Photophysical properties of porphyrin derivatives: Influence of the alkyl chains in homogeneous and micro-heterogeneous systems

Article abstract of DOI:10.1142/S1088424615500650

This work is focused on the evaluation of the photophysical properties of protoporphyrin-IX derivatives with different alkyl chains. The goal of this work is to understand the physicochemical properties and select prototypes of photosensitizers for photodynamic therapy. Solvents with different dielectric constants were used to obtain relevant data on different physical and chemical processes, such as fluorescence quantum yield and emission level of aggregation in nonpolar media. In addition, studies were conducted in micro-heterogeneous systems through the interaction of porphyrin derivatives with different surfactants, such as cetyltrimethylammonium bromide, sodium dodecyl sulfate and octyl phenol ethoxylate. The compounds were characterized by electronic absorption spectroscopy in the ultraviolet-visible region and fluorescence emission spectroscopy. The comparative study between the different chemical environments evaluated in the present study represents a relevant contribution to the comprehension of the properties acquired by the porphyrins in different micro-heterogeneous systems, which allows us to understand various interactions that these compounds are subject in biological medium, such as the influence of the polarity of the medium in the reactivity of porphyrins with nonpolar substituents. These results show the decisive physicochemical influence of the alkyl chains on the properties of the tetraazomacrocycle porphyrin.

Full text of DOI:10.1142/S1088424615500650

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2015; 19: 1–14
DOI: 10.1142/S1088424615500650
Published at http://www.worldscinet.com/jpp/
Photophysical properties of porphyrin derivatives:
Influence of the alkyl chains in homogeneous and
micro-heterogeneous systems
Sandra Cruz dos Santosa, Leonardo Marmo Moreirab, Diogo La Rosa Novoa,
Luiza Rosimeri Romano Santina, Daniela Bianchinia, Juliano Alves Bonacinc,
Ana Paula Romanid, Adjaci Uchoa Fernandese, Maurício S. Baptistaf
and Hueder Paulo Moisés de Oliveira*g
a Centro de Ciências Químicas, Farmacêuticas e de Alimentos, Universidade Federal de Pelotas (UFPel), Campus
Universitário S/N, 96160-000 Capão do Leão-RS, Brazil
b Departamento de Zootecnia (DEZOO), Universidade Federal de São João Del Rei (UFSJ), 36301-360 São João
Del Rei-MG, Brazil
c Instituto de Química, Universidade de Campinas, UNICAMP, P.O. Box 6154, 13083-970 Campinas-SP, Brazil
d Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas, Universidade Federal do ABC (UFABC), Avenida
dos Estados 5001, Bairro Bangu, 09210-170 Santo André-SP, Brazil
e Universidade Camilo Castelo Branco, São José dos Campos-SP, Brazil
f Instituto de Química by Instituto de Química, Universidade de São Paulo USP, São Paulo-SP, Brazil
g Centro de Ciências Naturais e Humanas, Universidade Federal do ABC (UFABC), Avenida dos Estados 5001,
Bairro Santa Terezinha, 09210-580 Santo André-SP, Brazil
Received 16 September 2014
Accepted 16 March 2015
ABSTRACT: This work is focused on the evaluation of the photophysical properties of protoporphyrin-
IX derivatives with different alkyl chains. The goal of this work is to understand the physicochemical
properties and select prototypes of photosensitizers for photodynamic therapy. Solvents with different
dielectric constants were used to obtain relevant data on different physical and chemical processes,
such as fluorescence quantum yield and emission level of aggregation in nonpolar media. In addition,
studies were conducted in micro-heterogeneous systems through the interaction of porphyrin derivatives
with different surfactants, such as cetyltrimethylammonium bromide, sodium dodecyl sulfate and octyl
phenol ethoxylate. The compounds were characterized by electronic absorption spectroscopy in the
ultraviolet-visible region and fluorescence emission spectroscopy. The comparative study between the
different chemical environments evaluated in the present study represents a relevant contribution to the
comprehension of the properties acquired by the porphyrins in different micro-heterogeneous systems,
which allows us to understand various interactions that these compounds are subject in biological
medium, such as the influence of the polarity of the medium in the reactivity of porphyrins with nonpolar
substituents. These results show the decisive physicochemical influence of the alkyl chains on the
properties of the tetraazomacrocycle porphyrin.
KEYWORDS: protoporphyrin-IX, fluorescence, solvatochromism, photodynamic therapy (PDT),
solvent effects, micellar systems.
INTRODUCTION
Photosensitizers (PS) are compounds that play a vital
role in photodynamic therapy (PDT). Activation of PS
*Correspondence to: Hueder Paulo Moisés de Oliveira, email:
hueder.paulo@ufabc.edu.br, +55 11-4996-0155
Copyright © 2015 World Scientific Publishing Company

2
S.C.D. SANTOS ET AL.
using a radiation of appropriate wavelength is applied
for treating diseases characterized by uncontrolled cell
growth, such as cancer, and various types of microbial
infections including diseases caused by bacteria, fungi
and protozoa [1, 2].
Porphyrins and their derivatives, due to their interesting
photophysical, photochemical and electrochemical
properties [3, 4], as well as their ability to be accumulated
intissuesofrapidgrowthasthetumortissue[5,6],inrecent
years have aroused great interest as drugs in PDT [7–12].
A typical example is the Photophrin®, a commercial PS
consisting basically of porphyrin derivatives, whose use
is approved by the FDA, the regulatory authority in the
United States [13, 14].
However, a drawback concerning the applications of
porphyrins in biological environment is the formation of
aggregates in aqueous media, due to p–p* interactions
and the hydrophobicity of these compounds [7, 15].
This feature affects the ability of porphyrins to be used
as photoactive principle in PS biologically friendly [16].
To minimize this problem, many groups have studied the
associationofporphyrinswithmodelsofmicellarsystems,
polymeric nanoparticles, dendrimers and liposomes [7,
17, 18], in order to investigate the interactions between
photosensitizers and potential carriers of drugs, since
the aforementioned micro-heterogeneous systems can
mimic biological systems, e.g. plasma membranes and
biological tissues [19, 20]. The transport of porphyrins in
biological medium through micro-heterogeneous systems
has been considered an interesting alternative to enabling
the delivery of the PS [7, 18]. Thus, it is necessary to
evaluate prototypes of the associations of porphyrin-
micro-heterogeneous systems before developing efficient
tests in vivo, so that the most suitable candidates to a set
of tests on animals can be known beforehand.
Moreover, such studies allow the acquisition of
physicochemical information, which is prerequisites to
obtain an adequate interpretation regarding the results in a
biological environment, i.e. to an effective understanding
of biological-chemical action that the porphyrin micro-
heterogeneous system may produce in a biological
environment.
