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Femtosecond Dynamics and Photoconversion of a H-Bonded Dye within Mesoporous Silicate Materials Michaz Gil, Cristina Martin, and Abderrazzak Douhal* Departamento de Quimica Fisica, Facultad de Ciencias Ambientales y Bioquimica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S.N. 45071 Toledo, Spain
bS Supporting Information ABSTRACT: In this study, we report on the steady-state UVvisible absorption, reflectance, and emission spectra and femtosecond dynamics of 7-hydroxyquinoline (7HQ) interacting with purely siliceous (R-MCM41) and Al-doped (Al-MCM41) mesoporous silicate material in dichloromethane solution, as well as its photoconversion upon 320-nm irradiation. The results show that changes in the electrostatic field inside the nanochannels induced by Al ions significantly affect the ground- and excitedstate photobehavior of the 7HQ structures H-bonded to the framework. For the AlMCM41 sample, the obtained decay times (0.470.60 and 5.86.6 ps and a rise time of 3.3 ps) in the ultrafast dynamics are assigned to the formation and relaxation processes of anionic (A) and zwitterionic (Z) structures of 7HQ. These times are longer than those observed for the regular MCM-41 sample (0.260.32 and ∼3.5 ps and a rise time of 0.75 ps). The difference is explained in terms of different nonspecific (electrostatic) interactions of 7HQ with the nanohosts. In contrast, we found that the influence of the channel diameter (in the 2540-Å range) is weak. Additionally, the 320nm irradiation study showed that the main conversion route leads excited E to form A, which is stabilized in the ground state, and lowers the transformation into the Z and neutral keto (K) populations. For 7HQ/R-MCM41, the rate constants of the E f A f (Z, K) photoconversions were found to be 8.8 ((0.23) 103 and 3.2 ((0.18) 103 min1, respectively. The K form is not produced when unstructured silica particles were used, indicating the effect of nanoconfinement on its formation and stability. We observed the same trend in the photoconversion of E to A and Z (K) in a solid-state sample, suggesting that the solvent does not play a key role in their formation. Our results demonstrate how the confinement and interactions of 7HQ with the MCM-41 nanomaterial affect the steady-state and time-resolved emission spectroscopy of a dye, as well as the equilibrium, stability, and photoconversion of its tautomeric and ionic structures. Such a rich photobehavior might be used in designing H-bonded guest: MCM-41 photoactive materials, such as light-emitting diodes, memories, and gates to cite a few possible applications in nanophotonics.
1. INTRODUCTION Proton- (or H-atom-) transfer reactions are among the most ubiquitous processes in nature, as well as among the most studied.13 Aromatic molecules with proton- (or hydrogen-atom-) accepting or donating groups are often used as model compounds to investigate proton-relay reactions.46 7-Hydroxyquinoline (7HQ, Scheme 1) is one such molecule whose hydroxyl and imino groups become significantly more acidic and basic, respectively, in the S1 state than they are in the ground state. However, the distance between the two involved centers (∼5 Å) prevents a direct (intramolecular) photoinduced proton-transfer reaction, and the process requires mediation of protic molecules, such as water, alcohol, acetic acid, or ammonia, and stabilization of the formed structures.712 In the gas phase, complexes of 7HQ with water involving one or two molecules do not show any proton-transfer reaction,13 whereas those involving several ammonia molecules undergo a photoinduced proton-transfer reaction.14 Numerous reports studying the excited-state proton-transfer (ESPT) reaction in 7HQ in solution have been published over the past two decades.1525 They showed that tautomeric and/or ionic equilibria between structures of 7HQ can produce enol (E), anion r 2011 American Chemical Society
(A), cation (C), zwitterion (Z), and keto (K) forms of the dye (see Scheme 1), depending on the medium used. In water, enol-group deprotonation and subsequent imine protonation lead to K or Z structures through anionic intermediates.2325 In bulk alcohols, the first ESPT step is slow reorganization of the solvent and formation of cyclic H-bonded complexes with two alcohol molecules, followed by a fast proton transfer.22 In turn, in polyhydroxy alcohols (ethylene glycol, glycerol), formation of a 1:1 complex can give rise to ESPT.20 Additionally, the ESPT reaction has been studied for 7HQ in heterogeneous environments, such as polymers, cyclodextrins, reverse micelles, and structured silicates.2630 Such media exert various degrees of confinement, which affects the stability of the tautomeric forms of 7HQ and their photobehavior. Thus, within the cavity of β-cyclodextrin (β-CD), an increase of the E-form population is observed mostly at the expense of the Z population.29 In turn, the water/surfactant concentration ratio in reverse micelles strongly Received: November 30, 2010 Revised: June 1, 2011 Published: June 02, 2011 14687
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The Journal of Physical Chemistry C Scheme 1. (A) Molecular Structures of 7-Hydroxyquinoline (7HQ) in Enol (E), Anion (A), Keto (K) and Zwitterion (Z) Forms and (B) Schematic Representation of MCM-41 Silicates
affects the excited-state dynamics of 7HQ, because of the presence of micelle regions containing water molecules with different mobilities.28 Recently, the photochemistry of 7HQ monomers isolated in argon matrixes at 20 K was reported.31,32 The studies showed the formation of the keto structure as the result of an intramolecular hydrogen-atom transfer in an argon cage, as well as production of 7-keten and 6-ketene by elimination of a hydrogen molecule. In room-temperature aprotic solutions, 7HQ is photostable when irradiated into the E absorption band. However, this behavior can be changed when the dye is in hydroxylic media. In the 1990s, we reported on the photoreaction of 7HQ in a rigid polymeric matrix of 2-hydroxyethyl methacrylate (PHEMA).26 Upon irradiation at 313 nm, a decrease in the E absorption intensity (at ∼323 nm) occurs, along with a simultaneous increase in a broad and unresolved absorption band at wavelengths longer than 342 nm. This effect was mainly assigned to the photoconversion of the E form into the K tautomer, but the production of single and double H-bonded structures of 7HQ was also suggested.26 Our recent study reported on the ground- and excited-state proton-transfer reaction of 7HQ within the channels of MCM-41.30 This material has a highly ordered SiO framework forming hexagonal channels of relatively uniform diameter (ranging from ∼10 to 200 Å). Its surface has lateral SiOH groups that can interact with guest molecules.33 Moreover, the presence of these silanol groups makes MCM-41 susceptible to chemical modifications, further increasing its potential to interact with guest molecules or to be used in important fields, such as heterogeneous catalysis and nanophotonics. The study showed that, upon adsorption of 7HQ within the nanochannels of MCM-41, significant populations of the A and Z structures are stabilized in the ground state through H-bonding interactions with the host material. These forms show characteristic absorption bands at longer wavelengths than are found
