Triplet Energy Acceptors Research Articles

Antioxidative Triplet Excitation Energy Transfer in Bacterial Reaction Center Using a Screened Range Separated Hybrid Functional.

Excess energy absorbed by photosystems (PSs) can result in photoinduced oxidative damage. Transfer of such energy within the core pigments of the reaction center in the form of triplet excitation is important in regulating and preserving the functionality of PSs. In the bacterial reaction center (BRC), the special pair (P) is understood to act as the electron donor in a photoinduced charge transfer process, triggering the charge separation process through the photoactive branch A pigments that experience a higher polarizing environment. At this work, triplet excitation energy transfer (TEET) in BRC is studied using a computational perspective to gain insights into the roles of the dielectric environment and interpigment orientations. We find in agreement with experimental observations that TEET proceeds through branch B. The TEET process toward branch B pigment is found to be significantly faster than the hypothetical process proceeding through branch A pigments with ps and ms time scales, respectively. Our calculations find that conformational differences play a major role in this branch asymmetry in TEET, where the dielectric environment asymmetry plays only a secondary role in directing the TEET to proceed through branch B. We also address TEET processes asserting the role of carotenoid as the final triplet energy acceptor and in a mutant form, where the branch pigments adjacent to P are replaced by bacteriopheophytins. The necessary electronic excitation energies and electronic state couplings are calculated by the recently developed polarization-consistent framework combining a screened range-separated hybrid functional and a polarizable continuum mode. The polarization-consistent potential energy surfaces are used to parametrize the quantum mechanical approach, implementing Fermi's golden rule expression of the TEET rate calculations.

Quasi-Two-Dimensional Perovskite Nanosheets Based on the Triplet Energy Acceptor Molecule with Pure Green Emission Light

Quasi-two-dimensional hybrid perovskites have proved to be promising next-generation semiconductor materials for light-emitting diodes and photovoltaic applications. However, these materials usually contain a mixture of phases that their photoluminescence is not monochrome. Here, we synthesize a series of quasi-two-dimensional organic–inorganic hybrid perovskite nanosheets (NSs) with the triplet energy acceptor molecule naphthalene methylamine (NMA), that is, (NMA)2(MA)⟨n⟩–1Pb⟨n⟩Br3⟨n⟩+1 (NMA = C11H9NH3, ⟨n⟩ = 2,3,4). It was found that with the organic layer, the light color of the NSs is pure green on account of energy transfer from the inorganic layer whose band gap is beyond triplet states of NMA (∼2.6 eV). Also, we demonstrated that the triplet energy transfer lifetime is ∼7 ps for the ⟨n⟩ = 2 and ∼9 ps for the ⟨n⟩ = 3 quasi-two-dimensional hybrid perovskite NSs by spectroscopy.

Intramolecular triplet energy transfer in two-dimensional hybrid perovskite nanosheets

A series of two-dimensional hybrid perovskite nanosheets containing organic triplet energy acceptor, (NEA)2PbBr4−xIx(NEA = C12H11NH3), were synthesized. Steady-state and transient spectra show that nanosheets with the appropriate energy level, the intramolecular triplet energy transfer will occur across the inorganic/organic interface. The first step of triplet energy transfer is the transfer of holes from inorganic part to organic part, and the rate is 1.55×1012s-1 in (NEA)2PbBr4 nanosheets. Then the electron transfer with rate of 5.5×1010s-1 takes place to complete the triplet energy transfer process.

Triplet-triplet annihilation photon-upconversion in hydrophilic media with biorelevant cholesteryl triplet energy acceptors

