π-Extended push-pull azo-pyrrole photoswitches: synthesis, solvatochromism and optical band gaps

Article abstract of DOI:10.1039/c9ob02410g

A new family of push-pull biphenyl-azopyrrole compounds 3b-g and 4b-d was efficiently obtained via a Suzuki cross-coupling reaction between 2-(4′-iodophenyl-azo)-N-methyl pyrrole (1a) or 3-(4′-iodophenyl-azo)-1,2,5-trimethyl pyrrole (2a) and 4′-substituted phenyl boronic acids in excellent yields. The influence of the π-biphenyl backbone and pyrrole pattern substitution was correlated with their optical properties. Solvatochromic studies via UV-visible spectrophotometry revealed that the inclusion of a 4′-nitro-biphenyl fragment favors a red-shift of the main absorption band in these azo compounds compared with their non-substituted analogues. Likewise, optical band-gaps were estimated by means of electronic absorption spectra and correlated with TD-DFT studies. The pyrrole pattern substitution and the π-conjugated backbone exhibit a clear influence on their thermal isomerization kinetics at room temperature. In all cases, biphenylazo-pyrrole compounds lead to the formation of J-type aggregates in binary MeOH:H₂O solvents. Under these conditions, compounds 3b-c undergo a water-assisted cis-to-trans isomerization at room temperature.

Full text of DOI:10.1039/c9ob02410g

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Villarreal, B. J. López Mayorga, D. Gallardo-Rosas, R. A. Toscano, M. D. P. Carreon-Castro, V. A. Basiuk, F.
Cortes-Guzman, J. G. López-Cortés and M. C. Ortega-Alfaro, Org. Biomol. Chem., 2020, DOI:
10.1039/C9OB02410G.
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DOI: 10.1039/C9OB02410G
ARTICLE
p
-Extended push-pull azo-pyrrole photoswitches: Synthesis,
solvatochromism and optical band gap.
J. A. Balam-Villarreal,a B. J. López-Mayorga,a D. Gallardo-Rosas,a R. A. Toscano,b M. P. Carreón-
Received 00th January 20xx,
Accepted 00th January 20xx
Castro,a V. A. Basiuk,a F. Cortés-Guzmán,b J. G. López-Cortés,*b and M. C. Ortega-Alfaro*a
DOI: 10.1039/x0xx00000x
A new family of push-pull biphenyl-azopyrrole compounds 3b-g and 4b-d was efficiently obtained via a Suzuki cross-coupling
reaction, between 2-(4’-Iodophenyl-azo)-N-methyl pyrrole (1a) or 3-(4’-Iodophenyl-azo)-1,2,5-trimethyl pyrrole (2a) and 4’-
substituted phenyl boronic acids in excellent yields. The influence of p-biphenyl backbone and pyrrole pattern substitution
was correlated with their optical properties. Solvatochromic studies via UV-Visible spectrophotometry revealed that
inclusion of a 4’-Nitro-biphenyl fragment favors a red-shift of the main absorption band in these azo compounds compared
with their non-substituted analogues. Likewise, optical Band-gaps were estimated by means of electronical absorption
spectra and correlated by TD-DFT studies. The pyrrole pattern substitution and the p-conjugated backbone exhibit a clear
influence in their thermal isomerization kinetics at room temperature. In all cases, biphenylazo-pyrrole compounds tend to
the formation of J-type aggregates in binary MeOH:H2O solvents. Under these conditions, compounds 3b-c undergo a water-
assisted cis-to-trans isomerization at room temperature.
important red shifts in their electronic absorption spectra.7-13, 16
Likewise, molecular polarization of push-pull systems has key
implications for optical response at molecular level and the
Introduction
Azo compounds belong to an extensive family of photochromic
compounds with a widespread of applications in many areas of
science. Some recent applications involve their use as
photopharmacological agents for photodynamic therapy
(PDT),1 and drug delivery agents,2 optical storage media,3 liquid
crystals,4 photochemical molecular switches5 and electronic
devices.6 Particularly, azoheteroarenes including nitrogen-
based heterocycles such as pyridine,7 pyrrole,8,9a pyrazole,9
imidazole,10 thiazole,11 benzothiazole12 and indole13 have been
studied as interesting photoswitches displaying nearly
quantitative E-Z isomerization with tunable thermal
lifetimes,9,13 and as promising precursors in material
science.3,4,6,8
Likewise, azo compounds containing a push-pull system
incorporated into their structure have been the focus of intense
research in the last years because of their interesting nonlinear
optic and solvatochromic properties and their great potential in
opto-electronic materials due to their fast thermal cis-to-trans
isomerization kinetics at room temperature.12,14 Experimental
studies have demonstrated that the incorporation of easily
delocalizable five-membered nitrogen-based heterocyclic ring
enhances their molecular hyperpolarizability15 and provokes
optimization of the
p-conjugated linkers, electron-donor and
electron acceptor groups are determining design variables to
take into account. 17
On the other hands, C-C coupling reactions are by far one of the
most used synthetic approach for extending the
p- conjugated
backbone in a variety of push-pull systems, but this tool has not
been used in arylazo-compounds due to the strong coordinating
behavior of azo-group to the conventional palladium sources
used in these reactions, that usually give rise to the formation
of palladacycles via C-H activation.18 Recently, we resolved this
drawback using a robust catalytic system that allowed us to
synthesize stilbenyl azo-pyrrole compounds in excellent yield,
using a Heck coupling reaction. These compounds exhibited
reversible photoisomerization with a good yield of Z-isomer in
PSS, being possible to control the photo switching process with
high spatial precision through non-linear optical excitation. 19 As
a part of a project directed to the synthesis of dyads D-p-A as
molecular photoswitches with potential applications as drug
delivery agents in physiological conditions.20 We have
envisaged to synthesize a new family of push-pull azo
compounds, containing as donor group, a pyrrole ring and a
p-
backbone formed by an azo group and a 4-substituted biphenyl
moiety with diverse electron withdrawing groups (Fig. 1). The
synthetic approach includes an azo coupling reaction followed
by a Suzuki-Miyaura cross-coupling reaction catalyzed by a
ferrocenyl palladacycle, which proved an excellent performance
in this catalytic reaction.21 To compare the optical properties of
this family of biphenyl-azopyrroles, analogues with a phenyl
a. Instituto de Ciencias Nucleares, UNAM, Circuito Exterior, Ciudad Universitaria,
Coyoacán C.P. 04510, Ciudad de México, México. E-mail:
carmen.ortega@nucleares.unam.mx
b. Instituto de Química UNAM, Circuito Exterior, Ciudad Universitaria, Coyoacán C.P.
04510, Cuidad de México, México. E-mail: jglcvdw@unam.mx
Experimental procedures and characterization data of all compounds. CCDC-
1950956-1950959 (1c, 2a, 2b and 4d). For crystallographic data in CIF format See
DOI: 10.1039/x0xx00000x
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Journal Name
I
properties, we also prepared the 3-substituted analogues 2a-c
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starting from 1,2,5-trimethylpyrrole. InDaOllI: c10a.s1e03s9a/Cz9oO-pBy0r2r4o1l0eGs
dyes are obtained as orange or red solids in moderated to
excellent yields. However, the reaction to obtain 2d was not
successful, even when the methods A or B were tested.
The structures of 1c,22 2a23 and 2b24 were confirmed by X-ray
single crystal diffraction (Fig. 2 and Fig. SI-1-2). In all cases, these
compounds showed orientational disorder in crystals.
Molecular structures are almost coplanar and, the azo group
displays an E geometry. The bond lengths N(2)-(N3) and N(3)-
C(6) for 1c are very similar to those observed in orange disperse
EWG
π−conjugated
N
R
N
backbone
R
N
+
CH3
B(OH)2
EWG
N
N
Push-Pull
R
N
R
azo-compound
CH3
61 [1.250(7)
Å
and 1.419(7) Å, respectively] and
Fig. 1. Structural design of new push-pull biphenyl-azopyrrole dyes.
phenylazopyrazole [1.254(2) and [1.427(2)] but are longer to
that observed in azobenzene [N-N, 1.237(2)].25 This behavior
confirms the push-pull nature of these arylazopyrroles.
