Meta-Alkoxy-Substituted Difluoroboron Dibenzoylmethane Complexes

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Meta-Alkoxy-Substituted Difluoroboron Dibenzoylmethane Complexes as Environment-Sensitive Materials Margaret L. Daly,‡ Caroline Kerr,‡ Christopher A. DeRosa, and Cassandra L. Fraser* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States S Supporting Information *

ABSTRACT: 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 oxygendonating 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 metasubstitution 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, iodosubstituted 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, solvatochromism, fluorescence, room-temperature phosphorescence, poly(lactic acid), nanoparticles, oxygen sensing

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INTRODUCTION Luminescent materials are useful for many applications, particularly in biological imaging. Biological sensing applications commonly utilize luminescent compounds because the emission is highly sensitive to analytes and can be easily detected by biologically noninvasive methods (e.g., color cameras).1 Environment-sensitive optical properties have also been used for biological imaging. For example, materials have been developed that are responsive to the pH, viscosity, or polarity of a solvent.2−4 Also, oxygen quenching of phosphorescence can be exploited for optical oxygen-sensing applications to monitor hypoxic environments.5 Insights into fluorescent and phosphorescent materials can yield candidates for biological sensing of polarity, viscosity, or oxygen concentration.6,7 In particular, boron-functionalized luminogens are excellent stimuli-responsive materials for these purposes.8−11 Difluoroboron β-diketonates (BF2bdks) have emerged as a leading class of functionalized organic luminogens. Given their colorful and multifaceted optical properties, they are a focus of numerous studies and have been incorporated into many emissive technologies. New insights into BF2bdk structure− property relationships in different material environments can advance many optical materials fields as any one dye has the potential to be used in a number of ways. For biosensing, charge-transfer BF2bdks designed by Tang and co-workers © XXXX American Chemical Society

resulted in near-infrared, solvent-sensitive emission, ultrasensitive to the polarity in mitochondria, which could be quantified by ratiometric techniques.12 In another study, Klymchenko and co-workers engineered red-emitting BF2bdks as viscosity-sensitive fluorophores for wash-free imaging of membranes.13 In modern organic light-emitting diodes (OLEDs), harnessing energy from the triplet manifold may yield the brightest, most efficient devices.14 Here, BF2bdks excel as well, as Adachi and co-workers were able to generate bright, color tunable OLEDs through excimer-tunable emission of difluoroboron avobenzone (BF2AVB) in devices.15 Finally, when dyes are embedded in a rigid matrix, room-temperature phosphorescence is activated.16 Consequently, oxygen sensing through phosphorescence quenching can be achieved with BF2bdks. Optical oxygen-sensing techniques are becoming increasingly competitive with commonly used electrodes.17 The advancement of metal-free boron dyes as oxygen-sensing materials can have distinct advantages in nontoxicity and costs compared to other phosphors in this growing field. Furthermore, because of the importance of oxygen in many medical conditions, such as cancer,18 diabetes,19 and poor Received: May 16, 2017 Accepted: August 17, 2017

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DOI: 10.1021/acsami.7b06910 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces wound healing,20 new materials can help physicians with patient diagnosis, treatment, and care in clinical settings.21 Given these many uses of BF2bdks, as small-molecule imaging agents, efficient device emitters, and macromolecular oxygen sensors, increased understanding and control of structuredependent properties can benefit many applied materials fields. Oxygen-sensing BF2bdks have been investigated by our group for a number of years. These molecules exhibit fluorescence (F) and room-temperature phosphorescence (RTP) when the dyes are confined to a rigid matrix, such as biodegradable PLA.16 Other optical properties of BF2bdks include large extinction coefficients, two-photon absorption, and high quantum yields.22 In these materials, the fluorescence serves as an internal reference (oxygen insensitive) as the RTP intensity changes based on the oxygen level (oxygen sensitive), enabling ratiometric sensing via a single-component material. Nanoparticles fabricated with these dual-emissive boron dye− polymers can be used as single-component, ratiometric imaging agents for generating oxygen maps of biological systems such as hypoxic tumors and wounds.23,24 Synthetic procedures for these polymeric materials have been developed. Dyes are prepared as alcohol initiators for the controlled ring-opening polymerization of lactones25−28 or as phenol couplers for conjugation to biocompatible polymers.29 Other reports on the structure−property relationships of difluoroboron dibenzoylmethane dyes have shown emission sensitive to the substituent position. Cano et al. studied asymmetric alkoxy substitution on both aryl moieties of difluoroboron dibenzoylmethane complexes and found that the asymmetric substitution pattern yielded compounds with larger Stokes shifts and longer emission lifetimes.30,31 Kononevich et al. studied alkoxysilyl derivatives of BF2dbm and showed that para- and meta-substitution of alkoxysilyl groups resulted in compounds with higher quantum yields and more red-shifted emission compared to ortho-alkoxysilyl substitution.32 In a computational study, Mirochnik et al. used alkoxy donors to change the energy level of the HOMO and nitro-accepting groups to adjust the energy level of the LUMO in BF2dbm complexes, where greater red shift in emission resulted from para-substituted nitro acceptors in BF2dbm.33 A decrease in the HOMO−LUMO energy gap with para-substituted accepting groups is also achieved with halide substitution.34 Typically, para-substituted ketone and ester building blocks are employed in β-diketonate ligand synthesis, which results in π−π* absorption in the corresponding boron dibenzoylmethane dyes. 35 In this report, BF 2 dibenzoylmethane (BF2dbm) dyes were synthesized with dodecylalkoxy chains in either the para-position (4-OC12H25; pX, where X = H, F, Cl, Br, or I) or the meta-position (3-OC12H25; mX, where X = H, F, Cl, Br, or I) on one aryl moiety of the dye, while the opposing phenyl ring was substituted with a halide (F, Cl, Br, I), as shown in Figure 1. The para-alkoxy-substituted derivatives were previously synthesized and characterized in CH2Cl2 solution and as solid-state powders.36 Because difluoroboron has acceptor behavior in these BF2dbm scaffolds, alkoxy chains were introduced as donors. The dodecylalkoxy chain is also hypothesized to increase dye solubility and aid in assembly in hydrophobic environments.37 Halide substitution increases acceptor ability and enhances intersystem crossing (ISC) via the heavy-atom effect for oxygen-sensing applications.38,39 The BF2dbm dyes were characterized in CH2Cl2 solution and PLA thin films. Solvatochromism experiments are

