Photochemistry of Fluorescent Azobenzenes Substituted with Azulenylpyridine Moiety

Article abstract of DOI:10.1080/15421406.2014.967653

Fluorescent properties and the photoisomerization behaviour of 4-azulenylpyridine-substituted azobenzenes were investigated. All studied compounds have shown fluorescent emission in solution, originating from excited singlet state S2 → S0 of azulene moiety. The substitution of the azobenzene chromophore has affected the preservation of the fluorescence emission in thin films prepared in PMMA matrix. Correlation of absorption spectra with ¹H-NMR spectroscopy revealed that for all compounds the E → Z isomerization takes place upon exposure to UV light.

Full text of DOI:10.1080/15421406.2014.967653

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Photochemistry of Fluorescent
Azobenzenes Substituted with
Azulenylpyridine Moiety
a
a
b
Eugenia Andreea Dragu , Anamaria Hanganu , Ileana Rau , Francois
b
b
a
a
Kajzar , Alexandrina Tane , Alexandru C. Razus & Simona Nica
a
Institute of Organic Chemistry “C. D. Nenitzescu” of the Romanian
Academy, Bucharest, Romania
b
POLITEHNICA University of Bucharest, Faculty of Applied Chemistry
and Materials Science, Bucharest, Romania
Published online: 15 Dec 2014.
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To cite this article: Eugenia Andreea Dragu, Anamaria Hanganu, Ileana Rau, Francois Kajzar,
Alexandrina Tane, Alexandru C. Razus & Simona Nica (2014) Photochemistry of Fluorescent
Azobenzenes Substituted with Azulenylpyridine Moiety, Molecular Crystals and Liquid Crystals, 604:1,
41-51, DOI: 10.1080/15421406.2014.967653
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Mol. Cryst. Liq. Cryst., Vol. 604: pp. 41–51, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.967653
Photochemistry of Fluorescent Azobenzenes
Substituted with Azulenylpyridine Moiety
1
1
EUGENIA ANDREEA DRAGU, ANAMARIA HANGANU,
2
2
ILEANA RAU, FRANCOIS KAJZAR, ALEXANDRINA
TANE, ALEXANDRU C. RAZUS, AND SIMONA NICA
2
1
1,∗
1Institute of Organic Chemistry “C. D. Nenitzescu” of the Romanian Academy,
Bucharest, Romania
2
POLITEHNICA University of Bucharest, Faculty of Applied Chemistry and
Materials Science, Bucharest, Romania
Fluorescent properties and the photoisomerization behaviour of 4-azulenylpyridine-
substituted azobenzenes were investigated. All studied compounds have shown fluores-
cent emission in solution, originating from excited singlet state S
2
→ S
0
of azulene
moiety. The substitution of the azobenzene chromophore has affected the preservation
of the fluorescence emission in thin films prepared in PMMA matrix. Correlation of
1
absorption spectra with H–NMR spectroscopy revealed that for all compounds the
E → Z isomerization takes place upon exposure to UV light.
Keywords Azobenzene; azulenylpyridine; fluorescence; photoisomerization.
Introduction
Azo compounds are versatile molecules, and have received considerable attention in both
fundamental and applied research areas. One of the most intriguing properties of these
compounds is the reversible photoisomerization process between the E and Z isomers
of different stability [1–4]. This property has been exploited in numerous applications
such as switchable sensors [5–7], ion channels [8–10], catalysts [11–13], liquid crystals
[14–16], or enhanced by their successful incorporation in various polymeric matrixes,
both through covalent linkage and/or host/guest doping providing systems with direct
application in biochemistry and optical devices [17–20]. In general, azo compounds show
strong electronic absorption bands which can be shifted from UV to the visible region by
chemical modification, thus allowing for the fine-tuning of color. When the appropriate
electron donor/acceptor ring substitution was chosen, the azo chromophores yielded high
optical nonlinearity with application in the field of optoelectronics and photonics [21–25].
The photoisomerization of azobenzene has been used to drive functional changes in
biomolecules like for example the photo-control of coiled coil proteins in cell culture
and ion channels on extracellular surfaces in vivo. [26, 27] In order to function as tools
for controlling molecular events in complex living systems however, the photoswitching
∗
Address correspondence to Simona Nica, Institute of Organic Chemistry “C. D. Nenitzescu”
of the Romanian Academy, Splaiul Independentei 202B, 060023, Bucharest, Romania. E-mail:
snica@cco.ro
[297]/41

42/[298]
E. A. Dragu et al.
properties of the azobenzene chromophore must involve non-destructive external stimuli
for the biological environment. Investigations into azobenzenes chemistry showed that
these molecules can be modified such as that, new properties may be achieved making
them appropriate for applications in biological systems. Azobenzenes are known to be
non-fluorescent in solutions owing to their ability to undergo fast E → Z isomerization [9].