Likewise, the crystal lattice structure of 1c shows two
conformers with an antiparallel distribution of molecules in
direction of axe b, stabilized by an O2 H1C intermolecular
interaction [2.605 Å], that maximize electron donor–acceptor
π–π interactions.
group as
p-linker were prepared. Likewise, a comparison using
2- or 3-susbtituted pyrrole as donor group is included. The
optical properties of these series of azo compounds were
determined by means of UV/Visible spectroscopies and
solvatochromic studies. We also determined their thermal
behavior upon photoisomerization studies. In addition, to have
an approach on the behavior of these azo compounds in
physiological conditions, we have also studied their aggregation
in binary solvents. Photoisomerization of biphenylazo pyrroles
under aggregation conditions modifies their kinetic parameters
giving rise to a faster water-assisted cis-to-trans isomerization
at room temperature.
…
To extend the
p-backbone in these arylazopyrrole compounds,
we used the Suzuki-Miyaura reaction to introduce the biphenyl
moiety. Initially, we performed the reaction between 1a and
phenylboronic acid using Pd(AcO)2 in the presence of PPh3
(Table 1, entry 1). However, we obtained the coupling product
2b in low yield, maybe because of the strong coordinative
character of the azo group included in the substrate, which can
be coordinated to the Pd (II) source,18 dropping the quantity of
the palladium active specie to promote the cross-coupling
reaction.
Results and discussion
Initially, we synthetized the arylazo(2-pyrrole) dyes 1a-d using
as precursor N-methylpyrrole, and the corresponding p-
substituted aniline (Scheme 1). To compare their optical
NH2
R'
Method A or B
N
Method A or B
N
N
N
N
R
R
N
N
R'
R'
R= H, Me
1a, R' = I (83%),
Method A
2a, R' = I (60%),
Method B
1b, R' = H (79%), Method A
1c, R' = NO2 (62%), Method B
1d, R' = CF3 (68%), Method B
2b, R' = H (95%), Method A
2c, R' = NO2 (62%), Method B
Scheme 1. Synthesis of arylazopyrrole dyes 1a-d and 2a-c through azo coupling reaction. Method A: 1) HCl, MeOH, 0 °C; 2) NaNO2 ,0 °C. Method B: 1) NaNO2, HCl, MeOH, 0 °C; 2)
Pyrrole compound, Pyridine, MeOH, 0 °C.
a)
b)
Fig. 2. (a) ORTEP view of 1c with thermal ellipsoids at 30% probability level. Selected bond lengths (Å) and angles (°): N(1)-C(1) 1.467(8); N(2)-N(3) 1.269(6); N(2)-C(2) 1.358(5); N(3)-
C(6) 1.408(5); N(4)-C(9) 1.449(5) N(4)-O(1) 1.222(6) C(5)-N(1)-C(2) 110.0(4), C(5)-N(1)-C(1) 125.2(4); C(2)-N(1)-C(1) 124.8(4), N(3)-N(2)-C(2) 114.6(3); N(2)-N(3)-C(6) 113.5(3); C(9)-
N(4)-O(1) 118.7(4); O(1)-N(4)-O(2) 122.8(4).(b) Crystal lattice structure of compound 1c.
2 | J. Name., 2012, 00, 1-3
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Table 1 Suzuki-Miyaura cross-coupling of 1a and different phenyl boronic acids.a
View Article Online
DOI: 10.1039/C9OB02410G
EtO
B(OH)2
S
Cl
N
N
Pd
N
N
[Pd], K2CO3
PPh3
N
N
+
[Pd] =
MeOH, 90 °C
Fe
I
1a
3
R
R
Entry
Compound
R
H
Heating Conditions
Time (min)
Yield (%)b
28c
TONf
280
980
960
-
TOF (h-1)g
560
MW
MW
IR
1
2
3
4
5
6
7
8
9
3b
3b
3b
3b
3c
3d
3e
3f
30
11
H
98
96d
Tracese
5345
480
H
120
1200
40
H
Thermal
MW
MW
MW
MW
MW
-
NO2
CF3
94
940
960
960
890
980
1410
2743
2880
4450
9800
21
96
SO2Me
COMe
NMe2
20
96
12
89
3g
6
98
a All reactions were performed with 1 mmol of the 1a, 1.2 mmol of the phenyl boronic acid, MeOH (5 mL) and 1.2 mmol of K2CO3, 0.1% mol of [Pd] at 90 °C. b Isolated
c
yield after SiO2 column chromatography. Reaction performed with 0.1% mol of Pd(OAc)2 / 2 PPh3. d Reaction conducted under infrared irradiation at 65 °C, using an
Osram lamp (bulb model Thera-Therm, 250 W, 125 V). For controlling the temperature, a Digi-Sense variable-time power controller was used. e Reaction conducted at
reflux of MeOH. f TON = ratio of moles of product formed to moles of catalyst used. g TOF = TON/t.
Table 2 Suzuki-Miyaura cross-coupling of 2a and different phenyl boronic acids.a
R
EtO
I
S
B(OH)2
Cl
Pd
N
[Pd], K2CO3
PPh3
N
[Pd] =
+
N
MeOH, 90 °C
MW
Fe
N
N
N
R
2a
4
Yield (%)b
TON
TOF (h-1)
Entry
Compound
R
Time (min)
1
2
3
4b
4c
4d
H
NO2
CF3
6
120
60
95
64
70
950
640
700
9500
320
700
a All reactions were performed with 1 mmol of the 2a, 1.2 mmol of the phenyl boronic acid, MeOH (5 mL) and 1.2 mmol of K2CO3, 0.1% mol of [Pd] at 90 °C. b Isolated
yield after SiO2 column chromatography.
Recently, we reported the catalytic applications of a ferrocenyl we only recovered traces of the coupling product, observing the
palladacycle, which demonstrated to be robust and efficient to degradation of azopyrrole precursor 1a (Table 1, entry 4).
promote the Suzuki coupling in soft reaction conditions.21 With Analyzing these results, we consider that the focused and
this in mind and looking for improve this reaction step, we sudden heating provided by MW (or IR irradiation) favors the
tested this palladium complex as catalytic precursor, using the complete transformation of 1a in this coupling reaction.
reaction conditions previously established in a model reaction Once optimized the coupling reaction, we used different phenyl
between 4-iodoaryl compounds and different aryl boronic boronic acids p-substituted with four different EWG, –N(Me)2 as
acids, where excellent yields and high TONs and TOFs values donor group, and hydrogen as reference. We also conducted
ranging from 1.8 to 9.8 x 103 were obtained.21 Following this the synthesis of series 4b-d, using as precursor 2a obtaining the
methodology, we reacted the compound 1a with diverse corresponding coupling products in good to excellent yields
phenylboronic acids (Table 1).
(Table 2). (HR-MS and NMR spectroscopies were used to
The palladium complex proved to be effective for the Suzuki confirm the structural identities of these series of compounds.
coupling, affording the coupling product in yields ranging from The structure of 4d26 was full confirmed by X-ray diffraction
89 to 98%, without presence of by-products, in short reaction analyses (Fig. SI-3). The azo group displays an E geometry and
times. As in model reaction to obtain biphenyl compounds, we the bond lengths N(2)-(N3) and N(3)-C(6) are very similar to
also tested if this reaction could be conducted under infrared those observed in the series of azopyrroles 1 and 2. As expected
irradiation, finding the formation of 3b in excellent yield, but the biphenyl unity is non-coplanar and this fact provokes a
the reaction time increases to 120 minutes (Table 1, entry 3). slipping of azo-pyrrole units in the crystalline packing avoiding
When the same reaction was conducted under thermal heating, the
p-p stacking observed in its analogue 1c.
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UV-Visible studies of arylazopyrrole dyes. Electronic spectrum intramolecular charge transfer observed in push-pull azo-
View Article Online
of 1b shows an intense absorption band around 386 nm compounds.14 Similar results are obtained across the series 2b-
DOI: 10.1039/C9OB02410G
assigned to
shoulder on the
observed for 1b in acetonitrile.9c-d When a strongest EWG such Fig. 4. Compound 3b displays a
as NO2 is included in the structure of this azo-compound (1c), a and n- * transition around 440 nm as a shoulder on the
red-shift and biggest values are observed in comparison with transition band. When, a strongly electron-acceptor group such
its counterpart 1b, the position of both transitions * and n- as -NO2 is included in the para-position of biphenyl moiety (3c),
p
-
p
* and the n-
* around 427 nm. A similar behavior was azopyrroles 3b-g and 4b-d acquired in chloroform are shown in
* transition near to 400 nm
p* transition appears as a small c (Table 3). The UV/vis spectra of p-extended biphenyl-
p-p
p-p
p
p-p*
e
p-p
p
* can interchange according to the expected behavior for most a biggest intensity and red-shift of both transition bands are
push-pull azocompounds,27 being practically non-possible to observed in comparison to 3b.
distingue them. This behavior correlates well with the expected
Table 3. UV-Vis data for pyrrole azo dyes 1, 2, 3 and 4 in different solvents.
