Photochromic benzo[ g ]quinoxalines

Article abstract of DOI:10.1139/V10-124

The photochromism of two 2,3-diarylbenzo[g]quinoxaline derivatives was explored. Irradiating 2,3-diphenylbenzo[g]quinoxaline with 350 nm light led to the formation of its photodimer, which could be reverted to its monomers either thermally or photochemically.The resulting photodimer absorbs at a longer wavelength than analogous dianthracene derivatives. 2,3-Dipyridylbenzo[g] quinoxaline was also observed to undergo photodimerization. The ability of these readily prepared benzoquinoxaline derivatives to undergo [4 + 4]-photocycloaddition reactions makes them attractive alternatives to anthracene derivatives.

Full text of DOI:10.1139/V10-124

297
Photochromic benzo[g]quinoxalines
Muriel Rakotomalala, Michael Katz, Emilie Voisin, Tamara C. S. Pace,
Cornelia Bohne, and Vance E. Williams
Abstract: The photochromism of two 2,3-diarylbenzo[g]quinoxaline derivatives was explored. Irradiating 2,3-diphenylben-
zo[g]quinoxaline with 350 nm light led to the formation of its photodimer, which could be reverted to its monomers either
thermally or photochemically.The resulting photodimer absorbs at a longer wavelength than analogous dianthracene deriva-
tives. 2,3-Dipyridylbenzo[g]quinoxaline was also observed to undergo photodimerization. The ability of these readily pre-
pared benzoquinoxaline derivatives to undergo [4 + 4]-photocycloaddition reactions makes them attractive alternatives to
anthracene derivatives.
Key words: cycloaddition, photochromism, benzoquinoxaline.
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Resume : On a etudie la photochromie de deux derives de 2,3-diarylbenzo[g]quinoxaline. L’irradiation de la 2,3-diphenyl-
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benzo[g]quinoxaline avec de la lumiere a 350 nm conduit a la formation de son photodimere qui peut revenir a sa forme
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monomere d’une fac¸on soit thermique ou photochimique. Le photodimere qui en resulte absorbe a une longueur d’onde
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plus grande que celle des derives dianthracenes analogues. On a observe que la 2,3-dipyridylbenzo[g]quinoxaline subit
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aussi une photodimerisation. L’habilite de ces derives de la benzoquinoxaline facilement accessibles a subir des reactions
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de [4 + 4]-photocycloaddition en font des alternatives attrayantes aux derives de l’anthracene.
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Mots-cles : cycloaddition, photochromie, benzoquinoxaline.
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[Traduit par la Redaction]
Introduction
Since the discovery of anthracene photochromism in the
19th century,1 extensive studies have been carried out on
the photodimerization of this and related molecules.2,3
Although these reversible reactions have been exploited in a
variety of contexts,4–10 their practical utility is limited by the
pronounced tendency of anthracene derivatives to photo-oxi-
dize3,11–13 and the short wavelengths (<300 nm) needed to
photolyze the dimers back into their monomers.3 Functional
groups at the 2 and 3 positions of anthracene can shift the
absorption of both the monomer and dimer to longer wave-
lengths, but such derivatives are often difficult to synthe-
size.14–18 In contrast, 2,3-disubstituted benzo[g]quinoxalines
can be prepared in a single step from commercially avail-
able starting materials19 and, given their structural similar-
ities to anthracene, are also expected to undergo [4 + 4]-
photocycloaddition reactions. Although numerous benzo[g]-
quinoxaline derivatives have been prepared,20,21 their photo-
chemistry has not been studied; indeed, the photochromism
of nitrogen-containing analogues of anthracene remains
largely unexplored.22–29
These considerations prompted us to investigate the pho-
tochemistry of two 2,3-diarylbenzo[g]quinoxaline deriva-
tives, PhB and PyB. The presence of either phenyl or
pyridyl groups at the 2 and 3 positions of these derivatives
was anticipated to confer several advantages on these chro-
mophores; not only are these groups expected to shift the
absorbance to longer wavelengths, but they may also im-
prove the solubility by inhibiting aggregation between the
benzo[g]quinoxaline cores. The pyridyl derivative PyB was
of further interest for its potential ability to complex to met-
als, which suggests a means to modulate the photoreactivity
through chemical stimuli.
