Spectral and structural characterization of 2-(fluorophenylamino)- and 2-(nitrophenylamino)-1,4-naphthoquinone derivatives

Article abstract of DOI:10.1016/j.molstruc.2014.03.044

Naphthoquinone amino derivatives exhibit interesting physicochemical properties and are of interest for potential medicinal purposes. The preparation of novel 2-(nitrophenylamino)-1,4-naphthoquinones derivatives was achieved by reaction of nitroanilines with 1,4-naphthoquinone with a catalytic amount of FeCl₃ or by direct nitration of 2-(phenylamino)-1,4-naphthoquinone (PAN). Structural and photophysical properties of a series of NO₂PANs and FPANs derivatives are examined using computational and spectroscopic methods. Absorbance and emission spectra are measured in a range of solvent environments to examine the impact of solvent-solute interactions. Additionally quantum calculations are used to evaluate the electronic nature of the spectral transitions and compare structures of the different PAN derivatives. The lowest energy electronic transitions have charge transfer character, and show the most sensitivity to solvent and substituents. Higher energy π- π^* transitions are relatively insensitive to both factors. Computational predictions are in good agreement with the experimental spectra, and provide molecular-level insight variations amongst the different aniline-substituents.

Full text of DOI:10.1016/j.molstruc.2014.03.044

Journal of Molecular Structure 1068 (2014) 1–7
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Spectral and structural characterization of 2-(fluorophenylamino)- and
2-(nitrophenylamino)-1,4-naphthoquinone derivatives
b
b
Elisa Leyva b, Sarah J. Schmidtke Sobeck a, , Silvia E. Loredo-Carrillo , Diego A. Magaldi-Lara
⇑
a College of Wooster, Department of Chemistry, 943 College Mall, Wooster, OH, USA
b Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, Nava 6, San Luis Potosí, SLP 78290, Mexico
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
A series of novel 2-(nitrophenylamino)-1,4-naphthoquinones derivatives was synthesized, and their
photophysical properties examined using computational and spectroscopic methods.
ꢀ
Novel 2-(nitrophenylamino)-1,4-
naphthoquinones derivatives were
synthesized and characterized.
Solvent impact on absorbance and
emission napthoquinone derivatives
is assessed.
ꢀ
ꢀ
ꢀ
The charge transfer transition is most
sensitive to solvent and substituents.
Quantum calculations confirm the
spectral assignments and substituent
effects.
a r t i c l e i n f o
a b s t r a c t
Article history:
Naphthoquinone amino derivatives exhibit interesting physicochemical properties and are of interest for
potential medicinal purposes. The preparation of novel 2-(nitrophenylamino)-1,4-naphthoquinones
derivatives was achieved by reaction of nitroanilines with 1,4-naphthoquinone with a catalytic amount
of FeCl3 or by direct nitration of 2-(phenylamino)-1,4-naphthoquinone (PAN). Structural and photophys-
ical properties of a series of NO2PANs and FPANs derivatives are examined using computational and spec-
troscopic methods. Absorbance and emission spectra are measured in a range of solvent environments to
examine the impact of solvent–solute interactions. Additionally quantum calculations are used to evalu-
ate the electronic nature of the spectral transitions and compare structures of the different PAN deriva-
tives. The lowest energy electronic transitions have charge transfer character, and show the most
Received 28 September 2013
Received in revised form 20 March 2014
Accepted 21 March 2014
Available online 28 March 2014
Keywords:
Solvatochromism
Electronic spectroscopy
Quantum structure calculations
Naphthoquinones
sensitivity to solvent and substituents. Higher energy p–
p* transitions are relatively insensitive to both
factors. Computational predictions are in good agreement with the experimental spectra, and provide
molecular-level insight variations amongst the different aniline-substituents.
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
Naphthoquinones are a class of molecules that are naturally
occurring chromatic pigments in plants, fungi and some animals,
and exhibit potentially useful medicinal properties [1]. They have
⇑
Corresponding author. Tel.: +1 330 263 2359; fax: +1 330 263 2386.
