Viologen embedded polyaromatic hydrocarbons (VPAH2+): Synthesis, computational, photophysical, and electrochemical characterizations of 3,8-diazaphenanthrenyl viologens

Article abstract of DOI:10.1016/j.tetlet.2015.08.049

Several new dicationic viologen sensitizers are synthesized and characterized both photo- and electro-chemically. Benzo-annulation of methyl viologen increases its fluorescence lifetime by a factor of 6.6. Similar lifetime increases were observed for methylated derivatives, that have the added advantage of enhanced solubilities. These electron transfer sensitizers can oxidize organic substrates with oxidation potentials of approximately 2.4 eV and lower.

Full text of DOI:10.1016/j.tetlet.2015.08.049

Tetrahedron Letters 56 (2015) 5591–5594
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
2+
Viologen embedded polyaromatic hydrocarbons (VPAH ): synthesis,
computational, photophysical, and electrochemical characterizations
of 3,8-diazaphenanthrenyl viologens
⇑
Thomas Bakupog, Edward L. Clennan , Xiaoping Zhang
Department of Chemistry, University of Wyoming, Laramie, WY 82071, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Several new dicationic viologen sensitizers are synthesized and characterized both photo- and electro-
chemically. Benzo-annulation of methyl viologen increases its fluorescence lifetime by a factor of 6.6.
Similar lifetime increases were observed for methylated derivatives, that have the added advantage of
enhanced solubilities. These electron transfer sensitizers can oxidize organic substrates with oxidation
potentials of approximately 2.4 eV and lower.
Received 30 June 2015
Revised 15 August 2015
Accepted 20 August 2015
Available online 21 August 2015
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Viologen
Sensitizer
Electron-transfer
Photochemistry
(MV2 ) has a singlet lifetime of 1.00 ± 0.04 ns in acetonitrile pre-
+ ⁄
Introduction
9
cluding all but ultrafast quenching events.
Viologens have been known and studied since 1882 when Wei-
1
0
ð1Þ
del and Russo first reported the synthesis of the 4,4 -viologen,
2+
methyl viologen, or paraquat, MV . However, interest in these
compounds did not pique until the mid 1950s when it was
revealed that diquat, DQ2 , functioned as a potent herbicide. In
more recent years these fascinating molecules have found use in
a wide-range of applications including as components of pho-
+
2
ð2Þ
We report here a photophysical, computational, and electro-
chemical study of viologens 1 –4 . The results demonstrate that
p-system of MV by ring fusion is a viable strategy
to produce new viologens with attributes more suitable for their
3
2+
2+
tochromic/electrochromic materials, in supramolecular assem-
blies,4 in molecular machines, and as electron shuttles and
sensors.
5
6
2+
extending the
7
1
0
use as electron transfer sensitizers.
Results and discussion
The syntheses of 12+–42+ are depicted in Scheme 1. The first step
involves a Pd-catalyzed cross coupling between two equivalents of
the 3-bromopyridine and triethoxyvinylsilane in water to give the
Organic dications have also been used as electron transfer pho-
tosensitizers since the initial photo-induced electron transfer pro-
duces a radical-cation/radical-cation pair (Eq. 1) whose rapid
repulsive separation competitively inhibits energy wasting return
11
1,2-bis-(3-pyridyl)ethenes in modest 19.6–56.8% yields.
Mallory photocyclizations occurs in very good 84% and 78% yields
The
1
2
to give the 3,8-diazaphenanthrenes 2 and 4, respectively. The
absence of a methyl group on the pyridyl rings a to nitrogen and
ortho to the vinyl group in the ethenes allowed alternative photocy-
clizations to give 1,8- and 1,10-diazaphenanthrenes that reduced
the yields of the desired 3,8-isomers 1 and 3 to 60% and 30%,
respectively. (See Supplementary material) Treatment of the 3,8-di-
8
2+
electron transfer. Viologens (V ) are especially attractive for this
application because of the remarkable stability of both their dica-
1
tion and radical-cation redox partners. (Eq. 2) Unfortunately,
⇑
Corresponding author. Tel.: +1 307 766 6667; fax: +1 307 766 2807.
E-mail address: clennane@uwyo.edu (E.L. Clennan).
3 3 4
azaphenanthrenes with excess Meerwein’s salt, (CH ) OBF , gave
http://dx.doi.org/10.1016/j.tetlet.2015.08.049
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
0

