Adiabatic deprotonation as an important competing pathway to ESIPT in photoacidic 2-phenylphenols

Article abstract of DOI:10.1039/c9cp02028d

ESIPT (Excited State Intramolecular Proton Transfer) to C atom in 2-phenylphenol is known to be an intrinsically inefficient process. However, to the best of our knowledge, a structure-ESIPT efficiency relationship has not been elucidated yet. Here, we show that there exists a competitive interplay between photoacidity and ESIPT efficiency for the 2-phenylphenol system. The attachment of electron withdrawing groups to the phenol moiety promotes adiabatic deprotonation in the excited state and diminishes the charge transfer character of the excitations, and both these factors contribute in decreasing the ESIPT reaction yield. On the other hand, unfavorable conformational distribution in the ground state also appears as another important aspect responsible for the low ESIPT extent of 2-phenylphenol. A new derivative bearing electron donating, bulky substituents at ortho and para positions of the phenol ring shows an outstanding ESIPT performance, which demonstrates that the efficiency of the process can be significantly enhanced by modifying the substitution pattern. We anticipate that our results will help to guide the molecular designing of new compounds with high ESIPT efficiency.

Full text of DOI:10.1039/c9cp02028d

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DOI: 10.1039/C9CP02028D
ARTICLE
Adiabatic deprotonation as an important competing pathway to
ESIPT in photoacidic 2-phenylphenols
Leandro D. Mena,*a D. M. A. Vera,b Maria T. Baumgartnera and Liliana B. Jimenez*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
ESIPT (Excited State Intramolecular Proton Transfer) to C atom in 2-phenylphenol is known to occur as an intrinsically
inefficient process; however, to the best of our knowledge, a structure-ESIPT efficiency relationship has not been
elucidated yet. Here we show that there exists a competitive interplay between photoacidity and ESIPT efficiency for 2-
phenylphenol system. The attachment of electron withdrawing groups to the phenol moiety promotes adiabatic
deprotonation in excited state and diminish the charge transfer character of excitations, both factors contributing to
decrease the ESIPT reaction yield. On the other hand, unfavorable conformational distribution in ground state also appears
as another important aspect responsible for the low ESIPT extent of 2-phenylphenol. A new derivative bearing electron
donor, bulky substituents at ortho and para positions of the phenol ring shows an outstanding ESIPT performance, which
demonstrates that the efficiency of the process can be significatively enhanced by modifying the substitution pattern. We
anticipate our results will help guide molecular design to produce new compounds with high ESIPT efficiency.
water,[7] although a few reports regarding superphotoacids
with ability to experience ESPT in nonaqueous environments
can be found in literature.[8] Moreover, it must be recalled that
many of these processes take place on an ultrafast time scale,
Introduction
Photoacids are organic light-absorbing molecules that present
an exacerbated tendency to act as proton donors upon
photoexcitation, becoming more acidic in excited state than in
ground state.[1] This acidity enhancement results in a large pKa
drop, thus excited-state pKa (pKa*) can reach slightly greater
than zero values in the case of “normal” photoacids, or even
negative values for the so-called “super” photoacids.[2] By far,
hydroxyarenes (phenols, naphthols, hydroxypyrenes, among
others) are the most studied group of photoacidic
compounds.[3] One of the most prominent examples is 1-
naphthol, whose pKa* is 0.4, considerably smaller than its
ground-state pKa of 9.2.[4] Because of this peculiar behaviour
and its potential applications, hydroxyarene photoacids have
attracted much attention in last decades.[5] Photoinduced
dissociation of hydroxyarenes in solution phase generally
occurs as an intermolecular event known as Excited State
Proton Transfer (ESPT),[6] in which a solvent molecule (or any
basic molecule) can act as the proton acceptor. Naturally, most
studies dealing with photoacids have been conducted in
making its experimental study
a
challenging task.[9] In
molecules having both proton donor and acceptor in close
proximity the excited state proton transfer can proceed
through an intramolecular mechanism termed ESIPT, besides
the aforementioned intermolecular process (Scheme 1).[9] As
with acidity, basicity also increases in excited state; thus, a
group presenting poor ground-state basicity can easily accept
a proton once the molecule has been excited.[10] A good
example of this is ESIPT to sp2 or sp hybridized carbon atoms,
in which a C atom (weakly basic in S0) becomes able to receive
a proton in excited state.[11] This case is quite peculiar, since
*
OH
Ph
O
*
K
*
a
+
H
Ph
ESPT
ESIPT
R
R
(ArOH)*
(ArO )*
H
O
H
h
o-QM
R
OH
O
a. INFIQC, Departamento de Química Orgánica
Facultad de Ciencias Químicas, Universidad Nacional de Córdoba
Ciudad Universitaria, Córdoba, Argentina
back-PT
Ka
+
H
Ph
Ph
E-mail: lmena@fcq.unc.edu.ar, ljimenez@fcq.unc.edu.ar
R
R
b. QUIAMM-INBIOTEC- Departamento de Química
ArO
ArOH
Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata
Mar del Plata, Argentina
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Scheme 1. ESPT and ESIPT processes of p-substituted 2-phenylphenols.
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most ESIPT processes usually involve carbonylic oxygen or performing the irradiation of a solution of 1 in a 3:1
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heterocyclic nitrogen atoms as the basic sites.[12]
CH3CN/D2O mixture with 254 nm light for 1 h we obtained a
DOI: 10.1039/C9CP02028D
In their seminal work,[13] Lukeman and Wan described ESIPT in 40% D-incorporation at the ortho position of the adjacent
2-phenylphenol, where the hydroxylic proton of phenol is phenyl ring.[17] In this work we are focusing only in intrinsic
transferred to an aromatic carbon atom at the ortho position ESIPT to the ortho position of the phenyl ring, without
of the adjacent phenyl ring, to give an ortho quinone methide addressing for water- mediated proton transfer to distal sites
(o-QM). Since then, many similar systems were reported, in
which ESIPT occurs together with other processes such as
water-mediated intramolecular proton transfer to distal sites
on the same molecule or proton transfer to bulk solvent.[14]
However, the connection between photoacidity and ESIPT
reactivity in those systems remains scarcely explored yet.
Unlike other examples in which an increase of the photoacidity
leads to barrierless, exergonic ESIPT,[15] the case of 2-
phenylphenol seems to be substantially different. A quantum
yield of deuterium incorporation (an indirect measure of ESIPT
efficiency) of 0.041 for 2-phenylphenol suggests that a
competition with other more efficient pathways is highly
possible. To confirm this assumption, we explored the
relationship between ESPT and ESIPT in a new family of
photoacidic 2-phenylphenols, and at the same time we were
able to obtain “normal” to “super” photoacids through simple
molecular modifications. This paper presents the detrimental
effect on ESIPT performance induced by increasing the
photoacidity of 2-phenylphenol derivatives and clarifies the
role of ESPT as a competing photochemical pathway.
Fig. 1 Plot of D-incorporation percentage vs. p Hammett parameter for
compounds 1-5.
