Four ternary lanthanide(III) complexes, namely [Eu(Anpp)(TTA)₃] (1), [Tb(Anpp)(TTA)₃] (2), [Eu(Pypp)(TTA)₃] (3) and [Tb(Pypp)(TTA)₃] (4), [TTA = 1,1,1-trifluoro-3-(2-theonyl)acetone, Anpp = 3-(anthracen-9-yl)-1-(pyridine-2-yl)prop-2-en-1-one, Pypp = 3-(pyren-1-yl)-1-(pyridine-2-yl)prop-2-en-1-one] were designed, synthesized and characterized by various spectroscopic techniques like FT-IR, ¹H-NMR, ESI-MS, UV-Vis and spectral emission studies. [Eu(Pypp)(TTA)₃] (3) was structurally characterized, showing an eight coordinated {EuN₁O₇} square antiprismatic geometry with one N,O-donor chalcone and three O,O-donor β-diketonate ligands. Detailed photophysical properties like excited state lifetime (τ) and hydration number (q) were determined. The presence of three TTA ligands inhibits non-radiative deactivation with lifetimes in H₂O in the range of 0.306–0.445 ms. The calculated hydration number (q) values (0.37–0.63) from the excited state lifetime measurements show the absence of any bound H₂O molecules in the coordinatively saturated complexes (q < 1). Complexes 1–4 were studied for their biological interactions with DNA and protein and their DNA photocleavage activity was investigated. The binding constants with CT-DNA (K_b = 1.15–8.8 × 10⁵ M⁻¹) and HSA (K_HSA = 6.5–8.1 × 10⁵ M⁻¹) of complexes 1–4 show their significant affinity towards these biological targets. Incorporation of chalcone ligands containing anthracene or pyrene as a photosensitizing moiety in these complexes were effective in generating reactive oxygen species (ROS). Complexes 1–4 display moderate photocleavage of supercoiled (SC)-DNA to its nicked circular (NC) form on exposure to UV-A light of 365 nm through the generation of singlet oxygen (¹O₂) and hydroxyl radicals (˙OH) under physiological conditions.