Excited-state absorption of meso-tetrasulfonatophenyl porphyrin: Effects of pH and micelles

Optical Materials 42 (2015) 516–521

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Excited-state absorption of meso-tetrasulfonatophenyl porphyrin: Effects of pH and micelles D.S. Correa a, L. De Boni b, G.G. Parra c, L. Misoguti b, C.R. Mendonça b, I.E. Borissevitch c, S.C. Zílio b, N.M. Barbosa Neto d,e, P.J. Gonçalves f,⇑ a

EMBRAPA Instrumentação, Rua XV de Novembro, 1452, CP 741, 13560-970 São Carlos, SP, Brazil Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil Departamento de Física, FFCLRP, Universidade de São Paulo, Av. Bandeirantes 3900, 14040-901 Ribeirão Preto, SP, Brazil d Instituto de Física, Universidade Federal de Uberlândia, Av. João Naves de Ávilla, 2121, Bloco 1X, 38.400-902 Uberlândia, MG, Brazil e Programa de Pós-graduação em Física, Universidade Federal do Pará, P.O. Box 479, 66075-110 Belém, PA, Brazil f Instituto de Física, Universidade Federal de Goiás, Caixa Postal 131, 74001-970 Goiânia, GO, Brazil b c

a r t i c l e

i n f o

Article history: Received 20 December 2014 Received in revised form 28 January 2015 Accepted 29 January 2015 Available online 18 February 2015 Keywords: Molecular symmetry Protonation Z-scan Laser flash photolysis Excited state absorption Optical limiting

a b s t r a c t The influence of the environment on the excited state transitions of meso-tetrakis(p-sulfonatophenyl) porphyrin (TPPS) is reported. TPPS was investigated in protonated and non-protonated forms, and in the presence of the cationic cetyltrimethylammonium bromide (CTAB) micelles. The singlet excited-state absorption spectra were measured by using the white-light continuum Z-scan technique and the triplet– triplet absorption spectra were acquired employing an association of laser flash photolysis and Z-scan techniques. Our results show that the perseveration of the molecular symmetry, upon excitation, depends on the state of multiplicity of the molecules, as well as on the environment and structural characteristics of the porphyrin. Additionally, it was observed that for excited molecules, the ring distortion caused by the protonation of porphyrin ring has great influence on the changes observed for the symmetry and vibronic structure. The results clearly show that the porphyrin investigated is a promising candidate for optical limiting applications for all investigated environments. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Porphyrins and porphyrin-like compounds possess intense absorption in the visible spectral region, presenting high and fast nonlinear optical response [1–3]. The excited-state absorption (ESA) properties are object of several investigations due to their importance for optical limiting [2], switching [3] and others, making them promising candidates for photonic applications. In addition, the study of ultrafast excited-state dynamics is essential to clarify the mechanisms of electronic and vibronic excited-state deactivation, including electron and energy transfer. These mechanisms play an essential role in various natural processes, in which porphyrins and porphyrin-like compounds participate, such as in photosynthesis and photodynamic effect against fungi, bacteria, cancer and others [4–6]. Therefore, the study of the excited states of porphyrins, such as the characterization of the absorption cross-sections, lifetimes and quantum yields is extremely important [1,2,7,8]. ⇑ Corresponding author. Fax: +55 62 3521 1014x247. E-mail address: [email protected] (P.J. Gonçalves). http://dx.doi.org/10.1016/j.optmat.2015.01.047 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

Previous works demonstrated that the interaction of porphyrins with nanostructures, such as synthetic and natural polymers, molecular multilayers, biological membranes and micelles, are capable of affecting their photophysical properties [9–13]. Due to their structural simplicity, micelles have been considered as an adequate model of a biological membrane, in particular, regarding the issue of location of foreign molecules (e.g. drugs) in an organized system [10,12]. In this way, information about the effect of environment and interaction with nano organized structure on photosensitizer characteristics of porphyrin can be considered keystone knowledge. It is well established that, under acidic conditions, free-base porphyrins can be protonated, through the binding of two additional hydrogens to the central nitrogen atoms. Protonated porphyrins have been found to exhibit distinct photophysical properties compared to their nonprotonated parent compounds, including substantial decrease in quantum yield of triplet state formation, changes in electronic absorption spectra, and increase in Stokes-shifts of the fluorescence spectra [14–16]. Moreover, protonated porphyrins typically present nonplanar structure due to the steric hindrance and electrostatic repulsion of the central hydrogen

