Electrochemical, spectroscopic, and (1)O2 sensitization characteristics of 10,10-dimethylbiladiene complexes of zinc and copper.

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethylbiladiene tetrapyrrole bearing ancillary pentafluorophenyl groups at the 5- and 15-meso positions (DMBil1) is presented. This nonmacrocyclic tetrapyrrole platform is robust and can serve as an excellent ligand scaffold for Zn(2+) and Cu(2+) centers. X-ray diffraction studies conducted for DMBil1 along with the corresponding Zn[DMBil1] and Cu[DMBil1] complexes show that this ligand scaffold binds a single metal ion within the tetrapyrrole core. Additionally, electrochemical experiments revealed that all three of the aforementioned compounds display an interesting redox chemistry as the DMBil1 framework can be both oxidized and reduced by two electrons. Spectroscopic and photophysical experiments carried out for DMBil1, Zn[DMBil1], and Cu[DMBil1] provide a basic picture of the electronic properties of these platforms. All three biladiene derivatives strongly absorb light in the visible region and are weakly emissive. The ability of these compounds to sensitize the formation of (1)O2 at wavelengths longer than 500 nm was probed. Both the free base and Zn(2+) 10,10-dimethylbiladiene architectures show modest efficiencies for (1)O2 sensitization. The combination of structural, electrochemical, and photophysical data detailed herein provides a basis for the design of additional biladiene constructs for the activation of O2 and other small molecules.

Introduction

Tetrapyrrole macrocycles such as porphyrins, corroles, and pthalocyanines are among the most well-studied class of organic chromophores and redox cofactors.[1] Linear tetrapyrroles such as biladienes and biliverdins have also been prepared and studied as organic chromophores with interesting absorption and photophysical properties.[2] Although linear polypyrroles do not typically absorb light as strongly as their conjugated macrocyclic homologues,[3] these chromophores serve an essential role in phytochrome photoreceptors, which work to regulate seed germination, flowering, and stem growth in plants.[4] In addition to serving several important roles in Nature, linear tetrapyrroles can also serve as ligands toward main group elements and transition metals[5] that include copper,[6,7] nickel,[8,9] and cobalt,[10,11] among others.[12] Although the coordination chemistry of several linear tetrapyrroles has received attention, the preparation and study of nonmacrocyclic tetrapyrrole complexes have not been pursued to nearly the same extent as porphyrinoid coordination chemistry.

The coordination chemistry of oligopyrroles has trailed that of porphyrinoids in part due to the preparation and study of linear tetrapyrrole derivatives having been hampered by their inherent instability. For example, a,c-biladiene derivatives in which two protons are connected to the sp3-hybridized 10-position of the tetrapyrrole can rapidly decompose in the presence of light and air.[13] When the biladiene is unsubstituted at the 1- and 19-positions (the termini of the tetrapyrrole), decomposition often involves cyclization to generate the corresponding corrole macrocycle, which is aromatic and very stable (Scheme 1).[14−17] The inherent instability of most linear tetrapyrroles is reflected by the ease with which such systems can be oxidized as bilirubin and related compounds display several redox waves from ∼0.5–0.8 V versus SCE,[18] which is roughly 1 V lower than that observed for homologous porphyrinoid macrocycles.

Scheme 1

Cyclization of a,c-Biladienes to Generate Corroles

Given that deprotonation at the 10-position of the a,c-biladiene framework is required for cyclization and aromatization of the final corrole macrocycle,[19,20] addition of alkyl groups at the 10-position of the biladiene framework lends stability to this class of tetrapyrrole.[21] On the basis of this precedent, we rationalized that 10,10-dimethylbiladienes would represent a stable class of nonmacrocyclic tetrapyrroles that could serve as excellent ligands for transition-metal centers. Moreover, by adapting the synthetic methods developed for the preparation of phlorins[22−25] and other porphyrinoids[26−29] containing sp3-hybridized meso positions, we sought to integrate pentafluorophenyl substituents onto the 5- and 15-positions of the biladiene framework. We now report that this strategy provides a convenient route to novel 10,10-dimethylbiladiene architectures that are easily metalated and offer a combination of chemical stability and synthetic availability. We have also characterized the structural, redox, and photophysical properties of these assemblies and show that they are modestly efficient sensitizers of 1O2 using visible light.

Experimental Section

General Materials and Methods

Reactions were performed in oven-dried round-bottomed flasks unless otherwise noted. Reactions that required an inert atmosphere were conducted under a positive pressure of N2 using flasks fitted with Suba-Seal rubber septa or in a nitrogen-filled glovebox. Air- and moisture-sensitive reagents were transferred using standard syringe or cannula techniques. Reagents and solvents were purchased from Sigma-Aldrich, Acros, Fisher, Strem, or Cambridge Isotopes Laboratories. Solvents for synthesis were of reagent grade or better and were dried by passage through activated alumina and then stored over 4 Å molecular sieves prior to use.[30] 5,5-Dimethyldipyrromethane and 5,5-dimethyl-1,9-bis(pentafluorobenzoyly)-dipyrromethane were prepared using previously described methods.[22−25] All other reagents were used as received.

