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Novel isoindigo derivatives bearing long-chain N-alkyl substituents: Synthesis and self-assemble behavior
Chemical Physics Letters 594 (2014) 69–73
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Novel isoindigo derivatives bearing long-chain N-alkyl substituents: Synthesis and self-assemble behavior Andrei V. Bogdanov ⇑, Tatiana N. Pashirova, Lenar I. Musin, Dmitry B. Krivolapov, Lucia Ya. Zakharova, Vladimir F. Mironov, Alexander I. Konovalov A. E. Arbuzov Institute of Organic and Physical Chemistry of Kazan Scientific Center of the Russian Academy of Sciences, 8 Akad. Arbuzov st., Kazan 420088, Russia
a r t i c l e
i n f o
Article history: Received 2 December 2013 In final form 22 January 2014 Available online 31 January 2014
a b s t r a c t A mild, catalyst-free and simple synthesis of 1,10 -di(n-alkyl)-isoindigo via the reaction of substituted isatins with tris(diethylamino)phosphine is reported. Their self-assembly in water–DMF (50% v/v) solution was investigated. Unexpectedly, no surface activity was observed for compound 2h. Spectrophotometry and fluorimetry allowed determining critical micelle concentration (0.1 mM and 0.8 mM respectively). Aggregation numbers of 8 and sizes of aggregates were determined. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Isoindigo, along with indigo and indirubin, is a representative of the indigoid bisindoles which at the present time are in the focus of the pharmacy and chemistry of functional materials. Recent studies have shown that a lot of compounds containing indolin2-on-3-ylidene moiety have a great anticancer potency [1–6]. Such isoindigo derivatives as 1-methylisoindigo (Meisoindigo) and 1-(tri-O-acetylxylopyranosyl)-isoindigo (Natura) are successfully used in the treatment of leukemia (Figure 1) [7–9]. In addition, in recent years isoindigo scaffold have become attractive building blocks in the synthesis of conjugated polymers. The polymers containing fluorene and isoindigo units are used in the design of organic memory devices. The synthesis of low band gap semiconductor materials on the basis of isoindigo with naphthalene and anthracene copolymers have also been reported [10,11]. Isoindigo based polymers are of importance in organic photovoltaics as components of active layers of organic solar cells [12–14]. Ongoing studies deal with the molecular design of amphiphiles bearing photoactive components and polyfunctional biological markers. This may be promising for the creation of materials with improved photoelectric and optic properties [15–18]. For example, amphiphilic derivatives of azobenzene, carbazole, acridine, isatin or cyan dyes are well proved to be such promising structures [19–27]. The design of systems which can be useful in the diagnosis and (or) treatment of severe diseases is also actual [28]. The viruses, liposomes, core–shell nanoparticles, micelles, carbon nanotubes and hybrid nanostructures are promising drug
⇑ Corresponding author. Fax: +7 (843) 273 22 53. E-mail address: [email protected] (A.V. Bogdanov). http://dx.doi.org/10.1016/j.cplett.2014.01.026 0009-2614/Ó 2014 Elsevier B.V. All rights reserved.
delivering devices for modern medicine. Undoubted merit of novel amphiphilic isoindigos is their potential ability to bind with nucleic acids which is due to the presence of bis-indole scaffold as purine analogue [29]. This can have great potential in gene therapy for an obtaining of new type of non-viral vectors. In addition, this class of compounds is attractive because the CMC (the critical micelle concentration) of the dimeric surfactants are always much smaller than that of the monomeric ones [30,31]. Meanwhile, it is known that dimeric surfactants can form threadlike micelles with lesser curvature and low toxicity [32]. In this Letter, we describe the synthesis of novel isoindigo derivatives bearing long-chain alkyl substituents with the aim of investigating their structural behavior in aqueous solution.
