Towards More Photostable, Brighter, and Less Phototoxic Chromophores: Synthesis and Properties of Porphyrins Functionalized with Cyclooctatetraene

Article abstract of DOI:10.1002/chem.202001804

Free base and zinc porphyrins functionalized with cyclooctatetraene (COT), a molecule known as a good triplet-state quencher, have been obtained and characterized in detail by structural, spectral, and photophysical techniques. Substitution with COT leads to a dramatic decrease of the intrinsic lifetime of the porphyrin triplet. As a result, photostability in oxygen-free solution increases by two to three orders of magnitude. In non-degassed solutions, improvement of photostability is about tenfold for zinc porphyrins, but the free bases become less photostable. Similar quantum yields of photodegradation in free base and zinc porphyrins containing the COT moiety indicate a common mechanism of photochemical decomposition. The new porphyrins are expected to be much less phototoxic, since the quantum yield of singlet oxygen formation strongly decreases because of the shorter triplet lifetime. The reduction of triplet lifetime should also enhance the brightness and reduce blinking in porphyrin chromophores emitting in single-molecule regime, since the duration of dark OFF states will be shorter.

Full text of DOI:10.1002/chem.202001804

Accepted Article
Accepted Article
Title: Towards more photostable, brighter, and less phototoxic
chromophores: Synthesis and properties of porphyrins
functionalized with cyclooctatetraene
Authors: Jacek Waluk, Jakub Ostapko, Aleksander Gorski, Joanna
Buczyńska, Barbara Golec, Krzysztof Nawara, Anastasiia
Kharchenko, Arkadiusz Listkowski, Magdalena Ceborska, and
Mariusz Pietrzak
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To be cited as: Chem. Eur. J. 10.1002/chem.202001804
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01/2020

10.1002/chem.202001804
Chemistry - A European Journal
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Towards more photostable, brighter, and less phototoxic
chromophores: Synthesis and properties of porphyrins
functionalized with cyclooctatetraene
Jakub Ostapko,*[a] Aleksander Gorski,[a] Joanna Buczyńska,[a] Barbara Golec,[a] Krzysztof Nawara,[a],[b]
Anastasiia Kharchenko,[a] Arkadiusz Listkowski,[a],[b] Magdalena Ceborska,[a] Mariusz Pietrzak,[a] Jacek
Waluk*[a],[b]
[a]
Dr. Jakub Ostapko, Dr. Aleksander Gorski, Dr. Joanna Buczyńska, Dr. Barbara Golec, Dr. Krzysztof Nawara, Dr. Anastasiia Kharchenko, Dr. Arkadiusz
Listkowski, Dr. Magdalena Ceborska, Dr. Mariusz Pietrzak, Prof. Jacek Waluk
Institute of Physical Chemistry
Polish Academy of Sciences
Kasprzaka 44/52, 01-224 Warsaw, Poland
j.waluk@ichf.edu.pl
[b]
Dr. Krzysztof Nawara, Dr. Arkadiusz Listkowski, Prof. Jacek Waluk
Faculty of Mathematics and Science
Cardinal Stefan Wyszyński University
Dewajtis 5, 01-815 Warsaw, Poland
Supporting information for this article is given via a link at the end of the document.
Abstract: Free base and zinc porphyrins functionalized with
cyclooctatetraene (COT), a molecule known as a good triplet state
quencher, have been obtained and characterized in detail by
structural, spectral, and photophysical techniques. Substitution with
COT leads to a dramatic decrease of the intrinsic lifetime of the
porphyrin triplet. As a result, photostability in oxygen-free solution
increases by 2-3 orders of magnitude. In non-degassed solutions,
improvement of photostability is about tenfold for zinc porphyrins, but
the free bases become less photostable. Similar quantum yields of
photodegradation in free base and zinc porphyrins containing the
COT moiety indicate a common mechanism of photochemical
decomposition. The new porphyrins are expected to be much less
phototoxic, since the quantum yield of singlet oxygen formation
strongly decreases because of shorter triplet lifetime. The reduction
of triplet lifetime should also enhance the brightness and reduce
Usually, the most important photophysical parameters that
influence photostability are the lifetime and energy of the lowest
triplet state T1. A molecule being in this state may participate in
photodegradation via electron or energy transfer processes or a
reaction with oxygen present in the environment. The longer the
triplet state lasts, the more likely these processes are. Radicals
formed in such a way lead to a cascade of reactions, resulting in
the dye decomposition and thereby the bleaching.
