Formation Of Singlet Excited States Research Articles

Evidence for the Formation of 1,2-Dioxetane as a High-Energy Intermediate and Possible Chemiexcitation Pathways in the Chemiluminescence of Lophine Peroxides.

A kinetic study of the chemiluminescent (CL) reaction mechanism of lophine-derived hydroperoxides and silylperoxides induced by a base and fluoride, respectively, provided evidence for the formation of a 1,2-dioxetane as a high-energy intermediate (HEI) of this CL transformation. This was postulated using a linear Hammett relationship, consistent with the formation of negative charge on the transition state of HEI generation (ρ > 1). The decomposition of this HEI leads to chemiexcitation with overall low singlet excited state formation quantum yield (ΦS from 1.1 to 14.5 × 10-5 E mol-1); nonetheless, ΦS = 1.20 × 10-3 E mol-1 was observed with both peroxides substituted with bromine. The use of electron-donating substituents increases chemiexcitation efficiency, while it also reduces the rate for both formation and decomposition of the HEI. Different possible pathways for HEI decomposition and chemiexcitation are discussed in light of literature data from the perspective of the substituent effect. This system could be explored in the future for analytical and labeling purposes or for biological oxidation through chemiexcitation.

Investigation of the push–pull effects on β-functionalized zinc porphyrin coordinated to C60 donor–acceptor conjugates

Two novel β-functionalized push–pull zinc porphyrins with amine or phenyl push-groups and cyclic imide or carboxylic esters pull-groups have been newly synthesized for light energy harvesting applications. The ethynylphenyl spacers extended the conjugation of the porphyrin π-systems, as reflected by their red-shifted absorbance and fluorescence spectra. Computational studies performed at the B3LYP/6-31G* level indicated no steric hindrance between the porphyrin π-system and the substituents. The calculated HOMO and LUMO orbitals displayed significant delocalization in 1, where the electron density in the HOMO was localized over the push groups, whereas the LUMO was extended over the porphyrin π- and pull-groups. On the other hand, 2 did not display significant orbital segregation, which is attributed to its weaker push–pull character. Electrochemical studies revealed smaller HOMO–LUMO gaps for the push–pull zinc porphyrins. As a consequence of the push–pull effects, reduction in fluorescence intensity and lifetime was observed. Femtosecond transient absorption spectral studies revealed successful formation of singlet excited state in both of the push–pull porphyrins. Donor–acceptor conjugates were subsequently built and characterized by coordinating an electron acceptor, C60Im, via metal–ligand axial coordination. Efficient photo-induced charge separation in both donor–acceptor conjugates was witnessed wherein the charge-separated states persisted tens of nanoseconds prior returning to the ground state.

Peroxyoxalate High-Energy Intermediate is Efficiently Decomposed by the Catalyst Imidazole.

The peroxyoxalate reaction is a highly efficient chemiluminescence system, its chemiexcitation process involving the intermolecular interaction between an activator (ACT) and the high-energy intermediate (HEI) of the reaction. Typically, the HEI is generated through the reaction of an oxalate ester with H2 O2 , in the presence of a basic/nucleophilic catalyst, such as imidazole (IMI-H). IMI-H, besides catalyzing the formation of the HEI, is also known to decompose this peroxidic intermediate. Despite that, up to now, no rate constant value has been determined for such significant interaction. Through kinetic measurements, we have observed that IMI-H is roughly four times more efficient than 9,10-diphenylanthracene (DPA), a classic ACT, in catalyzing the decomposition of the HEI by a bimolecular electron transfer reaction through a Chemically Initiated Electron Exchange Luminescence-like process. For instance, when IMI-H and DPA are at the same concentration, 78% of the generated HEI is actually consumed by the nonemissive bimolecular interaction with IMI-H. We have obtained an average singlet excited state formation quantum yield, at infinite ACT concentration, of (5.5±0.5)×10(-2) Emol(-1) , determined at five different IMI-H concentrations. This ultimately suggests that the yield of formation of HEI actually does not depend on the IMI-H concentration.

