Electrochemical, Spectroscopic, and 1O2 Sensitization Characteristics

Oct 9, 2017 - Electrochemical, Spectroscopic, and 1O2 Sensitization Characteristics of Synthetically Accessible Linear Tetrapyrrole ... Phone: 302-831...
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Cite This: Inorg. Chem. 2017, 56, 12703-12711

Electrochemical, Spectroscopic, and 1O2 Sensitization Characteristics of Synthetically Accessible Linear Tetrapyrrole Complexes of Palladium and Platinum Andrea M. Potocny, Allen J. Pistner, Glenn P. A. Yap, and Joel Rosenthal* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene (DMBil1) linear tetrapyrrole supporting PdII or PtII centers is presented. Both of these nonmacrocyclic tetrapyrrole platforms are robust and easily prepared via modular routes. X-ray diffraction experiments reveal that the Pd[DMBil1] and Pt[DMBil1] complexes adopt similar structures and incorporate a single PdII and PtII center, respectively. Additionally, electrochemical experiments revealed that both Pd[DMBil1] and Pt[DMBil1] can undergo two discrete oxidation and reduction processes. Spectroscopic experiments carried out for Pd[DMBil1] and Pt[DMBil1] provide further understanding of the electronic structure of these systems. Both complexes strongly absorb light in the UV−visible region, especially in the 350−600 nm range. Both Pd[DMBil1] and Pt[DMBil1] are luminescent under a nitrogen atmosphere. Upon photoexcitation of Pd[DMBil1], two emission bands are observed; fluorescence is detected from ∼500−700 nm and phosphorescence from ∼700−875 nm. Photoexcitation of Pt[DMBil1] leads only to phosphorescence, presumably due to enhanced intersystem crossing imparted by the heavier PtII center. Phosphorescence from both complexes is quenched under air due to energy transfer from the excited triplet state to ground state oxygen. Accordingly, irradiation with light of λ ≥ 500 nm prompts Pd[DMBil1] and Pt[DMBil1] to photosensitize the generation of 1O2 (singlet oxygen) with impressive quantum yields of 80% and 78%, respectively. The synthetic accessibility of these complexes coupled with their ability to efficiently photosensitize 1O2 may make them attractive platforms for development of new agents for photodynamic therapy.

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INTRODUCTION Macrocyclic tetrapyrrole architectures, which include porphyrins, corroles, and phthalocyanines, represent some of the most well-studied organic chromophores (Chart 1).1 Nonmacrocyclic linear tetrapyrroles such as biladienes and biliverdins have also been prepared and studied. Such platforms support a unique set of photophysical properties as compared to more traditional tetrapyrrole constructs.2,3 Some linear tetrapyrrole scaffolds can also serve as ligand platforms that can support main group elements and transition metals4 including copper,5−7 nickel,8,9 and cobalt,10,11 among others.12 In general, however, the coordination chemistry of nonmacrocyclic oligopyrrole constructs has been much more limited than that which is known for porphyrins, corroles, phthalocyanines, and other polypyrrole macrocycles. Studies of the coordination chemistry of many linear tetrapyrroles have been partially hampered by the inherent instability of such frameworks, which, unlike traditional macrocyclic tetrapyrroles, are nonaromatic. For instance, a,cbiladiene derivatives (Chart 1), in which two protons are connected to an sp3-hybridized meso-position bisecting the molecule, can rapidly decompose in the presence of air.13 Moreover, if the a,c-biladiene is not functionalized at the tetrapyrrole termini (i.e., the 1- and 19-positions), decom© 2017 American Chemical Society

position often proceeds via cyclization to yield the corresponding aromatic corrole macrocycle.14−17 Given that deprotonation and/or hydrogen atom abstraction from the 10-position of the a,c-biladiene framework is associated with the oxidative instability of this molecule,18,19 it may be expected that addition of alkyl groups at the 10position of the biladiene framework lends stability to this class of tetrapyrrole.20 Along these lines, we recently prepared a 10,10-dimethylbiladiene (DMBil) framework as a stable nonmacrocyclic tetrapyrrole. In addition to probing the basic light absorbing and redox properties of the DMBil platform, we also showed that this platform can ligate zinc(II) and copper(II) centers.5 In this present study, we have sought to further expand the coordination chemistry of the DMBil scaffold, with an eye toward development of metalated biladiene complexes that support triplet excited state chemistry for potential applications related to 1O2 (singlet oxygen) sensitization. Given that such excited state dynamics are predicated on intersystem crossing from the DMBil singlet excited state to the triplet excited state, we have worked to build on the work of other researchers who Received: April 10, 2017 Published: October 9, 2017 12703

