One- and two-photon excited dual fluorescence properties of
zinc(II) and cadmium(II) complexes containing 4-dipropylamino-
benzaldehyde thiosemicarbazone
Zhao-Ming Xue,*a Yu-Peng Tian,*a,b,c Dong Wangb and Min-Hua Jiangb
a Department of Chemistry, Anhui University, Hefei, 230039, China.
E-mail: zmxue@ahu.edu.cn; yptian@ahu.edu.cn; Fax: 86-551-5106000
b State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, China
c State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, 210093, China
Received 1st October 2002, Accepted 10th February 2003
First published as an Advance Article on the web 28th February 2003
A new ligand, 4-dipropylaminobenzaldehyde thiosemicarbazone (HL) and its complexes (ZnL2, CdL2), which
exhibit intense two-photon excited (TPE) dual fluorescence using 800 nm laser pulses in the femtosecond regime,
were synthesized and fully characterized. The measured power dependence of the fluorescence signals provides
direct evidence for TPE. The two-photon excited dual fluorescence spectra were compared and contrasted with
the corresponding results obtained from one-photon excitation. Emission peaks of the ligand and its complexes
were assigned with the aid of PM3 calculations.
another (generally higher-energy) photon at the characteristic
Introduction
wavelength of fluorophore emission. This induced fluorescence
signal displays a squared dependence on the exciting optical
power. The basic equation relating the fluorescence signal FI2
(in the absence of saturation, self-quenching, photobleaching,
or stimulated emission) to the experimental parameters for TPE
is given by eqn. (1):16
Shortly after the experimental verification of two-photon
absorption1 by Kaiser and Garret with a CaF2:Eu2ϩ crystal in
1961, two-photon processes have been used to create a number
of chemical or physical processes including optical data
storage,2 optical waveguiding,3 lithographic fabrication,4 and
fluorescence imaging.5 In fluorescence imaging, two-photon
excitation (TPE) has developed as an important alternative
to traditional one-photon excitation (OPE) in fluorescence
microscopy and spectroscopy.6,7 The intrinsic advantages of
two-photon excitation include reduced background fluor-
escence from fluorophores outside the focal volume, decreased
photobleaching, inherent optical sectioning capability, and
lower photodamage of sensitive biological samples.8 Recently,
selective recognition and in situ sensing of biologically import-
ant molecules using a dual fluorescent intramolecular charge
transfer (ICT) fluorophore are of considerable significance in
host–guest chemistry.9–11
2
FI2 = k(Φ2/2)n2σ2lρ2
(1)
where Φ2 is the fluorescence quantum yield of the molecule,
n2 is the fluorophore number density, σ2 is the two-photon
absorption cross-section (cm4 s), l is the path length, ρ2 is the
incident photon flux density in photons cmϪ2 Ϫ1), and k is a
s
dimensionless constant that depends on the optical setup. The
factor 2 in the denominator reflects the fact that two photons
are required for each absorption event. Since TPE is essentially
an instantaneous process, the peak incident photon flux density
of a pulsed laser source ρpeak determines the excitation rate.
This area, perceived until recently to be of only academic
interest, is now offering numerous opportunities both for
fundamental research and for new application development.
However, most of the reported materials which exhibit a
strong TPE fluorescence or OPE dual fluorescence are
organic chromophores.12–15 To our knowledge, metal complexes
with strong two-photon absorption are less studied. The
organic chromophore, 4-dipropylaminobenzaldehyde thio-
semicarbazone, which is a π-electron delocalized system
containing mixed sulfur and nitrogen donors, was considered
and prepared in our laboratory for two-photon exciting dual
fluorescence. Following the success of our initial work, we were
particularly interested in further using it as a ligand to coord-
inate to transition metal ions to form metal complexes that
show efficient TPE dual fluorescence. We selected Zn2ϩ and
Cd2ϩ as the metal ions, which have d10 structures, to form metal
complexes with the ligand. In this paper, we present the
two-photon properties of the ligand and its complexes that
exhibit intense two-photon excited dual fluorescence.
2
Dual fluorescence
4-(N,N-Dimethylamino)benzonitrile (DMABN) is the proto-
type of a group of organic donor–acceptor compounds that
can undergo intramolecular charge transfer (ICT) in the excited
singlet state. In polar solvents, it exhibits dual fluorescence:
apart from “normal” emission out of the locally-excited (LE)
state, which is also present in the gas phase and in nonpolar
solvents, in polar solvents a second, “anomalous”, strongly
red-shifted band is observed in the spectrum, attributed to a
charge-transfer (CT) state. This phenomenon has been mostly
explained by the twisted ICT (TICT) mechanism. In the
“twisted intramolecular charge transfer” (TICT) model, a 90Њ
twist of the dimethylamino group with respect to the phenyl
ring was considered to have occurred in the CT state of
DMABN. The dimethylamino group was thereby assumed to
be electronically decoupled from the rest of the molecule: the
“principle of minimum overlap”. The CT/LE fluorescence
quantum yield ratio ΦЈ(CT)/Φ(LE) increases with the efficiency
of the ICT reaction.
Theory
Experimental
1
Two-photon excited fluorescence
1
Chemicals
TPE of a fluorophore involves the absorption of two photons
in the same quantum event generating an electronically excited
state, followed by the subsequent spontaneous emission of
The new ligand (HL), and its complexes (ZnL2 and CdL2) were
synthesized (Scheme 1) and fully characterized by elemental
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 1 3 7 3 – 1 3 7 8
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