Triplet Energy Transfer Mechanism Research Articles

Triplet Energy Transfer Mechanism in Copper Photocatalytic N- and O-Methylation.

Methylation reactions are chemically simple but challenging to perform under mild and non-toxic conditions. A photochemical energy transfer strategy was merged with copper catalysis to enable fast reaction times of minutes and broad applicability to N-heterocycles, (hetero-)aromatic carboxylic acids, and drug-like molecules in high yields and good functional group tolerance. Detailed mechanistic investigations, using kinetic analysis, aprotic MS, UV/Vis, and luminescence quenching experiments revealed a triplet-triplet energy transfer mechanism between hypervalent iodine(III) reagents and readily available photosensitizers.

Green Light Promoted Iridium(III)/Copper(I)-Catalyzed Addition of Alkynes to Aziridinoquinoxalines Through the Intermediacy of Azomethine Ylides.

This manuscript describes the development of alkyne addition to the aziridine moiety of aziridinoquinoxalines using dual Ir(III)/Cu(I) catalytic system under green light-emitting diode (LED) photolysis (λmax =525 nm). This mild method features high levels of chemo- and regioselectivity and was used to generate 30 highly functionalized substituted dihydroquinoxalines in 36-98 % yield. This transformation was also carried asymmetrically using phthalazinamine-based chiral ligand to provide 9 chiral addition products in 96 : 4 to 86 : 14 e.r. The experimental and quantum chemical explorations of this reaction suggest a mechanism that involves Ir(III)-catalyzed triplet energy transfer followed by a ring-opening reaction ultimately leading to the formation of azomethine ylide intermediates. These azomethine intermediates undergo sequential protonation/copper(I) acetylide addition to provide the products.

Green Light Promoted Iridium(III)/Copper(I)‐Catalyzed Addition of Alkynes to Aziridinoquinoxalines Through the Intermediacy of Azomethine Ylides

AbstractThis manuscript describes the development of alkyne addition to the aziridine moiety of aziridinoquinoxalines using dual Ir(III)/Cu(I) catalytic system under green light‐emitting diode (LED) photolysis (λmax=525 nm). This mild method features high levels of chemo‐ and regioselectivity and was used to generate 30 highly functionalized substituted dihydroquinoxalines in 36–98 % yield. This transformation was also carried asymmetrically using phthalazinamine‐based chiral ligand to provide 9 chiral addition products in 96 : 4 to 86 : 14 e.r. The experimental and quantum chemical explorations of this reaction suggest a mechanism that involves Ir(III)‐catalyzed triplet energy transfer followed by a ring‐opening reaction ultimately leading to the formation of azomethine ylide intermediates. These azomethine intermediates undergo sequential protonation/copper(I) acetylide addition to provide the products.

Ultrafast spectroscopy of size-dependent hole-transfer-mediated triplet energy transfer in a quantum dot-molecule complex

Triplet energy transfer (TET) from quantum dots to molecules has attracted tremendous attention in the last decade because the long-lived triplet state of molecules benefits various photophysical or photochemical processes and quantum dots (QDs), as the triplet sensitizer, provide prominent optical absorption and emission tunable through flexible size and composition control. Although several dynamical models for some certain QD-molecule complexes have been proposed, the TET mechanism in such complexes is still incomplete due to intricate energy level configuration. More specifics of TET being revealed is advantageous for its further applications. In this paper, a series of QD-molecule complexes composed of CdSe QDs with different diameters and 5-tetracene carboxylic acid (TCA) are assembled. The precise size control of CdSe QDs is realized by combining the transmission electron microscope and steady-state spectra. All CdSe-TCA complexes display expected Type-II like band alignment, which favors the hole-transfer-mediated TET confirmed by a series of spectral measurements. On the surface, ultrafast spectroscopy shows the charge dynamics, specifically the stepwise hole and electron transfer during TET, slow down with increasing size of the CdSe QDs. However, comprehensive analysis of the experimental results reveals that the hole transfer and the following electron transfer essentially depend on the energy level difference and wavefunction overlap between the QD donor and molecule acceptor rather than simply the QD size. The conclusions can facilitate our understanding of the TET mechanism in a QD-molecule complex and help optimize the performance of TET-related photoelectronic devices designed based on size control.

