Intermediate Triplet State Research Articles

Triplet Excitation and Electroluminescence from a Supramolecular Monolayer Embedded in a Boron Nitride Tunnel Barrier.

We show that ordered monolayers of organic molecules stabilized by hydrogen bonding on the surface of exfoliated few-layer hexagonal boron nitride (hBN) flakes may be incorporated into van der Waals heterostructures with integral few-layer graphene contacts forming a molecular/two-dimensional hybrid tunneling diode. Electrons can tunnel through the hBN/molecular barrier under an applied voltage VSD, and we observe molecular electroluminescence from an excited singlet state with an emitted photon energy hν > eVSD, indicating upconversion by energies up to ∼1 eV. We show that tunneling electrons excite embedded molecules into singlet states in a two-step process via an intermediate triplet state through inelastic scattering and also observe direct emission from the triplet state. These heterostructures provide a solid-state device in which spin-triplet states, which cannot be generated by optical transitions, can be controllably excited and provide a new route to investigate the physics, chemistry, and quantum spin-based applications of triplet generation, emission, and molecular photon upconversion.

Theoretical Study of the Light-Induced Spin Crossover Mechanism in [Fe(mtz)6]2+ and [Fe(phen)3]2.

The deactivation pathway of the light-induced spin crossover process in two Fe(II) complexes has been studied by combining density functional theory calculations for the geometries and the normal vibrational modes and highly correlated wave function methods for the energies and spin-orbit coupling effects. For the two systems considered, the mechanism of the photoinduced conversion from the low-spin singlet to the high-spin quintet state implies two intersystem crossings through intermediate triplet states. However, for the [Fe(mtz)6]2+ complex, the process occurs within few picoseconds and involves uniquely metal-centered electronic states, whereas for the [Fe(phen)3]2+ system the deactivation channel involves both metal to ligand charge transfer and metal-centered states and takes place in a femtosecond time scale.

Climbing up the Ladder: Intermediate Triplet States Promote the Reverse Intersystem Crossing in the Efficient TADF Emitter ACRSA

It is usually assumed that the reverse intersystem crossing (rISC) in organic TADF emitters occurs at the microsecond time scale. However, the underlying dynamics can be enhanced in the presence of low-lying nπ* states as stipulated by the El-Sayed rule. Here, we report a theoretical study of the short-time quantum dynamics of the ACRSA dye in different solvents following the electrical excitation. The wave packet propagation in multidimensional excited state potentials reveals the intersystem crossing (ISC) rate to be one or 2 orders of magnitude faster compared to the rISC process depending on the environment. Upon the molecular geometry distortions, the system rapidly converts the initial vibrational density to the valence excited-state potentials with subsequent change of spin state owing to spin–orbit interaction. We show that a local triplet ππ* state, residing on the acceptor moiety, mediates the nonradiative passage from triplet to singlet, thus increasing the quantum yield of light harvesting in ...

Optimized Finite Difference Method for the Full-Potential XANES Simulations: Application to Molecular Adsorption Geometries in MOFs and Metal-Ligand Intersystem Crossing Transients.

Accurate modeling of the X-ray absorption near-edge spectra (XANES) is required to unravel the local structure of metal sites in complex systems and their structural changes upon chemical or light stimuli. Two relevant examples are reported here concerning the following: (i) the effect of molecular adsorption on 3d metals hosted inside metal-organic frameworks and (ii) light induced dynamics of spin crossover in metal-organic complexes. In both cases, the amount of structural models for simulation can reach a hundred, depending on the number of structural parameters. Thus, the choice of an accurate but computationally demanding finite difference method for the ab initio X-ray absorption simulations severely restricts the range of molecular systems that can be analyzed by personal computers. Employing the FDMNES code [Phys. Rev. B, 2001, 63, 125120] we show that this problem can be handled if a proper diagonalization scheme is applied. Due to the use of dedicated solvers for sparse matrices, the calculation time was reduced by more than 1 order of magnitude compared to the standard Gaussian method, while the amount of required RAM was halved. Ni K-edge XANES simulations performed by the accelerated version of the code allowed analyzing the coordination geometry of CO and NO on the Ni active sites in CPO-27-Ni MOF. The Ni-CO configuration was found to be linear, while Ni-NO was bent by almost 90°. Modeling of the Fe K-edge XANES of photoexcited aqueous [Fe(bpy)3](2+) with a 100 ps delay we identified the Fe-N distance elongation and bipyridine rotation upon transition from the initial low-spin to the final high-spin state. Subsequently, the X-ray absorption spectrum for the intermediate triplet state with expected 100 fs lifetime was theoretically predicted.

