Excited State Emission Research Articles

Current Context of Designing Phototheranostic Cyclometalated Iridium (III) Complexes to Open a New Avenue in Cancer Therapy.

Photo-induced chemotherapy offers the best option for the selective treatment of cancer among all the prevailing modalities. Iridium (III) complexes, flourished with excellent photophysical and photochemical properties, have been considered to be superior for undergoing photo-responsive cancer therapy. Large Stokes shift, long-lived triplet excited state, photostability, and tuneable emission have rendered its excellence as a phototheranostic agent. In particular, the cyclometalated Ir (III) complexes and their respective nanoparticles have made a strong niche in the arena of cancer therapy. In recent years, Ir (III) based complexes have shown promising utilities as both imaging and therapeutic agents as well. Therefore, this review summarises the recent advances in the strategic designing of cyclometalated Ir(III) complexes to augment their phototheranostic applications in precision medicine.

Experimental and Computational Investigation of the Photophysical Properties of a Series of Luminescent D−A−D’ and D−A−A’ Pyrimidines

AbstractPyrimidines, a prominent isomer class of diazines, are promising molecular constituents of electron‐deficient luminescent and hole‐transport materials. Here, we report the fluorescence lifetimes and describe the electronic structures and character of low‐lying emissive excited states of a series of synthetically accessible donor‐acceptor‐donor (D−A−D′) and donor‐acceptor‐acceptor (D−A−A′) type pyrimidines, including both all‐organic and hybrid organic‐organometallic analogues. We find that strategically varying the aryl substituents on a pyrimidine enables tuning of the character of the luminescent excited states from charge‐transfer (CT) to locally excited (LE) states, which then differ in their sensitivity to their environment. This study suggests structural handles for controlling the photophysics of arylpyrimidines relevant to a variety of applications for luminescent materials.

Exploring the interaction between derivatives of urea with resorcinol-based acridinedione dyes by employing fluorescence methods and molecular docking approach

Akyl urea derivatives, either symmetrical or unsymmetrical in nature possessing a free N-H or N-alkyl moieties in the molecular framework of urea governs the excited state properties of acridinedione dye (ADR1) which is a resorcinol-based derivative. Fluorescence enhancement (FE) of ADR1 dye is observed on the addition of unsymmetrical urea derivatives, whereas a decrease in the fluorescence intensity resulted upon the addition of symmetrical urea derivatives. The nature of the urea derivatives controls the suppression of photoinduced electron transfer (PET) via space, which is the cause of the FE. FQ results from the donor and acceptor moieties of ADR1 dye promoting the PET process. Urea (U), symmetrical and unsymmetrical urea derivatives (except Tetramethyl urea (TMU)) results no significant shift of the localized excited (LE) state emission of ADR1 dye. Studies of the ADR1 dye’s fluorescence lifetime in the presence of urea compounds show a significant increase in lifetime. This phenomenon is explained by a significant difference in the hydrogen-bonding (HB) pattern, which causes variationin the microenvironment created by thesolute molecules in the aqueous phase. Both the hydrophobic effects of the alkyl group substituents in the urea molecular framework and the HB interactions between the solute and solvent influence the excited state features of ADR1 dye. Further, the role of urea derivatives and ADR1 dye (both are considered as competitive guest molecules) energetics and molecular interactions were studied in the presence of host molecule, Human Serum Albumin (HSA). The binding energy (BE) and several bimolecular interactions driven by urea derivatives in the presence of ADR1-HSA complex in comparison with that of urea derivatives-HSA complex were investigated by molecular docking (Mol.Doc) methodology. Incorporating theoretical research along withsteady state and time-resolved fluorescence investigations proved to be an effective method to evaluate the interactions of competing solutes comprising bothhydrophobic and HB functional groups.

Interferometric excitation fluorescence lifetime imaging microscopy

Fluorescence lifetime imaging microscopy (FLIM) is a well-established technique with numerous imaging applications. Yet, one of the limitations of FLIM is that it only provides information about the emitting state. Here, we present an extension of FLIM by interferometric measurement of fluorescence excitation spectra. Interferometric Excitation Fluorescence Lifetime Imaging Microscopy (ixFLIM) reports on the correlation of the excitation spectra and emission lifetime, providing the correlation between the ground-state absorption and excited-state emission. As such, it extends the applicability of FLIM and removes some of its limitations. We introduce ixFLIM on progressively more complex systems, directly compare it to standard FLIM, and apply it to quantitative resonance energy transfer imaging from a single measurement.

Fine-Tuning the Microstructure and Photophysical Characteristics of Fluorescent Conjugated Copolymers Using Photoalignment and Liquid-Crystal Ordering.

