Solvatochromic Probe Research Articles

Kinetics and polarity effects in the synthesis of the ionic liquid, 1-hexyl-3-methyl-imidazolium bromide, using compressed CO2

Compressed carbon dioxide, as a solvent medium, offers unique benefits for the synthesis of ionic liquids (ILs) especially for the ability to combine reaction and separation. The second-order reaction rate constants were determined for the synthesis of 1-hexyl-3-methylimidazolium bromide ([HMIm][Br]) at 313.15K and 333.15K and 30–150bar from the reactants, 1-methylimidazole and 1-bromohexane. Two different scenarios were encountered based upon the pressure: a one-phase supercritical mixture of the reactants becoming a vapor-liquid IL/CO2 system at complete conversion; or a two-phase CO2-expanded liquid (CXL) that yielded a three-phase vapor-liquid-liquid equilibrium (VLLE) at intermediate conversions. Despite multiphase phenomena, the vast majority of the reaction occurs in the reactant-rich liquid phase. The kinetic rate constants were comparable with traditional organic solvents. The reaction rates were higher in the CXL mode than in the initially supercritical mixtures. The changes in the kinetic rate constant with pressure (CO2 composition) could be explained and quantified using Kamlet–Taft (KT) polarity parameters that were measured in the mixtures using solvatochromic probes.

Solvatochromic probe response within ionic liquids and their equimolar mixtures with tetraethylene glycol.

Synergism in a probe response within a mixture hints at the presence of strong interactions involving the solvent constituents of the mixture and possibly the probe. Unusual and rare "hyperpolarity" resulting from the synergism in probe response exhibited by ionic liquid (IL) mixtures with glycol family solvents is investigated in detail for equimolar mixtures of tetraethylene glycol (TEG) with many structurally different ILs using several UV-vis absorbance and fluorescence solvatochromic probes. Thirteen different ILs, of the same cation 1-butyl-3-methylimidazolium and different anions, of the same anion bis(trifluoromethylsulfonyl)imide and different cations, and of C2 methyl-substituted imidazolium cations, are used to assess the structural dependence of the IL on synergism exhibited by (IL + TEG) mixture. Responses from UV-vis absorbance probes are used to obtain ET [dipolarity/polarizability and/or H-bond donating (HBD) acidity] and Kamlet-Taft parameters [π* (dipolarity/polarizability), α (HBD acidity), and β (HB accepting basicity)] within (IL + TEG) mixtures. The band I-to-band III fluorescence intensity ratio of dipolarity probe pyrene along with the lowest energy fluorescence band maxima of pyrene-1-carboxaldehyde (PyCHO, a probe for the permittivity of the medium), coumarin-153 and N,N-dimethyl-6-propionyl-2-naphthylamine PRODAN (neutral photoinduced charge-transfer fluorescence probes), and 6-p-toluidine-2-naphthalenesulfonic acid (TNS) and l-anilinonaphthalene-8-sulfonate (ANS) (ionic photoinduced charge-transfer fluorescence probes) are used to assess whether synergism is exhibited by (IL + TEG) equimolar mixtures. Probe responses within TEG equimolar mixtures with ILs are compared to those with common organic solvents. An attempt is made to establish a correlation between the synergism observed in the probe response within an (IL + TEG) mixture and the structural features of the cation and anion of the IL, such as acidity of the protons of the cation, aromaticity of the cation, and size, shape, and coordinating ability of the anion. It is established that the solvatochromism exhibited by the probes within (IL + TEG) mixtures is due to complex coupling of several different interactions and dynamical processes involving the probe as well as IL and TEG within the mixture.

Polymeric chain dependent anomalous solvatochromism of ionic liquid + poly(ethylene glycol) mixtures

To alter the physicochemical properties of ILs with green cosolvents, molecular level understanding of the resultant mixtures is hotbed of the current research. In the present study, changes in the physicochemical properties of the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) on the addition of different polyethylene glycols (PEGs) (average MW 200, 400, 600, and 1500) by means of solvatochromic probes are investigated. The addition of the PEGs to IL [bmim][Tf2N] changes the physicochemical properties of the solvent mixtures in unusual and favorable fashion. The response of the solvatochromic probes Reichardt dye 33 and Nile Red hints about the “hyperpolarity” of the IL+PEG mixtures which is rare in the literature. The Kamlet–Taft parameters, hydrogen bond donor ability (HBD) (α), hydrogen bond acceptor ability (HBA) (β), and dipolarity/polarizability (π*) display nonideality, and show strong and unusual “hyperpolarity” behavior on the addition of PEGs to [bmim][Tf2N]. A solvent–solvent along with solute–solvent interaction hypothesis is proposed to explain the “hyperpolarity” of the mixtures. Further, the solvation models are used to delineate the solute–solvent/solvent–solvent interactions present in the binary mixtures. Combined nearly ideal binary solvent/Redlich–Kister (CNIBS/R–K) equation was demonstrated to predict the solvatochromic parameters satisfactorily.

