Photophysics, Dynamics, and Energy Transfer in Rigid Mimics of GFP-based Systems

Symmetry-Breaking Charge Transfer in Metal-Organic Frameworks

High quantum-yield charge carrier generation from the initially prepared excitons defines a key step in the light-harvesting and conversion scheme. Photoinduced charge transfer in molecular electron donor-acceptor assemblies is driven by a sizable ΔG0, which compromises the potential of the generated carriers. Reminiscent of the special pair at the reaction center of the natural light-harvesting complex, symmetry-breaking charge transfer (SBCT) within a pair of identical struts of metal-organic framework (MOF) will facilitate the efficient generation of long-lived charge carriers with maximized potentials without incorporating any foreign redox species. We report SBCT in pyrene-based zirconium metal-organic framework (MOF) NU-1000 that leads to efficient generation of radical ions in a polar solvent and bound CT states in a low-polar solvent. The probe unveils the role of the low-lying non-Franck-Condon excitonic states as intermediates in the formation of the SBCT state from the initially prepared Franck-Condon S1 states. Ultrafast and transient spectroscopy─probed over 200 fs-30 μs time scale─evinces a kSBCT = (110 ps)-1 in polar media (εs = 37.5) forming solvated radical ions with recombination rate kCR = (∼45 ns)-1. A slower rate with kSBCT = (203 ps)-1 was recorded in low-polar (εs = 7.0) solvent manifesting a bound [TBAPy•+ TBAPy•-] state with kCR ≈ (17 μs)-1. This discovery, along with other unique photophysical features relevant to light harvesting, should define a MOF-based platform for developing heterogeneous artificial photon energy conversion systems.

Fluorescence Amplification of Unsaturated Oxazolones Using Palladium: Photophysical and Computational Studies

Weakly fluorescent (Z)-4-arylidene-5-(4H)-oxazolones (1), Φ_PL < 0.1%, containing a variety of conjugated aromatic fragments and/or charged arylidene moieties, have been orthopalladated by reaction with Pd(OAc)₂. The resulting dinuclear complexes (2) have the oxazolone ligands bonded as a C^N-chelate, restricting intramolecular motions involving the oxazolone. From 2, a variety of mononuclear derivatives, such as [Pd(C^N-oxazolone)(O₂CCF₃)(py)] (3), [Pd(C^N-oxazolone)(py)₂](ClO₄) (4), [Pd(C^N-oxazolone)(Cl)(py)] (5), and [Pd(C^N-oxazolone)(X)(NHC)] (6, 7), have been prepared and fully characterized. Most of complexes 3-6 are strongly fluorescent in solution in the range of wavelengths from green to yellow, with values of Φ_PL up to 28% (4h), which are among the highest values of quantum yield ever reported for organometallic Pd complexes with bidentate ligands. This means that the introduction of the Pd in the oxazolone scaffold produces in some cases an amplification of the fluorescence of several orders of magnitude from the free ligand 1 to complexes 3-6. Systematic variations of the substituents of the oxazolones and the ancillary ligands show that the wavelength of emission is tuned by the nature of the oxazolone, while the quantum yield is deeply influenced by the change of ligands. TD-DFT studies of complexes 3-6 show a direct correlation between the participation of the Pd orbitals in the HOMO and the loss of emission through non-radiative pathways. This model allows the understanding of the amplification of the fluorescence and the future rational design of new organopalladium systems with improved properties.

Excited State Energy Transfer in Metal-Organic Frameworks

Excited state energy transfer in metal-organic frameworks (MOFs) is of great interest due to potential application of these materials in photocatalysis and fluorescence sensing. In photocatalysis, a light-harvesting antenna of MOFs can collect energy from a much larger area than a single reaction center and efficiently transport the energy to the active site to enhance photocatalytic efficiency, mimicking nature photosynthesis. In fluorescence sensing, excited state traveling on the framework can search for analyte quencher molecules to give amplified fluorescence quenching, so that one quencher turns off multiple excited states to enhance signal. Key to these designer performances is highly efficient energy transfer on these framework materials that are determined by types of excited states, dimension of the materials, and structure of the frameworks. Advancement of MOF synthetic chemistry provides new tools to control the rate and directionality of energy transfer in these materials, opening opportunities in manipulating excited states at an unprecedented level.