2. Materials and methods 2.1. Materials 1-Phenylazo-2-naphthol (Sudan I, Acros Organics, New Jersey, USA), pyrene for fluorescence (Sigma, Switzerland) (P99%), Cetylpyridinium bromide (CPB) (AppliChem, BioChemica, Germany), (99.2%) and dimethylsulfoxid-d6, 99.9% (Deutero GmbH) were used. Dimethylformamide (DMF) was purified according to conventional procedures [33]. Compounds 2a–h were synthesized as follows and are detailed in Supplementary data. To a solution of isatin derivative (1.16 mmol) in anhydrous CH2Cl2 (10 mL) under bubbling of argon at 60 °C a solution of tris(diethylamino)phosphine (287 mg, 1.16 mmol) in anhydrous CH2Cl2 (2 mL) was added dropwise. The reaction mixture was allowed to reach the room temperature in
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A.V. Bogdanov et al. / Chemical Physics Letters 594 (2014) 69–73 H N O
O
N H
O N H
N H O N H
indigo
H N
isoindigo
O
H N
O
H N
O
indirubin
O
O
O
N
N O
meisoindigo
OAc
Natura OAc
OAc
Figure 1. Examples of compounds containing the bis-indole moiety.
an inert atmosphere. The precipitated products 2 were filtered off and air-dried. 2.2. Solution preparation procedure Compound 2h was dissolved in DMF at 60 °C followed by preheated (60 °C) water addition to form 1:1 mixture with desired concentration. Purified water (18.2 MO cm resistivity at 25 °C) from Direct-Q 5 UV equipment was used for all solution preparation. 2.3. Tensiometry Surface tension measurements were performed using the du Nouy ring detachment method (Kruss K6 tensiometer). The experimental details are described elsewhere [34]. 2.4. Dynamic light scattering Size of aggregates was determined by dynamic light scattering using Zetasizer Nano (Malvern Instruments Limited Grovewood Road, Malvern, Worcestershire, UK). Laser source was a He–Ne gas laser with power of 4 mW and wavelength of 633 nm. The scattering intensity at a specific angle is detected using a sensitive avalanche photodiode detector. The Intensity changes are analysed with a digital autocorrelator which generates a correlation function. The measured autocorrelation functions were analyzed by Malvern DTS software, and the second-order cumulant expansion methods. The effective hydrodynamic radius (RH) was calculated according to the Einstein–Stokes relation: D = kBT/6pgRH, in which D is the diffusion coefficient, kB is the Boltzmann constant, T is the absolute temperature, and g is the viscosity. The diffusion coefficient was measured at least three times for each sample. The average error in these experiments was approximately 4%. The solutions were filtered with Millipore filters, to remove dust particles from the scattering volume.
350–500 nm. The thickness of the cell was 10 10 mm. The scanning speed was 120 nm/min. Fluorescence intensity of the first peak at 373 nm (I1), of the third peak at 384 nm (I3), and of the peak at 394 nm were found from the spectra. Determination of aggregation numbers was performed by the most intense peak at 394 nm. The N values were found from the dependence ln(I0/ I) = [N/(Csur CMC)][Q], where I0 and I are the intensities of fluorescence of pyrene in the absence and presence of quencher, respectively; Csur is total surfactant concentration; [Q] is concentration of the quencher. Cmc value was determined from the dependence of the intensity ratio of the first and third peaks of the surfactant concentration. 2.6. Dye solubilization study The solubilization experiments were performed by adding an excess of crystalline dye Sudan I to solutions. These solutions were allowed to equilibrate for about 48 h at room temperature. They were filtered, and their absorbency was measured at 495 nm using Specord UV–vis spectrophotometer. Quartz cuvettes containing the sample were used, with a cell length of 0.1 cm. 2.7. Spectroscopy NMR spectra were recorded on a Bruker AVANCE 400 spectrometer at a temperature of 303 K. The operating frequency of the spectrometer was 400 MHz (1H) and 100.6 MHz (13C). Chemical shifts are given on the d-scale relative to residual signal of the solvent (DMSO d(1H) = 2.50 ppm, d(13C) = 39.5 ppm). IR spectra were recorded on a Vector 22 Fourier spectrometer (Bruker, Germany) in the range 400–4000 cm1. Crystalline samples were studied as suspensions in Nujol. Mass spectra were recorded on a DFS Thermo Electron Corporation (USA) mass spectrometer operating at an ionization potential of 70 eV. Elemental analyzes were performed on a Carlo Erba elemental analyzer EA 1108.
