Accessing the Long-Lived Triplet Excited States in Transition-Metal Complexes: Molecular Design Rationales and Applications

The Emissive and Electrochemical Properties of Hypervalent Pyridine-Dipyrrolide Bismuth Complexes

We present a series of six hypervalent bismuth complexes Bi(R1PDPR2)X bearing ligands characterized by the pyridine-2,6-bis(pyrrolide) (PDP) structural motif. While bismuth holds considerable potential for facilitating efficient intersystem crossing (ISC), reports on phosphorescent molecular bismuth complexes are still scarce and mostly based on systems that exhibit inter- or intraligand charge transfer character of their optical excitations. Herein, the UV/vis absorptive, luminescent, and electrochemical properties of complexes Bi(R1PDPR2)X are explored, where the substituents R1 and R2, as well as the halide ligand X are varied. These compounds are characterized by an intense HOMO→LUMO transition of mixed ligand-to-metal charge transfer (LMCT) and interligand charge transfer (LL'CT) character, as shown by time-dependent density functional theory (TD-DFT) calculations. At 77 K in a 2-MeTHF matrix, these compounds exhibit red, long-lived phosphorescence with lifetimes ranging from 479 to 14 μs. Cyclic voltammetry measurements and TD-DFT calculations show that the substituents influence HOMO and LUMO energies to almost equal extent, resulting in nearly constant emission wavelengths throughout this series. Single-crystal X-ray diffraction studies of four of the six complexes exemplify the inherent Lewis acidity of the coordinated Bi3+ ion, in spite of its hypervalency.

Tunability of triplet excited states and photophysical behaviour of bis-cyclometalated iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline

This is a comprehensive study of the photophysical behaviour of heteroleptic iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline (imphen) as an ancillary ligand, represented by the general formula [Ir(N∩C)2(imphen)]PF6. As cyclometalating ligands, 2-phenylpyridine (Hppy), 2-phenylquinoline (Hpquin), 2-phenylbenzothiazole (Hpbztz), and 2-(2-pyridyl)benzothiophene (pybzthH) were used. The impact of structural modifications of cyclometalating ligands was widely explored by a combination of steady-state and time-resolved optical techniques accompanied by theoretical calculations. We evidenced that the cyclometalating ligands induce essential changes in the nature of the emissive excited state and the emission characteristics of [Ir(N∩C)2(imphen)]PF6. While the complex [Ir(ppy)2(imphen)]PF6 (1) is a typical 3MLLCT emitter, the lowest triplet states of [Ir(pquin)2(imphen)]PF6 (2), [Ir(pbztz)2(imphen)]PF6 (3) and [Ir(pybzth)2(imphen)]PF6 (4) have a predominant 3LCN∩C character. The phosphorescence colour of the investigated Ir(III) complexes changes from greenish-yellow to red, their quantum yields vary from 56 to 2%, and their triplet excited-state lifetimes fall in the 743-3840 ns range. The highest photoluminescence quantum yield was revealed for 2 in CH2Cl2, while complex 3 in MeCN shows the most pronounced increase in the lifetime. Both complexes 2 and 3 show an increased efficiency of singlet oxygen generation. The herein discussed structure-property relationships are of high significance for controlling photoinduced processes in heteroleptic iridium(III) complexes with the imphen-based ancillary ligand, and making further progress in effectively tuning the emission energies, quantum yields and excited-state lifetimes of these systems by structural modifications of cyclometalating ligands, especially the π-conjugation, the position of the N-donor and the presence of sulfur heteroatoms.

Luminescent Platinum(II) Complexes with Stimuli-Responsive Flexible Lewis Pair Ligands: Spectroscopic and Computational Studies

A series of new luminescent bimetallic platinum(II) complexes with stimuli-responsive flexible Lewis pair (FlexLP) ligands are described. The FlexLP ligands consist of a dimesitylboron Lewis acid and diphenylphosphine oxide Lewis base which are in equilibrium between the unbound open form and the Lewis adduct, controlled by the hydrogen bond donating strength of the solvent. Spectroscopic techniques and density functional theory (DFT) calculations were used to interpret the photophysics of the platinum(II) complexes. All complexes exhibit tunable absorption in the region of 300-500 nm and green to orange photoluminescence, depending on the ratio of weak (THF) to strong (MeOH) hydrogen bond donating solvent employed. Spectroscopic and computational data shows that phosphine and peripheral acetylide ligands on the platinum(II) centers have limited influence on the emission energy, indicating the emission originates from the FlexLP-dominated fluorescence. Using time-resolved transient absorption spectroscopy it is shown that the complexes undergo intersystem crossing (ISC) to the triplet excited state upon photoexcitation, and the ISC efficiency is affected by the peripheral acetylide ligands. The triplet excited state lifetime can also be manipulated by the state of the FlexLP ligand, with the closed form complexes having longer lifetimes than the open form complexes.

