Synthesis, characterization and photophysical studies of self-assembled azo biphenyl urea derivatives

Article abstract of DOI:10.1039/c5pp00357a

We reported the synthesis of a new series of azobiphenyl based urea derivatives 7 and their stimulus-responsive supramolecular structures in the form of sheet like self-assembled formations. The self-assembled nanostructural formations of azo derivatives

Full text of DOI:10.1039/c5pp00357a

Photochemical &
Photobiological Sciences
View Article Online
View Journal
PAPER
Synthesis, characterization and photophysical
studies of self-assembled azo biphenyl urea
Cite this: DOI: 10.1039/c5pp00357a
derivatives†
Jayaraman Sivamani, Rajendiran Balasaravanan, Kumaraguru Duraimurugan and
Ayyanar Siva*
We reported the synthesis of a new series of azobiphenyl based urea derivatives 7 and their stimulus-
responsive supramolecular structures in the form of sheet like self-assembled formations. The self-
assembled nanostructural formations of azo derivatives 7 are strongly dependent on the nature of the
solvent present in the systems. Further, we found that the amide hydrogen played a crucial role in
hydrogen bonding interactions to form a sheet like morphology upon stimulus responsive self-assembly.
This was confirmed by transmission electron microscopy and atomic force microscopy.
Received 29th September 2015,
Accepted 13th December 2015
DOI: 10.1039/c5pp00357a
www.rsc.org/pps
tions of molecules, because it is intrinsically fast, easily tune-
able in energy and intensity and, notably, light induced
1. Introduction
Supramolecular self-assembly is of great importance for appli- processes are often reversible.8 The reversible control of the
cations in materials science and biology, and to achieve nano- self-assembly of non-covalent building blocks by photo-
meter-scale molecular manipulations which cannot be irradiation is a key process for the synthesis of photorespon-
accomplished using conventional approaches. The self-assem- sive supramolecular materials. As we know, azobenzene-type
bly of molecules into a highly ordered supramolecular struc- derivatives can undergo an efficient and reversible photo-
ture is driven by many intermolecular forces, i.e. aromatic π–π isomerization reaction upon UV and visible light irradiation.9
stacking, hydrophobic forces, van der Waals forces, and hydro- Therefore, photoresponsive supramolecular materials based
gen bonding interactions.1 Self-assembly through hydrogen on azobenzene-type derivatives that have been widely reported
bonding interactions is a widely observed phenomenon in in recent years, especially in supramolecular architectures,
nature, because of moderately strong and directional pro- could be exploited for drug delivery.10–14
perties.2 The hydrogen bonding interactions between the
In this context, two different biphenyl based azo molecules
hydrogen atom and electro negative atoms especially nitrogen 7a and 7b containing both photoresponsive azo and urea
and oxygen atoms have been proven to be a useful and power- groups have been designed and synthesized in order to study
ful organizing force and were utilized for the formation of the photoinduced supramolecular self-assembled frameworks.
supramolecules.3,4 Urea and its derivatives are ubiquitous in (Fig. 1) Moreover, the target molecules have been explored for
biological and biochemical structures and functions and thus photoresponsive behaviour and the process of supramolecular
deserve special attention for the construction of dimensional engineering in the self-assembly of molecular frameworks
frameworks in recent years.5,6
through aggregation studies has also been investigated.
The development of functional materials composed of Further the self-assembled molecules are investigated by
macroscopic molecular assemblies that bring about reversible Transmission Electron Microscopy (TEM) and Atomic Force
changes of their shape and/or physical properties with external
stimuli has attracted much attention in materials science.7 For
convenience, light is used as an external stimulus to trigger
chemical systems, photoreactions and structural isomeriza-
Department of Inorganic Chemistry, School of Chemistry, Madurai Kamaraj
University, Madurai-21, Tamilnadu, India. E-mail: drasiva@gmail.com,
ptcsiva@yahoo.co.in; Tel: +91451-2458471
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5pp00357a
Fig. 1 Chemical structure of the target molecule.
This journal is © The Royal Society of Chemistry and Owner Societies 2015
Photochem. Photobiol. Sci.

View Article Online
Paper
Photochemical & Photobiological Sciences
Scheme 1 Schematic representation of the synthesis of compound 7(7a/7b).
Microscopy (AFM). In the first section, we synthesized two diborane, PdCl2(PPh3)2, Pd2(dpa)3 and P(O-tolyl)3 was obtained
different biphenyl based azo molecules 7 consisting of two from Sigma Aldrich. KMnO4, FeSO4·7H2O, were obtained from
different urea moieties (7a and 7b, Scheme 1) and all the com- Merck and all the solvents were obtained from laboratories
pounds were well characterized by various spectral techniques and were of analytical grade.
such as NMR and Mass spectroscopy. In the second section,
The melting points were measured in open capillary tubes
we focused on the exploration of systems engineered exclu- and are uncorrected. The 1H and 13C NMR spectra were
sively to undergo efficient and reversible photochemical reac- recorded on a Bruker (Avance) 300 & 400 MHz NMR instru-
tions between two stable isomers featuring markedly different ment using TMS as an internal standard, and CDCl3 and
properties (Fig. 3, Schemes 1 and 2 in the ESI†). In the third DMSO as solvents. Standard Bruker software was used
section, we have discussed the systems self-assembled through throughout. Chemical shifts are given in parts per million
the formation of hydrogen-bonds that are capable of under- (δ-scale) and the coupling constants are given in hertz. Silica
going large conformational changes upon aggregation. gel-G plates (Merck) were used for TLC analysis with a mixture
Further, we investigated the Density Functional Theory (DFT) of n-hexane and ethylacetate as an eluent. Column chromato-
calculation and Cyclic Voltammetry (CV) studies for the Intra- graphy was carried out in silica gel (60–120 mesh) using pet-
molecular Charge Transfers (ICT) of trans and cis isomers 7 roleum ether and ethylacetate as an eluent. Electrospray
(7a/7b).
