Solvent effects in the absorption spectra of the Ninhydrin chromophore

Visible spectra of the Ninhydrin chromophore, Ruhemann's purple, were studied in dimethyl sulfoxide (DMSO), formamide, N, N-dimethylformamide (DMF), and pyridine, as well as in mixed aqueous-nonaqueous solvent media. Large differences in both the position of absorption maxima and extinction coefficients for the two bands in the visible spectra in the various solvent media were observed. Both the absorption maxima and the extinction coefficients of the Ninhydrin chromophore were a linear function of the composition of dimethyl sulfoxide-H₂O solvent media. The experimental evidence allows predictions of values for the two absorption maxima of Ruhemann's purple as a function of the nature of the solvent medium. In nonaqueous aprotic solvents (i.e., DMSO and DMF), the maxima should be near 605 and 420 mμ; in nonaqueous aprotic solvents capable of undergoing charge-transfer interactions (i.e., pyridine), near 550 and 420 mμ; and in nonaqueous protic solvents (i.e., formamide), near 575 and 410 mμ. The maxima in aprotic media will be displaced to about 575 mμ (higher wavelength band) and 410 mμ (lower wavelength band) on dilution with protic solvents.     

Solvent effects on two-photon absorption of dialkylamino substituted distyrylbenzene chromophore

Solvent effects on the two-photon absorption of a symmetrical diamino substituted distyrylbenzene chromophore have been studied using the density functional response theory in combination with the polarizable continuum model. It is shown that the dielectric medium has a rather small effect both on the bond length alternation and on the one-photon absorption spectrum, but it affects significantly the two-photon absorption cross section. It is found that both one- and two-photon absorptions are extremely sensitive to the planarity of the molecule, and the absorption intensity can be dramatically reduced by the conformation distortion. It has led to the conclusion that the experimentally observed anomalous solvent effect on the two-photon absorption of dialkylamino substituted distyrylbenzene chromophores cannot be attributed to the intrinsic properties of a single molecule and its interaction with solvents.     

Solvent effect on the excited-state dynamics of analogues of the photoactive yellow protein chromophore

We previously reported that two analogues of the Photoactive Yellow Protein chromophore, trans-p-hydroxycinnamic acid (pCA(2-)) and its amide derivative (pCM-) in their deprotonated forms, undergo a trans-cis photoisomerization whereas the thioester derivative, trans-p-hydroxythiophenyl cinnamate (pCT-), does not. pCT- is also the only one to exhibit a short-lived intermediate on its excited-state deactivation pathway. We here further stress the existence of two different relaxation mechanisms for these molecules and examine the reaction coordinates involved. We looked at the effect of the solvent properties (viscosity, polarity, solvation dynamics) on their excited-state relaxation dynamics, probed by ultrafast transient absorption spectroscopy. Sensitivity to the solvent properties is found to be larger for pCT- than for pCA(2-) and pCM-. This difference is considered to reveal that either the relaxation pathway or the reaction coordinate is different for these two classes of analogues. It is also found to be correlated to the electron donor-acceptor character of the molecule. We attribute the excited-state deactivation of analogues bearing a weaker acceptor group, pCA(2-) and pCM-, to a stilbene-like photoisomerization mechanism with the concerted rotation of the ethylenic bond and one adjacent single bond. For pCT-, which contains a stronger acceptor group, we consider a photoisomerization mechanism mainly involving the single torsion of the ethylenic bond. The excited-state deactivation of pCT- would lead to the formation of a ground-state intermediate at the "perp" geometry, which would return to the initial trans conformation without net isomerization.     

Solvent interactions in the allylation of piperidine

The reaction between allylbromide and piperidine has been studied in different protic and aprotic solvents. The reaction is first order with respect to [allylbromide] and [piperidine]. A correlation analysis of the rate data with solvent properties shows that polarity (Y), polarizability (P), and electrophilicity (E) of the solvent simultaneously influence the rate of reaction. From the regression analysis, information regarding the relative solvation of the reactants and the activated complex is obtained and a solvation model is proposed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 421–425, 2009     

Hydrogen bonding interactions between indole and benzenoid-pi-bases

The NH-pi interactions of indole with benzene, naphthalene, phenanthrene, toluene, m-xylene, and mesitilene, in carbon tetrachloride solutions, have been studied by Fourier transform infrared spectroscopy. The experiments, carried out on the NH stretching band of indole, prove the formation of 1:1 complexes in which the NH bond of indole is engaged. The NH frequency shifts are independent of the number of rings in the base, but they progressively increase as the electron density is enhanced by methylation. The association constants increase with the increase of both, the number of rings and the methyl groups on the base. At higher base concentrations, further shifts on the free NH and associated bands indicate the formation of 1:2 complexes, which suggest hybride NH-pi and van der Waals interactions between one indole ring and two benzene acceptor molecules.     

