Photophysical Properties of [60] Fullerenes and Phthalocyanines

Mesoporous Silica Films Annealed at Various Temperatures. Selvaraj Subbiah and Robert Mokaya*. School of Chemistry, UniVersity of Nottingham, UniVersi...
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J. Phys. Chem. B 2005, 109, 5079-5084

5079

Photophysical Properties of [60]Fullerenes and Phthalocyanines Embedded in Ordered Mesoporous Silica Films Annealed at Various Temperatures Selvaraj Subbiah and Robert Mokaya* School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, United Kingdom ReceiVed: August 28, 2004; In Final Form: December 3, 2004

The photophysical properties of fullerene and/or phthalocyanine dyes embedded in ordered mesoporous silica films and the influence of annealing temperature on the nature of the immobilized dye molecules has been investigated using photoluminescence (PL) and diffuse reflectance (DR) studies. The PL and DR studies show that fullerene (C60) and/or zinc phthalocyanine (ZnPc) molecules incorporated into transparent mesoporous silica films, via either sol-gel or grafting routes, exist predominantly in monomeric form. Careful choice of annealing temperature, between 25 and 225 °C, can further enhance monomeric dispersion. For C60-containing films, monomeric dispersion of fullerene was observed for annealing temperatures up to 175 °C for sol-gel derived films and 225 °C for grafted films. Both sol-gel and grafted ZnPc-containing films showed evidence of monodispersed phthalocyanine for annealing temperatures up to 225 °C. In general, annealing temperatures in the range 125-175 °C were found to yield optimal monodispersion of the dye molecules. When both C60 and ZnPc were incorporated into the silica films, no evidence of interaction between the dyes, i.e., chargetransfer transitions or the formation of fullerene/phthalocyanine charge-transfer complexes, was observed. This suggests that embedded fullerene and phthalocyanine molecules may be used for the preparation of solid-state optical limiters, based on reverse saturable absorption, where monomeric dispersion of the dye molecules is important.

1. Introduction During the past decade much interest has been focused on the development of solid-state optical limiters.1,2 Among the materials studied, phthalocyanines and fullerene have been found to be the most promising for optical limiting applications.1-6 These molecules are extremely photostable and exhibit strong nonlinear optical effects due to reverse saturable absorption (RSA). Phthalocyanines are porphyrin analogues that exhibit a number of unique properties due to an extensive delocalized π-system. Their centrosymmetric structure and electron-donating ability make them of great interest in many scientific and technological areas.7 The excellent electrophilic (electron acceptor) nature of fullerenes makes them interesting building blocks of more complex systems. For optical limiting studies, phthalocyanines and fullerenes are normally dissolved in organic solvents such as toluene and are used as colored solutions.6,8,9 Some studies have also been carried out on the optical limiting properties of either fullerene or phthalocyanines embedded in solid matrixes. Combinations of phthalocyanines and fullerenes are promising for applications in electronics, in optoelectronics, and as nonlinear optical materials.10 Compared with phthalocyanine or fullerenes alone, the combination of these optical limiting molecules in a single optical limiter may have some advantages. For example, combining the optical limiting properties of phthalocyanines and fullerenes may achieve broad-band optical limiting especially if the dye molecules are noninteracting with respect to their optical limiting capability. It is, however, worth noting that, since fullerenes are good electron acceptors, it is possible that they may interact with electron donors such as phthalocyanine through charge-transfer interactions. Guldi and co-workers11 have reported on charge-transfer states in strongly

