Excitation Energy Transfer Supported Amplified Charge-Transfer Emission in an Anthracenedicarboxylate- and Bipyridophenazine-Based Coordination Complex

Article abstract of DOI:10.1021/acs.inorgchem.7b02698

A highly luminescent tetrameric zinc(II) complex, {[Zn₄(adc)₃(bpz)₆(HCOO)₂]·2H₂O} (1; adc = 9,10-anthracenedicarboxylate and bpz = bipyridophenazine), was synthesized by a solvothermal technique and characterized by single-crystal X-ray diffraction. The linear tetrameric units extend into three dimensions via π stacking of adc/bpz and bpz/bpz and multiple CH-π interactions. The compound shows strong red emission at 597 nm (λ_ex = 480 nm), which is attributed to charge-transfer (CT) emission within an adc/bpz donor-acceptor pair. This is also supported by density functional theory computations. Interestingly, the CT emission is amplified by energy transfer from another adc linker that is not involved in the CT interaction.

Full text of DOI:10.1021/acs.inorgchem.7b02698

Communication
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Excitation Energy Transfer Supported Amplified Charge-Transfer
Emission in an Anthracenedicarboxylate- and Bipyridophenazine-
Based Coordination Complex
Komal Prasad,†,‡ Debabrata Samanta,†,‡ Ritesh Haldar,† and Tapas Kumar Maji*,†,§
§
†Molecular Materials Laboratory, New Chemistry Unit, and Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for
Advanced Scientific Research (JNCASR), Jakkur, Bangalore 560064, India
S
* Supporting Information
coordination polymer.16 The work demonstrates the initial
ABSTRACT: A highly luminescent tetrameric zinc(II)
complex, {[Zn4(adc)3(bpz)6(HCOO)2]·2H2O} (1; adc =
9,10-anthracenedicarboxylate and bpz = bipyridophena-
zine), was synthesized by a solvothermal technique and
characterized by single-crystal X-ray diffraction. The linear
tetrameric units extend into three dimensions via π
stacking of adc/bpz and bpz/bpz and multiple CH−π
interactions. The compound shows strong red emission at
597 nm (λex = 480 nm), which is attributed to charge-
transfer (CT) emission within an adc/bpz donor−acceptor
pair. This is also supported by density functional theory
computations. Interestingly, the CT emission is amplified
by energy transfer from another adc linker that is not
involved in the CT interaction.
absorption of light by 2,6-naphthalenedicarboxylate, a linker of
the coordination polymer, which transfers the excitation energy
to a CT complex formed by another ligand, o-phenanthroline,
and an encapsulated aromatic amine guest in the framework.
However, it would be advantageous to have such energy-transfer
phenomena in stable and simple coordination complexes for
practical application in an optoelectronic device. To the best of
our knowledge, an example of amplified CT emission by an
energy-transfer process in discrete coordination complexes is yet
to be documented (Scheme 1).
Scheme 1. Schematic Representation of the Amplified CT
Emission for the Present Work
ature is a constant inspiration for chemists to build artificial
N
tools by mimicking its sophisticated working principles.
For instance, the photosynthetic system harvests light energy by
absorbing sunlight, which is transferred to the catalytic reaction
center and is finally stored as chemical energy.1 Over the decades,
scientists have been utilizing energy-transfer phenomena in
multichromophoric organic systems, metal−organic frameworks,
organic polymers, and inorganic quantum dots to build artificial
light-harvesting and optoelectronic materials.2−14
Because of the versatile optical applications of such materials, a
novel coordination compound, [{Zn4(adc)3(bpz)6(HCOO)2}·
2H2O] (1), has been synthesized utilizing 9,10-anthracenedi-
carboxylate (adc, a donor) and bipyridophenazine (bpz, an
acceptor) chromophores in the backbone. The formation of a
CT complex, bpz/adc/bpz, and the energy-transfer phenomen-
on from a neighboring adc to the CT complex were observed.
Moreover, the system showed tunable emission depending upon
the solvent polarity.
Coordination complexes are widely used as efficient light-
harvesting materials because they can hold chromophores in
close proximity inside the lattice with the highest degree of
order.9−11,15−20 Moreover, coordination-driven spatial organ-
ization of multichromophoric ligands results in novel photo-
physical properties in coordination complexes.21 For example,
coordination complexes, synthesized by combining electron-rich
and -deficient chromophoric ligands, often produce donor−
acceptor or charge-transfer (CT) complexes with exciting
properties like exciplex or CT emission.22−30 Moreover, in a
few cases, excitation energy-transfer phenomena were also
observed from a neighboring chromophore to the exciplex or
CT complex, leading to an enhanced and tunable emissive
material.16,17 Significant spectral overlap between the donor
emission and acceptor absorbance as well as an optimum donor−
acceptor distance are the primary requirements to realize the
energy-transfer process. Such materials provide the additional
advantage of absorbing light from a wider spectral range. An
example of such energy-transfer phenomena was reported by our
group, utilizing a guest-encapsulated metal−organic porous
Complex 1 has been synthesized by the self-assembly of ZnII
and bpz and adc linkers in dimethylformamide under
solvothermal conditions at 90 °C, yielding block-shaped dark-
red single crystals (see the Supporting Information, SI). Single-
crystal structure determination revealed that compound 1
crystallizes in the monoclinic P21/n space group and the
asymmetric unit contains two ZnII (Zn1 and Zn2) centers, one
formate, and 1.5 adc (adc1 and adc2) linkers (Figure 1). Each
octahedral Zn1 center is chelated to two bpz linkers through N1,
N2 and N3, N4 atoms, and two other positions are filled by a
chelated carboxylate group (O3 and O4) from adc1. Each
Received: October 19, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02698
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complex 1 is stable up to 300 °C. The TGA profile also showed
1.2 wt % loss in the temperature window of 80−170 °C, and this
corresponded to the loss of two guest water molecules from the
crystal (calcd 1.23 wt %; Figure S3).