Photoresponsive frameworks: energy transfer in the spotlight

In this paper, spiropyran-containing metal- and covalent-organic frameworks (MOFs and COFs, respectively) are probed as platforms for fostering photochromic behavior in solid-state materials, while simultaneously promoting directional energy transfer (ET). In particular, Förster resonance energy transfer (FRET) between spiropyran and porphyrin derivatives integrated as linkers in the framework matrix is discussed. The photochromic spiropyran derivatives allow for control over material optoelectronic properties through alternation of excitation wavelengths. Photoinduced changes in the material electronic profile have also been probed through conductivity measurements. Time-resolved photoluminescence studies were employed to evaluate the effect of photochromic linkers on material photophysics. Furthermore, "forward" and "reverse" FRET processes occurring between two distinct chromophores were modeled, and the Förster critical radii and ET rates were estimated to support the experimentally observed changes in material photoluminescence.

Photoinduced Charge Transfer with a Small Driving Force Facilitated by Exciplex-like Complex Formation in Metal-Organic Frameworks

Photoinduced charge transfer (PCT) is a key step in the light-harvesting (LH) process producing the redox equivalents for energy conversion. However, like traditional macromolecular donor-acceptor assemblies, most MOF-derived LH systems are designed with a large ΔG⁰ to drive PCT. To emulate the functionality of the reaction center of the natural LH complex that drives PCT within a pair of identical chromophores producing charge carriers with maximum potentials, we prepared two electronically diverse carboxy-terminated zinc porphyrins, BFBP(Zn)-COOH and TFP(Zn)-COOH, and installed them into the hexagonal pores of NU-1000 via solvent-assisted ligand incorporation (SALI), resulting in BFBP(Zn)@NU-1000 and TFP(Zn)@NU-1000 compositions. Varying the number of trifluoromethyl groups at the porphyrin core, we tuned the ground-state redox potentials of the porphyrins within ca. 0.1 V relative to that of NU-1000, defining a small ΔG₀ for PCT. For BFBP(Zn)@NU-1000, the relative ground- and excited-state redox potentials of the components facilitate an energy transfer (EnT) from NU-1000* to BFBP(Zn), forming BFBP(Zn)_S1* which entails a long-lived charge-separated complex formed through an exciplex-like [BFBP(Zn)_S1*-TBAPy] intermediate. Various time-resolved spectroscopic data suggest that EnT from NU-1000* may not involve a fast Förster-like resonance energy transfer (FRET) but rather through a slow [NU-1000*-BFBP(Zn)] intermediate formation. In contrast, TFP(Zn)@NU-1000 displays an efficient EnT from NU-1000* to [TFP(Zn)-TBAPy], a complex that formed at the ground state through electronic interaction, and thereon showed the excited-state feature of [TFP(Zn)-TBAPy]*. The results will help to develop synthetic LHC systems that can produce long-lived photogenerated charge carriers with high potentials, i.e., high open-circuit voltage in photoelectrochemical setups.

Confinement-guided photophysics in MOFs, COFs, and cages

In this review, the dependence of the photophysical response of chromophores in the confined environments associated with crystalline scaffolds, such as metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and molecular cages, has been carefully evaluated. Tunability of the framework aperture, cavity microenvironment, and scaffold topology significantly affects emission profiles, quantum yields, or fluorescence lifetimes of confined chromophores. In addition to the role of the host and its effect on the guest, the methods for integration of a chromophore (e.g., as a framework backbone, capping linker, ligand side group, or guest) are discussed. The overall potential of chromophore-integrated frameworks for a wide-range of applications, including artificial biomimetic systems, white-light emitting diodes, photoresponsive devices, and fluorescent sensors with unparalleled spatial resolution are highlighted throughout the review.

