Inorganic

14924

J. Phys. Chem. B 2004, 108, 14924-14932

White-Light Emission of Amine-Functionalized Organic/Inorganic Hybrids: Emitting Centers and Recombination Mechanisms L. D. Carlos,*,†,‡ R. A. Sa´ Ferreira,†,‡ R. N. Pereira,§ M. Assunç a˜ o,† and V. de Zea Bermudez# Departamento de Fı´sica, UniVersidade de AVeiro, 3810-193 AVeiro, Portugal, CICECO, UniVersidade de AVeiro, 3810-193 AVeiro, Portugal, Institut for Fysik og Astronomi, Århus UniVersitet, Ny Munkegade, 8000 Århus C, Danmark, and Departamento de Quı´mica and CQ-VR, UniVersidade de Tra´ s-os-Montes e Alto Douro, Quinta de Prados, Apartado 202, 5001-911 Vila Real Codex, Portugal ReceiVed: March 2, 2004; In Final Form: July 20, 2004

This work discusses the recombination mechanisms and the chemical nature of the emitting centers subjacent to the white-light emission of sol-gel derived amine-functionalized hybrids lacking metal activator ions, such as those based on 3-aminopropyl)triethoxysilane (APTES), 3-glycidyloxypropyltrimethoxysilane (GPTES), and on urea and urethane precursors. The white-light photoluminescence (PL) results from a convolution of the emission originated in the NH (NH2) groups of the urea or urethane bridges (APTES- and GPTES-based hybrids) with electron-hole recombinations occurring in the siloxane nanoclusters. These two components reveal a radiative recombination mechanism typical of donor-acceptor pairs, mediated by some localized centers. Photoinduced proton-transfer between defects such as NH3+ and NH- (GPTES- and APTES-based hybrids) or NH2+ and N- (di-ureasils and di-urethanesils) is proposed as the mechanism responsible for the NH-related component. Electron paramagnetic resonance data suggest that the specific PL mechanism subjacent to the component associated with the siliceous nanodomains involves oxygen-related defects.

I. Introduction The birth of “soft” inorganic chemistry processes, in particular the sol-gel route, allows the chemical design of pure and wellcontrolled multifunctional hybrid materials in which organic, inorganic, and even biological components are mixed at a nanosize level.1,2 The resulting synergies open up exciting directions in materials science research and related technologies with extraordinary implications in the processing of novel multifunctional advanced materials with innovative and unparalleled performances. In particular, the sol-gel approach offers the flexibility necessary for implementing innovative chemical design strategies for developing advanced photonic hybrid materials. Increasing attention has been focused in the past decade on the photonic features of organic/inorganic hybrid matrixes, particularly of the siloxane-based type.2 Although no commercial products are available yet, a significant number of innovative siloxane-based hybrids have been synthesized with advanced optical properties, such as hybrid optical switching and data storage devices,2a photoelectrochemical cells and coatings for solar energy conversion,3 hybrid materials having high laser efficiencies and good photostability,4 photopattern waveguiding structures for integrated optics,5 and electroluminescent diodes.6 In particular, the hybrid concept has also been employed to synthesize stable and efficient white-light photoluminescent amine-functionalized hybrids lacking metal activator ions, such as those obtained from 3-aminopropyl)triethoxysilane (APTES) and 3-glycidyloxypropyltrimethoxysilane (GPTES),7-11 from * Corresponding author, e-mail: [email protected]; phone 351 234370946, fax: 351 234 424965. † Departamento de Fı´sica, Universidade de Aveiro. ‡ CICECO, Universidade de Aveiro. § Århus Universitet. # Universidade de Tra ´ s-os-Montes e Alto Douro.

urea [NHC(dO)NH] and urethane [NHC(dO)O] cross-linked xerogels, classed as di-ureasils and di-urethanesils, respectively (Scheme 1),12-18 and from layered perovskite-type APTESPbCl4 hybrids.19 The APTES hybrid with formic acid is one of the most efficient phosphors known among those not containing activator metal ions, with a photoluminescence (PL) quantum yield of 35 ( 1%.7 For APTES-acetic acid phosphors, lower limits of 21 and 12% were estimated for the quantum yields of the two distinct emissions reported.8 Similar values were reported for the di-ureasils and di-urethanesils.11c,17 Despite the potential technological relevance of the aminefunctionalized hybrids, their room temperature (RT) quantum yields are higher than those reported for amorphous porous silicon, p-Si,20 and are similar to the typical values of the most efficient conjugated polymers.21 The origin of their efficient white-light intrinsic PL is not completely clarified.10,11,17 We demonstrate here that the radiative recombination mechanisms present in sol-gel-derived urea and urethane cross-linked organic/inorganic xerogels and in APTES-derived hybrids (classed here as aminosils) (Scheme 1), are typical of donoracceptor (D-A) pairs, mediated by some localized centers. In particular, photoinduced proton-transfer between defectsssuch as NH3+ and NH- for the aminosils, and NH2+ and N- for the di-ureasils and the di-urethanesilssand radiative recombinations involving oxygen-related defects are proposed as the responsible mechanisms of the white-light PL. The conclusions may be easily generalized for the di-ureasil organic precursor compounds (e.g., the diamines) and for other amine-functionalized hybrids based on APTES and GPTES.7-11,19 Furthermore, for di-ureasils and di-urethanesils the quantum yields of the NHrelated PL component is quantitatively associated with the extent and magnitude of the supramolecular interactions resulting from the self-assembly of urea or urethane groups via hydrogen bonding, supporting the previously suggested photoinduced

