The synthesis and structures of arene-substituted azadipyrromethenes

Article abstract of DOI:10.1016/j.poly.2015.09.034

We present the synthesis of several azadipyrromethenes with four peripheral arene units which can be readily generated via the well-established chalcone synthetic method. The stability and yields of these azadipyrromethenes is highly dependent on the nature of the arene substituent, with the bis-pyridine systems exhibiting the highest degree of instability. The structures of several of the compounds and their BF₂ adducts are presented; intramolecular hydrogen bonding is observed in the free base ADPMs. We also present a study into the absorption and emission properties of the ADPMs and their BF₂ adducts and observe that they are also highly dependent on the identity of the substituents.

Full text of DOI:10.1016/j.poly.2015.09.034

Accepted Manuscript
The synthesis and structures of arene-substituted azadipyrromethenes
Kullapa Chanawanno, Abed Hasheminasab, James T. Engle, Christopher J.
Ziegler
PII:
S0277-5387(15)00536-7
DOI:
http://dx.doi.org/10.1016/j.poly.2015.09.034
POLY 11548
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
2 July 2015
15 September 2015
Please cite this article as: K. Chanawanno, A. Hasheminasab, J.T. Engle, C.J. Ziegler, The synthesis and structures
of arene-substituted azadipyrromethenes, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.09.034
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The
synthesis
and
structures
of
arene-substituted
azadipyrromethenes
Kullapa Chanawanno, Abed Hasheminasab, James T. Engle and Christopher J.
Ziegler*
Department of Chemistry, University of Akron, Akron, OH 44325-3601, USA
ABSTRACT: We present the synthesis of several azadipyrromethenes with four peripheral arene units
which can be readily generated via the well-established chalcone synthetic method. The stability and
yields of these azadipyrromethenes is highly dependent on the nature of the arene substituent, with the
bis-pyridine systems exhibiting the highest degree of instability. The structures of several of the
compounds and their BF2 adducts are presented; intramolecular hydrogen bonding is observed in the free
base ADPMs. We also present a study in to the absorption and emission properties of the ADPMs and
their BF2 adducts and observe that they are also highly dependent on the identity of the substitutents.
KEYWORDS: Azadipyrromethene, boron difluoride, chalcone, X-ray crystallography, UV-visible
absorption, fluorescence
* Correspondence to: Department of Chemistry, University of Akron, Akron, OH 44325-3601 USA. Fax: (+001) 330-972-6085; Tel:
(+001) 330-972-2531 ; E-mail: ziegler@uakron.edu

1. INTRODUCTION
The dipyrromethene and azadipyrromethene families of compounds (1 and 2, Fig. 1) can be considered as “half-
macrocycles:” bidentate chelate analogs of porphyrins and azaporphyrins respectively. Both types of chelates can be
used to coordinate a variety of main group and transition metal ions [1-3], and their chemistry with boron has been
extensively explored, such as in the BODIPY family of fluorophores [4-6]. The dipyrromethenes can be synthesized
from pyrroles and aldehydes, the same reagents used to make porphyrins. Azadipyrromethenes (ADPMs) were first
synthesized in the 1940s by Rogers [7-8] and are produced via very different synthetic methodologies than the
structurally similar phthalocyanines. One common route for generating these compounds is via chalcone intermediates
(Scheme 1), which involves the synthesis of a chalcone via condensation, followed by a Michael addition to the
chalcone and formation of the desired chelates using ammonium acetate.
The chelating properties of the ADPMs and in particular the photophysical properties of their BF2 adducts has
spurred synthetic work on these ligand systems [9-12]. However, in contrast to the dipyrromethenes, less work has been
carried out on ADPM systems, and the chalcone-based syntheses exhibit different challenges. In this report, we present
a study into the synthesis of several arene substituted azadipyrromethenes (compounds 5) and their difluoroboron
adducts (compounds 6). The arene groups, positioned at the 2 and 4 positions on the pyrroles, include pyridines and
phenyl groups both with and without substitutions. The 2-pyridyl ADPM compounds, also recently reported by Bessette
et al. [13], often rearrange to form terpyridines, but can be stabilized with an electron donating group [14]. For the
remaining compounds, some have been synthesized but not structurally characterized via X-ray methods, and others
have not been probed for their photophysical properties. All compounds and intermediates have been fully
characterized, and we report the structures of several intermediate compounds along with the resultant ADPMs and
their BF2 adducts. With the increasing importance of BODIPY analogues in both fundamental and applied studies, we
hope to highight the synthetic challenges of ADPM and azaBODIPY synthesis, as well as the structural and
photophysical properties of these compounds.
2. MATERIALS AND METHODS
2.1. Materials and General Procedures
Chemicals were obtained commercially and used without further purification. Basic alumina (50−200 µm) was
purchased from Sorbent Technologies. NMR spectra were recorded on a Varian Mercury 300 MHz and Varian NMRS
500 MHz (2 channel with automation) spectrometers. Chemical shifts were reported with respect to residual solvent
peaks as internal standard (1H: CDCl3, δ = 7.26 ppm, d6-DMSO, δ = 2.50 ppm; 13C: CDCl3, δ = 77.36 ppm, d6-DMSO, δ
= 39.7 ppm). Electrospray MS (ES-MS; positive mode) spectra were recorded using a Bruker HCT-ultra ETD II Ion
Trap mass spectrometer. UV-visible spectroscopy was carried out on a Hitachi U-3010 spectrometer. Fluorescence
measurements were made on a Horiba Jobin Yvon FluoroMax-4 spectrofluorometer. Absorption and emission
measurements were carried out using standard 1 cm pathlength quartz cuvettes. Oxazine 170 was used as the standard
(φ = 63% in methanol) [15]. Due to possible decomposition of compounds, all spectroscopic experiment was using
freshly prepared solution samples.

