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Open AccessCCS ChemistryRESEARCH ARTICLE1 Dec 2020

Controllable Fragrance Release Mediated by Spontaneous Hydrogen Bonding with POSS–Thiourea Derivatives

    CCS Chem. 2020, 2, 478–487
    https://doi.org/10.31635/ccschem.020.202000166

    The purpose of achieving the long-lasting fragrance perception leads to nanocarrier-based profragrances in perfume applications. Herein, we report a family of novel profragrance systems based on polyhedral oligomeric silsesquioxane (POSS) derivatized thioureas (POSS thioureas) that enable linkage of volatile carbonyl fragrances with the spontaneous formation of fragile hydrogen bonds. This profragrance platform addresses the dilemma of the volatile nature of aroma-materials on the one hand, and the desired long-lasting effects on the other. Their releasing performance as profragrances is investigated by headspace solid-phase microextraction (SPME) in combination with gas chromatography (GC) analysis under water as the external humidity stimulus, indicating that the fragrance concentration released from the POSS–thiourea-based profragrance is up to four times higher than the neat reference of the corresponding perfume aldehydes. Furthermore, deposition of the novel profragrance system onto wallpaper results in excellent retentive capacity for volatile aldehydes. Given the low essential toxicity, the POSS–thiourea system has been demonstrated as a suitable profragrance for practical application to perfume delivery.

    Introduction

    Nowadays there are growing demands for fragrance-releasing technology in personal care or household products such as aromatic wallpaper, leathers, or textile fabrics.1,2,3 The highly volatile nature of traditional fragrances with effective sensory response causes easy loss in a short time. Thus, achieving a long-lasting perception of the aroma volatiles becomes a challenging task in search of either controllable delivery systems or novel fragrance ingredients.4–7 An attractive strategy to efficiently increase the long-lastingness of volatile aldehyde or ketone fragrances is encapsulation technique5, which relies on the so-called “core-shell structure” model.8–13 For instance, the fragrance molecules are enclosed in polymeric materials that can release the fragrances upon the breakage or diffusion of shells.14–26 In contrast to physical encapsulation, chemical approaches of profragrances have been developed by Herrmann and others, with chemically covalent binding of volatile molecules to the proper substrates during the formation of profragrance non-volatiles,1,27–33 wherein the volatile perfume molecules are chemically released with cleavage of a covalent bond or a weak interaction such as hydrogen bond.

    Polyhedral oligomeric silsesquioxane (POSS) is a silsesquioxane molecule of nanoscopic size, approximately 1–3 nm in diameter when the vertex groups are included. POSS has been intensively studied in biomaterial applications, nanocomposite separation membranes, and textiles.34–37 Given that POSS is very chemically stable and unreactive, it is therefore an excellent material for long-term applications where there is a demand for bio-safety and compatibility. However, to the best of our knowledge, the precise functionalization of POSS to investigate its controllable fragrance-release properties is rare. Moreover, the porosity of surfaces of natural fibers such as wallpaper, leathers, or silk textiles is deemed to adsorb the nanoparticles by surface deposition.38

    External stimuli such as hydrolysis or humidity, exposure to light, oxidation, elevated temperature, or pH are naturally ideal to trigger the controllable release of fragrances.30,32,3940–46 Given the specific dual hydrogen bonds with carbonyl functional groups, thiourea derivatives have emerged as efficacious catalysts for a variety of reactions in the connectivity with substrates of aldehydes or ketones.48,49 Of note, volatile organic compounds that bear carbonyl groups such as aldehydes, ketones, or esters are the largest group of perfume molecules.46,50 Provided that the spontaneous formation of hydrogen bonds can serve to bind and release the fragrance molecules of carbonyl compounds with thioureas in dynamic equilibria, we herein reported the POSS thioureas as profragrance substrates to bind with aldehydes for the sake of realizing the controllable fragrance release. As illustrated in Figure 1, the dual hydrogen bonds formed in the POSS–thiourea profragrance are unstable and fragile under atmospheric moisture, thus guaranteeing the breakdown of hydrogen bonds to release perfume molecules. Using water as an external humidity stimulus, we compared the releasing performance of five different POSS–thiourea substrates by headspace solid-phase microextraction (SPME) in combination with gas chromatography (GC),51–55 demonstrating that the perfume concentration released from the profragrance POSS thioureas is up to four times higher than the neat reference of the corresponding perfume aldehydes, suggestive of the prolonged fragrance release. The deposition of the novel profragrance system onto wallpaper can also exert excellent retentive capacity for volatile perfume aldehydes. Besides its low essential toxicity, the POSS–thiourea profragrance system has been for the first time demonstrated as a suitable precursor for practical application to fragrance delivery.

