Potassium Poly(Heptazine Imide): Transition Metal-Free Solid-State Triplet Sensitizer in Cascade Energy Transfer and [3+2]-cycloadditions

Article abstract of DOI:10.1002/anie.202004747

Polymeric carbon nitride materials have been used in numerous light-to-energy conversion applications ranging from photocatalysis to optoelectronics. For a new application and modelling, we first refined the crystal structure of potassium poly(heptazine imide) (K-PHI)—a benchmark carbon nitride material in photocatalysis—by means of X-ray powder diffraction and transmission electron microscopy. Using the crystal structure of K-PHI, periodic DFT calculations were performed to calculate the density-of-states (DOS) and localize intra band states (IBS). IBS were found to be responsible for the enhanced K-PHI absorption in the near IR region, to serve as electron traps, and to be useful in energy transfer reactions. Once excited with visible light, carbon nitrides, in addition to the direct recombination, can also undergo singlet–triplet intersystem crossing. We utilized the K-PHI centered triplet excited states to trigger a cascade of energy transfer reactions and, in turn, to sensitize, for example, singlet oxygen (¹O₂) as a starting point to synthesis up to 25 different N-rich heterocycles.

Full text of DOI:10.1002/anie.202004747

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Potassium Poly(Heptazine Imide) – Transition Metal-free Solid
State Triplet Sensitizer in Cascade Energy Transfer and [3+2]-
cycloadditions
Authors: Aleksandr Savateev, Nadezda V. Tarakina, Volker Strauss,
Tanveer Hussain, Katharina ten Brummelhuis, José Manuel
Sánchez Vadillo, Yevheniia Markushyna, Stefano Mazzanti,
Alexander P. Tyutyunnik, Ralf Walczak, Martin Oschatz, Dirk
Guldi, Amir Karton, and Markus Antonietti
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202004747
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10.1002/anie.202004747
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RESEARCH ARTICLE
Potassium Poly(Heptazine Imide) – Transition Metal-free Solid
State Triplet Sensitizer in Cascade Energy Transfer and [3+2]-
cycloadditions
Aleksandr Savateev,*,[a] Nadezda V. Tarakina,[a] Volker Strauss,[a] Tanveer Hussain,[b] Katharina ten
Brummelhuis,[a] José Manuel Sánchez Vadillo,[c] Yevheniia Markushyna,[a] Stefano Mazzanti,[a]
Alexander P. Tyutyunnik,[d] Ralf Walczak,[a] Martin Oschatz,[a] Dirk Guldi,[e] Amir Karton[b] and Markus
Antonietti*,[a]
[a]
[b]
Dr. Aleksandr Savateev,* Dr. Nadezda V. Tarakina, Dr. Volker Strauss, Katharina ten Brummelhuis, Yevheniia Markushyna, Stefano Mazzanti, Dr. Ralf
Walczak, Dr. Martin Oschatz, Prof. Markus Antonietti
Department of Colloid Chemistry
Max-Planck Institute of Colloids and Interfaces
Am Mühlenberg 1, 14476 Potsdam, Germany
E-mail: oleksandr.savatieiev@mpikg.mpg.de
Dr. Tanveer Hussain, Prof. Amir Karton
School of Molecular Sciences
The University of Western Australia
35 Stirling Highway, 6009, Perth, Western Australia, Australia
José Manuel Sánchez Vadillo
Institute of Secondary Education Maimónides
Calle Alfonso XIII, 4, 14001 Córdoba, Spain
[c]
[d]
Dr. Alexander P. Tyutyunnik
Institute of Solid State Chemistry
Ural Branch of the Russian Academy of Sciences
91 Pervomayskaya str., 620990, Ekaterinburg, Russia
Prof. Dirk Guldi
[e]
Department of Chemistry and Pharmacy, Interdisciplinary Center for Molecular Materials (ICMM)
Friedrich-Alexander University of Erlangen-Nürnberg
Egerlandstr. 3, 91058 Erlangen, Germany
Supporting information for this article is given via a link at the end of the document.