Figure 1. Chemical structures of alkoxy-substituted difluoroboron dibenzoylmethane dyes.

supported by density functional theory (DFT) calculations to provide insight into the excited-state transitions for each derivative. The iodide-substituted samples were co-nanoprecipitated with a PLA−PEG block copolymer, and oxygen calibrations were performed with the resulting nanoparticles to assess the oxygen-sensing capabilities and potential for biological imaging.

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EXPERIMENTAL DETAILS

Materials. The poly(lactic acid) (PLA) and PLA−PEG were prepared as previously described.22,40 The boron dyes were prepared via three-step synthesis: Williamson ether synthesis to produce the dodecylalkoxylated ketone, Claisen condensation to obtain the βdiketones, and boronation to yield the BF2bdks. The para-substituted dodecylalkoxy derivatives were prepared as previously described.36 Solvents CH2Cl2 and THF were dried and purified over 3 Å molecular sieves activated at 300 °C.41 All other chemicals were reagent grade from Sigma-Aldrich and were used without further purification. The βdiketone ligands used in the boronation reactions were purified by recrystallization. Thin films of dye/PLA blends were prepared by dissolving dyes and PLA in CH2Cl2 to form a homogeneous solution in a glass vial. The vial was slowly rotated under a stream of nitrogen to evaporate the solvent. The solution-cast films were then dried in vacuo overnight before measurements were performed. Boron dye nanoparticles were formed by dissolving the dye and PLA−PEG in DMF and then adding the DMF solution dropwise into water with stirring (i.e., nanoprecipitation). The PLA−PEG was prepared as previously reported.42 1H NMR (600 MHz) spectra were recorded on a Varian VMRS 600/51 instrument in CDCl3 or D6-DMSO. 1H NMR spectra were referenced to the residual signals for protiochloroform (7.26 ppm), protioDMSO (2.50 ppm), and protioacetone (2.09 ppm). Coupling constants are given in hertz. The UV−vis spectra were recorded on a Hewlett−Packard 8452A diode array spectrophotometer. Luminescence Characterization. Steady-state fluorescence emission spectra were recorded on a Horiba Fluorolog-3 Model FL3-22 spectrofluorometer (double-grating excitation and doublegrating emission monochromator). A 2 ms delay was used when recording the delayed emission spectra. Time-correlated single-photon counting (TCSPC) fluorescence lifetime measurements were performed with a NanoLED-370 (λex = 350 nm) excitation source and a DataStation Hub as the SPC controller. Phosphorescence lifetimes were measured with a 1 ms multichannel scalar (MCS) excited with a flash xenon lamp (λex = 369 nm; duration < 1 ms). Lifetime data were analyzed with DataStation v2.4 software from Horiba Jobin Yvon. Fluorescence spectra and lifetimes were obtained under ambient conditions (e.g., air, ∼21% oxygen). For phosphorescence measurements, solution-cast films in vials were fitted with a 12 mm PTFE/silicone/PTFE seal (Chromatography Research Supplies) connected by a screw cap. Vials were continuously purged with analytical grade N2 (Praxair) during measurements. Fluorescence and phosphorescence lifetimes were fit to double- or triple-exponential decays in solid-state films. Computational Modeling. The boronated samples were modeled with the Gaussian 09 suite of programs using DFT and B3LYP/631+G(d) to simulate the B, O, and C atoms.43 The C12 alkoxy chain was replaced with a C3 unit to reduce computational demand.44 All B


