Synthesis, Structure, and Photochemical Behavior of [5]Heli-viologen Isomers

Article abstract of DOI:10.1021/acs.joc.6b00835

The syntheses of isomeric helical viologens that have potential applications in supramolecular chemistry and catalysis have been developed. The structures of the molecules and their solid-state packing motifs have been determined by X-ray crystallography. Computational studies demonstrate that the magnitude of their racemization barriers is primarily determined by the identity of the helical scaffold and is insensitive to the placement of the viologen functional group. The isomers are similar in their photophysical behavior but very different in their photochemical behavior.

Full text of DOI:10.1021/acs.joc.6b00835

Article
pubs.acs.org/joc
Synthesis, Structure, and Photochemical Behavior of [5]Heli-viologen
Isomers
Xiaoping Zhang, Edward L. Clennan,* Navamoney Arulsamy, Rachael Weber, and Jacob Weber
Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, United States
S
* Supporting Information
ABSTRACT: The syntheses of isomeric helical viologens that have potential applications in supramolecular chemistry and
catalysis have been developed. The structures of the molecules and their solid-state packing motifs have been determined by X-
ray crystallography. Computational studies demonstrate that the magnitude of their racemization barriers is primarily determined
by the identity of the helical scaffold and is insensitive to the placement of the viologen functional group. The isomers are similar
in their photophysical behavior but very different in their photochemical behavior.
diaza[5]helicene bis-tetrafluoroborate27 and 3,8-dimethyl-3,8-
INTRODUCTION
■
diaza[5]helicene bis-tetrafluoroborate (12+ and 22+).27 These
Viologens are the diquaternary salts of 4,4′-bipyridine (e.g.,
MV2+).1 Their applications have dramatically expanded from
their early use as herbicides2−4 to their use as electron transfer
relays,5−7 as DNA photocleaving agents,8 as components of
charge-transfer complexes,9−13 in supramolecular assemblies,14
in polymers,15 and as integral components of molecular
electronic devices,16 such as sensors17−19 and electrochromic
switches.20
represent rare examples of chiral viologens15,28−31 and the first
examples that we are aware of in which the chirality is due to
the polyaromatic backbone. These compounds also complete
the link, first established by the fascinating helquats,32−35
between helicenes and viologens.
The interest in viologens is likely to remain high given the
recent recognition that fluorescent organic salts have many
favorable characteristics21,22 for device construction and the
reports that viologens may function as potent and novel
photoacids.23−25 Unfortunately, despite the fact that photo-
reduction of MV2+ has been known for many years,
photophysical and photochemical studies of viologens are
rare. In addition, Lin and Zhou26 have recently pointed out that
there is a paucity of viologens with extended π-conjugation, a
design element that can be used to fine-tune their photo-
chemical and electrochemical properties.
RESULTS AND DISCUSSION
■
Synthesis. Heli[5]viologen 12+ was readily available by
alkylation of the known 5,10-diaza[5]helicene36 that was
prepared by the slightly modified route shown in Scheme 1a.
1,2-Bis(3-quinoyl)ethene (3) was generated by a Hiyama−
Heck Pd-catalyzed cross-coupling reaction between triethox-
yvinylsilane and 2 equiv of 3-bromoquinoline.37,38 Photolysis of
3 with a Pyrex-filtered medium-pressure Hanovia lamp
generated a mixture of 5,10-diaza[5]helicene (4) and benzo-
[b]-1,8-diaza[4]helicene (5), which was separated and purified
by column chromatography. Alkylation of the major product 4
In order to address this gap in the structural landscape of
viologens, we report here the synthesis, structural character-
ization, photophysics, and photochemistry of a novel isomeric
pair of π-extended helical viologens, 5,10-dimethyl-5,10-
Received: April 13, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Scheme 1. Synthesis of 12+ and 22+
a
Table 1. Comparison of X-ray and B3LYP/6-311+G(2d,p) Structural Parameters
b
12
4
12+
11
22+
Θ
Θ
Θ
14,14a,14b,14c, deg
18.2 (17.5)
30.2 (30.3)
18.1 (17.3)
−
2.63 (2.54)
2.96 (2.90)
1.43 (1.42)
1.36 (1.35)
1.40 (1.40)
1.37 (1.36)
1.36 (1.35)
17.0
27.9
17.0
1.45
2.49
3.00
1.38
1.30
−
17.1 (17.2)
29.8 (33.8)
17.0 (17.2)
1.45 (1.44)
2.57 (2.65)
3.01 (3.03)
1.40 (1.40)
1.32 (1.31)
−
19.2
28.0
15.9
1.45
2.53
2.99
−
21.7 (16.0)
27.9 (26.8)
17.2 (21.9)
1.46 (1.45)
2.66 (2.49)
2.99 (2.91)
−
14a,14b,14c,14d, deg
1,14d,14c,14b, deg
c
d
d
d
py‑py, Å
1H,14H, Å
1,14, Å
d
d4a,5 (d4a,N),
Å
d
d5,6 (dN,6),
Å
−
−
e
d2,3 (d2,N) Å
1.35
1.31
1.30
1.37
1.37 (1.37)
1.33 (1.32)
1.32 (1.32)
e
d3,4 (dN,4), Å
−
−
−
_
e
d7,8 (d7,N), Å
−
−
e
d8,8a (dN,8a) Å
1.42 (1.42)
1.39 (1.39)
a
b
c
d
e
X-ray data in parentheses. [5]Helicene. Length of bond connecting two pyridinium rings. Carbon−nitrogen bond lengths in 12+. Carbon−
nitrogen bond lengths in 22+.
with Meerweins’ salt39 (trimethyloxonium tetrafluoroborate)
generated the heli-viologen 12+ in a modest but acceptable 73%
yield. Attempts to alkylate 4 with methyl iodide or with
dimethyl sulfate [(MeO)2SO2] failed. In each case, the
monomethylated helicene was obtained even in the presence
of excess alkylating agent. The same monomethylated helicene,
The synthetic route used to make 3,8-dimethyl-3,8-diaza[5]-
helicene (22+) is depicted in Scheme 1b. The key intermediate
in this reaction sequence is (3-methyl)benzo[f ]quinoline (6),
which was made in a ruthenium-catalyzed cyclocondensation
reaction between 2-naphthylamine (7) and 2-methyl-1,3-
propanediol.40 Conversion of 6 to (E)-1-(benzo[f ]quinolin-3-
yl)-2-(3-pyridyl)ethene (8) was accomplished by the three-step
Wittig reaction protocol shown in Scheme 1b. Including
gaseous HBr in the free-radical bromination step (NBS/AIBN)
−
albeit with a different counterion (BF4 ), was obtained by
refluxing a solution of 12+ in aqueous sodium hydroxide.
B
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Figure 1. Electrostatic potential energy surfaces (B3LYP/6-311+G(2d,p)) for heli-viologens 12+ and 22+.
Figure 2. Stacking motifs A, B, and C adopted by aza- and diaza[5]helicenes.
was required in order to protonate the nitrogen and suppress
oligomerization. The (3-bromomethyl)benzo[f ]quinoline hy-
drobromide product was then dried and used directly in the
next step to make the phosphonium salt that was subsequently
used in the Wittig reaction with 3-pyridine carboxaldehyde. The
Mallory photocyclization41 of 8 generated 1,8-diaza[5]helicene
(9), naphtha[2,1-b][1,8]phenanthroline (10), and 3,8-diaza[5]-
helicene (11), in 35%, 18%, and 38% yields, respectively, after
chromatographic separation. Helicene 11 was then converted
into 22+ in 70% yield by alkylation with trimethyloxonium
tetrafluoroborate.
bipyridine precursors, 4 → 12+ and 11 → 22+, is associated with
the viologen torsional angle, Θ14a,14b,14c,14d and Θ1,14d,14c,14b
,
respectively. In the case of 4 → 12+, the viologen torsional angle
increases by 1.9° (27.9° → 29.8°), while the other two fjord
region dihedral angles stay the same or only increase by 0.2°
(17.0° → 17.2°). In the case of 11 → 22+, the viologen torsional
angle increases by 1.3° (15.9° → 17.2°), while the other two
fjord dihedral angles decrease by 0.1° and increase by 2.5°.
These changes in the viologen torsional angle reflect the
electronic driving force to minimize the overlap between the
two positively charged N-methylpyridinium groups. In the case
of the unconstrained methyl viologen, MV2+, with weakly
Structural Characterization. The structures of 12+ and 22+
are completely consistent with their spectral data (see
Experimental Section) and were confirmed by X-ray crystallog-
raphy. The key crystal structure features of 12+ and 22+ are
compared to those of [5]helicene and to B3LYP/6-311+G-
(2d,p)-computed values for [5]helicene and for the bipyridine
precursors 4 and 11 of viologens 12+ and 22+, respectively, in
Table 1.
