Synthesis, structure, spectral properties and DFT quantum chemical calculations of 4-aminoazobenzene dyes. Effect of intramolecular hydrogen bonding on photoisomerization

Article abstract of DOI:10.1016/j.saa.2016.12.005

In this paper three different “push-pull” 4-aminoazobenzene dyes have been synthesized in order to characterize their photochromic behavior in different solvents. The molecular geometry was optimized by DFT/B3LYP functional combined with the standard 6–31

Full text of DOI:10.1016/j.saa.2016.12.005

Accepted Manuscript
Synthesis, structure, spectral properties and DFT quantum
chemical calculations of 4-aminoazobenzene dyes. Effect of
intramolecular hydrogen bonding on photoisomerization
Anton Georgiev, Emil Bubev, Deyan Dimov, Denitsa Yancheva,
Ivaylo Zhivkov, Jozef Krajčovič, Martin Vala, Martin Weiter,
Maria Machkova
PII:
DOI:
Reference:
S1386-1425(16)30721-1
doi: 10.1016/j.saa.2016.12.005
SAA 14818
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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Revised date:
Accepted date:
30 October 2016
27 November 2016
7 December 2016
Please cite this article as: Anton Georgiev, Emil Bubev, Deyan Dimov, Denitsa Yancheva,
Ivaylo Zhivkov, Jozef Krajčovič, Martin Vala, Martin Weiter, Maria Machkova , Synthesis,
structure, spectral properties and DFT quantum chemical calculations of
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ACCEPTED MANUSCRIPT
Synthesis, Structure, Spectral Properties and DFT Quantum Chemical Calculations
of 4-aminoazobenzene Dyes. Effect of Intramolecular Hydrogen Bonding on
Photoisomerization
Anton Georgiev a,*, Emil Bubev a, Deyan Dimov b, Denitsa Yancheva c, Ivaylo Zhivkov b,d, Jozef
Krajčovič d, Martin Vala d, Martin Weiter d, Maria Machkova a
aDepartment of Organic and Physical Chemistry, 1756 Sofia, 8 St. Kliment Ohridski Blvd, University of
Chemical Technology and Metallurgy, Bulgaria.
bDepartment of Nanostructured Materials and Technology, 1113 Sofia, 109 Acad. G. Bonchev Blvd,
Institute of Optical Materials and Technologies, Bulgarian Academy of Science, Bulgaria.
cLaboratory of Structural Organic Analysis, Sofia 1113, 9 Acad. G. Bonchev Blvd, Institute of Organic
Chemistry with Centre of Phytochemistry, Bulgarian Academy of Science, Bulgaria.
dMaterials Research Centre, Faculty of Chemistry, Purkyňova 118, Brno, Brno University of
Technology, Czech Republic.
a *Corresponding author: Anton Georgiev, Department of Organic and Physical Chemistry, 1756 Sofia
8 St. Kliment Ohridski Blvd, University of Chemical Technology and Metallurgy, Bulgaria, Tel: +359
887 94 06 83 e-mail: antonchem@abv.bg
Abstract
In this paper three different "push-pull" 4-aminoazobenzene dyes have been synthesized in order to
characterize their photochromic behavior in different solvents. The molecular geometry was optimized
by DFT/ B3LYP functional combined with the standard 6-31+G(d,p) basis set for trans (E) and cis (Z)
isomers and the energy levels of HOMO and LUMO frontier orbitals were computed using IEFPCM
solvation in CHCl3 and DMF. The calculated results were compared to the experimental optical band
gap and HOMO values of cyclic voltammetry. The intramolecular six-membered hydrogen bond was
formed in both isomers of the synthesized dyes. The thermodynamic parameters such as total electronic
energy E (RB3LYP), enthalpy H298 (sum of electronic and thermal enthalpies), free Gibbs energy G298
(sum of electronic and thermal free Gibbs energies) and dipole moment μ were computed for trans (E)
and cis (Z) isomers in order to estimate the ΔEtrans→cis, Δμtrans→cis, ΔHtrans→cis, ΔGtrans→cis and ΔStrans→cis
values. The NBO analysis was performed in order to understand the intramolecular charge transfer and
energy of resonance stabilization. The solvatochromic shift was evaluated by UV-VIS spectroscopy in
CHCl3 (nonpolar), EtOH (polar protic) and DMF (polar aprotic) solvents to determine the electron
withdrawing and donating properties of the substituents on electron transitions energy. Through the
increasing solvent polarity a strong bathochromic shift is observed. The photoisomerization experiments
have been performed in two solvents CHCl3 (nonpolar) and DMF (polar aprotic) by UV light irradiation
with λ = 365 nm at equal concentrations and time of illuminations. The electronic spectra were
computed by TD-DFT after geometry optimization using IEFPCM solvation in CHCl3 and DMF. The
degree of photoisomerization was calculated for the three azo chromophores in both solvents. By using
first derivative of the UV-VIS spectra it was possible to resolve the overlapped electron transitions
absorption bands. The existing intramolecular hydrogen bond in the azo chromophores was discussed in
relation to the isomerization mechanisms and relative stability of the cis (Z) isomers.
Keywords: 4-aminoazobenzene dyes; DFT quantum chemical calculations; HOMO and LUMO energy
levels; solvatochromism; photoisomerization; UV-VIS spectroscopy;

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1. Introduction
Azobenzene derivatives (ABs) contain chromophore -N=N- (azo) group, which undergoes reversible
trans-cis-trans photoisomerization cycle. The main characteristic UV-VIS absorption bands are at λmax
~ 440 nm related to the n→π* transition or S1 excited state and at λmax ~ 340 nm related to the π→π*
transition or S2 excited state [1,2]. The trans-cis isomerization can be driven by ultraviolet (UV) light
irradiation usually at λ ~ 365 nm, the reverse cis-trans isomerization occurs spontaneously in the dark,
under thermal treatment or by exposure to visible (VIS) light. The trans (E) isomer is more
thermodynamically stable than cis (Z) one due to higher energy of cis (Z) twist conformation related to
the orbitals repulsion between aromatic rings and substituents [2,3]. Geometrically, the cis-ABs are
characterized by 90° twisting of the aromatic rings relative to plane of the azo bond and 44 % reduction
in the distance between the 4 and 4' positions ~ (trans and ~ 5 5 (cis) [4]. Fig. 1 illustrates the
photoisomerization of ABs and the potential energy levels of electron transitions on ground (singlet)
state and first S1 and second S2 excited states.
Fig. 1. Schematic representation of photoisomerization of the AB derivatives and the potential energy
levels of ground (singlet) state and first S1 and second S2 excited states, adapted from Y. M. Riyad et al.
[3].
According to the spectral properties, nature and position of the substituents in the aromatic rings
azobenzene derivatives can be classified (by Rau) [5] as: (i) azobenzene derivatives (ABn); (ii)
aminoazobenzene derivatives (aAB); (iii) pseudostilbenes (pSB). Fig. 2 presents the nature and position
of the functional groups in substituted ABs.
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Fig. 2. General types of the substituted ABs.
