Solvent effects on photophysical properties of merocyanine 540

Absorption and corrected fluorescence emission spectra of merocyanine 540 (MC 540) were recorded in solvents with different physico-chemical parameters – dielectric constant ε_r, refractive index n, dipolarity/polarizability π^*, hydrogen-bonding ability α. Hypsochromic shifts of their maxima positions did not obey Lippert–Mataga equation. However, a correlation with π^* and α was observed. Relative quantum yields φ_f and rate constants of nonradiative deactivation processes k_nr of the dye in different solvents based on steady-state measurements were estimated. Dependence on hydrogen-bonding ability was indicated – with increasing α, φ_f decreased and k_nr increased. The interaction between MC 540 and solvent molecules is discussed in terms of the different interactions contributing to the solvent stabilization of merocyanine dyes. Our results indicate that hydrogen bonding may contribute to solvent stabilization of MC 540.     

The deexcitation of the S1 state of aminoanthraquinones: A steadystate and timeresolved study

The non-radiative processes of deactivation from the lowest singlet excited state of aminoanthraquinones have been studied using steady-state and time-resolved methods. The fluorescence decay rate constant, k_f correlates well with the solvent polarity parameter, E_T(30), in nonhydrogen bonding solvents. Large deuterium isotope effects in fluorescence lifetimes (τ_f) and quantum yields (ϕ_f) are observed in the case of 1-amino (AAQ) and 1-methylaminoanthraquinones (MAQ), where the S₁ state is mainly deactivated through internal conversion to the ground state. The temperature-dependence of the fluorescence quantum yields of various aminoanthraquinones was also investigated. The ϕ_f and τ_f exhibited strong temperature-dependence in the case of 1-acetylaminoanthraquinone (ACAQ). In the case of ACAQ, the intersystem crossing to the triplet state is a major deactivation channel from the S₁ and in this derivative a close-lying T₂ state seems to be responsible for the high k_isc rate. The fluorescence properties of 1,5-diaminoanthraquinone (DAQ) are affected by intermolecular hydrogen bonding with alcohols. Increasingn-alkyl chain length in the case of l-(n-alkyl)aminoanthraquinones from methyl to butyl does not produce any change in the fluorescence properties, whereas a hydroxypropyl substitution results in a small decrease of ϕ_f and τ_f in these compounds, indicating an interaction of the hydroxyl group with the carbonyl group of the aminoanthraquinones.     

Does Excited‐State Proton‐Transfer Reaction Contribute to the Emission Behaviour of 4‐Aminophthalimide in Aqueous Media?

4‐Aminophthalimide (AP) is an extensively used molecule both for fundamental studies and applications primarily due to its highly solvent‐sensitive fluorescence properties. The fluorescence spectrum of AP in aqueous media was recently shown to be dependent on the excitation wavelength. A time‐dependent blue shift of its emission spectrum is also reported. On the basis of these findings, the excited‐state solvent‐mediated proton‐transfer reaction of the molecule, which was proposed once but discarded at a later stage, is reintroduced. We report on the fluorescence behaviour of AP and its imide‐H protected derivative, N‐BuAP, to prove that a solvent‐assisted excited‐state keto–enol transformation does not contribute to the steady‐state and time‐resolved emission behaviour of AP in aqueous media. Our results also reveal that the fluorescence of AP in aqueous media arises from two distinct hydrogen‐bonded species. The deuterium isotope effect on the fluorescence quantum yield and lifetime of AP, which was thought to be a reflection of the excited‐state proton‐transfer reaction in the system, is explained by considering the difference in the influence of H₂O and D₂O on the nonradiative rates and ground‐state exchange of the proton with the solvent.     

Deuterium isotope effects in liquid–liquid phase diagrams of aniline + cyclohexane mixtures

Liquid–liquid miscibility temperatures as a function of composition and deuterium substitution have been experimentally determined for binary mixtures of aniline and cyclohexane and their various deuterated forms: aniline-d₂, aniline-d₅, cyclohexane-d₁₂. In all cases studied, the deuteration leads to the upward shift of the upper critical solution temperature. This effect is discussed in the frame of the theory of the condensed phase isotope effects coupled with the phenomenological g^E thermodynamic model of solutions.     

Determination of the lipophilicity of some aniline derivatives by reversed-phase thin-layer chromatography. The effect of the organic phase in the eluent

Summary The lipophilicity of aniline and 36 ring-substituted aniline derivatives was determined by reversed-phase thin-layer chromatography using methanol, acetone and acetonitrile as the organic phase. The R_M value of each compound linearly decreased with increasing concentration of the organic solvent, Acetonitrile showed the highest and methanol the lowest solvent strength, however, the difference between the solvent strength of methanol and acetone was negligible. The site of the substitution considerably influenced the lipophilicity particularly in the case of –NO₂ groups. Good correlation was found between the R_M value extrapolated to zero organic phase concentration and the partition coefficient between n-octanol:water. The change in the R_M value caused by a 1% increase of the organic phase concentration also correlated with the partition coefficient, however, it was of secondary importance. The lipophilicity parameters determined by reversed-phase thin-layer chromatography correlated to a lesser extent with the calculated lipophilicity values of aniline derivatives.     

