On the photophysical and photochemical behavior of fenbufen: A study in homogeneous media and micellar environments

Article abstract of DOI:10.1039/b100164g

The photochemistry of fenbufen, γ-oxo-[1,1′-biphenyl]-4-butanoic acid (FB), has been investigated in homogeneous solutions and in micellar environments. The photochemical and photophysical properties of the drug are mediated by its lowest excited triplet states. The presence of a low-lying π,π* triplet is responsible for the inefficient Norrish Type I photocleavage, whereas an upper n,π* triplet promotes a large intersystem crossing yield. FB interacts with both anionic and cationic micelles. The different binding mode of the drug with the microheterogeneous environments can significantly change its photoreactivity. These changes are rationalized on the basis of "micellar cage effects".

Full text of DOI:10.1039/b100164g

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On the photophysical and photochemical behavior of fenbufen:
a study in homogeneous media and micellar environments
Salvatore Sortino,*a Lydia J. Mart•nez*b and Giancarlo Marconic
a Dipartimento di Scienze Chimiche, Universita di Catania, V iale Andrea Doria 8, I-95125,
Catania, Italy. E-mail: ssortino=mbox.unict.it
b Department of Chemistry, Union College, Schenectady, NY 12308, USA
c Istituto di Fotochimica e Radiazioni dÏAlta Energia del CNR, Area della Ricerca, V ia Piero
Gobetti 101, I-40129, Bologna, Italy
Received (in Montpellier, France) 22nd December 2000, Accepted 23rd April 2001
First published as an Advance Article on the web 1st June 2001
The photochemistry of fenbufen, c-oxo-[1,1@-biphenyl]-4-butanoic acid (FB), has been investigated in homogeneous
solutions and in micellar environments. The photochemical and photophysical properties of the drug are mediated
by its lowest excited triplet states. The presence of a low-lying p,p* triplet is responsible for the inefficient Norrish
Type I photocleavage, whereas an upper n,p* triplet promotes a large intersystem crossing yield. FB interacts with
both anionic and cationic micelles. The di†erent binding mode of the drug with the microheterogeneous
environments can signiÐcantly change its photoreactivity. These changes are rationalized on the basis of ““micellar
cage e†ectsÏÏ.
Drug photochemistry has been the object of a considerable
amount of attention, especially in the last decade. Such inter-
est has been promoted by the fact that even though many of
these compounds are excellent therapeutical agents, they are
also known to often cause phototoxic and/or photoallergic
phenomena.1h3 Drug photodegradation reactions, leading to
transient species and stable photoproducts, as well as drug
photoinduced energy transfer processes, giving rise to singlet
oxygen photosensitization, are mainly responsible for such
undesirable side e†ects. Due to the awareness among the
scientiÐc community of the higher levels of solar UV radiation
reaching the earth, interest in the present topic has recently
increased markedly. Several reviews concerning the mecha-
nisms of both drug photodegradation and photosensitization
were recently published.4h6
compartmentalization in a biological microenvironment.7,8
Among the several biological mimetic systems, micelles rep-
resent one of the most exploited; photochemical investigations
in such an environment are always an active area of research.
Fenbufen (FB), c-oxo-[1,1@-biphenyl]-4-butanoic acid, is
believed to act as a prodrug for active anti-inÑammatory
metabolites such as 4-biphenylacetic acid and its hydroxylated
derivatives, which are inhibitors of the cyclooxygenase
enzymes.9 Although the incidence of erythema photoinduced
by FB is low, studies have shown that this prodrug stimulates
the production of interleukin-1 in keratinocytes exposed to
UV radiation.10 Moreover, it has been shown that FB is
capable of inducing both photohemolysis of red blood cells
and the photoperoxidation of unilamellar phosphatidylcholine
liposomes upon UVA irradiation.11
From this picture, it emerges that shedding light on the
photophysical properties and photochemical behavior of these
therapeutic agents is a prerequisite for an understanding of
the molecular basis of both their photostability and photo-
sensitized biological e†ects. Moreover, beyond the concern for
both drug photostability and phototoxicity, from a strict
chemical point of view this subject o†er opportunities to gain
more insight into fundamental aspects related to mechanisms
of photoinitiated processes in homogeneous solution,
organized systems and constrained media. Actually, for an
appropriate correlation between the phototoxicity and the
photochemical properties of a given drug to be obtained, an
adequate study should investigate the photochemistry of the
photosensitizer both in homogeneous solution and in the
presence of either a biological target or simpler model
systems. The physical properties of the photosensitizer (i.e.,
hydrophobicity, geometry, size, shape and charge) often lead
to the formation of supramolecular aggregates with bio-
molecules. These aggregates can display di†erent photo-
chemical properties compared to those of the ““free
photosensitizerÏÏ. The nature of the lowest excited states, the
efficiency of the photophysical and photochemical pathways,
the redox potentials, the fate of the reaction intermediates and
the opening of new photoreactive channels are some of the
properties of the photosensitizer that can be modulated by its
This paper reports on the spectroscopic and photo-
tochemical behavior of FB in homogeneous solution and in
micellar environment by coupling both steady-state and time-
resolved techniques to theoretical calculations.
