SOLVENT DEPENDENCE OF PYRIMIDINE DIMER SPLITTING IN A COVALENTLY LINKED DIMER-INDOLE SYSTEM

Cyclobutadipyrimidines (pyrimidine dimers) undergo splitting that is photosensitized by indole derivatives. We have prepared a compound in which a two-carbon linker connects a dimer to an indolyl group. Indolyl fluorescence quenching indicated that the two portions of the molecule interact in the excited state. Intramolecular photosensitization of dimer splitting was remarkably solvent dependent, ranging from phi spl = 0.06 in water to a high value of phi spl = 0.41 in the least polar solvent mixture examined, 1,4-dioxane-isopentane(5 : 95). A derivative with a 5-methoxy substituent on the indolyl ring behaved similarly. These results have been interpreted in terms of electron transfer from the excited indolyl group to the dimer, which would produce a charge-separated species. The dimer anion within such a species could split or undergo back electron transfer. The possibility that back electron transfer is in the Marcus inverted region can be used to rationalize the observed solvent dependence of splitting. In the inverted region, the high driving force of a charge recombination exceeds the reorganization energy of the solvent, which is less for solvents of low polarity than those of high polarity. If this theory is applicable to the hypothetical charge-separated species, a slower back electron transfer, and consequently higher splitting efficiencies, would be expected in solvents of lower polarity. Photolyases may have evolved in which a low polarity active site retards back transfer of an electron and thereby contributes to the efficiency of the enzymatic dimer splitting.     

Ultrafast photoisomerization of photoactive yellow protein chromophore analogues in solution: influence of the protonation state.

We investigate solvent viscosity and polarity effects on the photoisomerization of the protonated and deprotonated forms of two analogues of the photoactive yellow protein (PYP) chromophore. These are trans-p-hydroxybenzylidene acetone and trans-p-hydroxyphenyl cinnamate, studied in solutions of different polarity and viscosity at room temperature, by means of femtosecond fluorescence up-conversion. The fluorescence lifetimes of the protonated forms are found to be barely sensitive to solvent viscosity, and to increase with increasing solvent polarity. In contrast, the fluorescence decays of the deprotonated forms are significantly slowed down in viscous media and accelerated in polar solvents. These results elucidate the dramatic influence of the protonation state of the PYP chromophore analogues on their photoinduced dynamics. The viscosity and polarity effects are, respectively, interpreted in terms of different isomerization coordinates and charge redistribution in S(1). A trans-to-cis isomerization mechanism involving mainly the ethylenic double-bond torsion and/or solvation is proposed for the anionic forms, whereas "concerted" intramolecular motions are proposed for the neutral forms.     

Efficient Excited‐State Symmetry Breaking in a Cationic Quadrupolar System Bearing Diphenylamino Donors

We report a joint experimental and theoretical investigation of a quadrupolar D–π–A⁺–π–D system, the electron donors being diphenylamino groups and the electron acceptor being a methylpyridinium, in comparison with the dipolar D–π–A⁺ system. The emission spectra of the two compounds overlap in all the investigated solvents. This finding could be rationalized by TD‐DFT calculations: the LUMO–HOMO molecular orbitals involved in the emission transition are localized on the same branch of the quadrupolar structure that becomes the fluorescent portion, corresponding to that of the single‐arm compound. Excited‐state symmetry breaking has been rarely observed for quadrupolar systems showing negative solvatochromism and is here surprisingly revealed, even in low polarity solvents. Femtosecond transient absorption measurements revealed that an efficient photoinduced intramolecular charge transfer takes place in the quadrupolar chromophore, more efficient than in its dipolar analogue. This result is promising in view of the application of these compounds as novel two‐photon absorbing materials.     