Pyrene fluorescence quenching by aromatic azides

Pyrene fluorescence quenching by phenylazide derivatives with donor and acceptor substituents has been studied by fluorescence spectroscopy and flash photolysis. The rate constants of quenching (k _q) in acetonitrile ((0.2–1.2) × 10¹⁰ l mol^?1 s^?1) are found to be close to a diffusion limit; the rate constants were somewhat higher for perfluoro-substituted arylazides. It is found that k _q does not depend on solvent polarity; the formation of the pyrene cation in the course of pyrene fluorescence quenching by tolylazide was not detected. Pyrene fluorescence quenching occurred by an energy-transfer mechanism; this is supported by the coincidence of the quantum yields of the direct and sensitized photodecomposition of tolylazide. As estimated, energy transfer in rigid media occurs at characteristic distances of about 10 Å.     

Thermally assisted and heavy-atom assisted intersystem crossing in meso-substituted anthracenes

A single linear correlation between the activation energy of S₁—T intersystem crossing and the logarithm of the bimolecular fluorescence quenching rate constant by haloalkanes for meso-substituted anthracenes in fluid solutions is established. It is concluded that both the monomolecular and the heavy-atom-assisted deactivation of S₁ depend in the same way on the change of S₁ energy by substitution or solvent relative to the energy of the accepting triplet T_n. The anomalous increase of fluorescence of Br-substituted anthracenes in heavy-atom containing solvents is consistent with a widening of the S₁—T_n energy gap due to the changing solvent.     

Theoretical insight into the photodeactivation pathway of the tetradentate Pt (II) complex with different inductive substituents

Density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) both were used to explore the impacts of different inductive substituents on the photophysical properties, radiative/nonradiative processes and photodeactivation mechanism for the Pt (II) complex with novel spiro‐arranged tetradentate ligand. Spectrum simulations show that the electron donor methoxyl (‐OCH₃) group can cause the emission wavelength to red‐shift but have little effect on the absorption spectrum. In the simulation of the radiative decay process for the tetradentate Pt (II) complex, the singlet‐triplet splitting energy is reduced by the introduction of substituents with strong electron‐releasing capability (i.e., from the original trifluoromethyl (‐CF₃) group to ‐OCH₃ group), accompanied with a lower radiative rate constant (k_r). The analyses of non‐radiative decay processes show that the substitution of ‐OCH₃ group on azole rings reduces the energy barriers of thermally activated non‐radiative photodeactivation pathway, which in turn increases the temperature‐dependent non‐radiative rate constants (k_nr(T)). In addition, the substitution of ‐CF₃ by ‐OCH₃ group slightly weakens molecular rigidity and enhances the Huang‐Rhys factor, but decreases the SOC between the triplex excited (T) state and the ground (S₀) state. Thereby, the two complexes may have the similar temperature‐independent non‐radiative rate constant (k_nr’). This work offers theoretical guidance for the design and optimization of the efficient organic light emitting diode (OLED) materials based on the structure of tetradentate Pt (II) complexes.     

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.     

Revisiting fluorenone photophysics via dipolar fluorenone derivatives

The nonradiative decay of four dipolar fluorenone derivatives (FODs) was systematically investigated using steady state and time-resolved UV-vis absorption and fluorescence measurements combined with cyclic voltammetry. Analysis of the frontier orbital localization of the global minimum geometry and the vertical transitions was carried out from DFT calculations. The first singlet excited state was found to be π-π* in all derivatives regardless of the polarity of the solvent. Charge separation/recombination dominates the singlet excited state deactivation for carbazole-containing FODs. Intersystem crossing (ISC) operates exclusively in the 3,6-disubstituted variants as evidenced by phosphorescence experiments. In the case of CPAFO36, ISC competes disadvantageously with CT deactivation.     

