Multichromic Bis-Axially Extended Perylene Chromophore with Schiff Bases: Synthesis, Characterization and Electrochemical Studies

Article abstract of DOI:10.1007/s10895-016-1920-2

In the present paper a novel way of symmetric conjugation extension along molecular axes of perylene dianhydride chromophore has been devised to achieve lengthy delocalized electronic species exhibiting red shifted absorption and emission of UV-Visible ra

Full text of DOI:10.1007/s10895-016-1920-2

J Fluoresc
DOI 10.1007/s10895-016-1920-2
ORIGINAL ARTICLE
Multichromic Bis-Axially Extended Perylene Chromophore
with Schiff Bases: Synthesis, Characterization
and Electrochemical Studies
Ghulam Shabir1 & Aamer Saeed1 & Muhammad Arshad2 & Muhammad Zahid1
Received: 19 June 2016 /Accepted: 26 August 2016
#
Springer Science+Business Media New York 2016
.
.
.
Abstract In the present paper a novel way of symmetric con-
jugation extension along molecular axes of perylene
dianhydride chromophore has been devised to achieve lengthy
delocalized electronic species exhibiting red shifted absorption
and emission of UV-Visible radiations. During synthetic path-
way free amino Schiff bases of novel aldehydes with 4-amino
acetanilide have been condensed with perylene dianhydride in
quinoline at high temperature. Bis perylene diimide Schiff ba-
ses (5a-e) have been synthesized which showed absorption
Keywords Multichromic Bis-axially extended Perylene
Schiff Ba
Introduction
Functional dyes have increasing importance from previous 20–
30 years due to their use in high tech applications in everyday
life. Perylene based dyes are well suited as functional dyes due
to their wavelength span which ranges from 200 to 1500 nm.
More recently, perylene molecules and their derivatives have
attracted more and more attention in the past decade due to not
only their outstanding thermal and photochemical stabilities
[1, 2], but also their large application potential in organic opto-
electronic or electronic devices [3–6], such as field effect tran-
sistors, solar cells [7–12], light-harvesting arrays [13–17] and
light-emitting diodes [18–20]. Several perylene dyes have been
used as sensitizers in DSSCs, however, they exhibited very low
overall power conversion efficiencies [21–23]. Li and
Edvinsson reported diphenylamino-substituted perylene
monoanhydrides as sensitizers with power conversion efficien-
cy 3.9 % [24], later on they reported 6.8 % power conversion
efficiency with new perylene sensitizer bearing two thiophenol
groups [25]. The layered growth of these planar molecules
makes it possible to prepare fairly well ordered thin films of
perylenes on different substrates [26, 27]. The charge transport
and luminescence properties strongly depend on the structural
properties of these thin films [28–30]. Perylene dyes are well
known as the key chromophores among the metal-free dyes
[31], which have the advantage of low cost production because
they do not involve the precious rare earth metals. Their solu-
bility, absorption, and emission behavior can be efficiently con-
trolled using a variety of synthetic procedures, which include
functionalization of peri- or bay-positions of perylene core.
Systematic tuning of HOMO and LUMO levels of perylene
λ
max at 461–526 nm and emission at 525–550 nm. Structures
of newly obtained compounds have been confirmed by 1H and
13C-NMR studies. Cyclic voltammetric analysis of these dyes
exhibited oxidation and reduction peaks which provide indirect
evidence for their potential utility as n-type material for sensi-
tization of semiconductors in solar cells. LUMO and HOMO
energy levels were found in the range of −4.21 to −5.20 and
−6.75 to −7.57 eV, respectively.
Research Highlights
• Condensation of Schiff bases with perylene dianhydride was carried out
to bis-axially extended symmetric fluorescent chromophores.
• Schiff base extended chromophores showed yellow fluorescence while
extension with azo dyes produced pigments exhibiting reddish yellow
fluorescence in DMSO.
• Electrochemical study of these dyes was pursued by cyclic voltammetry
which showed half wave potential (E1/2) in the range + 0.229 to
−0.590 eV.
• LUMO and HOMO energy levels were found in the range of −4.21 to
−5.20 and −6.85 to −7.57 eV, respectively.
