Structureless Emission Research Articles

Conformational dynamics of the pyrene excimer.

The conformational dynamics of the pyrene excimer play a critical role in its unique fluorescence properties. Yet, the influence of multiple local minima on its excited-state behavior remains underexplored. Using a combination of time-dependent density functional theory (TD-DFT) and unsupervised machine learning analysis, we have identified and characterized a diverse set of stable excimer geometries in the first excited state. Our analysis reveals that rapid structural reorganization towards the most stable stacked-twisted conformer dominates the excimer's photophysics, outcompeting radiative relaxation. This conformer, which is primarily responsible for the characteristic red-shifted, structureless fluorescence emission, reconciles experimental observations of long fluorescence lifetimes and emission profiles. These findings provide new insights into the excited-state dynamics of excimers. They may inform the design of excimer-based materials in fields ranging from organic electronics to molecular sensing.

Excimer formation and site selectivity in single pyrene microcrystals

Thin microcrystals of pyrene have been recrystallized by using a surfactant. With a size-range of several hundred nm to a few micrometers, the crystals have been characterized by using standard techniques of X-ray and Raman spectroscopy besides the atomic force and scanning electron microscopy. The fluorescence microscopy of single microcrystal shows fluorescence due to monomer as well as the red-shifted broad, structureless excimer-type emission. The observed decay profiles of the red-shifted emission reveal existence of dimer and higher aggregates in the crystalline state when excited at the red-edge of the absorption band. This is in contrast to the excimer emission observed at higher energy excitation range corresponding to the monomer absorption. Distinctly different nature of time-resolved decay profiles differentiates the two mechanisms. The observations have been used to propose existence of different sites of crystal packing under two different excitations (370 nm and 408 nm). The structural anisotropy of these crystals makes them useful as a probe in crystal structure based molecular and biological sensing.

The upward propagating ionospheric hiss waves during the seismic time observed by the China seismo-electromagnetic satellite

The ionospheric hiss wave is a broadband incoherent and structureless electromagnetic emission. They appear in a relatively narrower frequency range between −0.1 and 1.5 kHz. However, according to previous observations, abnormal electromagnetic emissions during seismic activities also preferentially appear in the same frequency range of ionospheric hiss. This work studies the propagation features of the ionospheric hiss during seismic time based on the observations from the CSES (China Seismo-Electromagnetic Satellite). The wave vector analysis shows that during seismic activities, except for the downward propagating ionospheric hiss which is a common phenomenon in the ionosphere, there are upward propagating emissions mixed with the downward propagating ionospheric hiss. We made a statistical analysis of the shallow strong earthquakes (M ≥ 6.0, depth below 30 km) that occurred in mainland China from 2019 to 2022. We selected the ionospheric hiss events recorded by orbits passing over the epicenters within a time window (the 1-month prior to and 1-week after the main shock). We found that although most of the events are the typical downward propagating ionospheric hiss waves, however, there are certain events mixed with the upward propagating emissions. The statistical distribution analysis of wave propagation parameters shows that the major part of wave normal angles vary from 40 to 60, the azimuthal angles predominately attain below 40, and the ellipticity shows a more complicated feature varying around ± 0.5, and the planarity values predominate at values between 0.6 and 1. The frequency band of the upward propagating ionospheric hiss mostly varies between 300 Hz and 800 Hz. To further study the behavior of such upward propagating ionospheric hiss wave during the seismic time, we compared the wave activities under non-seismic activity and quiet space weather conditions, and the results confirm that the occurrence rate of the upward propagating emissions under quiet conditions is far less than that in the seismic time. We suggest that there is a link between the upward propagating ionospheric hiss and the seismic activity, but the physical reason behind it still remains a puzzle to us.

Intramolecular dimer formation reveals anomalous solvation within fluorous solvent perfluorodecalin.