Photophysical Properties of Intramolecular Charge Transfer in a Tribranched Donor–π–Acceptor Chromophore

The photophysical properties of intramolecular charge transfer (ICT) in a novel tribranched donor–π–acceptor chromophore, triphenoxazine‐2,4,6‐triphenyl‐1,3,5‐triazine (tri‐PXZ‐TRZ), with thermally activated delayed fluorescence character was investigated in different aprotic solvents by steady‐state spectroscopy and femtosecond and nanosecond transient absorption spectroscopy measurements. Increasing the solvent polarity led to a significant increase in the Stokes shift. The large Stokes shift in highly polar solvents was attributed to ICT properties upon excitation; this resulted in a strong interaction between the tri‐PXZ‐TRZ molecule and the surrounding solvent, which led to a strong solvation process. Quantum‐chemical calculations and changes in the dipole moment showed that this compound has a large degree of ICT. Furthermore, an apolar environment helped to preserve the symmetry of tri‐PXZ‐TRZ and to enhance its emission efficiency. The femtosecond and nanosecond transient absorption spectroscopy results indicated that the excited‐state dynamics of this push–pull molecule were strongly influenced by solvent polarity through the formation of a solvent‐stabilized ICT state. The excited‐state relaxation mechanism of tri‐PXZ‐TRZ was proposed by performing target model analysis on the femtosecond transient absorption spectra. In addition, the delayed fluorescence of tri‐PXZ‐TRZ was significantly modulated by a potential competition between solvation and intersystem crossing processes.     

Solvent‐Polarity‐Tuned Morphology and Inversion of Supramolecular Chirality in a Self‐Assembled Pyridylpyrazole‐Linked Glutamide Derivative: Nanofibers,Nanotwists, Nanotubes,and Microtubes

The self‐assembly of a low‐molecular‐weight organogelator into various hierarchical structures has been achieved for a pyridylpyrazole linked L ‐glutamide amphiphile in different solvents. Upon gel formation, supramolecular chirality was observed, which exhibited an obvious dependence on the polarity of the solvent. Positive supramolecular chirality was obtained in nonpolar solvents, whereas it was inverted into negative supramolecular chirality in polar solvents. Moreover, the gelator molecules self‐assembled into a diverse array of nanostructures over a wide scale range, from nanofibers to nanotubes and microtubes, depending on the solvent polarity. Such morphological changes could even occur for the xerogels in the solvent vapors. We found that the interactions between the pyridylpyrazole headgroups and the solvents could subtly change the stacking of the molecules and, hence, their self‐assembled nanostructures. This work exemplifies that organic solvents can significantly involve the gelation, as well as tune the structure and properties, of a gel.     

Physical Properties of 8‐Substituted 5,7‐Dichloro‐2‐Styrylquinolines as Potential Light Emitting Materials

Derivatives of 5,7‐dichloro‐2‐styrylquinoline ( 1 ), modified at position 8 of quinoline moiety with a methyl ether ( 4 , DCSQM) or acetate ( 5 , DCSQA), were synthesized and investigated. Both compounds exhibited high thermal stability (Td > 320 °C). The UV‐vis absorption of DCSQM and DCSQA varied only slightly in different solvents, whereas the emission spectra showed pronounced red shifts with increasing solvent polarity, suggesting the intramolecular charge transfer character of the emission state. Compounds 4 and 5 can emit lights from blue to green color in different solvents. The solvent polarity dependent electronic transitions are attributed to efficient intramolecular charge transfer (ICT) processes, in which the HOMOs and LUMOs are localized on the styrene‐based ring and the quinoline‐based moiety, respectively. The quinoline‐based LUMO provides compelling evidence that the first reduction site occurs on the electron‐deficient quinoline moiety.     

Effect of solvents on propylene epoxidation over TS-1 catalyst

Solvents have an important effect on the epoxidation of propylene catalyzed by TS-1. The experimental results show that, in different solvents, the catalytic activity of epoxidation is in the following order: methanol > 2-propanol > 2-butanol > acetonitrile > acetone > tetrahydrofuran. Based on the reaction mechanism, the effects of solvents on the epoxidation were studied from eight aspects, which included the electronic effect, the steric effect, the polarity of solvent, the effect of solvent on sorption and diffusion of reactant, the oxidation of alcohol, the etherification of PO, the deactivation of TS-1 and the solubility of propylene in the solvents. The electronic effect, steric effect and the polarity of solvent were considered to be the main aspects. This work may provide theoretical guidance for choosing solvents for these kinds of reactions and also may serve as basis for further industrialization. __________ Translated from Journal of Tsinghua University (Science and Technology), 2007, 47(9): 116–120 [译自: 清华大学学报(自然科学版]     

