Photoassisted "gate-lock" fluorescence "turn-on" in a new Schiff base and coordination ability of E-Z isomers

Article abstract of DOI:10.1021/ol2032043

Rapid photoresponse (1.0-8.0 min) through fluorescence "turn-on" signaling displayed by a novel Schiff base (L) creating "gate lock" via intramolecular C-H???N interaction in photoisomerized product (L′) has been described. Coordination chemistry of pre-

Full text of DOI:10.1021/ol2032043

ORGANIC
LETTERS
2012
Vol. 14, No. 2
592–595
Photoassisted “Gate-Lock” Fluorescence
“Turn-on” in a New Schiff Base and
Coordination Ability of EꢀZ Isomers
Rampal Pandey,† Rakesh Kumar Gupta,† Pei-Zhou Li,‡ Qiang Xu,‡ Arvind Misra,† and
Daya Shankar Pandey*,†
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi -221
005 (U.P.), India, and National Institute of Advanced Industrial Science and Technology
(AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
dspbhu@bhu.ac.in
Received December 1, 2011
ABSTRACT
Rapid photoresponse (1.0ꢀ8.0 min) through fluorescence “turn-on” signaling displayed by a novel Schiff base (L) creating “gate lock” via intramolecular
CꢀH N interaction in photoisomerized product (L0) has been described. Coordination chemistry of pre- and postirradiated species demonstrated a
3 3 3
drastic change in the reactivity which has been supported by NMR, HRMS, UVꢀvis, emission, electrochemical, and complexation studies.
Light-regulatable fluorescent compounds that undergo
structural, conformational, and dynamic changes triggered
by light excitation find wide applications as temporal
probes.1 The light-induced structural changes can be
easily evaluated if they are directly related to a fluorescent
readout.2 Photoactivation of the compounds in biological
systems frequently leads to fluorescence “turn-on” by
release of fluorophores with reasonable photostability.3
Considering their electrical and optical properties, some
chemical, biological, gas, and UV-light sensors have been
developed.4 Among these, UV-responsive compounds
are in high demand because of their potential applications
in diverse areas.5a,6 Although “offꢀon” UV-light switches
containing ZnO, CdS quantum dots/graphene, and a few
retinal-linked Schiff bases have been reported, a selective
and sensitive UV-light “offꢀon” switch based on a free
Schiff base has not been described.5,7
Further, metallo-supramolecular entities, metallo-hosts,
and related systems have received substantial interest
owing to their potential application in various fields.8
Creation of such a system is directed by the relative
position of nitrogen atoms in bis-imine Schiff bases.9,10 To
our knowledge, Schiff basesderived fromthe condensation
(1) (a) Patterson, G. H.; Lippincott-Schwartz, J. Science 2002, 297,
1873. (b) Lee, H. M.; Larson, D. R.; Lawrence, D. S. ACS Chem. Biol.
2009, 4, 409. (c) Mayer, G.; Heckel, A. Angew. Chem., Int. Ed. 2006, 45,
€
4900. (d) Grobner, G.; Burnett, I. J.; Glaubitz, C.; Choi, G.; Mason,
A. J.; Watts, A. Nature 2000, 405, 810.
(2) (a) Gurskaya, N. G.; Verkhusha, V. V.; Shcheglov, A. S.;
Staroverov, D. B.; Chepurnykh, T. V.; Fradkov, A. F.; Lukyanov, S.;
Lukyanov, K. A. Nat. Biotechnol. 2006, 24, 461. (b) Masuda, S.;
Hasegawa, K.; Ishii, A.; Ono, T.-A. Biochemistry 2004, 43, 5304.
(3) (a) Theriot, J. A.; Mitchison, T. J. Nature 1991, 352, 126. (b)
Kobayashi, T.; Urano, Y.; Kamiya, M.; Ueno, T.; Kojima, H.; Nagano,
T. J. Am. Chem. Soc. 2007, 129, 6696.
(5) (a) Fang, X.; Bando, Y.; Liao, M.; Gautam, U. K.; Zhi, C.;
Dierre, B.; Liu, B.; Zhai, T.; Sekiguchi, T.; Koide, Y.; Golberg, D. Adv.
Mater. 2009, 21, 2034.
(6) Spudich, J. L.; Yang, C.-S.; Jung, K.-H.; Spudich, E. N. Annu.
Rev. Cell Dev. Biol. 2000, 16, 365.
