NIR-Absorbing Donor–Acceptor Based 1,1,4,4-Tetracyanobuta-1,3-Diene (TCBD)- and Cyclohexa-2,5-Diene-1,4-Ylidene-Expanded TCBD-Substituted Ferrocenyl Phenothiazines

Article abstract of DOI:10.1002/asia.201700879

A series of unsymmetrical (D-A-D₁, D₁-π-D-A-D₁, and D₁-A₁-D-A₂-D₁; A=acceptor, D=donor) and symmetrical (D₁-A-D-A-D₁) phenothiazines (4 b, 4 c, 4 c′, 5 b, 5 c, 5 d, 5 d′, 5 e, 5 e′, 5 f, and 5 f′) were designed and synthesized by a [2+2] cycloaddition–electrocyclic ring-opening reaction of ferrocenyl-substituted phenothiazines with tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The photophysical, electrochemical, and computational studies show a strong charge-transfer (CT) interaction in the phenothiazine derivatives that can be tuned by varying the number of TCNE/TCNQ acceptors. Phenothiazines 4 b, 4 c, 4 c′, 5 b, 5 c, 5 d, 5 d′, 5 e, 5 e′, 5 f and 5 f′ show redshifted absorption in the λ=400 to 900 nm region, as a result of a low HOMO–LUMO gap, which is supported by TD-DFT calculations. The electrochemical study exhibits reduction waves at low potential due to strong 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and cyclohexa-2,5-diene-1,4-ylidene-expanded TCBD acceptors. The incorporation of cyclohexa-2,5-diene-1,4-ylidene-expanded TCBD stabilized the LUMO energy level to a greater extent than TCBD.

Full text of DOI:10.1002/asia.201700879

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: NIR Absorbing Donor-Acceptor Based 1,1,2,2-
Tetracyanobutadiene (TCBD) and Cyclohexa-2,5-Diene-1,4-
Ylidene-Expanded TCBD Substituted Ferrocenyl-Phenothiazines
Authors: Rajneesh Misra and Madhurima Poddar
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Asian J. 10.1002/asia.201700879
Link to VoR: http://dx.doi.org/10.1002/asia.201700879
A Journal of
A sister journal of Angewandte Chemie
and Chemistry – A European Journal

Chemistry - An Asian Journal
10.1002/asia.201700879
NIR Absorbing Donor–Acceptor Based 1,1,4,4–Tetracyanobuta–1,3–Diene (TCBD) and
Cyclohexa–2,5–Diene–1,4–Ylidene–Expanded
TCBD
Substituted
Ferrocenyl
Phenothiazines
Madhurima Poddar and Rajneesh Misra*
Department of Chemistry, Indian Institute of Technology Indore, Indore 453552, India
TOC:
1
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Chemistry - An Asian Journal
10.1002/asia.201700879
Abstract
A series of unsymmetrical (D–A–D , D –π–D–A–D and D –A –D–A –D ) and symmetrical
1
1
1
1
1
2
1
(
D –A–D–A–D ) type of phenothiazines 4b, 4c, 4c, 5b, 5c, 5d, 5d, 5e, 5e, 5f and 5fwere
1
1
designed and synthesized by [2 + 2] cycloaddition–electrocyclic ring-opening reaction of
ferrocenyl substituted phenothiazines with tetracyanoethylene (TCNE) and 7,7,8,8–
tetracyanoquinodimethane (TCNQ). The photophysical, electrochemical and computational
studies show strong charge-transfer (CT) interaction in the phenothiazine derivatives which can
be tuned by the variation of number of TCNE/TCNQ acceptors. The phenothiazines 4b, 4c, 4c,
5
b, 5c, 5d, 5d, 5e, 5e, 5f and 5f show red shifted absorption in 400–900 nm region, resulting in
low HOMO–LUMO gap which is supported by TD-DFT calculations. The electrochemical study
exhibits reduction waves at low potential due to strong 1,1,4,4–tetracyanobuta–1,3–diene
(TCBD) and cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD acceptors. The incorporation of
cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD stabilizes the LUMO energy level to greater
extent as compared to TCBD.
Introduction
-Conjugated molecular systems containing sulfur (S) and nitrogen (N) atoms are of
1
significant interest for various optoelectronic applications. A wide variety of S and N based
heterocyclic units such as thiazoles, benzothiazoles, benzothiadiazole, phenothiazines and many
more have been explored for non-linear optics (NLO), organic light emitting diodes (OLEDs),
2
organic photovoltaics (OPVs) and organic field-effect transistors (OFETs). The tuning of the
photonic properties of these systems can be achieved by altering the strength of donor or
2
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Chemistry - An Asian Journal
10.1002/asia.201700879
3
acceptor units, and the connecting -linker. Our group is interested in the design and synthesis
4
of small molecule based heterocyclic -conjugated molecular systems for organic photovoltaics.
