Novel aza-BODIPY derivatives substituted with heavy atoms such as bromine and iodine were synthesized, and their triplet and singlet oxygen generation efficiencies have been investigated. These derivatives showed absorption in the NIR region with high molar extinction coefficients. The dye substituted with four iodine atoms showed yields of Φ(T) = 0.78 and Φ((1)O(2)) = 0.70, which are the highest values so far obtained for the aza-BODIPY derivatives.

With the objective of developing small molecule based probes for proteins, interactions of polyhydroxyl-substituted squaraine dye (SQ) with bovine serum albumin (BSA) have been investigated by absorption, steady-state and time-resolved fluorescence, circular dichroism (CD), cyclic voltammetry (CV), 1H NMR, scanning electron, and tapping mode atomic force microscopic techniques. Increase in addition of BSA resulted in increase in absorbance and fluorescence quantum yields (80-fold) of SQ, along with significant bathochromic shifts in the absorption and fluorescence maxima. Half-reciprocal analysis of the absorption data gave a 1:1 stoichiometry for the complex between BSA and SQ with high association (Kass) constant of (1.4 +/- 0.1) x 106 M-1 and change in free energy of -35 kJ/mol. The complex formation was further confirmed by observation of induced CD signal corresponding to the SQ chromophore at 610 nm, upfield shift (about Deltadelta 0.1 ppm) of aromatic protons of SQ in 1H NMR spectra, and decrease in current intensity (CV) of SQ when bound to BSA. The picosecond time-resolved fluorescence studies indicated that the BSA-SQ complex exhibits biexponential decay with significantly enhanced lifetimes of 0.5 and 1.5 ns when compared to the lifetime of SQ (tau = 121 ps) in the absence of BSA. Employing displacement cum fluorimetry using site-specific binding ligands, such as dansylproline and dansylamide, indicated that SQ binds with protein selectively at site II involving hydrophobic, hydrogen bonding, and electrostatic interactions. The uniqueness of this molecular system is that it interacts with BSA selectively at site II and signals the binding event through dual mode recognition of "visual color" change and "turn on" fluorescence mechanism.

The design and development of novel squaraine dyes as sensitisers for photodynamic therapy (PDT) applications has grown tremendously in the last decade from the time when a squaraine dye was proposed to be a potential candidate, to-date when the use of such dyes have been demonstrated in animal models for skin cancer. This perspective article highlights the basic design, tuning of absorption, triplet excited state and two-photon absorption properties and recent developments of the squaraines as PDT sensitisers.

A new series of aza-BODIPY derivatives (4 a-4 c, 5 a,c, and 6 b,c) were synthesized and their excited-state properties, such as their triplet excited state and the yield of singlet-oxygen generation, were tuned by substituting with heavy atoms, such as bromine and iodine. The effect of substitution has been studied in detail by varying the position of halogenation. The core-substituted dyes showed high yields of the triplet excited state and high efficiencies of singlet-oxygen generation when compared to the peripheral-substituted systems. The dye 6 c, which was substituted with six iodine atoms on the core and peripheral phenyl ring, showed the highest quantum yields of the triplet excited state (Φ(T) =0.86) and of the efficiency of singlet-oxygen generation (Φ(Δ) =0.80). Interestingly, these dyes were highly efficient as photooxygenation catalysts under artificial light, as well as under normal sunlight conditions. The uniqueness of these aza-BODIPY systems is that they are stable under irradiation conditions, possess strong red-light absorption (620-680 nm), exhibit high yields of singlet-oxygen generation, and act as efficient and sustainable catalysts for photooxygenation reactions.

A water-soluble derivative of N-confused porphyrin (NCP) was synthesized, and the photodynamic therapeutic (PDT) application was investigated by photophysical and in vitro studies. High singlet oxygen quantum yield in water at longer wavelength and promising IC(50) values in a panel of cancer cell lines ensure the potential candidacy of the sensitizer as a PDT drug. Reactive oxygen species (ROS) generation on PDT in MDA-MB 231 cells and the apoptotic pathway of cell death was illustrated using different techniques.