Nonradiative Rate Constants Research Articles

A phosphorated spirobi[thioxanthene] host matrix enables high-efficiency simple white thermally activated delayed fluorescence diodes

Host matrix is crucial for optimizing exciton allocation in all Thermally activated delayed fluorescence (TADF) white Organic light-emitting diodes (OLED). However, only few spiro hosts are competent since unbalanced steric and electronic effects. Herein, a new spiro core 9,9′-spirobi[thioxanthene] (ST) based host STDPO is reported, whose two peripheral diphenylphosphine oxide (DPPO) groups are incorporated to construct tridimensional steric hindrance and Intermolecular hydrogen bond (IHB) network. As consequence, STDPO achieves the high enough first triplet (T1) energy level of 2.93 eV and favorable electron mobility (µe) by the level of 10−5 cm2 V−1 s−1. Simultaneously, the well-modulated intermolecular interactions in STDPO matrix give rise to the synergistic effect of blue and yellow TADF dopants in exciton utilization. The increased rate constants and efficiencies of radiative transitions and reduced nonradiative rate constants render a photoluminescence quantum yield as high as 90% and 100% exciton utilization efficiency of dually doped white TADF film. STDPO further endowed its single-emissive-layer TADF WOLEDs with the record ηPE of 71.8 lm W−1 achieved by spiro hosts.

Effects of the Surface Charge Density of Clay Minerals on Surface-Fixation Induced Emission of Acridinium Derivatives.

Surface-fixation induced emission is a fluorescence enhancement phenomenon, which is expressed when dye molecules satisfy a specific adsorption condition on the anionic clay surface. The photophysical behaviors of two types of cationic acridinium derivatives [10-methylacridinium perchlorate (Acr+) and 10-methyl-9-phenylacridinium perchlorate (PhAcr+)] on the synthetic saponites with different anionic charge densities were investigated. Under the suitable conditions, the fluorescence quantum yield (Φf) of PhAcr+ was enhanced 22.3 times by the complex formation with saponite compared to that in water without saponite. As the inter-negative charge distance of saponite increased from 1.04 to 1.54 nm, the Φf of PhAcr+ increased 1.25 times. In addition, the increase in the negative charge distance caused the increase in the integral value of the extinction coefficient and the radiative deactivation rate constant (kf) and the decrease in the nonradiative deactivation rate constant. It should be noted that the 2.3 times increase in kf is the highest among the reported values for the effect of clay. From these results, it was concluded that the photophysical properties of dyes can be modulated by changing the charge density of clay minerals.

Transparent Ion-Exchange Membrane Exhibiting Intense Emission under a Specific pH Condition Based on Polypyridyl Ruthenium(II) Complex with Two Imidazophenanthroline Groups.

The development and the photophysical behavior of a transparent ion-exchange membrane based on a pH-sensitive polypyridyl ruthenium(II) complex, [(bpy)2RuII(H2bpib)RuII(bpy)2](ClO4)4 (bpy = 2,2′-bipyridine, H2bpib = 1,4-bis([1,10]phenanthroline[5,6-d]-imidazol-2-yl)benzene), are experimentally and theoretically reported. The emission spectra of [(bpy)2RuII(H2bpib)RuII(bpy)2]@Nafion film were observed between pH 2 and pH 11 and showed the highest relative emission intensity at pH 5 (λmaxem = 594.4 nm). The relative emission intensity of the film significantly decreased down to 75% at pH 2 and 11 compared to that of pH 5. The quantum yields (Φ) and lifetimes (τ) showed similar correlations with respect to pH, Φ = 0.13 and τ = 1237 ns at pH 5, and Φ = 0.087 and τ = 1014 ns and Φ = 0.069 and τ = 954 ns at pH 2 and pH 11, respectively. These photophysical data are overall considerably superior to those of the solution, with the radiative- (kr) and non-radiative rate constants (knr) at pH 5 estimated to be kr = 1.06 × 105 s−1 and knr = 7.03 × 105 s−1. Density functional theory calculations suggested the contribution of ligand-to-ligand- and intraligand charge transfer to the imidazolium moiety in Ru-H3bpib species, implying that the positive charge on the H3bpib ligand works as a quencher. The Ru-Hbpib species seems to enhance non-radiative deactivation by reducing the energy of the upper-lying metal-centered excited state. These would be responsible for the pH-dependent “off-on-off” emission behavior.

