the ZL channel is determined by two factors: shape volume constraints and relative
strength of competitive interactions among confined species. This work deepens the
understanding of host-guest interactions in dye-ZL composites, a key requirement to
master the finely tuned mechanisms governing supramolecular organization in
confined nano-spaces.
The orientation of oxonine Ox
+ and pyronine Py
+ inside of channels of ZL
crystals was investigated by means of fluorescence microscopy and single-crystal
imaging. The result was a cone-shaped distribution of the ETDM with a half-cone
angle of 72
[149]. This result was not well understood based on geometrical
arguments because the ZL structure gives room for only two possible arrangements
of the molecules’ long axis: a half-cone angle of up to 40
for Ox
+ and an angle of
about 90
with respect to the c-axis of ZL. The measurements were therefore
reinvestigated by probing Ox
+
-ZL composites using two-photon fluorescence polarimetric microscopy. The result obtained over several points measured on several
crystals was a half-cone angle distribution of 70–85
, thus confirming the result of
the initial study [165]. How can this be? The puzzle was solved using first-principles
quantum chemical, molecular dynamics calculations [153]. It turned out that an
angle in the range of 70–85
is not an energetically favored orientation, and it might
only be visited transiently by Ox
+ at room temperature. The results of the calculations suggested different ways to interpret the optical microscopy data, namely, the
experimental results derive from the superposition of differently oriented Ox
+
. The
most stable orientation in the hydrated system, which corresponds to the optical
microscopy experiment conditions, is 88
. However, other conformations, with Ox
+
nearly aligned to the channel, are quite close in energy and can be easily accessed at
room temperature. Following this argument, the actual distribution of Ox
+ inside the
ZL channel is understood to result from a larger portion of Ox
+ molecules perpendicularly oriented and a smaller fraction approximately aligned to the channel. The
optical microscopy data are therefore interpreted as the resultant, i.e., the “vector
sum” of components arising from an Ox
+ population characterized by non-uniform
orientation, mainly perpendicular to the channel but contaminated by a fraction of
molecules approximately aligned to the channel. The results for Py
+ are similar
[153]. Even though the parallel orientation is not the equilibrium arrangement for the
hydrated Ox
+
-ZL composite, the low mobility of Ox
+ in the presence of water makes
it likely that a small though appreciable number of Ox
+ molecules could remain
frozen in such an arrangement for some time. Moreover, the low Ox
+ mobility,
detected along the simulations, is in line with the experimental observation that
diffusion of Ox
+ inside ZL is very slow even at 100
C [43]. Furthermore, such a
fraction of nearly parallel Ox
+ and Py
+ is responsible for the J aggregate coupling
observed in ZL, a fact which cannot be understood otherwise [166]. This demonstrates the enormous power of the combination of confocal microscopy and
advanced computational techniques for understanding the structure of guest-host
composites.
The electronic S 0 –S 1 (1La) transition of methylacridine MeAcr
+ is relatively
weak and oriented parallel to the y-axis, as indicated in Fig. 16a), while the more
intense S 0 –S 2 is parallel to z (1Lb). Both transitions are of πÀπà type
Guests in Nanochannels of Zeolite L
31
strength of competitive interactions among confined species. This work deepens the
understanding of host-guest interactions in dye-ZL composites, a key requirement to
master the finely tuned mechanisms governing supramolecular organization in
confined nano-spaces.
The orientation of oxonine Ox
+ and pyronine Py
+ inside of channels of ZL
crystals was investigated by means of fluorescence microscopy and single-crystal
imaging. The result was a cone-shaped distribution of the ETDM with a half-cone
angle of 72
[149]. This result was not well understood based on geometrical
arguments because the ZL structure gives room for only two possible arrangements
of the molecules’ long axis: a half-cone angle of up to 40
for Ox
+ and an angle of
about 90
with respect to the c-axis of ZL. The measurements were therefore
reinvestigated by probing Ox
+
-ZL composites using two-photon fluorescence polarimetric microscopy. The result obtained over several points measured on several
crystals was a half-cone angle distribution of 70–85
, thus confirming the result of
the initial study [165]. How can this be? The puzzle was solved using first-principles
quantum chemical, molecular dynamics calculations [153]. It turned out that an
angle in the range of 70–85
is not an energetically favored orientation, and it might
only be visited transiently by Ox
+ at room temperature. The results of the calculations suggested different ways to interpret the optical microscopy data, namely, the
experimental results derive from the superposition of differently oriented Ox
+
. The
most stable orientation in the hydrated system, which corresponds to the optical
microscopy experiment conditions, is 88
. However, other conformations, with Ox
+
nearly aligned to the channel, are quite close in energy and can be easily accessed at
room temperature. Following this argument, the actual distribution of Ox
+ inside the
ZL channel is understood to result from a larger portion of Ox
+ molecules perpendicularly oriented and a smaller fraction approximately aligned to the channel. The
optical microscopy data are therefore interpreted as the resultant, i.e., the “vector
sum” of components arising from an Ox
+ population characterized by non-uniform
orientation, mainly perpendicular to the channel but contaminated by a fraction of
molecules approximately aligned to the channel. The results for Py
+ are similar
[153]. Even though the parallel orientation is not the equilibrium arrangement for the
hydrated Ox
+
-ZL composite, the low mobility of Ox
+ in the presence of water makes
it likely that a small though appreciable number of Ox
+ molecules could remain
frozen in such an arrangement for some time. Moreover, the low Ox
+ mobility,
detected along the simulations, is in line with the experimental observation that
diffusion of Ox
+ inside ZL is very slow even at 100
C [43]. Furthermore, such a
fraction of nearly parallel Ox
+ and Py
+ is responsible for the J aggregate coupling
observed in ZL, a fact which cannot be understood otherwise [166]. This demonstrates the enormous power of the combination of confocal microscopy and
advanced computational techniques for understanding the structure of guest-host
composites.
The electronic S 0 –S 1 (1La) transition of methylacridine MeAcr
+ is relatively
weak and oriented parallel to the y-axis, as indicated in Fig. 16a), while the more
intense S 0 –S 2 is parallel to z (1Lb). Both transitions are of πÀπà type
Guests in Nanochannels of Zeolite L
31