Triplet-singlet Energy Transfer Research Articles

Selective Triplet–Singlet Förster‐Resonance Energy Transfer for Bright Red Afterglow Emission (Adv. Funct. Mater. 11/2023)

Selective Förster-Resonance Energy Transfer In article number 2211604, Shuzo Hirata and co-workers demonstrate the efficient triplet-singlet Förster-resonance energy transfer from the N-fused chromophore to an acceptor leads to the bright red afterglow emission. The selective triplet-singlet energy transfer is due to the high radiation rate from the lowest triplet state facilitated by the out-of-plane molecular vibrations in the N-fused chromophore and near-zero fluorescence suppressing the singlet-singlet energy transfer pathways.

PH-tunable phosphorescence and light harvesting in cucurbit[8]uril host-guest assemblies.

Host-guest assemblies of halo-phenyl pyridine derivatives and cucurbit[8]uril (CB[8]) exhibited pH-responsive room temperature phosphorescence (RTP) in aqueous media. Moreover, they acted as efficient light-harvesting systems demonstrating triplet-singlet energy transfer to various acceptor dyes.

Plasmon effect on triplet-singlet energy transfer in the dye-doped Langmuir-Blodgett films

The effect of silver plasmonic nanoparticles on triplet-singlet energy transfer in the donor-acceptor pair of organic dyes was studied. Layered dye films were prepared on the surface of glass and island silver films using the Langmuir-Blodgett method. Amphiphilic analogs of Rose bengal and polymethine dye were used as donor and acceptor of energy. A polymer monolayer was deposited between monolayers of donor and acceptor molecules to exclude the competing triplet-triplet energy transfer. The spectra of delayed fluorescence and phosphorescence of donor-acceptor films and the delayed luminescence lifetime of donor in these films have been measured. It is shown that a threefold increase in the fluorescence intensity and a twofold increase in the donor phosphorescence intensity are observed on silver island films. The successful triplet-singlet energy transfer is evidenced by both the quenching of donor centers and appearance of sensitized delayed fluorescence of the acceptor with the duration close to the donor triplet lifetime. In the presence of silver nanoparticles, in addition to enhancement in the intensity of the donor emission, an increase in the efficiency of tripletsinglet energy transfer was observed. The obtained results can be used in various optical devices.

Plasmon effect on simultaneous singlet-singlet and triplet-singlet energy transfer

The influence of plasmonic effect of Ag nanoparticles (NPs) on singlet-singlet (S–S) and triplet-singlet (T–S) energy transfer in the same donor–acceptor pair of organic molecules was studied. It was established that a 7-fold increase in the fluorescence and phosphorescence intensity of the energy donor (Rose bengal) and a 4-fold increase in the fluorescence intensity of the acceptor (polymethine dye) are observed on silver island films. The changes are associated with an increase in the emission rate of dye molecules, as evidenced by a decrease in the lifetime of their luminescence in the presence of Ag NPs. An increase in the efficiency of both types of energy transfer was recorded in the plasmon field of Ag NPs. The plasmon effect has almost the same influence on both S–S and T–S energy transfer. Estimation of the energy transfer rates using of the model of electric dipole interaction showed a good correlation between the experimental and calculated data for the S–S energy transfer.

Mechanism of the Formation of Electronically Excited Species by Oxidative Metabolic Processes: Role of Reactive Oxygen Species.

It is well known that biological systems, such as microorganisms, plants, and animals, including human beings, form spontaneous electronically excited species through oxidative metabolic processes. Though the mechanism responsible for the formation of electronically excited species is still not clearly understood, several lines of evidence suggest that reactive oxygen species (ROS) are involved in the formation of electronically excited species. This review attempts to describe the role of ROS in the formation of electronically excited species during oxidative metabolic processes. Briefly, the oxidation of biomolecules, such as lipids, proteins, and nucleic acids by ROS initiates a cascade of reactions that leads to the formation of triplet excited carbonyls formed by the decomposition of cyclic (1,2-dioxetane) and linear (tetroxide) high-energy intermediates. When chromophores are in proximity to triplet excited carbonyls, the triplet-singlet and triplet-triplet energy transfers from triplet excited carbonyls to chromophores result in the formation of singlet and triplet excited chromophores, respectively. Alternatively, when molecular oxygen is present, the triplet-singlet energy transfer from triplet excited carbonyls to molecular oxygen initiates the formation of singlet oxygen. Understanding the mechanism of the formation of electronically excited species allows us to use electronically excited species as a marker for oxidative metabolic processes in cells.

