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Synlett 2024; 35(01): 125-129
DOI: 10.1055/a-2138-7080
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Functional Dyes

Tunable Optical Properties and Self-Assemblies of a Water-Soluble Perimidinium Imide Dye

Tirupati Roy
,
Indraneel Debnath
,
Kingsuk Mahata  ∗

The work is funded by the Science and Engineering Research Board (CRG/2021/000017), Govt. of India.
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Abstract

The synthesis of unsymmetrically peri-annulated naphthalene dyes, perimidinium imides (PrIm), is reported. Compared with the symmetrical and popular analogous naphthalenediimide dyes, PrIm showed a red-shifted absorption maximum. The water-soluble dyes showed tunable self-assembly behaviors and optical properties. The dyes retain their photoluminescence properties in water.


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Key words

perimidinium imides - dyes - water-soluble - self-assembly

Owing to proximity effects and its uniqueness,[1] peri-annulation of naphthalene with appropriate functional groups is an important tool to realize a diverse range of fascinating dye molecules with exciting properties and applications. For example, naphthalene-1,4,5,8-tetracarboxylic acid diimide and its derivatives, commonly known as NDIs (Figure [1]),[2] have been used in many different areas such as organic semiconductors,[3] photovoltaics,[4] sensors,[5] energy storage,[6] supramolecular chemistry,[7] and medicinal chemistry.[8] Accordingly, numerous synthetic modifications have been carried out to produce a wide range of dyes with tunable optical and electrochemical properties. Whereas research on NDIs is well elaborated and very successful, the chemistry of other double peri-annulated naphthalenes remains underdeveloped. In fact, a limited number of different types of examples are known. Naphthalene bisamides (NBAs), a key building block for semiconducting polymers, have been reported only in recent times (Figure [1]).[9] The NBA building blocks have also been used in copolymers with improved semiconducting properties.[10] A mixed-type peri-annulated naphthalene, the lactam imide,[11] in which both the imide and the amide groups are attached at peri-positions, has been obtained from NDIs through treatment with a base (Figure [1]). Unsymmetrically peri-annulated naphthalenes with two different heterocycles, a pyrrole and a pyrimidine, were reported in 2015.[12] However, considering the vast chemistry of NDIs, examples of other doubly peri-annulated naphthalenes are scarce. Therefore, annulation of naphthalene with new functional groups is necessary both from the structural perspective and from the point of view of applications.

Zoom Image
Figure 1 Structures of the compounds used in the study along with relevant previous example

Among the singly peri-annulated naphthalenes, besides 1,8-naphthalimide,[13] perimidines[14] are keenly sought by various scientific communities. With peri-fused N-heterocycles, perimidines are versatile scaffolds and a fascinating class of compounds that have evolved over the last few decades.[15] The tremendous demand from such diverse areas as materials chemistry, medicinal chemistry, life science, and coordination chemistry, has led to the generation of a wide variety of derivatives with adjustable properties, reactivities, and electronic characters.[16] In most of the perimidines, there is substantial polarization of the π-electron cloud through the expulsion of a formally excessive π-electron from the six-membered N-containing ring to the naphthalene fragments. However, the situation can be altered by appropriate changes. The polarity can also be altered by converting the scaffolds into perimidinium salts.[17] Inspired by the diverse nature of the perimidine moiety and the widespread chemistry of naphthalimides, we wanted to amalgamate the two fragments, and herein we report a series of unsymmetrical peri-fused naphthalene dyes, the perimidinium imides (PrIm). We used a perimidinium salt to achieve an amphiphilic character in the dyes to tune their self-assembly properties.

