Spectroscopic characterization of intramolecular charge transfer of sodium 4-(N,N-dimethylamino)naphthalene-1-sulfonate

In this paper, a new dual fluorescent N,N-dimethylaminonaphthalene derivative, sodium 4-(N,N-dimethylamino)naphthalene-1-sulfonate (SDMDNS), was reported. It was found that SDMDNS emits dual fluorescence only in highly polar solvent water but not in organic solvents such as methanol, dioxane and acetonitrile. Only a single broad band emission at ca. 420 nm was observed in the short wavelength region in organic solvents. The dual fluorescence of SDMDNS in water was found at 423 and 520 nm, respectively. Introduction of organic solvent as ethanol into aqueous solution of SDMDNS leads to blue shift of the long-wavelength emission, and this was evidently supported by introduction of cyclodextrin or surfactant in the aqueous solution. It indicates that a highly polar solvent was required to bring out dual fluorescence; furthermore, the short wavelength fluorescence is emitted from locally excited (LE) state and the long wavelength fluorescence is emitted from charge transfer (CT) state. The pH dependence of the dual fluorescence of SDMDNS demonstrates that the neutral form of the molecular has a higher ratio of CT band intensity to LE band. Temperature effect on the excited state of SDMDNS was also examined and gave stabilization enthalpy (-DeltaH ) of the CT reaction 8.7 kJ mol(-1).     

Unusual Solvatofluorochromism of the Asymmetrically Substituted Aromatic Acetylene Derivative CNAacDMA**

The photophysical properties of the newly synthesized unsymmetrically substituted aromatic acetylene derivative 9-(2-(4-(N,N-dimethylamino)phenyl)ethynyl)anthracene-10-carbonitrile (CNAacDMA) were investigated with the steady-state and time-resolved fluorometry. In saturated hydrocarbon solvents, only fluorescence from a locally-excited state (LE) is recorded. In more polar solvents however, excitation of this dye leads to a charge transfer state (CT). In moderate polar solvents (ϵ=4–8) dual emission is observed as a result of competition between structural change and intramolecular charge transfer in the excited state. In polar solvents only one emission band, at shorter wavelength than CT emission, is observed, indicating a bidirectional solvatofluorochromism.     

Solvation processes on phenyl‐bonded stationary phases—The influence of polar functional groups

A series of phenyl‐bonded stationary phases with incorporated polar functional groups was subjected to an adsorption investigation. Measurement of acetonitrile and methanol adsorption was obtained using the minor disturbance method. It was observed that adsorption of organic solvent strongly depends on the presence of polar functional groups in the bonded phases that influence the hydrophobicity and polarity of the stationary phase surface. Additionally, relative adsorption of acetonitrile and methanol confirms earlier observations, that the presence of amine and amide groups in the stationary phase changes the relative elution strength of organic solvents. The heterogeneous surface of the stationary phase makes it possible to observe the competitiveness of the water and organic solvent adsorption.     

Excited-State Dynamics of Thienoguanosine,an Isomorphic Highly Fluorescent Analogue of Guanosine

Thienoguanosine (^thG) is an isomorphic analogue of guanosine with promising potentialities as fluorescent DNA label. As a free probe in protic solvents, ^thG exists in two tautomeric forms, identified as the H1, being the only one observed in nonprotic solvents, and H3 keto–amino tautomers. We herein investigate the photophysics of ^thG in solvents of different polarity, from water to dioxane, by combining time-resolved fluorescence with PCM/TD-DFT and CASSCF calculations. Fluorescence lifetimes of 14.5–20.5 and 7–13 ns were observed for the H1 and H3 tautomers, respectively, in the tested solvents. In methanol and ethanol, an additional fluorescent decay lifetime (≈3 ns) at the blue emission side (λ≈430 nm) as well as a 0.5 ns component with negative amplitude at the red edge of the spectrum, typical of an excited-state reaction, were observed. Our computational analysis explains the solvent effects observed on the tautomeric equilibrium. The main radiative and nonradiative deactivation routes have been mapped by PCM/TD-DFT calculations in solution and CASSCF in the gas phase. The most easily accessible conical intersection, involving an out-of plane motion of the sulfur atom in the five-membered ring of ^thG, is separated by a sizeable energy barrier (≥0.4 eV) from the minimum of the spectroscopic state, which explains the large experimental fluorescence quantum yield.     

