Synthesis and charge-transfer dynamics in a ferrocene-containing organoboryl aza-bodipy donor-acceptor triad with boron as the hub

Article abstract of DOI:10.1021/acs.inorgchem.5b00494

A N,N′-bis(ferroceneacetylene)boryl complex of 3,3′-diphenylazadiisoindolylmethene was synthesized by the reaction of an N,N′-difluoroboryl complex of 3,3′-diphenylazadiisoindolylmethene and ferroceneacetylene magnesium bromide. The novel diiron complex w

Full text of DOI:10.1021/acs.inorgchem.5b00494

Article
pubs.acs.org/IC
Synthesis and Charge-Transfer Dynamics in a Ferrocene-Containing
Organoboryl aza-BODIPY Donor−Acceptor Triad with Boron as the
Hub
Eranda Maligaspe,†,‡ Tom J. Pundsack,‡,§ Lauren M. Albert,§ Yuriy V. Zatsikha,† Pavlo V. Solntsev,†,§
David A. Blank,*,§ and Victor N. Nemykin*,†
†Department of Chemistry and Biochemistry, University of MinnesotaDuluth, 1039 University Drive, Duluth, Minnesota 55812,
United States
§Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
S
* Supporting Information
ABSTRACT: A N,N′-bis(ferroceneacetylene)boryl complex
of 3,3′-diphenylazadiisoindolylmethene was synthesized by
the reaction of an N,N′-difluoroboryl complex of 3,3′-
diphenylazadiisoindolylmethene and ferroceneacetylene mag-
nesium bromide. The novel diiron complex was characterized
by a variety of spectroscopic techniques, electrochemistry, and
ultrafast time-resolved methods. Spectroscopy and redox
behavior was correlated with the density functional theory
(DFT) and time-dependent DFT calculations. An unexpected
degree of coupling between the two Fc ligands was observed.
Despite a lack of conjugation between the donor and acceptor, the complex undergoes very rapid (τ = 1.7
0.1 ps)
photoinduced intramolecular charge separation followed by subpicosecond charge recombination to form a triplet state with a
lifetime of 4.8 0.1 μs.
nation. Suppression of through-bond coupling was also
intended to allow characterization of through-space coupling
between the two Fc moieties. The resulting molecular
electronic states present more isolation between the donor
and acceptor. However, the proximity and energetic alignment
of charge-transfer and triplet excited states results in an
unexpected, very rapid charge recombination to form a long-
lived triplet excited state with very high efficiency.
Herein we report the preparation and characterization of the
first organoboryl aza-BODIPY donor−acceptor triad 3
(Scheme 1) with B(CCFc)2 fragments. Electronic coupling
between the two Fc ligands is observed despite the relatively
large separation distance. Direct binding of the electron donor
to the acceptor at the boron atom results in photoinitiated
charge transfer in <2 picoseconds (ps) even in the absence of
conjugation between the donor and acceptor. Subsequent
charge recombination takes place even faster. The result is
efficient access to the triplet state mediated by charge transfer,
which extends the excited-state lifetime by 3 orders of
magnitude without the need for heavy atoms to enhance
spin−orbit coupling.
INTRODUCTION
■
The preparation of highly efficient functional materials for solar
energy conversion is of fundamental interest.1−5 Because of
their outstanding absorption properties, numerous porphyr-
inoids coupled with organic or organometallic donor frag-
ment(s) have been investigated in dye-sensitized solar cells and
organic photovoltaics.6−11 The photophysical properties of
ferrocenylporphyrins, phthalocyanines, tetraazaporphyrins, and
subphthalocyanines have recently been investigated by several
research groups, and the ferrocene (Fc)-based substituents
were found to be prominent electron-donating groups.12−33
BODIPYs and aza-BODIPYs (where BODIPY = dipyrrome-
theneboron difluoride) have recently emerged in this field
because of their unique photophysical properties.34−45 Several
Fc-containing BODIPY conjugates with a Fc group linked via a
spacer located at the α- or β-pyrrolic or meso position have
been reported.46−52 However, to the best of our knowledge, no
reports are available on BODIPYs and aza-BODIPYs with Fc
substituents connected via the central boron atom using direct
organometallic B−C bonds. Bonding the Fc ligands at the
boron atom leaves them in close proximity to aza-BODIPY but
without conjugated through-bond coupling. This work was
motivated by the potential to exploit a new balance between the
donor−acceptor coupling for the enhancement of excited-state
characteristics favorable for optoelectronic applications, for
example, attempting to maintain efficient intramolecular charge
separation with the inhibition of subsequent charge recombi-
Received: March 3, 2015
© XXXX American Chemical Society
A
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of stirring, the mixture was quenched by adding 5 mL of water. After
the addition of DCM, the organic layer was washed with water, dried,
and evaporated under reduced pressure. The crude compound was
purified by column chromatography on silica gel with CH2Cl2.
