Structure and Photochemistry of N-Salicylidene-p-carboxyaniline Isolated in Solid Argon

Article abstract of DOI:10.1021/acs.jpca.5b06516

Infrared matrix isolation spectroscopy and DFT/B3LYP/6-311++G(d,p) calculations have been used to characterize the conformational space of the enol-imine and keto-amine tautomers of N-salicylidene-p-carboxyaniline (SCA) in both their E and Z isomeric forms. Monomers of SCA were isolated in an argon matrix (15 K), which was shown to contain only the most stable conformer of the E-enol isomer of the compound. The matrix-isolated E-enol was then subjected to in situ UV irradiation (λ = 335; 345 nm, provided by a laser/MOPO system, or λ > 235 nm, provided by a Hg(Xe) broad-band source), and the photoinduced processes probed by infrared spectroscopy. Two photoreaction channels were observed, with a branching ratio of ～1:1, corresponding to E-enol → Z-enol isomerization and E-enol → E-keto tautomerization. Both processes were found to be rather effective, with practically complete consumption of the reactant after broad-band irradiation by 1 min only. Identification among the photoproduced species of the Z-enol conformer that differs from the reactant only by E-to-Z isomerization suggests the initial photoproduction of this conformer, which subsequently decays into the lowest energy Z-enol conformer (also identified experimentally). The E-enol → E-keto tautomerization requires an excited state intramolecular proton transfer and twisting about the exocyclic CC bond of the molecule. These processes most probably take place sequentially. However, in the present study the Z-keto isomer, which should act as intermediate in this sequence of processes, could not be detected, most probably due to its short lifetime under the used experimental conditions. On the contrary, the detailed structural and vibrational characterization of the photoproduced E-keto form was successfully achieved. (Figure Presented).

Full text of DOI:10.1021/acs.jpca.5b06516

Article
pubs.acs.org/JPCA
Structure and Photochemistry of N‑Salicylidene‑p‑carboxyaniline
Isolated in Solid Argon
†
‡,§
†
,‡
Mihaela Avadanei, Nihal Kus,
̧
Vasile Cozan, and Rui Fausto*
†
“
P. Poni” Institute of Macromolecular Chemistry, 700487 Iasi, Romania
‡
Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal
§
Department of Physics, Anadolu University, 26470 Eskise
̧ hir, Turkey
*
S Supporting Information
ABSTRACT: Infrared matrix isolation spectroscopy and DFT/
B3LYP/6-311++G(d,p) calculations have been used to characterize
the conformational space of the enol-imine and keto-amine
tautomers of N-salicylidene-p-carboxyaniline (SCA) in both their E
and Z isomeric forms. Monomers of SCA were isolated in an argon
matrix (15 K), which was shown to contain only the most stable
conformer of the E-enol isomer of the compound. The matrix-
isolated E-enol was then subjected to in situ UV irradiation (λ = 335;
345 nm, provided by a laser/MOPO system, or λ > 235 nm, provided
by a Hg(Xe) broad-band source), and the photoinduced processes
probed by infrared spectroscopy. Two photoreaction channels were
observed, with a branching ratio of ∼1:1, corresponding to E-enol →
Z-enol isomerization and E-enol → E-keto tautomerization. Both processes were found to be rather effective, with practically
complete consumption of the reactant after broad-band irradiation by 1 min only. Identification among the photoproduced
species of the Z-enol conformer that differs from the reactant only by E-to-Z isomerization suggests the initial photoproduction
of this conformer, which subsequently decays into the lowest energy Z-enol conformer (also identified experimentally). The E-
enol → E-keto tautomerization requires an excited state intramolecular proton transfer and twisting about the exocyclic CC bond
of the molecule. These processes most probably take place sequentially. However, in the present study the Z-keto isomer, which
should act as intermediate in this sequence of processes, could not be detected, most probably due to its short lifetime under the
used experimental conditions. On the contrary, the detailed structural and vibrational characterization of the photoproduced E-
keto form was successfully achieved.
1
. INTRODUCTION
following the electronic excitation of the E-enol form with UV
light (Scheme 1).
Salicylidene aniline derivatives change from yellow to red
upon irradiation with UV light and back to the original yellow
upon exposure to heat (thermal fading) or visible light. The
photochromic cycle can be described by the succession: light
The N-salicylidene anilines (SA; Scheme 1) belong to the
o‑hydroxyaryl Schiff bases family and constitute a special class
of organic materials showing switchable properties triggered by
light, such as photochromism and photoluminescence, which
can be efficiently exploited in practical applications like fast
molecular photoswitches, optical storages, and molecular
absorption to the first excited (S , π, π*) state of the E-enol →
1
4−7
excited state proton transfer (ESIPT),
leading to the Z-keto
1
−3
machines.
These molecules have an internal hydrogen
form → isomerization to the chromophoric E-keto form/
emission → thermal isomerization and back proton transfer in
the ground state, with recovery of the proton by the phenolic
bond involving the phenolic and the azomethine groups that
facilitates intramolecular proton transfer, thus allowing the
existence of two tautomeric forms: enol-imine and keto-amine.
In turn, each of these tautomers has two geometric isomers
8
−13
group and restoring of the initial enolic form.
The proton
transfer process triggers a suite of geometrical changes, starting
with the loss of aromaticity in the phenolic moiety through
tautomerization, and ending with the isomerization into the
final chromophoric E-keto structure.
(
E and Z forms), defined around the exocyclic CN (enol-
imine tautomer) or CC bond (keto-amine tautomer), which
may possess several conformational isomers.
