Triplet Excited State Research Articles

Twisted Benzothieno-Fused BOPHY Dyes as Triplet Photosensitizers with Long-Lived Triplet Excited States.

The synthesis of a series of photostable [b]-benzothieno-fused BOPHY derivatives is reported via one-pot condensation of formylated isoindoles or formylindoles with hydrazine and subsequent boron complexation. These dyes show strong absorption in the deep red region and acceptable fluorescence quantum yields (∼30%). The two fused benzothiophene moieties are slightly deviated from the BOPHY core (with dihedral angles of 6.1 and 10.2°). This slightly twisted conformation brings an enhanced spin-orbit coupling and a reduced energy band gap between singlet and triplet states. The enhanced intersystem cross process endows these series of dyes with a good singlet oxygen quantum yield (up to 63%), a high triplet-state quantum yield (up to 78%), and a long lifetime value (up to 127 μs). Density functional theory calculations indicate that the transition from S1 to T2 states is crucial for triplet-state formation, highlighting their high efficiency in intersystem crossing. The calculated triplet electron spin surface reveals a widespread distribution of triplet states across the conjugated molecular structure, which enhances the Dexter mechanism for triplet energy transfer in these BOPHY photosensitizers. These findings are helpful for thorough understanding of the fundamental ISC process and developing triplet photosensitizers.

Luminescence Changeable CO2-Storage Cylinder: Triple-Stranded Helical Eu(III)/Tb(III) Fluorinated MOFs with Amide Linkers.

Triple-stranded helical lanthanide MOFs with CO2 adsorption properties were investigated. Lanthanide MOFs ([Eu0.1Tb0.9(hfa)3(dpa)]n) are composed of lanthanide luminophores (Eu(III) and/or Tb(III) ions), fluorinated antenna ligands (hfa: hexafluoroacetylacetonate), and polyamide-type linker ligands (dpa: 4-(diphenylphosphoryl)-N-(4-(diphenylphosphoryl)phenyl)benzamide). The cylindrical structure was characterized by single-crystal X-ray analysis, thermogravimetric analysis, and gas adsorption measurements. The inner surfaces of the cylindrical channels were covered with the fluorine atoms of the hfa ligands. The emission intensity ratio (IEu/ITb) in [Eu0.1Tb0.9(hfa)3(dpa)]n is affected by the CO2 gas adsorption behavior. The change in IEu/ITb value was caused by the intermolecular interactions between the CO2 gas molecules and the fluorinated ligands, resulting in an electronic structural change of the lowest triplet excited state in the photosensitized hfa ligands.

1,4-Benzodioxane Substituted Aza-BODIPY: Towards Photostable yet Efficient Triplet Photosensitizer.

We report herein the synthesis of aza-BODIPY substituted with 1,4-benzodioxane-6-yl substituents at 3,5 positions of the chromophore system. Both pyrrole rings of the aza-BODIPY in question were substituted with bromine atoms in order to induce highly desirable photophysical properties, such as highly populated excited triplet state (T1) and long excited triplet-state lifetime (τT) of 21 μs. The photosensitized oxygenation of a model compounds, viz. DPBF, points to a high singlet oxygen and/or other ROS formation quantum yield of 0.42. The photosensitizer studied exhibited an absorption band within the so-called "therapeutic window", with λabs 678 nm. As estimated by CV/DPV measurements the 1,4-benzodioxane-6-yl substituted aza-BODIPYs studied exhibited a multi-electron oxidations at a relatively low potentials (Eox), pointing to the very good electron-donating properties of these molecules. High photostability and thermal stability was observed for all compounds studied. The good singlet oxygen quantum yield measured combined with an exceptional photostability makes this aza-BODIPY a promising candidate for applications such as photocatalysis and photodynamic therapy (PDT).

Face-to-Face Gold Porphyrins.