5
592
T. Bakupog et al. / Tetrahedron Letters 56 (2015) 5591–5594
Scheme 1.
the viologens, 12+, 22+, 32+, and 42+, that were purified by trituration
with methanol.
The geometries of 12+, 22+, 32+, and 42+ at the DFT B3LYP/6-311
+
G(2d,p) computational level do not differ significantly. However,
as anticipated the C–N bond lengths in the viologens are
.034 ± 0.011 Å longer than the corresponding C–C bonds in all
their carbon phenanthrene homologs, and the peri methyl–hydro-
0
2
+
gen interaction in 2 destabilizes it by 4.8 kcal/mol in comparison
2
+
to its 3 isomer. The most unexpected computational outcome is
the observation that the H –H10 distance decreases by 6% in the
from 2.137 Å to 2.006 Å. A similar
1
2
+
2+
2+
2+
series 1 > 3 > 2 > 4
decrease of 4% and 3% is also observed in the corresponding methyl
substituted 3,8-diazaphenanthrene (1–4) and phenanthrene series
suggesting that methyl buttressing interactions play the critical
1
role in changing the through space H –H10 distance.
The photophysical data for 12 –4 are given in Table 1. The
+
2+
UV–vis spectra are characterized by S ? S transitions centered
0
1
at 368, 363, 380, and 386 nm, for 12 , 2 , 3 , and 4 , respectively,
+
2+ 2+
2+
Figure 1. Absorption spectra for viologens and phenanthrenes.
ꢀ
1
with vibrational spacing of 1200 to 1600 cm as anticipated for
several in-plane C–H bend and ring breathing modes. (Fig. 1a) In
addition, all 4 viologens also exhibit vibrationally rich S
0
? S
2
tran-
(96–97%) contributor to the excited state density or wave function
sitions centered at 278 (12 ), 284 (2 ), 287 (3 ), and 292 nm (4 ).
These assignments were verified with remarkable precision by
TD-DFT (Time Domain Density Functional Theory) calculations
+
2+
2+
2+
for the S ? S absorption bands. The wavelengths of S ? S bands
0
1
0
2
were all predicted to within 8 to 17 nm of the observed values. The
dominant contributors to these higher energy absorption bands are
the HOMO ? LUMO + 1 transitions (71% to 85%) with less signifi-
predicting the wavelength of the S
0
? S
1
transitions to within 5,
2
+
2+
2+
2
, and 8 nm for 1 , 2 , and 4 , respectively. The calculated
cant
HOMO ꢀ 2 ? LUMO transitions.
The diazaphenanthrenes 1–4 also exhibit S ? S and S ? S2
contributions
from
the
HOMO ꢀ 1 ? LUMO
and
2
+
0 1
S ? S transition in 3 deviated somewhat from these very
precise predictions and was 26 nm bathochromic of the observed
0
1
0
value. The HOMO ? LUMO determinant is the dominant
vibrational clusters very similar to that observed in the viologens.
Fig. 1) The effects of N-methylation of 1–4 on the wavelengths of
the S ? S and S ? S transitions are very different. The S ? S
transitions experience bathochromic shifts of (calculated/observed)
45 nm/+30 nm, +48 nm/+23 nm, +51 nm/+32 nm, and +55 nm/
(
0
1
0
2
0
1
Table 1
Photophysical data for diazaphenanthrenyl viologens 12+–42+a
+
2
+
2+ 2+
2+
12+
22+
32+
42+
+38 nm for the conversions of 1, 2, 3, and 4 to 1 , 2 , 3 , and 4
,
)b
respectively. In dramatic contrast, the S ? S transitions are very
insensitive to N-methylation exhibiting very small shifts of (calcu-
0
2
k
UV–vis
(
e
387(5700)
390(7900)
371(6900)
355(3100) 336(800) 361(3000)
408
415
400(7400)
380(6100)
406(7600)
386(6700)
368(3400)
424
416
1046
3
3
67(4800)
50(2400)
lated/observed) +3nm/ꢀ1 nm, +2 nm/ꢀ2 nm, ꢀ3 nm/0 nm,
b
F
k
k
407
429
1270
6.6 ± 0.1
419
430
1134
6.1 ± 0.2
2+
2+
2+
ꢀ
3 nm/ꢀ2 nm for the conversions of 1, 2, 3, and 4 to 1 , 2 , 3
,
b
Phos
2+
c
and 4 , respectively. The reason for this stark difference is evident
upon examination of the orbitals involved in the transitions. The
N-methyl groups in the HOMO and LUMO + 1 orbitals that are
Stokes shift
1131
5.7 ± 0.1
d
s
U
k
E(S
F
4.8 ± 0.1
F
e
0.34 ± 0.01 0.38 ± 0.01
0.30 ± 0.01 0.29 ± 0.01
7
7
7
7
F
5.2 ꢁ 10
6.7 ꢁ 10
4.9 ꢁ 10
6.0 ꢁ 10
0 2
involved in the S ? S transitions are located at nodes on the orbi-
tal surface and would not be expected to influence orbital energy.
On the other hand, the nitrogen atoms bearing the N-methyl groups
0 1
in the LUMOs, which are important players in the S ? S
transitions, have substantial orbital coefficients. Consequently, the
N-methyl group would influence the energy of the LUMO but not
the HOMO substantially changing the HOMO–LUMO gap and as a
)f
72.4
71.7
69.9
68.9
1
f
E(T
1
)
68.4
68.9
66.8
68.6
a
b
c
d
e
f
In CH
In nm.
3
CN.
ꢀ1
In wavenumbers (cm ).
In nanoseconds (ns).
ꢀ1
In s
.
In kcal/mol.
0 1
result the wavelengths of the S ? S transitions.