The incorporation of heterocyclic moiety into the chromophore backbone leads to higher
5
chemical and thermal robustness. Phenothiazines are interesting mainly because of their
inherent folded conformation (folding angle of 158.58°). It can be transformed into a planar
conformation by the substitution of different functionalities in the N position of the
6
phenothiazine moiety. Phenothiazine allows variety of reactions including electrophilic
substitution at the aromatic position, nucleophilic reaction at the N position, oxidation at the
6
c
sulfur, etc. In addition, phenothiazines possess low reversible oxidation potential which makes
7
them suitable as electrophores in organic materials. The ferrocene is a strong electron donor and
its derivatives play an important role in NLO, superconductor, magnetic, semiconductor, and
8
redox catalyst materials. We were interested to incorporate cyano based 1,1,4,4–tetracyanobuta–
1
,3–diene (TCBD) and cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD acceptors on the
ferrocenyl phenothiazine derivatives to study the effect of the acceptors on photonic and
electrochemical properties of ferrocenyl phenothiazine. Cyano-based acceptors are one of the
9
most powerful unit for the application in organic electronic devices. The [2+2] cycloadditions
of tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) with the electron
rich alkynes followed by the electrocyclic ring-opening results in donor–acceptor type molecular
1
0,11,12
systems.
They have been used to form charge transfer (CT) complexes with a variety of
electron-rich organic and organometallic compounds which exhibit a number of interesting
1
0
properties such as electric conductivity. Diederich et al. are pioneer in the field of TCBD and
cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD chemistry and have studied a large variety
1
1,12
of TCBD and cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD derivatives.
3
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Chemistry - An Asian Journal
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Michinobu et al. have extensively explored the TCBD and cyclohexa–2,5–diene–1,4–
ylidene–expanded TCBD substituted polymers which are supposed to be promising materials for
1
3
photovoltaic applications. Shoji et al. have reported donor–acceptor based TCBD and
cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD molecules as redox active ICT
1
4
´
chromophores. Butenschoen et al. have reported a variety of 1,1 -disubstituted ferrocenyl
1
5
TCBD derivatives. Nakamura and coworkers have studied carbazole and its TCBD
1
6
derivatives. Our group has reported a vast variety of TCBD functionalized chromophores for
4
f, 17
organic electronics.
In continuation of our work on cyano based (TCNE/TCNQ) donor–acceptor materials,
herein we report the design and synthesis of unsymmetrical and symmetrical TCBD and
cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD chromophores where phenothiazine and
ferrocene are acting as strong donors. In this manuscript our objective was to improve the
photonic and electronic properties of ferrocene substituted phenothiazines by incorporating
TCNE and TCNQ in between phenothiazine and ferrocene building blocks. We have further
explored a comparative photophysical and electrochemical studies by varying the number of
TCBD and cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD acceptor as well as ferrocene
donor on the phenothiazine moiety. Additionally, theoretical calculations were performed in
order to study the conformation and the photonic properties of phenothiazines.
Results and Discussion
The TCBD and cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD substituted
ferrocenyl phenothiazines 4b, 4c, 4c, 5b, 5c, 5d, 5d, 5e, 5e, 5f and 5f were designed and
synthesized by the [2 + 2] cycloaddition–electrocyclic ring-opening reaction of ferrocenyl
4
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Chemistry - An Asian Journal
10.1002/asia.201700879
phenothiazines 4a and 5a with TCNE and TCNQ (Scheme 1, Scheme 2 and Scheme 3). The
ferrocenyl phenothiazines 4a and 5a were synthesized by the Sonogashira cross-coupling
reaction of 3-bromo-10-propylphenothiazine and 3,7-dibromo-10-propylphenothiazines with the
ethynyl ferrocene. The Pd-catalyzed Sonogashira cross-coupling reaction of phenothiazines 3a
and 3b with ethynyl ferrocene at 60 °C resulted ferrocenyl phenothiazines 4a and 5a in 50% and
5
1% yields respectively (Scheme S1).
In order to explore the effect of number of TCBD and cyclohexa–2,5–diene–1,4–ylidene–
expanded TCBD acceptors on the ferrocenyl phenothiazines the mono-(4b, 4c, 4c, 5b and 5d,
d) and di-(5c, 5e, 5e, 5f and 5f) substituted ferrocenyl phenothiazines were synthesized. The
5
precursors 4a and 5a undergo the [2 + 2] cycloaddition–electrocyclic ring-opening reaction with
TCNE at room temperature within 4 hours in CH Cl solvent, resulted TCBD functionalized
2
2
phenothiazines 4b and 5b in 83% and 85% yield, respectively (Scheme 1 and Scheme 2). The
ferrocenyl phenothiazine 5a undergoes a similar transformation by using excess amount of
TCNE which resulted phenothiazine 5c in 90% yield at 40 °C, for 12 hours in CH Cl solvent.
2
2
The derivatives of cyclohexa–2,5–diene–1,4–ylidene–expanded TCBDs were obtained as non-
separable regioisomeric mixtures. The reaction of ferrocenyl phenothiazine 4a with 1 equivalent
of TCNQ at 40 °C, for 12 hours in CH Cl solvent results in 40.5:59.5 regioisomeric
2
2
phenothiazines 4c and 4c in 85% yield. The reaction of ferrocenyl phenothiazine 5a with 1
equivalent of TCNQ at 40 °C, for 12 hours in CH Cl solvent results in 37.5:62.5 regioisomeric
2
2
phenothiazines 5d and 5d in 80% yield. The phenothiazines 5e and 5e was obtained in
5.1:54.9 regioisomeric mixtures by the similar reaction of excess amount of TCNQ with
phenothiazine 5a at 40 °C, for 4 days in CH Cl solvent and resulted in 70% yield (Scheme 2).
