Quenching and photobleaching of excited polycyclic aromatic hydrocarbons by carbon tetrachloride

The singlet quenching and photobleaching of several aromatic polycyclic hydrocarbons by carbon tetrachloride was investigated. The quenching rate constants were determined from fluorescence quenching measurements. The values obtained in ethanol range from 1.2 × 10¹⁰ M⁻s⁻¹ to 2 × 10⁶ M⁻¹ s⁻¹ for fluorene and coronene respectively. These values depend markedly on the solvent polarity (e.g. the quenching rate constant of perylyne increases from 2 × 10⁷ M⁻¹ to 1.24 × 10⁹ M⁻¹ s⁻¹ from cyclohexane to acetonitrile) and the characteristics of the substituents (e.g values of 2 × 10¹⁰ M⁻¹ s⁻¹ and 1.2 × 10⁸ M⁻¹ s⁻¹ were obtained for the quenching of 1-methoxynaphthalene and 1-cyanonaphthalene respectively). In most cases investigated, the singlet quenching leads to efficient photocleavage of the hydrocarbon.     

Theoretical study of the excited singlet and triplet states of alloxan

The possibility of excited‐state protomeric shifts in the biologically important molecule, alloxan, is investigated. We have focused on the S₁ and T₁ excited states of alloxan and its hydroxy tautomers. Modifications brought in by excitation on the relative stabilities, activation barriers, and optimized geometries, computed at the MNDO, AM1, and PM3 levels of approximation, have been discussed for both excited electronic states. The absorption and fluorescence spectra for the three tautomers are also discussed. Results show significant changes in the geometries on excitation, although the changes are similar for the singlet and triplet excited states. Though the relative stability orders do not change, the 2‐hydroxy tautomer is stabilized, while the 4‐hydroxy tautomer gets destabilized on excitation. The excited states are (n,π*) states, involving the promotion of a nonbonding oxygen lone pair from the CO? CO? CO moiety, which explains why the oxygens of this group become less basic and the 4‐hydroxy tautomer gets destabilized on excitation. However, the activation barriers do not reduce significantly on excitation, and this precludes the possibility of ground‐ or excited‐state proton transfer in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Photophysics of monodisperse platinum-acetylide oligomers: delocalization in the singlet and triplet excited states

A series of monodisperse Pt-acetylide polymers that contain the [-CC-(p-C6H4)-CC-(t-Pt(PBu3)2)-]n repeat unit has been prepared for n = 1, 2, 3, 4, 5, and 7. The photophysical properties of the series provide information concerning the relationship between the oligomer length and delocalization in the singlet and triplet excited states of the pi-conjugated electron system. The results imply that the singlet excited state is delocalized over approximately 6 repeat units; however, the triplet state is considerably more localized. The triplet energy is almost invariant with oligomer length, but the phosphorescence spectra and triplet nonradiative decay rates indicate that the electron-vibrational coupling in the triplet state decreases with increasing oligomer length.     

Theoretical analysis of singlet and triplet excited states of nickel porphyrins

Local density and generalized gradient approximation time-dependent density functional methods have been used for calculation of the singlet and triplet excited states of nickel-porphine, Ni-tetraphenyloporphine, and Ni-octaethyloporphyrine. Special attention is paid to metal-ligand transitions and d-d transitions. It is shown that the lowest exited singlet states of the three compounds can be described as a transfer of an electron from the porphine ring to the d(x2-y2) orbital of the nickel atom. On the other hand, the lowest excited triplet state arises from promotion of an electron between two nickel d orbitals, an occupied d(z2) and an empty d(x2-y2). It is proposed that a rapid quenching of the excited singlet states is due to an ultrafast intersystem crossing between 1Eg)and 3Eg or 3B1g states.     

Direct measurement of optical spin orientation in the excited triplet state of aromatic hydrocarbons at room temperature

Optically induced magnetization has been observed in polycrystals and solutions of various aromatic carbonyls, quinones, and aza-aromatics at room temperature using a simple pickup-coil detector. The magnetic-field dependence of the magnitude of the induced magnetization provided clear evidence of the creation of spin polarization in the photo-excited triplet state. The dependence of the relaxation rate of the magnetization on the magnetic field and the temperature has been measured.     

Characterization of the singlet and triplet excited states of 3-chloro-4-methylumbelliferone

