Lifetime Of Singlet State Research Articles

Excited states of ketones as electron donors—ketone—iodonium salt systems as photoinitiators for radical polymerization

The excited states of the aromatic ketones benzophenone, 4,4′-bis(dimethylamino)benzophenone (Michler's ketone), xanthone, thioxanthone, acridone and 10-methylacridone sensitize the decomposition of diphenyl-iodonium salts via electron transfer processes. Owing to the long lifetime of the excited singlet state, at high concentrations of iodonium salts both the singlet and triplet states can function as electron donors. In the presence of methylmethacrylate an additional quenching takes place. Determination of the processes which occur in the excited states allows the photoinitiation step of the reaction to be discussed. The quantum yields of the sensitized photolysis of diphenyliodonium salts and of the polymerizations initiated by the ketone—iodonium salts systems have been determined. Some of these systems are effective photoinitiators for radical polymerization.

Photophysical properties and photodynamic activity in vivo of some tetrapyrroles

Some of the photophysical properties (stationary absorbance and fluorescence, fluorescence decay times and singlet oxygen quantum yields) of pheophorbide a, metal-free, ClAl-, Cu- and Mg- t-butyl-substituted phthalocyanines, metal-free, ClAl- and Cu- t-butyl-substituted naphthalocyanines and of a number of tetraphenylporphyrins (5,10,15,20-tetraphenylporphyrin, 5,10,15,20-tetra( m-hydroxyphenyl)porphyrin, 5,10,15,20-tetra( p-hydroxyphenyl)porphyrin) have been studied in comparison with hematoporphyrin IX in order to select potent photosensitizers for the photodynamic treatment of cancer. The photodynamic activity of these compounds was investigated using Lewis lung carcinoma in mice. As a consequence of the photophysical parameters (relatively short singlet state lifetimes, and high singlet oxygen quantum yields) the photodynamic activities of pheophorbide a, t-butyl-substituted ClAl-phthalocyanine and ClAl-naphthalocyanine were selected for study in greater detail. Under the conditions employed in the present study, pheophorbide a was found to be the most effective sensitizer, as judged from its strong absorption at the excitation wavelength as compared with the hematoporphyrin derivative and greater singlet oxygen quantum yield relative to the phthalocyanines and naphthalocyanines. The photodynamic activity was observed to be strongly dependent on the photophysical parameters of the compounds. The primary mechanism underlying the photodynamic activity of these sensitizers probably consists of energy transfer from the lowest triplet state of the dyes to molecular oxygen, resulting in the formation of singlet oxygen (type II of photosensitization).

The location of organic guests within X-type faujasite zeolites via external heavy-atom induced phosphorescence

Heavy-atom induced phosphorescence of naphthalene and related aromatic guest molecules included in X-type faujasite zeolites (M +X −, M + = Li, + Na, + K +, Rb +, Cs +, Tl +)_has been studied and evidence has been obtained for direct interactions between the heavy-atom cations present within the zeolite supercage and the included organic guest. As expected for an external heavy-atom perturbed excited state, both singlet and triplet excited state lifetimes and emission efficiencies depend upon the identity and accessibility of the cation. The use of photophysical probes to monitor the guest's local environment and approaches to monitoring guest diffusion processes within zeolites via. phosphorescence are discussed.

Energy transfer to low energy chlorophyll species prior to trapping by P700 and subsequent electron transfer

It is found that the two singlet state lifetimes observed in medium sized isolated Photosystem One reaction centres belong to two distinct sets of particles. The nanosecond lifetime is due to PS1 particles in which P700 does not trap excitation energy, and the excitation energy is homogeneously distributed within the antennae of these particles. The spectral features of the picosecond component show that excitation energy in the antenna has become largely concentrated in one or more low energy (red) chlorophyll species within 3.5 ps. Antennae which have become decoupled from P700 also appear to be decoupled from these red "ancillary" chlorophylls, and this suggests that some substructure or level of organisation links them to P700.The rate of quenching of antenna singlet states appears to be independent of the redox state of P700 under the conditions used here, and oxidising P700 does not prevent excitation energy from reaching the red chlorophyll species in the antenna.We find no evidence in the data presented here of a chlorophyll molecule acting as a "metastable" primary acceptor (A0). The lower limit for the detection of such a species in these data is 20% of the optical density of the transient P700 bleach.

ChemInform Abstract: The Effect of the Nature of the Lowest Unoccupied Molecular Orbital (LUMO) of Some Arylmethylsulfonium Salt Derivatives on Photochemical and Redox Behavior.

