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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x
On the Interaction of Triarylmethane Dye Crystal Violet with Laponite Clay: Using Mineral Nanoparticles to Control the Dye Photophysics. C. Ley*a, J. Brendléb, A. Waltera, P. Jacquesa, A. Ibrahima, X. Allonasc
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The combination of organic dye with clays leads to very interesting hybrid materials with original properties. It is found that Laponite nanoparticles impact the photophysical properties of Crystal Violet dye inducing a kinetic stabilization of its excited emissive state, turning this non emissive dye into a fluorescent compound. Organic dyes are known to interact with clay minerals, like 1,2 smectites which belongs to the phyllosilicates family, by 2-4 adsorption or ion exchange. Original properties could be obtained with applications in water depollution,5 2,6 solubilization, control of photochemical reactions7-10 or optical properties.11,12 With this in mind we explore in this paper the interaction of a cationic triarylmethane dye (TAM) molecules with a synthetic commercially available 2:1 (TOT where T stands for Tetrahedral and O for Octahedral) phyllosilicate, namely Laponite (Scheme 1).13 Each Laponite platelet is composed of a central sheet of magnesium ions in octahedral coordination (O) with oxygen anions and hydroxyl groups and two outer tetrahedral sheets (T). The isomorphic substitution of lithium for magnesium cations in the octahedral sheet induces a negative charge which is neutralized by sodium cations located in the interlayer space. The average dimensions of the Laponite layers (diameter of 25 nm and a layer thickness of 0.92 nm) confers a much more higher edge area to surface ratio than the one usually found for natural smectite, making therefore Laponite suitable for edge modifications. Moreover, in addition to grafting, surface modifications can also be performed by ion exchange, allowing therefore to obtain multifunctional materials. Another point is that, at suitable concentration, Laponite could lead to gel formation in aqueous systems by electrostatic interactions between negatively charged faces and positively charged edge
of the nanoplatelets, imparting the systems outstanding 14 suspension, thixotropy and other unique properties. In such conditions the clay mineral is delaminated leading to suspension of individual nanoparticles as shown in 15,16 Scheme 1. Moreover, as Laponite is a 2:1 phyllosilicate the two external surfaces are identical siloxane planes which are 13 non-polar and not able to form hydrogen bonds.
Scheme 1: Molecular structure of Laponite RD and Crystal Violet (CV+)
17
Triarylmethane dyes (TAM) are cationic dyes known for 18-20 21,22 decades and used in many fields from biology to 23 19 industrial tints and inks. Among this family, Crystal Violet + (CV ) (Scheme 1) absorbs in the red region with high molar 18,24 absorption coefficient and exhibits very complex ultrafast photophysics in which excited states decay in a few 25-32 picoseconds. This fast excited state deactivation is solvent 33-35 36-38 viscosity dependent in a barrierless kinetic and involves phenyl ring rotations. Associated to the phenyl ring rotation, the formation of a non-emissive charge transfer (i.e. dark 25,31,35,39 state) was discussed in literature. Moreover, it has been shown that interactions with bovine serum albumin 40,41 (BSA) reduce the fast deactivation to some extent. Thus + CV photophysics and photochemistry are dependent on the molecule environment on a microscopic scale. Moreover, as isomorphic substitution in the octahedral Laponite sheets leads to negatively charged sites, in the presence of positively
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Journal Name
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0.0 g.L -1 0.5 g.L -1 1.0 g.L -1 2.0 g.L
spectra, small blue shift of the emission spectra can be seen -1 for Laponite concentration between 0.5 and 2 g.L (Table 1). Moreover, it can be seen that emission and absorption spectra present very good mirror image symmetry. Furthermore, the -1 excitation spectra in presence of 2 g.L of Laponite matches very well the corresponding absorption spectrum and the -1 Stokes shift is around 2000 cm . These facts clearly indicate that the state reached upon photon absorption is the emissive state, and that ground and excited states are very close in their 43,44 molecular structure.
0.00
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-0.10 15000
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-1
Wavenumber (cm ) -0.15
Figure 1: Normalized absorption (plain lines), emission (dotted lines) and excitation (circles) TDM spectra of CV+ in water as a function of Laponite concentration. Inset: evolution of raw fluorescence spectra as a function of Laponite content.
+
Table 1: Spectroscopic properties of CV in presence of Laponite nanoparticles, νmax(Fluo) and νmax(Abs) maximum wavenumbers (cm-1) of fluorescence and absorption respectively, FWHM full width at half maximum of absorption spectra and ∆ν is the Stokes shift
Laponite (g.L-1) νmax(Fluo)
0
0.5
1
2
n.m.
