Triplet States Of Organic Molecules Research Articles

Tetrathienothiophene Porphyrin as a Metal-Free Sensitizer for Room-Temperature Triplet-Triplet Annihilation Upconversion.

Optically excited triplet states of organic molecules serve as an energy pool for the subsequent processes, either photon energy downhill, such as room temperature phosphorescence, or photon energy uphill process—the triplet–triplet annihilation upconversion (TTA-UC). Manifestation of a high intersystem crossing coefficient is an unavoidable requirement for triplet state formation, following the absorption of a single photon. This requirement is even more inevitable if the excitation light is non-coherent, with moderate intensity and extremely low spectral power density, when compared with the light parameters of 1 Sun (1.5 AM). Coordination of a heavy atom increases substantially the probability of intersystem crossing. Nevertheless, having in mind the global shortage in precious and rare-earth metals, identification of metal-free organic moieties able to form triplet states becomes a prerequisite for environmental friendly optoelectronic technologies. This motivates us to synthesize a metal-free thienothiophene containing porphyrin, based on a condensation reaction between thienothiophene-2-carbaldehyde and pyrrole in an acidic medium by modified synthetic protocol. The upconversion couple tetrathienothiophene porphyrin/rubrene when excited at λ = 658 nm demonstrates bright, delayed fluorescence with a maximum emission at λ = 555 nm. This verifies our hypothesis that the ISC coefficient in thienothiophene porphyrin is efficient in order to create even at room temperature and low-intensity optical excitation densely populated organic triplet ensemble and is suitable for photon energy uphill processes, which makes this type of metal-free sensitizers even more important for optoelectronic applications.

Excited Triplet States of Organic Molecules and Reactive Free Radicals in Polymers

Excited Triplet States of Organic Molecules and Reactive Free Radicals in Polymers

Features of molecules accumulation in the triplet state at excitation of the organic compounds by rectangular pulses

Theoretical research of the time dependence on molecules concentration in triplet state under excitation organic compounds have been carried out by light pulses with high. The computer simulation of the molecular dependence on the triplet level population of time during the excitation pulse and after its ending have been realized by using the obtained analytic expressions. It is shown that for a certain relation between the rate constants of intra molecular transitions and the excitation pulse duration, the triplet level population can increase several times after excitation ending. It is due to the transition of molecules which it were in the excited singlet state at the end of the excitation pulse to the triplet state. Note that this effect must be considered in the study of dynamic of photo physical processes and photochemical reactions taking place with the participation of triplet states of organic molecules when they are excited by short light pulses of high intensity.

Screening mixing GW/Bethe–Salpeter approach for triplet states of organic molecules

We recently proposed a screening mixing many-body ansatz (Ziaei and Bredow 2017 Phys. Rev. B 96 195115) to decrease the typical overestimation of the Hartree–Fock based GW/Bethe–Salpeter equation (BSE) singlet–singlet excitation energies in molecular systems. Now we have evaluated the accuracy of the proposed scheme for triplet states of a set of 20 organic molecules known as the Thiel set. We show that by mixing different screenings into GW and BSE calculated within random phase approximation (in order to ensure best gap and an optimal exciton binding energy), the total mean absolute error of 0.59 eV in the standard Hartree–Fock based eigenvalue GW/BSE approach is reduced to 0.26 eV for 63 triplet states. We further demonstrate that the quasi-particle self-consistent GW/BSE approach in which orbitals and energies are updated in the Green’s function and the dynamically screened interaction mostly and considerably underestimates the excitation energies as shown for a few molecules.

Accessibility of Protein-Bound Chlorophylls Probed by Dynamic Electron Polarization.

The possibility to probe the accessibility of sites of proteins represents an important point to explore their interactions with specific substrates in solution. The dynamic electron polarization of nitroxide radicals induced by excited triplet states of organic molecules is a phenomenon that is known to occur in aqueous solutions. The interaction within the radical-triplet pair causes a net emissive dynamic electron polarization of the nitroxide radical, that can be detected by means of time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. We have exploited this effect to prove the accessibility of chlorophylls bound to a protein, namely, the water-soluble chlorophyll protein WSCP. The results have important implications for topological studies in macromolecules.