E-mail address: ssobeck@wooster.edu (S.J. Schmidtke Sobeck).
http://dx.doi.org/10.1016/j.molstruc.2014.03.044
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

2
E. Leyva et al. / Journal of Molecular Structure 1068 (2014) 1–7
a wide range of biological properties such as significant antimicro-
bial activity [2]; for example, several plant-derived derivatives have
antibacterial effects on some species of aerobic and anaerobic
organisms [1]. Antiparasitic [3] and antifungal properties, particu-
larly against species of Candida, of naphthoquinones have also been
reported [4–7]. Of great potential interest for this type of com-
pounds is their use as anticancer agents with applications in chemo-
therapy [8].
The biological activity of quinones has been related to their
redox properties [9–11] and their capacity to accept one or
two electrons to form the corresponding radical-anion (QÅ) and
hydroquinone radical dianion (Q2À). This capacity and several
physicochemical properties of a given quinone can be modified
by the addition of a substituted aniline to the quinone system
[12]. The electron donating or withdrawing properties of the
substituents on the aniline modify their redox properties, there-
by modifying the ability of the system to exhibit a charge trans-
fer from the substituent to the quinone upon photo-excitation
[13].
Fig. 1. Structure of PAN derivatives, where Rn are substituents, where the number
indicates the position and n = H, F, NO2, or OCH3. The dihedral angles and bond
distances of interest are labeled.
Table 1
Several studies of these molecular systems have been reported,
and the nature of the electronic spectrum of 1,4-naphthoquinone
has been widely investigated [14]. UV–Vis electronic absorption
PAN derivatives under investigation.
Compound
R2
R3
R4
R5
m.p. (°C)
p–
p* electron transitions
PANa
2FPAN
3FPAN
4FPAN
23FPAN
24FPAN
25FPAN
34FPAN
H
F
H
H
F
F
F
H
H
H
F
H
F
H
H
F
H
H
H
F
H
F
H
F
F
F
H
NO2
NO2
F
H
OMe
H
H
H
H
H
H
F
H
H
F
H
H
H
H
H
H
188–18920
155–15613
200–20113
244–24513
154–15513
205–20613
190–19113
260–26113
261–26213
202–20313
205–207
spectra of PAN derivatives show the
bands associated with benzene and naphthoquinone in the regions
around 203–211 nm and 265–273 nm. A weak n–p* transition
band is observed at 310–330 nm. A broad, low energy band can
be observed in the visible region centered between 438 and
480 nm for some naphthoquinone derivatives [15]. This latter
absorption is quite common for amino-substituted quinones and
has been previously assigned to have character corresponding to
a charge-transfer (CT) transition and weak n–p* electron transi-
tions of the carbonyl group in the quinone [16].
The impact of solvent environment upon spectral transitions
can be used to gain a better understanding about the nature of
the electronic states, and types of interactions between solvent
and solute. Furthermore, it is interesting to examine the effect of
a wide range of solvents varying in properties such as solvent
polarity and hydrogen-bonding ability, where the solvent can be
proton donor or acceptor [17–19]. Comparison of the solvatochro-
mic behavior of a series of naphthoquinone derivatives provides
insight into the specific interactions of different substituents with
the solvent and the impact the substituent has upon photo-physi-
cal properties of the system. Along with absorbance spectroscopy,
fluorescence has proven to be a versatile tool for studying molecu-
lar interactions, and can be used to provide a greater understand-
ing about the excited state properties [20].
Theoretical studies are very useful for spectral interpretation and
to gain a molecular level understanding about the nature of elec-
tronic and structural properties [14]. Interactions between solute
and solvent like hydrogen bonding, solvent–solute complexation,
changes in the electronic charge distribution and excited-state reac-
tions are important to understand when interpreting spectral
events [19,21].
