László Biczók

Spectroscopic properties of aromatic dicarboximides. Part1.—N—H and N-methyl-substituted naphthalimides

The photophysical properties of the N—H and N-methyl derivatives of 1,2-, 2,3- and 1,8-naphthalimides have been studied. The shift of the fluorescence emission position as a function of the solvent polarity indicates only a weak variation of dipole moment for the excited state compared with the corresponding value in the ground state (5.7 D for 2b, 2.8 D for 3b and <2 D for 4b, 1 D ≈ 3.335 64 × 10–30 C m, and 2b, 3b and 4b are N-methyl-1,2- naphthalimide, N-methyl-2,3-napthalimide and N-methyl-1,8-naphthalimide). However, important modifications of the photophysical properties are observed which depend on the relative position of the dicarboximide moiety on the naphthalene ring: the intersystem crossing rate constant of 4b increases dramatically by three orders of magnitude compared with that of 2b; simultaneously, the fluorescence quantum yield decreases from 0.77 to 0.03, although the corresponding rate constant, kf, increases. This difference is found to arise from the energy gap between the lowest1(π, π*) singlet excited state and the upper 3(n,π*) triplet state, which is of the order of 9 kcal mol–1 for 2b and less than 2 kcal mol–1 for 4b in acetonitrile solution. Protic solvents increase the energy difference between the n,π* and π,π* states thus decreasing the mixing of the two levels; as a consequence, the lifetime of 4b is increased, i.e. from <60 ps in hexane to 2.1 ns in trifluoroethanol. A triplet–triplet annihilation process occurs with the N-methyl derivatives 3b and 4b which leads to a monomer delayed fluorescence with the former, and mainly to a delayed excimer emission with the latter.

Fluorescence lifetime of Nile Red as a probe for the hydrogen bonding strength with its microenvironment

Abstract The fluorescence lifetime of Nile Red (NR) is not sensitive to dielectric solvent–solute interactions but markedly decreases with the increase of the hydrogen bond donating ability in alcohols because vibrations associated with hydrogen bonding are involved in the deactivation process. The negligible viscosity effect indicates that twisting of the diethylamino moiety of NR does not play significant role in the dissipation of the excitation energy.

Considerable Change of Fluorescence Properties upon Multiple Binding of Coralyne to 4-Sulfonatocalixarenes

The interaction of coralyne, an analogue of natural protoberberine alkaloids, with 4-sulfonatocalixarenes (SCXn) was studied in aqueous solution at pH 2 to reveal the major factors determining the stability, stoichiometry, and fluorescent properties of the species formed. Addition of SCXn to coralyne solution brought about remarkable fluorescence intensity diminution and hypochromism in the 300-440 nm absorption domain. SCXn hosts were capable of binding as many coralyne molecules as the number of their hydroxybenzenesulfonate units. The SCXn-promoted interaction among coralyne molecules was evidenced by the appearance of a long-lived fluorescence component. In dilute alkaloid solution, 1:1 and 1:2 coralyne/4-sulfonatocalix[4]arene complexes were formed, but only 1:1 association occurred with 4-sulfonatocalix[8]arene. Time-resolved fluorescence measurements demonstrated that photoinduced electron transfer from a hydroxybenzenesulfonate moiety to the singlet-excited coralyne can compete efficiently with the other deactivation processes.

Spectroscopic properties of aromatic dicarboximides part 3: Substituent effect on the photophysical properties of N-phenyl-2,3-napthalimides

Abstract In the present article we describe the effect of the substitution on the photolysis properties of a series of N -phenyl-2,3-napthalimides. It is found that the decrease in the electron-donating character of the substituent on the N -phenyl ring changes the fluorescence emission from a weak, broad and short-lived long-wavelength (LW) emission into a structured and long-lived fluorescence localized at the short wavelengths (SW) which behaves like that of the unsubstituted 2,3-napthalimide. Both SW and LW emissions can be observed in some cases. The different results can be explained on the basis of a three-level scheme where the vibrationally relaxed Franck-Condon state populates two different excited states, the one (SW) emitting at short wavelengths, and the other (LW) emitting at long wavelengths. The substitution on the phenyl ring influences mainly the energy of the LW excited state. The nature of the substituent on the N -phenyl ring has a determining influence also on the internal conversion process by virtue of the solvent- and rotation-induced pseudo-Jahn-Teller coupling of the two excited states.

The role of intersystem crossing in the deactivation of the singlet excited aminofluorenones

Solvent and substituent effects on the competition between internal conversion and triplet formation were studied systematically for aminofluorenones and their N-methylated derivatives. Intersystem crossing (ISC) was found to be the dominant process for the singlet excited 1-amino- and 1-methylaminofluorenone in all solvents. The short fluorescence decay time of these compounds does not originate from intramolecular hydrogen bonding induced internal conversion but it is due to the fast triplet formation. Rather slow (kISC⩽4.8 × 107 s−1) and solvent insensitive intersystem crossing characterizes the photophysical behavior of 2-, 3- and 4-aminofluorenones but their internal conversion rate strongly increases with solvent polarity. The change of the internal conversion rate constants with molecular structure and solvent can be rationalized in terms of the energy gap law.