Experimental
Reagents
Fenbufen and benzophenone were purchased from Sigma
Chemical Company. The latter was recrystallized twice from
ethanol. Sodium dodecyl sulfate (SDS) and cetyltrimethyl-
ammonium chloride (CTAC) were purchased from Fluka.
Water was puriÐed through a Millipore Milli-Q system. The
phosphate bu†er (10~2 M, pH 7.4) was prepared from reagent
grade products. All the organic solvents were of the highest
purity available and used without further puriÐcation.
Steady-state experiments
Irradiations were performed using monochromatic light
obtained from a Series 200 HeÈCd 325 nm laser (Liconix, St.
DOI: 10.1039/b100164g
New J. Chem., 2001, 25, 975È980
975
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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Clara, CA, USA). The incident photon Ñux was ca. 5 ] 1015
quanta s~1. The experimental irradiation procedures and the
light intensity measurements have been described pre-
viously.12,13
Instruments. The determination of the O (1* ) quantum yield
2
g
(U ) was carried out using optically matched oxygen-saturated
*
solutions of FB and phenalenone (U \ 0.98)17 by comparing
*
the linear slopes of plots for the dependence of the signal
The photodegradation quantum yields of FB, both in the
absence and in the presence of micelles, were determined
through high performance liquid chromatography (HPLC)
analysis, from the disappearance of the starting compound up
to 25% conversion. Due to the low quantum yield and the
necessity of analyzing at a low conversion percentage, the
values of U obtained in aqueous and in micellar solutions
were a†ected by a fairly large error (^20%). HPLC analysis
was performed on a Hewlett Packard 1100 chromatograph
equipped with an on-line photodiode array detector (DAD).
The chromatograms were recorded by monitoring at 230, 260
and 290 nm. The quantitative separation of the photoproducts
was achieved on a LiChroCart RP-18 column (5 lM packing,
intensity (extrapolated to zero time) against the laser energy.
Results and discussion
Absorption and emission
Fig. 1 shows the absorption and the Ñuorescence spectra of
FB recorded in bu†er solution at pH 7.4. The absorption
spectrum is characterized by a main band centered at 284 nm
(e \ 20 800 M~1 cm~1) and a weak tail extending beyond 350
nm. A shift towards shorter wavelengths was observed as the
solvent polarity decreased. In particular, the main band
shifted to 280 nm in both acetonitrile and isopropanol. The
Ñuorescence spectrum in bu†er solution exhibits a band cen-
4 ] 250 mm; HP) by eluting with a linear gradient of CH CN
3
in 0.01 M phosphate bu†er (pH 7) from 0 to 75% in 15 min,
tered at 400 nm. The Ñuorescence quantum yield (U ) of FB
and a Ñow rate of 1 mL min~1.
flu
under these experimental conditions is ca. 0.001. A red shift of
The absorption and Ñuorescence spectra were recorded with
a Beckman DU 650 spectrophotometer and a Spex Fluorolog-
2 (model F-111) spectroÑuorimeter, respectively. Fluorescence
quantum yields were obtained using quinine sulphate in 1 N
H SO (U \ 0.54) as the standard. The absorbance values of
ca. 10 nm of the Ñuorescence maximum accompanied by a
4-fold reduction of U was observed in acetonitrile whereas
flu
in isopropanol solution FB was basically non-Ñuorescent. The
values of U obtained are much lower than those reported
flu
2
4
flu
for biphenyl in similar solvents (U B 0.2), accounted for by a
the samples at the excitation wavelength were lower than 0.1
for a 1 cm pathlength. The Ñuorescence lifetime in degassed
solution was determined by means of a time-correlated single
photon counting system (IBH Consultants Ltd.). A nitrogen-
Ðlled lamp was used for excitation at 337 nm. The phosphor-
escence emission spectrum was measured at 77 K in an
ethanol glass using a Perkin-Elmer LS 50. The triplet energy
was determined from the onset of the phosphorescence spec-
trum.
flu
low-lying and scarcely emitting n,p* singlet state (vide infra).