We synthesized several novel NO2PAN derivatives and were
interested in comparing their physicochemical properties with
the FPAN derivatives previously synthesized [15]. We present a
combination of spectroscopic and computational analysis of a ser-
ies shown in Fig. 1 and Table 1. Spectral properties in a wide range
of solvents were examined to understand the nature of molecular
excited states and the effect of different solvents on solute–solvent
interactions. Both absorbance and emission spectroscopy are pre-
sented, in combination with computational studies, to gain a great-
er understanding of the nature of the spectral transitions. A
fundamental understanding about the molecular nature of some
physicochemical properties of the naphthoquinone derivatives as
a function of solvent and substituent provide a basic knowledge
35FPAN
H
H
F
245FPAN
2NO2PANb
4NO2PAN
2,4NO2PANb
4F2NO2PANb
2OMePANa
4OMePANa
H
H
H
H
H
H
H
NO2
H
NO2
NO2
Ome
H
337–33921
264–266
246–248
147–14820
154–15520
a
These compounds were prepared for comparison.
These compounds have not been previously described.
b
to exploit potentially useful applications of this family of
compounds.
Experimental
Spectral studies
Sample preparation
Solutions (2 Â 10À4 M) were made in five solvents: acetonitrile
(Pharmco-AAPer, 99.9% pure), dichloromethane (Pharmco-AAPer,
99.5%), hexane (Pharmco-AAPer, 99% pure), methanol (Pharmco-
AAper, 100% pure) and 1-propanol (Sigma–Aldrich, 99.9%). Stock
solutions were diluted to ensure the maximum absorbance in the
UV region was between 0.1 and 1. For fluorescence measurements
solutions were diluted such that the absorbance maximum at the
lowest energy absorbance band was between 0.1 and 0.2.
UV–Vis absorbance measurements
UV–Vis spectra were obtained at room temperature on a Varian
Cary 50 Bio Spectrophotometer. Spectra were corrected for solvent
background by calibrating the instrument to the blank solvent.
Spectra were taken in the range of 200–600 nm at a scan rate of
600 nm/min using the dual beam mode.
Fluorescence measurements
The emission spectra were obtained using a Varian Cary
Eclipse fluorescence spectrophotometer. Samples were excited

E. Leyva et al. / Journal of Molecular Structure 1068 (2014) 1–7
3
at the lowest energy absorbance, as determined by the UV–Vis
measurement. The PMT setting was adjusted to achieve sufficient
signal to noise ratio, typically at 1000 V. The scans were run at
600 nm/min. For all scans a sample of just the solvent was mea-
sured at the same excitation wavelength, PMT setting and scan
speed. The background signal from the neat solvent was sub-
tracted from the sample prior to analysis.
Preparation of 2-(2,4-dinitrophenylamino)-1,4-naphthoquinone
This derivative was prepared by the method previously re-
ported with some modifications [29]. A mixture of concentrated
acids HNO3 (7 mL) and H2SO4 (1.5 mL) was prepared and PAN
(0.9 mmol) was added. The reaction mixture was stirred for four
hours at room temperature. A volume (100 mL) of cold water
(100 mL) was added to induce precipitation. The resulting solid
was filtered and washed with NaHCO3 (5%), distilled water and
cold ethanol. The product was purified by chromatography.
Computational detail
Purification and elemental analysis
The PAN derivatives were optimized using Gaussian 09 [22].
The optimizations were performed using DFT calculations at the
B3LYP level of theory [23–25] with the Midi! basis set [26]. Fre-
quency calculations were performed to verify the nature of the sta-
tionary points.
Excitation energy calculations were conducted for each mole-
cule in the gas and solution phase, smd solvent model for acetoni-
trile, using their respective optimized structures. The first ten
singlet excitations were also calculated using the TD-B3LYP/6-
31+G(d) method. For comparison excitation energy calculations
were also conducted using semiempirical ZINDO methods in the
gas phase. The excitations for the first ten singlet transitions were
calculated. For comparison, all excitation energy calculations were
carried out using Gaussian 09 [22] and spectra visualized using the
Gaussview 5 interface [27].
To obtain analytical samples some products were recrystallized
several times in ethanol or CHCl3. Other products were passed
through a small silica column using CHCl3 as the eluent solvent.
Elemental analyses (Table 2) were performed in the laboratories
of the Universidad Nacional Autonoma de Mexico or Universidad
Autonoma Metropolitana.