2.5. Fluorescence measurements 2.8. X-ray diffraction analysis The fluorescence spectra of pyrene (1 106 mol L1) in H2O/ DMF (50% v/v) solutions of 2h in the absence and presence of quencher (cetylpyridinium bromide) were recorded at 25 °C in a Varian Cary Eclipse spectrofluorimeter. Sample excitation was at a wavelength of 335 nm. Emission spectra were recorded in
X-ray diffraction analysis for the crystals of compound 2c was collected at 293 K using graphite monochromated MoKa (k 0.71073 Å) radiation. The structures were solved by direct method and refined by the full matrix least-squares using SHELXTL [35]
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and WinGX [36] programs. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in idealized positions. Data collections: images were indexed, integrates, and scaled using the APEX2 [37] data reduction package. All Figures were made using ORTEP [38]. Crystallographic data for compound 2c in this Letter have been deposited with the Cambridge Crystallographic Data centre as supplemental publication No. CCDC 948378. Copies of the data can be obtained, free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 01223 336033 or email: [email protected]).
3. Results and discussion Synthetic approach presented here is based on our recent studies and involve the reaction of certain isatins with tris(diethylamino)phosphine. It proceeds in mild conditions and allows to obtain symmetrically substituted isoindigos with high yields in one step (Scheme 1) [39]. The structures of products 2a–h were established from their elemental and spectroscopic analyses including IR, 1H NMR, 13C NMR, and mass spectral data. The down-field shift of H-4 signal from about 7.51 ppm in starting isatins to 9.19 ppm in relative isoindigo 2a–h is observed in all cases. In addition, as a result of the C@C bond formation a signal of C-3 carbon atom appears at 133.5–133.6 ppm. The structure of compound 2c was finally determined by single crystal X-ray analysis (Figure 2). Since these compounds contain hydrophilic bis-indole moiety and hydrophobic alkyl chains, i.e. demonstrate amphiphilic character, their surface active and aggregation properties were examined. Solution behavior of the homological series is exemplified by 2h. Due to the poor solubility in water this compound was dissolved in H2O/DMF (50% v/v) mixture. Unexpectedly, no surface activity was observed for this sample, as it is evidenced from Figure S15 (see Supplementary Data). Sensitive probes for the aggregation behavior of amphiphiles are hydrophobic dyes. Using Sudan as an insoluble in water probe
Figure 3. Dependence of the dye absorbance at 495 nm on 2h concentration in H2O/DMF (50% v/v) (25 °C; optical length 0.1 cm).
allowed us to study its solubilization by spectrophotometry technique. It was shown that the absorbance of the probe at 495 nm markedly increases (Figure 3) at concentrations of 2h higher than 0.1 mM. This is probably due to the formation of aggregates with hydrophobic interior capable of binding the probe. The critical micelle concentration (cmc) of 2h corresponding to the sharp increase of the absorbance is about 0.1 mM. To support this result, the fluorescence technique was involved with the use of the insoluble in water pyrene as a probe. This method makes it possible to determine the onset of aggregation by monitoring the pyrene (I1/I3) ratio, i.e. the intensities of the first and second vibronic peaks in the pyrene spectrum [40–43]. As
R O
CH2Cl2 O
N
+ P(NEt 2)3
N O
-60 οC to 20 οC, 15 min - O=P(NEt2)3 O
R
N
1a-h
2a-h R R = C8 H17 (a), C9 H19 (b), C 10H 21 (c), C 12 H25 (d), C 14 H 29 (e), C 15 H31 (f), C 16 H33 (g), C 18 H 37 (h)
Scheme 1. Synthesis of isoindigo derivatives 2a–h.
Figure 2. ORTEP diagram of 2c.
