Charge transfer complexes of electron-rich naphthalene peri-dichalcogenides with TCNQ

Article abstract of DOI:10.1016/j.tetlet.2012.01.068

The electronic structures of several highly electron-rich 2,7-dimethoxynaphthalene peri-dichalcogenides were evaluated using optical and electrochemical methods, as well as by DFT calculations. Charge transfer complexes were formed with tetracyanoquinodimethane and resulted in absorption features that span from 300 nm to 1600 nm and HOMO-LUMO energy gaps as low as 0.8 eV.

Full text of DOI:10.1016/j.tetlet.2012.01.068

Tetrahedron Letters 53 (2012) 1603–1605
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Charge transfer complexes of electron-rich naphthalene peri-dichalcogenides
with TCNQ
⇑
⇑
David J. Press, Thomas G. Back , Todd C. Sutherland
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4
a r t i c l e i n f o
a b s t r a c t
Article history:
The electronic structures of several highly electron-rich 2,7-dimethoxynaphthalene peri-dichalcogenides
were evaluated using optical and electrochemical methods, as well as by DFT calculations. Charge trans-
fer complexes were formed with tetracyanoquinodimethane and resulted in absorption features that
span from 300 nm to 1600 nm and HOMO–LUMO energy gaps as low as 0.8 eV.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 23 November 2011
Revised 16 January 2012
Accepted 18 January 2012
Available online 28 January 2012
Keywords:
Chalcogens
Charge transfer complex
UV–vis spectroscopy
Cyclic voltammetry
Interest in novel organic charge transfer donors was initiated by
the synthesis of tetrathiafulvalene (TTF), its stable radical cation
and dication,1 along with the observation of the metallic-like con-
ductivity of the electron transfer complex of TTF with tetracyano-
quinodimethane (TCNQ).2 The heavier and more polarizable
selenium and tellurium analogues of sulfur-containing donor mol-
ecules are expected to reduce coulombic repulsions and improve
charge transfer (CT) properties, in part through differences in ion-
ization potentials and increased HOMO energy levels.3 Two of the
first such examples include tetraselenafulvalene4 and tetrame-
thyltetraselenafulvalene.5 CT complexes are usually weak and re-
sult from the ground-state mixing of the HOMO of the donor
with the LUMO of the acceptor.
constraint of the dihedral angles of diaryl dichalcogenides from
nearly orthogonal to either larger or smaller angles, as in the case
of the nearly planar11 naphthalene peri-dichalcogenides, is accom-
panied by a bathochromic shift that results from destabilization of
the ground state and a decrease in the energy of the HOMO–LUMO
transition.12 Electron-donating substituents on the naphthalene
ring are expected to increase the HOMO energies further. Recently,
electron-rich 2,7-dimethoxynaphathalene peri-dichalcogenides 3
and 4 were prepared in our laboratory13 for investigation of their
glutathione peroxidase-like activity, while the known unsubstitut-
ed diselenide 114 was also prepared for comparison (Scheme 1).
The synthesis of disulfide 2 was performed similarly and was inde-
pendently reported by Grainger et al.15 for use as a sulfur monoxide
transfer reagent. We now report the remarkable electronic proper-
ties of 2–4 and their comparison with 1.
Systems that display CT bands can act as semi-conductors,
which are fundamentally important components of organic photo-
voltaic (OPV) cells,6 organic field effect transistors (OFETs),7 and or-
ganic light emitting diodes (OLEDs).8 Moreover, because typical
CT-bands are broad, this enhances the photon-harvesting proper-
ties for OPV applications. Alternatively, such compounds can be-
have as charge carriers in OFET or OLED applications.
Br
Br
X
X
n
-BuLi
R
R
R
R
Although many TTF-based donors are known,9 less has been
reported on chalcogenoacenes.10 Molecules containing the naph-
tho[1,8-cd]-1,2-dithiole structure, along with its Se and Te analogues
and higher order peri-dichalcogenoacenes, have been investigated as
donors in organic complexes displaying CT behaviour.9,10 The
S, Se or Te
THF
1 R = H; X = Se (19%)
2 R = OMe; X = S (50%)
3 R = OMe; X = Se (64%)
4 R = OMe; X = Te (50%)
NC
NC
CN
⇑
= TCNQ
Corresponding authors. Tel.: +1 403 220 6256; fax: +1 403 289 9488 (T.G.B.);
tel.: +1 403 220 7559; fax: +1 403 289 9488 (T.C.S.).
E-mail addresses: tgback@ucalgary.ca (T.G. Back), sutherlt@ucalgary.ca
(T.C. Sutherland).