coupled phthalocyanine-fullerene ensembles. A study of fullerene (C60) doped zinc phthalocyanine (ZnPc) composites, by Chen et al.,12 showed that photoconductivity was enhanced due to the formation of an intermolecular charge-transfer complex between C60 and ZnPc. Zhu et al.13 have studied the optical limiting properties of phthalocyanine-fullerene derivatives by reacting asymmetrically substituted copper phthalocyanine (CuPc) with C60 via a Diels-Alder reaction. Although the CuPc and C60 were covalently bonded in one compound, their individual nonlinear optical properties were not affected; they independently contributed to the observed reverse saturable absorption.13 The optical limiting properties of fullerenecontaining polyimides has also been investigated, and the best performances have been achieved with samples doped with fullerenes and malachite green dye simultaneously.14 The combination of phthalocyanines and fullerenes may also be achieved via immobilization of the dyes in a suitable host matrix. The use of a host matrix to immobilize optical limiting molecules, thus providing solid-state optical limiter composite materials, is an interesting area of current research.15,16 The importance of the nature of the host matrix was recently illustrated by Venturini and co-workers,16 who incorporated fullerene into a poly(methyl methacrylate) (PMMA) polymer matrix and studied the optical limiting properties of the immobilized fullerene. They found that the optical limiting properties of the resulting composite material were compromised by the degradation/decomposition of the polymer matrix.16 The search for stable, yet suitably transparent host materials is therefore a desirable research goal. We are currently involved in a study on the immobilization of optical limiting dye molecules in transparent mesoporous silica hosts.17,18 We have studied the immobilization of fullerene and phthalocyanine dyes

10.1021/jp0461089 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/23/2005

5080 J. Phys. Chem. B, Vol. 109, No. 11, 2005 in transparent and ordered mesoporous molecular sieve (MMS) silica films whereby the incorporation of the dye molecules was achieved via sol-gel processes or postsynthesis impregnation.17,18 Here we report on the immobilization of a combination of phthalocyanine and fullerene dyes in transparent mesoporous silica thin films. We investigate the nature of the immobilized dyes and in particular the extent of their aggregation as opposed to monomeric dispersion. The extent of aggregation of fullerenes supported on amorphous silicon oxide films has been shown to depend on the annealing temperature.19 Hasegawa and Nonomura19 reported that C60 molecules doped onto a silicon oxide film aggregated to larger size clusters with increasing annealing temperature. We have therefore investigated the influence of annealing temperature on the nature of the immobilized dye molecules and present evidence, from photoluminescence and diffuse reflectance studies, regarding the distribution of fullerene (C60) and/or phthalocyanine (ZnPc) molecules in transparent mesoporous silica monolithic films prepared via various methods. We also present preliminary data on the optical limiting properties of the dye/MMS composite materials. 2. Materials and Methods 2.1. Materials. The dye/MMS composite materials were prepared via either a sol-gel route or postsynthesis grafting. For the sol-gel route, 1 mol of surfactant, cetytrimethylammoniun bromide (CTAB), was dissolved in a mixture of ethanol and toluene (ethanol/toluene molar ratio ) 4.25) and added to a stirred solution of 5 mol of tetraethyl orthosilicate (TEOS) in 0.74 g each of 0.1 N HCl and water. The required quantities of C60 and/or ZnPc in toluene were then added, and the resulting solution was refluxed at 80-90 °C for 5 h. (To avoid phase separation for materials embedded with both C60 and ZnPc, we used carefully selected concentrations of C60 (2.0 µM) and ZnPc (1.2 µM).) The refluxed mixture was then subjected to some solvent removal using a rotary evaporator at 50 °C, and the resulting viscous liquid was transferred onto a Petri dish and subsequently dried at room temperature for 24 h. We also prepared dye/MMS composites via a postsynthesis adsorption (i.e., grafting) route. For this route, calcined (550 °C for 5 h) dye-free transparent monolithic films of MMS were immersed in C60/toluene, ZnPc/toluene, or C60/ZnPc/ toluene solution for 2 h and subsequently dried at room temperature for 24 h. The amount of dye(s) added during the grafting route was similar to that used in the sol-gel route, i.e., 2.0 µM C60 and 1.2 µM ZnPc. The air-dried composite materials (from both the sol-gel route and the grafting route) were subsequently annealed at 125, 175, and 225 °C for 1 h and stored under moisture-free conditions. Using thermogravimetric analysis (TGA), we confirmed that the CTAB starts to decompose at a temperature of ca. 230 °C, while C60 and ZnPc are stable up to 600 and 450 °C, respectively. We have used the following designations to represent the samples prepared. Samples for which the incorporation of C60, ZnPc, and both (C60 and ZnPc) was via the sol-gel route are designated as C60-MMS, ZnPc-MMS, and (C60/ZnPc)-MMS. Samples in which the dyes were incorporated via postsynthesis grafting (i.e., dyes adsorbed on template-free MMS after calcination at 550 °C for 5 h) are designated as C60/MMS, ZnPc/MMS, and (C60/ZnPc)/MMS. 2.2. Characterization. Powder X-ray diffraction (XRD) data were obtained on a Philips PW 1830 diffractometer with Cu KR radiation (40 kV, 40 mA), 0.02° step size, and 1 s step time. Photoluminescence (PL) measurements were carried out