−
interacting counterions, such as BF4 , the optimal dihedral
angle that provides a balance between energetically favorable
resonance overlap and energetically destabilizing positive
charge interaction appears to be near 40°.14
The X-ray data reveal that the C1−C14 (d1,14) and H1−H14
(dH1,H14) distances are both substantially shorter in the 3,8-
viologen than in the 5,10-isomer. The location of the viologen
functional group within the [5]helicene scaffold coupled with
its preferred torsional angle (vide supra) undoubtedly
contributes to this difference. However, we cannot unequiv-
ocally rule out a small contribution from a more favorable
electrostatic interaction between the terminal A and E rings in
the 3,8-viologen than in the 5,10-viologen (Figure 1).
The structural features of the helicenes listed in Table 1 are
very similar in many respects. The only substantial differences
are in the fjord regions, the boundaries of which are defined by
the dihedral angles Θ14,14a,14b,14c, Θ14a,14b,14c,14d, and Θ1,14d,14c,14b
.
The middle dihedral angle, Θ14a,14b,14c,14d, in the fjord regions of
all the helicenes is substantially larger than the outer adjacent
dihedral angles, Θ14,14a,14b,14c and Θ1,14d,14c,14b, consistent with
the middle aromatic ring of the helicene suffering the greatest
deformation from the preferred planar geometry. The largest
change in the fjord regions that occurs upon alkylation of the
Macchi and co-workers42 recently reported a study of the
crystalline packing of a large number of aza- and diaza[5]-
helicenes, including that of 5,10-diaza[5]helicene (4). They
C
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Figure 3. Crystal packing projections for 12+ (I) and 22+ (II) and their stacking motifs Id and IId.
Figure 4. Side-on (A) and end-on (B) views showing close contacts between crystal-packing molecular pairs 12+/12+ and 22+/22+.
reported that these azahelicenes frequently adopted one of
three stacking motifs, as shown in Figure 2.
not centered directly over the pyridinium ring but are displaced
toward the periphery of the viologen so that the nearest
interaction is with the CH ring carbon α to nitrogen at a
distance of 3.41 Å. This simultaneously places the BF4− 4.85 Å
above the external rim of the terminal ring of an adjacent
helicene in the −M−P−M−P− columnar array of heli-
Heli-viologens 12+ and 22+, however, do not adopt any of
these established packing motifs but instead the unusual
packing arrangements shown in Figure 3. Heli-viologen 12+ has
C2 molecular symmetry and forms a crystal that belongs to the
centrosymmetric orthorhombic space group Pbcn (Figure 3, I).
Heli-viologen 22+ lacks the C2 molecular symmetry and forms a
crystal that belongs to the centrosymmetric monoclinic space
group C2/c. Both crystals are achiral. Three projections of the
crystal packing of an array of 16 molecules of 12+ (a, b, and c)
are depicted in the left-hand box in Figure 3. The heli-viologens
are arranged in three straight columns that alternate in
stereochemistry from top to bottom, as shown to the left of
projection Ib. The horizontal rows consist of a single
enantiomer, M or P, with the fjord regions pointing in the
−
viologens (Figure 3, Ib). In addition, all the BF4 are located
adjacent to the interstitial space between the columns and
experience two additional “in-plane” cross-column B−CH
interactions of 3.85 and 4.30 Å.
Three projections of the crystal packing of an array of 18
molecules of 22+ are depicted in the right-hand box in Figure 3.
−
The BF4 counterions are disordered and have been removed
from the packing array for clarity. A striking feature of the
crystal, which distinguishes it from 12+, is the pairwise packing
of the 22+ molecules. Both molecules in the pair have the same
stereochemistry and alternate, (M/M)−(P/P)−(M/M), along
the length of the column. Two molecules of acetonitrile, the
−
opposite direction in adjacent rows. Two BF4 are located on
opposite faces of 12+ at a B−N distance of 3.45 Å. The BF4− are
D
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
solvent used to grow the crystal of 22+, are intercalated into the
void in the crystal structure between each 22+/22+ pair. Three of
these acetonitrile partners are identified in projection IIb by
enclosing them in ovals. The columns, unlike that observed for
12+, are not straight but adopt a herringbone-like structure, as
depicted in projection IIa, which is a view down the diagonal of
the 18 molecule array, as shown by the arrow on projection IIb.
The stacking motifs for 12+ and 22+ (d in both boxes in
Figure 3) are characterized by enhanced molecular surface area
contact via π−π stacking in comparison to the aza-helicenes (A,
B, and C in Figure 2). Perfect sandwich-like packing is not
observed for the heli-viologens (see Figure 4) because of the
nonplanar topography of the helicenes, however, several π-
surface contacts of less than 4 Å are observed (Figure 4). The
near parallel arrangements of the bipyridinium ion functional
groups in the packing motifs (Figure 3, Id and IId) suggest that
the alignment of molecular dipoles and electrostatic inter-
actions plays a role in the crystal packing. In each case, the
electron-rich portion of the heli-viologen, as determined by the
electrostatic potential energy surfaces shown in Figure 1,
overlaps with the electron-poor surface of its partner. These
attractive interactions compensate for the repulsive closed-shell
π-cloud interactions and are the driving force for adoption of
these unusual packing arrangements.43
Table 2. B3LYP/6-311+G(2d,p) Racemization Barriers for
Helicenes
a
a
compd
ΔGc⧧alc (kcal mol−1
)
compd
ΔGc⧧alc (kcal mol−1
)
b
d
12
23.1
19.6
20.0
21.2
14
20.5
19.6
20.2
20.3
e
4
11
c
e
13+
22+
12+·BF4
ef
,
12+
−
a
Gibbs free energies at 25 °C calculated from potential energies and
b
adjusted for zero-point energies and thermal corrections. [5]-
Helicene. N-Methyl-5,10-diaza[5]helicene. 5,10-Diaza[5]helicene
c
d
e
bis-N-oxide. Chiral transition state entropic correction of R ln 2.
f
anti-Bis-tetrafluoroborate salt (see the text).
(STQN) method in conjunction with the B3LYP/6-311+G-
(2d,p) computation method. The activation barriers for the 3,8-
diaza[5]helicenes 11, 22+, and 22+·BF4 were adjusted by an
−
entropic contribution of R ln 2 to correct for reaction through
enantiomeric transitions states.
The gas-phase racemization barrier for [5]helicene (12)
(Table 2) is 1 kcal/mol lower than the experimental value
measured in naphthalene at 188 °C.47 Replacement of two
benzene rings with pyridines to give 5,10-diaza[5]helicene (4)
decreased the racemization barrier by 3.5 kcal/mol. Vacek
Dynamic Behavior. Heli-viologens 12+ and 22+, like their
carbocyclic analogues, are chiral as a result of their helical
structures. However, in analogy to other hetero[5]helicenes,
rapid room-temperature conformational deformations are likely
to limit the enantiomer lifetimes to a few minutes at room
temperature, making studies of resolved samples difficult.44
Consequently, we have initially chosen to limit our studies of
this important process with 12+ and 22+ to a series of
computational investigations. Previous computational studies
of [5]helicene racemizations have focused on planar (C2v) and
saddle-like (Cs) transition states that can relax with equal
probability to the P and M enantiomers. We will not consider
potential reversible bond-making−bond-breaking racemization
mechanisms, despite the fact that they have been considered45
and experimental evidence for a chemical pathway for
racemization has even been reported.46 These chemical
pathways are exceedingly rare and unlikely to occur with
these heli-viologens.
̌ ́
Chocholousova
and co-workers44 reported a B3LYP/cc-pVTZ
racemization barrier of 22.2 kcal/mol for 2-aza[5]helicene, a
decrease of 1.4 kcal/mol from their calculated value for 12,
which involved replacement of one terminal benzene with a
pyridine ring. The benzene rings in 12 and the nitrogen-
containing rings in both 4 and 2-aza[5]helicene are deformed
in their saddle-like racemization transition states from their
idealized planar geometries. Consequently, the decrease in the
racemization barriers in the aza-helicenes in comparison to the
carbocyclic [5]helicene could reflect the 8 kcal/mol smaller
pyridine resonance energy than that of benzene.48 The
racemization barrier for the monomethylated N-methyl-5,10-
diaza[5]helicene (13+), is 1.2 kcal/mol smaller than that for the
corresponding viologen (12+) but only 0.4 kcal/mol larger than
the barrier for its bipyridine precursor (4). On the other hand,
̌ ́
Vacek Chocholousova
and co-workers44 reported that the
racemization barrier for monoprotonated 2,13-diaza[5]helicene
is 5.7 kcal/mol smaller than that for its free base. The origin of
The transition states for racemizations of 12+ and 22+ were
located with the B3LYP/6-311+G(2d,p) computational
method and adopt saddle-like structures, as depicted in Figure
5. The B3LYP/6-311+G(2d,p) barriers for racemization of 12+
and 22+ along with those for several other [5]helicene
homologues are collected and compared in Table 2.
The activation barriers (Table 2) were calculated from the
differences in energies of the optimized helicene and
corresponding transition state and are corrected for zero-
point energies and thermal effects. The transition states were
located using the synchronous transition-guided quasi-Newton
̌ ́
the difference between Vacek Chocholousova et al.’s results and
ours is not entirely clear; however, we note that they report that
the transition state for racemization of protonated 2,13-
diaza[5]helicene is far from the common saddle-shaped (Cs)
structure and could be best characterized as a distorted helix. In
contrast, as shown in Table 3, when the nitrogens are not in the
terminal ring of the [5]helicene, both the B3LYP/6-311+G-
(2d,p) mono- and dimethylated transition states adopt the
idealized saddle-like topology. The four structural features in
the transition states that deviate most from those observed in
the P and M helices are the H-1/H-14 distance (d1H,14H) and
the three internal dihedral angles in the fjord region
(Θ14a,14b,14c,14d, Θ1,14d,14c,14b, and Θ14,14a,14b,14c) (Table 3).
