Excimer Luminescence Research Articles

Piezochromic luminescence of excimers formed by anthracene plane slip

Excimer materials are frequently applied to organic chemistry, optoelectronic devices and stimulus response. Specific research on the entire process of excimer generation in solid materials, as well as the intricate mechanism of piezochromic luminescence based on excimer, are still absent and need to be examined in-depth through comprehensive experiments. Therefore, we designed an anthracene-based molecule CZPAN, which adopts no overlap area between anthracene molecules in its crystal at ambient conditions, to investigate the piezochromic luminescence behavior and expect the anthracene group to form excimers under high-pressure stimulation. As pressure increases, CZPAN gradually changes from blue monomer luminescence to green excimer luminescence accompanied with the considerably red-shifted emission spectra, and exhibits a discontinuity of fluorescence quenching during the formation of excimers (in the pressure range of 5.3–6.6 GPa). Moreover, the pressure relief process is fully reversible. This study presents abundant piezochromic luminescence of CZPAN and proves that the formation of excimers requires a certain overlap area between molecules. Pursuing the appropriate strength of π–π interactions will thus be the focus of future research on excimer materials of high luminescence efficiency, so that excimers can be better applied in sensors and imaging devices.

Tailoring pyrene excimer luminescence via controlled sulfur oxidation

Manipulation of the oxidation degree of sulfur is carried out to synthesize sulfide-, sulfoxide- and sulfone-substituted pyrenes, fine-tune the electronic effect and stiffness of the molecules, and obtain optimal luminous efficiency of the excimers.

Chiral H-aggregation-induced large stokes shift with CPL generation assisted by α-helical poly(L-lysine) substructure.

Fluorescent materials with large Stokes shifts have significant potential for use in optical applications. Typically, a synthetic design strategy is utilized for this purpose. In this study, we demonstrated a novel method by binding a chiral template to a nonchiral fluorescent agent without chemical modification. Specifically, α-helical poly(L-lysine) was employed as the chiral template, which interacted with a disulfonic fluorescent dye, such as NK2751. The dye caused excimer luminescence by inducing the formation of a chirally H-aggregated dimer only when poly(L-lysine) was in an α-helical shape. The result was a Stokes shift of 230 nm. Similar effects were not observed when the chiral template was in a random coil condition and the Stokes shift was less than 40 nm. These findings imply that H-aggregated dimerization, which often results in quenching, permits the electronic transitions necessary for fluorescence events by the formation of the chirally twisted state. In addition, we introduce for the first time the generation of circularly polarized luminescence using the chirality induction phenomena in a dye supported by poly(L-lysine).

Nondestructive All-Optical Readout through Photoswitching of Intramolecular Excimer Emission.

We report the first intramolecular excimer photoswitching induced by molecular motion within a dithienylethene (DTE) molecule without destructive readout. The photochromic compound DTE bears two pyrene chromophores, judiciously positioned to face each other in the DTE's open form. The close proximity of the pyrenes in the open form is confirmed by NMR experiments and geometry optimization. Intense pyrene excimer luminescence is recorded, upon both one- and two-photon excitation (OPE and TPE). The photocyclization reaction of the DTE core induces a molecular motion of one pyrene moiety which thus prevents the possibility of formation of an excimer. Our DTE-based pyrene is stable upon TPE irradiation and shows a high photocyclization quantum yield. Such property specifications allow us to report the original nondestructive readout fluorescence by alternating exposure to OPE and TPE.

Sonoluminescence in solutions of organic aromatic luminophores: Monomer and excimer luminescence of benzene, toluene and p-xylene

This paper considers the spectra of glow that emerges during the sonolysis of benzene, toluene and p-xylene solutions in ethylene glycol under the action of a translationally motionless bubble levitating in the ultrasonic standing wave with a frequency of ~27 kHz or a moving bubble (single-bubble, or moving single-bubble sonoluminescence, SBSL, or m-SBSL, respectively). The m-SBSL spectra against the background of the broadband solvent continuum demonstrate the presence of characteristic luminescence bands of luminophores in the range of 270–305 nm (singlet excited monomers) and a band of singlet excited dimers (excimers) of these molecules with a maximum of 320 nm at a luminophores concentration in solution being 10−3-10−2 M. For 1.25∙10−3 M benzene solution, there were also recorded the bands of СН and С2 radicals occurred as a result of decomposing this luminophore. At concentrations higher than 10−2 M, luminophores luminescence is quenched and the m-SBSL spectra reveal only the continuum, the intensity of which also decreases with an increase in the luminophores concentration. This continuum is recorded without any significant change in its shape and intensity, with no bands of luminophores and radicals for the luminophores concentrations specified, and during SBSL. The obtained data are indicative of the penetration of luminophores molecules, in the m-SBSL regime, from the solution into the moving and deformed bubble according to the known sonochemical mechanism to inject solution nanodroplets and also their further collisional excitation in the nonequilibrium intrabubble plasma which repeatedly formed during oscillations of bubble in the acoustic field. The nonequilibrium plasma state determines the presence of luminescence both luminophore molecules (gas temperature about 103 K or less) and radical products of their decomposition (gas temperature is thousands K).

