Solvatochromic effects in the electronic absorption and nuclear magnetic resonance spectra of hypericin in organic solvents and in lipid bilayers

The natural product hypericin was tested in recent years as a biological photosensitizer with a potential for viral and cellular photodamage. We thus studied extensively its spectroscopy and membrane partitioning. Absorption, fluorescence excitation and emission spectra of the sodium salt (HyNa) were measured in 36 protic and aprotic, polar and apolar, solvents. Electronic transition bands as well as vibrational progressions were identified. Aggregation in some nonpolar solvents and protonation in organic acids were demonstrated. Modeling solvatochromism was done by Lippert equation, by the ET(30) parameter and by the Taft multiparameter approach. In all cases, separation into protic and aprotic solvents gave much better fits to the models. 13C chemical shift data could also be correlated with solvent polarity. They correlated best with Lippert's delta f polarity measure, but tended to fall into two distinct solvent groups--each along different lines--corresponding to protic and aprotic media, respectively. This interesting phenomenon suggests that in the case of the charged and slightly water soluble HyNa, two mechanisms of solvation are involved, each resulting in its own line equation. In aprotic media, dipole-dipole interaction is the predominant solvation mechanism. In protic solvents, the most effective means of solvation is likely to be hydrogen bonding. When intercalated into the liposomal phospholipid bilayer, HyNa is oriented at an angle to the interface, thus experiencing a gradient of solvent polarities: a highly polar environment (similar to methanol) for C-2/5, suggesting that they lie not far from the interface; a moderately polar environment (similar to that of n-propanol) for C-6a/14a, which are somewhat deeper within the bilayer; and a more lipophilic environment (akin to n-hexanol) for C-10/11. The fluorescence excitation peak in liposomes also correlates with an aprotic medium of relatively high polarity, as might be excepted from a molecule in a shallow position in the bilayer.     

The styrylpyridine dye for the silane sol-gel transition studies by time-dependent fluorescence

A trans-4-(p-N,N-dimethylaminostyryl)-N-vinylbenzylpyridinium chloride (vbDMASP) fluorescence probe was optimized in ground and excited state as a function of change in the microenvironment polarity, using the Amsol HyperChem program package. In the calculations, protic and aprotic solvents were used. On this basis a change in the molecule geometry after excitation, depending on the surrounding solvent, was determined. Absorption and steady-state fluorescence spectra of vbDMASP in the solvent of different polarity and in the model water-glycerol solutions were also recorded. On the basis of Stokes' shift change with the Onsager polarity scale a change in the dipole moment of the probe during transition from ground to excited state, in protic and aprotic solvents was determined. Since during the sol-gel transition of tetraethylorthosilane in the acidic environment both polarity and viscosity of the microenvironment change the vbDMASP probe was applied and fluorescence time-resolved measurements were done. On this basis the correlations between the results of time-resolved measurements for the multichromophoric probe applied in the gelation process and molecular optimization data are discussed.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXI. Solvent Effect on the Rate of 1-Methyl-1-chlorocyclopentane Heterolysis. Correlation Analysis of Solvation Effects

Heterolysis of 1-methyl-1-chlorocyclopentane in protic and aprotic solvents occurs by the E1 mechanism. The reaction rate in aprotic solvents or in a set of protic and aprotic solvents is satisfactorily described by the parameters of the polarity and electrophilicity or ionizing power of the solvents. In protic solvents, the reaction rate grows with increasing polarity or ionizing power of the solvent and decreases with increasing nucleophilicity.     

Solvent polarity induced structural changes in 2,6-diamino-9,10-anthraquinone dye

Photophysical properties of 2,6-diamino-9,10-anthraquinone (2,6-DAAQ) dye have been investigated in different solvents and solvent mixtures. The fluorescence quantum yields, fluorescence lifetimes, radiative rate constants, nonradiative rate constants and absorption and fluorescence spectral characteristics show unusual deviations in the lower polarity aprotic solvents in comparison to those in other aprotic solvents of medium to higher polarities. The results indicate that the dye exists in different structural forms in the lower and in the medium to higher polarity solvents. Drawing an analogy with the results reported for other amino-substituted dyes, it is inferred that 2,6-DAAQ dye adopts a planar intramolecular charge transfer (ICT) structure in medium to higher polarity solvents, where the amino lone pairs are in good resonance with the anthraquinone pi-cloud. In the lower polarity solvents, however, the dye is inferred to exist in a nonplanar structure where the amino lone pairs are not in good resonance with the anthraquinone pi-cloud. Due to these structural differences, the dye displays significantly different photophysical behavior in the lower polarity solvents than in the other solvents of medium to higher polarities. Supportive evidence for the above structural changes has been obtained from ab initio quantum chemical calculations on the structures of the dye under different conditions. Unusual deviations in the photophysical properties of 2,6-DAAQ dye in protic solvents in comparison to those in aprotic solvents of similar polarities are attributed to the intermolecular hydrogen bonding effect involving the OH groups of the protic solvents and the quinonoid oxygens of the dye.     