Energy-transfer in CsPbBr3 nanocrystals: Sensitization of porphyrin triplets

Sensitizing molecular triplets by colloidal nanocrystals via triplet energy transfer is important for applications such as upconversion or organic synthesis. Typically two step triplet energy transfer (TET) are included in these applications: firstly the triplet energy stored in nanocrystals are extracted into surface ligands, and then the ligands further transfer triplet energy into molecules in bulk solution. Here we report one-step TET application from CsPbBr3 perovskite nanocrystals (NCs) to surface-anchored metalloporphyrin derivative molecules (MP). Compared to conventional two-step TET, the one-step TET mechanism possess lower energy loss and higher TET efficiency which is more generally implementable. In this scheme, photoexcitation of CsPbBr3 NCs leads to the sensitization of MP ligands triplets which efficiently emit phosphorescence. The enhanced light absorption of MP ligands and down-shifted photon emission can be useful in devices such as luminescent solar concentrators.

Unambiguous spectral characterization on triplet energy transfer from quantum dots mediated by hole transfer competing with other carrier dynamics.

Triplet generation by quantum dots (QDs)-sensitized molecules emerges great potential in many applications. However, the mechanism of triplet energy transfer (TET) is still fuzzy especially due to the complicated energy level alignment of QDs and molecules or trap states in QDs. Here, CdSe QDs and 5-tetracene carboxylic acid (TCA) molecules are selected as the triplet donor and acceptor, respectively, to form a TET system. By tuning the band gap of CdSe, the CdSe-TCA complex is exactly designed to present a Type-II like alignment of relative energetics. Coupling the transient absorption and time-resolved fluorescence spectra, all carrier dynamics is distinctly elucidated. Quantitative analysis demonstrates that hole transfer persisting for ∼ 2 ps outcompetes all other carrier dynamics such as electron trapping (∼100 ps level), charge recombination (∼ 5 ns) and the so-called "back transfer charge recombination" (∼50 ns), and thus leads to a hole-transfer-mediated TET process. The low TET yield (∼34.0%) ascribed to electron behavior can be further improved if electron trapping and charge recombination are efficiently suppressed. The observation on distinguishable carrier dynamics attributed to legitimate design of energy level alignment facilitates a better understanding of the TET mechanism from QDs to molecules as well as further development of photoelectronic devices based on such TET systems.

Triplet energy transfer between inorganic nanocrystals and organic molecules

Merging the light- harvesting and emitting capability of inorganic materials with the versatile excited-state properties of organic molecules leveraging triplet energy transfer (TET) has recently become an important design strategy for light-driven applications ranging from energy conversion, biomedicine, to photoredox catalysis, and understanding the rules that govern such phenomena is essential. Recent years have witnessed some breakthroughs of TET as well as significant advances for regulating this photochemical process, but it still needs more integrated research at the center. In particular, efficient TET between semiconductor nanocrystals (or quantum dots) and organic molecules, which combines nanoscale size effect of the former and molecular versatility of the latter, has been demonstrated as a promising enabler for breaking the Shockley-Queisser limit in solar energy conversion, controlling photon emission, and driving photochemical reactions in the years to come. Here we review the fundamentals of TET mechanism and some of its emerging applications, with special focus on recent progress towards the understanding and application of TET in nanocrystal-molecule hybrids and highlight some challenges for the field. Work is currently aimed at providing insights in controlling TET dynamics with a view to making the process as efficient as possible according to expected designs.

Triplet Energy Transfer Mechanism of Ternary Organic Hybrid Thin Films of PFO/MEH-PPV/CsPbBr3 Perovskite Quantum Dots.

The triplet energy transfer mechanism of novel poly(9,9-di-n-octylflourenyl-2,7-diyl) (PFO)/poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/CsPbBr3 perovskite quantum dot (PQD) hybrid thin films was comprehensively investigated. The concentrations of PFO and MEH-PPV in all the specimens were fixed, while the PQD content was varied with various weight ratios and premixed by a solution blending method before it was spin-coated onto glass substrates. The triplet non-radiative Förster resonance energy transfers (FRETs) in the PFO/MEH-PPV/PQDs ternary blend, the dual FRET from PFO to both PQDs and MEH-PPV, and the secondary FRET from PQDs to MEH-PPV were observed. The values of the Förster radius (Ro) of FRET from PFO to MEH-PPV in the presence of various PQD contents (Case I) increased from 92.3 to 104.7 Å, and they decreased gradually from 68.0 to 39.5 Å for FRET from PFO to PQDs in the presence of MEH-PPV (Case II). These Ro values in both cases confirmed the dominance of FRET in ternary hybrid thin films. Upon increasing the PQD content, the distance between the donor and acceptor molecules (RDA) and the conjugation length (Aπ) in both cases gradually decreased. The small values of Ro, RDA, and Aπ with a decrease in the energy transfer lifetime (τET) due to an increase in the PQD contents in both Cases I and II confirmed the efficient FRET in the hybrid. To prevent intermolecular transfer in PFO, the concentrations of MEH-PPV (Case I) and PQDs (Case II) should be decreased to a range of 0.57–0.39 mM and increased in the range of 1.42–7.25 mM.

