X-ray photoelectron spectroscopy and tunable photoluminescence study of gold nanoparticles embedded in PVA films.

This article reports the systematic photoluminescence study of the various contents of gold nanocomposites in polyvinyl alcohol (PVA) films. The variations in the gold content in PVA film were 0.2, 0.5, 1.0, and 1.5 wt%. All the samples were excited at two selected wavelengths; those are at 400 nm and 532 nm. On exciting the gold-PVA nanocomposite films at 400 nm the photoluminescence was observed in the region of 430-500 nm in comparison to pure PVA films that show an emission at 400 nm. However, on exciting the gold-PVA nanocomposites at 532 nm, the emission was observed at 560-650 nm with a long tail till 700 nm that is unlike the pure PVA films that do not show any emission peak in this region. This suggests that emission between 430 and 500 nm regions is due to the coordination of PVA with gold nanoparticles because PVA has an emission at 400 nm. However, the emission peak between 560 and 650 nm is entirely due to the gold nanocomposite particle. The peak also shows a smaller red-shift that is usually with the increasing nanoparticles size with the increasing content in the PVA films. The formation of gold nanoparticles was justified by X-ray diffraction (XRD) analysis which is further supported by X-ray photoelectron spectroscopy (XPS) analysis.

Intramolecular charge transfer and solvation dynamics of push-pull dyes with different π-conjugated linkers.

The solvation-dependent excited state dynamics of two push-pull fluorophores with donor-π-acceptor (D-π-A) structures were investigated using steady-state and ultrafast transient absorption (TA) spectroscopy, backed by theoretical calculations. Identical D and A groups were present in both dyes, which differed only in the structure of their central π-conjugated linkers. Dye 1 features a p-phenylenediethynyl linker, while dye 2 contains a 2,5-diethynylthiophene linker. From the steady-state spectra, no appreciable shifts in absorption bands were observed, whereas large red-shifts in emission were seen with increasing solvent polarity, which indicated that the excited states were more polar than the ground state. Theoretical calculations support charge transfer from the triphenylamine (TPA) donor to the pentafluorosulfanyl (SF5) acceptor viaπ-conjugated linkers to form an intramolecular charge transfer (ICT) state. TA spectra revealed that a solvation-stabilized conformationally relaxed intramolecular charge transfer (ICT') state was formed in polar solvents, but only an ICT state was observed in nonpolar solvent. The SE band was quenched within 1 ps in high-polarity solvent, which corresponds to the low fluorescence quantum yield. It can be concluded that the dye with the p-phenylenediethynyl π-linker (i.e., dye 1) exhibits a larger degree of ICT than the thiophene analogue (i.e., dye 2). These findings demonstrate how solvation can fine-tune the photophysical properties of push-pull dyes, and this study highlights the importance of π-conjugated linkers in the excited state ICT process.

Ultrafast Charge-Separation in Triphenylamine-BODIPY-Derived Triads Carrying Centrally Positioned, Highly Electron-Deficient, Dicyanoquinodimethane or Tetracyanobutadiene Electron-Acceptors.

A series of new triphenylamine (TPA)-substituted BODIPYs 1-3 have been designed and synthesized through the Pd-catalysed Sonogashira cross-coupling and [2+2] cycloaddition-retroelectrocyclization reactions in good yields. This procedure yielded highly electron-deficient tetracyanobutadiene (TCBD) or dicyanoquinodimethane (DCNQ) electron-acceptor units centrally located at the TPA-BODIPY system. As a consequence, significant perturbation of the photonic and electronic properties was observed. The triads 2 and 3 showed red-shifted absorption, in addition to a strong charge-transfer-type absorption in the case of 3. The electrochemical studies revealed multi-redox processes involving the TPA, TCBD or DCNQ and BODIPY entities. The computational studies were performed at the B3LYP/6-31G** level to elucidate the geometry and electronic structures. An energy level diagram established for triads 2 and 3 revealed that the photoinduced charge-separation from the 1 BODIPY* is thermodynamically possible. In addition, charge transfer from TPA to TCBD in 2 and DCNQ in 3 was also possible. These charge transfer mechanisms were confirmed by photochemical studies performed using time-resolved emission and femtosecond-transient-absorption studies in solvents of varying polarity. Ultrafast charge-separation has been witnessed in these closely spaced, strongly interacting triads. The charge-separated state returned to the ground state without populating the 3 BODIPY*.

Unsymmetrical and Symmetrical Push-Pull Phenothiazines.

