Langmuir 1994,10, 2683-2687
2683
Photophysics of Rhodamine 6G-Doped Ti02 Particles during Drying Using Steady-State Spectroscopy and Variable-Frequency Phase and Modulation Data David Levy,* Manuel Ocaiia, and Carlos J. Serna Instituto de Ciencia de Materiales, CSIC, Serrano 115, 28006 Madrid, Spain Received February 17, 1994. In Final Form: May 11, 1994@ Variable-frequency (0.01-120 MHz) phase and modulation analyses are used to monitor the spectral properties of rhodamine 6G-doped T i 0 2 spherical particles, yielding information about the entrapping medium. This informationis useful in studying the identity of the different rhodamine 6G emitting species and the interaction mechanism on the T i 0 2 surface. The steady-state fluorescencespectra, as well as the phase and modulation data (lifetimes and fractional intensities and their uncertainties), were also used to study the rhodamine 6G changing properties during the solvent evacuation process (thermal treatment at 70 "C under vacuum conditions) of the doped particles. One- and two-component fits between the experimental and calculated data were compared for the lifetime resolution of the emitting rhodamine 6G molecules, and the results indicate that a two-componentfit in the decay law is justified by the experimental phase and modulation data. Great effort was taken to resolve the lifetime and the fractional intensities of the two different rhodamine 6G molecules, which were attributed to the emission states of adsorbed and dissolved species in the particles.
Introduction Traditionally, new materials were developed in response to the requirements of a n emerging device technology. In the past 10 years, sol-gel gel glasses doped with organic dyes have been prepared with some potential applications such as optical and electrooptical d e v i ~ e s . l -Recently, ~ we have shown the hydrolysis of liquid aerosols to be a n alternative synthesis route to multicomponent systems.6 Thus, spherical T i 0 2 , SiOz,ZrO2,andAl203 particles doped with several organic fluorescent dyes, such as rhodamine 6G (R6G), rhodamine B, and fluorescein, have been prepared by hydrolyzing aerosols consisting of the corresponding metal alkoxides to which a solution of the dye had been previously a d m i ~ e d . These ~ , ~ composite materials showed strong absorption and emission bands in the visible region. However, a detailed analysis of their spectral features has not been carried out. In this paper, we study the optical properties of the R6G-Ti02 system, as prepared and during thermal treatment, on the basis of steady-state fluorescence data. Although considerable information is available from steady-state measurements, it is usually informative to examine the time-resolved decays of fluorescenceintensity. As the decay time of a single fluorophore is often sensitive to its surrounding environment, the time-resolved data can indicate whether this fluorophore is present on one or more environments, whether the fluorophore undergoes a n excited-state reaction, and whether the dynamic properties of the environment result in time-dependent emission spectra.8-10 Fluorescence emission decay kinetics was also used in this work as a n analytical tool to study the spectral characteristics of the R6G-Ti02 ~~
~~
~
* Abstract published in Advance A C S Abstracts, June 15,1994.
(1)Levy, D.J.Non-Cryst. Solids 1992,147,148,508. (2)Avnir, D.;Levy, D.; Reisfeld, R. J. Phys. Chem. 1984,88,5956. Reisfeld, R.;Avnir, D. ChemPhys. Lett. 1984,109,593. (3)Levy, D.; (4)Levy, D.;Avnir, D. J. Phys. Chem. 1988,92,4734. (5)Levy, D.;Avnir, D. J.Photochem. Photobiol. A : Chem. 1991,57, 41. (6)Ocafia, M.; Levy, D.; Serna, C . J. J.Non-Cryst. Solids 1992,147, 148,621. Ocafia, M.; Serna, C. J. J.Sol-Gel Sci. Technol.,in press. (7)Levy, D.; ( 8 )Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum: New York, 1983;p 485. (9)Badea, M. G.;Brand, L.Methods Enzymol. 1979,61,378. (10)Lakowicz, J. R. J. Biochem. Biophys. Methods 1980,2,91.
0743-7463/94/2410-2683$04.50lO
particles. For such a n analysis, we propose using phaseresolved fluorescence spectroscopy (PRFS),11J2which has been widely used in biochemical research,13-17in chemical analysis,18and in clinical r e s e a r ~ h To . ~our ~ ~knowledge, ~~ this is the first time this technique has been applied to study inorganic solid samples. It will be shown that PRFS is a technique that can be used to resolve the lifetime of solid samples, and that this technique provides additional information about the photophysical properties of the R6G-Ti02 particles. With this technique, one may reliably determine the R6G lifetime and the fractional intensity (fi) of the different trapped species in such particles, as well as their evolution during the drying process.