室温下非极性溶剂中氧鎓盐的定域激发

The spectral properties of 8-(4-methoxyphenyl)methylene-2-phenyl-4-(4-methoxy) phenyl-5,6,7,8-tetrahydrobenzo[b]pyrylium salt in polar solvents (acetonitrile or 1,2-dichloroethane) and in non-polar solvents (toluene or benzene) have been compared. It has been shown that there are dual fluorescence emissions in non-polar solvent at room temperature, one of which corresponds to CT (charge transfer) emission, the other to LE (local excitation) emission, whereas there is only CT fluorescence emission in polar solvent at room temperature.     

卤代苯溶剂对镓咔咯配合物光物理性质的影响:外重原子效应

利用紫外-可见光谱、稳态和时间分辨荧光光谱以及飞秒瞬态吸收光谱探测了不同卤代苯溶剂对三种五氟苯基取代的镓咔咯(1-Ga、2-Ga、3-Ga)光物理性质的影响,结果表明卤代苯溶剂的色散力对于镓咔咯电子光谱吸收峰位置的影响起着主要作用;溶剂外重原子效应能显著降低镓咔咯的荧光量子产率。飞秒瞬态吸收光谱表明,光激发下,镓咔咯与卤代苯溶剂之间可发生电子转移反应,溶剂的重原子效应可以减缓电荷分离态复合物电荷重组速率。     

On the development of sensor molecules that display Fe(III)-amplified fluorescence

Incorporation of a tailor-made size-restricted dithia-aza-oxa macrocycle, 1-oxa-4,10-dithia-7-aza-cyclododecane, via a phenyl linker into two fluorescent sensor molecules with electronically decoupled, rigidly fixed, and sterically preoriented architectures, a 1,3,5-triaryl-Delta2-pyrazoline and a meso-substituted boron-dipyrromethene (BDP), yields amplified fluorescence in the red-visible spectral range upon binding of Fe(III) ions. The response to Fe(III) and potentially interfering metal ions is studied in highly polar aprotic and protic solvents for both probes as well as in neat and buffered aqueous solution for one of the sensor molecules, the BDP derivative. In organic solvents, the fluorescence of both indicators is quenched by an intramolecular charge or electron transfer in the excited state and coordination of Fe(III) leads to a revival of their fluorescence without pronounced spectral shifts. Most remarkably, the unbound BDP derivative shows dual emission in water and can be employed for the selective ratiometric signaling of Fe(III) in buffered aqueous solutions.     

Ultrafast photoinduced intramolecular charge transfer in push-pull distyryl furan and benzofuran: solvent and molecular structure effect

The excited state deactivation pathways of push-pull distyryl furan and benzofuran derivatives in several organic solvents were investigated in detail by using time-resolved transient absorption and fluorescence spectroscopies, with nano- and femto-second time resolution. Solvent polarity was found to play a key role in determining the efficiencies of fluorescence, intersystem crossing and internal conversion. The triplet yield gradually decreased, while the internal conversion increased upon increasing the solvent dielectric constant. However the fluorescence showed a different solvent polarity effect in the low and high solvent polarity region, with a reversal of the trend of fluorescence properties (quantum yield and lifetime). This fact points to an emitting state of a different nature (smaller and larger dipole moments) in the two cases, as also suggested by the huge fluorosolvatochromism. In fact the ultrafast spectroscopic investigation evidenced the presence of two transients characterized by peculiar spectral shapes assigned to a locally excited (LE) and a charge transfer (CT) state. In the more polar solvents the CT state was the longer lived, fluorescent one and an intramolecular charge transfer process was found to be operative and to become faster (up to ～200-250 fs) in the higher polarity media. On the contrary, distyrylfuran, which exhibits the same molecular skeleton without the push-pull character showed a similar excited state dynamics in solvents of different polarities.     

