Effect of nitro groups on the photo physical properties of benzimidazolone: a solvatochromic study

Electronic absorption and fluorescence spectra of mono, di, and tri-nitro benzimidazolones are measured at room temperature (298 K) in nine solvents with different polarities and the observed shifts are compared with benzimidazolone. Ground and excited state electric dipole moments are determined using the solvatochromic method based on the bulk solvent properties, F(1)(ε, n) and F(2)(ε, n). A reasonable agreement is observed between the experimental and ab initio dipole moments. Change in dipole moment is also determined using the solvatochromic method based on the microscopic solvent polarity parameter, (E(T)(N)), which considers the polarization changes due to hydrogen bonding in different solvents. It has been observed that the correlation of the solvatochromic Stokes shifts with the parameter (E(T)(N)), is superior to that derived using bulk solvent polarity functions for all the benzimidazolones reported in the present study. Calculated difference between excited state and ground state dipole moments seems to be a good measure of the effect of nitro group when correlated with (E(T)(N)).     

Solvatochromism of 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]alanine methyl ester. A new fluorescence probe

The photophysical properties of 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]alanine methyl ester (1b) and its Boc derivative (1a) were studied in a series of solvents. Its UV-Vis absorption spectra are less sensitive to the solvent polarity than the corresponding fluorescence spectra which show pronounced solvatochromic effect leading to large Stokes shifts. Using an efficient solvatochromic method, based on the molecular-microscopic empirical solvent polarity parameter E(T)(N), a large change of the dipole moment on excitation has been found. From an analysis of the solvatochromic behaviour of the UV-Vis absorption and fluorescence spectra in terms of bulk solvent polarity functions, f(epsilon(r),n) and g(n), a large excited-state dipole moment (mu(e) = 11D), almost perpendicular to the smaller ground-state dipole moment, was observed. This demonstrates the formation of an intramolecular charge-transfer excited state. Large changes of the fluorescence quantum yields as well as the fluorescence lifetimes with an increase of a solvent polarity cause that the new non-proteinogenic amino acid, 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]-alanine methyl ester, is a new useful fluorescence probe for biophysical studies of peptides and proteins.     

Solvent effect on absorption and fluorescence spectra of coumarin laser dyes: evaluation of ground and excited state dipole moments

The effect of solvents on absorption and fluorescence spectra and dipole moments of coumarin 307 (C307) and coumarin 522B (C522B) have been studied extensively in various solvents, viz., general solvents, alcohols and binary mixtures (acetonitrile-benzene) at 298K. The bathchromic shift observed in absorption and fluorescence spectra of C307 and C522B with increasing solvent polarity indicates that transition involved are pi-->pi*. Solvatochromic correlations were used to obtain the ground and excited state dipole moments. The excited state dipole moments are observed to be greater than their ground state counterparts in all the solvents studied. Further, the experimentally obtained Deltamu were compared with those using normalized polarity terms E(T)(N) from Reichardt equation.     

Excited-state dipole moments of some hydroxycoumarin dyes using an efficient solvatochromic method based on the solvent polarity parameter, EN(T)

Excited-state dipole moments of some hydroxycoumarins, extensively used as laser dyes, have been determined using the solvatochromic method based on the microscopic solvent polarity parameter EN(T). Agreement between experimental and Austin model 1 (AM 1) calculated dipole moment changes has been found to be close in most of the cases. Our results are expected to be quite reliable in view of the fact that the correlation of the solvatochromic Stokes shifts is superior to that obtained using bulk solvent polarity functions. The dipole moments in the excited state, for all the molecules investigated, are higher than the corresponding values in the ground state. The increase in dipole moment upon excitation has been explained in terms of the nature of emitting state and resonance structure.     

Solvatochromic study of excited state dipole moments of some biologically active indoles and tryptamines

Absorption and fluorescence spectra of some biologically active indole and tryptamine derivatives have been recorded at room temperature in solvents of different polarities. The interest in the photophysical properties of these molecules arises mainly from their utility in medicinal chemistry as neurotransmitter and hallucination/hallucinic agents. Excited-state dipole moments of these molecules have been estimated from solvent-dependent Stokes shift data using a solvatochromic method based on a microscopic solvent polarity parameter (ETN). All indoles show a substantial increase in the dipole moment upon excitation to the emitting state. These results are generally consistent with the Parametric Method 3 (PM3) calculations, and are found to be quite reliable in view of the fact that the correlation of the solvatochromic Stokes shifts with the microscopic solvent polarity parameter (ETN) is superior to that obtained using bulk solvent polarity functions.     