Due to the unusual properties of fluorine, a fluorous solvent obtained by replacing hydrogen atoms with fluorine atoms in a hydrocarbon solvent may afford unusual solute solvation and aggregation. A fluorescent dipyrenyl probe, 1,3-bis(1-pyrenyl)propane (BPP), was utilized to explore intramolecular aggregation within perfluorodecalin (PFD), a prototypical fluorous solvent. Steady-state fluorescence emission and excitation along with excited-state emission intensity decay of BPP were acquired in the temperature range of 293.15 to 333.15 K in fluorous solvent PFD and n-hexane-added PFD. The probe BPP is known to form an intramolecular excimer in common hydrocarbon solvents, which is characterized by a broad structureless low-energy emission band; the formation of intramolecular excimers exclusively in the excited state was confirmed via excitation scans along with the intensity decay of the monomer and excimer, respectively. While intramolecular dimerization of BPP does occur in PFD, emission wavelength-dependent excitation spectra and fits of the intensity decay data reveal that intramolecular aggregation takes place in the ground state as well. This ground-state heterogeneity is reduced as the temperature is increased. In the presence of as small as 0.1 mole fraction of n-hexane, the ground-state aggregation vanishes, and intramolecular dimerization again takes place exclusively in the excited state. The data hint at the poor solvation of BPP in PFD to be responsible for intramolecular ground-state aggregation. In PFD-rich (PFD + n-hexane) mixtures, probe pyrene (Py) was found to be preferentially solvated by n-hexane, further corroborating the above-mentioned proposition. Clear differences in solute solvation within a fluorous solvent as compared to hydrocarbon solvents leading to unexpected solubilization and aggregation in fluorous media are amply demonstrated.

Disentangling Excimer Emission from Chiral Induction in Nanoscale Helical Silica Scaffolds Bearing Achiral Chromophores.

The synthesis and characterization of diketopyrrolopyrroles and perylenemonoimidodiesters linked to a substituted benzoic acid in the ortho, meta, and para positions, are reported. Grafting of these dyes on the surface of chiral silica nanohelices is used to probe how the morphology of the platform at the mesoscopic level affects the induction of chiroptical properties onto achiral molecular chromophores. The grafted structures are weakly (diketopyrrolopyrroles) or strongly (perylenemonoimidodiesters) emissive, exhibiting both locally-excited state emission and a broad, structureless emission assigned to excimers. The dissymmetry factors obtained using circular dichroism highlight optimized supramolecular organization between the chromophores for enhancing the chiroptical properties of the system. In the ortho- derivatives, poor organization due to steric hindrance is reflected in a low density of chromophores on walls of the silica-nanostructures (<0.1 vs. >0.3 and up to 0.6 molecules/nm2 for the ortho and meta or para derivatives, respectively) and lower gabs values than in the other derivatives (gabs <2×10-5 vs 6×10-5 for the ortho and para derivatives, respectively). The para derivatives presented a better organization and increased values of gabs . All grafted chromophores evidence varying degrees of excimer emission which was not found to directly correlate to their grafting density.

Tuning the Photophysical Properties of Homoleptic Tris-Cyclometalated Ir(III) Complexes by Facile Modification of the Imidazo-Phenanthridine and Their Application to Phosphorescent Organic Light-Emitting Diodes.

To explore the excited-state electronic structure of the blue-emitting Ir(dmp)3 dopant material (dmp = 3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridine), which is notable for durable blue phosphorescent organic light-emitting diode (PhOLED), a series of homoleptic dmp-based Ir(III) complexes (DMP–R, tris[3-(2,6-dimethylphenyl)-7-R-imidazo[1,2-f]phenanthridin-12-yl-κC12,κN1]iridium, R = H, CH3, F, and CF3) were prepared by introducing an electron-donating group (EDG; −CH3) or an electron-withdrawing group (EWG; −F and −CF3) at the 7-position of the imidazo-phenanthridine ligand. The photophysical analysis demonstrated that the alteration from EDG to EWGs led to redshifted structureless emission profiles, which were correlated with variations in the 3MLCT/3ILCT ratio in the T1 excited state. From electrochemical studies and density functional theory calculations, it turned out that the excited-state nature of the dmp-based Ir(III) complexes was significantly affected by the inductive effect of the 7-substituent of the cyclometalating dmp ligand. As a result of the lowest unoccupied molecular orbital energy stabilization by the EWGs that suppressed the non-radiative pathway from the emissive triplet excited state to the 3d–d state, the F- and CF3-modified Ir(dmp)3 complexes (DMP–F and DMP–CF3) showed quantum yields of 27–30% in the solution state, which were at least 4- or 5-fold higher than those shown by DMP–H and DMP–CH3. A PhOLED device based on DMP–CF3 [CIE chromaticity (0.17, 0.39)], which demonstrated a distinct 3MLCT characteristic, exhibited better electroluminescent efficiencies with an external quantum efficiency of 13.5% than that based on DMP–CH3.

