RESUMEN

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.

RESUMEN

Bright afterglow room-temperature phosphorescence (RTP) soon after ceasing excitation is a promising technique for greatly increasing anti-counterfeiting capabilities. The development of a process for rapid high-resolution afterglow patterning of crystalline materials can improve both high-speed fabrication of anti-counterfeiting afterglow media and stable afterglow readout compared with those achieved with amorphous materials. Here, the high-resolution afterglow patterning of crystalline materials via cooperative organic vapo- and photo-stimulation is reported. A single crystal of (S)-(-)-2,2'-bis(diphenylphosphino)-5,5',6,6',7,7'8,8'-octahydro-1,1'-binaphthyl [(S)-H8-BINAP] doped with (S)-(-)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl [(S)-BINAP] shows green afterglow RTP. Crystals of (S)-BINAP-doped (S)-H8-BINAP changed to an amorphous state with no afterglow capability on weak continuous photoirradiation under dichloromethane (DCM) vapor. Photoirradiation induced oxidation of the (S)-H8-BINAP host molecule in the crystal. The oxidized (S)-H8-BINAP forms on the crystal surface strongly interacted with DCM molecules, which induces melting of the (S)-BINAP-doped (S)-H8-BINAP crystal and trigger formation of an amorphous state without an afterglow capability. High-resolution afterglow patterning of the crystalline film is rapidly achieved by using cooperative organic vapo- and photo-stimulation. In addition to the benefit of rapid afterglow patterning, the formed afterglow images of the crystalline film can be repeatedly read out under ambient conditions without DCM vapor.

RESUMEN

Afterglow room-temperature emission that is independent of autofluorescence after ceasing excitation is a promising technology for state-of-the-art bioimaging and security devices. However, the low brightness of the afterglow emission is a current limitation for using such materials in a variety of applications. Herein, the continuous formation of condensed triplet excitons for brighter afterglow room-temperature phosphorescence is reported. (S)-(-)-2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl ((S)-BINAP) incorporated in a crystalline host lattice showed bright green afterglow room-temperature phosphorescence under strong excitation. The small triplet-triplet absorption cross-section of (S)-BINAP in the whole range of visible wavelengths greatly suppressed the deactivation caused by Förster resonance energy transfer from excited states of (S)-BINAP to the accumulated triplet excitons of (S)-BINAP under strong continuous excitation. The steady-state concentration of the triplet excitons for (S)-BINAP reached 2.3 × 10-2  M, producing a bright afterglow. Owing to the brighter afterglow, afterglow detection using individual particles with sizes approaching the diffraction limit in aqueous conditions and irradiance-dependent anticounterfeiting can be achieved.

RESUMEN

Triplet harvesting is important for high-efficiency optoelectronics devices, time-resolved bioimaging, sensing, and anti-counterfeiting devices. Förster resonance energy transfer (FRET) from the donor (D) to the acceptor (A) is important to efficiently harvest the triplet excitons after a variety of excitations. However, general explanations of the key factors of FRET from the singlet state (FRETS-S) via reverse intersystem crossing and FRET from the triplet state (FRETT-S) have not been reported beyond spectral overlap between emission of the D and absorption of the A. This feature article gives an overview of FRET involving the triplet state. After discussing the contribution of the radiation yield from the state of the D considering spin-forbidden factors of FRET, a variety of schemes involving triplet states, such as FRETS-Svia reverse intersystem crossing from the triplet state, dual FRETS-S and FRETT-S, and selective FRETT-S, are introduced. Representative examples, including the chemical structure and FRET for triplet harvesting, are highlighted using emerging applications in optoelectronics and afterglow imaging. Finally, recent developments of using FRET involving triplet states for high-efficiency optoelectronic devices and time-resolved bioimaging are discussed. This article provides crucial information for controlling state-of-the-art properties using FRET involving the triplet state.

RESUMEN

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.

