Our Research Expertise and Publications

Establishing the correlation between varied excited states of dimeric structures with their unusual optoelectronic behavior comprised the central goal of our Research Centre. This in turn may open up the possibility to design improved compounds in terms of excited state behavior tunability through substitutions. In view of the above, tailor-making the advantageous stacking arrangements via chemical substitution is proposed for various congeners of acenes, perylene bisimides, and donor-pie-acceptor ased molecular systems.

 

Carbon Nanosheets by Morphology-Retained Carbonization of Two-Dimensional Assembled Anisotropic Carbon Nanorings

Angew Chem. Int. Ed. 2018, 57, 9679-9683

Two-dimensional (2D) carbon nanomaterials possessing promising physical and chemical properties find applications in high-performance energy storage devices and catalysts. However, large-scale fabrication of 2D carbon nanostructures is based on a few specific carbon templates or precursors and poses a formidable challenge. Now a new bottom-up method for carbon nanosheet fabrication using a newly designed anisotropic carbon nanoring molecule, CPPhen, is presented. CPPhen was self-assembled at a dynamic air–water interface with a vortex motion to afford molecular nanosheets, which were then carbonized under inert gas flow. Their nanosheet morphologies were retained after carbonization, which has never been seen for low-molecular weight compounds. Furthermore, adding pyridine as a nitrogen dopant in the self-assembly step successfully afforded nitrogen-doped carbon nanosheets containing mainly pyridinic nitrogen species.

Unusual Nonemissive Behavior of Rubrene J-Aggregates: A Rare Violation

J. Phys. Chem. B 2017, 121, 3190 − 3201

Structure–property correlations in rubrene (RB) colloidal J-aggregates were unravelled by steady state and time-resolved spectroscopy in conjunction with excited state density functional calculations. The RB J-aggregate with a slippage angle θ = 30.4°, estimated from the monomeric transition dipole moment directions, exhibited a broad fwhm of 1073 cm–1 and a 5 nm red-shifted absorption band carrying a transition dipole moment (M⃗λagg = 1.80 D) almost equivalent to the monomeric dye (M⃗λmon = 1.89 D). A significantly low magnitude of exciton coupling energy, ΔEexc = −358 cm–1 for the rhombic-RB colloidal J-aggregates resulted owing to the weaker electronic communication between the largely separated RB subunits (r = 7.2 Å) and a restricted exciton delocalization over the RB J-dimer (N = 2). The RB J-dimer exhibited a perfect balance between the computed singlet (2.53 eV) and the triplet (1.29 eV) exciton energies for singlet fission (SF). Supporting this, the PL decay profile of the J-aggregates revealed a delayed fluorescence, substantiating triplet pair formation via SF. The experimental evidence for the long-lived triplet formation was furthermore confirmed by its transient absorption (T1 → TN) at 530 nm. Consequently, a high probability for SF and a low probability for triplet–triplet recombination, leading to a dramatic lowering in photoluminescence quantum yield from 0.172 down to 0.035 was noted. The electronic structure calculations for the RB J-dimer followed TD-DFT-M062X/6-31G+(d,p) level of theory following integral equation formalism polarizable continuum model (IEFPCM) in water. S1 excited state for RB J-dimer was carefully analyzed using integral overlap of electron and hole density distribution (ϕ) and the defined t-indexes along all three spatial directions, and was found to be of locally excited in character.

A New Class of Nitroanilinic Dimer, the PNA O–Dimer: Electronic Structure and Emission Characteristics of O–Dimeric Aggregates

J. Phys. Chem. A 2015, 119, 8388−8399

p-Nitroaniline (PNA) has been reported as a “J” aggregate species. In retrospect, this communication confirms a radically different “oblique” orientation of the PNA units in all three solid, liquid, and gas phases of the dimer, the O-dimer. The nonvanishing transition dipole moments (TDM) associated with the allowed electronic excitations of the O-dimer, computed using electron–hole pair density distribution (EDD and HDD) analyses ascertained the two monomers to be inclined at slippage (θ) and polarization (α) angles of 18.5° and 55.4°, respectively. A detailed structure–property relationship of the PNA O-dimeric aggregate was carried out using UV–vis absorption and matrix scan emission spectroscopy, supported by electronic structure calculations at DFT-M062X/6-31G+(d,p) level using integral equation formalism polarizable continuum model (IEFPCM). The computed potential energy surface (PES) implied the global minimum of the PNA O-dimer stabilized by 4.8 kcal.mol–1, owing to bifurcated intermolecular hydrogen bonding. In the excited PNA O-dimeric aggregate, an exchange of excitation energy between the monomeric units resulted in two distinct electronic states separated by an interaction energy of −1644 cm–1. The TD-DFT computed excited state equilibrium structures of the PNA O-dimer corroborated the experimentally observed pronounced Stoke’s shift to internal conversion following vibrational relaxation of the allowed electronic excited states. On the basis of the detailed structural analysis of PNA O-dimer, the observed energy shifts in optical absorption spectroscopy were evident within the framework of exciton coupling model.

