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Rhodamine Fluorescent Dimers Adsorbed on the Porous Surface of Silica Gels Francisco del Monte,‡,† John D. Mackenzie,‡ and David Levy*,§ Department of Materials Science, School of Applied Science and Engineering, University of California, Los Angeles, Los Angeles, California 90095, Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, Madrid 28049, Spain, and Laboratorio de Instrumentacio´ n Espacial-LINES, Instituto Nacional de Tecnologı´a Aeroespacial-INTA, Torrejo´ n de Ardoz, Madrid 28850, Spain Received April 10, 2000. In Final Form: June 10, 2000 Rhodamine 6G (R6G)-doped silica gels were prepared through the sol-gel process using dye concentrations ranging from 4.00 × 10-6 to 6.40 × 10-4 M. The adsorption of the R6G on the porous surface of the silica gels determined the formation of R6G fluorescent J-dimers as concentration increased, in concordance with the behavior reported for other xanthene molecules as rhodamine B (RB) and rhodamine 110 (R110) also doped in silica gels. The photophysical study of the R6G fluorescent dimers doped in silica gels was done through the recording of the excitation and emission fluorescence spectra as well as the measurement of the lifetime values. The exciton theory was used for the elucidation of the adsorption angle (θ), the angle between the monomer units (R), and the separation distance between the molecules (R) of the R6G fluorescent dimers. The angle between the monomer units (R) and the separation distance between the molecules (R) were also determined for RB and R110 fluorescent dimers doped in silica gels.
* To whom correspondence should be addressed. ‡ University of California, Los Angeles. § CSIC and Instituto Nacional de Tecnologı´a Aeroespacial-INTA. † Current address: Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain.
to the spectroscopic studies achieved in these works, RB and R110 dimers or aggregates were formed on the porous surface of the silica gels. It is known that the formation at the adsorbed state of nonfluorescent or fluorescent dimers (called H-dimers and J-dimers, respectively) depends on the geometry adopted by the monomer constituents on the surface.14-20 Thus, the fluorescent character of the RB or R110 dimers in silica gels was determined through the determination of the angle, θ, formed between the longitudinal axis of the molecules and the adsorption surface (below 55° for fluorescent J-dimers, Figure 1).12,13 Moreover, oblique or coplanarinclined configurations were also differentiated for fluorescent dimers adsorbed on the porous surface of silica gels.13 In this paper, we attempt to study the feasibility of a different xanthene-type molecule, rhodamine 6G (R6G), to form J-dimers adsorbed on the porous surface of gelglasses. Thus, both spectral and dynamic fluorescence properties of R6G-doped gel-glasses were studied for a wide range of R6G concentrations. This study has been undertaken in order to determine the relationship between the molecular structure and its adsorption behavior (Figure 2). Furthermore, we also studied the geometry adopted by the R6G, RB, and R110 J-dimers when adsorbed on the porous cage of the silica gels. Thus, the exciton theory21-23
(1) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956. (2) Avnir, D.; Kaufman, V. R. Langmuir 1986, 2, 717. (3) Leveau, B.; Herlet, N.; Livage, J.; Sanchez, C. Chem. Phys. Lett. 1993, 206, 15. (4) del Monte, F.; Levy, D. Chem. Mater. 1995, 7, 292. (5) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San Diego, 1990. (6) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33. (7) Ulrich, D. R. J. Non-Cryst. Solids 1988, 100, 174. (8) Levy, D.; Avnir, D. J. Phys. Chem. 1988, 92, 4734. (9) Narang, U.; Bright, F. Chem. Mater. 1996, 8, 1410. (10) Innozenci, P.; Kozuka, H.; Yoko, T. J. Phys. Chem. B 1997, 101, 2285. (11) Martini, I.; Hartland, G. V.; Kamat, P. V. J. Phys. Chem. B 1997, 101, 4826. (12) del Monte, F.; Levy, D. J. Phys. Chem. B 1998, 102 (41), 8036. (13) del Monte, F.; Levy, D. J. Phys. Chem. B 1999, 103 (38), 8080.