Hence, it would be very useful to have photoswitching azobenzenes that include fluorescent
readout to monitor their activity in complex structures. Modification of the chromophore
skeleton with electron donating/accepting groups at para-positions can yield derivatives
emitting green, yellow, orange and red fluorescence with high quantum yields when the
azo-bond is successfully “locked” in the excited state [29, 30].
Among various types of electron donating/accepting groups, azulene derivatives can
play an important role. Azulene is a non-bezenoid aromatic hydrocarbon with unique
redox properties [31–33], reason why has been used to prepare semiconductive materials
[34, 35]. This aromatic hydrocarbon is a bicyclo[5.3.0]decapentene with five- and seven-
membered rings carrying negative and positive charges, respectively. Therefore, azulene
can react both with electrophile and nucleophile, being versatile fragment for the design of
organic materials [36]. Moreover, azulene and its derivatives exhibit fluorescent emission
originating from excited singlet state S2 → S0 with relatively high fluorescence quantum
yield, and S1 → S0 fluorescence with extremely low fluorescence quantum yield [37, 38]. In
this context, the use of azulene in the design of fluorescent azobenzenes is quite promising.
Owing to our continuing interests related to π-conjugated azulene derivatives [39–42],
we have recently reported the synthesis, optical and electrochemical properties of azophenyl
substituted 4-(azulen-1-yl)-pyridines 1–4 (Scheme 1) [43]. The aim of the present work is to
investigate the fluorescence properties of these compounds both in solution and poly(methyl
methacrylate) (PMMA) thin films, as well as their photochemical E → Z isomerization in
solution.
Scheme 1. Chemical structure of the compounds under investigation.

Photochemistry of Substituted Azobenzenes
Experimental Techniques and Characterization
[299]/43
The azo coupling reactions for isolation of compounds 1 – 4 (Scheme 1) was recently
reported [43]. The numbering of atom positions used for the characterization of the com-
pounds is described in Figure 7. The fluorescence measurements were performed on Jasco
FP 6500 fluorimeter using solvent of fluorescence grade. The UV-Vis spectra were recorded
on Varian Cary 100 spectrophotometer. Thin films were obtained using spin-coating tech-
nique on Laurell –WS– 400B – 6NPP/LITE apparatus. The photoisomerization reaction
was performed in the dark by using a Micro photochemical reaction assembly with quartz
well and UV lamp of 5.5 W, equipped with a water jacket. The monitoring of the E →
Z reaction progress was followed spectroscopically in a quartz cell with pass length of
1.0 cm. The thin films were prepared using PMMA (purchased from Sigma Aldrich). The
concentration of the 4-azulenylpyridine-substituted azobenzene films were made by mixing
the indicated concentration of dye with PMMA in TCE as solvent. The NMR spectra in
CDCl3 containing TMS as internal standard were recorded on Bruker Avance DRX400
spectrometer; chemical shifts (δ) are expressed in ppm, and J values are given in Hz. The
full characterization of the stable E – isomers of the herein investigated compounds was
1
previously reported [43]. The H-NMR data of the two E and Z isomers (obtained after 30
minutes of irradiation with a UV lamp of 254 nm (5.5 W)) are presented here.
