Stimuli-Responsive Reversible Switching of Intersystem Crossing in Pure Organic Material for Smart Photodynamic Therapy

Article abstract of DOI:10.1002/anie.201905129

Photosensitizers (PSs) with stimuli-responsive reversible switching of intersystem crossing (ISC) are highly promising for smart photodynamic therapy (PDT), but achieving this goal remains a tremendous challenge. This study introduces a strategy to obtain

Full text of DOI:10.1002/anie.201905129

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Accepted Article
Title: Stimuli Responsive Reversible Switching of Intersystem Crossing
in Pure Organic Material for Smart Photodynamic Therapy
Authors: Wenbo Hu, Tingchao He, Hui Zhao, Haojie Tao, Runfeng
Chen, Lu Jin, Junzi Li, Quli Fan, Wei Huang, Alexander Baev,
and Paras N. Prasad
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10.1002/anie.201905129
Angewandte Chemie International Edition
RESEARCH ARTICLE
Stimuli Responsive Reversible Switching of Intersystem Crossing
in Pure Organic Material for Smart Photodynamic Therapy
Wenbo Hu,[a,b,c] Tingchao He,[d] Hui Zhao,[a] Haojie Tao,[a] Runfeng Chen,[a] Lu Jin,[a] Junzi Li,[d] Quli
Fan,* [a,e] Wei Huang,*[a,c,e] Alexander Baev,[b] and Paras N. Prasad*[b]
Abstract: Photosensitizers (PSs) with stimuli responsive reversible
switchable of intersystem crossing (ISC) is highly promising for
smart photodynamic therapy (PDT), but achieving this goal remains
a tremendous challenge. This study introduces a strategy to obtain
such reversible switching of ISC in a new class of PSs, which exhibit
stimuli initiated twisting of conjugated backbone. We present a
multidisciplinary approach that includes femtosecond transient
absorption spectroscopy and quantum chemical calculations to show
remarkably enhanced ISC efficiency (ФISC), switching from nearly 0
to 90%, via increasing the degree of twisting, providing an innovative
mechanism to promote ISC in these organic structures. This leads
us to propose here and demonstrate the concept of smart PDT,
where pH-induced reversible twisting maximizes the ISC rate, and
thus enables strong photodynamic action only under pathological
stimulus (such as pH, hypoxia or enzyme). The ISC process is
turned off to deactivate PDT ability, when the PS is transferred or
metabolized away from pathological region.
Introduction
ISC is a radiationless transition from a singlet excited state of a
molecule to its triplet excited state, which underlies a variety of
Figure 1. Schematic illustration of reversible switching of ISC and potential
applications in many diverse fields such as in PDT and
application in smart PDT. (a) Novel strategy to design of a POM with pH-
phosphorescence imaging as well as in photovoltaics and
photocatalytic reactions.[1] The current ISC-related studies
mainly focus on the ФISC enhancement and/or activatable ISC
reversible conjugated backbone twisting (CBT) for reversible switching of ISC.
(b) Only inside the tumor the ISC of smart photosensitizer (PS) can be
switched on to generate cytotoxic singlet oxygen (1O2) for PDT and switched
off when PS is transferred or metabolized away from the tumor region.
with single-time switching feature, because activatable ISC with
the enhanced ФISC improves efficiency and precision of
theranostics.[2] Further development of reversible ISC that allows
a repeatable and selective tuning of triplet excited state towards
certain stimulus is particularly valuable for theranostics
applications. It is of outmost importance, if reversibly switchable
ISC could be achieved in response to pathological stimulus,
such as pH, hypoxia or enzyme. This would greatly benefit the
development of more advanced PDT that is currently infeasible
by using activatable ISC. The underlying reason is that
reversibly switchable ISC has an ability to be turned off to
deactivate PDT action when a PS is transferred or metabolized
away from pathological region. This will help overcome the
general limitations plaguing the current PDT paradigm, such as
unavoidable damage to normal tissues and long lasting
cutaneous photosensitivity. To the best of our knowledge,
obtaining such reversibly switchable ISC remains a tremendous
challenge and no demonstration of such smart PDT therapy, as
proposed here, has been reported.
[a]
Dr. W. Hu, Dr. H Zhao, H Tao, Prof. R. Chen, L. Jin, Prof. Q. Fan,
Prof. W. Huang
Key Laboratory for Organic Electronics and Information Displays &
Institute of Advanced Materials (IAM), Jiangsu National Synergetic
Innovation Center for Advanced Materials (SICAM), Nanjing
University of Posts & Telecommunications, Nanjing 210023, China.