(14) Kemnitz, K.; Yoshihara, K. J. Phys. Chem. 1991, 95, 6095. (15) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J. Phys. Chem. 1986, 90, 5094. (16) Muto, J. J. Phys. Chem. 1976, 80, 1342. (17) Itho, K.; Chiyokawa, Y.; Nakao, M.; Honda, K. J. Am. Chem. Soc. 1984, 106, 1620. (18) Drexhage, K. H. Topics in Applied Physics; Scha¨fer, F. P., Ed.; Springer: Berlin, 1973; Vol. 1, p 144. (19) Rohatgi, K. K. J. Mol. Spectrosc. 1968, 27, 545. (20) Nakashima, N.; Yoshihara, K.; Willig, F. J. Chem. Phys. 1980, 73, 3553. (21) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (22) McRae, E. G.; Kasha, M. Physical Processes in Radiation Biology, 1st ed.; Academic Press: New York, 1964. (23) McRae, E. G.; Kasha, M. J. Chem. Phys. 1961, 11, 38.
Introduction Incorporation of organic molecules into silica gels through the sol-gel process can be easily done because it begins at a solution stage and it runs at room temperature.1-7 When fluorescent dyes are incorporated within the porosity of silica gels, they usually show deviations from the characteristic fluorescent behavior observed in solution, which is typically considered as optimum. There are numerous experimental works reporting on the difficulties involved in retaining the optimum spectroscopic properties of organic dyes when incorporated in silica gels.3,4,8-11 Retention of such optical properties is typically achieved through the modification of the interactions established among the organic dyes and the porous surface.1,2,8-11 An important parameter also affecting the fluorescent behavior of organic dyes is the relationship that molecules can establish with neighbor molecules to form dimers or aggregates within the porosity of the silica gel, as recently reported for silica gels with rhodamine B (RB) and rhodamine 110 (R110) incorporated at different dye concentrations.12,13 According
10.1021/la000540+ CCC: $19.00 © 2000 American Chemical Society Published on Web 08/17/2000
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del Monte et al. Table 1: Concentration of R6G-Doped Gels 1-7a gel 1 gel 2 gel 3 gel 4 gel 5 gel 6 gel 7 [R6G] (M) × a
10-6
4
10
40
160
320
480
640
Concentrations are also valid for RB- and R110-doped gels 1-7.
Experimental Section
Figure 2. Molecular structure of R6G, R110, and RB.
Materials. R6G, R110, and RB from Aldrich were used as received without further purification. Tetramethyl orthosilicate (TMOS) from Aldrich, spectrophotometric grade methanol from Merck, and distilled and deionized water (DDW) were also used in the sample preparation. Sample Preparation. Silica gels (gels 1-7) were prepared from 9.1 mmol of TMOS, 36.4 mmol of H2O (water-to-monomer ratio, rw/m ) 4), and 33 mmol of methanol. A variable amount of dye (R6G, R110, or RB) was added to the solution without further prehydrolysis (Table 1). The mixture was allowed to stir for 30 min to get a homogeneous phase. The polymerization was carried out at room temperature in glass bottles covered with aluminum foil. After gelation occurred, the aluminum foil was perforated to allow the slow evaporation of the solvents until dried xerogels formed. The samples were kept in the dark during the study. Fluorescence Spectroscopy and Lifetimes. Multifrequency modulation, phase analysis, and fluorimetric measurements were performed at 25 °C on a 48000s (T-Optics) spectrofluorometer from SL-Aminco. The instrument is configured for software-controlled variable-frequency light modulation from 100 to 120 MHz. Spectroscopic and temporal properties were measured by reflection (front-face mode) on finely ground samples which were packed into quartz cells having a 1-mm path length. Fluorescence measurements were always recorded in the backward direction.25 A front-face sample holder for the samples was used for data collection and oriented at 60° in order to minimize the specular reflection of the excitation beam on the cooled R-928 photomultiplier tube. Appropriate filters were also used to eliminate Rayleigh and Raman scatters from the emission. A D-glycogen scatter solution was used as the reference for lifetime measurements. Excitation and emission spectra were corrected for the wavelength dependence of the 450-W xenon-arc excitation but not for the wavelength dependence of the detection system. No attempt was made to remove adsorbed or dissolved molecular oxygen from the doped materials, because lifetimes are in the range of a few nanoseconds. Reference samples that did not contain any fluorescent dopant were used to check the background and optical properties of the silica gels. Scattered light, both in the excitation and in the emission spectra, was avoided through the careful selection of the range of wavelengths and the respective emission or excitation wavelength selected for the recording of the spectra. Kubelka-Munt26-28 treatment of the spectra in order to eliminate scattered light was considered unnecessary.25,29 Measurement of relative fluorescent quantum yields was achieved on bulk samples with cube shape (0.5 × 0.5 × 0.5) cm3 in right-angle mode following the standard procedure described elsewhere.30 Phase-resolved fluorescence spectroscopy31 (PRFS) was used for lifetime measurements. The method provides two separated determinations of the fluorescence lifetimes due to the phase and modulation, which were separated measurements. Each phase and modulation value is the average of 100-200 readings.