Asunto(s)

RESUMEN

The suppression of thermally driven triplet deactivation is crucial for efficient persistent room-temperature phosphorescence (pRTP). However, the mechanism by which triplet deactivation occurs in metal-free molecular solids at room temperature (RT) remains unclear. Herein, we report a large pRTP intensity change in a molecular guest that depended on the reversible amorphous-crystal phase change in the molecular host, and we confirm the large contribution made by the rigidity of the host in suppressing intermolecular triplet quenching in the guest. (S)-(-)-2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl ((S)-BINAP) was doped as a guest into a highly purified (S)-bis(diphenylphosphino)-5,5',6,6',7,7',8,8'-octahydro-1,1'-binaphthyl ((S)-H8-BINAP) host. It was possible to reversibly form the amorphous and crystalline states of the solid by cooling to RT from various temperatures. The RTP yield (Φ p) originating from the (S)-BINAP was 6.7% in the crystalline state of the (S)-H8-BINAP host, whereas it decreased to 0.31% in the amorphous state. Arrhenius plots showing the rate of nonradiative deactivation from the lowest triplet excited state (T1) of the amorphous and crystalline solids indicated that the large difference in Φ p between the crystalline and amorphous states was mostly due to the discrepancy in the magnitude of quenching of intermolecular triplet energy transfer from the (S)-BINAP guest to the (S)-H8-BINAP host. Controlled analyses of the T1 energy of the guest and host, and of the reorganization energy of the intermolecular triplet energy transfer from the guest to the host, confirmed that the large difference in intermolecular triplet quenching was due to the discrepancy in the magnitude of the diffusion constant of the (S)-H8-BINAP host between its amorphous and crystalline states. Quantification of both the T1 energy and the diffusion constant of molecules used in solid materials is crucial for a meaningful discussion of the intermolecular triplet deactivation of various metal-free solid materials.

RESUMEN

Highly efficient persistent (lifetime > 0.1 s) room-temperature phosphorescence (pRTP) chromophores are important for futuristic high-resolution afterglow imaging for state-of-the-art security, analytical, and bioimaging applications. Suppression of the radiationless transition from the lowest triplet excited state (T1) of the chromophores is a critical factor to access the high RTP yield and RTP lifetime for desirable pRTP. Logical explanations for factor suppression based on chemical structures have not been reported. Here we clarify a strategy to reduce the radiationless transition from T1 based on chemical backbones and yield a simultaneous high RTP yield and high RTP lifetime. Yellow phosphorescence chromophores that contain a coronene backbone were synthesized and compared with yellow phosphorescent naphthalene. One of the designed coronene derivatives reached a RTP yield of 35%, which is the best value for chromophores with a RTP lifetime of 2 s. The optically measured rate constant of a radiationless transition from T1 was correlated precisely with a multiplication of vibrational spin-orbit coupling (SOC) at a T1 geometry and with the Franck-Condon chromophore factor. The agreement between the experimental and theoretical results confirmed that the extended two-dimensional fused structure in the coronene backbone contributes to a decrease in vibrational SOC and Franck-Condon factor between T1 and the ground state to decrease the radiationless transition. A resolution-tunable afterglow that depends on excitation intensity for anticounterfeit technology was demonstrated, and the resultant chromophores with a high RTP yield and high RTP lifetime were ideal for largely changing the resolution using weak excitation light.

RESUMEN

A chiral chromophore with thermally activated delayed fluorescence characteristics is designed. A model describing vibrations of the dihedral angle between donor and acceptor units allowed at room temperature explains the enhanced fluorescence rate of the molecule, including a nearly 100% photoluminescence quantum yield and the absence of circularly polarized emission characteristics from enantiomers of the chromophore.

