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Ultrafast Dynamics of Nile Red Interacting with Metal Doped Mesoporous Materials Cristina Martin, Piotr Piatkowski, Boiko Cohen, Micha# Gil, Maria Teresa Navarro, Avelino Corma, and Abderrazzak Douhal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02296 • Publication Date (Web): 13 May 2015 Downloaded from http://pubs.acs.org on May 22, 2015

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The Journal of Physical Chemistry

Ultrafast Dynamics of Nile Red Interacting with Metal Doped Mesoporous Materials Cristina Martín,1 Piotr Piatkowski,1 Boiko Cohen, 1 Michal Gil, 1≠ María Teresa Navarro, 2 Avelino Corma,2 and Abderrazzak Douhal*,1

1

Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica,

and Inamol, Universidad de Castilla-La Mancha, Avda. Carlos III, S.N., 45071 Toledo, Spain. 2

Instituto de Tecnología Química, UPV-CSIC, Avenida de los Naranjos s/n, 46022 Valencia, Spain

* corresponding author: Abderrazzak Douhal, email: [email protected], Phone number: +34-925-265717

≠ current direction: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01224 Warsaw, Poland,

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Abstract

We report on ultrafast studies of Nile Red (NR) interacting with MCM41 mesoporous materials doped by Al, Ga, Zr and Ti in dichloromethane suspensions. The steady-state results showed a significant red shift and broadening of the diffuse transmittance and the emission spectra upon interaction with the MCM41-based materials. These findings are explained in terms of H-bonds with the host, the different Brønsted/Lewis interactions with the matrix and formation of H- and J-aggregates, in addition to weakly and strongly adsorbed monomers. The pico- to nanosecond time-resolved data support this explanation, showing a significant shortening in the emission lifetimes where NR is interacting with metal-doped MCM41. The femtosecond dynamics of NR loaded into X-MCM41 (X= Si, Al, Ga) indicate that the chargeseparated state (CS) is formed at the S1 state in ∼350 fs. While for Zr- and Ti- MCM41 hosts the intramolecular charge transfer (ICT) occurs in less than 200 fs, and a subsequent electron injection to Ti or Zr trap states happens in ∼ 250 fs. Our studies reveal a strong interaction between the NR species and the framework of MCM41 materials at both the S0 and S1 states.


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1. Introduction Silica is one of the most abundant materials in the Earth’s crust and it has a key role in many applications due to its chemical and thermal stability, relatively easy processing and modification.1-2 Therefore nowadays, when the number of applications is rapidly growing, new silica based materials with higher control of their structures and larger specific surface area are demanded.3 The development of ordered porous silica materials,4-5 like zeolites,6 emerged as one of the most attractive alternatives to amorphous silica material, due to their unique physical and chemical properties.7 The zeolite molecular sieves are characterized by large specific surface areas, ordered pore systems and well-defined pore radius distributions.67

However, the pore size of many of these materials is restricted to only few nm,8-9 which

restrict their application when encapsulation of large molecules is desired. The discovery of a family of mesoporous silica materials known as M41S,10 from which the hexagonal-face MCM41 is probably the most popular one,11-12 has allowed overcoming such limitation.13-14 The versatility of these materials lies in the simplicity of the synthetic approaches, which allow their easy modification and functionalization, thus leading to more controlled properties, such as tunable pore size (10-200 Å) or presence of specific reaction centers.15-16 As a result complex MCM41 materials have been made and being used in a broad range of applications: for example sensors, solar cells, drug delivery or catalysis. 17-21 A large number of reports have demonstrated that functionalization of the MCM41 surface by introducing different tri- and tetra-valent metals (X-MCM41),15,

22-23

such as

aluminum24 or titanium,25 increases their catalytic activity.26-29 This behavior was explained in terms of the influence of the metal (X) and the Si/X atomic ratio of the MCM41 physicochemical properties that induce different acidity on the MCM41 surface.15,

22-28

Although the influence of these parameters on the MCM41 catalytic properties for different reactions has been studied in terms of the Brønsted and Lewis acidities,15,28,

30

there is no

evidence of their effect on the key factors, which govern the molecule-matrix interactions during the involved reactions. Thus, additional studies are needed to understand the influence of the acid-base properties at the host framework on the interactions with the loaded molecules, which in principle would facilitate the targeted design of composite materials. The knowledge and the understanding of the interactions between the organic dye molecules and silica-based materials help to determine the key processes that govern the molecules adsorption, and therefore to comprehend the reaction mechanisms involved in hybrid complexes. Several methodologies have been proposed to unravel the physical and chemical properties of pure and doped MCM41 materials.7,


11, 15

In this respect, the 3


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spectroscopic techniques proved to be very useful tools by taking advantage of the spectral and dynamical changes of the fluorescent guest molecules to research various properties of MCM41, such as acidity, polarity, porosity or surface charge

31-32

The optimal probe dye

molecule should be highly sensitive to the microenvironment nature of the mesoporous material leading to distinct absorption and/or emission spectral response.33 Among the organic probes, that are suitable to monitor their interactions with MCM41, Nile Red (NR, Scheme 1) is a good candidate due to its high sensitivity to the polarity of the medium and hydrogenbonding interactions.33-34 Several experimental and theoretical works have reported that the dependence of the NR photobehavior on the solvent polarity is due to the intramolecular charge-transfer character in its electronically first singlet excited-state.35-37 This is a result of the presence of and electron donor (diethylamino) and an electron acceptor (quinoid) moties in the molecular structure.38 Since the mid-1980’s, the NR spectroscopic behavior has been widely employed to assess the chemical properties and structural changes in heterogeneous and/or organized media, such as ionic liquids,39 polymeric nanoparticles (NPs),40-41 micelles,42-43 core-shell systems,44 cyclodextrins,45 intracellular lipids,46 zeolites47-48 and proteins.49 Furthermore, several studies of NR interacting with silica materials, like silica-solgel or SBA-15, have demonstrated that NR forms different aggregates as a result of interactions with the host matrix.50-54 Despite these studies, the relaxation dynamics of NR in complex systems has been characterized only on a picosecond time scale, even though the electronic relaxation in the hybrid complexes is often determined by the very first events (femto- and picosecond (fs-ps) time regime) occurring upon the photon absorption.31, 55 As far as we know there is no ultrafast study of NR interacting with silica based materials. Hence, the use of laser-based techniques with higher temporal resolution would provide further insights into the ultrafast processes occurring within the NR/MCM41 systems as we have shown for other hybrid materials.56-61 The objective of this work is to gain a deep understanding of the photophysical properties of hybrid NR doped MCM41 materials by probing the interactions within the formed complexes (with and without low percent of doping metals (X) in the framework, X = Al, Ga, Ti, Zr) in dichloromethane (DCM) suspensions. We studied the steady-state UVvisible absorption and emission spectra together with femto- to nanosecond emission dynamical behavior of the formed complexes. The results indicate that the ground- (S0) and excited- (S1) state properties of the NR are sensitive to the nature of the mesoporous microenvironment reflecting the H-bonding ability and intramolecular charge transfer character of the dye, and the Brønsted and Lewis acidities of the host. More specifically, a


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large bathochromic shift of the absorption (~1850 cm-1) and emission (∼ 1322 cm-1) bands, as well as a fluorescence quenching are observed in MCM41 materials in dichloromethane (DCM) suspensions when compared to the NR in pure DCM solution. We found that the fluorescence lifetime for the monomers decreases when the molecule interacts with these materials from 4.4 ns in DCM solution to ∼1.9 - 2.7 ns in presence of the MCM41 hosts. The H- and J-aggregates have lifetimes in ∼0.16 - 0.32 ns and 0.5 - 1.2 ns, respectively. The early time dynamics of NR molecules are also affected by the nature of the doping metal, where the time constants of the intramolecular charge transfer (ICT) and solvation processes occurring in DCM solution (∼1 ps) are reduced to ∼350 fs for R-MCM41, Al-MCM41 and GaMCM41samples. For NR interacting with Ti-MCM41 or Zr-MCM41, in addition to the ICT processes, an electron injection (∼ 250 fs) from the excited NR to the transition metal centers is also observed. These results are explained in terms of the different acidity (in terms of Brønsted and Lewis definitions) of the doped MCM41 framework, which results in different interactions between the host framework and the NR molecules. The findings provide the basis for a better understanding of the reaction mechanisms involved in these complexes, and may help in the design of sensors to the acidity of silicabased materials, and of nanophotonic devices or understanding photocatalytic mechanisms using this kind of supports.

