Grazyna Wenska

Intra- and Intermolecular Electronic Relaxation of the Second Excited Singlet and the Lowest Excited Triplet States of 1,3-Dimethyl-4-thiouracil in Solution¶

Intramolecular processes of deactivation of 1,3‐dimethyl‐4‐thiouracil (DMTU) from the second excited singlet (S2) (π, π*) and the lowest excited triplet (T1) (π, π*) states have been studied using perfluoro‐1,3‐dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (τ  S 2) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of τ  S 2 in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern–Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (τ0  T 1) and rate constants of self‐quenching (ksq). The lifetimes, τ0  T 1, of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group.

Photophysical and photochemical properties of 4-(1,2,4-triazol-1-yl)-pyrimidin-2(1H)-ones

Abstract The solvent and pH dependence of the absorption and fluorescence spectra, fluorescence quantum yields, fluorescence lifetimes, low temperature luminescence spectra and quantum yields and photochemical reactivity of a series of 4-(1,2,4-triazol-1-yl)-pyrimidin-2(1H)-ones have been examined and the data obtained were compared with the results reported previously for uracil and thymine nucleosides. Stacking interaction occurs between triazolyl-pyrimidinones and common nucleic acid bases and was detected by UV absorption and fluorescence spectroscopy. The compounds studied undergo [2+2] photocycloaddition. This was determined on the basis of photochemical studies of synthesized dinucleotide analogues.

Photophysical and photochemical properties of C-linked ribosides of pyridin-2-one

Abstract Absorption and emission properties as well as photochemical behavior of C-ribosides bearing pyridone as the aglycone, namely 5-(β- d -ribofuranosyl)-pyridin-2-one ( 1 ) and 3-(β- d -ribofuranosyl)-pyridin-2-one ( 2 ) and pyridone-uridine bichromophoric compound ( 6 ) have been investigated. Photocycloisomerization of the pyridone ring leading to diastereomeric Dewar type photoproducts is the major photochemical pathway both in C-nucleosides 1 , 2 and the bichromophoric compound 6 . In the case of the photocycloisomerization of 1 diastereomeric excess of 26% was determined by HPLC.

Synthesis and Fluorescent Properties of 6-(4-Biphenylyl)-3,9-dihydro-9-oxo-5H-imidazo[1,2-A]purine Analogues of Acyclovir and Ganciclovir

Abstract Tricyclic (T) analogues of acyclovir (ACV, 1) and ganciclovir (GCV, 2) carrying the 3,9-dihydro-9-oxo-5H-imidazo[1,2-a]purine system [i.e., 6-(4-BrPh)TACV, 5 and 6-(4-BrPh)TGCV, 6] were transformed into 6-[(4′-R2)-4-biphenylyl] derivatives of TACV (7–9) and TGCV (10–12) by Suzuki cross coupling with 4-substituted phenylboronic acids. Compound 11 (R2 = CH2OH) showed a high (∼1000) selectivity index against herpes simplex virus type 1 (HSV-1) together with advantageous fluorescence properties (emission in visible region, little overlap with absorption and moderate intensity).

Spectral and photophysical properties of some imidazo[1,2-a]purine derivatives related to acyclovir

Abstract Spectral and photophysical properties of 3,9-dihydro-3-[(2-hydroxyethoxy)methyl]-6-phenyl-9-oxo-imidazo[1,2-a]purine 7b (congener of acyclovir) and its three substituted derivatives 7a, c and 8a in organic solvents and in aqueous buffer solutions have been determined. Compound 7c has an ester group in the para position of the phenyl ring, compounds 7a and 8a are derivatives of 7b with a methyl group attached to imidazole N(5) or pyrimidine N(4) ring of the tricyclic skeleton, respectively. Experimental study has been supplemented with theoretical calculations of the singlet electronic transition energies and oscillator strengths by the time dependent density functional theory (TD-DFT). Combination of experimental and computational results have indicated that compounds 7b and c having mobile hydrogen exist in the ground state in a single N(5)H tautomeric form. The lowest excited S1 state of all compounds investigated has the π, π∗ configuration. The spectral and photophysical properties of 7a–c are similar. The fluorescence behaviour of the compounds depends on the solvent polarity and pH of the aqueous medium in the range of pH: 0–4 and 6–12. The pH dependence is a consequence of the protonation of the molecules (pKa=2.2 for 7a and pKa=2.0 for 7b and c) and their deprotonation in the ground state (pKa=8.0 and 8.2 for 7b and c, respectively). The absorption and fluorescence properties of the ionic forms have been determined. In contrast to 7a–c, the spectral and photophysical properties of 8a are not significantly influenced by the solvent polarity and they are independent on pH of aqueous solution within 4–12 range.

