Multiplicity conversion based on intramolecular triplet-to-singlet energy transfer

Phosphorescent acyclic cucurbituril solid supramolecular multicolour delayed fluorescence behaviour

Organic photoluminescent macrocyclic hosts have been widely advanced in many fields. Phosphorescent hosts with the ability to bind organic guests have rarely been reported. Herein, acyclic cucurbituril modified with four carboxylic acids (ACB-COOH) is mined to present uncommon purely organic room-temperature phosphorescence (RTP) at 510 nm with a lifetime of 1.86 μs. Its RTP properties are significantly promoted with an extended lifetime up to 2.12 s and considerable quantum yield of 6.29% after assembly with a polyvinyl alcohol (PVA) matrix. By virtue of the intrinsic self-crimping configuration of ACB-COOH, organic guests, including fluorescence dyes (Rhodamine B (RhB) and Pyronin Y (PyY)) and a drug molecule (morphine (Mor)), could be fully encapsulated by ACB-COOH to attain energy transfer involving phosphorescent acyclic cucurbituril. Ultimately, as-prepared systems are successfully exploited to establish multicolor afterglow materials and visible sensing of morphine. As an expansion of phosphorescent acyclic cucurbituril, the host afterglow color can be readily regulated by attaching different aromatic sidewalls. This study develops the fabrication strategies and application scope of a supramolecular phosphorescent host and opens up a new direction for the manufacture of intelligent long-lived luminescent materials.

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.

Modulation of Delayed Fluorescence Guided by Conformational Effect-Mediated Thermally Enhanced Phosphorescence in Phenothiazines-Quinoline-Cl Conjugates

Triplet energy harvesting via thermally activated delayed fluorescence (TADF) from pure organic systems has attracted great attention in organic light-emitting diodes, sensing, and photocatalysis. However, the realization of thermally enhanced phosphorescence (TEP)-guided efficient TADF with a high rate of reverse intersystem crossing (kRISC) still needs to be discovered. Herein, we report two phenothiazine-quinoline conjugates (P2QC, P2QMC) comprising two phenothiazine donors covalently attached to the chlorine-substituted quinolinyl acceptor. Spectroscopic analysis in conjunction with quantum chemistry calculations reveals that TEP in P2QC originated due to slow internal conversion from higher-lying triplet to lowest triplet (T2' → T1') of the quasi-axial (QA) conformer and TADF (kRISC = 1.44 × 108 s-1) originated from the quasi-equatorial (QE) conformer caused by a low singlet-triplet gap (ΔES1-T1 = 0.11 eV) and triplet energy transfer from QA to QE owing to the degenerate ground state of the conformers. In contrast, TADF (kRISC = 0.74 × 108 s-1) and dual phosphorescence under ambient conditions are observed in P2QMC. This study provides a sustainable guideline for developing efficient TADF emitters via conformation effects and energy transfer mechanisms.

Better Covalent Connection in a Molecular Triad Enables More Efficient Photochemical Energy Storage

Numerous studies have explored the kinetics of light-induced charge separation and thermal charge recombination in donor-acceptor compounds, but quantum efficiencies have rarely been investigated. Here, we report on two essentially isomeric molecular triads, both comprising a π-extended tetrathiafulvalene (ExTTF) donor, a ruthenium(II)-based photosensitizer, and a naphthalene diimide (NDI) acceptor. The key difference between the two triads is how the NDI acceptor is connected. Linkage at the NDI core provides stronger electronic coupling to the other molecular components than connection via the nitrogen atoms of NDI. This change in molecular connectivity is expected to accelerate both energy-storing charge separation and energy-wasting charge recombination processes, but it is not a priori clear how this will affect the triad's ability to store photochemical energy; any gain resulting from faster charge separation could potentially be (over)compensated by losses through accelerated charge recombination. The new key insight emerging from our study is that the quantum yield for the formation of a long-lived charge-separated state increases by a factor of 5 when going from nitrogen- to core-connected NDI, providing the important proof of concept that better molecular connectivity indeed enables more efficient photochemical energy storage. The physical origin of this behavior seems to root in different orbital connectivity pathways for charge separation and charge recombination, as well as in differences in the relevant orbital interactions depending on NDI connection. Our work provides guidelines for how to discriminate between energy-storing and energy-wasting electron transfer reactions in order to improve the quantum yields for photochemical energy storage and solar energy conversion.

