Triplet Excited State Properties Research Articles

Unraveling the Configuration Modulation in Spiro-Based Through-Space Charge Transfer Materials.

Thermally activated delayed fluorescence (TADF) materials hold promise for optoelectronic applications. Among various design strategies, through-space charge transfer (TSCT) systems offer the potential for enhanced performance. However, the relationship between molecular configuration and TSCT properties remains unclear compared to traditional through-band charge transfer materials. In this study, we investigated the influence of spatial configuration on TSCT features and electronic properties of triplet excited states in these TSCT materials. By manipulating the spatial arrangement between donor and acceptor segments using different spiro skeletons, a series of TSCT materials (DMB2-DMB5) was synthesized. Together with the parent molecule, DM-B, these materials exhibited completely different TADF characteristics, demonstrating the impact of spatial arrangements on their optoelectronic properties. Thus, the external quantum efficiency of these materials ranged from as high as 28.0 % (DMB2) to as low as 3.6 % (DMB5) due to variations in their TADF characteristics. Our findings highlight the significance of spatial configuration, beyond distance alone, in influencing TSCT properties when donor and acceptor segments are sufficiently close. This insight provides valuable guidance for developing advanced TSCT materials and advancing TADF systems with improved performance and functionality.

Aggregation-induced phosphorescence emission-active heteroleptic Ir(III) complexes: Synthesis, photophysics, and latent fingerprint detection applications

Three heteroleptic Ir(III) complexes (Ir-TPA, Ir-CzPh and Ir-PhCz) with different electron-donating aryl groups on C^N ligands were synthesized. Their optical properties were systematically investigated by spectroscopic methods and theoretical calculations to elucidate the effects of introducing different electron-donating groups on their triplet excited state properties. All three complexes possess aggregation-induced phosphorescence emission (AIPE) behaviors. Critically, the relationship between the size of organic ligands and AIPE behaviors for these complexes has been characterized. Their significant AIPE properties predicted that they could be used as efficient latent fingerprint (LFP) developers. Rapid (within 10 s) incubation in Ir(III) complex solution produced high-resolution, naked-eye visible and mobile-photographable LFP images, which could provide fast, reproducible and identifiable analyses of level 3 details for LFPs accurately. In addition, complex Ir-PhCz shows the best imaging of the LFP due to its green light emission and high quantum yield (24.5 %) in solid state. This rapid and practical LFP visualization methodology based on AIPE developers could provide a reliable solution for LFPs detection and investigations in actual scenarios.

Enabling Photo‐Crosslinking and Photo‐Sensitizing Properties for Synthetic Fluorescent Protein Chromophores

AbstractSynthetic fluorescent protein chromophores have been reported for their singlet state fluorescence properties and applications in bioimaging, but rarely for the triplet state chemistries. Herein, we enabled their photo‐sensitizing and photo‐crosslinking properties through rational modulations. Extension of molecular conjugation and introduction of heavy atoms promoted the generation of reactive oxygen species. Unlike other photosensitizers, these chromophores selectively photo‐crosslinked aggregated proteins and uncovered the interactome profiles. We also exemplified their general applications in chromophore‐assisted light inactivation, photodynamic therapy and photo induced polymerization. Theoretical calculation, pathway analysis and transient absorption spectroscopy provided mechanistic insights for this triplet state chemistry. Overall, this work expands the function and application of synthetic fluorescent protein chromophores by enabling their triplet excited state properties.

Enabling Photo-Crosslinking and Photo-Sensitizing Properties for Synthetic Fluorescent Protein Chromophores.

Synthetic fluorescent protein chromophores have been reported for their singlet state fluorescence properties and applications in bioimaging, but rarely for the triplet state chemistries. Herein, we enabled their photo-sensitizing and photo-crosslinking properties through rational modulations. Extension of molecular conjugation and introduction of heavy atoms promoted the generation of reactive oxygen species. Unlike other photosensitizers, these chromophores selectively photo-crosslinked aggregated proteins and uncovered the interactome profiles. We also exemplified their general applications in chromophore-assisted light inactivation, photodynamic therapy and photo induced polymerization. Theoretical calculation, pathway analysis and transient absorption spectroscopy provided mechanistic insights for this triplet state chemistry. Overall, this work expands the function and application of synthetic fluorescent protein chromophores by enabling their triplet excited state properties.

