The experimental studies on the determination of the ground and excited state dipole moments of some hemicyanine dyes

The ground state (mu(g)) and excited state (mu(e)) dipole moments of 15 hemicyanine dyes were studied at room temperature. They were estimated from solvatochromic shifts of the absorption and the fluorescence spectra as function of the solvent dielectric constant (varepsilon) and refractive index (n). In this paper we applied the Stokes shift phenomena not only for the determination of the difference in the dipole moment of excited state and ground state, but to determine the value of polarizability alpha as well. The obtained results show that excited state dipole moments of hemicyanine dyes are in the range from 5 to 15 Debye, and the difference between the excited and ground state dipole moments vary from 1 to 7 Debye. The excited and ground state dipole moments difference (mu(e)-mu(g)) obtained for selected dyes are positive. It means that the excited states of the dyes under the study are more polar than the ground state ones. Additionally, the value of both polarizability (alpha) and the Onsager radius (a) of each investigated hemicyanine dye molecule are determined, and the ratio of alpha/a(3) for each dye were calculated, which oscillate from 0.29 to 5.21. The increase in dipole moment has been explained in terms of the nature of excited state and its resonance structure.     

Ground state and excited state dipole moments of 6,8-diphenylimidazo[1,2-α]pyrazine determined from solvatochromic shifts of absorption and fluorescence spectra

Electronic absorption and dual fluorescence spectra of 6,8-diphenylimidazo[1,2-α]pyrazine (68DIP) was recorded in various solvents with different polarity at room temperature. The ground state (μg) and the excited state (μg) dipole moments of 68DIP were estimated from solvatochromic shifts of absorption and fluorescence spectra as a function of the dielectric constant (?) and refractive index (n). The results show that the value of excited state dipole moment in SE: μeSE=2.8772 D and twisted intramolecular charge transfer (TICT) excited equilibrated state dipole moment value of μeLE=2.9744 D was found. The solvent dependent spectral shifts in absorption and fluorescence spectra were analyzed by the polarizability-polarity and Kamlet-Taft parameters.     

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.     

A novel fluorophore with dual fluorescence: local excited state and photoinduced electron-transfer-promoted charge-transfer state.

Absorption and emission spectra of 9-N,N-dimethylaniline decahydroacridinedione (DMAADD) have been studied in different solvents. The fluorescence spectra of DMAADD are found to exhibit dual emission in aprotic solvents and single emission in protic solvents. The effect of solvent polarity and viscosity on the absorption and emission spectra has also been studied. The fluorescence excitation spectra of DMAADD monitored at both the emission bands are different. The presence of two different conformation of the same molecule in the ground state has lead to two close lying excited states, local excited (LE) and charge transfer (CT), and thereby results in the dual fluorescence of the dye. A CTstate involving the N,N-dimethylaniline group and the decahy droacridinedione chromophore as donor and acceptor, respectively, has been identified as the source of the long wavelength anomalous fluorescence. The experimental studies were supported by ab initio time dependent-density functional theory (TDDFT) calculations performed at the B3LYP/6-31G* level. The molecule possesses photoinduced electron transfer (PET) quenching in the LE state, which is confirmed by the fluorescence lifetime and fluorescent intensity enhancement in the presence of transition metal ions.     

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.     

Solvatofluorochromic properties of 2,7-dimethyl-9-(ditolylamino)acridine

Electronic absorption and fluorescence spectra of the acridine dye 2,7-dimethyl-9-(ditolylamino)acridine were studied at room temperature in solvents of different polarity (hexane, toluene, chloroform, tetrahydrofuran, acetonitrile, etc.). The obtained data on the shift of the fluorescence band maximum depending on the solvent polarity were used for the estimation of the dipole moment of 2,7-dimethyl-9-ditolylaminoacridine in the ground and the first excited state.     

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.     

Excited state characteristics of acridine dyes: acriflavine and acridine orange

The magnitude of the Stokes shift (frequency shifts in absorption and fluorescence spectra) is observed on changing the solvents and further has been used to calculate experimentally the dipole moments (ground state and excited state) of acriflavine and acridine orange dye molecules. Theoretically, dipole moments are calculated using PM 3 Model. The dipole moments of excited states, for both molecules investigated here, are higher than the corresponding values in the ground states. The increase in the dipole moment has been explained in terms of the nature of the excited state. Acriflavine dye overcomes the non-lasing behaviour of acridine orange due to quaternization of the central nitrogen atom.     

