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Preferred Formation of Coplanar Inclined Fluorescent J-Dimers in Rhodamine 101 Doped Silica Gels Francisco del Monte,* Maria L. Ferrer, and David Levy*,† Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı´ficas (ICMM-CSIC), Cantoblanco, Madrid, 28049, Spain Received March 7, 2001. In Final Form: May 7, 2001 Rhodamine 101 (R101) doped silica gels were prepared through the sol-gel process with dye concentrations ranging from 4.0 × 10-6 to 6.4 × 10-4 M. Excitation and emission fluorescence spectra and lifetime measurements were recorded for R101 doped silica gels in the whole range of concentrations. The photophysical behavior of the gel-glasses doped with high concentrations of R101 resembles that of fluorescent J-dimers in a coplanar inclined configuration. The formation of fluorescent J-dimers in a coplanar inclined rather than in an oblique configuration is discussed in terms of the molecular structure of R101.
Introduction The formation of aggregates of rhodamines has been widely studied in solutions.1-9 Moreover, the nonfluorescent or fluorescent character of the resulting aggregates (called H-dimers and J-dimers, respectively) has mainly been ascribed to the type of interaction forces established between the monomeric constituents, which is also affected by the characteristics of the surrounding environment. Thus, while H-dimers have been observed in solutions of polar and low-viscosity solvents (i.e., water at room temperature), J-dimers may be formed in weakly polar solvents or in the adsorbed state.1,10-15 Formation of rhodamine dimers or aggregates has also been widely studied for dyes absorbed both on organic as well as on inorganic supports, for example, LangmuirBlodgett films,16-23 clays,24-31 porous silica matrixes prepared through the sol-gel process,32-44 and others.45-59 † Also, Instituto Nacional de Tecnologı´a Aeroespacial-INTA, Laboratorio de Instrumentacio´n Espacial-LINES, Torrejo´n de Ardoz, Madrid, 28850, Spain.
(1) Drexhage, K. H. Topics in Applied Physics, Vol. 1; Scha¨fer, F. P., Ed.; Springer: Berlin, 1973; p 144. (2) Selwyn, J. E.; Steinfeld, J. I. J. Phys. Chem. 1972, 76, 762. (3) Faraggi, M.; Peretz, P.; Roshental, I.; Weinraub, D. Chem. Phys. Lett. 1984, 103, 310. (4) Obermueller, G.; Bojarski, C. Acta Phys. Pol. 1977, A52 (3), 431. (5) Lopez-Arbeloa, F.; Ruiz-Ojeda, P.; Lopez-Arbeloa, I. J. Lumin. 1989, 44, 105. (6) Ruiz-Ojeda, P.; Katime-Amashta, I. A.; Ochoa, J. R.; LopezArbeloa, I. J. Chem. Soc., Faraday Trans. 1988, 84 (1), 1. (7) Lopez-Arbeloa, F.; Ruiz-Ojeda, P.; Lopez-Arbeloa, I. J. Chem. Soc., Faraday Trans. 1988, 84 (12), 1903. (8) Lopez-Arbeloa, I.; Ruiz-Ojeda, P. Chem. Phys. Lett. 1981, 79, 347. (9) Chambers, R. W.; Kajiwara, T.; Kearns, D. R. J. Chem. Phys. 1974, 78, 380. (10) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J. Phys. Chem. 1986, 90, 5094. (11) Kemnitz, K.; Yoshihara, K. J. Phys. Chem. 1991, 95, 6095. (12) Itho, K.; Chiyokawa, Y.; Nakao, M.; Honda, K. J. Am. Chem. Soc. 1984, 106, 1620. (13) Nakashima, N.; Yoshihara, K.; Willig, F. J. Chem. Phys. 1980, 73, 3553. (14) Muto, J. J. Phys. Chem. 1976, 80, 1342. (15) Rohatgi, K. K. J. Mol. Spectrosc. 1968, 27, 545. (16) Biesmans, G.; Van der Auweraer, M.; Cathry, C.; Meerschaut, D.; De Schryver, F. C.; Storck, W.; Willig, F. J. Phys. Chem. 1991, 95, 3771. (17) Pevenage, D.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1999, 15, 4641. (18) Slyadneva, O. N.; Slyadnev, M. N.; Tsukanova, V. M.; Inoue, T.; Harata, A.; Ogawa, T. Langmuir 1999, 15, 8651. (19) Pevenage, D.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1999, 15, 8465.
