Solvatochromism and Nonradiative Decay of Intramolecular Charge-Transfer Excited States: Bands-of-Energy Model, Thermodynamics, and Self-Organization

The magic of biaryl linkers: the electronic coupling through them defines the propensity for excited-state symmetry breaking in quadrupolar acceptor-donor-acceptor fluorophores

Charge transfer (CT) is key for molecular photonics, governing the optical properties of chromophores comprising electron-rich and electron-deficient components. In photoexcited dyes with an acceptor-donor-acceptor or donor-acceptor-donor architecture, CT breaks their quadrupolar symmetry and yields dipolar structures manifesting pronounced solvatochromism. Herein, we explore the effects of electronic coupling through biaryl linkers on the excited-state symmetry breaking of such hybrid dyes composed of an electron-rich core, i.e., 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP), and pyrene substituents that can act as electron acceptors. Experimental and theoretical studies reveal that strengthening the donor-acceptor electronic coupling decreases the CT rates and the propensity for symmetry breaking. We ascribe this unexpected result to effects of electronic coupling on the CT thermodynamics, which in its turn affects the CT kinetics. In cases of intermediate electronic coupling, the pyrene-DHPP conjugates produce fluorescence spectra, spreading over the whole visible range, that in addition to the broad CT emission, show bands from the radiative deactivation of the locally excited states of the donor and the acceptors. Because the radiative deactivation of the low-lying CT states is distinctly slow, fluorescence from upper locally excited states emerge leading to the observed anti-Kasha behaviour. As a result, these dyes exhibit white fluorescence. In addition to demonstrating the multifaceted nature of the effects of electronic coupling on CT dynamics, these chromophores can act as broad-band light sources with practical importance for imaging and photonics.

Photoinduced charge transfer in push-pull pyrazoline-based chromophores - Relationship between molecular structure and photophysical, photovoltaic properties

The manuscript describes the effect of molecular structure on the photophysical and photovoltaic properties of the pyrazoline-based donor-branched-π-system-acceptor compounds decorated with two end groups: phenyl or thiophene. Although the absorption to the first singlet excited state is strongly allowed, the emission quantum yield is low in all studied solvents. This behaviour was explained by the existence of two non-radiative deactivation channels: the back electron transfer process, especially operated in polar solvents, and internal conversion realized as the rotation of flexible rotors (cyano, keto phenyl or thiophene). The feasibility of the photoinduced electron transfer process was corroborated by electrochemical, spectroelectrochemical measurements as well as DFT calculations. DFT calculations also support the existence of multiple conformations in the ground state, which differ from one another in terms of charge distribution and the values of ground state dipole moment. Finally, the mechanism of the singlet excited state deactivation of the studied compounds was determined by ultrafast pump-probe measurements. Our studies revealed that charge/electron transfer process may undergo over carbonyl bridge, included in branched π-system. Moreover, the thiophene decorated pyrazoline is characterized by a better photovoltaic power conversion efficiency, while the phenyl-ended pyrazoline can be applied as a viscosity sensor.

Expeditious fluorimetric detection of bilirubin by simple imidazole derived luminophore and it's pragmatic applicability in spiked biological fluids

Bilirubin is an indispensable biomarker for liver diseases. Utilizing organic molecules as sensor platform for effective detection of bilirubin are little. In addition, the reported fluorophores required longer incubation time for detection. Hence, herein we have attempted to design an imidazole derivative 4-(3H-imidazo[4,5-b]pyridin-2-yl)-N,N-diphenylaniline (IMI) from triphenylamine and pyridine units which could detect bilirubin swiftly without any incubation period. IMI manifested an instant quenching of emission in presence of bilirubin with limit of detection (LOD) 11.74 × 10^-6 mol L^-1. The mechanistic aspect of detection involves coexistence of both static and dynamic quenching which was suitably justified. Finally, the pragmatic application of IMI was performed in bio-fluids.

