The optical properties of meta-alkoxy-substituted difluoroboron dibenzoylmethane dyes were investigated in solution and in the solid state. Meta-alkoxy substitution induced strong intramolecular charge transfer (ICT) from the oxygen-donating substituent to the halide and boron acceptors in the excited state, as compared to the π–π* transition that is observed with para-alkoxy substitution. The optical properties of para- and meta-substituted alkoxy boron dyes were evaluated by calculations, in dilute solution, and in solid-state films. When embedded in amorphous matrixes (e.g., PLA, PMMA, PS, cholesterol), all dyes showed fluorescence (F) and phosphorescence (P) emission. In this report, we show that meta-substitution resulted in enhanced solvatochromism and an increased phosphorescence-to-fluorescence ratio in solid-state films compared to analogous para-substituted samples. With enhanced phosphorescence intensity via the heavy-atom effect, iodo-substituted dyes were further studied in PLA–PEG nanoparticles. Oxygen calibrations revealed stronger phosphorescence and a greater oxygen-sensing range for the meta- versus para-alkoxy-substituted dyes, features that are important for oxygen-sensing materials design.Keywords: difluoroboron β-diketonate complexes; fluorescence; nanoparticles; oxygen sensing; poly(lactic acid); room-temperature phosphorescence; solvatochromism;

Luminescent difluoroboron β-diketonate poly(lactic acid) (BF2bdkPLA) materials serve as biological imaging agents. In this study, dye structures were modified to achieve emission colors that span the visible region with potential for multiplexing applications. Four dyes with varying π-conjugation (phenyl, naphthyl) and donor groups (−OMe, −NMe2) were coupled to PLLA–PEG block copolymers (∼11 kDa) by a postpolymerization Mitsunobu reaction. The resulting dye–polymer conjugates were fabricated as nanoparticles (∼55 nm diameter) to produce nanomaterials with a range of emission colors (420–640 nm). For increased stability, dye-PLLA-PEG conjugates were also blended with dye-free PDLA–PEG to form stereocomplex nanoparticles of smaller size (∼45 nm diameter). The decreased dye loading in the stereoblocks blue-shifted the emission, generating a broader range of fluorescence colors (410–620 nm). Tumor accumulation was confirmed in a murine model through biodistribution studies with a red emitting dimethyl amino-substituted dye–polymer analogue. The synthesis, optical properties, oxygen-sensing capabilities, and stability of these block copolymer nanoparticles are presented.

Difluoroboron β-diketonate (BF2bdk) compounds show environment-sensitive optical properties in solution, aggregation-induced emission (AIE) and multi-stimuli responsive fluorescence switching in the solid state. Here, a series of 4-azepane-substituted β-diketone (bdk) ligands (L-H, L-OMe, L-Br) and their corresponding difluoroboron dyes (D-H, D-OMe, D-Br) were synthesized, and various responsive fluorescence properties of the compounds were studied, including solvatochromism, viscochromism, AIE, mechanochromic luminescence (ML) and halochromism. Compared to the β-diketones, the boron complexes exhibited higher extinction coefficients but lower quantum yields, and red-shifted absorption and emission in CH2Cl2. Computational studies showed that intramolecular charge transfer (ICT) dominated rather than π–π* transitions in all the compounds regardless of boron coordination. In solution, all the bdk ligands and boron dyes showed red-shifted emission in more polar solvents and increased fluorescence intensity in more viscous media. Upon aggregation, the emission of the β-diketones was quenched, however, the boronated dyes showed increased emission, indicative of AIE. Solid-state emission properties, ML and halochromism, were investigated on spin cast films. For ML, smearing caused a bathochromic emission shift for L-Br, and powder X-ray diffraction (XRD) patterns showed that the “as spun” and thermally annealed states were more crystalline and the smeared state was amorphous. No obvious ML emission shift was observed for L-H or L-OMe, and the boronated dyes were not mechano-active. Trifluoroacetic acid (TFA) and triethylamine (TEA) vapors were used to study halochromism. Large hypsochromic emission shifts were observed for all the compounds after exposure to TFA vapor, and reversible fluorescence switching was achieved using the acid/base pair.

