Solvatochromism as an efficient tool to study N,N‐dimethylamino‐ and cyano‐substituted π‐conjugated molecules with an intramolecular charge‐transfer absorption

A representative data set has been gained by the measurement of the electronic absorption spectra of 12 systematically selected push–pull systems with an intramolecular charge‐transfer (CT) absorption and the general structure D–π–A (D = donor, A = acceptor) featuring electron‐withdrawing CN groups, electron‐donating N(CH₃)₂ groups, and various π‐conjugated backbones in 32 solvents with different polarities. The longest‐wavelength absorption maxima λ_max and the corresponding wavenumbers $\tilde {v}_{{\rm max}} $ were evaluated from the UV/Vis spectra measured in 32 well‐selected solvents. The D–π–A push–pull systems were further characterized by quantum‐chemical quantities and simple structural parameters. Structure–solvatochromism relationships were evaluated by multidimensional statistic methods. Whereas solvent polarizability and solvent cavity size proved to be the most important factors affecting the position of λ_max, the solvent polarity was less important. The most important characteristics of organic CT compounds are the energy of the LUMO, the permanent dipole moment, the COSMO (COnductor‐like Screening MOdel) area, the COSMO volume, the number, and ratio of N,N‐dimethylamino and cyano groups, and eventually the number of triple bonds (π‐linkers). A relation between the first‐order polarizability α, the longest‐wavelength absorption maxima λ_max, and the structural features has also been found. The higher‐order polarizabilities β and γ are not related to the observed solvatochromism. Copyright © 2010 John Wiley & Sons, Ltd.     

On the use of β‐carotene as a probe for solvent polarizability

The solvatochromism of β‐carotene confirms its high sensitivity not only to the polarizability of the medium, but is also contaminated by additional solute/solvent interactions due to its dipolarity and acidity, as well as to changes in its molecular structure in some solvents. A thermochromic analysis of β‐carotene dissolved in 2‐methylbutane and 1‐chlorobutane (ClB) revealed the influence of the solvent dipolarity on its UV/Vis‐spectroscopy behavior in these solvents. Applying Abe's method to the solvent‐induced shift of the first Vis absorption band of β‐carotene in ClB revealed that the electronic excitation substantially increases its polarizability and its dipole moment. Other experimental evidence also confirms that β‐carotene is not a suitable polarizability probe of the medium. Copyright © 2013 John Wiley & Sons, Ltd.     

Tuning the absorption spectra and nonlinear optical properties of D‐π‐A azobenzene derivatives by changing the dipole moment and conjugation length: a theoretical study

The optical properties of several azobenzene derivatives were modulated by varying the dipole moments and conjugation lengths of the D‐π‐A systems. The relationship between the structure and absorption spectrum and polarizability was studied in the gas phase, THF and MeOH solutions, respectively, by using the density functional theory. The calculated absorption spectra and second‐order polarizabilities are in good agreement with the available experimental observations. In comparison with the D‐π‐A monomer, the H‐shaped D‐π‐A dimer almost doubles the dipole moments and hence increases the second‐order polarizabilities, without a significant shift in the maximum absorption bands. The addition of another azobenzol group between electron‐donating and ‐accepting groups increases the second‐order polarizabilities by 4–6 times, but leads to an evident red‐shift of about 65–80 nm in spectra. The relative second‐order polarizability of the halogen‐substituted derivatives is in the sequence of ? CF₃ > ? F > ? Cl > ? Br, without obvious substituent effects on the optical transparency. The D‐π‐A chromophores with the strong electron‐donating (amino) and ‐accepting (acetyl) substituent present the larger second‐order polarizabilities, at the cost of about 20 nm red‐shift of the maximum absorption lengths relative to the halogen‐substituted species. It is also demonstrated that both the linear and nonlinear optical properties augment with the increase in solvent polarity, accompanied by a red‐shift in the wavelengths of maximum absorption by about 18 and 23 nm, respectively, in THF and MeOH solutions. The changes in optical properties upon the structural modifications are further rationalized by the electronic structures of various H‐shaped dimers. Copyright © 2010 John Wiley & Sons, Ltd.     