* Aamer Saeed
asaeed@qau.edu.pk; aamersaeed@yahoo.com
1
Department of Chemistry, Quaid-I-Azam University,
Islamabad 45320, Pakistan
2
Chemistry Division, Directorate of Science, PINSTECH, Nilore,
Islamabad 45320, Pakistan

J Fluoresc
dyes improve both light harvesting properties and electron in-
jection capabilities to TiO2 conduction band for obtaining the
high PCE [32].
Keeping in view the above importance and efficiency of
rylene dyes as advanced materials we have synthesized axially
bis dimides of perylenetetracarboxi-3,4,9,10-bisanhydride
with Schiff bases, which have modified their hues and also
made them potentially applicable for dye lasers and solar cells.
was added aromatic aldehyde (2a, 0.001 mol). The reaction
mixture was heated at reflux temperature for 12 h in the pres-
ence of glacial acetic acid catalyst (0.5 ml). Completeness of
reaction was observed by TLC (4:1, pet ether: ethyl acetate).
At the completion of reaction, the reaction mixture was rotary
evaporated and collected the solid residue which was further
purified by recrystallization (50:50 ethyl acetate: ethanol).
Schiff base (1a) was suspended in water (30 ml) in a 250 ml
round bottomed flask and was added conc. HCl (1.5 ml).
Started stirring and heating at 90 °C, for 2 h, until the reaction
was completed as determined from TLC of reaction mixture
(4:1, pet ether: ethyl acetate). The reaction mixture was cooled
and the deprotected the Schiff base was separated from water
on standing for 1 h, filtered and dried in oven at 60 °C. In this
way other Schiff bases were synthesized by treating aldehydes
2b-e with p-aminoacetanilide (1).
Experimental
Materials
1,6,8,12-tetrachloroperylene 3,4,9,10-dianhydride was ob-
tained from Honest Joy Holdings Limited China. 4-Nitro-4′-
hydroxy azobenzene, 4-Methoxy-4′-hydroxy azobenzene, 2-
Methoxy-4′ -hydroxy azobenzene, 4-Chloro-4′ -
hydroxyazobenzene,4-Methyl-4′-hydroxy azobenzene, 3-
Nitro-4′-hydroxyazobenzene, and 4-hydroxy azobenzene-2-
carboxylic acid were obtained from Sigma-Aldrich.
Potassium carbonate was purchased from Daejing Korea.
Solvents such as ethanol, ethyl acetate, DMF, and methanol
were common laboratory grade chemicals.
b) Condensation of Schiff bases with Perylene dianhydride
Schiff base (3a, 0.002 mol) was dissolved in quinoline
(20 ml) 250 ml round bottomed flask and started stirring.
Then perylene dianhydride (4) (0.001 mol) was added to the
above solution and the reaction mixture was heated at 165 °C.
Heating was continued for 20 h for the completion of the
reaction, until it was determined from its TLC (4:1, pet ether:
ethyl acetate) the synthesis of perylene diimide Schiff base 5a.
On cooling the reaction mixture dye was precipitated from
quinoline which filtered, dried and recrystallized from DCM.
In this way other dyes 5b-e were synthesized.
Methods
All the chemicals used for analysis were of 99–100 % purity.
1HNMR spectra for all the compounds were conducted on a
300 MHz Bruker NMR spectrometer in DMSO solvent.
Splitting patterns were as follows: s (singlet), d (doublet), dd,
(double doublet), t (triplet), m (multiplet) and br (broad).
Chemical shifts were represented in δ (ppm). The FTIR spectra
were taken in the single beam Nicolet FT-IR 100. Ultraviolet-
visible (UV-VIS) spectra were recorded on a double beam
Perkin-Elmer Lambda 900 UV-VIS-NIR spectrophotometer.
The data were used to calculate the molar extinction coefficients
for the compounds. Whatman Filter Paper 40 was used to mon-
itor the entirety phenolic azo dyes synthesis. TLC plate Silica
Gel 60 F254 was utilized to test the progress of SJA hybrid azo
dyes. Luminescence was determined by using L. S 100 m and
electrochemical analysis was made by Electrochemical Analyzer
CH1830C. Fluorescence quantum yields were determined using
reference fluorescein having quantum yield 95 % in water.
