Kinetics of C-C and C-H bond cleavage in phenyl alkane radical cations generated by photoinduced electron transfer

Article abstract of DOI:10.1021/jp508556z

Employing nanosecond laser flash photolysis, we determined the relative importance of two fragmentation modes, namely, C-C bond cleavage and deprotonation, for the radical cation of 1,1,2,2-tetraphenylethane photogenerated by electron transfer to cyanoaromatic singlet excited states in acetonitrile at room temperature. Analysis of the kinetic data for this phenyl alkane suggests that the C-C bond cleavage dominates over the deprotonation by a ratio of about 2:1. In addition, the deprotonation kinetics of diphenylmethane, 1,1-diphenylethane, triphenylmethane, and several phenyl-substituted alcohols have been investigated. To aid identification and characterization, experiments based on two laser pulses in tandem (308 and 337.1 nm) were performed to probe fluorescence and photochemistry of the transient radicals formed as products of radical ion fragmentation. The first-order rate constants for growth of transient absorptions due to fragmentation-derived radicals were measured to be ≥1 × 10⁶ s^-1. Activation parameters, with activation enthalpies in the range 10-18 kJ/mol and activation entropies between -60 and -91 J/(mol.K), are also reported for fragmentation kinetics of radical cations of several systems under study. (Graph Presented).

Full text of DOI:10.1021/jp508556z

Article
pubs.acs.org/JPCA
Kinetics of C−C and C−H Bond Cleavage in Phenyl Alkane Radical
Cations Generated by Photoinduced Electron Transfer
†
Douglas Cyr and Paritosh Das*
Physical Sciences Department, Cameron University, Lawton, Oklahoma 73505, United States
*
S Supporting Information
ABSTRACT: Employing nanosecond laser flash photolysis, we determined the relative
importance of two fragmentation modes, namely, C−C bond cleavage and
deprotonation, for the radical cation of 1,1,2,2-tetraphenylethane photogenerated by
electron transfer to cyanoaromatic singlet excited states in acetonitrile at room
temperature. Analysis of the kinetic data for this phenyl alkane suggests that the C−C
bond cleavage dominates over the deprotonation by a ratio of about 2:1. In addition,
the deprotonation kinetics of diphenylmethane, 1,1-diphenylethane, triphenylmethane,
and several phenyl-substituted alcohols have been investigated. To aid identification and
characterization, experiments based on two laser pulses in tandem (308 and 337.1 nm)
were performed to probe fluorescence and photochemistry of the transient radicals
formed as products of radical ion fragmentation. The first-order rate constants for
growth of transient absorptions due to fragmentation-derived radicals were measured to
6
−1
be ≥1 × 10 s . Activation parameters, with activation enthalpies in the range 10−18
kJ/mol and activation entropies between −60 and −91 J/(mol.K), are also reported for fragmentation kinetics of radical cations
of several systems under study.
1
. INTRODUCTION
alcohols to the lowest singlet excited state of two cyanoar-
omatics, namely, DCN and TCB. In particular, this undertaking
was desirable to resolve an issue regarding the fate of the radical
cation of 1,1,2,2,-tetraphenylethane (TPE). In ref 24, the radical
Chemistry via photoinduced electron transfer (PET) can be
very rich owing to the involvement of a variety of transient
intermediates, namely, singlet and triplet excited states, radical
1
−18
cation of TPE was claimed to undergo C−C bond cleavage to
ion pairs, exciplexes, solvated radical ions, and radicals.
In
+
diphenylmethyl carbocation (Ph CH ) and diphenylmethyl
addition, secondary transient products can be formed and
implicated as a result of bimolecular interaction (quenching) of
these intermediates with various species, including oxygen. In
2
•
radical (Ph
CH ), with the latter contributing to the growth of
2
an intense transient absorption observed at 320−335 nm.
Reference 34 contains a comment by its authors that the
observation of the growth of the short-wavelength transient
absorption could possibly be interpreted in terms of
6
,15−29
the recent past, considerable interest has been shown
in
using time-resolved techniques such as laser flash photolysis
and pulse radiolysis to elucidate the mechanisms of chemical
transformations arising from photoassisted electron transfer.
deprotonation of TPE radical cation to diphenyl-
6
,11,15−18
•
Much of the focus
in recent fast-kinetic studies of
(
diphenylmethyl)methyl radical (Ph C −CHPh ), which
2
2
photoinitiated electron transfer has been on the dynamics of
contact and solvent-separated radical ion pairs in picosecond
and shorter time domains. However, to obtain a full picture of
the chemistry arising from PET, there is still a need for studying
processes involving solvated radical ions in the nanosecond and
longer time domains.
The work presented in this report was performed in a
continuation of our interest in time-resolved kinetic inves-
tigations of photochemical reactions initiated by electron
•
would be spectrally indistinguishable from Ph CH . Apparently,
2
this comment followed from the fact that the C−C bond
34
cleavage of TPE was not observed in the course of its 1,4-
dicyanobenzene-photosensitized steady-state photolysis in
acetonitrile−methanol mixture at ambient temperatures.
The donor substrates under study are as follows: (1) 1,1,2,2-
tetraphenylethane (TPE), Ph CH−CHPh ; (2) diphenyl-
2
2
methane (DPME), Ph CH ; (3) 1,1-diphenylethane (1,1-
2
2
1
9−29
DPE), Ph CH−CH ; (4) triphenylmethane (TPME), Ph CH;
2
3
3
transfer (ET).
In our work as well as that by several
(
5) benzyl alcohol (BZA), PhCH OH; (6) diphenylmethanol
2
other groups, cyanoaromatic systems, such as 1,4-dicyanonaph-
thalene (DCN), 1,2,4,5-tetracyanobenzene (TCB), and 9,10-
dicyanoanthracene, have been widely used as excited-state
(
DPML), Ph CHOH; (7) triphenylmethanol (TPML),
2
Ph COH.
3
15−29,34−37
electron acceptors.
In the work reported herein,
using nanosecond laser flash photolysis, we studied the
fragmentation behavior of radical cations produced as a result
of electron transfer from several phenyl-substituted alkanes and
Received: August 24, 2014
Revised: October 27, 2014
Published: October 27, 2014
©
2014 American Chemical Society
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The Journal of Physical Chemistry A
. EXPERIMENTAL SECTION
.1. Materials. 1,4-Dicyanonaphthalene (DCN) was
Article
2
the nitrogen gas, the temperature, monitored with a
thermocouple probe, could be controlled within ±1 °C.
2
prepared by heating 1,4-dibromonaphthalene (Eastman) with
3
. RESULTS
.1. Transient Absorption Spectra and Kinetics. In
cuprous cyanide and pyridine according to a modification of the
3
7,38
procedure
described for conversion of 1-bromonaphthalene
3
to 1-cyanonaphthalene. The crude product was purified by
column chromatography on neutral alumina followed by
recrystallization from toluene. 1,2,4,5-Tetracyanobenzene
acetonitrile, the lowest energy band systems of the ground-state
absorption spectra of the two cyanoaromatics we studied,
namely, 1,4-dicyanonaphthalene (DCN) and 1,2,4,5-tetracya-
nobenzene (TCB), occupy the spectral region 265−350 and
(
1
TCB) (Aldrich) was recrystallized twice from ethanol.
,1,2,2-Tetraphenylethane (TPE), purchased from Columbia
2
75−325 nm, respectively. Thus, 308 nm laser pulses were
Organic Chemicals, was recrystallized from methanol. Diphe-
nylmethane (DPME), 1,1-diphenylethane (1,1-DPE), and
benzyl alcohol (BZA), all from Aldrich, were used as received.
