Photoinduced electron-transfer reactions: Highly efficient cleavage of C-N bonds and photogeneration of tertiary amines

Article abstract of DOI:10.1021/jp972855c

The photocleavage of tertiary amine from tetraphenylborates complexed, via ammonium ions, to various chromophore electron acceptors has been studied by laser-flash photolysis. Electron transfer from the borate anions to acceptor excited states is determined to be the primary photochemical step leading to homolytic C-N bond scission and concomitant formation of the corresponding tertiary amine. This pathway was established by direct observation of p-benzoylbenzyl (7a), p-acetylbenzyl (7b), β-naphthylmethyl (7c), and 7-methoxycoumarin-4-methyl (7d) after laser-flash photolysis from N-(4-benzoyl)benzyl-N,N,N-tributylammonium tetraphenylborate (1a), N-(4-acetyl)benzyl-N,N,N-trimethylammonium tetraphenylborate (1b) N,N,N-tributyl-N-(2-methylnaphthalene)ammonium tetraphenylborate (1c), and N,N,N-tributyl-N-(4-methyl-7-methoxycoumarin)ammonium tetraphenylborate (1d), respectively. The presence of radicals (7a-7d) was also suggested by the reaction products. Comparison of the rate constants for quenching of the triplet states from excited (1a, 1b) and singlet states from (1c, 1d) reveals the efficiency of C-N bond cleavage. Quenching constants in the range of 10⁹-10¹² M^-1 s^-1 were observed. Differences are accounted for in terms of variations in the electron-accepting excited states and structural characteristics of the chromophore acceptors.

Full text of DOI:10.1021/jp972855c

J. Phys. Chem. A 1998, 102, 5375-5382
5375
Photoinduced Electron-Transfer Reactions: Highly Efficient Cleavage of C-N Bonds and
Photogeneration of Tertiary Amines1
Ananda M. Sarker, Yuji Kaneko, Alexander V. Nikolaitchik, and D. C. Neckers*
Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403
ReceiVed: September 2, 1997; In Final Form: December 31, 1997
The photocleavage of tertiary amine from tetraphenylborates complexed, via ammonium ions, to various
chromophore electron acceptors has been studied by laser-flash photolysis. Electron transfer from the borate
anions to acceptor excited states is determined to be the primary photochemical step leading to homolytic
C-N bond scission and concomitant formation of the corresponding tertiary amine. This pathway was
established by direct observation of p-benzoylbenzyl (7a), p-acetylbenzyl (7b), â-naphthylmethyl (7c), and
7-methoxycoumarin-4-methyl (7d) after laser-flash photolysis from N-(4-benzoyl)benzyl-N,N,N-tributylam-
monium tetraphenylborate (1a), N-(4-acetyl)benzyl-N,N,N-trimethylammonium tetraphenylborate (1b) N,N,N-
tributyl-N-(2-methylnaphthalene)ammonium tetraphenylborate (1c), and N,N,N-tributyl-N-(4-methyl-7-
methoxycoumarin)ammonium tetraphenylborate (1d), respectively. The presence of radicals (7a-7d) was
also suggested by the reaction products. Comparison of the rate constants for quenching of the triplet states
from excited (1a, 1b) and singlet states from (1c, 1d) reveals the efficiency of C-N bond cleavage. Quenching
constants in the range of 109-1012 M-1 s-1 were observed. Differences are accounted for in terms of variations
in the electron-accepting excited states and structural characteristics of the chromophore acceptors.
Introduction
SCHEME 1: Synthesis of Amine Precursors
One method of generating radical ions involves deposition
of light energy into acceptor or donor molecules, thus converting
them into potent redox reagents capable of undergoing electron
transfer with ground-state substrates.2 Radical ions may undergo
unimolecular fragmentation reactions to radicals and ions.3,4
Rapid bond-cleavage reactions that take place after electron
transfer represent an excellent route for the photogeneration of
a variety of reactive intermediates.5 If the system is self-
destructive such that the concomitant/subsequent bond-cleavage
reaction is rapid, energy-wasting back electron transfer processes
between the initially formed redox pair can, in principle, be
avoided.
Though extensive studies have been reported on the frag-
mentation of C-C bonds in photogenerated radical ions,6 C-N
bond scission from photogenerated reactive intermediates is less
common.7 Photolabile amine precursors are, however, reported
to be useful as photocleavable amino acid precursors in
biological systems.8 Certain photoreactive9 carbamates10 are
known to generate both primary and secondary amines. Since
tertiary amines are better catalysts than primary and secondary
amines,11 particularly in acyl transfer reactions where the
formation of undesirable amide-related products need be
avoided,12 we pursued systems that dissociate by C-N cleavage
to produce tertiary amines and have recently shown them to be
a useful component in imaging thin polymer films.13
The experimental system selected was based on previous work
and involved tetraphenylborate as the electron donor. Different
chromophores (Scheme 1) served as the electron acceptors. A
simple unimolecular C-N bond scission in the formed radical
ions served as a model for tertiary amine formation while
providing an internal probe of photoelectron-transfer dynamics.14
Synthesis and Steady-State Spectroscopy
Precursors of N-(4-benzoyl)benzyl-N,N,N-tributylammonium
tetraphenylborate (1a), N-(4-acetyl)benzyl-N,N,N-trimethylam-
monium tetraphenylborate (1b), N,N,N-tributyl-N-(2-methyl-
naphthalene)ammonium tetraphenylborate (1c), and N,N,N-tri-
butyl-N-(4-methyl-7-methoxycoumarin)ammonium tetraphen-
ylborate (1d) were synthesized according to the procedures
shown in Scheme 1. Bromomethyl precursors were converted
to ammonium bromide salts 3a-d by reaction with tertiary
amines in 78-86% yield. The reactions were performed in
* To whom correspondence should be addressed.
