Intramolecular Triplet Energy Transfer Research Articles

Efficient Spin-Orbit Charge-Transfer Intersystem Crossing and Slow Intramolecular Triplet-Triplet Energy Transfer in Bodipy-Perylenebisimide Compact Dyads and Triads.

Bodipy (BDP)-perylenebisimide (PBI) donor-acceptor dyads/triad were prepared to study the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). For BDP-PBI-3, in which BDP was attached at the imide position of PBI, higher singlet oxygen quantum yield (ΦΔ =85 %) was observed than the bay-substituted derivative BDP-PBI-1 (ΦΔ =30 %). Femtosecond transient absorption spectra indicate slow Förster resonance energy transfer (FRET; 40.4 ps) and charge separation (CS; 1.55 ns) in BDP-PBI-3, while for BDP-PBI-1, CS takes 2.8 ps. For triad BDP-PBI-2, ultrafast FRET (149 fs) and CS (4.7 ps) process were observed, the subsequent charge recombination (CR) takes 5.8 ns and long-lived 3 PBI* (179.8 μs) state is populated. Nanosecond transient absorption spectra of BDP-PBI-3 show that the CR gives upper triplet excited state (3 BDP*) and subsequently, via a slow intramolecular triplet energy transfer (14.5 μs), the 3 PBI* state is finally populated, indicating that upper triplet state is involved in SOCT-ISC. Time-resolved electron paramagnetic resonance spectroscopy revealed that both radical pair ISC (RP ISC) and SOCT-ISC contribute to the ISC. A rare electron spin polarization of (e, e, e, e, e, e) was observed for the triplet state formed via the RP ISC mechanism, due to the S-T+1 /T0 states mixing.

UV light-driven late-stage skeletal reorganization to diverse limonoid frameworks: A proof of concept for photobiosynthesis.

Late-stage skeletal reorganization (LSSR) is a type of fascinating organic transformation processes in natural product total synthesis. However, few facile and effective LSSR methodologies have hitherto been developed. Here, LSSR of limonoid natural products via photochemical cascades is first reported. Starting from xyloelves A and B, nine distinct limonoid products with five unprecedented scaffolds are generated. The photocascade pathways of these natural products and mechanistic rationale via intramolecular triplet energy transfer are revealed by quantum mechanical calculations. Most notably, ultraviolet light-driven transannular and stereoselective C→C 1,4-acyl migration is first found as a photochemical approach, particularly for LSSR of natural products. This approach holds promise for designing LSSR strategies to access bioactive cage-like molecules. Besides that, our findings provide a clear proof of concept for natural product photobiosynthesis. Xyloelf A, substantially ameliorating concanavalin A-induced liver injury in mice, could be used as a unique molecular template for hepatoprotective drug discovery.

Synthesis and Photophysics of Phenylene Based Triplet Donor–Acceptor Dyads: ortho vs. para Positional Effect on Intramolecular Triplet Energy Transfer

Two phenylene based geometrical/isomeric triplet ortho - and para –dyads ( o – 3 and p – 3 , respectively) were synthesized and fully characterized using advanced photophysical tools and computations. In dyad o – 3 , the through-space donor-acceptor interactions led to simultaneous triplet energy transfer and charge transfer with identical kinetics. On the other hand, in the dyad p – 3 , it was found that the phenylene spacer favors a fast triplet energy delocalization over the charge transfer process. Furthermore, analysis of the results from the present investigation indicates that the deactivation of the photo-excited species ( o – 3 )* occurs through both the intrinsic channel viz. S 0 ←S 1 and charge recombination. In the case of dyad p – 3 , the results indicate that the primary deactivation pathway is self-quenching or triplet-triplet annihilation involving the acceptor unit(s).

Osmium Complex-Chromophore Conjugates with Both Singlet-to-Triplet Absorption and Long Triplet Lifetime through Tuning of the Heavy-Atom Effect.