Low‐Threshold Organic Semiconductor Lasers with the Aid of Phosphorescent Ir(III) Complexes as Triplet Sensitizers

AbstractOrganic semiconductor lasers (OSLs) have emerged as particularly challenging. One of the major issues preventing the successful realization of lasing from organic emitters under electrically pumped conditions is the inevitable population of triplet excitons. Herein, a novel concept is presented to construct triplet–singlet guest–host gain systems with incorporating iridium complexes as the triplet sensitizers and a fluorescent conjugated polymer as the gain media to achieve light amplification. The direct triplet–singlet energy transfer process is confirmed by photoluminescence excitation spectra, photoinduced absorption spectroscopy, and fluorescence transients of the blend samples. Successful light amplification with a threefold lower amplified spontaneous emission threshold and much better lasing performance is demonstrated for the resulting triplet–singlet guest–host system as compared with the corresponding gain system without triplet sensitizers. Moreover, under electrically driven conditions, the fluorescent organic light‐emitting diodes (OLEDs) based on the triplet–singlet guest–host systems with “triplet sensitizers” exhibit enhanced electrical performance relative to those without. The work suggests an effective general methodology to utilize both the singlet and triplet excitons to contribute to the light amplification with excellent electrical performance in OLEDs, opening prospects toward attempting electrically pumped OSLs.

Ping-Pong Energy Transfer in a Boron Dipyrromethane Containing Pt(II)-Schiff Base Complex: Synthesis, Photophysical Studies, and Anti-Stokes Shift Increase in Triplet-Triplet Annihilation Upconversion.

A boron dipyrromethane (BDP)-containing Pt(II)-Schiff base complex (Pt-BDP), showing ping-pong singlet-triplet energy transfer, was synthesized, and the detailed photophysical properties were investigated using various steady-state and time-resolved transient spectroscopies. Femtosecond/nanosecond transient absorption spectroscopies demonstrated that, upon selective excitation of the BDP unit in Pt-BDP at 490 nm, Förster resonance energy transfer from the BDP unit to the Pt(II) coordination center occurred (6.7 ps), accompanied by an ultrafast intersystem crossing at the Pt(II) coordination center (<1 ps) and triplet-triplet energy transfer back to the BDP moiety (148 ps). These processes generated a triplet state localized at BDP, and the lifetime was 103.2 μs, much longer than the triplet-state lifetime of Pt-Ph (3.5 μs), a complex without the BDP moiety. Finally, Pt-BDP was used as a triplet photosensitizer for triplet-triplet annihilation (TTA) upconversion through selective excitation of the BDP unit or the Pt(II) coordination center at lower excitation energy. An upconversion quantum yield of up to 10% was observed with selective excitation of the BDP moiety, and a large anti-Stokes shift of 0.65 eV was observed upon excitation of the lower-energy band of the Pt(II) coordination center. We propose that using triplet photosensitizers with the ping-pong energy-transfer process may become a useful method for increasing the anti-Stokes shift of TTA upconversion.

Boosting the Efficiency of Near‐Infrared Fluorescent OLEDs with an Electroluminescent Peak of Nearly 800 nm by Sensitizer‐Based Cascade Energy Transfer

AbstractA sensitization‐based cascade energy transfer channel is proposed to boost the electroluminescent performances of the solution‐processed near‐infrared organic light‐emitting devices (OLEDs) featuring an electroluminescent peak of 786 nm from a new fluorescent emitter of N4,N4,N9,N9‐tetra‐p‐tolylnaphtho[2,3‐c][1,2,5]thiadiazole‐4,9‐diamine (NZ2mDPA) with unique aggregation‐induced emission (AIE) property. The optimized device is composed of 4,4′‐N,N‐dicarbazole‐biphenyl (CBP) as the host, bis(2‐phenyl‐1,3‐benzothiozolato‐N,C2′)iridium (Ir(bt)2(acac)) as the sensitizer, and NZ2mDPA as the emitter, where the cascade energy transfer can occur via two steps realizing unexpected triplet–singlet energy transfer by the Förster mechanism. The first step features efficient triplet harvesting from CBP to Ir(bt)2(acac), and then the second step involves in resonant energy transfer from the phosphorescent sensitizer to the near‐infrared AIE emitter of NZ2mDPA, which finally endows two channels of harvesting singlet and triplet excitons. The unique scheme achieves not only more efficient Förster energy transfer but also the higher utilization efficiency of triplet excitons. As a result, the near‐infrared OLEDs can realize a factor of 2.7 enhancement of external quantum efficiency by employing the phosphor‐sensitized AIE lumogen compared with the commonly used binary host–guest system.

