We report a joint theoretical−experimental study on the optical properties of 5-N-succinimidyl-2,2‘-bithiophene (NS-2T), a prototype system for a new class of biomarkers. Time-dependent density functional theory (TD-DFT) and approximate coupled-cluster single and doubles (CC2) calculations are performed in the ground and excited states. Theoretical results are compared with absorption, photoluminescence (PL), time-resolved PL, and PL quantum efficiency measurements. The excited state of NS-2T has a larger dipole moment as compared to that of the ground state, explaining the experimental shift of the PL peak in solvents of different polarity, and a smaller intersystem crossing (ISC) rate as compared to that of isolated bithiophene (2T), explaining the increased PL quantum efficiency. We also studied two model systems to describe the effects of the covalent binding of NS-2T to biomolecules and proteins with …