Characterization of 2-(2-nitrophenylamino)-1,4-naphthoquinone
It was obtained as an orange solid with m.p. 205–207 °C; IR
(KBr, cmÀ1) 3304 (NH), 1671, 1641 (C@O), 1500, 1337 (NO2),
1576 (NAH aromatic), 1278 (CAN aromatic); UV–Vis (CH3OH,
nm) 275, 343, 450; 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.74
(1H, bs, NAH), 8.21 (1H, dd, J = 8.2, 1.17 Hz, aromatic H), 8.1 (1H,
dd, J = 8.1, 1.17 Hz, aromatic H), 7.99 (1H, dd, J = 7.8, 1.17 Hz, aro-
matic H), 7.91 (1H, td, J = 7.42, 1.17 Hz, aromatic H), 7.84 (1H, td,
J = 7.42, 1.17 Hz, aromatic H), 7.83 (1H, dd, J = 7.42, 1.17 Hz, aro-
matic H) 7.81 (1H, td, J = 8.2, 1.17 Hz), 7.45 (1H, td, J = 8.2,
1.17 Hz), 6.26 (1H, s, vinyl H); HRMS calcd for C16H10N2O4 was
294.0641, found 294.0635.
Characterization methods
Melting points were measured with a Fisher Johns apparatus.
UV–Vis spectra were obtained on a Shimadzu UV-2401 PC spectro-
photometer. IR spectra were recorded on a Nicolet IS10 Thermo
Scientific FTIR spectrophotometer. NMR spectra were obtained on
a Varian Mercury 400 MHz spectrometer. Mass spectra were re-
corded on a Finnigan MAT 8200.
Characterization of 2-(4-nitrophenylamino)-1,4-naphthoquinone
It was obtained as a red solid with m.p. 337–339 °C; IR (KBr,
cmÀ1) 3185 (NH), 1671, 1641 (C@O), 1497, 1330 (NO2), 1570
(NAH aromatic), 1290 (CAN aromatic); UV–Vis (CH3OH, nm)
265, 338, 450; 1H NMR (400 MHz, DMSO-d6) d (ppm): 9.56 (1H,
bs, NH), 8.33 (1H, dd, J = 8.20, 1.37 Hz, aromatic H), 8.17 (1H, dd,
J = 8.5, 1.47 Hz, aromatic H), 7.89 (1H, td, J = 7.03, 1.17 Hz, aromatic
H), 7.83 (1H, td, J = 7.03, 1.17 Hz, aromatic H), 7.46 (2H, d,
J = 8.98 Hz, aromatic H), 7.22 (2H, d, J = 8.59 Hz, aromatic), 6.53
(1H, s, vinyl H); HRMS calcd for C16H10N2O4 was 294.0641, found
294.0631.
Synthetic procedures
Preparation of 2-(R-phenylamino)-1,4-naphthoquinones by method A
PAN derivatives were prepared by the method previously re-
ported with some modifications [15]. 1,4-Naphthoquinone
(1 mmol) was dissolved in ethanol (30 mL). A solution of a given
aniline (1 mmol) in ethanol (30 mL) was slowly added and the
reaction mixture was refluxed for 7 days. The reaction vessel was
equipped with a condenser to minimize ethanol losses during the
experiment. The solution turned deep red or orange-yellow when
the corresponding 2-(R-phenylamino)-1,4-naphthoquinone was
formed. The resulting solid was filtered, washed with cold etanol
and recrystallized in ethanol.
Table 2
Elemental analyses of PAN derivatives.