Polymer Electrochemiluminescence Featuring Thermally Activated Delayed Fluorescence

Electrochemiluminescence (ECL) based on conjugated polymers or oligomers is persistently being pursued owing to its huge application scope ranging from ultra-sensitive bioanalysis to ultra-resolution imaging and spectroscopy. Because of the theoretical limit in radiative exciton generation yield (typically ∼25 %) of those polymers or oligomers, the corresponding ECL efficiency is still limited, which hampers its ECL performance and its related applications. Herein, we report ECL based on a thermally activated delayed fluorescence (TADF) polymer scaffold, which is characteristic of all-exciton harvesting in the ECL process, and thus potentially capable of achieving ∼100 % ECL efficiency. These desired properties of the TADF polymer ECL is attributed to a fast and efficient up-conversion process from non-radiative triplet to radiative singlet states under thermal activation, which is absent in conventional fluorescent polymers/oligomers, such as F8BT. In this study, various ECL modes, including annihilation or co-reactant mode using TPrA or S₂ O₈ ^2- as co-reactant, are confirmed for our model TADF polymer ECL system, which was different from fluorescent polymer ECL counterpart. Furthermore, solid-state ECL sensing on L-cysteine (an important marker of disease) is also evaluated by using the model TADF polymer. Ultralow detection limit in combination with high sensitivity and good specificity are achieved for this model system, indicative of a high potential of the TADF polymer scaffold for applications in the broad field of ECL.

Structure-Property Relationship in Amber Color Light-Emitting Electrochemical Cell with TFSI Counteranion: Enhancing Device Performance by Different Substituents on N∧N Ligand

Amber color emitting novel Ir(III) complexes were synthesized: [Ir(Meppy)₂(Fpbpy)][PF₆] (1bPF₆), [Ir(Meppy)₂(Fpbpy)][TFSI] (1bTFSI), [Ir(Meppy)₂(MeObpy)][PF₆] (2bPF₆) and [Ir(Meppy)₂(MeObpy)][TFSI] (2bTFSI), where Meppy = 2-(p-methylphenyl)-pyridine (b), Fpbpy = 4,4'-bis(4-fluorophenyl)-2,2'-bipyridine (1), and MeObpy = 4,4'-bis(4-methoxy)-2,2'-bipyridine (2). The photophysical and X-ray results showed that the complexes have aggregation-induced phosphorescent emission (AIPE) and a salt-induced polymorphism effect. The highest photoluminescence intensity was observed in complex 2bTFSI compared to other complexes in the solid state. Their theoretical absorption and phosphorescence emission transitions in acetonitrile were also investigated by using double- and triple-ζ basis sets with B3LYP and PBE0 hybrid functional. The best light-emitting electrochemical cell (LEC) performance was exhibited by complex 2bTFSI, and the data obtained were as follows: Luminance, current density, luminous efficiency, turn-on time, power efficiency, and external quantum efficiency were measured as 16 156 cd/m², 554 mA/cm², 8.49 cd/A, 17 s, 3.95 lm/W and 6.37%, respectively. The investigation of crystallographic characteristics have shown that the LEC performance of these complexes depends on cationic-anionic interaction which has a significant influence on molecular stacking of the molecules. Because, complex 2bTFSI, with weak cationic-anionic interactions, shows strong π···π stacking interactions between the adjacent molecules, it is the best lighting application candidate among the complexes.

Simultaneous 3D surface profile and pressure measurement using phase-shift profilometry and pressure-sensitive paint

A simultaneous three-dimensional (3D) surface profile and pressure measurement method that integrates phase-shift profilometry and pressure-sensitive paint (PSP²) is proposed. The advantages of this novel technique over previous 3D pressure-sensitive paint (3D-PSP) techniques include a simplified system with low cost, no interference with PSP coatings, high spatial resolution, and high accuracy. A modified digital light-processing (DLP) projector-structured light generator is used to encode ultraviolet light and generate fringe projection to excite the pressure-sensitive paint. The 3D profile is reconstructed using four phase-shifting emission images. Meanwhile, the surface intensity ratio distribution is obtained. The PSP² method is applied to a nitrogen jet impingement experiment onto a spherical model. The intensity ratio results obtained using the PSP² method differ little from the conventional PSP results obtained using uniform excitation. The phase distortion due to the emission intensity fluctuation leads to errors in surface profile measurement, and the fringe projection with high contrast improves surface profile measurement accuracy. In most of the final results, the average total errors between the reconstructed 3D surface and the CAD geometry are less than 0.1 mm.

