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Hollow organosilica spheres as hosts: Photoinduced electron transfer between Ru(bpy)32+ and methylviologen
Inorganica Chimica Acta 360 (2007) 1017–1022 www.elsevier.com/locate/ica
Hollow organosilica spheres as hosts: Photoinduced electron transfer between RuðbpyÞ32þ and methylviologen Bele´n Ferrer, Francesc X. Llabre´s i Xamena, Hermenegildo Garcı´a
*
Instituto de Tecnologı´a Quı´mica CSIC-UPV and Departamento de Quı´mica, Universidad Polite´cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain Received 10 July 2006; accepted 28 July 2006 Available online 16 August 2006 Dedicated to Professor Vincenzo Balzani.
Abstract New organosilica hollow spheres containing the RuðbpyÞ3 2þ complex inside the walls have been prepared. This new material exhibits the typical phosphorescence of the photolumophore with a long emission lifetime, while laser flash photolysis measurements has allowed us the detection of photoejected electrons. The addition of MV2+ on the external surface neither quenches nor alters the transient optical spectrum on the submillisecond time scale. However, when an aqueous solution of MV2+ containing triethylamine is photolyzed in the presence of the ruthenium-containing spheres, the build up of a MV+ radical cation and eventually MV0 is observed. This proves that photoejected electrons initially located in the walls of the hollow spheres can be trapped at long time scale by an external quencher, the organosilica walls of the hollow spheres acting as electron relay. 2006 Elsevier B.V. All rights reserved. Keywords: Photoinduced electron transfer; Organosilica spheres; Viologen; RuðbpyÞ3 2þ
1. Introduction Zeolites and other related porous silicates have been widely used as solid hosts for encapsulating organic photo-active molecules [1]. The aim of this research was to control the photochemical properties of the guest through interactions with the host and confinement effects [2]. Thus for instance, organic pyrylium and thiapyryliumtype photosensitizers have been successfully prepared inside the supercages of faujasite zeolites, yielding hybrid organic–inorganic photocatalysts with increased performance and stability as compared to the pure organic dye in solution [3–5]. Besides their role of protecting the embedded guest against the attack of external nucleophiles, zeolites can also assist photoinduced electron transfer processes by providing a highly polar and rigid environment, minimizing back electron transfer and increasing the life*
Corresponding author. Tel.: +34 963877807; fax: +34 963879349. E-mail address: [email protected] (H. Garcı´a).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.07.107
time of charge-separated species [1,6–8]. One of the factors that has been invoked to explain the tendency of the incorporated guests to undergo photoinduced electron transfer processes in zeolites is the active role of the silicate walls acting as electron acceptors or electron donors [1]. Examples of photoinduced electron transfer processes in which the zeolite framework plays an active role are the cases of viologens encapsulated in zeolites [9–11] (in which photochemically excited organic guests abstract an electron from the basic sites of the inorganic host), and anthracene encapsulated in MFI zeolites [12] (in which the aromatic compound transfers an electron to the inorganic host), to name only a few. There are many related amorphous (micro)porous silica that have structural similarities with zeolites, but lack the crystalline framework. In contrast to the wealth of information about the photochemistry of organic guests incorporated in zeolites [1,8], there is a paucity of information about the efficiency of photoinduced electron transfer in these host-guest systems, studies on this area being few
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B. Ferrer et al. / Inorganica Chimica Acta 360 (2007) 1017–1022
and far between. As alternative to zeolites as host, we have now addressed encapsulation of photoactive molecules inside microporous spherical particles of amorphous SiO2. Specifically, we have chosen tris(bipyridyl)ruthenium(II) chloride, (Ru(bpy)3)Cl2, as a case study to illustrate the potential of these spherical silica particles as hosts. The aim of this work is to investigate the interaction between the encapsulated molecule and the (amorphous) organosiliceous walls of the spheres. In particular, we were interested in determining whether or not the protective role of the inorganic matrix enhances stabilizing the lifetime of the excited state of the guest. One important advantage of using monodisperse spheres instead of zeolites (with crystallites of irregular size and shape), is the ease with which they form highly regular and homogenous layers in the micrometer range. This remarkable morphological uniformity of the host particles can be a key factor in the development of sensors or optoelectronic devices using these host-guest materials, for which the use of very uniform and thin layers of the active material is a preferred or even a mandatory requirement. Moreover, the use of monodisperse spheres for encapsulating photoactive materials opens up the possibility of preparing opal-based photonic crystals with remarkable properties including non-linear optical effects [13,14]. RuðbpyÞ3 2þ is a well-known photosensitizer which acts as an electron donor in many types of assemblies or devices. Its photophysical properties have been studied in a large variety of media and solid supports. Some of its derivatives used in combination with TiO2 find application in inorganic dye-sensitized solar cells [15]. Encapsulation of RuðbpyÞ3 2þ within zeolite Y containing TiO2 nanoclusters gives rise to materials that present interesting photocatalytic activity [16]. Herein, we report the preparation of organosilica hollow spheres containing RuðbpyÞ3 2þ . We will study the photophysical properties of this photolumophore incorporated in the hollow spheres and also establish that the walls of hollow amorphous silica can participate as electron acceptor and electron relay.