Characterization of M-laurdan, a versatile probe to explore order in lipid membranes

Microdomains corresponding to localized partition of lipids between ordered and less ordered environments are the subject of intensive investigations, because of their putative participation in modulating cellular responses. One popular approach in the field consists in labelling membranes with solvatochromic fluorescent probes such as laurdan and C-laurdan. In this report, we describe a high-yield procedure for the synthesis of laurdan, C-laurdan and two new fluorophores, called MoC-laurdan and M-laurdan, as well as their extensive photophysical characterization. We find that the latter probe, M-laurdan, is particularly suited to discriminate lipid phases independently of the chemical nature of the lipids, as measured by both fluorescence Generalized Polarization (GP) and anisotropy in large unilamellar vesicles made of various lipid compositions. In addition, staining of live cells with M-laurdan shows a good stability over time without any apparent toxicity, as well as a wider distribution in the various cell compartments than the other probes.

Role of Protonation State and Solvation on the pH Dependent Optical Properties of Bromocresol Green.

pH sensors play a key role in many industrial and diagnostic applications. Mostly their usage is based on experience, and in many cases the working mechanisms of these sensors are not known in detail, thereby hindering a systematic improvement of such sensors for specific applications. In this report, we present results from combined quantum chemical and molecular mechanics calculations of molecular structures and optical absorption properties of bromocresol green (BRG) in aqueous solution with varying pH value. In the acidic pH range, this chromophore has an intense band with absorption maximum at 444 nm and in the basic pH regime the absorption spectra show a redshift toward 613 nm. In order to identify the molecular structures responsible for this pH dependent optical behavior the closed and open forms of BRG are studied using static approaches considering in each case the three possible protonated states namely, neutral, anionic, and dianionic. For the most significant forms, i.e. the open forms of BRG, extensive modeling based on the integrated approach has been carried out, where the structure and dynamics were studied using hybrid QM/MM molecular dynamics, while the excitation energy calculations were carried out using time dependent density functional theory wherein the surrounding solvent was described as polarizable continuum, semicontinuum, or via a molecular mechanics force-field. The anionic and dianionic forms of BRG have been recognized as molecular forms responsible for its acidic and basic pH behavior, respectively. In contrast to the case of solvatochromic probes, the different protonation states determine the optical behavior in different pH values for pH probes. Hence, the level of solvent description appears to be of minor importance. Independent of the level of theory used to describe the solvent, all models reproduce the spectral features of BRG in different pH and also the pH induced redshift in good agreement with experiment.

Fluorescence quenching of (dimethylamino)naphthalene dyes Badan and Prodan by tryptophan in cytochromes P450 and micelles.

Fluorescence of 2-(N,N-dimethylamino)-6-propionylnaphthalene dyes Badan and Prodan is quenched by tryptophan in Brij 58 micelles as well as in two cytochrome P450 proteins (CYP102, CYP119) with Badan covalently attached to a cysteine residue. Formation of nonemissive complexes between a dye molecule and tryptophan accounts for about 76% of the fluorescence intensity quenching in micelles, the rest is due to diffusive encounters. In the absence of tryptophan, fluorescence of Badan-labeled cytochromes decays with triexponential kinetics characterized by lifetimes of about 100 ps, 700–800 ps, and 3 ns. Site mutation of a histidine residue in the vicinity of the Badan label by tryptophan results in shortening of all three decay lifetimes. The relative amplitude of the fastest component increases at the expense of the two slower ones. The average quenching rate constants are 4.5 × 108 s–1 (CYP102) and 3.7 × 108 s–1 (CYP119), at 288 K. Cyclic voltammetry of Prodan in MeCN shows a reversible reduction peak at −1.85 V vs NHE that becomes chemically irreversible and shifts positively upon addition of water. A quasireversible reduction at −0.88 V was observed in an aqueous buffer (pH 7.3). The excited-state reduction potential of Prodan (and Badan) is estimated to vary from about +0.6 V (vs NHE) in polar aprotic media (MeCN) to approximately +1.6 V in water. Tryptophan quenching of Badan/Prodan fluorescence in CYPs and Brij 58 micelles is exergonic by ≤0.5 V and involves tryptophan oxidation by excited Badan/Prodan, coupled with a fast reaction between the reduced dye and water. Photoreduction is a new quenching mechanism for 2-(N,N-dimethylamino)-6-propionylnaphthalene dyes that are often used as solvatochromic polarity probes, FRET donors and acceptors, as well as reporters of solvation dynamics.