Diphenylmethanol (DPML), from Aldrich, was sublimed twice
before use. Triphenylmethanol (TPML), also from Aldrich, was
recrystallized from ethanol. Di-tert-butyl peroxide (DTBP)
from MC&B was purified by passing through an alumina
column. Acetonitrile (Aldrich, Gold Label) was distilled over
P O under an atmosphere of nitrogen. Benzene and methanol
were of spectral grade. Pyridine (Aldrich) was distilled under
reduced pressure. Lithium perchlorate, tetra-n-butyl ammonium
perchlorate, and tetraethylammonium perchlorate, all from
Aldrich, were used as received.
suitable for photoexcitation of these two cyanoaromatics at less
than millimolar concentrations in the presence of the donor
substrates under study at 10−60 mM without interference from
direct excitation of the latter. In addition, we performed several
dual-laser-pulse experiments with the purpose of identifying the
radical cation-derived transient products (absorbing strongly at
3
20−340 nm) based on their fluorescence and photochemical
behavior. In these experiments, in tandem with the primary 308
nm “synthesis” laser pulses, we used 337.1 nm “photolysis” laser
pulses delayed from the former by 5−10 μs.
2
5
24
In a previous study, it has been shown that fluorescence of
DCN in acetonitrile is quenched by TPE and related aryl alkyl
substrates (e.g., aryl pinacols and pinacol ethers) with
2.2. Nanosecond Laser Flash Photolysis. For nano-
s
bimolecular rate constants (k ) in the limit of diffusion control.
q
second laser flash photolysis experiments (performed at the
Radiation Laboratory, University of Notre Dame), appropri-
ately attenuated outputs from the following two laser systems
were used: UV-400 Molectron nitrogen (337.1 nm, 2−3 mJ, ∼8
ns) and Lambda-Physik EMG 101 excimer (Xe−Cl, 308 nm,
s
Specifically, the Stern−Volmer quenching constant (K ) is 94
SV
−1
42
9
−1 −1
s
M
for TPE; this gives a value of 9.9 × 10 M
s
for k .
q
On the basis of these data, TPE at concentrations of 20−30
1
mM would quench the DCN singlet ( DCN*) to the extent of
1
6
5−74%. The extents of quenching of DCN* by other donors
≤
20 mJ, ∼20 ns) . A description of the kinetic spectropho-
at the concentrations used in this work are estimated to be
similar (i.e., above 50%). As for TCB, its lowest singlet excited
state reduction potential is more favorable for electron
tometer and the data collection system is available in previous
39−41
publications from the Radiation Laboratory.
The time
domains for observation of transient species were typically in
the range from 100 ns to 100 μs. Usually a front-face
configuration was employed at an angle of ∼20° between the
directions of laser pulses and the monitoring light. To minimize
interference from fluorescence, in some experiments use was
made of 1 cm × 1 or 1 cm × 0.3 cm quartz photolysis cells in
which the laser excitation pulses and the monitoring light met
at a right angle. Time-resolved fluorescence or transient
absorption spectra were measured wavelength by wavelength
using a flow system in which a rectangular quartz photolysis cell
4
3
1
transfer than that of DCN*. In support of this, we measured
s
−1
the Stern−Volmer quenching constant (K ) to be 262 M for
SV
TPME as a quencher for steady-state TCB fluorescence in
acetonitrile (compared to 44 M− with DCN*). In addition,
1
1
48
with TCB, formation of ground-state electron donor−acceptor
(
EDA) complexes with the donor substrates under study is
expected to be facile. Photoexcitation of such complexes would
also lead to radical ions concomitantly with bimolecular
quenching of the uncomplexed excited state by the donors.
In the following subsections, we present results on transient
absorption phenomena on the nanosecond to microsecond
time scales following 308 nm laser pulse excitation of DCN and
TCB in the presence of 10−50 mM concentrations of various
phenyl-substituted alkanes and alcohols in acetonitrile at 24 °C.
On the basis of evidence from supporting experiments, we also
offer plausible interpretations of these phenomena in relation to
steps 1−7 (below) that show the various relevant processes in
the nanosecond to microsecond time domain following
photoexcitation of the cyanoaromatics by nanosecond laser
pulses. In eqs 1−7, TPE and DCN have been used as
representative donor and excited-state acceptor, respectively.
(path length ≈ 3 mm) was continuously fed with the solution
under experiment from a reservoir. Kinetics measurements at
specific monitoring wavelengths made use of static cells (quartz,
path length ≈ 2 or 10 mm) containing 2−4 mL of the solutions
that were shaken between laser shots. Except when the effect of
oxygen was meant to be studied, the solutions were
deoxygenated by bubbling high-purity argon or nitrogen.
Unless otherwise noted, all laser flash photolysis experiments
were performed in fluid solutions at room temperature (24 °C).
The ground-state absorbances of the solutions at laser
wavelengths were typically 0.1−0.3. The ground-state absorp-
tion spectra were recorded in a Cary 219 spectrophotometer
with a 1 nm band pass. The steady-state fluorescence
quenching experiments were carried out in a PerkinElmer LS-
1
1
−•
+•
DCN* + TPE → (DCN /TPE )
Formation of radical ion pair)
5
spectrofluorimeter with a perpendicular configuration for
(
(1)
excitation and emission.
For temperature-dependence studies, the photolysis cell was
surrounded by a Dewar-type quartz jacket fitted with flat
windows. Preheated or precooled nitrogen gas was allowed to
flow into the jacket around the cell. By regulating the flow of
1(
DCN /TPE ) → DCN + TPE
Back electron transfer)
−•
+•
(
(2)
1
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The Journal of Physical Chemistry A
Article
1(
DCN /TPE ) → DCN* + TPE
ET‐assisted intersystem crossing)
−•
+•
3
(
(3)
(4)
1(
−•
+•
2
−•
2
+•
DCN /TPE ) → DCN + TPE
(
Ion separation)
2TPE+• → Ph CH + Ph CH
2
•
+
(CC bond cleavage)
2
2
(5)
2TPE+• → Ph C CHPh + H
2
•
+
(Deprotonation)
2
2
(6)
2TPE+• + DCN → DCN + TPE
Diffusional back electron transfer)
.1.1. 1,1,2,2-Tetraphenylethane. The transient absorption
2
−•
(
(7)
3
spectra observed at 0.25 and 1.3 μs following 308 nm laser
pulse excitation of DCN in the presence of 20−30 mM TPE in
deoxygenated acetonitrile are shown in Figure 1. These spectra
Figure 2. Representative kinetic traces of transient absorption at (A)
3
25, (B) 385, and (C) 440 nm following 308 nm laser pulse excitation
of DCN in the presence of 20 mM TPE in acetonitrile.
from the emission could be substantially reduced to reveal the
fast, in-pulse formation of the 385 nm species.
The transient species observed with a sharp peak at 385−390
nm (Figure 1) is assigned to the solvated radical anion of DCN
formed as a result of the fast dissociation of the ion pair,
Figure 1. Transient absorption spectra observed at 0.25 and 1.3 μs
following 308 nm laser pulse excitation of DCN in the presence of 20
mM TPE in acetonitrile. (Inset) Kinetic trace of decay of fluorescence
resulting from subsequent 337.1 nm laser pulse excitation of the
transient absorbance produced by 308 nm laser pulse excitation of
DCN in the presence of 25 mM TPE in acetonitrile.
−
•
+•
DCN /TPE in the subnanosecond time domain (step 4).
This assignment is based on the similarity of the absorption
spectral features, decay kinetic behavior, and oxygen sensitivity
2
3,24
with those reported in the literature
for the radical anion.
