Phosphorescence Quantum Efficiency Research Articles

Intermolecular energy transfer in mixed laser dyes: photophysical properties of triplet states

Triplet-singlet energy transfer in laser dyes have been studied in EPA at 77K using N2 laser as an excitation source. Phosphorescence of the donor (D) and the delayed fluorescence of the acceptor (A) and their lifetimes have been measured for coumarin 102 (D)-rhodamine B(A) and 9(10H)-acridone (D)-rhodamine 6G(A) dye systems as a function of acceptor concentration. These data yield energy transfer rate constants of ∼103 dm3 mol−1 s−1 for the donor acceptor combinations, consistent with the Forster mechanism. The phosphorescence quantum efficiency and other spectral parameters are also reported.

Davydov splitting and two-photon excitation of cadmium tungstate (CdWO 4) single crystal phosphorescence

Phosphorescence characteristics of CdWO 4 excited by one-photon ( λ = 308 nm) and two-photon ( λ = 570–590 nm) processes were measured. A Davydov splitting of 120 ± 20 cm −1 was obtained in the phosphorescence spectra, suggesting a diffusion coefficient of about 1.2 × 10 −2 cm 2 s −1, and a diffusion length of about 3.1 × 10 −4 cm for the room temperature measured lifetime of 8μs. The phosphorescence quantum efficiency was less than ∼2% at low temperatures (only ∼0.25% at room temperature), indicating that the dominant decay mechanism was radiationless. The radiative lifetime was thus estimated as 1–2 ms. The two-photon phosphorescence excitation is characterized by an absorption cross-section of the order of 10 −49cm 4s.

Quantitative evaluation of soluble polymeric photosensitizers V: norbornadiene—quadricyclene conversion photosensitized by polymer-bound (benzyloxy)-benzaldehyde

Soluble polymeric photosensitizers based on o- and p-(benzylloxy)- benzaldehyde and the corresponding monomeric model compounds were studied as triplet energy donors to norbornadiene. The results are interpreted with assumption that a triplet exciplex is formed as an intermediate in the route to quadricyclene. The subsequent exciplex decomposition yields triplet norbornadiene which finally isomerizes to quadricyclene. The studied o- and p-substituted monomeric model compounds give exciplex decomposition efficiencies of about 0.5. On the other hand, the polymeric samples give lower exciplex formation efficiencies than the free sensitizers, in parallel with previous results obtained with stilbene. Phosphorescence spectra were measured at 77 K. The para compound has a phosphorescence quantum efficiency much greater than the ortho compound, very likely owing to the intrmolecular cyclization undergone by the triplets of the ortho compounds. Polymer binding enhances the phosphorescence of both o- and p- substituted chromophores.

Solvent effect on the phosphorescence of 2,5-diphenyloxazole at 77 K

The luminescence properties of the 2,5-diphenyloxazole (PPO) at 77 K are very sensitive to complex formation with hydroxyl-containing solvents. At the triplet level complex formation affects the internal conversion to the ground state as shown by adding large salt concentration in ethanol solutions. Salt addition induces an increase in the phosphorescence quantum efficiency and a blue-shift of this emission.

Excitation, Dissipative, and Emissive Mechanisms in Biochemicals

Triplet states, because of their long lifetimes and reactive properties, are of fundamental concern to radiation biologists. Our interest in the role of these states in radiation damage was stimulated by the observation that the ratios of fluorescence to phosphorescence (F/P) excited by x irradiation of the aromatic amino acids and trypsin are less, by a factor of about 10 to 100, than those observed for ultraviolet (uv) excitation of the lowest-lying excited states (1). This implies that x irradiation greatly enhances the relative population of excited triplet states or that fluorescence or phosphorescence quantum efficiencies are changed. Three mechanisms were proposed to account for this enhanced triplet population: (1) intersystem crossing, from singlet to triplet manifold during the energetic cascade from high-lying excited states to the lowest-energy, emitting states; (2) the interaction and breakup of collective excitations; or (3) direct spin exchange between incident, slow electrons and orbital electrons in the absorbing molecules. Possible methods to investigate these mechanisms are, respectively, (1) irradiation with subionizing vacuum uv (4 to 10 eV); (2) irradiation with high-energy vacuum uv (-25 eV); and (3) irradiation with slow electrons (<100 eV). Preliminary evidence bearing on the first and third possibilities has come recently from studies in our two laboratories and is summarized below. Experimental details are given in companion reports (2, 3). Following the initial absorption of energy, a series of relaxation and emissive events will occur which ultimately leave the absorbing system (which now may be altered chemically) in its ground state. The individual processes may be very fast ('10-15 second for light emission) or very slow (100 seconds for decay of triplet