The effect of substituent position and solvent on thermal Z‒E isomerization of dihydroquinolylazotetrazole dyes: kinetic, thermodynamic, and spectral approaches.

Kinetic and thermodynamic parameters have been investigated for the thermal Z‒E isomerization of dihydroquinolylazotetrazole dyes with alkyl substituents (Me, t-Bu, and Adm) at positions 1 (dyes 2) and 2 (dyes 3) of the tetrazole moiety in two solvents of different polarity, acetonitrile (MeCN) and toluene. The experimental results show crucial dependence of these parameters on a substituent position in the tetrazole moiety and on a solvent. For dyes 2, Eact and ΔH‡ are lower in MeCN than in toluene that results in a high increase in the lifetimes of the Z isomers: from milliseconds in MeCN to minutes in toluene. For dyes 3, the difference in Eact and ΔH‡ in the two solvents is opposite: Eact and ΔH‡ are by more than 20 kJ mol-1 higher in MeCN, nevertheless, the rate constants for 3 in toluene are comparable with those in MeCN at the ambient temperature and the difference in the behavior is determined by the value of negative entropy of activation. Quantum-chemical calculations of the thermal Z‒E isomerization show the possibility of the process to occur via crossing from the S0 to the thermally induced T1 state. The contribution of this path is highest for 3 in toluene. The analysis of the absorption spectra demonstrates that for the E isomers, the n‒π* and π‒π* transitions are within the long-wavelength absorption band and their positions relative each other are opposite in the solvents: the n‒π* transition is blue-shifted relative to the π‒π* transition in MeCN and is red-shifted in toluene.

Photoactivated biscarbocyanine dye with two conjugated chromophores: complexes with albumin, photochemical and phototoxic properties.

Complexes of photosensitizers with blood proteins play an essential role in their delivery to the cell, as well as in the efficacy of photodynamic therapy. Biscarbocyanine dye non-covalently binds human serum albumin (HSA), the dissociation constant of the dye with albumin being Kd = (1.7 ± 0.1) × 10-5 M. According to time correlated single photon counting (TCSPC) fluorescence lifetime spectroscopy data, two types of complexes with lifetimes of 1.0 ns and 2.5 ns are formed between the dye and HSA. Confocal fluorescence microscopy has unambiguously shown the penetration of biscarbocyanine into endoplasmic reticulum, lysosomes, mitochondria and nuclei of the cells. The dye demonstrates photocytotoxicity towards the colon carcinoma HCT116 cells with IC50 = 0.3 µM. Hydrophobicity of the polymethine chain and the presence of two positive charges on the dye molecule contribute to the effective binding of the dye with HSA and the penetration into cells. These facts allow considering the biscarbocyanine dye as a promising agent for the photodynamic therapy of cancer.

Novel hetarylazo dyes containing tetrazole and hydroquinoline moieties: spectral characteristics, solvatochromism and photochemistry.

Novel hetarylazo dyes containing tetrazole and tetra- or dihydroquinoline moieties were synthesized and their spectral properties in solvents of different polarities and H-bonding abilities were examined. The dyes exhibit solvatochromism dramatically depending on the proton accepting ability of solvents: (DMSO > H2O > MeOH > ACN > CH2Cl2) and the dye concentration. Upon dilution the absorption maximum of the visible band shows a blue shift and the absorption coefficient of the maximum decreases. This was accounted for by complex formation either between the dye molecules or between the dye and solvent molecules. The H-bond with partial proton transfer is formed between the acidic NH group of the tetrazole moiety of a dye molecule and the basic NH group of the hydroquinoline moiety of the other dye molecule or with a solvent with proton accepting ability. Upon dilution the equilibrium shifts to the complex with a solvent. The coexistence of several forms of the dye molecules with different absorption spectra was demonstrated in pulse photolysis upon excitation by light with different wavelengths. Three forms of cis-isomers were registered. The photogenerated cis-isomers decay with lifetimes from 200 µs to 5 ms. The fast cis-trans dark isomerization determines the photostability of the dyes.

Complex formation of albumin with tricarbocyanine dyes containing phosphonate groups.

The spectral characteristics, binding constants with bovine (BSA) and human serum albumin (HSA) and lifetimes of fluorescence in PBS and EtOH solutions and in the presence of BSA in PBS were measured for novel indotricarbocyanine dyes bearing remote phosphonate groups. These parameters are close to those for indocyanine green (ICG) indicating that the Coulomb interaction does not play a significant role in complex formation, and the binding is determined by the interaction of the dye polymethine chain with albumin. The fluorescence lifetimes of the complexes with BSA strongly indicate the formation of complexes of two types with different lifetimes. The complex with a longer fluorescence lifetime (740-800 ps) and major contribution (up to 88%) is bound to the more hydrophobic site and that with a shorter fluorescence lifetime (300-340 ps) to the more hydrophilic site.