Evaporation of the solvent yielded the metal-incorporated aza-
Scheme 1. Preparation of the Target aza-BODIPYs
1
BODIPY (aza-BODIPY/BF2; 170 mg, 60%). H NMR (500 MHz,
CDCl3): δ 8.12 (d, J = 8.05 Hz, 2H; α-benzo), 7.88 (m, 4H; ortho),
7.63 (d, J = 8.20 Hz, 2H; α-benzo), 7.53, 7.50 [m, meta (4H), para
(2H), β-benzo (2H)], 7.32 (dd, J1 = 7.45 Hz, J2 = 7.53 Hz, 2H; β-
benzo). 13C NMR (125 MHz, CDCl3): δ 153.5 (α-pyrrole), 140.0 (α-
pyrrole), 134.0 (β-pyrrole), 131.6 (β-pyrrole), 131.0 (β-benzo), 130.7
(para), 130.5 (ortho), 128.8 (meta), 127.6 (β-benzo), 124.5 (α-
benzo), 121.3 (α-benzo). UV−vis (toluene): λmax = 715 nm.
Synthesis of N,N-bis(ferrocenylboroyl)[N-(3-phenyl-2H-iso-
indol-1-yl)-N-(3-phenyl-1H-isoindol-1-ylidene)amine] (3).
Boron trichloride (1.3 equiv, 228 μL of a 1.0 M solution in hexanes)
was slowly added to a solution of 1 (1.0 equiv, 70 mg, 0.17 mmol) in
anhydrous toluene (20 mL) under an argon atmosphere. The reaction
mixture was stirred at room temperature for 1 h. Subsequently,
ethylmagnesium bromide (3 mol equiv, 0.36 mmol, 0.362 mL) was
added to a solution of ethynylferrocene (3.5 equiv, 170 mg, 0.809
mmol) in anhydrous THF (5 mL). The resulting mixture was heated
at 60 °C. After 2 h of refluxing, the Grignard mixture was cooled to
room temperature and then transferred via a cannula to a solution
mixture of aza-BODIPYH and BCl3. The resulting mixture was stirred
for 5 h. The solvent was evaporated, and the product was purified
using a chromatography column packed with flash neutral alumina,
using hexanes/DCM (25:75, v/v) as the eluent. Yield: 35.2 mg (21%).
Elem anal. Calcd for C52H36BN3Fe2·0.17CH2Cl2·0.16C6H14: C, 74.79;
EXPERIMENTAL METHODS
■
Reagents and Materials. All reactions were performed under an
argon atmosphere using standard Schlenk techniques. Solvents were
purified by standard methods: tetrahydrofuran (THF) was distilled
over a sodium−potassium alloy, toluene and diethyl ether were
distilled over sodium metal, dichloromethane (DCM) and hexanes
were distilled over calcium hydride under reduced pressure, and
methanol was distilled over magnesium under an argon atmosphere.
Bromobenzene, magnesium, NH4Cl, diisopropylethylamine, BF3·OEt2,
a BCl3 solution in hexanes, ethylmagnesium bromide, and
ethynylferrocene were purchased from Aldrich and used without
further purification. 1,2-Dicyanobenzene was purchased from Acros
Organics and used without further purification. Silica gel (60 Å, 60−
100 μm) was purchased from Dynamic Adsorbents Inc. Neutral
alumina (50−200 μm) was purchased from Sorbent Technologies.
