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Color-Tunable Fluorescence and White Light Emission from Mesoporous Organosilicas Based on Energy Transfer from 1,8-Naphthalimide Hosts to Perylenediimide Guests Fabiane de Jesus Trindade, Eduardo Rezende Triboni, Bruna Castanheira, and Sergio Brochsztain J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 03 Nov 2015 Downloaded from http://pubs.acs.org on November 4, 2015

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The Journal of Physical Chemistry

Color-Tunable Fluorescence and White Light Emission from Mesoporous Organosilicas Based on Energy Transfer from 1,8-Naphthalimide Hosts to Perylenediimide Guests

Fabiane de Jesus Trindade,1 Eduardo Rezende Triboni,2 Bruna Castanheira,1 and Sergio Brochsztain1*

1

Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas, Universidade Federal do ABC, Avenida dos Estados, 5001, Santo André-SP, 09210-170, Brazil.

2

Escola de Engenharia de Lorena, Universidade de São Paulo, Estrada Municipal do Campinho, Lorena-SP, 12602-810, Brazil.

*Correspondence author. E-mail: [email protected]. Phone/Fax: (55) (11) 49968260.

ACS Paragon Plus Environment


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Abstract The present work reports Förster resonance energy transfer (FRET) from 1,8naphthalimide (NI) donors bound to the pore walls of mesoporous silicas to perylenediimide (PDI) acceptors doped into the mesochannels. Mesoporous organosilicas containing covalently bound NI were synthesized by co-condensation of tetraethylorthosilicate (TEOS) with N-(3(triethoxysilyl)propyl)-1,8-naphthalimide (TEPNI) in the presence of a block copolymer surfactant as a template. The resulting materials were highly ordered, presenting a 2D-hexagonal structure, and displayed easily tunable optical properties, which could be controlled by the amount of NI in the sample. A sample prepared from a diluted TEPNI solution (SBANId) presented a blue, monomer-like emission. On the other hand, when a concentrated TEPNI solution was used, the resulting material (SBANIc) displayed a green, excimer-like emission. For the FRET studies, N,N´-bis(2,6-dimethylphenyl)-3,4,9,10-perylenediimide was doped into the pores of the SBANI samples from chloroform solutions. When excited at the NI absorption maximum (350 nm), PDI-doped SBANIc showed intense quenching of the NI emission band, even at very low PDI doping, with quenching efficiencies reaching nearly 80% with only 0.6 mol% PDI (PDI/NI ≈ 1/170). The emission of PDI was observed at higher doping ratios, even though the PDI hardly absorbs at 350 nm, thus evidencing FRET from the host NI to the guest PDI. SBANI materials with a suitable amount of the PDI dopant displayed a white emission, spanning the whole visible spectrum.


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Introduction Dye-doped mesoporous organosilicas have been recently explored as color-tunable systems based on Förster resonance energy transfer (FRET) between different fluorescent dyes.13

A myriad of advanced applications have been recently reported for these FRET based

mesoporous organosilicas, ranging from light-harvesting materials4,5 to ratiometric sensors,6-8 multi-channel traceable drug delivery systems,9-11 photocatalysts,11,12 and sensitizers for photodynamic therapy against cancer.13 FRET dyes can be incorporated by simple diffusion of both donor and acceptor into the channels of inorganic mesoporous silicas.14 Nevertheless, a better performance was obtained when the donor was placed in the pore walls, with the energy transferred to an acceptor doped as a guest in the pore interior.1-5 Several methods are known for the synthesis of dye-functionalized mesoporous organosilicas,15-17 including post-synthetic functionalization (grafting), co-condensation of an inorganic silica source (usually tetraethyl orthosilicate (TEOS) = Si(OEt)4) together with an organic trialkoxysilane (R–Si(OR´)3) and periodic mesoporous organosilicas (PMO), where the synthesis is made using up to 100% of a bridged organosilane of the type (R´O)3Si–R– Si(OR´)3.15-19 Most reported systems for FRET have employed PMO, where the donor dye is integral part of the pore wall. PMO containing biphenyl,4,5,9 naphthalene20,21 or anthracene7,9 as donors have been reported. However, when bulky organic chromophores, such as tetraphenylpyrene2 or perylenediimides,22 were incorporated into PMO, disruption of the mesoporous order was observed at relatively low dye doping. For large organic dyes, the cocondensation method is an attractive alternative, since the organic modifier protrudes from the pore walls towards the pore center in this method, and therefore is closer to the guest dye in the mesopores.15