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ACS Applied Materials & Interfaces vibrational frequencies were positive, assuring that the geometries are at least a local minimum. Single-point energy calculations were used to generate the molecular orbital diagrams utilizing B3LYP/6-31+G(d) for B, O, and C atoms. Time-dependent DFT, TD-B3LYP/6311+G(d) for B, O, and C atoms, was employed for an estimate of the absorption spectrum at the optimized ground-state geometry. A Tomasi polarized continuum for dichloromethane solvent was used in each calculation.45 Molecular orbital diagrams were depicted using GaussView 5 software.46 The coordinates in Tables S1, S3, and S5 are given in Cartesian, in Angstroms. Synthesis. 1-(3-(Dodecyloxy)phenyl)ethan-1-one. The metaOC12H25 ketone was prepared as previously described for the parasubstituted derivatives,36 but 3-hydroxyacetophenone was used in place of 4-hydroxyacetophenone, to yield a white crystalline powder, 1.56 g, 35%. 1H NMR (600 MHz, CDCl3): δ 7.50 (d, J = 12, 1H, 6-PhH), 7.46 (s, 1H, 2-Ph-H), 7.34 (t, J = 12, 1G, 5-Ph-H), 7.08 (d, J = 6, 1H, 4-Ph-H), 3.98 (t, J = 12, 2H, Ar-OCH2CH2CH2C8H16CH3), 2.58 (s, 3H, OC−CH3), 1.79 (p, J = 6, 2H, Ar-OCH2CH2CH2C8H16CH3), 1.47 (p, J = 6, 2H, Ar-OCH2CH2CH2C8H16CH3), 1.28 (m, broad, 16H, Ar-OCH2CH2CH2C8H16CH3), 0.86 (t, J = 6, 3H, ArOCH2CH2CH2C8H16CH3). β-Diketones (L-mX). The meta-diketone dye precursors were prepared as previously described for para-substituted dyes, with the exception of the ketone, 1-(3-(dodecyloxy)phenyl)ethan-1-one, used in place of 1-(4-(dodecyloxy)phenyl)ethan-1-one.22 The β-diketone ligands and the ketone/ester starting substrates were similar in polarity and solubility and proved difficult to isolate. Thus, crude ligands were used to generate the boron dyes in the next step. Difluoroboron-1-(3-(dodecyloxy)phenyl)-3-phenylpropane-1,3dione (mH). The phenyl dodecyloxyphenyl ligand (L-mH) (263 mg, 0.57 mmol) was weighed into an oven-dried 250 mL round-bottom flask and dissolved in anhydrous CH2Cl2 (100 mL). Boron trifluoride diethyl etherate was then added to the solution via a syringe (105 μL, 0.85 mmol). The reaction proceeded at room temperature, under nitrogen, and was monitored by TLC until consumption of the ligand substrate was complete (5 h). Excess boron reagent was quenched by K2CO3(s) (100 mg, 0.71 mmol), and the reaction mixture was stirred for an additional 30 min, followed by filtration to remove solids. Rotary evaporation to remove CH2Cl2 produced a yellow solid that was purified via recrystallization from hexanes/acetone to yield a yellow powder, 151 mg, 58%. 1H NMR (600 MHz, CDCl3): δ 7.97 (d, J = 6, 2H, 2, 6-Ph-H), 7.54 (d, J = 6, 2H, 3, 5-Ph-H), 7.50 (d, J = 6, 1H, 6-o-Ph-H), 7.48 (s, 1H, 2-o-Ph-H), 7.44 (t, J = 6, 1H, 5-o-Ph-H), 7.07 (d, J = 6, 1H, 4-o-Ph-H), 6.82 (s, 1H, COCHCO), 4.02 (t, J = 6, 2H, Ar-OCH 2 CH 2 CH 2 C 8 H 1 6 CH 3 ), 1.81 (p, J = 6, 2H, ArOCH2CH2CH2C8H16CH3), 1.46 (p, J = 6, 3H, ArO C H 2 C H 2 C H 2 C 8 H 1 6 C H 3 ), 1 .31 (m, b road, 22H , A rOCH2CH2CH2C8H16CH3), 0.86 (t, J = 6, 3H, ArOCH2CH2CH2C8H16CH3). HRMS (ESI, TOF) m/z calculated for C27H36BO3F2 457.2726 [M + H]+; found 457.2721. Difluoroboron-1-(3-(dodecyloxy)phenyl)-3-(4-fluorophenyl)propane-1,3-dione (mF). The fluoro-substituted boron dye was made as previously described for mH, using the fluorophenyl dodecyloxyphenyl ligand L-mF (122 mg, 0.28 mmol) instead of L-mH. The crude product was purified via recrystallization from hexanes/acetone to yield a yellow powder, 89 mg, 65%. 1H NMR (600 MHz, D6DMSO): δ 8.47 (m, 2H, 2, 6-f-Ph-H), 7.92 (m, 2H, 2, 6-o-Ph-H), 7.81 (s, 1H, COCHCO), 7.54 (t, 1H, 5-o-Ph-H), 7.51 (t, 2H, 3, 5-f-Ph-H), 7.36 (d, J = 6, 1H, 4-o-Ph-H), 4.08 (t, 2H, ArOCH2CH2CH2C8H16CH3), 1.74 (p, 2H, ArOCH2CH2CH2C8H16CH3), 1.41 (p, 2H, ArO C H 2 C H 2 C H 2 C 8 H 1 6 C H 3 ), 1 .23 (m, b road, 16H , A rOCH2CH2CH2C8H16CH3), 0.82 (t, 3H, ArOCH2CH2CH2C8H16CH3). HRMS (ESI, TOF) m/z calculated for C27H35O3F3B 475.2631 [M + H]+; found 475.2630. Difluoroboron-1-(3-(dodecyloxy)phenyl)-3-(4-chlorophenyl)propane-1,3-dione (mCl). The chloro-substituted boron dye was made as previously described for mH, using the chlorophenyl dodecyloxyphenyl ligand L-mCl (301 mg, 0.68 mmol) instead of LmH. The product was purified by column chromatography (12:1, 6:1,

