Photophysical properties of β-substituted free-base corroles

Article abstract of DOI:10.1021/ic502860g

Corroles are an emergent class of fluorophores that are finding an application and reaction chemistry to rival their porphyrin analogues. Despite a growing interest in the synthesis, reactivity, and functionalization of these macrocycles, their excited-st

Full text of DOI:10.1021/ic502860g

Article
pubs.acs.org/IC
Photophysical Properties of β‑Substituted Free-Base Corroles
Christopher M. Lemon, Robert L. Halbach, Michael Huynh, and Daniel G. Nocera*
Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United
States
S
* Supporting Information
ABSTRACT: Corroles are an emergent class of fluorophores that are finding an application and
reaction chemistry to rival their porphyrin analogues. Despite a growing interest in the synthesis,
reactivity, and functionalization of these macrocycles, their excited-state chemistry remains
undeveloped. A systematic study of the photophysical properties of β-substituted corroles was
performed on a series of free-base β-brominated derivatives as well as a β-linked corrole dimer.
The singlet and triplet electronic states of these compounds were examined with steady-state and
time-resolved spectroscopic methods, which are complemented with density functional theory
(DFT) and time-dependent DFT calculations to gain insight into the nature of the electronic
structure. Selective bromination of a single molecular edge manifests in a splitting of the Soret
band into x and y polarizations, which is a consequence of asymmetry of the molecular axes. A
pronounced heavy atom effect is the primary determinant of the photophysical properties of
these free-base corroles; bromination decreases the fluorescence quantum yield (from 15% to
0.47%) and lifetime (from 4 ns to 80 ps) by promoting enhanced intersystem crossing, as
evidenced by a dramatic increase in knr with bromine substitution. The nonbrominated dimer exhibits absorption and emission
features comparable to those of the tetrabrominated derivative, suggesting that oligomerization provides a means of red-shifting
the spectral properties akin to bromination but without decreasing the fluorescence quantum yield.
milieu. Free-base corroles have appreciable fluorescence
quantum yields and excited-state lifetimes of several nano-
seconds.40,43 The effect of substitution at the meso positions of
the corrole appears to have a more pronounced influence on
photophysical properties than it does for porphyrins. Spurred
by these observations, we were interested in delineating the
effect of β-substitution on the electronic structure and excited-
state properties of corroles. We find that the electronic spectral
properties are governed by the (x,y) asymmetry of the corrole
ring. As observed by Gross and co-workers for Al and Ga
corroles,54 β-substitution with halogen causes a severe
perturbation to the photophysics of the corrole, arising from
a pronounced heavy atom effect. Corrole dimerization at the β-
position provides a means of red-shifting the spectral properties
akin to bromination but without decreasing the fluorescence
quantum yield. Such design principles are useful for the
implementation of corroles for sensing applications.
INTRODUCTION
■
Porphyrins possess unique electronic and photophysical
properties that engender their use as imaging agents,1−4
photosensitizers for photodynamic therapy,5−7 light-harvesting
antennas,8−11 and optical sensors.12−15 A related class of
molecules is corroles, which, like porphyrins, are 18 π-electron
tetrapyrrole macrocycles but with a contracted 23-atom core
versus the 24-atom core of porphyrins. This contracted core
arises from a direct pyrrole−pyrrole linkage to yield three meso
carbons bridging pyrrole units, unlike the four meso carbons of
a porphyrin. Corroles have been historically difficult to
synthesize with the first preparations involving the cyclization
of the linear precursor biladiene-ac.16,17 With the advent of
facile one-pot methods,18−21 corrole chemistry has expanded
with emphasis on both the synthesis and peripheral
functionalization of the macrocycle.22−27 The availability of
corroles as a result of these synthetic methods has led to many
applications,28−30 including their function as molecular
catalysts,31−36 photosensitizers,37−39 singlet oxygen photo-
generators,40,41 and imaging agents.42 Despite these advances,
there have been comparatively few photophysical studies of
free-base corroles. Some of these studies include the elucidation
of photophysics as a function of meso-substitution,40,41,43−46
N−H tautomerization,47−50 and higher order architectures such
as corrole−fluorophore dyads.51−53
EXPERIMENTAL SECTION
■
Materials. The following materials were used as received: hexanes,
acetone, ethanol (EtOH), methanol (MeOH), chloroform (CHCl3),
dichloromethane (CH2Cl2), pentane, inhibitor-free tetrahydrofuran
(THF), acetonitrile (MeCN), toluene, benzonitrile (PhCN), dimethyl
sulfoxide (DMSO), pyrrole, indium(III) chloride (InCl3), sodium
hydroxide (NaOH) beads, methyl 4-formylbenzoate, 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ), N-bromosuccinimide (NBS), and
pentafluorobenzaldehyde from Sigma-Aldrich; sodium bicarbonate
(NaHCO3), sodium sulfate (Na2SO4), and sodium chloride (NaCl)
A desirable property of any fluorophore is the ability to tune
its absorbance and emission wavelengths, as well as the excited-
state lifetime. Modulation of spectral features to achieve a
desired optical window is important for any imaging or sensing
application, especially for those that function in the biological
Received: December 4, 2014
© XXXX American Chemical Society
A
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from Mallinckrodt; hydrochloric acid (HCl) from EMD; silica gel 60 Å
230−400 mesh ASTM from Whatman; and chloroform-d (CDCl3)
from Cambridge Isotope Laboratories. Argon gas (Airgas) was passed
over a Drierite column prior to use. The compounds 5-
pentafluorophenyldipyrromethane (1)55,56 and 5,10,15,20-tetraphenyl-
porphyrin (H2TPP)57 were prepared according to literature
procedures or slight modifications thereof.
fractions were identified as monobrominated corroles (2-bromo-2 and
3-bromo-2), as well as putative decomposition products on the basis of
1H NMR.
2,3-Dibromo-10-(4-methoxycarbonylphenyl)-5,15-bis-
1
(pentafluorophenyl)corrole (4). H NMR (500 MHz, CDCl3) δ 4.08
(s, 3H), 8.26 (d, J = 8.1 Hz, 2H), 8.44 (d, J = 8.1 Hz, 2H), 8.57 (d, J =
4.4 Hz, 1H), 8.62 (d, J = 4.8 Hz, 1H), 8.66 (overlapping d, J = 5.0 Hz,
1H), 8.67 (overlapping d, J = 4.8 Hz, 1H), 8.81 (d, J = 4.7 Hz, 1H),
9.39 (d, J = 4.4 Hz, 1H). 19F NMR (376 MHz, CH2Cl2) δ −164.64
(m, 2F), −163.42 (m, 2F), −155.02 (m, 1F), −154.07 (m, 1F),
−140.31 (d, J = 27.2 Hz, 2F), −140.11 (d, J = 27.1 Hz, 2F). Anal.
Calcd for (M + H)+, M = C39H16Br2F10N4O2: 922.9460; Found ESI-
MS: 922.9462. UV−vis (toluene), λ in nm (ε in 103 M−1 cm−1): 427
(93), 528 (7.5), 565 (15), 597 (8.9), 648 (14).
Corrole Synthesis. In a 1 L round-bottom flask, 1.58 g of 5-
pentafluorophenyldipyrromethane (1) (5.06 mmol) and 0.41 g of
methyl 4-formylbenzoate (2.5 mmol) were dissolved in 250 mL of
MeOH. A solution of HCl was prepared by dissolving 12.5 mL of
concentrated HCl (12 M) in 250 mL of H2O. The acid solution was
added to the MeOH solution and stirred at room temperature for 1 h.
