Intersectional Effects of Crystal Features on the Actuation Performance of Dynamic Molecular Crystals

Despite being researched for decades, shape-shifting molecular crystals have yet to claim their spot as an actuating materials class among the primary functional materials. While the process for developing and commercializing materials can be lengthy, it inevitably starts with building an extensive knowledge base, which for molecular crystal actuators remains scattered and disjointed. Using machine learning for the first time, we identify inherent features and structure–function relationships that fundamentally impact the mechanical response of molecular crystal actuators. Our model can factor in different crystal properties in tandem and decipher their intersectional and combined effects on each actuation performance. This analysis is an open invitation to utilize interdisciplinary expertise in translating the current basic research on molecular crystal actuators into technology-based development that promotes large-scale experimentation and prototyping.


Data collection:
The data utilized in this study have been primarily sourced from published literature between 2006 and 2020 with 68 unique studies, encompassing a range of actuation performance metrics and various crystal features for molecular crystals. In instances where values on the actuation performance were not explicitly stated in the literature, we employed editing software to extract information such as the dimensions of crystals or response and recovery times of the actuation, enabling us to calculate the additional properties and better populate the dataset. In instances where physical, chemical, or mechanical properties were not explicitly specified in the same literature, we resorted to publicly available datasets or alternative literature that report on the same molecule or single crystal. X-ray diffraction information was obtained from the Cambridge Structural Database (CSD) on the same single crystal systems. Where available, we used the crystallography information from the same study that reports on actuation performance. Given that actuation performance studies on molecular single crystals do not always conduct X-ray different analyses, we used the most recently published data that more closely matches the experimental conditions followed during actuation.
We have made every effort to ensure the accuracy of the data collected and the integrity of our analysis. However, it is important to note that we did not conduct any additional experimentation or validation beyond the information provided in the literature. We rely on the accuracy of the original published data, and as such, cannot be held responsible for any discrepancies or inaccuracies that may arise from the source material. By using the data and findings presented in this paper, the reader acknowledges and agrees that the authors are not liable for any errors, discrepancies, or inaccuracies in the original published data. In case any such discrepancies arise or in case the reader has any questions or concerns, please reach out directly to the corresponding author.
We recognize the value of continuous improvement and expansion of the dataset used in this study. As such, we invite and encourage collaboration within the research community to build upon the current data set with new examples of tested and characterized molecular crystals. By populating areas where data may be sparse, refining existing data points through re-experimentation, and incorporating new information, we can collectively enhance the learning capabilities and predictive accuracy of the machine learning model employed in this work.
Dynamic molecular crystals sample: The molecular crystals included in the dataset encompass a wide range of photoresponsive and thermoresponsive single crystals. While we recognize there are other examples of dynamic molecular crystals such mechanoresponsive, electroresponsive, and humidity-responsive crystals, these types of crystals are not as widely and extensively reported on and thereby are excluded from our dataset. Additionally, we recognize the variety of underlying chemical transformations and molecule types within photoresponsive (ex: t-type and p-type photochromic molecules) and thermoresponsive crystals. These differences are accounted for through different crystal features such as chemical families, reaction groups, and classification of single crystals by whether they undergo phase transition transformations, martensitic phase transition transformations, or not. Similarly, and due to few examples reported in the literature, crystals that undergo bending, contraction/expansion, and jumping represent the majority of the samples captured in the data. While the dataset does include crystals that undergo twisting, due to the small sample size, we excluded this type of deformation from the analysis.
Note: A molecular crystal actuation event defined in this work represents the introduction of a single stimulus at a time and a full actuation cycle starts with the introduction of a stimulus and ends after the crystal has gone back to its original condition or has reached steady state after which no motion can be recorded without the introduction of an additional stimulus.
Ashby Plots: The Ashby plots in this work were constructed using python and the dataset was collected through a wide literature review on microactuators and a range of soft to hard actuators reported on in recent studies. Additional examples of commercially available actuators were extracted from the ANSYS Granta Materials database.

Statistical Analysis and Machine Learning code:
The code python files have been uploaded to a public github repository and can be accessed through this link: https://github.com/jmah13/Dynamic-Molecular-Crystals.

Phase transition
Phase transition crystal transformations -Yes -No

Organometallic
Presence of an organometallic complex -Yes -No

Source of actuation
Type of stimulus driving the response -Photo -Thermo

Crystal habit
Geometric appearance of the crystal

Crystal family
Crystal systems based on unit cell

Reversibility
The crystal is able to retain its original state once stimulus is removed