Showing papers on "Triplet state published in 2012"

Quantum beats in crystalline tetracene delayed fluorescence due to triplet pair coherences produced by direct singlet fission.

Abstract: A detailed analysis of the oscillations seen in the delayed fluorescence of crystalline tetracene is presented in order to study the mechanism of singlet fission. Three quantum beat frequencies of 1.06 ± 0.05, 1.82 ± 0.05, and 2.92 ± 0.06 GHz are resolved, which are damped on a time scale of 20 ns. The effects of sample morphology, excitation wavelength, and temperature are examined. A density matrix model for singlet fission is developed that quantitatively describes the frequencies, amplitudes, and damping of the oscillations. The model assumes a direct coupling of the initially excited singlet exciton to the triplet pair manifold. There is no electronic coherence between the singlet and triplet pair states, but the rapid singlet decay time of ∼200 ps in solution-grown single crystals provides the impulsive population transfer necessary to create a coherent superposition of three zero-field triplet pair states |xx⟩, |yy⟩, and |zz⟩ with overall singlet character. This superposition of the three states gi...

Efficient organic light-emitting diodes through up-conversion from triplet to singlet excited states of exciplexes

Abstract: Enhanced electroluminescence efficiency is achieved in organic light-emitting diodes through delayed fluorescence of the exciplex state formed between 4,4′,4′′-tris[3-methylphenyl(phenyl)amino]-triphenylamine (m-MTDATA) as an electron-donating material and 2,8-bis(diphenylphosphoryl)dibenzo-[b,d]thiophene (PPT) as an electron-accepting material. The devices exhibited maximum external electroluminescence quantum and power efficiencies of 10.0% and 47.0 lm/W, respectively.

Highly luminescent and thermally stable lanthanide coordination polymers designed from 4-(dipyridin-2-yl)aminobenzoate: efficient energy transfer from Tb3+ to Eu3+ in a mixed lanthanide coordination compound.

Abstract: Herein, a new aromatic carboxylate ligand, namely, 4-(dipyridin-2-yl)aminobenzoic acid (HL), has been designed and employed for the construction of a series of lanthanide complexes (Eu3+ = 1, Tb3+ = 2, and Gd3+ = 3). Complexes of 1 and 2 were structurally authenticated by single-crystal X-ray diffraction and were found to exist as infinite 1D coordination polymers with the general formulas {[Eu(L)3(H2O)2]}n (1) and {[Tb(L)3(H2O)].(H2O)}n (2). Both compounds crystallize in monoclinic space group C2/c. The photophysical properties demonstrated that the developed 4-(dipyridin-2-yl)aminobenzoate ligand is well suited for the sensitization of Tb3+ emission (Φoverall = 64%) thanks to the favorable position of the triplet state (3ππ*) of the ligand [the energy difference between the triplet state of the ligand and the excited state of Tb3+ (ΔE) = 3ππ* – 5D4 = 3197 cm–1], as investigated in the Gd3+ complex. On the other hand, the corresponding Eu3+ complex shows weak luminescence efficiency (Φoverall = 7%) due t...

Quantum Theory of Chemical Reactions

Abstract: The Supermolecule Approach to the Solvation Problem.- Local Field Representation of Surrounding Medium Effects. From Liquid Solvent to Protein Core Effects.- The Effects of a Polarizable Environment Represented by the Generalized Born Formula in Self Consistent Quantum Chemical Calculations: Application to the Study of Ambident Reactions.- Chemical Reactivity in Inert Matrices.- Theory of Environmental Effects on Atoms and Molecules Trapped in Rare Gas Matrices.- Organometallic 1.2 Shift Type Migration Reactions.- A MO Treatment of the Regioselectivity of Nucleophilic Addition to ?-Enones.- An Electrophilic Addition Reaction in Solution: C2H4 + Cl2. Ab Initio Studies.- Theoretical Study of a Homogeneous Catalytic Reaction: The Chlorotris-(Triphenylphosphine)Rhodium(I)-Catalyzed Hydrogenation of Olefins.- Energy Surfaces in Quantum Chemistry.- Overlap in Binuclear Complexes: A Topological Approach of the Exchange Interaction.- The Evolution of Electronically Excited Molecules.- Study of Photochemical Reaction with Natural Correlation Method.- Theoretical Analysis of the Role of Rydberg States in the Photochemistry of Some Small Molecules.- Reappraisal of the Sudden Polarization in Olefins.- Photochemical Attachment of Aliphatic Hydrocarbons to Polynuclear Aromatic Hydrocarbons (PAH) in Rigid Alkane Matrices at 77K. Evidence for a Two-Photon Process Involving an Upper Excited Triplet State.- Index of Subjects.- Index of Names.

A Paramagnetic Bonding Mechanism for Diatomics in Strong Magnetic Fields

Abstract: Elementary chemistry distinguishes two kinds of strong bonds between atoms in molecules: the covalent bond, where bonding arises from valence electron pairs shared between neighboring atoms, and the ionic bond, where transfer of electrons from one atom to another leads to Coulombic attraction between the resulting ions. We present a third, distinct bonding mechanism: perpendicular paramagnetic bonding, generated by the stabilization of antibonding orbitals in their perpendicular orientation relative to an external magnetic field. In strong fields such as those present in the atmospheres of white dwarfs (on the order of 10(5) teslas) and other stellar objects, our calculations suggest that this mechanism underlies the strong bonding of H(2) in the (3)Σ(u)(+)(1σ(g)1σ(u)*) triplet state and of He(2) in the (1)Σ(g)(+)(1σ(g)(2)1σ(u)(*2)) singlet state, as well as their preferred perpendicular orientation in the external field.