Ionization Mass Spectrometry (ESI-MS) analyses were recorded
in LCQ Fleet, Thermo Fisher Instruments Limited, USA.
ESI-MS was performed in the positive ion mode. The collision
voltage and ionization voltage were −70 V and −4.5 kV, respecti-
vely, using nitrogen as the atomization and desolvation gas.
The desolvation temperature was set at 300 °C. The relative
amount of each component was determined from the LC-MS
chromatogram, using the area normalization method. UV
absorption was recorded on a JASCO UV-630, spectroflurom-
2. Experimental section
2.1 Materials and methods
All the chemicals and reagents used in this work were of
analytical grade. 4-Bromoaniline, 4-iodoaniline, phenyl chloro-
formate were obtained from Alfa Aesar. Bis(pinacolato)
Photochem. Photobiol. Sci.
This journal is © The Royal Society of Chemistry and Owner Societies 2015

View Article Online
Photochemical & Photobiological Sciences
Paper
eter were recorded on an Agilent 8000 and photoisomerization graphy on silica gel, using petroleum ether and ethyl acetate
of the cis isomer was performed with a Blak-Ray Long Wave (5%) as the eluent to give the corresponding compound as an
Ultraviolet Lamp, Model B-100 (λ = 365 nm).15 Cyclic voltam- orange solid (96% yield); mp 160 °C. 1H NMR (300 MHz,
metry was carried out using a CHI 680 electrochemical work- CDCl3) δH 7.96 (d, J = 8.4 Hz, 4H), 7.90 (d, J = 8.5 Hz, 4H), 1.37
station at RT with a scan rate of 50 mV s−1. The ground-state (s, 24H). 13C NMR (75 MHz, CDCl3) δC 154.23, 135.36, 121.76,
geometries were optimized using density functional theory 83.18, 24.75.
with the B3LYP hybrid Functional at the basis set level of
2.2.3 Synthesis of compound 5. Compound (5) was syn-
6-31G. All the calculations were performed using the Gaussian thesised according to the procedure described in the litera-
03 package. The nanoaggregated sample was prepared by the ture.17 To a stirred solution of 4-iodoaniline 4 (2 g, 0.91 mmol)
following method. A stock solution of 7 (7a/7b) in THF with a and pyridine (0.88 mL, 1.12 mmol) in CH2Cl2 at 0 °C was
predetermined concentration of 1 × 10−4 M was prepared. added phenyl chloroformate (1.38 mL, 1.12 mmol) dropwise
Aliquots of the samples were transferred into a 5 mL volu- over 5 min. The mixture was stirred at 0 °C for 10 min, washed
metric flask. After adding the appropriate amount of THF, with 5% aqueous citric acid, dried over anhydrous NaSO4 and
water was added dropwise under vigorous stirring to furnish 1 concentrated under vacuum. The crude material was recrystal-
× 10−5 M solutions with different water fractions. The absorp- lized by toluene, to give colourless needles in 91% yield; mp
1
tion and emission spectra of these resultant mixtures were 161 °C. H NMR (300 MHz, CDCl3): δH 7.61 (d, J = 8.8 Hz, 2H),
measured immediately. All the low resolution TEM images 7.39 (t, J = 7.9 Hz, 2H), 7.25–7.15 (m, 5H), 6.95 (s, 1H). 13C
were obtained from the deposition onto copper grids (with NMR (75 MHz, CDCl3): δC 151.64, 150.62, 138.19, 137.41,
carbon layer coating, 200 mesh) of dispersions (10−5 M) of 7 at 129.59, 125.98, 121.69, 120.94, 87.22.
40% H2O and THF; these grids were analyzed with a Philips
2.3 General procedure A for synthesis of compound 6
208 electron microscope at 200 kV, and the resulting images
were collected with an AMT high-resolution digital imaging To a stirred solution of 5 (1 eq.) and Et3N (1.1 eq.) in CH2Cl2 at
camera. The 3D topography of the prepared samples was eluci- 0 °C was added alkylamine (1.1 eq.) dropwise over 5 min; the
dated by atomic force microscopy (AFM) (non-contact mode, mixture was stirred at RT for 2 h. After the completion of the
A100 SGS, APE Research).
reaction, the mixture was concentrated and the crude product
was taken into ether and stirred for 10 min, filtered and
washed with ether, and dried to obtain the pure material.
