Currently, there is a great need for green and clean energy sources to meet the needs of the ever-increasing population in the world. Consequently, scientists across the globe are paying much attention to the development of low cost and high performance renewable energy devices. Dye-sensitized solar cells (DSSCs) are the front-runners among new solar cell technologies owing to their low production cost and high efficiency. Dyes (sensitizers) are one of the essential components of DSSC devices in which the widely used sensitizers are Ru(II) polypyridyl complexes. Despite the fact that Ru(II) polypyridyl complexes show high efficiency, they have limitations due to technical constrains. Hence, research has been accelerated in the design and synthesis of various dyes based on non-ruthenium metal complexes, porphyrins, phthalocyanines and metal-free organic compounds. In recent years, tetrathiafulvalenes (TTFs), dithiafulvalenes (DTFs) and their derivatives have been found to be best alternatives to Ru(II) polypyridyl complexes based on their easy synthesis, and electronic and thermal properties. In this review, we summarize the recent progress in metal-free organic dyes using TTF and DTF scaffolds for dye-sensitized solar cells. The physical properties of devices can be tuned via the strategic design of sensitizers, which in turn help in increasing the performance of the devices. Herein, special attention is paid to correlate the structure activity relationship of the components of D–π–A systems to gain insight into the efficient design strategies.

A phenothiazine (PTZ)–borondipyrromethene (BODIPY) based electron donor–acceptor dyad, 1, was designed and used for the sensitive and selective detection of hypochlorite. Upon excitation of the BODIPY moiety, the fluorescence of 1 was observed to be quenched (∼90%) due to photo-induced electron transfer from PTZ to ¹BODIPY*. However, upon the addition of incremental amounts of NaOCl, the fluorescence of 1 was enhanced, indicating inhibition of the reductive PET process from PTZ to BODIPY, which was quickly restored upon treatment with H₂S, indicating the reversibility of the probe for the continuous detection of hypochlorite. Parallel time-resolved fluorescence studies showed a decrease in the lifetime of the BODIPY moiety compared to the pristine BODIPY, and upon the addition of NaOCl, the lifetime of the BODIPY moiety (0.7 ns) in 1 was observed to be recovered highlighting the real-time applications of the probe in sensing NaOCl. Steady-state fluorescence experiments at varied pH have suggested that 1 can detect NaOCl in the pH range of 7–8, similar to physiological conditions, and at a pH of 7.6, 1 displayed high selectivity for hypochlorite over other reactive oxygen species. To understand the chemical and photophysical mechanisms involved in the fluorescence “turn-on” event, 1 was treated with NaOCl and subjected to ESI-MS analysis. Mass spectrometry showed the formation of oxidized products, sulfoxide and sulfone, and the density functional theory studies confirmed that the formation of these products was responsible for the inhibition of the reductive PET process from the PTZ group to the ¹BODIPY* moiety. Also, 1 exhibited enhanced fluorescence upon treatment with commercially available sodium hypochlorite disinfectants, indicating the applications of the probe to real samples. Cytotoxicity studies performed on HEK 293 cells suggested that, up to a 5 μM concentration of 1, the cell viability is above 80%, implicating the applications of the probe for biological samples.

A series of carbazole (CBZ)–boron dipyrromethene (BODIPY) based donor–acceptor dyads, CB1, CB2, and CB3, with CBZ as an energy donor, tethered together with spacers of varied sizes i.e., phenyl bridge, biphenyl bridge and diphenylethyne bridge, respectively, are reported. The newly synthesized dyads were characterized by various spectroscopic techniques. A comparison of the absorption and electrochemical data of the dyads with their reference compounds (i.e., 9-phenyl-9H-carbazole (C0) and N,N′-difluoroboryl-1,3,7,9-tetramethyl-5-phenyldipyrrin (B0)) revealed minimal ground-state interactions between the chromophores. Selective excitation of CBZ in the dyads at 290 nm resulted in the quenching of the CBZ emission followed by the appearance of BODIPY emission, revealing efficient energy transfer from singlet excited CBZ (¹CBZ*) to BODIPY. The photoinduced energy transfer phenomenon was studied in three different solvents of varying polarity. The driving forces for energy transfer (ΔG_EN) for all the dyads were found to be exothermic. The rate constants for energy transfer, k_ENT, measured by the femtosecond transient absorption technique in toluene were found to be in the range of 0.8–2.0 × 10¹⁰ s⁻¹, depending on the type of spacer between the CBZ and BODIPY entities, and were in close agreement with the theoretically estimated rates according to the Förster energy transfer model. In contrast, selective excitation of BODIPY in these dyads at 485 nm resulted in small quenching of the BODIPY emission, suggesting a lack of major photochemical events originating from ¹BODIPY*.