Triplet Formation of 4-Thiothymidine and Its Photosensitization to Oxygen Studied by Time-Resolved Thermal Lensing Technique

Abstract

Excited-state dynamics of 4-thiothymidine (S4-TdR) and its photosensitization to molecular oxygen in solution with UVA irradiation were investigated. Absorption and emission spectra measurements revealed that UVA photolysis of S4-TdR gives rise to a population of T1(pipi*), following S2(pipi*) --> S1(npi*) internal conversion. In transient absorption measurement, the 355 nm laser photolysis gave broad absorption (380-600 nm) bands of triplet S4-TdR. The time-resolved thermal lensing (TRTL) signal of S4-TdR containing the thermal component due to decay of triplet S4-TdR was clearly observed by the 355 nm laser excitation. The quantum yield for S1 --> T1 intersystem crossing was estimated to be unity by a triplet quenching experiment with potassium iodide. In the presence of molecular oxygen, the photosensitization from triplet S4-TdR gave rise to singlet oxygen O2 (1Deltag) with a quantum yield of 0.50. Therapeutic implications of such singlet oxygen formation are discussed.

Photochemistry of Nucleic Acid Bases and Their Thio- and Aza-Analogues in Solution

“…S 1 (nπ*) electronic transitions, respectively [10,18,19,34,200,201]. These assignments are supported by quantum chemical calculations [11,17,18,196,[202][203][204]. Additional absorption bands are observed closer to 200 nm (see, for example, Fig.…”

“…The absorption spectra shift from the UVC region in the nucleic acid monomers out to the UVA region in the thiobases. Furthermore, sulfur substitution inhibits ultrafast internal conversion to the ground state and dramatically increases the rate of intersystem crossing to the triplet manifold by enhanced spin-orbit coupling [11,17,18,27,33], which leads to near-unity triplet yields in these nucleic acid base analogues [11,16,18,22,31,144,195,196].…”

“…Suzuki and co-workers [11] investigated the excited-state dynamics of 4-thiodeoxythymidine by using a combination of time-dependent density functional theory (TD-DFT) calculations, time-resolved thermal lensing, and nanosecond transient absorption spectroscopy. Excitation was performed using 6-ns laser pulses at 355 nm.…”

“…A triplet quantum yield of 1.0 AE 0.1 was reported in acetonitrile. Based on vertical TD-DFT calculations, the authors proposed a kinetic model in which internal conversion from the S 2 (ππ*) to the S 1 (nπ*) state is followed by intersystem crossing to the T 1 (ππ*) state with a quantum yield of unity [11]. However, the authors were unable to corroborate the proposed mechanism because of the nanosecond time resolution of their experimental setup.…”

Pulse train fluorescence technique for measuring triplet state dynamics

“…The signal generated by the heatrelease pathway is characterized by lifetimes in the range from 10 ns to 20 μs. The TL is based on a two-color pump-probe scheme to provide information on the dynamics of molecules in the excited-state by monitoring the thermal lens signal [14,15]. Both PAC and TL are especially powerful for the investigation of nonfluorescent molecules and do not distinguish if the origin of nonradioactive processes is from singlet or triplet states.…”