Self-Assembly of a β-Cyclodextrin Bis-Inclusion Complex into a Highly Crystalline Fiber Network. An Effective Strategy for Null Aggregate Design

Cross-Transition-Dipole Stacking of Conjugated Organic Molecules: Structure, Exciton Behavior and Optoelectronic Property

Organic conjugated molecules have gained widespread application as organic semiconductors due to their unique optoelectronic properties. The rigidity of these large conjugated structures facilitates strong intermolecular interactions, which significantly influence their properties in the solid state through various molecular arrangements. The study of the relationship among molecular arrangement, exciton behavior, and optoelectronic properties is an eternal research topic. Cross-dipole stacking is a specific molecular arrangement that demonstrates unique characteristics and has received continuous attention over the past decades. This mini-review will first discuss the unique exciton behaviors in cross-dipole stacking based on exciton models, including weak exciton coupling and suppression of Förster resonance energy transfer. These exciton behaviors, determined by molecular stacking arrangements, are fundamental to the optoelectronic properties of cross-dipole stacking systems. Next, we will introduce well-defined cross-dipole systems and summarize their design principles from a molecular structure perspective. Finally, we will present the specific optoelectronic properties of cross-dipole stacking systems and their outstanding performance, such as high solid-state luminescence, good charge carrier mobility, and significant CD/CPL. Through this mini-review, we hope to enhance the understanding of cross-dipole stacking, contributing to the construction of such systems, the exploration of excited-state behaviors, and the discovery of high-performance materials.

Keeping the chromophores crossed: evidence for null exciton splitting

Fundamental understanding of the supramolecular assemblies of organic chromophores and the development of design strategies have seen endless ripples of interest owing to their exciting photophysical properties and optoelectronic applications. The independent discovery of dye aggregates by Jelley and Scheibe was the commencement of the remarkable advancement in the field of aggregate photophysics. Subsequent research warranted an exceptional model for defining the exciton interactions in aggregates, proposed by Davydov, Kasha and co-workers, independently, based on the long-range Coulombic coupling. Fascinatingly, the orthogonally cross-stacked molecular transition dipole arrangement was foretold by Kasha to possess null exciton interaction leading to spectroscopically uncoupled molecular assembly, which lacked an experimental signature for decades. There have been several attempts to identify and probe atypical molecular aggregates for decoding their optical behaviour. Herein, we discuss the recent efforts in experimentally verifying the unusual exciton interactions supported with quantum chemical computations, primarily focusing on the less explored null exciton splitting. Exciton engineering can be realized through synthetic modifications that can additionally offer control over the assorted non-covalent interactions for orchestrating precise supramolecular assembly, along with molecular editing. The task of attaining a minimal excitonic coupling through an orthogonally cross-stacked crystalline architecture envisaged to offer a monomer-like optical behaviour was first reported in 1,7-dibromoperylene-3,4,9,10-tetracarboxylic tetrabutylester (PTE-Br2). The attempt to stitch molecules covalently in an orthogonal fashion to possess null excitonic character culminated in a spiro-conjugated perylenediimide dimer exhibiting a monomer-like spectroscopic signature. The computational and experimental efforts to map the emergent properties of the cross-stacked architecture are also discussed here. Using the null aggregates formed by the interference effects between CT-mediated and Coulombic couplings in the molecular array is another strategy for achieving monomer-like spectroscopic properties in molecular assemblies. Moreover, identifying supramolecular assemblies with precise angle-dependent properties can have implications in functional material design, and this review can provide insights into the uncharted realm of null exciton splitting.

One-Pot Synthesis and Excited-State Dynamics of Null Exciton-Coupled Diketopyrrolopyrroles Oligo-Grids

Exciton coupling in molecular aggregates plays a vital role in impacting and fine-tuning optoelectronic materials and their efficiencies in devices. A versatile platform to decipher aggregation-property relationships is built around multichromophoric architectures. Here, a series of cyclic diketopyrrolopyrrole (DPP) oligomers featuring nanoscale gridarene structures and rigid bifluorenyl spacers are designed and synthesized via one-pot Friedel-Crafts reaction. DPP dimer [2]Grid and trimer [3]Grid, which are cyclic rigid nanoarchitectures of rather different sizes, are further characterized via steady-state and time-resolved absorption and fluorescence spectroscopies. They exhibit monomer-like spectroscopic signatures in the steady-state measurements, from which null exciton couplings are derived. Moreover, in an apolar solvent, high fluorescence quantum yields and excited-state dynamics that resembled DPP monomer are gathered. In a polar solvent, the localized singlet excited state on a single DPP dissociates into the adjacent null coupling DPP with charge transfer characteristics. This pathway facilitates the evolution of the symmetry-broken charge-separated state (SB-CS). Notable is the fact that the SB-CS of [2]Grid is, on one hand, in equilibrium with the singlet excited state and promotes, on the other hand, the formation of the triplet excited state with a yield of 32% via charge recombination.

