A Dual pH‐ and Light‐Responsive Spiropyran‐Based Surfactant: Investigations on Its Switching Behavior and Remote Control over Emulsion Stability

Abstract

A cationic surfactant containing a spiropyran unit is prepared exhibiting a dual‐responsive adjustability of its surface‐active characteristics. The switching mechanism of the system relies on the reversible conversion of the non‐ionic spiropyran (SP) to a zwitterionic merocyanine (MC) and can be controlled by adjusting the pH value and via light, resulting in a pH‐dependent photoactivity: While the compound possesses a pronounced difference in surface activity between both forms under acidic conditions, this behavior is suppressed at a neutral pH level. The underlying switching processes are investigated in detail, and a thermodynamic explanation based on a combination of theoretical and experimental results is provided. This complex stimuli‐responsive behavior enables remote‐control of colloidal systems. To demonstrate its applicability, the surfactant is utilized for the pH‐dependent manipulation of oil‐in‐water emulsions.

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

“…Two snapshots of the optimized geometries of spiropyran TMAB-C3-SP and merocyanine TMAB-C3-MC are shown in Figure 4. For both SP and MC, several geometries belonging to local minima on the potential energy surface can be distinguished, for example, two closed-form structures, differing by spatial arrangement of the O atom with respect to the indoline moiety, two open-form structures, differing by rotation of the benzopyran around the adjacent C=C bond, as noticed by some authors [16,59,61,64], the open forms with cisoid or transoid C=C double bond [89], or additional pairs of structures due to flexibility of the substituents, e.g. as discussed by Eilmes [69].…”

“…The geometry optimization of SP and MC is performed in a gas phase using the B3LYP/DGDZVP basis set with a tight self-consistent field convergence threshold. This functional is widely and successfully employed in studies of spiropyran-containing molecules [16,17,58,[68][69][70][71]. Next, the ground state geometries are optimized by applying the integral equation formalism variant of the polarizable continuum model (IEFPCM) to simulate the solvent environment [75].…”

“…The moieties that are utilized to render amphiphiles the photoresponsive nature are azobenzene [3][4][5][6], stilbene [7][8][9][10][11], dithienylethene [12][13][14] and spiropyran [15][16][17][18], to name a few [2]. Such amphiphiles have exhibited, for example, (i) smart principles for the dynamic control of surface tension of aqueous solutions [19], (ii) phenomenon of the light-driven diffusioosmosis upon which one can remove, gather or pattern a particle assembly at a solid-liquid interface [20], (iii) the photo-controlled stability and breakage of foams [21,22], (iv) new macroscopic functions such as liquid droplet transport [23] and bubble manipulation [15], (v) photoswitchable catalytic properties [24], (vi) photochemical OFF/ON cytotoxicity switching [25], etc.…”

“…The first group represents surfactants with spiropyran/merocyanine introduced as the head group (Figure 1a) [15,25,[31][32][33][34][35][36][37][38][39][40][41][42][43][44]. For the second one, SP/MC is a part of the tail of a photoresponsive amphiphile included at its different positions: at the end of the hydrocarbon chain [17,[26][27][28]30,[45][46][47][48][49][50][51][52] (Figure 1b) or in its middle [16]. The photoisomerization causes a dramatic change of the hydrophilic-hydrophobic balance of the amphiphile: apparently, MC itself is a zwitterionic form and is more hydrophilic than the nonionic SP state, as shown in experiments [26,44].…”

“…For most calculations the propyl sulfonate group of a photoacid was truncated to a methyl group to facilitate geometry optimisation. Very recently, Reifarth and others [16] have investigated switching behavior and remote control over emulsion stability of a dual pH and light-responsive SP surfactant with the photochrome segment included in the middle of the hydrophobic tail. Again, the density functional theory was utilized for prediction of the interconversion mechanism and the energy barriers of the mechanism's stages, which are in fair agreement with experimental value of the activation energy.…”