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.     

Triplet state quenching by ruthenocene

Ruthenocene quenches triplet states of organic molecules with energies greater than 24000 cm^?1 in benzene solution at a diffusion controlled rate , (6 ± 1) × 10⁹ dm³ mol^?1 s^?1. For triplets with energies less than this the efficiency of quenching is dependent on the energy of the triplet state being quenched but drops off less acutely than expected for endothermic energy transfer following the Arrhenius equation. This is in agreement with the lowest triplet state of ruthenocene being geometrically distorted as expected from the previously observed large Stokes shift between absorption to and emission from its lowest triplet state. Similarities to ferrocene quenching of triplet states are discussed. Quenching of the triplet state of benzil by ruthenocene does not fall on the smooth curve which exists between the quenching rate constants k_q and the energy of the triplet state being quenched. Queching of triplet benzil by ruthenocene is therefore attributed to favourable charge-transfer interactions, also in this case the behaviour is analogous to quenching of triplet methylene-blue by ferrocene where at least a proportion of electron transfer following quenching has been previously established.     

Excitonic Coupling Strength and Coherence Length in the Singlet and Triplet Excited States of meso–meso Directly Linked Zn(II)porphyrin Arrays

Excitation‐energy transport (EET) phenomena in meso–meso directly linked Zn(II )porphyrin arrays in the singlet and triplet excited states were investigated with a view to electronic coupling strength and coherence length by steady‐state and time‐resolved spectroscopic measurements. To investigate energy transfer in the triplet states, we modified the Zn(II )porphyrin arrays with bromo substituents at both ends. The coupling strength of the Soret bands of the arrays was estimated to be about 2200 cm^?1, and that of the Q bands is about 570 cm^?1. The coherence length in the S₁ state of the Zn(II )porphyrin arrays was determined to be 4–5 porphyrin units, which is comparable to that of the well‐ordered two‐dimensional circular structure B850 in the peripheral light‐harvesting antenna (LH2) in photosynthetic purple bacteria. This indicates that the Zn(II )porphyrin arrays are well suited for mimicking natural light‐harvesting antenna complexes. On the other hand, the rate of energy transfer in the triplet state is estimated to be on the order of 100 μs^?1, and the very weak coupling between the triplet states (ca. 0.003 cm^?1), indicates that the triplet excitation energy is essentially localized on a single porphyrin moiety.     

Quenching of triplet states by molecular oxygen and the role of charge-transfer interactions

The rate constants for oxygen quenching in benzene solution of the triplet states of several organic compounds with relatively high triplet energies have been measured in laser photolysis and pulse radiolysis experiments. The previously observed trend for aromatic hydrocarbons where the quenching rate constants decrease from a limiting value of about one ninth of that expected for a diffusion controlled reaction to lower values for triplet states with increasing triplet energy was not observed for the triplet states of certain aromatic ketones and amines. The higher rate constants observed, e.g. oxygen quenching of triplet N-methyl indole has k_Q = 1.4 × 10¹⁰ dm³ mol^?1 s^?1, are interpreted as being due to the presence of low lying triplet charge-transfer states which enhance the efficiency of quenching.     

Factor group splitting in the lowest triplet state of p-benzoquinone-d4 crystals

Polarized Stark-modulated Zeeman absorption experiments on p-benzoquinone-d₄ single crystals at 2 K show the factor group splitting in the origin of the lowest B_1g (nπ^*) triplet state at 18649 cm^?1 to be 0.62±0.06 cm^?1. The ordering of the crystal states is such that the orbital plus state lies at higher energy. The absence of a measurable factor group solitting in the ³A_u (nπ^*) state at 12.1 cm^?1 from the origin is taken as a further confirmation of the vibronic nature of this state. The ZFS parameter D of this level is found to be ?10±3 GHz.     

Ruthenium(II)–Polyimine–Coumarin Light‐Harvesting Molecular Arrays: Design Rationale and Application for Triplet–Triplet‐Annihilation‐Based Upconversion