2. Experimental part Nile Red (NR), purely siliceous and aluminium doped MCM41 material (R-MCM41 and Al-MCM41, respectively) and Dichloromethane (DCM, spectroscopic grade > 99.5%) were purchased from Sigma-Aldrich and used without further purification. was also used as received. Galium (Ga-MCM41), Titanium (Ti-MCM41) and Zirconium (Zr-MCM41) doped MCM41

were

prepared

using

hexadecyltrimethylammonium

bromide

amorphous

silica

(C16TMABr

(Aerosil200,

98%wt,

Degussa),

Sigma-Aldrich),

and

tetramethylammonium hydroxide (25% wt TMAOH in water, Sigma-Aldrich). The sources of titanium, zirconium and gallium were titanium tetraethoxide (99%, Sigma-Aldrich), zirconyl chloride octahydrate (98%, Sigma-Aldrich) and gallium oxide (99.9%, Sigma-Aldrich), respectively. The synthesis gels were prepared with the following molar composition: SiO2:0.15C16TAB:0.26TMAOH:0.015Ti(C2OH5)4:24.3H2O

for

Ti-MCM41

sample;

SiO2:0.15C16TAB:0.26TMAOH:0.01ZrOCl2.8H2O:24.3H2O for Zr-MCM41 sample and SiO2:0.15C16TAB:0.26TMAOH:0.013 Ga2O3:24.3H2O for Ga-MCM41 sample. In general, ACS Paragon Plus Environment

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the TMAOH solution and the source of corresponding metal were added to an aqueous solution of C16TAB. When the solution was homogenized, the silica was added. The homogeneous mixture was stirred at room temperature for 1 hour and, subsequently, is heated at 135ºC for 24 hours at the autogeneous pressure in Teflon lined stainless steel autoclaves without rotation. The as-prepared Me-MCM41 sample was recovered by filtration and extensively washed with distilled water (2 litres/g solid) and then the material was dried at 60ºC overnight. The occluded surfactant was removed by heating the sample at 813K under a continuous flow of N2 for 1 h, followed by 6 h treatment in a flow of air at the same temperature. The final solid presents MCM41 structure (XRD not shown). The metal loading and the textural properties of the metal-MCM41 (X-MCM41) were evaluated (Table 1). The synthesized materials were characterized by powder X-ray diffraction (XRD) with Philips X´Pert X-ray diffractometer equipped with a graphite monochromator and operating at 45 kV and 40 mA and using CuKα radiation (λ=0.1542 nm). Chemical analyses were done using a ICP optical Emission Spectrometer Varian 715-ES, after dissolution of the solids in a HNO3/HCl/HF aqueous solution. Textural properties were determined by N2 adsorption isotherms measured at 77K with a Micromeritics ASAP 2020 volumetric adsorption analyzer. The Brunauer–Emmett–Teller (BET) specific surface area62 was calculated from the nitrogen adsorption data in the relative pressure range from 0.04 to 0.2. The total pore volume63 was obtained from the amount of N2 adsorbed at a relative pressure of about 0.99. The pore diameter was evaluated using the Barret-Joyner- Halenda (BHJ) method64 on the adsorption branch of the isotherms. The obtained values for each material are listed in Table 1. The composite samples in all the cases were prepared by adding 100 mg of dried MCM41 host materials to 10 ml of dichloromethane (DCM) solution containing NR (10-4, 10-5 or 10-6 M) and stirring at room temperature for 15 h. The obtained material was washed several times with DCM, in order to remove weakly adsorbed dyes. Steady-state UV-visible absorption and diffuse transmittance spectra were recorded on Jasco V-670 equipped with a 60 mm integrating sphere ISN-723. Emission spectra were recorded using Fluoromax-4 (Jobin-Yvone). The emission lifetimes were measured using a picosecond

(ps)

time-correlated

single-photon-counting

(TCSPC)

spectrophotometer

(FluoTime 200, PicoQuant) described elsewhere.65 The samples was excited by a 40 pspulsed (< 1 mW, 40 MHz repetition rate) diode-laser (PicoQuant) centered at 635 nm (instrument response function, IRF ~ 80 ps). The fluorescence signal, gated at magic angle (54.7°), was monitored at a 90° angle to the excitation beam at discrete emission wavelengths. Decay data was analyzed using the FluoFit software package (PicoQuant). Exponential decay


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functions were convoluted with the experimental IRF and fit to the experimental decay. The quality of the fits, as well as the number of exponentials, were carefully selected based on the reduced χ2 values (which were ≤ 1.1) and the distributions of the residuals. The femtosecond emission transients have been collected using the fluorescence upconversion technique.66 The system consists of a femtosecond optical parameter oscillator (Inspire Auto 100) pumped by 820 nm pulses (90 fs, 2.5 W, 80 MHz) from a Ti:sapphire oscillator MaiTai HP (Spectra Physics) to generate the different excitation beams at 562, 600, 616 nm (∼0.1 nJ). The polarization of the latter was set to magic angle with respect to the fundamental beam. The sample has been placed in a 1-mm thick rotating cell. The fluorescence was focused with reflective optics into a 0.3 mm BBO crystal and gated with the fundamental femtosecond beam. The IRF of the apparatus (measured as a Raman signal of pure solvent) was ∼200 fs (full width at half maximum, fwhm) for all the excitation wavelengths. To analyze the decays, a multiexponential function convoluted with the IRF was used to fit the experimental transients. In all the cases, the errors for the calculated time components were smaller than 15%. All the experiments were performed at room temperature (293 K).

3. Results and Discussion 3.1. Steady-State UV-visible Absorption and Emission Studies 3.1.1. Steady state absorption and emission studies of free NR in DCM and interacting with R-MCM41 in DCM suspensions To begin with, we have studied the UV-visible steady-state behavior of NR interacting with different MCM41 host materials, for which the samples were prepared using the same initial concentration of NR in DCM (∼1x10-5M) and the same mass of the host. Figure 1A shows a comparison between the normalized absorption of the dye in an anhydrous DCM solution and interacting with the R-MCM41 in a DCM suspension. In a pure DCM solution, the spectrum presents an unstructured band with the maximum intensity of absorption at ∼ 537 nm, previously assigned to the S0→S1 (π,π*) transition.33-37 The band shape and spectral position for the NR/R-MCM41 sample suspension significantly differ from that in DCM, which suggests the presence of strong interactions between NR and the R-MCM41 framework, thus resulting in different NR populations at the ground state. The large bathochromic shift (~1850 cm-1) and the structure of the absorption band for NR interacting with R-MCM41 cannot be explained by simple solvatochromic effects. To understand the origin of the changes, the normalized diffuse transmittance (DT) spectrum was deconvoluted ACS Paragon Plus Environment

