Synthesis, Structure and Photochemistry of Dibenzylidenecyclobutanones

Abstract

A series of symmetrical dibenzylidene derivatives of cyclobutanone were synthesized with the goal of studying the physicochemical properties of cross-conjugated dienones (ketocyanine dyes). The structures of the products were established and studied by X-ray diffraction and by NMR and electronic spectroscopy. All the products had E,E-geometry. The oxidation and reduction potentials of the dienones were determined by cyclic voltammetry. The potentials were shown to depend on the nature, position, and number of substituents in the benzene rings. A linear correlation was found between the difference of the electrochemical oxidation and reduction potentials and the energy of the long-wavelength absorption maximum. This correlation can be employed to analyze the properties of other compounds of this type. Quantum chemistry was used to explain the observed regularities in the electrochemistry, absorption, and fluorescence of the dyes. The results are in good agreement with the experimental redox potentials and spectroscopy data.

Theoretical Study of Structure and Photophysics of Homologous Series of Bis(arylydene)cycloalkanones

“…The calculations demonstrated that the fractions of (E,Z) and (Z,Z) isomers is negligible in the ground state. This agrees with the experiment [1][2][3]22], where no electronic transitions corresponding to (E,Z) and (Z,Z) isomers were observed in the absorption spectra. The ground-state rotation barriers are ~50-60 kcal/mol, which indicates that trans-cis isomerization in the ground state is impossible at any reasonable temperature.…”

“…This results in blue-shifted spectra of the cyclohexanone derivatives. The HOMO and LUMO energies correlate with the oxidation and reduction potentials [1][2][3]. The HOMO-LUMO gaps also correlate with the experimental E ox -E red gaps (refer Figures S2 and S3 of Supplementary Materials).…”

“…The intensity of this transition decreases cyclobutanone to cyclohexanone (Figure S4 of Supplementary Materials). This is ported by the experimental data [1][2][3]: in cyclobutanone 1e the long-wave absor band consists of two distinct peaks that cannot be attributed to the vibronic progres while in 2e and 3e, the long-wave absorption band is only broadened. The position o band changes only slightly, which can be explained by the fact that HOMO-1 is n fected either by the substituents or by the central moiety.…”

“…The depths of the local minima associated with the rotation of the phenyl rings (shown by arrows in Figure 2) differ by a fraction of kilocalorie, and these minima are separated by low barriers, which can easily be overcome at room temperature, ensuring slightly hindered rotation of these groups. The 1H NMR experiments indicate that dyes (1-3)c exist as mixtures of rotamers [1][2][3].…”

“…Bis(arylidene)cycloalkanones (Figure 1), also known as symmetric cross-conjugated dienones, ketocyanine dyes, or diarylidene ketone derivatives, are D-π-A-π-D dyes with interesting photochemistry and photophysics. The electrochemistry, photophysics, and photochemistry of bis(arylidene)cycloalkanone series have been studied extensively [1][2][3]. The compounds were found to exhibit efficient trans-cis photoisomerization reactions.…”