Electrochemical, spectroscopic, and1O2 sensitization characteristics of 10,10-dimethylbiladiene complexes of zinc and copper

Article abstract of DOI:10.1021/jp506412r

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 ro

Full text of DOI:10.1021/jp506412r

Article
pubs.acs.org/JPCA
1
Electrochemical, Spectroscopic, and O2 Sensitization Characteristics
of 10,10-Dimethylbiladiene Complexes of Zinc and Copper
Allen J. Pistner,§ Rachel C. Pupillo,§ Glenn P. A. Yap,§ Daniel A. Lutterman,† Ying-Zhong Ma,†
and Joel Rosenthal*,§
§Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
†Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
S
* Supporting Information
ABSTRACT: 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
Zn2+ and Cu2+ 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 1O2 at wavelengths longer than 500 nm was probed. Both the free base and Zn2+ 10,10-
1
dimethylbiladiene architectures show modest efficiencies for 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.
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
INTRODUCTION
■
Tetrapyrrole macrocycles such as porphyrins, corroles, and
pthalocyanines are among the most well-studied class of
organic chromophores and redox cofactors.1 Linear tetrapyr-
roles 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 metals5 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.
Scheme 1. Cyclization of a,c-Biladienes to Generate Corroles
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.
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
Special Issue: Current Topics in Photochemistry
Received: June 27, 2014
Revised: August 30, 2014
Published: September 4, 2014
© 2014 American Chemical Society
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Given that deprotonation at the 10-position of the a,c-
biladiene framework is required for cyclization and aromatiza-
tion 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 phlorins22−25 and other porphyrinoids26−29 containing sp3-
hybridized meso positions, we sought to integrate pentafluor-
ophenyl 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 architec-
tures that are easily metalated and offer a combination of
chemical stability and synthetic availability. We have also
characterized the structural, redox, and photophysical proper-
ties of these assemblies and show that they are modestly
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
1
mg of the title compound in 53% yield. H 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.
1
efficient sensitizers of O2 using visible light.
EXPERIMENTAL SECTION
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
■
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.
1
title compound in 70% yield. H 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 spec-
tra 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) Quanta-
Master 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
1
Compound Characterization. H 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
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= 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 below33
CH3OH) as a reference sensitizer and the expression below,
1
where Φ O and Φref are the singlet oxygen sensitization
2
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.
2
⎛
⎝
⎞
⎠
⎛
⎜
⎝
⎞⎛
⎞
⎟
⎠
η
Aref Iem
em
⎜
⎜
⎟
⎟
Φ
= Φ
⎟⎜
⎛
⎝
⎞
⎠
em
ref
⎛
⎜
⎝
⎞
⎟
⎠
msample
mref
εref
A
I
η
em ⎠⎝
ref
⎜
⎜
⎟
⎟
1
ref
Φ O = Φ
ref
2
ε
sample
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.
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 decamethylferroce-
nium−decamethylferrocene internal standard of 1 mV versus
Ag/AgCl.36
Time-Resolved Spectroscopy. Picosecond time-corre-
lated 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 contribu-
tion. 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.
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 contribu-
tions 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
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-diphenyli-
sobenzofuran. This was accomplished by determining the
decrease in integrated emission intensity from unreacted furan
1
every 5 min for a total of 30 min. O2 sensitization quantum
yields were determined using [Ru(bpy)3]Cl2 (Φref = 0.81 in
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Scheme 2. Synthesis and Metallation of 10,10-Dimethylbiladiene
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
C33H18F10N4
660.5
triclinic
P-1
9.762(2)
Zn[DMBil1]·CH3OH
Cu[DMBil1]
C33H16F10N4Cu
722.04
orthorhombic
Pna2(1)
10.4143(3)
formula
Fw
C34H20F10N4OZn
877.48
crystal system
space group
a (Å)
triclinic
P-1
9.4815(3)
14.8240(5)
26.6756(10)
104.113(2)
98.585(2)
91.238(2)
3588.9(2)
4
b (Å)
9.830(2)
15.163(4)
100.996(4)
93.420(4)
96.674(4)
1414.7(6)
2
19.3742(5)
c (Å)
14.0031(4)
α (deg)
β (deg)
γ (deg)
V (Å3)
90
90
90
2825.39(14)
4
Z
temp (K)
200(2)
200(2)
200(2)
Dcalcd (g/cm−3
)
1.551
1.624
1.697
2θ range (deg)
μ (Mo Kα) (mm−1
4.22−54.92
0.139
6.92−147.66
3.326
7.78−136.46
1.999
)
relections
unique
R(int)
R1
18532
6426
0.0235
116052
15559
4610
0.0182
13894
0.0683
0.0396
0.0924
0.0424
0.237
wR2
0.1080
0.0659
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 LANL2DZ43
pseudopotential was used for Zn. All calculations used the SMD
universal continuum model44 with CH2Cl2 as the solvent (ε =