powder was collected from the dispersion by repeated centrifugation and thoroughly washed with distilled H2O, and it was then dried at room temperature in a dessicator. In order to ensure that no RuðbpyÞ3 2þ was unspecifically adsorbed at the external surface of the organosilica spheres, the material was exhaustively extracted with water using a Soxhlet equipment, until a colorless liquid was obtained. At this point, the ruthenium content at the external surface of the samples was below the detection limit of XPS. Following this method, samples containing two different amounts of Ru(bpy)3Cl2 (namely, 5 and 25 mg) were prepared. The samples will be hereafter referred to as Ru-5 and Ru-25, respectively. For quenching studies, addition of MV2+ to the ruthenium-containing spheres was accomplished by incipient wetness impregnation at room temperature of 0.4 mmol g1 of MVCl2 salt dissolved in 300 ll of H2O, followed by drying at room temperature under vacuum. Scanning electron micrographs (SEM) were obtained with a JEOL JSM-5410 operated at 20 kV. Diffuse reflectance UV–Vis spectra were recorded with a Varian Cary 5 G spectrophotometer using a praying mantis attachment and BaSO4 as a standard. Steady-state fluorescence measurements were performed on an Edinburgh FS900CDT spectrophotometer having a solid sample accessory and Czerny Turner monochromators. Diffuse reflectance laser flash photolysis measurements were made using the third (355 nm) harmonic pulse from a Surelite Nd:YAG laser (<10 ns pulse width; <20 mJ pulse1) as excitation source. Signals from the photomultiplier tube were captured and digitalized by a Tektronix 2440 transient digitizer and transferred to a PC programmed in the LabView environment. Details of similar time-resolved diffuse reflectance systems have been described elsewhere [18]. The powders were placed in a 3 · 7 mm2 Suprasil quartz cuvette capped with septa and the solids were purged with N2 before recording the spectra. Steady-state irradiation was carried out using a 355 nm laser beam as light source and the formation of viologen radical cation followed by transmission optical spectroscopy. 3. Results and discussion
2. Experimental The method for the preparation of RuðbpyÞ3 2þ -containing organosilica hollow spheres was adapted from the twostep method proposed by Hah et al. [17], which was modified to include the ruthenium complex in the synthesis route. Briefly, the desired amount of Ru(bpy)3Cl2 was dissolved in 75 ml of an aqueous HNO3 0.0066 M solution and held at 60 C. To this solution, 866 ll (4.5 mmoles) of phenyl trimethoxysilane (PTMS) (from Aldrich) was added under gentle magnetic stirring (300 rpm) and allowed to hydrolyze for 2 min. Finally, 20 ml of a 1.44 M NH4OH aqueous solution was added to assist condensation of PTMS, and the system was kept at the reaction temperature for 1 h. The resulting red–orange
The morphology of the ruthenium-containing samples was examined by SEM. Fig. 1 shows a SEM micrograph representative of sample Ru-5 (analogous results were obtained for sample Ru-25). This sample is characterized by well formed, highly monodispersed spherical particles of about 930 nm in diameter (less than ca. 5% of dispersion), although a small fraction of spheres with significantly smaller size can also be appreciated. In the inset of Fig. 1, a broken sphere with a hole is shown, in which the hollow structure of the spheres is evidenced. Spectroscopic characterization of the ruthenium samples by diffuse reflectance UV–Vis spectroscopy showed a structured band peaking at 259, 265 and 271 nm, which can be assigned to the phenyl groups of the organosilica
B. Ferrer et al. / Inorganica Chimica Acta 360 (2007) 1017–1022
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Table 1 Emission lifetimes of the samples studied obtained from the best fit of the emission decay at 600 nm sem (ls) RuðbpyÞ3 2þ @Ya Ru-25 (N2) Ru-25 (O2) Ru-25/MV0.4 a
Fig. 1. SEM micrograph of sample Ru-5. The inset shows an enlarged view of a broken particle, in which the hollow structure of the material is evidenced.
F(R) (a.u.)
I (a.u.)
spheres, and a broad absorption at 466 nm with a shoulder at 433 nm due to the characteristic metal-to-ligand charge transfer band of the RuðbpyÞ3 2þ complex (Fig. 2). Upon excitation at the UV–Vis absorption maximum (466 nm), the characteristic phosphorescence emission of the triplet excited state of the RuðbpyÞ3 2þ complex is observed at kem = 602 nm (see inset of Fig. 2). In view of the reported long-lifetime of RuðbpyÞ3 2þ complex encapsulated within zeolite Y [16], time-resolved emission studies of our hollow spheres containing ruthenium have been performed by using nanosecond 355 nm laser excitation. The phosphorescence half life of the excited state of RuðbpyÞ3 2þ complex inside the hollow spheres, obtained from the fitting of the emission decay recorded at 600 nm under N2 atmosphere to a monoexponential kinetics, has been found to be 1.35 ls (Table 1). If we compare this value with that obtained for the same polypyridine complex encapsulated within zeolite Y (two first-
0 550
600 650 700 Wavelength (nm)
750
0 300
400
500 600 Wavelength (nm)
700
800
Fig. 2. Diffuse reflectance UV–Vis spectrum (plotted as the Kubelka– Munk function F(R)) of the sample Ru-25. The inset shows the emission spectrum of this sample recorded under N2 atmosphere upon 466 nm excitation.
s1 = 0.88 (60%)/s2 = 0.37 (40%) 1.35 1.03 1.35
Data taken from Ref. [16].
