Near-Infrared Photoactive Aza-BODIPY: Thermally Robust and Photostable Photosensitizer and Efficient Electron Donor

Article abstract of DOI:10.1002/cphc.202000117

We report herein the synthesis of aza-BODIPY substituted with strongly electron-donating p-(diphenylamino)phenyl substituents (p-Ph₂N?) at 3,5-positions. The presence of p-Ph₂N? groups lowers the energy of the singlet excited state (

Full text of DOI:10.1002/cphc.202000117

Accepted Article
Title: Near infrared photoactive aza-BODIPY: thermally robust and
photostable photosensitizer and efficient electron donor
Authors: Łukasz Łapok, Igor Cieślar, Tomasz Pędziński, Katarzyna M.
Stadnicka, and Maria Nowakowska
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To be cited as: ChemPhysChem 10.1002/cphc.202000117
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Graphical abstract
Near infrared photoactive aza-BODIPY: thermally robust and
photostable photosensitizer and efficient electron donor
Dr. Łukasz Łapok1*, Igor Cieślar1, Dr. Tomasz Pędziński2,3, Prof. Dr. Katarzyna M. Stadnicka1,
Prof. Dr. Maria Nowakowska1
1Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland.
2Faculty of Chemistry, Adam Mickiewicz University in Poznań, 89b Umultowska, 61-614 Poznań,
Poland.
3Center for Advanced Technology, Adam Mickiewicz University in Poznań, 10, 61-614 Poznań, Poland
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Table-of-contents
Abstract
1. Introduction
2. Results and discussion
2.1.Synthesis and characterization
2.2.Optical properties
2.3.Time-resolved spectroscopy
2.4.Photosensitization
2.5.Electrochemistry and HOMO/LUMO energy levels
2.6.Photostability and thermal stability
3. Discussion
4. Experimental
4.1.Materials and methods
4.2.Synthesis
5. Conclusions
6. Acknowledgements
7. References
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ChemPhysChem
Near infrared photoactive aza-BODIPY: thermally robust and
photostable photosensitizer and efficient electron donor
Dr. Łukasz Łapok1*, Igor Cieślar1, Dr. Tomasz Pędziński2,3, Prof. Dr. Katarzyna M. Stadnicka1,
Prof. Dr. Maria Nowakowska1
1Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland.
2Faculty of Chemistry, Adam Mickiewicz University in Poznań, 89b Umultowska, 61-614 Poznań,
Poland.
3Center for Advanced Technology, Adam Mickiewicz University in Poznań, 10, 61-614 Poznań, Poland
*Corresponding author: lukaszlapok@chemia.uj.edu.pl, Tel.: +48 12 686 25 31, Fax: + 48 12 686 27
50.
Keywords: aza-BODIPY; triplet photosensitizer; excited-triplet state; near infrared
absorption; photosensitization,
Abstract
We report herein a synthesis of aza-BODIPY substituted with strongly electron-donating p-
(diphenylamino)phenyl substituents (p-Ph2N-) at 3,5-positions. The presence of p-Ph2N-
groups lowered the energy of the singlet excited state (Es) to 1.48 eV and induced NIR
absorption with λabs at 789 nm in THF. The compound studied was weakly emissive with the
emission band (λf) at 837 nm and with the singlet lifetime (τS) equal 100 ps. Nanosecond
laser photolysis experiments of aza-BODIPY in question revealed T1→Tn absorption spanning
from ca. 350 nm to ca. 550 nm with the triplet lifetime (τT) equal 21 μs. By introducing a
heavy atom (Br) into the structure of the aza-BODIPY we managed to turn it into a NIR
operating photosensitizer. The photosensitized oxygenation of the model compound –
diphenylisobenzofuran (DPBF) – proceeded via Type I and/or Type III mechanism without
formation of singlet oxygen (1O2). As estimated by CV/DPV measurements, the p-Ph2N-
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substituted aza-BODIPYs studied exhibited oxidation processes at a relatively low oxidation
1
potentials (Eox ), pointing to the very good electron-donating properties of these molecules.
Extremely high photostability and thermal robustness up to approximately 300°C were
observed for the p-Ph2N- substituted aza-BODIPYs.
1. Introduction
Dyes active in the near-infrared part of the spectrum (700 – 1100 nm) are highly sought-after
for both biomedical and high-tech applications [1,2]. Exemplarily, NIR active fluorophores
are promising molecular probes for both in vivo and in vitro bioimaging [3-5]. The advantage
of imaging in the NIR region stems from the fact that at wavelengths above 700 nm tissue
autofluorescence is significantly reduced and tissue components such as hemoglobin and
skin melanin exhibit negligible absorbance and scattering of near-infrared light.
Furthermore, triplet sensitizers capable of absorbing near-infrared light are highly sought-
after for biomedical applications - in particular for photodynamic therapy of cancer (PDT)
[6,7]. It is well-established that the depth of light penetration into a tissue strongly depends
on the light wavelength and it increases with the increase of the light wavelength.
Exemplarily, light with a wavelength shorter than 550 nm is unsuitable for PDT, however,
ability of light to penetrate tissue doubles from 550 nm to 650 nm and it doubles further as
the wavelength shifts to 700 nm. Further increase of light wavelength from 700 nm to 800
nm results in an increase of the ability of light to penetrate tissue of 10 %. These empirical
observations have some practical implications, viz. in order to treat more deeply seated
tumors, triplet photosensitizers possessing long-wavelength absorption are needed.
Consequently, the “therapeutic window” for PDT was defined as 600-1100 nm [8]. Typical
NIR absorbing dyes are squarines [9], diketopyrrolopyrroles [10], some derivatives of
porphyrins [11] and phthalocyanines [12], cyanines [13,14] and rylenes [15].
An interesting group of organic chromophores that have gained widespread attention in
recent years as red/NIR active fluorophores and photosensitizers are aza-BODIPYs (aza-4,4-
difluoro-4-bora-3a,4a-diaza-s-indacene) [16,17]. Structurally, aza-BODIPY dyes are
derivatives of better known BODIPY dyes (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)
obtained by replacing the meso-carbon atom that connects both pyrrole rings with an aza-
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bridge [18-27]. Replacement of the meso-carbon atom with an aza-bridge results in a
bathochromic shift of both absorption (λabs) and emission (λfl) bands of around 80 nm. Owing
to their advantageous photophysical properties (triplet lifetimes τT, triplet state quantum
yields ΦT and triplet state energies ET) combined with tunable singlet oxygen quantum yields
(ΦΔ) and ability to absorb light from the “therapeutic window”, aza-BODIPYs have been
proposed as potential triplet photosensitizers for both anti-microbial and anti-cancer
photodynamic therapy [18,28-49]. Furthermore, aza-BODIPYs were considered as
fluorescent chemosensors for the detection of ammonia [50], cysteine, homocysteine and
glutathione in living cells [51], Hg2+ [52-54], F- [55,56], in vivo detection of H2S and NO [57],
carbon dioxide [58], glucose in whole blood [59] and pH-sensitive sensors [60]. Aza-BODIPYs
have also been tested as photoactive components in photovoltaic devices [61-63] and
nonlinear optical materials [64].