Synthesis and Fluorescence Studies of Diborene Coinage Metal Complexes.

Nine different coinage metal (Cu, Ag) π complexes of diborenes with various anionic diborene (aryl, heteroaryl) and metal substituents (Cl, Br, C6F5, C2SiMe3), stabilizing neutral donors (N-heterocyclic carbene = NHC, phosphine), configurations (cis/trans, acyclic/cyclic diborene), and charges (neutral, cationic) were synthesized and characterized by multinuclear NMR spectroscopy and X-ray crystallographic analyses. Their optical properties were investigated by UV-vis absorption and steady-state as well as time-resolved luminescence spectroscopy in solution and the solid state to gain insights into the excited-state behavior of this unusual class of photoactive compounds and to provide structure-property relationships. The structural and electronic modification of the (B═B)···M motif greatly influences not only the visible light absorption but also the photostability and quantum yields, which can reach high values of up to f = 0.42. The lifetimes are found in the nanosecond regime, providing estimated radiative rate constants over a wide range of kr = 1.3-14 × 107 s-1, indicative of fluorescence. Intersystem crossing (ISC) is sufficiently slow for prompt emission from the S1 state to be observed, while the spin-orbit coupling in the T1 state is too weak for phosphorescence to occur at room temperature. ISC can be accelerated, however, by modifying diborene ligand substitution and the coinage metal center, hinting at the potential for exploiting the properties of long-lived triplet excited states of metal diborene complexes in the future.

Fluorene-decorated Ir(III) complexes: synthesis, photophysics and tunable triplet excited state properties in aggregation.

Two new heteroleptic cationic Ir(III) complexes bearing benzothiazole and pyridine motifs on fluorene groups were synthesized and characterized. Complexes Ir1 and Ir2 exhibit 1π,π* transitions below 430 nm, with broad but weak metal-to-ligand and ligand-to-ligand charge transfer absorption bands above 430 nm. Both complexes possess long-lived emissions (τem = 0.37 μs for Ir1, τem = 5.41 μs for Ir2) and triplet excited states (τTA = 0.14 μs for Ir1, τTA = 6.06 μs for Ir2). Their optical properties in solution and aggregated states were also investigated. Both Ir(III) complexes exhibit aggregation-induced phosphorescence emission behavior in an acetonitrile-water mixture. As the water content increased to 90%, the emission intensities of complexes Ir1 and Ir2 increased nearly 3 times and 2.8 times, respectively. Moreover, nonlinear transmittance experiments were performed in CH3CN and a CH3CN-H2O mixture (fw = 90%), and the strength of the reverse saturable absorption (RSA) at 532 nm followed the trend: Ir2 (CH3CN) ≥ Ir1 (CH3CN) > Ir2 (CH3CN-H2O) > Ir1 (CH3CN-H2O). Abundant triplet state excitons participated in the radiative transition progress, which inhibited the RSA process. These results indicate that the fluorene-decorated Ir(III) complexes are suitable for aggregation emission and optical power limiting applications.

Modulating the intersystem crossing mechanism of anthracene carboxyimide-based photosensitizers via structural adjustments and application as a potent photodynamic therapeutic reagent.

