Introduction
The realm of organic photoluminescence (PL) materials has seen continuous advancements over the past decades, largely driven by their application in optoelectronic devices, chemo-/bio-probes, solar cells, and environmental sensors. Traditional PL materials operate primarily through π-conjugated units like phenyl, thiophene, fluorene, and carbazole. Despite extensive exploitation, these materials fall short in certain areas: their complex synthetic processes, cytotoxicity, immunogenicity, and aggregation-caused quenching (ACQ) effects hinder practical applications, particularly in biological systems.
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Researchers are exploring nonconventional chromophores that lack classic conjugation and rigid units but exhibit intrinsic fluorescence and phosphorescence under specific conditions.
The Fundamentals of Unconventional Chromophores
Recent discoveries have highlighted that materials lacking typical chromophores, sometimes even devoid of (hetero)cyclic aromatic structures, can exhibit intrinsic fluorescence and phosphorescence under specific conditions. These unconventional chromophores include a diverse array of subgroups such as amino (-NH-), acylamino (-NH-CO-), phosphonate (-P(O)(OR)2), carbamate (-NH-COO-), and other functional groups. Such chromophores, especially in the form of oligomers and polymers, offer distinct advantages, including better hydrophilicity, structural diversity, low synthesis cost, high chain flexibility, and good biocompatibility.
Despite their promising potential, the photoluminescence (PL) mechanisms of these materials remain a topic of ongoing research and debate. Various mechanisms have been proposed, including oxidation/acidification, aggregation of carbonyl units, formation of hydrogen bonds, and spatial electron delocalization. A profound understanding of these mechanisms is crucial to advance the field and unlock further practical applications.
Fluorescence Mechanisms in Nonconventional Chromophores
Dendritic and hyperbranched poly(amido amine)s (PAMAMs) are among the earliest unconventional polymeric materials studied for their atypical fluorescence. Initial research suggested that PAMAMs emit fluorescence due to oxidation, but later studies demonstrated that changes in end groups and molecular conformation also significantly influence luminescence. These studies revealed that the rigidification of molecular chains, induced by Coulombic repulsion, hydrogen bonding, or chemical modifications, enhances fluorescence. Key findings indicated that conformational rigidification and aggregation at higher concentrations are essential for strong luminescence, aligning with the principles of aggregation-induced emission (AIE).
Fig. 1 The mechanism of PAMAM breakdown (Jiang N., et al. 2021).
Ongoing research has extended the understanding of AIE systems beyond traditional conjugated chromophores to less conventional luminogens. For instance, poly(ethylenediamine)s (PEIs) and hyperbranched polyamidoamines showed enhanced fluorescence upon aggregation, highlighting that dendritic or hyperbranched structures are not strictly necessary for fluorescence, but the presence of aliphatic amine groups plays a crucial role.
Cluster-Triggered Emission (CTE) and its Implications
The concept of cluster-triggered emission (CTE) was introduced to explain why certain materials emit fluorescence upon aggregation. This phenomenon occurs when nonconventional chromophores form clusters, leading to effective short contacts, overlap of electron clouds, and extended conjugation. The rigidified conformations resulting from these clusters effectively enhance luminescence. Starch, fibronectin, and polymers like polyether epoxy (EHBPE) have been shown to exhibit CTE, demonstrating intrinsic fluorescence from clusters of oxygen and other functional groups.
Further research has demonstrated that hydrogen bonds and other intermolecular interactions facilitate the clustering and stabilization of luminescent units, enhancing emission. The work on polyisobutene succinic anhydrides (PIBSA) and acetone-formaldehyde sulfonate (SAF) provided further evidence for the pivotal role of carbonyl clustering in luminescence, validating the CTE mechanism.
Room Temperature Phosphorescence (RTP) in Unconventional Chromophores
Unconventional luminescent materials are not limited to fluorescence; they also exhibit room temperature phosphorescence (RTP) and even ultra-long-lifetime RTP. Research has shown that polymers such as polyacrylonitrile (PAN) exhibit green phosphorescence at low temperatures due to the formation of cyano clusters, which promote spatial electron delocalization and rigidify the molecular conformation.
Other studies have identified that hydrogen bonding plays a crucial role in stabilizing triplet excitons, thereby facilitating RTP. For example, polyurethane derivatives and common polymers like polyacrylic acid (PAA) and poly(n-isopropyl acrylamide) (PNIPAM) have demonstrated significant RTP, attributed to intramolecular and intermolecular interactions that stabilize the chromophores' conformations.
Design Strategies for Enhanced Luminescence
The development of efficient nonconventional luminescent materials involves optimizing molecular structures and aggregation behavior. Researchers have explored several strategies to achieve this:
Molecular Weight and Flexibility
Increasing molecular weight and flexibility can promote more interactions between chains, facilitating aggregate formation and extending conjugation for red-shifted emissions. The synthesis of alternating copolymers of N-vinyl pyrrolidone and maleic anhydride demonstrated that lower rigidity and enhanced aggregation lead to stronger red luminescence.
Chemical Modifications
Peripheral modifications, like the inclusion of mannose units in PAMAMs, have been shown to inhibit chain movement, enhancing fluorescence. Modifications to side chains using electron-rich heteroatoms have also proven effective in tuning luminescence properties.
Hydrogen Bonding and Ionic Interactions
Enhancing hydrogen bonding and ionic interactions can increase conformational rigidification and stabilize triplet states. Studies on cellulose derivatives (MCC, HEC, HPC) revealed that higher hydrogen bond strength correlates with stronger emission and RTP.
Applications and Future Directions
Unconventional luminescent materials are poised to revolutionize various fields, particularly in biomedical applications, where their excellent biocompatibility and diverse emission properties are highly beneficial. Their ability to exhibit multi-color emissions based on excitation wavelength and aggregation state makes them ideal candidates for bioimaging, bacterial detection, and optoelectronic sensors.
Furthermore, advancements in understanding the CTE and AIE mechanisms continue to drive the design of new materials. For instance, recent studies on fluorine-substituted polymers and hydrated polymers like PNVCL have expanded the scope of unconventional chromophores, offering improved photostability and mechanical flexibility.
The exploration of triplet exciton generation and stabilization in these materials also opens new avenues for improving RTP efficiency. The synthesis of phosphoramidic acid oligomers and thiourea derivatives with high RTP efficiency exemplifies the potential of combining clustering with strong electron interactions.
Conclusion
The realm of nonconventional chromophores, with their unique structures and luminescence mechanisms, presents a fertile ground for innovative research and applications. By leveraging aggregation-induced emissions and cluster-triggered emission mechanisms, researchers have developed materials with remarkable fluorescence and phosphorescence properties. Ongoing investigations into the role of molecular interactions and structural design hold promise for creating advanced materials tailored for specific applications, thereby accelerating future developments in the field of luminescent materials. With continued exploration and optimization, unconventional chromophores are poised to redefine the landscape of photoluminescent materials and their applications.
Reference
- Jiang N., et al. Recent advances in oligomers/polymers with unconventional chromophores. Materials Chemistry Frontiers. 2021, 5(1): 60-75.
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