Photophysics and photochemistry of naphthoxazinone derivatives

Article abstract of DOI:10.1021/jo800039r

(Chemical Equation Presented) The photophysics and photochemistry of a series of naphthoxazinones have been studied using a combination of methods ranging from steady-state and time-resolved spectroscopic techniques to product analysis. The photophysics o

Full text of DOI:10.1021/jo800039r

Photophysics and Photochemistry of Naphthoxazinone Derivatives
Santi Nonell,† Lourdes R. Ferreras,† Alvaro Can˜ete,‡ Else Lemp,‡ German Günther,‡
Nancy Pizarro,‡ and Antonio L. Zanocco*,‡
Grup d’Enginyeria Molecular, Institut Qu´ımic de Sarria`, UniVersitat Ramon Llull, Via Augusta 390,
E-08017, Barcelona, Espan˜a, and Facultad de Ciencias Qu´ımicas y Farmace´uticas, Departamento de
Qu´ımica Orga´nica y Fisicoqu´ımica, UniVersidad de Chile, Casilla 233, Santiago - 1, Santiago, Chile
azanocco@ciq.uchile.cl
ReceiVed January 17, 2008
The photophysics and photochemistry of a series of naphthoxazinones have been studied using a
combination of methods ranging from steady-state and time-resolved spectroscopic techniques to product
analysis. The photophysics of naphthoxazinone derivatives is very dependent on the structure:
phenanthrene-like compounds exhibit higher fluorescence quantum yield than the less aromatic anthracene-
like homologous. The latter, exhibit a substantial degree of charge transfer in the excited singlet state.
These compounds are fairly photostable in the absence of additives, yielding a single photoproduct arising
from the triplet state. The presence of electron donors such as amines increases the photoconsumption
quantum yield and changes the product distribution, the primary photoproduct being a dihydronaphthox-
azinone that photoreacts further yielding ultimately an oxazoline derivative.
Introduction
cloadducts were formed (Scheme 1). Formation of the photo-
cycloadducts was completely quenched by oxygen and trans-
stilbene suggesting a reaction involving the triplet excited state.
No cycloadduct formation was observed when the methyl
substituent of the benzoxazinone was replaced with a phenyl
Benzoxazinone derivatives are a class of compounds exhibit-
ing spectral and photophysical properties of great interest such
as broad first absorption band, emission in the red, intense
fluorescence in both organic solutions and crystalline state, large
dipole moment increase in the excited state, large Stokes shifts,
and short fluorescence lifetimes.1–4 Regarding these properties,
it has been suggested that this type of compounds can be
employed as quantum counters, wavelength shifters, fluorescent
solar concentrators, fluorescent probes for biological systems,
and laser dyes.5–13 In spite of this interest, few studies addressing
the photochemistry of aryloxazinones have been carried out.
Light induced reactions of 1,4-benzoxazin-2-ones with electron-
deficient olefins have been described by Nishio et al.14,15
Irradiation of a mixture of 3-methyl-1,4-benzoxazin-2-one and
an excess of methacrylonitrile under nitrogen atmosphere gave
two stereoisomeric azetidine derivatives. In the presence of an
excess methyl methacrylate also two stereoisomeric photocy-
(1) Le Bris, M. T. J. Heterocyclic Chem. 1984, 21, 551–555.
(2) Le Bris, M. T. J. Heterocyclic Chem. 1985, 22, 1275–1280.
(3) Le Bris, M. T.; Mugnier, J.; Boursons, J.; Valeur, B. Chem. Phys. Lett.
1985, 1, 124–127.
(4) Le Bris, M. T. J. Heterocyclic Chem. 1989, 26, 429–433.
(5) Dupuy, F.; Rullie`re, C.; Le Bris, M. T.; Valeur, B. Opt. Commun. 1984,
51, 36–40.
(6) Mugnier, J.; Dordet, Y.; Pouget, J.; Le Bris, M. T.; Valeur, B. Sol. Energy
Mater. 1987, 15, 65–75.
(7) Monsigny, M.; Midoux, P.; Le Bris, M. T.; Roche, A. C.; Valeur, B.
Biol. Cell 1989, 67, 193–200.
(8) Monsigny, M.; Midoux, P.; Depierreux, C.; Bebear, C.; Le Bris, M. T.;
Valeur, B. Biol. Cell 1990, 70, 101–105.
(9) Depierreux, C.; Le Bris, M. T.; Michel, M. F.; Valeur, B.; Monsigny,
M.; Delmotte, F. FEMS Microbiol. Lett. 1990, 67, 237–243.
(10) Fery-Forgues, S.; Le Bris, M. T.; Guette, J. P.; Valeur, B. J. Phys. Chem.
1988, 92, 6233–6237.
(11) Fery-Forgues, S.; Le Bris, M. T.; Guette, J. P.; Valeur, B. J. Chem.
Soc., Chem. Commun. 1988, 5, 384–385.
(12) Khochkina, O. I.; Sokolova, I. V.; Loboda, L. I. Russ. Phys. J. 1988,
31, 510–513.
