Functional monomers and polymers. XCIII. Photochemical reactions on the acrylic and methacrylic copolymers containing thymine bases

Photodimerization of Polyacrylic and polymethacrylic derivatives with different pendant thymine unit content was studied in dimethylformamide solution. The quantum efficiency of thymine base for the photodimerization increased with increasing thymine content in the copolymers. The quenching study, which used isoprene as the quencher, revealed that the photodimerization resulted from excited singlet state increases with increasing thymine content. The photochemical results were discussed in terms of self-association of thymine bases in the polymer chain.     

Functional monomers and polymers. XCVI. Photochemical reactions on oligo- and polyethyleneimines which contain pendant thymine bases

The photodimerization reaction of oligo- and polyethyleneimine derivatives which contain pendant thymine bases in various amounts was studied in aqueous solution in a wide pH range and in N, N-dimethylformamide solution. The photodimerization reaction of these derivatives in poly(methyl methacrylate) was also studied. It was found that its quantum efficiency tended to increase as thymine units were added to the oligomers and copolymers. This result is discussed in terms of the effect of the nearest neighboring thymine units and singlet energy migration, particularly in the polymers.     

THEORETICAL INVESTIGATION OF EXCIMER AND EXCIPLEX STATES OF URACIL AND HALOGEN DERIVATIVES: EFFECT OF NONPARALLELISM OF BASES

Theoretical modeling of initial steps of the photodimerization mechanism of uracil, 5-methyl-and 5-halogen derivatives was performed. The interaction energy of bases in stacked dimers in the ground and lowest excited states was calculated as a function of the distance between the base planes and of the rotation angles within the perturbation theory for the extended Hückel treatment. The existence of excimer and exciplex region on the potential surface of the excited state was revealed. The excimer (exciplex) geometry has the planes nonparallel with more close contact of the C5-C6 bonds as compared to the ground state of dimers. The results provide new information useful for understanding the photodimerization mechanism of bases and testifies that the singlet excimer state can be a precursor of the photodimerization reaction.     

Functional monomers and polymers. XCIV. Photochemical reactions on the synthetic polymers containing thymine bases in the presence of adenine derivatives

Photodimerization reactions of polyacrylate and polymethacrylate derivatives and the dimer model compound containing thymine bases were studied in the presence of adenine derivatives in dimethyl sulfoxide; N,N-dimethylformamide; and dimethyl sulfoxide–ethylene glycol solutions. The photodimerization of thymine bases both in the polymers and in the dimer model compound was found to be quenched by the addition of adenine derivatives. Base-base interaction in the ground state was also studied by ultraviolet (UV) spectroscopy in the three solvents. The quenching of the photodimerizationof thymine bases in the presence of adenine derivatives was discussed in terms of the specific interaction between adenine and thymine bases both in ground and excited states.     

Functional monomers and polymers. LXXXVI. Photochemical reactions on synthetic polymers with pendant thymine bases: Effect of solvents

Photodimerization of thymine bases present on the side chain of acryloyl and methacryloyl-type polymers was studied in dimethyl sulfoxide, dimethylformamide, and dimethyl sulfoxide—ethylene glycol mixture. Quantum efficiencies for photodimerization and quenching with isoprene were influenced by the solvents. The self-association of thymine bases estimated from their ultraviolet (UV) spectra and intrinsic viscosity were related to the effect of solvent on the photodimerization.     

Functional monomers and polymers. CVIII. Photodimerization of pendant thymine bases in thymine containing poly-lysine derivatives

The photodimerization reaction of pendant thymine bases in thymine-containing poly-lysine derivatives was studied over a wide range in aqueous solution. It was found that the quantum yield of the photodimerization of pendant thymine bases is affected mainly by the conformation of the polymers in solution. The differences in photoreaction behavior were discussed for poly-D -, poly-L -, and poly-DL -lysine derivatives.     

Functional monomers and polymers. LXI. Photochemical reactions on the synthetic polymers containing thymine bases

Photodimerization of N-2-isobutyloxyethyl thymine (T-M), bis[2-(5-methyl-1-primidinyl)-ethyl] glutarate (T-T), poly(N-2-methacryloyloxyethyl thymine) (polyMAOT), and poly(N-2-acryloyloxyethyl thymine) (poly AOT) was studied in dimethylformamide solution. The rate of intramolecular photodimerization of polyMAOT and polyAOT was 5.7 and 7.2 times faster, respectively, than that of T-T. In T-M, however, intermolecular photodimerization did not occur under the reaction condition used. Quenching studies with isoprene suggest that the photodimerization of T-T occurs predominantly in an excited triplet state, whereas that of polyMAOT and polyAOT occur in singlet and triplet states. It was concluded that the photodimers of T-T, polyMAOT, and polyAOT are two syn-fused cyclobutane-type dimers (cis-syn and trans-syn). Quantum efficiencies of thymine base disappearance were determined by ultraviolet (UV) analysis to be 0.0012 for T-T, 0.0084 for polyMAOT, and 0.010 for polyAOT.     