E-mail: iamqlfan@njupt.edu.cn
[b]
[c]
Dr. W. Hu, Dr. A. Baev, Prof. P. Prasad
Institute for Lasers, Photonics and Biophotonics and the Department
of Chemistry, University at Buffalo, State University of New York,
Buffalo, New York 14260, USA.
E-mail: pnprasad@buffalo.edu
Dr. W Hu, Prof. W. Huang.
Key Laboratory of Flexible Electronics (KLOFE) & Institute of
Advanced Materials (IAM), Jiangsu National Synergetic Innovation
Center for Advanced Materials (SICAM), Nanjing Tech University
(Nanjing Tech), Nanjing 211816, China.
E-mail: iamwhuang@njtech.edu.cn
Dr. T. He, J Li
College of Physics Science & Technology, Shenzhen University,
Shenzhen 518060, China.
Prof. Q. Fan, Prof. W. Huang
Shaanxi Institute of Flexible Electronics, Northwestern Polytechnical
University, Xi’an 710072, China.
[d]
[e]
The key challenge to achieve reversible switching of ISC lies in
the inability to shut off ISC after activation, because current
strategies to enhance ISC leave little room for other
functionalities. In addition, majority of materials with high ФISC
are noble metal complexes (such as iridium and platinum
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.201905129
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RESEARCH ARTICLE
Figure 2. (a) Molecular structure of PE1-4 and their corresponding ФISC. (b)
Normalized absorption (solid) and fluorescence (dash dot) spectra.
complexes), that generally suffer from intrinsic problems such as
high costs and metal-induced toxicity.[3] Considering the inherent
biocompatibility and low cost,[4] obtaining reversible switching of
ISC in pure organic materials (POMs) is more desirable for
practical applications. For an efficient ISC to exist in POMs, their
chemical structure typically requires incorporating specific
moieties: aromatic carbonyls, heteroatoms, or halogens.[5]
Aromatic carbonyls exhibit strong spin-orbit coupling (SOC) at
the carbonyl oxygen which allows for intrinsic ISC.[6] According
to the El-Sayed rule, presence of heteroatoms (such as nitrogen
or sulfur), can enhance SOC because of the involvement of
different type orbitals in radiationless transitions between singlet
and triplet manifolds, 1(n, π*) →3(π, π*) being a typical
example.[7] Halogens, which combine the benefits of aromatic
carbonyls and heavy atom effect, have also been widely used in
enhancing ISC. Clearly, rigid incorporation of these moieties into
conjugated backbone makes achieving reversible switching of
ISC especially challenging, despite the ever-growing demands in
reversible switching of ISC for the emerging applications.
In a ground breaking research reported here, we introduce the
concept of smart PDT, where we show by study of the structure-
property relationships in a series of p-phenyleneethynylene-
based derivatives (named PE1-4), that exceptionally enhanced
ISC can be effected by a twist in the conjugated backbone. We
present a design of a small POM (PE5) which exhibits reversible
switching of ISC through pH-reversible conjugated backbone
twist (CBT). Figure 1 schematically presents this design and the
concept of smart PDT which is pH-activatable.
Design and basic optical properties. Our previous studies
found that POMs with side-chain ammonium group could be
used as a highly-efficient photosensitizer for PDT,[8] which
means that these POMs may have considerable ФISC for efficient
1O2 generation. However, the underlying mechanism of highly
1
efficient O2 generation in these POMs so far remained unclear.
To reveal a relationship between the molecular structure and
ISC, a series of similar POMs (PE1-4) were designed and
synthesized (Figure 2a, Scheme S1). PE1 and PE2 were used
to study the ISC properties that were individually influenced by
carboxyl and ammonium salt. PE3 was used to validate the
interaction between carboxyl and ammonium salt, and PE4 was
used to verify the effect of side-chain length on this interaction.
The steady-state absorption and fluorescence spectra exhibited
similar absorption and emission bands owing to their
comparable conjugated backbone (Figure 2b). Both the
absorption and the PL spectra of PE1, PE3, and PE4 were
slightly red-shifted compared to those of PE2. This is because of
a
considerably extended conjugated system after the
introduction of carboxyl groups. All photophysical parameters
are presented in Table 1.
