Formation Of Excited Singlet States Research Articles

Electrospun fibers functionalized with phthalocyanine-gold nanoparticle conjugates for photocatalytic applications

The heterogeneous photocatalytic activity of Zn(II) phthalocyanine-gold nanoparticle conjugates immobilized on electrospun polystyrene fibers was investigated using 4-chlorophenol and Orange G as model pollutants. The phthalocyanine-gold nanoparticle conjugates were synthesized via a ligand exchange reaction and characterized using spectroscopic and microscopic techniques. The interaction between the gold nanoparticles and phthalocyanines did not restrict the formation of excited singlet states, triplet states and hence the formation of singlet oxygen required for photocatalysis. The conjugates showed significantly higher singlet oxygen quantum yields and enhanced photocatalytic activity compared to the phthalocyanines. It was concluded that hybrid electrospun fibers provide great potential as active photocatalysts for oxidizing organic pollutants using singlet oxygen as an oxidant.

Beneficial role of gold nanoparticles as photoprotector of magnesium tetraphenylporphyrin

The photoprotection of porphyrins is of immense importance for their utilisation in several photo-related applications. In the present study, we have employed gold nanoparticles (AuNPs) to photoprotect a porphyrin, magnesium tetraphenylporphyrin (MgTPP). Absorption, fluorescence and X-ray photoemission spectroscopic (XPS) studies of MgTPP under illumination in the absence and in the presence of AuNPs have been carried out to assess their effect on the photostability of MgTPP. The results show that with an appropriate concentration of AuNPs, virtually 100% photoprotection can be provided to MgTPP. The protecting ability of AuNPs is the result of their binding with MgTPP at its nitrogen sites, inhibiting, thus, the binding of reactive oxygen species (ROS) at these sites, known to cause the photodegradation of MgTPP and several other porphyrins under aerobic conditions. Further, the decreased formation of excited singlet states of MgTPP, as a result of its binding with AuNPs, also suppresses the generation of ROS and improves the photostability of MgTPP. The binding between MgTPP and AuNPs may also be responsible for the photoprotection provided to MgTPP by AuNPs under anaerobic conditions too. The use of gold nanoparticles thus offers us a new strategy to photoprotect various porphyrins.

Kinetic and spectroscopic study of triplet state and ionic pathways in the laser-induced photoexcitation of N-ethylcarbazole in fluid solutions

When fluid solutions of N-ethylcarbazole (NEC) are irradiated by 308-nm pulses from an excimer laser, transient triplets and ions may be observed in the microsecond time regime. The formation of ions is favored in more polar solvents such as acetonitrile (AcN) and that of triplets in nonpolar solvents such as paraffin oil. The quantum yield of triplet formation from initial excitation is 0.23. Triplet states decay through concurrent first- and second-order channels: the second-order process, triplet-triplet annihilation, leads to formation of excited singlet states with an efficiency of 0.12. Transient carbazolyl cations and anions display double-exponential decays. To account for this behavior, a mechanism involving initial electron donation by the multiply excited chromophore to a nearby NEC molecule or solvent molecule is proposed; subsequent decay occurs primarily by germinate recombination of the resulting ion pair. The quantum yield for cation production is 0.037. Delayed fluorescence is AcN also decays by a double-exponential decay curve with rate constants, suggesting that the emission arises mainly from ion recombination.

Primary photosynthetic processes in Heliobacterium chlorum at 15K

Absorbance changes induced by 25-ps laser flashes were measured in membranes of Heliobacterium chlorum at 15 K. Absorbance difference spectra, measured at various times after the flash showed negative bands in the Qy region at 812, 793 and 665 nm. The first of these bands was attributed to the formation of excited singlet states of a long-wavelength form of antenna bacteriochlorophyll g (BChl g 808). Absorbance changes of shorter wavelength absorbing antenna BChls g were at least an order of magnitude smaller, indicating rapid excitation energy transfer (i.e. within the time resolution of the apparatus) from these BChls to BChl g 808. Excited BChl g 808 showed a bi-exponential decay with time constants of 50 and 200 ps. The bands at 793 and 665 nm may be attributed to the primary charge separation and reflect the photooxidation of the primary electron donor P-798 and photoreduction of a primary electron acceptor absorbing near 670 nm, presumably a BChl c or Chl a-like pigment. The bleaching of this pigment reversed with a time constant of 300 ps at 15 K and of 800 ps at 300 K. This indicates that electron transfer from the primary to the secondary electron acceptor is approximately 2.5 times faster at 15 K than at room temperature.

Picosecond pulse radiolysis studies on the formation of excited singlet states of solute molecules in several saturated hydrocarbon and ethanol solvents

Two kinds of the formation components of excited singlet states of solute molecules have been observed in the picosecond time region in methylcyclohexane, 3-methylpentane, and iso-octane as well as in cyclohexane, as reported previously. The ratios of two components are significantly dependent on solute concentrations and kinds of solvents. A fairly large amount of the slower formation component has been observed in cyclohexane and methylcyclohexane, little in iso-octane, and none in ethanol. The slower component is mainly due to the solute geminate ion recombination and partly due to the energy transfer from the excited singlet states of solvent molecules. The faster component has not been made clear, but could be due to Čerenkov light. The life-time of the excited singlet state of cyclohexane was directly measured to be 1.1 ± 0.1 ns.

Direct formation of excited singlet states in the electrogenerated chemiluminescence of mixed systems involving N-aryl-2-pyrazolines

The direct formation of the excited singlet states 1P * in the electron transfer reaction between the cation radicals ▪ of certain N-aryl-2-pyrazolines and the anion radicals ▪ of some polycyclic aromatic hydrocarbons, and aryl-substituted oxazoles and oxadiazoles is observed in electrogenerated chemiluminescence studies using measurements of the relative emission intensities and investigations in a magnetic field.

Spectra and decay kinetics of trimethyldioxetane chemiluminescence

Abstract Comparison of the spectra of chemiluminescence and photoluminescence of trimethyldioxetane and study of the influences of triplet quenchers and a radical scavenger lead to the conclusion that the formation of excited singlet states by dioxetane decomposition proceeds via excited biradicals and triplet states and not via a concerted bond cleavage.

Formation of excited singlet states in irradiated aromatic liquids

Formation of excited singlet states in irradiated aromatic liquids