Cassandra L. Fraser

In vivo optical molecular imaging and analysis in mice using dorsal window chamber models applied to hypoxia, vasculature and fluorescent reporters

Optical techniques for functional imaging in mice have a number of key advantages over other common imaging modalities such as magnetic resonance imaging, positron emission tomography or computed tomography, including high resolution, low cost and an extensive library of available contrast agents and reporter genes. A major challenge to such work is the limited penetration depth imposed by tissue turbidity. We describe a window chamber technique by which these limitations can be avoided. This facilitates the study of a wide range of processes, with potential endpoints including longitudinal gene expression, vascular remodeling and angiogenesis, and tumor growth and invasion. We further describe several quantitative imaging and analysis techniques for characterizing in vivo fluorescence properties and functional endpoints, including vascular morphology and oxygenation. The procedure takes ∼2 h to complete, plus up to several weeks for tumor growth and treatment procedures.

Aromatic Difluoroboron β‑Diketonate Complexes: Effects of π‑Conjugation and Media on Optical Properties

Aromatic difluoroboron β-diketonate complexes (BF2bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. To gain a better understanding of their emissive properties in both solution and polymer matrices, BF2bdks with varying aromatic groups were synthesized and their photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Absorption spectra showed systematic variations that are well correlated with structural features, including the size of the aryl substituent and the presence of a para electron-donating methoxy substituent. Computational modeling of the absorption spectra with the TD-B3LYP/6-311+G(d)//B3LYP/6-31G(d) formulation of density functional theory and a polarizable continuum model of dichloromethane solvent shows that all systems show intense π-π* one-electron excitations, usually from one of the highest occupied molecular orbitals (HOMO - k, k = 0, 1, 2) to the lowest unoccupied molecular orbital (LUMO). Emission properties are sensitive to the dye structure and medium. Based on spectroscopic and lifetime studies, BF2bdks exhibit comparable fluorescence properties in both solutions and polymers when the diketonate group is functionalized with smaller aromatic ring systems such as benzene. For BF2bdks with larger arene ring systems, such as anthracene, emission from a strong intramolecular charge-transfer (ICT) state was also noted in both solution and in PLA. There are differences in relative intensities of peaks arising from π-π* and ICT excitations depending upon dye loading in PLA. Substituent effects were also observed. Electron-donating methoxyl groups on the aromatic rings lead to enhanced fluorescence quantum yields. For certain dyes, phosphorescence is detected at low temperature or under a nitrogen atmosphere in PLA matrices.

Alkyl chain length effects on solid-state difluoroboron β-diketonate mechanochromic luminescence

Solid-state difluoroboron β-diketonate dyes display reversible mechanochromic luminescence (ML). To test the effects of alkyl chain length on solid state photoluminescence and ML, a series of dyes, BF2dbmOR, with dibenzoylmethane (dbm) ligands and alkoxyl substituents (–OR) were prepared, where R = CnH2n+1 and n = 1, 2, 3, 5, 6, 12, 14, 16, 18. Emission properties were investigated in solution and in the solid state. Fluorescence spectra and lifetimes were nearly identical for dyes in CH2Cl2 solution; whereas, in the solid state, as powders, thin films or spin cast films, emission maxima, and lifetimes were different among the samples. Solid-state ML emission spectra were monitored at room temperature as a function of time for smeared powders on quartz surfaces. The recovery time generally increased with alkyl chain length, ranging from minutes (n = 3) to days (n = 18). Longer chain analogues (n = 6, 12, 14, 16, 18) did not fully return to the original annealed emissive state even after months on quartz, though the dynamics are substrate dependent. Solid-state dyes were also investigated by XRD and DSC (powders), and by AFM (spin cast films).

Boron Polylactide Nanoparticles Exhibiting Fluorescence and Phosphorescence in Aqueous Medium

Difluoroboron dibenzoylmethane-polylactide, BF(2)dbmPLA, a biocompatible polymer-luminophore conjugate was fabricated as nanoparticles. Spherical particles <100 nm in size were generated via nanoprecipitation. Intense blue fluorescence, two-photon absorption, and long-lived room temperature phosphorescence (RTP) are retained in aqueous suspension. The nanoparticles were internalized by cells and visualized by fluorescence microscopy. Luminescent boron biomaterials show potential for imaging and sensing.