Numerous strategies for enhancing photostability have been
proposed.2-9 One of them is to decrease the triplet state formation
yield, e.g., by putting the chromophore in
a plasmonic
environment.10 We have shown that minimizing triplet formation
works for molecules that form hydrogen bonds with the solvent,
which leads to rapid depopulation of the lowest singlet state,
either by internal conversion or excited state double proton
blinking in porphyrin chromophores emitting in single molecule regime, transfer. In this way, generation of the triplet state is avoided and
since the duration of dark OFF states will be shorter.
the photodegradation quantum yield decreases by a factor of 200
in protic vs nonprotic solvents.11,12
For single molecule fluorescence studies in polymer films,
choosing an appropriate environment is crucial. It was
demonstrated that photodegradation quantum yields of terrylene
diimide vary by two orders of magnitude depending on the
polymer matrix.13 The highest photostability was observed for
polymers which exhibit the lowest oxygen mobility.
Definitely, the most popular strategy for improving
photostability is based on shortening the lifetime of the
photoreactive species or its precursor.2,6-8,14-26 For this purpose,
one can use either external agents or moieties chemically bound
to the chromophore (“self-healing chromophores”),8,21,26 thus
avoiding the diffusion limit.
In the context of emitter instability, seeking of photostable and
bright molecules seems to be crucial for obtaining satisfying time
and spatial resolutions in single molecule experiments. A general
strategy of using energy acceptors that quench the triplet state of
the emitter has been proposed in the ’70s and ’80s for stabilization
of laser dyes which are exposed to enormous flux of photons and
thereby may undergo photodegradation. A recent review gives a
Introduction
Since the introduction of the confocal fluorescence microscopy,
high spatial resolution imaging of structure and dynamics of
biological systems became possible. Microscopic techniques
employing fluorescence enable observation of processes on a
single-molecule level. This regime allows insight into the specific
environment of just one emitting molecule, which is in contrast to
the bulk experiments which average dye response over the
ensemble of observed chromophores. Still, the microscopic
techniques suffer from some limitations. The most important ones
are associated with high intensity of excitation, which causes
instability of the observed chromophore. It is manifested as
emitter bleaching or blinking. These signs of emitter instability
significantly hamper or even disable interpretation of experimental
results.1,2
1
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summary of triplet quenchers which have been tried so far.27 They
can be divided into two main classes. The first includes the
quenchers which operate by energy transfer (usually via Dexter
mechanism): cyclooctatetraene, cyclohexadiene, cyclohepta-
triene, 1,3-diketonate transition-metal chelates, KI, Ni2+). The
other class is based on redox processes. Here, such substances
have been used as Trolox, DABCO, combination of methyl
viologen-ascorbic acid, 4-nitrobenzyl alcohol, phenylenediamine,
β-mercaptoethanol, β-mercaptoethylamine, phenylenediamine,
n-propyl gallate, and sodium dithionite.
Among various tested laser dye additives, 1,3,5,7-
cyclooctatetraene (COT) was found to be a universal quencher,
stabilizing laser dyes of a broad scope of ground-excited state
energy gap.28,29 Nowadays, cyclootatetraene is considered as a
stabilizing agent of emitters used in imaging of biological
samples.2,5-8,20,22-25 Due to the diversity of commonly applied
bioimaging fluorophores and the required low cytotoxicity and
high photostability, the question of COT versatility arises. It has
been reported that the improvement in photostability strongly
depends on the nature of the chromophore.6
COT is a cyclic annulene possessing eight π electrons. This
number formally meets the requirement for antiaromaticity,
however the molecule itself is not antiaromatic in the ground state,
having nonplanar geometry of D2d symmetry due to Jahn-Teller
effect.30-33 The tub (D2d) shape in the ground state was elucidated
by gas-phase electron diffraction34-36 and by single-crystal X-ray
diffraction of the octamethyl derivative.37
At room temperature, COT undergoes tub shape inversion
and double bond migrations (Figure 1), as has been evidenced by
NMR experiments32,38,39 and other techniques.30,40 These
processes are thermally activated and require 10-13
kcal/mol30,32,39,40 and 2-4 kcal/mol,32,38,39 respectively. According
to calculations30,32,41 and experimental results,30 the transition
state of the D2d to D2d inversion is the planar structure possessing
D4h symmetry. Another process, delocalization of double bonds
and their subsequent shift requires planar geometry of D8h
symmetry. It was theoretically predicted42 and experimentally
evidenced30 that this transition state is an open-shell singlet,
violating the Hund’s rule.
The triplet state of COT obeys Baird’s rule, according to which
the 4N π electron annulene is aromatic in the excited state.43 The
aromaticity of D8h symmetry structure of COT has been predicted
theoretically. The calculated NICS(0) values for the T1 state of D8h
symmetry are highly negative (-12 ppm),44,45 which is indicative of
the presence of diatropic ring current, characteristic for aromatic
systems. The aromatic character of D8h symmetry S1 state with
NICS(0) of -8.87 ppm, antiaromatic character of D8h symmetry S0
state with NICS(0) of 40.7 ppm and nonaromatic character of D2d
S0 state with NICS(0)=1.16 ppm have been postulated.45 The
detailed discussion concerning aromatic vs antiaromatic
character of planar cyclooctatetraene can be found in a review
paper.33
These two factors: (i) conformational dynamics of the COT
ground state with D4h symmetry attributed to the maximum of the
ring inversion potential energy and (ii) the low energy of the
aromatic T1 state of D8h symmetry are the key factors making the
S0-T1 energy gap of cyclooctatetraene exceptionally low.