order decays s1 = 0.88 ls and s2 = 0.37 ls, as reported in Ref. [16]), we can observe a significant enhancement of the emission lifetime of the excited state of RuðbpyÞ3 2þ complex when it is incorporated inside the hollow spheres. The monoexponential fitting suggests a uniform location of RuðbpyÞ3 2þ through the spheres. In the zeolite Y, the RuðbpyÞ3 2þ complex is encapsulated inside the highly polar cages and there is empty space or coadsorbed water inside the cavities. In the hollow spheres, the RuðbpyÞ3 2þ complex could be incorporated in the hydrophobic walls or in the core of the spheres but in both cases there should be more restrictions for the movement than when it is encapsulated inside the cavities of zeolite Y. The hydrophobicity of the inner walls of the spheres is believed to be the main driving force to impart the spherical shape to the particles when they are formed in water. Then, this hydrophobicity and/or the rigidity of the environment can explain the significant enhancement of the emission lifetime of the RuðbpyÞ3 2þ complex incorporated in the hollow spheres. The emission of the excited state of RuðbpyÞ3 2þ complex incorporated in the hollow spheres is quenched upon purging with O2 (see Table 1). The 24% decrease in the emission lifetime indicates that a dynamic quenching is taking place in the system under O2 atmosphere. The relatively minor effect of the O2 and the relationship between the dynamic quenching and the diffusion suggest that the RuðbpyÞ3 2þ complex is located in the walls or inner core of the hollow spheres. Laser flash photolysis measurements of the organosilica hollow spheres containing the RuðbpyÞ3 2þ complex have also been performed. Laser excitation (355 nm) of the sample Ru-25 gives rise to the generation of a long-lived transient species (s > 3 ms) characterized by a broad absorption band from 600 to 750 nm (Fig. 3). Moreover, the diffuse reflectance transient absorption spectrum recorded 350 ls after the laser excitation shows a broad negative band at kmax = 450 nm due to the transient bleaching of the ground state. Exposure of the Ru-25 sample to the laser flash for hours did not lead to observable changes in the optical spectrum of the solid, indicating that the photochemical excitation does not result in the formation of photoproducts and only transient species are involved. The transient absorption spectrum shown in Fig. 3 is unique and different than those reported in the literature for the triplet excited state of the RuðbpyÞ3 2þ complex and for the RuðbpyÞ3 2þ transient species encapsulated in zeolite Y [19,20].
Δ O. D. (a.u.)
B. Ferrer et al. / Inorganica Chimica Acta 360 (2007) 1017–1022
Δ O.D. (a.u.)
1020
0
h
a 0
b
e0
200 time μs
400
e-
e-
MV2+ ~ [Ru(bpy)32+] ~ [Ru(bp 300
400
500
600
700
Wavelength (nm)
Scheme 1. Pictorial representation of the location of RuðbpyÞ3 2þ complex in the hollow spheres and of the trapping of photoejected electrons in the walls of the hollow spheres by MV2+ as electron acceptor.
Fig. 3. Transient diffuse reflectance spectra of the sample Ru-25 recorded 22 (d) and 350 (s) ls after 355 nm laser excitation under N2 atmosphere. The inset shows the signals monitored at 600 nm after purging with N2 (a) and CH2Cl2 (b).
In order to gain a deeper knowledge on the nature of the long-lived transient species generated after the excitation of the hollow spheres containing the RuðbpyÞ3 2þ complex, we have performed a quenching experiment using CH2Cl2, which is a well known quencher of electrons. The longlived transient species is quenched by CH2Cl2 (see inset of Fig. 3). Based on this observation, we propose that the long-lived transient species generated after the excitation of the organosilica hollow spheres containing the RuðbpyÞ3 2þ complex are ejected electrons which would be located on the walls of the hollow spheres. This assignment is in agreement with the absorption spectrum of electrons in silicas and aluminosilicates reported in the literature [21]. The generation of transient electrons provides firm support for the occurrence of photoinduced electron transfer between excited RuðbpyÞ3 2þ complex as donor and the walls of the spheres as acceptors. Upon laser excitation, the triplet excited state of the RuðbpyÞ3 2þ complex is formed and apparently a fraction of them are able to donate one electron to the walls of the sphere, forming RuðbpyÞ3 3þ in the process. This indicates that the walls of the hollow spheres can act as electron acceptors (Scheme 1). However, in contrast to what has been observed here and in spite of the well known role of zeolite frameworks as electron acceptors or donors [1], when the RuðbpyÞ3 2þ complex is encapsulated inside the cavities of zeolite Y, no transient electrons are detected (see Fig. 4) and the whole transient spectrum is dominated by the phosphorescence. Maybe the explanation to this fact is, in the case of the hollow spheres, the intimate contact with the hydrophobic walls or its close proximity to them. Also the presence of organic phenyl groups in the walls may play a role. Taking into account a previous work about the photoluminescence quenching of RuðbpyÞ3 2þ excited states by MV2+ incorporated within zeolites in which it has been established that the occurrence of static or dynamic processes is dependent on the extra- or intra-zeolitic location
O.D. (a.u.)
a 0
b
0
200
400
time ( s)
Fig. 4. Transient decay of the signal monitored at 600 nm for the samples Ru-25 (a) and ½RuðbpyÞ3 2þ @Y (b) recorded after 355 nm laser excitation under N2 atmosphere.
of the RuðbpyÞ3 2þ complex [22], it was of interest to study the quenching with MV2+ to address the location of the polypyridine complex in the hollow spheres. Upon addition of MV2+ (0.4 mmol g1) to the hollow spheres containing RuðbpyÞ3 2þ , no changes were observed either on the phosphorescence half life of the excited state of the RuðbpyÞ3 2þ complex inside the organosilica hollow spheres (see Table 1) or on the diffuse reflectance transient absorption spectrum (see Fig. 5). Time-resolved diffuse reflectance laser flash photolysis measurements of the organosilica hollow spheres containing the RuðbpyÞ3 2þ complex, Ru-25, after the addition of MV2+ allowed us to record a transient absorption spectrum characterized by a broad absorption band from 500 to 800 nm. This transient absorption spectrum is very similar to that obtained in the absence of MV2+ (see Fig. 3) that was assigned to photoejected electrons. Then, we can conclude that on the microsecond time scale the photoinduced electron transfer between the RuðbpyÞ3 2þ complex inside the organosilica hollow spheres and the MV2+ on the external surface can not be detected. This lack of quenching supports the location of the RuðbpyÞ3 2þ complex into the walls
Δ O.D. (a.u.)