[2,9-Bis(4-((Pyren-1-Ylmethylene)Amino)Phenyl)
Anthra[2,1,9-Def:6,5,10-d’e’f’]Diisoquino
Line-1,3,8,10(2H,9H)-Tetraone] (5a)
1
Brownish yellow crystals (67 %), m.p > 300 °C. H-NMR
(300 MHz, DMSO-d6) δ (ppm): 8.71 (s, 2H), 8.39 (2H, m),
8.35 (2H, d, J = 8.1 Hz), 8.29 (d, 4H, J = 8.6 Hz ), 8.26 (d, 4H,
J = 8.6 Hz), 8.20–8.04 (10H, m), 7.92 (2H, d, J = 8.1 Hz), 7.70
(d, 2H,), 7.59(d, 4H, J = 8.5 Hz), 7.56 (d, 4H, J = 8.5 Hz). 13C-
NMR (75 MHz, DMSO-d6) δ (ppm): 167.05, 158.3, 133.6,
131.2, 130.9, 129.6, 128.4, 128.3, 126.6, 126.3, 126.1, 125.6,
125.2, 125.1, 124.0, 124.5, 124.1, 122.5, 133.3, 135.4. FTIR
(Neat, cm−1) νmax: 3057, 1706, 1625, 1583, 1530, 1447, 1262,
1078, 947, 857, 842 cm−1. Anal. Calcd. For C70H36N4O4 C,
84.32; H, 3.64; N, 5.62; Found: C, 84.13; H, 3.70; N, 5.43.
General Procedure for Synthesis of Schiff Base and Azo
Extended Perylene Chrmophores
[2,9-Bis(4-((9H-Fluoren-9-Ylidene)Amino)Phenyl)
Anthra[2,1,9-Def:6,5,10-d’e’f’]Diisoquino Line
1,3,8,10(2H,9H)-Tetraone] (5b)
a) Synthesis of Schiff bases
1
Brownish yellow crystals (71 %), m.p > 300 °C. H-NMR
To the well stirred solution of 4-aminoacetanilide (1.52 g,
0.001 mol) in 25 ml ethanol in 250 ml round bottomed flask
(300 MHz, DMSO-d6) δ (ppm): 9.24 (s, 1H), 8.58 (d, 4H,
J = 8.7 Hz), 8.16 (d, 4H, J = 8.7 Hz), 8.02 (d, 4H,

J Fluoresc
Table 1 Absorption maxima of perylene schiff base diimide dyes (5a-e)
J = 8.6 Hz), 7.94 (d, 4H, J = 8.6 Hz), 7.68 (d, 4H, J = 8.6 Hz),
7.57 (d, 4H, J = 8.6 Hz). 13C-NMR (75 MHz, DMSO-d6) δ
(ppm): 170.31, 162.96, 146.58, 140.00, 138.74, 136.48,
135.62, 134.34, 131.18, 130.58, 129.57, 129.40, 128.39,
128.06, 125.60, 124.53, 124.29, 124.01, 122.27. FTIR
(Neat, cm−1) νmax: 3081, 1682, 1631, 1584, 1545, 1501,
1457, 1260, 1087, 952, 868 cm−1. Anal. Calcd. For
C62H32N4O4 C, 83.02; H, 3.60; N, 6.25; Found: C, 82.97;
H, 3.66; N, 6.17.
S. No
Compounds
λmax (Absorption)
λmax (Emission)
1
2
3
4
5
5a
5b
5c
5d
5e
515
487
492
526
523
537
550
525
Nil
533
2,9-Bis(4-((4-Nitrobenzylidene)Amino)Phenyl)
Anthra[2,1,9-Def:6,5,10-d’e’f’]Diisoquinoline
1,3,8,10(2H,9H)-Tetraone (5c)
1521, 1457, 1417, 1267, 1171, 1080, 907, 870, 828, 810,
780 cm−1. Anal. Calcd. For C50H26N6O10 C, 68.97; H, 3.01;
N, 9.65; Found: C, 68.88; H, 3.07; N, 9.56.
