Maria Fumanal

Luminescent diradicals with magnetic field‐sensitive emissive properties, aka magnetoluminescence ML, have gained great attention due to their potential for spin‐optical manipulation technologies. Recent studies provide evidence of ML in non‐Kekulé triarylmethyl‐based diradicals, ascribed to the modulation of the ground state singlet and triplet populations at low temperatures. The ML in these systems originates in a small singlet‐triplet gap and the different emissive properties of the singlet and triplet excited states, which may display distinct emission wavelengths, intensities, and/or bandwidths, depending on the constituent units. Understanding the mechanism behind a particular ML effect is crucial to advancing the development of novel diradicals with target ML properties. In this work, quantum chemical calculations are employed to elucidate the nature of the emissive states of the first reported ML‐active diradical. This system comprises two m‐PyBTM’ ((2‐chloro‐3‐pyridyl)bis(2,4,6‐trichlorophenyl)methyl) units covalently linked through a THDBA (5,6,8,9‐tetrahydro‐7‐phenyldibenz[c,h]acridine) bridge, specifically designed to bring the radicals into proximity while suppressing through‐bond spin‐exchange interactions. Our results provide evidence of a through‐space radical‐to‐radical charge transfer in the low‐lying singlet exciton of this system and unravel the non‐innocent role of the bridge in tuning its ML response. These findings contribute to the collective effort to understand and control ML in molecular materials.

Donor–acceptor (D–A) covalent organic frameworks (COFs) have gained great attention in the fields of optoelectronics due to their ability to promote charge transfer (CT) and charge transport, leading to long-lived charge carriers and thus better device performances. The modularity of these materials makes them ideal for computational molecular design based on electronic structure calculations, able to predict their excited state and photoconductive properties in silico. However, the characterization of CT transitions in D–A COFs inherits the same difficulties as their analogous molecular systems. This poses a challenge for standard cost-effective electronic structure methods, whose reliability needs to be carefully assessed. Moreover, strong CT is usually associated with localized states, whereas band dispersion is ascribed to delocalized states. Thus, whether the CT and the in-plane photoconductivity can be enhanced simultaneously still needs to be addressed. In this work, we propose 12 chemical modifications for two families of 1,3,6,8-tetraphenylpyrene (Py) 2D-COFs with potential for light-induced CT and highly dispersive bands. Based on DFT/TD-DFT calculations, we characterize the low-lying excited state properties of the 26 monolayers and expose the limitations of the most common approximations to provide reliable data. Ultimately, we analyze the CT versus band shape correlation and identify two possible candidates with improved features for optoelectronic applications.

Polyyne bridges have attracted extensive interest as molecular wires due to their shallow distance dependence during charge transfer. Here, we investigate whether triplet energy transfer from cadmium selenide (CdSe) quantum dots (QDs) to anthracene acceptors benefits from the high conductance associated with polyyne bridges, especially from the potential cumulene character in their excited states. Introducing π-electron rich oligoyne bridges between the surface-bound anthracene-based transmitter ligands, we explore the triplet energy transfer rate between the CdSe QDs and anthracene core. Our femtosecond transient absorption results reveal that a rate constant damping coefficient of β is 0.118 ± 0.011 Å−1, attributed to a through-bond coupling mechanism facilitated by conjugation among the anthracene core, the oligoyne bridges, and the COO⊖ anchoring group. In addition, oligoyne bridges lower the T1 energy level of the anthracene-based transmitters, enabling efficient triplet energy transfer from trapped excitons in CdSe QDs. Density-functional theory calculations suggest a slight cumulene character in these oligoyne bridges during triplet energy transfer, with diminished bond length alternation. This work demonstrates the potential of oligoyne bridges in mediating long-distance energy transfer.