The mechanochromic luminescent (ML) properties of boron coordinated β-diketones (BF2bdks) have been widely studied, however the stimuli responsive properties of uncoordinated β-diketones (bdks) are less known. While bdk dyes show promising properties, including high contrast ML, rapid room temperature self-erasure, solvatochromism, and aggregation induced emission (AIE), previously reported dyes exhibit emission over a narrow range of wavelengths (∼420–500 nm). To tune luminescence over a broader color range, dimethylamino (DMA) substituted β-diketones were synthesized with a series of electron donating and withdrawing substituents. Solvatochromism, aggregation induced emission (AIE), ML, and thermochromism were investigated. Most dyes showed positive solvatochromic shifts and were responsive to mechanical and thermal stimuli. The solid-state emission wavelengths correlated with the electron withdrawing strength of the para substituent and ranged from blue to orange (488–578 nm). Structural and thermal characterization was performed by powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) respectively. These data indicate that stimuli responsive properties are the result of a crystalline to amorphous phase transition. Additionally, the aggregation induced emission (AIE) properties of iodo and cyano substituted dyes were measured in THF/H2O solutions. Both showed intense emission resulting from aggregation. The sensitivity of these dyes toward matrix polarity was further investigated by fabricating polystyrene films with increasing concentrations of dye and camphoric anhydride, a non-emissive polar dopant. A red-shift in emission was observed in these films which demonstrated that matrix effects can be used to tune the emission of DMA-substituted diketones.

Difluoroboron β-diketonate (BF2bdk) compounds exhibit solid-state switchable luminescence under excitation by UV light. This property is usually manifested as a blue-shift in emission when dye films are thermally annealed followed by a red-shift in response to mechanical shear (i.e. mechanochromic luminescence). Here we report thiophene and furan heterocycle-substituted dyes bearing short methoxy and long dodecyloxy chain substituents. Optical properties of the dyes were investigated in solution, pristine powders, and films on paper and glass. The structural and thermal properties of the dyes were also investigated by powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. The methoxy-substituted dyes exhibited neither thermal nor mechano-responsive behavior, however, addition of a longer C12 alkoxyl chain substituent resulted in stimuli-responsive behavior. The furan and thiophene dodecyloxy-substituted dyes both exhibit high-contrast reversible luminescence switching between crystalline, blue-shifted and amorphous red-shifted emissive states. For the furan dye, gentle heating produced a green emissive form while melting followed by rapid cooling produced an orange emissive form. The thiophene dye, on the other hand, exhibited blue-shifted emission when annealed below its melting temperature and red-shifted emission when smeared (mechanochromic luminescence). These transformations for both dyes were found to be completely reversible, making them potential candidates for applications requiring reusable, functional materials.

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.Keywords: aggregation induced emission; boron complex; mechanochromic luminescence; solvatochromism; β-diketone

Lifetime-based oxygen imaging is useful in many biological applications but instrumentation can be stationary, expensive, and complex. Herein, we present a portable, cost effective, simple alternative with high spatiotemporal resolution that uses a complementary metal oxide silicon (CMOS) camera to measure oxygen sensitive lifetimes on the millisecond scale. We demonstrate its compatibility with difluoroboron β-diketonate poly(lactic acid) (BF2bdkPLA) polymers which are nontoxic and exhibit long-lived oxygen sensitive phosphorescence. Spatially resolved lifetimes of four BF2bdkPLA variants are measured using nonlinear least squares (NLS) and rapid lifetime determination (RLD) both of which are shown to be accurate and precise. Real-time imaging in a dynamic environment is demonstrated by determining lifetime pixel-wise. The setup costs less than $5000, easily fits into a backpack, and can operate on battery power alone. This versatility combined with the inherent utility of lifetime measurements make this system a useful tool for a wide variety of oxygen sensing applications. This study serves as an important foundation for the development of dual mode real time lifetime plus ratiometric imaging with bright, long lifetime difluoroboron β-diketonate probes.