RESUMEN

The radiationless transition rate based on intramolecular vibrations from the lowest excited triplet state (T1) at room temperature [knr(RT)] is crucial for triplet energy harvesting in optoelectronics and photonics applications. Although a decrease of knr(RT) of chromophores with strong intermolecular interactions is often proposed, scientific evidence for this has not been reported. Here we report a method to predict knr(RT). We optically estimated knr(RT) of various molecularly dispersed chromophores with a variety of transition characteristics from T1 to the ground state (S0) under appropriate inert liquid or solid host conditions. Spin-orbit coupling (SOC) without considering molecular vibrations was not correlated with the estimated knr(RT). However, the estimated knr(RT) was strongly correlated with a multiplication of SOC considering vibrations freely allowed at room temperature and the Franck-Condon factor. This correlation revealed that knr(RT) of many heavy-atom-free chromophores with a visible T1-S0 transition energy and local excited T1-S0 transition characteristics is intrinsically less than 100 s-1 even when vibrations freely occur. This information will assist researchers to appropriately design materials without limitations regarding intermolecular interactions to control T1 lifetime at room temperature and facilitate triplet energy harvesting.

RESUMEN

We report a novel material containing a rare metal-free dopant chromophore with controlled electronic symmetry, which exhibits strong optical limiting (OL) capabilities under weak, continuous, sky-blue irradiance. Electron-donating substituents at positions C2 and C7 of pyrene allow significant triplet generation because of the symmetrically forbidden transition between the ground state and the lowest singlet excited state, which leads to accumulation of triplet excitons in the dopant chromophore. This also leads to a small ground state absorption coefficient and induces greater absorption of sky-blue wavelengths when triplet excitons of the chromophore accumulate. Consequently, molecular glass doped with the designed chromophore displays stronger OL characteristics compared with those of the rare element-containing glass that previously demonstrated the greatest OL performance under continuous sky-blue irradiance at sunlight power levels. The described approach to developing cost-effective, state-of-the-art OL materials is crucial for nonlinear optical applications working at a large scale under sunlight or room lights.

RESUMEN

Persistent (lifetime > 100 ms) room-temperature phosphorescence (pRTP) is important for state-of-the-art security and bioimaging applications. An unclear relationship between chromophores and physical parameters relating to pRTP has prevented obtaining an RTP yield of over 50% and a lifetime over 1 s. Here highly efficient pRTP is reported under ambient conditions from heavy atom-free chromophores. A heavy atom-free aromatic core substituted with a long-conjugated amino group considerably accelerates the phosphorescence rate independent of the intramolecular vibration-based nonradiative rate from the lowest excited triplet state. One of the designed heavy atom-free dopant chromophores presents an RTP yield of 50% with a lifetime of 1 s under ambient conditions. The afterglow brightness under strong excitation is at least 104 times stronger than that of conventional long-persistent luminescence emitters. Here it is shown that highly efficient pRTP materials allow for high-resolution gated emission with a size of the diffraction limit using small-scale and low-cost photodetectors.

RESUMEN

Halide perovskites nanocrystals (NCs) are being explored as promising materials for optoelectronic applications, such as light-emitting devices or lasers. However, electroluminescence devices prepared from such NCs have long suffered from low efficiency and there has been no systematic study on the nanoscale origin of the poor efficiencies. Here, we use single-particle spectroscopy to compare electroluminescence and photoluminescence on the level of individual NCs of the perovskite CsPbBr3. The NCs form aggregates in a conducting matrix used as an emission layer in an electroluminescence device. In electroluminescence, only a small fraction of the NCs within the aggregate is emitting as a result of efficient charge migration, accumulation and selective recombination on larger NCs, leading to pronounced blinking and decreased efficiency. Under the condition of comparable excitation rates in both electroluminescence and photoluminescence, the intrinsic quantum yield in electroluminescence is on average 0.36 of that in photoluminescence.