To obtain the target dye, we wanted to introduce the cationic perimidinium functionality at the end of the synthetic scheme to avoid difficulties in purification. We started our synthesis with 5-bromo-5-nitro-1,8-naphthalic anhydride (1) (Scheme [1]).[18] Anhydride 1 was converted into corresponding imides 2a and 2b by treatment with the appropriate amines. For the aliphatic amine 2-ethylhexylamine, the imidization was carried out in ethanol at 80 °C to give imide 2a in 68% yield. Treatment of imide 2a with methylamine hydrochloride in the presence of K2CO3 in 1,4-dioxane produced imide 3a in 40% yield through nucleophilic aromatic substitution. Finally, the perimidinium ring was completed by treating compound 3a with triethyl orthoformate in the presence of HCl under an inert atmosphere at 70 °C to give the desired aPrIm dye as an orange-yellow solid in 72% yield.[19] aPrIm was characterized by a combination of several analytical techniques, such as 1H and 13C NMR spectroscopy, high-resolution mass spectrometry (HRMS), and IR spectroscopy. The HRMS data for aPrIm showed an intense peak at m/z = 378.2168, attributed to the perimidinium imide after the loss of the Cl– counteranion. Proton resonances of the dye in CDCl3 showed two doublets at δ = 8.69 and 7.24 ppm for the naphthalene rings. The doublet at the higher field is associated with H(2) and H(7). The other multiplet at a lower field was assigned to H(3) and H(6). The observation of only two signals for the naphthalene suggested a structure in which the positive charge is delocalized between the two perimidinium nitrogens. The singlet at δ = 11.91 ppm was attributed to the iminium proton. The number of signals for the aliphatic protons matched the proposed structure. By changing the imide chain and alkyl substituents in the iminium part, we synthesized another derivative, bPrIm, in a comparable yield. Details of its synthesis are given in the Supporting Information (SI).

Zoom Image
Scheme 1 Synthetic route for the preparation of the PrIm dyes

Having established their structure, we investigated the optical properties of the dyes. The absorption properties in various solvents were also used to monitor the self-assembly processes. Due to their amphiphilic nature, with a hydrophobic naphthalene monoimide moiety and a hydrophilic iminium part, the dyes are soluble in most common organic solvents, including nonpolar toluene, as well as in polar water. To assess the self-assembly behaviors of the dyes, we investigated their UV/vis spectra in solvents of various polarities. Because both the dyes showed similar optical properties (SI; Figure S24), we used only aPrIm for further investigations. Besides toluene and water, two solvents of opposite polarities, we recorded the absorption spectra of aPrIm in solvents of intermediate polarity, such as dichloromethane (DCM), acetonitrile, ethanol, and chloroform (Figure [2a]). In all the solvents, apart from toluene and ethanol, the dye showed very broad spectra. However, quite distinct spectra were observed in toluene and ethanol. The spectra were vibronically resolved and showed a bathochromically shifted absorption maxima compared with solutions in other solvents. In toluene, besides the absorption maximum at 434 nm (12780 cm–1 M–1), a shoulder was observed at about 411 nm (10820 cm–1 M–1). In comparison to NDIs, the absorption maximum of aPrIm is bathochromically shifted by 92 nm.[20] A vibronically resolved spectrum was also observed in ethanol at a 5 μM concentration. The spectral features in toluene and ethanol could be attributed to the molecularly dissolved state. On the other hand, the spectra in the other solvents indicated the existence of an aggregated state. To confirm this, we first investigated the concentration-dependent and variable-temperature (VT) absorption properties of aPrIm in toluene (SI; Figure S17). The spectral features did not change significantly in the temperature range 20–80 °C at a concentration of 50 μM, confirming the molecularly dissolved state of the dye. However, noticeable changes could be seen from the concentration-dependent studies (SI; Figure S16). The optical absorption was found to be different at a 1 mM concentration. Besides the appearance of a new shoulder at 424 nm, a broadening of the spectrum occurred. The full width at half maximum value increased from 2577 cm–1 at a 10 μM concentration to 2854 cm–1 at a 1 mM concentration. This change is ascribed to a self-assembly process. Unfortunately, absorption spectra could not be recorded at higher concentrations due to the limitations of the instrument.

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Figure 2 (a) Normalized absorption spectra of aPrIm in various solvents. (b) Absorption spectra of aPrIm at various concentrations in ethanol.