Table 1. Deuterium incorporation in the photolysis of compounds
1-5[a]
Results and Discussion
Compound
D-incorporation
2-Phenylphenol derivatives 1-5 were obtained from Suzuki-
Miyaura cross-coupling reactions in moderate to good yields
and purified by column chromatography. The introduction of R
groups was carried out exclusively at para position of phenol
moiety, in order to facilitate the analysis of substituent
electronic effect. The indirect detection of ESIPT in 1-5 was
performed by monitoring the regiospecific incorporation of
deuterium upon irradiation in a protic solvent, according to
the method reported by Wan and co-workers (Scheme 2).[16]
As ESIPT to carbon atoms is a process that occurs within
femtoseconds, Wan et al. designed an alternative, simple
methodology for ESIPT detection that consists in performing
the irradiation of the compounds in a protic deuterated
solvent. In such an environment, the exchange of hydroxylic
proton by deuteron takes place; then, upon photoexcitation,
the deuteron is transferred to the adjacent phenyl ring to give
the keto tautomer. After relaxation and back-tautomerization,
the starting material is recovered isotopically labelled at the
1
2
40
45
46
11
4
3
4[b]
5
[a]
Percentage deuterium exchange at the 2’ position of 2-
phenylphenol derivatives following photolysis in a 3:1 CH3CN/D2O
mixture, measured by 1H NMR (400 MHz).
photodegradation of compound 4, the carboxylic acid derivative was
employed instead.
[b]
To avoid
(i.e. 4´ position). In Table 1 the results of photolyses of
derivatives 1-5 are displayed. As can be seen, the ESIPT
reaction yield decreases as the electron withdrawing character
of substituent R rises, becoming almost negligible when R=CN.
A
plot of D-incorporation extent vs. the p Hammett
parameter shows a straight linear correlation, indicating the
existence of a close relationship between the nature of R
substituent and the ESIPT performance (Figure 1).
It is well-known that acidity (and, simbatically, photoacidity) of
phenols can be modulated by incorporating electron
withdrawing/donating substituents on the ortho and para ring
positions, [18] which allows pKa and pKa* to be finely tuned. An
enhancement in photoacidity implies that the proton can be
released more easily, but this does not necessarily result in a
3'
D
2'
4'
5'
OH
OH
1
h

1'
6
254 nm
2
6'
5
3
CH3CN:D2O (3:1)
Ar
4
R
R
1
2
3
4
5
R= H ( ), OMe ( ), t-Bu( ), COOMe ( ), CN ( )
higher
ESIPT
performance.
To
quantify
both
phenyl ring; following the deuterium incorporation by 1H NMR
and MS, the ESIPT reaction extent can be estimated. By
acidity/photoacidity in our 2-phenylphenol series, aqueous pKa
Scheme 2 Photochemical reaction of p-substituted 2-phenylphenols
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and pKa* values for 1, 2, 4 and 5 were determined (3 derivative Interestingly, pKa’s remain almost unchanged if compared with
View Article Online
DOI: 10.1039/C9CP02028D
was excluded because of its poor solubility in water). Ground- the corresponding values from the phenol series (without the
state pKa measurement was carried out using UV-Vis phenyl ring, Table 2),[20] but it can be noted that 2-
spectrophotometry, recording the absorption spectra for each phenylphenols are more acidic in S1 state than their simpler
compound in a pH range of 4-13. The pKa* determination was analogues. This may be due to the strong OH…. interaction
performed employing the Förster cycle (Eq. 1, see ESI for that 2-phenylphenols present in excited state, which weaken
spectroscopic details). [19]
the O-H bond; the existence of this interaction is known to be
a prerequisite for ESIPT to occur.
0.625 ArOH - ArO -
(
)


∆pKa * = pKa - pKa *
=
ESIPT in 1 can be enhanced by shifting the acid-base
equilibrium towards the undissociated form of 2-
phenylphenol, as can be deduced from the 50% of D-
incorporation obtained from the photolysis of 1 in a 3:1
CH3CN/D2O mixture at pH(D) 1 (adjusted with concentrated
DCl). When the same experiment was performed for 5, no
change in the D-exchange yield with respect to the experiment
conducted at pH(D) 7 was observed, suggesting that 5 possess
a pKa* lower than 1, which is consistent with the measured
pKa* of -1.62. Furthermore, steady state fluorescence
measurements are in accordance with the observed trend. The
spectrum of 1 in CH3CN revealed a single emission band
corresponding to the phenol form, whereas in the 3:1
CH3CN/H2O mixture a shoulder at 420 nm was detected as the
T
Table 2. Experimental pKa and pKa* values for 2-phenylphenol
derivatives (1-5) and literature values for p-substituted phenol
analogues (without phenyl ring).
2-phenylphenol series
Phenol series
R
[a]
pKa
pKa*
pKa
pKa*[b]
H (1)
10.04±0.03
1.59± 0.09
9.99
4.0
OMe (2)
10.36± 0.04
2.96± 0.03
10.27
5.6
t-Bu (3)
COOMe (4)
CN (5)
-
-
10.30
8.47
7.96
-
-
8.13± 0.06
7.88± 0.03
-1.57± 0.03
-1.62± 0.03
3.3
characteristic emission of the phenolate anion formed by an
[a] Ref. 20. [b] Ref. 1.
[16]
adiabatic proton transfer to the solvent (SI 3.4).
On the
other hand, emission spectra of 5 confirms the presence of the
anion even in neat CH3CN, which illustrate the character of 5
as a superphotoacid. In the CH3CN/D2O mixture the amount of
anion is greater, as seen in the emission spectrum (Figure 2).
Time-resolved fluorescence spectroscopy was then used to
characterize the lifetime of the undeprotonated and
deprotonated species for each compound (1-5, Table 3).
Fluorescence lifetimes in water decrease as the electron
withdrawing character of the substituents rise for both
undeprotonated and deprotonated species, which could be
related to electron effects of EWG groups. Nevertheless, this
trend is more marked for undeprotonated species, which can
also be related -at least partially- to the fact that ESPT to
solvent (a relevant deactivating pathway from S1) is favoured
for EWG-substituted compounds.[16]
In Table 2 the obtained pKa and pKa* values for each
compound are presented. As expected, both ground and
excited state acidities rise with the attaching of electron
withdrawing groups (EWG) to the molecule. pKa* values go
from 2.96 to -1.62, indicating that the series include normal (1
and 2) and superphotoacids (4 and 5). t-Butyl-containing
derivative 3 is expected to act as a normal photoacid (vide
infra). The obtained pKa* of 1.59 for 1 agrees with the
Table 3. Fluorescence lifetimes of undeprotonated (AH) and
deprotonated (A-) forms of compounds 1, 2 and 5 in water.[a]
Compound
 (AH)/ns (em / nm)
0.256 ± 0.003 (348)
1.881 ± 0.006 (384)
0.102 ± 0.005 (335)
 (A-) /ns (em / nm)
3.722 ± 0.006 (414)
5.079 ± 0.009 (455)
1.07 ± 0.04 (395)
1
2
5
[a]
Fluorescence lifetimes of aerated solutions measured at pH 1 (AH)
and at pH 13 (A-). exc = 267 nm.