Kinetic and thermodynamic control of tetraphenylethene aggregation‐induced emission behaviors

AbstractMany aggregation‐induced emission (AIE) systems exhibit broad and structureless luminescence emission spectra resembling the Gaussian distribution, which is likely due to kinetically locked molecular conformers in the condensed phase. To verify the hypothesis, a series of tetraphenylethene (TPE) derivatives are synthesized and characterized as aqueous nanoparticle suspensions. It is found that the unsubstituted TPE exhibits reduced fluorescence intensity accompanied by a blueshift of the emission maximum, after the temperature of the aqueous suspension is elevated and cooled to room temperature again. For a naphthalimide‐substituted TPE compound, thermal treatment of the AIE aqueous suspension results in complete, irreversible aggregation‐caused quenching (ACQ) of fluorescence, which can be restored by a redissolving‐precipitation process of thermally treated aggregates. The phenomenon is ascribed as a relative population shift of a kinetic AIE (k‐AIE) state to a thermodynamic AIE (t‐AIE) or ACQ state, evidenced by differential scanning calorimetry, dynamic light scattering, and scanning electron microscopy. The phenomenon may be universal for many other AIE systems and could be explored as stimuli‐responsive materials.

A red-orange carbazole-based iridium(III) complex: Synthesis, thermal, optical and electrochemical properties and OLED application

A novel heteroleptic iridium(III) acetylacetonate (acac) complex, (L-5-CHO)2Ir(acac) (3b), was synthesised from 2-(9’-hexylcarbazole-3’-yl)-5-formylpyridine (L-5-CHO) (1b). The complex 3b was determined to be thermally and electrochemically stable. The photoluminescence properties of the compound were studied, with a dichloromethane solution of 3b giving structureless emission at 662 nm, showing that the formyl group red-shifted the emission by 151 nm compared to the parent complex. Complex 3b was also shown to possess a moderate photoluminescence quantum yield (67%) and a short emission lifetime (τ = 280 ns). Organic light-emitting diodes (OLEDs) were fabricated with a solution-processed emissive layer (EML) consisting of poly(N-vinylcarbazole) (PVK), 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) and iridium complex (3b). The OLEDs containing complex 3b showed red-orange electroluminescence (EL) at 624 nm. The influence of the host materials was studied and the best performance was achieved with both PVK and PBD in the emissive layer, with the resulting OLEDs exhibiting a current efficiency of 0.84 cd/A, a power efficiency of 0.20 lm/W, and an external quantum efficiency (EQE) of 0.66% at a brightness of 2548 cd/m2.

