Abstract
Singlet fission (SF) is a photophysical process in which one photoexcited singlet exciton is converted into two triplet excitons, offering a promising route to overcome the Shockley-Queisser efficiency limit of conventional single-junction photovoltaic devices. Despite significant progress in recent years, the microscopic mechanism of singlet fission and the role of intermediate dark electronic states in the formation of triplet pairs remain incompletely understood, particularly in emerging classes of small poly-heteroaromatic chromophores. The SiFi-MYSTERY project addressed these challenges through an integrated approach combining molecular synthesis, ultrafast spectroscopy and first-principles theoretical modelling. Newly synthesized chromophores functionalized with electron-donating and electron-withdrawing substituents were investigated in solution using state-of-the-art ultrafast transient absorption and two-dimensional electronic spectroscopy, while their excited-state electronic structure and relaxation pathways were described using advanced multireference electronic-structure methods, multidimensional quantum dynamics and transient spectroscopy simulations. This combined experimental and theoretical strategy enabled a detailed characterization of the excited-state photophysics of isolated chromophores and established the foundation for understanding singlet-fission processes in molecular aggregates and thin films. The project brought together complementary expertise in theoretical chemistry, ultrafast spectroscopy and synthetic chemistry. The University of Bologna (Artur Nenov and Michele Mancinelli) coordinated the theoretical modelling activities and the synthesis of the target chromophores, while the Politecnico di Milano (Margherita Maiuri) carried out the ultrafast spectroscopic investigations. This multidisciplinary collaboration enabled the direct interpretation of the experimental observations through advanced quantum-chemical calculations and quantum-dynamical simulations. The knowledge generated within the project provides new mechanistic insight into the elementary photophysical processes preceding singlet fission and contributes to the rational design of next-generation organic materials for photovoltaic energy conversion.
Results achieved
The SiFi-MYSTERY project successfully combined synthetic chemistry, state-of-the-art ultrafast spectroscopy and advanced theoretical modelling to establish the photophysical mechanisms governing two emerging classes of singlet-fission chromophores, benzodipyrrolidone (BDPP) and benzodithiophene-monothiophene dioxide (BTDO). The project achieved its primary objective of providing a molecular-level understanding of the electronic structure and excited-state dynamics of these systems, thereby establishing the scientific basis for future investigations of singlet fission in molecular aggregates and thin films. Optimized synthetic protocols enabled the preparation of the parent chromophores together with a series of electron-donating and electron-withdrawing derivatives used throughout the project. In parallel, synthetic strategies for BTDO derivatives were substantially improved despite the intrinsic instability of the oxidized thiophene building blocks, providing the molecular platforms required for the spectroscopic and theoretical investigations. Ultrafast transient absorption spectroscopy with femtosecond time resolution provided a detailed picture of the photoinduced dynamics in solution. For the BDPP family, the experiments revealed that subtle chemical functionalization and solvent polarity strongly modify the excited-state relaxation pathways and the lifetime of the photoexcited state. For BTDO derivatives, the measurements identified distinct relaxation regimes and revealed the formation of long-lived triplet states through intersystem crossing in selected derivatives. These measurements supplied an extensive experimental benchmark for the theoretical investigations. The main scientific achievement of the project was the complete reconstruction of the electronic structure and relaxation mechanisms of the BDPP and BTDO monomers using state-of-the-art multireference electronic-structure methods and quantum dynamics simulations. For BDPP, the calculations demonstrated that the experimentally observed ultrafast dynamics originate from internal conversion between the bright excited state and a lower-lying dark excited state, while disproving the previously proposed involvement of a higher-lying multiexcitonic state in the relaxation mechanism. The rate of the internal conversion can be controlled by chemical functionalization. The project revealed that higher-lying nπ^*states transiently mediate the internal conversion process and that their accessibility can be controlled through solvent polarity. This establishes the long-lived bright excited state, rather than the previously hypothesized dark multiexcitonic state, as the most likely precursor for singlet-fission processes occurring in molecular aggregates. For BTDO, the theoretical calculations likewise revised the currently accepted description of the excited-state manifold by showing that the energy separation between bright and dark states is considerably larger than previously believed. The combined experimental and theoretical analysis demonstrated that the low-lying dark state does not participate in the primary relaxation of the monomer, while the long-lived transient signal observed experimentally originates from intersystem crossing to the triplet manifold. These results provide the first consistent microscopic description of the excited-state dynamics of BTDO monomers. Beyond the study of these specific chromophores, the project also delivered a new computational methodology for simulating ultrafast photoinduced processes in complex environments. A hybrid quantum-dynamics/molecular-mechanics framework was developed that combines fully quantum-mechanical treatment of both electrons and nuclei in the chromophore with an explicit classical description of the surrounding environment. This methodology significantly extends the capabilities available for modelling ultrafast excited-state dynamics in realistic condensed-phase systems and will find applications well beyond singlet-fission materials. Overall, the project successfully achieved its objectives by unraveling the electronic structure and ultrafast relaxation dynamics of the BDPP and BTDO monomers, providing a revised microscopic picture of their excited-state photophysics and resolving several open questions concerning the role of dark electronic states. These results represent an essential step towards understanding singlet fission in molecular aggregates and provide rational design principles for the development of next-generation organic materials for photovoltaic energy conversion.Project details
Unibo Team Leader: Artur Nenov
Unibo involved Department/s:
Dipartimento di Chimica Industriale "Toso Montanari"
Coordinator:
ALMA MATER STUDIORUM - Università di Bologna(Italy)
Total Eu Contribution: Euro (EUR) 239.682,00
Total Unibo Contribution: Euro (EUR) 150.640,00
Project Duration in months: 24
Start Date:
30/11/2023
End Date:
28/02/2026