Solvatochromic and Fluorescence Behavior of Sulfisoxazole

The Fluorescence spectroscopic and solvatochromic behavior of Sulfisoxazole, a sulfa drug with antimicrobial activities, in various pure solvents of different polarity and hydrogen bonding capability is reported. The fluorescence emission spectrum of sulfisoxazole was found to be solvent polarity dependent, where a notable red shift in emission maximum was observed with increasing solvent polarity as well as hydrogen bonding capability. The effects of the latter two solvent parameters were quantitatively investigated using the methods of Lippert–Mataga and solvatochromic comparison method (SCM) that is based on the Kamlet-Taft equation. Particularly, the Lippert–Mataga method was applied to estimate the dipole moment of the excited state (μ_e) upon plotting Stokes shift versus solvent polarizability (Δf), where a value of 11.54 Debye was obtained. On the other hand, applying the multiple regression analysis to the SCM method revealed that solvent polarizability (π*) and hydrogen-bond donor capability (α) approximately equally stabilize sulfisoxazole in the excited state with minor destabilization contribution by the hydrogen-bond acceptor capability (β). These findings revealed that the excited state of sulfisoxazole is stabilized by polar solvents, indicating that this drug molecules exhibit larger dipole moment in the excited state than in the ground state, which in turn implies that a potential intramolecular charge transfer (ICT) occurs after excitation.     

新型三腈基呋喃衍生物光谱特性的研究

通过两步反应合成了一种带有新型电子给体的D-π-A型三腈基呋喃衍生物(DCDHF-2-V),并采用旋涂法制备出与聚甲基丙烯酸甲酯(PMMA)共混均匀的聚合物薄膜.采用UV1700紫外可见分光光度计和F-4500荧光分光光度计研究了该化合物在不同极性溶剂以及薄膜状态下的吸收光谱和荧光光谱特性.结果表明,在薄膜状态下化合物的吸收峰有一定蓝移,吸收带变宽.随溶液极性的增加,荧光光谱的最人峰值波长逐渐红移,分子的荧光量子产率以及斯托克斯位移也有较大变化.据此计算出DCDHF-2-V分子激发态与基态偶极矩之差为35.68×10-30C·m,并根据双能级模型确定了分子的二阶非线性极化率β随波长的变化情况,当激光基频波长为1064 nm时,β为3323.4×10-40m4/V.     

Synthesis,properties, and solvatochromism of 1,3‐dimethyl‐5‐{(thien‐2‐yl)‐[4‐(1‐piperidyl) phenyl]methylidene}‐(1H,3H)‐pyrimidine‐2,4,6‐trione

A new merocyanine dye, 1,3‐Dimethyl‐5‐{(thien‐2‐yl)‐[4‐(1‐piperidyl)phenyl]methylidene}‐ (1H, 3H)‐pyrimidine‐2,4,6‐trione 3 , has been synthesized by condensation of 2‐[4‐(piperidyl)benzoyl]thiophene 1 with N,N′‐dimethyl barbituric acid 2 . The solvatochromic response of 3 dissolved in 26 solvents of different polarity has been measured. The solvent‐dependent long‐wavelength UV/Vis spectroscopic absorption maxima, v_max, are analyzed using the empirical Kamlet–Taft solvent parameters π* (dipolarity/polarizability), α (hydrogen‐bond donating capacity), and β (hydrogen‐bond accepting ability) in terms of the well‐established linear solvation energy relationship (LSER): (1) The solvent independent coefficients s , a , and b and (v_max)₀ have been determined. The McRae equation and the empirical solvent polarity index, E_T(30) have been also used to study the solvatochromism of 3 . Copyright © 2007 John Wiley & Sons, Ltd.     