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can be seen (Figure 4), a decrease in fluorescence intensity occurs with an increase in the concentration of 2h. Figure 5 demonstrates a marked decrease of the I1/I3 value with an increase in the concentration of 2h, thereby indicating that a decrease in the probe microenvironment is observed. It is reasonable to assume that the incorporation of pyrene into the hydrophobic region of the 2h aggregates occurs. Besides, additional binding of the probe by heterocyclic core may be assumed. This technique is successively used for the determination of cmc as a break point in the I1/I3 versus concentration plot [44,45]. According to fluorescence data (Figure 5) cmc value was found to be 0.85 mM. However, this value is higher than cmc determined by spectrophotometry technique. This disagreement may be explained by the fact that due to low cmc value compound 2h forms aggregates with low aggregation number, small hydrophobic core, and low solubilization capacity. As a result, the partition of pyrene between hydrophobic and bulk phases may exist as reported [44–46]. Reducing the I1/I3 value at
concentrations above the cmc of 2h may indicate the formation of loose structures allowing the water penetration. Such behavior is characteristic for gemini surfactants and for the compounds bearing multipolar head groups [47,48]. On the other hand, this may be due to the fact that compound 2h possesses quencher properties (Figure S16). The aggregation number (Nagg) of 2h were determined through the well-established steady-state fluorescence quenching method with cetylpyridinium bromide (CPB) used as a quencher (Figure S17) [49,50]. Figure 6 shows ln I0/I versus CPB concentration plot, from which the aggregation number of 8 was calculated using ln(I0/I) = [N/(Csur CMC)][Q] equation. The self- assembled structures formed in the isoindigo 2h solutions were investigated by dynamic light scattering (DLS). The size distribution is shown in Figure 7. It was found that the hydrodynamic diameter (D, nm) of the 2h particles depends on its concentration and increases from 80 nm to 140 nm. It should be stressed
Figure 4. Fluorescence of pyrene (Cpyrene = 1 106 M) in 2h H2O/DMF (50% v/v) solutions, 25 °C, C2h, mM: 0 (1); 0.006 (2); 0.01 (3); 0.02 (4); 0.04 (5); 0.1 (6); 0.2 (7); 0.4 (8); 0.6 (9); 0.7 (10); 0.8 (11); 1.4 (12); 2 (13).
Figure 6. Dependence of ln I0/I in 2h solutions on CPB concentration, C2h = 1 mM, k = 394 nm.
Figure 5. Dependence of the intensity ratio (I1/I3) of the first and third peaks of pyrene on the 2h concentrations, Cpyrene = 1 106 M, 25 °C.
Figure 7. The size distribution analysis of 2h particles in H2O/DMF (50% v/v) solutions by the number parameter, 25 °C, C2h, mM: 0.1 (1); 0.2 (2); 0.4 (3); 0.5 (4); 0.6 (5); 0.8 (6); 1 (7); 2 (8).
A.V. Bogdanov et al. / Chemical Physics Letters 594 (2014) 69–73
that the low polydispersity (PDI) of the samples is observed not exceeding 0.17 (Table S1). 4. Conclusion We have described the synthesis of novel amphiphilic isoindigo derivatives and their ability to form self-assembled structures in H2O/DMF (50% v/v) solutions. Aggregation properties obtained as well as an easy and high-yield synthetic approach allows us to propose them as candidate for the creation of the nanodevices with interesting optical properties and for the drugs delivery systems. Further studies to determine the influence of alkyl substituent length on self-association of whole isoindigo 2a–h series are underway. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2014.01. 026. References [1] C.R. Prakash, S. Raja, Mini Rev. Med. Chem. 12 (2012) 98. [2] X.K. Wee, T. Yang, M.L. Go, ChemMedChem 7 (2012) 777. [3] D. Marko, S. Schatzle, A. Friedel, A. Genzlinger, H. Zankl, L. Meijer, G. Eisenbrand, J Cancer 84 (2001) 283. [4] M.J. Moon, S.K. Lee, J.W. Lee, W.K. Song, S.W. Kim, J.I. Kim, C. Cho, S.J. Choi, Y.C. Kim, Bioorg. Med. Chem. 14 (2006) 237. [5] E. Damiens, B. Baratte, D. Marie, G. Eisenbrand, L. Meijer, Oncogene 20 (2001) 3786. [6] V. Myrianthopoulos, P. Magiatis, Y. Ferandin, A.L. Skaltsounis, L. Meijer, E.J. Mikros, Med. Chem. 50 (2007) 4027. [7] Z.J. Xiao, Y.S. Hao, B.C. Liu, L.S. Qian, Leukemia Lymphoma 43 (2002) 1763. [8] Z.J. Xiao, L.S. Qian, B.C. Liu, Y.S. Hao, J. Br. Haematol. 111 (2000) 711. [9] F. Chen et al., Leukemia Res. 34 (2010) 75. [10] X. Xu, L. Li, B. Liu, Y. Zou, Appl. Phys. Lett. 98 (2011) 0633031. [11] P. Sonar, H.-S.h. Tan, Sh. Sun, Y.M. Lam, A. Dodabalapur, Polym. Chem. 4 (2013) 1983. [12] G. Zhang, Y. Fu, Zh. Xie, Q. Zhang, Macromolecules 44 (2011) 1414. [13] W. Ying, F. Guo, J. Li, Q. Zhang, W. Wu, H. Tian, J. Hua, ACS Appl. Mater. Interfaces 4 (2012) 4215. [14] K. Vandewal et al., Adv. Funct. Mater. 22 (2012) 3480.