Table 1. Summary of key optical parameters for the POMs in this study.
[a]
[d]
[c]
[d]
λabs
λFL
tISC
kISC
POM
ФISC
1%
3%
83%
90%
(nm)
(nm)
(ps)
(×1010s-1)
PE1
PE2
PE3
PE4
380
356
370
356
433
408
440
414
197
87
1.3
1.0
0.5
1.1
77
Results and Discussion
100
[a] Lowest-energy absorption maximum wavelength. [b] Fluorescence
maximum wavelength. [c] ISC lifetime [d] ISC rate.
To get a solid evidence of the ISC process, we deciphered the
excitation dynamics of all samples using femtosecond transient
absorption (fs-TA) spectroscopy. Figure 3a shows 2D color-
coded fs-TA spectra of PE1-4 upon excitation at 350 nm, from
which we can identify the evolution of the primary excited state.
For all the samples, the initial signal was dominated by a
negative absorption band (white box). These negative
absorption bands should be attributed to stimulated emission
because their profiles are consistent with corresponding
fluorescence spectra (red dashed line at the bottom of Figure
3a). The variations in the position of stimulated emission were
caused by differences in the fluorescence onset for each POM
(Figure 2b). Any more detailed information associated with the
negative absorption bands is unattainable because of strong
instrumental noise. This is however not the key point of the
present study. Over time, PE1-4 showed a broad positive
absorption band that emerges quickly within 2 ps timeframe
(Figure 3a). This positive absorption band was assigned to
corresponding excited-state absorption (ESA). To distinguish
singlet ESA from triplet ESA, fs-TA plots at different delay times
were extracted from Figure 3a and presented in Figure S1 and
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RESEARCH ARTICLE
Figure 3. Experimental evidence of ISC. (a) 2D pseudo-colour fs-TA spectra of PE1-4 obtained with time delays from 0 to 2900 ps (350 nm, 0.3 mW). The
fluorescence spectrum of corresponding sample is presented in red dashed line to show the stimulated emission band. (b) fs-TA plots of PE1-4 at different pump-
probe delay times. Rising arrow represents population of triplet excited state while drop down arrow represents the depopulation of singlet excited state. The
presence of an isosbestic point (marked by black dotted circle) indicates the appearance of two different species, which usually represents the occurrence of ISC.
(c) Population dynamics of PE1-4, which illustrate the dynamics of the singlet excited state formation followed by the singlet to triplet transformation through ISC,
and triplet decay. Green ball and blue star represent the singlet and triplet-excited state population.
Figure 3b. Within a 2 ps timeframe (Figure S1), PE1-4 exhibited
red-shifted yet intensity-growing ESA, along the prolonged
delayed time, indicating the formation of new ESA species.
Upon further increase in delay time (Figure 3b), PE3 and PE4
exhibited two distinctive ESA bands, including a gradually
decreased peak around 535 nm and a continuously rising peak
around 580 nm. Dynamic decay trace of PE3 at 500 nm
revealed two decay components at 1.1 ps and 1216 ps (Figure
3c). The appearance of these long-lived components is
consistent with the 1.78 ns fluorescence lifetime obtained by
time-correlated single photon counting (Figure S2). Therefore,
the former peak of PE3 around 535 nm was assigned to singlet
ESA. The peak around 535 nm almost totally decays within 1 ps
(Figure 3a), while the peak around 580 nm lived up to 1000 ps
(Figure S3). Considering that a triplet excited state has a longer
lifetime than a singlet excited state, it is reasonable to assign the
latter peak around 589 nm to the triplet ESA. With same method,
in Figure 3a, we figured out the singlet (green box) and the
triplet (black box) ESA in PE1, PE2, and PE4. Notably, the
occurrence of an obvious isosbestic point in PE3 and PE4 is of
particular interest. The positive nature of the isosbestic point is
usually an indication of a singlet-to-triplet ISC process.[9] Overall,
these results presented a solid evidence for ISC in both PE3 and
PE4.
