Solvent and media effects on the photophysics of naphthoxazole derivatives

Article abstract of DOI:10.1111/php.12133

The photophysical properties of 2-phenyl-naphtho[1,2-d][1,3]oxazole, 2(4-N,N-dimethylaminophenyl)naphtho[1,2-d][1,3]oxazole and 2(4-N,N- diphenylaminophenyl) naphtho[1,2-d][1,3]oxazole were studied in a series of solvents. UV-Vis absorption spectra are insensitive to solvent polarity whereas the fluorescence spectra in the same solvent set show an important solvatochromic effect leading to large Stokes shifts. Linear solvation energy relationships were employed to correlate the position of fluorescence spectra maxima with microscopic empirical solvent parameters. This study indicates that important intramolecular charge transfer takes place during the excitation process. In addition, an analysis of the solvatochromic behavior of the UV-Vis absorption and fluorescence spectra in terms of the Lippert-Mataga equation shows a large increase in the excited-state dipole moment, which is also compatible with the formation of an intramolecular charge-transfer excited state. We propose both naphthoxazole derivatives as suitable fluorescent probes to determine physicochemical microproperties in several systems and as dyes in dye lasers; consequence of their high fluorescence quantum yields in most solvents, their large molar absorption coefficients, with fluorescence lifetimes in the range 1-3 ns as well as their high photostability.

Full text of DOI:10.1111/php.12133

Photochemistry and Photobiology, 20**, **: *–*
Solvent and media effects on the photophysics of naphthoxazole
derivatives†
Manuel Curitol1, Xavier Ragas2, Santi Nonell2, Nancy Pizarro3, María V. Encinas4, Pedro Rojas1,
1
1
1
1
ꢀ
€
Renzo P. Zanocco , Else Lemp , German Gunther and Antonio L. Zanocco*
1
ꢀ
ꢀ
Facultad de Ciencias Químicas y Farmaceuticas, Departamento de Química Organica y Fisicoquímica, Universidad de
Chile, Santiago, Chile
Grup d’Enginyeria Molecular, Institut Químic de Sarria, Universitat Ramon Llull, Barcelona, Espana
2
3
ꢁ
~
ꢀ
Facultad de Ecología y Recursos Naturales, Departamento de Ciencias Químicas, Universidad Andres Bello, Santiago,
Chile
4Facultad de Química y Biología, Universidad de Santiago de Chile, Santiago, Chile
Received 28 March 2013, accepted 2 July 2013, DOI: 10.1111/php.12133
ABSTRACT
and its derivatives (substituted with electron donor groups in the
para position on the phenyl ring) possess well-resolved vibrational
structure in nonpolar solvents. This is the result of a bond order
higher than one between C-2 in the heterocyclic ring and the phe-
nyl substituent, which restricts rotation of two aromatic subunits
of 2-phenylbenzoxazole (11). The presence of an electron donor
substituent in a phenyl ring of 2-phenylbenzoxazole shifts both
the absorption and emission spectra to longer wavelengths. The
major shift of the fluorescence spectrum can be explained in terms
of charge transfer in the excited state from the phenyl ring to the
benzoxazole moiety (11). The highly favorable photophysical
properties of this type of heterocyclic compounds, high fluores-
cence quantum yield, photostability, as well as their easy tuning
of photophysical properties (by changing a substituent at position
two of benzoxazole ring) made this compounds very useful in the
search of new fluorophores. Consequently, benzoxazole deriva-
tives have been used as organic plastic scintillators (12) and opti-
cal fiber sensors (13,14). Moreover, some of the derivatives are
biologically active, showing activity as cytotoxic (15,16), antimi-
crobial (17,18), inhibitors of both eukaryotic DNA topoisomerase
I and II (19) and genotoxic (20). They have also been used as
fluorescent probes for thiols (21–24) and metal cations (25,26)
quantitation in biological samples. The long-wavelength absorp-
tion and emission bands and high fluorescence quantum yield of
benzoxazol-5-yl-alanine derivatives allow their use as fluorescent
markers in biological systems, even in the presence of tryptophan
(27,28). Benzoxazole derivatives also are an efficient tool for
monitoring and exploring the properties of micelles and the
hydrophobic interactions in human serum albumin (29), and to
produce polymeric nonlinear optical materials with large hyperpo-
larizability values (30,31). Furthermore, polybenzoxazole deriva-
tives have been proposed as positive photosensitive precursors for
applications in microelectronics (32), in electroluminescent
devices (2,33), and for the generation of linearly polarized light
using the autoorganization capability of liquid crystalline media
(34). In spite of the wide range of basic and innovative research
that has been developed having as center the emissive properties
of benzoxazole, no reports describe the photophysics and/or the
photochemistry of compounds with the oxazole heterocycle con-
densed to extended aromatic systems. There are a variety of syn-
thetic methods to obtain naphthoxazole derivatives (35–40),
The photophysical properties of 2-phenyl-naphtho[1,2-d][1,3]
oxazole,
2(4-N,N-dimethylaminophenyl)naphtho[1,2-d][1,3]
oxazole and 2(4-N,N-diphenylaminophenyl) naphtho[1,2-d]
[1,3]oxazole were studied in a series of solvents. UV–Vis
absorption spectra are insensitive to solvent polarity whereas
the fluorescence spectra in the same solvent set show an
important solvatochromic effect leading to large Stokes shifts.