D.S. Correa et al. / Optical Materials 42 (2015) 516–521

atoms [17,18]. Therefore, protonated porphyrins provide unique prototypes to study the influence of nonplanarity on the physicochemical properties of macrocycles. Nonplanar conformations of porphyrins and related compounds are of great importance for their potential function in photosynthetic reaction centers, for instance, altering the dynamics of light harvesting in photosynthesis [19]. Moreover, it is widely known that the protonation of freebase porphyrins provides a convenient way to construct ordered molecular assemblies through the aggregation process [20,21]. Additionally, such aggregates change the photophysical properties of porphyrins [22,23], rendering interesting optical properties for technological applications [24–26]. In this way, any information on the changes of the excited-state characteristics is important for practical applications and may help to explain the mechanisms of molecule environment interaction. For porphyrins, the excited-state absorption (ESA) can be attributed, at resonance conditions, to excited singlet or triplet states absorption, or both combined. The excited singlet state absorption spectra of molecules can be obtained through the use of an extension of Z-scan technique, called white-light continuum Z-scan technique (WLCZ-scan) [27]. Recently, we proposed a new procedure, which combines the pulse train Z-scan technique together with absorbance spectroscopy and laser flash photolysis, to obtain the absorption cross-section spectra of triplet–triplet states in porphyrin-like molecules [28]. This approach allowed us to investigate features of excited-states absorption of meso-tetrakis(methylpyridiniumyl) porphyrin (TMPyP), revealing the influence of solvents on the vibronic structuration of the transitions assigned to triplet-states [28]. Here, we employ this approach to investigate the singlet and triplet excited-state absorption spectra (S1 ? Sn and T1 ? Tm) of free-base meso-tetrakis(p-sulfonatophenyl) porphyrin (TPPS) in its protonated and nonprotonated states interacting with the cationic micelles cetyltrimethylammonium bromide (CTAB) surfactants. Our findings are important to advance the understanding of the photophysical behavior of porphyrins-like molecules aiming at applications in pharmacokinetics, and photonic devices.

2. Experimental section The water soluble anionic meso-tetrakis(p-sulfonatophenyl) (TPPS) porphyrin (see Fig. 1) was purchased from the Porphyrin Products Inc. The protonation of TPPS can be controlled through the adjustments of pH values. For example, at pH > 6.0 TPPS is in the nonprotonated form while in pH < 4.5 it is protonated [7]. In the present work it was chosen pH values of 7.0 and 3.0, which were achieved by the addition of HCl or NaOH stock solutions. The micelles were formed from a cationic cetyltrimethylammonium bromide (CTAB), obtained from Sigma Aldrich Co. To assure the

Fig. 1. Molecular structure of TPPS porphyrin.

517

micellar formation, the TAB surfactant was used in a concentration of 20 mM, which is higher than the critical micellar concentration (CMC). All solutions were prepared in the Milli-Q quality water, and the porphyrin concentrations were controlled by means of a spectrophotometer. The porphyrin linear absorption was monitored with a Cary 17 spectrometer using a 2 mm quartz cuvette. The excited-state absorption, assigned to singlet states, was measured using the open-aperture WLC Z-scan technique, which is based on the traditional single wavelength Z-scan technique. The difference lies in the fact that in the standard Z-scan, a single wavelength excitation pulse is employed, while the WLC Z-scan uses a white broadband pulse. Further details can be found elsewhere [27,28]. The transient absorption spectra, DA(k), involving the triplet– triplet absorption was determined by laser flash photolysis (LFP), obtained with the second harmonic (532 nm) of a Nd:YAG SL400 Spectrum Laser System, with 4 ns of pulse duration, in association with pulse train Z-scan, according to Ref. [28]. 3. Results Fig. 2 shows the normalized transmittance (NT) spectra obtained with the WLC Z-scan technique for: TPPS/pH 3 (dotted line), TPPS/pH 7 (continuous line) and TPPS/pH 3 with CTAB (dashed line). These spectra were obtained from Z-scan signatures at different wavelengths, as shown in Fig. 3 and described elsewhere [27–29]. One notices that NT < 1 along the wavelength range studied for all molecules, except for TPPS/pH 3. Such NT decrease is a consequence of the higher absorption cross-sections of the excited-state in comparison to the ground-state, leading to reverse saturable absorption (RSA). For TPPS/pH 3 without micelles, the NT spectrum shows an increase in the transmittance (NT > 1) between 630 and 700 nm. For this spectral region the excited-state absorption cross-section is smaller than that of the ground-state, leading to saturable absorption (SA). Fig. 3 displays typical Z-scan curves for both SA (open squares) and RSA (open circles), acquired for different wavelengths (TPPS/pH 3 at 530 nm (s), 630 nm (D) and 645 nm (h)). In particular, when the excited- and ground-states present similar cross-sections, the normalized transmittance approaches a flat line, as represented by the open triangles of Fig. 3. In order to obtain the excited-state absorption cross-sections, r1n(k), we employed a model based on a three-energy-level diagram, depicted in the inset of Fig. 3, to fit the Z-scan curves for each wavelength and consequently simulates the NT spectra. This model, which includes the ground-state (S0) and two excited singlet states (S1 and Sn), is a good approximation to describe the