We report two new biorelevant cholesteryl-based triplet energy acceptors, derivatives of DPA or 9,10-diphenylanthracene (C–DPA and C2–DPA). Using two different triplet sensitizers: QDN (ET ≈ 1.67 eV in PEG200) and PdTPP (ET ≈ 1.78 eV in PEG200), we were able to achieve both endothermic (with QDN) and exothermic (with PdTPP) triplet sensitization of DPA, C–DPA and C2–DPA in hydrophilic PEG200 media. While the maximum rate of triplet energy transfer (TET) was achieved with PdTPP and DPA (kTET = 4.7 × 107 M−1 s−1), for the cholesteryl-based acceptors, we found that the kinetic of the TET process was dependent upon the concentration of the acceptor. For PdTPP/C–DPA pair, the rate of the dynamic triplet energy quenching was kTET = 1.9 × 107 M−1 s−1; however, at higher concentrations of the quencher, the system reached a stationary state due to formation of self-assembled sub-domains of C–DPA that likely slowed the TET process. It was also found that this aggregation of C–DPA in PEG200 led to a 3.5 folds increase in the Ith compared to 133 mW cm2 for DPA. Subsequently, we estimated the ΦUC for these donor/acceptor pairs: QDN/DPA, PdTPP/DPA, and PdTPP/C–DPA. With respect to the estimated threshold intensity (Ith), we found that the quantum yields of TTA-UC were 2 ≤ QYUC≤12%.

Control of Triplet Blinking Using Cyclooctatetraene to Access the Dynamics of Biomolecules at the Single‐Molecule Level

AbstractTo explore the dynamics of biomolecules, tracing the kinetics of photo‐induced chemical reactions via the triplet excited state (T1) of probe molecules offers a timescale that is about 106 times wider than via the singlet excited state (S1). Using cyclooctatetraene (COT) as a triplet energy acceptor and at the same time as a photostabilizer, the triplet–triplet energy transfer (TTET) kinetics governed by oligonucleotide (oligo) dynamics were studied at the single‐molecule level by measuring fluorescence blinking. TTET kinetics measurement allowed us to access the length‐ and sequence‐dependent dynamics of oligos and realize the single‐molecule detection of a model microRNA biomarker. In sharp contrast to the singlet–singlet Förster resonance energy transfer (FRET) that occurs in the 1–10 nm range, TTET requires a Van der Waals contact. The present method is thus a complementary method to FRET and provides direct information on biomolecular dynamics on the μs to ms timescale.

Control of Triplet Blinking Using Cyclooctatetraene to Access the Dynamics of Biomolecules at the Single‐Molecule Level

AbstractTo explore the dynamics of biomolecules, tracing the kinetics of photo‐induced chemical reactions via the triplet excited state (T1) of probe molecules offers a timescale that is about 106 times wider than via the singlet excited state (S1). Using cyclooctatetraene (COT) as a triplet energy acceptor and at the same time as a photostabilizer, the triplet–triplet energy transfer (TTET) kinetics governed by oligonucleotide (oligo) dynamics were studied at the single‐molecule level by measuring fluorescence blinking. TTET kinetics measurement allowed us to access the length‐ and sequence‐dependent dynamics of oligos and realize the single‐molecule detection of a model microRNA biomarker. In sharp contrast to the singlet–singlet Förster resonance energy transfer (FRET) that occurs in the 1–10 nm range, TTET requires a Van der Waals contact. The present method is thus a complementary method to FRET and provides direct information on biomolecular dynamics on the μs to ms timescale.

Simultaneously High Upconversion Efficiency and Large Anti‐Stokes Shift by Using Os(II) Complex Dyad as Triplet Photosensitizer

AbstractTo extend the triplet lifetime of photosensitizer, two new osmium complex dyads, with Os(phen)3 as triplet energy donor and 9,10‐diphenylanthracene (DPA) as energy acceptor, namely Os(phen)3‐DPA and Os(phen)3‐BDPA, are synthesized and characterized. The triplet lifetime of Os(phen)3‐DPA dyad is significantly extended to 1.1 μs by introducing triplet energy acceptor DPA to activate the intramolecular triplet energy transfer from the Os(phen)3 to DPA units. By developing the triplet–triplet annihilation upconversion piars of Os(phen)3‐DPA and 9‐phenyl‐10‐(p‐tolyl)anthracene, the deep‐red light (at 663 nm) is upconverted to the blue‐violet emission (at 415 nm) in 1,2‐dichloroethane solution, and the highest upconversion quantum yield of 9.7% is achieved with a large anti‐Stokes shift of 1.12 eV.