Compound Characterization

1H NMR and 13C NMR spectra were recorded at 25 °C on a Bruker 400 MHz spectrometer. 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).[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. LR-GCMS data were obtained using an Agilent gas chromatograph consisting of a 6850 Series GC system equipped with a 5973 Network mass-selective detector. LR-ESI MS data were obtained using either a LCQ Advantage from Thermofinnigan or a Shimadzu LCMS-2020. High-resolution mass spectrometry analyses were performed by the Mass Spectrometry Laboratory in the Department of Chemistry and Biochemistry at the University of Delaware.

10,10-Dimethyl-5,15-dipentfluorophenylbiladiene (DMBil1)

To a solution of 5,5-dimethyl-1,9-bis(pentafluorobenzoyly)-dipyrromethane (1) (281 mg, 0.50 mmol) dissolved in 40 mL of THF and MeOH (3:1) was added 946 mg of NaBH4 (25.0 mmol). The NaBH4 was added as a solid in a single batch at room temperature. The resulting mixture was stirred at room temperature for 2 h, following which the reaction was quenched with H2O and extracted with dichloromethane. The organic layer was washed sequentially with H2O and brine and dried over Na2SO4. The solvent was then removed via rotary evaporation, and the resulting residue was dissolved in 200 mL of dichloromethane and combined with InCl3 (15 mg, 68 μmol) and pyrrole (100 μL, 1.44 mmol). The reaction was stirred at room temperature under air for 30 min, after which time 180 mg of DDQ (0.8 mmol) was added to the stirred solution. After stirring the reaction for an additional 5 min, 14 mL of triethylamine (100 mmol) was added, and the mixture was stirred for an additional 2 h. Following removal of the solvent under reduced pressure, the crude product was purified by chromatography on silica using a mixture of hexanes and CH2Cl2 (2:1) as the eluent to give 160 mg of the title compound in 53% yield. 1H NMR (400 MHz, CDCl3, 25 °C) δ/ppm: 12.43 (s, 2H), 7.18 (s, 2H), 6.63 (d, J = 4.6 Hz, 2H), 6.57 (d, J = 4.6 Hz, 2H), 6.29–6.24 (m, 2H), 6.22 (d, J = 3.7 Hz, 2H), 1.81 (s, 6H). 13C NMR (101 MHz, CDCl3, 25 °C) δ/ppm: 177.47, 148.05, 144.98 (d, J = 248 Hz), 141.58 (d, J = 254 Hz), 137.55 (d, J = 252 Hz), 133.02, 132.27, 130.45, 124.61, 122.00, 120.24, 112.62, 111.68, 42.16, 26.00. HR-LIFDI-MS [M]+m/z: calcd for C33H18N4F10, 660.1372; found, 660.1372. Anal. Calcd for C33H18F10N4: C, 60.01; H, 2.75; N, 8.48. Found: C, 59.60; H, 3.04; N, 8.17.

Zn[DMBil1]

To a solution of DMBil1 (100 mg, 0.15 mmol) dissolved in 30 mL of DMF was added 665 mg of Zn(OAc)2 (3 mmol). This solution was heated at 60 °C for 4 h. Following removal of the solvent under reduced pressure, the resulting residue was dissolved in CH2Cl2 and filtered through Celite. The filtrate was washed with brine and dried over Na2SO4. Following removal of the CH2Cl2 by rotary evaporation, the crude product was redissolved in MeOH and concentrated under reduced pressure to deliver 76 mg of the title compound in 70% yield. 1H NMR (400 MHz, CDCl3, 25 °C) δ/ppm: 7.12 (s, 2H), 6.58 (d, J = 4.1 Hz, 2H), 6.54 (d, J = 3.9 Hz, 2H), 6.50 (d, J = 4.2 Hz, 2H), 6.38 (d, J = 3.7 Hz, 2H), 1.74 (s, 6H). 13C NMR (101 MHz, CDCl3) δ: 170.55, 150.72, 144.85 (d, J = 248.7 Hz), 141.22 (d, J = 248.7 Hz), 141.07, 139.75, 137.20 (d, J = 253.1 Hz), 131.44, 129.23, 128.15, 116.90, 116.54, 39.43, 30.21. HR-EI-MS [M]+m/z: calcd for C33H16N4F10Zn, 722.05064; found, 722.04983. Anal. Calcd for C33H16F10N4Zn + 1/2 CH3OH + DMF: C, 54.42; H, 2.89; N, 8.05. Found: C, 54.23; H, 2.57; N, 7.94.