* To whom correspondence should be addressed. Phone: 56-2-6782877. Fax:
(13) Fery-Forgues, S.; Le Bris, M. T.; Mialocq, J.-C.; Pouget, J.; Rettig, W.;
Valeur, B. J. Phys. Chem. 1992, 96, 701–710.
56-2-6782868.
† Universitat Ramon Llull.
‡ Universidad de Chile.
(14) Nishio, T.; Omote, Y. J. Org. Chem. 1985, 50, 1370–1373.
(15) Nishio, T. J. Chem. Soc., Perkin Trans. I 1990, 565–570.
10.1021/jo800039r CCC: $40.75
Published on Web 06/14/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 5371–5378 5371

Nonell et al.
TABLE 1. Solvent Dependence of the Absorption and
Fluorescence Transitions
SCHEME 1
a
Em
Abs
λ
(ε)/λ
max
max
1
2
3
hexane
392 (19140)/453 350 (23290)/500, 483 398 (12110)/459
benzene
398 (19020)/467 356 (23150)/514
396 (12370)/478
396 (11270)/490
396 (11045)/511
acetonitrile 392 (18820)/475 348 (24140)/538
methanol
394(18650)/476 346 (23630)/548
a
Abs
Em
λ
, λ
in nm; ε in M-1 cm-1
max
max
the Franck-Condon transitions) overestimate λmax by about of
20 nm, however analysis of molecular orbital indicates a π-π*
transition in all cases. The results are comparable to those
reported for benzoxazinone derivatives2 and structurally related
compounds such as coumarins.16 Values of λ
and molar
Abs
max
absorption coefficient are collected in Table 1. The polarity of
the solvent has a large effect on the fluorescence spectrum of
compounds 1 - 3. The red shift is quite large in polar solvents:
Em
for compound 2, the position of the emission maximum (λ
)
max
shifts from 500 nm in n-hexane to 548 nm in methanol, while
Em
for compound 3 λ
shifts from 459 nm in n-heptane to 506
max
nm in ethylene glycol. Representative values of λEmmax in selected
solvents are collected in Table 1.
These shifts can be analyzed in terms of various solvato-
chromic scales. We have chosen the LSER equation (eq 1)
introduced by Kamlet et al.:17,18
FIGURE 1. Chemical structures of the studied naphthoxazinones.
group and when the photolysis was conducted in the presence
of amines such as triethylamine. In the last case the main
photoreaction products are reductive dimers in moderate yields
(Scheme 1). A reaction mechanism involving the formation of
an exciplex between the oxazinone triplet and the olefin was
proposed, whereby the exciplex evolves to a 1,4-birradical
intermediate which ultimately leads to the observed products.14
In the presence of amines, electron transfer from the amine to
the benzoxazinone triplet and subsequent hydrogen abstraction
leading to the products has been suggested as the reaction
mechanism.
Building on the fact that the benzoxazinone excited states
have substantial charge-transfer character, we hypothesized that
substitution of the benzo- with a naphtho-group could substan-
tially affect the photophysical and photochemical properties of
the aryloxazinones. In this work, we report on the synthesis
and photochemistry of three naphthoxazinone derivatives (Figure
1). These compounds all contain a phenyl group at the
carbonyl’s R position, which should prevent the formation of
cyclic photoproducts, and differ in the position where the
oxazinone ring is fused to the naphtho group and in their relative
orientation, resulting in significant changes in their photophysical
properties. The properties evaluated for these compounds
suggest that they are valuable candidates for technological
applications, such as dyes, quantum counters, or fluorescent
probes.
P ) P0 + aπ * + bR + cꢀ + dFH2
(1)
where P is an energy-dependent property, π* accounts for
dipolarities and polarizabilities of solvent, R is related to the
hydrogen bond donor solvent ability, ꢀ indicates the solvent
capacity as hydrogen bond acceptor, and F2H is the square of
Hildebrand parameter, which accounts for the solvent cohesive
energy density and models the cavity effects.17,19,20 Compared
to other empirical scales such as ET(30),21 equation 1 was
established studying the behavior of many different chro-
mophores and therefore offers the advantage of its wider
applicability. Also, it enables the separation of specific hydrogen-
bonding effects.
The studied naphthoxazinone derivatives are not hydrogen-
bond donors but only hydrogen-bond acceptors mainly on the
carbonyl and the imino groups; hence, the dependence of λ
on solvent is expected to involve only two parameters, the
dipolarity/polarizability π* and the hydrogen-bonding ability
R.17 Indeed, the results of multilinear regression analysis by
employing StatView 5.0 statistical software confirm that the
values of λ depend only on the microscopic solvent param-
Em
max
Em
max
eters π* and R, showing red-shifts in solvents able to stabilize
charges and dipoles as well as in solvents with high R values
(Table 2).
Correlation equations result from purely statistical criteria.
The overall correlation equation quality is indicated by the
sample size, N, the product correlation coefficient, R, and the
Results and Discussion
(16) Kovac, B.; Novak, I. Spectrochim. Acta 2002, 58, 1483–1488.
(17) Taft, R.W.;.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2886–2894.