Herein, a series of compact anthracene carboxyimide (ACI) based donor-acceptor dyads were prepared by substituting bulky aryl moieties with various electron-donating ability to study the triplet-excited state properties. The ISC mechanism and triplet yield of the dyads were successfully tuned via structural manipulation. Efficient ISC (ΦΔ ≈ 99%) and long-lived triplet state (τT ≈ 122 μs) was observed for the orthogonal anthracene-labeled ACI derivative compared to the Ph-ACI and NP-ACI dyads, which showed fast triplet state decay (τT ≈ 7.7 μs). Femtosecond transient absorption study demonstrated the ultrafast charge separation (CS) and efficient charge recombination (CR) in the orthogonal dyads and ISC occurring via spin-orbit charge transfer (SOCT) mechanism (AN-ACI: τCS = 355 fs, τCR = 2.41 ns; PY-ACI: τCS = 321 fs, τCR = 1.61 ns), while in Ph-ACI and NP-ACI dyads triplet populate following the normal ISC channel (nπ* → ππ* transition), no CS was observed. We found that the attachment of suitable aryl donor moiety (AN- or PY-) to the ACI core can ensure the insertion of the intermediate triplet state, resulting in a small energy gap among charge separated state (CSS) and triplet state, which leads to efficient ISC in these derivatives. The SOCT-ISC-based AN-ACI dyad was confirmed to be a potent photodynamic therapeutic reagent; an ultra-low IC50 value (0.27 nM) that was nearly 214 times lower than that of the commercial Rose Bengal photosensitizer (57.8 nM) was observed.

Tuning the Triplet Excited State of Bis(dipyrrin) Zinc(II) Complexes: Symmetry Breaking Charge Transfer Architecture with Exceptionally Long Lived Triplet State for Upconversion.

Zinc(II) bis(dipyrrin) complexes, which feature intense visible absorption and efficient symmetry breaking charge transfer (SBCT) are outstanding candidates for photovoltaics but their short lived triplet states limit applications in several areas. Herein we demonstrate that triplet excited state dynamics of bis(dipyrrin) complexes can be efficiently tuned by attaching electron donating aryl moieties at the 5,5'-position of the complexes. For the first time, a long lived triplet excited state (τT =296 μs) along with efficient ISC ability (ΦΔ =71 %) was observed for zinc(II) bis(dipyrrin) complexes, formed via SBCT. The results revealed that molecular geometry and energy gap between the charge transfer (CT) state and triplet energy levels strongly control the triplet excited state properties of the complexes. An efficient triplet-triplet annihilation upconversion system was devised for the first time using a SBCT architecture as triplet photosensitizer, reaching a high upconversion quantum yield of 6.2 %. Our findings provide a blueprint for the development of triplet photosensitizers based on earth abundant metal complexes with long lived triplet state for revolutionary photochemical applications.

Spectroscopic characterization of dipicolinic acid and its photoproducts as thymine photosensitizers.

Dipicolinic acid (DPA), present in large amount in bacterial spores, has been proposed to act as an endogenous photosensitizer in spore photoproduct formation. The proposed mechanism involves a triplet-triplet energy transfer from DPA to thymine. However, up to now, no spectroscopic studies have been performed to determine the interaction between the endogenous compound and the nucleobase, probably due to its photolability in aqueous solutions. Here, triplet excited state properties of DPA are reported together with its bimolecular quenching rate constant by thymidine, kq of ca. 5.3×109M-1s-1. To run more reliable studies, a stable methyl ester derivative of DPA, which exhibits the same spectroscopic properties as the parent compound, is also described. Finally, DPA photoproducts are characterized. Studies of their triplet excited state properties have demonstrated that, interestingly, one of them is able to photosensitize thymidine triplet excited state formation.

AIPE-active Pt(ii) complexes with a tunable triplet excited state: design, mechanochromism and application in anti-counterfeiting

A series of AIPE-active Pt(ii) complexes exhibit tunable triplet excited state properties, mechanochromic behavior and potential application in anti-counterfeiting.