Manipulating Charge‐Transfer Excitons by Exciplex Matrix: Toward Thermally Activated Delayed Fluorescence Diodes with Power Efficiency beyond 110 lm W−1

AbstractThe understanding of the external enhancement effects from host matrixes on thermally activated delayed fluorescence (TADF) emitters is crucial but incomprehensive at present. Herein, a series of phosphine oxide (PO) acceptors mDBSOSPO (m = 2, 3, and 4, corresponding to PO substitution position) and 4,4'‐bis(9‐carbazolyl)‐2,2'‐dimethylbiphenyl (CDBP) as donor is used to construct CDBP:mDBSOSPO exciplex matrixes with typical TADF behaviors. After doped with a conventional yellow TADF emitter 4CzTPNBu, the exciplex matrixes dramatically elevate the reverse intersystem crossing (RISC) efficiencies up to 99%, effectively reduce triplet nonradiative rate constant, and tenfold increase singlet radiative/nonradiative ratio beyond 30 in the case of CDBP:2DBSOSPO:3% 4CzTPNBu. The time‐resolved photoluminescence and electroluminescence (EL) spectra demonstrate that in contrast to single‐molecular hosts, besides the additional RISC channel for TADF facilitation, the exciplexes support the charge preseparation for the step‐by‐step charge transfer‐based energy transfer featuring effective quenching suppression. These external enhancement effects of the exciplex matrixes lead to the state‐of‐the‐art EL performances of their yellow TADF diodes, including the recording power and quantum efficiencies of 114.9 lm W−1 and 30.3% to date.

Sulfur atom manipulates geometric isomerism of diphosphinine oxides for efficient delayed fluorescence diodes

Geometric isomerism has significant influences on intermolecular interactions. Therefore, despite the challenge, geometry manipulation is desired for developing high-performance optoelectronic materials. Herein, we demonstrate that heteroatom incorporation is a feasible approach to control cis- and trans- configurations of diphosphinine oxide compounds. Molecular geometries are highly dependent on heteroatom position, which is manifested by a cis- (cDPDPbDTO) and a trans- dithiophene-fused diphosphinine dioxide (tDPDPcDTO). It is showed that effects between S and P atoms bonded with the same C atoms bend the fused ring of cDPDPbDTO, giving rise to its cis-configuration. In contrast to trans-featured tDPDPcDTO containing a planar fused ring, cDPDPbDTO reveals more compact packing mode, accompanied by limited π-π stacking, and remarkably condensed excited states. Consequently, in addition to higher carrier mobility, cDPDPbDTO is much more effective in quenching suppression. Doped with a bluish green thermally activated delayed fluorescence (TADF) dye 2CzPN, cDPDPbDTO hosted film achieves fourfold increased radiative rate constant, and tenfold and sixfold decreased singlet and triplet nonradiative rate constants, respectively, as well as a 40%-improved reverse intersystem crossing (RISC) efficiency (97%). As the result, cDPDPbDTO endows its TADF systems with the state-of-the-art photoluminescence and electroluminescence quantum efficiencies of 95% and 26%, respectively.

Investigating phosphorescence capability of halogen-substituted metal-free organic molecules: A theoretical study

The radiative and non-radiative decay processes of five compounds are investigated through a comprehensive computational approach, for the aim of investigating the effect of different halogen substituents to the phosphorescent emission. Their optimal configurations at the ground (S0) and lowest triplet excited (T1) states are obtained and the calculated phosphorescent emission spectra are comparable with the experimental values. For 1,4-di(9H-carbazol-9-yl)benzene (PDCz), the electronic transition is between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), while for the four halides, the electronic transitions are attributed to several molecular orbitals. According to calculations, 9,9′-(2,5-diiodo-1,4-phenylene)bis(9H-carbazole) (PDICz) possesses the largest radiative decay rate constant (kr) and non-radiative decay rate constant (knr), which can be attributed to the strong spin-orbital coupling from the heavy iodine atom. However, the phosphorescent quantum efficiency (Φ) of PDICz is lower than that of 9,9′-(2,5-dibromo-1,4-phenylene)bis(9H-carbazole) (PDBCz), implying that a comprehensive consideration is necessary. Furthermore, by analyzing the vibrational mode, we have confirmed that the reorganization energies are also influenced by the different halogen atoms. While the dominated factor that determines the kr and knr comes from the spin-orbital coupling. We expect that our research findings will be beneficial to the newly designed organic phosphorescent materials in the future.