(18) Kamlet, M. J.; Hall, T. N.; Boykin, J.; Taft, R. W. J. Org. Chem. 1979,
44, 2599–2604.
Absorption and Fluorescence Spectra. The absorption
spectra of compounds 1-3 are nearly independent of solvent
polarity. Compound 2 exhibits the maximum of the lowest-
(19) Kamlet, M. J.; Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J. Am.
Chem. Soc. 1977, 99, 6027–6038.
Abs
energy absorption band (λ ) centered around 350 nm whereas
max
(20) Kamlet, M. J.; Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J. Org.
Chem. 1983, 48, 2877–2887.
for compounds 1 and 3 the maxima appear between 392 and
398 nm. Spectra calculations employing DFT formalism (B3LYP-
6311 g+ for structure optimization and ZINDO-S to calculate
(21) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry, 3rd
ed.; WILEY-VCH: Weinheim, 2003.
5372 J. Org. Chem. Vol. 73, No. 14, 2008

Photophysics and Photochemistry of Naphthoxazinone
TABLE 2. LSER Correlation Equations (υj ) υj + aπ* + br + cꢀ
TABLE 3. Difference of Dipole Moments between the Excited-
and Ground States of Compounds 1, 2, and 3
Em
+ dG2H) for the Dependence of the λ
(Expressed in cm-1) on the
max
Solvent Parameters
compound
Onsager radius/Åa
µ* - µ0/Debye
νj0
a
b
1
2
3
4.9
5.1
4.9
6.1
11.1
7.9
1
Coeff
(
t-stat
P(2-tail)
21951.4
42.6
517.0
<0.0001
N ) 12
-1079.9
69.9
-15.5
<0.0001
R) 0.99
-266.0
53.3
-4.9
0.0008
F ) 181.4
a Calculated using computational methods.
the difference of dipole moments between the excited- and
ground states (µ* - µ0). The results collected in Table 3 were
obtained employing values of the Onsager cavity radius
calculated from the minimized structures (B3LYP-6311 g+)
under Gaussian 03 environment.
The dipole moment differences between the S1 and S0 states
are similar for compounds 1 and 3 and substantially larger for
compound 2. The results are comparable to those determined
for coumarin derivatives,24 though smaller than those reported
for styryl derivatives of aminobenzoxazinones.13 Mulliken
charge analyses show charge separation mainly involving atoms
in the oxazinone ring with the dipole in the molecular plane
oriented close to the carbonyl bond axis. The dependence of
the fluorescence spectrum on the solvent parameters π* and R
and the large increase in dipole moment upon excitation suggest
an substantial charge-transfer character of the first excited singlet
state, particularly for compound 2.
2
3
Coeff
(
t-stat
P(2-tail)
20220.7
211.4
95.6
<0.0001
N ) 14
-1667.1
319.1
-5.2
0.0003
R) 0.91
-1016.4
166.8
-6.1
<0.0001
F ) 26.7
Coeff
(
t-stat
P(2-tail)
21627.9
76.7
281.9
<0.0001
N ) 17
-1452.3
114.5
-12.7
<0.0001
R) 0.97
-641.6
77.2
-8.3
<0.0001
F ) 120.5
Fluorescence Quantum Yield and Lifetime. The fluores-
cence quantum yields, ΦF, of compounds 1 and 3 are high and
similar in polar solvents, decreasing to different extents as the
solvent polarity is lowered (Table 4). In contrast, compound 2
has a very low fluorescence quantum yield, close to the detection
limit of our measurement system. This is consistent with its
higher triplet yield (Vide infra) and is related to structural and
energetic factors that contribute to the molecule aromaticity
(benzoannelation phenomena).25,26 Values of the fluorescence
lifetime in benzene, measured by employing the phase demodu-
lation method, are likewise collected in Table 4. Naphthoxazi-
nones 1 and 3 are compounds with phenanthrene-like structure
whereas naphthoxazinone 2 has an anthracene-like geometry.
Fluorescence data indicates that the inclusion of the heterocyclic
ring produces substantial differences when the same parameters
for the aromatic compounds, anthracene and phenanthrene, are
considered. Anthracene fluorescence lifetime and fluorescence
quantum yield are a factor twelve and a factor two larger than
those reported for phenanthrene, respectively,27 being the first
one more aromatic than the angular catacondensed homolo-
gous.26 Various studies indicate that a highly fluorescent
molecule must have rigid structure with an extensive delocalized
π double bond system, because π-π* transitions possess high
values of the molar absorbility coefficient and low decay half-
times other processes such as intersystem crossing cannot
compete with the fluorescence.28,29 These features are ac-
complished by aromatic molecules so that the vast majority of
luminophores are aromatic. Furthermore, from semiempirical
calculations has been suggested that, in coumarins, quinoxali-
nones, and benzoxazinones, the intersystem crossing from the
Em
FIGURE 2. Plot of νj (exp) vs νj (calc) for the dependence of the λ
max
on the solvent for compound 3. (Inset) Plot of the residuals of νj vs νj.