Linear solvation energy relationships were employed to cor-
relate the position of fluorescence spectra maxima with
microscopic empirical solvent parameters. This study indi-
cates that important intramolecular charge transfer takes
place during the excitation process. In addition, an analysis
of the solvatochromic behavior of the UV–Vis absorption and
fluorescence spectra in terms of the Lippert–Mataga equation
shows a large increase in the excited-state dipole moment,
which is also compatible with the formation of an intramolec-
ular charge-transfer excited state. We propose both naphth-
oxazole derivatives as suitable fluorescent probes to
determine physicochemical microproperties in several systems
and as dyes in dye lasers; consequence of their high fluores-
cence quantum yields in most solvents, their large molar
absorption coefficients, with fluorescence lifetimes in the
range 1–3 ns as well as their high photostability.
INTRODUCTION
There are many studies focused on the photophysical properties of
benzoxazole derivatives, from a basic perspective to the develop-
ment of molecules with suitable properties to be employed in
diverse applications (1–4). Early studies explore the absorptive
and emissive characteristics of 2-phenylbenzoxazole and its
p-substituted derivatives with the aim of accounting for their
capacities as near-UV laser generators. This application is strongly
supported by the high fluorescence quantum yields, the efficient
laser output and the extreme photostability of this class of dyes
(5–10). Absorption and emission spectra of 2-phenylbenzoxazole
*Corresponding author email: azanocco@ciq.uchile.cl (Antonio L. Zanocco)
†This article is part of the Special Issue dedicated to the memory of Elsa Abuin.
© 2013 The American Society of Photobiology
1

2
Manuel Curitol et al.
148.5, 131.7, 131.5, 129.3, 128.9, 127.9, 127.8, 127.4, 127.0, 126.4,
125.8, 122.7, 111.2.
however, the only study related to naphthoxazole photophysics
involves the use of 1,2-naphthozaxoles as substitutes for the benz-
oxazole residue in merocyanine 540. This change increases the
fluorescence and triplet yields and decreases concomitantly the
yield for photoisomerization (41). Also, naphthoxazole derivatives
have been used to fine-tune the phosphorescence color in a series
of heteroleptic Ir(III) complexes (42). Building on the fact that the
benzoxazole excited states have substantial charge-transfer charac-
ter, we hypothesized that substitution of the benzo- with a naphtho
group could markedly affect the photophysical properties of the a-
ryloxazoles. In addition, inclusion of an electron donor substituent
also should enhance the intramolecular charge transfer in the
excited singlet state. With this aim, we synthesized three naphth-
oxazole derivatives (Fig. 1) and studied the solvent effect on their
absorption and emission. The properties evaluated for these com-
pounds suggest that they are valuable candidates for technological
applications such as laser dyes, quantum counters or fluorescent
probes.
2(4-N,N-dimethylaminophenyl)naphtho[1,2-d][1,3]oxazole (NX2). Using
the same procedure described above, 2-nitrosonaphthol, 180 mg
(1.03 mmol) and p-dimethylaminobenzaldehyde, 223 mg (1.5 mmol)
yield 38% of 2(4-N,N-dimethylaminophenyl) naphtho[1,2-d][1,3]oxazole,
m.p. 182–183°C. Lit. (41) 177–179°C 1H-NMR (CDCl3) d: 8.57 (d, Jd
8.2 Hz, 1H), 8.20 (d, Jd 8.8 Hz, 2H), 7.95 (d, Jd 8.2 Hz, 1H), 7.72 (d, Jd
8.8 Hz, 2H), 7.64 (m, 1H), 7.50 (t, Jt 7.5 Hz, 1H), 6.81 (d, Jd 8.8 Hz,
2H), 3.07 (s, 6H). 13C-NMR (CDCl3) d: 167.7, 155.2, 150.7, 141.0,
134.2, 131.9, 129.7, 129.4, 128.1, 127.7, 125.4, 120.1, 117.9, 114.8,
113.8, 43.2.
