Arun K. Pal

Precise modulation of charge transfer (CT) characteristics in the excited state of aggregation-induced emission (AIE)-active organic emitters is essential for optimizing their performance mainly in optoelectronic applications such as organic light emitting diodes (OLEDs). In this work, two tetraphenylbuta-1,3-diene (TPB)-based donor–π–acceptor (D–π–A) derivatives, TPB–CHO–CBZ and TPB–CHO–PXZ, are designed by modifying the previously reported TPB–CHO–TPA, with the aim of systematically tuning the CT characteristics of the TPB backbones. Comprehensive photophysical and computational investigations reveal that the variation of the donor strength enables the fine control over the intramolecular CT behavior of these TPB derivatives, thereby influencing their excited-state properties such as solvatochromism, photoluminescence quantum yield, and emission lifetimes in both solution and solid states. Interestingly, temperature-dependent emission results provide an unusual enhancement of emission intensity accompanied by a blue shift for TPB–CHO–PXZ, suggesting the involvement of a twisted intramolecular charge transfer (TICT) state. Polarity and viscosity-dependent transient absorption spectroscopic studies have confirmed the presence of TICT character and its linear dependence of donor strength. Overall, the present findings demonstrate for the first time the significant structure–property correlations in TPB-based AIE systems, contributing to the rational design of high-performance optoelectronic materials for next-generation OLEDs and molecular electronics.

Silver nanocluster assemblies are an emerging class of fluorescent and stable solid materials by virtue of their structural tunability and stability compared to isolated clusters. The photophysical properties of the cluster nodes are influenced by an intra- or intercluster interaction. In this work, we show two-dimensional (2D) [Ag₅(C₂^tBu)₂(CF₃COO)₃(C₄H₄N₂)](CHCl₃) and three-dimensional (3D) [Ag₄(C₂^tBu)(CF₃COO)₃(C₄H₄N₂)] silver nanocluster assemblies obtained by tuning the molar ratios of the same protecting ligands and linker. The 2D assembly possesses a one-dimensional (1D) chain held together through the Ag···Ag interactions, whereas in the 3D assembly, Ag₄ units are linked through trifluoroacetate ligands to form the 1D chain with Ag₄-O-Ag₄ and Ag₄-O-C-O-Ag₄ connectivity. This varying effect of argentophilic interaction influences photoluminescence properties. In the 2D assembly, the emission was found to be dominated by the cluster-centered state owning to enhanced argentophilic interactions, whereas in 3D, the emission is from the linker. The theoretical investigations also revealed that the lowest unoccupied molecular orbitals (LUMOs) of the 3D assembly are localized on the pyrazine linker. In contrast, the LUMOs of the 2D assembly are mostly localized on the metal atoms within the cluster. This work highlights not only the importance of synthesis but also emphasizes the understanding of the cluster structure-luminescence relationship.

A series of bay-substituted PDI-based conjugated copolymers, which are composed of a linker as the donor moiety and perylene core as the strong p-electron acceptor moiety, have been designed to investigate the modulation of the photophysical properties with increasing donor–acceptor strength. After successful synthesis, all copolymers (namely, Benz-PDI, Btz-PDI, TzTz-PDI and NH-PDI) have been characterized by ¹³C NMR, MALDI-ToF and FT-IR analyses. The copolymers are moderately stable upon heating and are semi-crystalline in nature (except TzTz-PDI and NH-PDI). The optical characteristics of the copolymers are determined by absorption and emission studies. The redox properties of these copolymers are observed to change with increasing donor–acceptor strength, which is further supported by DFT calculations. Initial electrochemical studies reveal that the designed copolymers have the potential to be used as pseudocapacitive materials for energy storage applications. Among the four copolymers, charge–discharge studies indicate that NH-PDI produces the highest specific capacitance of 363 F g⁻¹ with outstanding cyclic stability of 15 000 cycles under a three-electrode setup. A symmetric supercapacitor device made of NH-PDI produced a specific capacitance of 134.2 F g⁻¹ with relatively higher values of specific energy density (E) and specific power density (P_max) at a current density of 0.5 A g⁻¹. The excellent result of PDI-embedded conjugated donor–acceptor polymers reveals a novel design strategy as well as the subsequent application of a new type of a stable polymeric electrode material for high-performance supercapacitors.