then 2:1 hexanes/ethyl acetate) to yield a yellow powder, 72 mg, 22%. H NMR (600 MHz, D6-DMSO): δ 8.74 (d, J = 3, 2H, 2, 6-Cl-Ph-H), 8.28 (s, 1H, 2-o-Ph-H), 8.27 (d, J = 3, 1H, 6-o-Ph-H), 8.17 (s, 1H, COCHCO), 8.08 (d, J = 6, 2H, 3, 5-Cl-Ph-H), 7.90 (t, 1H, 5-o-Ph-H), 7.73 (d, J = 3, 1H, 4-o-Ph-H), 4.43 (t, 2H, ArOCH2CH2CH2C8H16CH3), 2.08 (p, 2H, ArOCH2CH2CH2C8H16CH3), 1.77 (p, 2H, ArO CH 2 C H 2 CH 2 C 8 H 1 6 CH 3 ), 1.6 5 (m, b road, 16H, A rOCH2CH2CH2C8H16CH3), 1.17 (t, 3H, ArOCH2CH2CH2C8H16CH3). HRMS (ESI, TOF) m/z calculated for C27H35BO3F2Cl 491.2336 [M + H]+; found 491.2333. Difluoroboron-1-(3-(dodecyloxy)phenyl)-3-(4-bromophenyl)propane-1,3-dione (mBr). The bromo-substituted boron dye was made as previously described for mH, using the bromophenyl dodecyloxyphenyl ligand L-mBr (228 mg, 0.47 mmol) instead of LmH. The product was purified via recrystallization from hexanes/ acetone to yield a yellow powder, 141 mg, 56%. 1H NMR (600 MHz, D6-DMSO): δ 8.32 (d, J = 6, 2H, 2, 6-Br-Ph-H), 7.95 (s, 1H, 2-o-PhH), 7.93 (d, J = 3, 1H, 6-o-Ph-H), 7.89 (d, J = 6, 2H, 3, 5-Br-Ph-H), 7.83 (s, 1H, COCHCO), 7.57 (t, 1H, 5-o-Ph-H), 7.40 (d, J = 3, 1H, 4o-Ph-H), 4.11 (t, 2H, Ar-OCH2CH2CH2C8H16CH3), 1.77 (p, 2H, ArOCH2CH2CH2C8H16CH3), 1.45 (p, 2H, ArO CH 2 C H 2 CH 2 C 8 H 1 6 CH 3 ), 1.3 3 (m, b road, 16H, A rOCH2CH2CH2C8H16CH3), 0.85 (t, 3H, ArOCH2CH2CH2C8H16CH3). HRMS (ESI, TOF) m/z calculated for C27H35BO3F2Br 535.1831 [M + H]+; found 535.1832. Difluoroboron-1-(3-(dodecyloxy)phenyl)-3-(4-iodophenyl)propane-1,3-dione (mI). The iodo-substituted boron dye was made as previously described for mH, using the iodophenyl dodecyloxyphenyl ligand L-mI (587 mg, 1.1 mmol) instead of L-mH. The product was purified via recrystallization from hexanes/acetone to yield a yellow powder, 418 mg, 65%. 1H NMR (600 MHz, CDCl3): δ 7.93 (d, J = 6, 2H, 2, 6-i-Ph-H), 7.83 (d, J = 6, 2H, 3, 5-i-Ph-H), 7.68 (d, J = 6, 1H, 6o-Ph-H), 7.63 (s, 1H, 2-o-Ph-H), 7.43 (t, J = 6, 1H, 5-o-Ph-H), 7.22 (d, J = 6, 1H, 4-o-Ph-H), 7.11 (s, 1H, COCHCO), 4.03 (t, J = 6, 2H, Ar-OCH 2 CH 2 CH 2 C 8 H 1 6 CH 3 ), 1.81 (p, J = 6, 2H, ArOCH2CH2CH2C8H16CH3), 1.47 (p, J = 6, 2H, ArO CH 2 C H 2 CH 2 C 8 H 1 6 CH 3 ), 1.2 8 (m, b road, 16H, A rOCH2CH2CH2C8H16CH3), 0.87 (t, J = 6, 3H, ArOCH2CH2CH2C8H16CH3). HRMS (ESI, TOF) m/z calculated for C27H35O3IF2B 583.1692 [M + H]+; found 583.1700. 1

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RESULTS AND DISCUSSION Synthesis. The boron β-diketonate dyes were prepared via the three-step synthesis previously reported for para-alkoxy derivatives.36,47 The alkoxy ketones were prepared via Williamson ether synthesis, with 3-hydroxyacetophenone for the meta-OC12H25 ketone and 4-hydroxyacetophenone for the para-OC12H25 ketone. The ketones were then combined with the appropriately substituted benzoate (X = H, F, Cl, Br, I) to produce diketones via Claisen condensation, followed by recrystallization of the crude product from hexanes. Isolation of the β-diketone from the ketone-C12 precursor (1-(3(dodecyloxy)phenyl)ethan-1-one) proved difficult given the similarities in solubility and polarity of the compounds; thus, crude β-diketone ligands were used to make complexes that could be separated from ligand impurities in the next step. The boron-coordinated compounds were prepared by adding boron trifluoride diethyl etherate to a stirred CH2Cl2 solution of the β-diketones. The boronation reactions were quenched with K2CO3(s), and the products were purified either by recrystallization from acetone/hexanes or by column chromatography (hexanes/ethyl acetate eluent). The boron complexes are more polar and less soluble than the ligands, thus simplifying the isolation of the dyes via recrystallization or column chromatography. C


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ACS Applied Materials & Interfaces Optical Properties in Solution. The dyes were analyzed in dilute CH2Cl2 solutions (1.0 × 10−5 M) in air at room temperature, and the optical properties are summarized in Table 1. The dyes absorbed light in the UV to violet range

Comparison of the solution properties of the para-OC12H25 and the meta-OC12H25 samples points to a change in the excited-state transition. The meta-OC12H25 samples have redshifted emission by about 60 nm compared to the paraOC12H25 analogues (i.e., pCl: λF = 443 nm; mCl: λF = 507 nm). While the para-OC12H25 derivatives have Stokes shifts of about 40 nm, the meta-OC12H25 derivatives have larger Stokes shifts of around 110 nm in CH2Cl2. The quantum yields also appear to be affected by the change in alkoxy-donor position. The para-OC12H25 dyes have quantum yields above 0.60, while the meta-OC12H25 dyes have quantum yields ranging from 0.20 to 0.30. The radiative lifetimes of dyes were also calculated (τrad = τF/ ΦF), showing much longer radiative lifetimes for the metasubstituted derivatives than the para-OC12H25 dyes (pCl: τrad = 1.2 ns; mCl: τrad = 21.8 ns). Longer radiative lifetimes for the meta-OC12H25 dyes point to charge-transfer excited-state transitions. This is also suggested by the DFT calculations of the highest-occupied and lowest-unoccupied molecular orbitals (HOMOs/LUMOs) for pCl and two rotamers of the mCl dye shown in Figure 2. Additional data are provided in Tables S1− S6 and Figures S1−S3. For the mX dyes, the electron density is localized on the alkoxy-substituted ring in the HOMO, whereas in the LUMO it is predominantly on the dioxaborine moiety and the halide substituent. The electron density localization and energies of the HOMO and LUMO are not greatly affected by the mCl rotamer conformation (Figure 2). One rotamer of mCl (Rotamer 1) has a 3-OC12H25 substituent, while the other rotamer (Rotamer 2) has a 5-OC12H25 substitutent, using clockwise numbering from the diketonate site to distinguish these species. The enhanced electron density localization on the alkoxysubstituted ring is similar for both rotamers, as shown by the calculated absorption wavelengths from the HOMO (Rotamer 2: λabs = 423 nm; Rotamer 1: λabs = 427 nm) (Tables S4 and