T. Bakupog et al. / Tetrahedron Letters 56 (2015) 5591–5594
5593
All the viologens fluoresce and phosphoresce (Table 1 and
Fig. 1a). The fluorescence spectra are reasonable mirror images of
the absorption spectra but with broadened vibrational fine struc-
ture. The phosphorescence spectra exhibit well-defined vibrational
fine structure that allows easy assignment of the 0–0 band. (Fig. 1a
inset) Consequently, the singlet energies, E(S
from the crossing point of the absorption and fluorescence spectra
and the triplet energies, E(T ), from the 0–0 band in the phospho-
1
), were determined
1
rescence spectra. The singlet triplet energy gaps in the viologens
are very small and decrease with addition of methyl groups from
.0 kcal/mol in 1 to only 0.3 kcal/mol in tetra-methylated 42+
.
2
+
4
(Table 1) The phenanthrenes also all fluoresce but at shorter wave-
lengths (see Table S1 in the Supplementary material) leading to
singlet energies 7.7 to 8.4 kcal/mol higher than that of the corre-
sponding viologen. (80.1, 79.6, 77.7, and 77.3 kcal/mol for 1, 2, 3,
and 4, respectively) The Stokes shifts for the viologens are approx-
imately twice as large as for the corresponding phenanthrene
Figure 2. Methyl antibonding interactions in 22+, 32+, and 42+. B3LYP/6-311+G(2d,
(Table S1) reflecting a larger magnitude relaxation of the Franck
p).
Condon populated excited singlet state.
The fluorescence lifetimes for 12 –4 are factors of 4.8 to 6.6
+
2+
2+
times larger than observed in methyl viologen, MV . The Einstein
1
3
2
equation
(
(<x > = 6Dt), where D is the diffusion coefficient
estimated as 2 ꢁ 10 cm s ),14 and <x > is the mean squared
ꢀ5
2
ꢀ1
2
molecular displacement, predicts that an excited viologen with a
lifetime of 6 ns can migrate approximately 85 Å within its lifetime.
Consequently, we conservatively anticipate that diffusive encoun-
ter of the viologen sensitizer and the substrate will readily occur at
ꢀ2
15
substrate concentrations greater than 10 M.
The fluorescence quantum yields for the viologens and the
phenanthrenes (Table S1 in Supplementary material) are very sim-
ilar and fall in the narrow range of 0.26 to 0.38. The magnitude of
Figure 3. Syn- and anti-deformations in 2-electron reduced viologens.
7
ꢀ1
the rate constants of fluorescence, k
are those anticipated for a pp character of the S
F
=
U
F
/
s
F
[(3.5–7.3) ꢁ 10 s ],
⁄
1
excited state.
0
0
The reduction potentials for the viologens appear at ꢀ0.45,
syn 18.9 ; anti 18.7 ) that lack methyl groups on the 4 and 7
ꢀ
0.58, ꢀ0.52, and ꢀ0.62 V for reduction to the radical cation and
positions of the phenanthrene nucleus. For comparison, the corre-
0
0
0
at ꢀ0.94, ꢀ1.07, ꢀ0.96, and ꢀ1.19 V versus SCE for reduction to
sponding dihedral angles in MV are 17.5 for the syn- and 16.5 for
the anti-isomer (see Supplementary material for more structural
information).
the neutral redox partners in 12 , 2 , 3 , and 4 , respectively.
see Supplementary information for the CV curves) The value for
reduction to the radical cation for the unsubstituted phenanthrenyl
+
2+
2+
2+
(
2
+
viologen 1 is remarkably close to that reported for methyl violo-
Conclusion
1
6
17
gen (ꢀ0.45 and ꢀ0.43 V vs SCE). On the other hand, the reduc-
tions to both the radical cation and the neutral redox partners
become more difficult when hydrogens on the phenanthrene ring
are replaced with electron donating methyl groups. The 60 mV
more difficult reduction to the radical cation and 110 mV more dif-