(
E)-[3-(2,6-dimethyl-pyridin-4-yl)azulen-1-yl]-phenyl-diazene (1). UV-Vis (chloroform):
1
λmax (log ε) 247 (4.35), 327 (4.40), 430 (4.47) nm. H NMR (CDCl3): δ 2.66 (s, 6H, 2-Me,
6
4
ꢀ
-Me), 7.24 (s, 2H, 3-H, 5-H), 7.39 (t, 1H, J = 9.6 Hz, 5 -H), 7.41 (t, J = 7.5 Hz, 1H,
ꢀꢀ
ꢀ
ꢀꢀ
ꢀꢀ
-H), 7.48 (t, 1H, J = 10.0 Hz, 7 -H), 7.56 (t, 2H, J = 7.6 Hz, 3 -H, 5 -H), 7.82 (t, 1H,
ꢀ
ꢀꢀ
ꢀꢀ
ꢀ
J = 10.0 Hz, 6 -H), 8.03 (dAB, 2H, J = 7.6 Hz, 2 -H, 6 -H), 8.44 (s, 1H, 2 -H), 8.59 (d, 1H,
ꢀ
ꢀ
1
J = 9.6 Hz, 4 -H), 9.44 (d, 1H, J = 9.6 Hz, 8 -H) ppm. (Z)- 1: H NMR (CDCl3): δ 2.93
ꢀ
ꢀ
ꢀꢀ
(
s, 6H, 2-Me, 6-Me), 7.36 (t, 1H, J = 9.6 Hz, 5 -H), 7.46 (t, 3 H, J = 10.0 Hz, 7 -H, 3 -H,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀꢀ
5
6
2
-H), 7.57 (s, 2H, 3-H, 5-H), 7.64 (t, J = 8.4 Hz, 1H, 4 -H), 7.73 (t, 1H, J = 10.0 Hz,
ꢀꢀ
ꢀꢀ
ꢀ
-H), 8.01 (dAB, 2H, J = 7.6 Hz, 2 -H, 6 -H), 8.61 (d, 1H, J = 9.8 Hz, 4 -H), 8.85 (s, 1H,
ꢀ
-H), 9.45 (d, 1H, J = 9.6 Hz, 8 -H) ppm.
(
E)-[3-(2,6-dimethyl-pyridin-4-yl)-4,6,8-trimethyl-azulen-1-yl]-phenyl-diazene, (2). UV-
1
Vis (chloroform): λmax (log ε) 252 (4.61), 324 (4.44), 438 (4.46) nm. H NMR (CDCl3):
δ 2.47 (s, 3H, 6 -Me), 2.57 (s, 6H, 2-Me, 6-Me), 2.64 (s, 3H, 4 -Me), 3.41 (s, 3H, 4 -Me),
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀꢀ
7
7
2
6
1
2
.02 (s, 2H, 3-H, 5-H), 7.12 (s, 1H, 5 -H), 7.32 (s, 1H, 7 -H), 7.36 (t, 1H, J = 7.6 Hz, 4 -H),
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
.48 (t, 2H, J = 7.2 Hz, 3 -H, 5 -H), 7.85 (dAB, 2H, J = 7.6 Hz, 2 -H, 6 -H), 8.04 (s, 1H,
ꢀ
1
ꢀ
ꢀ
-H) ppm. (Z)- 2: H NMR (CDCl3): δ 2.47 (s, 3H, 6 -Me), 2.67 (s, 3H, 4 -Me), 2.94 (s,
ꢀ
ꢀ
H, 2-Me, 6-Me), 3.38 (s, 3H, 4 -Me), 7.22 (s, 1H, 5 -H), 7.32 (s, 2H, 3-H, 5-H), 7.37 (t,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀꢀ
ꢀꢀ
H, J = 7.2 Hz, 4 -H), 7.45 (s, 1H, 7 -H), 7.48 (d, 2H, J = 7.6 Hz, 3 -H, 5 -H), 7.82 (dAB,
ꢀꢀ
ꢀ
H, J = 7.6 Hz, 2 -H, 6 -H), 8.12 (s, 1H, 2 -H) ppm.
(
E)-[3-(2,6-dimethyl-pyridin-4-yl)azulen-1-yl]-(4-nitro-phenyl)diazene, (3). UV-Vis (chlo-
1
roform): λmax (log ε) 247 (4.44), 301 (4.46), 480 (4.42) nm. H NMR (CDCl3): δ 2.66 (s,
6
9
(
(
7
(
9
ꢀ
H, 2-Me, 6-Me), 7.29 (s, 2H, 3-H, 5-H), 7.53 (t, 1H, J = 9.8 Hz, 5 -H), 7.65 (t, 1H, J =
ꢀ
ꢀ
ꢀꢀ
ꢀꢀ
.8 Hz, 7 -H), 7.93 (t, 1H, J = 9.8 Hz, 6 -H), 8.09 (dAB, 2H, J = 9.2 Hz, 2 -H, 6 -H), 8.39
ꢀ
ꢀ
ꢀ
ꢀꢀ
ꢀ
ꢀ
dAB, 2H, J = 8.8 Hz, 3 -H, 5 -H), 8.43 (s, 1H, 2 -H), 8.64 (d, 1H, J = 9.6 Hz, 4 -H), 9.46
1
d, 1H, J = 10.0 Hz, 8 -H) ppm. (Z) -3: H NMR (CDCl3): δ 2.77 (s, 6H, 2-Me, 6-Me),
ꢀ
ꢀ
.35 (s, 2H, 3-H, 5-H), 7.56 (t, 1H, J = 9.8 Hz, 5 -H), 7.68 (t, 1H, J = 9.8 Hz, 7 -H), 7.96
ꢀ
ꢀꢀ
ꢀꢀ
t, 1H, J = 10.0 Hz, 6 -H), 8.08 (dAB, 2H, J = 8.8 Hz, 2 -H, 6 -H), 8.38 (dAB, 2H, J =
ꢀ
ꢀ
ꢀꢀ
ꢀ
ꢀ
.2 Hz, 3 -H, 5 -H), 8.44 (s, 1H, 2 -H), 8.64 (d, 1H, J = 9.6 Hz, 4 -H), 9.48 (d, 1H, J =