Photophysics of benzazole derived push–pull butadienes: A highly sensitive fluorescence probes

Two new homologues of 1,4-diphenylbutadiene, namely, 1-(2-benzazolyl)-4-(p-dimethylaminophenyl)buta-1,3-diene have been synthesized and their absorption and fluorescence properties have been investigated in different organic solvents. The absorption spectra are less sensitive to the solvent polarity than the corresponding fluorescence spectra, which show dual emission and high solvatochromic effect in polar solvents. Using an efficient solvatochromic method, a large excited state dipole moment parallel to the smaller ground state dipole moment was calculated. Other properties of the lowest excited state such as the planar ICT B_u nature, fluorescence quantum yield and the basicity of the two nitrogen atoms (of the benzoxazole or benzothiazole ring as well as the amino group) were studied by spectroscopic techniques and semiempirical AM1 quantum chemical calculations. The findings have been presented and discussed along with the promising fluorescence probing and pH-sensing properties of these two dienes. The main spectroscopic properties of the two derivatives have been also compared.     

Revisiting the photophysical properties and excited singlet-state dipole moments of several coumarin derivatives

The solvent effects on the electronic absorption and fluorescence emission spectra of several coumarins derivatives, containing amino, N,N-dimethyl-amino, N,N-diethyl-amino, hydroxyl, methyl, carboxyl, or halogen substituents at the positions 7, 4, or 3, were investigated in eight solvents with various polarities. The first excited singlet-state dipole moments of these coumarins were determined by various solvatochromic methods, using the theoretical ground-state dipole moments which were calculated by the AM1 method. The first excited singlet-state dipole moment values were obtained by the Bakhshiev, Kawski-Chamma-Viallet, Lippert-Mataga, and Reichardt-Dimroth equations, and were compared to the ground-state dipole moments. In all cases, the dipole moments were found to be higher in the excited singlet-state than in the ground state because of the different electron densities in both states. The red-shifts of the absorption and fluorescence emission bands, observed for most compounds upon increasing the solvent polarity, indicated that the electronic transitions were of π-π* nature.     

Solvatochromic behavior on the absorption and fluorescence spectra of Rose Bengal dye in various solvents

The absorption and fluorescence spectra of Rose Bengal dye were studied in various solvents. It was found that solvent effects on the absorption wavelength are consistent with the solvatochromic model of Kamlet, Abboud and Taft. The solvent polarizability value pi* was found to have a linear relationship with the absorption wavelength of the dye in various solvents. Additionally, the normalized transition energy value (E(T)(N)) showed some scattering when plotted versus Deltanu(af). Density functional calculations were used to assign the absorption in the region 540-570 nm to a pi-pi* transition between the HOMO and LUMO of the anion. Experimental ground state and excited state dipole moments were calculated by using the solvatochromatic shifts of absorption and fluorescence spectra as a function of the dielectric constant (epsilon) and refractive index (n). The dipole moment for Rose Bengal was found to be 1.72 Debye in the ground state, whereas this value was 2.33 Debye in the excited state.     

Solvatochromism of a novel betaine dye derived from purine

A novel solvatochromic betaine dye has been synthesized from xanthosine and characterized spectroscopically by UV-vis in a broad range of solvents. The dye 9-(2',3',5'-tri-O-acetyl-beta-d-ribofuranosyl)-2-(pyridinium-1-yl)-9H-purin-6-olate, 1a, exhibits solvent-induced spectral band shifts that are (2)/(3) as large as that of the betaine known as Reichardt's dye, which forms the basis of the E(T)(30) solvent polarity scale. Moreover, the dye 1a is a ribonucleoside and hence has the potential application as a polarity probe for application in RNA oligonucleotides. The isomeric dye 6-(pyridinium-1)-yl-9H-purin-2-olate, 2a, has also been synthesized and exhibits slightly smaller solvatochromic band shifts. The new betaine dyes have also been studied by comparing the experimental and calculated solvatochromic shifts based on the calculation of the UV/vis absorption spectra, using a combination of methods with density functional theory (DFT). The COSMO continuum dielectric method, an applied electric field term in the Hamiltonian, and time-dependent density functional theory (TD-DFT) methods were used to obtain absorption energies, ground-state dipole moments, and the difference dipole moment between the ground and excited states. The calculations predict a lower energy absorption band of charge-transfer character that is highly solvatochromic, and a higher energy absorption band that has pi-pi character which is not solvatochromic, in agreement with the experimental data. For Reichardt's dye the difference dipole moment between the ground and excited state (Deltamu = mu(e) - mu(g)) was also calculated and compared to experiment: Deltamu(calcd) = -6 D and Deltamu(exptl) = -9 +/- 1 D.(1) The ground-state dipole moment was found to be mu(g)(calcd) = 18 D and mu(g)(exptl) = 14.8 +/- 1.2 D.(1).     