Triplet Energy Transfer from Polymethine Dimers in the Complexes with Cucurbit[8]urils

Effect of cucurbit[8]uril on the efficiency of triplet–triplet energy transfer between the donor dimer and the acceptor monomer or dimer has been studied. The efficiency depends on the rate constant of quenching of donor delayed fluorescence by the triplet energy acceptor, on the lifetime of the donor triplet state in the absence of a quencher, and on the acceptor concentration. Triplet–triplet energy transfer between the 3,3'-diethylthiacarbocyanine dimer (donor) and indodicarbocyanine (acceptor) occurs with a rate constant of 1.5 × 108 L mol–1 s–1 . Replacing the indodicarbocyanine monomer by the thiadicarbocyanine dimer decreases the energy transfer rate constant by a factor of 4.5. In the case of the 3,3'-dimethylthiacarbocyanine dimer as a donor, the quantum yield of energy transfer to the indodicarbocyanine monomer decreases fourfold.

Homo- or Hetero-Triplet–Triplet Annihilation? A Case Study with Perylene-BODIPY Dyads/Triads

The photophysical processes of intramolecular “ping-pong” energy transfers in the iodinated reference dyad BDP-I2-Py, as well as the uniodinated dyad BDP-Py and triad BDP-2Py, were studied. For BDP-I2-Py, a forward Forster resonance energy transfer (FRET) from the perylene (Py) unit to the diiodoBDP unit (τ = 7 ps) and a backward triplet energy transfer (TTET, τ = 3 ns) from the diiodoBDP unit to the Py unit were observed. For the BDP-Py and BDP-2Py systems, a FRET (τ = 5–8 ps) and a photoinduced electron transfer (PET) (τ = 1–1.5 ns) were observed in acetonitrile. The uniodinated dyad and triad were used as the triplet energy acceptor and emitter for a triplet-triplet annihilation (TTA) upconversion with palladium tetraphenyltetrabenzoporphyrin as the triplet photosensitizer. A maximum upconversion quantum yield of 12.6% was observed. Given that the dyad (BDP-Py) contains one BDP unit and one Py unit while the triad (BDP-2Py) contains two Py units and one BDP unit and based on the results from steady-sta...

Increased vis-to-UV upconversion performance by energy level matching between a TADF donor and high triplet energy acceptors

An efficient visible-to-UV photon upconversion was achieved by combining a TADF sensitizer and UV emitters with the highest T1and S1energy levels.

Triplet-Triplet Energy Transfer Study in Hydrogen Bonding Systems.

The 2,6-diiodoBodipy-styrylBodipy hydrogen bonding system was prepared to study the effect of hydrogen bonding on the triplet-triplet-energy-transfer (TTET) process. 2,6-DiiodoBodipy linked with N-acetyl-2,6-diaminopyridine (D-2) was used as the triplet energy donor, and the styrylBodipy connected with thymine (A-1) was used as triplet energy acceptor, thus the TTET process was established upon photoexcitation. The photophysical processes of the hydrogen bonding system were studied with steady-state UV-vis absorption spectroscopy, fluorescence spectroscopy, fluorescence lifetime measurement and nanosecond time-resolved transient absorption spectroscopies. The TTET of the intramolecular/hydrogen bonding/intermolecular systems were compared through nanosecond transient absorption spectroscopy. The TTET process of the hydrogen bonding system is faster and more efficient (kTTET = 6.9 × 10(4) s(-1), ΦTTET = 94.0%) than intermolecular triplet energy transfer (kTTET = 6.0 × 10(4) s(-1), ΦTTET = 90.9%), but slower and less efficient than intramolecular triplet energy transfer (kTTET > 10(8) s(-1)). These results are valuable for designing self-assembly triplet photosensitizers and for the study of the TTET process of hydrogen bonding systems.

Diiodobodipy-styrylbodipy Dyads: Preparation and Study of the Intersystem Crossing and Fluorescence Resonance Energy Transfer.

