Tuning the Properties of Azadipyrromethene-Based Near-Infrared Dyes Using Intramolecular BO Chelation and Peripheral Substitutions

Article abstract of DOI:10.1021/acs.inorgchem.1c01597

Tetraphenylazadipyrromethenes (ADPs) are attractive near-infrared (NIR) dyes because of their simple synthesis and exceptional optical and electronic properties. The typical BF2 and less explored intramolecular BO coordination planarize the molecule, making them promising π-conjugated materials for organic electronic applications. However, their use has been mostly limited to vacuum-deposited devices. To improve the properties, we synthesized and characterized a series of ADP complexes and used density functional theory calculations to further explain the properties. Hexyloxy solubilizing groups increase the complexes' solubility in organic solvents and enable film formation from solution. Phenylethynyls at the pyrrolic positions extend πconjugation, red-shift absorption and emission peaks, and increase the ionization potential (IP) and electron affinity. When the properties of complexes with hexyloxy and phenyethynyl substitutions are compared, the BO complex is more planar and has a smaller IP than the corresponding BF2 complex because of increased electron density on the proximal phenyls. The BO complex has an unusual combination of properties: a solution λmax of 781 nm, emission at 805 nm, a small Stokes shift, and a quantum yield of 6%. It forms transparent films with a low optical gap of 1.22 eV. This new complex is a promising candidate for transparent solar cells and NIR photodetectors.

Full text of DOI:10.1021/acs.inorgchem.1c01597

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Article
Tuning the Properties of Azadipyrromethene-Based Near-Infrared
Dyes Using Intramolecular BO Chelation and Peripheral
Substitutions
̀
Jayvic C. Jimenez, Zehao Zhou, Arnold L. Rheingold, Shane M. Parker, and Genevieve Sauvé*
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ABSTRACT: Tetraphenylazadipyrromethenes (ADPs) are attractive
near-infrared (NIR) dyes because of their simple synthesis and
exceptional optical and electronic properties. The typical BF2 and less
explored intramolecular BO coordination planarize the molecule,
making them promising π-conjugated materials for organic electronic
applications. However, their use has been mostly limited to vacuum-
deposited devices. To improve the properties, we synthesized and
characterized a series of ADP complexes and used density functional
theory calculations to further explain the properties. Hexyloxy
solubilizing groups increase the complexes’ solubility in organic
solvents and enable film formation from solution. Phenylethynyls at
the pyrrolic positions extend π conjugation, red-shift absorption and
emission peaks, and increase the ionization potential (IP) and electron
affinity. When the properties of complexes with hexyloxy and
phenyethynyl substitutions are compared, the BO complex is more planar and has a smaller IP than the corresponding BF2
complex because of increased electron density on the proximal phenyls. The BO complex has an unusual combination of properties:
a solution λmax of 781 nm, emission at 805 nm, a small Stokes shift, and a quantum yield of 6%. It forms transparent films with a low
optical gap of 1.22 eV. This new complex is a promising candidate for transparent solar cells and NIR photodetectors.
nitrogen. This substitution red-shifts the absorption spectra
and increases the electron affinity (EA). The easiest ligand to
synthesize is the 1,3,5,7-tetraphenylazadipyrromethene (ADP),
shown in Figure 1.15 This ligand has an absorption peak at 600
nm in solution. Coordination with BF2 red-shifts the
absorption peak to 650 nm and increases the emission
quantum yields (QYs) and EA.12,16 Using intramolecular BO
complexation further red-shifts absorption of the unsubstituted
ADP to 728 nm.17,18 Extending the conjugation of aza-
BODIPY [or BF2(ADP) using our nomenclature] with
phenylethynyl groups at the pyrrolic positions also red-shifts
the absorption significantly to 732 nm.19 An ADP molecule
that combines the effect of BO intramolecular coordination
with extended conjugation of phenylethynyl groups has not
been published to our knowledge, but one would expect
further red-shifting.
INTRODUCTION
■
Near-infrared (NIR) dyes are of great interest for many
applications including biological sensing and imaging, photo-
dynamic therapy, NIR photodetection, and solar energy
conversion.1−4 In particular, dipyrromethene- and azadipyrro-
methene-based dyes have attracted a lot of attention because of
their tunable and strong absorption in the visible-to-NIR range
and fluorescence properties.5−8 These are π-conjugated
bidentate ligands that are typically coordinated with difluor-
oboryl (BF2, known as BODIPY and aza-BODIPY) but can
also be coordinated with other Group 13 elements and
transition metals.5,9,10 Approaches to red-shifting the optical
properties include introducing electron-rich/electron-poor
functional groups to create a push−pull effect, extending
conjugation, and rigidifying the molecule via fused rings.11,12
One underexplored strategy to red-shifting the absorption
spectra is to use intramolecular BO bonds. Here, hydroxide
groups are added to the ortho position of the proximal phenyls,
n−1
Received: June 2, 2021
resulting in a tetradentate N2O2 ligand that can coordinate
with transition metals or Group 13 elements such as boron,
thus rigidifying the structure by restricting the proximal
phenyls from rotating.13,14
Azadipyrromethenes differ from dipyrromethenes in that the
carbon that connects the two pyrrole rings is replaced with
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Figure 1. Molecular structure of ADP bearing chemically modifiable
positions: proximal phenyls, distal phenyls, pyrrolic positions, and
chelation. Some approaches to red-shift the absorption spectra are as
follows: BF2 chelation, intramolecular BO chelation, and a
combination of BF2 chelation and extension of conjugation with
phenylethynyls at the pyrrolic positions.
Figure 2. New complexes synthesized in this work.
coordination for molecules with phenylethynyls at the pyrrolic
position, we synthesized an analogue of 6a, molecule 8, and
used the previously published analogue of 6b, BF2(WS3).19
The thermal, structural, optical, and electrochemical properties
of the four newly synthesized complexes were examined, and
density functional theory (DFT) calculations were used to
further understand the effect of these chemical modifications
on the properties.
In terms of applications, BF2-chelated ADPs have been
explored for several biological applications, as well as active
materials in organic solar cells.20,21 The BO intramolecular
bonds are unstable to water and alcohols because these can
compete with intramolecular boron coordination. This limits
its use in biological applications but is still of interest for
organic electronic applications such as photovoltaics and
photodetectors, where water is usually avoided. For example,
BO-chelated ADPs were successfully used in vacuum-
deposited organic solar cells.22,23 In terms of solar cell
applications, BF2- and BO-chelated ADPs have mostly been
limited to vacuum-deposited devices because of low solubility
in organic solvents. One exception is a report where NIR-
absorbing benzannulated aza-BODIPY dyes were used in
ternary solution-processed solar cells to increase NIR solar
harvesting.24 Our group has tested BF2(ADP) and BF2(WS3)
(shown in Figure 1) in organic solar cells by blending with the
common donor poly(3hexylthiophene) in a 1:1 ratio. In our
hands, the cells did not show any significant photovoltaic
effect, most likely because of poor film formation and
unfavorable blend morphology.19 To enable these systems to
form good films and work in solution-processable solar cells, it
is necessary to introduce solubilizing groups. While function-
alization with hydrophilic groups has been explored to increase
the water solubility of ADP-based complexes for biological
applications,25,26 we are not aware of any reports exploring the
use of hydrophophic solubilizing groups on the BF2- or BO-
chelated ADP complexes to enable their use in solution-
processed organic electronic devices.