正己烷中对二烷基氨基苯甲酸的分子内电荷转移双重荧光

在非极性溶剂正己烷中,观察到了系列对二烷基氨基苯甲酸的双重荧光.皮秒激光诱导时间分辨荧光和溶剂极性效应研究证实:该双重荧光系由于激发态分子内电荷转移过程形成的电荷转移态和局部激发态所发射.超快反应动力学研究表明:在非极性溶剂中,对二烷基氨基苯甲酸分子内电荷转移过程是由较低的反应活化能所致.     

Long-range corrected time-dependent density functional study on fluorescence of 4,4'-dimethylaminobenzonitrile

Dual fluorescence of 4,4(')-dimethylaminobenzonitrile (DMABN) was theoretically investigated on the basis of long-range corrected time-dependent density functional theory. Excited-state geometry optimization states and single-point energy calculations with and without solvent effect were carried out. It has been explained that DMABN emits dual fluorescence only in polar solvents through locally excited (LE) and charge transfer (CT) states. It was, however, concluded from this study that although the main spectrum of dual fluorescence in acetonitrile solvent is clearly due to twisted intramolecular CT fluorescence, small secondary fluorescence in acetonitrile may also emanate from CT fluorescence during the DMABN twisting process. This conclusion is supported by an experimental interpretation on polarization spectroscopy. It was also found that the optimized DMABN geometries have certain wagging angles for the CT state and no wagging angle for the LE state. This may support an early experimental hypothesis that the dual fluorescence of DMABN is induced by the wagging mode due to vibronic coupling between LE and CT states. Consequently, the authors propose a fluorescence mechanism of DMABN in gas phase and in acetonitrile solvent: the main absorption proceeds to the CT state in both situations. In gas phase, single fluorescence is chiefly emitted from the LE state through the internal conversion from CT to LE states. Dual fluorescence in acetonitrile solvent may only be emitted from the CT state.     

室温下非极性溶剂中氧鎓盐的定域激发

氧鎓盐类化合物因具有较强的亲电性，在有机合成[1]及电照相体系[2]中存在着广泛的应用前景．由于它们还表现出较强的荧光发射能力［3］，因而在作为激光工作材料［4，5］方面也受到重视．正因如此，这类化合物的光物理和光谱特性引起了许多化学家的兴趣．氧钠盐化合物溶液的荧     

Water-induced fluorescence quenching of aniline and its derivatives in aqueous solution

Nonradiative deactivation processes of excited aniline and its derivatives in aqueous solution were investigated by steady-state and time-resolved fluorescence measurements to reveal characteristic solvent effects of water on the relaxation processes of excited organic molecules. The magnitude of nonradiative rate (k_nr) of excited aniline derivatives increased significantly in water compared to that in organic solvents (cyclohexane, ethanol, and acetonitrile). The fluorescence lifetime measurements in organic solvent/H₂O mixed solvents suggested that the fluorescence quenching in water was not due to exciplex formation but due to interactions with a water cluster. From temperature effect experiments on the fluorescence lifetime and quantum yield of aniline, N-methylaniline, and N,N-dimethylaniline, the apparent activation energies for the nonradiative deactivation rate in water were determined as 21, 30, and 41 kJ mol^-1, respectively. Upon substitution of hydrogen atoms in the aromatic ring of aniline derivatives for deuterium atoms resulted in normal deuterium isotope effect in cyclohexane, i.e. k_nr decreased by deuterium substitution, while in water the same deuterium substitution led to an increase in k_nr (the inverse isotope effect). The inverse isotope effects implied that a direct internal conversion to vibrationally higher excited states in the electronically ground state is not a dominant mechanism but the transition to a close-lying energy level, e.g. the relaxation to charge transfer to solvent (ctts) state, would be associated with the quenching mechanism in water.     