In this sense this work is focused on the influence
of the hydrophobic alkyl chains, which were obtained
by the esterification of propionic acid groups of the
protoporphyrin-IX (Pp-IX), in homogeneous systems
(polar and nonpolar solvents, donors or receptors of
hydrogen bonds) and micellar structures (consisting
of active agents on surfaces, i.e. “surfactants”) in the
photophysical properties of these compounds in order to
provide a model for drug carrier systems.
number of organic solvents as well as in the presence
of surfactants using techniques of steady state, such as
electronic absorption spectroscopy in ultraviolet-visible
(UV-vis) region and fluorescence emission spectroscopy.
The study of the phenomena involved in the excited
state is a key step to understand the photophysical
behavior of these compounds as a function of different
porphyrin-surfactant interactions. Therefore, studies
were conducted in micro-heterogeneous systems through
the interaction of porphyrin derivatives with different
surfactants, such as cetyltrimethylammonium bromide
(CTAB, cationic surfactant), sodium dodecyl sulfate
(SDS, anionic surfactant) and octyl phenol ethoxylate
(Triton X-100, non-ionic surfactant). The instrumental
analyzes associated with measurements of fluorescence
lifetimes have contributed to understand the mechanisms
of photophysical emission of each porphyrin derivative.
RESULTS AND DISCUSSION
Spectroscopic studies of porphyrins in homogeneous
systems
Studies of the influence of solvent on the spectroscopic
properties of porphyrins were conducted in this work.
Changes in absorption intensity as a function of the solvent
and the type of porphyrin can be observed in the electronic
absorption spectra, which are shown in Fig. 1. The band of
higherabsorptivityobservedaround405nmisdenominated
Soret band or B0-0 band. Four other bands, which are known
as Q-bands (Qx; Qy), can be observed in the electronic
spectra of porphyrins in regions of lower energies. Both
the Soret band and the Q-bands are transitions of p→p*
type [21, 22]. The Q-bands are situated between 480 and
700 nm, being related to transitions from the ground state
to vibrational levels (Qx0,0, Qx0,1) and (Qy0,0, Qy0,1) of excited
state S1 [23]. Although the intensities of the Q-bands are
10 to 20 times smaller than the Soret band, the majority
of clinical studies on PDT require the use of spectral
regions around 630 nm, which is known as “therapeutic
window.” This occurs due to a great penetration of light
into tissues by the longer wavelengths, which means that
this absorption band has greater efficiency in treatments
by photodynamic therapy, since a larger portion of the dye
can be excited [24]. The “therapeutic window” is explained
by the presence of endogenous dyes in different biological
tissues, which compete with PS by the photons emitted by
the source. In fact, two of the most important biological
dyes are heme moieties of hemoproteins and the melanin,
being the last one responsible for the characteristic
pigmentation of the human epidermis. Both biological
dyes have significant absorption in the same spectral range
where porphyrin dyes have the greater absorption, in other
words, around 400 nm (Soret band).
The goal of generate several derivatives of Pp-IX
is understand the structure-activity relationships and
find new candidates for PDT. It should be mentioned
that porphyrins have a well-established photodynamic.
Spectroscopic and photophysical properties of the
porphyrins were investigated in detail in a significant
The emission spectra of fluorescence of the porphyrins
in ACN, EtOH and DMSO are shown in Figs. 1b, 1d
Copyright © 2015 World Scientific Publishing Company
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PHOTOPHYSICAL PROPERTIES OF PORPHYRIN DERIVATIVES
3
Fig. 1. Electronic absorption spectra in the UV-vis region to the porphyrins dissolved in different organic solvents: (a) ACN,
(c) EtOH, (e) DMSO; Fluorescence emission spectra of the porphyrins in different organic solvents: (b) ACN, (d) EtOH, (f) DMSO.
lexc = lmaxAbs. Molar concentration: 5.0 × 10-6 M. The insets show the expanded region of Q-bands in the UV-vis spectra to the
porphyrins dissolved in different organic solvents
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S.C.D. SANTOS ET AL.
and 1f, respectively. In these spectra, an emission band
with relatively high intensity is observed near 630 nm.
Significant difference is observed in the spectral shapes,
which indicates that the solvent has influence on the
energy level of the orbitals involved in electronic
transitions p→p*.
The solvatochromism observed in this study was
evaluated through the solvent parameters shown in
Table 1 — scale ET(30), dipole moment (m), dielectric
constant (e), absolute viscosity (h) and refractive index
(n) — in order of increasing polarity.
The absorption and emission wavelength are strongly
dependent on the structure and solvent. For example,
Fig. 2 shows the normalized intensities of absorption and
emission of Pp12 in different solvents. In this case the
absorption wavelengths (at the maximum intensity —
lA) show a displacement in the following order: DMSO
> THF = DMF > EtOH > MeOH = ACN). On the other
hand, the emission wavelengths (at the maximum
intensity — lF) is THF > EtOH > DMF > DMSO =
MeOH > ACN. Based on these results, an order of
maximum intensity for the Soret band, according to the
solvent, can be constructed: THF > DMF > DMSO >
ACN > EtOH > MeOH. A significant interference in
electronic absorption of compounds is observed through
a bathochromic effect of the bands in the spectral range
between 400 and 407 nm, according to the polarity of
the solvent. The different solvents studied have shown a
bathochromic effect (positive) as the polarity decreases.
Thus, the bathochromic effect of the compounds is higher
in non-protic solvents, which indicates rearrangement of
thechargeofsolutemoleculesunderexcitationelectronics
[27]. An identical spectral behavior was observed for the
other porphyrin derivatives. In most cases, the bands are
shifted to shorter wavelengths as the solvent polarity is
increased, indicating a partial reduction of aggregate
formation of type J or H [28].
The band intensity (intensity of absorbed light) of the
Pp12 is more affected than other porphyrin derivatives
by the change of solvent. In addition, the intensity of
absorbance of Pp12 in all studied solvents is very low
when compared to other porphyrin derivatives. This
could be explained by the great influence of the nonpolar
chains inserted into the porphyrin, which would reduce
Fig. 2. Spectra of Pp12 with normalized intensities, obtained in different organic solvents (5.0 × 10-6 M): (a) absorption spectra of
UV-vis, (b) emission spectra of fluorescence
Table 1. Physicochemical parameters of the solvents
Solvent
ET(30) (kcal/mol) [25]
m (Debye) [26]
e [26]
h (cP) [26]
n [26]
MeOH
EtOH
ACN
55.4
51.9
45.6
45.1
43.2
37.4
1.7
1.7
32.6
22.4
37.5
46.6
36.7
7.6
0.6
1.326
1.359
1.342
1.476
1.427
1.404
1.08
0.38
2.0
3.2
DMSO
DMF
THF
3.96
3.8
0.82
0.55
1.75
ET(30) = solvent polarity (25°C); m = dipole moment; e = dielectric constant (20°C); h = absolute
viscosity (25°C); n = refractive index (25°C).