DOI: 10.1021/acs.inorgchem.7b00796 Inorg. Chem. 2017, 56, 12703−12711

Article

Inorganic Chemistry Chart 1. Some Classes of Tetrapyrrole Scaffolds

eluent to deliver 106 mg of the title compound as a bright red solid (yield = 68%). 1H NMR (400 MHz, CDCl3, 25 °C) δ/ppm: 7.41 (s, 2H), 6.67 (d, J = 4.5 Hz, 2H), 6.57 (dd, J = 9.9, 4.4 Hz, 4H), 6.49 (dd, J = 4.3, 1.4 Hz, 2H), 1.81 (s, 6H). 13C NMR (101 MHz, CDCl3, 25 °C) δ/ppm: 166.87, 152.30, 144.96 (d, J = 251.5 Hz), 141.72 (d, J = 251.5 Hz), 137.47 (d, J = 257.6 Hz), 135.03, 134.40, 131.10, 128.94, 128.41, 118.41, 116.70, 112.34, 41.78, 31.09. 19F NMR (565 MHz, CDCl3, 25 °C) δ/ppm: −138.15 (dd, J = 21.9, 6.3 Hz, 4F), −152.07 (t, J = 20.9 Hz, 2F), −160.64 (td, J = 21.5, 6.2 Hz, 4F). HR-ESI-MS: [M + H]+ m/z calcd for C33H17N4F10Pd, 765.0328; found, 765.0345. Anal. (C33H16N4) Calcd: C, 51.81; H, 2.10; N, 7.33. Found: C, 51.68; H, 1.72; N, 6.98. Platinum 10,10-Dimethyl-5,15-bis(pentafluorophenyl)biladiene (Pt[DMBil1]). This synthesis was accomplished by modifying a published procedure used for metalation of a porphyrin triad.32 The tetrapyrrole ligand DMBil1 (122 mg, 0.185 mmol) was dissolved in 29.6 mL of benzonitrile, and the resulting solution was sparged with N2 for 10 min. To the flask was added 1.2 equiv of PtCl2(PhCN)2 (105 mg, 0.222 mmol), and the reaction vessel was then heated to 130 °C under N2 for 48 h. After cooling the reaction to room temperature, the solvent was removed under reduced pressure, and the residue was redissolved in CH2Cl2 and filtered through Celite to remove any insoluble materials. The desired product was purified via column chromatography on silica using hexanes and CH2Cl2 (3:1) as the eluent to deliver 67 mg of the title compound as a dark red solid (yield = 42%). 1H NMR (400 MHz, CDCl3, 25 °C) δ/ppm: 7.51− 7.45 (m, 2H), 6.74 (d, J = 4.6 Hz, 2H), 6.65 (d, J = 4.7 Hz, 4H), 6.55 (dd, J = 4.4, 1.7 Hz, 2H), 1.81 (s, 6H). 13C NMR (101 MHz, CDCl3, 25 °C) δ/ppm: 166.14, 152.19, 144.99 (d, J = 255.5 Hz), 141.74 (d, J = 257.6 Hz), 137.48 (d, J = 257.6 Hz), 133.74, 132.79, 130.63, 128.82, 128.27, 117.94, 116.24, 112.15, 41.78, 31.28. 19F NMR (565 MHz, CDCl3, 25 °C) δ/ppm: −138.03 (dd, J = 21.9, 5.9 Hz, 4F), −151.94 (t, J = 20.8 Hz, 2F), −160.56 (dt, J = 20.7, 10.0 Hz, 4F). HR-LIFDIMS: [M+] m/z calcd for C33H16N4F10Pt, 853.0863; found, 853.0892. Anal. Calcd for C33H16N4F10Pt: C, 46.43; H, 1.89; N, 6.57. Found: C, 46.39; H, 1.65; N, 6.25. X-ray Structural Solution and Refinement. Crystals of Pd[DMBil1] were obtained via slow evaporation of a concentrated solution of the PdII complex in dichloromethane/pentane; crystals of Pt[DMBil1] were obtained via slow evaporation of a concentrated solution of the PtII complex in dichloromethane/hexane containing a drop of pyridine. Crystals were mounted onto a plastic mesh using viscous oil, and cooled to the data collection temperature. Data were collected on a Bruker-AXS APEX II DUO CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å) monochromated with graphite for Pt[DMBil1] and Cu Kα radiation (λ = 1.54178 Å) focused with Goebel mirrors for Pd[DMBil1]. Unit cell parameters were obtained from 36 data frames, 0.5° ω, from three different sections of the Ewald sphere. The systematic absences in the diffraction data for both complexes are uniquely consistent with P21/c. The data sets were treated with multiscan absorption corrections (Apex3 software suite, Madison, WI, 2005). The structures were solved using direct methods and refined with full-matrix, least-squares procedures on F2.33 Two symmetry unique compound molecules were located in the asymmetric unit. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were treated as idealized contributions with geometrically calculated positions and with Uiso equal to 1.2, or 1.5 for methyl, Ueq of the attached atom. Atomic scattering factors are contained in various versions of the

have exploited the heavy atom effect to facilitate intersystem crossing to the triplet state.21−24 In so doing, we sought to incorporate heavier metal centers into the DMBil1 ligand scaffold and have synthesized dimethylbiladiene complexes of palladium(II) (Pd[DMBil1]) and platinum(II) (Pt[DMBil1]), which are the first dimethylbiladiene complexes supporting second and third row transition metals, respectively. In addition to detailing the synthesis and molecular structure of these two new DMBil1 complexes, we also highlight their electrochemical and photophysical properties, which are significantly different from those of the free base DMBil1, as well as the Zn[DMBil1] and Cu[DMBil1] complexes. In particular, we show that Pd[DMBil1] and Pt[DMBil1] show long-wavelength phosphorescence, which is sensitive to the presence of air, and can sensitize the formation of 1O2 with impressive quantum yields, which is in contrast to such systems that do not support heavy metal centers.