2.2. Syntheses
3a-f: Chalcone synthesis: All chalcones syntheses were carried out using a similar procedure, and the following is
representative of the synthesis of 3a-f: An ethanolic solution (15 mL) of both acetylpyridine (10 mmol) and
benzaldehyde or p-substituted-benzaldehyde (10 mmol) were mixed in a 250-mL Erlenmeyer flask and 2 mL of a 30%
aqueous NaOH solution was then added dropwise (using a Pasteur pipet). In most cases, the reaction mixture turned
yellow or orange immediately after NaOH was added. After 12 hours, an excess of cold water was added to the flask
while the mixture was stirred vigorously until a solid precipitate was observed. The solid was collected, washed with
cold water, and air-dried. The purity of the crude product was determined at this point using 1H NMR spectroscopy. In
all cases, the spectroscopic characteristics were in good agreement with those found in the literature literature with
percent yields above 60% for all, which is similar to what has been reported earlier [16]. All chalcones could be used
immediately in the next step.
4a-f: Nitroketone synthesis: All nitroketone syntheses were carried out using a similar procedure and the following
is representative of the synthesis of 4a-f: The chalcone (3.9 mmol), nitromethane (20 mmol), and triethylamine (35
mmol) were combined in 30 mL of ethanol and heated to reflux for 48 h. The ethanol was removed, and the resulting oil
was portioned between CH2Cl2 (50 mL) and water (50 mL). The organic layer was washed with DI water (2 × 50 mL),
and the organic fractions were dried over anhydrous magnesium sulfate. The solvent was removed by rotary
evaporation, and the resulting oily brown product was use in next process without further purification. Compounds 4b,
4c and 4f are new. Single crystals of 4b were grown by slow evaporation from ethanol. All nitroketones were
produced in moderate to good yield: 4a: 1.02 g, 39%, 4c: 1.12 g, 44%, 4d: 2.03 g, 71%, 4e: 1.46 g, 42%, 4f: 1.50 g,
35%.
1
4b: Yield: 1.70 g, 68%. H (CDCl3) δ (ppm) 2.92 (s, 6H, CH3), 3.59 (dd, 1H, J = 18,9 Hz, CH2); 3.81 (dd, 1H, J =
18,9 Hz, CH2); 4.17 (quintet, 1H, J = 9 Hz, CH); 4.63 (dd, 1H, J = 18,9 Hz, CH2), 4.75 (dd, 1H, J = 18,9 Hz, CH2); 6.67
(d, 2H, J = 9 Hz, H in benzene); 7.17 (d, 2H, J = 9 Hz, H in benzene), 7.45-7.50 (m, 1H, H in pyridine), 7.82 (td, 1H, J
= 9,3 Hz, H in pyridine), 7.99 (dt, 1H, J = 9,3 Hz, H in pyridine), 8.67 (d, 1H, J = 9 Hz, H in pyridine).
5a-f Azadipyrromethene synthesis: All azadipyrromethenes were synthesized using a similar method as follows:
An ethanolic solution of the nitroketone (10 mmol) and ammonium acetate (0.30 mol) were refluxed for 48-72 h. After
the solution was cooled to room temperature, the solvent was removed by rotary evaporation and crude product was
dissolved in CH2Cl2 and washed with water. The CH2Cl2 layer was collected, dried over Na2SO4 and evaporated to
dryness. The resulting green or blue oil was purified by column chromatography using basic alumina and 5%
methanol/CH2Cl2 as the mobile phase. Compounds 5a, 5c, 5d, 5e and 5f were isolated as light blue solids and 5b as
dark blue solid. The absorption/emission data of all compounds are summarized in Table 2. Single crystals of 5b, 5d,
and 5f were grown by slow evaporation from dichloromethane.
5a: Yield: 5.0 mg, 0.11%. 1H (CDCl3) δ (ppm) 8.55 (d, 1H, J = 6.6 Hz, H in pyridine); 8.50 (d, 1H, J = 6.6 Hz, H in
pyridine); 8.01 (br t, 1H, J = 6.6 Hz, H in pyridine); 7.71-7.69 (m, 2H, H in pyridine and benzene); 7.55-7.62 (m, 4H, H
in benzene); 7.34 (s, 1H, H in pyrrole). 13C (CDCl3) δ (ppm) 143.50, 141.17, 135.10, 135.00, 130.73, 129.74, 128.97,
128.47, 127.38, 120.71, 120.03, 113.99, 109.94. m/z (ESI) calcd [M-C4H5 (pyridine dissociation)]+ for C26H16N5 398.1,
found 398.3.
1
5b: Yield: 57 mg, 1.1%. H (CDCl3) δ (ppm) 8.63 (d, 1H, J = 6.0 Hz, py); 8.06 (d, 1H, J = 6.0 Hz, H in pyridine);
7.96 (d, 2H, J = 9.0 Hz, H in benzene); 7.70 (m, 1H, H in pyridine); 7.18 (s, 1H, H in pyrrole); 7.17 (m, 1H, H in
pyridine); 6.68 (d, 2H, J = 9.0 Hz, H in benzene); 2.94 (s, 6H, 2xCH3). 13C (CDCl3) δ (ppm) 174.21, 154.88, 150.93,

