Solvatochromism of an Aminobenzodifuranone: An Unprecedented Positive Wavelength Shift

Copper(II) Ion Promoted Reverse Solvatochromic Response of the Silatrane Probe to Spectral Shifts: Preferential Solvation and Computational Approach

The unique solvatochromic attitude of an analyte owing to its coordination with metal ions in solvents of different polarities is challenging. Herein, we introduce two new solvatochromic 4-(pentan-3-yl) benzaldehyde-based triazolyl silatrane probes (5 and 6). The solvatochromic behavior of both probes 5 and 6 was studied using Reichardt's E (30) and the Kamlet-Taft empirical scale by UV-visible spectra in 14 solvents (hydrogen-bond donor (HBD) and non-HBD), and the results show that probes 5 and 6 exhibit reverse solvatochromism. Probe 5 witnessed an enhancement in this behavior upon coordination with the Cu²⁺ ion in MeCN/MeOH solvents due to the intramolecular charge transfer (ICT) process. Interestingly, the binding of probe 5 with Cu²⁺ ions resulted in an instant color change in MeCN and MeOH from pale yellow to light blue and brown-red, respectively, which can be easily detected by the "naked eye". A solvatochromic study of the complex 5-Cu²⁺ in binary mixtures of polar aprotic and polar protic solvents (MeCN/MeOH) discloses that the latter are more preferred over polar aprotic solvents in the solvation microsphere. The entire metal coordination process of probe 5 toward the Cu²⁺ ion can be visualized and was further evaluated by UV-vis/fluorescence spectral titrations, Fourier transform infrared (FT-IR) spectroscopy, and theoretical calculations employing density functional theory (DFT) and time-dependent-DFT (TD-DFT). The proposed analytical approach is believed to play a crucial role in the solvatochromic study of higher coordinated silicon compounds, which may be utilized to develop a solvent-dependent sensor.

Colour and constitution of conjugate bases of benzodifurantrione, its ring-opened derivatives and benzodifuranone dye analogues

The observation of ready deprotonation of the phenylogous enol of benzodifurantrione (BDT) to give a bright violet conjugate base has led to two follow up explorations. Extension of BDT enol by insertion of a p-phenylene unit into the enol C-O bond gives the known bright red 4-hydroxylated benzodifuranone dyes. Their deprotonation results in previously unreported near infrared-absorbing conjugate bases. These appear to aggregate in solution, the more so in less polar solvents. Ring-opened derivatives of BDT containing α-dicarbonyl substituents also give coloured conjugate bases, but α-keto-ester and -anilide derivatives differ substantially (ester yellow; anilide intense red). Investigation of this nonintuitive difference leads to the conclusion that while the anilide is essentially planar the ester is nonplanar. The contrast in conformation impacts on the auxochromic effects of the otherwise closely related α-dicarbonyl substituents and thus the variation in colour. The latter observation has potential across colour chemistry in general. In contrast to the readily observed BDT enol, no evidence has been adduced for enol tautomers amongst the ring-opened analogues.

The influence of the cation structure on the basicity-related polarity of ionic liquids

UV/Vis absorption data of (E)-4-(2-[5-{4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl}thiene-2-yl]vinyl)-2-(dicyano-methylene)-3-cyano-5,5-dimethyl-2,5-dihydrofuran (ThTCF) as a solvatochromic probe is applied to examine the anion coordination strength (e.g. of N(CN)₂, BF₄, PF₆, N(Tf)₂, CF₃COO) as a function of the cation structure of ionic liquids. Several 1-n-alky-3-methylimidazolium- and tetraalkylammonium CH₃-NR₃⁺-based ILs with different n-alkyl chain lengths (R = -C₄H₉, -C₆H₁₁, -C₈H₁₇, -C₁₀H₂₁) are considered. UV/Vis absorption data of ThTCF show subtle correlations with hydrogen bond accepting (HBA) ability-related measurands such as Kamlet-Taft β, Freire's E_HB, and Laurence β₁ parameter as a function of anion and cation structure. The different influence of the n-alkyl chain length of imidazolium- and tetraalkylammonium-based ILs on the dipolarity and HBA strength is confirmed by comparison with the ¹⁴N isotropic hyperfine coupling constants (A_iso) of a positively (CATI) and negatively charged spin probe (TSKCr) of TEMPO-type [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] and quantum chemically derived dipoles of the cations. The A_iso values correlate with the absorption energy of ThTCF and E_HB, but in different ways depending on the anion or charge of the spin probe. In a final discussion of the β, E_HB, and β₁ scales in relation to ThTCF, the importance of the molar concentration N of ionic liquids for the physical significance of the respective parameters is discussed.

