Photochemistry and Photophysics of (1-Naphthoyl)diphenylphosphine Oxide

Article abstract of DOI:10.1021/jp991128+

(l-Naphthoyl)diphenylphosphine oxide (1) was synthesized and characterized and its photochemistry investigated using emission spectroscopy, pulse radiolysis, and nanosecond laser flash photolysis. Fluorescence quantum yields are low in aprotic polar and nonpolar solvents. In methanol, as a result of hydrogen-bonding, change of S₁ from an n,π* state to a π,π* state leads to the decrease in the rate constant for intersystem crossing and results, finally, in an increase in fluorescence. Preferential solvation was evaluated using the E_T indicator "Reichardt's dye" (RD). E_T^N values were determined by gradually increasing the concentrations of methanol in methanol/acetonitrile mixtures. Fluorescence quantum yields correlate with the E_T^N values. Photolysis of 1 yields diphenyl[l-naphthoyl)oxy]phosphine (6), formed mainly via cage recombination of radicals. No radicals were detected by either nanosecond laser flash photolysis or pulse radiolysis of 1 in aprotic solvents. However, photolysis in methanol yields radicals when 1 is excited at 266 nm. The phosphinoyl radical can be quenched by either methyl methacrylate (MMA) or oxygen (k_q = 5.0 × 10⁷ and 5.3 × 10⁸ M^-1 s^-1, respectively). Such radical generation most likely results from the singlet excited state.

Full text of DOI:10.1021/jp991128+

J. Phys. Chem. A 1999, 103, 7757-7765
7757
ARTICLES
Photochemistry and Photophysics of (1-Naphthoyl)diphenylphosphine Oxide
†
†
Ningning Zhao, Bernd Strehmel, Anthony A. Gorman, Ian Hamblett, and
Douglas C. Neckers*,‡
1
Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403, and
Chemistry Department, UniVersity of Manchester, Manchester, M13 9PL, U.K.
ReceiVed: April 2, 1999; In Final Form: June 11, 1999
(1-Naphthoyl)diphenylphosphine oxide (1) was synthesized and characterized and its photochemistry
investigated using emission spectroscopy, pulse radiolysis, and nanosecond laser flash photolysis. Fluorescence
quantum yields are low in aprotic polar and nonpolar solvents. In methanol, as a result of hydrogen-bonding,
change of S
1
from an n,π* state to a π,π* state leads to the decrease in the rate constant for intersystem
crossing and results, finally, in an increase in fluorescence. Preferential solvation was evaluated using the E
indicator “Reichardt’s dye” (RD). E
methanol in methanol/acetonitrile mixtures. Fluorescence quantum yields correlate with the E
T
N
T
values were determined by gradually increasing the concentrations of
N
T
values.
Photolysis of 1 yields diphenyl[1-naphthoyl)oxy]phosphine (6), formed mainly via cage recombination of
radicals. No radicals were detected by either nanosecond laser flash photolysis or pulse radiolysis of 1 in
aprotic solvents. However, photolysis in methanol yields radicals when 1 is excited at 266 nm. The phosphinoyl
7
8
radical can be quenched by either methyl methacrylate (MMA) or oxygen (k
M
q
) 5.0 × 10 and 5.3 × 10
-
1
-1
s , respectively). Such radical generation most likely results from the singlet excited state.
Introduction
obtain more accurate values for the kinetic behavior of the triplet
state of 1. Fluorescence and phosphorescence experiments have
considered the emission properties of the excited states of 1 as
well.
In this paper, we report the first example of an acyl phosphine
oxide, 1, that undergoes an efficient disappearance but yields
no radicals that can be detected either by laser flash photolysis
In the past few years, acyl and bis(acyl)phosphine oxides have
become important UV photoinitiators for the free-radical
polymerization of acrylates.2-16 Both aryl acyl and bis(acyl)-
phosphine oxides undergo rapid R-cleavage from the triplet
excited state producing both carbon- and phosphorus-centered
radicals.5
,11c,12,14,15
The radical species formed
5-16
have been
or pulse radiolysis. Compound 1 behaves differently from the
detected by laser flash photolysis using both infrared and UV
2-16
analogous phenyl derivatives
which are known to efficiently
detection,5
,13,16
time-resolved ESR, CIDNP, and picosecond
11
12
produce radicals. The triplet state is formed in benzene on
transient laser excitation as confirmed by pulse radiolysis
experiments. Rate constants for the reactions of the diphe-
nylphosphinoyl (4) and naphthoyl radical (5) with methyl
methacrylate (MMA) and O2 have been determined. Photo-
products obtained after irradiation of 1 have been isolated.
15
pump-probe spectroscopy. Thus 2,4,6-trimethylbenzoyl-
diphenylphosphine oxide (2)5
,12-14
and bis(2,6-dimethoxyben-
zoyl)-2,4,4-trimethylpentylphosphine oxide (3)1
2,14
are shown
to generate both carbon- and phosphorus-centered radicals by
a-cleavage of the triplet excited state.
Results and Discussion
Fluorescence and Phosphorescence. The fluorescence quan-
tum yield (φf) of 1 strongly depends on the nature of the solvent
(Table 1). The emission spectra of 1 observed in polar solvents,
for example, acetonitrile and methanol (Figure 1), clearly
demonstrate that fluorescence is much more intense in a
hydrogen-bonding solvent than in non-hydrogen-bonding sol-
vents.
Rate constants for intersystem crossing (kisc) in non-hydrogen-
bonding solvents, estimated according to eqs 1 and 2, indicate
Global analysis has become a standard technique for the
examination of fluorescence in biological systems but not for
the evaluation of flash photolysis data. We have used this to
17
an efficient transition from singlet to triplet state occurs because
0
†
‡
of the relatively low values of k in aprotic solvents (Table
University of Manchester.
