Boroquinol Complexes with Fused Extended Aromatic Backbones: Synthesis and Optical Properties

Article abstract of DOI:10.1002/chem.201604421

Boron-based dyes are attractive synthetic targets due to their large variability of absorption and emission wavelengths. Through Pictet–Spengler cyclizations, followed by oxidation, π-extended boroquinols have been synthesized. During optimization of the

Full text of DOI:10.1002/chem.201604421

A Journal of
Accepted Article
Title: Boroquinol Complexes with fused Extended Aromatic Backbones
- Synthesis and Optical Properties
Authors: Michael Mastalerz, Sven Elbert, Frank Rominger, Philippe
Wagner, and Thines Kanagasundaram
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Boroquinol Complexes with fused Extended Aromatic Backbones
– Synthesis and Optical Properties
Sven M. Elbert,[a] Philippe Wagner,[a] Thines Kanagasundaram,[a] Frank Rominger[a] and Michael
Mastalerz*[a]
Abstract: Boron-based dyes are attractive synthetic targets due to
their large variability of absorption and emission wavelengths. By
Pictet-Spengler-cyclizations, followed by oxidation π-extended
boroquinol have been synthesized. During optimization of the reaction
conditions an unusual dearylation has been found and mechanistically
investigated. For two of the synthesized boroquinols,
mechanochromic effects with bathochromic shifts up to 50 nm were
found upon grinding.
additional fused aromatic rings.[3a] This bears the advantage that
in contrast to tuning the emission with substituents with rotational
freedom, the rigidity of the system is retained, resulting usually in
higher quantum yields.
Formally, boroquinols are fused π-extended boranils
(Figure 1).[7b] Boroquinols show large Stokes shifts (>5000 cm-1),
[7b, 12]
which are desirable due to the decreased reabsorption of
the emitted light and are not common for BODIPYs which usually
have Stokes shifts around 500 cm-1. They are also emissive in the
solid state, which is advantageous for applications as
electroluminescent materials.[13]
Introduction
Among fluorescent dyes, those based on boron organic
complexes moved more and more into the focus of research
because of their facile synthetic accessibility, high quantum yields
(Φ) in different media and chemical and light stability.[1] Within this
class, boron-dipyrromethenes (BODIPYs, Figure 1)[1] are one of
the most popular and most investigated families.[1c, 2] The optical
properties of BODIPYs can be modified by attaching various
substituents at the conjugated backbone.[1a, 3] A large variety of
BODIPY derivatives[4] and related systems with N,N-,[1e, 5] O,O-,[6]
Figure 1. Core structures of selected BF2-complexes and the target structures
of this work.
7]
and N,O-chelating[1e,
bidentate ligands for four-coordinate
fluorescent boron complexes have been introduced and
broadened the potential fields of application. Due to the ease of
preparation and their excellent optical properties, boranils
(Figure 1) with bidentate N,O-chelating ligands stand out of this
vast amount of different groups.[8] The core structure was first
described in 1969[9] and its emitting properties were already
mentioned in 1973 by Hohaus and co-workers, who described it
as a compound with blue fluorescence.[10] In 2011, Ziessel and
co-workers rediscovered the boranils and synthesized a number
of derivatives.[11] They obtained highly fluorescent complexes (Φ
up to 90%) and reported about post-functionalizations by
palladium-catalyzed cross-coupling reactions. The ease of their
synthesis and their good optical properties make boranils
interesting alternatives to BODIPY dyes.
Most interestingly, only a few boroquinol-derivatives have been
published to date.[7b, 14] Donor-acceptor type fused boroquinols
have shown emissions with λ>600 nm, and theoretical
calculations have suggested that they have potential application
as n-type semiconductors.[15] In 2012, a series of boroquinol
complexes was reported with different substituents on both, the
phenolic and the quinolic backbone. These complexes showed
excellent quantum yields (Φ up to 86%) and a remarkable
solvochromic behavior.[12]
Here we describe the synthesis of extended boroquinol
analogues (Figure 1) with even larger π-systems containing three
or more annulated aromatic rings such as phenanthridines,
thioquinolines and benzothienoquinolines by a one-pot Pictet-
Spengler-cyclization[16] as a key step for the formation of the N,O-
ligand.
It is known for BODIPYs that the emission properties, such as the
HOMO-LUMO energy gap, can be fine-tuned not only by
substituents,[1a] but also by extension of the π-system through
Results and Discussion
[a]
S. M. Elbert, P. Wagner, T. Kanagasundaram, Dr. F. Rominger,
Prof. Dr. M. Mastalerz
Synthesis and Characterization
Organisch-Chemisches Institut
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 273, 69120 Heidelberg, Germany
E-mail: michael.mastalerz@oci.uni-heidelberg.de
In 2009, the utilization of the Pictet-Spengler reaction in the
synthesis of phenanthridines was described. The reaction
occurred under harsh conditions (TFA, 120-140 °C, 1.5 - 7 days),
giving the products in isolated yields between 20% and 91%.[17]
Supporting information for this article is given via a link at the end of
the document.