Polarity of tetraalkylammonium-based ionic liquids and related low temperature molten salts

Solvatochromic measurements and ¹H NMR spectroscopy have been used to investigate the overall polarity of tetraalkylammonium based ionic liquids and low temperature molten salts.

Adsorption of poly(vinyl formamide-co-vinyl amine) (PVFA-co-PVAm) polymers on zinc, zinc oxide, iron, and iron oxide surfaces

The adsorption of poly(vinyl formamide) (PVFA) and the statistic copolymers poly(vinyl formamide-co-vinyl amine) (PVFA-co-PVAm) onto zinc and iron metal particles as well as their oxides was investigated. The adsorbates were characterized by means of XPS, DRIFT spectroscopy, wet chemical analysis, and solvatochromic probes. Dicyano-bis-(1,10-phenanthroline)-iron(II) (1), 3-(4-amino-3-methylphenyl)-7-phenyl-benzo-[1,2-b:4,5-b']difuran-2,6-dione (2), and 4-tert-butyl-2-(dicyano-methylene)-5-[4-(diethylamino)-benzylidene]-Δ(3)-thiazoline (3) as solvatochromic probes were coadsorbed onto zinc oxide to measure various effects of surface polarity. The experimental findings showed that the adsorption mechanism of PVFA and PVFA-co-PVAm strongly depends on the degree of hydrolysis of PVFA and pH values and also on the kind of metal or metal oxide surfaces that were employed as adsorbents. The adsorption mechanism of PVFA/PVFA-co-PVAm onto zinc oxide and iron oxide surfaces is mainly affected by electrostatic interactions. Particularly in the region of pH 5, the adsorption of PVFA/PVFA-co-PVAm onto zinc and iron metal particles is additionally influenced by redox processes, dissolution, and complexation reactions.

Synthesis, characterization and spectroscopic investigation of a novel phenylhydrazone Schiff base with solvatochromism

A novel Schiff base of 4,5-diazafluorene-9-p-nitrophenylhydrazone (DAFND) has been synthesized and characterized. The crystal structures of DAFND and its analogue 4,5-diazafluorene-9-phenylhydrazone (DAFPD) were determined by single crystal X-ray diffraction method. X-ray analyses reveal that DAFPD comprise of a nonplanar molecule and all atoms of DAFND are essentially coplanar. The color of DAFND changes from brown to blue when heated, so called thermochromism and the spectroscopic properties of the two compounds are investigated by electronic absorption spectra, showing DAFND possess solvatochromism, while DAFPD does not have thermochromic and solvatochromic properties. The lambda(max) of DAFND within various pure solvents are different ranging from 370 nm in toluene to 614 nm in pyridine. The imaginable mechanisms of thermochromism and solvatochromism are proposed.

Unique interactions between the chromophore and glutamate 16 lead to far-red emission in a red fluorescent protein

mPlum is a far-red fluorescent protein with emission maximum at approximately 650 nm and was derived by directed evolution from DsRed. Two residues near the chromophore, Glu16 and Ile65, were previously revealed to be indispensable for the far-red emission. Ultrafast time-resolved fluorescence emission studies revealed a time dependent shift in the emission maximum, initially about 625 nm, to about 650 nm over a period of 500 ps. This observation was attributed to rapid reorganization of the residues solvating the chromophore within mPlum. Here, the crystal structure of mPlum is described and compared with those of two blue shifted mutants mPlum-E16Q and -I65L. The results suggest that both the identity and precise orientation of residue 16, which forms a unique hydrogen bond with the chromophore, are required for far-red emission. Both the far-red emission and the time dependent shift in emission maximum are proposed to result from the interaction between the chromophore and Glu16. Our findings suggest that significant red shifts might be achieved in other fluorescent proteins using the strategy that led to the discovery of mPlum.

Photonic Logic Gates Based on Control of FRET by a Solvatochromic Perylene Bisimide

New perylenebisimide derivatives hydroxyperylenebisimide and naphthoperylenebisimide were obtained and applied to construct a new solvatochromic dyad 9. The solvatochromic behavior of hydroxyperylenebisimide was studied, and the structure of naphthoperylenebisimide was determined by X-ray crystallography. The spectral studies indicated that the hydroxyperylenebisimide and naphthoperylenebisimide units of dyad 9 were strongly coupled in the ground state, and as a result the fluorescence of the naphthoperylenebisimide unit was almost quenched and that of the hydroxyperylenebisimide unit was greatly enhanced due to the fluorescence resonance energy transfer (FRET). As we expected, this FRET process could be tuned with the addition of protons, base, and ferric ions. This behavior of dyad 9 could be interpreted by a two-input INH logic gate, while in the presence of Fe(III), the ion complex of 9 could execute a two-input XOR logic gate. By changing the output signal, a combinational logic circuit with three inputs could also be interpreted.