Ru^II–bis‐pyridine complexes typically absorb below 450 nm in the UV spectrum and their molar extinction coefficients are only moderate (ε<16 000 M ^?1 cm^?1). Thus, Ru^II–polyimine complexes that show intense visible‐light absorptions are of great interest. However, no effective light‐harvesting ruthenium(II)/organic chromophore arrays have been reported. Herein, we report the first visible‐light‐harvesting Ru^II–coumarin arrays, which absorb at 475 nm (ε up to 63 300 M ^?1 cm^?1, 4‐fold higher than typical Ru^II–polyimine complexes). The donor excited state in these arrays is efficiently converted into an acceptor excited state (i.e., efficient energy‐transfer) without losses in the phosphorescence quantum yield of the acceptor. Based on steady‐state and time‐resolved spectroscopy and DFT calculations, we proposed a general rule for the design of Ru^II–polypyridine–chromophore light‐harvesting arrays, which states that the ¹IL energy level of the ligand must be close to the respective energy level of the metal‐to‐ligand charge‐transfer (M LCT) states. Lower energy levels of ¹IL/³IL than the corresponding ¹M LCT/³M LCT states frustrate the cascade energy‐transfer process and, as a result, the harvested light energy cannot be efficiently transferred to the acceptor. We have also demonstrated that the light‐harvesting effect can be used to improve the upconversion quantum yield to 15.2 % (with 9,10‐diphenylanthracene as a triplet‐acceptor/annihilator), compared to the parent complex without the coumarin subunit, which showed an upconversion quantum yield of only 0.95 %.     

Theoretical study of spin–orbit coupling and kinetics in spin-forbidden reaction between Ta(NH2)3 and N2O

The activation mechanism of the nitrous oxide (N₂O) with the Ta(NH₂)₃ complex on the singlet and triplet potential energy surfaces has been investigated using the hybrid exchange correlation functional B3LYP. The minimum energy crossing point (MECP) is located by using the methods of Harvey et al. The rate-determining step of the N–O activation reaction is the intersystem crossing from ¹ 2 to ³ 2. The reacting system will change its spin multiplicities from the singlet state to the triplet state near MECP-1, which takes place with a spin crossing barrier of 32.5 kcal mol^?1, and then move on the triplet potential energy surface as the reaction proceeds. Analysis of spin–orbit coupling (SOC) using localized orbitals shows that MECP-1 will produce the significant SOC matrix element, the value of SOC is 272.46 cm^?1, due to the electron shift between two perpendicular π orbitals with the same rotation direction and the contribution from heavy atom Ta. The rate coefficients are calculated using Non-adiabatic Rice-Ramsperger-Kassel-Marcus (RRKM). Results indicate that the coefficients, k(E), are exceedingly high, k(E) > 10¹² s^?1, for energies above the intersystem crossing barrier (32.5 kcal mol^?1); however, in the lower temperature range of 200–600 K, the intersystem crossing is very slow, k(T) < 10^?6 s^?1.     

THE TRIPLET EXTINCTION COEFFICIENTS OF SOME BACTERIAL CAROTENOIDS

The extinction coefficients of the triplet states of the bacterial carotenoids, neurosporenc (Λ_max 489 nm). sphcroidene (Λ_max 510 nm). spheroidenone (Λ_max 550 nm) and spirilloxanthin (Λ_max 550 nm) in cyclohexane have been determined to be 27.4. 30.9. 6.06 and 9.20 × 10⁴ dm³ mol^?1 cm¹, respectively. These values were obtained by an energy transfer technique using a range of carotenoid concentrations. For the three that had been studied earlier, the extinctions now obtained are suhstantially higher than prcviously reported.     

Vibrational energy dependence of line broadening in the quinoxaline spectrum

The linewidths as a function of the number of quanta of the totally symmetric progression forming mode, 525 cm^?1 (509 - quinoxaline-d₆) have been measured at high resolution in the vapor phase absorption spectrum of quinoxaline. The line broadening is attributed to efficient intersystem crossing between the lowest single (nπ*) and triplet (ππ*) states. The increased broadening observed with increased energy is consistent with theoretical predictions and with the general results of parallel lifetime studies of individual vibronic levels.     

Relativistic molecular orbital theory of zero-field splittings in triplets

A method of calculating matrix elements of 1/r₁₂ in a basis of Dirac scattered-wave (DSW) orbitals is outlined. In the limit c → ∞, this method reduces to that described by Cook and Karplus for non-relativistic orbitals. For triplet states that can be described by a single configuration with two unpaired electrons, the relativistic exchange integrals give not only the singlet—triplet splittings (as in non-relativistic theory), but also the spin—orbit contributions to the triplet zero-field splittings. Results are reported for the ³ (n → π*) excited state of γ-thiopyrone (4H-pyran-4-thione), which has a very large value of D (calc. ?31 cm^?1, exp. ?24 to ?28 cm^?1).     

Simulated T ← S spectrum of 2,4,5-trimethylbenzaldehyde in durene

The T_1,2 ← S₀ phosphorescence excitation spectrum of 2,4,5-trimethylbenzaldehyde in durene has been simulated using forty-five zero-order Born-Oppenheimer product states of which thirty-two belong to T₁ (ππ*), the others to T₂ (nπ*). The spectrum is very complicated in the region 400–600 cm^?1 above the T₁ (ππ*) ←3 S₀ origin band at 24150 cm^?1. In this tangled region conventional vibrational analysis is not useful. Several comments on the physical properties of the excited triplet states of 2,4,5-trimethylbenzaldehyde are given.     