For most of N-salicylidene anilines, the ground state is
populated by the E-enol form, which is stabilized by an
OH···N intramolecular hydrogen bond that also locks the
salicylidene unit in a nearly planar conformation. The rich
photochemistry, in particular the photochromism exhibited by
these compounds, relies on a complicated succession of events,
The phototautomerization can be accompanied by an E → Z
isomerization around the azomethine CN bond, which
Received: July 7, 2015
Revised: August 4, 2015
©
XXXX American Chemical Society
A
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Scheme 1. General Scheme of the Photochemistry of N-Salicylidene Anilines
brings a twisted conformation of the enol form. Predicted
theoretically for the salicylidene aniline parent compound,
the Z-enol isomer has been connected with a remnant negative
One of the most important features of the photochromic
compounds is the fact that their photodynamics and photo-
reactivity can be controlled by molecular environment and
chemical substitution. From this perspective, minimizing the
intermolecular interactions and inhibiting molecular diffusion is
desirable to investigate the photophysical events without
external influences. These conditions are readily available by
isolating the molecules of the target compounds in low
temperature cryogenic noble gas matrixes. However, studies on
matrix isolated Schiff bases are scarce and have been essentially
centered on the analysis of conformational characteristics of the
6
,11
band observed in the transient absorption spectra, on the UV
1
4
wing, for N,N′-bis(salicylidene)-p-phenylenediamine, bis-
(
(
(phenylimino)methyl)hydroquinone, and 4-methoxy-2,5-bis-
(phenylimino)methyl)phenol. An infrared spectroscopic
15
proof of the twisted geometry has been recently reported for
2
trix.
-(1-(methylimino)methyl)phenol isolated in an argon ma-
1
6
The chemical structure of the colored photoproduct,
although widely recognized as the E-ketonic species, is still a
matter of some debate. It has been considered that the neutral
22−24
molecules and hydrogen bonding aspects.
To the best of
our knowledge, only a few studies have been concerned with
16,25
17
the photochemical transformations of these molecules.
quinoid form can coexist with a zwitterionic form, and that
the dominance and lifetime of one particular structure depend
on the molecular surroundings.
The present investigation focused on the photoinduced
structural changes of N-salicylidene-p-carboxyaniline (abbre-
viated, SCA) isolated in a solid argon matrix. Although the
photochromic behavior of SCA in the crystalline state is known,
The characteristic photodynamics of N-salicylidene anilines
can become more complex due to the existence of further
isomers and conformers of both the starting compound and the
transient species, and also due to competing radiative and
nonradiative channels. Thus, creation of the Z-enol and the
chromophoric E-keto photoproducts takes place through two
S /S conical intersections (CI), reached in a barrierless mode
26
and shown to be governed by the packing arrangement, the
unimolecular photochemistry of SCA had never been addressed
hitherto.
The well-known photochromism exhibited by the parent
27
compound, salicylidene aniline, in the solid phase, is
1
0
considered to be related with the large deviation (36°) of the
two aromatic rings from planarity, which “prepares” the
molecule for the E−Z isomerization. On the contrary, due to
the terminal −COOH group, SCA has the ability to self-
by rotation around the exocyclic CN and CC bonds,
respectively (Scheme 1). Deactivation of the S (π,π*) state of
1
the Z-keto form through fluorescence is concurrent with the
26
internal conversion through the S /S CI, and the preferred de-
1
0
associate in the solid state as dimers. The presence of the
excitation pathway is determined by details of the chemical
structures and medium factors.
neutral E-enol form in the ground state was confirmed by X-ray
26
diffraction and infrared spectroscopy data. In the crystalline
state, the SCA molecules adopt an almost flat geometry, the
dihedral angle between the two aromatic rings being only
Because of the multiple events at the excited state level, the
photochromic cycle of SA derivatives has been mainly studied
by electronic absorption and fluorescence spectroscopy, using
7.8(3)°. The claim resulting from former studies that SCA did
1
8,19
27−29
steady state or time-resolved techniques. Conventional
time-resolved infrared spectroscopy
and
not exhibit photochromism in solid state
was then
20,21
were used to identify
attributed to the almost planar conformation and crowded
packing in the crystal. However, in contrast to those studies,
our recent study has revealed that the compound is effectively
the photoproducts of irradiated salicylidene aniline in solution,
polycrystalline film and encaged in zeolites.
B
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2
6
photochromic in the solid state. The relative good quantum
3. RESULTS AND DISCUSSION
.1. Geometrical Features and Energies of SCA
2
6
yield of fluorescence in the crystalline state (0.115),
3
combined with the photochromic behavior, seems to indicate
a quite unusual behavior under light excitation that is worth
investigating.
In the present study, SCA was isolated in an argon matrix, at
5 K, and irradiated with laser light (λ = 335, 345 nm) or with
the full wavelength range of a high-pressure Hg(Xe) lamp.
Establishing the structure of SCA species present in the
cryogenic matrix and the photoproducts identification were
assisted by quantum chemical calculations carried out at the
DFT/B3LYP/6-311++G(d,p) level of approximation. Besides
the detailed structural analysis of the potential energy surface of
the compound and of the vibrational spectra of its possible
isomeric forms, this study provides relevant details on the
photochemical transformations of the isolated molecule of
SCA, thus also contributing to further our understanding of the
general photochemistry of N-salicylidene anilines.
Tautomeric Forms. The enol form of SCA has six internal
axes of rotation that may give rise to different isomers. Three of
these axes are defined within the salicylidene−azomethine
fragment [(C H OH)CHN], corresponding to internal
6
4
1
rotations around the CO phenolic and CN azomethinic
bonds, as well as around the CC bond connecting the
azomethine moiety to the phenolic ring. In the p-carboxyphenyl
fragment [(C H )COOH], two internal rotations shall be
6
5
considered, the first being defined around the CC bond
connecting the carboxylic substituent to the ring, and the
second corresponding to the rotation around the carboxylic
CO bond. The sixth internal rotation axis corresponds to the
NC bond connecting the salicylidene−azomethine and
p‑carboxyphenyl fragments. Though a priori the molecule
appears to be rather complicated in terms of possible geometry
arrangements, the situation is in fact relatively simple: (i) The
internal rotation about the carboxylic CO bond leads only to
two minimum energy conformations, where the OCO
H dihedral takes the values of 0° (cis) or 180° (trans), the latter
corresponding to a high-energy configuration with a relative
energy above the most stable geometry that exceeds 40 kJ
2
. EXPERIMENTAL AND COMPUTATIONAL METHODS
2
.1. Synthesis. The synthesis of SCA has been described in
26
a previous publication. Briefly, stoichiometric amounts of
salicylaldehyde and carboxybenzaldehyde were allowed to react
in ethanol, at room temperature, for 5 h. The solvent was
removed, and the remained solid product was washed with
ethanol three times, affording a pure and yellow crystalline
powder. Ethanol (Chimopar SA) and the starting materials
−1
34−36
mol and, thus, being of no practical relevance.
