Hydrogen Bonding and Photochemical Processes of a Cinnamic Acid Derivative in Langmuir−Blodgett Films

Fourier-transform infrared reflection absorption (IRA) spectroscopy was applied to study the structures and UV-light-induced photochemical processes of a cinnamic acid derivative (C 22 CA) in Langmuir−Blodgett (LB) films on a silver substrate. The IRA spectra of the LB films of C 22 CA with the number of monolayers ( L ) of 1−9 prepared by changing surface pressures (10−30 mN/m) gave four sets of ν(C O) and ν(C C) bands at 1730 and 1641, 1684 and 1624, 1674 and 1624, and 1715 and 1638 cm -1 . The first set was assigned to the C 22 CA molecules forming a hydrogen-bonded chain of carboxyl groups or a “lateral hydrogen-bonded state”, the second and third sets to “cis hydrogen-bonded dimers” (the cis conformation is defined in terms of the position of C O and C C groups around the C−C bond connecting the two groups), and the fourth set to the molecules forming a hydrogen-bonding between neighboring LB monolayers or a “face-to-face hydrogen-bonded state”. The lateral hydrogen-bonded state exists only in a one-monolayer LB film ( L = 1) and in the first monolayer of multimonolayer LB films (L ≥ 3), while the cis hydrogen-bonded dimers are formed in the multimonolayer LB films assuming a structure similar to that of a crystalline state of C 22 CA. When the multimonolayer LB films were annealed at 100 °C, the face-to-face hydrogen-bonded state was converted to the hydrogen-bonded dimer with the trans conformation, giving the set of the ν(C O) and ν(C C) bands at 1694 and 1632 cm -1, respectively. Upon UV irradiation, the C 22 CA molecules in the lateral hydrogen-bonded state do not show any photochemical process, while the molecules forming the cis and trans hydrogen-bonded dimers undergo photodimerization. On the other hand, the irradiation causes the conversion from the face-to-face hydrogen-bonded state to an irregular state consisting of the lateral hydrogen-bonded state and a non-hydrogen-bonded state. The conversion to the trans hydrogen-bonded dimer caused by the thermal treatment and the conversion to the irregular state due to the irradiation indicate that the face-to-face hydrogen bonded state is thermodynamically in a metastable (or a kinetically trapped) state. Thus, the IRA spectroscopy characterized a variety of structures of the cinnamic acid derivative forming LB monolayer assemblies and elucidated the correlation between the structures and photochemical processes induced by UV irradiation.