Triplet Energy Transfer from CsPbBr3 Nanocrystals Enabled by Quantum Confinement.

The spectral properties of lead halide perovskite nanocrystals (NCs) can be engineered by tuning either their sizes via the quantum confinement effect or their compositions using anion and/or cation exchange. To date, the latter is more frequently adopted, primarily because of the ease of ion exchange for lead halide perovskites, making the quantum confinement effect seemingly redundant for perovskite NCs. Here we report that quantum confinement is required for triplet energy transfer (TET) from perovskite NCs to polycyclic aromatic hydrocarbons (PAHs). Static and transient spectroscopy measurements on CsPbBr3 NC-pyrene hybrids showed that efficient TET occurred only for small-sized, quantum-confined CsPbBr3 NCs. The influences of the size-dependent driving force and spectral overlap on the TET rate were found to be negligible. Instead, the TET rate scaled linearly with carrier probability density at the NC surface, consistent with a Dexter-type TET mechanism requiring wave function exchange between the NC donors and pyrene acceptors. Efficient TET funnels the excitation energy generated in strongly light-absorbing perovskite NCs into long-lived triplets in PAHs, which may find broad applications such as photon upconversion and photoredox catalysis.

Heterogeneous photocatayltic deperoxidation with UV and visible light

AbstractA physical‐organic study is described on the photodecomposition of dicumyl peroxide co‐adsorbed with the sensitizers 4,4′‐dimethylbenzil or chlorin e6 on fumed (nonporous) silica. Dicumyl peroxide was decomposed by the heterogeneous photosensitization and monitored by the desorption of products acetophenone, 2‐phenylpropan‐2‐ol, and α‐methylstyrene using proton nuclear magnetic resonance and gas chromatography‐mass spectrometry. Dicumyl peroxide and sensitizer were co‐adsorbed on silica in 1:4 up to 200:1 ratios, with high peroxide destabilization occurring in ratios of about 10:1. This increased photodecomposition corresponded to sensitizer‐peroxide distances of 6 to 9 Å. A Dexter triplet energy transfer mechanism is proposed that explains the short sensitizer and peroxide separation distances for higher peroxide OO bond homolysis efficiencies on silica. This biphasic (gas/solid) system can thus serve both to destabilize and stabilize a peroxide, which may be of practical use for the delivery of alkoxy radicals for bacterial disinfection.

A Bio-Inspired, Catalytic E → Z Isomerization of Activated Olefins.

Herein, Nature's flavin-mediated activation of complex (poly)enes has been translated to a small molecule paradigm culminating in a highly (Z)-selective, catalytic isomerization of activated olefins using (-)-riboflavin (up to 99:1 Z/E). In contrast to the prominent Z → E isomerization of the natural system, it was possible to invert the directionality of the isomerization (E → Z) by simultaneously truncating the retinal scaffold, and introducing a third olefin substituent to augment A1,3-strain upon isomerization. Consequently, conjugation is reduced in the product chromophore leading to a substrate/product combination with discrete photophysical signatures. The operationally simple isomerization protocol has been applied to a variety of enone-derived substrates and showcased in the preparation of the medically relevant 4-substituted coumarin scaffold. A correlation of sensitizer triplet energy (ET) and reaction efficiency, together with the study of additive effects and mechanistic probes, is consistent with a triplet energy transfer mechanism.