2(4-N,N-diphenylaminophenyl)naphtho[1,2-d][1,3]oxazole (NX3). Employ-
ing a similar method as described above, 2-nitrosonaphthol, 180 mg
(1.03 mmol) and p-diphenylaminobenzaldehyde, 410 mg (1.5) mmol,
yield 52% of 2(4-N,N-diphenylaminophenyl) naphtho[1,2-d][1,3]oxazole,
m.p. 204–206°C. 1H-NMR (CDCl3) d: 8.42 (d, Jd 8.5 Hz, 1H), 8.12 (m,
3H), 7.95 (s, 2H), 7.71 (t, Jt 7.2 Hz, 1H), 7.60 (t, Jt 7.2 Hz, 1H), 7.42 (t, Jt
8.9 Hz, 6H), 7.20 (d, Jd 8.2 Hz, 2H), 7.06 (d, Jd 8.6 Hz 4H).
Instrumental methods. Fluorescence quantum yields (ΦF) were mea-
sured by the comparative method described by Eaton and Demas et al.
(43,44), using quinine sulfate in 0.1 N sulfuric acid (ΦF = 0.55) as refer-
ence. Optical densities of sample and reference solutions were set below
0.2 at the excitation wavelength and the fluorescence spectra were cor-
rected using rhodamine G as reference. Sample quantum yields were
evaluated by using Eq. (1):
MATERIALS AND METHODS
General. All solvents used in syntheses, spectroscopic and kinetic mea-
surements were of reagent grade, spectroscopic or high performance
liquid chromatography quality. Nuclear magnetic resonance (NMR) spec-
tra were recorded in a Bruker Advance 300 MHz spectrometer. Chemical
shifts are referred to the internal standard tetramethylsilane (TMS).
A Hewlett Packard 5890 gas chromatograph equipped with a nitrogen-
phosphorus detector and a capillary column was employed in gas-liquid
chromatography experiments. Steady-state fluorescence spectra were
recorded in a Fluorolog 2 Tau 2 spectrofluorometer. Fluorescence lifetime
measurements were carried out with the time correlated single-photon
counting technique using an Edinburgh Instruments OB-900 or a Pico-
Quant Fluotime 200 fluorescence lifetime spectrometers.
ꢀ
ꢁꢀ
ꢁ
Gradx
GradAct
gx2
g2Act
Ux ¼
UAct
ð1Þ
where Gradx and GradAct are the slope of integrated fluorescence vs.
absorbance plots for the sample and the actinometer, and gx and gAct are
the refractive index of sample and actinometer solutions respectively. All
measurements were carried out in nitrogen-purged solutions at
(20.0 ꢀ 0.5)°C. Geometry optimizations, energy evaluation and
Franck–Condon transition calculations were carried out with Gaussian
09W (45).
Chemical synthesis. Naphthoxazole derivatives were synthesized employ-
ing a modification in a method previously described in the literature (40).
2-phenyl-naphtho[1,2-d][1,3]oxazole (NX1). A solution of 2-nitroso-
naphthol, 180 mg (1.03 mmol), in absolute ethanol (EtOH) (ca. 30 mL)
was stirred with 10% (w/w) Pd/C catalyzer under hydrogen at room tem-
perature. The mixture was protected from light, and the reaction progress
was followed by TLC. After complete consumption of the starting mate-
rial (ca. 2 h), the excess of hydrogen was removed by flushing with a
nitrogen current. Then, 0.15 mL (1.48 mmol) of benzaldehyde in abso-
lute EtOH was added and the mixture was refluxed for 20 h under nitro-
gen. After cooling down to room temperature, 1.5 mmol of 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) was added carefully and addition-
ally stirred for 5 min at room temperature. Then, the solid residue was
filtered out and the ethanolic solution extracted with methylene chloride.
The extract was dried over magnesium sulfate and the solvent was evapo-
rated under reduced pressure. Purification by column chromatography
yields 53% of 2-phenyl-naphtho[1,2-d][1,3]oxazole, m.p. 123–124°C. Lit.