Cu[DMBil1]

To a solution of DMBil1 (100 mg, 0.15 mmol) dissolved in 40 mL of CH3CN was added 45 mg of Cu(OAc)2 (0.225 mmol). This solution was heated to 60 °C for 4 h. Following removal of the solvent under reduced pressure, the crude material was purified by chromatography on silica using hexane and CH2Cl2 (2:1) as the eluent to give 100 mg of the title compound in 92% yield. HR-ESI-MS [M + H]+m/z: calcd for C33H17N4F10Cu, 722.0590; found, 722.0595. Anal. Calcd for C33H16F10N4Cu: C, 54.89; H, 2.23; N, 7.76. Found: C, 54.90; H, 2.01; N, 7.48.

Steady-State Spectroscopy

UV/visible absorbance spectra were acquired on a StellarNet CCD array UV–vis spectrometer using screw cap quartz cuvettes (7q) of 1 cm path length from Starna. All absorbance spectra were recorded at room temperature. All samples for spectroscopic analysis were prepared in dry CH2Cl2 within a N2-filled glovebox.

Steady-state fluorescence 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. Samples for fluorescence analysis were prepared in an analogous method to that described above for the preparation of samples for UV–vis spectroscopy. Samples of DMBil1, Zn(DMBil1), and Cu(DMBil1) were excited at λex = 450, 475, and 465 nm, respectively, and emission was monitored with a step size of 0.5 nm and integration time of 0.25 s. Reported spectra are the average of at least five individual acquisitions.

Emission quantum yields were calculated using [Ru(bpy)3]Cl2 in acetonitrile (Φref = 0.062)[32,33] as the reference actinometer using the expression below[33]where Φem and Φref are the emission quantum yield of the sample and the reference, respectively, Aref and Aem are the measured absorbances of the reference and sample at the excitation wavelength, respectively, Iref and Iem are the integrated emission intensities of the reference and sample, respectively, and ηref and ηem are the refractive indices of the solvents of the reference and sample, respectively.

Time-Resolved Spectroscopy

Picosecond time-correlated single-photon counting (TCSPC) measurements were performed using a commercial femtosecond Ti:sapphire oscillator (Coherent Mira 900F) as the light source, which generates frequency-tunable pulses with a typical full width at half-maximum (fwhm) of 150 fs and a 76 MHz repetition rate. The output at a selected central wavelength was frequency-doubled using a 1.5 mm thick BBO crystal to obtain the excitation pulses centered at ∼455 nm. Fluorescence emission was selected using a 10 nm (fwhm) band-pass filter centered at 520 nm for DMBil1 and at 540 nm for Zn[DMBil1], which were chosen according to the peak wavelengths of their fluorescence emission spectra. The detection system has been described in detail previously,[34] which involves an actively quenched single-photon avalanche photodiode (PDM 50CT module, Micro Photon Devices) and a TCSPC module (PicoHarp 300, PicoQuant). The instrument response function (IRF) showed a fwhm of 70 ps as recorded using a dilute water suspension of coffee creamer directly at these chosen detection wavelengths to avoid pronounced spectral dependence of the detector in the spectral region of our interest. A 4.0 ps channel time was chosen, and at least 7000 counts were collected in the peak channel for the samples in order to obtain an acceptable signal-to-noise ratio. The polarization of the excitation beam was set to the magic angle (54.7°) with respect to an emission linear polarizer, which eliminates any depolarization contribution. Quantitative analysis of the time-resolved fluorescence data was performed by employing a least-squares deconvolution fitting algorithm with explicit consideration of the finite IRF (FluoFit, PicoQuant), and a reduced chi-squares (χ2) value was used to judge the quality of each fit.

Singlet Oxygen Sensitization

Quantification of singlet oxygen generation was carried out using the fluorescent probe 1,3-diphenylisobenzofuran as a trapping agent for 1O2.[35] Fluorescence measurements were recorded for CH3OH solutions that were 10 μM in sensitizer and 10 μM in 1,3-diphenylisobenzofuran. The solutions (2.0 mL total volume) were contained in screw cap quartz cuvettes (7q) of 1 cm path length and were irradiated with light passed through either a 500 or 550 nm band-pass filter. The rate of 1O2 production was determined by monitoring consumption of the 1,3-diphenylisobenzofuran. This was accomplished by determining the decrease in integrated emission intensity from unreacted furan every 5 min for a total of 30 min. 1O2 sensitization quantum yields were determined using [Ru(bpy)3]Cl2 (Φref = 0.81 in CH3OH) as a reference sensitizer and the expression below, where Φ and Φref are the singlet oxygen sensitization quantum yields for the sample and reference, respectively, msample and mref are the slopes of the decrease in furan fluorescence for the sample and the reference, respectively, and εsample and εref are the molar absorptivities at the irradiation wavelength (500 nm) for the sample and reference, respectively.