Elements of Organic Photochemistry

Abstract: 1 Basic Photophysical and Photochemical Concepts.- 1.1. Introduction.- 1.2. Energy Distribution in the Excited Molecule.- 1.2a. Light Absorption.- 1.2b. Internal Conversion and Intersystem Crossing.- 1.2c. Fluorescence and Phosphorescence.- 1.3. Photochemical Kinetics: Concentrations, Rates, Yields, and Quantum Yields.- 1.4. Classification of Molecular Electronic Transitions and Excited States.- 1.4a. ? ? ?* Transitions.- 1.4b. n ? ?* and l? a? Transitions.- 1.4c. Intramolecular Charge-Transfer Transitions (CT).- Problems.- 2 Photochemical Techniques and the Photodimerization of Anthracene and Related Compounds.- 2.1. Absorption and Emission Spectra.- 2.1a. Transition Probability.- 2.1b. Polarization Spectra.- 2.1c. The Measurement of Fluorescence Spectra and Fluorescence Quantum Yields.- 2.1d. The Measurement of Fluorescence Lifetimes.- 2.2. The Photodimerization of Anthracene and Related Compounds.- 2.2a. Structural Aspects: The Effect of Substituents on the Photodimerization.- 2.2b. Preparative Photochemical Techniques.- 2.2c. Kinetic and Mechanistic Aspects of the Anthracene Photodimerization.- 2.3. The Anthracene Triplet State.- Problem.- References.- 3 Photochemical Techniques and the Photochemistry of Ketones.- 3.1. The Photoreduction of Aryl Ketones: Nature of the Excited State.- 3.2. Flash Photolysis.- 3.3. The Photoreduction of Aryl Ketones: Structural Aspects.- 3.4. The Photoreduction of Aryl Ketones: Secondary Reactions.- 3.5. The Photoreduction of Aryl Ketones: Synthetic Applications.- 3.6. The Photoreduction of Alkanones.- 3.7. Intramolecular Hydrogen Abstraction by Ketones (Type II Cleavage).- 3.7a. The Multiplicity of the Excited State.- 3.7b. Stereoelectronic Effects.- 3.7c. Substituent Effects.- 3.7d. Synthetic Applications.- 3.8. Hydrogen Abstraction by Groups Other Than the Carbonyl.- Problems.- References.- 4 The Photochemistry of Simple Carbonyl Compounds: Type I Cleavage and Oxetane Formation.- 4.1. Type I Cleavage.- 4.1a. The Nature of the Excited State: Part I.- 4.1b. Some Examples and Synthetic Applications of Type I Cleavage Reactions.- 4.1c. Type I Cleavage Reactions Resulting in Loss of Carbon Monoxide.- 4.1d. ?-Cleavage of Cyclopropyl Ketones.- 4.1e. The Nature of the Excited State: Part II.- 4.2. The Formation of Oxetanes from Carbonyls and Olefins.- 4.2a. Oxetane Formation from Olefins and Aryl Ketones and Aldehydes.- 4.2b. Synthetic Applications of Oxetane Formation.- 4.2c. Oxetane Formation from Olefins and Aliphatic Aldehydes and Ketones.- 4.2d. Perturbational Molecular Orbital Theory (PMO) Applied to Oxetane Formation.- Problems.- References.- 5 The Triplet State.- 5.1. Introduction.- 5.1a. The Identity of the Phosphorescent State as a Triplet.- 5.1b. The Definition and Properties of a Triplet State.- 5.2. Determination of Triplet Energy Levels.- 5.2a. Phosphorescence Spectroscopy.- 5.2b. Singlet ? Triplet Absorption Spectra.- 5.2c. Phosphorescence Excitation Spectroscopy.- 5.2d. Flash Photolysis.- 5.2e. Electron Excitation.- 5.2f. The Lowest Triplet Levels of Organic Molecules.- 5.3. Determination of the Efficiency of Intersystem Crossing.- 5.3a. Flash Photolysis.- 5.3b. Triplet-Sensitized Isomerization.- 5.3c. Photooxidation.- 5.3d. Delayed Fluorescence.- 5.3e. Electron Spin Resonance Spectroscopy.- 5.3f. Intersystem Crossing Quantum Yields of Organic Molecules.- 5.4. Determination of Triplet Lifetimes.- 5.4a. Flash Photolysis.- 5.4b. Luminescence Decay.- 5.4c. The Effect of Deuteration on Triplet Lifetime.- 5.4d. Triplet Lifetimes of Various Organic Molecules.- 5.5. Excited State Geometry.- 5.6. Spin-Orbit Coupling and Intersystem Crossing.- 5.6a. The Nature of Spin-Orbit Coupling.- 5.6b. Effect of Heavy Atoms on Intercombinational Transitions in Aromatic Compounds.- 5.6c. Effect of Heavy Atoms on Intercombinational Transitions in Carbonyl and Heterocyclic Compounds.- 5.6d. External Heavy-Atom Effects and Charge Transfer.- References.- 6 Electronic Energy Transfer.- 6.1. Excitation Transfer within a Chromophore System.- 6.1a. Internal Conversion and Intersystem Crossing Theory.