Difluoroboron β-diketonate poly(lactic acid) materials exhibit both fluorescence (F) and oxygen sensitive room-temperature phosphorescence (RTP). Introduction of halide heavy atoms (Br and I) is an effective strategy to control the oxygen sensitivity in these materials. A series of naphthyl-phenyl (nbm) dye derivatives with hydrogen, bromide, and iodide substituents were prepared for comparison. As nanoparticles, the hydrogen derivative was hypersensitive to oxygen (0–0.3%), while the bromide analogue was suited for hypoxia detection (0–3% O2). The iodo derivative, BF2nbm(I)PLA, showed excellent F to RTP peak separation and a 0–100% oxygen sensitivity range, unprecedented for metal-free RTP emitting materials. Due to the dual emission and exceptionally long RTP lifetimes of these O2 sensing materials, a portable, cost-effective camera was used to quantify oxygen levels via lifetime and red/green/blue (RGB) ratiometry. The hypersensitive H dye was well matched to lifetime detection; simultaneous lifetime and ratiometric imaging was possible for the bromide analogue, whereas the iodide material, with intense RTP emission and a shorter lifetime, was suited for RGB ratiometry. To demonstrate the prospects of this camera/material design combination for bioimaging, iodide boron dye-PLA nanoparticles were applied to a murine wound model to detect oxygen levels. Surprisingly, wound oxygen imaging was achieved without covering (i.e., without isolating from ambient conditions, air). Additionally, wound healing was monitored via wound size reduction and associated oxygen recovery, from hypoxic to normoxic. These single-component materials provide a simple tunable platform for biological oxygen sensing that can be deployed to spatially resolve oxygen in a variety of environments.Keywords: difluoroboron β-diketonate complexes; fluorescence; poly(lactic acid); ratiometric oxygen sensing; RGB camera imaging; room-temperature phosphorescence; wound healing

Difluoroboron β-diketonate compounds exhibit solid-state luminescence phenomena. Among these are reversible mechanochromic luminescence (ML), aggregation induced emission (AIE), and mechanochromic luminescence quenching (MLQ). These properties can be tuned by alterations to the molecular structure. Dyes with varying halide substituents exhibit tunable ML, MLQ, and solid-state emission with high quantum yields. A series of difluoroboron dibenzoylmethane (BF2dbm) dyes with iodide and alkoxyl substituents (BF2dbm(I)OR) were synthesized where R = CH3 (C1), C5H11 (C5), C6H13 (C6), C12H25 (C12), and C18H37 (C18)). The 4-iodo parent compound BF2dbm(I) (H) was made for comparison. By keeping the heavy atom static, the dependence of ML properties on alkyl chain length was probed. The hydrogen derivative is only weakly emissive as a solid and exhibited minimal mechanoresponsive behavior. In contrast, alkoxy dyes exhibited tunable ML and MLQ properties depending on the length of the alkyl chain. Longer chain dyes corresponded to smaller singlet triplet energy gaps, greater triplet emission enhancement (77 K), and longer recovery times after smearing under ambient conditions. Shorter chain dyes have a much greater affinity for the ordered emissive state, as confirmed by atomic force microscopy (AFM). Powder X-ray diffraction (XRD) was performed on pristine dye powders as well as drop-cast films to gauge crystallinity in various forms. Single crystal XRD analysis of the H and C5 dyes revealed significant differences in crystal packing and π–π stacked dimers for dyes bearing alkyl chains and dyes without alkyl chains. Unique I–I and F−π close interactions were discovered in the dye crystals.

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.

Reversible mechanochromic luminescence (ML) is known for difluoroboron β-diketonates (BF2bdks). Here, we report ML for the methoxy-substituted dinaphthoylmethane (dnmOMe) ligand even without coordination to boron. In addition, dnmOMe shows solvatochromism and aggregation-induced emission (AIE). Diketone thin films showed high contrast emission when smeared, and rapid recovery of the pre-smeared state at room temperature.