TD-DFT calculations were performed to gain a deeper insight
into the ESIPT performance of the 2-phenylphenol series.
Although previous computational studies involved mostly
multi-reference calculation methods,[21] it has been
demonstrated that TD-DFT (a less computationally expensive
Fig. 2 Fluorescence emission spectra of p-CN-2-phenylphenol (5) in CH3CN and in a
CH3CN:H2O (3:1) mixture as solvents. The emission band at 419 nm in neat CH3CN is
assigned to the deprotonated form (5-).
estimated 1.15 reported in literature. [16]
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vertical absorption energies and main geometric parameters
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Table 4. Theoretically predicted pKa and pKa*.[a]
are presented in Table 5. DespiteDOCI:A1M0.1-0B339L/YCP9CPs0l2ig0h28tlDy
overestimates the vertical absorption energies for 1 and 2, the
charge transfer character of the overall process makes
necessary the use of a long-range corrected functional.
Compound
pKa
pKa*
1
2
3
4
5
9.59 (0.45)
9.18 (1.18)
9.68 (-)
2.07 (-0.48)
2.74 (0.22)
2.22 (-)
In Fig. 3 the potential energy surfaces (PES) in both ground and
first singlet states of 1 are shown. The enol minimum in S1 lies
0.64 eV below vertical excitation (Table 5), in accordance with
Xia et al. results.[21a] The dihedral angle between the two rings
is reduced from 58.2° in S0 to 21.5° in the S1 state, resulting in
a shortening of Cacceptor-H bond distance from 2.44 Å (S0) to
2.16 Å (S1). These geometrical changes lead to a stronger
OH… interaction in S1 state, as was stated elsewhere.[16] NCI
6.50 (1.63)
5.85 (2.03)
-1.51 (-0.05)
-1.73 (0.11)
[
a]
Difference with respect to experimental values are given in
parentheses, as pKaExp (pKa*Exp) - pKaTheo (pKa*Theo). See ESI for more
details.
method) gives quite reliable results for the proton transfer
process in the * state, despite the single-reference
character of the method.[22] Our theoretically predicted pKa
and pKa* values agree well with our experimental results and
confirm that photoacidity can be strongly affected by the
substitution pattern (Table 4). An MSE (mean signed error with
respect to the experimental values) of 1.32 units illustrates the
accuracy of the employed method for the pKa computation.
Theoretical calculation of pKa* was achieved using the Förster
cycle in the same manner as in the experimental work. The
agreement with experimental data is excellent, with an MSE
value of -0.04. Since the Förster cycle requires the
determination of absorption and emission maxima
wavelengths, the success of the theoretical prediction relies on
the quality of the chosen method to reproduce the
experimental spectroscopic information. In this regard, the use
of a long-range corrected functional such as CAM-B3LYP for
the computation of vertical absorption and emission energies
was expected to be the most reliable approach, considering
the possibility of a marked charge transfer character for most
electron donor substituted species (see below).
Table 5. Selected geometric and energetic parameters for
compounds 1, 2 and 5 computed at CAM-B3LYP/6-31+G(d) level in
acetonitrile.
Compound
1 (H)
2 (OMe)
5 (CN)
Geometric
parameters
C1-C2-C1’-C2’
angle enol
C1-C2-C1’-C2’
angle keto
O22H23
S0
58.2
0.0
S1
S0
S1
29.4
84.3
S0
S1
21.5
82.6
57.3
0.1
59.2
0.0
19.4
74.2
distance
(Å) enol [a]
0.97
2.44
0.98
2.16
0.97
2.44
0.98
2.13
0.97
2.44
0.98
2.24
C12H23
distance
(Å) enol [b]
The molecular modeling of ESIPT process was performed at
TD-DFT CAM-B3LYP/6-31+G(d) level of theory, in acetonitrile.
Energetic
S0
S1
S0
S1
S0
S1
parameters
Relative energy
enol (eV) [c]
Relative energy
keto (eV) [c]
Vertical
0.00
1.55
4.29
3.43
0.00
1.61
4.07
3.17
0.00
1.53
4.36
3.58
4.94
4.52
4.94
excitation (eV)
K* relaxation
energy (eV) [d]
0.80 (0.0018)
0.38 (0.0005)
1.28 (0.019)
[a]
O22 and H23 are the atoms forming the phenolic OH, according to
[b]
[c]
G09 input numbering.
C12=carbon atom acceptor.
Calculated
energies relative to the enol form in ground state for each compound.
[d] Relaxation energies of keto excited species, with oscillator strength
in parentheses.
(non-covalent interactions) analysis (SI 4.3.3) results show the
existence of a non-covalent interaction between the phenolic
OH and the  system in S0 that becomes stronger in the S1
Fig. 3 Potential energy curves of the S0 and S1 states of 1 in acetonitrile obtained
from relaxed scans along the OH stretching coordinate (step length=0.05 Å). Proton
transfer energy barriers and E for the keto form with respect to TS are indicated.
For the sake of comparison, only compounds 1, 2 and 5 were
studied. Energies of enol/keto minima in both S0 and S1 states,
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DOI: 10.1039/C9CP02028D
Fig. 4 Density difference plots (isovalue= 0.018 a.u.), ESP maps and dipole moment vectors at critical points (enol form, highest energy point and keto form) along the S1 potential
energy surface for 1. E* and K* address for excited-state enol form and twisted keto tautomer, respectively.
singlet state, in agreement with the information collected from relaxation of keto tautomer to a twisted state was also
our theoretical IR frequencies analysis. According to the previously described in other typical ESIPT systems as a decay
calculations, the O-H stretching vibrational frequency channel competing with radiative transitions and leading to
decreases from 3742 cm-1 in S0 to 3625 cm-1 at the S1 enol (E*) fluorescence quenching.[24] Unlike those systems, in which the
minimum, indicating
strengthening upon excitation. The same trend was found for for the characteristic Stokes-shifted ESIPT emission, 2-
derivatives 2 and 5, as expected (see ESI 4.3.4). phenylphenol shows no proton-transfer emission, suggesting
a
considerable OH… interaction excited keto form exists as a planar minimum in S1 responsible
Ground-state intramolecular proton transfer (GSIPT) exhibits the absence of a planar keto tautomer stable enough to emit
an energy barrier of 1.8 eV, too large for allowing thermal PT. fluorescence, in accordance with the results presented herein.
