Toward a Generalized Treatment of the Solvent Effect Based on Four Empirical Scales: Dipolarity (SdP, a New Scale), Polarizability (SP), Acidity (SA), and Basicity (SB) of the Medium

Photophysics, excited-state double-proton transfer and hydrogen-bonding properties of 5-deazaalloxazines

The photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S0 -Si transitions were investigated using the ab initio methods [MP2, CIS(D), EOM-CCSD] with the correlation-consistent basis sets. Also the time-dependent density functional theory (TD-DFT) has been employed. The lowest singlet excited states of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine are predicted to have the π, π* character, whereas similar alloxazines have two close-lying π, π* and n, π* transitions. Experimental steady-state and time-resolved spectral studies indicate formation of an isoalloxazinic excited state via excited-state double-proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5-deazaalloxazine molecule. Solvatochromism of both 5-deazaalloxazine and its 1,3-dimethyl substituted derivative was analyzed using the Kamlet-Taft scale and four-parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute-solvent hydrogen-bonding interactions in their excited state.

A spectroscopic rule from the solvatochromism of aromatic solutes in nonpolar solvents

The UV-vis absorption spectroscopy for a series of selected symmetrical aromatic hydrocarbons, benzene, naphthalene, anthracene, 9,10-diphenylanthracene, perylene, and rubrene in the gas phase or in 2-methylbutane, shows the consistency of a new spectroscopic rule. From a thorough spectroscopic analysis with temperature variation (293-113 K) this rule states that "an aromatic molecule, on Franck-Condon excitation can hardly generate an excited electronic state with a lower polarizability than that of its ground electronic-state." We have not found any exceptions to this rule.

Spectral-luminescent properties of laurdan molecule

Quantum-chemical calculations of ground and excited states of fluorescent probe (laurdan) by ab initio and semiempirical methods were performed. The laurdan optimized geometries of S0 and S1 states were obtained. The influence of laurdan nonrigidity structure on dipole moments and location of energy levels were studied. The specific solvation centers of laurdan were obtained. The rate constants of photoprocesses and fluorescence quantum yield of laurdan in non-polar solvent were calculated.

Photophysical behavior of some aromatic poly(1,3,4-oxadiazole-ether)s derivatives

The change in electronic absorption and emission spectra of two fluorinated poly(1,3,4-oxadiazole-ether)s in neat and binary solvent mixtures has been studied. The optical properties, absorption and photoluminescence of these polymers were investigated in solution and in solid state. The results were discussed as a function of solvent nature, excitation wavelength and local solvent composition, X2. The Catalan solvent scale was used for describing the solvatochromic shifts of the absorption and emission bands. The following binary solvent mixtures were used: chloroform-N,N'-dimethylformamide (CHCl3-DMF), chloroform-dimethyl sulfoxide (CHCl3-DMSO), and dimethylformamide-dimethyl sulfoxide (DMF-DMSO) and the influence of their composition on the absorption and emission maxima has been analyzed. The preferential solvation parameters such as local mole fraction (X2(L)), excess function (δs2) and preferential solvation constant (KPS) were determined.

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 S(1) -LE to S(0) 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).

Photoswitchable fluorescent diheteroarylethenes: substituent effects on photochromic and solvatochromic properties

Photoswitchable fluorescent diheteroarylethenes are promising candidates for applications in super-resolution molecular localization fluorescence microscopy thanks to their high quantum yields and fatigue-resistant photoswitching characteristics. We have studied the effect of varying substituents on the photophysical properties of six sulfone derivatives of diheteroarylethenes, which display fluorescence in one (closed form) of two thermally stable photochromic states. Electron-donating substituents displace the absorption and emission spectra towards the red without substantially affecting the fluorescence quantum yields. Furthermore, ethoxybromo, a very electron-donating substituent, stabilizes the excited state of the closed isomer to the extent of almost entirely inhibiting its cycloreversion. Multi-parameter Hammett correlations indicate a relationship between the emission maxima and electron-donating character, providing a useful tool in the design of future photochromic molecules. Most of the synthesized compounds exhibit small bathochromic shifts and shorter fluorescence lifetimes with an increase in solvent polarity. However, the ethoxybromo-substituted fluorescent photochrome is unique in its strong solvatochromic behaviour, constituting a photoactivatable (photochromic), fluorescent and highly solvatochromic small organic compound. The Catalán formalism identified solvent dipolarity as the principal basis of the solvatochromism, reflecting the highly polarized nature of this molecule.