The substitution pattern of the AB molecules determines their photochemical behavior. Four
mechanisms have been proposed in order to understand the photoisomerization pathway: (i) rotation;
(ii) inversion; (iii) concerted inversion; (iv) inversion-assisted rotation. The rotational pathway involves
rupture of the N=N π-bond to allow free rotation about the N–N σ-bond. The rotation changes the C–N–
N–C dihedral angle while the N=N–C angle remains fixed at ~120 °. In the inversion mechanism, one
N=N–C angle increases to 180o while the C–N=N–C dihedral angle remains fixed at 0 °, where in a
transition S0→S1 (n→π* state leads to the inversion around one of nitrogen in the same molecular
plane and excitation to the S0→S2 (π→π* state provokes isomerization via rotation one of the phenyl
ring around the N=N double bond. For isomerization to occur by concerted inversion, both N=N–C
bond angles increase to 180 ° generating a linear transition state. In inversion-assisted rotation, large
changes in the C–N=N–C dihedral angle and smaller but significant changes in the N=N–C angles
occur simultaneously [1,6,7]. The described mechanisms have been confirmed by quantum mechanical
calculations and ultrafast time-resolved spectroscopy [8,9]. Multiple isomerization pathways have often
been involved to explain experimental photochemical behavior.
Several factors are known to affect the photoisomerization: (i) The nature of the substituents in the
aromatic rings. The presence of electron withdrawing (EW) groups like -CHO, -CN, -NO2 decrease the
energy of the electron transitions, where the π→π* and n→π* bands are well separated (ii) The solvent
properties are important for the photochromic behavior due to the dipole-dipole interaction with the azo
dye molecules, leading to solvatochromic shifting. Nonpolar solvents weakly interact with the
nonbonding electron pairs of the substituents and azo nitrogens, therefore they affect slightly the
isomerization. However, the polar solvents (protic or aprotic) interact with the azo chromophores and a
bathochromic shift (red shift) is observed related to the decrease of the electron transition energy. (iii)
Intermolecular hydrogen bonds can be formed between two or more azo molecules or between the
solvent and azo chromophores. The intramolecular hydrogen bonds can be formed as: a) proton transfer
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between azo nitrogens and ortho phenol hydroxyl (or other proton donors) group. In this case, the
intramolecular hydrogen bonding or H-transfer depend on the solvent polarity, pH medium and
electronic properties of the other substituents; b) polar interaction without proton transfer between azo
nitrogens and ortho proton donors like –CH2OH, –CH2NH2 , where the intramolecular hydrogen bonds
are not affected by the solvent properties. The properties and position of the substituents respect to the
azo nitrogens strongly affect the photoisomerization, which change the energy of electron transitions.
(iv) The irradiation wavelength applied for π→π* (S0→S2) transitions in UV region is usually at λ ~
365 nm, while for n→π* (S0→S1) transitions in VIS range is approximately at λ ~ 450 - 550 nm. The
intensity and time of irradiation influence the degree of isomerization. (v) Higher temperature favors the
cis→trans conversion, due to the thermodynamically more stable trans (E) isomer [1,10,11,12,13].
In previous reported works there are many useful quantum chemical calculations on azo chromophores
in order to explain their spectral properties and molecular geometry [11,12,13,14,15]. By using the
density-functional theory (DFT) with B3LYP hybrid functional the isomerization mechanisms have
been proved for different types ABs. A. Mahmood et al. have compared the capability of B3LYP hybrid
functional with four different basis set (6-31G, 6-31+G, 6-31+G* and 6-31+G**) to predict the electron
transitions spectra of several heterocyclic azo chromophores applied in dye sensitized solar cells
(DSSCs). They have found that 6-31+G and 6-31+G* basis sets have good fit to the experimental data
[14]. Six novel azo dyes with different π-spacers have been synthesized and investigated by S.B. Novir
et al. through time dependent density-functional theory (TD-DFT) and DFT with B3LYP hybrid
functional in order to predict π-conjugation effects, which are important for performance parameters of
DSSCs [15]. The authors have found good agreement between theoretical and experimental results.
Azo-containing materials exhibit photoactive properties with potential application as optical
photoswitches, reversible information storage, organic solar cells, bioengineering devices, nonlinear
optical materials, polyelectrolyte multilayers, liquid crystals and other photoactive and photomechanical
devices [2,4,16]. Photoinduced isomerization between trans (E) and cis (Z) isomers and the occurrence
of magnetic anisotropy are the main properties of these compounds, which determine their practical
applications. Therefore the aim of the present study was to synthesize and characterize the spectral
properties
(hydroxymethyl)phenyl)diazenyl)-5-nitrobenzonitrile
dinitrophenyl)diazenyl)phenyl)methanol (Azo-2)
of
three
"push-pull"
4-aminoazobenzene
dyes
(E)-2-((4-amino-2-
(Azo-1),
and
(E)-(5-amino-2-((2,4-
(E)-(5-amino-2-((4-bromo-2-
methylphenyl)diazenyl)phenyl)methanol (Azo-3). The specific aims were: (i) synthesis, characterization
and quantum chemical calculations by DFT/ B3LYP functional combined with the standard 6-
31+G(d,p) basis set of the synthesized azo dyes; (ii) experimental determination of HOMO and LUMO
energy levels and optical band gap by cyclic voltammetry and UV-VIS spectroscopy; (iii) investigation
of solvatochromic behavior and spectral properties through UV-VIS spectroscopy in different solvents
as well as in solid films; (iv) a photoisomerization study in nonpolar (CHCl3) and polar aprotic (DMF)
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solvents by UV-irradiation at λ = 365 nm, supported by TD-DFT quantum chemical calculations as
trans (E) and cis (Z) isomers.
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2. Experimental
2.1. Used Materials.
The 3-aminobenzyl alcohol (99 %), 2-amino-5-nitrobenzonitrile (95 %), 2,4-dinitroaniline (98 %) and
4-bromo-2-methylaniline (97 %) were purchased from Sigma Aldrich. The solvents, hydrochloric acid,
sodium nitrite, sodium carbonate and urea were supplied from local supplier Valerus Ltd. For the
spectral investigation all solvents were spectroscopy grade purity.
2.2. General Methodology for Synthesis of 4-aminoazobenzenes.
The starting substituted anilines (diazo components) were dissolved in 9 mL HClconc. and 20 ml H2O
was added. The solution was then cooled to T = 0 оС in a salt/ice bath with stirring. Sodium nitrite (0.88
g, 11 mmole) in water (12 mL) was gradually added to this solution over 15 min period at T = 0 ÷ 5 оС
with stirring. The mixture was stirred for an additional 45 min to 1 h while maintaining at temperature
of 0 ÷ 5 оС. The excess of nitrous acid was destroyed by adding urea, and the solution was filtered. The
resulting clear diazonium salt was used immediately in the azo-coupling reaction. The 3-aminobenzyl
о
alcohol (azo component) was dissolved in 12 ml acetic acid and cooled to 0 ÷ 5 С in a salt/ice bath.
The cold diazonium salt solution was added to this cooled solution over 20 min with vigorous stirring in
a drop wise manner, while maintaining the pH 4-6 by addition of saturated sodium carbonate solution.
о
The mixture was further stirred for 1 h at 0 ÷ 5 С and resulting solid was filtered, washed with cold
water, dried, and recrystallized two times in ethanol. TLC CHCl3:CH3COOH 9:1. The reactions for
preparation of azo dyes are presented on the Scheme 1.
Scheme 1. Synthetic pathway of the 4-aminoazobenzene dyes (Azo-1, Azo-2 and Azo-3).