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for the E form in solution. The populations of all adsorbed forms depend on the number of accessible hydroxyl groups.30 The ESPT reaction takes place at the femtosecond to picosecond time scale, which suggests that it occurs within already sterically prepared hostguest complexes. These findings brought us to investigate the possible photochromism of 7HQ embedded in mesoporous solid matrixes of silicate materials (MCM-41 type). As the intensity maxima of the absorption and emission bands of various forms of 7HQ are spectrally separated, the light-driven conversion between them and within a highly ordered structure of MCM-41 might offer interesting possibilities for designing photoactive materials and nanodevices. The obtained results show that photoconversion in these materials occurs at an irradiation wavelength of 320 nm, whereas the samples are photostable at lower energies. The main process is an E f A conversion, but the production of the Z form and of another red-emitting structure (probably a neutral keto) occurs with a lower efficiency. Additionally, we report on the ultrafast excited-state dynamics of 7HQ within aluminosilicate MCM-41 and compare the results with those previously reported for the MCM-41 host.30
2. EXPERIMENTAL DETAILS 7-Hydroxyquinoline (7HQ, 99%) was purchased from Across Organics. Purely siliceous (R-MCM41) and aluminum-doped [Al-MCM41, (SiO2)0.9875(Al2O3)0.0125] materials (MCM-41 type, 25-Å average pore diameter) were purchased from SigmaAldrich; we reconfirmed their mesoporous structures using isothermal nitrogen adsorption and powder X-ray diffraction. The size of the MCM-41 particles was determined using a laser diffraction particle size analyzer (Beckman Coulter LS 230). The mean value of the particle diameter was found to be 46 μm, and the total range of the size distribution was 1200 μm (Figure S1, Supporting Information). Composite samples were prepared by adding 100 mg of dried (4 h at 250 °C) MCM-41 to 15 mL of dichloromethane (DCM, anhydrous spectral, 99.9%, Aldrich) solution containing 7HQ (1 mM) and stirring the mixture at room temperature for 15 h. The obtained material was washed several times with DCM. The washing removed most of the dye loosely adsorbed to the external surface of MCM-41 grains, but some externally adsorbed molecules might have still been present. However, their contribution to the measured signals should be small, as the internal surface of MCM-41 is much larger than the external surface. To examine this possibility of adsorption, we also used silica particles as a host (Merck, particle size 0.0630.200 mm, used after consecutive washing with nhexane, tetrahydrofurane, and dichloromethane), having silanol groups but lacking the nanochannel structure of MCM-41. Finally, the samples were dried in a vacuum. The loading of 7HQ within R-MCM41 was 2.08 mmol per 1 g of silicate. UVvisible absorption and emission measurements of powder samples in DCM suspension were performed using 1-cm rectangular quartz cells with stirring. For emission experiments, a 90° configuration between the excitation and observation channels was used. Steady-state absorption and diffuse transmittance spectra were recorded on a Jasco V-670 instrument equipped with a 60-mm integrating sphere (ISN-723). For the reflectance spectra, the KubelkaMunk remittance function was used: F(R) = (1 R)2/ 2R, where R is the diffuse reflectance intensity from the sample. Emission and infrared absorption spectra were recorded using a Fluoromax-4 (Jobin-Yvon) and Jasco FT/IR-4100 spectrometers, respectively. The emission lifetimes were measured with a previously 14688
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Figure 1. UVvisible absorption (Abs.) spectrum ( ) of 7HQ in dichloromethane (DCM) solution and diffuse transmittance (DTS) and emission (Em.) spectra of 7HQ in R-MCM41 (—) and Al-MCM41 ( 3 3 ). The excitation wavelength was 370 nm. For diffuse transmittance, the KubelkaMunk remittance function [F(R)] was used: F(R) = (1 R)2/2R, where R is the diffuse reflectance intensity from the sample.
described picosecond time-correlated single-photon-counting (TCSPC) spectrophotometer (FluoTime 200).34 The sample was excited by a 40-ps pulsed (20 MHz) laser centered at 371 nm, and the instrument response function (IRF) was typically 70 ps. The emission signal was collected at the magic angle (54.7°) using a microchannel-plate photomultiplier. The femtosecond emission transients were collected using the fluorescence upconversion technique. The system35 consisted of a femtosecond Ti:sapphire oscillator (Tissa 100) pumped with a 5-W diode laser (Verdi 5) and an upconversion setup. The pulses (70 fs, 400 mW, 86 MHz) were centered at 750 or 796 nm and doubled using a 0.5-mm-thick BBO (β-BaB2O4) crystal. The pump beam (∼0.1 nJ at 375 or 398 nm) excited the powder-like sample in DCM suspension, and the emission was gated at the magic angle by mixing it with a delayed fundamental beam, using a 1-mm BBO crystal. The IRF of the apparatus (measured as a Raman signal of DCM) was ∼200 fs. To analyze transient signals, multiexponential functions were convoluted with the IRF and fitted (globally for picosecond decays and individually for femtosecond transients) to the emission decays. The quality of the fits was characterized in terms of the residual distribution and reduced χ2 values. The photoirradiation experiments were carried out at 320 nm using the light coming from the Fluoromax-4P spectrometer and a standard 1-cm-path-length quartz cell under stirring. The irradiation intensity at 320 nm (1.16 1010 einstein/s, 2.4 1028 W/cm2) was determined using a potassium ferrioxalate chemical actinometer.36 All measurements were made at ∼293 K.