Fig. 2. Normalized transmittance at different wavelengths for: TPPS/pH 3 (dotted line), TPPS/pH 7 (solid line) and TPPS/pH 3/CTAB (dashed line) obtained with the WLC Z-scan technique.

518

D.S. Correa et al. / Optical Materials 42 (2015) 516–521

Fig. 3. Normalized transmittance as a function of z position measured using WLC Zscan in TPPS/pH 3. Each curve represents a distinct wavelength: 530 (s), 630 (4) and 645 nm (h). The solid lines are the fitting obtained with the three-energydiagram shown in the inset.

excited-state absorption at picoseconds time scale (WLC temporal width), once the intersystem-crossing time is on the order of nanoseconds [7], and therefore the triplet state contribution can be neglected. According to this model, molecules lying in the lowest electronic state S0 can be promoted to a vibronic level of S1 via a one-photon process, with an absorption cross-section r01. From this vibronic level, they can relax to the bottom of S1 and then decay to S0, with a time constant of s10 (in the order of nanoseconds), or be re-excited to a vibronic level of Sn, with an absorption cross-section r1n. In accordance with literature, the relaxation time of Sn state is in the order of tens to hundreds of femtoseconds [30]. The time evolution of the ground (S0), the first (S1) and the higher singlet (Sn) excited state populations are given by the following equations:

dnS0 nS ¼ W 01 ðkÞðnS0  nS1 Þ þ 1 dt s10 dnS1 nS nS ¼ W 01 ðkÞðnS0  nS1 Þ  W 1n ðkÞðnS1  nSn Þ  1 þ n dt s10 sn1 dnSn nS ¼ W 1n ðkÞðnS1  nSn Þ  n dt sn1

ð1aÞ ð1bÞ ð1cÞ

where nS0 ; nS1 and nSn are, respectively, the population fractions in the ground (S0), first (S1) and higher (Sn) excited singlet levels.

Fig. 4. Ground (solid squares), first singlet excited (solid triangles) and first triplet states (open circles) absorption cross-sections acquired at different wavelengths for: (a) TPPS bi-protonated (pH 3.0), (b) TPPS nonprotonated (pH 7.0) and (c) TPPS (pH 3.0) in the presence of CTAB micelles. The insets show details of the spectra at the Q-band region.

W 01 ðk; tÞ ¼ r01 ðkÞIðt;kÞ and W 1n ðk; tÞ ¼ r1n ðkÞIðt;kÞ are the S0 ? S1 and hm hm S1 ? Sn transition rates, with r01 ðkÞ and r1n ðkÞ being the absorption cross-sections corresponding to these transitions. s10 and sn1 are the S1 and Sn state lifetimes, respectively. During excitation, the absorption coefficient time dependence, aðk; tÞ, is given by:

aðk; tÞ ¼ N½nSo ðtÞr01 ðkÞ þ n1 ðtÞr1n ðkÞ

ð2Þ 3

where N is the number of molecules/cm . The set of rate equations was numerically solved considering the Gaussian temporal profile of the laser pulse. The transmittance was calculated by integrating Beer’s law over the sample optical path and the pulse temporal profile. The normalized transmittance used to fit the experimental NT curves was calculated by normalizing the transmittance at the focus position by the linear one, acquired at far field. The excited singlet state cross-section spectra, r1n(k), obtained from the fitting of the normalized transmittance curves, and the ground-state cross-section spectra, r01(k), acquired from linear absorption measurements, are shown in Fig. 4, for TPPS at pH 3.0, in the presence and in the absence of micelles and at pH 7.0. During the relaxation of the S1 state, a fraction of molecules can relax to triplet states T1 via intersystem crossing, as depicted in the inset of Fig. 5, leading to a considerable population in it. In order to

Fig. 5. Transient absorption (DA) spectra obtained with LFP at 0.5 ls after excitation for TPPS at: pH 7(4), pH 3 (s) and pH 3/CTAB (d).

investigate the absorption contribution coming from the triplet state, we measured the differential spectra of all porphyrins by means of LFP experiments, in which the differential optical

D.S. Correa et al. / Optical Materials 42 (2015) 516–521

absorption, DA(k), is obtained as a function of wavelength (see Fig. 5). It is worth to mention that for the particular case of non-protonated TPPS in the absence of micelles, a great similarity is observed with previous DA results reported in the literature [31]. The observed variation of optical absorption of the solution is caused by the difference between absorption cross-sections assigned to the singlet–singlet (r01(k)) and the triplet–triplet (r34(k)) transitions. The differential optical absorption can be calculated from [28,31]:

DAðkÞ ¼

1 ½r34 ðkÞ  r01 ðkÞNT ‘ 2:303

ð3Þ

where NT is the population on triplet state and ‘ is the sample thickness (1 cm). In order to obtain the r34(k) spectra through Eq. (3) we employ a combination of different techniques, according to Ref. [28]. In such approach, we combine the LFP technique with pulse train Zscan technique (PTZ-scan), whose data provides the absorption cross-section values (r34) at 532 nm [29,32]. Also using r34 (at k = 532 nm) we can calibrate the LFP results and obtain the unknown NT value. However, to avoid light scattering originated from the excitation laser beam (532 nm) of FLP, we do not measure DA(k) at this wavelength. Instead, DA(k) for 532 nm is obtained from an extrapolation from their values at 525 and 540 nm. Thus, the value of NT can be determined from Eq. (3) at 532 nm. Later, we use the same equation in order to obtain the r34(k) spectra from the LFP transient absorption spectrum (see open circles at Fig. 4). 4. Discussion The typical electronic absorption spectra of all porphyrins have two major absorption bands. The Q-band is located in the spectral region between 500 and 650 nm, while the intense B band (or Soret band) is located in the UV region (360–410 nm). The lowest energy transitions in porphyrin spectra are interpreted according to the Gouterman’s four-orbital model and are based on symmetry considerations of the molecules [33]. For instance, metalloporphyrins with idealized D4h symmetry have a near-degenerate a1u, a2u HOMO pair, while LUMOs are a strictly degenerate eg pair. These orbitals give rise to two couples of degenerates transitions Q and B (Qx and Qy and two more intense and of higher energy Bx and By). The labels x and y refers to the direction of polarization: the Qx (or Bx) band is polarized along the N–H. . .H–N axis (see Fig. 1), while the Qy (or By) band has y-polarization along the N. . .N direction. In the case of free-base porphyrins, there is a lowering of symmetry (D2h) that increases the degeneracy of the eg pair and leads to the formation of four one-electron transitions, from au and b1u HOMOs to the b2g, b3g LUMOs. In this way, the degeneracy between the Qx, Qy and Bx, By transitions is removed. In addition, each Q-band transition further splits, by vibronic coupling, in pairs of bands; Qx(0,0) and Qy(0,0), Qx(1,0) and Qy(1,0). Since the Qbands transitions are very weak, some authors have recently proposed that these bands are dominated by the Herzberg–Teller contributions, while the Franck–Condon contributions are negligible [34,35]. The Herzberg–Teller mechanism results from vibronically induced coupling between electronic states by electronic dipole transition [36]. We noticed that all samples present Q-bands (S0 ? S1) in the 460–700 nm region (r01(k) spectra in Fig. 4). In addition, these porphyrins also have an intense Soret absorption band close to 400 nm (not shown) [7]. For bi-protonated TPPS, we observe (see dark full squares at Fig. 4a) that the Q-band presents a red shift and a change in the values of the oscillator strength if compared with TPPS in pH 7 (Fig. 4b). TPPS in pH 3 presents its most intense band located around 644 nm, while for TPPS/pH 7 this occurs at 515 nm. In the presence of CTAB micelles, the TPPS linear spectrum at pH

