Accelerated Recombination of Lophyl Radicals in Micelles: Rapid Controlled Self-Assembly of Micelles Formed by Amphiphilic Lophine Dimers and Release of Solubilized Substance by Photoirradiation

Abstract

Controlling the morphology of molecular assemblies formed by surfactants by photoirradiation enables the controlled release of incorporated substances, which can be applied to delivery systems for drugs and active ingredients. On the other hand, conventional photoresponsive surfactants and molecular assemblies have a slow response speed, making it difficult to control their functions at the desired time. In this review, I discuss our recent progress in the accelerated control of functions of photoresponsive molecular assemblies by using lophine dimer as a photochromic compound. The lophine dimer derivative dissociates into a pair of lophyl radicals that upon ultraviolet (UV) light irradiation, and these radical species thermally recombine although the recombination reaction is extremely slow due to the diffusion of lophyl radicals. By using the confined inner space of micelles formed by surfactants, the recombination reaction was extremely accelerated. With UV light irradiation, rapid morphological changes in micelles, formed by amphiphilic lophine dimers were observed by using in situ small-angle neutron scattering (in situ SANS) system. Moreover, the rapid controlled release of calcein as a model drug was achieved by UV light irradiation using the photoresponsive micelles. This rapid system can realize the controlled release of drugs truly at the desired time, developing an efficient and precise drug delivery system (DDS). Furthermore, it can be applied in a wide range of fields such as release control of active ingredients, efficient heat exchange control, and actuating systems.

1 Introduction

By introducing a substituent into an amphiphilic molecule (surfactant if it changes interfacial properties) that changes its properties in response to external stimuli, the structure of the molecular aggregates formed can be controlled¹⁾. This makes it possible, for example, to release active ingredients (drugs, fragrances, nutrients, etc.) incorporated into the molecular aggregates by photoirradiation at the desired time (Fig. 1 (a) , top) . In addition, this stimuli-responsive surfactant not only functions as a molecular assembly but can also control interfacial properties (surface/interfacial tension, wettability, dispersibility, etc.) at any desired time. Light, electricity, magnetic fields, and changes in pH and temperature have been used as external stimuli¹⁾. Among these, 〝light〟 is clean and has excellent spatial resolution and wavelength selectivity, and molecular assemblies that change function in response to light stimuli are attracting attention^2),3),4).

Fig. 1

(a) Controlled release of solubilizates with the micelles formed by photoresponsive surfactants as an example of stimuli-responsive systems (top) and their release rates of this and previous works by photoirradiation (bottom) . (b) Photochromism of azobenzene, diarylethene and spiropyrane as typical photochromic compounds.

Photochromic molecules are often used as photo-responsive moieties, which undergo a reversible structural change upon light irradiation, resulting in a change in color tone and other photo- and electronic properties (Fig. 1 (b) ) . Examples include photoisomerization of azobenzene (trans-cis forms) , stilbene (trans-cis, dimerization forms) , diarylethene (open-closed rings) , and spiropyran (closed spiropyran-opened merocyanine types) .

Shinkai et al. designed surfactants with an azobenzene moiety as a photoresponsive moiety and reported that the micelle structure formed changes upon photoirradiation^5),6). Hatton et al. found that the balance of hydrophilic and hydrophobic groups (chain length) of azobenzene-modified nonionic surfactants significantly affects the behavior of the surface tension of aqueous solutions upon photoirradiation⁷⁾. Our group also reported that control of solubilization capacity of model perfumes^8),9)and solution viscosity¹⁰⁾of photoresponsive micelles, formed by an azobenzene-based cationic surfactant. Furthermore, the structural change of the micelles was used to control the thermal conductivity of the solution by photoirradiation and found an application in efficient heat exchange systems by Zakin and Raghavan^11),12). Thus, by combining photoresponsive molecules with various characteristics in addition to the interfacial properties and molecular assembly ability of surfactants, photoresponsive surfactants and molecular assemblies with desired functions can be developed.