ꢀ
1
0.0 Hz, 8 -H) ppm.

44/[300]
E. A. Dragu et al.
Figure 1. Fluorescent spectra of 1 (solid line) and 2 (dashed line) in 1,1,2-trichloroethane (conc.
−
5
1
.3 · 10 M, excitation wavelength = 420 nm).
(E)-[3-(2,6-dimethyl-pyridin-4-yl)-4,6,8-trimethyl-azulen-1-yl]-(4-nitro-
phenyl)diazene, (4). UV-Vis (chloroform): λmax (log ε) 254 (4.49), 323 (4.37), 486
1
ꢀ
(
4.52) nm. H NMR (CDCl3): δ 2.49 (s, 3H, 6 -Me), 2.59 (s, 6H, 2-Me, 6-Me), 2.68 (s,
ꢀ ꢀ ꢀ
H, 4 -Me), 3.41 (s, 3H, 8 -Me), 7.03 (s, 2H, 3-H, 5-H), 7.26 (s, 1H, 5 -H), 7.46 (s, 1H,
3
7
9
2
ꢀ
ꢀꢀ
ꢀꢀ
ꢀ
-H), 7.90 (dAB, 1H, J = 8.8 Hz, 2 -H, 6 -H), 8.05 (s, 1H, 2 -H), 8.34 (dAB, 2H, J =
ꢀꢀ
ꢀꢀ
1
ꢀ
.2 Hz, 3 -H, 5 -H) ppm. (Z)- 4: H NMR (CDCl3): δ 2.51 (s, 3H, 6 -Me), 2.73 (s, 6H,
ꢀ
ꢀ
-Me, 6-Me), 2.95 (s, 3H, 4 -Me), 3.42 (s, 3H, 8 -Me), 7.33 (s, 2H, 3-H, 5-H), 7.38 (s, 1H,
Figure 2. Fluorescent spectra of 3 (solid line) and 4 (dashed line) in 1,1,2-trichloroethane (conc.
−
5
0
.6 · 10 M, excitation wavelength = 460 nm).

Photochemistry of Substituted Azobenzenes
[301]/45
ꢀ
ꢀ
ꢀꢀ
ꢀꢀ
ꢀ
5
(
-H), 7.58 (s, 1H, 7 -H), 7.92 (dAB, 1H, J = 9.2 Hz, 2 -H, 6 -H), 8.14 (s, 1H, 2 -H), 8.36
ꢀꢀ
ꢀꢀ
dAB, 2H, J = 8.8 Hz, 3 -H, 5 -H) ppm.
Results and Discussion
Synthesis
The synthesis of compounds 1–4 (Scheme 1) was previously reported and involves the
azo coupling reaction of 4-azulenylpyridines with the diazonium salts of aniline and p-
nitroaniline, respectively, in potassium acetate buffer [43]. The reaction proceed in mild
conditions with moderate to high yields, making the compounds easily accessible.
Fluorescence Investigations
Azobenzene molecules are in general not fluorescent. The attachement of the azulenyl moi-
ety has induced fluorescent properties of the herein described compounds. The fluorescence
measurements were performed in 1,1,2-trichloroethane (TCE) solutions. Differences in the
intensity and position of the emission peaks have been observed. The main fluorescence
emission was the usual S2 → S0 azulene transition. The position of the visible absorption
bands was influenced by the functional groups grafted to the azobenzene skeleton and, to
a lesser extent, by the alkyl substituents of the azulene moiety. Thus, the nitro-substituted
dyes 3 and 4 displayed intense absorption maxima at 465 and 480 nm, respectively, while
the unsubstituted azobenzene dyes 1 and 2 showed visible maxima at lower wavelengths of
423 and 426 nm, respectively, therefore the fluorescence spectra were recorded using dif-
ferent excitation wavelength. The emission spectra of TCE solutions are shown in Figures 1
and 2. First, excitation of compounds 1 and 2 at the related wavelength (420 nm) gave the
emission spectra with λ at 531 nm for compound 1 and 540 nm for compounds 2, respec-
tively, with shoulder like maximum around 480 nm in both cases. The electron-withdrawing
Figure 3. Fluorescent spectra of PMMA thin films containing compound 1 (up) and compound 2
(down) of different loadings: 2% (solid line), 5% (dashed line) and 10% (dotted line).