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Novel isoindigo derivatives bearing long-chain N-alkyl substituents: Synthesis and self-assemble behavior Andrei V. Bogdanov ⇑, Tatiana N. Pashirova, Lenar I. Musin, Dmitry B. Krivolapov, Lucia Ya. Zakharova, Vladimir F. Mironov, Alexander I. Konovalov A. E. Arbuzov Institute of Organic and Physical Chemistry of Kazan Scientific Center of the Russian Academy of Sciences, 8 Akad. Arbuzov st., Kazan 420088, Russia
a r t i c l e
i n f o
Article history: Received 2 December 2013 In final form 22 January 2014 Available online 31 January 2014
a b s t r a c t A mild, catalyst-free and simple synthesis of 1,10 -di(n-alkyl)-isoindigo via the reaction of substituted isatins with tris(diethylamino)phosphine is reported. Their self-assembly in water–DMF (50% v/v) solution was investigated. Unexpectedly, no surface activity was observed for compound 2h. Spectrophotometry and fluorimetry allowed determining critical micelle concentration (0.1 mM and 0.8 mM respectively). Aggregation numbers of 8 and sizes of aggregates were determined. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Isoindigo, along with indigo and indirubin, is a representative of the indigoid bisindoles which at the present time are in the focus of the pharmacy and chemistry of functional materials. Recent studies have shown that a lot of compounds containing indolin2-on-3-ylidene moiety have a great anticancer potency [1–6]. Such isoindigo derivatives as 1-methylisoindigo (Meisoindigo) and 1-(tri-O-acetylxylopyranosyl)-isoindigo (Natura) are successfully used in the treatment of leukemia (Figure 1) [7–9]. In addition, in recent years isoindigo scaffold have become attractive building blocks in the synthesis of conjugated polymers. The polymers containing fluorene and isoindigo units are used in the design of organic memory devices. The synthesis of low band gap semiconductor materials on the basis of isoindigo with naphthalene and anthracene copolymers have also been reported [10,11]. Isoindigo based polymers are of importance in organic photovoltaics as components of active layers of organic solar cells [12–14]. Ongoing studies deal with the molecular design of amphiphiles bearing photoactive components and polyfunctional biological markers. This may be promising for the creation of materials with improved photoelectric and optic properties [15–18]. For example, amphiphilic derivatives of azobenzene, carbazole, acridine, isatin or cyan dyes are well proved to be such promising structures [19–27]. The design of systems which can be useful in the diagnosis and (or) treatment of severe diseases is also actual [28]. The viruses, liposomes, core–shell nanoparticles, micelles, carbon nanotubes and hybrid nanostructures are promising drug
⇑ Corresponding author. Fax: +7 (843) 273 22 53. E-mail address: [email protected] (A.V. Bogdanov). http://dx.doi.org/10.1016/j.cplett.2014.01.026 0009-2614/Ó 2014 Elsevier B.V. All rights reserved.
delivering devices for modern medicine. Undoubted merit of novel amphiphilic isoindigos is their potential ability to bind with nucleic acids which is due to the presence of bis-indole scaffold as purine analogue [29]. This can have great potential in gene therapy for an obtaining of new type of non-viral vectors. In addition, this class of compounds is attractive because the CMC (the critical micelle concentration) of the dimeric surfactants are always much smaller than that of the monomeric ones [30,31]. Meanwhile, it is known that dimeric surfactants can form threadlike micelles with lesser curvature and low toxicity [32]. In this Letter, we describe the synthesis of novel isoindigo derivatives bearing long-chain alkyl substituents with the aim of investigating their structural behavior in aqueous solution.
