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Developing organic fluorescent molecules with multi-functional properties such as stimuli responses, fluorescence tuning, sensing, white light emission and bioimaging is highly desirable because of their application potential. Herein, we synthesized a triphenylamine (TPA)-based red-emitting ionic fluorophore, (E)-4-(4-(diphenylamino)styryl)-1-alkylpyridin-1-ium (TPA-P), with varied alkyl chain lengths (TPA-P-1 (methyl), TPA-P-7 (heptyl) and TPA-P-14 (tetradecyl)) and utilized them for fluorescence tuning, white light emission, chlorinated solvent sensing and latent fingerprinting applications. TPA-P ionic fluorophores showed anion-dependent tunable solid-state fluorescence from 564 to 671 nm. Single crystal structural studies suggested the anion exchange and modulation of molecular packing in the solid state. Further, TPA-P-1 revealed distinctly different colors and fluorescences towards sodium dodecyl sulphate (SDS) anions in an aqueous medium. Limit of detection (LOD) analysis indicated a sensing response up to 0.8 μM of SDS. TPA-P fluorophores also exhibited sensitive color changes in chlorinated solvents. TPA-P exhibited distinguishable fluorescence in ortho-dichlorobenzene (o-DCB) and meta-dichlorobenzene (m-DCB) isomers. The ionic nature of red-emitting fluorophores was exploited to generate a stable white light-emitting solution by mixing with blue fluorescent polystyrene sulphonate (PSS). CIE chromaticity indicated 0.32, 0.33 color coordinates, which are very close to pure white light emission. The red fluorescence and lipophilic alkyl chain of the TPA-P-14 ionic fluorophore were utilized for latent fingerprinting via powder dusting, which showed well-defined finger marks up to level 3 information. Thus, the quaternization of pyridine nitrogen in TPA π-conjugated fluorophores red shifted the fluorescence, and ionic functionality was further utilized to tailor functional fluorescence properties. [ABSTRACT FROM AUTHOR]

Exploring the organic molecular structural features and supramolecular interactions provide basis for developing stimuli-responsive organic solid-state fluorescent materials. Herein, we have synthesized methoxyphenyl-substituted carbazole-based π-conjugated derivatives (Cz-MN, Cz-ECA, Cz-CA, Cz-CAA and Cz-NBA) with different acceptor units and investigated their influence on solid-state fluorescence. All five compounds showed aggregation-induced emission (AIE) in the solid state with acceptor-dependent tunable fluorescence from 488 to 602 nm (quantum yield (Φf) = 7.2 to 1.9%). Cz-ECA exhibited blue-shifted fluorescence, whereas Cz-NBA showed red-shifted fluorescence. Cz-MN, Cz-ECA and Cz-CA revealed relatively strong solid-state fluorescence compared to Cz-CAA and Cz-NBA. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) calculations suggested intramolecular charge transfer (ICT) from the carbazole donor to the acceptor unit. Mechanofluorochromic studies of Cz-MN and Cz-ECA showed self-reversible fluorescence switching but the other three AIEgens did not show any stimuli–response. Single crystal structural analysis of Cz-MN, Cz-ECA and Cz-CA showed twisted molecular conformations with different intermolecular interactions. Both Cz-MN and Cz-ECA did not show any strong intermolecular interactions in the crystal lattice. In contrast, Cz-CA exhibited strong amide–amide complementary H-bonding and an extended network structure. Hence, the twisted molecular conformation and the absence of strong intermolecular interactions in Cz-MN and Cz-ECA contributed to self-reversible mechanofluorochromism (MFC). Thus, the present study is expected to provide further insight to design AIEgens with stimuli-responsive self-reversible fluorescent materials. [ABSTRACT FROM AUTHOR]

Zinc metal–organic frameworks (Zn-MOFs) of phthalic acid (ZnPA) and nitro-phthalic acid (ZnNPA) microcrystalline particles were fabricated and utilized for the removal of organic cationic dyes (crystal violet (CV), methylene blue (MB) and rhodamine B (RhB)) from water. PXRD studies confirmed the formation of crystalline materials, and BET indicated the mesoporous structure. HR-SEM studies showed the rod and plate morphologies for ZnPA and ZnNPA, respectively. ZnPA revealed low adsorption of dyes (CV, 20%) whereas ZnNPA showed significantly enhanced dye adsorption (CV, 99%). In particular, ZnNPA exhibited higher adsorption of CV (99%) and MB dye (83%) compared to RhB (54%). Nevertheless, ZnNPA could not be reused due to strong adsorption of dyes. Hence, a natural binder was introduced by dipping ZnNPA into tea extract, which also showed high adsorption of CV and MB dyes. Kinetics of adsorption exhibited best fitting with the pseudo-second-order model, indicating strong interactions between the dyes and adsorbent. Adsorption isotherm fitting suggested a good correlation coefficient (R2) with the Freundlich model, suggesting multi-layer adsorption. Interestingly, ZnNPA showed high adsorption of CV and MB dyes across the pH range between 3.0 and 11.0. The binder-introduced ZnNPA adsorbent was easily regenerated and demonstrated three cycles of reusability without significant change in the dye adsorption. Thus, the present work indicated the role of functional groups in the ligand and binder for improving the dye adsorption characteristics of MOFs. [ABSTRACT FROM AUTHOR]

The halogen substitution in an organic fluorophore exerted significant influence on the molecular organization via halogen intermolecular interactions and solid-state fluorescence. The carbazole core unit has been extensively utilized for developing optoelectronic materials because of its excellent fluorescence and phosphorescence properties. Herein, we have synthesized dihalogen substituted carbazoles (Cz-Cl, Cz-Br, Cz-I) and investigated their role in the supramolecular organization and solid-state fluorescence. Solid-state structural analysis revealed that Cz and halogen derivatives produced a herringbone structure but with varied intermolecular interactions and molecular orientation. Cz showed N–H⋯π intermolecular interactions between the amine hydrogen and middle ring of carbazole. Cz-Cl and Cz-Br also showed N–H⋯π intermolecular interactions but between the amine hydrogen and halogen decorated phenyl ring of carbazole. Cz-I showed only weak halogen–halogen and halogen–hydrogen intermolecular interactions. Cz showed strong fluorescence in solution as well as the solid-state (Φf = 55.9%). However, halogen substituted carbazole derivatives showed only solid-state fluorescence with strong reduction of fluorescence efficiency (Φf = 6.5% (Cz-Cl), 1.2% (Cz-Br)). Further, Cz-I did not show any fluorescence even in the solid-state. The significant reduction of fluorescence might be attributed to the heavy atom effect. Halogen substitution also caused varied electron density distribution in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Thus, halogen substitution influenced the molecular organization via halogen intermolecular interactions and solid-state fluorescence due to the heavy atom effect. [ABSTRACT FROM AUTHOR]