Compound
C(% T)a
C(% E)b
N(%T)a
N(%E)b
H(%T)a
H (%E)b
PAN
2FPAN
3FPAN
4FPAN
77.10
71.91
71.91
71.91
67.37
67.37
67.37
67.37
67.37
63.37
65.31
65.31
56.65
61.54
73.11
73.11
76.96
71.88
72.09
71.85
67.12
67.15
67.22
67.13
67.19
63.13
65.17
64.96
56.34
61.25
72.91
73.17
5.43
5.24
5.24
5.24
4.91
4.91
4.91
4.91
4.91
4.62
9.52
9.52
12.39
8.97
5.02
5.02
5.62
5.20
5.18
5.15
4.83
4.89
4.85
4.78
4.91
4.53
9.40
9.40
12.31
8.80
4.85
4.91
4.45
3.77
3.77
3.77
3.18
3.18
3.18
3.18
3.18
2.66
3.43
3.43
2.67
2.91
4.69
4.69
4.34
3.72
3.78
3.69
3.10
3.16
3.11
3.03
3.15
2.63
3.43
3.49
2.47
3.14
4.75
4.91
Preparation of 2-(R-phenylamino)-1,4-naphthoquinones by method B
PAN derivatives were also prepared by the method previously
reported with some minor modifications [15,28]. 1,4-Naphthoqui-
none (1 mmol) was dissolved in ethanol (10 mL) and an amount
(0.1 mmol) of the Lewis acid catalyst (FeCl3 or CeCl3Á7H2O) was
added. The reaction mixture was stirred for 15 min to allow the
reaction between the Lewis base (1,4-naphthoquinone) and the Le-
wis acid catalyst. A solution of a given aniline (1 mmol) in ethanol
(10 mL) was slowly added and the mixture was refluxed for four
hours. The reaction vessel was equipped with a condenser to min-
imize ethanol losses during the experiment. The solution turned
deep red or orange-yellow when the corresponding 2-(R-phenyla-
mino)-1,4-naphthoquinone was formed. The resulting solid was
filtered, washed with cold ethanol and recrystallized from ethanol.
23FPAN
24FPAN
25FPAN
34FPAN
35FPAN
245FPAN
2NO2PAN
4NO2PAN
2,4NO2PAN
4F2NO2PAN
2OMePAN
4OMePAN
a
Calculated value.
Observed value.
b

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E. Leyva et al. / Journal of Molecular Structure 1068 (2014) 1–7
Table 3
Characterization of 2-(2, 4-dinitrophenylamino)-1,4-naphthoquinone
Reaction conditions and yields of NO2PANs by oxidative-addition reaction.
It was obtained as an orange solid with m.p. 264–266 °C; IR
(KBr, cmÀ1) 3276 (NH), 1689, 1672 (C@O), 1500, 1339 (NO2),
1583 (NAH aromatic), 1278 (CAN aromatic); UV–Vis (CH3OH,
nm) 265, 315, 448; NMR (400 MHz, CDCl3) d (ppm): 9.04 (1H, bs,
NH), 8.56 (1H, d, J = 2.7 Hz, aromatic H), 8.23 (1H, dd, J = 9.1,
2.7 Hz, aromatic H) 8.22 (1H, dd, J = 8.8, 2.7 Hz, aromatic H), 8.15
(1H, td, J = 7.52, 1.17 Hz, aromatic H), 7.92 (1H, td, J = 8.78,
2.7 Hz, aromatic H), 7.88 (1H, dd, J = 8.78, 2.5 Hz, aromatic H),
7.82 (1H, td, J = 7.52, 1.27 Hz), 6.88 (1H, s, vinyl H); HRMS calcd
for C16H9N3O6 was 339.0491, found 339.0485.
Compound
Yield (%)a
Yield (%)b
Yield (%)c
PANd
60
–
traces
–
–
60
60
85
–
55
–
–
70
85
78
35
55
–
35
75
82
2NO2PAN
4NO2PAN
2,4NO2PANe
4F2NO2PAN
2OMePANd
4OMePANd
a
b
c
Without catalyst and 7 days.
With catalyst CeCl3Á7H2O and 4 h.
With catalyst FeCl3 and 4 h.
These compounds were prepared for comparison.
This compound was prepared by nitration with HNO3/H2SO4.
d
e
Characterization of 2-(4-fluoro-2-nitrophenylamino)-1,
4-naphthoquinone
not been previously reported we decided to investigate their prep-
aration. Reacting 1,4-naphthoquinone with an o-nitroaniline with a
catalytic amount of FeCl3 resulted in the formation of moderate
amounts of adducts (Table 3). Comparing the results, it is clear that
nitro-substituted anilines are generally very weak bases and do not
react very easily with naphthoquinone. In the case of o-nitroani-
lines, they are sterically hindered due to the presence of a large sub-
stituent next to the aniline. Therefore, in this particular reaction the
size of the catalyst is very important, a smaller size catalyst [32] like
FeCl3 works better than CeCl3. The effect of the catalyst on this addi-
tion-oxidation reaction has been previously explained [15,28]. First,
it induces the exclusive 1,4-Michael addition and then it favors oxi-
dation to obtain naphthoquinone derivatives (Scheme 1).