Singlet energy migration along an alternating block copolymer of oligothiophene and oligosilylene in solution

The singlet excited-state properties of the block copolymers of oligothiophene and oligosilylene in solution were investigated with several fast spectroscopic methods. Time-resolved fluorescence measurements at room temperature and in a glassy matrix revealed that the singlet excited states of the block copolymers are deactivated accompanying structural changes of the polymer. It became clear from the transient absorption spectroscopy that the absorption peak of the singlet excited state shifted to the longer wavelength side compared to that of the corresponding oligothiophenes because of the sigma-pi conjugation of the oligothiophene and oligosilylene. The intersystem crossing process generating the triplet excited state was also revealed by the transient absorption spectroscopy. Energy migration along the polymer chain was revealed by the fluorescence anisotropy measurements. The time constant for the energy migration became faster as the size of the oligothiophene in the polymer repeating unit became shorter. From comparison with the F?rster theory, the energy migration process was attributed to an incoherent hopping mechanism.     

Functional monomers and polymers. CXIII. Photoinduced conformational change of thymine containing poly-lysine derivatives

The photodimerization reaction between thymine bases in thymine-grafted poly-D - and poly-L -lysine derivatives was studied in an alkaline, aqueous, buffered solution. It was found that the helix content of the polymers changes by photodimerization. The result was discussed in terms of conversion, the rate of photodimerization, and the helix content of the polymers.     

Functional monomers and polymers. XCV. Photochemical reactions on synthetic polymers having pendant thymine bases in polymethyl methacrylate film

The photodimerization reaction of synthetic polymers which contain thymine bases was studied in polymethyl methacrylate film. The quantum efficiency for the photodimerization of the thymine bases of polyacrylate and polymethacrylate, fixed in the polymethyl methacrylate film was measured. The value of the film was higher than those measured in solution.     

Transient absorption spectra and quenching of coumarin excited states by nucleic acid bases.

Triplet-triplet absorption spectra of coumarin show different profiles and maxima in ethanol from those in water, which are very similar to those reported in benzene. Long-lived transient species other than triplet states were generated as initial photoproducts between coumarins and nucleic acid bases. The excited singlet and triplet states of coumarins were quenched by nucleic acid bases. Adenine base quenched the excited singlet state of coumarins more efficiently than thymine base. However, photocycloadducts of furocoumarins are formed predominantly with thymine, and not with adenine. Moreover, it is reported that the poly[dA-dT].poly[dA-dT] sequence region is the most favourable site for the photocycloaddition reaction of furocoumarins. The results imply that adenine contributes to singlet-state photocycloaddition reaction of furocoumarins with thymine, probably through an adenine-furocoumarin-thymine termolecular interaction.     

A gas phase ab initio excited state geometry optimization study of thymine, cytosine and uracil

Molecular geometries of the nucleic acid bases thymine, cytosine and uracil in the ground and the lowest two singlet excited states were optimized using the ab initio approach employing the 4-31G basis set for all the atoms except the amino group of cytosine for which the 6-311+G^* basis set was used. The excited state calculations were performed employing configuration interaction involving singly excited configurations (CIS). Vibrational frequencies were computed in order to examine the nature of the stationary points on the potential energy surfaces obtained by geometry optimization. While the ground state geometries of uracil and thymine (except the methyl group hydrogens) are planar, the corresponding excited state geometries were found to be significantly nonplanar. In the case of cytosine, the amino group is pyramidal and the rest of the molecule is only slightly nonplanar in the ground state, but the excited state geometries are appreciably nonplanar. In particular, consequent to the S₂(n–π^*) excitation of cytosine, the amino group plane is strongly rotated. While thymine is stable in the S₂(π–π^*) excited state, uracil appears to be dissociative in the corresponding excited state.     

THE EFFECT OF 9-ETHYLADENINE ON THE PHOTODIMERIZATION OF 1-CYCLOHEXYLTHYMINE

Abstract— The photodimerization of 1-cyclohexylthymine (l-CT).and the effect of adding 9-ethyladenine (9-EA) on this reaction were studied in chloroform and/or acetonitrile solutions. The photodimerization of 1-CT is shown to occur through an excited triplet state. 9-EA works as an inhibitor against this reaction by quenching the excited singlet state of 1-CT. It is suggested that a singlet exciplex made up from the excited state 1-CT and the ground state 9-EA by hydrogen-bonding may play an important role in the quenching process.     

INTERACTIONS AND PHOTOREACTIVITY OF 8-ALKOXYPSORALENS WITH THYMINE. A MODEL APPROACH

Abstract— To investigate the interactions and the photoreactions in solution between thymine and psoralen, model compounds have been synthesized in which a thymine molecule is linked to a psoralen ring by a polymethylene bridge. Two series of compounds have been studied and compared as models for the two major drugs 5-MOP and 8-MOP. We report here the results obtained in the 8-alkoxypso-ralen series. In water, intramolecular ring-ring stacking interactions were evidenced using UV and ¹H NMR spectroscopies. In organic solvents such as ethanol, these interactions disappear completely. The photoreactivity of the models was examined in relation to their ground state interaction properties. in water and in ethanol. Slow photolysis of the pyrone ring of psoralen occurs whatever the length of the linking chain (3 to 12 methylenes). No intramolecular thymine-psoralen photoaddition nor psoralen photodimerization were observed. These results are discussed with regards to the behavior of the 5-alkoxy models which lead selectively to the 3,4 psoralen-thymine photoadduct.     

