Breaking the photoswitch speed limit

Rapid switching between two or more discrete states in the solid state is crucial for the development of advanced materials and devices. Stimuli-responsive materials, capable of undergoing fast transitions between distinct states, are essential for applications ranging from artificial muscles and supercapacitors to optoelectronics, drug delivery, and information encryption. The speed of these transitions, particularly photoisomerization, significantly impacts the efficiency and performance of such systems. Current photochromic molecules, even in solution where molecular motion is less restricted, typically exhibit isomerization rate constants ranging from 10⁵ to 10⁹ s⁻¹. While manipulating solvent properties like polarity and viscosity can enhance these rates, the improvement is usually limited to one order of magnitude. The solid state poses further challenges, as close packing and intermolecular interactions (π-π stacking, hydrogen bonding) can significantly hinder isomerization, especially for molecules undergoing large conformational changes, such as spiropyran derivatives. This limitation has restricted the development of high-performance solid-state stimuli-responsive materials. This research addresses this challenge by introducing a novel approach to dramatically improve the photoswitching speed of sterically demanding spiropyran derivatives in the solid state. The strategy focuses on precisely controlling and tuning the environment of the photochromic moiety, achieving unprecedented rate enhancements and opening new possibilities for material design and applications.

The authors extensively reviewed the literature on photochromic materials and their applications, highlighting the limitations of existing photoswitches, particularly spiropyran, hydrazone, and diarylethene derivatives. They cite numerous studies demonstrating the typical photoisomerization rates of these molecules in solution and the significant decrease in rates observed in the solid state due to intermolecular interactions. The existing methods to improve photoisomerization rates, such as adjusting solvent properties, are acknowledged as having limited effectiveness. This review sets the stage for the innovative approach proposed in the current study, which aims to circumvent these limitations by focusing on environmental engineering rather than solely on molecular modification.

The study employed three distinct classes of photochromic molecules: spiropyran, hydrazone, and diarylethene derivatives. The photoisomerization kinetics of these molecules were initially investigated in solution as a baseline for comparison. The key innovation lies in the integration of these photoswitches into well-defined porous scaffolds, specifically metal-organic frameworks (MOFs). Two different synthetic strategies were used for integrating the photoswitches into the MOFs: a de novo method, where the photoswitches were incorporated during framework formation, and a post-synthetic modification approach, where the photoswitches were added after scaffold synthesis. The selection of MOFs was based on three criteria: (1) maintaining scaffold integrity upon modification and irradiation; (2) possessing sufficiently large pores to accommodate the structural changes associated with photoswitch isomerization; and (3) having unsaturated metal sites for photoswitch coordination. The specific MOFs used were UiO-67 (zirconium-based) and Zn2(3)(DBTD) (zinc-based). Photoisomerization kinetics were studied in both solution and within the MOFs, with experiments performed under ambient and solvent-free conditions (achieved by evacuating the MOF pores). The solvent-free condition is crucial for removing any possible solvent stabilization effects on the photoisomerization. UV-Vis absorbance and diffuse reflectance spectroscopy were used to monitor photoisomerization kinetics. The data were fit to first-order exponential equations to extract rate constants. Time-dependent density functional theory (TD-DFT) calculations were performed to investigate the effect of a rigid matrix on the excited-state energy barrier for spiropyran isomerization. Finally, to demonstrate the versatility of the approach, a MOF containing both spiropyran and hydrazone photoswitches was synthesized and characterized. This material allowed the study of complementary switching in the optical profile, showcasing the potential for fine-tuning material properties.

The study's central finding is a dramatic ~1000-fold increase in the photoisomerization rate of spiropyran derivatives (specifically compounds 2 and 3) when integrated into solvent-free MOFs, compared to their behavior in solution. This substantial enhancement highlights the significant role of the engineered environment in facilitating rapid isomerization. In contrast, the rate enhancements for hydrazone and diarylethene derivatives within the MOFs were much smaller, suggesting that the significant structural rearrangement and zwitterionic intermediate involved in spiropyran isomerization are particularly impacted by the solvent-free environment. The TD-DFT calculations did not reveal any significant alteration in the excited-state energy barrier upon integration into the rigid matrix, indicating that the rate enhancement primarily results from the removal of solvent stabilization of the merocyanine isomer. The synthesis and characterization of a MOF containing both spiropyran and hydrazone photoswitches (UiO-67+2+5) demonstrated the feasibility of achieving a broad dynamic range of photoisomerization rates within a single material. This dual-photoswitch MOF exhibits complementary switching in its optical profile across a wide range of the UV-Vis spectrum (300-650 nm). The rate enhancement in the solvent-free environment was also observed for the spiropyran component of the dual-photoswitch MOF (~1000-fold), while the hydrazone component showed a smaller enhancement (~2.5-fold), consistent with the previous findings.

The findings demonstrate that the 'speed limit' of photoisomerization in solid-state photoswitches can be significantly surpassed by carefully engineering the molecular environment. The results challenge the conventional wisdom that solution-like photoisomerization in solids is unattainable. The significant rate enhancement observed for spiropyran derivatives in the solvent-free MOFs is attributed to the suppression of electrostatic interactions that stabilize the zwitterionic merocyanine form. The ability to integrate multiple photoswitches with different isomerization rates in a single platform opens up exciting possibilities for the design of materials with precisely tunable optical and functional properties. The success of this approach suggests that tailoring the microenvironment of photoresponsive units in solid-state materials can unlock significantly improved performance and functionality across numerous applications.

This research presents a groundbreaking approach to enhance the photoisomerization rate of solid-state photoswitches by meticulously engineering their surrounding environment. The ~1000-fold increase in spiropyran isomerization rate in a solvent-free MOF represents a substantial advancement in the field of stimuli-responsive materials. The ability to integrate multiple types of photochromic molecules in one framework opens pathways to materials with diverse and tunable responses. Future research could focus on exploring other MOF structures or different confined spaces to further optimize photoisomerization rates and expand the range of applicable photoswitches. This platform could be particularly beneficial for applications in advanced optoelectronics, drug delivery, and information encryption.

While the study demonstrates remarkable rate enhancements, the specific MOFs used and the types of photoswitches investigated represent a limited subset of possibilities. The generalizability of the findings to other MOFs or photochromic molecules requires further investigation. The solvent-free conditions may also limit the applications of these materials in certain contexts. Further research is needed to explore the long-term stability and durability of these materials under various environmental conditions.