Charge-transfer Triplet State Research Articles

Magnetic field dependence of radical-pair decay kinetics and molecular triplet quantum yield in quinone-depleted reaction centers

Radical-pair decay kinetics and molecular triplet quantum yields at various magnetic fields are reported for quinone-depleted reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides R26. The radical-pair decay is observed by picosecond absorption spectroscopy to be a single exponential to within the experimental uncertainty at all fields. The decay time increases from 13 ns at zero field to 17 ns at 1 kG, and decreases to 9 ns at 50 kG. The orientation averaged quantum yield of formation of the molecular triplet of the primary electron donor, 3P, drops to 47% of its zero-field value at 1 kG and rises to 126% at 50 kG. Combined analysis of these data gives a singlet radical-pair decay rate constant of 5 · 10 7 s −1, a lower limit for the triplet radical-pair decay rate constant of 1 · 10 8 s −1 and a lower limit for the quantum yield of radical-pair decay by the triplet channel of 38% at zero field. The upper limit of the quantum yield of 3P formation at zero field is measured to be 32%. In order to explain this apparent discrepancy, decay of the radical pair by the triplet channel must lead to some rapid ground state formation as well as some 3P formation. It is proposed that the triplet radical pair decays to a triplet charge-transfer state which is strongly coupled to the ground state by spin-orbit interactions. Several possibilities for this charge-transfer state are discussed.

The fluorescence of some dipolar NN-dialkyl-4-(dichloro-1,3,5-triazinyl)anilines. Part 3. Intersystem crossing yields from intramolecular charge-transfer excited states and triplet state properties

The phosphorescence and triplet–triplet optical absorption characteristics of NN-diethyl-4-(dichloro-1,3,5-triazinyl)aniline have been examined in rigid and fluid media. A value of 31 000 ± 5 000 dm3 mol–1 cm–1 has been determined for the molar extinction coefficient of the triplet–triplet absorption maxima at 490 nm of the compound dissolved in ether–pentane–alcohol (5 : 5 : 2) glass at 77 K and in poly-(methyl methacrylate) at 293 K. Intersystem crossing yields in toluene (0.10), tetrahydrofuran (0.15), n-butanol (0.14), ethanol (0.07), and acetonitrile (0.05) have been related to the quenching of fluoresence induced by polar solvent, and rationalised on the basis of a previously proposed ‘twisted intramolecular charge-transfer state’ model.The spectral properties of other related NN-disubstituted anilines possessing an electron acceptor substituent [CO2H, CO2Me, CN, CHO, C3N3Cl(NMe2), and NO2] in the para-position have also been investigated.

Mechanism of the photocycloaddition of 1-aminoanthraquinones to olefins by visible light irradiation; oxetan formation via an exciplex

The mechanisms of the photocycloadditions of 1-aminoanthraquinone (1) and 1-amino-2,4-dibromoanthraquinone (2) to olefins by visible light irradiation have been investigated. Fluorescence of compound (1) was quenched by olefins without the appearance of a new emission and the Stern–Volmer constants accorded well with those obtained from the double reciprocal plots of the quantum yield against the concentration of olefin. The rate constant for the reaction was larger for olefins with lower ionization potentials, and was one order of magnitude larger in ethanol than in benzene. Exciplex quenching effects caused by amines such as triethylamine, butylamine, piperidine, and pyridine were also observed. All the results indicate that the reaction proceeds via exciplex formation between compound (1) in its excited singlet charge-transfer state and the olefin. The photocycloaddition of compound (2) in benzene was quenched by oxygen and gave a linear Stern–Volmer plot; the reaction in ethanol was only partially quenched. Detailed kinetic studies indicated that, in benzene, compound (2) had a single mode of reaction via the excited triplet charge-transfer state, while in ethanol it had a dual mode of reaction via both the singlet and triplet charge-transfer states.

Energies and kinetics of radical pairs involving bacteriochlorophyll and bacteriopheophytin in bacterial reaction centers