Novel Pulsed Electron Paramagnetic Resonance Techniques for the Studies of Structure and Dynamics of Photo‐excited Triplet State of Organic Molecules: A Professional Journey

We present a few novel pulsed electron paramagnetic resonance techniques developed in our laboratory for the studies of structure and dynamics of the photo‐excited triplet state of organic molecules. We discuss many aspects of these new techniques and the significances of these measurements: (1) enhancing NMR signal intensity by dynamic nuclear polarization ‐ integrated solid effect, (2) performing magnetic resonance in zero‐field and low‐field by pulsed microwave, (3) mapping molecular motion of organic crystals by pulsed zero‐field and low‐field experiments, (4) probing spin dynamics at level anti‐crossing by fast field switching, (5) measuring hyperfine interaction by electron spin echo envelop modulation and spin‐echo electron nuclear double resonance and (6) detecting spin dynamics, nuclear quantum oscillation, entanglements and new avenues for quantum computer. We have employed the highly electron spin polarized pentacene triplet state as the model system in all of our pulsed EPR experiments. We performed most of our experiments at room temperature. The goals of our studies are aiming to improve spin detectability, to probe molecular dynamics, to determine electronic structures, to measure molecular interaction and motion, and to examine quantum coherence and oscillation which may yield new avenues in the applications of pulsed EPR techniques to quantum computer.

X-ray absorption spectra of excited triplet states of organic molecules

SynopsisX-ray absorption spectra (XAS) have been measured for several organic molecules in the lowest excited triplet states (T1) prepared by UV irradiation. T1-XAS deduced from the UV-ON/OFF difference spectra show good agreement, at least in the lowest peak region, with calculated T1-XAS.

Measurement of lifetimes of triplet states of organic molecules by fluorescence recovery kinetics

The possibility of measuring the lifetime of triplet states of fluorescent complex organic molecules by fluorescence recovery kinetics after switching off the excitation source under saturation conditions is shown using the example of protoporphyrin IX.

Core-excitation from excited triplet states of organic molecules: X-ray absorption spectra and doubly excited potential curves

SynopsisSoft X-ray absorption spectra of the lowest excited triplet states of several organic molecules (quinones, benzaldehydes, and other carbonyls) and potential curves of valence- and core- doubly excited states are calculated for the O 1s region.

Hyperbranched unsaturated polyphosphates as a protective matrix for long-term photon upconversion in air.

The energy stored in the triplet states of organic molecules, capable of energy transfer via an emissive process (phosphorescence) or a nonemissive process (triplet-triplet transfer), is actively dissipated in the presence of molecular oxygen. The reason is that photoexcited singlet oxygen is highly reactive, so the photoactive molecules in the system are quickly oxidized. Oxidation leads to further loss of efficiency and various undesirable side effects. In this work we have developed a structurally diverse library of hyperbranched unsaturated poly(phosphoester)s that allow efficient scavenging of singlet oxygen, but do not react with molecular oxygen in the ground state, i.e., triplet state. The triplet-triplet annihilation photon upconversion was chosen as a highly oxygen-sensitive process as proof for a long-term protection against singlet oxygen quenching, with comparable efficiencies of the photon upconversion under ambient conditions as in an oxygen-free environment in several unsaturated polyphosphates. The experimental results are further correlated to NMR spectroscopy and theoretical calculations evidencing the importance of the phosphate center. These results open a technological window toward efficient solar cells but also for sustainable solar upconversion devices, harvesting a broad-band sunlight excitation spectrum.

Effects of optical pumping in the photo-excitation of organic triplet states

Upon the application of laser and microwave pulses, non-zero magnetic moment of a photo-excited triplet state of organic molecules is generated in zero-field (ZF). The time evolution of the transient magnetic moments can be measured by free induction decay (FID) in ZF. The observed ZF spectra become broadened and ZF transition shifted to lower frequencies when the repetition rate of laser excitation is increased, which are attributed to the optical pumping of nuclear polarization (ONP) effect and the associated nuclear spin lattice relaxation processes. The observed ONP effect is discussed in terms of the local field effect and spin diffusion processes in optical pumping.