2.3.1 Synthesis of compound 6a. Compound (6a) was syn-
2.2 Synthesis and characterization
2.2.1 Synthesis of compound 2. Compound (2) was syn-
thesized according to the procedure described in the litera- thesized using the above described general procedure A using
ture.16 4-Bromoaniline (3.5 g, 2 mmol) was taken into a compound 5 (1 g, 0.29 mmol), Et3N (0.45 mL, 0.32 mmol) and
250 mL round bottom flask, dissolved in dichloromethane and octylamine (0.54 mL, 0.32 mmol). White solid (96% yield); mp
1
into this a homogeneous mixture of the oxidant (20.0 g) was 138 °C. H NMR (400 MHz, DMSO-d6) δH 8.49 (s, 1H), 7.51 (d,
added (the homogeneous mixture of the oxidant was prepared J = 8.8 Hz, 2H), 7.23 (d, J = 8.8 Hz, 2H), 6.14 (s, 1H), 3.05 (dd,
by grinding an equal amount of KMnO4 (10 g) and J = 19.4, 6.7 Hz, 2H), 1.40 (t, J = 6.2 Hz, 2H), 1.25 (s, 10H), 0.85
FeSO4·7H2O (10 g) in a mortar gently). The heterogeneous (t, J = 6.6 Hz, 3H). 13C NMR (100 MHz, DMSO-d6) δC 154.97,
mixture was refluxed for 5 h. After the completion of the start- 140.53, 137.15, 119.89, 83.38, 31.26, 29.70, 28.76, 28.71, 26.39,
ing material, the reaction mixture was cooled to room tempera- 22.11, 13.98. ESI-MS: m/z calcd for C15H23IN2O [M]+ 374.0855;
ture and filtered through Celite, and the residue was washed found 374.0832.
with dichloromethane and dried over anhydrous sodium sul-
phate. The solvent was removed and purified by column synthesized by using the above described general procedure A
chromatography (60–120 mesh), using petroleum ether as the using compound (1 g, 0.29 mmol), Et3N (0.45 mL,
2.3.2 Synthesis of compound 6b. Compound (6b) was
5
eluent to give the corresponding compound as a red solid 0.32 mmol) and t-butoxycarbazate (0.43 g, 0.32 mmol). White
1
(96% yield); mp 207 °C. H NMR (300 MHz, CDCl3) δ 7.79 (d, solid (94% yield); mp 162 °C. 1H NMR (400 MHz, DMSO-d6) δH
J = 8.8 Hz, 4H), 7.65 (d, J = 8.8 Hz, 4H). 13C NMR (75 MHz, 8.77 (s, 1H), 8.60 (s, 1H), 7.90 (s, 1H), 7.52 (d, J = 8.3 Hz, 2H),
CDCl3) δ 151.40, 132.58, 125.98, 124.60.
7.30 (d, J = 7.7 Hz, 2H), 1.41 (s, 9H). 13C NMR (75 MHz, DMSO-
2.2.2 Synthesis of compound 3. Compound (3) was syn- d6) δC 155.91, 155.51, 139.74, 137.12, 120.65, 84.41, 79.22,
thesized according to the modified procedure described in the 28.06. ESI-MS: m/z calcd for C12H16IN3O3 [M]+ 377.0236; found
literature.15 To a degassed solution of dry dioxane, 4,4′-dibromo- 377.0202.
azobenzene
2 (1 g, 0.29 mmol), bis(pinacolatodiborane)
2.4 General procedure B for synthesis of compound 7
(Suzuki coupling)
(2.61 g, 1.02 mmol), PdCl2(PPh3)2 (0.052 g, 0.015 mmol, 5%
mol) were added and the mixture was degassed a second time.
Potassium acetate (1.44 g, 1.45 mmol) was then added and the To a degassed solution of 3 (1 eq.) in dry DMF, was added
reaction mixture was degassed one last time, before allowing K2CO3 (5 eq.) in water and the mixture was degassed a second
the whole mass to stir overnight at 90 °C, under a nitrogen time. Pd2(dpa)3 (0.05 mol%) and P(O-tolyl)3 (0.5 mol%) were
atmosphere. Then the crude mixture was filtered over Celite, added and the mixture was degassed one last time, before
concentrated under vacuum and purified by column chromato- allowing the whole mass to stir at 100 °C. To this was added
This journal is © The Royal Society of Chemistry and Owner Societies 2015
Photochem. Photobiol. Sci.

View Article Online
Paper
Photochemical & Photobiological Sciences
dropwise a degassed solution of 6 (2.2 eq.) in dry DMF under a
nitrogen atm, and the reaction mixture was stirred at 100 °C
under a nitrogen atm. After the completion of the reaction,
the crude was filtered over Celite, concentrated under vacuum
and purified by column chromatography on silica gel, using
petroleum ether and ethyl acetate as the eluent to give the
corresponding compound 7.
2.4.1 Synthesis of compound 7a (DPOU). Compound (7a)
was synthesized by using the above described general pro-
cedure B using compound 3 (0.3 g, 0.69 mmol), K2CO3 (0.48 g,
3.4 mmol), Pd2(dpa)3 (0.003 g, 0.035 mmol, 0.05 mol%),
P(O-tolyl)3 (0.01 g, 0.35 mmol, 0.5 mol%) and 6a (0.65 g,
1.56 mmol). Light yellow solid (67% yield); mp 105 °C. 1H
NMR (300 MHz, DMSO-d6) δH 8.37 (s, 4H), 7.36 (d, J = 7.6 Hz,
4H), 7.20 (t, J = 7.5 Hz, 4H), 6.86 (t, J = 7.3 Hz, 4H), 6.10 (t, J =
5.5 Hz, 4H), 3.05 (dd, J = 19.2, 6.6 Hz, 4H), 1.40 (t, J = 5.6 Hz,
4H), 1.25 (s, 20H), 0.85 (t, J = 6.0 Hz, 6H). 13C NMR (75 MHz,
DMSO-d6) δC 155.22, 140.64, 128.61, 120.87, 117.55, 31.30,
29.81, 28.81, 28.76, 26.44, 22.14, 13.97. ESI-MS: m/z calculated
for C42H54N6O2 [M]+ 674.4308; found 674.4321.