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In the present report, we prepared mesoporous organosilicas containing the 1,8naphthalimide (NI) chromophore (Figure 1) by co-condensation between TEOS and N-(3(triethoxysilyl)propyl)-1,8-naphthalimide, in the presence of triblock copolymer Pluronic P123, in the same conditions usually employed for the synthesis of inorganic mesoporous silica SBA15.23 Small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) revealed that the new materials, which will be designated as SBANI, were in a highly ordered 2Dhexagonal structure, similar to pristine SBA-15.23 We show that the optical properties of the new materials can be tuned by the synthetic conditions. A sample prepared with a lower imide content (SBANId; 0.10 mmol/g) showed a blue emission when irradiated with a hand-held UV lamp (365 nm), whereas a sample with higher NI load (SBANIc; 0.46 mmol/g) displayed an excimerlike green emission. Recent reports have described the functionalization of mesoporous silicas with 4-amino-1,8-naphthalimides (ANI) by post-synthetic grating.24-27 However, a nonhomogeneous dye distribution is usually obtained with the grafting method, because pore blockage prevents the molecules to diffuse deeply into the channels.15 Besides, the ANI are yellow dyes with quite different properties than the colorless NI employed here. Further control over the optical properties was acquired by doping the SBANI materials with a perylenediimide (PDI) red dye, which served as an energy acceptor (Figure 2). PDI derivatives have been usually employed as energy acceptors in covalently bound donor-acceptor FRET dyads.28,29 1,8-Naphthalimides, on the other hand, have been often employed as FRET donors in such systems.29,30 The PDI dye employed here efficiently quenched the fluorescence of the NI in the dye-doped SBANI materials. Furthermore, the emission spectrum of the acceptor (PDI) was observed when the system was excited at the absorption band of the donor (NI). These


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results indicate the occurrence of FRET from the NI anchored to the walls to the PDI inside the channels (Figure 2).

SBANId (λmaxem = 395 nm) A

SBANIc (λmaxem = 470 nm) B

Figure 1. Schematic structures of SBANId (A) and SBANIc (B).



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Figure 2. Schematic view of the PDI-doped SBANI materials, stressing the FRET process from NI to PDI.


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Experimental Section Materials. The following reagents were obtained from Sigma Aldrich: Pluronic P123, tetraethylorthosilicate (TEOS), 3-aminopropyltriethoxysilane (APTES), 1,8-naphthalic anhydride and N,N´-bis(2,6-dimethylphenyl)-3,4,9,10-perylenediimide (PDI). Ethanol and chloroform (HPLC grade) and hydrochloric acid were purchased from Baker. All the aqueous solutions were prepared with deionized water. Synthesis of N-(3-(triethoxysilyl)propyl)-1,8-naphthalimide (TEPNI). The silylated NI derivative was synthesized in ethanolic solution by the reaction between 1,8-naphthalic anhydride and APTES, according to the reported procedure.31 The compound was used immediately as obtained, without isolation, for the synthesis of the SBANI materials. Synthesis of SBANId and SBANIc. Pluronic P123 (4.1 g) was dissolved in water (30 g) at 35 0C with stirring. After dissolution, 120 g of a 2 M HCl solution was added, and the solution stirred for 1 h. TEOS (8.5 g, 41 mmole) and TEPNI were then added. Two different amounts of TEPNI were employed. In the sample with lower naphthalimide content (SBANId), 40 mg (0.1 mmole) of TEPNI were added to the reaction mixture in 5 mL of ethanol (TEOS:TEPNI = 400). In the sample with higher naphthalimide content (SBANIc), 200 mg (0.5 mmole) of TEPNI were added to the reaction mixture in 5 mL of ethanol (TEOS:TEPNI = 80). The mixture was stirred for 24 h at room temperature, and then placed in a mini-autoclave, which was heated at 100 0C for further 24 h. The mixture was cooled down, and 2 L of water were added. The precipitated white solid was filtered off, washed with 1 L of water and dried in an oven at 40 0C for 2 days. In order to remove the template surfactant, the resulting materials were washed with three portions (500 mL each) of a mixture of ethanol:HCl (1:1) and then dried as above.



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Doping of the PDI dye into SBANI. The PDI chromophore was doped into the pores of SBANIc by stirring the SBANIc powder (20 mg) in chloroform solutions of the PDI (10 mL) for 20 hours, followed by filtration. The solids were then washed with 10 mL CHCl3 to remove loosely bound dye molecules and dried in an oven (50 0C, 6 hours). Solutions with the following [PDI] were employed: 1 x 10-6 M, 1 x 10-5 M, 1 x 10-4 M and 1 x 10-3 M. Slightly reddish powders were obtained, with a PDI/NI ratio of 0.2 mol%, 0.6 mol%, 2 mol% and 9 mol%, respectively, for the above solutions (molar ratios were obtained from the reflectance spectra, see below). In the case of SBANId, the same procedure was used, but only one sample was prepared ([PDI] = 1 x 10-3 M). Equipment. SAXS measurements were performed at the National Laboratory of Synchroton Light (LNLS), using a beam with wavelength of 1.55 Å. High resolution transmission electron microscopy (HRTEM) images were obtained with a JEOL JEM 2100 microscope operating at 200 kV. Samples for HRTEM were prepared by drop-casting an aqueous suspension of the mesoporous materials over a carbon-coated copper grid, followed by drying under ambient conditions. Nitrogen adsorption isotherms were collected with a Nova 2200 Surface Area and Pore Size Analyzer (Quantachrome). Surface areas were obtained with the BET method. Pore size distributions were obtained with the BJH method (desorption branch). Elemental analysis (CHN) was carried out at the Analytical Center of Sao Paulo University. Thermogravimetric analysis was performed with a TGAQ500 (TA Instruments), under N2 purge (heating rate: 20 0C/min). Infrared spectra were collected with a Perkin Elmer Frontier FTIR System, with the samples pressed in KBr pellets. Absorption spectra were collected with a Varian Cary 50 UV-visible spectrophotometer. Diffuse reflectance spectra (DRS) were also collected using the Cary 50, with an external optic fiber accessory (Barrelino).