Characterization of M-laurdan, a versatile probe to explore order in lipid membranes.

Microdomains corresponding to localized partition of lipids between ordered and less ordered environments are the subject of intensive investigations, because of their putative participation in modulating cellular responses. One popular approach in the field consists in labelling membranes with solvatochromic fluorescent probes such as laurdan and C-laurdan. In this report, we describe a high-yield procedure for the synthesis of laurdan, C-laurdan and two new fluorophores, called MoC-laurdan and M-laurdan, as well as their extensive photophysical characterization. We find that the latter probe, M-laurdan, is particularly suited to discriminate lipid phases independently of the chemical nature of the lipids, as measured by both fluorescence Generalized Polarization (GP) and anisotropy in large unilamellar vesicles made of various lipid compositions. In addition, staining of live cells with M-laurdan shows a good stability over time without any apparent toxicity, as well as a wider distribution in the various cell compartments than the other probes.

Solvatochromic probe in molecular solvents: implicit versus explicit solvent model

Solvent-induced shifts in the absorption spectrum of N,N-diethyl-4-nitroaniline were studied by quantum-chemical methods in water, dimethylsulfoxide, acetonitrile and acetone. TDDFT methodology and semiempirical ZINDO/S and PM6-CIS approaches were used to calculate excitation energies. Solvent effect was modeled in implicit solvent model by different variants of the PCM approach. Classical molecular dynamics was applied to obtain solute–solvent geometries used in explicit solvent modeling. Most implicit solvent models fail to reproduce the sequence of solvatochromic shifts for four studied solvents, usually yielding too small effect for water. The best result of the PCM method was obtained with SMD atomic radii. Semiempirical quantum-chemical methods in explicit solvent model did not provide satisfactory description of solvatochromic shifts with the largest disagreement to experiment observed for water. TDDFT explicit solvent calculations performed the best in modeling of spectral shifts. Problems with reproduction of experimental data were attributed to specific interactions.

Effect of π-conjugated linkage on photophysical properties: Acetylene linker as the better connection group for highly solvatochromic probes

Fluorophores based in triphenylamine and fluorene (1–3) were synthetized and the effects of π-conjugated linkers (vinyl, ethynyl and 1,3-diynyl) on the solvatochromic properties were studied. The fluorescence emission spectra of the dyes were studied using solvents of different polarities, and the influence of specific solvent–solute interactions over emission properties was investigated through Lippert–Mataga model. In particular, we found that compound 2 (with an ethynyl group) exhibits significant advantages over compounds 1 and 3 with vinyl and 1,3-dynyl groups, respectively. It shows a better solvatochromism (140 nm from hexane to CH3CN), higher Stokes shifts (178 nm in CH3CN) and more sensitive quantum yields toward solvent polarity (0.09DMF–0.98THF), when compared with those to the reference probe (Prodan). Furthermore, this linker exhibits higher resistance against photodegratation under same conditions. The geometrical and electronic transitions were analyzed by DFT and TD-DFT, and the results are consistent with the experimental observations.

Solvatochromic Absorbance Probe Behavior within Mixtures of the Ionic Liquid 1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide + Molecular Organic Solvents

The solvent mixtures made up of ionic liquid and molecular solvents have potential applications in various fields such as chemical analysis and synthesis. Thus, in the present study, the role of the solvent microsphere in changing the behavior of the solvatochromic absorbance probe within binary solvent mixtures of ionic liquids with common molecular organic solvents is explored. Further, the behavior of the probes is explained with the help of solute–solvent and solvent–solvent interactions present in the mixtures. The binary mixtures of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim][Tf2N], with molecular solvents, polar protic (methanol, 1-butanol) and polar aprotic (dimethyl sulfoxide, and N,N-dimethylformamide), have been selected for this investigation. The effect of the addition of molecular solvents to ionic liquid’s polarity parameters, ETN, and Kamlet Taft parameters, the hydrogen bond donor ability (HBD) (α), hydrogen bond acceptor ability (HBA) (β), and d...