Physical properties of porphyrin-based crystalline metal‒organic frameworks

Metal‒organic frameworks (MOFs) are widely studied molecular assemblies that have demonstrated promise for a range of potential applications. Given the unique and well-established photophysical and electrochemical properties of porphyrins, porphyrin-based MOFs are emerging as promising candidates for energy harvesting and conversion applications. Here we discuss the physical properties of porphyrin-based MOFs, highlighting the evolution of various optical and electronic features as a function of their modular framework structures and compositional variations.

Unraveling the Effect on Luminescent Properties by Postsynthetic Covalent and Noncovalent Grafting of gfp Chromophore Analogues in Nanoscale MOF-808

Here, we demonstrate mimicking of photophysical properties of native green fluorescent protein (gfp) by immobilizing the gfp chromophore analogues in nanoscale MOF-808 and further exploring the bioimaging applications. The two virtually nonfluorescent gfp chromophore analogues carrying different functionalities, BDI-AE (COOH/COOMe) and BDI-EE (COOMe/COOMe) were immobilized in nanosized MOF-808 via postsynthetic modification. An ¹H NMR and IR study confirms that BDI-AE was coordinated in NMOF-808, whereas BDI-EE was just noncovalently encapsulated. Interestingly, the extremely weakly fluorescent monomers BDI-AE and BDI-EE (QY = 0.01-0.03%, lifetime = 0.01-0.03 ns) showed a 10²-fold increase in quantum efficiency with a significantly longer excited-state lifetime (QY = 1.8-5.6%, lifetime 0.89-1.49 ns) after immobilization in the NMOF-808 scaffold. Moreover, BDI-AE@MOF-808 has 4 times higher quantum efficiency as well as longer excited-state lifetime in comparison to BDI-EE@NMOF-808 due to the rigidity imposed in the chromophore upon coordination with Zr⁴⁺ in the former case. Further, a cell viability test performed for BDI-AE@NMOF-808 in HeLa cells confirmed the nontoxic nature of the material and, more importantly, bioimaging applications have also been explored successfully.

Ship-in-a-Bottle Preparation of Long Wavelength Molecular Antennae in Lanthanide Metal-Organic Frameworks for Biological Imaging

While metal-organic frameworks (MOFs) have been identified as promising materials for sensitizing near-infrared emitting lanthanide ions (Ln³⁺) for biological imaging, long-wavelength excitation of such materials requires large, highly delocalized organic linkers or guest-chromophores. Incorporation of such species generally coincides with fewer Ln³⁺ emitters per unit volume. Herein, the excitation bands of ytterbium-based MOFs are extended to 800 nm via the postsynthetic coupling of acetylene units to form a high density of conjugated π-systems throughout MOF pores. The resulting long wavelength excitation/absorption bands are a synergistic property of the composite material as they are not observed in the individual organic components after disassociation of the MOFs, thus circumventing the need for large organic chromophores. We demonstrate that the long wavelength excitation and emission properties of these modified MOFs are maintained in the biological conditions of cell culture (aqueous environment, salts, heating), pointing toward their promising use for biological imaging applications.

Rational design of a high-efficiency, multivariate metal-organic framework phosphor for white LED bulbs