The styrylpyridine dye for the silane sol–gel transition studies by time-dependent fluorescence

A trans-4-(p-N,N-dimethylaminostyryl)-N-vinylbenzylpyridinium chloride (vbDMASP) fluorescence probe was optimized in ground and excited state as a function of change in the microenvironment polarity, using the Amsol HyperChem program package. In the calculations, protic and aprotic solvents were used. On this basis a change in the molecule geometry after excitation, depending on the surrounding solvent, was determined.Absorption and steady-state fluorescence spectra of vbDMASP in the solvent of different polarity and in the model water–glycerol solutions were also recorded. On the basis of Stokes’ shift change with the Onsager polarity scale a change in the dipole moment of the probe during transition from ground to excited state, in protic and aprotic solvents was determined.Since during the sol–gel transition of tetraethylorthosilane in the acidic environment both polarity and viscosity of the microenvironment change the vbDMASP probe was applied and fluorescence time-resolved measurements were done. On this basis the correlations between the results of time-resolved measurements for the multichromophoric probe applied in the gelation process and molecular optimization data are discussed.     

Solute–Solvent Interactions of Methyl Violet in Different Solvents on Spectral Data

Spectroscopic studies of Methyl violet in protic (water, methanol, ethanol, isopropanol and n-butanol) and aprotic solvents (acetone, DMF) were carried out. UV-Visible absorption spectra of Methyl violet in protic solvents showed a hypsochromic shift, as the solvent polarity was changed from less polar to more polar, while a bathochromic shift was observed for aprotic solvents. Transition energy of Methyl violet in different solvents was correlated with solvatochromic parameters to study solute–solvents interactions. The Kamlet–Taft, Catalan and unified scale models were applied to investigate interactions between Methyl violet and solvents. The best agreement is found for the Catalan model.     

Effects of Hydrogen-bonding Interaction and Polarity on Emission Spectrum of Naphthalene-Triethylamine in Mixed Solvent

The effects of the protic and aprotic polar solvents on the emission spectrum of the naphthalene-triethyl-amine system in THF were studied under conditions of steady-state illumination. The fluorescence spectrum of the naphthalene-triethylamine system consists of two emission bands, the fluorescence band of naphthalene (band A, 329 nm) and the emission band of the exciplex(band B, 468 nm). The intensities of both the emission bands decrease with increasing the solvent polarity. The intensity of band B also decreases due to the hy-drogen-bonding interaction between triethylamine and protic solvent, while that of band A increases. It is thus suggested that the quenching of naphthalene fluorescence by triethylamine in THF occurs through the charge transfer and electron transfer reactions. The spectral changes upon the increase of solvent polarity can be explained by the dependences of the equilibrium constant between exciplex and ion-pair and the rate constant for the electron transfer reaction from triethyl.amine to the excited naphthalene on the rel.ative permittivity of solvent. It is shown that the formation of intermolecular hydrogen-bonding between triethylamine and protic solvent suppresses the quenching reaction by the decrease in free amine. Acetonitrile has only a polar effect and trichloroacetic acid only a hydrogen-bonding(or protonation) effect, while alcohols have both the effects. The effects of alcohols could be separated into the effects of solvent polarity and intermolecular hy-drogen-bonding interaction quantitatively.     

Photophysical properties of coumarin-7 dye: role of twisted intramolecular charge transfer state in high polarity protic solvents

