Direct Irradiation of Phenol and Para-Substituted Phenols with a Laser Pulse (266 nm) in Homogeneous and Micro-heterogeneous Media. A Time-Resolved Spectroscopy Study

Gaston Siano, Stefano Crespi, and Sergio M. Bonesi*

Article

Direct Irradiation of Phenol and Para-Substituted Phenols with a

Laser Pulse (266 nm) in Homogeneous and Micro-heterogeneous

Media. A Time-Resolved Spectroscopy Study

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sı Supporting Information

ABSTRACT: Direct irradiation of para-substituted phenols under N atmos-

phere in homogeneous (cyclohexane, acetonitrile, and methanol) and micellar

(

SDS) solution was investigated by means of time-resolved spectroscopy. After a

laser pulse (266 nm), two transient species were formed, viz. the para-substituted

phenol radical-cations and the corresponding phenoxy radicals. The radical-

cations showed a broad absorption band located between 390 and 460 nm, while

the phenoxy radicals showed two characteristic bands centered at 320 nm and

−1

00−410 nm. The deprotonation rate constant of radical-cations (k ) of 10 s

and the reaction rate constant of the phenoxy radicals (k ) in the order of 10 −

−1 −1

0 M ·s have been derived. The k rate constants gave good linear Hammett

correlation with positive slope indicating that electron-withdrawing substituents

enhance the radical-cation acidity. The binding constants (K ) of the para-

substituted phenols with the surfactant were also measured, and NOESY

experiments showed that phenols were located in the hydrophobic core of the micelle. Finally, computational calculations provided

the predicted absorption spectra of the transients and nice linear correlations were obtained between the theoretical and

experimental energy of the lower absorption band of these species.

INTRODUCTION

photoionization chemistry. A mechanistic study of photo-

■

oxidation of an interesting series of para-substituted phenols was

also carried out using TiO as the photocatalyst that induced the

photodegradation of the substituted phenols. The decay kinetics

of these phenols were found to be consistent with the

Langmuir−Hinshelwood model.

The photolysis of phenols gives rise to a variety of transients

such as the triplet phenol, phenol radical-cation, phenoxyl

radical, and solvated electrons. It had been suggested that the

excitation of phenols yields the radical-cation species through a

biphotonic process in alcohols and n-butyl chloride solu-

Phenols are substrates with excellent electron donor properties

and have generated much interest because of their role as

antioxidants in biological and industrial processes. Their action

as radical scavengers derives from their ability to react with

radical species by H atom transfer processes, providing stable

−7

phenoxyl radicals.

Sterically hindered phenols are con-

sequently employed as stabilizers in the production of polymers,

oils, and food. The reactivity of these phenols as antioxidants

involves a complex system of parallel processes, involving

physical and chemical pathways. Hence, these paths could

9b,10h,i

·+

tions.

The phenol radical cations (ArOH ) are the ﬁrst

imply (i) the reaction of phenol antioxidants with precursors of

radical peroxides, (ii) direct irradiation of phenolic antioxidants

generating stable phenoxyl radicals, or (iii) ionization of

phenolic antioxidants by electron transfer photosensitization

from the matrix providing parent radical ions. In all the processes

listed, phenoxyl radicals were the ﬁnal product observed.

There are several reports regarding the reactivity of phenols in

solution under pulse radiolysis and laser ﬂash photolysis

spectroscopies. Early studies of phenols showed that there are

several routes to the formation of phenoxyl radicals, including β-

cleavage reactions of α-aryloxyacetophenones, hydrogen

abstraction reactions of phenols, and the reaction of ketyl

detectable transients that subsequently deprotonate to yield the

phenoxyl radicals. The process is characterized by a low

activation barrier (see eq 1 ).

Received: August 22, 2020

radical derived from the photoreduction of aromatic ketones.

Likewise, alternative routes to furnish phenoxyl radicals

employed photoinduced electron-transfer reactions as well as

XXXX American Chemical Society

J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry

Article

Phenol radical-cations have been observed directly in matrices

results obtained from a systematic laser ﬂash photolysis

investigation (laser pulse of 266 nm) of a series of para-

substituted phenols (see Chart 1) in homogeneous (cyclo-

11b

at low temperature, strong acids, and gas-phase clusters.

Evidence of their existence has been provided by CINDP and

ESR spectroscopies. Several phenol radical-cations have been

characterized with both pulse radiolysis and laser ﬂash

photolysis spectroscopies in acetonitrile and n-butyl chloride

Chart 1. Structures of the Phenol Derivatives

9b,10i,h,15

at room temperature.

Oxidation of phenols by pulse

radiolysis in the presence of persulfate or azide anions in

acetonitrile has demonstrated the formation of radicals and

radical-cations, and in the last case, under basic conditions,

excitation of the corresponding phenolates provided the

hexane, acetonitrile, and methanol) and heterogeneous (SDS

micellar solution) media at room temperature and under

nitrogen atmosphere. Such a study should contribute to

knowledge about the stability, reactivity, and chemical

consequences of the transient species viz. phenol radical-cations

and phenoxyl radicals. Furthermore, the phenol radical-cations

are expected to behave like acid compounds, and the acidity of

such a transient can be tuned depending on the electronic

properties of the substituents. In fact, it is expected that phenol

radical-cations bearing electron-withdrawing substituents would

increase the acidity of the radical-cation transients generating

more readily the corresponding phenoxyl radicals. This behavior

will be demonstrated by applying the Hammett linear

correlation and the Brønsted plots.

phenoxyl radicals.

Direct irradiation (308 nm) of α-tocopherol in micellar

solutions of HTAC and SDS have shown the formation of the α-

tocopheroxyl radical and this transient was found to be relatively

long-lived in micellar solutions allowing the slow reaction with

glutathione. In addition, the same radical transient has been

detected in micellar media using time-resolved Raman and laser

17b

ﬂash photolysis spectroscopies.

Recently, α-tocopheroxyl

radical has been formed by reaction of the corresponding phenol

with photochemically generated tert-butoxyl radical in SDS

solution and has been detected by laser ﬂash photolysis and EPR

measurements. The transient decay was prolonged, showing a

rate constant of 75 M⁻s . However, in the presence of several

1 −1

green tea polyphenols, the rate constants increased signiﬁcantly,

regenerating the α-tocopherol.

17c

RESULTS

■

Resveratrol is another phenol that has been studied by laser

ﬂash photolysis technique in micellar solution using SDS,

DTCA, and TX-100 as surfactants. In these studies, the

resveratroxyl radical was generated eﬃciently under direct

Irradiation of Para-Substituted Phenols in Homoge-

neous Media with a Laser Pulse. Direct irradiation of a series

of para-substituted phenols (see Chart 1) in diﬀerent solvents

cyclohexane, acetonitrile and methanol) was carried out with a

66 nm laser pulse under nitrogen atmosphere. The experiments

(

irradiation with a laser pulse of 266 nm. Formation of this

transient has also been observed when direct irradiation of

resveratrol in aqueous cyclodextrin solution was carried out with

showed the formation of two transient species viz. the phenol

radical-cation and the phenoxyl radical. Both species have

distinct lifetimes: the phenol radical-cations show short lifetime

(lower than 10 μs) while the phenoxyl radicals are species with

lifetimes higher than 20 μs. Moreover, the transient radical-

cation showed a broad absorption band located between 390

and 490 nm while the phenoxyl radical showed two character-

istic bands centered at 320 nm and 400−410 nm (see Scheme 1)

55 nm light and, in the presence of ascorbate, regeneration of

the phenol occurred eﬃciently. In this particular example, the

resveratroxyl radical was formed within the hydrophobic core of

the cyclodextrin.