An extensive photophysical characterization of 3-chloro-4-methylumbelliferone (3Cl4MU) in the ground-state, S(0), first excited singlet state, S(1), and lowest triplet state, T(1), was undertaken in water, neutral ethanol, acidified ethanol, and basified ethanol. Quantitative measurements of quantum yields (fluorescence, phosphorescence, intersystem crossing, internal conversion, and singlet oxygen formation) together with lifetimes were obtained at room and low temperature in water, dioxane/water mixtures, and alcohols. The different transient species were assigned and a general kinetic scheme is presented, summarizing the excited-state multiequilibria of 3Cl4MU. In water, the equilibrium is restricted to neutral (N*) and anionic (A*) species, both in the ground (pK(a) = 7.2) and first excited singlet states (pK(a)* = 0.5). In dioxane/water mixtures (pH ca. 6), substantial changes of the kinetics of the S(1) state were observed with the appearance of an additional tautomeric T* species. In low water content mixtures (mixture 9:1 v:v), only the neutral (N*) and tautomeric (T*) forms of 3Cl4MU are observed, whereas at higher water content mixtures (water mole fraction superior to 0.45), all three species N*, T*, and A* coexist in the excited state. In the triplet state, in the nonprotic and nonpolar solvent dioxane, the observed transient signals were assigned as the triplet-triplet transition of the neutral form, N*(T(1)) → N*(T(n)). In water, two transient species were observed and are assigned as the triplets of the neutral N*(T(1)) and the anionic form, A*(T(1)) (also obtained in basified ethanol). The phosphorescence spectra and decays of 3Cl4MU, in neutral, acidified, and basified solutions, demonstrate that only these two species N*(T(1)) and A*(T(1)) exist in the lowest lying triplet state, T(1). The radiative channel was found dominant for the deactivation of the anionic species, whereas with the neutral the S(1) ? S(0) internal conversion competes with fluorescence. For both N* and A* the intersystem crossing yield represents a minor deactivation channel for S(1).     

Photophysical properties of the lowest excited singlet and triplet states of thio- and seleno-psoralens

The decay processes of the lowest excited singlet and triplet states of five heteropsoralens (HPS) were investigated by steady-state and shift-phase fluorometry and by laser-flash photolysis in different solvents. The emission spectra of HPS are detectable only in trifluoroethanol (TFE), where fluorescence lifetimes (τ_F) and quantum yields (φ_F) were measured. The triplet lifetimes (τ_T), triplet (φ_T) and singlet-oxygen production (φ_Δ) quantum yields were determined in benzene, ethanol and TFE by laser-flash photolysis. Semiempirical (INDO/1-CI) calculations allowed the nature of the lowest excited singlet and triplet states and transition probabilities to be obtained. Theoretical and experimental results indicate that the two lowest excited singlet states S₁ and S₂ of HPS are close-lying and different in nature (π,π* and n,π*). The "proximity effect" between these two states controls the photophysical properties of HPS as it does for the other furocoumarins. However, HPS have a peculiar behavior with respect to the related compounds because they are fluorescent and have, in three cases, detectable intersystem crossing only in TFE. This behavior can be tentatively explained by a different energy gap and/or order between the S₁ and S₂ states.     

Quenching of singlet oxygen by hindered amine light stabilisers. A flash photolytic study

Rate constants for the quenching of singlet oxygen (¹O₂, ¹δ_g) for a series of piperidines, piperidine-N-oxyl free radicals and some commercially used hindered amine light stabilisers (HALS) have been measured by a laser flash photolysis method. Quenching rate constants are in the order: piperidine-N-oxyl free radicals ≤ secondary piperidines < tertiary piperidines. For some commercial HALS, ¹O₂ quenching rate constants and the light protective effect towards polypropylene photo-oxidation have been compared. No correlation has been found between the stabilizing action and the quenching efficiency towards ¹O₂. The data obtained point to little contribution of singlet oxygen to the key steps of polyolefin photo-oxidation.     

Fluorescence from the second excited singlet of aromatic thioketones in solution

The spectral characteristics and the quantum yield of the fluorescence from the second excited singlet state S₂ of the aromatic thioketone molecules xanthione (XS) and thioxanthione (TXS) have been determined in solution at room temperature and 77 K. In 3-methylpentane, the measured quantum yields are φ_f (295 K) = 5.1 × 10^?3 and φ_f(77 K) = 1.0 × 10^?2 for XS, and φ_f (295 K) = 1.5 × 10^?3 and φ_f (77 K) = 2.5 × 10^?3 for TXS. Using the Strickler-Berg expression for the radiative lifetime, the decay rate of S₂ is derived. It is concluded that internal conversion S₂ ? S₁ is the dominating deactivation channel of S₂ with k^{77 K}_nr(S₂ ? S₁) = 1.0 × 10¹⁰ s^?1 for XS and k^{77 K}_nr (S₂→S₁) = 2.2 × 10¹⁰ s^?1 for TXS. Between 295 and 77 K, φ_f increases by a factor of about 2 following an Arrhenius type expression. This temperature dependence of φ_f is considered to be intramolecular in nature and is attributed to a temperature sensitive rate constant k_nr(S₂?S₁) with an activation energy of 190 ± 20 cm^?1 and a frequency factor k′_nr = 3 × 10¹⁰ s^?1 for the XS molecule in 3-methylpentane.     