Abstract: Polymeric carbon nitride materials have been successfully
used in numerous light-to-energy conversion applications ranging
from photocatalysis to optoelectronics. For a new application and
modelling, we first refined the crystal structure of potassium
poly(heptazine imide) (K-PHI) – a benchmark carbon nitride material
in photocatalysis – by means of X-ray powder diffraction and
nitride impacted work in related areas, such as
photoelectrochemical cells,[7] metal-free electrodes,[8],[9] and
electroluminescence devices.[10] Applications beyond the
aforementioned are autonomous actuators,[11] photodetectors
based on photon driven ion transport in asymmetric carbon nitride
membranes,[12] and light-driven ion pump.[13] Polymeric carbon
transmission electron microscopy. Using the crystal structure of K-PHI, nitride materials have also been recognized as versatile and
periodic DFT calculations were performed to calculate the density-of-
states and localize intra band states (IBS). IBS were found to be
responsible for the enhanced K-PHI absorption in the near IR region,
reliable heterogeneous photocatalysts for preparing value-added
organic compounds.[14],[15] Ghosh, König et al. showed, for
example, that mesoporous graphitic carbon nitride (mpg-CN)
to serve as electron traps, and to be useful in energy transfer reactions. enables various kinds of reactions and also enables one-pot C-H
Once excited with visible light, carbon nitrides, beside the direct
recombination, can also undergo singlet-triplet intersystem crossing.
We utilized the K-PHI centered triplet excited states to trigger a
cascade of energy transfer reactions and, in turn, to sensitize, for
example, singlet oxygen (1O2) as a starting point to synthesis up to 25
different N-rich heterocycles.
bifunctionalization of organic molecules.[16] Extending this first
generation catalysts, potassium poly(heptazine imide) (K-PHI), a
crystalline carbon nitride material, facilitates a remarkable number
of unique reactions. Leading examples are oxidative thiolation of
toluene at room temperature[17] and multiple tandem
reactions.[18],[19] It is also capable to store electrons.[20],[21]
In the context of carbon nitride photocatalysts, the overwhelming
majority of reactions is based on electron transfer. Energy transfer,
on the other hand, generates excited-state molecules rather than
charged radicals. Therefore, new reaction paths can be
designed.[22] In terms of dioxygen, one of the greenest oxidants in
organic synthesis and simplest bimodal reactant, one electron
reduction leads to the superoxide radical (O2•―), while energy
transfer from a triplet excited state sensitizer affords singlet
oxygen (1O2). The chemistry of O2•― and 1O2 could not be more
Introduction
Artificial photosynthesis has been the primary area of inorganic
semiconductors application in chemistry for many years.[1],[2],[3]
Similarly, carbon nitrides are traditionally associated with water
splitting and CO2 conversion.[4],[5],[6] As a matter of fact, the
evolution of hydrogen from water in the case of polymeric carbon
1
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RESEARCH ARTICLE
•―
different.[23]
O2
participates
in
proton
abstraction,
We refine the crystal structure of nanocrystaline K-PHI, giving
special attention to understanding the defects structure of this
compound, and provided unambiguous evidence that light-
excited K-PHI indeed undergoes singlet-triplet intersystem
crossing (ISC).
As such, we employ a cascade of energy transfer reactions
starting with the K-PHI triplet excited states to O2 followed by a
subsequent quenching with aldoximes. Interaction of 1O2 with
aldoximes gives nitrile oxides, which triggered our interest in
exploring the synthesis of various oxadiazoles-1,2,4 and
isoxazoles via dipolar [3+2]-cycloaddition.
