Synthesis, photophysical and electrochemical properties of bay-axial perylene diimide-subphthalocyanine dyads

Article abstract of DOI:10.1016/j.tetlet.2014.05.120

Two novel perylene diimide-subphthalocyanine dyads in which the two macrocycles are linked together by a B-O bond have been synthesized. The electronic communication between the two chromophores was investigated by photophysical and electrochemical studie

Full text of DOI:10.1016/j.tetlet.2014.05.120

Accepted Manuscript
Synthesis, Photophysical and Electrochemical Properties of Bay-Axial Perylene
Diimide-Subphthalocyanine Dyads
Lakshmi C. Kasi Viswanath, Laura D. Shirtcliff, S. Krishnan, Nisha V. Handa,
K. Darrell Berlin
PII:
S0040-4039(14)00953-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.05.120
TETL 44711
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
17 April 2014
26 May 2014
29 May 2014
Please cite this article as: Kasi Viswanath, L.C., Shirtcliff, L.D., Krishnan, S., Handa, N.V., Darrell Berlin, K.,
Synthesis, Photophysical and Electrochemical Properties of Bay-Axial Perylene Diimide-Subphthalocyanine
Dyads, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.05.120
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Synthesis, Photophysical and Electrochemical Properties of
Bay-Axial Perylene Diimide-Subphthalocyanine Dyads
Lakshmi C. Kasi Viswanath, Laura D. Shirtcliff, S. Krishnan, Nisha V. Handa, and
K. Darrell Berlin*
Department of Chemistry, Oklahoma State University, 107 Physical sciences, Stillwater, OK-74078
Abstract: Two novel perylene diimide-subphthalocyanine dyads in which the two macrocycles are linked
together by a B-O bond have been synthesized. The electronic communication between the two
chromophores was investigated by photophysical and electrochemical studies.
Keywords: Subphthalocyanine, Perylene Diimides, macrocycles.
Introduction
Substantial efforts have been contributed towards the broadening of research in multicomponent
molecular assemblies.1 The utilization of such systems in various molecular photoelectronic devices and
also as artificial light energy harvesting systems require efficient electron transfer via photoexcitation with
light of specific wavelength.2 The competence of electronic communication in a multicomponent
asesembly relies on the orientation, relative distance, types of linkages and the electronic characteristics of
the macrocycles involved.3
Perylene Diimides (PDIs) have recently emerged as a good platform for the synthesis of such
multicomponent systems.4 Although a variety of donor acceptor dyads involving PDIs5 with covalent
linkages have been reported in the literature, the poor solubility of such multicomponent systems remains a
major issue for their potential use in molecular devices. As a means to overcome this problem,
Subphthalocyanines6 (SubPcs) have been chosen as suitable candidates as their counterparts.
SubPcs possess high solubility in the resulting dyads due to their convex shaped geometry. To the best
of our knowledge, there is only one example of Perylene-SubPc dyad7 reported in the literature.
Nonetheless, the SubPc in this dyad is covalently linked in the imide region of the perylene diimide by
forming a B-N bond. Our expectation is that the fusion of a SubPc with a PDI in the bay region can
execute fast electron transfer and thereby can be utilized in various optoelectronic devices.
Results and Discussion
We report herein the synthesis, photophysical and electrochemical properties of dihydroxybenzene
derivatives appended PDIs (PDI-OH) and Perylene Diimide–Subphthalocyanine (PDI-SubPc) dyads where
the SubPc chromophore is linked to the PDI in the bay region through an axial B-O bond. The distance
between the two macrocycles is coordinated by the type of linkage involved (m-, and p-aryl substitution)
between the two moieties. Hence o-, m-, and p- substituted PDI-OHs were synthesized from commerically
available Perylene Tetracarboxylic Acid Dianhydride (PTCDA) (1).
* Corresponding author. Tel.: +001-405-744-5950; fax: +001-405-744-6007; email: kenneth.d.berlin@okstate.edu