Quantification of ISC. To evaluate ФISC we analyzed the singlet
and the triplet population dynamics of PE1-4, which is presented
in Figure 3c. The triplet ESA around 580 nm for PE3 and PE4
increased abruptly within 1 ps (Figure 3c), and remained almost
unchanged from 10 to 100 ps (Figure 3a). This phenomenon
indicates that ISC in PE3 and PE4 occurred quickly on a < 10 ps
timescale. In sharp contrast, PE1 and PE2 exhibited
a
considerably longer singlet decay and ISC process on 100 ps
time scale, indicating an unfavorable ISC in PE1 and PE2. It is
well-documented that the dynamic growth trace of triplet ESA
represents its ISC lifetime.[10] Take PE4 as an example, the
dynamic growth trace of PE4 at 589 nm showed that the triplet
excited state is populated within 0.98 ps, and the rate of ISC
(kISC) is calculated to be 1.0×1012 s-1 (1/kISC). Dynamic decay
trace of PE4 at 535 nm revealed a fast singlet decay with the
lifetime of 0.9 ps. The efficiency of ISC (ФISC=[1/t (T1, rise)]/[1/t
(S1, decay)]) was estimated based on previous studies.[11] As
shown in Table 1, PE4 has a maximum ФISC of 90%, which is a
little higher than that of PE3 (83%), and almost two orders of
magnitude greater than that of PE1 (1%) and PE2 (3%). From
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10.1002/anie.201905129
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RESEARCH ARTICLE
these experimental results, it is reasonable to deduce that (i) the
carboxyl group or ammonium salt alone has negligible effect on
the enhancement of ФISC in p-phenyleneethynylene-based
conjugated system, and (ii) the interaction of carboxyl and
ammonium salt groups is responsible for the exceptionally
enhanced ФISC in PE3 and PE4. Notably, ФISC for PE4 is the
largest among the ФISC values reported for organic compounds,
which is tremendously promising for triplet-state-based
applications.
In addition, this study has also shed light on the mechanisms of
switchable/reversible ISC: (i) Comparing the ΦISC of PE1 with
that of PE3 or PE4, the mechanism that facilitates ISC originated
from aromatic carbonyls which are put into specific conditions,
where aromatic carbonyl oxygen and π-conjugated system are
not in the same molecular plane. (ii) Negligible ΦISC in PE2
excluded the effect of the external heavy atom (bromine) on ISC,
although external heavy atom effects have been observed in
deuterated solvent (such as deuterated methanol and water).[12]
Theoretical verification of CBT-induced ISC. To elucidate the
mechanism of ultra-efficient ISC in PE3 and PE4, we carried out
time-dependent density functional theory (TD-DFT) calculations
of the singlet and the triplet excited states. According to the
and greater spin-orbit coupling (SOC) constants facilitate higher
[1c, 13]
ФISC
.
On the basis of simple energetic arguments, the
increased number of Tn states with small ΔEST (± 0.37 eV) that
was found in PE3 and PE4 (Figure 4a and Table S1-4), resulted
in a high ISC rate.[14] Combining the energetic arguments and
the analysis of SOC constants computed by means of the Dalton
electronic structure program,[15] the feasible S1 → Tn channels
were identified (Figure 4a and Tables S5-8). Clearly, the
increased number of potential S1 → Tn channels and several
orders of magnitude larger SOC constants in PE3 and PE4
theoretically predict their ultrahigh ФISC. The larger SOC
constant in PE4 theoretically accounts for its higher ФISC relative
to PE3.
Furthermore, the molecular geometries of PE1-4 were studied to
gain an insight into the underlying mechanism responsible for
the ultrahigh ФISC. As shown in Figure 4b and Figure S4, both
PE3 and PE4 display a large torsion angle between central
benzene ring and the adjacent benzoate groups, indicating a
significant CBT, while PE1 and PE2 display nearly planar
structure. The twisted geometry in PE3 and PE4 is attributed to
the electrostatic attraction between the positively-charged
quaternary ammonium salt and the negatively-charged carboxyl
moiety. From these experimental and theoretical insights, it was
perturbation theory, a smaller singlet-triplet energy gap (ΔEST
)
Figure 4. TD-DFT study of the CBT-dominant ISC. (a) Schematic diagrams showing the computed energy levels and the probable ISC channels from the S1 state
to higher- or lower-lying triplet states (Tn). The triplet states in gray region are those levels with energy higher than ES1 + 0.37 eV or lower than ES1 - 0.37 eV. The
triplet states marked in green contain the same transition configuration compositions as S1. Average spin-orbit coupling (SOC) constants between S1 and the
involved triplet states (the larger SOC, the higher ISC possibility) are also shown. (b) Optimized molecular geometries. Electrostatic interaction between positively-
charged side-chain ammonium salt and negatively-charged terminal carboxyl group twists the conjugated backbone. The CBT was evaluated roughly by the
torsion angle between central benzene moiety and adjacent benzoate groups. (c) Experimental validation of CBT-enhanced ISC. Flatting the twisted structure of
PE4 into planar one to switch off its ISC via host-guest complexion in which negative-charged water-soluble pillar[5]arene (WP5) shields the positively-charged
ammonium salt and thus disrupts the electrostatic interaction.