Over the course of the reaction, the solution became cloudy, and a red-
brown color developed. The reaction mixture was poured into a 2 L
separatory funnel, and the product was extracted with CHCl3 (200 mL
×3). The combined organics were washed with water (×2), dried over
Na2SO4, and the solution was diluted to a volume of 1 L with CHCl3;
1.71 g of DDQ (2.51 mmol) was added, and this solution immediately
turned dark. The reaction mixture was stirred overnight at room
temperature. The crude reaction mixture was concentrated to a
volume of ∼200 mL and then poured onto a silica gel column packed
with hexanes. All fluorescent material was collected using CHCl3 as the
eluent. The product was purified on a silica gel column packed with
hexanes, using hexanes then 1:1 hexanes/CH2Cl2 to elute 2 as a purple
solution. A second macrocyclic product (3) was identified at the top of
the column and was isolated using 100% CH2Cl2 as the eluent to
afford a green solution. After solvent removal, 0.33 g (17% yield) of 3
was isolated as a shiny purple solid. Compound 2, which was initially
obtained as a black solid, was further purified on a silica gel column
packed with hexanes, using hexanes then 1:1 hexanes/CH2Cl2 as the
eluent. After solvent removal, 0.98 g (51% yield) of 2 was obtained as
a purple solid.
10-(4-Methoxycarbonylphenyl)-5,15-bis(pentafluorophenyl)-
corrole (2). 1H NMR (500 MHz, CDCl3) δ 4.10 (s, 3H), 8.27 (d, J =
8.1 Hz, 2H), 8.45 (d, J = 8.1 Hz, 2H), 8.59 (bs, 2H), 8.66 (d, J = 4.8
Hz, 2H), 8.73 (d, J = 4.6 Hz, 2H), 9.14 (d, J = 4.3 Hz, 2H). 19F NMR
(376 MHz, CH2Cl2) δ −163.98 (m, 4F), −155.15 (m, 2F), −140.15
(dd, J1 = 28.9 Hz, J2 = 6.5 Hz, 4F). Anal. Calcd for (M + H)+, M =
C39H18F10N4O2: 765.1270; Found ESI-MS: 765.1252. UV−vis
(toluene), λ in nm (ε in 103 M−1 cm−1): 424 (99), 525 (7.3), 565
(16), 615 (9.3), 640 (5.8).
3 - [ 1 0 ′ - ( 4 - M e t h o x y c a r b o n y l p h e n y l ) - 5 ′ , 1 5 ′ - b i s -
(pentafluorophenyl)corrole-3′-yl]-10-(4-methoxycarbonylphenyl)-
5,15-bis(pentafluorophenyl)corrole (3). 1H NMR (500 MHz,
CDCl3) δ 4.07 (s, 6H), 8.21 (d, J = 7.7 Hz, 4H), 8.25 (d, J = 8.0
Hz, 2H), 8.41 (d, J = 8.2 Hz, 4H), 8.47 (d, J = 4.8 Hz, 2H), 8.60 (d, J
= 4.7 Hz, 2H), 8.71 (d, J = 4.7 Hz, 2H), 8.74 (d, J = 4.3 Hz, 2H), 9.16
(s, 2H), 9.17 (d, J = 4.2 Hz, 2H). 19F NMR (376 MHz, CH2Cl2) δ
−167.62 (bs, 2F), −165.29 (m, 2F), −163.97 (m, 4F), −157.03 (bs,
2F), −155.10 (m, 2F), −141.33 (bs, 2F), −139.98 (dd, J1 = 25.9 Hz, J2
= 6.5 Hz, 4F),−139.49 (bs, 2F). Anal. Calcd for (M + H)+, M =
C78H34F20N8O4: 1527.2384; Found ESI-MS: 1527.2322. UV−vis
(toluene), λ in nm (ε in 103 M−1 cm−1): 418 (144), 448 (91), 547
(20), 582 (34), 627 (27), 656 (34).
2,3,17-Tribromo-10-(4-methoxycarbonylphenyl)-5,15-bis-
1
(pentafluorophenyl)corrole (5). H NMR (500 MHz, CDCl3) δ 4.09
(s, 3H), 8.24 (d, J = 8.1 Hz, 2H), 8.45 (d, J = 8.0 Hz, 2H), 8.58 (d, J =
4.8 Hz, 1H), 8.63 (d, J = 4.6 Hz, 2H), 8.73 (d, J = 4.7 Hz, 1H), 9.34 (s,
1H). 19F NMR (376 MHz, CH2Cl2) δ −164.38 (m, 2F), −164.09 (m,
2F), −154.60 (m, 1F), −154.01 (m, 1F), −140.92 (d, J = 27.0 Hz, 2F),
−140.14 (d, J = 27.3 Hz, 2F). Anal. Calcd for (M + H)+, M =
C39H15Br3F10N4O2: 1000.8565; Found ESI-MS: 1000.8555. UV−vis
(toluene), λ in nm (ε in 103 M−1 cm−1): 423 (100), 437 (92), 531
(7.3), 570 (16), 598 (11), 650 (19).
2,3,17,18-Tetrabromo-10-(4-methoxycarbonylphenyl)-5,15-bis-
(pentafluorophenyl)corrole (6). In a 100 mL round-bottom flask, 80
mg (0.10 mmol) of 2 was dissolved in 15 mL of MeCN. A solution of
NBS was prepared by dissolving 87 mg (0.49 mmol) of the compound
in 15 mL of CH2Cl2. The NBS solution was slowly added to the
corrole solution, ∼1 mL at a time using a pipet over the course of ∼5
min. The resultant blue-green solution was stirred at room
temperature for 1 h. The crude reaction mixture was washed with
water, saturated NaHCO3, and brine, then dried over Na2SO4, and
brought to dryness. The residue was purified on a silica gel column
packed with hexanes using hexanes then 1:1 hexanes/CH2Cl2 as the
eluent. The product eluted as a blue-green solution. The solvent was
removed to afford 67 mg (62% yield) of the title compound as a
purple solid. 1H NMR (500 MHz, CDCl3) δ 4.08 (s, 3H), 8.20 (d, J =
8.0 Hz, 2H), 8.43 (d, J = 8.1 Hz, 2H), 8.51 (d, J = 4.7 Hz, 2H), 8.59
(d, J = 4.6 Hz, 2H). 19F NMR (376 MHz, CH2Cl2) δ −164.04 (m,
4F), −154.03 (m, 2F), −140.46 (dd, J1 = 27.9 Hz, J2 = 6.2 Hz, 4F).
Anal. Calcd for (M + H)+, M = C39H14Br4F10N4O2: 1080.7650; Found
ESI-MS: 1080.7699. UV−vis (toluene), λ in nm (ε in 103 M−1 cm−1):
423 (98), 440 (88), 538 (8.1), 575 (19), 607 (13), 655 (19).
1
Physical Measurements. H NMR spectra were recorded at 23
°C on a Varian Inova-500 NMR spectrometer at the Harvard
University Department of Chemistry and Chemical Biology Laukien-
Purcell Instrumentation Center. All spectra were internally referenced
to the residual solvent signal (δ = 7.26 for CHCl3 in CDCl3).58 19F
NMR spectra were recorded at 25 °C on a Varian Mercury-400 NMR
spectrometer and externally referenced to α,α,α-trifluorotoluene (δ =
−63.72). Mass spectra were recorded on a Bruker micrO-TOF-QII
LCMS ESI-TOF mass spectrometer in positive ion mode. All spectra
were externally calibrated with sodium formate. UV−vis absorption
spectra were acquired using a Cary 5000 spectrometer (Agilent).
Steady-state emission spectra were recorded on a Photon Technology
International (PTI) QM 4 fluorometer equipped with a 150 W Xe arc
lamp and a Hamamatsu R2658 photomultiplier tube. Relative
quantum yields of corroles in toluene were calculated using H2TPP
in toluene as the reference according to the following equation
Preparation of Bromocorroles. In a 100 mL round-bottom flask,
82 mg (0.11 mmol) of 2 was dissolved in 20 mL of CH2Cl2. A solution
of N-bromosuccinimide (NBS) was prepared by dissolving 52 mg
(0.29 mmol) of the compound in 10 mL of MeCN. The NBS solution
was slowly added to the corrole solution, ∼1 mL at a time, using a
pipet over the course of ∼5 min. The resultant blue-green solution was
stirred at room temperature for 1 h. The crude reaction mixture was
extracted with CHCl3, washed with water, saturated NaHCO3, and
brine, then dried over Na2SO4 and brought to dryness. The residue
was purified on a silica gel column packed with hexanes using 1:1
hexanes/CHCl3 as the eluent. Fractions were collected in 100 mL
aliquots. Fractions 1−3 were blue-green in color and were identified as
compound 5 by 1H NMR, affording 27 mg (25% yield) of the product.
