Kasha's Rule Research Articles

Stable Boron‐Containing Blue‐Photoluminescent Radicals

Main observation and conclusionIt is highly urgent to develop synthetic strategies to make new category of stable luminescent radicals with desired emission wavelength. In this study, we have isolated two dioxoborocyclic radicals (3 and 4) by a direct synthetic route. They were characterized by UV, EPR spectroscopy and SQUID measurements. Their structures were obtained by single‐crystal X‐ray diffraction. Both radicals produce blue‐photoluminescence (458 nm for 3 and 478 nm for 4) by radiative decay from higher excited states (D2/D3) to the ground state (D0) based on theoretical calculation, breaking Kasha rule. The work records a new kind of radical emitters and the first stable radicals with blue emission bands.

Linear and nonlinear optical properties of a quadrupolar carbo-benzene and its benzenic parent: The carbo-merization effect

Herein, the optical properties of thiophene-functionalized quadrupolar carbo-benzenes and a benzenic parent, of generic structure Th–CC–[core]–CC–Th, Th = R2C4HS, are comparatively investigated. Beyond the previously unknown dioctylthienylethynylbenzene (core = p-C6H4, R = nOct), two bis-dialkylthienylethynyl-carbo-benzenes (core = C18Ph4, R = nOct, nBu) are envisaged for the unique "carbo-aromatic" character of the C18 macrocycle. The three targets were synthesized from the corresponding ethynylthiophenes in 47, 20 and 10% yield, respectively, then characterized by classical methods such as NMR spectroscopy, and X-ray crystallography for one of the carbo-benzenes. Regarding linear and nonlinear optical properties, our results show that the carbo-merization induces a significant shift to lower energies of the one-photon electronic excitations accompanied by an 8-fold increase of the molar extinction coefficient compared to the parent molecule. Intriguingly, these excitations lead to a broad band of photoluminescence comprising decay transitions of the type S1 → S0 but also of the type S2 → S0. This phenomenon of emission from higher excited states, which is contrary to Kasha's rule, is assigned to - or revealed by - a reduction of the internal conversion efficiency between S2 and S1. Two-photon induced transitions are also enhanced, the two-photon absorption cross-section (σ2PA) being in average five times larger for the carbo-benzenes than for their benzene parent in the wavelength range 650–950 nm, with a maximum of σ2PA = 1430 GM (1 GM = 10−50 cm4 s/photon). Beyond a moderate nonlinearity, this comparative study provides quantitative insights about the way carbo-merization or insertion of a π-conjugated macrocycle between chromophoric functions (here thiophene rings) can tune optical properties of organic molecules. The optical properties of the bis-dialkylthienylethynyl-carbo-benzenes are also discussed in regard of recent reports on organic chromophores based on other types of π-conjugated macrocyclic cores.

Recent progress in hot exciton materials for organic light-emitting diodes.

According to Kasha's rule, high-lying excited states usually have little effect on fluorescence. However, in some molecular systems, the high-lying excited states partly or even mainly contribute to the photophysical properties, especially in the process of harvesting triplet excitons in organic electroluminescent devices. In the current review, we focus on a type of organic light-emitting diode (OLED) materials called "hot exciton" materials, which can effectively harness the non-radiative triplet excitons via reverse intersystem crossing (RISC) from high-lying triplet states to singlet states (Tn→ Sm; n≥ 2, m≥ 1). Since Ma and Yang proposed the hot exciton mechanism for OLED material design in 2012, there have been many reports aiming at the design and synthesis of novel hot exciton luminogens. Herein, we present a comprehensive review of the recent progress in hot exciton materials. The developments of the hot exciton mechanism are reviewed, the fundamental principles regarding molecular design are discussed, and representative reported hot exciton luminogens are summarized and analyzed, along with their structure-property relationships and OLED applications.

Breaching Kasha's rule for dual emission: mechanisms, materials and applications

Strategies for anti-Kasha emissions: in this review, we gave an overview of the recent progress for anti-Kasha emitters from three aspects: isomerization balancing, excited state balancing and emissive building block combination.

Singlet fission from upper excited singlet states and polaron formation in rubrene film.