Intrachain chromophore interactions in silanylene‐spaced divinylbenzene copolymers

A range of silanylene‐spaced divinylbenzene copolymers ( 1 ) and the corresponding monomers ( 2 ) have been synthesized by the rhodium‐catalyzed hydrosilylation of the corresponding bisalkynes with bissilyl hydrides, and the photophysical properties of 1 and 2 have been investigated. The silicon moiety in 1 serves as an insulating tetrahedral spacer that makes 1 highly folded. The two chromophores may be in close proximity such that a ground‐state intrachain interaction between two conjugated moieties through space might occur. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2218–2231, 2003     

The indole chromophore: Separation of the 1La and 1Lb components by a methyl substitution perturbation technique

Substitution of a methyl group around the indole ring has been used as a method for mildly perturbing the indole absorption spectrum. Comparison of the spectra of two methylindoles by a subtraction technique has allowed the separation of the overlapping ¹L_a and ¹L_b bands, whose oscillator strengths were found to be 0.123 and 0.045 respectively.     

Solvent effects on the vibrational activity and photodynamics of the green fluorescent protein chromophore: a quantum-chemical study

Vibrational activities in the Raman and resonance Raman spectra of the cationic, neutral, and anionic forms of 4'-hydroxybenzylidene-2,3-dimethyl-imidazolinone, a model compound for the green fluorescent protein chromophore, have been obtained from quantum-chemical calculations in vacuo and with the inclusion of solvent effects through the polarizable continuum model. It is found that inclusion of solvent effects improves slightly the agreement with experimental data for the cationic and neutral forms, whose spectra are qualitatively well-described already by calculations in vacuo. In contrast, inclusion of solvent effects is crucial to reproduce correctly the activities of the anionic form. The structural effects of solvation are remarkable both in the ground and in the lowest excited state of the anionic chromophore and influence not only the vibrational activity but also the photodynamics of the lowest excited state. CASPT2//CASSCF photoreaction paths, computed by including solvent effects at the CASSCF level, indicate a facile torsional deformation around both exocyclic CC bonds. Rotation around the exocyclic CC double bond is shown to lead to a favored radiationless decay channel, more efficient than that in gas phase, and which explains the ultrafast fluorescence decay and ground-state recovery observed in solution. Conversely, rotation around the exocyclic CC single bond accounts for the bottleneck observed in the ground-state recovery cycle. It is also speculated that the ultrafast radiationless decay channel would be hampered in protein for unfavorable electrostatic interactions and steric reasons.     

Solvent interactions with [Val(5)]angiotensin II in ethanol-water

Intermolecular NOE experiments have been used to explore the interactions of water and ethanol molecules in 35% ethanol/65% water (v/v) with the octapeptide hormone [val (5)]angiotensin II at temperatures from 0 to 25 degrees C. Magnetic dipole-dipole cross relaxation terms sigma(HH)(NOE) and sigma(HH)(ROE) for interaction of both solvent components suggest that ethanol molecules interact with the peptide backbone atoms strongly enough to associate for times comparable to the rotational correlation time of the peptide; comparison of observed ROE and NOE cross relaxation terms indicate that lifetimes of these interactions are of the order 0.4 ns at 5 degrees C. Formation of such peptide-ethanol complexes can also account for larger-than-expected values of the cross relaxation terms at higher temperatures. Alternative explanations of the observations reported are shown to be unlikely, primarily because they require unreasonable and highly localized concentrations of the ethanol near the peptide. Side chains of the peptide appear to experience no unusual interactions with ethanol. Cross relaxation terms for water-peptide backbone interactions indicate long-lived interactions of water with the backbone atoms although the nonpolar side chains of the peptide (Val3, Val5, Pro7, and possibly Phe8) do not interact in any specific way with water molecules. Cross relaxation terms for protons of the polar (Tyr4 and His6) side chains may reflect strong interactions with water, but analysis of these is confounded by solvent proton exchange and possible spin diffusion effects.     

Specific solute-solute and solute-solvent interactions in organic solutions of indole

The temperature dependence of the UV absorption spectra of indole and N-methylindole in propyl ether and in 3-methylpentane was investigated. The results indicate the existence of hydrogen bond interactions between indole and the ether, and also the existence of self-associations of indole through bonding in the non-polar solvent.     

Questioning the photophysical model for the indole chromophore in the light of evidence obtained by controlling the non-specific effect of the medium with 1-chlorobutane as solvent

In this work, we substantiate the change in the emitting state of indole caused by the dipolarity increase in the solvent 1-chlorobutane, observed on lowering the temperature from 293 to 133 K, accompanied by no significant changes in the corresponding excitation and absorption spectra. No similar changes in indole emission were observed over the temperature range 293-133 K for solutions of indole in 2-methylbutane in the presence and absence of 0.5 M 1-chlorobutane. The solvatochromism of indole in 1-chlorobutane at temperatures from 293 to 133 K allowed us to estimate the dipole moment and polarizability of the emission state of the chromophore and to detect two states (S(1) and ): one, the S(1), involving no significant change and the other, the , exhibiting a substantial change in the dipole moment of the chromophore upon electronic excitation (viz. μ(S(1)) = 2.5 and vs. μ(S(0)) = 2.13 D). The former state, S(1), is the major contributor to the structured emission of indole at temperatures from 293 to 193 K, as is the latter, S'(1) , to its structureless, red-shifted emission over the range 193-133 K. Although the emission changes of indole, dissolved in 1-chlorobutane at temperatures from 293 to 133 K, are seemingly consistent with the widely accepted photophysical model for inversion of its (1)L(b) and (1)L(a) states as the polarity of the medium is increased, below 133 K the emission becomes structured and blue-shifted, two typical features of indole above 193 K. Also, below 123 K is not feasible to photo-select the (1)L(a) state in spite of this state being the first excited electronic state of indole under large dipolarity conditions. Therefore, the established photophysical model cannot hold under these conditions and a new one accounting for these experimental facts is proposed instead.     