Replicating the microstructural basis and the near 100% excitation energy transfer efficiency in naturally occurring light-harvesting complexes (LHCs) remains challenging in synthetic energy-harvesting devices. Biological photosynthesis regulates active ensembles of light-absorbing and funneling chlorophylls in proteins in response to fluctuating sunlight. Here, use of long-range liquid crystal (LC) ordering to tailor chain orientation and packing structure in liquid crystalline conjugated polymer (LCCP) layers for bio-mimicry of certain structural basis and light-harvesting properties of LHCs is reported. It is found that long-range orientational ordering in an LC phase of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) copolymer stabilizes a small fraction of randomly-oriented F8BT nanocrystals dispersed in an amorphous matrix of F8BT chains, resembling a self-doped host-guest system whereby excitation energy funneling and photoluminescence quantum efficiencies are enhanced significantly by triggering 3D donor-to-acceptor Förster resonance energy transfer (FRET) and dominant intrachain emission in the nano-crystal acceptor. Further, photoalignment of nematic F8BT layers is combined with LC orientational ordering to fabricate large-area-extended monodomains exhibiting >60% crystallinity and ≈20nm-long interchain packing order. Remarkably, these monodomains demonstrate strong linearly polarized emission, whilst also promoting a new band-edge absorption species and an extra emissive interchain excited state as compared to the non-aligned films.

Electron Transfer Enhanced by a Minimal Energetic Driving Force at the Organic‐Semiconductor Interface

AbstractThe energetic driving force for electron transfer must be minimized to realize efficient optoelectronic devices including organic light‐emitting diodes (OLEDs) and organic photovoltaics (OPVs). Exploring the dynamics of a charge‐transfer (CT) state at an interface leads to a comprehension of the relationship between energetics, electron‐transfer efficiency, and device performance. Here, we investigate the electron transfer from the CT state to the triplet excited state (T1) in upconversion OLEDs with 45 material combinations. By analyzing the CT emission and the singlet excited‐state emission from triplet–triplet annihilation via the dark T1, their energetics and electron‐transfer efficiencies are extracted. We demonstrate that the CT→T1 electron transfer is enhanced by the stronger CT interaction and a minimal energetic driving force (<0.1 eV), which is explained using the Marcus theory with a small reorganization energy of <0.1 eV. Through our analysis, a novel donor–acceptor combination for the OLED is developed and shows an efficient blue emission with an extremely low turn‐on voltage of 1.57 V. This work provides a solution to control interfacial CT states for efficient optoelectronic devices without energy loss.

Electron Transfer Enhanced by a Minimal Energetic Driving Force at the Organic-Semiconductor Interface.

The energetic driving force for electron transfer must be minimized to realize efficient optoelectronic devices including organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). Exploring the dynamics of a charge-transfer (CT) state at an interface leads to a comprehension of the relationship between energetics, electron-transfer efficiency, and device performance. Here, we investigate the electron transfer from the CT state to the triplet excited state (T1) in upconversion OLEDs with 45 material combinations. By analyzing the CT emission and the singlet excited-state emission from triplet-triplet annihilation via the dark T1, their energetics and electron-transfer efficiencies are extracted. We demonstrate that the CT→T1 electron transfer is enhanced by the stronger CT interaction and a minimal energetic driving force (<0.1 eV), which is explained using the Marcus theory with a small reorganization energy of <0.1 eV. Through our analysis, a novel donor-acceptor combination for the OLED is developed and shows an efficient blue emission with an extremely low turn-on voltage of 1.57 V. This work provides a solution to control interfacial CT states for efficient optoelectronic devices without energy loss.

N-Perfluorooctane versus n-Octane: Pyrene Fluorescence to Compare and Contrast Solute Solvation.