Synthesis, electrochemical and photochemical properties of neutral Ir(III) complex based on 3-Iodopyridine-2-carboxylate ligand

In this work, we report the synthesis and characterization of mer-[Ir(ppy)2(Ipic)], where ppy = 2-phenylpyridine and Ipic = 3-iodopyridine-2-carboxylate, as well as, the electrochemical and photophysical properties in solution. The electronic absorption spectra profile can be divided into two main regions: intense bands in the high energy UV-region (200–300 nm) assigned to ligand centered (IL) transition on both the cyclometalated (ppy) and the ancillary (Ipic) ligands, and lower intensity absorption bands located in the less energetic region (320–460 nm) can be experimentally assigned to the spin-allowed metal-to-ligand charge transfer (1MLCT) transition and the spin-forbidden 3MLCT. From the electrochemical results, were obtained the oxidation peak (Ir(III)→Ir(IV) couple) at +0.59 V resulting in a HOMO of −5.09 eV, and the reduction potentials −1.65 and −2.00 V, which can be attributed to the Ipic and ppy, respectively. Additionally, it is possible to observe that the introduction of the halogen atom promotes a significant variation in LUMO energy (2.75 eV), while the HOMO level is almost unchanged. The complex displayed broad and structureless emission spectra in acetonitrile (λmax = 510 nm, φ = 0.0032 and τ = 160 ns) and dichloromethane (λmax = 510 nm, 525 nm) solution at room temperature but becomes structured (λmax = 490, 522 nm) in propionitrile:butyronitrile at 77 K, indicating that the emitting state at room temperature has a mixed 3MLCT-3LC character but at 77 K the emission has a strong 3LC character.

High-Efficiency Narrow-Band Electro-Fluorescent Devices with Thermally Activated Delayed Fluorescence Sensitizers Combined Through-Bond and Through-Space Charge Transfers

Organic light-emitting diodes utilizing thermally activated delayed fluorescence sensitizers and multiple-resonance (MR) dopants may simultaneously offer high efficiencies and narrow-band emissions...

Pressure-dependent distinct luminescent evolutions of pyrene and TPA-Py single crystals.

The effects of the high pressure on two single crystals, pyrene and N,N-diphenyl-4-(pyren-1-yl)aniline (TPA-Py), were studied by in situ fluorescent and Raman spectroscopies. During the compression, the pyrene with one structureless excimer emission band showed a continuous bathochromic-shift. In contrast, with the pressure increasing to 10.36GPa, TPA-Py previously dominated with the hybridized local and charge transfer (HLCT) excited state gradually exhibited a new band at longer wavelengths, which is assigned to a new excited state species with the intramolecular charge transfer (ICT) state, caused by the pressure-induced changes on its molecular configuration. Accompanied by the spectral changes, a sequential color variation from blue to cyan was observed, giving a change to yellow and then red. The significant broadening of the full-width half-maximum (FWHM) of the TPA-Py is observed due to the enhanced dipole-dipole interaction and the existence of pressure gradient. Both pyrene and TPA-Py showed the delayed recovery of the luminescence in the compression-decompression cycle, which results from the poor reversibility of electronic structure caused by the compression-induced piezochromic effect. Furthermore, the evolutions of the Raman spectra of pyrene and TPA-Py indicated that the pressure-induced reversible transformation is caused by the molecular conformational change. This study is a deeper understanding of the structure-property relation of the HLCT species and will be a helpful reference for the regulation of photoluminescence in these intramolecular electron donor-acceptor crystal materials.

Phosphorescence properties of anionic cyclometalated platinum(II) complexes with fluorine-substituted tridentate diphenylpyridine in the solid state

The phosphorescence properties of platinum(II) complexes bearing a fluorine-substituted tridentate ligand were investigated to understand their counterintuitive weak emission. The n-tetrabutylammonium salt, (n-Bu4N)[Pt(dFphpy)X] (H2dFphpy = 2,6-bis(2,4-difluorophenyl)pyridine, X = CN− (1), Cl− (2)) exhibited weak structureless solid state emission spectra at 77 and 298 K. Based on temperature-dependent emission lifetime analysis and computational studies, the emissive states of 1 and 2 were assigned to the metal-to-ligand charge transfer state (3MLCT). The small activation energies from the emissive state with a planar structure to a non-emissive excited state with a bent structure resulted in low emission efficiency even in the solid state.

The Important Role of Coordination Geometry on Photophysical Properties of Blue-Green Emitting Ruthenium(II) Diisocyano Complexes Bearing 2-Benzoxazol-2-ylphenolate.

