Light-Induced Reactions of Diazotetrahydrofuranone without Elimination of Nitrogen: Experimental and Mechanistic Study

Article abstract of DOI:10.1021/acs.joc.7b01848

The energies and lifetimes of the excited states (S₁, S₂, S₅, T₁) of a diazotetrahydrofuranone were determined using experimental and computational methods. It was shown that direction of the diazoketone photoch

Full text of DOI:10.1021/acs.joc.7b01848

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Article
Light-induced Reactions of Diazotetrahydrofuranone without
Elimination of Nitrogen: Experimental and Mechanistic Study
Liudmila L. Rodina, Victor I. Baranovskii, Olesia S. Galkina,
Valerij A. Nikolaev, Nina L. Tonogina, and Alexey V. Povolotskiy
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01848 • Publication Date (Web): 11 Oct 2017
Downloaded from http://pubs.acs.org on October 12, 2017
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Light-induced Reactions of Diazotetrahydrofuranone without Elimination of Nitrogen:
Experimental and Mechanistic Study
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Liudmila L. Rodina,* Victor I. Baranovskii, Olesia S. Galkina, Valerij A. Nikolaev, Nina L.
Tonogina, Alexey V. Povolotskiy
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Institute of Chemistry, Saint-Petersburg State University
Universitetskii prospect 26, St. Petersburg, Petergof, 198504, Russia
E-mail: lrodina@vn6646.spb.edu
Table of Contents/Abstract Graphic
O
N2
Ph
Ph
O
O
hv; T1
HN
hv; S1
N
O
O
N
N
o
- N2
hv; Sn
*
; S0
Ph
Ph
Ph
Ph
-N2
O
O
1,2 - Rearran-
gements
Abstract
The energies and lifetimes of the excited states (S1, S2, S5, T1) of a diazotetrahydrofuranone were
determined using experimental and computational methods. It was shown that direction of the
diazoketone photochemical transformations without elimination of nitrogen is determined by
multiplicity and energy of the excited state, generated by UV irradiation of diazo compound:
isomerization to α-ketodiazirine proceeds from the singlet S1 state, whereas the alternative
process of C-H-insertion with hydrazone formation occurs through the triplet T1 state. The most
probable excited state that leads to elimination of nitrogen and Wolff rearrangement is one of the
highest singlet excited states of diazotetrahydrofuranone.
Introduction
The development and application of “green chemistry” methods are the priority objectives of
modern organic chemistry.1–3 From this viewpoint one of the most attractive and promising
techniques for the initiation of chemical reactions is optical excitation which has unambiguous
advantages over the thermal and catalytic activation of chemical processes.4 That is why the
actuality of developing the effective synthetic methods and investigation of the mechanism of
light-induced formation of new bonds in organic molecules are of fundamental importance.
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The submitted research is devoted to investigation of the novel recently discovered
photochemical reaction of diazo compounds which provides a way for the creation of new C-N-
bonds in organic molecules.
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According to established opinion, irradiation of aliphatic diazo compounds with UV light
usually gives rise to elimination of N2, which excludes nitrogen atoms of diazo group from the
succeeding chemical transformations.5-8 The universally known light-induced reactions of
diazocarbonyl compounds without elimination of nitrogen are essentially limited to their
isomerisation to α-ketodiazirines. 9,10 At the same time, it was found lately that diazoketones of
tetrahydrofurane series, under distinct conditions of photochemical reaction, can avoid
elimination of nitrogen molecule. Instead, they lead to insertion of the terminal N-atom of diazo
group into C-H bonds of organic compounds, 11,12 that is can subject to light-induced processes
with conservation of nitrogen atoms of diazo group in the compounds formed (Scheme 1).