At the absorbed state, the electrostatic interaction of the molecule with the polar surface and with other monomer molecules governs the formation of aggregates.10-12,14 The formation of either nonfluorescent H-dimers or fluorescent (20) Ballet, P.; Van der Auweraer, M.; De Schyver, F. C.; Lemmtyinen, H.; Vuorimaa, E. J. Phys. Chem. 1996, 100, 13701. (21) Tamai, N.; Yamazaki, T.; Yamazaki, I. Can. J. Phys. 1990, 68, 1013. (22) Vuorimaa, E.; Lemmetyinen, H.; Van der Auweraer, M.; De Schryver, F. C. Thin Solid Films 1995, 268, 114. (23) Morgenthaler, M. J. E.; Meech, S. R. J. Phys. Chem. 1996, 100, 3323. (24) Tapia-Este´bez, M. J.; Lopez-Arbeloa, F.; Lopez-Arbeloa, T.; Lopez-Arbeloa, I. Langmuir 1993, 9, 3629. (25) Tapia-Este´bez, M. J.; Lopez-Arbeloa, F.; Lopez-Arbeloa, T.; Lopez-Arbeloa, I.; Schoonheydt, R. A. Clay Miner. 1994, 29, 105. (26) Tapia-Este´bez, M. J.; Lopez-Arbeloa, F.; Lopez-Arbeloa, T.; Lopez-Arbeloa, I. J. Colloid Interface Sci. 1994, 162, 412. (27) Lopez-Arbeloa, F.; Tapia-Este´bez, M. J.; Lopez-Arbeloa, T.; Lopez-Arbeloa, I. Langmuir 1995, 11, 3211. (28) Tapia-Este´bez, M. J.; Lopez-Arbeloa, F.; Lopez-Arbeloa, T.; Lopez-Arbeloa, I. J. Colloid Interface Sci. 1995, 171, 439. (29) Lopez-Arbeloa, F.; Lopez-Arbeloa, T.; Lopez-Arbeloa, I. J. Colloid Interface Sci. 1997, 187, 105. (30) Lopez-Arbeloa, F.; Herra´n-Martinez, J. M.; Lopez-Arbeloa, T.; Lopez-Arbeloa, I. Langmuir 1998, 14, 4566. (31) Chaudhuri, R.; Lopez-Arbeloa, F.; Lopez-Arbeloa, I. Langmuir 2000, 16, 1285. (32) Blonski, S. Chem. Phys. Lett. 1991, 184, 229. (33) Casalboni, M.; De Matteis, F.; Francini, R.; Prosposito, P.; Senesi, R.; Grassano, U. M.; Pizzoferrato, R.; Gnappi, G.; Montenero, A. J. Lumin. 1997, 72-74, 475. (34) Sathy, P.; Penzkofer, A. J. Photochem. Photobiol., A 1997, 109, 53. (35) Ammer, F.; Penzkofer, P.; Weidner, P. Chem. Phys. 1995, 192, 325. (36) Severin-Vantilt, M. M. E.; Oomen, E. W. J. L. J. Non-Cryst. Solids 1993, 159, 38. (37) Fujii, T.; Nishikiori, H.; Tamura, T. Chem. Phys. Lett. 1995, 233, 424. (38) Fujii, T.; Ishii, A.; Anpo, M. J. Photochem. Photobiol., A 1990, 54, 231. (39) Nishikiori, H.; Fujii, T. J. Phys. Chem. 1997, 101, 3680. (40) del Monte, F.; Levy, D. J. Phys. Chem. B 1998, 102 (41), 8036. (41) del Monte, F.; Levy, D. J. Phys. Chem. B 1999, 103 (38), 8080. (42) del Monte, F.; Mackenzie, J. D.; Levy, D. Langmuir 2000, 16 (19), 7377. (43) Innocenzi, P.; Kozuka, H.; Yoko, T. J. Non-Cryst. Solids 1996, 201, 26. (44) Hungerford, G.; Suhling, K.; Ferreira, J. A. J. Photochem. Photobiol., A 1999, 129, 71. (45) Vieria-Ferreira, L. F.; Rosario-Freixo, M.; Garcia, A. R.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1992, 88 (1), 15. (46) Vieria-Ferreira, L. F.; Rosario-Freixo, M.; Garcia, A. R.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1992, 88 (1), 15. (47) Vieria-Ferreira, L. F.; Cabral, P. V.; Almeida, P.; Oliveira, A. S.; Bothelo do Rego, A. M. Macromolecules 1998, 31, 3936. (48) Mubarekyan, E.; Santore, M. Langmuir 1998, 14, 1597.
