A new macrocyclic ligand, $L^3$, has been synthesised, based on the cyclen framework grafted with three phenacyl light-harvesting groups and a C5-alkyl chain bearing a carboxylic acid function as a potential linker for biological material. Acidity constants are determined by spectrophotometric titrations, as well as conditional stability constants for the resulting 1:1 complexes with trivalent lanthanide ions. The complexes have stabilities comparable to 1,4,7,10-tetrakis(carbamoylmethyl)- 1,4,7,10- tetraazacyclododecane (dtma) complexes, with $pLn \approx 12-13$. Photophysical properties of the ligand and of the $EuL^3$ and $TbL^3$ complexes have been determined for both microcrystalline samples and solutions in water and acetonitrile. They point to the metal ion being present in an environment with axial symmetry derived from the $C_4$ point group. The hydration number determined for $TbL^3$ decreases with increasing pH value and becomes fractional at pH 7.5, which points to an equilibrium between two differently solvated species and probably to the participation of the deprotonated carboxylic acid chain in the complexation. The quantum yields in water (1.9% for $Eu^{III}$, 3.4% for $Tb^{III}$) are smaller than those for complexes with the symmetrically substituted parent macrocycle, but efficient luminescence resonant energy transfer (LRET) was observed when Cy5 dye was added to the solutions. Finally, the influence of the $TbL^3$ complex on cell viability is tested on both malignant (5D10 mouse hybridoma, Jurkat human T leukaemia, MCF-7 human breast carcinoma) and non-malignant (Hacat human keratinocyte) cell lines. Cell viability after 24 h incubation at $37^ {\circ}C$ with $500\mu m$ $TbL^3$ was >90% for all cell lines, except Jurkat (>70%). All of these properties make $LnL^3$ complexes interesting potential probes for bioanalyses.