The interaction of square-planar metal complexes through space is of fundamental interest and relevant for the potential cooperative catalysis of two metal complex sites. In order to elucidate gold/gold, gold/porphyrin, and porphyrin/porphyrin interactions in the formal oxidation states +III and +II (after reduction) and in the excited triplet state after light excitation, a Pacman bis(gold(III)) complex [Au2(DPD)][PF6]2 with square-planar face-to-face gold(porphyrin) moieties has been prepared and characterized. Absorption and luminescence spectroscopy, cyclic voltammetry, and EPR spectroscopy on [Au2(DPD)]n+ and a mononuclear reference are complemented by DFT and TDDFT calculations.

Formation of reactive nitrogen species promoted by iron ions through the photochemistry of a neonicotinoid insecticide

Abstract. Nitrous acid (HONO) and nitrogen oxides (NOx=NO+NO2) are important atmospheric pollutants and key intermediates in the global nitrogen cycle, but their sources and formation mechanisms are still poorly understood. Here, we investigated the effect of soluble iron (Fe3+) on the photochemical behavior of a widely used neonicotinoid (NN) insecticide, nitenpyram (NPM), in the aqueous phase. The yields of HONO and NOx increased significantly when NPM solution was irradiated in the presence of iron ions (Fe3+). We propose that the enhanced HONO and NO2 emissions from the photodegradation of NPM in the presence of iron ions result from the redox cycle between Fe3+ and Fe2+ and the generated reactive oxygen species (ROS) by electron transfer between the excited triplet state of NPM and molecular oxygen (O2). Using the laboratory-derived parameterization based on kinetic data and gridded downward solar radiation, we estimate that the photochemistry of NPM induced by Fe3+ releases 0.50 and 0.77 Tg N yr−1 of NOx and HONO, respectively, into the atmosphere. This study suggests a novel source of HONO and NOx during daytime and potentially helps to narrow the gap between field observations and model outcomes of HONO in the atmosphere. The suggested photochemistry of NPM can be an important contribution to the global nitrogen cycle affecting the atmospheric oxidizing capacity and climate change.

Recent Advances in Thermally Activated Delayed Fluorescence-Based Organic Afterglow Materials.

Thermally activated delayed fluorescence (TADF)-based materials are attracting widespread attention for different applications owing to their ability of harvesting both singlet and triplet excitons without noble metals in their structures. As compared to the conventional fluorescence and room-temperature phosphorescence pathways, TADF originates from the reverse intersystem crossing process from the excited triplet state (T1) to the singlet state (S1). Therefore, TADF emitters enabling activated and long lifetime T1 excitons are potential candidates for generating long-lived afterglow emission, an effect that can still be observed for a while by the naked eye after the removal of the excitation light source. Recently, TADF-based organic afterglow materials featuring high photoluminescence quantum yields and long lifetimes above 100 ms under ambient conditions, have emerged for advanced information security, high-contrast biological imaging, optoelectronic devices, and intelligent sensors, whereas the related systematic review is still lacking. Herein, the recent progress in TADF-based organic afterglow materials is summarized and an overview of the photophysical mechanism, design strategies, and the performances for relevant applications is given. In addition, the challenge and perspective of this area are given at the end of the review.

Hypoxia-Active Iridium(III) Bis-terpyridine Complexes Bearing Oligothienyl Substituents: Synthesis, Photophysics, and Phototoxicity toward Cancer Cells.