According to a theoretical model,41,46 the triplet state of COT
should be accessible by combination of nonvertical triplet-triplet
energy transfer from an aromatic donor with thermal activation of
vibrational modes involved in inversion. The nonvertical S0-T1
transition in COT is not subject to the Franck-Condon rule, which
results in effectively small S-T gap, making this molecule a
versatile energy acceptor.41 A similar mechanism of nonvertical
transition has been postulated for other floppy molecules, e.g.,
cis-stilbene47-49 and 2,3-diphenylnorbornene.49 The energy of
nonvertical S-T transition in COT has been estimated by triplet-
energy-transfer experiments as 41 kcal/mol.49 A more direct
methodology, relying on the photodetachment of electron from a
planar COT radical anion, allowed to determine the energy gap
between the D4h symmetry S0 state and the T1 state of D8h
symmetry to be equal to 12 kcal/mol.30 Combining these values
with the height of the ring inversion barrier measured with NMR
techniques,30,32,39,40 one may estimate the energy of S0 (D2d) – T1
(D8h) nonvertical transition as ca. 22 kcal/mol. Very important, the
small triplet-singlet energy difference implies that the product of
COT excitation via triplet-triplet energy transfer may not able to
transfer energy to the triplet (ground state) oxygen with 22.5
kcal/mol51 S-T energy gap.
Moreover, it was shown that the ability of COT derivatives to
support planar geometry and further bonds delocalization strongly
depends on the COT substitution pattern. The presence of
substituents impede conformational transition and bond shift
processes, increasing the S0(D2d)-S0(D8h) activation energy from
13.7 kcal/mol for unsubstituted COT38 to 17 kcal/mol for singly,32
22 kcal/mol for multiply substituted,30 and at least to 24 kcal/mol
for bicyclo[2.2.2]octene annelated derivatives,52 as evidenced by
NMR experiments.
The vertical process of S-T excitation requires the energy of
65-70 kcal/mol, as estimated by electron energy loss
spectroscopy53 and by calculations.54
For the development of the architecture of photostable
emitters, we propose in this work
a set of porphyrins
functionalized with COT, a unit possessing low energy triplet state.
This potential energy acceptor was covalently linked with
porphyrin in order to mimic the condition of high concentration of
triplet state quencher. Porphyrin was chosen as the energy donor
due to its well-known photosensitizing ability, high triplet state
population efficiency and the singlet-triplet energy gap of 37.6
kcal/mol,55 high enough to generate singlet oxygen.
Figure 1. Schematic representation of the potential energy of COT in the lowest
singlet and triplet states.
2
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The COT-porphyrin conjugates of rigid, and thereby well-
defined structure have been synthesized (Figure 2). The COT
subunit was attached at different geometries with respect to the
porphyrin core to establish the optimal distance and orientation
for efficient donor-acceptor energy transfer. We present the
synthesis of the conjugates in their free-base and zinc complex
forms and the results of preliminary spectral, photophysical, and
photochemical measurements.
porphyrin 6 with 51% yield. Porphyrin 1-Zn was obtained using
Suzuki coupling of porphyrin 6 and bromocyclooctatetraene 7.
Results and Discussion
Synthesis
Porphyrin-COT systems 1 and 2 were obtained by attachment of
the COT subunit to the free meso position of tritolylporphyrin 4. In
these cases, the COT moiety is placed on the side of the
porphyrin core. Porphyrin-COT 3 realizes a different geometry,
having the COT acceptor subunit attached via xanthene spacer
and located directly above the porphyrin macrocycle (Figure 2).
Scheme 1. Synthesis of porphyrins 1 and 2: a) B2(pin)2, Pd(PPh3)4, K2CO3, (n-
Bu)4NCl, THF, H2O, 50%; b) Pd(PPh3)4, CuI, NEt3, 90oC, 37%; c) Pd(PPh3)4,
CuI, NEt3, 90oC, 60%. Ar = p-tolyl; d) Pd(PPh3)4, K2CO3, THF, H2O, reflux, 59%.
The synthesis of bromocyclooctatetraene 7 (Scheme 2) was
carried out adopting the literature methodology.57 The 2-
methylbut-3-yn-2-ol derivative of COT (8) was obtained by
Sonogashira coupling of 7 and 2-methyl-3-butyn-2-ol (Scheme
2a). Subsequent acetylene deprotection under basic conditions
afforded formation of ethynylcyclooctatetraene 9 (Scheme 2c).
Figure 2. The synthesized COT-containing porphyrin derivatives and the
reference compounds. Ar = p-tolyl.