Figure 1. Strategies employed to induce near-infrared absorption and emission in aza-BODIPYs.
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Typical strategies applied to induce near infrared light absorption in aza-BODIPYs include: a)
functionalization of the chromophore with strong electron-donating groups, b) rigidification
of the structure and c) extension of the π-electron system (fused aza-BODIPYs) (Figure 1)
[21,24,28,65-67].
Our interest in efficient yet stable organic triplet photosensitizes capable of light absorption
from the visible and near-IR part of the spectrum has lead us to the discovery of novel aza-
BODIPYs [68-72]. Herein, we report the five-steps synthesis and a comprehensive study on
the photophysical and electrochemical properties of aza-BODIPY substituted with strongly
electron-donating p-(diphenylamino)phenyl substituents (p-Ph2N) at 3,5-positions (see
Scheme 1). It is well established that the presence of strong electron-donating substituents
in the aza-BODIPY structure results in significant bathochromic shift of the absorption and
emission bands. In an effort to induce highly desirable photophysical and photochemical
properties important for biomedical applications we introduced a heavy atom – in this case
bromine – into the structure of the aza-BODIPY in question. Empirical observations indicate
that the presence of atoms with high atomic numbers (I, Br, Pt, Tl, Pb, etc.) induces a spin
orbit coupling, facilitates intersystem crossing (ISC) and increases the population of
molecules in their excited-triplet state (T1). In addition, the aza-BODIPY presented in this
work was equipped with p-tert-butylphenyl rings at 1,7 positions to improve solubility and
consequently facilitate purification and spectroscopic analysis. Understanding the
relationship between the aza-BODIPY structure and the triplet state properties (such as
triplet lifetime τT and triplet state energy ET) is of paramount importance for developing next
generation photosensitizers for biomedical applications, artificial light-harvesting systems,
efficient photocatalysts as well as non-linear optical materials. This paper aims at exploring
the possibility of triplet-state formation in aza-BODIPYs substituted with p-Ph2N groups by
measuring the triplet-state lifetimes (τT) through recording the decay of the transient T1→Tn
absorption in nanosecond time-resolved transient absorption spectra. Furthermore, via
1
1
electrochemical measurements (CV and DPV), we explored the redox properties (Eox , Ered
)
and we estimated the alignment of the HOMO/LUMO energy levels of these novel aza-
BODIPY derivatives. With the help of thin-layer spectroelectrochemical measurements we
tried to shed some light on the relative stabilities of the electrochemically generated cation
̶
radicals (aza-BODIPY●+) and anion radicals (aza-BODIPY● ) – a property of prime importance
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for evaluating the potential application of organic photoactive materials in organic
electronics. Also, the solution photo-stability and thermal stability of these materials were
evaluated.
2. Results and discussion
2.1. Synthesis and characterization
Scheme 1. Synthesis of mono-bromo aza-BODIPY 7. Reaction conditions: a) acetyl chloride, ZnCl2,
DCM, 0°C to reflux, 24 h, 45%; b) 4-tert-butylbenzaldehyde, NaOH, EtOH, 0°C to RT, 24 h, 69%; c)
MeNO2, K2CO3, MeOH, reflux, 17h, 79% ; d) CH3COONH4,n-butanol, 117°C, 3h, 63%; e) BF3·Et2O, DIPA,
DCM, 1h, 91%; f) NBS, DCM, 2h, 0°C, 96%.
The synthesis of aza-BODIPY under question is presented in Scheme 1. The synthesis started
with the preparation of the precursor compound, i.e. 1-(4-(diphenylamino)phenyl)ethan-1-
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one (2) [73]. Compound 2 was prepared by a Friedel-Crafts acylation of triphenylamine (1) in
the presence of zinc chloride as a Lewis acid in 45 % yield. While optimizing the reaction
conditions for 2 we tested several Lewis acids such as AlCl3 and FeCl3, with ZnCl2 giving the
best results in terms of purity and the yield of the final product 2. Also, acetyl chloride
proved to be a better source of the acetyl group in comparison to acetic acid anhydride. The
reaction of 4-tert-butylbenzaldehyde with ketone 2 in a base catalyzed aldol reaction led to a
chalcone (α,β-unsaturated ketone) 3 in 69 % yield. The nucleophilic addition of carbanion
formed from nitromethane to the α,β-unsaturated ketone 3 (Michael addition) resulted in
formation of 1,3-diaryl-4-nitro-butan-1-one 4 in very good yield of 79 %. Despite numerous
attempts we failed to get compound 4 in a pure form. Neither column chromatography nor
crystallization or sublimation under vacuum allowed purification of 4. We managed,
however, to isolate a small amount of pure 4 with the help of a preparative thin layer
chromatography (Figure S28). The sample obtained was used for the spectroscopic
characterization of 4 and to calculate the reaction yield. The unpurified compound 4 was
used, however, for the next reaction step. Refluxing of 1,3-diaryl-4-nitro-butan-1-one 4 in n-
butanol in the presence of NH4OAc resulted in aza-dipyrromethene 5 (ADPM) in 63 % yield.