This is not surprising given the molecular structure of FB,
which can be viewed either as a substituted biphenyl or as a
substituted phenyl alkyl ketone.
Fig. 2 shows the phosphorescence emission spectrum of FB
in an ethanol glass at 77 K. A triplet energy of ca. 60 kcal
mol~1 can be estimated from the onset of the band. This value
is quite low if compared to that reported for biphenyl itself
(ca. 70 kcal mol~1)18 but in good agreement with that report-
ed for acylbiphenyl derivatives (ca. 65 kcal mol~1).19 Liter-
ature data concerning the nature of the lowest triplet states of
para-substituted phenyl ketones suggest a predominantly
3p,p* symmetry for such electronic states.19 Similarly to what
Nanosecond laser Ñash photolysis and time-resolved detection
of singlet molecular oxygen
The laser Ñash photolysis system employed in these studies
has been previously described.14,15 FB solutions ranging in
concentration from 0.3 to 1.0 mM were excited with a Molec-
tron UV-24 nitrogen laser (337 nm). All transient spectra were
recorded employing a Ñow system with a 7 ] 7 mm2 Suprasil
quartz cell with a 2 mL capacity and were purged with N or
2
O for 30È45 min before as well as during data acquisition. A
2
Surelite Nd : YAG laser (355 nm, 8 ns pulse) was used to
excite 3.5 mM solutions of benzophenone (BP) in acetonitrile
with or without FB for the determination of the triplet molar
absorption coefficients (*e ) by TÈT energy transfer and inter-
T
ISC
system crossing yields (U ) by comparative techniques. The
literature values for benzophenone in acetonitrile are (*e \
T
6500 mol L~1 cm~1 at 520 nm and U \ 1.0.16 At the FB
ISC
concentrations employed, more than 99% of the BP triplets
Fig. 1 Absorption and Ñuorescence (j \ 310 nm) spectra of FB in
were quenched. Nevertheless, kinetic corrections for incom-
plete quenching of the benzophenone triplet (using an experi-
mentally determined value for the rate of quenching of the
benzophenone triplet by FB of 4 ] 109 M~1 s~1) were per-
exc
phosphate bu†er (1 ] 10~2 M, pH 7.4).
formed in order to accurately determine the *e for FB.
T
Time-resolved detection of the singlet molecular oxygen O
2
(1* ) phosphorescence at 1270 nm was achieved by exciting at
g
308 nm with a Lumonics EX-530 laser. An EG&G Judson J16
8SP ROM5 (5 mm) germanium photodiode was mounted on
a modiÐed BNC connector. A 3 mm silicon Ðlter (Oriel Cor-
poration, Model 54081) was positioned between the sample
cell and the detector to remove any shorter wavelength IR or
visible interference. The signal was ampliÐed using a Stanford
Research Systems preampliÐer equipped with short and long
bandpass Ðlters, and captured by a Tektronix 2440 digitizer.
Data acquisition and processing were done with software
developed in the LabVIEW 3.1.1 environment from National
Fig. 2 Phosphorescence emission spectrum (j \ 320 nm) of FB in
exc
an ethanol glass at 77 K.
976
New J. Chem., 2001, 25, 975È980

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has been observed in these cases, the presence of two close-
lying triplets of n,p* and p,p* symmetry can be suggested for
FB. This proposal is conÐrmed by both theoretical calcu-
lations and photochemical experiments (vide infra).
We performed semi-empirical quantum mechanical calcu-
lations (ZINDO/S) starting from a ground state optimized
geometry (MM ] force Ðeld)20 and obtained the results
reported in Table 1. In general, one can describe FB as a
biphenyl perturbed by a carbonyl substituent that plays a sub-
stantial role in the photophysics of the whole molecule by
providing low-lying states of an n,p* nature. The Ðrst two
triplets, calculated to be at 580 and 471 nm respectively, are of
a di†erent nature, the Ðrst being a p,p* and the second an n,p*
state. Although the calculated energy gap between these states
is large (0.5 eV), there is the possibility of a proximity e†ect
due to the fairly large vibronic couplings brought about by
the C2O stretching mode (typically up to 1500 cm~1 in car-
bonyl compounds).21
Fig. 3 Transient absorption spectra observed in a 1 ] 10~3 M N -
2
saturated solution of FB in acetonitrile (…) 0.40, (L) 1.28, (=) 3.20
and (K) 6.80 ls after excitation by a 2.0 mJ laser pulse at 337 nm. The
inset shows the decay trace monitored at 420 nm.