2. Materials and methods 2.1. Materials 1-Phenylazo-2-naphthol (Sudan I, Acros Organics, New Jersey, USA), pyrene for fluorescence (Sigma, Switzerland) (P99%), Cetylpyridinium bromide (CPB) (AppliChem, BioChemica, Germany), (99.2%) and dimethylsulfoxid-d6, 99.9% (Deutero GmbH) were used. Dimethylformamide (DMF) was purified according to conventional procedures [33]. Compounds 2a–h were synthesized as follows and are detailed in Supplementary data. To a solution of isatin derivative (1.16 mmol) in anhydrous CH2Cl2 (10 mL) under bubbling of argon at 60 °C a solution of tris(diethylamino)phosphine (287 mg, 1.16 mmol) in anhydrous CH2Cl2 (2 mL) was added dropwise. The reaction mixture was allowed to reach the room temperature in
70
A.V. Bogdanov et al. / Chemical Physics Letters 594 (2014) 69–73 H N O
O
N H
O N H
N H O N H
indigo
H N
isoindigo
O
H N
O
H N
O
indirubin
O
O
O
N
N O
meisoindigo
OAc
Natura OAc
OAc
Figure 1. Examples of compounds containing the bis-indole moiety.
an inert atmosphere. The precipitated products 2 were filtered off and air-dried. 2.2. Solution preparation procedure Compound 2h was dissolved in DMF at 60 °C followed by preheated (60 °C) water addition to form 1:1 mixture with desired concentration. Purified water (18.2 MO cm resistivity at 25 °C) from Direct-Q 5 UV equipment was used for all solution preparation. 2.3. Tensiometry Surface tension measurements were performed using the du Nouy ring detachment method (Kruss K6 tensiometer). The experimental details are described elsewhere [34]. 2.4. Dynamic light scattering Size of aggregates was determined by dynamic light scattering using Zetasizer Nano (Malvern Instruments Limited Grovewood Road, Malvern, Worcestershire, UK). Laser source was a He–Ne gas laser with power of 4 mW and wavelength of 633 nm. The scattering intensity at a specific angle is detected using a sensitive avalanche photodiode detector. The Intensity changes are analysed with a digital autocorrelator which generates a correlation function. The measured autocorrelation functions were analyzed by Malvern DTS software, and the second-order cumulant expansion methods. The effective hydrodynamic radius (RH) was calculated according to the Einstein–Stokes relation: D = kBT/6pgRH, in which D is the diffusion coefficient, kB is the Boltzmann constant, T is the absolute temperature, and g is the viscosity. The diffusion coefficient was measured at least three times for each sample. The average error in these experiments was approximately 4%. The solutions were filtered with Millipore filters, to remove dust particles from the scattering volume.
350–500 nm. The thickness of the cell was 10 10 mm. The scanning speed was 120 nm/min. Fluorescence intensity of the first peak at 373 nm (I1), of the third peak at 384 nm (I3), and of the peak at 394 nm were found from the spectra. Determination of aggregation numbers was performed by the most intense peak at 394 nm. The N values were found from the dependence ln(I0/ I) = [N/(Csur CMC)][Q], where I0 and I are the intensities of fluorescence of pyrene in the absence and presence of quencher, respectively; Csur is total surfactant concentration; [Q] is concentration of the quencher. Cmc value was determined from the dependence of the intensity ratio of the first and third peaks of the surfactant concentration. 2.6. Dye solubilization study The solubilization experiments were performed by adding an excess of crystalline dye Sudan I to solutions. These solutions were allowed to equilibrate for about 48 h at room temperature. They were filtered, and their absorbency was measured at 495 nm using Specord UV–vis spectrophotometer. Quartz cuvettes containing the sample were used, with a cell length of 0.1 cm. 2.7. Spectroscopy NMR spectra were recorded on a Bruker AVANCE 400 spectrometer at a temperature of 303 K. The operating frequency of the spectrometer was 400 MHz (1H) and 100.6 MHz (13C). Chemical shifts are given on the d-scale relative to residual signal of the solvent (DMSO d(1H) = 2.50 ppm, d(13C) = 39.5 ppm). IR spectra were recorded on a Vector 22 Fourier spectrometer (Bruker, Germany) in the range 400–4000 cm1. Crystalline samples were studied as suspensions in Nujol. Mass spectra were recorded on a DFS Thermo Electron Corporation (USA) mass spectrometer operating at an ionization potential of 70 eV. Elemental analyzes were performed on a Carlo Erba elemental analyzer EA 1108.