On the other hand, in previous reports, including us, light irradiation on the order of minutes to hours is required to change the structure of molecular assemblies, making it difficult to develop the desired function at any given moment. If this change become rapid, the functional expression at the truly desired moment will become possible, meaning 〝on-demand〟 control of function (Fig. 1 (a) , bottom) . For example, it is possible to release the required amount of drug to the affected area at the desired time, thus realizing an efficient and precise on-demand drug delivery system. In this paper, I introduce our attempt to control interfacial properties at the desired timing, focusing on the speed and dynamics of functional changes induced by photoirradiation.

2 Acceleration of Recombination of Lophyl Radicals in Inner Spaces of Molecular Assemblies

In order to control the structure and function of molecular assemblies at high speed, we first focused on lophine dimers as a photochromic molecule¹³⁾. Upon ultraviolet (UV) irradiation, this molecule dissociates into two lophyl radicals, and these active radical species thermally recombine in the dark (Fig. 2) . However, this recombination reaction is extremely slow because the lophyl radicals diffuse freely in solution or medium. For this reason, lophine dimers and their derivatives have not received much attention as photochromic molecules but have been applied as radical polymerization initiators due to their low sensitivity to oxygen¹⁴⁾.

Fig. 2

Photochromism of lophine dimer.

On the other hand, Abe et al. reported that by chemically cross-linking two lophine moieties to each other to inhibit the diffusion of the lophyl radical, the recombination reaction is accelerated from the millimeter to femtosecond order^15),16) (Fig. 3) . This is because the formation of the crosslinked structure fixed the spatial position between the lophyl radicals produced by photoirradiation, and the radical recombination proceeded rapidly. Furthermore, Strehmel et al. reported that the recombination of lophyl radicals is accelerated by utilizing the domains formed by ionic liquids as the reaction space^17),18). From these reports, it is clear that physically or spatially constraining the diffusion of lophyl radicals speeds up the recombination reaction.

Fig. 3

Photochromism of lophine dimer bearing a linker to inhibit diffusion of lophyl radicals.

Based on the above findings, the inside of the micelle formed by the surfactant as a confined nano space was utilized to suppress the diffusion of lophyl radicals and to improve the speed of the recombination reaction. When an alkylated lophine dimer derivative (C₆-LPD) was solubilized in an aqueous micellar solution of the cationic surfactant cetyltrimethylammonium bromide (C₁₆TAB) (Fig. 4 (a) ) , the recombination rate of the lophyl radicals was promoted more than 100 times faster than in an organic solvent where the radical species can diffuse freely¹⁹⁾. Subsequently, to increase the solubilization ability inside the micelles, a lophine dimer derivative (TEG-LPD) containing a triethyleneglycol (TEG) group, a nonionic hydrophilic group, was solubilized in C₁₆TAB micelles in the same manner, and the recombination rate was further enhanced²⁰⁾ (Fig. 4) . These results indicate that the space inside the micelle is effective in promoting the recombination of the lophyl radicals.

Fig. 4

Chemical structures of lophine dimer derivatives (a) and recombination rates of lophyl radicals in micelles and an organic solvent (b) .

Next, to confirm whether this radical recombination reaction depends on the micellar reaction site, we evaluated the effect of micelle size using cationic surfactants with different alkyl chain lengths (C₁₂TAB, C₁₄TAB, or C₁₆TAB) , which are hydrophobic groups (Fig. 5) . An enhancement of the recombination reaction was observed as the alkyl chains became shorter, with the largest value at C₁₂TAB²¹⁾. Small-angle neutron scattering (SANS) was used to evaluate the size of each micelle solubilized with TEG-LPD. The long and short radii of the elliptical micelles were 22.3 and 18.1 Å for C₁₂TAB, 28.9 and 21.0 Å for C₁₄TAB, and 35.3 and 24.2 Å for C₁₆TAB²¹⁾. The size of the micelle decreased as the alkyl chain became shorter. The micelle volume calculated from this size was also taken into account, and as the volume decreased, enhanced recombination was observed. Taken together, it can be inferred that the smaller the micelle space in which radicals can diffuse, the closer the average distance between radicals becomes, which in turn promotes recombination. In addition, the evaluation of thermodynamic parameters by temperature variation measurements showed that the activation energy (ΔE_a) decreased with smaller micelles. Furthermore, the activation entropy (ΔS^‡) showed an increase in the randomness with decreasing micelle size. This higher randomness effectively worked in the small micelle space, which may have improved reactivity.