Recently, the photophysical and photochemical properties of

resveratrol have been investigated in water and acetonitrile

under direct irradiation with a laser pulse of 266 or 355 nm. Both

the key transient species, viz. phenoxyl radical and phenol

,15

which are similar to those reported in the literature.

Representative transient absorption spectra of some para-

substituted phenols in acetonitrile under nitrogen atmosphere

recorded after the laser pulse of 266 nm are shown in Figure 1.

All the transient absorption spectra recorded in cyclohexane,

acetonitrile and methanol in the same experimental conditions

are collected in Figure S1 (see Supporting Information).

In order to establish whether the transient absorption spectra

is the overlapping of absorption spectra of the phenoxyl radical

and phenol radical-cation, respectively, we have performed

additional experiments. p-Phenoxyphenol was selected as a

radical-cation, were observed. The photoreaction involves the

singlet excited state and is a two-photon ionization process. In

acetonitrile the transient radical-cation deprotonates with a rate

−1

constant of 2.7 × 10 s yielding the long-lived phenoxyl radical.

Water (0.75%) stabilized the intermediate that decayed with

−1 −1

rate constant around 1 × 10 M s .

On the other hand, the photochemical process of 8-hydroxy-

,3,6-pyrenetrisulfonic acid (PyOH) involves the photoelectron

−

•

•−

ejection from PyO to produce PyO and PyO under visible

laser excitation (470 nm). The kinetic rate constants for

phenolic antioxidants with PyO were mostly reliant on the ionic

probe because the absorption of both transients are partially

•

9−11

overlapped.

Hence, we have irradiated a solution of p-

strength depending on the antioxidant phenolate/phenol

dissociation constant. Also, the study carried out in Triton-

X100 micellar solution showed that the apparent rate constants

depend on the partition of the phenolic antioxidants between

the micelles and the aqueous media. The observed rate constants

phenoxyphenol in an acetonitrile water (9:1) mixture in the

presence of sodium persulfate with a laser pulse of 266 nm under

nitrogen atmosphere (see Scheme 2). This methodology aﬀords

the selective formation of radical-cations in solution, as reported

16a

in the literature.

−1 −1

varied between 1.41 × 10 and 6.22 × 10 M s .

Thus, we have recorded the transient absorption spectra of p-

phenoxyphenol in acetonitrile in the absence (black solid line)

and the presence of K S O (blue solid line) under nitrogen

The above results show that there is considerable interest in

the photochemistry of phenol derivatives, but to our knowledge,

a systematic photochemical study on a series of para-substituted

phenols has not been performed yet. Therefore, fostered by our

results on the photo-Fries reaction, in this work we describe the

atmosphere with a laser pulse of 266 nm which are shown in

Figure 2a. In the same ﬁgure, the diﬀerential spectrum (red solid

line) shows the estimated absorption spectra of the p-

J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry

Phenol radical cation and phenoxyl radical transients.

Article

Scheme 1. Direct Irradiation (266 nm) of Para-Substituted Phenols in Homogeneous Media

Figure 1. Time-resolved transient absorption spectra recorded with a laser pulse (200 μs; 266 nm) of acetonitrile solutions (5.1 × 10⁻⁴M) of (a) p-tert-

butylphenol; (b) p-methoxyphenol; (c) p-chlorophenol; and (d) p-cyanophenol.

Scheme 2. Direct Irradiation (266 nm) of p-Phenoxyphenol in an Acetonitrile/Water (9:1) Mixture in the Presence of K S O

−

(

0.010 mol·dm )

phenoxyphenol radical-cation with a maximum wavelength

located at 470 nm. Then, the signal located at a maximum

wavelength of 405 nm was assigned to the p-phenoxyphenoxyl

radical which was formed by deprotonation of the radical cation.

These experiments clearly show that both transients, the radical-

cation and the radical, are formed immediately after the laser

pulse and that the radical-cation subsequently loses a proton to

provide the radical in the 10 μs time scale. Additionally, the

transient decay traces of a solution of p-phenoxyphenol in

acetonitrile in the presence of sodium persulfate were recorded

at 410 and 460 nm, respectively (see Figure 2b). The decay trace

at 460 nm shows a biexponential decay trace yielding two

lifetime values: τ_H8.1 μs and τ 26.4 μs, respectively. This

behavior can be interpreted considering that both transient

species, the phenol radical-cation and the phenoxyl radical, react

through consecutive and diﬀerent pathways, viz. release of a

proton from the phenol radical-cation and chemical reaction of

the phenoxyl radical (see Scheme 2) as it was observed in earlier

,16

reported data.

The deprotonation process of the phenol

radical-cation is a unimolecular pathway, and the rate constant

(k ) of this process can be calculated from the reciprocal of the

lifetime, k = 1/τ . The value of the rate constant k of the p-

−1

phenoxyphenol radical-cation was found to be 1.2 × 10 s .

Similar behavior was observed when the transient decay trace

J. Org. Chem. XXXX, XXX, XXX−XXX

Article

Figure 2. (a) Transient absorption spectra obtained at zero time after the laser pulse (200 μs; 266 nm) of solutions (5.1 × 10⁻⁴M) of p-phenoxyphenol

in acetonitrile (black line) and in acetonitrile in the presence of sodium persulfate (blue line) under nitrogen atmosphere and the diﬀerential spectrum

of the p-phenoxyphenol radical cation (red line). (b) Transient decay traces recorded at 410 and 460 nm after the laser pulse (λ : 266 nm) of a

exc

solution (5.1 × 10⁻M) of p -phenoxyphenol in acetonitrile in the presence of sodium persulfate under N atmosphere.

−

Figure 3. (a) Transient decay traces recorded at 410 nm after the laser pulse (200 μs; λ : 266 nm) of solutions (5.1 × 10 M) of p-phenoxyphenol in

exc

acetonitrile in the absence (blue solid line) and in the presence of sodium persulfate (black solid line) under N atmosphere and room temperature. (b)

Reciprocal plotting (1/concentration) vs time) in acetonitrile in the absence (blue circles) and in the presence of sodium persulfate (black circles). In

all the linear ﬁtting regressions: R > 0.99.

Table 1. Spectroscopic Data of Transients from 4-Substituted Phenol Measured by Laser Flash Photolysis (266 nm) in diﬀerent

Solvents under N Atmosphere

cyclohexane

acetonitrile

methanol

λ_a_b_s(nm)

τ_H(μs)

τ_R(μs)

λ_a_b_s(nm)

τ_H(μs)

τ_R(μs)

λ_a_b_s(nm)

τ_H(μs)

τ_R(μs)

OMe

OPh

t-Bu

NO₂

405

400

405

410

400

4.6

1.3

4.4

3.8

4.7

16.0

20.0

25.9

22.3

22.2

405

410

400

340

410

435

400

5.0

4.3

5.8

1.6

6.2

0.6

1.9

0.7

0.8

29.6

38.1

41.4

19.1

42.2

42.4

22.2

15.8

34.9

405

400

340

410

435

400

7.9

6.3

4.7

5.2

5.0

0.2

1.9

0.7

1.0

38.2

39.1

30.7

23.0

66.3

14.0

24.6

58.2

insoluble

2.8

1.0

insoluble

400

435

19.6

12.2

^aConcentration of 4-substituted phenols: 5.0 × 10⁻⁴M. Errors: ± 0.2.

was analyzed at 410 nm. A biexponential ﬁtting of the transient

traces at 460 nm and at 410 nm, respectively, were found to be

similar values within the experimental error.

