Room-Temperature Phosphorescence-active Boronate Particles: Characterization and Ratiometric Afterglow-sensing Behavior by Surface Grafting of Rhodamine B

Thermochromic Afterglow from Benzene-1,4-Diboronic Acid-Doped Co-crystals

The accurate thermosensing requires a minimum impact of autofluorescence and light scattering from the samples. In this study, we discovered that commercially available benzene-1,4-diboronic acid (BDBA) doped co-crystals with trimethylolpropane (TMP) exhibit excellent thermochromic dual phosphorescence properties over a wide temperature range from -132 to 40 °C, despite its simple structure that does not have any donor-acceptor linkage. The dual phosphorescence was consisted of monomeric benzene-1,4-diboronate (BDBA ester) and aggregation-stabilized species. With an increase in temperature, the emission intensity from the monomeric state significantly decreased, whereas that originating from the aggregated state remained almost constant owing to the difference in their thermal stabilities. Further investigation revealed that molecular distortions in singlet excited states enable efficient intersystem crossing, causing efficient phosphorescence from the monomeric state of BDBA ester.

Phosphorescence resonance energy transfer from purely organic supramolecular assembly

Phosphorescence energy transfer systems have been applied in encryption, biomedical imaging and chemical sensing. These systems exhibit ultra-large Stokes shifts, high quantum yields and are colour-tuneable with long-wavelength afterglow fluorescence (particularly in the near-infrared) under ambient conditions. This review discusses triplet-to-singlet PRET or triplet-to-singlet-to-singlet cascaded PRET systems based on macrocyclic or assembly-confined purely organic phosphorescence introducing the critical toles of supramolecular noncovalent interactions in the process. These interactions promote intersystem crossing, restricting the motion of phosphors, minimizing non-radiative decay and organizing donor-acceptor pairs in close proximity. We discuss the applications of these systems and focus on the challenges ahead in facilitating their further development.

Chemometrics-assisted mechanism study of the room-temperature phosphorescence on nanoscopic boronate assemblies

Boronate assemblies, derived from benzene-1,4-diboronic acid, showed room-temperature phosphorescence (RTP) in both the solid-state and dispersions, being quite sensitive to the preparation conditions. Our chemometrics-assisted quantitative structure-property relationship (QSPR) analysis between the nanostructure and RTP behavior of boronate assemblies led us not only to understand their RTP mechanism but also to predict the RTP properties of unknown assemblies from the PXRD patterns.

A Ratiometric Afterglow Response of Aluminium Ions in Methanol-Water

An afterglow is beneficial as an emission signal in the field of displays and imaging probes. Here, boronic acid-appended and spirolactam ring-containing rhodamine dye 1 was synthesized and grafted onto the surface of room-temperature phosphorescence-active boronate nanoparticles (BPs), composed of polymeric 3-benzo-2,4,8,10-tetraoxa-3,9-diboraspiro[5.5]undecane. The resultant ensemble, 1@BP, exhibited a greenish afterglow. However, the addition of Al³⁺ into the dispersion solution with 1@BP led to a change in the afterglow to grass green as a result of Förster-type energy transfer from the phosphorescent BP to the Al³⁺ -interacting rhodamine dye 1 on the surface. Based on the ratio of the two emission intensities, a linear response in the concentration range of 3.8-15.2 μM was observed, with a detection limit of 4.2 μM for Al³⁺ . A metal ion-dependent discernable color in afterglow was observed on a 1@BP-coated filter paper, which would be useful for not only film-based afterglow chemosensors but also encryption application.

In Situ Turn-On Room Temperature Phosphorescence and Vapor Ultra-sensitivity at Lifetime Mode

Organic room temperature phosphorescence (RTP) systems are rarely reported for vapor phase sensing due to the contradiction between vapor permeability and phosphorescence ability. Till now, almost all reported works are based on ″turn-off″ mode RTP detection by destroying the compact-packaging oxygen-free environment. ″Turn-on″ mode RTP detection owns additional anti-interference capability due to a lower initial RTP background signal, while its realization is even harder. In this research, in situ phosphorescence ″turn-on″ sensing was realized for methamphetamine (MPEA) vapor detection. Based on the formation of aromatic phenolic aldehyde-secondary amine ion pairs with air-stable RTP performance, the fluorescent tri-formyl phenol (TFP) film was converted into a stable RTP state after being exposed to the MPEA vapor, as supported by nuclear magnetic resonance (NMR) and mass spectrometry together with theoretical calculations. The red-shifted absorption and emission, enhanced emission intensity, and 49.7 μs phosphorescence lifetime allowed multiple mode MPEA vapor sensing from chromaticity to fluorescence to phosphorescence. The lifetime mode detection limit reached 0.4 ppt, 5 orders of magnitude lower than the intensity mode detection limit of 20.3 ppb.