    Figure 1

    Figure 1 | Controllable release of POSS–thiourea profragrance with the assistance of intermolecular dual hydrogen bonding, using water as the external humidity stimulus, which serves to bind and release the fragrance molecules of carbonyl perfumes.

    Experimental Method

    Materials and general methods

    Unless otherwise indicated, all materials were obtained from commercial sources and used as purchased without further purification. Aminopropylisobutyl POSS (98%) and cinnamaldehyde (99%) were purchased from Bidepharm. 4-Methoxyphenyl isothiocyanate (95%), 4-methylphenyl isothiocyanate (98%), phenyl isothiocyanate (98%), 4-nitrophenyl isothiocyanate (98%), and 3,5-bis(trifluoromethyl)phenyl isothiocyanate (98%) were purchased from Aladdin Reagent. Cyclamen aldehyde (92%) and helional (97%) were purchased from MACKLIN Reagent. Flash column chromatography was performed with silica gel of 200–300 mesh ASTM. Reactions were monitored by TLC under detection with UV light. 1H and 13C NMR spectra were recorded in CDCl3 on Bruker 400 MHz spectrometers, with TMS as internal standard. Mass spectra were obtained using an electrospray ionization (ESI-TOF) mass spectrometer.

    Synthesis and characterization of 4-methoxyphenyl substituted POSS thiourea (3a)

    General methods: Aminopropylisobutyl POSS (0.2 mmol, 1 equiv.) was dissolved in dichloromethane (2 mL); phenyl isothiocyanate (0.2 mmol, 1 equiv.) was dissolved in dichloromethane (3 mL). Both were added to a 25 mL Schleck tube and stirred for 12 hours at room temperature. After the reaction was completed, the solvent was evaporated under reduced pressure. The crude product was purified by column chromatography using ethyl acetate/petroleum ether as eluent to afford the desired product, the title compound 3a as a white solid. 92% yield. M. p. 175–177 °C. 1H NMR (400 MHz, CDCl3): δ 7.14 (d, J = 8.8 Hz, 2H), 6.94 (d, J = 8.9 Hz, 2H), 3.83 (s, 3H), 3.59 (dd, J = 13.0, 6.8 Hz, 2H), 1.84 (m, 7H), 1.73–1.57 (m, 2H), 0.94 (t, J = 6.4 Hz, 42H), 0.67–0.50 (m, 16H). 13C NMR (100 MHz, CDCl3): δ 181.19, 159.00, 127.77, 115.32, 55.49, 47.70, 25.69, 25.66, 23.88, 23.84, 22.47, 22.39, 9.36. HRMS (ESI-TOF): m/z [M + H]+ Calcd for C39H78N2O13SSi8H 1039.3457; found 1039.3448.

    Synthesis and characterization of 4-methylphenyl POSS thiourea (3b)

    White solid. 98% yield. M. p. 185–187 °C. 1H NMR (400 MHz, CDCl3): δ 7.70 (s, 1H), 7.23 (d, J = 8.0 Hz, 2H), 7.09 (d, J = 8.2 Hz, 2H), 5.96 (s, 1H), 3.60 (dd, J = 12.9, 6.8 Hz, 2H), 2.37 (s, 3H), 1.84 (m, J = 13.4, 9.8, 6.7 Hz, 7H), 1.66 (t, J = 7.4 Hz, 2H), 0.94 (t, J = 6.7 Hz, 42H), 0.59 (t, J = 7.1 Hz, 16H). 13C NMR (100 MHz, CDCl3): δ 180.75, 137.52, 133.34, 130.80, 125.50, 47.72, 25.69, 25.66, 23.88, 23.84, 22.48, 22.39, 21.04, 9.39. HRMS (ESI-TOF): m/z [M + H]+ Calcd for C39H78N2O12SSi8H 1023.3508; found 1023.3506.

