Research

[A] Energy Donor-Acceptor Conjugates of Corrole & Porphyrins

[B] Highly Emissive Boron-Dipyrromethens Dyes

[C] NIR Fluorescent Dyes based on Aza-BODIPYs

[D] Functionalized Carbaporphyrins 

[E] Metal Dipyrrinato Complexes

[A] Energy Donor-Acceptor Conjugates of Corrole & Porphyrins

Corroles are porphyrin analogue, where four pyrrole rings are linked to each other via three methine bridges and one direct a-a linkage. Corroles are aromatic macrocycle with smaller cavity size and the regular tautomer has three inner NH protons. Such qualities makes corroles better candidate to coordinate with varieties of metal ions. The functionalized corroles can be further utilized to prepare covalently linked D-p-A (donor-accepter) systems for various applications. Corroles are promising candidates for panchromatic dyes due to their intense absorption in visible range, high fluorescent quantum yields and high radiative rate constants, where they can be linked with other chromophores to construct donor-acceptor type conjugated (D-p-A) systems. Our group have synthesized and studied six dyads having one carbazole unit covalently attached to either corrole or porphyrin unit. Steady state emission studies indicated the efficient energy transfer from donor-carbazole unit to acceptor corrole/porphyrin unit in these dyads. Electrochemical studies of all the dyads revealed perturbation of electronic properties of corrole/porphyrin unit in these compounds. 

Part of the work published in New Journal of Chemistry (2015) and  in Dyes and Pigments  (2017). 

[B] Highly Emissive Boron-Dipyrromethens Dyes: The BODIPYs (Boron dipyrromethenes) are a class of boron chelates with dipyrrin units. This fluorescent dye has remarkable photophysical properties like:  intense absorption in the 480-600 nm region, high quantum yield, negligible triplet-state formation, good thermal and photochemical stability. The major limitations of such dyes are the emission range (about 600 nm) and small Stokes shifts (5-15 nm). A simple way to induce bathochromic shift in their absorption and emission bands (around 700 nm) is the introduction of electron donor moieties at the various position of BODIPY core. It’s a known fact, that carbazole is a good electron donor and that’s why its derivatives have been used for various photovoltaic (OLEDs) and photosensitizers applications. Our group has prepared four BODIPY dyes substituted with N-butylcarbazole moiety at their meso-position. The presence of N-Butylcarbazole group has affected the electronic properties of the BODIPYs. 

Electrochemical studies clearly indicate that it’s easier to reduce these BODIPYs as compared to the meso-tolyl BODIPY.  We have also reported the first crystal structures of meso-carbazole substituted boron-dipyrrin dye and its dibrormo derivative, where reduced dihedral angle between the boron dipyrrin plane and meso-carbazole plane reflects increased interaction between the two moieties. Optical spectroscopy measurements showed higher extinction coefficients and large Stokes shifts (111 nm to 168 nm) for all compounds that are unprecedented in BODIPY family.  Fluorescence studies indicated efficient energy transfer (85-90%) from donor (meso-carbazole unit) to (boron-dipyrrin unit). 

We have also prepared a series of bridged bis-BODIPYs linked via aromatic spacers. The single crystal X-ray analysis revealed interesting C(H)---p interactions in the solid state of these bis-BODIPYs (shown above). The DFT studies of these molecules were carried out in collaboration with Prof. P.C. Jha (Central Univ. of Gandhinagar, Gujarat). HOMO-LUMO energy levels and optical properties were calculated by DFT studies for these dyes.  We have also developed mercury sensor based on triazole based BODIPY, which showed 27 times fluorescence enhancement upon Hg2+ binding in solution with 24 mM sensitivity.  Bio-imaging and cell viability tests were carried out in collaboration with Porf. R. Vasita (Central Univ. of Gandhinagar, Gujarat). Having less toxicity to live cells, these fluorescent probes were successfully used to map the Hg2+ ions in live A549 cells.

