Poly(2-Oxazoline) Amphiphilicity Tunes the Excited-State Proton Transfer of Pyrenol-Based Polyphotoacids.

The ability of light to change the properties of light-responsive polymers opens avenues for targeted release of cargo with a high degree of spatial and temporal control. Recently, we established photoacid polymers as light-switchable macromolecular amphiphiles. In these systems, light-induced excited-state proton transfer (ESPT) causes changes in amphilicity. However, as the intermolecular process itself critically depends on the local environment of the photoacid unit within the polymer, the overall amphiphilicity directly influences ESPT. Thus, understanding the impact of the local environment on the photophysics of photoacidic side chains is key to material design. In this contribution we address both thermodynamic and kinetic aspects of ESPT in oxazoline-based amphiphilic polymers with pyrenol-based photoacid side chains. We will compare the effect of polymer design, i. e. statistical and block arrangements, i. e. in poly[(2-ethyl-2-oxazoline)-co-(1-(6/8-hydroxyperene)sulphonylaziridine)] and poly(2-ethyl-2-oxazoline)-block-poly[(2-ethyl-2-oxazoline)-co-(2-(3-(6-hydroxypyrene)sulphonamide)propyl-2-oxazoline), on the intermolecular proton transfer reaction by combining steady-state and time-resolved absorption and emission spectroscopy. ESPT appears more prominent in the statistical copolymer compared to a block copolymer with overall similar pyrenol loading. We hypothesize that the difference is due to different local chain arrangements adopted by the polymers in the two cases.

Excited-State Dynamics in Borylated Arylisoquinoline Complexes in Solution and in cellulo.

Two four-coordinate organoboron N,C-chelate complexes with different functional terminals on the PEG chains are studied with respect to their photophysical properties within human MCF-7 cells. Their excited-state properties are characterized by time-resolved pump-probe spectroscopy and fluorescence lifetime microscopy. The excited-state relaxation dynamics of the two complexes are similar when studied in DMSO. Aggregation of the complexes with the carboxylate terminal group is observed in water. When studying the light-driven excited-state dynamics of both complexes in cellulo, i. e., after being taken up into human MCF-7 cells, both complexes show different features depending on the nature of the anchoring PEG chains. The lifetime of a characteristic intramolecular charge-transfer state is significantly shorter when studied in cellulo (360±170 ps) as compared to in DMSO (∼960 ps) at 600 nm for the complexes with an amino group. However, the kinetics of the complexes with the carboxylate group are in line with those recorded in DMSO. On the other hand, the lifetimes of the fluorescent state are almost identical for both complexes in cellulo. These findings underline the importance to evaluate the excited-state properties of fluorophores in a complex biological environment in order to fully account for intra- and intermolecular effects governing the light-induced processes in functional dyes.

In Cellulo Light-Induced Dynamics in a BODIPY-Perylene Dyad.

BODIPY-based donor-acceptor dyads are widely used as sensors and probes in life science. Thus, their biophysical properties are well established in solution, while their photophysical properties in cellulo, i. e., in the environment, in which the dyes are designed to function, are generally understood less. To address this issue, we present a sub-ns time-resolved transient absorption study of the excited-state dynamics of a BODIPY-perylene dyad designed as a twisted intramolecular charge transfer (TICT) probe of the local viscosity in live cells.

Using Biological Photophysics to Map the Excited-State Topology of Molecular Photosensitizers for Photodynamic Therapy.

This study employs TLD1433, a RuII -based photodynamic therapy (PDT) agent in human clinical trials, as a benchmark to establish protocols for studying the excited-state dynamics of photosensitizers (PSs) in cellulo, in the local environment provided by human cancer cells. Very little is known about the excited-state properties of any PS in live cells, and for TLD1433, it is terra incognita. This contribution targets a general problem in phototherapy, which is how to interrogate the light-triggered, function-determining processes of the PSs in the relevant biological environment, and establishes methodological advances to study the ultrafast photoinduced processes for TLD1433 when taken up by MCF7 cells. We generalize the methodological developments and results in terms of molecular physics by applying them to TLD1433's analogue TLD1633, making this study a benchmark to investigate the excited-state dynamics of phototoxic compounds in the complex biological environment.

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.