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EXPERIMENTAL SECTION

General Materials and Methods. Reactions requiring an inert atmosphere utilized flasks fitted with Suba-Seal rubber septa and utilized standard Schlenk techniques. Air and moisture sensitive reagents were transferred using standard syringe or cannula techniques. All glassware was dried at 140 °C for at least 3 h. Reagents and solvents were purchased from Sigma-Aldrich, Acros, Fisher, Strem, Alfa Aesar, VWR, Matrix Scientific, Decon Laboratories, Inc., or Cambridge Isotopes Laboratories. The DMBil1 ligand was prepared using published methods.25−29 Solvents for synthesis were of reagent grade or better. Anhydrous solvents were dried by passage through activated alumina and then stored over 4 Å molecular sieves prior to use. Column chromatography was performed with 40−63 μm silica gel from Silicycle. Compound Characterization. 1H NMR and 13C NMR spectra were recorded at 25 °C on a Bruker 400 MHz spectrometer with a cryogenic QNP probe. Proton spectra are referenced to the residual proton resonance of the deuterated solvent (CDCl3 = δ 7.26), and carbon spectra are referenced to the carbon resonances of the solvent (CDCl3 = δ 77.16).30 19F spectra were recorded at 25 °C on a Bruker 600 MHz spectrometer with a 5 mm Bruker SMART probe. Fluorine spectra are referenced to an external trifluoroacetic acid standard (TFA = δ −76.55 in CD3CN).31 All chemical shifts are reported using the standard δ notation in parts-per-million; positive chemical shifts are to higher frequency from the given reference. Low-resolution MS data were obtained using an LCQ Advantage LC/MS system with an ion trap mass analyzer from Thermofinnigan. High-resolution mass spectrometry analyses were performed by the Mass Spectrometry Laboratory in the Department of Chemistry and Biochemistry. Palladium 10,10-Dimethyl-5,15-bis(pentafluorophenyl)biladiene (Pd[DMBil1]). The tetrapyrrole ligand DMBil1 (134 mg, 0.203 mmol) and 1.2 equiv of Pd(OAc)2 (55 mg, 0.245 mmol) were dissolved in 50 mL of acetonitrile. The reaction headspace was evacuated, and the reaction was stirred at 60 °C for 4 h under static vacuum. After cooling the reaction to room temperature, the solvent was removed under reduced pressure, and the resulting residue was redissolved in CH2Cl2 and filtered through Celite to remove any insoluble materials. The desired product was purified via column chromatography on silica using hexanes and CH2Cl2 (3:1) as the 12704

DOI: 10.1021/acs.inorgchem.7b00796 Inorg. Chem. 2017, 56, 12703−12711

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Inorganic Chemistry Scheme 1. Synthetic Routes Employed for the Synthesis of Pd[DMBil1] and Pt[DMBil1]

diphenylisobenzofuran (DPBF).37,38 Measurements were carried out on an automated Photon Technology International (PTI) QuantaMaster 40 fluorometer equipped with a 75 W xenon arc lamp, an LPS220B lamp power supply, and a Hammamatsu R2658 photomultiplier tube using quartz cuvettes (6Q) with 1.0 cm path length. Each cuvette contained 2.0 mL of methanol solution that was 10.0 μM in Pd[DMBil1], Pt[DMBil1], or [Ru(bpy)3][PF6]2 (used as a reference, Φ = 0.81),39 and 1.0 μM in DPBF. A fourth cuvette containing only methanol and 1.0 μM DPBF was used as a control. Consumption of DPBF was monitored by observing the change in its integrated emission intensity following irradiation with light from an Intralux 9000 light source (Volpi) fitted with a 10 nm (fwhm) bandpass filter centered at 500 nm (Thor Laboratories, FB500-10). The cuvettes were irradiated for 10 s intervals for a total of 40 s. DPBF emission spectra were obtained by exciting at λex = 405 nm and scanning from λem = 400 to 600 nm using a step size of 1 nm and an integration time of 0.25 s. Calibration curves of the integrated emission intensity versus the concentration of unreacted DPBF remaining in solution were generated to correct for absorption of the photosensitizers between 400 and 600 nm. Emission spectra were collected from 10 μM solutions of Pd[DMBil1], Pt[DMBil1], and [Ru(bpy) 3][PF6]2 containing DPBF concentrations of 0, 0.25, 0.50, 0.75, 1.00, 1.25, or 1.50 μM. Linear regression lines were fit to the calibration data from each solution, and the integrated emission intensity values obtained from the 1O2 experiments were then used for linear regression analyses, enabling the corresponding concentrations of unreacted DPBF to be obtained. A final plot of the concentration of unreacted DPBF versus irradiation time formed a straight line with slope m, which was used with the following equation to calculate the 1O2 quantum yields

