Excited-state Dynamics in Segregated Donor-acceptor Stacks Versus A Peri-bisdonor-acceptor System.

The investigation of impact of through-space/through-bond electronic interaction among chromophores on photoexcited-state properties has immense potential owing to the distinct emergent photophysical pathways. Herein, the photoexcited-state dynamics of homo-sorted π-stacked aggregates of a naphthalenemonoimide and perylene-based acceptor-donor (NI-Pe) system and a fork-shaped acceptor-bisdonor (NI-Pe2) system possessing integrally stacked peri-substituted donors was examined. Femtosecond transient absorption (fsTA) spectra of NI-Pe monomer recorded in chloroform displayed spectroscopic signatures of the singlet state of Pe; 1Pe*, the charge-separated state; NI-●-Pe+●, and the triplet state of Pe; 3Pe*. The examination of ultrafast excited-state processes of NI-Pe aggregate in chloroform revealed faster charge recombination (𝜏𝐶𝑅𝑎 = 1.75 ns) than the corresponding monomer (𝜏𝐶𝑅𝑚 = 2.46 ns) which was followed by observation of a broad structureless band attributed to an excimer-like state. The fork-shaped NI-Pe2 displayed characteristic spectroscopic features of the NI radical anion (λmax~450 nm) and perylene dimer radical cation (λmax~520 nm) upon photoexcitation in non-polar toluene solvent in the nanosecond transient absorption (nsTA) spectroscopy. The investigation highlights the significance of intrinsic close-stacked arrangement of donors in ensuring a long-lived photoinduced charge-separated state (𝜏𝐶𝑅 = 1.35 µs) in non-polar solvents via delocalization of radical cation between the donors.

Retaining Hückel Aromaticity in the Triplet Excited State of Azobenzene.

The implication of the potential concept of aromaticity in the relaxed lowest triplet state of azobenzene, an efficient molecular switch, using elementary aromaticity indices based on magnetic, electronic, and geometric criteria has been discussed. Azobenzene exhibits a major Hückel aromatic character retained in the diradical lowest relaxed triplet state (T1 ) by virtue of a twisted geometry with partial delocalization of unpaired electrons in the perpendicular p-orbitals of two nitrogen atoms to the corresponding phenyl rings. The computational analysis has been expanded further to stilbene and N-diphenylmethanimine for an extensive understanding of the effect of closed-shell Hückel aromaticity in double-bond-linked phenyl rings. Our analysis concluded that stilbene has Hückel aromatic character in the relaxed T1 state and N-diphenylmethanimine has a considerable Hückel aromaticity in the phenyl ring near the carbon atom while a paramount Baird aromaticity in the phenyl ring near the nitrogen atom of the C=N double bond. The results reveal the application of excited-state aromaticity as a general tool for the design of molecular switches.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor-Acceptor Conjugates.

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.

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.

Exciton Isolation in Cross-Pentacene Architecture.

Null aggregates are an elusive, emergent class of molecular assembly categorized as spectroscopically uncoupled molecules. Orthogonally stacked chromophoric arrays are considered as a highlighted architecture for null aggregates. Herein, we unveil the null exciton character in a series of crystalline Greek cross (+)-assembly of 6,13-bisaryl-substituted pentacene derivatives. Quantum chemical computations suggest that the synergistic perpendicular orientation and significant interchromophoric separations realize negligible long-range Coulombic and short-range charge-transfer-mediated couplings in the null aggregate. The Greek cross (+)-orientation of pentacene dimers exhibits a selectively higher electron-transfer coupling with near-zero hole-transfer coupling and thereby contributes to the lowering of charge-transfer-mediated coupling even at shorter interchromophoric distances. Additional investigations on the nature of excitonic states of pentacene dimers proved that any deviation from a 90° cross-stacked orientation results in the emergence of delocalized Frenkel/mixed-Frenkel-CT character and the consequent loss of null exciton/monomer-like properties. The retention of exciton isolation even at a short-range coupling regime reassures the universality of null excitonic character in perpendicularly cross-stacked pentacene systems. The null-excitonic character was experimentally verified by the observation of similar spectral characteristics in the crystalline and monomeric solution state for 6,13-bisaryl-substituted pentacene derivatives. The partitioned influence of aryl and pentacene fragments on interchromophoric noncovalent interactions and photophysical properties, respectively, resulted in the emergence of pentacene centric Kasha's ideal null exciton, providing novel insights toward design strategies for cross-stacked chromophoric assemblies. Identifying the Greek cross (+)-stacked architecture-mediated null excitons with a charge-filtering phenomenon for the first time in the ever-versatile pentacene chromophoric systems can offer an extensive ground for the engineering of functional materials with advanced optoelectronic properties.

