Fluorescence and phosphorescence from higher excited states of organic molecules

Aggregation-Induced Emission: New Vistas at the Aggregate Level

Aggregation-induced emission (AIE) describes a photophysical phenomenon in which molecular aggregates exhibit stronger emission than the single molecules. Over the course of the last 20 years, AIE research has made great strides in material development, mechanistic study and high-tech applications. The achievements of AIE research demonstrate that molecular aggregates show many properties and functions that are absent in molecular species. In this review, we summarize the advances in the field of AIE and its related areas. We specifically focus on the new properties of materials attained by molecular aggregates beyond the microscopic molecular level. We hope this review will inspire more research into molecular ensembles at and beyond the meso level and lead to the significant progress in material and biological science.

Substituent Effect in the First Excited Triplet State of Monosubstituted Benzenes

The structure of 30 monosubstituted benzenes in the first excited triplet T₁ state was optimized with both unrestricted (U) and restricted open shell (RO) approximations combined with the ωB97XD/aug-cc-pVTZ basis method. The substituents exhibited diverse σ- and π-electron-donating and/or -withdrawing groups. Two different positions of the substituents are observed in the studied compounds in the T₁ state: one distorted from the plane and the other coplanar with a quinoidal ring. The majority of the substituents are π-electron donating in the first group while π-electron withdrawing in the second one. Basically, U- and RO-ωB97XD approximations yield concordant results except for the B-substituents and a few of the planar groups. In the T₁ state, the studied molecules are not aromatic, yet aromaticity estimated using the HOMA (harmonic oscillator model of aromaticity) index increases from ca. -0.2 to ca. 0.4 with substituent distortion, while in the S₁ state, they are only slightly less aromatic than in the ground state (HOMA ≈0.8 vs ≈1.0, respectively). Unexpectedly, the sEDA(T₁) and pEDA(T₁) substituent effect descriptors do not correlate with analogous parameters for the ground and first excited singlet states. This is because in the T₁ state, the geometry of the ring changes dramatically and the sEDA(T₁) and pEDA(T₁) descriptors do not characterize only the functional group but the entire molecule. Thus, they cannot provide useful scales for the substituents in the T₁ states. We found that the spin density in the T₁ states is accumulated at the C_ipso and C_p atoms, and with the substituent deformation angle, it nonlinearly increases at the former while decreases at the latter. It appeared that the gap between singly unoccupied molecular orbital and singly occupied molecular orbital (SUMO-SOMO) is determined by the change of the SOMO energy because the former is essentially constant. For the nonplanar structures, SOMO correlates with the torsion angle of the substituent and the ground-state pEDA(S₀) descriptor of the π-electron-donating substituents ranging from 0.02 to 0.2 e. Finally, shapes of the SOMO-1 instead of SOMO frontier orbitals in the T₁ state somehow resemble the highest occupied molecular orbital ones of the S₀ and S₁ states. For several planar systems, the shape of the U- and RO-density functional theory-calculated SOMO-1 orbitals differs substantially.

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.

Sensitive distance-based paper-based quantification of mercury ions using carbon nanodots and heating-based preconcentration

This article reports the first fluorescent distance-based paper device coupled with an evaporating preconcentration system for determining trace mercury ions (Hg2+) in water. The fluorescent nitrogen-doped carbon dots (NCDs) were synthesized by a one-step microwave method using citric acid and ethylenediamine. The fluorescence turn-off of the NCDs in the presence of Hg2+ was visualized with a common black light, and the distance of the quenched fluorescence correlated to Hg2+ concentration. The optimal conditions for pH, NCD concentration, sample volume, and reaction time were investigated. Heating preconcentration was used to improve the detection limits of the fluorescent distance-based paper device by a factor of 100. Under the optimal conditions, the naked eye limit of detection (LOD) was 5 μg L-1 Hg2+. This LOD is sufficient for monitoring drinking water where the maximum allowable mercury level is 6 μg L-1 as established by the World Health Organization (WHO). The fluorescent distance-based paper device was successfully applied for Hg2+ quantification in water samples without interference from other cations. The proposed method provides several advantages over atomic absorption spectroscopy including ease of use, inexpensive material and fabrication, and portability. In addition, the devices are simple to fabricate and have a long shelf-life (>5 months).