was applied to the excitation spectra of fluorescent R6G, RB, and R110 J-dimers for the determination of the angle between the monomer units (R) and the separation distance between the molecules (R) (Figure 1). It appears that this is the first time that a complete study of the geometry (φ, R, R) of rhodamine fluorescent dimers (RB, R110, and R6G) adsorbed on the porous surface of a silicabased gel-glass has been undertaken. It is noteworthy that the novel application of the exciton theory to the excitation, rather than the absorption, spectra24 for the elucidation of such structural parameters has been possible due to the fluorescent character of the dimers.
(24) (a) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983; Chapter 2, p 20. (b) Steiner, R. F. Topics in Fluorescence Spectroscopy, Volume 2, Principles; Lakowicz, J. R., Ed.; Plenum Press: New York, 1991; Chapter 1. (25) Oelkrug, D. Topics in Fluorescence Spectroscopy, Volume 4, Probe Design and Chemical Sensing; Lakowicz, J. R., Ed.; Plenum Press: New York, 1994; Chapter 8. (26) Kubelka, P. J. Opt. Soc. Am. 1948, 38, 448. (27) Kortum, G.; Brown, W.; Herzog, G. Angew. Chem., Int. Ed., Engl. 1963, 2, 333. (28) Wilkinson, F.; Kelly, G. P. Handbook of Organic Photochemistry; Scaino, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 1, p 293. (29) Vieria-Ferreira, L. F.; Rosario-Freixo, M.; Garcia, A. R.; Wilkinson, F. J. Chem. Soc. Faraday Trans. 1992, 88 (1), 15. (30) Kubin, R. F.; Fletcher, J. J. Lumin. 1982, 27, 455. (31) Lakowicz, J. R.; Laczko, G.; Cherek, H.; Gratton, E.; Limkeman, M. Biophys. J. 1984, 46, 462.
Figure 1. Formation of nonfluorescent H-dimers or fluorescent J-dimers as a consequence of the geometry adopted by the transition dipoles on the adsorption surface of a porous silica gel. J-dimers can adopt two different configurations: (a) coplanar-inclined and (b) oblique. From Kemnitz et al.14
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The frequency-dependent phase and modulation data were analyzed using a nonlinear least-squares procedure that minimizes the squared deviations between the observed and the expected phase and modulation values. The values of the floating parameters (lifetimes and fractional intensities of each fluorophore contributing to the total fluorescence intensity) are varied in a direction that minimizes the value of the deviations between the model and the data, χ2R (the reduced error). Computation is finished with a number of iterations through the fitting algorithm, at which point a minimum is found. The uncertainty in any phase or modulation measurement could be decreased to very small levels averaging in minutes. Specifically, values of χ2R greater than unity may indicate either the presence of systematic errors32 or an inappropriate model. We collected phase and modulation data measured at 20-25 modulation frequencies, and additional measurements did not seem to improve the resolution. An uncertainty of 0.5 for the resolution of the phase and 0.005 for the resolution of the modulation was applied for the calculation of χ2R. The accuracy of the measured lifetime values was determined according to the lower χ2R value found in each individual measurement.33 UV-Vis Spectroscopy. A Varian UV-Vis spectrophotometer, model 2300, was used to measure the absorption spectra (reflectance mode) of finely ground samples which were packed into quartz cells having a 1-mm path length. Extinction coefficients of RB-, R6G-, and R110-doped gel 1 (∫(ν) dν) were measured in transmittance mode on bulk samples having a 0.5cm path length.
