CPL of Mellein and Related Natural Compounds : Analysis of the ESIPT Phenomenon

(R)-( )-Mellein, (3R,4R)-4-hydroxymellein and (3R,4S)-4-hydroxymellein obtained from fungi, i. e. from Diplodia globulosa, were investigated as a class of natural products presenting ESIPT (excited state intramolecular proton transfer) phenomenon, through fluorescence and CPL (circularly polarized luminescence). The study was preceded by the assessment of the absolute configuration through ECD and VCD (electronic and vibrational circular dichroism) spectroscopies in addition to NMR spectra. It is found that ESIPT takes place in these systems very rapidly, and no dual fluorescence has been observed. The experimental study is backed up by TD-DFT calculations of ECD and CPL spectra, plus MD calculations to follow proton transfer in the excited state and careful analysis of the puckering dynamics of the lactone ring. Deprotonated forms of the three compounds were also investigated by the same chiroptical experimental and theoretical methods, showing how one can find in natural compounds not only biological activity but also biologically compatible sensing probes.