14940
15200
15280
νmax(Abs)
16980
17150
17100
17100
FWHM
2545
2230
2280
2320
∆ν
n.a.
2210
1900
1820
However the steady state fluorescence intensity increases with increasing Laponite content (see inset in Figure 1) possibly due to the restriction of phenyl ring movement, thus preventing 24,25,30,34,40 the formation of the dark state. As for absorption
0.00
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∆A
In Figure 1 changes in optical properties of CV+ as a function of Laponite content are displayed. It can be seen that Laponite nanoparticles modify both the absorption and emission spectra of the dye. When the Laponite content increases, a slight blue shift of the absorption spectrum is noticed together with a small lowering of the FWHM with the disappearance of the shoulder on the blue side (see Table 1). Knowing that the shoulder was attributed to the possible presence of two ground state conformers in free molecules, this behavior could be due to a flattening of the dye molecules adsorbed on the nanoparticles surface favoring or forcing one single conformer.31,33 The effect on emission spectra is by far more impressive. Indeed, no emission was detected for free molecules in water due to very low quantum yields41 and to very short excited lifetime26-28 (we measured less than 10 ps in water, vide infra).
0,5 0,0
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charged Crystal Violet molecules in water, an ion exchange reaction can occur between these latter and interlayer sodium cations leading to possible adsorption of dye on mineral 13,40 nanoparticles. This work aimed at studying the influence of the interactions between Crystal Violet and nanoplatelets of Laponite on the photophysical behaviour of the dye.
-1,0 -1,5
KOP1 KOP2 KOP3
-2,0 -2,5 1
10
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550
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650
700
Wavelength (nm) Figure 2: Femtosecond pump-probe time resolved spectroscopy of CV+ in pure water. (up) short time scale from 0.6 ps to 2.50 ps pumpprobe delay. (down) long timescale from 2.50 ps to 32 ps pump-probe delay. (inset) first three orthogonal kinetics obtained by singular value decomposition of the spectrotemporal results.
As most of the studies in the literature were done in organic solvents, and no fluorescence was observed in neat water, we perform time-resolved ultrafast pump-probe femtosecond spectroscopy measurements in pure water solution (Figure 2). + Changes of CV transient spectra in water are in good 26,27 agreement with the ones obtained in organic solution. A positive absorption can be seen before 500 nm, while the bleaching and stimulated emissions (SE) give negative signal from 500 to 700 nm in three picoseconds. Quite early, the bleaching band rises up and the stimulated emission is overlapped by a positive absorption peak which was ascribed either to a dark CT state or to a twisted ground state, both of 26,27,33,36 them obtained by phenyl ring rotation. Then, in less than ten picoseconds, the bleaching reached zero and the positive 630 nm peak also decays to zero. No more signal is detectable, indicating that all molecules have returned back to the ground state. This is confirmed by kinetic analysis of the
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Table 2: Emission lifetime analysis of CV+ in presence of Laponite nanoplatelets obtained by TCSPC, emission lifetime τi are given in ns and is the corresponding mean value.
Lifetime (ns)(Rel. Ampl.) τ1
0.5 g.L-1
1.0 g.L-1
2.0 g.L-1
0.08(79.6)
0.09(70.62)
0.17(57.71)
τ2
1.47(11.96)
1.49(16.83)
1.61(27.20)
τ3
3.66(8.40)
3.61(12.55)
4.00(15.10)
0.54
0.77
1.14
+
To get more insights into CV photophysics in such conditions time-resolved pump-probe femtosecond transient absorption experiment were done (Figure 3). The spectra seem to evolve homotetically with much more slower kinetics compared to free dye molecules in water. Moreover, they are very close in shape to the short time spectrum in pure water. This indicates + that mainly one excited state is now observed when CV interacts with Laponite nanoparticles. The second major
difference is the absence of the positive absorption peak at 630nm, indicating that neither the dark state nor the ground twisted state is formed. And finally, the bleaching does not rise up to zero within the time window of the setup: a permanent bleaching and a permanent negative signal at 630nm can be observed as a consequence of the storage of light energy in a long-lived excited state.