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Two equivalents of the aldehyde have been used, with one
equivalent probably acting as an oxidant. Recently, a number of
2-substituted phenols with thieno-, pyrrolo-, furo- or
oxazoloquinoline backbones and N,O-chelating binding sites
have been introduced by Pictet-Spengler reactions and
investigated for the selective binding of fluoride and the use as
antitubercular compounds.[18] The reaction procedures varied
very largely from mild conditions (20 mol% acetic acid, DMF,
60 °C, 16 h) to harsh ones (e.g. 30 mol% copper(II) triflate, 30
mol% 1,10-phenanthroline, nitrobenzene, 200°C) yielding
between 40% and 77% of the corresponding compounds. For a
twofold Pictet-Spengler cyclization on a pyrrolo-pyrrole-based
system, toluene and TFA have been used as solvents.[19]
According to this method, we first reacted 2-amino biaryl 1a[20] and
salicylaldehyde 2 to thienoquinoline 4a (Scheme 1, Table 1, Entry
1) under similar conditions (PhMe, 20% TFA, 100 °C). Besides
the desired product 4a (59%) substantial amounts of dearylated
thienoquinoline 6a[21] (29%) have been formed. Furthermore, by
the analysis of the crude reaction mixture with 1H NMR
spectroscopy and UPLC-MS phenol 7 has been identified as
another by-product. To the best of our knowledge this kind of
dearylation has been mentioned for the first time just very recently
for a porphyrin system.[22] However, this side reaction has only
been described phenomenological and no suggestion for a
reaction mechanism has been given. In this context, it is worth
mentioning that in a recent contribution for the Pictet-Spengler
reaction of 2-amino biaryls with glyoxylic esters a decarboxylated
product similar as in our case was found; again the occurrence of
this side reaction has been only described phenomenological and
no further investigation towards elucidating the mechanism has
been discussed.[23] Since the dearylated product has formed in
substantial amounts, we were interested in getting a deeper
mechanistically understanding of this reaction to optimize the
reaction on a rational basis.
dearylation has to occur from the saturated Pictet-Spengler
intermediate 9 (see Scheme 2). Two plausible reaction pathways
from 9 compete with each other. Either an oxidation of
dihydrophenanthridine 9 leads to the desired product 4a or a
protonation of 9 to 10 takes place, followed by a tautomerisation
of the phenol unit to the keto form 11, allowing the compound to
undergo cleavage of the C-C bond (in principle a retro-Mannich
or retro-hetero-ene reaction[24]), resulting in dearylated
thienophenantridine 6a. As a consequence, the presence or
absence and type of oxidant as well as the acid concentration
should play major roles to shift the reaction outcome either to 4a
or 6a. Thus we first studied the influence of acid concentration
under consistent conditions.
In order to optimize the reaction conditions, the acid concentration
and different atmospheres were tested (Table 1).
Table 1. Optimization of the reaction condition of the Pictet-Spengler
reaction of biaryl 1a and aldehydes 2 or 3.
Entry
RCHO
Atm
PhMe:TFA[a]
Yield [%][b]
(equiv.)
4a/5a
6a
7/8[c]
1
2 (1.0)
2 (1.0)
2 (1.0)
2 (1.0)
2 (1.0)
2 (1.0)
2 (1.0)
2 (1.0)
2 (1.0)
3 (1.0)
2 (2.0)
3 (2.0)
2 (1.0)
Air
Air
Air
Air
Ar
80:20
90:10
95:05
99:01
50:50
80:20
90:10
95:05
99:01
90:10
90:10
90:10
90:10
59 (4a)
66 (4a)
55 (4a)
25 (4a)
18 (4a)
18 (4a)
31 (4a)
46 (4a)
21 (4a)
66 (4a)
75 (4a)
86 (4a)
84 (4a)
29
8 (7)
2
13
10 (7)
7 (7)
3
9
4
n.d.[d]
67
n.d. (7)
n.d. (7)
22 (7)
25 (7)
11 (7)
5 (7)
5
6
Ar
68
7
Ar
26
We undertook isolated phenanthridine 4a the same reaction
conditions and could not find any dearylated product 6a, which
verifies the suggestion already done by Paolesse et al.[22] that the
8
Ar
12
9
Ar
5
10
11
12
13
Air
Air
Air
O2
n.d.
17
n.d. (8)
12 (7)
n.d. (8)
n.d. (8)
n.d.
n.d.
[a] v/v. [b] Determined by 1H NMR spectroscopy, using mesitylene as
internal standard. [c] Note that 7 and 8 are volatile compounds and might be
partially lost during work up. [d] n.d.: not detected.
Under ambient atmosphere the conversions were between 55%
and 75% with increasing amount of dearylation at higher acid
concentrations (Entries 2-4). Lower acid concentration diminished
the amount of dearylation product 6a, but also the formation of
desired phenanthridine 4a. An acid concentration of 1% led to low
conversions to 4a (25%, Entry 4 in Table 1). To suppress the
possibility for the oxidation pathway, the reactions were
performed under various acid concentrations in argon
Scheme 1. Reaction of 2-amino biaryl 1a with aldehydes 2 or 3 under various
conditions (see Table 1).