Copyright © 2015 World Scientific Publishing Company
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PHOTOPHYSICAL PROPERTIES OF PORPHYRIN DERIVATIVES
5
its affinity for solvents with higher polarity. The intensity
of fluorescence emission of Pp12 is relatively low when
compared to other porphyrin derivatives, which indicates
that this porphyrin derivative is not the best alternative
to be tested as a prototype of photosensitizer in PDT.
Furthermore, it should be considered an additional
factor that decreases the potential use of this compound
as PS. The long hydrofobic alkyl chains of Pp12 confer
the lowest polarity among porphyrin derivatives, which
reduces its solubility in polar solvents, such as water.
This low solubility favors the aggregation and decreases
the quantum yield of emission.
Based on the values of maximum wavelength of
absorption and emission (Table 2), an hypsochromic
shift is observed as the solvent polarity is increased.
The parameter FF shows higher values in less polar
solvents, as observed to DMSO, DMF and THF. On the
other hand, FF has lower values in solvents with higher
polarity, as EtOH, MeOH and ACN. The Stokes shift
indicates the red shift of the maximum fluorescence
wavelength in relation to the maximum absorption
wavelength. Table 2 shows lower values of νA–νF as the
solvent polarity is decreased. This indicates a higher
extension of interactions of porphyrin derivatives with
polar solvents. Considering the bands of transitions p–p*
for monomeric units of porphyrin, which corresponds
to porphyrin not aggregated each other, the increasing
dielectric constant tends to shift the porphyrin spectrum
Table 2 shows the results of the maximum absorption
wavelength (lA), molar absorptivity coefficient (e),
maximum emission wavelength (lF), Stokes shift (νA–
νF), fluorescence quantum yield based on the absorptivity
of the anthracene (FF), fluorescence lifetime (lexc
414 nm and lems = 630 nm) (t), constant of radiative decay
(fluorescence decay, Kr) and constant of non-radiative
decay (Knr). Kr and Knr were calculated from Equations 1
and 2, respectively.
=
to red (bathochromic shift), since the orbitals p* are
more polarizable and accessible than orbitals p involved
in electronic transitions.
The hydrophobic chain length inserted in the
porphyrins has a great influence in the spectral behavior,
specially for the porphyrin derivative containing dodecyl
groups (Pp12). As the carbon chain increases, the polarity
of porphyrin derivatives decreases. Thus, the aggregate
formation is reduced due to the increased solubility of
porphyrin in less polar solvents. This can be evidenced
by the higher FF values for Pp12 in DMF and THF
compared with polar solvents, whose FF is close to zero.
In order to understand the characteristic mechanisms
of emission for each type of porphyrin, the fluorescence
decays were studied in different solvents: DMSO, EtOH
and ACN. Fig. 3 shows the decay profiles of Pp2 and
Pp12 for these solvents.
Considering measurements obtained in the same
medium, the profiles of the fluorescence decay for
porphyrin derivatives are very similar. It should be
considered that the length of the alkyl chain is not
a significant parameter. The results show a simple
exponential decay for all systems suggesting only one
fluorescent specie. The parameters obtained from the
fluorescence decay adjusted by a monoexponential
curve are shown in Table 2. It should be noticed that in
all solvents, the fluorescence decay parameters depend
on the molecular structure of the solvent, with special
emphasis on its polarity. In addition, it is well-known
that both the values of fluorescence lifetime and the
fluorescence quantum yield have a decreasing tendency
as the solvent polarity increases [30, 31].
In this study, we have also calculated the constants
of fluorescence decay and non-radiative decay from
theoretical data. The oscillator strength is an important
parameter to be evaluated for fluorescent compounds
and corresponds to the effective number of the electrons
that are promoted from ground to excited state. These
transitions give the absorption area in the electronic
spectrum [29]. Es1 corresponds to the energy of the singlet
state S1, which is calculated according to Equation 3. This
is defined as the intersection between the normalized
spectra of absorption and fluorescence with the oscillator
strength (f), which is calculated from Equation 4. In this
equation, n is the refractive index of the solvent, Dν1/2 is
the full width at half maximum of the absorption band
and e is the molar absorptivity coefficient. The natural
radiative lifetime of the excited singlet state is represented
~
by t0, which is shown in Fig. 3. The parameter Umax is the
frequency of maximum absorption and f is the oscillator
strength, which is calculated from Equation 4. The
calculated fluorescence lifetime (tF-calc) is obtained from
t0 and FF, according to Equation 6.
The calculated constant of fluorescence decay of
singlet state (Kr-calc) was obtained from Equation 7. The
calculated constant of non-radiative decay (Knr-calc) was
obtained from Equation 8.
Kr = FF/t
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
Knr = (1–FF)/t
The viscosity is another important parameter since
the fluorescence lifetime depends on the viscosity and
dielectric constant of the solvent. The influence of the
viscosity in the rotational lifetime was evaluated in
polar and nonpolar solvents and was verified a linear
relationship between the rotational relaxation times and
the solvent viscosity [32, 33]. This dependence indicates
that the deactivation process of the excited state in
solution becomes slower in more viscous media [32, 33].
Es1 = 11.96 × 104/[(lA + lF)/2]
f = 4.32 × 10-9Dν1/2emax/n
~
t0 = 1.5/(U2max f)
tF-calc = t0FF
Kr-calc = FF/(tF-calc
)
Knr-calc = 1-FF/(tF-calc
)
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S.C.D. SANTOS ET AL.