S1089-5639(97)02855-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/12/1998

5376 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Sarker et al.
SCHEME 2: Photolysis Products of 1a-d in CH3CN at
350 nm
no amine was formed in control experiments carried out under
similar reaction conditions (vide infra) with 2a-2d when the
nonreducing gegenion BF4- was present. Excited-state electron
transfer from tetraphenylborate to chromophore acceptor was
thus responsible for photocleavage of the C-N bond.
Figure 1. Absorption spectra of 1a-1d in acetonitrile.
TABLE 1: Product Studies for the Photolysis of 1a-1d in
Acetonitrile (2.5 × 10-2 M) for 1 h
The products formed from irradiation of 1a-d in acetonitrile
were identified by comparing HPLC retention times with
authentic samples, as well as from MS fragmentation patterns
(Scheme 2). Since trends in the formation of products were
similar in all cases, the mechanism of photocleavage of the
ammonium borates appeared to be a general process. When
reactions were performed on a preparative scale, the yields of
isolated products represent only lower limits but product
distributions were similar in both small-scale and large-scale
reactions. The respective tertiary amines were produced in
products (% yield)b
compd
conversion (%)a
5
6
1a
1b
1c
1d
95
70
12
98
38
40
34
43
16
15
11
9
1
a Determined by H NMR analysis, cf. text. b Calculated on basis
of conversion.
1
virtually quantitative yields from 1a-1d as determined by H
solvents from which the resulting ammonium bromide precipi-
tated under strictly anhydrous conditions. Borate complexes
1a-d were obtained by reaction of the ammonium bromide with
the sodium salt of tetraphenylborate in water. The borates were
stable for months at room temperature.
NMR spectroscopy. Major radical-coupling products were
formed by 7a-7d (Chart 1), while products 6a-6d derived by
CHART 1
The steady-state absorption spectra of 1a,b and 1d in
acetonitrile had n-π* transitions centered at 335-340 nm
though the molar absorption coefficients differed (Figure 1).
The spectra of 1a,b and 1d in nonpolar solvents were similar
in shape to those in acetonitrile, however, were significantly
shifted to the red, suggesting the lower excited singlet state was
an n-π* state. The molar extinction coefficient for 1d was
13 000 M-1 cm-1 (334 nm) in acetonitrile indicating that the
π-π* transition overlapped with the n-π* transition in this
region.15 The spectrum of 1c evidenced only π-π* transitions
with a substantial tail at longer wavelengths. All spectra were
independent of anion structure and essentially identical to those
of the chromophores in the absence of complexing agent in the
same solvent. Thus, there was no indication of charge-transfer
complex formation.
hydrogen abstraction were minor (Table 1). At room temper-
ature, photodimer 5d was quite insoluble in acetonitrile and
precipitated during the photolysis. This product was identified
by high-resolution mass spectrometry and elemental analysis.
Quenching Experiments
The primary process of C-N bond cleavage was further
studied by quenching experiments using, as the electron donor,
tetrabutylammonium tetraphenylborate (Ph4B-Bu4N+) which
successfully quenched the triplet states of 2a,b and the singlet
states of 2c,d in acetonitrile at room temperature. In the case
of triplet-state quenching, addition of Ph4B-Bu4N+ resulted in
a change in the form of the transient decay profile, while singlet-
state quenching resulted in a change of emission intensity
(excited at 320 nm for 2c and 355 nm for 2d). In all cases,
linear Stern-Volmer plots were observed (Figure 2). Rate
constants for quenching were determined by measuring the
effects of the additive on the lifetimes of the triplet or singlet
states and are shown in Table 2. The quenching rate is not
affected by the substitutents on the quaternary nitrogen atom.
On the basis of the oxidation potentials of tetraphenylborate,5a
the reduction potentials of 2a-d, and the energy of their excited
Photolysis Products
Nondegassed solutions (acetonitrile) were irradiated in a
Rayonet photoreactor (λmax ) 350 nm). The efficiency of C-N
bond cleavage in 1a and 1d proved to be high, and 95% and
98% conversions, respectively, were obtained after just 1 h.