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atmosphere (Entries 5-9). With acid concentrations as high as 50
vol% the dearylated product 6a becomes the main product,
formed in 67% besides 18% of 4a (Entry 5). With decreasing
amount of acid, the amount of dearylation product 6a decreases
and the ratio of 6a/4a switches towards 4a with a maximum at a
5% acid concentration
TFA, air), have been used for reactions with two equivalents of
both, 4a and 5a (Entries 11-12) to elevate the product formation
to 75% (4a) or 86% (5a). From an atom-economically point of view,
it is not ideal to sacrifice half of the salicylaldehyde as oxidant.
Therefore, the 1:1-stoichiometric reaction of 1a and 2 was finally
performed with molecular oxygen as oxidant in toluene (with 10
vol% TFA) at 100 °C, giving 91% of the desired product 4a and
only traces of dearylation products 6a and 7 (Entry 13).
After the successful elucidation of suitable reaction conditions, a
series of π-extended compounds was synthesized. Suzuki-
Miyaura cross-coupling reactions[26] of boronic acids 12a-d[27] with
bromoaniline 13 afforded the required 2-aminobiaryls 1a-d[20, 28] in
48-95% yield (Scheme 3 and Table 2). Biaryls 1a-d were then
reacted with aldehydes 2 or 3 under the optimized reaction
conditions (PhMe, TFA, O2, 100 °C), giving the ligands 4a-d in
52-76% isolated yield. Reactions under ambient atmosphere
showed the discussed dearylation for all systems with isolated
yields of 6a-d[21, 29] of 15-17% and lower yields of the desired
ligands 4a-d in comparison to the optimized conditions (Scheme
3 and Table 2).
For a detailed spectroscopical analysis of the final BF2-complexes
(14a-d) the phenol ethers 5a-5d were synthesized under the
same conditions in 54-84% isolated yields.
4a-d were treated with borontrifluoride etherate (BF3∙Et2O) and
triethylamine to give the BF2-complexes 14a-d in 32-84% yield
(Scheme 3 and Table 2). It was found that complex 14d with two
methoxy groups is decomposing in solution with time, which
probably is the reason for the lower yield in this case.
Scheme 2. Two possible pathways from intermediate 9. Pathway 1: Oxidation
to thienoquinoline 4a. Pathway 2: Proton catalyzed dearylation to
thienoquinoline 6a and phenol 7.
(Entry 8). With only 1 vol% of acid (Entry 9), both products 4a and
6a are generated in lower amounts, similar to the reaction done
in air (Entry 4). This clearly demonstrates that the reaction
pathway to the desired product 4a is driven by the oxidant, and
the one to the dearylation product by the amount of acid. To prove
the tautomerization and C-C-cleavage steps, salicyl aldehyde
ether 3 has been synthesized[25] (see Supporting Information) and
used in the reaction with biaryl 1a under the same conditions as
the free phenol (10 % TFA, air; Entry 10). As expected, no
formation of dearylated product could be observed, due to the fact
that a Pictet-Spengler intermediate with a phenol ether cannot
undergo no tautomerization and only the corresponding
phenanthridine 5a was detected to be formed in 66%. As
mentioned before, the original procedure of Youn et al. used 2.0
equivalents of aldehyde, most likely because one equivalent is
acting as oxidant. Therefore, the so-far best conditions (10 vol%
Scheme 3. Synthesis of complexes 14a-d. Reagents and conditions: a) Pd2dba3,
HPtBu3BF4, THF, K2CO3 (aq.) (1 M), 80 °C, 16 h; b) 2 or 3, PhMe, TFA, O2, 100 °C,
16 h; c) 2, PhMe, TFA, air, 100 °C, 16 h; d) R=H, BF3∙Et2O, Et3N, CH2Cl2, r.t.,
16 h. For isolated yields see Table 2.
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systems in solution on the NMR time-scale. The planarity is
increased through the formation of a hydrogen bond
between the hydroxyl proton and the nitrogen in ligand 4a
leading to a pronounced downfield shift of proton Hh of ∆δ =
+0.8 ppm from 5a (δ = 7.45 ppm) to 4a (δ = 8.26 ppm), which
can be explained by a resulting intramolecular C-H∙∙∙S
contact of proton Hh.[30] In complex 14a no rotational freedom
along the biaryl axis is possible and consequently this
intramolecular interaction is evidenced by a negligible
change of the chemical shift of proton Hh in 14a (δ = 8.17
ppm) compared to 4a (δ = 8.26 ppm). In addition, an
intramolecular hydrogen bond (C-H∙∙∙F interaction) between
Hf and one fluorine of the BF2-moiety of 14a is apparent by
a significant downfield shift of proton Hf in complex 14a (δ =
8.91 ppm) compared to 5a (δ = 8.32-8.35 ppm) or 4a (δ =
8.20 ppm).[31] The interaction to a single fluorine atom is
furthermore supported by the presence of two signals in the
corresponding 19F NMR spectrum (δ = -140.6 ppm and δ = -140.5
ppm for complex 14a) with a coupling of the fluorine to the carbon
bound proton Hf (δ = 125.4 ppm, J = 8.1 Hz) “through space”.[32]
The C-H∙∙∙S interactions can be also found for the
benzothienoquinoline ligand 4b and complex 14b as well as for
the extended systems 17 and 18 (for chemical shifts see Table 3)
and the C-H∙∙∙F hydrogen bond are observed for all complexes
14a-d and 18, but will not be discussed here in detail (chemical
shifts of corresponding protons and carbons in Table 3).
