Precise Design of Phosphorescent Molecular Butterflies with Tunable Photoinduced Structural Change and Dual Emission

Article abstract of DOI:10.1002/anie.201505185

Photoinduced structural change (PSC) is a fundamental excited-state dynamic process in chemical and biological systems. However, precise control of PSC processes is very challenging, owing to the lack of guidelines for designing excited-state potential en

Full text of DOI:10.1002/anie.201505185

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201505185
German Edition: DOI: 10.1002/ange.201505185
Photochemistry
Precise Design of Phosphorescent Molecular Butterflies with Tunable
Photoinduced Structural Change and Dual Emission
Chenkun Zhou, Yu Tian, Zhao Yuan, Mingu Han, Jamie Wang, Lei Zhu,
Maliheh Shaban Tameh, Chen Huang,* and Biwu Ma*
Abstract: Photoinduced structural change (PSC) is a funda-
mental excited-state dynamic process in chemical and biolog-
ical systems. However, precise control of PSC processes is very
challenging, owing to the lack of guidelines for designing
excited-state potential energy surfaces (PESs). A series of
rationally designed butterfly-like phosphorescent binuclear
platinum complexes that undergo controlled PSC by Pt–Pt
distance shortening and exhibit tunable dual (greenish-blue
and red) emission are herein reported. Based on the Bell–
Evans–Polanyi principle, it is demonstrated how the energy
barrier of the PSC, which can be described as a chemical-
reaction-like process between the two energy minima on the
first triplet excited-state PES, can be controlled by synthetic
means. These results reveal a simple method to engineer the
dual emission of molecular systems by manipulating PES to
control PSC.
time observation of the excited-state dynamics and comple-
mentary quantum-chemical calculations have provided
a clear picture of the electronic relaxation and ultrafast
structural distortion of these compounds. Another well-
known PSC is the ultrafast contraction of the Pt–Pt distance
in binuclear platinum(II) complexes.[2k–r] Experimental stud-
ies of PSC processes in several platinum complexes, such as
2À
[Pt2(pop)4]4À (pop = P2O5H2
)
and [Pt(ppy)(m-tBu2pz)2]
(ppy = 2-phenylpyridyl, tBu2pz = 3,5-di-tert-butylpyrazolyl),
have provided direct evidence for a Pt–Pt contraction of
0.2–0.4 Š in the excited state, which is consistent with
theoretical calculations.[2o–r] PSC processes have also been
observed in purely organic molecular systems, such as
azobenzenes and stilbenes.[1c–e] Despite significant advances
in our understanding of the PSC processes in these molecular
systems, to the best of our knowledge, the precise control of
PSC processes has not been demonstrated for any molecular
system thus far.
Recently, we reported a butterfly-like pyrazolate-bridged
binuclear platinum complex, BFPtPZ, which undergoes PSC
by shortening of the Pt–Pt distance.[3] Unlike the ultrafast
photoinduced flattening of CuI complexes and the Pt–Pt
contraction in typical platinum complexes that leads to one
excited-state energy minimum and single emission, BFPtPZ
undergoes PSC between two excited structures on the first
triplet excited-state potential energy surface (PES), which
results in dual emission, that is, greenish-blue emission from
the excited state with the long Pt–Pt distance and red emission
from the excited state with the short Pt–Pt distance. The effect
of the molecular structure on PSC has been studied for a few
pyrazolate-bridged dinuclear platinum complexes. Among
these complexes, BFPtPZ is the only one with dual emission;
molecules based on other cyclometallating ligands, such as
2-(2’-thienyl)pyridine or 2-phenylpyridine, do not exhibit PSC
or dual emission.[2q,r,3,4] By introducing bulky groups to the
3- and 5-positions of the pyrazolate bridges, the emission can
be shifted to significantly lower energies.[2q,r,3,4] Substituent
effects on PSC have also been observed for CuI complexes:
Derivatives with bulkier substituents at the 2- and 9-positions
of the phenanthroline ligand require longer periods of time
for flattening, 200 fs for [Cu(phen)2]+ (phen = 1,10-phenan-
M
olecular excited states obtained by photoexcitation are
the foundation for solar energy conversion, photocatalysis,
and molecular machines.[1] Detailed studies of the excited-
state properties of molecules, including their structures,
energetics, and decay pathways, provide a better understand-
ing of photoinduced chemical and biological processes, and
enable the development of new functional materials and
