Laser induced transient absorption in liquid pyrene

The transient absorption spectra, over the spectral range 430 to 1020 nm, are presented for pure liquid pyrene at 425°K. The absorption profiles show both short- and long-lived components. The long-lived transient has a lifetime much greater than the lifetime of the monitoring flash-lamp pulse, and it is suggested that this absorption can be assigned to the triplet state of the monomer. The short-lived component is unassigned.     

The temperature dependence of proton relaxation times in triplet exciton ion radical salts

Proton spin lattice relaxation times have been measured for a variety of single crystal orientations of the (φ₃AsCH₃)⁺(TCNQ)₂^? ion radical salt over the temperature range of approximately 90 to 370°K. It is shown that the relaxation rate is directly proportional to the triplet exciton density where the exciton production energy is significantly dependent on temperature. The data suggests that exciton—exciton exchange is an important aspect in the relaxation mechanism.     

Delayed fluorescence in a pyrene-doped naphthalene crystal: homofusion and heterofusion of triplet excitons

Prompt and delayed emissions from a naphthalene crystal doped with pyrene have been investigated in the temperature range 77–300 K. It is shown that delayed fluorescence (DF) in this crystal originates from heterofusion and homofusion of two different types of trap triplets at low temperature (T< 130 K) and from homofusion of pyrene triplets at high temperature (T >200 K). Pyrene concentration effects on DF suggest that free triplet excitons are trapped more efficiently at monomeric pyrene sites than at nascent excimeric pyrene sites.     

Quasi-elastic light scattering studies on T7 DNA in the presence of a sinusoidal electric field

The mobilities and lifetimes of electrons and holes as well as the triplet exciton lifetime have been measured in a range of samples originating from a long anthracene crystal grown from the vapour phase in the zone refining tube. The results have demonstrated the influence of chemical and physical imperfections on these parameters. The triplet lifetime has been found to be fairly insensitive to impurities, as opposed to the lifetime of electrons and holes. The measurements of the charge-carrier mobility as a function of temperature have revealed that hole transport in a defective anthracene crystal is controlled by structural traps 0.26–0.27 eV deep, whereas the electron mobility. contrary to theoretical predictions. is lattice controlled over the investigated temperature range. These localized states can be photochemically eliminated. which suggests that paired anthracene molecules (“incipient” dimers) are responsible for the observed trapping asymmetry. These observations, by analogy with dimeric species forming excimers while excited, have been interpreted in terms of new chemical species formed as a result of trapping at “incipient” dimer pairs.     

Time-resolved photoelectron spectroscopy of low-energy excitations of 4×4 C60/Cu(111)

Time-resolved two-photon photoemission is applied to investigate electron dynamics in multiple monolayers (MLs) of ordered fullerite on a copper substrate. The experimental data are analyzed assuming coupled excited state dynamics. Rate equations fitted to these dynamics yield lifetimes of about 80 ps for the lowest unoccupied molecular orbital (LUMO), about 1.2 ns for the singlet exciton and 22 μs for the triplet exciton at a surface temperature of 140 K. For trapped triplet excitons lifetimes up to 200 μs are observed. An increased excitation fluence reduces the lifetime of the excitons due to annihilation. An increased sample temperature slightly reduces the lifetime of the triplet exciton. There is no evident dependence of the exciton lifetimes on the pump photon energy in the range of hν = 2.9 to 3.3 eV. A dependence on the layer thickness (10-20 ML) is not observed as long as more than 9 ML are prepared.     

Low temperature mechanism for the formation of polycyclic aromatic hydrocarbons from the pyrolysis of cellulose

The formation of polycyclic aromatic hydrocarbons (PAHs) from the pyrolysis of cellulose over the temperature range of 300–650 °C has been investigated. Detectable amounts (microgram per gram) of 2–4 ring PAHs were observed at and above 400 °C. Benzo[a]pyrene and benz[a]anthracene were observed at and above 500 °C. Changing the gas phase residence time from 2 to 18 s and the sample size from 200 to 500 mg did not significantly affect the yields of PAHs formed over this low temperature range. The addition of oxygen to the carrier gas stream significantly reduced the yields of PAHs. The pathway to PAH formation in the 300–650 °C temperature range is believed to proceed via the carbonization process where the solid residue undergoes a chemical transformation and rearrangement to give a more condensed polycyclic aromatic structure. The evolution profiles of PAHs from the solid residue suggests that smaller 2–3 ring PAHs evolve first and pass through a maximum at a slightly lower temperature than the larger 4–5 ring PAHs. The yields of PAHs obtained from the pyrolysis of d-glucose and sucrose are comparable to those obtained from cellulose.     