RESULTS AND DISCUSSION
■
Synthesis. Synthesis of the complexes is shown in Scheme
1. First, the chalcones 3a and 3b were synthesized by grinding
the appropriate starting materials 1 and 2 in a mortar and
pestle similar to the published procedure.27 The crude
chalcones were recrystallized in methanol (MeOH), resulting
in either a bright-yellow crystalline solid with ∼45% yield for
the Y = OH substitution or an off-white crystalline solid with
∼64% yield for the Y = H substitution. The free ligands 4a−4c
were obtained by the Michael addition of nitromethane,
followed by annulation with ammonium acetate in 1-butanol.28
The free ligands were washed with MeOH and isolated in
moderate yields of ∼36−42%.
While reaction of the free ligands with boron trifluoride
usually forms a BF2 complex, the presence of an o-phenolic
group in 4a and 4b favors formation of the intramolecular
borate (Scheme 1B; called BO in this work).17 The BO
complexes can withstand Stille coupling conditions to append
the phenylethynyl groups at the pyrrolic positions, giving 6a
and 6b. The BF2 complex analogue to 6a was also synthesized
by installing phenylethynyl groups at the pyrrolic positions of
4c, followed by reaction with boron fluoride to give complex 8.
The final complexes were successfully purified by recrystalliza-
tion. All complexes were characterized by 1H and 13C NMR to
determine the chemical structure and matrix-assisted laser
desorption ionization time-of-flight (MALDI-TOF) mass
spectrometry to confirm the purity and molecular mass. The
presence of tetrahedral boron in both BO and BF2 complexes
was confirmed by 11B NMR, with a signal in the 0−2 ppm
Here, we have synthesized and characterized four new
complexes of ADP, shown in Figure 2. This series of molecules
was chosen to look at the effect of three different peripheral
substitutions on the properties of BO-chelated ADP: hexyloxy
solubilizing groups at the para position of the distal phenyls
(5a), hexyloxy on the distal phenyls plus phenylethynyls at the
pyrrolic positions (6a), and phenylethynyls at the pyrrolic
position (6b). Also, to directly compare the effect of
B
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a
Scheme 1. Synthesis of (A) ADP Compounds, (B) BO-Coordinated Complexes, and (C) the BF2 Complex
a
(a) NaOH, ground, (b) MeNO2, NEt3 in MeOH at 90 °C, (c) ammonium acetate in 1-BuOH, (d) BF3OEt3 in toluene at 110 °C, (e) NIS in
CHCl3, and (f) Pd(PPh3)4 and tributyl(phenylethynyl)tin in xylenes at 150 °C.
range (a singlet for BO and a triplet for BF2). The presence of
fluorine in the BF2 complex was confirmed by 19F NMR, with a
characteristic quartet signal around −130 ppm. All complexes
are stable in air in the solid state. The BO complexes are stable
in tetrahydrofuran (THF) or chlorinated solvents but tend to
degrade in the presence of water or alcohol. In contrast, the
BF2 complex is generally stable in solution, even in the
presence of water or alcohol.
Thermal Properties. Thermal properties were determined
by thermogravimetric analysis (TGA; Figure S1 and Table S1)
and differential scanning calorimetry (DSC; Figure 3). TGA
indicates that all complexes are thermally stable, with 5%
weight loss temperatures of >390 °C (see the Supporting
Information). For DSC measurements, we analyzed crystals
obtained from solvent diffusion in THF/MeOH, except for 6b,
which produced no resolvable crystals. To avoid sample
decomposition, the samples were heated to a maximum of 350
°C, a temperature well below the 5% weight loss temperatures
found by TGA. The DSC first heating cycles of crystals of 5a
and 6a have weak and broad endothermic peaks at ∼180 and
210 °C, respectively, suggesting thermal motion corresponding
Figure 3. DSC thermogram showing both the first heating (right-
to movement of the alkoxy solubilizing groups. Upon further
heating, all complexes except 6b exhibit a sharp melting peak,
followed by a broad exothermic peak (marked with an
asterisk). The melting temperatures were 236, 290, and 230
°C for 5a, 6a, and 8, respectively. Complex 6b (without
solubilizing groups) shows only a sharp exothermic peak at 342
°C (marked with an asterisk) with no melting peak. This is
expected because 6b did not produce crystals suitable for X-ray
diffraction (XRD). The solubilizing groups therefore facilitate
facing arrow) and cooling (left-facing arrow) cycles. The peaks
marked with asterisks indicate exothermic peaks.
crystallization. All four complexes do not show any thermal
features during cooling (Figure S2), suggesting that they have
assembled into a vitrified state during the endothermic phase
of the heating and are not recrystallizing at the cooling rate
used. A second heating cycle shown in Figure S2 shows the
C
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Figure 4. Ellipsoid plots of compounds 5a, 6a, and 8 (front and side views).
featureless heating profile for all complexes, further reinforcing
the amorphous state of all of the complexes.
φ rotation of 5a and 6a are in agreement with both crystal
structures and the solvent phase. This is because the BO
complexes are geometrically constrained, leading to limited
variation in the possible geometric configurations. Compound
8, however, features fewer constraints because the proximal
phenyls can freely rotate. The C−N−C (N is the bridging
nitrogen to the pyrrole rings) angles can also affect
planarization of the phenyls. The BO complexes in the crystal
show ∼1° less than the solvent phase. This leads to better
planarization of the distal phenyls, relieving steric effects, which
then allow for the pyrrolic phenyls to also planarize.
Geometric Characterization. Suitable crystals for single-
crystal XRD were obtained from solvent diffusion in THF/
MeOH, except for 6b, which produced no resolvable crystals.
Ellipsoid plots are shown in Figure 4. The side view shows that
all of the boron complexes have a planar ADP core, including
the phenylethynyl groups, and that the proximal phenyls of the
BO complexes are more in-plane with the core than that of the
BF2 complex. To quantitatively characterize the structural
features, we looked at the twisting angle of the phenyl rings out
of the plane of the ADP core (α twisting angle) and the
rotation angle of each phenyl ring with respect to the single
bond connecting the phenyl ring to the ADP core (φ rotation
angle), depicted in Figure 5 for the proximal phenyls.
Optical Properties. The optical properties are given in
Figure 6 and summarized in Table 2. The UV−vis spectra in
Figure 6a show that all complexes strongly absorb in the
visible-to-NIR range. In solution, 5a has a dark pink/purple
color and has a strong and narrow absorption between 600 and
775 nm, with a maximum at 727 nm. Upon installing
phenylethynyl groups (6a, dark blue in solution), this peak
red-shifts to 781 nm, which we attribute to extension of the
conjugation. When the intramolecular BO is replaced with BF2
in 8 (blue-green in solution), the absorption spectra blue-shifts
to 702 nm and significantly broadens. 6b with intramolecular
BO and phenylethynyls but no solubilizing groups is dark
pink/purple in solutions and has a narrow peak at 791 nm,
slightly red-shifted by 10 nm relative to 6a. All BO complexes
have a higher extinction coefficient than the BF2 complex 8.