A Ru(bipyridine)3[PF6]2 Complex with a Rhodamine Unit – Synthesis, Photophysical Properties, and Application in Acid‐Controllable Triplet–Triplet Annihilation Upconversion

A ruthenium(II) tris(bipyridine)–rhodamine triad was prepared to control the triplet‐state properties with external stimuli (acid). The rhodamine moiety is an acid‐responsive module, and the RuII coordination center is responsible for triplet‐state formation upon photoexcitation. Through steady‐state UV/Vis absorption and luminescence spectroscopy, electrochemical characterization, and nanosecond/femtosecond transient absorption spectroscopy, we found that the dyad gives a quenched triplet metal‐to‐ligand charge‐transfer (3MLCT) excited state (lifetime τT = 103.6 ns vs. τT = 1.58 µs) in the absence of acid, owing to a photoinduced electron transfer (PET) process. In the presence of acid, the rhodamine unit transforms reversibly from the spirolactam structure into the open‐amide structure; thus, the PET is inhibited, the triplet‐state lifetime (5.70 µs) is prolonged, and the T1 state is relocated to the rhodamine unit. This is the first time that the fundamental triplet‐excited‐state properties of a RuII complex, that is, the triplet lifetime and localization, have been controlled simultaneously. Intramolecular singlet–triplet energy transfer and triplet–triplet energy transfer (TTET, 3MLCT→3IL) were observed for the triad in the femtosecond and nanosecond transient absorption spectra, respectively. The dyad was used for acid‐controllable triplet–triplet annihilation upconversion.

Enhancement of Singlet-Triplet Energy Transfer Between Dyes in a Polymer Film by Surface Plasmons of Gold Nanoparticles

The effect of the plasmon energy generated in Ag citrate hydrosol and ablated Au nanoparticles on the singlettriplet electron energy transfer between rhodamine 6G (R6G) and acriflavine (ACF) molecules incorporated into polyvinyl alcohol (PVA) films is studied. The increased efficiency of non-radiative energy transfer in the presence of Au nanoparticles and the increased lifetime of excited states of molecules are established.

Intramolecular Triplet Energy Transfer in Anthracene-Based Platinum Acetylide Oligomers

Platinum acetylide oligomers that contain an anthracene moiety have been synthesized and subjected to photophysical characterization. Spectroscopic measurement and DFT calculations reveal that both the singlet and triplet energy levels of the anthracene segment are lower than those of the platinum acetylide segment. Thus, the platinum acetylide segment acts as a sensitizer to populate the triplet state of the anthrancene segment via intramolecular triplet-triplet energy transfer. The objective of this work is to understand the mechanisms of energy-transfer dynamics in these systems. Fluorescence quenching and the dominant triplet absorption that arises from the anthracene segment in the transient absorption spectrum of Pt4An give clear evidence that energy transfer adopts an indirect mechanism, which begins with singlet-triplet energy transfer from the anthracene segment to the platinum acetylide segment followed by triplet-triplet energy transfer to the anthracene segment.

Luminescence properties of pHEMA-TiO2 gels based hybrids materials

Photoluminescence (PL) of photochromic pHEMA-TiO2 gels-based hybrids was studied by means of time- and energy-resolved spectroscopy at temperatures between 300K and 10K. The PL band at 485nm is assigned to S0←T1 transition of methoxyphenol (organic molecule added to the commercial monomer hydroxyethyl methacrylate, HEMA and used as an inhibitor of spontaneous polymerisation) in the polymer environment, while the PL band at 600nm is assigned to the self-trapped exciton onto octahedral TiO6 site of the inorganic component. The mechanisms of the excited states population are discussed. In particular it is shown that both singlet-triplet energy transfer in methoxyphenol and methoxyphenol–TiO2 charge transfer are strongly affected by the material composition and temperature. The hypothesis about the photoexcited holes annihilation with the trapped electrons is confirmed to be one of main mechanisms limiting the Ti3+ centres concentration.