CN
Scheme 1. Synthesis of peri-dichalcogenides 1–4 and structure of TCNQ.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.068

1604
D. J. Press et al. / Tetrahedron Letters 53 (2012) 1603–1605
1:1 stoichiometry. Job plots of the remaining CT complexes are in-
cluded in the Supplementary data. Using the Job plot and fitting
the data to a 1:1 binding isotherm,16 estimates of association con-
stants and molar absorptivities of the charge transfer complex
were made. Table 1 summarizes the optical properties of dichalc-
ogenides 1–4. Moving down the period from S to Te in the series
2–4, there is a clear trend toward lower energy absorptions of
the CT band associated with complex formation in acetonitrile.
The red-shifted CT absorptions are likely caused by the increase
in HOMO energy of the peri-dichalcogenide with increasing chalco-
gen size. Moreover, the heavier Te analogue 4 forms complexes
with similar affinity as its Se analogue 3; however, the 4:TCNQ
complex does show a significantly higher molar absorptivity than
either the S or Se analogue. In general, the CT complexes are weak,
with association constants that range between 101 and 102 MÀ1
.
Thus, there are clear advantages in installing the 2,7-dimethoxy
substituents because they enhance synthetic access by increasing
yields (compare 1 and 3 in Scheme 1), they improve solubilities
compared to diselenide 1, and they add electron density to the aryl
systems, resulting in a further increase in HOMO energy levels and
the ability to absorb very low energy photons when complexed
with electron-deficient acceptors. The absorption features are
promising for low band-gap organic semiconductors. Moreover,
the small molecular approach is amenable to vacuum deposition
techniques, provides materials of analytical purity, and enables
the tuning of electronic properties, thus affording accurate struc-
ture–property relationships.
Further assessment of the donor properties of the naphthalene
peri-dichalcogenides was determined by cyclic voltammetry (CV).
Under anodic potentials, the donor molecules were oxidized to
the radical cations and the half-wave potentials (E1/2), shown in Ta-
ble 1, provide an estimate of HOMO energy levels when compared
to a standard ferrocene redox reaction. Figure 2 overlays the oxida-
tion of dichalcogenides 2–4 at 100 mV sÀ1 with the reduction of
TCNQ. The oxidations of 2–4 are quasireversible, as based on the
peak splitting and ratio of peak currents.17 The electrochemical
parameters are summarized in the Supplementary data. The
unsubstituted diselenide 1, at 0.36 V higher than the ferrocene re-
dox, is more difficult to oxidize than any of the dimethoxy dichalc-
Figure 1. (a) UV–vis absorption spectra of 1 (black dashed line), 2 (black solid line),
3 (blue line) and 4 (red line) in CH3CN at 25 °C. Inset: Charge transfer peak using the
continuous variation concentration method of TCNQ and 4 in CH3CN. (b) Job plot of
4 and TCNQ at 1205 nm.
Absorption spectra of compounds 1–4 are shown in Figure 1a.
All compounds absorbed strongly in the UV to near-visible region
with molar absorptivities near 104 MÀ1 cmÀ1. As expected, the
unsubstituted diselenide 1 displayed the weakest absorptivity
compared to the dimethoxy derivatives 2–4. The lowest energy
electronic transition for each of these compounds showed a bath-
ochromic shift down the period from sulfur to tellurium. None of
the dichalcogenides studied exhibited fluorescence in solution or
in the solid state, presumably due to heavy atom quenching of
the excited states. Importantly, these compounds show extremely
weak electronic transitions above 450 nm (2.8 eV) and the molar
absorptivity of the UV–vis absorptions did not change over a con-
centration range of 10À5 M to 10À3 M, indicating that negligible
intermolecular interactions occur in acetonitrile.
ogenides. The 2,7-dimethoxy substituents of disulfide
2 add
electron density to the naphthalene moiety, enabling easier oxida-
tion at 0.31 V. Replacement of the sulfur atoms with the larger,
more diffuse selenium atoms of 3 resulted in oxidation at 0.18 V
and, finally, oxidation of the tellurium derivative 4 required only
60 mV relative to ferrocene.