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into its constituting components (Figure 1S, panel A and Table 1S in SI), while the background from scattering was removed by subtracting a linear function from the experimental data. The NR/R-MCM41 absorption spectrum is composed of four bands centered at 556, 589, 618 and 646 nm, and their contributions to the total spectrum are 17, 23, 30 and 30 % respectively (obtained from the integral intensity). The spectral components are attributed to NR molecules interacting with the host framework. In order to assign these bands to interacting NR species, the guest and host structures must be considered.15,

67-69

Several

studies have reported that the acidity of R-MCM41 results mainly from its hexagonal structure, where intramolecular H-bonds between the silanol groups are formed.70-71 It was suggested that the H-bond interactions between the hydroxyl groups of R-MCM41 and NR, lead to strong polarization of the dye molecules interacting with the framework, and the most favorable site in the NR molecule for this type of interactions is the carbonyl group.54, 68 On the other hand, several studies of NR interacting with silica materials, like silica-sol-gel or SBA-15, have demonstrated the presence of different aggregates.50-54 Indeed, the limited space provided by the R-MCM41 host enhances the specific and non-specific interactions between the adsorbed molecules and therefore leads to two types of aggregates, face to face (H-aggregates) and face to tail (J-aggregates).15,

68, 72-74

These aggregates exhibit different

absorption bands, for which the exciton coupling theory assigns the bathochromic and the hypsochromic shifted ones to J- and H-aggregates, respectively.73-75 Figure 2S in SI shows that increasing the concentration of NR interacting with the MCM41 framework leads to a broadening of the absorption band at 556 nm, and thus the observed change can be attributed to an increase in the contribution of the NR species weakly interacting with MCM41 surface. Note that the spectral position of this band (∼556 nm) is not very different from that obtained for NR in DCM solution (∼537), reflecting that the surrounding solvent still influences its absorption. 35-37 It was reported that highly polar protic environment, like silica material, leads to a large shift of the absorption band of the monomer from ∼470 nm (in non-polar solvent) to ∼ 600 nm.

35-37

Thus, we suggest that the band centered at 618 nm is attributed to a monomer

strongly interacting with the framework through H-bonds.

50-54

Furthermore, according to

previous studies and following the excitonic theory the bands at 589 and 646 nm are due to the H- and J-type aggregates formed most likely inside the host framework, due to the high restriction in the cavity of the MCM41. 50-54 In general, the J-aggregates are characterized by narrow absorption band, broader.

77

76

but due to higher disorder in solution or films it may become

For the NR/ X-MCM41 samples the interactions between the host framework and

the guest molecules lead to higher heterogeneity (due to multiple positions/orientations of the ACS Paragon Plus Environment

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interacting organic dye), and thus, result in a broad J-aggregates band. Scheme 1 illustrates the different interactions of encapsulated monomers, H- and J-aggregates of NR with the RMCM41, where for clarity we show only the species inside the cavity. 3.1.2. Steady state absorption and emission studies of NR interacting with Al-MCM41and GaMCM41 in DCM suspensions We next studied how isomorphic substitution of the Si atom in R-MCM41 framework by tri- or tetra-valent elements influences the interactions between the dye and the silica surface and between the molecules themselves.69 Figure 1C shows the normalized DT spectra of NR interacting with pure silica (R-MCM41) and metal-doped materials (X-MCM41, X = Al, Ga, Ti, Zr) in DCM suspensions. Firstly, we consider the substitution of the tetravalent Si element in the R-MCM41 framework by ∼ 3% of trivalent elements such as Al or Ga. This replacement gives rise to charge defects in the host framework that result in an increase in the host acidity due to generation of new Brønsted acid sites.15, 22-23 Because of the complexity of the DT spectra (Figure 1C), we decomposed the NR/Al-MCM41 and NR/Ga-MCM41 UVvisible DT ones into several absorption bands (Table 1S and Figure 1S, panel B and 1S, panel C in SI). In similarity to the R-MCM41 composites (Figure 1S, panel A in SI), we assign the 558 nm band in the spectra of both materials to the dye weakly interacting with the host, the 594 nm band to H-aggregates, the 615 nm band to the monomers and the 641 nm band to the J-aggregates entrapped in the cavity of the X-MCM41. However, their relative contributions to the total spectrum intensity differ from those for NR/R-MCM41 sample, and in this case they are 7, 25, 13, 55 % and 9, 33, 5, 53 %, for NR/Al-MCM41 and NR/Ga-MCM41, respectively (Table 1S in SI). The changes can be explained in terms of the differences in the chemical framework composition due to the substitution of the 3% of Si by Al and Ga atoms in R-MCM41 that favor different type of interactions between the host and the NR molecules (Scheme 1).15,

22-23

Therefore, the increase in the surface acidity due to the presence of

Brønsted acid sites enhances the H-bonding interactions between the host and the NR molecules. 68 These changes modify the populations of the adsorbed dye molecules in the Alor Ga-MCM41 composites, and as a result there is a preferential formation of J- (641 nm) and H-aggregates (594 nm), as evidenced by the increase in their relative contributions to the overall DT spectra. Moreover, we observe that in the case of Al-MCM41 (20x1017 molecules / gMCM41) the dye loading efficiency is comparable to that of R-MCM41 (19x1017 molecules / gMCM41), while this value (7x1017 molecules / gMCM41) for Ga-MCM41 is much smaller (Table 1). These differences can be explained due to the variations of morphology and Brønsted acidity in the host materials, (BET area - 1000 m2g-1 and pore volume – 0.98 cm3g-1 for R-


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MCM41, 970 m2g-1 and 1 cm3g-1 for Al-MCM41 and 946 m2g-1 and 0.96 cm3g-1for GaMCM41). The increment of pore volume and BET area in Al-MCM41 and R-MCM41 results in higher NR loading efficiency in comparison with Ga-MCM41. Furthermore, the substitution of Si by Al atoms in the MCM41 framework results in higher Brønsted acidity than the substitution by Ga atoms, and as a consequence the host-guest interactions are stronger for Al-MCM41.22-23 3.1.3. Steady state absorption and emission studies of NR interacting with Ti-MCM41and ZrMCM41 in DCM suspensions Next, the complexes of NR with MCM41 doped by Ti or Zr metals were studied. It should be noted that for these hosts, although the electroneutrality of the MCM41 is not affected, the presence of d-orbitals of relatively low energy produces new Lewis acid sites, even at low Ti and Zr doping ratio (∼ 1 %).22, 25-27 Figure 1S, panel D and 1S, panel E and Table 1S (in SI) show the results from the deconvolution of the DT spectra. The contribution of the first component assigned to NR weakly interacting with the host surface (at ∼555 nm) is lower for NR/Zr-MCM41 in comparison to the NR/R-MCM41 system, while it is not presented for the NR/Ti-MCM41. In these materials the formation of J-aggregates is favorable as shown by the increase in the contribution of the band at 649 and 661 nm for ZrMCM41 and Ti-MCM41 composites, respectively. These results are in agreement with the increment of the framework acidity due to the presence of Zr and Ti atoms, which enhances the host guest interactions, as observed in the trivalent-doped MCM41. Furthermore, the NR loading efficiency for Ti-MCM41 (22x1017 molecules / gMCM41) is comparable than that of RMCM41 (19 x1017 molecules / gMCM41), while this value (8x1017 molecules / gMCM41) for ZrMCM41 is much smaller (Table 1). As we explained above these differences are correlated with the acidity (in this case the Brønsted and Lewis), the BET area and the pore size of the host materials (BET area - 1128 m2g-1 and pore volume – 0.81 cm3g-1 for Ti-MCM41, 993 m2g-1 and 0.70 cm3g-1 for Zr-MCM41). Figure 1D shows the stationary UV-visible emission spectra of NR/X-MCM41 in DCM suspensions upon excitation at 600 nm. The spectra are characterized by an emission band with a maximum of emission intensity at ∼670 nm and a shoulder at ∼725 nm. Although the shape and the spectral position of the main emission band for the NR/X-MCM41 complexes (X = Al, Ga, Ti and Zr) are comparable across the different X-MCM41 hosts, they are largely red shifted (∼ 1322 cm-1) in comparison to the emission spectrum of NR in pure DCM (Figure 1B). It was reported that H-bonding and polarity effects strongly modify the shape and the position of the NR emission band.36 In similarity with the NR-hybrid ACS Paragon Plus Environment