The nature of the morphology produced by the self-assembly process was analyzed by field-emission scanning electron microscopy (FESEM). The FESEM image (SI; Figure S27) of the dye on an aluminum surface, prepared by drop-casting a solution of aPrIm in toluene (c = 1 mM), showed the formation of nanofibers. The aggregates extended up to several microns in length. This observation can be explained by considering hydrophobic interactions. With an increase in concentration, the unfavorable interactions between the solvent molecules and the iminium part of the dye increase. To minimize these, the dye organizes in a head-to-tail manner to form linear aggregates in which the hydrophobic imide chain is exposed to the solvent system, and the hydrophilic parts are masked by each other. The process is summarized in Figure [3].

Zoom Image
Figure 3 Two different self-assembly processes involving aPrIm dye. FESEM images of the dye in DCM (left) and in ethanol (right) are also shown.

Similar aggregation was observed in the more-polar solvents DCM and chloroform. An FESEM image of the dye on a surface after evaporation from DCM by the drop-casting method also showed the formation of nanofibers (Figure [3]). Concentration-dependent UV-vis absorption studies (SI; Figure S18) did not show any significant changes, suggesting aggregation occurred at all concentrations. Therefore, in the spectroscopic concentration range (10–6 to 10–4 M), the dye exists as a monomer in toluene, but undergoes a self-assembly process in DCM. The spectral features in chloroform were found to be akin to those in DCM. Investigations of the concentration-dependent absorption property (SI; Figure S19), did not show any substantial change, suggesting the existence of aggregates at spectroscopic concentrations. The self-assembled state of the dye is quite stable and does not dissociate completely to a molecularly dissolved state even at elevated temperatures, as evidenced from VT studies in chloroform (SI; Figure S20).

We then switched our focus to polar protic solvent ethanol, and we investigated the VT and concentration-dependent absorption properties. At a concentration of 5 μM, aPrIm showed an absorption maximum at 444 nm (46350 cm–1 M–1) with a shoulder at 424 nm (35000 cm–1 M–1). The optical signature of the dye remained unaltered at various temperatures (SI; Figure S21), confirming the presence of a molecularly dissolved state in the specified conditions. However, the spectral features changed drastically in the concentration-dependent studies (Figure [2b]). With an increase in concentration, the intensity at maximum decreased along with the appearance of a higher-energy shoulder, and, at a concentration of 1 mM, a new maximum appeared at 390 nm. Therefore, the dye switched from a monomeric state to an aggregated state on changing the concentration. The morphology of the aggregates was assessed by an FESEM study, which showed the formation of nanospheres (Figure [3]). Although the optical signatures of aPrIm in DCM and ethanol are similar at a high concentration (mM), the different morphologies are not surprising. Unlike the case in DCM, hydrophilic interactions between the iminium part and the ethanol molecules are favored, whereas the unfavorable interactions between the hydrophobic parts and the polar solvents need to be minimized. To minimize the hydrophilic interactions, aPrIm organizes into cyclic assemblies in which the iminium part is exposed to the environment of the solvent. The overall process is summarized in Figure [3].

The self-assembly process could also be seen in water at a very low concentration. In the concentration range 1 μM to 1 mM, broad spectra were observed (SI; Figure S22). The disassembly of the dye was prevented due to the very strong hydrophobic interactions, as evidenced from VT investigations (SI; Figure S23). A similar driving force to that observed in the case of ethanol led to aggregation in water.

We then investigated the photoluminescence property of the dye in all the solvents used in the self-assembly process. The dye exhibited strong emissions in different solvents with maxima ranging from 451 nm in water to 522 nm in toluene (SI; Figure S25). Diverse Stokes shifts were also found in the various solvents: the largest value of 3884 cm–1 was calculated for toluene, whereas the smallest shift (2172 cm–1) was observed in water. The relative photoluminescent quantum yields were 31, 32, and 34% in DCM, toluene, and ethanol, respectively. In water, the dye maintained its emissive character, albeit with a diminished value (6%). A photoluminescent property, especially in water, is important for potential applications of the dye in biology.