On singlet exciton fission in anthracene and tetracene at 77°K

Relative variations of prompt flourescence yield, on application of a magnetic field are studied at 77°K for anthracene and tetracene crystals excited by light with wavenumbers up to 50,000 cm^-1. The results obtained for anthacene, as compared to those previously reported at 300°K, indicate a very small temperature dependence. The variations observed for tetracene at 77°K are comparable in magnitude to those for anthracene. The singlet exciton fission process, responsible for the experimental observations, does not involve a thermally relaxed lowest bound or charge transfer singlet exciton. The possible role of upper excited vibronic states is discussed.     

Temperature dependence of the lifetime of pyrene triplet excimer emission

The stabilisation enthalpies of two different kinds of triplet excimers of pyrene are calculated from the temperature dependence of the triplet lifetime. They are found to be 800 cm⁻¹ and 2000 cm⁻¹.     

Effect of thermal activation on the reaction of toluene with hydrogen atoms

The effect of temperature on the reaction of toluene with hydrogen atoms is interpreted by the RRKM(Marcus–Rice) unimolecular rate theory. It was found that the product distribution changed markedly with temperature in the range of 300–780°K. Methyl-cyclohexadienes and methylcyclohexenes were the main products in the vicinity of room temperature, which were taken over by benzene and ethylbenzene as temperature increased. The calculation based on RRKM theory shows that the experimental results are explained in terms of the effect of temperature on the unimolecular reactions of the chemically activated methylcyclohexadienyl radicals.     

Phosphorescence studies of relaxation effects in bulk polymers. I. Depolarization measurements of segmental relaxation in poly(methyl acrylate)

This paper reports the first measurements of macromolecular segmental relaxation times by phosphorescence depolarization. Steady-state phosphorescence polarization experiments were performed on samples of poly(methyl acrylate) incorporating 0.5 wt % copolymerized acenaphthylene or 1-vinyl naphthalene as phosphorescent probes over the temperature range 77 to 310°K. Depolarization of phosphorescence occurs with the onset of segmental motion of the polymer at ca. 278°K. Motion of either probe is characterized by an activation energy of 195 (±5) kJ mole^?1, which is in fair agreement with the mean value of 230 kJ mole^?1 estimated for the segmental relaxation of poly(methyl acrylate) by dielectric and mechanical relaxation techniques. Transient depolarization measurements confirm the absence of probe motion below the glass transition temperature. Phosphorescence intensity and triplet state lifetime data are capable of detection of a second transition in the polymer in accord with observations using more conventional techniques.     

On the origin of long-lived emission in some charge-transfer crystals

Temperature dependence of emission spectra, decay times and intensity of emission, for molecular (1:1) crystals of charge-transfer (CT) complexes of tetrachloro- and tetrabromophthalic anhydride with penta- and hexamethylbenzene, have been investigated over a wide temperature range (1.7–300 K). A long-lived emission, observed in those crystals, has been identified as CT phosphorescence from CT triplet traps. No delayed emission, controlled by triplet—triplet annihilation (P-type), has been found. An explanation of observations connected with the fact that these complexes belong to the class where the lowest triplet state is of charge-transfer character is offered.     

Excited state kinetics and stern-Volmer quenching plots in the photolysis of azoisopropane at 366 nm

The photochemistry of azoisopropane is reinvestigated at 366 nm over an extended pressure range by using n-butane as an added bath gas, and over a range of temperature from 277° to 180°C. The Stern-Volmer type plot of the N₂ product quantum yield is interpreted in terms of the decomposition of the vibrationally excited upper singlet and triplet states, with the onset of the dissociation of the vibrationally equilibrated triplet state as the temperature is increased. The energy barrier for the dissociation of the vibrationally equilibrated first triplet state is found to be 8.8 kcal/mole. Triplet sensitization experiments with biacetyl correlate with our observations, and it is suggested that the proposed mechanism is generally applicable to the photodissociation of acyclic azoalkanes at 366 nm, based on a comparison of our data with those of Wu and Rice on hexafluoroazomethane.     