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Scheme 1. Sensitized photochemical reactions of diazotetrahydrofuranones. 11,12
The main goal of our presented study is elucidation of the pathways of these light-
induced reactions without elimination of nitrogen using the example of 4-diazo-2,2-dimethyl-
5,5-diphenyltetrahydrofuran-3-one (Figure 1), determination of multiplicity and lifetimes of the
generated excited states, as well as their function in photochemical transformations of
diazoketone 1.
Figure 1. Objects of investigation: diazoketone 1 and α-ketodiazirine 2.
In addition, light-induced and thermal reactions of α-ketodiazirine 2 (the cyclic isomer of
diazoketone 1), were also studied to understand its role in the processes under examination. By
analogy with our previous studies,11,12 tetrahydrofuran was used in photochemical experiments
as the main reaction partner with C-H bonds.
Results and Discussion
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Electronic excited states of diazoketone 1 and α-ketodiazirine 2. The information on
the electronic excited states of diazoketone 1 were obtained using experimental spectra of
absorbance (Figure S1a), luminescence (Figure S2) and phosphorescence (Figure S4), observed
lifetimes of the excited states (Figure S3, Table S2), computational studies(Figure S1b, Table
S1) and some literature data. 13-19
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UV-Vis spectra of diazoketone 1 in various solvents demonstrate three main bonds: the
highest observed optical density at 252 nm, lower band at 302 nm and the smallest band around
410 nm, which was observed in concentrated solution of diazoketone 1 only (Figure S1). The
computational and literature data allow us to assign the shortwave absorption to π-π* transitions
of diazoketone 1, as well as the longwave band at 410 nm can be equally assigned to π-π* and n-
π* transitions (Table S1). Based on the experimental and computational spectra of absorbance,
fluorescence and phosphorescence kinetics curves of diazofuranone 1, the energy level diagram
for diazoketone 1 was created (Figure 2). The lifetimes of electronic excited states were also
determined, which were further used for interpretation of the observed transformations of
diazoketone 1.
Figure 2. Energy level diagram of diazoketone 1 with optical transitions and lifetimes of excited
states.
As can be seen from Figure 2, the observed absorption bands corresponds to the electron
transitions from the ground state to the singlet excited states S1, S2 and S5 of diazoketone 1. It
should be noted that the lifetime of the singlet state S2 is greater than with the state S1 (11.6•10-9
s and 3.5•10-9 s respectively), so the probability of photochemical reactions through the state S2
should be higher in comparison to S1.4,20 The lifetime of state S1 is less than S2, probably because
of the intersystem crossing (ISC) into the triplet state T1 (the energy difference between S1 and
T1 is only 2-4 kcal/mol). The lifetime of the T1 state is about 24•10-6 s, which provides a good
possibility for intermolecular processes.
Similarly to diazoketone 1, the energy level diagram of diazirine 2 was created (Figure 3)
based on the spectral data (Figure S8). Relying on the position of the energy levels in this
diagram, one can conclude that diazirine 2 should be rather stable when excited by 405 nm light,
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but could subject to photochemical reactions when excited by UV radiation with wavelengths
less than 370 nm.
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Figure 3. Energy level diagram of α-ketodiazirine 2.
Photochemical transformations of diazoketone 1. Excitation of diazoketone 1 for the
singlet states S1, S2 and S5 was carried out by monochromatic sources with wavelengths 405 nm,
308 nm and 266 nm respectively. Photolysis occurred at 20-25 оС in solution of tetrahydrofurane
with the addition of H2O (100/1) to trap the assumed ketene formed by Wolff rearrangement of
diazoketone 1.7,21-25 The reactions were monitored by IR (Figure S5) and 1H NMR
spectroscopies.
Optical excitation of diazoketone 1 with 266 nm or 308 nm light leads to formation of
three main reaction products: α-ketodiazirine 2, hydrazone 3 and oxetanecarboxylic acid 4 (or its
anhydride 5) in the yields of 10-30 % each (Scheme 2).
Scheme 2. Photochemical transformations of diazoketone 1.