Subbiah and Mokaya

Figure 1. (A) Powder X-ray diffraction patterns of sol-gel derived (C60/ZnPc)-MMS composite dried at room temperature (a) or annealed at 225 °C (b). (B) Powder X-ray diffraction patterns of various solgel derived composites annealed at 225 °C: (a) C60-MMS, (b) ZnPcMMS, and (c) (C60/ZnPc)-MMS.

with a Nicolet Almega Raman spectrometer. The excitation laser was a 532 nm CW diode pumped solid-state laser (JDS Uniphase G537). Diffuse reflectance UV-vis (DR-UV/vis) spectra in the range of 200-1000 nm were obtained on a Lambda 35 (Perkin-Elmer) spectrometer equipped with diffuse reflectance attachment RSA-PE-20. Measurement of optical limiting properties was performed using a Nd:YAG laser at a wavelength of 532 nm with 7 ns pulse widths. The laser beam was focused with a lens onto the sample, and a Neutral Dense filter was used to control the input energy. The transmitted pulse energy was measured with a Molectron energy meter. For comparison we measured the optical limiting properties of C60 in toluene using a 1 cm quartz cell with the laser beam focused to a diameter of 3 mm. Prior to the measurements, a calibration was carried out with an empty quartz cell to eliminate surface reflection effects. 3. Results and Discussion 3.1. Physical Characterization. Figure 1 shows the XRD pattern of sol-gel derived as-synthesized (i.e., air-dried) composite samples and after thermal treatment at 225 °C for 1 h. The XRD patterns clearly indicate that the composite materials are mesostructured with respect to the ordering of the silica (MMS) host.17,18 The typical basal (d100) spacing of assynthesized composite materials was ca. 3.5 nm. Annealing at temperatures of up to 225 °C did not affect the mesostructural ordering or basal spacing of the composite materials. This means that removal of solvent by annealing at 225 °C does not have any affect on the d spacing of the dye-containing transparent mesoporous silica monolithic films. For example, the assynthesized (C60/ZnPc)-MMS sample has a d spacing of ca. 3.5 nm, which remained virtually unchanged after annealing at 225 °C (Figure 1A). The level of mesostrutural ordering was comparable for all sol-gel derived composite materials (Figure 1B). The mesostructural ordering of dye-free MMS films was comparable to that of dye-containing MMS films. On calcination, at 550 °C for 6 h in air, the dye-free MMS films (which maintained their transparency) had XRD patterns similar to those in Figure 1. Calcination did, however, cause a decrease in the basal spacing of ca. 17%. Postsynthesis grafting of C60 and/or ZnPc onto calcined MMS films had no effect on mesostructural ordering.

C60 and ZnPc Dyes in Mesoporous Silica Films

Figure 2. Photoluminescence (PL) spectra of sol-gel derived dyefree MMS films and dye-containing MMS composites annealed at 125 °C (A) or 225 °C (B): (a) dye-free MMS, (b) ZnPc-MMS, (c) C60-MMS, and (d) (C60/ZnPc)-MMS.

Figure 3. Photoluminescence (PL) spectra of dye-containing MMS composites, obtained via postsynthesis grafting, annealed at 125 °C (A) or 225 °C (B): (a) ZnPc/MMS, (b) C60/MMS, and (c) (C60/ZnPc)/ MMS.