Results and Discussion The increase of the R6G concentration incorporated into the silica gels (gels 1-5, Table 1) gave rise to a progressive red shift of the emission spectra of the R6G-doped silica gels (Figure 3, Table 2). A single lifetime was observed for gel 1 through the application of PRFS (Table 2), indicating a very homogeneous environment around the dye molecules.34 The value of this lifetime was markedly larger than the that of the lifetime observed for R6G in ethanol/ water solutions (3.83 vs 3.0 ns,35 respectively), which denotes the rigid environment of the R6G molecules within the porosity of the silica gel. The lack of mobility limits its intramolecular rotation modes, with the subsequent reduction of the deactivation processes or quenching (S1 f S0 internal conversion processes).14,15,36-38 A gradual increase in R6G lifetime values was also observed with the increase of the dye concentration from gel 1 to 5 (Table 2). Red shifts of the emission spectra, as well as the increase of the lifetime values, were recently reported for RB- and R110-doped silica gels as characteristic of the appearance of fluorescent J-dimer species adsorbed on the porous surface of silica gels.12,13 According to singleexciton theory,21-23 the radiative rate constant of fluorescent dimers is related to the radiative rate constant of the monomer unit and the angle, θ, formed between the longitudinal axis of the molecules (monomer transition moments) and the adsorption surface (Figure 1) through eq 1:14,15
krD ) 2krM cos2 θ
(1)
where krD is the radiative rate constant of the fluorescent (32) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill, Inc.: New York, 1969; p 336. (33) Gratton, E.; Linkeman, M. Biophys. J. 1983, 44, 315. (34) Levy, D.; Ocan˜a, M.; Serna, C. J. Langmuir 1994, 10, 2683. (35) Quinn, M. S.; Al-Ajeel, M. S.; Al-Bahrani, F. J. Lumin. 1985, 33, 53. (36) Kemnitz, K.; Murao, T.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. Chem. Phys. Lett. 1983, 101, 337. (37) Itho, K.; Honda, K. Chem. Phys. Lett. 1982, 87, 213. (38) Eyal, M.; Reisfeld, R.; Cherniyak, V.; Kaczmarek, L.; Grabowska, A. Chem. Phys. Lett. 1991, 176, 531.
Figure 3. Emission spectra (λex ) 470 nm) of R6G-doped gels 1-5. Table 2: Maximum of Excitation (λmex) and Emission (λmem) Spectra, Lifetimes (τ), and Experimental Reduced Chi-Squared (χ2R) for R6G Gels 1-7 and Experimental Angle (θ) of Adsorption for R6G Gels 2-5a gel
λmex (nm)b
λmem (nm)c
τ (ns)d
χ2R
θ (deg)
1 2 3 4 5 6 7
530 530 530 530-506 532-505 530-505 530-505
547 553 558 566 567 567 567
3.83 ( 0.16 4.28 ( 0.16 4.59 ( 0.18 5.75 ( 0.22 5.90 ( 0.22 5.77 ( 0.21 5.57 ( 0.22
1.2 1.1 1.3 0.9 1.1 1.0 1.0
monomer 47.97 ( 1.10 49.74 ( 0.96 54.74 ( 0.83 55.32 ( 0.77 H-dimers H-dimers
a Errors were calculated from refs 39, 40. b The excitation wavelength used to record the emission spectra was 470 nm. c The emission wavelength used to record the excitation spectra was 610 nm. d The excitation wavelength used to measure the lifetimes was 470 nm.