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For this purpose, a small amount of the powder was placed on a sheet of white printing paper and pressed with a spatula to form a regular spot (the paper alone was used as the baseline). Steadystate fluorescence spectra were registered with a Varian Cary Eclipse Fluorimeter. Fluorescence decay curves were collected with a FLS-980 Fluorescence Spectrometer (Edinburgh Instruments), with a pulsed LED excitation source (330 nm) operating at 500 KHz. The emission was monitored at 470 nm. The fluorescence decay curves were analyzed for exponential decay with the software provided with the instrument. For the solid-state fluorescence measurements, the powders were placed in a capillary tube sealed at one end.



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Results and discussion Small angle X-ray scattering (SAXS). Figure 3 shows diffractograms of the two SBANI samples, before and after solvent extraction. The X-ray data are summarized in Table 1, in comparison with a sample of non-modified SBA-15. The diffraction patterns of the SBANI samples are typical of SBA-15 materials with 2D-hexagonal p6mm symmetry,23 showing that the presence of the NI moiety did not disrupt the mesoporous order of the materials. It can be noted that the peak positions moved to lower 2-theta values with increasing NI load, for both assynthesized and extracted samples (Figure 3), implying that the cell parameter a0 (which is the distance between neighboring pore centers) increased for higher imide loads (Table 1). These results suggest that the bulky NI chromophore had a swollen effect on the Pluronic micelles, as observed with other swollen agents usually added in the synthesis of SBA-15.23 The diffractograms in Figure 3 suggest that SBANIc was more well-structured than SBANId, both in the as-synthesized state and after surfactant removal. The peaks indexed as 110 and 200 were quite intense in SBANIc, and other higher order reflections were well visible (Figure 3, inset). The peaks in SBANId, on the other hand, were less intense, and higher order reflections were barely visible, indicating a lower degree of order. This finding is striking, since in most co-condensation reactions the order of the material decreases with increasing concentration of the organic counterpart.15 These results suggest that TEPNI acts as an adjuvant in the process of pore templating, probably by locating itself at the interface of the Pluronic micelles during the synthesis, resulting in highly organized micelles.


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Table 1. SAXS and nitrogen isotherms parameters.a Sample

2θ100

d100

a0b

SBET

Vpore

BJH pore size

wtc

(nm)

(nm)

(m2/g)

(cm3/g)

(nm)

(nm)

SBA-15

0.90

9.8

11.3

608

0.94

7.0

4.3

SBANId-as

0.98

9.0

10.4

-

-

-

-

SBANIc-as

0.95

9.4

10.8

-

-

-

-

SBANId-ex

0.96

9.2

10.6

401

0.51

3.8

6.9

SBANIc-ex

0.90

9.8

11.3

409

0.71

4.8

6.5

a

as: as-synthesized; ex: solvent extracted. ba0 = 2d100/√3. cwall thickness = a0 – pore size.



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300

100

1E-3

200

110

0.01

0.6

intensity (a.u.)

210

0.1

200

A

110

0.8

100

11

0.4

0.5

1.0

1.5

x 10

2.0

2.5

3.0

extracted samples SBANIc SBANId

0.2

0.0 1.0

1.5

2.0

1.5

B

100

2θ (degrees)

1

1.0

200

110

0.1

0.5

210

0.01

100

intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1E-3 0.5

1.0

1.5

2.0

2.5

3.0

as-synthesized samples SBANIc SBANId 0.0 1.0

1.5

2.0

2θ (degrees)

Figure 3. SAXS of the SBANI samples. (A) extracted samples. (B) as-synthesized samples. The insets show the same data plotted in a log scale, stressing the higher order reflections.


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Figure 4. TEM images of SBANIc (A and B) and SBANId (C and D).



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High resolution transmission electron microscopy (HRTEM). Figure 4 shows HRTEM images of the extracted samples SBANId and SBANIc. A regular arrangement of parallel channels can be clearly seen for both samples, as expected for a 2D-hexagonal arrangement.23 Furthermore, images acquired with the proper orientation revealed the honeycomb arrangement of pore openings typical of 2D-hexagonal SBA-15.23 Thus, the TEM images confirm the SAXS results, showing that the SBANI materials were highly organized and display the expected hexagonal symmetry. Nitrogen adsorption isotherms. Figure 5 shows nitrogen adsorption isotherms for the modified samples in comparison to a sample of non-modified SBA-15 as a reference. BJH pore size distributions are shown in the inset. All samples showed type-H1 hysteresis, typical of SBA15 materials.23 The parameters obtained from the isotherms are summarized in Table 1. The wall thickness (wt) of the materials was calculated with wt = a0 – pore size. It can be noticed a decrease in the surface areas and pore volumes of the SBANI samples relative to the reference SBA-15 (Table 1), indicating that the NI moieties occupy part of the void space within the channels. It can be observed in Table 1 that SBANIc and pristine SBA-15 have both the same value of a0 = 11.3 nm (a0 is the sum of the pore diameter plus the wall thickness). However, SBANIc displayed narrower pores and thicker walls than SBA-15 (the decrease of ca 2 nm in pore size in SBANIc was compensated by a similar increase in wall thickness, giving the same a0). These results suggest that the NI molecules formed a shell lining the pore walls, so increasing its thickness (Figure 1), as expected for the co-condensation method. This arrangement is quite favorable for efficient energy transfer to acceptor dyes inside the pores. The wall thickness increase of ca 2 nm in SBANIc as compared to SBA-15 corresponds to approximately 3 NI