Optical and electrochemical properties of ethynylaniline derivatives of phenothiazine, phenothiazine-5-oxide and phenothiazine-5,5-dioxide

Modifications in the sulfur oxidation state of phenothiazine results in dramatic changes to the excited-state geometry with little effect on the HOMO energy.

Solvatochromic properties of 3,6-di-tert-butyl-8H-indolo[3,2,1-de]acridin-8-one

The spectral and photophysical properties of newly synthesized 3,6-di-tert-butyl-8H-indolo[3,2,1-de]acridin-8-one were studied.

Solvatochromic studies of pull–push molecules containing dimethylaniline and aromatic hydrocarbon linked by an acetylene unit

Pull–push molecules containing dimethylaniline and aromatic hydrocarbon linked by an acetylene unit were studied using spectroscopic methods and DFT theoretical calculations.

New cyanopyridone based luminescent liquid crystalline materials: synthesis and characterization

A series of new luminescent mesogens carrying cyanopyridone, triazole, and polar groups were successfully synthesized. Also, their liquid crystalline and photophysical properties were investigated.

Spectroscopy and Photophysics of Monomethyl-Substituted Derivatives of 5-Deazaalloxazine and 10-Ethyl-5-Deaza-Isoalloxazine

Steady-state and time-resolved spectra were used to describe the singlet and triplet states of 8-methyl-5-deazaalloxazine (8-Me-5-DAll), 9-methyl-5-deazaalloxazine (9-Me-5-DAll) and 10-ethyl-5-deaza-isoalloxazine (10-Et-5-DIAll). Solvatochromic properties were described using different polarity scales, including Δf and the four-parameter scale proposed by Catalán. The results indicate that the Catalán scale shows a strong influence of solvent acidity (hydrogen-bond donating ability) on the emission properties of 8-Me-5-DAll and 9-Me-5-DAll. These results indicate the importance of intermolecular solute-solvent hydrogen-bonding interactions in the excited state of these compounds. Contrary to deazaalloxazines, solvent acidity affects the absorption spectra of 10-Et-5-DIAll. Fluorescence lifetimes and quantum yields and also transient absorption spectra were determined for all of the compounds studied. Electronic structure and S(0)-S(i), S(0)-T(i ), T(1)-T(i) transitions energies and oscillator strengths were calculated using the TD-DFT methods. Theoretical calculations were compared to experimental data.

Synthesis, photophysical properties and solvatochromism of meso-substituted tetramethyl BODIPY dyes

The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene fluorescent dyes (BODIPYs) were first synthesized almost 50 years ago; however, the exploration of their technological application has only begun in the last 20 years. These dyes possess interesting photophysical properties, increasing interest in their application as fluorescent markers and/or dyes. Herein, we report the synthesis of tetramethyl BODIPY and four meso-substituted dyes (2-thienyl, 4-pyridinyl, 4-fluorophenyl and 4-nitrophenyl derivatives). Their photophysical characterization (absorption spectra, emission spectra, fluorescence quantum yields and time-resolved fluorescence) and solvatochromic behavior were studied. Absorption and emission were barely affected by substituents, with a slightly higher stokes shift observed in the substituted dyes. Substitutions could be associated with a shorter fluorescence lifetime and lower quantum yields. Good correlations were observed between the Catalán solvent descriptors and the photophysical parameters. Also, better correlation was observed between the solvent polarizability descriptor (SP) and photophysical parameters. Overall, only slight solvatochromism was observed. The 4-pyridinyl derivative was the subject of a relatively significant solvatochromism regarding the wavelengths of the emission spectra, with the observation of a bathochromically shifted emission in methanol. The fluorescence quantum yield of the 4-nitrophenyl substituted BODIPY was approximately 30 times higher in hexane, which may be of interest for practical applications.

