Long-lived Triplet Excited States Research Articles

Investigating Light-Induced Processes in Covalent Dye-Catalyst Assemblies for Hydrogen Production

The light-induced processes occurring in two dye-catalyst assemblies for light-driven hydrogen production were investigated by ultrafast transient absorption spectroscopy. These dyads consist of a push-pull organic dye based on a cyclopenta[1,2-b:5,4-b’]dithiophene (CPDT) bridge, covalently linked to two different H2-evolving cobalt catalysts. Whatever the nature of the latter, photoinduced intramolecular electron transfer from the excited state of the dye to the catalytic center was never observed. Instead, and in sharp contrast to the reference dye, a fast intersystem crossing (ISC) populates a long-lived triplet excited state, which in turn non-radiatively decays to the ground state. This study thus shows how the interplay of different structures in a dye-catalyst assembly can lead to unexpected excited state behavior and might open up new possibilities in the area of organic triplet sensitizers. More importantly, a reductive quenching mechanism with an external electron donor must be considered to drive hydrogen production with these dye-catalyst assemblies.

Long-Lived Local Triplet Excited State and Charge Transfer State of 4,4'-Dimethoxy Triphenylamine-BODIPY Compact Electron Donor/Acceptor Dyads.

The spin-orbit charge transfer intersystem crossing (SOCT-ISC) and the formation of a long-lived charge transfer (CT) state were studied with a series of 4,4'-dimethoxy triphenylamine-BODIPY compact electron donor/acceptor dyads. Different torsion freedoms were applied in the dyads to tune the electronic coupling between the donor and acceptor, and a red-shifted CT absorption band was observed for one dyad. The dyads show solvent polarity-dependent singlet oxygen photosensitizing ability (quantum yields 3%-79%). Nanosecond transient absorption spectra of the dyad in nonpolar solvent confirm the formation of triplet states. The intrinsic triplet state lifetime is up to 383 μs (in fluid solution), which is much longer than that accessed with the heavy atom effect (276 μs). Intermolecular triplet photosensitizing of the dyads in a polar solvent produces a long-lived 3CT state (lifetime, τCT = 8.0 μs supported by the electron spin density surface analysis). The triplet state lifetime of the dyads doped in a Clear Flex 50 polymer film is exceptionally long (7.6-11.4 ms), and formation of a long-lived CT state (37 μs) was observed. Triplet-triplet annihilation upconversion was performed with the electron donor/acceptor dyads used as the triplet photosensitizer and perylene used as the triplet acceptor; the upconversion quantum yield is up to 15.8%.

Efficient Thermally Activated Delayed Fluorescence from All-Inorganic Cesium Zirconium Halide Perovskite Nanocrystals.

Thermally activated delayed fluorescence (TADF) is generally observed in solid-state organic molecules or metal-organic complexes. However, TADF in all-inorganic colloidal nanocrystals (NCs) is rare. Herein, we report the first colloidal synthesis of an air-stable all-inorganic lead-free Cs2 ZrCl6 perovskite NCs. The Cs2 ZrCl6 NCs exhibit long-lived triplet excited state (138.2 μs), and feature high photoluminescence (PL) quantum efficiency (QY=60.37 %) due to TADF mechanism. The emission color can be easily tuned from blue to green by synthesizing the mixed-halide Cs2 ZrBrx Cl6-x (0≤x≤1.5) NCs. Femtosecond transient absorption and temperature dependent PL measurements are performed to clarify the emission mechanism. In addition, Bi3+ ions are successfully doped into Cs2 ZrCl6 NCs, which further extends the PL properties. This work not only develops a new lead-free halide perovskite NCs for potential optoelectronic applications, but also offers unique strategies for developing new inorganic phosphors.

Highly efficient triplet-triplet annihilation upconversion in high viscosity phthalate ester media

Movement of molecules are always affected by the surrounding solvent molecules, so solvents have an essential effect in triplet-triplet annihilation (TTA) upconversion. In this article, the TTA upconversion system with Diiodo-BODIPY (MBDP-2I) as sensitizer and perylene as acceptor in six solvents of differing viscosity were investigated (toluene, 1,4-dioxane and four high viscosity phthalate ester solvents DMP, DBP, DNOP and DOP). The upconversion efficiency was positively correlated with the solvent viscosity. Among the six solvents, the highest upconversion quantum efficiency of 9.59% was observed in high viscosity DOP (η = 72.4 mPa s), almost 1.5 times higher than that in 1,4-dioxane (η = 3.2 mPa s, ΦUC = 6.36%). The efficient upconversion was attributed to the long-lived triplet excited states of MBDP-2I, efficient fluorescence quantum yield of perylene and efficient TTA in high viscosity phthalate esters solvents. The mechanism underlying the highly efficient TTA upconversion in the high viscosity phthalate esters solvents was investigated by solvent cage effect. In addition, significant room temperature phosphorescence of MBDP-2I at 765 nm was observed in the viscous phthalate esters solvents.