devices. Among major excited-state dynamic processes of
molecular systems, photoinduced structural change (PSC) has
recently become a very active research field owing to the
advent of ultrafast time-resolved spectroscopic methods and
X-ray spectroscopy.[2] Copper(I) complexes with phenanthro-
line derivatives, which exhibit photoinduced “flattening” in
the metal-to-ligand charge transfer (MLCT) excited state,
have been thoroughly studied by several groups.[2a–j] The real-
[*] C. Zhou, Dr. Z. Yuan, Dr. M. Han, Prof. B. Ma
Chemical and Biomedical Engineering
FAMU-FSU College of Engineering (USA)
E-mail: bma@fsu.edu
Y. Tian, Prof. C. Huang, Prof. B. Ma
Materials Science Program, Florida State University (USA)
E-mail: chuang3@fsu.edu
Dr. Z. Yuan, J. Wang, Prof. L. Zhu, Prof. B. Ma
Department of Chemistry and Biochemistry
Florida State University (USA)
throline),
2,9-dimethyl-1,10-phenanthroline),
660 fs
for
[Cu(dmphen)2]+
and
(dmphen =
920 fs for
M. S. Tameh, Prof. C. Huang
Department of Scientific Computing
Florida State University (USA)
[Cu(dpphen)2]+ (dpphen = 2,9-diphenyl-1,10-phenanthroli-
ne).[2c,d] All of these findings suggest that PSC processes
could be manipulated by precise control of the molecular
structure.
Supporting information for this article, including details on the
synthesis and characterization as well as DFT calculations of the
complexes studied herein, is available on the WWW under http://dx.
doi.org/10.1002/anie.201505185.
Herein, we report a series of rationally designed butterfly-
like phosphorescent binuclear platinum complexes that
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undergo controlled PSC by Pt–Pt distance shortening and
exhibit tunable dual emission of greenish-blue and red light in
the steady state. The unique butterfly-like structure of these
complexes offers two handles for changing the molecular
structure: the cyclometallating ligand (the butterfly wings)
and the pyrazolate bridge (the butterfly body). We demon-
strate that the energy barrier separating the two lowest energy
states on the first triplet excited-state PES can be precisely
tuned by simply adjusting the relative energy between these
two states, based on the Bell–Evans–Polanyi principle.[5] Such
a simple scheme serves to achieve very precise control over
the PSC and dual-emission processes.
three major steps previously reported for the preparation of
BFPtPZ (1).[6] The cyclometallating ligands were prepared by
Suzuki couplings.[7] Reacting the cyclometallating ligand with
potassium tetrachloroplatinate afforded the chloride mono-
mers, which reacted with the pyrazolate bridging ligands in
the presence of a base to yield pyrazolate-bridged binuclear
platinum complexes. The products were purified by column
chromatography and recrystallization. 1H NMR spectroscopy
and MALDI spectrometry were used to characterize the
synthesized products. Most of these molecules were isolated
as a mixture of the cis and trans isomers with respect to the
relative orientation of the two cyclometallating ligands,
except for molecule 7, which was only present as its cis
isomer, as suggested by 1H NMR spectroscopy.[4b,6] We
performed unrestricted Kohn–Sham density functional
theory (DFT) calculations to study the ground-state and
first triplet excited-state PESs of these
The chemical structures of seven pyrazolate-bridged
platinum binuclear complexes (major isomers), which can
also be considered as molecular butterflies, are shown in
Scheme 1. The syntheses of these molecules followed the
molecules.[8] Our approach is similar to
that of Sakaki et al. on molecules 1 and
3.[9] The product compositions for each
molecule can be explained by comparing
the DFT ground-state energies of the differ-
ent isomers. A small energy difference of
approximately 0.006 eV was observed for
the isomers of molecules 1–6, which is
consistent with the formation of products
1–6 as mixtures of isomers. On the other
hand, a relatively large energy difference
was calculated for the isomers of 7, that is,
the cis form is 0.189 eV lower in energy than
the trans form, which is consistent with the
sole formation of the cis isomer.
All seven molecules are light-yellow
solids. Their absorption spectra in dichloro-
methane (DCM) solution at room temper-
ature are of similar shapes and intensities
(Figure 1a). The lowest structured absorp-
tion at around 465 nm can be assigned to the
spin-forbidden mixed ligand center/metal-
to-ligand charge transfer (3LC/MLCT) tran-
sition, suggesting little to no Pt–Pt metal-
metal-to-ligand charge transfer (MMLCT).
