A Photoexcitation-Induced Twisted Intramolecular Charge Shuttle

Article abstract of DOI:10.1002/anie.201902766

Charge transfer and separation are important processes governing numerous chemical reactions. Fundamental understanding of these processes and the underlying mechanisms is critical for photochemistry. Herein, we report the discovery of a new charge-transfer and separation process, namely the twisted intramolecular charge shuttle (TICS). In TICS systems, the donor and acceptor moieties dynamically switch roles in the excited state because of an approximately 90° intramolecular rotation. TICS systems thus exhibit charge shuttling. TICSs exist in several chemical families of fluorophores (such as coumarin, BODIPY, and oxygen/carbon/silicon–rhodamine), and could be utilized to construct functional fluorescent probes (i.e., viscosity- or biomolecule-sensing probes). The discovery of the TICS process expands the current perspectives of charge-transfer processes and will inspire future applications.

Full text of DOI:10.1002/anie.201902766

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Accepted Article
Title: Photoexcitation Induced Twisted Intramolecular Charge Shuttle
(TICS)
Authors: Weijie Chi, Qinglong Qiao, Richmond Lee, Wenjuan Liu, Yock
Siong Teo, Danning Gu, Matthew J. Lang, Young-Tae Chang,
Zhaochao Xu, and Xiaogang Liu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902766
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Angewandte Chemie International Edition
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Photoexcitation Induced Twisted Intramolecular Charge Shuttle
TICS)
(
Weijie Chi , Qinglong Qiao , Richmond Lee, Wenjuan Liu, [b] Yock Siong Teo, Danning Gu,
+
[a]
+ [b]
[a]
[c]
[c]
Matthew John Lang,[c],[d] Young-Tae Chang,* [e],[f] Zhaochao Xu,* [b] and Xiaogang Liu* [a]
Abstract: Charge transfer and separation are important processes
governing numerous chemical reactions. Fundamental
wavelength etc.) provide critical information on the
thermodynamics and kinetics of the charge transfer and
separation processes in the excited state, along with a plethora
of other important photo-physical and -chemical information.
Accordingly, studies have been conducted to investigate charge-
understanding of these processes and the underlying mechanism is
critical for photochemistry. Herein we reported the discovery of a
new charge transfer and separation process, namely twisted
intramolecular charge shuttle (TICS). In TICS systems, the donor
and acceptor moieties dynamically switch roles in the excited state
as a result of ~90° intramolecular twisting, thus exhibit a “charge
shuttle” phenomenon. We showed that TICS existed in several
chemical families of fluorophores (such as coumarin, BODIPY, and
oxygen/carbon/silicon-rhodamine), and could be utilized to construct
functional fluorescent probes (i.e., viscosity or biomolecule probes).
The discovery of the TICS process expands the current perspectives
of charge transfer processes and will inspire future applications.
transfer
and
separation
mechanisms
in
fluorescent
compounds.[ It has been widely known that the intramolecular
charge transfer (ICT) could be modulated via adjusting the
“push-pull” effect in a compound consisting of an electron-
donating group (EDG) and/or an electron-withdrawing group
(EWG).[ In such compounds, ICT is further enhanced upon
photoexcitation. In particular, in a landmark study, Grabowski
and co-workers proposed that quasi-rigid Donor-Acceptor (D-A)
compounds could undergo ~90° intramolecular twisting in the
excited state, greatly reinforcing charge transfer and resulting in
6]
7]
+
- [8]
a charge separated state (D -A ). They named this mechanism
as twisted intramolecular charge transfer (TICT; Figure 1a).
Rationalization of the TICT mechanism[9] has greatly facilitated
Photo-induced charge transfer and separation is one of the
fundamental processes,[1] responsible for photosynthesis[2] and
applications in solar cells,[3] photocatalyst,[4] and fluorescence
probes.[5] Greater understanding of charge transfer and
separation processes is thus important to aid in improving
photochemistry. However, owing to fast photon-absorption rates
and short exited state lifetime, understanding charge transfer
and separation at a molecular level remains a significant
challenge.
the development of many functional materials and devices, such
as bright and photostable fluorophores,[10] dark quenchers,[11]
viscosity sensors[12] and polarity sensors.[6b] Notably, charge-
transfer and separation processes in both the ICT and TICT
states are unidirectional, i.e., from the donor (D) to the acceptor
(
A) upon photoexcitation.