46/[302]
E. A. Dragu et al.
Figure 4. UV-Vis spectra of PMMA thin films containing compound 1 (left side) and compound 2
right side) of different loadings: 2% (solid line), 5% (dashed line) and 10% (dotted line).
(
effect of the methyl groups in compound 2 caused a red shift of the emission wavelength
accompanied by a small decrease of the intensity value.
The electron-donating ability of the nitro-substituent has influenced the λEx of com-
pounds 3 and 4. The emission spectra are shown in Figure 2. The absorption maximum
appeared at long wave position, with emission spectra showing maximum peaks at 554
and 556 nm, for 3 and 4, respectively. Most likely the electronic effect of the nitro-group
decreases the HOMO-LUMO gap and makes λEm shift to red direction. Indeed, MOPAC
calculations for HOMO and LUMO orbital energy showed that through extension of the
electronic conjugation, the LUMO energy decreases, and in turn reduces the HOMO-LUMO
gap, moving the absorption band at higher wavelengths [43].
Fluorescence measurements were also performed in thin films in order to monitor the
assembly process of the azulenyl-pyridine substituted azobenzenes. Only for the unsubsti-
tuted azobenzene compounds 1 and 2, could be recorded the fluorescence spectra of the
PMMA thin films containing different chromophore loadings (2%, 5% and 10%) (Fig. 3).
The visible absorption peaks in thin films (∼430 nm) are well preserved, being close to
the absorption maxima recorded in solutions (Fig. 4) with a linear increase of intensity
with the dye loading. For these compounds, characteristic emission peaks are observed at
525 nm, close to emission position in the corresponding TCE solutions. This indicates that,
the characteristic emission of the 4-azulenylpyridine azobenzene molecules preserve after
incorporation in polymeric matrix.
The stable growth of these assemblies yields an increase of the fluorescence intensity
with the amount of azulene-based dye incorporated in the PMMA matrix for the first two
concentrations, 2% and 5% respectively. Further loading (10% dye) causes a decrease of the
fluorescence intensity. This partial quenching effect is a consequence of strong π-π contacts
established between azulenyl moieties, in agreement with the well-known polarization of
the azulene in solid state [44].
In the case of PMMA thin films containing dye-compounds 3 and 4, the fluorescence
spectra exhibit no measurable emission peaks when excited at wavelengths longer than
400 nm. This is in accordance with the observed red shift of the visible absorption maxima

Photochemistry of Substituted Azobenzenes
[303]/47
Table 1. UV-Vis spectra of compounds 1–4 before irradiation, in CHCl3 solution
Compound
λ1 (ε)
λ2 (ε)
λ3 (ε)
1
2
3
4
247 (22291)
252 (40760)
247 (27432)
254 (31405)
327 (25129)
324 (27480)
301 (29066)
323 (23500)
430 (29318)
438 (29150)
480 (26381)
486 (33081)
in solutions. The recorded bathochromic shift is caused by strong π-conjugation induced
through electron-donating nitro substitution. As previously reported, planar molecules with
long electron conjugation path show negligible fluorescence due to the fact that close π – π
stacking in the solid state is promoted, which in turns quenches the fluorescence [45, 46].
Photoisomerization Investigations
The photochemical E → Z isomerization of the azo bond of compounds 1–4 was inves-
tigated by UV-Vis and NMR spectroscopy in chloroform solutions. When the compounds
were in the thermally stable E configuration, the absorption spectra showed two strong
bands in the UV region (see Table 1) and one in the visible region with molar absorptivities
−1
−1
ranging from 26381 to 33081 M cm and not much effect of solvent polarity. Thus,
unsubstituted azobenzene dyes 1 and 2 showed visible maxima at lower wavelengths of
430 and 438 nm while the, nitro-substituted dyes 3 and 4 displayed intense absorption
maxima at 480 and 486 nm, respectively.
