N-Dansyl-carbazoloquinone; a chemical and electrochemical fluorescent switch

Article abstract of DOI:10.1016/j.tetlet.2006.05.139

A new 'push-pull' molecule having an efficient fluorophore (dansyl) in electronic communication with an active redox quencher (phenyl-carbazoloquinone) through an NH-bridge was designed and synthesized. This all-organic molecule is suggested as a highly r

Full text of DOI:10.1016/j.tetlet.2006.05.139

Tetrahedron Letters 47 (2006) 5543–5546
N-Dansyl-carbazoloquinone; a chemical and
electrochemical fluorescent switch
R. A. Illos,a D. Shamir,a L. J. W. Shimon,b I. Zilbermannc and S. Bittnera,*
aDepartment of Chemistry, Ben-Gurion University of the Negev, PO Box 653, Beer Sheva 84105, Israel
bX-ray Crystallography Laboratory, Department of Chemical Services, Weizmann Institute of Science, 76100 Rehovot, Israel
cChemistry Department, Nuclear Research Centre Negev, Beer Sheva, Israel
Received 7 February 2006; revised 14 May 2006; accepted 23 May 2006
Available online 15 June 2006
Abstract—A new ‘push–pull’ molecule having an efficient fluorophore (dansyl) in electronic communication with an active redox
quencher (phenyl-carbazoloquinone) through an NH-bridge was designed and synthesized. This all-organic molecule is suggested
as a highly reversible ‘on/off’ molecular switching system. Chemical and electrochemical inter-conversion between the quinone
acceptor and the dansyl donor were demonstrated via UV–vis, cyclic voltammetry and fluorescence measurements.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Chemical transducers, or molecular switching systems,
find wide interest in science and technology due to their
ability to undergo structural changes in response to a
respective stimulation. Fluorescent switching device sys-
tems find use in probes for the determination of local
environmental redox properties and as bio-sensor ele-
ments to study electron and energy transfer mecha-
nisms.1,2 The common molecular switching system has
two transformable state systems, resulting in four differ-
ent integrated configurations or detectable features.3
Quinones covalently linked to different chromophores
and fluorophores represent donor–acceptor systems
with reversible redox functions and potential ‘switching’
properties. In such quinone optical molecular switches,
the interconversion of the four distinct states is caused
by multiplication of two electrochromic redox states,
namely quinone and hydroquinone.4 This quinone/
hydroquinone redox couple can interconvert reversibly
by exchanging two protons and two electrons, thus it
can serve as the ‘antenna’ or ‘control’ subunit, which af-
fects absorption or emission optical properties. Covalent
linking of quinones to strong fluorophores results in
efficient quenching of the donor emissive state via intra-
molecular electron transfer from the excited fluorophore
to the adjacent quinone acceptor, or by transfer of exci-
tation energy to a low-lying non-emissive charge-trans-
fer state.
Known molecular fluorescent switching systems, operat-
ing via a redox couple, consist of a metal-centered redox
couple (M(n+1)+/Mn+ 5,6 or a luminescent ion core7 (e.g.,
)
[RuII(bpy)3]2+) encircled by a macrocyclic receptor8
(e.g., an azacyclam metal complex). In this last system,
the azacyclam metal complex represents the redox cen-
ter, connected to a dansyl fluorophore.8 The mode of
linking (spacer) between the fluorophore and the redox
sub-unit varies. It can be a direct link (e.g., via a tertiary
nitrogen or a methylenic group in the case of the aza-
cyclam ring),9 it can be an NH group bridging between
a quinone and a stilbenic fluorophore,10 or it can be a
non-conjugated spacer (e.g., ethylenic group) connecting
a luminescent ion core to a quinone.7
In a previous paper, we described the synthesis and both
the chemo and electrophotoswitching capabilities of
a novel three-component ‘all-organic’ redox molecular
switch.11 This novel switch was composed of 2-chloro-
naphthoquinone covalently bridged to 5-dimethylamino-
naphthalene (dansyl) via a non-conjugated piperazine
spacer. The aim of the present study is to investigate an-
other member of the same motif but with a rigid structure
and a conjugated spacer and to learn how these structural
changes will influence its switching photochemical
properties.
The new molecular switch consists of a rigid and pla-
nar carbazoloquinone moiety, covalently bridged to
5-dimethylaminonaphthalene (the fluorophore) via a
*
Corresponding author. Tel.: +972 8 6461195; fax: +972 8 6472943;
e-mail: bittner@bgumail.bgu.ac.il
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.05.139