The dual-emissive properties of solid-state difluoroboron β-diketonate-poly(lactic acid) (BF2bdkPLA) materials have been utilized for biological oxygen sensing. In this work, BF2dbm(X)PLA materials were synthesized, where X = H, F, Cl, Br, and I. The effects of changing the halide substituent and PLA polymer chain length on the optical properties in dilute CH2Cl2 solutions and solid-state polymer films were studied. These luminescent materials show fluorescence, phosphorescence, and lifetime tunability on the basis of molecular weight, as well as lifetime modulation via the halide substituent. Short BF2dbm(Br)PLA (6.0 kDa) and both short and long BF2dbm(I)PLA polymers (6.0 or 20.3 kDa) have fluorescence and intense phosphorescence ideal for ratiometric oxygen sensing. The lighter halide-dye polymers with hydrogen, fluorine, and chlorine substitution have longer phosphorescence lifetimes and can be utilized as ultrasensitive oxygen sensors. Photostability was also analyzed for the polymer films.Keywords: difluoroboron β-diketonate complexes; heavy atom effect; photostability; poly(lactic acid); room-temperature phosphorescence

Dual emissive properties of solid-state difluoroboron β-diketonate-poly(lactic acid) (BF2bdk-PLA) materials have been utilized as biological oxygen sensors. Dyes with red-shifted absorption and emission are important for multiplexing and in vivo imaging, thus hydroxyl-functionalized dinaphthoylmethane initiators and dye-PLA conjugates BF2dnm(X)PLA (X = H, Br, I) with extended conjugation were synthesized. The luminescent materials show red-shifted absorbance (∼435 nm) and fluorescence tunability by molecular weight. Fluorescence colors range from yellow (∼530 nm) in 10–12 kDa polymers to green (∼490 nm) in 20–30 kDa polymers. Room-temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) are present under a nitrogen atmosphere. For the iodine-substituted derivative, BF2dnm(I)PLA, clearly distinguishable fluorescence (green) and phosphorescence (orange) peaks are present, making it ideal for ratiometric oxygen-sensing and imaging. Bromide and hydrogen analogues with weaker relative phosphorescence intensities and longer phosphorescence lifetimes can be used as highly sensitive, concentration independent, lifetime-based oxygen sensors or for gated emission detection. BF2dnm(I)PLA nanoparticles were taken up by T41 mouse mammary cells and successfully detected differences in oxygen levels during in vitro ratiometric imaging.

Luminescent materials are important for imaging and sensing. Aromatic difluoroboron β-diketonate complexes (BF2bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. A series of BF2bdks with naphthyl and phenyl groups was synthesized, and photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Polymer molecular weight and dye attachment site along with bromide heavy atom placement were varied to tune optical properties of dye–PLA materials. Systems without heavy atoms have long phosphorescence lifetimes, which is useful for lifetime-based oxygen sensing. Bromine substitution on the naphthyl ring resulted in intense, clearly distinguishable fluorescence and phosphorescence peaks important for ratiometric oxygen sensing and imaging.

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.

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.

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).

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).

Difluoroboron avobenzone (BF2AVB), a simple boron complex of a commercial sunscreen product, exhibits morphology-dependent emission and mechanochromic luminescence in the solid state. When scratched, smeared, or even gently touched, the emission color of BF2AVB films is significantly red-shifted under UV excitation. In the rubbed regions, the fluorescence recovers slowly at room temperature or much faster with heating, resulting in a simple rewritable “scratch the surface” ink of potential commercial use.

Fluorescence spectroscopy has been widely used to monitor different polymer processes such as polymerization kinetics, chain entanglements, and thermal transitions. The solvent-free controlled ring-opening polymerization (ROP) of lactide is significant both commercially and for research; thus, monitoring this process with a simple fluorescence method can be very useful. Here, a fluorescent dye, difluoroboron 4-methoxydibenzoylmethane (BF2dbmOMe) is employed to probe lactide bulk ROP by measuring the emission from solidified reaction aliquots at room temperature. It was found that, through the course of polymerization, the fluorescence of BF2dbmOMe in the solid-state aliquots exhibited a systematic shift from yellow to green and then to blue, accompanied by a gradual reduction in the decay lifetime. The fluorescence color change is sensitive to the monomer percent conversion, not the polymer molecular weight. On the basis of these observations and experimental data, we propose that the long-wavelength emission with perceivably longer lifetimes arises from BF2dbmOMe dye aggregates (ground and/or excited states), while the dissolved individual dye molecules are responsible for the blue fluorescence with a shorter lifetime. This demonstration of the utility of BF2dbmOMe as a fluorescent probe for lactide polymerization could have important practical implications.Keywords: fluorescence spectroscopy; polylactide; ring-opening polymerization