Fisher index of equation reliability, F. The consistency of each
term is indicated by the standard error, (, the 2-tail probability,
P(2-tail), and the t-statistic. Good quality is indicated by higher
F and t-stat values, and by small SD ones. Descriptor coefficients
accepted in the correlation equation were those that had a
significance level g 0.95. As anticipated, all compounds show
a similar behavior. Figure 2 shows a plot of the experimental
Em
vs calculated λ
(expressed in wavenumbers) for the naph-
thoxazinone 3 computed according to the LSER equation in
Table 1.
max
The good agreement between the two sets of data confirms
the importance of π* and R in the solvent dependence of the
λEmmax values. The plot of residuals in the inset of Figure 2 shows
Em
a random distribution of residuals in the range of λ
values.
max
These results are strong evidence that a substantial charge-
separation occurs in the first excited singlet state.
The red-shift can be interpreted in terms of an increased dipole
moment of the naphthoxazinones upon chromophore excitation
and subsequent solvent relaxation. Indeed a Lippert-Mataga
plot22,23 of the Stokes shift vs the solvent polarity function yields
(24) Cave, R. J.; Castner, E. W. J. Phys. Chem. A 2002, 106, 12117–12123.
(25) Krygowski, T. M.; Cyranski, M. Tetrahedron 1996, 52, 13795–13802.
(26) Krygowski, T. M.; Cyranski, M. K. Chem. ReV. 2001, 101, 1385–1419.
(27) Dabestani, R.; Ivanov, I. N. Photochem. Photobiol. 1999, 70, 10–34.
(28) Nijegorodov, N.; Winkoun, D. P. Spectrochim. Acta 1997, 53, 2013–
2022, Part A
(22) Lippert, E. Z. Naturforsch. 1955, 10, 541–547.
(23) Mataga, N.; Chosrowjan, H.; Taniguchi, S. J. Photochem. Photobiol.
C: Photochem. ReV. 2005, 6, 37–79.
(29) Gikas, E.; Parissi-Poulou, M.; Kazanis, M.; Vavagiannis, A. J. Mol.
Struct.: Theochem. 2005, 724, 135–142.
J. Org. Chem. Vol. 73, No. 14, 2008 5373

Nonell et al.
TABLE 4. Summary of Photophysical Parameters for the Singlet Excited States
TEA
ΦF
K
/M-1
τ0S/ns
benzene
1kTqEA/1010 M-1s-1
SV
compound
hexane
benzene
CH3CN
CH3OH
1
2
3
0.12
<0.003
0.32
0.46
<0.003
0.46
0.56
<0.003
0.50
0.50
<0.003
0.45
27.3
15.7
40.1
2.2
1.2
6.0
1.2
1.3
0.7
FIGURE 3. (a) Dependence of the fluorescence quenching of naphthoxazinone 1 on the amine ionization potential in benzene as solvent: 1
triethylamine, b diethylamine, 2 n-butylamine. (b) Dependence of the fluorescence quenching of naphthoxazinone 1 by triethylamine on the solvent:
2 acetonitrile, 9 benzene.
S1(π,π*) state to the Tn(n,π*) state competes with the fluores-
cence, and their ΦF values can be predicted by calculating the
∆E(Tn(n,π*),S1(π,π*)) values (the energy gap between the
Tn(n,π*) and S1(π,π*) states). The compounds having a higher
Tn(n,π*) state (the lowest triplet n,π* state) than the S1(π,π*)
state (the fluorescent state) strongly fluoresce.29
the solutions. Typical Stern-Volmer plots for naphthoxazinone
1 obtained under several experimental conditions are shown in
Figure 3. The plots were linear in the quencher concentration
range assayed. Table 4 also gives the values of the bimolecular
rate constant for singlet quenching, 1kqTEA, calculated as 1kq
TEA
) KSVTEA/τ0 . They are close to the diffusion control limit for
S
Naphthoxazinone derivatives show an opposite trend than that
observed for aromatic hydrocarbons such as anthracene and
phenantrene, being larger the fluorescence quantum yield and
the fluorescence lifetime for the phenanthrene-like compounds
than those determined for the anthracene-like oxazinones. These
differences could be ascribed to the effect of the heterocyclic
ring on the system aromaticity and the triplet excited states
manifold. The B3LYP-6311g+ calculations show that com-
pounds 1 and 3 have extensive delocalization of molecular
orbitals HOMO and LUMO involving the naphthalene moiety,
the oxazinone ring and the phenyl substituent. For compound
2, it was found that the HOMO is mainly localized on the
oxazinone ring and the neighbor condensed aromatic ring
whereas the phenyl substituent practically does not contribute
to the HOMO delocalization. Consequently, from the aromaticity
point of view, compounds 1 and 3 would be more fluorescent
than 2. In addition, within the limitations of ZINDO calculations
conducted in vacuo, for the naphthoxazinone derivative 2, it
was found that the energies of the higher Tn(n,π*) states (T4,
T5, and T6) are lower than the energy of the S1(π,π*) state,
whereas for compounds 1 and 3, T3 is the lower triplet whose
energy is near to the S1(π,π*) state. Considering the larger
spin-orbit coupling for S1(π,π*) f Tn(n,π*) transitions, the
El-Sayed rules30,31 and Salem propositions,32,33 an efficient
intersystem crossing process can be expected for compound 2
concomitant with the very low fluorescence quantum yield
experimentally observed.
all compounds and is ca. 2-fold larger in acetonitrile relative to
benzene (Figure 3b) reflecting the higher diffusional limit in
acetonitrile. The Stern-Volmer constants KSV obtained from
the slope of plots of Figure 3a give a good lineal correlation
with the amine ionization potential, increasing as the ionization
potential of the amine decreases. This result would be indicative
of the electron-transfer mechanism for quenching of naphthox-
azinone derivatives by amines.