Efficient metal-free organic room temperature phosphors

An innovative transformation of organic luminescent materials in recent years has realised the exciting research area of ultralong room-temperature phosphorescence. Here the credit for the advancements goes to the rational design of new organic phosphors. The continuous effort in the area has yielded wide varieties of metal-free organic systems capable of extending the lifetime to several seconds under ambient conditions with high quantum yield and attractive afterglow properties. The various strategies adopted in the past decade to manipulate the fate of triplet excitons suggest a bright future for this class of materials. To analyze the underlying processes in detail, we have chosen high performing organic triplet emitters that utilized the best possible ways to achieve a lifetime above one second along with impressive quantum yield and afterglow properties. Such a case study describing different classes of metal-free organic phosphors and strategies adopted for the efficient management of triplet excitons will stimulate the development of better candidates for futuristic applications. This Perspective discusses the phosphorescence features of single- and multi-component crystalline assemblies, host-guest assemblies, polymers, and polymer-based systems under various classes of molecules. The various applications of the organic phosphors, along with future perspectives, are also highlighted.

Effect of π···π Interactions of Donor Rings on Persistent Room-Temperature Phosphorescence in D4-A Conjugates and Data Security Application

Organic room-temperature phosphorescence (RTP) materials with persistent RTP (PRTP) have attracted huge interest in inks, bioimaging, and photodynamic therapy. However, the design principle to increase the lifetime of organic molecules is underdeveloped. Herein, we show donor(D₄)-acceptor(A) molecules (TOEPh, TOCPh, TOMPh, TOF and TOPh) with similar orientation of donor rings in aggregates that cause a large number of noncovalent interactions. We observed that TOEPh, TOCPh, TOMPh and TOF showed PRTP, whereas TOPh showed only phosphorescence emission (Φ_P = ∼11%) with no PRTP property at ambient conditions. The spectroscopic and single-crystal X-ray analyses confirm the molecular assembly via J-aggregation with a face-to-face orientation of the donor rings. The crystal structure analysis (TOEPh, TOCPh, TOMPh, TOF) reveals that moderate π···π interactions (3.706 to 4.065 Å) between the donor rings cause the enhancement of the phosphorescence lifetime (26 to 245 ms), whereas the short phosphorescence lifetime (12 ms) of TOPh was observed because of the absence of π···π interactions. We found that TOEPh shows a long lifetime (245 ms) as compared to other derivatives because of the presence of ethoxy (-OEt) groups that enables spin-orbit coupling caused by strong lone pair (O)···π interactions present in the molecule. Utilizing the PRTP feature of TOEPh and the fluorescence emission of TOPh, we have shown data security applications in poly(methyl methacrylate).

Dual emission in purely organic materials for optoelectronic applications

Purely organic molecules, which emit light by dual emissive (DE) pathways, have received increased attention in the last decade. These materials are now being utilized in practical optoelectronic, sensing and biomedical applications. In order to further extend the application of the DE emitters, it is crucial to gain a fundamental understanding of the links between the molecular structure and the underlying photophysical processes. This review categorizes the types of DE according to the spin multiplicity and time range of the emission, with emphasis on recent experimental advances. The design rules towards novel DE molecular candidates, the most perspective types of DE and possible future applications are outlined. These exciting developments highlight the opportunities for new materials synthesis and pave the way for accelerated future innovation and developments in this area.

Interplay between Förster and Dexter Energy Transfer Rates in Isomeric Donor-Bridge-Acceptor Systems

The ability to direct the flow of excitons enable molecular systems to perform highly advanced functions. Intramolecular energy transfer in donor-bridge-acceptor systems can occur by different mechanisms, and the ability to control the excited state energy pathways depends on the capacity to favor one process over another. Here, we show an anticorrelation between the rates of Förster and Dexter types of energy transfer in two isomeric donor-bridge-acceptor systems. Both dyads display intramolecular Förster triplet-to-singlet and Dexter triplet-to-triplet energy transfers. However, as the bridge-acceptor connection point changes, the rate of one energy transfer process increases at the same time as the other one decreases, allowing us to control the energy flow direction. This work shows how rational design can be used to tune excited state energy pathways in molecular dyads, which is of importance for advanced functions such as multiplicity conversion in future molecular materials.

The Effect of Electron Donation and Intermolecular Interactions on Ultralong Phosphorescence Lifetime of 4-Carnoyl Phenylboronic Acids

Purely organic phosphors with persistent room-temperature phosphorescence (RTP) demonstrate promising potential applications in optoelectronic area, bioimaging, and chemical sensing. However, it is still a formidable challenge to further design new organic phosphors due to the unclear mechanism to produce ultralong phosphorescence lifetimes. This paper investigates the correlation between the ultralong phosphorescence lifetime and structure of a series of 4-carbonylphenylboronic acid derivatives in the crystal state. Experimental and calculation results reveal that the electron-donating effect of substituents makes the phosphorescence lifetime longer by not only weakening the vibration relaxation of the excited triplet state but also increasing the energy of T₁. Moreover, numerous intermolecular interactions for reducing nonradiative relaxation and the degree of the π-π stacking for stabilizing the triplet state are beneficial to the persistent RTP. The work is conducted to clarify the structure-property correlation of phosphorescent materials and design new persistent phosphors. Finally, an attempt is completed using phosphorescent materials to design two-dimensional or three-dimensional codes and anticounterfeiting applications.