Solvent Dependence on Structure and Electronic Properties of 7-(Diethylamino) - 2H -1- Benzopyran-2- one (C-466) Laser Dye

In this work, we studied influences on the absorption and fluorescence emission spectra of coumarin-4066 (C-466) with different solvent polarity scale. The spectral shifts reflect the effect of the equilibrium solvents association across the energized solute particle, which adjusts inertially as a result of quick charge realignment upon radiative deactivation to the lowest electronic state. The dipole moments of C-466 are determined by employing the Bakhshiev, Kawski-Chamma-Viallet, Lippert-Mataga and McRae relations. The results from all these methods are, excited state dipole moment of C-466 is higher than the ground state dipole moments and which indicates molecule is less polar in the ground state. Theoretical analysis was also carried out by Density Functional theory (DFT and TD –DFT) employing the BECKE-1998 (exchange)/STO-6G basic set in ethanol solvent and in vacuum medium. The HOMO-LUMO, Solvent Accessible Surfaces (SAS) and Molecular Electrostatic Potential (MEP) were analysed to acquire additional knowledge of the molecular arrangement and electronic properties of C-466. These photophysical properties suggest delineation can be mauled for laying out new luminescent tests for various solvents microenvironment.     

Aromatic nitro substitution reaction between 4‐nitro‐N‐n‐butyl‐1,8‐naphthalimide and n‐heptanethiol in water–methanol binary mixtures

The second‐order rate constants of thiolysis by n‐heptanethiol on 4‐nitro‐N‐n‐butyl‐1,8‐naphthalimide (4NBN) are strongly affected by the water–methanol binary mixture composition reaching its maximum at around 50% mole fraction. In parallel solvent effects on 4NBN absorption molar extinction coefficient also shows a maximum at this composition region. From the spectroscopic study of reactant and product and the known H‐bond capacity of the mixture a rationalization that involves specific solvent H‐donor interaction with the nitro group is proposed to explain the kinetic data. Present findings also show a convenient methodology to obtain strongly fluorescent imides, valuable for peptide and analogs labeling as well as for thio‐naphthalimide derivatives preparations. Copyright © 2008 John Wiley & Sons, Ltd.     

Computational (solute–solvent cluster + PCM) study of medium effects on the experimental 13C and 1H NMR chemical shifts of lactones and lactams

Solvent effects on the ¹H and ¹³C NMR chemical shifts of some lactones: β‐propiolactone, γ‐butyrolactone, δ‐valerolactone and ε‐caprolactone, as well as lactams: azetidin‐2‐one, pyrrolidin‐2‐one, δ‐valerolactam and ε‐caprolactam have been investigated and discussed in a wide range of solvents. The experimental results were compared with density functional calculations using a large basis set. Solvent effects were computed by means of an integrated approach including the polarizable continuum model and an optimum number of explicit solvent molecules surrounding the solute. The agreement between computed and experimental chemical shifts fully validates our integrated approach. In order to quantify and elucidate the origin of the solvent effects on the ¹H and ¹³C chemical shifts of the selected compounds, a multi‐linear regression analysis has been carried out using the empirical Kamlet–Abboud–Taft solvatochromic parameters. It has been found that there is a good correlation between the solvent‐induced chemical shifts of ¹³C and the π* scale of solvent dipolarity polarizability. ¹H chemical shifts are affected mainly by the dipolarity–polarizability and the basicity of the solvent. An excellent agreement has been obtained between the calculated and the experimental data. Copyright © 2011 John Wiley & Sons, Ltd.     

