Experimental study of triplet exciton diffusivity in crystalline pyrene at 300 K

Preliminary measurements of the triplet diffusivity tensor in crystalline pyrene at 300 K are reported. Maximum diffusivity occurs along the b axis and D_ab = (1.25 ± 0.3) × 10^?4 cm² s^?1. Diffusion in the ac′ plane is nearly isotropic with D ≈ (0.3 ± 0.1) × 10^?4 cm² s^?1. These results are tentatively interpreted as diffusion dominated by nonlocal scattering.     

Fast formation of aromatic cations in cyclohexane observed by pulse radiolysis

The fast formation of cations of benzophenone, biphenyl, naphthalene, anthracene and phenanthrene in liquid cyclohexane was studied by nanosecond pulse radiolysis. Rate constants between 2.3 × 10¹⁰ M^?1 s^?1 (for naphthalene) and not below 2.5 × 10¹² M^?1 s^?1 (for benzophenone) were derived and compared with the corresponding rate constants for anion formation.     

Energy transfer between benzophenone and biacetyl

Energy transfer from benzophenone to biacetyl in the gas phase was studied by measuring the intensity and decay time of phosphorescence and fluorescence of both compounds as a function of pressure and composition of the mixture. We have established that benzophenone transfers energy along two parallel channels: singlet→singlet with high efficiency, k_SS? 2 ×10⁸ torr^?1s^?1, and triplet→triplet with lower efficiency, k_TT?8 × 10⁵ torr^?1s^?1.     

ODMR and triplet state kinetics of a carbazolyl substituted diacetylene crystal

Carbazolyl substituted diacetylene (DCH) monomer crystals showing phosphorescence from four traps have been investigated by optically detected magnetic resonance (ODMR) at 1.2 K. These localized triplet states are attributed to the carbazolyl side groups. Their population and depopulation rate constants and zero field splitting parameters (0.097 <|D|< 0.1002 cm^?1; 0.0068 <|E|<0.0105 cm^?1) have been determined. The results suggest that the traps are disturbed substituents. The proposed interaction of the trap states with an exciton band at 23926 cm^?1 is supported by temperature dependent lifetime measurements.     

PHOTOCHEMISTRY ON SURFACES: TRIPLET-TRIPLET ENERGY TRANSFER ON MICROCRYSTALLINE CELLULOSE STUDIED BY DIFFUSE REFLECTANCE TRANSIENT ABSORPTION and EMISSION SPECTROSCOPY*

Triplet-triplet energy transfer has been studied between benzophenone and an oxazine dye (2,7-bis(diethyl-amino)-phenazoxonium chloride) co-adsorbed on the surface of microcrystalline cellulose. Ground state absorption and fluorescence measurements provide evidence for dimer formation of the oxazine dye when adsorbed on cellulose in contrast to the behaviour in ethanol solution where no dimerization is observed. The equilibrium constant for dimerization, which is found to be (1.0 × 0.1) × 10⁶ mol^?1 g (2560 × 250 dm³ mol^?1) for oxazine alone on cellulose decreases in the presence of co-adsorbed benzophenone. Fluorescence is detected from excited monomeric but not from excited dimeric oxazine. The absorption spectrum of the triplet state of oxazine adsorbed on cellulose was obtained and its extinction coefficient evaluated relative to that of triplet benzophenone which was used as a sensitizer. The lifetime of adsorbed triplet oxazine is 4.3 ms which is 300 times longer than that in acetonitrile solution. The efficiency of energy transfer from triplet benzophenone to oxazine on cellulose was studied using both time resolved sensitized absorption and phosphorescence intensity measurements as a function of oxazine concentration. Lifetime measurements show that the energy transfer process involves static quenching since the benzophenone lifetime is independent of oxazine loading at the surface. A mechanism is proposed to explain the results in which one oxazine molecule is suggested as being able to quench phosphorescence from a “pool” consisting of 2 to 3 benzophenone molecules.     