E-mail: neckers@photo.bgsu.edu.
f
18,19
1).
1
0.1021/jp991128+ CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/10/1999

7758 J. Phys. Chem. A, Vol. 103, No. 39, 1999
Zhao et al.
TABLE 1: Photophysical Properties of 1 in Different
Solvents
TABLE 2: Nanosecond Laser Photolysis Data of 1
benzene
acetonitrile
methanol
25
2.2 × 10
cyclohexane benzene acetonitrile methanol
τ
(µs)a
22
18
1.3 × 10
T
a
-3
-3
-4
k (O ) M s-1 a
-1
9
9
9
φf
ꢀ
0
f
4.2 × 10
9.4 × 10
8.1 × 10
0.21
1.2 × 10
q
2
max (n,π*) (M cm-1) 451
k (s
-
1
389
532
ꢀ (L mol cm ) b
-1
-1
T
5180
6
9
6
8
6
9
7f
8f
b
-1 b
4
.5 × 10
3.9 × 10
5.3 × 10
∼10
∼10
)
φT
E
T
(kcal/mol)b
0.97 ( 0.1
54.8 ( 0.1
-
1 c
kisc (s
)
1.0 × 10
4.0 × 10
6.3 × 10
d
τp (ms)
83
a
e
T
τ , triplet decay time in solvents at room temperature obtained from
φp
0.033
64.2
b
laser flash photolysis and fitted by global analysis.
T
ꢀ , extinction
Es (kcal/mol)
64.2
64.2
87
coefficients of triplet-triplet absorption by sensitized pulse radiolysis.
a
f
φ , fluorescence quantum yields in different solvents (fluorescence
φ , benzophenone in benzene as a standard. E , lowest triplet energy
T
T
2
2,23
standard
9,10-diphenylanthracene φ
f
) 0.95 (CH
3
CN), φ
f
) 0.90
determined from triplet-triplet energy transfer equilibrium with chry-
sene.
(
cyclohexane)). b k , estimated from absorption spectra 10 ꢀmax(n,π*).
0
4
f
c
k
isc: intersystem crossing rate constant calculated by the relation kisc
0
d
)
f
T f
φ k /φ . Phosphorescence decay time in EPA (ether:isopentane:
state resulting in different values. We also observed an increase
of 23 kcal/mol in singlet excited-state energy from switching
from an aprotic medium (64 kcal/mol) to a protic one (87 kcal/
e
ethanol ) 5:5:2) at 77 K. Phosphorescence quantum yield in EPA
2
4
f
(
p
standard: benzophenone in EPA φ ) 0.74 ( 0.02). This number
is an estimate and was calculated from τ
s
∼ 8 ns in methanol solution
mol).
of 1 at 266 nm excitation by nanosecond flash photolysis.
0
f
Thus, the rate constants for fluorescence, k , indicate it to be
19
dependent on surroundings. A further explanation is based on
excited state configuration. Since the change of the solvent from
an aprotic to a protic one results in a change of the nature in
the excited state, we would expect different transitions.20 In non-
hydrogen-bonding solvents, efficient spin-orbit coupling be-
1
3
tween (n,π)* and (π,π)* states results in rapid intersystem
crossing. In contrast, inefficient intersystem crossing occurs
1
3
between (π,π)* and (π,π)* in protic solvents. Therefore,
fluorescence competes with the reduced efficiency for inter-
system crossing, which leads finally to an increase in φf. Similar
effects were previously reported for naphthaldehydes.21
Groups attached to the C-P single bond in 1 can freely rotate.
As a result, at least two ground state conformations, for example
S-cis and S-trans, may exist in equilibrium at room temperature
because there is a low energy barrier. In the excited state of 1
this bond property may change from single bond to double bond
in a manner similarly reported for polyenes. Since rotation about
a double bond requires a higher activation energy than rotation
about a single bond, the energy barrier to interconversion of
the conformers by single bond rotation becomes higher in their
respective excited states, and an equilibrium process forming
the isomers of 1 becomes impossible. Therefore, different
conformers of the ground state can be excited to different singlet
excited states and the latter are not interconvertable.
Figure 1. (a) Fluorescence emission (1 and 2) and excitation (3 and
-
5
4
) spectra of 1 in acetonitrile (1.1 × 10 M) at room temperature (1;
λ
ex ) 322 nm; 2, λex ) 338 nm; 3, λem ) 379 nm; 4, λem ) 400 nm)
Excitation at different wavelengths into the mixed n,π* and
π,π* band in aprotic polar solvents (i.e., acetonitrile) clearly
provides evidence for two different emission spectra (e.g. curves
and (b) Fluorescence emission (5 and 6), delayed fluorescence (7) and
-
5
excitation spectra of 1 (8) in methanol (1.4 × 10 M) at room
temperature (5, λex ) 325 nm; 6, λex ) 296 nm; 7, after 1 µs delay, λex
1
and 2 in Figure 1), caused by depopulation of two different
)
2
296 nm; 8, λem ) 372 nm). Left axis top: 3 and 4; right top: 1 and
; left bottom: 8; right bottom: 5, 6, and 7; the intensity for delayed
excited singlet states. In contrast, changing the solvent to
methanol results, as reported above, in an increase of the energy
of excited singlet state and therefore, presumably, to the
excitation of one species.
fluorescence, 7, is multiplied by a factor of 450.