In S1 state the ESIPT process is almost barrierless, leading to In general, the dipole moment change for the S1S0 transition
the keto form (K*) in a highly exergonic fashion. TD-DFT is very large; to explain the smaller dipole moment of K* with
describes K* as a true minimum in the S1 PES, but since at that respect to the S0 keto tautomer some authors suggested that
geometry S1 and S0 are separated by 0.8 eV the probability of a twisted conformations in S1 possess biradicaloid nature.[24a]
A
S1S0 non-radiative decay to occur is quite high. Indeed, Xia et significant electron density redistribution is necessary to occur
al. reported the existence of a conical intersection (CI) in that for the enolketo tautomerization, making the phenyl ring
region.[21a] The characterization of such critical point lies basic enough to receive the incoming proton. Such electron
beyond the scope of the present work; nevertheless, it is density relocalization is absent in ground state, which explain
worth to note that TD-DFT is capable of giving reasonable the large energy barrier found. The change of electron density
results despite its well-known breakdown in the upon excitation can be seen on Figure 5. The most important
neighbourhood of a CI.[23] No planar keto minimum was feature is the gain of density of the phenyl ring (in red), which
located in S1 at TD-DFT level; actually, as the dihedral angle is connected with the stabilization of the keto tautomer.
between rings gets closer to 0°, the potential energy surface For compounds having EWGs this density increase is lower,
does not exhibit a minimum, but rather the opposite (Fig. S4). becoming almost negligible when R=CN. The installation of a
We observed a twisting in the dihedral angle between rings
occurring concomitantly with the proton translocation to the
2’-position of the phenyl ring in S1. These geometry changes
are in accordance with the increasing charge transfer character
along the O-H coordinate (Fig. 4), as observed by Basaric et al.
for 2-phenyl-1-naphthol at RI-CC2 level of theory.[21c] Once K*
twisted form decays to S0 the basal K* structure relaxes to
planar K (dihedral angle between rings=0°) or anti-K (dihedral
angle=180°) structures. It should be noted that the formation
of a planar ortho-quinone methide does not take place in S1,
but rather occurs after relaxation to S0. This torsional
Fig. 5 Density difference plot (isovalue=0.0009 a.u.). The red/blue zones indicate an
increase/decrease of the electronic density upon absorption of light for the S0S1
transition of 2 (left), 1 (middle) and 5 (right).
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relaxed enol form in S1 providing an additional non-radiative
Table 6. Dipole moment magnitudes and long-range extent for
compounds 1, 2 and 5 in acetonitrile at CAM-B3LYP/6-31+G(d)
level of theory.[a]
View Article Online
decay channel)[21a] must be considered. DInOIp: r1e0v.1i0o3u9s/Cw9CoPr0k2s,02th8De
unfavourable conformational distribution in ground state has
been pointed out as the most suitable explanation for the poor
ESIPT performance of 2-phenylphenol. Basarić et al. showed
that the anti conformer of 1 with one explicit water molecule
(1-W, Fig. S14) in S0 is more stable than the syn conformer by
1.46 kcal mol-1 at RI-CC2 level.[21c] Our calculations at
DFT/B3LYP/6-31+G(d,p) level gave a similar energy difference
in acetonitrile (1.16 kcal mol-1 for 1-W), very close to the
reported results obtained with the coupled cluster method.
The energy difference between syn and anti conformers for 2-
W and 5-W was 0.53 kcal mol-1 and 1.44 kcal mol-1,
respectively. The Boltzmann population of syn conformer in
each case (29% for 2-W, 12% for 1-W and 8% for 5-W) clearly
indicates that the presence of an EWG in the molecule tips the
balance in favor of the anti conformer. Therefore,
unfavourable conformation in ground state constitute another
factor contributing to the low ESIPT efficiency of the CN-
derivative. Nevertheless, the prevalence of anti conformer in
S0 seems to be an intrinsic limitation of 2-phenylphenol
system, certainly absent for 2-phenyl-1-naphthol since the 8-H
avoids OH positioning away from phenyl moiety, making the
anti isomer less stable. In order to overcome this limitation,
we designed a new 2-phenylphenol derivative (6, Scheme 3)
bearing bulky t-butyl groups at ortho and para position of
phenol ring in order to minimize adiabatic deprotonation
effects and to control conformer distribution. The photolysis of
6 in a 3:1 CH3CN/D2O mixture with 254 nm light yielded >99%
of D-incorporation at the ortho position of the phenyl ring
Compounds
1 (H)
2.4
2 (OMe)
4.7
5 (CN)
7.0
S0  E form (D)
S1  E*form (D)
S0  K form (D)
S1  K* form (D)
Long-range
0.3
0.9
7.2
6.2
4.1
9.4
5.3
7.6
1.7
0.698
0.624
0.705
extent Λ (a.u.)
[a]
Molecular geometries and dipole moment vectors are shown in ESI
4.3.5
strong EWG in the molecule greatly affect the charge transfer
process, diminishing the amount of electron density
transferred to the phenyl ring. The long-range extent (Λ)[25]
calculated by us through the computation of the overlap
integral between the Slater coefficients of the main transitions
contributing to vertical excitation for 1, 2 and 5 revealed a
reduction in the charge transfer degree as the electron
withdrawing character of substituent rises (Table 6). Because
of this effect, the 5 PES lies at higher energies than the others,
making ESIPT slightly difficult (Fig. S3). Also, the K* form of 5
exhibits the greatest separation from S0 (1.28 eV, Table 5)
which could result in a less efficient passing through a conical
intersection. Moreover, comparing the Merz-Kollman charges
of E* with those of K*, it can be noted that a quite small
amount of positive charge is transferred to phenyl ring when
phototautomerization takes place. The phenol moiety charge
varies from 0.071 qe for E* to -0.163 qe for keto tautomer,
indicating that a charge of 0.234 (a 23.4% of a full proton) is
transferred, whereas in ground state the phenol charge
difference obtained is about 0.302 qe. For 5, this trend
becomes greater: the difference in phenol charge is about
0.099 qe (9% of proton) in S1 and 0.446 qe (44.6%) in ground
state. According to these results, the studied photoprocess in
S1 can be catalogued as ESIHT (excited state intramolecular
hydrogen transfer) rather than ESIPT, as Luber et al. proposed
for a related system.[26] As a matter of fact, in a more strictly
sense, the process should be described as an excited-state
PCET (proton coupled electron transfer).[27] Together with our
results on photoacidity, these findings strongly suggest that
the attaching of an EWG in the phenol moiety of 2-
phenylphenol decline ESIPT performance as result of a double
effect, that is, intrinsic ESIPT partially hindering and
improvement of adiabatic ESPT to bulk solvent, depleting the
number of excited molecules able to experience ESIPT.
1
after 1 h. Mass spectra analysis agrees with H NMR results,
indicating the presence of dideuterated species as the major
product (83%). Theoretical calculations at ground state in
acetonitrile at B3LYP/6-31+G(d,p) level demonstrate that syn 6
conformer is indeed more stable than anti 6 by 3.0 kcal mol-1,
as expected. These results strongly support the previous
theoretical approach and represent the first example of ESIPT
to C-atom enhancement by using steric hindrance as a tool for
controlling ground-state conformer distribution, and it also
constitutes one of the few examples of ESIPT tuning via alkyl-
substituent perturbation.[28]
D
OH
D
OH
OH
h
254 nm
+
D
CH3CN:D2O
3:1
Ar
6
Scheme 3 Photochemical reaction of 3,5-di-tert-butyl-2-phenyphenol
Unlike other systems in which the intramolecular proton
transfer in S1 occurs very efficiently (e.g. 2-phenyl-1-naphthol,
with a D-exchange quantum yield of 0.73), ESIPT in 2-
phenylphenol is a rather inefficient process, with a low
reaction quantum yield (0.041). In this paper we have
addressed adiabatic deprotonation as one of the causes
contributing to the low ESIPT yield, but other factors (for
example, the location of a conical intersection nearby the
Conclusions
In summary, the competition existing between excited-state
proton transfer to bulk solvent and ESIPT was revealed by
tuning the photoacidity of a series of 2-phenylphenols. The
installation of an EWG in the molecule makes 2-phenylphenol
a superphotoacid, favoring adiabatic deprotonation in excited
state and thus hindering ESIPT. Additionally, the theoretical
6 | J. Name., 2012, 00, 1-3
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Physical Chemistry Chemical Physics
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ARTICLE
calculations demonstrated that the EWGs also diminish the extracted employing CH2Cl2 and water. The organic layer was
View Article Online
DOI: 10.1039/C9CP02028D
charge transfer character of vertical excitation to S1 singlet dried over MgSO4 and evaporated under reduced pressure.