3. RESULTS AND DISCUSSION Figure 1 shows diffuse transmittance (DTS) and emission spectra of 7HQ in regular and aluminum-doped MCM-41 hosts (R-MCM41 and Al-MCM41, respectively) suspended in dichloromethane (DCM), as well as the absorption spectrum of 7HQ in DCM solution (without nanohosts). In pure solution, the absorption band centered at ∼320 nm is due to the enol (E) tautomer, which is the only form present in this solvent.37 Upon confinement of the dye within R-MCM41, the E form is partially converted in the ground state into the anionic (A) and zwitterionic (Z) structures, as indicated by the absorption shoulder in the 350450-nm region. Recently, we showed that this
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conversion occurs through the interactions of E with silanol groups inside the channels and depends on the number of accessible OH groups.30 The populations of the A and Z forms were practically absent when unstructured SiO2 particles were used as a host, showing the importance of the confinement effect in their formation and stability. In this work, we used R-MCM41 material having smaller pore diameters (∼25 Å), which changed the ratio of A to Z absorption (observed at ∼370 and ∼410 nm, respectively) in comparison to that previously reported for MCM-41 having channels of ∼40 Å.30 This difference (Figure 1) can be affected by other factors, however, such as possible traces of water left upon drying of these materials. In turn, the DTS spectrum of 7HQ within Al-MCM41 shows a much stronger band at 370 nm, assigned to the increased population of A. Incorporation of aluminum atoms into the silicate framework introduces negatively charged sites into the structure, which strongly interacts with labile protons of the hydroxylic group of 7HQ. This interaction stabilizes the anionic form of the guest. The steady-state fluorescence spectrum of 7HQ in DCM solution (not shown) has a single band with an intensity maximum at ∼355 nm that corresponds to the emission of the E form.30 In turn, using R-MCM41, the fluorescence spectrum depends on the excitation wavelength, but even upon 320-nm excitation, the largest emission band (centered at ∼500 nm) is mainly due to Z. The excited-state reaction leading to the latter form is also dominant within Al-MCM41 (Figure 1), although the contribution of A emission is significant in this host, as shown by an increased intensity in the 400480-nm region. 3.1. Ultrafast Dynamics of 7HQ within Al-MCM41. The analysis of fluorescence transients recorded at the femtosecond time scale requires information about the lifetimes of relaxed structures. Thus, we recorded the fluorescence decays of 7HQ within R- and Al-MCM41 materials in DCM suspensions upon excitation at 371 nm. The decays were globally fitted using a three-exponential function model. Table 1 reports the obtained values of decay times (τi), normalized amplitudes (ai) and contributions (Ci = τiai). The time constants are τ1 = 0.29 ns, τ2 = 1.49 ns, and τ3 = 4.53 ns for R-MCM41 and τ1 = 0.32 ns, τ2 = 1.72 ns, and τ3 = 5.06 ns for Al-MCM41. For both host materials, the shortest decay time, τ1, has its maximum amplitude at the blue side of the emission spectrum, and its contribution becomes very weak at observation wavelengths above 500 nm. This tendency is even clearer for the Al-doped material, where the τ1 component disappears completely at longer observation wavelengths. The τ2 decay time has the largest contribution at observation wavelengths of 450500 nm, but is present also at the red side of the spectrum. The longest lifetime, τ3, has its maximum amplitudes above 520 nm, where it becomes a dominant component in both host materials. The spectral positions and the relative contributions of the lifetime components are shown in Figure S2 (Supporting Information) as decay-associated spectra (DAS), constructed using the stationary emission spectra and the amplitudes listed in Table 1. The obtained decay times (τ1, τ2, and τ3) are not very different from those previously observed for 7HQ within R-MCM41 having a larger channels (diameter ≈ 40 Å), which were assigned to the lifetimes of E, A, and Z forms, respectively, bound to the mesoporous material.30 Thus, we adopt the same assignment of the observed lifetimes. We note a clear presence of the decaying component related to the framework-bound E form even in Al-MCM41. In the steady-state spectra, the signal of this form is either masked by much larger absorption of the anion or (in emission) is weak in comparison to 14689
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Table 1. Values of Fluorescence Lifetimes (τi), Normalized (to 100) Pre-Exponential Factors (ai), Contributions (Ci = aiτi), and Reduced χ2 Values Obtained from a Global Fit of the Emission Decays of 7HQ Interacting with R-MCM41 and Al-MCM41 in a Dichloromethane (DCM) Suspensiona λem (nm)
τ1 (ns)
a1 (%)
C1 (%)
τ2 (ns)
C2 (%)
τ3 (ns)
450
0.29
47
10
1.49
470
30
4
38
41
4.53
15
49
1.25
34
23
36
73
1.05
500
18
2
520 540
14 13
1 1
30
16
52
82
1.10
30 29
15 14
56 58
84 85
1.04 1.03
560
11
580
10
1
29
14
60
85
1.07
1
30
14
60
85
600
1.08
9
1
30
14
61
85
630
1.1
10
1
31
15
59
84
1.1
40
7
41
39
19
54
1.14
470
27
3
38
26
35
71
1.02
500 520
11 5
1 1
29 26
14 11
60 69
85 88
1.08 1.08
540
0
0
23
10
74
90
1.04
560
0
0
21
8
79
92
1.09
580
0
0
20
8
80
92
1.06
600
0
0
20
8
80
92
1.03
630
0
0
22
9
78
91
1.09
a2 (%)
a3 (%)
C3 (%)
χ2
R-MCM41
Al-MCM41 450
a
0.32
1.72
5.06
Excitation wavelength, 371 nm; observation wavelengths (λem), as indicated.
Table 2. Values of Time Constants (τi) and Normalized (to 100) Pre-Exponential Factors (ai) of the Multiexponential Functions Used in Fitting the Femtosecond Emission Transients of 7HQ Interacting with Al-MCM41 and R-MCM41 in a DCM Suspension at Different Observation Wavelengths (λobs)a λobs (nm)
τ1 (fs)
430
470
30
5.76
460
600
17
a1 (%)
τ2 (ps)
τ3 (ns)
a3 (%)
22
0.30b
48
6.58
14
0.30b
69
0.69 3.32
()19 ()8
1.70b 1.70b
81 65
a2 (%)
Al-MCM41
550 580
200
()27
430
260
53
3.54
20
0.29b
27
460
320
29
3.17
21
0.29b
50
550
0.78
()15
0.60c
85
580
0.74
()16
0.60c
84
R-MCM41
a
Negative sign indicates a rise component in the transient. b Dominant fluorescence lifetime component taken from TCSPC experiment. c Mean value of the two shortest fluorescence lifetimes.