(ii) The
internal rotation about the CC bond connecting the
carboxylic substituent to the ring, for a cis arrangement of the
OCOH fragment, gives rise to only two minima, where
the carboxylic substituent is in the same plane of the aromatic
(
Sigma-Aldrich) were used as received.
.2. Matrix-Isolation FTIR Spectroscopy. SCA was
37
2
ring. The two minima can be expected to have close energies
and they must be both considered in the structural analysis of
the molecule. (iii) Both the orientation of the phenolic OH
group and the preferred geometry around the CC bond
connecting the azomethine and phenolic moieties are
essentially determined by the possibility of establishment of
the OH···N intramolecular hydrogen bond, so that the most
favorable arrangements of this fragment of the molecule can be
easily predicted, corresponding to a nearly planar geometry
where the intramolecular hydrogen bond participates in a
pseudoaromatic six-membered ring. (iv) Finally, considering
the internal planarity of both the salicylidene−azomethine and
p-carboxyphenyl fragments, the rotation about the NC bond
connecting these two moieties can be easily perceived to lead to
the same minimum energy structures as the internal rotation
about the CC bond connecting the carboxylic substituent to
the ring. The preferred geometry about the NC bond can be
expected to have the salicylidene−azomethine and p‑carbox-
yphenyl fragments in different planes, taking as reference what
has been found for other molecules bearing similar structural
deposited by sublimation from a specially designed Knudsen
cell together with a large excess of the matrix gas (Ar, N60
purity, from Air Liquide) onto a CsI window, cooled at 15 K.
An APD Cryogenics closed-cycle helium refrigeration system
with a DE-202A expander was used. The temperature of the
CsI window was measured directly at the sample holder by a
silicon diode sensor, connected to a digital temperature
controller (Scientific Instruments, model 9650-1), which
provides an accuracy of 0.1 deg.
The infrared spectra were recorded, in the 4000−500 cm−1
range, using a Nicolet 6700 FTIR spectrometer, equipped with
a Ge/KBr beam splitter and a deuterated triglycine sulfate
DTGS) detector. The spectral resolution used was 0.5 cm .
.3. Laser and Broad-Band UV Irradiation Experi-
ments. In a first experiment, the matrix was irradiated using
narrow-band tunable light (0.2 cm spectral width; repetition
rate = 10 Hz; pulse energy 1−2 mJ) provided by the frequency-
doubled signal-beam of a Quanta-Ray MOPO-SL optical
parametric oscillator, pumped with a pulsed (10 ns) Nd:YAG
laser. In a second experiment, the irradiation has been
performed through the outer KBr window of the cryostat (λ
235 nm) with the broad-band wavelength profile of a 150 W
high-pressure Hg(Xe) lamp (Osram XBO 150 W/CR OFR).
.4. Theoretical Calculations. All molecular geometries
were optimized at the DFT level using the B3LYP functional,
−1
(
2
−1
28,29
elements.
Considering points (i)−(iv) above, it can be
concluded that the number of relevant structures for each enol
isomer (E and Z forms, which are defined by the orientation of
the groups around the CN bond) is in fact rather small.
The situation is even simpler for the keto tautomers of SCA,
because the internal rotation associated with the phenolic OH
group is lost in this case. The analysis made above in relation to
the internal rotations related with the carboxylic substituent
applies also to the keto tautomer, as well as that concerning the
rotation around the NC bond. Because the geometry about
the exocyclic CC bond of the keto tautomer defines the
nature of each keto isomer (E and Z), only the rotation around
the central CN bond is distinctive in the keto forms
>
2
30
which includes the Becke’s gradient exchange−correction and
the Lee, Yang, and Parr correlation functionals, and the 6-
11++G(d,p) basis set. Vibrational spectra were calculated at
the same level of theory. The theoretical frequencies were
scaled down by a factor of 0.978 in the fingerprint region, and
by 0.945 in the high frequency range. All the calculations were
31
32
3
33
performed with the Gaussian 09 suite of programs.
C
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compared to the enol forms) and, thus, must be investigated
Article
(
Figure 1A) are comparatively much higher in energy (by over
−
1
with more detail.
35 kJ mol ; Table 1), mostly due to the breaking of the
intramolecular OH···N bond in the salycilidene−azomethine
fragment. The slightly lower relative energies of forms VII and
VIII, compared to the energies of forms III−VI, result from the
presence in forms VII and VIII of the energetically more
favorable CH···N and CH···OH interactions within the
salycilidene−azomethine fragment, as compared to the
CH···N and CH···H(O) interactions existing in forms V and
VI, or CH···HC and N···OH contacts shown by forms III and
IV.
The Z-enol isomer has also eight conformers, which correlate
directly with the eight conformers of the E-enol isomer above-
discussed. As for this latter isomer, the Z-enol conformers can
be grouped in pairs showing identical structures except in the
orientation of the carboxylic group. The members of each
group are also practically isoenergetical (Table 1). Compared
to the most stable E-enol form, all Z-enol conformers have
The DFT(B3LYP)/6-311++G(d,p) calculated relative en-
ergies of the relevant structures of SCA, for both enol and keto
forms, are given in Table 1. Graphical representations of the
optimized minimum energy structures are shown in Figure
1A−C.