The viologen sensitizers described here are sufficiently soluble
for use in polar solvents (CH CN, H O), which are known to support
3
2
1
9
2+
electron transfer processes. For example, viologen 3 sensitizes
2
0
decomposition of tetraphenyloxirane in acetonitrile (see Supple-
mentary information for details). They are easy to handle and
stable to irradiation in the absence or presence of substrate for at
least 2 h, and in contrast to pyrylium based dications are very
ficult reduction to the neutral redox partner in 22 than in 3 can
be attributed to antibonding interactions with the methyl groups
on the 4,7-carbons that raises the energy of the electron accepting
LUMO (Fig. 2). Similar destabilizing interactions are absent with
the methyls residing on the 2,9-carbons. The reduction to the
neutral redox partner is chemically irreversible or at best quasi-ir-
+
2+
2
1
resistant to hydrolysis. Electrochemically they form the radical-
cations in reversible/quasi-reversible processes that allow mea-
surement of thermodynamically significant reduction potentials.
2
+
2+
2+
2+
2+
2+
reversible in 1 and in 3 , which lack methyl groups on the 4,7-
carbons.
The S1 states of 1 , 2 , 3 , and 4 are thermodynamically cap-
able of oxidizing organic substrates with oxidation potentials
We do not know precisely why the 2-electron reduced violo-
lower than 2.68, 2.53, 2.51, and 2.37 eV, respectively. The T states
1
0
0
2+ 2+ 2+
2+
gens 1 and 3 are kinetically less stable than those formed in
of 1 , 2 , 3 , and 4 are thermodynamically capable of oxidizing
organic substrates with oxidation potentials lower than 2.51, 2.41,
2.38, and 2.35 eV, respectively. In addition, the triplet excited
states and radical-cation/radical-cation pairs have the added
advantages of long lifetimes that will enhance their migration
lengths and suppress energy wasting return electron transfer.
2
+
2+
the reductions of 2 and 4 . The neutral redox partners exists
as syn- and anti-isomers18 (Fig. 3) and are stable for all four violo-
gens at the B3LYP/6-311+G(2d,p) computational level. The length
of the formally double bond between the two six-membered nitro-
gen containing rings in the neutral redox partners are nearly iden-
0
0
0
tical in all four compounds (1 1.372 Å; 2 1.374 Å; 3 1.373 Å; and
0
0
4
1.376 Å) and only slightly shorter than in MV (1.380 Å).
However, the deviation from planarity is greatest for 2 (
Acknowledgment
0
H
MeNC4Me
0
:
0
0
0
syn 23.1 ; anti 22.4) and 4
(H
MeNC4Me: syn 32.2 ; anti 32.1 ) and
We thank the National Science Foundation – United States
(CHE-1147542) for their generous support of this research.
0
0
0
0
smaller for 1
MeNC4H
(HMeNC4H: syn 17.1 ; anti 16.8 ) and 3 (H :

5
594
T. Bakupog et al. / Tetrahedron Letters 56 (2015) 5591–5594
8.
(a) Clennan, E. L.; Liao, C.; Ayokosok, E. J. Am. Chem. Soc. 2008, 130, 7552–7554;
b) Clennan, E. L.; Welch, W.; El-Idreesy, T. T.; Arulsamy, N. Can. J. Chem. 2015,
3, 414–421.
Supplementary data
(
9
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.08.
9. Peon, J.; Tan, X.; Hoerner, J. D.; Xia, C.; Luk, Y. F.; Kohler, B. J. Phys. Chem. A 2001,
105, 5768–5777.
0
1
0. A singlet lifetime of 10.4 ns has been reported for N,N -2,7-diazapyrenium
dication but the generality of this observation has not been demonstrated.
Ballardine, R.; Credi, A.; Gandolfi, M. T.; Giansante, C.; Marconi, G.; Silvi, S.;
Venturi, M. Inorg. Chim. Acta 2007, 360, 1072–1082.
0
49.
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