Fig. 4. (a) Optimized structures of NP4C and NPM calculated at the B3LYP/6-311+G(d,p) level; (b) Gibbs free energy difference (ΔG_S–F) between NP4C and folded NP4C in the different mediums (ΔG_S–F = G(NP4C) – G(folded NP4C)); (c) theoretical calculations for PL mechanistic investigation.