Table 1. Optical Properties in CH2Cl2 BF2bdk

λabsa (nm)

λFb (nm)

εc (M−1 cm−1)

τFd (ns)

ΦFe

τradf (ns)

pH pF pCl pBr pI mH mF mCl mBr mI

399 400 405 406 408 384 385 389 392 394

436 436 443 444 444 497 492 507 508 502

51600 56000 51000 60000 57000 28000 35000 29000 35000 31000

2.0 1.77g 1.22g 1.95g 1.50g 7.52 4.73 7.64 7.18 7.65

1.00 0.99g 0.99g 0.95g 0.67g 0.35 0.37 0.35 0.34 0.22

2.0 1.8 1.2 2.1 2.2 21.5 12.8 21.8 21.1 34.8

a

Absorbance maxima. bFluorescence maxima (λex = 369 nm). Extinction coefficient. dFluorescence lifetime (λex = 369 nm LED). e Fluorescence quantum yield. fRadiative lifetime: τrad = τF/ΦF. gValues taken from ref 22. c

(384−408 nm). As the halide substituent becomes heavier, the absorbance red-shifted for meta-OC12H25 (mX) and paraOC12H25 (pX) dye derivatives (e.g., pF: λabs = 400 nm versus pI: λabs = 408 nm; mF: λabs = 385 nm versus mI: λabs = 394 nm). The extinction coefficients of the para-OC12H25 derivatives were greater than those of the meta-OC12H25 dyes for all samples except pH and mH (e.g., pF: ε = 56000 M−1 cm−1 versus mF: ε = 35000 M−1 cm−1). The dyes emit blue (para-OC12H25) and blue-green (metaOC12H25) fluorescence in CH2Cl2 solution, and the halide substituents have only minor influences on the color (pF: λF = 436 nm, pI: λF = 444 nm; mF: λF = 492 nm, mI: λF = 502 nm).

Figure 2. Molecular orbital diagrams (HOMO and LUMO) of pCl and two rotamers of mCl (Rotamer 1, center; Rotamer 2, right). D


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Figure 3. Solvatochromism. (A) Chemical structures of pCl and mCl; (B) total emission spectra under air of pCl and mCl dyes in solution (λex = 369 nm); (C) images of pCl and mCl dyes in toluene, 1,4-dioxane, dichloromethane, acetone, and acetonitrile under UV light (from left to right); (D) Lippert−Mataga plots for the pCl and mCl dyes in the same solutions.

S6). Changing the alkoxy donor to the para-position produced a characteristic π−π* transition predicted by the MO diagrams, where the electron density of the HOMO is delocalized across the molecule. To further investigate the charge-transfer excited-state transition, the solvatochromism of the dyes was studied in solvents of varying polarity (Figures S4−S7). The data for the Cl-substituted dyes are shown in Figure 3. As the solvent polarity was increased (toluene < dioxanes < dichloromethane < acetone < acetonitrile), the emission of mCl red-shifted. The emission spectra showed a bathochromic shift from 465 nm in toluene to 562 nm in acetonitrile, and the emission intensity was greater in nonpolar solvents. The data were fit according to Lippert−Mataga theory in order to further evaluate the solvatochromic behavior.48,49 The plot of the Stokes shift (Δν) versus the solvent polarity parameter (Δf), described by the solvent orientation polarizability,1 shows a positive trend with a slope of 10.59, indicating moderate solvatochromism. The para-OC12H25 dyes, with less charge-transfer character, are less sensitive to solvent polarity, as indicated by the Lippert− Mataga slope of 4.51. Optical Properties of Films. The optical properties of the pX and mX derivatives in 2.5 wt % dye/PLA thin films are summarized in Table 2. The images and total emission spectra of mCl films in air and N2 are shown in Figures S8 and S9. The emission color did not vary greatly when the halide substituents were changed. However, the emission did shift when the

Table 2. Optical Properties in 2.5% Dye/PLA Thin Films BF2bdk

λFa (nm)

τFb (ns)

λRTPc (nm)

τRTPd (ms)

ΔλFPe (nm)

pH pF pCl pBr pI mH mF mCl mBr mI

432 438 431 466 438 459 457 465 458 472

3.59 4.52 1.98 3.20 1.19 5.49 5.22 5.27 1.97 1.99

520 517 511 525 522 532 534 531 526 536

194 76.3 178 80.6 5.03 340 135 188 36.6 11.0

88 79 80 59 84 73 77 66 68 64

a

Fluorescence maxima (λex = 369 nm). bFluorescence lifetime (λex = 369 nm LED). cDelayed emission maxima under N2 (λex = 369 nm, 2 ms delay time). dPhosphorescence lifetime under N2, monitored at the delayed emission maxima (λex = 369 nm). eWavelength gap between the fluorescence and the phosphorescence.

position of the alkoxy chain substituent was altered. For the meta-OC12H25-substituted compounds, with the exception of mBr, the fluorescence (F) and phosphorescence (RTP) redshifted by about 20 nm, consistent with the emission properties in CH2Cl2 (i.e., pCl: λRTP = 511 nm; mCl: λRTP = 531 nm). For ratiometric oxygen sensing, distinct emission peaks for fluorescence and room-temperature phosphorescence are desirable.23 All of the compounds show differences between the singlet and triplet emission colors (≥60 nm) sufficient for E


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Figure 4. Phosphorescence in Matrixes. Total emission spectra under air and N2 of 2.5% mI in (A) PS and (B) cholesterol thin films (λex = 369 nm).