In summary, we have synthesized two new unsymmetrical peri-annulated naphthalene dyes aPrIm and bPrIm. The dyes are soluble in most common solvents, including water, and exhibit bathochromically shifted absorption maxima compared with the symmetrical analogous NDI dyes. Two different types of morphologies could be generated from aPrIm simply by changing the polarity of the solvent. Different optical properties could also be achieved as a function of the solvent and concentration. Most notably, both the dyes retained their emissive property in water, making them attractive as fluorescence probes for biological applications.[21]


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Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

We thank the Department of Chemistry and the Central Instrumental Facility of the Indian Institute of Technology (IITG) for various instrumental facilities. T.R. and I.D. thank IITG for their doctoral fellowships.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/a-2138-7080.
  • Supporting Information

  • References and Notes

    • 4a Rundel K, Maniam S, Deshmukh K, Gann E, Prasad SK. K, Hodgkiss JM, Langford SJ, McNeill CR. J. Mater. Chem. A 2017; 5: 12266
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  • 19 6-Bromo-2-(2-ethylhexyl)-7-nitro-1H-benzo[de]isoquinoline-1,3(2H)-dione (2a) A 100 mL round-bottomed flask was charged with anhydride 1 (450 mg, 1.40 mmol) and EtOH (15 mL). A solution of 2-ethyl-1-hexylamine (181 mg, 1.40 mmol) in EtOH (15 mL) was added dropwise at r.t., and the mixture was stirred at 80 °C for 3 h, then cooled to r.t. The precipitate was collected by filtration and found to consist of analytically pure imide 2a. An additional amount of 2a was collected from the filtrate by treatment with H2O–EtOAc and subsequent chromatographic separation (silica gel, 10% EtOAc–hexane). The imide was obtained as a yellow solid; yield: 410 mg (68%). IR: 2956, 2918, 2870, 1704, 1662, 1588, 1567, 1543, 1458, 1439, 1436, 1346, 1231, 963 cm–1. 1H NMR (500 MHz, CDCl3): δ = 8.70 (d, 3 J = 7.6 Hz, 1 H), 8.51 (d, 3 J = 8.0 Hz, 1 H), 8.21 (d, 3 J = 7.6 Hz, 1 H), 7.93 (d, 3 J = 8.0 Hz, 1 H), 4.16–4.06 (m, 2 H), 1.94–1.88 (m, 1 H), 1.39–1.36 (m, 4 H), 1.31–1.27 (m, 4 H), 0.93 (t, 3 J = 7.6 Hz, 3 H), 0.88 (t, 3 J = 8.8 Hz, 3 H). 13C NMR (125 MHz, CDCl3): δ = 163.2, 162.4, 151.3, 136.0, 132.4, 131.3, 130.6, 125.8, 124.1, 123.6, 122.5, 121.2, 44.6, 37.9, 30.7, 28.6, 24.0, 23.0, 14.1. MALDI (TOF) HRMS: m/z [M + H]+ calcd for C20H22BrN2O4: 433.0685; found: 433.193. 