Received 1 June 2010. Accepted 22 July 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 14 February 2011.
M. Rakotomalala, M. Katz, E. Voisin, and V.E. Williams.1 Department of Chemistry, Simon Fraser University, 8888 University Dr.,
Burnaby, BC V5A 1S6, Canada.
T.C.S. Pace and C. Bohne. Chemistry Department, University of Victoria, P.O. Box 3065, Victoria, BC V8W 3V6, Canada.
1Corresponding author (e-mail: vancew@sfu.ca).
Can. J. Chem. 89: 297–302 (2011)
doi:10.1139/V10-124
Published by NRC Research Press

298
Can. J. Chem. Vol. 89, 2011
Scheme 1.
Fig. 1. Structure of the head-to-tail photodimer of 2,3-diphenylben-
zo[g]quinoxaline (ht-di-PhB) determined by X-ray crystallography.
Results and discussion
PhB and PyB were prepared from the condensation of
2,3-diaminonaphthalene with benzil19 and pyridil,30 respec-
tively (Scheme 1). Pyridil was obtained in two steps via a
benzoin condensation of picolinaldeyde followed by oxida-
tion using a slightly modified literature procedure.31
The relatively long bridgehead bonds observed in the
crystal structure suggested that, like anthracene derivatives,
these dimers might thermally revert to monomers at elevated
temperatures. The thermal stability of ht-di-PhB was studied
by monitoring the UV–vis spectrum of a 1 mmol/L solution
heated in DMSO at a variety of temperatures. No decompo-
sition was observed for a sample heated at 120 8C for 5 h,
indicating that the dimer is thermally stable at moderate
temperatures. In contrast, heating this compound to 150 8C
led to a continuous decrease in absorption bands and an ac-
companying increase in peaks associated with the monomer
PhB. At this temperature, reversion of the dimer to PhB
was complete after 2.5 h.
To examine whether ht-di-PhB also undergoes photore-
version, a 0.1 mmol/L acetonitrile solution of the dimer was
irradiated with 328 nm light and the progress of the reaction
was monitored by UV–vis spectroscopy (Fig. 2). During ir-
radiation, the absorbance at 328 nm steadily decreased while
a new peak at 380 nm grew in, indicating that the product
dissociates to the corresponding monomer. The 1H NMR
spectrum confirmed that PhB was being formed during the
course of these experiments. Dissociation reached comple-
tion after 240 min of irradiation. The spectral changes with
time show that two isobestic points may be located at 321
and 342 nm. However, close inspection of these wavelength
regions (Fig. 2, inset) reveals a very small systematic drift
as time progresses, which suggests that only two major spe-
cies are present in solution, but a minor species, probably
present at very low concentrations, is also possibly formed.
It is unclear at this time what the nature of this additional
photoproduct is, although it may arise from the cyclization
of the peripheral phenyl groups to form a triphenylene-like
product.33
The UV–vis absorption spectrum of PhB in acetonitrile
reveals a strong absorbance centred at 270 nm and a less in-
tense set of bands at longer wavelengths (lmax = 380 nm).
Irradiation of a degassed 4.4 mmol/L acetonitrile solution of
PhB with lamps centered at 350 nm for 150 min led to the
formation of a single photoproduct in a 20% yield. The yield
of this reaction increased to 72% when a filter solution of
KNO3 was employed to block wavelengths below 340 nm,32
which suggests that the product may be undergoing photore-
version to PhB upon exposure to shorter wavelengths. This
conjecture was supported by the observation that the photo-
product has an absorption maximum at 328 nm, where there
is considerable output from the lamps employed (vide infra).
This photoproduct is thermally stable at room temperature
and was readily separated from the starting material.
The MALDI-TOF spectrum of this photoproduct exhibits
an intense peak at m/z 333 and a weaker peak at 665, which
correspond to the M + 1 peaks of PhB and its photodimer,
respectively. The low intensity of the dimer peak is in keep-
ing with the behaviour of anthracene photodimers, which
typically dissociate during mass spectrometry experiments.2
1
The H NMR spectrum of the product was consistent with
that of the head-to-tail (ht) photodimer, ht-di-PhB. The
most significant feature of this spectrum is a singlet at
5.06 ppm, which is within the range expected for protons
attached to the sp3-hybridized bridgehead carbons of
dianthracene-like photodimers. The observation of only a
single photoproduct from this reaction is likely due to the
steric bulk of the phenyl groups, which may inhibit forma-
tion of the head-to-head (hh) stereoisomer, hh-di-PhB. It is
also possible that this second isomer was formed in trace
quantities that could not be detected, or that it exhibits ap-
preciably lower thermal stability than the ht dimer and
therefore decomposes.