Photophysics and Photochemistry of z‐Chlorprothixene in Acetonitrile†

Chlorprothixene (CPTX, Taractan) is a low potency antipsychotic mainly used for the treatment of psychotic disorders (e.g. schizophrenia) and acute mania occurring as part of bipolar disorders. As in the case of other numerous drugs used in the treatment of psychiatric disorders, CPTX presents geometric isomerism. Therefore, in vitro irradiation induces a rapid Z/E isomerization, which can affect its pharmacokinetic properties. This photoisomerization is not dependent on the oxygen concentration. The Z/E quantum yields determined for zCPTX in acetonitrile are 0.22 and 0.21 in anaerobic and aerobic environments, respectively. In the presence of water, both isomers decompose to produce 2-chlorothioxanthone (CTX) after prolonged irradiation. This process strongly depends on the water concentration and the irradiation time, i.e. it is autocatalyzed by the CTX through a triplet-energy transfer mechanism. The protonation state of the terminal amino group, on the other hand, has no effect on the isomerization process, but inhibits the formation of CTX. These results indicate that the phototoxicity of zCPTX is somehow affected by the formation of CTX.

Donor/acceptor interactions in systematically modified Ru(II)-Os(II) oligonucleotides.

Donor/acceptor (D/A) interactions are studied in a series of doubly modified 19-mer DNA duplexes. An ethynyl-linked Ru(II) donor nucleoside is maintained at the 5' terminus of each duplex, while an ethynyl-linked Os(II) nucleoside, placed on the complementary strands, is systematically moved toward the other terminus in three base pair increments. The steady-state Ru(II)-based luminescence quenching decreases from 90% at the shortest separation of 16 A (3 base pairs) to approximately 11% at the largest separation of 61 A (18 base pairs). Time-resolved experiments show a similar trend for the Ru(II) excited-state lifetime, and the decrease in the averaged excited-state lifetime for each duplex is linearly correlated with the fraction quenched obtained by steady-state measurements. Analysis according to the Förster dipole-dipole energy transfer mechanism shows a reasonable agreement. Deviation from idealized behavior is primarily attributed to uncertainty in the orientation factor, kappa(2). Analyzing D/A interactions in an analogous series of doubly modified oligonucleotides, where the ethynyl-linked Ru(II) center is replaced with a saturated two-carbon linked complex, yields an excellent correlation with the Förster mechanism. As this simple change partially relaxes the rigid geometry of the donor chromophore, these results suggest that the deviation from idealized Förster behavior observed for the duplexes containing the rigidly held Ru(II) center originates, at least partially, from ambiguities in the orientation factor. Surprisingly, analyzing both quenching data sets according to the Dexter mechanism also shows an excellent correlation. Although this can be interpreted as strong evidence for a Dexter triplet energy transfer mechanism, it does not imply that this electron exchange mechanism is operative in these D/A duplexes. Rather, it suggests that systems that transfer energy via the Förster mechanism can under certain circumstances exhibit Dexter-like "behavior", thus illustrating the danger of imposing a single physical model to describe D/A interactions in such complex systems. While we conclude that the Förster dipole-dipole energy transfer mechanism is the dominant pathway for D/A interactions in these modified oligonucleotides, a minor contribution from the Dexter electron exchange mechanism at short distances is likely. This complex behavior distinguishes DNA-bridged Ru(II)/Os(II) dyads from their corresponding low molecular-weight and covalently attached counterparts.

Toward Photoswitchable Dendritic Hosts. Interaction between Azobenzene-Functionalized Dendrimers and Eosin

Two poly(propyleneimine) dendrimers bearing up to 32 photoisomerizable azobenzene groups in the periphery have been used as potential hosts for eosin Y (2‘,4‘,5‘,7‘-tetrabromofluorescein dianion). The all-E azobenzene dendrimers can be reversibly switched to their Z form by light excitation. Both the E and Z forms of the dendrimers quench the eosin fluorescence by a static mechanism. The quenching is most likely due to an electron-transfer reaction between the singlet excited state of eosin and the tertiary amine units present along the branches of the dendrimers. Quenching by the Z form of the dendrimers is more efficient than quenching by the E form. The E → Z and Z → E photoisomerization reactions of the azobenzene units of the dendrimers are sensitized by eosin via a triplet−triplet energy transfer mechanism. The results obtained indicate that eosin is hosted by the dendrimers and suggest that the Z forms are more efficient hosts than the E forms.

Kinetic study of the direct and photosensitized photo-isomerization of acetoxyisopiperitenone: solvent effect and evaluation for the photochemical storage of light energy

Abstract The direct and sensitized photo-isomerization of acetoxyisopiperitenone (AIP) were investigated at room temperature (298 K) in solvents of different polarities (cyclohexae, dioxane, acetonitrile, methanol, ethanol, n -propanol, n -butanol). The direct photo-isomerization overall first-order rat constant ( k g ) and quantum yield (φ p ) were found to depend markedly on solvent polarity, with k g values ranging between 0.13 and 0.23 min −1 and φ p values between 0.09 and 0.29. The structures and yields of photoproducts were elucidated by gas chromatography—mass spectrometry. The kinetics of sensitized photo-isomerization of AIP were studied using acetophenone and benzophenone as sensitizers. The existence of a triplet—triplet energy transfer mechanism was demonstrated. The performances of photo-isomerization of AIP for photochemical storage of light energy were evaluated.