10.1021/la010357w CCC: $20.00 © 2001 American Chemical Society Published on Web 07/11/2001
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Figure 2. Exciton band energy diagram for molecular dimers with oblique (a) or coplanar inclined (b) transition dipoles, from Kasha et al. (ref 60). Figure 1. Formation of nonfluorescent H-dimers or fluorescent J-dimers as a consequence of the geometry adopted by the transition dipoles. J-Dimers can adopt two different configurations: (b) coplanar inclined and (c) oblique, from Kemnitz et al. (ref 10).
J-dimers has been related to the geometry adopted by the monomer constituents on the surface of adsorption (Figure 1). This geometry is defined by the angle (θ) formed between the longitudinal axis of the molecules (monomer transition moments) and the adsorption surface. Thus, formation of J-dimers has been reported for θ values ranging 0-55°, while over those angle values (θ > 55°), H-dimers are formed (Figure 1).10,12 Further structural information regarding the configurations adopted by the dimer can be obtained from the band splitting of absorption bands and their relative intensities (Figure 2).15,60-64 Accordingly to the Exciton theory,60-64 in the parallel plane twist angle model the band placed at higher energies (named H-band) is always stronger than the band placed at lower energies (named J-band) while the opposite relation takes place in the inplane oblique angle model (Figure 2a). The geometry of the aggregates is defined by two different angles (though indeed related by R + 2θ ) 180°)42 in the in-plane oblique angle model: the angle of orientation (R) and of inclination (49) Morgenthaler, M. J. E.; Meech, S. R. J. Phys. Chem. 1996, 100, 3323. (50) Nasr, C.; Liu, D.; Hotchandani, S.; Kramat, P. V. J. Phys. Chem. 1996, 100, 11054. (51) Kanezaki, E. Mol. Cryst. Liq. Cryst. 1996, 275, 225. (52) Liang, Y.; Moy, P. F.; Poole, J. A.; Ponte Goncalves, A. M. J. Phys. Chem. 1984, 88, 2451. (53) Bojarski, P.; Matczuk, A.; Bojarski, C.; Kawski, A.; Kuklinski, B.; Zurkowska, G.; Diehl, H. Chem. Phys. 1996, 210, 485. (54) Vyshkvarko, A. A.; Kiselev, V. F.; Paschenko, V. Z.; Plotnitkov, G. S. J. Lumin. 1991, 47, 327. (55) Wang, H.; Harris, J. M. J. Phys. Chem. 1995, 99, 16999. (56) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. Chem. Phys. Lett. 1983, 101, 337. (57) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J. Phys. Chem. 1987, 91, 1423. (58) Kemnitz, K.; Yoshihara, K.; Tani, T. J. Phys. Chem. 1990, 94, 3099. (59) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1990, 94, 516. (60) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (61) McRae, E. G.; Kasha, M. Physical Processes in Radiation Biology, 1st ed.; Academic Press: New York, 1964. (62) McRae, E. G.; Kasha, M. J. Chem. Phys. 1961, 11, 38. (63) Kasha, M. Rev. Mod. Phys. 1959, 31, 162. (64) Sadkowski, P. J.; Fleming, G. R. Chem. Phys. Lett. 1978, 57, 526.