    Synthesis and characterization of phenyl POSS thiourea (3c)

    White solid. 97% yield. M. p. 183–185 °C. 1H NMR (400 MHz, CDCl3): δ 7.60 (s, 1H), 7.44 (t, J = 7.8 Hz, 2H), 7.31 (t, J = 7.5 Hz, 1H), 7.20 (d, J = 7.6 Hz, 2H), 6.03 (s, 1H), 3.62 (dd, J = 12.8, 6.8 Hz, 2H), 1.84 (m, 7H), 1.68 (m, 2H), 0.94 (t, J = 6.6 Hz, 42H), 0.64–0.51 (m, 16H). 13C NMR (100 MHz, CDCl3): δ 180.63, 136.13, 130.26, 127.32, 125.25, 47.75, 25.70, 25.66, 23.88, 23.84, 22.48, 22.39, 9.40. HRMS (ESI-TOF): m/z [M + H]+ Calcd for C38H76N2O12SSi8H 1009.3351; found 1009.3361.

    Synthesis and characterization of p-nitrophenyl POSS thiourea (3d)

    White solid. 85% yield. M. p. 179–182 °C. 1H NMR (400 MHz, CDCl3): δ 7.14 (d, J = 8.8 Hz, 2H), 6.94 (d, J = 8.9 Hz, 2H), 3.83 (s, 3H), 3.59 (dd, J = 13.0, 6.8 Hz, 2H), 1.84 (m, 7H), 1.73–1.57 (m, 2H), 0.94 (t, J = 6.4 Hz, 42H), 0.67–0.50 (m, 16H). 13C NMR (100 MHz, CDCl3): δ 181.19, 159.00, 127.77, 115.32, 55.49, 47.70, 25.69, 25.66, 23.88, 23.84, 22.47, 22.39, 9.36. HRMS (ESI-TOF): m/z [M + H]+ Calcd for C39H78N2O13SSi8H 1039.3457; found 1039.3448.

    Synthesis and characterization of 3,5-bis(trifluoromethyl)phenyl POSS thiourea (3e)

    White solid. 90% yield. M. p. 162–165 °C. 1H NMR (400 MHz, CDCl3): δ 8.03–7.56 (m, 3H), 6.16 (s, 1H), 3.81–3.35 (m, 2H), 1.97–1.68 (m, 9H), 0.94 (t, J = 6.6 Hz, 42H), 0.71–0.48 (m, 16H). 13C NMR (100 MHz, CDCl3): δ 180.67, 137.48 (t, J = 1.2 Hz), 130.24 (q, J = 34.5 Hz), 124.10 (q, J = 271.8 Hz), 121.39 (q, J = 4.5 Hz), 119.12 (m), 25.67, 25.61, 23.87, 23.84, 22.45, 22.39, 9.52. HRMS (ESI-TOF): m/z [M + H]+ Calcd for C40H74F6N2O12SSi8H 1145.3099; found 1145.3104.

    Headspace SPME

    SPME was applied to detect the fragrance molecules released from the composites. The extraction fiber diameter was 75 μm; it was purchased from Supelco, Inc. (Bellefonte, PA, USA) and painted with carboxen/polydimethylsiloxane (CAR/PDMS). Before adsorption of aroma substances, the extraction fiber was preconditioned in the injection port of the gas chromatograph at 250 °C for 30 min, with the flow rate of carrier gas (helium) maintained at 1.0 mL min−1.