of FB. In particular, it promotes intersystem crossing (ISC)
The Ðrst calculated singlet (414.8 nm) corresponds in orbital
and the photogeneration of singlet oxygen, O (1* ). We
nature to the second triplet T (n,p*) and has null oscillator
2
g
2
obtained values of ca. 0.85 and 0.45 for U and U , respec-
strength: it can be activated through vibronic coupling and
ISC
*
tively, in acetonitrile. The value of U is somewhat lower than
that reported by Navaratnam and Jones.22 The discrepancy
could be due to both the di†erent excitation wavelengths and
corresponds, in energy, to the Ñuorescent state. The Ðrst size-
able singlet (responsible for the main absorption band) is cal-
*
culated to lie at 298 nm as S (p,p*) and corresponds to the
3
temperatures. The U
reported for biphenyl (U B 0.6) and can be accounted for by
an assisted ISC mediated by the upper n,p* triplet state
value is clearly higher than that
intense 1A ] 1L of the parent biphenyl, which hides the less
ISC
a
intense 1A ] 1L transition (calculated to be 304.6 nm).
Taking into account the calculated dipole moments of the
ISC
b
(Scheme 1). Fast internal conversion to a lower triplet state of
states involved (Table 1), one can explain the shifts observed
in the absorption and emission bands with the decrease of the
solvent polarity.
p,p* symmetry can explain the relatively large value for U
and the inefficient a-cleavage undergone by this ketone (vide
*
infra). In addition, the value obtained for the efficiency of O
2
(1* ) formation (S \ 0.53) is of the order of magnitude
Laser Ñash photolysis
g
*
reported for biphenyl itself whose lowest triplet state has a
The transient absorption spectra obtained after 337 nm laser
p,p* nature (S \ 0.42).24 Several groups have tried to explain
*
excitation of FB in N -saturated acetonitrile solution and
the intermediate S values (0.35 \ S \ 0.85) for naphthalene,
2
*
*
recorded at various delay times with respect to the laser exci-
biphenyl, and other aromatic systems. They have proposed
that the equilibrium between the singlet and triplet states of
the charge-transfer complex formed between the photo-
sensitizer and oxygen could be slower than its decay.24,25
Although it would be tempting to suggest that for FB the
tation are shown in Fig. 3. The spectra are characterized by a
broad absorption band extending from 320 nm to beyond 500
nm, with a maximum centered at 420 nm. This transient
species decays mono-exponentially with a rate constant of
6.7 ] 104 s~1 (inset Fig. 3) and no relevant new bands form
concurrently with its decay. Moreover, this transient can be
value of S reÑects the fact that there is a considerable mixing
*
between triplets of n,p* and p,p* symmetry, this certainly does
quenched by oxygen (k \ 8.2 ] 108 M~1 s~1) and its forma-
not explain the trend observed for other aromatic systems for
which mixing between states does not occur (i.e. biphenyl).
Literature data report weak vibronic mixing between two
triplet states whenever they are as close as the n,p* and p,p*
q
tion can be sensitized by benzophenone (k \ 4 ] 109 M~1
q
s~1, see Experimental). These results are basically the same as
those obtained by Navaratnam and Jones in the same
solvent.22 Thus, this band can be safely attributed to the TÈT
absorption of FB. This triplet absorption maximum for FB is
red-shifted compared to the TÈT absorption spectra reported
for biphenyl,23 probably due to the additional conjugation
induced by the presence of the carbonyl substituent.
triplets, even for biphenyl ketones.26 The wave function W of
T1
the lowest triplet can be represented by a linear combination
of two unperturbed wave functions W, with the mixing coeffi-
cients a and b dependent on the energy gap between the two
pure states and on the availability and efficacy of mixing
vibrations of proper symmetry:26
As mentioned previously, the presence of this carbonyl sub-
stituent has profound e†ects on the photophysical properties
WT1 \ aW(p,p*) ] bW(n,p*)
Table 1 Calculated energies, dipole moments, oscillator strengths
and nature of the low-lying states of FB
Taking into account the calculated energy gap between the
two states and a reliable vibronic coupling induced by the
C2O stretching mode, we can infer that a mixing of b/a \ 0.37
can be expected.