8.93). All geometry optimizations were performed in C1
symmetry with subsequent vibrational frequency analysis to
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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.
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.
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]·CH3OH (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 octaethylformyl-
biliverdinate 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 signifi-
cantly 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
RESULTS AND DISCUSSION
■
Biladiene Synthesis. Synthesis of a 10,10-dimethylbila-
diene 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 penta-
fluorophenylbenzoyl 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).
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.
The 10,10-dimethylbiladiene tetrapyrrole ligand is conforma-
tionally 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
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Table 2. Photophysical and Electrochemical Data for DMBil1, Zn[DMBil1] and Cu[DMBil1]
1
compound
Eox(1,2)/V
Ered(1,2)/V
λabs/nm (ε × 103 M−1·cm−1
291(1.1), 423(41.4), 450(43.0)
)
λem/nm (Φem
)
Φ O
2
DMBil1
1.33, 1.49
1.04, 1.33
0.92, 1.24
−1.05, −1.21
−1.13, −1.31
−0.88, −1.34
543(1.7 × 10−3
)
)
1.5 × 10−2
2.6 × 10−2
<0.2 × 10−2
Zn[DMBil1]
Cu[DMBil1]
306(6.9), 368(6.6), 467(50.8), 513(8.2), 548(10.2)
308(11.2), 364(13.0), 474(59.4), 558(9.1)
573(7.0 × 10−4
clashing between H(33) and C(1). As a result, C(1) and C(33)
are separated by over 3.3 Å.
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,
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°.
Electrochemistry. 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.
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
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].
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
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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.
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
Φ
DMBIL1 = 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. Incorpo-
ration 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].
1
Sensitized Production of O2. Porphyrinoids have found
1
application for the sensitization of O2, which is the reactive
species produced by many light-absorbing agents that are
employed for photodynamic therapy (PDT).49−54 Further-
more, many tetrapyrrole derivatives that contain points of
saturation along the ligand backbone are particularly good
Figure 4. Representation of the molecular orbitals involved in the
major electronic absorption transitions for (a) DMBil1 and (b)
Zn[DMBil1] in CH2Cl2.
1
sensitizers of O2 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.
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
1
The quantum yields for sensitization of O2 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-
1
O2
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 O = 1.5 × 10−2 upon irradiation
1
2
at λirr = 500 nm. Although this quantum yield is only modest,
when compared to free base porphyrin architectures, which can
1
sensitize the production of O2 with quantum efficiencies that
approach Φ O = 0.8 in polar protic solvents,58,62 our results
1
2
clearly demonstrate that the 10,10-dimethylbiladiene tetrapyr-
1
role can be used to generate O2. 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
1
for the free base DMBil1 ligand. The value of Φ O for
2
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
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Notes
which the DMBil1 chromophore undergoes intersystem
crossing to the triplet manifold. This behavior has been
previously observed for conventional tetrapyrrole porphyr-
inoids.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
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.R. thanks Oak Ridge Associated Universities for a Ralph E.
Powe Junior Faculty Enhancement Award. Additional financial
support for this work was provided in part by the American
Chemical Society Petroleum Research Fund and NSF CAREER
Award CHE1352120. Work by D.A.L. and Y.-Z.M. was
supported by the U.S. Department of Energy, Office of
Science, Basic Energy Sciences, Chemical Sciences, Geo-
sciences, and Biosciences Division. NMR and other data were
acquired at UD using instrumentation obtained with assistance
from the NSF and NIH (NSF MIR 0421224, NSF CRIF MU
CHE0840401 and CHE0541775, NIH P20 RR017716).
CH3OH solutions of Cu[DMBil1] led to negligible production
1
of O2 (Φ O < 0.2 × 10−2) upon irradiation at λirr = 500 nm,
1
2
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 porphyr-
inoids. 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]).
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ASSOCIATED CONTENT
* Supporting Information
■
S
Spectroscopic, computational, and crystallographic data. This
material is available free of charge via the Internet at http://
pubs.acs.org. Crystallographic data are also available from the
Cambridge Crystallographic Data Centre (CCDC 996931-
996933).
AUTHOR INFORMATION
Corresponding Author
*E-mail: joelr@udel.edu. Tel: 302-831-0716.
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
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