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.