(4) (a) Wang, Z. L. Adv. Mater. 2003, 15, 432. (b) LaFratta, C. N.;
Walt, D. R. Chem. Rev. 2008, 108, 614.
(7) Nielsen, I. B.; Petersen, M. A.; Lammich, L.; Nielsen, M. B.;
Andersen, L. H. J. Phys. Chem. A 2006, 110, 12592.
r
10.1021/ol2032043
Published on Web 01/09/2012
2012 American Chemical Society

1
In the H NMR spectrum of L, H1 and H4 protons
of phenylene-1,3-diamine with pyridyl-n-carboxaldehydes
(n = 2/3/4, Scheme S1, Supporting Information) have not
beenreported.9,10 The objectiveofdesigninga selective and
sensitive photoresponsive compound can be achieved by
developing a conjugated π-electron system, which can
assume a more stable configuration by acquiring an appro-
priate amount of energy. Based on our earlier results and
those from other groups,10,11 through this contribution we
describe a new Schiff base 2,4,6-trimethyl[N,N0-bis-
(pyridin-2-ylmethylene)]benzene-1,3-diamine (L) which
acts as photoswitch and is blind to visible light. The UV-
triggered fluorescence enhancement via structural changes
and coordination chemistry of both photoisomers has not
been described to the best of our knowledge.
Figure 1 summarizes the synthetic strategy for prepara-
tion of L and its response toward UV-light and metal
ions. Schiff base L was synthesized by condensation of
pyridine-2-carboxaldehyde with 2,4,6-trimethylbenzene-
1,3-diamine (2: 1) in benzene (Scheme S1, Supporting
Information) in reasonably good yield (83%).
resonated as doublets at δ 8.72 and 8.29 ppm (Figure S1a,
Supporting Information) while H5 (ꢀCHdNꢀ) and H6 as
singlets at δ 8.35 and 6.98 ppm. The triplets at δ 7.84 and
7.40 ppm has been assigned to H3 and H2 proton reso-
nances. Methyl protons of the central mesitylene ring
resonated at δ 2.15 (H8/H9) and 1.99 ppm, (H7) suggesting
symmetrical orientation of the molecule. The aromatic pro-
tons (H1-H6) of L0 exhibited small upfield shift (Figure S2a,
Supporting Information). However, the signals due to methyl
protons displayed splitting and appeared at ∼δ 2.18ꢀ
1.98 ppm (downfield shift) indicating some sort of interaction
between methyl protons and pyridyl nitrogen (Figure 1).
Further, 2D COSY and DEPT NMR spectra of L0 exhibited
enhanced HꢀH and CꢀH coupling and the presence of
ꢀCH2 and ꢀCH aliphatic protons (Figure S4 and S5,
Supporting Information). On the basis of NMR spectral
studies we conclude that the intramolecular CꢀH
1
tons. Stacked H NMR spectra also supported UV-light
N
interactions are responsible for the splitting of methyl pro-
3 3 3
induced structural changes (Figure S3, Supporting Informa-
tion). The presence of molecular ion peak at m/z, 329 in the
HRMS of both L and L0 with different fragmentation
patterns strongly supported our scheme (Figure 2b and S6,
Supporting Information).
Figure 1. Structures of L and L0 and approach of metal ions
toward the coordination sites of both isomers.
Details about the synthesis of L/ L0, metallacycles 1ꢀ6,
and their characterization data is given in the Supporting
Information. The characterization of L has been achieved
by satisfactory elemental analyses, FT-IR, NMR [1H, 13C,
Figure 2. (a) Absorption spectra of L (5 μM) and after UV
irradiation. (b) HRMS spectra of L and L0.
UVꢀvis spectrum of L (10 μM, Supporting Infor-
mation) displayed transitions at 351, 271, and 238 nm
assignable to nfπ* and πfπ* transitions (Table S2,
Supporting Information). The absorption spectrum
of L in methanol also exhibited a pattern analogous to
that in H2O/MeCN system (Figure S7a, Supporting In-
formation, inset). After UV irradiation (01 min, 365 nm;
5 μM), the optical density of low energy band at 348 nm
(ε, 2.29 ꢁ 104 Mꢀ1 cmꢀ1) gradually increases while that
1
1
13
2D COSY (1Hꢀ H, Hꢀ C), and DEPT (135°/90°)],
UVꢀvis, fluorescence, and cyclic voltametric studies.