N^N Pt(II) Bisacetylide Complexes with Oxoverdazyl Radical Ligands: Preparation, Photophysical Properties, and Magnetic Exchange Interaction between the Two Radical Ligands.

To study the effect of a stable radical on the photophysical properties of a phosphorescent Pt(II) coordination framework and the intramolecular magnetic interaction between radical ligands in the N^N Pt(II) bisacetylide complexes, we prepared a series of N^N Pt(II) bis(acetylide) complexes with oxoverdazyl radical acetylide ligands. The linker between the Pt(II) center and the spin carrier was systematically varied, to probe the effect on the sign and magnitude of the spin exchange interactions between the radical ligands and photophysical properties. The complexes were studied with steady-state and femtosecond/nanosecond transient absorption spectroscopy, continuous-wave electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT) computations. The transient absorption spectral studies show that the doublet excited state of the radicals are short-lived (τD ≈ 2 ps) and nonfluorescent. Moreover, the intrinsic long-lived triplet excited state (τT = 1.2 μs) of the Pt(II) coordination center was efficiently quenched by the radical (τT = 6.9 ps for one representative radical Pt(II) complex). The intramolecular magnetic interaction between the radical ligands through the diamagnetic Pt(II) atom was studied with temperature-dependent EPR spectroscopy; antiferromagnetic exchange interaction (-J S1S2, J = -5.4 ± 0.1 cm-1) for the complex with the shortest radical-radical distance through bridge fragments was observed. DFT computations give similar results for the sign and magnitude of the J values. For complexes with larger inter-radical distance, however, very weak coupling between the radical ligands was observed (|J| < 0.7 cm-1). Our results are useful for the study of the effect of a radical on the photophysical properties of the phosphorescent transition-metal complexes.

Divergent Photocatalytic Reactions of α-Ketoesters under Triplet Sensitization and Photoredox Conditions.

The long-lived triplet excited states of transition metal photocatalysts can activate organic substrates via either energy- or electron-transfer pathways, and the rates of these processes can be influenced by rational tuning of the reaction conditions. The characteristic reactive intermediates generated, however, are distinct and can exhibit very different reactivity patterns. This mechanistic diversity available to photocatalytic reactions might thus offer an opportunity to engineer divergent reactions that give markedly different chemical outcomes under superficially similar conditions. Herein, we show that the photocatalytic reactions of benzoylformate esters with alkenes can be directed toward either Paternò-Büchi cycloadditions or allylic functionalization reactions under conditions favoring energy transfer or electron transfer, respectively. These studies provide a framework for designing other divergent photocatalytic methods that produce different sets of reaction outcomes under photoredox and triplet sensitization conditions.

New Selenonapthaquinone-Based Copper (II) Complexes as the Next-Generation Photochemotherapeutic Agents.

