Mechanism Of Excimer Formation Research Articles

The mechanism of excimer formation: an experimental and theoretical study on the pyrene dimer.

The understanding of excimer formation in organic materials is of fundamental importance, since excimers profoundly influence their functional performance in applications such as light-harvesting, photovoltaics or organic electronics. We present a joint experimental and theoretical study of the ultrafast dynamics of excimer formation in the pyrene dimer in a supersonic jet, which is the archetype of an excimer forming system. We perform simulations of the nonadiabatic photodynamics in the frame of TDDFT that reveal two distinct excimer formation pathways in the gas-phase dimer. The first pathway involves local excited state relaxation close to the initial Franck-Condon geometry that is characterized by a strong excitation of the stacking coordinate exhibiting damped oscillations with a period of 350 fs that persist for several picoseconds. The second excimer forming pathway involves large amplitude oscillations along the parallel shift coordinate with a period of ≈900 fs that after intramolecular vibrational energy redistribution leads to the formation of a perfectly stacked dimer. The electronic relaxation within the excitonic manifold is mediated by the presence of intermolecular conical intersections formed between fully delocalized excitonic states. Such conical intersections may generally arise in stacked π-conjugated aggregates due to the interplay between the long-range and short-range electronic coupling. The simulations are supported by picosecond photoionization experiments in a supersonic jet that provide a time-constant for the excimer formation of around 6-7 ps, in good agreement with theory. Finally, in order to explore how the crystal environment influences the excimer formation dynamics we perform large scale QM/MM nonadiabatic dynamics simulations on a pyrene crystal in the framework of the long-range corrected tight-binding TDDFT. In contrast to the isolated dimer, the excimer formation in the crystal follows a single reaction pathway in which the initially excited parallel slip motion is strongly damped by the interaction with the surrounding molecules leading to the slow excimer stabilization on a picosecond time scale.

H-bonding vs non-H-bonding in 100% pyrene methacrylate comb polymers: self-assembly probed by time-resolved emission spectra and temperature dependent fluorescence.

The differences in self-organization behavior in novel 100% pyrene labeled comb methacrylate polymers probed as a function of their varied origins of excimer formation are presented. The different structural variations in the polymers included the presence or absence of hydrogen bonding interactions in the form of urethane linkages, short or long alkyl spacer segments separating the pyrene units from the polymer backbone and linear versus kinked urethane linkage. The effect of variable concentration and temperature on the chemical shift of the NH proton of the urethane linkage was probed using (1)H NMR experiments conducted at temperatures varying from 25 to 70 °C at two different concentrations (2.5 and 25 mmol) in DMSO-d6 as solvent. The photophysical properties of the polymers in dilute DMF solutions were investigated by steady state emission, fluorescence decay studies, time-resolved emission spectra (TRES), and variable temperature emission studies. It was observed that the polymer poly(PBH) having a non-hydrogen-bondable ester linkage in the pendant chains formed an excimer completely via a static mechanism and the ground state aggregate species were not broken even at higher temperatures. The polymer poly(PIC) having a short hydrogen-bondable urethane linkage formed an excimer via a static as well as dynamic mechanism. The other hydrogen-bondable urethane methacrylate polymers having a linear linker poly(PHH) and kinked linker (PIHP) formed excimer mostly via a dynamic mechanism with a very small contribution from the static route. The TRES studies carried out for the polymers provided significant insight into the excimer formation mechanism in these polymers. The variable temperature fluorescence studies highlighted the differences in the H-bonded vs non-H-bonded polymer as a function of their excimer recovery upon cooling.

Processes of molecular association and excimeric emission in protonated N, N-dimethylformamide

Formation of associates of N, N-dimethylformamide (DMF) molecules was studied to clarify their role in photoluminescent activity of protonated DMF solutions. The association of DMF molecules was observed in dilute aqueous solutions at concentrations of DMF above ∼4x10 −2 M. The association is enhanced when the C O bond of the DMF molecule is activated by protonation with hydrochloric acid, which leads to appearance of an excimeric emission at ∼530 nm. The excitation spectrum of the excimeric emission showed the excitation maximum in the region of the absorption of DMF associates, which is a first evidence of a more complex mechanism of excimer formation originating from excitation of associated rather than monomeric molecules in the ground state. A simple approach was provided to evaluate a number of molecules in the excimer structure. An original theory has been developed, and it was calculated that the DMF excimer has a dimeric nature. A model of the excimer formation was proposed, which suggests that a hydrogen-bonded associate is an intermediate form leading to the excimeric structure upon excitation. It was observed that DMF possesses also a monomeric emission with the emission maximum at ∼385 nm, which was attributed to the intramolecular charge-transfer process. It has been found that the change in structure of the DMF associates via the liquid–solid phase transition affects both excitation and emission bands of excimers, so that the excimeric emission shifts to the blue region and intermixes with the emission of DMF monomers.