Solvent Effects on the Two-Photon Absorption of Distyrylbenzene Chromophores

A series of organic- and water-soluble distyrylbenzene-based two-photon absorption (TPA) fluorophores containing dialkylamino donor groups at the termini was designed, synthesized, and characterized. The central core was systematically substituted to modulate intramolecular charge transfer (ICT). These molecules allow an examination of solvent effects on the TPA cross section (delta) and on the TPA action cross section. In toluene, the delta values follow the order of ICT strength. The effect of solvent on delta is nonmonotonic: maximum delta was measured in an intermediate polarity solvent (THF) and was lowest in water. We failed to find a correlation between the observed solvent effect and previous theoretical predictions. Hydrogen bonding to the donor groups and aggregation of the optical units in water, which are not included in calculational analysis, may be responsible for the discrepancies between experimental results and theory.

Hydrogen-bond effects on the electronic absorption spectrum and evaluation of nonlinear optical properties of an aminobenzodifuranone derivative that exhibits the largest positive solvatochromism

In this work, for the first time, a theoretical approach to describing the influence of hydrogen-bond formation on the electronic absorption spectrum and nonlinear optical properties of an aminobenzodifuranone derivative (ABF) that exhibits the largest positive solvatochromic shift compared to other known chromophores is given. The solvent effect was included via the supermolecule (SM) method. The calculations were performed for a strong low-lying (pi-->pi*) transition based on the configuration interaction singles (CIS) and time-dependent DFT (TDDFT) methods. The first-order hyperpolarizabilities (beta) were computed using the finite-field (FF) technique combined with the Hartree-Fock (HF) theory. Reasonable agreement between theory and experiment was obtained for the solvatochromic shifts of the ABF molecule. Moreover, it was found that H-bond formation strongly influences the NLO response of the systems investigated. [Figure: see text]. The interaction difference-density maps of the systems studied: a II--complex ABF with NFTB; b III--complex ABF with HMPA. The red color designates an increase of the electron density caused by the intermolecular interactions, whereas blue indicates a corresponding decrease of the electron density. The isodensity contours were plotted for +/-0.01 electron/bohr3 (DFT/B3LYP/6-31G(d,p)).

Probing Surface Basicity of Solid Acids with an Aminobenzodifurandione Dye as the Solvatochromic Probe

Solvatochromism and sorptiochromism of the dye 3-(4-amino-3-methylphenyl)-7-phenyl-benzo1,2b:4,5b'difuran-2,6-dione (1) are studied with an extended set of solvents and various solid acids including silicas, aluminas, and alumosilicates. 1 shows a positive solvatochromism with increasing basicity and dipolarity/polarizability of the solvent; its solvent-induced bathochromic UV-vis absorption band shift ranges from formic acid (upsilon(max) = 21 630 cm(-1)) to hexamethylphosphoric acid triamide (upsilon(max) = 14 200 cm(-1)). Multiple square analyses of upsilon(max) of the solvent-dependent solvatochromic UV-vis absorption band of 1 with several empirical solvent polarity parameters prove that a composite of basicity, acidity, and dipolarity/polarizability of the environment must be taken into account. For the analysis of the solvent-dependent UV-vis shift of 1, the Kamlet-Taft and Catalan solvent parameters have been evaluated. It could be shown that the Catalan solvent parameter set is more suitable to reflect multiple solvation processes involving both strong basic and strong acidic solvents. Quantum chemical calculations indicate that an interaction of the silanol oxygen atom with the protons of the amino group is clearly favored over various acidic attacks of silanol groups upon 1. Accordingly, surface basicity of silica, alumina, and alumosilicates can be determined using the linear solvation energy relationship derived from the solvent-dependent UV-vis band of 1. An unambiguous interpretation of the UV-vis spectroscopic data of 1 adsorbed on surfaces containing Lewis-acid sites is sometimes difficult. UV-vis monitoring of 1-loaded solid acids during surface titration with 2,6-di-tert-butyl pyridine allows the discrimination of whether Brønsted- or Lewis-acid sites interfere with 1. Additionally, adsorbed water has an important influence on the actual surface basicity of solid acids. 1 is recommended as a sensitive probe for checking both basicity and acidity when directly compared with solvatochromism of the established hydrogen-bond-donating indicator (cis-dicyano)bis(1,10-phenanthroline)iron(II) (2).