    GC analysis

    GC spectrometry is an efficient, accurate, and fast modern instrumental analysis method, which plays a key role in fragrance analysis. The analysis of profragrances through GC (7890B, Agilent Technologies, New York, USA) was used to identify their concentration of aromatic constituents. The Agilent 7890B gas chromatograph was used with a DB-Innowax polar column (60 m × 0.25 mm i.d. × 0.25 m film thickness, Supelco, Bellefonte, PA, USA). The carrier gas was helium (purity = 99.999%) with a flow rate of 2.0 mL s−1. The injection was conducted in a splitless mode for 3 min at 250 °C. The temperature program was isothermal for 3 min at 80 °C and rose to 230 °C at a rate of 10 °C min−1. Finally, it was heated up to 250 °C and held for 10 min. Injection temperature was 250 °C.

    Fragrance-releasing analysis

    For the measurements, separate headspace bottles of 10 mL capacity were labeled with No. 1, No. 2, and so on. One of the bottles contained 0.04 mmol of thiourea and an equimolar amount of aldehyde; the mixture was dissolved in 1 mL diethyl ether with subsequent evaporation to create the hydrogen-bonding profragrance complex. For the reference, 0.04 mmol of aldehyde was added to the other bottle. Both were held at room temperature for 24 h. Then 0.5 mL of deionized water was added to both bottles, together with 20 μL of o-dichlorobenzene solution (100 μg mL-1 in acetone) to serve as an internal standard. After intense stirring, the bottles were sealed for SPME. The headspace vials were preserved at 35 °C with adsorption for 20 min.

    Results and Discussion

    Constructing the profragrance system of POSS thioureas

    Thioureas have been synthesized by numerous methods. Reactions of amines with various reagents, such as thiophosgene, carbon disulfides, thiazalidine-2-thiones, or isothiocyanates constitute the most widely used procedures.54,55 Among them, the condensation of a primary amine with isothiocyanates is the most efficient route since the resulting thioureas are generally clean with high yield. As shown in Figure 2, POSS–thiourea precursors were prepared by reaction of POSS-derivatized amine 1 with aryl-substituted thiocyanates 2. As expected, the reaction proceeded smoothly in dichloromethane as solvent at room temperature for 12 hours. The corresponding products 3a– 3e were obtained in excellent yields of 85–98%, suggestive of high reaction efficiency in a very clean manner. Five aryl intermediates of isothiocyanates 2, namely 4-methoxyl, 4-methyl phenyl, phenyl, 3,5-bis(trifluoromethyl)phenyl, and 4-nitrophenyl, were employed to construct the corresponding POSS thiourea 3a– 3e. These variants were used to test the electronic effects of substituents on the controllable release of hydrogen-bonded fragrance molecules.

    Figure 2

    Figure 2 | POSS–thiourea precusors (3a–3e) prepared by reaction of POSS-substituted amine with aryl thiocyanates; high reaction efficiency was achieved.

    Spontaneous formation of dual hydrogen bonds in dynamic equilibria

    To verify the spontaneous formation of dual hydrogen bonds between POSS thioureas and perfume aldehydes, conventional 1H NMR experiments were carried out with a mixture of POSS thioureas 3 and cyclamen aldehyde in 1/1 molar ratio, as well as the pure cyclamen aldehyde. Before the NMR measurement, the mixture of 3 and the aldehydes was dissolved in dry CDCl3 for 12 h. Figure 3 shows the superimposed NMR spectra of both the mixture and the pure cyclamen aldehyde for comparison. In the mixture of cyclamen aldehyde with 3c, the chemical shift δ value of the characteristic peak of –CHO was shifted by 0.002 ppm. For 3a, 3b, 3d, and 3e, δ values were shifted by 0.001, 0.004, 0.001, and 0.002 ppm (see in Supporting Information), respectively. It is strongly suggestive of the spontaneous formation of intermolecular hydrogen bonding between the two components. Peak shifts of cinnamaldehyde and helional (Figure 1) with 3d were also evident ( Supporting Information), with δ values shifted by 0.011 and 0.001 ppm, respectively. Moreover, the δ value of one of the characteristic N–H peaks of thiourea 3c, in the range of 7.0–8.0 ppm, was also shifted by 0.081 ppm. Similarly, the δ values were shifted by 0.059, 0.016, 0.011, and 0.039 ppm for 3a, 3b, 3d and 3e ( Supporting Information), respectively. As a consequence, these 1H NMR experiments give direct evidence for the spontaneous formation of dual hydrogen bonds between POSS thioureas and perfume aldehydes.