those of A and Z structures because of its shorter lifetime. Moreover, the contributions of the three components are present over a relatively broad range of observation wavelengths, which indicates a significant heterogeneity of the populations within these composite materials. Figure 2 shows femtosecond fluorescence upconversion transients of the 7HQ/Al-MCM41 sample in a DCM suspension at different observation wavelengths and upon excitation at 375 nm. It also shows a comparison of 7HQ emission transients within
two different nanohosts (Al- and R-MCM41), and within Al-MCM41 at two different excitation wavelengths, 375 and 398 nm. Table 2 contains the times (τi) and pre-exponential factors (ai) obtained from the multiexponential fit (in a 50-ps window). We discuss the transients following the relevance of the region in the photobehavior. At the blue side of the stationary emission spectrum (430460 nm), the decays were fitted using three components for each nanomaterial. The longest component (τ3) corresponds to the lifetime of bound-E forms, which makes a significant contribution in this region. For 7HQ in R-MCM41, we observed two ultrafast components giving τ1 = 0.260.32 ps and τ2 = 3.23.5 ps. The obtained values are very similar to those previously reported for 7HQ in R-MCM41 having a larger channel, which were assigned to the production of anion intermediate (τ1) and zwitterionic structure (τ2).30 This result indicates that the pore size (in the range 2540 Å) has no effect on the excited-state dynamics and that the absorption and emission spectral differences obtained for 7HQ in Al-MCM41 host are consequences of the modification of the silica framework. We note that the femtosecond emission transients obtained for the 7HQ/R-MCM41 sample in acetonitrile (not shown) were similar to those measured using DCM suspension. This observation suggests that, at the ultrafast time scale, solvent polarity weakly affects the excited-state behavior of the 7HQ/RMCM41 material. Thus, for the 7HQ/Al-MCM41 composite, we obtained the values τ1 = 0.470.60 ps and τ2 = 5.86.6 ps, in addition to the lifetime-related component τ3. These decay times are significantly longer than those obtained using R-MCM41 (Table 2). To explain this result, two points should be considered. First, the population of anions at S0 is much larger for the Al-MCM41 material, and this form is predominantly excited at 14690
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Figure 2. (A) Magic-angle femtosecond-emission transients of 7HQ interacting with Al-MCM41 in a DCM suspension. The femtosecond excitation was at 375 nm, and the observation wavelengths are indicated in the plot. The solid lines are the best multiexponential fits (Table 2), and the dashed signal is the instrument response function (IRF). Comparisons of transients gated at 580 nm for 7HQ in (B) R-MCM41 and Al-MCM41 excited at 375 nm and (C) Al-MCM41 upon excitation at 398 and 375 nm.
375 nm, although the contribution of the bound-E state also exists, as shown by the picosecond-time measurements discussed above. Second, the difference is the presence of additional nonspecific (electrostatic) interactions between the guest and the host due to the aluminum atoms in the framework. Recently, using both femtosecond-resolved and single-molecule fluorescence microscopy of a H-bond dye, we showed that Al doping of MCM-41 at a level of 0.51% greatly changes the spectral and time-domain behaviors.38,39 Taking these observations into account, we suggest that the τ1 component for the Al-MCM41 host is mainly due to an intramolecular vibrational-energy redistribution (IVR) within the large population of directly excited A species, but might also include the production of anions from a lower population of enols bound to the silica framework. Regarding the stationary emission of 7HQ in Al-MCM41 (Figure 1), we observed that the fluorescence intensity of A was relatively increased in this material. This suggests that the τ2 component observed in the 430460-nm region should be assigned to the vibrational relaxation/cooling
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processes in excited A and its conversion into the red-emitting Z form. The former process commonly occurs for comparable-size organic molecules at the time scale of several picoseconds.40 In the red region of gated wavelengths (550580 nm), we mainly observed the dynamics of the zwitterion form for both types of hosts. The transients of 7HQ in R-MCM41 show a single rise time component of ∼0.75 ps. However, a second (decaying) component (∼8 ps) reported in the previous work and assigned to the vibrational relaxation (VR) of the Z form was not observed in the 375-nm-pumped experiment in the present case. As the amplitude of this 8-ps component was small and the two (40- and 25-Å channel diameter) regular materials differ in the relative populations of A and Z forms at S0, it is possible that this VRrelated component is even weaker for the current experiments. In turn, for 7HQ in Al-MCM41, we observed a single rise time of ∼0.7 ps at 550 nm. However, the transient gated at the reddest observation wavelength (580 nm) shows two different rising components: τ1 ≈ 0.2 ps and τ2 = 3.3 ps. These new components have different amplitudes (a1 = 27%, a2 = 8%) and are clearly visible in Figure 2B, which compares the 580-nm-gated transients of 7HQ interacting with R- and Al-MCM41. The τ2 value is very similar to the decaying value observed for R-MCM41 in the blue region of observation wavelengths, which is assigned to the A f Z reaction dynamics. Based on this result, we suggest that the 3.3-ps rise time in Al-MCM41 is due to the above reaction and that the corresponding decay time at the blue region might be overlapped with the relaxation signals. Note also that it is probably the lack of a decaying component originating from VR processes in Z that makes possible the observation of this longer rise time. For the 0.2-ps rise (τ1), we suggest that it might be due to the production of a vibrationally hot A form from the excited E form, as the latter is present at the low-energy observation wavelengths. Our recent work30 reported that the rise time observed in the reddest part of emission spectrum (540580 nm) for 7HQ in 40-Å R-MCM41 depends on the excitation wavelength. The decays monitored upon 375-nm excitation showed a subpicosecond rise component that not observed in the case of 411-nm excitation.30 This result was explained in terms of differences in the excess of excitation energy and different predominant forms of 7HQ absorbing at these excitation wavelengths. Figure 2C shows a comparison of 580nm-gated emission transients of 7HQ in Al-MCM41 for excitation at 375 and ∼400 nm. At the lower excitation energy, the longer 3.3-ps rise component is absent, but the shorter (subpicosecond) component is still observed. Again, this result is most probably due to differences in the population of the Z form at these two wavelengths being very low at 375 nm, but having a significant contribution in absorption at 400 nm. 3.2. Photoconversion within Silicate Nanomaterials. Diffuse Transmittance Spectra. The excited-state reactions occurring upon 370-nm (or lower-energy) excitation and described previously do not provoke permanent changes in the sample. However, previous reports showed a photochemistry of 7HQ under various conditions.26,31,32,41,42 Of special interest is UV irradiation of 7HQ embedded in a hydroxylic polymeric matrix, where the enol forms of the dye are converted into the tautomeric keto structures.26b This result suggests the possibility of 7HQ photochromism in other heterogeneous materials. Figure 3 shows the spectral changes in the UVvisible diffuse transmittance of 7HQ in R-MCM41 and adsorbed on amorphous silica particles in DCM suspensions, upon irradiation at 320 nm. For the regular MCM41 material, the intensity of the 14691
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Figure 3. UVvisible DTS spectra of 7HQ interacting with (A) R-MCM41 and (B) amorphous silica in DCM suspensions after the indicated times of irradiation at 320 nm. The inset in A shows part of the IR absorption spectra of ( ) fresh and (—) 200-min-irradiated 7HQ/R-MCM41 samples.
