Increasing Solvent Viscosity Research Articles

Isomer-Specific Solvatochromic and Molecular Rotor Properties of ESIPT-Active Push-Pull Fluorescent Chalcone Dyes.

Aromatic chromophores possessing intramolecular hydrogen-bonds that can undergo excited-state intramolecular proton transfer (ESIPT) are critical tools for chemosensing/biosensing applications because they create large Stokes-shifted fluorescence with no overlap with the absorption spectrum to limit back-ground interferences. Classic ESIPT-active fluorophores, such as the 2-(2'-hydroxyphenyl) benzazole (HBX) series (X = NH, O, S), favor a ground-state (GS) enol (E) form that undergoes ESIPT to afford an excited-state (ES) keto (K) tautomer that generates red-shifted fluorescence. Herein, we have attached the HBX moiety to 6-methoxy-indanone (6MI) to create isomeric (ortho and para) ESIPT-active chalcone dyes and have characterized their photophysical properties in polar protic solvents (MeOH and glycerol (Gly)/MeOH mixtures) and a nonpolar aprotic (1,4-dioxane) solvent for comparison. The chalcones favor a GS E structure, which undergoes ESIPT in MeOH, Gly/MeOH mixtures, and dioxane to exclusively afford K emission with large Stokes shifts. The o-isomers possess expanded π-conjugation compared to their p-isomer counterparts, which diminishes their tendency to generate twisted intramolecular charge transfer (TICT) states. Consequently, the o-isomers have greater quantum yields and lack molecular rotor (MR) character with little K emission response to increased solvent viscosity. However, they possess strong positive solvatochromism, displaying significant blue wavelength shifts coupled with turn-on K emission in moving from polar protic MeOH to nonpolar dioxane. In contrast, the p-isomers display MR character with turn-on K emission in 75:25 Gly/MeOH compared to their emission in MeOH (up to 14-fold) due to a strong tendency for TICT. Mechanistic insight into the observed isomer-specific photophysical properties of the ESIPT-active chalcones was obtained through density functional theory (DFT) calculations. Implications for DNA biosensing applications are discussed.

Reaction monitoring of rose bengal photodegradation in alcohols using multivariate frequency-domain dynamic fluorescence

• Photodegradation of rose bengal (RB) exposed to simulated solar radiation evaluated. • Reaction progress monitored using matrix-formatted frequency domain fluorescence. • RB and photoproduct(s) spectra and kinetics resolved by multivariate data mining. • Facile RB dehalogenation in methanol; no photoproducts observed in cyclohexanol. • Multivariate approach insulates results from distortion by unknown concomitants. The photodegradation of rose bengal (RB) by simulated solar radiation in several air saturated alcohols, monitored using wavelength-resolved frequency-domain dynamic fluorescence measurements, is described. This work illustrates a novel strategy for characterizing photochemical reactions of fluorophores using a combination of reaction monitoring by multichannel frequency domain fluorescence measurements and data mining. Kinetic and mechanistic details of RB photodegradation in the selected alcohols are obtained by resolving the spectra and frequency domain decays of unreacted RB and its fluorescent photoproducts from the multivariate measurements. The analysis indicates that RB exhibits photochemical activities and photodegradation rates that are strongly dependent on solvent properties. An associated benefit of the data mining is the insulation of rate calculations from distortions produced by spectral overlap of undiscovered concomitants. This was most evident in RB photodegradation rates in methanol, which paradoxically, is a common solvent choice for photosensitizer efficiency measurements using RB as the standard reference. RB photodegradation in methanol was not only the fastest of all the solvents investigated at 10.0±1 μM/min, methanol supported the formation of three fluorescent photoproducts that had emission properties consistent with RB dehalogenation. In the remaining alcohols, the rates of RB degradation decreased with decreasing solvent relative polarity and increasing solvent viscosity and only one dehalogenated RB derivative was observed. In octanol, the photodegradation rate was faster than expected based on its dielectric constant at 0.249±0.006 μM/min. In cyclohexanol, no photoproduct formation was detected. These results suggest a role for diffusion control in photoinitiated RB dehalogenation.