2-(2-Ethylhexyl)-6,7-bis(methylamino)-1H-benzo[de]isoquinoline-1,3(2H)-dione (3a) A 25 mL sealed tube was loaded with imide 2a (400 mg, 923 μmol). MeNH2·HCl (312 mg, 4.61mmol), K2CO3 (1.61 g, 13.8 mmol), and 1,4-dioxane (7 mL) were added, and the tube was then tightly sealed and the mixture was refluxed for 24 h. The mixture was then cooled to r.t., and the reaction was quenched by pouring into H2O (250 mL). The resulting mixture was extracted with EtOAc (3 × 150 mL) and the organic extracts were dried (Na2SO4), filtered, and concentrated under reduced pressure. The crude mixture was purified by column chromatography (silica gel, DCM) to give a brown–yellow solid; yield: 135 mg (40%). IR: 2956, 2918, 1677, 1631, 1583, 1461, 1392, 1170, 969, 810, 748 cm–1. 1H NMR (400 MHz, CDCl3): δ = 8.40 (d, 3 J = 6.0 Hz, 2 H), 6.71 (d, 3 J = 8.4 Hz, 2 H), 5.83 (s, 2 H), 4.09–4.04 (m, 2 H), 2.99 (d, 3 J = 4.8 Hz, 6 H), 1.94–1.91 (m, 1 H), 1.36–1.28 (m, 8 H), 1.44–1.29 (m, 8 H), 0.90 (t, 3 J = 7.2 Hz, 3 H), 0.86 (t, 3 J = 7.2 Hz, 3 H). 13C NMR (125 MHz, CDCl3): δ = 165.0, 153.2, 133.7, 125.0, 112.3, 111.6, 106.4, 43.8, 38.0, 31.3, 29.7, 28.9, 24.1, 23.1, 14.1, 10.8. MALDI (TOF) HRMS: m/z [M+] calcd for C22H29N3O2: 367.226; found: 367.168 7-(2-Ethylhexyl)-1,3-dimethyl-6,8-dioxo-3,6,7,8-tetrahydropyrido[3,4,5-gh]perimidin-1-ium Chloride (aPrIm) Under inert conditions, 3a (100 mg, 272 µmol) was stirred with triethyl orthoformate (4 mL) and concd HCl (100 μL) at 70 °C for 10 h. When the reaction was complete, the resultant precipitate was collected by filtration, washed with Et2O (2 × 10 mL), and dried in vacuo to give a yellow solid; yield: 77.0 mg (72%). IR: 2956, 2921, 2870, 1698, 1671, 1596, 1499, 1463, 1378, 1351, 1267, 1154, 1095, 849, 817, 771, 619 cm–1. 1H NMR (400 MHz, CDCl3): δ = 11.91 (s, 1 H), 8.69 (d, 3 J = 8.4 Hz, 2 H), 7.24 (d, 3 J = 8.4 Hz, 1 H), 4.16–4.06 (m, 8 H), 1.92–1.88 (m, 1 H), 1.39–1.27 (m, 8 H), 0.92 (t, 3 J = 7.2 Hz, 3 H), 0.87 (t, 3 J = 7.2 Hz, 3 H). 13C NMR (125 MHz, CDCl3): δ = 162.9, 137.1, 134.2, 129.1, 119.4, 119.0, 108.7, 44.5, 37.7, 30.7, 28.6, 24.0, 23.0, 14.1, 10.6. HRMS (ESI): m/z [M+] calcd for C23H28N3O2: 378.2176; found: 378.2169.
  • 20 Erten Ş, Posokhov Y, Alp S, İçli S. Dyes Pigm. 2005; 64: 171
    CrossrefSearch in Google Scholar
  • 21 Hao Y, Zheng M, Chen Y. J. Mater. Chem. B 2014; 2: 7369
    CrossrefPubMedSearch in Google Scholar

Corresponding Author

Kingsuk Mahata
Department of Chemistry, Indian Institute of Technology
Guwahati, Assam 781039
India   