Interestingly, all these interactions are found also in the solid state
(see discussion below).
Table 2. Isolated yields of the cross-coupling, cyclization and complexation reactions
towards complexes 14a-d as depicted in Scheme 3.
Entry
Ar
Yield of [%]
6
1
4
5
14
1
2
3
4
a
b
c
d
73 (1a)
48 (1b)
95 (1c)
80 (1d)
74[a]/42[b] (4a)
52[a]/46[b] (4b)
60[a]/35[b (4c)
76[a]/56[b (4d)
0[a]/15[b] (6a)
0[a]/17[b] (6b)
0[a]/16[b] (6c)
0[a]/15[b (6d)
78 (5a)
54 (5b)
66 (5c)
84 (5d)
84 (14a)
80 (14b)
68 (14c)
32 (14d)
[a] Oxygen atmosphere. [b] Ambient atmosphere.
With the optimized procedure, a system with five annulated rings
and two N,O-chelating moieties was accessible (Scheme 4):
Phenylenediamine 15 was synthesized by Suzuki-Miyaura cross-
coupling (see Supporting Information) and subsequently reacted
with aldehydes 2 or 3 under the mentioned optimized conditions
(PhMe, TFA, O2) to afford 17 in 46% yield. In contrast, methyl
protected derivative 16 could be synthesized in higher yields of
76%. The ether functions in 16 can also be cleaved with BBr3
giving 17 in 72%, thus the combined yield for this two-step
approach is slightly higher with 54%. Ligand 17 was converted to
BF2-complex 18 with borontrifluoride etherate and di-
isopropylamine in 70% yield.
Scheme 4. Synthesis of complex 18. Reagents and conditions: a) PhMe, TFA,
O2, 100 °C, 3 h; b) BBr3, CH2Cl2, 0 °C-r.t., 20 h; c) BF3∙Et2O, iPr2NH, C2H4Cl2,
85 °C, 16 h.
1
A comparison of H NMR spectra of compounds with the same
Figure 2. Comparison of 1H NMR spectra of: a) 5a; b) 4a; c) 14a in CDCl3 at
600 MHz and room temperature with signal assignment.
aromatic backbones (here thienoquinolines 5a, 4a and 14a,
Figure 2 and Table 3) allows first insights on intramolecular
interactions. 5a shows no hints of coplanarity of its aromatic
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A
B
C
D
G
J
E
H
K
F
I
L
M
N
O
Figure 3. Capped sticks representations of crystal structures of molecules 5a, 4a and 14a (A-C), molecules 5b, 4b and 14b (D-F), molecules 5c, 4c and 14c (G-I),
molecules 5d, 4d and 14d (J-L) and molecules 16-18 (M-O). The given angles are measured as angle between mean planes of the aromatic systems Ar and Ar’.
The chemical shifts of carbon C* and proton H* and the lengths of the intramolecular C-H∙∙∙S, O-H∙∙∙N and C-H∙∙∙F contacts can be found in Table 3. Colors: Carbon:
grey; Oxygen: red; Nitrogen: blue; Boron: rose; Fluorine: yellow; Hydrogen: white.
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Table 3. Selected NMR spectroscopic and crystallographic data.
[a]
[b]
[c]
Compound
δH*
δC*
∡
Ar Ar‘
dC-H∙∙∙S
∡
C-H∙∙∙S
dO-H∙∙∙N
∡
NHO
dB-N
∡
NBO
dC-H∙∙∙F
∡
CHF
[ppm]
[ppm]
[°]
[Å]
[°]
[Å]
[°]
154.0
147.2
150.9
-
[Å]
[°]
[Å]
[°]
4a
4b
4c
4d
5a
8.20
128.2 (s)
129.2 (s)
126.9 (s)
129.0 (s)
130.2 (s)
31.7
38.4
32.9
89.1
56.3
2.506
2.543
-
120.0
1.726
-
-
-
-
-
-
-
-
-
-
-
-
8.28-8.25
8.09
115.4
1.612
-
-
-
-
-
1.687
-
-
8.03
-
-
-
-
-
8.35-8.32
(m)
-
-
-
-
5b[d]
5c
8.43
8.25
8.19
8.91
9.02
8.82
131.2 (s)
130.5 (s)
128.8 (s)
125.4 (t)
125.8 (t)
125.4 (t)
70.1
56.1
83.5
35.8
37.1
37.2
37.4
47.8
44.0
40.0
44.7
63.3
14.7
34.9
35.2
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
5d
-
-
-
-
-
-
-
-
14a
14b
14c[e]
2.501
116.1
-
-
1.639
1.619
1.618
1.624
1.613
1.589
1.627
1.618
-
106.4
108.7
109.8
108.9
109.5
110.3
108.4
109.1
-
2.166
2.282
2.355
2.383
2.332
2.365
2.214
2.320
-
119.0
112.6
115.7
111.5
112.4
109.7
115.6
111.0
-
2.443
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
14d[e]
8.81
125.0 (m)
-
-
-
-
-
-
-
-
-
-
-
-
16
9.32
9.07
9.89
123.9 (s)
122.9 (s)
120.2 (t)
-
-
-
-
17
2.354
2.538
2.506
135.9
120.6
117.3
1.618
151.2
-
-
-
-
18[f]
-
-
-
-
1.617
1.636
109.7
108.6
2.333
2.331
109.5
111.1
[a] The position of proton H* is depicted in Figure 3. [b] The position of proton C* is depicted in Figure 3. [c] Measured as angle of mean planes of the aromatic
systems Ar and Ar’ as depicted in Figure 3. [d] A crystal structure was obtained, but can be only seen as proof of constitution due to low quality. [e] Three
independent molecules can be found in the crystal structure. [f] Two independent molecules can be found in the crystal structure.