Photoactive transition metal complexes like copper complexes find great interest in promoting metal-based photochemotherapeutic agents. In the present study, we explored the photocytotoxic efficacy of new selenylnaphthoquinone-based copper (II) complexes that provide a phenomenal platform in making an effective photo-chemotherapeutic agent via PDT in the clinical field of cancer therapy. Three new copper(II) complexes (1-3) were synthesized in 40-60% yield and characterized analytically/ spectroscopically. ATCC® Normal Adult Human Primary Epidermal Keratinocytes were grown in Dermal Cell Basal Media supplemented with Keratinocyte Growth Kit components, to propagate keratinocytes in serum- free (not animal free) conditions. Anticancer activity of the complexes was studied using MTT (3- [4,5- dimethyltiazol-2-yl]-2,5-diphenyl-tetrazolium bromide) assay. The intracellular ROS (1O2) generation was studied by using Flow Cytometric Analysis (FACS) on HaCaT cells using cell accessible non-polar 2',7'- Dichlorofluorescein Diacetate (DCFH-DA) dye. The Acridine Orange/Ethidium Bromide (AO/EB) dual staining assay was performed for detecting apoptosis in HaCaT cells. Several photophysical studies probing the generation of singlet oxygen was also carried out. We have performed Time-Dependent Density Functional Theory (TD-DFT) calculations using unrestricted B3LYP to understand the mechanism of type-II process. All the complexes were remarkably cytotoxic in HaCaT cells with IC50, 1-4μM under visible light with comparing lower dark toxicity. The presence of low-lying and long-lived triplet excited state allowed effective intersystem crossing and subsequent generation of singlet oxygen, which was the primary cytotoxic species responsible for oxidative stress and apoptosis. The experimental findings are in good agrrement with the computational analysis (TD-DFT). The remarkably enhanced cytotoxicity of the new selenyl copper (II) complexes under the visible light probed the role of Se in photosensitized generation of singlet oxygen which was responsible for apoptosis in HaCaT cells. The results in the present work are of paramount importance in developing next generation copper(II)-based PDT agents.

Persistent Organic Room-Temperature Phosphorescence in Cyclohexane-trans-1,2-Bisphthalimide Derivatives: The Dramatic Impact of Heterochiral vs Homochiral interactions.

Persistent metal-free room-temperature phosphorescence (RTP) materials attract significant interest owing to the production of long-lived triplet excited states. Although several organic designs show RTP, the impact of intermolecular interactions on the triplet excitons stabilization and migrations remains hardly understood because obtaining different ordered intermolecular interactions while conserving identical molecular electronic properties is very challenging. We propose here a new strategy to circumvent this problem by taking advantage of the distinct molecular packing that can be found between enantiomer and racemic forms of a chiral molecule. Structural, photophysical, and chiroptical investigations of chiral cyclohexane bisphthalimide derivatives showed that heterochiral and homochiral dimer interactions play a crucial role on the triplet excited state stabilization, resulting in higher RTP efficiency for enantiopure systems than for racemic one. This study paves the way to the use of molecular chirality to rationalize supramolecular properties arising from subtle intermolecular interactions.

Visible-Light-Driven Triplet Sensitization of Polycyclic Aromatic Hydrocarbons Using Thionated Perinones.

Metal-free chromophores that efficiently generate triplet excited states represent promising alternatives with respect to transition metal-containing photosensitizers, such as those featuring metal-to-ligand charge transfer excited states. However, such molecular constructs have remained underexplored due to the unclear relationship(s) between molecular structure and efficient/rapid intersystem crossing. In this regard, we present a series of three thionated perinone chromophores serving as a newly conceived class of heavy metal-free triplet photosensitizers. We demonstrate that thionation of the lone C═O substituent in each highly fluorescent perinone imparts red-shifted absorbance bands that maintain intense extinction coefficients across the visible spectrum, as well as unusually efficient triplet excited state formation as inferred from the measured singlet O2 quantum yields at 1270 nm (ΦΔ = 0.78-1.0). Electronic structure calculations revealed the emergence of a low energy S1 (n → π*) excited state in the proximity of a slightly higher energy S2 (π → π*) excited state. The distinct character in each of the two lowest-lying singlet state manifolds resulted in the energetic inversion of the corresponding triplet excited states due to differences in electron exchange interactions. Rapid S1 → T1 intersystem crossing was thereby facilitated in this manner through spin-orbit coupling as predicted by the El Sayed rules. The lifetimes of the resultant triplet excited states persisted into the microsecond time regime, as measured by transient absorbance spectroscopy, enabling effective bimolecular triplet sensitization of some common polycyclic aromatic hydrocarbons. The synthetically facile interchange of a single O atom to an S atom in the investigated perinones resulted in marked changes to their photophysical properties, namely, conversion of dominant singlet state fluorescence in the former to long-lived triplet excited states in the latter. The combined results suggest a general strategy for accessing long-lived triplet excited states in organic chromophores featuring a lone C═O moiety residing within its structure, valuable for the design of metal-free triplet photosensitizers.

Strong Visible-Light-Absorbing Cuprous Sensitizers for Dramatically Boosting Photocatalysis.