In an effort to develop hypoxia-active iridium(III) complexes with long visible-light absorption, we synthesized and characterized five bis(terpyridine) Ir(III) complexes bearing oligothienyl substituents on one of the terpyridine ligands, i.e., nT-Ir (n = 0-4). The UV-vis absorption, emission, and transient absorption spectroscopy were employed to characterize the singlet and triplet excited states of these complexes and to explore the effects of varied number of thienyl units on the photophysical parameters of the complexes. In vitro photodynamic therapeutic activities of these complexes were assessed with respect to three melanoma cell lines (SKMEL28, A375, and B16F10) and two breast cancer cell lines (MDA-MB-231 and MCF-7) under normoxia (∼18.5% oxygen tension) and hypoxia (1% oxygen tension) upon broadband visible (400-700 nm), blue (453 nm), green (523 nm), and red (633 nm) light activation. It was revealed that the increased number of thienyl units bathochromically shifted the low-energy absorption bands to the green/orange spectral regions and the emission bands to the near-infrared (NIR) regions. The lowest triplet excited-state lifetimes and the singlet oxygen generation efficiency also increased from 0T to 2T substitution but decreased in 3T and 4T substitution. All complexes exhibited low dark cytotoxicity toward all cell lines, but 2T-Ir-4T-Ir manifested high photocytotoxicity for all cell lines upon visible, blue, and green light activation under normoxia, with 2T-Ir showing the strongest photocytotoxicity toward SKMEL28, MDA-MB-231, and MCF-7 cells, and 4T-Ir being the most photocytotoxic one for B16F10 and A375 cells. Singlet oxygen, superoxide anion radicals, and peroxynitrite anions were found to likely be involved in the photocytotoxicity exhibited by the complexes. 4T-Ir also showed strong photocytotoxicity upon red-light excitation toward all cell lines under normoxia and retained its photocytotoxicity under hypoxia toward all cell lines upon visible, blue, and green light excitation. The hypoxic activity of 4T-Ir along with its green to orange light absorption, NIR emission, and low dark cytotoxicity suggest its potential as a photosensitizer for photodynamic therapy applications.

Anomalous Pressure-Induced Blue-Shifted Emission of Ionic Copper-Iodine Clusters: The Competitive Effect between Cuprophilic Interactions and Through-Space Interactions.

Developing ionic copper-iodine clusters with multiple emitting is crucial for enriching lighting and display materials with various colors. However, the luminescent properties of traditional ionic copper-iodine clusters are often closely associated with low-energy cluster-centered triplet emission, which will redshift further as the Cu⋅⋅⋅Cu bond length decreases. This article utilizes a pressure-treated strategy to achieve an anomalous pressure-induced blue-shifted luminescence phenomenon in ionic Cu4I6(4-dimethylamino-1-ethylpyridinium)2 crystals for the first time, which is based on dominant through-space charge-transfer (TSCT). Herein, we reveal that the more advantageous through-space interactions in the competition between cuprophilic interactions and through-space interactions can lead to a blue-shifted luminescence. High-pressure angle-dispersive X-ray diffraction and high-pressure infrared experiments show that the enhanced through-space interactions mainly originate from forming new intermolecular C-H⋅⋅⋅I hydrogen bonds and the enhancement of van der Waals interactions between organic cations and anionic clusters. Theoretical calculations and experimental studies of excited-state dynamics confirm that the blue-shifted emission is due to the increased energy gap between the excited triplet and ground states caused by the electron delocalization under stronger through-space interactions. This work deepens previous understanding and provides a new avenue to design and synthetic ionic copper-iodine clusters with high-energy TSCT emission.

Anomalous Pressure‐Induced Blue‐Shifted Emission of Ionic Copper‐Iodine Clusters: The Competitive Effect between Cuprophilic Interactions and Through‐Space Interactions

AbstractDeveloping ionic copper‐iodine clusters with multiple emitting is crucial for enriching lighting and display materials with various colors. However, the luminescent properties of traditional ionic copper‐iodine clusters are often closely associated with low‐energy cluster‐centered triplet emission, which will redshift further as the Cu⋅⋅⋅Cu bond length decreases. This article utilizes a pressure‐treated strategy to achieve an anomalous pressure‐induced blue‐shifted luminescence phenomenon in ionic Cu4I6(4‐dimethylamino‐1‐ethylpyridinium)2 crystals for the first time, which is based on dominant through‐space charge‐transfer (TSCT). Herein, we reveal that the more advantageous through‐space interactions in the competition between cuprophilic interactions and through‐space interactions can lead to a blue‐shifted luminescence. High‐pressure angle‐dispersive X‐ray diffraction and high‐pressure infrared experiments show that the enhanced through‐space interactions mainly originate from forming new intermolecular C−H⋅⋅⋅I hydrogen bonds and the enhancement of van der Waals interactions between organic cations and anionic clusters. Theoretical calculations and experimental studies of excited‐state dynamics confirm that the blue‐shifted emission is due to the increased energy gap between the excited triplet and ground states caused by the electron delocalization under stronger through‐space interactions. This work deepens previous understanding and provides a new avenue to design and synthetic ionic copper‐iodine clusters with high‐energy TSCT emission.