Os(II) complexes showing singlet-to-triplet absorption are of growing interest as a new class of triplet sensitizers that circumvent energy loss during intersystem crossing, and they enable effective utilization of input photon energy in various applications, such as photoredox catalysis, photodynamic therapy, and photon upconversion. However, triplet excited-state lifetimes of Os(II) complexes are often too short (τ < 1 μs) to transfer their energy to neighboring molecules. While the covalent conjugation of chromophores has been known to extend the net excited-state lifetimes through an intramolecular triplet energy transfer (IMET), heavy-atom effects of the central metals on the attached chromophore units have rarely been discussed. Here, we investigate the relationship between the spin-density contribution of the heavy metals and the net triplet excited-state lifetimes for a series of Os(II) and Ru(II) bis(terpyridine) complexes modified with perylene units. Phosphorescence lifetimes of these compounds strongly depend on the lifetimes of the perylenyl group-localized excited states that are shortened by the heavy-atom effect. The degree of heavy-atom effect can be largely circumvented by introducing meta-phenylene bridges, where the perylene unit retains its intrinsic long excited-state lifetime. The thermal activation to the short-lived excited states is suppressed, thanks to sufficient but still small energy losses during the IMET process. Involvement of the metal center was also confirmed by the prolonged lifetime by replacing Os(II) with Ru(II) that possesses a smaller spin-orbit coupling constant. These results indicate the importance of ligand structures that give a minimum heavy-atom effect as well as the sufficient energy gap among the excited states and fast IMET for elongating the triplet excited-state lifetime without sacrificing the excitation energy.

Efficient Triplet-Triplet Annihilation Upconversion in Solution and Hydrogel Enabled by an S-T Absorption Os(II) Complex Dyad with an Elongated Triplet Lifetime.

A new Os(II) complex dyad featuring direct singlet-to-triplet (S-T) absorption and intramolecular triplet energy transfer (ITET) with lifetime up to 7.0 μs was designed to enhance triplet energy transfer efficiency during triplet-triplet annihilation upconversion (TTA-UC). By pairing with 9,10-bis(phenylethynyl)anthracene (BPEA) as a triplet acceptor, intense upconverted green emission in deaerated solution was observed with unprecedented TTA-UC emission efficiency up to 26.3% (with a theoretical maximum efficiency of 100%) under photoexcitation in the first biological transparency window (650-900 nm). Meanwhile, a 7.1% TTA-UC emission efficiency was acquired in an air-saturated hydrogel containing the photosensitizer and a newly designed hydrophilic BPEA derivative. This ITET mechanism would inspire further development of a highly efficient TTA-UC system for biological fields and renewable energy production.

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.

Intersystem Crossing and Electron Spin Selectivity in Anthracene‐Naphthalimide Compact Electron Donor‐Acceptor Dyads Showing Different Geometry and Electronic Coupling Magnitudes

Anthracene-naphthalimide (An-NI) compact electron donor-acceptor dyads were prepared, in which the orientation and distance between the two subunits were varied by direct connection or with intervening phenyl linker. Efficient intersystem crossing (ISC) and long triplet state lifetime (ΦΔ =92 %, τT =438 μs) were observed for the directly connected dyads showing a perpendicular geometry (81°). This efficient spin-orbit charge transfer ISC (SOCT-ISC) takes 376 fs, inhibits the direct charge recombination (CR) to ground state (1 CT→S0 , takes 3.04 ns). Interestingly, efficient SOCT-ISC for dyads with intervening phenyl linker (ΦΔ =40 % in DCM) was also observed, although the electron donor and acceptor adopt almost coplanar geometry (dihedral angle: 15°). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy shows that the electron spin polarization of the triplet state, i. e. the electron spin selectivity of ISC, is highly dependent on the dihedral angle and the linker. For the dyads showing weaker coupling between the donor and acceptors, the charge separation and the intramolecular triplet energy transfer are inhibited at 80 K (frozen solution), because both the 3 An and 3 NI states were observed and the ESP are same as compared to the native anthracene and naphthalimide, which unravel their origin. The dyads were used as triplet photosensitizers for triplet-triplet annihilation upconversion (TTA UC). High UC quantum yield (ΦUC =12.9 %) as well as a large anti-Stokes shift (0.72 eV) was attained by excitation into the CT absorption band.

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-Triplet Annihilation Photon Upconversion in Polymer Thin Film: Sensitizer Design.