Photophysics and electrochemistry of a platinum-acetylide disubstituted perylenediimide

The synthesis and photophysical study of a perylene diimide (PDI) functionalised with platinum acetylide units of the type, trans{-C≡C-Pt(PBu3)2-C≡C-Ph} and comparison with a phenylacetylide substituted model compound are reported. The model compound demonstrates typical perylene absorption and photoluminescence spectra characteristic of singlet excited state formation and decay. The Pt-substitution, however, appears to induce spin-orbit coupling into the chromophore and giving rise to a triplet excited state which was confirmed by transient absorption measurements. This excited state is quenched by oxygen, leading to the formation of singlet oxygen in dichloromethane, recorded by time-resolved near-infrared luminescence measurements.

Interstate Crossing-Induced Chemiexcitation as the Reason for the Chemiluminescence of Dioxetanones

The decomposition reaction of dimethyl-1,2-dioxetanone in dichloromethane was studied by using a DFT approach. The low efficiency of triplet and singlet excited-state formation was rationalised. A charge-transfer process was demonstrated to be involved in the chemiluminescence process. Present and previous results allow us to define an interstate crossing-induced chemiexcitation (ICIC) mechanism for the chemiluminescence of dioxetanones. Charge transfer is needed to reach a transition state, in the vicinity of which direct population of excited states is possible. The chemiexcitation process is then governed by singlet/triplet intersystem crossings. Structural modifications then modify the rate of these crossings and the singlet ground and excited-state interaction, thereby modulating the efficiency of this process and the spin of the resulting products.

Photoinduced interactions between coumarin 151 and colloidal CdS nanoparticles in aqueous suspension

The photophysical features of 7-amino-4-trifluoromethylcoumarin (C151) in colloidal suspension that contains CdS semiconductor nanoparticles were investigated by using molecular UV–Vis absorption, steady-state and time-resolved fluorescence, and electron spin resonance (ESR) spectroscopy techniques. For this aim, the fluorescence intensity quenching of C151 molecules in colloidal aqueous solution containing colloidal CdS nanoparticles were examined. It was found that electrostatic interactions of C151 molecules at the ground state with CdS nanoparticles cause adsorption of certain amount of dye molecules onto colloidal CdS nanoparticles. After that, molecular excitation of C151 in the colloidal suspension leads to the formation of singlet excited state of C151 molecules and then adsorbed C151 molecules, at the singlet excited state, onto surfaces of CdS nanoparticles cause charge injection process. Therefore, the apparent association constants (Kapp), occurring between C151 molecules and colloidal CdS nanoparticles, and charge injection rate constants (ket) were calculated and compared with the values reported in the literature.

Enhanced chemiluminescence of 6-(4-methoxyphenyl)imidazo[1,2- a]pyrazin-3(7 H)-one by attachment of a cyclomaltooligosaccharide (cyclodextrin). Attachment of cyclomaltononaose (δ-cyclodextrin)

2-Methyl-6-(4-methoxyphenyl)imidazo[1,2- a]pyrazin-3(7 H)-one (MCLA) is an oxygen-induced chemiluminescent compound. It has been shown that the chemiluminescence can be enhanced by forming a cyclomaltooligosaccharide (cyclodextrin)-bound MCLA, and therefore, in continuation of the survey of the types of cyclodextrins, in this study, MCLA was attached to the secondary hydroxyl face of δ-cyclodextrin, which consists of nine d-glucose units. Although the oxygen-induced chemiluminescence efficiency of δ-cyclodextrin-bound MCLA in a pH 8.0 aqueous phosphate buffer was 12 times greater than that of MCLA, the efficiency was markedly lower than that of γ-cyclodextrin-bound MCLA, which exhibited the highest chemiluminescence efficiency in the previous investigation. Although fluorescence efficiency and light-emitter formation efficiency for δ-cyclodextrin-bound MCLA were similar to those for γ-cyclodextrin-bound MCLA, singlet-excited state formation efficiency for δ-cyclodextrin-bound MCLA was lower than that for γ-cyclodextrin-bound MCLA. This study distinctly indicated the optimum cyclodextrin for construction of greatly luminescent cyclodextrin-bound MCLA is γ-cyclodextrin.