Fractions 5−9 were violet in color and were identified as compound 4
by 1H NMR, affording 35 mg (35% yield) of the product. Later-eluting
2
⎛
⎝
⎞
⎠
⎛
⎜
⎝
⎞
⎟
⎠
η
η
∇
sam
sam
⎜
⎜
⎟
⎟
Φ
= Φ
sam
ref
∇
ref
(1)
ref
where ∇ is the slope of the plot of integrated fluorescence intensity
versus absorbance (constructed using 5 points), η is the refractive
index of the solvent (taken to be 1.4961 for toluene),59 and Φref is the
emission quantum yield of the reference. Φref was taken to be 0.11 for
an aerated sample of H2TPP in toluene.60 Corrole samples for lifetime
(τ0) measurements and transient absorbance spectra were prepared
B
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Figure 1. Synthesis of free-base corrole 2 and subsequent bromination with NBS to afford a series of β-brominated corroles 4−6.
using three freeze−pump−thaw (fpt) cycles at vacuum pressures
below 10−4 Torr.
thereby correcting for both sample emission and extraneous
background light. To acquire these four spectra, pump and probe
beams were selectively exposed to the sample using electronically
controlled shutters (Uniblitz T132, Vincent Associates), which were
triggered using a Stanford Research Systems DG535 delay generator
synchronized to the Q-switch output of the laser. For TA single
wavelength kinetics, output signal from the sample was passed through
a 450 nm long pass filter and then amplified by a photomultiplier tube
(R928, Hamamatsu) and collected on a 1 GHz digital oscilloscope
(9384CM, LeCroy); acquisition was triggered using a photodiode to
collect scattered laser excitation light.
Computational Details. Density functional theory (DFT)
calculations were performed with the hybrid functional Becke-3
parameter exchange functional65−67 and the Lee−Yang−Parr nonlocal
correlation functional (B3LYP)68 as implemented in the Gaussian 09,
Revision D.01 software package.69 For light atoms (H, C, N, O, and
F), a polarized split-valence triple-ζ basis set that includes p functions
on hydrogen atoms and d functions on other atoms (i.e., 6-311G(d,p)
or 6-311G** basis set) was used. The LANL2DZ basis set was used
for bromine atoms with the inclusion of an effective core potential
(ECP).70,71 All calculations were performed with a polarizable
continuum (PCM) solvation model in toluene using a polarizable
conductor calculation model (CPCM).72,73 All geometries were
confirmed as local minima structures by calculating the Hessian
matrix and ensuring that no imaginary eigenvalues were present.
Excited-state calculations were performed using time-dependent DFT
(TD-DFT)74−78 with the same functionals, basis sets, and solvation
details as the ground state, but with the inclusion of diffuse functions
on all light atoms (i.e., 6-311++G** basis set). Excited-state energies
were computed for the 10 or 15 lowest singlet and triplet excited
states. All optimized geometries and molecular orbitals were rendered
in the program Avogadro.79 Simulated UV−vis spectra were generated
in the program Gauss View 5 by broadening transition lines with
Gaussian functions with a half width of 0.08 eV.
Femtosecond emission lifetime measurements were acquired using
a Libra-F-HE (Coherent) chirped−pulse amplified Ti:sapphire laser
system.61 The 800 nm laser output was used to pump an OperA Solo
(Coherent) optical parametric amplifier (OPA); excitation pulses of
400 nm were produced via fourth harmonic generation of the signal
using a BBO crystal, and the pulse power was attenuated to 2−6 mW
at the sample using neutral density filters. Emission lifetimes were
measured on a Hamamatsu C4334 Streak Scope streak camera, which
has been described elsewhere.62 The emission signal was collected
over a 140 nm window centered at 680 or 700 nm with 50, 20, 2, or 1
ns time windows using a Stanford Research Systems DG535 delay
generator.
Nanosecond transient absorption (TA) spectra of corroles were
acquired using a previously reported system.63,64 Pump light was
provided by the third harmonic (355 nm) of a Quanta-Ray Nd:YAG
laser (Spectra Physics) operating at 10 Hz. The pump light was passed
through a BBO crystal in an optical parametric oscillator (OPO),
yielding a visible frequency that was tuned to 430 nm to excite the
sample. Excitation light was attenuated to 100−300 μJ per pulse for all
experiments using neutral density filters. Probe white light was
generated using a 75 W Xe-arc lamp (PTI). The probe beam was
aligned with the sample, while the laser pump beam was positioned at
15° with respect to the white light probe, and both beams were
focused on the sample. After exiting the sample, the light entered
either a Triax 320 (Jobin Yvon Horiba) or a iHR320 monochromator
(Horiba Scientific) and was dispersed by a blazed grating (500 nm,
300 grooves/mm) centered at 500 nm. The entrance and exit slits of
the monochromator were set to provide a spectral resolution of 4 nm.
TA spectra were collected using a gated intensified CCD camera
(DH520−25F-01, Andor Technology). Acquisition delays and gate
times for the CCD were set using a Stanford Research Systems DG535
delay generator, which was synchronized to the Q-switch output of the
laser. The final data were calculated from a combination of four
spectra: I (pump on/probe on), IF (pump on/probe off), I0 (pump
off/probe on), and IB (pump off/probe off). The resultant TA spectra
were obtained using the following equation:
X-ray Crystallographic Details. Diffraction-quality crystals of 2
were obtained by slow evaporation of a 1:1 mixture of pentane and
THF under N2 at 20 °C, affording crystals as purple blocks; a single
crystal was cut from a larger one for the X-ray diffraction study.
Crystals of 3 and 6 were obtained by dissolving each compound in a
minimal amount of a 1:1 mixture of pentane and THF and cooling the
resultant solutions to −40 °C, affording crystals as purple blocks. Low-
temperature (100 K) X-ray diffraction data were collected on a Bruker
⎛
⎜
⎝
⎞
⎟
⎠
I − IF
I − I
ΔOD = −log
(2)
0
B
C
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three-circle platform goniometer equipped with an Apex II CCD
detector and an Oxford cryostream cooling device, performing φ and
ω scans. Radiation was generated from a graphite fine focus sealed
tube Mo Kα (0.710 73 Å) source. Crystals were mounted on a
cryoloop using Paratone-N oil. Data were processed and refined using
the program SAINT supplied by Siemens Industrial Automation.
Structures were solved by intrinsic phasing methods in SHELXT and
refined by standard difference Fourier techniques in the SHELXTL
program suite.80 Hydrogen atoms were located in the difference map
and were refined isotropically using a riding model; all non-hydrogen
atoms were refined anisotropically. The structure of 3 contains a
pentafluorophenyl group that is disordered over two positions in a
ratio of ∼0.6:0.4, as well as three molecules of disordered THF and
one molecule of disordered pentane per molecule of 3. Since the
solvent disorder could not be modeled satisfactorily, the associated
electron density was removed using the program SQUEEZE. The
structure of 6 contains two molecules of THF, one molecule of N-
methyl-2-pyrrolidone, and one disordered molecule of pentane per
two molecules of 6. Since the disordered pentane molecule of 6 could
not be satisfactorily modeled, this electron density was removed using
the program SQUEEZE. Unit cell parameters, morphology, and
solution statistics for the structure are summarized in Supporting
Information, Table S1. Thermal ellipsoid plots are drawn at the 50%
probability level with hydrogen atoms removed for clarity.
the extent of bromination is proportional to the amount of
added NBS.85 Unlike H3TPFC, we find that treatment of
corrole 2 with NBS is not straightforward. Addition of small
volumes of NBS to 2 prompted the reaction mixture to turn
from violet to blue-green, and the observed fluorescence
intensity of the solution, as observed using a hand-held UV
lamp (365 nm) as the excitation source, dramatically decreased.