Publication History

Received: 03 April 2023

Accepted after revision: 25 July 2023

Accepted Manuscript online:
25 July 2023

Article published online:
13 September 2023

© 2023. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

  • References and Notes

    • 4a Rundel K, Maniam S, Deshmukh K, Gann E, Prasad SK. K, Hodgkiss JM, Langford SJ, McNeill CR. J. Mater. Chem. A 2017; 5: 12266
      CrossrefSearch in Google Scholar
    • 4b Hong J, Ha YH, Cha H, Kim R, Kim YJ, Park CE, Durrant JR, Kwon S.-K, An TK, Kim Y.-H. ACS Appl. Mater. Interfaces 2017; 9: 44667
      CrossrefPubMedSearch in Google Scholar
    • 4c Valero S, Cabrera-Espinoza A, Collavini S, Pascual J, Marinova N, Kosta I, Delgado JL. Eur. J. Org. Chem. 2020; 5329
      Crossref
  • 9 Eckstein BJ, Melkonyan FS, Manley EF, Fabiano S, Mouat AR, Chen LX, Facchetti A, Marks TJ. J. Am. Chem. Soc. 2017; 139: 14356
    CrossrefPubMedSearch in Google Scholar
  • 10 Eckstein BJ, Melkonyan FS, Wang G, Wang B, Manley EF, Fabiano S, Harbuzaru A, Ortiz RP, Chen LX, Facchetti A, Marks TJ. Adv. Funct. Mater. 2021; 31: 2009359
    CrossrefPubMedSearch in Google Scholar
  • 12 Aksenov AV, Aksenov NA, Ovcharov DS, Aksenov DA, Griaznov G, Voskressensky LG, Rubin M. RSC Adv. 2016; 6: 82425
    CrossrefSearch in Google Scholar
  • 16 Kalle P, Kiseleva MA, Tatarin SV, Smirnov DE, Zakharov AY, Emets VV, Churakov AV, Bezzubov SI. Molecules 2022; 27: 3201
    CrossrefPubMedSearch in Google Scholar
  • 18 Puglisi R, Pappalardo A, Gulino A, Sfrazzetto GT. ACS Omega 2019; 4: 7550
    CrossrefPubMedSearch in Google Scholar
  • 19 6-Bromo-2-(2-ethylhexyl)-7-nitro-1H-benzo[de]isoquinoline-1,3(2H)-dione (2a) A 100 mL round-bottomed flask was charged with anhydride 1 (450 mg, 1.40 mmol) and EtOH (15 mL). A solution of 2-ethyl-1-hexylamine (181 mg, 1.40 mmol) in EtOH (15 mL) was added dropwise at r.t., and the mixture was stirred at 80 °C for 3 h, then cooled to r.t. The precipitate was collected by filtration and found to consist of analytically pure imide 2a. An additional amount of 2a was collected from the filtrate by treatment with H2O–EtOAc and subsequent chromatographic separation (silica gel, 10% EtOAc–hexane). The imide was obtained as a yellow solid; yield: 410 mg (68%). IR: 2956, 2918, 2870, 1704, 1662, 1588, 1567, 1543, 1458, 1439, 1436, 1346, 1231, 963 cm–1. 1H NMR (500 MHz, CDCl3): δ = 8.70 (d, 3 J = 7.6 Hz, 1 H), 8.51 (d, 3 J = 8.0 Hz, 1 H), 8.21 (d, 3 J = 7.6 Hz, 1 H), 7.93 (d, 3 J = 8.0 Hz, 1 H), 4.16–4.06 (m, 2 H), 1.94–1.88 (m, 1 H), 1.39–1.36 (m, 4 H), 1.31–1.27 (m, 4 H), 0.93 (t, 3 J = 7.6 Hz, 3 H), 0.88 (t, 3 J = 8.8 Hz, 3 H). 13C NMR (125 MHz, CDCl3): δ = 163.2, 162.4, 151.3, 136.0, 132.4, 131.3, 130.6, 125.8, 124.1, 123.6, 122.5, 121.2, 44.6, 37.9, 30.7, 28.6, 24.0, 23.0, 14.1. MALDI (TOF) HRMS: m/z [M + H]+ calcd for C20H22BrN2O4: 433.0685; found: 433.193. 