Optoelectronically Active Luminescent Valine-Substituted Perylene Diimide: Structure - Property Correlation via Spectroscopic and Density Functional Approaches

Journal of Physical Organic Chemistry (Wiley), 2020, Volume 33, Issue10, e4095

In literature, the applicability of solution-phase perylene diimides (PDIs) semiconductors are limited due to their restricted solubility in solvents. In contrast, we synthesized a highly soluble and novel valine-functionalized PDI derivative (perylene diimide diacid, PDIDA) whose optical and electrical properties were carefully assessed by experimental and density functional approaches. Notably, on valine substitution, the ultraviolet-visible absorption band centered at 524 nm was attributed to the predominant HOMO ➔ LUMO electronic transition (weighing coefficient = 99 %). Interestingly, the nonuniform variation (W-shaped) in absorption energy for HOMO ➔ LUMO electronic transition in PDIDA with solvent dielectric constant was experimentally witnessed. The latter was computationally attributed to the more S1 stabilization over So solvent stabilization, particularly in ethanol and dimethyl sulfoxide (DMSO). Furthermore, upon 525 nm excitation, the maximum fluorescence emission was observed at 533 nm with photoluminescence quantum yield as high as 0.77. Interestingly, similar to absorption studies, pronounced influence of solvent polarity was evident on the emission maximum particularly in ethanol and DMSO. Subsequently, electrochemical investigation proved that the PDIDA sustained the intrinsic n-type semiconductivity with a dielectric constant (εr) 5, a current of 0.54 mA at 5 V, and an electrical conductivity of 1.88 × 10−5 Sm−1. Owing to the above remarkable properties of the synthesized PDIDA, it holds potential applications in photovoltaics, fluorescence-based detectors and n-type channel field effect transistors, and so forth.

Dimeric conformation sensitive electronic excited states of tetracene congeners and their unconventional non-fluorescent behaviour

J. Chem. Sci. (Springer) 2019, 131:52

Unconventional non-fluorescent J-aggregates of Tetracene (TC) and Naphtho[2,1,8-qra]tetracene (NT) were witnessed and their consequent dramatic quenching was unravelled by a steady state, time-resolved and transient spectroscopy in conjunction with excited state density functional calculations. The TC O-aggregate with slippage angle θ=22.3∘<54.7∘θ=22.3∘<54.7∘ exhibited substantial transition dipole moment (TDM) for both lower (2.79 D) and higher (1.59 D) energy singlet excitations, while, NT formed an ideal J-aggregate (polarization angle, α∼0∘)α∼0∘) with a predominant TDM to only a lower excitonic state (2.69 D). Subsequently, their unusual quenching was quantified with large drops in the photoluminescence quantum yields (PLQY) from 0.116 to 0.002 upon TC O-aggregation and from 0.478 to 0.038 upon NT J-aggregation. These intense PL drops were systematically investigated for possible occurrence of excimer-like emission quenching and/or photo-degradation of the TC core unit. In view of the TC O-aggregates exhibiting a perfect energetic balance between the singlet (2.34 eV) and triplet (1.28 eV) energies for singlet fission (SF) and a concomitant delayed fluorescence signal, their S1S1 decay characteristics were attributed to SF followed by an inverse triplet-triplet recombination. In contrast, the energetic imbalance (E(S1)<2xE(T1)E(S1)<2xE(T1)) in NT J-aggregates permitted only forward process of SF and the resulting long-lived triplet formation was traced with a positive transient absorption (T1→Tn)T1→Tn) band at 500 nm. Accordingly, the singlet excited state (S1)(S1) dynamics of TC O- and NT J-aggregates, being largely dominated by SF, depicted a depleted S1S1 population, accounting for the large deviation from aggregation induced enhanced emission, exhibited by classical dye J-aggregates.

H- and J-aggregates : Deviations from Classical Behaviour, Lambert Academic Publishing, Germany

2020, ISBN :978-620-2-92296-8

Paranitroaniline (PNA), a prototype example of donor-π-acceptor nitroanilinic system, is known to possess large NLO response. However, in the literature, two PNA dimer conformations dimeric structure H and J-dimeric structure have accounted for the global minimum. Among polyacenes, tetracene (TC) and its derivatives represented a perfect energetic balance for both SF as well as its inverse process of triplet-triplet recombination. However, in literature, several other factors including superradiance and excimer formation have also been accounted for in explaining the behavior of complex Photolumniscence TCs. Therefore, the need for aggregate structure elucidation was more than warranted topical and unambiguous experimental and computational data. The work presented in this book focuses on understanding and controlling the aggregates structure, Bonding and electronic properties of nitroaniline and tetracene colloidal aggregates. Establishing the correlation between varied excited state dimeric structures with the unusual behavior PI comprised the other central goal of present work.