enol-band absorption at ∼320 nm gradually decreases, and a large increase of the anion-band absorption is observed at ∼360 nm. This E f A reaction is the main photochemical channel of the studied system. The changes are initially fast, but they become slower with time, and an A intensity saturation is observed after prolonged irradiation (∼200 min). In turn, the changes in absorbance observed at wavelengths longer than 380 nm are more complex, because of a lower intensity and an overlapping between the absorption bands. To follow the photoinduced changes, Figure S3 (Supporting Information) shows the difference between the spectra of the irradiated and initial samples. Before irradiation, the shoulder at ∼420 nm (Figure 3) is due to the absorption of the Z form.30 This conclusion is further supported by a close similarity between the spectral position of this shoulder and the absorption spectrum of N-methyl-7-hydroxyquinolinium (NM7HQþ) in alkaline aqueous solution, where NM7HQþ exists exclusively as a zwitterionic form.18 During the initial period of irradiation, this shoulder increases slightly. After ∼100 min of irradiation, we observed a clear decrease of Z intensity (Figure S3, Supporting Information). Additionally, the irradiation produced a new band at ∼490 nm that formed within about 100 min. After this time, some decrease in intensity was observed. However, in contrast to the behavior of the zwitterion band at 420 nm, this new band preserved its shape and position even upon prolonged irradiation. A band deconvolution to separate contributions of A and the two red-absorbing (420- and 490-nm) structures using Gaussian- or lognormal- (SianoMetzler-) type43 functions turned out to be difficult to do accurately, because of the broadness of the bands and the large disproportion in their intensities. Despite this difficulty, these results suggest that the photoproduced Z form is further transformed into the final form absorbing at 490 nm and having a keto structure (vide infra).
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In an attempt to identify the species absorbing at ∼490 nm and emitting at 570 nm, we measured the IR absorption spectrum of fresh and irradiated 7HQ/R-MCM41 material in the solid state (dried powder), using pure R-MCM41 as a reference sample. The inset of Figure 3A shows the IR spectra recorded between 1500 and 4000 cm1. In this spectral region, we observed two bands at 2360 and 1659 cm1. The former is due to SiH stretching mode, as 7HQ itself does not have an IR absorption band in this region, either in the condensed phase at room temperature or when isolated in a rare-gas matrix at 20 K.32 The intensity of this band remained constant during the photoirradiation. The second band is assigned to a CdO vibration, and its intensity largely increased after the irradiation. This result suggests that, in addition to the A form, a keto-type compound is photoproduced. Previously, it was postulated that the polar zwitterionic form in a series of hydroxyquinolines (5-, 6-, 7-, and 8HQ) is in equilibrium with its resonance structure of the quinonoid type (which corresponds to K form in Scheme 1).44 In this family, 7HQ exhibits the highest fluorescence quantum yield, which has been explained in terms of a largest contribution of the quinonoid (or keto) form in both the excited and ground states of 7HQ compared to the other mentioned derivatives. These observations led us to assign the 490-nm absorption and 570-nm emission bands of the irradiated 7HQ/R-MCM41 sample to a keto-type structure (K, Scheme 1). To assess the role of confinement in the photoconversion of 7HQ, Figure 3B shows the DTS absorbance changes of 7HQ adsorbed on nonstructured silica particles in DCM. Before 320-nm irradiation, the spectrum shows less intensity above ∼350 nm than that for MCM-41, especially in the A absorption region (∼360 nm). Upon irradiation, the changes in the E and A absorption regions are qualitatively similar to those observed for the porous nanohost, although the increase of the A absorption and decrease of the E absorption are less pronounced. Additionally, the complex behavior observed in R-MCM41 in the Z absorption region (420 nm) is not present for the silica gel support, and the intensity increase at 490 nm is much smaller. We note that the ratios of the initial intensities of the E and A bands in irradiated samples are different for the studied silicate materials (Figure 3). Also, the final population of major photoproduct assigned to A is smaller in the case of nonporous SiO2. This behavior is explained in terms of different formation yields and stabilizations of the A form of 7HQ interacting with these supports, as the supports have different H-bonding and confining properties. Additionally, electrostatic interactions and increased basicity of aluminum-containing MCM-41 stabilizes the A form, as already shown by the DTS spectra of nonirradiated samples (Figure 1). To quantify the photoconversion reaction, Figure 4 shows plots of the changes in the absorption of 7HQ interacting with R-MCM41 and SiO2 particles in DCM suspension monitored at 360 nm as a function of the irradiation time. At this wavelength, the spectral changes are exclusively due to an increasing population of A, and the influence from other forms of 7HQ, if present, is not significant. However, this kind of quantitative analysis cannot be done for the absorption bands of E at 320 nm and Z at 410420 nm, as both of these bands are largely affected by the growing signal of the anionic forms. For monitoring the anion, the rich spectral behavior observed during irradiation suggests that the reaction scheme is more complex than a simple conversion of E to A. The first attempt to fit the experimental points of the A population change using the first-order reaction E f A with a single rate constant was not successful for either 14692
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Figure 4. Changes in the concentration of the A form (calculated using UVvisible DTS absorption intensity at 360 nm) of 7HQ interacting with (b) R-MCM41 and (O) amorphous silica for different irradiation times. The solid lines show the best fits obtained using eq 3 (see text for details).