Femtosecond fluorescence up-conversion and transient absorption pump-probe setups are applied to study the relaxation dynamics of the lower and upper excited singlet electronic states in easy-to-make rubrene films. Upon 250 nm (4.96 eV) excitation, singlet fission was observed directly from S2 state bypassing S1 state within 30 fs i.e. breaking the classical Kasha rule. From the transient absorption measurements, polaron formation was also detected on the same time scale. Both singlet fission and polaron formation are accelerated from upper excited states compared with S1 state. Our work shows that rubrene films with low degree of crystallinity could display efficient singlet fission, notably in the case of excitation to upper lying electronic states. This can strongly expand the applications of rubrene in organic electronics. Moreover, our results will provide a new direction for synthesizing novel materials with optimized excited state properties for organic photovoltaic applications.

Water-stimuli-responsive dynamic fluorescent switch from Kasha's rule to anti-Kasha's rule based on a tetraphenylethene substituted Schiff base

Stimuli-responsive fluorescence switches have aroused enormous interest due to their wide prospects in dealing with safety, health, energy, etc. However, most stimuli-responsive fluorescent switches usually follow the Kasha’s rule and are static responsive, which cannot utilize high-energy excited states and provide relatively unitary luminescent signal limiting their wider applications. Herein, a tetraphenylethene-substituted Schiff base (TPENOCN) was synthesized, which exhibited expectedly water-induced fluorescent switch from Kasha’s rule emission to anti-Kasha’s rule emission accompanied by dynamic emission color changes from yellow to blue with an enhanced fluorescent quantum yield. Theoretical and experimental investigations revealed that the dynamic fluorescent switch is induced by the gradually formation of intermolecular hydrogen bond between TPENOCN and water. Furthermore, an eco-friendly rewritable paper for security writing/printing, multilevel anti-counterfeiting and data storage were developed by employing TPENOCN as imaging layer and water as ink. The unique water-stimuli-responsive characteristics of TPENOCN with dynamic emission changes may have promising application in data security, data protection, fluorescence detection and so on.

Breaking Kasha's Rule as a Mechanism for Solution-Phase Room-Temperature Phosphorescence from High-Lying Triplet Excited State.

Organic room-temperature phosphorescence (ORTP) has been demonstrated successfully in solids. In contrast, solution-phase ORTP is rarely achieved, because the T1 → S0 phosphorescence is too slow to compete against nonradiative decay and the oxygen-quenching effect. Here, we reported that suppression of Kasha's rule is a strategy to achieve solution-phase ORTP from the high-lying T2 state by spatially separating T2 and T1 on different parts of the molecule (CzCbDBT) composed of carbonyl (Cb), dibenzothiophene (DBT), and carbazole moiety (Cz). On one hand, intersystem crossing (ISC) is much faster from S1 to T2 than that to T1, owing to the small energy-gap ΔES1-T2 and large spin-orbital coupling ξS1-T2. On the other hand, T2 → T1 internal conversion is inhibited owing to spatial separation, i.e., T2 on CbDBT and T1 on Cz, respectively. Also, combination of very fast radiative decay from T2 to S0 owing to large ξT2-S0, the efficient solution-phase ORTP emission from the T2 state was finally achieved.

Photochemical Ring Opening of Oxirane Modeled by Constrained Density Functional Theory.

A constrained density functional theory/classical trajectory surface hopping study of the photochemical dissociation of oxirane (CH2)2O is presented. The calculations confirm the Gomer–Noyes mechanism for the initial reaction and agree largely with experimental photolysis data including reaction yields. The calculated yields, however, depend both on temperature and its modeling. The timescales of the various reaction steps are well below 100 fs, similar to previous time-dependent density functional calculations. At variance with those, however, the present calculations obey Kasha’s rule, i.e., the photoreaction is initiated in the energetically lowest excited state.

Application of two-dimensional correlation fluorescence spectroscopy to detect the presence of trace amount of substances.

Although fluorescence spectroscopy is a highly sensitive method, it is still rather difficult to identify a minor fluorescent component whose fluorescent peak is overlapped and masked by a dominant fluorescent component in a sample solution. Herein, we describe a two-dimensional correlation spectroscopy (2D-COS) approach based on the Kasha's rule to solve the above common problem. We initially suppose that a sample solution contains the major component only, and the spectral behavior of the major component obeys the Kasha's rule. Then, the shapes of emission spectra obtained under excitation lights of different wavelengths remain invariant. Under this condition, the introduction of a minor fluorescent component can be reflected by the changes on the shapes of emission peaks in the series of emission spectra. Moreover, subtle changes, which are difficult to be found in the original spectra, can be clearly visualized as cross peaks in 2D asynchronous spectrum constructed using a series of emission spectra. In addition, we demonstrate that the intensities of cross peaks can be enhanced by changing the sequence of the series of emission spectra. We utilize the approach on an aqueous solution containing eosin Y and a trace amount of bromocresol green. The presence of bromocresol green with the concentration as low as 400nM can be revealed via the cross peaks in the resultant 2D asynchronous spectra. In a preliminary study, we suggest that 2D disrelation spectrum might provide an alternative chance to reveal the presence of small amount bromocresol green.