Stress‐dependent macromolecular crowding in the mitochondrial matrix

Macromolecules of various sizes induce crowding of the cellular environment. This crowding impacts on biochemical reactions by increasing solvent viscosity, decreasing the water‐accessible volume and altering protein shape, function, and interactions. Although mitochondria represent highly protein‐rich organelles, most of these proteins are somehow immobilized. Therefore, whether the mitochondrial matrix solvent exhibits macromolecular crowding is still unclear. Here, we demonstrate that fluorescent protein fusion peptides (AcGFP1 concatemers) in the mitochondrial matrix of HeLa cells display an elongated molecular structure and that their diffusion constant decreases with increasing molecular weight in a manner typical of macromolecular crowding. Chloramphenicol (CAP) treatment impaired mitochondrial function and reduced the number of cristae without triggering mitochondrial orthodox‐to‐condensed transition or a mitochondrial unfolded protein response. CAP‐treated cells displayed progressive concatemer immobilization with increasing molecular weight and an eightfold matrix viscosity increase, compatible with increased macromolecular crowding. These results establish that the matrix solvent exhibits macromolecular crowding in functional and dysfunctional mitochondria. Therefore, changes in matrix crowding likely affect matrix biochemical reactions in a manner depending on the molecular weight of the involved crowders and reactants.

Monitoring viscosity in live cells based on the excited-state absorption signal in transient absorption spectroscopy

Transient absorption (TA) signals have the features of real-time multi-signals and high sensitivity for biological monitoring, and they are less disturbed by the biological environment compared to single fluorescence signals. Two intelligently regulated probes (1-VBM and 1-VBH) are designed and synthesized with time-resolved excited-state absorption (ESA) and stimulated emission (SE) signals in TA spectroscopy. In mixed solutions of glycerol and DMSO, the ESA lifetimes of the 1-VBM probe linearly increase (y = 4.5419x + 303.98) with an increase in solvent viscosity (2.79–190.35 cP), showing similar sensitivity as SE lifetimes (y = 4.6342x + 311.06). Therefore, these TA signals can be used to monitor the viscosity of live cells. As serum changes in viscosity, 1-VBM has a sensitive and specific time-resolved ESA signal. Interestingly, the ESA signal is several times stronger than the SE signal in the serum of both healthy mice and mice with hepatoma. The lifetime of ESA signal in hepatoma serum (958.6 ps) is longer than that in normal serum (632.8 ps), which is induced by the higher blood viscosity in hepatoma mice. This indicates that the ESA signals in TA spectroscopy are a reliable marker for detecting viscosity in biological samples, and can be a potential tool for clinical diagnosis.

Unravelling the thermodynamics and binding interactions of bovine serum albumin (BSA) with thiazole based carbohydrazide: Multi-spectroscopic, DFT and molecular dynamics approach

In the present manuscript, a novel 2-((4-chlorophenyl)amino)-N'-(4-(diethylamino)-2-hydroxybenzylidene)thiazole-4-carbohydrazide (H1) has been synthesized and characterized using various spectroscopic techniques including 1D, 2D NMR, FT-IR and single-crystal XRD. Various electrostatic parameters such as frontier molecular orbital (FMO), natural bond orbital (NBO), Fukui functions and nonlinear optical (NLO) properties were explored by Density Functional Theory (DFT) using B3LYP/6-311+G(d,p) level of theory. The effect of solvent polarity on electronic absorption spectra was investigated using Time dependent-DFT. The thermodynamic parameters such as heat capacity, entropy and enthalpy were also investigated at different temperatures. Moreover, photophysical studies were performed to examine the effect of solvent polarity and viscosity. A positive solvatochromism and a 7 fold fluorescence enhancement were observed with the increase of solvent polarity and viscosity, respectively. The compound exhibit aggregation-induced emission in THF:H2O system. The binding interaction of H1 with BSA and its mechanism has been investigated using UV-visible absorption and fluorescence quenching techniques. The thermodynamics of binding suggest the presence of spontaneous hydrogen bonding interactions, as –ve values of ΔG, ΔS and ΔH were observed. The experimental results obtained from BSA binding studies were validated through in silico molecular docking and dynamics techniques. Molecular modelling studies perfectly corroborate the experimentally obtained results.