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complexes DT spectra, the large Stokes shift and the spectral broadening, cannot be explained only by the polarity effect of the environment. 78 The result is explained by H-bond formation along with the presence of aggregates, which induce a substantial decrease in the global fluorescence intensity helped by ICT and self-quenching processes. Hence, the observed emission spectra are a result of a combined contribution from NR monomers and J-type aggregates encapsulated into the MCM41 framework. Following the excitonic theory, the Haggregates have forbidden excitonic transitions and therefore are characterized by short fluorescence lifetimes that inhibit a clear observation of the corresponding steady-state emission spectra.73-75 Thus, the blue part of the emission spectra (λmax ∼ 670 nm) is assigned to the NR monomers, while the red shoulder at ∼ 720 nm is attributed to the J-type aggregates. On the other hand, the observed small differences in the spectral shift between the NR/XMCM41 emission spectra, including that of R-MCM41, are due to the nature of the metal elements, where higher acidity of the media leads to larger bathochromic shift for NR interacting with Ti- and Zr-MCM41 hosts.22

3.2. Picosecond to Nanosecond Time-Resolved Measurements 3.2.1. Picosecond to nanosecond dynamics of free NR in DCM and interacting with RMCM41 in DCM suspensions To get information on the fluorescence lifetimes of the NR in DCM solution and interacting with the used MCM41 mesoporous materials in DCM suspensions, we performed picosecond to nanosecond emission measurements following excitation at 635 nm. Figure 2 shows the emission decays collected at 670 nm, and Table 2 gives the values of the time constants and their relative pre-exponentials factors obtained from multiexponential fits to the experimental data. The emission decay of NR in DCM solution shows a single component (τ = 4.4 ns) due to the emission of the charge separate state (CS) formed after NR excitation.35-36, 38

Interacting with R-MCM41 results in the appearance of three different fluorescence

lifetimes (τ1 = 0.32 ± 0.04 ns, τ2 = 0.92 ± 0.09 ns and τ3 = 2.5 ± 0.28 ns), independently on the emission wavelength. The amplitudes (pre-exponential factors) of these components are A1 ∼23 %, A2 ∼65 %, A3 ∼10 %. The longest lifetime (τ3) is attributed to the NR monomers attached to the surface of the host, while τ1 and τ2 are assigned to H- and J- aggregates, respectively. In a report on the (E)-2-(2’-hydroxybenzyliden)amino-4-nitrophenol (HBA4NP) interacting with two aluminosilicate zeolites (NaX and NaY), it has been reported that the emission of the dye caged inside the pores of the hosts can be described by three different fluorescence lifetimes (τ1 = 115 – 150 ps, τ2 = 0.9 – 1.1 ns and τ3 = 5.5 – 6.0 ns), instead of ACS Paragon Plus Environment

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the single time component (τ = 14 ps) observed in the DCM solution.72 The τ1 and τ2 components were ascribed to the emission of H- and J-aggregates, respectively, while the τ3 was attributed to the monomers in the supercage of the zeolites. Additionally, it is well known that the H-aggregates have much shorter lifetimes than the J-type ones due to their forbidden excitonic transition.72, 79-81 The amplitudes of τ2 (64 – 68%) are higher than those of τ1 (22 – 25%) and τ3 (8 – 12%), which indicates larger contribution from J-aggregates to the emission decays of the encapsulated NR. The relative amplitude of τ3 decreases while those of τ1 and τ2 slightly increase with increasing of the observation wavelength. The observed decrease the A3 amplitude is consistent with the fact that the adsorbed monomers emit mainly at shorter wavelengths, while the contribution of the aggregates increases towards the red part of the emission spectrum. To further confirm the origin of the observed components, we measured the ps-ns emission decays of NR interacting with R-MCM41 at different initial concentrations of the dye (10-4, 10-5 and 10-6 M) and upon excitation at 635 nm (Figure 3A). At this wavelength, we mainly excite caged monomers and J-aggregate types (Figure 1S in SI). The obtained values of the fluorescence lifetimes at 670 nm observation wavelength are presented in Table 3. In all the cases a three-exponential function convolved with the IRF was used to fit the emission decays. The time components for the lowest used initial concentration to form the complexes (C0 = 10-6 M) are τ1 = 0.39 ± 0.06 ns, τ2 = 1.4 ± 0.12 ns and τ3 = 4.1 ± 0.35 ns, while those for the most concentrated sample (C0 = 10-4 M) are τ1 = 0.23 ± 0.02 ns, τ2 = 0.76 ± 0.11 ns and τ3 = 1.9 ± 0.11 ns. While the values of τ1, τ2 and τ3 are reduced upon increasing the amount of loaded NR, the pre-exponential factors increase for A1 but decrease for A2 and A3. Note that for the diluted NR loaded into R-MCM41 cavity, the value of τ3 component (τ3 = 4.1 ns) is similar to the fluorescence lifetime of the free dye in DCM solution (τ3 = 4.4 ns). This observation suggests that τ3 arises from NR monomers interacting with R-MCM41, and which relative population (their amplitude) decreases when the initial NR concentration increases, leading to an increase in the J-and H-aggregate populations. We have observed a similar behavior for other dyes interacting with silica-based materials in terms of monomers and aggregates.72, 82-84 The shortening of τ3 and the decrease of A3 upon increasing the NR concentration is interpreted in terms of its interactions with neighboring adsorbed NR molecules leading to an emission quenching, as observed in other guest:silica-based materials.72,

82-84

Because τ1 is shorter than τ2, we assign the first lifetime to H-aggregates

while τ2 is assigned to J-aggregates, in agreement with the photophysical behavior of the