Energy/
kcal mol~1
State
Dipole/D
Osc. strength
Nature
S
0
0
6.93
7.49
2.79
7.04
2.60
7.93
8.65
7.70
7.08
4.85
4.85
12.73
T
1
49.3
60.6
67.0
68.9
77.0
79.2
81.5
84.4
84.8
93.9
95.5
p,p*
n,p*
p,p*
n,p*
p,p*
p,p*
p,p*
p,p*
n,p*
n,p*
p,p*
T
T
2
3
S
1
0.0
T
4
T
5
T
6
T
7
T
8
S
0.0008
1.074
2
S
3
Scheme 1
New J. Chem., 2001, 25, 975È980
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Our interpretation is conÐrmed by laser Ñash photolysis
experiments in bu†er solutions at pH 7.4. The transient spec-
trum recorded in 10~2 M phosphate bu†er saline (PBS)
singlet state and the low photodegradation quantum yield, it
is also valid to propose a singlet-mediated Norrish Type I
photocleavage. In this context, we measured an upper limit of
0.2 ns for the singlet lifetime of FB in aqueous solution, which
rules out any relevant quenching by oxygen on the singlet
state. Since the photodegradation quantum yield of FB
dropped threefold in oxygen-saturated solutions, it is reason-
able to propose a photodecomposition mediated by a long-
lived triplet state.
undergoes a 25 nm red shift (j \ 450 nm) and decays
max
mono-exponentially with a rate constant of 1.4 ] 105 s~1
(data not shown). The large red shift in the TÈT absorption
spectra is in agreement with the p,p* nature of the lowest
triplet state. In addition, the rate constant for the quenching
of the FB triplet by oxygen in PBS is 5 ] 108 M~1 s~1, which
is similar, within experimental error, to that obtained in ace-
tonitrile. Due to the low solubility of benzophenone in water,
Photoprocesses in micellar environments
we were not able to determine U by energy transfer, and
ISC
The ground state absorption and emission spectra for FB in
0.1 M SDS were identical to those recorded in aqueous solu-
tion, suggesting the absence of speciÐc interactions between
the surfactant and both the ground and excited states of FB.
Nevertheless, the association between FB and SDS was
demonstrated by Ñuorescence quenching experiments per-
formed by using iodide, a quencher that is conÐned mainly in
the aqueous environment. The quenching results, performed
at two di†erent concentrations of SDS, are reported in Fig. 5
and analyzed by using eqn. (1):33
ground state depletion methods are difficult to perform with
an N laser, yielding unreliable data. We determined a U
2
*
value for FB in D O of 0.74. Assuming the same U as in
2
ISC
acetonitrile would result in an S value of ca. 0.8, which is
*
closer to the value expected for aromatic systems having
lowest triplet states of p,p* symmetry. In a more polar
environment, mixing between the n,p* and p,p* triplet states
decreases (see Table 1) and the lowest triplet state of FB has a
predominantly p,p* nature.
Photoreactivity in aqueous solution
U0/(U0 [ U ) \ [(aU0 /bU0 ) ] 1][1 ] 1/k q0 [Q]] (1)
f
f
f
fm
fw
q fw
Fig. 4 shows the changes in the absorption spectra observed in
a nitrogen-saturated solution of FB at pH 7.4 at di†erent irra-
diation times. Previous work pointed out that these results
were consistent with the formation of a main stable photo-
product, 4-biphenylcarboxyaldehyde.11 An a-cleavage process
(Norrish Type I fragmentation) accounts for the formation of
this stable photoproduct. It is well known that the nature and
multiplicity of the excited state involved in the photo-
degradation are two of the parameters a†ecting the quantum
efficiency of the a-cleavage process. The rate constants for a-
cleavage are much faster for n,p* excited states than for p,p*
excited states.27 In the former case, the half-vacant non-
bonding orbital on oxygen can overlap with the a-bond
undergoing homolysis.28,29 Furthermore, a low activation
energy is expected for a triplet state mediated a-cleavage
process compared to the singlet state mediated process.30 As a
consequence, the most efficient Norrish Type I reactions are
expected to occur from triplet states with n,p* character. In
our case, we obtained a photodegradation quantum yield for
where U0 is the total Ñuorescence quantum yield in the
f
absence of quencher, U0 and U0 are the Ñuorescence
fw
fm
quantum yields in water and in the micelles, respectively, a
and b are the fractions of the concentration of the drug in the
micelle and water, respectively, and the product k q0 is the
q fw
SternÈVolmer quenching constant. From the ratio a/b derived
from the intercept of the two plots in Fig. 5, an association
constant
k
between FB and the SDS micelles of
assoc
(2.5 ^ 0.5) ] 103 M~1 was obtained by using eqn. (2):33
K
\ k /k \ a/b[M]~1
(2)
assoc
The low value for K
`
~
is not surprising since FB is present
assoc
in its anionic form at neutral pH (pK \ 4.5). Deep incorpor-
ation of the drug in the micelle core is not favored from a
a
thermodynamic point of view due to the coulombic repulsion
caused by the negatively charged micelle and the chromo-
phore. Besides, localization of FB in a deep region of the
micellar environment would have led to changes both in the
absorption and emission spectra according to the polarity
e†ect on the excited states of the drug (vide supra and infra).