10.1021/jp049052r CCC: $27.50 © 2004 American Chemical Society Published on Web 09/04/2004

Emitting Centers and Recombination Mechanisms

J. Phys. Chem. B, Vol. 108, No. 39, 2004 14925

SCHEME 1 : Chemical Structure of Di-Ureasils, Di-Urethanesils, and Aminosils

proton-transfer between defects. Finally, electron paramagnetic resonance (EPR) data obtained for di-ureasils indicate the presence of oxygen-related defects. This correlates well with the recombination mechanism and with the nature of the emitting centers previously proposed as responsible for the observed white-light PL of those amine-functionalized hybrids. II. Experimental Section Synthesis. The synthesis of the di-ureasils and di-urethanesils has been reported in detail elsewhere.12,14,17,18 The final materials have been identified by the designation d-U(Y), di-ureasils, and d-Ut(Y′), di-urethanesils, where Y and Y′ are related to the diamine and poly(ethylene glycol), PEG, average molecular weights, respectively. Three diamines, Jeffamine ED-2001, Jeffamine ED-900, and Jeffamine ED-600, with average molecular weights of 2000, 900, and 600 g‚mol-1, respectively, corresponding to approximately 40.5, 15.5, and 8.5 OCH2CH2 repeat units, were used to prepare the di-ureasils (Scheme 1).

The di-urethanesils were synthesized using PEGs with the average molecular weight of 2000 and 300 g‚mol-1, corresponding to 45 and 6 repeat units, respectively (Scheme 1). The aminosils (Scheme 1) were prepared using the same procedure employed by Rousseau et al.22 APTES was mixed, at RT, with water and methanol (molar ratio 1 Si:9.6 CH3OH: 1.5 H2O). Photoluminescence Spectroscopy. The RT emission spectra were recorded on a 1 m spectrometer (SPEX 1704) coupled to a R928 Hamamatsu photomultiplier. A 150 W Xe arc lamp, a 0.25 m monochromator (KRATOS GM-252), and a He-Cd laser (325 nm, 40 mW) were used as a continuous excitation source. PL detected under different power excitations was recorded using a He-Cd laser (325 nm) coupled to attenuators filters. The RT emission spectra of the aminosils were recorded on a modular double-grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Jobin Yvon-Spex), in the front face acquisition mode, coupled to a

14926 J. Phys. Chem. B, Vol. 108, No. 39, 2004 R928 Hamamatsu photomultiplier. While the spectra of the aminosils were corrected for optics and detection spectral response, all the others were only corrected for the spectral response of the photomultipliers. The emission spectral height was corrected for the lamp intensity. Absolute Emission Quantum Yield. The absolute emission quantum yields (φ) were measured at RT using the technique for powdered samples described by Wrigthon et al.24 and it was calculated by φ ) A/(Rs - RH), where A is the area under the di-ureasils or di-urethanesils emission spectra, and RS and RH are the diffuse reflectance (with respect to a fixed wavelength) of the hybrids and of the reflecting standard, respectively. Emission and diffuse reflectance spectra were corrected for the optics and detection spectral responses. As reflecting standard we used KBr previously stored in an oven at 353 K. Powder size and packing fraction are crucial factors because RS and RH intensity depend on them. Thus, the diffuse reflectance was first measured at a wavelength not absorbed by the nanohybrids (720 nm). The di-ureasils and di-urethanesils were thoroughly ground until RH totally overlapped with RS, indicating a similar powder size and packing fraction. To prevent insufficient absorption of the exciting radiation, a powder layer around 3 mm was used and the utmost care was taken to ensure that only the sample was illuminated, to diminish the quantity of light scattered by the front sample holder. Diffuse reflectance and emission spectra were recorded under the excitation of a Xe arc lamp (450 mW) on a SPEX Fluorolog spectrafluorimeter coupled to a R928 Hamamatsu photomultiplier. All the spectra were detected at a 24.5° angle, relative to the incident light using. The same experimental conditions, namely, position of the hybrids/ standard holder, excitation and detection monochromator’ slits (0.3 and 0.1 mm, respectively), and optical alignment, were fixed. Three measurements were carried out for each sample, indicating reproducibility within 10%. The errors in the quantum yield values associated with this technique were estimated within 25%.24 However, we have recently performed the quantum yield determination of the same samples using a different technique proposed by Brill et al.,25 involving the use of white and phosphors standards whose experimental error lies beyond 10%. Thus, we believe that the experimental error associated with our measurement can be estimated also around 10%. Electron Paramagnetic Resonance Measurements. EPR spectra were obtained using a Bruker ESP 300E spectrometer mounted with an X-Band (ν ≈ 9.5 GHz) microwavebridge and corresponding cylindrical TE011 microwave resonator. Care was taken to ensure that the EPR spectra were taken under no-saturating conditions. The measurements were performed at RT on a representative sample, d-U(600), that was grounded to ensure a random orientation of the paramagnetic species relative to the magnetic field. III. Results and Discussion Photoluminescence Spectroscopy. The PL features of diamines, non-hydrolyzed di-ureasil and di-urethanesil precursors, final di-ureasils and di-urethanesils, and APTES- and GPTESbased hybrids have been the subject of several reports recently,11,12,13,14,15,16,17,18 see, for instance, the PL spectra in Figure 1 of Carlos et al.17 The PL spectrum of the APTES-based hybrid also resembles those reported elsewhere for other APTESderived hybrids with formic (HCOOH), acetic (CH3COOH), lactic (CH3CHOHCOOH), and valeric (CH3(CH2)3COOH) carboxylic acids.7-9,11 Furthermore, both the di-ureasils prepared with HCl or NH4F catalysts13,16 and those prepared by means of acetic acid solvolysis (a procedure carried out in the absence of water)11 show identical PL features.