Aggregation-Induced Intersystem Crossing: Rational Design for Phosphorescence Manipulation

Phosphorescence of organic molecules has drawn extensive attention due to its potential applications in energy and life science. However, typically intersystem crossing (ISC) in organic molecules is slow due to the small spin-orbit couplings (SOC) and large energy gaps (ΔE_S-T) between different multiplicities. Molecular aggregation offers a practical strategy to manipulate phosphorescent characteristics. In this work, the impact of aggregation on the luminescence properties of π-conjugated benzophenone luminophore 1-dibenzo[b,d]thiophen-2-yl(phenyl)methanone (BDBT) are investigated theoretically using density functional theory (DFT) and time-dependent DFT. Molecular aggregation results in substantial energy splitting and variation of SOC, eventually changing the ISC rate. This is known as the "aggregation-induced intersystem crossing" (AI-ISC) mechanism. Different types of electron donating and withdrawing functional groups are further introduced into BDBT molecular system to tailor the phosphorescent efficiency. We find that functional groups can influence the SOC and energy gaps and further manipulate the phosphorescence efficiency. Molecular systems with donating functional groups have faster ISC rates, and dimers exhibit the best electronic luminescence due to the relatively large SOC and small ΔE_S-T. The AI-ISC mechanism accompanied by group functionalization provides a practical platform for phosphorescence enhancement.

De novo strategy with engineering anti-Kasha/Kasha fluorophores enables reliable ratiometric quantification of biomolecules

Fluorescence-based technologies have revolutionized in vivo monitoring of biomolecules. However, significant technical hurdles in both probe chemistry and complex cellular environments have limited the accuracy of quantifying these biomolecules. Herein, we report a generalizable engineering strategy for dual-emission anti-Kasha-active fluorophores, which combine an integrated fluorescein with chromene (IFC) building block with donor-π-acceptor structural modification. These fluorophores exhibit an invariant near-infrared Kasha emission from the S1 state, while their anti-Kasha emission from the S2 state at around 520 nm can be finely regulated via a spirolactone open/closed switch. We introduce bio-recognition moieties to IFC structures, and demonstrate ratiometric quantification of cysteine and glutathione in living cells and animals, using the ratio (S2/S1) with the S1 emission as a reliable internal reference signal. This de novo strategy of tuning anti-Kasha-active properties expands the in vivo ratiometric quantification toolbox for highly accurate analysis in both basic life science research and clinical applications.

Direct synthesis of 2-arylazulenes by [8+2] cycloaddition of 2H-cyclohepta[b]furan-2-ones with silyl enol ethers

We developed a procedure for the direct synthesis of 2-arylazulenes, which were obtained in moderate to excellent yields, by [8+2] cycloaddition of 2H-cyclohepta[b]furan-2-ones with aryl-substituted silyl enol ethers. The structures of some 2-arylazulenes were clarified by single-crystal X-ray analysis. The 2-phenylazulene derivatives obtained by this study showed noticeable fluorescence in acidic media.

Direct excitation of higher excited state and kinetics of photoreactions

Excited-state reactions (ESR) play an essential role in chemical, physical, and biological processes. The mathematical models are usually used to study ESR in kinetics and steady-state regimes. In these models, the excitation pulse populates the first excited state (the first singlet level) of the primary molecular form. Recently, researchers' paid growing attention to the reactions excited via the higher energy levels. We modeled these reactions using the system of linear differential equations. Exact analytical expressions of the kinetics of N* and P* populations were derived for the general case when excitation performed via the higher S_n singlet state by the delta pulse. The graphical forms of these expressions were N and P time-dependent pulses. We detected the changes of the pulses' shapes, their maxima locations, the time behavior of the populations, and the total yield of the P* population. The changes occur due to the populating of the product excited state in the kinetic and thermodynamic reaction regimes. Numerical analysis performed for different ESR parameters revealed peculiarities of the N* and P* populations. Kinetics properties of these population characterize systems with varying rates of reversible ESR and various contributions of anti-Kasha (AK) reaction (from the S_n state) to P* population. Modeling data presented in graphical form, allowed to understand better (a) the impact of the AK reaction on the kinetic properties of the excited states of the molecular systems operating in various mode of ESR (kinetic, reversible and intermediate); (b) the photochemical processes' mechanisms. Also, this modeling allowed establishing the criteria for revealing the effect of the AK reaction for improving the efficiency of anti-Kasha processes.