Tuning the SOCT-ISC of bodipy based photosentizers by introducing different electron donating groups and its application in triplet-triplet-annihilation upconversion

To study the effect of electron-donating ability on spin orbit charge transfer intersystem crossing (SOCT-ISC) efficiency, a series of bodipy derivatives (BDP-O, BDP-2-MN, BDP-2-EN) linked with electron-donating groups with different electron-donor abilities were synthesized. For BDP-2-MN and BDP-2-EN, both compounds show a sharp charge transfer emission band at 630 nm, and the local excited (LE) emission in both compounds were completely quenched due to photo-induced electron transfer (PET). By femtosecond transient absorption measurement, the triplet state population in both compounds was confirmed to occur through charge recombination (1996 ps ~2307 ps) process after the photo-induced electron transfer upon photoexcitation of bodipy moiety. A long-lived triplet excited state was observed for both compounds (lifetimes: 37 ~ 42 μs) via nanosecond transient absorption. The BDP-2-MN and BDP-2-EN can act as the triplet photosensitizers without heavy elements for the application of triplet-triplet-annihilation upconversion (TTA-UC, ΦUC can reach 4.88%). In case of BDP-O molecule, no triplet signal was observed for this compound because of poor electronic coupling between donor and acceptor. Therefore, the triplet excited state properties of bodipy chromophore was successfully regulated by adjusting electronic strength of donor moiety and implement these photosensitizers in the application of TTA-UC.

Novel Aza-BODIPY based turn on selective and sensitive probe for on-site visual detection of bivalent copper ions

3-Ethoxy-N,N-diethylaniline substituted aza-BODIPY dye (7) was designed and synthesized that could recognize Cu (II) ions through sequential binding interactions. The visible changes in the absorption and enhancement in fluorescence intensity with the addition of Cu (II) ions makes this dye as a selective and sensitive probe for on-site visual detection and analysis. The presence of 3-ethoxy-N,N-diethylaniline group at the peripheral positions shifted its absorption to the near-infrared region, ca. 640–890 nm of the electromagnetic spectrum. This dye showed a gradual shift in the absorption maximum towards red with the increase in polarity of the medium. The triplet excited state properties of this dye suggest that it shows negligible singlet oxygen generation efficiency. The dye possesses excellent photostability, low fluorescence quantum yields, sharp absorption bands in the near-infrared region (NIR) and exhibits selective interactions with Cu(II) ions. The limit of detection was found to be as low as 350 nm and selectivity was not interfered by other metal ions and anions, thereby demonstrating its use as a novel probe for the detection and analysis of copper ions through visual absorption changes and ‘turn on’ fluorescence intensity.

Tetraphenylethene 9,10-Diphenylanthracene Derivatives - Synthesis and Photophysical Properties.

A series of tetraphenylethene 9,10-diphenylanthracene (TPE-DPA) derivatives have been synthesized, and their photophysical properties studied. Photoluminescence measurements in PMMA, neat films and nanoparticle dispersions reveal that different aggregation states are formed, which leads to different photophysical behavior. The triplet excited state properties were studied using Pt(II) octaethylporphyrin (PtOEP) as triplet sensitizer. Upconverted emission from the DPA moiety is observed in nanoparticle dispersions of each derivative. A higher upconverted emission intensity is observed in aerated (compared to deaerated) solutions of the derivatives following irradiation, which is attributed to oxidation of the TPE moiety. These results provide valuable insight for the design of AIE luminogens for triplet-triplet annihilation upconversion (TTA-UC).

The DFT/MRCI method

In the past two decades, the combined density functional theory and multireference configuration interaction (DFT/MRCI) method has developed from a powerful approach for computing spectral properties of singlet and triplet excited states of large molecules into a more general multireference method applicable to states of all spin multiplicities. In its original formulation, it shows great efficiency in the evaluation of singlet and triplet excited states which mainly originate from local one‐electron transitions. Moreover, DFT/MRCI is one of the few methods applicable to large systems that yields the correct ordering of states in extended π‐systems where double excitations play a significant role. A recently redesigned DFT/MRCI Hamiltonian extends the application range of the method to bi‐chromophores such as hydrogen‐bonded or π‐stacked dimers and loosely coupled donor–acceptor systems. In conjunction with a restricted‐open shell Kohn–Sham optimization of the molecular orbitals, even electronically excited doublet and quartet states can be addressed. After a short outline of the general ideas behind this semi‐empirical method and a brief review of alternative approaches combining density functional and multireference wavefunction theory, formulae for the DFT/MRCI Hamiltonian matrix elements are presented and the adjustments of the two‐electron contributions are discussed. The performance of the DFT/MRCI variants on excitation energies of organic molecules and transition metal compounds against experimental or ab initio reference data is analyzed and case studies are presented which show the strengths and limitations of the method. Finally, an overview over the properties available from DFT/MRCI wavefunctions and further developments is given.This article is categorized under: Electronic Structure Theory > Density Functional Theory Electronic Structure Theory > Semiempirical Electronic Structure Methods Software > Quantum Chemistry