5,6,7,9-Tetrahydro-[1,3]dioxolo[4,5-h]carbazol-8-one: A solvatochromic PET-acceptor fluorescent probe

In the present work, 5,6,7,9-tetrahydro-[1,3]dioxolo[4,5-h]carbazol-8-one (TDCO) has been established as a solvatochromic probe and an acceptor of photoinduced electron transfer through electronic absorption spectroscopy, steady-state and time-resolved fluorescence measurements and also by using the laser flash photolysis technique. Fluorescence spectra of the compound are more sensitive than its absorption spectra towards the nature of solvent and the composition of solvent mixture. In solvents with higher dielectric constant and better H-bond formation ability, fluorescence maximum of the compound shifts toward red. The reasons behind such solvent sensitivity of TDCO in electronic excited state are its higher dipole moment in excited state and formation of hydrogen bond with solvents. TDCO interacts with aromatic amine bases N,N-dimethylaniline (DMA) and N,N-diethylaniline (DEA) in excited state and as a consequence its fluorescence intensity and lifetime are quenched. The quenching order is the same as the order of ionization potential of the amines. Transient absorption studies indicate the involvement of photoinduced electron transfer from the amines to TDCO as a result of which the corresponding radical cations and anions have been formed. Simulation studies show that TDCO has the tendency to remain in stacked conformation with DMA and DEA, which facilitates photoinduced electron transfer.

A novel water soluble solvatochromic probe as a micropolarity reporter for homogeneous and microheterogeneous media

A new water soluble donor–acceptor dye, sodium 4-(methyl((1E,3E)-3-(1-oxo-1H-inden-2(3H)-ylidene)prop-1-enyl)amino)benzoate (DN3) has been synthesized. Optical response of the solvatochromic dye (DN3) has been studied in various homogeneous (neat and mixed binary solvents) and heterogeneous (SDS and CTAB homomicelle and β-cyclodextrin nanocavity) media. To get information regarding the change in the solvation interaction of the dye with the alteration in its microenvironment in different media, the photophysical properties of the dye have been monitored in various media using steady state and time resolved spectral analysis. Results obtained for mixed binary solvents containing water indicate that the solute is preferentially solvated by one of the component solvents and solvent–solvent interactions are also important in determining the preference. Effect of variation of pH in aqueous medium on the spectroscopic parameters of the dye has been studied and its pKa has been estimated. Studies in homomicelles (SDS and CTAB) reveal that the dye distributes itself between the aqueous and the micellar phase and the values of distribution coefficient have been estimated from the fluorescence parameters. The dye is encapsulated in β-cyclodextrin nanocavity and a 1:2 dye-β-cyclodextrin host–guest interaction is indicated. Semi-empirical quantum chemical calculations have been carried out to support the experimental results.

UV–Vis spectroscopic study of preferential solvation and intermolecular interactions in methanol/1-propanol/acetonitrile by means of solvatochromic probes

Solvatochromic UV–Vis shifts of four indicators (4-nitroaniline, 4-nitroanisole, 4-nitrophenol and N,N-dimethyl-4-nitroaniline) have been measured at 298.15K in the ternary mixture methanol/1-propanol/acetonitrile (MeOH/1-PrOH/MeCN) in a total of 22mole fractions, along with 18 additional mole fractions for each of the corresponding binary mixtures, MeOH/1-PrOH, 1-PrOH/MeCN and MeOH/MeCN. These values, combined with our previous experimental results for 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (Reichardt’s betaine dye) in the same mixtures, permitted the computation of the Kamlet–Taft solvent parameters, α, β, and π*. The rationalization of the spectroscopic behavior of each probe within each mixture’s whole mole fraction range was achieved through the use of the Bosch and Rosés preferential solvation model. The applied model allowed the identification of synergistic behaviors in MeCN/alcohol mixtures and thus to infer the existence of solvent complexes in solution.Also, the addition of small amounts of MeCN to the binary mixtures was seen to cause a significant variation in π*, whereas the addition of alcohol to MeCN mixtures always lead to a sudden change in α and β. The behavior of these parameters in the ternary mixture was shown to be mainly determined by the contributions of the underlying binary mixtures.

How polar are choline chloride-based deep eutectic solvents?