Manipulating the Ultralong Organic Phosphorescence of Small Molecular Crystals

Ultralong organic phosphorescence (UOP) of metal-free organic materials has received considerable attention recently owing to their long-lived emission lifetimes, and the fact that they present an attractive alternative to persistent luminescence in inorganic phosphors. Enormous research effort has been devoted on improving UOP performance in metal-free organic phosphors by promoting the intersystem crossing (ISC) process and suppressing the non-radiative decay of triplet state excitons. This minireview summarizes the recent advances in the rational approaches for manipulating the UOP properties of small molecular crystals, such as phosphorescence lifetime, efficiency, and emission colors. Finally, the present challenges and future development of this field are proposed. This review will provide a guideline to rationally design more advanced metal-free organic phosphorescence materials for potential applications.

Synthesis of phthalimides cross-conjugated with an azulene ring, and their structural, optical and electrochemical properties

The preparation of phthalimides cross-conjugated with an azulene ring was established by a one-pot Diels-Alder reaction of the corresponding 2-aminofuran derivatives with several maleimides, without the isolation of the intermediately formed [4 + 2] cycloadducts. The structure, optical and electrochemical properties of the novel phthalimide derivatives were clarified by single-crystal X-ray analysis, UV/Vis and fluorescence spectra, spectroelectrochemistry and voltammetry experiments, and theoretical calculations. These results indicated that the substituents on the azulene ring greatly affect the optical and electrochemical properties of the molecules.

Annulation of imidazo[1,2-a]pyridines under metal-free conditions

Synthesis of benzo[a]imidazo[5,1,2-cd]indolizines from 2-arylimidazo[1,2-a]pyridines and benzyne precursors under metal-free conditions has been described.

Design of superior phototheranostic agents guided by Jablonski diagrams

This review summarizes how Jablonski diagrams guide the design of advanced organic optical agents and improvement of disease phototheranostic efficacies.

Azulene-Derived Fluorescent Probe for Bioimaging: Detection of Reactive Oxygen and Nitrogen Species by Two-Photon Microscopy

Two-photon fluorescence microscopy has become an indispensable technique for cellular imaging. Whereas most two-photon fluorescent probes rely on well-known fluorophores, here we report a new fluorophore for bioimaging, namely azulene. A chemodosimeter, comprising a boronate ester receptor motif conjugated to an appropriately substituted azulene, is shown to be an effective two-photon fluorescent probe for reactive oxygen species, showing good cell penetration, high selectivity for peroxynitrite, no cytotoxicity, and excellent photostability.

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.

Can Coumarins Break Kasha's Rule?

Coumarin C-2 was reported ( Signore et al., J. Am. Chem. Soc. , 2010 , 132 , 1276 and Brancato et al., J. Phys. Chem. B , 2015 , 119 , 6144 ) to break Kasha's rule. However, the two lowest excited singlet states of C-2 are separated by less than 0.5 eV. To slow down the S₂ → S₁ internal conversion and thus to enable the Kasha's rule-breaking S₂ fluorescence, a much larger energy separation seems to be necessary. Thus, the photophysical behavior reported for C-2 raised very basic questions concerning mechanisms of nonradiative transitions in organic molecules. Herein we reinvestigated luminescence of C-2 and found that thoroughly purified C-2 does not show any dual fluorescence in steady-state experiments, contrary to the previous findings. The higher-energy emission, previously erroneously assigned as S₂ → S₀ fluorescence of C-2, stems from persistent impurity of the synthetic precursor (C-1).

Barbier Hyperbranching Polymerization-Induced Emission toward Facile Fabrication of White Light-Emitting Diode and Light-Harvesting Film

Luminescent polymers are generally constructed through polymerization of luminescent moieties. Polymerization itself, however, is mainly used for constructing polymer main chain, and the importance of polymerization on luminescence has yet to be explored. Here, we demonstrate a polymerization-induced emission strategy producing luminescent polymers by introducing Barbier reaction to hyperbranching polymerization, which allows luminescent properties to be easily tuned from the traditional type to an aggregation-induced emission type by simply adjusting the monomer structure and the polymerization time. When rotation about the phenyl groups in hyperbranched polytriphenylmethanols (HPTPMs) is hindered, HPTPMs exhibit traditional emission property. When all phenyl groups of HPTPM are rotatable, i.e., p,p',p″-HPTPM, it exhibits interesting aggregation-induced emission property with tunable emission colors from blue to yellow, by just adjusting polymerization time. Further applications of aggregation-induced emission type luminescent polymers are illustrated by the facile fabrication of white light-emitting diode (LED) and light-harvesting film with an antenna effect >14. This Barbier hyperbranching polymerization-induced emission provides a new strategy for the design of luminescent polymers and expands the methodology and functionality library of both hyperbranching polymerization and luminescent polymers.