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14

molecules, indicating that some homocondensation of the TEPNI precursor took place. Curiously, SBANId displayed narrower pores and thicker walls than SBANIc (Table 1), in spite of containing a lower NI load. This phenomenon can be attributed to the presence of residual surfactant within the pores of SBANId, as confirmed by elemental and thermogravimetric analysis (see below).

800

1.0

700 600

0.5

500 3

Vads (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.0

400

30 40 50 60 70 80 90 100

300 200 SBA-15 SBANIc SBANId

100 0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0

Figure 5. Nitrogen adsorption isotherms of SBA-15 and SBANI samples. Inset: Pore size distribution (BJH, desorption branch).



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Elemental analysis. Table 2 shows the elemental analysis of the SBANI samples before and after extraction with ethanol. The presence of C and N in the samples confirms the incorporation of the organic molecules into the materials. The imide loads can be calculated from the nitrogen content (since the surfactant does not contain nitrogen) and are given in Table 2 as both weight percent and mmol/g. It can be seen that the concentrated sample incorporated about 4-5 times more imide than the diluted sample, consistent with the solution concentrations. It has been usually accepted that the maximum amount of organic compound that can be added in the co-condensation method without loss of structure is ca 20 mole percent.32 This estimate based on the molar fraction, however, is questionable, since a bulky molecule will occupy a larger mass (and volume) fraction of the material than a small molecule, even if the mole fraction is the same. In the present case, the amount of the bulky NI chromophore in SBANIc was over 10% on a mass basis (Table 2), even though the mole fraction of NI was less than 1%. Therefore, the highly ordered structure observed for SBANIc is striking for the high dye loading. The relative amounts of 1,8-naphthalimide and poly(alkylene oxide) surfactant can be estimated from the C/N ratios. A C/N ratio of 12.8 is expected for a sample containing only the imide, as calculated for N-propyl-1,8-naphthalimide, which is representative of the imide fragment bound to the silica. Table 2 shows that the C/N ratio of SBANIc was close to that expected for the imide, even before the extraction, indicating that all the triblock copolymer surfactant was removed during the filtration step at the end of the synthesis. In contrast, SBANId had very high C and H content before extraction, evidencing the presence of the surfactant. Even after the extraction it appears that some surfactant was retained in the pores of SBANId, since the C/N ratios are still higher than expected for the imide alone. This finding is in agreement


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with the nitrogen isotherm results (Table 1). The different affinity of the SBANI samples towards the surfactant is consistent with the surface polarity of the pore walls. The sample with lower NI has a great amount of surface hydroxyls exposed, which could form hydrogen bonds with the surfactant (Figure 1A). The sample with higher NI load, on the other hand, apparently have the pore walls fully lined with naphthalene rings from the NI molecules, rendering the surface hydrophobic, with low affinity towards the poly(alkylene oxide) surfactant (Figure 1B). Literature reports of PMO with the pore walls consisting of naphthalene groups were also shown to have hydrophobic pore surface.20,21

Table 2. Elemental analysis of the samples before and after extraction with acidic ethanol.a,b sample

a

%C

%H

%N

C/N

NI load

NI load

(weight %)c

(mmol/g)c

SBANId-as

30.6

5.4

0.12

255

2.1 %

0.086

SBANId-ex

7.0

2.6

0.14

50

2.4 %

0.10

SBANIc-as

6.8

1.6

0.82

8.3

13.9 %

0.58

SBANIc-ex

6.7

1.7

0.64

10.5

10.9 %

0.46

average of two measurements. bas: as-synthesized; ex: solvent extracted. cbased on N.