state, which has a considerable impact on the basicity of C The dry crude was analyzed by NMR and CG-MS. NMR spectra
atoms of phenyl ring. Both facts combined provide evidence and MS results are described in ESI.
that the observed lost in efficiency is the result of -at least- Computational details. All DFT and TD-DFT calculations were
two different factors related with ESPT to solvent and ESIPT performed the Gaussian 09 package.[29] The relevant stationary
intrinsic reactivity. Furthermore, unfavorable conformational points were fully optimized by using the range-separated correction
distribution in ground state leading to a major population of of B3LYP functional, CAM (Coulomb Attenuating Method)[30] with
anti isomer is another important cause of low ESIPT extent. the 6-31+G(d) basis set. The obtained stationary points were
This limitation was avoided by attaching bulky, EDG groups at characterized by Hessian diagonalization and harmonic frequency
the ortho and para positions of phenol ring, minimizing both analyses to obtain zero-point and thermal corrections for the
anti form prevalence and adiabatic deprotonation. The energies, enthalpies and free energies. Solvation effects were
remarkable ESIPT performance shown by such compound simulated with PCM model, using the dielectric constant of
clearly demonstrates that, besides photoacidity, other factors acetonitrile. Relaxed scans were computed by allowing all the
must be considered to develop a better understanding of the internal degrees of freedom to relax apart from the driving
process. Thus, the efficiency of ESIPT to C in a typical system coordinate (O-H distance, step length=0.05 Å). Vertical excitation
can be improved through a careful molecular design. We and emission energies were calculated within the LR-PCM (linear
expect our results to have a significant impact on the design of response) scheme. For the determination of pKa values the
new similar ESIPT compounds with improved performances for B3LYP/6-311+G(d,p)/SMD level of theory was employed in water.
future potential applications.
To calculate pKa* values, the vertical absorption and emission
energies for undeprotonated species were simulated using the
CAM-B3LYP functional with the 6-31+G(d) basis set. The absorption
and emission energies of deprotonated species were computed at
the same level of theory. The corresponding wavelengths are
informed in Table S2; some values were refined using the non-
equilibrium state specific correction[31] in order to gain deeper
accuracy. For NCI analysis the Multiwfn software was employed.[32]
For the calculation of the long-range extent, Λ was defined as
usual:[25]
Experimental
Chemicals. All reagents (phenols, phenyl boronic acid, Pd-
derived catalysts and bases) were obtained from Sigma-Aldrich
and used as received. Solvents used for Suzuki-Miyaura
reactions were also used as received. Acetonitrile for
irradiation was dried, distilled and stored under molecular
sieves (4 Å) and nitrogen atmosphere until its use. Deuterium
oxide was donated by Central Nuclear Embalse (Córdoba,
Argentina), and used as received.
2
|
||
|
휅푖푎 휑푖 (푟) 휑푎 (푟) 푑푟
∑
∫
Instrumentation. Gas chromatographic analysis was
performed on a Varian GC with a flame ionization detector,
and equipped with a VF-5 ms, 30 m x 0.25 mm × 0.25 mm
푖,푎
 =
휅2푖푎
∑
푖,푎
1
column. H NMR, 13C NMR and 2D NMR were recorded on a
400 MHz Bruker nuclear magnetic resonance spectrometer.
Fluorescence spectra of the samples dissolved in the described
solvent were recorded with an Agilent Cary Eclipse
Fluorescence Spectrophotometer. Time-resolved fluorescence
휅
where
are the coefficients of each contributing φ → φ orbital
i a
푖푎
transition, with 0 < Λ < 1 ( Λ =1 for a totally local transition; smaller
values signifies increasing charge transfer character); the integrals
were computed using Gabedit 2.4.8 and the cubman facility of
Gaussian09.[33] Visualization and graphics rendering were carried
out with GaussView 5.0.8,[34] Gabedit 2.4.8 and VMD 1.9.3.[35] The
electronic energies in Hartrees, zero-point energies and thermal
corrections are available in Table S5, as well as the whole set of
Cartesian coordinates for all the stationary points (SI 4.4).
measures were taken with
a
Deltaflex Horiba
Spectrofluorometer using a diode laser (λ = 267 nm) for the
excitation of the samples. UV-visible spectra of the compounds
in solution were recorded with
a Shimadzu UV-1800
Spectrophotometer. HRMS were recorded on a Bruker,
MicroTOF Q II equipment, operated with an ESI source in
(positive/negative) mode, using nitrogen as nebulizing and
drying gas and sodium formiate 10 mM as internal standard.
Gas Chromatographic/Mass Spectrometer analysis were
carried out on a Shimadzu GC−MS QP 5050 spectrometer
equipped with a VF-5 ms, 30 m × 0.25 mm × 0.25 mm column.
Steady-state photolysis. Solutions of acetonitrile:deuterium
oxide (3:1) containing p-substituted phenylphenols 1-5 (ca.
2.20 mM) were irradiated under argon atmosphere (irradiation
time = 1 h). The irradiation source was a Luzchem multilamp
photoreactor ORG-model containing 10 low-pressure Hg lamps
(max= 254 nm). After this time, the acetonitrile was
evaporated under reduced pressure and the leftover was
Synthesis and characterization of compounds 1-6.