Synthesis of a dual-emissive pyrene-based fluorescent probe for imaging intracellular viscosity

A pyrene-based fluorescent molecular probe (Py2-5EG) was synthesized for optical detection of viscosity variation. Py2-5EG was synthesized by the reaction of two hydroxyl groups at the two ends of pentaethylene glycol with pyrene via a two-step reaction. Unlike typical pyrene compounds, the monomer fluorescence of Py2-5EG was not observed even in a dilute solution. Instead, the monomer fluorescence gradually increased with the increase in solvent viscosity. Blue fluorescent emission was observed from the monomer in a more viscous medium, and greenish blue emission from excimer was dominant in a less viscous medium in which dynamic excimer formation was involved. The observed fluorescence change of Py2-5EG was ratiometric and depended on the solution viscosity. A microscopic change in cellular viscosity was clearly observed inside and outside the mitochondria of living cells via different fluorescence colors of blue and green. The viscosity inside and outside mitochondria was determined using the fluorescence colors. Py2-5EG featured noticeable fluorescence emission shift in response to viscosity, and this phenomenon can be applied as a fluorescent probe, estimating the viscosity of various solutions as well as inside living cells.

Impact of Glucose on the Nanostructure and Mechanical Properties of Calcium-Alginate Hydrogels.

Alginate is a polysaccharide obtained from brown seaweed that is widely used in food, pharmaceutical, and biotechnological applications due to its versatility as a viscosifier and gelling agent. Here, we investigated the influence of the addition of glucose on the structure and mechanical properties of alginate solutions and calcium-alginate hydrogels produced by internal gelation through crosslinking with Ca2+. Using 1H low-field nuclear magnetic resonance (NMR) and small angle neutron scattering (SANS), we showed that alginate solutions at 1 wt % present structural heterogeneities at local scale whose size increases with glucose concentration (15–45 wt %). Remarkably, the molecular conformation of alginate in the gels obtained from internal gelation by Ca2+ crosslinking is similar to that found in solution. The mechanical properties of the gels evidence an increase in gel strength and elasticity upon the addition of glucose. The fitting of mechanical properties to a poroelastic model shows that structural changes within solutions prior to gelation and the increase in solvent viscosity contribute to the gel strength. The nanostructure of the gels (at local scale, i.e., up to few hundreds of Å) remains unaltered by the presence of glucose up to 30 wt %. At 45 wt %, the permeability obtained by the poroelastic model decreases, and the Young’s modulus increases. We suggest that macro (rather than micro) structural changes lead to this behavior due to the creation of a network of denser zones of chains at 45 wt % glucose. Our study paves the way for the design of calcium-alginate hydrogels with controlled structure for food and pharmaceutical applications in which interactions with glucose are of relevance.

Liquid Transfer for Viscoelastic Solutions.

Viscoelastic liquid transfer from one surface to another is a process that finds applications in many technologies, primarily in printing. Here, cylindrical-shaped capillary bridges pinned between two parallel disks are considered. Specifically, the effects of polymer mass fraction, solution viscosity, disk diameter, initial aspect ratio, final aspect ratio, stretching velocity, and filling fraction (alike contact angle) are experimentally investigated in uniaxial extensional flow. Both Newtonian and viscoelastic polymer solutions are prepared using polyethylene glycol and polyethylene oxide, with a wide variety of mass fractions. The results show that the increase in polymer mass fraction and solvent viscosity reduces the liquid transfer to the top surface. Moreover, the increase in the initial and final stretching heights of the capillary bridge also decreases the liquid transfer for both Newtonian and viscoelastic solutions. Finally, the shape of the capillary bridge is varied by changing the liquid volume. Now, Newtonian and viscoelastic solutions exhibit opposite behaviors for the liquid transfer. These findings are discussed in terms of interfacial shape instability and gravitational drainage.

Ultrafast Excited-State Dynamics in trans-(N-Heterocyclic carbene)platinum(II) Acetylide Complexes.

This study probes femto- and picosecond excited-state dynamics of a series of N-heterocyclic carbene (NHC) ligand-containing platinum(II) complexes of the type trans-(NHC)2PtII(CC-Ar)2, where CC-Ar is an arylacetylide. By using femtosecond transient absorption spectroscopy, two dynamic processes are observed: an ultrafast singlet → triplet intersystem crossing (<0.3 ps), followed by geometric/electronic relaxation that takes place on a 2-10 ps time scale. The geometric/electronic relaxation is attributed to ligand torsional modes, mainly arising from twisting of the aryl units relative to the square-planar PtL4 unit. The dynamics of this relaxation process depend somewhat on steric constraints induced by substituent groups attached to the (benz)imidazole and phenyl ligands. The geometric relaxation dynamics slow with increasing solvent viscosity. The experimental studies also reveal that the different conformers can be photoselected by varying the excitation at different near-UV wavelengths. To corroborate the experimental findings, density functional theory calculations were conducted to probe the effects of geometry and steric hindrance on the ground-state energy surface. The calculations suggest that the barrier for torsion of the CC-Ar units increases as N-substituents on the NHC ligands increase in the order CH3 < cyclohexyl < n-butyl and as the CC-Ar units are substituted in the 3 and 5 positions with tert-butyl groups.