Spectra and structure of small ring compounds: Part XXXV. Vibrational spectra and ring-puckering and torsional potential functions of cyclobutylamine

The IR spectra (50–4000 cm^?1) of gaseous and solid cyclobutylamine and cyclobutylamine-N-d₂ and the Raman spectra (25–4000 cm^?1) of gaseous, liquid and solid cyclobutylamine and cyclobutylamine-N-d₂ have been recorded. Depolarization values were measured for both the gaseous and liquid states. Most of the thirty-six fundamental vibrations have been assigned and support for more than one molecular configuration is presented. In the low frequency region for the “light” compound, a series of four Q-branches have been assigned to transitions between energy levels of the ring-puckering vibration for the equatorial isomer. The transitional frequencies were fitted to an asymmetric single-minimum potential function of the form: V(X) = 0.474 × 10⁶X⁴ - 0.204 × 10⁵X² + 0.993 × 10⁵X³ with a reduced mass of 160 amu. The following torsional potential constants were determined for the “light” molecule- V¹ = 77.8 ± 17.0 cm^?1, V₃ = 784.0 ± 3.3 cm^?1. The trans conformation was found to be more stable than the gauche form by approximately 58 cm^?1 (0.17 kcal mol^?1). The barriers to trans-gauche, gauche-trans, and gauche-gauche interconversion are 803, 745 and 803 cm^?1, respectively.     

Spectral-kinetic characteristics of lutetium-containing tetrapyrroles

The spectral-kinetic characteristics of the triplet states of tetraphenylporphyrin and triphenylcorrole complexes with an aminopolycarboxylic acid (EDTA or DTPA) or its complex with lutetium as a substitute and the corrole complex with Ga(III) as the central atom have been studied. The transient absorption spectra of the complexes in the triplet excited state (effective maximum at 460–470 nm) and the rate constants of triplet quenching by oxygen at room temperature (2 × 10⁸–7 × 10⁸ L mol^?1 s^?1) have been measured. The quantum yields (0.44–0.55) and the molar absorption coefficients of the triplet state (log ?_T = 4.81–4.89) have been determined for some of the derivatives. The efficiency of population and deactivation kinetics of the triplet states are determined by the structure of the porphyrinoid, in particular by the central ion, and depends slightly on the presence of a heavy atom at the periphery of the molecule. Possible uses of the new compounds for designing various optical devices are discussed.     

Experimental study of triplet exciton diffusion in diphenyl and benzophenone crystals

Biphenyl crystals to which have been added to 10^?2 mole/mole of benzophenone and a various concentration of naphthalene, excited by radiation absorbed only by benzophenone molecules give naphthalene phosphorescence. This emission is interpreted as a consequence of energy migration from benzophenone molecules to the naphthalene molecules through the triplet exciton band of biphenyl. From the study, at 120 K, of the intensity of the naphthalene phosphorescence dependence on concentrations, a coefficient of diffusion of triplet excitons of biphenyl is measured (D ≈ 10^?6 cm² s^?1). Analogous kinetics applied to binary mixed crystals, naphthalene-benzophenone, give a coefficient of diffusion of triplet excitions for benzophenone (D ≈ 10^?7 cm² s^?1).     

Primary Photoprocesses in a Fluoroquinolone Antibiotic Sarafloxacin†

The photophysical properties of the fluoroquinolone antibiotic sarafloxacin (SFX) were investigated in aqueous media. SFX in water, at pH 7.4, shows intense absorption with peaks at 272, 322 and 335 nm, (? = 36800 and 17000 dm³ mol^?1 cm^?1, respectively). Both the absorption and emission properties of SFX are pH‐dependent; pK_a values for the protonation equilibria of both the ground (5.8 and 9.1) and excited singlet states (5.7 and 9.0) of SFX were determined spectroscopically. SFX fluoresces weakly, the quantum yield for fluorescence emission being maximum (0.07) at pH 8. Laser flash photolysis and pulse radiolysis studies have been carried out in order to characterize the transient species of SFX in aqueous solution. Triplet–triplet absorption has a maximum at 610 nm with a molar absorption coefficient of 17,000 ± 1000 dm³ mol^?1 cm^?1. The quantum yield of triplet formation has been determined to be 0.35 ± 0.05. In the presence of oxygen, the triplet reacts to form excited singlet oxygen with quantum yield of 0.10. The initial triplet (³A*) was found to react with phosphate buffer to form triplet ³B* with lower energy and longer lifetime and having an absorption band centered at 700 nm. SFX triplet was also found to oxidize tryptophan to its radical with concomitant formation of the anion radical of SFX. Hence the photosensitivity of SFX could be initiated by the oxygen radicals and/or by SFX radicals acting as haptens.     

