Effect of solvents on photophysical properties and quenching of 2‐{[3‐(1H‐benzimidazole‐2‐yl) phenyl] carbonoimidoyl}phenol

The effect of solvents of varying polarity on the absorption and fluorescence emission of the Schiff base, 2‐{[3‐(1H‐benzimidazole‐2‐yl) phenyl]carbonoimidoyl}phenol, was studied using Lippert‐Mataga bulk polarity function, Reichardt's microscopic solvent polarity parameter and Kamlet's multiple linear regression approach. The spectral properties follow Reichardt's microscopic solvent polarity parameter better than Lippert‐Mataga bulk polarity parameter, indicating the presence of both general solute–solvent interactions and specific interactions. Catalan's multiple linear regression approach indicates the major role of solvent polarizability/dipolarity influence compared with solvent acidity or basicity. The solvatochromic effect was utilized to calculate the dipole moments of ground and excited states of the Schiff base using different methods. Bathochromic shift in the emission spectrum and the increase in dipole moment in the excited state signifies the intramolecular charge transfer character in the emitting singlet state. Fluorescence quenching by aniline was also studied in 1,4‐dioxane and n‐butanol, and the results were analyzed using sphere of action static quenching and finite sink approximation models. Copyright © 2014 John Wiley & Sons, Ltd.     

Specific interactions of alcohols and non‐alcohols with a biologically active boronic acid derivative: a spectroscopic study

The photophysical properties of 4‐fluoro‐2‐methoxyphenyl boronic acid (4FMPBA) are characterized using absorption and fluorescence techniques in series of non‐alcohols and alcohols. The results are analyzed using different solvent polarity functions and Kamlet and Catalan's multiple regression approaches. The excited state dipole moment and change in dipole moment are calculated using both the solvatochromic shift method and Reichardt's microscopic solvent polarity parameter . The ground state dipole moment is evaluated using quantum chemical calculations. It is found that general solute–solvent and hydrogen bond interactions are operative in this system. A red shift of ~ 9 nm in the emission spectra is observed with an increase in the solvent polarity, which depicts π→π^* transitions, as well as the possibility of an intramolecular charge transfer (ICT) character in the emitting singlet state of 4FMPBA. The relative quantum yield, radiative and non‐radiative decay constants are calculated in alkanes and alcohols using the single point method. It is found that the quantum yield of the molecule varies from 16.81% to 50.79% with the change in solvent polarity, indicating the dependence of fluorescence on the solvent environment. Copyright © 2015 John Wiley & Sons, Ltd.     

Electric dipole moments of the fluorescent probes Prodan and Laurdan: experimental and theoretical evaluations

Several experimental and theoretical approaches can be used for a comprehensive understanding of solvent effects on the electronic structure of solutes. In this review, we revisit the influence of solvents on the electronic structure of the fluorescent probes Prodan and Laurdan, focusing on their electric dipole moments. These biologically used probes were synthesized to be sensitive to the environment polarity. However, their solvent-dependent electronic structures are still a matter of discussion in the literature. The absorption and emission spectra of Prodan and Laurdan in different solvents indicate that the two probes have very similar electronic structures in both the ground and excited states. Theoretical calculations confirm that their electronic ground states are very much alike. In this review, we discuss the electric dipole moments of the ground and excited states calculated using the widely applied Lippert–Mataga equation, using both spherical and spheroid prolate cavities for the solute. The dimensions of the cavity were found to be crucial for the calculated dipole moments. These values are compared to those obtained by quantum mechanics calculations, considering Prodan in vacuum, in a polarizable continuum solvent, and using a hybrid quantum mechanics–molecular mechanics methodology. Based on the theoretical approaches it is evident that the Prodan dipole moment can change even in the absence of solute–solvent-specific interactions, which is not taken into consideration with the experimental Lippert–Mataga method. Moreover, in water, for electric dipole moment calculations, it is fundamental to consider hydrogen-bonded molecules.     