Table 1. Calculated Relative Energies (ΔE), Zero-Point-
Corrected Energies [ΔE(0)], and Gibbs Energies at 298.15
K (ΔG°) for the Different Forms of SCA Enol and Keto
a
Tautomers
structure
ΔE
ΔE(0)
ΔG°
ΔE
ΔE(0)
ΔG°
E-enol
I
0.00
0.31
0.00
0.25
0.00
0.16
II
III
51.99
52.06
44.51
44.30
36.02
35.86
49.21
49.16
41.91
41.74
33.35
33.19
46.52
46.54
39.46
39.30
30.72
30.48
−1
IV
energies above 60 kJ mol . The Z-enol conformer differing
from the most stable E-enol conformer essentially by
isomerization about the CN bond (Z-enol I) is the third
more stable Z-enol form and bears a weak OH···π
intramolecular hydrogen bond, established between the
phenolic OH group and the p-carboxyphenyl aromatic ring.
This conformer is 2.6 kJ mol− higher in energy than the lowest
energy Z-enol conformer (Z-enol VII), which differs from Z-
enol I in the orientation of the OH phenolic group and by
rotation about the CC bond connecting the phenol moiety
to the azomethine group (Figure 1B). This arrangement grants
a more opened structure that is also stabilized by a
CH···O(H) contact. The pairs of Z-enol conformers III/IV
V
VI
VII
VIII
Z-enol
I
1
63.49
63.60
68.65
68.67
66.91
66.92
60.86
60.91
61.68
61.85
66.46
66.61
65.00
65.02
59.33
59.36
60.52
60.75
65.25
65.39
62.54
62.47
56.67
56.75
2.63
2.73
7.79
7.80
6.04
6.05
0.00
0.04
2.35
2.52
7.14
7.28
5.68
5.69
0.00
0.03
3.85
4.07
8.57
8.72
5.87
5.80
0.00
0.08
II
III
IV
V
VI
VII
VIII
E-keto
I
−1
and V/VI have higher energies (over 6 kJ mol above the most
stable Z-enol conformer) because they bear less favorable
HO···π and OH···H contacts, respectively. All Z-enol con-
formers are strongly nonplanar, with the planes between the
azomethine and p-carboxyphenyl fragments and between the
azomethine and phenol fragments twisted by 56−68° and 23−
62.48
61.96
87.48
87.21
61.32
60.85
87.56
87.29
57.86
57.59
85.49
85.16
0.51
0.00
0.48
0.00
0.27
0.00
II
III
25.51
25.25
26.72
26.44
27.90
27.57
IV
Z-keto
I
4
3°, respectively. The geometry around the CN double bond
19.70
20.23
94.61
94.18
18.39
19.09
94.47
16.57
17.18
94.20
93.83
0.00
0.53
0.00
0.70
0.00
0.61
is also slightly deviated from planarity in all forms (6−8°), as
II
6,11,14−16
suggested before for similar compounds.
On the
III
74.91
74.48
76.07
75.66
77.63
77.26
contrary, the phenolic OH group is deviated significantly
from the plane of the aromatic ring only in conformers I/II
IV
94.05
a
−1
All energies in kJ mol . For structures of the different forms, see
(
∼19°) and V/VI (∼8°), where it is pointing inside the
Figure 1A−C. The first set of energy values are relative to the most
stable form (E-enol conformer I); the second set are relative to the
most stable conformer of the specific isomer.
molecule, whereas in the Z-enol conformers III/IV and VII/
VIII, where it points outside the molecule, it is deviated from
the ring plane by less than 3°.
In the case of the keto tautomer of SCA, the calculations
indicate that the Z isomer is considerably lower in energy than
As shown in Table 1, and in consonance with the
expectations, the calculations predict the most stable conformer
of the E-enol form as the lowest energy species among all
possible structures. In this conformer, the phenolic group is
hydrogen bonded to the azomethine nitrogen atom, forming a
nearly planar pseudoaromatic six-membered ring (OH···N
distance: 1.755 Å), and the p-carboxyphenyl moiety is also
nearly planar, with the carboxylic group assuming the cis
configuration. The planes of the salycilidene−azomethine and
p-carboxyphenyl fragments are twisted by ∼39°, with the
carbonyl bond of the p-carboxyphenyl group pointing to the
same side of the CN bond (Figure 1A). As anticipated, the
E-enol form differing from the lowest energy structure only in
the orientation of the carboxylic substituent is almost
−1
the E form (by over 40 kJ mol ), as it could be anticipated by
taking into account that the stabilizing intramolecular hydrogen
bond (NH···O) present in the Z-keto form is not present
in the E-keto isomer (Figure 1C). As for the enol isomers, for
both E- and Z-keto isomers the conformers differing only in the
orientation of the carboxylic group are almost isoenergetic
(Table 1). The most stable pair of conformers of the two keto
isomers possesses a planar geometry (C symmetry point
s
group), with the two substituted rings placed in the opposite
sides of the central CN bond. The higher energy forms are
nonplanar and have much higher energies compared with the
−1
most stable pairs of conformers (by ca. 25 and 75 kJ mol , for
E- and Z-keto isomers, respectively; Table 1). These high
energies result from the significantly unfavorable interactions
between the cyclohexadienone and p-carboxyphenyl fragments,
−1
isoenergetic to this latter (ΔE = 0.31 kJ mol ). All other
conformers of the E-enol tautomer of SCA (forms III to VIII in
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Figure 1. Conformers of (A) the SCA E-enol isomer, (B) the SCA Z-enol isomer, and (C) the SCA E-keto and Z-keto isomers. B3LYP/6-311+
G(d,p) calculated geometries are provided in the Supporting Information.
+
−
1
which are particularly relevant in the high-energy Z-keto
isomers, where the oxygen atom of the first fragment is directed
toward the aromatic ring of the second.
these two forms (>30 kJ mol , if Gibbs energies are
considered), it can be expected that, upon sublimation,
conformers I and II coexist in the gas phase equilibrium. At
room temperature, the expected populations of these two
conformers, based on their calculated Gibbs energies (0.16 kJ
By comparison of the corresponding most stable conformers,
the calculated energy order of the four isomeric forms of SCA is
−1
−1
E-enol > Z-keto (∼20 kJ mol ) ≫ E-keto and Z-enol (∼60−
mol ; Table 1) are I = 52% vs II = 48%.