Photophysical studies on coumarin-7 (C7) dye in different protic solvents reveal interesting changes in the properties of the dye on increasing the solvent polarity (Deltaf; Lippert-Mataga solvent polarity parameter) beyond a critical value. Up to Deltaf approximately 0.31, the photophysical properties of the dye follow good linear correlations with Deltaf. For Deltaf > approximately 0.31, however, the photophysical properties, especially the fluorescence quantum yields (Phi(f)), fluorescence lifetimes (tau(f)) and nonradiative rate constants (k(nr)), undergo large deviations from the above linearity, suggesting an unusual enhancement in the nonradiative decay rate for the excited dye in these high polarity protic solvents. The effect of temperature on the tau(f) values of the dye has also been investigated to reveal the mechanistic details of the deexcitation mechanism for the excited dye. Studies have also been carried out in deuterated solvents to understand the role of solute-solvent hydrogen bonding interactions on the photophysical properties of the dye. Observed results suggest that the fluorescence of the dye originates from the planar intramolecular charge transfer (ICT) state in all the solvents studied and the deviations in the properties in high polarity solvents (Deltaf > approximately 0.31) arise due to the participation of a new deexcitation channel associated with the formation of a nonfluorescent twisted intramolecular charge transfer (TICT) state of the dye. Comparing present results with those of a homologous dye coumarin 30 (C30; Photochem. Photobiol., 2004, 80, 104), it is indicated that unlike in C30, the TICT state of the C7 dye does not experience any extra stability in protic solvents compared to that in aprotic solvents. This has been attributed to the presence of intramolecular hydrogen bonding between the NH group (in the 3-benzimidazole substituent) of the C7 dye and its carbonyl group, which renders an extra stability to the planar ICT state, making the TICT state formation relatively difficult. Qualitative potential energy diagrams have been proposed to rationalize the differences observed in the results with C7 and C30 dyes in high polarity protic solvents.     

Synchronous fluorescence spectroscopic study of solvatochromic curcumin dye

Curcumin, the main yellow bioactive component of turmeric, has recently acquired attention by chemists due its wide range of potential biological applications as an antioxidant, an anti-inflammatory, and an anti-carcinogenic agent. This molecule fluoresces weakly and poorly soluble in water. In this detailed study of curcumin in thirteen different solvents, both the absorption and fluorescence spectra of curcumin was found to be broad, however, a narrower and simple synchronous fluorescence spectrum of curcumin was obtained at Δλ=10-20 nm. Lippert-Mataga plot of curcumin in different solvents illustrated two sets of linearity which is consistent with the plot of Stokes' shift vs. the ET30. When Stokes's shift in wavenumber scale was replaced by synchronous fluorescence maximum in nanometer scale, the solvent polarity dependency measured by λSFSmax vs. Lippert-Mataga plot or ET30 values offered similar trends as measured via Stokes' shift for protic and aprotic solvents for curcumin. Better linear correlation of λSFSmax vs. π* scale of solvent polarity was found compared to λabsmax or λemmax or Stokes' shift measurements. In Stokes' shift measurement both absorption/excitation as well as emission (fluorescence) spectra are required to compute the Stokes' shift in wavenumber scale, but measurement could be done in a very fast and simple way by taking a single scan of SFS avoiding calculation and obtain information about polarity of the solvent. Curcumin decay properties in all the solvents could be fitted well to a double-exponential decay function.     

Fluorometric study of solvation characteristics of ketocyanine dyes in mixed binary solvents

The fluorescence characteristics of two ketocyanine dyes have been studied in six mixed binary solvents. Several parameters such as the maximum energy E₁₂(F) of fluorescence, the quantum yield Φ₁₂ of fluorescence and the normalized intensity of the fluorescence have been investigated as functions of solvent composition. In protic+aprotic binary mixtures a dramatic change at the aprotic end has been observed. The results point to a preferential solvation of the solute by the protic component.     

Studies of solvation of ketocyanine dyes in homogeneous and heterogeneous media by UV/Vis spectroscopic method

Solvation characteristics of ketocyanine dyes (I-VI) have been investigated in pure solvents and heterogeneous media by absorption and fluorescence studies. The dyes are good reporters of solvent polarity. In protic solvents they exist as equilibrium mixtures of bare and hydrogen-bonded form in the ground state (S0), the latter being the emitting species. In aprotic solvents of low polarity association of the S1 state of the dye takes place. In aqueous micellar media the dye resides at the micelle water interface. The binding constant for dye-micelle interaction has been determined. Fluorescence data in beta-cyclodextrine solution resemble that for that neutral micellar solution indicating that the interaction between the -OH group of the heterogeneous part (micelle/cyclodextrine cavity) and the carbonyl oxygen of the dye is important in both the cases.     