The lower extinction coefficient of the BF2 complex 8 is most
likely due to the increased conformational flexibility of 8,
which broadens the absorption and reduces the peak height.
Thin films were obtained by spin-coating from chloroform
solutions. Compounds 5a, 6a, and 8 resulted in visually
smooth films, while 6b shows aggregation. The solubilizing
groups are therefore important to obtain smooth and uniform
films. λmax values in the films in Figure 6b follow trends similar
to those in solution. λmax red-shifted from solution to film by
39−66 nm, consistent with π−π stacking in films. Compounds
with phenylethynyl groups had a much broader absorption
band than 5a, suggesting that these groups enhance π−π
stacking interactions in films. Interestingly, 6a, which combines
phenyethynyls at the pyrrolic positions and hexyloxy
solubilizing groups, showed shoulders on each side of the
main absorption peak, leading to the broadest peak and lowest
optical gap of 1.22 eV. This indicates that the two types of
substitutions combine to promote intermolecular interactions
in films. Compound 6b has a slightly higher optical gap of 1.26
eV, whereas 5a and 8 have higher optical gaps of 1.50 and 1.52
eV, respectively. The films of 5a and 6a are optically clear, with
relatively low absorption in the visible range. On the other
hand, the BF2 complex film has a slight blue-green color due to
some absorption above 550 nm.
Figure 5. Measured α and φ angles of the proximal position. The α
angle measures the degree to which the proximal phenyl ring twists
out of the plane of the ADP core, while the φ angle is the dihedral
angle between the plane of the proximal phenyl ring and the ADP
core. Distal (dis) and pyrrolic (pyr) phenyls are highlighted in red and
green, respectively.
Specifically, the α twist angle is defined by the angle of the
phenyl ring axis relative to the best-fit plane of the ADP core,
and the φ rotation is defined as the dihedral angle between the
plane of the phenyl ring and the best-fit plane of the ADP core.
The measured angles for the proximal phenyls (pro), distal
phenyls (dis), and pyrrolic phenyls (pyr) are collected in Table
1. The BO complexes tend to have smaller angles and thus are
more planar than the BF2 complex. To better understand the
geometries of these complexes, we also computationally
characterized all four compounds’ lowest-lying ground-state
geometries in the solvent phase with DFT. The computational
structural characteristics are given in Table 1. The α angle and
D
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Table 1. Summarized Geometric Measurements from the Solvent Phase and Crystal Structure of the Four Compounds
solvation (computational)
crystal (experimental)
compound
position
α twist, deg
ϕ rotation, deg
C−N−C angle, deg
α twist, deg
ϕ rotation, deg
C−N−C angle, deg
5a
pro
dis
pro
dis
pyr
pro
dis
pyr
pro
dis
26.2, 26.2
1.5, 1.5
26.0, 26.0
1.7, 1.7
6.4, 6.5
5.7, 5.7
13.3, 13.5
32.3, 32.3
14.0, 14.0
39.6, 39.6
14.3, 14.2
56.4, 56.4
44.9, 44.9
5.5, 5.3
119.1
30.6, 28.7
2.8, 0.1
24.4, 24.4
9.3, 9.3
3.4, 3.4
2.4, 11.4
0.0, 0.1
9.2, 12.1
24.8, 30.5
14.2, 14.2
10.6, 10.6
1.3, 1.3
56.3, 26.5
33.6, 39.7
29.5, 3.2
117.73
6a
8
119.1
118.42
120.49
120.7
119.0
1.2, 1.2
10.7, 10.7
26.3, 26.3
1.7, 1.7
1.4, 5.6
6b
14.0, 14.0
42.3, 42.3
14.4, 14.4
pyr
6.9, 6.9
The fluorescence spectra of chloroform solutions are shown
in Figure 6c, and the emission properties are summarized in
Table 3. All compounds showed fluorescence with a maximum
in the NIR. Similar to trends observed in the absorption
spectra, the emission maximum was the smallest for 5a, at 741
nm, and red-shifts with phenylethynyl groups to 805 and 821
nm for 6a and 6b, respectively. Unlike in the absorption
spectra, the BF2 complex 8 has an emission at longer
wavelength than 5a despite having phenylethynyls. This is
reflected in the large Stokes shift (SS) of 65 nm (or 1207
cm−1) found for the BF2 complex, compared to the very small
SS of 13 nm (or 260 cm−1) for 5a. The fluorescence QYs
varied from 1.0% for 8 to a high of 8.3% for 5a. The QYs
appear to inversely correlate with the observed SS: the lower
the SS, the higher the QY. Compound 6a shows good
solubility in organic solvents, with a relatively high NIR QY of
6.0% at 805 nm.
Electrochemistry. Cyclic voltammograms of the com-
plexes in dichloromethane (DCM) are shown in Figure 7, and
the electrochemical properties are summarized in Table 4. All
complexes show two reversible reduction processes and one
irreversible oxidation process. Compound 5a shows the anodic
peak potential (Epa) and first cathodic peak potential (Epc)
occurring at +0.56 and −1.01 V, respectively. Adding
phenylethynyl (6a) increases Epa to 0.70 V but does not
significantly change the first Epc (−0.98 V). Removing the
intramolecular bonding in compound 8 increases both Epa and
Epc. Removing the hexyloxy solubilizing groups (6b) increases
the first Epc compared to that of 6a.
The ionization potential (IP) and EA were estimated from
the onsets of the first reduction and oxidation waves and are
reported in Table 4 and Figure 8a. The IP and EA for
BF2(WS3), the BF2 analogue of 6b, are also included for
comparison. The negative values of the IP and EA are often
used to approximate the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels, respectively.29 The difference between the IP and
EA is taken as the fundamental gap.30 The results demonstrate
that the IP and EA can be fine-tuned using with the chelation
type and substitutions, with IP ranging from 5.26 eV for 5a to
5.53 eV for 8 and EA ranging from 3.89 eV for 5a to 4.15 eV
for 6b. The phenylethynyl pyrrolic groups increase both the IP
and EA (or decrease the HOMO and LUMO energy levels).
The intramolecular BO chelates decrease the IP and have very
little effect on the EA [compare 6a with 8 or 6b with
BF2(WS3)]. This results in decreased fundamental gaps for the
BO complexes. The hexyloxy solubilizing groups also slightly
decrease the IP and EA [compare 6a with 6b and 8 with
BF2(WS3)].
Computation: Ground-State Properties. Vertical IPs
and EAs were computed using the ΔSCF procedure (the
energy difference between the relevant neutral and charged
species of the ground-state geometry) and are summarized in
Table S3. As shown in Figure 8a, the calculated IP and EA
agree with the experimental values to within 0.24 eV.
Generally, the calculated IPs are larger than the experimental
values and the calculated EAs are smaller than the
experiemental values, resulting in a larger calculated
fundamental gap than the measured fundamental gap by
0.36−0.43 eV (Figure 8b). While the calculated and
experimental values are different, the trends observed with
the chemical structure are similar.