In other words, all of these molecular
Scheme 1. Chemical structures of pyrazolate-bridged binuclear platinum complexes with
difluorophenylpyridine (dfppy) based ligands as the butterfly wings and pyrazolate (PZ)
ligands as the butterfly body.
Figure 1. a) Absorption spectra of molecules 1–7 in DCM solution at room temperature, the dashed lines represent a 100-fold magnification of
the solid lines. b) i,ii) Normalized emission spectra of molecules 1–7 in the solid state (in PS, i) and in DCM solution (ii) at room temperature.
iii,iv) Photographs of molecules 1–7 in PS (iii) and in DCM solution (iv) under UV light.
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butterflies keep their wings spread in their ground states and
right after photoexcitation. This is consistent with the lowest
energy states on the calculated ground-state PESs featuring
long Pt–Pt distances (Supporting Information, Figure S4).
The emission spectra of all seven molecules in polystyrene
(PS) and in DCM solution (lex = 360 nm) are shown in
Figure 1b (i and ii); the corresponding photographs under UV
excitation are also shown (iii and iv). All seven molecules
show very similar greenish-blue emission in PS, as the
structures of the molecules and their dynamics in the excited
states are greatly constrained. Dual emission was clearly
observed for all molecules, apart from complex 7, in solution,
as the molecules can easily change their structures and their
excited-state dynamics are no longer constrained. It is note-
worthy that the emission behavior of all molecules is the same
at any excitation wavelength and independent of the solution
concentration. The photoluminescence quantum yields of the
seven molecules in DCM at room temperature were mea-
sured to be 1.92 (1), 2.75 (2), 2.84 (3), 1.32 (4), 1.36 (5), 2.18
(6), and 0.71% (7). By splitting the dual emission into
greenish-blue and red emission (Figure S1), we obtained
quantum yields of 0.41 and 1.51 (1), 0.19 and 2.56 (2), 0.10 and
2.74 (3), 0.74 and 0.58 (4), 0.28 and 1.08 (5), 0.12 and 2.06 (6),
and 0.71 and 0% (7) for greenish-blue and red emission,
respectively (Table S1). When the bulkiness of the cyclo-
metallating ligand was increased through attaching methyl
and tert-butyl groups to the 2-(2,4-difluorophenyl)pyridine
(dfppy) ligand, the red/blue emission ratio decreased. To
produce dual emission with higher red/blue ratios, we
increased the bulkiness of the pyrazolate bridge by replacing
simple pyrazole (PZ) with 3-methylpyrazole (PZMe) and 3,5-
dimethylpyrazole (PZDMe). It should be pointed out that the
introduction of methyl or bulky tert-butyl groups does not
significantly affect the polarity, the dipole moment, or the
electronic structure of the cyclometallating and bridging
ligands. The only parameter being changed is the steric bulk,
which affects the molecular space filling (Figure S5). Overall,
among the library of molecular butterflies with different
bodies (pyrazolate bridges) and wings (cyclometallating
ligands), molecule 3, with the bulkiest body (PZDMe) and
lightest wings (dfppy), emitted mostly red light; molecule 7,
with the lightest body (PZ) and bulkiest wings (tBu-dfppy),
emitted only greenish-blue light. Other molecules, which are
based on different combinations of bodies and wings, display
dual emission with different red/blue ratios. By means of
molecular engineering, we can thus precisely control the
luminescence properties of a family of molecular butterflies.
The dual emission of BFPtPZ (1) is due to the competition
between the two local minima on the first triplet excited-state
PES: a greenish-blue emission from the T1a excited state with
3LC/MLCT characteristics at a long Pt–Pt distance, and a red
emission from the T1b excited state with 3MMLCT character-
istics at a short Pt–Pt distance (Figure 2a).[3] The PSC can
therefore be considered as a chemical-reaction-like process
taking place on the first triplet excited-state PES, with T1a as
the reactant and T1b as the product. Considering the fact that
the greenish-blue emission and the red emission have almost
identical decay behaviors,[3] the PSC of BFPtPZ (1) is
believed to take place very rapidly after intersystem crossing,
Figure 2. Excited-state dynamics and potential energy surfaces of the
molecular butterflies. a) Schematic representation of the PSC process-
es, transitions between various electronic states, and dual emission. Ea
and Eb are the energy barriers, DET is the energy difference between
the T1a and T1b states, kPSC is the rate of PSC from T1a to T1b, k’PSC is the
rate of reverse PSC, kbr and krr are the radiative decay rates of the
greenish-blue and red emissions, and kbnr (without accounting for kPSC
)
and krnr (without accounting for k’PSC) are the non-radiative decay rates
for T1a and T1b. b) Calculated potential energy surfaces of the first
triplet excited state versus the Pt–Pt distance for the molecular
butterflies 1–7.