The Photophysics of 6-(1-Pyrenyl)hexyl-11(1-pyrenyl)undecanoate Dissolved in Organic Liquids and Supercritical Carbon Dioxide: Impact on Olefin Metathesis

The Leitner and Fürstner groups reported (Fürstner, A.; Koch, D.; Langemann, K.; Leitner, W.; Six, C. Angew Chem., Int. Ed. Engl. 1997, 36, 2466) on the ring closing metathesis (RCM) of a 16-membered diene dissolved in supercritical CO2 (scCO2). The authors found that the cyclic product, indicative of an intramolecular RCM event, was formed in excellent yield when the CO2 density was high, but oligomers were formed by an acyclic diene metathesis (ADMET) pathway at lower CO2 densities. These results suggest that changes in the CO2 density lead to changes in the intra- vs intermolecular interactions between the 16-membered diene dissolved in scCO2. To assess this issue in more detail, we have prepared 6-(1-pyrenyl)hexyl-11-(1-pyrenyl)undecanoate [1-Py(CH2)10COO(CH2)61-Py] in which we replaced the terminal alkenes of Letiner and Fürstner's original diene with the fluorophore pyrene. We have studied the pyrene excimer formation of 1-Py(CH2)10COO(CH2)61-Py when it is dissolved in five organic solvents (cyclohexane, dichloromethane, ethanol, acetonitrile, and dimethyl sulfoxide) and supercritical carbon dioxide (scCO2) to determine how the tail segments interact with each other. The result show that the excimer formation mechanism is completely different when 1-Py(CH2)10COO(CH2)61-Py is dissolved in scCO2 or organic liquids. In liquids, excimer formation is purely dynamic in nature, there are two formation pathways to the excimer, and all the rates can be understood with the help of Kamlet−Taft linear solvent energy relationships. In scCO2, we found that the 1-Py(CH2)10COO(CH2)61-Py excimer-to-monomer intensity ratio (E/M) correlates directly with (1) the observed RCM yield for Leitner and Fürstner's original 16-membered diene and (2) the solvent refractive index function. The steady-state and time-resolved fluorescence of 1-Py(CH2)10COO(CH2)61-Py dissolved in scCO2 show that there are two excimers that form in scCO2 and their relative contributions change in a systematic way with changes in the CO2 pressure/density. Interestingly, the typical dynamically formed excimer species that emits at 470−480 nm (E1) forms within 2 ns of optical excitation; however, it is not the dominant species at low CO2 densities. E1 is equivalent to the species that goes on to form the RCM product in Leitner and Fürstner's original reaction. The second excimer (E2) emits in the 410−440 nm region. E2 is associated with intermolecular preassociated forms of the pyrene residues within a collection of 1-Py(CH2)10COO(CH2)61-Py molecules, and this species dominates at low CO2 densities. E2 is equivalent to the species that goes on to form the oligomeric product in the original Leitner and Fürstner reaction. As the CO2 density increases, the E1 excimer contribution increases relative to the E2 excimer contribution. The combination of the fluorescence and reaction outcome results are used to explain Leitner and Fürstner's previous density-dependent RCM yields.

Solvent and concentration effects on the steady state fluorescence of fluorenone

The electronic absorption and steady state fluorescence spectra of fluorenone in the regions 280–340 and 320–420 nm respectively have been obtained at room temperature for various concentrations in a series of non-polar and polar solvents. The effects of fluorenone concentration in various solvents in the range 10 −6−10 −3 M on the spectral properties are discussed. In addition to the monomer fluorescence, we have also observed another fluorescence band at longer wavelength in the region 420–600 nm which has been ascribed to excimer formation. The monomer and excimer fluorescence lifetimes of fluorenone in typical solvents such as benzene and acetonitrile have been determined experimentally. By combining our results of steady state fluorescence and measurements of fluorescence lifetimes with previously reported studies of laser-induced fluorescence, the mechanism of excimer formation has been explained. The absence of excimer formation in hexane, cyclohexane and alcohols is discussed.

Excited state formation in pulse irradiated polyethylene doped with aromatics

The absorption/emission spectra and kinetics of excited states (singlets and triplets) and ions generated by pulse radiolysis method in the polyethylene-solute (dimethyldiphenyl, pyrene) systems were investigated. It was found that excited states of aromatic admixtures were formed as a result of ion recombination in micro/millisecond time scales at temperatures higher than T g. Solute excimer emission was observed from pulse-irradiated PE samples at room temperature. The mechanism of excimer formation was discussed.