Racemization leads to a significant decrease in the H-1 and
H-14 distances to the point where they become approximately
1 Å closer in the saddle-like transition states than in the helical
energy minima. Simultaneously, the central ring flattens
considerably and the arms of the helicene twist to form the
sides of the saddle, as revealed by the changes in the dihedral
angles in the fjord region (Table 3).
Figure 5. Planar (C2v) and saddle-like (Cs) transitions states (B3LYP/
6-311+G(2d,p) for racemizations.
E
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
structures, the BF4− are located on opposite faces of the helical
Table 3. Racemization Transition States (TS) and Helical
Precursors (P/M) Key Structural Feature Differences
dication. In anti-12+·2BF4 , the anions are displaced from the
−
center of the pyridinium rings out toward the periphery of the
helicene scaffold above the N5−C6 and N10−C9 bonds with B−
N contact distances of 3.59 Å and C6−F and C9−F closest
−
contact distances of 3.07 and 3.02 Å, respectively. The BF4
adopt remarkably similar locations relative to the pyridinum
rings in anti-22+·2BF4 , with B−N3 and B−N8 contact distances
−
of 3.61 and 3.58 Å, respectively, and identical C4−F and C7−F
close contact distances of 3.05 Å.
−
In the syn-structures, one BF4 adopts a location similar to
−
that observed in the anti- isomers, while the other BF4 is
found in the plane of a pyridinium ring in close contact distance
with N−CαH. In syn-12+·2BF4 , the in-plane anion exhibits a
−
B−N contact distance of 4.76 Å and an even shorter B−C6
contact distance of 3.95 Å. The syn-structural motif offers the
possibility of population of diastereomers. These diasteromeric
−
salts were located on the PES for syn-22+·2BF4 , as depicted in
Figure 6 as the syn- and syn′-structures. The syn′-isomer is the
more stable diastereomer by 0.09 kcal/mol. The anti-isomers in
both 12+·2BF4 and 22+·2BF4 were less stable than the syn-
−
−
isomers [ZPE-corrected stability order: syn′-22+·2BF4− (0 kcal/
mol) > syn-22+·2BF4 (0.09 kcal/mol) > syn-12+·2BF4 (0.73
−
−
−
−
In order to minimize computation expense, our initial
racemization barrier calculations with 13+, 12+, and 22+ were
gas-phase calculations in the absence of solvent and counterion
kcal/mol) > anti-22+·2BF4 (1.28 kcal/mol) > anti-12+·2BF4
(2.26 kcal/mol)].
The only transition state that we were able to locate for
racemization was for the P(anti-12+·2BF4 ) ⇄ M(anti-12+·
−
−
(e.g., BF4 ). Consequently, these barriers (Table 2) can be
−
considered intrinsic barriers that can be compared to
experimental values in order to determine the influence of
solvent and counterion on barrier height. We have done some
ion pair calculations and indeed found them to be difficult and
time-consuming optimizations. As anticipated, several tetra-
fluoroborate salt optimized structures for 12+ and 22+ can be
located, but we cannot be certain that we have located the
global minimum. The energy minima we have located are
shown in Figure 6.
2BF4 ) interconversion (Figure 7). The transition state adopts
a saddle-like structure similar to those shown in Table 3. A
comparison of the key structural parameters d1H,14H (1.55 Å),
Θ14a,14b,14c,14d (0.20°), Θ1,14d,14c,14b (36.94°), and Θ14,14a,14b,14c
−
(37.20°) to those in Table 3 reveals a small BF4 -induced
narrowing of the saddle, leading to a decrease in the N−N
−
distance from 6.80 to 6.72 Å. The BF4 counterions in the
transition state adopt nearly identical positions over the
pyridinium rings, as observed in the P or M enantiomer. One
−
anti- And syn-bis-tetrafluoroborate salts were identified as
BF4 is on the concave surface of the saddle (“in-the-saddle”)
energy minima in acetonitrile on both the 12+·2BF4 and 22+·
with a B−N contact distance of 3.59 Å, and the second BF4− is
on the convex surface of the saddle with a B−N contact
distance of 3.60 Å. The racemization barrier decreases by 0.5
−
−
2BF4 potential energy surfaces using B3LYP/6-311+G(2d,p)
and the polarizable continuum model. (Figure 6). In the anti-
Figure 6. B3LYP/6-311+G(2d,p)-optimized 12+ and 22+ bis-tetrafluoroborate salt energy minima in CH3CN.
F
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
−
Figure 7. (a) End-on, (b) face-on, and (c) side-on views of the anti-12+·2BF4 transition state for racemization.
kcal/mol in comparison to the counterion free gas-phase value
listed in Table 2.
major contributor to S1 (vide infra), can undergo conrotatory
closure without generating a phase discontinuity (i.e., only 22+
can maintain maximum overlap during the reaction) (Figure 9).
Photochemistry. The isomeric viologens 12+ and 22+
exhibit remarkably different photochemical behavior. The
5,10-viologen 12+ is photochemically inert when exposed to
either ambient light or light from a 600 W halogen lamp. The
3,8-viologen 22+, on the other hand, reacted under identical
conditions within a few hours to generate (2,7-dimethyl-2,7-
diaza)benzo[ghi]perylene bis-tetrafluoroborate (152+), in quan-
titative yield (eq 1). The formation of 152+ during irradiation
was easily detected by its bright green fluorescence. Very low
solubility precluded collection of the 13C NMR of 152+.
However, the identity of 152+ was established by comparing its
calculated [GIAO B3LYP/6-311+G(2d,p)] and experimental
1H NMR (Figure 8) and by a MALDI-TOF/TOF exact mass
determination (see Experimental Section).
Figure 9. LUMOs of 12+ and 22+.
Photophysical Characterization. The photophysical data
collected for 12+ and 22+ and their bipyridine precursors 4 and
11 are compared in Table 4. The UV−visible spectra of
viologens 12+ and 22+ are depicted as solid lines and those of
their bipyridine precursors 4 and 11 as dashed lines of the same
color in Figure 10. For comparison, the UV−visible spectrum
of 5,10-diaza[5]helicene bis-N-oxide (14) is also displayed in
Figure 10 as a green line.
Time domain DFT calculations at the B3LYP/6-311+G-
(2d,p) computational level were used to assign the most
bathochromic envelope of peaks to the S0 → S1 transitions.
Despite the different placement of nitrogens in the helical
scaffold, the wavelengths and fine structure for these envelope
of peaks are remarkably similar for the two viologens 12+ and
22+ (Table 4). In addition, the lowest energy absorption bands
for 12+ and 22+ are shifted bathochromically by a nearly
identical 68 and 67 nm, respectively, from the lowest energy
peak in their bipyridine precursors 4 and 11. The vibronic
progressions are more sharply defined in the bipyridines than in
the corresponding viologen. The energy separations are
approximately 1200−1300 cm−1, as anticipated for in-plane
C−H bends and/or aromatic C−C stretches.49
Figure 8. Calculated versus experimental chemical shifts of 152+. Inset:
Expanded downfield region.
The closure of 22+ can be viewed as a symmetry-allowed 6π
electron conrotatory electrocyclic reaction of the red 1,3,5-
hexatriene unit shown in eq 1. The trans-dihydro product of a
conrotatory closure is 7.0 kcal/mol more stable than the cis-
dihydro product, which would form in a disallowed disrotatory
closure of 22+ [B3LYP/6-311+G(2d,p)]. A conrotatory closure
also provides a rationale for the different photochemical
reactivity of 12+ and 22+; only the LUMO of 22+, which is the
The agreement between the experimental and DFT-
calculated positions of the S0 → S1 transitions in both 12+
and 22+ was very sensitive to the computational medium. In the
gas phase the lowest energy bands in the TD-DFT calculations
G
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
a
Table 4. Photophysical Data for Heli-Viologens 12+ and 22+ and Their Bipyridine Precursors 4 and 11
12+
22+
4
11
b
UV−vis (ε × 10−4
)
465 (0.40)
441 (0.38)
470 (0.60)
449 (0.49)
397 (0.30)
377 (0.38)
358 (0.15)
408, 431, 455
501, 531, 586
679
403 (0.53)
383 (0.42)
364 (0.21)
c
λF
504
566
420, 436, 463
513, 547, 602
1004
c
λPhos
623
616
d
Stokes shift
1664
3609
e
τF
5.8 0.6
0.066 0.004
0.028 0.002
11.4 × 106
59.3 0.1
10.2 0.6
0.087 0.001
0.088 0.002
8.5 × 106
56.5 0.1
6.3 0.6
0.14 0.01
1.7
2.2 × 107
71.0 0.1
4.2 0.2
ΦF
τPhos
0.18 0.02
1.2 0.01
4.3 × 107
69.7 0.1
f
g
kFo
h
ES
1
h
1
ET
54
2
54
2
57.1 0.2
55.7 0.2
h
ΔES‑T
5.3
2
2.5
2
13.9 0.2
2.92(4.01)
14.0 0.2
2.95(3.84)
i
band gap
2.48(3.03)
2.39(2.59)
a
d
h
b
c
In acetonitrile. Extinction coefficient, in 104 M−1 cm−1. Fluorescence wavelength (λF) and phosphorescence wavelength (λPhos), in nanometers.
e
f
g
In wavenumber, cm−1. Fluorescence lifetime, in nanoseconds. Phosphorescence lifetime, in seconds. Radiative rate constant, calculated as ΦF/τF.