4
2
2
5
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Chemistry - An Asian Journal
10.1002/asia.201700879
4
c
b
4
a
4
c
4
Scheme 1. Synthetic route for ferrocenyl phenothiazines 4b, 4c and 4c.
The reaction of isomeric cyclohexa–2,5–diene–1,4–ylidene–expanded TCBDs functionalized
phenothiazine 5d and 5d with TCNE at 40 °C, for 12 hours in CH Cl solvent results in
2
2
3
3.1:66.9 regioisomeric phenothiazine 5f and 5f in 80% yield. The phenothiazines 5f and 5f
was also synthesized from TCBD functionalized phenothiazine 5b with TCNQ at 40 °C, for 12
hours in CH Cl solvent which resulted in 33.1:66.9 regioisomeric mixtures with 81% yield,
2
2
(
Scheme 3). The ferrocenyl phenothiazines as well as TCBD and cyclohexa–2,5–diene–1,4–
ylidene–expanded TCBD functionalized ferrocenyl phenothiazines 4b, 4c, 4c, 5b, 5c, 5d, 5d,
e, 5e, 5f and 5f are soluble in common organic solvents such as dichloromethane, chloroform,
5
1
13
tetrahydrofuran, toluene and were well characterized by H NMR, C NMR and HRMS
techniques.
6
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Chemistry - An Asian Journal
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5
b
5
c
5
a
5
d
5
d
5
e
5e
Scheme 2. Synthetic route for ferrocenyl phenothiazines 5b, 5c, 5d, 5d, 5e and 5e.
5
b
5
f
5
d
5d
5
f
Scheme 3. Synthetic route for ferrocenyl phenothiazines 5f and 5f.
7
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Photophysical Properties
The electronic absorption spectra of the ferrocenyl phenothiazines and their TCBD and
cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD conjugates 4b, 4c and 5b–5f were recorded
in dichloromethane at room temperature (Figure 1), and the data are compiled in Table 1.
The TCBD functionalized phenothiazines 4b, 5b and 5c exhibit ICT transition at 531 nm,
5
47 nm and 537 nm respectively which indicates that the incorporation of TCBD unit results in
strong donor–acceptor interaction. On the other hand, the cyclohexa–2,5–diene–1,4–ylidene–
expanded TCBD substituted phenothiazines 4c, 5d and 5e exhibit two strong absorption bands
due to the strong electron accepting capability of cyclohexa–2,5–diene–1,4–ylidene–expanded
TCBD.
The absorption band between 454–468 nm and 737–794 nm can be attributed to the π–π*
transition band and CT band, respectively. The TCBD/cyclohexa–2,5–diene–1,4–ylidene–
expanded TCBD substituted phenothiazine 5f exhibit π–π* transition band and CT band at 524
nm and 800 nm respectively. It also shows a shoulder band at 439 nm which may be due to the
presence of two different acceptors in phenothiazine 5f. The incorporation of the TCBD and
cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD acceptor units resulted in strong donor–
acceptor interaction which was further explained by TD-DFT calculation in dichloromethane
phase.
8
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1.0
(
i) 0.8
(ii)
4
5
5
b
b
c
4c
5d
5e
0.8
0
0
0
0
.6
.4
.2
.0
5
f
0
0
0
0
.6
.4
.2
.0


4
00
500
600
700
800
900
4
00
500
600
700
800
900
1000
1100
Wavelength (nm)
Wavelength (nm)
Figure 1. The electronic absorption spectra of phenothiazines (i) 4b, 5b and 5c, and (ii) 4c, 5d,
-5
5
e and 5f in dichloromethane (1 ×10 M).
The optical band gap of phenothiazines 4b, 4c and 5b–5f follow the order
4
b>5c>5b>5f>4c>5e>5d. The trend clearly indicates the influence of TCBD and cyclohexa–
,5–diene–1,4–ylidene–expanded TCBD on ferrocenyl phenothiazines leading to the red shifted
2
electronic absorption and low optical band gap which is further explained by TDDFT
calculations where the effect of regioisomeric mixture on the electronic spectra is also discussed.
Electrochemical Properties
The electrochemical properties of ferrocenyl phenothiazines 4b, 4c and 5b–5f were explored by
cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in dry dichloromethane
(DCM) solution at room temperature using tetrabutylammonium hexafluorophosphate (TBAPF6)
as a supporting electrolyte. The electrochemical data are compiled in Table 1, and the
representative CV and DPV plots are shown in Figure 2, and Figure S22–S27.
9
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Chemistry - An Asian Journal
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1
8
In general, phenothiazine shows one reversible oxidation wave. The TCBD and cyclohexa–
2
,5–diene–1,4–ylidene–expanded TCBD substituted ferrocenyl phenothiazines show an
additional reversible oxidation wave which corresponds to the oxidation of ferrocenyl ring.