RESUMEN

Conjugated molecular crystals with persistent room-temperature phosphorescence (RTP) are promising materials for sensing, security, and bioimaging applications. However, the electronic structures that lead to efficient persistent RTP are still unclear. Here, the electronic structures of tetraphenylmethane (C(C6H5)4), tetraphenylsilane (Si(C6H5)4), and tetraphenylgermane (Ge(C6H5)4) showing blue-green persistent RTP under ambient conditions are investigated. The persistent RTP of the crystals originates from minimization of triplet exciton quenching at room temperature not suppression of molecular vibrations. Localization of the highest occupied molecular orbitals (HOMOs) of the steric and highly symmetric conjugated crystal structures decreases the overlap of intermolecular HOMOs, minimizing triplet exciton migration, which accelerates defect quenching of triplet excitons. The localization of the HOMOs over the highly symmetric conjugated structures also induces moderate charge-transfer characteristics between high-order singlet excited states (S m ) and the ground state (S0). The combination of the moderate charge-transfer characteristics of the S m -S0 transition and local-excited state characteristics between the lowest excited triplet state and S0 accelerates the phosphorescence rate independent of the vibration-based nonradiative decay rate from the triplet state at room temperature. Thus, the decrease of triplet quenching and increase of phosphorescence rate caused by the HOMO localization contribute to the efficient persistent RTP of Ge(C6H5)4 crystals.

RESUMEN

Organic-inorganic halide perovskites have emerged as promising materials for next-generation solar cells. In nanostructured form also, these materials are excellent candidates for optoelectronic applications such as lasers and light-emitting diodes for displays and lighting. While great progress has been achieved so far in optimizing the intrinsic photophysical properties of perovskite nanocrystals (NCs), in working optoelectronic devices, external factors, such as the effects of conducting environment and the applied electric field on exciton generation and photon emission, have been largely unexplored. Here, we use NCs of the all-inorganic perovskite CsPbBr3 dispersed polyvinyl carbazole, a hole-conductor, and in poly(methyl methacrylate), an insulator, to examine the effects of applied electric field and conductivity of the matrix on the perovskite photophysics at the single-particle level. We found that the conducting environment causes a significant decrease of photoluminescence (PL) brightness of individual NCs due the appearance of intermediate-intensity emitting states with significantly shortened lifetime. Applied electric field has a similar effect and, in addition, causes a nonlinear spectral shift of the PL maxima, a combination of linear and quadratic Stark effects caused by environment-induced polarity and field-related polarizability. The environment and electric-field effects are explained by ionization of the NCs through hole transfer and emission of the resulting negatively charged excitons.

RESUMEN

Persistent room-temperature phosphorescence (RTP) under ambient conditions is attracting attention due to its strong potential for applications in bioimaging, sensing, or optical recording. Molecular packing leading to a rigid crystalline structure that minimizes nonradiative pathways from triplet state is often investigated for efficient RTP. However, for complex conjugated systems a key strategy to suppress the nonradiative deactivation is not found yet. Here, the origin of small rates of a nonradiative decay process from triplet states of conjugated molecular crystals showing RTP is reported. Optical microscopy analysis showed that, despite a favorable molecular stacking, an aromatic crystal with strong RTP is characterized by small diffusion length and small values of the diffusion coefficient of triplet excitons. Quantum chemical calculations reveal a large overlap between the lowest unoccupied molecular orbitals but very small overlap between the highest occupied molecular orbitals (HOMOs). Inefficient electron exchange caused by the small overlap of HOMOs prevents triplet excitons from diffusing over long distances and consequently from quenching at defect sites inside the crystal or at the crystal surface. These results will allow design of comprehensive molecular structures to obtain molecular solids with more efficient RTP.