(θ) (Figure 1c,2a), which, as mentioned above, in the range 0° < θ < 54.7° defines the fluorescent character of the dimers. Since 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 a head-to-tail rearrangement of the molecules (oblique configuration) should always be needed for fluorescence. At the adsorbed state, fluorescent dimers can also be formed in a coplanar inclined configuration with an angle (θ) of inclination in the 0° < θ < 54.7° range (Figure 1c,2a). Such a configuration is characterized by just a red shift of the band peak with no sign of band splitting (Figure 2b). Over this angle value (θ > 54.7°), the rearrangement of the molecules changes the geometry, which is typically reflected in the absorption spectra by blue shifts of the band maximum and increases of the absorbance.18 As mentioned above, porous silica matrixes prepared through the sol-gel process have also been studied as suitable supports for the formation of adsorbed J-dimers and H-dimers. The special characteristics of the sol-gel process (it begins at a solution stage and it runs at room temperature)65-67 made possible the incorporation of the different dyes68-71 prior to the formation of the matrix. Thus, in sol-gel matrixes, the formation of either nonfluorescent H-dimers or fluorescent J-dimers has been studied both along the preparation process of the silica matrix32,33,36-39,43 as well as at the xerogel state.32-44 In these cases, dye concentration in the initial solution and surface density of adsorbed chromophores at the xerogel state are correlated.40-42 In this present work, we study the formation of R101 J-dimers within the porosity of gel-glasses at the xerogel state. The analysis of the fluorescent properties will show that fluorescent J-dimers are mostly in a coplanar inclined configuration. Preferred formation of J-dimers in a coplanar inclined configuration has been observed in previous works only when the experimental conditions of the gel-glass preparation were somehow forced.41 Oth(65) Brinker, C. J.; Scherer, G. W. Sol Gel science; Academic Press: San Diego, CA, 1990. (66) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33. (67) Ulrich, D. R. J. Non-Cryst. Solids 1988, 100, 174. (68) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956. (69) Avnir, D.; Kaufman, V. R. Langmuir 1986, 2, 717. (70) Leveau, B.; Herlet, N.; Livage, J.; Sanchez, C. Chem. Phys. Lett. 1993, 206, 15. (71) del Monte, F.; Levy, D. Chem. Mater. 1995, 7, 292.
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Figure 3. Absorption ()), excitation (2, λex ) 500 nm), and emission (2, λem ) 660 nm) spectra of R101 doped gel 1 (inset: molecular structure of R101). Table 1. Concentration of R101 Doped Gels 1-7 gel 1 gel 2 gel 3 gel 4 gel 5 gel 6 gel 7 [R101] (M) ×
10+6
4
10
40
80
160
320
640
erwise, oblique was reported as the preferred configuration.40,42 The different configuration observed in this work for R101 will be attributed to the large planarity of this dye (Figure 3). The angle formed between the longitudinal axis of the fluorescent R101 J-dimers and the adsorption surface (θ, in Figure 1) will also be determined through the application of the Exciton theory to lifetime measurements. The results found in this work, besides those already published for RB, R110, and R6G doped gelglasses,40-42 show the capability that the sol-gel process offers to tailor the fluorescent properties of a number of solid matrixes. Experimental Section Materials. R101, from Lambda Physics, was 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 R101 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 the formation of dried xerogels. The samples were kept in the dark throughout 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 SLM-Aminco.72,73 Emission and excitation spectra were recorded by reflection (front face mode) in the backward direction74 on pills of 50 µm thickness prepared from finely ground samples. The instrument was configured as described elsewhere.40-42 Lifetimes were measured by reflection (72) (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. (73) Lakowicz, J. R.; Laczko, G.; Cherek, H.; Gratton, E.; Limkeman, M. Biophys. J. 1984, 46, 462. (74) Oelkrug, D. Topics in Fluorescence Spectroscopy, Volume 4: Probe Design and Chemical Sensing; Lakowicz, J. R., Ed.; Plenum Press: New York, 1994; Chapter 8.