Tetrabutylammonium tetrakis(pentafluorophenyl)borate (TBAF) was
used in anhydrous DCM for electrochemical studies, after preparation
according to literature procedures.53,54
Synthesis of N-(3-Phenyl-2H-isoindol-1-yl)-N-(3-phenyl-1H-
isoindol-1-ylidene)amine (1). Compound 1 was prepared using a
slightly modified procedure that was reported previously.39 The
Grignard reagent of phenylmagnesium bromide was prepared from 2.0
g (83 mmol) of magnesium in dry ether (5 mL) and 8.5 mL of
bromobenzene at 0 °C (magnesium was activated by placing a small
crystal of iodine in the flask and heated to observe a violet color). The
resulting mixture was stirred for a further 1.5 h. The solution of
phenylmagneisum bromide was added via cannula to a vigorously
stirred dry toluene solution (30 mL) of 1,2-dicyanobenzene (4.25 g,
33.2 mmol) at 0 °C. After 2 h of stirring, 10 mL of a saturated
ammonium chloride solution was slowly added to the resulting
mixture. An additional 10 mL of water was added to the mixture, and
the residue was distilled with water steam. The residue was filtered
with hot water, and a blue solid was dried. The compound was purified
by silica gel column chromatography using hexanes/DCM (15:85, v/
v) as the eluent. Evaporation of the solvent yielded a blue solid. Yield:
1.55 g (20.1%). 1H NMR (500 MHz, CDCl3): δ 8.15 (d, J = 4.11 Hz,
2H; α-benzo), 8.04 (dd, J = 8.01 Hz, 4H; ortho), 7.97 (d, J = 4.01 Hz,
2H; α-benzo), 7.58 (m, 4H; meta), 7.50 (m, 2H; para), 7.43 (dd, J1 =
7.30 Hz, J2 = 7.22 Hz, 2H; β-benzo), 7.34 (dd, J1 = 7.13 Hz, J2 = 7.13
Hz, 2H; β-benzo). 13C NMR (125 MHz, CDCl3): δ 148.6 (α-pyrrole),
143.9 (β-pyrrole), 135.4 (β-pyrrole), 133.2 (phenyl), 129.9 (β-
pyrrole), 129.4 (para), 129.2 (meta), 127.7 (ortho), 127.6 (β-
benzo), 126.3 (β-benzo), 122.1 (α-benzo), and 121.3 (α-benzo).
UV−vis (toluene): λmax = 655 nm.
1
H, 4.56; N, 4.92. Found: C, 74.79; H, 4.56; N, 4.63. H NMR (500
MHz, CDCl3): δ 8.28 (d, J = 8.40 Hz, 4H; ortho), 8.17 (d, J = 8.10 Hz,
2H; α′-benzo), 7.62 (m, 8H; α″-benzo, para, meta), 7.51 (m, 2H; β′-
benzo), 7.31 (m, 2H; β″-benzo), 4.03 (5H, s, Cp), 3.98 (2H, m, α-
Cp), 3.90 (2H, m, β-Cp). 13C NMR (125 MHz, CDCl3): δ 152.6 (α-
pyrrole), 137.4 (β-pyrrole), 133.4 (β-pyrrole), 132.0 (β-pyrrole), 131.9
(para), 131.2 (β-benzo), 129.6 (ortho), 129.20 (meta), 128.3
(phenyl), 127.8 (β-benzo), 126.5 (α-benzo), 123.6 (α-benzo), 120.7
(ethynyl C), 77.4 (Cipso), 71.2 (α-Cp), 69.7 (β-Cp), 68.0 (ethynyl C).
UV−vis (toluene): λmax = 706 nm.
X-ray Crystallography. X-ray diffraction data were collected on a
Rigaku Rapid II image-plate diffractometer using graphite-monochro-
mated Mo Kα (λ = 0.71074 Å) and Cu Kα (λ = 1.54187 Å) radiation
at 123 K. Multiscan absorption corrections were applied to the data in
all cases. The structures were solved by direct methods, as
implemented in SIR-92,55 and refined by full-matrix least squares
based on F2 using SHELXL-201356 and SHELXLE57 software.
Compound 1 was found to be disordered over a symmetry element
(center of inversion); therefore, the occupancies for all atoms were
constrained to 0.5. The geometry was restrained by using a set of
geometrical restraints such as DFIX and SADI. Water molecules were
found to lie on a special position (31) and disordered. Unfortunately,
no reasonable model was obtained for the disorder. Therefore, the
contribution from the disordered water molecules into the diffraction
data was removed by using a SQUEEZE procedure, as implemented in
PLATON.58 All non-hydrogen atoms were refined using an anisotropic
approximation, while hydrogen atoms were idealized and refined using
riding mode: thermal displacement parameters for all aromatic protons
were constrained to the parent atom with Uiso(H) = 1.2Ueq(C), where
Ueq = 1/3(U11 + U22 + U33). Analysis of the structures and visualization
of the results were done using PLATON software.
Absorption, Fluorescence, Electrochemistry, and NMR.
Absorption data were obtained on a Jasco 720 spectrophotometer.
