Photoinduced Ba2+ release and thermal rebinding by an azacrown ether linked by an alkynyl pyridine to a (bpy)Re(CO)3 group

Article abstract of DOI:10.1039/b309436g

A novel complex in which an azacrown ether is linked by an alkynyl pyridine to a (bpy)Re(CO)₃ group acts as a light-controlled ion switch: Ba²⁺ bound to the azacrown is released in ca. 10 ns on UV excitation, and rebinds thermally from bulk solution in ca. 1 μs.

Full text of DOI:10.1039/b309436g

Photoinduced Ba2+ release and thermal rebinding by an azacrown ether
linked by an alkynyl pyridine to a (bpy)Re(CO)3 group†
Jared D. Lewis and John N. Moore*
Department of Chemistry, The University of York, Heslington, York, UK YO10 5DD.
E-mail: jnm2@york.ac.uk
Received (in Cambridge, UK) 8th August 2003, Accepted 8th October 2003
First published as an Advance Article on the web 22nd October 2003
A novel complex in which an azacrown ether is linked by an
alkynyl pyridine to a (bpy)Re(CO)3 group acts as a light-
controlled ion switch: Ba2+ bound to the azacrown is released
in ca. 10 ns on UV excitation, and rebinds thermally from
bulk solution in ca. 1 ms.
The intense band at 405 nm in the spectrum of 1 can be assigned
to an intraligand charge-transfer (ILCT) transition by compar-
ison with related complexes and ligands.11
Pulsed excitation of model complex 2 at 355 nm gave strong
emission peaking at ca. 570 nm and the TRVIS spectrum shown
in Fig. 2, both of which are characteristic of a MLCT excited
state;10 the transient kinetics in both cases fitted well to a single
exponential decay with a lifetime of 260 ns. Thus, the observed
photophysics of 2 are assigned to a Re ? bpy MLCT state.
Pulsed excitation of azacrown complex 1 at 355 nm gave very
weak emission, with a lifetime of < 5 ns, and no transient
absorption on the nanosecond time scale. This observation
indicates that any excited states created from exciting into either
the ground-state MLCT absorption band, or the overlying ILCT
absorption band, must decay to the ground state in < 5 ns. The
presence of the azacrown substituent clearly results in a MLCT
state lifetime that is significantly shorter in 1 than 2. One
possible explanation is that the MLCT state of 1 is quenched by
ultrafast electron transfer from the azacrown nitrogen to Re, to
create a ligand-to-ligand charge-transfer (LLCT) excited state,
as has been observed in related complexes;8 an alternative
explanation could be that internal conversion creates a short-
lived L ligand-centred state such as an ILCT or 3pp* state.12 In
either case, decay to the ground state then occurs in < 5 ns.
Addition of 0.1 mol dm23 Ba(ClO)4 to 1 (2 3 1025 mol
dm23) in acetonitrile resulted in a blue-shift of the intense
The development of systems which can bind metal cations in
solution and release them in response to light is relevant to the
design of molecular switches, sensors, and other photodevices.1
“Caged” compounds giving irreversible cation release have
been developed principally for biochemical studies.2 Chrom-
phoric crown ether compounds have been developed to act as
sensors, and some also to act as reversible cation switches with
designs commonly based on excited-state isomerisation or
electron-transfer mechanisms which are photochemically or
thermally reversible.3 Most photocontrolled ion-release sys-
tems reported to date have been all-organic,4–6 but the
incorporation of a transition-metal redox centre within such
systems may provide additional excited-state decay routes that
bring advantages and versatility in design. We have been
studying (bpy)Re(CO)3L complexes functionalised with an
azacrown ether7,8 to explore their potential to act as light-
controlled ion switches based on this general design.9 Here, we
report a novel switch of Ba2+ and demonstrate its effectiveness
by nanosecond time-resolved UV-visible absorption (TRVIS)
and emission spectroscopy.
We have prepared a novel (bpy)Re(CO)3L+ complex 1, in
which the ligand L contains an aza-15-crown-5 ether attached to
an alkynyl pyridine.‡ The rationale behind this design was that
the electronic effects of Re ? bpy metal-to-ligand charge-
transfer (MLCT) excitation might be communicated effectively
through a C·C bond to result in a lowering of electron density
at the azacrown nitrogen and, hence, in efficient cation release
in the excited state. Compound 2 is a model complex with no
azacrown.‡
The UV-visible absorption spectra of 1 and 2 in acetonitrile
are shown in Fig. 1. For each complex, the moderately intense
Re ? bpy MLCT absorption band (e ≈ 3500 mol21 dm3
cm21), which occurs at ca. 350 nm in similar complexes,10 is
obscured by intense bands due to transitions centred on the L
ligand: the spectra of the free L ligands contain similar bands.
Fig. 1 UV-visible absorption spectra of 1, 1-Ba2+ and 2 in acetonitrile.
† Electronic supplementary information (ESI) available: analytical data.
See http://www.rsc.org/suppdata/cc/b3/b309436g/
Fig. 2 TRVIS spectrum obtained 50 ns after 355 nm excitation (5 mJ, 10 ns)
of 2 in acetonitrile. Inset: TRVIS kinetic trace recorded at 380 nm.