Singlet energy (ES ), triplet energy (ET ), and singlet−triplet energy gap (ΔES−T), in kcal/mol. Optical gap, in eV. B3LYP/6-311+G(2d,p)-
i
1
1
calculated gap in eV is in parentheses.
and 13 nm hypsochromic of the experimental values. We
attribute the hypersensitivity of the viologens to computational
medium to the charge transfer character of the S0 → S1
electronic transitions (Figure 11). The charge transfer character
is unmistakable in the HOMO → LUMO excitations of 12+ and
22+ (Figure 11). Using these orbitals to assess the extent of
charge transfer is valid since the HOMO → LUMO
determinant is the primary contributor to the excited state
density or wave function (Figure 11).50 In both cases, the
charge transfer is from the relatively electron rich aryl rings to
the bypyridinium functional group. This charge transfer is
clearly more important in 22+ than in 12+, providing a rationale
for the much larger sensitivity to the computational medium of
the S0 → S1 absorption wavelength in 22+ (597 nm → 513 nm)
than in 12+ (500 nm → 479 nm).
The lowest energy envelope of peaks in the absorption
spectra of 5,10-diaza[5]helicene bis-N-oxide (14) (Figure 10) is
approximately 70 nm hypsochromic of the S0 → S1 absorption
band in 12+ and in appearance is more similar to the absorption
spectra of the bipyridine precursors than the viologens.
However, the peaks are 6.5 times greater in intensity than the
S0 → S1 absorption bands in 4 and 11. The S0 → S1 band
appears at 419 nm with a very low oscillator strength of only
0.0051 with TD-DFT contributions from the HOMO →
LUMO+1 and the HOMO−1 → LUMO determinants of 74%
Figure 10. UV−visible spectra in CH3CN.
were found at 500 and 597 nm, 35 and 127 nm bathochromic
of the experimentally observed wavelengths in 12+ and 22+,
respectively. When the polarizable continuum model was used
to include acetonitrile in the calculations, the computed
wavelengths (479 and 513 nm) were only 14 and 43 nm
bathochromic of the experimental values. In contrast, gas-phase
TD-DFT calculations for 4 and 11 predicted absorbances 22
Figure 11. Dominant determinant contributors to the S0 → S1 band in (a) 12+ and (b) 22+.
H
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Figure 12. Dominant determinant contributors to S0 → S1 and energy level diagram for 14 showing phantom triplet T2(ππ*) and detected triplet
T1(nπ*).
and 24%, respectively. In the spectrum of pyridine N-oxide, this
small band is hidden under a very strong S0 → S2 band.51,52
Consistent with a similar interpretation for the absorption
spectrum of 14 is the appearance of the TD-DFT S0 → S2 band
at 412 nm, with an oscillator strength 105 times greater than
that of the S0 → S1 band.
excited state of 12+ is a potent oxidant capable of oxidizing any
organic substrate with an ionization potential of less than 2.35
eV.
The heli-viologens 12+ and 22+, their bipyridine precursors 4
and 11, and bis-N-oxide 14 all undergo intersystem crossing to
triplet excited states that are phosphorescent in ethanol glass at
−196 °C (see Supporting Information). The emissions of the
heli-viologens are bathochromic of the emissions of their
bipyridine precursors. In addition, the phosphorescence
lifetimes of 12+ and 22+ are 28 and 88 ms, respectively,
significantly shorter than observed for 4 (1.7 s) and 11 (1.2 s).
The singlet−triplet energy gaps (ΔES−T) for 12+ and 22+, 5.3
and 2.5 kcal/mol, respectively, are significantly smaller than
The heli-viologens 12+ and 22+ and their heli-bipyridine
precursors 4 and 11 all fluoresce with modest but larger
quantum yields and lifetimes than those reported for methyl
viologen (ΦF = 0.03
0.01; τF = 1
0.04 ns).25 The
fluorescence quantum yields for both heli-viologens were 2.1
times smaller than for their heli-bipyridine precursor (Table 4).
On the other hand, the measured lifetime for 22+, 10.2 0.6 ns,
is substantially larger than the lifetime measured for 12+ (5.8
0.6 ns). The fluorescence lifetime measurement for 22+,
however, is complicated by the fact that the sample was
contaminated with approximately 3% of its photochemical
closure product, 152+. Pure 152+ has an emission maximum at
observed for 4 and 11, 13.9
0.2 and 14
2 kcal/mol,
respectively (Table 4). All of these ΔES−T are significantly
smaller than the 20−30 kcal/mol observed for many planar
aromatic hydrocarbons.55 On the other hand, the ΔES−T for the
helical bipyridines are comparable to those reported for
[5]helicene (ES = 71.2 kcal/mol; ΔES−T = 14.9 kcal/mol)
488 nm and a fluorescence quantum yield of 0.47
0.2.
1
Consequently, its contribution to the fluorescence intensity is
substantial and the two fluorescence contributors were first
deconvoluted (see Supporting Information) prior to calculation
of ΦF for 22+. We believe that this procedure leads to an
accurate determination of ΦF for 22+ for several reasons: (1)
the fluorescence lifetime, τF, of 152+ is 7. 0 ns, which is smaller
than the deconvoluted τF of 10.2 0.6 for 22+ precluding any
quenching of 1(22+)* by the contaminant, and (2) this
deconvoluted lifetime and all the lifetimes reported in Table
4, when coupled with the fluorescence quantum yields, give
natural radiative rate constants, koF, of the magnitude anticipated
for a ππ* transition.53 The fluorescence of 12+ was quenched
and for [6]helicene (ES = 69.6 kcal/mol; ΔES−T = 15.1 kcal/
1
mol).57 The absorption maxima [436 nm (ε = 0.32 × 104 M−1
cm−1)], 415 nm (ε = 0.30 × 104 M−1 cm−1)], fluorescence
wavelength (λF = 491 nm), phosphorescence wavelength (λPhos
= 525 nm), singlet energy (ES = 62.5 0.1 kcal/mol), and
1
triplet energy (ET = 55
2 kcal/mol) of N-methyl-5,10-
1
diaza[5]helicene tetrafluoroborate were distinctly different from
the values for 12+ (Table 4) and midway between the values of
12+ and its bipyridine precursor 4. This eliminates the
possibility that the observed emissions for 12+ were from a
small amount of monoalkylated diazahelicene contaminant.
The ΔES−T of 23 kcal/mol for the bis-N-oxide 14 (ES = 66
with electron-rich aromatic compounds such as benzene [ES =
1
1
110 kcal/mol, kq = (1.41 0.23) × 108 M−1 s−1], toluene [ES
kcal/mol; ET = 43
2 kcal/mol) is abnormally large in
1
1
comparison to the ΔES−T for 12+ and 22+, and on the basis of
expectations from the energy gap law,53 its quantum yield for
fluorescence (ΦF = 0.016) is abnormally small. It is tempting to
suggest that the bis-N-oxide has a spectroscopically unobserved
(phantom) triplet [T2(ππ*)] nearly isoenergetic with S1
(Figure 12). The S0 → S1 transition in 14 has a significant
amount of nπ* character (Figure 12), so intersystem crossing
to T2(ππ*) would be allowed by El-Sayed’s rules.58−60
Consequently, competing fluorescence would be more
effectively quenched than in the viologens 12+ and 22+ that
= 106 kcal/mol, kq = (8.68 0.16) × 109 M−1 s−1], anisole [ES
1
= 101 kcal/mol,54 kq = (1.68
0.14) × 1010 M−1 s−1],
anthracene [ES = 76 kcal/mol, kq = (2.5 1.7) × 1010 M−1
1
s−1], and naphthalene [ES = 92 kcal/mol, kq = (2.2 0.55) ×
1
1010 M−1 s−1].55 The singlet energies of all these quenchers are
larger than the singlet energy of 12+ (Table 4) and
consequently are electron/charge transfer events. In addition,
the reduction potential of 12+ (−0.22 V vs SCE)56 coupled to
its excited state energy (Table 4) suggests that the singlet
I
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
(m, 4H), 7.70 (dd, 1H, J = 7.4, 7.3 Hz) ppm. 13C NMR (400 MHz,
CDCl3): δ 152.5, 148.4, 148.3, 147.7, 147.6, 136.3, 130.6, 130.5, 130.4,
130.3, 129.0, 128.6, 128.3, 128.0, 127.8, 127.7, 127.4, 126.7, 126.1,
126.0 ppm. Anal. Calcd for C20H12N2: 85.69% C, 4.31% H, 9.99% N.