Solvent Effects on Spectral Property and Dipole Moment of the Lowest Excited State of Coumarin 343 Dye

Steady-state absorption and fluorescence spectra, and time-resolved fluorescence spectra of coumarin 343 (C343) were measured in different solvents. The effect of the solvent on the spectral properties and dipole moment of the lowest excited state of C343 were investigated. It was found that the absorption and fluorescence spectra red-shifted slightly and strongly with increasing solvent polarity, respectively, because the charge distribution of the excited state leaded to the increasing difference between the absorption and fluorescence spectra with increasing solvent polarity. The dipole moment of the lowest excited state of C343was determined from solvatochromic measurements and the quantum chemical calculation, and the results obtained from these two methods were fully consistent. Investigations of the time-resolved fluorescence of C343 in different solvents indicated that the fluorescence lifetimes increased nearly linearly with increasing solvent polarity from 3.09 ns in toluene to 4.45 ns in water. This can be ascribed to the intermolecular hydrogen bonding interactions between C343 and hydrogen donating solvents.     

Photoinduced intramolecular charge transfer (ICT) reaction in trans-methyl p-(dimethylamino) cinnamate: A combined fluorescence measurement and quantum chemical calculations

The photophysical behaviour of trans-methyl p-(dimethylamino) cinnamate (t-MDMAC) donor–acceptor system has been investigated by steady-state absorption and emission spectroscopy and quantum chemical calculations. The molecule t-MDMAC shows an emission from the locally excited state in non-polar solvents. In addition to weak local emission, a strong solvent dependent red shifted fluorescence in polar aprotic solvents is attributed to highly polar intramolecular charge transfer state. However, the formation of hydrogen-bonded clusters with polar protic solvents has been suggested from a linear correlation between the observed red shifted fluorescence band maxima with hydrogen bonding parameters (). Calculations by ab initio and density functional theory show that the lone pair electron at nitrogen center is out of plane of the benzene ring in the global minimum ground state structure. In the gas phase, a potential energy surface along the twist coordinate at the donor (–NMe₂) and acceptor (–CH = CHCOOMe) sites shows stabilization of S₁ state and destabilization S₂ and S₀ states. A similar potential energy calculation along the twist coordinate in acetonitrile solvent using non-equilibrium polarized continuum model also shows more stabilization of S₁ state relative to other states and supports solvent dependent red shifted emission properties. In all types of calculations it is found that the nitrogen lone pair is delocalized over the benzene ring in the global minimum ground state and is localized on the nitrogen centre at the 90° twisted configuration. The S₁ energy state stabilization along the twist coordinate at the donor site and localized nitrogen lone pair at the perpendicular configuration support well the observed dual fluorescence in terms of proposed twisted intramolecular charge transfer (TICT) model.     

Kinetics of intramolecular charge transfer with N-phenylpyrrole in alkyl cyanides