The combination of both quenching and spectroscopic data
suggest an incorporation mode of FB involving localization of
the biphenyl chromophore near the Stern layer with the car-
boxyl head mainly exposed to an aqueous environment. Inter-
actions of this kind have already been reported for other
negatively charged alkylÈaryl chromophores.33
FB (U
) in bu†er solution of 4 ] 10~4. This low value is in
~FB
agreement with the results concerning the nature of the lowest
excited triplet state of FB (vide supra), which is a scarcely reac-
tive p,p* triplet state. For similar molecules, the observed
reactivity has been attributed to mixing of the lowest triplet
with the upper n,p* triplet state31 and, as proposed by
Wagner and Leavitt,32 from an equilibrium concentration of
the upper n,p* triplet (Scheme 1). In our case, the Boltzmann
When the irradiation of FB was performed in the presence
of SDS micelles, a 1.5-fold reduction in the photodegradation
quantum yield compared to that reported in aqueous solution
was observed. While a strong e†ect on the energy gap of the
distribution calculated at 298 K (ca. 2% population of T ) is
consistent with the low value of the photodegradation
quantum yield obtained. Given the n,p* nature of the lowest
2
Fig. 4 Spectral changes of FB in phosphate bu†er solutions
(1 ] 10~2 M, pH 7.4) observed under anaerobic conditions after 0, 10,
30 and 60 min irradiation with 325 nm light excitation.
Fig. 5 Plots of the data according to eqn. (1) for the quenching of
the Ñuorescence of FB by I~ in (=) 0.05 and (…) 0.1 M SDS.
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low lying triplets seems unlikely on the basis of the rather
weak incorporation of the drug in the anionic micelles, a pos-
sible reason for the lower photoreactivity of FB in SDS micel-
les could be related to the ““cage e†ectÏÏ. In fact, it is known
that localization of the drug in the micellar environment pro-
motes radical pair recombination before their di†usion.34h36
Once produced, the radical pair originated by CÈC homolysis
can either recombine or evolve towards the formation of
photoproducts. In the micellar environment, the former
process plays a dominant role and, as a consequence, inter-
system crossing in the radical cage is competitive with rota-
tion of the caged radical pair and di†usion into the solvent.
The small reduction in the photodegradation quantum yield
of FB in SDS is in agreement with both the spectroscopic data
and the association constant, thus corroborating the hypothe-
sis that the chromophore localizes near the micellar surface.
Fig. 6(A) shows the absorption spectrum of FB in the pres-
ence of 0.1 M CTAC and a similar spectrum in aqueous solu-
tion. The observed blue shift in the maximum is in agreement
with what is observed in solvents of lower polarity than water
(vide supra) and indicates deep incorporation of the drug in
the cationic micelle. From the dependence of the absorbance
changes on the concentration of CTAC an association con-
out the formation of pre-micellar aggregates between mol-
ecules of the drug and surfactant.
The Ñuorescence emission of FB was also a†ected by CTAC
if its concentration was kept above the CMC. Fig. 6(B) shows
the relevant quenching of the Ñuorescence emission upon
incorporation in the micelles. The decrease in the Ñuorescence
intensity can be attributed to the e†ect of the hydrophobic
microenvironment on the relative energies of the excited states
involved. Consequently, other non-radiative deactivation
channels (i.e. ISC) might take place with higher efficiencies.