Table 1. Photophysical and electrochemical properties of phenothiazines 4b, 4c, 4c, 5b, 5c, 5d,
5
d, 5e, 5e, 5f and 5f.
a
d
Phenothiazines
Photophysical data
Theoretical
Electrochemical data
c
data
abs (nm)

Optical
HOMO-
Eox(V)
Ered(V)
−
1
(
M
Band Gap LUMO energy
−
1
b
cm )
(eV)
gap (eV)
4
b
531
454
547
65323
1.59
1.23
1.54
2.64
2.22, 2.04
2.49
0.71
0.55
–0.75
1.04
–
4
c, 4c
60222
68007
–0.56
0
.65
5
b
0.33
–0.76
–1.06
0
0
.68
.81
5
c
537
468
70166
77333
1.57
1.19
2.63
0.68
.95
–0.72
–1.07
0
5
d, 5d
2.03, 1.87
0.32
–0.55
0
0
.55
.68
5
e, 5e
448
524
80368
77158
1.21
1.24
2.24, 2.00
2.28, 2.11
0.54
.79
–0.56
0
5
f, 5f
0.56
–0.54
–0.83
–1.11
0
0
.67
.85
a
−5
b
Absorbance measured in dichloromethane at 110 M concentration; abs: absorption wavelength; : extinction coefficient.
c
determined from onset wavelength of the UV/Vis absorption; obtained from density functional theory calculations at B3LYP/6-
d
−1
3
1+G** level; recorded by cyclic voltammetry, in 0.1 M solution of TBAPF in DCM at 100 mV s scan rate versus SCE
6
electrode.
1
0
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The phenothiazines 4c, 5c and 5e show two oxidation potentials at (+0.68 V, +0.95 V), (+0.55 V,
0.66 V) and (+0.54 V, +0.79 V) respectively, where the first oxidation potentials are attributed
+
to the ferrocenyl moiety and the second oxidation potentials are due to the phenothiazinyl
moiety. The phenothiazine 4b exhibits only one oxidation wave at +0.71 V due to the
1
7a–c
simultaneous reversible oxidation of ferrocene and phenothiazine units.
b, 5d and 5f show three oxidation peaks at (+0.33 V, +0.68 V and +0.81 V), (+0.32V, +0.55V,
0.68 V) and (+0.56 V, +0.67V and +0.85 V), respectively where the first two oxidation
The phenothiazines
5
+
potentials correspond to the ferrocene moiety and the third oxidation potential can be attributed
to the phenothiazine moiety. The trend in the first oxidation potential of the phenothiazines 4b,
4
c and 5b–5f follows the order 4b>5c>5f>4c>5e>5b>5d.
3
2
1
0.0µ
0.0µ
0.0µ
0.0
2
1
0.0µ
0.0µ
5
c CV
5f CV
0
.0
-
-
-
10.0µ
20.0µ
30.0µ
-10.0µ
-20.0µ
-30.0µ
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential (V) vs SCE
Potential (V) vs SCE
Figure 2. Cyclic voltammograms of phenothiazines 5c and 5f 0.01 M in 0.1 M TBAPF in
6
−
1
°
dichloromethane at a scan rate of 100 mV s standard calomel electrode (SCE) at 25 C.
The TCBD functionalized phenothiazines 4b, 5b and 5c exhibit a reversible two–step reduction
wave attributed to one–electron transfer in each step and show the reduction potential value at (–
0
.75 V, –1.04 V), (–0.76 V, –1.06 V) and (–0.72 V, –1.07 V), respectively due to the TCBD
units. The two reduction waves correspond to the formation of radical anions and dianions. A
1
1
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Chemistry - An Asian Journal
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positive shift in the first reduction potential was observed in phenothiazine 5c because of the
presence of two electron withdrawing TCBD units. This indicates that increasing the number of
TCBD unit enhances the π-accepting properties. The cyclohexa–2,5–diene–1,4–ylidene–
expanded TCBD adduct of phenothiazines 4c, 5d and 5e show only one reduction wave, whose
potentials were identified at –0.56 V, –0.55 V and –0.56 V, respectively which is due to the
simultaneous electrochemical reduction of cyclohexa–2,5diene–1,4ylidene–expanded TCBD
17a–c
units.
The phenothiazine 5f shows three reduction wave at –0.54 V, –0.83 V, –1.11 V where
the first reduction potential value corresponds to the cyclohexa–2,5diene–1,4ylidene–expanded
TCBD moiety and potential at –0.83 V and –1.11 V could be attributed to the TCBD moiety.
Therefore the introduction of cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD adduct shows
lower reduction potential as compared to TCBDs. The result reveals that the cyclohexa–2,5–
diene–1,4–ylidene–expanded TCBD unit stabilizes the LUMO energy level to greater extent than
that of TCBD. The HOMO and LUMO energy levels are calculated from the onset oxidation and
reduction potentials. The corresponding HOMO and LUMO energy levels of phenothiazines 4b,
4
c, 5b, 5c, 5d, 5e and 5f are –4.98 eV, –4.83 eV, –4.53 eV, –4.91 eV, –4.62 eV, –4.84 eV, –4.86
eV and –3.82 eV, –3.94 eV, –3.79 eV, –3.81 eV, –3.96 eV, –4.00 eV, –3.88 eV, respectively.