Found: 85.52% C, 4.47% H, 9.83% N.
must intersystem cross by an El-Sayed disallowed S1(ππ*) →
T1(ππ*) process.
CONCLUSIONS
■
The first example of isomeric helical viologens has been
reported, and their structures, racemization rates, photo-
chemistry, and photophysical properties have been presented
and compared. These new molecules have a variety of potential
applications, including their incorporation into molecular
machines and their use as chiral hosts11,61 for electron-rich
guests.62
N,N′-Dimethyl-5,10-diaza[5]helicene Bis-tetrafluoroborate
(12+). 5,10-Diaza[5]helicene (4) (40 mg, 0.14 mmol) was dissolved
in 30 mL of dichloromethane saturated with dry N2 gas.
Trimethyloxonium tetrafluoroborate (55 mg, 0.37 mmol) was then
added. The mixture was stirred in a nitrogen atmosphere for 24 h. The
solvent was then removed under reduced pressure. The yellow crude
product was triturated in warm 95% ethanol. It was then dried in
1
vacuum for 12 h. Yield: 50 mg (0.10 mmol, 73%). H NMR (400
MHz, CD3CN): δ 10.07 (d, 2H, J = 2.2 Hz), 8.75 (d, 2H, J = 8.3 Hz),
8.71 (d, 2H, J = 1.7 Hz), 8.61 (d, 2H, J = 8.8 Hz), 8.27 (ddd, 2H, J =
8.6, 7.1, 1.2 Hz), 7.89 (ddd, 2H, J = 8.3, 7.1, 0.9 Hz), 4.86 (s, 6H)
ppm. 13C NMR (600 MHz, DMSO-d6): δ 154.9, 135.3, 133.6, 131.5,
130.9, 129.3, 128.6, 127.9, 125.6, 119.7, and 46.1 ppm. Anal. Calcd for
C22H18N2B2F8: 54.59% C, 3.75% H, 5.79% N. Found: 54.35% C,
3.74% H, 5.82% N. It decomposes with a gradual color change starting
at 200 °C to finally form a black solid without ever clearly melting
within the temperature range of our melting point apparatus.
N-Methyl-5,10-diaza[5]helicene Tetrafluoroborate (13+).
N,N′-Dimethyl-5,10-diaza[5]helicene bis-tetrafluoroborate (12+) (30
mg, 0.07 mmol) was added to 10 mL of deionized water and refluxed
for 1 h. After the solution was cooled to room temperature, the solid
was removed by filtration. The solvent was then removed under
vacuum. The residue was recrystallized from ethanol and dried in
vacuum. A yellow solid (5 mg) was collected. Yield: 22% contaminated
with approximately 10% of 12+. 1H NMR (400 MHz, CD3CN): δ 9.90
(s, 1H), 9.68 (s, 1H), 8.95 (d, 1H, J = 8.5 Hz), 8.55 (d, 1H, J = 8.3
Hz), 8.52 (d, 1H, J = 8.7 Hz), 8.51 (d, 1H, J = 8.4 Hz), 8.49 (d, 1H, J
= 8.3 Hz), 8.37 (dd, 1H, J = 8.3, 1.2 Hz), 8.18 (ddd, 1H, J = 8.6, 7.2,
1.3 Hz), 7.93 (ddd, 1H, J = 8.3, 7.0, 1.3 Hz), 7.84 (ddd, 1H, J = 8.3,
7.1, 1.1 Hz), 7.59 (ddd, 1H, J = 8.4, 7.0, 1.4 Hz), 4.77 (s, 3H) ppm.
13C NMR (400 MHz, CD3CN): δ 154.6, 153.5, 147.4, 136.2, 134.8,
134.2, 131.7, 131.6, 131.2, 130.9, 130.0, 129.9, 129.5, 129.2, 127.8,
127.6, 127.4, 127.2, 124.3, 120.0, 46.9. HRMS (TOF/TOF MALDI)
m/z: [M]+ calcd for C21H15N2 295.1235, found 295.1210.
EXPERIMENTAL SECTION
■
General. Fluorescence spectra were recorded in acetonitrile by
irradiation into the S0 → S1 absorption band using excitation and
emission slit widths of 5 nm. Fluorescence quantum yields were
determined using diphenylanthracene (Φ = 0.90) and anthracene (Φ
= 0.27) as standards.63,64 Phosphorescence spectra and lifetimes were
collected at −196 °C in ethanol glass. Fluorescence lifetimes were
collected using a pulsed 340 nm LED lamp and a 400 nm cutoff filter.
The fluorescence lifetime instrument response function (IRF) was
collected with a dilute colloidal silica solution. High-resolution mass
spectra were collected using TOF-TOF-MALDI. The X-ray data were
measured at 150 K with a CCD detector system equipped with a
graphite monochromator and a Mo Kα fine-focus sealed tube operated
at 1.5 kW power (50 kV, 30 mA). The structure of 22+ was solved by
direct methods using the SHELXTL (V. 2014.11−0) software
package. All of the non-hydrogen atoms were refined anisotropically,
and the hydrogen atoms were placed in calculated positions and
refined isotropically by adapting a riding model with fixed thermal
parameters. The final refinement parameters are R1 = 0.0745 and wR2
= 0.2156 for data with F > 4σ(F) giving the data to parameter ratio of
24. The refinement for all data are R1 = 0.0989 and wR2 = 0.2443. All
calculations were done using either the Gaussian 03 or Gaussian 09
program packages (see Supporting Information). Geometry optimiza-
tions were done using DFT in conjunction with the B3LYP functional
and the 6-311+G(2d,p) basis set. All structures reported are stationary
points, as verified by frequency calculations that revealed either one or
zero negative frequencies. When solvent was included, the polarizable
continuum model and polarizable conductor calculations as
implemented in the Gaussian computation package were used.
2-Acetonaphthone Oxime.65 2-Acetonaphthone (20 g, 0.118
mol), hydroxylamine hydrochloride (8.3 g, 0.12 mol), and sodium
acetate (9.8 g, 0.12 mol) were dissolved in a mixture of 30 mL of
absolute ethanol and 9 mL of water. The mixture was then heated at
50 °C for 20 min. The white precipitate was collected by filtration and
washed with deionized water (20 mL × 3). It was then dried in
vacuum. Yield: 20.3 g (0.111 mol, 94%). 1H NMR (400 MHz,
CDCl3): δ 7.95 (s, 1H), 7.77−7.87 (m, 4H), 7.44−7.50 (m, 2H), 2.33
(s, 3H) ppm.
(E)-1,2-Di(quinolin-3-yl)ethene (3).37,38 A mixture of 0.4 mL of
triethoxy(vinyl)silane and 15 mL of 0.5 M aqueous sodium hydroxide
was added to a 100 mL pressure vessel followed by 0.416 mL (3.0
mmol) of 3-bromoquinoline and 2.0 mg (9 μmol) of palladium(II)
acetate. The vessel was then sealed and warmed at 140 °C for 3−5 h.
When the reaction cooled to room temperature the green precipitate
was filtered and washed with ethyl acetate/chloroform 10:1. The
yellow solid was then dried in vacuum. Yield: 0.38 g (2.7 mmol, 91%).
1H NMR (400 MHz, CDCl3): δ 9.20 (d, 2H, J = 2.1 Hz), 8.26 (d, 2H,
2-Acetamidonaphthalene.65 2-Acetonaphthone oxime (18.5 g,
0.1 mol) was added slowly with mechanical stirring to 50 mL of
polyphosphoric acid in a 200 mL beaker. The mixture was heated to
70 °C and kept at this temperature for 2 h. The mixture was then
poured into 300 mL of deionized water. A white precipitate was
obtained by filtration. The white solid was washed with deionized
J = 2.1 Hz), 8.11 (d, 2H, J = 8.4 Hz), 7.86 (d, 2H, J = 7.9 Hz), 7.72
(ddd, 2H, J = 8.4, 7.0, 1.5 Hz), 7.58 (ddd, 2H, J = 8.1 Hz, 6.8 Hz, 0.9
Hz), 7.48 (s, 2H) ppm.
1
water and dried in vacuum. Yield: 14.0 g (0.076 mol, yield 76%). H
5,10-Diaza[5]helicene (4)36 and Benzo[b]-1,8-diaza[4]-
helicene. (5). (E)-1,2-Di(quinolin-3-yl)ethane (3) (0.282 g, 1.0
mmol) was dissolved in ethyl acetate (150 mL) in a Pyrex vessel open
to air. The solution was irradiated with UV light for 24 h. The
irradiation was then stopped and the solvent removed under reduced
pressure. The residue was purified by column chromatography (eluent
NMR (400 MHz, CDCl3): δ 8.17 (s, 1H), 7.77 (m, 2H), 7.35−7.50
(m, 4H), 2.22 (s, 3H) ppm.