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Thermogravimetric analysis (TGA). Figure S1 shows TGA and the corresponding differential thermogravimetric curves (DTG) of the SBANI samples after extraction with ethanol. The SBANId sample lost more weight (18%) than SBANIc (14%), in agreement with the presence of surfactant in the pores. An additional degradation process was observed in the temperature range 250 – 350 0C for SBANId (Figure S1B), which can be attributed to degradation of the triblock copolymer surfactant,23 since the imide starts degrading only above 350 0C. Another interesting feature is that the imide in SBANIc decomposed in three steps, the most steep one centered at 560 0C. With SBANId, on the other hand, the imide was degraded in a single step centered at 460 0C (Figure S1B). These results show that the NI molecules were stabilized in SBANIc, probably due to π–stacking of the aromatic rings. Infrared spectra. Figure S2 shows the infrared spectra of the samples after extraction with ethanol. In addition to the strong bands attributable to the silicate framework, the carbonyl region of the spectra (shown in detail in Figure S2B) shows two bands at 1690 and 1647 cm-1 for SBANIc, which can be attributed to the symmetric and asymmetric imide carbonyl stretches, respectively (another band at 1593 cm-1 is due to a vibration mode of the aromatic ring). The imide carbonyl bands are barely visible in the case of SBANId, which is due to the small amount of the dye in this sample and the overlap with the SiO2 band at 1637 cm-1. The bands in the region 3020-2840 cm-1 can be assigned to methylene C-H stretching due to the N-propyl linker connecting the imide ring to the pore walls, although in the case of SBANId the ethylene and propylene oxide chains from the surfactant could contribute. Bands due to the Pluronics surfactant can be seen for SBANId in the region 1500-1320 cm-1 (Figure S2A),33 which are quite intense in the infrared spectrum of the as-synthesized SBANId (Figure S3).


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Optical properties of SBANI. Figure 6 shows DRS spectra of the SBANI samples converted to absorption units. The spectra are quite similar to the absorption spectra of 1,8naphthalimide derivatives reported in the literature,31,34-36 with an intense band near 230 nm and a less intense, vibrationally structured band in the range 300 – 370 nm. The SBANIc sample absorbs more intensely than SBANId, in agreement with the higher chromophore density. Moreover, the spectrum of SBANIc was much broader than that of SBANId and a change in the relative intensity of the vibrational peaks was also observed in the lower energy absorption band. In SBANId, the peak with maximum at 342 nm was the most intense, whereas in SBANIc the most prominent peak was at 336 nm. These effects could also be noticed in the fluorescence excitation spectra (Figure 7). These results suggest that the NI moieties existed as isolated molecules in SBANId, but were packed together in SBANIc (Figure 1). The fluorescence spectra of the two samples in the solid state showed quite different behavior (Figure 7). SBANId displayed a typical monomer-like emission spectrum (λmax = 395 nm), which was a mirror image of the excitation spectrum (Figure 7, blue lines), with a low Stokes shift (42 nm). In contrast, the emission of SBANIc was characteristic of excimers (or aggregated) species.36,37 The emission maximum of SBANIc (λmax = 470 nm) was red-shifted by almost 80 nm relative to the diluted sample, presenting a quite large Stokes shift (135 nm). Furthermore, the SBANId powder showed a blue fluorescence when illuminated with a handheld UV lamp (365 nm), whereas SBANIc displayed a greenish emission. These results show that the optical properties of the SBANI materials can be easily controlled by varying the synthetic conditions. Excimer emission have also been observed in PMOs containing other chromophores, such as biphenyl4 and naphthalene.20



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19

0.8

x3 336 nm

0.6

absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

342 nm

0.4

0.2

SBANIc SBANId

0.0 200

250

300

350

400

450

wavelength (nm)

Figure 6. Diffuse reflectance UV-visible spectra of the SBANI samples (presented in absorbance units). The spectrum of SBANId was rescaled as indicated.


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20

1.2

336 nm 352 nm 395 nm

470 nm

1.0

normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.8

ex SBANIc (em = 470 nm) em SBANIc (ex = 350 nm) ex SBANId (em = 395 nm) em SBANId (ex = 340 nm)

0.6

0.4

0.2

0.0 300

350

400

450

500

550

600

wavelength (nm)

Figure 7. Fluorescence spectra of the SBANI samples.



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Doping of SBANI powders with the PDI dye. The fluorescent SBANI materials are suitable to study FRET from a donor on the pore walls to an acceptor inside the pores, as reported in the literature for other dyes.1-5 In the present work, we chose to use 2,6dimethylphenyl-3,4,9,10-perylenediimide (Figure S4), a sterically hindered PDI derivative, as the acceptor dye, since there is substantial overlap between the excimer-like emission spectrum of SBANIc and the absorption spectrum of the PDI (Figure S5). A good spectral overlap is an important condition for efficient energy transfer. The studies were performed mainly with SBANIc, because the overlap between the emission of SBANId and the absorption of PDI was much lower (Figure S5). Incorporation of the PDI dye into the pores of SBANIc was successful, as judged by the reddish color presented by the powders after exposing to PDI solutions. Furthermore, the DRS spectra of the powders (Figure 8), showed the appearance of the PDI absorption band (420 – 650 nm), which became more intense with increasing dye load. The molar ratio PDI/NI in SBANIc was obtained from the absorbance ratios in Figure 8 (details of the calculations are given in Table S1), considering the molar absorptivity coefficients (ε) of the compounds in solution (10,000 M-1cm-1 for N-butyl-1,8-naphthalimide34 and 60,000 M-1cm-1 for PDI). In the case of SBANId, the DRS spectra (Figure S6) showed that a lower amount of the PDI dye was incorporated for the same [PDI] in solution. This result suggests that the apolar PDI dye has a higher affinity towards SBANIc than SBANId, which is due to the low polarity pore walls of the former, lined with naphthyl groups. It can be noticed that the absorption spectra of PDI in SBANIc (Figure 8) was broadened compared to the spectrum of the same compound in chloroform solution (Figure S7). It is unlikely that the broadening was due to a strong NI-PDI ground-state interaction, since a similar


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broadening was reported for PDI derivatives bound to SBA-15 by the grafting method, without the presence of a second dye (Figure S7).38 Spectral broadening is common in the reflectance spectra of PDI in powdered samples (Figure S7), and can be attributed to irregular surfaces and scattering effects, and perhaps to some PDI self-aggregation (see below).