Fig. 5

Schematic of micellar structures formed between TEG-LPD and cationic surfactants and apparent reaction rates of recombination of the lophyl radicals (k') vs. the volume of the micelle with different alkyl chain length of cationic surfactants. Reprinted with permission from ref 22. Copyright 2023 Elsevier.

These results indicate that the use of the cohesive space inside the micelle promoted the recombination reaction of the lophyl radicals produced by photoirradiation. The recombination rate was found to be dependent on the concentration of the lophine dimer inside the micelle. This indicates that the distance between active radical species affects the collision frequency. From this, we can say that the aggregation space inside the molecular assemblies is a suitable reaction field for photoisomerization of the lophine dimer to achieve high speed.

Based on these results, we attempted to form micelles formed only from lophine dimers to further increase the concentration of lophine dimers inside the micelles and to reduce the diffusion distance of the produced lophyl radicals. For this purpose, an amphiphilic lophine dimer with multiple TEG groups (3TEG-LPD) was newly designed and synthesized²⁰⁾. From surface tension measurements of the synthesized 3TEG-LPD solution, critical micelle concentration (cmc) and γ_cmc were determined to be 0.80 μM and 46.5 mN/m, respectively. Dynamic light scattering measurements suggested that 3TEG-LPD forms micelles with a diameter of approximately 7.4 nm. Similarly, in aqueous solutions of 3TEG-LPD micelles, the recombination reaction of the lophyl radicals produced by UV irradiation was promoted about 800-fold compared to that in organic solvents; an approximately 4-fold enhancement was observed compared to the results for TEG-LPD in C₁₂TAB micelles. This indicates that self-assembly of the lophine dimer is effective, for rapid photochromic reactions. On the other hand, azobenzene, a common photochromic molecule, requires sufficient free volume for isomerization, and the reaction rate and isomerization rate decrease with increasing concentration and self-assembly. Lophine dimers are the opposite of this tendency and are the most suitable photoresponsive moieties for functionalization of self-assemblies such as molecular assemblies.

3 Rapid Control of Morphology of Micelles Formed by Amphiphilic Lophine Dimers

Next, the photochromism of 3TEG-LPD for rapid control of interfacial properties by photoirradiation was investigated. 5.0 mM 3TEG-LPD solution was irradiated with UV light, and the surface tension value decreased in the order of seconds²⁰⁾. When the light irradiation was stopped, the surface tension value recovered to the original value in a similar manner in a second. This suggests that the lophyl radicals generated by light irradiation pack tightly on the air/water interface and stabilize the interface. In addition, the proximity of these radical species in the Gibbs monolayer and molecular assemblies at the air/water interface suggests that the recombination reactions proceeded quickly. In conclusion, accelerating the recombination reaction of the lophyl radicals is successfully demonstrated and in the rapid control of the surface tension by utilizing inside the molecular assemblies by photoirradiation.

The micelle structures formed by amphiphilic lophine dimers was analyzed and traced their temporal changes upon photoirradiation. In order to track the rapid photoisomerization of lophine dimers and the micelle structure change upon UV light irradiation, a new system of SANS docked with a UV lamp and a UV/Vis absorption spectrometer for inducing and monitoring the photochromic reaction of amphiphilic lophine dimers is develoed²²⁾. Therefore, this system in the SANS instrument (BL-15, TAIKAN) installed was constructed in the large neutron source of the J-PARC (Japan Proton Accelerator Research Complex) research facility in Japan^23),24) (Fig. 6) . This system allows us to investigate in detail the time scale of micellar structural changes after photochromic reaction.

Fig. 6

Photograph and schematic image of in situ small-angle neutron scattering (SANS) set-up.

Figure 7a shows the SANS profile of a 10 mM 3TEG-LPD in D₂O. The analysis shows that the experimental results fit the ellipsoid model well, with a short radius of ～28 Å and a long radius of ～47 Ų⁵⁾. When the micelle solution was irradiated with UV light, the long radius immediately changed to about 70 Å. On the other hand, the short radius did not change. Next, when the UV light irradiation was stopped, the profile immediately returned to its original shape, indicating that the structure was quickly recovered. This indicates that the 3TEG-LPD elongates and contracts in the direction of the long radius upon UV light irradiation.