Electrochemical measurements were conducted using a CH Instru-
ments electrochemical analyzer utilizing a three-electrode scheme with
platinum working and auxiliary and Ag/AgCl reference electrodes in a
0.05 M solution of TBAF in DCM with redox potentials corrected
using an internal standard (decamethylferrocene, Fc*) in all cases. The
redox potentials were then corrected to Fc using the appropriate
oxidation potentials for Fc*/Fc*+ versus Fc/Fc+ in the DCM/TBAF
system. NMR spectra were recorded on a Varian INOVA instrument
Synthesis of N,N-Difluoroboryl-[N-(3-phenyl-2H-isoindol-1-
yl)-N-(3-phenyl-1H-isoindol-1-ylidene)amine] (2a). Compound
2a was prepared using a slightly modified procedure that was reported
previously.39 To a solution of the free-base aza-BODIPYH (1; 253 mg,
0.635 mmol) in dry toluene (35 mL) was added diisopropylethylamine
(335 μL, 1.90 mmol) under an argon atmosphere, and the mixture was
stirred for 15−20 min at room temperature. Then BF3·OEt2 (470 μL,
3.81 mmol) was added to the mixture at room temperature. After 3 h
B
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with a 500 MHz frequency for hydrogen and 125 MHz for carbon.
Chemical shifts are reported in parts per million (ppm) and referenced
to tetramethylsilane [Si(CH3)4] and deuterated chloroform (CDCl3)
(BP86)63 and hybrid B3LYP exchange-correlation functionals.64,65
Indeed, the use of a hybrid B3LYP exchange-correlation functional
results in the highest occupied molecular orbital (HOMO), which is a
purely aza-BODIPY-centered MO in disagreement with experimental
data, while that calculated using BP86 exchange-correlation functional
vertical excitation energies for metal-to-ligand charge-transfer
(MLCT) transitions was found to be severely underestimated. In all
calculations, the 6-311G(d) basis set was employed.66 Solvent effects
were modeled using the polarizable continuum model (PCM)
approach.67 In all time-dependent density functional theory
(TDDFT) calculations, the lowest 20 excited states were calculated
in order to cover experimentally observed transitions in the UV−vis
region.
1
as internal standards. In all cases, the final assignments of the H and
13C signals were made using COSY and HMQC spectra. Elemental
analyses were conducted by Atlantic Microlab. Steady-state
fluorescence data were collected using a Cary Eclipse fluorimeter at
room temperature. Fluorescence quantum yields were determined
against the external standard Rhodamine 800 in ethanol, which has a
quantum yield of 0.19.59
Time-Resolved Absorption and Emission. The nanosecond to
millisecond decay of 3 was measured using a flash-photolysis setup. A
solution of compound 3 in toluene was placed in a 1 cm quartz
cuvette. The sample was pumped using a pulsed 337 nm nitrogen laser
(PTI GL-3300) with a repitition rate of 10 Hz and a fluence of 500 μJ/
cm2. The optical density of the sample was 0.5 at 337 nm. The excited-
state absorption of the sample was probed at a right angle to the pump
with a continuous-wave 532 nm diode laser. The excited-state
absorption decay was measured by detecting the probe beam using
a photodiode and a digital oscilloscope.
Fluorescence lifetimes were measured using time-correlated single
photon counting. Samples in a 1 cm quartz cuvette were excited with a
635 nm, 40 MHz diode laser (driver, Picoquant PDL 800-B; head,
Picoquant LDH-P-635). Emission was directed through a double
monochromator (Jobin-Yvon TRIAX-320) and detected using an
avalanche photodiode (Picoquant MPD PDM). The instrument
response of the system was approximately 500 ps full width at half-
maximum (fwhm). The signal for compound 1 decayed faster than the
instrument response; no signal was observed for compound 3 because
of its low fluorescence quantum yield. The signal for compound 2a is
shown in Figure S8 in the Supporting Information (SI).
RESULTS AND DISCUSSION
■
Compound 3 was prepared as outlined in Scheme 1.
Organometallic 3 is a stable solid in air and can be purified
by conventional chromatographic methods. However, 3 slowly
degrades upon prolonged standing in solution under an
ambient atmosphere. All of our attempts to prepare 4, in
which the Fc groups are directly linked to the boron atom,
failed because of the steric bulk of the Fc fragment. Compound
3 was first characterized by 1H, 13C, COSY, and HMQC NMR
and UV−vis spectra (Figure 1). Because of the lack of a strong
Transient absorption (TA) spectra were obtained using a home-
built laser system consisting of a Ti:sapphire oscillator (powered by a
Spectra Physics Millenia Pro) and a regenerative amplifier (powered
by a Spectra Physics Empower 15). Pulse widths are controlled with a
grating-based stretcher/compressor. The system gives 805 nm pulses
with ∼60 fs (fwhm) widths at a repetition rate of 1 kHz. Excitation
pulses for TA were created by directing a portion of this light into a
home-built noncollinear optical parametric amplifier (NOPA). The
output of the NOPA is tunable from 480 to 700 nm with a pulse width
of ∼60 fs. For the experiments presented here, the NOPA was tuned
to a center wavelength of 630 nm. Probe pulses for TA were created
by focusing a small fraction of the 805 nm light (<1 MW) into a 2 mm
sapphire window, providing a white-light continuum (420−750 nm).