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2.2.1. Preparation of (E)-2-((4-amino-2-(hydroxymethyl)phenyl)diazenyl)-5-nitrobenzonitrile, Azo-1.
The Azo-1 was synthesized from 2-amino-5-nitrobenzonitrile 12.26 mmole (2 g) and 3-aminobenzyl
alcohol 12.26 mmole (1.51 g), yield 43 %, m.p. 157-160 °C. ATR-FTIR cm-1: ν -OH 3404, νas -NH2
1
3329, νs -NH2 3232, ν -CH2 2980, 2930, ν -C≡N 2229, δ -NH2 1642, ν -NO2 1494, ν -N=N- 1403. H-
NMR (DMSO-d6) ppm: 6.9-8.5 (m, 6H), 5.1 (s, 2H, -NH2), 4.5 (s, 1H, -OH), 1.9 (s, 2H, -CH2).
Elemental analysis calculated for C14H11N5O3: C, 56.56; H, 3.73; N, 23.56; O, 16.15, Found: C, 55.98;
H, 3.46; N, 24.06; O, 16.85.
2.2.2. Preparation of (E)-(5-amino-2-((2,4-dinitrophenyl)diazenyl)phenyl)methanol, Azo-2.
The Azo-2 was synthesized from 2,4-dinitroaniline 10.93 mmole (2 g) and 3-aminobenzyl alcohol 10.93
mmole (1.35 g), yield 61 %, m.p. 121-124 °C. ATR-FTIR cm-1: ν -OH 3453, νas -NH2 3404, νs -NH2
3330, ν -CH2 2945, 2865, δ -NH2 1628, ν -NO2 (doublet) 1521, 1412, ν -N=N- 1388. 1H-NMR (DMSO-
d6) ppm: 6.9-8.5 (m, 6H), 5.4 (s, 3H, -NH2, -OH), 2.8 (s, 2H, -CH2). Elemental analysis calculated for
C13H11N5O5: C, 49.22; H, 3.49; N, 22.07; O, 25.21, Found: C, 48.81; H, 3.32; N, 22.20; O, 25.44.
2.2.3. Preparation of (E)-(5-amino-2-((4-bromo-2-methylphenyl)diazenyl)phenyl)methanol, Azo-3.
The Azo-3 was synthesized from 4-bromo-2-methylaniline 10.75 mmole (2 g) and 3-aminobenzyl
alcohol 10.75 mmole (1.33 g), yield 41 %, m.p. 142-146 °C. ATR-FTIR cm-1: ν -OH 3404, νas -NH2
3311, νs -NH2 3175, ν -CH2 and CH3 2981, 2930, 2867, δ -NH2 1613, ν -N=N- 1399. 1H-NMR (DMSO-
d6), ppm: 6.5-7.8 (m, 6H), 5.2 (s, 2H, -NH2), 4.8 (s, 1H, -OH), 1.9 (s, 2H, -CH2), 1.1 (s, 3H, -CH3).
Elemental analysis calculated for C14H14BrN3O: C, 52.52; H, 4.41; Br, 24.96; N, 13.12; O, 5.00, Found:
C, 53.57; H, 4.26; N, 13.45; O, 5.11.
2.3. Quantum Chemical Calculations.
The optimization of molecular geometry and electronic structure of studied molecules were performed
by GAUSSIAN 09W software package using density-functional theory DFT/ B3LYP functional
combined with the standard 6-31+G(d,p) basis set both in vacuo and solvation in CHCl3 [17]. The
interactions between the molecule and the solvent were evaluated at the same basis set by polarizable
continuum model (PCM) using the integral equation formalism variant (IEFPCM). The Natural
Bonding Orbital (NBO) and charges distribution were computed after optimization of geometry at the
same basis set using CHCl3 solvent. The frequency analysis was made at the same level of theory to
characterize the stationary points on the potential surface and to obtain thermodynamic parameters such
as absolute enthalpy (H) and Gibbs free energy (G) at 298 K. The precomputed vibrational scaling
factor 0.964 was used according to the CCCBDB (Computational Chemistry Comparison
and Benchmark Data Base) of National Institute of Standards and Technology, U.S. Department of
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Commerce [18]. The TD-DFT calculations were performed after optimization of geometry in trans (E)
and cis (Z) azo dyes at the same basis set using CHCl3 and DMF solvents.
2.4. Preparation of Thin Films.
The thin films of Azo-1, Azo-2 and Azo-3 were prepared on quartz substrates by spin coating (speed
1500 rpm) of starting DMF solutions with concentration 5×10-6 mol L-1, containing 2 % (w/w)
poly(methyl methacrylate) (PMMA) matrix. After coating, the films were subjected to the thermal
annealing for 1 h at temperature 50 oC for solvent removal.
2.5. Photoisomerization Experiment.
The photoisomerization of the azo dyes was performed in CHCl3 and DMF solutions at room
temperature (concentration 3.7×10-6 mol L-1) by 4 Watt UVL-21 Compact UV lamp λ = 365 nm. The
UV-VIS spectra were collected per 2 minutes from 0 to 20 minutes. Before investigation the samples
were stored in dark overnight at room temperature.
2.6. Spectral Measurements.
ATR-FTIR spectra of the compounds were recorded on a Bruker Tensor 27 FTIR spectrophotometer in
the range 4400-600 cm-1 with resolution 2 cm-1 at room temperature. The external reflection diamond
crystal has been used and the samples were scanned 128 times. The UV-VIS spectra have been done on
the Cary 5E- UV-VIS-NIR. The first derivative spectra were performed by Savitzky–Golay algorithm
with smoothing polynomial factor 10 and width of the peaks 7. The calibration curves of the dyes were
plotted by several points of absorption measurements in different concentrations and molar extinction
coefficients (εmax) in CHCl3, EtOH and DMF were calculated. The NMR spectra were collected on
Spinsolve benchtop NMR spectrometer with frequency 43 MHz and resolution < 0.7 Hz.
2.7. Cyclic Voltammetry Measurement.
The voltammograms (CV) were recorded on a Metrohm 797 VA trace analyzer and a 797 VA stand.
The Ag/AgCl, (3 mol L-1) KCl electrode was used as the reference electrode, the hanging mercury drop
electrode as the working electrode, and a platinum wire as the auxiliary electrode. All the voltammetric
experiments were conducted in a high purity nitrogen atmosphere at room temperature (25 ± 1o C). A
0.1 mol.L-1 sodium acetate-acetic buffer (pH 5.5 ± 0.1) solution was used as a supporting electrolyte.
The stock solutions (50 μM L-1) of Azo-1, Azo-2 and Azo-3 were prepared by dissolution of the
compounds in CHCl3. Procedure: A 10 ml volume of sodium acetate-acetic buffer and 200 μl of Azo-1,
Azo-2 and Azo-3 stock solutions (50 μM L-1) were pipetted in the electrochemical cell. Oxygen was
removed by bubbling pure nitrogen through the solution for 10 minutes. The voltamograms were
registered at the following parameters: working electrode: HMDE with a drop size of ≈ 0.15 mm2,
Voltage step 5 mV, Sweep rate 0.100 mV s-1. Adsorption was carried at -1.2 V for 60 s with continuous
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stirring at speed of 2000 rpm. The stirrer was stopped and the solution was allowed to rest for 10 s, then
the voltammograms were recorded over reduction potential range from +0.020 to -1.2 V [19,20].