Developing and characterizing green solvents with low toxicity and cost is one of the most important issues in chemistry. Deep Eutectic Solvents (DESs), in this regard, have shown tremendous promise. Compared to popular organic solvents, DESs possess negligible VOCs and are non-flammable. Compared to ionic liquids, which share many characteristics but are ionic compounds and not ionic mixtures, DESs are cheaper to make, much less toxic and mostly biodegradable. An estimate of the polarity associated with DESs is essential if they are to be used as green alternatives to common organic solvents in industries and academia. As no one physical parameter can satisfactorily represent solute-solvent interactions within a medium, polarity of DESs is assessed through solvatochromic optical spectroscopic responses of several UV-vis absorbance and molecular fluorescence probes. Information on the local microenvironment (i.e., the cybotactic region) that surrounds several solvatochromic probes [betaine dye, pyrene, pyrene-1-carboxaldehyde, 1-anilino-8-naphthalene sulfonate (ANS), p-toluidinyl-6-naphthalene sulfonate (TNS), 6-propionyl-2-(dimethylaminonaphthalene) (PRODAN), coumarin-153, and Nile Red] for four common and popular DESs formed from choline chloride combined with 1,2-ethanediol, glycerol, urea, and malonic acid, respectively, in 1 : 2 molar ratios termed ethaline, glyceline, reline, and maline is obtained and used to assess the effective polarity afforded by each of these DESs. The four DESs as indicated by these probe responses are found to be fairly dipolar in nature. Absorbance probe betaine dye and fluorescence probes ANS, TNS, PRODAN, coumarin-153, and Nile Red, whose solvatochromic responses are based on photoinduced charge-transfer, imply ethaline and glyceline, DESs formed using alcohol-based H-bond donors, to be relatively more dipolar in nature as compared to reline and maline. The pyrene polarity scale, which is based on polarity-induced changes in vibronic bands, indicates reline, the DES composed of urea as the hydrogen bond donor, to be significantly more dipolar than the other three DESs. Response of pyrene-1-carboxaldehyde, a polarity probe based on inversion of n-π* and π-π* states, hints at maline to be the most dipolar of the four DESs. The molecular structure of the H-bond donor in a DES clearly controls the dipolarity afforded by the DES. H-bonding and other specific solute-solvent interactions are found to play an important role in solvatochromic probe behavior for the four DESs. The cybotactic region of a probe dissolved in a DES affords information on the polarity of the DES towards solutes of similar nature and functionality.

Solvatochromic probes for detecting hydrogen-bond-donating solvents

Hydrogen bonding heavily influences conformations, rate of reactions, and chemical equilibria. The development of a method to monitor hydrogen bonding interactions independent of polarity is challenging as both are linked. We have developed two solvatochromic dyes that detect hydrogen-bond-donating solvents. The unique solvatochromism of the triazine architecture has allowed the development of probes that monitor hydrogen-bond-donating species including water.

Unusual solvatochromic absorbance probe behaviour within mixtures of poly(ethylene glycol)-400 + ionic liquid, [bmim][Tf2N

The potentially green solvents made up of ionic liquids (ILs) and poly(ethylene glycols) may have wide range of the applications in many chemical and biochemical fields. In the present work, solvatochromic absorbance probe behaviour is used to assess the physicochemical properties of the mixtures composed of PEG-400+IL, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim][Tf2N]. Lowest energy intramolecular charge-transfer absorbance maxima of a betaine dye, i.e., ETN, indicates the dipolarity/polarizability and/or hydrogen-bond donating (HBD) acidity of the [bmim][Tf2N]+PEG-400 mixtures to be even higher than that of neat [bmim][Tf2N], the solution component with higher dipolarity/polarizability and/or HBD acidity. Dipolarity/polarizability (π∗) obtained separately from the electronic absorbance response of probe N,N-diethyl-4-nitroaniline, and the HBD acidity (α) of PEG-400+[bmim][Tf2N] mixtures are also observed to be anomalously high. A comparative study of the PEG+IL mixtures has also been done with PEG-400+molecular organic solvents (protic polar [methanol], aprotic polar [N,N-dimethylformamide], and non polar, [benzene]) mixtures, but these mixtures do not show this type of unusual behaviour. A four-parameter simplified combined nearly ideal binary solvent/Redlich–Kister (CNIBS/R–K) equation is shown to satisfactorily predict the solvatochromic parameters within PEG-400+different solvent mixtures.