Reaction of aza-dipyrromethene ligand 5 with an ethereal complex of boron trifluoride in
dichloromethane and in the presence of diisopropylamine (DIPA), followed by
chromatographic purification afforded the corresponding aza-BODIPY 6 in excellent yield of
11
19
91 %. The incorporation of boron into the ligand 5 was confirmed by B and F NMR
spectroscopy. The 11B resonance signal was found at δ = 1.23 ppm for 6 and it was seen as a
triplet (1J = 32.5 Hz) pointing to the 11B–19F interactions. The 19F resonance signal was seen as
a quartet at δ = - 111.01 ppm due to the 11B–19F interaction (1J = 32.8 Hz). In order to explore
the impact of heavy halogen atoms on the excited triplet state characteristics of the aza-
BODIPY 6 we looked at the possibility of introducing heavy atoms, such as iodine or bromine,
into the structure of 6. Our initial attempt to introduce iodine atoms into the structure of 6,
with N-iodosuccinimide (NIS) as the halogenating reagent, failed. Regardless of the reaction
conditions applied (reaction temperatures ranging from - 60°C to RT, different reaction times
and solvents, different molar ratios of 6 to NIS) we observed either decomposition of the
products or formation of an inseparable mixture of products with different degree of
halogenation. Much better results were obtained when NIS was replaced with N-
bromosuccinimide (NBS) in DCM as the source of bromonium cation Br+. However, the
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precise control of the reaction outcome still proved to be extremely challenging. After
numerous attempts with various reaction conditions we managed to regioselectively
introduce one bromine atom per molecule of 6. The ratio of NBS to aza-BODIPY 6 had to be
kept strictly as 1:1 as any increase of the amount of NBS resulted in an inseparable mixture
of products. The mono-bromo derivative 7 was isolated in 96% yield.
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Figure 2. 1H NMR spectra of ADPM 5, aza-BODIPYs 6 and its brominated derivative 7 in CDCl3.
The regioselective introduction of bromine into the 2-position of the pyrrole ring was
1
unambiguously confirmed by H NMR spectroscopy (see Figure 2 and Supporting
1
1
Information). The H NMR spectrum of 7 is much more complex comparing to the H NMR
spectrum of 6, indicating the loss of symmetry for 7. The diagnostic resonance of the pyrrole
1
ring proton at 6-position was observed in the H NMR spectrum of 7 as a singlet at δ = 7.14
ppm with the integration equal precisely to one proton. Also, in the high resolution ESI
spectrum a peak at m/z 1023.3702 confirmed formation of mono-bromo derivative 7. The
identity of all intermediates, ADPM 5, aza-BODIPY 6 and the mono-bromo derivative 7
11
presented in this work was supported by high resolution ESI mass spectrometry, B NMR,
19F NMR, 1H NMR, 13C NMR, FT-ATR-IR and elemental analysis (see Supporting Information),
respectively.
Figure 3. The conformation of the molecules ADPM 5 and aza-BODIPY 6 stabilized by intramolecular
hydrogen bonds marked by dashed blue line along with the atom numbering scheme.
The structures of 5 and 6 were unambiguously confirmed by single crystal X-ray analysis (see
Supporting Information and Figure 3). In turn, the mono-brominated derivative 7 did not
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form crystals that could be subjected to X-ray analysis. The conformations of the molecules 5
and 6 with the atomic displacement ellipsoids for non-hydrogen atoms were shown in
Figures S27 and S28, while the selected bond lengths, valence angles, torsion and dihedral
angles were presented in Table S2. Crystal structures of compounds 5 and 6 follow the
symmetry of space groups P -1 and P 21/c, respectively. The asymmetric unit of 5 contains
the ADPM molecule and dichloromethane in the ratio of 1:0.694 (Figure S30). The nitrogen
atoms N(37) and N(57) of ligand 5, as well as the nitrogen atoms N(57) and N(57’) of aza-
BODIPY 6, clearly show sp2 configuration (Figure 4).
Figure 4. A comparison of the dihedral angles (top) between the five-membered rings of the central
chromophore and the hybridizations of the nitrogen atoms of the amino groups for different ADPM
and aza-BODIPYs substituted with either p-(Me2N) or p-(Ph2N) groups [71].
Interestingly,
the
chromophore
moiety
C(1)C(2)C(3)N(3A)B(4)N(4A)C(5)C(6)C(7)C(7A)N(8)C(8A) of 6 is not planar (the root mean
square deviation, rms, of the fitted atoms from the best plane equals 0.1425 Å) (Figure 4).
The observed deviation from planarity is due to tetrahedral configuration of boron atom of
the BF2 moiety (the average value of the valence angles at B(4) is equal to 109.4°). Only
N(3A)N(4A)C(7A)N(8)C(8A) part of the central six-membered ring is planar (rms deviation of
fitted atoms = 0.011 Å) and together with B(4) atom deviating from the best plane by
0.186(7) Å define a half-chair conformation of the N(3A)B(4)N(4A)C(7A)N(8)C(8A) six-
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membered ring. The general conformation of both molecules is stabilized by intramolecular
hydrogen bonds: N(1)-H(1)…N(1’) in the case of 5 and C(36)-H(36)...F(41), C(52)-H(52)…F(41)
and C(12)-H(12)…N(8) in the case of 6. The geometry of all intra- and intermolecular
hydrogen bonds as well as the other selected weak interactions are given in Table S3. In
Figure 4 we compared both the geometry of the central chromophore, viz. the dihedral
angles formed between the adjacent pyrrole rings, the hybridization of the nitrogen atoms
of the amino group and the optical properties (λabs) of a set of aza-BODIPYs bearing p-
aminophenyl substituents at 3,5-positions. The crystallographic data and the optical
properties of two of the structures presented in Figure 4 were recently reported by us in our
ChemPhysChem paper [71].
2.2. Optical properties
Figure 5. Normalized absorption (left panel) and normalized corrected emission (right panel) spectra
of 5-7 in THF at room temperature (inset: THF solutions of compounds studied). Excitation
wavelength: λex = 650 nm for 5, λex = 700 nm for 6 and 7.
The absorption and emission spectra of ADPM 5 and aza-BODIPYs 6 and 7 in THF are
outlined in Figure 5 and summarized in Table 1. Compounds 5-7 share similar spectral
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features, viz. an intense absorption band in the visible or near-IR part of the spectrum (S0 →
S1), accompanied by another, blue-shifted, weaker absorption band ca. half the intensity of
the main absorption band. This additional absorption band we attributed to the π-π*
transitions involving orbitals below the HOMO or above the LUMO. A similar spectral feature
was observed for aza-BODIPY with dimethylamino groups (Me2N-) in para- position of the
phenyl rings at 3,5-positions of the chromophore core (Figure 4) [71]. THF solutions of aza-
dipyrromethene 5 exhibit a bright green color with λabs equal 673 nm (ε = 48 000 M-1 cm-1).
Incorporation of difluoroboron into 5 resulted in a pronounced red-shift of the absorption
band of 116 nm. The aza-BODIPY 6 absorbed light at λabs 789 nm with a high molar
absorption coefficient of ε = 83 000 M-1 cm-1 and exhibited an intense reddish/violet color
resulting from the additional absorption band at λabs 570 nm.