The kinetics of photochemical transformations of diazoketone 1 upon irradiation with 266
nm light is represented in Figure 4a. As is seen from these data, during irradiation of diazoketone
1 the concentration of diazirine 2 at first increases but then begins to decrease with time. Similar
behaviour of the kinetic curve is also observed in the case of oxetanecarboxylic acid 4. These
observations most likely point to a partial photochemical decomposition of the initially formed
reaction products upon continuous UV irradiation of the reaction mixture. 26-28
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Figure 4. Kinetic curves of the reaction mixtures composition upon diazoketone 1 irradiation by
light with wavelengths of (a) 266 nm*), (b) 405 nm; diazirine 2 irradiation by (c) 266 nm light.
*) Similar kinetic results were obtained upon irradiation of diazoketone 1 with 308 nm UV light.
Irradiation of diazoketone 1 with 405 nm light also leads to formation of three main
reaction products 2–4 (Scheme 2, Figure 4b). However in this case, contrary to photolysis at a
wavelength of 266 nm, the formation of hydrazone 3 in the reaction mixture begins only after a
considerable time from the beginning of irradiation (more than 100 min). It is probably that this
"induction" period is necessary for the accumulation in the reaction mixture of a sufficient
amount of benzophenone which is usually formed by photochemical decomposition of oxetan-
carboxylic acid 4 26,27 and is mediated by the formation of hydrazone 3. At the same time, after
total conversion of diazoketone 1 concentration of diazirine 2 remains practically unchanged,
since it does not absorb in the spectral region of about 405 nm.
It should be also noted that irradiation of diazoketone 1 in the oxygen atmosphere leads to
an almost threefold decrease of the yield of hydrazone 3 (Scheme 2). This observation can
evidently serve as an argument in favor of the participation of the triplet excited states of
diazofuranone 1 in the process, since its concentration in the presence of the "quenching" triplets
(O2) goes down significantly. 11
Thus, it was shown that the optical excitation of diazoketone 1 by monochromatic light
with wavelengths of 405 nm, 308 nm and 266 nm into singlet states S1, S2, S5 respectively leads
to formation of three basic reaction products of photolytic reaction: diazirine 2, hydrazone 3 and
oxetancarboxylic acid 4 (or anhydride 5). According to the kinetic data, upon irradiation of
diazoketone 1 with 405 nm light, a monotonic accumulation of diazirine 2 occurs (Figure 4b),
but when irradiation of diazoketone 1 proceeds with 266 nm light, the concentration of diazirine
2 is initially increased, and then decreased in the course of time.
Photochemical transformations of α-ketodiazirine 2. The irradiation of α-ketodiazirine
2 was carried out with 266 nm monochromatic light at 30-35 °C in solution of THF and
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monitored by IR spectroscopy. The main directions, as well as kinetics of photochemical
transformations of diazirine 2 and accompanying processes under the indicated conditions are
presented in the Scheme 3 and Figure 4c.
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Scheme 3. Photochemical transformations of α-ketodiazirine 2.
As evident from the kinetic data on Figure 4c, in the first 10-15 min of the diazirine 2
irradiation the main photochemical process is isomerization to diazoketone 1. At the same time,
the concentrations of hydrazone 3 and oxetanecarboxylic acid 4 at this initial stage of the process
(5-10 min) are close to zero and begin to increase only after diazoketone 1 appears in the
reaction mixture. It is pertinent to note here that photolysis of diazoketone 1 under similar
conditions leads to formation of oxetancarboxylic acid 4 in the reaction mixture immediately
after the beginning of irradiation (Figure 4a). Thus, it can be concluded that irradiation of
diazirine 2 with 266 nm light gives rise only to isomerization of it to diazoketone 1, which is
further converted upon irradiation to N-substituted hydrazone 3 and oxetanecarboxylic acid 4
(Scheme 3). It is also evident that α-ketodiazirine 2 is not an intermediate on the way from
diazoketone 1 to the products of photolytic reaction studied.