Experimental Section Materials and Sample Preparation. Fluorophores,ethanol, and chloroform were of the highest purity commercially
available and were not further purified. Rhodamine 6G, fluorescein,and rhodamine B (as the reference for spectroscopic measurements) were from Kodak, titanium(IV) ethoxide, 1,4bis(5-phenyloxazol-2-yl)benzene(POPOP), and D-glyCOgenwere from Aldrich. The R6G-doped T i 0 2 amorphous spherical particles (0.2-7 pm) were prepared by hydrolysis of aerosols, following the procedure previously described.6 As the startingliquid,we used a mixture composed of 45 mL of titanium(IV)ethoxide and 9 mL of a 4.2 x M solution of rhodamine 6G in chloroform. Drying was performed under vacuum at 70 "C for 50,80,and 180 h. The maximum intensities of the excitation and emission ~
(11)Suencer. R. D.:Weber. G. Ann. N.Y.Acad. Sci. 1969.158.361. (12)L'akowicz,J.R,'; Laczko, G.; Cherek, H.; Gratton, E.; Limkeman, M. Biophys. J . 46,463, 1984. (13)Taylor, D.L.; Waggoner,A.S.; Lanni, F.; Murphy, R. F.; Burga, R. R. Application ofFluorescence in the Biomedical Sciences:A. R. Liss: New Yirk, 1986. . (14)Lakowicz, J. R. PrincipZes of. Fluorescence Spectroscopy; _ . Plenum: New York, 1983. (15)Steiner, R. F.;Weinriyb, I. Excited States ofProteins and Nucleui Acids; Plenum: New York, 1971. (16)Jameson, D.M.; Reinhart, G. R. Fluorescent Biomolecules, Plenum: New York, 1989. (17)Lakowicz, J. R., Ed. Time-Resolwed Laser Spectroscopy in Biochemistry; SPIE 909;SPIE: Bellingham, WA, 1988;pp 1-471. (18)Schulman, S. G.Molecular Luminescence Spectroscopy; John Wiley and Sons: New York, 1985. (19)Wolfbeis, 0. S.Pure Appl. Chem. 1987,59,663. (20)Peterson, J. I.; Vurek, G. G. Science 1984,224,123.
0 1994 American Chemical Society
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2684 Langmuir, Vol. 10, No. 8, 1994
incident light. Under ideal experimental circumstances, it is generally accepted that both pulse- and phase-modulation techniques yield equivalent information. The greater the multifrequencyphase-modulationrange of the fluorometer,the better the information content and resolving power of the measurements. The basic principles of PRFS have been described in detail by several researcher^.^-^^ Briefly, the resolution of the lifetime was analyzed using a nonlinear least-squares procedure12that minimizes the squared deviations between the measured phase (4") and modulation (m,)values with those predicted on the basis of an assumed decay law and mm).The subscript w is an index of the modulation frequency (w = 2@, and c indicates calculated values. This analysis is an interactive process which attempts to fit an appropriately chosen model to the experimental multifrequency data by varying the parameters of the model (lifetime, fractional contributions, etc.) in a direction which minimizes the deviations between the model and the data. Moreover, the fitting procedure12 is used to test whether the decay is single exponential or multiexponential, and to determine the parameters associated with the model and the uncertainties associated with the parameters. The emission from a fluorescent species excited by sinusoidally ill be phase shifted and demodulated relative modulated light w to the excitinglight by an amount that is dependent on the lifetime of the fluorophore. Lakowicz12assumes that the impulse response function 1(1,t),at any emission wavelength 1and time t , can be represented by a sum of n exponential decays,
For a mixture of fluorophores the values of ti represent the individual lifetimes and the values of ai the preexponential factors. The fractional steady-state intensity, fi, of each component in the mixture is given by
The values of 4 and m may be obtained from the sine, N,, and cosine, D,, transforms of the impulse response function,
450
550
050
WAVELENGTH (nm) Figure 1. Emission (I,(exc) = 520 nm, - - -) and excitation (L,(em) = 640 nm, -1 spectra of rhodamine 6G-doped T i 0 2
For a multiexponential decay these transforms are
particles: initial sample as prepared (a) and after drying for 50 h (b), 80 h, (c), and 180 h (d).