Photophysical properties and study of the singlet oxygen generation of tetraphenylporphyrinato palladium(II) complexes

Three differently substituted Pd-tetraphenyl-porphyrins complexes were synthesized and their X-ray crystal and photophysical properties were studied. All compounds display intense phosphorescence in de-aerated solutions at room temperature with decay time in the μs range. An efficient spin transfer to oxygen in aerated solutions was monitored using singlet oxygen (1 O 2) luminescence at 1275 nm. For all compounds, the quantum efficiency for generating singlet oxygen was found to be close to unity. Interesting differences in the pathways from the excited singlet state to singlet oxygen via the intermediate triplet state were observed. The excitation spectra obtained while recording the phosphorescence at 1275 nm closely matched the absorption spectra with two bands: the Soret (or B) band near 410–420 nm and a split Q-band at 510–520 nm. Noticeable differences in the B/Q intensity ratio between the absorption and excitation spectra (for both phosphorescence and singlet oxygen luminescence) were determined, indicating different Sx → T1 pathways.

Ultrafast Relaxation Dynamics of Osmium–Polypyridine Complexes in Solution

We present steady-state absorption and emission spectroscopy and femtosecond broadband photoluminescence up-conversion spectroscopy studies of the electronic relaxation of Os(dmbp)3 (Os1) and Os(bpy)2(dpp) (Os2) in ethanol, where dmbp is 4,4′-dimethyl-2,2′-biypridine, bpy is 2,2′-biypridine, and dpp is 2,3-dipyridyl pyrazine. In both cases, the steady-state phosphorescence is due to the lowest 3MLCT state, whose quantum yield we estimate to be ≤5.0 × 10–3. For Os1, the steady-state phosphorescence lifetime is 25 ns. In both complexes, the photoluminescence excitation spectra map the absorption spectrum, pointing to an excitation wavelength-independent quantum yield. The ultrafast studies revealed a short-lived (≤100 fs) fluorescence, which stems from the lowest singlet metal-to-ligand-charge-transfer (1MLCT) state and decays by intersystem crossing to the manifold of 3MLCT states. In addition, Os1 exhibits a 50 ps lived emission from an intermediate triplet state at an energy ∼2000 cm–1 above that of the long-lived (25 ns) phosphorescence. In Os2, the 1MLCT–3MLCT intersystem crossing is faster than that in Os1, and no emission from triplet states is observed other than the lowest one. These observations are attributed to a higher density of states or a smaller energy spacing between them compared with Os1. They highlight the importance of the energetics on the rate of intersystem crossing.

Theoretical study of the absorption and nonradiative deactivation of 1-nitronaphthalene in the low-lying singlet and triplet excited states including methanol and ethanol solvent effects

The photophysics of the 1-nitronaphthalene molecular system, after the absorption transition to the first singlet excited state, is theoretically studied for investigating the ultrafast multiplicity change to the triplet manifold. The consecutive transient absorption spectra experimentally observed in this molecular system are also studied. To identify the electronic states involved in the nonradiative decay, the minimum energy path of the first singlet excited state is obtained using the complete active space self-consistent field//configurational second-order perturbation approach. A near degeneracy region was found between the first singlet and the second triplet excited states with large spin-orbit coupling between them. The intersystem crossing rate was also evaluated. To support the proposed deactivation model the transient absorption spectra observed in the experiments were also considered. For this, computer simulations using sequential quantum mechanic-molecular mechanic methodology was used to consider the solvent effect in the ground and excited states for proper comparison with the experimental results. The absorption transitions from the second triplet excited state in the relaxed geometry permit to describe the transient absorption band experimentally observed around 200 fs after the absorption transition. This indicates that the T(2) electronic state is populated through the intersystem crossing presented here. The two transient absorption bands experimentally observed between 2 and 45 ps after the absorption transition are described here as the T(1)→T(3) and T(1)→T(5) transitions, supporting that the intermediate triplet state (T(2)) decays by internal conversion to T(1).