Non-Aqueous Solvents Unlikely to Present Any Cost Benefit for Post-Combustion CO2 Capture

A water-lean solvent for post-combustion CO2 capture was techno-economically assessed and compared with its parent water-based solvent. At a first glance, the use of nonaqueous solvents, appears to offer lower energy requirements and reduced solvent degradation. However, it is necessary to look at the full picture to avoid rushing to develop a concept that may not deliver all that it promises. A nonaqueous solvent technology requires less regeneration energy compared with using standard aqueous amine solvents, especially if combined with a flash vessel instead of a regeneration column, and has a lower amine degradation rate compared with the aqueous solvent. However, using a nonaqueous formulation offers important technical challenges in terms of operability (increased solvent viscosity) and health, safety and environmental risks (higher amine emissions). The economic evaluation shows a hypothetical reduction of 15% in unit total cost based on savings in energy consumption and solvent refill. However, implementing measures to mitigate viscosity changes and amine emissions nullifies these gains. Therefore, no economic improvement over the benchmark is likely from using the nonaqueous version of a solvent. It is expected that the same risks apply for other nonaqueous technologies, so it is very unlikely that a magic bullet will be found to make nonaqueous solvents an attractive option for CO2 capture.

Diffusion of Polymethylene Chain Molecules in Nonpolar Solvents.

The translational diffusion constants, D, for solutes with polymethylene chains are compared with the predictions of a hydrodynamic bead model based on Kirkwood-Riseman theory. The solute-solvent combinations include (a) n-alkanes in n-alkanes; (b) n-alkanes in benzene, toluene, tetralin, decalin, and CCl4; (c) 1-alkenes in n-alkanes; (d) 1-phenylalkanes in n-alkanes; and (e) 1-phenylalkanes in isocetane (2,2,4,4,6,8,8-heptamethylnonane), pristane (2,6,10,14-tetramethylpentadecane), and squalane (2,6,10,15,19,23-hexamethyltetracosane). The bead model gives good overall agreement with an average difference of less than 3% between 207 experimental and calculated diffusion constants that include published data as well as new D values determined for 1-alkenes and 1-phenylalkanes using capillary flow techniques. The calculated values are obtained using chain element (bead) radii that decrease as the solvent viscosity increases. The bead model's results are comparable to those obtained using cylinder and lollipop diffusion for many of the same solute-solvent systems; the three models are compared and discussed. The results for the 1-alkenes and n-alkanes in the n-alkanes are the first from a Kirkwood-Riseman analysis in a homologous series of solvents.

Discovering a rotational barrier within a charge-transfer state of a photoexcited chromophore in solution.

Methylation occurs in a myriad of systems with protective and regulatory functions. 8-methoxypyrene-1,3,6-trisulfonate (MPTS), a methoxy derivative of a photoacid, serves as a model system to study effects of methylation on the excited state potential energy landscape. A suite of spectroscopic techniques including transient absorption, wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS), and fluorescence quantum yield measurements via steady-state electronic spectroscopy reveal the energy dissipation pathways of MPTS following photoexcitation. Various solvents enable a systematic characterization of the H-bonding interaction, viscosity, and dynamic solvation that influence the ensuing relaxation pathways. The formation of a charge-transfer state out of the Franck–Condon region occurs on the femtosecond-to-picosecond solvation timescale before encountering a rotational barrier. The rotational relaxation correlates with the H-bond donating strength of solvent, while the rotational time constant lengthens as solvent viscosity increases. Time-resolved excited-state FSRS, aided by quantum calculations, provides crucial structural dynamics knowledge and reveals the sulfonate groups playing a dominant role during solvation. Several prominent vibrational motions of the pyrene ring backbone help maneuver the population toward the more fluorescent state. These ultrafast correlated electronic and nuclear motions ultimately govern the fate of the photoexcited chromophore in solution. Overall, MPTS in water displays the highest probability to fluoresce, while the aprotic and more viscous dimethyl sulfoxide enhances the nonradiative pathways. These mechanistic insights may apply robustly to other photoexcited chromophores that do not undergo excited-state proton transfer or remain trapped in a broad electronic state and also provide design principles to control molecular optical responses with site-specific atomic substitution.

