Zinc-induced switching of the nonlinear optical properties of a functionalized bis(styryl)benzene

Article abstract of DOI:10.1021/ja049204w

A large variation of the nonlinear transmission properties of a cyclen-based bis(styryl)benzene can be induced by a small change of the linear absorption spectrum upon Zn²⁺ binding. This result has been interpreted in the frame of a sequential

Full text of DOI:10.1021/ja049204w

Published on Web 04/30/2004
Zinc-Induced Switching of the Nonlinear Optical Properties of a
Functionalized Bis(styryl)benzene
Graziano Fabbrini, Enzo Menna, Michele Maggini,* Alessandro Canazza, Gabriele Marcolongo, and
Moreno Meneghetti*
Dipartimento di Scienze Chimiche and ITM-CNR, UniVersita` di PadoVa, Via Marzolo, 1 I-35131 PadoVa, Italy
Received February 13, 2004; E-mail: michele.maggini@unipd.it; moreno.meneghetti@unipd.it
Scheme 1a
Multiphoton absorption of π-conjugated molecules is an interest-
ing nonlinear optical (NLO) property that has triggered an increasing
interest because of its rapidly emerging prospects in the field of
biophotonics and materials science. In this connection, the synthesis
and study of multiphoton absorbing fluorophores with large cross
sections, in particular two-photon (2PA) cross sections, is strongly
pursued in view of applications that include three-dimensional
fluorescence imaging,1 optical power limiting,2 and 3D optical data
storage,3 to name only a few. Several molecular design principles
have been proposed to enhance the nonlinear absorption within a
variety of conjugated molecular structures. Among them, the bis-
(styryl)benzene (BSB) structure serves as an efficient conjugated
bridge (π) for 2PA enhancement4 in which donor (D) and/or
acceptor (A) substituents, in a symmetrical (D-π-D, A-π-A) or
asymmetrical (D-π-A) arrangement, can be readily introduced at
the central and side benzene rings. In the present work, a new A-π-
D, BSB derivative (1, Scheme 1), in which the donor unit is a
cyclen-based receptor for metal cations,5 was prepared and studied
with the long-term objective to develop new zinc-specific probes
operative within cells.6 We have found that, upon complexation
a
Conditions: (a) 4-cyanobenzyl-diethylphosphonate, t-BuOK, DMF,
with Zn2+, the electronic properties of the cyclen ligand can be
25 °C, 12 h, 60%. (b) PBr3, THF, 0-25 °C, 2 h, 68%. (c) Triethyl phosphite,
160 °C, 6 h, 82%. (d) t-BuOK, DMF, 25 °C, 12 h, 69%.
modified, thus allowing the control of the multiphoton absorption
characteristics of the BSB derivative. Previously, the modulation
of molecular NLO properties was reported for electrochemical7 and
proton-transfer8 reactions.
The BSB 1 was synthesized by a four-step procedure starting
from 4-hydroxymethyl-2,5-dimethoxybenzaldehyde 2, which in turn
was prepared from 1,4-dihydroxymethyl-2,5-dimethoxybenzene in
90% yield by selective oxidation of one hydroxyl function with
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).9 A Wadsworth-
Emmons coupling of 2 with 4-cyanobenzyl-diethylphosphonate10
in N,N-dimethylformamide (DMF) gave 4-[2-(4-hydroxymethyl-
2,5-dimethoxyphenyl)-vinyl]-benzonitrile in 60% yield. Subsequent
bromination with phosphorus tribromide gave 4-[2-(4-bromomethyl-
2,5-dimethoxyphenyl)-vinyl]-benzonitrile 3 in 68% yield. Phos-
phonate 4 was obtained from 3 by a Michaelis-Arbuzov reaction
in 82% yield. The Wadsworth-Emmons coupling reaction of 4
with 511 gave BSB 1 in 69% isolated yield.
Figure 1 shows the absorption spectrum of 1 in CH3CN without
and in the presence of 2 equiv of Zn(ClO4)2 hexahydrate (1-
Zn2+),12 in which a clear shift to higher frequencies and a lower
oscillator strength of the characteristic low-energy absorption band
of BSBs,4 centered at 432 nm, can be observed. The difference
between the two spectra, in particular the lower oscillator strength
of the 532 nm transition for 1-Zn2+, can be understood, at least
partially, by considering that 1 is a push-pull molecular structure
with the lowest excited state which is involved in a charge-transfer
excitation from the cyclen unit (donor) to the cyano group
(acceptor). Complexation with Zn2+ induces a higher localization
of the electrons of the cyclen which, in turn, becomes a weaker
Figure 1. Absorption spectra of 1 and of 1-Zn2+in CH3CN.
donor. As a result, 1 changes its D-π-A characteristics to a more
symmetrical A′-π-A character. This change can justify the variation
of the linear absorption spectrum, recalling that for symmetrical
BSBs one can predict,4 in that spectral region, the presence of two-
photon-allowed electronic states which are not active in the linear
spectrum.
Multiphoton absorption properties of 1 and 1-Zn2+ were
obtained by nonlinear transmission measurements13 using 9 ns
pulses of a Nd:YAG laser at the frequencies of 1064 and 532 nm.
BSB 1 did not show any nonlinear absorption behavior at 1064
nm nor in the presence of an excess of metal cation. On the contrary,
at 532 nm a definite nonlinear absorption performance was recorded
for 1 in the absence of Zn2+ binding. Figure 2 shows the nonlinear