(39) 122–123°C. 1H-NMR (CDCl3) d: 8.60 (d, Jd 8.20 Hz, 1H), 8.34 (m,
2H), 7.99 (d, Jd 8.20 Hz, 1H), 7.82 (d, Jd 8.90 Hz,1H), 7.76 (d, Jd
8.90 Hz, 1H), 7.69 (m, 1H), 7.56 (m, 4H). 13C-NMR (CDCl3) d: 162.7,
RESULTS AND DISCUSSION
Absorption spectra
The absorption spectra of the naphthoxazole derivatives 2-phenyl-
naphtho[1,2-d][1,3]oxazole (NX1), 2(4-N,N-dimethylaminophenyl)
naphtho[1,2-d][1,3]oxazole (NX2) and 2(4-N,N-diphenylamin-
ophenyl)naphtho[1,2-d][1,3]oxazole (NX3) were determined in a
1.00
0.75
0.50
0.25
0.00
H3C
CH3
N
N
300
350
400
450
N
N
N
O
O
O
λ/nm
Figure 2. Absorption spectra of 2-phenyl-naphtho[1,2-d][1,3]oxazole
(NX1) (●),2(4-N,N-dimethylaminophenyl)naphtho[1,2-d][1,3]oxazole (NX2)
(■) and 2(4-N,N-diphenylaminophenyl)naphtho[1,2-d][1,3]oxazole (NX3)
(▲) in benzene and methanol as solvents. Open symbols correspond to the
spectra in methanol.
NX1
NX2
NX3
Figure 1. Chemical structures of the studied naphthoxazole derivatives.

Photochemistry and Photobiology
3
large set of solvents of different polarity and proton donating capac-
ity at room temperature. Figure 2 shows the absorption spectra in
benzene and methanol as examples of nonpolar and polar protic
solvents respectively. The determined corresponding spectral data
are summarized in Table S1.
by employing linear solvation relationship (LSER) and/or theo-
retical linear solvation relationship (TLSER) equations. We
employed the semiempirical solvatochromic equation (LSER) of
Kamlet et al. Eq. (2) (49–51):
ꢂmF ¼ ꢂm0 þ apꢂ þ dd þ ba þ cb þ hq2H
ð2Þ
NX1, NX2 and NX3 display absorption spectra with an
intense and broad low-energy band in the range 310–360, 320–
390 and 325–415 nm respectively. The molar extinction coeffi-
cients at the maximum wavelength are high, ranging between
(7.98–30.40) 9 103 Mꢁ1 cmꢁ1, (11.79–52.95) 9 103 Mꢁ1 cmꢁ1
and (35.22–61,09) 9 103 Mꢁ1 cmꢁ1 for NX1, NX2 and NX3
respectively. The maximum absorption wavelength is almost
insensitive to the solvent polarity, being more relevant to the
profuse vibrational fine structure observed in NX1 spectrum; this
behavior is probably due to the limited rotation of the bond
between the oxazole ring and the 2-phenyl substituent (bond
order greater than one) (7,11). We performed spectra calculations
employing the density functional theory (DFT) formalism
(B3LYP-6311g+ for structure optimization) and ZINDO-S, CIS
and TD-SCF to calculate the vertical Franck–Condon transitions.
Calculations performed with ZINDO-S method overestimate
kmax by about of 10–30 nm, whereas CIS (631 g basis) underesti-
mate kmax values by a larger amount. Better results were obtained
using TD-SCF (DFT 6311+g) level of theory, from which verti-
cal transitions at 339.8 nm (f = 0.61), 357.0 nm (f = 0.85) and
379.7 (f = 9.1) are predicted for NX1, NX2 and NX3, in good
agreement with the experimental values. In addition, molecular
orbital analysis indicates a p–p* transition for these molecules.
These results are consistent with those previously reported for
benzoxazole derivatives (46) and structurally related compounds
such as benzoxazinones (47) and coumarins (48).
In Eq. (2), p* accounts for polarizabilities and dipolarities of
solvent (52,53), d is a correction term for polarizability, a corre-
sponds to the hydrogen bond donor solvent ability, b indicates
solvent capability as a hydrogen bond acceptor and qH is the
Hildebrand parameter, a measure of disruption of solvent–solvent
interactions in creating a cavity (50). The coefficients of the LSER
equation (Eq. (2)) obtained by multilinear regression analysis of
ꢂ
the dependence of mF on the solvent parameters are given in Table
S3. This analysis is supported by purely statistical criteria. To sum
up, sample size, N, product correlation coefficient, R, standard
deviation, SD and the Fisher index of equation reliability, F indi-
cate the quality of the overall correlation equation. The reliability
of each term is indicated by t-statistic, t-stat and the variance infla-
tion factor (VIF). Suitable quality is indicated by large N, F and
t-stat values, small SD values and R and VIF values close to one
(54). Also, we analyzed the dependence of the fluorescence maxi-
mum on solvent using a theoretical set of correlation parameters
determined solely from computational methods (55–57). The
TLSER descriptors have been developed to give optimal correla-
tion with LSER descriptors. The generalized TLSER equation
proposed by Famini et al. (55–57), (Eq. 3) can be used to analyze
dependence emission maxima on solvent properties.