Influence of molecular weight and concentration of poly(2‐vinylnaphthalene) on the bulk polymerization of styrene under UV‐irradiation in the presence of a magnetic field

AbstractThe effect of molecular weight and concentration of poly(2‐vinylnaphthalene) (PVN) on the bulk polymerization of styrene initiated by 2,2′‐azoisobutyronitrile (AIBN) was studied under UV‐irradiation in the presence of a magnetic field. It was found that in the suitable range of molecular weights (<1,7 · 103) and concentrations (1,62 to 9, 12 · 10−3 mol/L) of PVN, the molecular weight and molecular weight distribution of polystyrene significantly varied with the change of the external magnetic field. A triplet energy transfer mechanism between PVN and AIBN in the PVN cages was suggested on the basis of the experimental results.

Energy transfer in “covalently linked” and ‘face-to-face” bisporphyrins

The excited-state lifetimes of three covalently linked and one “face-to-face” hybrid zinc and copper bisporphyrins were measured and compared. Their decays are explained by a triplet energy-transfer mechanism through an exchange coupling. The geometrical structure of these bisporphyrins appears to play an important role in the triplet deactivation.

On the triplet energy transfer mechanism in isotopic mixed molecular crystals

The transfer of triplet excitation energy between isotopic traps in benzene isotopic mixed crystals has been investigated using both steady state and pulsed excitation sources. It has been demonstrated for the first time that the excitation–tunneling transfer mechanism [H. Sternlicht, G. C. Nieman, and G. W. Robinson, J. Chem. Phys. 38, 1326 (1963)] predicts the correct dependence of the transfer rate on trap depth. Furthermore, it is shown that the Perrin approximation, which grossly simplifies the treatment of the experimental data, is a reasonable albeit not perfect approximation for this system. A large fraction of the energy transfer is completed during times much shorter than the benzene triplet state lifetime. This is consistent with the strong distance dependence of the transfer rate constant predicted by a tunneling mechanism. Using the Perrin approximation, it has been shown that the use of a linear chain tunneling mechanism predicts the excitation exchange integral to be of the correct magnitude, a conclusion that is also consistent with a strong distance dependence of the transfer rate constant. (Energy can be transferred through 2–3 intervening host molecules.) In this system there is no evidence for the existence of the intermolecular triplet energy relaxation mechanism recently shown to be operative in chemically mixed crystals [H. C. Brenner and C. A. Hutchison, Jr., Chem. Phys. 58, 1328 (1973)].

Triplet energy transfer mechanism in isotopic mixed molecular crystals

The transfer of triplet excitation energy between isotopic traps in benzene isotopic mixed crystals has been investigated using both steady-state and pulsed excitation sources. The prediction of the correct dependence of the transfer rate on trap depth by the excitation-tunneling transfer mechanism was demonstrated. It was shown that the Perrin approximation is a reasonable albeit not perfect approximation for this system. Using this approximation, it has been shown that the use of a linear-chain tunneling mechanism predicts the excitation exchange integral to be the correct magnitude. This conclusion is consistent with a strong distance dependence of the transfer rate constant. (BLM)

Photosensitized Decomposition of Trioxalatocobaltate (III) Ion by Organic Triplet Sensitizers

Abstract Sensitized decomposition of the trioxalatocobaltate(III) ion was studied in deoxygenated aqueous solution at room temperature, using biacetyl as an n-π* triplet sensitizer and 9-carboxyanthracene as a π-π* triplet sensitizer. In the presence of one of these sensitizers the photodecomposition rate of the complex was increased, and the plot of the reciprocal of the sensitization quantum yield as a function of the concentration of the complex gave a straight line for each sensitization system. On the other hand, the complex quenched the phosphorescence without affecting the fluorescence of the sensitizers, indicating a triplet energy transfer mechanism. A photokinetic treatment of the data for both the sensitized photolysis and the quenching process gave a result in line with proposed mechanism for bimolecular triplet energy transfer.