2.5. Fluorescence measurements 2.8. X-ray diffraction analysis The fluorescence spectra of pyrene (1 106 mol L1) in H2O/ DMF (50% v/v) solutions of 2h in the absence and presence of quencher (cetylpyridinium bromide) were recorded at 25 °C in a Varian Cary Eclipse spectrofluorimeter. Sample excitation was at a wavelength of 335 nm. Emission spectra were recorded in
X-ray diffraction analysis for the crystals of compound 2c was collected at 293 K using graphite monochromated MoKa (k 0.71073 Å) radiation. The structures were solved by direct method and refined by the full matrix least-squares using SHELXTL [35]
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and WinGX [36] programs. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in idealized positions. Data collections: images were indexed, integrates, and scaled using the APEX2 [37] data reduction package. All Figures were made using ORTEP [38]. Crystallographic data for compound 2c in this Letter have been deposited with the Cambridge Crystallographic Data centre as supplemental publication No. CCDC 948378. Copies of the data can be obtained, free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 01223 336033 or email: [email protected]).
3. Results and discussion Synthetic approach presented here is based on our recent studies and involve the reaction of certain isatins with tris(diethylamino)phosphine. It proceeds in mild conditions and allows to obtain symmetrically substituted isoindigos with high yields in one step (Scheme 1) [39]. The structures of products 2a–h were established from their elemental and spectroscopic analyses including IR, 1H NMR, 13C NMR, and mass spectral data. The down-field shift of H-4 signal from about 7.51 ppm in starting isatins to 9.19 ppm in relative isoindigo 2a–h is observed in all cases. In addition, as a result of the C@C bond formation a signal of C-3 carbon atom appears at 133.5–133.6 ppm. The structure of compound 2c was finally determined by single crystal X-ray analysis (Figure 2). Since these compounds contain hydrophilic bis-indole moiety and hydrophobic alkyl chains, i.e. demonstrate amphiphilic character, their surface active and aggregation properties were examined. Solution behavior of the homological series is exemplified by 2h. Due to the poor solubility in water this compound was dissolved in H2O/DMF (50% v/v) mixture. Unexpectedly, no surface activity was observed for this sample, as it is evidenced from Figure S15 (see Supplementary Data). Sensitive probes for the aggregation behavior of amphiphiles are hydrophobic dyes. Using Sudan as an insoluble in water probe
Figure 3. Dependence of the dye absorbance at 495 nm on 2h concentration in H2O/DMF (50% v/v) (25 °C; optical length 0.1 cm).
allowed us to study its solubilization by spectrophotometry technique. It was shown that the absorbance of the probe at 495 nm markedly increases (Figure 3) at concentrations of 2h higher than 0.1 mM. This is probably due to the formation of aggregates with hydrophobic interior capable of binding the probe. The critical micelle concentration (cmc) of 2h corresponding to the sharp increase of the absorbance is about 0.1 mM. To support this result, the fluorescence technique was involved with the use of the insoluble in water pyrene as a probe. This method makes it possible to determine the onset of aggregation by monitoring the pyrene (I1/I3) ratio, i.e. the intensities of the first and second vibronic peaks in the pyrene spectrum [40–43]. As
R O
CH2Cl2 O
N
+ P(NEt 2)3
N O
-60 οC to 20 οC, 15 min - O=P(NEt2)3 O
R
N
1a-h
2a-h R R = C8 H17 (a), C9 H19 (b), C 10H 21 (c), C 12 H25 (d), C 14 H 29 (e), C 15 H31 (f), C 16 H33 (g), C 18 H 37 (h)
Scheme 1. Synthesis of isoindigo derivatives 2a–h.
Figure 2. ORTEP diagram of 2c.
