Prabhat Gautam

Ferrocenyl BODIPYs: synthesis, structure and properties

A set of donor–π-acceptor–π-donor type ferrocenyl substituted BODIPYs were designed and synthesized by the Sonogashira coupling reaction. These compounds exhibit red shifted absorption and poor fluorescence. The electrochemical properties of these compounds exhibit strong donor–acceptor interactions. The crystal structure of 3b exhibits an extensive hydrogen bonded 2-D network.

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.

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.

Reversible mechanochromism in dipyridylamine-substituted unsymmetrical benzothiadiazoles

We report the design and synthesis of push–pull benzothiadiazoles (BTDs) of type D1–π–A–π–D2 and D1–π–A–D2. These BTDs show strong charge transfer interaction. BTD 3 shows reversible mechanochromism with color contrast between yellow (crystalline state) and orange (amorphous state). Photophysical and computational studies reveal that the planar orientation of the pyridyl and BTD unit in 2 results in no change in solid state emission whereas non-planar orientation of the dipyridylamine and BTD unit in 3 results in efficient mechanochromism.

β-Substituted triarylborane appended porphyrins: photophysical properties and anion sensing

β-Substituted triarylborane porphyrins were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The incorporation of triarylborane unit results in a red shifted absorbance and fluorescence. The sensing ability of these porphyrins was studied for different anions. The triarylborane porphyrin 4 and 5 selectively detects fluoride ions as shown by the UV/vis absorption and fluorescence titration experiments. The binding constants for triarylborane porphyrin 4 and 5 in dichloromethane at 25 °C were found to be 1.0 × 106 M−1 and 5.0 × 105 M−1 respectively.

Aryl-substituted symmetrical and unsymmetrical benzothiadiazoles

A set of benzothiadiazoles (BTD) of the type D–π–A–π–D and D1–π–A–π–D2 were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. Their photophysical and electrochemical properties were studied. The substitution of anthracene on BTD improves its thermal stability, and lowers the HOMO–LUMO gap, which results in a red shift of the electronic absorption. The experimental optical gap values show good agreement with the calculated HOMO–LUMO gap.

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.

Star shaped ferrocenyl substituted triphenylamines

This manuscript reports design and synthesis of star shaped ferrocenyl substituted triphenylamine conjugates (Fc-TPA) 3a–3c by the Pd-catalyzed Sonogshira cross-coupling reaction. Their photophysical and electrochemical properties were investigated, which are a function of the conjugation length. The time dependent density functional (TD-DFT) studies were performed to understand and support the experimental findings. The LUMO could be significantly stabilized by increasing the conjugation. The thermal stability of Fc-TPA 3a–3c can be improved by increasing the conjugation length. The single crystal X-ray structure of Fc-TPA 3a is reported, which show interesting supramolecular interactions leading to the formation of 2D-network.

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*.

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.