R-MCM41 or silica particles. Thus, taking into account the presence of the Z form and the new band appearing at ∼490 nm, we considered the possibility of two photochemical reactions occurring in parallel steps k1
EfA k2
E f Z, K
ð1Þ
k1
ð2Þ
or in consecutive steps k2
E f A f Z, K
For both reaction schemes, the second step involves production of the Z tautomer, which is probably further transformed into keto structures, as the CdO absorption band at ∼1660 cm1 (Figure 3A) was clearly observed at the end of the irradiation experiments (∼200 min). However, this longer-time irradiation evolution (Z f K) does not affect our analysis, as we are monitoring only changes of A absorption. Thus, we found that the latter approach (eq 2) gives a satisfactory quality of fit to the experimental data (Figure 4) using the following equation to describe the concentration of the anion at the monitored wavelength ½A ¼
k1 ½E0 ½expðk1 tÞ expðk2 tÞ k2 k1
ð3Þ
where [A] is the concentration of the anion form; [E]0 is the initial concentration of the enol form; t is the irradiation time; k1and k2 are the rate constants of first and second reactions, respectively, of eq 2. The values of [E]0 and [A] were extracted from the absorption intensity data using the DTS spectral intensities at 330 and 360 nm, respectively. To estimate the extinction coefficients of the E and A forms of 7HQ in H-bonding environments, we measured the extinction coefficients in pure and alkaline methanol solutions and used the resulting in our calculations. We obtained ε = 4180 M1 cm1 for the enol (at 330 nm) and ε = 4630 M1 cm1 for the anion (at 360 nm). The goodness of the fit shown in Figure 4 demonstrates that eq 3 successfully describes the photoconversion behavior of the systems. For 7HQ/R-MCM41, we obtained rate constants of k1 = 8.8 ((0.23) 103 min1 and k2 = 3.2 ((0.18) 103 min1 (R2 = 0.992). For the 7HQ/SiO2 sample, the values were k1 = 10.3 ((0.32)
Figure 5. UVvisible reflectance spectra of dried-powder samples of 7HQ/R-MCM41 before and after 320-nm irradiation performed in DCM suspension and in the solid state.
103 min1 and k2 = 2.7 ((0.14) 103 min1 (R2 = 0.993). For both the structured and amorphous host materials, the value of k1 was larger than that of k2, which results in a growing anion population as the reaction proceeds. Previous studies on various dyes interacting with mesoporous hosts have shown that the presence of a solvent can affect the photobehavior of the encapsulated guest molecules.45,46 To eliminate the solvent effect on the absorption of the formed structures, we measured reflectance spectra of dried solid-state samples of 7HQ/R-MCM41 upon irradiation in DCM suspension, as well as upon direct irradiation in the solid state. Figure 5 shows that the spectral features observed in the fresh sample in DCM suspensions (i.e., E, A, and Z absorptions at 330, 360, and 420 nm, respectively) were also present in the nonirradiated dried sample. However, the intensity ratio of the bands was different than in the case of the suspensions. This observation is explained in terms of absence of solvent effects in the formation of the phototautomer. The reflectance spectrum of 7HQ/RMCM41 irradiated (240 min) in DCM suspension shows a significant increase of the A absorption and the appearance of a new absorption band at ∼500 nm. Moreover, similar changes were observed when the irradiation was carried out in dry powder. These results show that the photoconversion between different structures of 7HQ is stimulated by the interactions with the silicate host material but does not depend on the presence of the solvents used. The latter might, however, have an impact on the relative populations of the involved structures and their emission quantum yields. Emission and Fluorescence Excitation Spectra. Figure 6 shows the fluorescence spectra of 7HQ/R-MCM41 composite in a DCM suspension upon excitation at 320 nm. Parts A and B present different time spans of irradiation (tIR = 060 and 60250 min, respectively) for easy tracking of the changes. The band at ∼320 nm is due to E-form emission. Its intensity is the largest at the beginning of irradiation and continuously decreases during the experiment. A similar trend (i.e., decrease) was observed for an intense emission band centered at ∼510 nm, which is due to the Z form populated at the excited state from E and A.30 Because of the relatively long fluorescence lifetime (∼4.5 ns) of Z and the very fast excited-state reaction toward this form (∼3 ps),30 its emission is predominant until ∼135 min of irradiation. Following the permanent photoconversion of E, the Z emission gradually decreases, and at later times, new emission bands at ∼435 and 570 nm become more intense. The former is due to emission from the directly excited population of A, created 14693
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Figure 6. Changes in the emission spectra of 7HQ interacting with R-MCM41 in a DCM suspension upon 320-nm irradiation for the indicated times. For clarity, we separated the changes in the spectra into two categories: (A) 060 and (B) 60250 min. The arrows indicate the trends of emission intensity changes.
during irradiation. The spectral position of this band is blueshifted in comparison with the emission of the anion in alkaline water (∼490 nm), but it is similar to that observed in modified (silylated) MCM-41 material.30 The intensity of this emission is relatively weak upon 320-nm excitation, compared to the initial (tIR = 0 min) emission from Z. This is due to the smaller absorption of the A form at this excitation wavelength. It might also indicate a lower fluorescence quantum yield of photoproduced A and the presence of an efficient nonradiative deactivation channel. The other new emission band observed upon longer irradiation is centered at ∼570 nm. This fluorescence is not present in fresh 7HQ/R-MCM41 sample, and its increase coincides with the appearance of a 490-nm-centered DTS band in Figure 3A. The relation between these two bands is clear upon comparison of the emission spectra of fresh (tIR = 0 min) and irradiated (tIR = 235 min) 7HQ/ R-MCM41 samples for at excitation 470 nm (Figure S4, Supporting Information). The initially observed Z emission is present because of the excitation of an absorption tail of this form, but it disappears upon irradiation, and the new emission band with an intensity maximum at ∼570 nm is exclusively observed. Details about the ground-state stability of the present forms of 7HQ and the relations between absorption and emission bands are available from an analysis of the fluorescence excitation spectra. Figure 7 shows the normalized spectra for 7HQ/ R-MCM41 at tIR = 0, 60, and 230 min. For nonirradiated material, the bands/shoulders characteristic of the free-E (320 nm), bound-E (∼360 nm), A (360375 nm), and Z (∼430 nm) structures appear in the spectra, as the observation wavelengths change from 370 to 550 nm. The band pattern observed here is very similar to that of 40-Å-diameter MCM-41.30 During irradiation, the relative intensities of these bands change, mostly in favor of the anion, but a new absorption is observed at ∼490 nm when
Figure 7. Fluorescence excitation spectra of 7HQ interacting with R-MCM41 in a DCM suspension (A) before irradiation and upon irradiation at 320 nm for (B) 60 and (C) 235 min. The observation wavelengths are indicated in the legends.