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organic dye aggregates.72, 82-84 Furthermore, the comparison of the ps- to nanosecond decays of NR interacting with R-MCM41 ([NR] = 10-5 M) and upon excitation at 560, 590, 610 and 635 nm indicates that the observed dynamics originates from H-, J-aggregates and monomers and the observed fluorescence lifetimes do not depend on the excitation wavelength (Table 2S in SI). The relative contribution of the τ1 component decreases (from 32 to 22 % and 35 to 26% at 670 and 730 nm observation wavelength, respectively), while those of the τ3 one increases (from 7 to 11% and 5 to 9 %) when the excitation wavelength changes from 560 to 635 nm. In addition to that, the excitation at longer wavelengths (from 590 to 635 nm) results in higher absorption of the entrapped monomers (absorption maximum ∼ 610 nm) and, at the same time, lower absorption of H-aggregates (absorption maximum ∼ 590 nm) (Figure 1S, panel A in SI), and as a result, the contribution of the excited monomers increases while that of H-aggregates decreases. Our findings demonstrate that increasing NR concentration leads to a better packing of its molecules within the MCM41 framework, thus enforcing the formation of H-and J-aggregates (Scheme 1). 3.2.2. Picosecond to nanosecond dynamics of NR interacting with Al-MCM41and GaMCM41 in DCM suspensions The specific and non-specific interactions of NR with Al-MCM41 and Ga-MCM41 materials only slightly affect the fluorescence lifetime values of the NR species (τ1 ∼ 0.31 ns, τ2 ~ 1.2 ns and τ3 ∼ 2.7 ns) in comparison with the NR/R-MCM41 system (Figure 2 and Table 2). Moreover, the values of the relative amplitudes are also comparable to those found for RMCM41 sample. In similarity with this one, the contribution of τ2 component in the emission signal (53 – 67% and 40 – 46 % for Al-MCM41 and Ga-MCM41 samples, respectively) is higher than those of τ1 (18 – 29% and 29 – 32%) and τ3 (12 – 18% and 30 – 22%). The observed weak effect on the emission lifetimes and relative amplitudes is probably due to the low doping of the Al and Ga atoms in the MCM41 (3.0% for Al-MCM41 and 3.7% for GAMCM41). Note that the substitution of Si by Al or Ga atoms leads to the formation of Brønsted acid sites in the MCM41 framework, which additionally stabilizes the CS state in the dye and, at the same time, changes the relative contribution of the various species (monomers and aggregates) (Scheme 1). Table 3 gives the obtained values from the fits to the emission decays of NR/Al-MCM41 in DCM suspension at different initial concentrations of the dye (Figure 3B). The fluorescence lifetimes for the concentrated sample (τ1 = 0.18 ± 0.02 ns, τ2 = 0.73 ± 0.07 ns and τ3 = 1.2 ± 0.18 ns) are shorter than those of the diluted one (τ1 = 0.34 ± 0.01 ns, τ2 = 1.5 ± 0.14 ns and τ3 = 4.0 ± 0.28 ns). Single molecule and theoretical ACS Paragon Plus Environment

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studies have shown the contribution of intersystem crossing processes (single-triplet transition, S-T) to the NR excited state depopulation.38, 85 The presence of metal atoms in the host matrix can affect the energy states of the organic guest and thus influence the efficiency of these processes. However, the changes of the τi and Ai values with the used initial NR concentration are similar to those observed in NR/R-MCM41, which further indicates a weak influence of the doping metals (Al, Ga) on the ps-dynamics of the monomers and aggregates. Thus, we conclude that the observed changes in each series are due to the enhancement of dye-dye interactions, in similarity with the results for NR/R-MCM41 sample. Moreover, the increment of the τ1 contribution and the reduction of the τ2 one upon increasing the dye concentration confirm that the formation of H-aggregates is favored (Table 3). 3.2.3. Pico to nanosecond dynamics of NR interacting with Ti-MCM41and Zr-MCM41 in DCM suspensions The specific and non-specific interaction of NR with Ti-MCM41 and Zr-MCM41 result in a notable decrease of the fluorescence lifetimes of the monomers, H- and Jaggregates (τ1 = 0.16 ± 0.01 and 0.18 ± 0.02 ns, τ2 = 0.72 ± 0.05 and 0.51 ± 0.06 ns, and τ3 ∼ 2.0 ns), when compared to the NR/R-MCM41 sample behavior (Figure 2 and Table 2). This indicates the presence of a new decay channel that leads to faster quenching of the emission of NR molecules interacting with these hosts containing transition metals (Ti and Zr). The interactions of the excited NR with the Ti or Zr embedded in the MCM41 lattice allow for electron injection (EI) into trap states formed by the d-orbitals of the guest metal atoms (Lewis acidity).86-87 It was reported that the EI from NR to TiO2 doped Y-zeolite decreases the fluorescence lifetime of the CS state from ~4.5 ns in acetonitrile (ACN) solution to 1.6 ns.88 The same study has reported on additional fluorescence lifetime (τ = 0.38 ns) assigned to the local excited (LE) state of NR entrapped in the TiO2-Y-zeolite. The appearance of the LE state emission was explained by the possible restriction of the NR molecular motion inside the host framework leading to inhibition of the CS state formation. However, it is expected that the increase in the polarity of the environment (as is the case with Y-zeolite) will favor the formation of the CS state, and thus the interactions between NR and Y-zeolite should lead to even more emission from the CS form in comparison with the dye in the ACN solution. Hence, here we suggest that the observed 0.38 ns component may be assigned to the fluorescence from the CS state of aggregated molecules, which is in agreement with our results suggesting formation of J-aggregates in presence of the host framework (~0.3 ns decay). The 1.6 ns component for NR interacting with TiO2-Y-zeolite is not very different from those of the NR/Ti-MCM41 and NR/Zr-MCM41 systems (τ3 ∼ 2 ns) indicating that the ACS Paragon Plus Environment

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process of the EI is involved in the decrease of the fluorescence lifetimes. Additionally, it was shown that the aggregation may increase both the light-harvesting and charge-transfer efficiencies of adsorbed organic dyes.89-90 According to these reports, the increased driving force

for

EI

to

TiO2

nanoparticles

occurring

faster

from

H-aggregates

of

chalcogenorhodamine dyes (2.3-fold rise) in comparison with the monomer due to an efficient excited state migration within monolayer.90 Thus, we suggest that the EI from H-, and Jaggregates, and monomers of NR to the trap states in Ti- and Zr-MCM41 (occurring with different efficiencies) affects the relaxation dynamics of the excited species. In contrast to NR interacting with the R-MCM41, the τ1 component has a higher contribution (41 – 49% and 50 – 54% for NR loaded into Ti-MCM41 and Zr-MCM41, respectively) than the τ2 component (35– 38% and 20 – 36%). The observed dependence is most probably caused by the EI occurring on a time scale below the resolution of the spectrometer, which contributes to the amplitude of the shortest fluorescence lifetime. The influence of the additional ultrafast processes on the relative contributions of the time components observed for picosecond to nanosecond dynamics was reported for several systems.91-92 It should be noted that the observed changes of the relative contributions do not reflect the real change of the distributions/populations of the various species interacting with the Ti- and Zr-MCM41 frameworks. In contrast to NR loaded into R-MCM41, the dynamics of the sample containing TiMCM41 shows weak dependence on the initial concentration of the dye (Table 3 and Figure 3C). The fluorescence lifetimes for the diluted sample are τ1 = 0.16 ± 0.01 ns, τ2 = 0.86 ± 0.09 ns and τ3 = 2.4± 0.23 ns, while those for the concentrated one are τ1 = 0.14 ± 0.02 ns, τ2 = 0. 59 ± 0.06 ns and τ3 = 1.6 ± 0.20 ns. The emission lifetimes of NR interacting with R-MCM41 are sensitive to the dye concentration (Table 3), for the system containing Ti, in which the major process responsible for the fluorescence lifetime shortening is the EI. We will explore this issue in the following part using fs-technique. 3.3. Femtosecond Emission Measurements We used femtosecond (fs) emission spectroscopy to investigate the ultrafast photodynamics and the related short events for NR in DCM solution, as well as interacting with the above X-MCM41 in DCM suspensions upon excitations at 562, 600, 616 nm. To avoid the dynamics from the free dye (not interacting with these hosts) in DCM, the initial concentration of NR used for preparation of the studied samples was 10-5 M (Figure 2S in SI). 3.3.1. Femtosecond dynamics of free NR in DCM and interacting with R-MCM41 in DCM suspensions ACS Paragon Plus Environment