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.

PIMT-Mediated Protein Repair: Mechanism and Implications

Amino acids undergo many covalent modifications, but only few amino acid repair enzymes have been identified. Protein-L-isoaspartate (D-aspartate) O-methyltransferase (PIMT), also known as L-isoaspartyl/D-aspartyl protein carboxyl methyltransferase (PCMT), methylates covalently modified isoaspartate (isoAsp) residues accumulated in proteins via Asn deamidation and Asp hydrolysis. This cytoplasmic reaction occurs through the formation of succinimide cyclical intermediate and generates either isoAsp or Asp from succinimide. Succinimide conversion into Asp is spontaneous, while isoAsp is restored by PIMT using S-adenosylmethionine as a methyl donor. PIMT transforms isoAsp into succinimide, thereby creating an opportunity for the later to be converted into Asp. Apart from normal cell physiology, formation of isoAsp in proteins is promoted by various stress conditions. The resulting isoAsp can form a kink or bend in the protein backbone thus making the protein conformationally and functionally distorted. Many PIMT-interacting proteins (proteins with isoAsp residues) have been reported in eukaryotes, but only few of them have been found in prokaryotes. Extensive studies in mice have shown the importance of PIMT in neurodegeneration. Detail elucidation of PIMT function can create a platform for addressing various disorders such as Alzheimer's disease and cancer.

Enhancing the performance of pure organic room-temperature phosphorescent luminophores

Once considered the exclusive property of metal complexes, the phenomenon of room-temperature phosphorescence (RTP) has been increasingly realized in pure organic luminophores recently. Using precise molecular design and synthetic approaches to modulate their weak spin-orbit coupling, highly active triplet excitons, and ultrafast deactivation, organic luminophores can be endowed with long-lived and bright RTP characteristics. This has sparked intense explorations into organic luminophores with enhanced RTP features for different applications. This Review discusses the fundamental mechanism of RTP in pure organic luminophores, followed by design principles, enhancement strategies, and formulation methods to achieve highly phosphorescent and long-lived organic RTP luminophores even in aqueous media. The current challenges and future directions of this field are also discussed in the summary and outlook.

Strong Duschinsky Mixing Induced Breakdown of Kasha's Rule in an Organic Phosphor

We present the novel observation that Duschinsky mixings can lead to the breakdown of Kasha's rule in a white light phosphor molecule, dibenzo[ b, d]thiophen-2-yl (4-chlorophenyl)methanone. Our theoretical analyses show the energy gap between the T₁ and T₂ states (0.48 eV) is too large to allow for any significant population of the T₂ state at room temperature and instead the faster intersystem crossing (ISC) between the S₁ and T₂ states is rather due to strong Duschinsky mixing, leading to the emission from the T₂ state as well. A second-order cumulant-based method has been used for the calculation of the ISC rate, which suggests 2 orders of magnitude faster ISC rates for S₁ → T₂ compared to those for S₁ → T₁. We found that the carbonyl moiety of the S₁ and T₂ states of the molecule is significantly different with respect to bond angle and dihedral angles, engendering large displacements in selective normal modes, thus giving rise to strong Duschinsky mixing.

Semiempirical study on the absorption spectra of the coronene-like molecular models of graphene quantum dots

Polycyclic aromatic hydrocarbons of the general formula C_6n²H_6n (coronene family) were used as molecular models of graphene quantum dots (GQDs). Absorption spectra of the model compounds were calculated by ZINDO/S method. The S₀ → S₁ transition energy (E₁) was found to decrease with n as E₁ = 4.75 × n^-0.633 eV. This transition is forbidden in symmetric compounds but 'switches on' upon symmetry breaking. The energy of the first bright optical peak (E_br) was found to decrease with n as E_br = 6.31 × n^-0.6 eV. The data obtained corroborate the earlier finding that the size-independent optical properties of GQDs are determined by relatively small isolated sp² clusters separated by sp³ (oxygen-contained) 'defects' rather than the whole (corrupted) graphene sheets; such nanoparticles actually are not quantum dots. GQDs of pure (without defects) graphene sheets with fully π-conjugated sp² systems should exhibit size-dependent optical properties due to the quantum confinement effect.