A lipid derivative of difluoroboron−iododibenzoylmethane (BF2dbm(I)OC12H25) was synthesized via Claisen condensation and boronation. Green photoluminescence is observed for the complex in the solid state. Unlike the previously reported difluoroboron−avobenzone (BF2AVB) complex, which exhibited significantly red-shifted fluorescence upon mechanical perturbation, the emission of a BF2dbm(I)OC12H25 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.

Responsive biomaterials play important roles in imaging, diagnostics, and therapeutics. Polymeric nanoparticles (NPs) containing hydrophobic and hydrophilic segments are one class of biomaterial utilized for these purposes. The incorporation of luminescent molecules into NPs adds optical imaging and sensing capability to these vectors. Here we report on the synthesis of dual-emissive, pegylated NPs with “stealth”-like properties, delivered intravenously (IV), for the study of tumor accumulation. The NPs were created by means of stereocomplexation using a methoxy-terminated polyethylene glycol and poly(d-lactide) (mPEG-PDLA) block copolymer combined with iodide-substituted difluoroboron dibenzoylmethane-poly(l-lactide) (BF2dbm(I)PLLA). Boron nanoparticles (BNPs) were fabricated in two different solvent compositions to study the effects on BNP size distribution. The physical and photoluminescent properties of the BNPs were studied in vitro over time to determine stability. Finally, preliminary in vivo results show that stereocomplexed BNPs injected IV are taken up by tumors, an important prerequisite to their use as hypoxia imaging agents in preclinical studies.Keywords: Boron dye; enhanced permeation and retention (EPR) effect; fluorescence; nanoparticles; phosphorescence; poly(ethylene glycol) (PEG); poly(lactic acid) (PLA) stereocomplexes

The polymeric metal complex iron tris(dibenzoylmethane-polylactide), Fe(dbmPLA)3, was synthesized from d,l-lactide and Fe(dbmOH)3, a hydroxyl functionalized metalloinitiator. Reactions are fast relative to those with the dbmOH ligand initiator even when no Sn(oct)2 or DMAP catalyst is added, indicating that iron serves as both a dbm protecting group and a catalyst for lactide ring-opening polymerization (ROP). Kinetics experiments show good molecular weight control to ∼60−70% conversion (e.g., PDIs < 1.1; no evidence of transesterification via 1H NMR spectroscopic analysis; continued chain growth when Fe(dbmPLA)3 is heated with additional lactide monomer). For comparison, Fe(bdk)3 (bdk = β-diketonate) complexes were tested as lactide ROP catalysts using dbmOH and benzyl alcohol as the initiators. The red-orange Fe(dbmPLA)3 polymers were analyzed by GPC, 1H NMR and UV-vis spectroscopy. Demetalation with dilute HCl yields dbmPLA macroligands for chelation to other metals. The polymerization mechanism using Fe(dbmOH)3 as an initiator and catalyst is also discussed and results suggest ligand exchange prior to initiation may be involved.

Oxygen is essential for many forms of life and its depletion in the body and the environment can lead to deleterious effects. Low oxygen conditions, even anoxia, are associated with eutrophication of lakes and rivers, wherein an over abundance of nutrients often caused by pollution result in excessive plant growth and decay, threatening water quality, ecosystem balance, and aquatic life. In the body, low oxygen conditions or hypoxia may be generalized, as can occur at high altitude or during strenuous exercise, or localized in particular tissues, when there is a mismatch between oxygen supply and demand. Hypoxia is present in many important diseases as well. Low oxygen levels in tumors are often associated with biochemical changes, increased invasiveness, cancer progression, and resistance to radiation and chemotherapies. Vascular blockage in strokes, heart attacks, and peripheral artery disease, which is common in diabetes, are other situations where oxygen levels can drop precipitously and cause great damage to affected tissues. Clearly, innovative sensing technologies that provide new insight into these many oxygen dependent processes can impact global society in significant ways.