Single Crystal X-Ray Analysis
Table 3) and additionally a C-H∙∙∙F contact with dC-H∙∙∙F = 2.166 Å
as indicated by 1H NMR spectroscopy (the chemical shifts of
protons H* and the multiplicity of carbon nuclei C* was discussed
before and is summarized in Table 3). Benzothienoderivatives 5b,
4b and 14b as well as methoxyphenanthridines 5c, 4b and 14c
show analogous trends as the ones discussed (Table 3).
Only the dimethoxyphenanthridines 5d, 4d and 14d show larger
torsion angles (5d: ∡Ar-Ar’ = 83.5°; 4d: ∡Ar-Ar’ = 89.1°) due to
sterical hindrance of the methoxygroup in 7-position. Ligand 4d is
therefore the only ligand without an intramolecular hydrogen bond
and is forming hydrogen bonded dimers. The smallest torsion
between Ar and Ar’ was found for extended bis-ligand 17 with
All compounds have been studied by single-crystal X-ray
diffraction analysis.
The comparison of the X-ray crystal structures of 5a, 4a and 14a
(Figure 3A-C) supports the conclusion drawn by 1H NMR
spectroscopy. Methoxy derivative 5a shows no intramolecular
interactions due to a torsion angle between its aromatic systems
Ar and Ar’ of ∡Ar-Ar’ = 56.3° (Figure 3B and Table 3). The hydrogen
bond (O-H∙∙∙N) with a length of dO-H∙∙∙N = 1.726 Å in ligand 4a forces
the aromatic systems Ar and Ar’ to planarize (∡Ar-Ar’ = 31.7°) and
causes the formation of a C-H∙∙∙S contact with dC-H∙∙∙S = 2.506 Å.
A comparable torsion angle (∡Ar-Ar’ = 35.8°) and C-H∙∙∙S contact
(dC-H∙∙∙S = 2.501 Å) can be found in complex 14a (Figure 3C and
∡
Ar-Ar’ = 14.7°.
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A
B
C
D
E
F
G
H
Figure 4. Capped sticks representations of packing motifs in x-ray crystal structures of: Complex 14a (A and B); Complex 14b (C and D); Complex 14c (E and F);
Complex 14d (G and H). C-H∙∙∙F contacts (C) and π-π interactions (B, D, F, H) are shows as dotted green lines. Colors: Carbon: grey; Oxygen: red; Nitrogen: blue;
Boron: rose; Fluorine: yellow; Hydrogen: white.
The observations made by single crystal x-ray analysis on the
intramolecular interactions in solid state are in good agreement to
the ones in solution made by 1H NMR spectroscopy and seem to
be not driven by packing effects.
Complexes 14a-d all show π-π stacking motifs. Complex 14a
packs in offset columns (Figure 5A and B) with a distance of
dπ-π = 3.277 Å that are aligned through C-H∙∙∙F and dispersion
interactions perpendicular to the direction of the columns.
Complex 14b forms dimers through C-H∙∙∙F interactions of
dC-H∙∙∙F = 2.561 Å with an offset π-π stacking of dπ-π = 3.407 Å
(Figure 5C and D). The so created sheets interact with solvent
molecules (CH2Cl2) through C-H∙∙∙F and Cl∙∙∙π interactions.
Complexes 14c and 14d pack in a comparable way with helical
A
columns of the three independent molecules with π-π interactions
of dπ-π = 3.34-3.51 Å which are aligned through C-H∙∙∙F and
C-H∙∙∙π and dispersion interactions (Figure 5E, F, G and H).
Complex 18 forms ribbons through intermolecular C-H∙∙∙F
interactions of one fluorine of a BF2 unit and the proton in α-
position to the sulfur atom of the extended backbone with
dC-H∙∙∙F = 2.399 Å (Figure 5A). These ribbons align in a herringbone
like manner via Cl-S, C-H∙∙∙F and C-H∙∙∙Cl interaction over solvent
molecules (CHCl3) as depicted in Figure 5B.