519

3.0 (Fig. 4c) is similar to the nonprotonated one (pH 7.0). This fact is attributed to the shift in its pK value caused by the TPPS bound to micelles [10]. TPPS/pH 7 with and without micelles presented no difference in ground as well as in excited states absorption-cross sections. Furthermore, we also observe that, at Q-band region, the profile of the r1n ðkÞ spectra is similar to those of r01 ðkÞ, for both TPPS/pH 7 and TPPS/pH 3 in the presence of micelles (see Fig. 4b and c). Indeed, it has been reported that porphyrins and their derivates present similar excited-state profiles obtained for transitions from ground and from the first singlet excited states [15,28,37], which is not the case for cyanines and phthalocyanines [38–40], Moreover, several works have corroborated the suggestion of a strong vibronic coupling between excited-states and ground-state in porphyrins. Due to the rigid structure of free base porphyrin core, the minimum energy of excited state equilibrium geometry (Qx-band) is rather similar to the ground electronic state [41]. In this way, it was reported that the ground-state normal modes structure remains in the S1 excited-state and that the vibrational frequencies of both saturated and unsaturated substituents were minimally perturbed [42]. These findings, together with the observed small Stokes shift and the resolved vibronic structure in emission spectra, indicates that the vibronic structure of the porphyrins is preserved upon excitation [43], for the singlet states. Such similarity between the ground and excited state spectra was observed in a previous investigation of TMPyP, when studied in different environments [28], suggesting that the environmental effect is small on the excited singlet state cross-section of porphyrins. These facts can be understood considering that the tetrapirrole ring of freebase porphyrins exhibits a planar and rigid equilibrium structure, which is not strongly influenced by the environment, and therefore helps preserving the porphyrins electronic properties for both ground and first excited singlet state. Differently, triplet states have shown to be more affected by environment properties [28]. On the other hand, bi-protonated TPPS (pH = 3) presents changes in its linear and nonlinear optical properties, in comparison to the others. In the absence of micelles, protonation causes a singlet and triplet excited-state absorption enhancement in the region from 450 nm to 620 nm when compared to the nonprotonated form. Additionally, r01 ðkÞ and r1n ðkÞ spectra profiles are distinct, as seen in Fig. 4a. r1n ðkÞ is higher than r01 ðkÞ in almost all the region studied, except for wavelengths longer than 630 nm, where the molecule presents a high linear absorption. The deviation of the profile for r1n(k) spectrum for the bi-protonated TPPS in the absence of micelles (Fig. 4a), compared to the other cases can be understood in terms of electronic effects and conformational distortions caused by protonation. The bi-protonation of the central nitrogen of porphyrin core alters its static and dynamic photophysical properties compared to the respective free-base. Such changes are caused by the fact that the protonation of the inner pyrrole nitrogen atoms can induce a remarkable distortion of the tetrapyrrole ring from the planarity, leading to the so-called ‘saddled’ conformation, with the pyrrole rings tilted alternatively up and down with respect to the average molecular plane. Geometry optimizations indicate that the pyrrole rings are tilted by 30o with respect to the average molecular plane [41]. The distortion of the tetrapyrrole ring generates some effects in excited and ground states, analogous to those that occur upon formation of a corresponding metal derivative [44]. It is included (i) a collapse of the four main visible region ground-state absorption bands [Qy(1,0), Qy(0,0), Qx(1,0), Qx(0,0)] of the free-base to only two bands [Q(1,0), Q(0,0)], (ii) a red shift of the Q(0,0) band of the diacid from the Qx(0,0) band of the free-base and (iii) a red shift of the B band of the diacid from the B band of the free-base [44]. Moreover, the ring distortion also may be accompanied by the increase of the conformational flexibility [45], implying that the