The growth of the intense transient absorption with a
maximum at 325−330 nm (Figure 2A), similar to that observed
24
have the following principal features: (a) a sharp, intense band
system with absorption maximum at 325−330 nm charac-
terized by a first-order growth that is completed over ∼3 μs, (b)
a sharp but relatively less intense peak at 385−390 nm due to a
transient species that is formed within the laser pulse and
decays slowly on a >microsecond time scale, (c) rapid growth
followed by decay over ∼2 μs of a transient species with
absorption maximum at 440 nm, and (d) minor, slowly
decaying transient absorptions with maxima at ∼460 nm
formed within the laser pulse. Three kinetic traces showing the
decay/formation characteristics of the 325, 385, and 440 nm
species are presented in Figure 2. It should be noted that the
apparent initial growth observed for the 385 nm species (Figure
in an earlier study, is attributable, at least in part, to the
•
diphenylmethyl radical (Ph CH ) produced as a result of C−C
2
bond cleavage in the TPE radical cation (step 5). The observed
transient absorption spectrum, decay kinetics on a longer time
scale, and oxygen sensitivity of the decay agree well with such
51−56
behaviors well known for this extensively studied radical.
The ratio of the maximum absorbances at 330 and at 390 nm is
found to be 2.1 ± 0.2, which agrees very well with the ratio of
•
−•
molar absorption coefficients of Ph CH and DCN (namely,
23,51,52
2
4
4
−1
−1
4.2 × 10 and 2.2 × 10 M cm , respectively).
Additional evidence in support of the occurrence of step 5 is
34
presented later. As pointed out by Okamoto and Arnold, the
radical formed through deprotonation of TPE radical cation
•
2
B) is due to a contribution from the tail of the unquenched
(i.e., step 6) should be spectrally similar to Ph CH . The
2
+
•
57−63
portion of DCN fluorescence; this was verified by laser flash
experiments with the same solutions in the absence of the
monitoring light (that is, with the monitoring light blocked
from entering into the photolysis cell.) Also, experiments in
square cells (1 cm × 1 cm) in a right-angle geometry between
laser pulses and monitoring light showed that the interference
hydrocarbon radical cations (CH ) are recognized
to be
superacids with pK values in the range from 0 to −30.
a
Specifically, the pK ’s of the phenyl-substituted methanes,
a
57
Ph CH and PhCH , have been estimated to be −25 and
2
2
3
−20, respectively (in dimethyl sulfoxide). The pK of toluene
a
radical cation in acetonitrile has also been reported to be very
1
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Article
6
1
maximum (2.2 × 10 and 4.4 × 10 M−1 cm−1
4
4
negative, namely, −12 by Green et al. and −13 by Nicholas
,
6
3
23,51,52
and Arnold. On the basis of a similarly negative pK of TPE
respectively)
and ΔA
and ΔA
are the
a
max,DCN−•.
max,Ph2CH+
radical cation, one would expect it to undergo facile
corresponding maximum absorbances. Using the absorbance
data from our experiments with DCN and TPE, we estimated a
value of 0.7 ± 0.1 for ϕC−C. Thus, the deprotonation (i.e., step
6) accounts for about 30% of the decay mode of TPE radical
cation. Interestingly, we obtained a similar value for ϕC−C on
•
deprotonation to the radical, Ph C −CHPh , the transient
2
2
•
spectrum of which would overlap strongly with that of Ph CH .
2
Thus, using transient absorption spectra alone, one cannot
decide if the observed band system at 325−330 nm is due to
Ph CH entirely, partially, or not at all.
Convincing evidence in support of C−C bond cleavage in
TPE radical cation comes from the observation of the transient
absorption with a maximum at ∼440 nm (Figure 1),
characterized by its initial growth followed by decay (Figure
C). This transient absorption is assignable to the diphenyl-
methyl carbocation, Ph CH , formed from step 5. In
•
the basis of a comparison of absorbances at 325−330 and 440
2
•
nm if the molar absorption coefficient of the radical, Ph C −
2
CHPh , from deprotonation (step 6) was assumed to be the
2
•
4
same as that of Ph CH at 325−330 nm (namely, 4.4 × 10
2
−1
−1 51,52
M
cm ).
2
The minor absorption band system observed at ∼460 nm
+
2
(Figure 1) is due to DCN triplet produced in part from direct
intersystem crossing and in part through electron-transfer-
induced intersystem crossing (step 3). Its spectral location,
decay kinetics, and oxygen sensitivity match similar properties
acetonitrile at room temperature, this carbocation is
5
1,64−68
known
to exhibit an absorption maximum at 435 nm
and decay by first-order kinetics with a rate constant of 3.2 ×
0 s . The first-order decay of diphenylmethyl cations in
acetonitrile has been attributed to formation of a nitrilium ion
eq 8) and shown to be sensitive to the presence of
nucleophiles such as water. The reported
maxima of Ph CH in other solvents, namely, water,
6
−1
23,24
1
noted for the DCN triplet in earlier studies.
68
The transient phenomena observed with TCB as the excited-
state electron acceptor and TPE as the ground-state donor are
essentially similar to those described above for DCN. TCB is
(
5
1,52,64−68
absorption
+
71
2
known to form ground-state electron-donor−acceptor (EDA)
concentrated sulfuric acid, nitromethane, and 1,2-dichloro-
ethane, are in the same spectral region (440−453 nm).
complexes with aryl alkanes. Thus, as noted earlier, the radical
cation of TPE in this case is probably produced in part from the
excitation of the EDA complex at the laser excitation
wavelength (308 nm) used. Presented in Figure S1 (see
Supporting Information), the transient absorption spectra
consist of an intense absorption band system at 325−330 nm
due to radicals derived from TPE radical cation and a minor
band system around 440 nm due to diphenylmethyl
carbocation. The TCB radical anion exhibits its major
absorption maximum at 460 nm. Additionally, minor bands
are indicated for TCB− at shorter wavelengths, namely, 440,
+
+
Ph CH + CH C N → CH C N CHPh
An analysis of the formation/decay kinetics of the ∼440 nm
species observed in our experiments (Figure 2C) was
performed in terms of two consecutive first-order processes
using a formation rate constant of (3.7 ± 0.6) × 10 s (i.e.,
same as that of formation of the 330 nm species, i.e., radical(s)
produced in parallel from TPE radical ion fragmentation) and a
(8)
2
3
3
2
6
−1
•
6
−1
decay rate constant of 3.2 × 10 s . The computed maximum
3
60, and 345 nm. These spectral features agree with those
of the 440 nm transient absorption was found to be at 0.29 ±
15,50,72,73
−•
reported in the literature
for TCB . It should be
0
.05 μs following the laser pulse; this agreed reasonably well
noted that the ratio (3:1) of the observed maximum
absorbances at 330 and 460 nm agrees well with that of the
molar absorption coefficients of Ph CH and TCB at their
wavelength maxima (4.4 × 10 M cm at 330 nm
.35 × 10 M cm at 460 nm, respectively).
A small but nonnegligible difference was noted between the
transient absorption phenomena from TCB and DCN in the
presence of some of the donors studied. A minor component of
the transient absorbance growing over a long microsecond time
scale was observed with the former but not with the latter.
Though small (∼10% of the total absorbance), this slow
component was particularly conspicuous at the maximum (i.e.,
with the location of the maximum, 0.23 ± 0.05 μs, observed in
several of our experiments. The best fit of the kinetic trace in
Figure 2C into consecutive decay/formation kinetics led to an
+
−•
2
4
−1
−1
51,52
6
6
−1
and
estimate of (4.1 ± 0.5) × 10 and (3.6 ± 0.4) × 10 s for the
4
−1
−1
72
6
9
1
formation and decay rate constant, respectively. The
computed formation rate constant agrees, within experimental
errors, with that measured for formation of the radical(s) at 330
nm. Thus, both absorption spectral and kinetic behavior of the
observed 440 nm species strongly supports its assignment as
+
the carbocation, Ph CH .
2
An estimate of the extent to which the C−C bond cleavage
mode (step 5) contributes to the fragmentation of the TPE
radical cation could be made on the basis of a calculation of the
maximum plateau absorbance that the diphenylmethyl cation
Ph CH ) would have if it did not decay following its
−•
4
60 nm) corresponding to TCB . We chose not to investigate
74
it in any detail. However, because of the complication arising
from it in experiments with TCB, the kinetic data associated
donor radical cations were obtained from experiments with
DCN only.