Photochemistry of acetylacetone isolated in parahydrogen matrices upon 266 nm irradiation

The photochemistry of the chelated enol form of acetylacetone (AcAc) was investigated by UV excitation of the S(2) state at 266 nm in parahydrogen matrices, complemented by experiments in neon and normal hydrogen matrices. Infrared (IR) spectroscopy, combined with theoretical calculations, was used to identify the photoproducts. Isomerization towards various non-chelated forms (no intramolecular H-bond) of AcAc is the dominant channel whereas fragmentation is very minor. The isomerization kinetics is monitored by IR spectroscopy. Among the seven non-chelated conformers of AcAc, only three are formed in parahydrogen matrices, whereas four are observed in normal hydrogen matrices. This difference suggests that an active tunnelling process between conformers occurs in parahydrogen but is quenched in normal hydrogen where guest-host interactions are stronger. Fragmentation and isomerization of excited AcAc are discussed in the light of these new data. The role of the intermediate triplet state in the S(2)→ S(0) relaxation is confirmed, as the importance of phonons in the condensed phase.

On Electron Spin Polarization Created in the Excited Triplet State of Accessory Chlorophyll via Photoinduced Charge-Recombination of the Photosystem II Reaction Center

We present a theoretical approach to investigate the electron spin polarization (ESP) of the excited triplet state that has been detected using the time-resolved electron paramagnetic resonance (TREPR) method in the photosystem II center of the plants. We show, using the stochastic Liouville equation, that the ESP pattern created in the accessory chlorophyll (ChlaccD1) which reside near the PD1 chlorophyll of the active branch is explained by one-step, concerted double electron transfer model, initiating from the singlet–triplet conversion of the light-induced charge-separated state composed of PD1 radical cation and pheophytin radical anion. We also considered the sequential ESP transfer model via the triplet charge-recombination (CR) and the triplet–triplet energy transfer processes. It has been clearly shown that the ESP created in the 3ChlaccD1* is dependent on the rate constant (k TT) of the triplet–triplet energy transfer from the intermediate triplet state created by the CR. Also we show that the relative orientation of the principal axes of the spin dipolar interaction in the intermediate triplet state (3PD1*, as an example) may play a role in the ESP pattern, when the k TT is smaller than the angular frequency of the Zeeman energy. We have theoretically shown that the TREPR measurement of the ESP is very powerful to investigate the primary chemical process and to characterize the intermediate as a signature of the stepwise ESP transfer.

Helical Aromatic Oligoamide Foldamers as Organizational Scaffolds for Photoinduced Charge Transfer

Here we report the synthesis and characterization of four quinoline-derived foldamers with increasing oligomeric length; dimer O2P, tetramer O4P, pentamer O5P, and nonamer O9P functionalized with on one end an oligo(p-phenylene vinylene) (OPV) and on the other end a perylene bisimide (PB) chromophore. (1)H NMR confirms the formation of the expected folded structures in both toluene and chloroform solution. The structural predictability and rigidity of the oligomeric series enabled us to investigate the effect of a helical bridge and chromophore position on the photoinduced processes in the electron OPV-PB donor-acceptor pair in chloroform and toluene. The helical properties of the bridge ensured that the chromophore separation distance through space is different from the separation distance through the bridge. For all foldamer-solvent combinations studied, excitation of either OPV or PB results in nearly quantitative quenching of the fluorescence indicating a fast charge separation reaction between the OPV and PB. Femtosecond photoinduced absorption measurements confirmed the fast formation of a charge-separated state. The recombination reaction involves a combination of direct decay to the ground state and the formation of an intermediate triplet state, with their balance depending on the foldamer-solvent combination. Molecular orbital calculations rationalize the fast photoinduced charge separation, by revealing that the bridging foldamer mediates the charge transfer from donor to acceptor via the superexchange mechanism. Remarkably low attenuation factors (beta(CS) approximately 10(-2) A(-1)) were obtained using either through space or through bridge separation distance. However, in these calculations only three of the four foldamers show the expected linear behavior between the logarithm of the charge separation rate constant and the distance between the chromophores. The combined results show when a helical bridge is separating the charge transfer couple, hampering the usefulness of a uniform description of the charge-separation phenomena.