ꢂmF ¼ ꢂm0 þ ap1 þ beb þ cqꢁ þ dea þ eqþ þ hqH2
ð3Þ
Emission spectra
In Eq. (3), the bulk/steric term is described by the Hildebrand
parameter, qH. The parameter p1 corresponds to the index of
polarizability and accounts for the ease of moving (or polarizing)
of the electron cloud, and is obtained from the ratio between polar-
izability volume and molecular volume. The hydrogen bond
acceptor basicity (HBA) involves covalent, eb, and electrostatic,
NX1, NX2 and NX3 exhibit intense emission spectra. The posi-
tion of the fluorescence maximum is strongly affected by the sol-
vent, a red shift being observed upon increasing the solvent
polarity (Fig. 3). Such behavior indicates that the fluorescent
state would be a highly polar state with an important charge-
transfer character. The relevant emission parameters in the
solvent set employed are included in Table S2.
A deeper rationalization of solvent-induced shift of the fluo-
rescence spectra can be obtained from the analysis of the fluores-
cence maximum dependence on microscopic solvent parameters
q , terms. Similarly, hydrogen bond donor acidity (HBD) includes
ꢁ
covalent, ea, and electrostatic, q+, terms (57). The coefficients of
the LSER and TLSER equations obtained by multilinear regres-
sion analysis for the dependence of the fluorescence maximum on
solvent parameters are given in Table S3.
Results (Table S3) show that not all descriptors are statisti-
cally relevant. Descriptor coefficients accepted in the correlation
equation were those having significance level ≥0.95. Accord-
ingly, the qH and b parameters were discarded in LSER correla-
tion for NX1 and NX3. Furthermore, qH was not included in
LSER correlation for NX2. According to the LSER coefficients
in Table S3, the energy of the fluorescent state decreases in sol-
vents with largest capacity to stabilize charges and dipoles and
in strong HBD solvents. In addition, for NX2 the energy of the
fluorescent state also decreases in HBA solvents. This results
support an emissive state with a strong intramolecular charge-
transfer character which is stabilized in p* solvents. Further
stabilization of the excited state is obtained from the interactions
of HBD solvents with the most negative center of the molecule,
probably the oxygen atom in the oxazole ring. Dependence of
the emission wavelength on the b parameter for NX2 can be
explained in terms of HBA solvent interactions with the most
Methanol
1.0
0.8
0.6
0.4
0.2
0.0
DMF
Dioxane
Hexane
400
450
500
λ/nm
Figure 3. Normalized fluorescence spectra of NX2 in solvents represen-
tatives of the polarity scale.

4
Manuel Curitol et al.
positive center of the excited molecule, probably the dimethyla-
fluorescence maxima depends on solvent polarity function
defined by dielectric constant (e) and refractive index (g) of the
solvent. The dipole moment change between excited- and ground
states can be experimentally calculated from the Lippert–Mataga
relationship, if polarizability of the solute can be neglected, i.e.
a = 0 (61,62):
mino substituent. Similarly, only q , ea and q+ were statistically
ꢁ
relevant in the TLSER study. The TLSER treatment (Table S3)
shows that HBD and HBA solvents increase the emission wave-
length for all NX1, NX2 and NX3. In the TLSER treatment π1
(not included in our correlation equations of Table S3) models
the solvent ability to stabilize dipole-induced dipole and induced
dipole-induced dipole interactions. No adequate TLSER model
for dipolarity itself has been found. The TLSER analysis indi-
cates that the influence of HBA solvents is mainly electrostatic
ꢀ
ꢁ
e ꢁ 1
g2 ꢁ 1
2
ꢂ
ꢂ
mA ꢁ mF ¼ m1f ðe; g Þ þ const: ¼ m1
ꢁ
þ const:
ð4Þ
2e þ 1 2g2 þ 1
because only q is included in the correlation equation, whereas
ꢁ
HBD solvents shift the fluorescence maximum by means of elec-
trostatic and covalent interactions. The dependence of the wave-
length of the fluorescence maximum on ea would be explained if
considerable negative charge is developed on the oxygen atom in
the heterocyclic ring, which HBD solvents can stabilize through
predominant hydrogen-bonding interactions. Furthermore, if the
positive charge in the excited state is delocalized on the whole
aromatic system, no formal bonding interactions with HBA sol-
vents can be expected.