2-Naphthylamine (7).65 2-Acetamidonaphthalene (14 g, 0.076
mol) was dissolved in a mixture of 20 mL of absolute ethanol and 8
mL of concentrated hydrochloric acid. The mixture was refluxed for 1
h and then cooled to room temperature. The solution was adjusted to
pH > 7 by a 6 M NaOH aqueous solution. The brown precipitate was
collected by filtration, washed with water, and then dried in vacuum
for 6 h. White crystals were obtained by sublimation. Yield: 10.0 g
(0.070 mmol, 92%). 1H NMR (400 MHz, CDCl3): δ 7.69 (d, 1H, J =
8.2 Hz), 7.66 (d, 1H, J = 8.7 Hz), 7.59 (d, 1H, J = 8.3 Hz), 7.39 (m,
1H), 7.24 (m, 1H), 6.99 (d, 1H, J = 7.0 Hz), 6.95 (dd, 1H, J = 8.5, 2.5
Hz), 3.75 (br s, 2H) ppm.
1
was ethyl acetate:hexane = 5:1). Yield: 0.186 g (0.66 mmol, 66%). H
NMR (400 MHz, CDCl3): δ 9.44 (s, 2H), 8.61 (dd, 2H, J = 8.3, 1.0
Hz), 8.28 (dd, 2H, J = 8.2, 1.2 Hz), 8.11 (s, 2H), 7.74 (ddd, 2H, J =
8.4, 7.1, 1.3 Hz), 7.41 (ddd, 2H, J = 8.5, 7.2, 1.3 Hz). Benzo[b]-1,8-
diaza[4]helicene (5) was found as a byproduct in the above reaction. It
was also isolated by column chromatography and recrystallized from
acetone. Yield: 0.028 g (0.10 mmol, 10%). 1H NMR (400 MHz,
CDCl3): δ 11.45 (d, 1H, J = 8.5 Hz), 9.45 (s, 1H), 8.87 (s, 1H), 8.58
(d, 1H, J = 8.6 Hz), 8.35 (d, 1H, J = 8.2 Hz), 8.11 (m, 2H), 7.85−8.01
(3-Methyl)benzo[f ]quinoline (6).40 A stirred solution of RuCl3·
xH2O (52 mg, 0.20 mmol), MgBr2·OEt2 (52 mg, 0.20 mmol), PBu3
J
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
(100 μL, 0.40 mmol), 2-naphthylamine (7) (564 mg, 4.0 mmol), and
2-methyl-1,3-propanediol (426 μL, 4.8 mmol) in anhydrous
mesitylene (2 mL) was refluxed under an argon atmosphere for 16
h. The reaction mixture was then cooled to room temperature and
purified by column chromatography (toluene:ethyl acetate = 1:1) on
silica gel pretreated with 0.1% of Et3N in toluene. Yield: 289 mg (1.50
mmol, 38%). 1H NMR (400 MHz, CDCl3): δ 8.80 (d, 1H, J = 1.8 Hz),
8.70 (s, 1H), 8.59 (d, 1H, J = 7.9 Hz), 7.86−7.98 (m, 3H), 7.58−7.70
(m, 2H), 2.60 (s, 3H) ppm. 13C NMR (400 MHz, CDCl3): δ 151.4,
146.5, 132.1, 131.0, 130.4, 130.0, 129.7, 128.9, 128.3, 127.4, 127.0,
125.3, 122.8, 19.2 ppm.
(3-Bromomethyl)benzo[f ]quinoline hydrobromide. (3-
Methyl)benzo[f ]quinoline (6) (289 mg, 1.50 mmol), NBS (294 mg,
1.65 mmol), and AIBN (25 mg, 0.15 mmol) were added to 40 mL of
CCl4. The solution was then irradiated with a 100 W tungsten lamp for
4 h. The floating solid was removed by filtration. The filtrate was then
kept in a refrigerator for 1 h and filtered again to remove more of the
byproduct succinimide. All attempts to isolate the product, 3-
bromomethyl)benzo[f ]quinolone, failed due to polymerization unless
it was first converted to the hydrobromide salt, in situ, by bubbling
HBr gas through the solution. The yellow precipitate was collected and
the filtrate treated with additional HBr gas to produce more yellow
precipitate. The precipitate fractions were combined and washed with
DCM. After drying in vacuum for 12 h, it was used directly for the next
step without characterization.
Yield 84 mg (0.30 mmol, 35%). 1H NMR (400 MHz, CDCl3): δ 9.41
(s, 1H), 8.80 (dd, 1H, J = 4.2, 1.8 Hz), 8.34 (m, 2H), 8.08−8.17 (m,
3H), 8.00 (d, 1H, J = 8.5 Hz), 7.97 (d, 1H, J = 8.0 Hz), 7.57 7.64 (m,
2H), 7.35 (ddd, 1H, J = 8.6, 7.2, 1.3 Hz) ppm. 13C NMR (400 MHz,
CDCl3): δ 151.1, 147.4, 146.8, 145.4, 136.0, 132.8, 131.7, 130.9, 130.7,
130.4, 128.8, 128.0, 127.4, 127.3, 127.2, 127.0, 125.8, 123.5, 123.4,
121.3 ppm. HRMS (TOF/TOF MALDI) m/z: [M]+ calcd for
C20H13N2 281.1079, found 281.1095. Data for benzo[4,3-h]pyrido-
[a]acridine (10): Yield 43 mg (0.15 mmol, 18%). 1H NMR (400
MHz, CDCl3): δ 9.46 (s, 1H), 9.32 (s, 1H), 9.26 (d, 1H, J = 5.4 Hz),
8.94 (d, 1H, J = 5.4 Hz), 8.85 (d, 1H, J = 8.0 Hz), 8.23 (d, 1H, J = 9.3
Hz), 8.09 (d, 1H, J = 9.2 Hz), 8.04 (d, 1H, J = 8.9 Hz), 8.00 (d, 1H, J
= 7.9 Hz), 7.91 (d, 1H, J = 8.9 Hz), 7.80 (dd, 1H, J = 7.6, 7.2 Hz), 7.74
(dd, 1H, J = 7.3, 7.2 Hz) ppm. The limited solubility of 10 prevents
acquisition of a 13C NMR spectrum in a reasonable amount of time.
HRMS (TOF/TOF MALDI) m/z: [M]+ calcd for C20H13N2
281.1079, found 281.1082.
N,N′-Dimethyl-3,8-diaza[5]helicene Bis-tetrafluoroborate
(22+). 3,8-Diaza[5]helicene (11) (91 mg, 0.33 mmol) was dissolved
in 20 mL of dry DCM. Trimethyloxonium tetrafluoroborate (147 mg,
1 mmol) was then added to this solution. The mixture was stirred
under a nitrogen atmosphere for 24 h. Ethanol (2 mL) was then added
to quench the excess trimethyloxonium tetrafluoroborate. The solid
was then filtered and triturated with absolute ethanol. The red solid
was then filtered and dried in vacuum for 12 h. Yield: 112 mg (0.23
3-((Triphenylphosphonio)methyl)benzo[f ]quinolin-4-ium
Dibromide.66 The (3-bromomethyl)benzo[f ]quinoline hydrobro-
mide collected from the previous step and triphenylphosphine (393
mg, 1.5 mmol) were added to 20 mL of acetonitrile and refluxed
overnight. The brown solid was filtered and washed with 5 mL of
acetonitrile and 5 mL of ethyl acetate. It was then air-dried and used
directly for the next step without characterization.
1
mmol, 70%). H NMR (400 MHz, CD3CN): δ 9.88 (s, 1H), 9.75 (s,
1H), 8.97 (d, 1H, J = 6.8 Hz), 8.72 (d, 1H, J = 9.2 Hz), 8.67 (d, 1H, J
= 8.7 Hz), 8.64 (d, 1H, J = 8.8 Hz), 8.53 (d, 1H, J = 8.7 Hz), 8.43 (d,
1H, J = 9.3 Hz), 8.39 (d, 1H, J = 6.9 Hz), 8.36 (d, 1H, J = 8.0 Hz),
7.98 (ddd, 1H, J = 7.6, 7.8, 0.9 Hz), 7.66 (dd, 1H, J = 7.9, 7.4, 1.0 Hz),
4.81 (s, 3H), 4.53 (s, 3H) ppm. 13C NMR (400 MHz, CD3CN): δ
151.8, 151.0, 138.7, 138.8, 137.6, 136.7, 134.4, 133.9, 132.5, 132.1,
131.0, 130.9, 130.1, 130.0, 129.7, 129.5, 128.9, 127.7, 126.3, 116.5,
49.8, 48.2 ppm.
