Pierre Valat

Spectroscopic properties of aromatic dicarboximides. Part1.—N—H and N-methyl-substituted naphthalimides

The photophysical properties of the N—H and N-methyl derivatives of 1,2-, 2,3- and 1,8-naphthalimides have been studied. The shift of the fluorescence emission position as a function of the solvent polarity indicates only a weak variation of dipole moment for the excited state compared with the corresponding value in the ground state (5.7 D for 2b, 2.8 D for 3b and <2 D for 4b, 1 D ≈ 3.335 64 × 10–30 C m, and 2b, 3b and 4b are N-methyl-1,2- naphthalimide, N-methyl-2,3-napthalimide and N-methyl-1,8-naphthalimide). However, important modifications of the photophysical properties are observed which depend on the relative position of the dicarboximide moiety on the naphthalene ring: the intersystem crossing rate constant of 4b increases dramatically by three orders of magnitude compared with that of 2b; simultaneously, the fluorescence quantum yield decreases from 0.77 to 0.03, although the corresponding rate constant, kf, increases. This difference is found to arise from the energy gap between the lowest1(π, π*) singlet excited state and the upper 3(n,π*) triplet state, which is of the order of 9 kcal mol–1 for 2b and less than 2 kcal mol–1 for 4b in acetonitrile solution. Protic solvents increase the energy difference between the n,π* and π,π* states thus decreasing the mixing of the two levels; as a consequence, the lifetime of 4b is increased, i.e. from <60 ps in hexane to 2.1 ns in trifluoroethanol. A triplet–triplet annihilation process occurs with the N-methyl derivatives 3b and 4b which leads to a monomer delayed fluorescence with the former, and mainly to a delayed excimer emission with the latter.

Photoluminescence of polycrystalline zinc oxide co-activated with trivalent rare earth ions and lithium. Insertion of rare-earth ions into zinc oxide

Abstract The photoluminescence of polycrystalline sintered ZnO codoped with Li + and trivalent rate-earth (RE 3+ ) ions (Dy 3+ , Er 3+ , Eu 3+ , Ho 3+ , Nd 3+ , Sm 3+ and Tm 3+ ) has been studied. The luminescence spectra depend upon the nature of the rare earth. The UV excitation of the Eu 3+ -, Dy 3+ -, Sm 3+ -and Tm 3+ -doped samples induces the luminescence of the RE 3+ ions: in addition to the broad ZnO pattern, the spectra show the characteristic lines of the 4f transitions of the RE 3+ ions. The UV excitation of the Er 3+ -, Ho 3+ -and Nd 3+ -doped samples induces only the luminescence of ZnO with a partial reabsorption of the light by the RE 3+ ions, no emission from the RE 3+ ions being observed. Furthermore, the presence of Li + ions increases the absorbance of the RE 3+ ions and enables the observation of their luminescence under direct 4f-4f photoexcitation. Additional experiments, X-ray analysis, scanning electron micrography and energy dispersive spectrometry indicate that the RE 3+ ions are mainly located outside the ZnO microcrystals in the grain boundaries of the polycrystalline matrix where they are closely associated with Li + ions. An energy transfer between ZnO and the RE 3+ ions is evidenced only in the case of Tm 3+ . It is proposed that this energy transfer is a consequence of an electron/hole pair recombination at the grain boundaries involving the Tm 2+ semireduced form.

Photoluminescence of semiconducting zinc oxide containing rare earth ions as impurities

Abstract The photoluminescence of zinc oxide sintered at 1100°C and doped with rare earths (Tm3+, Ho3+, Nd3+ and Er3+) has been investigated under such excitation conditions (around 390 nm) where the light is principally absorbed by ZnO. Contrary to previous reports, no emission from the rare earth could be observed and only the luminescence of the semiconducting substrate in which the rare earth put the fingerprint of its absorption has been characterized. Such partial reabsorption of the green emission of ZnO by the rare earth has been verified by the diffuse reflectance spectra of the same samples which show the same typical fine structures. Only holmium presents an emission when excited in its absorption band at 450 nm; no light emitted above 900 nm could be characterized in the present study.

Experimental determination of excitonic levels in α-oligothiophenes

The effects of intermolecular interactions on the optical spectra of oligothiophenes are examined. Absorption spectra of isolated molecules are calculated and experimentally recorded in a rigid host matrix whereby molecules are distributed randomly or organized unidirectionally. Absorption spectra of thin films in an ordered and disordered state are given. Ordered films have three principal spectral regions which are discussed in terms of classical exciton theory. Absorption spectra in transmission of single crystals of quinquethiophene and sexithiophene are analyzed. The lowest optically allowed transition in the crystal corresponds to the lowest Davydov component. It consists of a sharp peak that is observed for even-numbered oligothiophenes in b polarization, and absent for odd-numbered rings due to the perfect alignment of the transition dipole moment with the long molecular axis. The upper Davydov component is viewed in both thin film and single crystal spectra. In between the two principal Davydov c...