In an effort to better understand the mechanism of photo-
dimerization, the excited state lifetimes of PhB were deter-
mined by single-photon counting for the singlet excited state
and by laser flash photolysis for the triplet state. The singlet
state lifetimes were found to be 1.9 ns 0.1 ns in acetoni-
trile and <0.5 ns in cyclohexane.34 The triplet lifetimes are
long (‡10 ms) in acetonitrile and cyclohexane. In compari-
son, the singlet excited state of anthracene has a lifetime of
5.3 ns in nonpolar solvents.35 We observed no appreciable
difference in the efficiency for photodimerization when this
reaction was carried out in aerated solutions, which excludes
reactivity from the triplet excited state. In addition, unlike
The identification of the photoproduct as ht-di-PhB was
confirmed by X-ray crystallography (Fig. 1) of single crys-
tals obtained by slow evaporation of a 1:1 acetonitrile/
chloroform solution. Under these conditions, the dimer crys-
tallizes in the monoclinic space group P21\n. The bond
lengths between the sp3-hybridized bridgehead carbons are
˚
1.61(6) A, which are longer than a typical carbon–carbon
single bond, but similar to the lengths of the corresponding
2
˚
bonds in dianthracene (1.624 A) and related systems.
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Rakotomalala et al.
299
Fig. 3. Absorption spectra of dianthracene (di-A), the photodimer
of 2,3-diphenylanthracene (di-DPA), and the head-to-tail photodi-
mer of 2,3-diphenylbenzo[g]quinoxaline (ht-di-PhB) in acetonitrile.
Fig. 2. UV–vis absorption spectra of the photodissociation of a
0.1 mmol/L solution of the head-to-tail photodimer of 2,3-diphe-
nylbenzo[g]quinoxaline (ht-di-PhB) in CH3CN using a 328 nm
light. Inset: expansion of the 315–355 nm region.
1
150 min. The H NMR spectrum of the residue obtained by
many anthracene derivatives, PhB does not appear to be
prone to photoreactions with oxygen, as no additional prod-
ucts such as endoperoxides were observed when the reaction
was performed in air. These results suggest that photodimer-
ization occurs from the singlet excited state. The absence of
endoperoxide formation makes it possible to perform the re-
action in aerated solutions, which represents a distinct ad-
vantage of this system for practical applications.
evaporation of the reaction solvent showed a new set of
peaks in the aromatic region accompanied by a singlet at
5.2 ppm associated with the formation of the bridgehead
across the central ring of two benzo[g]quinoxaline moieties.
The MALDI-TOF mass spectrum of this mixture showed a
peak at m/z 669, corresponding to M(di-PyB) + 1, and a
more intense peak for M(PyB) + 1. These results are there-
fore consistent with the identification of the photoproduct as
di-PyB. The photoadduct formed could be separated from
PyB by slow evaporation of CH3CN, which led to selective
precipitation of the photodimer. A priori, we expect the ht
dimer, ht-di-PyB, to preferentially form as the major prod-
uct. This prediction is borne out by the observation of an
NOE between protons a and b (Fig. 4) in the NOESY spec-
trum of this photoproduct in CDCl3. This NOE would not be
expected for the hh dimer.
In addition to its photochromic properties, PyB is also ca-
pable of binding a variety of metal cations.37–40 It would
therefore be of interest whether the presence of species ca-
pable of binding to this compound would also alter its reac-
tivity. However, initial attempts to modulate the tendency of
this compound to undergo [4 + 4]-photocycloaddition reac-
tions by the addition of transition metals such as zinc have
been unsuccessful. Controlling the photoreactivity of this
compound by protonation of the pyridyl groups also proved
impractical, as PyB becomes insoluble at low pH. Further
investigations into the reactivity of this compound in the
presence of transition metals and other species are under-
way.
