Derek E. Williams

Energy Transfer on Demand: Photoswitch-Directed Behavior of Metal–Porphyrin Frameworks

In this paper, a photochromic diarylethene-based derivative that is coordinatively immobilized within an extended porphyrin framework is shown to maintain its photoswitchable behavior and to direct the photophysical properties of the host. In particular, emission of a framework composed of bis(5-pyridyl-2-methyl-3-thienyl)cyclopentene (BPMTC) and tetrakis(4-carboxyphenyl)porphyrin (H4TCPP) ligands anchored by Zn(2+) ions can be altered as a function of incident light. We attribute the observed cyclic fluorescence behavior of the synthesized porphyrin-BPMTC array to activation of energy transfer (ET) pathways through BPMTC photoisomerization. Time-resolved photoluminescence measurements show a decrease in average porphyrin emission lifetime upon BPMTC insertion, consistent with an ET-based mechanism. These studies portend the possible utilization of photochromic ligands to direct chromophore behavior in large light-harvesting ensembles.

Metal–Organic Frameworks as a Versatile Tool To Study and Model Energy Transfer Processes

Development of materials with efficient and directional energy transfer (ET) could significantly modify the existing energy and material landscape. Metal-organic frameworks (MOFs) are a unique tool to address the upcoming challenges related to the enhancement of ET efficiency and directional energy transport. To harness MOFs as a versatile platform, mechanistic and structural aspects governing ET efficiency should be elucidated. In this context, we review ET mechanisms and structural motifs based on the recent advances achieved in MOF chemistry and also highlight the possible practical applications that are enabled by these studies.

Mimic of the Green Fluorescent Protein β-Barrel: Photophysics and Dynamics of Confined Chromophores Defined by a Rigid Porous Scaffold

Chromophores with a benzylidene imidazolidinone core define the emission profile of commonly used biomarkers such as the green fluorescent protein (GFP) and its analogues. In this communication, artificially engineered porous scaffolds have been shown to mimic the protein β-barrel structure, maintaining green fluorescence response and conformational rigidity of GFP-like chromophores. In particular, we demonstrated that the emission maximum in our artificial scaffolds is similar to those observed in the spectra of the natural GFP-based systems. To correlate the fluorescence response with a structure and perform a comprehensive analysis of the prepared photoluminescent scaffolds, (13)C cross-polarization magic angle spinning solid-state (CP-MAS) NMR spectroscopy, powder and single-crystal X-ray diffraction, and time-resolved fluorescence spectroscopy were employed. Quadrupolar spin-echo solid-state (2)H NMR spectroscopy, in combination with theoretical calculations, was implemented to probe low-frequency vibrational dynamics of the confined chromophores, demonstrating conformational restrictions imposed on the coordinatively trapped chromophores. Because of possible tunability of the introduced scaffolds, these studies could foreshadow utilization of the presented approach toward directing a fluorescence response in artificial GFP mimics, modulating a protein microenvironment, and controlling nonradiative pathways through chromophore dynamics.

Redox‐Active Corannulene Buckybowls in a Crystalline Hybrid Scaffold

A porous crystalline corannulene-containing scaffold, which combines the periodicity, dimensionality, and structural modularity of hybrid frameworks with the intrinsic properties of redox-active π-bowls, has been prepared. Single-crystal and powder X-ray diffraction, ab initio density functional theory computations, gas sorption analysis, fluorescence spectroscopy, and cyclic voltammetry were employed to study the properties of the novel corannulene derivatives and the buckybowl-based hybrid materials. X-ray diffraction studies revealed the preservation of the corannulene bowl inside the prepared rigid matrix, which offers the unique opportunity to extend the scaffold dimensionality through the buckybowl curvature. Merging the inherent properties of hybrid frameworks with the intrinsic properties of π-bowls opens a new avenue for preparing redox-active materials and potentially improving charge transport in the scaffold.

A Bio‐inspired Approach for Chromophore Communication: Ligand‐to‐Ligand and Host‐to‐Guest Energy Transfer in Hybrid Crystalline Scaffolds

Efficient multiple-chromophore coupling in a crystalline metal-organic scaffold was achieved by mimicking a protein system possessing 100% energy-transfer (ET) efficiency between a green fluorescent protein variant and cytochrome b562. The two approaches developed for ET relied on the construction of coordination assemblies and host-guest coupling. Based on time-resolved photoluminescence measurements in combination with calculations of the spectral overlap function and Förster radius, we demonstrated that both approaches resulted in a very high ET efficiency. In particular, the observed ligand-to-ligand ET efficiency value was the highest reported so far for two distinct ligands in a metal-organic framework. These studies provide important insights for the rational design of crystalline hybrid scaffolds consisting of a large ensemble of chromophore molecules with the capability of directional ET.