It was obtained as an orange solid with m.p. 246–248 °C; IR
(KBr, cmÀ1) 3207 (NH), 1677, 1650 (C@O), 1523, 1336 (NO2),
1584 (NAH aromatic), 1250 (CAN aromatic), 1112 (CAF); UV–Vis
(CH3OH, nm) 265, 310, 450; NMR (400 MHz, DMSO-d6) d (ppm):
9.81 (1H, bs, NH), 8.23 (1H, d, Jortho H-F = 10.44 Hz, aromatic H),
8.02 (1H, dd, J = 9.4, 2.7 Hz, aromatic H), 8.00 (1H, dd, J = 9.4,
2.7 Hz, aromatic H), 7.83 (1H, td, J = 7.52, 1.56 Hz, aromatic H),
7.80 (1H, d, JorthoHF = 9.68 Hz, aromatic H), 7.79 (1H, td, J = 7.53,
1.56 Hz), 7.72 (1H, d, JmetaHF = 6.76), 6.36 (1H, s, vinyl H); HRMS
calcd for C16H9N2O4 was 312.0546, found 312.0540.
Results and discussion
Synthesis and reaction pathway
Electronic spectroscopy
The reaction of anilines with 1,4-naphthoquinone to give 2-
(phenylamino)-1,4-naphthoquinones has been known for several
years [15]. It is an example of a C(sp2)AH bond transformation to
C(sp2)AN bond via an addition-oxidation reaction sequence [30].
The mechanism for this coupling reaction (Scheme 1) involves a
Michael 1,4-addition to give an intermediate which easily tauto-
merizes and gets oxidized to give the corresponding derivative
[15]. However, due to the physicochemical properties of 1,4-naph-
thoquinone, several compounds are actually produced. In addition,
substitution with deactivating groups, such as a nitro-group, yield
decreased or no reactivity [31]. In some cases, performing this
reaction without an adequate catalyst results in the formation of
complex mixtures or no reaction at all.
The absorbance spectra for the FPAN derivatives have been pre-
viously reported in methanol and assignments were made on the
character of the transitions observed [15]. For consistency, the
transitions are labeled using the same notation. Table 4 summa-
rizes the lowest energy absorbance bands observed, characterized
as the CT + n–p* (k4) and
p–
p* (k2) transitions. The absorbance
energies are reported in methanol and hexanes, the two most
divergent solvents studied.
The high energy p–p
* absorbance band shows minimal solvato-
chromic shifts and solvent–solute interactions would not be antic-
ipated to have a great impact upon the relative stability of these
states. The lowest energy band, centered about 435–480 nm, does
show sensitivity to the solvent environment. This band was
Recently, some of us have demonstrated that the use of an oxida-
tion agent and a Lewis acid such as CeCl3 or FeCl3 results in a cleaner
reaction between a fluoroaniline and 1,4-naphthoquinone with bet-
ter yields [15,28]. However, since o-nitroaniline derivatives have
Table 4
Summary of absorbance maxima for the PAN derivatives.
Compound
k2 max (nm)
k4 max (nm)
CT + n–pÃ
p–
pÃ
MeOH
Hexanes
MeOH
Hexanes
PAN
2FPAN
3FPAN
4FPAN
273
269
273
270
268
267
269
270
273
266
266
259
262
278
273
269
270
269
269
270
262
269
–
–
–
–
466
448
459
461
439
443
450
454
448
450
451
437
448
476
479
450
442
440
445
428
439
430
–
–
–
–
–
2,3FPAN
2,4FPAN
2,5FPAN
3,4FPAN
3,5FPAN
2NO2PAN
4NO2PAN
2,4NO2PAN
4F2NO2PAN
2OMePAN
4OMePAN
a
–
–
280
268
–
468
458
a
Where no value is given, the compound is insoluble in hexanes.
Scheme 1. Mechanism for oxidative coupling of anilines with 1,4-naphthoquinone.