To gain direct insight into the electronic structure based on
the above single-point-energy calculations, we look at the
frontier orbitals. Figure 9 shows contour plots of the HOMO
and LUMO for the four complexes. A considerable portion of
the electron density of the frontier orbitals sits on the ADP
̈
core. This direct observation is supported by a Lowdin
population analysis of the frontier molecular orbitals.31 Table 5
shows the proportion of each frontier orbital contributed by
the 12 atoms comprising the ADP core (Figure 5, highlighted
in blue), as well as the net charge on the proximal phenyl rings.
The ADP core contributes to 54−60% of the HOMO and 66−
71% of the LUMO for all compounds. Interestingly, we find a
linear relationship between the proximal charge and the IP, as
shown in Figure 8c. This implies that greater electron density
in the proximal phenyl rings reduces the IP.
Computation: Excited-State Properties. We simulated
the optical absorption spectra using time-dependent DFT
within the random-phase approximation. The resulting
absorption spectra are shown in Figure 10. For all four
molecules, the bright peak around 750 nm corresponds to the
S0 → S1 transition, which corresponds primarily to a HOMO
→ LUMO transition (>94% by weight). The energy and major
transition components of the lowest four states are listed in
Table S4. The computed oscillator strength for the lower-
energy peak of the BF2 complex 8 is 1.116, higher than that of
the BO complex 6a at 0.809. These results are consistent with
our experimental results. BF2 has a lower extinction coefficient
than the BO complex because of the increased conformational
flexibility, which broadens the absorption and reduces the peak
height and, hence, the extinction coefficient. Taking this into
account, the transition in 8 is actually stronger than the
transition in the BO compounds. For instance, the full width at
E
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Table 2. Summary of the Optical Properties
shift upon
λmax
film,
nm
film
formation,
nm
λmax,abs, nm
λonset
nm
,
optical
gap, eV
compound (ε, L mol−1 cm−1
)
5a
331 (36000),
529 (17000),
727 (88000)
769
42
826
1.50
6a
325 (37000),
604 (14000),
781 (99000)
820
39
1010
1.22
8
315 (39000),
702 (65000)
350 (42000),
569 (12000),
791 (92000)
761
857
59
66
818
984
1.52
1.26
6b
a
Table 3. Summary of the Emission Properties
compound
λmax,em, nm
SS, nm
SS, cm−1
Φfluor
5a
6a
8
741
805
767
821
14
24
65
30
260
382
1207
462
0.083
0.060
0.010
0.043
6b
a
λmax,em is the maximum emission wavelength, SS is the Stokes shift,
and Φfluor is the QY obtained using HITCI (Φ = 0.283 in ethanol) as
the standard.
Figure 7. Cyclic voltammogram of compounds 1−4 with 0.1 M
TBAPF6 in DCM as an electrolyte solution. Fc/Fc+ was used as the
internal standard.
Figure 6. Optical properties of the compounds: (a) UV−vis spectra in
CHCl3 with solutions in the inset; (b) normalized UV−vis spectra of
thin films spin-coated from CHCl3; (c) normalized fluorescence
spectra in CHCl3.
single pair of transition orbitals with a singular value greater
than 0.97. The NTO hole and electron orbitals strongly
resemble the HOMO and LUMO, respectively, as expected for
transitions dominated by a single HOMO-to-LUMO tran-
sition. Because the NTO pairs are both concentrated on the
ADP core, we conclude that S1 has an excitonic character with
no significant intramolecular charge transfer.
half-maximum (fwhm) of the lowest-energy absorption in 8 is
108 nm, compared to 43 nm for 6a. The total transition
strength is determined by integrating the extinction coefficient
for a single peak, which is approximately proportional to the
peak height times of the fwhm. Using this approach, we find
that the total transition strength in 8 is 1.65 times stronger
than the total transition strength in 6a, which closely matches
the ratio of the computed oscillator strength of 1.38.
CONCLUSIONS
■
In this study, we found that the optical, structural, and
electrochemical properties can be tuned using hexyloxy
solubilizing groups, pyrrolic phenylethynyls, and the boron
coordination type. Hexyloxy groups on the distal phenyls
increase the solubility in organic solvents, enable smooth
We further performed natural transition orbital (NTO)
analysis,32 and the obtained contour plots are shown in Figure
11. For each compound, the S1 transition is dominated by a
F
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Table 4. Summary of the Electrochemical Properties from CV Using Fc/Fc+ as the Internal Standard
a
compound
E1/2 ox, V
Epa, V
E
1/2 red, V
Epc, V
Eox onset, V
E
red onset, V
IP, eV
EA, eV
Egap
,
eV
5a
6a
8
0.56
0.70
0.86
0.68
1.08
−0.97, −1.73
−0.88, −1.61
−0.80, −1.59
−0.78, −1.47
−0.79, −1.59
−1.01, −1.77
−0.98, −1.69
−0.84, −1.63
−0.82, −1.51
−0.95, −1.75
0.46
0.54
0.73
0.61
0.86
−0.91
−0.76
−0.77
−0.65
−0.70
5.26
5.34
5.53
5.41
5.66
3.89
4.04
4.06
4.15
4.10
1.37
1.30
1.47
1.26
1.56
6b
b
BF2(WS3)
a
b
Egap is the fundamental gap. Electrochemical properties reproduced from ref 19.
Figure 8. (a) Estimated energy levels of the complexes in solution, taking E1/2 of Fc/Fc+ as −4.8 eV versus vacuum with energy levels from ΔSCF
calculation, (b) fundamental gaps obtained from CV and ΔSCF calculations, and (c) computed IP versus computed proximal phenyl charge.
of the ADP core to red-shift the absorption and emission
spectra and increase the IP and EA. Intramolecular BO
complexation constrains the proximal phenyl rotation, resulting
in smaller rotation angles and thus planarization of the
conjugated molecule. DFT calculations show that BO
complexation also leads to better planarization of the distal
phenyls and relieves steric effects, which then allows for the
pyrrolic phenyls to also planarize. This significantly red-shifts
the absorption and emission spectra, resulting in smaller
optical gaps. The BO complexes tend to have smaller SSs and
larger fluorescence QYs than the BF2 complex. The S0 → S1
excitation was found to correspond to a HOMO-to-LUMO
transition with no significant intramolecular charge-transfer
character. For ADP functionalized with both hexyloxy and
phenylethynyl groups, the BO complex (6a) has NIR optical
properties with λmax of 781 nm in solution, an emission at 805
nm, and a QY of 6%. It forms films that look transparent with a
low optical gap of 1.22 eV. This makes 6a promising for
transparent organic solar cells and NIR photodetectors.
Changing the boron coordination to BF2 increases the optical
gap to 1.52 eV, mainly because of an increase in the IP. DFT
calculations showed that the electron density on the proximal
phenyls directly correlates with the new complex’s IP. This
work advances the development of NIR chromophores and
extends the application of ADP-based dyes.
Figure 9. Contour plots of the HOMO (left) and LUMO (right),
plotted with an isovalue of 0.02 Å−3.