- 6.1b. Radiationless Transitions: Phosphorescence Microwave Double Resonance.- 6.1c. Zero-Field Optically Detected Magnetic Resonance (ODMR).- 6.2. Theory of Excitation Transfer between Two Chromophores.- 6.2a. Radiative Transfer (Trivial Mechanism).- 6.2b. Resonance Transfer (Long-Range Transfer).- 6.2c. Energy Transfer via Exchange Interaction.- 6.2d. Exciton Transfer (Strong Coupling).- 6.3. Excitation Transfer between Two Chromophores.- 6.3a. Singlet-Singlet Energy Transfer (Forster Type).- 6.3b. Singlet-Singlet Energy Transfer via Collisions.- 6.3c. Intermolecular Triplet-Triplet Energy Transfer.- 6.3d. Application of Triplet-Triplet Energy Transfer.- 6.3e. Schenck Mechanism.- 6.3f. Intramolecular Triplet Energy Transfer.- 6.3g. Exciton Interaction.- 6.4. Exciplex Quenching.- References.- 7 Dienone and Enone Photochemistry.- 7.1. Dienone Photoreactions.- 7.2. Dienone to Cyclopropyl Ketone Formation.- 7.2a. 3-5 Bond Orders.- 7.2b. Zwitterionic vs. Diradical Intermediates.- 7.2c. Pivot vs. Slither Mechanism.- 7.3. Dienone to Hydroxy Ketone.- 7.4. Cyclopropyl Ketones.- 7.5. 2,4-Cyclohexadienones.- 7.6. Cyclohexenone Photorearrangements.- 7.6a. Aryl-Substituted Cyclohexenones.- 7.6b. Alkyl-Substituted Cyclohexenones.- Problems.- References.- 8 The Di-?-Methane Photorearrangement.- 8.1. Acyclic Di-?-Methane Photorearrangement.- 8.1a. Regiospecificity and Stereochemistry.- 8.1b. Substitution at the Central sp3 Carbon Atom and Di-?-Methane Reactivity.- 8.1c. Reaction Rate Constants.- 8.2. Aryl Di-?-Methane Photorearrangement.- 8.3. Bicyclic Di-?-Methane Photorearrangement.- 8.3a. Barrelene.- 8.3b. Benzobarrelene.- 8.3c. Naphthobarrelenes.- 8.3d. Anthrabarrelene.- 8.3e. Other Selected Examples.- 8.4. Oxa-di-?-Methane Rearrangement.- Problems.- References.- 9 Photochemical Cis-Trans and Valence Isomerization of Olefins.- 9.1. Introduction: Cis-Trans Isomerization of Stilbene.- 9.2. Potential Energy Diagrams.- 9.3. Photosensitized Stilbene Isomerization.- 9.4. Nonvertical Energy Transfer.- 9.5. Stilbene Isomerization via Direct Photolysis.- 9.5a. Vibrationally Excited Ground State.- 9.5b. Triplet State Mechanism.- 9.5c. Singlet State Mechanism.- 9.6. Substituted Stilbenes.- 9.7. Piperylene Photochemistry.- 9.8. Alkene Photoisomerization.- 9.9. Intramolecular Cycloaddition Reactions.- 9.9a. Theory.- 9.9b. Intramolecular (2 + 2) Cycloadditions and Cycloreversion Reactions.- 9.9c. Intramolecular (4 + 2) Photocycloaddition Reactions.- 9.10. Photoelectrocyclic Reactions.- 9.10a. Theory.- 9.10b. Examples of Electrocyclic Reactions.- Problems.- References.- 10 Photodimerization and Photocycloaddition Reactions Yielding Cyclobutanes.- 10.1. Photodimerization and Photocycloaddition Reactions of Olefins and Polyenes.- 10.1a. Photodimerization of Olefins and Polyenes.- 10.1b. Photocycloaddition Reactions of Olefins and Polyenes.- 10.2. Photodimerization and Photocycloaddition Reactions of Aromatic Compounds.- 10.3. Photodimerization and Photocycloaddition Reactions of ?,?-Unsaturated Carbonyls and Acid Derivatives.- 10.3a. Photodimerization of ?,?-Unsaturated Carbonyls and Acid Derivatives.- 10.3b. Photocycloaddition Reactions of ?,?-Unsaturated Carbonyls and Acid Derivatives.- 10.4. Dimerization in the Solid Phase.- References.- 11 Photoelimination, Photoaddition, and Photosubstitution.- 11.1. Photoelimination Reactions.- 11.1a. Photoelimination of Nitrogen.- 11.1b. Photoelimination of Nitric Oxide from Organic Nitrites.- 11.1c. Miscellaneous Photoeliminations.- 11.2. Photoaddition Reactions.- 11.2a. Photoaddition of Water, Alcohols, and Carboxylic Acids.- 11.2b. Miscellaneous Photoadditions.- 11.3. Photosubstitution Reactions.- References.- 12 An Introduction to Photobiology.- 12.1. Photosynthesis.- 12.1a. The Photosynthetic Apparatus.- 12.1b. A Mechanistic Model for Photosynthesis.- 12.2. The Photochemistry of Vision.- 12.2a. Anatomy of the Human Eye.- 12.2b. The Visual Pigments and the Chemistry of Vision.- 12.3. Phototaxis and Phototropism.- 12.4. Damage and Subsequent Repair by Light (Photoreactivation).- 12.4a. The Photochemistry of the Nucleic Acids.- 12.4b. Photoreactivation.- References.