Excitation polarization was kept at the magic angle (54.7°) relative
to probe polarization to isolate the isotropic dynamics. The delay time
between the excitation and probe pulses is controlled by a delay stage
consisting of a retroreflector on a motorized stage (Newport
UTM150PP.1). After the pulses were focused in the sample, the
probe beam was directed through a monochromator (Princeton
Instruments SP2150i) and detected using a 256 pixel diode array
(Hamamatsu silicon array), giving a resolution of 2 nm/pixel. The
pump beam was modulated at half the laser repetition rate, while the
probe beam was measured for every laser pulse, allowing for a change
in the optical density, ΔOD, induced by the pump to be calculated for
each pulse pair. For each time point, 62500 pulse pairs were collected.
The dependence of the ΔOD signal for pulse energies between 10 and
40 nJ was found to be linear. Data shown were collected with pulse
energies of 25−35 nJ. The samples had an optical density of 0.25 and
were continuously pumped through a 1 Mm flow cell during data
collection to ensure a fresh sample for each laser pulse. Absorption
spectra taken before and after laser illumination were indistinguishable,
suggesting minimal photodegradation of the samples.
Figure 1. Experimental (top, in DCM) and PCM−TDDFT-predicted
(bottom) UV−vis spectra of the target compounds. Zero-intensity
low-energy MLCT bands are indicated by the vertical red bars in the
upper right corner.
ring π current in 3, the Fc group proton and carbon signals are
shifted only slightly from those observed in ferrocenylacetylene.
The low-energy absorption band of 2a peaking at 715 nm is
significantly shifted from that observed in the parent boron-free
compound, 1, peaking at 655 nm. Transformation of the BF2
fragment in 2a into B(CCFc)2 substituents in 3 results in
only a small shift in energy, with a peak absorbance at 706 nm
(Figure 1).
Fluorescence. Although the absorption spectra of 2a and 3
(Figure 1) are very similar, their fluorescence spectra and
excited-state relaxation dynamics are substantially different. The
steady-state emission spectra for compounds 1−3 are presented
at the top of Figure 2. The addition of the Fc substituents
Calculations. All computations were performed using Gaussian 09
software running under Windows or UNIX OS.60 Molecular orbital
(MO) contributions were compiled from single-point calculations
using the VMOdes program.61 In all calculations, the TPSSh hybrid
(10% Hartree−Fock exchange)62 exchange-correlation functional was
used because it was found in a set of model gas-phase calculations that
it is superior over standard generalized gradient approximation
C
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irreversible reductions were observed. Compound 3 shows
three reversible oxidations and two irreversible reduction
processes in the DCM/TBAF system.53,54,75−78 DPV data
clearly suggest that the oxidation potentials for the Fc
substituents in 3 are separated by ∼100 mV, which is indicative
of their electronic coupling. The computational results
presented in the next section indicate that through-bond
coupling between the Fc ligands and the aza-BODIPY core is
weak. Although electronic coupling in boron-bridged diferro-
cene systems has been previously described,79−85 the
observation of electronic communication in compound 3 is
surprising given the separation distance. The computational
results predict through-bond (11.45 Å) and through-space
(9.85 Å) Fe−Fe distances that are much longer than those in
previously described systems.86
Calculations. The electronic structure and vertical ex-
citation energy calculations for compounds 1−3 (Figures 1, 4,
and S13−S18 in the SI) correlate well with the experimental
results. In the case of compounds 1 and 2a, π and π* MOs
dominate the HOMO−LUMO regions and, accordingly, their
UV−vis absorption spectra can be described as consisting
almost exclusively of π → π* transitions. In agreement with the
electrochemical data, there is relatively small spacing in energy
between the HOMO, HOMO−1, HOMO−3, and HOMO−4
MOs in complex 3. These orbitals are predominantly Fc-
centered, while HOMO−2 is an aza-BODIPY-centered π
orbital. Such an electronic structure results in the appearance of
four low-energy Fc-to-aza-BODIPY MLCT transitions pre-
dicted by the TDDFT calculations (Figure 1). Taking into
consideration that the (CCFc) substituents in 3 are not
efficiently coupled to the aza-BODIPY π system, TDDFT
calculations predict low intensities for such bands, in agreement
with the experimental data. The most intense transition in 3 is
dominated by HOMO−2-to-LUMO electron transfer and is π
→ π* in its nature. In general, TDDFT calculations are in
excellent agreement with the experimental UV−vis absorption
data on compounds 1−3.