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.

Electronic excitation energy transfer in poly-N-epoxypropyl-3,6-dibromocarbazole

The mechanisms of electronic excitation energy transfer in films of poly-N-epoxypropyl-3,6-dibromocarbazole doped with pyrene, rubrene, and bis[2-(2′-benzothienyl)-pyridinato-N,C3′](acetylacetonate)iridium(III) (Btp2Ir(acac)) have been studied in the range T = 5–295 K. It has been established that the mechanisms of transfer depend on the relative position of the S1 and T1 levels of the base and dopant: if the S1 level of the dopant lies between the S1 and T1 levels of the base and its T1 level is below than that of the base (pyrene molecules), only the triplet-triplet transfer takes place, whereas if the S1 level of the dopant lies below the T1 level of the base (pyrene and Btp2Ir(acac) molecules), the triplet-singlet transfer is observed in addition to the triplet-triplet transfer. It has been assumed that the spin-forbidden triplet-singlet energy transfer is allowed because of the strong spinorbit interaction induced by two heavy bromine atoms in chromophores of the polymer. This transfer proceeds through the long-range dipole-dipole interaction at low temperatures and the electron exchange interaction in the vicinity of room temperature.

Fast transient absorption spectroscopy of the early events in photoexcited chiral benzophenone–naphthalene dyads

Photoinduced intra-molecular energy transfer in two ketoprofen(KP)-naproxol(NPX) diastereomers proceeds via two pathways. Very fast singlet–triplet energy transfer ( k = 1.2 × 10 11 s −1) from KP to NPX occurs for a small percentage (6%) and the major pathway is triplet–triplet energy transfer ( k ∼ 3 × 10 9 s −1). This was shown with femtosecond transient absorption spectroscopy and global and target analysis. Whereas the NPX triplet decay is strongly stereospecific (ratio of 1.6), the NPX triplet state formation for both dyads is very similar (ratio of 1 for the fast process and 1.2 for the slower process).

Intramolecular energy-transfer processes in a bis(porphyrin)-ruthenium(ii) bis(2,2′:6′,2″-terpyridine) molecular array

A molecular triad has been synthesized comprising two free-base porphyrin terminals linked to a central ruthenium(II) bis(2,2':6',2''-terpyridine) subunit via meso-phenylene groups. Illumination into the ruthenium(II) complex is accompanied by rapid intramolecular energy transfer from the metal-to-ligand, charge-transfer (MLCT) triplet to the lowest-energy pi-pi* triplet state localized on one of the porphyrin subunits. Transfer takes place from a vibrationally excited level which lowers the activation energy. The electronic coupling matrix element for this process is 73 cm(-1). Selective illumination into the lowest-energy singlet excited state (S1) localized on the porphyrin leads to fast singlet-triplet energy transfer that populates the MLCT triplet state with high efficiency. This latter process occurs via Dexter-type electron exchange at room temperature, but the activation energy is high and the reaction is prohibited at low temperature. For this latter process, the electronic coupling matrix element is only 8 cm(-1).

Injecting electronic excitation energy into an artificial antenna system through an Ru2+ complex.

The Ru2+ complex [Ru(bpy)2(bpy-ph4-Si(CH3)3)]2+ can be electrostatically bound to the negatively charged channel entrances of dye-loaded zeolite L crystals where it acts as a functional stopcock molecule. Impressive electronic triplet-singlet excitation energy transfer from the Ru2+ complex to the acceptor dye oxazine 1 (Ox1) located inside the channels can be observed when the donor molecule is selectively excited. Time-resolved luminescence experiments have been performed on the separate components and on the assembled donor-acceptor material. The luminescence lifetime of the Ru2+ complex attached to the zeolite is reduced by a factor of 30 when Ox1 acceptor molecules are present. The fluorescence decay of Ox1 incorporated in zeolite L is single exponential with a lifetime of 3 ns. The much longer lifetime in zeolite L than in solution is due to the fact, that the diethyl groups are sterically restricted when the dye is inside the host.