In an attempt to better control the distribution of
brominated products, the amount of NBS was varied from
1.9 to 2.7 equiv (Figure 1). In all cases, a mixture of products
(mono-, di-, and tribromo corroles) was obtained. However,
this reaction, as with nearly all corrole peripheral functionaliza-
tion reactions,26 is regioselective, producing four unique
species: 2-bromo-2, 3-bromo-2, 4, and 5. This result suggests
that bromination is not sequential and that all products are
formed in the course of the reaction. Indeed, monobromo
corroles were identified in reaction mixtures that used more
than 3 equiv of NBS. The best results were achieved when
using 2.6 equiv of NBS. All of these species have very similar Rf
values by thin-layer chromatography (TLC, 1:1 hexanes/
CHCl3) and appear as a single spot on the TLC plate.
Fortunately, each species has a distinctive color, making it
possible to separate these compounds by column chromatog-
raphy. Compound 5 elutes first as a blue-green solution,
followed by 4 as a violet solution with a gold-amber hue (see
TOC image). These two products were obtained as pure
species by careful column chromatography. The two mono-
brominated corroles (violet-brown solution) eluted next as a
RESULTS
■
Corrole Synthesis and Structure. Bromination of the
corrole periphery affords a convenient method of functionaliza-
tion and a useful synthetic handle for cross-coupling chemistry
as a means of further elaboration of the corrole macro-
cycle.21,81−87 Compound 2 was prepared by the HCl-catalyzed
condensation of dipyrromethane 1 and methyl 4-formylben-
zoate (Figure 1), following the procedure of Koszarna and
Gryko.88 This method furnishes corroles in high yield (up to
32% yield for 5,10,15-triphenylcorrole) and facilitates the
preparation of trans-A2B corroles without scrambling (i.e.,
circumventing the formation of the cis-A2B isomer). The
desired product was obtained in 51% yield, giving quantities of
∼1 g in a typical synthesis. This yield is greatly improved over
previous preparative methods,51 which used TFA as the acid
catalyst and gave 2 in 18% yield (∼300 mg of isolated product).
An additional fluorescent macrocyclic product, obtained in
17% isolated yield, was assigned as the 3,3′ linked dimer 3 on
1
mixture, as identified by H HMR, along with decomposition
products. These late-eluting fractions were discarded because
each contained a mixture of several products, as evidenced by
1
multiple methyl ester resonances in the H HMR spectrum.
Monobrominated corrole isomers are inseparable,85 and
therefore no efforts were made to further purify or characterize
these compounds, as mixtures are not suitable for photo-
physical characterization. Small fraction volumes were collected
to ensure that pure compounds were isolated. We note that
reported yields represent the isolated yields of pure
compounds; the actual reaction yields are presumably higher,
as fractions identified as mixtures of products were discarded.
Compound 6 was isolated as the exclusive product when more
than 4 equiv of NBS were used. This suggests that the excess
NBS reacts with any remaining corrole with fewer than four
bromine atoms to give a single product. Moreover, this result
also demonstrates that the unique axis of the corrole (i.e., the 2,
3, 17, and 18 positions) is the most reactive and that excess
NBS does not react at the opposite side (i.e., the 7, 8, 12, and
13 positions), a result that is consistent with known corrole
reactivity.26
1
the basis of mass spectrometry and H NMR. Seven unique β-
1
pyrrole protons in the H NMR spectrum established that the
lower-symmetry 2,3′-linked dimer had not formed. The
chemical shift of the singlet (δ 9.16 ppm), attributed to the
proton adjacent to the corrole−corrole linkage, suggested a 3,3′
linkage as it compared well to δ 9.12 ppm for the 3,3′ dimer of
5,10,15-tris(pentafluorophenyl)corrole (H3TPFC).89 A reso-
nance comparable to that of the 2,2′ dimer of H3TPFC (δ 9.66
ppm) was also absent,90 precluding formation of this derivative
as a byproduct. H3TPFC has recently been observed to
undergo regioselective oxidative coupling in the presence of
quinone oxidants to give 3,3′-linked oligomers,91 thus offering a
plausible explanation for the formation of 3 as a byproduct in
the preparation of 2.
Serial bromination of 2 generated brominated corroles 4−6.
The majority of bromination reactions have been performed on
metallocorroles by using a significant excess of Br2 to deliver
the β-octabromo derivatives.81−83 A notable exception is
germanium corrole, which gave hexabromo, pentabromo, or
tribromo derivatives (each as a single regioisomer), depending
on the amount of Br2 added.84 As an alternative to Br2, there
has been a report on the bromination of H3TPFC using NBS;
Diffraction-quality crystals of compounds 2, 3, and 6 were
obtained as purple blocks from THF and pentane. The thermal
ellipsoid plots of the refined structures are shown in Figures 2,
3, and 4, and a summary of the crystallographic data is
presented in Supporting Information, Table S1. The solid-state
structure of 2 (Figure 2) exhibits structural metrics that fall
within the range observed for other C6F5-substituted free-base
corroles.19,92,93 Three of the four pyrrole rings are nearly
coplanar, while the fourth cants out of the plane and hydrogen
bonds with a THF solvent molecule. The macrocycle has a
mean deviation of 0.170 0.144 Å from the mean 23-atom
plane, with a maximum deviation of 0.501 Å for the pyrrole that
participates in hydrogen bonding. The solid-state structure of 3
unambiguously confirms the 3,3′ linkage of the dimer (Figure
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Figure 2. Solid-state structure of 2, showing a THF molecule
hydrogen bonded to the corrole core. Thermal ellipsoids are drawn at
the 50% probability level, and carbon-bound hydrogen atoms were
removed for clarity.
Figure 4. Solid-state crystal structure of compound 6 showing the two
independent corrole molecules in the asymmetric unit. Thermal
ellipsoids are drawn at the 50% probability level, and carbon-bound
hydrogen atoms were removed for clarity. One corrole is highly
nonplanar, exhibiting a molecule of N-methyl-2-pyrrolidone bound to
the core (upper), while the other corrole (lower) features a hydrogen
bonded THF molecule, akin to the structure of 2 (Figure 2).
nonplanarity. One molecule (Figure 4, top) is highly nonplanar,
exhibiting a mean deviation of 0.259 0.214 Å from the mean
23-atom plane, with a maximum deviation of 0.639 Å (a pyrrole
carbon bearing a bromine atom). This large deviation may
originate from a crystal packing effect, as this molecule has a
molecule of N-methyl-2-pyrrolidone hydrogen bonded to the
macrocycle core. Two pyrrole units cant out of the plane to
result in this large deviation from planarity. Conversely, the
other corrole (Figure 4, bottom) has a THF molecule hydrogen
bonded to the macrocycle core; it exhibits mean deviation of
Figure 3. Solid-state crystal structure of compound 3. Thermal
ellipsoids are drawn at the 50% probability level. The lower structure
highlights the π-aryl interaction by showing the centroids of the
relevant rings and their corresponding distances. To better illustrate
this conformation, one corrole unit is drawn with ellipsoids while the
other uses capped sticks. Additionally, the 3,3′ linkage is highlighted in
red. For clarity, hydrogen atoms and the 10 meso substituents were
removed from the lower structure.
0.126
0.111 Å from the mean 23-atom plane, with a
maximum deviation of 0.388 Å (a pyrrole carbon bearing a
bromine atom). This observation is consistent with what is
observed in the solid-state structure of 2 and may be more
representative of corrole conformations.
Absorbance Spectroscopy. The electronic absorption
spectra of corroles 2, 4, 5, and 6, which are shown in Figure 5a,
exhibit intense Soret or B bands in the near-UV and weaker Q
bands in the visible spectral regions. Supporting Information,
Table S2 lists the absorbance maxima for these compounds.
The Soret band of 2 is quite broad (43 nm fwhm), and the line
shape displays a plateau rather than a Gaussian profile,
suggestive of overlapping transitions (i.e., Bx and By polar-
izations), which is expected for lower-symmetry corrole
molecules. With the inclusion of two bromine atoms along
the same face of the corrole (compound 4), the Soret red-shifts
by 3 nm and broadens (46 nm fwhm), establishing that the Bx
and By transitions are energetically more separated upon
bromination. Sequential bromination further separates these
transitions such that the two bands are clearly resolved in 6
(Supporting Information, Figure S1a). In this progression of
compounds, the edge defined by the 2, 3, 17, 18 positions of
the molecule is perturbed, while the other edge (2, 3, 7, 8
positions) is much less perturbed. For clarity, the y axis is taken
3, top). The structure is highly contorted to enable a favorable
π-aryl interaction between a pentafluorophenyl ring and a
pyrrole unit of the macrocycle (Figure 3, bottom), giving 3.435
and 3.547 Å distances between the centroids of these rings.