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ground-state ILCT absorption band to 352 nm (Fig. 1),
consistent with Ba2+ binding to the azacrown to form 1-Ba2+.9
Pulsed excitation of 1-Ba2+ at 355 nm gave very weak emission
with a lifetime of < 5 ns and a TRVIS spectrum that was
constant in profile at > 20 ns and consisted of a bleach at
340–360 nm and an absorption band at ca. 410 nm (Fig. 3). The
kinetics at 410 nm fitted well to a dual exponential decay with
lifetimes of 510 ns and 110 ns.
obtained for binding of Ba2+ to a benzothiazolium styryl aza-
15-crown-5 dye in acetonitrile.6
The mechanism of photorelease from 1-Ba2+ is not revealed
by the present study, although the short-time data provide some
information about the MLCT and ILCT states populated on
excitation. The short-lived emission from 1-Ba2+ indicates that
its MLCT state lifetime is similar to that of 1, and shorter than
that of 2, suggesting that the ground-state azacrown-Ba2+
interaction may be lost in < 5 ns in the MLCT state. The short-
time TRVIS spectrum shows strong bleaching at ca. 350 nm and
absorption at ca. 400 nm, indicating that the ground-state ILCT
band is bleached and suggesting that an ILCT state is observed;
the short-time kinetics suggest that this ILCT state then decays
in 510 ns. The lifetime of this ILCT state is longer for 1-Ba2+
than for 1, for which no nanosecond transient spectrum was
observed, suggesting that Ba2+ may remain associated with 1 on
the nanosecond time scale. Studies of all-organic azacrown dyes
have shown that an azacrown nitrogen-cation bond can break in
< 10 ps on excitation to produce a longer-lived species in which
the cation is more loosely bound to the azacrown.5 The short-
lived features reported here may arise from such a species,
present before full release of Ba2+ from the azacrown; this
process requires large structural rearrangement of the azacrown
and full solvation of the cation.
The UV-visible signature for ion release is obtained by
subtracting the steady-state UV-visible spectrum of 1-Ba2+
from that of 1 (Fig. 3). The TRVIS spectrum obtained at > 20 ns
after excitation of 1-Ba2+ matches this difference spectrum (Fig.
3), demonstrating that Ba2+ has been released and the ground-
state of 1 has been formed in < 20 ns. This TRVIS spectrum
then decays to the baseline, indicating that Ba2+ rebinds to 1 to
form 1-Ba2+ again, restoring the starting equilibrium.
The dependence of the kinetics on metal cation concentration
were studied by repeating the TRVIS experiment on 1-Ba2+ at
[Ba2+] = 0.10–0.0018 mol dm23 (Fig. 4). The kinetics recorded
at every concentration fitted well to a dual exponential decay,
with the lifetime of the longer-lived component showing a
strong dependence on Ba2+ concentration and increasing to 1.8
ms at [Ba2+] = 1.8 3 1023 mol dm23. Both the time scale and
concentration dependence indicate that the cation rebinds from
bulk solution, rather than geminately, and thereby that excita-
tion of 1-Ba2+ releases Ba2+ into bulk solution. A plot of the rate
constant for the longer-lived component (k2) versus Ba2+
concentration (Fig. 4) gave a slope of 9.3 3 107 mol21 dm3 s21
for the observed second-order rate constant for rebinding of
Ba2+ to 1; it is comparable to that of 4 3 107 mol21 dm3 s21
In summary, we report a (bpy)Re(CO)3L+ complex which
binds Ba2+, releases it on excitation, and rebinds it thermally on
a time scale of 0.1–2 ms depending on the metal cation
concentration. This work contributes to the rational design of
efficient light-controlled ion switches and their potential
applications; further work will concentrate on the detailed
nature of the cation release mechanism and will extend the
present studies to other metal cations.
We acknowledge financial support from EPSRC.
Notes and references
‡ (4,7,10,13-tetraoxa-1-azacyclopentadecyl)benzene
(phenylaza-
15-crown-5) was formylated at the 4-position according to ref. 13 to yield
(4,7,10,13-tetraoxa-1-azacyclopentadecyl)benzaldehyde, from which
4-ethynyl(4,7,10,13-tetraoxa-1-azacyclopentadecyl)benzene was prepared
by a method based on refs 14 and 15 and used to synthesise ligand L 1
according to ref 16. Ligand L 2 was synthesised identically, starting from
2
benzaldehyde. (bpy)Re(CO)3L+PF6 complexes 1 and 2 were prepared
from the respective ligands L 1 and L 2, respectively, according to ref. 7;
analytical data are given as electronic supplementary information.
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Fig. 3 Top: TRVIS spectra obtained at 4, 18, 30, 50 and 100 ns after 355 nm
excitation of 1-Ba2+ in acetonitrile. Bottom: TRVIS spectrum at 100 ns
overlaid with the difference spectrum (red trace) obtained by subtracting the
steady-state UV-visible spectrum of 1-Ba2+ from that of 1 (scaled to 410
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Fig. 4 Selected TRVIS kinetics recorded at 410 nm after 355 nm excitation
of 1-Ba2+ in acetonitrile at Ba2+ concentrations of (purple) 7.5 3 1023
,
(green) 0.020, (red) 0.040, and (blue) 0.10 mol dm23. Inset: Plot of k2 (see
text) versus Ba2+ concentration, with a linear regression fit.
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