Carlos et al.

Figure 1. Emission spectra, excited at 365 nm (3.40 eV), of Jeffamine ED-2001 solutions (0.1 M) in (a) THF and (b) H2O. The inset shows, for both solutions, the dependence of the emission integrated intensity with the diamine concentration.

The hypothesis that the PL might arise from any contaminant present in the commercial diamines used in the synthesis of the di-ureasils was readily discarded on the basis of the recognition that the di-urethanesil nonhydrolyzed precursors and the aminosil presented similar photoluminescent features. Moreover, taking into account the molecular structure of APTES (Scheme 1) and the fact that pure PEG (the organic precursor of the di-urethanesils) is not luminescent, we are able to conclude that an intrinsic emission associated with the lone pair of electrons of the NH-related groups existent in all these aminefunctionalized materials is present. This suggestion was previously pointed out in several works dealing with the PL features of amine-based hybrids.7,8a,11,14b,14d,17 With the purpose of carrying out a detailed characterization of the white-light emission of the diamines, we measured the emission of a series of solutions of Jeffamine ED-2001, using water and THF as solvents, with concentrations in the range between 0.1 and 5 × 10-5 M. To ensure a qualitative comparison between the emission intensity of the spectra, the measurements were done consecutively and all the experimental conditions (optical setup, focalization point and illuminated cross-section, sample holder and slits width) were kept constant. Once these requirements are obeyed, we can discuss qualitatively the variation of the luminescence intensity as function of the concentration and solvent type. A quantitative study requires an integrating sphere or the use of a standard phosphor. We observed for all the solutions that (i) the THF solutions emitted at higher energies than the aqueous solutions; (ii) the emission intensity of the THF solutions was lower than that of aqueous solutions; and (iii) in both solvents, the emission intensity decreased supralinearly with the decrease in diamine concentration. Figure 1 shows the PL spectra of the more-concentrated solutions in both solvents. The spectra resemble those presented elsewhere (e.g., Figure 1 of Carlos et al.17) except in the lower energy side (below 2.4 eV), where a lower-intensity shoulder appears. Such occurrence may be related to the presence in solution of different size clusters, due to incomplete homogenization of the solution. This was also already reported for similar amine-functionalized hybrids.11a The inset presents the variation of the integrated intensity with the concentration



Figure 2. Emission spectra of A: Jeffamine ED-600, B: d-U(600), and C: APTES-based hybrid. Excitations wavelengths: (a) 330, (b) 350, (c) 365, and (d) 395 nm. The inset shows the fitted energies and error bars for the Jeffamine ED-600. The dashed line represents a linear fit to the data (R = 0.99).

decrease of the aqueous and THF solutions. For concentrations lower than 0.004 M, the emission intensity was too low, which made the spectra detection difficult. In these cases, the variation of the intensity was very small, although the intensity-decreasing trend with dilution was maintained. The decrease of the PL intensity may be related to the increase of the average distance between amine groups induced by the dilution, supporting, therefore, the assumption that the amine groups must be involved in the PL features of these materials. A similar suggestion was recently pointed out stressing that the emission properties of amine-functionalized hybrids are connected with the existence of clusters involving both the organic and the inorganic components.11a,16 Small-angle X-ray scattering (SAXS)26 and fluorescence probing15 were used to study cluster formation in di-ureasils and di-urethanesils. Moreover, as the main difference between the aqueous and THF solutions was the solvent polarity, the lower emission intensity produced by the less-polar THF solvent suggested that, if proton transfer really takes place between amine groups, the solvent might be playing the role of vehicle, carrying the proton from the donor to the acceptor groups. If so, distant NH groups were favored in water solutions. We will return to this point below. To support the discussion on the recombination mechanisms behind the hybrids’ white-light emission and on the chemical nature of the corresponding emitting centers, we will briefly review in the following two paragraphs the main PL results that are fundamental to the subsequent discussion. Two components are clearly observed in the PL spectra of the di-ureasils and the di-urethanesils, namely in the blue (≈2.50 eV) and in the purplish-blue (≈2.92 eV) spectral regions, whereas the diamines spectra display only one component, peaking at ≈2.50 eV. The corresponding excitation spectra support the presence of two components for the hybrids and one emission for the diamines.23 Furthermore, time-resolved spectroscopy shows that those two distinct emissions have different time decays.17 The blue-band emission was related to the lone pair of electrons of the NH-containing groups and the purplish-blue component (detected only after polycondensation and building up of the inorganic network) was associated with recombinations that occur within the nanometer-sized siliceous clusters.11c,17,18b For the diamines and the hybrids, it was observed that the RT emission spectra are excitation-wavelength dependent, as