0
f
2
4
k ) ꢀmaxνmax ν1/2 = 10 ꢀ
(1)
max
The n,π* band in the absorption spectrum of 1 can easily be
distinguished from the π,π* band, Figure 2. In aprotic solvents
such as cyclohexane, benzene, and acetonitrile, one observes a
band of low intensity as a long tail around 400 nm that shifts
to the blue as the solvent polarity increases. In methanol, this
band disappears likely because of the hydrogen bonding that
ꢀ
max is the molar absorption coefficient of n,π*, νmax maximum
of absorption energy, and ν1/2 half-width of the absorption band.
0
kisc ) φ (k /φ )
(2)
T
f
f
2
5
leads to a further blue shift.
φT is obtained from laser flash photolysis; (see Table 2), and in
aprotic solvents that φT are assumed the same).
Compound 1 exhibits phosphorescence emission with vibra-
tional fine structure in EPA (ether:isopentane:ethanol ) 5:5:2)
glass at 77 K upon excitation at 423 nm (Figure 3). The
difference between the (0,0) and (0,1) emission bands is 1456
cm , corresponding approximately to a stretching mode in the
IR spectrum. The (0,0) emission band at 528 nm corresponds
In hydrogen-bonding solvents such as methanol, an increase
0
f
of k and a decrease of kisc is observed. This can be understood
-
1
if one considers a change of singlet excited states in different
solvents with the nature and population of the lowest singlet

(1-Naphthoyl)diphenylphosphine Oxide
J. Phys. Chem. A, Vol. 103, No. 39, 1999 7759
ETN values27 were measured in mixtures of acetonitrile/
N
methanol, Figure 5. The relationship between the ET values
and the molar fraction of solvent x2 is given by eq 3, where
N1
N2
N
ET and ET are the ET values of the pure solvent and the
index 1 refers to acetonitrile and the index 2 to methanol. The
ratio f2/f1 is equal to the preferential solvation coefficient and
indicates the preferred solvation of the solute RD in the mixture.
This ratio further reports which solute possesses the better
solubility in each respective solvent. When this value approaches
unity, there is no preferred solvation; the solute is equally soluble
in either solvent.
Figure 2. Ground state absorption spectra of 1 in different solvents
-4
(
1.0 × 1.0 M) Inset: ground state absorption spectra magnified around
4
00 nm in cyclohexane, benzene, and acetonitrile 1.0 x 10-3 M).
f2
N1
N2
N1
ET + x2 ET - ET
(
)
f1
N
T
E )
(3)
f2
1
+ x
- 1
2
(
)
f1
2
8
Using Marquardt’s nonlinear fit procedure there is 13.4 times
relative preferential solvation for RD in methanol in comparison
to acetonitrile (Figure 5). Furthermore, a satisfactory correlation
was found between the fluorescence quantum yields of 1
measured in mixtures of methanol and acetonitrile, and the
N
N
Figure 3. Normalized phosphorescence of 1; excitation (1) λem ) 529
nm and emission (2) at λex ) 423 nm spectra in EPA (ether:isopentane:
ethanol ) 5:5:2 in volume) at 77 K.
solvent polarity parameter ET (inset Figure 5). The ET values
contain all parameters, including the effect of hydrogen bonding,
that describe the solvation of solutes on a microscopic scale.
Therefore, a linear change of this parameter leads to a change
of the quantity φf. Excitation at either 338 or 296 nm yields
similar results. We conclude, therefore, that 1 is preferentially
solvated by methanol rather than by acetonitrile. After saturation,
the quantum yield increases more slowly, Figure 4.
to a triplet energy 54.2 kcal/mol. The phosphorescence decay
time is 83 ms in EPA glass, which is in the range for compounds
3
possessing a lowest (π,π)* triplet excited state. It is well
accepted that the singlet-triplet splitting, ∆EST, of the n,π* state
is smaller than that of π,π* states. The experimental value ∆EST
for 1 is around 10 kcal/mol because of n,π nature of lowest
singlet excited state.18
Nanosecond Laser Flash Photolysis. Laser flash photolysis
of 1 (excitation 355 nm) shows similar transient absorption
spectra between 280 nm-700 nm both in benzene and in CH3-
CN (Figure 6). The peak from 350 nm to shorter wavelengths
evidences bleaching where 1 possesses strong ground state
absorption. Surprisingly, the transient species between 370 and
With the gradual addition of methanol to a solution of 1 in
acetonitrile the intensity of the absorption at 338 nm decreases
while that of the absorption at 296 nm increases. This indicates
that the nature of the singlet excited state changes gradually
from the non-hydrogen-bonding solvent to the hydrogen-bonding
solvent. Thus, the presence of the hydrogen-bonding solvent,
methanol, causes an increase in π,π* contribution in S1, and
7
00 nm with maximum absorptions at 380 and 580 nm decay
on a microsecond time scale with rather different kinetics. At
low laser power, the transient species at 580 nm decays
monoexponentially. We attribute this transient to the triplet of
2
3
also increases φf by a factor of approximately 10 -10 . From
plots of φf vs [MeOH], a critical point is observed when the
excitation is carried out either at 296 nm or at 338 nm, Figure
1
. Decays at the different wavelengths were fitted by global
29
analysis with monoexponential kinetics. Transient absorption
spectra (26 time profiles between 450 and 700 nm) were linked
to one data set where only the preexponential factors were a
function of the registration wavelength and the calculated decay
times were equal for each wavelength. Figure 6 shows a
representative decay for 1 in benzene at a registration wave-
length of 580 nm. In general, the change in optical density at
each wavelength can be described by eq 4, though decay times
are almost the same at each wavelength.
4
. Beyond this critical point, the fluorescence quantum yields
level off. Two lines with different slopes are obtained, and these
have an intersection point at about 5 wt % methanol.