The photochromic properties of PhB compare favourably
to those of 2,3-diphenylanthracene, DPA.35 DPA also under-
goes [4 + 4]-cycloaddition to form a single photodimer,
which also appeared to be the ht stereoisomer. The UV–vis
absorption spectra of DPA and PhB are similar, although
the absorption of the latter is redshifted by ~20 nm relative
to that of the former. A dramatic difference is also observed
between the absorption spectra of the corresponding photo-
dimers (Fig. 3). Whereas ht-di-PhB exhibits a large peak at
328 nm, the photodimer of DPA (di-DPA) has no appreci-
able absorption above 320 nm, and the parent compound, di-
anthracene (di-A) does not absorb above 300 nm. As a
consequence, switching between PhB and its dimer can be
induced with longer wavelengths than is achievable with
most anthracene derivatives. It is also worth noting that
although both PhB and DPA can be prepared from benzil,
PhB is prepared in a single step in a high yield, whereas
DPA is prepared in three steps in considerably lower overall
yields.15,36
Conclusion
In conclusion, we have demonstrated that both PhB and
PyB photochemically dimerize in solution. It was also
shown that the dimer ht-di-PhB can be dissociated back to
PhB using either heat or light. PhB offers several advan-
tages over anthracene derivatives to which it is structurally
related. This compound is straightforward to prepare, is not
prone to photooxidation, and can be switched between its
dimer and monomer forms using relatively long wavelengths
The photochemistry of the dipyridyl derivative PyB is
similar to that of PhB. A 1.33 mmol/L acetonitrile solution
of PyB was irradiated with lamps centered at 350 nm for
Published by NRC Research Press

300
Can. J. Chem. Vol. 89, 2011
˚ ˚
P 1 21/n1, a = 8.6132(9) A, b = 18.4915(17) A, c =
Fig. 4. Proposed structure of the photoproduct formed upon irradia-
tion of 2,3-dipyridylbenzo[g]quinoxaline (PyB).
3
˚
˚
10.9618(16) A, b = 96.550(6), V = 1734.5(4) A , T = 293
K, Z = 2, rcalcd = 1.273 g/cm–3, m (Cu Ka) = 0.580, 3014
unique reflections, 2639 observed (Io > 2.5s(Io)). The final
RF = 0.0436 and RWF = 0.0645 (observed data).
Synthesis
Synthesis of 2,3-dibenzylbenzo[g]quinoxaline (PhB)
A solution of 2,3-diaminonaphtalene (1.0 mmol) and ben-
zil (1.1 mmol) in acetic acid (10 mL) was refluxed over-
night. Reaction progress was monitored by TLC (eluent:
toluene). Once all 2,3-diaminonaphtalene was consumed,
the hot reaction mixture was poured over crushed ice. The
resulting off-brown suspension was then neutralized with
satd. NaHCO3. The aqueous solution was extracted with di-
chloromethane (3 Â 80 mL). The organic washes were com-
bined and dried over MgSO4, filtered, and dried in vacuo
yielding an off-yellow powder (88%). The product was puri-
fied by flash column chromatography (eluent: dichlorome-
of light. As such, benzoquinoxalines represent a promising
new class of photochromic compounds that overcome many
of the shortcomings of anthracene derivatives, and we are
currently investigating the photochemistry of other members
of this family of molecules.
Experimental
Materials
thane) to yield
a yellow crystalline powder (75%).
Uncorrected mp 184–185 8C (lit. value19 mp 187–189 8C).
UV–vis lmax (nm) (3 ((mol/L)/cm): 271 (61 292), 302
(3683), 381 (12 341) 88. IR (KBr, cm–1) n: 3053, 2919,
All chemicals were used as provided from the supplier.
Acetonitrile, 2,3-diaminonaphthalene, and benzil were pur-
chased from Caledon Laboratories Ltd., Sigma-Aldrich
Chemical Co., and Matheson Colleman & Bell, respectively.
Potassium nitrate, acetic acid, and toluene were purchased
from Anachemia.