Molecular-Microscopic Properties and Preferential Solvation in Protic Ionic Liquid Mixtures

Structural and molecular-microscopic properties of the solvatochromic probes 4-nitroaniline, 4-nitroanisole, and Reichardt’s dye were investigated in binary mixtures of ethylammonium propionate with methanol, ethanol, 1-propanol and 2-propanol. Solvatochromic parameters (α, hydrogen-bond donor acidity; β, hydrogen-bond acceptor basicity; π*, dipolarity/polarizability; \( E_{\text{T}}^{\text{N}} \), normalized polarity parameter) in different binary mixtures of ionic liquid with molecular solvents were determined with UV–Vis spectroscopy. The \( E_{\text{T}}^{\text{N}} \) parameters show nearly ideal trends in all solvent mixtures, but the other parameters show different behavior in the mixtures. The π* parameters show a negative deviation from ideality in the ionic liquid/methanol system. In contrast, the α parameters have severe positive deviations from ideal behavior in ionic liquid/1-propanol and ionic liquid/2-propanol solvent mixtures. A synergistic solvation effect is observed for the π* parameters in IL/methanol mixtures. Specific solute–solvent interactions or solvent–solvent interactions, which cause non-ideal trends in some parameters, are justified and interpreted by the preferential solvation model.

Theoretical computation of Betain B30 solvatochromism using a Polarizable Continuum Model

We report in this paper the computation of optical properties of a solvatochromic probe using Time-Dependent Density Functional Theory methodology. Our protocol includes the use of an implicit solvent model to take into account the surrounding effects on the physical behavior of the considered molecules. Because this compound presents a long-range net charge separation in the ground state, preliminary benchmark calculations involving meta-GGA, hybrid, range-separated and dispersion-corrected functionals were used to determine electronic structures and to evaluate the vertical transition energies. Since the molecular target exhibits a very large negative solvatochromism, its absorption range among fifty-four organic solvents was screened. Topological considerations based on natural transition orbitals concerning the involved excited states are also presented. Thenceforward the value of the Dimroth–Reichardt index, a macroscopic empirical quantity related to the light absorption of this compound was predicted. Finally, the dipole moment variation during the electronic transitions was characterized, explaining the noteworthy solvatochromism of these molecules.

Dicyanostilbene-based two-photon thermo-solvatochromic fluorescence probes with large two-photon absorption cross sections: Detection of solvent polarities, viscosities, and temperature

The unusually sensitive solvatochromic probes (P1 and P2) derived from 4-methyl-2,5-dicyano-4′-amino stilbene (DCS) whose emission maxima and two-photon absorption cross sections (δ) vary from 445 and 452nm, and 5560 and 6670 GM in cyclohexane to 641 and 604nm, and 980 and 2040 GM in DMSO, respectively are described. P1 and P2 exhibit very strong polarity-, viscosity-, and temperature-dependence of fluorescence, and can be used to detect polarities, viscosities, and temperature. P1 and P2 possess small molecule-size, wide solvatochromism (445–641nm, 452–641nm), large δ (980–5560 GM, 2040–6670 GM), and high fluorescence quantum yields, and are ideal two-photon fluorescence solvatochromic probes.

Photophysical properties of 3-[2-(N-phenylcarbazolyl)benzoxazol-5-yl]alanine derivatives—experimental and theoretical studies

Solvatochromic probes are often used in biophysical studies to obtain information about polarity of the microenvironment. As there is not much natural fluorophores with such properties, there is still need for new synthetic compounds such as 3-(2-benzoxazol-5-yl)alanine derivatives. Among this group of non-proteinogenic fluorescent amino acids especially interesting are 3-[2-(4-aminophenyl)benzoxazol-5-yl]alanine derivatives whose solvatochromism depends on the substituents on the nitrogen atom, as revealed by our recent studies. To expand them we synthesized two new derivatives with an N-phenylcarbazole moiety in position 2 of the benzoxazole ring and studied their photophysical properties in solvents of different polarity and ability to form hydrogen bonds using absorption and steady-state and time-resolved fluorescence spectroscopy. Applying single parameter and multi-linear correlations with different solvent parameters, the excited state dipole moments were determined as well as the influence of solvent parameters on each photophysical property was estimated. Moreover, the geometry of compounds and vertical absorption transition were theoretically calculated (DFT and TD DFT methods). It was found that the place of substitution of the N-phenylcarbazole part by the benzoxazole unit determines the character of the electron transition (π-π* or ICT) and thereby the spectral and photophysical properties of the compounds studied.