The photophysical and photochemical properties of the merocyanine dye 1-methyl-2-(4-hydroxystyryl)pyridinium betaine (M) have been studied in aqueous solution at the PM3-SCRF (SCRF = self-consistent
reaction field) level of theory. The trans isomer is more stable than the cis one by 6 kcal/mol, and the energy
gap decreases upon protonation to 2.4 kcal/mol. Protonation on nitrogen is energetically unfavorable and
costs at least 31 kcal/mol more energy than protonation on oxygen. The acidity of the O-protonated form
(MH+) soars up on excitation as inferred from the decrease in the proton affinity values. Potential energy
surfaces (PES) for the ground and lowest excited states for the O-protonated (MH+) and unprotonated (M)
forms of the dye have been constructed to explore the deactivation pathways of the excited states. Upon
excitation the protonated form adopts the quinonoid geometry of the central single bond, and the trans ⇌ cis
photoisomerization goes through a minimum, referred to as the phantom state, of 90° twisted molecular
architecture located on the potential energy surface of its first excited singlet state. For the unprotonated
form (M), the cis → trans isomerization is a downhill process of a quite negligible energy barrier, surmountable
thermally at room temperature. The photochemical/protolytic cycle Mtrans ⇌ MH+
trans ⇌ MH+
cis ⇌ Mcis →
Mtrans could be utilized in the storage of information and its subsequent carrier retrieval. The optical transitions
are xy-plane polarized π−π* transitions and are expected at 372 nm (experiment, 364 nm) and 428 nm
(experiment, 426 nm) for MH+ and M, respectively, in aqueous medium. Within the framework of the SCRF
model, the unprotonated form (M) exhibits a slight positive solvatochromism. The hydrogen bond donor
ability of the solvent is likely the key factor behind the experimentally observed negative solvatochromism.