Pentacene-Bridge Interactions in an Axially Chiral Binaphthyl Pentacene Dimer

Long-Lived Triplets from Singlet Fission in Pentacene-Decorated Helical Supramolecular Polymers

Singlet fission (SF), which involves the conversion of a singlet excited state into two triplet excitons, holds great potential to boost the efficiency of photovoltaics. However, losses due to triplet-triplet annihilation hamper the efficient harvesting of SF-generated triplet excitons, which limits an effective implementation in solar energy conversion schemes. A fundamental understanding of the underlying structure-property relationships is thus crucial to define design principles for cutting-edge SF materials, yet it remains elusive. Herein, we harness helical supramolecular polymers decorated with pentacene side groups to elucidate intermolecular SF dynamics in solution and promote the formation of long-lived mobile triplets. By leveraging the hydrogen bonding-driven assembly of benzene-1,3,5-tricarboxamide (BTA) cores into one-dimensional scaffolds, we direct the organization of appended pentacene motifs into long-range ordered helical frameworks. Dynamic interactions between weakly coupled SF pendants mediate singlet conversion within hundreds of picoseconds, affording triplet quantum yields well above 100%. Moreover, analysis of triplet dynamics with a Monte Carlo simulation model reveals that triplet diffusion along the supramolecular fibers is favored over annihilation, resulting in independent triplets exhibiting considerably slow decay on the time scale of tens of microseconds. The molecular packing within the assembly is tuned by subtle changes in monomer design to increase the rate and efficiency of SF while ensuring exceptionally long-lived mobile triplets, allowing to maintain triplet quantum yields exceeding 100% for at least 100 ns. This work opens new opportunities to exploit self-assembled supramolecular polymers as functional templates to achieve long-lived SF-generated triplets.

Chirality in Singlet Fission: Controlling Singlet Fission in Aqueous Nanoparticles of Tetracenedicarboxylic Acid Ion Pairs

The singlet fission characteristics of aqueous nanoparticles, self-assembled from ion pairs of tetracene dicarboxylic acid and various amines with or without chirality, are thoroughly investigated. The structure of the ammonium molecule, the counterion, is found to play a decisive role in determining the molecular orientation of the ion pairs and its regularity, spectroscopic properties, the strength of the intermolecular coupling between the tetracene chromophores, and the consequent singlet fission process. Using chiral amines has led to the formation of crystalline nanosheets and efficient singlet fission with a triplet quantum yield as high as 133% ±20% and a rate constant of 6.99 × 109 s-1. The chiral ion pairs also provide a separation channel to free triplets with yields as high as 33% ±10%. In contrast, nanoparticles with achiral counterions do not show singlet fission, which gave low or high fluorescence quantum yields depending on the size of the counterions. The racemic ion pair produces a correlated triplet pair intermediate by singlet fission, but no decorrelation into two free triplets is observed, as triplet-triplet annihilation dominates. The introduction of chirality enables higher control over orientation and singlet fission in self-assembled chromophores. It provides new design guidelines for singlet fission materials.

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies g_abs of up to 2.6 × 10^-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

Electronic circular dichroism from real-time propagation in state space

In this paper, we propose to compute the electronic circular dichroism (ECD) spectra of chiral molecules using a real-time propagation of the time-dependent Schrödinger equation (TDSE) in the space of electronic field-free eigenstates, by coupling TDSE with a given treatment of the electronic structure of the target. The time-dependent induced magnetic moment is used to compute the ECD spectrum from an explicit electric perturbation. The full matrix representing the transition magnetic moment in the space of electronic states is generated from that among pairs of molecular orbitals. In the present work, we show the ECD spectra of methyloxirane, of several conformers of L-alanine, and of the Λ-Co(acac)₃ complex, computed from a singly excited ansatz of time-dependent density functional theory eigenstates. The time-domain ECD spectra properly reproduce the frequency-domain ones obtained in the linear-response regime and quantitatively agree with the available experimental data. Moreover, the time-domain approach to ECD allows us to naturally go beyond the ground-state rotationally averaged ECD spectrum, which is the standard outcome of the linear-response theory, e.g., by computing the ECD spectra from electronic excited states.

Probing Through-Bond and Through-Space Interactions in Singlet Fission-Based Pentacene Dimers

Interchromophoric interactions such as Coulombic coupling and exchange interactions are crucial to the functional properties of numerous π-conjugated systems. Here, we use magnetic circular dichroism (MCD) spectroscopy to investigate interchromophoric interactions in singlet fission relevant pentacene dimers. Using a simple analytical model, we outline a general relationship between the geometry of pentacene dimers and their calculated MCD response. We analyze experimental MCD spectra of different covalently bridged pentacene dimers to reveal how the molecular structure of the "bridge" affects the magnitude of through-space Coulombic and through-bond exchange interactions in the system. Our results show that through-bond interactions are significant in dimers with conjugated molecules as bridging units and these interactions promote the overall electronic coupling in the system. Our generalized approach paves the way for the application of MCD in investigating interchromophoric interactions across a range of π-conjugated systems.

Axially chiral indolenine derived chromophore dimers and their chiroptical absorption and emission properties

Yamamoto homocoupling of two chiral oxindoles led to the atropo-diastereoselective formation of an axially chiral oxindole dimer. This building block served as the starting material for the syntheses of axially chiral squaraine and merocyanine chromophore dimers. These dimers show pronounced chiroptical properties, this is, outstandingly high ECD signals (Δε up to ca. 1500 M⁻¹ cm⁻¹) as a couplet with positive Cotton effect for the P-configuration around the biaryl axis and a negative Cotton effect for the M-configuration. All investigated dimers also exhibit pronounced circularly polarised emission with anisotropy values of ca. 10⁻³ cgs. Time-dependent density functional calculations were used to analyse the three contributions (local one electron, electric–magnetic coupling, and exciton coupling) to the rotational strength applying the Rosenfeld equation to excitonically coupled chromophores. While the exciton coupling term proves to be the dominant one, the electric–magnetic coupling possesses the same sign and adds significantly to the total rotational strength owing to a favourable geometric arrangement of the two chromophores within the dimer.

From an axially chiral oxindole, squaraine and merocyanine chromophore dimers with pronounced chiroptical properties were prepared.