Sunscreen-Based Photocages for Topical Drugs: A Photophysical and Photochemical Study of A Diclofenac-Avobenzone Dyad.

Photosensitization by drugs is a problem of increasing importance in modern life. This phenomenon occurs when a chemical substance in the skin is exposed to sunlight. Photosensitizing drugs are reported to cause severe skin dermatitis, and indeed, it is generally advised to avoid sunbathing and to apply sunscreen. In this context, the nonsteroidal anti-inflammatory drug (NSAID) diclofenac is a photosensitive drug, especially when administered in topical form. In this work, efforts have been made to design and study an innovative pro-drug/pro-filter system containing diclofenac and the UVA filter avobenzone in order to develop a safer use of this topical drug. The design is based on the presence of a well-established photoremovable phenacyl group in the avobenzone structure. Steady-state photolysis of the dyad in hydrogen-donor solvents, monitored by UV-Vis spectrophotometry and HPLC, confirms the simultaneous photorelease of diclofenac and avobenzone. Laser flash photolysis and phosphorescence emission experiments allow us to gain insight into the photoactive triplet excited-state properties of the dyad. Finally, it is shown that avobenzone provides partial photoprotection to diclofenac from photocyclization to carbazole derivatives.

A study on excited-state properties of conjugated polymers using the Pariser-Parr-Pople-Peierls model combined with configuration-interaction-singles

Singlet and triplet excited-state properties of a single poly-p-phenylenevinylene (PPV) chain are studied using the Pariser-Parr-Pople-Peierls (P-P-P-P) model combined with the configuration interaction singles (CIS) formalism. The analytic CIS energy gradient is derived using the coupled perturbed Hartree-Fock (CPHF) equations in order to perform excited-state geometry optimizations. According to whether the diagonal elements of transition density matrices being zero or not, all excited states can be classified into two types: type I with nonzero diagonal elements, and type II with zero diagonal elements. In general, type I excited states can be directly generated upon photoexcitation, and there is high probability to find the electron and the hole very close to each other. While type II excited states cannot be directly generated upon photoexcitation, and there is high probability to find the electron and the hole far to each other. Using the parameters developed for this model, we examine the energy levels, oscillator strengths, optical absorption spectrum, and optimized excited-state geometries of the excited states at low energies, and they are consistent with other theoretical work very well. This work provides a new perspective on the excited-state properties of conjugated polymers.

Singly and Doubly N-Confused Calix[4]phyrin Organoplatinum(II) Complexes as Near-IR Triplet Sensitizers

Organoplatinum(II) complexes of calix[4]phyrin analogues, singly N-confused calix[4]phyrin (Pt-2), and doubly N-confused calix[4]phyrin (Pt-3), were synthesized and characterized. The explicit structures of these organoplatinum(II) complexes were elucidated by single-crystal X-ray diffraction and spectroscopic studies. The introduction of N-confused pyrrole rings to the parent calix[4]phyrin scaffold was found to have profound effects on the photophysical properties, such as the bathochromic shifts of both the absorption and phosphorescence maxima. The triplet excited state properties of these platinum complexes were analyzed by DFT calculations at the B3LYP level. The organoplatinum(II) complexes derived from the deformed scaffolds can serve as potent triplet sensitizers for singlet oxygen generation under aerobic conditions.

Triplet Excited State of BODIPY Accessed by Charge Recombination and Its Application in Triplet-Triplet Annihilation Upconversion.