Full‐Color Tunability Room Temperature Phosphorescence from Inorganic Crystal Frameworks

AbstractRoom temperature afterglow (RTA) materials have aroused prodigious research interests owing to their unique long‐life luminescent properties. However, it is rare for RTA materials to be reported with full‐color afterglow emission. Herein, a universal strategy is designed to activate full‐color tunability RTA based on phenylboronic acid derivatives (BOH) and boric acid (BA) via an ingenious solvent‐free solid‐phase heating. As the conjugation degrees of aromatic groups of the guest molecule expanded, the BOH/BA composites showed tunable afterglow colors ranging from blue to red after ultraviolet (UV) irradiation. This strategy is to firmly anchor the guest molecule in a rigid framework of the inorganic metaboric acid matrix by abundant hydrogen‐bonding interactions between the host and guest, suppressing molecular motion and promoting the intersystem crossing (ISC) rate between the singlet and triplet excited states for enhanced afterglow emission. In addition, through mixing in multiple guest molecules into BA simultaneously, the obtained composite can be modulated in a wide excitation range from blue to red. This fascinating study provides a simple and universal method to fabricate full‐color tunability RTA composites, making them a promising candidate for applications in advanced information security and multi‐level encryption.

Bipyridyldicarboxamides and f-metals: the influence of electron effects on the structure, stability, separation, and photophysical properties of their complexes.

In this work, three isomeric fluorinated bipyridyldicarboxamides were studied to evaluate the impact of the fluorine atom position on the structure, stability, Am(III)/Ln(III) separation, and photophysical properties of their complexes. The complexes of the fluorinated amides have a metal-to-ligand composition of 1 : 1, which is independent of the fluorine atom position or lanthanide metal. The bipyridyl fragments in the fluorinated complexes are flattened compared with those in unsubstituted ones. Ln-to-heteroatom distances are more affected by steric hindrance in the ligand and further by lanthanide ion radius contraction. This leads to significant effectivity of heavy lanthanide extraction compared with the light ones, particularly for 4F diamide. Fluorination leads to a slight variation in the excited triplet state of the complexes, and hence, the effectiveness of luminescence increases for Eu, Sm, and Tb complexes. Moreover, fluorination significantly affects the CIE chromaticity coordinates for the complexes.

Photophysics and photochemistry of (n-Bu4N)2[Pt(NO3)6] in acetonitrile: ultrafast pump-probe spectroscopy and quantum chemical insight.

The ultrafast processes caused by photoexcitation of (n-Bu4N)2[Pt(NO3)6] complex in acetonitrile were studied by means of transient absorption (TA) pump-probe spectroscopy and verified by quantum chemical calculations. The primary photochemical process was found to be an inner-sphere electron transfer followed by an escape of an •NO3 radical to the bulk solution. The reaction occurs via the dissociative triplet excited LMCT state of the initial complex. Based on the experimental data and quantum chemical calculations, the mechanism of ultrafast photophysical and photochemical processes is proposed.

Unveiling the role of external electric field on the charge transport and excited-state properties of high T1 host for blue OLED devices

Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed to understand the effect of the external electric field (EEF) on the reorganization (λ) and the lowest triplet excited-state (T1) energies of high T1 blue host materials. Depending on the direction and the strength of the external electric field (EEF), the positive and negative changes of hole and electron λ(h and λ e) values were found in these materials. More importantly, λ e seems to be more sensitive than λ h values under the EEF. It is also noticed that the calculated T1 energies are meaningfully changed in the application of EEFx and EEFy. In contrast, the effect of EEFz on the T1 energies can be negligible. From the results of theoretical investigation, the obvious evidence related to the influence of EEF on the charge transport and excited-state properties of high T1 blue host materials were obtained. In the present work, we expect that our theoretical study will provide new insight into understanding the influence of EEF as a key player in manipulating essential properties of the high T1 blue host material during the electrical operation.

An Extension of the Stern-Volmer Equation for Thermally Activated Delayed Fluorescence (TADF) Photocatalysts.