We sought additional evidence in support of the occurrence
of C−C bond cleavage in TPE radical cation leading to
diphenylmethyl radical by carrying out experiments in which
the transient species with λmax at ∼330 nm was subjected to
photoexcitation with a second 337.1 nm laser pulse that was
delayed by a few microseconds with respect to the initial 308
nm laser pulse. Two representative traces of the relatively long-
lived fluorescence decay observed in experiments with DCN
and TCB as excited-state acceptors are shown in the insets of
+
(
2
6
formation. On the basis of the rate constants of 4.1 × 10
6
−1
and 3.5 × 10 s for formation and decay, respectively, this
extrapolated plateau absorbance was estimated to be 2.6 times
6
9,70
larger than that at the observed maximum at 440 nm.
factor was then used in eq 9 (below) to calculate the fraction
ϕc−c) of the decay of TPE radical cation precursor that
occurred via C−C bond cleavage.
This
(
ΔAmax,Ph CH+ εmax,DCN−•
ΔAmax,DCN−•
k5
2
ϕC−C
=
= 2.6
k5 + k
6
ε
max,Ph CH
+
(9)
2
In eq 9, εmax,DCN−• and ε
are molar absorption
Figures 1 and S1, Supporting Information. The doublet−
max,Ph2CH+
−
•
+
•
coefficients of DCN
and Ph CH at their absorption
doublet fluorescence of Ph CH , with lifetimes in the range of
2
2
1
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The Journal of Physical Chemistry A
Article
1
00−260 ns in fluid solutions at room temperature, has been
53−55,75,76
well characterized in the literature.
The fluorescence
spectrum of the 330 nm transient that we recorded in a point-
by-point manner under excitation by 337.1 nm laser pulses
agreed in both location (500−600 nm) and structural features
54
with that published by Bromberg et al. This combined with
the observed relatively long fluorescence lifetimes of 210−250
7
7
ns provided strong evidence in support of a substantial
•
contribution of Ph CH to the transient absorption at 325−330
2
nm. Unlike the remarkably long-lived doublet excited state of
Ph CH , its substituted analogs, e.g., Ph C R* (R = Ph, Me, or
cyclo-Pr), have been shown to possess short lifetimes at room
•
•
2
2
53−55,76
temperature in fluid solutions.
The short lifetimes are
attributed to a large contribution of intramolecular photo-
chemistry, namely, electrocyclic ring closure, facilitated by an
increased twist angle of the phenyl rings owing to substituents
53−55,76
•
Figure 3. Transient absorption spectra observed upon 308 nm laser
pulse excitation of (A) 10 mM DPME solution in benzene:DTBP
mixture (4:1, v/v), (B) 10 mM TPE solution in benzene:DTBP
mixture (4:1, v/v), and (C) 10 mM TPE in acetone:acetonitrile
mixture (1:9, v/v). Times at which the spectra were recorded were 1,
1.2, and 1.2 μs, respectively, following the laser pulse. (Insets) (a)
Kinetic trace of fluorescence at 560 nm resulting from 337.1 nm laser
pulse excitation of the transient absorbance produced by 308 nm laser
pulse excitation 10 mM DPME solution in benzene:DTBP mixture;
bulkier than H.
Accordingly, the radical (Ph C −
2
CHPh ) derived from deprotonation of TPE radical cation
2
could not be responsible for our observed long-lived
fluorescence. We present later the results of our two-laser
experiments in the absorption mode showing significant
•
depletion of the absorbance of the two radicals, Ph C R* (R
2
=
Me and Ph), produced via deprotonation of 1,1-DPE and
TPME radical cations, respectively, as well as concomitant
(b) kinetic trace of emission at 560 nm resulting from 337.1 nm laser
increase in absorbance at 450−500 nm due to photoproduct
pulse excitation of the transient absorbance produced by 308 nm laser
pulse excitation 10 mM TPE solution in benzene:DTBP mixture.
formation in both cases.
•
To examine the extent to which Ph CH is spectrally distinct
2
•
from Ph C −CHPh , we performed experiments in which the
2
2
radicals in question were generated via hydrogen abstraction by
two radicals was not sufficiently pronounced to be definitive in
distinguishing between them. Notably, however, the transient
spectra observed upon laser flash photolysis of DCN or TCB in
the presence of TPE in acetonitrile displayed the short-
78
tert-butoxy radical and acetone triplet. As in a previous study,
the tert-butoxy radical was conveniently produced by laser flash
photolysis of a mixture of benzene and di-tert-butyl peroxide
(
DTBP) (4:1, v/v). Similarly, the 308 nm laser flash photolysis
wavelength maximum at a wavelength closer to the λ
of
max
of a 1:9 mixture (v/v) of acetone and acetonitrile produced
•
Ph CH , indicating qualitatively that C−C bond cleavage in
2
acetone triplet that abstracted hydrogen from TPE.
TPE radical cation was more important than its deprotonation.
Employing two-laser excitations (308 and 337.1 nm), we
probed the emission characteristics of the transient radicals
•
t‐BuOOBu‐t + hv → 2t‐BuO
(10)
(11)
•
•
t‐BuO → CH COCH + CH
(first order)
3
3
3
produced from DPME and TPE by H abstraction by tert-butoxy
•
radical. As expected for Ph CH from DPME, a relatively long-
2
•
t‐BuO + Ph CHCHPh
lived, strong fluorescence with a lifetime of ∼300 ns (see inset a
in Figure 3) was observed at 500−600 nm. In contrast, in the
case of the radical from TPE, practically no emission was
detected (except for a very short-lived, weak emission of
spurious origin, see inset b in Figure 3). The lack of
2
2
•
→
t‐BuOH + Ph C CHPh
(12)
(13)
2
2
1
3
(
CH ) CO + hv → (CH ) CO* → (CH ) CO*
3
2
3
3
2
2
•
fluorescence in the case of the radical, Ph C −CHPh , from
3(
2
2
CH ) CO* + Ph CHCHPh
3
2
2
2
TPE is in agreement with similar behaviors of doublet excited
states of substituted radicals, Ph C R (R = Ph, Me, or cyclo-Pr),
•
•
•
2
→
(CH ) C OH + Ph C CHPh
(14)
3
2
2
2
discussed earlier in this subsection.
The transient absorption spectra observed at ∼1 μs following
laser flash photolysis (308 nm) of benzene/DTBP mixture and
acetone in the presence of TPE and of benzene−DTBP mixture
in the presence of diphenylmethane (DPME) are presented in
Figure 3. The radicals produced as a result of H abstraction
from both TPE and DPME are spectrally similar. These spectra
exhibit sharp, intense absorption maxima in the spectral region
In light of our conclusion that the deprotonation to the
•
substituted radical Ph C −CHPh constitutes about 30% of
2
2
TPE radical ion decay in acetonitrile, it was desirable to
conduct a few two-laser experiments (308 and 337.1 nm) in the
absorption mode on the radical products from TPE radical
cation. The purpose was to show the occurrence of
•
photobleaching expected for Ph C −CHPh . A few experi-
2
2
1
3
10−335 nm. Close inspection, however, reveals a small
ments performed with TCB* as the acceptor and TPE as the
donor in acetonitrile showed a small amount of 337.1 nm laser-
induced depletion of transient absorbance at 325 nm and a
concomitant increase of transient absorbance at 490 nm. At the
former wavelength, partial recovery of the transient absorbance
•
difference. While the radical from DPME (i.e., Ph CH ) shows
2
its absorption maximum at 330 nm in benzene−DTBP mixture,
•
λ
of the radical from TPE (i.e., Ph C −CHPh ) is slightly
max
2
2
blue shifted (i.e., at 315 nm in benzene−DTBP mixture and
25 nm in acetonitrile). Admittedly, in the light of the
3
was noticed on a nanosecond time scale that matched
•
experimental error (±5 nm) in our transient absorption
spectral measurements, the difference in the spectra of the
kinetically with the fluorescence decay of Ph CH *. Similarly,
2
at 490 nm, a portion of the increased absorbance from 337.1
1
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nm photoexcitation underwent decay on the time scale of that
at 450−500 nm (Figure 4). These were assigned to the DCN
triplet formed through direct as well as electron-transfer-
assisted intersystem crossing (step 3).