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3. Results and Discussion
3.1. Background of interpretation of the results.
Three 4-aminoazobenzene dyes (Azo-1, Azo-2 and Azo-3, see scheme 1) were synthesized
incorporating -CH2OH group connected via intramolecular hydrogen bond to the -N=N- group in order
to investigate their photochromic behavior. Positions 2'- and 4'- in the phenyl ring were selected for
introduction of electron withdrawing (EW) or electron donating (ED) substituents to obtain
unsymmetrical electron ("push-pull") distribution on the molecular backbone. In Azo-1 a -C≡N group
was introduced as an EW at 2'-position and a -NO2 group as a second EW at 4'-position. The Azo-2 was
designed with two -NO2 groups as EW at 2'- and 4'- positions. Finally, Azo-3 was synthesized with two
ED groups: -CH3 (+I effect) at 2'-position and -Br (‒I and +M effects) at 4'-position. The optimized
molecular geometry of the trans (E) and cis (Z) isomers were computed at DFT/ B3LYP 6-31+G(d,p)
level of theory in vacuo and solvation using IEFPCM in CHCl3 and DMF solvent. The NBO analysis
was performed to evaluation the charge transfers (CT) and resonance energy on molecular backbone of
trans (E) and cis (Z) azo dyes. The energy of frontier highest occupied molecular
orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and Eg (energy band gap) were
computed and experimentally estimated by cyclic voltammetry and UV-VIS spectroscopy. The
solvatochromic behavior was investigated in three different solvents CHCl3 (non polar), EtOH (polar
protic) and DMF (polar aprotic) to characterize the influence of the polarity on electron transitions
energy. The photoisomerization of the azo chromophores was performed in CHCl3 and DMF at room
temperature with λ = 365 nm UV-irradiation and light intensity 4 Watt at equal concentrations and
irradiation time. The electron transitions spectra and excitation energies of trans (E) and cis (Z) azo
dyes were computed by TD-DFT using IEFPCM in CHCl3 and DMF solvent in order to understand the
electron transitions and molecular contributions, which are referred to the experimental results.
3.2. Quantum chemical calculations and solvatochromism.
The molecular backbone of trans (E) isomers has a trigonal planar geometry, whereas the -CH2OH
(hydroxymethyl) group is out of the plane and angles between tetrahedral sp3 hybridized carbon, oxygen
and hydrogens atoms are approximately 106 °. This leads to a formation of intramolecular six-
membered ring hydrogen bond with nonbonding electron pairs of the azo nitrogens. The optimized cis
(Z) isomers are nonplanar, where the two phenyl rings are twisted in respect to the C–N=N–C with
dihedral angles: –14.32 ° (Azo-1), –17.86 ° (Azo-2) and –11.49 ° (Azo-3). The calculated bond
parameters and X,Y,Z coordinates of the azo chromophores as trans (E) and cis (Z) isomers are given in
Supplementary Information Tables S.1, S.2, S.3, S.4, S.5 and S.6. On Figs. 3 and 4 the molecular
geometry optimized at DFT/ B3LYP 6-31+G(d,p) level of theory using IEFPCM in CHCl3 solvent of
the trans (E) and cis (Z) Azo-1, Azo-2 and Azo-3 are presented.
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2.
2. 5
Azo-2
Azo-1
2.
Azo-3
Fig. 3. Molecular geometry and intramolecular H-bond of trans (E) Azo-1, Azo-2 and Azo-3 isomers
optimized at DFT/ B3LYP 6-31+G(d,p) level of theory using IEFPCM in CHCl3 solvent.
-17.86 °
-14.32 °
Dihedral angle
Dihedral angle
2.234
2.238
Azo-2
Azo-1
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-11.45 °
Dihedral angle
2.193
Azo-3
Fig. 4. Molecular geometry and intramolecular H-bond of cis (Z) Azo-1, Azo-2 and Azo-3 isomers
optimized at DFT / B3LYP 6-31+G(d,p) level of theory using IEFPCM in CHCl3 solvent.
The lengths of the intramolecular hydrogen bonds in trans (E) isomers are shortened with the decrease
of the conjugation and "push-pull" character of substituents in the phenyl rings: 2.212 (Azo-1) >
2.175 (Azo-2) > 2.128 (Azo-3). This means that nonbonding electron pair of Azo-3 is weakly
conjugated with the phenyl ring and has a great capacity to donate electrons for hydrogen bonding. The
EW -C≡N and -NO2 groups of Azo-1 and Azo-2 involve the nonbonding azo nitrogens electron pairs in
n-π conjugation. For this reason, EW substituents in ortho- and para- positions decrease the electron
density and lead to weaker intramolecular hydrogen bonds. The cis (Z) azo dyes are in twisted
conformation, where the lengths of the hydrogen bonds are bigger than in the trans (E) isomers. The -
NH2 hydrogens are out of the plane in the phenyl rings and n-π, π-π conjugation of azo groups with
aromatic systems are decreased. Table 1 presents the calculated total electronic energy E (RB3LYP) and
dipole moment μ of the azo chromophores in trans (E) and cis (Z) isomers.
Table 1. Theoretical calculations of the total electronic energy E (RB3LYP) and dipole moment μ of
trans (E) and cis (Z) isomers of the azo dyes by DFT/ B3LYP 6-31+G(d,p) level of theory using
IEFPCM in CHCl3 solvent.
Compounds
E (RB3LYP) [kcal mol-1]
μ [D]
Trans
14.355
14.616
7.341
*ΔEtrans→cis **Δμtrans→cis
[kcal mol-1]
26.9829
[D]
Trans
Cis
Cis
Azo-1
Azo-2
Azo-3
-652277.1922
-722718.7037
-2104141.62
-652250.2093
-722695.1094
-2104135.533
9.139
8.975
5.594
-5.216
-5.641
-1.747
23.5944
6.08685
*ΔE = Ef cis – Ei trans; **Δμ = μf cis – μi trans
;
The ΔEtrans→cis and Δμtrans→cis were defined as difference between final state (f) cis (Z) isomers
(products of the photochemical reaction) and initial state (i) trans (E) (reactants). For Azo-1 and Azo-
2 the ΔEtrans→cis have similar values, which indicate unfavorable energetic state (higher energy) in cis
(Z) isomers. The change of the energy of Azo-3 is smaller compared to the Azo-1 and Azo-2. The
reasons are significant decrease of the conjugation on the molecular backbone and orbital repulsions
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between substituents and phenyl rings. The molecular dipole moment is a vector sum of bond dipole
moments and mainly depends on molecular geometry (relative orientation of the bonds). Less dipole
moments in cis (Z) compared to trans (E) isomers means that the bonds polarity are decreased and the
direction of vector dipoles of the bonds are opposite and canceled each other. The Δμtrans→cis values of
Azo-1 and Azo-2 show significant decrease compared to the Azo-3 due to the polarization of the
molecular backbone by EW and ED groups. The calculated thermodynamic parameters absolute
enthalpy H298 (sum of electronic and thermal enthalpies) and free Gibbs energy G298 (sum of electronic
and thermal free Gibbs energies) related to the ΔHtrans→cis, ΔGtrans→cis and ΔStrans→cis are presented in
Table 2.