(8) (a) Lehn, J. M. Supramolecular Chemistry: Concepts and Perspec-
tives; VCH: Weinheim, 1995; Chapter 9. (b) Lehn, J. M. Science 2002, 295,
2400. (c) Yam, V. W. W.; Lo, K. K. W. Chem. Soc. Rev. 1999, 28, 323. (d)
Albrecht, M. Chem. Rev. 2001, 101, 3457. (e) Nabeshima, T. Coord.
Chem. Rev. 1996, 148, 151. Fujita, M.; Tominaga, M.; Hori, A.;
Therrien, B. Acc. Chem. Res. 2005, 38, 371.
(9) (a) Nakasuka, N.; Kunimatsu, M.; Matsumura, K.; Tanaka, M.
´
Inorg. Chem. 1985, 24, 10. (b) Pardo, E.; Ruiz-Garcıa, R.; Lloret, F.;
for high energy band (271 nm, ε, 6.20 ꢁ 104 Mꢀ1 cmꢀ1
)
ꢀ
ꢀ
Julve, M.; Cano, J.; Pasan, J.; Ruiz-Perez, C.; Filali, Y.; Chamoreau
Journaux, L.-M. Y. Inorg. Chem. 2007, 46, 4504. (c) Halder, P.;
Zangrando, E.; Paine, T. K. Dalton Trans. 2009, 5386.
decreases (ε, 6.07 ꢁ 104 Mꢀ1 cmꢀ1) (Figure 2a). Notably,
irradiation up to 2.0ꢀ8.0 min leads to a considerable
change in the absorption spectra wherein low energy band
showed hyperchromic shift (ε, 2.51 ꢁ 104 Mꢀ1 cmꢀ1) and
the high energy band a hypsochromic (ε,7.13ꢁ 104 Mꢀ1 cmꢀ1)
with red shift of ∼3ꢀ5 nm. The isosbestic point at 275 nm
(1.0 min exposure) disappeared upon increasing the
(10) (a) Pandey, R.; Kumar, P.; Singh, A. K.; Shahid, M.; Li, P.-Z.;
Singh, S. K.; Xu, Q.; Misra, A.; Pandey, D. S. Inorg. Chem. 2011, 50,
3189. (b) Lavalette, A.; Hamblin, J.; Marsh, A.; Haddleton, D. M.;
Hannon, M. J. Chem. Commun. 2002, 3040. (c) Lavalette, A.; Tuna, F.;
Clarkson, G.; Alcock, N. W.; Hannon, M. J. Chem. Commun. 2003,
2666.
(11) Koner, R. R.; Ray, M. Inorg. Chem. 2008, 47, 9122.
Org. Lett., Vol. 14, No. 2, 2012
593

irradiation time (2.0ꢀ8.0 min) and a new one appeared at
264 nm. Narrow spectral alterations with two consecutive
isosbestic points clearly suggested the presence of more
than one species.12 Closer examination of the absorption
spectra of L and L0 (Figure 2) indicated some structural
changes (i.e., orientation, geometry, etc.) retaining con-
jugation length. This can be rationalized by assuming the
movement of pyridyl ring (E,Z) to form an eight mem-
bered cyclic ring (Z,Z) involving CꢀH N hydrogen
3 3 3
bonding between methyl protons and nitrogen lone
pair.13 The effect of metal ions were investigated under
analogous conditions. Notably, interactions between L0 and
various metal ions led to insignificant changes (Figure S8,
Supporting Information) suggesting Lbecomesinert toward
the tested metal ions after UV exposure.
Figure 3. (a) Fluorescence spectra of L and L0 (c, 10 μM; λexc
350 nm). Inset shows structural and color changes upon irradia-
tion. (b) Fluorescence titrations of L to L0 (c, 10 μM). Inset shows
fluorescence enhancement of Lwith respect to fraction of UV-light/min.
,
The Schiff base L fluoresces weakly at 430 nm (λex,
350 nm; Φfl, 0.032)14 with a Stokes shift of 5316 cmꢀ1 (Figure3
and Table S2, Supporting Information), which may be attrib-
uted to photoinduced electron transfer process.14 To
examine the photostability of L fluorescence spectra of
the same solution was acquired repeatedly with 1 min
time interval which showed gradual increase in the rela-
tive emission intensity (∼91%). The light-induced fluo-
rescence enhancement motivated us to scrutinize the
cause of photoswitching ability. Therefore, fluorescence
titrations were performed using L as a ‘host’ and light as a
‘guest’. After each excitation and proper mixing (Δt,
1.0 min) L showed gradual increase in the fluorescence
intensity (Figure 3b) with insignificant change in the Stokes
shift (53 cmꢀ1). The trend continued and attained satura-
tion after 8.0 min (Figure 5, inset) with an increase in
quantum yield by a factor of ∼11 (Φfl, 0.377). Time versus
intensity plot gave a sigmoid curve indicating two step
structural changes (Figure 3b, inset). Fluorescence “turn-on”
without any significant change in Stokes shift suggested
the possibility of structural changes rather than photo-
cleavage. The interaction of solvent with L under influ-
ence of UV light cannot be ruled out; therefore, fluo-
rescence behavior of L was investigated in MeOH, THF,
DMF, DMSO, 1,4-dioxane, CH2Cl2, and C6H6. Notably,
it exhibited an analogous pattern in the aforesaid solvents
except a small decrease in relative intensity in C6H6
(Figure S7b, Supporting Information).