Thermal transformations of α-ketodiazirine 2. To recognize a possibility of parallel
thermal processes during photochemical reactions in hand, the thermolysis of α-ketodiazirine 2
о
was studied. The reaction was carried out at 102 4 С in DMSO-d6 solution with the addition
of H2O (100/1) to trap the product of Wolff rearrangement – ketene 6. The conversion of
diazirine 2 was controlled by TLC, reaction mixture was analyzed by 1H NMR spectroscopy. As
a result of thermolysis three basic reaction products were obtained - diazoketone 1,
oxetanecarboxylic acid 4 and vinylketone 7 (Scheme 4).
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Scheme 4. Thermal reactions of α-ketodiazirine 2.
In view of the fact that diazoketone 1 is thermally quite stable at these conditions,29,30 it
can be concluded that the products of diazirine 2 thermolysis (diazoketone 1, oxetanecarboxyic
acid 4 and vinyl ketone 7) are formed solely from diazirine itself. This means that, unlike
photochemical transformations, thermolysis of diazirine 2 occurs in two parallel directions:
isomerization to diazoketone 1 and elimination of nitrogen to produce Wolf rearrangement
product 4 and 1,2-aryl group migration 7.
General scheme of diazoketones and diazirines of tetrahydrofuran series
transformations. Based on the obtained experimental data, energy levels of the excited states of
diazoketone 1 and diazirine 2, as well as the calculated energies of the ground states of the
reactants and reaction products, the general diagram of the observed photochemical reactions is
proposed (Figure 5). Excitation of diazoketone 1 with light at wavelengths of 405 nm, 308 nm
and 266 nm into singlet states S1, S2 and S5 leads to formation of three main products of the
photolytic reactions studied: α-ketodiazirine 2 and hydrazone 3 (in reactions without nitrogen
elimination), as well as the products of 1,2-nucleophilic rearrangements 4-7 (in reactions with
elimination of N2).
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Figure 5. General diagram of the photochemical transformations of diazoketone 1 and α-
ketodiazirine 2. The diagram shows the wavelengths of the pump.
Two versions of diazoketone 1 excited state can be represented in the form of mesomeric
structures of biradical A,A’ formed upon the transition of one of the n-electrons from py-orbital
of the terminal nitrogen atom to disintegrating πx*-orbital (Scheme 5). Placed at the neighboring
atom N (mesomeric form A) this electron can promote abstraction of H-atom from hydrogen
donor to give eventually N-substituted hydrazone 3 (Scheme 5, right pathway).
Scheme 5. The assumed alternatives of the diazoketone 1 excited states stabilization without
elimination of nitrogen.
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In the alternative case with the location of this electron at the atom C of diazo group (as is shown
in A') the stabilization of such a biradical intermediate may be intramolecular recombination
with the formation of diazirinic heterocycle 2 (Scheme 5, left pathway).
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The resulting α-ketodiazirine 2 can relax from the excited states to vibrationally excited
sublevels of the ground state S0*. The subsequent stabilization of these "hot" states can
principally take place in three ways (Figure 5): a) vibrational relaxation to ground state of
diazirine 2; b) a large energy loss and conversion to the initial diazoketone 1; c) elimination of
nitrogen and subsequent rearrangements with the formation of ketene 6 or vinylketone 7. The
possibility of the last direction was confirmed by experimental results on diazirine 2 thermolysis,
which in this case gave rise to Wolff rearrangement products (4 and 5) and 1,2-migration of the
aryl group 7 (Scheme 4).
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Most efficiently diazirine 2 is produced by excitation of diazoketone 1 at the wavelength
of 405 nm (Scheme 2). As this takes place a monotonic increase in its concentration is observed
(Figure 4b) since diazirine 2 does not absorb in this spectral region and, obviously, does not
undergo further transformations. At the same time when diazoketone 1 is excited at the
wavelengths of 266 nm (and 308 nm), the growth of diazirine 2 concentration is observed at the
beginning of irradiation followed by a decrease with time (Figure 4a), which evidently points to
the reversibility of diazoketone 1 - diazirine 2 photoisomerization.