dimers and krM is the radiative rate constant of the monomer unit. Adsorption angles calculated from the R6Glifetime values are reported in Table 2 for gels 2-5. The mutual orientation of the transition moments of the monomer units and the adsorption surface determined the fluorescent character of dimers (θ < 54.7° for fluorescent J-dimers, while θ > 54.7° for nonfluorescent H-dimers). It is worthy to note that the θ angle calculated for gel 5 is at the limiting value accepted for the formation of J-dimers (θ ≈ 55°, Table 2). Thus, a further increase of concentration (gels 6, 7) should give rise to the formation of H-dimers. The J-H-dimer conversion was experimentally observed by the decrease of the lifetimes of gels 6 and 7 as compared to gel 5 (Table 2), that is, the formation of nonfluorescent H-dimers contributes to the quenching of the excited states through intersystem crossing processes.20,39-41 Additional data supporting the fluorescent character of the dimers formed in gels 2-5 upon adsorption onto the porous surface of silica gels can be found in previous works (e.g., the formation of coplanar-inclined R110 fluorescent J-dimers by a reduction of the adsorption surface rather than by the increase of dye concentration).12,13 It is noteworthy that the gradual increase of the dye concentration should give rise to a distribution of dimer geometries rather than to abrupt changes in the geometry. However, the differentiation of two or more different lifetimes within such a distribution, each one correspond(39) Liang, Y.; Moy, P. F.; Poole, J. A.; Ponte Goncalves, A. M. J. Phys. Chem., 1984, 88, 2451. (40) Nasr, C.; Liu, D.; Hotchandani, S.; Kramat, P. V. J. Phys. Chem. 1996, 100, 11054. (41) Ballet, P.; Van der Auweraer, M.; De Schyver, F. C.; Lemmtyinen, H.; Vuorimaa, E. J. Phys. Chem. 1996, 100, 13701
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Figure 4. Excitation spectra (λem ) 610 nm) of R6G-doped gels 1-5.
ing to a dimer geometry, as well as their partial contributions to the fluorescence intensity, can hardly be attempted. For this reason, therefore, the measured lifetimes reported above, hence, the adsorption angles calculated from eq 1, must be taken as averaged values for such a distribution of dimer geometries but never as discrete values representing a single dimer geometry. Measurements of relative quantum yields (φf) have also been achieved (at right-angle mode) to corroborate the strong fluorescent character of the fluorescent J-dimers that are formed as concentration increases from gel 2 to 5 (Table 2). Thus, an increase in the relative quantum yield was found for gel 2 as compared to gel 1 [φf(gel 2)/ φf(gel 1) ≈ 1.24]. This finding resembles reported results from Langmuir-Blodget (LB) films.42,43 Unfortunately, the reliable determination of the quantum yields for gels 3-5 was avoided due to reabsorption processes that typically take place for measurements achieved at rightangle mode on highly concentrated samples. Furthermore, though the use of front-face mode in the quantum-yields measurements would minimize the reabsorption processes, those measurements are unreliable because the characteristics of our samples (finely ground) highly increase the scattering processes.25 Regarding the excitation spectra, an increase in the intensity of a band placed at 456 nm and a shoulder resulting from the red-shift of the principal band placed at 503 nm was observed as R6G concentration increased from gel 1 to 5 (Figure 4, Table 2).44,45 The relative intensities of absorption bands as well as the dimer band’s splitting can also be explained by the exciton theory.19,21-23 Thus, structural information of the dimer regarding its simplest model can be obtained. The dimer absorption spectrum consists of two separate bands (Figure 5). The dimer geometry is reflected in the shape of this spectrum; that is, the relative intensity of the bands.19,21-23,46 When the band which is placed at higher energies is the most intense, dimers are adopting a sandwich-type geometry (parallel-plane twist-angle model), while when the most intense band is placed at lower energies, dimers are adopting a head-to-tail geometry (in-plane oblique-angle model). According to the exciton theory,21-23,47 the H-band (42) Blonski, S. Chem. Phys. Lett. 1991, 184, 229. (43) Morgenthaler, M. J. E.; Meech, S. R. J. Phys. Chem. 1996, 100, 3323. (44) Fujii, T.; Nishikiori, H.; Tamura, T. Chem. Phys. Lett. 1995, 233, 424. (45) Nishikiori, H.; Fujii, T. J. Phys. Chem. 1997, 101, 3680. (46) Sadkowski, P. J.; Fleming, G. R. Chem. Phys. Lett. 1978, 57, 526. (47) Tapia-Este´vez, M. J.; Lo´pez-Arbeloa, F.; Lo´pez-Arbeloa, T.; Lo´pezArbeloa, I.; Schoonheydt, R. A. Clay Miner. 1994, 29, 105.