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1.0

absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SBANIc (no PDI) -6 1 x 10 M PDI -5 1 x 10 M PDI -4 1 x 10 M PDI -3 1 x 10 M PDI 0.5

0.0 300

400

500

600

700

wavelength (nm)

Figure 8. Diffuse reflectance spectra of SBANIc (presented in absorbance units) doped with varying amounts of the PDI dye (given as the [PDI] in the solution used for the incorporation). The spectra were normalized to Amax = 1 (at the NI absorption band) to stress the PDI/NI absorption ratios.


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Energy transfer from NI to PDI. The good overlap between the emission spectrum of SBANIc and the absorption spectrum of PDI (Figure S5) suggests that FRET is likely to take place in this system. The Förster radius (R0) is defined as the donor-acceptor distance where the FRET efficiency is 50%.39 In other words, when the fluorescence intensity of the donor in the presence of the acceptor is quenched to half its initial value (in the absence of the acceptor), the mean donor-acceptor distance equals R0. The Förster radius can be readily estimated from the spectral overlaps in Figure S5 and the fluorescence quantum yield of the NI donor (ΦD) using Equations 1-3 (Supporting Information).39 Using the value ΦD = 0.1 reported for N-butyl-1,8naphthalimide in ethanol,34 one obtains R0 = 3.1 nm for SBANIc/PDI and R0 = 2.5 nm for SBANId/PDI (details of the calculations are given as Supporting Information). Considering the pore diameters given in Table 1 (3.8 nm for SBANId and 4.8 nm for SBANIc), the guest PDI would be positioned within Ro from the donor in both samples, even if it was located at the pore center. Therefore, FRET from the NI on the pore walls to the PDI within the pores is predictable to occur from these calculations. Emission spectra of the SBANIc powder doped with different amounts of PDI are shown in Figure 9A. The donor-acceptor system was excited at 350 nm, where the NI donor absorbs strongly, while the PDI acceptor does not (Figure S4). It can be noticed that PDI efficiently quenched the fluorescence emission of NI, suggesting the occurrence of FRET from the excited NI donor in SBANIc to the PDI acceptor (Figure 2), as expected from the spectral overlap in Figure S5. The quenching efficiency (1 – I/I0) for SBANIc doped with PDI was plotted against the PDI content in Figure 10. Efficient quenching was observed even at low PDI doping. A doping of 0.2 mol% (PDI/NI ratio of 1/500) was enough to reduce the fluorescence intensity to nearly half the initial value. This means that light absorbed by about 250 NI molecules was



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funneled to a single PDI molecule at 0.2 mol% doping, suggesting energy hopping between NI molecules. With 0.6 mol% PDI the emission intensity was already close to the maximum quenching observed (plateau at ca 80% efficiency, Figure 10). These results mean that the average donor-acceptor distance was less than R0 for samples with more than 0.6 mol% PDI. Furthermore, the emission spectra of SBANIc with high PDI contents spanned the whole visible range (Figure 11A), and the samples presented a white fluorescence under UV illumination, which is quite desirable for the construction of LEDs.40-42 Figure 11B shows the CIE 1931 chromaticity diagram with the points corresponding to the emission spectra (with λex = 350 nm) of PDI-doped, as well as non-doped SBANI samples. It can be seen that the points span a range of colors, as observed by the naked eye with the hand-held 5W UV lamp. Particularly, the point corresponding to SBANIc + 9 mol% PDI (point d in Figure 11B) is very close to the coordinates corresponding to true white light (x = 1/3; y = 1/3).40 The quenching efficiency observed here with the pair SBANIc-PDI (Figure 10) was of similar magnitude as that reported by Mizoshita et al, who observed a quenching efficiency of ca 80% at 1% PDI doping for energy transfer from a tetraphenylpyrene (TPPy)-PMO host to a perylenediimide guest.43 Note that the Förster radius for the pair SBANIc-PDI reported here (R0 = 3.1 nm) was shorter than that reported for the pair TPPy-PDI (R0 = 4.5 nm).43 According to Equation 1 (Supporting Information), the shorter the Förster radius, the weaker the donoracceptor interaction. Thus, a similar quenching efficiency in spite of the weaker interaction suggests that donor and acceptor are in closer proximity in SBANIc as compared to the TPPyPMO. This can be attributed to the co-condensation method employed here, which places the NI donors sticking towards the pore centers (Figure 2), in contrast to PMO, where the donors are inside the silica walls.15-17


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600

SBANIc (no PDI) SBANIc + 0.2 mol% PDI SBANIc + 0.6 mol% PDI SBANIc + 2 mol% PDI SBANIc + 9 mol% PDI

fluorescence (a.u.)