Table 2. Photophysical properties of the porphyrin derivatives
Solvent
lA, nm e, M-1.
lF, nm
νA–νF,
FF
t, ns Kr, 107
Knr,
107s-1
Es1
f
t0, ns
TF-calc
ns
,
Kr-calc
,
Knr-calc,
107 s-1
cm-1
cm-1
s-1
107 s-1
Pp1
MeOH
EtOH
ACN
401
403
401
407
406
406
70180
68956
632
9114.87 0.10
9016.11 0.12 12.33
9089.80 0.16 7.07
8747.24 0.43 17.57
—
—
0.97
2.26
0.43
—
—
7.14
11.88
2.45
—
231.56
230.89
231.78
230.22
230.33
229.89
0.96
0.83
1.42
1.56
1.94
2.39
2.50
2.93
1.69
1.60
1.27
1.03
0.25
0.35
0.27
0.69
0.50
0.33
39.96
34.18
59.02
62.65
78.46
96.75
359.65
250.68
309.83
83.051
122.71
205.60
633
137666
199536
231346
240920
631
DMSO
DMF
THF
632
632.5
634.5
8820.27 0.39
8870.10 0.32
—
—
—
—
Pp2
MeOH
EtOH
ACN
401
403
401
407
406
406
133880
171644
195100
201870
223730
242000
632
9114.87 0.17
—
—
1.58
2.37
0.45
—
—
6.31
11.59
2.58
—
231.56
230.89
231.78
230.22
230.33
229.89
1.27
1.57
1.83
1.85
1.90
2.06
1.89
1.55
1.32
1.34
1.30
1.20
0.19
0.19
0.21
0.58
0.51
0.39
52.80
64.56
75.80
74.50
77.04
83.20
475.17
473.40
397.97
98.75
633
9016.11 0.20 12.68
9089.80 0.17 7.16
8747.24 0.45 17.41
631
DMSO
DMF
THF
632
632.5
634.5
8820.27 0.40
8870.10 0.32
—
—
120.50
176.79
—
—
Pp4
MeOH
EtOH
ACN
401
403
401
407
406
406
147900
167002
208020
220224
223164
216520
632
9114.87 0.17
—
—
1.58
2.44
0.4
—
—
6.32
11.93
2.24
—
231.56
230.89
231.78
230.22
230.33
229.89
1.41
1.48
1.93
1.73
1.86
1.85
1.71
1.65
1.25
1.44
1.33
1.34
0.17
0.20
0.20
0.62
0.52
0.43
58.48
60.58
79.96
69.64
75.27
74.90
526.31
444.24
419.80
92.31
633
9016.11 0.20 12.65
9089.80 0.17 6.96
8747.24 0.40 17.78
631
DMSO
DMF
THF
632
632.5
634.5
8820.27 0.41
8870.10 0.33
—
—
117.73
159.17
—
—
Pp8
MeOH
EtOH
ACN
401
403
401
407
406
406
160460
197670
203660
208926
220428
242860
632
9114.87 0.18
—
—
1.66
2.46
0.41
—
—
6.24
11.99
2.33
—
231.56
230.89
231.78
230.22
230.33
229.89
1.49
1.75
1.76
1.70
1.85
2.04
1.62
1.39
1.37
1.46
1.33
1.21
0.16
0.17
0.22
0.63
0.52
0.39
61.63
71.70
72.85
68.42
75.02
82.42
554.69
525.82
382.48
90.70
633
9016.11 0.21 12.65
631
9089.80 0.17
8747.24 0.41
8820.27 0.40
8870.10 0.34
6.92
17.6
—
DMSO
DMF
THF
632
632.5
634.5
117.34
175.14
—
—
—
Pp12
MeOH
EtOH
ACN
401
403
401
407
406
406
17800
29070
26880
22068
69256
144300
632
9114.87 0.01
—
—
0.4
0.56
0.05
—
—
7.61
13.56
0.3
231.56
230.89
231.78
230.22
230.33
229.89
0.14
0.71
0.25
0.23
0.76
1.27
1.74
3.45
9.63
1.06
3.25
1.94
1.74
0.41
1.54
4.56
1.27
0.62
5.74
29.02
10.39
9.43
51.62
212.83
54.53
633
9016.11 0.05 12.47
9089.80 0.04 7.08
8747.24 0.05 16.45
631
DMSO
DMF
THF
632
12.50
632.5
634.5
8820.27 0.29
8870.10 0.29
—
—
—
30.75
51.49
48.09
—
—
109.42
The structures of the porphyrin derivatives are shown in Fig. 1; lA = maximum absorption wavelength; e = molar absorptivity coefficient; lF =
maximum emission wavelength; (νA–νF) = Stokes shift; FF = fluorescence quantum yield based on the absorptivity of the anthracene; t = fluorescence
lifetime (lexc = 414 nm and lems = 630 nm); Kr = constant of fluorescence decay; Knr = constant of non-radiative decay; Es1 = energy of the singlet
state S1; f = oscillator strength; t0 = natural radiative lifetime of the excited singlet state; tF-calc = calculated fluorescence lifetime; Kr-calc = calculated
constant of fluorescence decay of the singlet state; Knr-calc = calculated constant of non-radiative decay.
In this work the fluorescence lifetimes are in the range of
7.0 and 17.8 ns. The lifetime in the excited state is longer
for porphyrin derivatives dissolved in lower polarity
solvents than those dissolved in higher polarity solvents.
According to the data presented in Table 2, the values
of lifetime are in the following order: tACN < tEtOH <
tDMSO. The viscosity of the solvents, ACN < EtOH <
DMSO (Table 1), corroborates the results. In addition,
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PHOTOPHYSICAL PROPERTIES OF PORPHYRIN DERIVATIVES
7
Spectroscopic studies of porphyrins derivatives
in micro-heterogeneous systems: micelles
Porphyrins often form aggregates in aqueous medium,
being necessary its dispersion in micro-heterogeneous
systems in order to be employed in PDT. The best
relation porphyrin/surfactant should be known for this
use. Figs. 4 and 5 show the absorption and emission
spectra, respectively, obtained for porphyrin derivatives
in different micro-heterogeneous systems, where the
micellar structures prevail.
The maximum absorption wavelength in the presence
of neutral micelles (Triton X-100, Fig. 4a) can be
compared to the wavelength obtained in homogeneous
systems with less polarity (labs ~ 408 nm), independently
of the pH analyzed. The results demonstrate the affinity
of porphyrin derivatives for non-ionic surfactants. This
type of surfactant tends to favor the interaction with
nonpolar porphyrins.
Fig. 3. Profiles of fluorescence decay for Pp2 and Pp12
dissolved in different solvents. Pulse excitation at 416 nm and
emission at 630 nm
The absorption spectra suggest a better interaction
of the porphyrin derivatives with the cationic surfactant
(Fig. 4c) than with the anionic surfactant (Fig. 4b). This
can be evidenced through the Soret bands, which are
better defined in the cationic micelles. The displacement
observed for the wavelenghts can be attributed to the
different forms of interaction of porphyrin derivatives
with the surfactants [36].
Inthiscontext,itshouldbeemphasizedtwofundamental
contributions of the porphyrin-surfactant interactions:
the nonpolar interactions between the porphyrin
derivative and the nonpolar chain of the surfactants, and
the polar interactions between the porphyrin derivative
and the polar head of the surfactants. The former type
of interaction should have an important role considering
the nonpolar characteristics of the porphyrin derivatives
that are being studied. In the latter, the cationic surfactant
has greater interaction with the porphyrin rings than the
anionic surfactant. This occurs due the interaction of the
cationic moiety of the surfactant CTAB with the pyrrolic
nitrogens of the porphyrin ring. Indeed, the polar head of
the surfactant CTAB acts as an acid coordination center
(Lewis acid) for the lone electrons pair from pyrrolic
nitrogens of the tetraazomacrocycle of the porphyrin
(Lewis base).