Relaxation in the Triplet Manifold of 1-Nitronaphthalene Observed by Transient Absorption Spectroscopy

Previous phosphorescence and triplet quantum yield determinations indicate that the primary photophysical channel for 1-nitronaphthalene is the formation of its lowest energy triplet state. Also, previous direct measurements of the decay of the fluorescence from this compound indicated that the crossing between the singlet and triplet manifolds is ultrafast (sub-100 fs). In this contribution we present a sub-picosecond transient absorption study of the relaxation of photoexcited 1-nitronaphthalene in methanol and other solvents. Our measurements reveal the time scale in which the fully relaxed T(1) state is formed. We have observed that the spectral evolution associated with this process takes place in time scales from one to a few tens of picoseconds. Specifically, the appearance of the absorption spectrum of T(1) in the visible region is accompanied by the decay of transient signals at wavelengths below 400 nm. Since the fluorescence lifetime of this compound is sub-100 fs, we assigned the picoseconds decaying signals below 400 nm to an intermediate triplet state which acts as a receiver state in the intersystem crossing step and from which the T(1) population accumulates. From the details of the spectral evolution and the effects of different solvents, we also conclude that T(1) formation and vibrational cooling within this state occur in similar time scales of between 1 and 16 ps. Mainly, our results provide direct evidence in support of the participation of an upper triplet state in the mechanism for intersystem crossing in this molecule. This is considered to be common in the photophysics of several nitrated polycyclic aromatic compounds and the most determinant feature of their primary photochemistry.

Light-Induced Excited-State Spin Trapping in Tetrazole-Based Spin Crossover Systems

Ab initio calculations have been performed on Fe (II) (tz) 6 (tz = 1- H-tetrazole) to establish the variation of the energy of the electronic states relevant to (reverse) light-induced excited-state spin trapping (LIESST) as function of the Fe-ligand distance. Equilibrium distances and absorption energies are correctly reproduced. The deactivation of the excited singlet is proposed to occur in the Franck-Condon region through overlap of vibrational states with an intermediate triplet state or an intersystem crossing along an asymmetric vibrational mode. This is followed by an intersystem crossing with the quintet state. Reverse LIESST involves a quintet-triplet and a triplet-singlet intersystem crossing around the equilibrium distance of the high-spin state. The influence of the transition metal is studied by changing Fe (II) for Co (II), Co (III), and Fe (III). The calculated curves for Fe (III) show remarkable similarity with Fe (II), indicating that the LIESST mechanism is based on the same electronic conversions in both systems.

Photo-Induced State Conversion Mechanism of an Optically Durable Molecular Memory with Controlled Hydrogen Bonding: A Spin–Orbit CI Study of [{Co(2,2′-biimidazole)(C6H4O2)(NH3)2}2

Abstract The possibility for photon-mode memory writing of [{Co(Hbim)(C6H4O2)(NH3)2}2] (Hbim = 2,2′-biimidazole) is examined from results obtained by spin–orbit complete active space configuration interaction (SO-CASCI) calculations. The spin-crossover phenomenon in the proton–electron-coupled complex monomer [Co(Hbim)(C6H4O2)(NH3)2] is a key process in memory writing to the complex dimer. We found a possibility of an efficient spin-crossover in the monomer. UV (ca. 300 nm) irradiation of the monomer 1A′ state leads to a spin-crossover into the high-spin 5A′ state. The back photoreaction from the 5A′ to 1A′ state is also possible with vis (ca. 600–700 nm) excitation. In these spin-crossover and back spin-crossover reactions, the intermediate triplet states, 13A″ and 23A″, are found to participate.

Controlled Release of Perfumery Aldehydes and Ketones by Norrish Type-II Photofragmentation of -Keto Esters in Undegassed Solution

Alkyl or aryl α-keto esters of primary or secondary alcohols decompose upon irradiation at 350 – 370 nm from the intermediate triplet state into aldehydes or ketones in polar, as well as apolar solvents. The use of these keto esters as delivery systems for the controlled release of perfumery aldehydes and ketones was investigated by photoirradiation in the presence of oxygen with a Xe or UV lamp, as well as outdoor sunlight. Systematic GC/MS analysis of the irradiated solutions showed that, under these conditions, the desired Norrish type-II fragmentation of the ester side chain is the predominant reaction pathway in most of the cases. γ-H Abstraction from the alkyl side chain of alkyl keto esters, as well as an intramolecular Paternò-Büchi reaction or epoxidation of the alkene function in different citronellyl α-keto esters were identified as the most important side reactions. Some of the experimental findings have been rationalized on the basis of ab initio and density-functional calculations. (Cyclohexyl)oxoacetates and oxo(phenyl)acetates were found to be the most suitable precursors for the desired perfumery applications.

Exergonic electron-transfer reaction between [60]fullerene anion and carbazole cation

The fluorscence of C60 is observed in the reaction of C60 radical anion and carbazole cation; this is interpreted in terms of the Marcus free energy diagram indicating a C60 intermediate triplet state.