A more detailed understanding of such solvent effects is
2
6
obtained using the solvent indicator Reichardt’s dye (RD) to
evaluate the solvent polarity parameter ETN in the mixtures of
acetonitrile/methanol.

7760 J. Phys. Chem. A, Vol. 103, No. 39, 1999
Zhao et al.
Figure 4. Relationship of fluorescence quantum yield in acetonitrile/methanol mixture vs methanol wt %.
Figure 5. Nonlinear relationship of E N
values of Reichardt’s dye vs methanol (wt %) acetonitrile/methanol binary system with preferential solvation.
T
N
Inset: The relationship of fluorescence quantum yield in acetonitrile/methanol mixture solvent with E
T
(∇, λex ) 338 nm; (b) λex ) 296 nm).
n
the triplet state. There is a long-lived species in that region fitted
by a combination of first- and second-order kinetics because of
the overlap of the triplet absorption and that of the naphthoyl
radical. The naphthoyl radical is present in small amounts and
is likely generated from the singlet excited state.
The addition of either methyl methacrylate (MMA) or oxygen
reduced the decay time of the naphthoyl radical and results in
pseudo-first-order decay kinetics. Bimolecular quenching con-
stants (kq) for the naphthoyl radical were obtained from plots
of the pseudo-first-order rate constants for the decay of transient
absorption (kdecay) vs the concentration of added quenchers:
4
∆
OD(t,λ ) ) BG(λ ) + R (λ ) exp(-4.5 × 10 t) (4)
∑
i)1
i
i
1
i
where BG ) background, R are preexponential factors.
Evidence for formation of the triplet states of 1 in benzene
and acetonitrile was obtained from O2 quenching experiments,
sensitized pulse radiolysis, and from triplet-triplet energy
transfer experiments. Sensitized pulse radiolysis showed that
the triplet absorption covered the region from 370 to 700 nm.
No radical species were detected since none of the decay curves
behaved according to a second-order kinetic law.30 The triplet
is quenched by oxygen, yielding bimolecular quenching con-
stants that are on the order of one-ninth diffusion control. The
data obtained are compiled in Table 2.
kdecay ) k + k [Q]
(5)
0
q
O2 and MMA quench this radical with bimolecular rate
9
6
-1 -1
The species at 380 nm decays much more slowly than does
constants 1.1 × 10 and 2.9 × 10 M s , respectively.

(1-Naphthoyl)diphenylphosphine Oxide
J. Phys. Chem. A, Vol. 103, No. 39, 1999 7761
-
4
Figure 6. Time profile of transient absorption of 1 in benzene (1.02 × 10 M) monitored at 580 nm with fit residuals on the top. Inset: time-
resolved transient absorption spectra of 1 in benzene at 355 nm excitation: after several decays (in µs).
the radicals come from the singlet excited state. The simulta-
neous growing in of radical absorption from the triplet state is
not observed. Although the small S1-T1 splitting agrees with
the presence of an n,π* singlet, it may be that the triplet has a
large degree of π,π* character, possibly indicated by the long
phosphorescence lifetime. This electronic configuration and the
lower electronic energy relative to the corresponding phenyl
derivatives may limit carbon-phosphorus bond cleavage.
In addition, a triplet quantum yield of almost unity may
explain the inefficient formation of radicals since the formation
of the triplet can be considered as an effective deactivation
pathway of the singlet excited state that reduces the tendency
for homolytic bond cleavage.
Surprisingly, laser flash photolysis of 1 in methanol after
excitation at λex ) 266 nm gave a rather different transient
absorption spectrum from those obtained in either benzene or
acetonitrile (Figure 8).
Three transient signals were observed in methanol as (i) an
absorption maximum at 330 nm, (ii) an emission maximum at
Figure 7. Dependence of the triplet-triplet absorption of 1 at 600
3
70 nm, and (iii) a further absorption maximum at 450 nm.
nm and benzophenone at 532 nm; optically matched deaerated benzene
-1
-1
Second-order kinetics were observed for the 330 nm absorp-
solutions (OD355 ) 0.5), laser power; (ꢀBP(532) ) 8920 L mol cm ).
tion (Table 2). This was assigned to the phosphinoyl radical
It was possible to determine the triplet quantum yield, φT, of
34
using the known extinction coefficient giving a rate constant
1
using the T-T extinction coefficient at 600 nm obtained from
-1 -1
for decay of 70.5 M s . Bimolecular rate constants for
sensitized pulse radiolysis by laser excitation at 355 nm of
optically matched benzene solutions (OD355 ) 0.5) of 1 and of
benzophenone (BP). BP is a known standard having a φT of
quenching of the phosphinoyl radical by MMA and O2 were
7
8
-1 -1
5
.0 × 10 and 5.3 × 10 M s , respectively. These were
obtained by eq 5 using a similar procedure to that used with
the naphthoyl radical.
3
1,32
33
unity
(eq 6). Plots of the intensities of the triplets of 1
and BP (Figure 7) as a function of laser energy extrapolated to
time zero gave slopes which, together with the known extinction
coefficients, gave a φT for 1 of 0.97( 0.1. This is clearly in
agreement with the very low fluorescence quantum yield in this
solvent.
The absorption of the radical around 330 nm was not expected
because, in aprotic solvents, we observed few radicals in the
transient absorption spectra. As can be seen from Table 1, the
8
-1
intersystem crossing rate kisc in methanol is slow (∼10 s )
0
f
7
-1
and fluorescence rate k is fast (∼10 s ) in comparison with
9
6
-1
∆
OD (1)ꢀ (532)
that in other solvents (∼10 and 10 s ). As a result, in
T
BP
φT(1) )
φT(BP)
(6)
0
methanol kisc and kf become competitive pathways. Since
∆
OD (BP)ꢀ (600)
T 1
radical absorption rises at the same time as the absorption of
the triplet states, the formation of the radical most likely occurs
more often from the singlet excited state.