Single-Benzene-Based Solvatochromic Chromophores: Color-Tunable and Bright Fluorescence in the Solid and Solution States

The search for structurally simple chromophores with superior fluorescence brightness and a wide range of solvent compatibility is highly desirable. Herein, a new type of single-benzene-based solvatochromic chromophore with a symmetric bifunctional structure, in which azetidine and ethoxycarbonyl moieties serve as the electron-donating and -withdrawing groups, respectively, is reported. This chromophore exhibits an extraordinary wide range of solvent compatibility and preserves excellent fluorescence quantum yields from nonpolar n-hexane to polar methanol and even in water. Unusually, the symmetric structure of the chromophore shows a distinct color change from bright green to red with increasing solvent polarity and possesses large Stokes shifts (λ=132–207 nm) in the tested solvents. Moreover, this single-benzene-based chromophore displays good photochemical stability in both solution and solid states, and even exhibits reversible mechanochromic luminescence.     

Photophysical Properties of Coumarin‐30 Dye in Aprotic and Protic Solvents of Varying Polarities¶

Experimental results on various photophysical properties of coumarin‐30 (C30) dye, namely, Stokes' shift (Δv), fluorescence quantum yield (τ_f), fluorescence lifetime (τ_f), radiative rate constant (k_f) and nonradiative rate constant (k_nr), as obtained using absorption and fluorescence measurements have been reported. Though in most of the solvents the properties of C30 show more or less linear correlation with the solvent polarity function, Δf= [(ε ‐ 1)/(2ε+ 1) ‐ (n² ‐ 1)/ (2n²+ l)], they show unusual deviations in nonpolar solvents at one end and in high‐polarity protic solvents at the other end. From the solvent polarity and temperature effect on the photophysical properties of the dye, following inferences have been drawn: ( 1 ) in nonpolar solvents, the dye exists in a nonpolar structure, where its 7‐NEt₂ substituent adopts a pyramidal configuration and the amino lone pair is out of resonance with the benzopyrone π cloud; ( 2 ) in medium to higher polarity solvents, the dye exists in a polar intra‐molecular charge transfer structure, where the 7‐NEt₂ group and the 1,2‐benzopyrone moiety are in the same plane and the amino lone pair is in resonance with the benzopyrone π cloud; ( 3 ) in protic solvents, the dye‐solvent intermolecular hydrogen bonding influences the photophysical properties of the dye; and ( 4 ) in high‐polarity protic solvents, the excited C30 undergoes a new activation‐controlled nonradiative deexcitation process because of the involvement of a twisted intra‐molecular charge transfer (TICT) state. Contrary to most other TICT molecules, the activation barrier for this deexcitation process in C30 is observed to increase with solvent polarity. A rational for this unusual behavior has been given on the basis of the solvent polarity‐dependent stabilization and crossing of relevant electronic states and the relative propensity of interconversion among these states.     

TIME-RESOLVED FLUORESCENCE OF NITROBENZOXADIAZOLE-AMINOHEXANOIC ACID: EFFECT OF INTERMOLECULAR HYDROGEN-BONDING ON NON-RADIATIVE DECAY

The fluorescence kinetics of the nitrobenzoxadiazole (NBD) chromophore were studied at low concentrations in solvents with varying polarity and hydrogen-bonding donor strength. The emission decay was essentially single exponential in all solvents studied. While the absorption and fluorescence solvatochromism is determined largely by the solvent polarity, the S1 state decay kinetics are strongly modulated by the solvent H-bonding capacity. The NBD emission lifetime, generally approximately 7-10 ns in the aprotic solvents, is reduced to 0.933 ns in water. The solvent deuterium isotope effect on the fluorescence decay is substantial in D2O and in methanol-d4, but is insignificant in DMSO-d6. These results are consistent with acceleration of S1----S0 internal conversion through an accepting vibrational mode created by intermolecular hydrogen-bonding of the NBD chromophore to an H atom-donating solvent. This work bears on the practically of using NBD as a fluorophore in assays for estrogen and progesterone receptors.     