A series of unsymmetrical and symmetrical push-pull phenothiazines (3-7) were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction and subsequent [2 + 2] cycloaddition-retroelectrocyclization reaction with tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The effect of systematic variation of the number and nature of cyano-based acceptor TCNE and TCNQ units on the photophysical, electrochemical, and computational studies was investigated. The single-photon absorption on phenothiazines 3-7 reveals that substitution of 1,1,4,4-tetracyanobutadiene (TCBD) and a cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD unit results in strong intramolecular charge transfer and lowering of the LUMO energy level. The TCBD-linked and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD-linked phenothiazines 3-7 exhibit multiredox waves. The computational studies on phenothiazines 3-7 exhibit substantial stabilization of the LUMO with the increase in acceptor strength, which results in lowering of the HOMO-LUMO gap.

Pulling with the Pentafluorosulfanyl Acceptor in Push-Pull Dyes.

A new class of push-pull fluorophores featuring the pentafluorosulfanyl (SF5) group as a potent acceptor has been synthesized. Known for its excellent chemical and thermal stability, the unique SF5 functionality is also strongly electron-withdrawing but at the same time highly lipophilic. We report six new fluorescent dyes, which were characterized by UV-vis/fluorescence spectroscopy, single-crystal X-ray diffraction, and cyclic voltammetry. Notable dye properties include large Stokes shifts (>100 nm), pronounced solvatofluorochromic effects arising from intramolecular charge transfer, moderate fluorescence quantum yields in both solutions and thin films, and extensive supramolecular C-H···F interactions in their crystalline states. Reversible mechanofluorochromism was also observed in dye 5, where grinding and fuming of a solid sample gave blue- and red-shifted emissions, respectively. Postfunctionalization of dye 3 to afford a pair of strong visible-light absorbers was also demonstrated.

Dicyanoquinodimethane-substituted benzothiadiazole for efficient small-molecule solar cells.

Two unsymmetrical donor-acceptor-acceptor-π-acceptor type benzothiadiazoles (BTD3 and BTD4) functionalized with tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) modules, showing strong absorption in the visible region are reported. The bulk heterojunction solar cells based on BTD4:PC71BM and BTD3:PC71BM based active layers processed with chloroform (CF), thermal annealing and subsequent solvent vapor annealing, i.e. two step annealing (TSA), exhibited PCEs of up to 6.02% and 5.36%, respectively, which is significantly higher than those of the corresponding devices based on the as-cast blend active layer. This enhancement is related to the improvement in exciton dissociation efficiency and more balanced charge transport in the devices based on the active layer processed with TSA treatment.

A D-π-A1-π-A2 push-pull small molecule donor for solution processed bulk heterojunction organic solar cells.

Herein, benzothiadiazole (BTD), as an acceptor A1, has been used as a backbone to link triphenylamine (TPA) as donor and naphthalimide (NPI) as acceptor (A2) moieties through ethylene linkers to design a small molecule. The donor-π-acceptor-π-acceptor (D-π-A1-π-A2) type small molecule denoted as was synthesized. In order to use it as an electron donor for solution processed bulk heterojunction small molecule solar cells its photonic and electronic properties were explored. The small molecule organic solar cells based on the optimized blend of with PC71BM processed in chloroform showed a power conversion efficiency (PCE) of 2.21%, which was significantly improved up to 6.67%, when a two-step annealing (TSA) treated blend was used as an active layer. The increase in the PCE was due to the enhancement in both Jsc and FF. The improvement in Jsc was related to the enhancement in the light harvesting efficiency of a TSA treated active layer relative to the as-cast layer, which is reflected in a better IPCE and better charge collection. The TSA treatment also leads to better nanoscale morphology for exciton dissociation into free charge carriers and improved crystallinity for balanced charge transport.

Tuning of the HOMO-LUMO gap of donor-substituted symmetrical and unsymmetrical benzothiadiazoles.

This article reports the design and synthesis of donor-substituted symmetrical and unsymmetrical benzothiadiazoles (BTDs) of 5-12 type D-π-A-D, D1-π-A-D2, D1-A1-A2-D2, D-A1-A2-D and D-A1-A2-A1-D by Ullmann, Suzuki and cycloaddition-retroelectrocyclization reactions. The photophysical, electrochemical and computational properties were studied and show substantial donor-acceptor interaction. Their single photon absorption show strong charge transfer bands in the near-infrared (NIR) region and the electrochemical reduction show multiple reduction waves. The optical HOMO-LUMO gap of BTDs 5-12 was found to be a function of the number and nature of the acceptors. Computational studies reveal that strong cyano-based acceptors, dicyanoquinodimethane (DCNQ) and tetracyanobutadiene (TCBD) lower the LUMO level in BTDs 7-12, which results in a low HOMO-LUMO gap compared to acetylene linked BTDs 5 and 6. The BTDs with carbazole and single TCBD and DCNQ acceptors show better thermal stability.

Donor-acceptor ferrocenyl-substituted benzothiadiazoles: synthesis, structure, and properties.