Investigating the Nonradiative Decay Pathway in the Excited State of Silepin Derivatives: A Study with Second-Order Multireference Perturbation Wavefunction Theory.

The fluorescence quantum yield for fluorescent organic molecules is an important molecular property, and tuning it up is desired for various applications. For the computational estimation of the fluorescence quantum yield, the theoretical prediction of the nonradiative decay rate constant has become an attractive subject of study. The rate constant of thermally activated nonradiative decay is related to the activation energy in the photoreaction; thus, the accuracy and reliability of the excited-state potential energies in the quantum chemical computation are critical. In this study, we employed a second-order multireference perturbation wavefunction theory for studying the thermally activated decay via conical intersection (CI) of 1,1-dimethyldibenzo[b,f]silepin derivatives. The correlation between the computed activation energy to reach the CI geometry in the S1 state and the experimentally determined fluorescence quantum yield implied that silepins nonradiatively decay via the CI triggered by the twisting of the central C-C bond. Geometry optimization of the transition state using multireference perturbation theory drastically reduced the estimated activation energy. Our computation gave reasonable predictions of the activation free energies of photoexcited 1,1-dimethyldibenzo[b,f]silepin. The energy profiles and geometry optimizations using proper quantum chemical methods played a critical role in reliable estimation of the rate constant and fluorescence quantum yield.

Luminescent halido(aryl) Pt(IV) complexes obtained via oxidative addition of iodobenzene or diaryliodonium salts to bis-cyclometalated Pt(II) precursors.

The synthesis of bis-cyclometalated halido(aryl) Pt(IV) complexes [PtX(Ar)(C^N)2], with C^N = cyclometalated 4-(tert-butyl)-2-phenylpyridine (bppy), 2-(p-tolyl)pyridine (tpy), 2-(2-thienyl)pyridine (thpy), or 1-phenylisoquinoline (piq), X = I, Cl, or F, and Ar = Ph (for all C^N ligands) or t-BuPh (for C^N = tpy), and the photophysical properties of the chlorido and fluorido series is reported. The oxidative addition of iodobenzene to cis-[Pt(C^N)2] precursors is demonstrated to occur in MeCN under irradiation with visible light to give complexes [PtI(Ph)(C^N)2], presumably involving radical species that also produce the activation of the solvent to give cyanomethyl complexes [PtI(CH2CN)(C^N)2]. The introduction of an aryl ligand can also be achieved by reacting cis-[Pt(C^N)2] with (Ar2I)PF6 (Ar = Ph, t-BuPh), which affords cationic intermediates of the type [Pt(Ar)(C^N)2(NCMe)]+. The subsequent addition of an iodide or chloride salt gives the corresponding iodido- or chlorido(aryl) complexes. The fluorido(aryl) derivatives can be obtained from the iodido complexes by halide exchange using AgF. The chlorido- and fluorido(aryl) complexes display intense phosphorescence in deaerated CH2Cl2 solution and poly(methyl methacrylate) (PMMA) films at 298 K from triplet excited states primarily localized on the cyclometalated ligands (3LC) with a small MLCT admixture. Compared with the chlorido complexes, the fluorido derivatives consistently present significantly shorter emission lifetimes and higher radiative and nonradiative rate constants due to a greater MLCT contribution to the emissive state. In contrast, the introduction of the t-BuPh group did not induce significant changes in radiative rates with respect to the phenyl complexes.

