Triplet States Of Aromatic Hydrocarbons Research Articles

Tuning the Excited‐State Energy of the Organic Chromophore in Bichromophoric Systems Based on the RuII Complexes of Tridentate Ligands

A series of new heteroleptic and homoleptic Ru(II) complexes containing variously substituted bis(pyridyl)triazine ligands has been prepared and their absorption spectra, redox behaviour and luminescence properties (both in fluid solution at room temperature and in a rigid matrix at 77 K) have been investigated. For some compounds, X-ray structures have also been determined. The new bis(pyridyl)triazines incorporate additional chromophores, such as biphenyl, phenanthrene, anthracene and bromoanthracene derivatives, so the Ru(II) species can be considered as multichromophoric species. The absorption spectra and redox properties of all the metal complexes have been assigned to features belonging to specific subunits, thus suggesting that these species can be regarded as multicomponent, supramolecular assemblies from an electronic coupling point of view. Whereas most of the complexes exhibit luminescence properties that can be attributed to metal-to-ligand charge-transfer (MLCT) states involving the metal-based subunit(s), the species containing the anthryl and, even more, the brominated anthryl chromophores exhibit complicated luminescence behaviour. For example, 2 d (the anthryl-containing heteroleptic metal compound) exhibits MLCT emission at room temperature and emission from the anthracene triplet at 77 K; 2 e (the bromo-substituted anthryl-containing heteroleptic metal compound) exhibits anthryl-based emission at 77 K and MLCT emission at room temperature, but with a prolonged lifetime, thus suggesting equilibration between two triplet states that belong to different chromophores. The equilibration regime between MLCT and aromatic hydrocarbon triplet states is therefore reached by suitable substitution on the organic chromophore.

Singlet oxygen formation efficiencies following quenching of excited singlet and triplet states of aromatic hydrocarbons by molecular oxygen

Rate constants for quenching by molecular oxygen of excited singlet and triplet states, k S O 2 and k T O 2 , respectively, of 11 aromatic hydrocarbons in cyclohexane are reported. Measured values of k S O 2 are in the range 0.44 to 2.5×10 10 dm 3 mol −1 s −1 and are therefore in many cases close to k d, the diffusion controlled value of 2.8×10 10 dm 3 mol −1 s −1. The fraction of excited singlet states quenched by oxygen which result in triplet states, f T O 2 , was measured for all the compounds and found to be unity. The efficiencies of singlet oxygen production during oxygen quenching of the excited singlet and triplet states, f Δ S and f Δ T, respectively, were also measured. Values of f Δ S were shown to be 0, within experimental error, for all compounds except perylene for which f Δ S=0.28±0.05. Values of k T O 2 vary from 0.46 to 2.32×10 9 dm 3 mol −1 s −1. Thus, k T O 2 / k d was found to be less than one-ninth for all the compounds studied. Three different methods were used to measure f Δ T values, which were found to range from 0.57 to 1. Combination of the total quenching rate constants with the fractional efficiencies allows separate net quenching rate constants to be obtained for the various oxygen quenching pathways. Quenching of both excited singlet and triplet states, with and without energy transfer, are shown to be ‘charge-transfer assisted’ and mechanisms accounting for these results are discussed.

Charge Transfer Effects on the Efficiency of Singlet Oxygen Production Following Oxygen Quenching of Excited Singlet and Triplet States of Aromatic Hydrocarbons in Acetonitrile

Rate constants for quenching by molecular oxygen of excited singlet and triplet states, and , respectively, are reported for 12 aromatic hydrocarbons in acetonitrile. Measured values of , except in the case of fluoranthene for which = 6.6 × 109 dm3 mol-1 s-1, are in the range (2.3−4.3) × 1010 dm3 mol-1 s-1, i.e., close to 4.5 × 1010 dm3 mol-1 s-1, the average value obtained for kd, the rate constant for diffusion-controlled reactions of oxygen with aromatic hydrocarbons in acetonitrile. Values of vary from 0.24 to 5.6 × 109 dm3 mol-1 s-1. Thus, /kd was found to be less than one-ninth for 11 compounds. The efficiencies of singlet oxygen production during oxygen quenching of the excited singlet and triplet states, and , respectively, were also measured, as were the oxidation potentials of the hydrocarbons in acetonitrile. Values of were shown to be zero within experimental error for eight compounds and in the range of 0.27 ± 0.05 for the other four compounds. Three different methods, which gave good agreement, were used to measure values of which were found to vary from 0.41 in the case of acenaphthene to 0.85 for anthracene. The fraction of excited singlet states quenched by oxygen which result in triplet states was also measured for all compounds and found to vary from 0.49 to 1.0. Combination of the total quenching rate constants with the fractional efficiencies allows separate net quenching rate constants to be obtained for the various oxygen quenching pathways in acetonitrile. The reasons for variations in these net quenching rate constants and thus in the fractional efficiencies for quenching by the various quenching pathways are discussed. Quenching of both excited singlet and triplet states by energy transfer and by charge-transfer assisted pathways are established. The logarithm of the net rate constants for quenching of the triplet states without energy transfer to oxygen for 11 of the aromatic hydrocarbons shows a linear dependence on the free energy for full charge transfer from the triplet state, with a slope which indicates that the transition states for this quenching pathway only have about 13.5% charge-transfer character.