Kinetic Studies of Photopolymerization of Monomer‐Containing Deep Eutectic Solvents

AbstractThe kinetics of photopolymerization in deep eutectic solvents (DESs) is investigated through real‐time Fourier transform‐infrared spectrometry (RT‐FTIR). The systems of this study are choline chloride (ChCl) DESs made of acrylic acid and methacrylic acid (DES monomers) and a nonreactive analog isobutyric acid (IBA). The DES consisting of the analog also contains the monomer methyl methacrylate (MMA). Analysis of the evolution of conversion indicates dramatic increase in polymerization rate when comparing the polymerization of the DES monomers to pure monomer polymerization. A significant increase in polymerization rate is also seen in the systems that include a methyl ester derivative of the hydrogen bond donor monomer (e.g., MMA) in a nonpolymerizable DES (e.g., IBA−ChCl). It is proposed that the increased solvent viscosity, preorganization due to hydrogen bonding, and the polarity of the DES around the monomers play a role in the enhancement of the polymerization rate.

Molecular dynamics study of a solvation motor in a Lennard-Jones solvent.

The motions of a solvation motor in a Lennard-Jones solvent were calculated by using molecular dynamics simulation. The results were analyzed considering the large spatial scale effects caused by the motion of the solvation motor. A reaction site was located on the surface of the solvation motor and the attraction between the reaction site and the solvent molecules was varied for 100 fs. The motion of the motor was driven by solvation changes near the reaction site on the motor. Two finite-size effects were observed in the motion. One was the hydrodynamic effect and the other was the increase in solvent viscosity caused by heat generation. The latter affected not only the displacement of the motor caused by the reaction but also the wave propagation phenomena. Both effects reduced the motor displacement. Heat generation affects the displacement, in particular for small systems. By contrast, the hydrodynamic effect remained even for large systems. An extrapolation method was proposed for the displacement.

Diffusion of Radical Ions in Ionic Liquids Having Long Alkyl Chains.

Magnetic field effects on a radical ion pair have been studied to investigate the diffusion of the radical ions generated by a photoinduced electron transfer reaction in ionic liquids having short and long alkyl chains. The yield of an escaped radical ion was evaluated by using a nanosecond laser flash photolysis under various magnetic fields. The magnitude of the magnetic field effect on the yield of the escaped radical was linearly increased with increasing solvent viscosity. Such solvent viscosity dependence of the magnetic field effect can be explained with the solvent viscosity dependence of the escape rate of the radical ions from the pair. In the time window (>20 ns) of our measurements, the effect of long alkyl chain aggregation on the dynamics of the radical ions was not clearly observed.

Study of electron transfer process in aqueous methanol system by using tryptophan based short peptide – Amino acid pairs

The present study demonstrated the nature of synergistic solvation behaviour in water – methanol binary solvent mixture(s) and its role on photo-induced electron transfer (PET) process during the interactions of different tryptophan based peptide conjugates with selected non-aromatic amino acids and also with standard coumarins – amines pairs. The highest degree of solvation was obtained in water rich region near Xwater = 0.85 with ETN(30) value 1.05, which was observed more compared to both pure water (1.00) and methanol (0.76). An analytical solvent exchange model was used to estimate the extent of synergistic solvation and found ∼2 times stronger than the preferential solvation efficiency. Such unconventional associative solvation behaviour was confirmed by fluorescence study of the solvatochromic probe which suggested that the weak nature of intermolecular interaction is responsible for such behaviour. The selected pairs of different amino acids as donors, and established pre-synthesized new tryptophan based peptide conjugates as acceptors were used to check the PET event in the polar environment(s) to know the reorganization energy of the binary solvent mixture. An inverted region was observed and estimated the value of reorganization energy (λ) in Xwater = 0.85. The values of λ in water, methanol and Xwater = 0.85 solution were further used to determine its dependency on solvent parameters. Interestingly, our observations revealed that the solvent viscosity depicted linear dependence with reorganization energy (λ) where the value of λ increases with increasing solvent viscosity. Therefore, the photo-physical event i.e. PET, in present solvent system, is significantly dependent on the physical properties of polar solvents.