−1
6
2 kJ mol ). The optimized geometries of all the investigated
As a general rule, in a matrix isolation experiment the gas
phase conformational equilibrium existing in the gaseous beam
used to prepare the matrix can be frozen in the cryogenic
media. The populations of the different conformers in the
matrix would then be equal to those of the gas phase prior to
deposition. However, if two conformers of different energies
are separated by low energy barriers (of a few kJ mol ),
conformational cooling usually takes place and the less stable
conformer converts into the most stable form during
SCA forms are provided as Supporting Information (Tables S1
and S2).
3
.2. Infrared Spectrum of SCA in an Argon Matrix. The
26
X-ray diffraction data obtained for the SCA starting material
indicated that, in the crystalline phase, molecules exist in the
most stable E-enol conformer (I). Because conformer II is
almost isoenergetic to conformer I, and all the other
conformers of the E-enol have energies much higher than
−
1
E
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3
8,39
deposition.
This phenomenon makes the information on
conformer I shall be present in the as-deposited argon matrix
of SCA.
The infrared spectrum of the Ar-matrix isolated SCA is
presented in Figure 3 (for the high-frequency range, Figure S1
the energy barriers of great relevance for interpretation of the
spectra of matrix isolated species in terms of their conforma-
tional composition.
In the case of SCA, the two E-enol conformers populated in
the gas phase equilibrium at room temperature, forms I and II,
can be interconverted by two ways: (i) by rotation around the
C−C bond connecting the carboxylic substituent to the ring;
(
ii) by the rotation about the N−C bond connecting the
salicylidene−azomethine and p-carboxyphenyl fragments. The
potential energy profiles for these two different interconversion
pathways are given in Figure 2. As can be seen in this figure, the
−1
Figure 3. Infrared spectra (in the 1800−500 cm spectral range) of
SCA monomers trapped in an argon matrix (15 K; top) and the
B3LYP/6-311++G(d,p) calculated spectra of the most stable con-
formers of the E-enol (middle) and Z-keto tautomers (bottom). The
theoretical wavenumbers were scaled down by a factor of 0.978; bands
were simulated by Lorentzian functions centered at the scaled
−1
calculated frequency and with full-width-at-half-band of 5 cm .
Calculated spectra for the most stable conformer of all SCA tautomers
are shown in Figure S2 (Supporting Information).
in the Supporting Information). The proposed band assign-
ments are provided in Table 2. As it is shown, the experimental
spectrum agrees very well with the theoretical spectrum of the
most stable E-enol conformer I, doubtlessly confirming the
presence of this species in the matrix. The presence of both the
Z-enol and E-keto forms in the spectrum of the as-deposited
matrix can be safely excluded, because formation of these
species from the E-enol can only occur upon photoexcitation.
The absence of the Z-keto form in the matrix could also be
easily confirmed. Indeed, the predicted infrared spectrum of the
most stable conformer of the Z-keto form of SCA (the one
resulting from E-enol conformer I by simple proton transfer
from the phenol moiety to the nitrogen atom of the azomethine
group) has characteristic intense infrared bands at 1644, 1552,
1520, 1511, 1308, and 631 cm , which do not have any
experimental counterpart (Figure 3). On the contrary, the
presence of the E-enol conformer II in the matrix cannot be
ruled out a priori, because the infrared spectra of the E-enol
conformers I and II are practically coincident (Figure S3).
Nevertheless, as discussed above, its absence has been
suggested by the theoretical analysis of the potential energy
surface of the molecule. To address this question exper-
imentally, the matrix was annealed up to 45 K (in steps of 3 K),
to see if any spectral changes were observed that could be
Figure 2. Potential energy profiles for interconversion between the
two most stable conformers of the E-enol isomer. Top: by internal
rotation about the C−C bond connecting the carboxylic substituent to
the ring; Bottom: by internal rotation about the N−C bond
connecting the salicylidene−azomethine and p-carboxyphenyl frag-
ments. (I/I′ and II/II′ represent pairs of symmetry equivalent
conformers.)
barrier associated with the rotation around the C−C bond
connecting the carboxylic substituent to the ring is of ca. 30 kJ
−1
mol ; i.e., it is large enough to allow both conformers I and II
to be trapped in the matrix. The energy profile for rotation
around the N−C bond is more complex, because it shows three
different transition states: the two planar transition states
−1
(
Figure 2) are associated with the conversion between the two
pairs of equivalent-by-symmetry forms (I,I′ and II,II′) and
−1
correspond to energy barriers of ca. 4.5−5.0 kJ mol ; in turn,
the transition states where the salicylidene−azomethine and
p‑carboxyphenyl fragments are nearly perpendicular to each
other interconvert conformer I into II (or I′ into II′) and stays
ca. 7 kJ mol− above the minima. These energy barriers might
be low enough to allow for conformational cooling to take
place upon deposition of the matrix. Thus, the calculations
suggest that most likely only the E-enol lowest energy
1
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a
Table 2. Assignment of the Vibrational Spectra of As-Deposited SCA Ar-Matrix and Irradiated Matrix
a
−1
IR
Wavenumbers (ν), scaled by 0.978 and 0.945 in the high frequency and fingerprint spectral regions, respectively, in cm . Infrared intensities (I )
−1
b
in km mol . Approximate descriptions were obtained by visual inspection of the animated vibrations using the graphical interface of Gaussian 09. ν,
stretching; δ, bending; γ, rocking; τ, torsion; s, symmetric; as, antisymmetric. r1 refers to the phenolic (or hexacyclodienone) rings, r2 to the p-
carboxyphenyl ring. subscripts p, az, c, and k stand for phenol, azomethine, carboxylic, and keto, respectively. (o), (m) and (p) indicate positions
c
ortho, meta, and para in the phenolic ring, in relation to the azomethyne substituent. Experimental frequencies for the irradiated matrix are for E-
keto and Z-enol (forms I and VII). Note that there are several cases where the same band is assigned to more than one species.