Excited-State Dynamics. Excited-state relaxation dynamics
in compounds 1−3 were measured using a combination of
time-resolved emission and TA. TA spectra at selected time
delays after excitation at 620 nm are presented in Figure 5. On
the basis of the computational predictions, we assign this
excitation to the π → π* transition largely isolated on aza-
BODIPY. The spectra in all cases consist of TA (positive
ΔOD) at shorter wavelengths and a ground-state bleach (GB,
negative ΔOD) that mirrors the static absorption spectrum at
longer wavelengths. Absorption of the initially created excited
state appears within the 50 fs time resolution of the
experiments. The initial TA was broad and peaked at 530 nm
for 1 and at ∼590 nm for 2a and 3. Correlated loss of the
excited-state absorption and the GB in the cases of 1 and 2a
demonstrates that relaxation proceeds directly back to the
ground state. The TA spectra for 1 and 2a do not change shape
with the delay time, and 1 decays 35 times faster than 2a (Table
3). The 1.9 ns lifetime for 2a was determined by both TA and
time-correlated single photon counting (Figure S8 in the SI). A
significant reduction in the measured internal conversion rate
for 2a compared to 1 is consistent with the substantial increase
observed in the fluorescence quantum yield (see Table 1).
Compared with 2a, the excited-state dynamics are more
complicated for compound 3. The shape of the TA changes on
a picosecond time scale. Because the initial absorption peaking
at 590 nm decays with a time constant of 1.7 ps (k = 5.9 × 1011
Figure 2. Steady-state fluorescence spectra in toluene (top) and
fluorescence spectral change upon oxidation of 3 with Fe(ClO4)3
(bottom).
results in a dramatic loss of emission. The fluorescence
quantum yield drops by over 2 orders of magnitude in 3 relative
to 2a (Table 1). Emission from 3 was recovered upon oxidation
Table 1. Absorption and Fluorescence Details for
Compounds 1, 2a, and 3
Stokes
λabs,max
ε
λem,max
(nm)
shift
compound (nm) (M−1 cm−1
)
(nm)
Φfl
1
655
715
706
47800
87000
80000
709
744
728
54
29
22
0.007 0.001
0.21 0.03
<0.001
2a
3
with Fe(ClO4)3, as presented in the bottom of Figure 2.
Recovery of the emission following oxidation of the Fc ligands
suggests that emission quenching in the neutral molecule
proceeds via charge transfer from the Fc ligand to aza-BODIPY,
which is a commonly reported quenching mechanism in Fc-
containing compounds.68−74
Electrochemistry. The redox properties of compounds 1−
3 were studied using electrochemical cyclic voltammetry (CV)
and differential-pulse voltammetry (DPV) methods, and the
results are presented in Figure 3 and Table 2. In the cases of 1
and 2a, two irreversible oxidations and one reversible and one
Figure 3. Room-temperature DPV (top) and CV (bottom) data on
compound 3 in a DCM/0.05 M TBAF system (the internal standard
decamethylferrocene is indicated as Fc*).
D
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Table 2. Redox Potentials (V, vs FcH/FcH+) for Compounds 1, 2a, and 3 in DCM/0.05 M TBAF at Room Temperature
oxidation
ox2
reduction
compound
ox1
ox3
red1
−1.333
−1.062
red2
1
0.659 (irr)
1.076 (irr)
0.157
0.175 (irr)
0.558 (irr)
0.072
−1.921 (irr)
−1.837 (irr)
−1.909 (irr)
2a
3a
0.537
−1.190 (irr)
s−1; see Figure 6 and Table 3), a new absorption peaking at 520
nm appears and remains static over the experimental range of
Figure 4. DFT−PCM-calculated [TPSSh/6-311G(d)] orbital energies
for 1, 2a, and 3 with a pictorial representation of the frontier MOs.
Figure 6. (top) Time dependence at the peak of TA, as shown in
Figure 5. Note the broken time axis, which is linear up to 10 ps and
then logarithmic. (bottom) Time-resolved TA at 532 nm of
compound 3 in toluene following excitation at 308 nm. The circles
are the data points, and the lines are single-exponential fits to the data.
Time constants are reported in Table 1.
time delays probed in Figure 5. Rapid decay of the initial
excited state and the correlated appearance of the new
absorption at 520 nm indicates that the quenching mechanism
is not direct internal conversion to the ground state but rather
conversion to a new excited state. With a lack of evidence for
intersystem crossing in the parent, 2a, even on a nanosecond
time scale, relaxation mediated by the Fc substituents must
proceed by either an energy-transfer or an electron-transfer
mechanism. The lack of overlap between the absorption
spectrum of the Fc donor and emission spectrum of the aza-
BODIPY acceptor (Figure S8 in the SI) allows us to rule out
resonance energy transfer as the quenching mechanism (this is
discussed quantitatively in the SI). We conclude that electron
transfer is responsible for loss of the initially created excited
state. According to this mechanism, electron transfer from the
low-spin iron(II) center in the Fc ligand to the photoexcited
aza-BODIPY π* core is predominantly responsible for
fluorescence quenching in complex 3. In agreement with this
conclusion, we have shown that stepwise oxidation of the Fc
substituents by Fe(ClO4)3 results in the recovery of
fluorescence (Figure 2).