and the T1a and T1b states reach equilibrium while they emit
phosphorescence. Combining our DFT results, we attempted
to gain a more fundamental understanding of the excited-
state dynamics of these molecular butterflies. Precise control
over the dual emission of these molecular butterflies is
realized in two steps: 1) The PSC kinetics and excited-state
equilibria are controlled by precisely engineering the posi-
tions of the T1a and T1b states and the energy barriers
separating them on the first triplet excited-state PES; 2) a
theoretical connection between the energy barrier and the
dual-emission characteristics is made.
To precisely control the PSC processes and the dual
emission, a fine-tuning of the energy barrier Ea is required.
The T1a states of all molecules were shifted to the same energy
(Figure 2b). As a good approximation, the first triplet
excited-state PES of these molecules can be considered as
the superposition of the two parabolic PESs of T1a and T1b if
the position of the transition state as a function of the reaction
coordinate remains the same in this series of molecules.
Therefore, a nearly linear correlation between the energy
barrier Ea and the energy difference DET between T1a and T1b
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methyl substituents, the degree by which T1b is lowered
because of the bulky bridging ligand is similar to the degree
by which T1b increases in energy because of the bulky
cyclometallating ligand. Overall, the energy barriers and T1b
increase in the order of 3 < 6 < 2 < 5 < 1 < 4 < 7. These DFT
results are in line with our experimental findings: The red/
blue ratios of the dual emissions for molecules 1–7 decrease in
the order of 3 > 6 > 2 > 5 > 1 > 4 > 7. The bulky groups have
the same effects on the ground state (S0), that is, a bulkier
pyrazolate bridge shifts the lowest S0 state to a shorter Pt–Pt
distance (corresponding to T1a) and stabilizes S0 at a short
Pt–Pt distance (corresponding to T1b); bulkier cyclometallat-
ing ligands have little to no impact on the S0 state at long Pt–
Pt distances (corresponding to T1a), but destabilize S0 at short
Pt–Pt distances (corresponding to T1b). As different bulky
groups shift the singlet ground-state and triplet excited-state
PESs by approximately the same small amount of energy, we
observed similar peak positions for the dual emissions of the
different molecules. It is important to keep in mind that the
intensity ratio between the red and greenish-blue emissions is
determined by PSC.
To quantitatively describe how the potential energy
surface and PSC control the dual-emission spectra in solution,
we established the correlations between several emission
characteristics, namely the red/blue ratios, PSC rates, and
energy barriers. As the excited states can rapidly reach
equilibrium at T1a and T1b after photoexcitation when the
energy barriers are small (Figure 2a), the ratio of the
populations of T1b and T1a is equal to kPSC/k’PSC. The red/
greenish-blue emission intensity ratio (red/blue) is equal to
[(kPSC krr)/(krr + krnr)]/[(k’PSC kbr)/(kbr + kbnr)], where krr and kbr
are the radiative decay rates of the red and greenish-blue
emissions, and krnr and kbnr are the non-radiative decay rates
(without accounting for PSC) of the T1a and T1b states. As the
greenish-blue and red emissions have almost identical decay
curves (Figure S2), the red/blue ratio of the dual emission can
be considered to be proportional to the ratio of the PSC rates,
kPSC/k’PSC (see the Supporting Information). The correlations
between the PSC rates and the energy barriers can be
Figure 3. Theoretical modeling of the tuning of the energy barrier and
its correlation with the experimentally determined PSC rate. a) Bell–
Evans–Polanyi plots of the energy barrier (Ea) versus the energy
difference between T1a and T1b (DET). The inset illustrates the Bell–
Evans–Polanyi principle. b) Dependence of the experimentally deter-
mined red/blue luminescence intensity ratios in solution on the energy
difference between T1a and T1b (DET).