disproportionation, or nucleophilic substitution reactions,[24] while
the most prominent reactions of 1O2 are Diels-Alder cycloaddition
and formation of dioxetanes.[25],[26],[27]
Numerous small organic compounds have been reported to
sensitize 1O2: Ir(ppy)3,[28] Ru(bpy)3Cl2,[29] [Mes-Acr]+ClO4
― [23]
,
riboflavin tetraacetate (RFT),[30] just to name a few. Ease of
separation, higher thermo-chemical stability, and possible
application on large scale make solid-state sensitizers more
attractive than homogeneous analogues. π-conjugated triplet
sensitizers have been a topic of research in past decades and
have been quite successfully used in light energy conversion,
such as LEDs fabrication,[31],[32] photon upconversion,[33],[34],[35]
cells imaging[36] etc. However, only few examples are known on
using such materials in photocatalysis.[37] Notable is that their use
has been restricted to ‘model’ reactions.[38] Most of these solid
state sensitizers are made out of soft polymer matrices with
encapsulated platinum group metal complexes.[39]
To this day, the modus operandi of carbon nitrides has always
been linked to electron transfer reactions, while energy transfer
reactions in most cases have not been even considered.
Replacing hazardous and toxic oxidants by simpler and more
sustainable chemicals, such as 1O2, is a central topic of
contemporary research. By virtue of 1O2 mediated reactions,
which are surprisingly restricted to the synthesis of model
compounds only, many questions and challenges evolve around
the function of carbon nitrides. Finally, we envisioned bimodal
photocatalysis, featuring a redox mediator, on one hand, and a
photosensitizer, on the other hand, integrated into the same
material, as a tool to intensify research applied in the synthesis of
organic molecules.
Results and Discussion
K-PHI was prepared from 5-aminotetrazole in LiCl/KCl eutectics
using mechanochemical pre-treatment of the respective
precursors,[40] while details regarding its characterization are
given in Figure S1. At first glance, the collected X-ray powder
diffraction patterns of K-PHI revealed both anisotropy in the shape
of the Bragg peaks (in the 2theta range 20-42°) and a clearly
pronounced diffuse halo, thus pointing towards the presence of
defects/disorder in this compound (Figure 2a). Having a better
understanding of the real structure of K-PHI and the degree of
structural disorder is considered essential for explaining and
tuning K-PHI properties.
Herein, we address a number of questions using K-PHI (Figure 1).
Figure 2. (a) Experimental (crosses), calculated (solid line), and difference
(bottom line) XRD patterns of K-PHI, (b) crystal structure of K-PHI after
refinement of XRD data. Blue and black spheres indicate N and C sites,
respectively; brown, grey and green show positions of K(1), K(2) and K(3),
respectively.
To start building structural model, a high-resolution transmission
electron microscopy (HRTEM) study has been performed. K-PHI
powder consists of lamellar nanocrystallites, which form big
agglomerates (Figure 3a). Fast Fourier Transforms (FFTs)
obtained from the HRTEM images can be indexed in a hexagonal
lattice with unit cell parameters a = 11.4(8) Å and c = 3.7(2) Å.
HRTEM images reveal a layered structure with nanometer-sized
domains that display unit cell distortions, faults in the sequences
of CN-layer stacking, edge and screw dislocations as well as
rippling of CN-layers (Figure 3).
Figure 1. Topics covered in the current work and interconnections between
them. Abbreviations are explained in the text.
The XRD pattern of K-PHI can be correspondingly indexed in a
hexagonal lattice with unit cell parameters: a = 12.637(3) Å, c =
3.2998(3) Å, space group P31m (157), which is in good
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Figure 3. HRTEM images of K-PHI: (a) agglomerates of lamellar crystallites, (b,c) view along (001) direction and (d) corresponding FFT, (e, f) view along (010)
direction and (g) corresponding FFT. White arrows indicate dislocations and rippling of CN-layers.
agreement with TEM data. Based on HRTEM, XRD data and
general assumptions about the structure from earlier works,[41] the
starting model for refinement was proposed to consist of
heptazine units, which are placed on top of each other in a so-
called AAA stacking forming continuous channels along the c
direction. Models with different CN-layers stacking, in particular
ccp (ABCABC), hcp (ABAB) and mixed stacking(AABB, etc.)
allowed to partially describe the positions and the broadening of
the peaks, but did not give a better description of the XRD pattern,
suggesting the presence of stacking faults rather than a second
structural modification in the sample. Potassium atoms were
located closer to the center of the channels and in between layers;
their positions were corrected further during refinement based on
difference Fourier maps. The details of the refinement, refined
atomic coordinates and temperature factors can be found in
Tables S1 and S2.
eV, 2.43 eV correspond to CBM-to-VBM transitions and, on the
other hand, that at 2.05 eV is assigned to CBM-to-IBS transitions
(Figure 4b).