Figure 5. Time-resolved TA spectra of compounds 1−3 in toluene
following excitation at 630 nm.
Table 3. Exicited-State Lifetimes of Compounds 1, 2a, and 3
Dissolved in Toluene
compound
τ1 (ps)
53
τ2 (μs)
1
1
2a
3
1900 100
1.7 0.1
4.8 0.1
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Article
The same experiments were also conducted in the more
polar solvent acetonitrile (Figures S10−S12 in the SI). In
addition to a small shift in the absorption features to shorter
wavelengths, the initial charge-separation step slows slightly to a
time constant of 2.4 0.2 ps. The difference in energy between
the initially excited and charge-separated (CS) states of ∼0.5 eV
is slightly larger than typical reorganization energies in similar
systems.87 Under these conditions, the initial charge separation
is predicted to take place with a small barrier on the inverted
side of Marcus theory. In the inverted regime, additional
stabilization of the CS state (relative to the initial π* state) by
the more polar acetonitrile will increase the barrier and
decrease the rate of charge transfer. This is illustrated in Figure
7, where the gray parabola represents additional stabilization in
recombination step of ∼1 ps. An extremely rapid recombina-
tion step is consistent with the change in energy, which is very
similar to typical values for the reorganization energy in
analogous systems, leading to a prediction of near-barrierless
transfer.87 This is illustrated with the parabolas in Figure 7.
Excess energy will also accelerate charge recombination prior to
pseudoequilibration in the CS state. We conclude that the most
likely identity of the long-lived excited state is a triplet state. We
are not able to distinguish between the states where the triplet
resides primarily on aza-BODIPY or on one of the Fc ligands.
The addition of the (CCFc) ligands at the boron atom
leads to very rapid charge transfer and subsequent recombina-
tion in 3. The result is efficient intersystem crossing without the
inclusion of heavy atoms to enhance spin−orbit coupling. In
addition to the potential advantages for optoelectronic
applications that come with extending the excited-state lifetime
by 3 orders of magnitude, the possibility arises of using this
system as a triplet sensitizer for singlet-oxygen chemistry. When
the solution was saturated with O2, the lifetime of 3 was
reduced to 0.28 0.02 μs (Figure 6), indicating that the long-
lived triplet state is effectively quenched by O2. To examine the
potential for sensitizing photochemical reactions, the con-
version of dihydroxynapthalene to juglone in the presence of
O2 was attempted.88 The experiments were carried out at
multiple excitation wavelengths 700, 365, and 308 nm, and
upon illumination with a xenon lamp. Under the same
conditions where efficient conversion was observed using
standard triplet sensitizers such as methylene blue, no evidence
for conversion was observed when using 3 as the sensitizer. At
Figure 7. Energy-level diagram. “aB” is an abbreviation for aza-
BODIPY.
1
3
0.98 eV, O2 is similar in energy to (aza-BODIPY). The
difference between the measured oxidation potential of 3 and
the reduction potential for O2 is 0.8 eV, making oxidation of the
triplet the thermodynamically favored reaction, as shown in
acetonitrile relative to the blue parabola, which represents
toluene. The observed rate reduction with increasing solvent
polarity supports the conclusion that charge transfer takes place
in the Marcus inverted regime.
1
Figure 7. The absence of evidence for the production of O2
suggests that the triplet excited state is predominantly
oxidatively quenched by O2 rather than being quenched
through energy transfer.
The lifetime of the state with absorbance peaking at 520 nm
was determined using flash photolysis and measuring the decay
in TA at 532 nm. This is presented at the bottom of Figure 6.