We have further studied the photophysical properties of
complex 1 in detail. The absorbance spectrum of 1, in the solid
state, showed two partially overlapped bands at 340 and 380 nm
for bpz and adc chromophores, respectively (Figure 3a, red).
Figure 1. Crystal structure of complex 1 showing the coordination
environment of Zn1 and Zn2.
pentacoordinated Zn2 center is chelated to one bpz (N9 and
N10) and three monodentate carboxylate O atoms (O1, O5, and
O8) from two adc and one formate ligands. The Zn1 and Zn2
centers are bridged by adc1, and the resulting dimeric fragment
Zn1Zn2(bpz)3(adc1)(HCOO) is further linked to its symmetry-
related counterpart by the adc2 linker, forming a tetranuclear
complex [{Zn4(adc)3(bpz)6(HCOO)2}·2H2O]. In the complex,
the Zn−N and Zn−O bond distances are in the ranges of
2.099(4)−2.279(5) and 1.945(4)−2.294(4) Å, respectively, and
the separations between Zn1−Zn2 and Zn2−Zn2 bridged by
adc1 and adc2 are 11.01 and 10.95 Å, respectively. Because the
complex is composed of two different extended π-chromophoric
linkers (adc and bpz), the crystal structure extends through π−π
interactions. The tetramers are π-stacked via adc2/bpz in a zigzag
fashion (Figure 2), which extends the structure in one dimension
Figure 3. (a) Solid-state UV−vis spectrum of 1 (red) and PL of adc
(black). Inset: picture of 1 under visible light. (b) Solid-state PL of 1
upon excitation at 380 nm (black) and 480 nm (red). The excitation
spectrum collected at 597 nm is presented in blue. Inset: picture of 1
under UV light. (c) PL of 1 in different solvents along with photographs.
(d) Fluorescence lifetime spectrum of 1 monitored at 597 nm upon
excitation with both 376 nm (blue) and 532 nm (red) laser.
Additionally, a red-shifted band was also observed in the visible
region at 480 nm, which can be attributed to a ground-state CT
interaction due to the cofacial arrangement of adc/bpz
chromophores. Possibly, this interaction results in the red
color of 1 (Figure 3a, inset). Complex 1 exhibited a single
emission band at 597 nm (bright red) upon excitation at 480 nm,
corresponding to the CT complex (Figure 3b). The solid-state
excitation spectrum of 1 (λem = 597 nm) showed a band at 451
nm, along with two other bands at 403 and 335 nm, indicating
the origin of the emission is the CT complex (Figure 3b). The
spectrum clearly suggests the presence of adc/bpz CT
interaction, where bpz acts as an acceptor and adc as a donor.
The fluorescence lifetime was measured to be 6.72 ns upon
monitoring at 597 nm (λex = 532 nm; Figure 3d). Such a long
excited-state lifetime additionally supports an excited-state
photophysical process with CT character. The absolute quantum
yield of the solid sample was determined to be 8.6%.
In order to obtain more insight about the origin of the CT
band in the UV−vis spectrum and corresponding bright-red
emission, we employed time-dependent density functional
theory (TD-DFT) computations for three different model
systems (models 1−3, Figure 4) and computed their excited
states. Model 1 illustrates adc/bpz π stacking, model 2 shows
bpz/bpz stacking, and model 3 considers the possibility of CT
from nonstacked adc to a bpz chromophore. The CAM-B3LYP
method,31 in combination with the 6-31+G* basis set, was
utilized for computations in the gas phase, as implemented in
Gaussian09.32 Because the gas-phase computations showed large
deviations from the UV−vis/photoluminescence (PL) measured
Figure 2. Two-dimensional packing of complex 1 through adc2/bpz and
bpz/bpz π stacking.
along the a axis. Notably, the π-stacking distance between adc2
and bpz is 3.59 Å. The one-dimensional chains are further
assembled through bpz/bpz π stacking, which leads to two-
dimensional extension of the packing on the ac plane. The two
different π stacks, adc2/bpz and bpz/bpz, are positioned in an
alternative arrangement in the ac plane. Finally, the two-
dimensional layers are interdigitated by multiple CH−π
interactions, resulting in a three-dimensional extended network.