Because of the low absorption of 1b,c around 350 nm, 1b
showed moderate conversion (70%) while 1c gave only 12%
conversion (Table 1). Conversion to tertiary amine was
1
determined by H NMR from the disappearance of the meth-
ylene peaks around 4.40 ppm (for 1a) and the appearance of a
new peak at 2.43 ppm due to the formation of amine. To assess
if any products resulted from secondary photoprocesses, 1a was
irradiated to less than 20% conversion, and product distributions
were found to be identical under these conditions. Likewise,

Photoinduced Electron-Transfer Reactions
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5377
Figure 3. Transient absorption spectra obtained by laser photolysis
(355 nm) of 1a in acetonitrile (1 × 10-3 M): (a) 300, (b) 590, and (c)
3200 ns. Insert: time profile at 530 nm.
Figure 2. Triplet-state quenching of 2a,b and singlet-state quenching
of 2c,d by tetrabutylammonium tetraphenylborate in acetonitrile.
TABLE 2: Reduction Potentials (Ered), Free Energy
Changes (∆G°) for Electron Transfer, and Quenching Rate
Constants (kq) for the Excited States of 2a-2d with
Tetrabutylammonium Tetraphenylborate in Acetonitrile
Figure 4. Time profile of transient absorption (normalized) of 1a in
acetonitrile in the presence of oxygen (20-min bubbling).
compds Ered (V) vs SCE Es,t (eV) ∆G° (eV)
kq (M-1 s-1
)
photodecomposition Φd of 1a, 1b, 1c, and 1d in CH3CN were
0.49, 0.42, 0.20, and 0.07, respectively. In CH3CN, the
ammonium borates were in ionic equilibrium; thus, diffusion
of the ions was required for electron-transfer reaction. Partici-
pation of longer lived triplet states in 1a and 1b accounted for
higher values of Φd, while the shorter lifetimes of the singlet
state of both 1c and 1d decreased the value of Φd in acetonitrile.
Photodecomposition of borate 1d in PhH (3 × 10-4 M) was
found to proceed with Φd ) 0.20. In nonpolar solvents such
as PhH, borate 1d existed as a tight ion pair, thus eliminating
the diffusion-controlled step.
2a
2b
2c
2d
-1.49
-1.62
-2.10
-1.30
3.01(t)a
3.16(t)b
3.88(s)c
3.63(s)d
-0.62
-0.64
-0.88
-1.43
5.65 × 109
5.76 × 109
6.25 × 109 e
5.94 × 1012 f
a Reference 34. b Reference 35. c Reference 36. d Reference 37. e I0/I
) 1 + kqτs [Ph4B-]; τs ) 8.9 ns for 1-cyanonaphthalene.37 f I0/I ) 1
+ kqτs [Ph4B-]; τs ) 5.1 ps (Φf ) 0.022 for 2d); τf ) 0.23 ns for
4-methyl-7-methoxycoumarin in methanol.38
states, the free energy change (∆G°)16 associated with electron
transfer can be estimated (Table 2). Electrochemical measure-
ments were carried out in acetonitrile using cyclic voltammetry
with tetrabutylammonium perchlorate (TBAP) as the supporting
electrolyte. In all cases, wave characteristics indicated irrevers-
ible one-electron reduction at peak voltages (Table 2). This
was taken as evidence for rapid bond cleavage reaction (on the
time scale of the electrochemical experiments) of all reduced
chromophore tertiary ammonium cations.
Laser-Flash Photolysis
Laser-flash photolysis of 1a in degassed acetonitrile (1 ×
10-3 M) at 355 nm (7 ns, 70 mJ) lead to a transient with
absorption maxima at 335 and 530 nm (Figure 3). Transient
absorption decay at 530 nm consisted of two components. The
lifetime of the faster component was 200 ns on the basis of a
first-order kinetic law. The decay of the slower component was
consistent with a second-order rate law (k/ꢀ ) 1.17 × 106 cm
s-1). Both absorption bands (335 and 530 nm) were quenched
by oxygen; however, the time profiles were not the same (Figure
4), indicating that the absorptions occurred from two different
species. Under similar conditions, a solution of 2a displayed a
Quantum Yields
Quantum yields for the photodecomposition of borates 1a-d
in CH3CN (5 × 10-3 M) were determined at 366 nm in a quartz
cell having a path length of 10 cm. The quantum yields of

5378 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Sarker et al.
Figure 5. Transient absorption spectra recorded by 355-nm laser-flash
photolysis of 2a in acetonitrile (1 × 10-2 M): (a) 6, (b) 12, (c) 18, (d)
30, and (e) 75 µs. Insert: time profile at 520 nm.
Figure 6. Transient absorption spectra of 1c in acetonitrile (1 × 10-2
M) at 2.5-µs delay times after the laser pulse. Insert: time profile at
380 nm.
triplet-triplet absorption with maxima at 320 and 550 nm (τ
) 13.2 µs).17 The decay of both suggested a one-component
system, and the behavior toward oxygen was also similar.
4-Methylbenzophenone, whose triplet absorption spectra was
very similar to that of 2a, behaves in an analogous fashion.