To correlate the 600 ps FL
[17]
,
which is significantly shorter than
typically observed for carbon nitride materials, >1 ns, with the
photocatalytic activity of K-PHI (shown down below), we
performed
further
spectroscopic
characterization.
The
fluorescence internal quantum efficiency (IQE) of K-PHI is
0.072%. Relatively low IQE values speak for non-radiative
deactivations in carbon nitride once in the singlet excited state.
Taking the extremely short fluorescence lifetime of 600 ps into
account, we conclude that only 1 out of 1000 exciton separation
events leads to recombination and that the vast majority of
excitons reaches non-radiative trap states in a matter of sub-
nanosecond time frame. However, it still remains unclear what is
the quantum state of those.
Our results are in good agreement with the model presented by
Lotsch et al.[42] However, we believe that describing the
nanocrystalline sample based on the model obtained for larger
crystals[42] would not be totally correct in our case. We think that
a high symmetry description with defects and disorder included is
closer to the real structure of the nanocrystalline sample and
assists in explaining the high catalytic activity of K-PHI.
From Table S2 we derive that the highest probability of finding
any K atoms corresponds to the K(1) positions (brown spheres on
Figure 2), suggesting that K atoms tend to sit closer to the center
of the channels so that they can form bonds with bridging nitrogen
atoms N(1).
It is important to mention that the obtained structural model is still
idealized; in the real compound, there is a high degree of disorder,
which is associated with (1) the stacking of PHI layers, and (2) a
disordered distribution of K atoms (the low probabilities to find
K(2) and K(3) at particular position suggests that atoms can be
randomly distributed in between layers). Taking earlier reports
into account, an optimum size of K-PHI crystallites exists in terms
of highest-performing photocatalysis, which is based on electron
transfer, such as dehydrogenation of alcohols.[40]
Diffuse reflectance UV-vis (DRUV-vis) spectrum suggests that in
the K-PHI structure at least two band gaps exist (Figure 4a). Of
great relevance is the onset of absorption with an energy of
2.64 eV; it relates to the intrinsic optical band gap seen typically
for graphitic carbon nitrides.[4] A smooth onset at around 1.86 eV
stands for low-energy transitions, which involve intraband states
(IBS).[43]
By applying a 10 ms delay, the phosphorescence spectrum was
recorded (Figure 4b). The phosphorescence features are red-
shifted compared to the fluorescence. Given that fluorescence
spectrum of K-PHI comprises at least four transitions, with 47.7%
contribution of the peak at 2.05 eV to the total fluorescence (Table
S3), ISC is more favorable when additional states are associated
with this transition. In this view, the singlet-triplet energy gap in K-
PHI was calculated to be 0.2 eV (Figure 4b). Earlier reported
singlet-triplet energy gaps for graphitic carbon nitride range from
0.156 to 0.248 eV.[38]
To address the question on quantum state of excitons in non-
radiative traps and to obtain insights into the excited state
dynamics in K-PHI, we conducted transient absorption
measurements (Figure 4c and S6). Upon excitation at 387 nm, a
broad minimum centered at 525 nm and
a maximum at
λ>1100 nm evolve. The former is assigned to ground state
bleaching of the singlet excited state, while the latter is assigned
to excited state absorptions. Within a few hundred picoseconds,
all of the aforementioned features transform into a broad negative
transient stretching over the entire spectrum and minimizing at
~600 nm. This signal decays to zero after 10 µs. The steady
deactivation of the short-lived transient together with the evolution
of the long-lived transient points to a population of a triplet from
the former singlet excited state.
