Hi there, I am Alfy

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.

The repeating arrangement of tubulin dimers confers great mechanical strength to microtubules, which are used as scaffolds for intracellular macromolecular transport in cells and exploited in biohybrid devices. The crystalline order in a microtubule, with lattice constants short enough to allow energy transfer between amino acid chromophores, is similar to synthetic structures designed for light harvesting. After photoexcitation, can these amino acid chromophores transfer excitation energy along the microtubule like a natural or artificial light-harvesting system? Here, we use tryptophan autofluorescence lifetimes to probe energy hopping between aromatic residues in tubulin and microtubules. By studying how the quencher concentration alters tryptophan autofluorescence lifetimes, we demonstrate that electronic energy can diffuse over 6.6 nm in microtubules. We discover that while diffusion lengths are influenced by tubulin polymerization state (free tubulin versus tubulin in the microtubule lattice), they are not significantly altered by the average number of protofilaments (13 versus 14). We also demonstrate that the presence of the anesthetics etomidate and isoflurane reduce exciton diffusion. Energy transport as explained by conventional Förster theory (accommodating for interactions between tryptophan and tyrosine residues) does not sufficiently explain our observations. Our studies indicate that microtubules are, unexpectedly, effective light harvesters.

Topology of frontier molecular orbitals (FMOs) induce highly sensitive charge transfer coupling with variation in 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 D2h point group reveals that the Greek cross (+) stacked orientation, irrespective of the molecular candidate, exhibit 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 (HOMO) or lowest unoccupied molecular orbitals (LUMO) of the monomers possessing gerade symmetry. In the Greek cross (+) stacked alignment, the (4n + 2) π-electronic acene systems with 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 orthogonal acene stacks resulted in negligible short-range exciton coupling, inciting null exciton splitting at short interplanar distances. Engineering chromophores in precise angular orientations ensuing characteristic emergent properties can have tremendous potential in the rational design of advanced optoelectronic materials.

Null aggregates are 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 synergistic perpendicular orientation and significant inter-chromophoric separations realize, negligible long-range Coulombic and short-range charge transfer mediated couplings in the null aggregate. The Greek cross (+) orientation of pentacene dimers exhibit a selectively higher electron transfer coupling with near-zero hole transfer coupling and thereby contribute to the lowering of charge transfer mediated coupling even at shorter inter-chromophoric distances. Additional investigations on the nature of excitonic states of pentacene dimers proved that any deviation from 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 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. 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 in-sights towards the design strategies for cross-stacked chromophoric assemblies. Identifying Greek cross-stacked architecture mediated null excitons with 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.

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.

A series of π-conjugated tetrabenzoacenes (TBA), including nitrogen-(un)doped derivatives, are computationally evaluated to comprehend the correlation between intrinsic structural arrangements and charge-transport characteristics. The central charge-transport parameters such as reorganization energy and electronic coupling are individually tuned through peri substitutions, core substitutions, and/or π extension in TBA derivatives. On the basis of reorganization energies, nitrogen doping impeded the electron transport in TBA analogs owing to significant structural changes associated with the reduction process. Our approach employing mapping of dimeric arrangements of TBA, modulated via long (pitch) and short (roll) axes displacements of the molecular entities, versus charge transfer coupling disclosed potential charge-transport regions in addition to the ideal cofacial modes. Charge transport characteristics of molecular packing arrangements of TBA mimicking the different orientations of graphene bilayers were analyzed, providing insights into the possible material applicability of TBA derivatives. The transition from completely aligned graphitic AA packing sequence to slip-stacked AB and AA′ stacking domains revealed a dent in the charge-transport map owing to node–antinode interaction of the frontier molecular orbitals. TBA analogs encompassing the expanded π-system materialized highly displaced dimeric orientation from AB-type packing to occupy a hierarchy favoring higher charge transfer coupling than the AB type. Thus, realizing stable interchromophoric arrangements of small organic molecules through chemical or physical techniques to control their charge-transporting efficiencies is an indispensable step toward the generation of better organic electronic devices.

Exciton interactions in molecular aggregates play a crucial role in tailoring the optical behavior of π-conjugated materials. Though vital for optoelectronic applications, ideal Greek cross-dipole (α = 90°) stacking of chromophores remains elusive. We report a novel Greek cross (+) assembly of 1,7-dibromoperylene-3,4,9,10-tetracarboxylic tetrabutylester (PTE-Br2) which exhibits null exciton coupling mediated monomer-like optical characteristics in crystalline state. Contrastingly, nonzero exciton coupling in X-type (α = 70.2°, PTE-Br0) and J-type (α = 0°, θ = 48.4°, PTE-Br4) assemblies render perturbed optical properties. Additionally, the semi-classical Marcus theory of charge-transfer rates predicts a selective hole transport phenomenon in the orthogonally stacked PTE-Br2. Precise rotation angle dependent optoelectronic properties in crystalline PTE-Br2 can have consequences in the rational design of novel π-conjugated materials for photonic and molecular electronic applications.