SHELXTL program library. For the case of Pt[DMBil1], a severely disordered half molecule of hexanes in the asymmetric unit was treated as diffused contributions (Squeeze, Platon).34 Structural information has been deposited with the Cambridge Crystallographic Database under depositary numbers CCDC 966865 and 966866. Electrochemical Measurements. All electrochemistry was performed in a nitrogen-filled glovebox using a CHI-620D potentiostat/galvanostat and a standard three-electrode configuration consisting of a platinum working disc electrode (2.0 mm diameter), a platinum wire auxiliary electrode, and a silver wire quasireference electrode. Measurements were collected in quiescent dichloromethane solutions containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the electrolyte. Pd[DMBil1] and Pt[DMBil1] were analyzed at concentrations of 1.0 mM using a scan rate of 50 mV/s and a sensitivity of 10 μA/V. All potentials are referenced relative to Ag/AgCl using a decamethylferrocenium/decamethylferrocene internal standard of 1 mV versus Ag/AgCl.35 UV−Vis Absorption Experiments. All UV−vis absorbance spectra were collected at room temperature on a StellarNet CCD array UV−vis spectrometer using quartz cuvettes (6Q) with a 1.0 cm path length from Starna Cells, Inc. Absorption spectra were collected in methanol containing Pd[DMBil1] or Pt[DMBil1] at concentrations of 5.0, 10.0, 15.0, 20.0, and 25.0 μM. Emission Experiments. Emission spectra were recorded on an automated Photon Technology International (PTI) QuantaMaster 40 fluorometer equipped with a 75 W xenon arc lamp, an LPS-220B lamp power supply, and a Hamamatsu R2658 photomultiplier tube. All samples were prepared in screw cap quartz cuvettes of 1.0 cm path length from Starna Cells, Inc. Solutions of Pd[DMBil1] and Pt[DMBil1] in methanol (25 μM) were prepared in a nitrogen-filled glovebox. The samples were excited at λex = 500 nm, and emission was monitored from λem = 515 to 1000 nm using a step size of 1 nm and an integration time of 0.25 s. Emission spectra were also measured following exposure of each of the samples to air. Reported spectra are the average of five individual acquisitions. Emission quantum yields were calculated using a solution of [Ru(bpy)3][PF6]2 in nitrogen-saturated acetonitrile (Φref = 0.094)36 as the reference and the expression below

⎛ m ⎞⎛ ε ⎞ Φs = Φref ⎜ s ⎟⎜ ref ⎟ ⎝ mref ⎠⎝ εs ⎠ Here, Φs and Φref are the 1O2 sensitization quantum yields for the sample and [Ru(bpy)3][PF6]2 reference, respectively; ms and mref are the slopes of the concentration of DPBF versus irradiation time plots for the sample and reference; and εs and εref are the extinction coefficients at the wavelength of irradiation (500 nm) for the sample and reference, respectively. Reported 1O2 quantum yields were obtained from an average of three trials.

2 ⎛ I ⎞⎛ A ⎞⎛ η ⎞ Φs = Φref ⎜ s ⎟⎜ ref ⎟⎜⎜ s ⎟⎟ ⎝ Iref ⎠⎝ A s ⎠⎝ ηref ⎠

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where Φs and Φref are the emission quantum yield of the sample and the reference, respectively, Is and Iref are the integrated emission intensities of the sample and reference, respectively, As and Aref are the measured absorbances of the sample and reference at the excitation wavelength, and ηs and ηref are the refractive indices of the solvents used for the sample and reference, respectively. The concentration of the [Ru(bpy)3][PF6]2 reference solution used was selected such that its absorbance at 500 nm approximately matched the absorbance of the 25 μM Pd[DMBil1] and Pt[DMBil1] sample solutions at 500 nm. Singlet Oxygen Experiments. Generation of 1O2 was quantified by monitoring fluorescence from the 1O2 trapping agent, 1,3-

RESULTS The DMBil1 ligand was readily prepared from commercially available starting materials via a simple two-step synthetic pathway (Scheme 1),25,27,40,41 starting from 5,5′-dimethyldipyrrolemethane, which can be easily prepared on multigram scales42 or purchased from a number of chemical suppliers. Upon deprotonation, the DMBil1 ligand can serve as a dianionic tetrapyrrole ligand. Shown in Scheme 1 are the routes employed for metalation of DMBil1 with PdII or PtII precursors 12705

DOI: 10.1021/acs.inorgchem.7b00796 Inorg. Chem. 2017, 56, 12703−12711

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Inorganic Chemistry

Figure 1. Thermal ellipsoid plots of (a) Pd[DMBil1] and (b) Pt[DMBil1] as viewed from the top, and in profile. Ellipsoids are drawn at 50% probability. For structures shown in profile, all hydrogen atoms and disordered solvent molecules have been omitted for clarity.

