Photophysical Studies of the Trans to Cis Isomerization of the Push−Pull Molecule: 1-(Pyridin-4-yl)-2-(N-methylpyrrol-2-yl)ethene (mepepy)

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• 作者：Audrey Mokdad ; Jonathan L. Belof ; Sung Wook Yi ; Stephen E. Shuler ; Mark L. McLaughlin ; Brian Space ; Randy W. Larsen
• 刊名：Journal of Physical Chemistry A
• 出版年：2008
• 出版时间：September 11, 2008
• 年：2008
• 卷：112
• 期：36
• 页码：8310-8315
• 全文大小：259K
• 年卷期：v.112,no.36(September 11, 2008)
• ISSN：1520-5215

文摘

Organic molecules possessing intramolecular charge-transfer properties (D−π−A type molecules) are of key interest particularly in the development of new optoelectronic materials as well as photoinduced magnetism. One such class of D−π−A molecules that is of particular interest contains photoswitchable intramolecular charge-transfer states via a photoisomerizable π-system linking the donor and acceptor groups. Here we report the photophysical and electronic properties of the trans to cis isomerization of 1-(pyridin-4-yl)-2-(N-methylpyrrol-2-yl)ethene ligand (mepepy) in aqueous solution using photoacoustic calorimetry (PAC) and theoretical methods. Density functional theory (DFT) calculations demonstrate a global energy difference between cis and trans isomers of mepepy to be 8 kcal mol⁻¹, while a slightly lower energy is observed between the local minima for the trans and cis isomers (7 kcal mol⁻¹). Interestingly, the trans isomer appears to exhibit two ground-state minima separated by an energy barrier of ∼9 kcal mol⁻¹. Results from the PAC studies indicate that the trans to cis isomerization results in a negligible volume change (0.9 ± 0.4 mL mol⁻¹) and an enthalpy change of 18 ± 3 kcal mol⁻¹. The fact that the acoustic waves associated with the trans to cis transition of mepepy overlap in frequency with those of a calorimetric reference implies that the conformational transition occurs faster than the ∼50 ns response time of the acoustic detector. Comparison of the experimental results with theoretical studies provide evidence for a mechanism in which the trans to cis isomerization of mepepy results in the loss of a hydrogen bond between a water molecule and the pyridine ring of mepepy.