The transient decay trace of p-phenoxyphenoxyl radical in

acetonitrile (solid blue line in Figure 3a) and in a mixture of

acetonitrile and water (9:1) in the presence of sodium persulfate

decay trace was observed (see Figure 2(b); blue solid line) and a

−1

rate constant k_Hof 1.3 × 10 s was obtained from the

reciprocal of the short lifetime (τ ). Therefore, the rate

constants k obtained from the analyses of the transient decay

J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry

Article

Table 2. Deprotonation (k ) and Reaction (k ) Rate Constants of 4-Substituted Phenoxyl Radicals and 4-Substituted Phenoxyl

Radical-Cation Measured by Laser Flash Photolysis (266 nm) in Diﬀerent Solvents under N Atmosphere

acetonitrile

k_R(M⁻¹s⁻¹

methanol

cyclohexane

k_H(s⁻¹)

)

k_H(s⁻¹

)

k_R(M⁻¹s⁻¹

)

k_H(s⁻¹

)

k_R(M⁻¹s⁻¹

)

pK_a

1.3 × 10⁵

2.1 × 10

1.6 × 10⁵

2.2 × 10⁵

OMe

OPh

t-Bu

NO₂

2.0 × 10

2.3 × 10

1.7 × 10

6.3 × 10

1.6 × 10

5.3 × 10

1.5 × 10

1.6 × 10

2.1 × 10

1.0 × 10

2.2 × 10

7.4 × 10

5.1 × 10

1.9 × 10

2.3 × 10

2.9 × 10

1.7 × 10

2.1 × 10

5.4 × 10

4.7

5.0 × 10⁹

9.8 × 10⁹

7.7 × 10

2.3 × 10⁵

2.8 × 10

5.3 × 10

7.1

7.2

8.1

5.7

9.9

1.9 × 10

1.0 × 10

2.6 × 10

2.4 × 10

2.0 × 10⁵

5.0 × 10⁶

7.6 × 10⁹

2.1 × 10⁵

1.5 × 10

2.0 × 10

insoluble

7.1 × 10⁹

3.6 × 10⁵

1.0 × 10⁶

5.3 × 10

3.6 × 10

1.4 × 10⁶

1.0 × 10⁶

3.2 × 10

3.9 × 10

13.0

15.0

1.6 × 10

^aConcentration of 4-substituted phenols: 5.0 × 10⁻⁴M. Errors: ± 10%. Data taken from ref 11c .

Figure 4. (a) Time-resolved transient absorption spectra recorded with a laser pulse (200 μs; 266 nm) of p-methoxyphenol (5.1 × 10⁻⁴M) in SDS 0.10

−

M aqueous solutions. (b) Transient decay trace recorded at 400 nm after the laser pulse (λ : 266 nm) of p-methoxyphenol (5.1 × 10 M) in SDS 0.10

exc

M aqueous solutions under N atmosphere.

(

solid black line in Figure 3a) was measured at 410 nm showing

clearly a biexponential behavior. The bimolecular rate constants

k_R) were determined by plotting the reciprocal of the

deprotonation lifetime (τ ) values of the phenol radical-cation

depend on the nature of the substituents. In fact, electron-donor

substituents attached to the phenol moiety impose higher

lifetime values than the one registered for those phenols bearing

electron-withdrawing substituents.

(

concentration of the radical transient against time and excellent

linear correlations were observed (see Figure 3b). Then after

applying a linear regression ﬁtting, the reaction rate constants

Deprotonation rate constants (k ) and reaction rate constants

(

k ) were obtained from the slopes. The second-order rate

(k ) are reported in Table 2. All of the para-substituted phenols

−1 −1

constant (k ) was found to be 4.7 × 10 M ·s in acetonitrile

water (9:1) mixture in the presence of sodium persulfate, while

in neat acetonitrile a value of 1.0 × 10 M ·s was obtained. It

in cyclohexane, acetonitrile, and methanol show excellent ﬁtting

with the kinetic model proposed (see Figure S3 in the

Supporting Information ). The slopes obtained after application

−1 −1

is noteworthy that the bimolecular rate constants (k ) measured

of linear regression ﬁttings to those straight lines provided k

values of 10 −10 M ·s (Table 2).

−1 −1

in both media are slightly diﬀerent. We could attribute these

results to the diﬀerence in polarity and proticity of the solvents

used; it seems reasonable to propose that p-phenoxyphenyloxy

radical could diﬀuse diﬀerently within the bulk of both reaction

solvents.

The same data analysis was applied to the transient absorption

spectra of the remaining para-substituted derivatives (Chart 1),

recorded in polar and nonpolar solvents such acetonitrile,

methanol, and cyclohexane (see Figure S1 in the Supporting

Information ). The transient decay traces at the maximum

absorption wavelength were measured (see Figure S2 in the

Supporting Information ) showing a biexponential behavior. The

spectroscopic data thus obtained are collected in Table 1. The

absorption wavelength do not change signiﬁcantly from

nonpolar to polar solvents while it is apparent that the

It is apparent from Table 2 that the bimolecular rate constants

k_Rdo not show any substituent eﬀect on the reactivity of the

phenyloxy radicals because the values are under diﬀusion control

in each solvent, and we can suggest that as quickly as the

reactants (phenoxyl radical or solvent molecules) encounter

each other they react. However, the deprotonation rate

constants (k ) do depend on the nature of the substituents.

As can be seen in Table 2, the deprotonation rate constant values

−1

6 −1

change from 1.6 × 10 s to 5.7 × 10 s as the substituents

move from electron-donating to electron-withdrawing ones.

Irradiation of Para-Substituted Phenols in Micellar

Solution with a Laser Pulse. Direct irradiation of a series of

para-substituted phenols (for structures, see Chart 1) in

aqueous SDS (0.10 M) was carried out with a 266 nm laser

J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry

Supporting Information ).

Article

under nitrogen atmosphere, and also in this case, both the

phenol radical-cation and the phenoxyl radical were formed after

the laser pulse. The time-resolved transient absorption spectra of

p-methoxyphenol in SDS 0.10 M aqueous solutions is depicted

as an example in Figure 4a. Its transient radical-cation showed a

broad absorption band located between 390 and 490 nm, while

the p-methoxyphenoxyl radical showed two characteristic bands

centered at 320 nm and 400−410 nm, values similar to those

As for the homogeneous solutions, it is apparent from the

table that no clear substituent eﬀect is observed in SDS micellar

solution for the bimolecular rate constants k , in contrast to k .