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R. A. Illos et al. / Tetrahedron Letters 47 (2006) 5543–5546
simple NH spacer. This molecular switch (1) was pre-
pared in five steps (Scheme 1). In a Michael-like addi-
tion reaction, p-phenylenediamine (16 mM) was reacted
with 1,4-naphthoquinone (8 mM) to yield 2-(4-amino-
phenylamino)-1,4-naphthoquinone. Reaction with ex-
cess acetic anhydride under the catalysis of sulfuric
acid12 gave the acylated product, N-4-(3,8-dioxonaph-
thalene-1,4-dienylamino)-acetamide. Cyclization with
Pd(OAc)2 in acetic acid13,14 afforded the condensed
system, N-2-(6,11-dioxonaphthalene-5H-benzo[b]carb-
azol)-acetamide, which was deacetylated to the 2-
amino-5H-benzo[b]carbazole-6,11-dione. The final step
involved reaction with dansyl chloride in the presence
of DMAP to give the molecular switch, N-2-(6,11-
dioxonaphthalene-5H-benzo[b]carbazole)-5-dimethyl-
aminonaphthalen-sulfonamide 1.
same molecular plane. The torsion angles S1–N1–C1–
C2 and N1–S1–C17–C18 between the p-phenylenedi-
amine sub-system and the 5-dimethylaminonaphthalene
are 130.6ꢁ and 93.4ꢁ, respectively. The naphthoquinone
rings in the stack are parallel and related by an inversion
center. The distance between two such rings, as mea-
sured between corresponding pairs of atoms, is 4.04
˚
(3) A. Intermolecular hydrogen bonds are formed
between the carbazolic NH of one molecule with the
quinone oxygen of a neighboring molecule (2.08 and
2
˚
2.07 A), forming centrosymmetric R2 (10) hydrogen
bond rings (Fig. 1, right). Thus each of the anti-parallel
pairs of molecules in the stack is involved in three such
bonds.
The UV–vis spectrum (Fig. 2, left) of 1 shows four typ-
ical p–p* aromatic and amino-substituted quinonoid
absorptions as expected for such a molecule.17 The
absorption at 400 nm is attributed to intramolecular
charge transfer. Indeed, the reduction of quinone 1 to
its hydroquinone form 1a (3 · 10ꢀ3 M methanolic solu-
tion of NaBH4, reacting with 3.43 · 10ꢀ5 M of 1) is fol-
lowed by a complete disappearance of the absorption at
400 nm. This transformation is visible as the solution
changes from yellow to colorless. A blue shift from
340 nm to a more intense absorbance at 336 nm was also
observed. The process is reversible and unless kept un-
der nitrogen, reoxidation occurs within 15 min at room
temperature. Reversibility between the quinone and
hydroquinone is almost 100% as was detected in the
UV absorptions (Fig. 2, left).
The structure of this new molecular switch was derived
1
13
from its H and C NMR, DEPT and HRMS data.15
In the 13C spectrum of 1, the two typical quinone car-
bonyl resonances were observed at 178.5 and
180.5 ppm, all the aromatics between 112 and 152 ppm
1
and the dimethylamino signal at 45.5 ppm. In the H
spectrum, no quinone protons (usually between 5 and
6 ppm) were observed and all the aromatics appeared
between 7.1 and 8.6 ppm. HRMS gave the m/z at
495.125278 proving thereby the composition and struc-
ture of 1.
The structure of compound 1 was firmly established by
X-ray crystal analysis (Fig. 1).16
As can be seen in the ORTEP diagram, the dansyl and
carbazoloquinone ring systems do not reside on the
The intrinsic fluorescence emission of the dansyl excited
state is totally quenched (OFF) in molecule 1, probably
Scheme 1. Synthesis of molecular switch 1.