Reversal in solvatochromism in some novel styrylpyridinium dyes having a hydrophobic cleft

The influence of solvent polarity on the electronic transition of four different N-hexadecyl styrylpyridinium dyes has been investigated in 15 solvents. The E(T)(30) scale has been used to propose a quantitative approach towards the relative stability of the electronic ground and excited state species. The extents of contribution of dipolar aprotic solvents towards the solvation of the excited species have been determined to be 42-48% for some of the dyes. Instead of a steady solvatochromism, all the dyes suffer a reversal in solvatochromism. The transitions of the solvatochromism, referred to as solvatochromic switches, are found to be at E(T)(30) values of approximately 50 for methyl and N,N-dimethylamino substituted dyes while at 37.6 for hydroxyl substituted dye and approximately 45 for 4-(1-methyl-2-phenylethenyl) pyridinium dye. A reversal in the trend of solvent effect in the later dye corresponding to 4-(4-methyl styryl)pyridinium dye has been attributed to an analogy of series and parallel electron flow.     

Photophysical properties of safranine O in protic solvents

Spectroscopic and photophysical properties of safranine O (Sf) were investigated in binary water/solvent mixtures. It was found that these properties are strongly solvent-dependent. A blue shift is observed for both the ground-state absorption and the triplet-triplet main absorption band when the solvent polarity augments. At the same time a red shift of the fluorescence emission band takes place. These facts are interpreted in terms of higher dipole moment of the dye molecule in the S(1) state as compared with the S(0) state, while a decrease in the dipole moment of the triplet state T(n) with respect to the triplet state T(1) occurs. The Stokes' shift and the fluorescence lifetime shows a linear correlation with the E(T)(30) parameter, while a non-linear behavior is observed when a correlation with models of a continuous dielectric solvent is attempted. These results suggest the operation of strong specific interactions of Sf with solvent molecules, most likely hydrogen bonding. From fluorescence lifetime and quantum yield determinations, as well as intersystem-crossing quantum yields, the solvent dependence of the photophysical kinetic parameters were obtained. The radiative fluorescence rate constant can be adequately reproduced by calculations based on the UV-Vis absorption and emission spectra, as given by the Strickler-Berg equation.     

Solvent effects on the thioamide rotational barrier: an experimental and theoretical study.

The solvent effect on the C-N rotational barriers of N,N-dimethylthioformamide (DMTF) and N,N-dimethylthioacetamide (DMTA) has been investigated using ab initio theory and NMR spectroscopy. Selective inversion recovery NMR experiments were used to measure rotational barriers in a series of solvents. These data are compared to ab initio results at the G2(MP2) theoretical level. The latter are corrected for large amplitude vibrational motions to give differences in free energy. The calculated gas phase barriers are in very good agreement with the experimental values. Solvation effects were calculated using reaction field theory. This approach has been found to give barriers that are in good agreement with experiment for many aprotic, nonaromatic solvents that do not engage in specific interactions with the solute molecules. The calculated solution-phase barriers for the thioamides using the above solvents are also in good agreement with the observed barriers. The solvent effect on the thioamide rotational barrier is larger than that for the amides because the thioamides have a larger ground-state dipole moment, and there is a larger change in dipole moment with increasing solvent polarity. The transition-state dipole moments for the amides and thioamides are relatively similar. The origin of the C-N rotational barrier and its relation to the concept of amide "resonance" is examined.     

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

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

Spectroscopic investigation of 3-pyrazolyl 2-pyrazoline derivative in homogeneous solvents

How solvent conditions such as solvent polarity and hydrogen-bonding affect the fluorescence of a newly synthesized 3-pyrazolyl 2-pyrazoline derivative (Pyz) having pharmaceutical activity has been explored. The solvatochromic effect of Pyz is due to a change in dipole moment of the compound in the excited state. The relaxation of S1 state is perturbed in hydrogen-bonding solvents. The fluorescence properties of the systems are strongly dependent on the polarity of the media. The non-radiative relaxation process is facilitated by an increase in the polarity of the media. The photophysical response of Pyz in different solvents has been explained considering solute-solvent interactions.     