Figure 2 exemplifies for Jeffamine ED-600, d-U(600), and the APTES-based hybrid. To quantify the energy of those two distinct emissions, a deconvolution fitting procedure was applied to the emission spectra of the diamines, the di-ureasils and the di-urethanesils. For the latter materials, the curve-fitting method (described in detail elsewhere14b,17) revealed the presence of two Gaussian bands in the blue (2.50-2.60 eV) and in the purplishblue (2.80-3.10 eV) spectral regions, excitation energies between 3.10 and 4.00 eV, Figures 3A and 3B. Only the former component could be accounted for in the 2.95-3.10 eV excitation range (Figure 3C). The full width at half-maximum (fwhm) reached approximately 0.40 and 0.35 eV for the blue and purplish-blue bands, respectively. On the contrary, the curve-fitting method applied to the diamines PL revealed the presence of only one Gaussian band in the blue region (2.543.07 eV) with a fwhm of ca. 0.45 eV. The errors in the peak and fwhm resulting from the curve-fitting method are less than 5%. For the xerogels, it is the purplish-blue component that essentially originates the shift of the emission spectra as the excitation wavelength varies. The energy of the blue band remains approximately constant, 2.58(0.02 eV, for excitation energies between 2.95 and 3.88 eV, while for lower excitations, 2.76-2.88 ( 0.01 eV, a linear red-shift of the emission peak is observed as the excitation energy decreases (Figure 4). The purplish-blue band shifts linearly as the excitation energy decreases from 3.54 to 3.10 eV. For higher values (3.76-4.13 eV) the emission is invariably peaked around 3.00-3.10 ( 0.02 eV (Figure 4). The diamines emission component (blue band) presents a linear shift within the whole selected interval range of excitation wavelengths (inset of Figure 2A). The strong dependence of the emission energy with the excitation wavelength (inset of Figure 2A and Figure 4) is characteristic of all of the materials under analysis (diamines, non-hydrolyzed di-ureasil and di-urethanesil precursors, and final di-ureasil, di-urethanesil, and aminosil xerogels) providing strong evidence of disordered-related processes generally associated with transitions occurring within localized states in noncrystalline structures (e.g., p-Si,27 hydrogenated amorphous Si, a-Si:H,28a and As2Se3 glasses28b). An identical dependence was also reported for a nonhydrolyzed di-ureasil precursor16 and for APTES-based hybrids with carboxylic acids.11a

14928 J. Phys. Chem. B, Vol. 108, No. 39, 2004

Carlos et al.

Figure 3. Emission spectra of d-U(2000) for different excitation wavelengths A: 365, B: 400, and C: 420 nm. The respective Gaussian peak components are also represented, blue (dashed line), purplish-blue (dotted line), and fit envelope (solid circles).

Figure 4. Fitted energies and error bars for the NH-related (solid symbols) and siliceous nanodomains (open symbols) emission components. A: d-U(2000) (squares), d-U(900) (diamonds), and d-U(600) (up triangles) and B: d-Ut(2000) (down triangles) and d-Ut(300) (circles). The dashed lines represent linear fits to the data (R = 0.99).

Although no significant changes were detected in the energy of the blue component, the energy of the purplish-blue band for the di-ureasils and di-urethanesils depends on the polymer chain length, in such a way that an increase in the domain size results in a decrease of the corresponding energy gap. Figure 5 shows the dependence of the di-ureasils’ emission energy with the respective domain sizes determined by SAXS26 and by X-ray diffraction (XRD),14b designated as Rp and Lp, respectively. For each excitation wavelength used, the emission energy follows approximately the same trend presented in Figure 5. The different values found for Rp and Lp are due to the fact that these parameters were calculated from different experimental data and different structural models. However, the crucial point is that independent of the absolute value found for Rp and Lp, we observe that greater nanodomain size corresponds to smaller emission energies. A similar behavior was reported for other silicon-based nanostructured materials.29,30 There are also other reports referring to the theoretical calculation of the emission energy as a function of the nanodomains size.29a,29c,30 Absolute Emission Quantum Yields. Table 1 shows the

Figure 5. Dependence of the fitted energy and respective error bars for the purplish-blue component of d-U(2000) (squares), d-U(900) (diamonds), and d-U(600) (up triangles), with the size of the nanodomains determined by SAXS (solid symbols), Rp, and XRD (open symbols), Lp. The dashed lines are guides to the eyes.

