Photoisomerization of linear merocyanines in salt solutions

• Published in: Journal of the Chinese Chemical Society (Taipei) 2023-05, Vol.70 (5), p.1017-1028
• Main Authors: Wu, Ping‐Chien, Tan, Kui‐Thong, Chen, I‐Chia
• Format: Article
• Language: English
• Subjects: Anisotropy; Cesium iodides; Decay; Deceleration; Dimethyl sulfoxide; Dimethylformamide; Fluorescence; Ionic liquids; Isomerization; merocyanine; photoisomerization; Saline solutions; salt solution; Time constant
• ISSN: 0009-4536; 2192-6549
• DOI: 10.1002/jccs.202200532

The trans‐cis isomerization in the excited state of linear merocyanine L‐Mero4 and phenyl substituted linear merocyanine P‐L‐Mero4 in salt solution and in ionic liquid was investigated using frequency upconversion measurements. Strontium chloride and cesium iodide were added to solvent dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) to vary the ionic strength. The time‐resolved fluorescence curves of merocyanines displayed multiple exponential decay behavior. The second temporal component with time constant τ2 ≈ 2.8 (11) ps of L‐Mero4 (P‐L‐Mero4) in DMSO was assigned to the duration to reach the isomerization equilibrium between the trans and the twisted conformers. The τ2 increased at higher salt concentrations and was explained by the attachment of salt ions on the polar excited merocyanines decelerating the isomerization rate. The rotational correlation time constants obtained from the anisotropy decay of fluorescence were 360 and 240 ps in neat DMSO for L‐Mero4 and P‐L‐Mero4, respectively, and they increased to 790 and 450 ps in the most concentrated SrCl2. Using Perrin relation, we estimated the increase in the rotating volume at [SrCl2] = 536 mM, revealing ≈15 SrCl2 molecules surrounding L‐Mero4 and 7 SrCl2 on P‐L‐Mero4. The experimental data indicated that the ion–molecule interaction was stronger with SrCl2 and on L‐Mero4 than on P‐L‐Mero4. The photoisomerization of L‐merocyanine undergoes from the trans to the perpendicular form in the excited state surface. In salt solution, the polar excited state is proposed to be attached by salt ions to decelerate the isomerization rate.