新型可聚合双发色团电致发光材料的合成

自１９８７年Ｔａｎｇ［１］报道了高效、高亮度的双层有机薄膜电致发光器件以来,电致发光材料的研究受到了广泛的关注．接着在１９９０年,英国剑桥大学Ｂｕｒｏｕｇｈｅｓ等［２］首次利用聚亚苯基乙烯（Ｐｏｌｙ（ｐ ｐｈｅｎｙｌｅｎｅｖｉｎｙｌｅｎｅ）,ＰＰＶ）...     

Photodimerization of Thymine-Containing Polymers: Applicability to Reversible Photoresists

The intramolecular photodimerization of thymine bases present on the side chain of a variety of synthetic oligomers and polymers was studied in detail in various organic solvents, and particularly in the case of thymine-containing polylysines, over a wide range of pH. Effects of solvents, presence of the complementary base, and spacers on the photoreaction were examined, and the conformational changes of the polymers were discussed in relation to their functionalities. Intermolecular photodimerization was further studied, and its applicability to photoresists was shown.     

Electronic structure,spectra, and mechanism of photodimerization of pyrimidine bases

Various electronic indices calculated by the CNDO /s-CI method for uracil, 5-fluorouracil, thymine, and cytosine and the Woodward-Hoffmann rule applied to the photodimerization of these molecules lead us to the following conclusions: (i) decrease of the C₅? C₆ bond order consequent to excitation is generally correlated with the photodimerization reaction for all the biological pyrimidines; (ii) excited pyrimidine molecules may be nonplanar and the excitation would be delocalized over both the molecules which photodimerize; (iii) electrostatic interactions may play a significant role in the initial stages of the photodimerization reaction; (iv) singlet precursors to the stable triplet photodimers may exist; and (v) electron density does not, in general, increase in the C₅? C₆ bond in the excited state as compared to the ground state, contradicting the proposals of some earlier workers. Hybrid population densities and σ-bond orders have been calculated for the first time for the molecules. Fluorine in 5-fluorouracil perturbs the hybrid populations as compared to those of uracil mainly in its vicinity.     

A doorway state leads to photostability or triplet photodamage in thymine DNA

Ultraviolet irradiation of DNA produces electronic excited states that predominantly eliminate the excitation energy by returning to the ground state (photostability) or following minor pathways into mutagenic photoproducts (photodamage). The cyclobutane pyrimidine dimer (CPD) formed from photodimerization of thymines in DNA is the most common form of photodamage. The underlying molecular processes governing photostability and photodamage of thymine-constituted DNA remain unclear. Here, a combined femtosecond broadband time-resolved fluorescence and transient absorption spectroscopies were employed to study a monomer thymidine and a single-stranded thymine oligonucleotide. We show that the protecting deactivation of a thymine multimer is due to an ultrafast single-base localized stepwise mechanism where the initial excited state decays via a doorway state to the ground state or proceeds via the doorway state to a triplet state identified as a major precursor for CPD photodamage. These results provide new mechanistic characterization of and a dynamic link between the photoexcitation of DNA and DNA photostability and photodamage.     

Electronic structures of the ground state and excited singlet states of nucleic acid bases

The self-consistent field (SCF) method is used in the semiempirical approximation (SA) with allowance for singly excited configurations to derive the ground state and singlet excited states for five nucleic acid bases (NAB). Empirical values are used for the nondiagonal elements of the matrix for the two-electron Coulomb integrals. The methyl group in thymine is allowed for via an induction model. Theoretical values are given for the energies of the first four electronic transitions for each base. The locations of these agree well with spectroscopic data. The energies of the first two transitions in adenine are found to be very similar when allowance is made for configuration interaction, so the first absorption band of adenine arises from overlap of two transitions.     

Electronic aspects of the photodimerization of pyrimidine bases and their derivatives

Results of calculations for the lower excited triplet and singlet states of pyrimidine bases and a number of their derivatives, calculated by the self-consistent field (SCF) method for open shells and by the configuration interaction (CI) method for self-consistent orbitals, are presented. It is shown that excitation to the T₁ and S₁ states primarily affects the C₅—C₆ bond while triplet excitation is almost completely localized on this bond. This leads to exceptionally pronounced weakening of the C₅—C₆ bond in the T₁ state. All other conditions being equal, the conditions in the T₁ state are more favorable for the dimerization of the bases than in the S₁ state. A correlation between the degree of localization of the triplet excitation at the C₅—C₆ bond and the ease of photodimerization has been established. The experimental facts associated with the formation of thymine free radicals in DNA are explained.The authors thank G. G. Dyadyusha and O. V. Shramko for discussing our results. The program for calculation of the bond orders by the CI method was kindly given to us by G. M. Kagan and G. I. Kagan.