Kinetic Trapping of Photoluminescent Frameworks During High-Concentration Synthesis of Non-Emissive Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are porous, crystalline materials constructed from organic linkers and inorganic nodes with potential utility in gas separations, drug delivery, sensing, and catalysis. Small variations in MOF synthesis conditions can lead to a range of accessible frameworks with divergent chemical or photophysical properties. New methods to controllably access phases with tailored properties would broaden the scope of MOFs that can be reliably prepared for specific applications. Herein, we demonstrate that simply increasing the reaction concentration during the solvothermal synthesis of M2(dobdc) (M = Mg, Mn, Ni; dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) MOFs unexpectedly leads to trapping of a new framework termed CORN-MOF-1 (CORN = Cornell University) instead. In-depth spectroscopic, crystallographic, and computational studies support that CORN-MOF-1 has a similar structure to M2(dobdc) but with partially protonated linkers and charge-balancing or coordinated formate groups in the pores. The resultant variation in linker spacings causes CORN-MOF-1 (Mg) to be strongly photoluminescent in the solid state, whereas H4dobdc and Mg2(dobdc) are weakly emissive due to excimer formation. In-depth photophysical studies suggest that CORN-MOF-1 (Mg) is the first MOF based on the H2dobdc2- linker that likely does not emit via an excited state intramolecular proton transfer (ESIPT) pathway. In addition, CORN-MOF-1 variants can be converted into high-quality samples of the thermodynamic M2(dobdc) phases by heating in N,N-dimethylformamide (DMF). Overall, our findings support that high-concentration synthesis provides a straightforward method to identify new MOFs with properties distinct from known materials and to produce highly porous samples of MOFs, paving the way for the discovery and gram-scale synthesis of framework materials.

A brand-new type of excited-state proton transfer (ESIPT) molecule based on sulfoxide/sulfenic acid tautomerism

The excited-state proton transfer (ESIPT) behavior of organic fluorophores has attracted much attention due to their unique photophysical properties. So far, ESIPT studies have mainly focused on the transfer of hydrogen atoms between N-N, N-O, or O-O. In this work, a brand-new type of ESIPT molecule based on sulfoxide/sulfenic acid tautomerism has been thoroughly investigated. The sulfoxide/sulfenic acid tautomerization process requires one step and two steps in the ground and first excited singlet states, respectively. A range of density functional theory and time-dependent density functional theory methods have been employed to investigate these structures, and the changes in aromaticity may be responsible for obtaining the ESIPT process. This work presents a novel ESIPT process, showcasing molecules that exhibit distinctive properties compared to conventional ESIPT compounds. These findings are expected to expand the horizons of experimental research in ESIPT.

Color-Tunable Multifunctional Excited-State Intramolecular Proton Transfer Emitter: Stimulated Emission of a Single Dye

The excited-state intramolecular proton transfer chromophores were regarded as good materials for laser action generation due to their inherent four-level photocycle. The excitation-dependent properties of these compounds enable light amplification from two distinct forms: both enol and keto, making it possible to obtain dual fluorescence emission. Herein, we report that a third option is possible for the first time stimulated emission was realized with a deprotonated ESIPT molecule based on a novel rigidified 2-(2'-hydroxyphenyl)benzothiazole derivative, triggering the possibility to fabricate real-time tunable active material. Through the rational engineering of the ratio of each emissive species, a red-green-blue device was fabricated with the possibility of white light generation. The degenerated two-wave mixing setup was applied to construct a continuously tunable distributed feedback laser.

Benchmarking Density Functional Approximations for Excited-State Properties of Fluorescent Dyes