Multiple‐State Emissions from Neat, Single‐Component Molecular Solids: Suppression of Kasha's Rule

AbstractThree rigid and structurally simple heterocyclic stilbene derivatives, (E)‐3H,3′H‐[1,1′‐biisobenzofuranylidene]‐3,3′‐dione, (E)‐3‐(3‐oxobenzo[c] thiophen‐1(3H)‐ylidene)isobenzofuran‐1(3H)‐one, and (E)‐3H,3′H‐[1,1′‐bibenzo[c] thiophenylidene]‐3,3′‐dione, are found to fluoresce in their neat solid phases, from upper (S2) and lowest (S1) singlet excited states, even at room temperature in air. Photophysical studies, single‐crystal structures, and theoretical calculations indicate that large energy gaps between S2 and S1 states (T2 and T1 states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S2 excited states (phosphorescence from T2 excited states). These results, including unprecedented fluorescence quantum yields (2.3–9.6 %) from the S2 states in the neat solids, establish a unique molecular skeleton for achieving multi‐colored emissions from upper excited states by “suppressing” Kasha's rule.

Multiple-State Emissions from Neat, Single-Component Molecular Solids: Suppression of Kasha's Rule.

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)-3H,3'H-[1,1'-biisobenzofuranylidene]-3,3'-dione, (E)-3-(3-oxobenzo[c] thiophen-1(3H)-ylidene)isobenzofuran-1(3H)-one, and (E)-3H,3'H-[1,1'-bibenzo[c] thiophenylidene]-3,3'-dione, are found to fluoresce in their neat solid phases, from upper (S2 ) and lowest (S1 ) singlet excited states, even at room temperature in air. Photophysical studies, single-crystal structures, and theoretical calculations indicate that large energy gaps between S2 and S1 states (T2 and T1 states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S2 excited states (phosphorescence from T2 excited states). These results, including unprecedented fluorescence quantum yields (2.3-9.6 %) from the S2 states in the neat solids, establish a unique molecular skeleton for achieving multi-colored emissions from upper excited states by "suppressing" Kasha's rule.

Tailoring Hot Exciton Dynamics in 2D Hybrid Perovskites through Cation Modification

We report a family of two-dimensional hybrid perovskites (2DHPs) based on phenethylammonium lead iodide ((PEA)2PbI4) that show complex structure in their low-temperature excitonic absorption and photoluminescence (PL) spectra as well as hot exciton PL. We replace the 2-position (ortho) H on the phenyl group of the PEA cation with F, Cl, or Br to systematically increase the cation's cross-sectional area and mass and study changes in the excitonic structure. These single atom substitutions substantially change the observable number of and spacing between discrete resonances in the excitonic absorption and PL spectra and drastically increase the amount of hot exciton PL that violates Kasha's rule by over an order of magnitude. To fit the progressively larger cations, the inorganic framework distorts and is strained, reducing the Pb-I-Pb bond angles and increasing the 2DHP band gap. Correlation between the 2DHP structure and steady-state and time-resolved spectra suggests the complex structure of resonances arises from one or two manifolds of states, depending on the 2DHP Pb-I-Pb bond angle (as)symmetry, and the resonances within a manifold are regularly spaced with an energy separation that decreases as the mass of the cation increases. The uniform separation between resonances and the dynamics that show excitons can only relax to the next-lowest state are consistent with a vibronic progression caused by a vibrational mode on the cation. These results demonstrate that simple changes to the cation can be used to tailor the properties and dynamics of the confined excitons without directly modifying the inorganic framework.

Semiclassical Approach to Photophysics Beyond Kasha's Rule and Vibronic Spectroscopy Beyond the Condon Approximation. The Case of Azulene.

