In this Review article, we systematically summarize the design and applications of various kinds of long-lived emissive probes for bioimaging and biosensing via time-resolved photoluminescence techniques The probes reviewed, including lanthanides, transition-metal complexes, organic dyes, carbon and silicon nanoparticles, metal clusters, and persistent phosphores, exhibit longer luminescence lifetimes than that of autofluorescence from biological tissue and organs When these probes are internalized into living cells or animals, time-gated photoluminescence imaging selectively collects long-lived signals for intensity analysis, while photoluminescence lifetime imaging reports the decay details of each pixel Since the long-lived signals are differentiated from autofluorescence in the time domain, the imaging contrast and sensing sensitivity are remarkably improved The future prospects and challenges in this rapidly growing field are addressed

Long-Lived Emissive Probes for Time-Resolved Photoluminescence Bioimaging and Biosensing

Triplet excitons in organic molecules underscore a variety of processes and technologies as a result of their long lifetime and spin multiplicity Organic phosphorescence, which originates from triplet excitons, has potential for the development of a new generation of organic optoelectronic materials and biomedical agents However, organic phosphorescence is typically only observed at cryogenic temperatures and under inert conditions in solution, which severely restricts its practical applications In the past few years, room-temperature-phosphorescent systems have been obtained based on organic aggregates Rapid advances in molecular-structure design and aggregation-behaviour modulation have enabled substantial progress, but the mechanistic picture is still not fully understood because of the high sensitivity and complexity of triplet-exciton behaviour This Review analyses key photophysical processes related to triplet excitons, including intersystem crossing, radiative and non-radiative decay, and quenching processes, to illustrate the intrinsic structure–property relationships and draw clear and integrated design principles The resulting strategies for the development of efficient and persistent room-temperature-phosphorescent systems are discussed, and newly emerged applications based on these materials are highlighted Advances in molecular-structure design and modulation of the aggregation behaviour have enabled much progress in the observation of room-temperature phosphorescence from organic aggregates This Review analyses key photophysical processes related to triplet excitons, illustrating the intrinsic structure–property relationships and identifying strategies to design efficient and persistent room-temperature-phosphorescent systems

Room-temperature phosphorescence from organic aggregates

The development of single molecule white light emitters is extremely challenging for pure phosphorescent metal-free system at room temperature. Here we report a single pure organic phosphor, namely 4-chlorobenzoyldibenzothiophene, emitting white room temperature phosphorescence with Commission Internationale de l’Eclair-age coordinates of (0.33, 0.35). Experimental and theoretical investigations reveal that the white light emission is emerged from dual phosphorescence, which emit from the first and second excited triplet states. We also demonstrate the validity of the strategy to achieve metal-free pure phosphorescent single molecule white light emitters by intrasystem mixing dual room temperature phosphorescence arising from the low- and high-lying triplet states. The development of single molecule white light-emitters is extremely challenging for pure phosphorescent metal-free systems at room temperature. Here the authors show a single pure organic room temperature phosphor, 4-chlorobenzoyldibenzothiophene, utilizing the emission from both T1 and T2 states.

White light emission from a single organic molecule with dual phosphorescence at room temperature.

Rational Molecular Design for Achieving Persistent and Efficient Pure Organic Room-Temperature Phosphorescence

Room temperature phosphorescence (RTP), which has a much longer emission lifetime than fluorescence, often enables unique material characteristics and fabrication of state-of-the-art optoelectronic devices that cannot be realized using conventional fluorescent materials. The triplet exciton characteristics related to the appearance of efficient RTP also often provide intrinsic intra- and inter-molecular physical information about the materials. This article reviews recently reported aromatic materials that present RTP characteristics. Various aromatic materials with RTP characteristics are classified in terms of their radiative rates and radiationless rates from the lowest triplet excited state (T1) while taking the presence and absence of heavy atoms and charge transfer characteristics at T1 into consideration. Statistical arrangements based on physical factors of RTP materials indicate that the recent appearance of RTP in various metal-free aromatic structures is related to a large reduction in quenching caused by strong intermolecular interaction between the aromatics and the surrounding materials; additionally, intrinsic factors of the aromatics including the radiative rate and the nonradiative rate caused by intramolecular vibration are still not well controlled. Intrinsic control of these rates is important for overall control of the RTP yield and lifetime for potential material applications. Finally, recent applications using the RTP characteristics are highlighted.