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1
ascribed to conversion of conformer II into the most stable
form I. According to the Barnes’ relationship (which is a well-
established empirical correlation relating the size of the energy
barrier with the temperature required to start observing a
counterparts at 1366/1338 cm− (Fermi doublet involving
participation of the δOCO + τCC combination tone, whose
fundamentals are observed at 653 and around 700 cm , the
−1
last being a torsional mode of the p-carboxyphenl ring), 1187
40
−1
−1
rotamerization process for a matrix isolated molecule), for a
cm , and in the 1165−1151 cm range (complex feature with
two different groups of bands also resulting from Fermi
interaction with the carboxylic τOH first overtone). The
νCN stretching mode of the azomethine group is predicted
−1
barrier of 7 kJ mol the isomerization should start to be
observed at a temperature below 35 K. However, until T = 39
K, no relevant spectral changes were observed. Above this
temperature evaporation of the argon started to take place (at
at 1630 cm− and observed at ca. 1622 cm , whereas the CH
1
−1
4
5 K, all argon has evaporated), and aggregation of the
bending modes (in-plane, δCH, and out-of-the-plane, γCH) of
−1
compound occurred. Formation of molecular associates
through the terminal carboxylic group is suggested by the
the same fragment were predicted at 1367 and 984 cm and
−1
observed at 1359 and at 985 cm . The azomethine νC−H
−1
−1
disappearance of the ν(CO) absorption at ca. 1751 cm and
stretching mode is predicted at 2872 cm and is tentatively
−1
−1
its replacement with the broad band at 1699 cm . Similarly,
assigned to the bands at 2869/2862 cm , though the usual low
−1
the ν(OH) band at ca. 3574 cm converts into the wider and
intensity of the bands due to CH stretching modes in the
spectra of Ar-matrix isolated molecules prevents a definitive
assignment.
−1
asymmetric band centered at 3394 cm . These changes are
accompanied by the transformation of the entire spectrum, with
most of the bands observed at 45 K showing a correspondence
with those observed in the room temperature spectrum of the
SCA powder. No indication of conformational changes were
observed along all the annealing experiment, which supports
the theoretical suggestion (vide supra) of the sole presence in
the matrix of the most stable E-enol conformer I.
One additional note shall be made here, calling attention to
the fact that several bands in the SCA Ar-matrix spectrum are
observed as multicomponent peaks, as it occurs most of times
in the spectra of matrix-isolated molecules due to the existence
of multiple matrix trapping sites. This is observed all along the
spectrum of the compound, but it is particularly noticeable for
the most intense bands, which are in most cases associated with
the more polarized groups in the molecule, whose internal
vibrational potentials are more sensitive to interactions with the
matrix atoms (namely, the bands associated with the phenolic
OH group and the carboxylic substituent).
2
6
Considering the very good agreement between the
experimental and theoretically predicted spectra, the assign-
ment of the bands (Table 2) is straightforward. In the high-
frequency range (Figure S1), the carboxylic νO−H stretching
−1
band is observed at ca. 3574 cm , at a frequency typical of a
free OH carboxylic group adopting the cis conformation
3.3. Unimolecular Photochemistry of SCA in an Argon
Matrix. The photochemical reactions experienced by SCA
molecules in cryogenic conditions were the subject of two
experiments: by using tunable UV light from a laser/MOPO
system, or exposing the sample to the broad-band light emitted
by a high-pressure Hg(Xe) lamp through the outer KBr
window of the cryostat (λ > 235 nm). The first experimental
approach used irradiations at 335 and 345 nm, which lie near
the absorption maximum of the UV/visible absorbance
−1
41−43
(
predicted value for SCA: 3565 cm ).
The stretching
vibration of the hydrogen-bonded phenolic OH group is
predicted by the calculations to give rise to a very intense
−1
infrared band at 3072 cm . Due to the involvement in the
hydrogen bond, however, and as found in other similar
4
3,44
systems,
one can expect the corresponding experimental
band to be very broad and spread a wide wavenumber range.
Accordingly, a broad feature extended from ca. 3225 to 2625
−1
29
cm is observed experimentally, which is assigned to this
spectrum of SCA (E-enol form).
−1
vibration. The band observed at 3430 cm in the high-
frequency spectral range is ascribed to the first overtone of the
carbonyl stretching vibration, whose fundamental appears at
The changes observed in the infrared spectrum of SCA upon
the different irradiations (λ > 235, or λ = 335; 345 nm) were
found to be similar, though a faster consumption of the reactant
was observed upon broad-band irradiation. Figures 4−6 show
the IR difference spectra (in three different spectral regions)
resulting from subtracting the spectrum of the as-deposited
matrix, containing exclusively the E-enol form, from the
spectrum registered after 20 min of irradiation at λ = 335 nm
followed by 30 min of irradiation at 345 nm, and from the
spectrum obtained after 1 min of broad-band irradiation (λ >
235 nm). The results obtained upon laser/MOPO irradiation at
335 and 345 nm were qualitatively identical, with the shorter
wavelength being slightly more efficient in promoting the
photochemical transformations than the longer one. The broad-
band irradiation was very effective, with consumption of
practically all reactant in the first minute of irradiation.
Prolonged broad-band irradiation led to observation of
−
1
1
751 cm .