Multiphoton Ionization and Fragmentation Process of Benzene at 193 nm Involving Ionization of Neutral Fragments

Abstract Laser flux dependences of multiphoton ionization (MPI) of benzene and benzene-d6 were measured both in gaseous phase and in molecular beam with ArF (193 nm) and KrF (248 nm) excimer lasers. Several different behaviors were observed at 193 nm in comparison with the MPI at 248 nm : An unexpected enhancement of the MPI efficiency at high laser power involving three-photon ionization process, an unusually small ratio of (C4D4+• + C4D3+ + C4D2+•)/C3D3+ (1—2) compared with the ratio expected from the breakdown diagram (3—5) and a very large isotope effect on the relative intensity of C4H4+•/C4D4+• (0.65). To interpret these phenomena, we propose a new MPI and fragmentation scheme at 193 nm involving two-photon ionization of benzene via the intermediate triplet state(s), possibly T1 and/or T2, and one-photon ionization of C3H3 (C3D3) radical formed via two-photon dissociation of benzene. The observed laser flux dependences of MPI efficiency and intensities of several major ions were analyzed with the rate equation model derived from the proposed scheme. As a result, more than 80% of the observed C3D3+ ions were found to be produced from the C3D3 radical. The analyses also suggest the existence of at least two isomeric species for the C3D3 (C3H3) radical and the C4D4+• (C4H4+•) ions, respectively.

Study of singlet-triplet coupling in glyoxal by level anticrossing spectroscopy. VII. Complete assignment of the 0 0, 8 1, 6 17 1 and 4 1 anticrossing spectra

In paper VI of this series, we have made the statistical analysis of the singlet-triplet coupling matrix elements for ten N S = 0 singlet vibrational levels of glyoxal, without determining the triplet quantum numbers. In this paper we present the complete assignment (triplet rotational quantum numbers and vibrational symmetry) of each anticrossing observed from four singlet vibrational levels: 0 0, 8 1, 6 17 1 and 4 1 (which give respectively 36, 76, 155 and 145 anticrossings by 0 to 7.5 T scans). Therefore we determine the zero-field energy origin of most of the triplet vibrational levels which are located within 7 cm −1 of the four singlet levels studied. Two kinds of selection rules are found: the first stemming from direct vibronic spin-orbit interactions and the second from indirect ones (involving an intermediate triplet state). About half of the anticrossings are due to direct vibronic spin-orbit interactions, the mean value of their matrix elements is more than ten times larger than indirect matrix elements and thus are dominant in ϱ〈 V st〉. In conclusion, we confirm that ISC in glyoxal is governed by direct vibronic spin-orbit interactions.

ChemInform Abstract: MECHANISTIC STUDIES OF THE PHOTODECOMPOSITION OF ARYLMETHYL SULFONES IN HOMOGENEOUS AND MICELLAR SOLUTIONS

AbstractDer Mechanismus der Photozersetzung der Arylsulfone (I) ‐(III) in homogener Lösung (‐Aceto‐nitril; direkt oder photosensibilisiert) bzw. in micellarer Lösung (Na‐dodecylsulfat) wird unter Anwendung der "steady‐state"‐ und der "time‐resolved"‐Technik untersucht.

Mechanistic studies of the photodecomposition of arylmethyl sulfones in homogeneous and micellar solutions

The mechanism of photodecomposition of aryl sulfones has been investigated by using both steady-state and time-resolved techniques. Direct evidence for radical and triplet-state intermediates is provided. A correlation is found between the dynamics of reaction of the intermediate triplet states with the stabilities of the intermediate radicals.

Static and Dynamic Properties of Quinoxalines in the Phosphorescent Triplet State from Optically Detected Magnetic Resonance

Abstract The static and dynamic properties of the phosphorescent triplet states of quinoxaline and 2,3-disubstituted derivatives in hexane have been studied at 1.4 K using the methods of optically detected magnetic resonance. In addition, the absorption and phosphorescence excitation spectra have also been measured to attain the information about the low lying excited states. A systematic comparison has been made for the purpose of examining the effect of substitution. A significant enhancement of the pumping rates for the y spin sublevel observed in the chlorinesubstituted derivatives is concluded to be due to the additional indirect process of intersystem crossing which occurs from S1 of n,π* to T1 passing through the intermediate triplet state of π,π*. A detailed discussion on the observed phosphorescence spectra originating from the three sublevels has achieved a satisfactory success in understanding the mechanisms responsible for the radiative decay processes. It is concluded that the mechanisms due to vibronic coupling with closely lying n,π* triplet states are of little importance in the systems studies here.