A combined spectroscopic and TDDFT investigation of the solute‐solvent interactions of two coumarin derivatives

The UV/Vis absorption and fluorescence characteristics of 3‐cyano‐7‐hydroxycoumarin [ CHC ] and 7‐amino‐4‐methyl‐3‐coumarinylacetic acid [ AMCA‐H ] were studied at room temperature in several neat solvents and binary solvent mixtures of 1,4‐dioxane/acetonitrile. The effects of solvent on the spectral properties are analyzed using single and multi‐parameter solvent polarity scales. Both general solute/solvent interactions and hydrogen bond interactions are operative in these systems. The solvation of CHC and AMCA‐H dyes in 1,4‐dioxane/acetonitrile solvent mixtures has been studied. The solutes CHC and AMCA‐H are preferentially solvated by acetonitrile and a synergistic effect is observed for both molecules in dioxane/acetonitrile solvent mixtures. In addition, using the solvatochromic method the ground‐ and the excited‐state dipole moments of both the dyes were calculated. The ground‐ and excited‐state dipole moments, absorption and emission maxima and HOMO–LUMO gap were also estimated theoretically using B3LYP/6–311+ G (d,p) level of theory in the gaseous phase, dioxane and acetonitrile solvents. Furthermore, changes in dipole moment values were also calculated using the variation of Stokes shift with the molecular–microscopic empirical solvent polarity parameter ( ). The observed excited‐state dipole moments are larger than their ground‐state counterparts, indicating a substantial redistribution of the electron densities in a more dipolar excited state for both coumarins investigated.     

Comparative study of the photophysical properties of indoprofen photoproducts in relation with their DNA photosensitizing properties.

The photophysical properties of indoprofen photoproducts have been examined in various solvents by absorbance and emission spectroscopies in relation with their photosensitizing properties. The photophysical properties of 2-[4-(1-hydroxy)ethylphenyl]isoindolin-1-one (HOINP) and 2-(4-ethylphenyl)isoindolin-1-one (ETINP) are typical of a singlet excited state when the ones of 2-(4-acetylphenyl)isoindolin-1-one (KINP) are based on its triplet excited state according to previous work. The effect of solvent polarity on the absorption and fluorescence properties of HOINP and ETINP has been investigated as a function of Delta f, the Lippert solvent polarity parameter. A solvatochromic effect, function of the polarity region, has been observed for both photoproducts due to a change in the dipole moment of the compound upon excitation. In low-polarity regions, the excited state dipole moment of HOINP undergoes only a moderate increase (11.5 D) as compared to the dipole moment of the ground state (4.5 D) suggesting that the fluorescence arises from the locally excited state while in high-polarity regions it is strongly increased (42.9 D), which can imply that the emission takes place from a charge transfer state. In the case of ETINP, it would seem that the emitting state is rather a charge transfer state whatever the region is (16.9 and 31.8 D for the calculated excited-state dipole moments in the low and high-polarity regions, respectively). HOINP and ETINP do not produce thymine dimers by photosensitization but induce photooxidative damage via an electron transfer mechanism.     

Effects of solvent polarity on the absorption and fluorescence spectra of chlorogenic acid and caffeic acid compounds: determination of the dipole moments

The effects of solvent polarity on absorption and fluorescence spectra of biologically active compounds (chlorogenic acid (CGA) and caffeic acids (CA)) have been investigated. In both spectra pronounced solvatochromic effects were observed with shift of emission peaks larger than the corresponding UV‐vis electronic absorption spectra. From solvatochromic theory the ground and excited‐state dipole moments were determined experimentally and theoretically. The differences between the excited and ground state dipole moment determined by Bakhshiev, Kawski–Chamma–Viallet and Reichardt equations are quite similar. The ground and excited‐state dipole moments were determined by theoretical quantum chemical calculation using density function theory (DFT) method (Gaussian 09) and were also similar to the experimental results. The HOMO‐LUMO energy band gaps for CGA and CFA were calculated and found to be 4.1119 and 1.8732 eV respectively. The results also indicated the CGA molecule is more stable than that of CFA. It was also observed that in both compounds the excited state possesses a higher dipole moment than that of the ground state. This confirms that the excited state of the hydroxycinnamic compounds is more polarized than that of the ground state and therefore is more sensitive to the solvent. Copyright © 2015 John Wiley & Sons, Ltd.     