Ultrafast dynamics of the excited states of 1-(p-nitrophenyl)-2-(hydroxymethyl)pyrrolidine

The dynamics of the excited states of 1-(p-nitrophenyl)-2-(hydroxymethyl)pyrrolidine (p-NPP) has been investigated using the subpicosecond transient absorption spectroscopic technique in different kinds of solvents. Following photoexcitation using 400 nm light, conformational relaxation via twisting of the nitro group, internal conversion (IC) and the intersystem crossing (ISC) processes have been established to be the three major relaxation pathways responsible for the ultrafast deactivation of the excited singlet (S(1)) state. Although the nitro-twisting process has been observed in all kinds of solvents, the relative probability of the occurrence of the other two processes has been found to be extremely sensitive to solvent polarity, because of alteration of the relative energies of the S(1) and the triplet (T(n)) states. In the solvents of lower polarity, the ISC is predominant over the IC process, because of near isoenergeticity of the S(1)(ππ*) and T(3)(nπ*) states. On the other hand, in the solvents of very large polarity, the energy of the S(1)(ππ*) state becomes lower than those of both the T(3)(nπ*) and T(2)(nπ*/ππ*) states, but those of the T(1)(ππ*) state and the IC process to the ground electronic (S(0)) state are predominant over the ISC, and hence the triplet yield is nearly negligible. However, in the solvents of medium polarity, the S(1) and T(2) states become isoenergetic and the deactivation of the S(1) state is directed to both the IC and ISC channels. In the solvents of low and medium polarity, following the ISC process, the excited states undergo IC, vibrational relaxation, and solvation in the triplet manifold. On the other hand, following the IC process in the Franck-Condon region of the S(0) state, the vibrationally hot molecules with the twisted nitro group subsequently undergo the reverse nitro-twisting process via dissipation of the excess vibrational energy to the solvent or vibrational cooling.     

Photophysics of Nile red in solution: steady state spectroscopy

Spectroscopic properties of Nile red (NR) in organic solvents, binary solvent mixtures have been studied. Remarkable shifts in the emission band positions have been observed as a function of the polarity of the medium. In solvent mixtures, these shifts can be explained by the process of specific solvation known as dielectric enrichment. The displacement of the fluorescence band was also measured as a function of temperature to obtain the thermochromic shifts (15 cm(-1) K(-1) in methyltetrahydrofuran and 13.8 cm(-1) K(-1) in butanol). Excited state dipole moments were calculated from these shifts.     

Solvation in pure liquids: what can be learned from the use of pairs of indicators?

The solvation of six solvatochromic probes in a large number of solvents (33-68) was examined at 25 degrees C. The probes employed were the following: 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl) phenolate (RB); 4-[(E)2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePM; 1-methylquinolinium-8-olate, QB; 2-bromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr, 2,6-dichloro-4-(2,4,6-triphenyl pyridinium-1-yl) phenolate (WB); and 2,6-dibromo-4-[(E)-2-(1-methylpyridinium-4-yl)ethenyl] phenolate, MePMBr(2), respectively. Of these, MePMBr is a novel compound. They can be grouped in three pairs, each with similar pK(a) in water but with different molecular properties, for example, lipophilicity and dipole moment. These pairs are formed by RB and MePM; QB and MePMBr; WB and MePMBr(2), respectively. Theoretical calculations were carried out in order to calculate their physicochemical properties including bond lengths, dihedral angles, dipole moments, and wavelength of absorption of the intramolecular charge-transfer band in four solvents, water, methanol, acetone, and DMSO, respectively. The data calculated were in excellent agreement with available experimental data, for example, bond length and dihedral angles. This gives credence to the use of the calculated properties in explaining the solvatochromic behaviors observed. The dependence of an empirical solvent polarity scale E(T)(probe) in kcal/mol on the physicochemical properties of the solvent (acidity, basicity, and dipolarity/polarizability) and those of the probes (pK(a), and dipole moment) was analyzed by using known multiparameter solvation equations. For each pair of probes, values of E(T)(probe) (for example, E(T)(MePM) versus E(T)(RB)) were found to be linearly correlated with correlation coefficients, r, between 0.9548 and 0.9860. For the mercyanine series, the values of E(T)(probe) also correlated linearly, with (r) of 0.9772 (MePMBr versus MePM) and 0.9919 (MePMBr(2) versus MePM). The response of each pair of probes (of similar pK(a)) to solvent acidity is the same, provided that solute-solvent hydrogen-bonding is not seriously affected by steric crowding (as in case of RB). We show, for the first time, that the response to solvent dipolarity/polarizability is linearly correlated to the dipole moment of the probes. The successive introduction of bromine atoms in MePM (to give MePMBr, then MePMBr(2)) leads to the following linear decrease: pK(a) in water, length of the phenolate oxygen-carbon bond, length of the central ethylenic bond, susceptibility to solvent acidity, and susceptibility to solvent dipolarity/polarizability. Thus studying the solvation of probes whose molecular structures are varied systematically produces a wealth of information on the effect of solute structure on its solvation. The results of solvation of the present probes were employed in order to test the goodness of fit of two independent sets of solvent solvatochromic parameters.     