520

D.S. Correa et al. / Optical Materials 42 (2015) 516–521

molecules are able to access additional nonplanar conformations in the ground and/or excited electronic states. Such larger flexibility with respect to the planar species increases the internal conversion creating an excited-state configuration with a higher number of near isoenergetic structures. Such configuration provides groundand excited-states absorption cross-sections spectra with relative low resolution, when compared to those of nonprotonated state. Other differences in the photophysical characteristics between protonated and nonprotonated porphyrins due to the non-preservation of the vibronic structure upon excitation that is caused by a distortion of planarity and also its inherently greater flexibility has been previously reported in the literature [16]. It is also observed that in the region from 500 to 700 nm, DA values are small for all samples (see Fig. 5), implying that r34(k) values are close to r01(k), which is the same behavior observed when comparing r1n(k) and r01(k) values (see Fig. 4c). For biprotonated TPPS, the triplet–triplet absorption spectrum (r34(k)), at the region closed to 650 nm, shows an absorption band similar to that found for singlet spectra, indicating that the symmetry and electronic configuration is partly preserved for the S1 ? T1 transition. Besides that, a new intense band arises close to 500 nm. On the other hand, for nonprotonated TPPS, in both water and in the presence of micelles, the r34(k) spectra does not

have the same vibronic structure presented in singlet transitions (r1n(k)), with the magnitude of the r34(k) being augmented in the blue region of the spectrum, causing an intense RSA. Aiming at evaluating the efficiency of the samples for optical limiting applications in the time scale varying from femto to nanosecond, we show in Fig. 6 rij ðkÞ=r01 ðkÞ spectra (i = 1,3 and j = n,4), with the purpose of prospecting spectral regions with intense RSA signal. It is observed that biprotonated TPPS presents a RSA process in the region between 450 and 600 nm. In this region, r1n ðkÞ=r01 ðkÞ ratio reaches a peak value around 10 (close to 515 nm) in the femtosecond regime (S1 ? Sn transition), while r34 ðkÞ=r01 ðkÞ reaches a peak value around 42 in nanosecond scale (T1 ? Tn absorption). On the other hand, SA occurs between 600 and 700 nm, which is coincident with the linear absorption peak at 644 nm. For nonprotonated TPPS, the r1n ðkÞ=r01 ðkÞ ratio reaches a peak value close to 2.5 along the Q-band at femtosecond time scale, indicating a RSA process. The r34 ðkÞ=r01 ðkÞ ratio is higher in the 450–500 nm range, reaching a peak value of 30 in nonprotonated TPPS and 25 in the presence of CTAB micelles. The behavior of nonprotonated TPPS and bi-protonated TPPS in presence of CTAB micelles were very similar (see Fig. 6(b) and (c)). In this case, the micelles promote the deprotonation of TPPS, causing both spectra becomes very similar in profile. 5. Conclusions TPPS porphyrin in its non-protonated form, in aqueous solutions, both in the absence and in the presence of micelles, presented singlet excited state absorption cross-sections (r1n(k)) similar to the ground state one (r01(k)), indicating that D2h molecular symmetry of nonprotonated porphyrin can be preserved upon electronic excitation. The environment modification from water to micelle weakly affects the ground- and excited-state absorption spectra, for non-protonated porphyrins, demonstrating a weak interaction of the porphyrin pyrrole ring p-conjugated system with the environment. The results for the excited triplet state absorption shows that for nonprotonated TPPS, in both water and in the presence of micelles, T1 ? Tn transitions do not have the same vibronic structure presented in singlet ones. It is also observed that bi-protonation caused considerable changes in the nonlinear optical properties of the porphyrin, which was attributed to changes in the molecular symmetry upon excitation. As a consequence, the first singlet excited-state absorption cross-section spectra become distinct to the ground-state one. In this case, the ratio r1n(k)/r01(k) reaches 10 at 505 nm. The r34(k)/r01(k) ratio reaches 30 for nonprotonated TPPS and 42 for biprotonated in the same region. In this way, TPPS porphyrin displayed a high RSA between 450 and 500 nm, indicating its potential for applications in optical limiters, especially to act on nanosecond laser pulses. Acknowledgments We thank financial support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Fundação de Amparo à Pesquisa do Estado de Goiás (FAPEG), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and INCT/INFO. References

Fig. 6. r1n/r01 and r1n/r01 spectra acquired for all environments investigated: (a) TPPS bi-protonated (pH 3), (b) TPPS nonprotonated (pH 7) and (c) TPPS (pH 3) in the presence of CTAB micelles.