The triplet excited state properties of two BODIPY phenothiazine dyads (BDP-1 and BDP-2) with different lengths of linker and orientations of the components were studied. The triplet state formation of BODIPY chromophore was achieved via photoinduced electron transfer (PET) and charge recombination (CR). BDP-1 has a longer linker between the phenothiazine and the BODIPY chromophore than BDP-2. Moreover, the two chromophores in BDP-2 assume a more orthogonal geometry both at the ground and in the first excited state (87°) than that of BDP-1 (34-40°). The fluorescence of the BODIPY moiety was significantly quenched in the dyads. The charge separation (CS) and CR dynamics of the dyads were studied with femtosecond transient absorption spectroscopy (kCS = 2.2 × 1011 s-1 and 2 × 1012 s-1 for BDP-1 and BDP-2, respectively; kCR = 4.5 × 1010 and 1.5 × 1011 s-1 for BDP-1 and BDP-2, respectively; in acetonitrile). Formation of the triplet excited state of the BODIPY moiety was observed for both dyads upon photoexcitation, and the triplet state quantum yield depends on both the linker length and the orientation of the chromophores. Triplet state quantum yields are 13.4 and 97.5% and lifetimes are 13 and 116 μs for BDP-1 and BDP-2, respectively. The spin-orbit charge transfer (SO-CT) mechanism is proposed to be responsible for the efficient triplet state formation. The dyads were used for triplet-triplet annihilation (TTA) upconversion, showing an upconversion quantum yield up to 3.2%.

Intersystem crossing and triplet excited state properties of thionated naphthalenediimide derivatives

Thionated naphthalenediimide (NDI) derivatives with 2,6-dibromo and diaminoalkyl substituents are prepared by treating of 1,4,5,8-naphthalenetetracarboxylic dianhydride with Lawesson's reagent. Red shifted absorption was obtained for all thionated derivates as compared to the respective oxo-compounds. Thionated NDI derivative with 2,6-diamino substituents shows strong absorption at 674nm. Nanosecond time-resolved transient absorption spectroscopy confirmed the population of triplet excited state of the thionated compounds upon photoexcitation and the triplet state quantum yield was determined as ΦT = 58%. Thionated derivatives were used for singlet oxygen (1O2) photosensitizing, the compound shows high singlet oxygen quantum yield (ΦΔ = 56%) with excitation at 675nm. With femtosecond transient absorption spectroscopy, the intersystem crossing (ISC) of the thionated NDI was found to take 32ps. Based on TD-DFT computations, the ISC of these heavy atom-free NDI is attributed to the S1 → T2/T3 transition, which is in 1(π-π)* → 3(n-π)* feature, which obeys with El-Sayed rule. Moreover, the S1/T3 state energy gap is 0.02eV, much smaller than that of S1/T1 energy gap (0.86eV). These results are useful for the design of heavy-atom-free triplet photosensitizers.

Helical Self-Assembly-Induced Singlet-Triplet Emissive Switching in a Mechanically Sensitive System.

In nanoscience, chirality has shown a significant ability to tune materials' electronic properties, whereas imposing macrochirality into the regulation of singlet-triplet features of organic optoelectronics remains a challenging research topic. Since the tuning for singlet and triplet excited-state properties in a single π-functional molecule connects to its multicolor luminescent application and potential improvement of internal quantum efficiency, we here report that supramolecular chirality can be employed to toggle the singlet and triplet emissions in a well-designed asterisk-shaped molecule. Employing a hexathiobenzene-based single luminophore as a prototype and functionalizing it with chiral α-lipoiate side groups, we find that helical nanoarchitectures can accordingly form in mixed DMF/H2O solution. On this basis, switching between fluorescence and phosphorescence of the material can be realized upon helical self-assembly and dissociation. Such a behavior can be attributed to a helical-conformation-dependent manipulation of the intersystem crossing. Furthermore, reversible mechanoluminescence of the corresponding solid sample was also observed to rely on an analogous molecular self-assembly alternation. These results can probably provide new visions for the development of next-generation supramolecular chiral functional materials.