Solvatochromic Plots Research Articles

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 (Tg) relaxations in PMMA.

Tuning Photoinduced Intramolecular Electron Transfer by Electron Accepting and Donating Substituents in Oxazolones

The solvatochromic and spectral properties of oxazolone derivatives in various solvents were reported. Fluorescence spectra clearly showed positive and negative solvatochromism depending on substituents. The solvatochromic plots and quantum chemical computations at DFT-B3LYP/6-31 + G(d,p) level were used to assess dipole moment changes between the ground and the first excited singlet-states. The electron accepting nitro substituent at the para-position increased the π-electron mobility, however, the 3,5-dinitro substituent decreased the π-electron mobility as a result of inverse accumulation of the electronic density as compared with that of its ground state. Experimental and computational studies proved that the photoinduced intramolecular electron transfer (PIET) is responsible for the observed solvatochromic effects. We demonstrate that PIET can be finely tailored by the position of the electron accepting and donating substituents in the phenyl ring of the oxazolone derivatives. We propose that the photoactive CPO derivatives are new molecular class of conjugated push-pull structures using azlactone moiety as the π-conjugated linker and may find applications in photovoltaic cells and light emitting diodes.

Development of a quinoxaline-based fluorescent probe for quantitative estimation of protein binding site polarity

The compound 2-[(1E)-2-(1H-pyrrol-2-yl)ethenyl]-quinoxaline (PQX) is a promising fluorescent chromophore for the estimation of protein binding site polarity, due to its full-color solvatochromic fluorescence. A linear relationship was obtained between the peak emission wavenumber and E(T)(N) (normalized solvent polarity). The BSA binding site polarity was estimated from the solvatochromic plot.

2-Amino-7-nitro-fluorenes in Neat and Mixed SolventsOptical Band Shapes and Solvatochromism

Femtosecond transient absorption spectroscopy of amino-nitro-fluorenes in the UV-visible range shows that the dynamic Stokes shift of the emission band is sensitive to infrared-active modes of the solvent. Bandshapes for stationary absorption and emission are needed to quantify the observed spectral evolution. They are reported for 2-amino-7-nitro-fluorene (ANF), 2-dimethylamino-7-nitro-fluorene (dM-ANF), and 2-di(n-butyl)amino-7-nitro-9-di(n-propyl)-fluorene (dBdP-ANF) in a variety of solvents. Bands broaden systematically with increasing solvent polarity. This effect is taken into account in an improved location of band positions. The resulting solvatochromic plots differ significantly from those that use peak positions of absorption spectra and fluorescence quantum distributions. Absorption spectra were also measured in aqueous solvent mixtures, and shifts are described by binding curves for hydrogen bonding and stepwise solvent exchange.

Tuning the properties of M(diimine)(dithiolate) complexes – The role of the metal and solvent effect. A combined experimental, DFT and TDDFT study

A combined experimental and DFT/TDDFT study on a series of seven M(diimine)(dithiolate) complexes of Group VIII metals has been performed. This analysis focuses mainly on three aspects: (a) the role of the metal, (b) the connection of NLO properties, through the intrinsic hyperpolarizability, with the slope of solvatochromic plots and (c) the effect of solvation on the valence orbitals of the complexes. Besides, the molecular and the electronic structure as well as the bonding scheme of these complexes are also explored. The three aforementioned aspects were not satisfactorily clarified in the literature up to now. Moreover, as it is proved, they possess a central role in the experimental design of the reactions in which these complexes take place or in the design of the synthesis of compounds with pre-required properties.

Fluorescence monitoring of photoinitiated polymerization reactions: Synthesis, photochemical study and behaviour as fluorescent probes of new derivatives of 4′-dimethylaminostyryldiazines

The fluorescence emission of three new fluorescent probes has been studied in environments of different polarity and viscosity. The diazine moieties behave as electron acceptor and a good correlation has been found between their reduction potentials and the dipole moments in the excited state (estimated from solvatochromic plots using Lippert equations). The probes have shown to be sensitive to changes in both microviscosity and micropolarity of the surroundings. They have been used as fluorescent sensors to monitor the photopolymerization reactions of some mono- and difunctional methacrylate monomers in bulk. The fluorescence emission band of the probes showed an increase in intensity, as well as an hypsochromic shift, as the degree of conversion increases, throughout the entire polymerization range in the different systems checked.

Photoinduced intramolecular charge transfer to meta position of benzene ring in 6-aminophthalides

Substitution of non-fluorescent phthalide (Pd) with amino group at meta (6) position in relation to the electron-accepting part of the lactone ring completely changes Pd photophysics: a new long-wavelength absorption band arises and the molecule becomes highly fluorescent. The experimental data and the analysis of vertical electronic transitions with TDDFT method indicate that the first absorption band in 6-aminophthalides (6-APds) comprises a single CT transition to the S1 state. Almost equal absorption and emission transition dipole moments indicate that S0 <--> S1 transition in all 6-APds is not affected by any mixing with other electronic states, the excited-state vibrational relaxation is not accompanied by significant conformational changes and the Stokes shifts reflect mainly solvation energetics of these molecules. Excited state dipole moments obtained from solvatochromic plots and from CASSCF calculations confirm large charge displacement from amino group towards the meta position of the benzene ring upon excitation of 6-APds to S1 state. Long fluorescence lifetimes and high fluorescence quantum yields demonstrate efficient and stable excited state charge separation in 6-APds. Taken together with sensitivity of 6-APds to polarity and proticity of the environment these properties make them good candidates for fluorescent probes of long-time scale molecular dynamics.