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can be seen (Figure 4), a decrease in fluorescence intensity occurs with an increase in the concentration of 2h. Figure 5 demonstrates a marked decrease of the I1/I3 value with an increase in the concentration of 2h, thereby indicating that a decrease in the probe microenvironment is observed. It is reasonable to assume that the incorporation of pyrene into the hydrophobic region of the 2h aggregates occurs. Besides, additional binding of the probe by heterocyclic core may be assumed. This technique is successively used for the determination of cmc as a break point in the I1/I3 versus concentration plot [44,45]. According to fluorescence data (Figure 5) cmc value was found to be 0.85 mM. However, this value is higher than cmc determined by spectrophotometry technique. This disagreement may be explained by the fact that due to low cmc value compound 2h forms aggregates with low aggregation number, small hydrophobic core, and low solubilization capacity. As a result, the partition of pyrene between hydrophobic and bulk phases may exist as reported [44–46]. Reducing the I1/I3 value at
concentrations above the cmc of 2h may indicate the formation of loose structures allowing the water penetration. Such behavior is characteristic for gemini surfactants and for the compounds bearing multipolar head groups [47,48]. On the other hand, this may be due to the fact that compound 2h possesses quencher properties (Figure S16). The aggregation number (Nagg) of 2h were determined through the well-established steady-state fluorescence quenching method with cetylpyridinium bromide (CPB) used as a quencher (Figure S17) [49,50]. Figure 6 shows ln I0/I versus CPB concentration plot, from which the aggregation number of 8 was calculated using ln(I0/I) = [N/(Csur CMC)][Q] equation. The self- assembled structures formed in the isoindigo 2h solutions were investigated by dynamic light scattering (DLS). The size distribution is shown in Figure 7. It was found that the hydrodynamic diameter (D, nm) of the 2h particles depends on its concentration and increases from 80 nm to 140 nm. It should be stressed
Figure 4. Fluorescence of pyrene (Cpyrene = 1 106 M) in 2h H2O/DMF (50% v/v) solutions, 25 °C, C2h, mM: 0 (1); 0.006 (2); 0.01 (3); 0.02 (4); 0.04 (5); 0.1 (6); 0.2 (7); 0.4 (8); 0.6 (9); 0.7 (10); 0.8 (11); 1.4 (12); 2 (13).
Figure 6. Dependence of ln I0/I in 2h solutions on CPB concentration, C2h = 1 mM, k = 394 nm.
Figure 5. Dependence of the intensity ratio (I1/I3) of the first and third peaks of pyrene on the 2h concentrations, Cpyrene = 1 106 M, 25 °C.
Figure 7. The size distribution analysis of 2h particles in H2O/DMF (50% v/v) solutions by the number parameter, 25 °C, C2h, mM: 0.1 (1); 0.2 (2); 0.4 (3); 0.5 (4); 0.6 (5); 0.8 (6); 1 (7); 2 (8).
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that the low polydispersity (PDI) of the samples is observed not exceeding 0.17 (Table S1). 4. Conclusion We have described the synthesis of novel amphiphilic isoindigo derivatives and their ability to form self-assembled structures in H2O/DMF (50% v/v) solutions. Aggregation properties obtained as well as an easy and high-yield synthetic approach allows us to propose them as candidate for the creation of the nanodevices with interesting optical properties and for the drugs delivery systems. Further studies to determine the influence of alkyl substituent length on self-association of whole isoindigo 2a–h series are underway. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2014.01. 026. References [1] C.R. Prakash, S. Raja, Mini Rev. Med. Chem. 12 (2012) 98. [2] X.K. Wee, T. Yang, M.L. Go, ChemMedChem 7 (2012) 777. [3] D. Marko, S. Schatzle, A. Friedel, A. Genzlinger, H. Zankl, L. Meijer, G. Eisenbrand, J Cancer 84 (2001) 283. [4] M.J. Moon, S.K. Lee, J.W. Lee, W.K. Song, S.W. Kim, J.I. Kim, C. Cho, S.J. Choi, Y.C. Kim, Bioorg. Med. Chem. 14 (2006) 237. [5] E. Damiens, B. Baratte, D. Marie, G. Eisenbrand, L. Meijer, Oncogene 20 (2001) 3786. [6] V. Myrianthopoulos, P. Magiatis, Y. Ferandin, A.L. Skaltsounis, L. Meijer, E.J. Mikros, Med. Chem. 50 (2007) 4027. [7] Z.J. Xiao, Y.S. Hao, B.C. Liu, L.S. Qian, Leukemia Lymphoma 43 (2002) 1763. [8] Z.J. Xiao, L.S. Qian, B.C. Liu, Y.S. Hao, J. Br. Haematol. 111 (2000) 711. [9] F. Chen et al., Leukemia Res. 34 (2010) 75. [10] X. Xu, L. Li, B. Liu, Y. Zou, Appl. Phys. Lett. 98 (2011) 0633031. [11] P. Sonar, H.-S.h. Tan, Sh. Sun, Y.M. Lam, A. Dodabalapur, Polym. Chem. 4 (2013) 1983. [12] G. Zhang, Y. Fu, Zh. Xie, Q. Zhang, Macromolecules 44 (2011) 1414. [13] W. Ying, F. Guo, J. Li, Q. Zhang, W. Wu, H. Tian, J. Hua, ACS Appl. Mater. Interfaces 4 (2012) 4215. [14] K. Vandewal et al., Adv. Funct. Mater. 22 (2012) 3480.
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