PDI-SubPc dyads were obtained by the nucleophilic substitution of the PDI-OHs on
dodecafluorosubphthalocyanine (Scheme 1).
Scheme I
O
N
O
O
O
O
O
N
O
O
N
O
O
N
O
3-aminopentane
imidazole
Br2 / CH2Cl2
32 h / RT
Br
Br
Br
Br
+
+
Br
160 o
C
O
N
O
O
O
O
O
N
O
O
N
O
O
N
O
1
2
3c
3b
3a
Cl
O
N
O
F
F
F
F
F
N
F
F
Toluene:DMF (4:1)
K2 CO3, 120 o
F
F
CN
CN
N
N
OH
B
C
BCl3 / p- xylene
F
F
N
O
N
N
3c
140 oC, 30 min
HO
F
F
F
OH
4a, o- (48%)
4b, m- (54%)
4c, p- (54%)
F
F
O
N
O
5
O
N
O
O
N
O
Cl
F
F
F
F
Toluene, 180 o
C
O
F
F
N
OH
F
F
Sealed Tube
3 days
O
O
N
N
+
B
F
F
F
F
N
N
N
N
N
B
N
F
F
F
F
N
N
F
F
N
F
F
O
N
O
O
N
O
F
F
F
F
5
6a, m- (24%)
6b, p- (26%)
4b or 4c
Reaction of PTCDA with 3-aminopentane in imidazole at 160 ˚C afforded diimide 2 (84%).8 Bromination
was accomplished by heating the diimide 2 with excess of Br2 (32 equiv) for 32 h at room temperature.9
Three products were obtained as a mixture of 1,6- and 1,7-regioisomers of the dibromo PDI derivatives
(21%), monobromo PDI (62%), and unreacted PDI 2. Longer reaction times (2 days) and increased
temperature (50 ˚C) only increased the yields of undesired dibromo derivatives (86%). The nucleophilic
substitution of o-, m- and p-substituted dihydroxybenzenes, (catechol, resorcinol, and hydroquinone) on
monobromo PDI 3c was first attempted with hydroquinone in a 1:5 ratio with 18-crown 6 as the catalyst
and K2CO3 as the base in toluene. However, no useful yields were achieved even after longer reaction
times. As a means to increase the solubility of 3c and effective nucleophilic substitution, a mixture of
toluene:DMF (4:1) was employed, and the reaction mixture was heated to 120 ˚C for 3 h under Ar (Scheme
1). Under these conditions, we were able to isolate 4c (54%). Similar reaction conditions were followed
for the o- and m-substituted derivatives.
For the
synthesis of 5, commercially available
tetrafluorophthalonitrile was reacted with a 1.0 M solution of BCl3 in p-xylene under Ar for 30 min.6 The
final step was the nucleophilic attack of the PDI on the SubPc unit to form the dyads. Conventional
nucleophilic substitution of the chloride ion by the hydroxyl group utilized a 1:5 ratio of the substrate and