1
2849, 1436, 1346, 1249. H NMR (400 MHz, CDCl3) dH:
8.75 (s, 2H), 8.13 (q, 2H), 7.57 (m, 6H), 7.37 (m, 6H). 13C
NMR (100 MHz, CDCl3) dC: 154.1, 139.1, 137.9, 134.0,
129.8, 128.9, 128.5, 128.2, 127.5, 126.7. MS m/e (CI):
333.3 (M + 1). Anal. calcd for C24H16H2 (%): C 86.72, H
4.85, N 8.43; found: C 86.56, H 5.00, N 8.21.
Instruments
1H NMR and 13C NMR spectroscopic characterization
were carried out using a Bruker AMX-400 400 MHz spec-
trometer; IR spectra were obtained using a Thermo Nicolet
Nexus 670 FT-IR ESP spectrometer, performed in KBr.
Low-resolution EI mass spectrometry (70 eV) was carried
out using a Hewlett Packard 5985 mass spectrometer.
MALDI-TOF mass spectra were carried out using a Perspec-
tive Voyager-DE STR from PE Applied Biosystems with a
nitrogen laser (337 nm) and using 2,5-dihydroxybenzoic
acid as the matrix. Microanalysis (C, H, and N) were per-
formed at Simon Fraser University, Burnaby, British Colum-
bia, by M. Yang and S. Wong. Melting points were
determined using a Fisher melting point apparatus and are
uncorrected. UV–vis absorption spectra were obtained on a
Cary 300 biospectrophotometer.
Synthesis of 1,2-di(pyridin-2-yl)ethane-1,2-dione (pyridil)
Picolinaldehyde (3.22 g, 30.0 mmol) and potassium cya-
nide (1.92 g, 30.0 mmol) were dissolved in ethanol (5 mL)
and water (20 mL) and heated to reflux. After 4 h, TLC
analysis (hexane / ethyl acetate, 1:1) showed that all picoli-
naldehyde was consumed; the hot reaction mixture was
poured over crushed ice. The resulting brown suspension
was filtered with a Buchner funnel and washed with cold
ethanol to yield 2-hydroxy-1,2-di(pyridine-2-yl)ethanone
(95%), which was used without further purification. 1H
NMR (500 MHz, CDCl3, ppm) dH: 8.47 (d, J = 5.0 Hz,
2.0H), 7.93–7.88 (m, 2H), 7.84 (dt, J1 = 8.2, J2 = 1.8 Hz,
2H), 7.19 (ddd, J1 = 7.3, J2 = 5.0, J3 = 1.2 Hz, 2H).
Fluorescence spectra were measured with a PTI QM-2
spectrometer, and the singlet excited state lifetimes were
measured with an OB920 single-photon counter from Edin-
burgh. The excitation wavelength was 380 nm and emission
spectra were collected between 395 and 700 nm (slit band-
width of 3 nm), while decays were collected at 460 nm in
cyclohexane or 490 nm in acetonitrile (slit bandwidth of
16 nm). For a description of the laser flash photolysis sys-
tem, see ref. 33. Samples were excited with a Nd:YAG laser
at 266 nm, and the kinetics were monitored at 400–410 and
460 nm. For all photophysical experiments, samples were
deoxygenated by bubbling nitrogen for 20 min.
2-Hydroxy-1-(pyridine-2-yl)ethanone (1.611 g, 7.5 mmol)
was dissolved in pyridine (25 mL). To this solution was
added a solution of CuSO4 (1.870 g, 7.5 mmol) in H2O
(10 mL). Once addition was complete, the reaction mixture
was stirred at reflux for 1 h. The hot reaction was poured
over crushed ice and filtered to yield 0.857 g of pure pyridil
(54%) as a yellow crystalline solid; mp 155–157 8C (lit.
1
value31 mp 156–157 8C). MS m/z (CI): 213 (M + 1). H
NMR (500 MHz, CDCl3, ppm) dH: 8.60 (d, J = 4.7 Hz,
2.0H), 8.23 (d, J = 7.8 Hz, 2H), 7.94 (dt, J = 7.7, 1.8 Hz,
2H), 7.50 (ddd, J1 = 7.6, J2 = 4.8, J3 = 1.2 Hz, 2H).
