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. 2015 Feb 11;137(5):1916-28.
doi: 10.1021/ja511381f. Epub 2015 Jan 30.

Differences in the Location of Guest Molecules Within Zeolite Pores as Revealed by Multilaser Excitation Confocal Fluorescence Microscopy: Which Molecule Is Where?

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Differences in the Location of Guest Molecules Within Zeolite Pores as Revealed by Multilaser Excitation Confocal Fluorescence Microscopy: Which Molecule Is Where?

Christoph Sprung et al. J Am Chem Soc. .
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Abstract

A detailed and systematic polarized confocal fluorescence microscopy investigation is presented on three batches of large coffin-shaped ZSM-5 crystals (i.e., parent, steamed at 500 °C, and steamed at 700 °C). In total, six laser lines of different wavelength in the visible region are employed on two crystal positions and three orientations with respect to the polarization plane of the excitation laser light. A fluorescent probe molecule is generated inside the zeolite pores, originating from the acid-catalyzed oligomerization of 4-fluorostyrene. A thorough analysis of the polarization plane of emitting fluorescent light reveals insight into the orientation of the fluorescent probe molecule restricted by the highly ordered zeolite channel framework, thereby visualizing pore accessibility and clearly distinguishing the occupation of straight and sinusoidal channels by the probe molecule. Spectral features are, furthermore, observed to tell apart molecules situated in one or the other pore. Special focus was given on the rim and tip regions of the zeolite ZSM-5 crystals. On the basis of the confocal approach of the investigation, the aforementioned features are evaluated in three dimensions, while the degradation of the zeolite framework upon postsynthesis steam treatment could be visualized by occupation of the sinusoidal pores.

Figures

Figure 1
Figure 1
Large coffin-shaped ZSM-5 crystal: The model shows the six subunits and the internal pore architecture for sinusoidal and straight pores. A 4-fluorostyrene monomer is shown in the lower center, which was used to create linear and cyclic fluorescent dimeric carbocations, illustrated on the right side inside the ZSM-5 channel framework.
Figure 2
Figure 2
Summary of experiments and evaluation principle of the multilaser excitation confocal fluorescence microscopy approach used: Three sets of large coffin-shaped ZSM-5 samples (parent and steamed at 500 and 700 °C) were stained with 4-fluorostyrene as probe molecule. Both roof and gable positions at 0°, 45° (for parent and steamed 700 °C), and 90° orientation were investigated for their fluorescence during 405, 457, 488, 514, 561, and 642 nm laser excitation. Eighteen focal planes were recorded, each consisting of 32 real color images (i.e., integral intensity of a 10 nm range, 400–720 nm and 430–750 nm, respectively). Hence, the presented data set contains a total of 96 sets of focal planes. Afterward, during data evaluation, 3-D (i.e., depth dependent) spectra were extracted for each set of focal planes from a region of interest.
Figure 3
Figure 3
(a) Absorption spectra of the ZSM-5 crystals; (b) parent, (c) steamed 500 °C, and (d) steamed 700 °C presented as photoimages. Those were the very same crystals investigated in the fluorescence microscopy investigations in this manuscript and obtained after impregnation and reaction of 4-fluorostyrene at 180 °C. Polarization-dependent absorption spectroscopy investigations were presented for parent and steamed 700 °C in Sprung et al. and Figure S4, Supporting Information, respectively. Scale bars represent 10 μm in width.
Figure 4
Figure 4
Combined view on orientation and steaming related features of ZSM-5 crystals stained with 4-fluorostyrene: λex = 561 nm, highlights a–e are described in the text. Investigated polarization plane is north–south, i.e., vertical. Scale bars represent 10 μm in width.
Figure 5
Figure 5
Zeolite ZSM-5 crystal, steamed at 700 °C, viewed in roof and gable orientation, excited with a 642 nm laser. Schematic drawing of the crystal is completed by focal planes of the top, middle, and bottom plane (with respect to the viewing direction). Arrows indicate the area from which depth-dependent spectra were extracted: (a,c) left of center, (b,d) center, and (e) top of center. Double-headed arrows indicate the recorded polarization plane, i.e., perpendicular to the long crystal axis. Scale bars represent 10 μm in width.
Figure 6
Figure 6
(a,b) Location: the rim, which is fluorescent in the vertical crystal, is not fluorescent in the horizontal one. (a) Zeolite ZSM-5 crystal was steamed at 500 °C and excited at 488 and 561 nm in horizontal and vertical orientations, respectively. (c,d) Spectral features: crystals were steamed at 700 °C and excited at 488 nm; the schematic drawings highlight the polarization plane of recorded fluorescence light. (e−i) Fluorescence spectra extracted from the indicated positions. Scale bars represent 10 μm in width.
Figure 7
Figure 7
Parent zeolite ZSM-5 crystals in gable position and 90° oriented. Focal planes from the center of the crystal. Depth-dependent fluorescence spectra were extracted from (a) the tip, (b) body between tip and center, and (c) center region. Scale bars represent 10 μm in width.
Figure 8
Figure 8
Influence of steaming: ZSM-5 crystals (parent, steamed (st) 500 °C, and steamed (st) 700 °C) in gable position oriented 90° with respect to the analyzer position; excitation wavelength was 488 nm. Depth-dependent fluorescence spectra from (a–c) the center part of the crystal and (d) the yellow tip part. Scale bars represent 10 μm in width.
Figure 9
Figure 9
(a,b) Laser reflection spectra for 488, 514, 561, and 642 nm laser lines, recorded on a glass sample holder and on bare ZSM-5 crystal. Subtraction of laser reflection for (c) 642, (d) 561, and (e) 514 nm laser light at steamed 700 °C ZSM-5 crystals. The spot beneath the crystals of the images in the left column was originating from the laser reflected in the sample holder. The signal of this reflection was effectively subtracted from the images by the procedure described in the text (compare images in the left and right columns). Scale bars represent 10 μm in width.
Figure 10
Figure 10
Transmission of fluorescence light of dichroic mirrors and 488 nm excitation: (a−c) parent and (d–f) steamed 500 °C ZSM-5 crystal, spectra extracted for (a,d) dichroic mirror 405/488 and (b,e) dichroic mirror 405/488/561. (c,f) Combined values of the former two.

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