(E)-1-(Benzo[f ]quinolin-3-yl)-2-(3-pyridyl)ethene (8).66 So-
dium (92 mg, 4 mmol) was added slowly to 10 mL of methanol at
0 °C. After all the sodium reacted, the 3-((triphenylphosphonio)-
methyl)benzo[f ]quinolin-4-ium dibromide collected from the pre-
vious step was added in portions to the reaction mixture. The mixture
was stirred at 0 °C for 1 h before addition of 3-pyridinecarboxaldehyde
(162 mg, 1.5 mmol). The reaction mixture was then warmed to reflux
and kept under reflux overnight. Then 30 mL of aqueous 10% HCl
solution was added slowly to the reaction. The mixture was extracted
with ethyl acetate to remove most of the triphenylphosphine oxide (20
mL × 3). The aqueous layer was then basified by 2 M NaOH and then
extracted again with ethyl acetate (30 mL × 3). The organic layers
were combined and dried over MgSO4. The solvent was then removed
under reduced pressure. The residue was purified by column
chromatography (eluent was ethyl acetate: hexane = 3:1). Yield of
three steps: 242 mg (0.86 mmol, 57%). 1H NMR (400 MHz, CDCl3):
δ 8.79 (m, 2H), 8.57 (d, 1H, J = 1.9 Hz), 8.49 (dd, 1H, J = 4.9, 1.5
Hz), 8.33 (m, 1H), 7.90−8.01 (m, 3H), 7.64 (m, 2H), 7.58 (d, 1H, J =
8.0 Hz), 7.15 (dd, 1H, J = 7.7, 4.8 Hz), 6.97 (d, 1H, J = 12.2 Hz), 6.85
(d, 1H, J = 12.1 Hz) ppm. 13C NMR (400 MHz, CDCl3): δ 150.2,
150.1, 149.4, 148.8, 135.6, 132.6, 132.5, 131.9, 131.1, 130.2, 129.9,
129.5, 129.3, 129.0, 128.8, 127.9, 127.5, 127.3, 123.3, 122.4 ppm.
3,8-Diaza[5]helicene (11), 1,8-Diaza[5]helicene (9), and
Benzo[a]pyrido[4,3-h]acridine (10). (E)-1-(benzo[f ]quinolin-3-
yl)-2-(3-pyridyl)ethene (8) (242 mg, 0.86 mmol) was dissolved in
300 mL of ethyl acetate and irradiated with UV light for 24 h. The
solvent was then removed under reduced pressure. The residue was
then purified by column chromatography (eluent was ethyl
acetate:hexane = 3:1 gradually increased to 5:1). Three products
were isolated, i.e., 3,8-diaza[5]helicene (11), 1,8-diaza[5]helicene (9),
and benzo[a]pyrido[4,3-h]acridine (10). Data for 3,8-diaza[5]helicene
(11): Yield 91 mg (0.33 mmol, 38%). 1H NMR (400 MHz, CDCl3): δ
9.42 (d, 1H, J = 0.8 Hz), 9.40 (s, 1H), 8.51 (m, 2H), 8.42 (d, 1H, J =
6.0 Hz), 8.09−8.19 (m, 4H), 8.03 (dd, 1H, J = 8.0, 1.3 Hz), 7.64 (ddd,
1H, J = 8.1, 7.6, 1.2 Hz), 7.41 (ddd, 1H, J = 8.5, 8.1, 1.4 Hz) ppm. 13C
NMR (400 MHz, CDCl3): δ 152.4, 151.7, 146.4, 143.4, 133.2, 132.9,
130.5, 129.4, 129.2, 128.8, 128.7, 128.2, 127.7, 127.6, 127.5, 126.9,
126.1, 125.1, 120.9, 120.8 ppm. Data for 1,8-diaza[5]helicene (9):
(2,7-Dimethyl-2,7-diaza)benzo[ghi]perylene Bis-tetrafluoro-
borate (152+). N,N′-Dimethyl-3,8-diaza[5]helicene bis-tetrafluorobo-
rate (22+) (20 mg, 0.041 mmol) was dissolved in 20 mL of acetonitrile.
The solution was irradiated with visible light (ambient light) for 48 h.
The solution was concentrated to about 5 mL followed by addition of
5 mL of diethyl ether. The yellow precipitate was filtered and dried in
vacuum. Yield: 18 mg (0.037 mmol, 90%). 1H NMR (400 MHz,
CD3CN): δ 10.36 (s, 1H), 10.27 (s, 1H), 9.84 (s, 1H), 9.56 (d, 1H, J =
7.8 Hz), 9.11 (d, 1H, J = 9.5 Hz), 8.98 (d, 1H, J = 7.8 Hz), 8.90 (d,
1H, J = 9.1 Hz), 8.81 (d, 1H, J = 9.5 Hz), 8.76 (d, 1H, J = 9.0 Hz),
8.64 (dd, 1H, J = 8.0, 7.9 Hz), 5.06 (s, 3H), 4.89 (s, 3H) ppm. The
limited solubility of 152+ prevents acquisition of a 13C NMR spectrum
in a reasonable amount of time. HRMS (MALDI/TOF-TOF) m/z:
2+
calcd for C22H16N2 308.13135, found 308.1325.
5,10-Diaza[5]helicene N,N′-Bisoxide (14).67 Under vigorous
magnetic stirring, 3-chloroperbenzoic acid (m-CPBA) (345 mg, 2
mmol) in CH2Cl2 (5 mL) was added dropwise into a solution of 5,10-
diaza[5]helicene (280 mg, 1 mmol) in CH2Cl2 (5 mL) at 0 °C. After
the addition, the reaction was warmed to room temperature and
stirred overnight. An aqueous solution of saturated NaHCO3 was
added to the mixture to neutralize the unreacted m-CPBA. The
resulting mixture was extracted with CH2Cl2 (3 × 10 mL). The
organic phase were combined and washed with saturated NaCl
solution (3 × 5 mL). The organic layer was dried over anhydrous
MgSO4, filtered, and evaporated under reduced pressure to give crude
product. The crude product was purified by trituration with
chloroform followed by recrystallization from methanol and dried in
vacuum. Yield: 169 mg (0.55 mmol, 55%). 1H NMR (400 MHz,
CD3CN): δ 8.93 (s, 2H), 8.79 (d, 2H, J = 8.6 Hz), 8.57 (d, 2H, J = 8.3
Hz), 7.91 (s, 2H), 7.85 (m, 2H), 7.55 (m, 2H) ppm. 13C NMR (400
MHz, CD3CN): δ 141.8, 134.9, 131.8, 129.9, 129.4, 128.8, 127.9,
127.6, 123.7, 120.8 ppm.
K
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
(22) Boydston, A. J.; Vu, P. D.; Dykhno, O. L.; Chang, V.; Wyatt, A.
R.; Stockett, A. S.; Ritschdorff, E. T.; Shear, J. B.; Bielawski, C. W. J.
Am. Chem. Soc. 2008, 130, 3143.
(23) Henrich, J. D.; Suchyta, S.; Kohler, B. J. Phys. Chem. B 2015,
119, 2737.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.6b00835.
(24) Hohenstein, E. G. J. Am. Chem. Soc. 2016, 138, 1868.
(25) Peon, J.; Tan, X.; Hoerner, J. D.; Xia, C.; Luk, Y. F.; Kohler, B. J.
Phys. Chem. A 2001, 105, 5768.
(26) Lin, F.; Zhao, X. Tetrahedron 2015, 71, 1124.
(27) A preliminary accounting of some of the work reported here:
Zhang, X.; Clennan, E. L.; Arulsamy, N. Org. Lett. 2014, 16, 4610.
(28) Takashima, H.; Tanaka, M.; Hasegawa, Y.; Tsukahara, K. J. Biol.
Inorg. Chem. 2003, 8, 499.
Computational details; X-ray crystallographic analysis;
fluorescence deconvolution of 152+; fluorescence,
absorption, and phosphorescence spectra of N-methyl-
1
5,10-diaza[5]helicene tetrafluoroborate; and 13C and H
NMR spectra (PDF)
Crystallographic data in CIF format for 12+ and 22+
(CIF)
(29) Tsukahara, K.; Goda, M. Chem. Lett. 1998, 929.
(30) Tsukahara, K.; Kaneko, J.; Miyaji, T.; Abe, K. Tetrahedron Lett.
1996, 37, 3149.
AUTHOR INFORMATION
■
(31) Tsukahara, K.; Ueda, R.; Goda, M. Bull. Chem. Soc. Jpn. 2001,
Corresponding Author
*E-mail: clennane@uwyo.edu
74, 1303.
̌
(32) Adriaenssens, L.; Severa, L.; Sal
́
ova,
Saman, D.; Rocha, S. V.; Finney, N. S.; Pospísi
F. Chem. - Eur. J. 2009, 15, 1072.
(33) Nath, N. K.; Severa, L.; Kunetskiy, R. A.; Císaro
M.; Ruzicka, K.; Koval, D.; Kasicka, V.; Teply, F.; Naumov, P. Chem. -
Eur. J. 2015, 21, 13508.
́
T.; Císaro
̌
va,
̌
́
I.; Pohl, R.;
Notes
̌
̌
l, L.; Slavíce
k, P.; Teply,
́
The authors declare no competing financial interest.
̌
́
va, I.; Fulem,
̌
̌
̌
́
̊
̌
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE-1147542)
for their generous support of this research.
(34) Reyes-Gutierrez, P. E.; Jirasek, M.; Severa, L.; Novotna, P.;
Koval, D.; Sazelova, P.; Vavra, J.; Meyer, A.; Cisarova, I.; Saman, D.;
Pohl, R.; Stepanek, P.; Slavicek, P.; Coe, B. J.; Hajek, M.; Kasicka, V.;
Urbanova, M.; Teply, F. Chem. Commun. 2015, 51, 1583.
REFERENCES
■
̌
́ ̌ ́
́
ra, J.; Saman, D.; Císarova, I.; Teply, F.
(35) Sonawane, M. R.; Vav
Synthesis 2015, 47, 3479.