A 400

ex = 350 nm

200

0 400

450

500

550

600

650

wavelength (nm)

200

fluorescence (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

B

SBANIc (no PDI) SBANIc + 0.2 mol% PDI SBANIc + 0.6 mol% PDI SBANIc + 2 mol% PDI SBANIc + 9 mol% PDI

150

ex = 470 nm 100

50

0 550

600

650

700

wavelength (nm)

Figure 9. Emission spectra of solid SBANIc samples doped with increasing amounts of the PDI dye. (A) Excitation at the NI absorption band (λex = 350 nm). (B) Excitation at the PDI absorption band (λex = 470 nm).



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1.0

0.8

quenching efficiency (1 - I/I0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SBANIc SBANId homogeneous solution

0.6

0.4

0.2

0.0 0

1

2

3

4

5

6

7

8

9

10

PDI/NI (mol%)

Figure 10. Quenching efficiency (1 – I/I0) at λem = 473 nm (with λex = 350 nm) as a function of the amount of PDI for SBANIc (●) and SBANId (■). Data for the quenching of N-butyl-1,8naphthalimide by PDI in homogeneous solution of CHCl3 (▲) are also shown.


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Figure 12 shows time-resolved fluorescence decay traces for doped and undoped SBANIc, with excitation at 330 nm and emission monitored at 470 nm. No fluorescence buildup corresponding to excimer formation was observed for SBANIc in the time scale of Figure 12, confirming that the excimer–like emission in the SBANIc materials was due to ground-state aggregation of NI, which is in agreement with the changes in the reflectance spectra observed in Figure 6. It can be noticed a faster decay in the presence of the PDI, confirming the occurrence of radiationless energy transfer from the NI donor to the PDI acceptor. The traces could be fit as a by-exponential decay for SBANIc, with time constants τ1 = 6.6 ns and τ2 = 35.9 ns, and a triexponential decay for SBANIc + 9 mol% PDI, with τ1 = 2.2 ns, τ2 = 9.6 ns and τ3 = 48.0 ns. The two faster time constants can be attributed to monomer and aggregated (excimer-like) emission of NI, respectively, which is in agreement with literature reports of fluorescence lifetimes in the range of 2 – 3 ns for monomeric and about 30 ns for excimeric NI derivatives34,36 (the third time constant in the PDI-doped sample could be related to PDI emission). We are presently performing more detailed time-resolved studies, which will be published elsewhere. The quenching was less efficient in the SBANId powder, as compared to SBANIc (Figure 10), as expected for the lower spectral overlap (Figure S5). Fluorescence quenching of N-butyl-1,8-naphthalimide, a soluble NI derivative, by the PDI dye in homogeneous solution (chloroform) was also studied (Figure S8). Quenching in homogeneous solution is due to the trivial emission-reabsorption process (radiative energy transfer). As seen in Figures S8 and 10, the quenching efficiency for the radiative energy transfer was much lower than with the SBANI powders, confirming that FRET indeed occurred in the mesoporous donor-acceptor materials. It can be noticed in Figure 9A that, in spite of the strong quenching, the emission of the acceptor PDI was not detected until the amount of PDI reached 2 mol%. A possibility to be



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considered is that photoinduced electron transfer (PET) from the NI to the PDI occurred concomitantly with energy transfer, resulting in non-fluorescent PDI radical anions, since the PDI dyes are known as good electron acceptors.44,45 Mizoshita et al, for instance, observed both energy and electron transfer from TPPy in the walls of PMOs to perylenediimides inside the pores.43 Nevertheless, the 1,8-naphthalimides are also good electron acceptors, but not electron donors.31,35-37,46 Therefore, PET from NI to PDI is unlikely. The absence of PDI emission in the samples with low doping is most likely due to the small amount of PDI, out of the detection range. For 0.2 mol% doping, the PDI emission was hardly detected even when it was directly excited at the PDI absorption band at 470 nm (Figure 9B). Mizoshita et al, who studied FRET from TPPy-PMO to PDI guests, also found a weak emission of the PDI acceptor,43 in contrast to the strongly emitting behavior of other acceptor dyes in PMO, such as coumarin 1 and rhodamine 6G.2,4 Figure 9B shows the emission spectra of the PDI-doped SBANIc under direct excitation of the PDI chromophore at 470 nm. Note that the emission spectrum of PDI at low dye doping (0.6 mol% PDI) was typical of monomeric PDI (the higher energy vibronic band peaking at 542 nm was the most intense, with a shoulder at 580 nm).47 As the PDI content increased, however, a red-shift in the spectra and a concomitant change in the vibronic structure were observed. At high PDI doping, the lower energy band (peaking at 590 nm) became more intense than the high energy one (Figure 9B). These spectral effects could be partly due to self-absorption by PDI at high concentrations, but it is likely that PDI aggregates were formed at high doping levels, since the absorption spectrum of the sample with highest doping shows an absorption tail extending beyond 600 nm (Figure 8).