Theoretical Calculations
The density functional theory calculation was performed on phenothiazines 4b, 4c, 5b, 5c, 5d, 5e
and 5f to explore the structure and electronic properties at B3LYP/6-31+G** level for B, C, F,
1
9
H, N and S. The optimized structures of phenothiazines are nonplanar with twisted geometry
(Table S1–S3). The incorporation of strong cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD
acceptor unit lower the LUMO energy level to greater extent as compared to TCBD, which
results in low HOMO–LUMO gap and red shifted electronic absorption. The theoretically
1
2
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Chemistry - An Asian Journal
10.1002/asia.201700879
determined HOMO levels of phenothiazines 4b, 4c, 5b, 5c, 5d, 5e and 5f are –5.53 eV, –5.62
eV, –5.41 eV, –5.95 eV, –5.39 eV, –6.03 eV and –6.0 eV whereas LUMO levels are –2.89 eV, –
3
.40 eV, –2.92 eV, –3.31 eV, –3.36 eV, –3.79 eV and –3.72 eV (Figure 3). The comparison
between FMOs of the isomeric phenothiazines (4c and 4c), (5d and 5d), (5e and 5e) and (5f
and 5f) are shown in the Supporting Information (Table S4–S7), where the calculated HOMO
levels of 4c, 5d, 5eand 5f are –5.45 eV, –5.29 eV, –5.87 eV and –5.93 eV, and the LUMO
levels are –3.41 eV, –3.42 eV, –3.87 eV and –3.82 eV respectively. The data reveals that the
LUMO energy levels of the isomeric phenothiazines 4c, 5d, 5eand 5f are more stabilized as
compared to the 4c, 5d, 5e and 5f, respectively.
Figure 3. Energy diagram of phenothiazines 4b, 4c and 5b–5f estimated by DFT calculations.
The time-dependent DFT calculation was performed at the B3LYP/6-31G (d, p) level on
optimized phenothiazines in dichloromethane to evaluate the absorption properties. The
1
3
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Chemistry - An Asian Journal
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transitions with composition, oscillator strengths, and assignments are as shown in Table S8. The
molecular orbital diagrams are shown in Table S1–S3.
The TD-DFT calculation shows absorption band at 578, 625 and 602 nm respectively for
phenothiazines 4b, 5b and 5c due to the ICT, whereas the phenothiazines 4c, 4c, 5d, 5d, 5e, 5e,
5
f and 5fexhibit two absorption bands in the visible region which can be attributed to the π–π*
transition at shorter wavelength and ICT at longer wavelength (Table S8). The main ICT
transition for phenothiazine 4b occurs from HOMO→LUMO+1, and HOMO→LUMO for
phenothiazines 5b and 5c. The charge-transfer occurs from HOMO→LUMO+2, HOMO–
3
→LUMO, HOMO–2→LUMO and HOMO–1→LUMO in phenothiazines 4c, 5d, 5e and 5f,
respectively whereas for 4c, 5d, 5eand 5f the charge-transfer occurs from HOMO→LUMO.
The data shows that the isomeric phenothiazines 4c, 5d, 5e and 5f are red shifted as compared
to 4c, 5d, 5e and 5f, respectively. The theoretical electronic absorption wavelengths were found
to be higher than those of experimental values which might be due to various factors, e.g. solvent
effect, dipole moment and temperature.
Conclusion
In summary, a series of TCBD and cyclohexa–2,5–diene–1,4–ylidene–expanded TCBD
substituted phenothiazines 4b, 4c, 4c, 5b, 5c, 5d, 5d, 5e, 5e, 5f and 5f were synthesized by
[
2+2] cycloaddition–electrocyclic ring-opening reactions. Their electrochemical properties
reveals that the incorporation of strong acceptors TCBD and cyclohexa–2,5–diene–1,4–ylidene–
expanded TCBD facilitates the reduction process of the ferrocenyl-phenothiazines which leads to
low HOMO–LUMO gap . The electronic absorption spectra exhibit strong ICT at longer
wavelength and strong donor–acceptor interactions. In particular, DFT and TDDFT calculations
1
4
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Chemistry - An Asian Journal
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reveal a broad understanding of the electronic structure and absorption spectra of the
phenothiazine chromophores which reveal that the incorporation of TCBD and cyclohexa–2,5–
diene–1,4–ylidene–expanded TCBD acceptor group perturbs HOMO–LUMO gap of the
phenothiazines to a greater extent which is in a good agreement with the experimental values.
The results provide an important procedure of designing new donor–acceptor chromophores with
low band gaps. The photovoltaic applications of these phenothiazines are currently ongoing in
our laboratory.
Experimental Section
General Methods. Chemicals were used as received unless otherwise indicated. All the oxygen
1
or moisture sensitive reactions were carried out under argon atmosphere. H NMR spectra were
recorded using a 400 MHz spectrometer. Chemical shifts are reported in delta (δ) units,
expressed in parts per million (ppm) downfield from tetramethylsilane (TMS) using residual
1
3
protonated solvent as an internal standard CDCl , 7.26 ppm. C NMR spectra were recorded
3
using a 400 MHz spectrometer. Chemical shifts are reported in delta (δ) units, expressed in parts
per million (ppm) downfield from tetramethylsilane (TMS) using the solvent as internal standard
1

CDCl , 77.0 ppm. The H NMR splitting patterns have been described as “s, singlet; d,
3
doublet; t, triplet and m, multiplet”. UV/Vis spectrums of all compounds were recorded in
dichloromethane solution. Cyclic voltammograms were recorded on electrochemical analyzer
using Glassy carbon as working electrode, Pt wire as the counter electrode, and Saturated
-1
Calomel Electrode (SCE) as the reference electrode. The scan rate was 100mVs for Cyclic
Voltammetry. A solution of tetrabutylammonium hexafluorophosphate (TBAPF ) in CH Cl
2
6
2
(0.1M) was used as supporting electrolyte.