of the monomer and aggregates in Figure 1were measured after each thermal treatment. Instrumentation. Multifrequency modulation and phase analysis and fluorometricmeasurements at 25 "Cwere performed on a 48000s (T-Optics) spectrofluorometer from SLM-Aminco. The apparatus is configured to software-controlled variablefrequency light modulation from 0.01 to 120 MHz. The frontsurface sample holder for a 0.1 x 1 x 3 cm3 cuvette containing the doped particles or the D-glycogen scatterer solution (used as the reference) was built for data collection and oriented at 60" to light excitation to minimize scattered light on the cooled wideband rfhousing for the R928 tube. Appropriate filters were used to eliminate Raleigh and Raman scatter from the emission. Excitation and emission spectra were corrected for the wavelength dependence of the 450-Wxenon arc excitation, but not for the wavelength dependence of the detection system. Method. The excitation beam is modulated sinusoidally at a frequency comparable to the decay rates of the sample. Information concerning the decay law of the sample is obtained of the emission, from the phase shift (4) and the modulation (m) both measured relative to the phase and modulation of the
(6) where J = Eiaitj. The phase and modulation values can be calculated from Nu and D, and are given by
(7)
For a given sample, the estimated values of ai and ti are those that minimize x 2 , which is the error-weighted sum of squared deviations between the measured and calculated values. When (21)Gratton, E.;Linkeman, M. Biophys. J . l983,44,315. (22)Bright, F. V.Appl. Spectrosc. 1993,47,1152. (23)Gratton, E.;Jameson, D. M.; Hall, R. D. Annu. Rev. Biophys. Bioeng. 1984,13, 105.
Photophysics of Rhodamine 6G-Doped Ti02 Particles both phase and modulation are available,
x2
is given by
Langmuir, Vol. 10, No. 8, 1994 2685 Table 1. Relative Intensities (1-1)of the monomer (m) and Aggregates (a) with Thermal Treatment Zredm)a
Zrel(a)b
Anax =
where uw and am, are the estimated uncertainties in the phase and modulation data at each frequency, respectively. Lakowicz used the Marquardt algorithmZ4as described by Bevington25 for the minimization of the x 2 concerning the parameters a, and zi. The properties of a good fit between the experimental and the calculated data are given by the value of the reduced x2 ( x 2 ~ ) ,
where v, the number of degrees of freedom, is given by Nq - p , N is the number of modulation frequencies, q is the number of emission wavelength, andp is the number offloating parameters (lifetimes and fractional intensities). For the correct model and random experimental errors the value of x 2 R is expected to fluctuate near unity. The minimum ~ used as a measure of the probability that the value of x 2 is experimental data are described by the assumed model. Values of X ~ R which , are significantly greater than unity, indicate that the assumed model is probably inadequate to explain the data, or suggest that experimental errors are present. Phase and Modulation Experiments. Each phase and modulation value is the average of 100 phase or modulation readings. The frequency-dependent phase and modulation data were analyzed using a nonlinear least-squares procedure that minimizes the squared deviations between the observed and expected phase and modulation values.12 The values of the floating parameters (lifetimes and fractional intensities) are , varied in a direction which minimizes the value of x 2 ~ and computation is finished with a number of interactions through the fitting algorithm, when a minimum is found. The uncertainty in any phase or modulation measurement could be decreased to very small levels by averaging by minutes. Specifically,values of x 2 greater ~ than unity may indicate either the presence of systematic errors25 or an inappropriated model. We collected phase and modulation data measured at 16 modulation frequencies, and additional measurements did not seem to improve the resolution. An uncertainty of 0.5 will apply in phase and 0.05 in modulation to the calculation of the reduced x2. It has been observed that the resolving power is enhanced by measurements at a restricted frequency range and at selected excitation and emission wavelengths.26 Drawbacks, including the "color effect",27were treated using a fluorophore as the standard (rhodamine B or fluorescein), hoping to match the sample emission wavelength as close as possible and minimize the artifacts due to this effect. We acquired data from different fluorophores in which the decay laws are believed to be monoexponential, and which have spectral characteristics similar to those of the R6G-Ti02 particles. Ti02 used as a blank did not show any background fluorescence under the spectral conditions of the experiments, and no attempt was made to remove adsorbed or dissolved molecular oxygen from the doped particles.