The structural factors affecting triplet-triplet annihilation (TTA) at the molecular level are not well-understood. Here, our steady-state photoluminescence and transient absorption results demonstrate that the spin statistical factor, η, decreases from 0.60 to 0.46 and 0.14 going from 9,10-diphenylanthracene (DPA) to the 1,5-DPA and 2,6-DPA isomers, respectively, during photon upconversion with a platinum octaethylporphyrin sensitizer. Density functional theory (DFT) shows that η depends on the energetics of hot triplet states and molecular rigidity. The significantly high conical intersection energy between the S₀ and T₁ states for 9,10-DPA gives its longer triplet lifetime. Time-dependent DFT calculations show that 9,10-DPA and 1,5-DPA can undergo high-level reverse intersystem crossing from their T₂ and T₃ states, respectively, to the bright S₁ state, increasing the limit of the spin statistical factor. Both factors ultimately serve to enhance the TTA efficiency. This work provides insight into designing molecules for efficient light-emitting and photon upconversion applications.

The donor–acceptor copolymer strategy for intramolecular Singlet Fission is applied for the molecular design of functional 2D covalent organic frameworks.

Kinetic rate constants are evaluated computationally for the spin-forbidden thermal relaxations of SCO complexes, unveiling their mechanism, and retrieving accurate T_LIESST predictions.

Hot exciton materials have the potential to improve the quantum efficiency of organic light‐emitting diodes (OLEDs) by promoting high reversed intersystem crossing (hRISC) between a high‐lying triplet (T_n, n ≥ 2) and a radiative singlet (S_m). In recent years, donor–acceptor–donor (D–A–D) molecular systems have shown great promise in its ability to enhance the hRISC process under certain conditions. However, strategies to find appropriate D–A–D combinations beyond trial‐and‐error are still elusive. This work exposes the limited applicability of the current fragment‐based design rules and proposes high‐throughput screening as the optimal route to find candidates that fulfill the energy criteria for hRISC. The strategy consists of first establishing the thresholds for large triplet–triplet splitting and small singlet–triplet gap, then filtering combinations through rate comparison of competitive crossing pathways, and finally confirming hRISC with spin‐orbital coupling evaluation. Based on a dataset of 234 compounds, this protocol identifies 31 candidates with potential for hRISC, 4 of which are reported in the literature. Remarkably, while most of the promising systems show prominent hybridized local and charge transfer character, several candidates do not fulfill this condition, indicating that different routes are possible to design efficient OLED materials.

Singlet fission (SF) is a two-step process in which a singlet splits into two triplets throughout the so-called correlated triplet-pair (¹TT) state. Intramolecular SF (iSF) materials, in particular, have attracted growing interest as they can be easily implemented in single-junction solar cells and boost their power conversion efficiency. Still, the potential of iSF materials such as polymers and oligomers for photovoltaic applications has been partially hindered by their ability to go beyond the ¹TT intermediate and generate free triplets, whose mechanism remains poorly understood. In this work, the main aspects governing the ¹TT dissociation in donor-acceptor copolymers and the key features that optimize this process are exposed. First, we show that both thermodynamics and kinetics play a crucial role in the intramolecular triplet-pair separation and second, we uncover the inherent flexibility of the donor unit as the fundamental ingredient to optimize them simultaneously. Overall, these results provide a better understanding of the intramolecular ¹TT dissociation process and establish a new paradigm for the development of novel iSF active materials.

N - and S -heteroatom oxidation is shown to induce appropriate energy splitting between the first singlet and triplet excited states in conjugated building blocks, making singlet fission thermodynamically possible.

Donor–acceptor (D–A) copolymers have shown great potential for intramolecular singlet fission (iSF). Nonetheless, very few design principles exist for optimizing these systems for iSF, with very little knowledge about how to engineer them for this purpose. In recent work, a fundamental trade-off between the main electronic ingredients required for iSF capable D–A coplanar copolymers was revealed. Still, further investigations are needed to understand these limitations and learn how to bypass them. In this work, we propose to induce torsion as an effective way to circumvent the limits of the coplanar approach. We disclose the potential of noncoplanar copolymers with inherently low triplet energies that encompass all the characteristics required for iSF beyond the limiting values associated with fully coplanar systems. Our findings shed some light on the electronic structure aspects of D–A copolymers for iSF and offer a new avenue for the rational design of novel promising candidates.