Photophysical processes of an intramolecular charge transfer fluorescent dye with carbazole units

A carbazole‐based compound with intramolecular charge transfer (ICT) characteristics, 3,6‐bis‐((N‐ethylcarbazole‐3‐)‐propene‐1‐keto)‐N‐ethylcarbazole (BCzPCz) was synthesized by N‐alkylation, acetylation and aldol condensation. BCzPCz was further confirmed by IR and ¹ H NMR. The central N‐ethylcarbazole was connected with two N‐ethylcarbazole units through the propenone group in BCzPCz. N‐ethylcarbazole and carbonyl groups were electron donors (D) and acceptors (A), respectively. The UV–vis absorption and fluorescence characteristics of BCzPCz were also investigated in different solvents. Solvatochromism was attributed to ICT complex formation in singlet excited state. Magnitude of the change in the dipole moment was 24.78 D according to Lippert‐Mataga equation. Fluorescence of BCzPCz was significantly affected by pH and was quenched in acidic medium. Fluorescence quantum yield of BCzPCz was 0.516 in ethanol. Experimental results showed its potential use as a fluorescence probe and as two‐photon absorption material. Copyright © 2012 John Wiley & Sons, Ltd.     

Solvatochromic study of 3‐N‐(N′‐methylacetamidino)benzanthrone and its interaction with dopamine by the fluorescence quenching mechanism

The change in photophysical properties of the organic molecule due to solvatochromic effect caused by different solvent environments at room temperature gives information about the dipole moments of 3‐N‐(N′‐methylacetamidino)benzanthrone (3‐MAB). The quantum yield, fluorescence lifetime of 3‐MAB was measured in different solvents to calculate radiative and non‐radiative rate constants. The results revealed that the excited state dipole moment (μ_e) is relatively larger compared to the ground state dipole moment (μ_g), indicating the excited state of the dye under study is more polar than the ground state and the same trend is noticed with theoretical calculations performed using the CAM‐B3LYP/6‐311+G(d,p) method. Further, the study on preferential solvation was carried out for 3‐MAB dye in ethyl acetate–methanol solvent mixture. The fluorescence quenching method has been employed for the detection of dopamine using 3‐MAB as fluorescent probe, using steady‐state and time resolved methods at room temperature. The method enables dopamine in the micro molar range to be detected. Also, an attempt to verify the quenching process by employing different models has been tried. Various rate parameters are measured using these models, our results indicates the quenching process is diffusion limited.     

Quantum chemical computations and photophysical spectral features studies of two coumarin compounds

An attempt was made to determine the ground state and excited state dipole moments and quantum chemical computations of two coumarin compounds, namely 3‐hydroxy‐3‐[2‐oxo‐2‐(2‐oxo‐2H‐chromen‐3‐yl)‐ethyl]‐1,3‐dihydro‐indol‐2‐one (3HOCE) and 3‐[2‐(8‐methoxy‐2‐oxo‐2H‐chromen‐3‐yl)‐2‐oxo‐ethylidene]‐1,3‐dihydro‐indol‐2‐one (3MOCE). Both compounds displayed a red shift with enhancement in solvent polarity. The larger excited state dipole moment indicated the more polar nature of the selected compounds in the excited state than in the ground state. Kinetic stability and chemical reactivity of the selected compounds were studied with help of the quantum chemical properties of the compounds such as frontier molecular orbital analysis using density functional theory calculations with B3LYP/6–311+G (d, p) basis sets. Molecular electrostatic potential, Mulliken charges, natural bond orbital, and nonlinear optical properties were further studied. NBO analysis showed proton transfer within the selected donor–acceptor, depicting the large energy of stabilization for the compounds. The calculated Fukui function inferred the local softness and electrophilicity indices of used solute compounds.     

