J. Phys. Chem. C 2009, 113, 965–970
965
Photophysics of Rhodamine 6G Laser Dye in Ordered Surfactant (C12TMA)/Clay (Laponite) Hybrid Films S. Salleres, F. Lo´pez Arbeloa,* V. Martı´nez, T. Arbeloa, and I. Lo´pez Arbeloa Departamento de Quı´mica Fı´sica, UniVersidad del Paı´s Vasco (UPV/EHU), Apartado 644, 48080 Bilbao, Spain ReceiVed: July 24, 2008; ReVised Manuscript ReceiVed: October 28, 2008
With the aim of designing highly luminescent hybrid materials in ordered solid host systems, surfactant molecules are incorporated into the interlayer space of Laponite clay sheets which are parallel stacked in supported films. The posterior inclusion of the photoactive dye rhodamine 6G in low concentrations induces a macroscopic arrangement of fluorescent molecules in a nanostructure organophilic environment with an anisotropic photoresponse to the linearly polarized light. Introduction Commercially available tunable dye lasers require the use of a circulating liquid solution of a laser dye as the active medium. Generally the dyes are solved in toxic and inflammable organic solvents, limiting the practical use of these types of lasers. Therefore, there is a special demand for the development of dye lasers in the solid state, in which the active media consist of solid host matrixes doped with fluorescent dyes. In order to overcome the threshold of the resonator cavity, active media with high optical densities at the pumping wavelength are required to observe laser signals. However, high dye concentrations lead to dye aggregation, with the formation of nonfluorescent or poorly fluorescent aggregates, which act as efficient quenchers of the fluorescent emission from the monomers (self-quenching).1-5 On the other hand, a macroscopic arrangement of dye molecules is essential for the development of new nonlinear optics and optoelectronic devices. Thus, the inclusion of dyes with high hyperpolarizability into nanostructured host composites may induce a nonsymmetrical distribution of organic molecules, needed in the design of new second harmonic generation systems.6 For these reasons, the synthesis of ordered solid materials with light-emitting ability has been the focus of much research in the past few decades.7-10 Recently we have shown that the elaboration of thin films of layered Laponite (Lap) clay supported in glass substrates by the spin-coating technique is an adequate strategy to obtain the stacking of clay layers in a parallel disposition.11 The posterior intercalation of rhodamine 6G (R6G) cations, probably the mostapplied laser dye, induces a macroscopic organization of dye molecules. Indeed, R6G molecules are adsorbed with a preferential orientation with respect to the normal to the clay layers, leading to an anisotropic behavior of the absorption and fluorescence spectra with respect to linearly polarized light.12 From the evolution of the absorption and fluorescence dichroic ratios with respect to the twisting angle of the films, the orientation angle of the dye with respect to the clay layers can be determined.13-16 Thus, the preferential orientation of R6G in Lap films depends on the aggregation state of the dye, and consequently on the dye loading: R6G monomers are disposed with the long molecular axes forming an angle of around 62° * Corresponding author. Telephone: +34 94 601 5971. Fax: +34 94 601 3500. E-mail:
[email protected].
