Alcohol- and acid-causing reversible switching of near-infrared absorption and luminescence in a donor-acceptor conjugated system

Article abstract of DOI:10.1039/b823248b

2-Substituted 7-oxo-7,11b-dihydrodibenzochromenilium derivatives (1-ArPyl⁺s) undergo reversible changes in near-infrared absorption when Ar = ferrocenyl, and complete on-off switching of luminescence when Ar = p-tolyl, m-tolyl, or phenyl, by th

Full text of DOI:10.1039/b823248b

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Alcohol- and acid-causing reversible switching of near-infrared absorption
and luminescence in a donor–acceptor conjugated systemw
Mio Kondo, Maai Uchikawa, Shoko Kume and Hiroshi Nishihara*
Received (in Cambridge, UK) 24th December 2008, Accepted 3rd February 2009
First published as an Advance Article on the web 25th February 2009
DOI: 10.1039/b823248b
2-Substituted 7-oxo-7,11b-dihydrodibenzochromenilium deriva-
tives (1-ArPyl+s) undergo reversible changes in near-infrared
absorption when Ar
= ferrocenyl, and complete on–off
switching of luminescence when Ar = p-tolyl, m-tolyl, or phenyl,
by the reaction with methanol or methoxide ion and with acid.
New molecules that reversibly change their chemical and/or
physical properties in response to external stimuli have
received significant attention because of their potential
versatility in applications related to molecular memory and
switches.1–5 Donor–acceptor (D–A) conjugated molecules are
candidates for these purposes; determining the mechanism of
switching in the D–A interaction is the key issue in achieving
bistability which implies that a molecule can be resting in two
states. We previously studied a series of donor–acceptor
complexes, ferrocenylethynylanthraquinones (FcAqs),6,7 and
recently reported novel protonation-induced cyclocondensation
reactions of 1-arylethynylanthraquinones (1-ArAqs), which
produce tetracyclic compounds containing a pyrylium ring
(1-ArPyl+), expanding the p-conjugated system and displaying
a strong red color.8 This change occurs when the lowest
unoccupied molecular orbital (LUMO) level is lowered in
the protonated species. When the aryl group was the strong
donor ferrocene (1-FcAq), an intervalence charge transfer
(IVCT) band appeared and a temperature-dependent change
in the valence state was observed with the valence tauto-
merization (VT) that resulted when the LUMO level was
lowered by the expansion of the p-conjugation system of the
acceptor moiety, which led in turn to a strong donor–acceptor
interaction (Fig. 1). These compounds constitute a new class
of p-conjugated molecules, the properties of which can be
controlled by the degree of donor–acceptor interaction.
Fig. 1 Protonation-induced cyclocondensation of 1-FcAq to produce
1-FcPyl+ and the valence tautomerization of 1-FcPyl+. (a) Changes in
the chemical structure. (b) Energy diagram for 1-FcAq and 1-FcPyl+
.
(c) Cyclic voltammograms of 1-FcAq (dotted line) and
1-FcPyl+TFSIÀ (solid line) in 0.1 M Bu4NClO4–CH2Cl2 at 0.1 V sÀ1
.
molecular switching system. We found that the p-conjugated
structure of 1-ArPyl+ is abolished by the addition of methanol
and is regenerated by the addition of acid. This reversible
structural change causes complete on–off switching in the
appearance of the IVCT band of the ferrocenyl derivative
and in the luminescent properties of the tolyl and phenyl
derivatives. These reversible structural changes have not
previously been observed in monocyclic pyrylium compounds.
A series of 1-ArPyl+TFSIÀ (Ar = p-Tol, m-Tol, Ph, or Fc)
was synthesized by the cyclocondensation reaction of 1-ArAqs
with 1.5 eq. of bis(trifluoromethanesulfone)imide (TFSIH), as
reported previously.8 In aprotic solvents, the 1-ArPyl+ ions
exhibited a p–p* band around 500 nm, indicating the presence
of a large p-conjugated system, whereas this band was
completely lost in protic organic solvents, such as methanol,
ethanol, and isopropanol (see Fig. S1, ESIw). Pyrylium
compounds show ring opening/closing reactions in response
to base–acid stimuli,9–11 in which the ring opening of the
pyrylium moiety completely destroys the aromaticity of the
ring. Therefore, it is expected that 1-ArPyl+ undergoes a
neutralization reaction with alcohol, leading to the loss of
the aromaticity of the pyrylium ring. To confirm this assump-
tion, 1-p-TolPyl+ was reacted with a base, NaOMe. The
reaction was performed by the addition of excess NaOMe to
a dichloromethane solution of 1-p-TolPyl+. The UV–Vis
absorption spectrum of the isolated product agreed completely
with that of 1-p-TolPyl+ in methanol (see Fig. S2, ESIw). In an
1H nuclear magnetic resonance (NMR) spectrum of the
isolated compound in MeCN-d3 (Fig. 2 and S3, ESIw), a
new peak was observed at 3.00 ppm, which was assignable
In this study, we aimed to control the p-conjugation
structure (which is closely associated with the LUMO level
of 1-ArPyl+) via chemical stimuli to produce a remarkable
Department of Chemistry, School of Science, The University of
Tokyo, 7-3-1, Hongo, Bunkyo-Ku, Tokyo, 113-0033, Japan.