Effect of TiO2 nanoparticles on some photophysical characteristics of ketocyanine dyes

The effect of titanium dioxide (TiO₂) nanoparticles (NPs) on photophysical characteristics of 2,5‐di[(E)‐1‐(4‐dimethylaminophenyl) methylidine]‐1‐cyclopentanone (2,5‐DMAPMC) and 2,5‐di[(E)‐1‐(4‐diethylaminophenyl)methylidine]‐1‐cyclopentanone (2,5‐DEAPMC) ketocyanine dyes has been studied using absorption, steady‐state and time‐resolved fluorescence spectroscopy. The magnitudes of association constants determined based on modified absorption spectrum of dyes due to the presence of TiO₂ NPs indicate the interaction of TiO₂ NPs with dye molecules. The quenching of fluorescence intensity of dyes by TiO₂ NPs is observed and it follows linear Stern‐Volmer (S‐V) equation. The magnitude of quenching rate parameter suggests the involvement of static quenching mechanism. The involvement of electron transfer process in reducing fluorescence intensity of dyes has been discussed. Also, varying influence of TiO₂ NPs on two dyes is explained based on the presence of different alkyl substituent in two dyes.     

Photophysical properties of 2-amino-9,10-anthraquinone: evidence for structural changes in the molecule with solvent polarity.

The photophysical properties of 2-amino-9,10-anthraquinone (2AAQ) have been investigated in different solvents and solvent mixtures and correlated with the Lippert-Mataga solvent polarity parameter, Deltaf. In the low solvent polarity region with Deltaf < ca. 0.1, the dye shows unusually high fluorescence quantum yields (Phif) and lifetimes (tauf) in comparison to those in other solvents of medium to high polarities. Similarly, the radiative rate constants (kf) are relatively lower and the non-radiative rate constants (knr) are relatively higher in the low polarity solvents in comparison to those in the medium to high polarity solvents. The current results have been rationalized assuming that the dye adopts different structural forms below and above the Deltaf value of approximately 0.1. It is inferred that in the low solvent polarity region the dye exists in a non-planar structure, with its 2-NH2 plane away from that of the 9,10-anthraquinone moiety in the ground state. In solvents of medium to high polarities, the dye exists in a polar intramolecular charge transfer (ICT) structure, where the amino lone pair of the 2-NH2 group is in strong resonance with the anthraquinone pi-cloud in the ground state. In all the solvents, however the dye is inferred to exist in the ICT structure in its excited (S1) state. Supportive evidence for the above hypothesis has been obtained from the solvent polarity effect on the Stokes' shifts for the dye. Quantum chemical studies on the structures of 2AAQ dye in the gas phase also give qualitative support for the inferences drawn from the photophysical properties of the dye in different solvents.     

Temperature influence on the energy of nonspecific and specific interactions taking place between 4-aminophthalimide (4-AP) and homogeneous solvents.

Temperature induced spectral shifts of the 4-aminophthalimide (4-AP) emission spectra have been measured and compared to the predictions of the McRae solvent induced shift theory (J. Phys. Chem., 1957, 61, 562-572). Three moderately polar chloroalkanes selected as nonspecifically interacting media, and six hydrogen accepting or/and electron pair donating solvents have been used as the media in which the temperature influence on 4-AP-solvent interactions has been studied in the range of 180-320 K. Using the ab initio determined 4-AP ground state dipole moment and fitting appropriate expression originating from the mentioned theory to the shifts found in the chloroalkanes it has been possible to estimate the 4-AP excited state dipole moment, the probe excited state Onsager radius and its gas phase emission spectrum position. Using these values the thermochromic shifts of 4-AP emission spectra in hydrogen bond forming solvents have been predicted and compared to the experimental one. Temperature has been found to have different impact on the changes, upon excitation of the probe, in the mean values of the energies of different hydrogen bonds formed by 4-AP with solvents molecules.     

Solvent polarity effect on excited-state lifetime of carotenoids and some dyes

The theory demonstrating the role of medium at the fluorescence quenching of polar compounds in solutions is briefly presented. It has been shown, that the rate of S(1) --> X(n) nonradiative conversion between the intramolecular charge transfer states depends on the permanent dipole moments in the ground (S(0)) and excited (S(1), X(n)) states as well as on solvent polarity. A relation for the rate of nonradiative excited-state energy conversion has been obtained and employed to test the known literature data for solvent effect on the S(1)-state lifetime of some biologically significant carotenoids and dyes (phthalimides). For phthalimides, the solvent isotope effect on the S(1)-state energy conversion, when hydrogen is replaced by deuterium in the OH groups of alcohols and water, has been analyzed. Based on the data for fluorescence quenching in solvents of different polarity, the dipole moments in the intermolecular charge transfer S(1) state have been obtained for carotenoids (peridinin, fucoxanthin, uriolide acetate) and for hydrogen-bonding complexes, which are formed by 4-amino-, 4-methylamino-, and 4-dimethylamino-N-methylphthalimides in alcohols and water.     