Perovskite solar cells (PSCs) are promising photovoltaic (PV) technologies due to their high-power conversion efficiency (PCE) and low fabrication cost. This review article delves into the changing PSC landscape by analyzing the various dimensional diversity, which includes zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) structures, and exploring the potential of mixed-dimensional integrations to improve the stability and performance of these promising PV devices. To fine-tune the properties of perovskite materials, researchers have used cutting-edge methods like passivation strategies, interface engineering, and exact crystal growth control. As a result, significant improvements in open-circuit voltage (VOC), long-term stability, and PCE have been made. This thorough review also discusses the complex trade-offs involved in each dimensional integration, offering important insights into the complex interplay between material properties, device architecture, and fabrication techniques. Dimensional diversity in PSCs ultimately represents a dynamic path toward advancing the state-of-the-art in PV technology, providing invaluable direction to scientists and engineers attempting to realize the full potential of these next-generation solar cells. In conclusion, this review article offers a thorough analysis of the developments, difficulties, and potential of perovskite-based solar cells. It contributes to the knowledge and development of high-efficiency PSCs that hold great promise in the race for better photovoltaic performance by looking at printing techniques, stability issues, applications, and the special properties of perovskite materials. [ABSTRACT FROM AUTHOR]

Cobalt hydroxide Co(OH)2 is considered to be a potential material for electrocatalyst, especially for oxygen evolution reaction (OER), owing to the earth's abundance, environmentally benign nature, and redox-active properties of cobalt. Herein, we report the fabrication of pristine Co(OH)2 and AgNP-integrated Co(OH)2 (Ag–Co(OH)2) by simple wet chemical methods and explore the electrocatalytic OER activity in the alkaline medium. High-resolution transmission electro-microscopic (HR-TEM) analysis revealed featureless nanostructures for Co(OH)2 with coexisting amorphous and crystalline phases, as well as the inclusion of crystalline AgNPs in Ag–Co(OH)2. X-ray photoelectron spectroscopic (XPS) analysis confirmed the inclusion of metallic AgNPs and the presence of Co at a mixed oxidation state. Electrocatalytic OER studies indicated that pristine Co(OH)2 required the overpotential of 299 mV to achieve a geometric current density of 10 mA cm−2. The integration of AgNPs in Co(OH)2 (Ag–Co(OH)2) showed a gradual improvement in the OER activity. The optimized sample, Ag–Co(OH)2-5, required the overpotential of 253 mV to produce 10 mA cm−2 current density. Tafel slope analysis revealed a lower value upon AgNP integration and electrochemical impedance showed lower charge transfer resistance. The lower Tafel value and charge transfer resistance of Ag–Co(OH)2-5 indicated good chemical coupling and faster reaction kinetics at the electrode surface. AgNPs incorporation with Co(OH)2 also showed enhanced turn-over frequency, electrochemical active surface area, and double-layer capacitance. The fabricated hybrid Ag–Co(OH)2-5 catalyst also exhibited good stability over 60 h. Thus, the electrocatalytic activity of low-cost Co(OH)2 was improved by fabricating coexisting amorphous and crystalline phases and integrating noble silver nanoparticles. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) with two dimensions (2D) exhibit remarkable electronic and optoelectronic properties that have made them a highly promising platform for the development of photodetectors (PDs). Highly efficient PDs can be made possible by the remarkable properties of 2D materials, including their large charge carrier mobility, high transparency, and tunable electronic structure. The photodetection mechanism in 2D TMD-based PDs is covered in detail in this article, with an emphasis on the important characteristics that set them apart from PDs based on other integrated materials. A variety of 2D TMD-based PD configurations are examined in this review, such as TMD–quantum dot (QD) configurations, TMD–graphene (Gr) hybrids, TMD–MXene composites, TMD–perovskite heterostructures, and single TMDs. The special qualities and benefits of each configuration are examined in great detail. This paper highlights the remarkable performance capabilities of 2D TMD-based PDs and provides a thorough review of recent developments in this field. Using state-of-the-art techniques, these include an ultrafast photo response, ultrabroad detectivity, and ultrahigh photoresponsivity. The article concludes by discussing the current issues facing the field of PDs and outlining the prospects for ground-breaking discoveries in this quickly developing field of study. The outlook highlights the potential for 2D TMD-based PDs going forward and offers promising avenues for additional development and use. [ABSTRACT FROM AUTHOR]

The coordination environment and networks of metal–organic frameworks (MOFs) play a significant role in their functional properties, especially catalysis. In recent years, the use of MOFs for electrocatalytic water-splitting reactions to produce green hydrogen fuel has gained increasing attention. Herein, we report the preparation of Co-benzene dicarboxylic acid-based MOFs (Co-MOF-1 and Co-MOF-2) with different coordination environments and correlated their structure-controlled electrocatalytic OER activity in an alkaline medium. Both Co-MOF-1 and Co-MOF-2 showed 3D network structures with different coordination environments around the Co metal centre. In Co-MOF-1, the Co metal centre is coordinated with DMF and pyridine along with a benzene dicarboxylic acid (BDC) ligand, whereas the Co metal centre is coordinated only with BDC ligands in Co-MOF-2. The Co metal centres in Co-MOF-1 contain labile solvent coordination but have completely saturated coordination with the BDC ligand alone in Co-MOF-2. Interestingly, Co-MOF-1 showed relatively strong OER activity and required the overpotential (η) of 294 mV to produce 10 mA cm−2 current density. However, Co-MOF-2 required the overpotential of 343 mV to achieve the 10 mA cm−2 current density. Further Co-MOF-1 showed comparatively low Tafel slope and charge transfer resistance and high electrochemical surface area than Co-MOF-2 and supported the enhanced OER activity. XPS analysis of Co-MOF-1 before and after catalysis suggested the conversion of Co-MOF-1 to CoOOH during the electrocatalysis that boosted the OER activity. The presence of a labile coordination site in Co-MOF-1 might have facilitated the interaction of hydroxyl electrolyte with the metal centre and active site generation by forming CoOOH easily which enhanced the OER activity. Co-MOF-1 also showed excellent stability for over 48 h. Hence, the present work provides structural insight for designing new MOFs for electrocatalytic water-splitting reactions. [ABSTRACT FROM AUTHOR]