Ground and excited state prototropic behavior of 1-(purin-6-yl)-3-methylimidazolium chloride

Abstract The ground and excited state acid-base properties of the compound 1-(purin-6-yl)-3-methylimidazolium chloride, 2 , have been studied in aqueous solutions by means of UV absorption and fluorescence (steady state and time resolved) measurements. The ground state exhibits cation-zwitterion and cation-divalent cation equilibria with p K a = 7.25±0.05 and −1.3±0.1, respectively. Within the pH range 5.3–3, the cationic form of 2 undergoes deprotonation in the excited singlet state leading to a highly fluorescent zwitterionic species. However, the equilibrium is not established within the excited singlet state lifetime and it is estimated that only a fraction (≈12%) of the excited molecules undergoes deprotonation. Using the Forster cycle, an excited singlet state p K a * = 1.5 is estimated from the steady state absorption and fluorescence emission spectral data. In the range 1 3 , and 1-(3-methylpurin-6-yl)-3methylimidazolium chloride, 4 .

Photochemical transformations of 5-halogeno-4-thiouridines

A series of 2′,3′,5′-tri-O-acetyl-5-halogeno-4-thiouridines, where halogen = Br, Cl, F, are synthesized and their photochemical transformations upon irradiation with near-UV light (λ > 300 nm) in aqueous acetonitrile solutions investigated. The main photochemical pathways in each case involve intermolecular additions leading to three main photoproducts with different relative distributions. The photoproducts are isolated and their structures determined based on MS, 1H and 13C NMR and UV spectral data.

Properties of singlet excited N-(pyrimidin-2-one-4-yl)pyridinium chloride and structurally related organic cations

Abstract The UV absorption and fluorescence spectra, fluorescence quantum yields and lifetimes of four new fluorescent N -(pyrimidin-2-one-4-yl)pyridinium and N -(pyrimidin-2-one-4-yl)methylimidazolium cations were measured in several solvents at room temperature and in ethanol glasses at 77 K. The spectroscopic studies were supplemented by theoretical calculations of the electronic transition energies, oscillator strengths and dipole moment changes by the INDO/S CI method. It is concluded that the lowest excited singlet state of the compounds studied has charge transfer character. The significantly different charge distribution in the S 1 state and the rotational relaxation of the excited molecules are responsible for the large Stokes shift observed in the spectra of pyridinium cations. N -(Pyrimidin-2-one-4-yl)methylimidazolium cations are more efficient fluorophores than pyridinium cations and this is due to the increased rate of non-radiative decay in the latter compounds.

Pyridine assisted phosphorylations of nucleobase bis-lactam systems. Formation and reactivity of dipyridinium species.

Abstract Heteroaromatic bis-lactam systems of uracil, thymine, xanthine and O-acetylated xanthosine undergo pyridine assisted phosphorylations with formation of respective dipyridinium species 5–8. Xanthosine appeared to be most reactive. Structure of those new representatives of N-aryl-pyridinium species was established on the basis of 1H and 13C NMR spectra. Under alkaline conditions dipyridinium species undergo transformation into stable fluorescent monosubstituted pyridinium derivatives 10–13.

Synthesis and fluorescent properties of the tricyclic analogues of acyclovir linked with nitrogen hetbrocyclic units.

Tricyclic (T, 3,9-dihydro-9-oxo-5H-imidazo[l,2-α]purine) analogues of acyclovir (ACV, 1), substituted in the 6 position with pyrid-4-yl, 4-(pyrid-4′-yl)Ph, 4-(pyrimidin-5′-yl)Ph and 4-(thiazol-2′-yl)Ph units were synthesized. For the synthesis ofthe heteroarylphenyl derivatives, a convenient general route was developed, i.e., Suzuki cross-coupling between protected 6-(4-dihydroxyborylphenyl) TACV and easily available bromoheterocycles. Fluorescent properties of newly synthesized TACV aoalogues strongly depend on the nature of a solvent. This sensitivity of fluorescence makes the compounds promising probes of H-bonding in the environment.