Figure 4. Excitation spectra (λem ) 660 nm) of R101 doped gels 1 (2), 3 (b), and 5 ((). (front face mode) in the backward direction on finely ground samples packed into quartz cells with a 1 mm path length as described elsewhere.40-42,45-47 The frequency-dependent phase and modulation data were recorded and analyzed as described elsewhere.40-42 The frequency range (5-110 MHz) allows the determination of lifetimes ranging from 0.5 to 20 ns. The accuracy of the lifetime values measured was determined according to the lower χR2 value found in each individual measurement.75 Values of χR2 greater than unity may indicate either the presence of systematic errors76 or an inappropriate model. UV-Vis Spectroscopy. A Varian UV-Vis spectrophotometer model 2300 was used to measure the absorption spectra (reflectance mode) of samples finely ground and packed in quartz cells with a 1 mm path length.40-42,45-47
Results and Discussion The spectroscopic properties of R101 doped gel 1 are shown in Figure 3. The silica porous matrix provides an adequate support for the preservation of the R101 fluorescence. Compared to R110, R6G, or RB doped silica gel-glasses,40-42 a red shift of both the R101 excitation as well as the emission spectra is observed. Regarding R110 and even R6G, this behavior can be explained by a better delocalization of the positive charge over the xanthene moiety of the rhodamine dye, due to the better electron donating properties of the N-ethyl group19 (≈23 nm each40-42). Regarding RB, where two donating N-ethyl groups are already attached to the xanthene moiety, the further red shift observed for the emission spectra (≈10 nm) of R101 must be a consequence of the impeded rotation of the N-groups.47 The study of the formation of R101 dimers within the porosity of silica gels has been achieved in this work on xerogels with dye concentrations ranging from 4.0 × 10-6 M (gel 1) to 6.4 × 10-4 M (gel 7). To date, the best description of the aggregation behavior of dye chromophores is based on the exciton-splitting theory, which correlates different dimer geometries with the shape of the absorption spectrum.15,60-64 The exciton-splitting theory predicts that the interaction of the transition dipole moments of the molecule forming the dimers can result in the spectral shift or in the splitting of the absorption band. Such a behavior of the absorption band corresponds to the exciton splitting of the excited state, which depends on both the chromophore spacing and the relative spatial arrangement of the transition dipole moments. In previous works based on a number of xanthene type molecules as (75) Gratton, E.; Linkeman, M. Biophys. J. 1983, 44, 315. (76) Bevington, P. R. Data reduction and Error Analysis for the Physical Sciences; McGraw-Hill, Inc.: New York, 1969; p 336.
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Table 2. Maximum of the Excitation (λmex) and the Emission (λmem) Spectra,a,b Lifetime Values (τ),c,d and Experimental Reduced Chi-Squared (χR2) for R101 Gels 1-7 and Experimental Angle (θ) of Adsorption for R101 Gels 2-5 gel 1 gel 2 gel 3 gel 4 gel 5 gel 6 gel 7
λmex (nm)
λmem (nm)
τ (ns)
χR2
θ (deg)
578 578 580 581 582 582 582
590 592 596 599 603 606 606
3.8 ( 0.2 4.0 ( 0.2 4.6 ( 0.2 5.1 ( 0.2 5.3 ( 0.2 5.1 ( 0.2 4.8 ( 0.2
1.2 1.0 1.2 0.8 1.0 1.2 1.1
monomer 12.9 ( 1.1 24.6 ( 1.0 30.3 ( 0.8 32.1 ( 0.8 H dimers H dimers
a The excitation wavelength used to record the emission spectra was 500 nm. b The emission wavelength used to record the excitation spectra was 660 nm. c The excitation wavelength used to measure the lifetimes was 500 nm. d Errors were calculated as described elsewhere (ref 76).
R6G, RB, and R110 doped in gel-glasses, the absorption/ excitation spectra showed a band splitting characteristic of aggregates adopting an oblique configuration.37-42 However, in this work, the increase of R101 concentration from gels 1 to 5 just gave rise to a 4 nm red shift of the principal band of the excitation spectra (from 578 nm for gel 1 to 582 nm for gel 5), with no appearance of band splitting (Figure 4, Table 2). Such a behavior is characteristic of dimers in the coplanar inclined configuration,19,60-64 which when the angle (θ) of inclination is in the range 0° < θ < 54.7° must show fluorescent character.10,11 The flatness of the xanthene molecule rather than hydrophobicity seems to determine the preferred formation of R101 dimers in the coplanar inclined configuration.31 Regarding lifetime measurements, a single lifetime value of 3.8 ns was found for gel 1 (Table 2), which denotes that the dye molecules are surrounded by a homogeneous environment.77 Note that this lifetime value was similar (even slightly shorter) than the lifetime reported for R101 in ethanol and water solutions (4.3 and 4.2 ns,47 respectively). This behavior is opposite to the noticeable increase of the lifetime found for a number of xanthene type molecules (RB, R110, or R6G) when incorporated in silica gel-glasses (from 3.00 to 3.83 ns, from 0.95 to 4.04 ns, and from 2.38 to 3.58 ns, respectively). However, a closer look points out that the lifetime values of every rhodamine doped in silica gel-glass is within the 3-4 ns range, which is characteristic of adsorbed molecules assuming maximum interaction between dye and substrate.78 The different behavior comes from the lifetime values found in solution, that is, much shorter for RB, R110, or R6G than for R101 (R110 < R6G < RB < R101). In solution, intramolecular rotation modes (e.g., rotation around R1R2N- groups at both ends of the xanthene chromophore) shorten the lifetime values due to the enhancement of the internal conversion processes from the excited to the ground state.10-12,56,57,79,80 The different steric hindrance of the R1R2N- groups in RB, R110, or R6G determines their characteristic capability to rotate,1,81 but the molecular structure of R101 makes such a rotation impossible (Figure 3). The xanthene skeleton of R101 is therefore adopting a flat position not only when adsorbed on a substrate but also in solution, making both lifetime values quite similar. (77) Levy, D.; Ocan˜a, M.; Serna, C. J. Langmuir 1994, 10, 2683. (78) Snyder, L. R. J. Phys. Chem. 1963, 67, 2622. (79) Itho, K.; Honda, K. Chem. Phys. Lett. 1982, 87, 213. (80) Eyal, M.; Reisfeld, R.; Cherniyak, V.; Kaczmarek, L.; Grabwska, A. Chem. Phys. Lett. 1991, 176, 531. (81) Osborne, A. D.; Winkorth, A. C. Chem. Phys. Lett. 1982, 85, 513.