This hypothesis is based on our Ðndings of an n,p* symmetry
for the lowest excited singlet state of FB, which is reasonably
stabilized by its incorporation in the low-polarity environ-
ment of the micelle. This can lead to a strong spinÈorbit coup-
ling with the close-lying 3p,p*, promoting an increase in the
ISC efficiency. This behavior is similar to that observed in
solvents less polar than water (i.e. acetonitrile and isopro-
panol, vide supra) and for the FBÈb-cyclodextrin inclusion
complex.39
Irradiation of FB in the presence of CTAC micelles leads to
a completely di†erent behavior than that observed either in
aqueous solutions or in SDS micelles. The dependence of the
ratio between the photodegradation quantum yield of FB in
stant K
assoc
ing eqn. (3):37
\ (3.5 ^ 0.5) ] 104 M~1 was obtained by apply-
the presence, Um , and in the absence, U
, of CTAC as a
~FB
~FB
function of the surfactant concentration (Fig. 7) indicates a
marked increase of the photodegradation, which reaches a
limiting value ca. 3 times higher than that reported in the
absence of surfactant. The main photoproduct under these
conditions was the same as that observed in aqueous solution,
4-biphenylcarboxyaldehyde. From Fig. 7 it can be seen that
1/(A [ A0) \ 1/(A0 [ A0)(1 ] 1/K
[M])
(3)
w
m
w
assoc
where A0 is the absorbance in the absence of CTAC, A is the
w
absorbance in the presence of CTAC and A0 is the limiting
m
absorbance upon complete incorporation of FB in the micel-
the increase in U
occurs around the CMC, suggesting that
lar phase. The value of K
is higher than that found for
~FB
assoc
such an enhancement reÑects a photoprocess occurring in the
micellar environment, as reported in the literature.40,41 The
increase of the photodegradation efficiency in the presence of
CTAC can be rationalized on the basis of the dual role played
by the micellar environment, acting both as a solvent and a
reactant.
SDS and can be reasonably attributed to the additional elec-
trostatic interaction involving the opposite charges of the
chromophore and the external surface of the micelle. These
interactions can play a key role in decreasing the rate constant
for the exit of FB from the micellar cage. Since the association
of solutes with micelles is di†usion controlled, the di†erences
in equilibrium constants are related to di†erences in the exit
rate constants.38 Although no relevant di†erences in the
absorption spectrum of FB were observed when the concen-
tration of CTAC was kept below the CMC, we cannot rule
The spectroscopic results obtained in the CTAC micelles
and the high value of the association constant suggest deep
incorporation of FB into these structures. Besides the decrease
in the polarity induced by the incorporation of FB into the
micelles, we cannot exclude an additional contribution from
this microenvironment in promoting a slight increase in the
rotational barrier between the two phenyl rings. Although
planarization for biphenyl is expected to be very fast even in
the micelles, we believe that the electrostatic interaction
between FB and the CTAC micellar surface could make this
barrier moderately high. Such e†ects on the rotational barrier
have been reported before for biphenyl-like chromophores;
speciÐcally, a reduction in the rotational motion has been
observed in the case of positively charged molecules bearing a
biphenyl-like chromophore incorporated in anionic micelles.42
The combination of these two e†ects could lead to the same
Fig. 6 Absorption (A) and emission (B) spectra of FB in phosphate
bu†er (1 ] 10~2 M, pH 7.4) (ÈÈ) in the absence and (È È È) in the
presence of 0.1 M CTAC.
Fig. 7 Dependence of the ratio between the quantum yield of photo-
degradation of FB in the presence, Um , and in the absence, U , of
vFB
vFB
CTAC as a function of the surfactant concentration.
New J. Chem., 2001, 25, 975È980
979

View Article Online
4
5
6
G. Condorelli, L. L. Costanzo, G. De Guidi, S. Giu†rida, P.
Miano, S. Sortino and A. Velardita, EPA News., 1996, 58, 60.
F. Bosca and M. A. Miranda, J. Photochem. Photobiol. B, 1998,
43, 1.
phenomena as proposed by Yang and coworkers;31,43 an
increase of the vibronic mixing between the p,p* and n,p*
states caused by the smaller energy gap accompanied by the
higher efficiency of the a-cleavage photoprocess. Since this
process in the CTAC micelles is expected to take place in a
deeper region than in the SDS micelles, the formation of the
photoproduct could also be favored by the fact that the
benzoyl radical intermediate may abstract hydrogen from the
surfactant, leading to the formation of the stable photo-
product. Similar processes have been reported in the liter-
ature, where an increase in the yield of aldehyde derivatives
formed through a-cleavage processes in the presence of suit-
able hydrogen donors has been observed.44 Nonetheless, the
increase in the photodegradation quantum yield of FB in
Symposium-in-print on drug photochemistry and phototoxicity:
Photochem. Photobiol., 1998, 68, 633.