Electrochemistry

Cyclic voltammetry (CV) experiments were performed in a N2-filled glovebox using a CHI-620D potentiostat and a standard three-electrode assembly. CV scans were recorded for quiescent solutions using a platinum working disk electrode (2.0 mm diameter), a platinum wire auxiliary electrode, and a silver wire quasi-reference electrode. CV experiments were performed in MeCN containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. Concentrations of analyte were 1 mM, and a scan rate of 50 mV/s and sensitivity of 10 mA/V were maintained during data acquisition. All reported potentials are referenced relative to Ag/AgCl using a decamethylferrocenium–decamethylferrocene internal standard of 1 mV versus Ag/AgCl.[36]

X-ray Structural Solution and Refinement

X-ray structural analysis for DMBil1, Zn[DMBil1], and Cu[DMBil1]: Crystals were mounted using viscous oil onto a plastic mesh and cooled to the data collection temperature. Data were collected on a Bruker-AXS APEX 2 DUO CCD diffractometer with Cu Kα radiation (λ = 1.54178 Å) collimated and monochromated using Goebel mirrors or with Mo Kα radiation (λ = 0.71073 Å) monochromated with a graphite crystal. Unit cell parameters were obtained from 36 data frames, 0.5° ω, from three different sections of the Ewald sphere. No higher symmetry than triclinic was observed for DMBil1 and Zn[DMBil1]. Solution in P-1 yielded chemically reasonable and computationally stable results of refinement. The unit cell parameters and systematic absences in the diffraction data for Cu[DMBil1] are consistent with Pna21 and Pnam [Pnma]. However, the observed occupancy and the absence of either a molecular mirror or a molecular inversion center are consistent exclusively with the noncentrosymmetric space group, Pna21. The absolute structure parameter in Cu[DMBil1] refined to nil, indicating the true hand of the data, has been determined. An inspection of the packing diagram suggests no overlooked symmetry. The data sets were treated with numerical absorption corrections based on indexed crystal faces and dimensions (Apex2 software suite, Madison, WI, 2005). The structures were solved using direct methods and refined with full-matrix, least-squares procedures on F2.[37] A P–M pseudohelical isomer pair, with a 3.4 Å π–π offset parallel intermolecular distance, and severely disordered solvent molecules (two ethanol molecules and two chloroform molecules, one of which was located near the origin at half-occupancy) were located in the asymmetric unit of Zn[DMBil1]. Cocrystallized, noncoordinated, solvent molecules in Zn[DMBil1] were treated as diffused diffraction contributions using Squeeze.[38] All non-hydrogen atoms were refined with anisotropic displacement parameters. The amine hydrogen atoms in Zn[DMBil1] were assigned to be consistent with surrounding non-hydrogen atom geometry and allowed to refine in position but with the anisotropic parameter restrained to 1.2 Ueq of the attached nitrogen atom. All other hydrogen atoms were treated as idealized contributions. Atomic scattering factors are contained in the SHELXTL 6.12 program library. The CIF has been deposited under CCDC 996931-996933.

Computations

All density functional calculations were performed using the Gaussian 09 (G09) program package,[39] with the Becke three-parameter hybrid exchange and Lee–Yang–Parr correlation functional (B3LYP).[40−42] The 6-31G* basis set was used for C, N, and F atoms. The LANL2DZ[43] pseudopotential was used for Zn. All calculations used the SMD universal continuum model[44] with CH2Cl2 as the solvent (ε = 8.93). All geometry optimizations were performed in C1 symmetry with subsequent vibrational frequency analysis to confirm that each stationary point was a minimum on the potential energy surface. The vertical singlet transition energies of the complexes were computed at the time-dependent density functional theory (TD-DFT) level in CH2Cl2 within G09 by using the optimized ground-state structure.

Results and Discussion

Biladiene Synthesis

Synthesis of a 10,10-dimethylbiladiene ligand with aryl groups at the 5- and 15-positions is straightforward, as illustrated in Scheme 2. This synthesis starts with condensation of 5,5-dimethyldipyrromethane with pentafluorophenylbenzoyl chloride to yield the corresponding diacylated compound (1) in 30% yield.[22] The subsequent reaction with pyrrole using TFA as an acid catalyst neatly yields the free base dimethylbiladiene with ancillary C6F5 substituents (DMBil1).

Scheme 2

Synthesis and Metallation of 10,10-Dimethylbiladiene

Metalation of the biladiene ligand is readily accomplished by reaction with divalent metal acetates. Treatment of DMBil1 with Zn(OAc)2·2H2O in DMF at 60 °C for 4 h cleanly affords the corresponding zinc(II) complex (Zn[DMBil1]) in 70% yield. The Zn[DMBil1] complex was characterized using a combination of 1H and 13C NMR, high-resolution mass spectrometry, and elemental analyses. The homologous biladiene complex of copper(II) (Cu[DMBil1]) is similarly obtained in fine yield upon reaction of DMBil1 with Cu(OAc)2·2H2O in MeCN. Characterization of Cu[DMBil1] provided satisfactory mass spectrometry and elemental analyses.