Investigation of the triplet states of DNA by optical detection of magnetic resonance using a new slow passage technique

A new method to obtain slow passage ODMR signals under otherwise impeding conditions is described. The technique is applied to identify the triplet state of DNA as thymine. The zero field splitting constants measured are |D| = (0.203 ± 0.003) cm^?1, and |E| ? (0.0123 ± 0.0005) cm^?1. From microwave-induced delayed phosphorescence and slow passage measurements a preliminary kinetic scheme for the triplet state is derived.     

The phosphorescent state of γ-thiopyrones: Optically-resolved zero-field splittings

The phosphorescence spectra, ODMR, and triplet sublevel kinetics are reported for the thioketones, 4H-pyran-4-thione (PT) and 2,6-dimethyl-4H-pyran-4-thione (DMPT). Extremely large ZFS parameters,D = ?24, and ?28 cm^?1, are assigned to two distinct trap sites of PT in n-pentane and D = ?24 cm^?1 for the major site of DMPT in n-hexane.     

Lifetimes of triplet state alkylbenzenes in the vapor phase

Rate constants for triplet state decay of C₆H₆, C₆D₆ and 15 alkylbenzenes in the vapor phase have been found, using a flash-sensitized biacetyl phosphorescence technique, to range between 800 s^?1 and ? 18 000 s^?1. Only benzene has a significant positive activation energy for decay. Kinetic and spectroscopic evidence supports a photoisomerization decay channel in t-butylbenzene. Comparison of lifetimes with molecular size shows that increased density of rotational levels does not account for rapid decay of the triplet state. This contradicts a recent suggestion to explain short lifetimes of triplet state aromatic hydrocarbons in the vapor phase, relative to the long lifetimes of the same molecules in low temperat matrices. Evidence suggests that the dominant decay paths for triplet state alkylbenzenes are different in room temperature vapors and low temperature matrices.     

Temperature effects on the phosphorescence spectra and lifetimes of 2,4-, 2,5-, and 3,4-dimethylbenzaldehydes dispersed in durene single crystals

The phosphorescence spectra and lifetimes of 2,4-, 2,5-, and 3,4-dimethylbenzaldehydes dispersed in durene single crystals have been measured as a function of temperature between 10 and 200 K. For all the guests involved, the vibrational structures of the spectra are found to be temperature dependent. This is interpreted in terms of two emissions that proceed from a triplet state having predominantly a ππ* character at low temperatures and from a thermally populated triplet state having essentially a nπ* character at higher temperatures. The energy gaps ΔE_T between ³ππ* and ³nπ* states evaluated spectroscopically are found to be 100, 70, and 340 cm^?1, respectively for 2,4-, 2,5- and 3,4-dimethylbenzaldehydes.Activation energies ΔE* determined from the Arrhenius plots of the phosphorescence decay rate constants are in good agreement with the ΔE_T for the first two guests. In contrast, the ΔE* are higher than the ΔE_T for 3,4-dimethylbenzaldehydes as well as for 2,4,5-trimethylbenzaldehyde (where ΔE_T ≈ 400 cm^?1) because of the rapid increase of radiationless transitions in the temperature range where thermal population of the upper ³nπ* state is efficient. In the low and high temperature ranges, the phosphorescence decays for all these guests are exponential. In the intermediate range, these decays are non-exponential. The origin of these non-exponential decays is discussed.     

ESR study of the hyperfine structure of the triplet state in crystalline donor acceptor complexes: A probe for the degree of charge transfer in the excited state

The hyperfine tensors of the triplet states of several guest donor molecules in naphthalene/tetracyanobenzene host crystals have been measured by EPR spectroscopy. From these data the degree of charge transfer (b²) in the excited state can be derived in a straightforward way. Whereas anthracene yields values almost identical with those characteristic for the isolated molecule (b² ≈ 0.05), phenanthrene shown a significant degree of charge transfer in the triplet state (b² ≈ 0.47).     