Figure 5. Dye Loading Effects. Total emission spectra under air of (a) pCl/PLA and (b) mCl/PLA films with varying dye weight percents (0.1− 100%).

this application.26 However, mCl was selected as the optimal compound due to its useful combination of red-shifted emission, distinct F and RTP peaks, long F and RTP lifetimes, and solvatochromism. The heavy atom effect is evident in the decreasing fluorescence and phosphorescence lifetimes with increasing atomic weight of the substituent (e.g., mH: τRTP = 340 ms; mI: τRTP = 11.0 ms).42 Additionally, with the exception of mBr, the emission lifetimes are longer for the meta-OC12H25 compounds when compared to those of the para-substituted compounds with the same halide substituent. This trend is observed for both the F and RTP lifetimes (pCl: τF = 1.98 ns, τRTP = 178 ms; mCl: τF = 5.27 ns, τRTP = 188 ms). The RTP lifetimes are very long, including when compared to other halide-substituted BF2bdk dyes without alkoxy chain substituents (BF2dbm(Cl)PLA: τRTP = 114−168 ms; mCl: τRTP = 188 ms).42 For the pX compounds, the fluorescence−phosphorescence wavelength (i.e., energy) gaps are slightly larger than those for the mX dyes (pCl: ΔλFP = 80 nm; mCl: ΔλST = 66 nm). This is evidence of the way donor substituents can be used to change the fluorescence more substantially than the phosphorescence, because the singlet emission red-shifted more dramatically as a result of meta-substitution than the triplet emission. Because the mX derivatives exhibited solvatochromism and interesting optical properties in PLA films, they were studied in different matrixes to observe optical property trends in dye/ matrix thin films of varying composition. PLA, a semicrystalline matrix (Tg = 60 °C), was used as the standard for optical property comparison, given it is most extensively studied and reported by our group.16,50−52 Poly(styrene) (PS; Tg = 100 °C) was selected because it is an easily accessible, transparent, and photostable material that may solvate BF2dbm dyes due to πstacking between the aromatic dye and the aromatic polymer.53

Poly(methyl methacrylate) (PMMA; Tg = 120 °C) was chosen because it is an amorphous polymer, like PS, but it is less hydrophobic and is slightly less ordered due to its smaller ester side chains, providing a good comparison to PS. In previous reports, Vacha et al. used an estradiol steroid matrix to enable phosphorescence in organic compounds.54,55 Here, cholesterol was used as a monomeric amorphous matrix for comparison, given it is present in cell membranes and commonly used in drug delivery applications.56,57 Also, due to solvatochromism, the dye emission may be affected by polymers with different dielectric constants (PLA: εr = 3.549; PS: εr = 2.49−2.55; PMMA: εr = 3.49).58,59 The total emission spectra of 2.5% mH, mCl, and mI thin films made with PLA, PS, PMMA, or cholesterol were examined under air (Figures 4 and S10−12). Small differences in emission were observed across the dye PLA, PMMA, and PS matrixes under air (Figure S10). The total emission spectra of 2.5% mI/PS (polymeric, amorphous) and cholesterol (monomeric, amorphous) thin films under air and N2 are compared in Figure 4. The ratio of fluorescence to phosphorescence of the dye in PS was comparable to that of the dye in PLA. The fluorescence was blue-shifted in the PS matrix compared to that in PLA (mI/PS: λF = ∼450 nm; mI/PLA: λF = ∼470 nm), indicative of dye solvation in the solid-state matrix. The blueshifted emission may also result because of the lower dielectric constant of PS compared to PLA. The mI/PMMA film showed negligible changes in emission compared to the dye/PLA and dye/PS films (Figures 4 and S11). The cholesterol matrix enabled weak phosphorescence. Broad peaks were observed in dye/cholesterol films, possibly due to dye aggregation (Figure 4). The maximum emission wavelengths in air of pCl and mCl in PLA, PS, PMMA, and cholesterol matrixes compared to the dye F


