Selenospiropyrans incorporating appended pyrene chromophores

Article abstract of DOI:10.1016/j.tetlet.2008.04.134

The synthesis of two pyrene-based selenospiropyran dyads is described along with their absorption profiles in various solvents.

Full text of DOI:10.1016/j.tetlet.2008.04.134

Tetrahedron Letters 49 (2008) 4292–4295
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Tetrahedron Letters
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Selenospiropyrans incorporating appended pyrene chromophores
*
Andrew C. Benniston , Jérôme Fortage
Molecular Photonics Laboratory, School of Natural Sciences, Newcastle University, Newcastle-upon-Tyne NE1 7RU, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of two pyrene-based selenospiropyran dyads is described along with their absorption
profiles in various solvents.
Received 28 February 2008
Revised 16 April 2008
Accepted 23 April 2008
Available online 27 April 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Major developments have been seen over the past twenty years
or so in molecular-scale assemblies that are capable of using pho-
tons to initiate the passage of information in a controlled manner.
hν
1
-
+
P
SP
P
SP
The tenet is that molecular-based systems will find applications in
the emerging field of photonics where fast optical responses are
exciplex
2
required. The basic structure commonly comprises a donor–
relay–acceptor motif, in which the relay acts as the conduit for pas-
sage of, for example, electrons, but can be altered in some manner
to perturb information flow. In general, a switching unit incorpo-
hν
3
P
MC
rated into the relay is the controller, and can operate by breaking
4
p-conjugation, or perturbing the energy levels of the HOMO/LU-
energy transfer
5
MOs within the relay. Identification of ultimate switching units
has seen the evolution of different groups including fulgides, diary-
lethenes and dihydroindolizines. In particular, the spiropyran (SP)
Scheme 1.
6
group has been targeted since it is readily ring-opened by light to
the merocyanine (MC) form. The reverse process can be thermally
would compete with MC to SP ring closure. We noted that the only
previous examples of selenium-based molecular systems, reported
by Miyashita et al., showed enhanced yields of the MC form and
displayed good switching action. A second design feature was to
extend the nitro-containing ring to increase delocalization to see
if this had any effect on the strength of the spiro C–O bond; a
weaker bond would hopefully favour the MC over the SP
arrangement.
7
induced or initiated by a light of long wavelength. As well as the
clear colour change, the MC form also promotes increased conjuga-
tion through the double-bond backbone. In previous work, we
devoted attention to using such a switching action to manipulate
11
8
energy migration in a T-shaped array. An upshot from this work
was the development of pyrene-based systems that could be
ring-opened via an exciplex (Scheme 1).9 The interest here was
that direct illumination of the spiropyran was avoided, which
sometimes can be difficult in multi-chromophoric systems for
which the SP absorption band is obscured. However, one disadvan-
tage of the molecular system was the fast ring closure, MC to SP,
induced by energy transfer from the pyrene to the singlet state
of the MC unit. Having established that the singlet state of the
MC was predominantly responsible for efficient ring closure, the
next stage in development was to endeavour to circumvent this
problem. It is well established that heavy atom incorporation
The procedures used to prepare the two prototype selenospiro-
pyran derivatives, P1 and P2, are shown in Scheme 2. The starting
material 1 was specifically targeted since the 6,8 substitution pat-
tern at the pyrene centre is consummate for appending other redox
or photoactive groups. The first task was to convert the alcohol
group of 1 to a good leaving group for reaction with commercially
available 2-methylbenzoselenazole.
12
The compound 1 could be readily converted to several deriv-
atives containing OMs (2b), Cl (2c), I (2d) and Br (2e) in good
yields. All attempts to prepare 2a, the OTs derivative, failed to give
the desired product. Rather disappointingly, no conditions could be
found to convert 2b–d to the corresponding 2-methylselenazolium
(
e.g., S, Se) into merocyanine dyes promotes efficient singlet to
10
triplet inter-system crossing. Thus, we devised a strategy to pro-
duce selenospiropyrans in the expectation that triplet formation
13
salts. However, by careful control of the temperature and reac-
tion times, the heating of 2e with neat 2-methylbenzoselenazole
at 100 °C for 5 d produced a workable quantity of 3a as the bro-
mide salt. As illustrated in Scheme 2, the reaction also produced
*
Corresponding author. Tel.: +44 0191 222 5706; fax: +44 0191 222 6929.
E-mail address: a.c.benniston@ncl.ac.uk (A. C. Benniston).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.04.134