150.24, 149.81, 143.04, 136.47, 130.35, 123.36, 122.70, 121.62, 112.62, 112.02, 40.38. m/z (ESI) calcd [M+H]+ for
C34H31N7 538.7, found 538.2.
1
5c Yield: 6 mg, 0.13%. H (CDCl3) δ (ppm) 8.54 (d, 1H, J = 6.6 Hz, H in pyridine); 8.49 (d, 1H, J = 6.6 Hz, H in
pyridine); 7.99 (br t, 1H, J = 6.6 Hz, H in pyridine); 7.60-7.52 (m, 3H, H in pyridine and benzene); 7.39-7.42 (m, 3H, H
in benzene and pyrrole); 2.46 (s, 3H, CH3). 13C (CDCl3) δ (ppm) 143.47, 141.29, 135.07, 134.83, 130.42, 128.84,
128.49, 127.76, 124.42, 120.63, 119.99, 113.92, 109.47, 21.56. m/z (ESI) calcd [M-C4H5 (pyridine dissociation)]+ for
C28H20N5 426.2, found 426.3.
1
5d Yield: 250 mg, 5.2% H (CDCl3) δ (ppm) 7.99 (d, 1H, J = 5 Hz, H in benzene); 7.79 (d, 1H, J = 5 Hz, H in
benzene); 7.37-7.42 (m, 3H, H in benzene); 7.26-7.28 (m, 3H, H in benzene and pyrrole); 7.02 (d, 2H, J = 5 Hz, H in
benzene). 13C NMR data cannot be obtained due to solubility issue.
1
5e Yield: 479 mg, 7.9% H (CDCl3) δ (ppm) 7.96 (d, 1H, J = 7.0 Hz, H in benzene); 7.70 (d, 2H, J = 7.0 Hz, H in
benzene); 7.59 (br d, 2H, J = 7.0 Hz, H in benzene); 7.53 (br d, 1H, J = 7.0 Hz, H in benzene); 7.35 (t, 1H, J = 7.0 Hz, H
in benzene); 7.25-7.31 (m, 2H, H in benzene); 7.08 (s, 1H, H in pyrrole). 13C (CDCl3) δ (ppm) 175.79, 132.39, 132.04,
131.02, 129.05, 128.27, 127.80, 127.38, 124.47, 114.90, 104.99.
5f Yield: 111 mg, 1.4% 1H (CDCl3) δ (ppm) 8.15-8.26 (m, H in benzene); 7.80 (s, 1H, H in pyrrole); 7.72-7.83 (m,
3H, H in benzene); 7.49-7.55 (m, 3H, H in benzene). 13C NMR data cannot be obtained due to solubility issue.
6d-f Boron aza-dipyrromethenes synthesis: The azadipyrromethene (0.15 mmol, 1 eq.) was dissolved in
dichloromethane (15 mL) and diisopropylethylamine (0.27 mL, 1.50 mmol, 10 eq.) and BF3·OEt2 (0.29 mL, 2.25 mmol,
15 eq.) were added dropwise. The solution was stirred at room temperature for 24 h. The organic layer was evaporated
under vacuum. The crude product was purified by chromatography on silica gel (1:1 CH2Cl2: hexane) to afford dark
green solid. Single crystals of 6f were grown by slow evaporation from dichloromethane.
6d Yield: 36.7 mg, 50% 1H (CDCl3) δ (ppm) 8.16 (d, 2H, J = 7.2 Hz, H in benzene); 7.87 (dd, 1H, J = 7.2, 1.2 Hz, H
in benzene); 7.48 (m, 2H, H in benzene); 7.40-7.44 (m, 2H, H in benzene); 7.25 (s, 1H, H in pyrrole); 7.12 (t, 1H, J =
7.2 Hz, H in benzene); 7.03 (d, 1H, J = 7.2 Hz, H in benzene). 13C (CDCl3) δ (ppm) 155.89, 149.84, 142.24, 133.37,
132.39, 129.28, 129.02, 128.72, 126.60, 125.49, 120.95, 120.22, 118.88, 113.32.
1
6e Yield: 55 mg, 56% H (CDCl3) δ (ppm) 8.05-8.08 (m, 2H, H in pyridine); 7.92 (dd, 1H, J = 7.0, 1.0 Hz, H in
benzene); 7.87 (s, 1H, H in pyrrole); 7.74 (d, 1H, J = 7.0 Hz, H in benzene); 7.63-7.66 (m, 2H, H in benzene); 7.55 (t,
1H, J = 7.0 Hz, H in benzene); 7.46-7.51 (m, 1H, H in benzene). 13C (CDCl3) δ (ppm) 156.42, 150.69, 138.15, 131.96,
131.89, 129.39, 129.19, 128.68, 127.20, 123.68, 117.19.
6f Yield: 51 mg, 42% 1H (CDCl3) δ (ppm) 8.01-8.05 (m, 3H, H in benzene); 7.73 (br t, 1H, H in benzene); 7.50-7.52
(m, 3H, H in benzene); 7.44 (br t, 1H, H in benzene); 7.05 (s, 1H, H in pyrrole). 13C NMR data cannot be obtained due
to solubility issue.
3. RESULTS AND DISCUSSION
3.1 Synthesis
Work on ADPMs has primarily focused on the end products, and thus research in to their fundamental properties has
been sporadic. The most comprehensive study was carried out by Bessette and coworkers, who examined a series of
arene substituted ADPMs, which included the unmodified tetraphenyl and bis-pyridyl compounds [13-14]. We became
interested in pyridine-substituted ADPMs as possible tetradentate chelates, but we found that these compounds were
unstable and thus unsuitable for metal chelation. Bessette and coworkers reported the same issues, as will be noted
below. We have also investigated bis-phenol substituted ADPMs with greater success and have carried out some