Recent Advances in Materials with Room-Temperature Phosphorescence: Photophysics for Triplet Exciton Stabilization

Purely organic materials with room-temperature phosphorescence (RTP) are currently under intense investigation because of their potential applications in sensing, imaging, and displaying. Inspired by certain organometallic systems, where ligand-localized phosphorescence ((3) π-π*) is mediated by ligand-to-metal or metal-to-ligand charge transfer (CT) states, we now show that donor-to-acceptor CT states from the same organic molecule can also mediate π-localized RTP. In the model system of N-substituted naphthalimides (NNIs), the relatively large energy gap between the NNI-localized (1) π-π* and (3) π-π* states of the aromatic ring can be bridged by intramolecular CT states when the NNI is chemically modified with an electron donor. These NNI-based RTP materials can be easily conjugated to both synthetic and natural macromolecules, which can be used for RTP microscopy.

Versatile Room-Temperature-Phosphorescent Materials Prepared from N-Substituted Naphthalimides: Emission Enhancement and Chemical Conjugation.

A new strategy to use conjugated polymers to conduct fluorescent enhancement sensing has been developed. Chiral 1,1′-bi-2-naphthol-based binding sites are linked by p-phenylene units to construct a conjugated polymer whose fluorescence is quenched by the aldehyde groups introduced at each binding site. Interaction of this polymer with chiral amino alcohols in the presence of Zn(II) leads to highly enantioselective fluorescent enhancement. It is found that the chiral conjugated polymer shows greatly enhanced enantioselectivity over the corresponding small molecular sensor under the same conditions. This work provides the first example that a conjugated polymer is used to greatly increase the enantioselectivity of a small molecular sensor in chiral recognition. Simultaneous determination of the concentration and enantiomeric composition of chiral substrates by a fluorescent measurement has been achieved by combining the polymer with salicylaldehyde in the assay.

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Conjugated polymer-enhanced enantioselectivity in fluorescent sensing

Modulation of red organic room-temperature phosphorescence in heavy atom-free phosphors

A small fluorescence ratiometric probe consisting of a single dye species, N-methyl-6-hydroxyquinolinium (MHQ), and coupled enzymatic substrates, exhibits a dramatic colour change (deep blue to red) and possesses a huge response ratio (over 2000 fold) upon specific recognition of target enzymes. Such dramatic responses are attributed to the excited-state proton transfer processes of MHQ molecules in water. Here the detection of β-galactosidase and porcine pancreatic lipase is successfully demonstrated and this class of molecules has the potential to be developed as a “naked-eye” probe in vitro.

Small quinolinium-based enzymatic probes via blue-to-red ratiometric fluorescence.

Organic room‐temperature phosphorescent (RTP) materials are useful in optical imaging and sensing technologies. However, most organic phosphors, including red‐light emitting dyes, exhibit unusually large Stokes shifts, and require excitation with ultraviolet radiation, the high energy of which can induce substantial photo damage. Here a design concept of using dihydroxy‐functionalized naphthalenediimide (NDI, λabs < 400 nm) is demonstrated to initiate ring‐opening polymerizations of L‐lactide (PLA) and ε‐caprolactone (PCL), resulting in a linear polymer NDI‐PLA2/NDI‐PCL2. With the right combination of substituents, the design simultaneously realizes broadband visible light absorption (λabs = 450–650 nm) and near‐infrared RTP (λRTP = 700 nm) with millisecond‐long lifetimes. Polymer site isolations of the aggregation‐prone NDI phosphors may prevent aggregation‐caused quenching of RTP common in bulk, by forming various microscopic ground‐state aggregates, which exhibit triplet‐emitting states lower than that of the individual phosphor. The method can potentially be expanded onto other molecular RTP systems.

Jiajun Du

Papers

Versatile Room-Temperature-Phosphorescent Materials Prepared from N-Substituted Naphthalimides: Emission Enhancement and Chemical Conjugation.

Conjugated polymer-enhanced enantioselectivity in fluorescent sensing

Modulation of red organic room-temperature phosphorescence in heavy atom-free phosphors

Small quinolinium-based enzymatic probes via blue-to-red ratiometric fluorescence.

Broad‐Band Visible‐Light Excitable Room‐Temperature Phosphorescence Via Polymer Site‐Isolated Dye Aggregates