A. C. Benniston, J. Fortage / Tetrahedron Letters 49 (2008) 4292–4295
4293
OH
(i) TsCl, CH Cl , DMAP
R
2
2
(
(
(
(
ii) MsCl, CH Cl , Et N
2 2 3
(
(
(
(
(
i) R = OTs yield 0% 2a
ii) R = OMs yield 100% 2b
iii) R = Cl yield 58% 2c
iv) R = I yield 85% 2d
v) R = Br yield 61% 2e
iii) Pyridine, SOCl , benzene
2
iv) MsCl, NaI, CH Cl , Et N
2
2
3
v) PPh , CBr , CH Cl
3
4
2
2
Si
Si
Si
Si
1
Se
N
Se
Se
N
Se
N
+
-
+
Br
N
H
+
-
NaOH, acetone
Br
Si
Si
H O, RT
3b
2
Conditions 3a 3b
o
1
1
1
50 C, 8 d
0% 0%
6
4%
85%
o
00 C, 21 h 66% 34%
Si
o
Si
00 C, 5 d 79% 21%
4
THF, 80 oC
3a
OH
CHO
or 6
Se
N O
NO2
NO2
R
Si
R = no ring P1 28%
R = ring P2 75%
Si
OH
O
Me
(
i) MeI, K CO , acetone, Δ
2 3
OH O
O
O
H
(
i) HNO , AcOH, RT
3
(
ii) LiAlH , THF, RT
4
OH
H
(
ii) AlCl , benzene, 100 ˚C
3
(
iii) PDC, DCM, RT
NO2
5
6
6
5%
7
1%
Scheme 2.
1
the protonated salt side product 3b, which was carried through in
the preparation as its separation from 3a was difficult. Deprotona-
tion of the methyl group within 3a with NaOH in water/acetone
with H NMR spectroscopy and sharp melting points confirmed the
identity and purity of the two compounds (Supplementary data).
The two final compounds were soluble in a wide range of solvents,
especially very low polarity solvents (e.g., cyclohexane) presum-
ably because of the appended pyrene unit.
(
1:10) afforded the derivative 4 in excellent yield. Refluxing of 4
with 2-hydroxy-5-nitrobenzaldehyde in THF afforded, after thor-
ough chromatography (silica gel, DCM/MeOH, 100:0 then 98:2),
the desired derivative P1. Synthesis of the corresponding naphtha-
lene analogue required the preparation of compound 6. This mate-
The ratio of SP:MC in a steady-state mixture is highly depen-
dent, along with other factors (i.e., temperature), on the polarity
of the solvent. Inspection of the H NMR spectrum can be rather
revealing, since the J-coupling constants and chemical shifts for the
two adjacent alkene protons are indicative of a cis geometry (SP) or
trans geometry (MC). Thus, H NMR spectra were recorded for P1
3
and P2 in CDCl at room temperature. Considering the only main
difference between the two compounds is the additional benzene
ring, the H NMR spectra for the two are very informative in terms
1
5
1
1
4
rial was obtained by an adaptation of literature procedures
starting from 1-hydroxynaphthalene-2-carboxylic acid. Thus, in
three steps this material was converted to the aldehyde 5 in an
overall 71% yield. Under mild nitration conditions 5 was selectively
converted to 1-methoxy-4-nitronaphthalene-2-carbaldehyde and
finally deprotected to afford 6. The refluxing of 6 with 4 in THF
1
1
afforded after purification (silica gel, CH
2
Cl
2
) the compound P2 in
of ground-state geometries. Illustrated in Figure 1 is the spectrum
for P1 along with the corresponding peak assignments that were
corroborated by COSY. The J-coupling constants for the two signals
associated with the alkene protons (H15, H16) are 9.9 Hz, and
good overall yield (75%). The somewhat high yield of this latter
compound would suggest enhanced reactivity of the aldehyde for
the naphthalene precursor 6. Accurate MALDI-MS and ES-MS along