Developing strong visible-light-absorbing (SVLA) earth-abundant photosensitizers (PSs) for significantly improving the utilization of solar energy is highly desirable, yet it remains a great challenge. Herein, we adopt a through-bond energy transfer (TBET) strategy by bridging boron dipyrromethene (Bodipy) and a CuI complex with an electronically conjugated bridge, resulting in the first SVLA CuI PSs (Cu-2 and Cu-3). Cu-3 has an extremely high molar extinction coefficient of 162 260 m-1 cm-1 at 518 nm, over 62 times higher than that of traditional CuI PS (Cu-1). The photooxidation activity of Cu-3 is much greater than that of Cu-1 and noble-metal PSs (Ru(bpy)3 2+ and Ir(ppy)3 + ) for both energy- and electron-transfer reactions. Femto- and nanosecond transient absorption and theoretical investigations demonstrate that a "ping-pong" energy-transfer process in Cu-3 involving a forward singlet TBET from Bodipy to the CuI complex and a backward triplet-triplet energy transfer greatly contribute to the long-lived and Bodipy-localized triplet excited state.

(Invited) High-Yield Singlet Fission of Tetracene Derivatives Utilizing Organic-Inorganic Hybrid Systems for Light Energy Conversion

Singlet fission (SF) is highly expected to improve the solar energy conversion properties by generating two excitons from one photon absorption. Among various organic compounds, acene derivatives such as tetracene and pentacene are promising for occurrence of efficient SF. Pentacene derivatives sufficiently meet the energy-level matching condition between a singlet and two triplet states: [E(S1) ≥ 2E(T1)]. Although tetracene is well-kwown as a good candidate for SF, the number of high-yield and long-lived triplet excited states through SF is extremely limited because of the relative acceleration of the reverse triplet-triplet annihilation (TTA) considering the above-mentioned energy matching condition. Consequently, the precise control of orientation and distance between two neighboring Tc units is definitely required for long-lived individual triplets through SF but not correlated triplet pair. Because the longer lifetime of the individual triplet states is useful for efficient light energy conversion.Therefore, we first demonstrate the quantitative sequential process from intramolecular SF to intermolecular two-electron transfers using 2,2′-biphenyl-bridged tetracene dimer (Tet-BP-Tet). The obtained high-yield and long-lived triplet excited states of Tet-BP-Tet by intramolecular SF (quantum yield of individual triplet states: ΦT = 175 ± 5% and lifetime of individual triplet states: τT = 0.29 ms) resulted in the quantitative intermolecular two-electron transfer process with chloranil in benzonitrile (electron transfer yield: ΦET = 173 ± 5%).1 Then, we also propose a novel supramolecular strategy utilizing the mixed self-assembled monolayers (SAMs) with two different chain lengths. Specifically, mixed tetracene-SAMs on gold nanoclusters, which are prepared by a tetracene-modified hetero-disulfide with two different chain lengths, attain high-yield SF (Φ SF: ~90%) and individual triplet yields (Φ Τ: ~160%).2 References Nakamura, S.; Sakai, H.; Nagashima, H.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T. ACS Energy Lett. 2019, 4, 26-31.Saegusa, T.; Sakai, H.; Nagashima, H.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T. J. Am. Chem. Soc. 2019, 141, 14720-14727.

Triplet Population Dynamics of Single Conjugated Polymer Molecules and Nanoscale Assemblies

The formation of long-lived triplet excited electronic states has important ramifications for conjugated organic materials used in optoelectronic devices. In the case of polymers, unravelling vario...

Ultralong Organic Phosphorescent Nanocrystals with Long-Lived Triplet Excited States for Afterglow Imaging and Photodynamic Therapy.

The development of novel applications of ultralong organic phosphorescent (UOP) materials is highly desired. Herein, a series of UOP materials (EDCz, E = O, S, Se, and Te) for bacterial afterglow imaging and photodynamic therapy (PDT) is reported. By structurally bonding with the chalcogen atoms with π-conjugated scaffolds, EDCz not only absorbs visible light but also emits UOP with an efficiency of ca. 0.01-6.8% and a long lifetime of 0.08-0.318 s under ambient conditions. Benefiting from the long-lived triplet excited states, the SeDCz nanocrystals (NCs) possessed the best optical properties in the series, generating 1O2 under white light irradiation and performing as an agent for Staphylococcus aureus afterglow imaging and PDT at a low concentration (98 ng mL-1). The SeDCz NCs are also utilized as real-time UOP imaging agents and promoted healing of infected wounds in living mice. To the best of our knowledge, this study presents the first example of UOP-based bacterial photodynamic theranostic agents and creates a platform for the next-generation efficient UOP-based photosensitizers for bioimaging and skin regeneration.