Functionalization of aza-BODIPY 6 with bromine at 2-position of the pyrrole ring retained its
ability to absorb light from the near-IR part of the spectrum. Monobromo-derivative 7
absorbed light at λabs 769 nm with a molar absorption coefficient of ε = 55 000 M-1 cm-1 and
exhibited deep navy color.
Table 1. Summary of spectroscopic and photophysical data for compounds 5, 6, 7 and 8 in THF.
(DPBF)
(luminesc.)
Compound
λabs
λf
Φf
Es
ε
τS
τT
Φox
ΦΔ
[nm]
[nm]
[eV]
[M-1cm-1]
[ps]
[μs]
5
6
7
673
789
769
754
838
838
0.0065
0.0056
0.0076
1.65
1.48
1.48
48 000
83 000
55 000
108 ± 1
100 ± 1
99 ± 1
-
0.00
0.00
0.00
0.00
21 ± 2
22 ± 2
0.00
0.055 ± 0.002
λabs: absorption band maximum in THF, λf: fluorescence band maximum in THF at RT, Φf: fluorescence quantum
yield measured in THF (indocyanine green was used as a reference compound with Φf = 0.16 in MaOH for 6 and
7), Es: energy of singlet excited state, ε: molar absorption coefficient at λabs in THF, τS: lifetime of excited singlet
(luminesc.)
state in THF, τT: lifetime of excited triplet in THF, Φox(DPBF): quantum yield of DPBF oxidation in THF, ΦΔ
:
quantum yield of singlet oxygen in THF measured using the luminescence method.
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At room temperature in THF solutions, the fluorescence maximum was observed at λf = 754
nm (Stokes shift of 81 nm) for 5, at λf = 838 nm (Stokes shift of 49 nm) for 6 and at λf = 838
nm (Stokes shift of 69 nm) for 7. Compounds 5, 6 and 7 can be deemed nearly non-
fluorescent, because only negligible fluorescence was recorded with fluorescence quantum
yields (Φf) below 1%. Based on the fluorescence spectra the following singlet excited state
(S1) energies were calculated: ES = 1.65 eV for 5, ES = 1.48 eV for 6 and 7, respectively.
We believe, that the lack of torsional rigidity may induce the internal conversion in 5-7
leading to negligible emission. It has been described by several authors that rigidification of
the aza-BODIPY structure leads to an increase of fluorescence quantum yield (Φf) [21]. Also,
for the ADPM 5 additional phenomena may contribute to the lack of a noticeable
fluorescence, namely, the possible tautomerization of the aza-dipyrromethene (migration of
the proton between the adjacent pyrrole rings) that provides an efficient funnel for the non-
radiative deactivation of the excited singlet state (S1) [72]. We also speculate that the
intramolecular charge transfer (ICT) nature of the excited state in 5-7 exerted by the
presence of strongly electron-donating p-(diphenylamino)phenyl substituents enhances the
non-radiative decay and is responsible for the lack of strong emission in these compounds.
2.3. Time-resolved spectroscopy
We have investigated both the excited singlet (S1) and excited triplet (T1) state dynamics by
recording femtosecond and nanosecond time-resolved transient absorption spectra,
respectively. The transient absorption spectra, transient decays and singlet (τS) and triplet
(τT) state lifetimes of 5-7 are presented in Figures 6 and 7 and summarized in Table 1.
Femtosecond flash photolysis allowed observation of excited singlet-state transient
absorption spectra (S1 → Sn) and dynamics (S1 → S0) in THF (Figure 6). The transient
absorption spectra of 5-7 feature a positive S1 → Sn absorption band spanning from ca. 440
nm to ca. 600 nm, superimposed with ground state bleaching signal (S1 → S0). The decay of
the transient S1 → Sn absorption and the recovery of the ground state absorption S1 → S0
were used to calculate singlet lifetimes (τS) of the investigated compounds 5-7, with both
methods giving similar values of τS. Furthermore, for 5-7, mono-exponential decays of the
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transient absorption and recovery of the ground state absorption were observed. All
compounds studied had very short-living excited singlet state (τS) in the picosecond range.
ADPM 5 was found to have an excited singlet lifetime of (τS) 108 ps, aza-BODIPY 6 showed
excited singlet lifetime of (τS ) 100 ps while the brominated derivative 7 had nearly same
excited singlet lifetime as its non-brominated congener 6, viz. τS = 99 ps. These excited
singlet lifetimes (τS) results were in very good agreement with the excited singlet lifetimes
(τS) calculated from the fluorescence decays.
Figure 6. Femtosecond flash photolysis of 5, 6 and 7 in THF at room temperature (laser energy 2 μJ,
excitation wavelength λex 700 nm). Transient decays were monitored at λ 557 nm for 5, λ 467 nm for
6 and λ 465 nm for 7, respectively.
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In order to observe the excited triplet-state transient spectra (T1 → Tn) and dynamics we
changed the time domain of the measurements from femtosecond to nanosecond flash
photolysis. Upon laser photolysis (λex 355 nm) of deaerated THF solution of ADPM 5 no
observable transient absorption spectra were recorded. Introducing BF2 moiety into the
ADPM 5 to form aza-BODIPY 6 and further bromination of 6 to mono-bromo derivative 7
produced two compounds with pronounced and observable triplet state. The nanosecond
transient absorption spectra of 6 and 7 exhibit a broad positive T1 → Tn absorption band
spanning from ca. 350 nm to ca. 550 nm, superimposed with the ground state bleaching
signal. The mono-exponential decay of the T1 → Tn absorption was used to calculate the
triplet lifetimes (τT) of both 6 and 7. Both aza-BODIPYs had very similar triplet lifetimes: τT of
21 ± 2 μs was found for 6 and τT equaled to 22 ± 2 μs for 7. After saturating the THF solutions
of 6 and 7 with air for 1 minute the observed kinetics shortened substantially to
approximately τT = 1.5 μs for both aza-BODIPYs. This observation provides irrefutable
evidence that the observed transient absorptions originated from the excited triplet-state
(T1 → Tn).
Figure 7. Nanosecond transient absorption spectra of aza-BODIPY 6 and 7 measured in THF at room
temperature with excitation wavelength λex = 355 nm and laser energy 3.0 mJ/pulse. Transient
decays (at the bottom) monitored at λ = 410 nm with laser energy 0.5 mJ/pulse (to avoid triplet-
triplet annihilation).
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2.4. Photosensitization
Figure 8. Photosensitized degradation of 1,3-diphenylisobenzofuran (DPBF) initiated by aza-BODIPY 7
in THF.