Testing computational models of hyperpolarizability in a merocyanine dye using spectroscopic and DFT methods

The structural and electronic properties of a highly solvatochromic merocyanine dye, 2-(3-cyano-5,5-dimethyl-4-(3-(1-octadecylpyridin-4(1H)-ylidene)prop-1-enyl)furan-2(5H)-ylidene)malononitrile (pyr3pi), have been investigated using UV-vis, NMR, hyper-Rayleigh scattering, and Raman spectroscopies and further interpreted using computational chemistry. Spectroscopic data indicate that pyr3pi exists in its zwitterionic form even in low polarity solvents with electronic absorption spectra showing a hypsochromic shift with an increase in solvent polarity and NMR experiments indicating an increasingly zwitterionic structure in chloroform as the temperature is lowered. Raman spectra in increasingly polar solvents show small variations of the structure that are consistent with a change toward a structure with more zwitterionic character. However, comparison of the calculated and experimental vibrational energies and intensities and comparison of NMR coupling constants with calculated bond order indicate that calculations underestimate the amount of charge separation seen in low polarity solvents. Although for this system density functional theory (DFT) calculations and the two-state model qualitatively reproduce negative solvatochromism, they fail to reproduce the trends in hyperpolarizability seen experimentally. This is attributed to solvent field DFT calculations underestimating the degree of charge separation in reaction fields representing low polarity solvents.     

Vinyl monomers bearing chromophore moieties and their polymers. XI. Synthesis and photochemical behavior of carbazole‐containing methacrylic monomers and their polymers

Three carbazole‐containing methacrylic monomers, 2‐(N‐carbazolyl)ethyl methacrylate(CzEMA), 6‐(N‐carbazolyl)hexyl methacrylate(CzHMA), and 11‐(N‐carbazolyl)undecyl methacrylate (CzUMA), and their saturated model compounds, 2‐(N‐carbazolyl)ethyl isobutyrate, 6‐(N‐carbazolyl)hexyl isobutyrate, and 11‐(N‐carbazolyl)undecyl isobutyrate, were synthesized and polymerized. UV absorption spectra showed that there was either negligible or no interaction between the carbon–carbon double bond of the methacrylic group and the carbazolyl chromophore moiety in the ground state for these monomers. Fluorescence spectra of the monomers, their model compounds, and the polymers were recorded in the solvents with different polarities. CzEMA exhibited the fluorescence structural self‐quenching effect (SSQE), but CzHMA and CzUMA did not. In addition, the SSQE of CzEMA depended strongly on the polarity of the solvents. That is, the stronger the polarity of a solvent was, the more obvious the SSQE was. Therefore, the SSQE of CzEMA mainly was caused by the intramolecular charge‐transfer interaction between the excited electron‐donating carbazolyl chromophore moiety and the electron‐accepting carbon–carbon double bond of the methacrylic group. This was confirmed by the fluorescence‐decay curves and the fluorescence lifetimes of the monomers, their model compounds, and the polymers. The monomers, their model compounds, and the polymers initiated the photopolymerization of methyl methacrylate (MMA) upon UV irradiation. CzEMA showed greater initiation ability than the other two monomers and their model compounds; this was ascribed to the photoinduced intramolecular charge‐transfer interaction. The higher initiation efficiency of the homopolymers compared to that of the copolymers with MMA was interpreted as the result of singlet energy migration of the excited carbazolyl chromophores along the polymer chains. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 679–688, 2000     

A kinetic investigation of the addition of diazomethane to styrenes. Role of dipolar aprotic solvents in predicting direction of dipole orientation

The addition of diazomethane to a series of substituted styrenes has been kinetically investigated with emphasis on polar, solvent, and solvation effects. Although the reaction is generally insensitive to solvent polarity, a sizeable substituent effect (ρ = +0.90) has been observed. Also, a sizeable increase in rate was generally observed in protic solvents such as water, and a considerable rate enhancement occurred in the case of nitrostyrene in the dipolar aprotic solvent, dimethylformamide. The results are discussed in terms of a concerted mechanism involving a partially-charged transition state II, with negative charge formation on the α-carbon of the styrene. The methoxystyrene offers an exception to the Hammett equation and the unexpectedly high rate constant for this compound could be discussed in terms of a possible participation of the highly nucleophilic resonance form of p-methoxystyrene and an α-addition via the transition state III. However, solvation effects in dimethylformamide (failure to obtain the anticipated rate increase) and the formation of a 3-substituted pyrazoline adduct from the cycloaddition reaction indicate steric control to predominate over electronic factors and β-addition to prevail unequivocally. It is proposed that solvation effects in protic solvents can be utilized to advantage in cycloaddition reactions leading to 1-pyrazolines, for which no other routes are available at the present time.     