For the electron acceptor/donor molecule N-phenylpyrrole (PP), the fast intramolecular charge transfer (ICT) reaction accompanied by dual fluorescence from a locally excited (LE) and an ICT state is investigated in alkyl cyanide solvents as a function of temperature. After a comparison of the X-ray crystal structure of PP with calculations from the literature, absorption and fluorescence spectra of PP in a series of solvents over a wide polarity range are discussed. ICT with PP strongly depends on solvent polarity and starts to appear in solvents more polar than diethyl ether. From an analysis of the ICT/LE fluorescence quantum yield ratio Phi'(ICT)/Phi(LE), approximate data for the change in enthalpy -DeltaH of the ICT reaction of PP are obtained, ranging from 9 kJ/mol in acetonitrile (MeCN) to 4 kJ/mol in n-butyl cyanide (BuCN). From ICT and LE fluorescence decays of PP measured as a function of temperature, the forward (Ea = 9 kJ/mol in ethyl cyanide (EtCN) and 6 kJ/mol in MeCN) and backward (Ed = 16 kJ/mol in EtCN and MeCN) ICT reaction barriers are determined. From these data, -Delta H (7 kJ/mol (EtCN); 10 kJ/mol (MeCN)) is calculated, in good agreement with the results coming from Phi'(ICT)/Phi(LE). The data for Ea show that the forward ICT barrier becomes smaller with increasing solvent polarity, whereas the absence of change for Ed comes from the compensating increase of -DeltaH. Both observations are indicative of a late transition state for the LE --> ICT reaction. For PP in EtCN and MeCN, the ICT radiative rate constant k'(f)(ICT) increases with temperature. This is caused by the ICT low transition dipole moment and hence does not contain information on the molecular structure (twisted or planar) of the ICT state. The fast ICT observed with PP supports our previous conclusion, based on a comparison of PP with its planarized derivative fluorazene, that the pyrrole and phenyl moieties in the ICT state of PP are coplanar and possess substantial electronic coupling.     

ESIPT inspired fluorescent 2-(4-benzo[d]oxazol-2-yl)naphtho[1,2-d]oxazol-2-yl)phenol: experimental and DFT based approach to photophysical properties

ESIPT inspired fluorescent 2-(4-benzo[d]oxazol-2-yl)naphtho[1,2-d]oxazol-2-yl)phenol was synthesized from 1-amino-3-(1,3-benzoxazol-2-yl)naphthalen-2-ol. Photophysical behavior of the synthesized compound was studied using UV–visible and fluorescence spectroscopy in polar and non-polar solvents. The synthesized naphthoxazolyl benzoxazole is fluorescent and very sensitive to the micro-environment. It shows a single absorption and dual emission in non-polar solvents with large Stokes shift originating from Excited State Intramolecular Proton Transfer while in polar solvents only a single short wavelength emission is observed. Experimental absorption and emission wavelengths are in good agreement with those predicted using the Time-Dependent Density Functional Theory (TD-DFT) [B3LYP/6-31G(d)]. The largest wavelength difference between the experimental and computed absorption maxima was 16 nm (acetonitrile) and 7 nm (ethyl acetate, THF, and 1,4-dioxane) in the short and long wavelength regions, respectively. A largest difference of 25 nm was observed for the short wavelength emission in DMF and 22 nm for the longer wavelength emission in chloroform.     

Crystal structure, spectroscopic, and theoretical investigations of excited-state proton transfer in the doubly hydrogen-bonded dimer of 2-butylamino-6-methyl-4-nitropyridine N-oxide

The crystal structure of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M) was resolved on the basis of X-ray diffraction. Solid 2B6M occurs in the form of a doubly hydrogen-bonded dimer with squarelike hydrogen-bonding network composed of two intra- (2.556(2) A) and two intermolecular (2.891(2) A) N-H...O type hydrogen bonds. The molecule thus has both a protonable and a deprotonable group that led us to investigate the possibility of an excited-state proton transfer (ESIPT) reaction in different solvents by means of experimental absorption, steady state, and time-resolved emission spectroscopy. The results were correlated with quantum mechanical TD-DFT and PM3 calculations. Experimental and theoretical findings show the possibility of an ESIPT reaction in polar solvents. It is demonstrated that in particular the emission spectra of 2B6M are very sensitive to solvent properties, and a large value of the Stokes shift (about 8000 cm(-1)) in acetonitrile is indicative for an ESIPT process. This conclusion is further supported by time-resolved fluorescence decay measurents that show dual exponential decay in polar solvents. Vertical excitation energies calculated by TD-DFT reproduce the experimental absorption maxima in nonpolar solvents well. The majority of electronic transitions in 2B6M is of pi --> pi* character with a charge shift from the electron-donating to the electron-accepting groups. The calculations show that, due to the charge redistribution on excitation, the acidity of the amino group increases significantly, which facilitates the proton transfer from the amino to the N-oxide group in the excited state.     

