A dual-mode molecular switch based on a chiral binaphthol-coumarin compound

Article abstract of DOI:10.1016/S0040-4039(00)00576-1

A dual-mode, efficient molecular switching system based on photomodulation of optical rotational power and fluorescence emission of a chiral binaphthol-coumarin compound is presented. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00576-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 4025–4028
A dual-mode molecular switch based on a chiral
binaphthol-coumarin compound
Maria M. Birau and Zhi Yuan Wang ∗
Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, K1S 5B6 Canada
Received 23 March 2000; revised 5 April 2000; accepted 6 April 2000
Abstract
A dual-mode, efficient molecular switching system based on photomodulation of optical rotational power and
fluorescence emission of a chiral binaphthol-coumarin compound is presented. © 2000 Elsevier Science Ltd. All
rights reserved.
Keywords: binaphthol; coumarin; chiral; fluorescence; molecular switch.
Molecular switches are based on bistability of a molecule and the ability of reversible interconvertion
between two isomeric forms by external stimuli. Among many requirements the two isomers should
be at least switchable and detectable by non-destructive means.1 Between a single-mode and a dual-
mode molecular switch, the latter offers more advantages such as ‘writing’ by light at one wavelength
and ‘reading’ by light at another wavelength that does not destroy the bistability. Lehn and co-workers
reported a dual-mode molecular switch based on photochromic and electrochromic properties that are
regulated and integrated in a single structure.2 Another interesting dual-mode switching system reported
by Feringa and Huck employs chiral helical alkenes in which chirality and fluorescence can be reversibly
modulated by light.3
We report here a dual-mode switching system that is based on the photomodulation of fluorescence
and optical activity of a chiral binaphthol-coumarin compound (2). The system is designed to have
two functional moieties, chiral binaphthol and photoactive coumarin, incorporated in a single molecule
which can be switched in two modes based on the changes in the fluorescence intensity and optical
rotation. Photodimerization of coumarin and its derivatives has been intensively studied in solution,4
self-assembled monolayer5 and polymeric film,6 as well as a pendant group in a polymer.7 It is well
known that upon irradiation at 350 nm in solution coumarin forms [2+2] dimers. These photodimers can
be cleaved upon irradiation at wavelengths shorter than 300 nm. Another important feature associated
with the photodimerization of coumarin is the change in its emission intensity.8 Coumarin fluoresces
upon excitation at 320 nm while its dimers do not.5,8 If the chiroptical property and fluorescence of
∗
Corresponding author. E-mail: wangw@ccs.carleton.ca (Z. Yuan Wang)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00576-1
tetl 16855

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the newly designed system 2 are distinctly different at the two photostationary states (open and closed
forms), it could in principle be used as a dual-mode molecular switch.
Compound 2 was synthesized from 7-hydroxycoumarin in two steps (Scheme 1). The acid 19 was
first derived from 7-hydroxycoumarin and subsequently reacted with (R,S)-1,10-bi-2-naphthol to afford
the corresponding racemic 2 as a white solid (23.5%, mp 183–186°C).10 When (S)-1,10-bi-2-naphthol
was used in the reaction, the optically active 2 was obtained ([α]20D=−131, c=0.01, chloroform). The
photochemical studies of both racemic and enantiomeric 2 and the corresponding cyclic dimers 3 were
carried out in the chloroform solution (1.6×10−5 M) in a quartz cell and in the solid state (in KBr matrix).
Using a mercury lamp (300 W, 350 nm), the photocyclization in solution was complete in about 10 min
and the cyclic dimers 3 (as two regioisomers) were formed in quantitative yield as determined by HPLC
(Scheme 1). The complete photocleavage occurred in 2 min at 250 nm. This reversible switching process
could be repeated in many cycles similar to other reported coumarin systems (e.g. 100 on/off cycles)6
and easily monitored by UV–vis spectroscopy. Upon irradiation with 350 nm UV light the absorption
peak at 297 nm (ε=130 000 M−1 cm−1) for the lactone double bond in 2 decreased significantly while
the same peak went back to the initial intensity after irradiation at 250 nm (Fig. 1). The coumarin dimers
3 did not go back to the starting compound 2 for 4 days in chloroform solution at ambient temperatures.
In the solid state (2 mg of 2 in 200 mg of KBr as a pellet) the photodimers were also formed readily (10
min) in quantitative yield. Interestingly, the racemic 3 formed in KBr matrix did not go back to the initial
compound 2 after UV irradiation at 254 nm for 5 h, irradiation with white light for 1 h, or when kept in
a dark environment at 22°C for weeks. Thus, once the compound 3 is formed in the solid state, it could
potentially be detected or read many times, similar to a WORM memory system.
Scheme 1. Synthesis and photochemistry of (R,S)- and (S)-2. (i) K2CO3, acetone, 18-crown-6, reflux, 48 h; (ii) DMAP, DCC,
(R,S) or (S)-1,10-bi-naphthol, CH2Cl2, 36 h, 22°C
It was observed that before photocyclization compound 2 showed little fluorescence, due to the
quenching between the binaphthol and coumarin moieties. After photocyclization, its fluorescence
increased significantly (Fig. 1). This increase in fluorescence is solely contributed by the binaphthol
moiety, thus modulation of fluorescence emissions between the two states (2 and 3) can be achieved by
selective photoirradiation.
Optical rotations of optically active 2 in chloroform (10 mg in 1 mL) before and after photoirradiation
also changed noticeably. Typically, the specific optical rotation of 3 is half of that of enantiomeric 2 at a
given wavelength (Table 1). It is known that the more planar (or conjugated) a chiral binaphthyl molecule
is, the higher its optical rotation will be.11 Molecular modeling (HyperChem, version 4.0) showed that

