Abstract
Transition metal complexes (TMCs) play a central role in photocatalysis, molecular electronics, phototherapy, artificial photosynthesis and light-emitting devices. Their functionality originates from ultrafast photoinduced processes occurring within the first few femtoseconds after light absorption, when the electronic population is redistributed among ligand- and metal-centered excited states. Despite their importance, these primary processes remain only partially understood because they require both experimental techniques with unprecedented temporal resolution and theoretical methods capable of accurately describing the complex electronic structure of TMCs. The ASTRAL project combined advanced spectroscopy and multireference quantum chemistry to develop new experimental and computational tools for investigating these elementary photoinduced processes. On the experimental side, the project developed a new ultraviolet pump-extreme ultraviolet probe spectroscopy platform based on few-femtosecond ultraviolet excitation pulses and attosecond XUV probing. In parallel, advanced multireference electronic-structure methodologies were established for accurately describing electronically excited states of TMCs and for laying the foundations of first-principles simulations of transient spectroscopies spanning the ultraviolet to the X-ray spectral range. The project also provided new insight into the electronic structure of cyclometalated iridium complexes, an important class of compounds for optoelectronic and photocatalytic applications. The project was carried out through the collaboration between CNR - Istituto di Fotonica e Nanotecnologie (CNR-IFN, Milano, Rocio Borrego Varillas), responsible for the development of the ultrafast spectroscopic platform, the University of Bologna (Artur Nenov), which led the theoretical modelling activities, and CNR-SCITEC (Wojciech Mroz), which developed and optimized the preparation of samples for ultraviolet and XUV spectroscopy.
Results achieved
The ASTRAL project aimed at advancing the methodologies needed to observe and interpret electronic dynamics occurring in TMCs on the few-femtosecond timescale, where many of the fundamental processes responsible for the optical and photochemical properties of these compounds originate. A major achievement of the project was the development of a new ultraviolet source for ultrafast spectroscopy. Few-femtosecond ultraviolet pulses with energies in the microjoule range were generated and fully characterized over a broad spectral range, enabling resonant excitation of molecular systems with unprecedented temporal resolution. The project also demonstrated the generation of isolated attosecond extreme-ultraviolet pulses, providing new opportunities for implementing ultrafast UV-XUV pump-probe spectroscopy with simultaneous temporal and elemental sensitivity. Considerable effort was devoted to preparing molecular samples suitable for ultraviolet and XUV transmission experiments. Different polymeric substrates and deposition strategies were investigated and optimized to obtain mechanically stable free-standing thin films with adequate optical quality. These protocols were successfully applied to a family of cyclometalated iridium complexes. In parallel, the project developed advanced theoretical methodologies for describing nonadiabatic dynamics of TMCs based on model Hamiltonians approximating the potential energy surfaces through low order polynomials parametrized at the multireference level of theory. A computational protocol was developed to describe metal-centered core-excited electronic states within the same multireference framework adopted for the valence electronic structure. The resulting computational protocols provide a significantly more reliable description of excited electronic states in systems characterized by closely spaced excited states and strong electron correlation and constitute the basis for future simulations of transient absorption spectroscopies from the ultraviolet to the X-ray spectral range. These methodologies were applied to the technologically important cyclometalating ligands 2-phenylpyridine (ppy) and 2,2′‑bipyridine (bpy), as well as to the prototype phosphorescent complexes Ir(ppy)₃ and Ir[(ppy)2 (bpy)]+ widely employed in organic light-emitting devices and photocatalysis. The calculations revealed that the optical properties of the ligands originate from the interplay between several bright and dark excited states whose energetic ordering is highly sensitive to electron correlation. Furthermore, coordination to the iridium center was shown to substantially reorganize the ligand-centered excited-state manifold, while the electronic coupling between different ligands was found to have only a minor influence on the electronic structure. These results provide a comprehensive multireference description of the ligand-centered excited states in the TMC. The project originally included a combined experimental and theoretical investigation of the ultrafast excited-state dynamics of metalloporphyrins and iridium complexes. Although these investigations were not completed within the project timeframe, the necessary experimental infrastructure, sample preparation procedures, and computational methodologies were developed. These results will support future studies of ultrafast electronic dynamics in TMCs.Dettagli del progetto
Responsabile scientifico: Artur Nenov
Strutture Unibo coinvolte:
Dipartimento di Chimica Industriale "Toso Montanari"
Coordinatore:
CNR - Consiglio Nazionale delle Ricerche(Italy)
Contributo totale Unibo: Euro (EUR) 86.629,00
Durata del progetto in mesi: 24
Data di inizio
28/09/2023
Data di fine:
27/09/2025
