Green-light induced cycloadditions

Article abstract of DOI:10.1039/d1cc00340b

We introduce a red-shifted tetrazole that is able to undergo efficient nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) under blue and green light irradiation. We provide a detailed wavelength-dependent reactivity map, and employ a number of LEDs for high-conversion small molecule and polymer end-group modification.

Full text of DOI:10.1039/d1cc00340b

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Green-light induced cycloadditions†
ab
Philipp W. Kamm,abc James P. Blinco,
*
Andreas-Neil Unterreiner *c and
Cite this: Chem. Commun., 2021,
57, 3991
ab
Christopher Barner-Kowollik
*
Received 20th January 2021,
Accepted 17th March 2021
DOI: 10.1039/d1cc00340b
rsc.li/chemcomm
We introduce a red-shifted tetrazole that is able to undergo efficient
Whereas traditionally absorption spectra are used to determine
nitrile imine-mediated tetrazole–ene cycloaddition (NITEC) under the most efficient irradiation wavelength, our group recently
blue and green light irradiation. We provide a detailed wavelength- reported that the wavelength-dependent reactivity maximum is
dependent reactivity map, and employ a number of LEDs for high- often significantly red-shifted compared to the absorption maxi-
conversion small molecule and polymer end-group modification.
mum.16,17 To monitor wavelength-dependent reactivity, we
developed the action plot concept, where a wavelength-tuneable
Light induced reactions have found widespread use in both ns-laser setup is used as a monochromatic light source to trigger a
industrial and biomedical applications, as well as fundamental photo-induced reaction at specific wavelengths, with the laser
research due to their spatio-temporal control and non-invasive energy adjusted so that a constant photon count at each wave-
nature.1–4 An even higher level of control can be achieved by length is ensured. Subsequently, the reaction conversion is plotted
tuning the wavelength and intensity of the incident light versus the wavelength. Such precise reactivity maps are imperative
source. Through activation of different photoactive moieties to install truly l-orthogonal reaction systems, which – once
independently and in a sequential fashion, multicomponent, established – can be conducted using readily available LED setups
photo-induced reaction systems have been used to establish with a broader (but still controlled) spectral range.18
l-orthogonality as a versatile pathway for even more complex
A very promising and widely used UV and visible light
reaction routes.5–10 However, to date, the majority of photo- addressable reaction is the nitrile imine-mediated tetrazole–
reactions are triggered with UV light, which is associated with ene cycloaddition (NITEC), whereby a 2,5 substituted tetrazole –
severe limitations such as decreased penetration depth, harm- upon light irradiation – eliminates N2, forming a highly reactive
fulness to biological tissue and propensity for side reac- nitrile imine that can undergo a rapid 1,3-dipolar cycloaddi-
tions.11,12 Identifying and developing chromophore moieties tion.1 First described by Huisgen and Sustmann,19 it has since
that can be activated by visible light and react in an efficient been employed for a variety of applications, including protein
and specific manner therefore is a continuing pursuit. Several labelling,20 nucleic acid modification,21 polymer ligation,22
strategies have been developed to fulfil these challenging hydrogel assembly,23 and modification of nanoparticles and micro-
requirements, i.e. extending the conjugated system, introducing spheres.24,25 In recent years, researchers have worked on extending
electron-donating substituents to lower the HOMO–LUMO gap, the absorption profile of these compounds from UVB into the
or non-linear two-photon-absorption processes.13–15
visible light regime, mostly by introducing substituents or extended
conjugated chromophores at the N-phenyl ring.22,26,27 However, to
our knowledge, no NITEC reaction so far has been carried out with
wavelengths longer than 420 nm in a one photon process.
Herein, we report the synthesis and photoreactivity of a blue
to green light activated tetrazole equipped with a dimethylami-
nopyrenyl moiety (Scheme 1). The wavelength-dependent reac-
tivity is carefully assessed for an exemplary small-molecule
ligation reaction, and clean and high-conversion NITEC photoreac-
tions are reported for a number of dipolarophiles, including an
end-group functionalized poly(methyl)methacrylate based polymer
chain at wavelengths up to 515 nm.
a Centre for Materials Science, Queensland University of Technology (QUT),
2 George Street, Brisbane, QLD 4000, Australia. E-mail: j.blinco@qut.edu.au,
christopher.barnerkowollik@qut.edu.au
b School of Chemistry and Physics, Queensland University of Technology (QUT),
2 George Street, Brisbane, QLD 4000, Australia
c Molecular Physical Chemistry Group, Institute of Physical Chemistry, Karlsruhe
Institute of Technology (KIT), Fritz-Haber-Weg 2, Geb. 30.44, Karlsruhe 76131,
Germany. E-mail: andreas.unterreiner@kit.edu
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cc00340b
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Scheme 1 Synthetic routes for photo-induced NITEC reactions of APAT: Left arrow: Small molecule model reaction between APAT and diethylfu-
marate, using a tuneable ns-pulsed laser system in a wavelength range between 320 nm and 515 nm, to precisely monitor the wavelength dependent
reactivity. Right arrow: Polymer end-group modification, using three different LEDs, centred around 452 nm, 500 nm, and 515 nm, respectively.