Kasha's rule: a reappraisal

This work revises some anomalous cases reported in the literature, which seemingly violate Kasha's rule. To the contrary, apart from azulene, the remaining molecules fulfill Kasha's rule. Kasha's rule must be stated just as in the seminal paper (M. Kasha, Discuss. Faraday Soc., 1950, 9, 14-19): "The emitting electronic level of a given multiplicity is the lowest excited level of that multiplicity". Therefore, Kasha's rule focuses on the emission (photophysics) for complex molecules, in condensed phase, for the absorption of one photon per molecule under photostationary conditions, then a rapid internal conversion and a vibrational relaxation warrant that the corresponding emission comes from the first excited electronic level regardless of which electronic state of equal multiplicity is excited.

Assessing the accuracy of simplified coupled cluster methods for electronic excited states in f0 actinide compounds

We scrutinize the performance of different variants of equation of motion coupled cluster (EOM-CC) methods to predict electronic excitation energies and excited state potential energy surfaces in closed-shell actinide species.

White-light emission from a structurally simple hydrazone

Two hydrazones featuring a unique excitation wavelength-dependent dual fluorescence emission have been developed.

Modulating Photo- and Electroluminescence in a Stimuli-Responsive π-Conjugated Donor-Acceptor Molecular System

Functional organic materials that display reversible changes in fluorescence in response to external stimuli are of immense interest owing to their potential applications in sensors, probes, and security links. While earlier studies mainly focused on changes in photoluminescence (PL) color in response to external stimuli, stimuli-responsive electroluminescence (EL) has not yet been explored for color-tunable emitters in organic light-emitting diodes (OLEDs). Here a stimuli-responsive fluorophoric molecular system is reported that is capable of switching its emission color between green and orange in the solid state upon grinding, heating, and exposure to chemical vapor. A mechanistic study combining X-ray diffraction analysis and quantum chemical calculations reveals that the tunable green/orange emissions originate from the fluorophore's alternating excited-state conformers formed in the crystalline and amorphous phases. By taking advantage of this stimuli-responsive fluorescence behavior, two-color emissive OLEDs were produced using the same fluorophore in different solid phases.

Fluorescence of Nonaromatic Organic Systems and Room Temperature Phosphorescence of Organic Luminogens: The Intrinsic Principle and Recent Progress

Following the development of photoluminescence systems with various compositions, some nontraditional structures, including nonaromatic organic systems as fluorophores and organic luminogens as the source of phosphorescence emission at room temperature, have attracted considerable attention for their advantages in biological and medical applications, and for the updated photophysical understandings in science. In this Review, the recent progress in understanding these organic compounds or polymers for fluorescence and phosphorescence is briefly summarized, with the aim of exploring the intrinsic principle of these novel photoluminescence systems and providing reasonable constructs for molecular design. Finally, some prospects are suggested for further development of this continually expanding area of research, with the coined concept of Molecular Uniting Set Identified Characteristic (MUSIC).

Existence of Two Fluorescence Bands in all- trans-Polyenes with Six and Seven Double Bonds

From the analysis of UV/vis absorption, emission, and excitation of emission spectra of all- trans-α,ω-tetra- tert-butylpolyenes with six and seven double-bonds (3,14-di( tert-butyl)-2,2,15,15-tetramethyl-3,5,7,9,11,13-hexadecahexaene (ttbP6), and 3,16-di( tert-butyl)-2,2,17,17-tetramethyl-3,5,7,9,11,13,15-octadecaheptaene (ttbP7)) it is concluded that these hydrocarbons do not exhibit dual S₁/S₂ fluorescence, as could be inferred from the work of Christensen et al. [ J. Phys. Chem. A 2008 , 112 , 12629 - 12636 ]. The spectral data for ttbP6 and ttbP7 display a unique fluorescence from their S₁ states.