Temperature‐Dependent Behavior of the Dual Fluorescence of 2‐(3‐Fluorophenyl)‐2,3‐dihydro‐1H‐benzo[f]isoindole‐1,3‐dione

The fluorescence behavior of 2‐(3‐fluorophenyl)‐2,3‐dihydro‐1H‐benzo[f]isoindole‐1,3‐dione ( 1 ) was studied in solvents of different polarity and viscosity. Dual luminescence is observed and the short‐wavelength emission is found to increase considerably with the solvent polarity. The ratio of the fluorescence quantum yield of the two states emitting, the one (SW*) at short wavelength and the other (LW*) at long wavelength, shows a bell‐shaped dependence on the reciprocal of the temperature in diethyl ether, butyronitrile, and propane‐1,2,3‐triol triacetate (glycerol triacetate; GTA). This has been interpreted as the result of a reversible interconversion between the two states. The enthalpy difference between the SW* and LW* excited states, as deduced from the slope of the ln (Φ/Φ) vs. 1/T curves in the high temperature range, is found to be solvent polarity and solvent viscosity independent as the same value (−7.3 kJ/mol) is obtained in the three above‐mentioned solvents. The independence from polarity is the consequence of a similar difference in dipole moment between the ground‐state and the SW* and LW* excited states (4.5 and 4.9 D, respectively, derived from solvatochromy). The activation energy of the SW*→LW* step deduced from the low temperature measurements in the nonviscous solvents, increases with solvent polarity (11.6 and 17.5 kJ/mol for diethyl ether and butyronitrile, respectively); they are greater than the viscous‐flow activation energy of the solvents indicating that the resolvation of the excited dipole controls the kinetics. In the nonviscous solvents, the LW* state originates from the SW* state, while in the viscous GTA solution, both states are formed simultaneously within the 1‐ps laser pulse.     

Solvent effects on the absorption and fluorescence spectra of coumarins 6 and 7 molecules: determination of ground and excited state dipole moment

Absorption and fluorescence emission spectra of coumarins 6 and 7 were recorded in solvents with different solvent parameters, viz., dielectric constant epsilon and refractive index n. The fluorescence lifetime of these dyes were measured in butanol at higher values of viscosity over temperature. Experimental ground and excited state dipole moments are determined by means of solvatochromic shift method and also the excited state dipole moments are determined in combination with ground state dipole moments. It was determined that dipole moments of the excited state were higher than those of the ground state in both the molecules.     

The electronic spectra and excited state dipole moment of 2-fluoropyridine

The electronic absorption spectrum in the vapour state and in solution in different solvents in the region 3000–1900 Å and the fluorescence and phosphorescence emission spectra in ethanol or cyclohexane at 77 K have been studied for 2-fluoropyridine and analysed. Two systems of absorption band corresponding to the π→π* transition II and π→π* transition III have been observed and the excited state dipole moments have been determined from the solvent-induced shifts of the electronic absorption bands. The half-life of phosphorescence in cyclohexane at 77 K is found to be 3.5 s.     

Bischromophoric styrylpyridinium dyes. Spectroscopic properties of 1,3-bis-[4-(p-N,N-dialkylaminostyryl)pyridinyl]propane dibromides

The photophysical properties of newly synthesized bischromophoric solvatochromic stilbazolium dyes, 1,3-bis-[4-(p-N,N-dialkylaminostyryl)pyridinyl]propane dibromides (C1-C9), were studied in a series of solvents and their spectroscopic properties were compared with structurally related, monochromophoric styrylpyridinium dyes (SP1-SP9). The position of the UV-vis absorption spectra maximum of novel dyes is only slightly solvent polarity dependent in contrast to the fluorescence spectra that show pronounced solvatochromic effect demonstrated by a large Stokes shifts. The influence of the solvent on absorption and emission spectra, and the solvatochromic properties observed for both ground and first excited states for all the dyes were used for the evaluation of their excited state dipole moments. The ground state dipole moments of both mono- and bischromophoric dyes were established by applying ab initio calculations. The calculations and measurements unexpectedly show that the bischromophoric dyes are characterized by ground state dipole moments being equal to about half of that characterizing their monomeric equivalents, while the excited state dipole moments of bischromophoric dyes are about 10-25% higher in comparison to their monomeric equivalents.     