1
Yellowish red crystals (75 %), m.p > 300 °C. H-NMR
(300 MHz, DMSO-d6) δ (ppm): 9.82 (dd, 2H), 8.29(d, 4H,
J = 8.6 Hz), 8.11 (d, 4H, J = 8.6 Hz), 8.26 (d, 4H, J = 8.5 Hz),
8.00 (d, 4H, J = 8.5 Hz), 7.60 (d, 4H, J = 8.4 Hz), 7.54 (d, 4H,
J = 8.4 Hz). 13C-NMR (75 MHz, DMSO-d6) δ (ppm): 177.02,
162.96, 151.75, 149.88, 140.00, 139.97, 133.99, 131.49,
130.58, 128.67, 128.06, 125.64, 125.60, 124.60, 124.53 and
124.29. FTIR (Neat, cm−1) νmax: 3061, 1701, 1617, 1590,
1531, 1440, 1267, 1171, 1088, 925, 863, 830, 817 cm−1.
Anal. Calcd. For C50H26N6O8 C, 71.60; H, 3.12; N, 10.02;
Found: C, 71.45; H, 3.18; N, 10.00.
2,9-Bis(4-((4-(Dimethylamino)Benzylidene)Amino)Phenyl)
Anthra[2,1,9-Def:6,5,10-d’e’f’]
Diisoquinoline-1,3,8,10(2H,9H)-Tetraone (5e)
1
Yellowish red crystals (70 %), m.p > 300 °C. H-NMR
(300 MHz, DMSO-d6) δ (ppm): 9.21 (dd, 1H), 8.27 (d, 4H,
J = 8.6 Hz), 8.11 (d, 4H, J = 8.7 Hz), 7.65 (d, 4H, J = 8.6 Hz),
7.53 (d, 4H J = 8.6 Hz), 7.56 (d, 4H, J = 8.4 Hz), 6.68 (d, 4H,
J = 8.4 Hz), 2.91 (s, 12H). 13C-NMR (75 MHz, DMSO-d6) δ
(ppm): 171.02, 162.96, 155.51, 148.84, 140.28, 133.63, 131.23,
130.58, 128.67, 128.06, 125.64, 125.60, 124.53, 124.29, 122.95,
111.56, and 41.91. FTIR (Neat, cm−1) νmax: 3041, 2923, 1690,
1628 1582, 1541, 1511, 1457, 1262, 1088, 1011, 878, 830, 817,
791 cm−1. Anal. Calcd. For C54H38N6O4 C, 77.68; H, 4.59; N,
10.07; Found: C, 77.53; H, 4.64; N, 10.03.
2,9-Bis(4-((2-Hydroxy-6-Nitrobenzylidene)Amino)Phenyl)
Anthra[2,1,9-Def:6,5,10-d’e’f’] Diisoquinoline-1,3,8,10
(2H, 9H)-Tetraone (5d)
1
Yellowish red crystals (72 %), m.p > 300 °C. H-NMR
(300 MHz, DMSO-d6) δ (ppm): 9.05 (dd, 2H), 8.17 (d, 4H,
J = 8.7 Hz), 8.15 (d, 4H, J = 8.7 Hz), 7.94 (d, 2H, J = 8.5 Hz),
8.20 (t, 2H), 8.26 (t, 2H), 7.58 (d, 4H, J = 8.6 Hz), 7.50 (d, 4H,
J = 8.6 Hz). 13C-NMR (75 MHz, DMSO-d6) δ (ppm): 168.52,
162.96, 151.75, 149.88, 140.00, 139.97, 133.99, 131.49,
130.58, 128.67, 125.60, 124.60, 124.53, 124.29. FTIR
(Neat, cm−1) νmax: 3455, 3045, 1698, 1618 1585, 1537,
Results and Discussion
Synthesis of Perylene Schiff Base Azo Diimide Dyes (5a-e)
Synthesis of Perylene Schiff Base Azo Diimide Dyes (PDA)
dyes 5a-e has been achieved by two step procedure involving
2.5
Fig. 1 UV.visible Spectra of
perylene Schiff base diimide
dyes 5a-e in DMSO
2.0
5a
5b
5c
5d
5e
1.5
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)

J Fluoresc
Scheme 1 Synthesis of perylene
Schiff base diimide dyes 5a-e
the synthesis of Schiff bases and their condensation with
perylene dianhydride. Schiff bases synthesis was conducted
by reacting p-amino acetanilide (1) and substituted aldehydes
and ketones (2a-e). Condensation of p-amino acetanilide
(0.01 mol) with substituted aldehydes (0.01 mol) in ethanol
was catalyzed by acetic acid. The reaction mixture was stirred
for 10–12 h at reflux temperature until its completion was
observed by taking the TLC of reaction mixture (4:1 Pet ether:
ethyl acetate). Schiff bases were separated by rotary evapora-
tion of the reaction mixture and then recrystallized from eth-
anol and ethyl acetate. In this way a series of Schiff bases 3a-e
were synthesized and purified. Deacetylation of above synthe-
sized Schiff bases was done by hydrolysis of Schiff bases (3a-
e) in aqueous solution 20 ml catalyzed by conc. HCl (4 ml).