gain deep insight into wavelength triggered structural
changes, L was irradiated at 254 nm. It showed behavior
similar to that observed at 365 nm (Figure S9b, Support-
ing Information). Additionally, L0 remains unchanged in
dark (298 K, CH2Cl2, 24 h).
Based on our earlier findings,10 the report by Ray et al.,11
and NMR, HRMS, UVꢀvis, and fluorescence spec-
tral data, we propose that light-induced structural chan-
ges in L retain the extent of conjugation and controlled
by intramolecular CꢀH N interactions.13 The resulting
3 3 3
eight-membered ring creates rigidity in the molecule
which controls vibrational motion and in turn, fluores-
cence “turn-on”.15 One can speculate that L0 would be less
reactive or inert toward metal ions. To affirm our postu-
lation, nitrate salts of various metal ions were added to a
solution of L0 (Figure S9a, Supporting Information). It
was observed that the fluorescence spectrum of L0 re-
mains unchanged indicating inertness of L0 toward the
metal ions. In contrast, such type of ligands show good
coordination ability toward transition metals.10
Therefore, L and L0 were treated with various metal
ions in the absence of UV light in methanol at room
temperature. Notably, L reacted readily with most of the
metal nitrates and afforded binuclear metallacycles
[{M(C21H20N4)}2}2] (Xꢀ)n, (1ꢀ6), [Co (C12H8N3O2)2]-
PF6 H2O (2b), and [{Ni(C21H20N4)(H2O)2}2] (NO3)4
3
To understand the reversibility of structural changes in
the presence and absence of UV light, L (MeCN, 3 mL,
100 μM) was irradiated for different durations (10 min to
24 h, 365 nm) and monitored by emission spectral studies.
The spectral features were similar for a sample irradiated
for short (10 min) or long (24 h) time period suggesting
that ∼10 min is sufficient for the conversion of L to L0. To
(3a) (Scheme S2, Supporting Information), characterized
by elemental analyses and spectral (FT-IR, NMR, ab-
sorption, and emission) and electrochemical studies
(Figures S10ꢀS12, Supporting Information). Structures
of 1, 2b, 3a, and 5 have been determined crystallographi-
cally (Figure 4 and Figure S13 and Table S1, Supporting
Information). The crystal structure of 3a exhibited a
looped arrangement with double-stranded 12-membered
metallamacrocycle cation [{Ni(C21H24N4O2)}2]4þ along
with coordinated water molecules. In the cations of 3a
immediate coordination geometry about each Ni(II) is
distorted octahedral with NiN4O2 environment16 and N4O2
€
(12) Wohri, A. B.; Katona, G.; Johansson, L. C.; Fritz, E.;
Malmerberg, E.; Andersson, M.; Vincent, J.; Eklund, M.; Cammarata,
M.; Wulff, M.; Davidsson, J.; Groenhof, G.; Neutze, R. Science 2010,
328, 630.
(13) (a) Lin, C.-W.; Chou, P.-T.; Liao, Y.-H.; Lin, Y.-C.; Chen
C.-T.; Chen, Y.-C.; Lai, C.-H.; Chen, B.-S.; Liu, Y.-H.; Wang, C.-C.;
Ho., M.-L. Chem.;Eur. J. 2010, 16, 3770. (b) Harlow, R. L.; Li, C.;
Sammes, M. P. J. Chem. Soc., Chem. Commun. 1984, 818.
(14) (a) Deda, M. L.; Ghedini, M.; Aiello, I.; Grisolia, A. Chem. Lett.
2004, 33, 1060. (b) Williams, N. J.; Gan, W.; Reibenspies, J. H.;
Hancock, R. D. Inorg. Chem. 2009, 48, 1407.