On the dual emission of p-dimethylaminobenzonitrile and its photophysical implications

A solvatochromic analysis of available absorption and LE and TICT emission data in p-dimethylaminobenzonitrile (DMABN) was conducted. Applying the Abe method to the results of a thermochromic analysis of DMABN in 1-chlorobutane (ClB) allowed us to determine the polarizability and dipole moment of the excited electronic states involved in the absorption and emission transitions. As shown herein, the LE → TICT excited state reaction for DMABN is triggered by the solvent polarity but is additionally influenced by the viscosity. The experimental evidence negates the assumption that the radiative constant for the TICT fluorescence of DMABN is temperature-dependent.

Effect of solvents on the spectroscopic properties of LD-489 & LD-473: estimation of ground and excited state dipole moments by solvatochromic shift method

The absorption and fluorescence spectra of 6,7,8,9-tetrahydro-6,8,9-trimethyl-4-(trifluoro methyl)-2H-pyrano[2,3-b][1,8]naphthyridin-2-one (LD-489) and 1,2,3,8-tetrahydro-1,2,3,3,8-pentamethyl-5-(trifluoromethyl)-7H-pyrrolo[3,2-g]quinolin-7-one (LD-473) have been recorded at room temperature in different solvents and 1,4-dioxane-acetonitrile solvent mixtures. The UV-Visible absorption spectra are less sensitive to solvent polarity than the corresponding fluorescence spectra in both the dyes which show pronounced solvatochromic effect. The effects of solvents upon the spectral properties are analyzed using Lippert-Mataga polarity function, Richardts microscopic solvent polarity parameter and Catalán's multiple linear regression approach. Both general solute-solvent interactions and specific interactions are operative in these systems. The solvatochromic correlations are used to estimate excited state dipole moments using experimentally determined ground state dipole moments. The excited state dipole moment for both the dyes are found to be larger than their corresponding ground state dipole moment and is interpreted based on their resonance structures.

Local solvent acidities in β-cyclodextrin complexes with PRODAN derivatives

The local solvent acidities (SA scale) of six 6-carbonyl-2-aminonaphthalene derivatives as β-cyclodextrin complexes in water are determined through fluorescence quenching. The local polarities (E(T)(N) scale) are determined through the shift of the emission center-of-mass. The apparent SA values reflect the solvent structure surrounding the guest’s carbonyl group, whereas the apparent E(T)(N) values reveal the net polarity of the entire guest molecule. Comparison of these values affords greater insight into the structures of the host–guest complexes. Derivatives 1 and 5 show unusually large acidities, indicative of highly exposed carbonyl groups. The remaining compounds give emission intensities pointing to shielded carbonyl groups. In this study, PRODAN and its derivatives are functioning as dual channel sensors of their local environment.

QM/MM excited state molecular dynamics and fluorescence spectroscopy of BODIPY

Absorption and emission spectra arising from the lowest energy transition in BODIPY have been simulated in the gas phase and water using a quantum mechanics/molecular mechanics (QM/MM) approach. Kohn-Sham density functional theory (DFT) is used to calculate both ground (So) and first excited (S1) states using the maximum overlap method to obtain the S1 state. This approach gives ground and excited state structures in good agreement with structures found using multiconfigurational perturbation theory (CASPT2). Application of a post-self-consistent field spin-purification relationship also yields transition energies in agreement with CASPT2 and available experimental data. Spectral bands were simulated using many structures taken from ab initio molecular dynamics simulations of the ground and first excited states. In these simulations, DFT is used for BODIPY, and in the condensed phase simulations the water molecules are treated classically. The resulting spectra show a blue shift of 0.3 eV in both absorption and emission bands in water compared to the gas phase. A Stokes shift of about 0.1 eV is predicted, and the width of the emission band in solution is significantly broader than the absorption band. These results are consistent with experimental data for BODIPY and closely related dyes, and demonstrate how both absorption and emission spectra in solution can be simulated using a quantum mechanical treatment of the electronic structure of the solute.