Pyrene was inserted into a low-density polyethylene matrix. Fluorescence spectra as a function of temperature and the differential scanning calorimetry (DSC) trace were recorded simultaneously. Along with the usual vibronic bands, a low-intensity band at 365 nm appears at higher energies with respect to the 0−0 transition in the pyrene fluorescence spectra. The fluorescence intensity of this small band increased with temperature, and an isoemissive point was observed to occur at 368 nm.This emission was interpreted as arising from pyrene molecules located in the outer rigid interfacial region of polymer crystallites. Its temperature-dependent fluorescence was interpreted in terms of electron−phonon coupling; two phonons which coincide with fundamental vibrations of polyethylene were necessary to fit experimental data. Coupling with a high-energy phonon was possible at low temperature, whereas, above the β relaxation temperature, phonon coupling occurs with a lowerenergy phonon. The α relaxation was detected as a maximum in fluorescence intensity since above its characteristic temperature, nonradiative processes begin to operate. l. IntroductionLuminescent techniques have long been employed in biological systems. 1 However, only recently have these techniques begun to be considered as customary tools in synthetic polymer research. 2 , 3 The main benefits of using molecular fluorescent probes and labels are the multiple interactions that may occur between excited states and their environment. Using an adequate probe selection and insertion procedure, different research topics of current interest can be approached: polymer miscibility, 4 conformational dynamics in solution,S· 7 diffusion in polymers, 8 surface characterization,9 poly mer relaxation processes, 10,11 and monitoring of polym erization reactions. 12 · 13 Another important experimen tal advantage is that fluorescent probes can be incor porated either intrinsically or extrinsically with very low concentration and fluorescence can be detected with very high sensitivity.Secondary relaxation processes of polyethylene (PE}, although extensively studied, are not fully understood because of the morpholo i.\ ical and microstructural com plexity of this polymer. 1 , ts Using anthracene lumines cence, 1 6 one of the authors observed that the change of fluorescence intensity with temperature parallels the three main relaxations reported for polyethylene. Since this molecular probe was not located inside the crystal line region, fluorescent intensity changes were inter preted as due to relaxations in the amorphous bulk polymer and at the crystallite interfaces. In fact, it is currently thought that the interfacial crystalline struc ture is coupled with the motion within the crystallite interior. 14 Location of fluorescence probes at the crys tallite interfaces may prove to be a powerful tool for studying such kind of coupled crystallite and interfacial relaxation processes. In addition, because of the dif ferent time and length scales in which thermal, dynamic mech...

In this study two salicylidene ligands, N,N'-bis(salicylidene)-1,2-phenylenediamine and N,N'-bis(salicylidene)-4,5-diaminopyrimidine, and their respective aquo-zinc(II) coordination compounds were synthesized. Their characterization was performed by FTIR, proton and carbon NMR, elemental analysis,mass spectroscopy and cyclic voltammetry. Crystal structures of the ligands were determined by monocrystal X-ray diffraction. The photoluminescent properties under photostationary conditions indicate that the ligand emission predominates in both the pristine materials and their zinc(II)complexes. For both ligands, the coordination of a metal atom leads to a redshift of their emission bands in both solvent and solid state. Molecular structures and excitation energies of ligands and complexes were evaluated at DFT level using PBE0/aug-cc-pVDZ. Their ligand and complex electronic transitions can be assigned mainly to the intraligand π→π * type, mainly involving frontier molecular orbitals, with a small participation of the metal. According to our calculations, there is an increasing in the planarity of the ligand structure in the complex, which could explain the redshifting observed in the absorption and emission spectra. The dynamic photoluminescence suggests the occurrence of excited state intramolecular proton transfer from the oxygen to the nitrogen atoms in the coordination site of the sal-4,5-pym.Moreover, they are able to predict the occurrence of the excited state internal proton transfer for the sal-4,5-pym. The dynamic of this proton transfer is demonstrated by both, time resolved emission spectra (TRES) and studies in protic solvent (ethanol).

Phosphorescence from four aromatic ketones (xanthone, benzophenone, flavone, and 9-methoxyflavone) has been used to study secondary α-, β-, and γ-relaxation processes in polystyrene and poly(n-alkyl methacrylate)s (n-alkyl = methyl, ethyl, and butyl) in the temperature range 15−400 K. Plots of normalized phosphorescence intensity versus temperature and the respective Arrhenius curves showed changes of nonradiative deactivation efficiencies, which were attributed to polymer secondary relaxation processes. In addition to the phosphorescence emission, flavones in poly(n-alkyl methacrylate)s also exhibited fluorescence emission ascribed to the protonated moiety. The presence of these two emissions indicated a probe distribution sensing two different microenvironments:  protic (fluorescent) and nonprotic (phosphorescent) sites. Protic sites reveal the existence of acidic residues copolymerized to the methacrylic polymer chains. Plots of fluorescence intensity versus temperature have been used to determine the polymer relaxation processes of these acidic domains always revealing a higher onset temperature compared to the phosphorescence emission. The influence of both the size and the shape of these probes has also been analyzed, demonstrating that small molecules (xanthone and benzophenone) are more sensitive to shorter segmental motions occurring at lower temperatures.

The dependence of several aspects of the fluorescence from 9-anthryl groups, covalently attached by tethers of differing lengths to interior sites of five polyolefinic films whose crystallinities range from 0 to 74%, has been explored between 40 and 400 K. The data are employed to determine microscopically the onsets of various relaxation processes of the polymers, the sensitivity of the probes to changes in their local morphologies, and the distribution of the groups between amorphous and interfacial sites. The results are compared with those from noncovalently attached (doped) 9-methylanthracene molecules. Specifically, the relationship between the lengths of tethers for the 9-anthryl groups, as well as the chemical method by which each tether is attached to the polymer chains, and the ability of the fluorescence from the lumophores to detect the onsets of host relaxation processes are explored. The two attachment methods lead to different distributions of lumophores in the amorphous and interfacial regions of the polymers. Furthermore, very short (one atom; methylene) and very long (12 atoms) tethers allow the 9-anthryl groups to sense changes in the local environments more acutely than a tether of intermediate length (three atoms). The advantages of covalent attachment for this sort of study (specifically, the inability of the probe to diffuse within a film from site-to-site with time) and the limitations of the utility of the data are discussed.

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ESIPT for white-light generation from all-solution-processed OLEDs.

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