The results of the triplet quantum yield measurements and
transient absorption spectra of 1 confirm the assumption that

7762 J. Phys. Chem. A, Vol. 103, No. 39, 1999
Zhao et al.
Figure 9. Transient absorption measured at 580 nm after irradiation
-
3
-1
of a deaerated benzene solution of Ch (9.8 × 10 mol L ) and 1 (2.2
-
4
-1
x 10 mol L ) with a 20 ns electron pulse, 2 µs/division, 2.0%
absorption/division, A
absorption by 1*).
0
) 0.050, A ) 0. 035 (0.029 corrected for
e
3
-
2
-1
(
1
B) solution of 2-acetonaphthone (2-A; 10 mol L ) containing
-
4
-1
(10 mol L ) led to formation of a species with identical
spectral characteristics to that observed by laser flash photolysis
with λmax ∼ 580-600 nm. The species decayed cleanly with a
first-order rate constant at low radiation dose ∼55 µs. This
confirms the identity of the 580 nm species as the triplet state
of 1, produced here via the sequence of events summarized in
3
5
eqs 7-10. In addition, this experiment shows that the triplet
-
1
energy of 1 is below that of 2-A (59.0 kcal mol ).
Figure 8. Time-resolved transient absorption spectra of 1 in methanol
1
1
-
5
B* + 2-A f B + 2-A*
(7)
(8)
(9.29 × 10 M) at 266 nm excitation: after several decays (in µs).
3
3
B* + 2-A f B + 2-A*
The transient absorption at 450 nm reveals monoexponential
1
3
kinetics. Its rate of quenching by O2 is on the order of one-
ninth diffusion control quenching, indicating it to be the triplet.
It has a similar decay to that observed in benzene and acetonitrile
2-A* f 2-A*
(9)
3
3
2
-A* + 1 f 2-A + 1*
(10)
(Table 2). The transient species obtained at 370 nm is located
The extinction coefficient of the triplet state of 1 at 580 nm
in the same region as the steady state fluorescence in methanol
and there is no ground state absorption in this region so this
signal is assigned to fluorescence. After a few hundred
nanoseconds, the emission is still observed in the transient
absorption spectra. Before 6 µs, this emission grows in; after 6
µs, it disappears and decays at the same time as the triplet. We
assign this transient as delayed fluorescence. Triplet-triplet
annihilation produces the singlet excited state, which then gives
this delayed fluorescence which is probably of the P-type. This
is supported by time-resolved delayed fluorescence using the
Spex and obtained after 1 µs delay (Figure 1b). This emission
falls in the same region as regular fluorescence. Its lifetime is
calculated as 17 µs after monoexponential fitting of the decay
profile at 370 nm. This is less than the lifetime of triplet. This
postulate is further confirmed by emission spectra obtained by
nanosecond photolysis without analyzing light. The species
decay monoexponentially with a lifetime 13.3 µs, which is
nearly half the triplet lifetime.
has been determined using the technique of Bensasson and
Land. In these experiments two solutions of 2-A as above,
3
1
one with and one without 1, were subjected to pulse radiolysis.
3
3
The 2-A* and 1* yields in terms of OD at their respective
maxima, 430 and 580-600 nm, were obtained by extrapolating
their decays back to time zero. The rate constant for energy
9
-1 -1
transfer (cf. eq 10), determined to be 6.0 × 10 L mol
s
in
a separate experiment, and the rate constant for natural decay
3
4
-1
of 2-A*, 2.0 × 10 s , showed that >98% of the latter were
scavenged by 1. The ratio of the extrapolated OD’s and the
-
1
known T-T extinction coefficient of 2-A (10 300 L mol
cm ) thus gave a value for 1 of 5000 L mol cm at 580
nm. Repetition using naphthalene (N), T-T extinction coef-
ficient 13 200 L mol cm at 425 nm, instead of 2-A gave
a value of 5360 L mol cm , the average value being 5180 L
-
1 31
-1
-1
-
1
-1
32
-
1
-1
-
1
-1
mol cm .
We have routinely determined triplet energies (ET) in benzene
by establishing equilibria for triplet energy transfer with a
sensitizer of known ET (cf. ref 35 and references therein). A
particular advantage of this method is that large errors in K
result in small errors in ∆G due to their exponential relationship.
The kinetics at 370 nm show a fast growth of a transient
absorption signal followed by a slow decay to a long-lived
species. This absorption signal consists of complicated processes
because of the overlap of several species including the phos-
phinoyl radical, the triplet, and the fluorescence emission. The
slow decay kinetics at 370 nm was fitted to a decay time of 21
µs. This has the similar decay time as the triplet at 450 nm.
-
1
The two triplets need to be within ∼4.0 kcal mol in energy
for these experiments to work and the energy must be deposited
initially within the highest energy sensitizer, i.e.; it must ideally
35
have by far the higher concentration by a factor of 50 or more.