0.05
0.00
-0.05
∆A
0.28 ps 0.34 ps 0.37 ps 0.43 ps 0.50 ps 0.72 ps 1.03 ps 2.06 ps
-0.10
-0.15
0.05
0.00
-0.05
2.5
2.06 ps 4.18 ps 10.0 ps 24.2 ps 109 ps 300 ps 760 ps 1.56 ns
-0.10
-0.15
450
KOP1 KOP2 KOP3
2.0 1.5
AU.
results: a global multiexponential analysis of the data was 45,46 performed by singular value decomposition (SVD) and by fitting the first three orthogonal kinetics (KOP1, KOP2 and KOP3 inset Figure 2). As can be seen (logarithmic time scale), the kinetics are strongly non-exponential (in agreement with a possible barrierless deactivation model and with previous 24-26,29,30 works). Functions containing up to seven time constants were necessary to mathematically obtain a good fit. As these seven time constants only reveal the non-exponentiality of the + ultrafast CV excited state kinetics they are not relevant and consequently not reported. It should be noted that no delayed growth was observed which strongly indicate that the FC state reached upon laser excitation is close to S1. Accordingly, and taking into account previous works, a simplified mechanism + could be proposed for explanation of the CV photophysics in water (Scheme 2 left): after photoexcitation, the S1 (or a FC state) decays toward a dark twisted charge transfer state (CT) (kDS) which immediately recombines in the ground state (krec) or in a twisted ground state isomer in a few picoseconds 26,27,33,36 (kTGS), leading to a strong quenching of fluorescence emission. The picture is completely different in Laponite water solution. The fluorescence lifetimes as a function of Laponite concentration were recorded by TCSPC (Table 2): in the presence of Laponite, multiexponential emission decay are recorded and fitted with three time constants (up to 4 ns). These multiexponential decays are possibly due to heterogeneity in the dye-Laponite association. Moreover, as can be seen in Table 2, the relative amplitude of the slow components increase with increasing Laponite concentration. The fact that the mean lifetime increases with increasing content of Laponite is in line with the increase in the steadystate fluorescence intensity and indicates that interaction of + CV with Laponite nanoparticles favors the radiative deactivation of the emissive state of the dye by preventing the 41,42 phenyl ring rotations, as it was observed with BSA.
1.0 0.5 0.0 -0.5 -1.0 0.1
1
10
100
1000
Times (ps)
500
550
600
650
Wavelength (nm) Figure 3: Femtosecond pump-probe time resolved spectroscopy of CV+ in 2 g.L-1 Laponite water solution. (up) short time scale from 0.6 to 2.06 ns pump-probe delay. (down) long timescale from 2.50 to 1.56 ns pump-probe delay. (inset) first three orthogonal kinetics obtained by singular value decomposition of the spectrotemporal TAM results.
This is confirmed by looking at the global analysis of the first three orthogonal kinetics (KOP1, 2 and 3 inset Figure 3): it is clear that, even if not monoexponential due to heterogeneous dye-Laponite interaction, global kinetics are slow down and give permanent signal in the time window of our setup. The multiexponential fit requires three time constants (400 fs, 3.2 ps and 37 ps) and a step function corresponding to the long lived fluorescent state found by TCSPC measurements. Thus, it + appears that when CV molecules interact with Laponite nanoparticles, neither the twisted dark state nor the twisted ground state is observed. According to our results, we can + postulate the following mechanism (Scheme 2 right) when CV molecules are in interaction with Laponite nanoplatelets:
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7 Scheme 2: CV+ photophysics in water (left) and in presence of Laponite nanoparticles (right).
8
As the twisting of phenyl ring becomes more difficult, a more "planar" conformation of ground state CV+ molecules adsorbed at the surface of Laponite nanoparticles S0(L) is favored and certainly no more twisted ground states are accessible. It is also possible that the twisted dark state becomes strongly destabilized by the interaction with Laponite nanoplatelets due to the hindering of phenyl ring rotations. As a consequence, the CT state is hardly populated and the light absorbed energy is now kept in the emissive complexed excited S1(L) state, which relaxed to initial ground state S0(L) by fluorescence (kfluo) (and/or internal conversion). In this work, we have demonstrated that electrostatic interactions between negatively charged basal surface of Laponite nanoplatelets and CV+ cationic molecules can be used to tune the dye photophysics. Indeed, this strong electrostatic interaction hinders phenyl ring movements and consequently prevent the formation of either the twisted dark state or a twisted ground state isomer responsible of the fast decay of + CV excited state. As a consequence, light energy is stored in a singlet emissive bright state turning this non emissive dye into a fluorescent hybrid compound. EXPERIMENTAL METHODS Steady state UV-Visible spectra were obtained on a Cary 4000 spectrophotometer. Fluorescence and excitation spectra were performed with a Horiba-Jobin-Yvon fluoromax 4 equipped with a time correlated single photon counting (TCSPC) module. A 140 ns 607 nm nanoled was used as pulsed source. Spectra were corrected to take into account the non-reciprocity of 47,48 absorption and emission phenomena according to in a transition dipole moment representation (TDM). Femtosecond Pump-probe measurements were performed on a CDP corp Excipro system, the femtosecond laser excitation wavelength was adjusted to 590 nm with a pump-probe cross-correlation 49 about 200 fs as described elsewhere. Crystal Violet was purchased from Sigma-Aldrich, its structure was checked by NMR, and used as received. Laponite RD was obtained from BYK Additives &Instruments former Rockwood additives.