Efficient visible-to-UV photon upconversion via triplet-triplet annihilation (TTA) is accomplished in polyurethane (PU) films by developing new, powerful photosensitizers fully functional in the solid-state matrix. These rationally designed triplet sensitizers feature a bichromophoric scaffold comprising a tris-cyclometalated iridium(III) complex covalently tethered to a suitable organic small molecule. The very rapid intramolecular triplet energy transfer from the former to the latter is pivotal for achieving the potent sensitizing ability, because this process out-competes the radiative and nonradiative decays inherent to the metal complex and produces long-lived triplet excitons localized with the acceptor moiety readily available for intermolecular transfer and TTA. Nonetheless, compared to the solution state, the molecular diffusion is greatly limited in solid matrices, which even creates difficulty for the Dexter-type intramolecular energy transfer. This is proven by the experimental results showing that the sensitizing performance of the bichromophoric molecules strongly depends on the spatial distance separating the donor (D) and acceptor (A) units and that incorporating a longer linker between the D and A evidently curbs the TTA upconversion efficiency in PU films. Using a rationally optimized sensitizer structure in combination with 2,7-di-tert-butylpyrene as the annihilator/emitter, the doped polyurethane (PU) films demonstrate effective visible-to-UV upconverted emission signal under noncoherent-light irradiation, attaining an upconversion quantum yield of 2.6%. Such quantum efficiency is the highest value so far reported for the visible-to-UV TTA systems in solid matrices.

Intra-molecular triplet energy transfer is a general approach to improve organic fluorophore photostability.

Bright, long-lasting and non-phototoxic organic fluorophores are essential to the continued advancement of biological imaging. Traditional approaches towards achieving photostability, such as the removal of molecular oxygen and the use of small-molecule additives in solution, suffer from potentially toxic side effects, particularly in the context of living cells. The direct conjugation of small-molecule triplet state quenchers, such as cyclooctatetraene (COT), to organic fluorophores has the potential to bypass these issues by restoring reactive fluorophore triplet states to the ground state through intra-molecular triplet energy transfer. Such methods have enabled marked improvement in cyanine fluorophore photostability spanning the visible spectrum. However, the generality of this strategy to chemically and structurally diverse fluorophore species has yet to be examined. Here, we show that the proximal linkage of COT increases the photon yield of a diverse range of organic fluorophores widely used in biological imaging applications, demonstrating that the intra-molecular triplet energy transfer mechanism is a potentially general approach for improving organic fluorophore performance and photostability.

Iridium-Based High-Sensitivity Oxygen Sensors and Photosensitizers with Ultralong Triplet Lifetimes.

The photophysics of a series of bichromophoric molecules featuring an intramolecular triplet energy transfer between a triscyclometalated iridium(III) complex and covalently linked organic group are studied. By systematically varying the energy gap (0.1-0.3 eV) between the donor (metal complex) and acceptor (pyrene unit), reversible triplet energy transfer processes with equilibrium constant K ranging from ca. 500 to 40 000 are established. Unique photophysical consequences of such large K values are observed. Because of the highly imbalanced forward and backward energy transfer rates, triplet excitons dominantly populate the acceptor moiety in the steady state, giving rise to ultralong luminescence lifetimes up to 1-4 ms. Because the triscyclometalated Ir and triplet pyrene groups both impart relatively small nonradiative energy loss, decent phosphorescence quantum yields (Φ = 0.1-0.6) are attained in spite of the exceptionally prolonged excited states. By virtue of such precious combination of long-lived triplet state and high Φ, these bichromophoric molecules can serve as highly sensitive luminescent sensors for detecting trace amount of O2 and as potent photosensitizers for producing singlet oxygen even under low-oxygen content conditions.

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.

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...

Substituent effects on the photorearrangements of unsymmetrically substituted diazinobarrelenes

A series of diazinobarrelenes 8–15 engendered with alkyl functionalities at the barrelene skeleton were irradiated with 350 nm light under direct and acetone-sensitized reaction conditions. Under these conditions, all the barrelenes except barrelene 14 afforded semibullvalenes with varying degrees of regioselectivity and product distribution. Dicyanopyrazinobarrelenes 8–10 which furnished semibullvalenes 32–41via the aryl–vinyl initial bridging route were strongly controlled by the nitrile functionalities installed at the aromatic sites. Benzoquinoxalinobarrelenes 11–13 which afforded semibullvalenes 42–49, preferentially underwent photorearrangement via vinyl–vinyl bridging even if the compounds were excited at a wavelength where the quinoxaline moiety absorbed most of the light. Zimmerman's bridging hypothesis and the possibility for quinoxalines to undergo intramolecular triplet energy transfer could reasonably account for the observed regioselectivity. Barrelene 14 was insensitive to photorearrangement whereas benzo[f,h]quinoxalinobarrelene 15 preferentially underwent ADPM rearrangement affording semibullvalenes 50–52. Electronic and steric factors of alkyl substituents overwhelmingly controlled the product forming steps whereas localization and minimization of triplet energies greatly influenced the initial bridging interaction.