3.2 Photoluminescence Studies. 3.2.1. Fullerene C60. The photoluminescence (PL) spectra of fullerene (C60) and/or zinc phthalocyanine doped mesoporous silica monolithic films, annealed at 125 or 225 °C, is shown in Figures 2 and 3. Figure 2 shows the PL spectra of composite materials in which the dyes were incorporated via a sol-gel route, and Figure 3 shows the PL spectra for materials where the dyes were introduced onto the mesoporous silica matrix via postsynthesis grafting. We note that, under the conditions used, calcined (dye-free) mesoporous silica films did not exhibit any significant photoluminescence, and therefore the “blank” spectrum for calcined MMS is not shown in Figure 3. (It is, however, worth noting that as-synthesized dye-free MMS (blank) annealed at 125 °C shows a PL peak at 1.82 eV, shoulders at 1.64 and 1.73 eV, and a broad peak at ca. 2.0 eV (Figure 2A, spectrum a) and after annealing at 225 °C gives a broad peak centered at ca. 1.96 eV (Figure 2B, spectrum a).) After fullerene incorporation via the sol-gel route, we observed PL peaks at 1.7 and 1.9 eV for C60-MMS composites annealed at 125 °C (Figure 2A, spectrum c) and 225 °C (Figure 2B, spectrum c), respectively. It is worth emphasizing that pure C60 at room temperature is known to exhibit a PL band at ca. 1.7 eV and that the HOMOLUMO gap of C60 in the molecular state is 1.9 eV, while for C60 in the solid crystalline state the gap is 1.5 eV.20,21 The PL peak of C60 observed at 1.7 eV for the C60-MMS composite annealed at 125 °C therefore lies below the HOMO-LUMO gap of C60 in a molecular state. The assignment of the peak at 1.7 eV to C60 embedded in the MMS is consistent with previous

J. Phys. Chem. B, Vol. 109, No. 11, 2005 5081 reports.3,22-24 A PL peak at 1.7 eV has previously been ascribed to C60 in solution, powder, or thin film form.3,22-24 The fullerene PL peak appears to undergo a blue shift to higher energy (by 0.2 eV) from 1.7 to 1.9 eV as a result of increasing the annealing temperature from 125 to 225 °C. The blue shift to higher energy is accompanied by an increase in intensity (note that the intensity scale of Figure 2B is twice that of Figure 2A). An increase in annealing temperature of C60-doped amorphous silica films has previously been shown to result in the formation of C60 clusters whose size (i.e., number of molecules in the cluster) increases at higher annealing temperature.19 It is therefore likely that annealing at 225 °C may encourage the formation of larger C60 clusters which are more effectively trapped within the MMS silica host. In addition, a higher annealing temperature removes residual solvent molecules. Solvent molecules, when present, can quench emission from the fullerene molecules. The removal of solvent may also encourage aggregation of C60 and cluster formation. The sum total of these effects (i.e., cluster formation and removal of residual solvent molecules) may result in a higher quantum confinement effect (QCE) at 225 °C compared to 125 °C. A stronger QCE after annealing at 225 °C is consistent with the observed higher PL intensity and a blue shift in the PL peak.25 Previous studies have also shown the C60 trapped in silica hosts exhibits a PL peak at ca. 1.9 eV.26,27 The PL spectra of grafted C60/MMS composites (for which C60 was introduced onto the MMS host via postsynthesis grafting) are shown in Figure 3. The C60/MMS composite exhibits a PL peak at 1.73 eV after annealing at 125 °C (Figure 2A, spectrum b). The PL peak blue shifts to 1.76 eV (i.e., a blue shift of 0.03 eV) and the intensity increases when the annealing temperature is increased to 225 °C (Figure 2B, spectrum b). We attribute the increase in PL intensity to the removal of residual solvent molecules and consequently less quenching after annealing at 225 °C. The blue shift is much lower than that observed for the sol-gel derived C60-MMS composites, where the PL peak blue shifts by 0.2 eV when the annealing temperature is increased from 125 to 225 °C. For grafted C60/MMS composites, the MMS host is already calcined and is therefore not expected to shrink. There is therefore little change in the QCE when the annealing temperature is raised from 125 to 225 °C. Comparison between the PL spectra of sol-gel derived and grafted fullerene-MMS composites indicate that their method of preparation has an influence on the PL properties of the immobilized C60 molecules. 3.2.2. Phthalocyanine (ZnPc). For zinc phthalocyanine embedded into MMS via the sol-gel route, the resulting composite material (ZnPc-MMS), after annealing at 125 °C, gives a PL peak at 1.78 eV with shoulders at 1.81 and 1.73 eV and a broad peak at 1.98 eV as shown in Figure 2A, spectrum b. The peaks at 1.81, 1.73, and 1.98 eV are also observed for the “blank” MMS host (Figure 2A, spectrum a). We therefore attribute the peak at 1.78 eV to ZnPc molecules embedded in MMS. When the annealing temperature was raised to 225 °C (Figure 2B spectrum b), the ZnPc PL peak shifted to at ca. 1.82 eV (a blue shift of 0.04 eV compared to the sample annealed at 125 °C) and increased in intensity. The other peak observed, at 1.98 eV, can be ascribed to the MMS host (see Figure 2B, spectrum a). It has previously been reported that ZnPc in solution exhibits a PL peak at ca. 1.8 eV.11 Wark and co-workers have also reported that ZnPc incorporated within micelles in surfactant containing MMS hosts (i.e., similar to the sol-gel derived ZnPc-MMS composite) exhibits a PL peak at 1.84 eV.28 Our attribution of the peaks at 1.78 eV (for ZnPc-MMS composite