Zur theorie der α-ketosa¨uren: Mechanismus der enolisierung einiger brenztraubensa¨ureamide

The mechanism of enolisation of pyruvamide is discussed by the influence of substituents on the kinetic CH₃-acidity, by general base-catalysis of enolisation, by the enthalpy and entropy of activation and primary kinetic and kinetic solvent deuterium isotope effects respectively. A Bro¨nsted coefficient β = 0·71 has been obtained in the general base catalysis of pyruvdiethylamide enolisation. The effect of car☐ylsubstituents on the kinetic CH₃-acidity is produced not only by an inductive mechanism. The importance of solvent structure is demonstrated by a strong negative entropy of activation for the H₂O-catalysed reaction. In the H₂O-catalysed enolisation of pyruvdiethylamide a large kinetic deuterium solvent isotope effect k_o^H₂O/k_o^D₂O = 2·39) was obtained at 25°C. In contrast, when hydroxid is the catalyst, the primary kinetic deuterium isotope effect is unusually low (k_H/k_D = 3·5). Thus, in comparison to other keto compounds, a different mechanism of enolisation for the pyruvic acid derivatives must be postulated. Some aspects of this mechanism are discussed in the paper.     

Nonradiative decay mechanism of fluoren-9-ylidene malononitrile ambipolar derivatives

We report recent results on the nonradiative decay (NRD) of fluoren-9-ylidene malononitrile (FM) ambipolar derivatives (FMDs). 2,7- and 3,6-disubstituted FMDs present distinctive photophysics. Charge separation was found dominant for excited state relaxation. The radiative decay (RD) is sensitive to changes in temperature and solvent medium only for the case of 3,6-FMDs. Excited state deactivation of carbazole-containing 3,6-FMD (CPAFM36) was exclusively nonradiative in polar solvents with excited state lifetimes shorter than 10 ps. The charge separation/recombination mechanism of the corresponding FMDs is suggested to fall in the inverted Marcus region of electron transfer. Given the electron-withdrawing properties of the FM unit, its ambipolar derivatives are suggested as potential candidates for air-stable organic thin-film transistors and molecular organic photovoltaics.     

Fluorescence from the second excited singlet of aromatic thioketones in solution

The spectral characteristics and the quantum yield of the fluorescence from the second excited singlet state S₂ of the aromatic thioketone molecules xanthione (XS) and thioxanthione (TXS) have been determined in solution at room temperature and 77 K. In 3-methylpentane, the measured quantum yields are φ_f (295 K) = 5.1 × 10^?3 and φ_f(77 K) = 1.0 × 10^?2 for XS, and φ_f (295 K) = 1.5 × 10^?3 and φ_f (77 K) = 2.5 × 10^?3 for TXS. Using the Strickler-Berg expression for the radiative lifetime, the decay rate of S₂ is derived. It is concluded that internal conversion S₂ ? S₁ is the dominating deactivation channel of S₂ with k^{77 K}_nr(S₂ ? S₁) = 1.0 × 10¹⁰ s^?1 for XS and k^{77 K}_nr (S₂→S₁) = 2.2 × 10¹⁰ s^?1 for TXS. Between 295 and 77 K, φ_f increases by a factor of about 2 following an Arrhenius type expression. This temperature dependence of φ_f is considered to be intramolecular in nature and is attributed to a temperature sensitive rate constant k_nr(S₂?S₁) with an activation energy of 190 ± 20 cm^?1 and a frequency factor k′_nr = 3 × 10¹⁰ s^?1 for the XS molecule in 3-methylpentane.     

On the role of intramolecular vibrations in the nonradiative decay of excited charge-transfer states of molecular complexes

An attempt for a theoretical treatment of radiationless transitions from excited charge-transfer states in molecular complexes is made within the framework of the statistical limit of radiationless transitions theory. This work deals with the S₁ → S₀ internal conversion in charge-transfer complexes of tetracyanoethylene (an electron acceptor) with benzene and toluene and their perdeuterated analogues. A dominant role of the high-frequency totally symmetric intramolecular vibrational modes in the nonradiative decay of excited charge-transfer states is assumed (this was inferred from the experimentally observed deuterium isotope effect on radiationless S₁ → S₀ transitions). Calculated absolute rate constants for internal conversion are found to be in good agreement with experimental ones. The results of our calculations reflect very well the observed moderate deuterium isotope effect.     

Large reaction radii for the quenching of fluorescent excited states of liquid cis-decalin and cyclohexane

The large rates of quenching by different solutes of the fluorescent solvent excited state in liquid cis-decalin and cyclohexane as observed by pulse radiolysis are explained by large reaction radii. Values of 14, 13, and 15 Å are found for the quenching of the excited state in cis-decalin by CCl₄, in cyclohexane by CCl₄, and by O₂ respectively.     

Influence of Excitation of the Lanthanide 4 fShell on Complexation with Organic Substrates in Solutions: 2. Isotope Effects in Complexation of Tris(heptafluorodimethyloctanedionato)europium(III) with Dimethyl Sulfoxide in Benzene Solutions

The effect of deuteration of dimethyl sulfoxide on the fluorescence quantum yield, temperature quenching of fluorescence, and its complexation with tris(heptafluorodimethyloctanedionato)europium(III) (Eu(fod)₃) in the ground and excited states was studied. It was found that excitation of f–ftransitions in Eu(III) increases the stability of Eu(fod)₃complexes with sulfoxides but has a very insignificantly influence on isotope effects. The deuterium effect is displayed to a small extent in altering the quantum yield and is accompanied by a decrease in the efficiency of temperature quenching of Eu(fod)^* ₃fluorescence.     