Fig. 7

In-situ SANS profiles and UV/Vis absorption data. (a) SANS profiles (actual data and fitted curves) of 10 mM 3TEG-LPD in D₂O before and after 2 min UV light irradiation and 4 min in the dark. (b) The integrated scattering intensity of the SANS profiles of 3TEG-LPD in the q-range of 0.01-0.05 Å^－1 during multiple cycles of irradiation and idle periods in the dark. (c) UV/Vis absorption spectra before and after UV irradiation and idle storage in the dark, and the temporal changes in absorption at 580 nm during the cycle using an in-situ UV/Vis absorption spectrometer.

To follow the temporal changes in the micelle structure, the irradiation cycle of 2 min light irradiation/4 min in the dark was repeated several times. To evaluate the intensity of the SANS profiles, the intensity integrals of the profiles every 60 seconds at q values from 0.01 to 0.05 Å^－1 were plotted against time (Fig. 7 (b) ) . The results show that the processes of elongation and contraction of the micelle structure upon UV light irradiation ON-OFF are both completed within 60 seconds.

In addition, the UV-visible absorption spectra were simultaneously measured. The absorption peak at 580 nm derived from the lophyl radical increased upon UV light irradiation (Fig. 7c) . The change in absorbance with light irradiation ON-OFF was almost consistent with the change in SANS integrals. This indicates that the change in micelle structure occurs without a time lag from the photoisomerization reaction of the lophyl radical. In conclusion, it was found that the micelle formed by 3TEG-LPD undergoes a rapid and reversible structural change upon UV light irradiation.

Recently, time-resolved structural analysis of the 3TEG-LPD micelle by the stroboscopic SANS revealed the morphological change with a time width of 0.5 s in the presence and absence of UV light irradiation²⁶⁾. This revealed that the morphological change in the micelles is faster than the recombination of the lophyl radicals. The result implies that more about the dynamics of the photochromic reaction and self-assembly needs to be understood in order to realize more rapid control of micelle morphology and its functions.

4 Rapid Controlled Release

We investigated the controlled release of solubilized substance using these rapid photoresponsive micelles. As a fluorescent model drug, calcein^27),28)was solubilized in an aqueous solution of 3TEG-LPD micelles. Fluorescence spectra showed a calcein-derived peak at 545 nm, which was shifted to a longer wavelength and decreased in intensity upon UV light irradiation (Fig. 8 (a) ) . This indicates that calcein is released from the micelles upon photoirradiation, resulting in aggregation and precipitation.

The fluorescence peak intensity plotted against time showed that the fluorescence change was completed earlier than 60 seconds upon UV irradiation (Fig. 8 (b) ) . This indicates that the change in solubilizing capacity is sufficient for the release of calcein before the UV-irradiated micelle structural change reaches a steady state. Similarly, calcein release experiments were conducted using an amphiphilic lophene dimer with 12 TEG groups (6TEG-LPD) , which is known not to undergo photo-induced micellar morphological changes. As a result, no fluorescence change was observed. This confirms that the fluorescence change in the 3TEG-LPD micelle system is not caused by photo-degradation of calcein, but is induced by photo-induced changes in the micelle structure. From this, it was found that 3TEG-LPD micelles can rapidly release solubilized model drugs.

Fig. 8

(a) Variations in fluorescence spectra of 1.0 mM calcein/5.0 mM 3TEG-LPD aqueous solution under UV light irradiation. (b) The normalized transient changes in fluorescence intensity at the peaks for 1.0 mM calcein/5.0 mM 3TEG-LPD and 6TEG-LPD aqueous solutions during UV irradiation. The image of photo-induced controlled release of calcein from 3TEG-LPD micelle.

5 Conclusion

This review focused on control of interfacial properties, focusing on the speed and dynamics of functional changes induced by photoirradiation. The recombination reaction of lophyl radicals produced from lophine dimer upon ultraviolet (UV) light irradiation was acerated using the inner space of micelles formed by surfactants. Depending on the internal space size of the micelle and its effective concentration of lophyl radicals, the recombination reaction of lophyl radicals is significantly accelerated. It was revealed that it is important to control the distance by inhibiting the diffusion of lophyl radicals. Furthermore, by using a newly developed in situ small-angle neutron scattering (in situ SANS) system, it was found that the ellipsoidal micelles formed by the amphiphilic lophine dimer (3TEG-LPD) reversibly extend and shrink in the order of seconds upon UV light irradiation. The rapid photoresponsive lophine dimer micelles realized rapid controlled release of model drugs, solubilized in the micelle.