B. Ferrer et al. / Inorganica Chimica Acta 360 (2007) 1017–1022
0
400
500
600
700
800
Wavelength (nm) Fig. 5. Transient diffuse reflectance spectrum of the sample Ru-25 upon addition of MV2+ recorded 350 ls after 355 nm laser excitation (20 mJ pulse1) under N2 atmosphere.
Absorbance (a.u.)
or inside the core of the organosilica hollow spheres. The inefficient oxygen quenching of the triplet excited state emission of the RuðbpyÞ3 2þ complex incorporated in the organosilica particles is also compatible with the RuðbpyÞ3 2þ complex being located in the walls or at the inner core of the hollow spheres. To assess whether or not these photoejected electrons can reach and be captured at the exterior of the particle, we have performed the steady-state irradiation of a dispersion of the material Ru-25 in an aqueous solution of MV2+ (2 · 102 M) containing an excess of triethylamine (5 · 102 M) as sacrificial electron donor under N2 purging. Note that the concentration of MV2+ is lower than that used for the time resolved quenching experiment. After 15 min of irradiation, the solution becomes blue and the UV–Vis spectrum of the irradiated system (curve b of Fig. 6) shows the presence of the methylviologen radical cation MV+. This provides a firm evidence that the photogenerated electrons, initially in the walls of the spheres, can be trapped by exter-
b c a
0 300
400
500
600
700
800
Wavelength (nm) Fig. 6. UV–Vis absorption spectra of an aqueous solution of MVCl2 0.03 M containing an excess of triethylamine and 50 mg of the material Ru-25 before (a) and after 355 nm laser irradiation for 15 (b) and 30 (c) minutes under N2 atmosphere.
1021
nal electron acceptors. In this case, the addition of triethylamine favors formation of methylviologen radical cation because the aliphatic amine acts as a sacrificial electron donor giving one electron to the RuðbpyÞ3 3þ , and thus recovering the initial RuðbpyÞ3 2þ . Thus, the presence of triethylamine thwarts deactivation through back electron transfer from the walls or MV+ to RuðbpyÞ3 3þ . As it can be deduced from the absorption spectrum recorded after 30 min irradiation (curve c of Fig. 6), further irradiation of the system causes the subsequent reduction of the MV+ to the MV0 characterized by an absorption peak at 390 nm. In summary, we have prepared new organosilica hollow spheres containing the [RuðbpyÞ3 2þ ] complex which exhibit the typical phosphorescence of the photolumophore with a long emission lifetime.The Laser flash photolysis of the ruthenium complex inside the walls has allowed us the detection of photoejected electrons. These photoejected electrons in the walls of the spheres can be trapped by external MV2+ in deaerated aqueous solutions, thus indicating the active role of the organosilica walls of the hollow spheres as electron relay. Acknowledgements Financial support by the Spanish DGES (CTQ 20066828) is gratefully acknowledged. B.F. (Juan de la Cierva contract) and F.X.L.X. (Ramo´n y Cajal contract) thank the Spanish Ministry of Science and Education for two research associate contracts. References [1] (a) H. Garcı´a, H.D. Roth, Chem. Rev. 102 (2002) 3947; (b) S. Hashimoto, J. Pohotochem. Photobiol. C, Photochem. Rev. 4 (2003) 19; (c) J.K. Thomas, Chem. Rev. 93 (1993) 301. [2] F. Marquez, H. Garcia, E. Palomares, L. Fernandez, A. Corma J. Am. Chem. Soc. 122 (2000) 6520. [3] M. Alvaro, E. Carbonell, V. Fornes, H. Garcia, New J. Chem. 28 (2004) 631. [4] M. Alvaro, E. Carbonell, H. Garcia, C. Lamaza, M. Narayana Pillai, Photochem. Photobiol. Sci. 3 (2004) 189. [5] A. Sanjuan, M. Alvaro, G. Aguirre, H. Garcia, J.C. Scaiano, J. Am. Chem. Soc. 120 (1998) 7351. [6] K.B. Yoon, Chem. Rev. 93 (1993) 321. [7] K. Kalyansundaram, Photochemistry in Microheterogeneous Systems, Plenum Press, New York, 1987. [8] J.C. Scaiano, H. Garcı´a, Acc. Chem. Res. 32 (1999) 783. [9] M. Alvaro, H. Garcia, S. Garcia, F. Marquez, J.C. Scaiano, J. Phys. Chem. 101 (1997) 3043. [10] M. Alvaro, A. Corma, B. Ferrer, H. Garcia, E. Palomares, Phys. Chem. Chem. Phys. 6 (2004) 1345. [11] K.T. Ranjit, L. Kevan, J. Phys. Chem. B. 106 (2002) 1104. [12] H. Vezin, A. Moissette, C. Bre´mard, Angew. Chem. 115 (2003) 5745. [13] W. Wang, B. Gu, L. Liang, W.A. Hamilton, J. Phys. Chem. B. 107 (2003) 12113. [14] Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 12 (2000) 693. [15] M. Gra¨tzel, A. Hagfeldt, Chem. Rev. 95 (1995) 49. [16] G. Cosa, M.N. Chretien, M.S. Galletero, V. Fornes, H. Garcia, J.C. Scaiano, J. Phys. Chem. B. 106 (2002) 2460.
