Background and Originality
Content
Fluorescence, after more than a century of development, has become an
indispensable technique in various fields, such as organic
light-emitting diodes (OLEDs), chemical probes, and
bioimaging.[1-3] Most of the traditional
fluorophores show π-conjugated polycyclic aromatic structures, whose
photophysical performance is regulated by the intramolecular
through-bond conjugation (TBC), for example, increasing the
π-conjugation length or introducing electron-donating/-withdrawing
groups.[4-5] However, recently, researchers
discovered that some non-conjugated and non-aromatic polymers, such as
polyamidoamine [6], maleic anhydride/vinyl acetate
alternating copolymers (PMV),[7-8] polyethylene
glycol,[9]polyacrylonitrile,[10] and
polyesters,[11-12] were found to exhibit intrinsic
fluorescence at clustering state. The similar phenomenon has also been
observed in natural polymers, including rice, starch,
cellulose,[13] and
proteins.[14] These non-conjugated structures do
not fluoresce in isolated state but show anomalous fluorescence upon
aggregation, known as clusteroluminescence
(CL).[15-21] Luminogens with CL (CLgens) hold
significant technological, theoretical, and practical value. CLgens are
also known for their low toxicity, biocompatibility, and abundant raw
materials.[22-24] Currently, carbonyl-based
polymers, such as polypeptides and polyesters, constitute an important
class of CLgens and have been widely studied. However, most of these
CLgens predominantly emit blue emission with low
efficiency.[25] Unlike traditional conjugated
fluorophores based on TBC theory, through-space interactions (TSIs)
among different subunits were revealed to play an important role in
CL.[26-32] But a systematic photophysical theory
based on TSIs was absent, which makes it a challenge to regulate the CL
performance.
In nature, numerous fluorescent materials are constructed based on
“complexation”, including green fluorescent protein (GFP) and the
bioluminescence exhibited by fireflies.[33]Besides the natural materials, recent reports have highlighted the
significant impact of complexation on the photophysical properties of
artificial luminophores, particularly in phosphorescent systems. For
example, a few thousandth doping levels could induce ultralong organic
phosphorescence from fluorescent molecules.[34]Additionally, one of the strategies to develop white light-emitting
materials is the complexation among several molecular components with
different emission colors.[35] Compared with the
strategy of structural modification based on chemical reaction, the
complexation is easier and more efficient in adjusting the luminescent
properties. Therefore, complexation emerges as a novel strategy for the
construction of luminescent materials. Inspired by the complexation
phenomenon observed in natural and artificial luminophores, we are
wondering whether such an engineering strategy could be used to
manipulate the CL performance of carbonyl-based polymers. Actually, in
recent studies, scientists have discovered intriguing complexation
phenomena involving carbonyl-based polymers and organic bases. For
example, PMV showed an intrinsic emission peak around 440 nm, but an
extra long-wavelength emission peak around 600 nm was observed when it
was dissolved in N -Methylpyrrolidone (weak organic base
solvent).[8] This raises the question of whether
complexation is universal in carbonyl-based polymers and organic bases,
especially for the manipulation of CL properties.
In this work, we have systematically investigated the complex formation
between carbonyl-based polymers and organic bases. We investigated the
complexation-induced clusteroluminescence phenomenon in various
carbonyl-based polymers, including polyamide, polyester, polycarbonate,
poly(monothiocarbonate), in conjunction with the electron-rich organic
base, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Figure 1 illustrates
the fluorescent photos of different polymer-DBU complexes recorded at
different time intervals under 365 nm ultraviolet (UV) excitation. Our
findings demonstrate that upon complexation with DBU, different types of
carbonyl-based polymers exhibit an increase in intrinsic emission
intensity around 440 nm and,[36-37] importantly,
the emergence of a new long-wavelength CL peak. This article unveils the
potential of carbonyl-based polymer-DBU complexation as a novel strategy
to modulate the clusteroluminescent properties in these systems.