AbstractFluorescence quantum yield, excited singlet state lifetimes, and electrochemical peak potentials for reduction of (I) and (II) support the conclusion from MO calculations that the nature of the LUMO switches from σ* to π* for (Ib) and (IIb).

ACRYLAMIDE QUENCHING OF TRYPTOPHAN PHOTOCHEMISTRY AND PHOTOPHYSICS

Studies of acrylamide quenching of tryptophan (Trp) fluorescence, photochemistry, and photoionization have been conducted. Quenching of Trp fluorescence in aqueous solution by addition of acrylamide in the concentration range 0.0-0.5 M was measured and resulted in a Stern-Volmer quenching constant of KSV = 21 +/- 3 M-1. Photolysis experiments were performed in which Trp was photolyzed at 295 nm in the presence of varying concentrations of acrylamide. The loss of Trp was monitored using reverse-phase high performance liquid chromatography (RP-HPLC) and was observed to follow first order kinetics. Production of N-formylkynurenine (NFK) was observed by RP-HPLC in irradiated Trp samples both in the presence and absence of added acrylamide. In addition, no new photochemical product was detected. This was taken as evidence that acrylamide did not alter the photochemical pathway but just reduced the reaction rate as expected for a physical quenching mechanism. Plotting the reciprocal of photolysis rate constant versus acrylamide concentration produced a Stern-Volmer constant for quenching of Trp photochemistry of KSV = 6 +/- 2 M-1. The KSV values for both fluorescence quenching and photolysis quenching were thus large, implying efficient quenching of both processes by acrylamide. Assuming an excited singlet state lifetime of 2.8 ns, the calculated second-order quenching rate constants for fluorescence and photolysis were kq = 7.5 x 10(9) and 2.1 x 10(9) M-1 s-1 respectively. The possible involvement of photoionization in the photolysis mechanism was investigated by studies of acrylamide quenching of voltage transients produced by xenon flash lamp excitation of Trp at aqueous/teflon or aqueous/mica interfaces.(ABSTRACT TRUNCATED AT 250 WORDS)

ChemInform Abstract: Photophysical Properties of a C6 Hydrocarbon Linked Porphyrin Dimer.

The synthesis and characterisation of a C6 hydrocarbon linked porphyrin dimer and its zinc complex is described. From fluorescence quantum yields and excited singlet and triplet state lifetimes, recorded for the dimers and the corresponding monomer species, it is suggested that the dimeric porphyrins exist in solution in open and closed conformations. The open conformations retain photophysical properties similar to those of the relevant monomeric species but the closed conformations do not fluoresce.

ChemInform Abstract: Dependence of the Lifetime of the Twisted Excited Singlet State of Tetraphenylethylene on Solvent Polarity

ChemInform Abstract: Dependence of the Lifetime of the Twisted Excited Singlet State of Tetraphenylethylene on Solvent Polarity

Photophysical properties of a C6 hydrocarbon linked porphyrin dimer

AbstractThe synthesis and characterisation of a C6 hydrocarbon linked porphyrin dimer and its zinc complex is described. From fluorescence quantum yields and excited singlet and triplet state lifetimes, recorded for the dimers and the corresponding monomer species, it is suggested that the dimeric porphyrins exist in solution in open and closed conformations. The open conformations retain photophysical properties similar to those of the relevant monomeric species but the closed conformations do not fluoresce.

Photochemical and Photophysical Studies of Rose Bengal Poly(vinylpyrrolidone) — Application to the Photoreduction of Water

The photophysics and photochemistry of the xanthen dye rose bengal have been studied both free in aqueous solution and when bound to poly( vinylpyrrolidone ) ( pvp ). Fluorescence, picosecond spectroscopy and laser flash photolysis measurements have been undertaken. The singlet and triplet excited state lifetimes of the dye are increased and the dye photostability is enhanced upon binding to the polymer. These observations may be explained on the basis of changes in the microenvironment surrounding the dye and restrictions on molecular diffusion in the polymer. The reduced rate of non- radiative decay and higher photostability of rose bengal in the presence of polymer leads to improved overall yields of hydrogen production during the photoreduction of water in reaction systems which incorporate the dye and pvp as a light-harvesting pigment. One of the most efficient systems for the photoreduction of water at pH 9 and above is reported.