Kinetics of rearrangement of solvation sheath of an excited molecule of the fluorescent probe 4"-dimethylaminochalcone

Processes accompanying the quenching of the fluorescent probe 4"-dimethylaminochalcone by hydroxyl groups of the proton-donor solvent 1-butanol have been studied. The kinetics of the deactivation of the excited state of 4"-dimethylaminochalcone has been monitored from the transition absorption spectra at a time resolution of 50 fs and fluorescence decay at a time resolution of 30 ps. The data obtained allow thinking that the next picture occurs in 1-butanol. At first stage, the 4"-dimethylaminochalcone molecule in its ground state forms a hydrogen bond with an alcohol molecule. At the second stage, the absorption of light quantum and corresponding rise of the dipole moment of 4"-dimethylaminochalcone take place, the initially existing hydrogen bond is retained. The third stage consists in the rearrangement of the 4"-dimethylaminochalcone solvation shell formed by alcohol dipole molecules due to an increase of the dipole of moment 4"-dimethylaminochalcone; this rearrangement takes an energy of about 24 kJ/mol, the arrangement time constant is close to 40 ps; the initial hydrogen bond is retained. The fourth stage involves processes that lead to fluorescence quenching; their time constant is about 200 ps. Taking into account that the quenching is a much slower process than the relaxation of the solvation shell, it was supposed that the quenching is not a direct consequence of the solvation shell relaxation or the existence of the hydrogen bond formed prior to excitation. Then the fluorescence quenching of 4"-dimethylaminochalcone can be accomplished through some other processes that are observed in other fluorescent molecules: (a) rearrangement of the initial hydrogen bond from a conformation that cannot quench the fluorescence of 4"-dimethylaminochalcone to a more "effective" conformation, (b) charge transfer between the excited of molecule 4"-dimethylaminochalcone and alcohol, or (c) solvent-induced twist of the 4"-dimethylaminochalcone amino group (its withdrawal from the molecule plane) by the action of the solvent.     

Solvent‐dependent ultrafast optical response of conjugated push–pull chromophores

Two new difluoroboron β‐carbonyl cyclic ketonate complexes C2B and DC2B were investigated using several spectroscopic methods. Relative to the absorption spectra, the fluorescence spectra were more affected by the polarity of the solvent. Also, compound C2B showed a more pronounced Stokes’ shift after solvent polarity increased. Transient absorption measurements then demonstrated the relaxation behaviour of the excited state compound molecule. The kinetic results showed that the excited state C2B in tetrahydrofuran (THF) can return from the intramolecular charge‐transfer (ICT) state and the initial excited state to the ground state. The kinetic relaxation pathway after THF was replaced by dimethyl sulfoxide became single. When the carbazole unit was introduced, DC2B also exhibited an ICT state but there was no significant difference in the excited state relaxation path after solvent polarity was changed. The results indicated that C2B is more susceptible to solvent polarity regulation. The global fit results revealed that an increase in the solvent polarity prolonged the lifetime of the ICT state of compound C2B and had the opposite effect on compound DC2B. These results provide guidance for understanding the relationship between solvent polarity and the designing and synthesizing advanced compound materials.     

Fluorescence modulation of 1,7‐bis(4‐hydroxy‐3‐methoxyphenyl)‐1,6‐heptadiene‐3,5‐dione by silver nanoparticles and its possible analytical application

The effects of silver nanoparticles on the photophysical properties of 1,7‐bis(4‐hydroxy‐3‐methoxyphenyl)‐1,6‐heptadiene‐3,5‐dione, popularly known as curcumin, have been investigated using optical absorption and fluorescence techniques. Although absorption spectroscopy suggests a ground‐state complex formation, fluorescence quenching data confirms a simultaneous static and dynamic quenching, inferring ground as well as excited‐state complex formation. The recovery of fluorescence quenching of the curcumin–silver nanoparticle complex in the presence of ascorbic acid or uric acid emphasizes a strong interaction between the silver nanoparticles and ascorbic acid/uric acid, suggesting that fluorescence recovery after the quenching of curcumin–silver nanoparticle complexes has potential for ascorbic acid or uric acid assay development. Copyright © 2011 John Wiley & Sons, Ltd.     