Designing High-Triplet-Yield Phenothiazine Donor–Acceptor Complexes for Photoredox Catalysis

Phenothiazine, owing to its ease of oxidation and modularity with respect to facile functionalization, is an attractive central chemical unit from which to construct highly reducing organic photoredox catalysts. While design improvements have been made in the community, the yield of intersystem crossing (ΦISC), which determines access to the long-lived triplet excited state, has yet to be systematically optimized. Herein, we explore the impacts of N-aryl substituent variation on excited-state dynamics using picosecond to millisecond transient absorption and emission spectroscopies. Design principles are uncovered that center on controlling the energy of an intermediate charge transfer (CT) state within the singlet excited-state manifold, which, in turn, dictates the yield of CT-state formation and the rate constants for its depletion. Ultimately, we find ΦISC to be highly sensitive to the electron-withdrawing character of the N-aryl electron acceptor in the aforementioned CT state, with ΦISC ranging from ∼0 to 0.96.

联三吡啶铂(Ⅱ)配合物的非线性吸收和光限幅（特邀）

The reported work in 2003-2019 on the reverse saturable absorption (RSA) or two-photon absorption (TPA) and/or optical limiting (OPL) of platinum(II) terpyridine complexes was summarized in this minireview. Photophysical properties, including the ground-state absorption (GSA), excited-state absorption (ESA), excited-state lifetimes, and the quantum yields of triplet excited-state formation, RSA/OPL at 532 nm for ns laser pulses, TPA characteristics in the near-IR spectral regions, and the structure-property correlations were reviewed. This paper is composed of four sections. First, the current status of OPL materials and devices, the general requirements for reverse saturable absorbers and two-photon absorbing materials, and the different types and characteristics of square-planar platinum(II) complexes were briefly introduced. Then the photophysics and RSA/OPL of six series of Pt(Ⅱ) terpyridine-analogous complexes and the structure-property correlations were discussed. Following it the TPA of five series of Pt(Ⅱ) terpyridine complexes and the impacts of structural variations on the TPA cross sections (σ2) were reviewed. Finally, brief conclusions were drawn based on the reported studies. A general trend discovered was that the charge transfer absorption band(s) and the ESA can be readily tuned by substituents on the acetylide or the terpyridine ligand. Introducing electron-donating substituent to the acetylide or terpyridine ligand or improving the coplanarity between the aromatic substituent and the terpyridine ligand red-shifted the ground-state charge-transfer absorption band(s) at the price of decreasing/quenching the triplet ESA, which consequently reduced the RSA/OPL at 532 nm. Extending the π-conjugation of the terpyridine ligand dramatically improved the σ2 values of the Pt(Ⅱ) terpyridine complexes. Incorporation of electron-withdrawing π-conjugated aromatic substituent restrained the GSA to < 500 nm while keeping a long-lived triplet excited state with broadband ESA in the visible spectral regions and moderately strong TPA in the NIR regions. This approach could provide a solution for developing broadband OPL materials.

Near-Infrared-Excitable Organic Ultralong Phosphorescence through Multiphoton Absorption.

Organic ultralong room-temperature phosphorescence (OURTP) with a long-lived triplet excited state up to several seconds has triggered widespread research interests, but most OURTP materials are excited by only ultraviolet (UV) or blue light owing to their unique stabilized triplet- and solid-state emission feature. Here, we demonstrate that near-infrared- (NIR-) excitable OURTP molecules can be rationally designed by implanting intra/intermolecular charge transfer (CT) characteristics into H-aggregation to stimulate the efficient nonlinear multiphoton absorption (MPA). The resultant upconverted MPA-OURTP show ultralong lifetimes over 0.42 s and a phosphorescence quantum yield of ~37% under both UV and NIR light irradiation. Empowered by the extraordinary MPA-OURTP, novel applications including two-photon bioimaging, visual laser power detection and excitation, and lifetime multiplexing encryption devices were successfully realized. These discoveries illustrate not only a delicate design map for the construction of NIR-excitable OURTP materials but also insightful guidance for exploring OURTP-based nonlinear optoelectronic properties and applications.