the emission is gated at a far red side of the spectrum. Upon longer irradiation, the relative intensity of A absorption becomes predominant for all observation wavelengths. This indicates that the structures are still related to common photochemical channels in the excited state, as an excitation of A at 350 nm results in red emission observed at 650 nm. A different fluorescence behavior was observed upon irradiation of 7HQ adsorbed on the amorphous surface of silica gel particles in DCM. Figure 8 shows the fluorescence spectra recorded during this experiment upon excitation at 320 nm. Before irradiation, a dominant emission band is centered at ∼360 nm, but it is significantly broader than the one observed in DCM solution without a silicate material. This band is due to emission of E forms bound to silica particles, and the increased breadth shows a diverse strength of interactions (heterogeneity) on the surface. The band observed at 520 nm is due to the Z tautomer populated through an excited-state reaction, as shown by the weak absorption of this form in the DTS spectrum (Figure 3B). During irradiation, the intensity of the E emission largely decreases until it practically disappears. Concomitantly, an increase of the A fluorescence intensity is observed, initially as a red shift of the main emission band and then as a separate band with a maximum at ∼430 nm. In turn, the Z-form fluorescence band (at 520 nm) decreases during the first 30 min of irradiation, but an 14694
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Figure 8. Changes in the emission spectra of 7HQ interacting with amorphous silica in a DCM suspension upon 320-nm irradiation for the indicated times. For clarity, we separated the changes in the spectra into two categories: (A) 030 and (B) 30200 min. The arrows indicate the trends of emission intensity changes.
increase in intensity is observed at tIR > 40 min. The increase is not due to an overlap with the growing A fluorescence. The observed behavior is explained by different origins of emitting Z structures at different periods of photoconversion. The initial decrease is due to the disappearance of the E form on the silica surface, as the ESPT reaction in this form gives excited Z structures emitting at 520 nm. Note also that the photocreated A (stabilized at S0) produces Z through an A f Z proton-transfer reaction. The fluorescence excitation spectra in Figure S5 (Supporting Information) show that a small population of Z at S0 is present also in this material and gives the fluorescence observed in the 520-nm region. To examine the effect of solvent on the emission of the caged 7HQ structures, Figure 9 shows the fluorescence spectra of dried solidstate samples of 7HQ/R-MCM41. For the nonirradiated sample (Figure 9A), we observed an intensity maximum at ∼500 nm, which corresponds to the emission of Z form. The position of the spectrum is similar to that obtained in DCM suspension before irradiation (Figure 6A), and it is the same for excitation wavelengths of 370 and 420 nm. However, the fluorescence spectra of the irradiated materials show a dominant emission band at ∼580 nm and less intense bands at shorter observation wavelengths. The large 580-nm band is due to a new photoproduced form, observed also in DCM suspension at tIR > 135 min (Figure 6B). Comparison of the emission spectra obtained from dried samples that were irradiated in DCM suspension and in the solid state (Figure 9B,C) shows that the position of this band is independent of the presence or absence of a solvent within the channels of R-MCM41.
Figure 9. Fluorescence spectra of dried-powder samples of 7HQ/RMCM41: (A) before irradiation and after 320-nm irradiation (B) in DCM suspension (240 min) and (C) in the solid state (180 min).
Note that the changes in the emission intensities observed for the studied materials are not suitable for the calculation of the rate constants of the photoconversion reactions for the following reasons: (i) reabsorption of the emission (mainly for the E region), (ii) overlap of emission signals of the different forms of 7HQ, (iii) and different and unknown fluorescence quantum yields of these structures when interacting with silicate materials. In addition, the relative contributions in emission change upon irradiation, making it impossible to follow the kinetics using fluorescence signals. Origin of Photobehavior of 7HQ Interacting with MCM-41. From the data discussed in the preceding sections, it is clear that the photobehavior of 7HQ interacting with silicate materials mainly consists of two separate routes that can be referred to photochemical pathways (introducing a permanent change) and photophysical ones (relaxing back to the initial ground-state structures). For the former, the main process occurring during irradiation is E f A conversion, and it produces stable groundstate tautomeric structures with different absorption spectra having isosbestic points at 314 and 332 nm (Figure 3). The permanent photoconversion depends on the excitation wavelength and is not observed for wavelengths longer than ∼370 nm. 14695
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This restriction is not present in the case of second pathway, which leads from an excited E to A and then to Z tautomers in consecutive ESPT steps.30 Using this route, any of the forms (E, A, Z/K) can relax to the ground state without producing a permanent change. These observations suggest that the energy barrier on the excited-state potential-energy surfaces of the system is larger for the permanent (photochemical) reaction but that it is much lower for the other route. The rather short ESPT times (200 fs4 ps) observed for 7HQ in R-MCM41 in this work and in our previous study support this supposition.30 Closely related remains the question about the high stability of photochemically produced ground-state forms of 7HQ. In fact, such species already exist in freshly prepared composite samples. We can suggest only that their interactions with the silica framework of the host (SiO, OH) differ from those of the rest of 7HQ. Note that the absorption and emission bands are relatively broad, and heterogeneity in the interactions of a H-bonded guest has been reported for these types of materials.38,39,47,48 The substantial increase in the anion absorption observed in Al-MCM41 indicates that electrostatic forces play an important role in forming and stabilizing this structure within the nanotube. Very recently, a photoreaction of 7HQ enol deposited in a low-temperature (20 K) argon matrix was investigated using IR and UVvis spectroscopies.31,32 These results showed that, under these cryogenic conditions, the main products of irradiation were 7-quinolinoxyl radical and 7-ketene, in addition to a smaller population of the keto form of 7HQ. For the K structure, the CdO stretching band has been observed at 1633 cm1 (and calculated by DFT method to appear at 1642 cm1).32 In the electronic spectrum, this form appeared as a broad and weak band with an intensity maximum at ∼460 nm.31 The other structure observed in the cryomatrix and formally having a CdO group is 7-ketene. This species can be safely rejected from our considerations, because it absorbs at 260 nm in the electronic spectrum and in the IR spectrum its CdCdO group has a band at 2117 cm1.32 Finally, 7-quinolinoxyl radical has a UVvis absorption band at 360 nm, which corresponds to that observed for the A form within MCM-41 hosts. This compound cannot be stable at room temperature, however, and its formation requires hydrogen-atom elimination, in opposition to proton displacement for generation of the anionic structure. Scheme 2 summarizes our findings regarding photoconversion of 7HQ/R-MCM41 material. The 320-nm irradiation of the confined enol form leads to the formation of a ground-state population of the anionic and tautomeric structures. This route, however, is not efficient upon lower-energy excitation (370 nm), which suggests the presence of a barrier on the potential energy surface of the excited state. To obtain more information about the photochemical routes of this rather complex nanosystem, we