The Journey to Precious-Metal-Free Organic Phosphors from Single-Benzene-Cored Fluorophores

This article takes a look back over our research on the development of organic fluorophores that efficiently emit light in the solid state and precious-metal-free phosphors that emit light at room temperature. In particular, we place an emphasis on the prehistory of each project and the relationship between the established molecular designs. Our story starts from the serendipitous discovery of a luminophore with a single benzene core and follows the molecular design and characterization of 2,5-dipiperidyl-1,4-bis(acceptor-substituted ethenyl)benzenes that exhibit solid-state fluorescence with high-to-excellent quantum yields in the blue-to-red region. In addition, the design concepts, luminescence characteristics, and applications of eight novel classes of fluorophores are described, and the discovery, design, and luminescent properties of precious-metal-free compounds that show efficient room-temperature phosphorescence are presented.

Efficient Emission of Ultraviolet Light by Solid State Organic Fluorophores: Synthesis and Characterization of 1,4-Dialkeny-2,5-dioxybenzenes

The design and development of organic luminophores that exhibit efficient ultraviolet (UV) fluorescence in the solid state remains underexplored. Here, we report that 1,4-dialkenyl-2,5-dialkoxybenzenes and 1,4-dialkenyl-2,5-disiloxybenzenes act as such UV-emissive fluorophores. The dialkenyldioxybenzenes were readily prepared in three steps from 2,5-dimethoxy-1,4-diacetylbenzene or 2,5-dimethoxy-1,4-diformylbenzene via two to four steps from 1,4-bis(diethoxyphosphonylmethyl)-2,5-dimethoxybenzene. The dialkenyldioxybenzenes emit UV light in solution (λ_em =350-387 nm) and in the solid state (λ_em =328-388 nm). In addition, the quantum yields in the solid state were generally higher than those in solution. In particular, the adamantylidene-substituted benzenes fluoresced in the UV region with high quantum yields (Φ=0.37-0.55) in the solid state. Thin films of poly(methyl methacrylate) doped with the adamantylidene-substituted benzenes also exhibited UV emission with good efficiency (Φ=0.27-0.45). Density functional theory calculations revealed that the optical excitation of the dialkenyldimethoxybenzenes involves intramolecular charge-transfer from the ether oxygen atoms to the twisted alkenyl-benzene-alkenyl moiety, whereas the dialkenylbis(triphenylsiloxy)benzenes were optically excited through intramolecular charge-transfer from the oxygen atoms and twisted π-system to the phenyl-Si moieties of each triphenylsilyl group.

Synthesis and optical properties of mono- and diaminocorannulenes

A simple synthesis of aminocorannulenes leads to the discovery of surprising properties: tunable visible emission; solid-state fluorescence; water-soluble fluorescent corannulenes.

Synthesis, characterization, aggregation-induced enhanced emission and solvatochromic behavior of dimethyl 4′-(diphenylamino)biphenyl-3,5-dicarboxylate: experimental and theoretical studies

Red shift in the emission wavelength by 149 nm as a result of increased solvent polarity.