Phosphorescence Spectra and Triplet Energy. Emission
spectra were recorded in both methyltetrahydrofuran (MTHF)
and in methyl cyclohexane/1,2-diiodoethane (4:3) at 77 K.
Phosphorescence could only be observed in the heavy-atom
containing solvent and only for 2. From the onset of the
phosphorescence at ca. 640 nm, we estimate the triplet energy
level of 2 at 188 ( 5 kJ·mol-1 (Figure 4).
Triplet Absorption Spectra and Lifetime. Nanosecond-laser
flash excitation of argon-saturated benzene solutions of the
compounds resulted in transient absorbance changes, which
could be quenched by oxygen and thus were attributed to their
triplet state. The differential triplet-minus-singlet absorption
spectra are shown in Figure 5. The lifetime values are collected
in Table 5. Similarly as found for the singlet state, naphthox-
azinone 3 has also a remarkably longer triplet lifetime than the
other two compounds examined.
Singlet Oxygen Photosensitization. The triplet lifetime
decreases significantly in the presence of oxygen (Table 2),
indicating efficient energy transfer to this molecule. Using time-
resolved singlet oxygen O2(1∆g) phosphorescence detection at
1270 nm, the quantum yield of O2(1∆g) production, Φ∆, has
been readily determined. The values for the three compounds
are collected in Table 5 and are consistent with the fluorescence
quantum yields, that is, Φ∆ is highest for the nonfluorescent
compound 2, indicating that intersystem-crossing is a major
Fluorescence Quenching by Amines. The fluorescence is
quenched by adding increasing amounts of various amines to
(30) El-Sayed, M. A. J. Chem. Phys. 1963, 38, 2834–2838.
(31) El-Sayed, M. A. J. Chem. Phys. 1964, 41, 2462–2467.
(32) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972, 11, 92–
111.
(33) Salem, L. Pure Appl. Chem. 1973, 33, 317–328.
5374 J. Org. Chem. Vol. 73, No. 14, 2008

Photophysics and Photochemistry of Naphthoxazinone
Photochemical Reactions. Irradiation of compounds 1-3 (λ
) 366 nm) under Ar in benzene lead to a decrease in the
concentration of the naphthoxazinones. No consumption was
observed when the same experiment was carried out in O2-
purged solutions. In the presence of TEA, the rate of naph-
thoxazinone-derivative consumption increased in a TEA-
concentration dependent way. The photoconsumption quantum
yields were extrapolated to zero irradiation time and measured
under several experimental conditions. The self-sensitized
photoxygenation of 9,10-dimethylanthracene was used as
actinometer.
Product distribution of naphthoxazinone 1 photoreaction in
benzene, was analyzed by obtaining GC-MS chromatograms
before and after exhaustive photolysis of compound 1 under
Ar. The time zero chromatogram shows a single peak at 15.94
min, corresponding to naphthoxazinone 1 as verified by its mass
spectra. The photolyzed solution shows a single additional peak
at 13.22 min. The identity of the single photoproduct (not
isolated) was established as the naphthoxazole derivative from
its mass spectra that shows a molecular ion of mass 245. In
addition, retention time and mass spectra matching to that
obtained in identical chromatographic conditions for the pure
compound synthesized previously.34 The formation of a single
naphthoxazol derivative was also observed in the photolysis of
compounds 2 and 3.
FIGURE 4. Emission spectrum of 2 in methyl cyclohexane/1,2-
diiodoethane (4:3) at 77 K.
In the presence of amines such as triethylamine (TEA) the
consumption quantum yield increases depending on the amine
concentration and the product distribution is more complex. A
sequence of chromatograms was obtained upon photolysis of
naphthoxazinone 2 in Ar-saturated benzene at various irradiation
times. At short irradiation times, besides the signal with retention
time 7.62 min, with a mass spectra corresponding to the
nonphotolized naphthoxazinone 2, only one photoproduct with
retention time of 7.90 min could be observed. The spectrum of
compound with retention time 7.90 min shows a molecular ion
with m/z 275, two mass units larger than napthoxazinone 2,
indicating the formation of a dihydronaphthoxazinone derivative.