Intramolecular Energy Transfer in Molecular Dyads Comprising Free-base Porphyrin and Ruthenium(II) Bis(2,2‘:6‘,2‘ ‘-terpyridine) Termini

A molecular dyad has been synthesized in which free-base porphyrin and ruthenium(II) bis(2,2':6',2-terpyridine) subunits are linked via a meso-phenylene group. The distal terpyridine ligand bears a single phenylethynylene group. Selective illumination into the metal complex is followed by rapid intramolecular triplet-triplet energy transfer from the metal-to-ligand charge-transfer (MLCT) triplet to the lowest-energy π,π* triplet state localized on the porphyrin. This process is characterized by a reorganization energy of 0.14 eV and an electronic coupling matrix element of 76 cm - 1 . Because of the alkynylene substituent, the initially produced MLCT state is centered on the distal terpyridine. Therefore, triplet energy transfer must cross the proximal terpyridine ligand. At low temperature, nuclear tunneling renders the rate of triplet energy transfer activationless. Upon selective illumination into the lowest-energy singlet (S 1 ) state localized on the porphyrin, fast singlet-triplet energy transfer occurs, to populate the MLCT triplet with high efficiency. This process happens by way of Dexter-type electron exchange at room temperature, and the MLCT triplet can be identified as a reaction intermediate at low temperature. The activation energy for singlet-triplet energy transfer is only 0.05 eV, because of the smaller energy gap, and the electronic coupling matrix element is decreased to 11 cm - 1 , because energy transfer is spin-forbidden. At low temperature, dipole-dipole energy transfer becomes the main mechanism for decay of the porphyrin S 1 state. Excitation into the Soret band of the porphyrin is followed by rapid internal conversion to S 1 without energy or electron transfer to the appended metal complex.

Photophysics of pyridylporphyrin Ru(II) adducts: heavy-atom effects and intramolecular decay pathways.

Eight adducts between different pyridylporphyrins and ruthenium complexes, MPyP[RuCl(2)(DMSO)(2)(CO)], c-DPyP[RuCl(2)(DMSO)(2)(CO)](2), TrPyP[RuCl(2)(DMSO)(2)(CO)](3), TPyP[RuCl(2)(DMSO)(2)(CO)](4), (MPyP)(2)[RuCl(2)(DMSO)(2)], [c-DPyP[RuCl(2)(DMSO)(2)]](2), MPyP[RuCl(2)(CO)(3)], and [c-DPyP[RuCl(2)(CO)(2)]](2), have been investigated. The results show that in all the adducts the porphyrin singlet is quenched, to a greater or lesser extent, relative to the parent-free molecule. This study provides insight into the mechanisms of singlet quenching in the adducts. Two mechanisms for singlet quenching, both related to the "heavy-atom effect" of the ruthenium center and experimentally distinguishable by transient spectroscopy, are examined. Enhanced intersystem crossing within the porphyrin chromophore is demonstrated for the series of adducts MPyP[RuCl(2)(DMSO)(2)(CO)], c-DPyP[RuCl(2)(DMSO)(2)(CO)](2), TrPyP[RuCl(2)(DMSO)(2)(CO)](3), and TPyP[RuCl(2)(DMSO)(2)(CO)](4), where a nice correlation is observed between the magnitude of the effect and the number of ruthenium centers attached to the pyridylporphyrin chromophore. Singlet-triplet energy transfer from the pyridylporphyrin chromophore to the ruthenium center(s) is an additional efficient quenching channel for adducts containing ruthenium centers with weak field ligands and low triplet energies, such as (MPyP)(2)[RuCl(2)(DMSO)(2)] and [c-DPyP[RuCl(2)(DMSO)(2)]](2).

Triplet-singlet energy transfer in dye-doped tris(phenylquinoxaline) studied by photoinduced absorption spectroscopy

Energy transfer from fluorescent and phosphorescent tris(phenylquinoxaline) to a phosphorescent dye has been studied by means of photoinduced absorption spectroscopy. The frequency response of the donor's triplet-triplet absorption signal has been measured to gain information about the time scale and kinetics of the triplet-singlet energy transfer.