Reactivity of ZrO(MFP) and ZrO(RP) Nanoparticles with LnCl3 for Solvatochromic Luminescence Modification and pH-Dependent Optical Sensing

The luminescence of the inorganic-organic hybrid nanoparticles ZrO(MFP) (MFP=methylfluorescein phosphate) and ZrO(RP) (RP=resorufin phosphate) was modified by addition of different rare earth halides LnCl3 . The resulting composite materials form dispersible nanoparticles that exhibit modified nanoparticle fluorescence depending on the rare earth ion. The resulting chromaticity of the luminescence is further variable by the employment of different solvents for ZrO(MFP)-based composite systems. The strong solvatochromic effect of the MFP chromophore leads to different luminescence chromaticities of the composite materials between green, yellow, and blue in THF, toluene, and dichloromethane, respectively. The luminescence of ZrO(RP)-based composite particles can be modified between the red and blue spectral regions in dependence on the applied reaction temperature. Beside a luminescence shift that is derived from nanoparticle modification by LnCl3 , a strong turn-on effect of ZrO(RP) particles results after contact with different Brønsted acids and bases in combination with a respective chromaticity shift. Both effects enable the potential employment of such particles as highly sensitive optical pH sensors.

Development of three ways molecular logic gate based on water soluble phenazine fluorescent 'selective ion' sensor

New hydrophilic fluorescent selective ion sensor based on phenazine and phthalazine moieties, 1,1'-(phenazine-2,3-diyl)-bis(3-(1,4-dihydroxyphthalazin-6-yl)urea) (1), has been designed, synthesized and characterized. Interestingly, sensor 1 exhibits prominent "turn-on" and "turn-off" fluorogenic signaling at 580 nm towards Fe²⁺ & AcO^- and Sr²⁺ & Cu²⁺, respectively. The fluorescence titration experiments shed light on the nature of the interaction between 1 and guest molecules (Fe²⁺, Sr²⁺, Cu²⁺ and AcO^-), which divulge that 1 is flexible enough to orient itself according to the size of the guest molecule. Water mediated excited-state intramolecular proton transfer (ESIPT) and photo-induced electron transfer (PET) mechanisms are responsible for the dual behavior of 1, which binds with guest molecules in 1:1 stoichiometry. Based on the significant duplex fluorescence response of 1, a molecular logic gate keypad lock with sixteen "on" passwords for a storage system has been developed.

White light emission in water through admixtures of donor–π–acceptor siblings: experiment and simulation

White light emission through binary admixtures of push–pull stilbenes.

Transition Metal Substitution Effects on Metal-to-Polyoxometalate Charge Transfer

A series of hetero-bimetallic transition metal-substituted polyoxometalates (TMSPs) were synthesized based on the Co(II)-centered ligand [Co(II)W11O39](10-). The eight complex series, [Co(II)(M(x)OHy)W11O39]((12-x-y)-) (M(x)OHy = V(IV)O, Cr(III)(OH2), Mn(II)(OH2), Fe(III)(OH2), Co(II)(OH2), Ni(II)(OH2), Cu(II)(OH2), Zn(II)(OH2)), of which six are reported for the first time, was synthesized starting from [Co(III)W11O39](9-) and studied using spectroscopic, electrochemical, and computational techniques to evaluate the influence of substituted transition metals on the photodynamics of the metal-to-polyoxometalate charge transfer (MPCT) transition. The bimetallic complexes all show higher visible light absorption than the plenary [Co(II)W12O40](6-) and demonstrate the same MPCT transition as the plenary complex, but they have shorter excited-state lifetimes (sub-300 ps in aqueous media). The decreased lifetimes are rationalized on the basis of nonradiative relaxation due to coordinating aqua ligands, increased interaction with cations due to increased negative charge, and the energy gap law, with the strongest single factor appearing to be the charge on the anion. The most promising results are from the Cr- and Fe-substituted systems, which retain excited-state lifetimes at least 50% of that of [Co(II)W12O40](6-) while more than tripling the absorbance at 400 nm.

A naphthalimide-based fluorescent sensor for halogenated solvents

AMNis the first sensor to have the ability to differentiate CCl₄, CHCl₃, CH₂Cl₂and CHBr₃from halogenated solvents.

Gas-phase energy of the S2←S0 transition and electrostatic properties of the S2 state of carotenoid peridinin via a solvatochromic shift and orientation broadening of the absorption spectrum

The solvent effect on the position and the shape of the absorption spectrum of peridinin for 12 protic and aprotic solvents as well as the temperature effect for methanol were studied using a solvatochromic theory based on the Onsager sphere cavity model.