The condensation reaction between naphthylacetonitrile isomers and anthracene aldehyde produced unexpected highly twisted AIEgens (2-(naphthalen-1-yl)-2-(10-oxo-9,10-dihydroanthracen-9-yl)acetonitrile (1) and (Z)-3-(anthracen-9-yl)-2-(naphthalen-2-yl)acrylonitrile (2)). 1 and 2 exhibited strong solid-state fluorescence and mechanical stimuli-induced reversible fluorescence switching. Crystallization of 2 in acidic CHCl3–CH3OH produced Cl substituted anthracene (2-Cl), whereas in CH2Cl2 pure crystals of 2 were produced. Single crystal analysis of 1 revealed a twisted conformation in the middle ring of anthracene due to carbonyl introduction. The carbonyl oxygen and cyano nitrogen are involved in the H-bonding interactions in the crystal lattice. 2 showed slipped stacking between naphthyl units whereas 2-Cl exhibited intermolecular H-bonding between Cl and anthracene hydrogens. 1 exhibited strong solid-state fluorescence (λmax = 541 nm, quantum yield (Φf) = 18.2%), whereas 2 and 2-Cl displayed tunable and relatively low fluorescence efficiency (λmax = 547 (2), 489 nm (2-Cl), Φf = 6.3 (2) and 3.6% (2-Cl)). Computational studies suggested clear intramolecular charge transfer (ICT) in 1 compared to 2 and 2-Cl. Further, 1 and 2-Cl showed mechanical crushing and heating induced reversible fluorescence switching but 2 did not show any fluorescence switching. Powder X-ray diffraction indicated a reversible phase transformation upon crushing and heating that caused reversible fluorescence switching. Hence, the present study provides insight into the subtle structural impact on intermolecular interactions and solid-state fluorescence. [ABSTRACT FROM AUTHOR]

The versatility of metal coordination geometry and organic linkers is used to derive metal–organic frameworks (MOFs) with an intriguing network structure in the solid-state, exerting significant control over its functional application. In this study, we prepared Co-benzene dicarboxylic acid based MOFs with 1D and 2D network structures with similar coordination environment and studied their electrocatalytic OER activity in alkaline medium. The use of bidentate amines (2,2′-bipyridine and DABCO) controlled the dimensionality and produced water coordinated 1D (Co-MOF-1D) and 2D (Co-MOF-2D) network structures. Although both Co-MOFs showed strong electrocatalytic OER activity, Co-MOF-2D exhibited relatively strong OER activity (required overpotential (η) of 211 mV to produce 10 mA cm−2 current density) compared to Co-MOF-1D (η10 = 237 mV). Co-MOF-2D exhibited excellent stability with a low Tafel slope, charge transfer resistance, and high electrochemical surface area that further supported the enhanced OER activity. [ABSTRACT FROM AUTHOR]

Terpyridine (TP) integrated with tetraphenylethylene (TPE-TP), pyrene (Py-TP) and triphenylamine (TPA-TP) exhibited structure controlled molecular packing and distinct fluorescence in the solid state. TPE-TP exhibited infinite aromatic π⋯π stacking between terpyridine units, Py-TP produced an isolated pyrene dimer, and TPA-TP displayed helical assembly via weak intermolecular H-bonding in the crystal lattice. The different molecular assembly of TPE-TP, TPA-TP and Py-TP produced a locally excited (LE) state, charge transfer (CT) state and excimeric emission at 460, 520 and 568 nm, respectively, in the solid state. Interestingly, the different molecular packing of TPE-TP, Py-TP and TPA-TP produced varied mechanofluorochromism. TPA-TP exhibited on–off reversible fluorescence switching upon mechanical crushing and heating, whereas TPE-TP and Py-TP displayed switching between two fluorescence states but in opposite directions. Crushing caused red shift of emission in TPE-TP and blue shift in Py-TP. Both TPA-TP and Py-TP showed strong fluorescence in solution and crystalline states, but TPE-TP showed typical aggregation induced emission (AIE). Further, Py-TP and TPE-TP showed selective fluorescence sensing of picric acid in solution and solid matrix, respectively. TPA-TP exhibited sensing of nitroaromatics both in solution as well as in the solid state. Solid state fluorescence of TPA-TP and TPE-TP was further utilized for fabricating PVA polymer composite thin films and demonstrated reusable fluorescence sensing of nitroaromatics in aqueous medium. The PVA–fluorophore composite film exhibited clear fluorescence quenching upon immersing the thin film into 10−7 M picric acid. Thus, the present study demonstrates the role of molecular structure in developing stimuli-responsive fluorescence materials and reusable sensors. [ABSTRACT FROM AUTHOR]

Metal–organic frameworks (MOFs) with their tunable topology, functionality and coordination environment have been considered as potential materials for various applications including electrocatalysis. Herein, we have synthesised a water coordinated nickel based 2D metal–organic framework (Ni-MOF) and a coordination complex (Ni-C) and investigated their electrocatalytic OER activity. The Ni-MOF showed a 2D sheet structure with one water coordination whereas a four water molecule coordinated charged complex was formed in the Ni-C. Thermogravimetric analysis (TGA) confirmed their water coordination and good thermal stability. Interestingly, electrocatalytic OER studies showed strongly enhanced activity for the Ni-MOF and that it required a low overpotential (194 mV) to produce a geometric current density of 10 mA cm−2. The Ni-C required 225 mV to produce 10 mA cm−2. The post-catalytic analysis suggested that the Ni-MOF and Ni-C are converted to nickel hydroxides/oxyhydroxides during electrocatalysis and acted as the catalytic centre. The low Tafel slope and charge transfer resistance further supported the higher activity of Ni-MOF based nickel hydroxides/oxyhydroxides. Chronoamperometric studies revealed the excellent stability of the Ni-MOF based catalyst over 72 h. The present study revealed the potential of developing highly active electrocatalysts based on Ni-MOFs by optimizing the topology and coordination environment. [ABSTRACT FROM AUTHOR]

Metal organic framework (MOF) based electrocatalysts have received increased attention in recent years because of their tailorable structure, functionality and porosity. Herein, cobalt-terephthalic acid (CoTPA) based microcrystalline MOFs particles are synthesized in the presence of four different organic amines (DABCO (D), butylamine (B), hexylamine (H) and triethylamine (T)) and the electrocatalytic oxygen evolution reaction was explored. Powder X-ray diffraction (PXRD) analysis confirmed the formation of crystalline CoTPA MOFs with a similar structure but with different faceted orientation. Further, morphological studies revealed the formation of CoTPA MOFs microparticles with different morphologies depending on the amine utilized in the reaction. BET analysis confirmed the porosity with a different surface area and pore diameter. Thermogravimetric analysis indicated the water coordination in all four MOFs. X-ray photoelectron spectroscopy (XPS) analysis indicated the coexistence of Co2+ and Co3+ in the MOFs. All four MOFs exhibited strong OER activity in alkaline medium. CoTPA-D exhibited relatively higher activity and required an overpotential of 273 mV to produce the benchmark current density of 10 mA cm−2 compared to CoTPA-B (281 mV), CoTPA-H (287 mV) and CoTPA-T (278 mV). The low Tafel slope and charge transfer resistance indicated the faster reaction kinetics whereas the higher electrochemical surface area and double layer capacitance (Cdl) suggested the availability of more active sites in CoTPA-D MOFs. The post catalytic analysis suggested the conversion of CoMOFs into CoOOH during electrocatalysis, which resulted in the enhanced OER reaction. Thus, the present work reports the facile preparation of CoMOFs with varied porosity and functionality that resulted in different electrocatalytic OER activity. [ABSTRACT FROM AUTHOR]