This article reports the design, and synthesis of D-π1-A-π2-D unsymmetrical, and D-π1-A-π2-A-π1-D symmetrical type of ferrocenyl-substituted benzothiadiazoles by the Pd-catalyzed Sonogashira, and Stille coupling reactions. The photophysical and electrochemical behavior of the ferrocenyl-substituted benzothiadiazoles show strong donor-acceptor interaction. The increase in the number of acceptor benzothiadiazole unit, results in the lowering of the energy gap, which leads to the bathochromic shift of the absorption spectrum. The single crystal X-ray structures of 3a, 5a, and 5g were obtained which show interesting supramolecular interactions.

Aryl-substituted unsymmetrical benzothiadiazoles: synthesis, structure, and properties.

A family of unsymmetrical donor-acceptor, ferrocenyl-substituted benzothiadiazoles of types D1-π-A-π-D2, D1-π-A1-π-A2, D1-A-π-D2, and D1-A1-A2-D2, bearing a variety of electron-donating and electron-withdrawing groups, were designed and synthesized. Their photophysical, electrochemical, and computational properties were explored, which show strong donor-acceptor interaction. The presence of electron-rich units anthracene (6f) and triphenylamine (6h), and an electron-deficient unit 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) (9b) results in lowering of the band gap, which leads to a red shift of the absorption spectrum in these benzothiadiazole systems. The single crystal structures of 6c, 6g, 7a, and 7b are reported, which show marvelous supramolecular interactions.

Colorimetric and fluorimetric detection of fluoride ion using thiazole derived receptor.

Thiazole based receptor 3, was designed and synthesized by condensation reactionof5-chlorosalicylaldehyde with 4-(4-phenylthiazol-2-yl)semicarbazide for colorimetric and fluorimetric detection of fluoride ion. Receptor 3 was characterized by 1H NMR, 13C NMR, and HRMS, and shows absorption in 280-400 nm region with emission at 442 nm in tetrahydrofuran (THF). Addition of fluoride ion to the THF solution of receptor 3 results in color change from colorless to yellow with significant change in UV-Visible absorption. The receptor-anion interaction occurs via hydrogen bonding followed by deprotonation which results in large bathochromic shift in absorption spectra and naked-eye color change. The colorimetric changes show selective response for fluoride ions over other anions. Fluorescence studies exhibit remarkable enhancement in emission intensity upon addition of fluoride ion with a limit of detection (LOD) of 8.6 nM. The 1H NMR titration studies exhibit deprotonation of the -OH proton of the salicylaldimine moiety resulting significant colorimetric and fluorimetric changes.

Water-Dispersible Bismuth-Organic Materials with Computed Tomography Contrast Properties.

Two bismuth-organic network polymers were synthesized by means of a one-step polycondensation reaction between an aromatic dithiol/trithiol and triphenylbismuth. The materials were characterized by solid-state UV-vis spectroscopy, Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, powder X-ray diffraction, elemental microanalysis, and thermogravimetric analysis. Uniform dispersion of the hydrophobic and water-insoluble bismuth-containing polymers in aqueous media was achieved by the addition of 2 kDa poly(ethylene glycol) methyl ether thiol. This enabled quantitative phantom imaging experiments on a clinical computed tomography (CT) scanner, which showed that the coordination polymers possessed strong CT contrast properties. The observed X-ray attenuation properties of each coordination polymer were correlated with its bismuth payload. The X-ray opacity, thermal and chemical stabilities, and aqueous dispersibility of this novel class of bismuth-organic materials make them potentially useful as biomedical CT contrast agents and radiopaque materials.

Donor-acceptor-acceptor (D-A-A) type 1,8-naphthalimides as non-fullerene small molecule acceptors for bulk heterojunction solar cells.

Donor-acceptor-acceptor (D-A-A) type 1,8-naphthalimide based small molecules SM1 and SM2 functionalized with tetracyanobutadiene (TCBD) and dicyanoquino-dimethane (DCNQ) modules, showing strong absorption in the visible and near-infrared (NIR) region are reported. TCBD and DCNQ linked SM1 and SM2 exhibit multi-redox waves. The electrochemical and optical HOMO-LUMO gaps show similar trends. These SMs exhibit a broad absorption profile which is complementary to the D-A copolymer P donor and also possess an appropriate lowest unoccupied molecular orbital (LUMO) to serve as an acceptor with P with a LUMO level of -3.33 eV. The organic solar cells based on P:SM1 and P:SM2 exhibit a PCE of 4.94% and 6.11%, respectively. The higher value of the PCE for the SM2 based organic solar cells has been attributed to the broader absorption profile, more balanced charge transport and lower photon energy loss. The values of Voc of the organic solar cells for the SM1 acceptor (1.06 V and 1.02 V without and with solvent additive) are the highest values reported for devices based on non-fullerene acceptors to the best of our knowledge. The energy loss (Eloss) of 0.56 eV and 0.48 eV for SM1 and SM2 based devices, respectively is one of the smallest reported for BHJ organic solar cells.