Triplet vs Singlet Energy Transfer in Organic Semiconductors: The Tortoise and the Hare

Abstract: Current bilayer organic photovoltaics cannot be made thick enough to absorb all incident solar radiation because of the short diffusion lengths (≈10 nm) of singlet excitons. Thus, the diffusion length sets an upper bound on the efficiency of these devices. By contrast, triplet excitons can have very long diffusion lengths (as large as 10 μm) in organic solids, leading some to speculate that triplet excitonic solar cells could be more efficient than their singlet counterparts. In this paper, we examine the nature of singlet and triplet exciton diffusion. We demonstrate that although there are fundamental physical upper bounds on the distance singlet excitons can travel by hopping, there are no corresponding limits on triplet diffusion lengths. This conclusion strongly supports the idea that triplet diffusion should be more controllable than singlet diffusion in organic photovoltaics. To validate our predictions, we model triplet diffusion by purely ab inito means in various crystals, achieving good agreeme...

Intersystem crossing and dynamics in O(3P) + C2H4 multichannel reaction: Experiment validates theory

Abstract: The O(3P) + C2H4 reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet- and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Primary products from five competing channels (H + CH2CHO, H + CH3CO, H2 + CH2CO, CH3 + HCO, CH2 + CH2O) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-ionization mass-spectrometric detection at a collision energy of 8.4 kcal/mol. As some of the observed products can only be formed via ISC from triplet to singlet PESs, from the product BRs the extent of ISC is inferred. A new full-dimensional PES for the triplet state as well as spin-orbit coupling to the singlet PES are reported, and roughly half a million surface hopping trajectories are run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential cross-sections. Both theory and experiment find almost equal contributions from the two PESs to the reaction, posing the question of how important is it to consider the ISC as one of the nonadiabatic effects for this and similar systems involved in combustion chemistry. Detailed comparisons at the level of angular and translational energy distributions between theory and experiment are presented for the two primary channel products, CH3 + HCO and H + CH2CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.

Driving Unidirectional Molecular Rotary Motors with Visible Light by Intra- And Intermolecular Energy Transfer from Palladium Porphyrin

Abstract: Driving molecular rotary motors using visible light (530-550 nm) instead of UV light was achieved using palladium tetraphenylporphyrin as a triplet sensitizer. Visible light driven rotation was confirmed by UV/vis absorption, circular dichroism and (1)H NMR spectroscopy and the rotation was confirmed to be unidirectional and with similar photostationary states, despite proceeding via a triplet instead of a singlet excited state of the molecular motor. Energy transfer proceeds in both inter- and intramolecular fashion from the triplet state of the porphyrin to the motor. Stern Volmer plots show that the rate of intermolecular quenching of the porphyrin excited state by the molecular motor is diffusion-controlled.

Azo anion radical complex of rhodium as a molecular memory switching device: isolation, characterization, and evaluation of current-voltage characteristics.

Abstract: Two rare examples of azo anion diradical complexes of Rh(III) are reported. These complexes showed excellent memory switching properties with a large ON/OFF ratio and are suitable for RAM/ROM applications. Their electronic structures have been elucidated using a host of physical methods, including X-ray crystallography, variable-temperature magnetic susceptibility measurement, cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory. The results indicate a predominant triplet state description of the systems with two ferromagnetically coupled radicals.

Long-lived 1H singlet spin states originating from para-hydrogen in Cs-symmetric molecules stored for minutes in high magnetic fields.

Abstract: Nuclear magnetic resonance (NMR) is a very powerful tool in physics, chemistry, and life sciences, although limited by low sensitivity. This problem can be overcome by hyperpolarization techniques dramatically enhancing the NMR signal. However, this approach is restricted to relatively short time scales depending on the nuclear spin-lattice relaxation time T(1) in the range of seconds. This makes long-lived singlet states very useful as a way to extend the hyperpolarization lifetimes. Para-hydrogen induced polarization (PHIP) is particularly suitable, because para-H(2) possesses singlet symmetry. Most PHIP experiments, however, are performed on asymmetric molecules, and the initial singlet state is directly converted to a NMR observable triplet state decaying with T(1), in the order of seconds. We demonstrate that in symmetric molecules, a long-lived singlet state created by PHIP can be stored for several minutes on protons in high magnetic fields. Subsequently, it is converted into observable high nonthermal magnetization by controlled singlet-triplet conversion via level anticrossing.

On the Mechanisms of Cyanine Fluorophore Photostabilization.

Abstract: Cyanine fluorophores exhibit greatly improved photostability when covalently linked to stabilizers, such as cyclooctatetraene (COT), nitrobenzyl alcohol (NBA) or Trolox. However, the mechanism by which photostabilization is mediated has yet to be determined. Here we present spectroscopic evidence that COT, when covalently linked to Cy5, substantially reduces the lifetime of the Cy5 triplet state, and that the degree of triplet state quenching correlates with enhancements in photostability observed in single-molecule fluorescence measurements. By contrast, NBA and Trolox did not quench the Cy5 triplet state under our conditions suggesting that their mechanism of photostabilization is different from COT and does not target the fluorophore triplet state directly. These findings provide insights into the mechanisms of fluorophore photostabilization that may lead to improved fluorophore designs for biological imaging applications.