This study presents an extensive analysis of the predictive power of time-dependent density functional theory in determining the excited-state properties of two groups of important fluorescent dyes, difluoroboranes and hydroxyphenylimidazo[1,2-a]pyridine derivatives. To ensure statistically meaningful results, the data set is comprised of 85 molecules manifesting diverse photophysical properties. The vertical excitation energies and dipole moments (in the electronic ground and excited states) of the aforementioned dyes were determined using the RI-CC2 method (reference) and with 18 density functional approximations (DFA). The set encompasses DFAs with varying amounts of exact exchange energy (EEX): from 0% (e.g., SVWN, BLYP), through a medium (e.g., TPSSh, B3LYP), up to a major contribution of EEX (e.g., BMK, MN15). It also includes range-separated hybrids (CAM-B3LYP, LC-BLYP). Similar error profiles of vertical energy were obtained for both dye groups, although the errors related to hydroxyphenylimidazopiridines are significantly larger. Overall, functionals including 40-55% of EEX (SOGGA11-X, BMK, M06-2X) ensure satisfactory agreement with the reference vertical excitation energies obtained using the RI-CC2 method; however, MN15 significantly outperforms them, providing a mean absolute error of merely 0.04 eV together with a very high correlation coefficient (R2 = 0.98). Within the investigated set of functionals, there is no single functional that would equally accurately determine ground- and excited-state dipole moments of difluoroboranes and hydroxyphenylimidazopiridine derivatives. Depending on the chosen set of dyes, the most accurate μGS predictions were delivered by MN15 incorporating a major EEX contribution (difluoroboranes) and by PBE0 containing a minor EEX fraction (hydroxyphenylimidazopiridines). Reverse trends are observed for μES, i.e., for difluoroboranes the best results were obtained with functionals including a minor fraction of EEX, specifically PBE0, while in the case of hydroxyphenylimidazopiridines, much more accurate predictions were provided by functionals incorporating a major EEX contribution (BMK, MN15).

Substitution Effect on 2-(Oxazolinyl)-phenols and 1,2,5-Chalcogenadiazole-Annulated Derivatives: Emission-Color-Tunable, Minimalistic Excited-State Intramolecular Proton Transfer (ESIPT)-Based Luminophores

Minimalistic 2-(oxazolinyl)-phenols substituted with different electron-donating and -withdrawing groups as well as 1,2,5-chalcogenadiazole-annulated derivatives thereof were synthesized and investigated in regard to their emission behavior in solution as well as in the solid state. Depending on the nature of the incorporated substituent and its position, emission efficiencies were increased or diminished, resulting in AIE or ACQ characteristics. Single-crystal analysis revealed J- and H-type packing motifs and a so-far undescribed isolation of ESIPT-based fluorophores in the keto form.

Dual-state efficient chromophore with pH-responsive and solvatofluorochromic properties based on an asymmetric single benzene framework

We present a unique single-benzene-based chromophore with asymmetric structure that emits efficient luminescence in both solid and solution states and shows solvatofluorochromism, large Stokes shift (154-219 nm) and an extraordinary wide range of solvent compatibility. The asymmetric structural feature endows this chromophore with significant ratiometric pH-dependent fluorescence for accurate and fast pH sensing over a pH range from 4.0 to 8.2, which could be applied for visual identification of different brands of drinking water. It may find broad applications in environmental sensing and pH-associated physiological processes.

The Journey to Precious-Metal-Free Organic Phosphors from Single-Benzene-Cored Fluorophores

This article takes a look back over our research on the development of organic fluorophores that efficiently emit light in the solid state and precious-metal-free phosphors that emit light at room temperature. In particular, we place an emphasis on the prehistory of each project and the relationship between the established molecular designs. Our story starts from the serendipitous discovery of a luminophore with a single benzene core and follows the molecular design and characterization of 2,5-dipiperidyl-1,4-bis(acceptor-substituted ethenyl)benzenes that exhibit solid-state fluorescence with high-to-excellent quantum yields in the blue-to-red region. In addition, the design concepts, luminescence characteristics, and applications of eight novel classes of fluorophores are described, and the discovery, design, and luminescent properties of precious-metal-free compounds that show efficient room-temperature phosphorescence are presented.