Characterization of the Excited States of Indigo Derivatives in their Reduced Forms

A comprehensive characterization of the electronic spectral and photophysical properties of the leuco (reduced) form of several indigo derivatives, including indigo and Tyrian Purple, with di‐, tetra‐, and hexa‐substitution, was obtained in solution. The characterization involves absorption, fluorescence, and triplet–triplet absorption spectra, together with quantitative measurements of quantum yields of fluorescence, ?_F (0.46–0.04), intersystem crossing, ?_T (0.013–0.034), internal conversion, ?_IC, and the corresponding lifetimes. The position and degree of substitution promote differences in the spectral and photophysical properties displayed by the investigated leuco derivatives. The ?_F values are about two orders of magnitude higher than those previously obtained for the corresponding keto forms. Also in contrast with the behavior found for the keto forms, the S₁～～→T₁ intersystem crossing is an efficient route for the excited‐state deactivation channel. These findings strengthen the fact that, in contrast to keto indigo where the internal conversion dominates the deactivation of the excited‐state, with leuco indigo (and derivatives), the excited state deactivation involves competition between internal conversion, triplet state formation, and fluorescence. A time‐resolved investigation of one of the compounds in glycerol showed the presence of a photoisomerization process.     

Photophysical properties of some phenetidines in non-polar and hydrogen bonding solvents at room temperature and 77 K

The present study was undertaken to investigate the photophysical processes in o-, m- and p-phenetidines, when dissolved in nonpolar and hydrogen bonding solvents, in their ground state and excited electronic state S₁, both at 300 and 77 K. In the ground as well as in the S₁ state it is proposed that the o-phenetidine molecule possesses a structure in which NH₂ and OC₂H₅ groups are away from each other, both in nonpolar cyclohexane (CH) and H-bond acceptor solvent triethylamine (TEA). The formation of a transient or nonemissive charge transfer (CT) complex resulting from strong excited state hydrogen bonding interaction with TEA is found to be responsible for the observed fluorescence quenching of the proton donor phenetidines at 300 K. From the room as well as low (77 K) temperature electronic absorption and steady state fluorescence studies, it was deduced that nonplanarity in the structure of the molecules increases as one moves from aniline to the phenetidines. It is suggested that in the solvent stiffening temperature 77 K, triplet states of all the phenetidines (o-, m- and p-) acquire some nπ* character due to conformational changes, whereas ππ* character is retained in their S₁ state. This facilitates a larger intersystem crossing (ISC) rate in phenetidines relative to the situation in aniline where both S₁ and T₁ possess the same nπ* nature at 77 K due to its more planar structure. However, ISC efficiency in phenetidines at 77 K is found to be impeded, especially in the case of o- and m-isomers, in the presence of TEA as inferred from the lowering of φ_p/φ_⨍ values and the increment of τ_p. In p-phenetidine, rapid equilibrium between a triplet state hydrogen bonded species and free molecules during the triplet excited state lifetime is suggested.     

Intramolecular Proton Transfer in 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine: Evidence for Tautomer in the Ground State

The intramolecular proton transfer in a newly synthesized molecule, 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine (HPOP) is studied using UV‐visible absorption, fluorescence emission, fluorescence excitation and time‐resolved fluorescence spectroscopy. In the ground state, the molecule exists as cis‐ and trans‐enol in all the solvents. However, in dioxane, alcohols, acetonitrile, dimethylformamide and dimethylsulfoxide the keto tautomer is also observed in the ground state. Dual fluorescence is observed in HPOP where the large Stoke shifted emission is due to emission from the excited‐state intramolecular proton transfer product, whereas the other emission is the normal emission from enol form. The fluorescence (both normal and tautomer emission) of HPOP is less than those of corresponding benzoxazole and imidazopyridine derivatives. This reveals that the nonradiative decay becomes more efficient upon substitution of electronegative atom on the charge acceptor group. The pH studies substantiate the conclusion that (unlike in its imidazole analog) the third ground state species is the keto tautomer and not the monoanion. The effect of temperature on cis‐enol‐trans‐enol‐keto equilibrium and the nonradiative deactivation from the excited state are also investigated.     

Application of the energy gap law to the decay of charge transfer excited states,solvent effects

Values of non-radiative decay rate constants (k_nr) and emission energies (E_cm) have been obtained for Os(Phen₃)²⁺ in a series of solvents and the results are consistent with the energy gap law. For hydroxylic solvents like water or methanol related studies suggest the existence of strong, specific contributions to the vibrational trapping energy of the solvent.