Fig. 2 Absorption (a) and PL (b) spectra of compound 7 in THF (7a (5 ×
10−5 M), 7b (1 × 10−5 M)).
maxima, attributed to the π–π* and n–π* transitions of the
trans isomer of 7. In the absorption spectra of 7a, absorption
maxima appear at 277 nm and 370 nm, and 7b exhibited them
at 293 nm and 375 nm. Compound 7b (λemi = 392 nm, λex
=
331 nm) exhibits emission at higher wavelengths when com-
pared to 7a (λemi = 383 nm, λex = 330 nm) due to the presence
of two carbonyl groups in 7b, which in turn increases the
absorption and emission values compared to 7a.
2.4.2 Synthesis of compound 7b (DPBHU). Compound
(7b) was synthesized by using the above described general pro-
cedure B using compound 3 (0.3 g, 0.69 mmol), K2CO3 (0.48 g,
3.4 mmol), Pd2(dpa)3 (0.003 g, 0.035 mmol, 0.05 mol%),
P(O-tolyl)3 (0.01 g, 0.35 mmol, 0.5 mol%) and 6b (0.57 g,
1.56 mmol). Light yellow solid (65% yield); mp 138 °C. 1H
NMR (400 MHz, DMSO-d6) δH 8.82 (s, 2H), 8.66 (s, 4H),
7.98–7.86 (m, 4H), 7.56 (d, J = 8.7 Hz, 4H), 7.44 (d, J = 7.9 Hz,
2H), 7.31 (d, J = 7.8 Hz, 4H), 7.24 (t, J = 8.2 Hz, 2H), 1.40 (s,
18H). 13C NMR (100 MHz, DMSO-d6) δC 156.06, 156.00, 139.73,
137.16, 128.62, 121.78, 120.82, 79.25, 28.09. ESI-MS: m/z calcd
for C36H40N8O6 [M]+ 680.3071; found 680.3073.
Further, the photophysical properties of compounds 7
(7a/7b) were studied in various organic solvents (Fig. S1 and S2
in the ESI†) to find out the solvent effect and all the characteri-
stic data are listed in Tables 1 and 2 (in the ESI†). The polarity
of solvents is strongly influenced by the photophysical pro-
perties of compounds 7 (7a/7b), due to the presence of the
donor and acceptor groups in all the molecules. The PL
spectra of compounds 7 (7a/7b) are red shifted significantly
with increasing solvent polarity (Fig. S1 and S2 in the ESI†),
due to an increase in charge separation in the excited state,
resulting in an increased dipole moment. The observed solva-
tochromic effects for the azo derivatives 7 (7a/7b) result from
an internal charge transfer after excitation, which causes a
highly polar charge separated emission state that is stabilized
most effectively by the polar solvents.18 When comparing the
stoke shift in all the solvents (Tables 1 and 2 in the ESI†), it
gradually increases from non-polar to polar solvents; such a
behaviour is consistent with the stabilization of the highly
polar emitting exited state by polar solvents, i.e. the electronic
redistribution occurring upon excitation.19
3. Results and discussion
3.1 Synthesis
We have synthesized two different biphenyl based azo mole-
cules 7 (7a and 7b) consisting of two different urea moieties
such as octyl (6a) and tert-butylcarbazate (6b), and the syn-
thetic routes of all the compounds are depicted in Scheme 1.
The compounds 2 ,16 3 15 and 5 17 were synthesized by pre-
viously reported procedures and further, compound 5 was
reacted with octylamine and tert-butylcarbazate to give the
corresponding urea 6 (6a/6b). Finally compound 3 was Suzuki
coupled with compounds 6 (6a/6b) to afford compounds 7
(7a/7b) with yields of 67% (7a) and 65% (7b). All the com-
pounds were well characterized by 1H, 13C NMR and mass
spectra and all spectral data are given in the ESI.†
3.3 Photoisomerization
UV/Vis spectroscopy is further used to study the photorespon-
siveness of the incorporated azobenzenes.20 The photoisomeri-
zation of 7 in THF is studied upon irradiation with 365 nm UV
light. The absorption spectra are recorded over different time
intervals until photostationary states are reached (Fig. 3).
Photoisomerization from trans- to cis-azobenzene 7a in THF
(5 × 10−5 M) displayed absorption bands at 277 nm and
370 nm, attributed to the π–π* transition of the cis and trans
3.2 Photophysical properties
The absorption and PL spectra of biphenyl based azo com- isomers respectively and the band at 484 nm corresponds to
pounds 7a and 7b in THF are somewhat different to one the n–π* transition of the cis isomer. UV irradiation causes a
another (Fig. 2), due to the presence of different functional progressive trans to cis isomerization as noted by the decrease
groups ((3-octylurea) 6a and (3-tert-butylcarbamateurea) 6b, of the 370 nm band and the increase of the 277 nm and
Scheme 1). Both the molecules exhibit two absorption 484 nm bands, with the photostationary state (PSS) being
Photochem. Photobiol. Sci.