structural studies. Additionally, there has been interest in bromine-substituted ADPMs, since they can be readily
functionalized via C-C bond forming reactions.
The synthesis of ADPMs was performed by starting with the synthesis of chalcone derivatives via a Claisen-Schmidt
condensation of an aldehyde and acetophenone using base as a catalyst. The desired chalcones can be produced in good
yields from the reaction at room temperature in ethanol. The chalcones were then used in a Michael addition reaction
with nitromethane to yield the corresponding nitroketones. Both the chalcones and the nitroketones were isolated as
stable crystalline materials; we were able to elucidate the structures of two of the intermediate structures (compounds
3b and 4b) and both exhibit the expected features (see supplemental information).
ADPMs were subsequently obtained by reaction of the nitroketones with ammonium acetate in alcoholic solvent
under reflux. A relatively long reaction time (up to three days) is required in most cases. All ADPMs can be purified
with column chromatography. Basic alumina gel was a better stationary phase than silica gel, in particular for the
pyridine compounds, due to their lack of mobility on silica. In general, low yields (less than 10%) of 5a-5c were
observed, which is comparable with literature observations [17-18]. Thus, despite the facility of the chalcone synthetic
method, the low yield of the final step can limit the usefulness of this strategy. Moreover, the instability of 5a and 5c
preclude us from synthesizing their aza-BODIPY analogues. The yields of both nitroketones as well as ADPMs likely
depends on the identity of the R groups attached to phenyl rings (5a; R=H, 5b; R=N(CH3)2, 5c; R=CH3). Strongly
electron donating groups, such as R=N(CH3)2, resulted in the highest yields for compound 4b and 5b. This implies the
ability of electron donating/withdrawing ability of R can induce the production efficiency of these ADPMs.
All compounds showed moderate to good solubility in several organic solvents e.g. chloroform, dichloromethane,
acetone, and alcohols. The NMR spectra of all ADPMs were in accordance with the data reported in the literature [19-
20]. The aromatic proton signals were found in the range between 6.0 ppm and 9.0 ppm. The diagnostic peak for the
pyrrolic C-H group appears as a singlet and found around 7.20-7.30 ppm. The solvent signal from CDCl3 can possibly
overlap with this peak, however. In solution phase, all compounds tend to decompose over time as observed by the
1
gradual disappearance of H signals in NMR. Bessette and coworkers have demonstrated that the similar compound as
5a containing proximal pyridines underwent decomposition especially in solution phase.[13] We observed degradation
to form tripyrranes, which would often crystallize out of solutions of 5a. The degradation of this product possibly
resulted from the increased reactivity of the adjacent carbons of the N-bridge due to conjugation with electron-poor
pyridine substituents. However, an electrochemistry study on a free azadipyrromethene with electron-rich anisole
groups also reported similar instability [4].
The more stable compounds 5d-5f were further reacted with BF3·OEt2 and compounds 6d-6f were obtained. Of these
compounds, compound 6d and 6e have been prepared previously; only 6f is a new compound. Unlike the ADPM
freebase syntheses, reactions to insert BF2 proceed reasonably easily and afford products in decent yield; our
observations on these compounds are in agreement with literature values.
3.2 X-Ray crystallography
Both the ADPMs and their BF2 complexes readily form single crystals and can often be elucidated by X-ray
crystallography. Of the compounds that we investigated, the structure of 6e has been previously presented in the
literature [21], and a disordered structure of 5a was also elucidated by Bessette and coworkers [13]. We were also able
to obtain a structure of compound 5a, but encountered the same problems and were not able to get an improved
structure. However, we were able to obtain good X-ray data for the dimethylaminophenyl analog compound 5b. The
bis-phenol ADPM 5d was structurally characterized and is similar to several systems also presented by Bessette and
coworkers [13-14]. In addition, we were able to elucidate the structure of the tetrabromo compound 5f and its BF2