Fig. 4 also shows a great dependence of the spectral
properties with the pH in micellar systems. The spectra
obtained in anionic micelles (SDS), except the spectrum
obtained at pH 2.0, show a splitting of the Soret band. In
these spectra, the Soret band exhibits components which
are shifted to blue wavenumber and other components
which are shifted to red wavenumber when compared to
the monomeric form in non-ionic micelles. This indicates
the presence of different intermolecular interactions and
conformationofporphyrins,probablyduetotheformation
of aggregates. These results are according to those
reported by Inamura et al. [37] and Scolaro et al. [38],
which have studied the stability of the protoporphyrin-IX
the results of tF-calc have a similar tendency observed for
values obtained experimentally, with higher values in
DMSO.
Other relevant results are the constants of fluorescence
decay, Kr, and the constants of non-radiative decay, Knr,
which are shown inTable 2. Kr and Knr were obtained from
the fluorescence quantum yield, FF, and the fluorescence
lifetime, t, according to Equations 1 and 2. It is worth
mentioning that only one emitting specie or only the
dominant specie was considered [34]. Kr is associated
with the deactivation process of an electron p promoted
from the singlet excited state (S1) to fundamental singlet
state (S0). This process corresponds to fluorescence
emission of a molecule p conjugated. Kr values increase
as the alkyl chains of porphyrin derivatives increase.
This behavior is related with the relaxation process of
the molecules in the respective solvents. The constant Kr
can be compared with that of the calculated fluorescence
decay, Kr-calc, which is obtained from the absorption
.
spectra by 1/t0 Kr-calc is higher than Kr but the tendency
associated to solvent polarity is the same for both. On the
other hand, the Knr is assigned to transitions by internal
conversion and intersystem crossing. Thus, Knr is a
constant for non-radiative decay due to electron transfer
(ET) from negative charge to positive one. This process is
related to an intramolecular electronic transfer from S1 to
S0 [35]. These results suggest that the relation between Kr
and Knr depends on the p extension of the S1 excited state
of the molecule with the solvent.
It should be mentioned that the quantum yields
in polar solvents are lower than those observed in
solvents with lower polarity. In Table 2, the porphyrin
derivatives show that Kr is higher than Knr. This occurs
due the affinity of the porphyrin derivatives for less polar
solvents, where the appropriated micro-polarity and the
viscosity suppress the non-radiative processes. Thus, the
fluorescence quantum yield and Kr are larger.
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8
S.C.D. SANTOS ET AL.
Fig. 4. Electronic absorption spectra of the UV-vis for Pp2 (5.0 × 10-6 M) in micro-heterogeneous systems: (a) Triton X-100, (b) SDS
and (c) CTAB in different pH values; and maximum intensity of absorption for each porphyrin derivative (5.0 × 10-6 M): (d) Triton
X-100, (e) SDS and (f) CTAB at different pH values
Copyright © 2015 World Scientific Publishing Company
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PHOTOPHYSICAL PROPERTIES OF PORPHYRIN DERIVATIVES
9
Fig. 5. Fluorescence emission spectra (lexc = 408 nm): (a) Triton X-100, (b) SDS and (c) CTAB at pH = 7.0; (d) Pp2 in Triton X-100,
SDS and CTAB in different pH values. Molar concentration of porphyrin derivatives: 5.0 × 10-6 M
in aqueous solutions with different pH. The authors have
observed the formation of great aggregates in moderately
acidic conditions (pH = 4.8), being the aggregates
identified by the splitting in the Soret band between 300
and 500 nm, as it was also observed in our studies for
the anionic micelles. This behavior was explained by the
formation of dimers of porphyrin, which are bonded by
axial interactions of stacking p–p and by strong lateral
hydrophobic “edge-to-edge.” In addition, these systems
can be stabilized by hydrogen bonds through side chains
[36]. As seen in the literature, the monomeric form of
protoporphyrin-IX is observed in the pH range between
0 and 3.0, the dimeric form is observed in pH > 8.0 and
great aggregates are observed in the pH range between
3.0 and 7.0 [38].
in the intensity of the Soret band when compared to
the protoporphyrin-IX. It indicates that the increase of
nonpolar chains enhances the affinity between porphyrin
and surfactant. Thus, considering the great flexibility of
the porphyrin ring, especially in the free bases form, the
extension ofVan derWaals interactions are representative,
which favor the stabilization by nonpolar interactions.
This suggests that probably the hydrophobic interaction
between the nonpolar chain of the ionic surfactant and
the porphyrin is the main type of supramolecular contact.
This monomeric form observed in more acidic media
is associated to the efficient protonation of pyrrolic
nitrogens, which are inside the porphyrin ring [37]. In
our study, it was also observed a better solubility of the
monomeric form of protoporphyrin-IX derivatives by
anionic micelles with higher acidity.
The spectra obtained from different porphyrins in the
same medium were similar but with different absorption
intensities (Fig. 4). In our results, we can observe that
porphyrins with longer alkyl chains have shown higher
definition in the absorption spectra and an increase
The emission spectra of the five compounds, with
exception of SDS micelles at pH 2.0, exhibit relatively
high intensity band between 620 and 660 nm. The
maximum emission wavelength was changed according
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S.C.D. SANTOS ET AL.
to the type of surfactant used or the size of the porphyrin
derivative chains. In these media, lF values were close to
those obtained in THF, which confirms that the emitting
species in micelles are located in less polar environment.
In ionic surfactants medium, a large suppression of
fluorescence was observed, especially in CTAB micelles,
where increasing alkyl chain has direct influence in the
increase of fluorescence suppression. However, this
suppression was not very efficient at pH 2.0. Another
interesting result was the displacement of the band to
shorter wavelengths in the anionic media at pH 2.0.
The fluorescence quantum yields (Table 3) were
calculatedforeachsolutioninordertoobtainanindication
of efficiency of fluorescence. Higher values of FF were
obtained in the neutral micelles (Triton X-100), while a
large decrease was observed in cationic micelles (CTAB)
for chains containing more than two carbon atoms. The
chain size in the protoporphyrin-IX derivatives affects
directly the values of FF, being possible to observe
values extremely small for the Pp12 in all micellar media.
Considering anionic micelles (SDS), the highest FF value
was observed for the Pp4 porphyrin derivative.