•
of Ph CH emission. These nanosecond absorption phenom-
2
•
ena, attributable to Ph CH *, were followed by small, longer
2
lived components of depletion and of increased absorbance at
the two wavelengths, respectively. Apparently, these longer
lived absorbance changes were due, in a large part, to the
3.1.3. 1,1-Diphenylethane. We undertook the study of 1,1-
diphenylethane (1,1-DPE) since its radical cation could
presumably undergo both deprotonation and C−C bond
cleavage. The transient absorption spectra observed upon 308
nm laser flash photolysis of DCN in the presence of 27 mM
1,1-DPE in acetonitrile are presented in Figure 5. The inset of
•
photochemistry of Ph C −CHPh . The magnitudes of these
2
2
•
changes, however, were small, understandably because Ph C −
CHPh was a minor component of the mixture of transients
that were photoexcited at 337.1 nm. In addition, Ph C −
CHPh with its absorption maximum blue shifted relative to
2
2
•
2
2
•
that of Ph CH probably absorbed less light at the 337.1 nm
2
laser excitation wavelength.
3.1.2. Diphenylmethane. The 308 nm laser flash photolysis
of DCN or TCB in the presence of 25−30 mM diphenyl-
methane (DPME) in acetonitrile results in formation of
transient species that are assignable to cyanoaromatic radical
•
anions and diphenylmethyl radical, Ph CH . The latter is
2
formed as a result of deprotonation of DPME radical cation (eq
15) on a microsecond time scale.
+
•
•
+
Ph CH2 → Ph CH + H
(15)
2
2
Figure 4 shows the transient spectra observed at 0.5−3 μs
•
following laser pulse excitation of DCN. The radical, Ph CH ,
2
Figure 5. Transient absorption spectra observed at 0.5 and 1.5 μs
following 308 nm laser pulse excitation of DCN in the presence of 27
mM 1,1-DPE in acetonitrile. (Inset) Kinetic trace for formation of
transient absorption at 325 nm following 308 nm laser pulse excitation
of DCN in the presence of 27 mM 1,1-DPE in acetonitrile.
this figure shows a kinetic trace of the first-order growth of the
transient absorption at 325 nm. The sharp, intense absorption
maximum (325 nm) in the short wavelength region is
assignable to either or both of the radicals that could be
formed from the radical cation of 1,1-DPE (eqs 16 and 17).
•
+
•
+
Ph CHCH3 → Ph C CH + H
(16)
(17)
2
2
3
•
+
•
+
Ph CHCH → Ph CH + CH
2
3
2
3
However, two-laser experiments (based on 308 and 337.1 nm
in tandem) showed that the observed radical product from 1,1-
DPE radical cation was nonemitting. Thus, it could not possibly
be the diphenylmethyl radical, ruling out C−C bond cleavage
Figure 4. Transient absorption spectra observed at 0.5 and 1.5 μs
following 308 nm laser pulse excitation of DCN in the presence of 30
mM DPME in acetonitrile. (Inset) Kinetic trace of growth of transient
absorption at 325 nm following 308 nm laser pulse excitation of DCN
in the presence of 30 mM DPME in acetonitrile.
(
eq 17) as a fragmentation mode. In addition, as detailed in
1
Figure 6 with TCB* as acceptor, the transient absorption
spectra recorded at ∼0.2 μs before and at ∼0.2 μs after the
337.1 nm “photolysis” laser pulse showed considerable
depletion of absorbance at 315−340 nm, accompanied by
enhancement of absorbance at 450−500 nm. Thus, 337.1 nm
photoexcitation of the 325 nm transient species resulted in
formation a photoproduct with an absorption band system at
430−500 nm. This is distinctly indicated by the difference
spectrum C in Figure 6. In the insets of Figure 6 are presented
kinetic traces that also clearly show the depletion of absorbance
at 330 nm and its enhancement at 460 nm. These results point
to the pronounced photochemistry of the radical product
formed from deprotonation (step 16) and support its
is characterized by its sharp, intense absorption maximum at
1
330 nm (Figure 4). The transient spectra observed with TCB*
as acceptor are presented in Figure S2, Supporting Information.
The inset in Figure 4 shows its first-order growth over ∼3 μs.
The radical anions of DCN and TCB, formed within the laser
pulse, display their major maxima at 390 and 460 nm,
respectively (Figures 4 and S2, Supporting Information).
Identification of these transients was confirmed on the basis
of spectral and kinetic similarity with those reported in the
1
5,22,23
literature.
Two-laser experiments based on 308 and 337.1
nm laser pulses in tandem led to the observation of the
•
relatively long-lived fluorescence (500−600 nm, τ ≈ 300 ns)
assignment as Ph C −CH (in line with the discussion
F
2
3
characteristic of the diphenylmethyl radical. A representative
kinetic trace of the emission is presented in the inset of Figure
S2, Supporting Information.
presented earlier for substituted diphenylmethyl radicals).
3.1.4. Triphenylmethane. As expected, the 308 nm laser
flash photolysis of DCN or TCB in the presence of
1
As in the case of TPE (discussed earlier), with DCN* as the
triphenylmethane (TPME) in acetonitrile resulted in facile
•
acceptor, additional minor transient absorptions were observed
formation of the radical, Ph C , via fast deprotonation of
3
1
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Figure 7. Transient absorption spectra observed at 1.2 and 1.75 μs
following 308 nm laser pulse excitation of TCB in the presence of 25
mM TPME in acetonitrile with a 337.1 nm laser pulse photoexciting
the transients at 1.4 μs after the 308 nm pulse. (C) Difference of
transient absorptions after and before 337.1 nm photolysis. (Insets)
Representative kinetic traces showing (a) bleaching at 320 nm and (b)
photoproduct formation at 490 nm as a result of 337.1 nm laser
photolysis of transient(s).
Figure 6. Transient absorption spectra observed at (A) 3.5 and (B) 4.0
μs following 308 nm laser pulse excitation of TCB in the presence of
25 mM 1,1-DPE in acetonitrile with a 337.1 nm laser pulse
photoexciting the transient at 3.7 μs after the 308 nm pulse. (C)
Difference of transient absorptions after and before 337.1 nm
photolysis. (Insets) Representative kinetic traces showing (a)
bleaching at 330 nm and (b) photoproduct formation at 460 nm as
a result of 337.1 nm laser photolysis of transient(s).
we performed laser flash photolysis experiments with three
1
phenyl-substituted alcohols as electron donors for TCB* and
1
TPME radical cation. As shown in Figure S3, Supporting
Information, the radical is characterized by a sharp absorption
maximum at 335 nm. The spectral and kinetic characteristics
observed by us agreed with those reported in the literature for
DCN* in acetonitrile. With benzyl alcohol (BZA), the electron
transfer to both of the two singlet excited-state cyanoaromatic
acceptors resulted in formation of phenyl hydroxymethyl
•
radical, PhC HOH. This occurred within the laser pulse,
5
2,79−81
this well-known radical.
In our flash photolysis experi-
suggesting that deprotonation of BZA radical cation to phenyl
hydroxymethyl radical was too fast to be observed over
nanoseconds. The transient absorption spectra are shown in
ments, the radical was formed within the laser pulse, suggesting
that the deprotonation took place on a time scale shorter than a
nanosecond. Experiments at subambient temperatures (e.g.,
1
Figure S4 (Supporting Information) with TCB* as acceptor.