5082 J. Phys. Chem. B, Vol. 109, No. 11, 2005 annealed at 125 °C) and 1.82 eV (for ZnPc-MMS composite annealed at 225 °C) to ZnPc molecules immobilized in the MMS host is therefore consistent with these previous reports.11,28 Grafted ZnPc/MMS composite (prepared via a postsynthesis grafting route), annealed at 125 °C, exhibits a main PL peak at 1.82 eV and weak shoulders at 1.67 and 1.76 eV (Figure 3A, spectrum a). When the annealing temperature is increased to 225 °C, the grafted ZnPc/MMS composite exhibits a single PL peak at ca. 2.0 eV (Figure 3B, spectrum a). It is noteworthy that, after annealing of the grafted ZnPc/MMS composite at 225 °C, there is no indication of a prominent PL peak at ca. 1.8 eV. A PL peak at ca. 1.8 eV is characteristic of ZnPc molecules in sol-gel derived phthalocyanine-MMS composites.28 The absence of a prominent PL peak at ca. 1.8 eV for the ZnPc/MMS annealed at 225 °C (i.e., after removal of solvent) is an indication that the peak at ca. 1.8 eV is characteristic of ZnPc immobilized within the MMS host in the presence of either micelles or residual solvent molecules. The overall picture that emerges from Figures 2 and 3 is that all C60-containing MMS composites (except for sol-gel derived C60-MMS annealed at 225 °C; Figure 2B, spectrum c) show evidence of the presence of C60 dispersed in monomeric form, i.e., a PL peak at ca. 1.7 eV. Such a peak is characteristic of C60 in solution where the fullerene is known to be monodispersed.17,29,30 For ZnPc-containing composites, all samples (with the exception of grafted ZnPc/MMS annealed at 225 °C; Figure 3B, spectrum a) show clear evidence (a PL peak at ca. 1.8 eV) of the presence of ZnPc monodispersed in the MMS host. 3.2.3 Combination of Fullerene (C60) and Phthalocyanine (ZnPc). Sol-gel derived (C60/ZnPc)-MMS composites (which contain both fullerene and phthalocyanine molecules) annealed at 125 °C exhibit PL peaks at 1.82, 1.78, 1.73, and 1.64 eV (Figure 2A, spectrum d). We tentatively attribute these peaks as follows: C60 (1.73 eV) and ZnPc (1.78 eV). It is likely that the other two peaks (at 1.82 and 1.64 eV) are due to the MMS host, since they are also observed for the “blank” dye-free MMS film (Figure 2A, spectrum a). On increasing the annealing temperature to 225 °C (Figure 2B, spectrum d), the PL spectrum shows evidence of both ZnPc and C60, with a peak at ca. 1.8 eV (ZnPc) and shoulders at 1.9 eV (C60) and 1.64 eV (MMS host). We attribute the PL peak at 1.8 eV to ZnPc, although we cannot rule out contribution of the MMS host to this peak. The peaks at 1.9 and 1.64 eV are due to C60 and the MMS host, respectively. The grafted (C60/ZnPc)/MMS composite annealed at 125 °C (Figure 3A, spectrum c), on the other hand, exhibits a peak (due to C60) at 1.74 eV and two other peaks at 1.67 and 1.82 eV due to ZnPc. The ZnPc peak at 1.82 eV has a particularly high intensity. After annealing at 225 °C (Figure 3B, spectrum c), the main peaks observed are at 1.82 eV (ZnPc) and 1.76 eV (C60). The overall picture that emerges from the PL studies of MMS films containing both C60 and ZnPc is that there is little interaction between the immobilized fullerene and phthalocyanine molecules. The embedded dyes appear to exhibit “independent” PL spectra. This observation is important in the context of preparing broad-band solid-state optical limiters where the dye molecules (fullerene and phthalocyanine) should contribute separately to the total optical limiting capacity. 3.3. Diffuse Reflectance Studies. 3.3.1 Fullerene C60. To characterize the samples further, we used diffuse reflectance spectroscopy to study the nature of the C60 and/or ZnPc embedded in MMS. Diffuse reflectance spectra of the C60MMS composites before and after annealing at various temperatures are shown in Figure 4A. The spectra show the typical