Molecular structural tailoring is one of the versatile approaches for developing functional organic fluorescent molecules with desired properties for practical applications. In this manuscript, we report the synthesis of triphenylamine based boron complexes (TPA-BF2) with unusual thermofluorochromism and alkyl chain length controlled molecular fluorescent liquids/solids. TPA-BF2 complexes with ethyl, propyl, butyl and pentyl groups produced fluorescent solids whereas with hexyl, heptyl and octyl groups yielded fluorescent liquids. The longer-alkyl chain compounds significantly improved the fluorescence efficiency (quantum yield (Φf = 12.6%)) by inhibiting close arrangement of fluorophores. TPA-BF2-2 and TPA-BF2-3 showed very weak fluorescence (Φf = 1.9%). On the other hand, all complexes exhibited strong solvent polarity dependent tunable fluorescence in the solution state. Importantly, the ability to form multiple excited states (locally excited (LE), charge transfer (CT) and twisted intramolecular charge transfer (TICT) states) resulted in unusual fluorescence tuning and reversible thermofluorochromism. The conversion of CT emission at RT to LE state emission at 77 K (under liquid nitrogen) of TPA-BF2 complexes in CHCl3/toluene resulted in strong blue shifting, while warming towards RT, the TPA-BF2 CHCl3 solution showed unusual fluorescence tuning. The fluorescence λmax of TPA-BF2-2 was red shifted from 476 to 600 nm and then blue shifted to RT fluorescence (550 nm). Interestingly, TPA-BF2 complexes in solution and in the PMMA matrix showed clear phosphorescence at 77 K. The unusual fluorescence tuning while warming from 77 K is attributed to the phosphorescence emission coupled with fluorescence at room temperature. Notably, the fluorescence intensity at RT was relatively stronger compared to red shifted fluorescence intensity while warming. TPA-BF2 complexes in toluene showed strong linear enhancement of fluorescence intensity (1.9 to 2.5 times) with a blue shifted peak position upon increasing the temperature from 0 to 100 °C. The alkyl chain length exhibited a strong influence on the rate and extent of fluorescence tuning with temperature. Thus, alkyl group variations in simple Schiff base–boron complexes produced unusual thermofluorochromism and molecular fluorescent liquids. [ABSTRACT FROM AUTHOR]

Highly efficient and stable Earth abundant transition metal electrocatalysts are in great demand for the oxygen evolution reaction (OER), a bottleneck process involved in the water splitting reaction and metal–air batteries. Herein, we have demonstrated a single step direct fabrication of cobalt hydroxide (Co(OH)2) nanowires doped with vanadium(V) in a less stable +4 oxidation state and fluoride (F) ions (V–Co(OH)2) on a carbon cloth electrode that showed highly enhanced OER activity under alkaline conditions. V–Co(OH)2 nanowires synthesized under the optimized conditions produced excellent OER activity with an ultralow overpotential of 136 mV at 10 mA cm−2 (scan rate 1 mV s−1), a small Tafel slope (51.6 mV dec−1) and good stability over 72 h. To the best of our knowledge, this is the lowest overpotential reported for cobalt-based electrocatalysts to achieve a geometric current density of 10 mA cm−2. The controlled synthesis and HR-TEM studies revealed the formation of hybrid nanostructures (nanowires along with spherical assembly of nanoparticles) and codoping of V and F ions played an important role in enhancing the OER activity. The detailed chemical composition and oxidation state analysis by X-ray photoelectron spectroscopy (XPS) confirmed the doping of V4+ and ionic F in V–Co(OH)2 with mixed valence states of Co2+/Co3+ and a higher Co2+ ratio. The outstanding OER activity of V–Co(OH)2 is attributed to the formation of a spherical assembly of nanoparticles with nanowires, which provided a high number of catalytically active sites with enhanced charge transport, and doping of higher valence V4+ and strongly electronegative F in V–Co(OH)2 with a higher ratio of Co2+/Co3+ promoted OOH* intermediate generation and significantly boosted the OER activity. Overall, the present work highlights the possibility of achieving highly active Earth abundant OER electrocatalysts by controlling the mixed oxidation state of Co with a judicious choice of dopants along with maintaining optimal nanostructure morphologies. [ABSTRACT FROM AUTHOR]

Fluorine and nitrogen codoped cobalt hydroxide–graphene oxide nanocomposites (N,F-Co(OH)2/GO) were synthesized by a simple hydrothermal method and demonstrated highly enhanced oxygen evolution activity in an alkaline medium. N,F-Co(OH)2/GO synthesized under optimized reaction conditions required an overpotential of 228 mV to produce the benchmark current density of 10 mA cm−2 (scan rate 1 mV s−1). In contrast, N,F-Co(OH)2 without GO and Co(OH)2/GO without fluorine required higher overpotentials (370 (N,F-Co(OH)2) and 325 mV (Co(OH)2/GO)) for producing the current density of 10 mA cm−2. The low Tafel slope (52.6 mV dec−1) and charge transfer resistance, and high electrochemical double layer capacitance of N,F-Co(OH)2/GO compared to N,F-Co(OH)2 indicate faster kinetics at the electrode–catalyst interface. The N,F-Co(OH)2/GO catalyst showed good stability over 30 h. High-resolution transmission electron microscope (HR-TEM) images showed good dispersion of polycrystalline Co(OH)2 nanoparticles in the GO matrix. X-ray photoelectron spectroscopic (XPS) analysis revealed the coexistence of Co2+/Co3+ and the doping of nitrogen and fluorine in N,F-Co(OH)2/GO. XPS further revealed the presence of F in its ionic state and being covalently attached to GO. The integration of highly electronegative F with GO stabilizes the Co2+ active centre along with improving the charge transfer and adsorption process that contributes to improved OER. Thus, the present work reports a facile method for preparing F-doped GO–Co(OH)2 electrocatalysts with enhanced OER activity under alkaline conditions. [ABSTRACT FROM AUTHOR]

As the heaviest chalcogen atom, tellurium (Te) is endowed with a high Seebeck coefficient and low lattice thermal conductivity, which are the key factors that contribute to a high ZT value in thermoelectricity. The most studied thermoelectric element is Te, and it was theoretically and experimentally studied to be an ideal thermoelectric element. However, the instability and mechanical endurance of Te thermoelectric modules cannot be guaranteed, especially in flexible electronic devices. A focused Te/polymer strategy was recently developed and implemented to address these problems, so combining nanostructured Te with flexible polymers can be an efficient way to construct stable, efficient and flexible thermoelectric generators. In this review, the use of nanostructured Te for thermoelectricity is summarized in brief, and then the preparation methods of Te/polymers are reviewed. Subsequently, the thermoelectric properties and applications of Te/polymers are highlighted, followed by detailed polymer types, including PEDOT:PSS, PANI, P3HT and PPy. Finally, based on current progress and achieved results, the prospects and future directions of Te/polymers are highlighted and discussed. [ABSTRACT FROM AUTHOR]