Table 2. Theoretical calculations of the thermodynamic parameters of trans (E) and cis (Z) isomers of
the azo dyes by DFT/ B3LYP 6-31+G(d,p) level of theory using IEFPCM in CHCl3 solvent.
Compounds
H298 [kcal mol-1]
Trans Cis
-652113.3926 -652086.6707 -652157.1828 -652130.9842 26.7219
-722552.2235 -722526.7732 -722597.851 -722572.2002 25.4503
G298 [kcal mol-1]
Trans Cis
***ΔStrans→cis
[cal K-1 mol-1]
1.7563
*ΔHtrans→cis **ΔGtrans→cis
[kcal mol-1] [kcal mol-1]
Azo-1
Azo-2
Azo-3
26.1985
25.6507
21.1741
-0.6724
-0.3053
-2103967.922 -2103946.839 -2104009.826 -2103988.652 21.0831
*ΔH = Hf cis – Hi trans; **ΔG = Gf cis – Gi trans; ***
The parameters of H298 and G298 between cis (Z) and trans (E) isomers are given by ΔHtrans→cis and
ΔGtrans→cis, where the differences in energies of Azo-1 and Azo-2 are higher than Azo-3. The ΔHtrans→cis
>0 and ΔGtrans→cis >0 show that E→Z isomerisation is reversible photochemical reaction and does not
occur spontaneous without UV irradiation to a less stable cis (Z) isomer. The ΔStrans→cis of Azo-1
increases, while for Azo-2 and Azo-3 decrease. The ΔStrans→cis is related to the order of the macroscopic
system and probability of photoisomerization reactions to cis (Z) isomers. For Azo-1 the change of
entropy has a positive value because of higher ΔHtrans→cis and ΔGtrans→cis. This indicates that Azo-1 and
Azo-2 are more stable in trans (E) isomers, while for Azo-3 the photoisomerization to cis (Z) isomer is
favored by the kinetic factor. The computed and experimental values of vibrational bands of the
chromophores are given in Supplementary Information Tables S.7, S.8 and S.9 and Figs. S.1, S.2 and
S.3 [21]. The frequencies of the IR bands of ν -OH, ν -NH2 for cis (Z) isomers are higher compared to
the trans (E) isomers The calculated bands of ν -N=N- for trans (E) isomers are in a good agreement
with the experimental results, whereas for the cis (Z) isomers are not registered (Azo-1 and Azo-2) due
to the symmetric vibrations, where . The computed HOMO and LUMO frontier orbitals and
surface electron distributions of the azo chromophores are presented in Supplementary Information
Figs. S.4, S.5 and S.6.
The natural charges of atoms in functional groups provide information about molecular polarizability,
electronic structure and charge affects on dipole moment. The natural bond orbitals (NBO) analysis and
second order Fock matrix are carried out by considering all possible interactions between (donor) Lewis
13

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NBOs and (acceptor) non Lewis NBOs. The intramolecular charge transfers (CT) or conjugations on
molecular backbone are the main features of photochromic properties of the molecules. For each donor
NBO (i) and acceptor NBO (j), the resonance stabilization energy associated with i→j delocalization
can be evaluated as (Eq. 1),
(Eq. 1)
where qi is the donor orbital occupancy, εj and εi are diagonal elements and F(i,j) is the off diagonal NBO
Fock matrix element [21,22,23]. The natural charges and NBOs analysis of azo dyes are presented on
Fig. 5 and Table 3. Significant changes of charges and resonance energy of the azo nitrogens (-N=N-)
acceptor and lone pair (LP) -NH2 group donor from trans (E) to cis (Z) conversion for all azo dyes are
observed. The changes are decreased in order: (-N=N-) Azo-2>Azo-1>Azo-3 and (-NH2) Azo-3>Azo-
1>Azo-2. This means that conjugation and CT in cis (Z) isomers are decreased compared to the trans
(E) isomers. The stabilization energy E(2) indicates the intensive interaction between electron donors
and electron acceptors and greater the extent of conjugation of the whole system in trans (E) isomers.
This indicates that Azo-1 and Azo-2 are more stable in trans (E) isomers because of the energy of
resonance stabilization and effectively π-π conjugation The relative higher stability of Azo-3 in cis (Z)
could be explained by the lower resonance energy and π-π conjugation It should be remarked that azo
LP(1) N10 and hydroxymethyl LP*(1) H 24 (-OH) have E(2) ~ 5 – 6 kcal mol-1, while the azo LP(1)
N11 and hydroxymethyl LP*(1) H 24 (-OH) this energy is approximately zero. This fact demonstrates
that there are an intramolecular six-membered hydrogen bonds in azo dyes.
14

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-
+
-0.027
0.064
-0.189
-0.149
-0.034
Donor
0.316
-0.267
0.354
0.224
0.326
-0.204
0.097
-0.314
Acceptor
cis Azo-1
0.044
0.057
trans Azo-1
-0.291
0.064
-0.256
-0.272
0.522
-
+
-0.165
Donor
0.308
-0.142
0.094
-0.249
0.233
0.562
-0.204
-0.307
Acceptor
0.057
cis Azo-2
0.050
-0.299
trans Azo-2
0.051
0.061
-
+
0.062
-0.181
-0.181
Donor
0.219
0.079
-0.226
0.040
0.262
-0.014
0.060
-0.225
0.054
Weak donor
cis Azo-3
trans Azo-3
0.041
0.023
Fig. 5. NBO charges of trans (E) and cis (Z) azo dyes isomers calculated at DFT/ B3LYP 6-31+G(d,p)
level of theory using IEFPCM in CHCl3 solvent. Charges transfer on molecular backbones are
presented as summing the atoms charges in functional groups and phenyl rings. The charges
distribution are marked as donor and acceptor parts of the dyes connected via azo bridge.
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Table 3. Second order perturbation theory analysis of Fock matrix in NBO basis of azo dyes calculated
by DFT/ B3LYP 6-31+G(d,p) level of theory using IEFPCM in CHCl3 solvent.