Dual emission of chalcone-analogue dyes emitting in the red region

Abstract The photophysical properties of new synthesized chalcones namely; 1-(4′-R-phenyl)-5-(4′-dimethylaminophenyl)-2,4- pentadien-1-one, [R=H (1), Cl (2) and OCH3 (3)] were studied in different solvents by using steady-state absorption and emission spectroscopy. The fluorescence spectra of these chalcones exhibit dual emission in medium and polar solvents. The dual emission was attributed to population of a polar locally excited (LE) state and a highly dipolar intramolecular charge transfer (ICT) state. The changes in dipole moments upon excitation were calculated from the solvatochromic plots. The total fluorescence quantum yields (φf) were also determined, and their values are strongly dependent on the nature of substitutent and the solvent polarity. Semiempirical molecular orbital calculations using the atom superposition and electron delocalization molecular orbital (ASED-MO) method were also performed to investigate the molecular and electronic structures of these chalcones in both the ground and excited state. The change of the dipole moment upon excitation was explained on the basis of changes in the charge redistribution over the whole skeleton of the molecules, which agree well with the experimental results. Also, the nature and energy of the electronic transitions were elucidated and discussed in relation to the experimental data.

Dual Fluorescence of Phenyl and Biphenyl Substituted Pyrene Derivatives

The photophysical properties of several acceptor substituted 1-arylpyrene derivatives were investigated. The fluorescence spectra strongly depend on the nature of the aryl moiety and the position and number of methoxycarbonyl acceptor groups. Dual fluorescence, originating from a locally excited and a charge transfer state, was observed for the diester derivatives. The solvent dependence of the dual fluorescence and the slightly curved solvatochromic plots indicate a change of the character of the excited states from solvents of low to high polarity. The rate constants for fluorescence and nonradiative decay were calculated to reveal the nature of the excited-state relaxation, that is, the increase of the mesomeric interactions by geometrical flattening or stabilization of the CT state by further twisting toward perpendicularity. Flattening is the major relaxation pathway of the diester phenylpyrene derivative in nonpolar solvents whereas in highly polar solvents the low value for the fluorescence transit...

Preferential solvation of solvatochromic benzothiazolinic merocyanines in mixed binary solvents

The preferential solvation of α- and γ-oxobenzothiazolinic merocyanines was studied using UV-visible spectroscopy in binary solvent mixtures containing either water or methanol and cosolvents (formamide, ethanol, 2-propanol, DMF, CH 3CN and 1,4-dioxane). In binary mixtures containing water, the dyes are preferentially solvated by the organic cosolvent over the whole composition range, only in the case of the α-oxo derivative in H 2O–CH 3CN mixtures the solvatochromic plot showed a biphasic behavior. In binary solvent mixtures comprising MeOH, the dyes showed no preference for either solvents in MeOH–EtOH mixtures, while in MeOH–CH 3CN and 1,4-dioxane–MeOH mixtures the former solvent was the preferred one. Also, the results showed that γ-oxo derivative is preferentially solvated by the cosolvent in both MeOH-2-propanol and MeOH–DMF mixtures. Although the α-oxo analogue showed an ideal behavior in MeOH-2-propanol mixtures as well as in the MeOH rich region in MeOH–DMF mixtures, it is solvated preferentially by MeOH in the DMF rich side. Also, the role of solute-solvent and solvent-solvent interactions in determining the preferential solvation was discussed.

Photochemistry of Locally Excited and Intramolecular Charge Transfer States of (E)-4-Cyano-4‘-(pentamethyldisilanyl)stilbene

The photophysical properties and photochemistry of (E)-4-cyano-4‘-(pentamethyldisilanyl)stilbene ((E)-3) and the corresponding monosilylated stilbene (E)-5 have been studied. Solvatochromic plots of (E)-3 show separate slopes for ether solvents and halocarbons. The emission in ethers is assigned to an LE state of similar polarity to that of (E)-5. In halocarbons a more polar intramolecular charge transfer (CT) state apparently contributes to the emission. This component is completely quenched by nucleophilic ethers. In contrast to (E)-5, Φf and Φ(E,Z) of (E)-3 strongly decrease in CH3CN, consistent with the solvent polarity induced LE → CT process and nucleophilic quenching of the CT state by solvent. Alcohols also quench Φf and Φ(E,Z) with similar Stern−Volmer constants, linking the two processes to the same state (LE). Such quenching results in formation of (E)-4‘-(hydrodimethylsilyl)-4-cyanostilbene (4) by regioselective nucleophilic cleavage of the Si−Si bond, according to deuterium labeling studies. ...

Photochemistry of Locally Excited and Intramolecular Charge Transfer States of a Disilane Bearing a Cyclic Arylethenyl Electron Acceptor Substituent

1-(p-cyanophenyl)-2-(pentamethyldisilanyl)cyclopentene (1) exhibits a fluorescence at λ>400 nm, which strongly shifts to the red with increasing polarity of the solvent. The corresponding solvatochromic plot is indicative of an emissive, lowest energy charge-transfer (CT) state. Stern-Volmer quenching of the fluorescence by dilute methanol in pentane gives a linear plot with a unit intercept and a slope k Q τ ct =0.70. Quenching is accompanied by formation of (arylcyclopentenyl)dimethylsilane 7, the major product observed upon photolysis of 1 in the presence of methanol. This is evidenced by the linear double-reciprocal plot of quantum yields of 7 versus methanol concentration, which gives a k Q τ ct value in agreement with fluorescence quenching