Table 5. ADP Core Percentage Contributions (%) to the
EXPERIMENTAL SECTION
■
̈
Molecular Orbital, Lowdin Atomic Charges of the ADP
Characterization. 1H, 19F, 11B, and 13C NMR spectra were
recorded using a 500 MHz Bruker Ascend Advance III HDTM
spectrometer equipped with a Prodigy ultrahigh-sensitivity Multi-
nuclear Broadband CryoProbe. Glassware was precleaned and flame-
dried prior to use. MALDI-TOF mass spectrometry spectra were
acquired in reflective negative mode on a Bruker Autoflex III
Smartbeam MALDI-TOF spectrometer with terthiophene as the
matrix. Elemental analysis was performed by Robertson Microlit
Laboratories.
Core, and Simulated IPs
compound
HOMO
LUMO
proximal charge, e−
IP, eV
5a
6a
8
60
54
58
55
68
7
72
68
−0.28
−0.22
−0.11
−0.21
5.46
5.53
5.75
5.57
6b
Thermal Measurements. Thermal measurements were con-
ducted with DSC measurements and taken with a DSC Q100 series
from TA Instruments with a ramp rate of 10 °C/min. Samples were
analyzed as crystalline solids. All samples were hermetically sealed
solution-processed films, facilitate crystallization, slightly blue-
shift the absorption and emission peaks, and slightly lower the
IP and EA. The pyrrolic phenylethynyls extend the conjugation
G
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Figure 10. Simulated UV−vis spectra (dashed) compared to measured spectra (solid). Simulated spectra are broadened by a Gaussian with a fwhm
of 0.1 eV and uniformly shifted by −0.40 eV.
compounds were dissolved with HPLC-grade solvent, and measure-
ments were taken without any inert gas sparging. Equation 1 was used
to calculate the QYs:35
2
Slopesample
Slopest
η
sample
ΦF
= ΦF
sample
st
2
η
(1)
st
where Slopest is the slope of a graph of integrated fluorescence
intensity versus absorbance plot for the standard (HITCI), Slopesample
is the slope of a graph of integrated fluorescence intensity versus
absorbance plot for the sample, ηst is the refractive index of the solvent
used for the standard (here, ethanol, 1.36), and ηsample is the refractive
index of the solvent used for the sample (here, chloroform, 1.45).
Electrochemistry. Cyclic voltammetry measurements were
obtained using an Eco Chemie AutoLab PGSTAT302N potentiostat
at room temperature at a scan rate of 0.1 V/s. Glassware was
thoroughly cleaned and dried in the oven overnight prior to use.
Extra-dry DCM obtained from Acros Organics was the solvent.
Ferrocene was purified via sublimation, and tetra-n-butylammonium-
hexafluorophosphate (TBAPF6) was recrystallized from MeOH,
which was then used to make the supporting electrolyte (0.1 M).
The three-electrode configuration used a freshly polished glassy
carbon electrode as the working electrode, a platinum electrode as the
counter electrode, and a freshly polished glassy carbon electrode as
the reference electrode. The IP and EA were determined by
extrapolation of the onsets for the first oxidation and reduction
events, using the value of −4.8 eV for Fc/Fc+. The estimated energy
levels were calculated using
Figure 11. NTO hole and electron orbitals for the S0 → S1 excitation
of four compounds (S0 → S1 singular values are 0.974, 0.974, 0.977,
and 0.975 for compounds 5a, 6a, 8, and 6b, respectively), plotted with
isovalue = 0.02 Å−3.
EIP = Eonset(Ox) + 4.8 eV
EEA = Eonset(Red) + 4.8 eV
using aluminum pans, and a hermetically sealed blank aluminum pan
was used as the reference. TGA was performed using a Q500 series
with a ramp rate of 10 °C/min. The substrate was a platinum pan that
was incinerated with a butane torch prior to use. Both Tm and Td5
(temperature at 5% weight loss) were determined from thermal
measurements.
where Eonset(Ox) is the onset of the first oxidation, Eonset(Red) is the
onset of the first reduction, and 4.8 eV is the electrochemical half-
potential of Fc/Fc+.
Crystallography. Suitable crystals of all compounds were
obtained from solvent diffusion from THF and MeOH as the
antisolvent. Care was taken that the carefully layered solvents were
not disturbed; otherwise, the crystals would degrade. Data were
collected at 100 K using either Cu or Mo rotating anode sources on
Bruker diffractometers equipped with microfocus optics. In most
cases, high-resolution data were not available because of very weak
diffraction at high 2θ values. Nonetheless, data provided unambiguous
structural characterizations despite the frequent presence of slightly-
to-highly disordered alkyl chains.
Optical Properties. Film and solution UV−vis measurements
were collected on a Cary 5000 UV−vis−NIR spectrophotometer with
high-performance liquid chromatography (HPLC)-grade chloroform
in a quart cuvette. Solutions were made from 1 mg of material with 1
mL of HPLC-grade chloroform. Thin films of the compounds were
prepared using a 2 mg/mL solution in CHCl3, which was stirred at 50
°C overnight. Solutions were casted onto cleaned borosilicate glass
using
a syringe and passed through a 0.22 μm poly-
(tetrafluoroethylene) filter. The glass substrates were spin-coated
with a Laurell WS-650 spin coater at 2000 rpm for 30 s. Fluorescence
measurements were obtained on a Cary Eclipse fluorimeter using
quartz cuvettes with a Teflon cap. Solutions were obtained from a 1
mg/mL stock solution and then diluted so that the absorbance values
were below 0.1 to minimize inner-filter effects. QYs were obtained
using 97% 1,1′,3,3,3′,3′-hexamethylindotricarbocyanine iodide
(HITCI) as the standard (st) because λabs is 743 nm and λem is
772 nm, suitable for our complexes (ΦF = 0.283 in ethanol).33,34 All
Geometry Optimization. First, all of the hexyloxy groups were
replaced with methoxy groups to streamline the calculations. Then we
used the Conformer-Rotamer Ensemble Sampling Tool (xtb-CREST)
with the GFN2 force field to generate low-lying conformers.36−38
Next, all conformers were optimized using DFT. We applied tight self-
consistent-field (SCF) convergence thresholds (“TightSCF” option,
grid 4, final grid 6), with the TPSS density functional with D3
dispersion corrections using the Becke−Johnson damping scheme
[D3(BJ)] and the def2-TZVP basis set.39−42 In all cases, the
H
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resolution of the identity was used to approximate the Coulomb
integrals in combination with the def2/J auxiliary basis set.43,44
Solvation effects were included with the conductor-like polarizable
continuum model (CPCM) using dielectric constant 8.93 (DCM).45
For each molecule, the conformer with the lowest TPSS energy was
finally optimized with ωB97X-D3BJ/def2-TZVP/CPCM with ex-
change-type two-electron integrals approximated using the chain of
spheres (COSX) with GRIDX8 and using “TightSCF” and “Tight-
Opt” as the SCF and geometry optimization criteria, respectively.46−48
All results reported use these optimized structures. All other
conformers found to be within 1 kcal/mol (by TPSS) of the
lowest-energy conformer are presented in the Supporting Informa-
tion. For all calculations, we used ORCA 4.2.49
The resulting product is a red metallic solid (1.52 g, 2.23 mmol,
36.2% yield). MALDI-TOF: m/z 681.31. Calcd: m/z 681.36. 1H
NMR (500 MHz, chloroform-d): δ 7.91−7.85 (m, 4H), 7.70 (d, J =
7.8 Hz, 2H), 7.19 (t, J = 7.6 Hz, 2H), 7.13 (s, 2H), 7.00 (d, J = 8.3
Hz, 2H), 6.96−6.87 (m, 6H), 4.03 (t, J = 6.6 Hz, 4H), 1.85 (p, J = 6.8
Hz, 4H), 1.51 (p, J = 6.8 Hz, 4H), 1.41−1.35 (m, 8H), 0.94 (t, J = 6.9
Hz, 6H). 13C NMR (126 MHz, CDCl3): δ 159.88, 157.33, 154.45,
142.42, 131.94, 131.27, 128.65, 126.54, 120.88, 117.63, 117.14,
114.68, 113.82, 77.67, 77.41, 77.16, 68.52, 32.06, 29.72, 26.21, 23.05,
14.48.
