Advanced Photoresponsive Materials Using the Metal–Organic Framework Approach

Abstract

In recent years, a particularly attractive and novel strategy has become available for the fabrication of photoresponsive, porous, and crystalline molecular solids from appropriately functionalized chromophoric linkers. The crystallinity of these materials allows a rather straightforward description and analysis using theoretical methods, thus tremendously accelerating the design of novel materials. This new class of crystalline molecular solids is referred to as metal-organic frameworks, MOFs (or porous coordination polymers, PCPs). [1] MOFs are constructed from metal-/metal-oxo nodes and organic linkers (Figure 1). The incorporation of photoactive species into MOFs may be realized by using them as linkers (method L) or attaching them to a linker (method A), as shown in Figure 1a. In addition, the porosity of MOFs allows the loading of chromophoric compounds as guests (method G) into the pores of this interesting framework material.In this review article, we mainly focus on organic photoactive species, which are either simply loaded as guests into porous MOFs or used after appropriate functionalization as building blocks for the construction of the framework (methods L, A, and G). [2] It is important to note that in the context of photoresponsive behavior, MOFs carry a potential which by far exceeds that of nonporous coordination polymers. This is because simple loading of guest/solvent molecules of different size/polarity/functionality in the MOF pores can impart a large optical response, which is useful, e.g., for sensing applications. [3] As an example, the solvent-dependent optical response or solvatochromism [4] is illustrated in Figure 1b. Such effects cannot be realized for nonporous coordination polymers.The different types of photoresponsive molecules addressed in this review can be grouped into two classes. The first contains molecules where the structure remains essentially unchanged upon light-induced electronic excitation, and the second contains molecular species that change their structure or conformation upon absorption of photons (molecular switches).Light with a wavelength in the range 200-800 nm (6.2-1.5 eV) can excite a molecule to a transient excited electronic state. The transient state then decays to a low-energy state, either the parent ground state or another longer-lived excited state. Decay time scales are in the range 10 −12 -10 1 s, and the released energy can be radiative or nonradiative in nature. For many applications, nonradiative energy loss is unwanted, When fabricating macroscopic devices exploiting the properties of organic chromophores, the corresponding molecules need to be condensed into a solid material. Since optical absorption properties are often strongly affected by interchromophore interactions, solids with a well-defined structure carry substantial advantages over amorphous materials. Here, the metal-organic framework (MOF)-based approach is presented. By appropriate functionalization, most organic chromophores can be converted to function as linkers, which can coo...

Chemistry of Soft Porous Crystals: Structural Dynamics and Gas Adsorption Properties

“…The pressure and energy associated with the phase transition can be tuned chemically or by mechanical pressure . Adsorption heat transformation by SPCs highlights a potential technology for cooling and heating in adsorption heat pump systems based on environmentally friendly working fluids, such as water, methanol, and ethanol. Photoresponse The coexistence of a ligand and guest molecule that are both light sensitive causes a structural phase transition upon light irradiation . For example, [Zn 2 (bdc) 2 (dabco)] accommodating distyrylbenzene (DSB) can selectively adsorb CO 2 over other atmospheric gases, such as N 2 , O 2 , and Ar, with the response involving a fluorescence change of the host.…”

“…The coexistence of a ligand and guest molecule that are both light sensitive causes a structural phase transition upon light irradiation . For example, [Zn 2 (bdc) 2 (dabco)] accommodating distyrylbenzene (DSB) can selectively adsorb CO 2 over other atmospheric gases, such as N 2 , O 2 , and Ar, with the response involving a fluorescence change of the host.…”

“…In such systems, SPCs relax to the initial ground state once the energy input is switched off and thus provide a fast responsive out‐of‐equilibrium system with defined dynamics. Recent examples of the incorporation of artificial molecular machines and switches in porous crystals demonstrate the feasibility of local dynamics activation by light . The combination such local out‐of‐equilibrium dynamic properties with the cooperativity of phase transitions in SPCs provides opportunities for generating unprecedented and potentially counterintuitive guest–host properties. Defects and disorders Structural defects and disorders in SPCs are expected to introduce kinetic aspects to structural transitions as previously explained in the example on crystal size effects (see Section 4.3).…”

Electroactive Organic Building Blocks for the Chemical Design of Functional Porous Frameworks (MOFs and COFs) in Electronics

“…The virtually infinite possibilities for tuning the electronic and opticalp roperties of MOFs and COFs based on the appropriate selection, combination and modification of the organic building blocks, have prompted the interest in the implementation of such functionalporous frameworks in optoelectronics [28,30,146] and photonics. [24,147,148] The synthetic versatility of organic chemistry allows manipulating the electronic nature of different light-responsive organic buildingb locks, becoming an important strategy to fine-tune the performance of, for example, new MOF-and COF-based luminescents ensors, [24,149] photovoltaic [30] and electroluminescent devices, [150,151] with entire control over their physical properties.…”

Die Chemie verformbarer poröser Kristalle – Strukturdynamik und Gasadsorptionseigenschaften

“…In solchen Systemen relaxieren SPCs zum Ausgangszustand, wenn der Energieeintrag unterbrochen ist, was ein schnelles, responsives System ermöglicht, das außerhalb des Gleichgewichts mit definierten Dynamiken arbeiten kann. Kürzlich veröffentlichte Beispiele demonstrieren die Möglichkeit, künstliche molekulare Maschinen und Schalter in poröse Kristalle einzubauen und damit die Aktivierung lokaler Dynamik mit Licht zu erlauben . Die Kombination solcher lokalen dynamischen Eigenschaften mit dem kooperativen Phasenübergang, der durch das Kristallgitter diktiert ist, lieferte bis dato unbekannte Möglichkeiten zum Erhalt responsiver, potenziell kontraintuitiver Wirt‐Gast‐Eigenschaften. Defekte und Unordnung Strukturdefekte und ‐unordnung in SPCs können kinetische Aspekte bei Strukturübergängen verursachen, wie am Beispiel von Kristallgrößeneffekten erläutert wurde (siehe Abschnitt 4.3).…”