Solvatochromic study of 1,2-dihydroxyanthraquinone in neat and binary solvent mixtures

Preferential solvation of a solvatochromic probe has been studied in binary mixtures comprising of a non-protic and a protic solvent. The non-protic solvents employed are carbon tetrachloride (CCl(4)), acetonitrile (AcN) and N,N-dimethyl formamide (DMF) and the protic solvents are methanol (MeOH) and ethanol (EtOH). The probe molecule exhibits different spectroscopic characteristics depending upon the properties of the solubilizing media. The observed spectral features provide an indication of the microenvironment immediately surrounding the probe. Solvatochromic shifts of the ground and excited states of the probe were analysed by monitoring the charge transfer absorption band and the fluorescence emission spectra in terms of the solute-solvent and solvent-solvent interactions. Fluorescence emission spectra show the dual emission due to excited state proton transfer nature of the probe molecule. The effect of solvent and the excitation energy on dual emission are also studied. The observed magnitude of the Stokes shift in the above solvents has been used to deduce experimentally the dipole moment ratio of the probe molecule for the excited state to the ground state. The dipole moment of excited state is higher than the ground state.     

Excited state dynamics of Michler's ketone: a laser flash photolysis study

Steady state absorption and fluorescence as well as the time resolved absorption studies in the pico and subpicosecond time domain have been performed to characterize the excited singlet and triplet states of Michler's ketone (MK). The nature of the lowest excited singlet (S₁) and triplet (T₁) states depends on the polarity of the solvent - in nonpolar solvents they have either pure nπ * character or mixed character of nπ * and ππ * states but in more polar solvents the states have CT character. Concentration dependence of the shapes of the fluorescence as well the excited singlet and triplet absorption spectra provide the evidence for the association of the MK molecules in the ground state.     

Dual Solvatochromism of Neutral Red

Abstract— The effect of solvent polarity on the electronic absorption and fluorescence properties of neutral red (NR), a phenazine-based dye of biological importance has been investigated in several neat and mixed solvents. An unusual dual solvatochromic behavior has been observed that reveals the existence of two closely spaced electronic excited states in NR. In low-polarity solvents the fluorescence of the NR is mainly emitted from the localized excited state, whereas in high-polarity solvents the emission from the charge transfer state dominates. The dipole moments of the localized and charge transfer states of NR have been estimated from the solvatochromic shifts. The dipole moment of the localized excited state (4.8 D) is only slightly higher than that of the ground state (2.0 D), while that of the charge transfer state is drastically higher (17.5 D). Fluorescence quantum yields and the life-times of NR have been determined in different solvents and correlated with the solvatochromic shifts.     

Dipole moments studies of fluorenone and 4-hydroxyfluorenone

Electronic absorption, excitation and fluorescence spectra of fluorenone and 4-hydroxyfluorenone were recorded at room temperature in several aprotic solvent of varying polarities. The ground (mu(g)) and excited (mu(e)) state dipole moments of both molecules were estimated from solvatochromic shifts of absorption and fluorescence spectra as a function of the dielectric constant (epsilon) and refractive index (n). These experimental results were completed with theoretical results of quantum chemical calculations (AM1). The experimental and theoretical dipole moments in the ground state were compared. It was determined that dipole moments of excited state were higher than those of the ground state for both molecules.     

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.     

Solvatochromic behavior of donor-acceptor-polyenes: dimethylamino-cyano-diphenylbutadiene

4-(Dimethylamino)-4'-cyano-1,4-diphenylbutadiene (DCB) and 4-(dimethylamino)-2,6-dimethyl-4'-cyano-1,4-diphenylbutadiene (DMDCB) have been characterized spectroscopically. Quantum chemical calculations were performed for comparison. Solvatochromic shifts of the fluorescence were strong and showed a linear dependence on the solvent polarity parameters, whereas shifts in the absorption spectra are very weak only correlate better with the polarizability of the solvents. Excited state dipole moments derived from fluorescence using the Onsager model are very large and similar for both compounds. It is concluded that a strongly allowed and highly dipolar pi, pi* state is the lowest excited state in polar solvents. The strong difference in absorption and fluorescence solvatochromic slopes suggests that the simple Onsager model with a point dipole approximation is not sufficient here.     

2-氨基苯并噻唑的结构及激发态质子转移动力学

通过傅里叶变换红外光谱（FTIR）、傅里叶变换拉曼（FT-Raman）和488 nm拉曼光谱,结合密度泛函理论（DFT）计算研究了2-氨基苯并噻唑（ABT）在晶态和溶剂中的二聚体结构,并解释了质子性溶剂中ABT二聚体与溶剂分子间的氢键作用.电子光谱实验揭示了ABT二聚体的光物理和光化学反应;紫外吸收和荧光发射光谱结果表明,溶剂、激发波长和pH值对ABT二聚件激发态衰变具有调控作用;含时密度泛函理论（TD-DFT）解释了ABT二聚体双荧光现象,提出了高激发态的质子转移机理.