Design of ratiometric monoaromatic fluorescence probe via modulating intramolecular hydrogen bonding: A case study of alkaline phosphatase sensing

Monoaromatic molecules are a category of molecules containing a single aromatic ring which generally emit light in the ultraviolet (UV) region. Despite their facile preparation, the UV emission greatly limits their application as organic probes. In this study, we developed a general method to red shift the emission of monoaromatic molecules. Significant fluorescence red-shift (∼100 nm per intramolecular hydrogen bonding) can be achieved by introducing intramolecular hydrogen bonding units to benzene, a typical monoaromatic molecule. Upon increasing the number of hydrogen bonding units on the benzene ring, UV, blue, and green emissions are screened, which are switchable by simply breaking/restoration the intramolecular hydrogen bonding. As a demonstration, with the breaking of one intramolecular H-bonding, the green emission (λ_em^max = 533 nm) of 2,5-dihydroxyterephthalic acid (DHTA) changed to cyan (λ_em^max = 463 nm) upon the formation of its phosphorylated form (denoted as PDHTA), which, in the presence of alkaline phosphatase (ALP), hydrolyzed and recovered the green emission. By taking advantage of the switchable emission colors, ratiometric in vitro and endogenous ALP sensing was achieved. This general approach offers a great promise to develop organic probes with tunable emissions for fluorescence analysis and imaging by different intramolecular hydrogen bonding.

Structurally-thrifty and visible-absorbing fluorophores

Fluorophores with a minimal push-pull backbone are actively pursued due to their potentials in biological labelling. Herein a series of structurally-thrifty and visible-absorbing fluorophores (SDXs) were successfully constructed following the D'D-π-A design strategy, in which a secondary donor (D') was introduced in conjugation with the donor (D) to enhance its electron donating capability. For a very small scaffold, SDXs exhibit a surprisingly long-wavelength absorption band in the visible spectral range (λ_abs = 420 nm) and a strong green fluorescence emission (λ_em = 530 nm) with a fluorescence quantum yield up to 0.84. Notably, fluorescence of SDXs was quenched in hydrogen-bonding solvents, e.g. MeOH and H₂O. This phenomenon renders SDXs feasibility for imaging of cellular non-hydrogen-bonding microenvironment, as demonstrated with BEAS-2B cells. These results proved that the D'D-π-A is a powerful design strategy to construct novel structurally-thrifty fluorophores.

A highly efficient solution and solid state ESIPT fluorophore and its OLED application

Readily accessible and functionalized ESIPT dyes with high fluorescence quantum yield in solution, including water, and in crystalline state are presented.

fromage: A library for the study of molecular crystal excited states at the aggregate scale

The study of photoexcitations in molecular aggregates faces the twofold problem of the increased computational cost associated with excited states and the complexity of the interactions among the constituent monomers. A mechanistic investigation of these processes requires the analysis of the intermolecular interactions, the effect of the environment, and 3D arrangements or crystal packing on the excited states. A considerable number of techniques have been tailored to navigate these obstacles; however, they are usually restricted to in‐house codes and thus require a disproportionate effort to adopt by researchers approaching the field. Herein, we present the FRamewOrk for Molecular AGgregate Excitations (fromage), which implements a collection of such techniques in a Python library complemented with ready‐to‐use scripts. The program structure is presented and the principal features available to the user are described: geometrical analysis, exciton characterization, and a variety of ONIOM schemes. Each is illustrated by examples of diverse organic molecules in condensed phase settings. The program is available at https://github.com/Crespo-Otero-group/fromage.

Single-Benzene-Based Solvatochromic Chromophores: Color-Tunable and Bright Fluorescence in the Solid and Solution States

The search for structurally simple chromophores with superior fluorescence brightness and a wide range of solvent compatibility is highly desirable. Herein, a new type of single-benzene-based solvatochromic chromophore with a symmetric bifunctional structure, in which azetidine and ethoxycarbonyl moieties serve as the electron-donating and -withdrawing groups, respectively, is reported. This chromophore exhibits an extraordinary wide range of solvent compatibility and preserves excellent fluorescence quantum yields from nonpolar n-hexane to polar methanol and even in water. Unusually, the symmetric structure of the chromophore shows a distinct color change from bright green to red with increasing solvent polarity and possesses large Stokes shifts (λ=132-207 nm) in the tested solvents. Moreover, this single-benzene-based chromophore displays good photochemical stability in both solution and solid states, and even exhibits reversible mechanochromic luminescence.