B
Figure 5. Capped sticks representations of packing motifs in the x-ray crystal
structures of complex 18. The green dotted lines in A illustrate C-H∙∙∙F contacts.
Colors: Carbon: grey; Oxygen: red; Nitrogen: blue; Boron: rose; Fluorine:
yellow; Hydrogen: white.
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Table 4. Spectroscopic properties.
Compound
4a
Solvent
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
C6H14
PhMe
THF
λabs (lg ε) [nm][a]
364 (3.95)
384 (4.02)
359 (3.76)
363 (3.87)
339 (3.86)
366 (3.86)
346 (2.84)
355 (3.64)
395 (4.02)
403 (3.73)
398 (3.97)
392 (4.00)
385 (4.02)
λem (λex) [nm][a]
Stokes Shift [cm-1]
Φ[b]
-
Τ [ns]
-
kf (108 s-1)[c]
knr (108 s-1)[d]
[e]
-
-
-
-
[e]
4b
-
-
-
-
-
-
[e]
4c
-
-
-
-
-
-
[e]
4d
-
-
-
-
-
-
[e]
5a
-
-
-
-
-
-
5b
388 (324)
1549
-
0.07
-
0.70
-
1.00
-
13.3
-
[e]
5c
-
[e]
5d
-
-
-
-
-
-
14a
14a
14a
14a
14a
14a
14b
14b
14c
14c
14d
14d
16
523 (396)
512 (356)
517 (398)
523 (392)
533 (385)
-
6196
5283
5783
6390
7212
-
0.32
0.29
0.37
0.24
0.13
-
5.90
5.43
5.71
5.36
3.56
-
0.54
0.53
0.65
0.45
0.36
-
1.15
1.31
1.10
1.42
2.44
-
MeCN
MeOH
CHCl3
PhMe
CHCl3
PhMe
CHCl3
PhMe
CHCl3
CHCl3
CHCl3
PhMe
[f]
-
417 (4.06)
417 (4.10)
385 (3.97)
387 (3.98)
399 (3.92)
395 (3.97)
392 (4.34)
433 (4.44)
464 (4.45)
462 (4.50)
543 (417)
534 (417)
521 (387)
515 (392)
530 (399)
522 (395)
411, 432 (373)
5565
5254
6780
6422
6195
6159
1179
-
0.28
0.11
0.24
0.28
0.18
0.26
0.12
-
2.03
1.89
4.99
4.59
4.57
4.80
0.53
-
1.38
0.58
0.48
0.61
0.39
0.54
2.26
-
3.55
4.71
1.52
1.57
1.79
1.54
16.6
-
[e]
17
-
18
555 (464)
537 (422)
3534
3023
0.15
0.20
2.03
1.81
0.74
1.10
4.19
4.42
18
[a] Measured at room temperature. [b] Determined by the absolute method. [c] Radiative rate constant kf is calculated as kf = Φ/Τ. [d] Non-radiative rate constant
knr is calculated as knr = (1-Φ)/Τ. [e] Non fluorescent. [f] Not stable in Methanol.
optical properties depending on the substituent in 2’-position
(Figure 6A and Table 4) in chloroform solutions. A methoxy
substituent in this position, as present in molecule 5a prevents the
co-planarization of the aromatic systems (comparable to the
observations in solid state as discussed above) leading to the
most redshifted absorption at λabs = 339 nm. For the phenol
analogue 4a the most redshifted maximum is bathochromically
Optical Properties in Solution and Solid State
Since the synthesized compounds contain different aromatic
backbones and show structural differences in the solid state, they
were investigated by UV/Vis and emission spectroscopy. A
comparison of compounds with the same backbone (e.g.
thienoquinolines 4a, 5a and 14a) shows a significant change in
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shifted about ∆λ = +25 nm to λmax = 364 nm, which can be
explained with a higher co-planarity of the two aryl systems as a
result of the intramolecular cyclic hydrogen bond.[33] Upon the
introduction of the BF2-centers another bathochromical shift of
∆λ = +31 nm to λabs = 395 nm is observed for complex 14a. Similar
trends exist for all investigated series (see Table 4 and Supporting
Information) and are not further discussed in detail. The most
extended systems (16-18) show absorptions between
λabs = 392 nm (16) and λabs = 462 nm (complex 18) (Table 4). A
comparison of the five boroquinols (Figure 6B) reveal that the
three complexes with three annulated rings (14a, 14c and 14d)
have comparable low energy absorption bands with maxima
between λabs = 385 nm (14c) and λabs = 399 nm (14d). Complex
14b with four annulated rings has its most redshifted absorption
maximum at λabs = 417 nm and the extended bis-complex 18 at
λabs = 464 nm (Figure 6B and Table 4). In contrast to negligible
emissions of some of the precursors (5b and 16), boroquinols
14a-d and 18 show emission in chloroform solution with maxima
between λem = 521 nm (14c) and λem = 555 nm (18) (Figure 6C
and Table 4) and quantum yields between Φ = 0.15 (18) and
Φ = 0.32 (14a). Other boranils and smaller boroquinols had to be
decorated with electron donating or electron donating and
accepting groups (push-pull systems) to emit at comparable
wavelenghts (boranils: λem = 445-539 nm),[11] whereas for
example unsubstituted smaller boroquinols emit at λem = 476
nm.[7b]
decreasing solvent polarity ranging from Φ = 0.13 in acetonitrile
to Φ = 0.37 in toluene. Interestingly, the quantum yield in hexane
(Φ = 0.29) is rather low considering the non-polar character of the
solvent. This and the lower extinction coefficient compared to
other solvents (see Table 4) could be attributed to the low
solubility in hexane accompanied by formation of aggregates. In
methanol
a deborylation was observed (see Supporting
Information), which is in agreement to observations made before
for other boroquinols.[7b] Complex 14b exhibits a quantum yield of
Φ = 0.28 in chloroform and is within this series the only complex
showing
a lower quantum yield in toluene (Φ = 0.11) in
comparison to chloroform. The boroquinols with methoxy
substituted phenanthridine backbones (14c and 14d) show
comparable quantum yields of Φ = 0.24 (14c) and Φ = 0.18 (14d)
in chloroform and 0.04-0.06 higher quantum yield in toluene (see
Table 4).