to generate Pd[DMBil1] and Pt[DMBil1], respectively. In brief, reaction of the free base DMBil1 ligand with Pd(OAc)2 in acetonitrile at 60 °C for 4 h allowed for isolation of the PdII complexed tetrapyrrole in 68% yield. The Pt[DMBil1] complex was obtained in 42% yield via reaction of PtCl2(PhCN)2 with DMBil1 in benzonitrile at 120 °C for approximately 2 days under a nitrogen atmosphere. The composition and structures of the PdII and PtII complexes shown in Scheme 1 were verified by 1H, 13C, and 19 F NMR; high-resolution mass spectrometry; elemental analysis; and single crystal X-ray diffraction. Both complexes crystallized with two chemically identical molecules per unit cell. Figure 1 shows both top and profile views of the molecular structures obtained for Pd[DMBil1] and Pt[DMBil1]. The corresponding crystallographic parameters are summarized in Table 1, and fully labeled thermal ellipsoid plots are available in the Supporting Information (Figures S1 and S2). The thermal ellipsoid plots shown in Figure 1 reveal that although the PdII and PtII metal cations coordinate to the nitrogen atoms of the four pyrrole units, the complexes do not rigorously adopt the square planar geometry commonly encountered for tetrapyrrole complexes of d8 metals. This behavior is attributed to steric interactions between H(1) and H(33) of the DMBil1 scaffold, which preclude the terminal pyrrole units from occupying a common plane. A similar deviation from planarity has also been observed for another nonmacrocyclic tetrapyrrole complex of palladium.43 Additionally, as has been seen with palladium tetraphenylporphyrin44 and platinum tetrakis(perfluorophenyl)porphyrin,45 the meso pentafluorophenyl groups of both Pd[DMBil1] and Pt[DMBil1] are nearly perpendicular relative to the dipyrromethene groups to which they are joined. Table 2 summarizes the relevant dihedral angles, which differ slightly for each of the two metalated dimethylbiladiene molecules present in the unit cells of the Pd[DMBil1] and Pt[DMBil1] crystals. The Pd−N bond lengths for Pd[DMBil1] were found to range between 2.000 and 2.043 Å, in close agreement with those observed for another linear tetrapyrrole palladium(II) complex43 and for palladium(II) tetraphenylporphyrin.44 The Pt−N bond lengths of Pt[DMBil1] show a range 2.005−2.033 Å, which is similar to the average value reported for platinum(II) tetrakis(pentafluorophenyl)porphyrin.45

Table 1. Crystallographic Data for Pd[DMBil1] and Pt[DMBil1] empirical formula fw cryst syst space group a b c α β γ V Z temp Dcalcd 2θ range μ reflns unique R(int) R1 wR2

Pd[DMBil1]

Pt[DMBil1]

C33H16F10N4Pd 764.90 monoclinic P21/c 29.6552(10) Å 18.6692(6) Å 11.1727(4) Å 90° 100.821(2)° 90° 6075.7(4) Å3 8 200(2) K 1.672 g/cm−3 3.034-75.256° 5.774 mm−1 (Cu Kα) 74768 12437 0.0964 0.0534 0.1424

C36H23F10N4Pt 896.67 monoclinic P21/c 29.458(4) Å 18.655(2) Å 11.2461(14) Å 90° 100.623(2)° 90° 6074.4(13) Å3 8 200(2) K 1.961 g/cm−3 1.781−27.586° 4.718 mm−1 (Mo Kα) 79012 14006 0.0472 0.0260 0.0558

Table 2. Relevant Dihedral Angles Measured for Pd[DMBil1] and Pt[DMBil1] Pd[DMBil1] dihedral angles between interior pyrrolesa terminal pyrrolesb dipyrromethene units and meso C6F5 groups

Pt[DMBil1]

molecule 1 molecule 2 molecule 1 molecule 2 44.73° 52.74° 83.02° 83.80°

40.78° 56.73° 72.31° 67.04°

43.83° 52.33° 82.69° 83.71°

40.59° 57.18° 71.18° 66.52°

a

The DMBil1 interior pyrroles are the pyrroles connected to the sp3hybridized meso carbon. bThe DMBil1 terminal pyrroles are the pyrroles on the open end of the tetrapyrrole scaffold.

In an effort to probe the electronic properties of the palladium(II) and platinum(II) DMBil1 complexes, cyclic voltammetry (CV) and differential pulsed voltammetry 12706