All-Visible (>500 nm)-Light-Induced Diarylethene Photochromism Based on Multiplicity Conversion via Intramolecular Energy Transfer

Photoswitching molecules that reversibly switch upon visible-light irradiation are some of the most attractive targets for biological and imaging applications. In this study, we found a diarylethene (DAE) derivative having a covalently attached perylenebisimide (PBI) unit (DAE-PBI dyad) underwent an unexpected cyclization reaction upon irradiation with green (500-550 nm) light, where the DAE unit has no absorbance. The photoreactivity was enhanced in solvents containing heavy atoms and in the presence of oxygen. As inferred from the solvent dependence and the calculated excited-state energies of DAE and PBI units, it was suggested that the probable mechanism for this unique visible-light-induced cyclization reaction is multiplicity conversion based on intramolecular energy transfer from the excited singlet state of the PBI unit to the triplet state of DAE units (i.e., DAE-¹[PBI]* → ³[DAE]*-PBI). Such a unique photoreaction mechanism with the assistance of oxygen will pave the way for new molecular design for the development of visible-light switching molecules.

High-Performance Narrowband Pure-Red OLEDs with External Quantum Efficiencies up to 36.1% and Ultralow Efficiency Roll-Off

High-color-purity blue and green organic light-emitting diodes (OLEDs) have been resolved thanks to the development of B/N-based polycyclic multiple resonance (MR) emitters. However, due to the derivatization limit of B/N polycyclic structures, the design of red MR emitters remains challenging. Herein, a series of novel red MR emitters is reported by para-positioning N-π-N, O-π-O, B-π-B pairs onto a benzene ring to construct an MR central core. These emitters can be facilely and modularly synthesized, allowing for easy fine-tuning of emission spectra by peripheral groups. Moreover, these red MR emitters display excellent photophysical properties such as near-unity photoluminescence quantum yield (PLQY), fast radiative decay rate (k_r ) up to 7.4 × 10⁷ s^-1 , and most importantly, narrowband emission with full-width at half-maximum (FWHM) of 32 nm. Incorporating these MR emitters, pure red OLEDs sensitized by phosphor realize state-of-the-art device performances with external quantum efficiency (EQE) exceeding 36%, ultralow efficiency roll-off (EQE remains as high as 25.1% at the brightness of 50 000 cd m^-2 ), ultrahigh brightness over 130 000 cd m^-2 , together with good device lifetime.

Two-dimensional transition metal dichalcogenides as an emerging platform for singlet fission solar cells

Singlet fission, a rapid exciton doubling process via inverse Auger recombination, is recognized as one of the most practical and feasible means for overcoming the Shockley-Queisser limit. Singlet fission solar cells are generally developed by integrating photon downconversion organic semiconductors into conventional photovoltaic devices to break the maximum photovoltaic response of the host semiconductors by virtue of extra triplet excitons. In this regard, proper matching of two different semiconductors and heterointerface engineering are both crucial for highly efficient singlet fission solar cells. Therefore, the aim of this study is to review the prerequisite conditions for efficient triplet transfer at the heterointerfaces and thus highlight the robust spin and valley degrees of freedom of transition metal dichalcogenides with the ultimate goal of stimulating research into next-generation singlet fission solar cells.