(15) Grobner, G.; Burnett, I. J.; Glaubitz, C.; Choi, G.; James, A.;
Watts, M. A. Nature 2000, 405, 810.
ꢁ
(16) Palacios, M. A.; Rodrıguez-Dieguez, A.; Sironi, A.; Herrera,
´
J. M.; Mota, A. J.; Cano, J.; Colacio, E. Dalton Trans. 2009, 8538.
Org. Lett., Vol. 14, No. 2, 2012
594

Figure 5. Fluorescence spectra of 1ꢀ6 (c, 10 μM; H2O/MeCN,
7: 3). Inset shows fluorescence enhancements from L to L0.
Figure 4. Crystal structures of 1, 3a, and 5 (30% thermal
probability).
donor sites contains opposite (Δ, Λ) chirality. It represents
the first Ni(II) octahedral complex containing a Schiff
base derived from substituted benzene-1,3-diamine.16
To have a clear correlation between L and correspond-
ing metallacycles, the absorption and fluorescence beha-
viors of 1ꢀ6 were investigated (Figure 5 and Figure S14
and Table S2, Supporting Information). Among these, 1,
2b, 3a/3b are nonfluorescent, while 4ꢀ6 displays strong
fluorescence with significant Stokes shift relative to L/L0 due
to the chelation enhanced fluorescence (CHEF) process.17
On the other hand, the reaction between L0 and metal
nitrates under only visible light do not yield any complex
at room temperature. Further, L0 was extracted from the
reaction mixture (96%) using dichloromethane. Marked
difference in the reactivity of L and L0 supported crea-
tion of a cyclic structure after UV irradiation, wherein
the approach of metal is restricted by intramolecular
Figure 6. Cyclic voltammograms of L (black) and L0 (red) in
H2O/MeCN (7:3) in anodic (a) and cathodic (b) windows.
by comparing optical properties of L, L0 and model
system L1. The UVꢀvis spectra of L1 exhibited changes
due to EꢀZ isomerization, while fluorescence spectra
remains unaltered upon UV-irradiation (Figure S23,
Supporting Information).
CꢀH N interactions “gate-lock” (Figure 1).13
3 3 3
Redox behavior of L, L0 (Figure 6 and Table S2, Support-
ing Information) and metallacycles 1ꢀ6 (Figures S15ꢀS22,
Supporting Information) have been followed by cyclic
voltammetry. The cyclic voltammogram of L displayed one
irreversible wave at þ1.63 V (Epa) in the anodic potential
window. It has been tentatively assigned to oxidation of
π-conjugated Schiff base (HOMO) to mono cationic
(HOMOþ) species.18 The quasi-reversible waves at Epc
ꢀ1.38 and ꢀ1.97 V may be assigned to ligand based
reductions.18 Conversely, L0 displayed an oxidative wave
at þ1.72 V and two quasi-reversible reductions ꢀ1.45,
and ꢀ2.43 V. High positive and negative E1/2 values for L0
are consistent with its stabilization relative to L. The
observation is consistent with the conclusions drawn
from optical studies.
Through this work we have presented a new Schiff base
L exhibiting UV-triggered fluorescence ‘turn-on’ due
to EꢀZ structural changes controlled by intramolec-
ular CꢀH N interactions leading to a gate-lock for
3 3 3
metal ion coordination. Unambiguous support has
been provided by NMR, HRMS, UVꢀvis, emission,
and electrochemical studies and complexation behavior.
Both L and L0 are stable under visible light and exhibit
marked difference in the reactivity toward various
metal ions.
Acknowledgment. Thanks are due tothe Department of
Science and Technology (DST) and the Council of Scien-
tific and IndustrialResearch(CSIR), New Delhi, India, for
providing financial assistance through Schemes SR/S1/IC-
15/2006 and 9/13(288)/2010-EMR-I.
Light induced EꢀZ isomerization in L and the presence
of CꢀH N interactions in L0 has further been supported
3 3 3
Supporting Information Available. Schemes, NMR
spectra, ORTEP view, optical plots, cyclic voltammograms,
and tables. This material is available free of charge via the
Internet at http://pubs.acs.org.
(17) Burdette, S. C.; Lippard, S. J. Coord. Chem. Rev. 2001, 216ꢀ217,
333.
(18) Bu, X. H.; Liu, H.; Du, M.; Wong, K. M. C.; Yam, V. W. W.;
Shionoya, M. Inorg. Chem. 2001, 40, 4143.
Org. Lett., Vol. 14, No. 2, 2012
595