Inspired by the high photoconversion efficiency observed in natural light-harvesting systems, the hierarchical organization of molecular building blocks has gained impetus in the past few decades. Particularly, the molecular arrangement and packing in the active layer of organic solar cells (OSCs) have garnered significant attention due to the decisive role of the nature of donor/acceptor (D/A) heterojunctions in charge carrier generation and ultimately the power conversion efficiency. This review focuses on the recent developments in emergent optoelectronic properties exhibited by self-sorted donor-on-donor/acceptor-on-acceptor arrangement of covalently linked D–A systems, highlighting the ultrafast excited state dynamics of charge transfer and transport. Segregated organization of donors and acceptors promotes the delocalization of photoinduced charges among the stacks, engendering an enhanced charge separation lifetime and percolation pathways with ambipolar conductivity and charge carrier yield. Covalently linking donors and acceptors ensure a sufficient D–A interface and interchromophoric electronic coupling as required for faster charge separation while providing better control over their supramolecular assemblies. The design strategies to attain D–A conjugate assemblies with optimal charge carrier generation efficiency, the scope of their application compared to state-of-the-art OSCs, current challenges, and future opportunities are discussed in the review. An integrated overview of rational design approaches derived from the comprehension of underlying photoinduced processes can pave the way toward superior optoelectronic devices and bring in new possibilities to the avenue of functional supramolecular architectures.

The design of highly efficient supramolecular architectures that mimic competent natural systems requires a comprehensive knowledge of noncovalent interactions. Halogen bonding is an excellent noncovalent interaction that forms halogen–halogen (X2) as well as trihalogen interacting synthons. Herein, we report the first observation of a symmetric radial assembly of chromophores (R3̅c space group) composed of a stable hexabromine interacting synthon (Br6) that further push the limits of our understanding on the nature, role, and potential of noncovalent halogen bonding. Contrary to the destabilization proposed for Type-I X2 interactions, Br6-synthon-possessing Type-I X2 interactions exhibit a stabilizing nature owing to the exchange-correlation component. The radial assembly of chromophores is further strengthened by intermolecular through-space charge transfer interaction. Br6-synthon-driven 3-fold symmetric radial assembly render a lattice structure that reminisces the chromophoric arrangement in the light harvesting system 2 of purple bacteria.

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Excimers exhibit through-space aromatic character.

Among the various donor-acceptor (D-A) charge-transfer co-crystals investigated in the past few decades, tetrathiafulvalene-tetracyanoquinodimethane (F⋅Q, popularly known as TTF⋅TCNQ)-based co-crystals have fascinated materials chemists owing to their exceptional conducting and magnetic properties that arise from the packing in crystal structures. Here, crystallographic information files of eighteen F⋅Q-based co-crystals are extracted from the Cambridge Structural Database (CSD) and classified into Class 1 (D-on-D and A-on-A segregated stacks; F⋅Q, F1⋅Q-F6⋅Q, and F⋅Q1), Class 2 (-A-D-A-D-A-D- mixed stacks; F6a⋅Q-F11⋅Q and F⋅Q2), and Class 3 [-A-D-A-A-D-A-; Class 3a (F12⋅Q and F13⋅Q) and -D-D-A-A-; Class 3b (F14⋅Q)] systems according to their packing modes. Hirshfeld surface analysis, PIXEL energy calculations, and quantum theory of atoms in molecules (QTAIM) analysis are performed on the selected multicomponent charge-transfer crystals for the first time, in an attempt to explore the driving forces that give rise to different classes of 3 D crystal packing, which in turn mandates the expedient electronic properties exhibited by the investigated co-crystals. PIXEL calculations reveal that the dispersion energy component makes the maximum contribution to the total lattice energy for most of the F⋅Q-based co-crystals under study. Although the Q-on-Q dimer is the energetically most favored dimer in F⋅Q, the substituents on F capable of forming hydrogen-bonding, C⋅⋅⋅S, and other weak intermolecular interactions result in the greater stability of the F-on-F dimer for F1⋅Q-F6⋅Q (except F2⋅Q). The C⋅⋅⋅S, C ⋅⋅⋅S, S⋅⋅⋅N, and π⋅⋅⋅π interaction-driven D-on-A dimer is found to be the most stable dimer of all the Class 2 co-crystals. Band structure and density-of-state calculations of the representative co-crystals in each class indicate different electronic structures according to the packing arrangement. F⋅Q and F6⋅Q with a high interaction of electronic orbitals between D-on-D and A-on-A in segregated stacks are found to be metal-like (bandgap, E =0.003 eV) and metallic (overlapping bands in the Fermi level), respectively, whereas the polymorph of F6⋅Q belonging to Class 2 (F6a⋅Q) displays a semiconductor-type band structure (E =0.053 eV). F12⋅Q of Class 3a exhibits a metal-like band structure (E =0.001 eV). The fine tuning of chromophores with diverse functional substituents capable of triggering weak intermolecular interactions that give rise to the desired packing and charge-transfer properties has the potential to open floodgates of opportunity for research in the chemistry of materials and fabrication of efficient electronic devices.

The sophisticated, yet ingenious, supramolecular architectures in nature have often inspired the design of synthetic molecular frameworks mimicking the efficacious emergent properties nurtured by these systems. Herein, the unique crystalline assembly of a dibromonaphthalimide derivative, 1,8-dibromonaphthalene­(3,5-dimethoxyphenyl)­imide (NIBr2OMe), forming base-pair-like dimers via a stabilizing parallelogram-type Br4 synthon, that further slip-stack to form segregated donor-acceptor arrays, is reported. The peculiar arrangement of the covalently linked donor-acceptor (D-A) moieties with HOMO/LUMO localized on the donor/acceptor part and the peri-peri halogen-halogen interactions imparts higher hole and electron transfer couplings for stacked and halogen–halogen bonded dimers of NIBr2OMe, respectively. The theoretical calculation of anisotropic mobility displayed orthogonal trajectories for maximal hole and electron transport along the slip-stacked and halogen–halogen bonded edge-to-edge directions, respectively. Thus, the unnarrated crucial role of interhalogen interactions in modulating intermolecular electronic couplings and hence the directionality of charge transport is revealed. The study is the first indication for the pre-proposed orthogonal electron and hole transport character in a crystalline organic donor-acceptor system providing novel strategies toward designing archetypical organic materials with charge carrier transport in predetermined trajectories for advanced optoelectronic applications.

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