Multi-wavelength analysis of the time-absorption profiles yields
five lifetimes (Figure 4d). The ground state recovery from the
singlet excited state occurs with two lifetime components of 1 =
1.4 ps and 2 = 12.1 ps. A triplet excited state is populated via
intersystem crossing within a lifetime of 3 = 470 ps. The triplet
finally deactivates with two lifetime components 4 = 161 ns and
Room temperature steady state PL spectrum of K-PHI is more
complex compared to, for example, g-C3N4,[43] and involves at
least 4 major transitions. On one hand, the peaks at 2.76 eV, 2.53
3
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5 = 1.9 µs. The presence of two lifetime components indicates the
presence IBS.
Such a complex excited state dynamics of K-PHI is very different
from a carbon nitride reported by Wu et al. and Durrant et al., who
described the excited state deactivation by a single exponential
fitting function.[44],[43] On the other hand, Xie et al. also observed
by TAS formation of triplet states in carbon nitride, albeit
intersystem crossing occurs within a few picoseconds.[45] Figure
4e summarizes the excited state dynamics in K-PHI.
To explore the potential of the non-radiative triplet excited states
in K-PHI to sensitize, for example, 1O2, we monitored the emission
of K-PHI suspension in MeCN saturated with O2 at 1271 nm.
Indeed, we recorded the distinct 1O2 fluorescence pattern (Figure
4f). mpg-CN was also able to produce 1O2, albeit with significantly
lower efficiency. Sensitization of 1O2 thereby evidences that triplet
states in carbon nitride materials may be used for energy transfer
in photocatalytic reactions as will be shown below.
To characterize the non-radiative trap states, we invoked
theoretical modelling of K-PHI structure. To this date, the density
functional theory (DFT) modeling has been carried out for diverse
carbon nitrides.[43],[46] Considering, however, the unique chemical
structure of K-PHI, we want to obtain a cohesive picture of its
electronic structure.
Using the refined crystal structure, we performed periodic DFT
calculations to obtain the total and partial density of states (DOS)
of K-PHI. For our calculations, we took the K-PHI structure
bearing three K(1) potassium atoms (those with a fraction 0.7032
in Table S2, brown spheres on Figure 2b). The DOS profile
(Figure 5a) reveal that the conduction band of K-PHI is formed by
s- and p-orbitals of nitrogen (55%) and carbon (26%) followed by
potassium s, p and d-orbitals (19%) with the edge located at
―0.81 eV. The experimentally determined flat-band potential is
―0.50 V vs NHE.[47]
Figure 4. Spectroscopic characterization of K-PHI. (a) Solid state DRUV-vis
spectrum of K-PHI. (b) Room temperature steady-state fluorescence spectrum
(black curve, λex
= 360 nm) and deconvoluted fluorescence spectrum.
Phosphorescence spectrum (light blue, λex = 360 nm, delay 10 ms). The asterisk
at 1.72 eV denotes 2nd order excitation light diffraction. (c) Transient absorption
spectra of K-PHI suspension in MeCN (1.7 mg mL-1, λex = 387 nm). (d) Time-
absorption profiles of the TAS monitored at 530 nm and 1300 nm. Lifetime
components obtained by fitting the data with exponential functions are shown.
(e) Summary of the excited state dynamics in K-PHI. G – ground state, S –
excited singlet state, T – excited triplet state. (f) 1O2 fluorescence (λex = 320 nm,
OD320nm = 0.35). Suspension of carbon nitride in MeCN purged with O2.
Figure 5. Band structure of K-PHI. (a) Partial (C, N, K) and total DOS in K-PHI
ground state. Fermi energy is located at 0 eV. (b) UPS of K-PHI. (c)
Experimental band structure of K-PHI and that derived from DFT calculations.
TDOS on the right is given as a guide for CBM, IBS and VBM. Experimental
data set: CB potential determined by Mott-Schottky analysis.[47] Uncertainty of
IBS onset and VBM determination due to smooth signal onset in UPS and
presence of several emissive states near CBM (Figure 4b) are denoted by
closely lying levels.
4
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By means of ultraviolet photoelectron spectroscopy (UPS), the
valence band maximum is determined at +2.68 eV (Figure 5b).