Influence of Triplet State Multidimensionality on Excited State Lifetimes of Bis-tridentate Ru(II) Complexes: A Computational Study

Abstract: Calculated triplet excited state potential energy surfaces are presented for a set of three bis-tridentate Ru(II)-polypyridyl dies covering a wide range of room temperature excited state lifetimes: [Ru(II)(tpy)(2)](2+), 250 ps; [Ru(II)(bmp)(2)](2+), 15 ns; and [Ru(II)(dqp)(2)](2+), 3 mu s (tpy is 2,2':6',2 ''-terpyridine, bmp is 6-(2-picoly1)-2,2'-bipyridine, and dqp is 2,6-di(quinolin-8-yl)Fridine). The computational results provide a multidimensional view of the (3)MLCT-(3)MC transition for the investigated complexes. Recently reported results of significantly prolonged (3)MLCT excited state lifetimes of bis-tridentate Ru(II)-complexes, for example [Ru(II)(dqp)(2)](2+), are found to correlate with substantial differences in their triplet excited state multidimensional potential energy surfaces. In addition to identification of low-energy transition paths for (3)MLCT-(3)MC conversion associated with simultaneous elongation of two or more Ru-N bonds for all investigated complexes, the calculations also suggest significant differences in (3)MLCT state volume in the multidimensional reaction coordinate space formed from various combinations of Ru-N bond distance varix:ions. This is proposed to be an important aspect for understanding the large differences in experimentally observed (3)MLCT excited state lifetimes. The results demonstrate the advantage of considering multidimensional potential energy surfaces beyond the Franck-Condon region in order to predict photophysical and photochemical properties of bis-tridentate Ru(II)-polypyridyl dyes and related metal complexes.

[Ru(bpy)3]2+ as a reference in transient absorption spectroscopy: differential absorption coefficients for formation of the long-lived 3MLCT excited state

Abstract: Transient absorption spectroscopy and other time-resolved methods are commonly used to study chemical reactions and biological processes induced by absorption of light. In order to scale the signal amplitude or to compare results obtained under different conditions, it is advisable to use a reference system, a standard of convenient and well-defined properties. Finding Tris(bipyridine)ruthenium(II), [Ru(bpy)3]2+, a suitable candidate for a transient-absorption spectroscopy reference due to its favourable photochemical properties, we have determined accurate relative values of differential molar absorption coefficients (Δe) for light-induced formation of the metal-to-ligand charge transfer (MLCT) excited triplet state at several relevant wavelengths (wavelengths of commercially available lasers) in the UV and visible regions. We have also attempted to determine the absolute value of Δe close to the wavelength of maximum bleaching (∼450 nm) and we propose to narrow down the interval of conceivable values for Δe450 from the broad range of published values (−0.88 × 104 M−1cm−1 to −1.36 × 104 M−1cm−1) to −1.1 × 104 M−1cm−1 ± 15%. Having ourselves successfully applied [Ru(bpy)3]2+ as a standard in a recent time-resolved study of enzymatic DNA repair, we would like to encourage other scientists to use this convenient tool as a reference in their future spectroscopic studies on time scales from picoseconds to hundreds of nanoseconds.

Intersystem crossing involving strongly spin exchange-coupled radical ion pairs in donor-bridge-acceptor molecules.

Abstract: Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor-bridge-acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ(+•)-PI(-•) or DMJ(+•)-NI(-•) quantitatively in 1-4 (τ(CS) ≤ 10 ps), and that charge recombination occurs with τ(CR) = 268 and 158 ps for 1 and 3, respectively, and with τ(CR) = 2.6 and 10 ns for 2 and 4, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (2J) between DMJ(+•) and PI(-•) or NI(-•), giving 2J > 600 mT for 1-3 and 2J = 170 mT for 4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of (3)*An upon charge recombination occurs by spin-orbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 2J determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in 1-3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1-4 depend on the donor-acceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum.

Photochemistry of fac-[Re(bpy)(CO)3Cl].

Abstract: The photochemistry of fac-[Re(bpy)(CO)3Cl] (1 a; bpy=2,2′-bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding mer-isomer 1 b. However, in the presence of a small amount of MeCN, the main product was the CO-ligand-substituted complex (OC-6-24)-[Re(bpy)(CO)2Cl(MeCN)] (2 c; bpy=2,2′-bipyridine). In MeCN, two isomers, 2 c and its (OC-6-34) form (2 a), were produced. Only 2 c thermally isomerized to produce the (OC-6-44) form 2 b. A detailed investigation led to the conclusion that both 1 b and 2 c are produced by a dissociative mechanism, whereas 2 a forms by an associative mechanism. A comparison of the ultrafast transient UV-visible absorption, emission, and IR spectra of 1 a acquired by excitation using higher-energy light (e.g., 270 nm) and lower-energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of 1 a. Irradiation of 1 a using the higher-energy light resulted in the generation of the higher singlet excited state with τ≤25 fs, from which intersystem crossing proceeded to give the higher triplet state (3HES(1)). In THF, 3HES(1) was competitively converted to both the triplet ligand field (3LF) and metal-to-ligand charge transfer (3mLCT) with lifetimes of 200 fs, in which the former is a reactive state that converts to [Re(bpy)(CO)2Cl(thf)]+ (1 c) within 10 ps by means of a dissociative mechanism. Re-coordination of CO to 1 c gives both 1 a and 1 b. In MeCN, irradiation of 1 a by using high-energy light gives the coordinatively unsaturated complex, which rapidly converted to 2 c. A seven-coordinate complex is also produced within several hundred femtoseconds, which is converted to 2 a within several hundred picoseconds.