Photochromism in cucurbit[8]uril cavity: inhibition of hydrolysis and modification of the rate of merocyanine-spiropyran transformations.

The effect of inclusion complex formation on the photochromic behavior of a spirobenzopyran dye and its merocyanine isomer was studied in aqueous solution using cucurbit[8]uril (CB8) as a host. The merocyanine (MC) and protonated merocyanine (MCH(+)) were the most stable forms both in water and inside the cavity of CB8. The equilibrium constant of 1:1 complexation with CB8 was found to be 1.7 × 10(5) and 2.0 × 10(6) M(-1) for the former and latter species, respectively. The encapsulation led to significant change in the rate of the photoinduced and thermal photochromic transformations and hindered the hydrolysis of MC. The effect of CB8 on the reaction kinetics strongly altered with pH. The transition from the spiropyran form to trans-MC in a thermal reaction had 33 kJ mol(-1) lower activation energy and more than 5 orders of magnitude smaller Arrhenius pre-exponential factor in CB8 than in water.

Kinetics and Thermodynamics of Berberine Inclusion in Cucurbit[7]uril

The kinetics and thermodynamics of berberine inclusion in cucurbit[7]uril was studied by stopped-flow method, fluorescence titrations, and isothermal calorimetry in neat water. The ~500-fold fluorescence intensity enhancement upon encapsulation was exploited to monitor the complex formation in real time at various temperatures. The increase in the fluorescence intensity could be fitted well by assuming a simple 1:1 binding equilibrium without any intermediate formation. For the rate constants of association and dissociation, (1.9 ± 0.1) × 10(7) M(-1) s(-1) and (0.81 ± 0.08) s(-1) were found at 298 K, respectively. The ingression into the cavity of CB7 had 32 ± 2 kJ mol(-1) activation enthalpy, implying a constrictive binding, whereas 69 ± 2 kJ mol(-1) was obtained for the activation enthalpy of the egression. Substantial structural change had to occur when berberine passed through the tight carbonyl-rimmed portal of the macrocycle to reach the transition state. An enthalpy-driven complexation took place with a slight entropy gain.

4-Sulfonatocalix[6]arene-induced aggregation of ionic liquids

The interaction of 4-sulfonatocalix[6]arene (SCX6) macrocycle with 1-alkyl-3-methylimidazolium type of ionic liquids possessing dodecyl, tetradecyl, or hexadecyl substituent was studied in aqueous solution at 298 K. Host-guest complexation promoted the spontaneous self-assembly into nanoparticles of 7:1 ionic liquid:SCX6 stoichiometry. Positively charged and stable nanoparticles were produced in solutions of 7-200-fold excess of ionic liquid as compared to the amount of SCX6. The negatively charged nanoparticles formed in solutions having 2-7 ionic liquid:SCX6 molar ratios evolved into larger species. The stability of the nanoparticles increased with the lengthening of aliphatic chain of the ionic liquid. Cryo-TEM experiments showed dense particles generally with spherical shape and multilayered structure, which has been confirmed by small-angle neutron scattering.

Spectroscopic properties of aromatic dicarboximides. Part 2.—Substituent effect on the photophysical properties of N-phenyl-1,2-naphthalimide

Absorption and fluorescence spectra, fluorescence decay times, fluorescence quantum yields and triplet yields have been determined for N-phenyl-1,2-naphthalimide and its phenyl-substituted methyl derivatives in different solvents. N-Phenyl-1,2-naphthalimide emits long-wavelength fluorescence in hexane (λmaxf= 550 nm) which is red shifted by methyl substitution at the meta and para positions of the phenyl ring and by using a solvent of higher polarity. The fluorescence decays on the sub-nanosecond timescale. When the N-phenyl-1,2-naphthalimide has o-methyl substituents in the phenyl group, they emit dual fluorescence. The location of the short-wavelength component is constant while that of the long-wavelength component is blue shifted as a result of o-methyl substitution. Ortho substitution also increases the decay time of the long-wavelength fluorescence. The results are explained in terms of vibronic interaction between the S1(ππ*) and S2(nπ*) excited states (pseudo-Jahn–Teller effect) which is enhanced by solvent relaxation and twisting of the phenyl ring towards a coplanar geometry.

Thermodynamics of inclusion complex formation between 1-alkyl-3-methylimidazolium ionic liquids and cucurbit[7]uril

The thermodynamics of 1:1 inclusion complex formation between 1-alkyl-3-methylimidazolium type ionic liquids and cucurbit[7]uril was studied by isothermal titration calorimetry in aqueous solution at 298 K. The encapsulation proved to be enthalpy driven for all cations used. The enthalpy change upon binding (ΔH) became more negative when the 1-alkyl moiety of the imidazolium ring was gradually lengthened reaching the most exothermic association with the hexyl derivative. Further increase of the number of carbon atoms in the aliphatic chain led to less negative ΔH values. The much smaller entropy change followed the trend of ΔH. The slope of the linear enthalpy–entropy correlation found in the present work is significantly smaller than that reported previously for cyclodextrin complexes, because the more rigid CB7 macrocycle cannot undergo significant conformational change upon complexation.