Solvent and temperature effects in the electron transfer quenching of triplet states of aromatic hydrocarbons by nitrobenzenes

The triplet quenching of anthracene and a,h-dibenzanthracene by four nitrobenzenes was studied by laser flash photolysis. Bimolecular quenching rate constants were determined in acetonitrile and methanol. They follow the expected free energy correlation for an outer sphere electron transfer process. The rate constants in methanol are generally higher than those in acetonitrile. The activation enthalpies and entropies were also measured in both solvents. The activation entropies are more negative in acetonitrile and the activation enthalpies are lower in acetonitrile with negative values being observed in some cases. Transient absorption spectra show that in the quenching process free radical ions are formed in methanol, but in acetonitrile free ions are almost absent. These results are compared with the current models for outer sphere electron transfer reactions. They cannot be explained if the solvent is considered as a dielectric continuum.

Mechanism on the Efficient Formation of Singlet Oxygen by Energy Transfer from Excited Singlet and Triplet States of Aromatic Hydrocarbons

Abstract The formation efficiencies of singlet oxygen for the oxygen quenching in excited singlet of aromatic hydrocarbons; A and for the quenching in the triplet T1; B were estimated. The hydrocarbons were classified into three types: (i) A = 1 and B = 1 for dicyanoanthracene i.e., total 200% efficiency, (ii) A ≈ 0.3 and B = 1 for anthracene, (iii) A ≈ 0 and B = 1 for pyrene, as examples. It is suggested that the energy levels of T2 and a dissociative exciplex; (T11Δg) correlated to A and B values from the energetic considerations.

Quenching of triplet states of aromatic hydrocarbons by xenon heavy atom perturber

Reactions of triplet excited diphenyl, p-terphenyl, naphthalene and pyrene molecules were studied in benzene solvent by measuring the triplet-triplet absorption and the delayed fluorescence arising from triplet-triplet annihilation. It was found that the triplet-triplet reaction occurs practically with a diffusion limited rate. The first order decay rate parameters of triplet molecules in 5 mmol dm -3 benzene solutions were measured around 1x10 4s -1. In presence of Xe this rate parameter increased, which indicated a definite heavy atom effect on the triplet. The rate parameters were found in the order 10 5-10 6mol -1dm 3s -1.

Quenching of triplet states of aromatic hydrocarbons by copper(II) 1,3-diketonates in solution

The rate constants for quenching of the triplet states of eleven different aromatic hydrocarbons by copper(II) bis (2,4-pentanedionate) and copper(II) bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate) in solution have been measured by laser flash photolysis. The involvement of electron- and energy-transfer mechanisms in the quenching process is elaborated and corresponding transmission coefficients and intrinsic barriers are obtained:

ChemInform Abstract: Quenching of Triplet States of Aromatic Hydrocarbons by Tertiary Amines.

ChemInform Abstract: Quenching of Triplet States of Aromatic Hydrocarbons by Tertiary Amines.

Quenching of the triplet states of aromatic hydrocarbons by tertiary amines

Rate constants for the quenching of the triplet states of 4-chlorobiphenyl, 1-chloronaphthalene, 9-chloroanthracene and anthracene have been determined. Whilst the quenching efficiency of the more energetic triplet states (ca. 60 kcal mol–1) by amines appears to be related to the ionisation potential of the latter, no clear trend was seen for the anthracenes. The degree of charge transfer appears, therefore, to decrease as the triplet energy of the hydrocarbon is reduced. By use of α, ω-diaminoalkanes it was found that the triplet states do not participate in complex formation involving simultaneous interaction of the two amino groups and the aromatic hydrocarbon. The quenching efficiency of the diamines increases as the chain linking the amino group is lengthened, suggesting that these compounds undergo multicollisional quenching. The fluorescence of 3,3′-bipyrenyl is quenched by N,N′-dimethylaniline (KSV= 75 ± 1 dm3 mol–1 in acetonitrile and 2.8 ± 0.1 dm3 mol–1 in cyclohexane), but in acetonitrile no evidence for triplet formation could be found. It is suggested that conformational linking in the bipyrene aids radiationless decay to the ground state.

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.

Electron Spin-Spin Interaction in Excited Triplet States of Aromatic Hydrocarbons with a Trigonal Symmetry Axis

Abstract Zero-field splitting (ZFS) parameters of benzene, triphenylene, and coronene molecules in their lowest and second excited triplet states are calculated from the electron spin-spin interaction using semi-empirical self-consistentfield π-electron molecular orbitals. The results show that in obtaining a good agreement of the evaluated ZFS parameters with the experimental values the selection of the basis functions in LCAO-MO’s with zero-differential overlap is actually important. The ZFS parameters in some distorted structures are also calculated.