Benzene
ET(30)c=34.3
a
1,4-Dioxane
ET(30)c= 36.0
a
THF
Chloroform
ET(30)c=39.1
a
DMSO
Methanol
ET(30)c=55.4
a
ET(30)c=37.4
ET(30)c=45.1
Entry
Compound
a
a
lmax
eb
lmax
eb
lmax
eb
lmax
eb
lmax
eb
lmax
eb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1b
1c
1d
2b
2c
3b
3c
3d
3e
3f
389
422
396
363
414
401
413
404
405
406
423
378
399
381
2.8
3.4
2.3
2.1
2.0
2.5
2.2
2.1
3.3
2.4
3.4
3.0
2.7
2.8
386
419
394
362
411
400
410
402
403
405
421
377
396
380
2.3
2.6
2.2
1.9
2.2
2.4
2.3
3.1
2.7
3.1
2.9
3.0
3.0
2.7
387
422
395
363
418
400
411
402
403
405
421
378
400
383
2.6
3.6
2.8
1.9
2.3
3.3
3.9
3.9
3.1
2.9
2.7
3.1
2.8
2.8
387
424
394
364
420
400
411
401
404
405
423
378
398
381
2.2
2.6
2.0
1.9
2.2
2.4
3.0
2.9
1.7
1.4
1.2
2.5
3.0
2.6
391
433
400
370
434
406
420
406
409
412
452
387
---d
2.1
2.5
2.2
1.9
1.8
2.5
3.0
2.6
3.2
3.2
1.4
2.4
---d
1.9
386
420
393
364
419
398
409
400
402
403
425
380
395
382
2.7
3.0
2.6
2.2
2.1
2.6
3.5
2.2
1.9
2.5
2.8
2.9
2.7
2.9
3g
4b
4c
4d
390
almax (nm) assigned to p-p* transition, be (104 M-1cm-1), c ET(30) (Kcal, mol-1), d Non soluble
Fig. 3 UV-Vis absorption spectra of compounds 3b-g (a) and 4b-d (b) in CHCl3.
Fig. 4. (a) Absorption spectra of 1b-d (dash lines) and 3b-d (solid lines) in chloroform. (b) Absorption spectra of 2b-d (Solid lines) and 4b-d (dash lines) in chloroform. (c)
Comparison of absorption spectra of 3c in different solvents.
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DOI: 10.1039/C9OB02410G
However, although weakly electron-acceptor groups provoke a
slight red-shift, the intensity of this transition band decreases
(Table 3, entries 8-10). Conversely, when a strongly electron-
releasing group such as -NMe2 is included in the para-position
of biphenyl moiety (3g), its electronic absorption spectrum
changes. Both p-p* and n-p* transition bands are red-shifted
and occur with a similar intensity in comparison to those
observed in 3b.
N
N
N
R
MeOH
λmax = 416 nm
St-3c,(19) R = NO2 λmax = 426 nm
St-3b,(19) R = H
Comparing the absorption spectra of series 1(b-d) with the
extended series 3(b-d), we observe that the extension of the
p
p
-
-
R
N
N
linker produces both hyperchromic and a red-shift on the
absorption spectra. However, when we compare the optical
behavior of 1c versus 3c, although the intensity of both
transition bands increases, a blue-shift is observed (Dl» 13 nm),
(Tables 3, entries 2, 7 and Fig. 4). This blue shift on the
absorption spectrum of 3c, can be attributed to two factors, a)
the strong electron acceptor character of nitro group that
N
R'
DMSO
MeOH
λmax = 365 nm
5c,(13c) R = NO2, R'= H λmax = 407 nm
6b,(13c) R = H, R'= Me λmax = 369 nm
5b,(13c) R = H, R'= H
λmax = 372 nm
λmax = 422 nm
λmax = 380 nm
overtake the
-CF3: sp =0.53)28 and, b) the torsion angle in biphenyl moiety
that limits the appropriate overlapping of the -backbone as
revealed the X-ray diffraction analysis of 4d (Fig. SI-3). Given the
absorption spectra of 3d, it seems that the -CF3 behaves as best
acceptor group in these push-pull biphenyl-azo-(2-pyrrole)
compounds, being the inductive effect the main responsible of
its behavior. When we analyze the optical properties of series
2(b-c) versus 4(b-d), we observe a similar tendency (Fig. 5).
p-donor characteristics of pyrrole (-NO2: sp =0.81;
Fig 5. Examples of related push-pull heteroaryl azo-compounds.
p
similar behavior as its counterpart 2b-c, but the presence of N-
methyl group in 6b gives a best match with the 2b denoting the
best donor character of the N-methylpyrrole fragment.
Optical band gap. UV-Vis electronic absorption spectroscopy is
a well-known technique to evaluate the optical absorption band
gap of compounds.30 The band gap in molecular compounds
could be estimated as the energy difference between the
HOMO and LUMO orbitals, and can be determined by
performing an extrapolation on the absorption spectrum,
finding the wavelength at which the material begins to absorb.31
The optical band gaps for all aryl and biaryl- azopyrroles were
estimated from the first optical absorption edge. Eg is the band
gap corresponding to a particular absorption of low photon
energy (hν) and express the radiation energy in eV using the
following equation (1). Where h is Planck constant and c is the
speed of light in vacuum. The optical absorption spectra of all
samples were acquired from CHCl3 solutions, so that they can
be directly compared to the results obtained from
computational studies.
Comparing the electronic absorption spectra of these azo-
pyrrole dyes with that of disperse orange 3, a well-known push-
pull azo dye, this azo-compound exhibits
a similar p-
p
*transition at 413 nm (
e
= 23203 M-1 cm-1) in CHCl3 (Fig. S65).
In general, push-pull dyes tend to respond to solvent polarity
changes, we thus decided to study this behavior and the
absorption spectra of all compounds were acquired using six
solvents with different polarity characteristics according to the
empirical Reichardt parameter ET(30).29 Absorption spectra
data are summarized in table 3. As expected, azo-compounds
containing a phenyl or a biphenyl group (1b, 2b, 3b and 4b)
display a similar absorption spectrum when different solvents
are used. In some cases, the intensity of the transition
absorption was affected by non-polar solvent like benzene,
Eg (eV)= hν = hc/λ………… (1)
which can be attributed to
p-stacking solvent-analyte
interactions. Compounds 1c, 2c, 3c and 4c behave as push-pull
30
systems, and a clear positive solvatochromic effect is observed
25
2c
(Figure 5). Thus, a red shift of the
when we compare their spectra acquired in polar solvents such
as THF ( = 7.6), MeOH ( =32.6) and DMSO ( = 46.8). This
behavior can be attributed to a considerable charge-transfer
(CT) character favored by these polar solvents stabilizing both
p-p* transition is evident
20
15
10
e
e
e
5
508 nm
the
p-p* and n-p* excited state. A similar behavior was
0
250
300
350
400
450
500
550
600
observed in other related push-pull azo-compounds systems
such as 5b-c (Figure 5).13c In this case, the presence of an indole
Wavelength (nm)
Fig 6. Absorption spectrum in CHCl3 and optical onset for 2c, (Optical Band gap: 2.44 eV).
fragment as p-extended heterocyclic donor group leads a
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Fig. 6 shows the absorption spectrum of 2c where the
corresponding wavelength to the band gap energy can be provoking important differences in terms of dipole moments. In
calculated from the cross point of absorption onset and general, largest dipole moments are found for the E isomers,
corrected base line. Table S6 summarizes the optical band gaps being more evident for those containing -NO2 as strongly
estimated for all azopyrrole compounds synthesized. In all electron withdrawing group (1c, 2c, 3c and 4c), revealing the
cases, azopyrroles containing CF3 or NO2 group display the push-pull nature of these azo-pyrrole compounds.
smaller value of band gap, demonstrating their push-pull Likewise, we observe that the electronic polarization when the
Journal Name
Z
isomers display different conformational arrangement
View Article Online
DOI: 10.1039/C9OB02410G
character.
