Pressure-induced structural and electronic variations of Se helical chains confined in nano-channels of zeolite single crystals are studied. Raman and optical absorption spectra provide us with evidence of softened vibration modes and band gap reduction under high pressure. We reveal that the Se chains elongate under hydrostatic compression by ab initio calculations. The changes in morphology lead to a softening of phonons and narrowing of energy gaps consistent with observations from optical measurements. The present investigation demonstrates negative compressibility of Se chains in one-dimensional channels of a zeolite framework. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.