Understanding the photophysics of stercobilin-Zn(II) and urobilin-Zn(II) complexes towards faecal pigment analysis

A detailed photophysical study of two faecal pigments (FPs), Urobilin (UB) and Stercobilin (SB), and their zinc complexes [FP-Zn(II)] was carried out. The enhancement of UB and SB fluorescence resulting from the formation of their Zn(II) complexes was attributed to the complexation-induced rigidity of the chromophoric units, and the corresponding decrease of nonradiative decay rate constants of the excited singlet states (knr). The effect of various physicochemical environments was also studied in detail in order to understand the fluorescence behaviour of the Zn(II) complexes. FP-Zn(II) complexes have a lower solubility in water that results in the formation of molecular aggregates. The aggregation-induced loss of fluorescence of FP-Zn(II) complexes could be overcome by using the appropriate mixture of ethanol and water (70:30). Molecular orbital calculations on the FP-Zn(II) complexes provided a good idea of the geometry of the complexes and helped rationalise the enhancement of fluorescence after complexation. This study could pave the way towards developing a convenient non-extraction aqueous phase analytical procedure for detection of FPs using Zn(II) complexation method.

Double-Stranded DNA Matrix for Photosensitization Switching

Photosensitization, originated from the activation of triplet states, is the basis of many photodynamic applications, but often competes with a series of nonradiative processes. Herein, we communic...

Density Functional Studies on Photophysical Properties of Boron-Pyridyl-Imino-Isoindoline Dyes: Effect of the Fusion.

In this work, to make out the aryl-fusion effect on the photophysical properties of boron-pyridyl-imino-isoindoline dyes, compounds 1–5 were theoretically studied through analyses of their geometric and electronic structures, optical properties, transport abilities, and radiative (kr) and non-radiative decay rate (knr) constants. The highest occupied molecular orbitals of aryl-fused compounds 2–5 are higher owing to the extended conjugation. Interestingly, aryl fusion in pyridyl increases the lowest unoccupied molecular orbital (LUMO) level, while isoindoline decreases the LUMO level; thus, 4 and 5 with aryl fusion both in pyridyl and isoindoline exhibit a similar LUMO to 1. Compounds 4 and 5 show relatively low ionization potentials and high electron affinities, suggesting a better ability to inject holes and electrons. Importantly, the aryl fusion is conducive to the decrease of kIC. The designed compound 5 exhibits a red-shifted emission maximum, low λh, and low kIC, which endow it with great potential for applications in organic electronics. Our investigation provides an in-depth understanding of the aryl-fusion effect on boron-pyridyl-imino-isoindoline dyes at molecular levels and demonstrates that it is achievable.

Phosphorescent Cationic Heterodinuclear IrIII /MI Complexes (M=CuI , AuI ) with a Hybrid Janus-Type N-Heterocyclic Carbene Bridge.

A novel class of phosphorescent cationic heterobimetallic IrIII /MI complexes, where MI =CuI (4) and AuI (5), is reported. The two metal centers are connected by the hybrid bridging 1,3-dimesityl-5-acetylimidazol-2-ylidene-4-olate (IMesAcac) ligand that combines both a chelating acetylacetonato-like and a monodentate N-heterocyclic carbene site coordinated onto an IrIII and a MI center, respectively. Complexes 4 and 5 have been prepared straightforwardly by a stepwise site-selective metalation with the zwitterionic [(IPr)MI (IMesAcac)] metalloproligand (IPr=1,3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene) and they have been fully characterized by spectroscopic, electrochemical, and computational investigation. Complexes 4 and 5 display intense red emission arising from a low-energy excited state that is located onto the "Ir(C^N)" moiety featuring an admixed triplet ligand-centered/metal-to-ligand charge transfer (3 IL/1 MLCT) character. Comparison with the benchmark mononuclear complexes reveals negligible electronic coupling between the two distal metal centers at the electronic ground state. The bimetallic systems display enhanced photophysical properties in comparison with the parental congeners. Noteworthy, similar non-radiative rate constants have been determined along with a two-fold increase of radiative rate, yielding brightly red-emitting cyclometalating IrIII complexes. This finding is ascribed to the increased MLCT character of the emitting state in complexes 4 and 5 due to the smaller energy gap between the 3 IL and 1 MLCT manifolds, which mix via spin-orbit coupling.

