Theoretical Study on Photophysical and Photochemical Properties of a Merocyanine Dye

Abstract

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.

Thermo‐Solvatochromism of Merocyanine Polarity Probes – What Are the Consequences of Increasing Probe Lipophilicity through Annelation?

“…Suggested protolytic/photochemical cycle for MePM. [32] cle is shown in Figure 4. [32] Protonation of the trans isomer (I) produces H + trans (II), which undergoes light-induced isomerization to H + cis (III).…”

“…[32] cle is shown in Figure 4. [32] Protonation of the trans isomer (I) produces H + trans (II), which undergoes light-induced isomerization to H + cis (III). This, after deprotonation, produces (IV), which after absorption of light is transformed back into (I); this step is irreversible.…”

Photoisomerization of simple merocyanines: a theoretical and experimental comparison with polyenes and symmetric cyanines

“…More recently, the electronic states of both cyanine and merocyanine dyes have been the subject of several MO SCF CI studies. 1,[16][17][18][45][46][47][48][49][50] In this section we focus our attention on the strongly allowed long-wavelength absorption which, for the three chromogen types, mainly arises from electronic excitation from the p H to the p* L orbitals. For the sake of brevity, we only report in Fig.…”

Merocyanine dyes: dimeric aggregation in solution, film structure and pH sensing

“…a red shift of the visible absorption band as the solvent polarity decreases, this common feature of merocyanine derivatives being frequently attributed to a zwitterionic ground state. [9][10][11][12][13] However, contrary to this, Hamman and El-Nahas 15 have suggested that the hydrogen-bonding ability of the donor is a vital factor and Alberti and Echave 16 have reported nucleophile-carbonyl interactions (e.g. in acetone) and hydrogenbonding (e.g.…”