On completion of reaction deacetylated Schiff bases were sep-
arated, purified and dried. Deacetylated Schiff bases
(0.01 mol) were condensed with perylene dianhydride
(0.005 mol) in quinoline at temperature 165 °C and the reac-
tion was catalyzed by zinc acetate Zn(CH3COO)2 (0.5 g).
Reaction mixture was stirred for 24 h at this temperature, until
completition of reaction was observed by TLC (pure dichlo-
romethane). On cooling the reaction mixture, products were
separated which were then filtered and dried in oven at
80 °C. In this way a series of perylene Schiff base
diimides 5a-e were synthesized [33].
Spectral Characterization of Perylene Schiff Base Azo
Diimide Dyes (5a-e)
The UV, FTIR, 1H-NMR and 13C-NMR studies were done to
elucidate the structures of newly synthesized perylene Schiff
base diimide dyes. The U. V/visible absorption spectra of the
dyes (1 × 10−5 M) were taken at room temperature in dimethyl
sulfoxide (DMSO) (Fig. 1) and the selected spectral data is
summarized in Table 1.
Three absorption maxima (λmax), bands in UV-visible spec-
tra of dyes (5a-e) were observed during electronic transition of
dye molecules in DMSO, at 430–450, 470–490 and 510–
530 nm. The λmax for all the dyes was the result of π-π*
transitions of the molecules indicative of the existence of
Table 2 Singlet energies and oscillation strengths of symmetric
diimide dyes (5a-e)
Compound
λ
max (Ao)
Es (kcal/mol)
єmax (L/mol.cm)
f
5a
5b
5c
5d
5e
5150
4870
4920
5260
5230
55.5
58.7
58.1
54.3
54.6
13,880
17,220
56,800
23,880
42,940
0.091
0.18
0.11
0.12
0.15

J Fluoresc
Fig. 2 1H-NMR spectrum of 5c
alkenic and imine linkages present in dyes. The λmax in range
510–530 nm is due to π-π* transitions of imine linkages
C = N as well as conjugated C = C bonds of perylene ring.
For dyes 5a-e effect of substituents is not high, depicted from
their λmax which is not too much different from one dye to
another. This phenomenon was observed due to the difference
in the conjugational or migrating ability of the electron lone
pairs on nitrogen atoms (Scheme 1). This also indicates that
the electronic behavior of the nitrogen atoms of imine group is
to some extent different in different derivatives. Using the
wavelength of maximum absorptions (λmax) singlet energies
of compounds 5a-e were calculated and this was in the range
54–58 kcal/mol and oscillation strengths were also determined
which varied from 0.091 to 0.15.
(5a-e). Singlet energy depends upon the wavelength of max-
imum absorption, which is different in different derivatives as
it is clear from Fig. 1 and Table 2. Oscillator strength is de-
pendent on the half width (ν1/2) of the selected absorption
band and sharp absorption band is seen in the UV spectrum
of this compound, so the oscillator strength is lowest for com-
pound 5a and highest for compound 5e.