Ab initio time-domain study of the triplet state in a semiconducting carbon nanotube: intersystem crossing, phosphorescence time, and line width.

Abstract: Motivated by recent experiments (J. Am. Chem. Soc. 2011, 133, 17156), we used nonadiabatic (NA) molecular dynamics implemented within ab initio time-domain density functional theory to investigate the evolution of the excited electronic singlet and triplet states in the (6,4) carbon nanotube (CNT). The simulation simultaneously included the NA electron–phonon interaction and the spin–orbit (SO) interaction and focused on the intersystem crossing (ISC) from the first excited singlet state (S1) to the triplet state (T1) and subsequent relaxation to the ground electronic state (S0). For the first time, the state-of-the-art methodology (Phys. Rev. Lett. 2005, 95, 163001; Phys. Rev. Lett. 2008, 100, 197402) has been advanced to include triplet states. The S1–T1 ISC was calculated to occur within tens of picoseconds, in agreement with the experimental data. This time scale is on the same order as the S1–S0 nonradiative decay time obtained previously for the (6,4) CNT. The homogeneous phosphorescence line width,...

Polymer triplet energy levels need not limit photocurrent collection in organic solar cells.

Abstract: We study charge recombination via triplet excited states in donor/acceptor organic solar cells and find that, contrary to intuition, high internal quantum efficiency (IQE) can be obtained in polymer/fullerene blend devices even when the polymer triplet state is significantly lower in energy than the intermolecular charge transfer (CT) state. Our model donor system comprises the copolymer PIDT-PhanQ: poly(indacenodithiophene-co-phenanthro[9,10-b]quinoxaline), which when blended with phenyl-C71-butyric acid methyl ester (PC71BM) is capable of achieving power conversion efficiencies of 6.0% and IQE ≈ 90%, despite the fact that the polymer triplet state lies 300 meV below the interfacial CT state. However, as we push the open circuit voltage (VOC) higher by tailoring the fullerene reduction potential, we observe signatures of a new recombination loss process near VOC = 1.0 V that we do not observe for PCBM-based devices. Using photoinduced absorption and photoluminescence spectroscopy, we show that a new reco...

Electronic Structure of Lithium Peroxide Clusters and Relevance to Lithium–Air Batteries

Abstract: The prospect of Li–air(oxygen) batteries has generated much interest because of the possibility of extending the range of electric vehicles due to their potentially high gravimetric density. The exact morphology of the lithium peroxide formed during discharge has not been determined yet, but the growth likely involves nanoparticles and possibly agglomerates of nanoparticles. In this article, we report on density functional calculations of stoichiometric lithium peroxide clusters that provide evidence for the stabilization of high spin states relative to the closed shell state in the clusters. The density functional calculations indicate that a triplet state is favored over a closed shell singlet state for a dimer, trimer, and tetramer of lithium peroxide, whereas in the lithium peroxide monomer, the closed shell singlet is strongly favored. Density functional calculations on a much larger cluster, (Li2O2)16, also indicate that it similarly has a high spin state with four unpaired electrons located on the ...

Structural Optimization by Quantum Monte Carlo: Investigating the Low-Lying Excited States of Ethylene

Abstract: We present full structural optimizations of the ground state and of the low lying triplet state of the ethylene molecule by means of Quantum Monte Carlo methods. Using the efficient structural optimization method based on renormalization techniques and on adjoint differentiation algorithms recently proposed (Sorella, S.; Capriotti, L. J. Chem. Phys. 2010, 133, 234111), we present the variational convergence of both wave function parameters and atomic positions. All of the calculations were done using an accurate and compact wave function based on Pauling's resonating valence bond representation: the Jastrow Antisymmetrized Geminal Power (JAGP). All structural and wave function parameters are optimized, including coefficients and exponents of the Gaussian primitives of the AGP and the Jastrow atomic orbitals. Bond lengths and bond angles are calculated with a statistical error of about 0.1% and are in good agreement with the available experimental data. The Variational and Diffusion Monte Carlo calculations estimate vertical and adiabatic excitation energies in the ranges 4.623(10)− 4.688(5) eV and 3.001(5)−3.091(5) eV, respectively. The adiabatic gap, which is in line with other correlated quantum chemistry methods, is slightly higher than the value estimated by recent photodissociation experiments. Our results demonstrate how Quantum Monte Carlo calculations have become a promising and computationally affordable tool for the structural optimization of correlated molecular systems.

Ligand-localized triplet-state photophysics in a platinum(II) terpyridyl perylenediimideacetylide.

Abstract: The synthesis, electrochemistry, and photophysical behavior of a Pt(II) terpyridyl perylenediimide (PDI) acetylide (1) charge-transfer complex is reported. The title compound exhibits strong (e ≈ 5 × 104 M–1cm–1) low-energy PDI acetylide-based π–π* absorption bands in the visible range extending to 600 nm, producing highly quenched singlet fluorescence (Φ = 0.014 ± 0.001, τ = 109 ps) with respect to a nonmetalated PDI model chromophore. Nanosecond transient absorption spectroscopy revealed the presence of a long excited-state lifetime (372 ns in 2-methyltetrahydrofuran) with transient features consistent with the PDI–acetylide triplet state, ascertained by direct comparison to a model Pt(II) PDI–acetylide complex lacking low-energy charge-transfer transitions. For the first time, time-resolved step–scan FT-IR spectroscopy was used to characterize the triplet excited state of the PDI–acetylide sensitized in the title compound and its associated model complex. The observed red shifts (∼30–50 cm–1) in the C═...