This has a small void space that hosts two water molecules, as
realized from thermogravimetric analysis (TGA), Fourier
transform infrared (FT-IR), and elemental analyses.
The PXRD pattern of the as-synthesized complex 1 matched
well with the simulated pattern of a single crystal, suggesting
phase purity of the bulk product (Figure S2). TGA revealed that
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at the CT absorption maximum. The fluorescence lifetime was
found to be very low (7.75 ps) upon monitoring the emission at
420 nm, whereas the value is higher (0.29 ns) in the monomeric
state (solution). In order to further cross-check the energy-
transfer process, we attempted to break the adc/bpz π-stacked
supramolecular association by dissolving complex 1 in various
solvents (Figure 3c). Indeed, in dichloromethane, the emission
band at 597 nm completely vanishes and, consequently, an
emission band appears at 400−440 nm, corresponding to the adc
chromophore. The liquid chromatography−mass spectrometery
of the solution showed two distinct peaks at 472.5 and 708.9 Da
for [1 + 6H]6+ and [1 + 4H]4+, respectively (Figure S4). This
indicates that complex 1 remains stable but disaggregates in
solution, resulting in breakage of the CT complex and the
disappearance of the CT emission. The energy-transfer efficiency
was calculated to be 29.3% with an energy-transfer rate constant
of 3.37 × 107 s−1 (SI). The above results suggest that adc1 acts as
an energy-transfer antenna for amplifying the CT emission.
In summary, we have synthesized a highly luminescent three-
dimensional supramolecular metal−organic complex,
[{Zn4(adc)3(bpz)6(HCO2)2}·2H2O] (1), which showed a CT
transition at 480 nm due to adc/bpz π stacking in the solid state
and a corresponding emission in the red region at 597 nm. TD-
DFT computations on the adc/bpz π-stacked model system
reveals that an electron transition occurs from the adc to bpz
chromophore. Most interestingly, this study represents a classic
example where the CT emission is amplified by energy transfer
from the surrounding nonstacked anthracene moiety. Our results
will help in the design of a new type of light-harvesting hybrid
material by assembling multichromophoric ligands through
coordinative interaction.
Figure 4. Models 1−3 represented with computed molecular orbital
diagrams that have contributed to the lowest-energy electronic
transition.
in the solid phase, we analyzed the origin of the CT band and
emission by comparing experimental and computational Stokes
shifts to cancel out the error due to the phase difference. The
computed electronic transition of model 3 displayed the lowest
energetic transition from HOMO (π orbital of adc) to LUMO+1
(π orbital of carboxylate) at 370 nm (f = 0.41), whereas the
emission appeared at 444 nm with a Stokes shift of 74 nm (Figure
4). There was no electronic transition from adc to bpz observed
in this model. Model 2 showed an electronic transition having a
major contribution from HOMO to LUMO at 307 nm (f =
0.59), but the emission band corresponding to the transition
appeared at 319 nm with a Stokes shift of 12 nm only. Therefore,
bpz/bpz-stacked model 2 and linear model 3 could not explain
such a huge Stokes shift of 117 nm obtained in the experiment.
As expected, model 1 with adc/bpz stacking showed a CT
transition from HOMO (π orbital of adc) to LUMO+2 (π orbital
of bpz) at 374 nm and corresponding emission at 508 nm with a
Stokes shift of 134 nm matching well with the experiment.
Therefore, the dark-red emission was generated as a result of
excitation at the adc/bpz CT complex.
Furthermore, a closer inspection of the crystal structure shows
that adc1 moieties are not involved in the CT complex (Figure
2), and, hence, a separate emission band should be expected in
the region of 400−440 nm for the adc1 chromophore upon
excitation at 380 nm. Strikingly, no emission band was observed
in the region upon excitation at several wavelengths in the range
of 340−400 nm in the solid state. The absence of adc1 emission
may be attributed to complete excitation energy transfer from the
adc1 chromophore to the adc/bpz CT complex. This is feasible
because the adc emission is completely overlapped with the CT
band, as observed in UV−vis spectrum of 1 (Figure 3a).
Interestingly, the emission intensity at 597 nm is higher when
excited at 380 nm than that of 480 nm, indicating that the CT
emission is amplified by energy transfer from adc1. This is further
supported by the excitation spectrum collected at 597 nm, which
shows the highest intensity at 403 nm, which is the absorption of
the adc linker. The fluorescence lifetime was also measured to
support the energy-transfer process (Figure 3d). In the solid
state, the fluorescence lifetime was found to be 8.69 ns upon
excitation at 376 nm (at adc) and monitoring the emission at 597
nm (CT emission), whereas the lifetime is 6.72 ns when excited
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02698.
Experimental details and additional results (PDF)
Accession Codes
CCDC 1581938 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tmaji@jncasr.ac.in.
■
ORCID
Tapas Kumar Maji: 0000-0002-7700-1146
Author Contributions
‡K.P. and D.S. contributed equally. All authors have given
approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
T.K.M. and D.S. acknowledge DST, India (MR-2015/001019
and TRC DST/C.14.10/16-2724, JNCASR), and JNCASR for
funding.
C
DOI: 10.1021/acs.inorgchem.7b02698
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