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are carrying out studies at the single-molecule level. Such an approach will allow for the study of the importance of population heterogeneity in the photobehavior of 7HQ interacting with silica-based nanomaterials, as it will enable the examination of individuals in the desired population, as we recently reported.39
4. CONCLUSIONS In this work, we reported on the steady-state UVvisible absorption and fluorescence spectroscopy, as well femtosecond nanosecond time-resolved emission behavior, and photoconversion of 7HQ interacting with R-MCM41, Al-MCM41, and amorphous silica particles in dichloromethane solution. The stabilization of the ground-state populations of various forms of the guest molecule (enol E, anion A, zwitterion Z, and keto K) depends on the type of host material. This result is explained in terms of different H-bonding, electrostatic, and confining host/ guest interactions. The picosecondnanosecond emission experiments on 7HQ within the MCM-41 hosts in dicholoromethane show a weak dependence of the fluorescence lifetimes on the channel diameter (in the 2540-Å range) and on the presence of metal ions within the host framework. The lifetimes of 7HQ/AlMCM41 (0.32, 1.72, and 5.06 ns) were not very different from those obtained using R-MCM41 host (0.29, 1.49, and 4.53 ns), and were assigned to bound E, A, and Z forms, respectively. In contrast, at the ultrafast (femtoseconds to a few picoseconds) time scale, the Aldoped nanomaterial affects the formation and relaxation of the A and Z structures within the channels. The decay times obtained for 7HQ within Al-MCM41 were τ1 = 0.470.60 ps and τ2 = 5.86.6 ps. In addition, a rise time of 3.3 fs was observed when the red part of the emission spectrum was gated. The τ1 time was assigned to the formation of excited A and intramolecular vibrational relaxation within this structure, whereas τ2 was assigned to vibrational relaxation/cooling of A. In turn, the 3.3-ps rise time is evidence of the A f Z excited-state reaction. These times are longer than the values observed for R-MCM41 (0.260.32 and ∼3.5 ps and a rise of 0.75 ps), which is explained in terms of different nonspecific (electrostatic) interactions of 7HQ with the framework of regular and Al-doped nanomaterials. The irradiation of 7HQ within R-MCM41 and amorphous silicate materials in DCM at 320 nm leads to conversion of the E form into a significant population of ground-state stabilized A as the main photochemical channel. Further consecutive photoreactions to produce Z and then the keto form absorbing at 490 nm and emitting at 570 nm occur with a lower efficiency in R-MCM41. This last reaction (Z f K) was not observed when using nonporous silica particles, which shows the importance of H-bond interactions and the confinement effect. The photoconversion of E also occurred in a solid-state sample, leading to an increase of the A and Z (K) populations. This result suggests that the used (aprotic) solvent does not play a key role in the formation of these structures. The production of a new form of the functional dye embedded in mesoporous silica channels using light as a triggering tool reveals a material with some interesting properties. First, the K form is created efficiently only upon higher-energy irradiation, whereas longer-wavelength light can be used to probe the sample. Second, the absorption and emission of the created structures are relatively well-separated from those of initial forms of the dye. Third, the populations of particular forms of the dye can be tuned by modifying the silica framework or by introducing substituents into the channels, such as methyl groups.30 Finally, the photoproduced form is stable once created, and we did not 14696
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The Journal of Physical Chemistry C observe any reverse conversion reaction, upon either spectral or thermal activation. We believe that the findings of this work open up interesting possibilities in using H-bonded guests interacting with silicabased materials for designing potential photoactive devices such as molecular switches/gates or memories, to propose a few.
’ ASSOCIATED CONTENT
bS
Supporting Information. Decay-associated spectra in regular and aluminum-doped hosts, difference DTS spectra of irradiated samples, comparison of emission spectra of fresh and irradiated 7HQ/R-MCM41 samples, and fluorescence excitation spectra of 7HQ interacting with amorphous silica. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: þ34-925-268840.
’ ACKNOWLEDGMENT This work was supported by the JCCM and MICINN through Projects PCI08-5868, Consolider Ingenio 2010 (CSD20090050, MULTICAT), and MAT2008-01609, respectively. We thank Maria Rosaria Di Nunzio for her help in the irradiation experiment and Jacinto Alonso Azcarate for determination of the particle distribution. C.M thanks the MEC for the FPU fellowship. ’ REFERENCES (1) (a) Douhal, A.; Kim, S. K.; Zewail, A. H. Nature 1995, 378, 260. (b) Douhal, A.; Lahmani, F.; Zewail, A. H. Chem. Phys. 1996, 207, 477. (2) Hydrogen-Transfer Reactions; Hynes, J. T., Klinman, J. P., Limbach, H. H., Schowen, R. L., Eds.; Wiley-VCH: Weinheim, Germany, 2006; Vols. I and II. (3) Ultrafast H-Bonding Dynamics and Proton-Transfer Processes in the Condensed Phase; Elsaesser, T., Bakker, H. J., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2002. (4) Garczarek, F.; Gerwert, K. Nature 2004, 439, 109. (5) Stoner-Ma, D.; Jaye, A. A.; Matousek, P.; Towrie, M.; Meech, S. R.; Tonge, P. J. J. Am. Chem. Soc. 2005, 127, 2864. (6) Inoue, J.; Tomioka, N.; Itai, A.; Harayama, S. Biochemistry 1998, 37, 3305. (7) Bach, A.; Tanner, C.; Manca, C.; Frey, H.-M.; Leutwyler, S. J. Chem. Phys. 2003, 119, 5933. (8) Kohtani, S.; Tagami, A.; Nakagaki, R. Chem. Phys. Lett. 2000, 316, 88. (9) Matsumoto, Y.; Ebata, T.; Mikami, N. J. Phys. Chem. A 2002, 106, 5591. (10) Chou, P.-T.; Wei, C.-Y.; Wang, C.-R. C.; Hung, F.-T.; Chang, C.-P. J. Phys. Chem. A 1999, 103, 1939. (11) Bardez, E. Isr. J. Chem. 1999, 39, 319. (12) Fang, W.-H. J. Phys. Chem. A 1999, 103, 5567. (13) Lahmani, F.; Douhal, A.; Breheret, E.; Zehnacker-Rentien, A. Chem. Phys. Lett. 1994, 220, 235. (14) Tanner, C.; Manca, C.; Leutwyler, S. Science 2003, 302, 1736. (15) Park, S.-Y.; Lee, Y.-S.; Kwon, O.-H.; Jang, D.-J. Chem. Commun. 2009, 926. (16) Bhattacharya, B.; Samanta, A. J. Phys. Chem. B 2008, 112, 10101. (17) Kwon, O.-H.; Lee, Y.-S.; Yoo, B. K.; Jang, D.-J. Angew. Chem., Int. Ed. 2006, 45, 415. (18) Kim, T. G.; Topp, M. R. J. Phys. Chem. A 2004, 108, 10060.
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