A class of aryl trifluoromethyl-containing β-diketones were synthesized via one step Claisen condensation. These π-conjugated diketones exhibit strong solvatochromism from intramolecular donor-acceptor charge transfer (CT). In addition, fluorescence quantum yields (ϕf) and lifetimes (τf) were measured in different solvents. Diketones exhibit bathochromic shifts in emission spectra with increasing solvent polarity. Fluorescence changes upon Group II metal binding were also studied. Despite the relatively simple structure, the anthracene-CF3 diketone, atm, has strong binding affinity for Mg2+. A 70 nm blue shift and sixfold increase in intensity were observed upon addition of only one equivalent MgCl2 in ethanol solution. It also shows selectivity for Mg2+ binding even in the presence of excess Ca2+. Association constant (Ka) calculations suggest atm has two orders of magnitude stronger chelation for divalent magnesium than for calcium. These findings make atm an attractive starting point for molecular probe and light emitting material design.

Difluoroboron β-diketonate−polymer conjugates have remarkable solid-state luminescent properties that are useful in a variety of fields including multiphoton microscopy, cell biology, and tumor hypoxia imaging. Despite the successful applications of these systems, the role of boron in these polymeric materials has not been thoroughly investigated. Here we explore a boron-free model system with dibenzoylmethane chromophores in poly(lactic acid) (PLA) for comparison. The hydroxyl-functionalized aromatic diketone, dibenzoylmethane (dbmOH), is weakly fluorescent in the solid state and nonfluorescent in solution while its difluoroboron complex (BF2dbmOH) is highly emissive in both states. Using dbmOH and BF2dbmOH as initiators, well-defined end-functionalized polylactides, dbmPLA and BF2dbmPLA, were obtained via tin-catalyzed controlled ring-opening polymerization. Boronation of the dbmOH initiator affects the polymerization kinetics and the photophysical properties of the resulting BF2dbmPLA material. Both dbmPLA and BF2dbmPLA are dual emissive in the solid state, exhibiting both fluorescence and room-temperature phosphorescence (RTP), whereas only BF2dbmPLA is luminescent in solution. These results suggest that boron plays two roles: (1) as a protecting group in the polymerization and (2) as an emission enhancer. Finally, the presence of dual emission for both polymers indicates that it may be the diketone core structure rather than the difluoroboron that is essential for RTP in a rigid PLA matrix.

Difluoroboron β-diketonate dyes are noted for their intense fluorescence, one- and two-photon absorption, solvatochromism, and good aqueous stability. Incorporation of these dyes into polymers leads to processable optical materials, which in some cases display additional emissive features. For example, previously we reported that difluoroboron dibenzoylmethane−polylactide (BF2dbmPLA) exhibits both fluorescence and room temperature phosphorescence (RTP). Typically RTP is ascribed to a matrix effect, where the rigid ordered medium restricts dye molecular motions, and excited-state decay via thermal pathways is hindered in favor of radiative decay. To test polymer effects on dye emissive properties, this study explores BF2dbmPCL (PCL = poly(ε-caprolactone), a semicrystalline polymer) of different molecular weights ranging from ∼3 to 18 kDa. PCL materials exhibit the expected intense fluorescence and molecular weight dependent emission but, surprisingly, no RTP. Phosphorescence is evident at lower temperatures (0 to −78 °C), however, and both intensities and lifetimes increase as the temperature is dropped. Polymer thermal properties were also examined. Aside from in the lowest molecular weight sample, the dye end group showed little effect on melting and glass transition temperatures. These dye-modified materials lend useful optical imaging capability to PCL, a polymer commonly used in drug delivery and tissue engineering applications.