pyrrole ring is 3-substituted by the azo-group is more effective,
as result of the “partial” heteroarene conjugation into the
Computational studies. To gain more insight about the azophenyl moiety.9c Conversely, in the case of compounds 1b,
intramolecular charge transfer properties due to the electronic 2b, 3b and 4b, we observe that their Z isomers display the
excitation, we analyzed the HOMO-LUMO gap energies, the slightly larger dipole moments, which reveals the electron
molecular dipole moments, the electronic population and donor properties of pyrrole ring relative to azo group (pyrrole:
energy changes of donor and acceptor groups as evidences of σp= +0.39; PhN=N- σp= +0.34).28
the push-pull nature of these aryl and biphenylazo-pyrroles
(Figure 7 and table S6). All the possible conformers of E and Z Tables S7 summarize the energetic and electronic features of
isomeric forms were fully optimized, where the E isomer is the excited state associated with the λmax of each molecule, in
always the most stable and their structures agree well with both configuration (E and Z) and the donor and acceptor groups
those determined by X-ray diffraction analyses for 1c, 2b and within them. In general, it is possible to observe that E-isomers
4d. The HOMO and LUMO of E isomers are clearly of π-nature, present λmax at lower excited states, with larger oscillator
similar to those obtained at the PBE0/6-31G** level of theory strength and fewer orbital transitions involved, but in these
using CPCM-Grimme.32 Likewise, all Z isomers exhibit a isomers the intramolecular transfer electron is smaller. The
perpendicular T-shaped conformation, similar to those electron transfer from the donor the acceptor groups within the
reported by M. J. Fuchter et al.9c for compound 1b. From the molecules provokes, in general, a destabilization of these two
molecular orbital isosurfaces of these compounds (Figure 7), we groups that account to the molecular excited state energies.
observe that the HOMO is mainly localized on pyrrole ring, Also, in general, the HOMO→LUMO transition is the main
whereas the LUMO is observed around the 4-susbstituted component of the excitation, except for the Z-2c case, where
phenyl-EWG moiety. In most of cases, the HOMO and LUMO of the HOMO-1→LUMO is the more important (Table S7).
E and Z- isomers are of a p-nature. As expected, calculated E and
Fig. 7. Frontier molecular orbital in the minimum energy orientation for Z and E isomers of 1c, 2c, 3c and 4c calculated at the PBE-D/DNP level of theory, using CHCl3 as implicit
solvent (COSMO model). Calculated frontier orbital gaps, optical energies gaps (Data obtained from experimental UV-visible spectra acquired in CHCl3) and molecular dipole
moments.
6 | J. Name., 2012, 00, 1-3
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View Article Online
DOI: 10.1039/C9OB02410G
The excited state energies (ΔE) correlate linearly with the The solutions of E-3b, E-3c, E-4b and E-4c showed to be stable
electrons transferred from the donor to the acceptor group for at least ten irradiation-relaxation cycles and do not show any
(ΔN) following the equation 2. The constants values (m and b) appreciable photobleaching (Figure 8 and S66-S68 in Supporting
are shown in the supporting information (Fig. S-81-82). In the Information). From the PSS spectra and the deconvolution of
four families, the nitro group produces larger electron transfers the cis/trans contributions, we identified the absorption
and lower excitation energies, whereas the H and CF3 groups maxima for each Z-isomer around 340 nm, which was assigned
gives similar effects in the families 1 and 3 but more scattered to a p-p* transition. We also observe that the 3b-c present a
in the other families. The azopyrrole dyes with NO2 and H slower isomerization in comparison with the series of biphenyl-
substituents present larger destabilization of donor groups than azopyrroles 4b-c as reveal their kinetic data (Table 4). The most
of acceptor ones (from 3 to 6 times), whereas the CF3 generates important differences in both molar absorption coefficients
the opposite behavior where the destabilization of the acceptor between the E and Z isomers are detected in the compounds 3c
group is 8 to 11 times than that of the donor group. There are and 4c. A similar tendency was observed with its
p-conjugated
four special cases where the acceptor groups stabilize but the counterpart St-3c,19 revealing the push-pull nature of these
other group has larger destabilizations.
biphenyl-azopyrrole compounds. Following the methodology of
M. J. Fuchter et al,9c Z/E ratios were obtained from the UV-Vis
measurements and are included in table 4. It can be noted that
the amount of Z- isomer in the PSS is higher in 3b-c than in 4b-
the c. These results also indicate that when a NO2 group is included
ΔE = -m ΔN + b
(2)
Photoisomerization.
Likewise,
we
studied
photoisomerization of compounds 3b-c and 4c-d, upon in the structure, the thermal stability of the Z- isomer slightly
irradiation into the first absorption band. Thus, a 2 x 10-5
decreases in comparison with the results obtained for St-3c
M
solution of these compounds in methanol was irradiated with a (Table 4, Figure 5).19 This behavior can be attributed to the poor
5W white led for 1 minute (time required to stablish a electronic communication through the biphenyl linker included
photostationary state, PSS). The UV-Vis spectrum of E-3c and in these series of azopyrrole compounds. On the other hand,
the respective PSS after irradiation, as well as several when we compared the thermal behavior with their analogues
intermediate spectra are showed in Figure 8. The spectra 5b-c and 6b, a drastic change is observed in terms of their
obtained for compounds 3a and 4b-c are included in the SI thermal cis-trans return giving t1/2 in the scale of miliseconds to
(Figures S66-S68).
microseconds.13
Aggregation Studies. Since push-pull azo dyes are highly
versatile units for optics and photonics materials, their
aggregation behavior can modify their optical properties. Thus,
azo dyes with attached aryl groups, are one example of a dye
system whose organization can be characterized using the
position of the p-p
*absorption band of its chromophore.33 As
the optical properties of dye aggregates vary with the relative
orientation of the monomers and the size and structure of the
molecular aggregate, precise control over these parameters is
highly desirable. J- and H-aggregates34 are the terms coined for
one-dimensional arrangements of strongly coupled monomers
that exhibit absorption spectra distinct from those of their
monomeric forms. All azopyrroles synthesized showed limited
solubility in water, but in partially aqueous medium they can be
forced to an “arrested crystallization” process, which can be
followed by UV/visible absorption spectroscopy.35 To study
their possible aggregation, we used MeOH:H2O as binary
solvent at the same sample concentration; in the second
experiment we modified the molarity at the same MeOH:H2O
ratio. Results obtained for 3b are shown in figure 9. 3b exhibits
a main absorption band at
lmax= 398 nm, but when the
proportion of water increases, the absorption band adopts an
asymmetrical shape with a relative inflexion around 435 nm,
suggesting the formation of J- aggregates, which are
characterized by an arrangement of monomeric molecules in
one dimension to achieve a parallel orientation of their
transition moments with a zero angle between the transition
moments and the line joining the molecular centers.34
Fig. 8. (a) UV-Vis spectra for 3c (3 x 10-5 M, MeOH) after irradiation (PSS) and cis-
trans return. Spectra were acquired each 5 minutes. (b) 3c was irradiated ten
times for 1 minute (5.0 mW).
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Journal Name
View Article Online
DOI: 10.1039/C9OB02410G
Table 4. Half-life (t1/2, min) rate constant (k, s-1) cis ® trans thermal isomerization, cis/trans percents in photoisomerization state.a
PPS (%)
Z isomer
e (M-1 cm-1)
1.0 x 104
1.1 x 104
1.9 x 104
2.1 x 104
3.0 x 104
4.1 x 104
2.8 x 104
4.1 x 104
Compound
3b
t1/2, (min)
24.2
2.8
k (s-1)
Z
E
l
max (nm)
340
342
342
345
343
341
359
355
4.7 x 10-4
4.13 x 10-3
6.8 x 10-4
2.5 x 10-3
6.1 x 10-3
9.4 x 10-3
1.3 x 10-3
9.7 x 10-4
75
67
75
61
44
40
72
83
25
33
25
39
56
60
28
17
3bb
3c
3cb
16.9
4.7
4b
1.9
4c
1.2
St-3b19
St-3c19
8.6
12.1
a UV-Vis spectra acquired in MeOH at 2 x 10-5 M, E® Z PSS irradiation conducted with a white led. b UV-Vis spectra acquired in MeOH:H2O (80:20) at 2 x 10-5 M, E® Z PSS
irradiation conducted with a white led.
Fig 9. (a) Absorption spectra of 3b at 7 x 10-5 M, using MeOH: H2O in different ratios. (b) Absorption spectra of 3b at different concentrations using MeOH/H2O (80:20).
(c) UV-Vis spectra for 3b (5 x 10-5 M, MeOH/H2O 80:20) after irradiation (PSS) and cis-trans return. Spectra were acquired each 5 minutes.