7
8
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9
10 K. D. Rainsford, C. Ying and D. OÏShaughnessy, Agents Actions,
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12 G. De Guidi, R. Chillemi, L. L. Costanzo, S. Giu†rida S. Sortino
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CTAC micelles (Um B 1.2 ] 10~3) still compares well with
~FB
those reported for aryl alkyl ketones characterized by lowest
3p,p* states that are weakly mixed with upper 3n,p* states.44
In conclusion, this study has demonstrated that the photo-
reactivity of FB is mediated by its lowest excited triplet state
of p,p* nature and having an upper n,p* triplet as precursor.
The presence of the carbonyl substitutent promotes an
assisted intersystem crossing, resulting in high yields for triplet
population compared to biphenyl itself. The mixing between
the two low-lying triplets is modulated by solvent polarity and
plays a key role in the efficiency of singlet oxygen photoge-
neration. In light of the high quantum yield of this transient
species, a Type II photosensitization mechanism seems to be
the most relevant pathway to explain the phototoxic e†ect of
FB.
17 R. Schimdt, C. Tanielian, R. Dunsbach and C. Wol†, J. Photo-
chem. Photobiol. A, 1994, 79, 11.
18 P. J. Wagner, J. Am. Chem. Soc., 1967, 89, 2820.
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Soc., 1973, 95, 5604.
20 Hyperchem package, Hypercube Inc., Waterloo, Canada, 1996.
21 G. Marconi and G. Orlandi, Croat. Chim. Acta., 1983, 56, 225.
22 S. Navaratnam and S. A. Jones, J. Photochem. Photobiol. A, 2000,
132, 175.
The formation of carbon-centered radicals results from an
inefficient cleavage process (Norrish Type I). The photo-
degradation of the drug can be altered by its incorporation
into micellar structures and depends on the nature of the sur-
factant. These Ðndings suggest that localization of the drug in
speciÐc regions of a biosubstrate with particular hydropho-
bicity and charge can lead to an increase in the photo-
degradation efficiency and, as a consequence, to an increase in
the formation of free radicals.
23 I. Carmichael and G. L. Hug, in Handbook of Organic Photo-
chemistry, ed. J. C. Scaiano, CRC Press, Boca Raton, FL, USA,
1989, vol. 1, pp. 369È404.
24 R. W. Redmond and S. E. Braslavsky, Chem. Phys. L ett., 1988,
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27 N. J. Turro, Modern Molecular Photochemistry, The Benjamin/
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28 H. E. Zimmerman, Adv. Photochem., 1963, 1, 183.
29 D. I. Schuster, R. G. Underwood and T. P. Knundes, J. Am.
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Acknowledgements
30 M. F. Mirbach, M. J. Mirbach, K. C. Liu and N. J. Turro, J.
Photochem., 1978, 8, 299.
Financial support from MURST ““CoÐnanziamento di Pro-
grammi di Ricerca di Rilevante Interesse NazionaleÏÏ
(Progetto: Meccanismi di Processi Fotoindotti in Sistemi
Organizzati) and from Istituto Superiore di Sanita (Progetto:
Proprieta Chimico-Fisiche dei Medicamenti e loro Sicurezza
dÏUso) are gratefully acknowledged. We are also thankful for
the Ðnancial support of the Department of Chemistry at
Union College. We wish to express our sincere thanks to Prof.
J. C. Scaiano of the University of Ottawa, where part of this
work was carried out while S. S. and L. J. M. were a visiting
researcher and a post-doc student, for the use of the time-
resolved spectroscopic apparatus and for his encouragement.
Special thanks are also due to Prof. G. Condorelli for his criti-
cal reading of the manuscript.
31 N. C. Yang and R. L. Dusenbery, J. Am. Chem. Soc., 1968, 90,
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32 P. J. Wagner and R. A. Leavitt, J. Am. Chem. Soc., 1970, 92, 5806.
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35 N. J. Turro and W. R. Cherry, J. Am. Chem. Soc., 1978, 100, 7431.
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38 M. Almgren, F. Grieser and J. K. Thomas, J. Am. Chem. Soc.,
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39 S. Sortino, G. De Guidi, S. Giu†rida, S. Fazio, G. Salemi and S.
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