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Notes and references
33 34 35
1
36
2
S. Yarlv, A. Nasser, P. Bar-on, J. Chem. Soc. Faraday Trans., 1990, 86, 1593. M. M. Lezhnina, T. Grewe, H. Stoehr, U. Kynast, Angew. Chem. Int. Ed., 2013, 51, 10652.
37 38
F. L. Arbeloa, V. Martinez Martinez, T. Arbeloa, I. L. Arbeloa, J. Photochem. Photobiol. C, 2007, 8, 85. S. Salleres, F. L. Arbeloa, V. Martinez Martinez, T. Arbeloa, I. L. Arbeloa, Langmuir, 2010, 26, 930. Q. Zhang, T. Zhang, T. He, L. Hen L., Appl. Clay Sci., 2014, 90, 1. T. Felbeck, T. Behnke, K. Hoffmann, M. Grabolle, M. M. Lezhnina, U. H. Kynast, U. Resch-Genger, Langmuir 2013, 29, 11489. T. Shichi, S. J. Takagi, J. Photochem. photobiol. C, 2000, 1, 113. J. Bujdak, A. Czimerova, F. L. Arbeloa, J. Colloid Interface Sci., 2011, 364, 497. Y. Ishida, T. Shimada, H. Tachibana, H. Inoue, S. Takagi, J. Phys. Chem., A 2012, 116, 12065. T. Shinsuke, S. Tetsuya, I. Yohei, F. Takuya, M. Dai, T. Hiroshi, E. Miharu, H. Inoue, Langmuir, 2013, 29, 2108. A.U. Ferreira, A. L. Poli, F. Gessner, M.G. Neumann, C. C. Schmitt Cavalheiro, J. Lum., 2013, 136, 63. Y. Suzuki, Y. Tenma, Y. Nishioka, K. Kamada, K. Ohta, J. Phys. Chem. C, 2011, 115, 20653. Handbook of clay science, developments in clay science 1, F. Bergaya, B. K. G. Theng, G. Lagaly Eds; 2006 Elsevier, Oxford UK. Laporte Industries, Ltd. Laponite Technical Bulletin L104/90/A 1990, 1-15. 1Tritschler, U.; Zlotnikov, I.; Zaslansky, P.; Aichmayer, B.; Fratzl, P.; Schlaad, H.; Cölfen, H.;. Langmuir 2013, 29, 1109311101. 1.Daniel, L. M.; Frost, R. L.; Zhu, H. Y; J. Colloid. Interface Sci. 2007, 316, 72-79. Industrial Dyes: Chemistry, Properties, Apllications; K. Hunge, Ed.; Wiley-VCH, Weiheim 2003. E. Q. Adam, L. Rosenstein, J. Am. Chem. Soc., 1914, 36, 1452. D.F. Duxbury, Chemical Reviews 1993, 93, 381. Advances in Photochemistry, Vol. 26, C. D. Neckers, G. von Bünau, D. H. Volman Eds., 2001 J. Wiley and Sons. T. J. Beveridge, Biotech. Histochem .2001, 76, 111. M. K. Pal, J. K. Ghosh, Spectrochim. Acta, Part A, 1994, 50, 119. Industrial Dyes: Chemistry, Properties, Applications; K. Hunge Ed., 2003, J. Wiley and Sons, Weinheim. C. S. Oliveira, K. P. Branco, M. S. Baptista, L. I. Guilherme, Spectrochim. Acta, Part A, 2002, 58, 2971. M. M. Martin, E. Breheret, F. Nesa, Y. H. Meyer, Chem. Phys. 1989, 130, 279. M. M. Martin, P. Plaza, Y. H. Meyer, J. Phys. Chem., 1991, 95, 9310. M. M. Martin, P. Plaza, Y. H. Meyer, Chem. Phys., 1991, 153, 297. H. B. Lueck, J. L. mac Hale, W. D. Edwards, J. Am. Chem. Soc., 1992, 114, 2342. Y. Maruyama, M. Ishikawa, H. Satozono, J. Am. Chem. Soc., 1996, 118, 6257. Y. Maruyama, O. Magnin, H. Satozono, M. Ishikawa, J. Phys. Chem. A, 1999, 103, 5629. M. Ishikawa, J. Y. Ye, Y. Maruyama, H. Nakatsuka, J. Phys. Chem. A 1999, 103, 4319. Y. Nagasawa, Y. Ando, D. Kataoka, H. Matsuda, H. Miyasaka, T. Okada, J. Phys. Chem. A, 2002, 106, 2024. D.A. Cremers, W. Windsor, Chem. Phys. Lett., 1980, 71, 27. D. Ben-Amotz, C. B. Harris, Chem. Phys. Lett., 1985, 119, 305. V. Sundstrom, T. Gollbro, H. Bergstrom, Chem. Phys., 1982, 73, 439. B. Bagchi, V. Sundstrom, U. Aberg, Chem. Phys. Lett., 1989, 162, 227. V. Sundstrom, T. Gilbro, J. Chem. Phys., 1984, 81, 3463. D. Ben-Amotz, C. B. Harris, J. Chem. Phys., 1987, 86, 4856.