Fluorescence quenching experiments are essential mechanistic tools in photoredox catalysis, allowing one to elucidate the first step in the catalytic cycle that occurs after photon absorption. Thermally activated delayed fluorescence (TADF) photocatalysts, however, yield nonlinear Stern-Volmer plots, thus requiring an adjustment to this widely used method to determine the efficiency of excited state quenching. Here, we derive an extension of the Stern-Volmer equation for TADF fluorophores that considers quenching from both the singlet and triplet excited states and experimentally verify it with fluorescence quenching experiments using the commonly employed TADF-photocatalyst 4CzIPN, and multiple-resonance TADF-photocatalyst QAO with three different quenchers in four solvents. The experimental data are perfectly described by this new equation, which in addition to the Stern-Volmer quenching constants allows for the determination of the product of intersystem and reverse intersystem crossing quantum yields, a quantity that is independent of the quencher.

Photoexcitation and One-Electron Reduction Processes of a CO2 Photoreduction Dyad Catalyst Having a Zinc(II) Porphyrin Photosensitizer.

We have explored the photophysical properties and one-electron reduction process in the dyad photocatalyst for CO2 photoreduction, ZnP-phen=Re, in which the catalyst of fac-[Re(1,10-phenanthoroline)(CO)3Br] is directly connected with the photosensitizer of zinc(II) porphyrin (ZnP), using time-resolved infrared spectroscopy, transient absorption spectroscopy, and quantum chemical calculations. We revealed the following photophysical properties: (1) the intersystem crossing occurs with a time constant of ∼20 ps, which is much faster than that of a ZnP single unit, and (2) the charge density in the excited singlet and triplet states is mainly localized on ZnP, which means that the excited state is assignable to the π-π* transition in ZnP. The one-electron reduction by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole occurs via the triplet excited state with the time constant of ∼100 ns and directly from the ground state with the time constant of ∼3 μs. The charge in the one-electron reduction species spans ZnP and the phenanthroline ligand, and the dihedral angle between ZnP and the phenanthroline ligand is rotated by ∼24° with respect to that in the ground state, which presumably offers an advantage for proceeding to the next CO2 reduction process. These insights could guide the new design of dyad photocatalysts with porphyrin photosensitizers.

Lowering of the singlet-triplet energy gap via intramolecular exciton-exciton coupling

Organic dyes typically have electronically excited states of both singlet and triplet multiplicity. Controlling the energy difference between these states is a key factor for making efficient organic light emitting diodes and triplet sensitizers, which fulfill essential functions in chemistry, physics, and medicine. Here, we propose a strategy to shift the singlet excited state of a known sensitizer to lower energies without shifting the energy of the triplet state, thus without compromising the ability of the sensitizer to do work. We covalently connect two to four sensitizers in such a way that their transition dipole moments are aligned in a head-to-tail fashion, but, through steric encumbrance, the delocalization is minimized between each moiety. Exciton coupling between the singlet excited states considerably lowers the first excited singlet state energy. However, the energy of the lowest triplet excited state is unperturbed because the exciton coupling strength depends on the magnitude of the transition dipole moments, which for triplets are very small. We expect that the presented strategy of designed intramolecular exciton coupling will be a useful concept in the design of both photosensitizers and emitters for organic light emitting diodes as both benefits from a small singlet-triplet energy gap.

Theoretical Investigation of Novel Nitrogen-Heterocyclic Iridium(III) Polypyridyl Complexes as Photosensitizers for Two-Photon Photodynamic Therapy.

Two-photon photodynamic therapy (TP-PDT) has become a major cancer treatment due to its larger tissue penetration depth, good spatial selectivity, and less damage to normal cells. In this contribution, a series of novel photosensitizer molecules (Ir-2, Ir-2-1∼Ir-2-4) have been designed based on the experimentally demonstrated photosensitizer [Ir(ppy)2(osip)] (PF6) by fine tuning the π-conjugated structure and introducing different nitrogen-heterocyclic substituents. The electronic structures, one- and two-photon absorption spectra, triplet excited state lifetime, solvation-free energy, and photosensitizing performance were evaluated by means of density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results suggested that the molecule Ir-2, incorporating thiophene as the π-connecting group, exhibits a higher probability of triplet state formation, enhanced two-photon absorption cross-section, and prolonged triplet state lifetime. Furthermore, the four designed nitrogen-heterocyclic complexes Ir-2-1∼Ir-2-4 demonstrate favorable photosensitizing properties, with two-photon absorption cross-sections reaching up to 110 GM and triplet excited state lifetimes exceeding 1000 μs for Ir-2-4.