Spontaneous gel formation of iron(II) tris(bipyridine)-centered poly(ethylene glycol) methacrylate ([Fe{bpy(PEG-MA)2}3]2+) was observed without the addition of a cross-linking agent. BpyPEG2 macroligands were first modified with methacrylate groups using methacrylic anhydride and then combined with FeSO4 to produce [Fe{bpy(PEG-MA)2}3]SO4. End group analysis by 1H NMR spectroscopy verified quantitative methacrylation of the PEG hydroxyl chain ends. A series of experiments and control reactions were performed to investigate the conditions required for gel formation. Hydrogels of [Fe{bpy(PEG-MA)2}3]SO4 were produced both in the presence and in the absence of a photoinitiator. Controls using MA-PEG-MA also formed hydrogels in the presence of [Fe(bpy)3]2+; however, the addition of a radical scavenger, TEMPO, prevented formation of a polymer network, suggesting radical involvement. Treatment of preformed hydrogels of bpy(PEG-MA)2 with aqueous solutions of FeSO4, CuBr2, and CoCl2 also produced materials with color changes indicative of complexation.

Luminescent materials result when boron dyes are combined with polyester biomaterials. Previously, it was reported that difluoroboron dibenzoylmethane polylactide, BF2dbmPLA, exhibits intense fluorescence as well as unusual room temperature phosphorescence (RTP) in the solid state. To gain a better understanding of polymer effects on photophysical properties, a poly(ε-caprolactone) (PCL) analogue, BF2dbmPCL, was synthesized by tin catalyzed ring-opening polymerization and employed as a macroinitiator in combination with d,l- and l-lactide for the preparation of BF2dbmPCL-PLA and BF2dbmPCL-PLLA block copolymers, respectively. Reasonably well-defined materials with low polydispersity indices (PDIs) were obtained in good yield. 1H NMR and UV−vis spectroscopy and thermal analysis are reported. Luminescence spectra and lifetimes were measured for polymer solutions and films. All materials are intensely fluorescent with high quantum yields. Although no RTP is noted for BF2dbmPCL films, curiously, this property is restored for PCL-polylactide block copolymer analogues in the solid state.

Ruthenium polypyridyl complexes are incorporated into polymers for sensing and light emitting materials applications. Coupling reactions between metal complexes and polymers are one route to polymeric metal complexes. In an effort to increase conjugation efficiency, tune materials properties, and introduce a responsive crosslink, ruthenium tris(bipyridine) derivatives with sulfur substituents were synthesized and compared to oxygen analogues. Difunctional thiols, thioesters, thioethers, and disulfides, as well as hexafunctional nonpolymeric model systems, were explored. Upon exposure to oxygen, the thiol derivative was readily oxidized. These studies guided Ru(bpy) 3 PEG coupling reactions with disulfide and thioether linkages, which proceeded to ∼80% and ∼60% yield, respectively. The luminescence properties of the Ru PEG derivatives and model systems were investigated. The emission spectra and lifetimes for all complexes in CH 3CN under an inert atmosphere are comparable to [Ru(bpy) 3]Cl 2. Lifetime data for nonpolymeric analogues fit to a single exponential decay indicating heterogeneity, suggesting sample homogeneity, whereas data for polymers fit to a multiexponential decay. In contrast to certain [Ru(bpy) 3] 2+/thiol mixtures, no intramolecular quenching by the sulfide is observed for [Ru(bpy) 2{bpy(CH 2SH) 2}](PF 6) 2. Emission spectra red shift and multiexponential decay are noted for the oxidized Ru thiol product. The rates of oxygen quenching are slower for Ru PEG derivatives than those for nonpolymeric analogues, which may be attributed to shielding effects of the polymer chain.