Synthesis, photophysical and electroluminescent properties of a novel bright light-emitting Eu 3+ complex based on a fluorene-containing bidentate aryl phosphine oxide

A novel bidentate aryl phosphine oxide (APO) ligand with fluorene as the chromophore (9,9-diethyl-9H-fluorene-2,7-diyl)bis(diphenylphosphine)oxide (EFDPO), as well as the corresponding Eu 3+ complex 1 were designed and synthesized. The strong absorption antennae effect of the functional APO ligands was proved. The effect of EFDPO in the intramolecular singlet and triplet energy transfer was also investigated. It is showed that the rigid structure and the appropriate energy levels of EFDPO not only reduce the energy loss induced by the structure relaxation, but also restrain the solvent quenching and facilitate the energy transfer from EFDPO to Eu 3+. The thermal analysis was also performed to prove the improved thermal stability and phase stability of the complex. The complex exhibited good electroluminescent (EL) performance, such as maximum brightness more than 600 cd m −2, E.Q.E. around 2% and stable monochromic red emission at 616 nm.

The fragment spin difference scheme for triplet-triplet energy transfer coupling

To calculate the electronic couplings in both inter- and intramolecular triplet energy transfer (TET), we have developed the "fragment spin difference" (FSD) scheme. The FSD was a generalization from the "fragment charge difference" (FCD) method of Voityuk et al. [J. Chem. Phys. 117, 5607 (2002)] for electron transfer (ET) coupling. In FSD, the spin population difference was used in place of the charge difference in FCD. FSD is derived from the eigenstate energies and populations, and therefore the FSD couplings contain all contributions in the Hamiltonian as well as the potential overlap effect. In the present work, two series of molecules, all-trans-polyene oligomers and polycyclic aromatic hydrocarbons, were tested for intermolecular TET study. The TET coupling results are largely similar to those from the previously developed direct coupling scheme, with FSD being easier and more flexible in use. On the other hand, the Dexter's exchange integral value, a quantity that is often used as an approximate for the TET coupling, varies in a large range as compared to the corresponding TET coupling. To test the FSD for intramolecular TET, we have calculated the TET couplings between zinc(II)-porphyrin and free-base porphyrin separated by different numbers of p-phenyleneethynylene bridge units. Our estimated rate constants are consistent with experimentally measured TET rates. The FSD method can be used for both intermolecular and intramolecular TET, regardless of their symmetry. This general applicability is an improvement over most existing methodologies.

1971-1976 - Profiting from a New Beginning in Geneva.

A few projects of organic photochemistry, which have been investigated in Geneva in 1971-76, are reviewed summarily. They include excited state-selective processes from the n,π* and π,π* triplets and selectively occurring π → π* induced reactions of α,β-unsaturated cyclic ketones, oxadi-π-methane rearrangements and 1,3-acetyl shifts of β,γ-unsaturated methyl ketones, and di-π-methane rearrangements of benzoylnaphthobarrelene compounds. Furthermore, intramolecular triplet energy transfer dependence on rigid donor-acceptor syn-anti topologies between indanone and naphthalene moieties most likely reflects competition between radiationless energy dissipation by through-space aromatic π interaction (amounting to intramolecular exciplex action) and through-σ-bond exchange transfer. Finally, electronically integrating actinometry and time-resolved IR spectroscopy of excited states and reaction intermediates are mentioned.