[1] M.O. Senge, M. Fazekas, E.G.A. Notaras, W.J. Blau, M. Zawadska, O.B. Locos, E.M.N. Mhuircheartaigh, Adv. Mat. 19 (2007) 2737–2774. [2] P. Gautam, B. Dhokale, V. Shukla, C.P. Singh, K.S. Bindra, R. Misra, J. Photochem. Photobiol. A 239 (2012) 24–27. [3] C.P. Singh, K.S. Bindra, B. Jain, S.M. Oak, Opt. Comm. 245 (2005) 407–414.

D.S. Correa et al. / Optical Materials 42 (2015) 516–521 [4] L.M. Almeida, F.F. Zanoelo, K.P. Castro, I.E. Borissevitch, C.M.A. Soares, P.J. Gonçalves, Photochem. Photobiol. 88 (2012) 992–1000. [5] D.A. Caminos, M.B. Spesia, E.N. Durantini, Photochem. Photobiol. Sci. 5 (2006) 56–65. [6] R. Bonnett, Chem. Soc. Rev. 24 (1995) 19–33. [7] P.J. Gonçalves, L. De Boni, N.M. Barbosa Neto, J.J. Rodrigues Jr., S.C. Zílio, I.E. Borissevitch, Chem. Phys. Lett. 407 (2005) 236–241. [8] P. Kubat, J. Mosinger, J. Photochem. Photobiol. A 96 (1996) 93–97. [9] C. Pavani, A.F. Uchoa, C.S. Oliveira, Y. Iamamoto, M.S. Baptista, Photochem. Photobiol. Sci. 8 (2009) 233–240. [10] S.C.M. Gandini, V.E. Yushmanov, I.E. Borissevitch, M. Tabak, Langmuir 15 (1999) 6233–6243. [11] D.S. Neto, A. Hawe, M. Tabak, Eur. Biophys. J. 42 (2013) 267–279. [12] P.S. Santiago, D.S. Neto, S.C.M. Gandini, M. Tabak, Colloid Surf B: Biointerface 65 (2008) 247–256. [13] P.J. Gonçalves, D.S. Corrêa, P.L. Franzen, L. De Boni, L.M. Almeida, C.R. Mendonça, I.E. Borissevitch, S.C. Zílio, Spectrochim. Act. A 112 (2013) 309–317. [14] V.S. Chirvony, A. van Hoek, V.A. Galievsky, I.V. Sazanovich, T.J. Schaafsma, D. Holten, J. Phys. Chem. B 104 (2000) 9909–9917. [15] Z.B. Liu, Y. Zhu, Y.Z. Zhu, J.G. Tian, J.Y. Zheng, J. Phys. Chem. B 111 (2007) 14136–14142. [16] A. Marcelli, P. Foggi, L. Moroni, C. Gellini, P.R. Salvi, I.J. Badovinac, J. Phys. Chem. A 111 (2007) 2276–2282. [17] B. Cheng, O.Q. Munro, H.M. Marques, W.R. Scheidt, J. Am. Chem. Soc. 119 (1997) 10732–10742. [18] Z.Y. Li, H.L. Wang, T.T. Lu, T.J. He, F.C. Liu, D.M. Chen, Spect. Chim. Act. A 67 (67) (2007) 1382–1391. [19] Y.C. Cheng, G.R. Fleming, Annu. Rev. Phys. Chem. 60 (2009) 241–262. [20] N.C. Maiti, M. Ravilkanth, S. Mazumdar, N. Periasamy, J. Phys. Chem. 99 (1995) 17192–17197. [21] J.M. Ribó, J. Crusats, J.A. Farrera, M.L. Valero, J. Chem. Soc. Chem. Commun. (1994) 681–682. [22] P.J. Gonçalves, N.M. Barbosa Neto, G.G. Parra, L. de Boni, L.P.F. Aggarwal, J.P. Siqueira, L. Misoguti, I.E. Borissevitch, S.C. Zílio, Opt. Mater. 34 (2012) 741– 747. [23] A. Miura, Y. Shibata, H. Chosrowjan, N. Mataga, N. Tamai, J. Photochem. Photobiol. A 178 (2006) 192–200. [24] E. Collini, C. Ferrante, R. Bozio, J. Phys. Chem. B 109 (2005) 2–5. [25] E. Collini, C. Ferrante, R. Bozio, A. Lodi, G. Ponterini, J. Mater. Chem. 16 (2006) 1573–1578.