Upon addition of TCNQ to any of the dichalcogenides 1–4, a new
absorption feature emerged at longer wavelength, attributed to a
CT interaction. In particular, TCNQ addition to ditelluride 4 re-
sulted in an exceptionally broad CT peak ranging from ca. 850–
1600 nm and centerd at 1205 nm, as shown in the inset of Figure 1a
(4:TCNQ binding) and the Job plot in Figure 1b, which confirms a
For comparison, tetrathiafulvalene and tetraselenafulvalene
have oxidation potentials of 0.33 V and 0.48 V versus SCE
(ꢀ75 mV and 225 mV vs Fc/Fc+).18 The diffusion coefficients
trended with molecular size, suggesting that minimal aggregation
effects occurred, while electron transfer rates (kets) were similar
for 1–4, as expected for such analogous structures. It should be
Table 1
Optical and electrochemical properties of 1–4 and their charge transfer complexes with TCNQ
Abs onset/nm (eV)
D–A CT banda/nm (eV)
CT band onsetb/nm (eV)
Kac/MÀ1
eD–Ad/MÀ1 cmÀ1
E1/2e/V
EHOMOf/eV
1
2
3
4
400 (3.1)
400 (3.1)
415 (3.0)
440 (2.8)
1100 (1.1)
1117 (1.1)
1125 (1.1)
1205 (1.0)
1300 (0.9)
1550 (0.8)
1600 (0.8)
50
80
850
560
400
600
350
0.36
0.31
0.18
0.06
À5.2
À5.1
À5.0
À4.9
>1600 (<0.8)
1600
a
b
c
d
e
f
Donor–acceptor charge transfer kmax values.
Onset of absorption of CT peak.
Association constant of donor–acceptor complex in CH3CN.
Molar absorptivity of donor–acceptor complex.
Potential reported versus Fc/Fc+.
HOMO energy levels calculated from Fc/Fc+ redox at À4.8 eV.

D. J. Press et al. / Tetrahedron Letters 53 (2012) 1603–1605
1605
those of the Te analogue are shown in Figure 3b. The HOMO is a
p
-system delocalized over the naphthalene ring and chalcogen
atoms, whereas the LUMO is localized to a r⁄ orbital between
the chalcogens.
In summary, the incorporation of electron-donating methoxy
substituents into naphthalene peri-dichalcogenide donor mole-
cules significantly improves their photovoltaic properties. The
products display remarkably broad and low energy absorptions
down to 0.8 eV, which is essential for harvesting low energy pho-
tons. Additionally, they serve as potent donors and form CT com-
plexes in solution with 1:1 binding stoichiometry with TCNQ.
The improved properties of the methoxy derivatives relative to
the unsubstituted peri-dichalcogenides offer promise in organic
electronic applications.
Figure 2. Cyclic voltammagrams of chalcogenoacenes 2–4 and TCNQ in CH3CN
containing 0.1 M (nBu)4NPF6 at 100 mV sÀ1 at a glassy carbon working electrode, a
Ag/Ag+ quasi reference electrode and a Pt counter electrode in a conventional 3-
electrode cell.
Acknowledgments
We thank the Natural Sciences and Engineering Research
Council (NSERC) of Canada for financial support. D. J. P. thanks
NSERC and Alberta Innovates-Technology Futures for scholarships.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.01.068.
References and notes
1. Wudl, F.; Smith, G. M.; Hufnagel, E. J. J. Chem. Soc. D 1970, 1453–1454.
2. Ferraris, J.; Cowan, D. O.; Walatka, V.; Perlstein, J. H. J. Am. Chem. Soc. 1973, 95,
948–949.
3. (a) Villar, H. O.; Otto, P.; Dupuis, M.; Ladik, J. Synth. Meth. 1993, 59, 97–110; (b)
Heeney, M.; Zhang, W.; Crouch, D. J.; Chabinyc, M. L.; Gordeyev, S.; Hamilton,
R.; Higgins, S. J.; McCulloch, I.; Skabara, P. J.; Sparrowe, D.; Tierney, S. Chem.
Commun. 2007, 5061–5063.
4. Engler, E. M.; Patel, V. V. J. Am. Chem. Soc. 1974, 96, 7376–7378.
5. Bechgaard, K.; Cowan, D. O.; Bloch, A. N. J. Chem. Soc., Chem. Commun. 1974,
937–938.
6. Deibel, C.; Strobel, T.; Dyakonov, V. Adv. Mater. 2010, 22, 4097–4111.
7. (a) Gao, X.; Qiu, W.; Liu, Y.; Yu, G.; Zhu, D. Pure Appl. Chem. 2008, 80, 2405–
2423; (b) Guo, Y.; Yu, G.; Liu, Y. Adv. Mater. 2010, 22, 4427–4447; (c) Mori, T. J.
Phys.: Condens. Matter 2008, 20, 184010.
Figure 3. (a) Frontier molecular orbital (FMO) energy levels of 1–4 calculated at the
B3LYP level of theory using the 6-31G+(d) basis set. (b) FMOs of 4.