Phosphorescent Platinum Dyads with Cyclometalated Ligands: Synthesis, Characterization, and Photophysical Studies

The synthesis, characterization, and photophysical properties are reported for a series of (C^N)Pt dyad complexes with cyclometalated ligands (C^N: F = 2-(4′,6′-difluorophenyl)pyridyl, tpy = 2-(4′-methylphenyl)pyridyl, thpy = 2-(2-thienyl)pyridyl, and btp = 2-(2-benzothienyl)pyridyl). The dyads are connected with a bridging ligand, sym-tetraacetylethane (tae), that consists of two 2,4-pentadionate units covalently linked at the 3-position. Stepwise synthesis was applied to obtain both homoleptic and heteroleptic dyads that have been characterized by 1H NMR and elemental analysis. X-ray crystallographic analysis shows a near orthogonal orientation of the two diketonate moieties in the di-tpyPt (84°) and FPt-thpyPt (89°). An investigation of the photophysical properties of the dyads has been carried out. For homodyads, the emission characteristics are governed by the nature of the cyclometalating ligand, allowing the emission to be tuned throughout the visible spectrum. The di-FPt and di-tpyPt dyads show a modest decrease in luminescent lifetimes compared to their mononuclear analogs. The di-FPt complex undergoes efficient self-quenching by an excimer that is presumed to have only π−π interactions. The heterodyads exhibit efficient intramolecular triplet energy transfer leading to the luminescence almost exclusively from the lower-energy moiety. The energy transfer is presumed to be mediated through the bridging tae ligand by a Dexter-like mechanism involving a combination of hopping and superexchange processes.

Photochemistry synthesis. Part 1: Syntheses of xanthine derivatives by photolysis of 1-(5′-oxohexyl)-3,7-dimethyl-3,7-dihydro-1 H-purine-2,6-dione (pentoxifylline): an ambident chromophore

We investigated the use of photochemistry to make novel derivatives of pentoxifylline. Under conditions that favour singlet excited states, we obtained 1-allyl-3,7-dimethyl-3,7-dihydro-1 H-purine-2,6-dione, ( R ∗, R ∗)-(±)-1-{[2-hydroxy-2-methylcyclobutyl]methyl}-3,7-dimethyl-3,7-dihydro-1 H-purine-2,6-dione and 1-(5-hydroxyhexyl)-3,7-dimethyl-3,7-dihydro-1 H-purine-2,6-dione. Naphthalene or molecular oxygen increases the yields and triplet sensitisers (acetophenone, benzophenone and acetone) decrease the yields. Efficient intramolecular triplet energy transfer from the carbonyl to the xanthine moiety allows the carbonyl moiety to react from a singlet excited state only. In solvents with an α-hydroxyalkyl hydrogen under conditions that favour triplet excited states, we obtained 8-substituted pentoxifylline derivatives: 8-(1-hydroxy-1-methylethyl)-3,7-dimethyl-1-(5-oxohexyl)-3,7-dihydro-1 H-purine-2,6-dione in isopropanol, 8-(1-hydroxymethyl)-3,7-dimethyl-1-(5-oxo-hexyl)-3,7-dihydro-1 H-purine-2,6-dione in methanol and 8-(1-hydroxyethyl)-3,7-dimethyl-1-(5-oxohexyl)-3,7-dihydro-1 H-purine-2,6-dione in ethanol. The xanthine moiety reacts from a triplet state via a radical mechanism and yields are considerably improved by the addition of catalytic amounts of di- tert-butyl peroxide.

Intramolecular Triplet Energy Transfer via Higher Triplet Excited State during Stepwise Two-Color Two-Laser Irradiation

We studied the energy transfer processes in the molecular array consisting of pyrene (Py), biphenyl (Ph2), and bisphthalimidethiophene (ImT), (Py-Ph2)2-ImT, during two-color two-laser flash photolysis (2-LFP). The first laser irradiation predominantly generates ImT in the lowest triplet excited state (ImT(T1)) because of the efficient singlet energy transfer from Py in the lowest singlet excited state to ImT and, then, intersystem crossing of ImT. ImT(T1) was excited to the higher triplet excited state (Tn) with the second laser irradiation. Then, the triplet energy was rapidly transferred to Py via a two-step triplet energy transfer (TET) process through Ph2. The efficient generation of Py(T1) was suggested from the nanosecond-picosecond 2-LFP. The back-TET from Py(T1) to ImT was observed for several tens of microseconds after the second laser irradiation. The estimated intramolecular TET rate from Py(T1) to ImT was as slow as 3.1 x 104 s-1. Hence, long-lived Py(T1) was selectively and efficiently produced during the 2-LFP.