−
50 °C) did not reveal any growth of the transient absorption
The phenyl hydroxymethyl radical is characterized by a strong
absorption band system at 260−300 nm with a sharp maximum
at 275 nm (Figure S4, Supporting Information). This spectrum
at ≥50 ns time domains.
•
We examined the photochemistry of Ph C by performing
3
two-laser experiments (308 and 337.1 nm in tandem) with
agrees with the published transient spectra of the ketyl radical,
1
•
TCB* as the electron acceptor from TPME. The results are
PhC (CH )OH, produced as a result of H abstraction by the
3
8
2,83
presented in Figure 7. The transient absorption spectra at ∼0.2
μs before the 337.1 nm laser pulse and at ∼0.2 μs after it
display, respectively, a pronounced decrease of absorbance at
acetophenone triplet.
The absorption band systems at
longer wavelengths in Figure S4, Supporting Information, are
primarily due to TCB− and in a small part due to PhC HOH.
•
•
3
20−350 nm and a substantial, concomitant increase in
The results of our experiments with diphenylmethanol
1
1
absorbance at 400−550 nm. The difference spectrum (C in
Figure 7) clearly reveals formation of a photoproduct with an
absorption spectrum at 400−550 nm. The kinetic traces in the
insets of Figure 7 demonstrate the loss and enhancement of
absorbance in the two spectral regions as a result of the 337.1
nm photolysis of Ph C . Dual-laser experiments on Ph C in
aqueous solution by Faria and Steenken showed that as a
(DPML) as a donor for TCB* and DCN* were similar to
those observed with benzyl alcohol. The transient absorption
spectra from 308 nm laser flash photolysis of TCB in the
presence of 50 mM DPML in acetonitrile are presented in
Figure S5 (Supporting Information). The observed transient
•
•
absorption spectra are composed of those of diphenyl
3
3
81
•
hydroxymethyl radical (Ph C OH) with band systems at
2
result of photolysis by 248 or 308 nm laser pulses the radical
320−340 and 500−580 nm and TCB radical anion with band
systems at 440−470 and 340−390 nm. For both, the observed
spectra are in agreement with those reported in the
+
underwent extensive photoionization to Ph C with an
3
efficiency of 75−85%. This work also revealed a small extent
of photochemical transformation, i.e., formation of the
cyclization product, 4a,4b-dihydro-9-phenylfluorenyl radical,
1
5,72,84−86
literature.
At room temperature, deprotonation of
DPML radical cation occurred within nanoseconds after the
laser pulse. At subambient temperatures (i.e., at and below −15
°C), however, we were able to observe the fast growth of the
absorbance of the radical with lifetimes ranging from 100 to
250 ns as the temperature was lowered from −15 to −45 °C.
In the case of triphenylmethanol (TPML), electron transfer
5
3,54
in agreement with the work of Bromberg et al.
The
features of the difference spectrum (Figure 7C) observed in our
experiment in acetonitrile demonstrate that photocyclization is
favored in this solvent.
3.1.5. Benzyl Alcohol (BZA), Diphenylmethanol (DPML),
1
1
and Triphenylmethanol (TPML). For the sake of comparison,
to TCB* or DCN* occurred, as evidenced by formation the
1
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cyanoaromatic radical anions. The transient spectra with
1
TCB* as acceptor are presented in Figure 8. In addition to
Figure 9. Eyring plots for the temperature dependence of C−C bond
cleavage and/or deprotonation kinetics of the radical cations of (I)
TPE, (II) DPME, (III) 1,1-DPE, and (IV) DPML.
different. A close look at the plots corresponding to TPE (I in
both Figures 9 and S6, Supporting Information) reveals a slight
curvature. However, our attempt to fit the data to an
appropriate double-exponential equation did not yield any
meaningful results.
Figure 8. Transient absorption spectra observed at 0.5 and 2.0 μs
following 308 nm laser pulse excitation of TCB in the presence of 25
mM TPML in acetonitrile.
3
.3. Quenching Studies. A limited number of experiments
the absorbances at 400−480 nm due to TCB radical cation,
prominent transient absorptions are evident at 330−450 nm.
The latter are best assigned to the radical cation of TPML
formed within the laser pulse.
were carried out to study the effects of several reagents, namely,
perchlorate salts, methanol and pyridine, which could
potentially act as quenchers for some of the steps involved in
radical cation formation and fragmentation processes. The
objective of this work was to corroborate the assignments of the
observed transient species while keeping in mind that the
effects of the quenchers might originate in multifarious
manners, e.g., interactions with photoexcited states of
cyanoaromatics, radical cations and their ion-pair precursors,
and products of radical ion fragmentation.
3
.2. Temperature Dependence of Radical Cation
Fragmentation Kinetics. The first-order rate constants for
growth of radicals as a result of the fragmentation of the radical
cations from TPE, DPME, and 1,1-DPE at room temperature
(
24 °C) are given in Table 1. As our kinetic data suggest,
fragmentation of TPE radical cation comprises C−C bond
cleavage and deprotonation in parallel. For DPME and 1,1-DPE
radical cations, deprotonation is the sole mode of fragmenta-
tion. Also included in Table 1 is the rate constant for
deprotonation of the radical cation of DPML at the highest
temperature (−17 °C) at which we could measure the rate
reliably. It should be noted that the rate constant data in Table
3.3.1. Perchlorate Salts. We studied the effects of three
perchlorate salts, namely, lithium perchlorate, triethylammo-
nium perchlorate (TEAP), and tri-n-butyl ammonium per-
chlorate (TBAP), at 0.10 M on DCN-photosensitized
formation and fragmentation of radical cations of TPE, 1,1-
DPME, and DPM in acetonitrile. The effects of the three salts,
presumably arising from the ion pairing of the radical cation
with perchlorate anion, were nearly identical. Under 308 nm
laser pulse excitation of DCN + 20 mM TPE in acetonitrile, the
presence of lithium perchlorate, TEAP, and TBAP, each at 0.10
M in the solution, caused a slight enhancement (4−9%) in the
transient absorption due to radical products at 325 nm. The
first-order rate constants for growth of the same transient
absorption also showed a small increase, namely, by 18−30%.
In the presence of the salts, the growth−decay phenomena of
1
were obtained from experiments that were carried out in
optically dilute solutions at relatively low 308 nm laser pulse
intensities for excitation. In experiments in which high
concentrations of the transient radicals were produced, the
rate constants obtained from best fits of the growth profiles to
first-order kinetics were found to be 5−30% higher.
We studied the temperature dependence of the fragmenta-
tion kinetics by measuring the rate constants at various
temperatures in the range from −45 to 75 °C. The Eyring and
Arrhenius plots are presented in Figures 9 and S6 (Supporting
Information), respectively. The activation parameters obtained
from these plots are compiled in Table 1. In light of the two
parallel modes contributing to fragmentation of the TPE radical
cation, one would expect a nonlinear Arrhenius (or Eyring) plot
if the activation parameters for the two modes are substantially
transient absorbance due to the diphenylmethyl carbocation
+
(Ph CH ) at 440 nm disappeared completely, suggesting that
2
the carbocation either was not formed in the presence of the
salts or was kinetically quenched by them. The carbocation is
known to be quenched by various inorganic anions (e.g.,
halides and nitrate) with diffusion-controlled rate constants in
Table 1. First-Order Rate Constants (at 24 °C) and Activation Parameters for C−C Bond Fragmentation of Radical Cations in
Acetonitrile
6
−1
a
‡
‡
radical cation substrate
,1,2,2,-tetraphenylethane
fragmentation mode
k at 24 °C, 10 s
Ea, kJ/mol
ΔH , kJ/mol
ΔS , J/mol-K
1
C−C cleavage and deprotonation
deprotonation
3.7
1.5
1.4
19.4 ± 0.7
18.1 ± 0.6
13.2 ± 0.4
12.0 ± 1.8
17.7 ± 0.7
15.8 ± 0.6
10.9 ± 0.4
10.1 ± 1.6
−60 ± 2
−73 ± 2
−91 ± 1
−72 ± 7
diphenylmethane
,1-diphenylethane
1
deprotonation
b
diphenylmethanol
deprotonation
7.8
a
b
With ±15%. At −17 °C.