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39 S. Saikan, J. Sei, J. Chem. Phys., 1983, 79, 4154. 40 M.S. Baptista, G. L. Indig, J. Phys. Chem. B, 1998, 102, 4678. 41 A. C. Bhasikuttan, A. V. Sapre, L. V. Shastri, J. Photochem. Photobiol. A: Chem., 2002, 150, 59. 42 T. Hiemstra, W. H. Riemsdjik, J. Colloid Interface Sci., 1996, 179, 488. 43 J. R. Lakowicz, Principles of fluorescence spectroscopy, Kluwer Academic/Plenum publishers, New York, USA,1999. 44 B. Valeur, Molecular Fluorescence Principles and Applications, Wiley-VCH Verlag GmbH, Weinheim, 2002. 45 N. P. Ernsting , S. A. Kovalenko, T. Senyushkina , J. Saam , V. M. Farztdinov, J. Phys. Chem. A, 2001, 105, 3443. 46 J. Brazard, C. Ley, F. Lacombat, P. Plaza, M. M. Martin, G. Checcucci, F. Lenci, J. Phys. Chem. B 2008, 112, 15182. 47 G. Angulo, G. Grammp, A. Rosspeinter, Spectrochim. Acta, Part A, 2006, 65, 727. 48 G. Angulo, EPA Newsletter, 2007, 26. 49 C. Ley, P. Bordat, L. H. di Stefano, L. Remongin, A. Ibrahim, P. Jacques, X. Allonas, Phys.Chem.Chem.Phys., 2015, 17, 59825990.
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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x
On the Interaction of Triarylmethane Dye Crystal Violet with Laponite Clay: Using Mineral Nanoparticles to Control the Dye Photophysics. C. Ley*a, J. Brendléb, A. Waltera, P. Jacquesa, A. Ibrahima, X. Allonasc
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The combination of organic dye with clays leads to very interesting hybrid materials with original properties. It is found that Laponite nanoparticles impact the photophysical properties of Crystal Violet dye inducing a kinetic stabilization of its excited emissive state, turning this non emissive dye into a fluorescent compound. Organic dyes are known to interact with clay minerals, like 1,2 smectites which belongs to the phyllosilicates family, by 2-4 adsorption or ion exchange. Original properties could be obtained with applications in water depollution,5 2,6 solubilization, control of photochemical reactions7-10 or optical properties.11,12 With this in mind we explore in this paper the interaction of a cationic triarylmethane dye (TAM) molecules with a synthetic commercially available 2:1 (TOT where T stands for Tetrahedral and O for Octahedral) phyllosilicate, namely Laponite (Scheme 1).13 Each Laponite platelet is composed of a central sheet of magnesium ions in octahedral coordination (O) with oxygen anions and hydroxyl groups and two outer tetrahedral sheets (T). The isomorphic substitution of lithium for magnesium cations in the octahedral sheet induces a negative charge which is neutralized by sodium cations located in the interlayer space. The average dimensions of the Laponite layers (diameter of 25 nm and a layer thickness of 0.92 nm) confers a much more higher edge area to surface ratio than the one usually found for natural smectite, making therefore Laponite suitable for edge modifications. Moreover, in addition to grafting, surface modifications can also be performed by ion exchange, allowing therefore to obtain multifunctional materials. Another point is that, at suitable concentration, Laponite could lead to gel formation in aqueous systems by electrostatic interactions between negatively charged faces and positively charged edge
of the nanoplatelets, imparting the systems outstanding 14 suspension, thixotropy and other unique properties. In such conditions the clay mineral is delaminated leading to suspension of individual nanoparticles as shown in 15,16 Scheme 1. Moreover, as Laponite is a 2:1 phyllosilicate the two external surfaces are identical siloxane planes which are 13 non-polar and not able to form hydrogen bonds.