−1

Indeed, the k values change from 1.1 × 10 s to 5.6 × 10 s

as the substituents move from electron-donor to electron-

withdrawing ones. This eﬀect was quantiﬁed plotting log(k /

R=H

k_H) versus the σ Hammett parameters (see Figure S5 in

the Supporting Information ), and after linear regression ﬁttings

of the straight line a ρ value of 1.01 was obtained. This value

indicated that electron-withdrawing substituents accelerate the

deprotonation reaction of the para-substituted phenols, and the

substituent eﬀect was transmitted eﬃciently like in substituted

benzoic acids.

,15

reported in the literature

as well as to those recorded in

homogeneous media (see Figure S1 in the Supporting

Information ). Furthermore, the spectroscopic feature in terms

of shape and band location for p-methoxyphenol transients in

micellar solution are also observed and found similar to those

time-resolved absorption spectra recorded for the other para-

substituted phenol in micellar solution (see Figure S1 in the

Quantiﬁcation of the Substituent and Solvent Eﬀects.

The substituent eﬀect was quantiﬁed employing the linear

Hammett correlation and the best linear correlation ﬁttings were

Figure 4b depicts a transient decay trace recorded at 400 nm

after the laser pulse (λ_e_x_c266 nm) of p-methoxyphenol in SDS

obtained when σ Hammett parameters were used (see Figure

4a). The Hammett linear free-energy relationship (LFER) is

described by eq 2

.10 M aqueous solution under N atmosphere showing a

biexponential behavior. After ﬁtting, the short lifetime τ (lower

than 10 μs) was assigned to the para-substituted phenol radical-

log(k (R))/log(k (H)) = ρ·σ

(2)

cations while the phenoxyl radicals are those species showing

where k (R) is the deprotonation rate constant of para-

lifetimes higher than 20 μs (τ ). The lifetime (τ and τ ) and the

substituted phenol radical-cations, k (H) is the deprotonation

absorption wavelength (λ ) of the band associate to the lower

abs

rate constant of unsubstituted phenol radical-cation, ρ is the

energy transitions are collected in Table 3. It is apparent from

slope of the linear regression and σ are the Hammett

substituent constants. Figure 5a shows a linear correlation

R=H

^T^a^b^l^e³^.^S^p^e^c^t^r^o^s^c^o^pⁱ^c^,^Lⁱ^f^e^tⁱ^m^e^,^Aⁿ^d^D^e^p^r^o^t^oⁿ^a^tⁱ^oⁿ⁽^kH⁾

between the [log(k_H)/log(k_H)] and the substituent

and Reaction (k ) Rate Constants of 4-Substituted Phenoxyl

constant (σ ) providing a ρ value of 0.61 (R > 0.90). This

Radicals and 4-Substituted Phenoxyl Radical-Cation

positive value indicates that the deprotonation rate constants

Measured by Laser Flash Photolysis (266 nm) in Micro-

(

k_H) are sensitive to the electronic eﬀects of the substituents

heterogeneous Media (SDS 0.10 M) under N Atmosphere

attached to the aromatic ring. Thus, electron-donating groups

cause a decrease of the deprotonation rate constants due to a

noticeable stabilization of the phenol radical-cation by

resonance eﬀect while electron-withdrawing groups cause an

increase of the rate constants. However, the sensitivity observed

in para-substituted phenols is lower than what is observed in

para-substituted benzoic acids: the ρ value <1 indicates a lower

transmission of the substituent eﬀect.

k_H₍_s₋1

k_R(M⁻¹_s−1

λ_a_b_s(nm)

τ_H(μs)

τ_R(μs)

)

7.0 × 10⁹

2.6 × 10⁹

OMe

OPh

t-Bu

NO₂

435

452

425

428

425

395

433

460

450

9.1

1.6

4.6

4.4

1.8

21.3

3.6

9.1

0.2

47.2

50.9

34.6

31.6

20.9

141.5

45.3

75.0

1.1 × 10

6.3 × 10

2.2 × 10

2.3 × 10

5.6 × 10

4.7 × 10

2.8 × 10

1.1 × 10

5.6 × 10

1.2 × 10

6.3 × 10

7.7 × 10⁸

1.4 × 10⁹

The radical-cation deprotonation process (see Scheme 1)

accounts for the acidity of such transients and appears to parallel,

quite regularly, the thermodynamic acidity. Indeed, the plot of

2.2 × 10

11c

the deprotonation rate constants k as pk vs pK of the para-

_C_o_n_c_e_n_t_r_a_t_i_o_n_o_f_p_a_r_a_-_s_u_b_s_t_i_t_u_t_e_d_p_h_e_n_o_l_s_:₅_.₀_×₁₀−4 M. Errors: ±

.2 in lifetime values and ±10% in rate constants.

substituted phenol radical-cations (Brønsted plot) is satisfac-

torily linear (see Figure 5(b)). The Brønsted coeﬃcient α values

are quite similar for the two sets of reaction solvents used, viz. α

= 0.10 in MeCN and MeOH, while α = 0.13 in cyclohexane and

SDS 0.10 M. The similarity of the α values demonstrates the

these data that the deprotonation lifetime (τ ) values of the

phenol radical-cations depend on the nature of the substituents.

In fact, electron-donor substituents attached to the phenol

moiety show lifetime values higher than those phenols bearing

electron-withdrawing substituents. Otherwise, no clear sub-

stituent eﬀect was observed for the τ_Rvalues or for the

absorption wavelengths (λ_a_b_s).

reliability of the pK ’s reported in Table 2. In our experiments,

we suggest that the possible bases for the deprotonation to occur

could be the residual H O for the experiments carried out in

MeCN and MeOH, while in cyclohexane or micellar solution

(SDS) the bases are the para-substituted phenols themselves.

Finally, we can also suggest that the base does not play a

signiﬁcant role in the transition-state structure due to the similar

Brønsted coeﬃcients exhibited by such structurally diﬀerent

As already described, the deprotonation rate constants (k ) of

the para-substituted phenol radical-cations were easily calcu-

lated from the reciprocal of the short lifetime values (k = 1/τ )

and are also shown in Table 3. On the other hand, plotting the

reciprocal of the concentration of the para-substituted phenoxyl

radicals versus time provided an excellent linear ﬁt (see Figure

S3 in Supporting Information ). Then the reaction rate constants

bases as H O and para-substituted phenols.

Finally, we have studied the solvent eﬀect on the

deprotonation pathway of the para-substituted phenol radical-

cations using the E (30) Reichardt’s solvent polarity parame-

(

k_R) for all the para-substituted phenols were obtained from the

ter, while the E (30) parameter for a micellar solution of SDS

in water was calculated from the E solvent parameter reported

in the literature. Figure 6 came of plotting log(k ) values of

slopes after application of linear regression ﬁttings to those

straight lines. The k values of 10 −10 M ·s are also

−1 −1

collected in Table 3.

some para-substituted phenols bearing electron-donor and

J. Org. Chem. XXXX, XXX, XXX−XXX

cyclohexane and SDS 0.1 M is represented by the red line.

Article

Figure 5. (a) Linear Hammett correlation of deprotonation rate constants (k ) of para-substituted phenol radical-cations versus σ in homogeneous

media. (b) Brønsted linear correlation between acid constant (pK ) and deprotonation rate constants (pk ) of para-substituted phenol radical-cations

in homogeneous and micro-heterogeneous media. Best linear ﬁtting in MeCN and MeOH is represented by the black line while the best linear ﬁtting in

Then, the binding of the para-substituted phenols to the SDS

surfactant that took place within the hydrophobic core of the

micelle was demonstrated, analyzing the bathochromic and

hyperchromic shifts of the lower energy absorption band of the

phenols in water by addition of increasing amounts of surfactant.