1
5
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Chemistry - An Asian Journal
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Synthesis of 4b. Tetracyanoethylene (TCNE) (28.2 mg, 0.22 mmol) was added to a solution of
compound 4a (98.8 mg, 0.22 mmol) in CH Cl (50 mL). The mixture was stirred at room
2
2
temperature for 4 h. After the completion of the reaction the solvent was removed in vacuum,
and the product was purified by column chromatography with CH Cl as the eluent which yield
2
2
1
4
b as a dark violet colored solid. Yield: 106.9 mg, 83%. H NMR (400 MHz, CDCl ): δ = 7.59
3
(d, J = 7.28 Hz, 1H), 7.30 (s, 1H), 7.16 (t, J = 7.56 Hz, 1H), 7.06-6.95 (m, 2H), 6.87-6.81 (m,
2
2
1
1
H), 5.18 (s, 1H), 4.95 (s, 1H), 4.84 (s, 1H), 4.70 (s, 1H), 4.45-4.37 (m, 5H), 3.82 (t, J=7.04 Hz,
1
3
H), 1.85-1.79 (m, 2H), 1.01 (t, J = 7.28 Hz, 3H); C NMR (100 MHz, CDCl ): δ = 173.3,
3
63.5, 150.8, 142.2, 129.7, 128.6, 127.9, 127.6, 127.5, 125.3, 124.3, 124.2, 122.6, 116.1, 114.9,
13.7, 112.8, 112.5, 80.3, 79.1, 75.5, 75.4, 74.9, 72.7, 72.5, 72.0, 71.9, 71.5, 49.9, 19.9, 11.1;
+
HRMS (ESI-TOF): m/z calculated for C H FeN S= 578.1097 [M+H] , measured 578.1082
3
3
23
5
+
[
M+H]
Synthesis of 4c and 4c. Tetracyanoquinodimethane (TCNQ) (44.9 mg, 0.22 mmol) was added
to a solution of compound 4a (98.8 mg, 0.22 mmol) in CH Cl (50 mL). The mixture was
2
2
refluxed at 40 °C for 12 h. After the completion of the reaction the solvent was removed in
vacuum and the product was purified by column chromatography with CH Cl as the eluent
2
2
which yield 4c and 4c in 40.5:59.5 calculated regioisomeric mixtures as a dark brown colored
1
solid. Overall Yield: 122.0 mg, 85%. H NMR (400 MHz, CDCl ): δ = 8.31 (d, J = 9.28 Hz, 1H),
3
7
9
7
.55 (d, J = 7.52 Hz, 1H), 7.45 (d, J = 9.52 Hz, 1H), 7.38 (d, J = 9.52 Hz, 1H), 7.23–7.10 (m,
H), 7.02–6.95 (m, 4H), 6.89–6.77 (m, 5H), 4.97–4.76 (m, 7H), 4.42–4.26 (m, 11H), 3.85 (t, J =
1
3
.04 Hz, 2H), 3.80 (t, J = 7.04 Hz, 2H), 1.87–1.77 (m, 4H), 1.04–0.98 (m, 6H); C NMR (100
MHz, CDCl ): δ = 177.4, 167.4, 157.1, 154.6, 154.2, 150.4, 149.6, 148.5, 142.9, 142.3, 134.7,
3
1
35.2, 134.1, 131.6, 131.4, 130.6, 130.5, 129.9, 129.0, 128.9, 128.2, 127.9, 127.6, 126.7, 126.0,
1
6
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Chemistry - An Asian Journal
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1
8
7
25.6, 125.4, 125.1, 124.8, 124.2, 124.0, 123.8, 115.0, 114.9, 114.3, 113.8, 113.7, 113.5, 113.3,
0.0, 79.6, 79.5, 79.4, 77.8, 76.2, 76.1, 75.5, 75.0, 74.9, 74.8, 74.7, 73.9, 72.8, 72.6, 72.5, 72.4,
1.8, 71.1, 49.8, 49.4, 20.1, 20.0, 11.2, 11.1; HRMS (ESI-TOF): m/z calculated for
+
+
C H FeN S= 676.1229 [M+Na] , measured 676.1210 [M+Na]
39
27
5
Synthesis of 5b. Tetracyanoethylene (TCNE) (19.2 mg, 0.15 mmol) was added to a solution of
compound 5a (98.6 mg, 0.15 mmol) in CH Cl (50 mL). The mixture was stirred at room
2
2
temperature for 4 h. After the completion of the reaction the solvent was removed in vacuum,
and the product was purified by column chromatography with CH Cl as the eluent to yield 5b as
2
2
1
a dark violet colored solid. Yield: 100.10 mg, 85%; H NMR (400 MHz, CDCl ): δ = 7.58 (d, J =
3
8
4
2
1