2,6-Diiodobodipy-styrylbodipy dyads were prepared to study the competing intersystem crossing (ISC) and the fluorescence-resonance-energy-transfer (FRET), and its effect on the photophysical property of the dyads. In the dyads, 2,6-diiodobodipy moiety was used as singlet energy donor and the spin converter for triplet state formation, whereas the styrylbodipy was used as singlet and triplet energy acceptors, thus the competition between the ISC and FRET processes is established. The photophysical properties were studied with steady-state UV-vis absorption and fluorescence spectroscopy, electrochemical characterization, and femto/nanosecond time-resolved transient absorption spectroscopies. FRET was confirmed with steady state fluorescence quenching and fluorescence excitation spectra and ultrafast transient absorption spectroscopy (kFRET = 5.0 × 10(10) s(-1)). The singlet oxygen quantum yield (ΦΔ = 0.19) of the dyad was reduced as compared with that of the reference spin converter (2,6-diiodobodipy, ΦΔ = 0.85), thus the ISC was substantially inhibited by FRET. Photoinduced intramolecular electron transfer (ET) was studied by electrochemical data and fluorescence quenching. Intermolecular triplet energy transfer was studied with nanosecond transient absorption spectroscopy as an efficient (ΦTTET = 92%) and fast process (kTTET = 5.2 × 10(4) s(-1)). These results are useful for designing organic triplet photosensitizers and for the study of the photophysical properties.

Switching of the triplet-triplet-annihilation upconversion with photoresponsive triplet energy acceptor: photocontrollable singlet/triplet energy transfer and electron transfer.

A photoswitchable fluorescent triad based on two 9,10-diphenylanthracene (DPA) and one dithienylethene (DTE) moiety is prepared for photoswitching of triplet-triplet annihilation upconversion. The DPA and DTE moieties in the triad were connected via Click reaction. The DPA unit in the triad was used as the triplet energy acceptor and upconverted fluorescence emitter. The fluorescence of the triad is switched ON with the DTE moiety in open form [DTE-(o)] (upconversion quantum yield ΦUC = 1.2%). Upon UV irradiation, photocyclization of the DTE-(o) moiety produces the closed form [DTE-(c)], as a result the fluorescence of DPA moiety was switched off (ΦUC is negligible). Three different mechanisms are responsible for the upconverted fluorescence photoswitching effect (i.e., the photoactivated fluorescence resonance energy transfer, the intramolecular electron transfer, as well as the photoactivated intermolecular triplet energy transfer between the photosensitizer and DTE-(c) moiety). Previously, the photoswitching of TTA upconversion was accomplished with only one mechanism (i.e., the triplet state quenching of the photosensitizer by DTE-(c) via either the intermolecular or intramolecular energy transfer). The photophysical processes involved in the photochromism and photoswitching of TTA upconversion were studied with steady-state UV-vis absorption and fluorescence emission spectroscopies, nanosecond transient absorption spectroscopy, electrochemical characterization, and DFT/TDDFT calculations.

Perylene-Derived Triplet Acceptors with Optimized Excited State Energy Levels for Triplet–Triplet Annihilation Assisted Upconversion

A series of perylene derivatives are prepared as triplet energy acceptors for triplet-triplet annihilation (TTA) assisted upconversion. The aim is to optimize the energy levels of the T1 and S1 states of the triplet acceptors, so that the prerequisite for TTA (2E(T1) > E(S1)) can be better satisfied, and eventually to increase the upconversion efficiency. Tuning of the energy levels of the excited states of the triplet acceptors is realized either by attaching aryl groups to perylene (via single or triple carbon-carbon bonds), or by assembling a perylene-BODIPY dyad, in which the components present complementary S1 and T1 state energy levels. The S1 state energy levels of the perylene derivatives are generally decreased compared to perylene. The anti-Stokes shift, TTA, and upconversion efficiencies of the new triplet acceptors are improved with respect to the perylene hallmark. For the perylene-BODIPY dyads, the fluorescence emission was substantially quenched in polar solvents. Moreover, we found that extension of the π-conjugation of BODIPY energy donor significantly reduces the energy level of the S1 state. Low S1 state energy level and high T1 state energy level are beneficial for triplet photosensitizers.