The triplet state of 2a was found to be quenched by Ph4B-Bu4N+
(1 × 10-3 M, acetonitrile, kq ) 5.65 × 109 M-1 s-1) with
concomitant observation of absorptions at 335 and 530 nm. The
530-nm band showed similar behavior to that of 1a, indicating
the presence of similar species.
4-(Bromomethyl)benzophenone was studied as a model
compound under similar flash photolysis conditions. A transient
spectrum was observed with bands at 335 and 530 nm. The
530-nm band decayed with second-order kinetics (k/ꢀ ) 2.49
× 108 cm s-1) that differed from those observed for the 335-
nm absorption. The only possible radical product that resulted
following laser-flash photolysis of 4-(bromomethyl)benzophe-
none was p-benzoylbenzyl radical, 7a. This assignment was
in complete agreement with the results of photolysis product
studies of 1a wherein a coupled product 5a from the p-
benzoylbenzyl radical 7a was isolated. We thus assigned the
reactive intermediate observed at 530 nm upon laser-flash
photolysis of 1a and the mixture (2a and Ph4B-Bu4N+) to be
the p-benzoylbenzyl radical 7a. The absorption at 335 nm was
from residual absorption due to a stable coupling product of
the radicals.
Figure 7. Transient absorption spectra of 1d in acetonitrile (1 × 10-4
M): (a) 0.59, (b) 1.2, (c) 1.8, and (d) 5.6 µs. Insert: time profile at
640 nm.
assigned to the 2-naphthylmethyl radical 7c.19 This assignment
was confirmed from an independently generated spectrum of
the radical obtained from the photolysis of 2-(bromomethyl)-
naphthalene.
The time-resolved transient spectrum for 1b (1 × 10-2 M,
acetonitrile) showed (Figure 5) two absorption bands at 380
and 520 nm, respectively. Decay of the 520-nm absorption was
fitted by second-order kinetics (k/ꢀ ) 1.75 × 106 cm s-1). Laser-
flash photolysis of 2b in acetonitrile gave only one absorption
at 360 nm assigned to the triplet state.18 This triplet state was
quenched by Ph4B-Bu4N+ with a rate constant kq ) 5.76 ×
109 M-1 s-1 and was accompanied by the appearance of a new
absorption at 520 nm. The decay behavior was practically
identical to that of 1b (520-nm band); therefore, we concluded
we were observing the p-acetylbenzyl radical 7b. This assign-
ment was consistent with the results of product studies which
show that the product 5b derived from radical 7b was the major
product.
Laser-flash photolysis of 1d in acetonitrile (5 × 10-4 M) gave
the transient absorption spectra shown in Figure 7. Following
the excitation laser pulse, transient absorption bands at 480 and
600-700 nm were observed. The 480-nm band had an
exponential decay with a lifetime of 1.5 µs. The broad
absorption band 600-700 nm decayed with second-order
kinetics (k/ꢀ ) 1.3 × 107 cm s-1). Both transient species were
quenched by the addition of oxygen. The short-lived species
absorbing at 480 nm was assigned to the triplet state of 1d by
comparing with literature data.18 Further confirmation of this
assignment was achieved from independent generation of
transient spectra by photolysis of 4-methyl-7-methoxycoumarin
in benzene (τ ) 1.2 µs, max ) 470 nm). Similar results were
obtained with 2d (1 × 10-4 M) under identical experimental
conditions where no oxidizable group was present. The
absorption at 480 nm was unaffected by the addition of
Ph4B-Bu4N+ (acetonitrile, 2 × 10-3 M). However, a new broad
absorption at 600-700 nm appeared upon photolysis of 2d with
decay behavior similar to that observed for 1d at approximately
Figure 6 shows the transient absorption of 1c (1 × 10-2 M,
acetonitrile) with a maximum at 380 nm decaying in a
bimolecular reaction (k/ꢀ ) 1.07 × 108 cm s-1). Triplet
absorption was observed at 420 nm for 2-methylnaphthalene
under identical conditions. Therefore, the band at 380 nm was

Photoinduced Electron-Transfer Reactions
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5379
640 nm. This broad absorption at 600-700 nm was assigned
to the radical 7d by comparison to spectra generated from
4-bromomethyl-7-methoxycoumarin under similar experimental
conditions.
SCHEME 3: Proposed Mechanism for Photocleavage
Reaction
Picosecond transient absorption studies of 1d in benzene
(3 × 10-4 M) showed the presence of broad 600-700 nm
absorption forming immediately after a 30-ps laser-excitation
pulse. The presence of radical 7d in the reaction system on
this time scale indicated a fast rate of C-N bond cleavage in
the reduced 7-methoxycoumarin tertiary ammonium cation.
Discussion
The rate constants for bimolecular quenching of the excited
states of 2a-d by Ph4B-Bu4N+ are given in Table 2 along with
reduction potentials and excited-state energies. Rate constants
(kq) for the quenching of each of the excited states were high
and approached diffusion-controlled limits. Though the reduc-
tion potential of 2a was lower than that of 2b, the triplet-state
energy of 2b was higher than that of 2a, thus compensating for
the reactivity toward bimolecular quenching and providing a
nearly constant driving force for forward electron transfer.