Aggregation-induced emission luminogens for latent fingerprint detection

For over a century, fingerprints have served as a pivotal tool for identification of individuals owing to their enduring characteristics and easily apparent features, particularly in the realm of criminal investigations. Latent fingerprints (LFPs) are "invisible fingerprints" that are most commonly available at crime scenes and require a rapid, selective, sensitive, and convenient method for detection. However, existing fingerprint development techniques harbour limitations, prompting the exploration of novel approaches that prioritize investigator safety and environmental sustainability. Leveraging the unique photophysical properties of aggregation-induced emission luminogens (AIEgens) has emerged as a promising strategy for on-site analysis of LFP visualization. In this highlight, we have presented a comparative analysis of various AIEgens (organic compounds, metal complexes, nanoparticles, and polymers) for the development and detection of LFPs. Through this examination, insights into the efficiency and potential applications of AIE-based fingerprint development techniques are provided. In addition, several strategies have been proposed for circumventing the limitations of existing AIEgens. We hope that this highlight article will encourage more researchers to investigate AIEgens in LFP detection, contributing to forensic science.

Structural and Photophysical Trends in Rhenium(I) Carbonyl Complexes with 2,2':6',2″-Terpyridines

This is the first comprehensive review of rhenium(I) carbonyl complexes with 2,2':6',2″-terpyridine-based ligands (R-terpy)-encompassing their synthesis, molecular features, photophysical behavior, and potential applications. Particular attention has been devoted to demonstrating how the coordination mode of 2,2':6',2″-terpyridine (terpy-κ2N and terpy-κ3N), structural modifications of terpy framework (R), and the nature of ancillary ligands (X-mono-negative anion, L-neutral ligand) may tune the photophysical behavior of Re(I) complexes [Re(X/L)(CO)3(R-terpy-κ2N)]0/+ and [Re(X/L)(CO)2(R-terpy-κ3N)]0/+. Our discussion also includes homo- and heteronuclear multicomponent systems with {Re(CO)3(R-terpy-κ2N)} and {Re(CO)2(R-terpy-κ3N)} motifs. The presented structure-property relationships are of high importance for controlling the photoinduced processes in these systems and making further progress in the development of more efficient Re-based luminophores, photosensitizers, and photocatalysts for modern technologies.

Expanding the Anti-Stokes Shift of TTA-UC to 1.04 eV by Chemical Tuning of BODIPY-Based Organic Photosensitizers

A series of BODIPY-derivatives B-1 ∼ B-5 were synthesized as metal-free photosensitizers in triplet-triplet annihilation (TTA) upconversion (UC) with the purpose of finely tuning the excited state properties to reduce the energy loss during intersystem crossing (ISC), and hence to expand the anti-Stokes shift of TTA-UC. It was revealed that B-5 showed a ΔEST of as small as 0.28 eV, thus, providing a large anti-Stokes shift of 1.04 eV in TTA-UC with organic sensitizers. A deep red-blue upconversion emission with UC quantum yields of up to 10.7% (out of a 50% maximum) was observed.

Further Insights into the Impact of Ligand-Localized Excited States on the Photophysics of Phenanthroline-Based Rhenium(I) Tricarbonyl Complexes