The fingerprint region (Figure 3) shows an excellent
correspondence between the experimental and theoretical
spectra. The main deformation vibrations associated with the
hydrogen-bonded phenolic OH group are predicted at 1417
−
1
−1
cm (δOH) and 820 cm (τOH) and have correspondence
−1
with the bands at 1420/1418 and 810−795 cm in the
experimental spectrum. The displacement of the τ(OH) mode
of the phenolic OH group toward considerably higher
wavenumbers relatively to the analogous vibration of the
carboxylic OH group (observed around 571 cm , matching the
calculated value; Table 2) is related to the involvement of the
first in the intramolecular hydrogen bond. The shift of this type
of vibration to higher wavenumbers upon hydrogen bond
formation is indeed a general rule, which has been
−1
−
1
46
characteristic bands of CO (ca. 2340 cm ) and CO
2
45
−1 47
comprehensively addressed elsewhere. The δOH in-plane
bending vibration of the carboxylic group contributes to several
absorptions in the fingerprint region, mainly to those predicted
(about 2130 cm ), demonstrating occurrence of both
photodecarboxylation and photodecarbonylation reactions.
These bands were, however, completely absent from the
infrared spectrum obtained after 1 min of broad-band
−1
at 1334, 1185, and 1155/1151 cm , which have experimental
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Figure 4. From bottom to top: calculated infrared spectra of
conformers I (black) and VII (blue) of the Z-enol; calculated infrared
spectra of E-keto (bands pointing up) and E-enol most stable
conformers; simulated difference spectrum built from the calculated
infrared spectra: 0.5 × E-keto + 0.5 × (Z-enol I + Z-enol VII) − E-
enol; experimental infrared difference spectrum: matrix irradiated for 1
min with the broad-band light of the Hg(Xe) lamp (λ > 235 nm)
minus as-deposited matrix; experimental infrared difference spectrum:
matrix irradiated for 20 min of irradiation at λ = 335 nm followed by
Figure 5. From bottom to top: calculated infrared spectra of
conformers I (black) and VII (blue) of the Z-enol; calculated infrared
spectra of E-keto (bands pointing up) and E-enol most stable
conformers; simulated difference spectrum built from the calculated
infrared spectra: 0.5 × E-keto + 0.5 × (Z-enol I + Z-enol VII) − E-
enol; experimental infrared difference spectrum: matrix irradiated for 1
min with the broad-band light of the Hg(Xe) lamp (λ > 235 nm)
minus as-deposited matrix; experimental infrared difference spectrum:
matrix irradiated for 20 min of irradiation at λ = 335 nm followed by
30 min at λ = 345 nm with laser light minus as-deposited matrix. All
calculated spectra were obtained at the B3LYP/6-311++G(d,p) level
of theory and the wavenumbers were scaled by 0.978; the bands were
simulated by Lorentzian functions centered at the scaled calculated
30 min at λ = 345 nm with laser light minus as-deposited matrix. All
calculated spectra were obtained at the B3LYP/6-311++G(d,p) level
of theory and the wavenumbers were scaled by 0.945; the bands were
simulated by Lorentzian functions centered at the scaled calculated
−1
−1
frequency and with full-width-at-half-band of 5 cm .
frequency and with full-width-at-half-band of 5 cm .
irradiation, indicating that the photodecomposition reactions
occurred from the initially formed photoproducts and not from
the E-enol reactant (this does not mean that the E-enol species
cannot also undergo photodecarboxylation or photodecarbo-
nylation, but just that these are much slower processes
compared to those occurring extensively during the first
minute of irradiation).
Systematic comparison of the bands due to the photo-
produced species with the calculated spectra of all SCA
isomeric species (all investigated conformers of E- and Z-enol
and E- and Z-keto forms; see Table S3 in the Supporting
Information for calculated spectra for all these species) allowed
us to identify the products of the photoinduced reactions. This
identification was made on the basis of the full spectral range
the following discussion, the above-mentioned fact that all pairs
of structures differing only in the orientation of the carboxylic
substituent are spectroscopically indiscernible will be taken into
account.
The band at 3632 cm− appears at a characteristic frequency
1
of an OH stretching vibration of a non-hydrogen-bonded
48,49
phenolic moiety,
being indicative of photoproduction of an
enol-type species where the OH phenol group is free.
Conformers III−VIII of the E-enol and III−VIII of the Z-
enol obey this condition. However, the E-enol forms can be
safely discarded, because all of them have characteristic intense
bands (according to the theoretical predictions) that are not
present in the spectra of the irradiated matrixes: for the E-enol
−
1
conformers III/IV bands at 1580 and 1102 cm , for
(
assignments are given in Table 2). However, the bands
conformers V−VIII bands in the 1595−1585 and 1230−1220
−1
−1
−1
observed at 3632 cm and in the 3540−3490 cm region
cm regions (all these bands corresponding to vibrations with
−1
(Figure 4), at 1531, 1516, and 1258 cm (Figure 5), and at
significant contributions from the phenolic δCOH or νC−O
coordinate). The remaining intense bands of these forms are
12 cm− (Figure 6) proved to be instrumental for this task. In
1
7
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The Journal of Physical Chemistry A
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(
(
simultaneous or subsequent) conformational rearrangement
Figure 1A,B).
The bands at 1258 cm− (Figure 5) and 712 cm−1 (Figure 6)
1
in the spectra of the irradiated matrixes are also characteristic of
Z-enol forms (according to the calculations they cannot be
ascribed to E-enol or the keto isomers), being ascribed to the
phenolic νC−O stretching and one of the torsional modes of
the p-carboxyphenyl ring, and are a further experimental
evidence of photoproduction of the Z-enol isomer in the
described experiments.
In turn, the pair of bands observed in the spectra of the
irradiated matrixes at 1531 and 1516 cm− cannot be explained
by any of the enol isomers (Figure 5), being a particularly clear
evidence of the occurrence of the E-enol → E-keto photo-
tautomerization upon irradiation of matrix-isolated SCA.
Indeed, besides these two bands (originated in vibrations
with significant contributions from the cyclohexadienone ring
stretchings), most of the predicted bands for the E-keto most
stable pair of conformers (E-keto I/II forms) are experimentally
observed, as can be easily noticed in Figures 4−6.