Despite its high reduction potential, the reactivity of the excited
singlet state of 2c was higher than that for either 2a or 2b. This
can be accounted for in terms of the higher excited-state energy
of the singlet state of 2c compared to the triplet energy of both
2a and 2b. The quenching of 2d exceeded diffusion-limited
rates due to its low reduction potential as well as its large
excited-state energy. In each case, ∆G°, the driving force for
electron transfer, was negative indicating a highly exothermic
process. Despite the similar reactivities of the triplet and singlet
excited states of the respective compounds (Table 2), it seems
noteworthy that the efficiency of amine formation (conversion
percent) of 1c was significantly lower as a result of competitive
deactivation of the singlet state.20 However, the conversion
(percent) and yield of products in the case of 1d were much
higher, indicating that structural characteristics are also impor-
tant.
Spectroscopic evidence suggests that electron transfer takes
place from the donor (Ph4B-Bu4N+) to the excited triplet or
singlet states depending on the acceptor. As a result, C-N bond
cleavage occurs homolytically in the corresponding reduced
tertiary ammonium cations, and the radicals 7a-d are formed.
This bond-cleavage pattern was confirmed by direct observation
of radicals 7a-7d by laser-flash photolysis, and each radical
was identified by comparison with identical radicals as produced
by independent, known routes. All transient absorption spectra
have the common decay characteristics of bimolecular reactions.
Additional support for formation of radicals 7a-7d comes
from the isolated photolysis products (Scheme 2). Photolysis
of 1a-1d in acetonitrile at 350 nm in the presence of oxygen
resulted in the formation of 5a-d and 6a-6d as the major and
minor products, respectively. In the absence of oxygen, a
similar distribution of products results though yields are higher.
The minor products, in all cases, were formed via hydrogen
abstraction from CH3CN, a poor but known hydrogen donor.21
This was confirmed using CD3CN in the photolysis, and
observing a molecular ion peak indicated that deuterium had
been incorporated in 6a-6d. That radical coupling products
are the major products indicates relatively long lifetimes for
7a-7d.
not clear. Trapping experiments employing TEMPO (2,2,6,6-
tetramethyl-l-piperidinyloxy) failed to detect phenyl radical,
though in the absence of oxygen the yield of biphenyl was
significantly lower. It seems reasonable to conclude that
biphenyl23 is formed as a result of the intramolecular coupling
of two phenyls from tetraphenylborate, a result consistent with
other experiments.24
A representative mechanism proposed for the photocleavage
of 1a-1d is represented with 1b in Scheme 3. According to
Scheme 3, radical 7b is formed from the triplet radical pair 8
generated after the electron transfer via pathways a and b. We
were unable to detect transient absorbance corresponding to the
triplet radical pair 8 formed on the picosecond time scale, nor
could we detect transient 9 as suggested in pathway a for 7b
using picosecond flash photolysis. The bond-breaking process
leading to the formation of radicals in each case is coupled to
the primary electron-transfer process and occurs on the sub-
picosecond time scale. Back electron transfer to the tetraphen-
ylboranyl radical (τ ) 45 ps) would likely be impossible on
this time scale because of the rapid C-N bond cleavage.25,26
Since back electron transfer to regenerate starting material is
also spin-forbidden in triplet radical pairs, and the triplet radical
pair must undergo intersystem crossing to the singlet state, which
subsequently collapses forming ground state, it is not surprising
that the product yields reveal that back electron transfer is minor
for the triplet reactions (1a,b) where the efficiency of the amine
formation approaches unity.
Back electron transfer may be significant for the photoreaction
of 1c, in which the singlet radical pair was formed.27 This
assumption is consistent with low quantum yield for the
disappearance of 1c, which is smaller than the others. The
overall efficiency of photoreduction, therefore, is low. Another
explanation for the lower efficiency of amine generation is the
low molar extinction coefficient at 350 nm for 1c. It shows
only a tail absorption beyond 325 nm.
Structural features in 1d and its efficient photocleavage invite
additional comments. Although electron transfer occurred in
the excited singlet state in 1d, the yield of products and rate of
conversion to amine suggested a relatively minor role for back
electron transfer and the importance of nuclear vibrational
motion in electron-transfer processes. Compound 1d having
an electron-donating methoxy group in the 7-position and an
electron-withdrawing substituent in the 4-position provided
positional push-pull substituents and was the main reason for
ultrafast electron transfer. The drastic change in reaction rate
Heterolytic C-N bond scission was ruled out because of the
absence of products that could be attributed to ionic intermedi-
ates.22 Thus, the acetamide would be an expected product in
acetonitrile. The source of biphenyl in the product mixture was

5380 J. Phys. Chem. A, Vol. 102, No. 28, 1998
Sarker et al.
as a function of the push-pull substituents on the coumarin
has been previously observed28 wherein amino and trifluoro-
methyl substituents were at 7- and 4-position, respectively.