Efficient structural modification of electron‐withdrawing substituents on Pt(II) complexes for red emitters: A theoretical study

In this study, the electronic structures and optical properties of a cyclometalated Pt(II) complex (M1) and a series of derivatives (M1–F, M1–CF3, and M1–CN) with electron‐withdrawing substituents (–F, –CF3, and –CN) at the carbazole moiety were theoretically investigated by density functional theory and time‐dependent density functional theory. The calculation results reveal that these Pt complexes display deep red phosphorescence emission above Λ = 640 nm. When the 3MLCT/π → π* to triplet metal‐centered 3MC/d–d state decay mechanism is taken into consideration, the nonradiative decay rate constant (knr) decreased in the order M1 > M1–CF3 > M1–F > M1–CN. The <T1|HSOC|Sm> and kr values of M1‐F are similar with those of M1, however the Knr rate ofM1‐F is larger than that of M1. M1–F is expected to have improved quantum yields. Moreover, through the analyses of the HOMO/LUMO level and triplet energy, it is found that the introduction of –F and –CN substituents in M1 results in efficient energy transfer from the host material 4,4′‐N,N′‐dicarbazole‐biphenyl to these complexes. In view of the electroluminescent applications in organic light‐emitting diodes, M1–F can serve as efficient deep‐red guest materials with improved electron injection and transport ability.

Electrochemical Switching of a Fluorescent Molecular Rotor Embedded within a Bistable Rotaxane.

We report how the nanoconfined environment, introduced by the mechanical bonds within an electrochemically switchable bistable [2]rotaxane, controls the rotation of a fluorescent molecular rotor, namely, an 8-phenyl-substituted boron dipyrromethene (BODIPY). The electrochemical switching of the bistable [2]rotaxane induces changes in the ground-state coconformation and in the corresponding excited-state properties of the BODIPY rotor. In the starting redox state, when no external potential is applied, the cyclobis(paraquat-p-phenylene) (CBPQT4+) ring component encircles the tetrathiafulvalene (TTF) unit on the dumbbell component, leaving the BODIPY rotor unhindered and exhibiting low fluorescence. Upon oxidation of the TTF unit to a TTF2+ dication, the CBPQT4+ ring is forced toward the molecular rotor, leading to an increased energy barrier for the excited state to rotate the rotor into the state with a high nonradiative rate constant, resulting in an overall 3.4-fold fluorescence enhancement. On the other hand, when the solvent polarity is high enough to stabilize the excited charge-transfer state between the BODIPY rotor and the CBPQT4+ ring, movement of the ring toward the BODIPY rotor produces an unexpectedly strong fluorescence signal decrease as the result of photoinduced electron transfer from the BODIPY rotor to the CBPQT4+ ring. The nanoconfinement effect introduced by mechanical bonding can effectively lead to modulation of the physicochemical properties as observed in this bistable [2]rotaxane. On account of the straightforward synthetic strategy and the facile modulation of switchable electrochromic behavior, our approach could pave the way for the development of new stimuli-responsive materials based on mechanically interlocked molecules for future electro-optical applications, such as sensors, molecular memories, and molecular logic gates.

Effect of Main Versus Ancillary Ligand Substitution on the Photophysical Properties of a Series of Ir(III) Complexes: A Detailed Theoretical Investigation.

Herein, we report the effects of different electron-withdrawing groups (EWG) (-F) and electron-donating groups (EDG) (-OMe and -NH2) on main ligands (ppy) and ancillary (acac) of [Ir(ppy)2(acac)] [ppy = 2-phenylpyridine; acac = acetylacetonato] using seven complexes by DFT and TDDFT calculations. We find that irrespective of the substituents, absorption of ppy-substituted complexes is blue-shifted, while for the acac-substituted complexes, it is red-shifted. The calculations also show that the substitution of EWGs causes an overall drop in the frontier molecular orbital energy levels; however, we observed a reverse effect for EDGs. To calculate the radiative rate kr, we considered the spin-orbit coupling matrix element (SOCME) (⟨T1|HSOC|Sn⟩) between Sn (n = 1, 2, etc.) excited state and T1, transition dipole moment (μ(Sn)), and the energy difference between excited singlet states Sn and T1 state (ΔE(Sn - T1)). To compare the temperature-independent nonradiative process, we considered SOC between T1 and S0 (⟨T1|HSOC|S0⟩) and the energy gap between optimized T1 and S0 states. Furthermore, to formulate the temperature-dependent nonradiative rate, we computed the activation barrier (E1) for the metal-to-ligand state (3MLCT) to a metal-centered state (3MC) conversion. The emission peaks show that the changes of triplet state T1 from 3MLCT → 3MC via transition states (3TS) and 3MLCT → 1GS (GS = ground state) via the 3MC/1GS minimum energy crossing point are not much affected by the nature of substituents in the ancillary and the main ligand. The order of E1 for the investigated complexes indicates that electron-donating substituents -OMe at both ppy and acac ligands can cause a decrease in nonradiative rate constants. Natural transition orbitals of the complexes show that they are mainly localized on the main ligand ppy and the Ir atoms and hardly on the ancillary ligand acac.