Figure 5. Reflectance spectra of R101 doped gels 5 ((), 6 (0), and 7 (4).
The measurement of the lifetime values of gels 1-5 can also be used in the calculation of the adsorption angles accordingly to the Exciton theory.60-63 Equation 1 relates the radiative rate constant of the monomer with the angle (θ) formed between the fluorescent dimers in the coplanar inclined configuration and the adsorption surface (Figure 1):10,11
krD ) krM cos2 θ
(1)
where krD is the radiative rate constant of the fluorescent dimers and krM is the radiative rate constant of the monomer unit. The radiative rate constants of the monomer and the fluorescent dimers (krM and krD, respectively) were directly calculated from the R101 lifetime measurements of gels 1-5, gel 1 for the monomer and gels 2-5 for the fluorescent dimers, which is valid approximation taking into account that the R101 quantum yield of fluorescence (φf) is ∼1.82 The adsorption angle θ values for gels 2-5 are reported in Table 2. The highest angle θ in which R101 fluorescent dimers are adsorbed corresponds to gel 5. The decrease of the lifetime value observed for a further increase of concentration (gels 6 and 7, Table 2) is ascribed to the appearance of H-dimers. The occurrence of the band splitting in the absorption spectra of gels 6 and 7 (Figure 5) readily indicates the change of the geometry from J-dimers in the coplanar inclined configuration to H-dimers in the oblique configuration by a head-to-tail rearrangement of the molecules.18,40,42 Furthermore, the formation of nonfluorescent H-dimers contributes to the quenching of the excited states through intersystem crossing processes,20,50,52 which explains the observed decrease of the lifetime values from gels 5-7. Any trace of fluorescence should be vanished by a complete J-dimer to H-dimer conversion, and the experimental measurements of lifetime values are therefore indicating the presence of a distribution of dimer geometries. Unfortunately, the discrimination of two or more different lifetimes within such a distribution, each one corresponding to a dimer geometry, as well as their partial contributions to the fluorescence intensity can hardly be attempted with our experimental setup. Therefore, the measured lifetimes reported above, and 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. (82) Karstane, T.; Kobe, K. J. Phys. Chem. 1980, 84, 1871.
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Figure 6. Emission spectra (λex ) 500 nm) of R101 doped gels 1 (2), 3 (b), and 5 (().