Singlet exciton energy transfer in tetracene-doped p-terphenyl single crystals

A picosecond laser system has been employed to study the kinetics of singlet exciton energy transfer in p-terphenyl single crystals doped with tetracene over the temperature range 77–300 K. First-order kinetics prevail and exciton migration from host to guest is controlled by thermal activation over a potential energy barrier.     

anisotropy of exchange interactions in crystalline nitroxide radicals

The paramagnetic susceptibility of three crystalline nitroxide radicals has been measured in the temperature range 1.3–300°K, in order to determine the exchange intermolecular coupling. It was found that for different radicals the parameter θ varies from ?16 to 0.5°K, indicating the possibility of both antiferromagnetic and ferromagnetic coupling in these crystalline radicals. It is shown that this possibility of both types of coupling, and the observed order of magnitude of its strength, may be explained assuming direct exchange and using the asymptotic method of calculating the exchange integrals proposed by Herring.     

Heat capacity of amorphous polyhexene-1 between 20 and 300°K: Intramolecular vibrational contribution to Cv below the glass transition

The heat capacity of polyhexene-1 was measured between 20 and 300°K. The apparatus, an adiabatic calorimeter giving results with a random error of 0.2–0.4%, is briefly described. The characterization of the sample by x-ray diffraction patterns established that it was amorphous at all temperatures. Gold foil was incorporated with the sample to increase the apparent thermal diffusivity and so to decrease the time needed for the measurements. The glass transition temperature was found to be 215.5 ± 1°K. On the C_p curve, no subglass anomaly was detected, unlike the results of experiments described elsewhere. The calculation of C_v is discussed, and an explanation is given for the choice of the number of intramolecular vibrational modes per monomer which are assumed to contribute to C_v. A linear continuum model with characteristic temperature θ₁ = 736°K allows us to fit the experimental curve over a temperature range of 140°K.     

Singlet-Triplet Energy Inversion in Carbon Nitride Photocatalysts

Time-resolved EPR (TR-EPR) demonstrates the formation of well-defined spin triplet excitons in carbon nitride. This permits to experimentally probe the extent of the triplet wavefunction which delocalizes over several tri-s-triazine units. Analysis of the temperature dependence of the TR-EPR signal reveals the mobility of the triplet excitons. By employing monochromatic light excitation in the range 430–600 nm, the energy of the spin triplet is estimated to be ≈0.2 eV above the conduction band edge, proving that the triplet exciton lies above the corresponding singlet. Comparison between amorphous and graphitic forms establishes the singlet-triplet inversion as a general feature of carbon nitride materials.     

Modeling the time and temperature dependence of the oPs formation probability in polystyrene and its derivatives

Positron annihilation lifetime spectroscopy measurements of polystyrene and its derivatives have been performed in the temperature range 20–300 K. The ortho‐positronium lifetime calculated from the lifetime spectrum analysis is related to the mean free‐volume hole size. However, the ortho‐positronium yield in the medium is dependent on the polymer chemistry and also on the irradiation effect induced by the positron source. We proposed a model based on two simple processes that cause, respectively, enhancement and inhibition of positronium formation to fit the experimental data over a broad temperature range. Using this model, intrinsic parameters for the polymers under study, such as the shallowly trapped electron recombination energy and the free‐radical recombination energy, were calculated and discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2063–2073, 2009     

Cadmium‐1,3,6,8‐tetrakis(4‐carboxylphenyl)pyrene Framework as a Thermometer for Fluorescence Sensing of Temperature

Compounds exhibiting tunable fluorescence emission upon adjusting the temperature are considered smart materials. Herein a three‐dimensional structure of {[Cd(TBAPy)(H₂O)₂] · 4(H₂O)}_n ( 1 ) constructed from Cd^II and 1,3,6,8‐tetrakis(4‐carboxylphenyl)pyrene (TBAPy^4–) ligand was synthesized and characterized. Temperature‐dependent fluorescence studies showed the emission intensities for compound 1 decrease as temperature increasing. The emission intensity is linearly related to temperature in the range of 100 to 300 K. This findings demonstrate that compound 1 is a good candidate for temperature sensitive sensor over a wide temperature range. Such a fluorescence behavior is closely related to the highly conjugated organic ligand with a pyrene core.     

Picosecond relaxation dynamics in polydiacetylene-pTS

Time-resolved absorption spectroscopy as well as transient grating measurements were performed on polydiacetylene-pTS. Singlet exciton lifetimes of 0.8 ± 0.2 ps at 300 K and 1.1 ± 0.2 ps at 5 K were measured. We propose that the observed non-radiative relaxation proceeds through a conformationally relaxed singlet state which lives for roughly 1.0 and 3.4 ps at 300 and 5 K, respectively. Data pertinent to the formation of triplet excitons are also presented and discussed.