The photochromic behaviour of compounds 1–4 was spectrophotometrically monitored
by following the changes in the absorption spectra while irradiating at 254 nm (5.5 W UV
lamp, at room temperature). As expected, all compounds displayed E → Z isomerization
upon exposure to UV light. In the first 30 seconds of irradiation, both the nitro-substituted
and the unsubstituted dyes showed a hypsochromic shift of the visible band (Figs. 5 and
6), while upon prolonged irradiation they displayed different absorption profiles. Thus,
3
Figure 5. Absorption spectra of compounds 1 and 2 in CHCl solutions under 254 nm irradiation.
The exposure time is shown in the inset legends.

48/[304]
E. A. Dragu et al.
3
Figure 6. Absorption spectra of compounds 3 and 4 in CHCl solutions under 254 nm irradiation.
The exposure time is shown in the inset legends.
the unsubstituted dyes 1 and 2 showed an increase in the absorption at around 400 nm
accompanied by the appearance of a new absorption band in the visible region at 514
and 534 nm, respectively (Fig. 5). On the other hand, prolonged irradiation of the nitro-
substituted dyes 3 and 4 produced the same hypsochromic shift of the absorption maximum,
whereas the appearance of the new visible band was not as clear (Fig. 5). However, for all
four compounds, the photochemical E → Z isomerization proceeded to approximately the
same extent, with the absorbance variation after UV irradiation ranging between 0.43 and
0.53 absorbance units. Exposure of the irradiated solutions to sunlight for a day caused
the disappearance of the newly formed visible band and the partial recovery of the UV-Vis
spectra recorded after 30 seconds of UV irradiation.
1
Correlation of absorption spectra with H–NMR data elucidated the spectral assign-
ments. Examination of the NMR data for the pre-irradiated solutions of all azobenzenes in
CDCl3 (around 5 mM for NMR experiments, but diluted to 0.025 mM for absorption spec-
tra) revealed the absence of the Z isomer. In contrast, the NMR spectra of the samples after
30 minutes of UV irradiation (5.5 W, UV lamp) were comprised of peaks corresponding
only to the Z stereoisomer, thus demonstrating that the E → Z isomerization occurred.
The differences in chemical shifts between the E and Z isomers are more pronounced
for the pyridyl protons and less for the azulenyl moiety. A summary of the NMR data and
spectral assignments for the E and Z isomers of compounds 1–4 is given in Table 2. The
spectral changes occurring after the E → Z isomerization can be summarized as follows:
1. The pyridyl protons, both aromatic and methyl protons, are consistently more
deshielded in the Z than in the E isomer.
ꢀ
2
. The protons at the 2 -position of the azulene ring (see Fig. 7 for atom numbering)
resonate at significantly different chemical shifts in the two isomers, depending on
the substitution of both the azobenzene core and the azulene moiety.
For the sake of clarity, the NMR spectral changes induced by irradiation will be
detailed for compound 2 in which the alkyl substitution of the azulene moiety simplifies the
1
assignment of aromatic protons. Thus, the H-NMR spectrum of the pre-irradiated solution
of 2 showed singlet peaks at 2.57 and 7.02 ppm, corresponding to the protons at the 2/6- and

Photochemistry of Substituted Azobenzenes
[305]/49
Table 2. Relevant chemical shifts of pyridinium ring and azulen protons for compounds
–4
1
Entry
1
Compound
1–E
δCH3–2.6 (ppm)
δH–3.5 (ppm)
δ
ꢀ
(ppm)
H–2
2.66
2.93
2.57
2.94
2.66
2.77
2.59
2.73
7.24
7.57
7.02
7.32
7.29
7.35
7.03
7.33
8.44
8.61
8.04
8.12
8.43
8.44
8.05
8.14
1
–Z
2–E
–Z
3–E
–Z
4–E
–Z
2
3
4
2
3
4
3/5-positions of the pyridyl moiety. Upon UV irradiation, these two peaks shifted downfield
to 2.94 and 7.32 ppm, respectively (Figure 7). This significant downfield chemical shift is
in accordance with the proposed photoisomerization reaction and can be correlated with
ꢀ
the loss of symmetry in the resulting Z isomer. Furthermore, the proton at the 2 -position of
azulene also shifted downfield by 0.08 ppm because of the deshielding effect of the phenyl
group which is located closer to the azulenyl moiety in the Z than in the E isomer.
Figure 7. 1H-NMR spectra of 2 in CDCl3, before (down) and after UV irradiation (top). The NMR
scale is truncated for better comparison.