    Figure 3

    Figure 3 | Change of chemical shift in the spontaneous formation of dual hydrogen bonds between POSS thiourea 3c and perfume cyclamen aldehyde (1H NMR, CDCl3).

    Controllable release behavior of profragrances

    The moisture in a high-humidity environment is the most desirable factor or stimulus to drive the dissociation of aldehydes by breaking the fragile dual hydrogen bonds. We propose that, in the POSS–thiourea system, the perfume molecules of cyclamen aldehyde can be controllably released by exposure of the profragrance to water,56 which competes for the hydrogen-bonding adsorption sites on POSS thiourea.47 GC-FID analysis equipped with a SPME system was carried out for testing the releasing performance of profragrances. Specifically, the mixtures of profragrances with aldehydes were dissolved in diethyl ether with a subsequent evaporation for the hydrogen-bonding POSS–thiourea system. The vials with the profragrance systems and pure references of aldehydes were then kept open to volatilize the aldehydes for 24 h at room temperature. Furthermore, to quantitatively evaluate the performance of the dynamic mixture, o-dichlorobenzene was introduced into the system before the vials were heated and served as the internal standard. The headspace vials were sealed and preserved at 35 °C, which contained originally 0.04 mmol of profragrances and 20 μL (0.14 mmol) of o-dichlorobenzene. The concentrations of released perfume component (μg mL−1) were well described by Equation 1:

    c = A 1 × 100 × 20 A 2 × 10 (Equation 1)
    where A1 represents the peak area of released perfume component in GC-FID analysis. A2 the peak area of released internal standard of o-dichlorobenzene. Constants 100 and 20 are characteristic for the concentration (μg mL−1) and the volume (μL) of internal standard; the number of 10 is the capacity of the headspace vial (mL).

    Figure 4 shows the concentration of cyclamen aldehyde released from 0.04 mmol mixtures of POSS thioureas bearing different substituents with cyclamen aldehydes. One line in the figure represents the release curve of the pure cyclamen aldehyde after being volatilized for 24 h, the other line shows the fragrance-releasing trend with addition of POSS thioureas. Several features are interesting. As expected, after volatilizing for 24 h, the rest cyclamen aldehydes released from the mixtures quickly reached dynamic equilibria and the concentrations were 405, 490, 498, 534, and 582 μg L−1 for 3a, 3b, 3c, 3d, and 3e, respectively. The concentration of pure cyclamen aldehyde was 183 μg L−1, which was lower than the corresponding profragrance systems. The results are consistent with the expected formation of the dual hydrogen bonds (Figure 1), along with prolonging the slow release of perfume aldehydes. Furthermore, the profragrance of POSS thioureas can increase the headspace concentrations of aromatic aldehydes up to four times greater than the neat reference of the corresponding aldehydes.

    Figure 4

    Figure 4 | (a) SPME analysis in combination with gas chromatography (GC), with comparisons of headspace curves of free aldehyde and volatile fragrance released from profragrance 3a (b), 3b (c), 3c (d), 3d (e), and 3e (f) treated with water.

    Moreover, the substituent effect on POSS thioureas seems to be another important parameter in the releasing efficiency. For instance, the POSS thioureas substituted with electron-withdrawing groups, such as nitro and trifluoromethyl groups, gave rise to higher headspace concentrations of fragrances than those substituted with electron-donating groups, such as methyl and methoxyl, and the lowest headspace concentrations were measured in the presence of precursor 3a bearing a methoxyl group. This phenomenon could be attributed to the fact that stronger hydrogen bonds might be formed between the thiourea substituted with electron-withdrawing group and perfume aldehydes.