However, signal starts to develop at around +1.45 eV. This is
indicative for the existence of IBS. Taking UPS and DRUV-vis
data as well as theoretical DOS modeling of K-PHI (Figure 5a)
into concert, we conclude that the IBS are located at ca. +1.45 eV.
In the DOS of K-PHI, IBS are observed as an ‘island’ emerging at
+1.00 eV and stretching up to +2.42 eV (Figure 5a and 5c).
Presence of a ‘valley’ between +2.42 eV and +2.63 eV underlines
that the states are well-separated from the VB. The IBS are not
caused by the introduction of potassium itself, as their
corresponding contributions in the range of energies from +0.93 V
to +2.5 V are negligibly small (0.48 %). Instead, we ascribe them
to n-π* transitions.[48] In K-PHI, the heptazine units are arranged
in large macrocycles and each of them is formed by 6 heptazine
units (Figure 2b). Therefore, the K-PHI structure is more flexible
compared to the conventional graphitic carbon nitride, where
heptazine units either form a rigid conjugated 2D structure or are
best described by chains of heptazine units bound by hydrogen
bonds.[49] Therefore, ensembles of heptazine units in K-PHI can
adopt out-of-plane conformations, which, in turn, facilitates the
otherwise restricted n-π* transitions.[50]
Figure 5c summarizes the experimentally determined CBM and
VBM in K-PHI with that obtained from the DFT modelling. The
difference between CBM (‒0.50 eV, calculated taking into
account Mott-Schottky plots) and VBM (+2.68 eV, determined
from UPS) is 3.18 eV – the energy gap or transport gap, is larger
than the optical band of 2.64 eV.[51]
The preliminary results of 1O2 addition to 9,10-
diphenylanthracene to undergo endoperoxide formation point to
higher selectivity of the heterogeneous photocatalysis versus
homogeneous system (Table S4). To test the K-PHI assisted 1O2
sensitization in the synthesis of N-rich heterocycles, we examine
the synthesis of oxadiazoles-1,2,4 in greater details due to the
importance of this class of organic compounds for medicinal
chemistry and material science.[52],[53] We expect formation of
nitrile oxides from the corresponding aldoximes upon 1O2
quenching followed by [3+2] cycloaddition to the C≡N-group of a
nitrile.
Figure 6. Photocatalytic synthesis of oxadiazoles-1,2,4 and isoxazoles.
Conditions: oxime 50 µmol, K-PHI 5 mg, nitrile 3 mL (acetonitrile 3 mL was used
in isoxazoles synthesis), air balloon 1 bar, 461 nm (88 mW·cm-2), T = 35°C, 24 h.
a Blue phosphorescent OLEDs precursor.[53] b A potential drug for treatment of
c
Alzheimer’s disease.[54] A pharmaceutical drug (PTC124) precursor to target
genetic disorders.[55] d NMR yield.
(Figure 7a). Acetonitrile is stable against oxidation and reduction
in the range from −2.1 V to +1.9 V vs. Fc/Fc+. Relative to the
·―
aforementioned, the standard redox potential of the O2/O2
Thorough reaction conditions screening is given in Tables S5-S12.
A series of oxadiazoles-1,2,4 (1-23, Figure 6) was prepared by
coupling different oximes with nitriles. Generally, the yields of
oxadiazoles were higher combining aromatic nitriles with electron
deficient oximes (Figure S10). For ethylcyanoacetate, cyclization
toward oxadiazoles 18 and 19 competes with the Knoevenagel
condensation. Using the developed method, we prepared several
applied oxadiazoles derivatives.[53],[54] An attempt to couple oxime
of 3-formylbenzoic acid with 2-fluorobenzonitrile in order to
prepare PTC124 – a pharmaceutical drug to target genetic
disorders – was not successful.[55] Instead, we succeeded to
synthesize the corresponding methyl ester 22. The ester group of
the oxadiazole is subjected to hydrolysis under alkaline
conditions.[56] Furthermore, we found that the intramolecular
cyclization in chalcone oximes is triggered by K-PHI under blue
light irradiation. Here, the tentative intermediate, namely 4,5-
dihydroisoxasole, is further oxidized to isoxazole 24.