Photoinduced spontaneous and stimulated emission in sexithiophene single crystals

Abstract Temperature dependent steady-state and transient photoluminescence (PL) measurements were carried out on sexithiophene (6T) single crystals. As the temperature is lowered, three different sets of equally spaced peaks appear on the steady state PL spectrum. This behavior is interpreted in the frame of the molecular exciton theory. The first energy set is attributed to transitions to the lower level of the Davydov splitting of the 1B u excited state of the isolated molecule, whereas the two other sets are ascribed to the crystal defects. An analysis of the PL emission and excitation spectra allowed us to establish a general exciton energy scheme of sexithiophene crystal. At low excitation levels, the transient PL follows a single exponential decay, with a decay time of 1.8 ± 01 ns. When the excitation energy is increased, a second, much faster component is added. Concurrently, the PL spectrum narrows to a single line centered at the origin of the low energy set of the steady state PL. The width of the emission line is 13 cm −1 at 10 K, and tends to widen and shift towards lower energies when the temperature is raised. All these observations are accounted for in term of stimulated emission.

THE FOUR-LEVEL STIMULATED EMISSION IN SEXITHIOPHENE SINGLE CRYSTALS

Single crystal of conjugated sexithiophene oligomer shows stimulated emission when excited with a low-energy photonic pulse. This phenomenon is interpreted in terms of the excitonic energy diagram of the crystal, which presents a four-level pathway for the photoexcitation and emission, similar to the one observed in classical inorganic-based laser materials.

Transient electroluminescence of monolayer and bilayer sexithiophene diodes

Abstract The transient electroluminescence of monolayer and bilayer sexithiophene-based diodes has been measured. The delay time of the luminescence onset of the monolayer diode corresponds to a hole mobility of 5 × 10−6 cm2 V−1 s−1 which is considerably lower than that obtained by field-effect measurements. This is interpreted in terms of strong transient trapping. The bilayer diode presents a twofold time-resolved response which is attributed to the different mobility of its constituent layers.

Spectroscopic properties of aromatic dicarboximides part 3: Substituent effect on the photophysical properties of N-phenyl-2,3-napthalimides

Abstract In the present article we describe the effect of the substitution on the photolysis properties of a series of N -phenyl-2,3-napthalimides. It is found that the decrease in the electron-donating character of the substituent on the N -phenyl ring changes the fluorescence emission from a weak, broad and short-lived long-wavelength (LW) emission into a structured and long-lived fluorescence localized at the short wavelengths (SW) which behaves like that of the unsubstituted 2,3-napthalimide. Both SW and LW emissions can be observed in some cases. The different results can be explained on the basis of a three-level scheme where the vibrationally relaxed Franck-Condon state populates two different excited states, the one (SW) emitting at short wavelengths, and the other (LW) emitting at long wavelengths. The substitution on the phenyl ring influences mainly the energy of the LW excited state. The nature of the substituent on the N -phenyl ring has a determining influence also on the internal conversion process by virtue of the solvent- and rotation-induced pseudo-Jahn-Teller coupling of the two excited states.

Laser effect of a series of variously substituted pyrylium and thiopyrylium salts

Abstract The laser effect of 47 pyrylium salts and 6 thiopyrylium salts has been investigated by excitation with a pulsed (5 Hz) nitrogen laser (337 nm) and compared to that of rhodamine 6G. The thiopyrylium derivatives as well as the pyrylium salts which contain nitro substituents show a rather poor effect if any. Out of the pyrylium series, 19 compounds have an important response as laser dyes. For most of them, the lasing range is larger than that of rhodamine 6G and for several of them (19, 20, 21, 32, 35, 39, 47 and 46) this range covers more than 80 nm. The behaviour of 35 and 47 is particularly striking.

Quenching of zinc oxide photoluminescence by d- and f-transition metal ions

Abstract The photoluminescence of semiconducting zinc oxide doped with nickel, cobalt or neodymium ions has been studied. It is shown that the d-transition metal ions strongly deactivate the global emission of zinc oxide by two quenching processes: energy transfer to the transition metal ion (mainly) and trivial reabsorption of ZnO luminescence. By doping with neodymium ions the principal deactivation process is light reabsorption, but due to multiple internal reflection or the mobility of the charge carriers in the semiconducting samples, a mismatch between the theoretical approach and the experimental results is observed. Using samples of different thickness (0.1–1 mm) evidences are shown for mean path lengths of light longer than those predicted by the theory.