Cubic Nanogrids for Counterbalance Contradiction among Reorganization Energy, Strain Energy, and Wide Bandgap

Molecular cross-scale gridization and polygridization of organic π-backbones make it possible to install 0/1/2/3-dimensional organic wide-bandgap semiconductors (OWBGSs) with potentially ZnO-like fascinating multifunctionality such as optoelectronic and piezoelectronic features. However, gridization effects are limited to uncover, because the establishment of gridochemistry still requires a long time, which offers a chance to understand the effects with a theoretical method, together with data statistics and machine learning. Herein, we demonstrate a state-of-the-art 3D cubic nanogridon with a size of ∼2 × 2 × 1.5 nm³ to examine its multigridization of π-segments on the bandgap, molecular strain energy (MSE), as well as reorganization energy (ROE). A cubic gridon (CG) consists of a four-armed bifluorene skeleton and a thiophene-containing fused arene plane with the Csp³ spiro-linkage, which can be deinstalled into face-on or edge-on monogrids. As a result, multigridization does not significantly reduce bandgaps (E_g ≥ 4.03 eV), while the MSE increases gradually from 4.72 to 23.83 kcal/mol. Very importantly, the ROE of a CG exhibits an extreme reduction down to ∼28 meV (λ⁺) that is near the thermal fluctuation energy (∼26 meV). Our multigridization results break through the limitation of the basic positively proportional relationship between reorganization energies and bandgaps in organic semiconductors. Furthermore, multigridization makes it possible to keep the ROE small under the condition of a high MSE in OWBGS that will guide the cross-scale design of multifunctional OWBGSs with both inorganics' optoelectronic performance and organics' mechanical flexibility.

On-demand modulating afterglow color of water-soluble polymers through phosphorescence FRET for multicolor security printing

Developing full-color organic ultralong room temperature phosphorescence (OURTP) materials with continuously variable afterglow emission is of considerable practical importance in diverse optoelectronic applications but remains a formidable challenge. Here, we present an effective strategy for on-demand engineering of afterglow color in water-soluble polymeric systems via efficient phosphorescence Förster resonance energy transfer. Using a blue afterglow emitting water-soluble polymer as host and a series of fluorescent emitters with varied emissive colors as guests, afterglow emission is rationally modulated, conferring the full-color afterglow emission ranging from blue to red and even white with ultralong lifetimes up to 4.2 s and photoluminescence quantum yields of 36%.These water-soluble multicolor-emitting polymeric afterglow systems can function as OURTP security inks, and multilevel information encryption was successfully established by RGB-based multicolor security printing. These results present important guidance in developing high-performance afterglow polymers with on-demand color tuning ability for remarkable optoelectronic applications.

Completely aqueous processable stimulus responsive organic room temperature phosphorescence materials with tunable afterglow color

Many luminescent stimuli responsive materials are based on fluorescence emission, while stimuli-responsive room temperature phosphorescent materials are less explored. Here, we show a kind of stimulus-responsive room temperature phosphorescence materials by the covalent linkage of phosphorescent chromophore of arylboronic acid and polymer matrix of poly(vinylalcohol). Attributed to the rigid environment offered from hydrogen bond and B-O covalent bond between arylboronic acid and poly(vinylalcohol), the yielded polymer film exhibits ultralong room temperature phosphorescence with lifetime of 2.43 s and phosphorescence quantum yield of 7.51%. Interestingly, the RTP property of this film is sensitive to the water and heat stimuli, because water could destroy the hydrogen bonds between adjacent poly(vinylalcohol) polymers, then changing the rigidity of this system. Furthermore, by introducing another two fluorescent dyes to this system, the color of afterglow with stimulus response effect could be adjusted from blue to green to orange through triplet-to-singlet Förster-resonance energy-transfer. Finally, due to the water/heat-sensitive, multicolor and completely aqueous processable feature for these three afterglow hybrids, they are successfully applied in multifunctional ink for anti-counterfeit, screen printing and fingerprint record.