The ability of halogenated aza-BODIPY 7 to sensitize the formation of singlet oxygen (1O2)
and/or other reactive forms of oxygen (ROS) was considered and quantitatively assessed as
singlet oxygen quantum yield (ΦΔ) and quantum yield of DPBF oxidation (Φox(DPBF)),
respectively. In order to determine the value of Φox(DPBF) and ΦΔ and of 7 we resorted to two
methods, viz. the indirect method based on a chemical quencher and the direct method
based on the observation of singlet oxygen phosphorescence [74]. In the first method, the
progress of the reaction between the reactive oxygen species (ROS) formed upon
photosensitization with the compound studied and the well-known chemical quencher (viz.
1,3-diphenylisobenzofuran, DPBF) was followed by the observation of the decay of the
absorption band of DPBF at λ 417 nm. The second method relied on the observation of the
intensity of the luminescence of singlet oxygen (1O2) at λ 1270 nm formed upon
photosensitization with the compound studied and the standard compound with known
value of singlet oxygen quantum yield (ΦΔ). The luminescence method allows exclusive
detection of singlet oxygen (1O2), while in the method utilizing a chemical quencher, the
DPBF employed reacts indistinguishably with both singlet oxygen (1O2) and the radicals
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•
•-
formed (viz. OH•, HO2 , O2 , R•, R•+, ROO•, RO•) [75]. Comparison of the values obtained by
both methods gives an insight into the relative contribution of Type II (singlet oxygen
formation), Type I (hydrogen atom abstraction) and Type III (electron transfer) mechanisms
of the photo-oxygenation initiated by the photosensitizers tested [76]. Interestingly, when
the oxygen saturated THF solution of aza-BODIPY 7 was excited at its λabs a gradual depletion
of the absorption band of the chemical quencher was observed pointing to the formation of
(DPBF)
singlet oxygen and/or other ROS (Figure 8). A quantum yield of DPBF oxidation Φox
=
0.055 ± 0.002 was found for mono-brominated aza-BODIPY 7. Importantly, no luminescence
originating from singlet oxygen (1O2) was discernible at λ 1270 nm upon excitation of the
oxygen saturated THF solution of 7. A comparison of these two experiments clearly indicates
that the photo-oxygenation of DPBF with aza-BODIPY 7 as the photosensitizer does not
occur via Type II mechanism but it involves either hydrogen abstraction (Type I) and/or
electron transfer (Type III) mechanism.
2.5. Electrochemistry and HOMO/LUMO energy levels
Table 2. The frontier energy levels [eV] and redox potentials [V] vs. Fc/Fc+ for 5, 6 and 7 obtained
from differential pulse voltammetry (DPV). Glassy carbon electrode was used as a working electrode.
Data collected in 1,2-dichlorobenzene with 0.1 M tetrabutylammonium tetrafluoroborate as
supporting electrolyte.
1
2
1
2
a
b
Compound
Eox
Eox
Ered
Ered
ELUMO
[eV]
EHOMO
[eV]
Eg (electro) c
[eV]
[V]
[V]
[V]
[V]
5
6
7
+ 0.06
+ 0.23
- 1.54
- 2.13
- 3.56
- 4.00
- 4.06
- 5.16
- 5.28
- 5.41
1.60
1.28
1.35
+ 0.18
+ 0.43
+ 0.53
- 1.10
- 1.04
- 1.91
- 1.76
+ 0.31
+
+
a ELUMO= - (E[first reduction vs. Fc/Fc ] + 5.1) [eV], b EHOMO= - (E[first oxidation vs. Fc/Fc ] + 5.1) [eV], c E(electro) = electrochemical
gap.
Differential pulse voltammetry (DPV) and cyclic voltammetry (CV) measurements were
1
performed in an effort to estimate the electrochemical properties of compounds 5 to 7 (Eox
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and Ered1), to estimate the HOMO/LUMO energy levels and to comprehend the impact of
both para-diphenylamino groups (Ph2N-) and halogen substituents on the electrochemical
properties of the compounds studied. The results were summarized in Table 2 and
presented in Figure 9. In order to elucidate the nature of the redox processes observed and
to shed more light on the relative stabilities of the reduced and oxidized species formed, a
thin-layer spectroelectrochemistry was performed (Figure 10).
Both the DPV and CV measurements indicated the presence of two oxidation processes
1
2
1
within the potential range applied (Eox , Eox ) with the first oxidation potential (Eox )
occurring at a relatively low potential - indicating an ease to undergo oxidation process (low
ionization potential). This observation is in line with the electron-donating nature of the
para-diphenylamino groups (Ph2N-). ADPM 5 was the easiest of all the compounds studied
1
to oxidize with the first oxidation potential (Eox ) found at a value of + 0.06 V vs. Fc/Fc+.
1
Controlled potential electrolysis of AcCN solution of 5 at + 0.4 V vs. Ag/Ag+, where Eox
occurs, resulted in the emergence of redshifted band at λ 970 nm at the expense of the band
at λ 690 nm. Such spectral changes are associated with the formation of a cation radical
(ADPM●+). Switching the potential to a more negative allowed the recovery of the initial
absorption spectrum of ADPM 5, meaning that the first electrochemical oxidation was a
chemically reversible process. In turn, controlled potential electrolysis of 5 at a potential of -
1.1 V vs. Ag/Ag+, where Ered1 occurs, resulted in irreversible degradation of the reduced form
of 5 (Figure S1). Incorporation of difluoroboron moiety (BF2) into the binding pocket of the
1
ligand 5 had the effect of shifting the first oxidation potential (Eox ) to a more positive value,
viz. + 0.18 V vs. Fc/Fc+ - an observation in accordance with the electron-deficient nature of
the BF2 moiety. Controlled potential electrolysis of AcCN solution of 6 at + 0.43 V vs. Ag/Ag+,
viz. at Eox1, resulted in the emergence of NIR band at λ 1035 nm at the expense of the bands
at λ 805 nm and λ 580 nm (Figure 10). The observed spectral changes were ascribed to the
formation of the cation radical (aza-BODIPY●+). Switching the potential to a more negative
allowed recovery of the initial absorption spectrum of 6, meaning that the first
electrochemical oxidation was a chemically reversible process within the timeframe of the
experiment. Controlled potential electrolysis of AcCN solution of 6 at the potential where
first reduction occurs (viz. - 1.0 V vs. Ag/Ag+) resulted in anion radical formation, viz. aza-
BODIPY●-, marked by evolution of two absorption bands at λ 870 nm and λ 460 nm (a
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10.1002/cphc.202000117
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reversible process upon applying a more positive potential). Adding the bromine atom to
one of the pyrrole rings at 2 position (Scheme 1) further shifted the first oxidation potential
1
(Eox ) to a more positive value for compound 7, viz. + 0.31 V vs. Fc/Fc+.
