for both samples but differs enormously in intensity. The shape corresponds to the
absorption band of TTFA. The about 100 times larger emission intensity of the
stopcock modified samples is manifested when comparing the red and the black
emission bands. The photographic images (C) and (D) provide an intuitive impression of the effect exerted by the stopcock modification. Similar observations were
made when using DBM as a ligand.
What is the reason for this unexpected important luminescence enhancement? It is
known that protonation of diketonates competes with full coordination to Eu
3+ and
that incomplete coordination affects the luminescence yield. Both TTFA and DBM
can be protonated under the acidic environment of hydrated ZL. DBM to an even
larger extent, as its pK a value is larger than that of TTFA, with values of 8.95 and 6.33,
respectively. This provides the explanation for the remarkable influence of the stopcock modification with BuImz
+
Sil on the luminescence intensity of the β-diketonatebased composites. It is caused by a decrease of the proton activity inside of the NZL
channels. Each stopper attached to the channel entrance exchanges one of the cations
that control the proton strength, as explained above. The effect is enhanced by the fact
that the imidazolium acts as a weak base [185, 186]. The effect weakens with
increasing size of the ZL crystals. The ratio of channel entrances and u.c. of a ZL
crystal of equal length and diameter d in nm is equal to 1.485/d. This means that, for
example, a 30 nm NZL consists of about 20 times more u.c. than channel entrances. It
is, however, useful to remember that the BuImz
+
Sil also acts like a cork on a bottle: it
seals the ZL channel as illustrated in Fig. 10 [100]. Li et al. applied the understanding
provided by these observations for designing highly sensitive rare earth-ZL composites as sensing devices for detecting basic molecules [99, 187].
Hostasol Red (HR, Table 2) is a solvatochromic dye of brilliant color. It can be
inserted easily into the channels of ZL where its bright fluorescence is largely
quenched, essentially because of protonation in the first excited state [172, 178]. It is
remarkable that exchange of only one half out of the 3.6 charge compensating
potassium cations per u.c. by the imidazolium cation IMZ
+ (Table 3) causes a jump
from about 3% fluorescence quantum yield observed in potassium HR-ZL composites
to a yield of nearly 100% [83]. The reason is the same, as we have seen above in
Fig. 20 upon addition of the BuImz
+
Sil stopcock to [Eu
3+
TTFA n ]-NZL. IMZ
+ reduces
the proton activity inside the channels of ZL sufficiently, so that the quenching caused
by excited state protonation is suppressed. We indicate the number and type of charge
compensating cations as follows: ZL.p((3.6-x)M1
+
,xM2
+
), where p is the loading of
HR; see Eqs. (3) and (4). Figure 21 shows representative data that illustrate the
influence of IMZ
+ on HR-ZL composites. The bathochromic shift of the absorption
and fluorescence bands manifests the solvatochromism of HR. The photographic
images give a visual impression on the influence of the IMZ
+
. It is remarkable that
the color and the luminescence intensity of the sample HR-ZL.5-(2.6 K
+
,1.0IMZ
+
) did
not change upon prolonged immersion in water [83].
IMZ
+ is the only organic cation the influence of which on the properties of guest-ZL
composites was studied to some extent as substitute for part of the charge compensating metal cation. Many other options exist, still waiting to be explored, with
the potential of fundamentally influencing properties of many guest-ZL composites.
42
G. Calzaferri
absorption band of TTFA. The about 100 times larger emission intensity of the
stopcock modified samples is manifested when comparing the red and the black
emission bands. The photographic images (C) and (D) provide an intuitive impression of the effect exerted by the stopcock modification. Similar observations were
made when using DBM as a ligand.
What is the reason for this unexpected important luminescence enhancement? It is
known that protonation of diketonates competes with full coordination to Eu
3+ and
that incomplete coordination affects the luminescence yield. Both TTFA and DBM
can be protonated under the acidic environment of hydrated ZL. DBM to an even
larger extent, as its pK a value is larger than that of TTFA, with values of 8.95 and 6.33,
respectively. This provides the explanation for the remarkable influence of the stopcock modification with BuImz
+
Sil on the luminescence intensity of the β-diketonatebased composites. It is caused by a decrease of the proton activity inside of the NZL
channels. Each stopper attached to the channel entrance exchanges one of the cations
that control the proton strength, as explained above. The effect is enhanced by the fact
that the imidazolium acts as a weak base [185, 186]. The effect weakens with
increasing size of the ZL crystals. The ratio of channel entrances and u.c. of a ZL
crystal of equal length and diameter d in nm is equal to 1.485/d. This means that, for
example, a 30 nm NZL consists of about 20 times more u.c. than channel entrances. It
is, however, useful to remember that the BuImz
+
Sil also acts like a cork on a bottle: it
seals the ZL channel as illustrated in Fig. 10 [100]. Li et al. applied the understanding
provided by these observations for designing highly sensitive rare earth-ZL composites as sensing devices for detecting basic molecules [99, 187].
Hostasol Red (HR, Table 2) is a solvatochromic dye of brilliant color. It can be
inserted easily into the channels of ZL where its bright fluorescence is largely
quenched, essentially because of protonation in the first excited state [172, 178]. It is
remarkable that exchange of only one half out of the 3.6 charge compensating
potassium cations per u.c. by the imidazolium cation IMZ
+ (Table 3) causes a jump
from about 3% fluorescence quantum yield observed in potassium HR-ZL composites
to a yield of nearly 100% [83]. The reason is the same, as we have seen above in
Fig. 20 upon addition of the BuImz
+
Sil stopcock to [Eu
3+
TTFA n ]-NZL. IMZ
+ reduces
the proton activity inside the channels of ZL sufficiently, so that the quenching caused
by excited state protonation is suppressed. We indicate the number and type of charge
compensating cations as follows: ZL.p((3.6-x)M1
+
,xM2
+
), where p is the loading of
HR; see Eqs. (3) and (4). Figure 21 shows representative data that illustrate the
influence of IMZ
+ on HR-ZL composites. The bathochromic shift of the absorption
and fluorescence bands manifests the solvatochromism of HR. The photographic
images give a visual impression on the influence of the IMZ
+
. It is remarkable that
the color and the luminescence intensity of the sample HR-ZL.5-(2.6 K
+
,1.0IMZ
+
) did
not change upon prolonged immersion in water [83].
IMZ
+ is the only organic cation the influence of which on the properties of guest-ZL
composites was studied to some extent as substitute for part of the charge compensating metal cation. Many other options exist, still waiting to be explored, with
the potential of fundamentally influencing properties of many guest-ZL composites.
42
G. Calzaferri