8. (a) Chen, S.; Deng, L.; Xie, J.; Peng, L.; Xie, L.; Fan, Q.; Huang, W. Adv. Mater.
2010, 22, 5227–5239; (b) Gather, M. C.; Köhnen, A.; Meerholz, K. Adv. Mater.
2011, 23, 233–248.
9. (a) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. Rev. 2004, 104, 4891–4945;
(b) Gorgues, A.; Hudhomme, P.; Sallé, M. Chem. Rev. 2004, 104, 5151–5184; (c)
Iyoda, M.; Hasegawa, M.; Miyake, Y. Chem. Rev. 2004, 104, 5085–5114; (d)
Jeppesen, J. O.; Becher, J. Eur. J. Org. Chem. 2003, 3245–3266; (e) Segura, J. L.;
Martín, N. Angew. Chem., Int. Ed. 2001, 40, 1372–1409; (f) Wallis, J. D.; Griffiths,
J.-P. J. Mater. Chem. 2005, 15, 347–365.
10. Zhang, L.; Fakhouri, S. M.; Liu, F.; Timmons, J. C.; Ran, N. A.; Briseno, A. L. J.
Mater. Chem. 2010, 21, 1329–1337.
11. Aucott, S. M.; Milton, H. L.; Robertson, S. D.; Slawin, A. M. Z.; Woollins, J. D.
Heteroatom Chem. 2004, 15, 530–542.
12. (a) Back, T. G.; Codding, P. W. Can. J. Chem. 1983, 61, 2749–2752; (b) Field, L. In
Organic Chemistry of Sulfur; Oae, S., Ed.; Plenum Press: New York, 1977. Chapter
7; (c) Kuder, J. E. In Organic Selenium Compounds: Their Chemistry and Biology;
Klayman, D. L., Günther, W. H. H., Eds.; Wiley Interscience: New York, 1973.
Chapter 15B; (d) Seino, J.; Honda, Y.; Hada, M.; Nakatsuji, H. J. Phys. Chem. A
2006, 110, 10053–10062.
13. Press, D. J.; Back, T. G. Org. Lett. 2011, 13, 4104–4107.
14. Meinwald, J.; Dauplaise, D.; Wudl, F.; Hauser, J. J. J. Am. Chem. Soc. 1977, 99,
255–257.
15. Grainger, R. S.; Patel, B.; Kariuki, B. M.; Male, L.; Spencer, N. J. Am. Chem. Soc.
2011, 133, 5843–5852.
16. Connors, K. A. Binding Constants: the Measurement of Molecular Complex
Stability; Wiley: New York, 1987.
noted, however, that ditelluride 4 decomposed after 3 to 4 potential
scans on the glassy carbon electrode surface, precluding ket deter-
mination. The E1/2 values are converted to an electron volt scale
and compared to vacuum by subtracting ferrocene’s experimentally
determined EHOMO at 4.8 eV.19 The CV results of the donor molecules
support a high HOMO energy level, which is a requirement for good
donor systems. TCNQ, shown in blue in Figure 2, underwent facile
two sequential one-electron reductions at 0.22 V and 0.78 V versus
ferrocene, implying a very low energy LUMO.
Additional insight into the donor molecule properties was gar-
nered from DFT calculations (see Supplementary data). Optimiza-
tions and frequency calculations were carried out for compounds
1–4 and TCNQ, and the resulting frontier orbital energy levels are
shown in Figure 3a. A step-wise increase in HOMO energy levels
is apparent through the series 1–4, which parallels the experimen-
tally determined HOMO energy levels and lends support to the
chosen basis set. Concomitantly, the ELUMO of the donors also in-
creases throughout the period, resulting in a HOMO–LUMO energy
gap that is nearly constant (ꢀ2.5 eV), but does not match the opti-
cally derived value using the absorption onset, possibly because
the HOMO–LUMO transition is forbidden. The HOMO and LUMO
orbitals have nearly identical shapes for dichalcogenides 2–4 and
17. Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and
Applications, 2nd ed.; Wiley: New York, 2001.
18. (a) Engler, E. M.; Kaufman, F. B.; Green, D. C.; Klots, C. E.; Compton, R. N. J. Am.
Chem. Soc. 1975, 97, 2921–2922; (b) Coffen, D. L.; Chambers, J. Q.; Williams, D.
R.; Garrett, P. E.; Canfield, N. D. J. Am. Chem. Soc. 1971, 93, 2258–2268.
19. Scharber, M. C.; Muehlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A.
J.; Brabec, C. J. Adv. Mater. 2006, 18, 789–794.