We have successfully established such an equilibrium between
Pulse Radiolysis in Benzene. Pulse radiolysis in benzene is
an excellent technique for producing triplet excited states of
solute molecules in high yield.35 Thus, irradiation of a benzene
chrysene (Ch) and 1 (cf. eq 11 and Figure 9), the former clearly
3
3
Ch* + 1 h Ch + 1*
(11)

(1-Naphthoyl)diphenylphosphine Oxide
J. Phys. Chem. A, Vol. 103, No. 39, 1999 7763
3
SCHEME 1: Photochemistry of 1 (5 × 10- M) in
The contradiction is that 1 has a highly efficient disappearance
upon irradiation but yields no radicals that can be detected by
laser flash photolysis. Since it also proves to be a poor
Acetonitrile at 355 nm Irradiation
3
8
photoinitiator, this further implies that few radicals are
generated. The efficient disappearance of 1 is attributed to the
photoreaction of both singlet and triplet excited states. That is,
the photoproducts may still arise from triplet excited state but
do so by rearrangement in solvent cage. Major product 6
suggests the primary step may be carbon-phosphorus bond
cleavage, but that subsequent recombination dominates in the
solvent cage rather than cage escape. Thus, one recombination
process gives both the starting material and product 6. Such a
recombination process commonly occurs with benzene deriva-
tives 2 and 3, although both the carbonyl radical and phosphi-
noyl radical can be detected by laser flash photolysis of UV
and IR.
Conclusion
The photochemistry of 1 is dominated by a solvent cage
process. Even though no radicals are detected except in
methanol, the triplet has been detected by nanosecond laser flash
photolysis as well as pulse radiolysis, and further characterized
by triplet-triplet energy transfer. Hydrogen bonding has a
dramatic effect on the fluorescence spectrum of 1 with a high
fluorescence quantum yield being observed only in methanol.
This is attributed to the elevation of the singlet excited state
from n,π* to π,π* which then leads to a decreasing rate constant
of intersystem crossing. The solvent indicator Reichardt’s dye
gives the relative preferential solvation in methanol as being
having the larger triplet energy. Thus, pulse radiolysis of a
deaerated benzene solution of Ch (ET ) 56.6 kcal mol ; 9.8
-
1
13.4 times larger than that observed in acetonitrile, and suggests
-
3
-1
-4
-1
×
10 mol L ) and 1 (2.2 × 10 mol L ) resulted in
the dominant nature of methanol solvation. All of these facts
clearly demonstrate that the extension of the chromophoric
system alone in aryl-diphenylphosphine oxides is not sufficient
in order to produce compounds with high quantum yields for
R-cleavage.
3
“
immediate” formation of Ch*, monitored at 580 nm. Since
* also absorbs at this wavelength but significantly less than
3
1
-
1
-1
chrysene (29 800-5180 L mol cm ) a simple correction
could be made. The decay (Figure 9) was biexponential, showing
fast decay to equilibrium followed by slow bleeding of that
equilibrium. Extrapolation of the fast and slow decays to time
zero gave optical densities corresponding to the concentrations
Experimental Part
3
of Ch* initially, A0, and at equilibrium, Ae (after correction).
Materials and Solvents. (1-Naphthoyl)diphenylphosphine
Because decay to equilibrium is fast relative to subsequent decay
39
oxide was prepared according to a literature procedure and
(cf. Figure 9), the equilibrium constant is given by K ) [CH]-
recrystallized from cyclohexane before use.
[
0
A0 - Ae]/[1][Ae] ) 22 ( 5, corresponding to ∆G ) -1.8 (
Unless mentioned, all other chemicals and solvents (spec-
trophotometric grade) were obtained from Aldrich and used as
received.
-
1
.2 kcal mol . The standard assumption of no entropy change
-
1
gives an ET value for 1 of 54.8 ( 0.1 kcal mol . This is in
excellent agreement with the value of 54.2 kcal mol obtained
-1
General Methods. Melting points were determined with a
Thomas-Hoover capillary melting point apparatus and are
uncorrected. NMR spectra were obtained either with a Varian
Gemini 200 NMR spectrometer or Varian Unity Plus 400 NMR
spectrometer. Chemical shifts are in ppm with TMS as the
from phosphorescence data.
Steady State Photolysis and Product Analysis. Steady state
irradiation of 1 in cyclohexane, benzene, or acetonitrile causes
ground state bleaching and an isosbestic point in the UV spectra
at 312 nm. Quantum yields for the disappearance of 1, φd,
determined by UV absorption spectroscopy using a standard
solution 0.1 M benzophenone with 0.1 M benzhydrol in benzene
with photoreduction quantum yield of 0.68,36 possess consider-
ably higher values in benzene (0.29) and acetonitrile (0.57).
Photolysis of an aerated (0.005 M) acetonitrile solution of 1
gives the products shown in Scheme 1. Diphenyl[1-naphthoyl)-
oxy]phosphine (6)3 is the major product. 1-Naphthaldehyde (7)
and diphenylphosphine oxide (8) are minor products. Each was
identified by comparison of NMR and GC/MS analysis with
authentic samples. The structures of two isomeric diphosphorus
1
13
internal standard ( H NMR and C NMR) or H3PO4 as the
31
external standard ( P NMR). GC/MS and MS spectra were
obtained on a Hewlett-Packard 5988 mass spectrometer coupled
to an HP5880A GC, interfaced to an HP 2623A data processor.
HPLC were taken by a Hewlett-Packard high-performance liquid
chromatograph with a HP 1046A UV-absorbance detector.
Quantitative HPLC analysis and separation was carried out on
a RP-18 column and a mobile phase of a mixture of acetonitrile
and water. Infrared spectra were taken with a Galaxy series 6020
FTIR spectrometer. Thin-layer chromotography was performed
with Whatman silica gel coated TLC plates. Aldrich silica gel
60 Å (70-270 mesh) was used in column chromatography.
Elemental analysis was performed by Atlanta Microlab, Inc.