Enhanced reactivity was rationalized in terms of the static and
dynamic roles of the rotational motion of the amino group. A
similar effect of the amino group on the rate of electron transfer
is often discussed in the studies of twisted intramolecular charge
transfer (TICT), such as in the case of N,N-dimethylaminoben-
zonitrile.29
In the present system, rotational motion of the 7-methoxy
group plays an important role. Change in the vibrational
character of the methoxy group by substitution may cause a
change in the electron-transfer rate. A change in electron-
transfer dynamics from changing the vibrational parameters has
been explained in terms of the extended Sumi-Marcus model.30
However, we considered that the presence of nonreversible
electron transfer was due to both electronic effects as well as
vibrational motion. The presence of an electron-withdrawing
substituent in the 4-position and an electron-donating substituent
in the 7-position of coumarin allowed for a large value of the
electron-transfer rate constant. This ultrafast reaction took place
from a nonreversible state successfully competing with back
electron transfer.
General Procedure for Synthesis of Ammonium Bromide
Salts (3a-3c). Tertiary amine was added in excess with stirring
at room temperature to a solution of the respective bromomethyl
compound in chloroform-ether (1:2). A white precipitate
formed within 12 h (in some cases, almost immediately) and
was separated by filtration and dried. After recrystallization from
a suitable solvent, white crystals were obtained (70-80% yield).
N-(4-Benzoyl)benzyl-N,N,N-tributylammonium Bromide (3a).
Mp 160-161 °C (ethyl acetate/acetone). 1H NMR (acetone):
δ 7.90 (m, 4H), 7.68 (m, 1H), 7.58 (m, 4H), 5.11 (s, 2H), 3.50
(m, 6H), 2.06 (m, 6H), 1.43 (m, 6H), 1.00 (t, J ) 7 Hz, 9H).
13C (CD3OD): δ 199.18, 142.61, 139.99, 136.11, 135.73,
134.91, 133.38, 132.91, 131.54, 64.40, 61.50, 27.02, 22.54,
15.86. Anal. Calcd for C26H38BrNO: C, 67.86; H, 8.26; Br,
17.37. Found: C, 67.79; H, 8.28; Br, 17.43.
N-(4-Acetyl)benzyl-N,N,N-trimethylammonium Bromide (3b).
Mp 196-196.5 °C (acetone/methanol). 1H NMR (CD3CN): δ
8.13 (d, J ) 8.4 Hz, 2H), 7.75 (d, J ) 8.4 Hz, 2H), 4.68 (s,
2H), 3.18 (s, 9H), 2.66 (s, 3H). 13C NMR (CD3OD): δ 201.44,
141.89, 136.38, 135.71, 131.87, 71.33, 55.29, 28.86. Anal.
Calcd for C12H18BrNO: C, 52.98; H, 6.62; N, 5.15; Br, 29.38.
Found: C, 52.90; H, 6.60; N, 5.16; Br, 29.42.
N,N,N-Tributyl-N-(2-methylnaphthalene)ammonium Bromide
(3c). Mp 120-121 °C (ethyl acetate/ethanol). 1H NMR (CD3-
CN): δ 8.00 (m, 4H), 7.72 (m, 3H), 4.63 (s, 2H), 3.15 (m, 6H)
1.82 (m, 6H), 1.36 (m, 6H), 1.00 (t, J ) 7.4 Hz, 9H). 13C NMR
(CD3OD): δ 134.31, 130.25, 129.43, 129.38, 129.01, 128.85,
128.32, 126.12, 63.30, 59.41, 25.13, 20.74, 14.00. Anal. Calcd
for C23H36NBr: C, 68.01; H, 8.86; Br, 19.60. Found: C, 68.07;
H, 8.87; Br, 19.64.
Conclusion
We have presented a new sequence of reactions involving
excited-state electron transfer followed by C-N bond cleavage
which successfully competes with back electron transfer. The
reactions fulfill the requirements of high efficiency in the
generation of reactive tertiary amines. Release of amine from
borate-complex precursors is a rapid process, assuming the rate
constant for decay of the excited states, 109-1012M-1 s-1. These
desirable criteria for the new generation of phototriggers provide
a promising future for their application to polymer curing,
imaging, and other applications.
N,N,N-Tributyl-N-(4-methyl-7-methoxycoumarin)ammoni-
um Bromide (3d). 4-(Bromomethyl)-7-methoxycoumarin (1.0
g, 3.72 mmol) was suspended in methanol (20 mL), and
tributylamine (2.75 g, 14.88 mmol) was added to it. The
mixture was refluxed overnight during which time the reaction
mixture became clear. The solution was concentrated by
evaporating the solvent. After adding hexanes (30 mL), it was
kept at 0 °C overnight. The solid was filtered and recrystallized
from ethyl acetate-ethanol. 3d as white crystals (1.05 g) was
obtained (75% yield); mp 157-158 °C. 1H NMR (CD3CN):
δ 8.21 (d, J ) 8.6 Hz, 1H), 6.97 (d, J ) 2.5 Hz, 1H), 6.92 (d,
J ) 2.5 Hz, 1H), 6.58 (s, 1H), 4.87 (s, 2H), 3.89 (s, 3H), 3.42
(m, 6H), 1.75 (m, 6H), 1.29 (m, 6H), 0.91 (t, J ) 7.2 Hz, 9H).
