The spectroscopic behavior of anthracene encapsulated in porous zeolitic zinc imidazolate framework ZIF-8 was studied. Nanoparticles of ZIF-8 with high crystallinity and large specific surface areas were prepared in one-pot synthesis. Monitoring single anthracene encapsulation in a ZIF-8 cell was done by careful control of the synthesis of the hybrid. The molecular electronic spectra of anthracene adsorbed on the external surface of ZIF-8 were compared with those of encapsulated anthracene. It was demonstrated that the encapsulation of anthracene induced unexpected confinement effects including the restriction of intramolecular vibrations in its absorption spectrum. It was shown that vibronic components in the high-energy part of the S0 → S1 absorption band totally collapse upon encapsulation. Our study highlights the mechanism of confinement-induced electronic excitation limitation related to the restriction of intramolecular vibrations, which evidences the role of host–guest interaction in infrared absorption and fluorescence emission properties.