Solvent and Media Effects on the Photophysics of Naphthoxazole Derivatives

The photophysical properties of 2‐phenyl‐naphtho[1,2‐d][1,3]oxazole, 2(4‐N,N‐dimethylaminophenyl)naphtho[1,2‐d][1,3]oxazole and 2(4‐N,N‐diphenylaminophenyl) naphtho[1,2‐d][1,3]oxazole were studied in a series of solvents. UV–Vis absorption spectra are insensitive to solvent polarity whereas the fluorescence spectra in the same solvent set show an important solvatochromic effect leading to large Stokes shifts. Linear solvation energy relationships were employed to correlate the position of fluorescence spectra maxima with microscopic empirical solvent parameters. This study indicates that important intramolecular charge transfer takes place during the excitation process. In addition, an analysis of the solvatochromic behavior of the UV–Vis absorption and fluorescence spectra in terms of the Lippert–Mataga equation shows a large increase in the excited‐state dipole moment, which is also compatible with the formation of an intramolecular charge‐transfer excited state. We propose both naphthoxazole derivatives as suitable fluorescent probes to determine physicochemical microproperties in several systems and as dyes in dye lasers; consequence of their high fluorescence quantum yields in most solvents, their large molar absorption coefficients, with fluorescence lifetimes in the range 1–3 ns as well as their high photostability.     

Study on the fluorescence properties of a new intramolecular charge transfer compound 1,5-diphenyl-3-(N-ethylcarbazole-3-yl)-2-pyrazoline

The fluorescence properties of a newly synthesized compound, 1,5-diphenyl-3-(N-ethylcarbazole-3-yl)-2-pyrazoline (DEP) have been studied. On excitation at 352 nm, the fluorescence spectrum exhibits a large red shift with an increase in the polarity of solvents. The intensity of the band is different in different solvents as well. The change in the dipole moment in various solvents at room temperature has been characterized by the absorption and steady state fluorescence techniques and calculated based on the Lippert-Mataga equation. DEP has an increase of dipole moment of 2.83 D units on excitation to the lowest singlet state. It is concluded that photo-induced charge transfer from N (1) to C (3) actually exists in the excited state of the pyrazoline moiety. Its fluorescence property is relative to viscosity and temperature of solvents. The phi(f) of DEP in neutral medium or basic medium is higher than acidic medium. In addition, when the concentration of DEP is higher than 10(-3) M, its fluorescence is quenched by the collision of each molecule. The red shift of the maximum emission of DEP attributes to the formation of aggregates and the conjugate system is strengthened.     

(1)H NMR investigation of solvent effects in aromatic stacking interactions

One of the marquis challenges in modern Organic Chemistry concerns the design and synthesis of abiotic compounds that emulate the exquisite complex structures and/or functions of biological macromolecules. Oligomers possessing the propensity to adopt well-defined compact conformations, or foldamers, have been attained utilizing hydrogen bonding, torsional restriction, and solvophobic interactions.(1) In this laboratory, aromatic electron donor--acceptor interactions have been exploited in the design of aedamers--foldamers that adopt a novel, pleated secondary structure in aqueous solution. Herein is reported detailed (1)H NMR binding studies of aedamer monomers that were carried out in solvents and solvent mixtures covering a broad polarity range. Curve-fitting analysis of the binding data using a model that incorporated the formation of higher order and self-associated complexes yielded a linear free energy relationship between the free energy of complexation and the empirical solvent polarity parameter, E(T)(30). From these studies, the association of electron-rich and electron-deficient aedamer monomers was seen to be driven primarily by hydrophobic interactions in polar solvents. However, the magnitude of these interactions is modulated to a significant extent by the geometry of the donor--acceptor complex, which, in turn, is dictated by the electrostatic complementarity between the electron-deficient and electron-rich aromatic faces of the monomers.