2
ðle ꢁ lgÞ
ja3
m1 ¼
ð5Þ
where le and lg are the dipole moments of the fluorophore in
the excited- and the ground state respectively; a is a radius of
the Onsager cavity, assumed to be a sphere and j is a universal
constant equal to 1.10511 9 10ꢁ35 C2. For those spectra in
which the maximum is not well defined due to the fine structure
or shoulders appearance, the maximum wavelength of the lower
energy band was obtained using a convolution method. A repre-
sentative Lippert–Mataga plot for NX1 is shown in Fig. 4. At
this point, it must be stated that, according to the results of the
solvent effect, the stabilization of fluorescent excited state not
only depends on electric factors. Solvent acidity and basicity also
lower the energy (affecting the maximum wavelength). So, deter-
minations by using Lippert–Mataga relationship in this case,
probably overestimate the magnitude of excited dipole moments.
Based on the calculated values of Onsager’s cavity radius and
the ground-state dipole moment for NX1, NX2 and NX3 using
DFT B3LYP (6311+g) to minimize the molecular geometry, the
excited-state dipole moments were calculated from the slopes of
Lippert–Mataga plots. Results obtained from these experiments,
summarized in Table 1, show a significant increase in the dipole
moment in the singlet excited state. Thus, a relatively large
charge transfer takes place during the excitation of the oxazole
derivatives. The larger dipole moments obtained for both NX2
and NX3 for the ground and the excited state reflect the donor
effect of the dimethyl and diphenylamino group on the phenyl
substituent, implying that both naphthalene moiety and phenyl
substituent in position 2 are involved in the charge-transfer
Fluorescence quantum yields for NX1 and NX2 are close to 1
in most solvents (Table S2). Furthermore, fluorescence lifetimes
are near to 2 and 1.3 ns for NX1 and NX2, respectively, and are
essentially solvent independent because the predominant radiative
process is mainly dominated by the aromatic nature of the mole-
cule. For NX3, fluorescence quantum yield shows solvent depen-
dence, diminishing from 1 in nonpolar solvents to around of
0.65 in polar solvents. Concomitantly, the fluorescence lifetimes
increase from 1.3–1.5 ns in nonpolar solvents such as hexane or
benzene to 3.7–4.2 ns in polar solvents such as methanol or pro-
pylencarbonate. These results can be explained if we consider
that /F = kF/(kF + knr) and sF= 1/(kF + knr) and the formation of
a twisted intramolecular charge transfer (TICT) that opens a non-
radiative deactivation pathway due to the rotation of the disubsti-
tuted amino group (58). It is well known that the energy barrier
for the formation of a TICT state from the locally excited state
decreases as the polarity of the medium increases (59,60). As the
dipole moment of the TICT state is very large, the dipolar inter-
actions of the TICT state will raise with the increase in the
solvent polarity. Thus, the diminution of the fluorescence
intensity of NX3 in polar solvents could be due to the large sta-
bilization of the highly polar TICT state by strong dipole–dipole
and/or hydrogen-bonding interactions and consequent rapid non-
radiative transition to the ground state. This fact diminishes kF
and increases knr, but the radiative deactivation path remains as
the prevailing process in the photophysics of di-N,N-substituted
aminophenylnaphthoxazoles. The effect is evident for NX3, but
not clearly observed for NX2 because the TICT formation
depends on two main factors, the planarity of the molecule and
the donor–acceptor character of the substituent, both advanta-
geous for the molecule with the diphenylamino substituent. Inde-
pendent of these results, the large fluorescence quantum yields
and short fluorescence lifetimes found for the naphthoxazole
derivatives in a representative set of solvents, make these com-
pounds very promising fluorescent probes to be used in a wide
range of microenvironments.
3000
2000
1000
0.0
0.1
0.2
0.3
f( )
ε,n2
Excited-state dipole moment determination
Figure 4. Lippert–Mataga plot of the difference between the maximum
of the absorption and fluorescence on the solvent polarity function, f(n2,
e) = (((e ꢁ 1)/(2e + 1))ꢁ((n2 ꢁ 1)/(2n2 + 1))) for 2-phenyl-naphtho[1,2-
d][1,3]oxazole.
The spectral shift of the fluorescence, or more rigorously, the
difference between the positions of the absorption and

Photochemistry and Photobiology
5
Table 1. Ground (lg) and excited dipolar moments (le) for NX1, NX2
and NX3.