Merging of a Perylene Moiety Enables a RuII Photosensitizer with Long-Lived Excited States and the Efficient Production of Singlet Oxygen

Multichromophoric systems based on a Ru^II polypyridine moiety containing an additional organic chromophore are of increasing interest with respect to different light‐driven applications. Here, we present the synthesis and detailed characterization of a novel Ru^II photosensitizer, namely [(tbbpy)₂Ru((2‐(perylen‐3‐yl)‐1H‐imidazo[4,5‐f][1,10]‐phenanthrolline))](PF₆)₂RuipPer, that includes a merged perylene dye in the back of the ip ligand. This complex features two emissive excited states as well as a long‐lived (8 μs) dark state in acetonitrile solution. Compared to prototype [(bpy)₃Ru]²⁺‐like complexes, a strongly altered absorption (ϵ=50.3×10³ M⁻¹ cm⁻¹ at 467 nm) and emission behavior caused by the introduction of the perylene unit is found. A combination of spectro‐electrochemistry and time‐resolved spectroscopy was used to elucidate the nature of the excited states. Finally, this photosensitizer was successfully used for the efficient formation of reactive singlet oxygen.

Bioinspired Luminescent Europium-Based Probe Capable of Discrimination between Ag+ and Cu. Inorg Chem 2021;60:10791-10798. [PMID: 34236828 DOI: 10.1021/acs.inorgchem.1c01486]

Due to their similar coordination properties, discrimination of Cu⁺ and Ag⁺ by water-soluble luminescent probes is challenging. We have synthesized LCC4^Eu, an 18 amino acid cyclic peptide bearing a europium complex, which is able to bind one Cu⁺ or Ag⁺ ion by the side chains of two methionines, a histidine and a 3-(1-naphthyl)-l-alanine. In this system, the naphthyl moiety establishes a cation-π interaction with these cations. It also acts as an antenna for the sensitization of Eu³⁺ luminescence. Interestingly, when excited at 280 nm, LCC4^Eu behaves as a turn-on probe for Ag⁺ (+150% Eu emission) and as a turn-off probe for Cu⁺ (-50% Eu³⁺ emission). Shifting the excitation wavelength to 305 nm makes the probe responsive to Ag⁺ (+380% Eu³⁺ emission) but not to Cu⁺ or other physiological cations. Thus, LCC4^Eu is uniquely capable of discriminating Ag⁺ from Cu⁺. A detailed spectroscopic characterization based on steady-state and time-resolved measurements clearly demonstrates that Eu³⁺ sensitization relies on electronic energy transfer from the naphthalene triplet state to the Eu³⁺ excited states and that the cation-π interaction lowers the energy of this triplet state by 700 and 2400 cm^-1 for Ag⁺ and Cu⁺, respectively. Spectroscopic data point to a modulation of the efficiency of the electronic energy transfer caused by the differential red shift of the naphthalene triplet, deciphering the differential luminescence response of LCC4^Eu toward Ag⁺ and Cu⁺.

Control of Multicolor and White Emission by Triplet Energy Transfer

A new strategy by manipulating the progress of triplet energy transfer (TET) is developed to realize adjustable multicolor and pure white emission. Donor phosphorescent molecules emits light when encapsulated into polyvinyl alcohol (PVA) through hydrogen bond interactions, and acceptor fluorescent molecules emits light when doped into PVA through cation-π interactions and hydrogen bond interactions. In addition, the triplet to singlet energy transfer process and mechanism are proved using the energy diagram and lifetime. The broadband emission color of the obtained composite film can be easily modulated by simply adjusting the amount and component of dyes, especially the white emission with CIE coordinates of (0.339, 0.337). This work provides a facile and versatile method for the development of multicolor and pure white-light-emitting diodes, which uses the interactions to light up luminescence properties, and can further aid in the wide development of applications for TET in various other fields.