Synthesis of PyB
This compound was synthesized according to literature
procedure.30 The product was recrystallized from hot ethanol
yielding PyB (73%) as shiny yellow crystals; mp 183–
185 8C (lit. value30 mp 184–185 8C). MS m/z (CI) 335
X-ray crystallography
Crystal data for ht-di-DPBQ
C48H32N4, MW = 664.79, monoclinic, space group =
Published by NRC Research Press

Rakotomalala et al.
301
1
(M + 1). H NMR (400 MHz, CDCl3, ppm) dH: 8.80 (s, 2H),
Supplementary data
8.36 (d, J = 4.8 Hz, 2H), 8.15 (m, 2H), 8.09 (d, J = 7.8 Hz,
2H), 7.87 (dt, J1 = 1.7, J2 = 7.7 Hz, 2H), 7.61 (m, 2H), 7.26
(m, 4H).
Supplementary data (UV–vis absorption spectra of PhB
with ht-di-PhB and spectra showing the progress of the
thermal dissociation experiment) for this article are available
on the journal Web site (canjchem.nrc.ca). CCDC 653177
contains the X-ray data in CIF format for this manuscript.
These data can be obtained, free of charge, via http://www.
ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.
cam.ac.uk).
Photochemical experiments
All photodimerization experiments were carried out in
glass tubes in a Rayonet photoreactor fitted with seven
RPR3500A lamps.
For the experiments in which a filter solution was em-
ployed, 3.32 mg PhB was dissolved in 0.75 mL of CH3CN.
The solution was transferred to an NMR tube and immersed
into a test tube filled with a 2.0 mol/L solution of KNO3 in
water. The sample was irradiated in a Rayonet for 150 min.
The solvent was then removed in vacuo and redissolved in a
minimum volume of CDCl3. The 1H NMR sprectrum
showed a 4:1 ratio of dimer to monomer.
Acknowledgement
The authors gratefully acknowledge the Natural Sciences
and Engineering Research Council of Canada (NSERC), the
University of Victoria, Victoria, British Columbia, and Si-
mon Fraser University, Burnaby, British Columbia, for fund-
ing; T.C.S.P. thanks NSERC for a Canada Graduate
Scholarships – Doctoral (CGS-D) scholarship; and M.-K.
Yang and F. Haftbaradaran are acknowledged for carrying
out the microanalysis.
Synthesis of ht-di-PhB
PhB (3.3 mg) was dissolved in spectral grade benzene
(1.5 mL) and deoxygenated. The sample was then irradiated
in a Rayonet fitted with seven 350 nm lamps for 2 h. After
irradiation, a white powder was precipitated on the side of
the glass. The solution was decanted and the white product
on the side of the glass vessel was collected. MALDI-TOF
m/z: 665 (M + 1). UV–vis lmax (nm) (3 (mol/L)/cm): 230
(47 543), 270 (17 967), 308 (23 337), 328 (29 242). IR
(KBr, cm–1) n: 3057, 2959, 2919, 2849, 1389, 1144. 1H
NMR (400 MHz, CDCl3, ppm) dH: 7.23 (m, 16H), 7.14 (m,
8H), 7.03 (m, 4H), 5.05 (s, 4H). 13C NMR (100 MHz,
CDCl3, ppm) dC: 156.3, 148.7, 140.4, 138.8, 129.4, 128.1,
128.0, 126.9, 52.7. (Note: due to the limited solubility of
this compound, we were unable to observe the signals for
quaternary carbons.) Anal. calcd for C48H32N4 (%): C
86.72, H 4.85, N 8.43; found: C 86.61, H 4.98, N 8.15.
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100 mL volumetric flask. The solution (20 mL, 1.00 Â
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Synthesis of ht-di-PyB
ht-di-PyB was prepared using the method described
above for ht-di-PhB to afford a white solid that precipitated
from the reaction solution. MALDI-TOF m/z: 669 (M + 1).
1H NMR (500 MHz, CDCl3, ppm) dH: 8.43 (d, J = 4.8 Hz,
2H), 7.69–7.63 (m, 2H), 7.45–7.37 (m, 2H), 7.25 (dd, J1 =
5.4, J2 = 3.3 Hz, 2H), 7.21–7.14 (m, 1H), 7.00 (dd, J1 =
5.4, J2 = 3.2 Hz, 2H), 5.16 (s, 2 H). 13C NMR (125 MHz,
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