E(j)-E(i)
[a.u]
F(i,j)
[a.u]
Donor
NBO (i)
Acceptor
NBO (j)
E(2)
Compounds
[kcal mol-1]
LP(1) N25
LP(2) O23 σ* C –C20
π C –C3 π* N10–N11
π N 0–N11 π* C12–C13
LP(1) N10 LP*(1) H 24
LP(1) N11 LP*(1) H 24
π* C5–C6
48.74
6.92
27.91
13.39
5.15
0.20
8.92
8.61
19.08
31.62
2.50
34.32
8.26
22.27
3.47
4.36
0.28
0.70
0.21
0.38
0.61
0.60
0.95
0.94
0.37
0.13
1.13
0.30
0.71
0.22
0.39
0.60
0.94
0.92
0.37
0.13
1.14
0.28
0.70
0.21
0.38
0.61
0.61
0.95
0.93
0.15
0.68
0.13
1.14
0.107
0.063
0.070
0.068
0.056
0.011
0.083
0.081
0.081
0.061
0.048
0.096
0.068
0.065
0.036
0.051
0.076
0.052
0.066
0.063
0.046
0.107
0.063
0.06
trans (E)
Azo-1
LP(1) N10
σ* C1–C3
LP(1) N11 σ* C12–C13
π C 4–C16 π* C3 –N33
π C 5–C17 π* N28–O30
σ C 5–C17 σ* N28–O29
LP(1) N25
LP(2) O23 σ* C –C20
π C –C2 π*N10–N11
π N 0–N11 π* C12–C13
LP(1) N10 LP*(1) H 24
LP(1) N10
LP(1) N11 σ* C12–C13
π C 4–C16 π* C –N30
π C 5–C17 π* N31–O32
σ C 5–C17 σ* N31–O32
LP(1) N25
LP(2) O23
π C1–C3
π N10–N11 π* C –C13
LP(1) N10 LP*(1) H24
LP(1) N11 LP*(1) H24
π* C4–C6
cis (Z) Azo-1
σ* C1–C3
7.54
3.52
12.65
32.56
2.33
48.43
6.99
27.15
11.39
6.10
0.23
8.84
7.23
20.72
1.14
π* C5–C6
σ* C –C20
π* N 0–N11
0.063
0.061
0.012
0.083
0.074
0.054
0.027
0.065
0.047
trans (E)
Azo-2
LP(1) N10
σ* C1–C3
LP(1) N11 σ* C12–C13
π C14–C16 π* N3 –O33
π C14–C16 σ* N3 –O34
π C15–C17 π* N –O29
π C16–C17 σ* N –O29
34.68
2.45
π* C5–C6 or
π* C4–C6
LP(1) N25
-
-
-
LP(2) O23
σ C –C2
π N10–N11 π* C –C13
LP(1) N10 σ* O 3–H24
LP(1) N10
LP(1) N11 π* C –C14
π C12–C14 π* N3 –O34
π C12–C14 σ* N3 –O33
π C16–C17 π* N –O30
σ C16–C17 σ* N –O31
σ* C –C20
σ* N 0–N11
8.37
2.78
3.03
2.60
7.70
18.79
26.43
2.12
28.30
2.31
0.71
1.26
0.38
0.89
0.94
0.37
0.15
1.15
0.14
1.15
0.17
0.069
0.053
0.034
0.044
0.077
0.081
0.059
0.044
0.061
0.046
0.120
cis (Z) Azo-2
σ* C1–C3
trans (E)
LP(1) N25
LP*(1) C6
69.37
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Azo-3
LP(2) O23
LP(2) Br29 σ* C 5–C17
LP(1) N10 σ* O 3–H24
σ* C –C20
6.80
3.04
4.71
8.40
8.05
2.30
37.21
1.42
3.52
3.15
4.43
67.66
8.14
3.04
3.13
7.67
8.80
2.88
3.08
1.49
3.42
2.45
4.36
0.71
0.85
0.90
0.97
0.96
1.26
0.05
0.73
0.52
0.52
1.06
0.17
0.71
0.85
0.88
0.94
0.41
1.26
0.41
0.73
0.52
0.52
1.06
0.062
0.045
0.059
0.081
0.097
0.048
0.068
0.032
0.042
0.039
0.061
0.119
0068
0.045
0.048
0.077
0.058
0.054
0.035
0.032
0.042
0.035
0.061
LP(1) N10
LP(1) N11 σ* C –C13
σ C1–C3 σ* N 0–N11
π N 0–N11 π* C12–C13
π C 5–C17 LP*(1) Br29
σ C30–H31 π* C14–C16
σ C30–H32 π* C14–C16
σ C30–H33 σ* C12–C14
LP(1) N25
LP(2) O23
LP(2) Br29 σ* C 5–C17
LP(1) N10 σ* O 3–H24
LP(1) N10
LP(1) N11 π* C –C14
σ C1–C2 σ* N 0–N11
π N 0–N11 π* C12–C14
π C 6–C17 LP*(1) Br29
σ C30–H31 π* C12–C14
σ C30–H32 π* C12–C14
σ C30–H33 σ* C12–C14
σ* C1–C3
LP*(1) C6
σ* C –C20
σ* C1–C3
cis (Z) Azo-3
The investigation of the electronic spectral features of the azo compounds is important in order to
estimate the electron transitions energy, optical band gap and HOMO-LUMO energy levels of frontier
orbitals. These parameters determine the possible application of the molecules for optoelectronic
devices such as optical photoswitches, optical storage information and solar cells. The energy of
HOMO frontier orbitals and the optical band gap are determined by cyclic voltammetry (Supplementary
Information, Fig. S.7) and UV-VIS spectroscopy by the absorption edge. In the cyclic voltammetry
ferrocence is used as a known reference value of -4.4 eV to calculate the energy of the HOMO by Eox
onset (Eq. 2):
HOMO = -e [EOx(onset) + 4.4] (Eq. 2)
where, Eox onset represents the experimentally determined value of the oxidation potential.
In organic molecules, the energy levels of the electronic states correspond to the energy carried by UV
or VIS radiation. At resonance, the molecules can absorb quantified energy transported by the
electromagnetic radiation, and promote an electron from the low energy molecular orbital to the higher
one [24,25]. These transitions can be measured using a UV-VIS spectrophotometer, where Eg
corresponds to the energy of the long wavelength edge of the excitation absorption band [24,25,26]. The
longest absorption wavelength λonset is used to calculate the optical gap energy, Eg, according to the
equation 3 [26]:
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(Eq 3
The LUMO energy was calculated according to the equation 4:
LUMO = HOMO + Eg (Eq. 4)
It is possible to define the optical band gap as a difference between the HOMO and LUMO of a species
created by photoexcitation of electrons. From the other hand, the electrochemical band gap is a
difference, between reduction and oxidation potentials of the species. Electrochemical reduction
requires higher energy than LUMO and also for oxidation the energy should be lower than HOMO.
These extra energy requirements for red-ox produce a difference between the electrochemical band gap
and optical band gap. The reason could be the exciton binding energy, if electrochemical band gap
energy is less than optical band gap, other factors such as solvation, dipole-dipole interactions and
electrode surface phenomena might play a role [27]. The schematic representation of the differences
between optical and electrochemical band gap is illustrated on Fig. 6.
Fig. 6. Schematic illustration between optical and electrochemical band gap, adapted from S.N.
Inamdar et al. [27].
Fig. 7 and Table 4 present the computed and experimentally estimated energy levels of HOMO and
LUMO frontier orbitals and Eg of azo dyes.
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Fig. 7. Computed and experimentally estimated in CHCl3 energy levels of HOMO and LUMO frontier
orbitals of the azo dyes.
The computed and experimental energy levels are quite different, especially for HOMO levels. This
could be attributed to absence of direct electron transitions between HOMO and LUMO. The solvation
of the molecules, dipole-dipole interactions and the surface properties of the electrode are the other
influencing factors. The computed and experimental Eg for Azo-1 and Azo-2 are close, while for Azo-3
the difference is 0.9 eV.
Table 4. Theoretically and experimentally estimated HOMO and LUMO energy levels of frontier
orbitals of the trans (E) azo compounds in CHCl3.