The structural assembly of organic fluorescent molecules exerts a significant influence on solid-state fluorescence properties, including fluorescence tuning and switching. Herein, we have synthesized a Schiff base based organic fluorophore, 2-((E)-((E)-(4-hydroxybenzylidene)hydrazono)(phenyl)methyl)phenol (1), with intra- and intermolecular H-bonding hydroxyl functionalities, and studied its solid state structural arrangement and fluorescence properties. 1 exhibited aggregation induced enhanced emission (AIEE) in the solid state. The crystallization of 1 produced geometrical (E/Z) isomers (1a (E-isomer) and 1b (Z-isomer)) with completely different intra- and intermolecular H-bonding. 1a showed typical intramolecular H-bonding with an adjacent nitrogen and showed six membered planar rings while 1b revealed unusual intramolecular H-bonding with seven membered non-planar rings. The intermolecular H-bonding of the additional hydroxy group in 1a produced a hexagon arrangement with the inclusion of a water molecule whereas a 2D network structure is formed in 1b. However, both 1a and 1b isomers showed large Stokes shifted solid state fluorescence at 570 and 565 nm, respectively, due to the intramolecular H-bonding facilitated excited state intramolecular proton transfer (ESIPT) process. Interestingly, both 1a and 1b exhibited self-reversible fluorescence switching after crushing without showing any structural transformation. 1a exhibited a relatively slow fluorescence recovery compared to 1b. Computational studies showed that intramolecular H-bonding/conformational twisting controlled the electron density distribution in the HOMO. PXRD studies confirmed the structural stability and complete regeneration of the crystalline phase after mechanical crushing. Thus, the simple interplay of intra/intermolecular H-bonding in 1 led to the formation of geometrical isomers with distinct self-reversibility. [ABSTRACT FROM AUTHOR]

Integrating an intramolecular H-bonding functionality in an organic fluorophore produces an excited state intramolecular proton transfer (ESIPT) mechanism facilitating large Stokes shifted fluorescent materials. The tunable and environmental sensitive fluorescence of ESIPT molecules makes them potential candidates for white light emitting, chemo-sensing and bioimaging applications. Herein, we designed organic fluorophores (2-((E)-((E)-(2-hydroxybenzylidene)hydrazono)(phenyl)methyl)phenol (1), 2-((E)-(((E)-(2-hydroxyphenyl)(phenyl)methylene)hydrazono)methyl)-4-methoxyphenol (2), 2-((E)-((E)-((2-hydroxyphenyl)(phenyl)methylene)hydrazono)methyl)-5-methoxyphenol (3) and 5-(diethylamino)-2-((E)-((E)-((2-hydroxy phenyl)(phenyl)methylene)hydrazono)methyl)phenol (4)) with two intramolecular H-bonding functionalities and unsymmetrical structures (salicylaldehyde-imine and benzophenone-imine) and explored their substituent-dependent structural assembly and solid-state fluorescence. 1–4 showed tunable solid-state fluorescence between 536 and 645 nm dependent on the inter/intramolecular H-bonding. 3 produced yellow (3-Y) and red (3-R) fluorescent E/Z geometrical isomers. Solid-state structural studies revealed the formation of intramolecular H-bonding at both functionalities in 1, 2a and 3-Y, whereas 2b, 3-R and 4 displayed one intra- and another intermolecular H-bonding. The formation of intra- and intermolecular H-bonding significantly influenced the solid-state fluorescence. Mechanofluorochromic (MFC) studies showed self-reversible fluorescence switching for 1–3 and stimuli-induced reversible fluorescence switching for 4. Importantly, the geometrical isomers 3-Y and 3-R exhibited distinct fluorescence switching without showing any structural transformation upon crushing/heating. Computational studies suggested that the 3-R isomer is more stable by 9.1 kcal mol−1 compared to 3-Y. Powder X-ray diffraction studies further confirmed its reversible phase transition and structural stability. Overall, the present work explored the structural versatility of ESIPT molecules and its impact on fluorescence tuning and switching. [ABSTRACT FROM AUTHOR]

Interlinked aggregation induced emissive (AIE) fluorophores (triphenylamine (TPA) donor (D) and indanedione acceptor (A)) via non-conjugated spacers (1–3) showed concentration-dependent tunable fluorescence from blue to deep red/NIR in solution and deep red emission in the solid-state. 1–3 exhibited weak broad solid-state fluorescence covering the blue to deep red region (433 to 706 nm, ΦF = 2.3 to 5.3%). It is noted that the non-interlinked TPA-indanedione fluorophore was non-emissive in solution but showed strong solid-state fluorescence at 611 nm (ΦF = 17.7%). In contrast, 1–3 showed strong fluorescence in the solution state (ΦF = 0.001 to 0.09 compared to fluorescein). Importantly, 1–3 exhibited concentration-dependent tunable fluorescence from blue (457 nm) to deep red/NIR (720 nm) in CHCl3 due to varied aggregation of fluorophores. Dynamic light scattering (DLS) studies confirmed the molecular aggregation even at very low concentrations (10−7 M). The increasing concentration lowered the solubility and produced bigger aggregates with wide size distribution that caused tunable fluorescence. The deep red fluorescence of 1 was utilized for live-cell imaging applications using E. coli cells. Fluorescence microscopic images revealed clear red fluorescent E. coli cells with a rod morphology. Furthermore, the non-toxic nature of 1 was confirmed by in vitro as well as in vivo studies. Thus, deep red/NIR emissive fluorophores were achieved by interlinking two AIEgens using a non-conjugated spacer and utilized for live cell imaging. [ABSTRACT FROM AUTHOR]