transmission data at 532 nm of a 9.8 × 10-4 M solution of 1 in
CH3CN and of the same solution with 2 equiv of Zn(ClO4)2
9
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10.1021/ja049204w CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
induced by a small change of the linear absorption spectrum upon
Zn2+ binding. This result has been interpreted in the frame of a
sequential two-photon process in which one photon is absorbed
from the ground state and one photon is absorbed from an excited
state.
The on-off switching of the nonlinear transmission character-
istics of 1 induced by Zn2+ binding is, to our knowledge, a new
phenomenon which holds promises in the field of bio-imaging since
a high transmission in the nonlinear regime is recorded only when
Zn2+ is present.
Acknowledgment. We thank the University of Padova (GR/
No. CPDA012428) and MIUR (GR/No. 2002032171 and GR/No.
60A03-5413/01) for financial support, G. Saielli and F. Mancin
for advice, and M. Garbin for his help in carring out some of the
photophysical characterizations of 1.
Figure 2. Nonlinear transmittance of 1 and 1-Zn2+ in CH3CN at 532
nm. The solid line shows a model calculation for a sequential two-photon
absorption process. The dashed line shows the same calculation but with a
lower ground-state one-photon absorption cross section (see text).
hexahydrate. The on-off switching of the nonlinear transmittance,
induced by Zn2+ coordination, is evident.
Supporting Information Available: Details for the synthesis of 1
and NLO measurements (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
It has been recently reported,14 following theoretical studies, that
multiphoton absorption phenomena may underlie different mech-
anisms for long (nanosecond) and short (femtosecond) laser pulses.
In particular, long nanosecond pulses make possible a real popula-
tion of excited states. Therefore, the usual large nonlinear optical
response observed in this time regime can be related to sequential
processes with absorption from excited states. Since we used
nanosecond pulses for the nonlinear absorption measurements, we
expect that excited-state absorptions are active in our system.
Therefore the fitting, shown in Figure 2, was achieved by using a
model for the dynamics of the excited states that was based on
kinetic equations15 that could be written, and solved numerically,16
for one-photon, two-photon, and higher order absorptions. The
model took into account that, at 532 nm, 1 has a weak linear
absorption (see inset to Figure 1), which is indeed sufficient to
drive the BSB toward an excited state. A good fitting has been
obtained by considering the simplest excited-state process, namely,
a one-photon excited-state absorption. The calculated curve, reported
in Figure 2 for the nonlinear trasmittance of 1, gives a very
satisfactory account of the experimental data by using 0.14 ns for
the relaxation of the first excited state, a fast relaxation (1 ps) for
the excited-state reached after the first excitation, a ground-state
References
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(1)
absorption cross section (σGS ) 1.26 × 10-18 cm2 equivalent to
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ꢀGS ) 330 mol-1 l cm-1 at 532 nm) obtained from the linear
spectrum (see inset to Figure 1), and an absorption cross section
(1)
for the one-photon excited state (σex ) 8.0 × 10-16 cm2, ꢀex
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To understand why the 1-Zn2+ complex shows a different
behavior, one first observes that its linear absorption at 532 nm is
much weaker than that of 1 (see inset to Figure 1). We therefore
(10) 4-Cyanobenzyl-diethylphosphonate was prepared in 95% isolated yield
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Michaelis-Arbuzov procedure.
(11) The aldehyde 5 was prepared from N-phenyl-4,7,10-trimethyl-1,4,7,10-
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(1)
repeated the calculation, changing only the linear absorption (σGS
) 7.65 × 10-20 cm2, ꢀGS) 20 mol-1 l cm-1). The calculated curve,
reported in Figure 2 (dashed line), accounts for the experimental
data up to 70 × 1024 ph cm-2 s-1. This shows the importance of
the linear absorption for the nonlinear behavior of 1-Zn2+ and
strengthens the role of excited-state populations recalled above for
nanosecond pulses. For the fitting at higher intensities, a σex(1) value
(σex(1) ) 8.0 × 10-17 cm2, ꢀex ) 2.09 × 104 mol-1 l cm-1) 1 order
of magnitude lower than that previously used for 1 adequately
accounts for the observed behavior up to the highest explored
intensities.
(12) Two-fold molar excess of Zn2+ over the cyclen ligand has been employed
to ensure the full formation of the 1-Zn2+ complex.
(13) Nonlinear transmission is an NLO technique by which the variation of a
sample transmission is recorded against the intensity of a laser pulse at a
definite wavelength.
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In summary, we have shown how a large variation of the
nonlinear transmission properties of a cyclen-based BSB could be
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