E-mail: nisihara@chem.s.u-tokyo.ac.jp; Fax: +81 (0)3 5831 8063
w Electronic supplementary information (ESI) available: Details in
determination of the structure of 1-p-TolPyl-OMe, experimental and
calculated excitation energies to the lowest excited state for 1-p-
TolPyl-OMe (Table S1), UV-Vis spectra of 1-p-TolPyl in different
solvents (Fig. S1), UV-Vis spectra of 1-p-TolPyl in methanol and the
isolated compound in reaction of 1-p-TolPyl and NaOMe, a 1H NMR
spectrum of 1-p-TolPyl-OMe (Fig. S3), the optimized molecular
structure of 1-p-TolPyl-OMe (Fig. S4), molecular orbitals of 1-p-
TolPyl-OMe (Fig. S5), UV-Vis-NIR spectra of 1-ArPyl+ (Fig. S6),
and the changes of absorbance at 536 nm of a 1-p-TolPyl+ solution
upon the alternate addition of TBACl and TFSIH (Fig. S7). See DOI:
10.1039/b823248b
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extended in the tricyclic moiety. Structure 2 indicates that the
reaction of the 1-ArPyl+s with base generates a base-adducted
species instead of a ring-opened compound, unlike the mono-
cyclic pyrylium salts. This is because the enol species is largely
stabilized by the stilbene-like structure in 1-ArPyl-OMe, which
prevents the ring-opening reaction.
We next studied the reversibility of the changes in physical
properties caused by the alternation of the p-conjugated
system between 1-ArPyl+ and 1-ArPyl-OMe. As shown in
Fig. 3(a), the pp* band of the pyrylium ring disappeared
with the stepwise addition of methanol to dichloromethane
solutions of 1-ArPyls. All spectral changes exhibited isosbestic
points indicating the absence of side reactions, producing
solely 1-ArPyl-OMe. The further addition of TFSIH to the
solution recovered the appearance of the pp* band at 500 nm,
with again the isosbestic points shown in Fig. 3(b), indicating
the completely reversible change in chemical structure between
1-ArPyl+ and 1-ArPyl-OMe.
It should be noted that the 1-ArPyl+s, except 1-FcPyl+,
exhibit emission bands at around 600 nm (lem = 650 for Ar =
p-Tol, 620 for m-Tol, and 600 nm for Ph) (Fig. 3(c)). The band
shapes are mirror images of the p–p* absorption bands
(see Fig. S6, ESIw) and the Stokes’ shifts are about 100 nm,
indicating that the emission is assignable to fluorescence from
the S1 excitation state. The fluorescence quantum yields of the
1-ArPyl+s were determined to be 0.09 (p-Tol), 0.20 (m-Tol),
and 0.19 (Ph) in dichloromethane at room temperature.12
Fig. 2 1H NMR spectra of 1-p-TolPyl-OMe in acetonitrile-d3.
(a) Chemical structures of 1 and 2. HMQC (b) and HMBC (c) spectra
and their enlarged spectra for (d) and (e), respectively.
to a methoxy group. The molar ratio of the methoxy group to
1-p-TolPyl+ was 1 : 1. An upfield shift of Ha of the pyrylium
ring was also observed, supporting the disappearance of the
p-conjugated system of the pyrylium moiety. With these
results, we confirmed the progression of the neutralization
reaction, which formed only one product. As shown in Fig. 2,
the 2 and 6 positions of the pyrylium ring underwent nucleo-
philic attack by the base and the ring-opening reaction
proceeded.2 In the case of 1-p-TolPyl+, which has an
asymmetric pyrylium ring, a methoxide ion can attack both
Ca and Cb; consequently, two structures of the product are
possible, 1 and 2, as shown in Fig. 2. With heteronuclear
multiple-bond correlation (HMBC) measurements, a cross
peak between Ha and Ca was observed, and the chemical
structure of the isolated compound was thus determined to
be 2 in Fig. 2 (for details, see ESIw). Thus, 1-RPyl+ completely
lost the aromaticity of the pyrylium ring with the addition of
the methoxide ion.