Water splitting electrolysis is a promising sustainable methodology to produce clean and green hydrogen fuel, which is considered as a prominent replacement for conventional fossil fuels. Developing highly active, stable and low-cost electrocatalysts plays a critical role for achieving efficient production of hydrogen by electrolysis for practical applications. Herein, we report the synthesis of Co3O4 spinel oxides by simple calcination of commercially available cobalt salts at different temperatures (400, 600 and 800 °C) and the disordered structure induced highly enhanced hydrogen evolution reaction (HER) in an alkaline medium (1.0 M KOH). Powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) indicated the formation of a normal spinel Co3O4 structure at 400 (Co3O4-400) and 600 °C (Co3O4-600) calcination, whereas disordered Co3O4 at 800 °C (Co3O4-800). HR-TEM analysis revealed the formation of spherical crystalline particles in Co3O4-400 and featureless aggregated crystalline particles at higher temperatures. The d-spacing analysis using PXRD and the lattice fringe pattern showed a decrease of the spacing with increasing calcination temperature, which suggested higher strain for Co3O4-800. The electrocatalytic HER activity of Co3O4-400 and Co3O4-600 in an alkaline medium showed moderate activity. Co3O4-400 and Co3O4-600 required an overpotential of 293 and 240 mV, respectively, to achieve a geometric current density of 10 mA cm−2. In contrast, disordered Co3O4-800 required a very low overpotential (93 mV) for producing the geometric current density of 10 mA cm−2. Electrochemical impedance studies showed faster kinetics and charge transfer for disordered spinel Co3O4-800 and supported the enhanced HER activity. The electrocatalytic stability studies for Co3O4-800 were tested up to 48 h at 100 mA cm−2 current density and confirmed the excellent stability. Thus, a low-cost earth abundant HER electrocatalyst was fabricated by a simple method. [ABSTRACT FROM AUTHOR]

The fluorophore structure and supramolecular interactions play important roles in the molecular conformation and packing in the solid state that strongly influence the solid-state fluorescence properties. Herein, we report the synthesis of pyridylacetonitrile integrated anthracene fluorophores ((Z)-3-(anthracen-9-yl)-2-(pyridin-2-yl)acrylonitrile (2-APy), (Z)-3-(anthracen-9-yl)-2-(pyridin-3-yl)acrylonitrile (3-APy) and (Z)-3-(anthracen-9-yl)-2-(pyridin-4-yl)acrylonitrile (4-APy)) and pyridine nitrogen position controlled stimuli-responsive fluorescence switching. 3-APy exhibited strongly enhanced solid state fluorescence (ΦF = 45.83%) compared to 2-APy (ΦF = 1.68%) and 4-APy (ΦF = 6.05%). Moreover, 3-APy displayed reversible pressure/heating induced off–on fluorescence switching whereas the quenched fluorescence of 2-APy and 4-APy by crushing did not recover after heating. Solid state structural analysis revealed a twisted molecular conformation between anthracene and pyridylacetonitrile in all three molecules. However, the pyridine nitrogen position significantly influenced the extent of twisting and molecular packing. The highly twisted molecular conformation and intermolecular interactions facilitated good separation of anthracene molecules without π–π stacking in the crystal lattice of 3-APy. In contrast, 2-APy showed a face-to-face arrangement of pyridine rings whereas anthracene and pyridine rings were overlapped in the 4-APy crystal lattice contributing to weak fluorescence. Powder X-ray diffraction (PXRD) of 3-APy showed reversible transformation between crystalline and amorphous phases upon crushing and heating, causing reversible off–on fluorescence switching. The strong solid-state fluorescence and pyridine unit of 3-APy were used to demonstrate halochromic reversible fluorescence switching. Further the weak fluorescence of 2-APy and 4-APy was strongly enhanced via supramolecular co-crystal formation with p-toluenesulfonic acid (PTSA) which hindered aromatic overlapping and close packing in the crystal lattice. Overall, the present work studied the impact of subtle positional change of nitrogen on the molecular aggregation and solid-state fluorescence properties. [ABSTRACT FROM AUTHOR]

pH-responsive fluorescence molecules are interesting candidates for achieving tunable as well as white light emission from a single molecule. Herein, we have synthesized pyridine-integrated carbazole and triphenylamine donor–acceptor isomers, (Z)-3-(4-(9H-carbazol-9-yl)phenyl)-2-(pyridinyl)acrylonitrile (1–3) and (Z)-3-(4-(diphenylamino)phenyl)-2-(pyridinyl)acrylonitrile (4–6), and studied the effects of the structure and solvent on acid-responsive fluorescence modulation in the solution state. Both carbazole (1–3) and triphenylamine (4–6) isomers produced twisted intramolecular charge transfer (TICT) states and showed tunable fluorescence depending on the solvent polarity. Highly blue shifted fluorescence was observed in non-polar toluene, while polar DMF showed large red-shifted fluorescence. Further, 1–3 isomers exhibited relatively strong fluorescence compared to 4–6. Interestingly, 1–6 exhibited organic acid-dependent (trifluoroacetic acid (TFA) and p-toluene sulphonic acid (PTSA)) and solvent-dependent fluorescence tuning. Carbazole isomers (1–3) showed wide fluorescence colour tuning from blue to red as well as white light emission in toluene, whereas green to red transition was observed in CHCl3. Addition of TFA, a stronger acid, to 1 and 3 afforded blue, green and red emission along with white emission in toluene, whereas weak PTSA produced blue and red emission along with white emission. In contrast, addition of TFA or PTSA to the triphenylamine isomers (4–6) exhibited complete quenching of fluorescence in all solvents. The meta-isomers did not show significant fluorescence tuning with acids. The wide fluorescence tuning of 1 in toluene with TFA has also been successfully transformed into a PMMA polymer matrix, which also exhibited clear fluorescence tuning from blue to red. Density functional theoretical (DFT) studies indicated high molecular twisting in the excited state and clear separation of charge transfer from the donor carbazole to the acceptor unit compared to the propeller-shaped triphenylamine isomer; this may contribute to the wide fluorescence tuning in the carbazole isomers. The present studies could be useful for choosing appropriate molecular structures and solvent systems for achieving wide fluorescence tuning and white light emission from a single donor–acceptor molecule. [ABSTRACT FROM AUTHOR]

Stimuli-responsive organic solid state fluorescent materials are considered as potential candidates for optoelectronic application as well as in the biomedical field. Molecular design and supramolecular interaction controlled organization in the solid state played an important role in producing switchable and tunable fluorescent materials. Excited state intramolecular proton transfer (ESIPT) mechanism-based solid state fluorescent materials showed unique photophysical properties such as a large Stokes shift and local environment (pH, polarity, ions and viscosity) responsive fluorescence modulation. The unique photophysical properties of ESIPT molecules made them interesting for various fields including laser dyes, molecular probes, optoelectronics, white emissive materials and optical information storage materials. Systematic fluorophore structural engineering has been performed over the years to gain insight on the ESIPT mechanism in order to improve the quantum efficiency and introduce desirable material attributes for functional applications. The substitutional unit, molecular conformation and supramolecular interactions played a significant role in transforming planar ESIPT fluorophores to stimuli-induced fluorescence switching materials either between two states or off–on states. In this review article, we have presented the recent developments in ESIPT-based solid state fluorescent materials and external stimuli-induced fluorescence switching. [ABSTRACT FROM AUTHOR]