Results and Discussion Analysis of the Steady-State Spectra. The extent
of the cage effect on the fluorescence is well reflected in the steady-state spectra. The spectral behavior of the R6G in the as-prepared sample (Figure l a ) shows a n excitation spectrum with Am= = 547 nm and an emission (24)Marquardt, D. W. J. SOC.Ind. Appl. Math. 1963,11, 431. (25) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill, Inc.: New York, 1969;p 336. (26) Levy, D. Manuscript in preparation. (27)Ware, W. R.; Pratinidhi, M.; Bauer, R. K. Rev. Sci. Instrum. 1983,54,1148.
&mx
547nm 1.00 0.45 0.60 0.40
as-prepared safnple after drying for 50 h after drying for 80 h after drying for 180 h
=
Amax
471nm 0.27 0.32 0.42
=
504nm 0.47 0.54 0.61
a Monomer intensity relative to the intensity of the monomer in the as-prepared Sam le bAggre ate intensity relative to the ~ l e e s f i ~ j v i & ~ ~ % " t m m a~ "~ saree
1
'0
1
3200
1
1
1
1
1
1
2600 2000 1400 WAVENUMBER(cm-1)
1
1
1
800
Figure 2. IR spectra in KJ3r of the R6G-doped T i 0 2 particles as prepared (a) and after drying at 70 "C for 180 h (b).
ascribed to the absorption and emission bands, respectively, of the monomer form of the R6G. Two additional bands are also observed a t Am= = 471 and 504 nm in the excitation spectra (Figure la) that are probably due to aggregated species of the R6G. These bands are commonly M.2 It observed in solutions at dye concentrations > should be noted that the R6G concentration in the starting solution which is added to the titanium alkoxide to form M. the liquid aerosol was 4.2 x After 50 h of drying (Figure lb, Table l ) , the relative intensity of the monomer bands was appreciably reduced when compared to that of the as-prepared sample. However, after 80 h a slight increase of the relative intensity of the monomer was detected (Table 1). On the other hand, the relative intensity of the bands of the aggregated species (at Amax = 471 and 504 nm) was found to increase after heating for 50 and 80 h (Table 1, Figure lb,c). With longer (180 h) heating time the monomer and aggregate bands were found to decrease and to disappear, respectively (Figure Id). This behavior can be explained by a partial desorption ofthe coadsorbates (i.e.,the removal of some physisorbed water and organic solvents) which starts upon mere activation at room temperature, and is more pronounced with the subsequent increase of temperature to 70 "C and time to 180 h. This process allows disaggregation and R6G adsorption on vacant sites of the Ti02 surface. The release of solvents is clearly supported by the IR spectra of the doped particles before and after the thermal treatment, which show a decrease of the water (3360 and 1620 cm-l) and organic solvent (2800-3000, 1300-1500, and 1000-1150 cm-l) bands (Figure 2). This decrease is also supported by the weight loss ofthe ample,^
Levy et al.
2686 Langmuir, Vol. 10,No. 8, 1994
-
1 .oo
1 .oo
100
a
Q)
I
0.80
-%
One-component fit
0.60 0.40
0.20
0
P s w
6
s
-I
3
80 60
0.80
Two-component lit
5
-
0.60 0
-
0.40
c
r
40
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2 B
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20
0.20
0
0.00 0.04
0.04
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r
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0
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1-3.00 3
10 FREQUENCY (MHz)
E
0
e
e
I
P 0
0
z I 0 + 0.00 5
5
100
Figure 3. (a) One-component fit of the frequency-dependent ( 0 )measured phase and (0)measured modulation data of the
z 0
3
2 0.00
0.00
5
2r0
0
W
3
m
5
YI -0.04
O-3.00 3
10 FREQUENCY (MHz)
P
100
Figure 4. (a) Two-component fit of the frequency-dependent ( 0 )measured phase and ( 0 ) measured modulation data of the
as-preparedR6G-Ti02 particles and the (0)calculated phase and (+)calculatedmodulationbased on the best one-component fit. (b)Frequency dependence deviationsbetween the measured and calculated (0)phase angles and calculated (0)modulations.
as-prepared R6G-Ti02 particles and the ( 0 )calculated phase and (+) calculated modulationbased on the best one-component fit. (b)Frequency dependence deviationsbetween the measured and calculated (0)phase angles and calculated ( 0 )modulations.