C2-Symmetric ferrocenyl bisthiazoles: synthesis, photophysical, electrochemical and DFT studies.

A series of donor-acceptor ferrocenyl substituted bisthiazoles 3-8 were designed and synthesized by the Pd-catalyzed Suzuki, Heck, and Sonogashira cross-coupling reactions. Their photophysical, electrochemical and computational studies reveal strong donor-acceptor interactions. The photonic and electrochemical studies show that the ferrocenyl bisthiazoles with vinyl linkage ferrocenyl-bisthiazole 4, show better electronic communication compared to rest of the ferrocenyl bisthiazoles. The time dependent density functional theory (TD-DFT) calculation at B3LYP on the ferrocenyl substituted bisthiazoles 3-5 was performed, in which the ferrocenyl-bisthiazole 4 shows strong donor-acceptor interactions compared to the Fc-bisthiazoles 3 and 5. The thermal stability of the ferrocenyl substituted bisthiazoles 3-8 is reported, in which Fc-bisthiazole 8 shows high thermal stability. The single crystal structures of ferrocenyl-bisthiazoles 3 and 5 are reported.

Unsymmetrical Donor-Acceptor-Acceptor-π-Donor Type Benzothiadiazole-Based Small Molecule for a Solution Processed Bulk Heterojunction Organic Solar Cell.

A D1-A-A'-π-D2 type (D = donor; A = acceptor) unsymmetrical small molecule denoted as BTD3 containing different end group donor moieties has been designed and synthesized for use as a donor in the solution processable bulk heterojunction (BHJ) solar cell. The BTD3 exhibits a low HOMO-LUMO gap of 1.68 eV and deeper HOMO energy level (-5.5 eV). Its LUMO energy level (-3.65 eV) is compatible with the LUMO level of PC71BM to facilitate the electron transfer from BTD3 to PC71BM in the BHJ solar cell. The solution processed BHJ solar cell with optimized BTD3:PC71BM active layer processed with THF solvent exhibited a PCE of 3.15% with Jsc = 7.45 mA/cm(2), Voc = 0.94 V, and FF = 0.45. Moreover, the device with optimized concentration of 3 vol. % 1-chloronaphthalene (CN) additive, i.e., CN/THF, showed significant enhancement in PCE up to 4.61% (Jsc = 9.48 mA/cm(2), Voc = 0.90 V, and FF = 0.54). The improvement in the PCE has been attributed to the appropriate nanoscale phase separation morphology, balance charge transport, and enhancement in the light harvesting ability of the active layer.

Carbazole-BODIPY conjugates: design, synthesis, structure and properties.

A set of carbazole substituted BODIPYs 2a-2c were designed and synthesized by the Pd-catalysed Sonogashira cross-coupling reaction. The effects of variation in the donor strength of various carbazoles were investigated by photophysical, electrochemical and computational studies. The electronic absorption spectra of BODIPYs 2a and 2c show charge transfer bands, which show red shift in polar solvents. The BODIPYs 2a-2c are highly fluorescent in nonpolar solvents (emission from the localized state) and poorly fluorescent in polar solvents (emission from the charge transfer state). The photophysical and electrochemical studies reveal strong donor-acceptor interaction between carbazole and BODIPY and follows the order 2a > 2c > 2b. The computational calculations show good agreement with the experimental results. The single crystal structures of BODIPYs 2a-2c are reported, which exhibit interesting supramolecular interactions. The packing diagrams of 2a show a zigzag 3D structural arrangement, whereas 2b and 2c show complex 3D structural motifs.

ß-Substituted ferrocenyl porphyrins: synthesis, structure, and properties.

ß-Substituted ferrocenyl porphyrins were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The UV-vis absorption, emission, and cyclic voltammetric results indicate strong electronic communication between ferrocene and porphyrin. The porphyrin 4a is non-emissive in nature, while 4b and 4c show reduced fluorescence quantum yield. The single crystal X-ray structure of 4b is reported, which shows extensive C-H-π interactions.

Donor-acceptor, ferrocenyl substituted BODIPYs with marvelous supramolecular interactions.

A series of donor-acceptor ferrocenyl substituted BODIPYs have been designed and synthesized via palladium catalyzed Suzuki and Sonogashira coupling reactions. The UV-visible absorption results indicate intramolecular charge transfer from the ferrocene to the BODIPY. The fluorescence quantum yield was drastically reduced, where the ferrocenyl group was directly attached to the BODIPY. The single crystal X-ray structures of 1', 2, 3, and 4 show marvelous supramolecular interactions. The crystal structure of 1' shows an extensive hydrogen bonded 2D network, 2 shows sheet like structure, 3 shows zigzag packing along the a-axis, whereas 4 shows sheet like structure in which both the surfaces of the sheet are covered with the ferrocenyl group.