Time-Dependent Emission Enhancement of the Ethynylpyrene-o-Carborane Dyad and Its Application as a Luminescent Color Sensor for Evaluating Water Contents in Organic Solvents.

The time-dependent emission enhancement (TDEE) phenomena of the 1-(o-carboran-1-yl)ethynylpyrene dyad were reported. It was found that the emission intensity from the dyad increased in tetrahydrofuran (THF), acetone and dichloromethane with increasing incubation time. From the mechanistic studies, it was suggested that agglomeration of the dyad gradually proceeded in these media, followed by expression of excimer luminescence. Additionally, it was shown that the rates of TDEE of the dyad were sensitively accelerated in the presence of a trace amount of water. Based on this fact, a detection system for water contents in acetone was constructed. Before and after incubation for 96 h at room temperature, time courses of changes in optical properties were monitored. Finally, water contents in acetone can be estimated by the degrees of TDEE and emission color changes in the range from 0.1 wt % to 2.0 wt % and from 2.0 wt % to 20 wt %, respectively.

Luminescence and sensory properties of PMMA films doped with a boron chelate

The optical sensory properties of polymethylmethacrylate films doped with 2,2-difluoro-4-methylnaphtho-[2,1-e]-1,3,2-dioxaborine (1) for water vapor are found. A mechanism of the optical response is proposed: water molecules contribute through hydrogen bonds to the dissociation of aggregates of 1 having intense excimer luminescence of J-aggregates. A sharp quenching of luminescence is observed. The spectrum of the films after exposure to water vapor corresponds to the monomer luminescence of 1.

New polymers containing BF2-benzoylacetonate groups. Synthesis, luminescence, excimer and exciplex formation

In the present study, a new synthetic method for the functionalization of polystyrene (PS) and (styrene-methyl methacrylate) copolymer has been developed. Using the new method, polymers containing BF2-benzoylacetonate groups have been obtained through double acylation by acetic anhydride with boron trifluoride. Luminescence of the produced polymers in solutions and films has been studied. Quantum yields of polymer solution luminescence are significantly higher than those of the low-molecular-weight analog – boron difluoride benzoylacetonate. For the polymer, in which styrene fragments are separated by methyl methacrylate groups, at low concentrations of the polymer in solution one observes the monomer luminescence of BF2-benzoylacetonate groups, while at high concentrations – the excimer luminescence. In case of PS-based polymers, in which BF2-benzoylacetonate groups and phenyl rings are not separated, in diluted solutions one observes the fluorescence of the intramolecular exciplexes, while at the concentration increase – the luminescence of intermolecular exciplexes. The ability of excimer formation is responsible for the increased photostability of the produced polymers.

Amorphous porphyrin glasses exhibit near-infrared excimer luminescence

The amorphous nature of a series of zinc–porphyrins bearing two 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups at the meso-positions, named “porphyrin glass”, were tolerant of π-conjugation engineering in ethynylene-linked dimers.

Nitrogen-Containing Analog of Dibenzoylmethanate of Boron Difluoride: Luminescence, Structure, Quantum Chemical Modeling, and Delay Fluorescence.

Boron difluoride of 3-amino-1,3-diphenyl-2-propene-1-onate (1) has been synthesized and its crystal structure has been determined. The comparative studies of 1 and its oxygen analog 1,3-diphenyl-1,3-dionate (dibenzoylmethanate) of boron difluoride (2) have been performed using the methods of stationary and time-resolved spectroscopy and quantum chemical modeling. It was established that at the transition from solutions to crystals, a bathochromic shift of the spectra and a significant increase of luminescence intensity of 1 take place. The luminescent properties of solutions of 1 and 2 are similar. The peculiarities of crystal packings of 1 and 2 are responsible for differences in crystals luminescent properties. For crystals of 2, one observes the luminescence of J-aggregates and excimers, while for 1, in which a dimer is an elementary structural fragment, only the excimer luminescence is registered. A delayed excimer fluorescence of the P-type was observed for crystals of 1 and 2 at room temperature. The intensity of the delayed fluorescence of 1 is 300-fold higher than that of 2. Graphical Abstract Luminescence of J-aggregates and the formation of excimers in crystals of 1 and 2.

