The Role of H-Bonds in the Excited-State Properties of Multichromophoric Systems: Static and Dynamic Aspects

Given their importance, hydrogen bonds (H-bonds) have been the subject of intense investigation since their discovery. Indeed, H-bonds play a fundamental role in determining the structure, the electronic properties, and the dynamics of complex systems, including biologically relevant materials such as DNA and proteins. While H-bonds have been largely investigated for systems in their electronic ground state, fewer studies have focused on how the presence of H-bonds could affect the static and dynamic properties of electronic excited states. This review presents an overview of the more relevant progress in studying the role of H-bond interactions in modulating excited-state features in multichromophoric biomimetic complex systems. The most promising spectroscopic techniques that can be used for investigating the H-bond effects in excited states and for characterizing the ultrafast processes associated with their dynamics are briefly summarized. Then, experimental insights into the modulation of the electronic properties resulting from the presence of H-bond interactions are provided, and the role of the H-bond in tuning the excited-state dynamics and the related photophysical processes is discussed.

Recent Advances in Photorelease Complexes for Therapeutic Applications”

Photorelease complexes represent a class of agents for which UV-visible light triggers the expulsion of a specfic molecule that is intrinsically part of the inner coordination sphere or held in...

A Ru-anthraquinone dyad with triple functions of PACT, photoredox catalysis and PDT upon red light irradiation

Phototherapy for cancer treatment has received much attention in recent years, and compounds with multiple anticancer mechanisms upon irradiation are particularly appealing. In this work, a nitro-anthraquinone group was attached to a biq (2,2'-biquinoline) ligand based Ru(ii) complex, endowing the resultant Ru1 compound with multiple anticancer mechanisms upon 600 nm light irradiation. Ru1 can undergo biq ligand photodissociation, showing its potential as a photoactivated chemotherapy (PACT) agent. Moreover, a Ru(iii) centre and an anthraquinone anion centre may be generated upon irradiation, which can further oxidize NADH/NADPH and generate O2˙-, successfully eliciting photoredox catalysis and photodynamic therapy (PDT). Compared to the control complex Ru2 without the nitroanthraquinone group, Ru1 exhibited much enhanced photocytotoxicity towards a series of cancer cell lines and 3D multicellular spheroids upon red light irradiation.

The effect of hydrogen bonds on the ultrafast relaxation dynamics of a BODIPY dimer

The influence of hydrogen bonds (H-bonds) in the structure, dynamics, and functionality of biological and artificial complex systems is the subject of intense investigation. In this broad context, particular attention has recently been focused on the ultrafast H-bond dependent dynamical properties in the electronic excited state because of their potentially dramatic consequences on the mechanism, dynamics, and efficiency of photochemical reactions and photophysical processes of crucial importance for life and technology. Excited-state H-bond dynamics generally occur on ultrafast time scales of hundreds of femtoseconds or less, making the characterization of associated mechanisms particularly challenging with conventional time-resolved techniques. Here, 2D electronic spectroscopy is exploited to shed light on this still largely unexplored dynamic mechanism. An H-bonded molecular dimer prepared by self-assembly of two boron-dipyrromethene dyes has been specifically designed and synthesized for this aim. The obtained results confirm that upon formation of H-bonds and the dimer, a new ultrafast relaxation channel is activated in the ultrafast dynamics, mediated by the vibrational motions of the hydrogen donor and acceptor groups. This relaxation channel also involves, beyond intra-molecular relaxations, an inter-molecular transfer process. This is particularly significant considering the long distance between the centers of mass of the two molecules. These findings suggest that the design of H-bonded structures is a particularly powerful tool to drive the ultrafast dynamics in complex materials.

Proton-Coupled Electron Transfer for Photoinduced Generation of Two-Electron Reduced Species of Quinone

Purpose-built molecules that follow the fundamental process of photosynthesis have significance in developing better insight into the natural photosynthesis process. Quinones have a significant role as electron acceptors in natural photosynthesis, and their reduction is assisted through H-bond donation or protonation. The major challenge in such studies is to couple the multielectron and proton-transfer process and to achieve a reasonably stable charge-separated state for the elucidation of the mechanistic pathway. We have tried to address this issue through the design of a donor-acceptor-donor molecular triad (2RuAQ) derived from two equivalent [Ru(bpy)₃]²⁺ derivatives and a bridging anthraquinone moiety (AQ). Photoinduced proton-coupled electron transfer (PCET) for this molecular triad was systematically investigated in the absence and presence of hexafluoroisopropanol and p-toluenesulfonic acid (PTSA) using time-resolved absorption spectroscopy in the ultrafast time domain. Results reveal the generation of a relatively long-lived charge-separated state in this multi-electron transfer reaction, and we could confirm the generation of AQ^2- and Ru^III as the transient intermediates. We could rationalize the mechanistic pathway and the dynamics associated with photoinduced processes and the role of H-bonding in stabilizing charge-separated states. Transient absorption spectroscopic studies reveal that the rates of intramolecular electron transfer and the mechanistic pathways associated with the PCET process are significantly different in different solvent compositions having different polarities. In acetonitrile, a concerted PCET mechanism prevails, whereas the stepwise PCET reaction process is observed in the presence of PTSA. The results of the present study represent a unique model for the mechanistic diversity of PCET reactions.

Hydrogen peroxide assisted photorelease of an anthraquinone-based ligand from [Ru(2,2'-bipyridine)2(9,10-dioxo-9,10-dihydroanthracen-1-olate)]Cl in aqueous solution

A new class of light-activated ruthenium(ii) complex was designed as a potential blocker of biological functioning, especially for targeting redox reactions within mitochondria under light activation. Based on our concepts the complex [Ru(bipy)2(1-hydroxyanthra-9,10 quinone)]Cl (RU1) was prepared and studied to understand the preliminary reaction mechanisms and its excited state behaviour through a series of stability tests, electrochemistry, UV-Visible kinetics and femtosecond transient absorption spectroscopy experiments. Under white light in the presence of H2O2 two different reactions (fast and slow) appear to take place. The complex loses the quinone-based ligand and a resulting Ru(iii) or Ru(v) species is produced. The complex RU1 shows potential to consume H2O2 from the one carbon metabolism in mitochondria, and hence may cut the energy cycle pathway of tumor cells.