Intersystem crossing at singlet conical intersections

The energy of the lowest triplet state of organic molecules is intermediate between the ground state and the first excited singlet. At the S 1/ S 0 conical intersection, the two singlet states are degenerate. It is shown that for some molecules (ethylene, benzene, toluene and pyrrole) the T 1 state is also degenerate with the two singlet states. Moreover, the spin orbit coupling matrix element at this structure is necessarily large, so that intersystem crossing can be quite efficient. If the lowest triplet state is repulsive (as in the studied molecules) it may significantly contribute to the dissociation yield under certain experimental conditions.

A quasioptical transient electron spin resonance spectrometer operating at 120 and 240 GHz

A new multifrequency quasioptical electron paramagnetic resonance (EPR) spectrometer is described. The superheterodyne design with Schottky diode mixer/detectors enables fast detection with subnanosecond time resolution. Optical access makes it suitable for transient EPR (TR-EPR) at 120 and 240 GHz. These high frequencies allow for an accurate determination of small g-tensor anisotropies as are encountered in excited triplet states of organic molecules like porphyrins and fullerenes. The measured concentration sensitivity for continuous-wave (cw) EPR at 240 GHz and at room temperature without cavity is 1013spins∕cm3 (15 nM) for a 1 mT linewidth and a 1 Hz bandwidth. With a Fabry-Perot cavity and a sample volume of 30 nl, the sensitivity at 240 GHz corresponds to ≈3×109 spins for a 1 mT linewidth. The spectrometer’s performance is illustrated with applications of transient EPR of excited triplet states of organic molecules, as well as cw EPR of nitroxide reference systems and a thin film of a colossal magnetoresistance material.

Energy transfer between triplet states of organic molecules in a vapour phase

Energy transfer between triplet states of organic molecules in a vapour phase

Measurements of energy gap at the level anti-crossing region of the photo-excited triplet state of organic molecules

By the application of zero-field magnetic resonance (ZFMR)/FID technique and fast-field sweeping around the level anti-crossing region with fixed rf frequency, we obtained the (minimum) energy gaps between the two crossing levels as 16.7±2.0 and 14.3±3.7 MHz for pentacene-h 14 in p-terphenyl and pentacene-d 14 in p-terphenyl, respectively, when B 0∥ x. The minimum energy gaps occur at 14 mT for both systems. The hyperfine interaction contributes only partially to the observed energy gap.

A combination of Kohn–Sham density functional theory and multi-reference configuration interaction methods

An effective Hamiltonian in a basis of spin- and space-symmetry adapted configuration state functions (CSF), which includes information from Kohn–Sham density functional theory (DFT), is used to calculate configuration interaction (CI) wave functions for the electronic states of molecules. The method emphasizes on states of multiconfigurational character which cannot be represented by conventional DFT. The CI matrix elements are constructed empirically by using the exact operator and corrections from DFT. Both the optimized KS orbitals from the parent determinant and the corresponding KS potential from the parent state density are used. Depending on their energy gap the CI off-diagonal elements between CSF are exponentially scaled to zero to avoid double counting of electron correlation. The selection of the most important CSF describing nondynamical correlation effects and the use of an approximate resolution of the identity (RI) for the evaluation of the two-electron integrals allows a very efficient DFT/MRCI treatment of molecules with several hundreds of electrons. As applications, the prediction of excitation energies for singlet and triplet states of organic molecules and transition metal complexes, the calculation of electronic circular dichroism spectra and investigations of the energetics of diradicals are presented. It is found, that the new DFT/MRCI approach gives results of high accuracy (rms errors for relative energies <0.2 eV) comparable to those from sophisticated ab initio treatments.