2-(2-Ethylhexyl)-6,7-bis(methylamino)-1H-benzo[de]isoquinoline-1,3(2H)-dione (3a) A 25 mL sealed tube was loaded with imide 2a (400 mg, 923 μmol). MeNH2·HCl (312 mg, 4.61mmol), K2CO3 (1.61 g, 13.8 mmol), and 1,4-dioxane (7 mL) were added, and the tube was then tightly sealed and the mixture was refluxed for 24 h. The mixture was then cooled to r.t., and the reaction was quenched by pouring into H2O (250 mL). The resulting mixture was extracted with EtOAc (3 × 150 mL) and the organic extracts were dried (Na2SO4), filtered, and concentrated under reduced pressure. The crude mixture was purified by column chromatography (silica gel, DCM) to give a brown–yellow solid; yield: 135 mg (40%). IR: 2956, 2918, 1677, 1631, 1583, 1461, 1392, 1170, 969, 810, 748 cm–1. 1H NMR (400 MHz, CDCl3): δ = 8.40 (d, 3 J = 6.0 Hz, 2 H), 6.71 (d, 3 J = 8.4 Hz, 2 H), 5.83 (s, 2 H), 4.09–4.04 (m, 2 H), 2.99 (d, 3 J = 4.8 Hz, 6 H), 1.94–1.91 (m, 1 H), 1.36–1.28 (m, 8 H), 1.44–1.29 (m, 8 H), 0.90 (t, 3 J = 7.2 Hz, 3 H), 0.86 (t, 3 J = 7.2 Hz, 3 H). 13C NMR (125 MHz, CDCl3): δ = 165.0, 153.2, 133.7, 125.0, 112.3, 111.6, 106.4, 43.8, 38.0, 31.3, 29.7, 28.9, 24.1, 23.1, 14.1, 10.8. MALDI (TOF) HRMS: m/z [M+] calcd for C22H29N3O2: 367.226; found: 367.168 7-(2-Ethylhexyl)-1,3-dimethyl-6,8-dioxo-3,6,7,8-tetrahydropyrido[3,4,5-gh]perimidin-1-ium Chloride (aPrIm) Under inert conditions, 3a (100 mg, 272 µmol) was stirred with triethyl orthoformate (4 mL) and concd HCl (100 μL) at 70 °C for 10 h. When the reaction was complete, the resultant precipitate was collected by filtration, washed with Et2O (2 × 10 mL), and dried in vacuo to give a yellow solid; yield: 77.0 mg (72%). IR: 2956, 2921, 2870, 1698, 1671, 1596, 1499, 1463, 1378, 1351, 1267, 1154, 1095, 849, 817, 771, 619 cm–1. 1H NMR (400 MHz, CDCl3): δ = 11.91 (s, 1 H), 8.69 (d, 3 J = 8.4 Hz, 2 H), 7.24 (d, 3 J = 8.4 Hz, 1 H), 4.16–4.06 (m, 8 H), 1.92–1.88 (m, 1 H), 1.39–1.27 (m, 8 H), 0.92 (t, 3 J = 7.2 Hz, 3 H), 0.87 (t, 3 J = 7.2 Hz, 3 H). 13C NMR (125 MHz, CDCl3): δ = 162.9, 137.1, 134.2, 129.1, 119.4, 119.0, 108.7, 44.5, 37.7, 30.7, 28.6, 24.0, 23.0, 14.1, 10.6. HRMS (ESI): m/z [M+] calcd for C23H28N3O2: 378.2176; found: 378.2169.
  • 20 Erten Ş, Posokhov Y, Alp S, İçli S. Dyes Pigm. 2005; 64: 171
    CrossrefSearch in Google Scholar
  • 21 Hao Y, Zheng M, Chen Y. J. Mater. Chem. B 2014; 2: 7369
    CrossrefPubMedSearch in Google Scholar

   Permissions and Reprints All articles of this category
Zoom Image
Figure 1 Structures of the compounds used in the study along with relevant previous example
Zoom Image
Scheme 1 Synthetic route for the preparation of the PrIm dyes
Zoom Image
Figure 2 (a) Normalized absorption spectra of aPrIm in various solvents. (b) Absorption spectra of aPrIm at various concentrations in ethanol.
Zoom Image
Figure 3 Two different self-assembly processes involving aPrIm dye. FESEM images of the dye in DCM (left) and in ethanol (right) are also shown.