Photon Upconversion by Triplet–Triplet Annihilation in Ru(bpy)3- and DPA-Functionalized Polymers

Two photophysically active polymers, each containing one absorbing Ru(bpy)3 triplet sensitizing core and multiple 9,10-diphenylanthracene (DPA) triplet energy acceptors, are shown to exhibit noncoherent photon upconversion by triplet–triplet annihilation (TTA) in both fluid solution and in thin solid films. An analysis of the kinetics and absorbed power dependencies of the upconverted fluorescence demonstrates that, in fluid solution, TTA is occurring in the low-power, pseudo-first order limit, and that the energy transfer mechanism involves fast, efficient intramolecular triplet energy transfer from the excited Ru(bpy)3 core to the DPA acceptors in the macromolecule’s pendant arms, followed by intermolecular TTA. Intermolecular triplet energy transfer, diffusion coefficient, and quantum yield measurements using Ru(bpy)3 and DPA controls suggest that, although a minority of the intermolecular encounters result in TTA in this one absorbing core system, efficient intramolecular TTA could be realized in a po...

Design and Photophysical Properties of Zinc(II) Porphyrin‐Containing Dendrons Linked to a Central Artificial Special Pair

The click chemistry synthesis and photophysical properties, notably photo-induced energy and electron transfers between the central core and the peripheral chromophores of a series of artificial special pair-dendron systems (dendron = G1, G2, G3; Gx = zinc(II) tetra-meso-arylporphyrin-containing polyimides) built upon a central core of dimethylxanthenebis(metal(II) porphyrin) (metal = zinc, copper), are reported. The dendrons act as singlet and triplet energy acceptors or donors, depending on the dendrimeric systems. The presence of the paramagnetic d(9) copper(II) in the dendrimers promotes singlet-triplet energy transfer from the zinc(II) tetra-meso-arylporphyrin to the bis(copper(II) porphyrin) unit and slow triplet-triplet energy transfer from the central bis(copper(II) porphyrin) fragment to the peripheral zinc(II) tetra-meso-arylporphyrin. If bis(zinc(II) porphyrin) is the central core, evidence for chain folding is observed; this is unambiguously demonstrated by the presence of triplet-triplet energy transfer in the heterobimetallic systems, a process that can only occur at short distances.

Experimental and Theoretical Study of Triplet Energy Transfer in Rigid Polymer Films

With the judicious selection of triplet energy donor (D) and acceptor (A) pairs, a laser flash photolysis procedure has provided a sensitive method for the study of triplet energy transfer in rigid polymer films. By monitoring changes in triplet-triplet (T-T) absorptions the kinetics of triplet energy transfer were evaluated at short time scales, and overall energy-transfer quantum yields were also obtained. Combinations of xanthone- or thioxanthone-type donors and polyphenyl acceptors were particularly suited to these measurements because the former have high intersystem-crossing quantum yields and the latter have very high extinction coefficients for T-T absorption. For exothermic transfer most of the energy transfer that occurred within the lifetime of triplet D ( (3)D) took place in less than a few microseconds after (3)D formation in poly(methyl methacrylate), and triplet A yields were limited largely by the number of A molecules in near contact with (3)D. The kinetics of triplet energy transfer were modeled using a modified Perrin-type statistical arrangement of D/A separations with allowance for excluded volume in combination with a Dexter-type formula for the distance-dependent exchange energy-transfer rate constant. Experimental observations were best explained by constraining D/A separations to reflect the dimensions of intervening molecules of the medium. Rate constants, k 0, for exothermic energy transfer from (3)D to A molecules in physical contact are approximately 10 (11) s (-1) and very similar to triplet energy-transfer rate constants determined from solution encounters. Energy-transfer rate constants, k( r), fall off as approximately exp(-2 r/ 0.85), where r is the separation distance between D and A centers in angstroms. Exchange energy transfer is not restricted to (3)D and A in physical contact, but at </=0.4 M A at least 85% of the energy transfer arises from interaction of (3)D with a single nearest-neighbor A molecule. The modified Perrin model was also applied to quantum yields of quenching in rigid media. Comparison to the simple Perrin model for quenching shows that the latter may be adequate as long as molecular volumes are accommodated in the Perrin expression. Under these conditions the critical radius, r c, corresponds to the (3)D/A separation at which the effective rate constant for energy transfer equals the inverse of the (3)D lifetime.