Structural Characterization of DMBil1

The solid-state structures of DMBil1, Zn[DMBil1] and Cu[DMBil1] are shown in Figure 1, and crystallographic parameters for each structure are provided in Table 1. Each of the atoms that make up the core of the free base biladiene ligand and metal complexes is numbered in the standard fashion (note: fully labeled and numbered thermal ellipsoid plots of DMBil1, Zn[DMBil1], and Cu[DMBil1] are provided in the Supporting Information), and structural metrics for these systems are tabulated in Tables S1–S6. Structural highlights of the crystal structures obtained for the systems studied are summarized below.

Figure 1

Comparative views of crystal structures of (a) DMBIL1, (b) Zn[DMBil1]·MeOH, and (c) Cu[DMBil1]. Thermal ellipsoid plots are shown from above (top) and in profile (bottom) for each structure. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except those on coordinated solvent molecules) are omitted for clarity.

Table 1

Crystallographic data for DMBil1, Zn[DMBil1] and Cu[DMBil1]

	DMBil1	Zn[DMBil1]·CH3OH	Cu[DMBil1]
formula	C33H18F10N4	C34H20F10N4OZn	C33H16F10N4Cu
Fw	660.5	877.48	722.04
crystal system	triclinic	triclinic	orthorhombic
space group	P-1	P-1	Pna2(1)
a (Å)	9.762(2)	9.4815(3)	10.4143(3)
b (Å)	9.830(2)	14.8240(5)	19.3742(5)
c (Å)	15.163(4)	26.6756(10)	14.0031(4)
α (deg)	100.996(4)	104.113(2)	90
β (deg)	93.420(4)	98.585(2)	90
γ (deg)	96.674(4)	91.238(2)	90
V (Å3)	1414.7(6)	3588.9(2)	2825.39(14)
Z	2	4	4
temp (K)	200(2)	200(2)	200(2)
Dcalcd (g/cm–3)	1.551	1.624	1.697
2θ range (deg)	4.22–54.92	6.92–147.66	7.78–136.46
μ (Mo Kα) (mm–1)	0.139	3.326	1.999
relections	18532	116052	15559
unique	6426	13894	4610
R(int)	0.0235	0.0683	0.0182
R1	0.0396	0.0424	0.237
wR2	0.0924	0.1080	0.0659

The 10,10-dimethylbiladiene tetrapyrrole ligand is conformationally flexible, and the dipyrromethane units are significantly distorted from coplanarity in the solid-state structure of DMBil1. The dihedral angle between the two dipyrromethane units bridged by the saturated dimethyl-substituted sp3 center is 66.08°. This bent structure is consistent with calculated global energy minima for related 1,19-unsubstituted a,c-biladienes, for which the planar corrole-like conformation has been predicted to be roughly 20 kcal/mol higher in energy.[45] The pentafluorophenyl substituents are also canted with respect to the planes of the individual dipyrromethane moieties with average dihedral angles of 67.57°. Finally, the alternating short and long bond lengths of the pyrroles adjoined through the central sp3 hybridized center (C(10)) are consistent with the tautomeric structure shown in Figure 1, in which the terminal pyrrole positions bear protonated nitrogens. The C(1)–N(1) versus C(4)–N(1) and C(30)–N(4) versus C(33)–N(4) bond distances differ slightly by 0.029 and 0.017 Å, respectively, consistent with the assigned protonation.

Metalation of DMBil1 leads to coordination by all four pyrrole moieties of the biladiene ligand bind to a single central metal. Zn[DMBil1]·CHOH (Figure1b, Table 1) is formed upon crystallization of the zinc(II) complex from a concentrated solution of CHCl3 and CH3OH (1:1). Under these crystallization conditions, 1 equiv of methanol is bound to the metal with trigonal bipyramidal coordination geometry. The equatorial plane of Zn[DMBil1]·MeOH is comprised of pyrrole nitrogen atoms, N(2) and N(4), and the methanol ligand. The overall structure of Zn[DMBil1]·MeOH is reminiscent of a previously reported zinc octaethylformylbiliverdinate complex bearing an apical aquo ligand;[46] however, because the bilverdinate ligand is conjugated across all four pyrrole units, this tetrapyrrole complex adopts a square-pyramidal geometry instead. The structure of Zn[DMBil1]·MeOH is also distinct from that reported for a zinc complex of decamethyl-a,c-biladiene, which does not have a bound solvato ligand in the apical position.[47]