Comprehensive Photophysical Behaviour of Ethynyl Fluorenes and Ethynyl Anthracenes Investigated by Fast and Ultrafast Time‐Resolved Spectroscopy

Detailed investigations by time‐resolved transient absorption and fluorescence spectroscopies with nano‐ and femtosecond time resolutions are carried out with the aim of characterising the lowest excited singlet and triplet states of three ethynyl fluorenes ( 1 – 3 ) and three ethynyl anthracenes ( 4 – 6 ) in solvents of different polarity. The solvent is found to modify the deactivation pathways of the lowest excited singlet state of compounds 1 – 4 , thus changing their fluorescence, intersystem crossing and internal conversion efficiencies. The fluorescence and triplet yields gradually decrease, while the internal conversion quantum yield increases upon increasing the solvent dielectric constant. These experimental results, coupled with the marked fluorosolvatochromic effect, point to the involvement of an emitting state with a charge‐transfer (CT) character, strongly stabilised by polar solvents. This is proved by ultrafast spectroscopic studies in which two transients, distinguished by characteristic spectral shapes assigned to locally excited (LE) and CT states, are detected, the CT state being the longer lived and fluorescent one in highly polar solvents. The intramolecular LE→CT process, operative in highly polar media, becomes particularly fast (up to ≈300 fs) in the case of the NO₂ derivative 1 . No push–pull character is found for 5 and 6 , which exhibit different photophysical behaviour; indeed, the solvent polarity does not modify significantly the dynamics of the lowest excited singlet states. Quantum mechanical calculations at the TDDFT level are also used to determine the state order and nature of the lowest excited singlet and triplet states and to rationalise the different photophysical behaviour of fluorine and anthracene derivatives, particularly concerning the intersystem crossing process.     

Photophysics of a Coumarin in Different Solvents: Use of Different Solvatochromic Models

This study reported the photophysics of 7‐(diethylamino)coumarin‐3‐carboxylic acid N‐succinimidyl ester (7‐DCCAE) in different neat solvents of varying polarity using steady‐state absorption, fluorescence emission and picosecond time‐resolved spectroscopy. In nonpolar solvents, the dye molecule predominantly exists in nonpolar structure and exhibits very low value of nonradiative decay rate constant (k_nr), demonstrating the emission takes place from S₁‐LE to S₀ ground state. The fluorescence quantum yields, lifetime values of 7‐DCCAE in different solvents are rationalized on the basis of intramolecular charge transfer (ICT) followed by twisted intramolecular charge transfer state formation (TICT) as well as specific solute–solvent interactions. Several solvatochromic models (such as Lippert, Dimroth, Kamlet–Taft, Catalán 3P and Catalán 4P models) were used to analyze the solvatochromic shift of 7‐DCCAE in different solvents. The different empirical models show that the observed results are better correlate for nonchlorinated solvents and provide statistically significant best‐fit result. A comparison was done between comparatively new solvatochromic model (Catalán 3P and Catalán 4P model) with Kamlet–Taft model. The ground state structure of the said molecule was optimized by using Density Functional Theory (DFT).     

Efficient Simmons–Smith cyclopropanation with Zn/Cu and CH2I2

An appropriate solvent to perform the original Simmons–Smith reaction was reinvestigated. Among available solvents, cyclopentyl methyl ether (CPME), a recently commercialized ethereal solvent, was found to be the best so far. Compared with Et₂O under reflux – the commonest conditions – reaction completion in CPME at 50 °C was about 10 times faster. The product yields and selectivities were mostly identical to those with Et₂O, but were better in some cases; e.g. 13–56% with 2‐cyclohexenol. The good performance of CPME should be mainly due to its moderate polarity and high boiling point. Copyright © 2012 John Wiley & Sons, Ltd.     