8
Hz, 1H), 7.28 (s, 2H), 7.16 (s, 1H), 6.83–6.76 (m, 2H), 5.29 (s, 3H), 5.18 (s, 1H), 4.95 (s, 1H),
.85 (s, 1H), 4.71 (s, 1H), 4.45 (s, 6H), 6.94 (s, 6H), 3.81 (t, J = 6.04 Hz, 2H), 1.83–1.79 (m,
1
3
H), 1.02 (t, J = 6.76 Hz, 3H); C NMR (100 MHz, CDCl ): δ = 173.3, 163.4, 150.2, 141.4,
3
30.9, 130.0, 129.7, 127.5, 124.8, 124.5, 122.6, 120.1, 115.8, 115.1, 113.7, 112.8, 112.7, 112.4,
9.5, 84.2, 80.6, 79.1, 75.5, 75.0, 72.5, 72.0, 71.4, 70.0, 68.9, 64.9, 49.9, 19.9, 11.1; HRMS
+
+
(
ESI-TOF): m/z calculated for C H Fe N S= 824.0633 [M+K] , measured 824.0630 [M+K]
45 31 2 5
Synthesis of 5c. Tetracyanoethylene (TCNE) (38.4 mg, 0.3 mmol) was added to a solution of
compound 5a (98.6 mg, 0.15 mmol) in CH Cl (50 mL). The mixture was stirred at room
2
2
temperature for 12 h. After the completion of the reaction the solvent was removed in vacuum,
and the product was purified by column chromatography with CH Cl as the eluent which yield
2
2
1
5
c as a dark violet colored solid. Yield: 123.4 mg, 90%; H NMR (400 MHz, CDCl ): δ = 7.54 (t,
3
J = 8.28 Hz, 2H), 7.22 (d, J = 6.04 Hz, 2H), 6.89–6.86 (m, 2H), 5.30 (d, J = 7.76 Hz, 2H), 5.01
(s, 2H), 4.88 (s, 2H), 4.59 (d, J = 5.28 Hz, 2H), 4.46 (s, 8H), 3.82 (t, J = 7.04 Hz, 2H), 1.84–1.79
1
3
(
m, 2H), 1.03 (t, J = 7.28 Hz, 3H); C NMR (100 MHz, CDCl ): δ = 172.6, 163.5, 147.9, 147.8,
3
1
7
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Chemistry - An Asian Journal
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1
29.8, 127.5, 127.4, 126.2, 124.3, 116.1, 113.6, 112.8, 112.2, 111.9, 82.9, 78.8, 74.5, 72.7, 72.2,
+
7
1.8, 50.3, 19.9, 11.1; HRMS (ESI-TOF): m/z calculated for C H Fe N S= 952.0756 [M+K] ,
51 31 2 9
+
measured 952.1018 [M+K]
Synthesis of 5d and 5d. Tetracyanoquinodimethane (TCNQ) (30.6 mg, 0.15 mmol) was added
to a solution of compound 5a (98.6 mg, 0.15 mmol) in CH Cl (50 mL). The mixture was
2
2
refluxed at 40 °C for 12 h. After the completion of the reaction the solvent was removed in
vacuum and the product was purified by column chromatography with CH Cl as the eluent
2
2
which yield 5d and 5d in 37.5:62.5 calculated regioisomeric mixtures as a dark brown colored
1
solid. Overall Yield: 103.4 mg, 80%; H NMR (400 MHz, CDCl ): δ = 8.31 (d, J = 9.8 Hz, 1H),
3
7
.56 (d, J = 8.28 Hz, 1H), 7.46 (d, J = 9.52 Hz, 2H), 7.38 (d, J = 9.52 Hz, 1H), 7.20 (t, J = 8.76
Hz, 5H), 7.13 (s, 1H), 7.03 (d, J = 9.28 Hz, 1H), 6.88 (d, J = 8.52 Hz, 1H), 6.80–6.74 (m, 5H),
5
2
.28 (s, 2H), 4.93 (s, 1H), 4.83 (s, 3H), 4.76 (s, 1H), 4.45–4.20 (m, 31H), 3.84 (t, J = 7.0 Hz,
1
3
H), 3.79 (t, J = 7.04 Hz, 2H), 1.87–1.77 (m, 4H), 1.05–0.98 (m, 6H); C NMR (100 MHz,
CDCl ): δ = 167.3, 156.9, 154.6, 154.2, 149.8, 149.4, 147.9, 142.2, 141.5, 134.6, 134.1, 134.0,
3
1
1
1
7
31.5, 130.9, 130.5, 130.1, 130.0, 129.8, 129.2, 129.0, 128.2, 127.0, 126.1, 125.5, 125.1, 124.9,
24.6, 124.1, 122.9, 122.7, 119.9, 119.5, 115.7, 115.6, 115.3, 115.0, 114.3,113.8, 113.6, 113.4,
13.2, 89.5, 89.4, 89.2, 84.2, 80.3,79.5, 79.4, 77.9, 77.8, 76.1, 75.7, 74.9, 74.8, 73.8, 72.7, 72.6,
2.5, 72.4, 71.8, 71.3, 69.9, 49.8, 20.1, 19.9, 11.2, 11.1; HRMS (ESI-TOF): m/z calculated for
+
+
C H Fe N S= 861.1309 [M] , measured 861.1283 [M] .