Through-space aromatic character in excimers.

Aromaticity, though widely used to delineate diverse photochemical phenomena, remains to be examined in excimers, a fundamental and extensively studied entity in the excited states. Herein, the first theoretical evidence for the excited state through-space aromatic character in triplet state (T1) excimers of benzene, naphthalene and anthracene is reported using multiple aromaticity descriptors based on magnetic, electronic and geometric criteria. The calculated chemical shifts and induced current densities manifest the presence of transannular π-electronic currents in the excimers. The results open up enormous research potential from exploring the possibility of through-space aromatic character in singlet excimers to its possible implications in photoexcited state processes of aromatic supramolecular systems.

Distinct Crystalline Aromatic Structural Motifs: Identification, Classification, and Implications.

Spatial noncovalent helical organization of nucleobases in DNA and radial organization of chromophores in natural light-harvesting systems are fascinating yet enigmatic. Understanding the numerous weak interactions that drive the formation of elegant supramolecular architectures in native natural systems and developing bioinspired design strategies have seen a surge of interest in recent decades. Self-assembly of functional chromophores in the crystalline phase is a definitive strategy to identify novel molecule-molecule interactions, in particular, atom-atom interactions, and to understand the synergistic nature of noncovalent interactions that stabilizes the supramolecular organization. This Account narrates our recent efforts in developing desirable supramolecular motifs employing weak interaction-based strategies and our observation of deviations from the common motifs chartered in aromatic systems. Modulation of long-range aromatic interactions through chemical modifications (acylation, benzoylation, haloacylation, and alkylation of chromophores) to attain a preferred stacking (herringbone, lamellar, or columnar) is presented. Particular attention has been given to attaining lamellar or columnar packing possessing potential interchromophoric electronic coupling mediated high charge mobility. Supramolecular arrangements of noncovalently or covalently associated donor-acceptor systems that open up additional possibilities of packing modes (segregated, mixed etc.) are explored. Our persistent efforts yielded distinct twisted-segregated and alternate distichous stacks for the nonparallel covalently linked donor-acceptor systems that favor a long-lived photoinduced charge-separated state. We further move on to discuss the unconventional packing motifs that were identified recently. The highly sought-after Greek cross (+) stacking of chromophores in crystalline phase and an elegant crystalline radial arrangement of chromophores are examined. The Greek cross (+) stacked architecture exhibits monomer-like emission characteristics owing to the absence of exciton coupling across the orthogonally stacked chromophores. Crystalline helical chromophore assembly is yet another emerging motif with far-reaching applications in domains ranging from asymmetric catalysis to chiral smart materials and has been accounted here by citing certain phenomenal examples from literature. Thus, this Account demonstrates that identifying and classifying new structural motifs based on topological aspects, such as interchromophoric orientation (cross) and extended chromophore arrangement in the crystal lattice (radial, helical, etc.), are crucial since such fundamental characteristics dictate the properties emerging out of the corresponding motifs. Encouraged from ours and others' works, we propose the addition of new aromatic supramolecular structural motifs, namely, cross-stacked, helical, and radial arrangements, in order to expand the classification. We believe that identifying new emergent property-based supramolecular motifs and investigating the methods to achieve the desired motif will eventually have implications in fundamental crystal engineering, supramolecular chemistry, and biomimetic design of functional materials.

Anomalous Halogen-Halogen Interaction Assists Radial Chromophoric Assembly.

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 ( R3̅ c 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.

Structure-Packing-Property Correlation of Self-Sorted Versus Interdigitated Assembly in TTF⋠TCNQ-Based Charge-Transport Materials.

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, Csp ⋅⋅⋅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, Eg =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 (Eg =0.053 eV). F12⋅Q of Class 3a exhibits a metal-like band structure (Eg =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.