Characteristic luminescence of the anilinium ion and the pKa values of excited singlet and triplet states

Contrary to earlier observations a characteristic luminescence due to the C₆H₅NH⁺₃ ion was observed. The ion at 80 K, gives a fluorescence similar to that of benzene and a phosphorescence similar to that of aniline. The benzene type fluorescence changes to the aniline type on warming. It is suggested that an excited C₆H₅NH⁺₃ ion giving rise to the fluorescence becomes dissociated in the excited state. Computations for a change in the pKa values for excited singlet and triplet states of C₆H₅NH⁺₃ ion also suggest that is a larger probability of losing the proton in the excited state.     

Quenching enhancement of the singlet excited state of pheophorbide-a by DNA in the presence of the quinone carboquone

Changes in the emission fluorescence intensity of pheophorbide-a (PHEO) in the presence of carboquone (CARBOQ) were used to obtain the association constant, the number of CARBOQ molecules interacting with PHEO, and the fluorescence quantum yield of the complex. Excitation spectra of mixtures of PHEO and CARBOQ in ethanol (EtOH) show an unresolved doublet in the red-most excitation band of PHEO, indicating the formation of a loose ground-state complex. The 1:1 CARBOQ-PHEO complex shows a higher fluorescence quantum yield in EtOH (0.41 ± 0.02) than in buffer solution (0.089 ± 0.002), which is also higher than that of the PHEO monomer (0.28). Quenching of the PHEO fluorescence by DNA nucleosides and double-stranded oligonucleotides was also observed and the bimolecular quenching rate constants were determined. The quenching rate constant increase as the oxidation potential of the DNA nucleoside increases. Larger quenching constants were obtained in the presence of CARBOQ suggesting that CARBOQ enhances DNA photo-oxidation, presumably by inhibiting the back-electron-transfer reaction from the photoreduced PHEO to the oxidized base. Thus, the enhanced DNA-base photosensitized oxidation by PHEO in the presence of CARBOQ may be related to the large extent by which this quinone covalently binds to DNA, as previously reported.     

Quenching of the excited singlet state of acridine and 10-methylacridinium cation by thio-organic compounds in aqueous solution

In this work, the lowest excited singlet states of acridine (Acr), acridinium (AcrH⁺) and 10-methylacridinium (AcrMe⁺) are quenched by sulfur-containing amino acids and carboxylic acids in aqueous solution. Both steady-state and time-resolved fluorescence techniques were used to monitor the quenching of fluorescence. Stern–Volmer plots of the fluorescence intensity showed a static component (K_S) to the quenching. The experimental K_S values were compared to theoretical K_S values for outer-sphere complexes based on Debye–Hückel theory and the Fuoss equation. The general agreement between experimental and theoretical K_S values indicate that the static quenching can be attributed to non-fluorescing ion pairs associated as simple outer-sphere complexes. The computed values of the interionic distances of the ion pairs are consistent with the ion pairs of the Z_AZ_Q=−1 and −2 cases being solvent-separated ion pairs while those of the Z_AZ_Q=−3 case are contact ion pairs. The effect of the reactants’ charges on the quenching rate constants (dynamic component) was observed for the reactions of AcrMe⁺ with the anionic forms of the quenchers (having charges Z_Q=−1, −2 and −3). The rate constants (extrapolated to ionic strength, μ=0) for the quenching processes were determined to be 0.3–5.3×10¹⁰ M⁻¹ s⁻¹ depending on the ionic charge (Z_Q) of the quencher used. These trends in the quenching rate constants are rationalized with a quenching scheme for electron transfer. Analogous quenching rate constants for alanine and glycine were found to be at least an order of magnitude lower. Photoinduced electron transfer from the sulfur atom of the quencher molecule to the acridine excited singlet state is suggested to be the most likely mechanism of the process under discussion.     

Theoretical study of photoinduced singlet and triplet excited states of 4-dimethylaminobenzonitrile and its derivatives

Singlet and triplet low-lying states of the 4-dimethylaminobenzonitrile and its derivatives have been studied by the density functional theory and ab initio methodologies. Calculations reveal that the existence of the methyl groups in the phenyl ring and the amino twisting significantly modify properties of their excited states. A twisted singlet intramolecular charge-transfer state can be accessed through decay of the second planar singlet excited state with charge-transfer character along the amino twisting coordinate or by an intramolecular charge-transfer reaction involved with a locally first excited singlet state. Plausible charge-transfer triplet states and intersystem crossing processes among singlet and triplet states have been explored by spin-orbit coupling calculations. The intersystem crossing process was predicted to be the dominant deactivation channel of the photoexcited 4-dimethylaminobenzonitrile.     

Energy transfer in the course of triplet state quenching of aromatic hydrocarbons by nitroxyl radicals

Quenching of triplet states of aromatic hydrocarbons by nitroxyl radicals has been investigated by the flash photolysis method. There are two different mechanisms of triplet quenching: quenching occurs via enhanced intersystem crossing on exchange interaction with the radical for the triplet states of aromatic hydrocarbons which have low triplet energy (E_T < 14700 cm^?1); for very high triplet energies, energy transfer from the triplet molecule to the nitroxyl radical occurs. The energy of the excited nitroxyl radical was estimated to be 18000 cm^?1.