The fluorescence lifetimes of the complexes with three annulated
rings (14a, 14c and 14d) are with τ = 4.57-5.90 ns significantly
higher than the ones of complexes 14b or 18 (τ = 2.03 ns) (Table
4). The resulting radiative rate constants increase for all
boroquinols from chloroform to toluene (Table 4); again with 14b
as only exception. The difference in the decrease of the radiative
rate constant kf in comparison to the non-radiative rate constant
knr (Table 4) shows that the lower quantum yields of 14d and 18
are more likely to be attributed to an increase of non-radiative
processes than to a destabilization of the corresponding excited
states.
The emission properties of boroquinol 14a were extensively
investigated in various solvents (Table 4 and Supporting
Information) and show an increasing quantum yield with
A
B
C
D
E
F
Figure 6. A: UV/Vis spectra of compounds 4a, 5a and 14a and emission spectrum of 14a in CHCl3 at room temperature. B: UV/Vis spectra of complexes 14a-d and
18 in CHCl3 at room temperature. C: Emission spectra of complexes 14a-d and 18 in CHCl3 at room temperature. D: Emission spectra of complexes 14a-d in solid
state at room temperature. E: Emission spectra of complexes 14a in solid state heated (T = 200 °C) and ground. The dashed lines display a second circle of heating
and grinding. The inlet shows photographs of the solid materials irradiated at λ = 366 nm. F: PXRD patterns of 14a: top: mechanically ground material; middle:
heated material; calculated from single crystal x-ray analysis.
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In comparison to the emissions in solution (CHCl3), the emissions
in solid state (Figure 6C, 6D and 6E, Tables 4 and 5) are
hypsochromically shifted for the as-synthesized solid materials.
For 14a a large shift of ∆λ = -59 nm (λem solution = 523 nm; λem
solid = 464 nm) has been observed. Most interestingly, boroquinols
14a and 14d show a mechanochromic behaviour[34] in the solid
upon mechanical grinding of the as-synthesized solids.
of 14d was not investigated, because it decomposes at rather low
temperatures (T > 60 °C).
Complexes 14b and 14c only show negligible changes of their
emission wavelength upon grinding (Table 5 and Supporting
Information).
DFT Calculations
Table 5. Emission spectroscopy data of complexes 14a-d in solution and
solid state.
To obtain more detailed information about the influence of the π-
extension of the backbones of boroquinols 14a-d and 18 the
relative energy levels of frontier molecular orbitals have been
calculated at the B3LYP/6-311+G** level of theory.[36]
Cmp #
λem (CHCl3) [nm]
λem (solid) [nm][a]
λem (solid) [nm][b]
14a
14b
14c
14d
523
543
521
530
464
519
510
495
514
524
511
524
[a] As-synthesized solid material. [b] Mechanically ground solid material.
Again 14a shows the largest shift of ∆λ = +50 nm (λem
,
as-synthesized
= 464 nm; λem ground = 514 nm; Table 5, Figure 6E), which was fully
reversible upon heating (T = 200 °C). In general a change in
morphology of the fluorescent solid material has been pointed out
to be the reason for mechanochromic effects.[35] Boroquinol 14a
was investigated by powder x-ray diffraction (PXRD) to get further
insight to the origin of the observed effects. The as-synthesized
material has a high degree of crystallinity, which is largely
decreased upon grinding (Figure 6F). The reason for the change
in emission wavelength could be explained with the presence of
strong intermolecular interactions such as π-π-interactions in the
ordered (or as-synthesized) phase, as seen in the X-ray crystal
structure (as discussed above, Figure 4A-B). Since the measured
PXRD-pattern fits perfectly to one calculated from the crystal
structure, it can be concluded that these interactions elucidated
by single crystal x-ray analysis are also present in the as-
synthesized crystalline material. The shear forces applied by the
mechanical stimulus is potentially changing the intermolecular
interactions on a molecular level and therefore the degree of
crystallinity. The resulting meta-stable state and its accompanied
intermolecular interactions allow different transitions upon
excitation and cause the observed change of the emission
wavelength. The shift of the emission wavelength can be also
seen by pure eye as the colour changes from light blue (as-
synthesized or heated) to yellow (ground) as depicted in the inlet
of Figure 6E.