Azulene is a prototypical molecule with an anomalous fluorescence from the second excited electronic state, thus violating Kasha's rule, and with an emission spectrum that cannot be understood within the Condon approximation. To better understand the photophysics and spectroscopy of azulene and other nonconventional molecules, we developed a systematic, general, and efficient computational approach combining the semiclassical dynamics of nuclei with ab initio electronic structure. First, to analyze the nonadiabatic effects, we complement the standard population dynamics by a rigorous measure of adiabaticity, estimated with the multiple-surface dephasing representation. Second, we propose a new semiclassical method for simulating non-Condon spectra, which combines the extended thawed Gaussian approximation with the efficient single-Hessian approach. S1 ← S0 and S2 ← S0 absorption and S2 → S0 emission spectra of azulene, recorded in a new set of experiments, agree very well with our calculations. We find that accuracy of the evaluated spectra requires the treatment of anharmonicity, Herzberg-Teller, and mode-mixing effects.

A small bimetallic Ag3Cu2 nanocluster with dual emissions within and against Kasha's rule.

Single crystals of a small bimetallic Ag3Cu2 nanocluster protected by six ligands of 2,4-dimethylbenzene thiol are synthesized by a one-pot procedure of wet chemistry. This Ag3Cu2 nanocluster bears a trigonal bipyramid metallic core with two copper atoms located on both sides of a triangular Ag3. Interestingly, the six Cu-Ag side edges of the trigonal bipyramid are fully protected by the six ligands giving rise to reinforced stability and high chemical purity. More interestingly, this Ag3Cu2 cluster shows strong dual fluorescence emissions in both ultraviolet visible (UV-vis) and near infrared (NIR) regions. Theoretical calculations reproduce the absorption and fluorescence spectra where the NIR emission at 824 nm is assigned to the S1→ S0 transition, while the simultaneous emission in the visible band is due to the radiation of highly excited states and is against Kasha's rule.

Restriction of Flip-flop Motion as a Mechanism for Aggregation-Induced Emission.

Although the restriction of intramolecular motion (RIM) has been accepted as a general working mechanism for the aggregation-induced emission (AIE) phenomenon, some new mechanisms, such as suppression of Kasha's rule (SOKR), has also been proposed to explain the AIE of boron difluorohydrazone (BODIHY) derivatives. However, the understanding of the relation and difference between RIM and SOKR mechanisms is limited. To address this issue, we performed a theoretical study on the excited state decay of a series of BODIHY derivatives. Surprisingly, we found that the first excited state of BODIHY derivatives is a bright state and contradicts with the SOKR mechanism. Importantly, we proposed a new mechanism, termed as restriction of flip-flop motion, to explain the AIE of BODIHY derivatives. This mechanism involves the formation of an umbrella-like minimal energy conical intersection through flip-flop motion, which is easily accessible in low-viscosity solvents and will be restricted in high-viscosity solvents.

Multiple Anti-Kasha Emissions in Transition-Metal Complexes.

In this Letter, we present first-principles evidence that several higher-lying excited states are responsible for the emission spectrum of [M(CO)4(bpy)] (M = Cr, Mo, W and bpy = 2,2'-bipyrimidine) complexes. These results highlight the violation of Kasha's rule, which states that after irradiation, molecules emit light with appreciable yield only from their lowest energy excited state. Furthermore, in [W(CO)4(bpy)] and [Mo(CO)4(bpy)], the breaking of Kasha's rule is two-fold because at least two different excited states besides T1 are involved in emission. To our knowledge, these are the first transition-metal complexes unambiguously demonstrated to display simultaneous equilibrated and nonequilibrated anti-Kasha emissions. This work also highlights the complexity of the emissive processes of tetracarbonyl-diimine transition-metal complexes, which are controlled via a subtle interplay of electronic and geometrical effects along the excited-state deactivation dynamics.

A Simple Central Seven-Membered BOPYIN: Synthesis, Structural, Spectroscopic Properties, and Cellular Imaging Application.

A two-step, one-flask synthesis of central seven-membered borondifluoride-3,3-dimethyl-2-[2-(2-pyrrolyl)ethenyl] indole (BOPYIN) ligands has been developed by using the unexplored 3,3-dimethyl-2-[2-(2-pyrrolyl)ethenyl] indole. The simple synthetic approach has enabled modification of the electronic structure by changing the substituents on the indole unit. X-ray analysis indicated that conformations of the seven-membered BF2 complex including BOPYIN and diazaborepin differ from that of the five- and six-membered organoboron complexes. Interestingly, the bond angle of the N⋅⋅⋅B-N bond increases with the number of atoms in the core ring, based on Baeyer strain theory. These unsymmetric BOPYIN derivatives have excellent photophysical properties, including high fluorescence quantum yields, except for BOPYIN-4 in the solution state, large Stokes shifts, and good molar absorptivity. The dipole moment of BOPYIN-3 in the first excited singlet state and ground state was demonstrated by a linear Lippert-Mataga plot. The absorption and emission spectra were not mirror images for BOPYIN-1-3 and 5, in contradiction to Kasha's rule, as determined by TDDFT. The synthesized BOPYINs have been shown to be biocompatible fluorophores in cell bioimaging.