(1) Weidel, H.; Russo, M. Monatsh. Chem. 1882, 3, 850.
(2) Monk, P. M. S. The Viologens. Physicochemical Properties, Synthesis
and Applications of the Salts of 4,4′-Bipyridine; John Wiley & Sons:
Chichester, England, 1998.
(36) Bazzini, C.; Brovelli, S.; Caronna, T.; Gambarotti, C.; Giannone,
M.; Macchi, P.; Meinardi, F.; Mele, A.; Panzeri, W.; Recupero, F.;
Sironi, A.; Tubino, R. Eur. J. Org. Chem. 2005, 2005, 1247.
(37) Gordillo, A.; de Jesus, E.; Lopez-Mardomingo, C. Chem.
Commun. 2007, 4056.
(3) Summers, L. A. The Bipyridinium Herbicides; Academic Press:
New York, 1980.
(4) Summers, L. A. In Advances in Heterocyclic Chemistry; Katritzky,
A. R., Ed.; Academic Press: Orlando, FL, 1984; Vol. 35, p 281.
(5) Boβmann, S.; Seiler, M.; Durr, H. J. Phys. Org. Chem. 1992, 5, 63.
(6) Contreras Carballada, P.; Mourtzis, N.; Felici, M.; Bonnet, S.;
Nolte, R. J. M.; Williams, R. M.; De Cola, L.; Feiters, M. C. Eur. J. Org.
Chem. 2012, 2012, 6729.
(7) Xu, C.; Mochizuki, D.; Maitani, M. M.; Wada, Y. Eur. J. Inorg.
Chem. 2014, 2014, 1470.
(38) Gordillo, A.; Ortuno, M. A.; Lop
́
ez-Mardomingo, C.; Lledos
́
, A.;
̃
Ujaque, G.; de Jesus, E. J. Am. Chem. Soc. 2013, 135, 13749.
́
̈
(39) Meerwein, H. In Organic Syntheses; Baumgarten, H. E., Ed.; John
Wiley & Sons: New York, 1973; Collect. Vol. V, p 1096.
(40) Monrad, R. N.; Madsen, R. Org. Biomol. Chem. 2011, 9, 610.
(41) Mallory, F. B.; Mallory, C. W. In Organic Reactions; Dauben, W.
G., Ed.; John Wiley & Sons: New York, 1984; Vol. 30, p 1.
(42) Bazzini, C.; Caronna, T.; Fontana, F.; Macchi, P.; Mele, A.; Sora,
I. N.; Panzeri, W.; Sironi, A. New J. Chem. 2008, 32, 1710.
(43) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int.
Ed. 2003, 42, 1210.
́
(8) Lorente, A.; Fernandez-Saiz, M.; Herraiz, F.; Lehn, J.-M.;
Vigneron, J.-P. Tetrahedron Lett. 1999, 40, 5901.
(9) Bachrach, S. M. J. Phys. Chem. A 2013, 117, 8484.
(10) Bachrach, S. M.; Nickle, Z. O. M. J. Phys. Chem. A 2015, 119,
10613.
(11) Barnes, J. C.; Jurícek, M.; Strutt, N. L.; Frasconi, M.; Sampath,
S.; Giesener, M. A.; McGrier, P. L.; Bruns, C. J.; Stern, C. L.; Sarjeant,
A. A.; Stoddart, J. F. J. Am. Chem. Soc. 2013, 135, 183.
(12) Fahrenbach, A. C.; Barnes, J. C.; Lanfranchi, D. A.; Li, H.;
Coskun, A.; Gassensmith, J. J.; Liu, Z.; Benítez, D.; Trabolsi, A.;
Goddard, W. A.; Elhabiri, M.; Stoddart, J. F. J. Am. Chem. Soc. 2012,
134, 3061.
(13) Jalilov, A. S.; Patwardhan, S.; Singh, A.; Simeon, T.; Sarjeant, A.
A.; Schatz, G. C.; Lewis, F. D. J. Phys. Chem. B 2014, 118, 125.
(14) Clennan, E. L. Coord. Chem. Rev. 2004, 248, 477.
(15) Deng, J.; Zhou, C.; Song, N. Macromolecules 2009, 42, 6865.
(16) Coskun, A.; Spruell, J. M.; Barin, G.; Dichtel, W. R.; Flood, A.
H.; Botros, Y. Y.; Stoddart, J. F. Chem. Soc. Rev. 2012, 41, 4827.
(17) Kaur, K.; Mittal, S. K.; Kumar S K, A.; Kumar, A.; Kumar, S.
Anal. Methods 2013, 5, 5565.
(44) Vacek Chocholouso
̌
va,
al, M.; Stara, I. G.; Meyer, M.; Bourdillon, M.; Pospísi
L.; Stary, I. Chem. - Eur. J. 2014, 20, 877.
́
J.; Vacek, J.; Andronova, A.; Míse
̌
k, J.;
̌
Songis, O.; Sam
́
́
̌
l,
́
(45) Martin, R. H.; Marchant, M. J. Tetrahedron 1974, 30, 347.
(46) Adam, R.; Ballesteros-Garrido, R.; Vallcorba, O.; Abarca, B.;
Ballesteros, R.; Leroux, F. R.; Colobert, F.; Amigo, J. M.; Rius, J.
Tetrahedron Lett. 2013, 54, 4316.
́
(47) Goedicke, C.; Stegemeyer, H. Tetrahedron Lett. 1970, 11, 937.
(48) Joule, J. A.; Mills, K. Heterocyclic Chemistry, 5th ed.; Wiley-
Blackwell, 2010.
(49) Orlandi, G.; Zerbetto, F.; Henneker, W.; Zgierski, M. Z. Chem.
Phys. Lett. 1986, 129, 296.
(50) The contributions of the HOMO−LUMO determinant was
calculated by taking 2 times the square of the coefficient for the
configuration interaction expansion. This is an approximation that
ignores contributions from de-excitations.
(51) Nakagawa, Y.; Suzuka, I.; Ito, M. Chem. Phys. Lett. 1993, 208,
(18) Li, H.-Y.; Wei, Y.-L.; Dong, X.-Y.; Zang, S.-Q.; Mak, T. C. W.
453.
Chem. Mater. 2015, 27, 1327.
(52) Palmer, M. H.; Findlay, R. H.; Moyes, W.; Gaskell, A. J. J. Chem.
Soc., Perkin Trans. 2 (1972-1999) 1975, 841.
(53) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular
Photochemistry of Organic Molecules; University Science Books:
Sausalito, CA, 2010.
(19) Tong, J.; Guo, X.; Jia, L.; Tian, X. Mater. Lett. 2015, 158, 255.
(20) Sun, J.; Wu, Y.; Wang, Y.; Liu, Z.; Cheng, C.; Hartlieb, K. J.;
Wasielewski, M. R.; Stoddart, J. F. J. Am. Chem. Soc. 2015, 137, 13484.
(21) Boydston, A. J.; Pecinovsky, C. S.; Chao, S. T.; Bielawski, C. W.
J. Am. Chem. Soc. 2007, 129, 14550.
L
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
(54) Wintgens, V. In CRC Handbook of Organic Photochemistry;
Scaiano, J. C., Ed.; CRC Press, Inc.: Boca Raton, FL, 2000; Vol. II, p
405.
(55) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of
Photochemistry, 3rd ed.; CRC Press: Boca Raton, FL, 2006.
(56) Clennan, E. L., Zhang, X.; Petek, T. Unpublished results.
(57) Sapir, M.; Vander Donckt, E. Chem. Phys. Lett. 1975, 36, 108.
(58) El-Sayed, M. A. J. Chem. Phys. 1962, 36, 573.
(59) El-Sayed, M. A. J. Chem. Phys. 1963, 38, 2834.
(60) El-Sayed, M. A. J. Chem. Phys. 1964, 41, 2462.
(61) Sue, C.-H.; Basu, S.; Fahrenbach, A. C.; Shveyd, A. K.; Dey, S.
K.; Botros, Y. Y.; Stoddart, J. F. Chem. Sci. 2010, 1, 119.
(62) Bernardo, A. R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc.
1992, 114, 10624.
(63) Eaton, D. F. In CRC Handbook of Organic Photochemistry;
Scaiano, J. C., Ed.; CRC Press, Inc.: Boca Raton, FL, 2000; Vol. I, p
231.
(64) Hamai, S.; Hirayama, F. J. Phys. Chem. 1983, 87, 83.
(65) Zi, G.; Zhang, Z.; Wang, Q.; Ai, L. In Zhuanli Shenqing Gogkai
Shumingshu; 2010; Vol. CN 101740758 A 20100512.
(66) Dolusi
Colau, D.; Pochet, L.; Van den Eynde, B.; Masereel, B.; Wouters, J.;
Frederick, R. J. Med. Chem. 2011, 54, 5320.
(67) Li, G.; Jia, C.; Sun, K. Org. Lett. 2013, 15, 5198.
̌ ́
c, E.; Larrieu, P.; Moineaux, L.; Stroobant, V.; Pilotte, L.;
́
́
M
DOI: 10.1021/acs.joc.6b00835
J. Org. Chem. XXXX, XXX, XXX−XXX