Scheme 2. Synthesis of COT derivatives and COT-halide derivative
7
rearrangement:58 a) Pd(PPh3)4, CuI, NEt3, 90oC, 86%; b) Pd(PPh3)4, CuI, NEt3,
120oC, 40%; c) KOH, i-PrOH, reflux, 80%, d)KOH, i-PrOH, reflux, 80%.
For the synthesis of 1 and 2 (Scheme 1), zinc complex of 5-
tritolylporphyrin 4-Zn was obtained adopting the Lindsey protocol
(See SI).56 Porphyrin 4-Zn was then brominated at the meso
position, giving porphyrin 5, which was used directly in the
Sonogashira coupling with ethynylcyclooctatetraene 9. This
reaction resulted in the formation of porphyrin 2-Zn with 62%
yield; the compound was demetalated under standard conditions,
giving free base porphyrin 2-H. Zinc complex of 5-bromo-
tritolylporphyrin 5-Zn was also used for the synthesis of porphyrin
1. The -B(pin) group was introduced with the palladium catalyzed
reaction of bromoporphyrin 5 and bis(pinacolato)diboron, giving
It has to be noted here that the temperature of Sonogashira
reaction has to be strictly controlled. According to the report by
Cope and Burg,58 followed by mechanistic studies of Huisgen,59
halide derivatives of COT undergo rearrangement, giving
corresponding derivatives of Z-stilbene. The rearrangement-
Sonogashira coupling (Scheme 2, path b) and the direct
Sonogashira coupling (Scheme 2, path a) are two competitive
processes. According to the experiment, formation of 10 as a
product of the rearrangement-coupling sequence was the
3
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dominant process (40% yield, the crude mixture contained
significant amount of decomposed material) when the reaction
was run at 120oC. Lowering the reaction temperature to 90oC
allowed to favor direct coupling process (Scheme 2, path a),
giving the product 8 with good yield of 86%. Only traces of the
rearrangement product were then observed in the crude mixture.
Porphyrins 1 and 2 possess the COT moiety placed on the
side of the molecule close to the meso position of the macrocycle
core. Another topology of the triplet donor-acceptor system,
orienting COT above the macrocycle, was realized by porphyrin
3. We decided to construct porphyrin 3 utilizing the building blocks
that already possess the xanthene subunit functionalized with
COT. For this purpose, formyl derivative of bromoxanthene 14
was subjected to the palladium catalyzed coupling with
ethynylcyclooctatetraene 9, giving product 16 with good yield
(Scheme 3a). Next, formyl derivative of 16 was used for the
construction of dipyrromethane motive by acidic condensation
with pyrrole (Scheme 3b). Adopting the Lindsey procedure,56 the
obtained compound 17 was used in the condensation with
dicarbinol derived from 1,9-di-toluoyl-5-tolyldipyrromethane tin
complex 18 by reduction with sodium borohydride. Upon oxidation
with DDQ the targeted porphyrin 3-H was obtained with good yield
(19%). This product was then transformed into zinc complex 3-Zn
under standard conditions.
1H NMR spectra of 1-H are strongly dependent on
temperature. The spectra registered at 293 K (Figure 3) reveal
two broad (δ=9.97-9.30 ppm) signals in the low field region
assigned to the two β-pyrrolic protons located in the vicinity of the
COT subunit at positions 3 and 7. These signals are followed by
a set of narrow aromatic signals, corresponding to the remaining
β-pyrrolic and aromatic tolyl protons. The protons of the COT
moiety appear as a set of broad signals in the 6.92-5.95 ppm
range of the spectrum, not revealing discrete structure. The inner
NH protons appear in the high field region as a singlet with δ
= -2.83 ppm.
Heating of the porphyrin 1-H NMR sample up to 50oC resulted
in the coalescence of the signal from β-pyrrolic protons
corresponding to the positions 3 and 7 and a partial coalescence
of the signals from the COT moiety.
Figure 3. Top: 1H NMR (CDCl3, 600 MHz) spectra of 1-H at different
temperatures. Bottom: tautomerization and ring inversion dynamics in 1-H.
The spectra registered in the low temperature regime at -55oC
revealed a discrete structure of some signals. The peaks present
as two broad signals at room temperature and assigned to
positions 3 and 7 appeared as a set of two pairs of signals at 10.20,
9.36 and 10.11, 9.46 ppm. Similarly, broad signals observed at
room temperature and assigned to the COT moiety protons
appeared as a complex pattern of sharp signals in the range of
7.06-6.09 ppm. The temperature dependent response has been
also observed in the case of NH protons. The spectra recorded at
room temperature reveal one signal in the high-field region of the
spectrum at -2.72 ppm due to the fast exchange of inner NH
protons. Decreasing the temperature to -55oC allowed to slow
down the exchange dynamics and to observe the NH signals as
a set of four singlets. The observed temperature response is
caused by changes of magnetic environment of position 3 and 7
(β-pyrrolic protons), COT moiety protons, and the inner NH
protons of porphyrin core. It may be postulated that the
temperature response of the 1H NMR signals is associated with
Scheme 3. Synthesis of porphyrin 3: a) Pd(PPh3)4, CuI, NEt3, 90oC, 62%;
b) InCl3, pyrrole (excess), NaOH, 60%, c) (i) NaBH4, MeOH, THF,
(ii) Yb(OTf)3×H2O, DCM, (iii) DDQ, DCM, 19%.