The presence of this compound as the main photoreaction
product at low conversion was corroborated by comparison with
the mass spectrum and retention time of the CG-MS chromato-
gram measured for a genuine sample synthesized by catalytic
hydrogenation of naphthoxazinone 2 and characterized by its
1H NMR spectrum. At larger photolysis time (over 10 min),
new peaks become visible in the chromatogram at retention time
of 6.26 and 6.82 min. The signal appearing at 6.26 min shows
a mass spectra with a molecular ion m/z equal to 245 and a
fragmentation pattern corresponding to the oxazole derivative
of naphthoxazinone 2. In addition, the mass spectrum of the
peak with retention time of 6.82 min that shows a molecular
ion of m/z equal to 247. This last result is compatible with the
formation of an oxazoline derivative that could be a product of
photolysis of the dihydronaphthoxazinone (tR ) 7.90 min). The
dihydronaphthoxazinone apparently is the primary photoproduct,
which accumulates at the beginning of photolysis. In order to
corroborate that the oxazoline derivative is the only product in
the photolysis of the dihydronaphthoxazinone, a purified sample
of the dihydronaphthoxazinone, was irradiated under Ar in the
absence and the presence of TEA, employing identical experi-
mental conditions than those for the photolysis of 2. In both
experiments the GC-MS chromatograms show a single photo-
FIGURE 5. Transient absorption spectrum at 1 µs delay alter laser
flash excitation of Ar-saturated benzene solutions of 1, 2, and 3.
TABLE 5. Summary of Photophysical Parameters for the Triplet
State in Benzene
lifetime (Ar)/ µs lifetime (air)/ µs 3kqTEA/105 M-1 s-1
Φ∆
1
2
3
31
36
60
0.39
0.24
0.30
1.3 ( 0.1
3.1 ( 0.1
1.5 ( 0.1
0.26 ( 0.01
0.48 ( 0.03
0.15 ( 0.01
deactivation pathway for this naphthoxazinone. We also mea-
sured the compounds ability to quench singlet oxygen and found
∆
it to be negligible, with rate constants kq < 105 M-1 -1
s
for 1
∆
and 3, and kq < 104 M-1 -1
for 2.
s
Rate Constants for Triplet Quenching by TEA. Addition
of increasing amounts of TEA to Ar-saturated benzene solutions
of the compounds resulted in a concomitant increase in the decay
rate of their triple state. Analysis of Stern-Volmer plots using
3kobs )1/τΤ ) 1/τ0Τ + 3kqTEA[TEA] yielded the quenching rate
constants collected in Table 5, with compound 2 ca. twice as
reactive as compounds 1 and 3. The rate constants for quenching
of the triplet state are five orders of magnitude smaller than
those for the singlet state. A clear decrease in the amplitude of
the transients could also be observed, consistent with the
quenching of the singlet state precursor by TEA (see above).
(34) Saitz, C.; Rodr´ıguez, H.; Ma´rquez, A.; Can˜ete, A.; Jullian, C.; Zanocco,
A. L. Synthetic Commun. 2001, 31, 135–140.
J. Org. Chem. Vol. 73, No. 14, 2008 5375

Nonell et al.
SCHEME 2
SCHEME 3
reaction product with tR ) 6.82 min and the same mass spectrum
as the oxazoline derivative. In addition, the formation rate of
the oxazoline derivative was independent of the presence of
TEA. Photolysis of compounds 1 and 3 afforded the same
product distribution observed for 2, however in the photolysis
of 1 and 3 the primary dihydronaphthoxazinone photoreaction
product does not accumulate, indicating a smaller formation rate,
a larger photoreaction rate of the dihydronaphthoxazinone to
produce secondary products and/or an efficient back reaction
of the primary intermediates produced in quenching by amine.
The laser flash photolysis results allow us to disregard the first
possibility, as the quenching rate constants are of the same order
of magnitude for the three compounds but the photoconsumption
quantum yields, at least 1 order of magnitude smaller for 1 and
3, favor the last possibility.
The photoconsumption quantum yields, the rate constants for
quenching of singlet and triplet states with TEA, and the product
distribution allow us to explain the photochemical behavior of
naphthoxazinone derivatives. A low photoconsumption quantum
yield was determined in Ar-purged solutions for 1, 2, and 3 (in
the absence of additives), being one order of magnitude larger
for 2. This result is compatible with the very low fluorescence
quantum yield and the largest quantum yield of singlet oxygen
formation observed for 2 and could be understood in terms of
a more efficient intersystem crossing to the triplet excited states,
probably promoted by S1(π,π*)-Tn(n,π*) spin-orbit coupling.
In O2 saturated and/or air equilibrated benzene solutions, they
were stable up to 100 h of irradiation at 360 nm with a 150 W
black ray lamp. This indicates that, in the absence of additives
such as electron donors, the photoreactions of naphthoxazinone
derivatives arise from the triplet excited state. In fact, the
consumption quantum yields under argon in absence of addi-
tives, follow the same trend as the triplet yields. A simple
photoextrusion mechanism involving a diradical intermediate
is proposed to explain the formation of the naphthoxazole
(Scheme 2).