7
3
1
photoproducts (9, 10) have been tentatively assigned by
NMR spectroscopy.12
P

7764 J. Phys. Chem. A, Vol. 103, No. 39, 1999
Zhao et al.
Absorption spectra were recorded using a Hewlett-Packard
452A diode array UV-vis spectrophotometer. Fluorescence
by the McMaster Endowment to N. Zhao is also gratefully
acknowledged.
8
and phosphorescence spectra were measured using a Spex
Fluorolog 2 spectrophotometer. All spectra were measured using
identical conditions of the instrument. Phosphorescence experi-
ments were performed at 77 K in EPA (ether:isopentane:ethanol
References and Notes
(
(
(
(
1) Contribution 383 from the Center for Photochemical Sciences.
2) Jacobi, M.; Henne, A. J. Radiat. Curing 1983, 10, 16.
3) Jacobi, M.; Henne, A. Polym. Paint Color J. 1985, 175, 636.
4) Oldering, P. K. T. Chemistry and Technology of UV and EB
)
5:5:2) under argon. The source was a Xe lamp 3.0 ms pulse
passed through monochromator. The quantum yields of fluo-
rescence were determined relative to 9,10-diphenylanthracene.
Formulation for Coatings, Inks and Paints; SITA Technology Ltd.: London,
1991; Vol. 3, 168.
(
1-Naphthoyl)diphenylphosphine Oxide (1).39 (55%
1
(5) Sumiyoshi, T.; Schnabel, W.; Henne, A.; Lechtken, P. Polymer
985, 26, 141.
yield): yellowish solid mp 157-158 °C; H NMR (200 MHz,
CDCl3): δ 7.43-7.65 (m, 10 H), 7.86-7.92 (m, 3 H), 7.945
1
(6) Sumiyoshi, T.; Schnabel, W. Makromol. Chem. 1985, 186, 1811.
(
8
d, J ) 6.2 Hz, 1 H), 8.07 (d, J ) 8.0 Hz, 1 H), 8.881 (d, J )
.8 Hz, 1 H), 9.294 (d, J ) 6.6 Hz); 13C NMR (50 MHz,
CDCl3): δ 124.749, 125.350, 126.788, 128.498, 128.753,
(7) Majima, T.; Schnabel, W. J. Photochem. Photobiol. A: Chem. 1991,
58, 239.
(8) Sumiyoshi, T.; Schnabel, W.; Henne, A. J. Photochem. 1986, 32,
19.
1
1
1
29.345 (d, J ) 6.35 Hz), 130.136 (d, J ) 7.31 Hz), 131.337,
(
91.
9) Sumiyoshi, T.; Schnabel, W.; Henne, A. J. Photochem. 1986, 32,
1
31.865 (d, J ) 9.1 Hz), 132.329 (d, J ) 2.75 Hz), 133.293 (d,
3
1
J ) 50 Hz), 133.848, 135.886, 205.95 (d, J ) 78.25 Hz);
P
(
63.
10) Sumiyoshi, T.; Schnabel, W.; Henne, A. J. Photochem. 1985, 30,
NMR (CDCl3): δ 23.532. MS: m/e 356(4), 327(0.12), 249-
0.14), 201(8.8), 183(1.5), 155(100), 127(60.7), 123(0.7);
C23H17O2P requires C: 77.52; H: 4.81 (found C: 77.26; H:
.85).
Steady State Irradiation. (1-Naphthoyl)diphenylphosphine
(
11) (a) Roberts, B. P.; Singh, K. J. J. Organomet. Chem. 1978, 159,
(
3
1. (b) Kamachi, M.; Kuwata, K.; Sumiyoshi, T.; Schnabel, W. J. Chem.
Soc., Perkin Trans. 2, 1988, 961. (c) Turro, N. J.; Khudyakov, I. V. Chem.
Phys. Lett. 1992, 193, 546. (d) Kajiwara, A.; Konishi, Y.; Schnabel, W.;
Kuwata, K.; Kamachi, M. Macromolecules 1993, 26, 1656. (e) Kamachi,
M.; Kajiwara, A.; Saegusa, K.; Morishiwa, Y. Macromolecules 1993, 26,
7369.
4
oxide was dissolved in acetonitrile in Pyrex test tubes, sealed
with a rubber septum, bound by sticky Parafilm, and degassed
by bubbling dry argon gas through the solution for 10-15 min.
The solution was irradiated at 300-400 nm for 5 h in a Rayonet
RPR 100 photoreactor, equipped with 16 350 nm F8T5-BLB
UV lamps. Products were isolated by chromatography on silica
gel with hexanes-ethyl acetate (4:1 to 1:6) as the eluent they
were identified by comparison with authentic samples or
characterized by spectral means.
(12) Kolezak, U.; Rist, G.; Dietliker, K.; Wirz, J. J. Am. Chem. Soc.
1
996, 118, 6477.
13) Sluggett, G. W.; Turro, C.; George, M. W.; Koptyug, I. V.; Turro,
N, J. J. Am. Chem. Soc. 1995, 117, 5148.
14) Sluggett, G. W.; McGarry, P. F.; George, M. W.; Koptyug, I. V.;
(
(
Turro, N, J. J. Am. Chem. Soc. 1996, 118, 7367.
(15) Jockusch, S.; Koptyug, I. V.; McGarry, P. F.; Sluggett, G. W.;
Turro, N. J.; Watkins, D. M. J. Am. Chem. Soc. 1997, 119, 11495.
(
16) (a) Baxter, J. E.; Davison, R. S. Macromol. Chem. Rapid Commun.
987, 8, 311. (b) Baxter, J. E.; Davison, R. S. Macromol. Chem. 1988,189,
2769.