NMR characteristics
The obtained porphyrin-COT conjugates 1-3 were characterized
by NMR techniques. All of the porphyrin-COT derivatives
revealed temperature dependent dynamics in NMR experiments.
1
The H NMR and corresponding 2D spectra were recorded at
room and at low temperatures. Due to limited solubility of
porphyrins at low temperatures and unsatisfying 13C NMR signal
intensity, the carbon NMR characteristics were provided by 13C-
1H HSQC experiments.
4
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two kinds of dynamics. The first one is the NH tautomerism,
profoundly investigated by NMR techniques for both symmetric
and asymmetric porphyrins.60-63 It was shown that cooling of the
sample freezes the NH tautomerism and lowers the NMR-
effective symmetry of porphyrin. In the typical A3B-type porphyrin
one may expect a set of two NH signals in the 1H NMR spectrum
at the temperature at which tautomeric process is frozen. It is not
the case in the experiment carried out with porphyrin 1-H. Its low
temperature spectra reveal presence of four distinguishable NH
signals. The reason for this is the other dynamical process
involving conformational transitions of the COT moiety. Its tub
shape may assume two positions with respect to the porphyrin
core. Each of the orientation has a different impact on the
magnetic environment of the inner NH protons, resulting in the
splitting of two dynamically frozen signals into a set of four singlets.
Porphyrin 2-H, structurally similar to 1-H, possesses the COT
moiety separated from the porphyrin core by a triple bond. The
spatial separation influences temperature-dependent 1H NMR
spectra of 2-H. The room temperature spectrum reveals the
presence of a narrow doublet in the low field region of the
spectrum at 9.58 ppm (Figure 4). This signal is assigned to the
protons located at positions 3 and 7. Its discrete structure
contrasts with the broadened structure of the signals from protons
3 and 7 of 1-H. The presence of the narrow signal assigned to the
positions 3 and 7 means that the magnetic environment of this
positions is not strongly affected by the dynamics of the COT
moiety, contrary to the case of 1-H. Lowering the temperature
affects the structure of signals from protons 3 and 7 and results,
at -55oC, in the splitting into two broadened singlets. This
response is attributed to the freezing of NH-tautomerization
dynamics. The slow exchange of the two inner protons allows to
distinguish between the protonated and non-protonated pyrrole
subunit of the porphyrin core. Freezing of the dynamics is
evidenced by the NH signals temperature response.
The NH signals appearing as an averaged singlet at -2.34
ppm at room temperature split into two singlets at -2.47 and -2.52
ppm upon cooling to -55oC. The splitting of 2-H NH signal into two
signals is in contrast to the splitting of 1-H NH protons, which
appear as four singlets at low temperature. These differences
evidence lack of the magnetic interactions of the porphyrin core
protons and the COT moiety in the case of 2-H.
The protons of the COT in 2-H appear in the 6.90-5.95 ppm
range of the spectrum. The spectrum recorded at room
temperature reveals the presence of complex structure consisting
of a set of broad signals. A discrete structure arises upon
1
temperature decrease. The well-dissolved structure of COT H
NMR signals is observed at -800C, indicating that, similar to the
porphyrin 1-H, ring inversion dynamics is frozen.
1H NMR spectra of 3-H (Figure 5) were recorded in the range
of 25oC to -80oC. Again, the compound was found to reveal
dynamics due to both, the NH tautomerization and COT moiety
conformational changes.
Figure 5. Top: 1H NMR (CD2Cl2, 300 MHz) spectra of 3-H at different
temperatures; Bottom: tautomerization and ring inversion dynamics in 3-H.
1H NMR spectra recorded at room temperature show a low
field pseudo singlet, assigned to the eight β-pyrrolic protons, a set
of aromatic multiplets of tolyl groups and xanthene moiety. At δ =
4.70 a complex structure of three of the seven COT moiety
protons signals appears. These COT signals are followed by a set
of broad signals, assigned to the remaining four protons of the
COT subunit, of which the analysis is not possible due to
overlapping with sharp signals of the aliphatic groups. The NH
protons signals are found at δ = -2.68 ppm as a broad singlet. As
expected, the temperature response of the sample is profound.
Upon cooling to -80oC, β-pyrrolic protons are no longer
magnetically equivalent and appear as a complex structure of
overlapping doublets. The signals from protons assigned to the
Figure 4. Top: 1H NMR (CDCl3, 600 MHz) spectra of 2-H at different
temperatures. Bottom: tautomerization and ring inversion dynamics in 2-H.