Research Article

ACS Applied Materials & Interfaces loading (0.1−100% weight percent) are shown in Figure S12. As an example, the total emission spectra for the various dye loadings for the chloro-substituted derivatives in PLA are shown in Figure 5. The general trend was red-shifted emission with increasing dye loading, observed due to dye−dye interactions. This trend was apparent for both the para- and meta-alkoxy-substituted dyes; however, at higher dye loadings, the pCl showed deviations from this general trend (Figures 5a and S12). This is possibly due to crystal packing of the dye and phase separation at higher dye loadings.60,61 While previous studies have shown mechanochromic luminescence in the paraderivatives, preliminary studies of the meta-alkoxy-substituted dyes do not show mechanochromic luminescence.36,47 Therefore, crystallization might be hindered in the meta-derivatives at high dye loading, resulting in a controllable emission pattern in the mX derivatives (Figure 5b). Oxygen-Sensitive Nanoparticles. The mX dyes exhibit properties useful for in vivo oxygen sensing. Intramolecular charge transfer (ICT) red shifts fluorescence and phosphorescence, which is useful for minimizing background autofluorescence and increasing tissue penetration of light.62 The distinct emission colors of F and RTP (∼60−70 nm difference) and the longer fluorescence and phosphorescence lifetimes for the mX derivatives are ideal for ratiometric oxygen sensing. Measuring oxygen concentration in biological contexts benefits diagnosis. Healing and treatment response may also be monitored via oxygen sensing and imaging. Extreme concentrations of oxygen are associated with many health issues, such as cancer, cardiac ischemia, macular degeneration, chronic wounds, seizing brain tissue, and strokes.17 Nanoparticles are a convenient model for O2 calibration routinely performed in our lab and could potentially be used for biomedical applications.29 Oxygen-sensing dye PEG−PLA nanoparticles were prepared as previously described for dye− PLA conjugates.63 In this report, fabrication was achieved by codissolution of a PLA−PEG block copolymer (PLA= 10K, PEG = 2K; PDI = 1.05)64 and the lipid dye (1−6; 2.5 wt %) in dimethylformamide (DMF), followed by precipitation into distilled water.63 Formation of the nanoparticles was confirmed by dynamic light scattering (DLS; Figure S13 and Table S7). The average hydrodynamic radii (RH) for freshly prepared pI/ PLA−PEG and mI/PLA−PEG nanoparticles were 70.1 and 65.8 nm, respectively. When oxygen was removed from the aqueous nanoparticle environment, phosphorescence was observed. Because phosphorescence is only observed in a rigid matrix, such as PLA, not for dyes in solution, suspended in water, or in PEG, it is assumed that the lipid dyes are blended into the hydrophobic PLA core of the nanoparticles.65 These findings are in accord with analogous rhodamine alkyl chain derivatives in PLA.66 To test the oxygen sensitivity of the pI and mI samples, oxygen calibrations were performed by measuring the emission at various oxygen percentages (Figure 6). The mI derivative exhibited greater oxygen sensitivity than pI from 0−5% O2 (Figure S14−15). As is common for many BF2bdkPLA nanoparticles, the fluorescence to phosphorescence ratiometric calibrations did not show a strong linear correlation (R2 = 0.93; Figures S14 and S15) over the 0−21% O2 concentration range.24,26 This likely arises from a number of factors, such as sample heterogeneity and signal separation (e.g., fluorescence and phosphorescence).67,68 At 2.5% dye in PLA−PEG, there may be multiple O2-quenching species (e.g., monomers, aggregates, different environments), but the greater influence

Figure 6. Oxygen Calibrations. Total emission spectra of (a) pI/PLA− PEG and (b) mI/PLA−PEG nanoparticles at varying levels of oxygen concentration (0−100%). The arrows indicate decreasing phosphorescence intensity with increasing oxygen concentration.

on the ratiometric response is likely the fluorescence intensity at the phosphorescence wavelength where oxygen-sensitive emission is monitored. This can be addressed by isolating the fluorescence and phosphorescence signals through data processing. To obtain phosphorescence absent oxygen-invariant fluorescence, the total emission spectrum at 100% O2 (i.e., only fluorescence; phosphorescence quenched) is subtracted from the total emission spectra at various oxygen concentrations (Figure S16). Evaluation of the isolated phosphorescence intensity shows a Stern Volmer relationship with a linear correlation (R2 = 0.98) from 0 to 21% O2 (Figure S16). An advantage of the mI material is that the F and RTP correspond to the blue and green RGB channels of camera imaging systems, respectively, such that the blue channel serves as the reference and the green channel as the sensor.54,55 As previously shown by DeRosa et al., these features can be utilized in conjunction with a portable, cost-effective camera for tissue oxygen imaging.23 Data is collected on the RGB channels of the camera, and real-time ratiometric and lifetime oxygen measurements are possible for materials across a full range of oxygen sensitivity (0−100%).

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CONCLUSIONS It is invaluable in applied materials research to have a toolkit of molecular design strategies available for generating materials with properties tailored for a given application. This study contributes some noteworthy advances in this regard. In this report, the synthesis and characterization of 10 boron dyes with dodecylalkoxy groups and various halide substituents is described. Optical properties of these dyes were observed in CH2Cl2 solutions and PLA thin films. Calculations (i.e., DFT) comparing the chlorine derivatives suggest a π−π* transition to the excited state for the para-alkoxy-substituted dye and charge G