Theoretically calculated [eV] Eox(onset) Absorption Experimentally estimated [eV]
Compounds
[V]
edge [nm]
577
HOMO LUMO
Eg
HOMO
-4.50
-4.54
LUMO
-2.35
-2.34
Eg
Azo-1
Azo-2
Azo-3
-6.15
-6.18
-5.76
-3.58
-3.57
-2.63
2.57
2.61
3.13
-0.10
-0.14
-0.20
2.15
2.20
2.22
566
562
-4.60
-2.38
The behavior of the dyes in solvents with different polarity corresponds to interactions between the azo
dipoles and solvent molecules. The solvatochromic shifts in CHCl3, EtOH and DMF of Azo-1, Azo-2
and Azo-3 (Fig. 8) were investigated and molar extinction coefficient (εmax), oscillator strength (f) and
transition dipole moment (μ2) have been calculated form UV-VIS spectra (Table 5). Oscillator strength,
f, was calculated by the following equation (Eq. 5) [28,29]:
(Eq. 5)
where, ε molar absorptive [L mol-1 cm-1],ῡ wavenumber [cm-1]
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Transition dipole moment, μ2, was found from the equation (Eq. 6) [28,29]:
(Eq
where, is reduced Planck constant [eV·s]; me is mass of the electron [eV]; e is charge of the electron
[esu].
The electron transition π→π* (S0→S2 and n→π* (S0→S1) bands of Azo-1 and Azo-2 are well
separated due to the unsymmetrical electron distributions (push-pull) of EW groups, while in the spectra
of Azo-3 they are overlapped in a broad peak because of properties of the ED groups. In nonpolar
solvent CHCl3 Azo-1 has absorption of π→π* at λmax = 330 nm and n→π* at λmax = 462 nm. Increasing
the solvent polarity leads to a strong bathochromic shift in EtOH (π→π* at λmax = 34 nm and n→π* at
λmax = 512 nm) and DMF (π→π* at λmax = 360 nm and n→π* at λmax = 534 nm). The UV-VIS spectrum
of Azo-2 in CHCl3 is quite interesting because of two separated bands appearing in the UV region for
π→π* λmax1 = 3 nm and λmax2 = 378 nm, while in the VIS region n→π* is at λmax = 473 nm. The
existence of two bands in UV range are due to the competitive electron resonance distributions between
azo π system and ortho nitro group related to the HOMO→LUMO+ transition (Supplementary
Information Table S.14 and S.16). The solvent polarity also causes strong solvatochromic shift to lower
energy (red shift in EtOH (for π→π* λmax1 = 336 nm and λmax2 = 3 6 nm, for n→π* λmax = 512 nm) and
DMF (for π→π* λmax1 = 346 nm and λmax2 = 3 nm, for n→π* λmax = 533 nm). The overlapping π→π*
and n→π* electron transition bands of Azo-3 exhibit solvatochromic behavior by shifting of the
absorption maximum in CHCl3 at λmax = 384 nm, EtOH at λmax = 411 nm and DMF at λmax = 420 nm.
All azo compounds have shown strong solvation by dipole interactions with the solvents, which leads to
a decrease energy of the electron transitions. The electronic spectra of solid films of the azo
chromophores in PMMA matrix have similar energy to the spectra in EtOH. This means that there are
strong dipole-dipole interactions between the chromophores and matrix as well as the solid state
intermolecular dipole interactions between the azo molecules.
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Fig. 8. Normalized UV-VIS spectra of the azo dyes in CHCl3, EtOH, DMF solutions (conc. 5×10-6 mol
L-1) and solid films.
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Table 5. Experimentally calculated solvatochromic shifts (Δλ , molar extinctions (εmax), oscillator
strength (f) and transition dipole moment (μ2) of the azo chromophores in different polar solvents.
λmax [nm]
CHCl3 EtOH DMF [nm]
εmax [L mol-1 cm-1]
CHCl3 EtOH
3.145×104 2.902×104 2.483×104 0.412 0.368 0.230 2.2
f
μ2 [D]
Δλ
Compounds
DMF
CHCl3 EtOH DMF CHCl3 EtOH DMF
330
462
329
378
473
384
341
512
336
386
512
411
360
534
346
397
533
420
30
2.03 1.34
0.17 0.13
1.68 1.57
0.71 0.85
0.29 0.11
1.42 0.89
Azo-1
72 7.0257×102 2.320×103 2.03×103 0.01 0.020 0.016 0.15
17
19
60
36
2.475×104 2.403×104 2.243×104 0.324 0.308 0.280 1.64
1.126×104 1.355×104 1.205×104 0.129 0.114 0.131 1.99
8.721×102 3.84×103 1.67×103 0.01 0.035 0.013 0.16
1.343×104 2.045×104 1.272×104 0.151 0.214 0.130 0.98
Azo-2
Azo-3
It might be concluded that the oscillator strength f depends on the symmetry of frontier molecular
orbitals, where the electron transitions are allowed, moreover the electron levels are affected by the
solvents (Table 5). The f for n→π* (S0→S1) transitions are increased by the solvents polarity, whereas
for π→π* (S0→S2) are decreased. The transition dipole moment μ2 depends strongly on f and on the
nature of the molecules. It can be seen that solvation in protic polar solvent EtOH has a stronger
transition dipole moment, which may be due to the EtOH---Azo dyes hydrogen bonding.
3.3. Photoisomerization
The photoisomerization is an important characteristic of azo compounds and the energy of this
conversion depends on the previously discussed parameters - solvatochromism, f, μ2, Eg, and by energy
and intensity of the UV light and time of irradiation. The experiments were conducted for the three azo
dyes at identical concentrations and time of illumination in two solvents: CHCl3 and DMF. The aim was
to evaluate the influence of the solvents on the photochromic properties of the azo dyes in relation to the
energy of the electron transitions and intramolecular hydrogen bonding. The degree of
photoisomerization at the photostationary state, R, was evaluated according to the equation (Eq. 7) [30]:
(Eq
where A0 is the initial absorbance and A∞ is the absorbance at the photostationary state.
The absorbance bands of molar electron transitions are not narrow as atomic transitions. The reason for
this is overlapping of the ground state energy, oscillation energy and rotational energy of molecules as
well as the interaction of the dyes with the solvents. Derivative spectra enhance resolution of the
overlapped bands since changes in the gradient are examined. First derivative (FD) emphasizes band
widths, because the first derivative of most band shapes has maxima and minima near the half-height
points of the bands. The spectrum is expressed as absorbance, A, as a function of wavelength, λ, where
the zero and derivative spectra are [31]:
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Figs. 9, 10 and 11 present the absorption and first derivative (FD) UV-VIS spectra of the
photoisomerization of Azo-1, Azo-2 and Azo-3 in CHCl3 and DMF for 20 min UV-irradiation with λ =
365 nm. Table 6 summarizes the calculated values of the photoisomerization degree in both solvents.
Table 6. Experimentally estimated values of the photoisomerization degree, R, in CHCl3 and DMF.
R
CHCl3
R
DMF
Compounds
Azo-1
(π→π*) λmax = 330 nm
(n→π*) λmax = 462 nm
Azo-2
Azo-3
(π→π*) λmax = 384 nm
(n→π*)
Compounds
Azo-1
(π→π*) λmax = 360 nm 22.13
3.13
71.45
-
(n→π*) λmax = 534 nm
Azo-2
Azo-3
2.04
-
14.12
(π→π*) λmax = 420 nm 21.86
(n→π*)
23

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Fig. 9. Absorption and FD UV-VIS spectra of the photoisomerization experiment of Azo-1 in CHCl3 and
DMF performed by UV-irradiation with λ = 365 nm for 20 min.