5
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COT moiety become sharp and well-separated. Low temperature
1H NMR spectrum supported by COSY and NOESY experiments
allowed to locate COT protons signals (Figure 5; for COSY and
NOESY spectra see Figures S14 and S15) that are partially
overlapping with the signals from the aliphatic groups. The
interaction of the COT moiety with diatropic current of porphyrin
has a crucial impact on the chemical shifts of COT protons. The
protons located closer to the porphyrin ring are expected to be
strongly deshielded and to appear in the higher field region. It is
evidenced by H-H coupling of the COT signals located in the
range of 2.52-1.13 ppm with the NH signals in the NOESY
experiment. COT protons appearing in the lower field region do
not reveal the magnetic coupling with NH protons.
The inner NH protons appear as weakly separated four
singlets, indicating that the tautomerization is frozen and that the
NH protons are located in two magnetically distinguishable trans
conformations differing by the orientation of the COT moiety
(Figure 5).
Since the COT ring inversion is crucial for enabling effective
energy transfer, we decided to estimate the energy barrier of this
process for the sterically hindered system represented by
porphyrin 3 and for the case when the COT subunit is spatially
separated for porphyrin core, represented by porphyrin 2. For this
purpose we used a series of temperature dependent NMR spectra
of porphyrins 2 and 3. In order to simplify the experiment and to
exclude the influence of NH dynamics on the NMR spectra, the
NH protons were replaced by Zn2+ cation.
The hindrance of 3-H porphyrin-COT system, manifested in
the NMR experiments, is also evidenced by the structure
determined by X-ray crystallography. Porphyrins, as aromatic
compounds, possess a planar core ring. This planarity may be
perturbed by different factors, such as protonation or strong
spatial hindrance by substituents.64-66
The presence of a bulky COT moiety located above the
porphyrin core has an impact on the macrocycle shape. The
planarity of the core is disturbed, however the molecule does not
change its electronic properties significantly, as indicated by the
comparison of 3-H and tritolylporphyrin 4-H absorption spectra
(Figure 7). The molecule was found to be pseudoplanar, with the
highest deviation from the porphyrin core mean plane of 0.23 Å
found for 13C. The strain in 3-H is also evidenced by deviation
from linearity of the C≡C-C(COT) fragment by 8o. Another
parameter, important in the context of Dexter energy transfer, is
the donor-acceptor distance. In the case of 3-H the distance
between the centroid of the C8H7 COT subunit and the porphyrin
core mean plane is 4.72 Å at 100 K. This value meets the
requirement for short contact interaction, which limits the
maximum donor-acceptor separation to approximately 10 Å.
Unfortunately, attempts to grow crystals suitable to X-ray
diffraction measurement failed in the case of the five remaining
porphyrins.
Spectroscopy and photophysics
The kinetics of the ring inversion-double bond migration
process in COT, occurring via D8h symmetry structure (Figure 1),
were elucidated by the line shape analysis for porphyrin 2-Zn. The
same procedure could not be applied for porphyrin 3-Zn due to
Absorption and fluorescence spectra of all the investigated
compounds are shown in Figure 7. Table 1 presents the
photophysical parameters and the values of photodegradation
quantum yields measured in aerated and oxygen-free solutions.
1
1
the overlapping of analytical H NMR signals. As the H NMR
probe for porphyrin 2-Zn we used pairs of COT moiety signals: 2’,
8’ and 3’, 7’. The obtained kinetic constants were analyzed via
Arrhenius plot, giving respective values of the activation energy
(See SI, Figure S12). The energy barrier of COT ring inversion-
double bond migration in 2-Zn was found to be equal to 15.0±0.5
kcal/mol. The respective energy barrier for porphyrin 3-Zn was
estimated by comparison of signals coalescence temperature.
The barrier was estimated to be 2-3 kcal/mol lower than in the
case of 2-Zn. The effect of the reduction of the activation energy
is opposite to the previously observed influence of bulky
substituents on COT energetics, which rather increase the barrier.
This may be ascribed to the sterical interaction of the COT moiety
with the porphyrin core. The repulsion of these two subunits is
evidenced by deviation of the porphyrin core from planarity, as
shown by the single crystal structure (Figure 6). The ring inversion
process associated with adaptation of planar structure by COT
reduces the steric strain, which is reflected in the reduction of the
energy barrier.
4-H
4-Zn
x10
x10
3-Zn
3-H
x10
x10
2-Zn
2-H
x5
x5
1-H
1-Zn
x10
x10
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Figure 7. Absorption and emission spectra recorded in toluene at 293 K.