Computational studies of photophysical properties of porphin, tetraphenylporphyrin and tetrabenzoporphyrin.

Abstract: The molecular photonics of porphyrins are studied using a combination of first-principle and semi-empirical calculations. The applicability of the approach is demonstrated by calculations on free-base porphyrin, tetraphenylporphyrin, and tetrabenzoporphyrin. The method uses excitation energies and oscillator strengths calculated at the linear-response time-dependent density functional theory (TDDFT) or the corresponding values calculated at the linear-response approximate second-order coupled-cluster (CC2) levels. The lowest singlet excitation energies obtained in the TDDFT and CC2 calculations are 0.0–0.28 eV and 0.18–0.47 eV larger than the experimental values, respectively. The excitation energies for the first triplet state calculated at the TDDFT level are in excellent agreement with experiment, whereas the corresponding CC2 values have larger deviations from experiment of 0.420.66 eV. The matrix elements of the spin–orbit and non-adiabatic coupling operators have been calculated at the semi-empirical intermediate neglect of differential overlap (INDO) level using a spectroscopic parameterization. The calculations yield rate constants for internal conversion and intersystem crossing processes as well as quantum yields for fluorescence and phosphorescence. The main mechanism for the quenching of fluorescence in tetraphenylporphyrin and tetrabenzoporphyrin is the internal conversion, whereas for free-base porphyrin both the internal conversion and the intersystem crossing processes reduce the fluorescence intensity. The phosphorescence is quenched by a fast internal conversion from the triplet to the ground state.

Enhanced charge detection of spin qubit readout via an intermediate state

Abstract: We employ an intermediate excited charge state of a lateral quantum dot device to increase the charge detection contrast during the qubit state readout procedure, allowing us to increase the visibility of coherent qubit oscillations. This approach amplifies the coherent oscillation magnitude but has no effect on the detector noise resulting in an increase in the signal to noise ratio. In this letter, we apply this scheme to demonstrate a significant enhancement of the fringe contrast of coherent Landau-Zener-Stuckelberg oscillations between singlet S and triplet T+ two-spin states.

Bright green luminescent molecular terbium plastic materials derived from 3,5-bis(perfluorobenzyloxy)benzoate

Abstract: Luminescent lanthanide complexes (Eu3+ (1) or Tb3+ (2)) involving a highly fluorinated aromatic carboxylate, namely, 3,5-bis(perfluorobenzyloxy)benzoic acid which acts as an antenna chromophore and sensitizes the visible emitting lanthanides, have been synthesized and characterized and their photophysical properties investigated. The results demonstrated that the replacement of high-energy C–H vibrations with fluorinated phenyl groups in the 3,5-bis(benzyloxy)benzoate effectively improves the luminescence intensity and lifetimes of lanthanide complexes. It is interesting to note that the designed fluorinated carboxylate is well suited for the sensitization of Tb3+ emission (Φsen = 52%), thanks to a favorable position of the triplet state of the ligand as investigated in the Gd3+ complex. On the other hand, the corresponding Eu3+ complex shows weak luminescence efficiency (Φsen = 24%) due to poor match of the triplet state of the ligand with the emissive excited states of the metal ion. In the present work, efforts have also been made to isolate luminescent molecular terbium plastic materials by combining the unique optical properties of lanthanides with the mechanical characteristics, thermal stability, flexibility and film-forming tendency of polymers (PMMA). The photoluminescence quantum yields of polymer–lanthanide hybrid materials are significantly enhanced (53–65%) as compared to that of the Tb3+–3,5-bis(perfluorobenzyloxy)benzoate complex.

All-optical control of the spin state in the NV − center in diamond

Abstract: We describe an all-optical scheme for spin manipulation in the ground-state triplet of the negatively charged nitrogen-vacancy (NV) center in diamond. Virtual optical excitation from the ${}^{3}{A}_{2}$ ground state into the ${}^{3}E$ excited state allows for spin rotations by virtue of the spin-spin interaction in the two-fold orbitally degenerate excited state. We derive an effective Hamiltonian for optically induced spin-flip transitions within the ground state spin triplet due to off-resonant optical pumping. Furthermore, we investigate the spin qubit formed by the Zeeman sublevels with spin projection ${m}_{S}=0$ and ${m}_{S}=\ensuremath{-}1$ along the NV axis around the ground state level anticrossing with regard to full optical control of the electron spin.

Graphene as a quencher of electronic excited states of photochemical probes.

Abstract: Graphene sheets quench the singlet and triplet excited states of a series of six photochemical probes including pyrene, acridine orange, tris(2,2-bipyridyl)ruthenium(II) dichloride, methylene blue, meso-tetrakis(phenylsulphonate)porphyrin, and 5,10,15,20-tetraphenyl-21H,28H-porphine zinc. It was found that Stern–Volmer fluorescence quenching can fit to one or two different quenching regimes depending on the probe. In addition, the quenching can be either static or dynamic depending on the fluorophore. The occurrence of several quenching regimes has been interpreted considering that quenching arises from the crowding of the fluorophore on both graphene faces, or site isolation on the graphene sheets. Laser flash photolysis has shown that the triplet lifetime of the probes generally decreases due to graphene quenching and that no new transients appear except in the case of methylene blue, where a new absorption spectrum characterized by a continuous absorption band is observed and attributed to graphene ra...