analog 6f. The structures of these four compounds are shown in Figure 2 and the X-ray data collection and structure
parameters can be found in Table 1.
For compound 5b, the examination of the molecular structure revealed an orthorhombic system with the space group
Pna21. As shown in Figure 2, the dipyrrole unit is planar and each of the phenyl rings is slightly tilted from that plane
with the angles being ~29 and 34o. The pyridine rings are coplanar with the dipyrrole unit. Intramolecular hydrogen
bonding was observed between two pyrrole rings with an N-N distance of ~2.83 Å. The crystal packing exhibited a zig-
zag pattern with no significant strong intermolecular hydrogen bonds.
Compound 5d, which was solved in the monoclinic C2/c space group, also showed hydrogen bonding, but in this
case the strongest bonding was observed between the phenyl hydroxyl groups and the pyrrole nitrogen atoms. Both
interactions measure ~2.65 Å. Weaker hydrogen bonding interactions are also seen between the two nitrogen positions
and between the two oxygen atoms, measuring ~2.93 and ~2.96 Å respectively. No intermolecular hydrogen bonding is
observed in the packing structure, which exhibits a herringbone type pattern. The planes of the two phenyl rings are
slightly tilted from the plane of the dipyrrole, with angles of ~29 and ~24°. For the phenol rings, one is co-planar with
the dipyrrole while the second exhibits a slight deviation of ~8°.
The tetrabrominated compound 5f was solved in the triclinic P-1 space group and forms layers in the extended
packing structure. The hydrogen bonding between the pyrrolic nitrogen positions is a bit stronger than in 5b, measuring
~2.72 Å, and there are no intermolecular hydrogen bonding contacts. In 5f, all four of the phenyl rings are tilted away
from the planar dipyrrolic unit, with the brominated rings tilted at ~27 and ~21° away from the plane, and the
unsubstituted phenyls tilted at ~20 and ~27°.
We were also able to elucidate the structure of the tetrabromo ADPM BF2 adduct 6f. Like 5f, this compound adopts
the triclinic P-1 space group and forms layers in the solid. In this structure, there is a dicholormethane solvent molecule
in the void space. All four phenyl rings are tilted away from the plane of the dipyrrole unit, as seen in the free base
ADPMs: the two brominated phenyl rings form angles of ~27 and 44° away from this plane and the two unusbstituted
phenyls are at ~12 and ~29°. Due to the BF2 binding at the core, no intra- or intermolecular H-bond is observed in this
structure.
3.3 Photophysical properties
In spite of the interest in free base ADPMs and their boron complexes for photonic applications, the number of
fundamental studies on these simple chromophores has been limited. The most comprehensive study to date was
reported in the Bassette paper, which examined absorption characteristics and carried out DFT calculations to provide
insight into the nature of these transitions. We probed both the absorption and emission properties of both the free base
and BF2 ADPMs synthesized for this work. With regard to absorbance, our observations are in good agreement with the
results and conclusions from the Bessette manuscript. All of the azadipyrromethenes synthesized exhibited strong
absorbance bands around 600 nm and higher. Compounds 5 show absorptions with moderate molar absorptivities
ranging from ~15,000 to 40,000 M-1cm-1. In addition, all of the compounds exhibited spectra with similar lineshapes
with the exception of compound 5b, which displays significant broadening and loss of intensity of the primary
absorption band.
The free base ADPMs exhibited fluorescent emission above 640 nm when excited at their primary absorption band,
with the exception of compound 5b, which exhibited no fluorescence. The absorption/emission characteristics of both
5a and 5c are very similar to the previously reported heterocycles-containing ADPM (thiophene-ADPM) [22].
Compounds 5a and 5c exhibit quantum yields of ~6.5 and ~11 percent, respectively. The quantum yield values of the
aryl substituted ADPMs 5d-5f, with the exocyclic bromide and hydroxide substitutents, are much smaller than in 5a and
5c, and are only a fraction of a percent. One notable quality seen in the free base ADPM compounds are the magnitude