4
294
A. C. Benniston, J. Fortage / Tetrahedron Letters 49 (2008) 4292–4295
1
0
0
.0
.8
.6
CDCl3
H7 H1,3; H4,10
H + H9
5
H17
0.4
0.2
H15
H20
H16
H13
H19
H14
H11
H12
0.0
300
400
500
600
700
800
8.5
8.0
7.5
7.0
6.5
6.0
Wavelength / nm
ppm
Figure 3. Absorption spectra recorded for P2 in cyclohexane (black), toluene (grey)
and CH Cl (dashed) at room temperature.
Figure 1. Selected 300 MHz 1H NMR spectrum for P1 in CDCl
at rt, and the corr-
3
2
2
esponding peak assignments.
form.16 That the long-wavelength absorption band for P1 is only
just observable from the baseline is in agreement with the pro-
posed SP form dominating the steady-state mixture. The most sig-
nificant finding regarded the alteration in the absorption spectrum
for P2 when recorded in a very low polarity solvent (Fig. 3). Even
though it is recognized that solvent polarity plays an important
consequently consistent with the cis geometry, as shown, for the
closed SP form. There is some evidence of minor resonances on
the baseline (<5%) that almost certainly represent the open MC
form. In view of the NMR data, the limit for the SP:MC ratio is
assigned tentatively to 95:5. By comparison, the H NMR spectrum
3
for P2 in CDCl at room temperature (Supplementary data) con-
1
1
7
role in controlling the MC absorption maximum, the appearance
of a new near-IR band at ca. 700 nm in cyclohexane is unprece-
dented. It should be noted that the usual absorption band attrib-
uted to the MC is still visible. The cause of the new band is not
at this stage completely understood, but may arise from excimer/
exciplex formation involving the pyrene and naphthalene units.
Future studies will be required to resolve this issue.
tains no discernable resonances in the region 6–7 ppm, that corres-
pond to alkene protons H15 and H16. Furthermore, a number of
downfield signals are broad, which would indicate a dynamic pro-
cess taking place in solution. This hypothesis was tested by collect-
1
ing H NMR spectra for P2 in CD
2
Cl
2
at low temperatures; this
solvent was chosen to avoid aromatic signals being obscured by
the residual solvent peak. The 500 MHz 1H NMR spectrum at
In previous literature reports it has been shown that illumina-
tion at the MC long wavelength absorption band brings about ring
À50 °C (Fig. 2) contains far more detail and well-resolved peaks,
and their assignments are especially instructive. The proton reso-
nance for H15 is well downfield (9.0 ppm) and the J-coupling con-
stant is 13.5 Hz. The peak at 7.70 ppm is assigned readily to H16
with an identical J-coupling constant. These large J-values are fully
consistent with a trans alkene geometry, as expected for the open
MC form. Once again minor resonances on the baseline can be
assigned to the closed SP form. The SP:MC ratio is assigned
tentatively to 5:95, which is the converse of the case for P1. It
would appear that a relatively subtle structural change can greatly
influence the SP:MC ratio.
1
8
closure. Under steady state conditions the irradiation of P2 at ca.
00 nm in N -purged CH Cl , toluene or cyclohexane solutions
6
2
2
2
afforded no observable changes in the respective absorption spec-
tra. Illumination of a sample of P2 in cyclohexane at ca. 700 nm,
however, led to a gradual decrease in the intensity of this band
along with the band at 600 nm (Supplementary data). No precipi-
tate could be detected in the sample cuvette after prolonged irra-
diation times. Thus, it appears possible to selectively facilitate
the ring closure, MC to SP, by long-wavelength excitation of P2
in cyclohexane. Rather curiously, irradiation of the sample at
wavelengths between 350 and 450 nm did not bring about restora-
tion of the original absorption spectrum.
Two new pyrene-based molecular dyads have been produced,
which incorporate benzoselenazole in the basic structures. The full
extent of the effect of the selenium on controlling the SP:MC ratio
is not at present fully clear, and future work will be needed to
address this issue. Preliminary evidence is encouraging that the
new systems display disparate behaviour in solution and hence
should find applications in future photonic-based systems.
The unmistakable structural difference between P1 and P2 is
evident in their UV–visible spectra recorded in CH Cl (Supple-
2 2
mentary data). The structured absorption bands in the 300–
Ã
4
00 nm region are assigned readily to p–p transitions of the pyr-
ene group. The long wavelength broad absorption band at ca.
6
00 nm is associated with electronic transitions for the open MC
H1,3
H4,10
Acknowledgements
H5,9
H7
H17
We thank the Leverhulme Trust for financial support and the
ESPRC sponsored Mass Spectrometry Service at Swansea for col-
lecting electrospray mass spectra.
H19
H12, H13, H21
H15
H11
H14
H16
H20H22
Supplementary data
Detailed description of the synthesis and characterization for se-
9.0
8.5
8.0
ppm
7.5
7.0
lected intermediates and final products. Proton NMR of P2 in CDCl
3
,
absorption spectra for P1 and P2 in CH Cl and irradiation absorp-
2
2
Figure 2. Partial 500 MHz 1H NMR spectrum for P2 in CD
peak assignments.
tion spectra. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2008.04.134.
2
Cl
2
at À50 °C and the

A. C. Benniston, J. Fortage / Tetrahedron Letters 49 (2008) 4292–4295
4295
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