Thermal Hysteresis in the Photoresponsivity of a Langmuir Film of Amphiphilic Spiropyran

Spiropyran/Merocyanine Amphiphile in Various Solvents: A Joint Experimental–Theoretical Approach to Photophysical Properties and Self-Assembly

This joint experimental-theoretical work focuses on molecular and photophysical properties of the spiropyran-containing amphiphilic molecule in organic and aqueous solutions. Being dissolved in tested organic solvents, the system demonstrates positive photochromism, i.e., upon UV stimulus the colorless spiropyran form is transformed into colorful merocyanine isomer. However, the aqueous solution of the amphiphile possesses a negative photochromism: the orange-red merocyanine form becomes thermodynamically more stable in water, and both UV and vis stimuli lead to the partial or complete photobleaching of the solution. The explanation of this phenomenon is given on the basis of density functional theory calculations and classical modeling including thermodynamic integration. The simulations reveal that stabilization of merocyanine in water proceeds with the energy of ca. 70 kJ mol−1, and that the Helmholtz free energy of hydration of merocyanine form is 100 kJ mol−1 lower as compared to the behavior of SP isomer in water. The explanation of such a difference lies in the molecular properties of the merocyanine: after ring-opening reaction this molecule transforms into a zwitterionic form, as evidenced by the electrostatic potential plotted around the opened form. The presence of three charged groups on the periphery of a flat conjugated backbone stimulates the self-assembly of merocyanine molecules in water, ending up with the formation of elongated associates with stack-like building blocks, as shown in molecular dynamics simulations of the aqueous solution with the concentration above critical micelle concentration. Our quantitative evaluation of the hydrophilicity switching in spiropyran/merocyanine containing surfactants may prompt the search for new systems, including colloidal and polymeric ones, aiming at remote tuning of their morphology, which could give new promising shapes and patterns for the needs of modern nanotechnology.

Photoswitchable nanoparticles for triggered tissue penetration and drug delivery

We report a novel nanoparticulate drug delivery system that undergoes reversible volume change from 150 to 40 nm upon phototriggering with UV light. The volume change of these monodisperse nanoparticles comprising spiropyran, which undergoes reversible photoisomerization, and PEGylated lipid enables repetitive dosing from a single administration and enhances tissue penetration. The photoswitching allows particles to fluoresce and release drugs inside cells when illuminated with UV light. The mechanism of the light-induced size switching and triggered-release is studied. These particles provide spatiotemporal control of drug release and enhanced tissue penetration, useful properties in many disease states including cancer.

Photoreactions and lateral patterning in Langmuir and Langmuir–Blodgett films

Reversible morphological changes occur with photoisomerization of azobenzene in Langmuir-Blodgett (LB) films complexed with polycations, which contradicts an implicit assumption of the concept of free volume that two-dimensional film structures are preserved during the photoisomerization. J-aggregates of chromophores are formed by two processes. The first process is "light-induced J-aggregation" in which photoisomerized molecules form J-aggregates. The other process is "triggered J-aggregation," in which photoisomerization of one of the components triggers J-aggregation of another chemical species in the mixed films. Both processes of J-aggregation are in many cases accompanied by large morphological changes of the films. However, LB films fabricated using processes under isobaric conditions do not change their morphology during light-induced J-aggregation and are patterned with J-aggregates using ultraviolet illumination through a photomask. Phase separation in mixed LB films gives rise to two-dimensional patterns, which are used to fabricate templates by using an amphiphilic silane-coupling agent as one of the components in the mixed LB films. Nanopatterns are also fabricated.

Control of photoreaction of amphiphilic spiropyran/n-alkane Langmuir and Langmuir-Blodgett films using the phase transition of n-alkane

The structures and photoreactions of Langmuir and Langmuir-Blodgett (LB) films of an amphiphilic spiropyran, 1',3'-dihydro-3',3'-dimethyl-6-nitro-1'-octadecyl-8-(docosanoyloxymethyl)spiro[2H-1-benzopyran-2,2'-(2H)-indole] (SP), mixed with n-alkane are investigated. The mixing ratio was fixed at 1/2 for SP/n-alkane. The surface pressure-area isotherms of SP/octadecane are categorized into two regimes: a low-temperature regime where octadecane is packed with the alkyl chains of SP, and a high-temperature regime where the addition of octadecane does not influence the isotherms significantly. The temperature dividing the two regimes is related with the melting point of the n-alkane mixed with SP in the bulk. UV irradiation of the Langmuir film in the high-temperature regime gives rise to light-induced J-aggregation, whereas that in the low-temperature regime causes only the isomerization of SP to the corresponding merocyanine, indicating that J-aggregation is hindered by the presence of n-alkane in the low-temperature regime. IR external reflection spectroscopy of the Langmuir films shows that n-alkane is released from the film during J-aggregation. The structural changes of the mixed Langmuir and LB films during J-aggregation are almost the same with those of the films of pure SP.

Brewster angle microscopic observations of the Langmuir films of amphiphilic spiropyran during compression and under UV illumination

The structure of the Langmuir film of an amphiphilic spiropyran, 1',3'-dihydro-3',3'-dimethyl-6-nitro-l'-octadecyl-8-(docosanoyloxyme thyl)spiro[2H-1-benzopyran-2,2'-(2H)-indole] (SP), is investigated using Brewster angle microscopy (BAM). The BAM observations show that the Langmuir film of SP can be roughly categorized into three regimes: a low-temperature regime at 7-13 degrees C; a medium-temperature regime at 23-30 degrees C; a high-temperature regime at 40 degrees C. The low-temperature regime is characterized both by the domains that are formed just after the spreading and by the onset of the surface pressure when the domains are merged together to form continuous trilayers. In the medium-temperature regime, a continuous monolayer film is formed after the solvent evaporation, followed by the growth of "embryos" with compression. Around the phase transition point, the "embryos" serve as the "nucleation sites" of the circular trilayer domains. The characteristic features of the high-temperature regime are similar to the ones of the medium-temperature regime except for the absence of a steep rise in surface pressure after the plateau region and the absence of the circular trilayer domains. UV illumination of the Langmuir films leads to the isomerization of SP into merocyanine (MC). However, J-aggregates of MC are formed only when the circular trilayer domains are present. On the basis of the above results, we present a phase diagram of the Langmuir film of SP. The structure and photoreaction depend strongly on the phase of the Langmuir film, indicating that the area/molecule is not the only decisive parameter.