The present work shows the pivotal role of N-donor substituents attached to 1,10-phenanthroline at the 4,7-positions in perturbation of ground- and excited-state properties of fac-[ReCl(CO)3(R2phen)]. Excited-state processes occurring upon photoexcitation in the designed systems were thoroughly explored with a wide range of steady-state and time-resolved spectroscopic techniques, including transient absorption, as well as experimental results were complemented by theoretical studies based on the density functional theory (DFT). It was demonstrated that the attachment of six-membered heterocyclic amines (piperidine─ppr, morpholine─mor, and thiomorpholine─tmor) is a very effective tool for extending absorptivity and excited-state lifetimes of resulting fac-[ReCl(CO)3(R2phen)] due to the contribution of the excited state localized on the phenanthroline-based ligand. Both absorption and emission properties of these systems were attributed to configurationally mixed MLCT/IL excited states. Re(I) complexes with phenoxazine (pxz) and phenothiazine (ptz) substituents were shown to possess charge-separated excited states, clearly evidenced by the simultaneous presence of signals typical of phen-* and pxz+* or ptz+* in transient absorption spectra. Both complexes are rare examples of NIR light-emitting coordination compounds. The decoration of the phen framework with less polar 9,9-dimethyl-9,10-dihydroacridine (dmac) groups resulted in the formation of [ReCl(CO)3(R2phen)] with mixed 3MLCT/3ILCT triplet excited state.

Computational study of the photophysical properties and electronic structure of iridium(III) photosensitizer complexes with electron-withdrawing groups

A series of novel [Ir(tpy)(btp)Cl]+ complexes (Ir1-Ir4) have been reported to show excellent performance as photosensitizers. The introduction of electron-withdrawing groups increases visible light absorption and the lifetime of triplet states. To improve the photophysical properties, we theoretically design Ir5-Ir9 with electron-withdrawing groups (Cl, F, COOH, CN and NO2). Surprisingly, our findings indicate that the photosensitizer performance does not strictly increase with the electron-withdrawing ability of the substituents. In this work, the geometric and electronic structures, transition features, and photophysical properties of Ir1-Ir9 are investigated. The natural transition orbital (NTO) analysis indicates that the T1 and T2 states play a role in the photochemical pathways. Ultraviolet-visible (UV-vis) absorption spectra and charge-transfer spectra (CTS) have been investigated to show that the introduction of electron-withdrawing groups not only improves the visible light absorbing ability, but also changes the nature of electron excitation, providing a future molecular design strategy for similar series of photosensitizers. The rates of (reverse) intersystem crossing and the Huang-Rhys factors are evaluated to interpret the experimental results within the framework of Marcus theory. For complexes Ir1-Ir7, the introduction of electron-withdrawing groups leads to a lower efficiency of reverse intersystem crossing and a strong non-radiative process T2 → T1, resulting in a long triplet lifetime and excellent performance as a photosensitizer. Furthermore, some newly designed complexes (Ir7-Ir9) show great potential as thermally activated delayed fluorescence emitters, contrary to our initial expectations.

Pyrenyl-Substituted Imidazo[4,5-f][1,10]phenanthroline Rhenium(I) Complexes with Record-High Triplet Excited-State Lifetimes at Room Temperature: Steric Control of Photoinduced Processes in Bichromophoric Systems

Photochemical applications based on intermolecular photoinduced energy triplet state transfer require photosensitizers with strong visible absorptivity and extended triplet excited-state lifetimes. Using a bichromophore approach, two Re(I) tricarbonyl complexes with 2-(1-pyrenyl)-1H-imidazo[4,5-f][1,10]phenanthroline (pyr-imphen) and 1-(4-(methyl)phenyl)-2-(1-pyrenyl)-imidazo[4,5-f][1,10]phenanthroline (pyr-tol-imphen) showing extraordinary long triplet excited states at room temperature (>1000 μs) were obtained, and their ground- and excited-state properties were thoroughly investigated by a wide range of spectroscopic methods, including femtosecond transient absorption (fs-TA). It is worth noting that the designed [ReCl(CO)3(pyr-imphen)] (1) and [ReCl(CO)3(pyr-tol-imphen)] (2) complexes form a unique pair differing in the mutual chromophore arrangement due to introduction of a 4-(methyl)phenyl substituent into the imidazole ring at the H1-position, imposing an increase in the dihedral angle between the pyrene and {ReCl(CO)3(imphen)} chromophores. The magnitude of the electronic coupling between the pyrene and {ReCl(CO)3(imphen)} chromophores was found to be an efficient tool to tune the photophysical properties of 1 and 2. The usefulness of designed Re(I) compounds as triplet photosensitizers was successfully verified by examination of their abilities for 1O2 generation and triplet-triplet annihilation upconversion. The phosphorescence lifetimes, ∼1800 μs for 1 and ∼1500 μs for 2, are the longest lifetimes reported for Re(I) diimine carbonyl complexes in solution at room temperature.