50/[306]
E. A. Dragu et al.
Conclusions
Azobenzene molecules substituted with 4-azulenylpyridine moiety showed strong fluores-
cent properties in solution. It has been showed that, the attachment of the azulenyl fragment
to the non fluorescent azobenzene chromophore has induced fluorescent properties of the
herein described compounds. Differences in the intensity and position of the emission peaks
have been observed. For all compounds, photochemical E → Z isomerization of the azo
bond proceeded when UV wavelength of 254 nm was used. This was confirmed by UV-Vis
and NMR spectrsoscopy with differences in the position of the maximum absorption and
in chemical shifts between the E and Z isomers. The fluorescence data and the photoiso-
merization behaviour provide supporting evidence of the possible practical use of these
molecules in material science. Further investigation of their non linear optical properties is
in progress.
Funding
I.R., F.K. and A.T. acknowledge the financial support of Romanian Ministry of Education,
Research, Youth and Sports, through the UEFISCDI organism, under Contract Number
279/7.10.2011, Code Project PN-II-ID-PCE-2011-3-0505.
References
[
[
[
1] Browne, W. R., & Feringa, B. L. (2006). Nature, 1, 25.
2] Barret, C. J., Mamiya, J., Yagerc, K. G., & Ikeda, T. (2007). Soft Mater., 10, 1205.
3] Hedge, G., Kozenkov, V. M., Chigrinov, V. G., & Kwor, H. S. (2009). Mol. Cryst. Liq. Cryst.,
507, 41.
[
[
4] Khoshima, H., & Sabet, R. A. (2012). Mol. Cryst. Liq. Cryst., 559, 16.
5] Ferri, V., Elbing, M., Pace, G., Dickey, M. D., Zharnikov, M., Samor `ı , P., Mayor, M., & Rampi,
M. A. (2008). Angew. Chem.,120, 3455.
[
[
[
6] Duan, P., Qin, L., & Liu, M. (2011). Langmuir, 27, 1326.
7] Thomas, R., & Tamaoki, N. (2011). Org. Biomol. Chem., 9, 5389.
8] Hoppmann, C., Seedorff, S., Richter, A., Fabian, H., Schmieder, P., R u¨ ck-Braun, K., & Beyer-
mann, M. (2009). Angew. Chem., 48, 6636.
[9] Banghart, M., Borges, K., Isacoff, E., Trauner, D., & Kramer, R. H. (2004). Nature, 7, 1381.
[
10] Fortin, D. L., Banghart, M. R., Dunn, T. W., Borges, K., Wagenaar, D. A., Gaudry, Q., Karakos-
sian, M. H., Otis, T. S., Kristan, W. B., Trauner, D., & Kramer, R. H. (2008). Nature, 5, 331.
11] Stoll, R. S., Peters, M. V., Kuhn, A., Heiles, S., Goddard, R., Buhl, M., Thiele, C. M., & Hecht,
S. (2009). J. Amer. Chem. Soc., 131, 357.
[
[
[
[
12] Braunschweig, H., & Kupfer, T. (2008). J. Amer. Chem. Soc., 130, 4242.
13] Grirrane, A., Corma, A., & Garc ´ı a, H. (2008). Science, 232, 1661.
14] Baba, K., Ono, H., Itoh, E., Itoh, S., Noda, K., Usui, T., Ishihara, K., Inamo, M., Takagi, H. D.,
&
Asano, T. (2006). Chem. Eur. J., 12, 5328.
[
[
[
15] Branco, L. C., & Pina, F. (2009). Chem. Comm., 41, 6204.
16] Behring, A., Benkler, E., & Tsudi, T. (2010). Mol. Cryst. Liq. Cryst., 422, 219.
17] Hugell, T., Holland, N. B., Cattani, A., Moroder, L., Seitz, M., & Gaub, H. E. (2002). Science,
296, 1103.
[
[
18] Yager, K. G., & Barret, C. J. (2006). J. Photochem. Photobiol. A, 182, 250.
19] Urbas, A., Tondiglia, V., Natarajan, L., Sutherland, R., Yu, H., Li, J. H., & Bunning, T. (2004).
J. Amer. Chem. Soc., 126, 13580.
[20] Labruto, R., Gottarelli, G., Masiero, S., Spada, G., & Malthete, J. (2010). Mol. Cryst. Liq. Cryst.,
420, 1.

