A photoactive basket-like metal-organic tetragon worked as an enzymatic molecular flask for light driven H2 production

Article abstract of DOI:10.1039/c2cc37853a

A photoactive basket-like metal-organic tetragon Ce-ZL that contained a carbazole photosensitizer was developed to capture the biomimetic [FeFe]-H ₂ases for light driven H₂ production. The system exhibited enzymatic behaviour and its activity was inhibited by the encapsulation of ATP.

Full text of DOI:10.1039/c2cc37853a

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A photoactive basket-like metal–organic tetragon
worked as an enzymatic molecular flask for light
driven H2 production†
Cite this: Chem. Commun., 2013,
49, 627
Received 31st August 2012,
Accepted 21st November 2012
Cheng He, Jian Wang, Liang Zhao, Tao Liu, Jing Zhang and Chunying Duan*
DOI: 10.1039/c2cc37853a
www.rsc.org/chemcomm
A photoactive basket-like metal–organic tetragon Ce–ZL that contained
By incorporating a carbazole fragment8 as the photosensitizer
a carbazole photosensitizer was developed to capture the biomimetic within linear bridged bistridentate amide-containing chelating
[FeFe]-H2ases for light driven H2 production. The system exhibited systems, we report herein a basket-like tetragon Ce–ZL for encap-
enzymatic behaviour and its activity was inhibited by the encapsulation sulating a [FeFe]-H2ases model compound 1. This molecular basket
of ATP.
had a robust framework with a gated flexible hydrophobic cavity,
modelled on the naturally engineered pockets that potentially
interacted with 1 directly. We envisioned that the imparted stability
to the host–guest system would benefit photocatalytic H2 produc-
tion in aqueous solution. The reduction potential of the excited
carbazole moiety (ca. À2.3 V) would be negative enough to make
photo-induced electronic transfer from the sensitizer to 1 possible,9
and the confinement of the cavity would enforce the proximity
between the carbazole groups and the substrate, ensuring the
enhancement of the PET efficiency to avoid unwanted energy-
transfer or reverse-ET reactions (Scheme 1).10
Photocatalytic hydrogen production from water represents an
important process in sustainable solar energy conversion for the
future. Inspired by photosynthetic complexes in nature, this pro-
cess was realized by a homogeneous reaction environment toward
optimal availability of active catalytic sites for solar hydrogen
production.1 With the structural elucidation of [FeFe]-H2ases,2
scientists are working hard to develop long-lived homogeneous
artificial photo-synthetic systems that, in their excited states, can
reduce [FeFe]-H2ases directly and efficiently for H2 generation.3 The
naturally engineered pockets that bond the catalytic centres of
metalloenzymes impart stability to unusual molecular structures to
facilitate molecular transformation.4 The construction of host–
guest photosynthetic systems enforcing the electron transfer
process in a local microenvironment might be a promising
approach to increase the H2 production efficiency.
Ligand H4ZL was obtained by a simple Schiff-base reaction of
salicylaldehyde with 9-butyl-3,6-dicarbohydrazidecarbazole in a
methanol solution. Evaporating a DMF solution of H4ZL with
Metal–organic macrocycles represent a unique class of functional
molecular containers that display interesting recognition properties
and fascinating reactivity reminiscent of natural enzymes.5 The
architectures that generate well-defined cavities with gated pores
provide specific inner environments for the selective uptake and
bonding of guest molecules and the catalysis of their reactions.6
Without doubt, the major challenge for well-confined systems
applied to photocatalytic H2 production goes beyond achieving
molecular recognition of the [FeFe]-H2ases, but includes the mod-
ification of the excited state potential of the photo-sensitizer to
match the reduction potential of [FeFe]-H2ases, and the relative
positions between the photosensitizer and the [FeFe]-H2ases to avoid
unwanted energy-transfer.7
State Key Laboratory of Fine Chemicals, Dalian University of Technology Dalian,
116012, China. E-mail: cyduan@dlut.edu.cn
Scheme 1 Schematic representation of the fragment of the photoactive
basket-like metal–organic tetragon and host–guest complexation species for
photocatalytic H2 production. The green balls represent the cerium ions.