1.0
0.5
0.0
ꢃ
a0 (A)
Compound
lg/D
Δl/D
le/D
SDS
NX1
NX2
NX3
0.96
3.31
2.01
5.14
5.67
6.30
9.61 ꢀ 1.3
12.01 ꢀ 1.8
14.55 ꢀ 2.1
10.57 ꢀ 2.0
15.32 ꢀ 2.31
16.56 ꢀ 2.41
TX-100
Water
MPS
lg obtained from DFT B3LYP 6311g+dp minimized structure in gas
phase, a0 calculated from DFT B3LYP 6311g+dp minimized structure,
Δl obtained from the slope of the Lippert–Mataga plot.
process toward the oxazole acceptor center. Similar results for
the change in the dipole moment as consequence of an intramo-
lecular charge transfer have been reported for analogous
2-phenyl-benzoxazole derivatives (63,64).
400
500
λ/nm
600
Figure 5. Comparison of the normalized fluorescence spectra of 2(4-N,
N-diphenylaminophenyl)naphtho[1,2-d][1,3]oxazole in water and in
micellar solution of various surfactants.
Photostability
Considering the potential use as fluorescent dyes in dye lasers or
for single-molecule-based microscopy techniques of the studied
compounds, we also evaluated their photostability under extreme
irradiation conditions. Photoconsumption quantum yields were
evaluated in benzene as solvent, using Aberchrome 540 as acti-
nometer and Exalite 376 (rhodamine 110) as reference. Samples
were irradiated with the third harmonic pulses (355 nm) of a
Continuum-Surelite II Nd:Yag laser (6 ls, 15 mJ/pulse, 6000
pulses). The concentration changes were monitored by employ-
ing gas chromatography and/or UV–Vis spectrophometry. Photo-
consumption quantum yields, (the mean of six measurements)
ET(30) values corresponding to the microenvironment in which
NX3 locates in SDS, TX-100 and SuMP micellar solutions. With
this approximation, we can conclude that NX3 in SDS micellar
solutions senses a media similar to dimethylformamide or aceto-
nitrile whereas in TX-100 and SuMP micellar solutions the
microenvironment resembles dioxane or tetrahydrofuran, conse-
quently, both probes, NX2 and NX3, should be located near to
the micellar interface. Due to the intramolecular charge-transfer
nature of the excited NX2 and NX3 state, the disubstituted
amino group, –NR2 should have a more positive charge whereas
the electron withdrawing naphthoxazole moiety will have a more
negative charge. Hence, in SDS micelles, it could be suggested
that the –NR2 group is located in the Stern layer (containing the
negatively charged sulfate groups) and the naphthoxazole moiety
in the Gouy–Chapman layer (including the more positively
charged sodium counter ions) (see Figure S1). Similarly, in
SuMP micelles the positively charged amino substituent interacts
with the carboxyl oxygen of the acid moiety of the surfactant,
whereas the oxazole ring localizes between the sucrose head
groups where the interactions between the hydroxyl groups and
the oxazole ring heteroatoms are very favorable. On the contrary,
for the fluorescent probe, 2-(p-dimethylaminostyryl)benzoxazole
has been proposed that in TX-100 micelles the –NR2 group is
present in the Gouy–Chapman layer, whereas the benzoxazole
ring locates in the Stern layer (29). Consequently, red shifting in
SDS micelles can be understood in terms of two factors, the
stabilization of the probe ground state in water, a process mark-
edly dominated by hydrogen bond interactions, and the greater
stabilization of its excited singlet state in the SDS micellar inter-
face where the larger molecular dipole of the excited species
makes possible strong interactions with the charged interface.
The blue shift observed in TX-100 and SuMP micelles can be
explained in a similar fashion. The transference of the probe
from the water media to the non-ionic, although rich in hydroxyl
groups interface, destabilizes the probe ground state because
hydrogen bond interactions are broken. However, an even more
important destabilization of the excited probe in the interface
should take place, due to the increase in the probe dipole
moment with the excitation and the less-polar microenvironment
around it as result of the presence of structured water molecules
near to surfactant heads. A similar, less-polar, rich in structured
for NX1, NX2 and NX3, equal to (4.6 ꢀ 0.9) 9 10ꢁ5
,
(2.4 ꢀ 0.6) 9 10ꢁ5 and (2.1 ꢀ 0.5) 9 10ꢁ5, respectively, are
close to the measured for the extensively used dye rhodamine
110, equal to (6.7 ꢀ 1.4) 9 10ꢁ6. These results show that the
studied naphthoxazole derivatives are very photostable being
appropriate molecules to be used as fluorescent dyes in experi-
ments involving long-term irradiation.