Subbiah and Mokaya

Figure 4. Diffuse reflectance spectra of (A) sol-gel derived C60MMS and (B) grafted C60/MMS composites annealed at (a) room temperature, (b) 125 °C, (c) 175 °C, or (d) 225 °C.

features of C60 embedded in MMS.1-3,17 For the room temperature dried sample (Figure 4A, spectrum a), peaks at ca. 265, 330, and 405 nm and other less intense bands above 600 nm are clearly observed. The appearance of the spike-like peak at ca. 405 nm along with the other features shows that the solgel incorporated C60 is predominantly embedded in a monomeric form.17 As the annealing temperature increases, the intensity of the peaks characteristic of monomeric C60 decreases gradually. The “monomeric” peaks are observed up to an annealing temperature of 175 °C (Figure 4A, spectrum c). After annealing at 225 °C (spectrum d), some of the characteristic peaks for monomeric distribution of C60 are not observed. In particular, the spike-like peak at ca. 405 nm completely disappears, perhaps signifying that the fullerene is no longer in monomeric distribution. In contrast, the diffuse reflectance spectra of C60/MMS composites (C60 grafted on calcined MMS) shown in Figure 4B exhibit peaks characteristic of monomeric dispersion of C60 even after annealing at 225 °C. The spectra in Figure 4A therefore suggest that the fullerene embedded in C60-MMS composites prepared via sol-gel synthesis is monodispersed in the MMS host, perhaps with the aid of either the CTAB molecules or residual solvent. The apparent loss of monomer dispersion, after annealing at 225 °C, may be explained by removal of solvent and/or the formation of C60 clusters.19 However, it is interesting to note that, even though annealing at 225 °C obviously leads to a decrease in monomer dispersion, there is no evidence of extensive C60 agglomeration. Instead, what is obtained is a spectrum with features similar to those observed for a physical mixture of C60 in a matrix such as KBr (except for the broad band at ca. 330 nm).17 This observation is consistent with the PL spectra obtained for C60-MMS after annealing at 225 °C, i.e., a broad band at 1.9 eV representing a blue shift of 0.2 eV compared to the PL spectra after annealing at 125 °C. It is likely that the blue shift is caused by increase in cluster size at the higher annealing temperature of 225 °C.19 In contrast, grafted C60/MMS composites still exhibit all the characteristics of monomer dispersion even after annealing at 225 °C (Figure 4B). The annealing temperature therefore appears to have a lesser effect on the nature of fullerene embedded in grafted C60/MMS composites. This is in agreement with the PL spectra in Figure 3, where the annealing temperature has little effect on the position of the PL peak observed for grafted C60/ MMS composites, i.e., 1.73 and 1.76 eV after annealing at 125 and 225 °C, respectively. 3.3.2. Phthalocyanine (ZnPc). Diffuse reflectance spectra of sol-gel derived ZnPc-MMS composites annealed at various temperatures are shown in Figure 5A. The spectrum of the room temperature dried sample exhibits peaks at 690, 715, and 735 nm. These peaks, which cannot be ascribed to monomeric

C60 and ZnPc Dyes in Mesoporous Silica Films

Figure 5. Diffuse reflectance spectra of (A) sol-gel derived ZnPcMMS and (B) grafted ZnPc/MMS composites annealed at (a) room temperature, (b) 125 °C, (c) 175 °C, or (d) 225 °C. For comparison we have included a UV-visible spectrum (part A, spectrum e) of ZnPc in solution (toluene).

