Fig. 3. Results obtained for the HBO dye 7. (a) Density difference plot (isovalue = 0.0018 a.u.). The red/blue zones indicate an increase/decrease of the electronic density upon absorption of light. (b) Energy profile for proton transfer along the ground- and excited-states IRC. Energy scale is relative to the ground-state enol form and is calculated at the M06-2X/6-31G(d) level.114 Bond lengths are in Å, reaction coordinates are defined in mass weighted coordinates (Bohr 
).
