Metal ion-manipulated afterglow on rhodamine 6G derivative-doped room-temperature phosphorescent PVA films

The long-lived room-temperature phosphorescence (RTP) originating from thiophene boronate polyvinyl alcohol (PVA) has enabled the creation of metal-ion-responsive RTP films doped with spirolactam ring-containing rhodamine 6G (1). In this study, RTP-active PVA films, namely, TDB@PVA and ATB@PVA, were prepared through boronate esterification of thiophene-2,5-diboronic acid (TDB) and 5-acetylthiophene-2-boronic acid (ATB) with the diol units of PVA. The delayed emission properties were evaluated, revealing an emission band at 477 nm with a turquoise afterglow for TDB@PVA and at 510 nm with a green afterglow for ATB@PVA after UV light irradiation ceased. The photophysical properties were assessed using TD-DFT and DFT calculations at the B3LYP/cc-pVDZ level. N-(rhodamine-6G)lactam dye with a salicylimine unit (1) was doped into the RTP-based PVA films, producing a multicolored afterglow upon the addition of metal ions. This phenomenon is explained by a triplet-to-singlet Förster-type resonance energy transfer process from the cross-linked thiophene boronate in PVA to the metal-ion-activated colored form of 1. This photophysical feature finds applicability in encryption techniques. Notably, the reversible metal-ligand coordination of 1 in the PVA system enabled a write/erase information process.

Symmetry-Breaking Triplet Excited State Enhances Red Afterglow Enabling Ubiquitous Afterglow Readout

Molecular vibrations are often factors that deactivate luminescence. However, if there are molecular motion elements that enhance luminescence, it may be possible to utilize molecular movement as a design guideline to enhance luminescence. Here, the authors report a large contribution of symmetry-breaking molecular motion that enhances red persistent room-temperature phosphorescence (RTP) in donor-π-donor conjugated chromophores. The deuterated form of the donor-π-donor chromophore exhibits efficient red persistent RTP with a yield of 21% and a lifetime of 1.6 s. A dynamic calculation of the phosphorescence rate constant (kp) indicates that the symmetry-breaking movement has a crucial role in selectively facilitating kp without increasing nonradiative transition from the lowest triplet excited state. Molecules exhibiting efficient red persistent RTP enable long-wavelength excitation, indicating the suitability of observing afterglow readout in a bright indoor environment with a white-light-emitting diode flashlight, greatly expanding the range of anti-counterfeiting applications that use afterglow.

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.

Activating Molecular Room-Temperature Phosphorescence by Manipulating Excited-State Energy Levels in Poly(vinyl alcohol) Matrix

Poly(vinyl alcohol) (PVA) has been found as a wonderful matrix for chromophores to boost their room-temperature phosphorescence (RTP) character by forming abundant hydrogen bonding. Despite the well-utilized protective effect, the constructive role in accelerating the intersystem crossing is less investigated. Here, we focus on its role in manipulating the excited-state energy level to facilitate multiple intersystem crossing channels. Six benzoyl carbazole derivatives do not emit RTP in their solutions, powders, or crystals but exhibit significantly persistent RTP signals when embedded into the PVA matrix. Charge-transfer excited states were trapped by cofacial stacking in crystal, which blocks the intersystem crossing channels. In the PVA matrix, the allowed broad distribution of charge-transfer states covers the locally excited states, offering multiple intersystem crossing pathways via spin-vibronic orbit coupling. Consequently, efficient and persistent heavy-atom-free phosphors have been developed with the highest quantum yields of 7.7% and the longest lifetime of 2.3 s.

Molecular nanoarchitectonics: unification of nanotechnology and molecular/materials science

The development of nanotechnology has provided an opportunity to integrate a wide range of phenomena and disciplines from the atomic scale, the molecular scale, and the nanoscale into materials. Nanoarchitectonics as a post-nanotechnology concept is a methodology for developing functional material systems using units such as atoms, molecules, and nanomaterials. Especially, molecular nanoarchitectonics has been strongly promoted recently by incorporating nanotechnological methods into organic synthesis. Examples of research that have attracted attention include the direct observation of organic synthesis processes at the molecular level with high resolution, and the control of organic syntheses with probe microscope tips. These can also be considered as starting points for nanoarchitectonics. In this review, these examples of molecular nanoarchitectonics are introduced, and future prospects of nanoarchitectonics are discussed. The fusion of basic science and the application of practical functional materials will complete materials chemistry for everything.

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.

Enhanced Red Persistent Room-Temperature Phosphorescence Induced by Orthogonal Structure Disruption during Electronic Relaxation

Bright, persistent, room-temperature phosphorescence (RTP) at long wavelengths is crucial for high-resolution imaging in the absence of in vivo autofluorescence. However, efficient long-wavelength RTP is difficult. Here, enhanced red RTP based on a unique mechanism was observed from deuterated dibenzo[g.p]chrysenes substituted with a phenoxazine. The yield was 16%, with an average lifetime of 1.8 s. An orthogonal dihedral angle between the dibenzo[g.p]chrysene and the phenoxazine in the lowest excited singlet state allowed a forbidden fluorescence to increase triplet generation. When the dihedral angle changed, disengagement of the forbidden fluorescence from the excited singlet state occurred, and the lowest triplet excited state had a facilitated phosphorescence rate without increasing its nonradiative transition rate. The facilitated phosphorescence rate as well as the large triplet yield led to the enhanced red RTP.

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.

Stepwise Energy Transfer: Near-Infrared Persistent Luminescence from Doped Polymeric Systems

Organic near infrared (NIR) persistent-luminescence systems with bright and long-lived emission are highly valuable for applications in communication, imaging, and sensors. However, realizing these materials (especially lifetime over 0.1 s) is a challenge, mainly because of non-radiative quenching of their long-lived excitons. Herein, a universal strategy of stepwise Förster resonance energy transfer (FRET) for a bright NIR system with remarkable persistent luminescence (up to 0.2 s at 810 nm) is presented, based on a new triphenylene-dye-doped polymer (triphenylene-2-ylboronic acid@poly(vinyl alcohol) (TP@PVA)) with a persistent blue phosphorescence of 3.29 s. This persistent NIR luminescence is demonstrated for application not only in NIR anti-counterfeiting but also NIR bioimaging with penetrating a piece of skin as thick as 2.0 mm. By co-doping a red dye (such as Nile red) and an NIR dye Cyanine 7 (Cy7) into this doped PVA film, the shortage of spectral overlap between TP emission and Cy7 absorbance is successfully solved, through a stepwise FRET process involving triplet to singlet (TS)-FRET from TP to the intermediate red dye and then singlet to singlet (SS)-FRET to Cy7. It is noted that the efficiency of the upper TS-FRET is enhanced significantly by the lower SS-FRET, leading to high efficiencies for the continuous FRETs.