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10.1002/anie.201905129
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RESEARCH ARTICLE
reasonable to attribute the ultrahigh ФISC in PE3 and PE4 to the
twisted conjugated backbone.
radiationless transition involves a change in orbital type.[17] The
out-of-plane (molecular plane) π-type orbitals in a planar
geometry give the major contribution to the S1 and the closely
lying T2 and T3 states (Figure S6). Thus, the orbital angular
momentum operator cannot efficiently couple these states,
leading to small values of SOC constant. Meanwhile, in the
twisted geometry ISC is enhanced because of an in-plane
projection of π-character orbitals, giving the major contribution to
Demonstration of CBT-enhanced ISC. The CBT-induced ФISC
was theoretically confirmed by TD-DFT calculations (Figure S5),
in which the simplified conjugated backbone was extracted and
the electronic structure calculations for planar and twisted
geometries were carried out. In planar geometry ISC between S1
and Tn is suppressed owed to a large ΔEST between S1 and T1,4
,
and small SOC constant between S1 and T2,3. The EI-Sayed
rule[16] states that the rate of ISC is considerably large if the
T2
and
T3
states.
Figure 5. pH-reversible ISC and intracellular pH-activatable PDT. (a) Schematic illustration of pH-reversible structure. (b) pH-reversible ISC, which was
demonstrated by triplet excited state-induced assumption of ADMA. (c) Schematic illustration of the photophysical and photochemical processes taken place in
PE5-mediated PDT. After ISC, the excited T2,3,4 states decay back to T1 through internal conversion. The energy in T1 is then transferred to molecular oxygen to
generate 1O2 for PDT. (d) 1O2 emission generated by PE5 in D2O with different pH value. (e) Micrographs of intracellular 1O2 generation in HeLa cells. Scale bar:
40 μm. The non-fluorescent 1O2 probe can be transferred into green fluorescence by 1O2. 5,10,15,20-Tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-
toluenesulfonate) (TMPyP4) is a frequently used PS for PDT and here as a comparison to highlight the pH-activatable ability of PE5. (f) pH-activatable PDT of PE5.
Scale bar:160 μm. The green color represents live cells, and the red color represents dead cells.
The in- and out-of-plane orbitals can now be coupled (Figure S6), demonstrated the critical contribution of CBT to the exceptionally
resulting in a larger SOC constant. In addition, the S0→S2
transition is forbidden in the planar geometry, but weakly
allowed in the twisted geometry, opening up a potential S2→T4
ISC channel.
enhanced ΦISC.
pH-reversible ISC in POM. The availability of CBT-enhanced
ISC offers unique opportunities to construct reversibly
switchable ISC. Following this principle, we designed and
synthesized a small inner salt-type POM (named PE5) with
optimized isoelectric point of pH 6.0 (Figure 5a and Figures
S9,10), which enables a significant twisting at pH 6.0 for high
ΦISC while a slight twist in normal biological environment (pH 7.4)
with very low or negligible ΦISC. This is because PE5 exists in a
zwitterionic form at pH 6.0 (Figure S9) in which electrostatic
interaction results into a significant CBT leading to high ΦISC. At
pH 7.4, PE5 exists mainly in anionic form losing the electrostatic
interaction to drive the twisted geometry. The ISC on/off at pH
6.0 and 7.4 was experimentally confirmed by a typical triplet
states-induced photodegradation of anthracene-9,10-diyl-bis-
methylmalonate (ADMA) in the presence of PE5 and molecular
This CBT-enhanced ФISC was further experimentally validated by
host-guest complexion method. It is principally feasible to
decrease ФISC through flattening the twisted conjugated
backbone into a planar one. To validate this assumption, the
positive-charged quaternary ammonium salt in PE4 was
threaded into the cave of the negative-charged water-soluble
pillar[5]arene (WP5) via host-guest complexion, to flatten the
conjugated backbone (Figure 4c, Figure S7), because of the
repulsive electrostatic interaction.[18] The remarkably-decreased
ΦISC in as-prepared complex (WP5⊃PE4) from 90% to 0%
highlighted the CBT-enhanced ФISC (Figure S8). Taken together,
the computational and experimental data unambiguously
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10.1002/anie.201905129
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RESEARCH ARTICLE
oxygen (Figure 5b). The distinct consumption of ADMA
demonstrated the ISC on at pH 6.0 and ISC off at pH 7.4. Most
Organic Electronics and Information Displays, the Natural
Science Foundation of Jiangsu Province of China (No.
