Effects on the photophysical properties of naphthylamine derivatives upon their inclusion in cyclodextrin nanocavities

Unique structural effect on the fluorosolvatochromism and dual fluorescence emission of D-π-A+ cationic chromophores with furyl bridge. An approach to white light emitters

Photophysical behavior of two D - π - A+ cationic compounds with the same furyl bridge and nicotinamidine group as an electron acceptor moiety and two electron donating groups, namely methoxy (I) and N,N-dimethylamino (II) groups was examined using steady-state and time-resolved techniques in variety of solvents. Time-dependent density functional theory (TDDFT) calculations were performed in some representative solvents and compared with the experimental results. Steady state and time-resolved studies in different solvents reveal that fluorescence emission of (I) is ascribed to an emission from an excited state (ICT) with higher dipole moment than the ground state while the emission of (II) is a dual emission from a state with high charge transfer nature (ICT) in addition to the locally excited state (LE). The fluorescence emission spectra of (II) were found to depend on the excitation wavelength and an increase in the excitation wavelength led to the formation of a longer wavelength emission band with lower quantum yield. It has also been found that the fluorescence excitation spectra were dependent on the emission wavelength. The effect of solvent on the nature of dual emission was examined. Correlation of the photophysical properties of the excited states of (I) and (II) with the solvent polarity, ε, reveals the charge transfer nature of (I) and the long wavelength emission band of (II), while their correlation with the solvent polarity parameter (ETN) shows two different trends when the solvents are divided to aprotic and protic solvents. For precise investigation of the impact of each solvent parameter on each photophysical property, Catalán's and Laurence's four parametric linear solvation energy relationships were studied. We have found that the non-specific interactions of the solvent are primarily responsible for controlling the photophysical properties, as demonstrated by Catalán's and Laurence's treatments. DFT and TDDFT calculations were used to anticipate the dipole moments in the ground and excited states and geometry of both states.

Photophysical properties of push-pull monocationic D-π-A+ thiophene based derivatives: Fluorosolvatochromism and pH studies

Photophysical properties of two thiophene salts of the form D-π-A⁺ are studied in several solvents and at various pH values of the aqueous solution. The studied compounds embrace methoxy group as electron donating moiety at one end and cationic amidine group with and without fluorine atom at the ortho position of the amidine group as the electron withdrawing group at the other end of the molecules and separated by thiophene ring. The two thiophene derivatives are 4-(5-(4-methoxyphenyl)thiophen-2-yl)benzamidine hydrochloride salt (MOTB) and 2-fluoro-4-(5-(4-methoxyphenyl) thiophen-2-yl)benzamidine hydrochloride salt (FMOTB). The observed changes in the fluorescence emission spectra with the nature of the solvent were found to be much more pronounced than the corresponding absorption spectra which signify an emission from the intramolecular charge transfer state. The higher bathochromic shift in the fluorescence emission spectra than the absorption spectra indicates that the excited state dipole moment is larger than that of the ground state. It has also been observed that the presence of the fluorine atom in the electron withdrawing part does not show any changes in the absorption spectra while a clear bathochromic shift is observed in the fluorescence emission spectra indicating an enhanced strength of the electron withdrawing ability in case of FMOTB. Effect of pH was also studied and pK_a values were evaluated. The observed photophysical properties were correlated to the normalized solvent polarity parameter (E_T^N) when solvents are classified to protic and aprotic solvents. This designates the importance of hydrogen bonding interactions. We have also applied a couple of linear solvation energy relationships for better understanding of the exact contribution of each solvent parameter on each photophysical property. We have found that both Catalán's and Laurence's treatments show that the photophysical properties are mainly controlled by the solvent's non-specific interactions. However, these models were not sufficient to interpret the observed data without the inclusion of the participation of the specific interactions.

The relationship between solvatochromic properties and in silico ADME parameters of new chloroethylnitrosourea derivatives with potential anticancer activity and their β-Cyclodextrin complexes

In view of the anticancer effect of nitrosoureas a set of four new N-(2-chloroethyl)-N-nitrosourea (CENU) derivatives was synthesized. An in silico absorption, distribution, metabolism, excretion and toxicity (ADME/Tox) prediction study revealed that the CENU derivatives satisfied all the required criteria for oral administration and introduced them as remarkable anticancer candidates in the central nervous system (CNS). A comparative solvatochromic study including the Kamlet-Taft, Catalán and Laurence models indicated that the solvatochromic behavior of the CENUs depended on both, unspecific and specific solvent-solute interactions. In detail, the solvatochromic effect of the solvent polarity on the absorption and emission maxima was significant for all CENUs, whereas the solvatochromic effect of the solvent's ability to donate or accept hydrogen bonds on the absorption and emission maxima was critically dependent on the electron density of the N'-aryl group. From the solvatochromic comparison method, excellent correlations (r ≥ 0.890) were obtained between the ADME parameters and the solvatochromic regression coefficients obtained by the Catalán model. As potential stabilizers, inclusion complexes of the investigated CENU derivatives with β-cyclodextrin (β-CD) were also explored. The spectrofluorimetric host-guest experiments included double-reciprocal Benesi-Hildebrand plots as well as the molar ratio and continuous variation plots (Job's plots), which established a 1:1 β-CD to CENU binding stoichiometry and relatively high affinities of β-CD for CENU derivatives.

Photophysical properties and fluorosolvatochromism of D–π–A thiophene based derivatives

Solvation-dependent photophysical properties of two push–pull thiophene-based compounds with donor–π–acceptor (D–π–A) structures were investigated using absorption, fluorescence emission and time resolved spectroscopy, and supported by different solvation models. Intramolecular charge transfer characteristics of the structurally similar 2-fluoro-4-(5-(4-methoxyphenyl)thiophen-2-yl)benzonitrile (MOT) and 4-(5-(4-(dimethylamino)phenyl)thiophen-2-yl)-2-fluorobenzonitrile (DMAT) were investigated. Significant enhancement of intramolecular charge transfer strength has been observed through molecular structure modification of the electron donating group from a methoxy to dimethylamine group. Ground state absorption spectra show a small red shift of about 10 nm and 18 nm while the fluorescence emission spectra show a large red shift of about 66 nm and 162 nm on changing from the nonpolar cyclohexane to the aprotic polar DMSO for MOT and DMAT, respectively. Dipole moment change from the ground state to the charge transfer excited state is calculated to be 6.6 D in MOT and 9.0 D in DMAT. The fluorescence quantum yield, fluorescence lifetime and the derived radiative and non-radiative rate constants were found to be better correlated to the emission energy rather than any of the solvent properties. Three multi-parametric relationships were used in the interpretation of the specific versus non-specific solute–solvent interactions, namely, Kamlet–Taft, Catalán and Laurence et al. models. The findings of these approaches are used to extract useful information about different aspects of solvent effects on the photophysical properties of the two studied compounds. Kamlet–Taft solvatochromic model indicates that non-specific interactions are dominant in controlling the photophysical properties. Catalán's solvent dipolarity/polarizability parameter is found to play a significant role in solvatochromic behaviour which is also designated by the Laurence model.

Solvation-dependent photophysical properties of two push–pull thiophene-based compounds with donor–π–acceptor (D–π–A) structures were investigated using absorption, fluorescence emission and time resolved spectroscopy, and supported by different solvation models.