A series of blue-green emitting RuII diisocyano complexes containing 2-benzoxazol-2-ylphenolate (PBO) have been prepared. The complexes were isolated under varied reaction conditions in two isomeric forms, i.e., trans,trans,trans- (1) and cis,trans,cis- (2), with varied ligand coordination geometry above the RuII center. The photoluminescence of the isomeric complexes has been compared and tuned by the systematic variation of the electronic properties of the isocyanides. The cis,trans,cis- isomers exhibit structureless emission in the blue-green region (471-517 nm) upon excitation at λex > 400 nm in dichloromethane solution at room temperature. Both isomeric forms show similarly structured greenish emission at 499-523 nm on excitation at λex > 355 nm in a methanol/ethanol (4:1) glassy medium at 77 K. On careful comparison with the corresponding absorption and electrochemical data, it is suggested that the solution emission of the cis,trans,cis- isomers (2) at room temperature is originated from the metal-to-ligand charge transfer (MLCT), while a ligand-centered (LC) parentage is assigned for the emission in a glassy state for both isomeric forms. In line with the above experimental results, DFT calculation demonstrates the change in the nature and relative energy of the HOMOs and LUMOs with respect to the varied ligand coordination geometry and π-accepting ability of the isocyanides.

Abnormal temperature dependence of triplet-triplet annihilation in excimer-based devices

<p indent="0mm">Excimer is widely focused since Förster and Kasper discovered it in 1954. It is dimeric species formed by two identical molecules, which is associative in the excited electronic state and dissociative in the ground state. Usually, it has a long emission lifetime and exhibits red-shifted, broadened and structureless fluorescence emission relative to that of monomer, which leads to various applications, including laser devices, chemo-sensors and white-light organic light-emitting diodes (OLEDs). These applications are closely related to our life. Although excimer has many advantages, there are few probing tools about it and few reports that give a detailed understanding of it. More importantly, the microscopic mechanism of excimer is very significant for preparing photoelectric devices, improving emission efficiency of devices and its more applications. Thus, understanding the microscopic mechanisms of excimer is both challenging and needful for us. Recently, magneto-electroluminescence (MEL) has been shown to be a useful tool to understand the dynamics of the excited states within OLEDs. This technique without contact and harm uses an external magnetic field that can change the balance between singlet and triplet states and the EL intensity by affecting spin-related microcosmic processes within OLEDs. These microcosmic processes include intersystem crossing, reverse intersystem crossing, singlet fission and triplet-triplet annihilation (TTA). These microscopic processes possess their own characteristic MEL curves and are influenced by charge injection/recombination and the concentration and lifetime of excited states. As is known to all, the injection current used for an OLED can affect the concentration of excited states, while the working temperature of an OLED can influence the charge injection/recombination and the lifetime of the excited states. So, studying the current- and temperature-dependent MEL traces from OLEDs is an effective method to analyze intersystem crossing, reverse intersystem crossing, and TTA. Thus, microcosmic processes in the excimer-based OLEDs can be investigated by analyzing MEL curves of devices. Excimer-based OLEDs were fabricated, and their MEL curves were measured at different temperatures and injection currents. The MEL curves of excimer-based OLEDs are always composed of the low-field components (|<bold><italic>B</italic></bold>|≤50 mT) determined by intersystem crossing and the high-field components (50 mT&lt;|<bold><italic>B</italic></bold>|≤300 mT) governed by TTA. This indicates that TTA with delayed fluorescence always exists in the excimer-based devices at different temperatures and injection currents. It’s noteworthy that, under a fixed injection current, the temperature dependence of TTA in the excimer-based OLEDs is contrary to that of exciton-based OLEDs. TTA in the exciton-based OLEDs happens at low temperature and enhances with decreasing temperature. But TTA in the excimer-based OLEDs happens at room temperature and weakens with decreasing temperature. This abnormal temperature dependence of TTA in the excimer-based OLEDs overcomes the disadvantage that TTA can only be used at low temperature to improve the luminescent efficiency of devices in the past. Based on the analysis of the structure, current-voltage-brightness characteristics and spectra of devices, the abnormal temperature dependence of TTA in the excimer-based OLEDs is related to the long lifetime of the excimer and the inhibition of excimer formation by low temperature. This work discloses the abnormal temperature dependence of TTA in the excimer-based OLEDs by investigating MEL curves of devices at different temperatures and may be helpful for improving the luminescent efficiency of OLEDs.