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Figure 9. Differential pulse voltammetry and cyclic voltammetry of 5, 6 and 7. Data were collected on
glassy carbon working electrode in o-dichlorobenzene with 0.1 M nBu4NBF4 as the supporting
electrolyte at a scan rate of 100 mVs-1 (cyclic voltammetry) . Redox potentials are reported in
reference to the ferrocene/ferrocenium redox couple.
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Figure 10. UV-Vis spectra of aza-BODIPY 6 collected during electrolysis at two different potentials,
viz. - 1.0 V and + 0.43 V vs Ag/Ag+. Data collected in 1,2-dichlorobenzene with 0.1 M
tetrabutylammonium tetrafluoroborate as supporting electrolyte with Pt working electrode.
Two reduction processes were discernible for compounds 5-7. Both oxidation and reduction
processes observed for 5-7 could be deemed as electrochemically irreversible process in
terms of the electron transfer kinetics (ΔE = Ep,a – Ep,c, ca. 340 mV to 550 mV). ADPM 5 was
1
the hardest to reduce out of all compounds studied with the first reduction potential (Ered
)
observed at – 1.54 V vs. Fc/Fc+. Given the electron-deficient nature of the BF2 moiety,
incorporation of BF2 into the ligand 5 to form aza-BODIPY 6, significantly shifted the first
reduction potential (Ered1) to – 1.10 V vs. Fc/Fc+. Adding bromine atom to one of the pyrrole
rings at 2 position had virtually no impact on the first reduction potential (Ered1) of aza-
BODIPY 7, viz. Ered1 = - 1.04 V vs. Fc/Fc+.
1
The values of the first reduction (Ered1) and oxidation (Eox ) potentials were used to calculate
the energy levels of the LUMO (ELUMO) and HOMO (EHOMO), assuming that the formal
potential of the Fc/Fc+ redox couple equals – 5.1 eV in the Fermi scale (Table 2) [77-78]. The
+
following equations were used: ELUMO= - (E[first reduction vs. Fc/Fc ] + 5.1) [eV] and EHOMO= - (E[first
+
oxidation vs. Fc/Fc ] + 5.1) [eV]. Estimation of the ELUMO gave the value of - 3.56 eV for 5, - 4.00 eV
for 6 and - 4.06 eV for 7, respectively. The EHOMO was found to be located at - 5.16 eV for 5, -
5.28 eV for 6 and - 5.41 eV for 7, respectively. Adding BF2 moiety to the binding pocket of
the ADPM 5 had the effect of lowering the energy of the LUMO(ELUMO) and only slightly
affected the energy of the HOMO (EHOMO). Moreover, bromination of the pyrrole ring at 2
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10.1002/cphc.202000117
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position (compounds 7) had no impact on ELUMO and only slightly lowered the EHOMO. The
electrochemical energy gaps (Eg) calculated as the difference between the first oxidation and
first reduction potentials equaled 1.60 eV for the ADPM 5, 1.28 eV for aza-BODIPY 6 and 1.35
eV for the mono-brominated derivative 7.
2.6. Photostability and thermal stability
Figure 11. TGA profiles (left panel, heating rate 10°Cmin-1, aerobic conditions) and UV/Vis monitored
photostability of compounds 5, 6 and 7 in THF solutions exposed to the irradiation at λ > 473 (right
panel) and I = 1000±50 W/m2.
The stability of compounds 5-7 was tested in terms of both thermal stability and
photostability. The thermogravimetric profiles obtained for 5-7 under aerobic conditions
indicated good thermal stability up to approximately 300 °C (Figure 11). The THF solutions of
5-7 were exposed to the bright light of a xenon lamp (1000±50 W/m2) separated from the
samples with a 473 nm cut-off filter under aerobic conditions (Figure 11). UV/Vis spectra
were taken over time in an effort to assess the relative photostability of the ADPM 5 and
aza-BODIPYs 6 and 7 studied. After 10 h of irradiation no loss of the initial intensity of the
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10.1002/cphc.202000117
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absorption band was observed for compound 5 and merely 1% loss of the initial intensity of
the absorption band was detected for compounds 6 and 7, respectively. These findings point
to an outstanding photostability of 5-7 under the experimental conditions applied. This
finding is of prime importance in view of the fact that materials suitable for photocatalysis
and optoelectronic applications should be characterized by high durability under
photoirradiation.
3. Discussion
Synthesis of organic dyes capable of light absorption from the NIR part of the spectrum with
concomitant ability to sensitize formation of singlet oxygen (1O2) and/or other reactive forms
of oxygen (ROS) is a promising but challenging research field with only scarce literature
reports on such photosensitizers. One of the most frequently employed strategies to induce
NIR absorption in a given chromophore system relies on modifying the structure with
strongly electron-donating groups (amino groups in particular, see Introduction). Presence of
such groups lowers the energy of the singlet excited state (Es) and shifts the absorption band
(λabs) beyond the visible part of the spectrum. However, presence of strongly electro-
donating amino groups (either Me2N- or Ph2N-) may also result in some unfavorable
phenomena that diminish ability of such molecule to efficiently populate triplet excited state
(ET) and generate ROS. In one of our recent papers we reported synthesis and properties of
NIR absorbing aza-BODIPY substituted with p-(dimethylamino)phenyl groups – a structure
that closely resembles aza-BODIPY reported herein [71]. In the same paper we reported
properties of several poly-halogenated derivatives of p-(Me2N)Ph- substituted aza-BODIPY.
We observed a pronounced intramolecular charge transfer (ICT) exerted by the presence of
strongly electron-donating p-(Me2N)Ph- groups resulting in lack of highly populated triplet
excited state (T1), relatively low lifetime of triplet excited state (τT = 6 μs) and inability of
1
these aza-BODIPYs to sensitize formation of O2 and other ROS (ΦΔ = 0). Herein, we
demonstrated that by replacing the dimethylamino groups (Me2N-) with a diphenylamino
(Ph2N-) groups we managed to slightly diminish the extent of ICT, increase the excited triplet
(DPBF)
lifetime to τT = 22 μs and induce photosensitizing properties in compound 7 (Φox
=
(DPBF)
0.055). However, the value of Φox
that we observed for p-(diphenylamino)phenyl
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10.1002/cphc.202000117
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substituted aza-BODIPY reported herein is still too low for the dye to be used as a PDT
photosensitizer or photocatalyst. A work is in progress in our laboratory to improve the
photosensitizing properties of p-(diphenylamino)phenyl substituted aza-BODIPY while
maintaining its ability to absorb light from the NIR part of the spectrum.