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Inorganic Chemistry

contrast, both of the reductions observed by CV are largely reversible when the tetrapyrrole ligand is complexed to a PdII or PtII center. On the basis of the DPVs shown in Figure S3 and the CVs shown in Figure 2, redox waves were observed at Ered(1) = −0.96 V, Ered(2) = −1.25 V, and Ered(1) = −0.93 V, Ered(2) = −1.24 V for Pd[DMBil1] and Pt[DMBil1], respectively. The reversibility of the two oxidations observed for the PdII and PtII dimethylbiladiene complexes is also improved relative to the free base DMBil1 ligand. On the basis of the DPVs shown in Figure S3 and the CVs shown in Figure 2, redox events associated with oxidation of Pd[DMBil1] take place at Eox(1) = 1.15 V and Eox(2) = 1.49 V; oxidation waves for Pt[DMBil1] are observed at similar potentials of Eox(1) = 1.16 V and Eox(2) = 1.52 V. It is possible that the metal center stabilizes the ligand by precluding the splayed-open conformation available to free base DMBil1 and forces it to approach a pseudo-square-planar conformation, which may enhance charge delocalization by facilitating more efficient πconjugation. In addition to improving the reversibility of the dimethylbiladiene redox behavior, metalation of the tetrapyrrole increases the potential separation between the two oxidation waves as well as the potential separation between the two reduction waves. Consequently, the four redox couples observed for Pd[DMBil1] and Pt[DMBil1] are more discrete relative to those of the free base DMBil1 ligand. Complexation of the dimethylbiladiene tetrapyrrole to PdII or PtII shifts the first oxidation to less positive potentials compared to that of free base DMBil1 (Table 3) by ∼180−200 mV. By contrast, Pd[DMBil1] and Pt[DMBil1] only showed a small anodic shift of the second oxidation wave (∼30−50 mV) relative to that of DMBil1. The first reduction wave of Pd[DMBil1] and Pt[DMBil1] occurs at less negative potentials compared to that for the free base ligand by ∼200−250 mV, while the potential of the second reduction does not change. This behavior deviates slightly from that observed for Zn[DMBil1], which was found to display a much smaller shift toward less negative potentials for the first reduction and a very small cathodic shift in the second reduction. The UV−vis absorption spectra of Pd[DMBil1] and Pt[DMBil1] were recorded in MeOH and are shown in Figure 3. Much like previously studied DMBil1 and Zn[DMBil1], both the PdII and PtII dimethylbiladiene complexes strongly absorb light between 350 and 550 nm (Figure 3). As is the case for Zn[DMBil1], the introduction of PdII and PtII centers into the DMBil1 ligand gives rise to additional absorption features at longer wavelengths that are not present for the free base dimethylbiladiene ligand (Table 3 and Figure S4). The maximum extinction coefficients observed for Pd[DMBil1] and Pt[DMBil1] are slightly smaller than those of the free base and zinc analogues (Table 3) but are still quite strong (εmax ∼ 30000 M−1 cm−1). Pd[DMBil1] possesses three prominent

(DPV) experiments were carried out in dichloromethane containing 0.1 M TBAPF6 as supporting electrolyte. Figure 2

Figure 2. Cyclic voltammograms recorded for DMBil1, Zn[DMBil1], Pd[DMBil1], and Pt[DMBil1] in CH2Cl2 containing 0.1 M TBAPF6 and an internal decamethylferrocene standard. CVs were recorded under N2 using a platinum disc as the working electrode at a scan rate of 50 mV/s.

compares the cyclic voltammograms obtained for 1.0 mM solutions of Pd[DMBil1] and Pt[DMBil1] with those obtained previously for Zn[DMBil1] and the free base DMBil1.25 DPVs recorded for Pd[DMBil1] and Pt[DMBil1] are reproduced in the Supporting Information (Figure S3), and the apparent redox potentials for each of these four compounds are listed in Table 3. All four biladiene derivatives show two 1e− reductions and two 1e− oxidations which are reported versus Ag/AgCl. Given that the ZnII center of Zn[DMBil1] is inherently redox inactive, it can be concluded that the two redox waves observed for this complex at Ered(1) = −1.12 V and Ered(2) = −1.33 V are ligand-centered reductions. Likewise, the two irreversible oxidative redox features observed for Zn[DMBil1] at Eox(1) = 1.12 V and Eox(2) = 1.39 V are ligand-based oxidations. The similarities among the redox features observed for Pd[DMBil1], Pt[DMBil1], and Zn[DMBil1], along with the fact that the free base ligand (DMBil1) can itself undergo two oxidation and two reduction processes, suggest that the redox waves observed for the PdII and PtII dimethylbiladiene complexes are most likely ligand-based as well. This multielectron redox chemistry is characteristic of porphyrinoids bearing germinal dimethyl substituents at an sp3-hybridized meso position.46,47 Incorporation of a metal center into the dimethylbiladiene improves the reversibility of the tetrapyrrole architecture’s redox features. Each of the four redox waves observed for free base DMBil1 are irreversible, as judged by CV (Figure 2). By

Table 3. Electrochemical and Photophysical Data Recorded for DMBil1, Zn[DMBil1], Pd[DMBil1], and Pt[DMBil1]a

a

Eox(1,2)/V

Ered(1,2)/V

DMBil1 Zn[DMBil1]

1.37, 1.46 1.12, 1.39

−1.18, −1.25 −1.12, −1.33

Pd[DMBil1] Pt[DMBil1]