In this study, we report a bidirectional charge transfer process
This challenge could be overcome if the model systems under
study emit fluorescence (or other forms of luminescence).[1b] The
change in fluorescence output (i.e., intensity, lifetime, and
in fluorophores, whereby the roles of the donor and acceptor are
dynamically reversed to allow charge “shuttle” after
photoexcitation and ~90° intramolecular twisting (Figure 1a). The
proposed mechanism is named twisted intramolecular charge
shuttle (TICS), and we demonstrate that TICS is a general
charge transfer mechanism, predicted in a broad range of
[
a]
Dr. W. Chi, Dr. R. Lee, and Prof. Dr. X. Liu
Science and Math Cluster
Singapore University of Technology and Design
fluorophores,
such
as
coumarins,
BODIPYs,
and
8
Somapah Road, Singapore 487372, Singapore
oxygen/carbon/silicon-rhodamines. We will also demonstrate
that TICS enables us to develop effective fluorescent probes.
We began this work by observing that 1 and 2 exhibited
considerably different quantum yields (φ = 92% for 1 and 9% for
E-mail: xiaogang_liu@sutd.edu.sg
Dr. Q. Qiao, W. Liu, and Prof. Dr. Z. Xu
CAS Key Laboratory of Separation Science for Analytical Chemistry
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
[
b]
457 Zhongshan Road, Dalian 116023, China.
2
in methanol), as the meso-substituent changed from an amino
E-mail: zcxu@dicp.ac.cn
group to a dimethylamino group (Figure 1b). To understand if a
correlation between the amino group and the quantum yields
exists, we conducted data searches for similar compounds in
chemical databases (Table S1). The searches returned results
with many BODIPY derivatives showing the same pattern, that is,
attaching a dialkylated amino group at the meso-position led to
substantially low quantum yields (especially in polar solvents).
Interestingly, further searches showed that this relationship
holds true in rhodamine dyes (Table S2), such as in compounds
[
c]
d]
Y. S. Teo, D. Gu, Prof. Dr. M. J. Lang
Singapore-MIT Alliance for Research and Technology (SMART)
1
CREATE Way, Singapore 138602, Singapore.
[
Prof. Dr. M. J. Lang
Department of Chemical and Biomolecular Engineering and
Department of Molecular Physiology and Biophysics
Vanderbilt University
Nashville, TN 37235, United States.
Prof. Dr. Y. T. Chang
Center for Self-Assembly and Complexity
Institute for Basic Science (IBS)
Pohang 37673, Republic of Korea.
Prof. Dr. Y. T. Chang
[
e]
f]
2 2
3 (φ = 85%) and 4 (φ = 11% in CH Cl ; Figure 1b). This trend led
[
us to suspect that the low quantum yields in 2 and 4 were due to
N-C bond rotations of the dialkylated amino groups, as those in
the TICT mechanism.
Department of Chemistry
Pohang University of Science and Technology (POSTECH)
Pohang 37673, Republic of Korea
E-mail: ytchang@postech.ac.kr
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
[+
]
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Figure 1. (a) Schematic illustration of the twisted intramolecular charge transfer/shuttle (TICT/TICS) mechanisms; “D” and “A” denote electron-donating and
electron-accepting moieties, respectively. (b) Representative TICS compounds and their quantum yields (φ). (c) Optimized molecular structures, LUMO, HOMO,
and relative energy levels of the ground (S
0 1
) and excited (S ) states during light absorption (FC, upward arrows), emission (LE, downward arrows), and the TICS
state of 2 in ethanol. The energy levels of 2 was computed using the linear solvation method. Calculated potential energy surfaces of (d) 2 and (e) 1 in the S
1
state in ethanol. (f) Molecular structure and crystallographic asymmetric unit of 5 at room temperature with anisotropic displacement ellipsoids drawn at the 50%
probability level. Fluorescence intensity changes of (g) 5 and (h) 1, as a function of viscosity in the mixture of ethylene glycol (E) and glycerol (G). [1] = 10 μM; [5]
=
10 μM; excitation wavelength: 380 nm. (i) Transient absorption spectra of 5 in chloroform. Decay dynamics of the transient absorption spectra of 5 at (j) 485 nm
and (k) 555 nm.