Electronic interactions in a branched chromophore investigated by nonlinear optical and time-resolved spectroscopy

The third order nonlinear optical properties of a trimer branched chromophore system and its linear molecule analog are investigated. Two-photon absorption and degenerate four wave mixing measurements were carried out on both systems. An enhancement in the nonlinear optical effect is observed for the branched trimer molecule in comparison to the linear chromophore system. Ultrafast time-resolved measurements were carried out to probe the excited state dynamics in the branched structures. The time-resolved measurements suggest that the two important processes affecting the nonlinear optical properties in the trimer system, charge transfer stabilization and initial electronic delocalization, occur on two different time scales.     

Strong intramolecular chromophore interactions in novel bis([60]fullerene)-oligoporphyrin nanoarrays

Novel nanodimensional dyads were prepared by Bingel reaction of meso, meso-linked oligoporphyrin bis-malonates with C60. Distinct conformational effects were observed as a result of strong fullerene-porphyrin interactions, which also lead to efficient quenching of the porphyrin luminescence.     

Lanthanide complexes of new polyaminocarboxylates with the bis-pyrazolylpyrimidine chromophore

We describe the straightforward synthesis of a new polyacid ligand derived from the bis-pyrazolylpyrimidine motif that displayed excellent properties concerning its ability to sensitize the emission of lanthanides. Emission lifetimes were in the range of ms and quantum yields were 0.15 and 0.85 for Eu and Tb, respectively, among the largest measured by us in related compounds, thus making this ligand an excellent candidate for its application in multitagging time-resolved fluoroimmunoassays.     

Cyanine dyes with an isoindolenine ring in the chromophore

2-[3-(3-Methyl-2-benzothiazolinylidenyl)-1-methylisoindolinylidenyl]methyl-3-methylbenzothiazolium perchlorate was obtained by the condensation of 2-methylbenzothiazole methylmethosulfate with 1,1,3-trichloroisoindolenine. Similar dyes of the pyridine and quinoline series were also synthesized. The absorption bands of the cyanine dyes with an isoindolenine ring in the chromophore are shifted by 30 nm to the shortwave region in comparison with the corresponding dicarbocyanines. Acids protonate the dyes at the isoindolenine ring to form compounds with two conjugated chromophores. Condensation of 1,1,3-trichloroisoindolenine with N-substituted rhodanines yields 3-substituted 5-[3-(3-methyl-4-hydroxythiazoline-2,5-thionyl)-1-isoindolinylidene]rhodanines. Alkylation and hydrolysis of the N-methyl substituted dye yields 5-[3-(3-methyl-4-hydroxy-5-thiazolin-5-onyl)isoindolinylidene-1]-3-methylthiazolidine-2, 4-dione.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 454–458, April, 1971.     

Raman spectroscopy reveals direct chromophore interactions in the Leu/Gln105 spectral tuning switch of proteorhodopsins

Proteorhodopsins are an extensive family of photoactive membrane proteins found in proteobacteria distributed throughout the world's oceans which are often classified as green- or blue-absorbing (GPR and BPR, respectively) on the basis of their visible absorption maxima. GPR and BPR have significantly different properties including photocycle lifetimes and wavelength dependence on pH. Previous studies revealed that these different properties are correlated with a single residue, Leu105 in GPR and Gln105 in BPR, although the molecular basis for the different properties of GPR and BPR has not yet been elucidated. We have studied the unexcited states of GPR and BPR using resonance Raman spectroscopy which enhances almost exclusively chromophore vibrations. We find that both spectra are remarkably similar, indicating that the retinylidene structure of GPR and BPR are almost identical. However, the frequency of a band assigned to the retinal C13-methyl-rock vibration is shifted from 1006 cm (-1) in GPR to 1012 cm (-1) in BPR. A similar shift is observed in the GPR mutant L105Q indicating Leu and Gln residues interact differently with the retinal C13-methyl group. The environment of the Schiff base of GPR and BPR differ as indicated by differences in the H/D induced down-shift of the Schiff base vibration. Residues located in transmembrane helices (D-G) do not contribute to the observed differences in the protein-chromophore interaction between BPR and GPR based on the Raman spectra of chimeras. These results support a model whereby the substitution of the hydrophilic Gln105 in BPR with the smaller hydrophobic Leu105 in GPR directly alters the environment of both the retinal C13 group and the Schiff base.