Nevertheless, the angle value (θ) found for gel 5 is far below the limiting value accepted for adsorbed fluorescent J-dimers (32° versus 55°, respectively). As mentioned above, the flatness of the molecular structure of R101 preferentially promotes its aggregation in a coplanar inclined configuration, where the nitrogen atom of the bottom molecule favorably interacts with the carboxylic group of the upper molecule.18 Moreover, the carboxylic group of the bottom molecule is most likely responsible for the dimer adsorption on the porous surface through H-bonding with the silanol groups.83 The increase of concentration tends to increase the angle of absorption, weakening the interaction between the nitrogen and the carboxylic group of the dimer molecules, while closing the carboxyphenyl rings, which, at this stage, are perpendicular to the xanthene plane.1,18 Taking into account that the coplanar inclined configuration resembles a nontwisted sandwich arrangement, the steric hindrance between the carboxyphenyl rings largely increases when the inclination angle tends to perpendicular. In this case, inclination angles over 32° seem to close the carboxyphenyl rings of the R101 molecules enough to promote the rearrangement of the molecules in a different configuration, that is, nonfluorescent H-dimers in the oblique configuration as indicated by the band splitting in the absorption spectra. The rearrangement must also be promoted by the weakness of the H-bond interaction between the carboxylic group of the bottom molecule of the dimer and the porous surface that takes place with the increase of the angle of inclination. The length of the rigid xanthene plane of R101 is also supporting the low angle value (below 55°) at which the carboxylic group detaches from the porous surface. Regarding the emission spectra, the increase of the dye concentration caused a red shift of the emission maxima from 590 nm for gel 1 to 603 nm for gel 5, which is in concordance with the formation of fluorescent dimers, that emit at longer wavelengths than nonaggregate species (Figure 6, Table 2).10-12,18,19,59-63 A linear increase of the fluorescence intensity with dye concentration has been observed up to gel 5, while over this dye concentration the fluorescence intensity decreases as a consequence of the appearance of aggregates that contribute to the quenching of the excited states through intersystem crossing processes (Figure 7).20,50,52 Both red shifts of the emission maxima as well as linearity of fluorescence intensity with surface density of adsorbed chormophores have been (83) Pouxviel, J. C.; Dunn, B.; Zink, J. I. J. Phys. Chem. 1989, 93, 2134.
Figure 7. Fluorescence intensity of R101 doped gels 1-7 versus R101 concentration.
reported previously18,20 and explained as a consequence of the increase of the polarizability at the dye surrounding environment resulting from the close packing of xanthene moieties.10,18 This explanation of the environment is in good concordance with the formation of fluorescent or nonfluorescent dimers, depending on whether the dye concentration is below or above that of gel 5, respectively; that is, approach of the xanthene moieties is indeed required for the formation of any dimer. Moreover, in highly concentrated samples, red shifts of the emission spectrum have also been ascribed to reabsorption processes.84-87 The overlapping of the absorption and the emission spectra may cause the reabsorption of the low wavelengths of the emission spectrum along the optical pathway, resulting in the shift of the emission spectrum as a whole toward higher wavelengths (red shift). However, reabsorption processes have been described as negligible for the experimental conditions used in this paper for the recording of the emission spectra.40-42,44-47 Furthermore, the linearity of the fluorescence intensity with concentration described above also corroborates the lack of reabsorption processes in gels 1-5. Otherwise, fluorescence intensity would decrease with the increase of concentration due to the greater overlapping between absorption and emission spectra.84 Conclusions Spontaneous formation of fluorescent J-dimers in a coplanar inclined configuration has been reported for the first time for R101 doped silica gels. While the fluorescent character of the aggregates is a consequence of their geometry of adsorption on the porous silica surface, the preferred coplanar inclined configuration was adopted as a consequence of the molecular structure of the R101. In this case, the lack of mobility of the substituents linked to the nitrogen atom of the xanthene structure modifies the behavior of R101 as compared to any of the other rhodamines (R6G, RB, and R110) previously incorporated in silica gels, not only in reference to the configuration adopted upon adsorption but also to the angle at which J-dimer to H-dimer conversion is observed. (84) Lopez-Arbeloa, I. J. Photochem. 1982, 18, 161. (85) Hammond, P. R. J. Chem. Phys. 1979, 70, 3884. (86) Kawahigashi, M.; Hirayama, S. J. Lumin. 1989, 43, 207. (87) El-Daly, S. A.; Okamoto, M.; Hirayama, S. J. Photochem. Photobiol., A 1995, 91, 105.
Fluorescent J-Dimers in Doped Silica Gels
This work contributes to expanding the possibilities, mostly in terms of wavelengths, offered by rhodamine doped sol-gel-glasses as tunable solid laser dyes, thanks to the tailoring of different configurations that can be attempted through both the sol-gel reaction conditions as well as the molecular structure of the fluorescent dye.
Langmuir, Vol. 17, No. 16, 2001 4817
Acknowledgment. The authors are grateful to CICYT for Research Grant ESP98-1332-C04-04. Maria L. Ferrer is grateful to Comunidad Auto´noma de Madrid for a postdoctoral fellowship. LA010357W