6,8-Bis[4-(hexyloxy)phenyl]-14,15-dioxa-4b1,7,7a114-triaza-
14a14-borabenzo[6,7]indeno[3,4-ef ]aceanthrylene (5a; CCDC
2084913). In a 50 mL Schlenk flask attached to a reflux condenser,
4a (1.00 g, 1.47 mmol) was added and then purged with nitrogen
three times. Dry toluene (25 mL) and then BF3OEt2 (2.08 g, 14.67
mmol) were added via a syringe and then stirred for 30 min at reflux.
Dry diisopropylethylamine (3.79 g, 29.33 mmol) was added via a
syringe and then stirred for an additional 3 h. The solution was cooled
to room temperature and then evaporated via a rotary evaporator until
dry. The solids were recrystallized from vapor diffusion using THF/
pentanes. Red metallic crystals were obtained that were washed with
MeOH and then dried. The resulting solid was a metallic red solid
(0.75 g, 1.09 mmol, 74.2% yield). MALDI-TOF: m/z 689.427. Calcd:
Single-Point Calculation. The SCF energies of the neutral
species, cations, and anions were computed using ωB97X-D3BJ/def2-
TZVP/CPCM as described above.
Absorption Spectrum. For each spectrum, we computed the 15
lowest singlet excited states with ωB97X-D3BJ/def2-TZVP/CPCM
and broadened the stick spectrum using Gaussians with a fwhm of 0.1
eV. A uniform shift of −0.40 eV was applied to all simulated spectra to
assist in visual comparison.
NTO Analysis. We use TheoDORE to perform NTO analysis.50
The NTO pair with the largest singular value for the bright S1 state is
shown in Figure 11, along with the associated singular value.
Synthesis. 4-Hexyloxybenzaldehyde (1a). The synthesis was
done according to the literature.51 The compound was used directly
in the next step.
1
m/z 689.66. H NMR (500 MHz, chloroform-d): δ 8.16−8.09 (m,
4H), 7.84 (dd, J = 7.6 and 1.6 Hz, 2H), 7.39 (ddd, J = 8.6, 7.2, and 1.7
Hz, 2H), 7.12 (s, 2H), 7.10 (td, J = 7.5 and 1.1 Hz, 2H), 7.03−6.97
(m, 6H), 4.05 (t, J = 6.7 Hz, 4H), 1.84 (p, J = 6.7 Hz, 4H), 1.51 (p, J
= 7.2 Hz, 4H), 1.43−1.34 (m, 8H), 0.93 (t, J = 7.0 Hz, 6H). 13C
NMR (126 MHz, CDCl3): δ 160.55, 156.19, 149.73, 144.83, 142.20,
133.20, 130.67, 126.70, 125.26, 120.95, 120.33, 119.16, 114.96,
111.57, 77.41, 77.36, 77.16, 76.91, 68.34, 31.76, 29.38, 25.89, 22.78,
14.21. 11B NMR (160 MHz, CDCl3): δ −1.60. Elem anal. Found: C,
76.56; H, 6.39; N, 6.21. Calcd: C, 76.63; H, 6.43; N, 6.09. Tm = 236
°C; Td5 = 390 °C.
2′-Hydroxy-4-hexyloxychalcone (3a). 2′-Hydroxyacetophenone
(2a; 5.65 g, 27.39 mmol), 1a (3.73 g, 27.39 mmol), and sodium
hydroxide (3.29 g, 82.17 mmol) were added to a mortar and ground
using a pestle until an orange paste formed. The paste was left to dry
for 1 h and then ground into a fine orange powder. The powder was
dissolved in MeOH and then neutralized with 1 M HCl to precipitate
bright-yellow solids. The solids were filtered and then recrystallized in
MeOH as bright-yellow crystals (4.13 g, 12.7 mmol, 46.5% yield). 1H
NMR (500 MHz, chloroform-d): δ 12.95 (s, 1H), 7.96−7.88 (m,
2H), 7.65−7.59 (m, 2H), 7.54 (d, J = 15.4 Hz, 1H), 7.49 (td, J = 7.9
and 1.6 Hz, 1H), 7.02 (d, J = 8.4 Hz, 1H), 6.94 (dd, J = 8.0 and 5.7
Hz, 3H), 4.02 (t, J = 6.6 Hz, 2H), 1.85−1.76 (m, 2H), 1.60−1.41 (m,
2H), 1.41−1.30 (m, 4H), 0.91 (t, 3H). 13C NMR (126 MHz,
CDCl3): δ 193.84, 163.70, 161.84, 145.64, 136.26, 130.72, 129.67,
127.25, 120.29, 118.89, 118.74, 117.54, 115.15, 77.41, 77.36, 77.16,
76.91, 68.41, 31.71, 29.26, 25.83, 22.75, 14.19.
6,8-Bis[4-(hexyloxy)phenyl]-5,9-bis(phenylethynyl)-14,15-dioxa-
4b1,7,7a1-triaza-14a14-borabenzo[6,7]indeno[3,4-ef ]-
aceanthrylen-7a1-ium (6a; CCDC 2084912). 4a (750 mg, 1.09
mmol) and N-iodosuccinimide (562 mg, 2.50 mmol) were used,
resulting in a solid (630 mg, 0.890 mmol, 61.5% yield), of which 300
mg was directly used in the next step. In the second step,
tributyl(phenylethynyl)stannane (374 mg, 0.956 mmol) and Pd-
(PPh3)4 (37 mg, 0.032 mmol) were added in the same manner as that
of the previous compound. The solution turned from purple/pink to
dark cyan in color. The resulting solids were recrystallized in THF/
MeOH via solvent diffusion. The crystals were filtered and dried
under vacuum, resulting as a shiny, metallic blue-green solid (176 mg,
0.198 mmol, 62.1% yield). MALDI-TOF: m/z 889.638. Calcd: m/z
4-Hexyloxychalcone (3b). 4-Hexyloxybenzaldehyde1 (1b; 1.75 g,
14.54 mmol), acetophenone (2b; 3.00 g, 14.54 mmol), and sodium
hydroxide (640 mg, 16.00 mmol) were added to a mortar and ground
using a pestle until an off-white paste formed. The paste was left to
dry for 1 h and then further ground to a powder. The powder was
recrystallized from MeOH and then filtered as an off-white solid (4.49
1
889.40. H NMR (500 MHz, chloroform-d): δ 8.68 (d, J = 8.1 Hz,
2H), 8.41−8.36 (m, 4H), 7.63−7.57 (m, 4H), 7.43 (dt, J = 14.9 and
8.1 Hz, 8H), 7.18 (t, J = 7.5 Hz, 2H), 7.06−7.01 (m, 6H), 4.07 (t, J =
6.6 Hz, 4H), 1.85 (p, J = 6.9 Hz, 4H), 1.55 (s, 21H), 1.51 (p, J = 7.9
and 7.5 Hz, 3H), 1.38 (h, J = 3.8 Hz, 8H), 0.93 (t, J = 6.9 Hz, 6H).