Configuration of organic dye excimers in nanoporous SiO2 matrices

The effect of cyanine dye 3,3′-dioctadecyloxacarbocyanine perchlorate (DiO) and benzimidazole dye 4-dimethylamino-1,8-naphthoylene-1′,2′-benzimidazole (DNBI) accumulation in nanoporous silica matrices on the dyes luminescence properties has been studied. For both dyes, ground state dimer formation with perpendicular transition dipoles at high dye concentrations has been considered as a result of restricted geometry of the nanoscale pores. The dimer excitation leads to excimer formation revealing by appearance of new long-wavelength luminescence band and shortening the dye luminescence lifetime. In the excimer luminescence excitation spectra two additional bands have been observed, one of which is bathochromically shifted relatively to the absorption band and another one is hypsocromically shifted. Using the Kasha exciton model it was shown that the excimers possess oblique transition dipoles configuration.

Cross-shaped photoluminescence of excimers in perylene crystals

Cross-shaped excimer (self-trapped exciton) luminescence from α- and β-perylene single crystals of 50–100 μm was found when they were excited at the center of the crystals with a continuous-wave (cw) laser resonant with the exciton absorption. The cross shape is formed by the two lines which intersect at the excited position and are perpendicular to the sides of the crystals of parallelogram shape. Luminescence is emitted from the excited spot and 4 side edges in the cross shape. The most striking feature is that the luminescence intensity at the edges was as high as or higher than at the excited spot. The possibility of the exciton propagation or the waveguide effect is rejected both experimentally and theoretically. This phenomenon can be reasonably explained only when the radiative transition probability of excimers is significantly enhanced at the crystals side edges than at the center due to the lower symmetry.

Photoinduced excimer formation of boron difluoride β-diketonates in poly(methyl methacrylate)

The photochemical behavior of β-diketonates of boron difluoride in poly(methyl methacrylate) was studied. Excimers of boron chelates are formed in the course of UV irradiation of polymer compositions accompanied by a bathochromic shift of the luminescence spectrum. The intensification (quenching) of luminescence of the polymer compositions during irradiation depends on the ratio of the quantum yields of the monomer and excimer luminescence of boron difluoride β-diketonates.

Electronic communication between fluorescent pyrene excimers and spin crossover complexes in nanocomposite particles

The mechanical strain induced fluorescence lifetime variation in hybrid bistable [Fe(H-trz)2(trz)]BF4@SiO2@pyrene nanoparticles.

Effect of Local and Global Structural Order on the Performance of Perylene Diimide Excimeric Solar Cells

Herein, we present a detailed study of the structure-function relationship in the organic photovoltaic (OPV) blend film composed of N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) and the low energy gap copolymer of poly[4,8-bis-substituted-benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-4-substituted-thieno[3,4-b]thiophene-2,6-diyl] (PBDTTT-E-O). The hierarchical organization in the photoactive layers and in extruded fibers of PBDTTT-E-O:EP-PDI was studied by fluorescence optical microscopy, atomic force microscopy, and wide-angle X-ray scattering (WAXS). WAXS revealed a nanophase-separated structure where PBDTTT-E-O domains of 4.3 nm in size coexist with EP-PDI domains of 20 nm size. Thermal annealing results in an increase of the PBDTTT-E-O domains, but it does not affect the size of the EP-PDI domains. Only the length of the EP-PDI columns in each domain is increased by thermal treatment. The photophysical characterization of the PBDTTT-E-O:EP-PDI layers and the electrical characterization of the corresponding OPV and unipolar carrier devices were performed. The quenching of the EP-PDI excimer luminescence is correlated with the photocurrent generation efficiency of the OPV devices. At high annealing temperatures the EP-PDI columnar length becomes larger than the previously reported diffusion length of the PDI excimer, and fewer excimers dissociate at the EP-PDI/polymer interfaces, leading to reduced photocurrent generation. The charge transport properties of the PBDTTT-E-O:EP-PDI blend film were studied as a function of the active layer microstructure that was tuned by thermal treatment. Thermal processing increases electron mobility, but the poor connectivity of the EP-PDI domains keeps hole mobility six times higher. In respect to the as-spun OPV device, a 3-fold increase is found in the power conversion efficiency of the device annealed at 100 °C. The high surface roughness of the PBDTTT-E-O:EP-PDI photoactive layer impedes the efficient extraction of charges, and a thin and smooth perylene-3,4,9,10-tetracarboxylic bisbenzimidazole overlayer is required for increasing the device performance to a power conversion efficiency (PCE) ∼ 1.7%. The inversion in the polarity of the device contacts resulted in an inverted device with PCE ∼ 1.9%. We provide rational guidelines for the accurate tuning of the layer microstructure in PDI-based photoactive layers of efficient OPV devices. Local disorder in the EP-PDI aggregates is essential (i) for the optimum electron transport that is ensured by the efficient connectivity of the EP-PDI columns in adjacent EP-PDI domains and (ii) for preventing the stabilization of the neutral photoexcitations in the EP-PDI domains in the form of slowly diffusive excimers. The high photocurrent generation efficiency achieved suggests the EP-PDI excimers are formed faster than the activation of triplet states, and photocurrent losses are minimized.