R. A. Illos et al. / Tetrahedron Letters 47 (2006) 5543–5546
5545
Figure 1. Crystal structure (left) and stacking mode (right) of the molecular switch 1.
Figure 2. UV–vis absorption of 1 and its reduced form (hydroquinone) 1a in methanol (left). Fluorescence emission of compounds 1 and 1a in
methanol (right). Excitation 336 nm; emission 528 nm; quantum yield 0.018; Stokes shift 10,822 cmꢀ1 [measured using quinine sulfate in 0.1 M
H2SO4 as a standard].
due to collisionless intramolecular electron transfer from
the excited dansyl to the adjacent carbazoloquinone
acceptor. However, instant chemical ‘ON’ fluorescence
occurs upon reduction of the quinone with sodium
borohydride (Fig. 2, right). Fluorescence quenching of
1 is not solvent dependant, it is fully quenched in protic
as well as aprotic polar solvents such as DMSO, etha-
nol, methanol or acetonitrile/water. This result is in con-
trast to the behavior of our previous non-conjugated
switch11 where quenching is incomplete in alcoholic
medium and some emission is still observed even when
the compound is in its quinone form. The emission
quantum yield of 1a is rather modest compared to that
of our former model (0.018 and 0.40, respectively), while
the Stokes shifts are longer (10822 and 8640, respec-
tively). The differences might be due to the mode of con-
jugation18 existing between the excited fluorophore and
the rigid carbazolohydroquinone moiety, which are sep-
arated by only an NH spacer.
Figure 3. Cyclic voltammogram of 1 in 0.1 M TBAPF6 under an Ar
atmosphere, in acetonitrile–water (98:2) performed on a glassy carbon
electrode, with ferrocene–Ag/AgCl as a reference electrode.
(E21=2 ¼ ꢀ0:87 V vs Ag/AgCl) corresponds to the subse-
quent addition of a second electron to the semiquinone
anion radical, producing a hydroquinone dianion
(Q2ꢀ).19 Scan rate studies show a linear correlation of
I (lA) vs (scan rate)1/2 (Fig. 3, inset), indicating diffusion
controlled processes for both electrochemical redox
steps.
The reduction of quinones is a reversible process, which
can be induced both chemically (using reducing agents
like NaBH4) and electrochemically (using applied poten-
tial). Cyclic voltammetric measurements of 1 were per-
formed in acetonitrile/water (98:2) (Fig. 3) and two
one electron reversible processes were observed. The
first reversible reduction wave (E11=2 ¼ ꢀ0:68 V vs Ag/
AgCl) represents the addition of one electron to the
quinone moiety (Q) to form a semiquinone anion radical
(Qꢀꢀ). The second reversible reduction wave
Preparative electrolysis of compound 1 (3.03 · 10ꢀ5 M)
followed by UV–vis measurements was conducted by
applying potentials of ꢀ0.6 and ꢀ1.0 V (vs Ag/AgCl),
respectively (slightly ‘more’ cathodic than the peaks