† Electronic supplementary information (ESI) available: Experimental details
and additional spectroscopic data. CCDC 898733. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2cc37853a
c
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Ce(NO3)3Á6H2O in air led to the formation of crystalline solids of to reduce 1 (À1.7 V)9 forming the [FeIFe0] intermediate for
Ce–ZL in a yield of about 52%. Elemental analysis and powder X-ray electro- and photochemical H2 generation.
diffraction indicated the pure phase of the sample. Magnetic
The ESI-MS spectrum of Ce–ZL exhibited an intense peak at
susceptibility measurements (1.8–300 K) showed the diamagnetic m/z = 1914.08 with isotopic distribution patterns separated by
behaviour of the bulk sample, suggesting the presence of four CeIV 0.5 Da, assignable to the species [Ce4(HZL)6]2À, demonstrating
ions in Ce–ZL. The X-ray photoelectron spectroscopy spectrum of the formation of M4L6 species in solution. Upon the addition
Ce–ZL exhibited a distinct band at 916 eV assignable to the 4f0 of 1, the ESI-MS spectrum exhibited two new peaks at m/z
orbital transitions of the CeIV atom.11
2354.06 and 2372.50, assignable to [Ce4(HZL)6 * 12ÁDMFÁ2H2O]2À
Single crystal X-ray structural analysis of compound Ce–ZL and [Ce4(HZL)6 * 12ÁDMFÁ4H2O]2À, suggesting the formation of a
revealed the formation of a basket-like molecular structure. Each 1 : 2 stoichiometric host–guest complexation species in solution.
molecule of Ce–ZL comprised six deprotonated ligands and four The addition of 1 (up to 0.50 mM) caused the emission quenching
cerium ions. The cerium ions each coordinated to three tridentate of Ce–ZL (30 mM) with an intensity decrease to 5% of its original
chelating sites and were positioned at the corners of a square. Four intensity. The quenching behaviour, assignable to the photo-
of the six ligands were positioned at the edge of the square, and each induced electron transfer (PET) process from the excited state of
bridged two cerium centers alternately. The bridged CeÁÁÁCe separa- Ce–ZL* to 1, provided the possibility of Ce–ZL to activate 1 for H2
tion in one edge was about ca. 12.6 Å. The alkyl rings of the four production in solution.15
ligands positioned in the same direction interacted with each other
The light-driven H2 production of the Ce–ZL/1 system was
and acted as the handle of the basket. The other two ligands carried out in the presence of NiPr2EtHÁOAc as the sacrificial
positioned at the other side of the square acted as the bottom of electron donors.16 The thermostatic solutions were continuously
the basket. As shown in Fig. 1, the height of the basket is irradiated with light from a 500 W Xe lamp, using a water filter to
approximately 5.74 Å (from the center of the Ce4 square to the N4 absorb the heat. As shown in Fig. 2, the turnover number (TON) of
square formed by the four carbazole N atoms), allowing guests with H2 evolution (based on Ce–ZL) with Ce–ZL/1 in a 1 : 2 stoichiome-
suitable sizes to ingress and egress. The four flexible handles afford try was up to 30 in 4 h with the initial turnover frequency (TOF)
the possibility for the modulation of the cavity size to match about 11 hÀ1 for the first hour. The TON of our system was
the geometries of guest molecules. As the opening of the basket is comparable with the previously reported non-covalently connected
about 16.9 Â 8.9 Å2, which is larger than the size of 1 (7.5 Â 6.1 Å2), supramolecular systems.16 Control experiments with either the free
Ce–ZL thus was able to encapsulate compound 1 within the cavity. photoactive host Ce–ZL or free 1 demonstrated that both of them
The Ce–O (phenol) and Ce–O (amide) distances of 2.17 and 2.45 Å, together are essential for H2 generation. The absence of either
respectively, and the Ce–N distance of 2.64 Å were in good agree- of them yielded unobservable to insignificant amounts of H2.