Employing Long-Range Inductive Effects to Modulate Metal-to-Ligand Charge Transfer Photoluminescence in Homoleptic Cu(I) Complexes

Four Cu(I) bis(phenanthroline) photosensitizers formulated from a new ligand structural motif (Cu1-Cu4) coded according to their 2,9-substituents were synthesized, structurally characterized, and fully evaluated using steady-state and time-resolved absorption and photoluminescence (PL) measurements as well as electrochemistry. The 2,9-disubstituted-3,4,7,8-tetramethyl-1,10-phenanthroline ligands feature the following six-membered ring systems prepared through photochemical synthesis: 4,4-dimethylcyclohexyl (1), tetrahydro-2H-pyran-4-yl (2), tetrahydro-2H-thiopyran-4-yl (3), and 4,4-difluorocyclohexyl (4). Universally, these Cu(I) metal-to-ligand charge transfer (MLCT) chromophores display excited-state lifetimes on the microsecond time scale at room temperature, including the three longest-lived homoleptic cuprous phenanthroline excited states measured to date in de-aerated CH₂Cl₂, τ = 2.5-4.3 μs. This series of molecules also feature high PL quantum efficiencies (Φ_PL = 5.3-12% in CH₂Cl₂). Temperature-dependent PL lifetime experiments confirmed that all these molecules exhibit reverse intersystem crossing and display thermally activated delayed PL from a ¹MLCT excited state lying slightly above the ³MLCT state, 1050-1490 cm^-1. Ultrafast and conventional transient absorption measurements confirmed that the PL originates from the MLCT excited state, which remains sterically arrested, preventing an excessive flattening distortion even when dissolved in Lewis basic CH₃CN. Combined PL and electrochemical data provided evidence that Cu1-Cu4 are highly potent photoreductants (E_ox* = -1.73 to -1.62 V vs Fc^+/0 in CH₃CN), whose potentials are altered solely based on which heteroatoms or substituents are resident on the 2,9-appended ring derivatives. It is proposed that long-range electronic inductive effects are responsible for the systematic modulation observed in the PL spectra, excited-state lifetimes, and the ground state absorption spectra and redox potentials. Cu1-Cu4 quantitatively follow the energy gap law, correlating well with structurally related cuprous phenanthrolines and are also shown to triplet photosensitize the excited states of 9,10-diphenylanthracene with bimolecular rate constants ranging from 1.61 to 2.82 × 10⁸ M^-1 s^-1. The ability to tailor both photophysical and electrochemical properties using long-range inductive effects imposed by the 2,9-ring platforms advocates new directions for future MLCT chromophore discovery.

Carrier-free nanomedicines self-assembled from palbociclib dimers and Ce6 for enhanced combined chemo-photodynamic therapy of breast cancer

Palbociclib is the world's first CDK4/6 kinase inhibitor to be marketed. However, it is not effective in the treatment of triple negative breast cancer (TNBC) due to the loss of retinoblastoma protein expression. Thus, combinatorial chemotherapy is indispensable for TNBC treatment. Herein, a carrier-free nanomedicine self-assembled from palbociclib dimers and Ce6 for enhanced combined chemo-photodynamic therapy of breast cancer is reported. The dimeric prodrug (Palb-TK-Palb) was synthesized by conjugating two palbociclib molecules to the connecting skeleton containing a ROS-responsive cleavable thioketal bond. The Palb-TK-Palb/Ce6 NP co-delivery nanoplatform was prepared through the self-assembly of Palb-TK-Palb, Ce6 and DSPE-PEG2000. This novel carrier-free formulation as an efficient therapeutic agent showed efficient therapeutic agent loading capacity, high cellular uptake and huge therapeutic performance against breast cancer cells. The results of in vitro antitumor activity and cell apoptosis demonstrated that Palb-TK-Palb/Ce6 NPs presented a better inhibitory effect on the growth of cancer cells due to the palbociclib and Ce6 co-delivery nanomedicine-mediated synergistic chemo-photodynamic therapy. The IC₅₀ values of Palb-TK-Palb/Ce6 NPs in MDA-MB-231 cells were around 1-2 μM and 2 μM and the Palb-TK-Palb/Ce6 NPs showed an increase in apoptosis up to 91.9%. In general, the carrier-free nanomedicine self-assembled from palbociclib dimers and Ce6 provides options for combinatorial chemo-photodynamic therapy.