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[17] H.J. Hah, J.S. Kim, B.J. Jeon, S.M. Koo, Y.E. Lee, Chem. Commun. (2003) 1712. [18] F. Wilkinson, G. Kelly, in: J.C. Scaiano (Ed.), Handbook of Organic Photochemistry, CRC Press, Boca Raton, FL, 1989, p. 293. [19] (a) W. Turbeville, D.S. Robins, P.K. Dutta, J. Phys. Chem. B. 96 (1992) 5024;
(b) M. Alvaro, B. Ferrer, E. Palomares, V. Balzani, A. Credi, M. Venturi, J.F. Stoddart, S. Wenger, J. Phys. Chem. B. 107 (2003) 141319. [20] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. Von Zelewsky: Coord. Chem. Rev. 84 (1988) 277. [21] S.L. Murov, Handbook of Photochemistry, 1993. [22] Y.I. Kim, T.E. Mallouk, J. Phys. Chem. 96 (1992) 2879.
Hollow organosilica spheres as hosts: Photoinduced electron transfer between RuðbpyÞ32þ and methylviologen Bele´n Ferrer, Francesc X. Llabre´s i Xamena, Hermenegildo Garcı´a
*
Instituto de Tecnologı´a Quı´mica CSIC-UPV and Departamento de Quı´mica, Universidad Polite´cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain Received 10 July 2006; accepted 28 July 2006 Available online 16 August 2006 Dedicated to Professor Vincenzo Balzani.
Abstract New organosilica hollow spheres containing the RuðbpyÞ3 2þ complex inside the walls have been prepared. This new material exhibits the typical phosphorescence of the photolumophore with a long emission lifetime, while laser flash photolysis measurements has allowed us the detection of photoejected electrons. The addition of MV2+ on the external surface neither quenches nor alters the transient optical spectrum on the submillisecond time scale. However, when an aqueous solution of MV2+ containing triethylamine is photolyzed in the presence of the ruthenium-containing spheres, the build up of a MV+ radical cation and eventually MV0 is observed. This proves that photoejected electrons initially located in the walls of the hollow spheres can be trapped at long time scale by an external quencher, the organosilica walls of the hollow spheres acting as electron relay. 2006 Elsevier B.V. All rights reserved. Keywords: Photoinduced electron transfer; Organosilica spheres; Viologen; RuðbpyÞ3 2þ
1. Introduction Zeolites and other related porous silicates have been widely used as solid hosts for encapsulating organic photo-active molecules [1]. The aim of this research was to control the photochemical properties of the guest through interactions with the host and confinement effects [2]. Thus for instance, organic pyrylium and thiapyryliumtype photosensitizers have been successfully prepared inside the supercages of faujasite zeolites, yielding hybrid organic–inorganic photocatalysts with increased performance and stability as compared to the pure organic dye in solution [3–5]. Besides their role of protecting the embedded guest against the attack of external nucleophiles, zeolites can also assist photoinduced electron transfer processes by providing a highly polar and rigid environment, minimizing back electron transfer and increasing the life*
Corresponding author. Tel.: +34 963877807; fax: +34 963879349. E-mail address: [email protected] (H. Garcı´a).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.07.107
time of charge-separated species [1,6–8]. One of the factors that has been invoked to explain the tendency of the incorporated guests to undergo photoinduced electron transfer processes in zeolites is the active role of the silicate walls acting as electron acceptors or electron donors [1]. Examples of photoinduced electron transfer processes in which the zeolite framework plays an active role are the cases of viologens encapsulated in zeolites [9–11] (in which photochemically excited organic guests abstract an electron from the basic sites of the inorganic host), and anthracene encapsulated in MFI zeolites [12] (in which the aromatic compound transfers an electron to the inorganic host), to name only a few. There are many related amorphous (micro)porous silica that have structural similarities with zeolites, but lack the crystalline framework. In contrast to the wealth of information about the photochemistry of organic guests incorporated in zeolites [1,8], there is a paucity of information about the efficiency of photoinduced electron transfer in these host-guest systems, studies on this area being few
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B. Ferrer et al. / Inorganica Chimica Acta 360 (2007) 1017–1022
and far between. As alternative to zeolites as host, we have now addressed encapsulation of photoactive molecules inside microporous spherical particles of amorphous SiO2. Specifically, we have chosen tris(bipyridyl)ruthenium(II) chloride, (Ru(bpy)3)Cl2, as a case study to illustrate the potential of these spherical silica particles as hosts. The aim of this work is to investigate the interaction between the encapsulated molecule and the (amorphous) organosiliceous walls of the spheres. In particular, we were interested in determining whether or not the protective role of the inorganic matrix enhances stabilizing the lifetime of the excited state of the guest. One important advantage of using monodisperse spheres instead of zeolites (with crystallites of irregular size and shape), is the ease with which they form highly regular and homogenous layers in the micrometer range. This remarkable morphological uniformity of the host particles can be a key factor in the development of sensors or optoelectronic devices using these host-guest materials, for which the use of very uniform and thin layers of the active material is a preferred or even a mandatory requirement. Moreover, the use of monodisperse spheres for encapsulating photoactive materials opens up the possibility of preparing opal-based photonic crystals with remarkable properties including non-linear optical effects [13,14]. RuðbpyÞ3 2þ is a well-known photosensitizer which acts as an electron donor in many types of assemblies or devices. Its photophysical properties have been studied in a large variety of media and solid supports. Some of its derivatives used in combination with TiO2 find application in inorganic dye-sensitized solar cells [15]. Encapsulation of RuðbpyÞ3 2þ within zeolite Y containing TiO2 nanoclusters gives rise to materials that present interesting photocatalytic activity [16]. Herein, we report the preparation of organosilica hollow spheres containing RuðbpyÞ3 2þ . We will study the photophysical properties of this photolumophore incorporated in the hollow spheres and also establish that the walls of hollow amorphous silica can participate as electron acceptor and electron relay.