Photoproduct formation through the triplet state is enhanced
by the presence of TEA. As a typical example, when naph-
thoxazinone 2 was irradiated in benzene under Ar, the con-
sumption quantum yield increased by a factor of 32 when 1.4
× 10-3 M of TEA was added, whereas no photoreaction was
observed if the solution was saturated with O2, even under
extensive irradiation and in the presence of TEA. Also,
quenching of singlet state did not exceed 8% at the TEA
concentrations employed in these experiments.
We propose that the photoproducts observed upon photolysis
of naphthoxazinone derivatives in the presence of TEA arise
from electron-transfer from TEA to their triplet state (Scheme
3). The radical ion-pair formed can evolve to the starting
products or a proton could be transferred from the amine radical
cation to the naphthoxazinone radical anion to give a neutral
radical pair. The primary stable product, the dihydro-naphthox-
azinone, could thus be produced by disproportionation of this
radical pair within the solvent cage. An analogous mechanism
has been invoked to explain the formation of dihydroquinoxalin-
2-ones by irradiation of 3-phenylquinoxalin-2-ones in the
presence of amines.35 The lower consumption quantum yield
in 1 and 3 suggests that back electron transfer is more efficient
in phenanthrene-like naphthoxazinones.
(35) De la Fuente, J. R.; Can˜ete, A.; Zanocco, A. L.; Saitz, C.; Jullian, C. J.
Org. Chem. 2000, 65, 7949–7958.
5376 J. Org. Chem. Vol. 73, No. 14, 2008

Photophysics and Photochemistry of Naphthoxazinone
Conclusions
respectively. All measurements were carried out in nitrogen-purged
solutions at (20 ( 0.5) °C.
The photophysics of the naphthoxazinone derivatives studied
in this work is strongly dependent on the position of oxazolinone
ring on the naphthalene moiety. Phenantrene-like naphthoxazi-
nones exhibit fluorescence quantum yields higher than the less-
aromatic anthracene-like homologous. In contrast, the latter
show the strongest dependence of the emission maxima on the
solvent polarity, indicating an important charge separation in
the singlet excited state. Although all compounds are essentially
photostable in the absence of additives, formation of a naph-
thoxazole derivative from the triplet excited-state is observed,
albeit with very low quantum yield. The presence of electron
donors such as amines enhances the photoconsumption quantum
yield and changes the product distribution by opening a
photoreaction mechanism mediated by electron transfer to the
excited triplet state. Their photophysical properties and large
photostability in air-equilibrated solutions suggest that naph-
thoxazinone derivatives could be valuable dyes for dye laser
applications (3) as well as for singlet oxygen photosensitization
(2).
Photoconsumption quantum yields under several experimental
conditions were evaluated using the self-sensitized photooxygen-
ation of 9,10-dimethylanthracene in air saturated Freon 113 (Φ )
0.566).42,43 The photon flux, q/einstein s-1 was calculated according
to eq 3:
∆A(324 nm)V
Φ(λ)ε(324 nm)tl
q )
(3)
with t ) irradiation time, V ) volume of the solution, l ) optical
path length, Φ(λ) ) photo-oxigenation quantum yield, and ε(324
nm) ) molar absorption coefficient. Time-dependent naphthoxazi-
none photoconsumption was measured from its absorption spectra
and/or by the decreasing of the peak area in the GC-NPD
chromatogram. Additional experiments using the benzophenone/
benzhydrol actinometer44,45 were performed to confirm values in
selected experiments. Not more than 20% of a difference between
the two actinometers employed was observed for a given experiment.
The rate constant for quenching of the naphthoxazinone singlet
state by a series of amines (1kq) was determined from a Stern-Volmer
analysis of the amine effect on the naphthoxazinone steady-state
fluorescence intensity using eq 4:
Experimental Details
I0
I
Materials. All solvents used in the syntheses and spectroscopic
and kinetic measurements were of reagent grade, were of spectro-
scopic or HPLC quality, and were purified by the usual procedures
when necessary.36
) 1 + KSV[Q]
(4)
where I0 and I are the areas under the fluorescence spectrum at
zero and higher concentrations of the amine, respectively, and KSV
is the Stern-Volmer constant () kq τ0 , where τ0 is the singlet
Chemical Synthesis. Naphthoxazinones-derivatives were ob-
tained by employing a modification of the Moffet method.37
2-Phenyl-3H-naphtho[2,1-b][1,4]-oxazin-3-one (1).34,38 Freshly
distilled methyl benzoylformiate (2.09 mmol) and 1-aminonaphthol
(2.75 mmol) in 20 mL of etanol/pyridine were heated at 110 °C by
1 h in a flask equipped with a reflux condenser and a magnetic
stirrer. The precipitate obtained by cooling the mixture in ice-water
bath was filtered and washed at least three times each with: water,
diluted HCl, and water. Finally, the dry solid was recrystallized
from acetonitrile to yield 0.75 g (80%) of the product as a yellow
crystalline solid, mp 173.5-174.5 °C. Lit.32 174-175 °C. 1H NMR
(CDCl3): δ 7.37 [d, 1H, J ) 9.0 Hz]; 7.49 [m, 3H]; 7.53 [dd, 1H];
7.65 [dd, 1H]; 7.83 [d, 1H, J ) 8.1 Hz]; 7.90 [d, 1H, J ) 9.0 Hz];
8.46 [m, 2H]; 8.84 [d, 1H, J ) 8.35 Hz]. 13C NMR (CDCl3): δ
116.1; 123.3; 126.8; 127.1; 128.5; 128.7; 128.8d; 129.8d; 130.9;
131.5; 131.7; 132.8; 134.9; 145.2; 148.8. IR (KBr): cm-1 1725.7
(CdO); 1584.9 (CdN); Elem. anal. calcd %C: 79.11; %H: 4.06;
%N: 5.13; exp. %C: 79.11; %H: 4.08; %N: 5.43.