Photophysics ofβ-carotene and related compounds (review)

A review of data on the photophysics of carotenoids is presented. Results of investigations of spectroscopic, temporal, and energy parameters of excited S1 and S2 singlet states of β-carotene and related compounds are critically examined. These states give rise to extremely high probabilities (1011–1013 sec−1) of radiationless deactivation of the electronic excitation energy in carotenoids. Results of investigations of photophysical properties of triplet states of carotenoids are considered mainly from the standpoint of quenching of singlet oxygen and triplet states of organic molecules by carotenoids.

Solution thermochemistry of the radicals of glycine

Using gas phase thermochemical data, the following Gibbs energies of formation in aqueous solution (in kJ mol–1) have been estimated for radicals of glycine: H3N+CH2CO2˙– 93, H2N˙+CH2CO2H –163, H3N+CH˙CO2H –198, H2NCH˙C(OH)2+–268, H3N+CH2CO2––371, H2NCH2CO2˙–95, H2N˙+CH2CO2––158, HN˙CH2CO2H –148, H2NCH˙CO2H –246, HN˙CH2CO2––147 and H2NCH˙CO2––208. The uncertainty in these values is estimated to be ±20 kJ mol–1.In accord with earlier EPR studies, the H2NCH˙CO2H and H2NCH˙CO2– radicals are predicted to be the most stable. Non-equivalence of the NH protons of the latter can be rationalized by a strong internal H ⋯–OCO bond. Formation of the H3N+CH2CO2˙ and H2NCH2CO2˙ acyloxyl species is expected to require very strong oxidants (E° > 3 V). Production of H2N˙+CH2CO2H and H2N˙+CH2CO2– is proposed as a better explanation of H2NCH2˙ formation in SO4˙– oxidations. The H2N˙+CH2CO2– radical, which is also susceptible to loss of CO2, would lie above H2NCH2CO2˙ in the gas phase, but its Gibbs energy of formation in aqueous solution will be ca. 0.65 V less than that of H2NCH2CO2˙. E°(H2N˙+CH2CO2–/H2NCH2CO2–) is estimated to be near 1.6 V. This is in keeping with observed one electron oxidations of H2NCH2CO2– by triplet states of organic molecules with reduction potentials in the region of 1.5–1.8 V.

Optical Detection of NQR: Crystal Field and Matrix Effects on Halogenated Organic Molecules

Electron spin polarization or alignment achieved in the excited triplet state of organic molecules can be transferred into nuclear spin polarization (alignment) during an optical pumping cycle. Due to this effect, quadrupole resonance (NQR) investigations of nuclei with concentrations of 10-5 molar are possible in the electronic singlet ground and excited triplet state in one and the same experiment by the applied optical pumping cycle. By this conventional NMR sensitivity is enhanced by several orders of magnitude, enabling investigations of defect site structures in molecular crystals.

Sensitization and quenching processes of alkylcobalt(III) compounds

Ethyl(aquo)cobaloxime is found to be a potent quencher of a wide variety of excited states. Laser flash photolysis studies show that triplet states of organic molecules with ET > 170 kJ mol–1 are quenched at rates exceeding 109 dm3 mol–1 s–1, while those of rubrene and β-carotene, with ET < 110 kJ mol–1, are not effectively deactivated. A variety of excited states of inorganic molecules are also quenched at rates exceeding 109 dm3 mol–1 s–1 by ethyl(aquo)cobaloxime: the insensitivity of the quenching rate to the reduction potentials of the excited states concerned (differing by 1.8 V) indicates a mechanism of energy (rather than electron) transfer. Analogous kinetic studies have been carried out, by way of comparison, on tris(acetylacetonato)cobalt(III) and a few studies on methylcobalamin. A number of the quenchers examined have been shown to sensitize the photodecomposition of ethyl(aquo)cobaloxime, indicating the mechanism of energy transfer to be one of transfer to the photodissociative state of the alkylcobalt(III) compound: this is dissociated very efficiently by donors with energies as low as 170 kJ mol–1(14 210 cm–1 or 704 nm) compared with the energy of its longest wavelength transition of 262 kJ mol–1.