Quantification of Photocyclization Kinetics and Its Temperature Dependence in a Cofacial Metal–Organic Framework

Abstract

As complex materials are widely used in emerging technologies for environmental and energy applications, it is important to be able to quantify their stimuli-response behaviors. Light is a useful stimulus to modulate multifunctional electrochemical, magnetic, optical, and structural properties in metal− organic frameworks (MOFs); however, the underlying mechanisms and kinetics of light-induced structural changes are not well understood. Herein, a double [2 + 2] photocyclization in photoactive [Cd 2 (stil) 2 (Py 2 TTF) 2 ] (stil 2− = 4,4′-stilbenedicarboxylic acid, Py 2 TTF = 2,6-bis(4′-pyridyl)-tetrathiafulvalene) offers a powerful platform to quantitatively probe solid-state photocyclization kinetics. Variable-temperature Raman spectroscopy revealed a nonlinear temperature dependence of these parameters, which could be analyzed using the Johnson−Mehl−Avrami−Kolmogorov (JMAK) kinetic model to yield a maximum rate observed between 0 °C and 20 °C of approximately 0.172 s −1 . These results offer the first example of the quantification of the photocyclization kinetics in a MOF. Density functional theory (DFT) calculations support a singlet reaction mechanism for the double [2 + 2] photocyclization, which is facilitated by the cofacial alignment of Py 2 TTF ligands. Establishing mechanistic and kinetic models that can be applied to multistimuli-responsive materials provides a powerful platform for their future design for applications in sensing, switching, and molecular separations.

Variable-Temperature Photocyclization Kinetics in a Metal–Organic Framework (MOF): A Comparison of the Johnson–Mehl–Avrami–Kolmogorov and Finke–Watzky Models

“…The 'bell-shaped" relationship of reaction rate with temperature was observed for 1, as has been observed for other solid-state photocyclization reactions (Figure 2). 12,36,37 As expected, the reaction rate increased with increasing temperature until a maximum reaction rate was observed at −20 °C. The Avrami constant showed an inverse relationship, decreasing as the reaction rate increased.…”

“…The first employed a SCXRD snap-shotting experiment to follow the course of the reaction . The second was work previously carried out in our group using variable-temperature Raman spectroscopy to determine the temperature-dependent kinetics for a MOF containing cofacially aligned Py 2 TTF ligands …”

“…11 The second was work previously carried out in our group using variable-temperature Raman spectroscopy to determine the temperature-dependent kinetics for a MOF containing cofacially aligned Py 2 TTF ligands. 12 In the 1960s, Schmidt and co-workers established a set of topochemical principles that govern solid-state photocyclizations. 13−15 The topochemical principles suggest that a solidstate photocyclization will occur (1) with a minimal amount of molecular motion and (2) when the participating chemical bonds are aligned either parallel or antiparallel and are within 3.7−4.2 Å of each other.…”

“…As in previous work, variabletemperature Raman spectroscopy was used to probe the temperature dependence of the double [2 + 2] photocyclization observed in this framework. 12 Irradiation was performed using the built-in light microscope of a Qontor Raman spectrometer (Leica DM2700M microscope, containing a 4500K LED light with a maximum power of 15 W at an irradiation distance of 13 mm) operating at 60% intensity. The crystals were irradiated at 5 s intervals, and a Raman spectrum was collected after each irradiation period.…”

The Chemistry of Metal Organic Framework Materials

“…More recently, MOF glasses, a disordered form of MOFs, are finding niche applications. This virtual collection catalogues recent MOF advances featured in Chemistry of Materials from 2022 to 2023, − with an accompanying editorial by Laura Gagliardi and Omar Yaghi that outlines three critical future directions for MOFs …”