The triplet state of photosynthetic pigments. I. Pheophytins

ZFS constants (in cm^?1) and decay rate constants for the lowest triplet state of pheophytins have been determined by ESR: pheophytin a: D = 341 ± 3,E = 33 ± 3, K_T = 1050 s^?1; pheophytin b: D = 358 ± 8, E = 46 ± 5, K_T = 630 s^?1; bacteriopheophytin: D = 256 ± 4, E = 54 ± 5/37 ± 5, K_T ≈ 4000 s^?1. In addition values for the decay rate constants and relative populating rates of the individual spin levels have been obtained; these numbers turn out to be appreciably different from those for the corresponding chlorophylls. For the series pheophytin a, b and bacteriopheophytin we find parallel behaviour with the corresponding chlorophylls. The effects of side group substitution and pyrrole ring reduction on the ZFS constant D can be understood by including configuration interaction between the excited states using the 4-orbital model. The change of the mean triplet decay constant K_T upon side group substitution and pyrrole ring reduction follows an energy gap law. Substitution of the central Mg-ion by two protons, however, causes K_T to increase; this is attributed to the introduction of an extra promoting mode - of the NH-group - and/or to the presence of low lying nπ^* states in pheophytins.     

ODMR and triplet state kinetics of a carbazolyl substituted diacetylene crystal

Carbazolyl substituted diacetylene (DCH) monomer crystals showing phosphorescence from four traps have been investigated by optically detected magnetic resonance (ODMR) at 1.2 K. These localized triplet states are attributed to the carbazolyl side groups. Their population and depopulation rate constants and zero field splitting parameters (0.097 <|D|< 0.1002 cm^?1; 0.0068 <|E|<0.0105 cm^?1) have been determined. The results suggest that the traps are disturbed substituents. The proposed interaction of the trap states with an exciton band at 23926 cm^?1 is supported by temperature dependent lifetime measurements.     

Broadband Visible‐Light‐Harvesting trans‐Bis(alkylphosphine) Platinum(II)‐Alkynyl Complexes with Singlet Energy Transfer between BODIPY and Naphthalene Diimide Ligands

A heteroleptic bis(tributylphosphine) platinum(II)‐alkynyl complex ( Pt‐1 ) showing broadband visible‐light absorption was prepared. Two different visible‐light‐absorbing ligands, that is, ethynylated boron‐dipyrromethene (BODIPY) and a functionalized naphthalene diimide (NDI) were used in the molecule. Two reference complexes, Pt‐2 and Pt‐3 , which contain only the NDI or BODIPY ligand, respectively, were also prepared. The coordinated BODIPY ligand shows absorption at 503 nm and fluorescence at 516 nm, whereas the coordinated NDI ligand absorbs at 594 nm; the spectral overlap between the two ligands ensures intramolecular resonance energy transfer in Pt‐1 , with BODIPY as the singlet energy donor and NDI as the energy acceptor. The complex shows strong absorption in the region 450 nm–640 nm, with molar absorption coefficient up to 88 000 M ^?1 cm^?1. Long‐lived triplet excited states lifetimes were observed for Pt‐1 – Pt‐3 (36.9 μs, 28.3 μs, and 818.6 μs, respectively). Singlet and triplet energy transfer processes were studied by the fluorescence/phosphorescence excitation spectra, steady‐state and time‐resolved UV/Vis absorption and luminescence spectra, as well as nanosecond time‐resolved transient difference absorption spectra. A triplet‐state equilibrium was observed for Pt‐1 . The complexes were used as triplet photosensitizers for triplet–triplet annihilation upconversion, with upconversion quantum yields up to 18.4 % being observed for Pt‐1 .     

Stark effects on the low energy electronic states of p-benzoquinone single crystals

The Stark effect on the visible absorption spectra of p-benzoquinone single crystals has been measured using modulation techniques. The measurements of second order Stark effects confirm the presence of at least two close lying electronic nπ^* states in both the singlet and triplet systems. Both the origin of the first singlet-singlet and singlet—triplet transition appear as doublets separated by 3.5 and 18 cm^?1, respectively. For the triplet state the low energy component is Stark induced and has not been observed previously in absorption. The doublet components are mixed in the electric field showing that they are of different parity. They are tentatively assigned as inversion doublets of a symmetric double well potential produced by the vibronic interaction of two different electronic nπ^* states.