13C NMR (126 MHz, CDCl3): δ 160.67, 156.30, 149.87, 144.43,
141.54, 133.95, 132.01, 131.43, 128.77, 128.74, 127.99, 124.62,
123.45, 120.97, 120.13, 119.18, 114.56, 108.85, 98.56, 84.42, 68.33,
31.76, 29.39, 25.90, 22.78, 14.21. 11B NMR (160 MHz, CDCl3): δ
−1.64. Elem anal. Found: C, 80.93; H, 5.85; N, 4.59. Calcd: C, 80.98;
H, 5.89; N, 4.72. Tm = 290 °C; Td5 = 400 °C.
1
g, 14.5 mmol, 64.4% yield). H NMR (500 MHz, chloroform-d): δ
8.01 (dt, J = 7.1 and 1.4 Hz, 2H), 7.79 (d, J = 15.6 Hz, 1H), 7.60 (s,
1H), 7.59−7.54 (m, 2H), 7.50 (dd, J = 8.3 and 6.9 Hz, 2H), 7.41 (d, J
= 15.6 Hz, 1H), 6.95−6.90 (m, 2H), 4.00 (t, J = 6.6 Hz, 2H), 1.80 (p,
J = 6.7 Hz, 2H), 1.47 (p, 2H), 1.40−1.30 (m, 4H), 0.91 (t, 3H). 13C
NMR (126 MHz, CDCl3): δ 190.77, 161.48, 144.98, 138.70, 132.66,
130.38, 128.70, 128.56, 128.45, 127.51, 119.75, 115.06, 77.41, 77.16,
76.91, 68.35, 31.71, 29.27, 25.83, 22.75, 14.18.
(Z)-2-[3-[4-(Hexyloxy)phenyl]]-2-[[3-[4-(hexyloxy)phenyl]-5-(2-
hydroxyphenyl)-1H-pyrrol-2-yl]imino]-2H-pyrrol-5-yl]phenol (4a).
3a (4.00 g, 12.33 mmol), nitromethane (3.76 g, 61.65 mmol), and
triethylamime (5.24 g, 61.65 mmol) were added into a 200 mL
Schlenk flask with 100 mL of MeOH and then refluxed for 16 h. The
solution was cooled in an ice bath and then neutralized with 1 M HCl
until pH ∼2 and precipitates formed. The solids were filtered and
then dried under vacuum. The solids were added to a 200 mL round-
bottom flask with 100 mL of 1-BuOH and ammonium acetate (33.25
g, 431.30 mmol) and then refluxed for 16 h. The solution was a
heterogeneous mixture of a dark-blue-green solution with red
precipitates. The solids were filtered into a Hirsch funnel and then
washed with MeOH until the filtrate was clear and then with hexanes.
6,8-Diphenyl-5,9-bis(phenylethynyl)-14,15-dioxa-4b1,7,7a1-tria-
za-14a14-borabenzo[6,7]indeno[3,4-ef ]aceanthrylen-7a1-ium
(6b). 4b and 5b were made according to the literature.17 5b (482 mg,
0.650 mmol), tributyl(phenylethynyl)stannane (763 mg, 1.95 mmol),
and Pd(PPh3)4 (46 mg, 0.065 mmol) were added into a 200 mL
Schlenk flask. Once evaporated, the dried solids were washed with
MeOH and then redissolved in DCM, and ethyl acetate was then
added. The compound was precipitated from removal of DCM and
then filtered. The product was recrystallized from CHCl3/MeOH via
solvent diffusion as a metallic blue-green solid (336 mg, 0.487 mmol,
1
74.9% yield). MALDI-TOF: m/z 689.411. Calcd: m/z 689.23. H
NMR (500 MHz, chloroform-d): δ 8.70 (dd, J = 8.0 and 1.7 Hz, 2H),
I
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8.41−8.29 (m, 4H), 7.64−7.55 (m, 4H), 7.54−7.45 (m, 8H), 7.44−
7.37 (m, 6H), 7.23−7.16 (m, 2H), 7.05 (dd, J = 8.3 and 1.1 Hz, 2H).
13C NMR (126 MHz, CDCl3): δ 156.14, 150.02, 144.72, 141.74,
7.42 (m, 6H), 7.35−7.26 (m, 10H), 7.06−6.99 (m, 4H), 4.09 (t, J =
6.6 Hz, 4H), 1.86 (p, J = 6.8 Hz, 4H), 1.52 (p, J = 7.2 and 6.5 Hz,
4H), 1.44−1.34 (m, 8H), 0.94 (t, J = 6.6 Hz, 6H). 13C NMR (126
MHz, CDCl3): δ 161.56, 160.94, 144.68, 143.23, 132.38, 131.25,
130.96, 130.57, 130.31, 128.52, 128.49, 127.96, 124.64, 123.39,
114.46, 97.27, 84.16, 68.38, 31.77, 29.38, 25.92, 22.79, 14.22. 11B
NMR (160 MHz, CDCl3): δ 0.71, 0.52, 0.34. 19F NMR (471 MHz,
CDCl3): δ −130.69, −130.76, −130.83, −130.89. Elem anal. Found:
134.31, 131.79, 131.51, 130.43, 129.64, 128.96, 128.77, 128.47,
128.03, 123.24, 121.16, 120.19, 119.07, 110.77, 99.10, 83.96. 11B
NMR (160 MHz, CDCl3): δ −1.61. Elem anal. Found: C, 82.96; H,
3.49; N, 6.04. Calcd: C, 83.61; H, 4.09; N, 6.09. Td5 = 522 °C.