Developing rare-earth element (REE) free yellow phosphors that can be excited by 455 nm blue light will help to decrease the environmental impact of manufacturing energy efficient white light-emitting diodes (WLEDs), decrease their cost of production, and accelerate their adoption across the globe. Luminescent metal-organic frameworks (LMOFs) demonstrate strong potential for use as phosphor materials and have been investigated intensively in recent years. However, the majority are not suitable for the current WLED technology due to their lack of blue excitability. Therefore, designing highly efficient blue-excitable, yellow-emitting, REE free LMOFs is much needed. With an internal quantum yield of 76% at 455 nm excitation, LMOF-231 is the most efficient blue-excitable yellow-emitting LMOF phosphor reported to date. Spectroscopic studies suggest that this quantum yield could be further improved by narrowing the material's bandgap. Based on this information and guided by DFT calculations, we apply a ligand substitution strategy to produce a semi-fluorinated analogue of LMOF-231, LMOF-305. With an internal quantum yield of 88% (λ _em = 550 nm) under 455 nm excitation, this LMOF sets a new record for luminescent efficiency in yellow-emitting, blue-excitable, REE free LMOF phosphors. Temperature-dependent and polarized photoluminescence (PL) studies have provided insight on the mechanism of emission and origin of the significant PL enhancement.

Controlling Charge-Transport in Metal-Organic Frameworks: Contribution of Topological and Spin-State Variation on the Iron-Porphyrin Centered Redox Hopping Rate

Metal-organic frameworks (MOFs) are an emerging class of compositions for electro- and photoelectrocatalytic energy conversion processes. Understanding and improving the charge-transport processes within these high surface area molecular redox catalyst assemblies are critical since the charge carrier conductivity is inherently limited in MOFs. Here, we examine a series of four chemically identical but structurally different hydrolytically robust Zr^IV-MOFs constructed from tetrakis(4-carboxyphenyl)porphyrinato iron(III), TCPP(Fe^III) to understand how their topological construction affects redox hopping conductivity. While a structural variation fixes center-to-center distances to define the hopping rate, we probe that altering the central metal spin-state can further tune the TCPP(Fe^III/II) reorganization energy of the self-exchange process. Significant increase in the hopping rate was observed upon axial coordination of 1-methyl imidazole (MIM), which converts a weakly halide bound high-spin (HS) TCPP(Fe^III/II) to the six-coordinated low-spin (LS) complex. Our electrochemical and resonance Raman data reveal that pore geometry that defines the Fe-Fe distance in these frameworks dictate the steric demand to accommodate two MIM-molecules, and thus, the population of LS vs HS species is a function of topology in the presence of an excess ligand.

Red-Shifted Substrates for FAST Fluorogen-Activating Protein Based on the GFP-Like Chromophores

A genetically encoded fluorescent tag for live cell microscopy is presented. This tag is composed of previously published fluorogen-activating protein FAST and a novel fluorogenic derivative of green fluorescent protein (GFP)-like chromophore with red fluorescence. The reversible binding of the novel fluorogen and FAST is accompanied by three orders of magnitude increase in red fluorescence (580-650 nm). The proposed dye instantly stains target cellular proteins fused with FAST, washes out in a minute timescale, and exhibits higher photostability of the fluorescence signal in confocal and widefield microscopy, in contrast with previously published fluorogen:FAST complexes.

Pyridinium Analogues of Green Fluorescent Protein Chromophore: Fluorogenic Dyes with Large Solvent-Dependent Stokes Shift

Novel fluorogenic dyes based on the GFP chromophore are developed. The compounds contain a pyridinium ring instead of phenolate and feature large Stokes shifts and solvent-dependent variations in the fluorescence quantum yield. Electronic structure calculations explain the trends in solvatochromic behavior in terms of the increase of the dipole moment upon excited-state relaxation in polar solvents associated with the changes in bonding pattern in the excited state. A unique combination of such optical characteristics and lipophilic properties enables using one of the new dyes for imaging the membrane structure of endoplasmic reticulum. An extremely high photostability (due to a dynamic exchange between the free and absorbed states) and selectivity make this compound a promising label for this type of cellular organelles.

Photochemistry and photophysics of MOFs: steps towards MOF-based sensing enhancements

In combination with porosity and tunability, light harvesting, energy transfer, and photocatalysis, are facets crucial for engineering of MOF-based sensors.

Green applications of metal–organic frameworks

MOFs as green materials – a highlight of the environmentally conscious or “green” applications of MOFs.