Effect of the Molecular Conformation on Excitation Energy Transfer in Conformationally Constrained Boryl-BODIPY Dyads

We studied the dual emission characteristics of a series of boryl-BODIPYs (1-6) comprised of triarylborane (TAB) as an energy donor and BODIPY as an energy acceptor. The molecular conformations of dyads 1-6 were systematically tuned by judiciously changing the spacer that bridged the boryl and BODIPY moieties. Frontier molecular orbitals (FMOs) are localized in 3, 4, and 6 with a twisted molecular conformation. In contrast, FMOs are significantly delocalized in 1, 2, and 5 with the least-twisted molecular conformation. Dyads 1-6 showed dual emission features when they were excited at the TAB-dominated absorption band. However, the ratio between the two emission bands in 1-6 significantly varied depending on the molecular conformations. Systematic photoluminescence (PL) studies (both steady-state and time-resolved PL) together with computational, crystal structure, and anion binding studies established that the frustrated excited-state energy transfer from borane to BODIPY is the cause of the dual emission features in these molecular dyads. These studies also revealed that the energy transfer from borane to BODIPY can be elegantly tuned by modulating the dihedral angle between these two moieties.

Tracking light-induced electron transfer toward O2 in a hybrid photoredox-laccase system

Photobiocatalysis uses light to perform specific chemical transformations in a selective and efficient way. The intention is to couple a photoredox cycle with an enzyme performing multielectronic catalytic activities. Laccase, a robust multicopper oxidase, can be envisioned to use dioxygen as a clean electron sink when coupled to an oxidation photocatalyst. Here, we provide a detailed study of the coupling of a [Ru(bpy)₃]²⁺ photosensitizer to laccase. We demonstrate that efficient laccase reduction requires an electron relay like methyl viologen. In the presence of dioxygen, electrons transiently stored in superoxide ions are scavenged by laccase to form water instead of H₂O₂. The net result is the photo accumulation of highly oxidizing [Ru(bpy)₃]³⁺. This study provides ground for the use of laccase in tandem with a light-driven oxidative process and O₂ as one-electron transfer relay and as four-electron substrate to be a sustainable final electron acceptor in a photocatalytic process.

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An electron relay boosts photoreduction of laccase

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Superoxide is efficiently captured by laccase preventing formation of H₂O₂

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Light activation reveals information on elementary steps inside the enzyme

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Laccase enables O₂ as terminal electron acceptor for oxidative photocatalysis

Controlling Spin-Correlated Radical Pairs with Donor-Acceptor Dyads: A New Concept to Generate Reduced Metal Complexes for More Efficient Photocatalysis

One-electron reduced metal complexes derived from photoactive ruthenium or iridium complexes are important intermediates for substrate activation steps in photoredox catalysis and for the photocatalytic generation of solar fuels. However, owing to the heavy atom effect, direct photochemical pathways to these key intermediates suffer from intrinsic efficiency problems resulting from rapid geminate recombination of radical pairs within the so-called solvent cage. In this study, we prepared and investigated molecular dyads capable of producing reduced metal complexes via an indirect pathway relying on a sequence of energy and electron transfer processes between a Ru complex and a covalently connected anthracene moiety. Our test reaction to establish the proof-of-concept is the photochemical reduction of ruthenium(tris)bipyridine by the ascorbate dianion as sacrificial donor in aqueous solution. The photochemical key step in the Ru-anthracene dyads is the reduction of a purely organic (anthracene) triplet excited state by the ascorbate dianion, yielding a spin-correlated radical pair whose (unproductive) recombination is strongly spin-forbidden. By carrying out detailed laser flash photolysis investigations, we provide clear evidence for the indirect reduced metal complex generation mechanism and show that this pathway can outperform the conventional direct metal complex photoreduction. The further optimization of our approach involving relatively simple molecular dyads might result in novel photocatalysts that convert substrates with unprecedented quantum yields.

Role of Spin-Orbit Coupling in Long Range Energy Transfer in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are emerging as a promising platform for solar energy conversion applications. Their potential utilization as efficient chromophores in artificial photosynthesis is closely related to the understanding of light-harvesting and energy transfer processes that occur within these molecular scaffolds. Herein, we present the photophysical investigation of Ru(II), Ir(III), and Os(II) polypyridyl complexes incorporated into the backbone of UiO-67. In this work, we systematically study the effect of spin-orbit coupling on dipole-dipole energy transfer in MOFs using steady-state and time-resolved spectroscopic techniques. The results of our work indicate successful triplet-to-singlet energy transfer and a sizable increase in the transfer kinetics and critical distance, as direct consequences of strong spin-orbit couplings. Remarkably, the reported R0 value for OsDCBPY (R0 = 88 ± 10 Å) represents one of the largest Förster distances observed in an MOF. Collectively, this work contributes to the general knowledge of energy transfer in materials and provides groundwork for efficient utilization in artificial photosynthetic assemblies.

