Measuring the total photon economy of molecular species through fluorescent optical cycling.

The total photon economy of a chromophore molecular species represents a study of how absorbed photons partition among various electronic states and ultimately dissipate their excited energy into the environment. A complete accounting of these rates and pathways would allow one to optimize chromophores and their environments for applications. We describe a technique, fluorescent optical cycling (FOC), which allows for simultaneous observation of prompt and delayed emission during and after multiple pulsed excitation, ultimately granting access to multi-state photophysical rates. We exercise control over the excitation pulse train, which allows us to "optically shelve" long-lived intermediate states without the use of diode or flashlamp excitation. By recording all photon arrival times in the visible and shortwave infrared, we can simultaneously resolve fluorescence, phosphorescence, and singlet oxygen sensitization in a single experiment. We use FOC to examine the photophysics of dual emitting bis(di-R-phosphino)alkanethiophene-pyridine-platinum ([Pt(thpy)(dppm)]+) under different solvation conditions, revealing changes in intersystem crossing and phosphorescent rates induced by the external heavy atom effect. Coupling FOC with decay associated Fourier spectroscopy (DAFS), we demonstrate simultaneous correlated spectral and lifetime data in this dual emitting complex. Finally, FOC combined with superconducting nanowire single photon detectors (SNSPDs) allows us to observe the shortwave infrared region (SWIR) phosphorescence of singlet oxygen sensitized by Rose Bengal. Overall, FOC provides a powerful tool to simultaneously study multiple photophysics across timescales, even in weakly populated electronic states.

Metal-Catalyzed and Metal-Free Nucleophilic Substitution of 7-I-B18H21.

In this work, two pathways of reactivity are investigated to generate site-specific substitutions at the B7 vertex of the luminescent boron cluster, anti-B18H22. First, a palladium-catalyzed cross-coupling reaction utilizing the precursor 7-I-B18H21 and a series of model nucleophiles was developed, ultimately producing several B-N- and B-O-substituted species. Interestingly, the B-I bond in this cluster can also be substituted in an uncatalyzed fashion, leading to the formation of various B-N, B-O, and B-S products. This work highlights intricate differences corresponding to these two reaction pathways and analyzes the role of solvents and additives on product distributions. As a result of our synthetic studies, seven new B18-based clusters were synthesized, isolated, and characterized by mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. The photoluminescence properties of two structurally similar ether and thioether products were further investigated, with both exhibiting blue fluorescence in solution at 298 K and long-lived green or yellow phosphorescence at 77 K. Overall, this work shows, for the first time, the ability to perform substitution of a boron-halogen bond with nucleophiles in a B18-based cluster, resulting in the formation of photoluminescent molecules.

Improved synthesis of icosahedral carboranes containing exopolyhedral B-C and C-C bonds.

Carboranes are boron-rich molecular clusters possessing electronic characteristics that allow for orthogonal approaches to vertex-selective modifications. We report improved functionalization methods utilizing orthogonal chemistry to achieve efficient substitution at electron-rich B-vertices and electron-poor C-vertices of carborane. Functionalization of B-vertices with alkyl and (hetero)aryl groups using the corresponding Grignard reagents has been improved through the use of a Pd-based precatalyst featuring an electron-rich biaryl phosphine ligand, resulting in reduced reaction times. Importantly, this method is tolerant towards alkyl-based Grignard reagents containing ß-hydrogens. Furthermore, a transition metal-free approach to the substitution of carborane C-vertices with (hetero)aryl substrates has been developed under nucleophilic aromatic substitution (SNAr) conditions. The selective substitution of carboranes afforded by these methods holds potential for the rational synthesis of heterofunctionalized boron clusters with substituents on both boron and carbon-based vertices.

Optimization of a series of heterocycles as survival motor neuron gene transcription enhancers.

Spinal muscular atrophy (SMA) is a neurodegenerative disorder that results from mutations in the SMN1 gene, leading to survival motor neuron (SMN) protein deficiency. One therapeutic strategy for SMA is to identify compounds that enhance the expression of the SMN2 gene, which normally only is a minor contributor to functional SMN protein production, but which is unaffected in SMA. A recent high-throughput screening campaign identified a 3,4-dihydro-4-phenyl-2(1H)-quinolinone derivative (2) that increases the expression of SMN2 by 2-fold with an EC50 = 8.3 µM. A structure-activity relationship (SAR) study revealed that the array of tolerated substituents, on either the benzo portion of the quinolinone or the 4-phenyl, was very narrow. However, the lactam ring of the quinolinone was more amenable to modifications. For example, the quinazolinone (9a) and the benzoxazepin-2(3H)-one (19) demonstrated improved potency and efficacy for increase in SMN2 expression as compared to 2.

Synthesis and structural properties of a 2D Zn(II) dodecahydroxy-closo-dodecaborate coordination polymer.

In this work, we discuss the synthesis and characterization of a 2D coordination polymer composed of a dianionic perhydroxylated boron cluster, [B12(OH)122-], coordinated to Zn(II)-the first example of a transition metal-coordinated [B12(OH)12]2- compound. This material was synthesized via cation exchange from the starting cesium salt and then subjected to rigorous characterization prior to and after thermal activation. Numerous techniques, including XRD, FTIR, SEM, TGA, and solid-state NMR revealed a 2D coordination polymer composed of sheets of Zn(II) ions intercalated between planes of boron clusters. The as-synthesized material was then evacuated of solvent via thermal treatment, and atomic-level changes from this transformation were elucidated through a combination of 1D and 2D solid-state NMR analyses of 11B and 1H nuclei, suggesting the full removal of coordinated solvent molecules. Evidence also suggested that [B12(OH)122-] can adjust its coordination to Zn(II) in the solid-state through hemilability of its numerous -OH ligands.

Benchmarking the dynamic luminescence properties and UV stability of B18H22-based materials.

The dynamic photoluminescence properties, and potential quenching mechanisms, of anti-B18H22, 4,4'-Br2-anti-B18H20, and 4,4'-I2-anti-B18H20 are investigated in solution and polymer films. UV stability studies of the neat powders show no decomposition occurring after intense 7 day light soaking. In contrast, clusters incorporated into polymer films are found to degrade into smaller borane fragments under the same irradiation conditions. To highlight the utility of these compounds, we leverage their favorable optical properties in a prototype UV imaging setup.

A molecular boron cluster-based chromophore with dual emission.

Bromination of the luminescent borane, anti-B18H22, via electrophilic substitution using AlCl3 and Br2, yields the monosubstituted derivative 4-Br-anti-B18H21 as an air-stable crystalline solid. In contrast to the unsubstituted parent compound, 4-Br-anti-B18H21 possesses dual emission upon excitation with UV light and exhibits fluorescence at 410 nm and phosphorescence at 503 nm, with Φtotal = 0.07 in oxygen-free cyclohexane. Increased oxygen content in cyclohexane solution quenches the phosphorescence signal. The fluorescent signal intensity remains unaffected by oxygen, suggesting that this molecule could be used as a ratiometric oxygen probe.