Rotational Relaxation of Benzophenone in the Vapour Phase

The excited singlet state lifetimes of benzophenone vapour have been measured at shorter wavelengths than previous studies and with picosecond time resolution. Excitation was at a series of wavelengths from 313.5 nm to 284 nm, which gave S2(ΠΠ*) decay times of tens of picoseconds. We also report measurements of the polarisation anisotropy decay due to free rotation of excited state benzophenone. Time-dependent anisotropies calculated by both quantum mechanical and classical formulae are in good agreement with the experimental data and demonstrate benzophenone to behave as a rigid rotor. From the residual anisotropy, at long times, it is concluded that no rotation to vibrational energy transfer occurs during the excited state lifetime.

Dynamical studies of triplet-singlet pairs for bound excitons related to complex defects in GaP

Transient tunable dye-laser luminescence spectroscopy has been applied to study the kinetics of carrier recombination at those complex defects in GaP which have a triplet-singlet (TS) pair of states as the lowest energy bound exciton configuration. The characteristic photoluminescence decay has been measured for several such BE systems, and from these date the lifetimes of both singlet and triplet states can be deduced. A typical singlet lifetime in GaP is of the order of 1–10 μs, while the triplet lifetime often is about a factor of 100 larger. A typical case of a thermalized TS BE system is demonstrated with the (Cu-Li) I BE with a triplet line at 2.3062 eV. it is noted that for many BE systems excitation transfer is quite important, and complicates the evaluation of data.

Dynamics of geminate ions in the pulse radiolysis of cyclohexane and n-hexane containing aromatic solutes studied by streak camera measurements of fluorescence and absorption

The nature of geminate ion reactions in cyclohexane or n-hexane solutions irradiated with 30 ps electron pulses has been investigated by observing solute excited state fluorescence and absorption of solute ions. For the fluorescence measurements, 4-Br,4″hexa(5-decyl)- p-terphenyl(BHTP) was used as the solute. The short fluorescence lifetime (0.3 ns) of the excited singlet state of this compound (BHTP 1) facilitates the interpretation of the results because BHTP 1 formed in an excitation process occuring during the pulse decays rapidly enough that the slower production of BHTP 1 by reaction of BHTP ⨪ with the radical cation of the solvent is more easily discernible than in the case of scintillators with longer lifetimes. Comparison of the results with a model indicates a qualitative correspondence with the known properties of ions and electrons in the two solvents. In particular, the results support a higher mobility cation derived from the solvent in cyclohexane, compared with n-hexane. Quantitatively, however, the experimental data show a greater amount of excitation occuring within the first 20 ps (during the pulse) than is expected from the model. Results on the formation of the pyrene radical cation in cyclohexane and n-hexane solutions of pyrene also show significant differences between the two solvents which indicate a slower formation of the pyrene cation in n-hexane.

Determination of femtosecond lifetimes of higher excited singlet states by means of transient photophysical hole-burning. The S 2 state of phenanthrene

Transient photophysical hole-burning is studied as a method of measuring femtosecond lifetimes of higher excited electronic states of polyatomic molecules. For the 0,0 transition to the S 2 state of phenanthrene in isopentane/2-methylpentane glass at 4 K a homogeneous linewidth of 67 ± 7 cm −1 was found corresponding to a lifetime of 79 ± 8 fs for this state.

AN UNUSUAL EXCITED STATE SPECIES OF ortho‐ HYDROXY‐CINNAMIC ACID*

AbstractThe fluorescence of ortho‐hydroxycinnamic acid was studied in aqueous solution over the pH 0‐14 range and in methanol. At pH values above 9.7, intense anion fluorescence is observed. In the pH 4‐9 range emissions are from both the monoanion (albeit weakly) and from the dianion due to photodissociation. In the pH 0‐4 range a blue and a green fluorescence was found, which is attributed to emissions from the unchanged molecule and from an unusual excited state tautomer respectively. The latter is assumed to be the phenolate anion of cinnamic acid and represents a tautomer with no equivalent in the electronic ground state. It is assumed to be formed by adiabatic photodissociation during the lifetime of the first excited singlet state.

Direct measurement of the lowest excited singlet state lifetime of all-trans-β-carotene and related carotenoids

A 4 ps, 510 nm laser pulse was used to electronically excite toluene solutions of all-trans-β-carotene, canthaxanthin, and β-8'-apocarotenal. The 430–510 nm electronic absorption band of each carotene bleaches immediately upon excitation and recovers with single-exponential kinetics: τ = 8.4 ± 0.6 ps for all-trans-β-carotene, 5.2 ± 0.6 ps for canthaxanthin, and 25.4 ± 0.2 ps for β-8 '-apocarotenal. The results are discussed in terms of the excited singlet state properties of carotenoids.