Switching the Mechanism of NADH Photooxidation by Supramolecular Interactions.

A series of three Ru(II) polypyridine complexes was investigated for the selective photocatalytic oxidation of NAD(P)H to NAD(P)+ in water. A combination of (time-resolved) spectroscopic studies and photocatalysis experiments revealed that ligand design can be used to control the mechanism of the photooxidation: For prototypical Ru(II) complexes a 1 O2 pathway was found. Rudppz ([(tbbpy)2 Ru(dppz)]Cl2 , tbbpy=4,4'-di-tert-butyl-2,2'-bipyridine, dppz=dipyrido[3,2-a:2',3'-c]phenazine), instead, initiated the cofactor oxidation by electron transfer from NAD(P)H enabled by supramolecular binding between substrate and catalyst. Expulsion of the photoproduct NAD(P)+ from the supramolecular binding site in Rudppz allowed very efficient turnover. Therefore, Rudppz permits repetitive selective assembly and oxidative conversion of reduced naturally occurring nicotinamides by recognizing the redox state of the cofactor under formation of H2 O2 as additional product. This photocatalytic process can fuel discontinuous photobiocatalysis.

A Molecular Photosensitizer in a Porous Block Copolymer Matrix-Implications for the Design of Photocatalytically Active Membranes.

Recently, porous photocatalytically active block copolymer membranes were introduced, based on heterogenized molecular catalysts. Here, we report the integration of the photosensitizer, i. e., the light absorbing unit in an intermolecular photocatalytic system into block copolymer membranes in a covalent manner. We study the resulting structure and evaluate the orientational mobility of the photosensitizer as integral part of the photocatalytic system in such membranes. To this end we utilize transient absorption anisotropy, highlighting the temporal reorientation of the transition dipole moment probed in a femtosecond pump-probe experiment. Our findings indicate that the photosensitizer is rigidly bound to the polymer membrane and shows a large heterogeneity of absolute anisotropy values as a function of location probed within the matrix. This reflects the sample inhomogeneity arising from different protonation states of the photosensitizer and different intermolecular interactions of the photosensitizers within the block copolymer membrane scaffold.

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.

An Iridium Complex as Bidentate Halogen Bond-Based Anion Receptor Featuring an IncreasedOptical Response.

We present a luminescent Ir(III) complex featuring a bidentate halogen bond donor site capable of strong anion binding. The tailor-made Ir(III)(L)2 moiety offers a significantly higher emission quantum yield (8.4 %) compared to previous Ir(III)-based chemo-sensors (2.5 %). The successful binding of chloride, bromide and acetate is demonstrated using emission titrations. These experiments reveal association constants of up to 1.6×105 M-1. Furthermore, a new approach to evaluate the association constant by utilizing the shift of the emission was used for the first time. The experimentally observed characteristics are supported by quantum chemical simulations.

Diiodo-BODIPY Sensitizing of the [Mo3S13]2- Cluster for Noble-Metal-Free Visible-Light-Driven Hydrogen Evolution within a Polyampholytic Matrix.

We report on a photocatalytic setup that utilizes the organic photosensitizer (PS) diiodo-BODIPY and the non-precious-metal-based hydrogen evolution reaction (HER) catalyst (NH4)2[Mo3S13] together with a polyampholytic unimolecular matrix poly(dehydroalanine)-graft-poly(ethylene glycol) (PDha-g-PEG) in aqueous media. The system shows exceptionally high performance with turnover numbers (TON > 7300) and turnover frequencies (TOF > 450 h-1) that are typical for noble-metal-containing systems. Excited-state absorption spectra reveal the formation of a long-lived triplet state of the PS in both aqueous and organic media. The system is a blueprint for developing noble-metal-free HER in water. Component optimization, e.g., by modification of the meso substituent of the PS and the composition of the HER catalyst, is further possible.

Monitoring excited-state relaxation in a molecular marker in live cells-a case study on astaxanthin.

Small molecules are frequently used as dyes, labels and markers to visualize and probe biophysical processes within cells. However, very little is generally known about the light-driven excited-state reactivity of such systems when placed in cells. Here an experimental approach to study ps time-resolved excited state dynamics of a benchmark molecular marker, astaxanthin, in live human cells is introduced.

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.