Anal. Calcd for C23H36BrNO3: C, 60.82; H, 7.93; N, 3.08;
Br, 17.60. Found: C, 60.25; H, 8.00; N, 2.99; Br, 17.22.
General Procedure for Synthesis of Tetraphenylborate
Complexes (1a-1d). The ammonium bromide salt was dis-
solved in a minimum of water, and undissolved dust materials,
if present, were filtered. An equivalent amount of the sodium
tetraphenylborate in water was added dropwise at room tem-
perature with stirring. A thick white precipitate formed im-
mediately. This was diluted by the addition of water and
filtered. After recrystallization, white needles of the tetraphen-
ylborate complex (75%-85% yields) were obtained.
Experimental Section
General. Melting-point determinations were carried out on
a Thomas capillary melting point apparatus; all temperatures
were uncorrected. Elemental analysis was performed by
Atlantic Microlab, Inc., Georgia. 1H NMR spectra and 13C
NMR spectra were taken on Gemini GEM-200 (200 MHz) and
Gemini GEM-200 (50 MHz) spectrometer, respectively. UV-
vis and emission spectra were recorded on a Hewlett-Packard
8452 diode array spectrophotometer and SPEX Fluorolog
spectrophotometer, respectively. GC-MS was performed on a
Hewlett-Packard 5988 mass spectrometer coupled to a HP
5880A GC, interfaced to a HP 2623A data processor. GC
measurements were carried out on a Hewlett-Packard (HP) 5890
gas chromatograph.
Nanosecond-flash-photolysis studies were performed using
a kinetic spectrophotometric detection system previously de-
scribed.31 The excitation pulse (7-ns duration, 355 nm) of light
was the third harmonic of a Q-switched Nd:YAG laser. Typical
excitation pulse amplitudes were a few millijoules per pulse.
Transients produced were followed temporally and spectrally
by a computer-controlled kinetic spectrophotometer. Picosecond
transient absorption measurements were performed on a setup
described by Logunov and Rodgers.32
N-(4-Benzoyl)benzyl-N,N,N-tributylammonium Tetraphenyl-
borate (1a). Mp 197-198 °C (ethanol/acetone). 1H NMR
(acetone, δ): 7.83 (m, 4H), 7.68 (m, 1H), 7.54 (m, 4H), 7.26
(m, 8H, ortho to B), 6.99 (t, J ) 7.4 Hz, 8H, meta to B), 6.83
(t, J ) 7.4 Hz, 4H, para to B), 4.39 (s, 2H), 3.05 (m, 6H), 1.75
(m, 6H), 1.37 (m, 6H), 0.99 (t, J ) 7.4 Hz, 9H). Anal. Calcd
for C50H58BNO: C, 85.87; H, 8.29; N, 2.00. Found: C, 85.87;
H, 8.41; N, 2.08.
Reduction potentials were measured on a BAS-100 poten-
tiostat equipped with BAS PA-1 preamplifer. The reference
electrode was a Ag/AgNO3 in tetrabutylammonium perchlorate
solution (0.1 M in acetonitrile). The working electrode was a
platinum wire.
N-(4-Acetyl)benzyl-N,N,N-trimethylammonium Tetraphenyl-
borate (1b). Mp 205-206 °C (ethanol/acetonitrile). 1H NMR

Photoinduced Electron-Transfer Reactions
J. Phys. Chem. A, Vol. 102, No. 28, 1998 5381
(CD3CN): δ 8.05 (d, J ) 8.4 Hz, 2H), 7.57 (d, J ) 8.4 Hz,
2H), 7.28 (m, 8H), 7.00 (t, J ) 7.0 Hz, 8H), 6.85 (t, J ) 7.0
Hz, 4H), 4.34 (s, 2H), 2.93 (s, 9H), 2.60 (s, 3H). Anal. Calcd
for C36H38BNO: C, 84.58; H, 7.43; N, 2.74. Found: C, 84.62;
H, 7.41; N, 2.78.
General Procedure for Determination of Quantum Yield
of Photodecomposition of the Borates. A solution of a borate
in CH3CN or benzene (35 mL) was placed in a quartz cell (10-
cm path length), degassed with bubbling N2 for 30 min, and
sealed. The concentration of borates 1a, 1b, 1c, and 1d was 5
× 10-3 M in CH3CN and 3 × 10-4 M for 1d in benzene.