3H‐1,2‐Dithiole‐3‐thione derivatives as novel solvatochromic dyes

The UV–Vis spectrum of 5‐(1‐butylthio)‐3H‐1,2‐dithiole‐3‐thione (1a) and that of the chromium pentacarbonyl complex of 5‐methyl‐3H‐1,2‐dithiole‐3‐thione (3) present significant changes with the solvent polarity. The two absorption bands shown by the compounds in the region above 300 nm were identified by theoretical calculations. For Compound 1a these are n→π^* and →π^* transitions and for Compound 3 the longest wavelength absorption corresponds to a charge transfer band and shows a remarkably negative solvatochromism. Not only has the wavelength of maximum absorption changed with the solvent but also the ratio of the absorbances at the two wavelengths. The effect of solvents was correlated with solvatochromic parameters such as π^* and α. The spectrum of 5‐(1‐butylthio)‐3H‐1,2‐dithiole‐3‐one ( 2 ) was also measured in different solvents but in this case the changes observed are less significant than for the other two compounds. The spectra of 1a and 3 were also determined in the presence of anionic (SDS), cationic (CTAB), and neutral surfactants (Brig‐35) and it is shown that these compounds can be used as probes for the polarity of the binding sites of organized assemblies. Copyright © 2008 John Wiley & Sons, Ltd.     

Quantum dot as spin filter and spin memory

We present the first measurement of the polarizability and the permanent dipole moment of isolated KC60 molecules by molecular beam deflection technique. We have obtained a value of 2506+/-250 A(3) for the polarizability at room temperature. The addition of a potassium atom enhances by more than a factor of 20 the polarizability of a pure C60 molecule. This very high polarizability and the lack of observed permanent dipole show that the apparent polarizability of KC60 is induced by the free skating of the potassium atom on the C60 surface, resulting in a statistical orientation of the dipole. The results are interpreted with a simple model similar to the Langevin theory for paramagnetic systems.     

Unsymmetrical dyes derived from 7,8‐dihydrobenzo[c,d]furo[2,3‐f]indole

On the basis of the 7,8‐dihydrobenzo[c,d]furo[2,3‐f]indole nucleus, a number of unsymmetrical carbocyanines as well as styryl dyes have been synthesized and their absorption spectra have been measured. Starting from the deviations of long‐wavelength maxima, the value of electron‐donor ability D has been estimated for the heterocycle under study and a number of dye end groups have been ranked by their electron‐donor properties. Experimental inferences are supported by the quantum chemically calculated bond length alternations and energy levels for the dyes concerned. Copyright © 2009 John Wiley & Sons, Ltd.     

Excitation-wavelength Dependent Fluorescence of Ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate

The excitation wavelength dependence of the steady-state and time-resolved emission spectra of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) in tetrahydrofuran (THF) at room temperature has been examined. It is found that the ratio of the fluorescence intensity of the long-wavelength and short-wavelength fluorescence bands strongly depends on the excitation wavelength, whereas the wavelengths of the fluorescence excitation and fluorescence bands maxima are independent on the observation/excitation wavelengths. The dynamic Stokes shift of fluorophore in locally excited (LE) and intramolecular charge transfer (ICT) states has been studied with a time resolution about 30 ps. The difference between Stokes shift in the LE and ICT states was attributed to the solvent response to the large photoinduced dipole moment of EAADCy in the fluorescent charge transfer state. On this base we can state that, the relaxation of the polar solvent molecules around the fluorophore was observed.     

Photophysical properties of a series of 4‐aryl substituted 1,4‐dihydropyridines

In this article, a series of Hantzsch 1,4‐dihydropyridines with different substituted aryl groups were synthesized and its spectral data obtained by UV–Vis absorption and fluorescence emission spectroscopies in solution. The dihydropyridines present absorption located around 350 nm and fluorescence emission in the blue–green region. A higher Stokes’ shift could be observed for the derivative 3b because of an intramolecular charge transfer in the excited state from the dimethylaniline to the dihydropyridine chromophores, which was corroborated by a linear relation of the fluorescence maxima (ν_max) versus the solvent polarity function (Δf) from the Lippert–Mataga correlation. A comparison between the experimental data and time‐dependent density functional theory‐polarizable continuum model calculations of the vertical transitions was performed to help on the elucidation of the photophysics of these compounds. For these calculations, the S₀ and S₁ states were optimized using Becke, three‐parameter, Lee–Yang–Parr/6‐31 G* and Configuration Interaction Singles/6‐31 G*, respectively. The predicted absorption maxima are in good agreement with the experimental; however, the theoretical fluorescence emission maxima do not match the experimental, which means that the excited specie cannot be related to neither a locally excited state nor to an aromatized structure. Copyright © 2012 John Wiley & Sons, Ltd.     