Part of the work published in:

 Sensors and Actuators B: Chemical, 244, 673–683, 2017 

Luminescence, 1-5, 2017

Journal of Fluorescence, 1-14,  2017

[C] NIR Fluorescent Dyes based on Aza-BODIPYs: Aza-BODIPYs are sub-class of  BODIPYs  and very promising candidates as NIR dyes due to their strong absorption and emission properties around 600-800 nm range and versatile structure. Near-infrared (NIR) light can penetrate deep into biological tissue non-invasively as compared to UV-vis light, thus NIR fluorescent dyes have important roles in biological imaging, sensing and  photo-dynamic therapy (PDT). The aza-BODIPYs have found applications in optoelectronics, bio-imaging, sensing and PDT. Aza-BODIPYs derivatives have also been tested as florescent pH indicators in physiological and alkaline pH range. Methoxy- and hydroxyl-phenyl substituted aza-BODIPYs have been investigated for decent two photon absorption (TPA) properties for future applications in optical limiting, optical data storage, TPA imaging microscopy etc. Aza-BODIPYs have also been studied as photosensitizer for application in vacuum deposited organic solar cells. The combination of redox active group and fluorophore unit in a single molecule has great interest for applications in molecular electronic devices. Our group have prepared Aza-BODIPYs having two ferrocene units at their 1,7-positions.  The absorption maxima are in the range of 614-638 nm and emission maxima are around 672-685 nm. Ferrocenyl substituted Aza-BODIPYs also exhibited interesting Fe3+ sensing properties in solution. 

 A series of NIR aza-BODIPYs having different energy donor groups (naphthalene, N-phenylcarbazole and N-butylcarbazole) were synthesized and characterized. Energy transfer studies using steady state fluorescence revealed 93% energy transfer from donor groups to aza-BODPY core in these dyes. Electrochemical studies were carried out and HOMO-LUMO energy levels were derived from CV data and compared with the data obtained from DFT studies of these molecules.

[D] Functionalized Carbaporphyrins: N-Confused porphyrins belongs to the class of carbaporphyrins which have one –CH moiety instead of -NH group inside the macrocyclic ring. This alteration renders very interesting spectral change in the N-Confused porphyrins with red shifted absorption and emission maxima. Recently our group has reported the synthesis of a series of difunctionalized N-confused porphyrins and their outer N-methylated derivatives via simple [3+1] condensation method. Also, some novel key precursors like: tripyrranes and 2,4-pyrrole diols were prepared to utilized the full potential of [3+1] porphyrin condensation method which was not explored earlier. These difunctionalized N-confused porphyrins contains pentafluorophenyl group at their C5 and C20 positions  and the substituent at their C10 and C15 positions are varied from p-bromophenyl, p-nitrophenyl, p-iodophenyl, m-bromophenyl and  p-tolyl groups. All the N-confused porphyrins were studied by fluorescence and cyclic voltammetry techniques. Emission studies indicated reasonable quantum yields with blue shifted emission maxima and smaller Stokes shifts. Electrochemical analysis indicated easier oxidation of N-methylated N-confused porphyrin compared to other difunctionalized porphyrins. Such difunctionalized N-confused porphyrin could be very useful for the construction of D-A type of systems in future.

 [E] Metal Dipyrrinato Complexes: Dipyrrin molecule has two pyrrole ring linked to each other at their a –position by meso-aryl group. Due to the presence of pyrrolic NH hydrogens, it can act as excellent ligand for variety of metals. In our group we have synthesized a series of triphenylamine substituted dipyrrinato metal complexes both mono- and binuclear ones, having Zn(II), In(III), Pd(II), Ni(II) and Co(II) metal ions. Detailed synthesis, photophysical and electrochemical studies have been described for all eight complexes. Dipyrrin with triphenylamine group at their meso-positions were utilized for the first time to make such metal complexes. Replacing the simple phenyl/ mesityl groups on the dipyrrin ligand with bulky electron donating triphenylamine group has affected the properties of these metal complexes. Most of the metal complexes exhibited blue shifts in their absorption maxima due to the presence of electron rich triphenylamine moieties on the dipyrrin ligands. Absorption maxima were blue shifted in all the compounds as compared to their corresponding meso-phenyl dipyrrinato metal complexes. Emission maxima were red shifted with reasonable Stokes shifts (4000-7000 cm-1) and singlet state lifetimes were in the range of 2-4 nano-seconds. Electrochemical studies revealed dipyrrin based reversible/ quasi reversible oxidation potentials for all the compounds.