Also, via the electrochemical measurements we demonstrated that these compounds can be
1
used as efficient electron donors with very low first oxidation potential (Eox ). The p-
(diphenylamino)phenyl substituents at 3,5 positions can act as an “electron reservoir” with
the electron delocalization of the free electron pair of the Ph2N- group being the driving
force for the efficient electron donation. Research and development dedicated to organic
light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and dyes sensitized
solar cells (DSSC) have been the subject of considerable attention in recent decades. Devices
like organic electronics and optoelectronics require either electron-donating (p-type) or
electron-accepting (n-type) materials. Given the very strong electron-donating character of
the p-(Ph2N)Ph- substituted aza-BODIPY/ADPM described in this paper combined with high
thermal stability, remarkable photostability, high molar absorption coefficient (ε) and low
ionization potential, we postulate that these compounds maybe of great interest as
photoactive/electroactive components of various organic electronics or optoelectronics. In
Figure 12 we compared a set of aza-BODIPYs and aza-dipyrromethenes with both electron-
donating and electron-accepting groups prepared and reported by our research group
[69,71,72,79,80].
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Figure 12. A comparison of optical (λabs, in THF) and electrochemical (Eox1, Ered1 in 1,2-
dichlorobenzene or THF) properties of aza-dipyrromethenes and aza-BODIPYs bearing either
electron-withdrawing or electron-donating substituents at 5,5’ positions (aza-dipyrromethene) or 3,5
positions (aza-BODIPY) prepared in our laboratory [69,71,72,79,80].
4. Experimental
4.1. Materials and methods
NMR spectra were recorded on Bruker Avance III HD 400 MHz in deuterated solvents.
Electrospray mass spectrometer microTOF II (Bruker Daltonics, Bremen/Germany) with a
resolution of > 16 500 FWHM and a mass accuracy of < 2 ppm was used to obtain the high-
resolution mass spectra. FT-ATR-IR spectra were recorded using a Nicolet iS10 spectrometer.
UV-Vis absorption spectra at RT were measured using a Hewlett-Packard 8452A diode-array
spectrophotometer equipped with a HP 89090A Peltier temperature control accessory or
HITACHI U-2910. Fluorescence spectra were collected using Cary Eclipse Agilent
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spectrophotometer. For the elemental micro analysis, the CHNS Vario Micro Cube analyzer
combined with the electronic microbalance was used. Compounds were analysed using a
Thermo-gravimetric analyzer Mettler Toledo TGA/SDTA 851e and the analysis was carried
out using samples of approximately 3-5 mg in weight and a heating rate of 10 °C min-1 under
air. Reaction progress was controlled using TLC on silica gel (Supelco). 1,2-Dichlorobenzene
was dried with CaH2 and freshly distilled prior to use. Triphenylamine (1) was purchased
from TCI Europe and used as supplied.
4.2. Synthesis
1-(4-(diphenylamino)phenyl)ethan-1-one (2): The described synthesis of 2 is a modification
of the procedure reported elsewhere [73]. Acetyl chloride (6.4 g, 5.8 ml, 81.53 mmol) was
added dropwise via syringe to a slurry of triphenylamine (1; 20 g, 81.53 mmol) and ZnCl2
(11.11 g, 81.53 mmol) in dry dichloromethane (80 ml) at 0 °C. When the addition of acetyl
chloride was completed, the ice bath was removed and the reaction mixture was kept under
refluxed for 24 h. The reaction mixture was poured over crushed ice and a mixture of
concentrated hydrochloric acid (20 ml) and water (60 ml) was added. After 0.5 h of stirring
the acidify mixture was neutralized with a solution of Na2CO3. The crude product was
extracted with a dichloromethane and the organic layer was washed with water (3 x 100 ml).
The crude product was pre-adsorbed onto a silica gel and twice chromatographed on a
column filled with a silica gel (5 cm x 18 cm). A mixture of toluene and hexane (1:1) was used
as an eluent. The purity of fractions collected was controlled with a TLC (silica gel;
acetone/hexane, 2:8). Yield: 10.56 g (45 %), white solid. 1H NMR (600 MHz, CDCl3) δ: 7.80 (d,
J = 8.9 Hz, 2H), 7.32 (dd, J = 8.4, 7.4 Hz, 4H), 7.17 – 7.12 (m, 6H), 6.99 (d, J = 8.9 Hz, 2H), 2.53
13
(s, 3H). C NMR (151 MHz, CDCl3) δ: 196.6, 152.3, 146.6, 130.0, 129.9, 129.8, 126.1, 124.8,
119.8, 26.4. HR-ESI-MS: calcd for C20H17NONa [M+Na+]+ 310.1202. Found: 310.1202. FT-ATR-
IR: ν (cm-1) = 3324, 3036, 3005, 1669, 1579, 1486, 1416, 1356, 1329, 1262, 1170, 1114,
1073, 951, 842, 819, 695. Anal. Calcd for C20H17NO: C, 83.59 %, H, 5.97 %, N, 4.88 %. Found:
C, 83.39 %, H, 6.00 %, N, 4.77 %.
3-(4-(tert-butyl)phenyl)-1-(4-(diphenylamino)phenyl)prop-2-en-1-one (3): NaOH (4.5 g,
11.25 mmol) was dissolved in EtOH (50 ml). This solution was slowly added to a vigorously
stirred mixture of 4-tert-butylbenzaldehyde (6.1 g, 37.6 mmol) and 1-(4-
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(diphenylamino)phenyl)ethan-1-one (2; 10.563 g, 36.76 mmol) in ethanol (550 ml). The
temperature of the reaction mixture was kept at 0 °C during addition of the base. After 0.5 h
the ice bath was removed and the reaction mixture was allowed to reach the room
temperature. The reaction mixture was stirred at room temperature for 24h. The formation
of a yellow, crystalline solid was observed. Water was added dropwise to the reaction
mixture under vigorous stirring and the precipitate formed was filtered and washed with a
copious amount of water, and then twice with a mixture of acetonitrile/water (45 ml/55 ml).
The crude product was purified by column chromatography (silica gel; toluene/hexane, 1:1).