[1,1'-Biphenyl]-2-ol (2-Phenylphenol) (1).[36] 2-Iodo-phenol (1
equiv, 1 mmol) and phenylboronic acid (1 equiv, 1 mmol) were
added to a Schlenk tube with 10 mL of distilled water. Then, K2CO3
(4 equiv, 4 mmol) and Pd/C (2% mol, 21 mg) were added to the
mixture and stirred. The system was heated to 80 °C for 16 h. For
the extraction process diethyl ether (3 x 10 mL) and acidified water
were used. The organic extract was dried with Na2SO4, filtered, and
evaporated under reduced pressure. Products were first identified
by GC and GC−MS. The compound was isolated from the crude by
column chromatography as a colorless oil, employing a mixture of
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Physical Chemistry Chemical Physics
Page 8 of 10
ARTICLE
Journal Name
pentane/CH2Cl2 (90:10) as eluent. The product was obtained with product was obtained as oil, 50% yield. A third part oVfiethwiAsrtaiccleidOwnlianes
1
1
DOI: 10.1039/C9CP02028D
40% yield, 70 mg. H NMR (400 MHz, (CD3)2CO, 25 ºC): δ= 8.27 (s, used for the photolysis at 254 nm in CH3CN/D2O. H NMR (400 MHz,
1H, OH); 7.59 (d, J(H,H)= 7.9 Hz, 2H); 7.39 (t, J(H,H)= 7.9 Hz, 2H); (CD3)2CO, 25 ºC): δ= 9.24 (s, 1H, OH); 8.00 (ds, 2J(H,H)= 2.2 Hz, 1H);
7.31-7.28 (m, 2H); 7.18 (dt, J(H,H)= 8.1 Hz, J(H,H)= 1.4 Hz, 1H); 7.90 (dd, J(H,H)= 8.4 Hz, J(H,H)= 2.2 Hz, 1H); 7.62 (d, J(H,H)= 8.5
6.99 (d, J(H,H)= 8.1 Hz, 1H); 6.93 (t, J(H,H)= 7.6 Hz, 1H). MS (EI): Hz, 2H); 7.44 (t, J(H,H)= 7.7 Hz, 2H); 7.34 (t, J(H,H)= 7.6 Hz, 1H);
m/z: 170 (100%), 169 (74%), 141 (39%), 115 (33%), 63 (6%).
1
1
1
2
1
2
1
1
1
1
1
1
7.09 (d, J(H,H)= 8.5 Hz, 1H). When (CD3)2SO is used as solvent, the
5-Methoxy-[1,1'-biphenyl]-2-ol (2).[37] It was followed the signal of the proton for the COOH group is detected at δ 12.47 ppm.
procedure described by Camargo Solórzano et al. with slightly MS (EI): m/z: 214 (100%), 197 (34%), 169 (18%), 141 (28%), 115
modifications.[38] 2-Bromo-4-methoxyphenol (1 equiv, 0.67 mmol) (24%), 98 (10%). The rest was dissolved in 10 mL of methanol and
and phenylboronic acid (1.3 equiv, 0.9 mmol) were added to a put in a round bottom flask, then, 50 L of concentrated H2SO4 was
Schlenk tube with a 5 mL of dioxane:water (4:1) under inert added. The reaction was refluxed overnight. The crude was purified
atmosphere (N2). Then, K3PO4.H2O (3 equiv, 2 mmol), PPh3 (10% over silica gel using an eluent gradient (pentane/ethyl acetate)
mol) and Pd(dba)2 (5% mol, 19 mg) were added to the mixture and starting from 100 % of pentane. The product was obtained as white
stirred by 24 hs at 90 ºC. For the extraction process diethyl ether (3 solid, 50% yield, 44 mg. 1H NMR (400 MHz, (CD3)2CO) δ= 9.26 (s, 1H,
2
1
x 10 mL) and acidified water were used. The organic extract was OH); 7.965 (ds, J(H,H)= 2.3 Hz, 1H); 7.86 (dd, J(H,H)= 8.4 Hz,
dried with Mg2SO4, filtered, and evaporated under reduced 2J(H,H)= 2.3 Hz, 1H); 7.61-7.59 (m, 2H); 7.44 (t, 1J(H,H)= 7.4 Hz, 2H);
1
1
pressure. Products were first identified by GC and GC−MS. The 7.34 (t, J(H,H)= 7.4 Hz, 1H); 7.08 (d, J(H,H)= 8.4 Hz, 1H); 3.85 (s,
compound was isolated from the crude by column chromatography 3H, CH3). 13C NMR (100 MHz, (CD3)2CO) δ= 167.1 (q, C=O); 159.5 (q,
(hexane/CH2Cl2 at 9.5:0.5 ratio) followed by a preparative TLC with CAr-OH); 138.7 (q, CAr); 133.2 (CAr-H); 131.1 (CAr-H); 130.1 (2 CAr-H);
hexane/CH2Cl2 (8:2) as eluent. The product was obtained as a pale 129.4 (q, CAr); 128.9 (2 CAr-H); 128.0 (CAr-H); 122.9 (q, CAr); 116.9
brown oil, 18% yield, 24 mg. 1H NMR (400 MHz, (CD3)2CO, 25 ºC): δ= (CAr-H); 51.9 (CH3). MS (EI): m/z: 228 (61%), 197 (100%), 141 (21%),
7.83 (s, 1H, OH); 7.60 (d, J(H,H)= 7.7 Hz, 2H); 7.39 (t, J(H,H)= 7.8 115 (22%), 98 (21%), 70 (16%). HRMS (ESI-TOF) m/z [M + Na]+ Calcd
Hz, 2H); 7.29 (t, J(H,H)= 7.5 Hz, 1H); 6.91 (d, J(H,H)= 8.7 Hz, 1H); for C14H12O3Na: 251.0679; Found: 251.0660.
6.86 (ds, J(H,H)= 3.0 Hz, 1H); 6.77 (dd, J(H,H)= 8.7 Hz, J(H,H)= 3.0 6-Hydroxy-[1,1'-biphenyl]-3-carbonitrile (5).[41] It was followed the
1
1
1
1
2
1
2
Hz, 1H); 3.77 (s, 3H, CH3). MS (EI): m/z: 200 (100%), 185 (83%), 157 procedure described by Zhao et al. et al. with slightly
(34%), 128 (34%), 129 (18%), 115 (10%), 102 (8%), 77 (8%).
modifications.[41] 2-Bromo-4-methoxyphenol (1 equiv, 0.67 mmol)
5-(tert-Butyl)-[1,1'-biphenyl]-2-ol (3).[39] It was followed the and phenylboronic acid (1.3 equiv, 0.9 mmol) were added to a
procedure described by Camargo Solórzano et al. with slightly Schlenk tube with a 5 mL of dioxane:water (4:1) under inert
modifications.[38] 2-bromo-4-(tert-butyl)phenol (1 equiv, 0.5 mmol) atmosphere (N2). Then, K3PO4.H2O (3 equiv, 2 mmol), PPh3 (10%
and phenylboronic acid (1.1 equiv, 0.55 mmol) were added to a mol) and Pd(dba)2 (5% mol, 19 mg) were added to the mixture and
Schlenk tube with 5 mL of distilled water. Then, (n-Bu)4N+Br− (3% stirred by 24 hs at 90 ºC. For the extraction process diethyl ether (3
equiv), CsF (4 equiv, 2 mmol) and Pd/C (2% mol, 10 mg) were added x 10 mL) and acidified water were used. The organic extract was
to the mixture. The system was heated to 80 °C and stirred by 2.5 h. dried with Mg2SO4, filtered, and evaporated under reduced
For the extraction process diethyl ether (3 x 10 mL) and acidified pressure. Products were first identified by GC and GC−MS. The
water were used. The organic extract was dried with Na2SO4, compound was isolated from the crude by column chromatography
filtered, and evaporated under reduced pressure. Products were (hexane/CH2Cl2 at 9.5:0.5 ratio) followed by a preparative TLC with
first identified by GC and GC−MS. The compound was isolated from hexane/CH2Cl2 (8:2) as eluent. The product was obtained as pale
the crude by column chromatography as pale yellow oil, employing brown oil, 18% yield, 24 mg. 1H NMR (400 MHz, (CD3)2CO, 25 ºC): δ=
2
a linear gradient of eluent composed by pentane and CH2Cl2, from a 9.51 (s, 1H, OH); 7.675 (ds, J(H,H)= 1.9 Hz, 1H); 7.62-7.58 (m, 3H);
1
1
ratio 90:10 ratio to 70:30. The product was obtained with 36% yield, 7.44 (t, J(H,H)= 7.3 Hz, 2H); 7.37 (t, J(H,H)= 7.3 Hz, 1H); 7.16 (d,
41 mg. H NMR (400 MHz, (CD3)2CO, 25 ºC): δ= 8.06 (s, 1H, OH); 1J(H,H)= 8.4 Hz, 1H). MS (EI): m/z: 195 (100%), 194 (82%), 167
7.59 (d, J(H,H)= 7.8 Hz, 2H); 7.40 (t, J(H,H)= 7.8 Hz, 2H); 7.32-7.27 (13%), 166 (25%), 140 (20%), 139 (16%), 115 (6%), 84 (8%).