Indeed, the binding of the para-substituted phenols to the

micelle can be described according to eq 3 where K is the

binding constant, S represents the phenols, Surf the surfactants,

and [S-Surf] the complex formed between phenols and the

surfactant.

^Kb

[S‐Surf]

[

S] + [Surf] F [S‐Surf] K =

[

S][Surf]

(3)

Application of Lambert−Beer law on eq 3 provided eq 4

Figure 6. Correlation of the log(k ) of para-substituted phenol radical-

where A and A are the absorbances at the maximum wavelength

cations against the E (30) solvent polarity parameter.

in the absence and presence of surfactant, respectively, ε is the

molar absorptivity of the complex, and ε is the para-substituted

phenols molar absorptivity. Rearranging eq 4 gave eq 5 where a

electron-withdrawing substituents against the solvent E (30)

values but no linear correlation was observed.

This behavior indicates that the solvent is not involved in the

deprotonation pathway of the radical-cations because, for each

linear relationship is observed between [A /(A − A )] and the

reciprocal of the concentration of the surfactant.

(

A − A₀)

ε K [Surf]

para-substituted phenol, the k values are similar in nonpolar,

^A0

ε (1 + K [Surf])

(4)

(5)

polar aprotic, and polar protic solvents as well as in the

hydrophobic core of SDS micelle. However, as it was previously

pointed out in the text, the phenol radical-cation deprotonation

process depends on the acidity of such transients and correlates

(A − A₀)

ε_S

ε_C

ε_S

ε K [Surf]

A₀

satisfactorily with the pK values according to the Brønsted plots

(

see Figure 5b), providing lower Brønsted coeﬃcient α values in

Figure 7 shows the data obtained for some para-substituted

phenols and SDS, including the best linear regression curves,

while the straight lines and the corresponding linear regression

curves for the other para -substituted phenols are collected in

two sets of solvents, viz. α = 0.10 in MeCN and MeOH while α =

.13 in cyclohexane and SDS 0.10 M.

Binding Constants (K ) of Para-Substituted Phenols in

Micellar Media. Micellar solutions, which are often considered

as micro-photochemical reactors, are micro-heterogeneous

systems where photoreactions can be performed. In this regard,

we have conducted UV−vis and 2D H NMR spectroscopy

studies of para-substituted phenols in micellar solution in order

Figure S6 (see the Supporting Information ). The K values for

the para-substituted phenols were calculated from the ratio of

the slope and the intercept of the regression curve (Table 4).

Likewise, the calculation of the error of the binding constant K_b

values were determined from the intercept, slope and their

corresponding error values according to the mathematical

method clearly described in the Supporting Information (data

to know the positioning of the reactant within the micelles.

Thus, we have determined the binding constant (K ) between

the SDS micelles and para-substituted phenols employing UV−

shown in Table S1). The K values obtained for the para-

vis spectroscopy applying a methodology that has been

substituted phenols are typical of aromatic solutes as pointed out

previously reported for aryl acetamide and aryl benzoates.

by Quina, Treiner, and co-workers, and estimation of K values

J. Org. Chem. XXXX, XXX, XXX−XXX

Article

TD-DFT Calculations for Prediction of UV−vis Absorp-

tion Spectra of Para-Substituted Phenol Radical-Cations

and Radicals. The results of TD-DFT calculations for the UV−

vis absorption spectra of para-substituted phenol radical-cations

and radicals, respectively, in terms of predicted maximum

absorption wavelength values but reported as energy values

(

ΔE) have been collected in Table 5 together with those

experimental ΔE values obtained from the transient absorption

spectra recorded after the laser pulse (266 nm). The TD-DFT

simulation was carried out using Gaussian 16 (Revision B.01

software package), and the geometry of the molecules was

optimized and the themochemistry was obtained using the

(

U)ωB97XD/def2SVP level of theory. The UV−vis spectra

were simulated at the SMD-(U)ωB97XD/def2TZVP level

solvent: vacuum, cyclohexane, and acetonitrile), computing

Figure 7. Plot of the A /(A − A ) values of some selected para-

(

substituted phenols vs the reciprocal concentration of the micelle in

SDS (0.10 M).

the lowest 25 singlet transitions and the UV−vis band was

plotted as ε vs λ (excitation wavelength in nm) with the peaks,

furnished by the calculation, assuming a Gaussian band shape

(characterized by a standard deviation σ = 0.4 eV) and using the

equation for the simulated spectra according to the one

₁₀₀_M₋1 in SDS for weakly hydrophobic substrates have also

been reported. Likewise, similar K values have been reported

≤

described in the Gaussian White Papers. All the predicted

UV−vis absorption spectra of para-substituted phenol radical-

cations are collected in Figure S8 (see the Supporting

Information ) while those belonging to the para-substituted

phenoxyl radicals are shown in Figure S9 (see the Supporting

Information ).

for the binding of anionic, cationic, and neutral surfactants and a

series of benzoyl chloride derivatives, aryl acetanilides, and aryl

benzoates, respectively.

26,29

Once the binding constants of the para-substituted phenols

have been measured, the next step was to conﬁrm qualitatively

the location of the phenols within the hydrophobic core of the

micelle employing 2D NMR spectroscopy. Determination of the

extent of coaggregation in water between two diﬀerent kinds of

surfactants as well as the localization of substrates inside the

Next, we attempted to build a correlation between the

theoretical ΔE

values and the experimental ones (ΔE(exp)⁾^,

(theo)

the corresponding plots being shown in Figure 9. Fairly good

linear correlations have been obtained for all the para-

substituted phenol transients excluding the p-nitrophenol

radical-cation as well as the p-nitro- and p-phenylphenoxy

radical in each linear regressions (see the corresponding arrows

in Figure 9).

6,30

micelle NOESY experiments have been often used.

Satisfactory results are obtained from NOESY experiments

when cross-peaks between diagnostic signals of the substrate and

the surfactants, respectively, are noticed in the corresponding

contour plots. Thus, Figure 8 shows the NOESY experiment

The linear correlations between the predicted and exper-

imental lower absorption band of para-substituted phenol

radical-cation and para-substituted phenoxyl radicals have been

obtained with slopes close to unity and acceptable R-square

values (see Table 6).

performed in D O for a solution of SDS (100 mM) in the

presence of p-cyano phenol (7 mM) at room temperature and in

the same ﬁgure the labels of the protons of the surfactant SDS

and the p-cyano phenol are also depicted. The inset red frames

recognize the NOE (Nuclear Overhauser Eﬀect) between the

signals belonging to the aromatic protons (H-2/H-6 and H-3/

H-5) of p-cyano phenol and the signals of the surfactant SDS

such as α, β, ω, and the bulk protons. Similar spectroscopic

These results demonstrate that the prediction of the ΔE(theo)

using the adopted theoretical approach is fairly accurate leading

to a good estimation of the energies values. However, these

results deserve some comments. As for para-substituted phenol

radical-cations (Figure 9a), their energy values span a quite wide

range, viz. from 2.73 to 3.24 eV, and the points seem to distribute

quite uniformly across the whole range. An exception is

constituted by compound 9 showing an important deviation

from the linear regression (see the data points indicated with an

arrow in Figure 9a). The observed deviation of the data points of

compound 9 can be attributed to the theoretical prediction of a

nonplanarity of the nitro group with respect to the aromatic

plane and thus, canceling the expected bathochromic shift of the

absorption band that the theoretical data should correctly be

results have been obtained for solutions of SDS in D O in the

presence of p-methoxy phenol (see Figure S7 in the Supporting

Information ). The cross-peaks of diagnostic signals observed in

the 2D NMR contour plots are in agreement with and reinforce

the UV−vis spectroscopic analyses. However, we cannot

estimate precisely the location of the phenols with accuracy

but we can suggest that the para-substituted phenols are located

within the hydrophobic core of the micelle because the proton

nuclei of the phenols correlate nicely with the proton nuclei of

the surfactant as can be seen through the cross-peaks of the

contour plots.