This journal is © The Royal Society of Chemistry and Owner Societies 2015

View Article Online
Photochemical & Photobiological Sciences
Paper
with maxima at 383 nm (7a) and 392 nm (7b) in pure THF
solutions, whereas with an increase in the percentage of the
water content from 10 to 30% (v/v), gradually the emission
intensity increases with a red shift similar to that of the
absorption spectra. However the fluorescence intensity signifi-
cantly increased in the 40% water–THF mixture, and red
shifted to 469 nm (7a) and 468 nm (7b). Furthermore, the
addition of the water fraction leads to a decrease in the absorp-
tion and emission intensity. We believe that in aqueous media
intramolecular rotations are restricted because of the for-
mation of nanoaggregates, which block the non-radiative chan-
nels and populate the radiative excitations, thereby making the
molecules emissive in their aggregated state. When the water
content increases, the solute molecules can aggregate into two
kinds of nano-particle suspensions: one being crystal particles
and the other one being amorphous particles. The former
results in an enhancement in the PL intensity, while the latter
leads to a reduction in emission intensity.22 A decrease in the
emission intensity upon the addition of more than 40% of the
water fraction may be caused by the smaller number of emit-
ting molecules which govern the fluorescence enhancement
due to the restriction-in-rotation process and thereby the net
fluorescence intensity is decreased for more than 40% of the
water–THF mixture.23 Further the fluorescence change that
ensued from aggregation is indicated by the plot of
fluorescence peak intensity against the corresponding water
fractions (Fig. S4†).
Fig. 3 Monitoring 7 as it shrinks upon UV irradiation (λ = 365 nm). Time
evolution of the UV/Vis absorption spectra of a solution of 7 until the
PSS was reached, (a). 7a in THF (in ca. 20 s, 5 × 10−5 M), (b). 7b in THF (in
ca. 20 s, 1 × 10−5 M).
reached after 20 s (Fig. 3a). A clear isosbestic point was also
observed at 330 nm and 448 nm. Similar reactions were also
observed for 7b (1 × 10−5 M), where the absorption bands at
293 nm and 375 nm are attributed to the π–π* transition of the
cis and trans isomers respectively, while the band at 482 nm is
attributed to the n–π* transition of the cis isomer. UV
irradiation causes a progressive trans to cis isomerization as
noted by the decrease in the intensity of the π–π* band at
375 nm as the reaction proceeded, whereas the bands at
293 nm and 482 nm are raised simultaneously, with clear iso-
sbestic points at 331 nm and 457 nm. The photostationary
state (PSS) was reached after 20 seconds (Fig. 3b). A similar
trend was also reported by Zhang,20a Elbing,20b and Liu et al.21
Moreover, these two compounds
7 (7a/7b) in 40%
H2O : THF exhibit higher fluorescence intensity and bathochro-
mic shift when compared to those in the pure THF solution.
The 40% water–THF system has unique spectral properties in
the aggregation studies. Further increasing the concentration
of water by 50 to 70% decreases the absorption maxima. This
is due to the formation of crystalline particles of the aggre-
gates. This can be confirmed by the electron diffraction
microscopy analysis of the aggregates formed in the solvent
mixtures with the 40% water fraction displaying many clear
diffraction spots (Fig. 5, Fig. S5 in the ESI†). The studies show
that the self-assembled molecules of 7a and 7b in 40%
H2O : THF yield nanosheets. The thickness of the nanosheets
is found to be approximately 30 nm for 7a and 20 nm for 7b
3.4 Aggregation studies
To determine whether the compounds have aggregation-
induced emission properties, the UV absorption and PL emis-
sion behaviours of their diluted mixtures were studied in
different water–THF fractions. Compound 7 (7a/7b) is freely
soluble in THF, and thereby the absorption and emission be-
haviour of the compound is studied through the aggregation
process by increasing the percentage of water in the THF solu-
tion (Fig. S3 and S4 in the ESI†). Fig. S3† shows the absorption
behaviour of 7 (7a/7b) in pure THF and in THF/water mixtures.
Interestingly, the increasing percentage of water (0–40%) leads
to an increase in absorbance intensity and further increasing
the percentage of water (50–70%) leads to a decrease in absor-
bance. The PL spectra (Fig. 4) do not obviously show emissions
Fig. 4 Emission spectra of compound 7a (a) and 7b (b) in different
THF : H2O fractions excited at 330 nm and 331 nm respectively.
Fig. 5 TEM image of self-assembled molecules of 7a (a) and 7b (b) in
the 40% water–THF mixture.
This journal is © The Royal Society of Chemistry and Owner Societies 2015
Photochem. Photobiol. Sci.

View Article Online
Paper
Photochemical & Photobiological Sciences
(Fig. S6 in the ESI†). Furthermore, the self assembly of 7
(7a/7b) was confirmed by theoretical studies (Scheme 3; ESI†).
From these studies, we observed the formation of a sheet like
(7) morphology through hydrogen bonding upon aggregation.
The formation of hydrogen bonding was confirmed by the
FT-IR spectra (Fig. S7 in the ESI†) before and after irradiation
of light (365 nm).