Photoisomerization of Enediynyl Linker Leads to Slipped Cofacial Hydroporphyrin Dyads with Strong Through-Bond and Through-Space Electronic Interactions

Photoisomerization of 3,4-di(methoxycarbonyl)-enediyne linker in hydroporphyrin (chlorin or bacteriochlorin) dyads leads to thermally stable cis isomers, where macrocycles adopt a slipped cofacial mutual geometry with an edge-to-edge distance of ∼3.6 Å (determined by density functional theory (DFT) calculations). Absorption spectra exhibit a significant splitting of the long-wavelength Qy band, which indicates a strong electronic coupling with a strength of V = ∼477 cm-1 that increases to 725 cm-1 upon metalation of hydroporphyrins. Each dyad features a broad, structureless emission band, with large Stokes shift, which is indicative of excimer formation. DFT calculations for dyads show both strong through-bond electronic coupling and through-space electronic interactions, due to the overlap of π-orbitals. Overall, geometry, electronic structure, strength of electronic interactions, and optical properties of reported dyads closely resemble those observed for photosynthetic special pairs. Dyads reported here represent a novel type of photoactive arrays with various modes of electronic interactions between chromophores. Combining through-bond and through-space coupling appears to be a viable strategy to engineer novel optical and photochemical properties in organic conjugated materials.

Solvent-Dependent Sensitization of Ytterbium and Neodymium via an Intramolecular Excimer.

We report the synthesis of a di(1-pyrenyl)phosphoryl acetophenone ligand containing two pyrenyl moieties linked by a single phosphorus atom. The ligand exhibits solvent-dependent emission: in nonpolar solvents, typical monomeric pyrene emission is observed, whereas in polar solvents, an additional broad and structureless emission appears. The emission in polar solvents is concentration independent and is attributed to the emission of an intramolecular excimer. The coordination of the di(1-pyrenyl)phosphoryl acetophenone ligand as well as the corresponding deprotonated anionic di(1-pyrenyl)phosphoryl acetophenonate ligand was studied with the near-infrared emitting lanthanides, neodymium and ytterbium. Solvent-dependent sensitization of both lanthanides was observed and correlates with the presence of the excimer emission. Sensitization of ytterbium is more efficient than neodymium, and the overall quantum yields were found to be 12.8 and 1.9% for ytterbium and neodymium, respectively.

Synthesis, characterization and optical properties of novel Ir(III) complexes bearing N-heterocycle substituents

A series of cationic heteroleptic Ir(III) complexes with different N-heterocycle groups (N-phenothiazinyl, N-indolyl, N-carbazolyl, 3,6-di-tert-butyl-N-carbazolyl) were synthesized and characterized. The photophysical properties of these complexes were systematically investigated. All complexes exhibit strong 1π-π∗ absorption bands in the UV region and broad 1MLCT absorption bands in the visible region. In addition, these complexes exhibit weak absorption after 500 nm, which could be attributed to 3π,π∗/3CT transition. All complexes exhibit broad and structureless emission bands from 568 nm to 627 nm at room temperature, which are originated from 3MLCT/3LLCT excited states. The electron donating substituents attached on the 2-phenylpyridine ligands cause a pronounced red-shift of the emission band. Except 1d, all complexes show strong triplet transient absorptions from UV to visible region, which were assigned to the 3MLCT/3π,π∗ excited state. In addition, complexes 1a-1c all exhibit reverse saturable absorption (RSA) at 532 nm, which follows the trend of 1a>1b > 1c. The photophysical properties of these Ir(III) complexes can be influenced drastically by the substituents on 2-phenylpyridine ligands, which would be useful for rational design of optical functional materials.