    Next, we employed several release kinetic models to fit the performance of the profragrance systems. Dissolution–diffusion mathematical kinetic models (zero-order, first-order, Higuchi, Korsmeyer–Peppas, Hixson–Crowell, and Bhaskar) were adopted to describe the release performance of the fragrance delivery system in solution (Figure 5). Through the correlation coefficients (R2) of corresponding kinetic models, it is evident that the release dynamics of profragrances conform to a first-order kinetic model because all R2 values were greater than 0.95, and even up to 0.99. According to a first-order kinetic model, the release rate is linearly dependent on the initial concentration of POSS–thiourea profragrances.

    Figure 5

    Figure 5 | Kinetic models to fit the release performance of POSS–thiourea profragrance systems. (a–f) The release kinetics simulation and (g) mathematical models of fragrance release, and the corresponding best-fit constants and correlation coefficients.

    To test the potential application of functional perfume in aromatic wallpaper, the profragrance mixture of POSS thiourea 3c with cyclamen aldehyde was deposited onto base paper, as illustrated in Figure 6. POSS thiourea 3c with an equimolar amount of cyclamen aldehyde was added into the beaker, followed by addition of alcohol. The 1 cm × 1 cm sample of base paper was soaked in the aforementioned solution for 48 h and taken out and dried in a dish. At the same time, a sample of pure cyclamen aldehyde was deposited as reference in a similar manner. The dried wallpapers were investigated by SPME analysis using water as the trigger to represent a humid environment. The amount of desorbed fragrance of cyclamen aldehyde was determined by the volatiles as internal compounds from a sealed system. The integrated peak areas of cyclamen aldehydes in the GC analysis were plotted against the time. Figure 6 shows the releasing property on the wallpaper to check the release performance of POSS-thiourea profragrance systems. Since the cellulose bearing plenty of hydroxyl groups, which also formed hydrogen-bonding with aldehydes. Although it is very difficult to discuss the precise rate constants, the profragrance has a greater rate constant, and “release kinetics,” in proportion to its greater slope when the concentrations are the same. In comparison of Figure 5, the concentration from wallpaper is much lower than the pure examples, which is well demonstrated as our target of controllable release.

    Figure 6

    Figure 6 | The fragrance release curves of POSS thioureas in wallpaper. (a) The process by which wallpaper is treated with precursor 3. Comparisons of headspace curves of free aldehyde and volatile fragrance released from profragrance 3a (b), 3b (c), 3c (d), 3d (e), 3e (f) using water as the trigger to represent a humid environment.

    Conclusion

    We have for the first time achieved controllable fragrance release mediated by the spontaneous formation of dual hydrogen bonding with POSS thioureas, especially for the purpose of long-lastingness of perfumes. The POSS precursors are nanoscale, nontoxic, and have high affinity to wallpaper fabrics. As demonstrated, the moisture condition in the presence of high-humidity environment is the most desirable factor or stimulus to influence the dissociation of aldehydes by breaking the fragile dual hydrogen bonds. Several mathematical models were adopted to fit the kinetic performance of the release property in the dynamic systems, in which the first-order kinetic model gave acceptable correlation coefficients (R2 > 0.95). GC analysis with SPME provided a promising quantitative analysis method for the study of profragrance release. We also explored the headspace concentrations of perfume aldehydes released from a mixture of POSS thioureas and cyclamen aldehyde using water as the humidity trigger, thereby achieving the long-lasting release of fragrance from wallpaper. Taken together with the convenience of the spontaneous hydrogen bond formation, the POSS–thiourea derivatives provide a straightforward platform to build proper profragrance systems for practical applications in perfume release.

    Supporting Information

    Supporting Information is available.

    Conflict of Interest

    The authors declare no conflicts of interest.

    Funding Information

    This work was supported by National Key Research and Development Program (2016YFA0200300), NSFC/China (21788102 and 21636002), Shanghai Municipal Science and Technology Major Project (2018SHZDZX03), Shanghai Municipal Science and Technology Major Project (2018SHZDZX03), Innovation Program of Shanghai Municipal Education Commission, Programme of Introducing Talents of Discipline to Universities (B16017), Croucher Foundation (Hong Kong) in the form of a CAS-Croucher Foundation Joint Laboratory Grant, and Fundamental Research Funds for the Central Universities.

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