Solvent dependent study of carbonyl vibrations of 3‐phenoxybenzaldehyde and 4‐ethoxybenzaldehyde by Raman spectroscopy and ab initio calculations

A Raman spectroscopy investigation of the carbonyl stretching vibrations of 3‐phenoxybenzaldehye (3Phbz) and 4‐ethoxybenzaldeheyde (4Etob) was carried out in binary mixtures with different polar and nonpolar solvents. The purpose of this study was twofold: firstly, to describe the interaction of the carbonyl groups of two solute molecules in terms of a splitting in the isotropic and anisotropic components and secondly, to analyze their spectroscopic signatures in a binary mixture. Changes in wavenumber position, variation in the anisotropic shift and full width half maximum were investigated for binary mixtures with different mole fractions of the reference systems. In binary mixtures, the observed increase in wavenumber with solvent concentration does not show linearity, indicating the significant role of molecular interactions on the occurrence of breaking of the self‐association of the solute. In all the solvents, a gradual decrease in the anisotropic shift reflects the progressive separation of the coupled oscillators with dilution. Γ_i(η_c), 3Phbz—solvent mixtures, exhibit a gradual decrease with decrease in the concentration of the solute which is an evidence on the influence of micro viscosity on linewidth. For 4Etob, the carbonyl stretching vibration shows two well‐resolved components in the Raman spectra, attributed to the presence of two distinct carbonyl groups: hydrogen‐bonded and free carbonyl groups. The intensity ratio of the carbonyl stretching vibration of these two types of carbonyl groups is studied to understand the dynamics of solute/solvent molecules owing to hydrogen bond interactions. Ab initio calculations were employed for predicting relevant molecular structures in the binary mixtures arising from intermolecular interactions, and are related to the experimental results. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis of hydroxyoligophenylenes containing electron‐donating,electron‐accepting groups,or π‐deficient aromatic ring and their solvatochromic behavior

We have reported oligo(p‐phenylene)s (OPPs) with an OH group located at one end, namely, OPP(n)‐OHs (where n is the number of benzene rings). The OPPs exhibited significant solvatochromism; the deprotonation of the OH groups of OPP(n)‐OHs , when treated with NaH, caused a bathochromic shift of absorption maxima (λ_max) that increased with the donor numbers (DNs) of the solvents. We assumed that the solvatochromism exhibited by OPP(n)‐ONa was attributed to an intramolecular charge shift from the sodium phenoxy group(s) to the adjacent rings. In this study, to investigate the assumption, hydroxyoligophenylenes ( R‐OPP(n)‐OH ) with an electron‐donating dimethylamino group (n = 3, R = NMe₂), an electron‐accepting nitro group (n = 3, R = NO₂), and a π‐deficient pyridine ring (n = 2, R = Py) were synthesized by the Suzuki coupling reaction. The deprotonation of the OH group of by treatment with NaH caused a bathochromic shift (Δλ) of λ_max of R‐OPP(m)‐ONa . The Δλ of the deprotonated species increased with the DNs of the solvents. The emission peak positions of R‐OPP(m)‐ONa depended on the DNs of the solvents; therefore, the emission color could be tuned by changing the solvent. R‐OPP(m)‐OH received an electrochemical oxidation of the OH group and OPP unit. The data related to the remarkable solvatochromic behavior of R‐OPP(n)‐ONa will be useful information for the development of new luminescent materials.     