absolute emission quantum yields of the di-ureasils and diurethanesils, measured at 2.95 and 3.10 eV. For all the samples a maximum φ value was obtained at 3.10 eV excitation energy. This particular energy maximizes the emission quantum yield because it corresponds to the largest overlap between the excitation spectra of the two hybrid-emitting centers (NHcontaining groups and siliceous nanodomains). For 2.95 eV the fitting procedure applied to the steady state emission spectra revealed the presence of only the NH-related band, Figure 3C. Therefore, the absolute emission quantum yield measured at this specific excitation wavelength is connected only to the blue emission. For the luminescence originated in the siliceous nanodomains it is not possible to quantify the respective φ value independently, because at the excitation wavelength range used there is no selective excitation for that emission. In each series, di-ureasils or di-urethanesils, an increase in the polymeric chain length corresponds to an increase in the emission quantum yield (Table 1). Recombination Mechanisms. Despite the previous argu-



TABLE 1: Absolute Emission Quantum Yield (%) for Di-Ureasils and Di-Urethanesils Measured at 400 (3.10) and 420 nm (2.95 eV) 400 nm (3.10 eV) 420 nm (2.95 eV)

d-U(2000)

d-U(900)

d-U(600)

d-Ut(2000)

d-Ut(300)

8.2 7.4

8.0 8.3

6.6 6.0

19.2 13.4

10.2 7.2

ments that ascribed the hybrids’ white-light emission to the convolution of a longer-lived emission originating in the NHcontaining groups with shorter-lived electron-hole recombinations occurring in the siliceous nanodomains, the actual detailed physical mechanisms are not well described. To shed light on this issue, our subsequent discussion will be based on the recombination mechanisms known for crystalline semiconductors, c-SC, with relevance to amorphous materials. The distinction between the various near band-edge transitions (free exciton, bound exciton, free carrier to neutral acceptordonor, and D-A pairs) that occurs in c-SC can be made through the analysis of the behavior of the emission intensity as the excitation power is varied.31b,32 The emission intensity, I, depends on the excitation power, L, according to the power law I ∝ Lk. When 1 < k < 2 we are in the presence of exciton-like transitions; k e 1 is characteristic of free-to-bound and D-A pairs.31b,32 Deviations from this behavior were observed either when L was varied more than two orders of magnitude or when the selected excitation energy was resonant with the semiconductor band gap.31b Figure 6 represents the integrated intensity of the d-U(2000) di-ureasil steady-state emission calculated for different magnitudes of the excitation power. The integrated intensity of both components was obtained independently using the curve-fitting procedure described previously.14b,17 The integrated intensity depends sublinearly on the power excitation, with a k value of 0.91 ( 0.07 and 0.89 ( 0.06, for the blue and purplish-blue components, respectively (Figure 6). This is a clear indication that both emissions revealed a recombination mechanism typical of D-A pairs, mediated by some localized centers.31b,32 Similar dependence with the power excitation, k ) 0.98 (0.03, is

Figure 6. Integrated intensity for the d-U(2000) and Jeffamine ED2001 emission components measured at different power excitations. The dashed lines correspond to linear fits to the data (R = 0.99). The inset shows the shift of the NH-related emission between the energy of the maximum intensity of the time-resolved spectra and the corresponding value measured under continuous excitation. The spectra were measured at 375 nm for a fixed acquisition window of 10.00 × 10-3 s and for DT between 8 × 10-5 and 10-1 s. The line is a guide to the eyes.