Substitution with COT does not significantly affect the
absorption of the porphyrin core. The largest effect is observed
for 2-H and 2-Zn, which exhibit a red shift of about 20 nm with
respect to free base and zinc tetraphenylporphyrin (TPP and
ZnTPP, shown in Table 1 for comparison). Such behaviour could
have been expected because of the additional 휋 electrons in the
ethyne linker. Also the fluorescence properties remain similar,
except for 2-H and 2-Zn, which exhibit much shorter fluorescence
Figure 6. X-ray molecular structure of 3-H, perspective views. Substituents
were omitted for clarity.
6
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10.1002/chem.202001804
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FULL PAPER
lifetimes. In 2-Zn, one also notes, both in absorption and emission, whereas the signal was very weak for COT-containing
an inverse intensity ratio of the two Q bands as compared to other
zinc porphyrins.
compounds.
In the context of photostability, a crucial parameter is the
porphyrin triplet lifetime (T). Inspection of Table 1 clearly shows
that substitution with COT leads to dramatic decrease of T: in
degassed solutions, the lifetimes measured for COT derivatives
are three orders of magnitude shorter than those of the reference
compounds 4-H and 4-Zn. The difference between porphyrins
with and without COT is also clearly observed for non-degassed
solutions, where the oxygen efficiently quenches the triplet state.
In COT derivatives, the triplet lifetimes are several times shorter,
indicating that the intramolecular quenching effectively competes
with quenching by oxygen. The triplet lifetimes in the presence of
COT become as short as 100 ns or less. For 3-Zn, practically the
same, very short lifetime of about 50 ns was obtained for both
degassed and non-degassed solutions, which means that in this
molecule the intramolecular quenching dominates over energy
transfer to oxygen. For other molecules, both types of quenching
contribute approximately equally, as may be deduced from the
comparison of triplet lifetimes measured in non-degasssed and
degassed solutions.
Photodegradation studies
In order to assess the effect of introducing COT on photostability,
toluene solutions (both degassed and non-degassed) of all new
porphyrins have been irradiated for specified time periods. The
decrease of the chromophore concentration with time was
determined by measuring changes in the absorption spectra. A
detailed description of the procedures used for the determination
of photodegradation quantum yield (deg) is given in the
Supplementary Information. The values of deg are shown in
Table 1.
Large differences in photostability are observed for aerated
solutions of free base and zinc porphyrins that do not contain the
COT moiety. 4-H (and also TPP) are much more photostable than
the zinc derivative 4-Zn. For the latter, the quantum yield of
photodegradation is two orders of magnitude larger.
Substitution with COT has a strong influence on the
photostability. It is enhanced, by an order of magnitude, in zinc
derivatives 2-Zn and 3-Zn, while it remains the same in 1-Zn. In
contrast, free base porphyrins become less photostable upon
introducing COT. The quantum yields of photodegradation are
now similar for both, free base and zinc porphyrins, which
suggests a common mechanism of photodecomposition.
One can expect that shortening of triplet lifetimes should lower
the efficiency of singlet oxygen generation. This was confirmed by
initial studies. Phosphorescence from singlet oxygen could be
readily observed in non-degassed solutions of 4-H and 4-Zn,
Table 1. Spectral and photophysical parameters for toluene solutions at 293 K.
Abs max[a]
fl
fl[c] [ns]
T1[d] [ns]
T1[d] [ns]
deg
deg
[b]
[e]
[e]
non-degassed
degassed
non-degassed
degassed
1-H
421.5 (35.0)
649.0 (0.32)
0.096
8.7
106
179
5.3×10-6
2.5×10-7
1-Zn
2-H
425.5 (36.2)
590.0 (0.31)
0.031
0.067
0.096
0.11
1.5
3.3
0.2
9.6
0.9
9.9
2.2
103
78
177
186
2.6×10-5
1.7×10-6
1.2×10-6
1.1×10-6
2.0×10-6
2.2×10-7
2.5×10-5
3.1×10-7
8.0×10-7
3.5×10-8
1.2×10-7
9.0×10-8
3.0×10-7
3.3×10-6
1.4×10-3
438.5 (30.8)
672.5 (1.12)
2-Zn
3-H
444.5 (33.5)
615.5 (2.51)
75
194
422.5 (44.1)
648.5 (0.39)
84
191
3-Zn
4-H
425.0 (49.5)
589.0 (0.39)
0.018
0.090
0.042
0.10[f]
48
58
415.0 (41.4)
641.5 (0.25)
247
362
196[f]
207 000
266 000
>10 000[f]
4-Zn
TPP
418.5 (40.6)
582.5 (0.20)
419.0 (48.7)
650.0 (0.52)
9.6
9.3g
ZnTPP
422.8 (57.4)[g]
589.0 (0.52)
0.033[f]
0.037[h]
2.0g
309[f]
> 10 000[f]
[a] Transition energy corresponding to the Soret and lowest Q bands, respectively; in parentheses, values of /104. [b] Fluorescence quantum yield measured with
respect to TPP (fl = 0.10 ref. 67 ), accuracy: 20%. [c] Fluorescence lifetime, accuracy: ±0.1 ns. [d] Triplet lifetime. [e] Photodegradation quantum yield, estimated
accuracy: 30%. [f] Ref. 67 [g] Ref.68 [h] Ref. 69
7
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10.1002/chem.202001804
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FULL PAPER
Removal of oxygen has a huge effect on photostability.