Synthesis of main chain polymeric benzophenone photoinitiator via thiol‐ene click chemistry and Its use in free radical polymerization

Main chain polymeric benzophenone photoinitiator (PBP) was synthesized by using “Thiol‐ene Click Chemistry” and characterized with ¹H NMR, FTIR, UV, and phosphorescence spectroscopies. PBP as a polymeric photoinitiator presented excellent absorption properties (ε₂₉₄ = 28,300 mol^?1L^?1cm^?1) compared to the molecular initiator BP (ε₂₅₂ = 16,600 mol^?1L^?1cm^?1). The triplet energy of PBP was obtained from the phosphorescence measurement in 2‐methyl tetrahydrofurane at 77 K as 298.3 kJ/mol and according to phosphorescence lifetime, the lowest triplet state of PBP has an n‐π* nature. Triplet–triplet absorption spectrum of PBP at 550 nm following laser excitation (355 nm) were recorded and triplet lifetime of PBP was found as 250 ns. The photoinitiation efficiency of PBP was determined for the polymerization of Hexanedioldiacrylate (HDDA) with PBP and BP in the presence of a coinitiator namely, N‐methyldiethanolamine (MDEA) by Photo‐DSC. The initiation efficiency of PBP for polymerization of HDDA is much higher than for the formulation consisting of BP. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

The triplet state of magnesiumporphin · ethanol in an n-octane crystal studied by microwave induced changes in the fluorescence intensity

The zero-field splitting of the triplet state of magnesiumporphin solvated by ethanol is represented by D = 0.035 cm^?1 and |E| = 0.010 cm^?1. The decay rates of the upper two spin components both are found to be about 20 s^?1, while that of the bottom component (where the spin lies in the molecular plane) is about 2 s^?1.     

Luminescence studies in polymers—X: Triplet energy migration in copolymers styrene-vinylbenzophenone. Decay-time and its relation to phosphorescence emission and excitation spectra

The decay of the phosphorescence of copolymers styrene-vinylbenzophenone excited with u.v. light of 365 nm either in glassy solution or as films at 77°K has been analysed. In glassy solution, the decay is exponential for a wide range of copolymer composition (τ = 5·10^?3 sec). The decay of films reveals the presence of a second type of trap with a longer life-time (τ = 2·10^?2 sec). The importance of these traps increases with the vinylbenzophenone content. The efficiency of triplet energy migration also increases with vinylbenzophenone content as a result of exchange interactions between benzophenone groups. The results are in agreement with Voltz's theory at moderate vinylbenzophenone content (<50 mole %).     

Stark effects on the phosphorescence spectra of p-benzoquinone

Stark effects on the phosphorescence spectra of p-benzoquinone have been studied in naphthalene crystals at temperatures between 1.7 and 30 K. The experimental data establish that the band splitting of 24 cm^?1 arises from a double minimum potential in the T₁ state.     

Diffusion in a unidimensional zeolite pore system: Propane in AlPO4-5

Self-diffusion and diffusive transport of propane in large (≈ 180 μm) crystals of AlPO₄-5 have been studied by ZLC and tracer ZLC methods. In contrast to the results of recent studies of the self-diffusion of CH₄ and CF₄ in the same zeolite by the PFG NMR method the results of the present study show a rather fast one-dimensional Fickian diffusion process (D ≈ 10⁻⁵–10⁻⁶ cm² s⁻¹ at 300 K) with no evidence of single-file behavior. A possible explanation for this apparent discrepancy is suggested.     

Vibrational relaxation of triplet chlorotoulenes studied by photosensitized phosphorescence of biacetyl

The photosensitized phosphorescence of biacetyl by chlorotoulene vapors has been investigated under stationary conditions. Quantum yields of stable triplet formation for chlorotoulenes increased with increasing foreign gas (ethane) pressure, suggesting that vibrational relaxation of initially formed triplet levels is competitive with photodissociation of the CCl bond. Foreign-gas pressure effects on the triplet formation yield by the 0-0 excitation has lead to photodecomposition rate constants of chlorotoluenes ( ≈ 10² s^?1). Step-ladder collisional relaxation processes have been postulated for the excitation at shorter wavelengths.     