For aromatic dyes, this arrangement would yield a side-by-side presence of water in the medium accelerates the cis-trans
positioning of the aromatic rings. Intermolecular interactions return, also decreasing the t1/2. This behavior can be attributed
involving the substituents on the aromatic rings act to stabilize to the presence of the J aggregate formed under these
J-aggregates which results in a red shifted (often narrower36) conditions, where the possible side-by-side intermolecular
absorption spectrum relative to that of the monomer. A similar interactions modify the associated energy of both individual
behavior is observed across this series of compounds. For 4d trans and cis isomers. We also observe, that when a mixture
(Figure SI-64), we observe that when the ratio of water MeOH:H2O (60:40) was used, in both cases, the relaxation rate
increases, a hyperchromic effect occurs, and the absorption was much faster and happened within a few seconds therefore
band also adopts an asymmetrical shape with a small inflexion plots of thermal relaxation could not be obtained. Likewise, we
around 420 nm, suggesting the presence of J- aggregates, but in observe that both 3b and 3c maintain the organization of the J
this case the change is less evident. Correlating this behavior aggregate during the thermal relaxation cis-to-trans (Figure 9
with that observed in the solid state, the
between monomers in solution must be less effective.
p
-interactions and S69). The faster thermal relaxation observed in these
conditions reveals that the photoisomerization process can be
As mentioned before, some related heteroaryl azo compounds water-assisted, and this strategy should extend the scope of
have been used as photopharmacological agents,9f where the light-responsive systems in material and life sciences. A similar
physiological medium is mainly composed by water. In such behavior has been observed in other push-pull heteroaryl azo
conditions, aggregation behavior can be determining to control dyes which exhibit reversible switching in a subnanosecond
their photoisomerization process. Taking this in mind, we time scale.12-13
explored the photoisomerization of compounds 3b and 3c,
using MeOH:H2O (80:20 and 60:40) as binary solvent. In both
Experimental section
Materials and methods
cases, we observe the detailed thermal relaxation that occurs
after illumination of compounds 3b and 3c with a white led for
1 minute. For instance, the spectra of 3b clearly exhibit
isosbestic points and the cis isomer relaxes back completely to
the trans isomer (Figure 9). The kinetic data revealed that the
All reagents and solvents were obtained from commercial
suppliers and used without further purification. Column
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chromatography was performed using 70–230 mesh silica gel. temperature. A solution of 10% NaOH was added until pH=10
View Article Online
DOI: 10.1039/C9OB02410G
The palladacycle used as palladium precursor for the Suzuki- and then, the mixture was extracted with CH2Cl2 and dried with
Miyaura cross-coupling reactions was synthesized using the anhydrous Na2SO4. The crude product was purified by column
methodology previously described elsewhere.21 Microwave chromatography on silica gel with hexane.
irradiation experiments were performed using a Monowave Method B: In a 100 mL round-flask, 4-Iodoaniline (2.51 g, 11.2
300 single-mode microwave reactor. The reaction temperature mmol) was dissolved in 5 mL of MeOH and mixed at 0 °C with
is monitored by an internal fiber-optic (FO) temperature probe HCl [1M] (10 mL, 0.01 mol); then, an aqueous solution of sodium
(ruby thermometer) protected by a borosilicate immersion well nitrite (0.93 g, 13.4 mmol in the minimum quantity of water)
inserted directly into the reaction mixture. Reaction times refer was added dropwise. After 30 minutes stirring at 0 °C a mixture
to the hold time at the desired set temperature and not to total of N-methylpyrrole (1 mL, 11.2 mmol), pyridine (2.7 mL, 33.6
irradiation time. A hydraulic sensor integrated in the swiveling mmol) and 5 mL of methanol were added and stirred for 30
cover of the instrument performs pressure sensing. The minutes at 0 °C and after 30 minutes more at room
reusable 10 mL Pyrex vial is sealed with PEEK snap caps and temperature. A saturated solution of Na2CO3 was added until
standard PTFE coated silicone septa. Reaction cooling is pH=10 and then, the mixture was extracted with CH2Cl2 and
performed by compressed air automatically after the heating dried with anhydrous Na2SO4. The crude product was purified
period has elapsed. Melting points were obtained on a Stuart by column chromatography on silica gel with hexane.
Melting Point Apparatus SMP10 and they are uncorrected.
All compounds were characterized by IR spectra, recorded on a 2-((4’-iodophenyl)diazenyl)-N-methyl-pyrrole (1a). Orange
Perkin-Elmer Spectrum 100 FT-IR spectrophotometer provided solid, 80%, mp 97 °C. IRnmax (ATR, cm-1): 2923, 2851, 2693, 2551,
with an ATR polarization attachment and all data are expressed 2390 (C-H), 1343 (C=C). 1H NMR (300 MHz, CDCl3, ppm):
d
3.93
[s, 3H, H1], 6.29 [m, 1H, H3], 6.72 [m, 1H, H4], 6.94 [m, 1H, H5],
7.65 [dd, 4H, J= 8.7Hz, H7 and H8]. 13C NMR (75 MHz, CDCl3,
NMR spectra were measured on a Bruker Avance 300 ppm): 33.5 [C1], 95.1 [C9], 100.7 [C4], 110.6 [C3], 123.7 [C7],
in wave numbers (cm-1).
d
Spectrometer, at an operating frequency of 300 MHz for 1H and 127.6 [C5], 138.1 [C8], 146.4 [C2], 153.0 [C6]. MS-EI m/z: 311[M+],
75 MHz for 13C, using tetramethylsilane (TMS) as internal 232 [M+- CH3NC4H3], 203 [M+- CH3NC4H3N2], 108 [M+- C6H4I], 93
reference and CDCl3 as solvent; chemical shifts values are given [M+- NC6H4I], 80 [M+- N2C6H4I]. HR-MS (FAB+) m/z for C11H10N3I:
in parts per million (ppm), relative to TMS. Mass spectrometry calculated 310.9919, found 310.9923.
analyses were obtained on a JEOL SX102A (EI+ and FAB+) and on 2-(phenyldiazenyl)-N-methylpyrrole (1b).9c 87%, Yellow liquid
AccuTOF JMS-T100LC (DART), the values of the signals are at RT. IRn
expressed in mass/charge units (m/z). UV-Vis absorption 1497 (C=C). H NMR (300 MHz, CDCl3, ppm):
(ATR, cm-1): 3104, 3061, 3032, 2945, 2708 (C-H),
max
1
d 3.95 [s, 3H, H1],
spectra were recorded at 298 K on a Varian Cary 100 UV-Vis 6.29 [dd, 1H, J= 3Hz, H3], 6.71 [d, 1H, J= 3Hz, H4], 6.92 [s, 1H, H5],
spectrophotometers, using spectrophotometric grade solvents 7.35 [t, 1H, H9], 7.45 [t, 1H, H8], 7.80 [d, 1H, H7]. 13C NMR (75
purchased from Sigma-Aldrich Co. and 1 cm quartz cell. The MHz, CDCl3, ppm):
d
33.4 [C1], 100.0 [C4], 110.2 [C3], 122.0 [C7],
solvatochromic study was performed using 10-5 M to 10-4
M
127.0 [C9], 129.0 [C8], 129.3 [C5], 146.0 [C2], 154.0[C6]. MS-DART
solutions of azopyrrole dyes in several solvents at room m/z: 186 [M + 1]+.
temperature. The aggregation study was carried out using 5x10- N-methyl-2-((4’-nitrophenyl)diazenyl)-pyrrole (1c).37 Red solid,
5 M solutions of azopyrrole dyes at different MeOH: H2O ratios, 62%, mp 142 °C. IRn (ATR, cm-1): 3280, 3116, 2920, 2851 (C-
max
1
from 100:0 to 20:80 at room temperature. ASTM type 1 ultra- H), 1602, 1583 (C=C), 1508 (NO2). H NMR (300 MHz, CDCl3,
pure water (Millipore-Q system, 18.2 M
solvent for the aggregation studies.
W
cm) was used as ppm):
J= 1.5Hz, H4], 7.08 [t, 1H, H5], 7.89 [d, 2H, J= 9Hz, H7), 8.31[d, 2H,
33.7 [C1], 102.6
d 3.99 [s, 3H, H1], 6.36 [dd, 1H, J= 2.4Hz, H3], 6.83 [dd, 1H,
Isomerization Kinetic measurements. UV-Vis absorption J= 9Hz, H8]. 13C NMR (75 MHz, CDCl3, ppm):
d
spectra were recorded in a Cary 100 spectrometer. The solvent [C4], 111.6 [C3], 122.4 [C7], 124.8 [C8], 129.7 [C5], 147.0 [C9],
used for all experiments was methanol HPLC grade (Sigma 147.3 [C2], 157.4 [C6]. MS-DART m/z: 231 [M + 1]+.