To achieve red-shifted reactivity, it is vital to add extended
conjugated systems or electron donating substituents in the N2
position.26 Adding a Me2N-group in the para position of the
N2-phenylring of a diaryltetrazole shifted the absorption maximum
from 302 nm to 336 nm. Similarly, by substituting the N2 phenyl
ring with a pyrene moiety, absorption was shifted to 343 nm.26,28
We hypothesize that a combination of both strategies allows for an
even more red-shifted photoactive molecule, hence, dimethyla-
mino pyrenyl aryl tetrazole (APAT) was synthesized in four steps
from pyrene-1-amine (Scheme S1, ESI†). This presents a modifica-
tion of pyrene aryl tetrazole (PAT), which was introduced by our
group previously.22 While introducing a second substituent on
the pyrene ring through nitration naturally leads to formation of
regio-isomers, only the 1,8-disubstituted species could be isolated
in sufficient quantities after chromotography (Scheme 1).
1
The positions of the substituents were confirmed via 2D H- and
13C-NMR spectroscopy (Fig. S1, ESI†).
Fig. 1 (a) Exemplary LC chromatogram (scan wavelength: 392 nm) after
photoreaction at 440 nm, showing the integrated peaks of APAT and CA1.
(b) Overlay of the absorption profiles of APAT (460 mM) and CA1 (40 mM)
(left y-axis) and the wavelength-dependent reactivity profile of the photo-
Investigation of the absorption properties of APAT in MeCN
revealed two overlapping broad absorption bands in the long
wavelength regime, showing two absorption maxima at 376 nm
and 402 nm, which we attributed to transitions into low-lying
induced NITEC reaction between APAT and diethylfumarate (right y-axis).
electronically excited singlet states and to a charge-transfer
state, respectively (Fig. 1b, left y-axis).28,29 However, absorbance
can be observed up to 500 nm and a slight tailing occurs even
well beyond 500 nm (up to 600 nm). Furthermore, two absorp-
tion maxima appear in the short wavelength region (237 nm
All experiments were conducted in MeCN and repeated two or three times
to obtain error bars. For a detailed description of the laser setup, data
processing and conditions of all action plot experiments refer to the ESI.†
and 291 nm), which can be attributed to transitions into higher (for chromatograms of all tested small molecules, refer to Fig. S4,
excited states. We found a fluorescence quantum yield of 0.39 ESI†), as well as fast conversion. Importantly, the photoreaction
in MeCN, with emission peaking at 533 nm, leaving a significant proceeded with high selectivity and without significant amounts
portion of absorbed photons potentially available to induce of side products (at wavelengths 340 nm and lower, degradation
photoreaction (for full absorption and emission spectrum refer of the phenyl tetrazole moiety was observed, yielding dimethyla-
to Fig. S2, ESI†).
mino pyrene). Critically, unlike previous literature observations,
To investigate the photoreactivity at various wavelengths, no cycloaddition between the nitrile imine and the solvent, MeCN,
small molecule model reactions were conducted in the wave- was observed.30 The relative amounts of CA1 and unreacted APAT
length range between 320 nm and 525 nm and analysed via allowed to calculate the reaction conversion. Since diethylfuma-
coupled liquid chromatography and high-resolution mass spec- rate is a symmetric molecule, it does not lead to different regio-
trometry (LC–MS). Gradient-free MeCN was used as the mobile isomers after cycloaddition. Even so, two different product
1
phase, which allowed us to quantify APAT and the cycloadduct species are present in the H-NMR spectrum (Fig. 2): the first
by correlating the chromatogram peaks with the respective corresponds to the pyrazoline adduct, which is expressed by two
e392 nm in MeCN, where 392 nm is the scanning wavelength of additional signals at d = 5.55 ppm and 4.84 ppm. The second
the UV detector. Furthermore, using diethylfumarate as a dipo- corresponds to the pyrazole adduct after rearomatization, hence
larophile proved to provide the best separation of APAT and loss of the two aforementioned signals. Photoreactions between
cycloadduct CA1 on the column, hence minimising peak overlap APAT and diethylfumarate were conducted in a wavelength range
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addition–fragmentation chain-transfer (RAFT) polymerization. Since
the o-terminal dithiocarbonyl group inherently incorporated during
this process is strongly coloured and prone to side reactions, it was
removed and exchanged for a hydroxy group (Scheme S2, ESI†). To
ensure sufficient end-group reactivity (higher for low molecular
weights), but also facilitate precipitation after the photoreaction, a
medium molecular weight (Mn = 5.2 kDa, Ð = 1.15, refer to ESI,† for
the full molecular weight distribution) was chosen for the polymer
chains. Four equiv. of APAT were irradiated with 1 equiv. of PMMA-
Mal in MeCN. Differences in output powers and photon energy of
the LEDs were accounted for by adjusting the irradiation time (for
detailed description of the setup refer to ESI†). Unreacted APAT was
separated from CA2 by using preparative size exclusion chromato-
graphy (SEC), and after precipitation CA2 was analysed by 1H-NMR
spectroscopy (Fig. 3 and Fig. S8, ESI†) and size exclusion
chromatography (Fig. S10, ESI†). The spectra reveal presence of
two different species of CA2, corresponding to the pyrazoline
cycloadduct and the rearomatized pyrazole cycloadduct. These
findings were confirmed by diffusion-ordered spectroscopy
Fig. 2 1H-NMR spectra before and after NITEC reaction using a 10 W
452 nm LED. Zoomed-in spectra are available in the ESI,† (Fig. S7 and S19).