This is atypical of meso-aryl corroles and porphyrins, which
usually have these substituents orthogonal to the macrocycle
plane. This close contact results in a 42.09° separation between
the mean 23-atom planes of the corrole units. This is
comparable to that observed for the 3,3′-linked dimer of
H3TPFC (CCDC No. 842519),91 which displays a 47.60°
separation of the corrole planes. Despite this close contact, the
corrole units adopt a highly planar structure, with three of the
pyrrole units being largely coplanar, while the fourth pyrrole
unit (i.e., the one participating in dimer formation) cants out of
the corrole plane. The crystal structure of 6 displays two
corroles in the asymmetric unit that display differing degrees of
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peaks when the compound is dissolved in solvents of increasing
dielectric constant (Figure 5b), as has also been observed for
both 5,10,15-triphenylcorrole (H3TPC) and 5,10,15-tris(4-
methoxyphenyl)-corrole (H3TMPC).43
The steady-state absorbance spectrum of dimer 3 (Figure 6)
is comparable to the monomeric corroles, except the optical
Figure 6. Steady-state absorption (red) and emission (blue) spectra
(λexc = 560 nm) of dimer 3 in toluene. Qualitatively, these spectra are
similar to that of the monomer 2, but the transitions are more intense;
an additional feature at ∼750 nm is observed, indicative of a dimeric
structure.
absorption transitions are considerably more intense than the
corrole monomer. This increased optical absorbance is due to
the presence of two fluorophores in a single molecule. The
Soret band displays a shoulder, indicative of two overlapping
transitions. The most intense peak of 3 is blue-shifted by 338
cm−1 relative to 2. Conversely, all of the Q transitions of 3 are
red-shifted relative to 2: 517 and 766 cm−1 for Qy transitions
and 311 and 381 cm−1 for Qx. Additionally, compound 3
exhibits a feature at 731 nm (ε = 5700 M−1 cm−1) that is
unique to a dimeric structure. A similar feature has been
observed for corrole dimers and higher order oligomers,89,91
and it is likely due to transitions between corrole subunits.
Fluorescence Spectroscopy. Steady-state emission spec-
tra (λexc = 440 nm) were recorded for toluene solutions of
compounds 2, 4, 5, and 6. Table 1 summarizes the emission
data for these corroles, and Figure 7a displays the normalized
emission spectra, which exhibit an intense band with a
vibrational shoulder to the red; expansion of the Q-band
region for compound 2 is presented in Supporting Information,
Figure S2. On the basis of the assignment of the Q-band
absorption profile, the intense fluorescence band is assigned to
Qx(0,0), and the shoulder is assigned to Qx(0,1). This
nomenclature is consistent with that used to describe
fluorescence transitions for free-base porphyrins.94 A constant
energy gap of ∼1300−1400 cm−1 between Qx(0,0) and Qx(0,1)
Figure 5. (a) Comparison of the steady state absorption spectra of a
series of corroles in toluene: 2 (red ), 4 (yellow ), 5 (green ),
and 6 (blue ). With an increasing extent of bromination, the Soret
band splits, and these transitions exhibit a spectral red shift. See
Supporting Information, Figure S1 for an expansion of the Soret and
Q-band regions. (b) Steady-state absorption spectra of 2 in toluene
(red − −), PhCN (maroon - - -), and DMSO (purple ).
as the C2 pseudorotational axis and is aligned through the 10
position, whereas the x axis is perpendicular to it in the plane of
the corrole and passes through the 5 and 15 positions.
All four corroles exhibit four Q bands in the 500−700 nm
region. Consistent with what is observed for porphyrins, there
are two fundamental bands Qx(0,0) and Qy(0,0) at 565 and 650
nm, respectively, each with vibrational overtones Qx(1,0) and
Qy(1,0) at 530 and 600 nm, respectively (Supporting
Information, Figure S1b). This nomenclature is in line with
that used to describe porphyrin transitions. An energy
difference of ∼1200−1350 cm−1 is observed for each Q(1,0)
and Q(0,0) pair in this series of molecules, with an average
energy separation of 1277 cm−1 63 cm−1; this is consistent
with the energy separation of ∼1250 cm−1 observed for
porphyrins.94 Additionally, there is an energy spacing of
∼2100−2200 cm−1 between corresponding x and y bands
(i.e., Qx(0,0) and Qy(0,0)) with an average separation of 2148
cm−1 64 cm−1. The splitting of x and y Q bands is ∼3000
cm−1 for porphyrins, and this separation is known to be smaller
for other tetrapyrrolic macrocycles.94
is observed, with an average separation of 1335 cm−1
32
cm−1, which is comparable to that observed between Qx(0,0)
and Qx(1,0) in the absorption spectra. The observed Stokes
shift between the Qx(0,0) absorption and emission features is
∼400 cm−1 for 2, 4, and 5, whereas that of 6 is ∼650 cm−1.
Paralleling trends in the absorption profile, the emission spectra
exhibit a red shift with increasing bromination. As observed for
the absorbance Q bands, the steady-state emission Q features
for 3 are red-shifted relative to 2 by 644 and 795 cm−1 for
Qx(0,0) and Qx(0,1), respectively. The fluorescence quantum
While the x and y polarizations of these compounds should
be distinguished as a result of symmetry, the Soret band of
corroles does not typically reflect an (x, y) polarization, and a
single, sometimes broad, absorption feature is observed, as in
the case of 2. The Soret band clearly resolves to two distinct
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Table 1. Photophysical Data for Free-Base Corroles
a
b
b
c
d
e
e
corrole
Qx(0,0)
Qx(0,1)
τo (ps)
Φf × 102
kr (s−1
)
knr (s−1
)
f
2
4
5
6
3
657
664
668
684
686
722
728
732
770
766
4111 24
15
4.06 × 107
4.74 × 107
4.15 × 107
5.66 × 107
8.86 × 107
2.30 × 108
4.26 × 109
8.10 × 109
1.20 × 1010
2.40 × 108
235
121
84
7
3
3
1.1
0.51
0.47
27
3285 20
a
b
c
Toluene solution, transition wavelengths are in units of nm. Observed transitions with λexc = 440 nm. Freeze−pump−thawed samples (<10−4
Torr) with λexc = 400 nm. Fluorescence quantum yield, relative to H2TPP in toluene (ϕf = 0.11). Calculated using eq 3. 95% confidence interval.
d
e
f
exponential-modified Gaussian (R2 > 0.99). Using eq 3, the
radiative and nonradiative rate constants kr and knr, respectively,
may be determined for each compound:
kr
Φ =
= krτ
kr + knr
(3)
where Φ is the is emission quantum yield, and τ is the natural
fluorescence lifetime. Whereas kr is effectively invariant for all
monomeric corroles, knr increases significantly with bromina-
tion. Although the quantum yield of 3 is nearly twice that of 2,
the radiative lifetime is similar, indicating that additional
deactivation pathways are present in the dimer.
Transient Absorption Spectroscopy. In the absence of
phosphorescence, nanosecond transient absorption (TA) was
utilized to probe the spectral features of the triplet state and its
relaxation dynamics. The TA spectra of corroles 2 and 4−6 in
toluene, shown in Figure 8 for aerated samples and Supporting
Information, Figures S4−S7 for freeze−pump−thawed (fpt)
samples, were recorded by pumping the Soret band of the
ground state absorption spectrum (λexc = 430 nm). The spectral
data is summarized in Table 2. The TA spectra are dominated
by bleaches of the ground state Soret and Q bands with a
concomitant growth of two features: one centered at ∼380 nm
and the other at ∼470 nm. The normalized triplet absorption
spectra of compounds 2 and 4−6 is presented in Supporting
Information, Figure S8. The two major features in the spectrum
red shift upon bromination, although the observed differences
among the brominated derivatives 4−6 are nominal. This
suggests that the nature of the triplet state is quite similar across
this series of corroles. Consistent with trends from the ground
state electronic absorption spectrum, the higher energy band of
3 in the TA spectrum (Supporting Information, Figures S9 and
S10) is nominally (∼140 cm−1) blue-shifted whereas the lower
energy band around 505 nm is significantly red-shifted (∼1660
cm−1) relative to that observed for 2.