1.15, 1.49 1.16, 1.52

−0.96, −1.25 −0.93, −1.24

λabs/nm (ε × 103 M−1 cm−1) 415 (26.2), 454 (31.4), 541 (1.8) 364 (9.3), 378 (9.0), 451 (40.4), 466 (37.5), 509 (9.1), 542 (12.5) 401 (17.4), 483 (31.9), 540 (7.5) 426 (29.8), 484 (20.3), 537 (12.0)

λfl/nm (Φfl)

λph/nm (Φph)

526 (4.3 × 10−3) 548 (3.9 × 10−4) 557 (1.3 × 10−4)

ΦΔ 1.5% 2.6%

753 (1.3 × 10−4) 656, 755 (6.3 × 10−4)

80% 78%

All redox potentials reported versus Ag/AgCl in CH2Cl2. All spectroscopic data recorded in MeOH. 12707

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Inorganic Chemistry

spectral positions, the long-wavelength emission bands observed for Pd[DMBil1] and Pt[DMBil1] are consistent with phosphorescence. The higher-energy luminescence (λem = 557 nm) observed for Pd[DMBil1] is presumed to be due to singlet emission (i.e., fluorescence). These spectral assignments are also consistent with the changes in emission intensity observed for Pd[DMBil1] and Pt[DMBil1] in the presence of air (vide inf ra). When the luminescence experiments described above were repeated under air, the emission spectra obtained for both Pd[DMBil1] and Pt[DMBil1] were notably different from those recorded under an atmosphere of N2. More specifically, as is shown in Figure S5, the long-wavelength emission features (∼700−875 nm) were essentially absent upon excitation (λex = 500 nm) of Pd[DMBil1] and Pt[DMBil1]. The emission band detected between ∼500 and 700 nm for Pd[DMBil1], however, remained unchanged in the presence of air (Figure S5). Both the spectral position and sensitivity to O2 suggested that the long-wavelength emission features observed for Pd[DMBil1] and Pt[DMBil1] are likely phosphorescence from the triplet excited states of these complexes, while the emission given off in the visible region for Pd[DMBil1] is fluorescence derived from a singlet excited state of this complex. As summarized in Table 3, the fluorescence quantum yield decreases across the sequence DMBil1 > Zn[DMBil1] > Pd[DMBil1], with no measurable fluorescence being detected for Pt[DMBil1]. No measurable phosphorescence was detected from DMBil1 or Zn[DMBil1];25 however, both the PdII and PtII dimethylbiladiene complexes showed significant triplet emission, as Pd[DMBil1] gave a phosphorescence quantum yield of Φph = 1.3 × 10−4, while that for Pt[DMBil1] was measured to be roughly 5 times larger (Φph = 6.3 × 10−4). The significant rise in Φph values for the metalated dimethylbiladiene complexes of ZnII ≪ PdII < PtII likely reflects the increased heavy atom effect imparted by the metal centers as one moves from left to right across this series. DMBil1 and Zn[DMBil1] can sensitize 1O2 with quantum yields of ΦΔ < 5%;25 however, the evidence of a heavy atom effect imparted by the presence of the PdII and PtII centers of Pd[DMBil1] and Pt[DMBil1] (vide supra) suggested that these two complexes may be more efficient photosensitizers for 1 O2 production. These hopes were buoyed by the observation that phosphorescence of Pd[DMBil1] and Pt[DMBil1] is actively quenched in the presence of oxygen. The extent to which this quenching is coupled to the production of 1O2 was quantified using 1,3-diphenylisobenzofuran (DPBF)37,38 as a 1 O2 trapping agent, in methanol using [Ru(bpy)3]2+ as an actinometer (ΦΔ = 0.81 in CH3OH).39 The quantum yields of 1 O2 sensitization by Pd[DMBil1] and Pt[DMBil1] upon irradiation with light of λex = 500 nm were measured to be ΦΔ = 80% and 78%, respectively. These impressive values are significantly higher than those obtained for DMBil1 (ΦΔ = 1.5%) or Zn[DMBil1] (ΦΔ = 2.6%), as shown in Table 2. We note that since the absorption spectra of PdII and PtII dimethylbiladiene complexes display spectral features that extend out to ∼600 nm, both of these photosensitizers provide a means to sensitize production of 1O2 using wavelengths of light commonly used for photodynamic therapy. Moreover, Pd[DMBil1] and Pt[DMBil1] display 1O2 quantum yields higher than or comparable to those of most photosensitizers currently used in photodynamic therapy,50,51 including Photo-

Figure 3. Electronic absorption (solid line) and normalized emission (dashed line) spectra of (a) Pd[DMBil1] and (b) Pt[DMBil1] recorded in MeOH at 298 K under a nitrogen atmosphere.