Role of excited state intramolecular charge transfer in the photophysical properties of norfloxacin and its derivatives

The photophysical properties of 1-ethyl-6-fluoro-7-(1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (norfloxacin, NFX) and some of its derivatives have been studied to evaluate the role of the free carboxylic acid and the nonprotonated piperazinyl group in the behavior of the 1,4-dihydro-4-oxoquinoline ring. Steady state and time-resolved fluorescence measurements at different pHs provide clear evidence in favor of singlet excited-state deactivation of NFX and its N(4')-methyl derivative pefloxacin (PFX) via intramolecular electron transfer from the N(4') atom of the piperazinyl ring to the fluoroquinolone (FQ) main system. This is a very efficient, energy-wasting pathway, which becomes dramatically enhanced in basic media. Acetylation at N(4') (as in ANFX) decreases the availability of the lone pair, making observable its fluorescence and the transient absorption spectrum of its triplet excited state even at high pH. It also reveals that the geometry of FQs changes from an almost sp3 hybridization of the N(1') of the piperazinyl substituent in the ground state to nearly sp2 in the singlet excited state (rehybridization accompanied by intramolecular charge transfer, RICT); accordingly, the singlet energy of ANFX is significantly lower than that of NFX and PFX. The fluorescence measurements using acetonitrile as a polar nonprotic organic solvent further support deactivation of the singlet excited state of nonacetylated NFX derivatives via intramolecular electron transfer from the N(4') atom.     

Dual fluorescence of omeprazole: effect of solvents and pH

Absorption, steady state fluorescence and time-resolved fluorescence spectra of omeprazole (OMP) have been studied in solvents of different polarity and pH. With an increase in the polarity of the solvents, blue shift is observed in the longer wavelength whereas red shift is noticed in the shorter wavelength band. The dual emission observed in non-polar solvents suggests that the energy of the twisted intramolecular charge transfer (TICT) state is lower than that of the locally excited (LE) state. The normal Stokes-shifted band originates from the LE state, and the large Stokes-shifted band is due to the emission from a TICT state. The Stokes shift of OMP is correlated with various solvent polarity scales like E_T(30) and f?(D,n).     

Solvent and pH dependent fluorescent properties of a dimethylaminostyryl borondipyrromethene dye in solution

Steady-state and time-resolved fluorescence techniques have been used to study the photophysical properties of the fluorescent BODIPY-derived dye 3-{2-[4-(dimethylamino)phenyl]ethenyl}-4,4-difluoro-8-(4-methoxyphenyl)-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene. This compound has been synthesized via a microwave-assisted condensation of p-N,N-dimethylaminobenzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted borondipyrromethene unit. The fluorescence properties of the dye are strongly solvent dependent: increasing the solvent polarity leads to lower fluorescence quantum yields and lifetimes, and the wavelength of maximum fluorescence emission shifts to the red. The Catalán solvent scales are found to be the most suitable for describing the solvatochromic shifts of the fluorescence emission. These are dominated by polarity/polarizability effects, as confirmed by quantum-chemical calculations performed in the dielectric continuum approximation. Fluorescence decay profiles of the dye can be described by a single-exponential fit in most solvents investigated, while two decay times are found in alcohols. The dye undergoes a reversible protonation-deprotonation reaction in the acidic pH range with a pK(a) of 2.25 in acetonitrile solution. Fluorimetric titrations as a function of pH produce fluorescence emission enhancements at lower pH. The fluorescence excitation spectra show a hypsochromic shift from 600 nm for the neutral amine to 553 nm for the ammonium form, so that ratiometric measurements can be used to determine pK(a).