Ab initio study of the solvent H-bonding effect on ESIPT reaction and electronic transitions of 3-hydroxychromone derivatives

The electronic transitions occurring in 4-(N,N-dimethylamino)-3-hydroxyflavone (DMAF) and 2-furanyl-3-hydroxychromone (FHC) were investigated using the TDDFT method in aprotic and protic solvents. The solvent effect was incorporated into the calculations via the PCM formalism. The H-bonding between solute and protic solvent was taken into account by considering a molecular complex between these molecules. To examine the effect of the H-bond on the ESIPT reaction, the absorption and emission wavelengths as well as the energies of the different states that intervene during these electronic transitions were calculated in acetonitrile, ethanol and methanol. The calculated positions of the absorption and emission wavelengths in various solvents were in excellent agreement with the experimental spectra, validating our approach. We found that in DMAF, the hydrogen bonding with protic solvents makes the ESIPT reaction energetically unfavourable, which explains the absence of the ESIPT tautomer emission in protic solvents. In contrast, the excited tautomer state of FHC remains energetically favourable in both aprotic and protic solvents. Comparing our calculations with the previously reported time-resolved fluorescence data, the ESIPT reaction of DMAF in aprotic solvents is reversible because the emitting states are energetically close, whereas in FHC, ESIPT is irreversible because the tautomer state is below the corresponding normal state. Therefore, the ESIPT reaction in DMAF is controlled by the relative energies of the excited states (thermodynamic control), while in FHC the ESIPT is controlled probably by the energetic barrier (kinetic control).     

Photophysics of representative ketocyanine dyes: dependence on molecular structure

Spectral properties of a new fluorescent ketocyanine dye have been discussed. The energy of maximum absorption/fluorescence of the dye exhibits bathochromic shift with increasing polarity of the medium. Both dipolarity-polarisability and hydrogen bond donation interaction contribute to solvation of the dye. Study of fluorescence parameters points to existence of different emitting states of the dye for aprotic and protic solvents. While the emitting state is the (1)(π, π*) state for aprotic solvents, fluorescence supposedly take place from a different emitting state involving H-bond formation in the excited state in protic solvents. Fluorescence parameters of the dye have been compared with those for a structurally similar symmetric ketocyanine dye. The faster decay of the dye relative to its symmetric counterpart has been explained as due to an increase of nonradiative decay.     

Photophysical Properties of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine

Spectral and photophysical investigations of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine (APT) have been performed in various solvents with different polarity and hydrogen-bonding ability.The emission spectra of APT are found to exhibit dual fluorescence in polar solvents, which attributes to the local excited and intramolecular charge transfer states, respectively. The two-state model is proven out for APT in polar solvent by the time-correlated single photon counting emission decay measurement. Interestingly, the linear relationships of different emission maxima and solvent polarity parameter are found for APT in protic and aprotic solvents, because of the hydrogen bond formation between APT and alcohols at the amino nitrogen N25. Furthermore, the effects of the complexation of the metal ion with tpy group of APT and the hydrogen bond formation between APT with methanol at the terpyridinenitrogen N4—N8—N14 are also presented. The appearance of new long-wave absorption and fluorescence bands indicates that a new ground state of the complexes is formed.     

A novel fluorophore with dual fluorescence: local excited state and photoinduced electron-transfer-promoted charge-transfer state.

Absorption and emission spectra of 9-N,N-dimethylaniline decahydroacridinedione (DMAADD) have been studied in different solvents. The fluorescence spectra of DMAADD are found to exhibit dual emission in aprotic solvents and single emission in protic solvents. The effect of solvent polarity and viscosity on the absorption and emission spectra has also been studied. The fluorescence excitation spectra of DMAADD monitored at both the emission bands are different. The presence of two different conformation of the same molecule in the ground state has lead to two close lying excited states, local excited (LE) and charge transfer (CT), and thereby results in the dual fluorescence of the dye. A CTstate involving the N,N-dimethylaniline group and the decahy droacridinedione chromophore as donor and acceptor, respectively, has been identified as the source of the long wavelength anomalous fluorescence. The experimental studies were supported by ab initio time dependent-density functional theory (TDDFT) calculations performed at the B3LYP/6-31G* level. The molecule possesses photoinduced electron transfer (PET) quenching in the LE state, which is confirmed by the fluorescence lifetime and fluorescent intensity enhancement in the presence of transition metal ions.     

Photophysical Properties of the Cationic Form of Neutral Red

Abstract— Photophysical properties of the cationic form of neutral red (NRH⁺), a phenazine-based dye of biological importance, have been investigated in several protic and aprotic solvents using optical absorption, steady-state and time-resolved fluorescence and picosecond laser flash photolysis techniques. Absorption and fluorescence characteristics of the dye in protic solvents indicate the existence of intermolecular hydrogen bonding between the NRH⁺ and solvent molecules in the ground state as well as in the excited state. Measurements of the fluorescence lifetime in normal and heavy water also support the formation of intermolecular hydrogen bonding. Time-resolved transient absorption spectra obtained in the picosecond laser flash photolysis experiments show only the absorption band due to the S_n← S₁ absorption. The picosecond transient absorption results do not indicate any spectral shifts attributable to the hydrogen bond formation dynamics between the excited NRH⁺ and the protic solvent molecules. It is inferred that the hydrogen bonding dynamics are much faster than the time resolution of our picosecond setup (∼35 ps).     