Figure 7. Steady-state emission for a series of corroles in toluene: 2
(red), 4 (yellow), 5 (green), and 6 (blue). (a) Normalized steady-state
emission spectra (λexc = 440 nm), demonstrating a red shift in the
emission maximum. (b) Absorbance-matched (A560 = 0.100 0.002)
emission spectra (λexc = 560 nm), reflecting fluorescence quenching
upon bromination. For clarity, the emission intensity of compounds 4,
5, and 6 was increased by a factor of 10.
The temporal evolution of the TA spectra was determined
from monitoring single wavelength kinetics for the prominent
spectral feature at ∼470 nm for each species. Lifetime data are
given in Table 2. Compounds 4−6 obey monoexponential
yield also exhibits a pronounced decrease with increasing
bromination. Figure 7b shows the steady-state emission spectra
(λexc = 560 nm) for absorbance-matched toluene solutions of 2,
4−6 at the excitation wavelength of λexc = 560 nm (A560 = 0.100
0.002). The fluorescence quantum yields are provided in
Table 1, where eq 1 was used to determine Φ with H2TPP in
toluene as the reference (Φref = 0.11). No phosphorescence was
observed from the triplet manifold of these corroles.
Consistent with emission intensity results, the emission
lifetime also exhibits a marked decrease with bromination. The
fluorescence lifetimes of nonbrominated corroles listed in Table
1 were determined from fitting the data with a monoexponen-
tial decay (R2 > 0.99). For the brominated derivatives, the
lifetimes were so short that the decay is on the order of the
instrument response function, as illustrated in Supporting
Information, Figure S3. As a result, these traces were fit with an
2
decay kinetics (Radj > 0.99) whereas 2 and 3 exhibit
biexponential decay kinetics with the shorter lifetime
component dominating the fit. The biexponential decay
indicates the presence of two distinct triplet states for 2 and
3. The lifetime of the triplet excited state decreases with
increasing bromination. Introduction of oxygen into solution
leads to significant quenching of the long-lived triplet feature in
all samples. The decreased lifetime furnishes a diffusional rate
constant (kq ≈ 2 × 10−9 M−1 s−1) for the quenching of the
triplet excited state by oxygen.
Density Functional Theory Calculations. DFT calcu-
lations were performed using the B3LYP functional to gain
insight into the nature of the electronic structure and the effect
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Figure 8. Nanosecond transient absorption (TA) spectra (λexc = 430 nm) of aerated toluene solutions of (a) 2, (b) 4, (c) 5, and (d) 6, showing the
bleach of the ground state with concomitant growth of the triplet state. The temporal evolution of each spectrum is from 20 to 735 ns in 65 ns
intervals. The corresponding spectra of fpt samples are presented in Supporting Information, Figures S4−S7.
Table 2. Summary of Transient Absorption Kinetics Data for Corroles
a
b
b
c
d
d
corrole
Abs1
Abs2
τo1 (μs), %
τo2 (μs), %
τair (ns)
kq (M−1 s−1
)
e
2
4
5
6
3
377
385
384
382
375
465
470
474
473
504
151 5 (21)
12.2 0.1 (100)
8.8 0.1 (100)
5.4 0.1 (100)
19.2 0.6 (79)
230
263
283
298
267
3
3
5
2
5
2.2 × 109
1.9 × 109
1.8 × 109
1.6 × 109
1.9 × 109
88.3 9.1 (29)
22.8 1.3 (71)
a
b
c
Toluene solution, transition wavelengths are in units of nm. Measured from transient absorption spectra with λexc = 430 nm. Freeze−pump−
d
e
thawed samples (<10−4 Torr). Relative contribution to the biexponential fit. 95% confidence interval.
of bromination on the frontier molecular orbitals. Ground-state
geometry optimization for compounds 2 and 4−6 was carried
out using the 6-311G** basis set and a CPCM solvation model
in toluene. The optimized structures, which are presented in
Supporting Information, Figures S11−S14 with the Cartesian
coordinates listed in Supporting Information, Tables S3−S6,
were verified as local minima by performing frequency
calculations and ensuring that there were no imaginary
frequencies. Qualitatively, the structures are quite similar and
exhibit a single pyrrole ring (the so-called A or D ring) that
largely deviates from the mean 23-atom plane, irrespective of
bromination. The calculated structure of 2 is consistent with
the solid-state structure of 2 and other free-base corroles, as
typified by H3TPFC.19,92,93 In these crystal structures, three of
the pyrrole units are nearly coplanar while the fourth (the A or
D ring) cants out of the plane. The root-mean-square (RMS)
deviation between the calculated and solid-state structures of 2
is 0.430 Å; the overlay of the structures is presented in
Supporting Information, Figure S15. Although the calculated
distortion of the macrocycle upon bromination is not as
pronounced as it is in the crystal structure (Figure 4, top), the
solid-state (Figure 4, bottom) and calculated structures of 6 are
quite similar (Supporting Information, Figure S16), with an
RMS deviation of 0.566 Å. This larger deviation relative to 2 is
primarily due to a different conformation of the 10-meso
substituent rather than the corrole ring itself.
The frontier molecular orbitals of the corroles (HOMO−1,
HOMO, LUMO, and LUMO+1) are shown in Figure 9 for 2
and Supporting Information, Figures S17−S19 for 4−6.
Qualitatively, the frontier orbitals are all π-type symmetry
with significant orbital density at the β positions. The LUMO
and HOMO−1 orbitals possess the greatest amount of density
at the β positions, suggesting that these two orbitals are most
affected by β substitution. As shown in Supporting Information,
Figure S20 and Table S7, the frontier orbitals are energetically
well-separated from the rest of the molecular orbital manifold.
The calculated HOMO−LUMO gap decreases with increasing
bromination, consistent with the observed red shift of spectral
features: 2.51 eV (494 nm) for compound 2 to 2.43 eV (510
nm) for compound 6 (see Supporting Information, Table S7).
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excited states, as well as the orbital contributions to each
transition, are presented in Supporting Information, Tables
S9−S16. The four lowest-energy singlet excited states give rise
to two Q-like transitions and two Soret-like transitions. These
states (S1−S4) for compounds 2, 4−6 exclusively involve the
four frontier orbitals (with the exception of S4 for 6, which has
an 18% contribution of HOMO−2 → LUMO). The deviation
between the calculated and experimental optical transitions
range from 0.05 to 0.28 eV and is in range with TD-DFT
benchmark studies on test sets of organic molecules, which
demonstrate an absolute mean error of 0.3−0.5 eV.95−97
Although the exact energies of the calculated transitions do not
align precisely with experiment, the predicted UV−vis spectra
(Supporting Information, Figures S25−S28) qualitatively
reflect experimental observations. The Soret region broadens
and subsequently splits with increasing bromination. As
observed experimentally, the Soret bands of 2 and 4 look like
a single, broad transition, while 5 displays a prominent
shoulder, and 6 exhibits two resolved transitions. Additionally,
the oscillator strength of the lower-energy Q-band increases
with increasing bromination, as observed experimentally.
Moreover, the two Q states (S1 and S2) and two Soret states
(S3 and S4) are orthogonally polarized (see Supporting
Information, Tables S17−S20 for the components of the
electronic dipole moment vectors for states S1−S4). This
implies that both the Q and B bands are split into x and y
polarizations due to the low symmetry of the corrole. Thus, the
split Soret band is due to the energetic separation of the Bx and
By transitions. Calculated triplet states are qualitatively similar
to absorbance bands observed experimentally by TA spectros-
copy (see Supporting Information, Figure S8), although the
exact wavelength maxima are slightly shifted.