Research Article


(5) Wang, X.; Wolfbeis, O. S. Optical Methods for Sensing and Imaging Oxygen: Materials, Spectroscopies and Applications. Chem. Soc. Rev. 2014, 43, 3666−3761. (6) Roussakis, E.; Li, Z.; Nichols, A. J.; Evans, C. L. Oxygen-Sensing Methods in Biomedicine from the Macroscale to the Microscale. Angew. Chem., Int. Ed. 2015, 54, 8340−8362. (7) Klymchenko, A. S. Solvatochromic and Fluorogenic Dyes as Environment-Sensitive Probes: Design and Biological Applications. Acc. Chem. Res. 2017, 50, 366−375. (8) Nawn, G.; McDonald, R.; Hicks, R. G. Synthesis and Characterization of Heterobimetallic (Pd/B) Nindigo Complexes and Comparisons to Their Homobimetallic (Pd2, B2) Analogues. Inorg. Chem. 2013, 52, 10912−10919. (9) Barbon, S. M.; Price, J. T.; Reinkeluers, P. A.; Gilroy, J. B. Substituent-Dependent Optical and Electrochemical Properties of Triarylformazanate Boron Difluoride Complexes. Inorg. Chem. 2014, 53, 10585−10593. (10) Smith, L. F.; Blight, B. A.; Park, H. J.; Wang, S. Sensitizing Tb(III) and Eu(III) Emission with Triarylboron Functionalized 1,3Diketonato Ligands. Inorg. Chem. 2014, 53, 8036−8044. (11) Mukherjee, S.; Thilagar, P. Stimuli and Shape Responsive “Boron-Containing” Luminescent Organic Materials. J. Mater. Chem. C 2016, 4, 2647−2662. (12) Xiao, H.; Li, P.; Zhang, W.; Tang, B. An Ultrasensitive NearInfrared Ratiometric Fluorescent Probe for Imaging Mitochondrial Polarity in Live Cells and in Vivo. Chem. Sci. 2016, 7, 1588−1593. (13) Karpenko, I. A.; Niko, Y.; Yakubovskyi, V. P.; Gerasov, A. O.; Bonnet, D.; Kovtun, Y. P.; Klymchenko, A. S. Push−pull Dioxaborine as Fluorescent Molecular Rotor: Far-Red Fluorogenic Probe for Ligand−receptor Interactions. J. Mater. Chem. C 2016, 4, 3002−3009. (14) Reineke, S. Organic Light-Emitting Diodes: Phosphorescence Meets Its Match. Nat. Photonics 2014, 8, 269−270. (15) Mo, H.-W.; Tsuchiya, Y.; Geng, Y.; Sagawa, T.; Kikuchi, C.; Nakanotani, H.; Ito, F.; Adachi, C. Color Tuning of Avobenzone Boron Difluoride as an Emitter to Achieve Full-Color Emission. Adv. Funct. Mater. 2016, 26, 6703−6710. (16) Zhang, G.; Chen, J.; Payne, S. J.; Kooi, S. E.; Demas, J. N.; Fraser, C. L. Multi-Emissive Difluoroboron Dibenzoylmethane Polylactide Exhibiting Intense Fluorescence and Oxygen-Sensitive Room-Temperature Phosphorescence. J. Am. Chem. Soc. 2007, 129, 8942−8943. (17) Wolfbeis, O. S. Luminescent Sensing and Imaging of Oxygen: Fierce Competition to the Clark Electrode. BioEssays 2015, 37, 921− 928. (18) Hockel, M.; Vaupel, P. Tumor Hypoxia: Definitions and Current Clinical, Biologic, and Molecular Aspects. JNCI J. Natl. Cancer Inst. 2001, 93, 266−276. (19) Yudovsky, D.; Nouvong, A.; Pilon, L. Hyperspectral Imaging in Diabetic Foot Wound Care. J. Diabetes Sci. Technol. 2010, 4, 1099− 1113. (20) Schreml, S.; Szeimies, R. M.; Prantl, L.; Karrer, S.; Landthaler, M.; Babilas, P. Oxygen in Acute and Chronic Wound Healing. Br. J. Dermatol. 2010, 163, 257−268. (21) Schreml, S.; Meier, R. J.; Kirschbaum, M.; Kong, S. C.; Gehmert, S.; Felthaus, O.; Küchler, S.; Sharpe, J. R.; Wöltje, K.; Weiß, K. T.; Albert, M.; Seidl, U.; Schröder, J.; Morsczeck, C.; Prantl, L.; Duschl, C.; Pedersen, S. F.; Gosau, M.; Berneburg, M.; Wolfbeis, O. S.; Landthaler, M.; Babilas, P. Luminescent Dual Sensors Reveal Extracellular pH-Gradients and Hypoxia on Chronic Wounds That Disrupt Epidermal Repair. Theranostics 2014, 4, 721−735. (22) Zhang, G.; Kooi, S. E.; Demas, J. N.; Fraser, C. L. Emission Color Tuning with Polymer Molecular Weight for Difluoroboron Dibenzoylmethane-Polylactide. Adv. Mater. 2008, 20, 2099−2104. (23) DeRosa, C. A.; Seaman, S. A.; Mathew, A. S.; Gorick, C. M.; Fan, Z.; Demas, J. N.; Peirce, S. M.; Fraser, C. L. Oxygen Sensing Difluoroboron β-Diketonate Polylactide Materials with Tunable Dynamic Ranges for Wound Imaging. ACS Sens. 2016, 1, 1366−1373.

transfer to the excited state for the meta-alkoxy-substituted dye. The differences in excited-state transitions are demonstrated in solvatochromism experiments, where the meta-OC12H25 boron dye exhibits emission color changes in solvents of varying polarity, whereas the para-OC12H25 dye displays blue emission regardless of solvent and is largely insensitive to polarity. This set of compounds gives new insight into donor/acceptor substitution trends that may be used to fine-tune boron βdiketonates for oxygen sensing and other applications. Fluorescence (F) and oxygen-sensitive room-temperature phosphorescence (RTP) are observed in 2.5% dye/PLA thin films. Boron dye emission ranges from ∼430 to 470 nm for F and ∼510 to 530 nm for RTP. The meta-OC12H25 compounds show red-shifted F and RTP and longer emission lifetimes compared to para-dyes. The heavy atom effect is used to enhance ISC for oxygen-sensing applications. Oxygen calibrations using the meta-OC12H25 iodide dye demonstrate oxygen sensitivity ranging from 0−5% O2 or 0−21% with data processing. The distinct F and RTP peaks, lifetimes, and oxygen sensitivity of the meta-OC12H25 boron dye derivatives show promise for biological oxygen imaging agents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06910. Computational details, solvatochromism data, total emission of films in air and N2, matrix-dependent emission, dye loading-dependent emission, nanoparticle data, and oxygen calibrations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Cassandra L. Fraser: 0000-0002-8927-4694 Author Contributions ‡

M.L.D. and C.K. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Institutes of Health (R01 CA167250) for support of this work. We gratefully acknowledge the UVA Center for Undergraduate Excellence for Harrison Undergraduate Research Awards to M.L.D. and C.K. and the Beckman Scholars Program for a fellowship to C.K. We thank Dr. Tristan Butler and Dr. James N. Demas for helpful discussions.

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Meta-Alkoxy-Substituted Difluoroboron Dibenzoylmethane Complexes

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