Next, we employed quantum chemical calculations to
understand the molecular origins of the low quantum yields in 2
Figures 1c and S1—S10). It is noted that the dimethylamino
group at the meso-position of 2 was partially positively charged
+0.36e) in the ground state (Figure S5) and acted as an
electron-withdrawing group (EWG) in the Franck–Condon (FC)
state during light absorption. For example, upon photoexcitation,
the electron-density of the dimethylamino group greatly
increases as substantial charge flows towards the meso-position
of BODIPY (Figure 1c). In the excited state, compound 2
(
(
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exhibited two stable conformations, and the most stable
conformation was exemplified by a ~90° rotation between the
dimethylamino group and the fluorophore scaffold. Moreover,
the dimethylamino group remains as an electron-withdrawing
group in the local excited (LE) state, similar to that in the FC
state (Figures S6 and S7). However, after the ~90° rotation, the
dimethylamino group switched role to become electron-donating
rotations. In other words, intramolecular rotations in TICT
compounds enhanced “charge transfer” but did not induce
“charge shuttle” (Figure S11). Moreover, TICT compounds
display
a positive solvatochromism, as charge transfer is
enhanced in the exited state. In contrast, TICS compounds often
exhibit a negative solvatochromism in UV-Vis absorption spectra,
as the amino group withdraws charge, and leads to a smaller
dipole moment upon photo-excitation (Tables S3 and S4,
Figures S11, and S25–S29).
To gain more insights into this distinct TICS mechanism, we
calculated the potential energy surface (PES) of 2 in the first
1
excited singlet state (S ) as a function of amino group rotations
(Figure 1d). This PES exhibited two stable conformations,
corroborating our previous results. Notably, the TICS
conformation is the global minima and the oscillator strength of
the TICS conformation is almost zero, indicating that TICS is a
dark state. The low quantum yield of 2 was thus ascribed to the
formation of the TICS state. In contrast, PES calculations on 1
(
Figures S8). Consequently, the electron-density of the
dimethylamino group substantially reduced in the excited state
as charge flowed back to the BODIPY scaffold (Figures 1c). In
other words, the ~90° rotation in 2 is accompanied by a “charge
shuttle” process, as the dimethylamino group switched role from
an EWG to an EDG. We thus term this process as twisted
intramolecular charge shuttle (TICS).
It is worth highlighting that the charge shuttle process clearly
differentiates 2 from the charge transfer in the TICT mechanism.
During TICT formation, a fluorophore (such as Coumarin 152;
Figure S11) also exhibited two stable conformations in the
excited state, and the TICT state was characterized by an
intramolecular rotation of ~90° between the fluorophore scaffold
and the amino group. However, the amino group remained as an
electron-donating group (EDG) both before and after molecular
showed no other minima, i.e., apart from the LE state in the S
1
(Figure 1e), and the quantum yield of 1 is high in the absence of
TICS.
Figure 2. (a) Optimized molecular structures, LUMO, HOMO, and relative energy levels of the ground (S
upward arrows), emission (LE, downward arrows), and the TICS state of 6 in ethanol; HOMO was obtained with corrections using the state-specific solvation.