(a) APAT before irradiation. (b) Cycloadduct after purification via column
chromatography. While in the spectrum both the pyrazoline and the
re-aromatized pyrazole species are present, for clarity, only the pyrazoline
cycloadduct without specification of the stereo-active centres is depicted.
between 320 nm and 525 nm and used to compile the wavelength- (DOSY) and correlation spectroscopy (COSY) (Fig. S8, ESI†).
dependent reactivity map. 1H-NMR and absorption spectra of CA1 The relative extent to which these two species are formed is
were obtained after synthesizing a larger quantity, using a 452 nm difficult to predict and varies in every experiment, which is why
centred 10 W LED, which then could be isolated and purified by both species are treated and quantified as one.
column chromatography. Interestingly, unlike reported in pre-
The 1H-NMR spectrum after 452 nm irradiation for 2.5 h (Fig. 3,
vious studies on NITEC reactions, CA1 is non-fluorescent and green line) shows no remaining PMMA-Mal peak (HCQCH) at
fluorescence intensity decreases by a factor of 100 during the d = 6.75 ppm anymore, indicating quantitative end-group conversion.
course of the photoreaction.
A significant amount of pyrazoline cycloadduct is still present. After
The action plot depicted in Fig. 1 reveals that the reactivity 500 nm irradiation for 3.7 h, peak ratios correspond to 50%
map does not follow the absorption spectrum. While an absorp-
tion minimum occurs at 335 nm, the reactivity minimum is
shifted to 360 nm, yielding 2% conversion. Whereas the absorp-
tion spectrum shows two maxima in the investigated wave-
length regime, only one broad reactivity band is observed,
peaking between 430 nm and 450 nm (14% conversion), which
is significantly red-shifted compared to the absorption maxi-
mum (402 nm). Since we were mainly interested in the photo-
reactive behaviour in the visible regime, more data points were
recorded at wavelengths between 410 nm and 525 nm. Apart from
an outlier at 470 nm, conversion gradually decreases with higher
wavelengths. While no conversion was observed at 515 nm and
above under the given conditions, significant conversion was
achieved when the number of incident photons was increased
by a factor of 25 (Fig. S5, ESI†), demonstrating that – while less
efficient than with blue light – the reaction can be triggered with
green light wavelengths. We refrained from driving the reaction to
higher conversions in the wavelength-dependence study, since
with higher conversion competing absorption through formation
of the cycloadduct is no longer negligible. Nevertheless, we are
able to demonstrate that high conversions are possible with a
substantially increased number of photons, driving the reaction to
89% conversion with 465 nm irradiation (Fig. S6, ESI†).
With the detailed action plot at hand, we proceeded to
employ APAT for polymer end-group modification (Scheme 1,
right arrow), using a range of different LEDs centred around
452 nm, 500 nm and 515 nm, respectively. To that end, a maleimide
Fig. 3 Polymer end-group modification and analysis. (a) Reaction scheme
of the 1,3-dipolar cycloaddition of a maleimide end-capped PMMA polymer
and APAT, forming the pyrazoline cycloadduct. (b) Zoom into the aromatic
region of 1H-NMR spectra of CA2 after photo-induced polymer end-group
modification. Reaction conversion was determined by comparing the
resonance integrals of the aromatic protons (a = 13 H) and remaining vinylic
protons of the maleimide end-group (b = 2 H). For reference, the spectrum
functional PMMA based polymer was synthesized via reversible of PMMA-Mal is depicted in blue. Full spectra are available in the ESI.†
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conversion (yellow line). Unreacted tetrazole could be recovered as Notes and references
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There are no conflicts to declare.
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