Table 4. Constants of Binding (K ) of the Para-Substituted Phenols Measured in Micro-heterogeneous Media (SDS 0.10 M)

Errors: calculations described in the Supporting Information.

J. Org. Chem. XXXX, XXX, XXX−XXX

Article

Figure 8. 2D NOESY contour plot of a solution of SDS (100 mM) and p-cyanophenol (7 mM) in D O at room temperature was recorded with a 500

MHz spectrometer.

Table 5. Experimental and DFT Calculation of the Lower

Absorption Band Energies (ΔE) of the Para-Substituted

Phenol Radical-Cations and Para-Substituted Phenol

Radicals Measured in Diﬀerent Solvents

para-substituted phenoxyl radicals (Figure 9b) a fairly linear

correlation was also obtained between the theoretical and

experimental energy values with exclusion of the data belonging

to p-nitro- and p-phenylphenoxyl radicals which are indicated

with black arrows in Figure 9b. This anomalous behavior can be

again ascribed to the peculiar nonplanar arrangement of the

nitro and phenyl groups with respect to the aromatic plane.

DISCUSSION

■

The results presented in this paper report for the ﬁrst time the

absolute rate constants for the deprotonation pathway (k ) of

para-substituted phenol radical-cations and the reaction path-

way (k ) of para-substituted phenoxyl radicals in diﬀerent

solvents such as homogeneous solvents (cyclohexane, acetoni-

trile, and methanol) and SDS micellar solution (Tables 2 and 3).

These data were obtained from the nonlinear regression analyses

of the transient decay traces showing biexponential decay

behavior (Figure S2 in the Supporting Information ). Indeed, the

observed transient decay behavior is attributed to the formation

of two transient species, viz. the phenol radical-cation and the

phenoxyl radical, after direct irradiation of a series of para-

substituted phenols with a laser pulse of 266 nm in diﬀerent

solvents and micellar solution. Furthermore, we were able to

characterize both transient species in terms of lifetime values

Measured by laser ﬂash photolysis (266 nm) under N atmosphere.

−

Concentration of para-substituted phenols: 5.0 × 10 M. Errors: ±

(

Tables 1 and 3) showing shorter lifetime values (lower than 10

.02. Values calculated with SMD-(U)ωB97XD/def2TZVP level

(

solvent: cyclohexane and acetonitrile).

μs) in the case of para-substituted phenol radical-cations, while

larger ones (higher than 20 μs) were obtained for para-

substituted phenoxyl radicals. Likewise, the time-resolved

absorption spectra of the para-substituted phenols were also

measured in homogeneous solvents and SDS micellar solution

(Figure 1 and Figure S1 in the Supporting Information ) and

resulted to be the overlapping of the absorption spectra of the

predicted. Indeed, the theoretical values that were calculated in

acetonitrile and cyclohexane, respectively, have predicted a

hypsochromic shift of the lower absorption band which is not

the spectroscopic shift observed experimentally. Moving to the

J. Org. Chem. XXXX, XXX, XXX−XXX

Article

Figure 9. Correlation of the theoretical and experimental energies of the lower absorption band of: (a) para-substituted phenol radical-cation and (b)

para-substituted phenoxyl radicals.

Table 6. Linear Correlation between Experimental and DFT

Calculations of the Lower Absorption Band Energies (ΔE) of

the Para-Substituted Phenol Radical-Cations and Para-

Substituted Phenol Radicals

methoxy, phenoxy, methyl, and tert-butyl groups stabilize the

radical-cations causing less spin density at the oxygen of the

phenol.

Conversely, electron-withdrawing groups (chloro, ketone,

cyano, and nitro) pull electrons away from the aromatic ring

destabilizing the corresponding radical-cations and leading to

higher spin density at the oxygen of the phenol. Furthermore,

the positive ρ values obtained in our experiments account for the

nucleophilic character of the para-substituted phenol radical-

cations during the deprotonation process. By contrast, the

ΔE₍_t_h_e_o₎= aΔE₍_e_x_p₎+ b

transient

R²

para-substituted phenol radical-cations

para-substituted phenyloxy radicals

1.20

1.17

−0.18

0.00

0.79

0.95

transient radical-cations with those of the absorption spectra of

the corresponding para-substituted phenoxyl radicals. The

spectra we have obtained agree well with those available in the

literature after making allowance for slight solvent-induced

substituent eﬀect on the k rate constants was also attempted,

but no linear relationship was observed because the rate

constants are almost diﬀusion-controlled rates, which render the

process insensitive to substituent eﬀects.

,15

shifts.

On the other hand, it was observed that the rate constants of

The eﬀect of the substitution on the absorption maxima of the

phenol radical-cations and the phenoxyl radicals but calculated

as energy values (ΔE) is interesting (Table 5). While a

bathochromic shift of the absorption maximum is observed for

all the substituents studied, a notable blue shift is observed for

the case of the phenyl group. Even though a trend in the energy

values moving to shorter values is noticed with increasing

deprotonation process (k ) of the para-substituted phenol

radical-cations correlate nicely with the thermodynamic acidity

(

pK ) leading to satisfactory Brønsted plots (Figure 5(b)). The

Brønsted coeﬃcient thus obtained (α = 0.10 in MeCN and

MeOH and α = 0.13 in cyclohexane and SDS 0.10 M) clearly

indicates a transition-state structure very close to that of the

reactants with a very small degree of O−H bond cleavage, as

Hammett σ values, no well-deﬁned relationship was found. The

expected for exergonic reactions. Interestingly, smaller

peculiar behavior of the phenyl group was observed in transients,

the radical-cation as well as the phenoxyl radical and can be

attributed to the nonplanarity of the phenyl group with respect

to the plane of the aromatic ring. Furthermore, this

spectroscopic behavior was observed in diﬀerent solvents

Brønsted coeﬃcients have been reported for the case of

deprotonation of polymethylbenzene radical-cations and α-

substituted p-methoxytoluene coming to the same conclusion

23b,c

regarding the corresponding transition-state.

Two possible bases could be involved during the deprotona-

(

cyclohexane, acetonitrile, and methanol) and also in SDS

tion process of the radical-cations, viz. H O or the para-

micellar solution.

substituted phenols, depending on the reaction solvent used

during the irradiations. Thus, we suggest that residual water is

the base present during the experiments carried out in MeCN

and MeOH while irradiations carried out in cyclohexane and

SDS micellar solution the bases are the respective para-

substituted phenols. In the former case, the substituted phenols

cannot be ruled out as possible bases together with water.

