Interaction with a Biomolecule Facilitates the Formation of the Function-Determining Long-Lived Triplet State in a Ruthenium Complex for Photodynamic Therapy.

TLD1433 is the first ruthenium (Ru)-based photodynamic therapy (PDT) agent to advance to clinical trials and is currently in a phase II study for treating nonmuscle bladder cancer with PDT. Herein, we present a photophysical study of TLD1433 and its derivative TLD1633 using complex, biologically relevant solvents to elucidate the excited-state properties that are key for biological activity. The complexes incorporate an imidazo [4,5-f][1,10]phenanthroline (IP) ligand appended to α-ter- or quaterthiophene, respectively, where TLD1433 = [Ru(4,4'-dmb)2(IP-3T)]Cl2 and TLD1633 = [Ru(4,4'-dmb)2(IP-4T)]Cl2 (4,4'-dmb = 4,4'-dimethyl-2,2'-bipyridine; 3T = α-terthiophene; 4T = α-quaterthiophene). Time-resolved transient absorption experiments demonstrate that the excited-state dynamics of the complexes change upon interaction with biological macromolecules (e.g., DNA). In this case, the accessibility of the lowest-energy triplet intraligand charge-transfer (3ILCT) state (T1) is increased at the expense of a higher-lying 3ILCT state. We attribute this behavior to the increased rigidity of the ligand framework upon binding to DNA, which prolongs the lifetime of the T1 state. This lowest-lying state is primarily responsible for O2 sensitization and hence photoinduced cytotoxicity. Therefore, to gain a realistic picture of the excited-state kinetics that underlie the photoinduced function of the complexes, it is necessary to interrogate their photophysical dynamics in the presence of biological targets once they are known.

Modulating the Excited-State Decay Pathways of Cu(I) 4H-Imidazolate Complexes by Excitation Wavelength and Ligand Backbone.

Cu(I) 4H-imidazolato complexes are excellent photosensitizers with broad and intense light absorption properties, based on an earth-abundant metal, and hold great promise as photosensitizers in artificial photosynthesis and for accumulation of redox equivalents. In this study, the excited-state relaxation dynamics of three novel heteroleptic Cu(I) 4H-imidazolato complexes with phenyl, tolyl, and mesityl side groups are systematically investigated by femtosecond and nanosecond time-resolved transient absorption spectroscopy and theoretical methods, complemented by steady-state absorption spectroscopy and (spectro)electrochemistry. After photoexcitation into the metal-to-ligand charge transfer (MLCT) and intraligand charge transfer absorption band, fast (0.6-1 ps) intersystem crossing occurs into the triplet MLCT manifold. The triplet-state population relaxes via the geometrical planarization of the N-aryl rings on the Cu(I) 4H-imidazolato complexes. Depending on the initial Franck-Condon state, the remaining small singlet state population relaxes into two geometrically distinct minima geometries with similar energy, S1/2,relax and S3/4,relax. Subsequent ground-state recovery from S1/2,relax and internal conversion from S3/4,relax to S1/2,relax take place on a 100 ps time scale. The internal conversion can be understood as hole transfer from a dyz-orbital to a dxz-orbital, which is accompanied with the structural reorganization of the coordination environment. Generally, the photophysical processes are determined by the steric hindrance of the side groups on the ligands. And the excited singlet-state pathways are dependent on the excitation wavelength.

It Takes Three to Tango - the length of the oligothiophene determines the nature of the long-lived excited state and the resulting photocytotoxicity of a Ru(II) photodrug.

TLD1433 is the first Ru(II) complex to be tested as a photodynamic therapy agent in a clinical trial. In this contribution we study TLD1433 in the context of structurally-related Ru(II)-imidozo[4,5-f][1,10]phenanthroline (ip) complexes appended with thiophene rings to decipher the unique photophysical properties which are associated with increasing oligothiophene chain length. Substitution of the ip ligand with ter- or quaterthiophene changes the nature of the long-lived triplet state from metal-to-ligand charge-transfer to 3ππ* character. The addition of the third thiophene thus presents a critical juncture which not only determines the photophysics of the complex but most importantly its capacity for 1O2 generation and hence the potential of the complex to be used as a photocytotoxic agent. ENTRY FOR THE TABLE OF CONTENTS: A low-lying triplet intraligand state (3IL) determines the properties of the long-lived excited states in a series of Ru(II) complexes. The 3IL state can be accessed by increasing the length of an oligothiophene chain. The 3IL state is extremely efficient at generating 1O2 and thus enhances the potency of the complexes as PDT agents.