Role of Interface Chemistry in Opening New Radiative Pathways in InP/CdSe Giant Quantum Dots with Blinking‐Suppressed Two‐Color Emission

AbstractInP/CdSe core/thick‐shell “giant” quantum dots (gQDs) that exhibit blinking‐suppressed two‐color excitonic emission have been synthesized and optically characterized. These type II heterostructures exhibit photoluminescence from both a charge‐separated, near‐infrared type II excitonic state, and a shell‐localized visible‐color excitonic state. Infrared emission is intrinsic to the type II QD, while visible emission can either be eliminated or enhanced through chemical modification of the InP surface prior to CdSe shell growth. Single‐QD photoluminescence measurements confirm that the dual color emission is from individual nanocrystals. The probability of observing dual emission from individual QDs and the extent of blinking suppression increases with shell thickness. Visible emission can be stabilized by the addition of a second shell of CdS, where the resulting InP/CdSe/CdS core/shell/shell nanocrystals afford the strongest blinking suppression, determined by analysis of the Mandel Q parameter. Transient absorption spectroscopy verifies that dual emission arises when hole relaxation from the shell to the core is impeded, possibly as a result of enhanced interfacial hole trapping at F− or O2− defect sites. Electron–hole recombination in the shell then competes with slower type II recombination, providing a different mechanism for breaking Kasha's rule and allowing two colors of light to be emitted from one nanostructure.

Measurement of 3-photon excitation and emission spectra and verification of Kasha’s rule for selected fluorescent proteins excited at the 1700-nm window

Fluorescent proteins are widely used to visualize structures and dynamics in various biological samples. Multiphoton microscopy is especially suitable for imaging structures expressing fluorescent proteins with subcellular resolution. 3-photon microscopy (3PM) excited at the 1700-nm window has proven to be promising for deep-tissue (such as brain) imaging expressing red fluorescent proteins. However, the 3-photon excitation and emission spectra of fluorescent proteins suitable at this window remain largely unknown, hampering protein selection and detection optimization. Here we demonstrate detailed measurement of 3-photon excitation and emission spectra for selected fluorescent proteins, suitable for 3-photon excitation at the 1700-nm window. The measured 3-photon excitation spectra will provide guidelines for protein and excitation wavelength selection. The measured 3-photon emission spectra and comparison with the 1-photon emission spectra, on one hand proves that the fundamental Kasha's rule is still valid for 3-photon fluorescence in these fluorescent proteins, on the other hand will be helpful for efficient fluorescence signal detection.

Nonaromatic Amine Containing Exciplex for Thermally Activated Delayed Fluorescent Electroluminescence

AbstractA novel nonaromatic amine containing hole‐transporting material, 3,3′‐(2‐methyl‐1,3‐phenylene)diphenoxathiine (mPXTMP), is developed and coevaporated with electron‐transporting materials to form comixed films. Pure‐blue, sky‐blue, and green thermally activated delayed fluorescent (TADF) exciplex emissions are observed. Though the mPXTMP:2,4,6‐tri([1,1′‐biphenyl]‐3‐yl)‐1,3,5‐triazine (T2T) bimolecular system exhibits a large singlet–triplet splitting energy (ΔEST) of 0.44 eV, its locally triplet excited states play vital roles in the cascade up‐conversion process, leading to an abnormal synergistic enhancement of TADF and phosphorescence from high‐lying triplet states at a low temperature, which violates Kasha's rule. Another bimolecular system, mPXTMP:PO‐T2T, achieves a fast up‐conversion rate of 106 s−1 benefiting from a small ΔEST of 0.10 eV and the enhanced spin–orbital coupling induced by the sulfur element rich environment. The corresponding organic light‐emitting diodes realize an exciton utilization ratio of 61.7–92.6%, verifying that these nonaromatic amine containing exciplex systems can be promising candidates toward purely organic electroluminescence with 100% internal quantum efficiency.