1
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the vicinity of 2 × 10 M s−1 in acetonitrile. In comparison
to TPE, in the presence of the salts, the enhancement of the
yields of radicals in terms of plateau absorbance at 325 nm and
of their first-order formation kinetics was spectacular in the case
DPME and 1,1-DPE. For 1,1-DPE, the increase in the rate
1
0
−1
87
For DPME radical cation, in particular, measurements of the
•
growth kinetics of Ph CH at varying submillimolar pyridine
2
concentrations (0−0.31 mM) led to an estimate of the rate
9
−1 −1
constant at 9.0 × 10 M
s
for the bimolecular quenching of
the radical cation by pyridine. In these measurements, a
•
•
constant for growth of Ph C −CH was about 3-fold (i.e., from
.1 × 10 to 3.2 × 10 s ) and that in the plateau absorbance
at 325 nm) was by ∼52%. In the case of DPME, the rate
constant for growth of Ph CH absorption also increased
dramatically from 2.0 × 10 s to ∼8 × 10 s and the
corresponding plateau absorbance increased by ∼46%. In the
absence of detailed data, it was premature to attempt to
interpret the effects of ion pairing. Nevertheless, the differences
between the salt effects on TPE radical cation and those on its
DPME and 1,1-DPE analogs were worth noting.
.3.2. Methanol. A brief study of the effects of methanol
MeOH) was of interest in light of the fact that it has been used
as a nucleophile to trap Ph C in electron-transfer photo-
The 308 nm laser flash photolysis of
DCN in the presence of 20 mM TPE and 0.082 M MeOH
showed that the first-order rate constant for growth of 325 nm
absorbance increased slightly from 3.3 × 10 (without MeOH)
to 3.7 × 10 s (with MeOH). This increase in rate, though
small, suggested modest reactivity of TPE radical cation with
the alcohol. The rate increase was accompanied by a decrease in
plateau absorbance to the extent of 18%. Pertinent to our
assignment of the 440 nm transient as diphenylmethyl
carbocation, the growth−decay profile absorbance at 440 nm
attributable to this species was totally absent in the experiment
with MeOH. Methanol is a moderately strong quencher of the
progressive decrease in the absorbance due to Ph CH was also
2
3
2
6
6
−1
1
noticed with increasing [Pyr].
(
•
2
4. DISCUSSION
6
−1
6
−1
One of the objectives of this work was to establish or rule out
C−C bond cleavage as a significant mode of fragmentation of
TPE radical cation produced under photoinitiated electron-
transfer sensitization by cyanoaromatics. Our detailed transient
spectral and kinetic data generated for this system combined
with the findings from two-laser experiments provided both
qualitative and quantitative evidence in support of the
occurrence of extensive C−C bond cleavage. At the same
time, the data also suggest that this fragmentation mode alone,
3
(
+
2
3
4,35
chemical studies.
•
+
leading to Ph CH and Ph CH , cannot account for all of the
2
2
observed transient absorbance observed at 325−330 nm. This
6
absorbance is assignable to either or both of the spectrally
•
•
6
−1
similar radicals, Ph CH and Ph C −CHPh , the latter being
2
2
2
the product of deprotonation. Our detailed kinetic analysis of
the absorbance data at 325−330 nm relative to that at the
absorption maximum of diphenylmethyl carbocation (440 nm)
led to the conclusion that the deprotonation mode constitutes a
minor component (30%) of the fragmentation. These results
regarding TPE agree with those reported in ref 35 on DCN-
sensitized photolysis (steady state) of TPE in acetonitrile−
methanol (3:1). However, in light of our finding that C−C
cleavage occurs even at subambient temperatures well below 10
87
9
−1
carbocation with a reported rate constant of 1.2 × 10 M
−1
s
in acetonitrile. With [MeOH] = 0.082 M, the lifetime of the
°
C, it is not clear why the product from C−C cleavage (i.e.,
carbocation would be shorter than 10 ns, rendering it
undetectable in our experiment. With DPME and 1,1-DPE
radical cations, the increase in the growth rate constant for 325
nm absorbance in the presence of 0.080 M MeOH was more
pronounced (i.e., by a factor of 2) than in the case of TPE. The
corresponding plateau absorbances in the presence of MeOH
were higher than those in its absence by 10−20%. The behavior
of TPE radical cation in the presence of MeOH was again
noticeably different from that of DPME and 1,1-DPE radical
cations.
34
methyl diphenylmethyl ether) was not observed in the course
of 1,4-cyanobenzene-senstized photolysis in acetonitrile−
methanol at 10 °C. It is possible that in the presence of the
very high concentration of methanol (6.2 M) used in the work
3
4,35
of Okamoto et al.
deprotonation competed more favorably
with the cleavage mode. The significant enhancement of decay
kinetics of DPME and 1,1-DPE radical cations observed by us
at much lower [MeOH], i.e., 0.08 M, lends support to this
explanation.
3
.3.3. Pyridine. Like methanol, pyridine (Pyr) is also
As discussed in the Results, the radical cations of hydro-
+
•
expected to act as a nucleophile interacting with diphenyl-
methyl cation. A few 308 nm laser flash photolysis experiments
with DCN and TPE in the presence of 3.1 mM pyridine
showed the disappearance of the growth/decay phenomena of
transient absorbance at 440 nm (due to Ph CH ) in the
presence of pyridine. Pyridine at 3.1 mM also caused a
noticeable decrease (by 25%) in the absorbance due to DCN
radical anion (monitored at 385 nm). This decrease could be
carbons (CH ) are recognized to be very strong acids with
negative pK values (from 0 to −30) in solutions of dimethyl
sulfoxide and acetonitrile.
of phenyl-substituted alkanes such as toluene and diphenyl-
methane have highly negative pK values in the range from −12
90
a
57−63
In particular, the radical cations
+
2
a
to −25. In light of this property, it is not surprising that we
observed very fast deprotonation kinetics of radical cations of
phenyl-substituted alkanes and alcohols with associated first-
1
order rate constants higher than 1 × 106
−1
explained in part in terms of the quenching of the DCN* by
s
(at room
8
8
pyridine. Note that the laser flash photolysis (308 nm) of
DCN in the presence of pyridine alone (i.e., with no TPE) did
not show any significant absorbance due to DCN radical anion
temperature). For TPE radical cation also, with one-third of the
fragmentation apportioned to deprotonation, the rate constant
6
−1
associated with it is ∼1 × 10 s . The reported proton loss rate
89
at 385−390 nm. Interestingly, however, at the millimolar
concentration of pyridine used, the yield of the transient radical
products at 325 nm was almost completely suppressed (i.e., by
constants for radical cations of methyl-substituted benzenes are
in the range from 10 to 107
temperature.
4
s
−1
in water at room
In particular, the rate constant for deproto-
nation of toluene radical cation in aqueous solution has been
6
1,91
∼
90%) and the usual fast growth of the transient absorption at
7
−1
91
this wavelength was replaced by a considerably slower growth
of much weaker absorption with a lifetime of ∼4 μs. Similar
effects of pyridine at millimolar concentrations were also noted
measured to be 1.0 × 10 s at 298 K. Our measured values
6
−1
of room-temperature deprotonation rate constants, ∼10 s ,
for phenyl alkane radical cations in the less nucleophilic (basic)
1
92
with DPME and 1,1-DPE as the donor substrate for DCN*.
solvent acetonitrile are very reasonable.