Aldrich). Photoisomerization was induced by irradiation using a N-methyl-2-((4’-trifluorophenyl)diazenyl)-pyrrole (1d). Orange
5W white led. Solutions of studied compounds were irradiated solid, 68%, mp 57 °C. IRnmax (ATR, cm-1): 3144, 3126, 2920, 2626
1
under continuous stirring in a quartz cell with 1 cm of optical (C-H), 1608 (C=C), 1094 (C-F). MS-DART m/z: 254 [M + 1]+. H
pathway.
Synthetic procedures
NMR (300 MHz, CDCl3, ppm):
J=2.7Hz, H3], 6.77 [dd, 1H, J= 1.5Hz, H4], 7.00 [s, 1H, H5], 7.78
[dd, 4H, J= 8.4Hz, H7 and H8]. 13C NMR (75 MHz, CDCl3, ppm):
33.6 [C1], 101.3 [C4], 110.9 [C3], 122.1 [C7], 124.2 [q, JC-F= 270Hz,
d 3.97 [s, 3H, H1], 6.33 [dd, 1H,
d
General procedure for synthesis of compounds 1a-d and 2a-c.
Method A: In a 100 mL round-flask, 4-Iodoaniline (2.51 g, 11.2
mmol) and N-methylpyrrole (1 mL, 11.2 mmol), were dissolved
in 6 mL of MeOH, then HCl [1M] (10 mL, 0.01 mol) was added
and the mixture was cooled at mixed at 0 °C. Then, an aqueous
solution of sodium nitrite (0.93 g, 13.4 mmol in the minimum
quantity of water) was added dropwise. The mixture was stirred
10 minutes at 0 °C and the 30 minutes more at room
C10], 126.2 [q, JC-F= 3.75Hz, C8], 128.3 [C5], 130.4 [q, JC-F
=
32.25Hz, C9], 146.6 [C2], 155.7 [C6]. HR-MS (ESI+) m/z for
C12H11F3N3: calculated 254.09051, found 254.08993.
3-((4’-iodophenyl)diazenyl)-1,2,5-trimethyl-pyrrole
(2a).
Orange solid. 60%, mp 123 °C. IRn (ATR, cm-1): 3117, 3062,
max
3044, 2934, 2913 (C-H), 1727, 1664, 1581, 1531 (C=C). 1H NMR
(300 MHz, CDCl3, ppm):
d
2.22 [s, 3H, H7], 2.55 [s, 3H, H6], 3.44
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Journal Name
1
[s, 3H, H1], 6.27 [s, 1H, H4], 7.63 [dd, 4H, J= 9Hz, H9 and H10]. 13
NMR (75 MHz, CDCl3, ppm): 9.9 [C7], 12.5 [C6], 30.4 [C1], 93.7 CDCl3, ppm):
C
2923, 2851 (C-H), 1591 (C=C), 1506 (NO2). H NMR (300 MHz,
View Article Online
d
d
3.99 [s, 3H, H1], 6.33 [dd,D1OHI:,1J0=.120.379H/Cz9,OHB30],2461.07G6
[C11], 95.4 [C4], 123.6 [C9], 131.0 [C5], 137.9 [C10], 140.0 [C2], [dd, 1H, J= 1.8Hz, H4], 6.98-6.99 [m, 1H, H5], 7.81 [dd, 4H, J=
153.3 [C8]. MS-DART m/z: 340 [M + 1]+. HR-MS (ESI+) m/z for 8.7Hz, H7 and H8], 8.04 [dd, 4H, J= 8.7Hz, H11 and H12]. 13C NMR
C13H15I1N3: calculated 340.03106, found 340.03116.
(75 MHz, CDCl3, ppm): d 33.5 [C1], 100.7 [C4], 110.7 [C3], 122.8
3-(phenyldiazenyl)-1,2,5-trimethylpyrrole (2b).9c Orange solid [C7], 124.2 [C12], 127.7 [C11], 128.1 [C8], 128.4 [C2], 139.1 [C5],
95%, mp 95 °C. IRnmax (ATR, cm-1): 3070, 3033, 2979, 2936, 2912, 146.7 [C6], 146.8 [C9], 147.2 [C13], 153.9 [C10]. MS-EI m/z:
2851 (C-H), 1531 (C=C). 1H NMR (300 MHz, CDCl3, ppm):
d
2.26 306[M+], 227[M+- CH3NC4H3], 152 [M+- CH3NC4H3N2NO2], 108
[s, 3H, H7], 2.61 [s, 3H, H6], 3.48 [s, 3H, H1], 6.31 [s, 1H H4], 7.31- [M+- (C6H4)2NO2], 80 [M+- N2(C6H4)2NO2]. HR-MS (FAB+) m/z for
7.35 [m, 1H, H11], 7.43-7.82 [m, 4H, H9 and H10]. 13C NMR (75 C17H14O2N4: calculated 306.1117, found: 306.1115.
MHz, CDCl3, ppm):
d 9.9 [C7], 12.5 [C6], 30.3 [C1], 95.2 [C4], 121.7 N-methyl-2-((4’-(4’’-trifluoro)-biphenyl)diazenyl)-pyrrole (3d).
[C9], 128.3 [C11], 128.8 [C10], 130.5 [C5], 135.9 [C2], 140.0 [C3], Orange solid, 96%, mp 123 °C. IRn
154.0 [C8]. MS-DART m/z: 214 [M + 1]+. HR-MS (ESI+) m/z for 2926, 2858 (C-H), 1727, 1612, 1493 (C=C), 1110 (C-F). H NMR
(KBr, cm-1): 3116, 2956,
max
1
C13H16N3: calculated 214.13442, found 214.13388.
(300 MHz, CDCl3, ppm): d 398[s, 3H, H1], 6.32 [s, 1H, H3], 6.75 [d,
3-((4’-nitrophenyl)diazenyl)-1,2,5-trimethylpyrrole (2c). Red 1H, 6.75Hz, H4], 6.96 [s, 1H, H5], 7.67-7.91 [m, 8H, H7, H8, H11
solid. 16%, mp 163 °C (dec). IRnmax (ATR, cm-1): 3098, 2936, 2916, and H12]. 13C NMR (75 MHz, CDCl3, ppm):
2850 (C-H), 1601, 1585 (C=C), 1507 (NO2). H NMR (300 MHz, 110.5 [C3], 122.7 [C7], 124.3 [q, JC-F= 270Hz, C14], 125.8 [q, JC-F
CDCl3, ppm): d 2.24 [s, 3H, H7], 2.59 [s, 3H, H6], 3.48 [s, 3H, H1], 3.75Hz, C12], 127.3 [C5], 127.4 [C11], 127.9 [C8], 129.5 [q, JC-F
6.28 [s, 1H, H4], 8.05 [dd, 4H, J= 9Hz, H9 and H10]. 13C NMR (75 32.25Hz, C13], 140.3 [C2], 144.0 [C6], 146.7 [C9], 153.4 [C10]. MS-
d
33.5 [C1], 100.4 [C4],
1
=
=
MHz, CDCl3, ppm):
d
10.0 [C7], 12.4 [C6], 30.5 [C1], 95.6 [C4], EI m/z: 329 [M+], 250 [M+- CH3NC4H3], 236 [M+- CH3NC4H3N],
122.1 [C9], 124.6 [C10], 131.9 [C8], 139.2 [C5], 141.1 [C2], 146.7 221 [M+- CH3NC4H3N2], 108 [M+- (C6H4)2CF3], 80 [M+-
[C3], 157.9 [C11]. MS-DART m/z: 259 [M + 1]+. HR-MS (ESI+) m/z N2(C6H4)2CF3]. HR-MS (ESI+) m/z for C18H15F3N3: calculated
for C13H15O2N4: calculated 259.11950, found 259.11862.
330.12181, found 330.12167.