Developing an efficient transition metal-based non-noble electrocatalyst is important for the industrial water splitting process and producing green sustainable hydrogen (H2) fuel. Particularly, the direct fabrication of electrocatalysts on an electrode surface can exhibit strong electronic coupling that results in increased catalytic activity. Herein, copper (Cu) and cuprous oxide (Cu2O) nanoparticles were directly deposited on a nickel foam (NF) substrate and the controlled calcination of the Cu produced mixed copper–copper oxides (CuO, Cu2O–CuO and Cu–Cu2O–CuO) that exhibited excellent composition-controlled bifunctional electrocatalytic (oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)) activity in an alkaline medium with good stability. Powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopic (XPS) studies confirmed the formation of pure and mixed copper–copper oxide nanomaterials on NF. Mixed copper oxide (Cu2O–CuO@NF) and pure CuO@NF exhibited excellent electrocatalytic OER and HER activity, respectively, in alkaline media. To the best of our knowledge, Cu2O–CuO@NF required an overpotential of 262 mV to achieve the lowest geometric current density (10 mA cm−2) among the pure copper-based electrocatalysts reported. Cu2O–CuO@NF also produced a high current density (130.9 mA cm−2) at a relatively low applied potential (1.56 V) compared to other catalysts. The low Tafel slope (45.8 mV dec−1) and charge-transfer resistance indicated that good electronic coupling and fast charge transport led to the higher activity. In the HER, pure CuO@NF required a much lower overpotential of 165 mV to produce geometric current density. It is worth noting that CuO@NF showed strong HER activity in the alkaline medium. The Tafel slope of 121.81 mV dec−1 for CuO@NF suggests that the desorption process could be the rate-determining step in the HER process. Cu2O–CuO@NF and CuO@NF exhibited good stability in the alkaline medium. The highly enhanced current density and good stability of Cu2O–CuO@NF in 6.0 M KOH at 60 °C highlights its potential for practical applications. Overall, an earth-abundant copper-based electrocatalyst with excellent OER and HER activity in alkaline media has been developed by directly fabricating on a conducting substrate and controlling the copper oxides composition. [ABSTRACT FROM AUTHOR]

Smart fluorescent materials exhibit a controlled fluorescence response to different external stimuli such as pressure, heat, light, pH, etc. Molecular structure and supramolecular assembly via weak intermolecular interactions strongly influence the fluorescence efficiency and colour. In particular, non-planar molecular structures play a significant role in developing solid state fluorescent materials. Triphenylamine (TPA), a typical non-planar propeller molecule with interesting optoelectronic properties, provides an excellent opportunity for developing a variety of molecular fluorescent materials by taking advantage of synthetic feasibility. For example, a donor–acceptor (D–A) aggregation induced emissive (AIE) fluorophore can be developed by integrating an acceptor group into the phenyl unit of a TPA donor. In this review, we focus exclusively on TPA-based smart fluorescent materials that exhibit distinct and reversible fluorescence switching towards different stimuli. Molecular engineering of the TPA fluorophore resulted in the development of different types of stimuli-responsive materials and the conformational flexibility of non-planar phenyl groups often produced polymorphism induced fluorescence tuning. A detailed discussion of the structure–property relationships of TPA derivatives could provide insights for developing new TPA-based smart fluorescent materials with versatile properties. [ABSTRACT FROM AUTHOR]

Copper–pyridinedicarboxylic acid produces concomitant polymorphs, a copper complex (Cu–C) and a coordination polymer (Cu–P), which have a similar coordination environment but completely different electrocatalytic activity. Cu–P displayed highly enhanced activity for the hydrogen evolution reaction (HER, η@10 mA cm−2 = 285 mV, jmax = 161.7 mA cm−2) compared to Cu–C (η@10 mA cm−2 = 444 mV, jmax = 48.7 mA cm−2) in a neutral medium. [ABSTRACT FROM AUTHOR]

Carbazole-based donor–acceptor derivatives of halogen-substituted positional isomers were synthesised and the effect of the halogen and position on the solid-state fluorescence and mechanofluorochromism was explored. All compounds showed good solid-state fluorescence (Φf = 15.67 to 31.32%). Interestingly, the crystallization of the positional isomers produced concomitant blue and green fluorescent polymorphs and tunable solid-state fluorescence. Solid-state structural analysis revealed a subtle conformational change in the acceptor unit, which led to polymorphism and tunable fluorescence. Halogen substitution induced a higher conformational twist and a large fluorescence shift between the polymorphs. The supramolecular interactions (H-bonding, C–H⋯π and π⋯π) observed in the crystal lattice rigidified the fluorophores and enhanced the solid-state fluorescence. Computational studies were performed to gain insights into the electronic energy level of the fluorophore and fluorescence modulation. All fluorophores showed mechanofluorochromism (MFC), reversible fluorescence switching, upon crushing and heating. The crushed solids also showed self-reversible fluorescence switching with a longer time. Interestingly, OFF–ON reversible fluorescence switching was also demonstrated by melting and warming/scratching. The powder X-ray diffraction studies supported self-reversible and crystallization-induced fluorescence switching. [ABSTRACT FROM AUTHOR]

The current study demonstrates a facile one-pot pyrolysis route to develop photoluminescent graphitic carbon nitride nanosheets (g-C3N4) for the fluorescence detection of hydrazine. The morphological images show that MnO2 nanotubes are decorated over the surface of g-C3N4 nanosheets, and the addition of hydrazine completely topples the MnO2 nanotubes; this uncovers the g-C3N4 nanosheets. The turn-off fluorescence was achieved with the decoration of MnO2 nanostructures on the g-C3N4-nanosheets via the fluorescence resonance energy transfer mechanism. Morphological influences of MnO2 nanostructures on the dynamic quenching of the fluorescence behavior of g-C3N4 nanosheets were analyzed. The fluorescence energy transfer between g-C3N4 and MnO2 was restricted with the addition of hydrazine; this led to the recovery of the fluorescence behavior of g-C3N4. The proposed sensor exhibits high sensitivity, the wide linear range of 2–55 μM, and the low detection limit of 0.12 μM. The developed nanocomposite exhibits excellent selectivity towards hydrazine over common metal ions and molecules with analogous structures. Furthermore, the prepared nanocomposite exhibits good recovery in environmental water samples; this unveils its practical competence in real samples. Conspicuously, this study provides an excellent platform for the development of a competent fluorescent probe and dynamic opportunities for the effectual monitoring of hydrazine in environmental water samples. [ABSTRACT FROM AUTHOR]