Reactions of aromatic sulphonyl chlorides with anilines : Studies of solvent effects by the approach of multiparameter empirical correlations

The reaction kinetics of 5-substituted 2-thiophenesulphonyl chlorides with anilines were studied in fourteen pure solvents (protic and aprotic) and in mixed solvents at 25°. The approach of multiparameter equations to describe solvent effects according to the Palm-Koppel and Krygowski-Fawcett models was unsuccessful. Instead satisfactory single parameter linear correlations, one for protic solvents with positive slope and another for aprotic solvents with negative slope, were found by using the dielectric constant ?. An S_AN mechanism for these reactions was proposed, bond-making being the rate-determining step for protic solvents and bond-breaking for aprotic ones. The analysis of some data for the reactions of benzenesulphonyl chloride showed that the mechanism is analogous also for this substrate and the rate-determining step is depending on both solvent and nucleophile. Hammett ρ-values for the reactions of substituted 2-thiophenesulphonyl chlorides with aniline are in accord with the proposed mechanism. ?-Values for the reactions of 2-thiophenesulphonyl chloride with substituted anilines are related to the solvent effects by equation ? = ? 15.7 f(?) + 0.113E + 3.94. The solvent effects on these values can be interpreted by the effect of the dielectric constant and the influence of H-bonding. Mixed solvents are characterized by the presence of a maximum rate.     

Solvent dependence of the one-electron reduction of substituted benzo- and naphthoquinones

Potentials for the one-electron reduction of a series of substituted benzo- and naphthoquinones were measured in 10 aprotic solvents by cyclic voltammetry and used to construct Hammett plots. Hammett reaction constants, ρ, were determined in each solvent and used as indicators of the solvent-mediated stabilization of the charged radical-anion product of the reduction reaction. Correlations of Hammett ρ values with solvent parameters suggest that the Lewis acidity of the solvent, in particular the solvent's ability to donate a partially positive proton, is a consistent predictor of the degree of charge stabilization of the quinone radical anion. Other mechanisms of charge stabilization, including solvent–solute charge transfer complexes and dipole–dipole interactions involving the molecular dipole moment (or an induced dipole moment) of the solvent, were found to be inconsistent predictors of the effect of solvent on one-electron quinone reduction in aprotic media.     

Synthesis and Spectral Properties of 4‐(2,5‐Dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO)

An interesting flourophore, 4‐(2,5‐dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) was synthesized by mixing an equivalent molar quantity of hippuric acid and 2,5‐dimethoxybenzaldehyde in acetic anhydride in the presence of anhydrous sodium acetate. The absorption and fluorescence characteristics of 4‐(2,5‐dimethoxy‐phenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) were investigated in different solvents. DMPO dye exhibits red shift in both absorption and emission spectra as solvent polarity increases, indicating change in the dipole moment of molecules upon excitation due to an intramolecular charge transfer interaction. The fluorescence quantum yield depends strongly on the properties of the solvents, which was attributed to positive and negative solvatokinetic effects. A crystalline solid of DMPO gave strong excimer like emission at 630 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B‐type class of Steven's Classification. DMPO displayed fluorescence quenching by triethylamine via nonemissive exciplex formation.     

Tuning the Nature of the Fluorescent State: A Substituted Polycondensed Dye as a Case Study

An extensive spectroscopic analysis is presented of an elongated polycondensed dye with a donor–acceptor substitution. The charge‐transfer (CT) state, polarized along the long molecular axis, is close in energy to a local excitation (LE) of the polycondensed system, roughly polarized along the short molecular axis, which makes this system particularly suitable to investigate the subtle LE/CT interplay. An essential‐state model is presented that quantitatively reproduces absorption and fluorescence spectra, as well as fluorescence emission and excitation anisotropy spectra collected in solvents of different polarity and viscosity, which sets a sound basis for the understanding of how solvent polarity and solvent relaxation affect the nature of low‐lying excitations. The markedly different fluorescence emission and excitation anisotropy spectra measured in glassy and liquid polar solvents unambiguously demonstrate the major role played by solvent relaxation in the definition of fluorescence properties of the dye.