ZnPc,18 gradually disappear as the annealing temperature increases. After annealing at 125 °C, a peak characteristic of monomeric ZnPc, at 680 nm, is observed.18 The intensity and prominence of this peak (slightly shifted to 676 nm) and that of the monomer overtone at ca. 610 nm increase at higher annealing temperature. This indicates that drying the sol-gel derived ZnPc-MMS composites induces monomeric dispersion of the embedded phthalocyanine. After annealing at 175 and 225 °C, the ZnPc is monodispersed with only a weak peak at ca. 645 nm, indicating a limited formation of dimers. Indeed, the spectrum of the ZnPc-MMS composites annealed above 125 °C is very similar to that of ZnPc in solution (as shown by Figure 5A, spectrum e), where the phthalocyanine is known to exist in monomeric form. We note that the spectrum of ZnPcMMS composites shows a strong red shift compared to that of ZnPc in toluene due to the polar environment within the MMS host.18 We also note that the limited effect of annealing temperature on the UV spectra of the sol-gel embedded ZnPcMMS composites is consistent with the corresponding PL spectra, which also show little change in the PL peak position, i.e., a PL peak at 1.78 eV at 125 °C that slightly shifts to 1.82 eV at 225 °C. Postsynthesis grafting of ZnPc on calcined MMS results in ZnPc/MMS composite materials with rather different characteristics compared to sol-gel derived ZnPc-MMS composites. As shown in Figure 5B, the grafted ZnPc/MMS composites show evidence of monomeric dispersion, i.e., peaks at ca. 676 and 620 nm. However, the intensity of these monomer peaks drastically reduces after annealing at 225 °C. It appears therefore that, even though the calcined template-free MMS matrix provides a suitable environment for the ZnPc molecules to be trapped monomerically, annealing at higher temperature (225 °C) leads to some aggregation of the phthalocyanine molecules. This observation is consistent with the differences in the PL spectra of grafted ZnPc/MMS composites annealed at 125 and 225 °C shown in Figure 3. The PL spectrum (see Figure 3) of ZnPc/MMS annealed at 225 °C exhibits a peak at ca. 2.0 eV, while that annealed at 125 °C gives a main peak at 1.82 eV. Increasing the annealing temperature from 125 to 225 °C appears to cause the PL peak to blue shift (by 0.18 eV); evidence from Figure 5B suggests that this blue shift may be due to a decrease in the extent of monomer distribution of the grafted phthalocyanine molecules. 3.3.3. Combination of Fullerene C60 and Phthalocyanine (ZnPc). The diffuse reflectance spectra of MMS films containing both C60 and ZnPc, after annealing at various temperatures, are shown in Figure 6A for sol-gel derived ((C60/ZnPc)-MMS) composites and in Figure 6B for grafted ((C60/ZnPc)/MMS)

J. Phys. Chem. B, Vol. 109, No. 11, 2005 5083

Figure 6. Diffuse reflectance spectra of (A) sol-gel derived (C60/ZnPc)-MMS and (B) grafted (C60/ZnPc)/MMS composites annealed at (a) room temperature, (b) 125 °C, (c) 175 °C, or (d) 225 °C.