with respect to the clay layer normal;13,14 similar orientations are observed for long-displaced collinear J-type aggregates, whereas short-displaced collinear H-type aggregates are disposed more to the perpendicular of the clay layers as the dye content is increased.13,15 The fluorescence intensity of these ordered R6G/Lap films strongly depends on the dye loading. For low-to-moderate dye contents, the absolute fluorescence intensity of the films increases with the dye loading because of the increment in the absorbance at the excitation wavelength. A posterior increase in the dye content, however, induces a decrease in the absolute fluorescence intensity. The apparent fluorescence efficiency, analyzed as the fluorescence intensity over the absorbance at the excitation wavelength, drastically decreases with the dye content by at least 3 orders of magnitude from the most diluted dye R6G/Lap film (that with a relative dye/clay concentration of 0.1% cation exchange capacity (CEC), percentage of the adsorbed R6G cations over the total cationic exchange capacity of the clay) to the highest concentrated dye R6G/Lap film (60% CEC).17 This fluorescence quenching is assigned mainly to the dye aggregation.17,18 R6G in Lap films can form nonluminescent H-type and fluorescent J-type aggregates, with the former acting as efficient quenchers of the fluorescent emission from monomers. J-Type aggregates emit at longer wavelengths than the monomer emission band.18 In order to reduce the dye aggregation and enhance the fluorescence intensity of dyes in clay systems, several authors have claimed the convenience of modifying the hydrophilic environment of the interlayer space of hydrated clays. Indeed, the aggregation of R6G in ethanol is around 2 orders of magnitude lower than in aqueous solution.19-21 An organophilic ambience in clay systems can be obtained by the intercalation of organic molecules in the clay. Some examples are the coadsorption of ethanol molecules together with the dye22 or the inclusion of surfactant molecules (i.e., long-chain alkyltrimethylammonium cation) in the clay.23-26 In this way, dye-dye interactions may be prevented by the presence of the surfactant molecules (acting as spacers for dye molecules) inhibiting the dye aggregation and, complementarily, may reduce the tendency of dye molecules to self-associate by hydrophobic forces. One method to elaborate dye/surfactant/clay films is the synthesis of the organophilic clay powder (i.e. by interchanging the exchangeable inorganic cations by cationic surfactant molecules in aqueous suspension and the posterior elimination
10.1021/jp806553p CCC: $40.75 2009 American Chemical Society Published on Web 12/30/2008
966 J. Phys. Chem. C, Vol. 113, No. 3, 2009
Salleres et al.
TABLE 1: Characterization of Elaborated Organophilic C12TMA/Lap Filmsa samples
synthesis conditions
νas(CH2) (cm-1)
νs(CH2) (cm-1)
C12TMA (% CEC)
2θ (deg)
∆(2θ) (deg)
R (Å)
C12TMA Lap
-
2918 -
2850 -
-
6.9
3.0
3.2
10 min, 2 × 10-4 M, 32 °C 20 min, 2 × 10-4 M, 32 °C 4 h, 2 × 10-4 M, 32 °C 20 h, 10-2 M, 42 °C 72 h, 10-2 M, 42 °C
2929 2928 2927 2924 2920
2856 2855 2854 2853 2851
30 46 70 120 170
6.6 6.5 6.4 6.2 6.1
2.1 2.1 1.9 2.1 2.2
3.8 4.1 4.3 4.7 5.0
OL1 OL2 OL3 OL4 OL5 a
Immersion conditions of Lap Films into C12TMA solutions; wavenumbers of symmetric and asymmetric CH2 stretching IR bands of C12TMA (νas and νs); C12TMA content (in % CEC) in Lap films evaluated from IR spectroscopy; 001 XRD peak position (2θ) and width (∆(2θ)); basal interlayer distance (R) derived from XRD data.
of water), the subsequent elaboration of the organoclay film by the spin-coating technique, and the final intercalation of the dye by immersing the organoclay film into an adequate dye solution. This methodology is recommended for the synthesis of organoclay films with specific surfactant contents. The validity of this approach was previously checked by our group for the specific case of Lap clay with dodecyltrimethylammonium (C12TMA) surfactant and R6G as fluorescent dye.27 It was proven that the presence of C12TMA molecules favors the formation of fluorescent J-aggregates of R6G, improving the fluorescent ability of R6G dye in Lap films. Unfortunately, the elaboration of thin films of organophilic C12TMA/ Lap films by the spin-coating technique did not provide an ordered stacking of clay layers,27 limiting the optical application of these dye/surfactant/clay films. Indeed, these R6G/C12TMA/ Lap films did not show any anisotropic photoresponse to linearly polarized light. In order to obtain ordered dye/surfactant/clay films, we have changed the elaboration methodology in the present paper. Since spin coating provides organized Lap films,11 we have first proceeded to elaborate ordered pure Lap films with a posterior intercalation of surfactant molecules by the immersion of the Lap films into aqueous/ethanol solutions of dodecyltrimethylammonium bromide. This synthesis method is similar to that proposed by Sasai et al.25 Experimental Section The sodium form of Laponite (Lap) clay was supplied by Lapporte Industries and was used as received. This synthetic clay is characterized by its high chemical purity and a very small particle size (