The FTIR spectra of rylene dyes (5a-e) exhibited absorp-
tion bands due to O-H, Ar-H, C = O of diimides, C = C and
C = N, stretching and bending vibrations at 3455, 3080–3120,
1680–1690, 1585–1630, 1430–1445, 1230–1250 and 770–
810 cm−1 respectively. In particular the peak observed at
1100–1150 cm−1 was as a result of C-O-C functionality. The
absorption bands at 1585-1630 cm−1 and 770–810 cm−1
depicted the presence of C = C stretching and bending
vibrations respectively. Schiff base linkage was con-
From the singlet energy study it is observed that there is a
change of 3–4 kcal/mol in the singlet energies of compounds
Fig. 3 13C-NMR spectrum of 5c

J Fluoresc
Fig. 4 Fluorescence spectrum of
compounds 5a-e
1000
800
600
400
200
0
5a
5b
5c
5d
5e
450
500
550
600
650
700
750
Wavelength (nm)
firmed by absorptions at 1430–1445 cm−1. Stretching
vibrations absorptions in between 3080 and 3120 cm−1
were because of C = C-H bonds of aromatic rings pres-
ent in dye molecules 5a-e.
Theoretical radiative lifetimes (To) depend upon molar extinc-
tion coefficient (єmax), mean frequency (V2max) and half-
width of the selected absorption (ΔV1/2). Radiative life
time is highest for compound 5a because of its sharp
absorption band in UV spectrum and it has the lowest
value of the fluorescence rate constant which has in-
verse relation with radiative life time (Table 3).
The radiative life time (To) of compound 5c is shortest
because To depends upon the square of mean frequency for
the absorption band (cm−1) and the half-width of the selected
absorption (cm−1) band through reciprocal way. The mean
frequency and halfwidth of absorption band are high for 5c
as apparent from broadness of UV-Visible absorption band so
radiative lifetime becomes shortest for aforementioned
compound.
1H-NMR and 13C-NMR studies proved the synthesis of
rylene dyes 5a-e. Dye 5c showed characteristic peak for imine
functionality at 8.90 ppm and a pair of doublet at 7.36 and
7.39 ppm due to phenylene ring sandwiched between nitrogen
atoms. A pair doublet peak patteren at δ 8.14 and 8.18 ppm is
to due to two chemically and magnetically nonequivalent pro-
tons of the naphthalene unit of perylene ring and doublet peak
and pair of triplets at 7.81, 7.71 and 7.53 ppm due to phenyl
ring attached with NO2 group. 13C-NMR spectrum of 5c
showed peak for carbonyl group of imide functionality at
162.4 ppm and imine carbon showed signal at 158.7 ppm
which are the distinguishing signals for these dyes. Fifteen
signals in the range 121-151 ppm are due to aromatic carbon
nuclei in the molecules (Figs. 2 and 3)
Electrochemical Properties of Perylene Schiff Base Azo
Diimide Dyes (5a-e)
Fluorescence Studies of Perylene Schiff Base Azo Diimide
Dyes (5a-e)
The electrochemical characterization of all compounds was
made in detail using cyclic voltammetry in aqueous solution
containing 0.1 M TBAPF6 as a supporting electrolyte (Fig. 5).
All dyes exhibited irreversible oxidation peaks in their volt-
ammograms and oxidation onset potential were
Fluorescence study of compounds 5a-e was conducted by
preparing their dilute solution in DMSO (1 × 10−5 M).
All the compounds exhibited strong fluorescence above
500 nm except 5d which showed no fluorescence that
might be attributed to aggregation of flat molecules of
this dye and electron withdrawing groups attached to this
molecule. Highest fluorescence was observed for dye 5a and
5b at wavelength 537 and 550 nm respectively due to the
highly conjugated system like pyrene and fluorene present in
these compounds [34] (Fig. 4).