Scheme 1: Molecular structure of Laponite RD and Crystal Violet (CV+)
17
Triarylmethane dyes (TAM) are cationic dyes known for 18-20 21,22 decades and used in many fields from biology to 23 19 industrial tints and inks. Among this family, Crystal Violet + (CV ) (Scheme 1) absorbs in the red region with high molar 18,24 absorption coefficient and exhibits very complex ultrafast photophysics in which excited states decay in a few 25-32 picoseconds. This fast excited state deactivation is solvent 33-35 36-38 viscosity dependent in a barrierless kinetic and involves phenyl ring rotations. Associated to the phenyl ring rotation, the formation of a non-emissive charge transfer (i.e. dark 25,31,35,39 state) was discussed in literature. Moreover, it has been shown that interactions with bovine serum albumin 40,41 (BSA) reduce the fast deactivation to some extent. Thus + CV photophysics and photochemistry are dependent on the molecule environment on a microscopic scale. Moreover, as isomorphic substitution in the octahedral Laponite sheets leads to negatively charged sites, in the presence of positively
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[Lap] g.L 1.0
Intensity
0.8
0.6
8x10
5
7x10
5
6x10
5
5x10
5
4x10
5
3x10
5
2x10
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1x10
5
-1
0.0 0.5 1.0 2.0 0.5 1.0 1.5 2.0 -1
0.0 g.L -1 0.5 g.L -1 1.0 g.L -1 2.0 g.L
spectra, small blue shift of the emission spectra can be seen -1 for Laponite concentration between 0.5 and 2 g.L (Table 1). Moreover, it can be seen that emission and absorption spectra present very good mirror image symmetry. Furthermore, the -1 excitation spectra in presence of 2 g.L of Laponite matches very well the corresponding absorption spectrum and the -1 Stokes shift is around 2000 cm . These facts clearly indicate that the state reached upon photon absorption is the emissive state, and that ground and excited states are very close in their 43,44 molecular structure.
0.00
0 550
600
650
0.4
700
750
800
Wavelength (nm)
-0.05
∆A
0.2
0.0
0.66 ps 0.72 ps 0.78 ps 0.88 ps 1.00 ps 1.25 ps 1.50 ps 2.00 ps 2.50 ps
-0.10 15000
20000
25000
30000
-1
Wavenumber (cm ) -0.15
Figure 1: Normalized absorption (plain lines), emission (dotted lines) and excitation (circles) TDM spectra of CV+ in water as a function of Laponite concentration. Inset: evolution of raw fluorescence spectra as a function of Laponite content.
+
Table 1: Spectroscopic properties of CV in presence of Laponite nanoparticles, νmax(Fluo) and νmax(Abs) maximum wavenumbers (cm-1) of fluorescence and absorption respectively, FWHM full width at half maximum of absorption spectra and ∆ν is the Stokes shift
Laponite (g.L-1) νmax(Fluo)
0
0.5
1
2
n.m.
14940
15200
15280
νmax(Abs)
16980
17150
17100
17100
FWHM
2545
2230
2280
2320
∆ν
n.a.
2210
1900
1820
However the steady state fluorescence intensity increases with increasing Laponite content (see inset in Figure 1) possibly due to the restriction of phenyl ring movement, thus preventing 24,25,30,34,40 the formation of the dark state. As for absorption
0.00
-0.05 1,0
∆A
In Figure 1 changes in optical properties of CV+ as a function of Laponite content are displayed. It can be seen that Laponite nanoparticles modify both the absorption and emission spectra of the dye. When the Laponite content increases, a slight blue shift of the absorption spectrum is noticed together with a small lowering of the FWHM with the disappearance of the shoulder on the blue side (see Table 1). Knowing that the shoulder was attributed to the possible presence of two ground state conformers in free molecules, this behavior could be due to a flattening of the dye molecules adsorbed on the nanoparticles surface favoring or forcing one single conformer.31,33 The effect on emission spectra is by far more impressive. Indeed, no emission was detected for free molecules in water due to very low quantum yields41 and to very short excited lifetime26-28 (we measured less than 10 ps in water, vide infra).