detected for the integrated emission intensity of Jeffamine ED2001 (Figure 6). Relevant additional experimental evidence allowing the identification of D-A transitions can be obtained from timeresolved spectroscopy.33 For longer delay times (DT), a red shift in the spectrum is expected because distant pairs have a smaller recombination probability. The respective energy levels therefore have larger lifetimes, a situation that is favored with the increasing delay time. As a consequence, the recombination takes place at lower energies.31a,33 For the d-U(2000) blue band the energetic shift (∆E) between the energy of the maximum intensity of the time-resolved spectra, measured for DT between 8 × 10-5 and 10-1 s, and the corresponding value detected under continuous excitation are shown in the inset of Figure 6. Within the interval 8 × 10-5 e DT e 2 × 10-2 s, the blue-band emission shifts toward the red and for 2 × 10-2 < DT e 10-1 s the recombination always occurs at ≈ 2.40 eV. The decrease of the emission energy with DT is consistent with D-A pair recombination.31c With respect to the purplish-blue band, the results are restricted to 8 × 10-5 e DT e 5 × 10-2 s because this emission occurs in a time scale two orders of magnitude lower than the blue-band recombination processes. Therefore, we are not able to access the corresponding stages observed for the blue component. In fact, the emission energy does not vary within this DT interval, peaking invariably around 2.94 eV, which is coincident with the steady-state emission, ∆E ) 0. Although ∆E * 0 was expected, this may not controvert a D-A pairrecombination mechanism. In fact, a similar behavior was already reported for silicon nanocrystals, where although the emission intensity presents a sublinear excitation power dependence (indicating the presence of D-A pairs), the RT timeresolved spectra remain unchanged as the DT increases from 0 to 5 × 10-5 s.32 Chemical Nature of the D-A Pairs. The next paragraphs discuss the possible chemical nature of the blue and the purplishblue D-A pairs-related emissions. Let us start with the discussion of the former component. It has been clearly demonstrated that the existence of oppositely charged defects in c-SC,33 amorphous semiconductors, a-SC,32,34 and organic compounds35,36 provides a good way to radiative electron-hole recombinations. In the 1970s it was reported that urea, OdC(NH2)2, and thiourea, SdC(NH2)2, crystals exhibited luminescence in the blue and green spectral regions, induced by X-ray radiation.35 It was proposed that the defects responsible for the emission were related to the rupture of intermolecular hydrogen bonds established between packed urea molecules. The oxygen atom of each urea molecule binds four hydrogen atoms of two adjacent molecules, forming weak hydrogen-type bonds (N-H‚‚‚OdC). When the urea crystals were irradiated, proton-transfer was induced and two defects with opposite charges, NH3+ and NH-, were formed. These two defects can act as electron and hole traps, respectively, recombine radiatively, and account for the observed emission. Photoinduced proton-transfer has also been reported in many others materials, e.g., semiconducting polymers,37 organic systems involving amine groups,36 and microporous zeolites.38 We propose that this same type of defect is the chemical

14930 J. Phys. Chem. B, Vol. 108, No. 39, 2004 species responsible for the PL blue component of diamines, aminosils, di-ureasils, and di-urethanesils, that is, NH3+/NHfor the diamines and aminosils, and NH2+/N- for the di-urea and di-urethane cross-linked xerogels. Furthermore, similar defects could also be responsible for the PL of APTES- and GPTES-based hybrids (NH3+/NH-) and of APTES-derived hybrids with carboxylic acids (NH3+/RCOO-, R ) H, CH3, CH3CHOH, or CH3(CH2)3, for formic, acetic, lactic, and valeric acids, respectively). It is interesting to note that the presence of NH3+/CH3COO- defects was recently proposed in xerogels formed by reactions of APTES, TEOS, and acetic acid.10a The above suggestion is consistent with the observed behavior of the emission intensity of Jeffamine ED-2001 solutions in water and THF. The two solvents differ in the polarity of their molecules. The water molecules are more polar than those of the THF. Therefore, water behaves as a more efficient vehicle for proton transport than THF, thus contributing to a more efficient luminescent mechanism. As a consequence, the PL intensity of the aqueous solutions is higher than that of the THF solutions. Moreover, the observed red shift of the PL spectra of the aqueous solutions, with respect to those of THF (characteristic of an emission-size-dependent) may also be explained on the basis of the differences in the polarity between the molecules of the two solvents. Considering the photoinduced proton-transfer model, we can suggest that the D-A pairs are formed with distinct NH2 groups and that the distance between them determines the emission energy in such a way that distant pairs emit at longer wavelengths. For the aqueous solutions the emission arising from more distant NH2 groups is then favored due to the presence of a more-efficient proton-transfer vehicle (greater polarity), resulting in a spectrum blue-shifted with respect to the emission measured for the THF solutions. Another experimental result that supports the previous suggestion is the variation of the emission quantum yield of the blue component with the length of the polymeric chain (Table 1). For the di-ureasil with shorter polymer chains, d-U(600), the strongest hydrogen bonds are formed14a that localize the proton and render difficult the induced proton transfer between NH groups, leading to the corresponding decrease of the quantum yield. In the di-ureasils containing longer polymer chains, d-U(900) and d-U(2000), the number of urea-urea hydrogen-bonded associations formed are minor and practically inexistent, respectively.14a Therefore, despite the larger distance between the NH groups, the protons have a greater degree of mobility; a situation that favors higher quantum yields. Close analysis of the NH-related emission quantum yield values obtained for the di-urethanesils allows one to infer that an increase (43%) for the longer polymer chain, d-Ut(2000) with respect to the di-ureasil with the same polymer chain length, d-U(2000), results (Table 1). The smaller quantum yield of the d-U(2000) may be induced by the markedly stronger hydrogen bonds established between adjacent urea groups that do not occur in the urethane linkages, as pointed out previously.14a,39 The number of hydrogen donor sites present in the urethane and urea groups is not the same; the urethane linkage contains just one NH group, whereas the urea moiety is composed of two NH groups. This implies that, while the CdO moiety of a urethane group may form a single hydrogen bond with the NH moiety of another urethane group,39d urea groups of neighbor molecules may be linked by means of planar bifurcated hydrogen bonds.14a,39a As a consequence, the hydrogen-bonding geometry of the urethane groups might be substantially different from that of the urea groups. Such a situation resulted, for