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Fig. 1. Fluorescence spectra and UV–vis spectra (inset) in chloroform (1.6×10−5 M) of: (a) racemic 2; and (b) racemic 3
the cyclic dimer is more twisted and hence less conjugated than the starting compound (open form),
which supports our result that after photodimerization the optical rotation is decreased.
Table 1
Specific optical rotations for (S)-2 and corresponding cyclic 3a
In conclusion, compound 2 acts as a molecular switch that can be addressed by UV light and has
two stable distinct states detectable in two modes by light (fluorescence and optical rotation) at the
wavelengths that will not destabilize either of the two isomeric molecules.
This work was supported by the Natural Sciences and Engineering Research Council of Canada.
References
1. Feringa, B. L.; Jager, W. F.; de Lange, B. Tetrahedron 1993, 49, 8267.
2. Kawai, S. H.; Gilat, S. L.; Lehn, J.-M. Chem. Commun. 1994, 1011.
3. Huck, N. P. M.; Feringa, B. L. Chem. Commun. 1995, 1095.
4. Leenders, L. H.; Schouteden, E.; De Schryver, F. C. J. Org. Chem. 1973, 38, 957.
5. Li, W.; Lynch, V.; Thompson, H.; Fox, M. A. J. Am. Chem. Soc. 1997, 119, 7211.
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9. A suspension of 7-hydroxycoumarin (3.000 g, 0.018 mol), chloroacetic acid (1.700 g, 0.018 mol), anhydrous K2CO3 (7.460
g, 0.054 mol), 18-crown-6 (1.200 g) in 100 mL of dry acetone was heated to reflux for 3 days. The suspension was filtered
and the yellowish precipitate was dissolved in water. The resulting solution was extracted with ethyl acetate (4×25 mL) and
the organic layer was washed with saturated NaHCO3 solution (4×20 mL). The washings were acidified with concentrated
HCl solution until it was slightly acidic. The resulting white precipitate was filtered and dried at 100°C in an oven for 10
1
h to afford the pure product 1: 2.49 g (61% yield); mp 223–226°C; H NMR (400 MHz, DMSO-d6): δ 7.98 (d, J=9.52
Hz, 1H), 7.62 (d, J=9.28 Hz, 1H), 6.94 (dd, J=7.4 Hz, 2.2 Hz, 2H), 6.29 (d, J=9.48 Hz, 1H), 4.81 (d, 2H); 13C NMR (100

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MHz, DMSO-d6): δ 169.7, 160.9, 160.2, 155.2, 144.3, 129.5, 112.8, 112.7, 101.5, 64.8, 54.9; HRMS calcd for C11H8O5:
220.0376; found: 220.0376.
10. 1,10-Bi-2-naphthol (1.430 g, 0.005 mol), 7-carboxymethoxycoumarin (1) (2.500 g 0.011 mol) and 3 spatula tips of 4-
N,N-dimethylaminopyridine (DMAP) were dissolved into 50 mL of anhydrous dichloromethane. After the addition of
N,N-dicyclohexylcarbodiimide (DCC) (4.726 g, 0.023 mol), the solution was stirred at room temperature for 36 h. The
white suspension was filtered and the organic phase was washed first with 150 mL of 1 M HCl solution, then with 150 mL
of 1 M NaOH solution and finally with distilled water (5×20 mL). The organic solution was dried over MgSO4 and the
solvent was removed in vacuo to afford 1.5 g of the crude product. After further washing with acetone, the pure product
2 was obtained: 0.811 g (23.5% yield); mp 183–186°C; 1H NMR (400 MHz, CDCl3): δ 7.94 (d, J=5.84 Hz, 1H), 7.84 (d,
J=8.16 Hz, 1H), 7.54 (d, J=10.4 Hz, 1H), 7.42 (d, J=8.96 Hz, 1H), 7.38 (d, J=6.96, 1H), 7.21–7.17 (m, 1H), 7.10 (d, J=11.2
Hz, 1H), 7.04 (d, J=8.48 Hz, 1H), 6.45 (dd, J=8.6 Hz, 2.48 Hz, 1H), 6.37 (d, J=2.48 Hz, 1H), 6.2 (d, J=9.48 Hz, 1H),
4.4–4.5 (m, 2H); 13C NMR (100 MHz, CDCl3): δ 166.7, 160.9, 160.4, 155.3, 145.8, 143.2, 133.1, 131.7, 130.3, 128.8,
128.1, 127.2, 126.2, 125.9, 122.9, 121.1, 113.7, 113.3, 113.2, 101.5, 64.7; MS (FAB, m/e, relative intensity) 691 (M+H+,
38%).
11. Gao, J. P.; Meng, X. S.; Bender, T. P.; MacKinnon, S.; Grand, V.; Wang, Z. Y. Chem. Commun. 1999, 1281.