3.5 Electrochemical experiments
Fig. 6 shows the cyclic voltammetry (CV) diagrams of the two
azo compounds 7 (7a/7b) in THF solution in the presence of
0.1 M Bu4NBr as the supporting electrolyte with a glassy
carbon working electrode, a platinum wire as the counter
electrode and Ag/AgCl as the reference electrode at a scan rate
of 50 mVs−1. The cyclic voltammogram of the two azo com-
pounds were recorded before and after light irradiation. From
this figure (Fig. 6), we observed that both the molecules (trans
form) exhibit good electrochemical behaviour. The oxidation
and reduction peaks of 7a are observed at −0.63, −0.21 V and
−0.346 V and those of 7b are at −0.67, −0.237 V and −0.25 V.
Further irradiation of light at 365 nm on 7 leads to a decrease
in both the anodic and the cathodic peak current.24 However,
on irradiation with visible light, the original CV curves are
effectively reproduced for both the compounds 7 (7a/7b).
Fig. 7 DFT-computed molecular frontier orbitals of 7b.
absorption band being accompanied by a charge transfer
nature of the π–π* transition from the HOMO to the LUMO
orbital through the space mechanism. In cis-DPOU (7a) the
HOMO is localized over both the biphenyl rings and the
LUMO is spread mainly over the di azo linkage. In trans-DPOU
(7a) the HOMO is localized over on the both the biphenyl rings
with the little contribution from the di-azo group and the
LUMO is predominantly spread over on the di-azo linkage with
little contribution from diphenyl units (Fig. S8 in the ESI†).
This is the cause for the change in the energy gap of the cis
and trans-DPOU (7a) isomers. The whole pi moiety behaves as
a HOMO and the biphenyl unit with a diazo unit behaves as a
LUMO in trans-DPBHU (7b) whereas in cis-DPBHU (7b) the
HOMO is localized on the pi moiety and the diazo unit
behaves as a LUMO with little contribution from one of the
benzene rings (Fig. 7). These electron density changes are
attributed to the changes in the absorption behavior of the
isomers. These calculated results well coincide with the experi-
mentally obtained results.
The TD-DFT calculations were carried out using the above
mentioned sets with the optimized geometries. The absorption
wavelengths and their corresponding oscillator strengths are
given in Table 3 in the ESI.† In this the cis form (7a) has three
transitions: the absorption at 466.28 nm is due to the
HOMO−1 to LUMO transition and those at 355.40, and
260.60 nm are mainly due to the HOMO to LUMO transition.
The oscillator strength of the transitions shows that these are
strong transitions. In the case of the trans form (7a) the absor-
bance wavelengths are 443.17, 374.57 and 274.39 nm, the oscil-
lator strength of the 443.17 nm transition is weak whereas the
other transitions are relatively strong. The difference in the
oscillator strength of the cis and trans isomers is related to the
different photophysical behaviors of the isomers. The tran-
sitions of cis and trans forms (7b) occur at 474.35, 363.17 and
257.11 nm and 454, 367.99 and 261 nm respectively. The tran-
sitions at 474.35 and 454 nm for cis and trans isomers are due
to the HOMO−1 to LUMO transition and the other transitions
are related to the HOMO–LUMO transitions. In the case of the
cis form the oscillator strength of the 474.35 nm transition is
strong when compared to the trans form; the oscillator
strength of the trans form is too weak.
3.6 Quantum chemical calculation
To investigate the geometric and electronic properties of all
compounds 7 (7a/7b), quantum chemical calculations have
been performed using the Gaussian 03 program.25 The geome-
tries of all the compounds were optimized by using the B3LYP/
6-31G (d, p) functions. TDDFT calculations were carried out
using the optimized geometries, subsequently the frontier
molecular orbitals were plotted. The calculated HOMO and
LUMO of 7 are illustrated in Fig. 7 (7b) and Fig. S8 (7a, see in
the ESI†). The electron density in the HOMOs of all the com-
pounds is mainly located on the entire pi moiety with low
coefficients on the azo group, that is present in the middle of
the compound 7 (7a/7b) and the LUMO is spread mainly over
the di azo linkage. The electron transition from HOMO to
LUMO in all compounds is directly through intramolecular
charge transfer (ICT) interactions and also due to the lowest
Fig. 6 Cyclic voltammograms of 7(7a/7b) before (solid line) and after
(dotted line) irradiation with a wave guided 365 nm light for 20 s. The
data were obtained from measurements of 7(7a/7b) immersed in 0.1 M
Bu4NBr with a potential sweep rate of 50 mV between 0.0 and −1.1 V.
Photochem. Photobiol. Sci.
This journal is © The Royal Society of Chemistry and Owner Societies 2015

View Article Online
Photochemical & Photobiological Sciences
Paper
strands: complexation induced nonsymmetry and dynamic
behaviour, J. Org. Chem., 2008, 73, 6369–6377.
4. Conclusions
In this work, we have successfully synthesized two different
biphenyl based azo molecules bearing a long alkyl chain and
tert-butyl at the peripheries. These two molecules show signifi-
cant photophysical properties, photoresponsive properties and
also exhibit self-assembly through hydrogen bonding upon
aggregation to form a sheet like morphology. The electro-
chemical and DFT theories reveal that both the compounds
exhibit intramolecular charge transfer of trans and cis isomers
upon excitation.
5 (a) G. B. W. L. Ligthart, H. Ohkawa, R. P. Sijbesma and
E. W. Meijer, Complementary quadruple hydrogen bonding
in supramolecular copolymers, J. Am. Chem. Soc., 2005,
127, 810–811; (b) L. S. Shimizu, S. R. Salpage and
A. A. Korous, Functional materials from self-assembled bis-
urea macrocycles, Acc. Chem. Res., 2014, 47, 2116–2127.