Photochemistry of Substituted Azobenzenes
[307]/51
[
[
[
[
[
[
21] Chen, Z., Zhong, C., Zhang, Z., Li, Z., Niu, L., Bin, Y., & Zhang, F. (2008). J. Phys. Chem. B,
12, 7387.
22] Facchetti, A., Annoni, E., Beverina, L., Morone, M., Zhu, P., Marks, T. J., & Pagani, G. A.
2004). Nature, 3, 910.
23] Cheng, Y. J., Luo, J., Huang, S., Zhou, X., Shi, Z., Kim, T. D., & Jen, A. K. Y. (2008). Chem.
Mater., 20, 5047.
1
(
24] Hrozhyk, U. A., Serak, S. V., Tabiryan, N. V., & Bunning, T. J. (2008). Adv. Funct. Mater., 17,
1735.
25] Niziol, J, Kajzar, F., Rau, I., Bednarz, M., Pielichowski, J., & Pielichowski, K. (2006). Mol.
Cryst. Liq. Cryst., 447, 167.
26] Zhang, F., Timm, K. A., Arndt, K. M., & Woolley, G. A. (2010). Angew. Chem. Int. Ed. Engl.,
49, 3943.
[
[
27] Beharry, A. A., & Woolley, G. A. (2011). Chem. Soc. Rev., 40, 4422.
28] Rau, H. (1990). In: Azocompounds. Durr, H., & Bouas-Laurent, H. (Eds.). Chapter Pho-
tochromism, Molecules and Systems, Elsevier: Amsterdam, 165.
[29] Dugave, C., & Demange, L. (2003). Chem. Rev., 103, 2475.
[30] Yoshino, J., Furuta, A., Kambe, T., Ito, H., Kano, N., Kawashima, T., Ito, Y., & Asashima, M.
(
2010). Chem. Eur. J., 16, 5026.
31] Dragu, E. A., Nica, S., Tecuceanu, V., Bala, D., Mihailciuc, C., Hanganu, A., & Razus, A. C.
2013). Eur. J. Org. Chem., 6601.
32] Dragu, E. A., Nica, S., Raicopol, M., Baran, A., Anghel, D. F., Cojocaru, B., Tarko, L., & Razus,
A. C. (2012). Tetrahedron Lett., 53, 2611.
[
[
[
(
33] Cristian, L., Nica, S., Pavel, O. D., Mihailciuc, C., Almasan, V., Coman, S. M., Hardacre, C., &
Parvulescu, V. I. (2013). Cat. Sci. Technol., 3, 2646.
[34] Wang, F., Lai, Y. H., Kocherginsky N. M., & Kosteski, Y. Y. (2003). Org. Lett., 5, 995.
[35] Murai, M., Amir, E., Amir, R. J., & Hawker, C. J. (2012). Chem. Sci., 3, 2721.
[36] Zeller, K. P. (1985). In: Houben weyl, Methoden der Organichen Chemie, Vol. 5/2c, Thieme:
Stuttgart, 127.
[
37] T e´ treault, N., Muthyala, R.S., Liu, R.S.H., & Steer, R.P. (1999). J. Phys. Chem. A., 103, 2524.
[38] Abou-Zied, O. K., Sinha, H. K., & Steer, R. P. (1997). J. Mol. Spectrosc., 183, 42.
[39] Razus, A. C., Dragu, E. A., Nica, S., & Nicolescu, A. (2011). Tetrahedron Lett., 52, 1858.
[40] Razus, A. C., Cristian, L., Nica, S., Zaharia, O., & Hanganu, A. (2011). Arkivoc, 38.
[41] Razus, A. C., Nica, S., & Dragu, E. A. (2009). Synth. Comm., 39, 3166.
[42] Razus, A. C., Pavel, C., Lehadus, O., Nica, S., & Birzan, L. (2008). Tetrahedron, 64, 1792.
[43] Razus, A. C., Nica, S., Cristian, L., Raicopol, M., Birzan, L., & Dragu, A. E. (2011). Dyes
Pigm., 91, 55.
[
[
[
44] Ion, A. E., Nica, S., Madalan, A. M., Maxim, C., Julve, M., Lloret, F., & Andruh, M. (2014).
CystEngComm., 16, 319.
45] Yeh, H. C.; Wu, W. C.; Wen, Y. S.; Dai, D. C.; Wang, J. K., & Chen C. T. (2004). J. Org. Chem.,
69, 6455.
46] Ooyama, Y.; Hayashi, A.; Okamoto, T.; Egawa, H.; Mamura, T., & Yoshida, K. (2008). Eur. J.
Org. Chem., 3085.