Spectral Signatures of Intramolecular Charge Transfer Process in β-Enaminones: A Combined Experimental and Theoretical Analysis

Abstract

In this paper, we present spectroscopic signatures of intramolecular charge transfer (ICT) and effects of solvent on the ICT process in 3-(phenylamino)-2-cyclohexen-1-one (PACO), a member of the well-known molecular family, the beta-enaminones. The dual fluorescence in the steady state emission spectra of the molecule in polar solvents indicates the occurrence of ICT, which is further supported by time-resolved studies, using time correlated single photon counting technique with picosecond resolution. To understand the nature of the charge transfer, pH dependent studies of the probe in water were performed, where a quenching of fluorescence was observed even in the presence of very low concentrations of acids. Solvent induced fluorescence quenching was observed in ethanol and methanol. The ICT process was also investigated by quantum chemical calculations. To understand the role of solvents in the ICT process, we have theoretically studied the macroscopic and microscopic solvation of the probe in water. The absorption spectra of the molecule in the gas phase as well as in water were simulated using time dependent density functional theory with cc-pVTZ basis set and self-consistent reaction field theory that models macroscopic solvation. The possibility of microscopic solvation in water was probed theoretically and the formation of 1:3 molecular clusters by PACO with water molecules has been confirmed. Our findings could have a bearing on pH sensing applications of the probe.

Investigation of contrasting hydrogen bonding pattern of 3-(phenylamino)-cyclohexen-1-one with solvents in the ground and excited states

“…The structure of the probe was confirmed through 1 H and 13 C NMR and IR spectra of the molecule, which have been reported elsewhere [22]. The purity of the compound has been checked for spectroscopic studies.…”

“…The probe molecule under investigation, 3-(phenylamino)-2-cyclohexen-1-one (PACO) was synthesized using the procedure described elsewhere [20,22]. The structure of the probe was confirmed through 1 H and 13 C NMR and IR spectra of the molecule, which have been reported elsewhere [22].…”

“…The minute amount of water contents (0.03-0.05%) in the solvents have been removed using the procedure described in Ref. [22]. Triply distilled water was used to prepare all aqueous solutions and was checked for residual fluorescence, if any, before use.…”

“…The vertical excitation energies and the oscillator strengths of corresponding transitions have been calculated using this geometry at the CI singles (CIS) level of theory and same basis set. We compare the obtained electronic structure with that we reported earlier [22]. The HF and CIS calculations were performed using a standard quantum chemical software, Gaussian 03 [31].…”

“…As both the structure and properties of an organic molecule may be susceptible to the nature of the media, studying the effects of solvents on benaminones could be an interesting topic of research. In a recent contribution [22], we have shown that 3-(phenylamino)-cyclohexen-1-one, which has been abbreviated as PACO (Scheme 1) is a novel intramolecular charge transfer probe. In this molecule, the transfer of electron occurs from the nitrogen to oxygen atom through the p-electron bridge (Scheme 2).…”