Fluorous solvents may offer distinctly different solvation environments to a solute compared to their hydrocarbon analogues due to the inherently high electronegativity associated with fluorine. Solute solvation within n-perfluorooctane (PFO) is compared with that in n-octane using the well-established polycyclic aromatic hydrocarbon (PAH) fluorescence probe pyrene in the temperature range of 288 to 318 K. Both density (ρ) and dynamic viscosity (η) of PFO are considerably higher than those of n-octane. UV-vis molecular absorbance, fluorescence emission/excitation, and excited-state emission intensity decay reveal the cybotactic region of pyrene to be more nonpolar in PFO than that in n-octane. Bimolecular quenching rate constants (kq) for the pyrene-nitrobenzene fluorophore-quencher pair adhere to the Stokes-Einstein formulation; however, they are considerably higher than the estimated rate constants for the diffusion-controlled process (kdiff). This is due to the high electron affinity of nitrobenzene leading to aromatic π-π interactions between pyrene and nitrobenzene. For a nonaromatic low electron affinity quencher, such as nitromethane, while kq < kdiff in n-octane, kq > kdiff in PFO. This is due to the fact that highly electronegative fluorines on PFO stabilize the partial positive charge (δ+) that develops on excited pyrene during electron/charge transfer to the quencher nitromethane, facilitating quenching in the process. Exciplex formation between pyrene and triethylamine (TEA) is more favored in PFO as opposed to n-octane although ηPFO > ηn-octane. The developing charge on the exciplex is stabilized by the electronegative fluorines of the PFO. The pyrene-TEA exciplex appears to form exclusively in the excited state of pyrene, and the kinetics of exciplex formation is in the subnanosecond regime. On the contrary, the efficiency of exciplex formation between pyrene and N,N-dimethylaniline (DMA) is comparable in PFO and n-octane, and the kinetics is slower in comparison to that of the pyrene-TEA exciplex. Certain ground-state heterogeneity is detected for the pyrene-DMA system in PFO due to the low solubilizing ability of the fluorous solvent. Highly electronegative fluorines on perfluorohydrocarbon solvents are found to offer unusual and unique solvation characteristics.

Deep Blue Emitter with a Combination of Hybridized Local and Charge Transfer Excited State and Aggregation-Induced Emission Features for Efficient Non-Doped OLED.

Herein, a deep blue emitter (PI-TPB-CN) with a synergistic effect of hybridized local and charge transfer excited state (HLCT) and aggregation-induced emission (AIE) properties is successfully designed and synthesized to improve the performance of deep blue organic light-emitting diodes (OLEDs). It is constructed using a 1,2,4,5-tetraphenylbenzene (TPB) as an π-conjugated AIE core being asymmetrically functionalized with a phenanthro[9,10-d]imidazole (PI) as a weak donor (D) and a benzonitrile (CN) as an acceptor (A), thereby formulating D-π-A type fluorophore. Its HLCT and AIE properties verified by theoretical calculations, solvatochromic effects, and transient photoluminescence decay experiments, bring about a strong blue emission (452 nm) with a high photoluminescence quantum yield of 74 % in the thin film. PI-TPB-CN is successfully employed as a blue emitter in OLEDs. Non-doped OLED with the structure of ITO/HAT-CN (6 nm)/NPB (30 nm)/TCTA (10 nm)/PI-TPB-CN (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) demonstrates excellent electroluminescence (EL) performance with blue emission (451 nm) and maximum external quantum efficiency (EQEmax) of 7.38 %. The device with a thinner layer of PI-TPB-CN (20 nm) and TPBi (30 nm) exhibits a deeper blue emission (444 nm) with CIE coordinates of (0.156, 0.096), a low turn-on voltage of 3.0 V, and EQEmax of 6.45 %.

Near-infrared luminescent open-shell π-conjugated systems with a bright lowest-energy zwitterionic singlet excited state.

Open-shell systems with extensive π-conjugation have fascinating properties due to their narrow bandgaps and spin interactions. In this work, we report neutral open-shell di- and polyradical conjugated materials exhibiting intriguing optical and magnetic properties. Our key design advance is the planarized geometry allowing for greater interaction between adjacent spins. This results in absorption and emission in the near infrared at 803 and 1050 nanometers, respectively, and we demonstrate a unique electronic structure where a bright zwitterionic excited state is the lowest-accessible electronic transition. Electron paramagnetic resonance spectroscopy and superconducting quantum interference device measurements reveal that our materials are open-shell singlets with different degrees of spin interactions, dynamics, and antiferromagnetic properties, which likely contributed to the formation of their emissive zwitterionic singlet excited state and near-infrared emission. In addition, our materials show reversible and stable electrochromic switching with more than 500 cycles, indicating their potential for optoelectronic and electrochemical energy storage applications.

Photophysical properties of benzo[i]dipyrido[3,2-a:2′,3′-c] phenazine (dppn) – A prospective ligand for light-activated anticancer complexes

Benzo[i]dipyrido[3,2-a:2′,3′c]phenazine (dppn) is a prospective ligand for constructing light-activated anticancer complexes of platinum metals. In spite of practical importance, only scarce information on the dppn photophysics and photochemistry was found in the literature. In this work photophysical properties of dppn in acetonitrile solutions were studied by means of stationary and laser flash photolysis, time-resolved luminescence, time-resolved singlet oxygen detection and ultrafast TA spectroscopy. Quantitative characteristics of luminescence (quantum yield 0.26, lifetime 52 ns in deaerated solutions) were measured. Triplet state of the initial compound 3dppn* is formed with the quantum yield estimated as 0.4 from the singlet oxygen production; the main channels of 3dppn* are quenching by dissolved oxygen and self-quenching by the ground state of dppn. Combining the data of ultrafast TA absorption and time-resolved luminescence, we detected the formation of “dark” (not emissive) excited state of dppn with the decay time ca. 150 ps. Two hypotheses on the nature of the dark state were put forward.