Highly Efficient Deep Blue Luminescence of 2-Coordinate Coinage Metal Complexes Bearing Bulky NHC Benzimidazolyl Carbene.

The structural, photophysical and electrochemical properties of three luminescent 2-coordinate coinage metal (i.e., M = Cu, Ag, Au) complexes bearing a sterically bulky benzimidazolyl carbene, 1,3-bis(2,6-diisopropylphenyl)-1-H-benzo[d]imidazol-2-ylidene (BZI), and carbazolide (Cz) as the anionic ligand were investigated. All the complexes emit in the deep blue region (~430 nm) with relatively narrow spectra (full width at half maximum = 44 nm, 2,300 cm−1) characterized by vibronic fine structure in nonpolar media (methylcyclohexane at room temperature), and with high photoluminescence quantum yields (ΦPL > 80%) and radiative rate constants (kr ~ 7.8 × 105 s−1). The luminescence is solvatochromic, undergoing a red-shift in a polar solvent (CH2Cl2) at room temperature that are accompanied by a decrease in quantum yields (ΦPL < 23%) and radiative rate constants (kr < 4.0 × 104 s−1), whereas the non-radiative rate constants remain nearly constant (knr ~ 1.0 × 105 s−1). The radiative rate is controlled via thermally assisted delayed fluorescence (TADF) and temperature-dependent luminescence studies of the gold complex (AuBZI) in methylcyclohexane solution reveal an energy difference between the lowest singlet and triplet excited states of 920 cm−1. An organic light-emitting diode (OLED) fabricated using AuBZI as a luminescent dopant has an external quantum efficiency of 12% and narrow, deep-blue emission (CIE = 0.16, 0.06).

SYNTHESIS AND STUDY OF SPECTRAL PROPERTIES OF AMINO ACIDS – BODIPY CONJUGATES

The present work describes direct one-pot synthesis of the two novel amino acids – boron-dipyrromethene (BODIPY) conjugates with histidine and tyrosine residues bound to fluorophore via amino group. The synthesized compounds were fully characterized by means of 1H and 11B nuclear magnetic resonance spectroscopy, infrared spectroscopy, time-of-flight mass spectrometry with matrix-activated laser desorption / ionization. All the data are in accordance with the proposed structures. The amino acids – BODIPY conjugates under study were investigated by means of absorbance and fluorescence spectroscopy. Absorption and emission spectra of the compounds in dimethyl sulfoxide (DMSO) – buffer solution (BS) mixture (1:1) were obtained. Moreover, a number of photophysical characteristics of the conjugates, namely: relative fluorescence quantum yield, molar extinction coefficient, relative fluorescence lifetime, radiative and non-radiative rate constants were calulated. 3D fluorescence spectra of the compounds in DMSO – BS mixture (1:1) were obtained. Absorption and emission titration of the compounds was carried out. The results of the titration allowed us to confirm additionally a stoichiometric composition of the conjugates. Quantum chemical analysis of structural and spectral properties of the compounds in a ground state was carried out. The results of quantum chemical calculations gave an insight to a nature of the BODIPY – amino acid interactions in the conjugates. Besides, the synthesized amino acids – BODIPY conjugates are found to be highly soluble in polar solvents. Thus, a range of unique properties of the objects of the present research enables us to use them for visualization of bioactive macromolecules as well as biochemical processes in living cells.