powder was collected from the dispersion by repeated centrifugation and thoroughly washed with distilled H2O, and it was then dried at room temperature in a dessicator. In order to ensure that no RuðbpyÞ3 2þ was unspecifically adsorbed at the external surface of the organosilica spheres, the material was exhaustively extracted with water using a Soxhlet equipment, until a colorless liquid was obtained. At this point, the ruthenium content at the external surface of the samples was below the detection limit of XPS. Following this method, samples containing two different amounts of Ru(bpy)3Cl2 (namely, 5 and 25 mg) were prepared. The samples will be hereafter referred to as Ru-5 and Ru-25, respectively. For quenching studies, addition of MV2+ to the ruthenium-containing spheres was accomplished by incipient wetness impregnation at room temperature of 0.4 mmol g1 of MVCl2 salt dissolved in 300 ll of H2O, followed by drying at room temperature under vacuum. Scanning electron micrographs (SEM) were obtained with a JEOL JSM-5410 operated at 20 kV. Diffuse reflectance UV–Vis spectra were recorded with a Varian Cary 5 G spectrophotometer using a praying mantis attachment and BaSO4 as a standard. Steady-state fluorescence measurements were performed on an Edinburgh FS900CDT spectrophotometer having a solid sample accessory and Czerny Turner monochromators. Diffuse reflectance laser flash photolysis measurements were made using the third (355 nm) harmonic pulse from a Surelite Nd:YAG laser (<10 ns pulse width; <20 mJ pulse1) as excitation source. Signals from the photomultiplier tube were captured and digitalized by a Tektronix 2440 transient digitizer and transferred to a PC programmed in the LabView environment. Details of similar time-resolved diffuse reflectance systems have been described elsewhere [18]. The powders were placed in a 3 · 7 mm2 Suprasil quartz cuvette capped with septa and the solids were purged with N2 before recording the spectra. Steady-state irradiation was carried out using a 355 nm laser beam as light source and the formation of viologen radical cation followed by transmission optical spectroscopy. 3. Results and discussion
2. Experimental The method for the preparation of RuðbpyÞ3 2þ -containing organosilica hollow spheres was adapted from the twostep method proposed by Hah et al. [17], which was modified to include the ruthenium complex in the synthesis route. Briefly, the desired amount of Ru(bpy)3Cl2 was dissolved in 75 ml of an aqueous HNO3 0.0066 M solution and held at 60 C. To this solution, 866 ll (4.5 mmoles) of phenyl trimethoxysilane (PTMS) (from Aldrich) was added under gentle magnetic stirring (300 rpm) and allowed to hydrolyze for 2 min. Finally, 20 ml of a 1.44 M NH4OH aqueous solution was added to assist condensation of PTMS, and the system was kept at the reaction temperature for 1 h. The resulting red–orange
The morphology of the ruthenium-containing samples was examined by SEM. Fig. 1 shows a SEM micrograph representative of sample Ru-5 (analogous results were obtained for sample Ru-25). This sample is characterized by well formed, highly monodispersed spherical particles of about 930 nm in diameter (less than ca. 5% of dispersion), although a small fraction of spheres with significantly smaller size can also be appreciated. In the inset of Fig. 1, a broken sphere with a hole is shown, in which the hollow structure of the spheres is evidenced. Spectroscopic characterization of the ruthenium samples by diffuse reflectance UV–Vis spectroscopy showed a structured band peaking at 259, 265 and 271 nm, which can be assigned to the phenyl groups of the organosilica
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Table 1 Emission lifetimes of the samples studied obtained from the best fit of the emission decay at 600 nm sem (ls) RuðbpyÞ3 2þ @Ya Ru-25 (N2) Ru-25 (O2) Ru-25/MV0.4 a
Fig. 1. SEM micrograph of sample Ru-5. The inset shows an enlarged view of a broken particle, in which the hollow structure of the material is evidenced.
F(R) (a.u.)
I (a.u.)
spheres, and a broad absorption at 466 nm with a shoulder at 433 nm due to the characteristic metal-to-ligand charge transfer band of the RuðbpyÞ3 2þ complex (Fig. 2). Upon excitation at the UV–Vis absorption maximum (466 nm), the characteristic phosphorescence emission of the triplet excited state of the RuðbpyÞ3 2þ complex is observed at kem = 602 nm (see inset of Fig. 2). In view of the reported long-lifetime of RuðbpyÞ3 2þ complex encapsulated within zeolite Y [16], time-resolved emission studies of our hollow spheres containing ruthenium have been performed by using nanosecond 355 nm laser excitation. The phosphorescence half life of the excited state of RuðbpyÞ3 2þ complex inside the hollow spheres, obtained from the fitting of the emission decay recorded at 600 nm under N2 atmosphere to a monoexponential kinetics, has been found to be 1.35 ls (Table 1). If we compare this value with that obtained for the same polypyridine complex encapsulated within zeolite Y (two first-
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Fig. 2. Diffuse reflectance UV–Vis spectrum (plotted as the Kubelka– Munk function F(R)) of the sample Ru-25. The inset shows the emission spectrum of this sample recorded under N2 atmosphere upon 466 nm excitation.
s1 = 0.88 (60%)/s2 = 0.37 (40%) 1.35 1.03 1.35
Data taken from Ref. [16].