Fully conjugated push–pull dendrons with high dipole moments in excited state; synthesis and theoretical rationalization

Novel fully conjugated push–pull dendrons were synthesized by a divergent approach to evaluate the performance of non‐conventional architectures like dendritic one in charge separation processes associated with photovoltaic events. The dipole moments in excited state were estimated by the solvatochromic method, to be related to the charge separation efficiency. A 1:2 ratio of donor–acceptor groups (methoxy and nitro groups respectively) promotes the largest dipole moment in both ground and excited state (up to 17 D), due to the efficient electron density transfer over the entire molecule, through the π‐electron system. The synthesized dendrons induce charge transfer on excitation as follows from UV–vis absorption–emission analysis and theoretical calculations. Copyright © 2015 John Wiley & Sons, Ltd.     

Solvatochromism of 2‐(N,N‐dimethylamino)‐7‐nitrofluorene and the natural dye β‐carotene: application for the determination of solvent dipolarity and polarizability

The effects of solvents on chemical phenomena (rate and equilibrium constants, spectroscopic transitions, etc.) are conveniently described by solvation free‐energy relationships that take into account solvent acidity, basicity and dipolarity/polarizability. The latter can be separated into its components by manipulating the UV–vis spectra of two solvatochromic probes, 2‐(N,N‐dimethylamino)‐7‐nitrofluorene (DMANF) and a di‐(tert‐butyl)‐tetramethyl docosanonaen probe (ttbP9) whose synthesis is laborious and expensive. Recently, we have shown that the natural dye β‐carotene can be conveniently employed instead of ttbP9 for the determination of solvent polarizability (SP) of 76 molecular solvents and four ionic liquids. In the present work, we report the polarizabilities of further 24 solvents. Based on the solvatochromism of β‐carotene and DMANF, we have calculated solvent dipolarity (SD) for 103 protic and aprotic molecular solvents, and ionic liquids. The dependence of SD and SP on the number of carbon atoms in the acyl‐ or alkyl group of several homologous series (alcohols; 2‐alkoxyethanols; carboxylic acid‐ anhydrides, and esters, ionic liquids) is calculated and briefly discussed. Copyright © 2013 John Wiley & Sons, Ltd.     

Optimization in Solvent Selection for Chlorin e6 in Photodynamic Therapy

The photophysical properties of chlorin e6 (Ce6) in twelve different protic, aprotic and non-polar solvents were investigated using ultraviolet–visible and fluorescence spectroscopic methods. Solvatochromic effects were determined by the changes in quantum yield, Stokes shift, fluorescence half-life and excited state dipole moments of Ce6 in the different solvents. The absorption shifts observed in different solvents were further analyzed using the Kamlet-Abboud-Taft model and the nature of solute-solvent interactions between Ce6 and different protic and aprotic solvents was elucidated. The quantum yields were found highest in protic solvents (except water), followed by aprotic and non-polar solvents. Solvent polarity parameters showed a linear increasing trend with Stokes shift and fluorescence half-life, which indicated the presence of Ce6-solvent interaction. Using the Kamlet-Abboud-Taft model, a direct correlation between the solvent polarity parameters and absorption shift was observed, which substantiated the existence of Ce6-solvent interaction by hydrogen bond formation. The excited state dipole moments in specific protic and aprotic solvents were found to be higher than the ground state dipole moments, implying a more polar nature of Ce6 during excited state transition.     

溶剂效应对β胡萝卜素分子电子振动耦合的影响

测量了10种典型溶剂中β胡萝卜素分子的紫外-可见吸收谱和共振拉曼光谱.结果表明:溶剂的极化率、介电常数都对β胡萝卜素分子的电子-振动耦合有影响;随着极化率的增大,β胡萝卜素分子的黄昆因子、电子-振动耦合常数减小,拉曼截面增加,且这些影响与溶剂极性无关;随着溶剂介电常数的增加,对于非极性溶剂,β胡萝卜素分子的黄昆因子、电子-振动耦合常数减小,拉曼截面增加,对于极性溶剂,没有获得比较好的规律.给出了溶剂性质对电子-振动耦合的影响规律,为分子的电子-振动耦合研究中溶剂的选择提供了参考.