1
Laser Flash Photolysis. Nanosecond laser flash photolysis
experiments were carried out on a setup described by Ford and
Rodgers using a Q-switched Nd:YAG laser as a pump light (355
(17) Beechem, J. M.; Gratton, E.; Amelot, M.; Knutson, J. R.; Brand,
L. In Topics in AdVanced Fluorescence Spectroscopy; Lakowicz, J. R., Ed.;
Plenum Press: New York, 1991; Vol. 2, p 241.
40
nm; 60 mJ/pulse; 7 ns pulse or 266 nm; 5 mJ/pulse; 7 ns pulse).
(18) Turro, N. J., Ed., Modern Molecular Photochemistry; The Benjamin/
Argon, oxygen, or air was bubbled continuously through the
sample solution during the measurements.
Cummings Publishing: Menlo Park, CA, 1978.
(
19) The origin of eq 1 belongs to the Strickler-Berg treatment. We
used eq 1 to discuss differences in our experiments, and this showed that
the kf data obtained in aprotic solvents are approximately of the same
Pulse Radiolysis. Single-wavelength experiments were ob-
tained essentially as described with the exception that data were
collected by means of a Tektronix TDS 754C digital storage
oscilloscope. Analytical routines being performed on a DAN
0
order. Data are not available for methanol because the molecule is
insufficiently solvated. The original reference can be found in: Strickler,
S. J.; Berg, R. J. J. Chem. Phys. 1962, 37, 814.
(20) El-Sayed, M. A. J. Chem. Phys. 1963, 38, 2834.
4
86 PC using in-house software.41
(21) Kitamura, M.; Baba, H. Bull. Chem. Soc. Jpn. 1975, 48, 1191.
The multiwavelength system was essentially as described, but
(22) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry, 2nd ed.; Marcel Dekker: New York, 1993.
with the prototype acrylic lens rod and optical fibers replaced
by quartz equivalents. Analytical software was written in-house
and routines performed on a DAN 486 PC.42
(23) Sanyo, H.; Hirayama, F. J. Phys. Chem. 1983, 87, 83.
(24) Scaiano, J. C., Ed. Handbook of Organic Photochemistry; CRC
Press: Boca Raton, FL, 1989; Vol. 1, p 237.
(
(
(
(
25) Breadley, G. J.; Kasha, M. J. Am. Chem. Soc. 1955, 77, 4462.
26) Reichardt, C. Chem. ReV. 1994, 94, 2319.
27) Bosch, E.; Roses, M. J. Chem. Soc., Faraday Trans. 1992, 88, 3541.
28) (a) Marquardt, D. W. Soc. Ind. Appl. Math. 1963, 11, 431. (b)
Acknowledgment. We thank the National Science Founda-
tion, Division of Materials Research (NSF 9526755), and the
Office of Naval Research Polymers Program (N00014-97-1-
Levenberg, K. Q. Appl. Math. 1944, 2, 164.
29) The program IGOR Pro 3.11 from Wavemetrics Inc. was used for
global analysis.
0
834) for financial support of this work. Conversations with
(
Prof. G. S. Hammond and Prof. M. A. J. Rodgers are gratefully
acknowledged. The authors also gratefully acknowledge the use
of transient facilities from the laboratories of Professor M. A.
J. Rodgers. Pulse radiolysis experiments were performed at the
Paterson Institute for Cancer Research Free Radical Research
Facility, the Christie Hospital NHS Trust, Manchester. The
Facility is funded under the European Commission TMR
PROGRAMMEsACCESS TO LARGE SCALE FACILITIES,
Grant ERBFMGECT 950084. A research fellowship granted
(
30) According to the second-order kinetic law where 1/c ) (1/c0) +
kt, our data were transformed such that they can be plotted according to
this treatment. It is easier to distinguish if the data belong to a second-
order kinetic law or not because this is a linear function.
(31) Gorman, A. A.; Hamblett, I.; Lambert, C.; Prescott, A. L.; Rodgers,
M. A. J.; Spence, H. M. J. Am. Chem. Soc. 1987, 109, 3091.
(
32) Bensasson, R. V.; Land, E. J. Photochem. Photobiol. ReV. 1978,
, 163.
33) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15(1),
3
(
1.

(1-Naphthoyl)diphenylphosphine Oxide
J. Phys. Chem. A, Vol. 103, No. 39, 1999 7765
(
34) Sumiyoshi, T.; Schnabel, W. J. Photochem. 1986, 32, 119.
(39) (a) Ger. Offen. 2830927 (1980), BASF AG.; invs.: Lechtken, P.;
Buethe, I.; Hesse, A. (b) Berlin, K. D.; Taylor, H. A. J. Am. Chem. Soc.
1964, 86, 3826.
(35) Gorman, A. A.; Hamblett, I.; Irvine, M.; Raby, P.; Standen, M. C.;
Yeates, S. J. Am. Chem. Soc. 1985, 107, 4404
(
(
36) Moore, W. M.; Ketchum, M. J. Am. Chem. Soc. 1962, 84, 1368.
37) Brierley, J.; Dickstein, J. I.; Trippett, S. Phosphorus, Sulfur Relat.
(
40) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1994, 98, 3822.
41) Gorman, A. A.; Hamblett, I. Chem. Phys. Lett. 1983, 97, 422.
(
Elem. 1979, 7, 167.
38) Hu, S.; Popielarz, R.; Neckers, D. C. Macromolecules 1998, 31,
107.
(
(42) Lubis, R.; Allan, D.; Hodgson, B. W.; Swallow, A. J. Radiat. Phys.
Chem. 1992, 39, 7.
4