Absorbance changes reflecting the formation of a transient radical-pair state, P(F), were measured in reaction centers from Rhodopseudomonas sphaeroides under conditions that blocked electron transfer to a later carrier (a quinone, Q). The temperature dependence of the absorbance changes suggests that P(F) is an equilibrium mixture of two states, which appear to be mainly (1)[P([unk])B([unk])] and (1)[P([unk])H([unk])]. P is a bacteriochlorophyll dimer, B is a bacteriochlorophyll absorbing at 800 nm, and H is a bacteriopheophytin. In the presence of Q([unk]), the energy of (1)[P([unk])B([unk])] is about 0.025 eV above that of (1)[P([unk])H([unk])], (1)[P([unk])H([unk])] can decay to a triplet state, P(R), which also is an equilibrium mixture of two states, separated by about 0.03 eV. The lower of these appears to be mainly a locally excited triplet state of P, (3)P; the upper state contains a major contribution from a triplet charge-transfer state, (3)[P([unk])B([unk])]. The temperature dependence of delayed fluorescence from P(R) indicates that (3)P lies 0.40 eV below the excited singlet state, P(*), which is about 0.05 eV above (1)[P([unk])H([unk])]. The (1,3)[P([unk])B([unk])] charge-transfer states thus appear to interact with the locally excited states of P and B to give singlet and triplet states that are separated in energy by about 0.35 eV. This is 10(6) times larger than the splitting between (1)[P([unk])H([unk])] and (3)[P([unk])H([unk])] and implies strong orbital overlap between P([unk]) and B([unk]). This is consistent with recent picosecond studies which suggest that electron transfer from P(*) to B occurs within 1 ps and is followed in 4 to 10 ps by electron transfer from B([unk]) to H.

Trapping of an inner-sphere charge-transfer triplet state in a single crystal of a uranyl complex

Irradiation of a single crystal of UO2(NO3)2·2OP(OEt)3 at 4·2 K produces a metastable triplet state whose e.s.r. characteristics indicate a localized electron transfer from an NO3– ion to the excited UO22+ ion.

Zero-phonon lines and electron-phonon coupling of charge transfer triplet states: Naphthalene-TCPA

Two different kinds of triplet trap systems in naphthalene-TCPA single crystals show very different electron-phonon coupling parameters. It is argued that this phenomenon is due to the different magnitude of charge transfer in the excited state; this assumption is supported by a model calculation based on a Coulombic excited state interaction.

Redox photochemistry of diethyldithiophosphatecobalt(III)

The photochemical behavior of Co(dtp) 3 [dtp − = (C 2H 5O) 2PS − 2] in CH 2Cl 2 solution investigated as a function of irradiating wavelength along all the absorption spectrum. The dependence on the triplet sensitizer energy was also investigated. The reaction Co(dtp) 3 → Co(dtp) 2 occurred by irradiating in the charge transfer or intrmolecular ligand spectral regions, whereas the complex was essentially inert by irradiating in the ligand field region. The quantum yields monotonicallt decreased by increasing the irradiation wavelength. The reaction was quenched by dicyclopentadienyliron. Benzophenone and 2′-acetonaphthone sensitized the redox reaction through an energy transfer mechanism; with the first sensitizer a diffusion controlled process occurred and therefore the energy gap between teh donor-acceptor triplet states is greater than 2.9 Kcal; with the second a probable reversible energy transfer was operating, suggesting that the 2′-acetonaphthone and complex triplet energies lies close to each other. With benzil as sensitizer the photoreactive state was not populated enough for the redox process to be observed. The overall results indicate that the photoreactive state is a charge transfer triplet state, whose population depends either on the intersystem crossing being faster than thermal equilibration from excited states or on the energy transfer from sensitizers having a triplet energy ⩾59 Kcal.

The symmetry of a triplet charge transfer state in phenanthrene-PMDA crystals: dimer versus trimer

The low symmetry of the unit cell of the charge transfer crystal phenanthrene-PMDA investigation of the symmetry of the lowest excited triplet state. The investigation is carried out on the basis of complete evaluation of the fine structure tensor of the triplet state. Additional information from the hyperfine structure of the localized triplet state leads to the conclusion that the complex symmetry corresponds to a trimer with the following structure: (PMDA) (phenanthrene) (PMDA).

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 × 10 10 dm 3 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.

High resolution esr experiments to determine the electron distribution in charge-transfer triplet states

The hyperfine tensors of the complexes pentamethylbenzene—1,2,3,4,5-tetracyanobenzene (TCNB) and hexacyanobenzene—napthalene dissolved in the charge-tranfer crystal napthalene—TCNB have been determined by ESR spectroscopy. The data yield the electron distribution in the charge transfer state, showing that, in the crystal, the triplet electrons are distributed over no more than two molecules. This proves that the excitation causes a breakdown of the inversion symmetry of the crystal. The spectra indicate molecular motion in the cyrstal.

