trum a), the characteristic monomer peaks (observed at ca. 680
and 625 nm) are red shifted by ca. 20 nm compared to ZnPc in
solution. We also note that the spectrum of a physical mixture of
ZnPc and MMS (spectrum c, Fig. 3) indicates, as expected, that
the phthalocyanine exists predominantly as dimers or ag-
gregates; the monomer peak at 680 nm is much reduced while
the dimer–aggregate peaks at ca. 640 and 720 nm are very
prominent. We can therefore infer from the spectra that
phthalocyanines in ZnPc/MMS composites are embedded
within the calcined MMS pores predominantly in monomeric
form. The strong red shift (ca. 20 nm) of the Q band observed
for ZnPc/MMS is consistent with a change in chemical
environment (compared to ZnPc in toluene solution); it is likely
that the ZnPc molecules are adsorbed onto the pores of the
MMS where their p electrons interact with the surface hydroxyl
groups of the calcined MMS host.12 Interestingly, an increase in
the amount of ZnPc adsorbed on the MMS leads to aggregation
of the embedded ZnPc, as shown in Fig. 3, spectrum b. At a
ZnPc content of 0.9 µM, the resulting ZnPc/MMS composite
exhibits a spectrum with reduced monomer peaks (at ca. 625
and 680 nm), an emerging dimer–aggregate peak at ca. 645 nm
and broad bands at ca. 550 and 720 nm (similar to those
observed for a physical mixture of ZnPc and MMS where the
ZnPc exists as dimers or aggregates — see Fig. 3). This allows
us to semi-quantitatively fit our spectra (and show a transforma-
tion from monomeric to dimeric–aggregated pthalocyanine as
ZnPc content increases) which confirms that at certain ZnPc
contents, the phthalocyanine in ZnPc/MMS composites exists
predominantly in monomeric form.
Fig. 2 (a) Absorption spectrum of ZnPc in toluene and (b) diffuse
reflectance spectrum of ZnPc embedded in MMS mesophase (i.e., as-
synthesized ZnPc–MMS).
660 nm. This is the (0–0) transition from the HOMO to the
LUMO.11 A second characteristic band lies at ca. 600 nm; this
is the (0–1) transition from HOMO to the first overtone of the
LUMO.11 The bands at ca. 600 and 660 nm are indicators of
monomeric phthalocyanine. The shoulder at ca. 630 nm is
usually attributed to dimers or aggregates. Fig. 2a is therefore
consistent with the fact that, in solution, ZnPc exists predom-
inantly in monomeric form. The diffuse reflectance spectrum of
the as-synthesized ZnPc–MMS mesophase is shown in Fig. 2b.
It is interesting to note the similarities (shape and number of
peaks) of the two spectra in Fig. 2. This similarity indicates that
the phthalocyanine in ZnPc–MMS exists in the same manner as
ZnPc in toluene solution, i.e., predominantly in monomeric
form. Such monodispersion of the phthalocyanine may be
attributed to the incorporation of the ZnPc in the hydrophobic
region of the surfactant micelles. The ZnPc may also interact
directly with the MMS mesostructure. For the ZnPc–MMS (Fig.
2b), the position of the Q band exhibits a 16 nm red shift
(compared to ZnPc in toluene solution) to ca. 676 nm due to a
change in the chemical environment. It is plausible that the
MMS mesophase provides a favourable environment to accom-
modate the ZnPc molecules effectively trapping them in a
manner that does not allow aggregation or intermolecular
interaction. The homogeneity of ZnPc distribution in the MMS
host can be verified from the sharpness of the Q-band.6 By
comparing the full width at half maximum (FWHM) of the Q
bands of spectrums a and b in Fig. 2, it can be concluded that the
ZnPc molecules are embedded uniformly into the MMS host.
Typical diffuse reflectance spectra of ZnPc/MMS composites
(at various ZnPc content) prepared via post-synthesis adsorp-
tion and a physical mixture of ZnPc and MMS are shown in Fig.
3. We first note that a typical spectrum for ZnPc/MMS
composites (e.g. containing ca. 0.45 µM ZnPc) exhibits peaks
characteristic of monomeric phthalocyanine. In Fig. 3 (spec-
We have shown that phthalocyanine in the directly prepared
ZnPc/MMS mesophases or ZnPc–MMS composites prepared
via a post-synthesis adsorption route exists predominantly in
monomeric form. The ordered high surface area MMS provides
an excellent host for monodispersion of phthalocyanines. This
is important for optical applications because aggregation is
known to damage the optical properties of such composites.
Furthermore, when the dye is held rigidly in a solid matrix,
many of the mechanisms that quench the triplet state are
reduced by the dye–matrix interaction. Consequently, the
optical nonlinearity of the material is increased.13,14
The authors are grateful to the EPSRC for financial
support.
Notes and references
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Fig. 3 Diffuse reflectance spectra of (a) ZnPc/MMS (0.45 µM ZnPc), (b)
ZnPc/MMS (0.9 µM ZnPc) and (c) physical mixture of ZnPc and MMS.
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