However, we have concluded that the base does not play a

signiﬁcant role in the transition-state structure due to the similar

Brønsted coeﬃcients exhibited by such structurally diﬀerent

The Hammett linear correlation of deprotonation rate

constants (k ) of the para-substituted phenol radical-cations

leads to ρ = 0.61 (Figure 5a) when the data were obtained in

cyclohexane, acetonitrile and methanol, whereas in SDS micellar

solution the ρ value was 1.01 (Figure S5 in the Supporting

Information). Both values are higher in magnitude than that

obtained for some para-substituted phenols in n-butyl chloride

(

ρ = 0.31). This behavior can be attributed to the diﬀerent

polarity and proticity of the solvents used in our study, including

the hydrophobic core of the SDS micellar solution. The

deprotonation behavior of the phenol radical-cations is

explained by electronic eﬀects of the substituents. Indeed, the

positive ρ value implies that electron-donating groups such as

bases as H O and para-substituted phenols. As far as we know,

these are the ﬁrst examples of Brønsted plots for the

deprotonation of para-substituted phenol radical-cations.

J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry

Article

We have demonstrated that micellar solutions are eﬀective

micro reactors where photochemical reaction of phenols can

occur. Indeed, the irradiation with a laser pulse of 266 nm of

para-substituted phenols provided transients radical-cations and

phenoxyl radicals as it was observed in homogeneous media.

The photochemical reaction of phenols in SDS micellar solution

does involve the deprotonation pathway of the radical-cations

and the consecutive reactivity of the phenoxyl radicals as can be

judged from the data collected in Table 3 and Figure S1

obtained in all the solvents we have studied (cyclohexane,

acetonitrile, methanol, and SDS micellar solution).

CONCLUSIONS

■

The photochemical behavior of a series of para-substituted

phenols under laser ﬂash photolysis with a pulse of 266 nm has

been examined in this paper and two transients; viz. phenol

radical-cations and phenoxyl radicals are formed after the laser

pulse in homogeneous and micellar media. These intermediates

were characterized in terms of UV−vis absorption spectra,

lifetimes (τ and τ ) and rate constants (k and k ).

Supporting Information) in terms of lifetime, rate constants

(k and k ) and time-resolved UV−vis absorption spectra of the

respective transients. Therefore, these spectroscopic data in

micellar media were successfully measured because para-

substituted phenols were solubilized within the hydrophobic

core of the micelle, a conﬁned environment, where the phenols

were able to absorb light of 266 nm, photoionized and provide

the corresponding radical-cations and the phenoxyl radicals

within the micelle. Spectroscopic measurements were carried

out to conﬁrm the binding of phenols the micelle and their

location within the hydrophobic core. Indeed, UV−vis spec-

The substituent eﬀect was analyzed according to Hammett

correlations, and good linear regressions were obtained when

log(k ) was plotted versus the substituent parameters σ

whereas k rate constants do not show any Hammett correlation

because are almost diﬀusion-controlled rates which render the

process insensitive to substituent eﬀects. The substituent eﬀect

on the deprotontion rate constants k eﬀectively demonstrated

that electron-withdrawing substituents increase the acidity of

the phenoxyl radical-cations generating the corresponding

phenol radicals whereas electron-donor substituents operate

troscopy was used to measure the binding constant (K ) of para-

substituted phenols with SDS surfactant according to eq 4 and

the values thus obtained (Table 4) tell us that are typical values

^o^p^p^o^sⁱ^t^e^l^y^.^F^u^r^t^h^e^r^m^o^r^e^,^t^h^e^d^e^p^r^o^t^oⁿ^a^tⁱ^oⁿ^r^a^t^e^c^oⁿ^s^t^aⁿ^t^s^kH

6−29

was also found to correlate nicely with the pK values in the

for aromatic solutes as it was reported in the literature.

−

Brønsted plots of the para-substituted phenol radical-cations.

The Brønsted coeﬃcients α values thus obtained from the slopes

clearly indicated a transition-state structure very close to that of

the reactants with a very small degree of O−H bond cleavage.

This experimental evidence on the early transition-state

structure is reported for the ﬁrst time demonstrating that the

acidity of the phenoxyl radical-cations are not so stronger as

would be expected.

The photochemical reaction of the para-substituted phenols

proceeded eﬃciently also in micellar solution, and the phenol

radical-cations, as well as the phenoxyl radicals, were formed

within the hydrophobic core of the micelle after the laser pulse of

Furthermore, estimation of K values ≤100 M in anionic,

cationic and neutral surfactants have been previously reported

pointing out that the binding of the phenols with the surfactant,

in our case surfactant SDS, proceeded eﬃciently. Likewise, cross

peaks of the 2D NMR spectra (Figure 8) clearly show that a nice

correlation exists between the aromatic protons (H-2/H-6 and

H-3/H-5) of p-cyanophenol with all the characteristic protons

of the surfactant (α, β, bulk and ω protons) meaning that the

aromatic protons are in close contact with any portion of the

aliphatic chains of the surfactant. Therefore, we suggest that the

disorder in the arrangement of the aliphatic chains within the

hydrophobic core of the micelle is responsible for the NOE

66 nm and were spectroscopically characterized. We have

(

Nuclear Overhauser eﬀect) between the aromatic protons and

demonstrated that the phenols bind to the micelle measuring the

all the surfactant protons. The contour of the 2D NMR spectra

led to recognize that after eﬃcient binding of the phenols to the

micelle, the phenols locate qualitatively within the micelle core.

The location of the phenols cannot be established with accuracy

because of the limitation of the 2D NMR methodology.

corresponding binding constants (K ) by using UV−vis

spectroscopy. Location of the para-substituted phenols within

the shell or the hydrophobic core of the micelle was achieved

employing 2D NOESY NMR spectroscopy. Therefore, these

results have demonstrated that the photochemical reaction takes

place eﬃciently within the micelle, which behaves like a

photochemical microreactor. On the other hand, the substituent

Finally, we were able to predict the UV−vis absorption

spectra of para-substituted phenol radical-cations and radicals,

respectively, by using TD-DFT calculations (Figures S8 and S9

in the Supporting Information), and then the predicted

maximum absorption wavelength values reported as energy

values (ΔE₍_t_h_e_o₎) have been correlated with those experimental

eﬀect on the deprotontion rate constants k measured in a

conﬁned hydrophobic enviorment showed a similar substituent

eﬀect as it was observed in homogeneous media demonstrating

once again that electron-withdrawing substituents increases the

acidity of the phenoxyl radical-cations also in micellar media.

Furthermore, Brønsted plots obtained for the phenol radical-

cations in SDS solution also evidenced the proposal of an early

transition-state structure because the Brønsted coeﬃcient α

value measured in micellar solution is similar to that obtained in

cyclohexane.