Binding of zinc(II) to the DMBil1 ligand leads to desymmetrization of the 10,10-dimethyl substituents. The six-membered ring comprised of Zn(1), N(2), C(15), C(10), C(19), and N(3) adopts a distorted boat-type conformation, resulting in C(18) being projected into an axial position and C(17) into an equatorial position. Moreover, the C(10)–C(18) bond is canted by only 25.22° with respect to the Zn(1)–O(1) axis that defines the zinc–methanol bond. The dihedral angle between the two pyrroles bridged by the saturated dimethyl-substituted sp3 center (C(10)) is significantly compressed (16.55°) in comparison to that for the free base ligand (vide supra). Nonetheless, the biladiene ligand is still highly distorted from a planar corrole-like conformation as the two terminal pyrrole units that make up the ligand are canted by 48.40° with respect to one another due to steric clashing between H(33) and C(1). As a result, C(1) and C(33) are separated by over 3.3 Å.

The copper(II) center of Cu[DMBil1] is four-coordinate; however, unlike in most typical copper(II) porphyrinoids, the coordination environment is significantly distorted from square-planar. Similar to the case for the Zn[DMBil1] derivative, steric repulsion between H(1A) and C(19) as well as H(19A) and C(1) on the terminal pyrroles causes the entire biladiene scaffold to twist. Atoms C(1) and C(19) are therefore separated by ∼3.04 Å, although this distance is closer than the two corresponding carbon atoms for the zinc(II) derivative by more than 0.25 Å. Similarly, the two terminal pyrrole units that make up the tetrapyrrole platform are canted by 50.59° with respect to one another. The bond distances between the copper(II) center and pyrrole nitrogens range from 1.969 to 1.980 Å, with an average distance of ∼1.972 Å. The ancillary pentafluorophenyl substituents are nearly orthogonal with respect to the planes of the individual dipyrromethane moieties that make up the biladiene scaffold of Cu[DMBil1], with average dihedral angles of 82.57°.

Polypyrrole scaffolds display rich redox properties and can often be oxidized and/or reduced by multiple electron equivalents. CV carried out for 1.0 mM solutions of DMBil1 in MeCN containing 0.1 M TBAPF6 shows that the biladiene can be both oxidized and reduced by two electrons. The CV trace recorded for DMBil1 (blue trace) is shown in Figure 2 and displays two irreversible one-electron oxidation (Eox(1) ≈ 1.33 V, Eox(2) ≈ 1.49 V) and two irreversible one-electron reduction waves (Ered(1) ≈ −1.05 V, Ered(2) ≈ −1.21 V). These redox properties are significantly perturbed upon metalation of the biladiene scaffold with either zinc or copper. Figure 2 juxtaposes the CV traces recorded for Zn[DMBil1] (red trace) and Cu[DMBil1] (green trace) with the free base biladiene (DMBil1). Both the zinc and copper biladiene scaffolds also display four one-electron redox waves that include two oxidation events. As summarized in Table 2, Zn[DMBil1] displays two irreversible one-electron oxidations at Eox(1) ≈ 1.04 V and Eox(2) ≈ 1.33 V. Cu[DMBil1] also displays two oxidation waves (Eox(1) ≈ 0.92 V and Eox (2) ≈ 1.24 V), which are more reversible than those observed for the zinc or free base biladiene derivatives. It is unclear as to why the oxidation of Cu[DMBil1] is more reversible on the CV time scale when compared to the oxidation of the free base and Zn[DMBil1] derivatives.

Figure 2

(a) Oxidative and (b) reductive cyclic voltammograms (CVs) recorded at a scan rate of 50 mV/s for DMBil1, Zn[DMBil1], and Cu[DMBil1] in MeCN containing TBAPF6 and an internal decamethylferrocene standard.

Table 2

Photophysical and Electrochemical Data for DMBil1, Zn[DMBil1] and Cu[DMBil1]

compound	Eox(1,2)/V	Ered(1,2)/V	λabs/nm (ε × 103 M–1·cm–1)	λem/nm (Φem)	Φ1O2
DMBil1	1.33, 1.49	–1.05, −1.21	291(1.1), 423(41.4), 450(43.0)	543(1.7 × 10–3)	1.5 × 10–2
Zn[DMBil1]	1.04, 1.33	–1.13, −1.31	306(6.9), 368(6.6), 467(50.8), 513(8.2), 548(10.2)	573(7.0 × 10–4)	2.6 × 10–2
Cu[DMBil1]	0.92, 1.24	–0.88, −1.34	308(11.2), 364(13.0), 474(59.4), 558(9.1)		<0.2 × 10–2