Room-temperature phosphorescence and aggregation behavior in chiral heavy-atom-free organic molecules

Purely organic room temperature phosphorescence materials (RTP) have attracted much attention recently, but most of them are substituted with heavy atoms to enhance the intersystem crossing (ISC), which requires complicated design and synthesis. Herein, we report four chiral heavy-atom-free small molecules which integrate properties of aggregation and long-lifetime room temperature phosphorescence. The phosphorescence lifetime of synthesized chiral molecules is measured to be 150 ms, and the phosphorescence quantum yield reaches 15 % at room temperature. The twisted chiral conformation of four molecules not only affect aggregation photoluminescence properties but also can synergistically stabilize triplet exciton in the triplet excited states for excellent ISC efficiency. This strategy enriches the application fields of chiral aggregated long-lifetime room temperature phosphorescent materials.

Electron Spin Dynamics of the Intersystem Crossing in Aminoanthraquinone Derivatives: The Spectral Telltale of Short Triplet Excited States.

We studied the excited state dynamics of two bis-amino substituted anthraquinone (AQ) derivatives, with absorption in the visible spectral region, which results from the attachment of a electron-donating group to the electron-deficient AQ chromophore. Femtosecond transient absorption spectra show that intersystem crossing (ISC) takes place in 190-320 ps, and nanosecond transient absorption spectra demonstrated an unusually short triplet state lifetime (2.06-5.43 μs) for the two AQ derivatives. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show an inversion of the electron spin polarization (ESP) phase pattern of the triplet state at a longer delay time after laser flash. Spectral simulations show faster decay of the Ty sublevel than the other two sublevels (τx = 15.0 μs, τy = 1.50 μs, τz = 15.0 μs); theoretical computation predicts initial overpopulation of the Ty sublevel, and rationalizes the short T1 state lifetime and the ESP inversion. Theoretical computations taking into account the electron-vibrational coupling, i.e., the Herzberg-Teller effect, successfully rationalize the TREPR experimental observations.

Triad photosensitizers based on iodo-aza-Bodipy and naphthalenediimides (NDI) with broadband absorption: Study of resonance energy transfer and electron transfer

Two organic photosensitizers based on resonance energy transfer (A-1 and A-2) are prepared. Naphthalenediimides was used as intramolecular energy donor and iodo-aza-Bodipy was used as intramolecular energy acceptor/spin converter. A-1(ε = 42800 at 618 nm and ε = 59600 at 674 nm) and A-2 (ε = 33300 at 514 nm and ε = 68300 at 674 nm) give the two strong absorption band and both of them show broadband absorption in visible region. The photophysical properties of the compounds were studied with steady state and time-resolved spectroscopy in detail. With steady state and time-resolved spectroscopy, we found that photoexcitation into the energy donor was followed by singlet energy transfer, then via the intersystem crossing (ISC) of the energy acceptor. By nanosecond time-resolved transient absorption spectroscopy and DFT calculation, triplet excited state is localized on the iodo-aza-Bodipy moiety. Quantum yield of singlet oxygen of A-1 and A-2 is 39.7% and 40.9 %, respectively. Based on fluorescence lifetime quenching, the efficiency of intramolecular energy transfer is estimated to be 15.9% and 27.7% for A-1 and A-2. And PET is responsible for the relatively low efficiency, which is identified by the calculation of kcs / kFRET and the Gibbs free energy change(△G0cs). A-1 and A-2 were used for singlet oxygen (1O2) mediated photooxidation of 1,5-dihydroxylnaphthalene and the photosensitizing ability are more efficient than the triplet photosensitizers with the mono-chromophore photosensitizer.