BK20171020), the China Postdoctoral Science Foundation (No.
2018T110488), the Shenzhen Basic Research Project of
notably, PE5 exhibited
a
pH-reversible ISC for smart
applications requiring switchable ISC (Figure 5b), which was
unobtainable in previous studies. In addition, the excellent
photostability and moderate absorption of PE5 was also
demonstrated in Figure S11 and 12, highlighting their practical
application in PDT.
Science
and
Technology
under
Grant
(No.
JCYJ20170302142433007), open research fund of Key
Laboratory for Organic Electronics and Information Displays,
and Synergetic Innovation Center for Organic Electronics and
Information Displays.
pH-activatable PDT. Given the pH-reversible ISC, PE5 was
further utilized as a smart organic PS for the proof-of-concept
application in pH-activatable PDT (Figure S13). It is theoretically
conceivable that PE5 can only produce 1O2 in tumor
microenvironment (acid) due to the switch-on of ISC at pH 6.0.
Conflict of interest
1
As anticipated, characteristic O2 emission of PE5 at 1270 nm
The authors declare no conflict of interest.
1
demonstrated its pH-activatable O2 generation (Figure 5c). The
1O2 quantum yield (Figure S14) of PE5 was calculated to be
48% at pH 6.0 and 5% in a neutral environment (pH = 7.4). We
stimulated the tumor microenvironment by adjusting the pH
Keywords: intersystem crossing • reversible switching • pure
organic material • smart photodynamic therapy • ultrafast
spectroscopy
1
value of a culture solution.[19] Intracellular O2 imaging in Figure
5d demonstrated the pH-activatable 1O2 of PE5. In contrast,
commercial PS, TMPyP4, exhibited undifferentiated 1O2
generation, which causes unwanted damage to healthy cells or
tissues. Finally, the in vitro PDT effect of PE5 was evaluated by
assessing the cellular phototoxicity, in which green-emissive
calcein AM (living cell) and red-emissive propidium iodide (PI,
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dead cell) cellular viability kit was used to distinguish the dead
20]
cells from the live cells (Figure 5e).[8b,
Compared with the
unselective damage of TMPyP4, PE5 possesses outstanding
selectivity for the potential application in more precise
photodynamic therapy of tumor cells. We would like to mention
here that the application of PE5 to PDT in vivo may be limited by
the fact that the excitation wavelength does not fall into the
biological transparency window (750-900 nm). However, there
are many plausible ways to overcome this hindrance via, for
instance, using the upconverting nanophosphorsor even in situ
photon upconversion in tissue and two-photon excitation in near-
infrared region.[20-21]
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Conclusion
J. Z. Zhao, W. H. Wu, J. F. Sun, S. Guo, Chem. Soc. Rev. 2013, 42,
5323-5351.
We have presented experimental demonstration and theoretical
verification of CBT-enhanced ISC in p-phenyleneethynylene-
based POMs, which not only reveals a concise yet reliable
approach to design novel POMs with high ΦISC but also enables
a deeper understanding of ISC switchability. Moreover, easily-
tuneable ISC not only allows an advanced PDT paradigm with
precision not currently achievable by available PS, but also
opens up fascinating opportunities for triplet-state-related
applications such as in switchable photochemical synthesis.
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RESEARCH ARTICLE
A strategy to obtain reversible
switching of ISC in a new class of
organic small molecular
photosensitizers by stimuli initiated
twisting of conjugated backbone for
Wenbo Hu, Tingchao He, Hui Zhao,
Haojie Tao, Runfeng Chen, Lu Jin, Junzi
Li, Quli Fan,* Wei Huang,* Alexander
Baev, and Paras N. Prasad*
Page No. – Page No.
smart photodynamic therapy.
((Insert TOC Graphic here))
Stimuli Responsive Reversible
Switching of Intersystem Crossing in
Pure Organic Material for Smart
Photodynamic Therapy
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