Interestingly, it is opposite for COT-containing and not containing
porphyrins. For the latter, degassing results in a spectacular
decrease of photostability, i.e., much larger values of deg. In
contrast, for both zinc and free base porphyrins substituted with
COT, oxygen removal leads to strong, about an order of
magnitude increase of photostability. These results reveal a dual
role of oxygen. First, it can act as a triplet quencher, decreasing
the probability of a photoreaction, as is clearly observed in the
case of 4-H and 4-Zn. Second, it can also act as a reactant in
redox processes, in which case the photostability will decrease.
The latter functionality becomes important in the regime when the
triplet quenching by oxygen is no longer relevant, i.e., for COT-
substituted porphyrins.
photophysical and redox properties of the COT moiety. Work
along these lines is in progress.
Porphyrins with intrinsically short triplet lifetimes are very
good candidates for single molecule fluorescence applications,
for two reasons: (i) good photostability and (ii) reduced OFF
periods, which should result in the absence or strong reduction of
blinking.
The strategies of using porphyrins for bioconjugation are well
recognized.70-71 In principle, it should be possible to synthesize
analogues of the presented porphyrins possessing polar groups
enabling their solubility, and anchoring groups for biomolecule
attachment. However, in the case of COT-porphyrin conjugates it
is expected that the nonpolar COT subunit will hamper solubility
of the porphyrin dye, even when functionalized with polar groups.
Moreover, technical problems related to solubility and reagents
compatibility (reactivity and stability) with polar and anchoring
groups may arise. To conclude, the potential COT-porphyrin dyes
equipped with polar and anchoring groups are too complex to
assume in advance that their synthesis and isolation is achievable.
To address the question of the possibility of COT-porphyrin
bioconjugation, a synthetic attempt should be undertaken.
Finally, one should mention that the novel porphyrins should
be less phototoxic, since the ability to generate singlet oxygen via
energy transfer from the porphyrin triplet has been strongly
reduced because of dramatic shortening of the triplet lifetime.
Conclusion
The instability of emitters is limiting the spatial and temporal
resolution of experiments relying on single molecule fluorescence.
The stability may be enhanced by efficient quenching of triplet
states, which population is responsible for initiation of
photodegradation paths. One of potential quenchers is
cyclooctatetraene COT, an annulene possessing low energy
triplet state. We presented a synthesis of series of its conjugates
with porphyrins of different geometry. These model systems
consist of the efficient energy donor (porphyrin) and acceptor
(cyclooctatetraene). We monitored the acceptor functionality by
1H NMR as well as stationary and transient electronic
spectroscopy methods. Temperature-dependent 1H NMR spectra
revealed that the COT subunit of the conjugate undergoes ring
inversion dynamics at room temperature. This process is not
hampered by the sterical hindrance caused by vicinity of porphyrin,
however, it may be frozen at approximately -40oC. According to
the theory, this process is essential for efficient energy transfer,
because the transition state of the ring inversion process has a
similar geometry (D4h) to the geometry of T1 state (D8h). Adopting
this geometry, COT minimizes the S0-T1 energy gap. The
measured triplet lifetimes of porphyrin-COT systems 1-3 in the
aerated solvent revealed values several times smaller with
respect to the reference tritolylporphyrin 4-H and its zinc complex
4-Zn. The shortest triplet lifetime (48 ns) was observed in the case
of porphyrin 3-Zn possessing the COT acceptor moiety placed
directly above the porphyrin core. This value changes only slightly
in the aerated solvent (58 ns), meaning that the conditions for
efficient intramolecular triplet state quenching have been fulfilled
and the COT-assisted quenching process is much faster than
quenching by molecular oxygen. Spectacular improvement in
Acknowledgements
This work was supported by the Polish National Science Centre
(NCN) grant 2016/22/A/ST4/00029. We are grateful to prof.
Sławomir Szymański for his help in the analysis of NMR data.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Photostability • Porphyrin synthesis
• Cyclooctatetraene • Triplet quenching • Conformational
dynamics
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Entry for the Table of Contents
Insert text for Table of Contents here. Efficient triplet state quenching has been achieved in porphyrins functionalized with
cyclooctatetraene (COT), leading to enhanced photostability of zinc porphyrins. The energy barrier of COT ring inversion-double
bond migration depends on the COT moiety location with respect to the porphyrin core. These novel compounds are good models for
studying nonvertical energy transfer and the competition between energy and electron transfer processes.
Institute and/or researcher Twitter usernames:
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