The potential energy curves along the reaction coordinate diazirine – diazoketone for
ground state S0 and excited singlet S1, S2 and triplet T1 states were calculated to clarify the
question which excited energy level of diazoketone 1 is responsible for diazirine 2 formation
(Figure S6). It was found that the levels of S1 and T1 were very close and there was a conical
intersection of the states S0 and T1. Thus it is reasonable to assume, that the formation of
biradical A,A’ can occur both in the singlet S1 and in the triplet Т1 states, populated by ISC
(Figure S6). Further stabilization of this intermediate can occur as a typical diabatic process,
proceeding first on the potential surface of the excited state S1 or T1 and passing (without
emission of light) onto the ground state of diazirine 2. At the same time no effect of oxygen on
the yield of diazirine 2 upon irradiation of diazoketone 1 was found. Therefore, one can conclude
that α-ketodiazirine 2 is formed from the singlet state S1 of diazoketone 1.
It is evident that formation of С-Н insertion product (hydrazone 3) occurs with the
participation of the triplet excited state (with lifetime in microsecond region) as a result of
intermolecular interaction of the diazocarbonyl biradical A,A’ with С-Н donor, tetrahydrofurane
(Scheme 5). As it was already mentioned, the triplet state can arise from the singlet state S1 of
diazoketone 1 via ISC, which should be very effective, since the energy difference between the
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levels S1 and T1 is only about 2-4 kcal/mol (Figure 5). Additional indirect argument in favour of
participation of the triplet intermediates in these processes originates from the results of
photolysis of tetramethylsubstituted analogue of diazoketone 1, diazotetrahydrofuranone 8
(Scheme 6). 11
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Scheme 6. Photochemical transformations of diazoketone 8 under various conditions. 11
In contrast to diazoketone 1, the direct irradiation of tetramethylsubstituted analogue 8 failed to
produce the product of the C-H insertion 9. Instead, only Wolff rearrangement product С was
isolated from reaction mixture (more than 90%). At the same time, hydrazone 10 was obtained
from diazoketone 8 (in the yields of up to 69 %), but under conditions of sensitized photolysis. 11
Similar alternative apparently is realized upon irradiation of diazoketone 1 with 266 nm
and 308 nm light, which also leads to formation of hydrazone 3 (Scheme 2). As it was
established earlier 26,27 during direct photolysis of diazoketone 1 a partial decomposition of the
resulting diphenylsubstituted oxetane derivatives occurs to produce benzophenone. The latter
promotes the appearance of diazoketone 1 in the triplet state, which thereafter reacts with H-
donors to furnish the products of C-H insertion under conditions of direct irradiation.
The participation of triplet states in the formation of C-H-insertion products 3 is also
confirmed by photochemical experiments in the atmosphere of oxygen – a “quencher” of triplet
states. Under these conditions, the yield of hydrazone 3 notably decreases, while the efficiency
of Wolff rearrangement is growing (Scheme 2). 11
As for the products of 1,2-nucleophilic rearrangements 4-7, they are evidently formed
from the singlet excited states of diazoketone 1.17,18,20,31-35 The potential energy curves along the
reaction coordinate of diazoketone 1 - ketene 6 at Figure S7 shows that in order to obtain ketene
6 from diazoketone 1, it is necessary to overcome the energy barrier of 60 kcal/mol. Since the
energy of the S0-S1 transition is about 70 kcal/mol (Figure S7), this process can be realized
through any electronically excited state of diazoketone 1, starting with S1. This preliminary
estimation is confirmed by experimental results, demonstrating the possibility of Wolff
rearrangement upon irradiation of diazoketone 1 with 405 nm light (Scheme 2), which
corresponds to the energy of the S0-S1 transition.