Thefluorescencelifetimewasalsomeasuredinmicellar
systems (Table 3). The excitation wavelength was 416 nm
and the emission wavelength was 630 nm at 20°C. Only
one fluorescence lifetime was obtained for each type of
porphyrin or surfactant, suggesting the presence of only
one emitting specie. Comparing porphyrins in the same
micro-heterogeneous systems, where micellar structure
is predominant, the lifetimes were similar. The lifetime
of the micelles was observed in the following order: tCTAB
< tSDS < tTriton X-100. This order is in agreement with the
values obtained for fluorescence quantum yields, which
are dependent on the different micellar environments.
These results are very close to those reported by Maiti
et al. [39] for Pp1. The authors have assigned the increase
in the lifetime of this porphyrin in micellar systems when
compared to the value obtained in pure solvents (THF)
and the reduction in fluorescence suppression by oxygen
in micellar samples, which occurs due to the limited
Table 3. Photophysical properties of the porphyrin derivatives dispersed in micro-heterogeneous systems
Surfactant
lA, nm
e, M-1.cm-1
lF, nm
νA–νF, cm-1
FF
t, ns
tr, ns
Kr, 107 s-1
Knr, 107 s-1
Pp1
Triton X-100
SDS
408
408
408
36320
18200
16100
663
665
664
8712.02
8761.77
8736.93
0.20
0.12
0.18
18.74
16.67
13.95
11.39
2.50
2.34
1.07
0.72
1.29
4.27
5.28
5.88
CTAB
Pp2
Triton X-100
SDS
408
409
409
89200
28520
40220
633.5
634
8724.48
8677.01
8664.56
0.35
0.11
0.08
18.75
17.07
13.88
13.47
1.76
2.84
1.87
0.64
0.58
3.47
5.21
6.63
CTAB
633.5
Pp4
Triton X-100
SDS
408
409
409
75840
34200
19820
634
8736.93
8689.44
8689.44
0.31
0.22
0.03
18.65
16.80
14.04
14.10
2.17
3.13
1.66
1.31
0.21
3.70
4.64
6.91
634.5
634.5
CTAB
Pp8
Triton X-100
SDS
408
409
410
62392
23440
25700
634
8736.93
8689.44
8666.97
0.28
0.21
0.03
18.70
16.56
14.11
13.31
1.39
3.63
1.50
1.27
0.21
3.85
4.78
6.87
634.5
636
CTAB
Pp12
Triton X-100
SDS
412
406
411
3800
6400
964
634
635
633
8498.97
8882.51
8533.11
0.01
0.02
0.01
18.31
16.73
13.88
14.37
1.48
4.51
0.05
0.12
0.07
5.41
5.86
7.13
CTAB
The structures of the porphyrin derivatives are shown in Fig. 1; CTAB: cetyltrimethylammonium bromide; SDS: sodium dodecyl
sulfate; Triton X-100: octyl phenol ethoxylate; lA = maximum absorption wavelength; e = molar absorptivity coefficient; lF =
maximum wavelength of the emission; (νA–νF) = Stokes shift; FF = fluorescence quantum yield based on the absorptivity of the
anthracene; t = fluorescence lifetime (lexc = 414 nm and lems = 630 nm); tr = rotational correlation time; Kr = constant of fluorescence
decay; Knr = constant of non-radiative decay.
Copyright © 2015 World Scientific Publishing Company
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PHOTOPHYSICAL PROPERTIES OF PORPHYRIN DERIVATIVES
11
diffusion of this suppressor. It can be also observed an
increase in the fluorescence lifetimes of the porphyrin
derivatives with respect to protoporphyrin-IX.
porphyrin can be physically accommodated within the
micelles. However, it is well-known that porphyrins
are not usually in the core of the micelles, but rather
in a spatial position nearest the surface [39]. Thus, the
prevalence of nonpolar porphyrin derivatives studied in
this work suggests that the spatial arrangement of these
dyes can occur in an intermediate position between the
polar heads and the micellar center.
As previously mentioned the predominant micro-
heterogeneous system is the micelle. However, a
significant amount of monomeric aggregates and pre-
micellar aggregates of free surfactants in the solvent can
be interacting with monomers or aggregates of porphyrin.
This mention is relevant since the systems evaluated
in this work are not trivial chemical equilibria and as
well as the biological systems, suggests the coexistence
of many species. It is possible that the increase in full
width at half maximum of some bands, as well as some
less characteristic results and even slightly differentiated
from that similar studies in the literature, are related to
the peculiarities of the interactions between porphyrin
systems and monomeric units of surfactants as well as
the pre-micellar aggregates of surfactants.
The rotational correlation times (tr) of the porphyrin
derivatives were determined for micellar systems in order
tostudyinmoredetailtheformationofdimers/aggregates.
This study was developed to understand the rotational
movement of porphyrins, which can play a significant
role in fluorescence spectroscopy. The anisotropy decay
profiles of the molecules in media containing surfactants
were obtained from four measurements of fluorescence
decay by using crossed polarizers. The best fit of data
was obtained for the decay of a monoexponential curve,
which has identified a fast rotational correlation time
for porphyrins in media of CTAB and SDS as well as
a relatively higher rotational correlation time for Triton
X-100, as shown in Table 3.
EXPERIMENTAL
Protoporphyrin-IX and alcohols were obtained from
Aldrich and Fluka, respectively. Other commercial
chemicals were used without further purification.
Derivatives of protoporphyrin-IX, Fig. 6, were obtained
by Fischer esterification reaction, whose procedure
was described previously [40, 41]. The reactions were
performed at room temperature for six hours (6 h) in
excess of the correspondent alcohol. After reaction time,
the crude product was neutralized with ammonia solution
and extracted with dichloromethane. The products were
purified by column chromatography on basic alumina
(Merck: Brockmann Grade V), with dichloromethane/
methanol = 1000:1 (v/v), dissolved in dichloromethane
and precipitated by methanol addition. All compounds
were obtained with a yield exceeding 80%. The
products were characterized by mass spectrometry
(MicroTOF lc Bruker Daltonics, Capillary: 4000 V,
Nebulizer: 0.4 bar, Dry gas rate: 5.0 L.min-1, Temp:
180°C); UV-vis spectroscopy (Shimadzu UV-2401PC
spectrophotometer); 1H NMR spectroscopy (Bruker DRX
500, 500.13 MHz, CDCl3). Unequivocal assignments of
1H NMR spectra were made by 2D COSY (1H/1H), see
the figures in the Supporting information (SI).
Values close to 0.1 were obtained for the static
anisotropy in this study. According to Maiti et al. [39] the
depolarization of the probe fluorescence intercalated in
the micelle can be assigned to three mechanisms: (i) the
rotational momentum of the probe into the micelle, (ii) the
micelle rotation as a whole and (iii) the translational
diffusion of the probe along the surface of the micelle.