Thermodynamic energies of donor and acceptor triplet states

Thermodynamic energies ( E T) of lowest triplet states (T 1) have been measured for 60 triplet energy donors and acceptors. Laser flash photolysis procedures were used to monitor the equilibration between the triplet–triplet absorptions of reference and unknown compounds in ethyl acetate at 295 K. These measurements provide triplet free energies that reflect both enthalpy and entropy differences between T 1 and the ground state (S 0). These thermodynamic energies are the basis of quantitative models for triplet energy transfer kinetics in solution. For comparison, T 1 energies were also measured via phosphorescence at 295 K for most of the donors and many of the acceptors of this study. The phosphorescence of naphthalene at 295 K served as a reference for relating the thermodynamic and phosphorescent triplet energies ( E P). For most rigid molecules the E T and E P values are nearly identical. However, for molecules with considerable rotational or conformational freedom, such as ketocoumarin and α-diketone donors and polyphenyl acceptors, E T values tend to be substantially greater than E P values. This difference partially reflects the tendency for reduced rotational freedom in T 1 relative to S 0, which leads to a reduction in entropy and a corresponding increase in free energy (from − TΔ S) in T 1. For nonrigid chromophores triplet energies obtained from low-temperature measurements tend to approximate E T values in some cases, because the energy increases associated with emission from unrelaxed triplets tend to parallel the positive contributions of decreased T 1 entropy to E T. Triplet–triplet absorption maxima and extinction coefficients are provided for all of the donors and acceptors. Intersystem crossing quantum yields, also measured by laser flash photolysis, are provided for the triplet energy donors.

Phosphorescent Resonant Energy Transfer between Iridium Complexes

The mechanism for triplet energy transfer from the green-emitting fac-tris[2-(4'-tert-butylphenyl)pyridinato]iridium (Ir(tBu-ppy)3) complex to the red-emitting bis[2-(2'-benzothienyl)pyridinato-N,C3')(acetylacetonato)iridium (Ir(btp)2(acac)) phosphor has been investigated using steady-state and time-resolved photoluminescence spectroscopy. [2,2';5,'2' ']Terthiophene (3T) was also used as triplet energy acceptor to differentiate between the two common mechanisms for energy transfer, i.e., the direct exchange of electrons (Dexter transfer) or the coupling of transition dipoles (Förster transfer). Unlike Ir(btp)2(acac), 3T can only be active in Dexter energy transfer because it has a negligible ground state absorption to the 3(pi-pi*) state. The experiments demonstrate that in semidilute solution, the 3MLCT state of Ir(tBu-ppy)3 can transfer its triplet energy to the lower-lying 3(pi-pi*) states of both Ir(btp)2(acac) and 3T. For both acceptors, this transfer occurs via a diffusion-controlled reaction with a common rate constant (ken = 3.8 x 10(9) L mol-1 s-1). In a solid-state polymer matrix, the two acceptors, however, show entirely different behavior. The 3MLCT phosphorescence of Ir(tBu-ppy)3 is strongly quenched by Ir(btp)2(acac) but not by 3T. This reveals that under conditions where molecular diffusion is inhibited, triplet energy transfer only occurs via the Förster mechanism, provided that the transition dipole moments involved on energy donor and acceptor are not negligible. With the use of the Förster radius for triplet energy transfer from Ir(tBu-ppy)3 to Ir(btp)2(acac) of R0 = 3.02 nm, the experimentally observed quenching is found to agree quantitatively with a model for Förster energy transfer that assumes a random distribution of acceptors in a rigid matrix.

Laws of donor radiation reabsorption by triplet energy acceptor molecules

Results of investigations into the influence of the donor radiation absorption by acceptor molecules in the triplet state on the phosphorescence intensity of the energy donor in solid solutions of organic compounds are presented. A comparison of the results of theoretical calculations with experimental data demonstrates their good agreement.