Table 3 Theoretical Radiative Lifetime and Fluorescence rate
constants of Perylene Schiff base Diimides (5a-e)
Compound
ΔV1/2 (cm−1
)
Vmax(cm−1
)
To (ns)
kf (108/s )
5a
5b
5c
5d
5e
1089
2047
1264
1453
1819
19,417
20,533
20,325
19,011
19,120
61.4
23.5
11.8
27.9
12.2
0.16
0.42
0.84
0.35
0.81
Theoretical radiative lifetimes and fluorescence rate con-
stants were calculated using the standard equation which var-
ied from 11.8 to 64.4 ns and 0.16 to 0.84/s respectively.

J Fluoresc
Fig. 5 Cyclic Voltammogram of
perylene Schiff base diimide azo
dyes 5a-e in DMSO
5a
5b
5c
5d
5e
0.00002
0.00001
0.00000
-0.00001
-0.00002
-0.00003
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Volt
determined from their CV curves. Redox potential (Eox)
of dyes 5a-e were different from each other and were in the
range 0.229 to −0.590 Vas shown below in the Table 4. Redox
potential was highest for dye 5e and lowest for 5a. The Eox
were used to determine the energy levels of highest occupied
molecular orbital (HOMO) by following the empirical Bredas
equations [35].
LUMO energy levels were in −4.21 to −5.20 eV, and
highest energy LUMO levels seen for 5a dye bearing
pyrene moiety and lowest were in 5e bearing
dimethylamino group at phenylene ring at imide position
of perylene ring. These results were in accordance with
the general phenomenon observed electrochemical studies
of compounds that the electron donating groups decrease
the energy gap between HOMO and LUMO levels and
electron withdrawing groups increase this gap of energy.
HOMO energy levels in the dyes have energy range − 6.85
to 7.57 eV and lowest HOMO levels are present in 5e and
highest for 5a because in compound 5a there is more
availability of electrons due to highly rich pyrene ring
which increases the energy of HOMO levels and thereby
decreases the energy gap. Optical band gap energies were
calculated by standard procedure and were in the range
2.37 to 2.64 eV. For compound 5e lowest highest energy
gap is observed which is due dimethyl amino group at
Schiff base condensed along the molecular axis, but com-
parison cannot be made among compounds as these have
diverse Schiff bases condensed with perylene dianhydride
[36–38] (Table 4).
Thermal Properties of Perylene Schiff Base Azo Diimide
Dyes (5a-e)
Thermogravimetric analysis is used to evaluate the ther-
mal stability of a material. In a desired temperature range,
if species is thermally stable, there will be no observed
mass change [39, 40]. Thermogravimetric study of dyes
was made to determine the thermal stability of synthe-
sized dyes 5a-e upto 500 °C. All perylene Schiff base
diimide dyes showed no considerable weight loss during
the applied temperature range. From the TGA thermo-
grams it is apparent that synthesized dyes are highly ther-
mally stable. For all perylene Schiff base diimide dyes
(5a-e) start up weight loss temperature was observed at
about 450 °C and very loss occurred at this temperature as
indicated from Fig. 6.
Conclusions
Table 4 Half wave potential (E1/2) and LUMO energy levels of
perylene Schiff base diimide azo dyes (5a-e)
Synthesis of some new bis-axially extended symmetric
multichromic fluorescent perylene schiff base and azo diimides
chromophores was accomplished in excellent yields.
Synthesized derivatives exhibited yellowish and reddish yellow
fluorescence with emission peaks in the range 525–550 nm with
nearly unit fluorescence quantum yields. LUMO and HOMO
energy levels were found in the range of −4.21 to −5.20 and
−6.85 to −7.57 eV, respectively with band gap energy 2.35–
2.69/eV. High thermal stability was shown by these dyes in
S. No. Compounds E1/2 (V) Eg (eV) HOMO (eV) LUMO (eV)
1
2
3
4
5
5a
5b
5c
5d
5e
-0.590
-0.135
2.64
2.38
-6.85
-7.04
-7.68
-7.37
-7.57
-4.21
-4.66
-5.16
-5.02
-5.20
+0.360 2.52
+0.229 2.37
+0.400 2.35

J Fluoresc
Fig. 6 TGA thermogram of dye (5c)
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