In the absence of molecular oxygen, the lifetime was 4.8 0.1
μs in toluene. As shown in Figure 5, there is only a single
transformation observed following photoexcitation, with a clean
isosbestic point at 630 nm. Given the prior conclusion that
deactivation is initiated by electron transfer from the Fc ligand,
there are two possibilities: the long-lived state is the initially
created CS state, or the CS state is converted to a subsequent
state on a time scale faster than the 1.7 ps initial charge
separation. The latter case has been proposed to occur in
analogous systems, with rapid recombination suggested to
result in the formation of a triplet state.46 A triplet state
localized on either aza-BODIPY or Fc would be located around
∼1 eV above the ground state, placing it roughly 0.3 eV below
the CS state, as shown in the energy-level diagram in Figure
7.46
A relatively short charge-separation distance of only ∼6.5 Å
between the aza-BODIPY and Fc iron centers makes a CS
lifetime of >1 μs difficult to justify. In addition, there are two
energetically accessible triplet states, with the triplet located on
either aza-BODIPY or Fc, that allow for subsequent trans-
formation of the CS state via charge recombination. This would
have to take place more rapidly than the initial charge-
separation step because no intermediate was observed in the
TA measurements, placing an upper limit on the charge
CONCLUSION
■
The first aza-BODIPY complex with Fc-containing organo-
boron fragments was synthesized and characterized. Surpris-
ingly, the Fc ligands in this compound are electronically
coupled to each other despite unfavorable geometry, a lack of
conjugation in their connectivity, and a separation distance of
10 Å. Photoinitiated charge separation takes place very rapidly
and is followed by even faster charge recombination to form a
triplet state in 1.7 ps in toluene. This efficient charge-transfer-
mediated internal conversion results in a large increase in the
excited-state lifetime from 2 ns to 4.8 μs. An energy-level
diagram summarizing the excited-state dynamics is presented in
Figure 7. Efficient access to the triplet state without the need to
incorporate heavy atoms is not common, and in this sense, this
complex does exceed the performance of other reported Fc-
BODIPY conjugates, limiting the through-bond coupling
between the Fc ligands and aza-BODIPY via bonding at the
boron while balancing the through-space coupling with
orientation and spatial proximity offers a new paradigm for
tuning the electronic structures and excited-state dynamics in
this class of molecules. Using charge transfer to enhance the
intersystem crossing provides an additional design principle for
materials targeted at optoelectronic devices.
F
DOI: 10.1021/acs.inorgchem.5b00494
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Inorganic Chemistry
Article
(19) Lukyanets, E. A.; Nemykin, V. N. J. Porphyrins Phthalocyanines
2010, 14, 1−40.
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format, X-ray, NMR, DFT
data, absorption and time-resolved spectroscopy for compound
3 in acetonitrile, and a complete analysis of the potential for
resonance energy transfer. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
(20) Maiya, G. B.; Barbe, J. M.; Kadish, K. M. Inorg. Chem. 1989, 28,
2524−2527.
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166.
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Maximov, A. Y.; Polshin, E. V.; Koposov, A. Y. Dalton Trans. 2007,
3378−3389.
AUTHOR INFORMATION
(23) Nemykin, V. N.; Galloni, P.; Floris, B.; Barrett, C. D.; Hadt, R.
G.; Subbotin, R. I.; Marrani, A. G.; Zanoni, R.; Loim, N. M. Dalton
Trans. 2008, 4233−4246.
■
Corresponding Authors
*E-mail: blank@umn.edu.
*E-mail: vnemykin@d.umn.edu.
(24) Nemykin, V. N.; Purchel, A. A.; Spaeth, A. D.; Barybin, M. V.
Inorg. Chem. 2013, 52, 11004−11012.
Author Contributions
(25) Poon, K. W.; Yan, Y.; Li, X. Y.; Ng, D. Organometallics 1999, 18,
3528−3533.
‡E.M. and T.J.P. contributed equally to this work.
(26) Rohde, G. T.; Sabin, J. R.; Barrett, C. D.; Nemykin, V. N. New J.
Chem. 2011, 35, 1440−1448.
Notes
The authors declare no competing financial interest.
(27) Shoji, O.; Tanaka, H.; Kawai, T.; Kobuke, Y. J. Am. Chem. Soc.
2005, 127, 8598−8599.
ACKNOWLEDGMENTS
■
(28) Solntsev, P. V.; Sabin, J. R.; Dammer, S. J.; Gerasimchuk, N. N.;
Nemykin, V. N. Chem. Commun. 2010, 46, 6581−6583.
(29) Sugimoto, H.; Tanaka, T.; Osuka, A. Chem. Lett. 2011, 40, 629−
631.
This work was supported by Grants NSF CHE-1110455, NSF
DMR-1006566, CHE-1401375, and NSF MRI CHE-0922366,
the Minnesota Supercomputing Institute, and a University of
Minnesota Grant-in-Aid to V.N.N.
(30) Suijkerbuijk, B. M. J. M.; Klein Gebbink, R. J. M. Angew. Chem.,
Int. Ed. 2008, 47, 7396−7421.
(31) Vecchi, A.; Galloni, P.; Floris, B.; Nemykin, V. N. J. Porphyrins
Phthalocyanines 2013, 17, 165−196.
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