Null Exciton-Coupled Chromophoric Dimer Exhibits Symmetry-Breaking Charge Separation

A comprehensive understanding of the structure-property relationships in multichromophoric architectures has pushed the limits for developing robust photosynthetic mimics and molecular photovoltaics. The elusive phenomenon of null exciton splitting has gathered immense attention in recent years owing to the occurrence in unique chromophoric architectures and consequent emergent properties. Herein, we unveil the hitherto unobserved null exciton coupling assisted highly efficient photoinduced symmetry-breaking charge separation (SB-CS) in a Greek cross (+)-oriented spiro-conjugated perylenediimide dimer (Sp-PDI₂). Quantum chemical calculations have rationalized the infrequent manifestation of null exciton coupling behavior in Sp-PDI₂. Negligible contribution of long-range Coulombic and short-range charge-transfer mediated coupling renders a monomer-like spectroscopic signature for Sp-PDI₂ in toluene. The Greek cross (+)-arranged Sp-PDI₂ possesses a selective hole-transfer coupling, facilitating the ultrafast dissociation of null excitons and evolution of the charge-separated state in polar solvents. Radical cationic and anionic spectroscopic signatures were characterized by employing femtosecond transient absorption spectroscopy. The substantial hole transfer electronic coupling and lower activation energy barrier of Sp-PDI₂ accelerated the charge separation rate. The rate of charge recombination (CR) markedly decelerated due to falling into the inverted region of the Marcus parabola, where the driving force of CR is larger than the total reorganization energy for CR. Hence, the ratio of the rates for SB-CS over CR of Sp-PDI₂ exhibited an unprecedently high value of 2647 in acetonitrile. The current study provides impeccable evidence for the role of selective charge filtering in governing efficient SB-CS and thereby novel insights towards the design of biomimics and advanced functional materials.

Self-Assembly and Photochemistry of a Pyrene-Methyl Viologen Supramolecular Fiber System

This paper reports the self-assembly of a donor-acceptor system into nanoscopic structures and the photo processes taking place within these structures. The donor employed is pyrene linked to two β-cyclodextrin molecules (CD-PY-CD), and adamantane-linked methyl viologen attached to the three arms of mesitylene (Ms-(MV²⁺-AD)₃) is the acceptor. CD-PY-CD and Ms-(MV²⁺-AD)₃ when dissolved in water self-assembled into vesicles, which joined together to give long fibers. The self-assembly was studied using spectroscopic and microscopic techniques. Fluorescence of the pyrene chromophore was quenched within the self-assembled system due to efficient photoinduced electron transfer to methyl viologen. Photoinduced electron transfer within the assembly is confirmed through identification of product radical ions in flash photolysis experiments. Steady-state irradiation of the self-assembled system in an optical bench led to the formation of methyl viologen radical cation, which was stable for a few hours. Longevity of the radical cation was attributed to the fast reaction of pyrene radical cation with adjacent pyrene to give an unstable adduct, which slows down the back electron transfer process.

Mutually exclusive hole and electron transfer coupling in cross stacked acenes

The topology of frontier molecular orbitals (FMOs) induces highly sensitive charge transfer coupling with variation in the intermolecular arrangement. A consistent optoelectronic property correlated to a specific aggregate architecture independent of the nature of the monomer is a rare phenomenon. Our theoretical investigation on stacked dimeric systems of linear [n]acenes (n = 2-5) and selected non-linear acenes with a D2h point group reveals that the Greek cross (+) stacked orientation, irrespective of the molecular candidate, exhibits mutually exclusive hole and electron transfer couplings. The deactivation of either hole or electron transfer coupling is a consequence of the zero inter-orbital overlap between the highest occupied molecular orbitals (HOMOs) or lowest unoccupied molecular orbitals (LUMOs) of the monomers possessing gerade symmetry. In the Greek cross (+) stacked alignment, the (4n + 2) π-electronic acene systems with an odd number of benzenoids exhibit exclusive electron transfer coupling, while the even numbered acenes exhibit selective hole transfer coupling. The trend is reversed for representative 4n π-electronic acene systems. The effect of mutually exclusive charge transfer coupling in the hopping regime of charge transport was evaluated using semiclassical Marcus theory, and selective charge carrier mobility was exhibited by the Greek cross (+) stacks of the considered acene candidates. Additionally, the characteristic charge transfer coupling of the orthogonal acene stacks resulted in negligible short-range exciton coupling, inciting null exciton splitting at short interplanar distances. Engineering chromophores in precise angular orientations ensuring characteristic emergent properties can have tremendous potential in the rational design of advanced optoelectronic materials.

Generation of Long-Lived Photoinduced Charge Separation in a Supramolecular Toroidal Assembly

Efficiencies of artificial photosynthetic and photocatalytic systems depend on their ability to generate long-lived charge-separated (CS) states in photoinduced electron transfer (PET) reactions. PET, in most cases, is followed by an ultrafast back electron transfer, which severely reduces lifetime and quantum yield of CS states. Generation of a long-lived CS state is an important goal in the study of PET reactions. Herein, we report that this goal is achieved using a hierarchically self-assembled anthracene-methyl viologen donor-acceptor system. Anthracene linked to two β-cyclodextrin molecules (CD-AN-CD) and methyl viologen linked to two adamantane units (AD-MV²⁺-AD) form an inclusion complex in water, which further self-assembled into well-defined toroidal nanostructures. The fluorescence of anthracene is highly quenched in the self-assembled system because of PET from anthracene to methyl viologen. Irradiation of the aqueous toroidal solution led to formation of a long-lived CS state. Rational mechanisms for the formation of the toroidal nanostructures and long-lived photoinduced charge separation are presented in the paper.