Calculated potential energy surfaces of (b) 6 and (c) 7 in the S state in ethanol. (d) In-silico designed TICS compounds. (e) Viscosity dependence of emission
0 1
) and excited (S ) states during light absorption (FC,
1
intensities of 15 ([15] = 10 μM) in ethylene glycol (E) and glycerol (G) mixtures. (f) Reaction mechanism of 15 and GSH. (g) and (h) Confocal microscope images
of HeLa cells stained with 15 and Hoechst 33342 for 120 min ([15] = 5 μM; [Hoechst 33342] = 3 μM. In (h) cells were pre-treated with 1 mM NMM for 20 min to
remove GSH, followed by dye staining for 120 min. Blue Channel: excitation wavelength = 405 nm; emission filter band-path 420−470 nm; Red Channel:
excitation wavelength = 543 nm; emission filter band-path 570−620 nm. Scale bar = 20 μm.
To experimentally validate the TICS hypothesis in BODIPY
dyes, we next synthesized and characterized two BODIPY
derivatives, including 1 and 5 (Figure 1b, f). BODIPY 5 (a close
analogue of 2) exhibited low quantum yields (Table S3).
Molecular structure of 5 derived from its crystal structure (Figure
1f) also yielded excellent agreement with our DFT calculations
(Tables S7 and S8). Moreover, as we increased solvent
viscosity by introducing more glycerol in the ethylene glycol
/glycerol mixtures, we noticed considerable enhancement of
fluorescence intensities by up to ~6 times in 5 (Figures 1g).
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These observations indicated that the low quantum yields of 5
were related to molecular rotations. In contrast, BODIPY 1
exhibited high quantum yields (Table S4) and their emission
intensities were not sensitive to solvent viscosities (Figure 1h).
These results again corroborate our theoretical calculations.
We continued to perform transient absorption spectroscopy
measurements on 5 (Figures 1i and S38—S41). After photo-
excitation, our experimental data revealed the co-existence of
two excited states. The first excited state produced a simulated
emission band at ~480 nm, with a short decay lifetime of 3.0 ps
Finally, significant rotations of the dialkylated-amino moieties
and the associated fluorescence quenching in the TICS state
can be employed in many practical applications.[ Inspired by
the pioneering work of de Silva et al.,[5b] we are interested in
applying TICS compounds as fluorescent probes. Since the
shuttle mechanism involves rotations of molecules, they are
good candidates for quantifying solvent viscosities (such as by 5
and 15; Figures 1g and 2e). Since TICS compounds possess
low-polarity, they could be insensitive to solvent polarity, making
them dedicated viscosity probes.
14]
(
Figure 1j). We assigned this band to the LE state. The second
We also expected that replacement/removal of the amino
group at the meso-position could eliminate TICS and enhance
emission intensities. Indeed, the reaction between 15 and
glutathione (GSH) removed the meso-aniline group and turned
on bright red emissions peaked at 611 nm (Figure 2f). In
addition, our in vitro experiments showed that 15 was highly
selective to GSH (Figures S34—S37). Inspired by these
successes, we next proceeded to perform live cell experiments.
By staining HeLa cells with 5 μM Probe 15 and 3 μM Hoechst
33342 (a nucleus stain) for 2 h, we observed bright emissions
from both the red channel and the blue channel (due to the
presence of GSH and nucleus, respectively; Figure 2g). In
contrast, by pre-treating cells with 1 mM N-Methylmaleimide
(NMM) to remove GSH, we observed no red emission in this
control group, but only blue emissions from the nucleus (Figure
2h). These experiments demonstrated that TICS compound 15
is a promising GSH probe for both in vitro and in vivo
experiments. Interestingly, we also noted many existing NO and
phosgene fluorescent probes actually belonged to the TICS
family (Figures S23 and S24).
In conclusion, we have discovered a new charge transfer and
separation process, namely twisted intramolecular charge
shuttle (TICS), through chemical database searches, quantum
chemical rationalizations, and experimental validations. In TICS
compounds, the dialkylated-amino groups acted as an electron-
withdrawing group upon light absorption but became an
electron-donating group after the ~90° intramolecular rotation.