couple is centered at –1.27 V. Given that the potential of the
conduction band of K-PHI is located at −0.9 V vs. Fc/Fc+ (−0.5 V
•―
vs. NHE), reduction of O2 to afford O2
renders
thermodynamically challenging. Therefore, electrochemical
measurements also suggest that 1O2 sensitization is the
preferential path of O2 activation. Depending on the oxime
structure, the onset of irreversible oxidation occurs in the range
from +0.51 V, for pyrrole-2-carbaldehyde oxime 28a, to +1.67 V
vs. Fc/Fc+, for pivalaldehyde oxime 9a (Figure S12). Reaction in
the presence of electron scavengers, such as nitrobenzene and
S8,[17] excluded participation of the photogenerated holes in the
synthesis of oxadiazoles-1,2,4 (Table S6). Addition of DMPO to
the reaction mixture did not lead to the formation of the DMPO-
O2•― adduct as evidenced by EPR spectroscopy (Figure S7). On
the contrary, synthesis of 9,10-diphenylantracene endoperoxide
(Table S4), 1O2 fluorescence detection (Figure 4f), and detection
of TEMPO radical (Figure 7b) formed in situ from 2,2,6,6-
tretramethylpiperidine univocally confirmed participation of 1O2 in
the reaction.[57]
The reaction between benzaldehyde oxime and acetonitrile has
been chosen to investigate the mechanism. The apparent
quantum yield (AQY) of oxadiazole 11 reached (5.8±0.4)·10-5
%
When present in equal amount, benzaldehyde oxime 2a is
converted to oxadiazole 11 2.91 ± 0.15 faster compared to 2a-d1
after 6 h of irradiation (Figure S11). The redox potentials of the
reagents were determined by cyclic voltammetry (CV) in MeCN
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Even though the vast majority of oximes quench 1O2 by means of
dehydrogenation, a number of electron rich oximes including 26a-
31a fail to undergo click-heterocyclization (Figure S13). Instead,
aldehydes are formed as the products of C=N bond cleavage. A
likely rationale is based on the formation of an intermediate
charge transfer complex between the electron rich π-conjugated
system (27a-30a) and 1O2.[59],[60] Once formed, the enriched
electron density on dioxygen facilitates nucleophilic addition to the
C=N bond followed by recovery of the aldehyde. Notably,
carboxylic or phenolic protons as part of oximes 26a and 31a
inhibit the pathway A.[61]
Taking the experimental data into account, we gather a tentative
mechanism of 1O2 quenching by oximes, which is summarized in
Figure 7d. Depending on the chemical structure of oxime, its
interaction with 1O2 is likely to follow two different pathways,
namely oxidation to the nitrile oxide (pathway A) or addition to the
C=N bond (pathway B). Pathway A is operative in most of the
studied cases. Pathway B requires electron rich oximes or oximes
bearing acidic protons. Independent support for our mechanistic
conclusions came from solvent screening tests (Table S10). In the
absence of, for example, click-cyclizable nitriles, pathway B for
benzaldehyde oxime 2a exclusively is activated. In stark contrast,
solvents, which are capable of effectively quenching 1O2, such as
1,4-dioxane, anisole, DMSO, DMF, inhibit both pathways. In the
case of 1,4-dioxane, the conversion of oxime 2a is 46% next to
19% of 3,5-diphenyl-1,2,4-oxadiazole, which evolves as the
product of nitrile oxide dimerization.