6 (a) P. Brocorens, M. Linares, C. G. Duhayon, R. Guillot,
B. Andrioletti, D. Suhr, B. Isare, R. Lazzaroni and
L. Bouteiller, Conformational plasticity of hydrogen
bonded bis-urea supramolecular polymers, J. Phys. Chem.
B, 2013, 117, 5379–5386; (b) K. Yabuuchi, E. M. Owusu and
T. Kato, A new urea gelator: incorporation of intra-and
intermolecular hydrogen bonding for stable 1D self-assem-
bly, Org. Biomol. Chem., 2003, 1, 3464–3469.
7 (a) A. Lendlein, H. Jiang, O. Junger and R. Langer, Light-
induced shape-memory polymers, Nature, 2005, 434, 879–
882; (b) J. Lee, C. Y. Koh, J. P. Singer, S. Jeon, M. Maldovan,
O. Stein and E. L. Thomas, 25th anniversary article: ordered
polymer structures for the engineering of photons and
phonons, Adv. Mater., 2014, 26, 532–569; (c) S. Yagai,
M. Yamauchi, A. Kobayashi, T. Karatsu, A. Kitamura,
T. Ohba and Y. Kikkawa, Control over hierarchy levels in
the self-assembly of stackable nanotoroids, J. Am. Chem.
Soc., 2012, 134, 18205–18208; (d) V. Balzani, M. C. Leon,
A. Credi, B. Ferrer, M. Venturi, A. H. Flood and
J. F. Stoddart, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 1178–
1183.
Acknowledgements
This work was funded by the Department of Science and Tech-
nology, New Delhi, India (Grant No. SR/F/1584/2012-13) and
also supported by the Council of Scientific and Industrial
Research, New Delhi, India (Grant No. 01(2540)/11/EMR-II).
Notes and references
1 (a) L. Brunsveld, B. Folmer, E. Meijer and R. Sijbesma,
Supramolecular polymers, Chem. Rev., 2001, 101, 4071–
4097; (b) T. Aida, E. Meijer and S. Stupp, Functional supra-
molecular polymers, Science, 2012, 335, 813–817;
(c) R. R. Sinden, DNA Structure and Function, Academic
Press, Inc., New York, 1994; (d) J. M. Lehn, Supramolecular
polymer chemistry-scope and perspectives, Polym. Int.,
2002, 51, 825–839.
2 (a) L. J. Prins, D. N. Reinhoudt and P. Timmerman, Non-
covalent synthesis using hydrogen bonding, Angew. Chem.,
Int. Ed., 2001, 40, 2382–2426; (b) G. A. Jeffrey, An introduc-
tion to hydrogen bonding, Oxford University Press, Oxford,
1997; (c) C. B. Aakeroy and K. R. Seddon, The hydrogen
bond and crystal engineering, Chem. Soc. Rev., 1993, 22,
8 H. M. D. Bandara and S. C. Burdette, Photoisomerization
in different classes of azobenzene, Chem. Soc. Rev., 2012,
41, 1809–1825.
9 S. Dawn, M. B. Dewal, D. Sobransingh, M. C. Paderes,
A. C. Wibowo, M. D. Smith, J. A. Krause, P. J. Pellechia and
L. S. Shimizu, Self-assembled phenylethynylene bis-urea
macrocycles facilitate the selective photodimerization of
coumarin, J. Am. Chem. Soc., 2011, 133, 7025–7032.
397–407; (d) J. W. Steed and J. L. Atwood, Supramolecular 10 Y. C. Wang, J. Wu, Y. Li, J. Z. Du, Y. Y. Yuan and J. Wang,
Chemistry, John Wiley and Sons Ltd, Chichester, 2000.
3 (a) P. S. Corbin and S. C. Zimmerman, Self-association
without regard to prototropy. A heterocycle that forms
Engineering nanoscopic hydrogels via photo-crosslinking
salt-induced polymer assembly for targeted drug delivery,
Chem. Commun., 2010, 46, 3520–3522.
extremely stable quadruply hydrogen-bonded dimers, 11 J. Gao, Y. He, H. Xu, B. Song, X. Zhang, Z. Wang and
J. Am. Chem. Soc., 1998, 120, 9710–9711; (b) P. S. Corbin
and S. C. Zimmerman, Complexation-induced unfolding of
heterocyclic ureas: A hydrogen-bonded, sheet like heterodi-
X. Wang, Azobenzene-containing supramolecular polymer
films for laser-induced surface relief gratings, Chem.
Mater., 2007, 19, 14–17.
mer, J. Am. Chem. Soc., 2000, 122, 3779–3780; (c) T. Park, 12 (a) J. Gao, Y. He, F. Liu, X. Zhang, Z. Wang and X. Wang,
S. C. Zimmerman and S. Nakashima, A highly stable
quadruply hydrogen-bonded heterocomplex useful for
supramolecular polymer blends, J. Am. Chem. Soc., 2005,
127, 6520–6521.
4 (a) T. Park, E. M. Todd, S. Nakashima and
S. C. Zimmerman, A quadruply hydrogen bonded hetero-
Azobenzene-containing supramolecular side-chain polymer
films for laser-induced surface relief gratings, Chem.
Mater., 2007, 19, 3877–3881; (b) D. Gust, J. Andreasson,
U. Pischel, T. A. Moore and A. L. Moore, Data and signal
processing using photochromic molecules, Chem.