Tetrahedral Silicon-Centered Dibenzoylmethanatoboron Difluorides: Synthesis, Crystal Structure, and Photophysical Behavior in Solution and the Solid State.

Four tetrahedral silicon-centered derivatives of dibenzoylmethanatoboron difluoride (DBMBF2 ) were synthesized and characterized. Their structural and optical features both in solution and the solid state were investigated by using X-ray crystallography, steady-state and time-dependent spectroscopy, and DFT-based calculations. In dilute solutions, the molar absorption coefficient increases from 40500 to 175200 M-1 cm-1 as the number of DBMBF2 fragments in a molecule increases from one to four, while, in contrast, the nonradiative rate constant of fluorescence decay decreases from 0.49 to 0.34. In the solid state, absorption and emission spectra depend on the degree of crystallinity and microcrystal size. The tris-DBMBF2 derivative forms fully overlapping dimeric structures that exhibit excimer-like fluorescence, which is accurately predicted by the quantum-chemical calculations. The mono-DBMBF2 derivative exhibits fully reverse mechanofluorochromic behavior.

Restriction‐Induced Luminescence Enhancement in 2D Interlayer Supramolecular Infinite Solid Solution for Cell Imaging

AbstractConsidering the anionic intercalation capacities of layered double hydroxides (LDH), 2‐hydroxy‐4‐methoxybenzophenone‐5‐sulfonate (BP4) and decanesulfonate (DES) anions are cointercalated into the interlayers of Mg2Al‐LDH to form DES–BP4 (x%)/LDH composites, which can be regarded as 2D interlayer supramolecular infinite solid solutions (2D ISISSs), according to a structural analysis. Compared to those of pristine BP4, the fluorescence properties of DES–BP4 (x%)/LDH 2D ISISSs are significantly improved and their emission peak wavelengths are adjustable in the range of 491–510 nm by varying x. DES–BP4 (8%)/LDH exhibits the optimal fluorescence properties. Its quantum yield, lifetime, and fluorescence intensity are increased ≈14, 4, and 45 times, respectively. Moreover, its nonradiative relaxation rate constant is decreased five times, while its radiative relaxation rate constant is increased three times. The luminescence enhancement can be attributed to the formation of the 2D ISISSs system, which effectively restricted the intramolecular movements of BP4 anions. Owing to its enhanced fluorescence properties, DES–BP4 (8%)/LDH exhibits outstanding performances for cell penetration and confocal fluorescence imaging and lifetime imaging of lysosome in HeLa cells. Therefore, the 2D ISISSs based on the LDH matrix provide a novel route to optimize the performances of functional molecules and expand their applications.

Interplay of Radiative and Non Radiative Rate Constants in the Photophysics of Fluorescent Proteins

Genetically encodable fluorophores like Fluorescent Proteins (FPs) have been engineered to provide better contrast, localization and specificity. Red-FPs (RFPs) are known to have an extended chromophore through an acylimine moiety with respect to their blue shifted and generally brighter counterparts. The molecular brightness is based on the product of the extinction coefficient (EC) and fluorescence quantum yield (QY). Development of brighter FPs has focused on increasing the QY, which is directly proportional to fluorescence lifetime (FL), a time constant corresponding to the inverse sum of the ensemble radiative and non-radiative decay rate constants. We have performed directed evolution of RFPs through random and site-directed mutagenesis on a multi-parametric microfluidic sorting system capable of FL and brightness selections. We also applied site directed mutagenesis on RFPs based on structural analysis to investigate the structural effects on FL and QY. The selected mutants derived from mCherry and FusionRed show that the improvement in FL brings about an expected linear increase in QY (∼3.2 fold for mCherry and ∼2.3 fold for FusionRed). This can be attributed to significant decrease in the non-radiative rate constant, and a modest improvement in radiative rate constant that is supported by hypsochromic shifts and changes in absorption profiles. Both approaches reveal that the improvement in molecular brightness with increased FL is mainly governed by a decrease in the non-radiative rate constant in conjunction with improved absorption (narrower absorption spectrum and higher ECmax). This strategy forms the basis of the best photophysical route to engineer brighter RFPs.