(m, 2H); 7.23 (dd, J(H,H)= 8.5 Hz, J(H,H)= 2.2 Hz, 1H); 6.91 (d, 3,5-di-tert-butyl-[1,1'-biphenyl]-2-ol (6). 2,4-di-tert-Butylphenol (1
1J(H,H)= 8.5 Hz, 1H); 1.32 (s, 9H, 3xCH3). MS (EI): m/z: 226 (24%), equiv, 1 mmol) and phenylboronic acid (1.2 equiv, 0.55 mmol) were
1
1
1
1
2
211 (100%), 183 (12%), 165 (8%), 152 (8%), 91 (21%).
added to a Schlenk tube with 5 mL of distilled water. Then, (n-
6-Hydroxy-[1,1'-biphenyl]-3-carboxylic acid and Methyl 6-hydroxy- Bu)4N+Br− (3% equiv), CsF (4 equiv, 4 mmol) and Pd/C (2% mol, 20
[1,1'-biphenyl]-3-carboxylate (4).[40] 3-Bromo-4-hydroxybenzoic mg) were added to the mixture. The procedure followed is the
acid (1 eq, 1.16 mmol), phenylboronic acid (1.16 mmol, 1 eq.), same one used for the synthesis of compound 3, until to obtain the
palladium(II)acetate (0.035 mmol, 0.03 eq.) and 1.5M cesium pure compound. The product was obtained with 16% yield, 44 mg.
carbonate (aqueous) (2.3 mL) were dissolved in DMF (5 mL) at room 1H NMR (400 MHz, (CD3)2CO, 25 ºC): δ= 7.45 (d, J(H,H)= 4.4 Hz, 4H);
2
temperature under nitrogen then heated at 45° C for 24 hours. 7.37-7.33 (m, 2H); 7.06 (ds, J(H,H)= 2.4 Hz, 1H); 6.69 (s,1H, OH);
Worked up by adding water (10 mL) then adjusting to pH=3 with 1N 1.47 (s, 9H, 3 CH3); 1.32 (s, 9H, 3 CH3). 13C NMR (100 MHz, (CD3)2CO)
HCl. Extracted the acidic aqueous 3 times with ethyl acetate. The δ= 150.3 (q, CAr-OH); 142.6 (q, CAr); 139.8 (q, CAr); 136.9 (q, CAr);
ethyl acetate layers were combined and rinsed 3 times with water 130.5 (2 CAr-H); 130.1 (q, CAr); 129.7 (2 CAr-H); 128.1 (CAr-H); 125.8
(10 mL). The ethyl acetate layer was then dried over Na2SO4 and (CAr-H); 123.9 (CAr-H); 35.8 (q, CCH3); 34.8 (q, CCH3); 31.9 (3 CH3);
then, the solvent was evaporated under reduced pressure. The oil 30.2 (3 CH3). HRMS (ESI-TOF) m/z [M + Na]+ Calcd for C20H26ONa:
was purified over silica gel in 1:1 hexanes/ethyl acetate. The 305.1876; Found: 305.1867.
8 | J. Name., 2012, 00, 1-3
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Page 9 of 10
Physical Chemistry Chemical Physics
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ARTICLE
16 M. Lukeman, P. Wan, J. Am. Chem. Soc., 2002, 124, 9458-
Conflicts of interest
There are no conflicts to declare.
View Article Online
9464.
17 The discrepancy between our resuDltOIa: n10d.10t3h9e/C9pCreP0vi2o0u2s8lDy
published 55% value may be due to imponderable
experimental details; however, the obtained result is a valid
reference control for all subsequent measurements.
18 G. Granucci, J. T. Hynes, P. Millié, T.-H. Tran-Thi, J. Am. Chem.
Soc., 2000, 122, 12243-12253.
19 (a) A. Weller, Prog. React. Kinet., 1961, 1, 187; (b) T. Förster,
Z. Elektrochem., 1950, 54, 531.
20 (a) J. Jover, R. Bosque, J. Sales, QSAR Comb. Sci., 2007, 26,
385-397; (b) A. Albert, E. P. Serjeant, The Determination of
Ionization Constants. A Laboratory Manual., 3rd ed.,
Chapman and Hall, New York, 1984.
Acknowledgements
This work was partly supported by the Consejo Nacional de
Investigaciones Científicas y Técnicas (CONICET), Secretaría de
Ciencia
y Tecnología, Universidad Nacional de Córdoba
(SECyT), and the Agencia Nacional de Promoción Científica y
Técnica (ANPCyT). L. D. Mena gratefully acknowledges a
fellowship from CONICET.
21 (a) S.-H. Xia, B.-B. Xie, Q. Fang, G. Cui, W. Thiel, Phys. Chem.
Chem. Phys., 2015, 17, 9687-9697; (b) N. Basarić, N. Doš ić, J.
Ivković, Y. H. Wang, J. Ve jković, K. M inarić-Majerski, P. Wan,
J. Org. Chem., 2013, 78, 1811-1823; (c) N. Basarić, N. Doš ić,
J. Ivković, Y.-H. Wang, M. Mališ, P. Wan, Chem. Eur. J., 2012,
18, 10617-10624.
22 C. Schriever, M. Barbatti, K. Stock, A. J. A. Aquino, D. Tunega,
S. Lochbrunner, E. Riedle, R. d. Vivie-Riedle, H. Lischka,
Chem. Phys., 2008, 347, 446-461.
References
1
(a) V. Balzani, P. Ceroni, A. Juris, in Photochemistry and
Photophysics. Concepts, Research, Applications, Wiley-VCH,
Verlag GmbH & Co. KGaA, Weinheim, 2014, ch. 4; (b) D.
Pines and E. Pines, in Handbook of Hydrogen Transfer, ed. R.
23 B. G. Levine, C. Ko, J. Quenneville, T. J. Martinez, Mol. Phys.,
2006, 104, 1039-1051.
L. Schowen, WILEY-VCH Verlag GmbH
& Co. KGaA,
Weinheim, 2006, ch. 12; (c) J. F. Ireland, P. A. H. Wyatt, Adv.