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The emission spectra of SBANIc containing 9 mol% PDI with excitation at 350 nm and 470 nm are compared in Figure 11A. Some interesting features can be observed. Let us first examine the emission of NI (350 - 500 nm). The excimer-like emission (470 nm band), corresponding to aggregated NI, was quenched preferentially over the monomer emission (400 nm band), corresponding to isolated NI molecules. SBANIc still contained a certain proportion of monomeric NI, as judged by the shoulder at ca 400 nm seen in Figure 9A. With the addition of PDI, the excimer-like band was quenched more strongly than the monomer band (Figure 9A). At 9 mol% doping, the two bands have nearly the same intensity (Figure 11A). These results suggest that the stacked NI molecules, rather than the isolated ones, are the responsible for energy transfer to the PDI guests. When the emission of PDI (500 – 700 nm) was considered, it can be observed a monomer-like spectrum when the excitation was via FRET from NI (ex = 350 nm), as compared to the direct excitation of PDI at 470 nm, which yielded an emission spectrum with vibronic structure typical of stacked PDI (Figure 11A).47 These results suggest that the energy was transferred preferentially from stacked NI molecules, and it was channeled preferentially to isolated PDI molecules. The effect of the excitation wavelength on the emission spectra of PDI in homogeneous solution was quite different from that observed with SBANIc (Figure S9). In a chloroform solution of PDI (Figure S9A), emission with the excitation at 470 nm was by far more intense (ca 7 x) than that with excitation at 350 nm, as expected, considering that the PDI dye absorbs only very slightly at 350 nm (Figure S4). However, in SBANIc containing 9 mol% PDI the opposite trend was observed (Figure S9B). The PDI emission was more intense when excited at the NI absorption band at 350 nm than direct excitation of PDI at 470 nm, providing further



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evidence for efficient FRET from NI to PDI. Furthermore, in the excitation spectrum of SBANIc doped with 9 mol% PDI (Figure S10), the NI excitation band was detected, even when the emission was fixed at 550 nm, where the PDI emits, confirming that excitation of NI resulted in the emission of PDI.

Conclusions Highly structured mesoporous silicates functionalized with 1,8-naphthalimides were successfully prepared by co-condensation with TEOS. The color of the fluorescence could be tuned by varying the amount of NI in the materials, thus controlling the aggregation state of the dye. The SBANI materials still contained void volume within the pores, where a PDI dye was successfully doped. Efficient energy transfer was observed from the NI donors at the pore walls to the PDI acceptors inside the channels. The PDI-doped SBANI materials showed emission over most of the visible spectrum, rendering the materials strong candidates for white emitting LEDs.

Acknowledgments S. B. acknowledges the support of Brazilian agencies FAPESP (grant No 2012/16358-9) and CNPq (grant No 480189/2011-0). E. R. T. was supported by FAPESP (grant No 2015/06064-6). F. J. T. thanks FAPESP for a doctoral fellowship (process number 2007/087201). The authors wish to thank the LNLS for allowing the use of a synchrotron beam for the SAXS measurements (Research Proposal SAXS1 - 13709).


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aggregated NI

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aggregated PDI

monomeric NI

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

monomeric PDI

0.5

ex 350 nm ex 470 nm

SBANIc + 9 mol% PDI 0.0 350

400

450

500

550

600

650

wavelength (nm)

Figure 11. (A) Normalized emission spectra of SBANIc doped with 9 mol% PDI, with excitation either at the NI absorption band (λex = 350 nm) (−) or at the PDI absorption band (λex = 470 nm) (−). (B) CIE 1931 chromaticity diagram showing the points corresponding to the emission spectra (with λex = 350 nm) of PDI-doped SBANIc (●) and SBANId (□). (a) non-doped SBANIc; (b) SBANIc + 0.6 mol% PDI; (c) SBANIc + 2 mol% PDI; (d) SBANIc + 9 mol% PDI; (e) non-doped SBANId; (f) SBANId + 6 mol% PDI.



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SBANIc SBANIc + 9% PDI

I (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

t1 = 6.6 ns t2 = 35.9 ns

2500

t1 = 2.2 ns t2 = 9.6 ns t3 = 48.0 ns 0 0

10

20

30

40

50

t (ns)

Figure 12. Time-resolved fluorescence traces monitored at 470 nm for undoped SBANIc and SBANIc doped with 9 mol% PDI (excitation = 330 nm). The solid lines represent the best fit for exponential decay.


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Supporting Information Available Thermogravimetric analysis (Figure S1) and infrared spectra (Figures S2 and S3) of SBANIc and SBANId samples. Excitation and emission spectra (Figure S4) of N,N´-bis(2,6dimethylphenyl)-3,4,9,10-perylenediimide (PDI) in solution. Spectral overlap (Figure S5) between the emission of solid SBANI samples and the absorption spectrum of PDI in solution. Reflectance spectra (Figure S6) of SBANId undoped and doped with the PDI dye. Reflectance spectra (Figure S7) of PDI incorporated in SBA-15 with and without NI. Emission spectra (Figure S8) showing the quenching of N-butyl-1,8-naphthalimide by PDI in solution. Emission spectra (Figure S9) of PDI in solution and in the SBANIc powder with excitation at different wavelengths. Excitation spectra (Figure S10) of SBANIc + 9 mol% PDI with the emission fixed at the emission band of PDI. Calculation of Förster radius (Figure S11). This information is available free

of charge via the Internet at http://pubs.acs.org



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