Figure 7. Optical limiting behavior of C60-toluene solution (3) and sol-gel derived dye-MMS composites; C60-MMS annealed at 125 °C (b) and ZnPc-MMS annealed at 225 °C (9).

composites. In both cases, the spectra show evidence of monodispersed C60 and ZnPc. Indeed a comparison with Figures 4 and 5 shows that spectra in Figure 6 have peaks attributable to both dyes. For C60, the monomer peak at ca. 405 nm is observed up to an annealing temperature of 175 °C. An annealing temperature of 225 °C must therefore be considered as unsuitable (too high) for monodispersion of fullerene molecules. However, other C60 peaks, at 250 and 330 nm, are observed at all annealing temperatures up to 225 °C. The peaks due to monodispersed ZnPc are much more clearly defined at all temperatures. Annealing at 225 °C does not appear to have any detrimental effect on the monodispersion of the phthalocyanine molecules. 3.4. Optical Limiting Properties. Figure 7 shows the optical limiting behavior of sol-gel derived C60-MMS and ZnPcMMS composites annealed at 125 and 225 °C, respectively. For comparison we have included the optical limiting data for (9.5 mM) solution of C60 in toluene. The dye-MMS composites show optical limiting effects; at low input energy the transmission for both dye composites is linear, but at higher input energy the transmission decreases and a nonlinear relationship exists between the output and input energy. The data in Figure 7 show that, in general, the optical limiting behavior of the composites is comparable to that of C60 in toluene. The optical limiting capability of the dye-MMS composites is consistent with the diffuse reflectance (DR) and photoluminescence (PL) data discussed above. Both the DR and PL studies suggest that the fullerene and phthalocyanine molecules in the dye-MMS composites exit in monomeric form. The lack of significant aggregation for the immobilized dyes implies that the dyes

5084 J. Phys. Chem. B, Vol. 109, No. 11, 2005 should be effective for optical limiting. The expected optical limiting capability of the monomerically immobilized dyes is clearly illustrated in Figure 7. In particular, the C60-MMS composite shows remarkable optical limiting with a limiting threshold below 0.2 J/cm2. The optical limiting behavior of the ZnPc-MMS composite is similar to what has previously been observed for phthalocyanines immobilized in disordered silica xerogels.15c,d We are currently investigating more fully the effect of the synthesis route (sol-gel or grafting) and annealing temperature on the optical limiting properties of the dye-MMS composites. 4. Conclusions Fullerenes and phthalocyanine molecules may be incorporated into ordered mesoporous molecular sieve (MMS) silica films either during formation of the silica (sol-gel route) or via postsynthesis grafting onto calcined silica films. We have investigated the photophysical and optical limiting properties of fullerene- and/or phthalocyanine-containing MMS composite materials and probed the influence of annealing temperature (room temperature to 225 °C) on the nature of the embedded fullerene (C60) and phthalocyanine (zinc phthalocyanine, ZnPc) molecules. The incorporation of the dye molecules does not have any effect on the mesostructural ordering of the mesoporous silica hosts. Photoluminescence (PL) and diffuse reflectance (DR) studies indicate that the embedded C60 and ZnPc molecules exist predominantly as monomers. We observed no evidence of significant molecular aggregation. For C60-containing MMS films, the PL and DR spectra of all composites (except for solgel derived C60-MMS annealed at 225 °C) showed evidence of the presence of C60 dispersed in monomeric form. For ZnPccontaining MMS films, all composites showed evidence of the presence of ZnPc monodispersed in the MMS host. For all composites, annealing temperatures between 125 and 175 °C were found to yield optimal monodispersion of the dye molecules. When both C60 and ZnPc were incorporated into the MMS films, we observed no evidence of interaction between the dyes, i.e., charge-transfer transitions or the formation of fullerene/phthalocyanine charge-transfer complexes. We have demonstrated that the dye-MMS composites exhibit optical limiting properties that are consistent with monomeric dispersion of the immobilized fullerene and phthalocyanine molecules. These observations are important in the context of preparing solid-state optical limiters, based on reverse saturable adsorption, where monomeric dispersion of the dye molecules (fullerene and phthalocyanine) is important. Acknowledgment. We thank the EPSRC for financial support and are grateful to Dr Xue-Zhong Sun and Prof. M. George for assistance with some of the PL and optical limiting measurements. References and Notes (1) Schulz-Ekloff, G.; Wohrle, D.; van Duffel, B.; Schoonheydt, R. A. Microporous Mesoporous Mater. 2002, 51, 91.

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