Phthalocyanines are ideal candidates as photosensitizers
for photodynamic
therapy (PDT) of cancer due to their favorable chemical and photophysical
properties. However, their tendency to form aggregates in water reduces
PDT efficacy and poses challenges in obtaining efficient forms of
phthalocyanines for therapeutic applications. In the current work,
polyvinylpyrrolidone (PVP) and micellar formulations were compared
for encapsulating and monomerizing a water-soluble zinc phthalocyanine
bearing four non-peripheral triethylene glycol chains (Pc1). 1H NMR spectroscopy combined with UV–vis absorption
and fluorescence spectroscopy revealed that Pc1 exists
as a mixture of regioisomers in monomeric form in dimethyl sulfoxide
but forms dimers in an aqueous buffer. PVP, polyethylene glycol castor
oil (Kolliphor RH40), and three different triblock copolymers with
varying proportions of polyethylene and polypropylene glycol units
(termed P188, P84, and F127) were tested as micellar carriers for Pc1. 1H NMR chemical shift analysis, diffusion-ordered
spectroscopy, and 2D nuclear Overhauser enhancement spectroscopy was
applied to monitor the encapsulation and localization of Pc1 at the polymer interface. Kolliphor RH40 and F127 micelles exhibited
the highest affinity for encapsulating Pc1 in the micellar
core and resulted in intense Pc1 fluorescence emission
as well as efficient singlet oxygen formation along with PVP. Among
the triblock copolymers, efficiency in binding and dimer dissolution
decreased in the order F127 > P84 > P188. PVP was a strong binder
for Pc1. However, Pc1 molecules are rather
surface-attached and exist as monomer and dimer mixtures. The results
demonstrate that NMR combined with optical spectroscopy offer powerful
tools to assess parameters like drug binding, localization sites,
and dynamic properties that play key roles in achieving high host–guest
compatibility. With the corresponding adjustments, polymeric micelles
can offer simple and easily accessible drug delivery systems optimizing
phthalocyanines’ properties as efficient photosensitizers.