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We report the design and synthesis of push-pull benzothiadiazoles (BTDs) of type D 1 -p-A-p-D 2 and D 1 -p-A-D 2 . 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.Scheme 1 Synthesis of BTDs 2 and 3. Reaction conditions: (i) PdCl 2 (PPh 3 ) 2 , CuI, THF-TEA (1 : 1), 60 C, 24 h; (ii) CuSO 4 , K 2 CO 3 , 1,2-dichlorobenzene, 180 C, 48 h.

Two new ferrocenyl substituted triphenylamine based donor–acceptor dyes D1 and D2 were synthesized and used as sensitizers for dye sensitized solar cells (DSSCs).

We have designed and synthesized ferrocenyl (donor) bisthiazole linked triphenylamine (donor) based donor-π-acceptor-π-donor-acceptor (D-π-A)-π-D-A type dyes D1 and D2 by using Pd-catalyzed Sonogashira cross-coupling and Knoevenagel condensation reactions. Their photophysical, electrochemical and computational studies reveal strong donor-acceptor interaction. Dye sensitized solar cells (DSSCs) based on D1 and D2 exhibit power conversion efficiencies (PCE) of 6.33% and 5.03%, respectively. The higher PCE value of the D1 based DSSC is attributed to its enhanced short-circuit current (J) and open-circuit current (V) and fill factor (FF) values because of the strong binding of the anchoring cyanoacrylic acid with the TiO surface as compared to the dicyanovinyl unit in D2. Time dependent density functional theory (TD-DFT) calculations at B3LYP level on dyes D1 and D2 were performed, which reveal that both dyes show HOMO-1 → LUMO as a major transition. Computational photovoltaic calculations also reveal that dye D1 has better electron injection (ΔG) from E to the conduction band (CB) of TiO as compared to dye D2, which is in good agreement with experimental results.

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.