Confinement of the Triplet States in π‐Conjugated BODIPY Dimers Linked with Ethynylene or Butadiynylene Bridges: A Different View on the Effect of Symmetry

Understanding the impact of the excited state wavefunction confinement is crucial for the engineering of the photophysical properties and applications of organic chromophores. In the present contribution, the localization of the triplet state wavefunctions of some symmetric ethyne/butadiyne bridged BODIPY dimers and asymmetric BODIPY derivatives presenting extended π-conjugation frameworks is studied with time-resolved electron paramagnetic resonance spectroscopy and time-dependent density functional theory computations. Based on the Zero Field Splitting D parameters, we conclude that the triplet state wavefunctions are highly localized on one BODIPY unit in the symmetric dimers, which is consistent with the ab initio modelling that finds delocalized triplet state destabilized by 12-14 kcal mol^-1 as compared to its localized counterpart. The result provides a new insight into the study of triplet excited state confinement and the design of molecular wires or photosensitizers for photovoltaics and photocatalysis.

Metal Coordination Effects on the Photophysics of Dipyrrinato Photosensitizers

Within this work, we review the metal coordination effect on the photophysics of metal dipyrrinato complexes. Dipyrrinato complexes are promising candidates in the search for alternative transition metal photosensitizers for application in photodynamic therapy (PDT). These complexes can be activated by irradiation with light of a specific wavelength, after which, cytotoxic reactive oxygen species (ROS) are generated. The metal coordination allows for the use of the heavy atom effect, which can enhance the triplet generation necessary for generation of ROS. Additionally, the flexibility of these complexes for metal ions, substitutions and ligands allows the possibility to tune their photophysical properties. A general overview of the mechanism of photodynamic therapy and the properties of the triplet photosensitizers is given, followed by further details of dipyrrinato complexes described in the literature that show relevance as photosensitizers for PDT. In particular, the photophysical properties of Re(I), Ru(II), Rh(III), Ir(III), Zn(II), Pd(II), Pt(II), Ni(II), Cu(II), Ga(III), In(III) and Al(III) dipyrrinato complexes are discussed. The potential for future development in the field of (dipyrrinato)metal complexes is addressed, and several new research topics are suggested throughout this work. We propose that significant advances could be made for heteroleptic bis(dipyrrinato)zinc(II) and homoleptic bis(dipyrrinato)palladium(II) complexes and their application as photosensitizers for PDT.

BINOL blocks as accessible triplet state modulators in BODIPY dyes

BINOL moieties of different electronic demand are useful blocks for enabling the photo-production and modulation of triplet excited states in readily-accesible BINOL-based O-BODIPY dyes from standard F-BODIPY precursors. The rapid and rational development of smarter triplet-enabling BODIPY dyes on the basis of this strategy (e.g., TADF biomarker 4a or room temperature phosphor 4g) paves the way for advancing photonic applications based on organic triplet photosensitizers.

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.

Enhanced phosphorescence of Pd(II) and Pt(II) complexes adsorbed onto Laponite for optical sensing of triplet molecular dioxygen in water

Two isoleptic complexes involving Pd(II) and Pt(II) centres with a tetradentate luminophoric ligand were adsorbed onto Laponite (LAP), yielding H2O-dispersible nanohybrids with remarkable photoexcited state properties. The adsorption promoted rigidification of the coordination compounds and suppression of roto-vibrational deactivation pathways, thus enhancing the performance of the molecular species. Interestingly, room temperature phosphorescence was achieved in the case of the Pd(II)-containing nanohybrid along with a marked 3O2 sensitivity, whereas the photoluminescence of the Pt(II)-based material showed only a mild dependence on the presence of 3O2 while reaching up to 37% quantum yield.

Impact of iodine loading and substitution position on intersystem crossing efficiency in a series of ten methylated-meso-phenyl-BODIPY dyes