0,5 0,0
-0.10
2.50 3.00 4.00 7.50 10.0 15.6 31.6
-0.15
500
-0,5
A.U.
A.U.
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charged Crystal Violet molecules in water, an ion exchange reaction can occur between these latter and interlayer sodium cations leading to possible adsorption of dye on mineral 13,40 nanoparticles. This work aimed at studying the influence of the interactions between Crystal Violet and nanoplatelets of Laponite on the photophysical behaviour of the dye.
-1,0 -1,5
KOP1 KOP2 KOP3
-2,0 -2,5 1
10
100
Time (ps)
550
600
650
700
Wavelength (nm) Figure 2: Femtosecond pump-probe time resolved spectroscopy of CV+ in pure water. (up) short time scale from 0.6 ps to 2.50 ps pumpprobe delay. (down) long timescale from 2.50 ps to 32 ps pump-probe delay. (inset) first three orthogonal kinetics obtained by singular value decomposition of the spectrotemporal results.
As most of the studies in the literature were done in organic solvents, and no fluorescence was observed in neat water, we perform time-resolved ultrafast pump-probe femtosecond spectroscopy measurements in pure water solution (Figure 2). + Changes of CV transient spectra in water are in good 26,27 agreement with the ones obtained in organic solution. A positive absorption can be seen before 500 nm, while the bleaching and stimulated emissions (SE) give negative signal from 500 to 700 nm in three picoseconds. Quite early, the bleaching band rises up and the stimulated emission is overlapped by a positive absorption peak which was ascribed either to a dark CT state or to a twisted ground state, both of 26,27,33,36 them obtained by phenyl ring rotation. Then, in less than ten picoseconds, the bleaching reached zero and the positive 630 nm peak also decays to zero. No more signal is detectable, indicating that all molecules have returned back to the ground state. This is confirmed by kinetic analysis of the
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Table 2: Emission lifetime analysis of CV+ in presence of Laponite nanoplatelets obtained by TCSPC, emission lifetime τi are given in ns and is the corresponding mean value.
Lifetime (ns)(Rel. Ampl.) τ1
0.5 g.L-1
1.0 g.L-1
2.0 g.L-1
0.08(79.6)
0.09(70.62)
0.17(57.71)
τ2
1.47(11.96)
1.49(16.83)
1.61(27.20)
τ3
3.66(8.40)
3.61(12.55)
4.00(15.10)
0.54
0.77
1.14
+
To get more insights into CV photophysics in such conditions time-resolved pump-probe femtosecond transient absorption experiment were done (Figure 3). The spectra seem to evolve homotetically with much more slower kinetics compared to free dye molecules in water. Moreover, they are very close in shape to the short time spectrum in pure water. This indicates + that mainly one excited state is now observed when CV interacts with Laponite nanoparticles. The second major
difference is the absence of the positive absorption peak at 630nm, indicating that neither the dark state nor the ground twisted state is formed. And finally, the bleaching does not rise up to zero within the time window of the setup: a permanent bleaching and a permanent negative signal at 630nm can be observed as a consequence of the storage of light energy in a long-lived excited state.
0.05
0.00
-0.05
∆A
0.28 ps 0.34 ps 0.37 ps 0.43 ps 0.50 ps 0.72 ps 1.03 ps 2.06 ps
-0.10
-0.15
0.05
0.00
-0.05
2.5
2.06 ps 4.18 ps 10.0 ps 24.2 ps 109 ps 300 ps 760 ps 1.56 ns
-0.10
-0.15
450
KOP1 KOP2 KOP3
2.0 1.5
AU.