There is growing demand for the utilization of first-row transition metal complexes in light-driven processes instead of their conventional noble metal counterparts due to the greater sustainability of first-row transition metal complexes. However, their major drawback is the ultrafast lifetime of the electronic excited states of these first-row transition metal complexes, particularly those of d⁸ square-planar systems such as Ni(II) complexes, wherein low-lying metal-centered (MC) states provide the deactivation pathway. To increase the excited-state lifetime and broaden their applications, it is important to develop sterically bulky, strong field ligands with low-lying π* orbitals and a highly σ-donating nature to augment the energy of MC states. The current strategy relies on synthetically carbene-based ligands, which are substitutionally cumbersome and act as σ-donors only. In this work, we introduce a bis-indole pyridine (H₂BIP) ligand framework, whose dianionic congener (BIP) demonstrates the ability to form stronger covalent bonds with a Ni(II) center compared to neutral donors like carbene and its effect on the complex to produce a less distorted excited-state structure. When conjoined with ancillary ligands such as pyridine or lutidine, the BIP ligand orchestrates the formation of low-energy ³CT states, which decay in ∼40 ps.

The mechanism of palladium-catalyzed annulation reactions of benzamide- and anilide-type aromatic systems with maleimides is investigated using density functional theory. Double C-H bond activation is key to forming the desired annulation product. The first C-H bond activation for anilide- and amide-type ligands can occur at the ortho and benzylic C-H bonds, while the second C-H activation occurs at the meta carbon of the aromatic rings. For the anilide-type system, ortho and benzylic C-H bond activations occur via four- and five-membered palladacycles, respectively. In contrast, for the benzamide-type system, ortho and benzylic C-H bond activations occur via five- and six-membered palladacycles, respectively. The energy span model suggests that the initial C-H bond activation step at the benzylic position determines the turnover frequency for both anilide- and benzamide-type systems. Energy decomposition analysis and distortion-interaction/activation-strain analyses are employed to understand the electronic and steric factors controlling the turnover frequency-determining transition state.

In photodynamic therapy (PDT) treatment, heavy-atom-free photosensitizers (PSs) are a great source of singlet oxygen photosensitizer. Reactive oxygen species (ROS) are produced by an energy transfer from the lowest energy triplet excited state to the molecular oxygen of cancer cells. To clarify the photophysical characteristics in the excited states of a few experimentally identified thionated (>C=S) molecules and their oxygenated congeners (>C=O), a quantum chemical study is conducted. This study illustrates the properties of the excited states in oxygen congeners that render them unsuitable for PDT treatment. Concurrently, a hierarchy is presented based on the utility of the lowest-energy triplet excitons of thionated compounds. Their non-radiative decay rates are calculated for reverse-ISC and inter-system crossover (ISC) processes. In addition, the vibronic importance of C=O and C=S bonds is clarified by the computation of the Huang–Rhys factor, effective vibrational mode, and reorganization energy inside the Marcus–Levich–Jörtner system. ROS generation in thionated PSs exceeds their oxygen congeners as kf ≪ kISC, where radiative decay rate is designated as kf. As a result, the current work offers a calculated strategy for analyzing the effectiveness of thionated photosensitizers in PDT.