In contrast to the free base biladiene ligand, both Zn[DMBil1] and Cu[DMBil1] display reversible reductive redox events. Zn[DMBil1] displays two reversible one-electron reduction events at Ered(1) ≈ −1.13 V and Ered(2) ≈ −1.31 V, and Cu[DMBil1] displays two reversible one-electron reduction events at Ered(1) ≈ −0.88 V and Ered(2) ≈ −1.34 V. Given that the two reductive redox couples observed for each of the above DMBil1 derivatives occur at similar potentials, it is likely that both reductive waves observed for the Zn[DMBil1] and Cu[DMBil1] complexes correspond to reduction of the tetrapyrrole ligand. We note that while the Ered(2) values obtained for both the Zn2+ and Cu2+ biladiene derivatives are centered at around −1.3 V, the first reduction of these two derivatives occurs at vastly different potentials. As shown in Figure 2b, Cu[DMBil1] displays an Ered(1) value that is ∼0.25 V more positive than that recorded for Zn[DMBil1]. This difference may be due to adsorption of the Cu[DMBil1] at the electrode surface upon reduction, as judged by the broad polarization features that are observed in the CV for this compound from −0.55 to −0.75 V (Figure 2b).

Spectroscopy

Electronic spectra of DMBil1 and its zinc(II) and copper(II) complexes are consistent with other tetrapyrrole frameworks containing a point of saturation. Figure 3 compares the electronic absorption spectra of DMBil1, Zn[DMBil1], and Cu[DMBil1] in CH2Cl2. The spectral profile of DMBil1 is reminiscent of that of homologous tetrapyrroles with a 5,5-diproteo substitution pattern. For instance, similar to DMBil1, the free base form of 1,19-dideoxybiladienes-ac demonstrates a strong absorption at roughly 520 nm with weaker bands at wavelengths between 300 and 400 nm.[19] By comparison, DMBil1 displays two closely separated, strongly absorbing bands at 423 (ε ≈ 41 480) and 450 nm (ε ≈ 43 060). TD-DFT calculations performed for this tetrapyrrole derivative revealed that these intense absorption bands are correlated to transitions between the four molecular frontier orbitals. As shown in Figure 4, both of these π–π* transitions involve the near-degenerate HOMO and HOMO–1 levels, as well as the LUMO and LUMO+1.

Figure 3

Electronic absorption (solid curves) and emission (dashed curves) spectra recorded at 298 K in CH2Cl2 of (a) DMBil1, (b) Zn[DMBil1], and (c) Cu[DMBil1].

Figure 4

Representation of the molecular orbitals involved in the major electronic absorption transitions for (a) DMBil1 and (b) Zn[DMBil1] in CH2Cl2.

Metalation of the DMBil1 scaffold with Zn2+ is manifest in significant changes to the structure and corresponding electronic absorption spectrum of this chromophore. As opposed to the free base DMBil1 ligand, which displays two intense bands of similar intensity between ∼420 and 450 nm, CH2Cl2 solutions of Zn[DMBil1] display a single intense transition at 467 nm (ε = 50 790). TD-DFT calculations indicate that this absorption is largely attributable to electronic transitions between the four frontier orbitals between the HOMO–1 and LUMO+1. The electron density of the molecular orbitals is confined to the biladiene π-system. In contrast to the free ligand, Zn[DMBil1] also displays two low-energy absorptions at 513 (ε = 8180) and 548 nm (ε = 10 170). These absorption bands are weaker than the primary π–π* transition at 467 nm but serve to extend the ability of the chromophore to absorb light at longer wavelengths. TD-DFT calculations revealed that the two low-energy transitions observed for Zn[DMBil1] can be primarily attributed to electron promotion from the near-degenerate HOMO and HOMO–1 levels to the closely spaced LUMO and LUMO+1. The UV–vis absorption profile of Cu[DMBil1] is qualitatively similar to that of the corresponding Zn2+ derivative. The photophysical properties of the DMBil1 derivatives detailed above are summarized in Table 2.

Not unlike other tetrapyrrole derivatives, DMBil1 produces strong fluorescence upon photoexcitation. Excitation into the absorption bands of DMBil1 induces a broad emission profile between approximately 500 and 700 nm that is centered at 543 nm (τobs = 15.3 ps). The quantum yield for emission upon excitation of DMBil1 in deaerated CH2Cl2 at λex = 450 nm is ΦDMBIL1 = 1.7 × 10–3. Zn[DMBil1] is also luminescent (τobs = 22.3 ps); however, the emission from this zinc complex is shifted to lower energies compared to the free base biladiene ligand. The fluorescence lifetimes of DMBil1 and Zn[DMBil1] are significantly shorter than analogous porphyrin derivatives, which typically range from approximately 2 to 15 ns at room temperature in typical organic solvents.[48] The emission lifetimes recorded for DMBil1 and Zn[DMBil1] are similar in value to those observed for phlorin tetrapyrrole architectures that contain a single sp3 hybridized meso position.[22−25]

Similar to the trend observed for zinc versus free base porphyrins, the quantum yield for emission from Zn[DMBil1] is reduced by more than 50% to ΦZn[DMBIL1] = 7.0 × 10–4 as compared to the value observed for DMBil1. The emission observed for DMBil1 and Zn[DMBil1] is presumed to originate from the excited singlet state of these compounds because the emission quantum yields are not attenuated by introduction of oxygen to the luminescent samples. Incorporation of Cu2+ into the biladiene core abolishes the luminescence observed for this platform. This observation is typical of tetrapyrrole complexes with open d shells, as the placement of d–d states energetically below the π–π states of the conjugated ligand circumvents luminescence for Cu[DMBil1].

