Nicolae Filipescu

Spectra and quantum states of europium β-diketone chelates in polymethylmethacrylate

Abstract An analysis of the absorption spectra of different Eu β-diketone chelates led to a detailed spectroscopic investigation of Eu tris (4,4,4-trifluoro-1,2 thienyl-1,3 butanedione) chelate dissolved in polymethylmethacrylate and other hosts, at different temperatures and concentrations. The mechanism responsible for the line fluorescence is attributed to an intramolecular energy transfer from the triplet states of the organic chelate to the 5 D 0 and 5 D 1 resonance energy levels of the central europium ion. The concentration, temperature and host dependent fluorescence spectrum of Eu chelate have been investigated and interpreted. The absorption and phosphorescence spectra of the identical gadolinium chelate in different hosts were related to the overall transfer mechanism and the lowest-lying triplet states. The “molecular field splitting” effects are observed at low temperatures for the 7 F 0 → 5 D 2 transition and interpreted based on structural criteria. The intramolecular energy transfer has been tentatively formulated based on the admixture of the biradical triplet state of the chelate with the emitting levels of the europium ion.

Excimer emission from dibenzofuran and substituted fluorenes

Abstract The concentration- and temperature-dependent emission quantum yields of four new excimer-forming compounds - 1-methylfluorene, 2-fluorofluorene, 2-methoxyfluorene, and dibenzofuran - were examined. The photokinetic analysis of the fluorescence data allowed approximate evaluation of the enthalpies and entropies of photoassociation, monomer-excimer equilibrium constants, and the repulsive potential energies.

Analysis of Fluorescence Spectra of Rare‐Earth Chelates. II. Dipyridyl Complexes of Europic Chloride

The emission spectra of two dipyridyl complexes of EuCl3 were analyzed in detail treating the ligand field as a perturbation on the free‐ion levels. The ligand‐field parameters were calculated from the splitting in the 7F1 and 7F2 levels of the Eu3 + ion without assuming a known molecular geometry and the model suggested by these parameters is discussed. In addition, the ligand‐field parameters are used to calculate interatomic distances and to compare the coordinative properties of water and dipyridyl ligands. The method developed for deriving and verifying the geometrical arrangement and interatomic distances from fluorescence spectra proves to be a powerful tool.

Coumarinium–europium energy transfer: efficient triplet migration between two cations

Intermolecular triplet energy transfer from the coumarinium cation to the tripositive europic ion was demonstrated.

SPECTROPHOTOMETRIC STUDY OF DIENESTROL PHOTOISOMERIZATION

Abstract— Photolysis of synthetic estrogen dienestrol resulted in formation of a 4a,4b‐dihydrophenanthrene, a rare example of a stable intermediate of the classical stilbene → phenanthrene photocyclization. Time‐lapse spectrometric investigation suggested an initial cis‐trans isomerization in the dimethylbuta‐dime moiety of dienestrol followed by a photochemical [1, 51 sigmatropic proton transfer which converted the two cross‐conjugated p‐hydroxystyrene halves into a vinyl‐stilbene. A subsequent electrocyclic ring‐closure in an electronically excited state generated a cyclic nonaromatic heptenediol. This last photoproduct tautomerized rapidly in the dark to a stable, crystalline, tricyclic pentenedione. The final diketone product is an example of a 4a.4b‐dihydrophenanthrene self‐stabilized by a double ketoenol tautomerism. The three sequential photoisomerizations of dienestrol are made possible by a fortuitous coincidence of steric and orbital symmetry requirements.

Interaction of nonconjugated chromophores in rigid model compounds

Abstract Spectroscopic techniques and photochemistry were used to evaluate the interaction between an electronically excited chromophore and a different one in its ground state in rigid model compounds. These inflexible compounds are better suited for transfer work than binary mixed solutions of donor and acceptor since both the separation distance and the mutual orientation of the two chromophores are known, and complicating factors such as intermolecular transfer and complex formation are excluded by work at high dilution. Phenanthrene (P) and p-dimethoxybenzene (D) were rigidly connected to an inert spirocyclopropane-norbornane σ-frame. The S 1 → S 0 and T 1 → S 0 emissions of the two chromophores are found at different wavelengths and therefore can be detected separately. The emission and excitation spectra of the model compound were compared to those of an equivalent one-to-one mixture of each chromophore rigidly attached to the same inert frame. The dilute mixture shows the fluorescence and phosphorescence of each of the two components. In the model compound, however, the D fluorescence is definitely absent. The mechanism is consistent with efficient singlet energy transfer from the selectively excited D to P combined with partial back transfer from the T 2 state of P to the lowest triplet of D.

Lanthanide ions as sensitive probes. Part III. Triplet-energy transfer from 2-acetonaphthone and 2-acetylfluorene to europic ions in frozen solution

Transfer of triplet energy from 2-acetonaphthone and 2-acetylfluorene to tervalent europium ions was investigated in rigid glass by measuring the quantum yields of donor phosphorescence and acceptor fluorescence and the decay of donor emission under selective excitation of the ketone at different Eu3+ concentrations. It is suggested that transfer originates in the n,π* triplet, which is only slightly above the lowest π,π* triplet of the sensitizer. A suggested photokinetic scheme, consistent with the presumed exchange mechanism, explains the observed behaviour and yields transfer rate constants of 2·3 and 10 | mol–1 s–1 for the two sensitizers. Spectroscopic observables predicted by statistical treatments were in very good agreement with experiment and were consistent with the assumed exchange mechanism. Alterations in the predicted time-dependence of phosphorescence intensity by donor self-quenching and by changes in the microscopic decay probability expression are considered.

A new actinometer: decafluorobenzophenone–isopropyl alcohol

The clean photoreduction in propan-2-ol of decafluorobenzophenone to decafluorobenzhydrol and the ease of g.l.c. determination of the extent of conversion suggest this system as a new actinometer.

Nearest-neighbour model for transfer of electronic excitation energy

Triplet–triplet, singlet–singlet, and triplet–singlet energy-transfer data are interpreted with a statistical model which assumes transfer from excited donor only to its nearest acceptor. Predictions of the model agree well with experimental luminescence-yield and lifetime measurements. Analysis of potential experimental errors shows the results from the nearest-neighbour model to be virtually indistinguishable from those of the best previous model, developed by Inokuti and Hirayama. This agreement lends quantitative support to the hypothesis that transfer probability decreases rapidly with donor–acceptor separation. The shape of the nearest-neighbour distribution function explains the partial success of the sphere-of-quenching model applied to co-solute exchange-transfer experiments.

Photochemical reactions of 4,4′-di-t-butylbenzophenone in isopropyl alcohol

The photoreduction of 4,4′-di-t-butylbenzophenone in deaerated isopropyl alcohol was investigated by (a) monitoring absorption spectra following short intervals of irradiation, (b) successive determinations of absorption spectra throughout the dark period following irradiation, and (c) periodic recording of emission and excitation spectra during or after irradiation. The reactions taking place in the dark were examined separately from those during irradiation, and the second intermediate was allowed to react with sodium isopropoxide and sulphuric acid. Quantitative kinetic calculations were used to determine the orders of reaction for the individual steps and their rate constants. Extinction coefficients for the intermediates were likewise calculated, although their structures were not identified.