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At the same time, excitation at the higher singlet states of diazoketone 1 leads to a more
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efficient Wolff rearrangement. Thus, it was shown earlier that the yield of oxetancarboxylic acid
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4 at short-wavelength excitation (> 210 nm) increases by almost an order of magnitude in
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comparison to the longwave irradiation (> 310 nm) (Scheme 2).
This experimental data
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indicates that the Wolff rearrangement of diazoketone 1 does not only occur through the lower
level of S1 when irradiated with 405 nm light, but also through the upper singlet excited states
populated by the short-wave irradiation of diazoketone 1.
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The conversion of diazirine 2 to ketene 6 upon photoexcitation is not realized since there
are no conical intersection of S0 and T1 on the potential energy surfaces of photochemical
reactions. Thus, one can conclude that diazirine 2 can not directly participate in photochemical
Wolff rearrangement, but only through diazoketone 1, which is formed via photochemical
isomerization of diazirine 2.
Conclusion
The effect of multiplicity and energy of the excited states of diazotetrahydrofuranone on the
direction and pathways of photochemical transformations was established using optical and
theoretical methods of investigation. Photochemical reactions of diazoketone without elimination
of nitrogen lead to isomerization into α-ketospirodiazirine or to insertion of the terminal nitrogen
atom of diazogroup into C-H bond of the solvent to produce N-substituted hydrazone.
Isomerization of diazoketone to α-ketodiazirine proceeds in the singlet S1 excited state,
whereas the alternative reaction giving rise to C-H insertion products occurs in the triplet T1
state. The latter arises during the course of photochemical reaction, either by ISC of the
diazoketone S1 into T1 state, or by sensitization of diazoketone with benzophenone, which is
formed in reaction media due to decomposition of the primary reaction products. The formation
of α-ketodiazirine from diazoketone is a diabatic process occuring on the excited potential
surface and terminating without emission of photons on the potential surface of the ground state
of diazirine.
Irradiation of α-ketodiazirine with 266 nm UV light does not cause elimination of
nitrogen, but gives rise solely to reversible isomerization to diazoketone. By contrast,
thermolysis of α-ketodiazirine at 102 °C, mainly leads to elimination of nitrogen and 1,2-
nucleophilic rearrangements, as well as to partial isomerization to diazoketone.
In photochemical processes studied the most probable excited state leading to elimination
of N2 and rearrangements is one of the highest singlet states of diazotetrahydrofuranone.
Experimental Section
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General. H NMR and C NMR spectra were recorded in CDCl3 or DMSO-d6 solutions on a
Bruker Avance 400 MHz spectrometer using tetramethylsilane as an internal standard. HRMS
experiments were performed on a Bruker micrOTOF instrument. Melting points were recorded
on a Büchi B-540 apparatus. Column chromatography was carried out on the neutral silica gel
(70-230 mesh) with hexane and acetone as eluents in gradient regime.
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UV-Vis spectroscopy. Measurement of the UV-Vis spectra was carried out with a Perkin Elmer
Lambda 1050 spectrophotometer. The samples of diazoketone 1 or diazirine 2 were dissolved in
THF at the 1 mg/ml concentration and placed in a quartz cuvette with the 1 cm optical path
length. A cuvette with pure THF was placed in the comparison channel.
Calculation of the absorption spectra was carried out by TD DFT. Using the calculated data, the
energies of optical transitions from the ground state of molecules to excited singlet states are
determined.
The luminescence and luminescence excitation spectra were measured with a Horiba
Fluorolog-3 spectrometer. The samples of diazoketone 1 or diazirine 2 were dissolved in THF
and brought to solutions concentrations with the optical density of 0.05 cm-1 at 252 nm
wavelength.
Triplet states determination. The energy difference between the basic S0 and the triplet T1
states of the molecules was determined from the phosphorescence spectra. The phosphorescence
spectra were measured at a liquid nitrogen temperature with 390 nm excitation.