1
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Article
3
4,35
The steady-state photolysis work
on 1,4-dicyanobenzene-
scale to produce an equilibrated mixture of the complex,
•
+
photosensitized C−C bond-cleavage reaction of TPE in
acetonitrile:methanol mixture (3:1) gave an estimate of the
activation energy of the rate-determining step at 7.2 kcal/mol
i.e., 30.1 kJ/mol). The activation energy measured with DCN
as photosensitizer was slightly lower (28.0 kJ/mol). It was
Ph CH and C H NH (eq 19).
2 5 5
+
•
Ph CH
+ C H N
2
2
5
5
(
•
+
35
→ Complex ⇌ Ph CH + C H NH
(19)
2
5
5
3
5
suggested that these energies represented a minimum value,
close to the dissociation energy of the central C−C bond in
TPE radical cation. The Arrhenius activation energy (19.4 kJ/
mol) that we measured for fragmentation of the TPE radical
cation in acetonitrile is considerably lower. Obviously, a
significant difference in the relative contributions of the two
modes of fragmentation in the different solvents used in the
two studies could be responsible for the observed lack of
agreement on activation energy. Notably, the activation
energies measured for deprotonation of the radical cations of
DPME, 1,1-DPE, and DPML are smaller than the activation
energy of fragmentation of the TPE radical cation (Table 1).
This suggests that the extent of increase in the rate of C−C
bond cleavage in the TPE radical cation due to an increase in
temperature would be more pronounced than that in its
deprotonation rate.
The intermediacy of a complex formed between pyridine and
radical cations of phenylthioacetic acids has, in fact, been
implicated in the mechanism of electron-transfer-induced
decarboxylation of these thioacids. Obviously, the pyridine
quenching effects on phenyl alkane radical cation fragmentation
behaviors deserve further investigation.
4
93
5. CONCLUSIONS
Using nanosecond laser flash photolysis, a kinetic study was
performed on the fragmentation behaviors of radical cations of
several phenyl-substituted alkanes and alcohols. Radical cations
were generated in acetonitrile via electron transfer to singlet
excited states of 1,4-dicyanonaphthalene (DCN) and 1,2,4,5-
tetracyanobenzene (TCB). The major findings from this study
are as follows.
The C−C bond cleavage and deprotonation (i.e., C−H
cleavage) in the radical cations studied in this work result from
intramolecular electron transfer from the σ orbitals associated
with these bonds to the highest singly occupied molecular
orbital (SOMO) of the phenyl groups in the radical cations.
For the most efficient overlap between these orbitals that would
facilitate the electron transfer, the σ bond should be in a
perpendicular orientation with respect to the plane of the
(
1) Fragmentation of the radical cation of 1,1,2,2-tetraphe-
nylethane (TPE) in acetonitrile at 24 °C occurs with a
6
−1
first-order rate constant of ∼4 × 10 s via both
carbon−carbon bond cleavage and deprotonation. The
former mode dominates over the latter by a factor of 2:1
approximately.
(
2) The radical cations of diphenylmethane (DPME) and
1,1-diphenylethane (1,1-DPE) decay solely by deproto-
nation with first-order rate constants in the vicinity of 1
4
,94
phenyl rings.
The σ-bond cleavage kinetics of a radical
cation would be relatively slow if its most stable conformation
6
−1
×
10 s (at room temperature).
does not correspond to the favorable conformation for the σ-
4
,94
(3) The deprotonation kinetics of triphenylmethane
(TPME), diphenylmethanol (DPML), and benzyl
alcohol (BZA) at room temperature were too fast to
bond/π-system overlap.
In order to shed light on these
95
structural aspects, we have undertaken a computational study
of the structures and energetics of the phenyl alkane radical
cations. Our preliminary results from DFT/UB3LYP/6-31g-full
optimization calculations (solvent: acetonitrile) using Gaussian-
7
be resolved in our study (with rate constants > 1 × 10
s ).
−1
(
4) The activation energies (Arrhenius) and activation
enthalpies (Eyring) of fragmentation kinetics are in the
ranges 12−19 and 10−18 kJ/mol, respectively. The
activation entropies (Eyring) range from −60 to −91 J/
(mol·K)
09 show that in the most stable conformation of the TPE
radical cation the C−C bond in the β position with respect to a
phenyl moiety is nearly perpendicular to the plane of the
phenyl ring, while in the most favorable case, the β-C−H bond
is off from such orientation by nearly 30°. This can explain, at
least partially, why C−C bond cleavage is favored over
deprotonation in the case of TPE radical cation. DFT
calculations on other phenyl alkane systems and using more
extensive basis sets are currently in progress.
(5) The quenching effects of pyridine on the decay behaviors
of radical cations of TPE, DPME, and 1,1-DPE suggest
the involvement of a radical cation/pyridine complex in
the quenching mechanism.
While our data from the quenching studies are not
comprehensive enough to explain most of the observations
we made, some discussion is in order for the results we
obtained on pyridine quenching. As a proton acceptor, pyridine
is expected to contribute to the radical cation deprotonation
step (eq 18).
ASSOCIATED CONTENT
■
* Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
S
+
•
•
+
Ph CH2 + C H N → Ph CH + C H NH
(18)
2
5
5
2
5
5
AUTHOR INFORMATION
■
In agreement, our experiments with the DCN/DPME system
showed an enhancement in growth kinetics of the radical in the
presence of submillimolar pyridine concentrations. However,
the involvement of eq 18 should not result in a decrease in the
plateau absorbance due to the product radical, Ph CH . At the
low concentrations used, the quenching effect of pyridine on
DCN* was negligible. It is possible that pyridine initially forms
Corresponding Author
E-mail: pdas@cameron.edu.
*
Present Address
Chevron Products Company, Richmond, California 94801,
United States.
†
•
2
1
Notes
a complex with the radical cation which decays on a longer time
The authors declare no competing financial interest.
1
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The Journal of Physical Chemistry A
Article
(
18) Wang, Y.; Haze, O.; Dinnocenzo, J. P.; Farid, S.; Farid, R. S.;
ACKNOWLEDGMENTS
■
Gould, I. R. Bonded Exciplexes. A New Concept in Photochemical
Generous financial support for this work was obtained from the
Harvard L. and Judith D. Tomlinson of Duncan Endowed
Lectureship in Physical Sciences and the Frontiers in Chemistry
Endowed Lectureship in Chemistry at Cameron University.
The experimental work presented in this report utilized laser
flash photolysis equipment and other instrumental facilities at
the Notre Dame Radiation Laboratory (NDRL). P.D. is
thankful to Drs. Gordon Hug, Ian Carmichael (Director,
NDRL), and G. N. R. Tripathi for help and hospitality during
part of the work.
Reactions. J. Org. Chem. 2007, 72, 6970−6981.
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1
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1
1
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4
5,47
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8,49
by
ΔG(kcal/mol) = 23.06[E
− E
] + W , where E
A*/A−• p D+•/D
D/D+•
is the oxidation potential (in V) of the donor ground state, E
is
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7
bringing together the product ions to form the ion pair (estimated to
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4,37,45
1
1
(
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⎝
⎞
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(
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⎟
⎟
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The Journal of Physical Chemistry A
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74) It is possible that the slowly growing minor component of
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quenching of TCB*. Using 2.80 eV (i.e., 64.6 kcal/mol) as the
4
9
energy of the lowest triplet state of TCB and the equation for Gibbs
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77) The observed lifetime in acetonitrile at room temperature varied
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noticeably shorter; this is explainable in terms of TCB acting as an
•
efficient electron-transfer quencher for Ph CH *. Cyr, D. R.; Das, P.
2
K. Manuscript in preparation.
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88) Steady-state fluorescence measurements in acetonitrile gave
−1
values of 94 and 148 M for Stern−Volmer constants for the pyridine
quenching of TCB and DCN fluorescence, respectively.
(
89) Pyridine is known to interact with singlets of cyanoaromatics,
1
8
e.g., 9,10-dicyanoanthracene, through formation of a bonded exciplex
that does not lead to ion separation to any appreciable extent.
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