Efficient Intramolecular Singlet Fission in Spiro-Linked Heterodimers

Electronic Structure and Vibrational Properties of Indenotetracene-Based Crystal

Asymmetrically substituted indenotetracene crystals are promising nonfullerene electron transport materials for organic photovoltaics, offering potential improvements in efficiency and stability. In this work, we present a first-principle investigation of the electronic and vibrational properties of a diarylindenotetracene system functionalized with two methoxy groups (hereafter DimethoxyASI). Single-crystal X-ray diffraction analysis [reported in J. Org. Chem. 2018, 83, 4, 1828] reveals a monoclinicP2 1 / c $$ {\mathrm{P}2}_1/\mathrm{c} $$ structure with an interplanar distance of 3.76 Å, providing insight into the molecular packing and intermolecular interactions that govern the solid-state organization. Notably, for the first time, in this work we identify two distinct dimeric species within the crystalline lattice by a structural and electronic analysis, each exhibiting different intermolecular arrangements that significantly influence both the electronic structure and vibrational properties of the material. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations provide insight into the molecular packing, electronic states, and vibrational characteristics of the crystal. The theoretical absorption spectrum, obtained from TDDFT calculations, features three main electronic transitions centered at 530, 360, and 275 nm, displaying a mixed character of localized excitations and charge-transfer contributions. The vibrational properties, investigated through phonon density of states calculations at the DFT level, highlight well-defined spectral features. While most vibrational modes remain consistent between monomeric and dimeric configurations, significant deviations emerge in the low-frequency region, where intermolecular interactions and crystal packing effects play a crucial role. Furthermore, the two dimeric species exhibit distinct electronic properties beyond their geometric differences. A key distinguishing factor is the transition electric dipole moments (TEDMs), which governs the probability and polarization of electronic transitions. Our analysis reveals that the TEDMs magnitude and orientation vary significantly between the two dimeric species, suggesting that they may interact differently with polarized light. These differences provide new insight into the role of molecular aggregation in shaping the optical response of organic semiconductors and highlight the impact of polymorphism on their electronic properties. Overall, this study underscores the intricate relationship between molecular packing, electronic structure, and vibrational properties in indenotetracene-based materials, contributing to a deeper understanding of their potential applications in optoelectronic devices.

Individually Tunable Energy Levels of Oligomers Based on N-B←N Units

Opto-electronic properties and device performance of organic semiconductors are mainly determined by energy levels of their frontier molecular orbitals, e.g. lowest unoccupied molecular orbital (ELUMO) and highest occupied molecular orbital (EHOMO) in the ground state, first singlet state (ES1) and first triplet state (ET1) in the excited state. These energy levels are always intricately intertwined. Herein, we report a series of monodisperse oligomers based on double B←N bridged bipyridine (BNBP) units. With the increasing number of repeating units, the oligomers exhibit gradually downshifted ELUMO and nearly unchanged EHOMO due to the different distribution of the frontier molecular orbitals of the oligomers. Moreover, the oligomers exhibit gradually decreasing ES1 and nearly unchanged ET1 because of the different contributions of the charge transfer component in the excited state. This work provides new insight into energy level tuning of organic semiconductors, which is important for high-performance organic opto-electronic devices.

Diverse quantum interference regimes in intramolecular singlet fission chromophores with thiophene-based linkers

An array of thiophene-based π-conjugated linkers in covalently linked pentacene dimers allow us to access diverse quantum interference (QI), modulating nonadiabatic coupling (NAC) in the singlet fission (SF) process. Simulations show that structural isomerism in terms of S atom orientation substantially alters NAC with relatively marginal impacts on energies. Extended curly arrow rules (ECARs) reveal sensitive dependence of QI on SF linker topologies and connectivity, categorizing regimes of constructive, destructive, and previously unrealized in SF research, shifted destructive QI. Drastic NAC changes in terms of S atom orientation are rationalized based on the nature of QI. Our results from nonequilibrium Green's function calculation using density functional theory corroborate the classification of QI regimes based on ECARs. Moreover, we found that the extent of charge resonance contribution in electronic states relevant to multiexciton formation and the appearance of optically allowed charge transfer excitation strongly depends on the operative QI regime. Notably, the magnitude of NAC effectively captures this influence. Our findings show that QI can rationalize and semi-quantitatively correlate with NAC for the multiexciton formation step in the SF process.

The Lack of Triplet Fusion for an Intramolecular Singlet Fission Chromophore: The Expected, the Unexpected, and a Reconciliation

Singlet fission (SF) has the potential to play a key role in photovoltaics since it generates a larger number of longer-lived triplet excitons after photoabsorption. Intramolecular SF (iSF) is of special interest since it enables tuning of SF efficiency by adjusting interchromophore configuration through covalent interaction. However, as elaborated in the present work, iSF chromophores are doomed to dissatisfy one general thermodynamic criterion for all SF chromophores, intramolecular or not: E(T2) ≥ 2E(T1), and therefore, the fusion of two triplet excitons to one triplet exciton is thermodynamically favorable. In our nonadiabatic quantum dynamics simulation for a model iSF chromophore, this expected fusion does not occur, because of the inefficient intersystem crossing hidden under the cover of internal conversion of the triplet fusion. A reconciliation is achieved between the dissatisfaction of E(T2) ≥ 2E(T1) and the large tetraradical character for general iSF chromophores.