Direct and oxygen-mediated triplet–triplet annihilation of tetraphenylporphyrin in multilayers of decanol on the external surface of NaA zeolite

The kinetics of triplet state decay and concomitant delayed fluorescence (DF) of meso-tetraphenylporphyrin (TPP) adsorbed on the external surface of NaA zeolite covered by coadsorbed n-decanol (DEC) were studied by the laser flash photolysis. In deoxygenated solid samples of NaA with coadsorbed DEC, the diffusion-controlled bimolecular triplet–triplet annihilation (TTA) dominates in contrast to what is observed on the dry NaA surface. The TTA which results in DF can be described in terms of two reaction channels: (a) an energy transfer pathway with singlet excited state formation; (b) enhanced intersystem crossing in the triplet encounter complex. Whilst the TTA in multilayers occurs in DEC microphase, the reaction parameters change significantly when adsorbed ground-state molecules are involved. A fast TTA rate constant found at large TPP concentration in solid samples with DEC was attributed to triplet energy migration in TPP assemblies. Singlet oxygen works as an efficient mobile energy carrier between TPP triplets in both solid and liquid aerated samples. The parameters of this reaction may also be modified by changing the ratio [TPP]/[DEC] in multilayers, which in general act as microreactor systems with supramolecular organization of reactants.

Chemiluminescence of 2-methyl-6-arylimidazo-[1,2-a]pyrazin-3(7H)-one in protic solvents: electron-donating substituent effect on the formation of the neutral singlet excited-state molecule.

2-Methyl-6-arylimidazo[1,2-a]pyrazin-3(7H)-ones with a substituent such as phenyl, 4-methoxyphenyl or 4-trifluoromethoxyphenyl at the 6-position of the imidazo[1,2-a]pyrazin-3(7H)-one ring system, produced chemiluminescence emission in mixtures of water and DMF and in several mixtures of MeOH and DMF under neutral conditions. Under these protic luminescence conditions, the respective light emissions were generated from neutral singlet excited-state molecules. The electron-donating effect of the 4-methoxy substituent on the phenyl group increased the efficiency of the neutral singlet excited state formation, whereas non-substitution and a 4-trifluoromethoxy group having no electron donating ability decreased the efficiency. The compound having the electron-donating methoxy group substituent showed two chemiluminescence emitters, which generated light at lambda(max) 410-420 nm and 460 nm. It was determined that the neutral molecules in the excited state generating light emission at the shorter wavelengths are neutral singlet excited-state molecules suitable for highly efficient singlet excited-state formation. A role of the electron-donating effect of the methoxy group is postulated to be generation of the special neutral singlet excited-state molecules.

Fluorescent solvent excited states from hole-electron recombination in cis- and trans-decalin irradiated with high energy electrons

The formation of the fluorescent solvent excited states in cis- and trans-decalin in the presence of various electron and hole scavengers has been studied by measuring the fluorescence resulting from pulsed irradiation. In order to be able to determine the effect of the charge scavengers on the yield of formation of the fluorescent excited state the quenching of the excited state by the scavengers has been studied in detail. Diffusion coefficients of the solutes have been determined and reaction radii for the quenching reactions are given. The ratio of the depression of the yield of formation of fluorescent excited states and the yield of charge scavenging has been found to be approx. 0.9 for both electron and hole scavengers and shows a tendency to decrease at higher concentrations. The results are consistent with earlier independent measurements of the mobilities of electron holes and excess electrons. The ratio of approx. 0.9 represents the average probability of singlet excited state formation from ion recombination for the relatively longer lived ion pairs, which indicates a relatively large contribution of single ion pairs. The average probability of singlet formation from the shorter lived ion pairs is expected to be lower.