Subpicosecond intersystem crossing in mono- and di(organophosphine)gold(I) naphthalene derivatives in solution.

Abstract: Femtosecond-to-microsecond broadband transient absorption experiments are reported for Cy3PAu(2-naphthyl) (1), (Cy3PAu)2(2,6-naphthalenediyl) (2), and (Cy3PAu)2(2,7-naphthalenediyl) (3), where Cy = cyclohexyl. Global and target analyses of the data, based on a sequential kinetic model, reveal four spectral components. These components are assigned to (1) excited state absorption (ESA) of the ligand-centered S1 state; (2) ESA of a receiver ligand-to-metal or metal-to-ligand charge transfer triplet state (τ1 ≤ 300 fs); (3) ESA of the vibrationally excited, ligand-centered T1 state (τ3 = 7–10 ps); and (4) ESA of the relaxed T1 state. Intersystem crossing (ISC) occurs in hundreds of femtoseconds, while internal conversion (IC) in the triplet manifold is slow (τ2 ≈ 2 ps). The relaxed T1 state shows biphasic decay kinetics in 2 and 3 with lifetimes of hundreds of picoseconds and hundreds of nanoseconds in air-saturated conditions, while only monophasic decay is observed in 1 under identical conditions. The prim...

Experimental and theoretical studies of the O(3P) + C2H4 reaction dynamics: Collision energy dependence of branching ratios and extent of intersystem crossing

Abstract: The reaction of O((3)P) with C(2)H(4), of importance in combustion and atmospheric chemistry, stands out as paradigm reaction involving not only the indicated triplet state potential energy surface (PES) but also an interleaved singlet PES that is coupled to the triplet surface. This reaction poses great challenges for theory and experiment, owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Crossed molecular beam (CMB) scattering experiments with soft electron ionization detection are used to disentangle the dynamics of this polyatomic multichannel reaction at a collision energy E(c) of 8.4 kcal∕mol. Five different primary products have been identified and characterized, which correspond to the five exothermic competing channels leading to H + CH(2)CHO, H + CH(3)CO, CH(3) + HCO, CH(2) + H(2)CO, and H(2) + CH(2)CO. These experiments extend our previous CMB work at higher collision energy (E(c) ∼ 13 kcal∕mol) and when the results are combined with the literature branching ratios from kinetics experiments at room temperature (E(c) ∼ 1 kcal∕mol), permit to explore the variation of the branching ratios over a wide range of collision energies. In a synergistic fashion, full-dimensional, QCT surface hopping calculations of the O((3)P) + C(2)H(4) reaction using ab initio PESs for the singlet and triplet states and their coupling, are reported at collision energies corresponding to the CMB and the kinetics ones. Both theory and experiment find almost an equal contribution from the triplet and singlet surfaces to the reaction, as seen from the collision energy dependence of branching ratios of product channels and extent of intersystem crossing (ISC). Further detailed comparisons at the level of angular distributions and translational energy distributions are made between theory and experiment for the three primary radical channel products, H + CH(2)CHO, CH(3) + HCO, and CH(2) + H(2)CO. The very good agreement between theory and experiment indicates that QCT surface-hopping calculations, using reliable coupled multidimensional PESs, can yield accurate dynamical information for polyatomic multichannel reactions in which ISC plays an important role.

Complexes with a Single Metal–Metal Bond as a Sensitive Probe of Quality of Exchange-Correlation Functionals

Abstract: The electronic structure of the vanadium dimer complex [V(C5H5)]2Pn with a single metal-metal bond was characterized, and the energies of higher spin states were evaluated. To simplify evaluation of orbital contributions to bonding between atoms and fragments, occupancy-perturbed bond orders were introduced. The structure and experimentally determined singlet-triplet gap in this complex can be used to test the quality of modern exchange-correlation functionals. Most generalized gradient approximation (GGA) functionals were determined to be quite suitable to reproduce the metal-metal distance and the single-triplet energy gap in [V(C5H5)]2Pn. Further accuracy improvement can be achieved by using empirical dispersion corrections. Hybrid exchange-correlation functionals, including the B3LYP functional, performed poorly for both structural and energy predictions. The hybrid functionals significantly overestimate the stability of the singlet state with the antiferromagnetically coupled high-spin metal ions relative to the lowest-energy triplet state and the singlet state with stronger metal-metal interactions. Thus, these XC functionals are not quite suitable for computational studies of multinuclear 3d transition metal complexes with weak-to-intermediate metal-metal bonding.

Spectral- and Pulse-Shape Discrimination in Triplet-Harvesting Plastic Scintillators

Abstract: In this work, we describe a method to control the relative proportion of prompt and delayed luminosity of organic-based scintillators via direct and exponential emission from an extrinsic triplet state. This approach involves the incorporation of triplet-harvesting heavy metal complexes in plastic scintillator matrices to convert intrinsically non-luminescent host states to highly emissive guest states. Measurements on these plastic scintillators indicate improved light yields over the undoped polymers and the ability to perform neutron/gamma particle-discrimination. A similar extent of molecular-level control is not possible in traditional organic materials due to complex decay kinetics and the absence of spectral information for the delayed triplet-derived emission. The materials described here address these limitations through efficient host-guest triplet harvesting, which enables particle discrimination according to conventional pulse-shape discrimination (PSD) and a previously unreported spectral-shape discrimination (SSD) scheme.