Block copolymers incorporating biocompatible poly(ethylene glycol) (PEG) and poly(lactide) (PLA) polymers and labile metal complexes can serve as stimuli responsive biomaterials. Bipyridine-centered PEG-PLA materials were prepared via ring opening polymerization of ethylene oxide and lactide (d,l and l). Macroligands and their iron complexes [Fe{bpy(PEG-PLA)2}3]Cl2 were characterized by gel permeation chromatography, 1H NMR, and UV−vis spectroscopy. Solvent and counterion effects were also explored by UV−vis spectroscopy. Extinction coefficients drop 3-fold in CH2Cl2; values typical for iron(II) tris(bipyridine) complexes are restored by addition of very small quantities of MeOH. Thermal properties of bpy(PEG-PLA)2 and [Fe{bpy(PEG-PLA)2}3]2+ were investigated by thermogravimetric analysis and differential scanning calorimetry, showing values typical for the polymer blocks and little effect of incorporated ligand or metal sites. Micelle-like nanoparticles of block copolymer macroligands and iron star polymer complexes were prepared via nanoprecipitation. Dynamic light scattering and transmission electron microscopy analyses confirm the presence of spheroid particles (macroligands, 111−253 nm in diameter; iron complexes, 37−75 nm in diameter).

Elaborate biocompatible block copolymer architectures are achievable via controlled ring opening polymerization (ROP). Selective incorporation of donor groups results in macroligands that can coordinate metals to form site-isolated metallopolymers. A hydroxyl-functionalized bipyridine, bpy(CH2OH)2, was employed as an initiator for the ROP of ethylene oxide, producing bpyPEG2. Subsequent reaction with ε-caprolactone followed by lactide (d,l- and l-) results in well-defined bpy-centered triblock PCL-PEG-bpy-PEG-PCL and pentablock PLA-PCL-PEG-bpy-PEG-PCL-PLA and PLLA-PCL-PEG-bpy-PEG-PCL-PLLA macroligands, respectively, with low polydispersity indices (PLA = d,l-polylactide, PCL = poly(ε-caprolactone), PEG = poly(ethylene glycol), PLLA = l-polylactide). These systems were combined with FeCl2 to produce iron-centered star blocks [Fe{bpy(PEG-PCL)2}3]Cl2, [Fe{bpy(PEG-PCL-PLA)2}3]Cl2, and [Fe{bpy(PEG-PCL-PLLA)2}3]Cl2 with six diblock and triblock arms. Materials were characterized by gel permeation chromatography (GPC), 1H NMR, and UV−vis spectroscopy, verifying that synthetic targets were achieved. Thermal gravimetric analysis shows decomposition temperatures corresponding to PEG, PCL, and PLA blocks, and melting temperatures are typical for these materials.

Difluoroboron dibenzoylmethane-polylactide, BF2dbmPLA, 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.Keywords: boron dye; fluorescence; nanoparticles; phosphorescence; poly(lactic acid) (PLA)

Difluoroboron β-diketonate (BF2bdk) compounds exhibit solid-state switchable luminescence under excitation by UV light. This property is usually manifested as a blue-shift in emission when dye films are thermally annealed followed by a red-shift in response to mechanical shear (i.e. mechanochromic luminescence). Here we report thiophene and furan heterocycle-substituted dyes bearing short methoxy and long dodecyloxy chain substituents. Optical properties of the dyes were investigated in solution, pristine powders, and films on paper and glass. The structural and thermal properties of the dyes were also investigated by powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC), respectively. The methoxy-substituted dyes exhibited neither thermal nor mechano-responsive behavior, however, addition of a longer C12 alkoxyl chain substituent resulted in stimuli-responsive behavior. The furan and thiophene dodecyloxy-substituted dyes both exhibit high-contrast reversible luminescence switching between crystalline, blue-shifted and amorphous red-shifted emissive states. For the furan dye, gentle heating produced a green emissive form while melting followed by rapid cooling produced an orange emissive form. The thiophene dye, on the other hand, exhibited blue-shifted emission when annealed below its melting temperature and red-shifted emission when smeared (mechanochromic luminescence). These transformations for both dyes were found to be completely reversible, making them potential candidates for applications requiring reusable, functional materials.

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.

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.

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).

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).

Reversible mechanochromic luminescence (ML) is known for difluoroboron β-diketonates (BF2bdks). Here, we report ML for the methoxy-substituted dinaphthoylmethane (dnmOMe) ligand even without coordination to boron. In addition, dnmOMe shows solvatochromism and aggregation-induced emission (AIE). Diketone thin films showed high contrast emission when smeared, and rapid recovery of the pre-smeared state at room temperature.