Four core and six distyryl-extended methylated-meso-phenyl-BODIPY dyes with varying iodine content were synthesized. The influence of iodine loading and substitution position on the photophysical properties of these chromophores was evaluated. Selective iodine insertion at the 2- and 6-positions of the methylated-meso-phenyl-BODIPY core, rather than maximum iodine content, resulted in the highest intersystem crossing efficiency. Iodination of the distyryl-extended BODIPY core afforded intersystem crossing quantum yields comparable to 2,6-diiodo-BODIPY. Inclusion of an iodine at the para-meso-phenyl position generally enhanced non-radiative decay in the BODIPY excited-state, leading to lower fluorescence and intersystem crossing quantum yield values. Iodine substitution at the styryl-positions resulted in negligible changes to the excited-state dynamics. This study highlights: (1) the rate of radiative decay is similar in all ten derivatives (on the order of 1 × 108 s-1), (2) iodination of the 2,6-positions results in the greatest enhancement of intersystem crossing efficiency, (3) care must be taken when modifying the para-meso-phenyl position as it could have detrimental effects on the excited-state dynamics, (4) the excited-state is negligibly affected by iodination of the styryl groups, potentially enabling orthogonal functionalization without modifying the molecular photophysics, (5) distyryl extension of the chromophore core diminishes rates of non-radiative decay and intersystem crossing, resulting in higher fluorescence quantum yields and lower intersystem crossing yields in the π-extended derivatives compared to the core BDP derivatives, and (6) DFT calculations provide insight into the electronic and structural factors regulating intersystem crossing and vibrational relaxation in these molecules.

Ligand-controlled and nanoconfinement-boosted luminescence employing Pt(ii) and Pd(ii) complexes: from color-tunable aggregation-enhanced dual emitters towards self-referenced oxygen reporters

In this work, we describe the synthesis, structural and photophysical characterization of four novel Pd(ii) and Pt(ii) complexes bearing tetradentate luminophoric ligands with high photoluminescence quantum yields (Φ L) and long excited state lifetimes (τ) at room temperature, where the results were interpreted by means of DFT calculations. Incorporation of fluorine atoms into the tetradentate ligand favors aggregation and thereby, a shortened average distance between the metal centers, which provides accessibility to metal-metal-to-ligand charge-transfer (3MMLCT) excimers acting as red-shifted energy traps if compared with the monomeric entities. This supramolecular approach provides an elegant way to enable room-temperature phosphorescence from Pd(ii) complexes, which are otherwise quenched by a thermal population of dissociative states due to a lower ligand field splitting. Encapsulation of these complexes in 100 nm-sized aminated polystyrene nanoparticles enables concentration-controlled aggregation-enhanced dual emission. This phenomenon facilitates the tunability of the absorption and emission colors while providing a rigidified environment supporting an enhanced Φ L up to about 80% and extended τ exceeding 100 μs. Additionally, these nanoarrays constitute rare examples for self-referenced oxygen reporters, since the phosphorescence of the aggregates is insensitive to external influences, whereas the monomeric species drop in luminescence lifetime and intensity with increasing triplet molecular dioxygen concentrations (diffusion-controlled quenching).

Charge separation, charge recombination and intersystem crossing in orthogonal naphthalimide-perylene electron donor/acceptor dyad

We prepared an orthogonal electron donor/acceptor dyad (NI-Py) with perylene (Py) as electron donor and 4-aminonaphthalimide (NI) as an electron acceptor. The molecule adopts orthogonal geometry due to the steric hindrance exerted by the 4-amino substituents on the NI moiety. The photophysical properties of dyad were studied by steady-state UV-Vis absorption and fluorescence spectroscopies, femtosecond/nanosecond transient absorption spectroscopies and DFT computations. Ground state interaction between the NI and Py units is negligible; however, charge separation occurs upon photoexcitation, indicated by the quenching of the fluorescence of the dyad in polar solvents, i.e. fluorescence quantum yield (Φ_F) is 61.9% in toluene and Φ_F = 0.2% in methanol. Spin-orbit-coupled charge transfer-induced intersystem crossing (SOCT-ISC) was confirmed by femtosecond transient absorption spectroscopy (charge separation takes 1.7 ps and charge recombination takes 6.9 ns, in CH₂Cl₂). Nanosecond transient absorption spectra indicated the formation of perylene-localized triplet state, and the triplet state lifetime (175 μs) is much longer than that accessed with the heavy atom effect (3-bromoperylene; 16 μs). The singlet oxygen quantum (Φ_Δ) yield of the dyad is 2.2% in hexane and 9.5% in dichloromethane. The low SOCT-ISC efficiency as compared to the previously reported analogue (Φ_Δ = 80%) is attributed to the mismatch of the ¹CT/T_n state energies, and/or the orientation of the NI and Py units, i.e. orthogonal geometry is not sufficient for achieving efficient SOCT-ISC in compact electron donor/acceptor dyads.