E. Leyva et al. / Journal of Molecular Structure 1068 (2014) 1–7
5
Table 5
Summary of optimized geometries of the PAN derivatives. Bond angles and distances
are defined in Fig. 1.
Compound
U
1 (°)
U2 (°)
R1 (Å)
R2 (Å)
PAN
2FPAN
3FPAN
4FPAN
33.0
0.0
30.6
37.6
0.0
23.6
0.0
22.1
26.4
0.0
1.98
1.96
1.97
1.98
1.95
1.95
1.96
1.97
1.96
1.95
2.06
1.95
2.05
2.06
1.98
1.99
2.34
2.12
2.36
2.38
2.13
2.13
2.12
2.39
2,3FPAN
2,4FPAN
2,5FPAN
3,4FPAN
3,5FPAN
2,4,5FPAN
2NO2PAN
4NO2PAN
2,4NO2PAN
4F2NO2PAN
2OMePAN
4OMePAN
0.0
0.0
0.0
0.0
34.9
27.0
0.0
35.8
24.8
33.8
37.1
À14.3
37.4
24.7
19.6
0.0
24.6
18.3
23.8
25.4
À10.9
26.2
2.34
2.14
2.51 (1.82)a
2.29
2.49 (1.80)
2.52 (1.83)
2.06
2.38
a
For the 2NO2 compounds the length in parentheses is from the amine H to the
nitro O.
Fig. 2. Normalized absorbance spectra in a range of solvents for 4FPAN, 4NO2PAN,
and 4OMePAN. The solvents are acetonitrile (black), methanol (blue), propanol
(green), dichloromethane (orange), and hexanes (red). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
Fig. 3. Normalized and solvent subtracted emission spectra in a range of solvents
for 2FPAN. The solvents are acetonitrile (black), methanol (blue), propanol (green),
dichloromethane (orange), and hexanes (red). (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this
article.)
Fig. 4. Representative calculated absorbance spectra for 4FPAN, 4OMePAN, and
4NO2PAN. The results are shown for the TD-DFT calculations with acetonitrile
solvation.
previously assigned to a combination of a charge transfer (CT) to
the carbonyl and weak n–p* of the quinone system [15]. The PAN
derivatives, which are soluble in hexanes, exhibit a blue shift in
hexanes relative to polar solvents. This is most likely due to the
lower stability and higher energy of the CT state in a nonpolar
environment, since a nonpolar solvent provides only weak induced
dipole interactions to stabilize the polarized solute. In contrast, po-
lar solvents have stronger permanent dipole–dipole interactions
and are better able to stabilize a charge-separated excited state.
There are no significant differences in the absorbance energies of
this band amongst the polar solvents studied indicating that
specific interactions with the solvent, such as hydrogen bonding,
do not affect the relative energies of the electronic states.
Sample spectra illustrating the solvatochromism of representa-
tive derivatives are presented in Fig. 2. An interesting feature of the
spectra is the effect of aniline substituent. It is clearly observed
that the FPANs and OMePANs have similar absorbance profiles.
In comparison, there is a shift in the relative intensities of the
absorbance bands for the NO2PANs. The absorbance around
340 nm (k3) has a much greater intensity in the nitro-derivative,

6
E. Leyva et al. / Journal of Molecular Structure 1068 (2014) 1–7
Fig. 5. Selected molecular orbitals for 4FPAN: HOMO (a), LUMO (b), and LUMO + 2(c). For comparison the LUMO + 2 of 4NO2PAN is shown (d).
comparable to the k2 and k4 bands. Computational analysis of the
excitation energies and visualization of the molecular orbitals is
used to further examine this phenomenon, in the subsequent
section.
bands in the nitro-substituted PAN species (Fig. 2). Overall the
experimentally predicted spectra capture the key features of the
experimental spectra. The complete table of the first ten excitation
energies and oscillatory strengths calculated using TD-DFT and
ZINDO methods are reported in the supplementary materials.