Sensitized Production of 1O2

Porphyrinoids have found application for the sensitization of 1O2, which is the reactive species produced by many light-absorbing agents that are employed for photodynamic therapy (PDT).[49−54] Furthermore, many tetrapyrrole derivatives that contain points of saturation along the ligand backbone are particularly good sensitizers of 1O2 upon excitation with light toward the low-energy end of the visible region of the electromagnetic spectrum.[55−59] As such, we rationalized that biladiene complexes may also be able to serve in such a role.

The quantum yields for sensitization of 1O2 by DMBil1, Zn[DMBil1], and Cu[DMBil1] were measured using [Ru(bpy)3]2+ as a standard (Φ = 0.81 in CH3OH)[60] and 1,3-diphenylisobenzofuran as a trapping agent for the detection of 1O2.[35,61] The quantum yield of 1O2 sensitization by DMBil1 in CH3OH was measured to be Φ = 1.5 × 10–2 upon irradiation at λirr = 500 nm. Although this quantum yield is only modest, when compared to free base porphyrin architectures, which can sensitize the production of 1O2 with quantum efficiencies that approach Φ = 0.8 in polar protic solvents,[58,62] our results clearly demonstrate that the 10,10-dimethylbiladiene tetrapyrrole can be used to generate 1O2. Because the absorbance profile of Zn[DMBil1] displays spectral features from 500 to 575 nm, this complex provides a means to sensitize 1O2 production using longer-wavelength light than that required for the free base DMBil1 ligand. The value of Φ for Zn[DMBil1] was measured to be 2.6 × 10–2 in CH3OH (λirr = 550 nm). The enhanced photosensitization observed for the zinc derivative likely stems in part from the presence of the Zn2+ center, which enhances the kinetics and efficiency with which the DMBil1 chromophore undergoes intersystem crossing to the triplet manifold. This behavior has been previously observed for conventional tetrapyrrole porphyrinoids.[63] Moreover, this rationale is consistent with the decreased fluorescence quantum yield observed for Zn[DMBil1] as compared to the free base ligand. Irradiation of CH3OH solutions of Cu[DMBil1] led to negligible production of 1O2 (Φ < 0.2 × 10–2) upon irradiation at λirr = 500 nm, consistent with rapid relaxation of the excited state of this complex via the low-energy copper d–d states.

Concluding Remarks

Linear tetrapyrrole architectures have interesting redox, spectroscopic, and photophysical properties. To date, however, the pace at which such systems have been prepared and studied has significantly lagged behind that of macrocyclic porphyrinoids. This discrepancy is due in part to the arduous synthesis and inherent instability of a,c-biladienes and other linear tetrapyrroles. Substitution of the 10-position of the general biladiene architecture with germinal dimethyl groups allows for the convenient construction, isolation, and study of a 10,10-dimethylbiladiene platform with pentafluorophenyl substituents at the 5- and 15-meso positions (DMBIl1). Moreover, the straightforward synthesis and excellent stability of DMBIL1 lends itself to the study and use of this tetrapyrrole scaffold as a ligand for transition-metal centers such as Zn2+ (Zn[DMBil1]) and Cu2+ (Cu[DMBil1]).

Structural analysis confirms that DMBil1 binds a single metal center within the tetrapyrrole core. Additionally, DMBil1 and its Zn2+ (Zn[DMBil1]) and Cu2+ (Cu[DMBil1]) derivatives support multiple redox states. Each of these 10,10-dimethylbiladiene derivatives can be both oxidized and reduced by two electrons as judged by CV. Furthermore, DMBil1 strongly absorbs light in the visible region, and the Zn[DMBil1] and Cu[DMBil1] complexes are able to collect photons at wavelengths approaching 600 nm. The ability of these tetrapyrrole systems to sensitize the formation of 1O2 at wavelengths longer than 500 nm was probed. Both the free base and Zn2+ 10,10-dimethylbiladiene architectures show modest efficiencies for 1O2 sensitization, demonstrating that the DMBil1 framework may be a competent platform for the construction of new PDT agents. With these results in hand, we are now poised to further develop the coordination chemistry of DMBil1 and to investigate these and related nonmacrocyclic tetrapyrrole species for various applications in the photochemical and catalysis arenas.