To interrogate the nature of the excited states of 3, TD-DFT
calculations were performed. Single-point excited-state energy
calculations were performed to determine the 15 lowest-energy
singlet and triplet states using the same methods as those used
with compounds 2 and 4−6. The simulated UV−vis spectrum
is presented in Supporting Information, Figure S29 and the
results of these calculations are summarized in Supporting
Information, Tables S21−S23. Since the dimer has twice the
number of molecular orbitals and they come in nearly
degenerate pairs, there are many more states calculated for
this compound. For example, 15 singlet states appear in the 630
to 390 nm range, whereas there are only four for the
monomeric compounds 2 and 4−6. Thus, it is not surprising
that there are several nearly degenerate states calculated for the
dimer. This phenomenon is particularly prevalent for the triplet
states. Despite the preponderance of states, there are only four
Q-like states and four Soret-like states that have substantial
(>0.05) oscillator strengths, giving a qualitative correspondence
to experimental observations.
Figure 9. Four frontier molecular orbitals calculated for compound 2,
illustrating that all four orbitals possess electron density at the β
positions.
Whereas the trend is captured, the absolute values of the
calculated HOMO−LUMO gap deviate from the observed
energies of the lowest-energy electronic transitions. Conversely,
the observed and calculated spectra are in better agreement
with TD-DFT calculations (vide infra).
The ground-state geometry optimization of the dimer 3 was
performed as a full molecule without truncation (presented in
Supporting Information, Figure S21 and Cartesian coordinates
in Supporting Information, Table S8) and validated as a local
minimum by a frequency calculation. The calculated structure
displays a more splayed geometry than that of the solid-state
structure, with a 65.46° separation between the mean 23-atom
planes of the corrole (as compared to 42.09° of Figure 3). The
π-aryl interaction of the pentafluorophenyl and pyrrole rings is
not captured in the calculation, with a 4.119 Å distance between
the centroids of these rings (compared to 3.435 Å in Figure 3).
This results in a longer center-to-center distance of the corrole
macrocycles: 9.199 Å for the computed structure compared to
8.304 Å for the solid-state structure. The molecular orbitals of 3
are similar to those of the monomer 2 (Supporting
Information, Figure S22), although there are two dimer orbitals
per monomer orbital owing to the presence of two distinct π
systems in the dimer. As a result, there are eight frontier orbitals
that are analogous to the four orbitals for monomeric corroles.
The two nearly degenerate orbitals only differ by the nature of
the lobe along the 3−3′ bond that forms the dimer (see
Supporting Information, Figures S23 and S24). For example,
the only difference between HOMO−3 and HOMO−2 of 3 is
the nature of the lobe along the 3−3′ bond: the HOMO−3 has
a continuous lobe, whereas the HOMO−2 has a node along the
bond. This phenomenon is observed for each pair of molecular
orbitals in the dimer 3.
DISCUSSION
■
Since the introduction of one-pot corrole synthetic method-
ologies,18−21 there has been a renewed interest in corrole
chemistry with regard to the coordination chemistry, reactivity,
and applications of these molecules. Given the diverse and
numerous reports of corroles, H3TPFC has become the
archetypical corrole, just as 5,10,15,20-tetraphenylporphyrin
(H2TPP) is the prototypical porphyrin. The corrole core is
inherently more electron-rich than porphyrin98 and, hence, is
particularly prone to decomposition by prolonged exposure to
light and oxygen. In this regard, H3TPFC has three electron-
TD-DFT calculations were performed using the B3LYP
functional to interrogate the nature of the excited states of these
compounds and correlate the electronic structure to the
absorption spectra. Single-point excited-state energy calcula-
tions determined the 10 lowest-energy singlet and triplet states
for compounds 2, 4−6 using the 6-311++G** basis set and a
CPCM solvation model in toluene. Simulated UV−vis spectra
of these compounds are presented in Supporting Information,
Figures S25−S28. The energy of these singlet and triplet
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Figure 10. The four frontier orbitals for a porphyrin with D4h symmetry, adapted from ref 94. With a descent in symmetry from porphyrin with D4h
symmetry to corrole with C2v symmetry, the degeneracy of the LUMOs is lifted. The schematics for the corrole frontier orbitals are adapted from
DFT calculations reported in refs 103 and 104; the orbital ordering is based on the calculations shown in Figure 9 and ref 105.
withdrawing pentafluorophenyl groups to reduce the electron
density of the tetrapyrrole core, rendering this molecule one of
the most stable (i.e., resistant to decomposition) corroles.99 To
harness the stability of H3TPFC while still featuring a synthetic
handle for further derivatization or conjugation, we targeted the
preparation of corrole 2 and its brominated analogues. This
molecule features two pentafluorophenyl groups for stability as
well as a methyl ester that may be hydrolyzed to a carboxylic
acid, which may subsequently react with a free amine to furnish
more elaborate molecular architectures via amide bond
formation.
nitrogen atoms, thus supporting the assignment of the HOMO
as the orbital of b1 symmetry (Figure 10) and the HOMO−1 of
a2 symmetry.105
The series of compounds 2 and 4−6 provides insight into the
correlation of the absorption spectra with the molecular orbital
picture of Figure 10. A unique feature of corroles is the
observance of the splitting of the Soret into two bands under
various experimental conditions. Deprotonation of a pyrrole
nitrogen has been implicated as the origin of this phenomenon
in polar solvents.18,20,43,92,106 However, there are several
counterexamples in which a split Soret band is observed in
nonpolar solvents such as toluene or dichloromethane.40,107 In
the case of 2, two bands do resolve with increasing polarity of
the solvent (Figure 5b). However, a very strong acid would be
formed if solvent were to deprotonate the macrocycle (e.g.,
pKa(PhCNH+) = −10).108 Alternatively, unique Soret tran-
sitions have been ascribed to the presence of two tautomers in
solution.47,92,107 While tautomerization is fast at room temper-
ature on the NMR time scale,109 the energy difference between
tautomers is too small (ES0 = 33 cm−1) to account for the 970
cm−1 observed splitting.47 Moreover, DFT calculations have
demonstrated that each tautomer displays several Soret-like
transitions.107 Finally, it has been postulated that the split Soret
may be ascribed to nonplanarity of the macrocycle, engendered
by sterically demanding meso substituents,40,110 though
corroles with unhindered substituents also exhibit a split
Soret band.107 Supporting Information, Table S24 provides
structural metrics to assess the planarity of meso substituted
free-base corroles without β substituents. The example with
mesityl substituents, which has the smallest deviation from the
23-atom plane and the smallest dihedral angles of Table S24,
displays a split Soret band in toluene.111 Conversely, corroles
with pentafluorophenyl groups do not exhibit split Soret bands
although they display a greater degree of nonplanarity.112,113 As
a result, there is no strong correlation between the degree of
nonplanarity and splitting of the Soret band. A better
correlation is the asymmetry induced along the x and y axes
of the corrole macrocycle. Differentiation of the x and y axes,
either by structural distortion of the macrocyclic ring, or more
directly by chemical substituents, leads to an energetic
β-Substitution on the pyrrole units of the corrole ring
significantly perturbs the electronic structure by modulating the
relative energies of the HOMO and LUMO levels. A
convenient benchmark for the corrole electronic structure is
the four orbital model of porphyrins shown in Figure
10.94,100−102 The two HOMOs (a1u and a2u) and the two
LUMOs (eg) of π symmetry are energetically well-separated
from all other molecular orbitals. Transitions associated with
the (a2ueg) and (a1ueg) electron configurations, which are π−π*
in nature, lead to the characteristic Soret and Q bands. Ghosh
and co-workers have demonstrated that this four orbital model
holds for corroles as well.103,104 With a descent in symmetry
from D4h for a metalloporphyrin to C2v for a metallocorrole, the
x and y axes of the molecule are inequivalent, resulting in
splitting of both the Q and B bands into x and y components.