Recent results also revealed that the hydrophilic group structure of the amphiphilic lophine dimer affects the recombination rate, which in turn significantly affects the release behavior of the model drug. From this, a search for suitable substituents on the lophine dimer skeleton is still possible. In addition, our micelle system is approximately 2-fold slower than the cross-linked lophine dimer by Abe et al. This indicates that ultrafast photoresponsive molecular assemblies are expected to be constructed by optimizing the driving force of self-assembly and molecular design. Furthermore, this system requires a unified understanding of hierarchical chemical phenomena at different scales, such as photochromic reactions, micellar morphology, and solubilizing ability. In the future, it is expected that SANS stroboscopic analysis²⁶⁾will be used to understand the system and to search for the optimal structure.

This system is expected to contribute not only to drug delivery systems (DDS) , but also to a wide range of fields such as release control of active ingredients, efficient heat exchange control, and actuating systems.

Acknowledgement

I am grateful to thank prof. Hideki Sakai and prof. Kenichi Saka (Tokyo University of Science) for their supervision and support of these studies. I would like to thank Dr. Hiroki Iwase (CROSS) for the collaborative work in neutron scattering measurements. This research was partly supported by JSPS KAKENHI Grant-in-Aid for Early-Career Scientist (Grant Number JP20K15248) .