as well as the lifetime measurements in the next sections. It should be noted that steady-state experiments could not explain the decrease of the monomer relative intensity in Table 1. Comparing part a with parts b and c of Figure 1, it is observed that the sample heated for 50-80 h produces a noticeable broadening (Figure lb,c) of the fluorescence band of the emitting R6G. This effect is not peculiar to a decompositionprocess, as no such broad band is observed on similar treatments in solution.28 Besides, R6G molecules are not appreciably perturbed by temperature a t these conditions of the thermal treatment. This broadening effect could be somehow related to the heterogeneous contribution of emitting species in different adsorption sites of the particle, which should determine the profile and intensity of the spectrum. Moreover, a n additional contribution to that broad band could be due to the residual amount of solvent molecules (with different solubility properties) that are interacting with the adsorbed species, and to the liquid-like physisorbed phase (containing dissolved R6G emitting molecules). The result of these interactions is a n appreciable shift of the Am= of R6G in the emission spectrum, as shown in Figure lb,c, which clearly does not show a unique liquid-like spectral behavior of R6G.29*30 The broadening effect of the emission band is reduced after 180 h of drying, as shown in Figure Id. Under this situation, R6G molecules adopt a more uniform behavior on the Ti02 particles. But, one would anticipate that there is evidence by liftime measurements in the next sections that some ofthe R6G molecules were adsorbed in different adsorption sites of the Ti02 particles.
Lifetime Analysis of One- and Two-Component Mixtures. The results obtained from the phase and modulation measurements for the as-prepared R6G-Ti02 particles are shown in Figure 3a. The lifetime, t,estimated from the least-squares procedure, on the basis of a onecomponent fit, was ~ 3 - 1 2 0=~ 2.39 ~ ~ ns. The calculated values of 4, and m , based on the best one-component fit are also shown in Figure 3a. The one-component fit gives an uncertainty value of x 2 =~ 60-130 obtained for this analysis, which is obviously unacceptable. In addition, the deviations between the measured and calculated phase angles and between the measured and calculated modulation were as large as A#, = 3" and Am, = 4%,respectively (Figure 3b), and changed with frequency. These deviations, distributed far away from zero, also indicate that there is not a good fit t o a single-exponential decay. The presence of two components in the decay time is clearly evident from the frequency dependence of the phase and modulation values as may be seen by comparing Figures 3a and 4a. This fitting dramatically decreases x 2 from ~ 60-130 in the one-component fit to 0.6-1.5 in this case. We found lifetimes for the two-component fit of z1= 1.34ns and t 2 = 3.64 ns. The fractional intensities of the components were fil = 0.20 and f i 2 = 0.80. In this case, the deviations of A#, and Am, shown in Figure 4b appeared to be randomly distributed around zero, and these are of a magnitude comparable with the experimental random errors. Such results indicate that including a n additional component in the decay law is justified by the experimental phase and modulation data. Analysis of the Lifetime with Drying. Our observations on the steady-state spectra in the study of the dynamics of the drying process were supported by the measurement of the evolution of the fluorescence lifetime with thermal activation. The R6G lifetimes and fractional intensities and their uncertainties in the as-prepared R6Gdoped TiOz particles are shown in Table 2 and Figure 5.
~~~
(28) &to, D.; Sugimura, A. Opt. Commun. 1974,10, 327. (29) Schafer, F. P.Dye lasers; Springer: Berlin, 1977. (30) Drexhage, K.H.J.Res.Nut1. Bur. Stund.,A: Phys. Chem. 1976, 8OA, 421.
Photophysics of Rhodamine 6G-Doped Ti02 Particles
Langmuir, Vol. 10,No. 8, 1994 2687
Table 2. One- and Two-Component Analyses for the As-Prepared Sample and after a Vacuum Thermal Treatment at 70 "Cfor 180 h one-componentfit as-prepared sample after drying for 180 h a fil
t (ns)
X2R
tl (ns)
tz (ns)
F1"
X2R
2.40 1.59
60-130 80-100
1.10 0.52
3.20 2.29
0.10-0.20 0.30-0.40
0.6-1.5 0.6-1.6
+ f i z = 1.0.
-
0.60
2.40
0.50
2.16
0.40
1.92
r:
a c
I*
.-2 L
Q
'L A
z
z
(D
1.68
0.30
o'20
0.10
t'
0
80
a
a
v 40
-
1
1.44
I 1.20
120
160
200
Drying time (hr)
Figure 5. Variation of the two-component-fitlifetimes, tl ( x ) and TZ (O), with the drying time (lifetimes in parentheses correspond to values of the as-prepared sample).
The same approach as in the above sections can be applied to the dried samples for the analysis of the lifetime for a one-component fit, t,and for a two-component fit, tl and ZZ
two-component fit
.