Environmentally sensitive fluorescence of the topical retinoid adapalene.

Intrinsic fluorescence of drugs brings valuable information on their localization in the organism and their interaction with key biomolecules. In this work, we investigate the absorption and emission properties of the topical retinoid adapalene in different solvents and biological media. While the UVA/UVB absorption band does not exhibit any significant solvent-dependent behavior, a strong positive solvatochromism is observed for the emission. These results are in line with molecular modeling and simulations that show the presence of two quasi-degenerate states, i.e., a local π-π* and an intermolecular charge-transfer (ICT) state. However, molecular modeling also revealed that, whatever the solvent, at the corresponding equilibrium geometry the lowest and emissive excited state is the local π-π*. Finally, the potential of adapalene to act as a biological probe is demonstrated using albumin, DNA and micelles.

Efficient chiral hydrogel template based on supramolecular self-assembly driven by chiral carbon dots for circularly polarized luminescence

Since the chiral emission of excited states is observed on carbon dots (CDs), exploration towards the design and synthesis of chiral CDs nanomaterials with circularly polarized luminescence (CPL) properties has been at a brisk pace. In this regard, the “host and guest” co-assembly strategy based on the combination of CDs and chiral templates has been of unique interest recently for its convenient operation, multicolor tunable CPL, and wide application of prepared CDs-composited materials in optoelectronic devices and information encryption. However, the existing chiral templates that match perfectly with chiral CDs exhibiting optical activity both in ground and excited states are rather scarce. In this work, we synthesize the chiral CDs that could induce the spontaneous supramolecular self-assembly of N-(9-fluorenylmethox-ycarbonyl) (Fmoc)-protected glutamic acid to form chiral hydrogels with helical nanostructure. The co-assembled hydrogels show powerful chiral template function, which not only enable chiral CDs with a luminescence dissymmetry factor (glum) up to 10-2, but also have universal chiral transfer to inserted dye molecules, realizing full-color CPL and Förster resonance energy transfer (FRET) CPL as well as the distinction between left and right circularly polarized light. This CPL-active template based on chiral CDs enriches the design scenario of chiral functionalized nanomaterials.

Numerical investigation on nonlinear dynamics of two-state quantum dot laser subject to optical injection

Based on the cascade relaxation model of the quantum dot (QD) lasers, the nonlinear dynamics of a two-state QD laser subject to optical injection is numerically studied. The simulation results show that, for a two-state QD laser operating at the ground-state (GS) and excited-state (ES) emission simultaneously, through adjusting the injection parameters, the two emission states can display the stable (S), injection locking (IL), suppression (SP), period-one (P1), period-two (P2), multi-period (MP), and chaotic pulse (CP) states when the frequency of the injection light is close to the GS lasing frequency. Further mapping of these dynamic states in the parameter space of frequency detuning and injection strength, it is found that the complex dynamic behaviors are mainly distributed in the negative frequency detuning region. In addition, the complexity of the chaotic signal is qualified by the permutation entropy (PE) calculating, and the high complexity signal with PE value over 0.95 is obtained. Moreover, within a certain frequency detuning range, the ES emission can be effectively suppressed by increasing the injection strength.

DPPM-Bridged Binuclear Pt(II) Pincer Complexes: Chemistry, Structure, and Photophysics in Solution Revisited.

A series of luminescent binuclear ([dppm{Pt(NNC)}2]2+) and mononuclear ([PPh3Pt(NNC)]+) complexes containing pincer ligands were synthesized and characterized. Photophysical characteristics of both types of complexes were studied in dichloromethane solution. In the solid phase, the binuclear compounds adopt a syn configuration where the {Pt(NNC)} fragments are held together due to intramolecular Pt-Pt bonding and π-stacking of the pincer ligand aromatic systems. Analysis of the complexes' molecular structure in solution by multinuclear NMR spectroscopy showed that the stacked intramolecular configuration is retained in fluid media, which is in complete agreement with a considerable red shift of the emission wavelength due to formation of the intramolecular Pt-Pt bond, leading to the transformation of an emissive excited state to 3MMLCT. It was also found that triethylamine quenches the emission of both types of complexes; the mechanism of quenching is a combination of dynamic and static channels of excited-state deactivation. In the case of binuclear complexes, deprotonation of the dppm methylene bridge by triethylamine also contributes to the chromophore quenching. To explain the observed chemistry of binuclear complex interactions with Et3N, a chemical equilibrium scheme was suggested, which was confirmed by quantitative monitoring of the 31P signal variations as a function of triethylamine concentration.