An Activatable Triplet Sensitizer Based on Triplet Electron Transfer and Its Application for Triplet-Triplet Annihilation Upconversion

Activatable triplet photosensitization refers to a photosentization process which can be turned on/off easily by external stimulus. Activatable triplet photosensitizations are normally achieved by interfering with the singlet excited state before the intersystem cross process (ISC), i.e., the formation process of triplet states of sensitizer. To achieve novel activatable triplet photosensitization, a disulfide-bridged porphyrin zinc(II) dyad (ZnPor-S-S-ZnPor) is prepared. Although fast ISC can be conducted in this dyad, an extremely low efficiency is obtained when employing this dyad as a triplet donor in triplet-triplet annihilation upconversion (TTA-UC) for sensitizing perylene. This is because of the presence of electron transfer from the triplet state of the porphyrin zinc(II) unit to the disulfide bond, which quickly quenches the triplet state of the porphyrin zinc(II) unit. This electron transfer process can be stopped by the cleavage of the disulfide bond in the presence of thiol, and TTA-UC efficiency can be enhanced significantly. Our result demonstrates for the first time that the disulfide bond can act as not only an easy cleavage linker but also a triplet electron acceptor. Furthermore, quenching the triplet states of sensitizer by triplet electron transfer provides an alternative protocol for designing activatable triplet sensitizers except controlling the singlet excited state before the ISC process.

A Platinum(II) Complex of Heptamethine Cyanine for Photoenhanced Cytotoxicity and Cellular Imaging in Near-IR Light

Controlled generation of cytotoxic agents with near-IR light is a current focus of photoactivated cancer therapy, including that involving cytotoxic platinum species. A heptamethine cyanine scaffolded PtII complex, IR797-Platin exhibits unprecedented Pt-O bond scission and enhancement in DNA platination in near-IR light. This complex also displayed significant singlet oxygen quantum yield thereby qualifying as a near-IR photodynamic therapeutic agent. The complex showed 30-60 fold enhancement of cytotoxicity in near-IR light in various cancer cell lines. The cellular imaging properties were also leveraged to observe its significant co-localization in cytoplasmic organelles. This is the first demonstration of a near-IR light-initiated therapy involving the cytotoxic effects of both active cisplatin and singlet oxygen.

Recent strategies to improve boron dipyrromethene (BODIPY) for photodynamic cancer therapy: an updated review

BODIPYs are photosensitizers activatable by light to generate highly reactive singlet oxygen (¹O₂) from molecular oxygen, leading to tissue damage in the photoirradiated region. Despite their extraordinary photophysical characteristics, they are not featured in clinical photodynamic therapy. This review discusses the recent advances in the design and/or modifications of BODIPYs since 2013, to improve their potential in photodynamic cancer therapy and related areas.

Novel ruthenium and iridium complexes of N-substituted carbazole as triplet photosensitisers

Novel mono- and di-nuclear Ru(ii) and Ir(iii) complexes, bearing a modified carbazole moiety are synthesised. In comparison to their mononuclear analogues, the homonuclear diatomic complexes (RuCRu and IrCIr), in which the carbazole containing-ligand functions as a bridge, display increased absorbance in the visible region, and give rise to higher singlet oxygen quantum yields.

Luminescent Cyclometalated Gold(III) Alkyl Complexes: Photophysical and Photochemical Properties

A series of luminescent cyclometalated gold(III) complexes having alkyls as auxiliary ligands has been prepared. The alkyl ligand was found to effectively increase the emission quantum yields and lifetimes of luminescent cyclometalated gold(III) complexes by circumventing the population of LLCT excited states that are found in complexes supported by arylacetylide ligands. These gold(III) alkyl complexes exhibit emission quantum yields and lifetimes of up to 0.40 and 180 μs, respectively, in solution at room temperature. The triplet emission color of these complexes is tunable from yellow to sky blue by modifying the cyclometalating ligand.

Recent development of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents, anticancer drugs, and antibacterial agents

This Perspective summarizes recent advances in the biological applications of luminescent rhenium(i) tricarbonyl polypyridine complexes.

Influence of amino acid side chains on the formation of two component self-assembling nanofibrous hydrogels

Supramolecular co-assembly of amino acid derivatives capped with NDI and Py moieties undergoing supramolecular hydrogelation was developed.