Irradiation was carried out using 366-nm light from a 200-W
Hg lamp isolated from the spectrum with two-color glass filters
(cutoff at 340 nm and at 400 nm) that was passed through a
10-cm water filter and focused into the sample. The irradiation
time varied from 30 s for 1d in PhH to 15 min for 1d in CH3-
CN. The maximum degree of photodecomposition was 25%
for 1a in CH3CN. The intensity of 366-nm light was determined
using anthracene dimerization reaction in O2 free benzene (1.05
× 10-2 M, 25 °C, quantum yield of anthracene disappearance
is 0.0706).33 The concentration of the remaining borates was
determined by HPLC.
N,N,N-Tributyl-N-(2-methylnaphthalene)ammonium Tetraphen-
ylborate (1c). Mp 174-175 °C (ethanol). 1H NMR (CD3CN):
δ 8.00 (m, 4H), 7.63 (m, 2H), 7.50 (m, 1H), 7.26 (m, 8H, ortho
to B), 6.99 (t, J ) 7.0 Hz, 8H, meta to B), 6.83 (t, J ) 7.2 Hz,
4H, para to B), 4.46 (s, 2H), 3.06 (m, 6H), 1.79 (m, 6H), 1.37
(m, 6H), 1.00 (t, J ) 7.0 Hz, 9H). Anal. Calcd for C47H56BN:
C, 87.48; H, 8.68, N, 2.17. Found: C, 87.40; H, 8.72; N, 2.18.
N,N,N-Tributyl-N-(4-methyl-7-methoxycoumarin)ammoni-
um Tetraphenylborate (1d). Mp 149-150 °C (ethanol). 1H
NMR (CD3CN): δ 7.71 (d, J ) 9.8 Hz, 1H), 7.26 (m, 8H),
6.98 (m, 10H), 6.83 (m, 4H), 6.19 (s, 1H), 4.42 (s, 2H), 3.90
(s, 3H), 3.21 (m, 6H), 1.17 (m, 6H), 1.26 (m, 6H), 0.93 (t, J )
7.0 Hz, 9H). Anal. Calcd for C47H56BNO3: C, 81.42; H, 8.08;
N, 2.02. Found: C, 81.28; H, 8.07; N, 2.05.
N,N,N-Tributyl-N-(4-methyl-7-methoxycoumarin)ammoni-
um Tetrafluoroborate (2d). Bromide 3d (0.30 g, 0.66 mmol)
was dissolved in water (20 mL) and undissolved dust materials
were filtered, if present. Aqueous fluoroboric acid (1.0 mL,
48 wt %) was added dropwise with stirring. A white precipitate
was formed immediately. Water (50 mL) was added, and the
solution was stirred for 15 min. The product (0.15 g, 50% yield)
was filtered and washed with water; mp 160-162 °C. Anal.
Calcd for C23H36BF4NO3: C, 59.91; H, 7.81; N, 3.04; F, 16.48.
Found: C, 60.03; H, 7.83; N, 3.06; F, 16.45.
N-(4-Benzoyl)benzyl-N,N,N-tributylammonium Tetrafluorobo-
rate (2a). Prepared as above (80% yield); mp 78-79 °C. Anal.
Calcd for C26H38BF4NO: C, 66.86; H, 8.14; Br, 0.0. Found:
C, 66.93; H, 8.19; Br, 0.0.
N-(4-Acetyl)benzyl-N,N,N-trimethylammonium Tetrafluorobo-
rate (2b). Prepared as 2d (83% yield); mp 152-53 °C. Anal.
Calcd for C12H18BF4NO: C, 51.67; H, 6.45; N, 5.02; F, 27.25.
Found: C, 51.72; H, 6.42; N, 5.08; F, 27.20.
N,N,N-Tributyl-N-(2-methylnaphthalene)ammonium Tetraflu-
oroborate (2c). Prepared as 2d (62% yield); mp 147-48 °C.
Anal. Calcd for C23H36BF4N: C, 66.88; H, 8.72; N, 3.39; F,
18.40. Found: C, 66.93; H, 8.70; N, 3.41; F, 18.37.
HPLC Analysis of the Borates. Typically, 1 M aqueous
HCl (3 drops) was added to the borate solution (25 µL), which
was then diluted to 5 mL with MeOH. This pretreated sample
was analyzed on a Hewlett-Packard 1050 HPLC equipped with
Nucleosil AB C18 column (15.0 cm × 4.6 mm, Altech) and a
UV absorbance detector set of 370 nm. The eluent was MeOH.
Acknowledgment. This work was supported by the National
Science Foundation (DMR-9013109) and by the Office of Naval
Research (N00014-93-1-0772). Support from the McMaster
Endowment and the Center for Photochemical Sciences is also
acknowledged with gratitude as well as helpful discussions with
Professor M. A. J. Rodgers and his students.
References and Notes
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41-(Bromomethyl)acetophenone. 41-Methylacetophenone
(3.0 g, 22.35 mmol), N-bromosuccinimide (4.38 g 24.60 mmol),
and AIBN (30 mg) in carbon tetrachloride (50 mL) was refluxed
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General Procedure for Photolysis. Preparative-scale pho-
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