of their Stokes shifts, which are larger than that of similarly structured BODIPY compounds such as tetramethyl-
BODIPY (which is less than 10 nm) [23]. In particular, very large Stokes shifts were seen in the brominated analogs 5e
(49 nm) and 5f (48 nm).
The aza-BODIPY compounds 6d-6f exhibited more intense fluorescence emissions that are bathochromically shifted
relative to the free base chelates. Compounds 6e and 6f exhibit similar energies of emission (~684 nm), but the
flurorescence from 6d is even lower in energy at ~741 nm, which follows the trends seen in the free base absorption
and emission spectra. The Stokes shifts, while larger than those seen in BODIPYs, are smaller than in the free base
compounds.
4. CONCLUSIONS
In conclusion, we have synthesized a series of ADPM compounds and several BF2 adducts, and present a study into
their structures and photophysical attributes. Although the chalcone method for synthesis of ADPMs is readily
accessible via easily prepared starting materials, this strategy can suffer from low yields in the final step. The stability
of the resultant ADPM materials is highly dependent on the identity and substitution of the phenyl groups, however,
when stable, free base ADPMs can be readily structurally elucidated. Both ADPMs and their BF2 complexes are
fluorescent, and in particular the BF2 adducts exhibit red shifted emissions relative to their BODIPY analogs. Our
observations are in good agreement with previously reported studies.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the University of Akron for support of this research. K.C. acknowledges
Dr.Natalie Romano for fluorescent measurement suggestions and the Royal Thai Government for financial support.
APPENDIX A. SUPPLEMENTARY DATA
CCDC 1063881-1063884 contain the supplementary crystallographic data for 5b, 5d, 5f and 6f. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/XXXXXXXXXXX.
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Fig. 1. The structures of dipyrromethene (1, left) and azadipyrromethene (2, right).