The lifetime of the excited states was measured with a Horiba Fluorolog-3 spectrometer using
pulsed LED excitation sources with 1 ns pulse duration. The kinetic curves of the luminescence
decay at different wavelengths of the molecules were recorded after pulsed optical excitation to
measure the lifetime excited states of the samples. The obtained experimental curves were
approximated by the following exponential function: y=A+B*exp(-x/τ) where A and B are the
approximation parameters, τ is the required lifetime of the excited state.
FTIR spectroscopy. Photochemical transformations of diazoketones in THF solutions have been
investigated under excitation by various radiation sources: THORLABS Mounted High-Power
LED 405 nm, Xe-Cl laser 308 nm and Coherent MBD-266 266 nm. Reaction mixtures were
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monitored by IR spectra at certain time intervals. The IR spectra were measured by Thermo
Scientific Nicolet 8700 FT-IR spectrometer equipped with Attenuated Total Reflectance (ATR)
sampling accessory.
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Most of the considered in the article compounds were obtained and fully characterized by
spectroscopic methods in earlier publications: 128,36, 311,28 , 411,28 , 627 , 711,26 , B12 , C11,12 , D11,12
.
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Diazoketone 1 was obtained according to the previously described protocols,28,36 α-ketodiazirine
2 was prepared for the first time by photochemical isomerization of diazoketone 1. 12
Synthesis of α-ketodiazirine 2. Solution of diazoketone 1 (730 mg, 2.5 mmol) in freshly
distilled cyclohexane (5 mL), placed in a small quartz vessel, was irradiated at 30–35°C by long-
wave UV light with λ 395−405 nm (two lamps UV LED Spotlight, 3 watt each) for 250 h under
air atmosphere. The solvent from reaction mixture was removed in vacuo and residue was
1
analyzed by H NMR spectroscopy (internal standard – Cl2CHCHCl2) followed by silica gel
column chromatography to afford α-ketodiazirine 2.
6,6-Dimethyl-4,4-diphenyl-5-oxa-1,2-diazaspiro[2.4]hept-1-en-7-on (2), yield 267 mg (46%;
based on 80% conversion of diazoketone 1). Colorless solid, mp 73-75 °C (hexane), Rf 0.67
(hexane/acetone 10:1). IR (neat), cm–1: 1754 (C=O). UV (THF), nm: 315, 328, 345, 367 (Figure
1
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S8). H NMR (400 MHz, CDCl3), δ, ppm : 7.37 – 7.30 (m, 10H), 1.39 (c, 6H) (Figure S9). C
NMR (101 MHz, CDCl3), δ, ppm: 210.0, 140.5, 128.5, 128.4, 127.3, 81.8, 80.8, 38.9, 25.1
(Figure S10). HRMS (ESI-TOF): m/z [M + Ag]+, calc for C18H16N2O2Ag+: 399.0257, found
399.0270.
Acknowledgments
The work was carried out using the equipment of the Research Park of St. Petersburg State
University: Magnetic Resonance Research Centre, Centre for Optical and Laser Materials
Research, Chemical Analysis and Materials Research Centre, Centre for X-ray Diffraction
Studies.
Supporting Information. UV-Vis spectra for compound 1, quantum-chemical calculations for
compound 1, luminescence spectrum for compound 1, kinetic of luminescence for compound 1,
phosphorescence spectrum for compound 1, observed lifetime of excited states for compound 1,
FTIR spectra of the reaction mixture, vibrational bands in the IR spectra, DFT calculation
coordinates for compound 1, DFT calculation coordinates for compound 2, DFT calculation
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coordinates for compound 6, DFT computation for reaction pathway 2 → 1, DFT computation
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for reaction pathway 1 → 6, UV-Vis spectrum for compound 2, Н NMR spectrum for
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compound 2, C NMR spectrum for compound 2. This material is available free of charge via
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the Internet at http://pubs.acs.org.
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