Commun., 2012, 48, 1947–1957.
complex displaying high-fidelity recognition, J. Am. Chem. 13 (a) X. Liang, X. Yue, Z. Dai and J. I. Kikuchi, Photorespon-
Soc., 2005, 127, 18133–18142; (b) Y. Yang, J. F. Xiang,
M. Xue, H. Y. Hu and C. F. Chen, Supramolecular substi-
tution reactions between hydrazide-based molecular duplex
sive liposomal nanohybrid cerasomes, Chem. Commun.,
2011, 47, 4751–7453; (b) Z. Yu and S. Hecht, Reversible and
quantitative denaturation of amphiphilic oligo(azobenzene)
This journal is © The Royal Society of Chemistry and Owner Societies 2015
Photochem. Photobiol. Sci.

View Article Online
Paper
Photochemical & Photobiological Sciences
foldamers, Angew. Chem., Int. Ed., 2011, 50, 1640–1643; 21 (a) N. Liu, Z. Chen, D. R. Dunphy, Y. B. Jiang, R. A. Assink
(c) I. Willerich and F. Grohn, Photoswitchable nanoassem-
blies by electrostatic self-assembly, Angew. Chem., Int. Ed.,
2010, 49, 8104–8108.
and C. J. Brinker, Photoresponsive nanocomposite formed
by self-assembly of an azobenzene-modified silane, Angew.
Chem. Int. Ed., 2003, 42, 1731–1734.
14 (a) O. Yaroshchuk and Y. Reznikov, Photoalignment of 22 Z. Y. Yang, Z. G. Chi, T. Yu, X. Q. Zhang, M. N. Chen,
liquid crystals: basics and current trends, J. Mater. Chem.,
2012, 22, 286–300; (b) W. Wu, L. Yao, T. Yang, R. Yin, F. Li
and Y. Yu, NIR-light-induced deformation of cross-linked
liquid-crystal polymers using upconversion nanopho-
sphors, J. Am. Chem. Soc., 2011, 133, 15810–15813.
B. J. Xu, S. W. Liu, Y. Zhang and J. R. Xu, Triphenylethylene
carbazole derivatives as a new class of AIE materials with
strong blue light emission and high glass transition temp-
erature, J. Mater. Chem., 2009, 19, 5541–5546.
23 (a) V. Bhalla, S. Kaur, V. Vij and M. Kumar, Mercury-modu-
lated supramolecular assembly of a hexaphenylbenzene
derivative for selective detection of picric acid, Inorg.
Chem., 2013, 52, 4860–4865; (b) S. Dong, Z. Li and J. Qin,
New carbazole-based fluorophores: synthesis, characteri-
zation, and aggregation-induced emission enhancement,
J. Phys. Chem. B, 2009, 113, 434–441.
24 Z. Wang, A. M. Nygard, M. J. Cook and D. A. Russell, An
evanescent-field-driven self-assembled molecular photo-
switch for macrocycle coordination and release, Langmuir,
2004, 20, 5850–5857.
15 E. Busseron, J. Lux, M. Degardin and J. Rebek Jr., Synthesis
and recognition studies with a ditopic, photoswitchable
deep cavitand, Chem. Commun., 2013, 49, 4842–4844.
16 D. C. Barman, P. Saikia, D. Prajapati and J. S. Sandhu,
Heterogeneous permanganate oxidations. a novel method
for the deamination using solid supported iron-permanga-
nate, Synth. Commun., 2002, 32, 3407–3412.
17 A. Yokoyama, T. Maruyama, K. Tagami, H. Masu,
K. Katagiri, I. Azumaya and T. Yokozawa, One-pot synthesis
of cyclic triamides with a triangular cavity from trans-
stilbene and diphenylacetylene monomers, Org. Lett., 2008, 25 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
10, 3207–3210.
M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr.,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas,
D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith,
M. A. A. Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian 03,
Revision E.01, Gaussian, Inc., Wallingford, CT, 2004.
18 M. H. Habibi, A. Hassanzadeh and A. Z. Isfahani, Spectro-
scopic studies of solophenyl red 3BL polyazo dye tautomer-
ism in different solvents using UV–visible, 1H NMR and
steady-state fluorescence techniques, Dyes Pigm., 2006, 69,
93–101.
19 J. Y. Si, K. Yang, M. Y. Jeong, H. C. Ahn, S. J. Jeon and
B. R. Cho, Bis-1,4-(p-diarylaminostryl)-2,5- dicyanobenzene
derivatives with large two-photon absorption cross-sec-
tions, Org. Lett., 2003, 5, 645–648.
20 (a) J. Zhang, J. K. Whitesell and M. A. Fox, Photoreactivity
of self-assembled monolayers of azobenzene or stilbene
derivatives capped on colloidal gold clusters, Chem. Mater.,
2001, 13, 2323–2331; (b) M. Elbing, A. Blaszczyk,
C. V. Hanisch, M. Mayor, V. Ferri, C. Grave, M. A. Rampi,
G. Pace, P. Samori, A. Shaporenko and M. Zharnikov,
Single component self-assembled monolayers of aromatic
azo-biphenyl: influence of the packing tightness on the
SAM structure and light-induced molecular movements,
Adv. Funct. Mater., 2008, 18, 2972–2983.
Photochem. Photobiol. Sci.
This journal is © The Royal Society of Chemistry and Owner Societies 2015