The effect of bromine substitution on the charge-transfer emission of the complex of phenanthrene and tetracyanobenzene

The absorption spectra of the charge-transfer complexes of sym-tetracyanobenzene (TCNB) with phenanthrene, 9-bromophenanthrene, and 9,10-dibromophenanthrene are measured in chloroform solutions at room temperature. The total emission and phosphorescence spectra of the donors and the complexes are measured at 77 K in rigid glasses. The phosphorescence decay lifetimes are determined for phenanthrene, TCNB, and for the phenanthrene-TCNB complex, and a decrease in the phenanthrene-TCNB complex lifetime relative to the lifetimes of the two components is observed. The luminescence spectra of the complexes exhibit both a red shift and a lack of structure as compared with the donor spectra. The results are interpreted, in agreement with the results of Iwata et al. for the phenanthrene-TCNB complex ( 1), as an indication that there is a considerable degree of charge-transfer character in the lowest triplet state ( T 1). Bromine substitution leads to a decrease in the energy of the phenanthrene triplet state. As a result, the energy gap between the donor molecule triplet state and the complex charge-transfer triplet state decreases from phenanthrene, to 9-bromophenanthrene, to 9,10-dibromophenanthrene. The results suggest that the proximity of these two triplet states in 9,10-dibromophenanthrene and its charge-transfer complex leads to some local donor triplet state character in the emitting complex triplet state.

The triplet states of complexes with tetrachlorophthalic anhydride and phthalic anhydride as electron acceptors

Triplet-triplet (T-T) absorption, E.S.R., and phosphorescence spectra were observed for tetrachlorophthalic anhydride and phthalic anhydride complexes with methyl substituted benzenes. T-T absorption bands characteristic of electron donor-acceptor interaction, in addition to those pertinent to the component molecules, were found for the tetrachlorophthalic anhydride complexes. The observed T-T absorption spectra can be understood systematically by considering the interaction among zero-order charge-transfer triplet states and zero-order locally (within the acceptor) excited triplet states. From the analysis of the results of E.S.R. spectra, the CT characters of the lowest triplet states were obtained to be 80, 46, 37, and 26 per cent for the tetrachlorophthalic anhydride complexes with durene, mesitylene, toluene, and benzene, respectively, while the lowest triplet states of complexes of phthalic anhydride with mesitylene and toluene were found to have the character of local excitation within the acceptor.

Reduction by the triplet charge-transfer state of tris(bipyridyl)ruthenium(II). Photochemical reaction between tris(bipyridyl) ruthenium(II) and thallium(III)

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTReduction by the triplet charge-transfer state of tris(bipyridyl)ruthenium(II). Photochemical reaction between tris(bipyridyl) ruthenium(II) and thallium(III)Gerald S. Laurence and Vincenzo. BalzaniCite this: Inorg. Chem. 1974, 13, 12, 2976–2982Publication Date (Print):December 1, 1974Publication History Published online1 May 2002Published inissue 1 December 1974https://doi.org/10.1021/ic50142a039RIGHTS & PERMISSIONSArticle Views207Altmetric-Citations56LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (977 KB) Get e-Alerts Get e-Alerts

ChemInform Abstract: THE TRIPLET STATES OF 1,3,5‐TRICYANOBENZENE COMPLEXES WITH METHYL SUBSTITUTED BENZENES

Triplet-triplet absorption, phosphorescence, and ESR spectra have been measured on the 1,3,5-tricyanobenzene complexes with toluene, mesitylene, durene, and hexamethylbenzene. Four triplet-triplet absorption bands were commonly observed in the wave number region of 10000–30000 cm−1 for all the complexes. The observed phosphorescence and T-T absorption spectra could be understood systematically by considering the interaction among zero-order charge-transfer triplet states and zero-order locally (within the acceptor) excited triplet states. From an analysis of the zero-field splitting values obtained from the observed ESR spectra, the contribution of the zero-order charge-transfer triplet state to the lowest triplet state was determined to be 76,65,25, and 9% for the 1,3,5-tricyanobenzene complexes with hexamethylbenzene, durene, mesitylene, and toluene, respectively. From the consideration on the results obtained for various polycyanobenzene complexes, the phosphorescence lifetime was found to decrease rem...

The Triplet States of 1,3,5-Tricyanobenzene Complexes with Methyl Substituted Benzenes

Abstract Triplet-triplet absorption, phosphorescence, and ESR spectra have been measured on the 1,3,5-tricyanobenzene complexes with toluene, mesitylene, durene, and hexamethylbenzene. Four triplet-triplet absorption bands were commonly observed in the wave number region of 10000–30000 cm−1 for all the complexes. The observed phosphorescence and T-T absorption spectra could be understood systematically by considering the interaction among zero-order charge-transfer triplet states and zero-order locally (within the acceptor) excited triplet states. From an analysis of the zero-field splitting values obtained from the observed ESR spectra, the contribution of the zero-order charge-transfer triplet state to the lowest triplet state was determined to be 76,65,25, and 9% for the 1,3,5-tricyanobenzene complexes with hexamethylbenzene, durene, mesitylene, and toluene, respectively. From the consideration on the results obtained for various polycyanobenzene complexes, the phosphorescence lifetime was found to decrease remarkably for the complexes with the charge-transfer character higher than 50% in their lowest triplet states.