Figure 7. DFT calculations of frontier molecular orbitals and energy levels at the
B3LYP/6-311+G** level of theory of boroquinols 14a-d and 18.
The calculated energy levels of the HOMOs of all five complexes
(Figure 7) are with EHOMO = -6.2 eV (18) to EHOMO = -5.9 eV (14d)
in a similar range. In all cases, the largest orbital coefficients of
the HOMO are localized on the former salicylaldehyde part, which
is the same for all compounds. The largest orbital coefficients of
the LUMOs can be found on the extended aromatic backbone.
Electron donating methoxy groups, as found in 14c and 14d
increase the energies of the LUMOs to ELUMO = -2.5 eV in both
cases, indicating that the influence of the methoxy group in 7-
position is negligible. 14a (ELUMO = -2.8 eV) and 14b (ELUMO = -2.9
eV) show comparable LUMO energies resulting in HOMO-LUMO
differences of ∆(ELUMO-EHOMO) = 3.2-3.3 eV. The LUMO of
complex 18 was found at ELUMO = -3.3 eV and the resulting
HOMO-LUMO difference of ∆(ELUMO-EHOMO) = 2.9 eV is the
smallest for all five complexes. This is reflected by the most
redshifted absorption and emission as discussed above.
The thermal reversibility of the obtained effects was proven by
subsequent heating and grinding of the solid material. Solid state
fluorescence spectroscopy proved the complete reversibility of
both, the emissions maxima and the shape of the emission bands
(Figure 6E). The reversibility can best be explained by
reorganisation of the metastable amorphous state by lattice
vibration upon heating and crystallization.[35a]
A similar mechanochromic effect was observed for 14d, which
shows a bathochromic shift of ∆λ =+28 nm upon grinding (Table
5 and Supporting Information). However, the thermal reversibility
Conclusions
We successfully applied the Pictet-Spengler reaction to
synthesize π-extended ligands with N,O-chelating moieties.
During the synthesis a barely described side-reaction was found
and a plausible mechanism suggested by evaluation of various
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[4] For recent examples of BODIPY derivatives see: a) J. Bartelmess, M.
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screening conditions. According to the suggested mechanism, the
conditions have finally been optimized, giving the ligands for the
boroquinols in high yields. The corresponding fluorescent
boroquinols emitted at wavelengths up to λem = 555 nm with
quantum yields up to Φ = 0.37. Remarkably, some complexes
showed large Stokes shifts (>7200 cm-1) or a mechanochromic
behavior in solid state by a transition between amorphous and
crystalline phases, which has been proved by PXRD. As has been
demonstrated with the larger compound with a heteropicene-
based backbone, the optimized reaction conditions opens the
possibility to larger structures containing more than one BF2-unit
and opens the possibility to be applied for typical products by
dynamic covalent chemistry contain salicyl imines, such as
macrocycles[37] or cage compounds,[38] which is currently
investigated in our laboratories.
Experimental Section
All crystallographic information files (CCDC-1503734 [4a],
1503735 [4b], 1503736 [4c], 1503737 [4d], 1503738 [5a],
1503739 [5b], 1503740 [5c], 1503741 [5d], 1503742 [14a],
1503743 [14b], 1503744 [14c], 1503745 [14d], 1503742 [16],
1503742 [17] and 1503742 [18]) have been deposited in the
Cambridge Crystallographic Data Centre and can be downloaded
free of charge via www.ccdc.camac.uk/data_request/cif.ls.
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Supporting Information (see footnote on the first page of this
article): For synthetic procedures, characterization and spectra of
the discussed molecules see Supporting Information.
Acknowledgements
S. M. E. is grateful to the Studienstiftung des deutschen Volkes
for a PhD scholarship. Hendrik Herrmann (Himmel Group) and
Petra Krämer (Hashmi Group) are acknowledged for PXRD and
IR measurements.
Keywords: fluorescence • boron complexes • extended π-
system • mechanochromism • Pictet-Spengler reaction
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10.1002/chem.201604421
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Stiff it up. By Pictet-Spengler
Sven M. Elbert, Philippe Wagner, Thines
Kanagasundaram, Frank Rominger and
Michael Mastalerz*
cyclisations a series of boroquinols
with extended π-backbones have
been made accessible. Some of them
showing reversible mechanochromical
effects upon grinding. During the
Pictet-Spengler cyclization a rarely
described fragmentation under C-C-
bond cleavage occurred, for which s
plausible mechanism could be
suggested on the basis of various
reaction conditions.
Page No. – Page No.
Boroquinol Complexes with fused
Extended Aromatic Backbones –
Synthesis and Optical Properties
This article is protected by copyright. All rights reserved.