Symmetry is further lowered for free-base corroles with their
three interior N−H protons. The four lowest-energy transitions
(in C2v symmetry) are (b1b1) and (a2a2) of A1 symmetry, as well
as (b1a2) and (a2b1) configurations of B2 symmetry;103 they
configurationally mix to result in intense B bands and weak Q
bands, analogous to those observed for porphyrins. Orbital
calculations of an unsubstituted gallium(III) corrole103 and
diboron corrole104 indicate that all four frontier orbitals possess
some electron density at the β-positions. The order of the
orbitals is consistent with the DFT calculations of Figure 9, as
well as the experimental electron paramagnetic resonance
spectrum of an oxidized Ga(III) corrole. This species displays
hyperfine coupling to the nitrogen atoms, demonstrating that
the unpaired electron resides in an orbital with density on the
J
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Article
separation of the x and y polarized transitions, manifesting in a
resolved split of the Soret band. In the cases of 2 and 4−6, this
differentiation is afforded by the bromination of one edge of the
corrole. More subtle effects can also influence the polarization
as well. As shown by the crystal structures of 2 and 6, polar
solvents may interact with the pyrroles of the corrole to lower
the overall symmetry of the system, thus also engendering
differences in the transition dipoles along the molecular x and y
axes.
The differentiation of the (x, y) transition dipole moments is
also reflected in the Q transitions. The lowest-energy Q(0,0)
band is the most perturbed by bromination, exhibiting a
significant red shift (358 cm−1 in going from 2 to 6) with a
concomitant increase in oscillator strength. This is contrasted
with the higher-energy Q(0,0) band, which has similar energy
and oscillator strength for 2, 4, and 5, with significant changes
only observed for 6. If the brominated edge of the macrocycle is
taken as the x axis (2, 3, 17, 18 positions), then the low-energy
Q-band (∼650 nm) is consistent with a Qx(0,0) assignment, as
it is most affected by bromination, whereas the higher-energy
Q-band (∼565 nm) is less affected and is assigned as Qy(0,0).
Similarly, the higher-energy band in the Soret manifold is less
perturbed, whereas the lower-energy band shifts to lower
energy with increasing bromination. These results are
consistent with a y-polarized higher-energy band and an x-
polarized lower-energy band. These band assignments are
consistent with the results of magnetic circular dichroism
spectroscopy.107
Calculated orbital energies are in accordance with the
contentions deduced from spectroscopic trends. TD-DFT
calculations corroborate that the visible absorbance bands
arise from transitions among the four frontier orbitals of Figure
9; the calculated singlet states for 3 constitute transitions
among the analogous eight frontier molecular orbitals of the
dimer. Although the predicted spectra are qualitatively similar
to experiment, the calculations do not accurately predict the
low-energy Q bands. A single reference method thus does not
account for the significant configuration interaction among the
frontier MO set, and as a result, multireference methods114 are
needed to capture the excited states more accurately.
Using the calculated molecular orbitals, one may attempt to
correlate electron density with the observed product distribu-
tion. Bromination by NBS may proceed by either an
electrophilic aromatic substitution (EAS) or radical mechanism.
If the EAS mechanism is operative, then the corrole HOMO is
likely to participate in the reaction. Conversely, the corrole
LUMO may be more prominent in a radical mechanism. This
orbital has similar density at the 2 and 3 positions (Figure 9)
and may explain the observance of both 2-bromo-2 and 3-
bromo-2. The radical mechanism is consistent with the
observance of a distribution of products when less than 4
equiv of NBS is used. While this correlation is satisfying, no
experiments were performed to determine which mechanism
(or if both) is operative in the bromination reaction, and thus a
correlation between molecular orbital density and reactivity is
tenuous at best.
fluorescence quantum yield with bromination. On the basis of
lifetime data and the natural lifetimes per eq 3, kr is relatively
constant across the series (∼4 × 107 s−1) for 2 and 4−6,
whereas there is a drastic increase in knr from 2 × 108 s−1 for 2
to 1 × 1010 s−1 for 6. Analysis of ln(knr) versus ΔE reveals that
that the nonradiative rate deviates positively from the linear
prediction of the energy gap law (Supporting Information,
Figure S30).115,116 The energy gap law underestimates knr by a
factor of ∼10 per bromine atom. Thus, the perturbation of the
energy of the frontier molecular orbital manifold by β-
substitution is insufficient to rationalize the significant increase
in knr with bromination. The enhanced knr is ascribed to a
bromine heavy atom effect; increased spin−orbit coupling with
increased bromination promotes intersystem crossing and thus
results in a shorter emission lifetimes and lower quantum yields.
In the absence of bromination, knr is well-behaved. These
results are in accordance with those of Gross and co-workers54
who observe a serial red shift of spectral features and a decrease
in fluorescence lifetime with increasing β-iodination of
aluminum and gallium corroles. Dimer 3 has approximately
the same nonradiative rate constant knr as monomer 2. The
higher fluorescence quantum yield of 3 is in line with an
increase in the radiative rate constant (τo = 1/kr + knr), as
reflected by the increased oscillator strength of the dimer.
The photophysical properties of these free-base corroles
form a reference for the properties of the corresponding metal
complexes. By examining the molecular orbitals of Figure 9, it is
likely that the HOMO will be most affected by the presence of
a metal, as this orbital does not have a node at the center. The
extent of perturbation of the absorption properties will depend
of the eletronegativity of the metal and the conformation it
imposes on the macrocycle. As with porphyrins, the emission
properties will be governed by the nature of the metal to render
complexes fluorescent, phosphorescent, or nonemissive. The
DFT calculations of this study indicate that the LUMO and
HOMO−1 are most affected by bromination, and hence serial
bromination of a metallocorrole should lead to a red shift of all
spectral features and an increase in the oscillator strength of the
lowest-energy absorption features, as observed for the free-base
corroles in this study. The precise extent of the perturbation of
bromination on the metallocorroles is under investigation.
CONCLUSIONS
■
The combined experimental and computational examination of
compounds 2−6 has provided insight into the basic photo-
physics of free-base β-substituted corroles. Serial bromination
of the corrole periphery at the β positions greatly perturbs the
four frontier molecular orbital manifold of N4 macrocycles. TD-
DFT calculations corroborate spectral assignments, which are
governed by two Q states and two B (Soret) states with
orthogonal polarization. The Soret band splits upon increasing
bromination as a result of the asymmetry along the x- and y-
axes of the corrole core. This study has helped to contextualize
previous observations of a split Soret band. We contend that
there are always two Soret transitions, as dictated by the
symmetry of the corrole framework. The ability to resolve the
bands depends on the electronic nature of the substituents and
the polarity of the solvent in which the spectrum is obtained.
In addition to the red shift of observed spectral features, the
inclusion of bromine atoms produces a pronounced heavy atom
effect, which is the primary determinant of the photophysical
properties of the free-base corrole. Bromination decreases the
fluorescence quantum yield and lifetime by promoting
Emission spectral features red shift with increasing
bromination. Calculations clearly establish that the primary
orbital contribution in the fluorescence emission is LUMO →
HOMO. The difference in the HOMO/LUMO gap between
compounds 2 and 6 (640 cm−1) is quite similar to the observed
energy difference (600 cm−1) of the Qx(0,0) emission feature
for 2 and 6. Of greater significance is the substantial decrease in
K
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Article
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increase in knr with bromine substitution. A nonbrominated
dimer exhibits absorption and emission features comparable to
the tetrabrominated derivative, suggesting that oligomerization
is a means of red-shifting the spectral properties akin to
bromination but without decreasing the fluorescence quantum
yield. With this understanding of the photophysical properties
of free-base corroles, we are currently exploring applications of
these platforms as energy-transfer acceptors within quantum
dot−corrole conjugates for optical chemosensing applications.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic procedures for the synthesis of compound 1 in
addition to 1H and 19F NMR spectra for all compounds.
Summaries of crystallographic data, TA spectral data, and
results of DFT calculations (including optimized geometries,
molecular orbital illustrations, simulated UV−vis spectra, tables
of Cartesian coordinates, and tables of TD-DFT results). This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: dnocera@fas.harvard.edu.
■
(27) Barata, J. F. B.; Santos, C. I. M.; Neves, M. G. P. M. S.; Faustino,
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Funding
This material is based upon work supported by the U.S.
Department of Energy Office of Science, Office of Basic Energy
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.M.L. acknowledges the National Science Foundation’s
Graduate Research Fellowship Program. We thank Bryce
Anderson and Andrew Maher for assistance with experiments
using the Libra-F-HE laser system as well as Casandra Cox and
Bon Jun . Koo for the images used in the TOC graphic.
Calculations were performed using the Odyssey cluster
supported by FAS Research Computing.
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DOI: 10.1021/ic502860g
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