Biochar (BC), prepared from the carbonization of neem wood, was modified with nickel ferrite (NiFe2O4) nanorod/poly(3,4-ethylenedioxythiophene) (PEDOT) composite and directly employed as a free-standing and binder-free anode in microbial fuel cells (MFCs). The morphological and X-ray diffraction analyses demonstrate that the cubic spinel-structured NiFe2O4 nanorods are homogeneously distributed over the sheet-like amorphous PEDOT. The bioelectrochemical activity of exoelectrogens is improved with PEDOT/NiFe2O4/BC as endorsed by the superior oxidation current in the voltammograms. Furthermore, the MFC with PEDOT/NiFe2O4/BC demonstrates a peak power of 1200 mW m−2 because of the unique catalytic nanoarchitectures, compact biofilm formation, continuous electron conduction paths, and elevated structural and chemical stabilities of PEDOT/NiFe2O4. The waste matter-derived electrode constituent, binder-free electrode modification, mediator-less electron transference, and considerable power generation associated with a proficient lifespan performance of PEDOT/NiFe2O4 open up a versatile avenue towards the development of cost-efficient, high-performance, and durable MFCs. [ABSTRACT FROM AUTHOR]

Correction for 'Cobalt metal–organic framework microcrystalline particles with strong electrocatalytic activity: amine controlled morphology and OER activity' by Gunasekaran Arunkumar et al., New J. Chem., 2023, https://doi.org/10.1039/d3nj01457f. [ABSTRACT FROM AUTHOR]

In the present quantum chemical investigation, for the first time, we shed light on the promising structure, charge transport and nonlinear optical properties (NLO) of oxygen-bridged planarized 4-aminopyridine (APY3O3). Density functional theory (DFT) and time dependent DFT methods are used to elucidate the novel structure, optical and NLO properties of open 4-aminopyridine (APY3) as well as oxygen-bridged closed APY3O3 systems. The calculated third-order polarizability 〈γ〉 for APY3 and APY3O3 is found to be 76.85 and 89.16 × 10−36 esu with the B3LYP method and the 6-311G** basis set, respectively, which is ∼10 and 12 times larger than that of para-nitroaniline (a prototype NLO molecule). Calculation of the electron affinity and reorganization energy suggests that good electron transfer properties are feasible by changing the geometry from open APY3 to closed APY3O3 through oxygen bridging. Interestingly, the configuration of a dimer having APY3O3 and oxygen-bridged triphenylborane (TPBO3) monomers demonstrates remarkable enhancement in NLO response through a novel non-covalent intermolecular charge transfer process. The present investigation not only evokes scientific interest regarding the promising structure of APY3O3 but also highlights its several optical and NLO properties that can be beneficial for its use as an advanced functional material in modern hi-tech applications. [ABSTRACT FROM AUTHOR]

The buckyferrocenes, synthesized through face-to-face fusion of ferrocene and fullerene fragments (C60Me10), are expected to enjoy the rich scientific heritage of ferrocene and fullerene with an extensively large π-conjugation network between the two Fe atoms [Y. Matsuo, K. Tahara and E. Nakamura, J. Am. Chem. Soc., 2006, 128, 7154]. However, the addition of pentamethyl groups at each end of the fullerene fragment breaks the π-conjugation path as well as metal–metal spin correlation between the two-ferrocene faces in a buckyferrocene. We found that the unblocking of π-conjugation from different positions in fullerene fragments have substantial effects on their topologies, spin densities, diradical characters as well as nonlinear optical (NLO) properties of these buckyferrocenes. We study the topological dependence of open-shell diradical character and second hyperpolarizability (γ), the third-order NLO properties at the molecular scale, in several buckyferrocenes. On the basis of their different diradical characters (yi), which are defined by the occupation number of the lowest unoccupied natural orbital (LUNO) + i (i = 0, 1,…), these buckyferrocenes are categorized into three groups, i.e., closed-shell (yi = 0), intermediate open-shell singlet (0 ＜ yi ＜ 1), and almost pure open-shell singlet (yi = 1) compounds. For example, we found that buckyferrocenes including (CpFe)2η5C60Me10 and (CpFe)2η5C70Me10 have closed-shell configurations. The buckyferrocenes (CpFe)2η5C60, (CpFe)2η5C70, (CpFe)2η5C70Me8, (CpFe)2η5C70Me4 and (CpFe)2η5C30 are intermediate open-shell singlet, while (CpFe)2η5C60Me4 and (CpFe)2η5C60Me8 are pure open-shell singlet complexes. Interestingly, the γzzzz amplitude of (CpFe)2η5C60, an open-shell intermediate diradical complex, is about 41 times and 13 times as large as those of its closed-shell ((CpFe)2η5C60Me10) and pure diradical ((CpFe)2η5C60Me8) counterparts, respectively. Similarly, the γzzzz amplitudes of (CpFe)2η5C70, (CpFe)2η5C70Me4, and (CpFe)2η5C70Me8 with intermediate diradical character are about 36, 17, and 9 times as large as that of their closed-shell (CpFe)2η5C70Me10 counterpart. The fact that larger γzzzz values are obtained for buckyferrocenes with intermediate diradical characters is in line with the “y–γ correlation” obtained from the valence configuration interaction (VCI) results for a two-site diradical model [M. Nakano, et al., Phys. Rev. Lett., 2007, 99, 033001] as well as for fullerene and graphene systems. The γzzzz density analysis shows that the large positive contributions originate from the large γzzzz density distributions on the upper- and lower-extended edges of the buckyferrocenes, between which significant spin polarizations appear within the spin-unrestricted DFT level of theory. These results demonstrate that such buckyferrocenes are potential candidates for a novel class of open-shell singlet NLO systems, where γzzzz values are modulated by tuning their diradical character through the use of suitably modified fullerene fragments. [ABSTRACT FROM AUTHOR]

Nonlinear optical (NLO) materials are the smartest materials of the era, and have the ability to generate new electromagnetic fields with changed frequencies, phases, and other physical properties. Recently, many cutting edge research reports have been focused on NLO materials especially on those which are composed of sp2 hybridized carbon nanostructures. As the carbon nanostructures are composed of abundant π-electrons and have significant delocalization, these are potential candidates for modern NLO materials. Generally, sp2 hybridized carbon nanostructures can be divided into zero-dimensional fullerenes, one-dimensional nanotubes and two-dimensional graphene nanoribbons and quantum dots etc. These dimensionally different carbon nanomaterials are promising candidates for a wide range of applications in next-generation nanotechnologies. In present feature article, we first briefly explain a theoretical structure–NLO property relationship based on perturbation theory and then elucidate the crucial factors to control the NLO responses. We put together the different random investigations of sp2 hybridized carbon nanostructures for NLO application by highlighting the importance of their several structural designs to tune NLO amplitudes. Furthermore, we make a comparative and updated analysis of the NLO properties of dimensionally different sp2 hybridized carbon nanomaterials i.e. fullerenes, carbon nanotubes, and graphene nanoribbons and quantum dots. Finally, we make a brief discussion about different aspects and opportunities to use the sp2 hybridized carbon nanomaterials as high performance NLO materials of the future. This review is a focused perspective based on different updated quantum chemical investigations about fullerenes, nanotubes and graphene nanoribbons and quantum dots for their possible use in nonlinear optical applications. [ABSTRACT FROM AUTHOR]