Excimer Luminescence From Nonresonantly Excited Pyrene and Perylene Molecules in Solution

By nonresonant excitation below the absorption edge of pyrene and perylene molecules in solution, efficient excimer luminescence is observed. For perylene in solution, there are two kinds of nonresonant excimer emission that closely resemble Y- and E-emissions in perylene crystal and perylene in polymer. The concentration dependence of the photon flux density of monomer and excimer emission is reasonably explained by a simple model with a few fitting parameters based on the ordinary excimer formation process, in which the excited-state monomer interacts with a ground-state monomer. Observed quantum efficiency of excimer formation is by orders of magnitude higher than that expected if a monomer is excited from thermally populated vibrational levels. The experimental evidence for direct resonant excitation of excimers in perylene nanocrystals is obtained from the absence of anti-Stokes emission. This is due to a symmetry-breaking effect close to crystal surface, suggesting that excimers are formed through a more efficient process in solution, such as resonant excitation of weakly interacting molecules with a symmetry-broken intermolecular configuration.

Optical properties of KDP crystals doped with pyrenetetrasulfonic acid salt

KDP crystals with incorporated molecules of the luminophore 1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt (SPA) were grown from KH2PO4 solutions by the temperature lowering method. The luminescence spectra of KDP/SPA crystals and SPA solutions in distilled water and in KH2PO4 solutions, were studied. The increase of the dye content gives rise to excimer luminescence in the solutions and in KDP crystals, which is due to the formation of associated dye molecules. SPA molecules possessing negative electrostatic potential are incorporated into the pyramidal growth sector of KDP crystal.

Size‐dependent fluorescence of dibenzoylmethanate and ditoluylmethanate of boron difluoride

Significant differences in the luminescence properties of dibenzoylmethanate and ditoluylmethanate of boron difluoride bulk crystals and microcrystals were detected. The size-dependent blue-to-green colour change of the luminescence is connected with the band intensity redistribution of monomer and excimer luminescence and fluorescence due to interdimer interaction.

Bis(benzoxazolyl)stilbene excimers as temperature and deformation sensors for biodegradable poly(1,4-butylene succinate) films

In this work, new and innovative polymeric film sensors based on excimer luminescence and responsive to both mechanical and temperature stress were obtained through the dispersion of moderate amounts (0.02–0.2 wt%) of the food-grade dye bis(benzoxazolyl)stilbene (BBS) into a thermoplastic aliphatic biodegradable polyester [poly(1,4-butylene succinate), PBS]. Emission from BBS excimers emerged with dye concentrations higher than 0.05 wt% conferring to the film a green luminescence (λexc. = 366 nm). On applying mechanical stress at rt, the PBS reorganization efficiently breaks the BBS arrangements leading to the prevalence of blue emission from the excited isolated chromophores. Moreover, the optical behaviour of PBS–BBS quenched blends was thermally affected in the range 50–80 °C, providing composite films characterized by a very sensitive temperature dependent luminescence response. The easy modulation of the luminescent properties of polyester films by varying the supramolecular organization of BBS dispersed molecules by thermal or mechanical perturbations suggests various applications in the field of smart and intelligent films from thermoplastic materials.

Photoluminescence of liquid-crystal azo derivatives in nanopores

The fluorescence of the nematic liquid crystal n-butyl-n′-methoxyazoxybenzene (BMAOB) in the form of a layer and in porous glasses with pores of different diameter is investigated. The fluorescence spectra of BMAOB in a layer and in small-diameter pores differ significantly from each other. A band peaked at 475 nm is observed in the stationary spectrum of a BMAOB layer, whereas the spectrum of BMAOB in pores smaller than 4 nm in diameter contains a band peaked at 550 nm. When the pore diameter decreases to 1 nm, the band at 475 nm is almost completely suppressed and only the band at 550 nm manifests itself. It is believed that the band at 550 nm is due to the excimer luminescence of azo molecules. The fluorescence of a BMAOB layer indicates the occurrence of efficient trans-cis isomerization upon UV excitation. At the same time, another mechanism of phototransformation is dominant for BMAOB in porous matrices. Most likely, it is photodestruction caused by interaction of BMAOB molecules with oxygen and water bound in pores. Fluorescence of porous glasses is also revealed. Their fluorescence band shifts to shorter wavelengths with a decrease in the pore diameter. The fluorescence of porous glasses with filled pores is not detected, apparently, due to nonradiative energy transfer to BMAOB molecules.