References

• 1)  Brown, P.; Butts, C.P.; Eastoe, J. Stimuli-responsive surfactants. Soft Matter 9, 2365 (2013) .
• 2)  Eastoe, J.; Vesperinas, A. Self-assembly of light-sensitive surfactants. Soft Matter 1, 338 (2005) .
• 3)  Chen, S.; Costil, R.; Leung, F.K.; Feringa, B.L. Self‐assembly of photoresponsive molecular amphiphiles in aqueous media. Angew. Chem. Int. Ed. 60, 11604-11627 (2021) .
• 4)  Yamaguchi, T.; Ogawa, M. Photoinduced movement: How photoirradiation induced the movements of matter. Sci. Tech. Adv. Mater. 23, 796-844 (2022) .
• 5)  Shinkai, S.; Matsuo, K.; Sato, M.; Sone, T.; Manabe, O. Photocontrol of micellar catalysis. Tetrahedron Lett. 22, 1409-1412 (1981) .
• 6)  Shinkai, S.; Matsuo, K.; Harada, A.; Manabe, O. Photocontrol of micellar catalyses. J. Chem. Soc., Perkin Trans. 2 No. 10, 1261 (1982) .
• 7)  Shang, T.; Smith, K.A.; Hatton, T.A. Photoresponsive surfactants exhibiting unusually large, reversible surface tension changes under varying illumination conditions. Langmuir 19, 10764-10773 (2003) .
• 8)  Orihara, Y.; Matsumura, A.; Saito, Y.; Ogawa, N.; Saji, T. et al. Reversible release control of an oily substance using photoresponsive micelles. Langmuir 17, 6072-6076 (2001) .
• 9)  Akamatsu, M.; FitzGerald, P.A.; Shiina, M.; Misono, T.; Tsuchiya, K. et al. Micelle structure in a photoresponsive surfactant with and without solubilized ethylbenzene from small-angle neutron scattering. J. Phys. Chem. B 119, 5904-5910 (2015) .
• 10)  Sakai, H.; Orihara, Y.; Kodashima, H.; Matsumura, A.; Ohkubo, T. et al. Photoinduced reversible change of fluid viscosity. J. Am. Chem. Soc. 127, 13454-13455 (2005) .
• 11)  Shi, H.; Wang, Y.; Fang, B.; Talmon, Y.; Ge, W. et al. Light-responsive threadlike micelles as drag reducing fluids with enhanced heat-transfer capabilities. Langmuir 27, 5806-5813 (2011) .
• 12)  Shi, H.; Ge, W.; Oh, H.; Pattison, S.M.; Huggins, J.T. et al. Photoreversible micellar solution as a smart drag-reducing fluid for use in district heating/cooling systems. Langmuir 29, 102-109 (2013) .
• 13)  Hayashi, T.; Maeda, K. Preparation of a new phototropic substance. Bull. Chem. Soc. Jpn. 33, 565-566 (1960) .
• 14)  Dumur, F. Recent advances on visible light triphenylamine-based photoinitiators of polymerization. Eur. Polym. J. 166, 111036 (2022) .
• 15)  Fujita, K.; Hatano, S.; Kato, D.; Abe, J. Photochromism of a radical diffusion-inhibited hexaarylbiimidazole derivative with intense coloration and fast decoloration performance. Org. Lett. 10, 3105-3108 (2008) .
• 16)  Kishimoto, Y.; Abe, J. A fast photochromic molecule that colors only under UV light. J. Am. Chem. Soc. 131, 4227-4229 (2009) .
• 17)  Berdzinski, S.; Horst, J.; Straßburg, P.; Strehmel, V. Recombination of lophyl radicals in pyrrolidinium-based ionic liquids. ChemPhysChem 14, 1899-1908 (2013) .
• 18)  Strehmel, V.; Wishart, J.F.; Polyansky, D.E.; Strehmel, B. Recombination of photogenerated lophyl radicals in imidazolium-based ionic liquids. ChemPhysChem 10, 3112-3118 (2009) .
• 19)  Akamatsu, M.; Suzuki, T.; Tsuchiya, K.; Masaki, H.; Sakai, K.; Sakai, H. Accelerated recombination of lophyl radicals solubilized in micelles. Chem. Lett. 47, 113-115 (2018) .
• 20)  Akamatsu, M.; Kobayashi, K.; Sakai, K.; Sakai, H. Accelerated recombination of lophyl radicals and control of the surface tension with amphiphilic lophine dimers. Chem. Commun. 55, 9769-9772 (2019) .
• 21)  Akamatsu, M.; Morita, T.; Kobayashi, K.; Iwase, H.; Sakai, K.; Sakai, H. Controlled recombination rate of lophyl radicals in cationic surfactant micelles. Colloids Surf. A: Physicochem. Eng. Asp. 638, 128319 (2022) .
• 22)  Kelly, E.A.; Houston, J.E.; Evans, R.C. Probing the dynamic self-assembly behaviour of photoswitchable wormlike micelles in real-time. Soft Matter 15, 1253-1259 (2019) .
• 23)  Sakaguchi, Y.; Kasai, S.; Oh-Uchi, K.; Morikawa, T.; Iwase, H.; Akamatsu, M. et al. Recent development of the sample environment for light irradiation experiments at the materials and life science experimental facility. JPS Conf. Proc. 33, 1-7 (2021) .
• 24)  Iwase, H.; Akamatsu, M.; Inamura, Y.; Sakaguchi, Y.; Morikawa, T. et al. New measurement system based on small-angle neutron scattering for structural analysis of light-responsive materials. J. Appl. Crystallogr. 56, 110-115 (2023) .
• 25)  Akamatsu, M.; Kobayashi, K.; Iwase, H.; Sakaguchi, Y.; Tanaka, R.; Sakai, K.; Sakai, H. Rapid controlled release by photo-irradiation using morphological changes in micelles formed by amphiphilic lophine dimers. Sci. Rep. 11, 10754 (2021) .
• 26)  Iwase, H.; Akamatsu, M.; Inamura, Y.; Sakaguchi, Y.; Kobayashi, K.; Sakai, H. Time-resolved structural analysis of fast-photoresponsive surfactant micelles by stroboscopic small-angle neutron scattering. Langmuir 39, 12357-12364 (2023) .
• 27)  Memoli, A.; Palermiti, L.G.; Travagli, V.; Alhaique, F. Effects of surfactants on the spectral behaviour of calcein (II) : A method of evaluation. J. Pharm. Biomed. Anal. 19, 627-632 (1999) .
• 28)  Memoll, A.; Palermitit, L.G.; Travaglif, V.; Alhaique, F. Effects of surfactants on the spectral behaviour of calcein. J. Pharm. Biomed. Anal. 12, 307-312 (1994) .