Synthesis and Photophysical Properties of Silver(I) Coordination Polymers Bridged by Dimethylpyrazine: Comparison of Emissive Excited States between Silver(I) and Copper(I) Congeners.

Highly luminescent silver(I) coordination polymers [Ag2X2(PPh3)2(Me2pyz)]n (X = I, Br, Cl; Me2pyz: 2,5-dimethylpyrazine) were prepared together with copper congeners [Cu2X2(PPh3)2(Me2pyz)]n (X = I, Br). All the complexes showed thermally activated delayed fluorescence from the charge-transfer states in the visible region, from blue to red. The isomorphous relationship among the complexes allowed a detailed discussion of the effect of halogenido ligands and crystal packing on their luminescence energy. The relaxation in the emissive excited states (ESs) was determined to be more remarkable in silver complexes than in copper complexes despite their isomorphous structures, and the electronic effect of halogenido ligands was comparable to the effect of relaxation in emissive ESs.

Single-molecule pursuit of daylight from multiple excited states emission

Single-molecule pursuit of daylight from multiple excited states emission

Photoluminescence of 3- and 4-coordinate platinum(0) phosphine complexes

Abstract Six platinum complexes bearing monodentate phosphine ligands were prepared and their structures and photophysical properties examined. The complexes were photoluminescent in the solid state, with a maximum quantum yield of 72%. The tris(phosphine) platinum complexes displayed more red-shifted emissions than the tetrakis complexes. The nature of the emissive excited states is discussed based on the results of density functional theory and time-dependent density functional theory calculations.

Room-Temperature Phosphorescence Materials Featuring Triplet Hybrid Local Charge Transfer Emission.

Room-temperature phosphorescence materials have found important applications in dissolved oxygen sensing, temperature monitoring, anticounterfeiting, etc., because of their prolonged phosphorescence lifetime. However, the known systems mainly utilize the triplet local excited state emission, which is generally less sensitive to microenvironment perturbation. In this work, we designed a series of 4-phenyl-1,8-naphthalimide (NMI) derivatives containing different numbers of carbazole (Cz) units (denoted as NMI-Cz, NMI-2Cz, and NMI-3Cz). Steady state and time-resolved spectroscopy studies determined that the compounds undergo intramolecular through-space charge transfer in solution, yielding a triplet hybrid local charge transfer state. Room-temperature phosphorescence emission was observed in compound-doped poly(methyl methacrylate) thin films upon ammonia treatment. Interestingly, emission from different films exhibited different persistence times. We believe a film-based, time-resolved luminescent ammonia sensor could be developed by making a device of the emissive films as fabricated.

Multicolor photoluminescence of Cu14 clusters modulated using surface ligands.

Copper nanoclusters exhibit unique structural features and their molecular assembly results in diverse photoluminescence properties. In this study, we present ligand-dependent multicolor luminescence observed in a Cu14 cluster, primarily protected by ortho-carborane-9,12-dithiol (o-CBDT), featuring an octahedral Cu6 inner kernel enveloped by eight isolated copper atoms. The outer layer of the metal kernel consists of six bidentate o-CBDT ligands, in which carborane backbones are connected through μ3-sulphide linkages. The initially prepared Cu14 cluster, solely protected by six o-CBDT ligands, did not crystallize in its native form. However, in the presence of N,N-dimethylformamide (DMF), the cluster crystallized along with six DMF molecules. Single-crystal X-ray diffraction (SCXRD) revealed that the DMF molecules were directly coordinated to six of the eight capping Cu atoms, while oxygen atoms were bound to the two remaining Cu apices in antipodal positions. Efficient tailoring of the cluster surface with DMF shifted its luminescence from yellow to bright red. Luminescence decay profiles showed fluorescence emission for these clusters, originating from the singlet states. Additionally, we synthesized microcrystalline fibers with a one-dimensional assembly of DMF-appended Cu14 clusters and bidentate DPPE linkers. These fibers exhibited bright greenish-yellow phosphorescence emission, originating from the triplet state, indicating the drastic surface tailoring effect of secondary ligands. Theoretical calculations provided insights into the electronic energy levels and associated electronic transitions for these clusters. This work demonstrated dynamic tuning of the emissive excited states of copper nanoclusters through the efficient engineering of ligands.