Methods. The fluorescence quantum yields (ΦF) were measured
by the comparative method described by Eaton and Demas,39,40
using quinine sulfate in 0.1 N sulfuric acid (ΦF ) 0.55)41 or
fluorescein in 0.1 N NaOH (ΦF ) 0,92)41 as references. Optical
densities of sample and reference solutions were set below 0.2 at
the excitation wavelength, and the fluorescence spectra were
corrected by using rhodamine G as reference. Sample quantum yield
was evaluated by using eq 2:
1
S
S
lifetime at zero amine concentration).
Fluorescence lifetimes, τ0 , were measured by employing the
S
phase demodulation method using fluorescein as reference (τ0
)
S
4.05 ns).46,47 Phase shift and emission demodulation were employed
to calculate τ0 according to eqs 5 and 6:
S
τp ) ω-1 tan φ
(5)
(6)
1/2
1
τm ) ω-1 2 - 1
(
)
m
where τp is the phase lifetime, τm the demodulation lifetime, ω the
circular frequency of the incident light, φ the phase angle displace-
ment, and m is the demodulation factor. Measured values gave τp
) τm ) τ0 , indicating monoexponential fluorescence decays.
S
Rate constants for quenching of the triplet state by TEA were
determined examining the effect of this amine on the rate of triplet
decay. In the presence of a quencher, the observed pseudofirst order
rate constant for triplet decay, 3kobs, increases linearly as shown in
eq 7:
3kobs ) 1 ⁄ τT ) 1 ⁄ τT0 + 3kTqEA[TEA]
Values of kTqEA were deduced from the slope of the linear plot
(7)
3
3
of kobs vs the concentration of TEA.
Gradx
ηx2
(40) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991–1024.
(41) Heller, C. A.; Henry, R. A.; McLaughlin, B. A.; Bliss, D. E. J. Chem.
Eng. Data 1974, 19, 214–219.
Φx )
ΦAct
(2)
(
)
2
(
)
GradAct
ηAct
(42) Adick, H.-J.; Schmidt, R.; Brauer, H.-D. J. Photochem. Photobiol. A:
Chem. 1988, 45, 89–96.
where GradX and GradAct are the slope of integrated fluorescence
vs absorbance plots for the sample and the actinometer, respectively,
and ηx and ηAct are the refractive index of sample and actinometer,
(43) Kuhn, H. J.; Braslavsky, S. E.; Schmidt, R. Pure Appl. Chem. 2004,
76, 2105–2146.
(44) Borderie, B.; Lavabre, D.; Levy, G. J. Photochem. Photobiol. A: Chem.
1991, 56, 13–23.
(36) Wiberg, K. B. Laboratory technique in organic chemistry; McGraw-
Hill: New York, 1960.
(45) Murov, S. L. Handbook of Photochemistry, 2nd ed.; Marcel Dekker,
New York, 1993; Sect. 13, p 307.
(37) Mofett, R. B. J. Med. Chem. 1966, 9, 475–478.
(38) Ma´rquez, A.; Saitz, C.; Can˜ete, A.; Rodr´ıguez, H.; Jullian, C. Mag.
Reson. Chem. 1998, 36, 449–453.
(46) Nunnally, B. K.; He, H.; Li, L.-C.; Tucker, S. A.; McGown, L. B. Anal.
Chem. 1997, 69, 2392–2397.
(47) Pant, S.; Tripathi, H. B.; Pant, D. D. J. Photochem. Photobiol. A: Chem.
1994, 81, 7–11.
(39) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107–1114.
J. Org. Chem. Vol. 73, No. 14, 2008 5377

Nonell et al.
Acknowledgment. Financial support from FONDECYT
(grants 1050796 and 7060251) is gratefully acknowledged.
13C NMR, and mass spectra. Absolute energies, lowest-energy
Franck-Condon transition, Cartesian coordinates, Mulliken
charge distribution, and HOMO/ LUMO localization of naph-
thoxazinone derivatives 1, 2, and 3. This material is available
free of charge via the Internet at http://pubs.acs.org.
Supporting Information Available: General experimental
methods, experimental synthetic procedures, spectroscopic and
analytical data for naphthoxazinone derivatives 2 and 3 and the
corresponding dihydronaphthoxazinone photoproducts, 1H NMR,
JO800039R
5378 J. Org. Chem. Vol. 73, No. 14, 2008