Absence of Intramolecular Charge Transfer with 4-Fluoro-N,N-dimethylaniline (DMA4F), Contrary to an Experimental Report Supported by Computations

Visualizing and characterizing excited states from time-dependent density functional theory

Time-dependent density functional theory (TD-DFT) is the most widely-used electronic structure method for excited states, due to a favorable combination of low cost and semi-quantitative accuracy in many contexts, even if there are well recognized limitations. This Perspective describes various ways in which excited states from TD-DFT calculations can be visualized and analyzed, both qualitatively and quantitatively. This includes not just orbitals and densities but also well-defined statistical measures of electron-hole separation and of Frenkel-type exciton delocalization. Emphasis is placed on mathematical connections between methods that have often been discussed separately. Particular attention is paid to charge-transfer diagnostics, which provide indicators of when TD-DFT may not be trustworthy due to its categorical failure to describe long-range electron transfer. Measures of exciton size and charge separation that are directly connected to the underlying transition density are recommended over more ad hoc metrics for quantifying charge-transfer character.

Probing of local polarity in poly(methyl methacrylate) with the charge transfer transition in Nile red

The permittivity of polymers and its spatial distribution play a crucial role in the behavior of thin films, such as those used, e.g., as sensor coatings. In an attempt to develop a conclusive approach to determine these quantities, the polarity of the model polymer poly(methyl methacrylate) (PMMA) in 600 nm thin films on a glass support was probed by the energy of the charge transfer transition in the oxazine dye Nile red (NR) at 25 °C. The absorption and fluorescence spectra of NR were observed to shift to the red with increasing solvent polarity, because of the intramolecular charge transfer character of the optical transition. New types of solvatochromic plots of emission frequency against absorption frequency and vice versa afforded the Onsager radius-free estimation of the ground and excited states dipole moment ratio. With this approach the values of these dipole moments of 11.97 D and 18.30–19.16 D, respectively, were obtained for NR. An effective local dielectric constant of 5.9–8.3 for PMMA thin films was calculated from the solvatochromic plot and the fluorescence maximum of NR observed in the PMMA films. The fluorescence band of NR in the rigid PMMA films shifted to the red by 130 cm⁻¹ with increasing excitation wavelength from 470 to 540 nm, while in a series of liquids the position of the emission maximum of NR remained constant within same range of the excitation wavelength. It is concluded that the fluorescence spectrum of NR in PMMA undergoes inhomogeneous broadening due to different surroundings of NR molecules in the ground state and slow sub-glass transition (T_g) relaxations in PMMA.

Spectroscopic and Theoretical Studies of Fluorescence Effects in 2-Methylamino-5-(2,4-dihydroxyphenyl)-1,3,4-thiadiazole Induced by Molecular Aggregation

The article presents the results of fluorescence analyses of 2-methylamino-5-(2,4-dihydroxyphenyl)-1,3,4-thiadiazole (MDFT) in an aqueous environment. MDFT dissolved in aqueous solutions with a pH value in the range from 1 to 4.5 yielded an interesting effect of two clearly separated fluorescence emissions. In turn, a single fluorescence was observed in MDFT dissolved in water solutions with a pH value from 4.5 to 12. As it was suggested in the previous investigations of other 1,3,4-thiadiazole compounds, these effects may be associated with conformational changes in the structure of the analysed molecule accompanied by aggregation effects. Crystallographic data showed that the effect of the two separated fluorescence emissions occurred in a conformation with the –OH group in the resorcyl ring bound on the side of the sulphur atom from the 1,3,4-thiadiazole ring. The hypothesis of aggregation as the mechanism involved in the change in the spectral properties at low pH is supported by the results of (Time-Dependent) Density Functional Theory calculations. The possibility of rapid analysis of conformational changes with the fluorescence spectroscopy technique may be rather important outcome obtained from the spectroscopic studies presented in this article. Additionally, the presented results seem to be highly important as they can be easily observed in solutions and biologically important samples.