Symmetry-breaking charge separation (SB-CS) has recently evolved as an emerging concept offering its potential to the latest generation of organic photovoltaics. However there are several concerns that need to be addressed to reach the state-of-the-art in SB-CS chemistry, for instance, the desirable molecular geometry, interchromophoric distance and extent of electronic coupling. To shed light on those features, it is reported herein, that ortho-functionalized perylene monoimide (PMI) constituted regioisomeric dimer and trimer derivatives with varied molecular twisting and electronic conjugation have been synthesized. In steady-state photophysical studies, all the dimers and trimer derivatives exhibit a larger bathochromic shift in the emission spectra and a significant reduction of fluorescence quantum yield in polar DMF. Among the series of multichromophores, ortho- and self-coupled dimers display the strikingly different optical feature of SB-CS with a very fast charge separation rate (τ_CS = 80.2 ps) upon photoexcitation in DMF, which is unveiled by femtosecond transient absorption (fs-TA) studies. The SB-CS for two dimers is well-supported by the formation of PMI˙⁺ and PMI˙⁻ bands in the fs-TA spectra. Further analysis of fs-TA data revealed that, among the other multichromophores the trimer also exhibits a clear charge separation, whereas SB-CS signatures are less prominent, but can not be completely disregarded, for the meta- and para-dimers. Additionally, the charge separation dynamics of those above-mentioned PMI derivatives are devoid of a kinetically favorable excimer or triplet formation. The evidence of a profound charge transfer phenomenon in the ortho-dimer is characterized by density functional theory (DFT) calculations on excited state electronic structures. The excitonic communications in the excited state electronic arrangements unravel the key role of dihedral twisting in SB-CS. The thermodynamic feasibility of CS (ΔG_CS) and activation barrier (ΔG^≠) of the derivatives in DMF are established from the Rehm–Weller equation and Marcus's theory, respectively. This work is an in-depth study of the effect of mutual orientation of PMIs and regioisomerism in determining sustainable guidelines for using SB-CS.

Triplet energy harvesting via thermally activated delayed fluorescence (TADF) from pure organic systems has attracted great attention in organic light-emitting diodes, sensing, and photocatalysis. However, the realization of thermally enhanced phosphorescence (TEP)-guided efficient TADF with a high rate of reverse intersystem crossing (k_RISC) still needs to be discovered. Herein, we report two phenothiazine-quinoline conjugates (P2QC, P2QMC) comprising two phenothiazine donors covalently attached to the chlorine-substituted quinolinyl acceptor. Spectroscopic analysis in conjunction with quantum chemistry calculations reveals that TEP in P2QC originated due to slow internal conversion from higher-lying triplet to lowest triplet (T2' → T1') of the quasi-axial (QA) conformer and TADF (k_RISC = 1.44 × 10⁸ s^-1) originated from the quasi-equatorial (QE) conformer caused by a low singlet-triplet gap (ΔE_S1-T1 = 0.11 eV) and triplet energy transfer from QA to QE owing to the degenerate ground state of the conformers. In contrast, TADF (k_RISC = 0.74 × 10⁸ s^-1) and dual phosphorescence under ambient conditions are observed in P2QMC. This study provides a sustainable guideline for developing efficient TADF emitters via conformation effects and energy transfer mechanisms.

Non-centrosymmetric molecular crystals have a plethora of applications, such as piezoelectric transducers, energy storage and nonlinear optical materials owing to their unique structural order which is absent in other synthetic materials. As most crystals are brittle, their efficiency declines upon prolonged usage due to fatigue or catastrophic failure, limiting their utilities. Some natural substances, like bone, enamel, leaf and skin, function efficiently, last a life-time, thanks to their inherent self-healing nature. Therefore, incorporating self-healing ability in crystalline materials will greatly broaden their scope. Here, we report single crystals of a dibenzoate derivative, capable of self-healing within milliseconds via autonomous actuation. Systematic quantitative experiments reveal the limit of mechanical forces that the self-healing crystals can withstand. As a proof-of-concept, we also demonstrate that our self-healed crystals can retain their second harmonic generation (SHG) with high efficiency. Kinematic analysis of the actuation in our system also revealed its impressive performance parameters, and shows actuation response times in the millisecond range.

We report for the first-time intramolecular singlet fission (SF) in the naphthalenediimide (NDI) scaffold constrained in a cyclophane architecture, while individually the NDI units does not satisfy the requisite energy criterion for SF.