Adsorption rate of Reactive Black 5 on chitosan based materials: geometry and swelling effects

The overall adsorption rate of Reactive Black 5 dye (RB5) on chitosan based materials was elucidated using diffusional models. Fundamental aspects, such as, geometry of the adsorbents and swelling effects were considered. Chitosan based materials (powder and film) were prepared from shrimp wastes and characterized regarding to the fundamental features for adsorption. Experimental decay curves were obtained under different conditions of stirring rate and initial dye concentration. The data were modeled according to the external mass transfer and diffusional models. The k_L (external mass transfer coefficient), D_ep (effective pore diffusion coefficient) and D_s (surface diffusion coefficient) values were estimated. For both adsorbents, it was found that the surface diffusion was the intraparticle diffusion mechanism governing the adsorption rate of RB5, since its contribution was higher than 92 % regardless the position and time. The D_s values ranged from 2.85 × 10^?11 to 5.78 × 10^?11 for chitosan powder and from 4.15 × 10^?11 to 12.12 × 10^?11 cm² s^?1 for chitosan films. The RB5 adsorption was faster when chitosan powder was used, mainly at higher stirring rates and initial dye concentrations. The swelling effect was most pronounced for the chitosan films, where, provided an increase of about 65 times in the D_s value.     

Rutherford backscattering spectrometry studies of iodine diffusion in polyimide

Rutherford backscattering spectrometry with a 2.1-meV He²⁺ion beam is used to measure the diffusion of iodine into polyamic acid and polyimide films. The iodine diffusion coefficient D decreases from its initial value of about 2X10^?13cm²/s in polyamic acid to approximately 1.4 × 10^?15cm²/s in partially cured polyimide, but then increases to a value of nearly 1.5 × 10^?14 cm²/s in fully cured polyimide. This dramatic increase in D cannot be attributed to the “in-plane” biaxial orientation of the polyimide molecules since indential D's were found with isotropic specimens. Microvoids less than 2 nm in size caused by water and carbon dioxide formation during imidization may, however, give rise to the observed behavior. The results demonstrate that Rutherford backscattering spectrometry with its excellent depth resolution (better than 30 nm) and good sensitivity (50 ppm iodine can be detected) is very useful for measuring the diffusion of slowly diffusing species in glassy polymers.     

Pulse polarography: IX. Proton diffusion in aqueous sulphate solution

Average diffusion currents, , of the ions H₃O⁺ and HSO₄^? in the system 0.1 M K₂SO₄+H₂SO₄ (pH=3.1?3.8) have been measured at 25°C both with a dropping mercury and a rotating disk electrode, using pulse techniques. The separate diffusion coefficients D_HSO4 and D_H have been estimated fromand it was found that D_H≤61×10^?6 cm²s^?1, which is definitely less than the value calculated when relaxation and (bulk) viscosity effects are taken into account. This is at variance with all the literature D_H values for non-sulphate systems and also with our experimental result D_H=(84.6±0.2)10^?6 cm²s^?1 for the system 0.1 M KCl+HCl, which is about 7% above the value from Onsager's theory. The peculiar behaviour of D_H in a K₂SO₄ solution is attributed to a strong decrease in rotational freedom of water molecules in such a medium.This paper also gives a critical evaluation of the principle of “independent diffusion” in excess neutral electrolyte if more than one species participates in the diffusion process.     

Deuteration effects on the phosphorescence of aromatic hydrocarbons

Observations of the phosphorescence decay of isotopic mixtures of naphthalene-h_s and -d_s, phenanthrene-h₁₀ and -d₁₀, and chrysene-h₁₂ and -d₁₂, in ethanol solutions at 77 K are analysed to determine the ratio k_PT^H/k_PT^D of the triplet radiative rate parameters of the perprotonated and perdeuterated compounds. The ratio is 1.20 (±0.07) for naphthalene, 1.39 (±0.06) for phenanthrene, and 0.98 (±0.04) for chrysene.