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Firstly, we present and discuss the photodynamical behavior of NR in a DCM solution and interacting with R-MCM41 in DCM suspensions, following excitation at 562 nm. Figures 4 and 3S in SI show the gated fs-fluorescence up-conversion transients of both samples, while Table 4 gives the parameters obtained from the multiexponential fits to the experimental data. The dynamics of NR in a DCM solution is characterized by a single rising component (τ = 0.96 ± 0.20 ps) and a long offset resolved by the ps- measurements (4.4 ns) in the range of 630 – 730 nm (Figure 3S in SI). The rising component is attributed to the formation of the CS state due to an ICT process in NR. It should be noted that the time of the observed rise is most probably affected by solvation dynamics, which occurs on similar time scale and presents comparable spectral and dynamical behavior. 93-95 Similar results have been reported for other systems undergoing ultrafast charge separation.93-95 Additionally, this assignment is in agreement with previous studies that have shown that the excitation of the NR molecules leading (within the excitation pulse) the LE state can be depopulated by emission and/or ICT from the diethylamino group to the carbonyl oxygen moiety producing the CS structure.35-36, 38

Figures 4 and 5A show the obtained transients for the NR/R-MCM41. At shorter wavelengths of the emission spectrum (630 – 650 nm) the signals decay biexponentially to a constant offset (310 ps, the shortest time in the ps-emission studies) with time constants of τ1 410 ± 30 fs and τ2 = 3.26 ± 0.25ps. At the red part of the emission (670 – 730 nm), the observed decays rise rapidly in 350 ± 40 fs. This value is similar to the decay component (τ1) observed at 630-650 nm, indicating a common channel for both dynamics and therefore it is assigned to ICT within NR (Scheme 2). The value of the rising component for NR interacting with R-MCM41 (350 fs) in a DCM suspension is almost three times smaller than that for the dye in DCM solution (0.96 ps). This result clearly indicates the influence of the environment (polarity and H-bonding interactions) on the CS state formation of NR. It should be noted that the ultrafast dynamics of the NR/mesoporous material complexes are most probably affected also by the solvent-host and solvent-guest interactions (solvation processes).

96-97

The NR

fluorescence in a nonpolar environment may result from both the LE and CS states due to the energy barrier between the two forms.38 The increase of the environment polarity decreases the value of the energy barrier between the LE and CS states leading to efficient formation of the CS form which, in turn, gives rise to the emission signal.38 Moreover, it is known that ICT process from the diethylamino group to the carbonyl one is promoted by H-bonding between the solvated molecule and the protic environment interacting with the carbonyl site.98 Since a large electron density is located at the carbonyl oxygen, this is the most favorable site for the ACS Paragon Plus Environment

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formation of H-bonds.36,

54

The ICT might be also affected by the processes occurring

between aggregated molecules (vibrational energy redistribution or exciton migration). However, the lack of excitation wavelength dependence of the value of the ultrafast time component suggests that these processes do not significantly affect the CS state formation (vide infra). We assign the second component (τ2 = 3.26 ps) to the process of vibrational cooling (VC) at S1 of the CS state. Several authors have reported that VC for organic molecules having a size comparable to that of NR occurs on a time scale of few picoseconds.99-101 For example, the VC times measured for molecules such as pyrogallol red, bromopyrogallol red and aurintricarboxylic acid range between 1 and 6 ps.100 The relative contributions of τ1 and τ2 decrease (from 30% and 14% to 7% and 6%, respectively) when the observation wavelength changes from 630 to 650 nm, and finally τ1 becomes a rising component while, τ2 disappears at 700 and 730 nm. The observed wavelength dependence of the time components further supports the assignment of τ1 to the LE→ CS process and τ2 to the VC process. The absence of τ2 at 700 and 730 nm is due to the fact that in this spectral region the emission occurs from relaxed S1 state. To study the influence of the excitation wavelength on the ultrafast dynamics of NR ([NR] = 10-5 M) interacting with R-MCM41, we recorded the fs-emission transients upon excitation at 562, 600, and 616 nm. Figure 4S, panel A in SI shows that the signals at 700 nm do not depend on the excitation wavelength. Upon excitation at different wavelengths, the various NR species interacting with R-MCM41 are promoted (H- aggregate, monomers, and J- aggregates, respectively) to the S1 state with different efficiencies as they absorb at different region (Fig. 2SA in SI). Although one can expect the differences between the obtained dynamics, the absence of excitation wavelength dependent behavior of the upconversion transients suggests that the ultrafast processes in NR/R-MCM41 occur in the same time regime or faster than the processes of energy redistribution between the different species of neighboring dye molecules and between NR and the MCM41 framework. 3.3.2. Femtosecond dynamics of NR interacting with Al-MCM41 and Ga-MCM41 in DCM suspensions The parameters obtained from the multiexponential fits to the experimental data for NR interacting with Al-MCM41 and Ga-MCM41 in DCM suspensions are comparable to those for NR/R-MCM41 system (Table 4 and Figures 4, 5B and 5S in SI). At 630 – 650 nm, the subpicosecond components for the NR/Al-MCM41 and NR/Ga-MCM41 systems are 347 ± 40 fs (Figure 4 and 5S in SI). The τ2 value for both samples is 3.48 ± 0.35 and 3.33 ± 0.20


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ps, respectively. The relative amplitudes (Ai) of these components decrease with the observation wavelength. At longer observation wavelengths (700 and 730 nm), a rising component (τ1) appears while τ2 disappears indicating that the origin of the ultrafast processes in NR/R-MCM41 and NR-Al/Ga-MCM41 samples are identical. At the reddest part of the emission spectrum (670 – 730 nm), a fs-rising component is observed having a time constant (317 ± 25 and 360 ± 35fs, respectively) similar to that of τ1 with the decay at the shortest wavelength region. Since the ultrafast behavior of NR interacting with the Al-MCM41 and Ga-MCM41 is similar to that of NR/R-MCM41, we assign the observed components to the dynamics of the LE state leading to formation of CS one (τ1) followed by VC (τ2) of the hot CS at the S1 state (Scheme 2). 3.3.3. Femtosecond dynamics of NR interacting with Ti-MCM41 and Zr-MCM41 in DCM suspensions Figures 4, 5C and 6S in SI, show the emission transients of NR interacting with TiMCM41 and Zr-MCM41 in DCM suspensions and upon excitation at 562 nm. The decays at the shortest wavelengths of the emission spectra (630 – 650 nm) are similar to those of the samples having Al and Ga dopants in MCM41 framework (Figure 4). The multiexponential fits give time constants of τ1 = 263 ± 25, 193 ± 20 fs; and τ2 = 2.26 ± 0.24, 2.11 ± 0.36 ps for both samples, respectively. The τ1 and τ2 values are shorter than for NR interacting with RMCM41, Al-MCM41 and Ga-MCM41. Additionally, the emission transients of NR/ZrMCM41 and NR/Ti-MCM41 samples at the reddest side of the emission spectrum (670 – 730 nm) do not present a rising component, contrary to the obtained dynamics using R-MCM41, Al-MCM41 and Ga-MCM41hosts. The values of τ2 (2.30 ± 0.31 and 2.42 ± 0.25 ps) are comparable to those present in the bluest region (Table 4). The absence of a rising component for the decays at the red part is due to the appearance of a new ultrafast decay channel in NR upon interaction with Ti-MCM41 and Zr-MCM41 materials. In the presence of Ti and Zr metals, we believe that the charge separation in NR occurs in < 0.2 ps, visible in the 630-650 nm region, and the rising fs-component is not observed due to the occurrence of EI from the CS state of NR to the traps of Zr and Ti (Scheme 3). Here, we suggest that the τ1 component results from the process of EI from the CS state of the excited dye molecule to the unoccupied d-orbitals of Ti or Zr embedded in the MCM41 host. As a result, Ti and Zr atoms form trap states in the MCM41 host which are located lower than the CS state energy level of NR.The presence of a new pathway of deactivation (EI) also affects the times at the VC of the S1 state of NR (2.30 and 2.42 ps), which now are shorter in comparison with those observed in R-