String-Attached Oligothiophene Substituents Determine the Fate of Excited States in Ruthenium Complexes for Photodynamic Therapy.

We explore the photophysical properties of a family of Ru(II) complexes, Ru-ip-nT, designed as photosensitizers (PSs) for photodynamic therapy (PDT). The complexes incorporate a 1H-imidazo[4,5-f][1,10]-phenanthroline (ip) ligand appended to one or more thiophene rings. One of the complexes studied herein, Ru-ip-3T (known as TLD1433), is currently in phase II human clinical trials for treating bladder cancer by PDT. The potent photocytotoxicity of Ru-ip-3T is attributed to a long-lived intraligand charge-transfer triplet state. The accessibility of this state changes upon varying the length (n) of the oligothiophene substituent. In this paper, we highlight the impact of n on the ultrafast photoinduced dynamics in Ru-ip-nT, leading to the formation of the function-determining long-lived state. Femtosecond time-resolved transient absorption combined with resonance Raman data was used to map the excited-state relaxation processes from the Franck-Condon point of absorption to the formation of the lowest-energy triplet excited state, which is a triplet metal-to-ligand charge-transfer excited state for Ru-ip-0T-1T and an oligothienyl-localized triplet intraligand charge-transfer excited state for Ru-ip-2T-4T. We establish the structure-activity relationships with regard to changes in the excited-state dynamics as a function of thiophene chain length, which alters the photophysics of the complexes and presumably impacts the photocytotoxicity of these PSs.

Intracellular Photophysics of an Osmium Complex bearing an Oligothiophene Extended Ligand.

This contribution describes the excited-state properties of an Osmium-complex when taken up into human cells. The complex 1 [Os(bpy)2 (IP-4T)](PF6 )2 with bpy=2,2'-bipyridine and IP-4T=2-{5'-[3',4'-diethyl-(2,2'-bithien-5-yl)]-3,4-diethyl-2,2'-bithiophene}imidazo[4,5-f][1,10]phenanthroline) can be discussed as a candidate for photodynamic therapy in the biological red/NIR window. The complex is taken up by MCF7 cells and localizes rather homogeneously within in the cytoplasm. To detail the sub-ns photophysics of 1, comparative transient absorption measurements were carried out in different solvents to derive a model of the photoinduced processes. Key to rationalize the excited-state relaxation is a long-lived 3 ILCT state associated with the oligothiophene chain. This model was then tested with the complex internalized into MCF7 cells, since the intracellular environment has long been suspected to take big influence on the excited state properties. In our study of 1 in cells, we were able to show that, though the overall model remained the same, the excited-state dynamics are affected strongly by the intracellular environment. Our study represents the first in depth correlation towards ex-vivo and in vivo ultrafast spectroscopy for a possible photodrug.

Unusually Short-Lived Solvent-Dependent Excited State in a Half-Sandwich Ru(II) Complex Induced by Low-Lying 3MC States.

A ruthenium complex with a half-sandwich geometry ([(p-cymene)Ru(Cl)(curcuminoid)]) was synthesized, characterized, and investigated regarding its ultrafast photophysics. These photophysical investigations of the complex revealed a weak and short-lived emission from the initially populated 1MLCT state and solvent-dependent photoinduced dynamics, where the secondarily populated 3MC state is stabilized by nonpolar solvents. Overall the decay of the 3dd-MC state to the ground state is completed within picoseconds. This short excited-state lifetime is in stark contrast to the typically observed long-lived 3MLCT states with lifetimes of nanoseconds or microseconds in unstrained, octahedral ruthenium complexes but is in good agreement with the findings for distorted octahedral complexes. This is pointing to the half-sandwich geometry as a new and easy approach to study these otherwise often concealed dd states.

Excited State Properties of Heteroleptic Cu(I) 4H-Imidazolate Complexes.