Fulleretic Well‐Defined Scaffolds: Donor–Fullerene Alignment Through Metal Coordination and Its Effect on Photophysics

Abstract Herein, we report the first example of a crystalline metal–donor–fullerene framework, in which control of the donor–fullerene mutual orientation was achieved through chemical bond formation, in particular, by metal coordination. The 13C cross‐polarization magic‐angle spinning NMR spectroscopy, X‐ray diffraction, and time‐resolved fluorescence spectroscopy were performed for comprehensive structural analysis and energy‐transfer (ET) studies of the fulleretic donor–acceptor scaffold. Furthermore, in combination with photoluminescence measurements, the theoretical calculations of the spectral overlap function, Förster radius, excitation energies, and band structure were employed to elucidate the photophysical and ET processes in the prepared fulleretic material. We envision that the well‐defined fulleretic donor–acceptor materials could contribute not only to the basic science of fullerene chemistry but would also be used towards effective development of organic photovoltaics and molecular electronics.

A5RE4X[TO4]4 crystal growth and photoluminescence. Hydroflux synthesis of sodium rare earth silicate hydroxides

Single crystals of Na5RE4(OH)[SiO4]4 (RE = Pr, Nd, Sm, Eu, Tb–Yb, Y) were grown using the hydroflux synthetic method. All compositions adopt the tetragonal I space group with lattice parameter ranges of a = 11.5275(4)–12.0588(3) A and c = 5.3951(4)–5.4846(13) A. Intense photoluminescent properties were observed for Na5Eu4(OH)[SiO4]4, Na5Gd4(OH)[SiO4]4, and Na5Tb4(OH)[SiO4]4. The magnetic susceptibility was measured for the magnetic rare earth containing compositions, where the terbium analogue displayed antiferromagnetic order at T = 2.8 K.

A5RE4X[TO4]4 crystal growth and photoluminescence. Fluoride flux synthesis of sodium and potassium rare earth silicate oxyfluorides

Crystals of Na5RE4F[SiO4]4 (RE = Pr, Nd, Sm–Tm) and K5Pr4F[SiO4]4 were grown using a fluoride flux synthetic technique. All compositions crystallize in the tetragonal space group I with lattice parameter ranges of a = 11.5094(2)–12.3745(2) A and c = 5.37000(10)–5.5011(2) A. Antiferromagnetic ordering is observed for Na5RE4F[SiO4]4 (RE = Tb, Dy), second harmonic generation for Na5RE4F[SiO4]4 (RE = Pr, Sm–Ho), and intense optical properties and fluorescence quantum yield for Na5RE4F[SiO4]4 (RE = Eu, Gd, Tb). The crystal structures and physical property measurements are discussed.

Flipping the Switch: Fast Photoisomerization in a Confined Environment

Stimuli-responsive materials are vital for addressing emerging demands in the advanced technology sector as well as current industrial challenges. Here, we report for the first time that coordinative integration of photoresponsive building blocks possessing photochromic spiropyran and diarylethene moieties within a rigid scaffold of metal-organic frameworks (MOFs) could control photophysics, in particular, cycloreversion kinetics, with a level of control that is not accessible in the solid state or solution. On the series of photoactive materials, we demonstrated for the first time that photoisomerization rates of photochromic compounds could be tuned within almost 2 orders of magnitude. Moreover, cycloreversion rates of photoresponsive derivatives could be modulated as a function of the framework structure. Furthermore, through MOF engineering we were able to achieve complete isomerization for coordinatively immobilized spiropyran derivatives, typically exhibiting limited photoswitching behavior in the solid state. For instance, spectroscopic analysis revealed that the novel monosubstituted spiropyran derivative grafted to the backbone of the MOF pillar exhibits a remarkable photoisomerization rate of 0.16 s-1, typical for cycloreversion in solution. We also applied the acquired fundamental principles toward mapping of changes in material properties, which could provide a pathway for monitoring material aging or structural deterioration.