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MCM41, Al-MCM41 and Ga-MCM41 (3.48 and 3.33 ps). The ultrafast EI to the localized trap levels formed by the Ti and Zr atoms in the doped MCM41 lattice occurs in ~230 fs, and thus it leads to hot CS state depopulation which, in turn, shortens the VC time (Scheme 3). Similar behavior was observed for the YD2-o-C8 porphyrin dye adsorbed on the TiO2 surface, where the VC time (~0.7 ps), was reduced in comparison with the porphyrin/Al2O3 (4 ps) due to the EI into titania.102 The appearance of a new deactivation channel is also in agreement with the shortening of the emission lifetimes. Similar to the NR interacting with RMCM41 in DCM solution, the emission dynamics of NR/Ti-MCM41 at 700 nm upon excitation at 562, 600 and 616 nm are analogous (Figure 4S, panel B in SI), which reflects that the different contributions of different NR forms (aggregates and monomers) excited at different wavelengths have similar ultrafast dynamics. The difference observed in the behavior of NR in the tri- and tetravalent metal doped MCM41 materials is related to the presence of different types of unoccupied orbitals. Although both the sp3- and the d-orbitals of the atoms embedded in the MCM41 lattice can form a sequence of donor and acceptor trap states levels within the band gap of the host, in the case of Ti and Zr the unoccupied d-orbitals have higher electron affinity, while for Al and Ga only unoccupied orbitals with less electron affinity are available.86-87, 103 In a report on the X- and Y-zeolites doped by the transition metals, it was shown that the trap states in the host framework were formed efficiently by d-orbitals of the guest atoms increasing the catalytic properties of the zeolite matrix.104 The ultrafast up-conversion emission experiments show that the EI occurs only for MCM41 doped by transition metals, while this process does not occur when NR is interacting with Al-MCM41 and Ga-MCM41, having then higher energy levels than that of the CS state of NR. As a result the electron injection process does not occur in this last family (Scheme 2 and Scheme 3).

4. Conclusion We investigated the S0 and S1 interactions of Nile Red with a series of MCM41 mesoporous materials doped by various metal atoms (Al, Ga, Ti and Zr) in DCM suspensions, using stationary absorption, emission and femtosecond to nanosecond time resolved spectroscopic techniques. The results show that the interaction of the dye with these materials leads to the formation of different populations of NR, including monomers (weakly and strongly interacting with the framework), J- and H-aggregates. The large bathochromic shifts observed in the steady-state absorption and emission spectra are explained in terms of Brønsted (H-bonds formation) and Lewis interactions between the dye and the host. In pure


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DCM solution, NR has a lifetime of ∼ 4.4 ns, while for the NR/R-MCM41, NR/Al-MCM41 and NR/Ga-MCM41 composites, the fluorescence lifetimes assigned to J- and H-aggregates are τ1 = ∼ 0.3 ns and τ2 = ∼1 ns, respectively, while that of the interacting monomers is τ3 = ∼2.6 ns. Using Ti-MCM41 and Zr-MCM41 as hosts, we obtained τ1 = ∼0.2 ns, τ2 = ∼ 0.6 ns, and τ3 ∼ 2 ns, which are reduced in comparison with those of NR/R-MCM41 sample, due to the process of electron injection (EI) to the trap states formed by the transition metals embedded in the MCM41 lattice. Moreover, the interactions between the dye molecules and Ti or Zr atoms result in a different distribution of the populations on the surface host (J-, Haggregates and monomers), relatively to the situation for NR/R-MCM41, NR/Al-MCM41 and NR/Ga-MCM41 systems. The femtosecond results using R-MCM41, Al-MCM41 and GaMCM41 hosts indicate that the initially formed LE state of NR populations give rise to a CS one within 320 – 410 fs. We also observed that the excited species lose their excess of vibrational energy in the range of 3.2 – 3.5 ps. For the NR/Ti-MCM41 and NR/Zr-MCM41 samples, the CS state formation occurs in a time shorter than 200 fs. Furthermore, in the presence of transition metals, the CS state of NR can be depopulated by EI in ∼ 250 fs) to the trap states formed by the d-orbitals of the dopants. As a result of EI, the vibrational cooling of the CS state is ∼ 1 ps, shorter than that observed in R-MCM41, Al-MCM41 and Ga-MCM41 hosts (∼ 3.5 ps). The present results provide relevant information to understand the interactions at S0 and S1 states of a well-known sensor for polarity (NR) with a family of metal doped MCM41 in DCM suspensions. These findings should help in designing better sensors of polarity, acidity/basicity of silica materials. Furthermore, the acquired knowledge about the physico-chemical properties of these metal doped materials will be useful for other applications such as photocatalysis, nanophotonics and drug delivery. Acknowledgment: This work was supported by the MINECO and JCCM through Projects Consolider Ingenio 2010 (CDS2009-0050), PRI-PIBIN-2011-1283, MAT2011-25472, MAT2012-38567-C02-01, PROMETEOII/2013/011 and PEII-2014-003-P. CM thanks MEC for the FPU fellowship. Supporting Information Figure 1S: The spectral deconvolution of absorption of NR interacting with X-MCM41. Figure 2S: UV-visible absorption spectra for NR interacting with R-MCM41 at two concentrations of the dye. Figure 3S: Femtosecond-emission transients of Nile Red in DCM upon excitation at 562 nm. Figure 4S: Excitation wavelength dependence (562, 600 and 616 nm) in the femtosecond-emission transients for NR/R-MCM41 and NR/Ti-MCM41. Figure ACS Paragon Plus Environment

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5S-6S: Femtosecond-emission transients for NR/X-MCM41suspensions upon excitation 562 nm at different observation wavelength. Table 1S list the values obtained from the absorption deconvolution. Table 2S: list of the values of fluorescence lifetimes and normalized (to 100) pre-exponential factors for NR/R-MCM41 upon excitation at different wavelengths. This material is available free of charge via the Internet at http://pubs.acs.org.


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References

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TOC

LE

CS Al, Ga

0.2 – 0.4 ps

S1

~250 fs

2.5 – 3.5 ps

Ti, Zr

562 nm

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

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~0.2 - 4ns

S0 (X

(X

M)

Si )

O

O

H

H

(X

H O

H O (Si

X) O H

Si)

(X)

(X

Si )

NR / X-MCM41 X = Si, Al, Ga, Ti, Zr


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Table and Figure Captions Table 1. Values of the characteristic textural properties of X-MCM41 (% metal contents, BET area and pore volume) and their NR loading efficiency. X= Si, Al, Ga, Ti, Zr. Table 2. Values of fluorescence lifetimes (τi) and normalized (to 100) pre-exponential factors (Ai) obtained from a global multiexponential fits of the emission decays of NR interacting with R-MCM41, Al-MCM41, Ga-MCM41, Ti-MCM41, Zr-MCM41 in DCM suspension upon excitation at 635 nm. Table 3. Values of fluorescence lifetimes (τi) and normalized (to 100) pre-exponential factors (Ai) obtained from a global multiexponential fits to the emission decays of NR at different concentrations interacting with R-MCM41, Al-MCM41, Ga-MCM41 in DCM suspensions. The excitation and observation wavelengths were at 635 and 670 nm, respectively. Table 4. Values of time constants (τi) and normalized (to 100) pre-exponential factors (Ai) of the multiexponential functions used to fit the transient absorption signals of Nile Red in DCM solution and Nile Red interacting with R-MCM41, Al-MCM41, Ga-MCM41, Ti-MCM41, ZrMCM41 in DCM suspensions, upon excitation at 562 nm. Negative values of the amplitudes indicate a rising components. * The τ3 time constants were fixed from the values obtained in ps-experiments. Scheme 1. Schematic presentation of the molecular NR structure and interacting monomers and H-and J-aggregates of NR within X-MCM41. X= Si, Al, Ga, Ti, Zr. Scheme 2. Schematic presentation (not in scale) of energy levels S0 and S1 states of NR interacting with R-MCM41, Al-MCM41 and Ga-MCM41 in a DCM suspension. The scheme gives the values of time constants related to the processes occurring in these systems. Scheme 3. Schematic presentation (not in scale) of energy levels S0 and S1 states of NR interacting with Ti-MCM41 and Zr-MCM41 in a DCM suspension. The scheme gives the values of time constants related to the processes occurring in these systems. Figure 1. Normalized to the maximum of intensity A) UV-visible absorption and B) emission spectra of NR in a DCM solution and interacting with R-MCM41 materials. Comparison of the C) UV-visible absorption and DT spectra and D) emission spectra of NR interacting with different X-MCM41: (1) R-MCM41, (2) Ga-MCM41, (3) Al-MCM41, (4) Zr-MCM41 and (5) Ti-MCM41 in DCM suspensions. The emission spectra for NR in DCM and interacting with X-MCM41 were excited at 470 nm and 600 nm, respectively. The inset in Figure 1A