The contraction of the p-conjugated system was also
supported by time-dependent density functional theory
(TD-DFT) calculations using B3LYP/6-31G+(p,d) basis sets
for all atoms (see the geometry of 1-p-TolPyl-OMe in Fig. S4,
ESIw). 1-p-TolPyl-OMe showed a weak band at around
385 nm, assignable to the transition from the highest occupied
molecular orbital (HOMO), consisting of the p-orbital
extended in the p-tolyl moiety and the adjacent six-membered
ring (see Fig. S5 and Table S1, ESIw), to LUMO, consisting of
the p* orbital extended in the tricyclic moiety, and a strong
band around 325 nm, which was assigned to the transition
from HOMO to LUMO + 1, consisting of the p* orbital
Fig. 3 Alcohol and acid responses of 1-ArPyl+. (a) UV-Vis–NIR
spectral changes in 1-p-TolPyl+ with the addition of 0 (solid line) to
100 mL of methanol (dashed line). (b) UV-Vis–NIR spectral changes in
1-p-TolPyl-OMe with the addition of 0 (solid line) to 1 eq. of TFSIH
(dashed line). (c) Excitation (i) and emission (ii) spectra of 1-p-TolPyl+
(solid lines), 1-m-TolPyl+ (dotted lines), and 1-PhPyl+ (dashed lines)
in dichloromethane at room temperature. (d) Changes in the emission
spectra of 1-p-TolPyl+ upon the addition of 0 (solid line) to 100 mL of
methanol (dashed line). (e) Photographs of 1-PhPyl+ in dichloro-
methane with 360 nm light irradiation with the alternate addition of
excess NaOMe and TFSIH.
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solution of 1-p-TolPyl+ quantitatively formed a chloro-
attached product, 1-p-TolPyl-Cl. This product was recon-
verted to 1-p-TolPyl+ by the addition of a stoichiometric
amount of TFSIH (1.0 eq.) (see Fig. S7, ESIw).
In conclusion, we have shown that 1-ArPyl+ can be rever-
sibly converted to 1-ArPyl-OMe and 1-ArPyl-Cl using bases
and acids as external stimuli. Furthermore, the conversion
causes a change in the p-conjugated system of the pyrylium
ring, leading to a switching of the fluorescence properties of
1-p-TolPyl+, 1-m-TolPyl+, and 1-PhPyl+, and IVCT absorp-
tion resulting from the change in the donor–acceptor inter-
action of 1-FcPyl+.
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 18655021 and 20245013) from MEXT, Japan,
and by a Research Fellowship from the Japan Society for the
Promotion of Science for Young Scientists.
Fig. 4 Alcohol and acid responses of 1-FcPyl+. (a) Switching of the
D–A interaction between 1-FcPyl+ and 1-FcPyl-OMe. (b) UV-Vis–
NIR spectral changes in 1-FcPyl+ upon the addition of 0 (solid line) to
80 mL methanol (dashed line). (c) UV-Vis–NIR spectral changes in
1-FcPyl-OMe upon the addition of 0 (solid line) to 1 eq. of TFSIH
(dashed line).
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As expected, these emissions of 1-ArPyl+s in dichloromethane
disappeared with the loss of p-conjugation with the stepwise
addition of methanol, as shown in Fig. 3(d). No emission
occurred in the solution with a sufficient amount of methanol.
Repetitive additions of NaOMe and TFSIH caused the
reversible and complete quenching and the recovery of the
fluorescence of 1-PhPyl+, as shown in Fig. 3(e).
No emission was observed for 1-FcPyl+ because the ferro-
cene moiety acts as a strong emission quencher.13,14 However,
the absorption band around 1000 nm, assigned to the IVCT
transition, responded to the addition of NaOMe and acid,
causing a reversible change in the chemical structure between
1-FcPyl+ and 1-FcPyl-OMe (Fig. 4a). Fig. 4b shows the
changes in the UV-Vis–near-infrared (NIR) spectra with the
stepwise addition of methanol to a solution of FcPyl+TFSIÀ
in dichloromethane, and Fig. 4c shows those spectra on the
further addition of TFSI. Both figures give the isosbestic
points and Fig. 4a shows the complete disappearance of the
NIR band, indicating the complete change between FcPyl+
and 1-FcPyl-OMe.
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 1993–1995 | 1995