2-((4’-(4’’-methansulfonyl)-biphenyl)diazenyl)-N-methyl-
pyrrole (3e). Yellow solid, 96%, mp 153 °C. IRn (ATR, cm-1):
General procedure for Synthesis of compounds 3a-g and 4b-c.
max
3123, 3062, 3025, 3005, 2956, 2923, 2851 (C-H), 1727, 1592,
1498 (C=C), 1325, 1299, 1148 (RSO2R’). 1H NMR (300 MHz,
CDCl3, ppm): d 3.10 [s, 3H, H14], 3.99 [s, 3H, H1], 6.33 [s, 1H, H3],
Representative procedure for the synthesis of 3b: In a 10 mL
microwave vial, were mixed 1a (311 mg,
1 mmol),
phenylboronic acid (139 mg, 1.2 mmol), K2CO3 (167.5 mg, 1.2
mmol), 0.1 % mol of palladium complex (0.68 mg) and methanol
as solvent (4mL). The mixture was irradiated in a microwave
reactor during 11 minutes at 90 °C. After the complete
consumption of 1a, judged by TLC (hexane: CH2Cl2, 9:1), the
crude was extracted with CH2Cl2 and dried with anhydrous
Na2SO4. The crude product was purified by flash column
chromatography on silica gel (hexane:CH2Cl2, 9:1).
Note: The entire vials used in each coupling reaction were
meticulously cleaned with aqua regia to avoid the presence of
unseen palladium catalyst. Attention: Aqua regia solution must
be prepared inside a fume hood with the sash down as much as
is practical to contain the vapors and protect against injury in
case of splashing or glassware breakage, following the safety
protocols for handling strong acids.
6.76 [d, 1H, J= 3.3Hz, H4], 6.99 [s, 1H, H5], 7.82 [dd, 4H, J= 8.1Hz,
H7 and H8], 7.93 [dd, 4H, J= 8.1Hz, H11 and H12]. 13C NMR (75
MHz, CDCl3, ppm):
d 33.5 [C1], 44.7 [C14], 100.6 [C4], 110.6 [C3],
122.7 [C7], 127.6 [C2], 127.9 [C12], 128.0 [C11], 128.0 [C8], 139.3
[C5], 139.5 [C6], 146.0 [C9], 153.4 [C13], 153.7 [C10]. MS-EI m/z:
339 [M+], 260 [M+- CH3NC4H3], 231[M+- CH3NC4H3N2], 142 [M+-
CH3NC4H3N2C6H4], 108 [M+- (C6H4)2SO2CH3], 80 [M+- N2(C6H4)2
SO2CH3]. HR-MS (ESI+) m/z for C18H18N3O2S1: calculated
340.11197, found 340.11139.
2-((4’-(4’’-acetyl)-biphenyl)diazenyl)-N-methyl-pyrrole
(3f).
Orange solid, 89%, mp 149 °C. IRn (ATR, cm-1): 3334, 3121,
max
3106, 2995, 2919, 2850 (C-H), 1672 (C=O), 1599 (C=C). 1H NMR
(300 MHz, CDCl3, ppm):
[dd, 1H, H3], 6.75 [dd, 1H, H4], 6.98 [t, 1H, H5], 7.71-8.06 [m, 8H,
H7, H8, H11 and H12]. 13C NMR (75 MHz, CDCl3, ppm):
26.7 [C15],
d 2.65 [s, 3H, H15], 3.99 [s, 3H, H1], 6.23
d
2-((4’-biphenyl)diazenyl)-N-methylpyrrole (3b). Orange solid,
33.5 [C1], 100.4 [C4], 110.5 [C3], 122.6 [C8], 127.1 [C12], 127.4
[C5], 127.9 [C11], 129.0 [C7], 136.0 [C2], 140.4 [C6], 145.0 [C9],
146.0 [C13], 153.4 [C10], 197.7 [C14]. MS-EI m/z: 303 [M+]. HR-MS
(ESI+) m/z for C19H18N3O1: calculated 304.14499, found
304.14509.
98%, mp 92 °C. IRnmax (ATR, cm-1): 3054, 3031, 2951 (C-H), 1496
1
(C=C). H NMR (300 MHz, CDCl3, ppm):
d 3.97 [s, 3H, H1], 6.30
[dd, 1H, J=2.6Hz, H3], 6.73 [dd, 1H, J= 1.5Hz, H4], 6.93-6.95 [m,
1H, H5], 7.36-7.38 [m 1H, H13], 7.43-7.48 [m, 2H, H12], 7.62-7.68
[m, 4H, H8 and H11], 7.87-7.90 [m, 2H, H7]. 13C NMR (75 MHz,
2-((4’-(4’’-dimethylamino)-biphenyl)diazenyl)-N-methyl-
pyrrole (3g). C19H20N4. 98%, mp 169 °C. IRnmax (ATR, cm-1): 3121,
2919, 2850, 2803 (C-H), 1734, 1665, 1604, 1588 (C=C), 1349
(ArNR2). MS-EI m/z: 304 [M+], 225 [M+- CH3NC4H3], 211 [M+-
CDCl3, ppm): d 33.5 [C1], 100.0 [C4], 110.3 [C3], 122.5 [C5], 127.1
[C8 and C11], 127.7 [C13], 128.9 [C7 and C12], 140.5 [C10], 142.0
[C9], 146.7 [C5], 152.8 [C6]. MS-EI m/z: 261 [M+], 181 [M+-
CH3NC4H3], 152 [M+- CH3NC4H3N2], 108 [M+- (C6H4)2], 80 [M+-
N2(C6H4)2]. HR-MS (ESI+) m/z for C17H16N3: calculated 262.13442,
found 262.13408.
CH3NC4H3N],
CH3NC4H3N2N(CH3)2],108 [M+- (C6H4)2 N(CH3)2], 80 [M+-
N2(C6H4)2 N(CH3)2]. 1H NMR (300 MHz, CDCl3, ppm):
2.98 [s, 6H,
196
[M+-
CH3NC4H3N2],
152
[M+-
d
N-methyl-2-((4’-(4’’-nitro)-biphenyl)diazenyl)-pyrrole
(3c).
H14], 3.94 [s, 3H, H1], 6.28 [s, 1H, H3], 6.70 [s, 1H, H4], 6.90 [s, 1H,
H5], 7.17 [dd, 4H, J= 7.8Hz, H7 and H8], 7.74 [dd, 4H, J= 7.5Hz H11
Orange solid, 94%, mp 136 °C. IRn
(KBr, cm-1): 3112, 3076,
max
10 | J. Name., 2012, 00, 1-3
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Page 11 of 13
Organic & Biomolecular Chemistry
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Journal Name
and H12]. 13C NMR (75 MHz, CDCl3, ppm):
99.6 [C4], 110.2 [C3], 112.8 [C12], 122.6 [C7], 126.5 [C11], 127.7 Data Centre (http://www.ccdc.cam.ac.uk).
[C8], 128.1 [C5], 142.2 [C10], 146.7 [C2], 150.2 [C9], 152.0 [C13].
HR-MS (ESI+) m/z for C19H21N4: calculated 305.17662, found
ARTICLE
d
33.4 [C1], 40.6 [C14], be obtained free of charge from the Cambridge Crystallographic
View Article Online
DOI: 10.1039/C9OB02410G
Computational details
305.17759.
The electronic structure of the push-pull azo-pyrrole
compounds at ground state were obtained at DFT level with the
DMol3 numerical-based density-functional module
3-(4’-biphenyldiazenyl)-1,2,5-trimethylpyrrole (4b). Orange
solid, 95%, mp 178 °C. IRnmax (ATR, cm-1): 3061, 3032, 2918, 2851
(C-H), 1729 (C=C). 1H NMR (300 MHz, CDCl3, ppm):
d 2.16 [s, 3H,
implemented in the Materials Studio 6.0 software package from
Accelrys Inc., by employing PBE (Perdew-Burke-Ernzerhof
correlation)40 functional within general gradient approximation
(GGA), in conjunction with Grimme's dispersion correction and
the double numerical basis set DNP, which has a polarization d-
function added on all non-hydrogen atoms and a polarization p-
function on all H atoms. The calculations involve the geometry
optimization of trans and cis isomers in vacuum and in
chloroform as implicit solvent (COSMO model), which yields the
energies of frontier (HOMO and LUMO) orbitals, HOMO-LUMO
gap energies, as well as the dipole moment. PBE is a hybrid DFT
approach, widely accepted for the determination of geometry
of molecules and calculations of their electronic properties with
a small positive bias.41 The use of the double numerical basis set
DNP,42 which is equivalent to the 6-31G** Gaussian-type basis
set, provides a good accuracy, further enhanced by applying the
dispersion correction by Grimme.32 In all calculations, the
quality (convergence criteria) was set to 'fine'.