Carlos et al. instance, in the tighter packing of adjacent chains observed in bis-urea compounds with respect to the bis-urethane analogues.39a The presence of the strongest hydrogen bonds contributes to localize the proton and render difficult the induced transfer of the hydrogen atoms between NH groups. Consequently, the NHrelated emission quantum yield will be smaller in the di-ureasil type hybrid. This same effect (e.g., quantum yields determined by the extent and magnitude of the supramolecular interactions resulting from the self-assembly of urea or urethane groups via hydrogen bonding) was recently described in di-ureasils prepared through carboxylic acid solvolysis for which the PL quantum yields increase around 30-35%, relative to analogous hybrids prepared by conventional sol-gel reactions.11c The existence of mobile (free) protons in the hybrids and in the diamines is further demonstrated by complex impedance measurements. At RT the ionic conductivity of d-U(2000), d-U(900), d-U(600), and Jeffamine ED-2001 is about 3.4 × 10-8, 5.0 × 10-9, 9.2 × 10-10, and 3.9 × 10-9 S/cm, respectively. These results show that the mobility of the protons decreases from d-U(2000) to d-U(600), confirming the conclusions drawn from quantum yield analysis. The PL features of the purplish-blue emission, namely its red shift as the excitation energy increases (Figure 4) and the typical Arrhenius behavior displayed by its lifetime as the temperature increases from 14 to 300 K (Figure 4 of Carlos et al.17), show undoubtedly the existence of localized states that might be induced either by the lack of long-range order or by the presence of oxygen defects in these nanodomains. The possible influence of oxygen defects in the emission features of other amine-functionalized hybrids was previously suggested for APTES-based hybrids containing carboxylic acids10a and TEOS- and MTEOS-based hybrids containing acetylacetonate (acac) and zirconium propoxide.10b For the former hybrids, it was observed that the luminescence intensity of such organic/inorganic hybrids increases with the increase of the heat treatment temperature between 20 and 200 °C. Furthermore, the luminescence intensity is greater in the xerogels treated under vacuum than in the hybrids treated in air.10a This finding was interpreted as an indication that higher temperatures and vacuum conditions favor the creation of oxygen-related defects.10a The TEOS- and MTEOS-based hybrids with acac and zirconium propoxide10b were studied by EPR, which demonstrated the existence of oxygen-related defects. Another defect type associated with carbon impurities was proposed as being the origin of the luminescence of sol-gel derived silica gels based on TEOS and TMOS incorporating a variety of carboxylic acids.7 Under adequate heat treatments (at least above 520 K) a carbon impurity is created in the -OSi-O- network forming -O-C-O- and/or -Si-C- bonds.7 However, the carbon-related defects are only luminescent active after a heat treatment as these xerogels are not luminescent prior to this procedure.7 This situation contrasts with that found with oxygen-related defects, as the luminescent centers are active prior to any thermal treatment. Since the aminosils, di-ureasils, and di-urethanesils discussed here are RT-efficient emitters, without needing a prior heat treatment, we can rule out carbondefect impurities as responsible for the siliceous nanodomains emission features, a fact that, unfortunately, is not sufficiently stressed in the literature. In what follows, we will show that EPR measurements furnish solid evidence in agreement with the existence of oxygen-related defects that are subjacent to the purplish-blue emission mediated by D-A pairs. Figure 7 depicts the EPR spectrum of d-U(600). The shape displayed by this spectrum is similar to that typically produced


J. Phys. Chem. B, Vol. 108, No. 39, 2004 14931 functionalized hybrids, an attractive model is a defect with a structure similar to the •O-O-SitO3 center in SiO2, but where the peroxy-bond silicon atom becomes coordinated to one carbon and two oxygen atoms: •O-O-Sit(CO2). The peroxyradical hole trap has been described in disordered systems, such as the amorphous silicon-oxide system, by assuming a distribution on the g-values.43,48 This is symptomatic of a strong dependence of the g-values on the local environment of the defect, that is, a small change in the environment of the silicon bonded to the peroxy-radical is expected to produce a significant change in the EPR parameters of the system. Such an observation supports the model proposed here for the EPR active defect. IV. Conclusions

Figure 7. EPR spectrum of the d-U(600) di-ureasil (modulation frequency 100 kHz, modulation amplitude 0.102 mT, and microwave power 5.004 mW).