Pure fraction were collected. Impure fractions were combined together and re-purified by a
column chromatography (silica gel; toluene/hexane, 1:1). Yield: 10.91 g (69 %), a yellow
solid.1H NMR (600 MHz, CDCl3) δ: 7.92 (d, J = 8.9 Hz, 2H), 7.81 (d, J = 15.6 Hz, 1H), 7.58 (d, J =
8.3 Hz, 2H), 7.51 (d, J = 15.6 Hz, 1H), 7.44 (d, J = 8.4 Hz, 2H), 7.33 (dd, J = 8.4, 7.5 Hz, 4H),
13
7.18 (dd, J = 8.5, 1.0 Hz, 4H), 7.15 (t, J = 7.4 Hz, 2H), 7.05 (d, J = 8.8 Hz, 2H), 1.35 (s, 9H). C
NMR (75 MHz, CDCl3) δ: 188.5, 154.0, 152.1, 146.7, 143.7, 132.6, 130.9, 130.2, 129.7, 128.3,
126.1, 126.0, 124.7, 121.2, 120.0, 35.0, 31.3. HR-ESI-MS: calcd for C31H29NO [M]+: 454.2141.
Found: 454.2141. FT-ATR-IR: ν (cm-1) = 3035, 2959, 2865, 1656, 1633, 1581, 1486, 1418,
1326, 1273, 1221, 1170, 1112, 1074, 1026, 973, 817, 751, 696. Anal. Calcd for C31H29NO: C,
86.27 %, H, 6.78 %, N, 3.25 %. Found: C, 85.54 %, H, 6.77 %, N, 3.12 %.
3-(4-(tert-butyl)phenyl)-1-(4-(diphenylamino)phenyl)-4-nitrobutan-1-one (4): Compound 3
(12.686 g, 29.41 mmol), nitromethane (2.153 g, 35.29 mmol), K2CO3 (1 g, 7.23 mmol) and
methanol (350 ml) were combined in a flask. The reaction mixture was kept under reflux for
17 hours. The solvent was removed on a rotary evaporator. The crude product was pre-
adsorbed onto a silica gel and purified by column chromatography (SiO2; toluene/hexane
2:1). The purity of the fractions was controlled with a TLC (silica gel; toluene/hexane, 2:1).
The chromatography performed allowed only partial purification. 15.135 g of impure
product in the form of oil was collected. Neither chromatography nor crystallization allowed
isolation of pure product. As all attempts to purify the product 5 failed it was decided to use
the impure product for the next reaction step. However, in order to spectroscopically
confirm formation of 4 a small sample of the impure product (123 mg) was purified by a
preparative TLC (MERCK, PLC Silica gel 60 F245, 2 mm, 20mx20cm) with a mixture of acetone
and hexane (1:9 v/v) as an eluent. The preparative TLC plate was developed four times
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10.1002/cphc.202000117
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before collecting the pure product. Yield: 93 mg, 79 %. 1H NMR (600 MHz, CDCl3) δ: 7.75 (d, J
= 8.9 Hz, 2H), 7.35 – 7.30 (m, 6H), 7.20 (d, J = 8.3 Hz, 2H), 7.17 – 7.12 (m, 6H), 6.95 (d, J = 8.9
Hz, 2H), 4.82 (dd, J = 12.5, 6.5 Hz, 1H), 4.67 (dd, J = 12.5, 8.2 Hz, 1H), 4.22 – 4.13 (m, 1H),
13
3.37 (dd, J = 17.3, 6.0 Hz, 1H), 3.30 (dd, J = 17.3, 7.9 Hz, 1H), 1.29 (s, 9H). C NMR (75 MHz,
CDCl3) δ: due to the overlap of many resonance signals we do not report the 13C NMR
spectrum for this compound. HR-ESI-MS: calcd for C32H32N2O3Na [M+Na+]+: 515.2305.
Found: 515.2305. FT-ATR-IR: ν (cm-1) = 3059, 2960, 2867, 1668, 1583, 1549, 1488, 1422,
1374, 1323, 1268, 1230, 1172, 1114, 1074, 978, 905, 829, 755, 729, 695.
ADPM (5): Compound 4 (4.98 g, given that the crude product contains 79% of the compound
4 it accounts for 7.99 mmol of 4), ammonium acetate (30 g, 389.2 mmol) and n-butanol (20
ml) were combined in a flask and heated under reflux (117 °C) for 1.5 h after which time
another portion of ammonium acetate was added to the reaction mixture and the stirring
was continued for an additional 1.5 h (total reaction time: 3 h). The solution was cooled and
the precipitate formed was filtered and washed with methanol. The crude product was
dissolved in acetone (40 ml) and water was added dropwise to the stirred acetone solution
until all compound precipitated. The precipitate formed was filtered and washed with water
and methanol. The crude product was pre-adsorbed onto a few grams of silica gel and
passed through a glass column (30 cm) filled with silica gel (flash chromatography). A
mixture of toluene and hexane (1:1, v/v) was used as an eluent. An intense blue fraction was
collected and the solvent was removed. The flash chromatography was repeated two more
times in the same manner. The partially purified product was dissolved in DCM (50 ml) and
methanol was added dropwise (50 ml) to the solution. The solution was left overnight to
form crystals of pure product (dark, shiny needles). The crystals formed were filtered and
1
thoroughly washed with methanol. Yield: 2.253 g (63%). H NMR analysis indicated that
compound 6 co-crystallized with DCM in a 1:1 ratio.
1H NMR (600 MHz, CDCl3) δ: 8.00 (d, J = 8.4 Hz, 4H), 7.77 (d, J = 8.7 Hz, 4H), 7.44 (d, J = 8.4
Hz, 4H), 7.30 (m, 8H), 7.16 (d, J = 7.6 Hz, 8H), 7.12 (d, J = 8.7 Hz, 4H), 7.10 (t, J = 7.4 Hz, 4H),
13
1.39 (s, 18H), the signal of the pyrrole rings overlaps with the triplet at 7.10 ppm (2H). C
NMR (75 MHz, CDCl3) δ: 154.0, 150.9, 149.7, 149.5, 147.2, 142.1, 131.4, 129.6, 129.1, 127.6,
125.7, 125.4, 125.2, 124.1, 122.6, 114.0, 34.8, 31.5. UV-Vis (THF) λ nm (ε [M-1cm-1]): 673 (48
000). HR-ESI-MS: calcd for C64H57N5 [M]+: 895.4608. Found: 895.4607. FT-ATR-IR: ν (cm-1) =
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