51
35
2
5
Synthesis of 5e and 5e. Tetracyanoquinodimethane (TCNQ) (61.2 mg, 0.3 mmol) was added to
a solution of compound 5a (98.6 mg, 0.15 mmol) in CH Cl (50 mL). The mixture was refluxed
2
2
at 40 °C for 4 days. After the completion of the reaction the solvent was removed in vacuum and
the product was purified by column chromatography with CH Cl as the eluent which yield 5e
2
2
1
8
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Chemistry - An Asian Journal
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and 5e in 45.1:54.9 calculated regioisomeric mixtures as a dark brown colored solid. Overall
1
Yield: 111.9 mg, 70%; H NMR (400 MHz, CDCl ): δ = 8.30–8.26 (m, 3H), 7.55–7.42 (m, 7H),
3
7
1
1
1
1
1
1
.31–7.28 (m, 4H), 7.22–7.18 (m, 6H), 7.13 (s, 1H), 7.02 (s, 2H), 6.91–6.73 (m, 8H), 5.28 (s,
H), 4.95–4.80 (m, 13H), 4.36–4.29 (m, 21H), 3.82 (t, J = 8.0 Hz, 2H), 3.75 (t, J = 9.52 Hz, 2H),
1
3
.87–1.74 (m, 4H), 1.05–0.97 (m, 6H); C NMR (100 MHz, CDCl ): δ = 167.2, 156.3, 156.0,
3
54.4, 153.9, 148.3, 148.1, 147.5, 147.4, 145.4, 134.0, 133.8, 132.1, 131.5, 130.6, 130.5, 130.1,
29.9, 128.9, 128.6, 127.8, 125.0, 124.2, 123.9, 115.9, 115.8, 115.6, 115.0, 114.9, 113.4, 113.1,
12.9, 112.8, 112.7, 82.1, 81.7, 79.3, 79.1; HRMS (ESI-TOF): m/z calculated for C H Fe N S=
6
3
39
2
9
+
+
088.1644 [M+Na] , measured 1088.1651 [M+Na] .
Synthesis of 5f and 5f. Tetracyanoethylene (TCNE) (15.4 mg, 0.12 mmol) was added to a
solution of compound 5d and 5d (103.4 mg, 0.12 mmol) in CH Cl (50 mL). The mixture was
2
2
refluxed at 40 °C for 12 h. After the completion of the reaction the solvent was removed in
vacuum, and the product was purified by column chromatography with CH Cl as the eluent
2
2
which yield 5f and 5f in 33.1:66.9 calculated regioisomeric mixtures as a dark colored solid.
1
Overall Yield: 95 mg, 80%; H NMR (400 MHz, CDCl ): δ = 8.29 (d, J = 9.6 Hz, 1H), 7.58–7.45
3
(m, 5H), 7.34–7.28 (m, 2H), 7.21–7.07 (m, 6H), 6.93–6.75 (m, 5H), 5.28–5.23 (m, 3H), 4.99–
4
2
.81 (m, 11H), 4.65 (d, J=7.28 Hz, 1H), 4.57 (s, 2H), 4.44–4.30 (m, 20H), 3.85 (t, J = 6.52 Hz,
1
3
H), 3.78 (t, J = 6.52 Hz, 2H), 1.84–1.75 (m, 4H), 1.05–0.99 (m, 6H); C NMR (100 MHz,
CDCl ): δ = 172.6, 172.5, 167.2, 163.5, 163.4, 158.2, 154.5, 153.9, 148.7, 148.3, 147.9, 147.4,
3
1
1
1
7
45.3, 134.2, 134.0, 133.9, 133.8, 132.1, 131.4, 130.7, 130.5, 129.9, 129.8, 129.7, 129.0, 128.7,
28.1, 128.0, 127.8, 127.5, 126.0, 125.8, 125.0, 124.2, 124.1, 123.9, 116.0, 115.9, 115.8, 115.0,
14.2, 113.7, 113.5, 113.4, 112.9, 112.8, 112.7, 112.4, 112.2, 111.9, 82.7, 82.6, 82.2, 82.1, 79.1,
8.9, 78.8, 75.8, 75.3, 75.1, 74.9, 73.9, 73.0, 72.7, 72.6, 72.1, 71.8, 71.7, 53.4, 50.2, 19.9, 19.8,
1
9
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+
1
1
1.1, 11.0; HRMS (ESI-TOF): m/z calculated for C H Fe N S= 1012.1330 [M+Na] , measured
58
39
2
9
+
012.1462 [M+Na] .
Supporting Information
1
13
General experimental methods and H and C NMR and HRMS spectra of all the new
compounds, cyclic voltammograms, computational results.
Acknowledgements
The work was supported by DST, INSA and CSIR Govt. of India, New Delhi. We are grateful to
the Sophisticated Instrumentation Centre (SIC), IIT Indore.
Keywords: Cross-coupling, Cycloaddition–Electrocyclic ring-opening, UV/Vis spectroscopy,
Cyclic Voltammetry, Density functional calculations, TD-DFT calculations.
Notes and References
Department of Chemistry, Indian Institute of Technology Indore, MP 453552, India. Fax: +91
7
312361482; Tel: +91 7312438710; E-mail: rajneeshmisra@iiti.ac.in
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