results: a global multiexponential analysis of the data was 45,46 performed by singular value decomposition (SVD) and by fitting the first three orthogonal kinetics (KOP1, KOP2 and KOP3 inset Figure 2). As can be seen (logarithmic time scale), the kinetics are strongly non-exponential (in agreement with a possible barrierless deactivation model and with previous 24-26,29,30 works). Functions containing up to seven time constants were necessary to mathematically obtain a good fit. As these seven time constants only reveal the non-exponentiality of the + ultrafast CV excited state kinetics they are not relevant and consequently not reported. It should be noted that no delayed growth was observed which strongly indicate that the FC state reached upon laser excitation is close to S1. Accordingly, and taking into account previous works, a simplified mechanism + could be proposed for explanation of the CV photophysics in water (Scheme 2 left): after photoexcitation, the S1 (or a FC state) decays toward a dark twisted charge transfer state (CT) (kDS) which immediately recombines in the ground state (krec) or in a twisted ground state isomer in a few picoseconds 26,27,33,36 (kTGS), leading to a strong quenching of fluorescence emission. The picture is completely different in Laponite water solution. The fluorescence lifetimes as a function of Laponite concentration were recorded by TCSPC (Table 2): in the presence of Laponite, multiexponential emission decay are recorded and fitted with three time constants (up to 4 ns). These multiexponential decays are possibly due to heterogeneity in the dye-Laponite association. Moreover, as can be seen in Table 2, the relative amplitude of the slow components increase with increasing Laponite concentration. The fact that the mean lifetime increases with increasing content of Laponite is in line with the increase in the steadystate fluorescence intensity and indicates that interaction of + CV with Laponite nanoparticles favors the radiative deactivation of the emissive state of the dye by preventing the 41,42 phenyl ring rotations, as it was observed with BSA.
1.0 0.5 0.0 -0.5 -1.0 0.1
1
10
100
1000
Times (ps)
500
550
600
650
Wavelength (nm) Figure 3: Femtosecond pump-probe time resolved spectroscopy of CV+ in 2 g.L-1 Laponite water solution. (up) short time scale from 0.6 to 2.06 ns pump-probe delay. (down) long timescale from 2.50 to 1.56 ns pump-probe delay. (inset) first three orthogonal kinetics obtained by singular value decomposition of the spectrotemporal TAM results.
This is confirmed by looking at the global analysis of the first three orthogonal kinetics (KOP1, 2 and 3 inset Figure 3): it is clear that, even if not monoexponential due to heterogeneous dye-Laponite interaction, global kinetics are slow down and give permanent signal in the time window of our setup. The multiexponential fit requires three time constants (400 fs, 3.2 ps and 37 ps) and a step function corresponding to the long lived fluorescent state found by TCSPC measurements. Thus, it + appears that when CV molecules interact with Laponite nanoparticles, neither the twisted dark state nor the twisted ground state is observed. According to our results, we can + postulate the following mechanism (Scheme 2 right) when CV molecules are in interaction with Laponite nanoplatelets:
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7 Scheme 2: CV+ photophysics in water (left) and in presence of Laponite nanoparticles (right).
8
As the twisting of phenyl ring becomes more difficult, a more "planar" conformation of ground state CV+ molecules adsorbed at the surface of Laponite nanoparticles S0(L) is favored and certainly no more twisted ground states are accessible. It is also possible that the twisted dark state becomes strongly destabilized by the interaction with Laponite nanoplatelets due to the hindering of phenyl ring rotations. As a consequence, the CT state is hardly populated and the light absorbed energy is now kept in the emissive complexed excited S1(L) state, which relaxed to initial ground state S0(L) by fluorescence (kfluo) (and/or internal conversion). In this work, we have demonstrated that electrostatic interactions between negatively charged basal surface of Laponite nanoplatelets and CV+ cationic molecules can be used to tune the dye photophysics. Indeed, this strong electrostatic interaction hinders phenyl ring movements and consequently prevent the formation of either the twisted dark state or a twisted ground state isomer responsible of the fast decay of + CV excited state. As a consequence, light energy is stored in a singlet emissive bright state turning this non emissive dye into a fluorescent hybrid compound. EXPERIMENTAL METHODS Steady state UV-Visible spectra were obtained on a Cary 4000 spectrophotometer. Fluorescence and excitation spectra were performed with a Horiba-Jobin-Yvon fluoromax 4 equipped with a time correlated single photon counting (TCSPC) module. A 140 ns 607 nm nanoled was used as pulsed source. Spectra were corrected to take into account the non-reciprocity of 47,48 absorption and emission phenomena according to in a transition dipole moment representation (TDM). Femtosecond Pump-probe measurements were performed on a CDP corp Excipro system, the femtosecond laser excitation wavelength was adjusted to 590 nm with a pump-probe cross-correlation 49 about 200 fs as described elsewhere. Crystal Violet was purchased from Sigma-Aldrich, its structure was checked by NMR, and used as received. Laponite RD was obtained from BYK Additives &Instruments former Rockwood additives.
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Notes and references
33 34 35
1
36
2
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