No abstract

Important note To cite this publication, please use the final published version (if applicable).Please check the document version above.

Significant influences of sulfur and aza substitution on excited-state dynamics in thymine analogues, 6-aza-2-thiothymine (ATT) and 2-thiothymine (2TT), were intensively studied by means of nanosecond transient absorption and time-resolved luminescence spectroscopy. Transient absorption spectral measurements gave distinct spectral features and sufficiently longer lifetimes for both molecules attributable to the T(1) (pi pi*) state under Ar-saturated condition. Additionally, another long-lived subsequent transient was also observed for ATT, which suggests hydrogen abstraction from a ground-state molecule itself takes place only for triplet ATT. Quantum yields for intersystem crossing (Phi(ISC)) were determined to be unity as well as other pyrimidine analogues we reported previously, and efficient photosensitized singlet oxygen O(2)* ((1)Delta(g)) formation was also observed in the presence of dissolved molecular oxygen with a quantum yield (Phi(Delta)) of 0.69 +/- 0.02 for ATT whereas it almost halved for 2TT. These findings have shown the combination of subtle substitutions can possibly control photophysical or photochemical properties such as O(2)* ((1)Delta(g)) formation or reactivity in excited states in addition to the substantial intersystem crossing caused by replacing oxygen O2 in thymine by sulfur (referred to S2 substitution henceforward). The experimental results were corroborated by quantum chemical calculation at the B3LYP/6-31+G(d,p)/PCM level.

Excited state characteristics of aza analogues of nucleic acid bases, 8-azaadenine (8AA), 5-azacytosine (5AC), 8-azaguanine (8AG), and 6-azauracil (6AU), in acetonitrile solution were comprehensively investigated with steady state absorption and emission spectra, transient absorption measurements, emission measurements for the singlet oxygen molecule, and time-dependent density functional theory (TD-DFT) calculations. The triplet-triplet absorption spectrum of 8AA whose peak was 455 nm was observed for the first time. Sensitized singlet oxygen formation of 8AA was also observed in O(2)-saturated acetonitrile with quantum yields of 0.15 +/- 0.02. It was concluded that there were two kinds of aza analogues of nucleic acid bases: type A had substantial quantum yield for the intersystem crossing and potential of O2 (1Delta(g)) formation (8AA and 6AU), and type B did not (5AC and 8AG). TD-DFT calculations indicated that type A molecules had a dark 1npi* state below the first allowed 1pipi* state, while both S1 and S2 states for type B molecules had a pipi* character. It strongly suggested that the dark 1npi* state below the 1pipi* state would play an important role in the ISC process of aza analogues of nucleic acid bases.