Solvent effects on the spectroscopic properties of aflatoxin B1

1. Solvent-induced changes in the spectral properties of aflatoxin B1 were investigated using protic and aprotic solvents. 2. The absorption data were less sensitive to solvent effects than the fluorescence emission data. 3. Stokes shifts in protic solvents were greater than those in aprotic solvents indicating hydrogen bond formation between solvent and the excited state of aflatoxin B1. 4. From the Stokes shift data for aprotic solvents, the dipole moment of aflatoxin B1 was estimated to increase by 15.7 Debye units upon excitation to the excited singlet state.     

Investigations on spectra and orientation of dipole moments of uracil and thymine by electrochromism

The direction of the ground stale dipole moment with respect to the transition moment to the lowest excited singlet state as well as the dipole moment in this excited state could be determined from the influence of an electric field on the light absorption of uracil and thymine in solution. By making use of results on the orientation of the transition moment in the molecular framework, the orientation of the ground-and excited-state dipole moments can also be fixed and compared with theoretical predictions. The agreement is fair. The measurements show that in both compounds a weak band is hidden under the longest-wavelength absorption band.     

Kinetics of the rearrangement of the solvation shell of an excited fluorescent probe 4″-dimethylaminochalcone

A study was made of the processes associated with the quenching of 4″-dimethylaminochalcone (DMAC) fluorescence by proton-donor solvent (1-butanol). The kinetics of deactivation of the DMAC excited state was assessed by transient absorption spectra with a time resolution about 50 fs and by fluorescence decay with ～30-ps resolution. The following sequence of events could thus be envisaged: (i) the DMAC molecule in the ground state (prior to excitation) makes a hydrogen bond with an alcohol molecule; (ii) absorption of a light quantum causes a corresponding increase of the DMAC dipole moment; the H-bond is retained; (iii) the solvation shell formed by alcohol dipoles is reorganized in response to the raise of the DMAC dipole moment, with an energy expenditure about 24 kJ/mol and a time constant about 40 ps; the initial H-bond is still retained; (iv) processes leading to fluorescence quenching occur with an effective time constant of nearly 200 ps. Since quenching is far slower than solvate rearrangement, one can suppose that it is not a direct consequence of shell relaxation or prior H-bonding. Thus, DMAC fluorescence quenching may involve different processes observed with other aromatic molecules: H-bond rearrangement from a nonquenching to a more ‘efficient’ conformation, charge transfer between the excited molecule and alcohol, or solvent-induced out-of-plane twist of the DMAC amino group.     

Ab initio calculations of molecular properties of low–lying electronic states of 2–cyclopenten–1–one – link with biological activity

Geometries, vibrational frequencies, vertical and adiabatic excitation energies, dipole moments and dipole polarizabilities of the ground and the three lowest electronic excited states, S(1)(n, π (*)), T(1)(n, π (*)), and T(2)(π, π (*)) of the 2-cyclopenten-1-one molecule (2CP) were calculated at the CCSD and CCSD(T) levels of approximation. Our results indicate that two triplets T(1)(n, π (*)) and T(2)(π, π (*)) are lying very close each to other, while the singlet S(1)(n, π (*)) is well above them. There are dramatic changes in dipole moments for (n, π (*)) excited states in respect to the ground state. On the other hand the T(2)(π, π (*)) state has a similar dipole moment as the ground state. These changes can be interpreted within the MO picture using electrostatic potential maps and changes in model IR spectra. Our CCSD(T) dipole moment data for the ground state and almost isoenergetic triplets T(1)(n, π (*)) and T(2)(π, π (*)) are 1.469?a.u., 0.551?a.u., and 1.124?a.u., respectively. Dipole polarizabilities of investigated excited states are much less affected by electron excitations than dipole moments. These are the first dipole moment and polarizability data of 2CP in the literature. The changes of molecular properties upon excitation to S(1)(n, π (*)) and T(1)(n, π (*)) correlate with the experimental data on the biological activity of 2CP related to the α, β-unsaturated carbonyl group.