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

Abstract

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)(ippy)] (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 (MLCT) state that is equilibrium with an intraligand (IL) state. The prolonged lifetime provides ample opportunity for bimolecular quenching of this state by oxygen; thus singlet oxygen (O) sensitization is very efficient. In simple aqueous solution, fast cooling within the MLCT manifold is followed by energy transfer to anIL 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.

Synthesis, Characterization and Photobiological Studies of Ru(II) Dyads Derived from α‐Oligothiophene Derivatives of 1,10‐Phenanthroline

“…, where I , A and η are the integrated emission intensity, the absorbance at the excitation wavelength and the refractive index of the solvent, respectively. Reference quantum yields used for [Ru(bpy) 3 ](PF 6 ) 2 were: Φ em = 0.012 at 298 K in aerated MeCN , Φ em = 0.095 at 298 K in deaerated MeCN , Φ em = 0.38 at 77 K in frozen 4:1 v/v EtOH:MeOH and Φ Δ = 0.56 in aerated MeCN . …”

“…The term metal-organic dyad has been used to describe these Ru(II) complexes that incorporate p-expanded organic chromophores that are tethered to one of the chelating ligands, and these systems were first studied by groups investigating triplet-triplet energy transfer in bichromophoric systems (13). The predicted utility (at the time) of such supramolecular systems was in the area of optoelectronic and luminescencebased technologies, but more recently, their potential in photobiological applications such as PDT/PCT has been recognized (7,(14)(15)(16)(17)(18)(19)(20).…”

Correlation of intracellular oxygen and cell metabolism by simultaneous PLIM of phosphorescent TLD1433 and FLIM of NAD(P)H

“…Based on their photophysical and electrochemical properties, Ru(II) complexes have been considered for light‐based applications and are under investigation as PSs for PDT . The wide range of different triplet excited states makes the Ru(II) complexes attractive for PDT.…”

Fast repetition rate fs pulsed lasers for advanced PLIM microscopy

“…13 Of main interest for the diagnostic purpose is the emissive 3 MLCT state, which shows phosphorescence, with a phosphorescence lifetime normally in the sub-s and s range, depending on the complex and transition metal used. [14][15][16][17][18] For example, the well-known [Ru(bpy) 3 ] 2þ (bpy ¼ 2,2 0 -bipyridine) has a lifetime of 620 ns in water. 13 The compound investigated within this work, TLD1433 (a Ru(II) complex derived from -terthienyl appended to imidazo [4,5-f] [1,10]-phenantroline) has entered a human clinical trial for PDT of nonmuscle invasive bladder cancer (ClinicalTrials.-gov Identi¯er NCT03053635).…”