by an ensemble of randomly oriented paramagnetic centers with S ) 1/2 and axial (or closely axial) symmetry.40 The analysis of the spectrum profile yields a parallel g-value of g| ) 2.035 ( 0.007 and an averaged perpendicular g-value of 〈g⊥〉 ) 2.018 ( 0.007. The detected spectrum is most probably produced by intrinsic defects, which originate from unsaturated bonds in the material, since we do not expect detectable amounts of unwanted foreign elements in the samples. It is difficult to establish unambiguously the structure of the involved defect solely based on the information provided by the spectrum of Figure 7. However, we can rule out some of the possibilities and advance some considerations about the most probable structures. The E′-type luminescent centers, which have been described both in crystalline and fused SiO2,41,42 are different variations of a fundamental intrinsic defect. The basic unit of this defect is an unpaired electron on a silicon-dangling bond, where the spin is highly localized in the silicon atom. In principle, such a type of defect may well be formed during the synthesis of the amine-functionalized materials studied here. The paramagnetic properties of the different types of E′ defects have been described with g-values that are within the range 1.999 < g < 2.002.42 Because the g-values shown in Figure 7 are considerably higher than those reported for the E′ centers, we exclude the possibility that the observed spectrum has been produced by a defect that complies a silicon dangling bond. Two other fundamental defects have been frequently detected by EPR in various forms of SiO2. They are the nonbridging oxygen hole center (NBOHC), usually denoted by •O-SitO3, and the peroxy-radical hole trap (•O-O-SitO3). The NBOHC has been characterized by a g-matrix of g1 ) 2.0010, g2 ) 2.0095, and g3 ) 2.078,43 whereas the peroxy-radical has been described with the following g-values: g1 ) 2.0014, g2 ) 2.0074, and g3 ) 2.067.44 The general disagreement between the g-values reported for the NBOHC and peroxy-radical defects in SiO2 and those determined from the spectrum of Figure 7 provides evidence that neither of the two defects are likely to be responsible for the observed EPR spectrum. However, it is interesting to note that the average g-value 〈g〉 ) (g1 + g2 + g3)/3 that is calculated from the NBOHC and peroxy-radical data and the average g-value 〈g〉 ) (g| + 2 × g⊥)/3 determined from the data provided by the present measurements are very close. Moreover, the g-values shown in Figure 7 are in the characteristic range of peroxy-radical defects discovered in other materials.10b,45,46,47 In this context, the spectrum shown in Figure 7 is attributable to an oxygen-related defect. For amine-

Radiative recombination mechanisms typical of D-A pairs are the origin of the bright RT white-light PL of sol-gel-derived siloxane-based hybrids lacking a metal activator and based on APTES, GPTES, urea, or urethane precursors. The white-light PL of di-ureasils, di-urethanesils, APTES- and GPTES-based hybrids results from a convolution of the emission originating from the NH (NH2) groups of the urea or urethane bridges (diamines, APTES- and GPTES-based hybrids) with electronhole recombinations that occur in the siloxane nanoclusters. Photoinduced proton-transfer between defects such as NH3+ and NH-, for the diamines, GPTES- and APTES-based hybrids, and NH2+ and N-, for the di-ureasils and the di-urethanesils, is the proposed mechanism thought to be responsible for the NHrelated component. For the emission ascribed to the siliceous nanodomains, the specific D-A PL mechanism involves oxygen-related defects. EPR measurements agree with a defect model similar to that of the well-known peroxy-radical in SiO2, but where the silicon is coordinated to one carbon and two other oxygen atoms: •O-O-Sit(CO2). The detailed characterization of those defect-related emitting centers will definitively contribute to the recognition of the paths needed for the development of other siloxane-based hybrids characterized by interesting PL features and high light emission efficiency. Acknowledgment. The authors would like to thank M. M. Silva (Chemistry Department, University of Minho, Portugal) for providing complex impedance data and J. Soares and M. M. Arau´jo de Oliveira (Physics Department, University of Aveiro) for their help in the EPR measurements. The financial support from FEDER and Fundaç aõ para a Cieˆncia e Tecnologia, POCTI program (CTM/33653/00), is gratefully acknowledged. References and Notes (1) (a) Brinker, C. J.; Scherer, G. W. Sol-gel Science, The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990. (b) Judeinstein, P.; Sanchez, C. J. Mater. Chem. 1996, 6, 511. (2) (a) Sanchez, C.; Ribot, F.; Lebeau, B. J. Mater. Chem. 1999, 9, 35. (b) Sanchez, C.; Lebeau, B. Materials Research Society Bull. 2001, 26, 377. (c) Sanchez, C.; Soler-Illia, G. J. de A. A.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061. (d) Andrews, M. P. Integrated Optics DeVices: Potential for Commercialization; Najafi, S. I., Armenise, M. N., Eds.; SPIEsThe International Society of Optical EngineeringsSeries: Bellingham, WA, 1997; Vol. 2997, p 48. (e) Seddon, A. B. IEEE Proc. Circuits DeVices Syst. 1998, 145, 369. (f) Schmidt, H.; Jonschker, G.; Goedicke, S.; Mennig, M. J. Sol-Gel Sci. Technol. 2000, 19, 39. (3) Li, H.; Inoue, S.; Ueda, D.; Machida, K.; Adachi, G. Electrochem. Solid State Lett. 1999, 2, 354. (4) Faloss, M.; Canva, M.; Georges, P.; Brun, A.; Chaput, F.; Boilot, J.-P. Appl. Opt. 1997, 36, 6760. (5) (a) Blanc, D.; Pelissier, S.; Saravanamuttu, K.; Najafi, S. I.; Andrews, M. P. AdV. Mater. 1999, 11, 1508. (b) Buestrich, R.; Kahlenberg, F.; Popall, M.; Dannberg, P.; M.-Fiedler, R.; Ro¨sch, O. J. Sol-Gel Sci. Technol. 2001, 20, 181.

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Inorganic

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