24

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Fig. 10. Absorption and FD UV-VIS spectra of the photoisomerization experiment of Azo-2 in CHCl3
and DMF performed by UV-irradiation with λ = 365 nm for 20 min.
25

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Fig. 11. Absorption and FD UV-VIS spectra of the photoisomerization experiment of Azo-3 in CHCl3
and DMF performed by UV-irradiation with λ = 365 nm for 20 min.
The UV-VIS spectra of Azo-1 in CHCl3 (Fig. 9) with progress of the irradiation time the n→π*
transitions decreased and reached zero absorbance with 20 min period The π→π* transitions manifest
only weak changes of the absorption band. This can be confirmed by FD spectra, where the transitions
in UV range at λmax = 309 and 353 nm exhibit no significant difference. In DMF solvent the n→π* (λmax
= 534 nm) absorption intensity increasing weakly with the illumination time, while for π→π* transition
(λmax = 360 nm) the decreased intensity of the absorbance band is considerable after 20 min period. The
results are visualized from FD spectra at λmax = 320 and 384 nm. For Azo-2 in CHCl3 (Fig. 10) there
was no isomerization, while in DMF a very small changes of the absorption bands at λmax = 346, 397
and 533 nm are registered. The reasons might be: (i) the differences between lengths (L) of
intramolecular hydrogen bonds of the trans (E) and cis (Z) isomers (Figs. 3 and 4)
decrease in order: 0 065 (Azo-3) > 0 05 ( zo- 0 0 6 (Azo-1). This means that Azo-2 has
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higher stability in trans (E) isomer compared to the Azo-1. (ii) steric effects of ortho -NO2 group in cis
(Z) isomer. This is confirmed by bigger C–N=N–C dihedral angle –17.86 ° compared to the Azo-1, –
14.32 ° and Azo-3, –11.45 °. (iii) fast reversible trans-cis-trans photoisomerazation cycle; (iv) low
intensity of the applied UV-light and short time of irradiation. After derivatization, the bands at λmax
=
326, 362, 420 and 572 nm are split. Interesting results are observed for Azo-3 in CHCl3, where after 2
min period of irradiation the absorbance was decreased fast with bathochromic shift to λmax = 391 nm
(Fig. 11). After this period there were no significant absorption changes. The isomerization in DMF,
after exposure for 2 min period the causes the absorption to decrease strongly without shifting. After
this irradiation time, the absorption was changed evenly for 20 min period. The FD indicated well
separated π→π* band at 3 nm and n→π* at 449 and 478 nm.
The electronic spectra of trans (E) and cis (Z) azo dyes calculated by TD-DFT after geometry
optimization using IEFPCM in CHCl3 and DMF are presented on Fig. 12. The electron transitions
energy and molecular contributions of trans→cis are given in Supplementary Information Tables S.10,
S.11, S.12, S.13, S.14, S.15, S.16, S.17, S.18, S.19, S.20, S.21. The Trans (E) isomers of Azo-1 and
Azo-2 are characterized with strong absorption in visible region, which referred to the HOMO→LUMO
and HOMO- →LUMO transitions. However, experimental UV-VIS spectra have shown weak
absorption bands and oscillator strength (Table 5) in this range because of direct S0→S1 (n→π*
transitions are unlikely compared to the favorable S0→S2 (π→π* transitions It should be noted that the
properties of single molecule are quite different from those when the molecules interacting each other.
The π→π* bands in ultraviolet region are consist of several energetically close transitions with different
molecular contributions: HOMO-2→LUMO, HOMO→LUMO+2 and HOMO- →LUMO+ . This
indicate that EW groups decrease the energy levels due to the conjugation on molecular backbones and
intramolecular electron transfers. The spectra of cis (Z) isomers in CHCl3 demonstrate forbidden
HOMO- →LUMO (450 nm Azo-1) or weakly HOMO→LUMO+ (448 nm Azo-2) transitions in
visible region, while in DMF the absorptions are increased. The results are in good agreement with
experimental spectra, especially for Azo-1. Direct HOMO→LUMO is most likely transition of trans (E)
Azo-3 in both solvents, which is in good agreement with experimental. Strong hypsochromic shift in cis
(Z) isomer is observed related to the π→π* transition (HOMO-1→LUMO) due to the loss of
polarization (-I effect) of the Br atom and n-π conjugation of the –NH2 group on molecular backbone. In
general, strong EW groups in ortho and para positions increase the conjugation and energy levels on
intramolecular electron transfers are decreased. The ED and EW with –I and +M effects give the direct
HOMO→LUMO transition as main absorption band in visible range.
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Fig. 12. Theoretically calculated UV-VIS spectra of the trans (E) and cis (Z) azo dyes by TD-DFT/
B3LYP 6-31+G(d,p) level of theory using IEFPCM in CHCl3 and DMF solvents.
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The presence of intramolecular hydrogen bond in the azo molecules allowed discussion on probable
inversion mechanisms from trans (E) to cis (Z) isomerization, based on the quantum chemical
calculations and photoisomerization experiment, where the –N(10)=N(11)–C(12) angle (114.70 ° for
Azo-1) increases to 180 ° and dihedral angle remains fixed at 0 °. The hydrogen bond occupies the
nonbonding orbital (electron pair) of azo N(10) and the formed dipole compensate the electron density
from neighboring N(11) (Fig. 5 and Table 3). For this reason the resonance electron distribution
between –CH2O–H----N=N–C is lower in energy compared to the non-hydrogen bonding. The rotation
mechanism is unlikely due to the stability of hydrogen bonding on -N=N- group. The bond length of the
trans (E) isomer –N(10)=N(11)– (for Azo-1 is 1.2785 Å) is shorter than =N(11)–C(12) (for Azo-1 is
1.3968 Å) (Supplementary Information Tables S.1, S.2 and S.3). In the cis (Z) isomer the resulting –
N(10)=N(11)–C(12) angle is 126.10 ° (for Azo-1), the bond length between –N(10)=N(11)– is 1.2559 Å
(for Azo-1) and =N(11)–C(12) is 1.4120 Å. (Supplementary Information Tables S.4, S.5 and S.6). The
Azo-2 and Azo-3 have similar bond parameters. The absorption of the n→π* (S0→S1) transition in
nonpolar CHCl3 solvent is decreased during conversion from trans (E) to cis (Z) isomer due to the
symmetry-forbidden transition and six-membered intramolecular hydrogen bonding [32]. Only in polar
protic or aprotic solvents the absorption of n→π* transition is increased in photoisomerization, because
of solvation the nonbonding electrons by dipole-dipole interactions with the solvents. The effect of
intramolecular hydrogen bonding is not depending on the solvent properties, but only on the polarity
and geometry of molecules. It should be noticed that phenol -OH group in ortho place to the azo group
causes a strong azo-hydrazone tautomerism dependent on the solvent polarity. In our molecules such
tautomerization is impossible because of the alkoxy anion is a strong base without possibility for
resonance stabilization by conjugation to aromatic π electrons likewise the phenol -OH group [33]. The
difference of tautomerization between phenol -OH and hydroxymethyl -CH2OH groups in ortho
position to the azo group is illustrated on scheme 2.
Scheme 2. Difference of azo-hydrazone tautomerization between phenol -OH and hydroxymethyl -
CH2OH groups.
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