Figure 5. Exciton-band energy diagram for molecular dimers with oblique transition dipoles. From Kasha et al.15
Figure 6. Absorption spectra, in reflectance mode, of RB-doped gels 2-7.
is always stronger than the J-band in the parallel-plane twist-angle model while the opposite relationship takes place in the in-plane oblique-angle model. Because the fluorescent character of dimers is due to the allowed transition from the J-dimer level to the ground level, sandwich-type dimers should not be fluorescent, and an oblique configuration should always be attributed to the fluorescent dimers. It is therefore concluded that oblique should be the configuration adopted by the R6G fluorescent J-dimers in gels 2-5, as reported for RB12 and R11013 fluorescent J-dimer-doped silica gels in a similar range of concentrations (gels 2-5). A further increase in concentration (gels 6, 7) should give rise to the conversion from J- to H-dimers which can be experimentally observed in the absorption spectra recorded for RB-doped gels 2-7; that is, the intensity of the band placed at higher energies increases slowly from gel 2 to gel 5, becoming more intense than that placed at lower energies for gels 6 and 7 (Figure 6).12 In accordance with the fluorescent character of the dimers adsorbed on the porous surface for R6G-doped gels 2-5, the exciton theory was applied to the excitation, rather than to the absorption, spectra of the aggregates.21-23,47-52 Thus, the R-angle in the dimer was calculated from the ratio between the areas of the long(48) Lo´pez-Arbeloa, F.; Herra´n Martı´nez, J. M.; Lo´pez-Arbeloa, T.; Lo´pez-Arbeloa, I. Langmuir 1998, 14, 4566. (49) Tapia-Este´vez, M. J.; Lo´pez-Arbeloa, F.; Lo´pez-Arbeloa, T.; Lo´pezArbeloa, I. J. Colloid Interface Sci. 1994, 162, 412.
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Figure 7. Excitation spectra of R110-doped gel 1 (monomer) and the bands resulting from the total excitation spectra of gels 2-5 after subtraction of the gel 1 excitation spectrum. The excitation spectra of gels 1-5 were normalized prior to subtraction.
wavelength and the short-wavelength dimer excitation bands (eq 2).47-52 The interaction energy (U) between the monomers in the aggregate was calculated from the longwavelength and the short-wavelength dimer excitation bands (eq 3).47-52 The ratio between the areas of the longwavelength and the short-wavelength dimer excitation bands as well as the wavelength values were experimentally calculated as described elsewhere (Figure 7).52
tan2(R/2) ) A1/A2
(2)
U ) (ν2 - ν1)/2
(3)
For the oblique configuration, the separation distance between the molecules (R, in Å) can be calculated from eq 4, where νm and ∫(ν) dν are the maximum wavenumber and the area of the monomer absorption band, respectively (Figure 8).47-52 No modifications were observed for νm, also calculated from the excitation spectrum of the gel 1.
R)
(
∫
)
(1.85)102 (ν) dν [cos(R) + 3 sin(R/2)] Uνm
Figure 8. Extinction coefficients of RB-, R6G-, and R110-doped gel 1 (∫(ν) dν ) 2.276 × 108 M-1 cm-1 for RB, 1.954 × 108 M-1 cm-1 for R6G, and 2.097 × 108 M-1 cm-1 for R110). Table 3: Values of Exciton Splitting (∆ν), Calculated Angle between the Monomer Units (r), and Calculated Separation Distance between the Molecules (R)a RB
R6G
∆ν R R ∆ν R gel (cm-1) (deg) (Å) (cm-1) (deg) 2 3 4 5
2522 2718 2874 2982 a
57.9 65.0 70.6 70.4
9.63 9.48 9.36 9.25
1772 1978 2860 3083
R110 R (Å)
69.4 10.24 67.0 9.85 66.5 8.71 67.2 8.51
∆ν R R (cm-1) (deg) (Å) 2356 3074 3539 3489
80.6 74.8 65.2 65.6
9.46 8.62 8.15 8.20
From eq 3.