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shows a picture of the samples for NR in DCM solution (pink) and the NR/R-MCM41 DCM suspension (blue). Figure 2. Normalized (to the maximun ofintensity) magic-angle emission decays of NR in (1) DCM solution and interacting with (2) R-MCM41, (3) Al-MCM41, (4) Ga-MCM41, (5) TiMCM41 and (6) Zr-MCM41 in DCM suspensions upon excitation at 635 nm. The solid lines are from the best multiexponential fits to the experimental data. IRF is the instrumental response function (~80 ps). The observation wavelength for NR in a DCM solution and interacting with different MCM41 in DCM suspensions was at 670 nm.

Figure 3. Normalized (to the maximun ofintensity) magic-angle emission decays of NR at different initial concentrations, interacting with: A) R-MCM41, B) Al-MCM41 and C) TiMCM41 in DCM upon excitation at

635 nm. The solid lines are from the best

multiexponential fits of the experimental data, and IRF is the instrumental response function (~80 ps).

Figure 4. Magic-angle femtosecond-emission transients of Nile Red in a DCM solution and interacting with different MCM41 materials in DCM suspensions observed at A) at 630 nm and B) 700 nm (λex = 562 nm). The solid lines are from the best multiexponential fits of the experimental data, and IRF is the instrumental response function (~200 fs). Figure 5. Magic-angle femtosecond-emission transients of NR interacting with A) RMCM41, B) Al-MCM41 and C) Ti-MCM41 in DCM suspensions at different emission wavelengths (λex = 562 nm). The solid lines are from the best multiexponential fits of the experimental data, and IRF is the instrumental response function (~200 fs).


30

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Table 1

Pore volume NRLoading 3 -1 17 (cm /g ) (10 molec. NR / g of MCM41)

Host

Doping (%)

BET Area (m2/g-1)

R-MCM41

0

1000

0.98

19

Al-MCM41

3.0

970

1.00

20

Ga-CM41

3.7

946

0.96

7

Ti-MCM41

1.2

1128

0.81

22

Zr-MCM41

1.8

993

0.70

8


31


Table 2

TiMCM41

GaMCM41

AlMCM41

RMCM41

Host

ZrMCM41

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

Page 32 of 42

λobs

τ1 / ns

A1 /%

τ2 / ns

A2 /%

τ3 / ns

A3 /%

650

24

64

12

670 700

22 23

67 67

11 10

0.32

0.92

2.5

730

25

66

9

750

24

68

8

650

29

53

18

670

21

62

17

700

0.31

18

1.17

67

2.7

15

730

23

63

14

750

21

67

12

650

30

40

30

670

0.30

29

1.02

44

2.6

27

700

32

46

22

650

41

35

24

670

45

35

20

700

0.16

48

0.72

36

1.9

16

730

48

37

15

750

49

38

13

650

50

20

30

670 700

0.18

54 51

0.51

28 36


2.0

18 13

32

Page 33 of 42

Host

NRa / M

τ1 / ns

A1 / %

τ2 / ns

A2 / %

τ3 / ns

A3 / %

R-MCM41

∼10-6

0.39

21

1.44

63

4.12

16

∼10-5

0.32

22

0.92

67

2.49

11

∼10-4

0.23

47

0.76

64

1.89

5

Al-MCM41

Table 3

∼10-6

0.34

14

1.45

71

4.02

15

∼10-5

0.31

21

1.17

62

2.69

17

∼10-4

0.18

51

0.73

34

1.17

15

Ti-MCM41

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


∼10-6

0.16

32

0.86

38

2.41

30

∼10-5

0.15

45

0.72

35

1.92

20

∼10-4

0.14

54

0.59

34

1.61

12


33


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

Page 34 of 42

Table 4

λem/nm

630

640

650

670

700

730

τi (Ai %)

DCM

τ1 / ps τ2 / ps τ3*/ ns τ1 / ps τ2 / ps τ3*/ ns τ1 / ps τ2 / ps τ3*/ ns τ1 / ps τ2 / ps τ3*/ ns τ1 / ps τ2 / ps τ3*/ ns τ1 / ps τ2 / ps τ3*/ ns

1.04 (-15) 4.4 (85) 1.04 (-16) 4.4 (84) 1.04 (-16) 4.4 (84) 0.87 (-20) 4.4 (80) 0.87 (-22) 4.4 (78) 0.87 (-21) 4.4 (79)

R-MCM41

Al-MCM41

Ga-MCM41

0.42 (30) 3.22 (14) 0.31 (56) 0.42 (17) 3.15 (10) 0.31 (73) 0.40 (7) 3.42 (6) 0.31 (87) 0.34 (-12) 3.35 (6) 0.31 (82) 0.34 (-5) 0.31 (95) 0.38 (-9) 0.31 (91)

0.35 (36) 3.23 (18) 0.29 (46) 0.36 (20) 3.21 (15) 0.29 (65) 0.33 (12) 4.02 (10) 0.29 (78) 0.30 (-6) 0.29 (94) 0.30 (-8) 0.29 (92) 0.35 (-5) 0.29 (95)

0.35 (39) 3.45 (14) 0.30 (47) 0.36 (20) 3.22 (11) 0.30 (69) 0.33 (11) 3.31 (10) 0.30 (79) 0.38 (-34) 0.44 (31) 0.30 (35) 0.34 (-5) 0.30 (95) 0.36 (-9) 0.30 (91)


Ti-MCM41 0.22 (54) 2.22 (19) 0.16 (27) 0.27 (35) 2.15 (21) 0.16 (44) 0.28 (25) 2.41 (22) 0.16 (53) 0.28 (13) 2.54 (19) 0.16 (68) 2.33 (25) 0.16 (75) 2.38 (26) 0.16 (74)

Zr-MCM41 0.18 (39) 2.25 (25) 0.19 (36) 0.2 (20) 1.85 (26) 0.18 (54) 0.2 (14) 2.22 (16) 0.18 (70) 1.87 (10) 0.18 (90) 2.51 (6) 0.18 (94) 2.53 (6) 0.18 (94)

34

Page 35 of 42

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


Scheme 1.

N

O

O

N


35


Scheme 2.

SiO2 CB LE

CS

S1

S0

Trap states

Al, Ga

~350 fs ~3.5 ps ~0.3 - 2.7ns

660 nm

562 nm

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

Page 36 of 42

NR/ X-MCM41 X = Si, Al, Ga


36

Page 37 of 42

Scheme 3.

SiO2 CB LE

CT

Ultrafast Dynamics of Nile Red Interacting with ... - ACS Publications

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