Conclusion
Our investigations started with determining the crystal structure of
K-PHI. We showed that it crystallizes in a hexagonal lattice with
unit cell parameters: a = 12.637(3) Å, c = 3.2998(3) Å, space
group P31m. We described stacking disorder in the PHI layers
and the non-uniform distribution of K+-ions in the structure. Still,
with highest probability of K atoms to be found in the center of the
channels so that they can form bonds with bridging nitrogen
atoms N(1). Once the crystal structure of K-PHI was defined, we
performed periodic DFT modelling to calculate, on one hand, the
electronic band structure and, on the other hand, the density of
states. We reached a sound agreement with the experiments in
terms of intraband electronic states at around +1.45 V as the
origin of sizeable absorption in the near-IR enabling energy
transfer reactions. Once excited with visible light, a broad range
of photochemical responses sets in. Most interestingly is the fact
that K-PHI undergoes significant singlet-triplet intersystem
crossing across a 0.20 eV energy gap affording a reasonably
long-lived triplet excited state. In terms of dynamics, the
combination of TAS and TCSPC enabled deriving 600 ps for the
radiative decay of the singlet excited state, 470 ps for the non-
radiative intersystem crossing, 2.07 µs for the non-radiative decay
of the triplet excited state.
Figure 7. Mechanism investigation. (a) Cyclic voltammetry study. From bottom
to top: electrolyte purged with argon; electrolyte saturated with O2; a solution of
benzaldehyde oxime 2a (2 mM) in electrolyte purged with Ar; a solution of
benzaldehyde oxime 2a (2 mM) in electrolyte saturated with O2. (b) EPR
spectrum of in situ generated TEMPO. (c) Primary and secondary KIE (mean ±
SD., n=3) of oxadiazoles-1,2,4 synthesis. (d) Proposed mechanism of cascade
energy transfer and 1O2 quenching via pathways A and B.
(primary kinetic isotope effect, KIE, Figure 7c). The results
suggest that breaking of the C―D bond is the rate limiting step.
When benzaldehyde oxime 2a was mixed with CH3CN and
CD3CN in a 1:1 ratio, under the photocatalytic conditions using K-
PHI, CH3-substituted oxadiazole 11 forms 1.11 ± 0.04 times faster
(secondary KIE) compared to the CD3-substituted 17 oxadiazole.
In the case of Ir(ppy)3, the secondary KIE is 1.31 ± 0.05.
Compared to the homogeneous catalysis, K-PHI reduces the
isotope effect due to the polarization of the C-D bonds induced by
the electrostatic field on the K-PHI surface. Zeta potential
measurements resulted in a value of ―40 mV for K-PHI.[58]
Our work was rounded off by utilizing the triplet excited states in
1
1
K-PHI to sensitize O2 and, in turn, to employ O2 in its reaction
with a broad range of oximes. Hereby, two pathways of 1O2
quenching exist: first (pathway A), it is the oxime dehydrogenation,
which affords nitrile oxide and, second (pathway B), it is the
addition to oxime C=N bond, which results in aldehyde formation.
When 1O2 quenching is performed via pathway A in, for example,
the presence of reactive multiple bonds (C≡N, C=C), the nitrile
6
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RESEARCH ARTICLE
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oxadiazoles-1,2,4 and isoxazoles were synthesized in 11-82%
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In the context of this work, the developed photocatalytic [3+2]-
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AS and MA are grateful the Deutsche Forschungsgemeinschaft
(DFG-An 156 13-1) and Max Planck Society for the financial
support. Dr. Bogdan Kurpil (developing conditions of oxadiazoles-
1,2,4 chromatographic purification), Prof. Burkhard König
(providing [Mes-Acr]+ClO4―), Olaf Niemeyer (the head of NMR
facility of the MPICI), Michael Born and Klaus Bienert (electric
workshop of the MPICI) are acknowledged for their contribution to
this project. This work was supported by resources provided by
the Pawsey Supercomputing Centre with funding from the
Australian Government and the Government of Western Australia.
AK acknowledges an Australian Research Council (ARC) Future
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Keywords: carbon nitride • potassium poly(heptazine imide) •
solid state sensitizer • singlet oxygen • organic photocatalysis
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Entry for the Table of Contents
Insert graphic for Table of Contents here.
New heterogeneous triplet sensitizer – potassium poly(heptazine imide), converts aldoximes to oxadiazoles-1,2,4 under visible light
irradiation… yet offers massive possibilities for interdisciplinary research.
Institute and/or researcher Twitter usernames: @AleksandrSavat3, @InnovativMPIKG, @MpiciPotsdam.
9
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