Fig. 2. Structures of 5b, 5d, 5f and 6f with 50% thermal ellipsoids. Non-ionizable hydrogen atoms have been oomitted for clarity.

Fig. 3. Absorption (left) and emission (right) spectra in CH2Cl2 for compounds
5 and 6.

Scheme 1. Synthetic pathways of ADPMs and their analogues, i: NaOH, rt in EtOH 24h; ii: CH3NO2, NEt3, reflux in EtOH 24-72 h;
iii: NH4OAc, reflux in EtOH 48-72 h; iv: BF3·OEt2, DIPEA, room temp in EtOH 24 h.

Compound
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
5b
5d
5f
6f
C34H31N7
537.66
orthorhombic
Pna21
C32H23N3 O2
481.53
monoclinic
C2/c
C32H19Br4N3
765.14
Triclinic
P-1
C33H20BBr4Cl2F2N3
897.87
Triclinic
P-1
18.7433(5)
14.6652(4)
10.1077(3)
90.00
40.455(4)
3.8342(4)
29.733(3)
90.00
7.2063(3)
13.4467(6)
14.3807(7)
79.159(2)
79.864(3)
85.742(2)
1346.05(11)
2
10.8200(15)
12.1002(15)
13.393(2)
83.532(5)
66.530(4)
78.096(3)
1572.9(4)
2
b (Å)
c (Å)
α (°)
β (°)
90.00
93.714(8)
90.00
γ (°)
90.00
Volume (Å3)
Z
2778.35(13)
4
4602.3(8)
8
Dc (Mg/m3)
µ (mm-1)
1.285
1.390
1.888
1.896
0.617
0.699
7.520
8.166
F(000)
1136.0
16683
2016.0
20736
744
872
Reflections collected
Data/Restraints/Parameters
GOF on F2
R1 (on Fo2, I > 2 (I))
wR2 (on Fo2, I > 2 (I))
R1 (all data)
wR2 (all data)
13997
17304
4544/1/374
1.193
3610/6/336
1.085
4213/0/352
1.044
4949 / 24 / 406
1.136
0.0325
0.0779
0.0389
0.0806
0.0396
0.1083
0.0433
0.1167
0.0456
0.1226
0.0485
0.1267
0.0368
0.1096
0.0369
0.1097
Table 1. X-ray data collection and structural parameters for 5b, 5d, 5f and 6f.

λ
abs (nm)
ε (M-1cm-1)
36,300
24,400
17,700
36,400
14,300
31,900
36,500
39,400
35,100
34,400
80,300
15,700
88,300
301,600
86,000
18,700
63,700
λ
emis (nm)
φ (%)*
compound
5a
Stokes shift (nm)
635
297
631
640
307
614
313
604
304
598
325
498
724
317
657
315
657
655
-
20
6.5
5b
5c
5d
-
-
670
643
30
29
11
0.029
5e
5f
653
646
49
48
0.033
0.064
6d
741
17
4.3
6e
6f
683
684
26
27
23
7.6
Table 2. Absorption and emission data for compounds 5 and 6 (in CH2Cl2). *The percentage values were achieved by fluorescent
study using oxazine 170 as the reference (ϕMeOH = 63%).

Graphical abstract synopsis
We present the synthesis of several azadipyrromethenes with four peripheral arene units which can
be readily generated via the well-established chalcone method. The structures of several of the
compounds and their BF2 adducts are presented; we also present a study into their absorption and
emission properties.

Graphical Abstract