521

[26] T. Ogawa, E. Tokunaga, T. Kobayashi, Chem. Phys. Lett. 408 (2005) 186–191. [27] L. De Boni, A.A. Andrade, L. Misoguti, C.R. Mendonça, S.C. Zílio, Opt. Express 12 (2004) 3921–3927. [28] N.M. Barbosa Neto, D.S. Correa, L. De Boni, G.G. Parra, L. Misoguti, C.R. Mendonça, I.E. Borissevitch, S.C. Zílio, P.J. Gonçalves, Chem. Phys. Lett. 587 (2013) 118–123. [29] N.M. Barbosa Neto, L. De Boni, J.J. Rodrigues Jr., L. Misoguti, C.R. Mendonça, L.R. Dinelli, A.A. Batista, S.C. Zilio, J. Porphyr. Phthalocya. 7 (2003) 452–456. [30] S.V. Rao, D.N. Rao, J. Porphyr. Phthalocya. 6 (2002) 233. [31] G.S. Nahor, J. Rabani, F. Grieser, J. Phys. Chem. 85 (1981) 697–702. [32] P.J. Gonçalves, L.P.F. Aggarwal, C.A. Marquezin, A.S. Ito, L. De Boni, N.M. Barbosa Neto, J.J. Rodrigues Jr., S.C. Zilio, I.E. Borissevitch, J. Photochem. Photobiol. A 181 (2006) 378–384. [33] M. Gouterman, G.H. Wagniére, L.C. Snyder, J. Mol. Spectrosc. 11 (1963) 108– 127. [34] F. Santoro, A. Lami, R. Improta, J. Bloino, V. Barone, J. Chem. Phys. 128 (2008) 224311. [35] B. Minaev, Y.H. Wang, C.K. Wang, Y. Luo, H. Ågren, Spectrochim. Act A 65 (2006) 308–323. [36] M. Klessinger, J. Michl, Excited states and Photochemistry of Organic Molecules, VCH Publishers Inc, New York, 1995. [37] R.V. Maximiano, E. Piovesan, S.C. Zílio, A.E.H. Machado, R. de Paula, J.A.S. Cavaleiro, I.E. Borissevitch, A.S. Ito, P.J. Gonçalves, N.M. Barbosa Neto, J. Photochem. Photobiol. A 214 (2010) 115–120. [38] L. De Boni, C.R. Mendonca, J. Phys. Org. Chem. 24 (2011) 630–634. [39] M.G. Vivas, E.G.R. Fernandes, M.L. Rodríguez-Méndez, C.R. Mendonça, Chem. Phys. Lett. 531 (2012) 173–176. [40] L. De Boni, E. Piovesan, L. Gaffo, C.R. Mendonça, J. Phys. Chem. A 112 (2008) 6803–6807. [41] R. Improta, C. Ferrante, R. Bozio, V. Barone, Phys. Chem. Chem. Phys. 11 (2009) 4664–4673. [42] R. Kumble, G.R. Loppnow, S. Hu, A. Mukherjee, M.A. Thompson, T.G. Spiro, J. Phys. Chem. 99 (1995) 5809–5816. [43] L. Moroni, C. Gellini, P.R. Salvi, J. Phys. Chem. A 112 (2008) 11044–11051. [44] P.J. Gonçalves, L. De Boni, I.E. Borissevitch, S.C. Zílio, J. Phys. Chem. A 112 (2008) 6522–6526. [45] A. Rosa, G. Ricciardi, E. Jan Baerends, A. Romeo, L.M. Scolaro, J. Phys. Chem. A 107 (2003) 11468–11482.