Studies in micellar systems
Self-association of long-chain amphiphilic molecules forms
micellar aggregates in aqueous solutions, above certain concen-
tration called critical micelle concentration (CMC). In this sec-
tion, NX2 and NX3 have been employed to determine the CMC
values for the commonly used surfactants sodium dodecyl sulfate
(SDS) triton X-100, TX-100 and sucrose monopalmitate (SuMP).
The CMC values were calculated from the variation in the rela-
tive fluorescence intensities (IF/IF0) with the surfactant concentra-
tion, however, the observed change in the fluorescence spectrum
with the surfactant addition depends on the surfactant chemical
nature. Addition of SDS to an aqueous solution of NX2 and
NX3 increases the fluorescence intensity and shifts the emission
maxima to the red (Fig. 5), whereas unexpectedly, when TX-100
and SuMP are added, the fluorescence maximum is blue shifted.
This behavior can be rationalized in terms of the NX2 and
NX3 localization in the micelle. It is obvious that the probe is
not incorporated into the micelle core, otherwise a noteworthy
blue shift of the emission maxima (up to approximately 410–
420 nm) would be observed due to the nonpolar nature of this
microenvironment. From the dependence of the fluorescence
maximum with the dielectric constant and the ET(30) parameter
of the solvent, we can estimate the dielectric constant and the

6
Manuel Curitol et al.
water microenvironment around the TX-100 head groups can be
also expected. This picture of the probe location in micellar solu-
tions is further supported by the larger increase in the fluores-
cence quantum yield in SuMP and TX-100 micelles (Fig. 6).
The increase of /F in SuMP and TX-100 micelles implies that
the probe is located in a constrained microenvironment where its
mobility is relatively reduced due to the presence of structured
water molecules, although probe incorporation in some extension
between the hydrophobic tails of surfactant molecules has been
also suggested by Fayed (29), studying the absorption and emis-
sion characteristics of 2-(p-dimethylaminostyryl) benzoxazole in
micellar solutions.
terms of the probe interactions with the hydroxyl groups of the
sucrose head of the surfactant and probably with SuMP aggre-
gates at the premicellization range (68).
CONCLUSIONS
The studied naphthoxazole derivatives show valuable spectro-
scopic and photochemical properties: large molar absorbability and
fluorescence quantum yield, short fluorescence lifetime and com-
parable photostability to commercially available laser dyes. These
compounds have a photophysic behavior similar to analogous
benzoxazoles, but with larger molar absorptivity’s, a small red
shift of both the absorption and emission maxima and a larger
Stokes shift in polar solvents. These properties in addition to the
large solvent–dependent changes in the dipole moment on excita-
tion make these molecules valuable candidates as fluorescent
probes to be used in fluorescence microscopy and to determine
micellar properties such as CMC or partition constants of added
additives (68).
Despite of the emission shift, the CMC values obtained for
SDS, TX-100 and SuMP are in good agreement with those
obtained using other well-known probes (3,65–67). Figure 6A–C
0
shows the plots obtained for the dependence of the values of IF/IF
on the SDS, TX-100 and SuMP concentration, monitored at 460,
430 and 441 nm, respectively, when the probe employed was NX3.
From these plots the CMC values included in Table 2 were
obtained. When SDS or TX-100 was the surfactant, fluorescence
intensity values increase abruptly at the CMC, if the fluorescence
changes are followed at the wavelength corresponding to the
maximum observed above the CMC and the fluorescence inten-
sity increase was independent of the probe employed. With
SuMP the behavior is more complicated as deduced from the
plot of IF/IF0. The fluorescence intensity increases gradually from
very low SuMP concentration. This behavior can be explained in
Acknowledgement—Financial support from FONDECYT (grants 1080410
and 1040573) is gratefully acknowledged.
SUPPORTING INFORMATION
Additional Supporting Information may be found in the online
version of this article:
Table S1. Detailed absorption characterization of NX1, NX2
and NX3 in solution.
Table S2. Detailed emission characterization of NX1, NX2
and NX3 in solution.
Table S3. Analysis of solvent effect on the emission of NX1,
NX2 and NX3 using LSER equations.
Figure S1. Schematic representation of NX3 location in SDS
and SuMP micelles.
25
C
B
A
10
8
25
20
15
10
5
20
15
10
5
6
4
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