Phys. Org. Chem., 1976, 12, 131-221.
K. M. Solntsev, D. Huppert, N. Agmon, L. M. Tolbert, J. Phys.
Chem. A., 2000, 104, 4658-4669.
(a) S. Kaneko, S. Yotoriyama, H. Koda, S. Tobita, J. Phys.
Chem. A., 2009, 113, 3021-3028; (b) D. Huppert, L. M.
Tolbert, S. Linares-Samaniego, J. Phys. Chem. A, 1997, 101,
4602-4605; (c) N. Agmon, D. Huppert, A. Masad, E. Pines, J.
Phys. Chem., 1991, 95, 10407-10413.
24 (a) F. A. S. Chipem, G. Krishnamoorthy, J. Phys. Chem. A,
2009, 113, 12063-12070; (b) M. Barbatti, A. J. A. Aquino, H.
Lischka, C. Schriever, S. Lochbrunner, E. Riedle, Phys. Chem.
Chem. Phys., 2009, 11, 1406-1415; (c) D. LeGourriérec, V.
Kharlanov, R. G. Brown, W. Rettig, J. Photochem. Photobiol.
A, 2000, 130, 101-111.
25 M. J. G. Peach, P. Benfield, T. Helgaker, D. J. Tozer, J. Chem.
Phys., 2008, 128, 044118: 1-8.
26 S. Luber, K. Adamczyk, E. T. J. Nibbering, V. S. Batista, J. Phys.
Chem. A, 2013, 117, 5269-5279.
2
3
4
S. P. Webb, L. A. Philips, S. W. Yeh, L. M. Tolbert, J. H. Clark, J.
Phys. Chem., 1986, 90, 5154-5164.
Y. Liao, Acc. Chem. Res., 2017, 50, 1956-1964.
27 C.-C. Hsieh, C.-M. Jiang, P.-T. Chou, Acc. Chem. Res., 2010,
43, 1364-1374.
5
6
7
8
P. Zhou, K. Han, Acc. Chem. Res., 2018, 51, 1681-1690.
G. W. Robinson, J. Phys. Chem., 1991, 95, 10386-10391.
(a) D. Maus, A. Grandjean, G. Jung, J. Phys. Chem. A., 2018,
122, 9025-9030; (b) O. Gajst, L. P. d. Silva, J. C. G. E. d. Silva,
D. Huppert, J. Phys. Chem. A., 2018, 122, 6166-6175; (c) B.
Finkler, C. Spies, M. Vester, F. Walte, K. Omlor, I. Riemann,
M. Zimmer, F. Stracke, M. Gerhards, G. Jung, Photochem.
Photobiol. Sci., 2014, 13, 548-552; (d) C. Spies, B. Finkler, N.
Acarb, G. Jung, Phys. Chem. Chem. Phys., 2013, 15, 19893-
19905.
28 Z.-Y. Liu, J.-W. Hu, C.-L. Chen, Y.-A. Chen, K.-Y. Chen, P.-T.
Chou, J. Phys. Chem. C., 2018, 122, 21833-21840.
29 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci,
G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F.
Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V.
N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A.
Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N.
Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V.
Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P.
Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox,
Gaussian 09, Revision E.01, Gaussian, Inc., Wallingford CT,
2009.
30 T. Yanai, D. P. Tew, N. C. Handy, Chem. Phys. Lett., 2004, 393,
51-57.
31 R. Improta, G. Scalmani, M. J. Frisch, V. Barone, J. Chem.
Phys., 2007, 127, 074504: 1-9.
32 T. Lu, F. Chen, J. Comput. Chem., 2012, 33, 580-592.
33 A.-R. Allouche, J. Comput. Chem., 2011, 32, 174-182.
34 GaussView, Version 5, Roy Dennington, Todd Keith, and John
Millam, Semichem Inc., Shawnee, Mission, KS, 2009.
35 Visual Molecular Dynamics (VMD), version 1.8.9,
http://www.ks.uiuc.edu/Research/vmd.
9
T. Kumpulainen, B. Lang, A. Rosspeintner, E. Vauthey, Chem.
Rev., 2017, 117, 10826-10939.
10 R. Pollard, S. Wu, G. Zhang, P. Wan, J. Org. Chem., 1993, 58,
2605-2613.
11 M. Lukeman, in Quinone Methides, ed. S. E. Rokita, John
Wiley & Sons, Inc., Hoboken, 2009, ch. 1.
12 V. S. Padalkar, S. Seki, Chem. Soc. Rev., 2016, 45, 169-202.
13 M. Lukeman, P. Wan, Chemical Communications, 2001, 0,
1004-1005.
14 (a) Y.-H. Wang, P. Wan, Photochem. Photobiol. Sci., 2013, 12,
1571-1588; (b) Y.-H. Wang, P. Wan, Photochem. Photobiol.
Sci., 2011, 10, 1934-1944; (c) M. Lukeman, M.-D. Burns, P.
Wan, Can. J. Chem., 2011, 89, 433-440; (d) N. Basarić, P.
Wan, Photochem. Photobiol. Sci., 2006, 5, 656-664.
15 (a) C. Azarias, S. Budzak, A. D. Laurent, G. Ulrich, D.
Jacquemin, Chem. Sci., 2016, 7, 3763-3774; (b) H. W. Tseng,
J. Q. Liu, J. A. Chen, C. M. Chao, K. M. Liu, C. L. Chen, T. C. Lin,
C. H. Hung, Y. L. Chou, T. C. Lin, T.-L. Wang, P.-T. Chou, J.
Phys. Chem. Lett., 2015, 6, 1477-1486; (c) A. J. Stasyuk, M. K.
Cyrański, D. T. Gryko, M. Solà, J. Chem. Theory Comput.,
2015, 11, 1046-1054.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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Page 10 of 10
ARTICLE
Journal Name
36 B. Xiao, T. J. Gong, Z. J. Liu, J. H. Liu, D. F. Luo, J. Xu, L. Liu, J.
Am. Chem. Soc., 2011, 133, 9250-9253.
View Article Online
DOI: 10.1039/C9CP02028D
37 (a) N. Haga, H. Takayanagi, J. Org. Chem., 1996, 61, 735-745;
(b) R. Beugelmans, M. Bois-Choussy, Tetrahedron Lett., 1988,
29, 1289-1292.
38 P. C. Solórzano, F. Brigante, A. B. Pierini, L. B. Jimenez, J. Org.
Chem., 2018, 83, 7867-7877.
39 B. Xiao, Y. Fu, J. Xu, T.-J. Gong, J.-J. Dai, J. Yi, L. Liu, J. Am.
Chem. Soc., 2010, 132, 468-470.
40 P. H. Carter, R. J. Cherney, D. G. Batt, J. V. Duncia, D. S.
Gardner, S. S. Ko, A. S. Srivastava, M. G. Yang, Vol. 7,829,571
B2, US, 2010, p. 147.
41 J. Zhao, Y. Wang, Y. He, L. Liu, Q. Zhu, Org. Lett., 2012, 14,
1078-1081.
10 | J. Name., 2012, 00, 1-3
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