1
In summary, the analysis of the results of the performed
irradiation experiments on matrix-isolated monomeric SCA E-
enol clearly demonstrates the competitive occurrence of simple
E-enol → Z-enol isomerization and E-enol → E-keto
tautomerization. Both processes were found to be rather
effective, with practically complete consumption of the reactant
after broad-band irradiation (λ > 235 nm) by 1 min only. With
all probability, the first process leads to the initial formation of
the Z-enol conformer I, which requires just the E-to-Z
isomerization to take place without any additional conforma-
tional rearrangent; conformational isomerization shall then
occur transforming the Z-enol I into the lowest energy Z-enol
conformer VII. Note that among the Z-enol conformers,
conformer VII (and its spectroscopically indiscernible pair,
VIII) is the only conformer with a lower energy than conformer
I, so that formation of the remaining Z-enol conformers seems
in fact not very probable. The E-enol → E-keto tautomerization
implies excited state intramolecular proton transfer and twisting
about the exocyclic CC bond of the molecule, transforming the
initial E-enol form into the E-keto form. The two events
Figure 6. From bottom to top: calculated infrared spectra of
conformers I (black) and VII (blue) of the Z-enol; calculated infrared
spectra of E-keto (bands pointing up) and E-enol most stable
conformers; simulated difference spectrum built from the calculated
infrared spectra: 0.5 × E-keto + 0.5 × (Z-enol I + Z-enol VII) − E-
enol; experimental infrared difference spectrum: matrix irradiated for 1
min with the broad-band light of the Hg(Xe) lamp (λ > 235 nm)
minus as-deposited matrix; experimental infrared difference spectrum:
matrix irradiated for 20 min of irradiation at λ = 335 nm followed by
30 min at λ = 345 nm with laser light minus as-deposited matrix. All
calculated spectra were obtained at the B3LYP/6-311++G(d,p) level
of theory and the wavenumbers were scaled by 0.978; the bands were
simulated by Lorentzian functions centered at the scaled calculated
(
proton transfer and CC twisting) most probably take place
sequentially, though there is no convincing spectroscopic
evidence of the presence in the irradiated matrixes of the Z-keto
form, which should act as intermediate in this sequence of
processes (Scheme 1). This can be attributed to the efficiency
and fast kinetics of the two processes under the irradiation
conditions used, and also because the E-keto form was found to
be rather photostable under those conditions, so that no
photostationary equilibrium could be reached between the two
keto isomers (back conversion should require use of
considerably longer excitation wavelength, in the visible
−1
frequency and with full-width-at-half-band of 5 cm .
nearly coincident with bands due to other species. On the
contrary, all the Z-enol forms III−VIII can a priori be present in
the irradiated sample, because they do not have any
characteristic intense band predicted by the theory at a
frequency where no bands are observed in the experimental
spectra. One can, however, suppose that, even if all these Z-enol
conformers are present (and, as it will be pointed out below,
there are reasons to think they are not), the lowest energy
conformer VII shall dominate.
26
range).
In an attempt to roughly estimate the branching ratio
between the E-enol → Z-enol isomerization and E-enol → E-
keto tautomerization photoinduced processes, one tried to
reproduce the experimentally observed infrared difference
spectra shown in Figures 4−6 using different combinations of
the theoretically predicted infrared spectra of the relevant
species (i.e., the reactant E-enol conformer I, and the identified
photoproducts, Z-enol conformers I and VII and E-keto form
I). The resulting simulated spectrum built by assuming the E-
enol → Z-enol:E-enol → E-keto branching ratio of 1:1 (with
the spectra of the two Z-enol conformers contributing equally)
−1
The broad feature observed in the 3540−3490 cm region
of the spectrum of the irradiated matrixes (Figure 4) can only
be originated in the Z-enol pair of conformers I/II, because
both its frequency range and the observed broad profile are
very characteristic of a stretching mode of a phenolic O−H
group involved in a weak hydrogen bond. Preferential
formation of Z-enol I from the irradiated E-enol I species
seems in fact to be rather plausible, because it requires just the
E → Z isomerization to take place without any other
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Article
is represented in Figures 4−6 and shows a very good agreement
with the experimental data, indicating that, under the used
experimental conditions, the E-enol → Z-enol isomerization
and E-enol → E-keto tautomerization occur with nearly equal
probability.
+G(d,p) calculated infrared spectra for the most stable
conformer of the four different tautomers of SCA,
B3LYP/6-311++G(d,p) calculated infrared spectra of the
E-enol conformers I and II (full range). Tables of the
most characteristic dihedral angles for the various
conformers of SCA, B3LYP/6-311++G(d,p) calculated
Cartesian coordinates for the optimized structures, and
infrared spectra of the various conformers of SCA (PDF)
4. CONCLUSION
The molecular structure and photochemistry of N-salicylidene-
p-carboxyaniline isolated in a cryogenic (15 K) solid argon
matrix was investigated. The detailed characterization of the
conformational space of the enol-imine and keto-amine
tautomers of the compound in both their E and Z isomeric
forms was undertaken. These data, together with the
theoretically predicted infrared spectra for the various con-
formers of the enol-imine and keto-amine SCA forms, were
used to interpret the infrared spectrum of the matrix-isolated
compound. It was demonstrated that the obtained Ar-matrixes
AUTHOR INFORMATION
Corresponding Author
*R. Fausto E-mail: rfausto@ci.uc.pt.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge Fundaca
̧
o
̃
para a Cien
̂
cia e
(
prepared from the vapors resulting from sublimation of
a Tecnologia (FCT), through the projects PEst-OE/QUI/
UI0313/2014 (Coimbra Chemistry Centre), cofunded by
QREN-COMPETE-UE. Financial support has been also
provided within the framework of the FCT postdoctoral
grant SFRH/BPD/88372/2012 (N.K.).
crystalline E-enol form) contain only the most stable conformer
of the E-enol isomer of the compound. Irradiation of the
matrixes (either with narrow- or broad-band UV light, at λ =
335; 345 nm, or λ > 235 nm, respectively) led to observation of
two competing photoreaction channels: E-enol → Z-enol
isomerization and E-enol → E-keto tautomerization (prolonged
broad-band irradiation was found to lead to photofragmenta-
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