Figure 2.

Extended Jabłonski diagram for molecular rotors. Like conventional fluorophores, a molecular rotor is elevated from the ground state (S₀) to the energetically higher first excited state (S₁) by photon absorption. Different vibrational states (indicated by parallel lines) cause some energy loss, and emission from the LE state occurs at a longer wavelength than the excitation (Stokes shift). For molecular rotors, the Jabłonski diagram needs to be extended, because the excited-state energy is lower in the TICT state, whereas the ground-state energy is higher in the TICT state than in the LE state. Therefore, the S₁ - S₀ energy gap is lower in the TICT state with a correspondingly lower relaxation energy. In the case of moelcular rotors with dual-band emission (such as DMABN), the TICT energy gap is slightly smaller than the LE energy gap (A). If the TICT energy gap is much smaller than the LE energy gap, for example in DCVJ, emission from the TICT state occurs without photon emission (B). This diagram does not reflect intersystem crossing, because triplet states do not play any significant role in the fluorescence of molecular rotors.