Electron transfer from the excited singlet and triplet states of aromatic hydrocarbons to methyl viologen

Evidence is presented that the excited singlet and triplet states of aromatic hydrocarbons are oxidised by methyl viologen (MV 2+). Rate constants for electron transfer from triplet naphthalene, anthracene, phenanthrene and fluoranthene are close to diffusion-controlled whereas those for triplet 1-cyanonaphthalene and 1-cyanoanthracene are about one order of magnitude lower.

The dependence of the ZFS parameter D on kind and degree of annellation in aromatic hydrocarbons

Highly localized C6-ring units with strong benzene like character have been proved to exist in the triplet state of aromatic hydrocarbons in correspondence with Clar's sextet concept. A number of rules could be inferred for the connection between the D-parameter and the molecule's topology. In this way new aspects for the interpretation of D are opened. The concept is presented along with the experimental D values of molecules of the pyrene-, naphthalene- and anthracene series.

Quenching of triplet states of aromatic hydrocarbons by quinones due to favourable charge-transfer interactions. Part 2.—Solvent effects

Rate constants for quenching of the triplet states of 1,2-benzanthracene and/or anthracene by 2,5-dimethyl-p-benzoquinone, 2-methyl-p-benzoquinone and p-benzoquinone have been measured in eight different solvents at room temperature. For those solvents with low dielectric constants, (< 10), i.e., toluene, cyclohexane, ethyl acetate and tetrahydrofuran, rate constants for quenching kQ, relative to those for diffusion controlled reactions kd, have been shown to depend on ΔGCT, the free energy change expected for charge transfer to give a dielectrically non-relaxed CT state, in the same manner and to show the same correlation between log (kQ/kd) and ΔGCT as found previously in similar studies for benzene as solvent. Quenching is relatively more efficient in polar solvents (acetonitrile, N,N-dimethyl-formamide and formamide) when ΔGCT > –0.1 eV but again there is no correlation with static dielectric constant or with solvent reorientation energies, and it is suggested that quenching may result in these solvents from only partial charge transfer.

Quenching of triplet states of aromatic hydrocarbons by quinones due to charge-transfer interactions

The rate constants for quenching of the triplet states of seven hydrocarbons by benzoquinone, toluquinone and xyloquinone have been measured in benzene solution at room temperature using the technique of nanosecond laser photolysis. The pairs of molecules were selected to exclude the possibility of electronic energy transfer and no transients were detected resulting from the quenching reaction. The quenching rate constants, kQ, which for the most efficient cases reach 1010 dm3 mol–1 s–1, were shown to increase with the increasing electron affinity of the quinone and decreasing ionisation potential of the triplet state being quenched. Various free energy relationships are examined in order to explain the dependence of log kQ on ΔG⊖ct, the standard free energy change expected for charge transfer in benzene as solvent.

External heavy atom effect on the triplet state of aromatic hydrocarbons: IV. Fluorene, triphenylene-d 12 and coronene

The phosphorescence decay functions and spectra of ethanol solutions of triphenylene- d 12, coronene and fluorene were observed in the presence of different KI concentrations at 77 K. The analysis of the dependence of phosphorescence (T 1 → S 0) rate constant, k PT, and intersystem crossing (T 1 ⇝ S 0) rate constant, k GT, on triplet energies E T of aromatic molecules and on the distance between the heavy atom and the aromatic molecule show that the external heavy atom effects on k PT and k GT are essentially of the second and third order, respectively. The phosphorescence spectra of triphenylene and coronene show the presence of the third order contributions in the radiative transition, which involves the second triplet state of aromatic molecule.

The influence of iodide ions on the radiative and radiationless decay of the triplet state of aromatic hydrocarbons

An analysis of the influence of I − ions on radiative and radiationless transitions from the lowest triplet state of aromatic hydrocarbons is presented. The distance dependence in the external heavy atom effects and the dependence of the enhancement of the T 1→S 0 radiative transition on the triplet energy is discussed in terms of perturbation theory.

External heavy atom effects on the decay of the triplet state of aromatic hydrocarbons III. The decay functions of fluorescence and phosphorescence of carbazole in the presence of KI

A description is given of the kinetics of the decay of fluorescence and phosphorescence of an aromatic molecule in the presence of a perturber in a rigid medium. Experimental data for fluorescence and phosphorescence decay functions and relative quantum yields of carbazole in the presence of KI are presented. We find that a Dexter-type formula gives a reasonable fit for the distance dependence of the intersystem crossing rate constants S 1 ≈≈→ T n , T 1 ≈≈→ S 0 and the radiative T 1→T 0 rate constant. The “effective Bohr radius” L, calculated from K PT, compares very well with that determined from triplet-triplet energy transfer studies.

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.

External heavy atom effect on decay of the triplet state of aromatic hydrocarbons. I. The decay functions of phosphorescence and of the population of the triplet state

The derivation of the decay functions of the phosphorescence and population of the triplet states of aromatic molecules in the presence of perturbers, under steady-state and flash excitation, taking into account the influence of perturbers on three rate constants: S 1 ⇝ T i , k TM; T 1 → S 0, k PT ; ⇝ S 0, k GT, valid in rigid media has been presented.