Considering the diameter of the porphyrin ring (~13 Å),
which is smaller than the diameter of the micelles SDS,
CTAB or Triton X-100, it is possible to assume that the
Anal. calcd. for Pp1 (21H,23H-Porphine-2,18-dipro-
panoicacid, 7,12-diethenyl-3,8,13,17-tetramethyl-, 2,18-
1
dimethyl ester). Yield 98%. H NMR (CDCl3, 500.13
MHz): d, ppm -3.77 (br s, 2H, H-21 and H-23); 3.27 (t,
4H, J = 7.5 Hz, H-132 and H-172); 3.60; 3.61; 3.68 and
3.69 (4s, 14H, H-2, H-7, H-12 and H-18); 3.66 (s, 6H, 134
and 174); 6.18 (d, 2H, J = 11.5 Hz, H-32a and H-82a); 6.36
NH
N
N
NH
N
N
ROH/H2SO4
HN
HN
R = CH3(Pp1)
R = CH2CH3 (Pp2)
R = CH2(CH2)2CH2(Pp4)
R = CH2(CH2)6CH2(Pp8)
R = CH2(CH2)10CH2(Pp12)
OH
O
HO
O
O
O
O
O
R
R
Fig. 6. Synthetic route and structural formulas of protoporphyrin-IX derivatives
Copyright © 2015 World Scientific Publishing Company
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12
S.C.D. SANTOS ET AL.
(d, 2H, J = 17.5 Hz, H-32b and H-82b); 8.27 (dd, 2H, J =
17.5 and 11.5 Hz, H-31 and H-81) 10.00, 10.04, 10.14 and
10.18 (4s, 4H, H-5, H-10, H-15 and H-20). ESI-MS-TOF:
m/z 591.2966 calcd. for C36H39N4O4+ (calcd. for [M + H]+
591.2970).
Anal. calcd. for Pp2 (21H,23H-Porphine-2,18-dipro-
panoicacid,7,12-diethenyl-3,8,13,17-tetramethyl-, 2,18-
diethyl ester).Yield 92%. 1H NMR (CDCl3, 500.13 MHz):
d, ppm -3.78 (br s, 2H, H-21 and H-23); 1.14 (t, 6H, J = 7.0
Hz, H-135 and H-175); 3.25 (t, 4H, J = 7.5 Hz, H-132 and
H-172); 3.60; 3.61; 3.67 and 3.70 (4s, 14H, H-2, H-7, H-12
and H-18); 4.14 (q, 4H, J = 7.0 Hz, H-134 and H-174);
4.38(t, 4H, J = 7.0 Hz, H-131 and H-171); 6.18 (d, 2H, J =
11.5 Hz, H-32a and H-82a); 6.36 (d, 2H, J = 18.0 Hz, H-32b
and H-82b); 8.26 (m, 2H, H-31 and H-81); 10.01, 10.03,
10.12 and 10.16 (4s, 4H, H-5, H-10, H-15 and H-20). ESI-
MS-TOF: m/z 619.3279 calcd. for C38H43N4O4+ (calcd. for
[M + H]+ 619.3275).
Anal. calcd. for Pp4 (21H,23H-Porphine-2,18-dipro-
panoicacid, 7,12-diethenyl-3,8,13,17-tetramethyl-, 2,18-
dibutyl ester). Yield 96%. 1H NMR (CDCl3, 300 MHz):
d, ppm -3.69 (br s, 2H, H-21 and H-23); 0.711 (t, 6H, J =
7.5 Hz, H-137 and H-177); 1.14–1.21 (m, 4H, H-136 and
H-176);1.42–1.51(m,4H,H-135 andH-175);3.23(t,4H,J=
7.5 Hz, H-132 and H-172); 3.56 (6H); 3.60 (3H) and 3.62
(3H) (3s, 12H, H-2, H-7, H-12 and H-18); 4.07 (t, 4H, J =
7.0 Hz, H-134 and H-174); 4.36 (t, 4H, J = 7.0 Hz, H-131
and H-171); 6.15 (two overlapping dd, 2H, J = 11.5 and
J = 2.0 Hz, H-32a and H-82a); 6.30 (two overlapping dd,
2H, J = 18.0 Hz, H-32b and H-82b); 8.16 and 8.22 (two dd,
2H, J = 17.5 and 11.5 Hz, H-31 and H-81) 9.91, 9.94 and
10.03 (4s, 4H, H-5, H-10, H-15 and H-20). ESI-MS-TOF:
m/z 675.3905 calcd. for C42H51N4O4+ (calcd. for [M +
H]+ 675.3912).
6H, 3H and 3H, H-2, H-7, H-12 and H-18); 4.03 (t, 4H, J
= 7.0 Hz, H-134 and H-174); 4.36 (t, 4H, J = 7.5 Hz, H-131
and H-171); 6.15 (d, 2H, J = 11.5 Hz, H-32a and H-82a);
6.33 (d, 2H, J = 17.0 Hz, H-32b and H-82b); 8.20 (dd, 2H, J
= 17.0 and 11.5 Hz, H-31 and H-81) 9.94, 9.96, 10.05 and
10.06 (4s, 4H, H-5, H-10, H-15 and H-20). ESI-MS-TOF:
m/z 899.6409 calcd. for C58H83N4O4+ (calcd. for [M + H]+
899.6403).
Spectroscopic and photophysical studies
The porphyrin solutions analyzed in this work were
obtained from a 1.0 × 10-3 M stock solution in dimethyl-
sulfoxide. The spectra of electronic absorption in the
ultraviolet-visible region and fluorescence emission of por-
phyrins were obtained in the concentration of 5.0 × 10-6 M
in different solvents (ethanol-EtOH; methanol-MeOH;
tetrahydrofuran-THF; acetonitrile-ACN; dimethylforma-
mide-DMF; dimethylsulfoxide-DMSO) and the respective
surfactants (Triton X-100, SDS, CTAB). Moreover, the
measurements involving the surfactants were obtained at
different pH values. The absorption spectra were obtained
using a UV-vis spectrophotometer (Lambda 25, Perkin
Elmer) and the fluorescence spectra were obtained using
a fluorescence spectrophotometer (LS55, Perkin Elmer).
The excitation wavelength used for measurements of
fluorescence was obtained from the length of maximum
absorption in the electronic absorption measurements in
the UV-vis. The fluorescence quantum yield (FF) was
determined in the solvents, making use of anthracene
in ethanol as standard solution, FF = 0.27 [42]. All
measurements were obtained at 20°C.