absorption features with an absorption maximum at 483 nm (ε = 31900 M−1 cm−1). This absorption band is flanked by a more modest transition at 401 nm (ε = 17400 M−1 cm−1) and a smaller shoulder around 540 nm (ε = 7500 M−1 cm−1). Pt[DMBil1] has a maximum extinction coefficient of 29800 M−1 cm−1 at 426 nm accompanied by a smaller local maximum at 484 nm (ε = 20300 M−1 cm−1) and a shoulder at 537 nm (ε = 12000 M−1 cm−1). For comparison, the electronic spectra of the Pd II and Pt II metal complexes of meso-tetrakis(perfluorophenyl)porphyrin each also show three major absorbance features between 360 and 560 nm.45,48 The maximum extinction coefficients associated with the most prominent absorption features of these two porphyrin compounds are significantly larger than those of Pd[DMBil1] and Pt[DMBil1], and the three principal absorption bands are significantly less broad due to Gouterman’s four orbital model of porphyrin electronic structure.49 The severe reduction in symmetry in Pd[DMBil1] and Pt[DMBil1], as compared to that of an aromatic porphyrin, is likely responsible for the broader overlapping features observed for the dimethylbiladiene complexes.45,48 Emission studies of Pd[DMBil1] and Pt[DMBil1] revealed that, like the free base and Zn[DMBil1] analogues, both the Pd II and Pt II dimethylbiladiene complexes are weakly luminescent. Following excitation at λex = 500 nm, a solution of Pd[DMBil1] in nitrogen-saturated methanol produces a broad spectrum with emission maxima at 557 and 753 nm. The emission spectrum recorded for Pt[DMBil1] was notably different from that for the PdII homologue; as opposed to luminescing across both the visible and near-IR regions (∼500−900 nm), excitation of deaerated solutions of Pt[DMBil1] at λex = 500 nm produced a single salient emission band spanning the region from λem = 700 to 875 nm, with a maximum at 755 nm (Figure 3). On the basis of their 12708

DOI: 10.1021/acs.inorgchem.7b00796 Inorg. Chem. 2017, 56, 12703−12711

Article

Inorganic Chemistry frin (89%),52 Foscan (43%),53 and Verteporfin (84%),50 suggesting that DMBil1 complexes may be useful platforms for construction of new PDT (photodynamic therapy) agents.

ing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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CONCLUSIONS AND FUTURE DIRECTIONS Photodynamic therapy holds great promise as a treatment for numerous types of cancer. The widespread implementation of this treatment type relies, in part, on the development of new agents that can be prepared via modular and scalable synthetic routes, are capable of absorbing light in the preferred therapeutic window (550−800 nm), and can generate 1O2 with high quantum yields. Previously, we introduced DMBil1, a synthetically accessible linear tetrapyrrole, which sensitizes 1O2 in very low quantum yields and absorbs light at wavelengths longer than 500 nm when complexed to Zn2+. In an effort to build upon this earlier work and obtain analogous complexes capable of more efficient 1O2 production, we have prepared the homologous PdII and PtII dimethylbiladiene complexes (Pd[DMBil1] and Pt[DMBil1]). As expected for tetrapyrroles containing sp3-hybridized meso-positions, these complexes exhibit a rich redox chemistry composed of two 1e− oxidations and two 1e− reductions. The electronic spectra of Pd[DMBil1] and Pt[DMBil1] show appreciable light absorption between 350 and 600 nm. Moreover, the influence of the heavy metal centers of these complexes is evident from studies of their emission properties and their ability to sensitize 1 O 2 production. In contrast to the free base DMBil1 and Zn[DMBil1] homologues, which only exhibit weak fluorescence, Pd[DMBil1] and Pt[DMBil1] show long-wavelength emission in the absence of oxygen. The spectral position of these emission bands together with their sensitivity to O2 is consistent with triplet emission. This phosphorescence, which is likely induced by enhanced intersystem crossing to the triplet excited state by the PdII and PtII centers, is efficiently quenched in the presence of air by energy transfer to ground state oxygen. As such, both Pd[DMBil1] and Pt[DMBil1] exhibit impressive quantum yields (ΦΔ ∼ 80%) for formation of 1O2, relative to DMBil1 and Zn[DMBil1] upon irradiation with light of λex > 500 nm. Indeed, the ease with which Pd[DMBil1] and Pt[DMBil1] can be synthesized, coupled with their abilities to photosensitize 1O2 with quantum yields that are comparable to those for many PDT agents, suggests that dimethylbiladiene complexes may be well-suited for use in photodynamic therapy. Such a goal will require the development of metalated DMBil1 derivatives that are soluble in aqueous solution and welltolerated by biological samples. Efforts to tailor the properties of DMBil1 architectures to achieve both of these goals remain major areas of emphasis in our laboratory.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 302-831-0716. ORCID

Joel Rosenthal: 0000-0002-6814-6503 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF CAREER Award CHE1352120 and NIH P20GM104316. NMR and other data were acquired at UD using instrumentation obtained with assistance from the NSF and NIH (NSF-MIR 0421224, NSFMIR 1048367, NSF-CRIF MU CHE-0840401 and CHE0541775, NIH P20 RR017716).

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00796. Crystallographic, electrochemical, and spectroscopic data (PDF) Accession Codes

CCDC 966865−966866 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by email12709

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on October 9, 2017, with errors in Chart 1. The corrected version was reposted on October 18, 2017.

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DOI: 10.1021/acs.inorgchem.7b00796 Inorg. Chem. 2017, 56, 12703−12711