CIDNP as a tool to unveil the reaction mechanism: interaction of mixed phosphonium-iodonium ylide with p-methoxyphenylacetylene.

In situ acquisition of the reaction between benzoyl phosphonium-iodonium ylide and p-methoxyphenylacetylene in an NMR spectrometer reveals the CIDNP effect in 31P and 1H NMR spectra of major products, substituted furan (emission) and phosphonium salt (enhanced absorption). The mechanism of products formation via radical pairs is discussed.

Stepwise versus concerted mechanism of photoinduced proton transfer in sec-1,2-dihydroquinolines: effect of excitation wavelength and solvent composition.

Excited-state proton transfer in 2,2,4,6-tetramethyl-1,2-dihydroquinoline was studied in dependence on solvent composition in binary mixtures of MeOH with nonpolar pentane and polar acetonitrile by an ultrafast pump-probe technique with λ(pump) = 350 nm and spectroscopic detection. The concerted mechanism was observed earlier at λ(pump) = 308 nm, whereas the stepwise mechanism is clearly observed at λ(pump) = 350 nm. The "fast" (200 fs) component is attributed to PT from MeOH molecule to the C(3) of the heterocycle to give the carbocationic structure in the highest vibrational ground state. This process is succeeded by the "slow" PT from the NH group to the MeO(-) anion in the cluster of MeOH molecules to give corresponding 2,3-dihydroquinoline. The former process is independent of the solvent composition, whereas the latter depends on the solvent composition in the MeOH-C5H12 mixtures with a characteristic time increasing from 15 ps in bulk MeOH to 150 ps in the mixture containing 2 vol % MeOH. This result is explained in terms of stabilization of the ionic species in the less polar microenvironment.

Ultrafast excited state proton transfer dynamics of 1,2-dihydroquinolines in methanol solution.

Femtosecond and picosecond dynamics of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) and 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline (2) were studied in MeOH, MeOD, and Pr(i)OH to probe the early events of the photoinduced proton transfer (PT) between 1,2-dihydroquinolines (DHQ) and a solvent. From studies in the two solvents MeOH and Pr(i)OH and by examining the effect of deuterium replacement of proton, it has been established that PT takes 150-200 fs in MeOH, but does not occur in Pr(i)OH. The formation of PT products in the ground state proceeds concurrently to the relaxation of the higher vibrational excited singlet state to the thermally equilibrated state S(1) of DHQ. The absorption spectrum of the S(1) state was registered, and the time constant of its decay in MeOH (ca. 1 ns) agrees well with the lifetime of fluorescence measured recently by single photon counting.

Mechanism of the photoinduced addition of methanol to the double bond of 2,2,4,6-tetramethyl- and 1,2,2,4,6-pentamethyl-1,2-dihydroquinolines.

The mechanism of the photoinduced Markovnikov addition of methanol to the double bond of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) and 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline (2) was studied by the flash photolysis technique over a wide range of acetic acid and KOH concentrations. The successive formation of two transients was observed in neutral solutions in the case of compound 1. These transients have lifetimes on ms time-scales and absorption spectra with maxima at 420 and 480 nm for the first and the second transient, respectively. The decay rate constants of these transients (k1 and k2) were measured over the temperature range from 10 to 45 degrees C and the activation energies E1 approximately 0 and E2 (31.4 +/- 3.1) kJ mol(-1) were determined. In MeOH acidified with acetic acid only the second transient (lambda(max) = 480 nm) was observed, and it is proposed that this is due to a carbocation. The deuterium kinetic isotope effects determined in MeOD are 2 and 1.3 for k1 and k2, respectively, indicating that the first reaction is proton transfer proceeding via the A-SE2 mechanism and the second reaction is the nucleophilic addition of the solvent to the carbocation. In alkaline media, only the first transient (lambda(max) = 420 nm) was observed with a lifetime of 250 ms at [KOH] > 2 x 10(-3) mol dm(-3). In the case of compound 2, the formation of a carbocation with lambda(max) = 490 mn and a lifetime of several ms is observed within 10 ns of excitation in neutral, acidic, and alkaline solutions. The carbocation is quenched with KOH with a rate constant of 7 x 10(6) mol(-1) dm(3) s(-1). The quantum yields of the reaction and of the fluorescence were measured and possible mechanisms discussed.