The lowest energy spectral transition with significant oscilla-
tory strength for all compounds is a HOMO to LUMO transition
with CT character. Representative MOs are shown in Fig. 5. The en-
ergy of lowest energy transition is in the visible region of the spec-
trum by ZINDO and TD-DFT methods, with higher wavelengths
predicted by the latter. The other dominant spectral band between
In solution PAN derivatives have very weak emission, even fol-
lowing degassing and slight cooling of samples (to a minimum of
10 °C). Representative emission spectra in the series of solvents
are presented in Fig. 3. These spectra were collected following exci-
tation of the low energy CT band and were verified by excitation
scans for the observed emission band. Collectively the variation in
substituent group and position did not have a significant impact
upon the emission energies nor intensities. A broad, weak emission
was observed in the range of 550–600 nm for the polar solvents. In
hexanes the band is blue shifted by approximately 50 nm. The lower
energy emission in polar solvents indicates a greater stabilization of
the CT excited state by dipole–dipole interactions with the sur-
rounding solvent molecules. As in the case of the absorbance spectra
no significant differences are observed between the polar solvents
studied. Further supporting the conclusion that specific interactions
with the solvent, such as hydrogen bonding, do not appear to influ-
ence the photophysical properties of the system.
250 and 300 nm is made up of excitations exhibiting significant
p
to p* character. Some of the weaker transitions in this spectral re-
gion have changes in the electron density about the carbonyl
groups of the naphthoquinone. The notable spectral change pre-
dicted by ZINDO and TD-DFT methods for 4NO2PAN, relative to
4FPAN and 4OMePAN, is due to a change in the energy of the
HOMO to LUMO + 2 transition. In the nitro-derivative there is CT
character accompanying the transition, in contrast to the dominant
p* character centered on the quinone system LUMO + 2 state of
4FPAN and 4OMePAN (Fig. 5). The result is a red shift for the sec-
ond (lowest) energy absorbance band in the nitro-derivatives.
In general, the calculations agree with the experimental spectra
showing minimal differences in the electronic transitions between
derivatives with the same substituents, regardless of the position.
The main variation that is observed is based upon the derivative it-
self, with the nitro-substituent being the most divergent. Substitu-
tion of a nitro group at either an ortho or para position of aniline
results in a quite defined series of absorbance bands, with spectral
changes attributed to the nature of the second excited electronic
state. Indeed, a nitro group has a strong electron-withdrawing
capacity in comparison to the electron-donating ability of methoxy
substituents and weak electron-withdrawing ability of a fluoro-
substituent [34]. The electronic states with electron density on
the aniline moiety of the NO2PANs are shifted higher in energy
than the other derivatives that are better able to support electron
density on the aniline moiety. It follows that higher energy CT
states, such as 4NO2PAN’s LUMO + 2 state (Fig. 5), are lower in en-
ergy for the NO2PANs than p* states of the aniline system.
Computational studies
The structures were analyzed with respect to relevant hydrogen
bonding structure between the two rings relative to the central
amine group, as labeled in Fig. 1. Bond lengths and angles are in Ta-
ble 5. The 2FPANs have a planar geometry for the two rings to max-
imize the hydrogen bonds between the fluorine substituent and
the amine hydrogen [15,33]. In contrast, other PAN compounds
with substituents that do not induce hydrogen bonding to the
central amine have dihedral angles in the range of 20–40°, quite
similar to the parent compound PAN. In the compounds with a ni-
tro-substituent in the two position, it is expected that a hydrogen
bond would be formed between an oxygen of the nitro and the
central amine hydrogen [15]. However, due to the size of the nitro
group this arrangement does not take place and the dihedral angles
between the rings do not change significantly from the ones ob-
tained with unsubstituted PAN.
Representative calculated UV–Vis spectra for 4FPAN, 4OMePAN,
and 4NO2PAN are shown in Fig. 4. Several PAN derivatives studied
exhibit similar spectral bands like that of 4FPAN and 4OMePAN,
with the exception of the NO2PANs. This difference is similar to
the experimental spectra, showing more closely spaced absorbance
Conclusions
Novel NO2PAN derivatives were prepared by direct nitration of
PAN or oxidative-addition of nitroanilines to 1,4-naphthoquinone

E. Leyva et al. / Journal of Molecular Structure 1068 (2014) 1–7
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