Excimer formation mechanism in gaseous krypton and Kr/N2 mixtures

A new approach for the understanding of the energy relaxation dynamics of excited atoms involving a long-lived molecular precursor is presented here for krypton. Excitation of the gas close to the 5s[3/2]2 metastable atomic level (E at. −E exc.<kT) is achieved with an intense VUV laser source (I ≈ 1012 photon/pulse) realized by resonantly enhanced 4-wave mixing (2ω1 + ω2) in room temperature mercury vapor (N Hg ≈ 1013 at./cm3). The decay of the II. continuum luminescence (145 nm) is studied. In the pressure range 200–500 mbar, decay rates depend linearly on pressure but have a negative zero-pressure intersect. We show here that this result can be understood as an effect of the exchange of energy between two different “reservoirs” of atomic (5s[3/2]2) and molecular (1g) nature, and can be an inherent peculiarity of the recombination kinetics of excited atoms with several product channels. The efficiency of the model is checked for the Kr/N2 system. Rate constants for relaxation processes are determined in pure krypton and in Kr/N2 mixtures.

Convolution analysis of the pyrene excimer formation in membranes

The mechanism of excimer formation in rigid and fluid membranes is analyzed from the fluorescence dynamics of both monomer and excimer emissions. We analyze the kinetics of the excimer emission using a convolution formalism. In the rigid state of distearyldimethylammonium chloride bilayers, we observed a discrepancy between the decay of the monomer and the excimer populations. This shows that a fraction of the pyrene probes is isolated and is not submitted to excimer formation. The convolution analysis enables us to estimate with good precision the fraction of isolated monomers. We have analyzed the monomer emission using transient diffusion models

Extended mechanism of excimer formation

A modified excimer formation mechanism using an encounter complex, in which the main reaction takes place, is presented and the rate coefficients of the elementary reaction steps are discussed. The kinetic description by ordinary rate equations fails in the case of excimer formation with a time-dependent rate coefficient caused by non-stationary diffusion. Convolution kinetics remain valid in that case and provide decay curves and quantum yields.

Excited-state annihilation process involving a cyclometalated platinum(II) complex

The Pt(tpy)(ppz) complex exhibits strong luminescence with a relatively long excited-state lifetime (15.3 {mu}s) in deaerated acetonitrile solution, at room temperature and at low excitation intensity, and can be easily involved in excited-state quenching processes. The {sub 3}CT excited state is, in fact, quenched (1) by oxygen (k{sub q} {congruent} 10{sup 9} M{sup {minus}1} s{sup {minus}1}), (2) by the ground-state complex (k{sub q} = 5.7 {times} 10{sup 7} M{sup {minus}1} s{sup {minus}1}), and (3) by another {sup 3}CT excited state in an annihilation process, which is practically diffusion controlled (k{sub 3} > 6 {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1}). The ground-state quenching and the annihilation process most probably occur via an excimer formation mechanism. 46 refs., 3 figs.

Excimer formation in irradiated high density polyethylene doped with aromatics

Luminescence from γ-irradiated high density polyethylene doped with naphthalene and pyrene was investigated. Excimer emission of the aromatic admixture was observed in the temperature range (200–230 K) of the high temperature radiothermoluminescence peak. During nanosecond pulse radiolysis, traces of excimer emission were recorded in the same temperature range. Pyrene excimer emission was observed from pulse-irradiated samples at room temperature. The mechanism of excimer formation is discussed.

Critical Examination of the Calculus Convolution Kinetics: The Influence of Nonstationary Diffusion on Excimer Formation with Dissociation

AbstractIf time dependent rate coefficients (TDRC) appear in reactions, ordinary rate equations (RE) fail. The calculus convolution kinetics (CK) provides consistent results in these cases, but its appliance is quite unfamiliar. For rate coefficient(s) (RC) that are time‐independent the results of CK are analytically identical with those of RE, though being obtained on a completely different way. – In this paper the failure of the RE method will be briefly demonstrated with an example. Furthermore the concept of CK will be presented in short and applied on excimer formation. On the level of synthesis data a comparison of the result of the rate equations' treatment with √t‐term obtained numerically, the CK result and measuring data will be carried out. For this the time resolved decay curves of “single photon counting” – measurements and synthesis data of both methods convoluted by flash will be graphically laid on top of each other. It turns out that curves from CK represent the measuring graph better than data of the RE calculated with the same parameters. Larger differences appear in the initial part of the curves with both methods. To explain these differences a new mechanism of excimer formation will be brought into discussion, but also the uncompletness of the Smoluchowski approximation for nonstationary diffusion will be taken into account. – The kinetical treatment given below is an example of excited state kinetics [1]; but CK is not limited on this kind of kinetics.