In view of the x z values ~ in Table 2, an additional component in the decay law is well justified. Both components and their different lifetimes can be readily identified spectroscopically, and the lifetimes can be accurately determinedz6with a low value of xz~. In the case of the as-prepared sample, most of the contribution to the fluorescence intensity is due to a kind of R6G emitting species having a fractional intensity of f i z = 0.8-0.9 with tz = 3.2 ns (Table 2). This lifetime is close to the value ofthe R6G lifetime31,3z in ethanol solution (3.89 ns). Therefore, in what follows, the results will be discussed only based on two-component analysis. The result of thermal activation at 70 "C for 50 h is a dramatic decrease in the lifetime of the two components from tl = 1.1 ns and tz = 3.2 ns to t1 = 0.13 ns and tz = 2.00 ns (Figure 51, respectively. This behavior is a consequence of the abundant evacuation of residual liquid, which is also supported by the IR spectra (Figure 21, and the increase of the aggregate (Table 1)intensity in the excited steady-state spectra of Figure l b . The lifetime decrease could be caused by quenching concentration since thermal activationleads t o a large increase of the R6G concentration inside the pores that could produce the fluorescence self-quenching. This energy transfer is also favored by a larger overlap between the absorption and emission bands (the broadening of the bands in Figure lb). This self-quenching (increase of the concentration) effect of singlet energy transfer agrees with the decrease of the monomer fluorescence intensity after drying at 70 "C for 50 h (Table 1). (31)Lakowicz, J. R.;Cherek, H.; Balter, A. J. Biochem. Biophys. Methods 1981,5, 131. (32)Kolber, Z.S.;Barkley, M. D. Anal. Chem. 1986,152, 6 . (33)Parker, C. A.Photoluminescenceof Solutions; Elsevier Publishing Co.: Amsterdam, 1968.
With further heating time the lifetimes, tl and t ~ , gradually increase (Figure 51, which parallels the spectral behavior of the fluorescence emission, and it is shown in Figure lb,c and Table 1. This can be explained as due to the abundant elimination of the residual amount of water and organic solvents from the Ti02 particles (Figure 2). Finally, after 180 h of thermal activation (Figure 51, the lifetimes increase to t1 = 0.52 ns and zz = 2.29 ns (a decrease compared to the as-prepared sample). A decline of the quenching effect can explain this lifetime behavior as a consequence of the migration of the R6G molecules from the liquid phase (more solvents were evacuated) to the surface of the particles. Under these conditions, we found that the contribution of the R6G molecules in the particles to the total fluorescence intensity was 30-40% for t1 and 60-70% for t 2 (Table 2). We attributed these results to the excited states from which the two emissions originate being populated independently. In fact, we conclude that, when organic solvents and water are present, R6G behaves as a dissolved molecule, and the residual solvents avoid physiadsorption of the R6G molecules on the surface of the Ti02 particle. The increased population of the shorter subnanosecond relaxation time (t1= 0.52 ns) may be associatedwith excited R6G molecules adsorbed on the T i 0 2 particle. The longer relaxation time (t2= 2.29 ns) is probably associated with the dissolved but quenched R6G molecules in the residual solvent inside the pores.
Conclusions We showed that phase-resolved fluorescence spectroscopy can be applied to study inorganic solid samples. The combination of variable-frequency phase and modulation and steady-state methods enable us to measure the photophysics of inorganic solids, such as Ti02 particles containing R6G molecules. The careful study of the spectral and dynamic properties in Ti02 doped particles has led to a n understanding of the dynamic properties of the R6G/TiOz interface during the different drying steps. We resolved two lifetimes of R6G in Ti02 particles, in which the shorter subnanosecond relaxation time (tl= 0.52 ns) is associated with excited R6Gmolecules adsorbed on the T i 0 2 particle, and the longer relaxation time (TZ = 2.29 ns) is associated with the dissolved R6G molecules in the residual solvent inside the pores. The phaseresolved method also allows the fractional intensity of each R6G species on the pores to be resolved. We found that, after drying, the contribution of the R6G molecules in the particles to the total fluorescence intensity was 30-40% for rl and 60-70% for 72.
Acknowledgment. This study was supported by the grant of the CICYT Mat-90/0791. We thank C A M (Comunidad Authoma de Madrid) for a SLM-Aminco spectrofluorometer. We thank Carlos Alonso for technical support,