Structural and luminescent characterisation of uraniferous fluorapatite and haematite associated with phosphatic rocks of the Bijawar group in Sagar District, Madhya Pradesh (India)

The structural and spectroscopic characteristics of phosphatic ferruginous shale samples from the Bijawar Group rocks from Sagar District of Madhya Pradesh (India) have been probed for identification of uranium species. Fluorapatite (\(\hbox {Ca}_{5}\hbox {(PO}_{4})_{3}\hbox {F}\), FAP) and haematite (\(\upalpha \)-\(\hbox {Fe}_{2}\hbox {O}_{3}\)) were identified as the main phases in the separated mineral concentrates. The photoluminescence (PL) and X-ray absorption near edge spectroscopy (XANES) studies pointed to a strong experimental evidence of both U(IV) and U(VI) oxidation states in the mineral concentrate portion obtained from the same parent host rock. The PL spectrum has confirmed the charge transfer (f–d) transition bands in UV and near-UV regions with emission peaks at ca. 290, 313, 336, 399 and 416 nm, which has been attributed to the substitution of \(\hbox {Ca}^{2+}\) ions by U(IV) in FAP and broad structureless emission due to stabilisation of U(VI) as \(\hbox {UO}_{6}^{6-}\) in haematite. Time-resolved spectroscopy studies have revealed biexponential decay components lasting 2–5 ns for U(IV) species and \(10\,\upmu \hbox {s}\) for U(VI) species. These characterisations revealed the fundamental information about the oxidation state and form of uranium in this region. Remediation measures for the Bijawar region are also suggested.

Enhanced Exciplex Emission of Pyrene Thin Films Doped by Perylene: Structural, Photophysical and Morphological Investigation.

The present work reports on the preparation of thin films of pyrene luminophors doped by varying amounts of perylene by spin coating technique. The structural, morphological, and photophysical properties of pyrene thin films have been investigated as a function of perylene contents. X-ray diffraction studies of doped thin films show well-defined peaks, and estimated crystallite size decreases with increasing perylene content, due to the formation of closed packed crystal structure. SEM images of pure pyrene revealed smooth, and compact and separated crystals with amounts of perylene. The result of absorption spectra showed decrease in intensity with perylene content. However, the fluorescence spectra of pyrene containing higher amounts of perylene showed broad and structureless exciplex emission at 510nm. The emission colour of pyrene luminophors tuned from blue to green by controlling the concentration of perylene. The phenomenon of change in colour was seen due to efficient excitation energy transfer from pyrene to perylene in thin films. It also found that the intensity of exciplex emission increases with increasing concentration of perylene is of high demand in fluorescent lamp as well as green organic light emitting diodes.

Fluorescent 1-Arylidene-1,3-dihydroisobenzofuran: Ligand-Free Palladium Nanoparticles, Catalyzed Domino Synthesis and Photophysical Studies

The present work describes synthesis and photophysical studies of a class of fluorophores containing biologically relevant 1,3-dihydroisobenzofuran scaffold. Ligand-free palladium nanoparticles have been utilized for domino synthesis of 1-arylidene-1,3-dihydroisobenzofurans from 2-iodobenzyl alcohol and terminal arylacetylenes under mild reaction conditions. Reaction of terminal alkylacetylenes with 2-iodobenzyl alcohol however, provided only the Sonogashira products (2-(alk-1-ynyl)phenyl)methanols. The 1-arylidene-1,3-dihydroisobenzofuran derivatives showed structured fluorescence spectra in non-polar cyclohexane, originated from a locally excited state and structureless emission in polar acetonitrile involving intramolecular charge transfer process. The vibronic structures in the locally excited emission were confined to the ring C=C stretching of the dyes. A detailed structure-property relationship of the 1-arylidene-1,3-dihydroisobenzofuran dyes is presented in this study.