Reversible solid-state mechanochromic fluorescence from a boron lipid dye

Difluoroboron β-diketone complexes are versatile light-emitting molecules that exhibit tunable emission in both solution and the solid state. Among this class of dyes, difluoroboron dibenzoylmethane-polylactide (BF2dbmPLA) polymers have been investigated for their molecular weight dependent fluorescence where the polymer chain plays an important role in BF2dbm solid-state emission. Here the substituent effects were further examined with a lipid chain replacing polylactide. Surprising process dependent and reversible mechanochromic fluorescence was discovered for the boron dodecane complex (BF2dbmOC12H25). A thermally annealed spin-cast film of the lipid dye on glass exhibited blue fluorescence under UV light but after shearing or scratching, the mechanically perturbed region turned yellow-green. The blue coloration could be rapidly recovered by thermal treatment of the film. The phenomena were investigated by steady-state fluorescence spectroscopy at room, low, and high temperatures, in situfluorescence microscopy, fluorescence lifetime measurements, and X-ray diffraction. Consistent with previous findings, the ordered-to-amorphous structural change that occurs upon mechanical perturbation may increase molecular rotational freedom, allowing for more efficient excimer emission, which typically occurs at longer wavelengths.

Mechanochromic Luminescence and Aggregation Induced Emission of Dinaphthoylmethane β-Diketones and Their Boronated Counterparts

Mechanochromic luminescence has been observed in many boron coordinated β-diketonate (BF2bdk) complexes. Recently, it was shown that the metal-free methoxy-substituted dinaphthoylmethane β-diketone (dnmOMe) also displayed aggregation induced emission (AIE), solvatochromism, and high contrast mechanochromic luminescence (ML) that recovered rapidly at room temperature. In order to understand how substituents and boron coordination affect solution and solid-state optical properties, a series of methoxy- and bromo-substituted derivatives (dnm, dnmOMe, dnmBr, and dnmBrOMe) and their corresponding boron complexes (BF2dnm, BF2dnmOMe, BF2dnmBr, and BF2dnmBrOMe) were synthesized and their AIE, ML, and room temperature recovery properties were compared. All boron complexes exhibited red-shifted absorption and emission, in addition to larger solution and solid-state quantum yields than β-diketones. While AIE studies show increased emission for dnmOMe and dnmBrOMe, the emission of corresponding boron complexes diminished upon aggregation. However, boron complexes were still strongly emissive in the solid state. ML properties were investigated using spin-cast films. Smearing resulted in the appearance of blue-green emission in ligands and a color change from green to yellow-orange in boron complexes. Bromide substituted derivatives showed increased room temperature recovery times compared to other dnm ligands, and boron complexes show only partial recovery over several days.

Arene effects on difluoroboron β-diketonate mechanochromic luminescence

Difluoroboron β-diketonate (BF2bdk) dyes display reversible mechanochromic luminescence (ML) in the solid state. A series of BF2bdk dyes with methyl, phenyl, naphthyl and anthracyl groups (i.e. arene substituents and bdk ligands: Me–Ph = mbm, Ph–Ph = dbm, Np–Ph = nbm, An–Ph = abm) were prepared to test the effects of π conjugation length and arene size on ML properties. Solid-state emission spectra were recorded for powders, spin-cast films, and dye coated weighing paper. The materials emit at various wavelengths from blue to red, depending on the conjugation length. Additionally, emission spectra were recorded for smeared solids and their recovery was tracked at room temperature over time. All dyes except for BF2mbm show emission changes upon mechanical perturbation, with increasing π conjugation correlating with more dramatic, redshifted fluorescence, however, their recovery is significantly affected by the aromatic substituents. The thermal and structural properties of the dyes in the solid state were also investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM).