Measurements of fluorescence decay intensity were
obtained by the method Single Photon Counting. The
laser pulses were obtained with a titanium-sapphire laser
(Tsunami 3950, Spectra Physics), which was pumped
by a solid-state laser (Millenia Xs, Spectra Physics).
The light output of this laser has an integrated power
in the range between 6.0 and 10.0 W. The frequency
of the pulses generated by the Tsunami is adjusted by
a Pulse Picker (3986, Spectra Physics), which generates
laser pulses in the range between 840 and 1000 nm.
A pulse selector controls the repetition frequency of
the pulses. The desired laser radiation can be obtained
by a generator of second and third harmonics. At the
specific case of porphyrins, the second harmonic with
excitation at 414 nm was used in the measurements. The
laser pulses of excitation are correlated temporally with
pulses of fluorescence emitted by the sample, being the
data detected and collected by a spectrometer (Edinburgh
Instruments).
Anal. calcd. for Pp8 (21H,23H-Porphine-2,18-dipro-
panoicacid, 7,12-diethenyl-3,8,13,17-tetramethyl-, 2,18-
1
diocyl ester). Yield 81%. H NMR (CDCl3, 300 MHz):
d, ppm -3.53 (br s, 2H, H-21 and H-23); 0.65 and 0.67
((t, 3H, J = 7.5 Hz) and (t, 3H, J = 7.5 Hz), H-1311 and
H-1711); 1.15–1.42 (m, 20H, Hs-136-1310 and Hs-176-
1710); 1.65 (m, 4H, J = 7.0 Hz, H-135 and H-175); 3.25
(t, 4H, J = 7.5 Hz, H-132 and H-172); 3.60; 3.66; and 3.67
(3s, 6H, 3H and 3H, H-2, H-7, H-12 and H-18); 3.97 and
3.98 (two overlapping t, 4H, J = 7.0 Hz, H-134 and H-174);
4.39 (t, 4H, J = 7.0 Hz, H-131 and H-171); 6.16 (d, 2H, J =
11.5 Hz, H-32a and H-82a); 6.35 (d, 2H, J = 17.0 Hz, H-32b
and H-82b); 8.24 (dd, 2H, J = 17.0 and 11.5 Hz, H-31 and
H-81) 10.01, 10.11 and 10.13 (3s, 2H, 1H and 1H, H-5,
H-10, H-15 and H-20). ESI-MS-TOF: m/z 787.5157
calcd. for C50H67N4O4+ (calcd. for [M + H]+ 787.5161).
Anal. calcd. for Pp12 (21H,23H-Porphine-2,18-
dipropanoicacid, 7,12-diethenyl-3,8,13,17-tetramethyl-,
The spectrometer has a sample holder thermostated
by a circulating water bath. The anisotropy decay was
measured using a compensator (Babinet-Soleil BSC,
Halbo Optics) in the excitation beam and a polarizer
(P920, Edinburgh Instruments) of prism Glan-Thompson
in the beam emission. The fluorescence lifetimes of
1
2,18-didodecyl ester). Yield 84%. H NMR (CDCl3,
300 MHz): d, ppm -3.70 (br s, 2H, H-21 and H-23); 0.82
(t, 6H, J = 7.0 Hz, H-1315 and H-1715); 0.84–1.31 (m, 40H,
H-alkyl); 1.38(m, 4H, J = 7.0 Hz, H-135 and H-175) 3.24 (t,
4H, J = 7.0 Hz, H-132 and H-172); 3.57; 3.62 and 3.63 (3s,
Copyright © 2015 World Scientific Publishing Company
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PHOTOPHYSICAL PROPERTIES OF PORPHYRIN DERIVATIVES
13
the porphyrins were determined in different solvents
(DMSO, EtOH and ACN) and in micro-heterogeneous
systems (in the presence of surfactants).
Acknowledgements
The authors are grateful to Prof. Dr. Amando Siuiti
Ito by the use of the equipment to measure the lifetime
with temporal resolution and by the financial support
of Chamada Pública MCT/CNPq/MEC/CAPES —
Ação transversal n° 06/2011 — Casadinho/Procad —
Process n° 552197/2011-4. Hueder Paulo Moisés de
Oliveira thanks to CNPq — Process 474019/2012-8 and
479655/2008-1. Maurício da Silva Baptista thanks to
FAPESP — Process 2012/50680-5.
Heterogeneous media were generated by the
dissolution of porphyrins in solutions of surfactants with
time of stirring of 15 min. The concentration of surfactant
was 40.0 × 10-3 M, which corresponds to a value higher
than the critical micelle concentration (CMC). This
indicates also that the micelle is the micro-heterogeneous
structure prevalent in this chemical system. The final
concentration of porphyrins in the micellar media was
1.0 × 10-5 M. To measure the time-resolved fluorescence,
the wavelengths of excitation and emission were 414 nm
and 630 nm, respectively. All measurements were
performed at 20°C.
Supporting information
Figures S1–S6 are given in the supplementary material.
This material is available free of charge via the Internet at
http://www.worldscinet.com/jpp/jpp.shtml.
CONCLUSION
Markeddifferenceswereobservedinthephotophysical
properties (absorption, emission, quantum yield,
fluorescence lifetime and orientational time) between
the porphyrin derivatives and the different chemical
environments. The porphyrin derivatives have shown a
decrease in quantum yield and fluorescence lifetime in
more polar solvents. Constants of fluorescence decay,
which were calculated from the absorption spectra,
are higher than those calculated from the lifetime data
obtained experimentally. However, both data show similar
tendency with respect to the solvent polarity. The size of
the hydrophobic chains of the porphyrin derivatives has a
significant influence in their interactions with surfactants,
especially with respect to sodium dodecyl sulfate. It was
also observed a large increase of FF generated by Pp12
in solvents with lower polarity (DMF and THF), when
compared to other solvents whose FF data were close
to zero. The incorporation of porphyrins in micellar
systems was confirmed by fluorescence decay through
a simple exponential curve for all porphyrins. This
model is in agreement with data in the literature, where
the porphyrin is confined in a region near the micellar
surface. No difference was noticed for the rotational time
as the alkyl chains of porphyrin become longer, which
indicates that the position of the porphyrin in the micelles
was not influenced by the presence of long nonpolar
chains. Protoporphyrin-IX derivatives can be considered
as a model of hydrophobic molecule with a high affinity
for non-ionic surfactants. This increased affinity can be
explained by the significant presence of Van der Waals
interactions, which influence directly the properties of
fluorescence and allow more effective stabilization of
porphyrins in neutral micelles when compared to its
stability in micelles consisting of ionic surfactants. In
this study the emitting species were identified as well as
its photophysical behavior as potential photosensitizer in
different chemical environments. These information are
fundamental to develop the most suitable prototype for
application in PDT.
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