Enhancing Photodynamic Therapy Efficacy of Upconversion-Based Nanoparticles Conjugated with a Long-Lived Triplet Excited State Iridium(III)-Naphthalimide Complex: Toward Highly Enhanced Hypoxia-Inducible Factor-1.

Metal-based photosensitizers are of great interest in photodynamic therapy (PDT) due to their tunable photophysicochemical characteristics and structure flexibility. Herein, an iridium-based photosensitizer (1) with a long-lived intraligand (3IL) excited state has been designed and synthesized, which shows significantly enhanced singlet oxygen (1O2) generation efficiency (∼45 folds) relative to that of the model iridium(III) complex (2) under 460 nm irradiation. In order to achieve deep tissue penetration, complex 1 was further covalently bonded to the upconversion nanoparticles (UCNPs). Besides, 1-benzyl-3-(5'-hydroxymethyl-2'-furyl)indazole (YC-1), an effective HIF-1α inhibitor, was physically adsorbed into the hydrophobic layer at the surface of UCNPs. Once upon near-infrared (NIR) irradiation, iridium complex 1-mediated toxic 1O2 was generated for PDT, whose efficient conversion of oxygen to 1O2 during the PDT would exacerbate the hypoxic condition of tumor tissue and lead to the upregulation of HIF-1α for the following HIF-1 targeting tumor therapy. This study highlights the potential for applying a nanoplatform composed of a long-lived iridium-based photosensitizer and an HIF-1α inhibitor in tumor therapy, which converts PDT-induced tumor hypoxia to a therapy advantage, thus opening up ideas to overcome the hypoxia in PDT therapy.

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.

Nucleus targeting anthraquinone-based copper (II) complexes as the potent PDT agents: Synthesis, photo-physical and theoretical evaluation

Present work explored the structural aspects of four new anthraquinone-based copper(II)complexes of the general formula [Cu(L1)B] (1, 2) and [Cu(L2)B] (3, 4) where L1 = 2-((2-mercaptophenylimino)methyl)phenol, L2 = 2-((2-mercaptophenylimino)methyl)-4,6-di-tert-butylphenol and B are 1,10-phenanthroline,dipyrido[3,2-d:2′,3′-f]quinoxaline-8,9-napthaquinone, in modulating in vitro photo-dynamic activities. Nucleus targeting complexes have shown remarkable cytotoxicity in visible-light to cancer cells (IC50 ∼ 2–11 µM) with reduced dark toxicity (IC50 > 50 µM) unlike other copper (II) complexes. Singlet oxygen generated on photo-sensitization of the complexes was the key cytotoxic species responsible for apoptotic damage of cancer cells. Degree of photo-cytotoxicity of the photo-activated complexes was related to the extent of 1O2 generation which was probed by several photo-physical studies along with TD-DFT calculations. Presence of low-lying, long-lived triplet excited state and hence increased ability to generate 1O2 from 3O2 through type-II photo-process was proposed to explain the degree of photo-cytotoxicity of the complexes. We observed dual photo-sensitization of S-coordination and anthraquinone moiety for the complex 4 leading to remarkable PDT effect to cancer cells with minimal dark toxicity. Overall, our investigations on exploring the structural aspects of copper (II) complexes for PDT were a phenomenal break-through in developing copper-based photo-chemotherapeutics in the clinical arena of cancer therapy.

Controlled Orientations of Neighboring Tetracene Units by Mixed Self-Assembled Monolayers on Gold Nanoclusters for High-Yield and Long-Lived Triplet Excited States through Singlet Fission.

Although tetracene (Tc) is well-known as a good candidate for singlet fission (SF), the number of high-yield and long-lived triplet excited states through SF is extremely limited because of the relative acceleration of the reverse triplet-triplet annihilation (TTA) considering the energy matching between a singlet and two triplet states. Systematic control of electronic interactions between two neighboring units using conventional covalent linkages and molecular assembly methods to optimize these kinetic processes is quite difficult because of the complicated synthesis and random orientations. In this study, we propose a novel supramolecular strategy utilizing mixed self-assembled monolayers (SAMs) with two different chain lengths. Specifically, mixed Tc-SAMs on gold nanoclusters, which are prepared using Tc-modified heterodisulfides with two different chain lengths, attain high-yield SF (ΦSF ≈ 90%) and individual triplet yields (ΦΤ ≈ 160%). The obtained ΦSF is the highest value among Tc derivatives in homogeneous solution to the best of our knowledge.