Estimation of fluorescence Quantum yield of a biologically active boronic acid-Analysis of Specific interactions with alcohols and non-alcohols

The efficient emission of fluorescence intensity by solutes is characterized in terms of what is known as the Quantum yield. Quantum yield gives the probability of the excited state being deactivated by fluorescence rather than by another, non-radiative mechanism. Knowledge of quantum yield gives better insight about various aspects like; structural changes induced by solvent polarity, solute-solvent interactions, their use as standards etc.The fluorescence quantum yield (Φ) of 2-methoxypyridin-3-yl-3-boronic acid [2MPBA] is evaluated using single point method. The value of Φ , radiative (kr) and non-radiative (knr) decay constants are calculated in non-alcohols and alcohols. The value of absorbance (OD) is decreasing with increase in solvent viscosity and it is attributed to the protonation of the excited state of fluorophores in the solvent. Also, it is observed that integrated fluorescence intensity (Fint) is more in solvents of high viscosity which is quite expected since solute molecules are slightly rigid in high viscous media. Φ increases from 31.7% in octanol to 78.3% in ethanol with a small exception for methanol. The higher quantum yield of 2MPBA is may be due to the presence of methoxypyridin group. The higher Φ values in alcohols are further supported by higher kr values and lesser knr values. The higher value of Φ for 2MPBA allows one to use it as a standard reference, for the estimation of relative quantum yield of other boronic acid derivatives

Equilibrium folding dynamics of meACP in water, heavy water, and low concentration of urea

Many proteins fold in apparent two-state behavior, as partially folded intermediates only transiently accumulate and easily escape detection. Besides a native form and a mainly unfolded form, we captured a partially unfolded form of an acyl carrier protein from Micromonospora echinospora (meACP) in the folding/unfolding equilibrium using chemical exchange saturation transfer NMR experiments. The C-terminal region of the partially unfolded form is mainly folded and the N-terminal is unfolded. Furthermore, to understand how the folding process of meACP is influenced by solvent environments, we compared the folding dynamics of meACP in D2O, H2O and low concentration of urea. As the environment becomes more denaturing from D2O to H2O and then to urea, the unfolded state becomes increasingly populated, and the folding rate decreases. Adding a small amount of urea, which does not change solvent viscosity, has little effects on the unfolding rates, while changing H2O to D2O reduces the unfolding rates possibly due to the increase of solvent viscosity. The quantified solvent effects on the protein folding Gibbs energy and activation energy suggest that the transition state of folding may have a similar structure to the native state of the protein.

Thermal diffusivity and lifetime studies of Styryl 7 dye on DNA-CTMA complex

This study reports the sensing of micro viscosity variation of Styryl 7(LDS 750) dye in different solvents and biological samples through thermal lens and fluorescence lifetime techniques. Fluorescence properties of Styryl 7 dye in different solvents and with DNA-CTMA (deoxyribonucleic acid attached with cetyltrimethyl-ammonium cationic surfactant molecules) were measured using fluorescence lifetime technique. Fluorescence lifetime of Styryl 7 in different solvents are found to increase with the viscosity of the samples. With increase in weight percentage of DNA-CTMA, the fluorescence lifetime increases from 0.17ns to1.47ns. Lifetime studies confirm that Styryl 7 dye is a good fluorescent probe for detecting the micro viscosity of the samples. Thermal lens technique, which is an efficient probe to measure non-radiative relaxation route were used to measure the thermal diffusivity of Styryl 7 in different solvents and with varying weight percentage of DNA-CTMA. Thermal diffusivity of the samples are found to decrease with increase in solvent viscosity and increased weight percentage of DNA-CTMA

Enhanced antitumor effect of poly(L-glutamic acid)-rose bengal conjugate nanoparticle

The photophysical properties of two N-salicylidene p-X-anilines (XCN, COOH) were studied in a series of solvents with varying polarity and proticity by steady state absorption and emission spectroscopy. The two compounds are dually fluorescent in solution due to an excited state proton transfer. The solvatochromic shifts were used to connect the solvent polarity parameter to the dipole moment in excited state using several approaches. The multiparameter Kamlet–Taft and Catalán solvent scales with four parameters have been employed to analyze the specific and non-specific solvent–solute interactions. The absorption spectra are less sensitive to the substitution at the aniline ring and to the solvent properties than the fluorescence spectra. Especially the ratio between the normal and abnormal Stokes shifted fluorescence intensities is dependent on the polarity and hydrogen bonding abilities of the solvent. The increasing solvent viscosity led to a decrease in the ratio of the intensities of the two emissions.