Mechanochromic luminescence of halide-substituted difluoroboron β-diketonate dyes

Many difluoroboron β-diketonate (BF2bdk) compounds display mechanochromic luminescence (ML) and substituents can influence their optical properties. For example, the difluoroboron iododibenzoylmethane derivative, BF2dbm(I)OC12H25, exhibits mechanochromic luminescence quenching (MLQ). When annealed films of this dye are smeared under air, the perturbed region becomes dim under UV light. In this study, effects of differing halide substituents on ML were investigated. A series of dyes of the form BF2dbm(X)OC12H25, where X = H, F, Cl, Br, and I, were synthesized. The luminescence properties of these dyes were studied in CH2Cl2 solution, as films on weighing paper, spin-cast films on glass substrates, and as bulk powders. All of the dyes exhibit ML and MLQ. Solid-state emissions from these dyes are also sensitive to annealing temperature. The F, Cl, and Br dyes required higher annealing temperatures than the H and I analogues to achieve ordered emissive states. Spin-cast films of the dyes on glass were studied using atomic force microscopy (AFM) and X-ray diffraction (XRD), revealing a change from an amorphous material in the as-spun state to an ordered, crystalline material after annealing, with the F and Cl dyes showing a propensity to form large lamellar crystallites. Powders were also investigated using XRD and showed changes in diffraction patterns corresponding to thermal annealing and grinding. Finally, halogen substituents greatly diminished the ability of the dye films to recover their ordered emissive states after smearing under ambient conditions. These dyes could be useful for applications in which a more permanent inscription is desirable.

Metal complexes with polymeric ligands: Chelation and metalloinitiation approaches to metal tris(bipyridine)-containing materials

Synthetic approaches to metal complexes with polymeric ligands are described. The development of efficient methods for preparing simple bipyridine (bpy) derivatives and their corresponding metal complexes has facilitated their use as initiators and coupling agents in polymer syntheses. Ligand reagents were utilized as initiators in controlled polymerization reactions to form poly(2-R-2-oxazolines) (R = methyl, ethyl, phenyl, undecyl), polystyrenes, poly(methyl methacrylates) (PMMA)s, poly(e-caprolactone)s, and poly(lactic acid)s with bipyridine chelates at the end or centers of the chains. Poly(ethylene glycol) macroligands were formed by a chain-coupling method. Detailed studies of reaction kinetics were performed to determine the scope and limitations of each reaction type with different catalysts and reaction conditions. These results are illustrated for bpyPMMA n (n = 1 or 2), which was prepared by atom transfer radical polymerization with a CuBr/1,4,4,7,7,10-hexamethyltriethylenetetraamine catalyst system. Results of the kinetics investigations performed with other ligands and metalloinitiators are summarized. Macroligands thus prepared were coordinated to a labile metal ion, Fe(II), with standard protocols. Ultraviolet-visible spectral data for selected Fe-centered polymers are provided that confirm the production of the targeted polymeric iron complex products. An inert metal, Ru(II), was used as a template for generating architectural diversity; polymeric complexes with one to six chains emanating from the central core, as well as different heteroarm star products, were prepared.

Mechanochromic luminescence quenching: force-enhanced singlet-to-triplet intersystem crossing for iodide-substituted difluoroboron-dibenzoylmethane-dodecane in the solid state.

A lipid derivative of difluoroboron-iododibenzoylmethane (BF(2)dbm(I)OC(12)H(25)) was synthesized via Claisen condensation and boronation. Green photoluminescence is observed for the complex in the solid state. Unlike the previously reported difluoroboron-avobenzone (BF(2)AVB) complex, which exhibited significantly red-shifted fluorescence upon mechanical perturbation, the emission of a BF(2)dbm(I)OC(12)H(25) solid film is quenched when the sample is smeared under air but becomes orange under nitrogen. Spectroscopic and lifetime studies suggest that smearing brings the singlet excited state closer to the triplet state, thus increasing the coupling between the two states. As a result, intersystem crossing from the singlet to the triplet excited state is facilitated, and the total luminescence intensity is quenched at room temperature.