The excited state properties of three heteroleptic copper(I) xantphos 4H-imidazolate complexes are investigated by means of femtosecond and nanosecond time-resolved transient absorption spectroscopy in dichloromethane solution. The subpicosecond spectral changes observed after excitation into the MLCT absorption band are interpreted as intersystem crossing from the singlet to the triplet manifold. This interpretation is corroborated by DFT and TD-DFT results, indicating a comparable molecular geometry in the ground state (and hence the nonrelaxed singlet state) and the excited triplet state. Population of the triplet state is followed by planarization of the N-aryl rings of the 4H-imidazolate ligand on a 10 ps time scale. The planarization strongly depends on the substitution pattern of the N-aryls and correlates with the reduced moment of inertia for the planarization motion. The triplet state subsequently decays to the ground state in about 100 ns. These results demonstrate that the excited state processes of copper(I) complexes depend on the specific ligand(s) and their substitution pattern. Thus, the work presented points to a possibility to design copper(I) complexes with specific photophysical properties.

Structure, Biosynthesis, and Biological Activity of the Cyclic Lipopeptide Anikasin.

The class of cyclic lipopeptide natural products consists of compounds with a diverse range of bioactivities. In this study, we elucidated the structure of the cyclic lipopeptide anikasin using X-ray crystallography, analyzed its biosynthetic gene cluster, and investigated its natural role in the interaction between the producer strain Pseudomonas fluorescens HKI0770 and protozoal predators. These results led to the conclusion that anikasin has dual functionality enabling swarming motility and acting as a niche amoebicide, which effectively inhibits the social amoeba Polysphondylium violaceum and protects the producer strain from protozoal grazing.

Excited State Dynamics of a Photobiologically Active Ru(II) Dyad Are Altered in Biologically Relevant Environments.

In this study femtosecond and nanosecond time-resolved transient absorption spectroscopy was used to investigate the influence of ionic strength and complexity on the excited state dynamics of a Ru(II)-based metal-organic dyad. The bis-heteroleptic complex [Ru(bpy)2(ippy)]2+ (1), where bpy = 2,2'-bipyridine and ippy = 2-(1-pyrenyl-1H-imidazo[4,5-f][1,10]phenanthroline, is a potent photosensitizer for in vitro photodynamic therapy (PDT) and photodynamic inactivation (PDI) of microorganisms owing to a long-lived triplet excited state derived from a metal-to-ligand charge-transfer (3MLCT) state that is equilibrium with an intraligand (3IL) state. The prolonged lifetime provides ample opportunity for bimolecular quenching of this state by oxygen; thus singlet oxygen (1O2) sensitization is very efficient. In simple aqueous solution, fast cooling within the 3MLCT manifold is followed by energy transfer to an 3IL state, which is facilitated by rotation of a pyrenyl unit about the imidazo-pyrenyl (ip) coannular bond. For solutions of 1 in high ionic strength simulated biological fluid (SBF), a more physiologically relevant solvent that contains a complex mixture of ions at pH 7.4, femtosecond studies revealed an additional excited state, possibly based on an ion-ligand interaction. This new state appearing in high ionic strength SBF was not observable in water, simple buffers, or low ionic strength SBF. These photoinduced dynamics were also affected by the presence of biomolecules such as DNA in simple buffer, whereby relaxation on the picosecond time scale was accelerated from 39 to 18 ps with DNA intercalation by 1. The increased rate of coplanarization of the pyrene and the imidazole units was attributed to DNA-induced conformational restriction of the pyrenyl unit relative to the ip bond. Quantitative changes to excited state decay rates of 1 in solutions of high ionic strength were also observed when probed on the microsecond time scale. Notably, the thermalized excited state decay pathways were altered substantially with DNA intercalation, with access to some states being completely blocked. Experimentally, this manifested in the absence of the slowest microsecond decay channel, which is normally observed for 1 in solution. The quantitative and qualitative observations from this study highlight the importance of employing biologically relevant solvents and potential biomolecule targets when the excited state dynamics and photophysical properties (under cell-free conditions) responsible for the potent photobiological effects are assessed in the context of photodynamic therapy and photodynamic inactivation.