nucleophile in dry toluene at reflux temperature for 3 days. In order to decrease the concentration of the
nucleophile (1:1 ratio of SubPc:nucleophile), a sealed tube was employed, and the temperature was
increased to 180 ˚C. Under such conditions, the products 6a (m-substituted) and 6b (p-substituted) were
isolated in yields of 26% and 24%, respectively. Both dyads possess good solubility in common organic
solvents and were readily isolated by chromatographic separation. Compounds 4a-c and 6a-b were fully
characterized by 1H and 13C NMR, MALDI-MS, and UV-Vis spectrosocopy.
The steady state absorption spectra recorded for the different PDI-OHs and PDI-SubPc dyads in CH2Cl2
at room temperature are shown in Figure 1. No significant change in the λmax values was observed for o-
and m-substituted PDI-OH derivatives. In contrast, the λmax value for 4c exhibited a remarkable
bathochromic shift of ~12 nm. The PDI-SubPc dyads 6a and 6b exhibited two major absorption bands at
~530 nm and ~572 nm corresponding to the PDI and SubPc moieties, respectively. A more pronounced
splitting of the Q band (PDI) was observed in 6b displaying two absorption peaks for the PDI moiety
appearing at 532 nm and 547 nm, confirming the electronic communication between the two macrocyles.
Figure 1. (a) UV-vis spectra of PDI-OHs (4a-c) and PDI-SubPc dyads in CH2Cl2 at room temperature (b)
Fluorescence emission spectra of PDI-SubPc dyads 6a-b in in CH2Cl2 at room temperature .
Figure 1 also shows the emission spectra of compounds 6a and 6b. Upon excitation of the dyads at the
max (530 nm), two emission bands (530 nm, and 580 nm) were observed for 6a. Excitation of 6b at the
λ
same wavelength resulted in virtual disappearance of the band at 530 nm with increased intensity of the
emission band for the SubPc unit ocurring at 584 nm. These observations are good indications for the
1
existence of electronic communication between the PDI and SubPc subunits. The H NMR spectra of
compounds 6a and 6b showed significant upfield shifts (5.7 ppm) for the linking phenoxy protons as a
consequence of the high diamagnetic ring current of the SubPc unit (supporting information).
Cyclic voltammetry (CV) was used to assess the electrochemical features of the synthesized compounds.
The CV of compounds 4a-c and 6a-b was carried out in CH2Cl2 containing 0.1 M TBPF6 as the supporting
electrolyte under deaerated conditions. A glassy carbon was used as the working electrode with Ag/AgNO3
as the reference electrode and a platinum wire as the counter electrode, in a standard 3 electrode
electrochemical cell. The cyclic voltammograms of all the reference compounds (Top) and the PDI-SubPc
dyads (Bottom) are shown in Figure 2. Table 1 presents the summarized electrochemical potentials and
energy levels of the synthesized compounds measured from the cyclic voltammograms. In the cathodic
direction (between +1 and -1.8 V), 6a and 6b shows multiple waves, out of which two corresponds closely
to the PDI systems [E1(red) and E2(red), Figure 2 (bottom)]. Additional irreversible reduction peaks were
observed only in the cases of the dyads centered at -1.16 V for 6a and -1.44 V for 6b and not in the parent
compounds.

A
5
4c
4b
4a
B
B
6a
6b
Figure 2. Cyclic voltammogram of (A) PDI-OH (4a-c) and (B) PDI-SubPc (6a-b) in CH2Cl2 and 0.1 M TBPF6 as a
supporting electrolyte.
Table 1. Electrochemical potentials and electronic energy levels of PDI-OHs and PDI-SubPcs.
Entry
4a
Eox (V)a
0.27
Ered (V)b
-0.46
HOMO (eV)
-4.42
LUMO (eV)
-3.69
0.34
-0.48
-4.49
-3.67
4b
0.17
-0.49
-4.32
-3.66
4c
0.30
-0.27
-4.45
-3.88
6a
0.18
-0.23
-4.33
-3.92
6b
aOnset oxidation potential. bOnset reduction potential.
The HOMO and LUMO energy levels10 calculated from the onset of the cathodic and anodic events show that the
dyads 6a and 6b have similar reduction potentials (similar LUMO energy values) and varied oxidation potentials
10
ec
(different HOMO energy values). Most importantly, the calculated electrochemical bandgap (Eg
)
values of the
dyads [6a = 0.57 eV; 6b = 0.41 eV) are relatively smaller than the corresponding PDI-OHs (4b = 0.82 eV and 4c =
0.66 eV). This unique feature indicate the formation of electronically coupled complex between PDI and
SubPc with energetically favored easy exchange of electrons between them.
In conclusion, we have synthesized three novel dihydroxy appended PDIs and two PDI-SubPc dyads
where the SubPc moiety is covalently linked in the bay region of the PDI through axial B-O bond.
Structures of the new compounds were characterized by NMR analysis. The synthesized dyads exhibit a
good energy transfer processes which is evident from the spectroscopic and electrochemical data.

Acknowledgment: This work was supported by Oklahoma State University. We thank Dr. Baskar
Nammalwar for discussions on this work.
Supporting Information Available Experimental procedures, characterization data, and 1H and 13C NMR
spectra for compounds 4a-c and 6a-b, and absorption and emission spectra of all the synthesized
compounds. This material is available free of charge via the Internet at xxxxxxx.
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Graphical Abstract
Synthesis, Photophysical and
Electrochemical Properties of Bay-Axial
Perylene Diimide-Subphthalocyanine Dyads
Leave this area blank for abstract info.
Lakshmi C. Kasi Viswanath, Laura D. Shirtcliff, Sadagopan Krishnan, Nisha V. Handa and K. Darrell Berlin*
N
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B
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