H7], 2.51 [s, 3H, H6], 3.36 [s, 3H, H1], 6.25 [s, 1H, H4], 7.25-7.30
[m, 1H, H15], 7.35-7.81 [m, 8H, H9, H10, H13 and H14]. 13C NMR (75
MHz, CDCl3, ppm):
d 9.8 [C7], 12.4 [C6], 30.2 [C1], 95.3 [C4],
122.11 [C13], 127.0 [C10], 127.3 [C15], 127.5 [C14], 128.7 [C9],
130.6 [C5], 136.0 [C2], 140.1 [C3], 140.7 [C11], 140.8 [C12], 153.1
[C8]. MS-DART m/z: 290 [M + 1]+. HR-MS (ESI+) m/z for C19H20N3:
calculated 290.16572, found 290.16520.
3-(4’-(4’’-nitro)-biphenyldiazenyl)-1,2,5-trimethylpyrrole (4c).
Orange solid. 64%, mp 197 °C (dec). IRn
(ATR, cm-1): 3060,
max
1
2920, 2851 (C-H), 1591(C=C), 1509 (NO2). H NMR (300 MHz,
CDCl3, ppm): 2.28 [s, 3H, H7], 2.63 [s, 3H, H6], 3.51 [s, 3H, H1],
6.33 [s, 1H, H4], 7.82 [dd, 4H, J= 9Hz, H9 and H10], 8.07 [dd, 4H,
J= 9Hz, H13 and H14]. 13C NMR (75 MHz, CDCl3, ppm):
10.0 [C7],
d
d
12.5 [C6], 30.4 [C1], 95.4 [C4], 122.5 [C9], 124.2 [C14], 127.6 [C13],
127.9 [C10], 131.0 [C5], 136.9 [C2], 138.1 [C3], 140.4 [C6], 147.0
[C11], 147.2 [C15], 154.3 [C12]. MS-DART m/z: 335 [M + 1]+. HR-
MS (ESI+) m/z for C19H19N4O2: calculated 335.15080, found
335.15009.
The calculation of the excited states of the push-pull azo-pyrrole
compounds was performed with the Cam-B3LYP functional43
and the 6-311++G(d,p) basis within the time-dependent DFT
formalism as implemented in Gaussian 09.44 The Cam-B3LYP
functional has demonstrated to properly reproduce excited
state properties such as absorption energies.45 50 states were
obtained with chloroform as implicit solvent modeled with PCM
method. Atomic population and energies were calculated
within the atoms-in-molecules approach at basal and excited
states, to determine the electron charge transfer and atomic
energy changes after the electronic excitation.46 The
calculations involve the geometry optimization of trans and cis
isomers.
3-(4’-(4’’-trifluoro)-biphenyldiazenyl)-1,2,5-trimethylpyrrole
(4d). Orange solid, 70%, mp 175 °C. IRn
(ATR, cm-1): 3075,
max
3033, 2919, 2851 (C-H), 1733, 1662, 1612, 1534 (C=C), 1108 (C-
F). 1H NMR (300 MHz, CDCl3, ppm):
2.23 [s, 3H, H7], 2.58 [s, 3H,
H6], 3.45 [s, 3H, H1], 6.31 [s, 1H, H4], 7.64-7.88 [m, 8H, H9, H10,
H13 and H14]. 13C NMR (75 MHz, CDCl3, ppm):
9.8 [C7], 12.4 [C6],
d
d
30.2 [C1], 95.4 [C4], 122.3 [C9], 124.3 [q, JC-F= 270Hz, C16], 125.7
[m, C14], 127.2 [C13], 127.7 [C10], 129.2 [q, JC-F= 32.25Hz, C15],
130.8 [C5], 136.4 [C2], 139.2 [C8], 140.3 [C3], 144.3 [C11], 153.8
[C12]. MS-DART m/z: 358 [M + 1]+. HR-MS (ESI+) m/z for
C20H19F3N3: calculated 358.15311, found 358.15252.
Structure determination by X-ray crystallography
Suitable X-ray quality crystals of 1c,22 2a,23 2b24 and 4d26 were
grown by slow evaporation of chloroform at room temperature.
Crystals of each compound were mounted on a glass fiber at
room temperature, and then placed on a Bruker Smart Apex
CCD (for 1c), Bruker D8 Venture κ geometry diffractometer
Conclusions
In summary, a new family of push-pull biphenyl-azopyrrole dyes
was successfully obtained through a Suzuki-Miyaura cross-
coupling reaction in excellent yields. The use of
a
208039-1 (for 2a, 2b, 4d), both equipped with Mo-K
a radiation;
ferrocenylpalladacycle as palladium precursor in combination
with microwaves as heating source was crucial when azo-
pyrrole compounds are used as demanding substrates. The
inclusion of strong electron withdrawing groups such as NO2 in
combination with pyrrole ring to each side of N=N azo group
induces an important red-shift to the visible region. The UV-vis
decay was negligible in both cases. Details of crystallographic
data collected for these compounds are provided in SI.
Systematic absences and intensity statistics were used in space
group determination. The structure was solved using direct
methods.38 Anisotropic structure refinements were achieved
using full matrix, least-squares technique on all non-hydrogen
atoms. All hydrogen atoms were placed in idealized positions,
based on hybridization, with isotropic thermal parameters fixed
at 1.2 times the value of the attached atom. Structure solutions
and refinements were performed using SHELXTL V6.10.39 The
experimental and refinement details of the X-ray
crystallographic structure of compounds 1c, 2a, 2b and 4d can
studies also revealed that the p-linker and the position of azo
group on pyrrole ring impart a significant influence on the
optical response mainly modifying their molar absorption
coefficient.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
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OrPglaenaisce&dBoionmotolaedcjuulastr Cmhaermgiinstsry
Page 12 of 13
ARTICLE
Journal Name
Nature 2017, 546, 632. (c) J. Garcia-Amorós, S. Nonell, D.
Likewise, the results indicated that the p-p* band of these push-
Velasco, Chem. Commun. 2011, 47, 4022-4024. View Article Online
DOI: 10.1039/C9OB02410G
pull biphenyl-azopyrrole compounds, around 410 nm shows
bathochromic shift as the polarity of solvent was increased.
The theoretical studies showed that biphenyl-azopyrroles
including a push-pull system have large values of dipole
moment, relative low Band-gapopt energies and an effective
electronic transfer, even when the azopyrrole moiety is not
totally coplanar with biphenyls in ground state. Biphenyl-
azopyrrole compounds can be photoisomerized upon
irradiation into the first absorption band using a white led. This
photoisomerization process can be water-assisted, tuning their
photoresponse, which should extend the scope of these light-
responsive systems in material and life sciences. Further
researches in this direction will be addressed in our
laboratories.
8
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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22 Crystal data 1c: C11H10N4O2, M = 230.23, Triclinic, a = 7.345(5), 44 Gaussian 09, Revision D.01, M. J. Frisch, G. W. Trucks, H. B.
b = 7.435(6), c = 11.625(9) Å, V =555.4 Å3, T = 298 K, space
group P-1, Z = 2, μ(Mo–K
) = 0.099 mm-1, 11624 reflections
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a
measured, 2445 unique (Rint =0.0812) which were used in all
calculations. The final R and wR(F2) were 0.0951 and 0.2184
respectively (all data), CCDC 1950956.
23 Crystal data of 2a: C13H14IN3, M = 339.17, Monoclinic, a =
11.538(10), b = 9.5869(9), c = 12.6216(11) Å, V =1340.4 Å3, T
= 298 K, space group P21/n, Z = 4, μ(Mo–K a
) = 2.372 mm-1,
16497 reflections measured, 3070 unique (Rint =0.0239) which
were used in all calculations. The final R and wR(F2) were
0.0307 and 0.0700 respectively (all data), CCDC 1950957.
24 Crystal data of 2b: C13H15N3, M = 213.28, Monoclinic, a =
8.564(4), b = 22.313 (11), c = 25.094 (12) Å, V =47788(4) Å3, T
= 298 K, space group P21/n, Z = 16, μ(Mo–K a
) = 0.073 mm-1,
112168 reflections measured, 10535 unique (Rint =0.135)
which were used in all calculations. The final R and wR(F2)
were 0.069 and 0.1283 respectively (all data), CCDC 1950959. 45 U. Salzner J. Chem. Theory Comput. 2013, 9, 4064−4073.
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26 Crystal data of 4d: C20H18F3N3, M = 357.37, Monoclinic, a =
14.409(2), b = 7.2189(10), c = 17.976(3) Å, V =1778.9(5) Å3, T
= 298 K, space group P21/c, Z = 4, μ(Mo–K a
) = 0.102 mm-1,
26053 reflections measured, 3932 unique (Rint =0.0336) which
were used in all calculations. The final R and wR(F2) were
0.047 and 0.1398 respectively (all data), CCDC 1950958.
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