The triplet-triplet absorption spectra of electron donor-acceptor complexes

Triplet-triplet (T-T) absorption spectra were measured in the wave-number region between 5000 and 30 000 cm-1 for the 1,2,4,5-tetracyanobenzene complexes with toluene, mesitylene, durene, and hexamethylbenzene. From the analysis of the observed T-T absorption spectra, with the aid of theoretical considerations based on the interaction among zero-order charge-transfer and locally excited triplet states, the assignment was made and the nature of the lower triplet states of the complexes was clarified. We conclude that the lowest zero-order charge-transfer triplet state and the lowest zero-order locally excited triplet states are close to each other for the complexes, and the charge-transfer character of the lowest state changes from 81 to 15 per cent by changing the electron donor from hexamethylbenzene to toluene.

ChemInform Abstract: IONIC PHOTODISSOCIATION OF ELECTRON DONOR‐ACCEPTOR COMPLEXES

Ionic photodissociation of electron donor-acceptor (EDA) complexes of s-tetracyanobenzene (TCNB) and pyromellitic dianhydride (PMDA) with 2-methyltetrahydrofuran (MTHF) has been investigated by means of nsec flash photolysis and transient photocurrent measurements. The solvent-shared ion-pair of TCNB anion and MTHF cation is formed from the excited charge-transfer (CT) singlet state of a TCNB–MTHF complex, dissociating into solvated ions. TCNB complexes with aromatic hydrocarbons dissociate directly into ions from the lowest excited CT singlet state. Dissociation of PMDA complexes occurs in the CT triplet state in nonpolar solvents and mainly in the excited CT singlet state in polar solvents. The relative rate constant of radiationless process, which competes with ionic dissociation in the excited singlet state, can be deduced from the effect of solvent on ionic photodissociation. It has been confirmed for the first time that the ionic dissociation of a TCNB–benzene–1,2-dichloroethane (DCE) system occurs ...

Ionic Photodissociation of Electron Donor-Acceptor Complexes

Abstract Ionic photodissociation of electron donor-acceptor (EDA) complexes of s-tetracyanobenzene (TCNB) and pyromellitic dianhydride (PMDA) with 2-methyltetrahydrofuran (MTHF) has been investigated by means of nsec flash photolysis and transient photocurrent measurements. The solvent-shared ion-pair of TCNB anion and MTHF cation is formed from the excited charge-transfer (CT) singlet state of a TCNB–MTHF complex, dissociating into solvated ions. TCNB complexes with aromatic hydrocarbons dissociate directly into ions from the lowest excited CT singlet state. Dissociation of PMDA complexes occurs in the CT triplet state in nonpolar solvents and mainly in the excited CT singlet state in polar solvents. The relative rate constant of radiationless process, which competes with ionic dissociation in the excited singlet state, can be deduced from the effect of solvent on ionic photodissociation. It has been confirmed for the first time that the ionic dissociation of a TCNB–benzene–1,2-dichloroethane (DCE) system occurs in the excited Franck-Condon (FC) state. It is pointed out that the latter dissociation mechanism is not inconsistent with all the results obtained for TCNB and PMDA complexes under various conditions.

Triplet Charge Transfer Complex between Excited Electron Acceptor and Ground State Electron Donor

The half-wave reduction and oxidation potentials, E1/2's, of carbonyl compounds and amines, respectively, are used to estimate the location of triplet charge transfer states (CT). A linear free-energy relationship between the calculated CT triplet energies and the corresponding carbonyl molecules triplet energies was obtained. It has also been found that the free energy change (ΔF) for CT complex formation correlate with the triplet carbonyl quenching rate constant kq.

The triplet-triplet absorption observed for some tetracyanobenzene complexes

The absorption bands due to the transitions from the charge-transfer triplet state to higher triplet states were observed in the visible and near infrared regions for the tetracyanobenzene complexes with durene and hexamethylbenzene. The results of these complexes, together with that of the tetracyanobenzene-toluene complex, show that the absorption maximum in the visible region, which can be assigned to the transition from the lowest triplet state of the complex to a higher locally excited triplet state, shifts to higher frequencies with the decreasing ionization potential of the electron donor. Another band in the near infrared region was assigned to the transition between the two states caused by the interaction between the lowest charge-transfer and locally excited configurations (triplet).