No Electron-Donating Substituent Effect on the Singlet Excited State Formation from the 5-(5-Aryl-2-pyrazinylamino)-1,2,4-trioxanes in Dimethyl Sulfoxide Triggered by Potassium t-Butoxide

Abstract 5-(5-Phenyl-2-pyrazinylamino)-1,2,4-trioxane derivatives having a substituent on the 4 position of the phenyl group gave chemiluminescence in dimethyl sulfoxide by addition of potassium t-butoxide. No electron-donating substituent effect was observed on the yield of singlet excited state formation. This may be explained by the negative charge formed on the nitrogen atom next to the transient dioxetane moiety, which may produce singlet excited-state molecule by the intramolecular CIEEL mechanism.

Pulse radiolysis studies on the formation of singlet excited states of coumarin 153 in cyclohexane solutions

By using the technique of nanosecond pulse radiolysis, pulsed electron beam induced light emission from coumarin dyes in hydrocarbon solvents has been studied. The emission spectra so obtained were similar to the optically excited fluorescence spectra. The emission lifetimes were of the same order as the fluorescence lifetimes in the respective solvents, showing that the emitting species are the same in both the cases viz. singlet excited states of the dyes. In one system viz. C 153 in cyclohexane experiments were carried out in presence of electron and hole scavengers and also the concentration dependence of emission intensity studied over a wide range. From these it is concluded that the solute excited states are formed mainly by energy transfer from the solvent excited states, part of which may arise from excitation by cerenkov light generated in the medium.

Excited states and primary photochemical reactions in the photosynthetic bacterium Heliobacterium chlorum

The charge separation and excited states of antenna bacteriochlorophyll in membrane fragments of the recently discovered photosynthetic bacterium Heliobacterium chlorum were studied by absorbance-difference spectroscopy. Formation of singlet excited states of bacteriochlorophyll g with a lifetime of 200 ps or less was observed as the disappearance of the ground state absorption bands. From the absorbance-difference spectra, it was concluded that the primary photochemical reaction consists of the transfer of an electron from the primary donor P-798 to a possibly bacteriochlorophyll c-like pigment absorbing at 670 nm. Electron transfer to the secondary acceptor occurred with a time constant of about 500 ps. The midpoint potential of this acceptor (between -450 and -560 mV) and the absence of significant absorbance changes in the near-infrared upon its reduction suggest that this acceptor is an iron-sulfur center. It is concluded that the primary photochemistry of H. chlorum is similar to that of green sulfur bacteria.

Geminate recombination kinetics of solute radical ions: Singlet excited state formation in cyclohexane solutions of biphenyl

Transient absorption spectra of the solute anion, cation and triplet state and the solute fluorescence in the pulse radiolysis of 0.1 mole 1 -1 biphenyl in cyclohexane were observed on a nanosecond timescale longer than 1 ns after a 20 ps pulse. The formation of the solute excited singlet state is mainly due to the geminate ion recombination reaction even in the high concentrated solutions. The decay of the solute ions obeys the reciprocal square root dependence on time longer than 10 ns from the end of a 10 ps pulse. The slope of this reciprocal square root plots agrees with the literature value on a longer timescale obtained by microwave absorption. The yield of free ions obtained from the intercept of the slope agrees also with the literature values obtained by the field clearing method. Ratio of the formation rate of the solute excited triplet state to the decay rate of the solute anion changes in a time range between 5 and 20 ns. It is very well correlated with a theoretical calculation of spin correlation decay of the germinate ion pairs by Brocklehurst, although the formation of the solute triplet state was observed even on a timescale shorter than 5 ns from the end of a 20 ps pulse, where loss of spin correlation is negligibly small.

Pulse radiolysis of solutions of stilbene. I. Evidence for triplet and singlet excited state formation

Pulse radiolysis of solutions of stilbene. I. Evidence for triplet and singlet excited state formation