ment with relative compounds,12 with valences of cerium centers
calculated as 3.9 and 4.0 for Ce(1) and Ce(2), respectively.13
Ce–ZL (15 mM) exhibited a characteristic absorption band for
the carbazole moiety at 395 nm (log e = 5.07) and an emission
band (quantum yield ca. 0.013) at about 470 nm, when excited at
335 nm. The energy difference between the LUMO and HOMO of
the carbazole group was calculated as 3.10 eV.14 Electrochemical
measurements exhibited a redox potential of E1/2(Ce–ZL+/Ce–ZL)
at 0.79 V (vs. SCE), assignable to the redox potential of the
carbazole moiety.8 The redox potential of the excited state species
E1/2(Ce–ZL+/Ce–ZL*), calculated as À2.3 V, was negative enough
Fig. 2 Family of emission spectra for Ce–ZL (30 mM) upon the addition of 1 (up to
0.50 mM) (a); cyclic voltammograms of Ce–ZL in a DMF solution containing
n-Bu4NClO4 (0.1 mol LÀ1) with a scan rate of 50 mVsÀ1 (b); photoinduced H2
evolution (TON, based on Ce–ZL) of Ce–ZL (0.2 mM) over time with different 1
concentrations of 0.2 (green), 0.4 (black), 0.6 (red) and 0.8 (blue) mM (c);
photoinduced H2 production of the Ce–ZL/1 systems and other control systems.
Fig. 1 Crystal structure of the Ce-based supramolecular host Ce–ZL. Solvent
molecules and anions are omitted for clarity. The metal, oxygen, nitrogen and
carbon are drawn in green, red, blue and grey respectively.
The red, green, blue and cyan bars show the TON value after irradiation for 3
hours of the blank, Ce–ZL/1, with the ATP and the ligand with 1 systems,
respectively (d).
c
628 Chem. Commun., 2013, 49, 627--629
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the TON value. All these complete experiments demonstrated that
molecules of ATP were true competitive inhibitors. The inhibition
was reversible, as upon addition of excess 1 to the above mentioned
ATP inhibiting system, photo-induced H2 production was recovered.
These results gave further proof that Ce–ZL is an interesting
molecular flask, within which 1 was activated.
In summary, a photoactive metal–organic basket comprising an
artificial protein environment and a chromophore was achieved for
the encapsulation of the [FeFe]-hydrogenase model. The host–guest
system displayed light-driven H2 production in aqueous media with
characteristic enzymatic dynamic behaviour and was inhibited by
the presence of ATP, demonstrating the bright future of metallcycles
acting as artificial molecular flasks for solar-driven splitting of water
for H2 production.
Fig. 3 1H NMR spectra (aromatic region) of free Ce–ZL (0.1 mM) (a), free ATP (0.2 mM)
(c) and of Ce–ZL and ATP (b) in a 1 : 2 molar ratio in d6-DMSO/D2O, respectively.
On the other hand, using ligand H4ZL as the photosensitizer
instead of Ce–ZL in the presence of 1 in same condition caused a
little H2 production. This result might be one of the indicators for
the fact that the formation of a host–guest complexation species
was essential to photo-activate 1. The small but significant blue
shift of the n(CO) vibration signals in the IR spectra gave further
proof for the formation of a host–guest complexation species.
Importantly, experiments carried out with different Ce–ZL : 1
mole ratios showed that the initial rate in first hour of H2
production increased with increasing Ce–ZL : 1 mole ratio, until
it reached 1 : 2. Almost the same TOF in the case of the Ce–ZL : 1
ratio over 1 : 2 suggested that the main species for the catalytic
reaction possibly was the 1 : 2 stoichiometric host–guest Ce–ZL *
12. In fact, the linear fitting of the fluorescence titration curve of
Ce–ZL upon addition of 1 demonstrated the 1 : 2 stoichiometric
host–guest behaviour and the associative constant Kass was calcu-
lated as 1.15 Â 108 dm6 mol2. In the presence of excess 1 (more
than 10 equivalent), our system exhibited a pseudo-zeroth-order
kinetic behaviour. It seems that the production of H2 is enzymatic-
like, governed by the Michaelis–Menten mechanism,17,18 in which
substrate binding is a first equilibrium prior to the rate-limiting
step of the reaction.
This work was supported by NSFC 21171029 and 21025102.
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thus was enzymatic-like and suggested that the H2 production
occurred within the cavity of Ce–ZL.18 In the case of [Ru(bpy)3]2+
as the photosensitizer, the TON value in a modulated system was
comparable to the literature work,19 but the addition of 10 mole
equivalent of ATP (2 mM) did not cause any significant change in
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 627--629 629