BioMAX - the first macromolecular crystallography beamline at MAX IV Laboratory

BioMAX is the first macromolecular crystallography beamline at the MAX IV Laboratory 3 GeV storage ring, which is the first operational multi-bend achromat storage ring. Due to the low-emittance storage ring, BioMAX has a parallel, high-intensity X-ray beam, even when focused down to 20 µm × 5 µm using the bendable focusing mirrors. The beam is tunable in the energy range 5-25 keV using the in-vacuum undulator and the horizontally deflecting double-crystal monochromator. BioMAX is equipped with an MD3 diffractometer, an ISARA high-capacity sample changer and an EIGER 16M hybrid pixel detector. Data collection at BioMAX is controlled using the newly developed MXCuBE3 graphical user interface, and sample tracking is handled by ISPyB. The computing infrastructure includes data storage and processing both at MAX IV and the Lund University supercomputing center LUNARC. With state-of-the-art instrumentation, a high degree of automation, a user-friendly control system interface and remote operation, BioMAX provides an excellent facility for most macromolecular crystallography experiments. Serial crystallography using either a high-viscosity extruder injector or the MD3 as a fixed-target scanner is already implemented. The serial crystallography activities at MAX IV Laboratory will be further developed at the microfocus beamline MicroMAX, when it comes into operation in 2022. MicroMAX will have a 1 µm × 1 µm beam focus and a flux up to 1015 photons s-1 with main applications in serial crystallography, room-temperature structure determinations and time-resolved experiments.

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.

Phosphorescence Energy Transfer: Ambient Afterglow Fluorescence from Water-Processable and Purely Organic Dyes via Delayed Sensitization

Ambient afterglow luminescence from metal-free organic chromophores would provide a promising alternative to the well-explored inorganic phosphors. However, the realization of air-stable and solution-processable organic afterglow systems with long-lived triplet or singlet states remains a formidable challenge. In the present study, a delayed sensitization of the singlet state of organic dyes via phosphorescence energy transfer from organic phosphors is proposed as an alternative strategy to realize "afterglow fluorescence". This concept is demonstrated with a long-lived phosphor as the energy donor and commercially available fluorescent dyes as the energy acceptor. Triplet-to-singlet Förster-resonance energy-transfer (TS-FRET) between donor and acceptor chromophores, which are co-organized in an amorphous polymer matrix, results in tuneable yellow and red afterglow from the fluorescent acceptors. Moreover, these afterglow fluorescent hybrids are highly solution-processable and show excellent air-stability with good quantum yields.

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

We found that boronate particles (BP), as a self-assembled system prepared by sequential dehydration of benzene-1,4-diboronic acid with pentaerythritol, showed greenish room-temperature phosphorescence (RTP). This emission was observed in both solid and dispersion state in water. To understand the RTP properties, X-ray crystallographic analysis, and density functional theory (DFT) and time-dependent DFT at M06-2X/6-31G(d,p) level were performed using 3,9-dibenzo-2,4,8,10-tetraoxa-3,9-diboraspiro[5.5]undecane (1) as a model compound. Our interest in functionalizing the RTP-active particles led us to graft Rhodamine B onto their surface. The resulting system emitted a dual afterglow via a Förster-type resonance energy transfer process from the BP in the excited triplet state to Rhodamine B acting as an acceptor fluorophore. This emission behavior was used for ratiometric afterglow sensing of water content in THF with a detection limit of 0.28 %, indicating that this study could pave the way for a new strategy for developing color-variable afterglow chemosensors for various analytes.