Finally, TD-DFT calculations led to predict the energy

(ΔE₍_t_h_e_o₎) values associated with the maximum absorption

wavelengths of the transients and were correlated with the

energy values (ΔE₍_e_x_p₎) experimentally obtained. Fairly good

linear regressions have been obtained with slopes close to unity,

ΔE

values obtained from the transient absorption spectra

recorded after the laser pulse (266 nm). Fairly good linear

correlations have been obtained with slopes closer to unity

(

exp)

reﬂecting that the prediction of the ΔE

using the adopted

(

theo)

theoretical approach leads to a good estimation of the energies

values. However, p-nitro- and p-phenylphenols deviate from the

linear regression, and we suggested that this behavior can be

attributed to the fact that both substituents are clearly twisted

respect to the plane of the aromatic ring when the analyses were

carried out on the transients radical-cation and the correspond-

ing phenoxyl radical. Furthermore, the twisting of the

substituents entails to a hypsochromic shift of the maximum

absorption wavelengths, canceling the bathochromic shift

expected experimentally. Noteworthy, a reasonably good linear

reﬂecting that the prediction of the ΔE

using the adopted

theoretical approach leads to a reasonable estimation of the

energies values.

(

theo)

correlations was observed for the experimental values (ΔE₍_e_x_p₎

)

J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry

Supporting Information ).

Article

Our study provides evidence for the eﬃcient formation of

para-substituted phenol radical-cations after the laser pulse that

collapses into the corresponding phenoxyl radicals. The

deprotonation rate constants of the radical-cations depend on

ε(λ) = ∑ ε(λ)

i=1

2Ñ|

n o

Å i

y Ño

f_i

Å j

z Ño

i z

Å j

z Ñ

∑ m1.3062974 × 10

expÅ−

Ñ}

the nature of the substituents and correlate well with the pK . A

Å j

z Ño

Å j

z Ñ

z o

Ño

i=1 o

Ño

systematic study on the photochemical behavior of the para-

substituted phenols was carried out for the ﬁrst time and we

hope that the spectroscopic data presented in this paper will be

useful for the readership in the understanding the photo-

chemistry of phenols.

3099.6

Å k

ÇÅ

3099.6

{ Ño

ÖÑ ~

(

1a)

with f_ithe computed oscillator strengths, referred to a speciﬁc

wavelength λ . For the sake of simplicity, only the information regarding

the ﬁ rst ﬁ ve transitions of the species analyzed are reported (see the

EXPERIMENTAL SECTION

■

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.joc.0c02031.

■

Materials and Equipment. Para-substituted phenols, potassium

persulfate, and sodium dodecyl sulfonate were obtained from

commercial sources. Spectroscopic grade solvents were used as

received. 2D NOESY spectra were recorded in D O on a 500 MHz

spectrometer, using a NOESY-ph pulse sequence with a 600 ms mixing

time and a recovery delay of 1.5 s. 2K data points were collected for 512

increments of 16 scans, using TPPI f1quadrature detection; chemical

shifts (δ) are reported in part per million (ppm), relative to the signal of

trimethylsilylpropionic acid, used as internal standard. The measure-

ments were carried out using standard pulse sequences. Structural

assignments were made with additional information from COSY,

HSQC, and HMBC experiments. The UV−vis spectra were measured

with a Shimadzu UV-1203 spectrophotometer using two-faced

stoppered quartz cuvettes (1 mm × 1 mm) at 298 K.

UV−vis absorption spectra under time-resolved spectros-

copy; transient decay traces at maximum absorption

wavelength (λ_m_a_x); determination of the rate constants k_H

and k_Rin homogeneous and heterogeneous media;

Bronsted plot; Hammett linear correlation plot in

heterogeneous media; determination of the constants of

binding K ; 2D NOESY NMR spectra in micellar media;

TD-DFT calculated UV−vis absorption spectra of para-

substituted phenol transients (PDF )

Determination of the Binding Constants (K ) of Para-

substituted Phenols in Micellar Media. Solutions of para-

AUTHOR INFORMATION

Corresponding Author

substituted phenols were prepared in deionized water (Milli-Q), and

■

their concentrations varied between 5.5 × 10⁻M and 1.0 × 10 M. An

aliquot (2 mL) of the para-substituted phenol solution was placed in a

ﬂuorescence-stoppered quartz cuvette provided with a stirring bar, and

the UV−vis spectrum was recorded. The initial absorbance value at the

−4

Sergio M. Bonesi − Universidad de Buenos Aires. Facultad de

Ciencias Exactas y Naturales, Departamento de Quımica

Organica, C1428EGA Buenos Aires, Argentina; CONICET −

Universidad de Buenos Aires. Centro de Investigaciones en

Hidratos de Carbono (CIHIDECAR), C1428EGA Buenos Aires,

Argentina; Dipartimento di Chimica, Sezione Chimica Organica,

University of Pavia, 27100 Pavia, Italy; orcid.org/0000-

maximum absorption wavelength (A ) was read. Subsequently, aliquots

(

10 μL) of concentrated surfactant solution (0.10 M) were added. The

UV−vis spectra were registered, recording for each solution the A value

at the maximum absorption wavelength. After each addition of

surfactant, the solution was stirred for 20 min before measuring the

absorbance. With the values of A and A in hand, the values of (A /(A −

003-0722-339X ; Phone: +54-11-45763346;

Email: smbonesi@qo.fcen.uba.ar

A )) versus the reciprocal of the concentration of the micellar surfactant

were plotted, and the data were ﬁtted with a linear regression program.

Authors

Gaston Siano − Universidad de Buenos Aires. Facultad de

Ciencias Exactas y Naturales, Departamento de Quımica

Organ

The K values were obtained by calculating the ratio of the slope and the

origin.

Laser Flash Photolysis. The laser pulse photolysis apparatus

consisted of a Flash lamp-pumped Q-switched SpitLight-100 Nd:YAG

laser from InnoLas used at the fourth harmonic of its fundamental

wavelength. The LP920-K monitor system (supplied by Edinburgh

Instruments), arranged in a cross-beam conﬁguration, consisted of a

high-intensity 450 W ozone free Xe arc lamp (operating in pulsed

wave), a Czerny−-Turner with triple grating turret monochromator,

and a ﬁve-stage dynode photomultiplier. The signals were captured by

means of a Tektronix TDS 3012C digital phosphor oscilloscope, and

the data were processed with the L900 software supplied by Edinburgh

Instruments. The solutions to be analyzed were placed in a ﬂuorescence

cuvette (d = 10 mm).

ica, C1428EGA Buenos Aires, Argentina; CONICET −

Universidad de Buenos Aires. Centro de Investigaciones en

Hidratos de Carbono (CIHIDECAR), C1428EGA Buenos Aires,

Argentina

Stefano Crespi − Stratingh Institute for Chemistry, University of

Groningen, 9747AG Groningen, The Netherlands; orcid.org/

000-0002-0279-4903

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.joc.0c02031

TD-DFT Calculations. The TD-DFT simulation was carried out

Notes

using Gaussian 16, Revision B.01 software package. The geometry of

the molecules was optimized and the themochemistry was obtained at

the (U)ωB97XD/def2-SVP level of theory. The UV/vis spectra were

simulated at the SMD-(U)ωB97XD/def2-TZVP level (solvent:

vacuum, cyclohexane, and acetonitrile), computing the lowest 25

singlet transitions.

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

Sergio M. Bonesi is a research member of CONICET.

■

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The UV/vis band was plotted as ε vs λ (excitation wavelength in nm)

with the peaks, furnished by the calculation, assuming a Gaussian band

shape (characterized by a standard deviation σ = 0.4 eV). The equation

for the simulated spectra follows the one described in the Gaussian

■

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