states is expected (Figure 3a) because of the Bell–Evans–
Polanyi principle.[5] Consequently, a desired energy barrier Ea
can simply be obtained by adjusting the relative energies of
T1a and T1b by molecular engineering. DFT calculations show
that the steric bulk of the pyrazolate bridging ligands
influences both the T1a and T1b states. As the steric bulk of
the bridging ligands increases, the T1a state (3LC/MLCT)
shifts to slightly shorter Pt–Pt distances and the T1b state
(3MMLCT) is stabilized to lower energies with slightly more
localized Pt–Pt molecular orbitals. This is due to the fact that
the bulky groups attached to the bridging ligands can force
the two butterfly wings closer together in the T1a state and
confine the Pt–Pt molecular orbitals in the T1b state by space
filling. On the other hand, the steric bulk of the cyclo-
metallating ligand has little to no impact on the T1a state,
because the cyclometallating ligands are far away from each
other with almost no interaction between the attached methyl
or bulky tert-butyl groups when the butterfly wings are open.
However, when the butterfly wings get closer to each other at
shorter Pt–Pt distances, bulkier cyclometallating ligands will
destabilize the T1b state to higher energies with slightly
stretched Pt–Pt molecular orbitals. Overall, the steric bulk
affects the molecular space filling and the energy of the
molecular orbitals, especially the Pt–Pt dz2 anti-bonding
orbitals in the T1b state (Figure S5). Interestingly, with
¼ AeÀE =ðRTÞ and
[10]
a
described by the Arrhenius equation,
k
PSC
0
k PSC ¼ AeÀE =ðRTÞ, where A is the pre-exponential factor, Ea is
the energy barrier going from T1a to T1b, Eb is the energy
barrier going from T1b to T1a, and R is the universal gas
constant. Based on this analysis, we expect an exponential
correlation between the red/blue ratios and the energy
difference (DET) between T1a and T1b. An Arrhenius plot is
given in Figure 3b, which clearly validates our simple scheme
for the design of the excited-state PES to control the PSC and
excited-state equilibrium, which consequently determine the
dual emission. This dependence of the emission spectra on the
molecular structures clarifies why simple pyrazole-bridged
binuclear platinum complexes with other ligands, such as
2-(2’-thienyl)pyridine or 2-phenylpyridine, do not show dual
emission.[2q,r,3,4] These complexes, based on cyclometallating
ligands with a lower energy than dfppy, have a lower T1a, but
a similar T1b state, resulting in a larger energy barrier Ea,
which prevents PSC from T1a to T1b.
b
In summary, we have demonstrated how molecular
engineering can be employed to achieve precise and wide-
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Phenom. 2001, 119, 161 – 174; f) L. X. Chen, W. J. H. Jager, G.
range control over the dual emission of a library of
phosphorescent molecular butterflies based on pyrazolate-
bridged binuclear platinum complexes. Our approach relies
on the fine-tuning of the energy barrier for the photoinduced
structural change on the excited-state potential energy sur-
face, based on the Bell–Evans–Polanyi principle. A quantita-
tive description of the precise manipulation of the energy
barrier for the control of the photoinduced structural change
and the dual emission has been established by treating
photoinduced structural change as a chemical reaction on the
excited-state potential energy surface from the initial wing-
spread state to the final wing-folded state. Complementary
experimental and computational efforts significantly
advanced our fundamental understanding of these processes.
It was highlighted how the excited-state dynamics of photo-
active molecular systems could eventually be controlled, and
the feasibility of using computational predictions to guide the
development of materials with desired properties in a rational
manner was demonstrated. Detailed studies of the excited-
state kinetics of these molecular butterflies using ultrafast
spectroscopic methods and investigations on their applica-
tions as molecular sensors and machines are underway.
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We acknowledge the Florida State University for financial
support through the Energy and Materials Initiative. L.Z.
acknowledges
the
National
Science
Foundation
(CHE1213574). We thank Dr. Kenneth Hanson for helpful
discussions.
Keywords: dual emission · excited states · molecular dynamics ·
photochemistry · platinum complexes
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Received: June 7, 2015
Revised: June 25, 2015
Published online: July 21, 2015
Angew. Chem. Int. Ed. 2015, 54, 9591 –9595
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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