This unique role switching and “charge shuttle” process
differentiated TICS from the TICT mechanism. We also
demonstrated that TICS was applicable to a wide range of
chemical families of fluorophores and could be employed to
construct useful fluorescent probes. The discovery of the TICS
mechanism provides an expansive view on charge transfer and
separation processes existing in nature and will inspire potential
applications.
excited state showed an excited state absorption band with a
peak and a shoulder at 555 nm and 521 nm, respectively, and a
much longer decay lifetime of 78.3 ps (Figure 1k). Note that in
the TICS state, a negatively charged BODIPY radical will form
upon the complete charge transfer from the amino group to the
BODIPY scaffold. It was reported that this BODIPY radical has a
transient absorption spectrum at ~550—580 nm (Figure S42).[13]
This is in good agreement with our observation. We thus
attributed the second excited state to the TICS state. The
transition rate from the LE to the TICS states was estimated to
be 3.3×1011 s-1.
Encouraged by the consistency between theoretical and
experimental results, we next explored if the TICS model was
also applicable to oxygen-rhodamines (Figures 2a and S12—
S14). Our calculations showed that dialkylated amino group
substituted rhodamine 6 exhibited two stable conformation in the
1
S state, and the ~90° twisted conformation was the global
minimum (Figure 2b). Moreover, 6 clearly exhibited the “charge
shuttle” character: the amino group at the meso-position served
as an EWG in the FC and LE states but switched to an EDG in
the TICS state upon photoexcitation. Our calculations also
showed that the primary amino substituted rhodamine 7 is not
prone to TICS formation, as the twisted state is unstable in the
PES of 7 (Figure 2c).
We next synthesized 6 and 7 and measured their spectral
properties (Tables S5 and S6; Figures S30—S33). As
predicated by our theoretical calculations, the quantum yields of
6
was very low due to substantial TICS formations, while 7
emitted bright fluorescence. These results also suggested that
the TICS mechanism was not limited to BODIPY dyes but could
exist in other dye families.
As such, we computationally explored if TICS was applicable
to various families of fluorophores. Our theoretical calculations
showed that TICS was energetically favourable in coumarin 8,
alkoxy BODIPY 9, oxygen-rhodamine 10 and 11, carbon-
rhodamine 12, and silicon-rhodamine 13 (Figures 2d and S15—
S20). We also found that in TICS compounds, the amino group
is not limited to dialkylated amino groups, but can be extended
to aniline groups as well, such as in BODIPY 14 and rhodamine
Acknowledgements
WC, RL and XL were indebted to the financial support from
SUTD and the SUTD-MIT International Design Centre (IDC)
1
5 (Figures S21 and S22). Overall, our results demonstrated
that TICS represented a general charge transfer and separation
process, governing many different types of fluorophores.
Among these predicted TICS compounds, we chose to
synthesize 15. Indeed, as solvent viscosity increased, the
emission intensities of 15 were greatly enhanced (Figure 2e).
These results confirmed the rotational nature of the TICS
compound 15.
[
T1SRCI17126, IDD21700101, IDG31800104]. QQ, WL and ZX
were supported by NSFC (21878286, 21502189) and DICP
DMTO201603, TMSR201601). YST, DG and MJL were
(
supported by NRF Singapore through SMART Centre’s BioSyM
IRG research program. YTC was supported by Institute for Basic
Science (IBS) [IBS-R007-A1]. The authors would like to
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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Twisted intramolecular charge
shuttle (TICS): in TICS systems, the
donor and acceptor moieties
Weijie Chi, Qinglong Qiao, Richmond
Lee, Wenjuan Liu, Yock Siong Teo,
Danning Gu, Matthew John Lang,
Young-Tae Chang,* Zhaochao Xu,*
and Xiaogang Liu*
dynamically switch roles in the
excited state as a result of ~90°
intramolecular twisting, thus exhibit
a “charge shuttle” phenomenon. We
showed that TICS existed in several
chemical families of fluorophores,
and could be utilized to construct
functional fluorescent probes.
Page No. – Page No.
Photoexcitation Induced Twisted
Intramolecular Charge Shuttle
(
TICS)
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