It is noteworthy that the R value found for RB in gel 5 is in good correlation with the R value previously reported for R3B in oblique configuration48 and larger than that observed for rhodamines in a sandwich-type configuration.47,49-52 These results corroborate the oblique configuration as the preferred configuration for the rhodamine fluorescent dimers incorporated in silica gels that have been studied in this work. Conclusions
1/3
(4)
The calculated angle between the monomer units (R) and the separation distance between the molecules (R) for R6G, R110, and RB gels 2-5 are shown in Table 3. The validity of these calculations can be evaluated by how close the sum of R + 2θ is to 180° (see Figure 1). Good correlation was mainly observed for R110 gels 2-5 and for R6G and RB gels 4-5. The calculated R values decrease as the concentration increases from gel 2 to 5, in correlation with the decrease of the θ values observed from the lifetimes for RB, R110, and R6G. Among them, the following tendency was observed: R(RB) > R(R6G) > R(R110). This behavior can be attributed to the increase of the size of the group that is linked to the nitrogen decreases (-N(CH2CH3)2 for RB, -NH(CH2CH3) for R6G, and -NH2 for R110), which increases the steric hindrance for the establishment of the head-to-tail interaction characteristic of the oblique configuration. (50) Tapia-Este´vez, M. J.; Lo´pez-Arbeloa, F.; Lo´pez-Arbeloa, T.; Lo´pezArbeloa, I. Langmuir 1993, 91, 3629. (51) Lo´pez-Arbeloa, F.; Tapia-Este´vez, M. J.; Lo´pez-Arbeloa, T.; Lo´pezArbeloa, I. Langmuir 1995, 11, 3211. (52) Bojarski, P.; Matczuk, A.; Bojarski, C.; Kawski, A.; Kuklinski, B.; Zurkowska, G.; Diehl, H. Chem. Phys. 1996, 210, 485.
Fluorescent J-dimers have been observed in R6G-, RB-, and R110-doped silica gels as a consequence of their adsorption geometry on the porous silica surface. The appearance of fluorescent dimers gives rise to a wide range of possible values for the maximum of the emission spectra as well as for the lifetime values. The large tunability observed in these systems, besides its strong fluorescent character, makes them good candidates for the preparation of solid tunable-laser dyes.53,54 Exciton theory has been first applied for the calculation of the geometry adopted by the R6G, RB, and R110 fluorescent species within the porous cage of the silica gels prepared by the sol-gel process, that is, for the angle θat which the monomer constituents are adsorbed on the porous silica surface, the angle R formed between the monomer constituents, and the separation between them R. It is remarkable that, thanks to the fluorescent character of the J-dimers, the exciton theory has been successfully applied on the excitation, rather than on the absorption, spectra. It has been observed that different sustituents linked to the nitrogen atom of the xanthene molecules (R6G, RB, and R110) do not modify the adsorption pattern on the porous silica surface. It is necessary, therefore, that the (53) Selwyn, J. E.; Steinfeld, J. I. J. Phys. Chem. 1972, 76, 762. (54) Rahn, M. D.; King, T. A. Appl. Opt. 1995, 34, 8260.
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adsorption of xanthene-type molecules on the porous surface of silica gels must take place through the carbonyl group, regardless whether it belongs to an acid or to an ester. Nevertheless, the sustituents linked to the nitrogen atom determine the separation between the molecules; that is, the higher the steric hindrance of the sustituents linked to the nitrogen atom, the larger the R values.
del Monte et al.
Acknowledgment. The authors are grateful to the CICYT (Spain), for the Research Grant ESP98-1332-C0404, and to the Air Force Office of Scientific Research (USA). The authors are also grateful to Carlos Alonso for technical support. F.d.M. is grateful to the Ministerio de Educacio´n y Cultura of Spain for a postdoctoral fellowship. LA000540+