order decays s1 = 0.88 ls and s2 = 0.37 ls, as reported in Ref. [16]), we can observe a significant enhancement of the emission lifetime of the excited state of RuðbpyÞ3 2þ complex when it is incorporated inside the hollow spheres. The monoexponential fitting suggests a uniform location of RuðbpyÞ3 2þ through the spheres. In the zeolite Y, the RuðbpyÞ3 2þ complex is encapsulated inside the highly polar cages and there is empty space or coadsorbed water inside the cavities. In the hollow spheres, the RuðbpyÞ3 2þ complex could be incorporated in the hydrophobic walls or in the core of the spheres but in both cases there should be more restrictions for the movement than when it is encapsulated inside the cavities of zeolite Y. The hydrophobicity of the inner walls of the spheres is believed to be the main driving force to impart the spherical shape to the particles when they are formed in water. Then, this hydrophobicity and/or the rigidity of the environment can explain the significant enhancement of the emission lifetime of the RuðbpyÞ3 2þ complex incorporated in the hollow spheres. The emission of the excited state of RuðbpyÞ3 2þ complex incorporated in the hollow spheres is quenched upon purging with O2 (see Table 1). The 24% decrease in the emission lifetime indicates that a dynamic quenching is taking place in the system under O2 atmosphere. The relatively minor effect of the O2 and the relationship between the dynamic quenching and the diffusion suggest that the RuðbpyÞ3 2þ complex is located in the walls or inner core of the hollow spheres. Laser flash photolysis measurements of the organosilica hollow spheres containing the RuðbpyÞ3 2þ complex have also been performed. Laser excitation (355 nm) of the sample Ru-25 gives rise to the generation of a long-lived transient species (s > 3 ms) characterized by a broad absorption band from 600 to 750 nm (Fig. 3). Moreover, the diffuse reflectance transient absorption spectrum recorded 350 ls after the laser excitation shows a broad negative band at kmax = 450 nm due to the transient bleaching of the ground state. Exposure of the Ru-25 sample to the laser flash for hours did not lead to observable changes in the optical spectrum of the solid, indicating that the photochemical excitation does not result in the formation of photoproducts and only transient species are involved. The transient absorption spectrum shown in Fig. 3 is unique and different than those reported in the literature for the triplet excited state of the RuðbpyÞ3 2þ complex and for the RuðbpyÞ3 2þ transient species encapsulated in zeolite Y [19,20].
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Δ O.D. (a.u.)
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Scheme 1. Pictorial representation of the location of RuðbpyÞ3 2þ complex in the hollow spheres and of the trapping of photoejected electrons in the walls of the hollow spheres by MV2+ as electron acceptor.
Fig. 3. Transient diffuse reflectance spectra of the sample Ru-25 recorded 22 (d) and 350 (s) ls after 355 nm laser excitation under N2 atmosphere. The inset shows the signals monitored at 600 nm after purging with N2 (a) and CH2Cl2 (b).
In order to gain a deeper knowledge on the nature of the long-lived transient species generated after the excitation of the hollow spheres containing the RuðbpyÞ3 2þ complex, we have performed a quenching experiment using CH2Cl2, which is a well known quencher of electrons. The longlived transient species is quenched by CH2Cl2 (see inset of Fig. 3). Based on this observation, we propose that the long-lived transient species generated after the excitation of the organosilica hollow spheres containing the RuðbpyÞ3 2þ complex are ejected electrons which would be located on the walls of the hollow spheres. This assignment is in agreement with the absorption spectrum of electrons in silicas and aluminosilicates reported in the literature [21]. The generation of transient electrons provides firm support for the occurrence of photoinduced electron transfer between excited RuðbpyÞ3 2þ complex as donor and the walls of the spheres as acceptors. Upon laser excitation, the triplet excited state of the RuðbpyÞ3 2þ complex is formed and apparently a fraction of them are able to donate one electron to the walls of the sphere, forming RuðbpyÞ3 3þ in the process. This indicates that the walls of the hollow spheres can act as electron acceptors (Scheme 1). However, in contrast to what has been observed here and in spite of the well known role of zeolite frameworks as electron acceptors or donors [1], when the RuðbpyÞ3 2þ complex is encapsulated inside the cavities of zeolite Y, no transient electrons are detected (see Fig. 4) and the whole transient spectrum is dominated by the phosphorescence. Maybe the explanation to this fact is, in the case of the hollow spheres, the intimate contact with the hydrophobic walls or its close proximity to them. Also the presence of organic phenyl groups in the walls may play a role. Taking into account a previous work about the photoluminescence quenching of RuðbpyÞ3 2þ excited states by MV2+ incorporated within zeolites in which it has been established that the occurrence of static or dynamic processes is dependent on the extra- or intra-zeolitic location
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Fig. 4. Transient decay of the signal monitored at 600 nm for the samples Ru-25 (a) and ½RuðbpyÞ3 2þ @Y (b) recorded after 355 nm laser excitation under N2 atmosphere.
of the RuðbpyÞ3 2þ complex [22], it was of interest to study the quenching with MV2+ to address the location of the polypyridine complex in the hollow spheres. Upon addition of MV2+ (0.4 mmol g1) to the hollow spheres containing RuðbpyÞ3 2þ , no changes were observed either on the phosphorescence half life of the excited state of the RuðbpyÞ3 2þ complex inside the organosilica hollow spheres (see Table 1) or on the diffuse reflectance transient absorption spectrum (see Fig. 5). Time-resolved diffuse reflectance laser flash photolysis measurements of the organosilica hollow spheres containing the RuðbpyÞ3 2þ complex, Ru-25, after the addition of MV2+ allowed us to record a transient absorption spectrum characterized by a broad absorption band from 500 to 800 nm. This transient absorption spectrum is very similar to that obtained in the absence of MV2+ (see Fig. 3) that was assigned to photoejected electrons. Then, we can conclude that on the microsecond time scale the photoinduced electron transfer between the RuðbpyÞ3 2þ complex inside the organosilica hollow spheres and the MV2+ on the external surface can not be detected. This lack of quenching supports the location of the RuðbpyÞ3 2þ complex into the walls
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0
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Wavelength (nm) Fig. 5. Transient diffuse reflectance spectrum of the sample Ru-25 upon addition of MV2+ recorded 350 ls after 355 nm laser excitation (20 mJ pulse1) under N2 atmosphere.