Red-Emissive Organic Crystals of a Single-Benzene Molecule: Elastically Bendable and Flexible Optical Waveguide

Organic crystals are easily cracked into pieces or powders under applied stress because of their intrinsic brittle nature. This undesired mechanical property directly limits their application in flexible optical and optoelectronic devices. Herein, we developed a compact single-benzene molecule dimethyl 2,5-bis((2-hydroxyethyl)amino)terephthalate, which was easily crystallized to form two polymorphs, A and B. Featuring a single-benzene π-system, both polymorphs A and B display red fluorescence in crystals. More importantly, crystals of polymorph A are flexible and can be elastically bent under mechanical force. Given these advantages, a flexible optical waveguide has been realized in the crystal of polymorph A with a bent shape, highlighting its potential application in flexible devices. In addition, the thermal transformation of crystals from polymorph A to polymorph B, which was accompanied by the change of optical property as well as mechanical elasticity, has been observed.

2,5-Dimethoxybenzene-1,4-dicarboxaldehyde: An Emissive Organic Crystal and Highly Efficient Fluorescent Waveguide

To identify the simplest organic structure for an emitter, we focused on 2,5-dimethoxybenzene-1,4-dicarboaldehyde. This symmetric molecule has a very low molecular weight (MW=194), a single benzene unit, and consists of only three elements (H, C and O). It forms highly efficient and pure emitting crystals (λem=499 nm, Φ^F =0.42, FWHM=42 nm) due to the rigid structure based on the single benzene framework and four intramolecular hydrogen bonds between electron-donating methoxy and electron-accepting aldehyde groups. This crystal acts as a good optical waveguide with pure green emission (FWHM=34 nm) and very low loss coefficient (0.00120 dB/μm).

Exploring Potential Energy Surfaces for Aggregation-Induced Emission-From Solution to Crystal

Aggregation-induced emission (AIE) is a phenomenon where non-luminescent compounds in solution become strongly luminescent in aggregate and solid phase. It provides a fertile ground for luminescent applications that has rapidly developed in the last 15 years. In this review, we focus on the contributions of theory and computations to understanding the molecular mechanism behind it. Starting from initial models, such as restriction of intramolecular rotations (RIR), and the calculation of non-radiative rates with Fermi's golden rule (FGR), we center on studies of the global excited-state potential energy surfaces that have provided the basis for the restricted access to a conical intersection (RACI) model. In this model, which has been shown to apply for a diverse group of AIEgens, the lack of fluorescence in solution comes from radiationless decay at a CI in solution that is hindered in the aggregate state. We also highlight how intermolecular interactions modulate the photophysics in the aggregate phase, in terms of fluorescence quantum yield and emission color.

Marrying multicomponent reactions and aggregation-induced emission (AIE): new directions for fluorescent nanoprobes

Recent development and progress for fabrication and applications of aggregation-induced emission polymers through multicomponent reactions have been summarized in this review.

Fluorene-containing tetraphenylethylene molecules as lasing materials

A series of star‐shaped oligofluorene molecules, each containing a TPE core, have been specifically designed and produced to show effective aggregation‐induced emission (AIE). Each molecule differs either in the number of fluorene units within the arms (e.g., 1 or 4, compounds 4 and 5), or the terminal group positioned at the end of each arm (e.g., H, TMS, or TPA, compounds 4, 6, and 7). Although they are all poor emitters in solution phase they become efficient yellow‐green luminogens in the condensed state. Their AIE properties were investigated in THF/H₂O mixtures, with each molecule exhibiting a clear emission enhancement at specific water contents. An all‐organic distributed feedback (DFB) laser was fabricated using compound 4 as the gain material and exhibited an average threshold energy fluence of 60 ± 6 μJ/cm² and emission in the green region. Furthermore, piezofluorochromism studies on a thin film of this material displayed a linear dependence of the amplified spontaneous emission (ASE) peak position on applied pressure, indicating potential applications as lasing‐based pressure sensors. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 734–746

Highly efficient blue solid emitters and tautomerization-induced ON/OFF fluorescence switching based on structurally simple 3(5)-phenol-1H-pyrazoles

3(5)-Phenyl-1H-pyrazoles are employed in this study to develop highly efficient organic crystalline solids with deep-blue ESIPT fluorescence as well as intriguing fluorescence “ON/OFF” switching.