Photocrosslinking of polymers containing cinnamoyl groups: The change in polymer coil dimension during the reaction in solution

Intra- and intermolecular photocrosslinking reactions of poly(2-cinnamoyloxyethyl methacrylate) (PCEMA) and copoly(CEMA/MMA) in dilute solution under u.v. light irradiation at 30°C were studied by u.v. absorption, light scattering, GPC and viscosity measurements. The contraction of polymer coils proceeds due to predominant intramolecular photodimerization in very dilute solution. When the polymer concentration increases, competition between intra- and intermolecular photodimerization is observed, and the volume of polymer coils increases gradually due to the increase in molecular weight caused by the intermolecular reaction overcoming the contraction caused by the intramolecular reaction. The contribution of intramolecular crosslinking is much more important in the presence of oxygen than in its absence, showing that the shorter lifetime of the excited singlet state makes intermolecular photodimerization difficult.

Photo-physics and photo-chemistry of aromatic oxanilides used as polymer light stabilizers

Aromatic oxanilides, used as polymer light stabilizers, show a remarkably low photo-reactivity both in oxygen-free and in air-saturated solutions. Their photo-physical properties have been investigated by a combination of experimental and theoretical methods: photo-electron spectroscopy, electron energy loss spectroscopy, ultraviolet-visible and luminescence spectroscopy, laser flash photolysis and MINDO/3 calculations. In fluid solvents, and even in a polymer at room temperature, the lifetime of the excited singlet state is of the order of a few ps; this is ascribed tentatively to a reversible intramolecular proton transfer. Aromatic oxanilides protect photo-degradable polymers largely by their inner filter effect. Their role as quenchers of excited species is relatively unimportant and they have no action as radical scavengers, although they are not easily attacked by free radicals in solution.

ChemInform Abstract: CONCERNING THE LIFETIME OF THE SECOND EXCITED SINGLET STATE OF ADAMANTANETHIONE

Chemischer InformationsdienstVolume 16, Issue 17 Physical Organic Chemistry ChemInform Abstract: CONCERNING THE LIFETIME OF THE SECOND EXCITED SINGLET STATE OF ADAMANTANETHIONE K. J. FALK, K. J. FALKSearch for more papers by this authorA. R. KNIGHT, A. R. KNIGHTSearch for more papers by this authorA. MACIEJEWSKI, A. MACIEJEWSKISearch for more papers by this authorR. P. STEER, R. P. STEERSearch for more papers by this author K. J. FALK, K. J. FALKSearch for more papers by this authorA. R. KNIGHT, A. R. KNIGHTSearch for more papers by this authorA. MACIEJEWSKI, A. MACIEJEWSKISearch for more papers by this authorR. P. STEER, R. P. STEERSearch for more papers by this author First published: April 30, 1985 https://doi.org/10.1002/chin.198517040AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume16, Issue17April 30, 1985 RelatedInformation

Dediazoniation of arene diazonium salts by triplet excited aromatic ketones: electron or energy transfer?

The triplet excited aromatic ketones benzophenone, xanthone, thioxanthone, fluorenone, benzil and 4,4′-dimethoxybenzophenone sensitize the dediazoniation of arene diazonium salts. The quantum yields of the photolyses of benzene diazonium tetrafluoroborate in the presence of these ketones and of 4-chlorobenzene diazonium tetrafluoroborate, 4-methyl-benzene diazonium tetrafluoroborate and 4-methoxybenzene diazonium tetrafluoroborate in the presence of benzophenone were determined. In almost all cases the reduction products are of the Ar—H type (Ar ≡ aryl) which is characteristic of the electron transfer mechanism. The photochemically induced dynamic nuclear polarization (photo-CIDNP) of 4-chlorobenzene diazonium tetrafluoroborate and 4-cyanobenzenediazonium-β- 15N tetrafluoroborate in the presence of the ketones under investigation was also studied. In all cases except fluorenone the signs of the photo-CIDNP effects clearly point to triplet precursors. The photo-CIDNP effect in the presence of fluorenone has the same sign as that or pyrene which is known to react via the excited singlet state. This is because of the long lifetime of the excited singlet state of fluorenone which is efficiently trapped by the diazonium salt. These results suggest that the dediazoniation of arene diazonium salts sensitized by aromatic ketones proceeds via electron transfer but not via energy transfer.