN donor units. The compounds have acceptable thermal stabilities and this confirms their suitability for high temperature poling in order to examine their macroscopic NLO response. These studies are currently underway. Acknowledgements This work has been supported by the New Zealand Ministry for Science and Innovation (Contracts C08X0704) and by grants from the University of Leuven (GOA/2006/03). References [1] Cho MJ, Choi DH, Sullivan PA, Akelaitis AJP, Dalton LR. Recent progress in second-order nonlinear optical polymers and dendrimers. Progress in Polymer Science 2008;33:1013e58. [2] Dalton LR, Benight SJ, Johnson LE, Knorr DB, Kosilkin I, Eichinger BE, et al. Systematic nanoengineering of soft matter organic electro-optic materials. Chemistry of Materials 2011;23:430e45. [3] Kanis DR, Ratner M, Marks TJ. Design and construction of molecular assemblies with large second-order optical nonlinearities. Quantum chemical aspects. Chemical Reviews 1994;94:195e242.

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