Monomeric and aggregated pyrene and 16-(1-pyrenyl)hexadecanoic acid in small, unilamellar phosphatidylcholine vesicles and ethanol—buffer solutions

Excimer formation of pyrene (PY) and 16-(1-pyrenyl)hexadecanoic acid (C 16PY) incorporated into small unilamellar vesicles of phosphatidylcholine (SUV-PC) was studied by absorption and fluorescence spectroscopy. We also examined the spectra of C 16PY dissolved in ethanol and in binary mixtures of ethanol and buffer to compare the characteristics of ground state aggregated and monomeric forms of the probe. The experiments performed in ethanol—buffer mixtures give evidence of aggregation of the pyrenyl moieties of C 16PY. In contrast, with up to about 4.0 mol.% of probe incorporated (PY or C 16PY) into SUV-PC, no ground state dimers, or any other type of molecular aggregate, could be detected. We conclude that under these experimental conditions the mechanism of excimer formation in SUV-PC is a purely dynamic process, most probably a diffusion-controlled probe—probe interaction. An important point to arise from this work is that the incorporated probe concentration relative to the membrane lipid content should be taken into account in order to distinguish between true excimer emission and excimer-like emission from ground state aggregates.

Characteristics of excimer formation in langmuir-blodgett assemblies of 1-palmitoyl-2-pyrenedecanoylphosphatidylcholine and dipalmitoylphosphatidylcholine

Langmuir-Blodgett film alloys of PPDPC (1-palmitoyl-2-[10-(pyren-1-yl)]decanoyl- sn-glycero-3-phosphocholine) and DPPC (1,2-dipalmitoyl- sn-glycero-3-phosphatidylcholine) were assembled in varying molar proportions on quartz glass slides. The transferred layers were then characterized according to their pyrene excimer to monomer fluorescence intensity ratio i E/ i M and fluorescence quantum yields as a function of film composition. The observed deviation from non-ideal mixing is considered to be due to the formation of regular distribution patterns of PPDPC in a DPPC lattice. The observed critical mole fractions of PPDPC evident as steps in I E/ I M versus X PPDPC plots can be accounted for by a model involving a trigonal distribution of pyrenedecanoyl chains in the phospholipid acyl chain lattice. The implications of the distribution of PPDPC as superlattices to the excimer formation mechanism are discussed.

Excimer formation of N-(1-pyrenesulfonyl)-dipalmitoyl-L-alpha-phosphatidylethanolamine at the lipid-water interface of fat cell plasma membranes.

Abstract— The mechanism of excimer formation of N‐(l‐pyrenesulfonyl)‐dipalmitoyl‐L‐α‐phosphatidylethanolamine incorporated into fat cell plasma membranes was studied by means of steady‐state and pulse fluorometry. It was found that the pyrenesulfonyl group at the lipid‐water interface of the membrane formed an appreciable amount of ground‐state dimers and that the excimer of the pyrenesulfonyl group was formed through two different processes: (i) the collisional interaction of an excited monomer with a ground‐state monomer, and (ii) the direct excitation of a ground‐state dimer (approximate ratio of the two processes, 1:1). Analysis of fluorescence decay curves revealed that the detailed mechanism of excimer formation by process (i) is essentially similar to the mechanism established for the reaction in organic solvents but not to that in a highly viscous medium such as the hydrocarbon core of the membrane.

On the mechanism of excimer formation in triplet—triplet annihilation

The effects of temperature and solvent on the magnetic field dependence of delayed monomer and excimer fluorescence of aromatics in solution are discussed. The field effect data elucidate the mechanism of excimer formation in triplet—triplet annihilation.

A study on mechanism of excimer formation through fluorescence quenching for isotactic and atactic polystyrenes and poly(p‐methylstyrenes) in solution

AbstractThe mechanism of excimer formation and deactivation for isotactic and atactic polystyrenes, and isotactic and atactic poly(p‐methylstyrenes) was studied by comparison of the rate parameters which were calculated from the fluorescence quenching and the decay time measurements of excimer emission band. The activation energy of excimer formation was obtained from the effect of temperature change on fluorescence intensity. The rate parameters of excimer formation for isotactic polymers are in the order of 109 s−1 and larger than those for atactic polymers. The activation energy for isotactic polymers is smaller than that for atactic polymers. These results show that isotactic polymers can form the excimer more easily than atactic polymers. The facility of excimer formation for isotactic polymers is interpreted as lower energy barrier required to achieve the excited excimer state.

Magnetic field dependence of monomer and excimer delayed fluorescence of aromatics in fluid solution

We have extended our study of the magnetic field dependence of monomer ( M M) and excimer ( I D) delayed fluorescence of aromatics in fluid solution. I M and I D show a monotonic decrease with increasing field strength at room temperature in agreement with theoretical predictions. At lower temperatures both I M and I D show an initial increase followed by a monotonic decrease. This marked temperature dependence can be attributed to a “freezing-out” of the effect of molecular motion on the field effect. At even lower temperatures the field effect curves of I M and I D start to diverge. Two models have been proposed to explain the mechanism of excimer formation in triplet—triplet annihilation. Qualitatively at least the divergence can be rationalized in terms of both models.