Absorbance (a.u.)
or inside the core of the organosilica hollow spheres. The inefficient oxygen quenching of the triplet excited state emission of the RuðbpyÞ3 2þ complex incorporated in the organosilica particles is also compatible with the RuðbpyÞ3 2þ complex being located in the walls or at the inner core of the hollow spheres. To assess whether or not these photoejected electrons can reach and be captured at the exterior of the particle, we have performed the steady-state irradiation of a dispersion of the material Ru-25 in an aqueous solution of MV2+ (2 · 102 M) containing an excess of triethylamine (5 · 102 M) as sacrificial electron donor under N2 purging. Note that the concentration of MV2+ is lower than that used for the time resolved quenching experiment. After 15 min of irradiation, the solution becomes blue and the UV–Vis spectrum of the irradiated system (curve b of Fig. 6) shows the presence of the methylviologen radical cation MV+. This provides a firm evidence that the photogenerated electrons, initially in the walls of the spheres, can be trapped by exter-
b c a
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Wavelength (nm) Fig. 6. UV–Vis absorption spectra of an aqueous solution of MVCl2 0.03 M containing an excess of triethylamine and 50 mg of the material Ru-25 before (a) and after 355 nm laser irradiation for 15 (b) and 30 (c) minutes under N2 atmosphere.
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nal electron acceptors. In this case, the addition of triethylamine favors formation of methylviologen radical cation because the aliphatic amine acts as a sacrificial electron donor giving one electron to the RuðbpyÞ3 3þ , and thus recovering the initial RuðbpyÞ3 2þ . Thus, the presence of triethylamine thwarts deactivation through back electron transfer from the walls or MV+ to RuðbpyÞ3 3þ . As it can be deduced from the absorption spectrum recorded after 30 min irradiation (curve c of Fig. 6), further irradiation of the system causes the subsequent reduction of the MV+ to the MV0 characterized by an absorption peak at 390 nm. In summary, we have prepared new organosilica hollow spheres containing the [RuðbpyÞ3 2þ ] complex which exhibit the typical phosphorescence of the photolumophore with a long emission lifetime.The Laser flash photolysis of the ruthenium complex inside the walls has allowed us the detection of photoejected electrons. These photoejected electrons in the walls of the spheres can be trapped by external MV2+ in deaerated aqueous solutions, thus indicating the active role of the organosilica walls of the hollow spheres as electron relay. Acknowledgements Financial support by the Spanish DGES (CTQ 20066828) is gratefully acknowledged. B.F. (Juan de la Cierva contract) and F.X.L.X. (Ramo´n y Cajal contract) thank the Spanish Ministry of Science and Education for two research associate contracts. References [1] (a) H. Garcı´a, H.D. Roth, Chem. Rev. 102 (2002) 3947; (b) S. Hashimoto, J. Pohotochem. Photobiol. C, Photochem. Rev. 4 (2003) 19; (c) J.K. Thomas, Chem. Rev. 93 (1993) 301. [2] F. Marquez, H. Garcia, E. Palomares, L. Fernandez, A. Corma J. Am. Chem. Soc. 122 (2000) 6520. [3] M. Alvaro, E. Carbonell, V. Fornes, H. Garcia, New J. Chem. 28 (2004) 631. [4] M. Alvaro, E. Carbonell, H. Garcia, C. Lamaza, M. Narayana Pillai, Photochem. Photobiol. Sci. 3 (2004) 189. [5] A. Sanjuan, M. Alvaro, G. Aguirre, H. Garcia, J.C. Scaiano, J. Am. Chem. Soc. 120 (1998) 7351. [6] K.B. Yoon, Chem. Rev. 93 (1993) 321. [7] K. Kalyansundaram, Photochemistry in Microheterogeneous Systems, Plenum Press, New York, 1987. [8] J.C. Scaiano, H. Garcı´a, Acc. Chem. Res. 32 (1999) 783. [9] M. Alvaro, H. Garcia, S. Garcia, F. Marquez, J.C. Scaiano, J. Phys. Chem. 101 (1997) 3043. [10] M. Alvaro, A. Corma, B. Ferrer, H. Garcia, E. Palomares, Phys. Chem. Chem. Phys. 6 (2004) 1345. [11] K.T. Ranjit, L. Kevan, J. Phys. Chem. B. 106 (2002) 1104. [12] H. Vezin, A. Moissette, C. Bre´mard, Angew. Chem. 115 (2003) 5745. [13] W. Wang, B. Gu, L. Liang, W.A. Hamilton, J. Phys. Chem. B. 107 (2003) 12113. [14] Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 12 (2000) 693. [15] M. Gra¨tzel, A. Hagfeldt, Chem. Rev. 95 (1995) 49. [16] G. Cosa, M.N. Chretien, M.S. Galletero, V. Fornes, H. Garcia, J.C. Scaiano, J. Phys. Chem. B. 106 (2002) 2460.
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