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R. A. Illos et al. / Tetrahedron Letters 47 (2006) 5543–5546
9. De Santis, G.; Fabbrizzi, L.; Licchelli, M.; Mangano, C.;
Sacchi, D. Inorg. Chem. 1995, 34, 3581–3582.
10. Sutovsky, Y.; Likhtenshtein, G. I.; Bittner, S. Tetrahedron
2003, 59, 2939–2945.
11. Illos, R. A.; Harlev, E.; Bittner, S. Tetrahedron Lett. 2005,
46, 8427–8430.
corresponding to the reduction of quinone and semiqui-
none radical anions, respectively). It was found that the
redox reaction was accompanied by changes in the UV–
vis spectra, demonstrating the electro-interconversion of
1 to 1a as well as in the opposite direction (95–100%)
(Fig. 3).
12. Hoover, J. R. E.; Day, A. R. J. Am. Chem. Soc. 1954, 76,
4148–4152.
In conclusion, a new ‘all-organic’ fluorescent switch was
designed and synthesized. In such a system, the redox
control, the spacer and the fluorophore subunits can
be changed at will to form a variety of new molecular
switching systems. Consequently, the emission and the
absorption properties can be manipulated and con-
trolled. The synthesis of such donor–acceptor dyads
with switchable function exhibiting a ‘logic’ behavior
is of great interest for applications in the field of mole-
cular electronics and molecular recognition. In this
letter, we have shown that changing the nature of the
spacer from a cyclic non-conjugated to a rigid conju-
gated structure induces conformational changes that
have direct repercussion on the electronic absorption
and emission as well as on the switching characteristics.
13. Bittner, S.; Krief, P.; Massil, T. Synthesis 1991, 1, 215–
216.
14. Luo, Y. L.; Chou, T. C.; Cheng, C. C. J. Heterocycl.
Chem. 1996, 33, 113–117.
1
15. 1: mp 211–212 ꢁC. H NMR (DMSO, 500 MHz): d 10.80
(br s, 1H, NH), d 8.42–8.44 (d, 1H, J = 9.0 Hz), 8.38–8.40
(d, 1H, J = 8.5 Hz), 8.18–8.20 (dd, 1H, J = 7.5, 1.1 Hz),
8.02–8.06 (dt, 2H, J = 9.0, 1.3 Hz), 7.87 (d, 1H,
J = 2.1 Hz), 7.75–7.83 (dsxt, 2H, J = 9.0, 8.5, 1.7,
1.3 Hz), 7.61–7.64 (t, 1H, J = 8.5 Hz), 7.53–7.57 (t, 1H,
J = 8.5 Hz), 7.37–7.39 (d, 1H, J = 9.0 Hz), 7.23–7.24 (d,
1H, J = 7.3 Hz), 7.17–7.19 (dd, 1H, J = 9.0, 2.1 Hz), 2.76
(s, 6H). 13C NMR (DMSO, 125 MHz): d 180.5 (C@O),
178.5 (C@O), 153.0, 138.0, 135.5, 135.2, 134.7, 134.6,
133.6, 133.0, 130.5, 130.2, 129.5, 129.4, 128.6, 126.5, 126.4,
124.6, 123.9, 121.2, 119.2, 117.4, 115.7, 115.0, 112.5, 45.5.
DEPT 13C NMR (DMSO, 125 MHz): 134.6, 133.6, 130.4,
130.1, 128.6, 126.5, 126.4, 123.9, 121.2, 119.1, 115.7, 114.9,
112.5, 45.5. HRMS (CI) (m/z): 495.125395 (M+), calcd
mass 495.125278 for C28H21N3O4S.
Acknowledgements
16. C28H21N3O4S, orthorhombic, space group Pca21; a =
˚
˚
˚
We wish to thank Ms. E. Solomon for skillful help, Dr.
Sofia Kolusheva for her help in fluorescence measure-
ments, Ms. A. Levkovitch for NMR measurements
and Ms. S. Whiskerman for help in X-ray analysis.
20.657 (3) A, b = 7.151 (9) A, c = 30.870 (13) A,
V = 4560.1 (6) A , Z = 8, d = 1.444 g · cmꢀ3, T = 120
3
˚
(2) K. Diffractometer scan mode: Nonius Kappa CCD,
monochromatized Mo-Ka radiation, 2436 unique reflec-
tions in the range 2.79 6 2h 6 20.81ꢁ. Full matrix least
squares refinement with 2436 reflections [I > 2r(I)] and
209 variables; R = 0.0584, RW = 0.1101; residual electron
density 0.251 e · Aꢀ3. Crystallographic data (excluding
˚
References and notes
structure factors) for the structure in this Letter have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication number CCDC
292197. Copies of the data can be obtained free of charge,
on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [fax: +44 (0)1223 33603 or e-mail:
deposit@ccdc.cam.ac.uk]. Each request should be accom-
panied by the complete citation of this publication.
17. Electronic absorption data in methanol, for 1: kmax (nm):
218, 270, 340, 400. Molar absorptivity log e: 4.46, 4.32,
3.55, 3.54, respectively, and for 1a: kmax (nm): 216, 256,
336, log e: 4.46, 4.22, 3.74, respectively.
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