RESUMEN

The nonstoichiometry of I-III-VI semiconductor nanoparticles, especially the ratio of group I to group III elements, has been utilized to control their physicochemical properties. We report the solution-phase synthesis of nonstoichiometric Ag-In-S and Ag-In-Ga-S nanoparticles and results of the investigation of their photoluminescence (PL) properties in relation to their chemical compositions. While stoichiometric AgInS2 nanoparticles simply exhibited only a broad PL band originating from defect sites in the particles, a narrow band edge PL peak newly appeared with a decrease in the Ag fraction in the nonstoichiometric Ag-In-S nanoparticles. The relative PL intensity of this band edge emission with respect to the defect-site emission was optimal at a Ag/(Ag + In) value of ca. 0.4. The peak wavelength of the band edge emission was tunable from 610 to 500 nm by increased doping with Ga3+ into Ag-In-S nanoparticles due to an increase of the energy gap. Furthermore, surface coating of Ga3+-doped Ag-In-S nanoparticles, that is, Ag-In-Ga-S nanoparticles, with a GaS x shell drastically and selectively suppressed the broad defect-site PL peak and, at the same time, led to an increase in the PL quantum yield (QY) of the band edge emission peak. The optimal PL QY was 28% for Ag-In-Ga-S@GaS x core-shell particles, with green band-edge emission at 530 nm and a full width at half-maximum of 181 meV (41 nm). The observed wavelength tunability of the band-edge PL peak will facilitate possible use of these toxic-element-free I-III-VI-based nanoparticles in a wide area of applications.

RESUMEN

A peculiarity of cyanobacterial Photosystem I (PSI) is the presence of so-called red chlorophylls absorbing at wavelengths longer than the reaction center P700. The origin and function of these chlorophylls have been debated in literature, but so far no consensus has been reached on either question. Here, we use plasmon-enhanced single-particle fluorescence spectroscopy to elucidate the origin of both short- and long-wavelength emitting species in monomeric PSI from Thermosynechococcus elongatus at room temperature. Polarized fluorescence spectra of single PSI complexes reveal a phase shift in the modulation of the short-wavelength (687 nm) and long-wavelength (717 nm) peaks. Numerical simulations show that this phase shift reflects a spatial angle of 15° between the transition dipole moments of the two forms. Quantum chemical calculations, together with reported X-ray structural and spectroscopic data, were used to assign the chlorophyll a monomer A3 as a candidate for the short-wavelength emitter and the B31-B32 chlorophyll dimer as a candidate for the long-wavelength emitter.

Asunto(s)

RESUMEN

The radiative rate ( kp) of the lowest triplet excited state (T1) and the nonradiative rate based on intramolecular vibrations at room temperature [ knr(RT)] from T1 for heavy atom-free conjugated structures are determined by considering the triplet yield and quenching rate from T1. Donor substitution did not strongly influence knr(RT) but greatly enhanced kp. The knr(RT) values were comparable between donor-substituted molecules and nonsubstituted molecules, which we explain by similar vibrational spin-orbit coupling (SOC) related to the transition from T1 to the ground state (S0). We attribute the enhancement of kp induced by donor substitution to the appearance of a large SOC between high-order singlet excited states (Sm) and T1 together with the large transition dipole moments of the Sm-S0 transitions. Knowledge of this mechanism is important for developing future efficient persistent room-temperature phosphorescence from doped aromatic materials and aromatic crystals.

RESUMEN

Ring expansion metathesis polymerisation (REMP) has proven to be a viable approach to prepare high purity cyclic polymers. Macrocyclic polymers with a fully conjugated defect free backbone are of particular interest as these polymers have no end groups that can act as charge traps. In this work soluble macrocyclic poly(p-phenylenevinylene)s ( cPPVs) have been prepared directly via the REMP of substituted paracyclophanedienes. Single-molecule spectroscopy of the two topological forms of PPV i.e., linear ( lPPV) and cyclic ( cPPV) revealed that lPPV exists in an extended conformation whereas the cPPV adopts a restricted ring-like conformation. Despite such large differences in the chain conformation, the spectral properties of the two compounds are unexpectedly very similar, and are dominated by torsional deformations in relatively short conjugated segments.