Atomistic Approach toward Selective Photocatalytic Oxidation of a Mustard-Gas Simulant: A Case Study with Heavy-Chalcogen-Containing PCN-57 Analogues

Here we describe the synthesis of two Zr-based benzothiadiazole- and benzoselenadiazole-containing metal-organic frameworks (MOFs) for the selective photocatalytic oxidation of the mustard gas simulant, 2-chloroethyl ethyl sulfide (CEES). The photophysical properties of the linkers and MOFs are characterized by steady-state absorption and emission, time-resolved emission, and ultrafast transient absorption spectroscopy. The benzoselenadiazole-containing MOF shows superior catalytic activity compared to that containing benzothiadiazole with a half-life of 3.5 min for CEES oxidation to nontoxic 2-chloroethyl ethyl sulfoxide (CEESO). Transient absorption spectroscopy performed on the benzoselenadiazole linker reveals the presence of a triplet excited state, which decays with a lifetime of 9.4 μs, resulting in the generation of singlet oxygen for photocatalysis. This study demonstrates the effect of heavy chalcogen substitution within a porous framework for the modulation of photocatalytic activity.

ortho and para chromophores of green fluorescent protein: controlling electron emission and internal conversion

Green fluorescent protein (GFP) continues to play an important role in the biological and biochemical sciences as an efficient fluorescent probe and is also known to undergo light-induced redox transformations. Here, we employ photoelectron spectroscopy and quantum chemistry calculations to investigate how the phenoxide moiety controls the competition between electron emission and internal conversion in the isolated GFP chromophore anion, following photoexcitation with ultraviolet light in the range 400-230 nm. We find that moving the phenoxide group from the para position to the ortho position enhances internal conversion back to the ground electronic state but that adding an additional OH group to the para chromophore, at the ortho position, impedes internal conversion. Guided by quantum chemistry calculations, we interpret these observations in terms of torsions around the C-C-C bridge being enhanced by electrostatic repulsions or impeded by the formation of a hydrogen-bonded seven-membered ring. We also find that moving the phenoxide group from the para position to the ortho position reduces the energy required for detachment processes, whereas adding an additional OH group to the para chromophore at the ortho position increases the energy required for detachment processes. These results have potential applications in tuning light-induced redox processes of this biologically and technologically important fluorescent protein.

A metal–organic framework as a flask: photophysics of confined chromophores with a benzylidene imidazolinone core

Photophysics and dynamics of chromophores with a benzylidene imidazolinone core, responsible for emission of green fluorescent protein variants, were studied as a function of host topology by three approaches.

Photoinduced Chemistry in Fluorescent Proteins: Curse or Blessing?

Photoinduced reactions play an important role in the photocycle of fluorescent proteins from the green fluorescent protein (GFP) family. Among such processes are photoisomerization, photooxidation/photoreduction, breaking and making of covalent bonds, and excited-state proton transfer (ESPT). Many of these transformations are initiated by electron transfer (ET). The quantum yields of these processes vary significantly, from nearly 1 for ESPT to 10^-4-10^-6 for ET. Importantly, even when quantum yields are relatively small, at the conditions of repeated illumination the overall effect is significant. Depending on the task at hand, fluorescent protein photochemistry is regarded either as an asset facilitating new applications or as a nuisance leading to the loss of optical output. The phenomena arising due to phototransformations include (i) large Stokes shifts, (ii) photoconversions, photoactivation, and photoswitching, (iii) phototoxicity, (iv) blinking, (v) permanent bleaching, and (vi) formation of long-lived intermediates. The focus of this review is on the most recent experimental and theoretical work on photoinduced transformations in fluorescent proteins. We also provide an overview of the photophysics of fluorescent proteins, highlighting the interplay between photochemistry and other channels (fluorescence, radiationless relaxation, and intersystem crossing). The similarities and differences with photochemical processes in other biological systems and in dyes are also discussed.