Effect of solvent on absorption and fluorescence spectra of a typical fluorinated azo dye for its acidic and basic structures

The effect of 15 polar solvents on absorption and fluorescence energies of a typical fluorinated azo dye, 4-(2,3,5,6-tetrafluoro-pyridin-4-yl azo)-phenol, was reported for its acidic, MH, and basic, M, structures.For MH, the absorption energy is described on the basis of multi-linear equation with Taft's π* (solvent polarity) and β (hydrogen bond acceptor) parameters while the fluorescence energy varies rectilinearly with free energy of transferring the proton to the surrounding solvent, ΔG_t°.For M, the hydrogen bonding donor ability of protic solvent, α, is a predominant factor which affects the absorption energy while in aprotic solvents, the absorption energy correlates linearly with Kirkwood function. As the ability of the solvent for hydrogen bonding increases, the absorption band width will increase in parallel with the transition energy.     

Spectroscopic characterization of phenazinium dye aggregates in water and acetonitrile media: effect of methyl substitution on the aggregation phenomenon

Absorption, fluorescence, and fluorescence excitation spectral studies of two planar, cationic phenazinium dyes, namely, phenosafranin (PSF) and safranin-T (ST), have been performed in protic and aprotic polar solvents. The studies reveal the formation of both J- and H-aggregates in concentrated solutions. The planarity of the phenazinium skeleton and the presence of a positive charge are attributed to be the driving force for this aggregation behavior. The aggregates are established to be dimers only. The positive inductive effect of the methyl substituents in safranin-T augments the aggregation process. The experiments reveal that for both dyes, the polar protic solvent favors the aggregation process more than the aprotic solvent.     

Spiropyran-azobenzene-DBU system as solvent indicator

A dyad bearing azobenzene and spiropyran units was synthesized and its applications in indicating the polarity and protic or aprotic properties of a solvent were explored. The spiropyran-azobenzene derivative (SPAB) can be induced to different forms in different miscellaneous solvents accompanied with different color changes and spectral characteristics at the presence of organic base DBU. In a nonpolar or low-polar solvent, SPAB exists in thermostable spiropyran form with yellow color output. While in an aprotic polar solvent, the spiropyran part isomerized to merocyanine form giving a blue color. When SPAB is subjected to a protic solvent, the alkylation reaction occurs at the oxygen generating the alkylated-SPAB with red color. This solvent-dependent property can be used for discriminating solvent type.     

Ultrafast vibrational dynamics of SCN(-) and N3(-) in polar solvents studied by nonlinear infrared spectroscopy

In this paper, we report on our investigation into the vibrational dynamics of the antisymmetric stretching modes of SCN(-) and N(3)(-) in several polar solvents. We used an infrared (IR) pump-probe method to study orientational relaxation processes. In two aprotic solvents (N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO)), the anisotropy decay shows a bimodal feature, whereas in other solvents the anisotropy decay can be fitted well by a single exponential function. We consider that the relative contribution of fast-decaying components is smaller in the other solvents than in DMF and DMSO. We discuss the possible origins of the different anisotropy decay behavior in different solvents. From the three-pulse IR photon echo measurements for SCN(-) and N(3)(-), we found that the time-correlation functions (TCFs) of vibrational frequency fluctuations decay on two different time scales, one of which is less than 100 fs and the other is approximately 3-6 ps. In aprotic solvents, the fast-decaying components of the TCFs on a <100 fs time scale play an important role in the vibrational frequency fluctuation, although the contribution of collective solvent reorganization in aprotic solvents was clearly observed to have small amplitudes. On the other hand, we found that the amplitude of components that decay in a few picoseconds and/or the constant offset of the TCF in protic solvents is relatively large compared with that in aprotic solvents. With the formation and dissociation of hydrogen bonds between ion solute and solvent molecules, the spectra of different solvated species are exchanged with each other and merged into one band. We considered that this exchange may be an origin of slow-decaying components of the TCFs and that the decay of the TCFs corresponds to the time scales of the exchange for protic solvents such as formamide. The mechanism of vibrational frequency fluctuations for the antisymmetric stretching modes of SCN(-) and N(3)(-) is discussed in terms of the difference between protic and aprotic solvents.