(Z)-3-[4-(Hexyloxy)phenyl]-N-[3-[4-(hexyloxy)phenyl]-5-phenyl-
1H-pyrrol-2-yl]-5-phenyl-2H-pyrrol-2-imine (4c). The procedure is
similar to the synthesis of 4a. 3b (2.50 g, 8.11 mmol), nitromethane
(2.47 g, 40.53 mmol), and trimethylamine (4.10 g, 40.53 mmol) were
added then in the next step with ammonium acetate (10.94 g, 141.92
mmol). The solution was refluxed for 16 h. The resulting reaction
produced a heterogeneous mixture of dark blue/purple solution with
a metallic yellow precipitate. The solution was evaporated with a
rotary evaporator into a dry solid. The solid was resuspended in
MeOH, which was filtered and then washed with MeOH and then
with hexanes. The resulting solid was dried under vacuum as a
metallic yellow/blue solid (0.55 g, 846 mmol, 41.7% yield). MALDI-
TOF: m/z 649.247. Calcd: m/z 649.37. 1H NMR (500 MHz,
chloroform-d): δ 12.74 (s, 1H), 8.06−7.99 (m, 4H), 7.97−7.92 (m,
4H), 7.53 (t, J = 7.6 Hz, 4H), 7.45 (t, J = 7.3 Hz, 2H), 7.11 (s, 2H),
7.00−6.93 (m, 4H), 4.04 (t, J = 6.6 Hz, 4H), 1.84 (p, J = 6.8 Hz, 4H),
1.51 (p, J = 7.3 Hz, 4H), 1.38 (p, J = 6.8 and 4.4 Hz, 8H), 0.93 (t, J =
6.9 Hz, 6H). 13C NMR (126 MHz, CDCl3): δ 159.45, 155.04, 149.66,
142.53, 132.51, 130.49, 130.01, 129.22, 126.63, 114.50, 113.57, 77.41,
77.36, 77.16, 76.91, 68.27, 31.79, 29.44, 25.93, 22.79, 14.22.
(Z)-3-[4-(Hexyloxy)phenyl]-N-[3-[4-(hexyloxy)phenyl]-5-phenyl-
4-(phenylethynyl)-1H-pyrrol-2-yl]-5-phenyl-4-(phenylethynyl)-2H-
pyrrol-2-imine (7). 4a (1.00 g, 1.54 mmol) and N-iodosuccinimide
(727 mg, 3.23 mmol) were added to a 100 mL Schlenk flask with 50
mL of degassed chloroform and 2 drops of glacial acetic acid. The
solution was stirred for 16 h, evaporated via a rotary evaporator,
resuspended in MeOH, and then filtered. The solids were then dried
under vacuum and used directly. Solids (500 mg) were added to a 100
mL Schlenk flask and then purged three times with nitrogen. The flask
was transferred into a glovebox, where Pd(PPh3)4 (64 mg, 0.055
mmol) was added. The flask was taken out of the glovebox and then
charged with 50 mL of dry xylenes and tributyl(phenylethynyl)-
stannane (651 mg, 1.66 mmol) of via a syringe. The solution was
heated to 135 °C for 16 h; meanwhile, the color went from dark
purple to dark blue. The resulting solution was evaporated via a rotary
evaporator, then resuspended in MeOH, filtered, and then washed
with hot ethyl acetate until no color persisted in the washes. The solid
was dried under vacuum, resulting in a metallic purple/red solid. The
solid was confirmed by NMR, with the molecular mass of the solids
confirmed by MALDI-TOF (345 mg, 0.406 mmol, 73.2% yield).
MALDI-TOF: m/z 849.507. Calcd: m/z 849.43. 1H NMR (500 MHz,
chloroform-d): δ 12.69 (s, 1H), 8.29−8.21 (m, 4H), 8.17 (d, J = 8.6
Hz, 4H), 7.55−7.40 (m, 10H), 7.38−7.28 (m, 6H), 6.93 (d, J = 8.6
Hz, 4H), 4.05 (t, J = 6.6 Hz, 4H), 1.93−1.78 (m, 4H), 1.53 (p, J = 7.0
Hz, 4H), 1.46−1.36 (m, 8H), 1.00−0.91 (m, 6H). 13C NMR (126
MHz, CDCl3): δ 159.62, 155.57, 149.09, 142.94, 132.42, 132.06,
131.22, 130.24, 128.84, 128.54, 128.22, 125.54, 123.97, 113.95,
110.16, 97.73, 86.17, 77.41, 77.16, 76.91, 68.21, 31.83, 29.49, 25.97,
22.82, 14.24.
C, 80.45; H, 6.27; N, 4.86. Calcd: C, 80.26; H, 6.06; N, 4.68. Tm
230 °C; Td5 = 413 °C.
=
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01597.
■
sı
TGA thermograms, DSC thermograms of the second
heat cycle, a summary of the thermal properties,
MALDI-TOF spectra of all new compounds, computa-
tional details including conformers’ geometry, ground-
and excited-state properties, and 1H and 13C NMR
spectra of all new compounds (PDF)
Accession Codes
CCDC 2084912−2084914 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
̀
Genevieve Sauvé − Department of Chemistry, Case Western
Reserve University, Cleveland, Ohio 44106, United States;
orcid.org/0000-0002-9721-0447;
Email: genevieve.sauve@case.edu
Authors
Jayvic C. Jimenez − Department of Chemistry, Case Western
Reserve University, Cleveland, Ohio 44106, United States;
orcid.org/0000-0002-2342-5648
Zehao Zhou − Department of Chemistry, Case Western
Reserve University, Cleveland, Ohio 44106, United States;
orcid.org/0000-0003-2185-6855
Arnold L. Rheingold − Department of Chemistry and
Biochemistry, University of California, San Diego, La Jolla,
California 92093, United States; orcid.org/0000-0003-
4472-8127
Shane M. Parker − Department of Chemistry, Case Western
Reserve University, Cleveland, Ohio 44106, United States;
orcid.org/0000-0002-1110-3393
Complete contact information is available at:
5,5-Difluoro-1,9-bis[4-(hexyloxy)phenyl]-3,7-diphenyl-2,8-bis-
(phenylethynyl)-5H-514-dipyrrolo[1,2-c:2′,1′-f ][1,3,5,2]-
triazaborinin-4-ium (8; CCDC 2084914). 4b (50 mg, 0.059 mmol)
was added to a 100 mL Schlenk flask with 50 mL of dry toluene for 30
min under reflux conditions. BF3OEt2 (84 mg, 0.59 mmol) and
diisopropylethylamine (78 mg, 0.588 mmol) were added dropwise,
then stirred for 3 h, and then cooled to room temperature. The
solution was washed with three aliquots of water (10 mL), then dried
with magnesium sulfate, and then passed through a plug of Celite.
The compound was evaporated and then recrystallized in DCM and
MeOH. The solids were dried under vacuum, resulting in crystalline
metallic purple solids (45 mg, 0.050 mmol, 85% yield). MALDI-TOF:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01597
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are thankful for the National Science Foundation (NSF;
Grant CHEM 19048678) for funding the experimental part of
■
1
m/z 897.650. Calcd: m/z 897.43. H NMR (500 MHz, chloroform-
d): δ 8.35−8.29 (m, 4H), 8.02 (dd, J = 6.6 and 2.9 Hz, 4H), 7.55−
J
https://doi.org/10.1021/acs.inorgchem.1c01597
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
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this work and Case Western Reserve University for funding the
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voltammetry instrumentation. This project was possible thanks
to Grant NSF MRI-0821515 for MALDI-TOF instrumentation
and Grant NSF MRI-1334048 for NMR instrumentation. This
work made use of the High Performance Computing Resource
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