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Exciton Isolation in Cross Pentacene Architectures
Alfy Benny, Lijina M. P., Remya Ramakrishnan, Mahesh Hariharan

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.

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Chem. Commun., 2020,56, 225-228

Through space aromatic character in excimers
Vishnu Vijay, Meera Madhu, Remya Ramakrishnan, Alfy Benny, Mahesh Hariharan

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.

J. Phys. Chem. C 2019, 123, 44, 26758-26768

In Silico Exploration for Maximal Charge Transport in Organized Tetrabenzoacenes through Pitch and Roll Displacements
Alfy Benny, Devika Sasikumar, Mahesh Hariharan

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.

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Angew.Chem.Int.Ed. 2018, 57, 15696–1570

Null Exciton Splitting in Chromophoric Greek Cross (+) Aggregate

Ebin Sebastian, Abbey M. Philip, Alfy Benny, Mahesh Hariharan

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.

Journal of Physical Chemistry C, Front Cover

Angewande Chemie, Inside Cover

Journal of American Chemical Society, Front Cover

© 2020 by Alfy Benny

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